Mixed-matrix membranes with molecular recognition windows for selective helium extraction from natural gas

Domain: chemistry
Published: 2026-02-19
Editorial outcome: major_revision

Abstract
Mixed-matrix membranes (MMMs) with high chain packing density by incorporating soluble macrocycle compounds represent a promising class of materials for gas separation. However, achieving the ultra-high selectivity (He/CH4 > 1000) for helium extraction from natural gas with ultra-low helium content remains a formidable challenge, especially for Matrimid membranes, which are commercially available but exhibit relatively low permeability and moderate selectivity. Herein, the cyclic Cyclen with specific intra-ring dimensions was incorporated into Matrimid as a pore-structure modifier to enhance the He/CH4 selectivity. The strong hydrogen bonding interactions between Cyclen and Matrimid chains induced a denser chain stacking and modulation of the interchain gap structures, which enables rapid mass transfer of small He gas molecules while hindering the diffusion of large CH4 gas molecules across the membrane, thereby significantly enhanced He/CH4 molecular sieving capacity. Molecular dynamics simulations indicate that the MMMs prepared using Cyclen as a filler exhibited tunable microporous and more efficient He transport channels. Notably, the He/CH4 selectivity reached up to an impressive value of 6788 after physical aging for 110 days, which outperformed almost all reported polymer-based membranes and was even comparable to that of some advanced carbon molecular sieve membranes.

Introduction
Helium (He) is an extremely scarce, non-renewable gas with a robust safety profile, making it an essential resource for the development of numerous high-tech industries, including aerospace, electronics, medical equipment, and scientific research1–3. The rapid expansion of these industries has driven the global consumption of helium to increase at an annual rate of 5–7%4. On Earth, He is primarily found in the atmosphere and in natural gas. Although the atmosphere contains a substantial amount of He, its ultra-low concentration (less than 5 ppm) makes extraction from the air extremely difficult5–7. By contrast, natural gas serves as the richest and most accessible source, despite the initial concentration of He in most natural gas reservoirs is still relatively low (around 0.3%), which also places a great challenge on the He extraction technology8–11. Currently, the most widely used and large-scale industrial He separation and purification methods are based on energy-intensive processes such as cryogenic distillation and pressure swing adsorption12. To reduce the overall energy consumption and enhance the separation efficiency, membrane technology has emerged as an effective alternative for He enrichment13,14. Membrane-based separation technologies are particularly attractive due to their low energy consumption, high efficiency, and operational simplicity in the field of He recovery from natural gas15–19.

The conventional polymer membranes for gas separation have favorable processability, high stability and low cost20. However, their gas transport performance is governed by an inherent trade-off relationship between permeability and selectivity, resulting in most commercially available polymer membranes having low permeability and moderate selectivity21–23. To extract helium from natural gas with ultra-low He content, the gas separation membranes must possess He/CH4 selectivity greater than 1000, a critical threshold for industrial applications6,24. Unfortunately, few polymer membranes meet this stringent requirement25. Membranes based on molecular sieve materials can easily overcome Robeson’s upper limit by relying on their ability to distinguish molecules based on size and shape, but these materials often suffer from poor mechanical properties26. Mixed matrix membranes (MMMs), which incorporate nanofillers with certain pore sizes into a processable polymer matrix, effectively improve the gas separation efficiency by integrating the advantages of both components27–29. Nonetheless, the disparity in physicochemical properties between filler and polymer often lead to poor interfacial compatibility, resulting in the formation of non-selective regions at the interface that ultimately reduce membrane selectivity30–32. Enhancing filler-polymer interactions can improve the interfacial compatibility of mixed matrix membranes. Common methods used in current research include filler surface functionalization30, morphology regulation33,34, in-situ growth35, solid-solvent processing approach36, ligand replacement37,38, ionic cross-linking39, covalent cross-linking40, and intermolecular force regulation41. However, most nanofillers show poor dispersibility in organic solvents, and the formation of nanofiller clusters in the polymer matrix will reduce the gas transport properties42. Even though the dispersion of nanofillers can be enhanced by surface functionalization43,44, it is still a great challenge to achieve acceptable selectivity for helium extraction from natural gas.

High-permeability polymers fail to provide the necessary selectivity for high purity He extraction. In this regard, Matrimid was selected as the polymer matrix for its dense stacking of polymer chains. Most of the Matrimid-based mixed matrix membranes used for He separation select porous materials as nanofillers (e.g., Cu-BTC, Cu-BDC, CNFs, C60, etc.)45–49, which provide special pores that only allow He to pass through, thereby enhancing its diffusion rate. In addition, these nanofillers provide moderate resistance to the mass transfer of larger gas molecules, thereby improving the molecular sieve nature of the membrane. However, the poor compatibility between these fillers and Matrimid induces an undesirable effect on membrane separation performance, making it difficult to meet the required separation capacity for He purification.

Macrocycles with specific nanostructures are organic molecules that can be well dispersed in organic solvents50. The functional groups of macrocycles can form strong intermolecular forces with the polymers, thereby improving their interfacial compatibility with the polymer matrix and minimizing their self-interaction tendency in the mixed solution51,52. In addition, the strong interactions between the macrocycle and the polymer chains tighten the polymer packing closer together and shrink the d-spacing values of polymer chains, which significantly improves the gas separation performance of the membranes52–54. Cyclen, an organic macrocyclic compound with flexible structure, has a cavity size of ~3.2–3.4 Å55,56. The abundant secondary amine groups in the Cyclen structure provide enough binding sites to modulate the membrane structure through the formation of chemical bonds or strong intermolecular forces, which is commonly used in the construction of molecule transport channels to enhance membrane densification and reduce interfacial defects by regulating the free volume of the membrane57–59.

In this study, we prepared He separation membranes with ultra-high permselectivity by incorporating a molecular recognition window (Cyclen) into Matrimid. Cyclen connected with Matrimid chains by hydrogen bonding (Fig. S1), which adjusted the submicroporous structure of the MMM membrane, tightened the chain spacing, and increased the transmembrane mass transfer resistance for large gases. At the same time, the Cyclen with a certain cavity size creates a special transport channel for small gases, increasing their permeability and counteracting the negative effect of increased chain stacking density (Fig. 1). In addition, the membrane showed exceptional performance after physical aging, exceeding that of almost all mixed matrix membranes.

Results
Characterizations of Matrimid-Cyclen membranes
The optical photograph showed that all Matrimid-Cyclen membranes are yellow and highly transparent (Fig. 2a). The cross-sectional microstructures of Matrimid-Cyclen membranes by the field emission scanning electron microscopy (FESEM) are shown in Fig. 2b. With the increase of Cyclen content, the vein structures gradually appeared and became dense, which indicates a continuous enhancement of the hydrogen-bonding interactions between the Matrimid chains and Cyclen. At low Cyclen loading (Cyclen loading ≤ 5%), the cross sections of Matrimid-Cyclen membranes demonstrated dense and smooth morphologies. With the gradual increase of Cyclen loading (10–15% Cyclen loading), more hydrogen bonding between Cyclen and Matrimid chains leads to the formation of more prominent vein structures in the membrane. All the Matrimid-Cyclen membranes display homogeneous cross section and surface at 50,000× electron magnification, without any significant aggregation or discrete particulate phases observed (Fig. 2c and Fig. S2), confirming the presence of only a continuous single phase within these membranes. Additionally, energy dispersive X-ray spectroscopy (EDS) mapping of the cross-sections of the matrimid-Cyclen membranes further verifies the uniform distribution of Cyclen as evidenced by the detection of characteristic nitrogen (N) signals (Fig. 2d).

The X-ray diffraction (XRD) patterns of pure Matrimid membrane, MMMs, and Cyclen are shown in Fig. 3a and Fig. S3. The pristine Matrimid membrane exhibit two broad amorphous diffraction peaks at ~15 and 22°, corresponding to d-spacings of 5.93 Å and 4.09 Å, respectively. These peaks indicate the existence of two major types of interchain gaps in Matrimid membranes. When Cyclen was incorporated at low loading (1–5% Cyclen loading), only two broad amorphous diffraction peaks were observed for the Matrimid-Cyclen membranes, while the crystallinity of Cyclen completely disappeared upon mixing with the polymers, which suggests that homogeneous mixture at the molecular level was achieved between Cyclen and Matrimid. The peak at 15° in Matrimid membrane corresponding to a d-spacing value of 5.93 Å and the broad peak at 22° corresponding to a d-spacing of 4.09 Å. At low loadings (1–5% Cyclen loading), these peaks shift to lower angles in the MMMs. Specifically, the d-spacing value of the broad peaks at 22° and 15° decreases from 4.09 to 3.96 Å and 5.93 to 5.54 Å, respectively. This shift is attributed to the hydrogen bonding interactions between Cyclen and the regularly arranged Matrimid chains reduces interchain spacing and densifies the overall packing, which is essential for improving the sieving ability of the membrane (Fig. 3c). However, two bigger interchain regions tend to undergo distinct expansion with further increasing Cyclen content (10–15% Cyclen mass loading), probably because an excessive number of Cyclen molecules are accommodated within the microporous region. In addition, a new diffraction peak attributed to Cyclen was observed, the peaks of Cyclen are evident in those of the MMMs without any peak shifts, indicating that the crystalline structure of Cyclen was maintained inside Matrimid, and that penetration of the Matrimid chains could be neglected.

The strong interaction between the filler and polymer often leads to peculiar changes in the Fourier-transform infrared-attenuated total reflectance (ATR-FTIR) profile, thus ATR-FTIR analysis of the Matrimid membrane, Matrimid-Cyclen membranes, and Cyclen were conducted. As shown in Fig. 3b and Fig. S4, the characteristic absorbance bands at 1775 cm−1 and 1717 cm−1 correspond to the symmetric and asymmetric stretching vibrations of the 5-membered imide ring carbonyl groups, whereas the band at 1670 cm−1 corresponds to the symmetric stretching vibration of benzophenone carbonyl. Compared with the Matrimid membrane, the characteristic C-C=O peak at 1670 cm−1 of all Matrimid-Cyclen membranes shows a red shift as the Cyclen loading increased, which was attributed to the formation of hydrogen bonding interaction between Matrimid and Cyclen. It is speculated that the preferred binding site for the -NH of Cyclen is the benzophenone carbonyl on the Matrimid backbone. After the formation of MMMs, the characteristic peaks attributed to the imine N-H stretching vibrations in the 3080–3340 cm−1 range of the Cyclen spectra completely disappeared (Fig. 3b and Fig. S4), which confirmed the formation of dense hydrogen bonding in the Matrimid-Cyclen membranes. The dense hydrogen bonding between Cyclen and Matrimid enhances their interfacial compatibility, reduces non-selective interfacial gaps and significantly improves the high gas separation performance of the MMMs21,32.

Figure 3d presents the thermogravimetric analysis (TGA) of Matrimid and Matrimid-Cyclen membranes. Cyclen begins to degrade at temperatures slightly above 100 °C (Fig. S5). The initial weight loss observed from 100 to 400 °C may be attributed to the evaporation of the residual organic solvent and the degradation of Cyclen within the MMMs. The weight loss gradually increases with increasing Cyclen mass loading. The second weight loss occurred between 400 and 550 °C, at which point the degradation of lateral methyl groups from Matrimid begins. The third stage is caused by the decomposition of the polymer main chains, which begins from ~550 °C. In general, the interactions between the polymer matrix and the filler restrict the thermal movement of the polymer chains, resulting in an increase in the energy required for chain movement and decomposition, thereby enhancing the thermal stability of these membranes. The FTIR analysis indicates that hydrogen bonding between Matrimid and Cyclen increased with the increase of Cyclen mass loading, the thermal stability of Matrimid-Cyclen membranes should be improved. However, this is not the case, as the decomposition temperature of Cyclen is significantly lower than that of Matrimid (Fig. S5). The decomposition of Cyclen creates micropores within the Matrimid-Cyclen membranes, which provide additional space for polymer chain movement and reduce the energy required for the decomposition of the polymer chains, ultimately decreasing the overall thermal stability of the membrane.

To further investigate the intermolecular forces in the membrane, the chemical composition and elemental states were analyzed using X-ray photoelectron spectroscopy (XPS). The full XPS survey spectra (Fig. 3e) reveal that the nitrogen (N) signal intensity progressively increases with Cyclen loading. The appearing C=O···H and N-H···O signals in the O 1s and N 1s XPS spectra of the Matrimid-Cyclen membrane as compared to the pristine Matrimid membrane provided evidence for the formation of hydrogen bonds (Fig. 3f and Fig. S6). In addition, the extent of hydrogen bonding reactions between secondary amine groups in Cyclen and the carbonyl in Matrimid backbone quantified by XPS results is found to increase from 5.96 to 15.37% as the Cyclen mass loading increases from 1 to 15% (Table S1), which further corroborates the existence of dense hydrogen bonding between Cyclen molecule and Matrimid chains. These findings are consistent with previous characterization results and the hydrogen bonding interactions between the N-H groups of Cyclen and the carbonyl (C=O) groups of Matrimid play a pivotal role in governing polymer chain packing. Dynamic mechanical thermal analysis (DMTA) was conducted for Matrimid-Cyclen samples to evaluate their mechanical property within a temperature range of 50–350 °C. As shown in Fig. 3g–i, the storage modulus (E′) (1800 MPa) of these membranes almost completely maintained before reaching the corresponding glass transition temperature (Tg). Actually, the higher Cyclen loadings resulted in more brittle membranes. The effect of Cyclen content on the peak intensity of damping (Tan δ) is shown in Fig. 3i. The peak intensity of Tan δ (damping value) decreases as Cyclen content increases. These results demonstrated that the Matrimid-Cyclen membranes exhibit both good mechanical properties and thermal stability, making them promising candidates for gas separation applications.

Gas separation performance of Matrimid-Cyclen membranes
To investigate the gas transport properties of Matrimid and Matrimid-Cyclen membranes, gas permeation tests for He (2.60 Å), H2 (2.89 Å), CO2 (3.3 Å), N2 (3.64 Å) and CH4 (3.80 Å) were performed using a constant-volume, variable-pressure system at 25 °C under a feed pressure of 1.0 bar (Fig. S7). The gas permeability and selectivity data were summarized in Table S2. As shown in Fig. 4a, the gas permeability was found to decrease with increasing gas kinetic diameter, and a distinct cut-off effect of the gas permeability was observed between CO2 and N2, demonstrating the presence of distinct gas molecular sieving characteristics in the Matrimid-Cyclen membranes. The incorporation of Cyclen enhances the molecular sieving effect, showing significant separation potential for He, H2, and CO2. The uniform pore size (3.2–3.4 Å) of Cyclen provides dedicated transport channel for small molecule gases (i.e., He, H2 and CO2), facilitating their selective transport across the membrane, while the large molecule gases (N2 and CH4) exhibit decreased gas permeability due to the tightening of polymer chain spacing.

The gas separation performance of the prepared Matrimid-Cyclen membranes was evaluated for several important gas pairs (He/N2, He/CH4, H2/N2, H2/CH4, CO2/N2 and CO2/CH4) as a function of filler concentration (Fig. 4c–e). With increasing Cyclen loading from 0 to 5 wt%, the He permeability increased from 34 to 66 Barrer, the H2 permeability increased from 27 to 62 Barrer, and the CO2 permeability increased from 15 to 20 Barrer, corresponding to increases of ~98%, 127% and 34%, respectively. In contrast, the permeability of larger gas molecules (e.g., N2 and CH4) steadily decreased. The inconsistent permeability changes of these gases make the membranes exhibit attractive ultra-high gas selectivity, the selectivities of He/N2 and He/CH4 are 8 and 19 times that of the pure membranes, respectively. The improved separation performance of Matrimid-Cyclen membranes indicated a reduction in interchain gaps with a significantly enhanced size-sieving effect, which is consistent with the XRD findings. XRD analysis shows that the increasing hydrogen bonding interactions between Cyclen and Matrimid will reduce the polymer chain spacing as the Cyclen content increases from 0% to 5%, which might typically result in a decrease in permeability for all gases. In fact, the permeabilities of He, H2 and CO2 continue to increase as the Cyclen content increases. This can be attributed to the molecular sieve effect of Cyclen, which selectively facilitates the passage of gases with molecular diameters smaller than the Cyclen pore size while hindering the transport of larger molecules such as N2 and CH4 to pass through more devious diffusion transport pathways. Therefore, higher Cyclen content leads to the formation of more efficient molecular sieving channels, enhancing the permeability of small gases (e.g., He, H2 and CO2). The strong hydrogen bonding between Cyclen and Matrimid tightens the d-spacing of Matrimid polymer chains and reduces the chance of defects in the membrane, resulting in longer and more tortuous transport pathways for larger gas molecules (e.g., N2 and CH4), thereby decreasing their permeability (Fig. 4b). This also increases the selectivity of multiple important gas pairs. The Matrimid-Cyclen-5% membrane exhibits a He permeability of 66 Barrer, and the separation selectivities of He/N2 and He/CH4 were 664 and 1660, respectively. These results are very attractive for industrial applications, as improving selectivity is more critical than enhancing permeability for obtaining high-purity He. With Cyclen content increased from 5 to 15%, all the gas permeabilities increased, the He/CH4 selectivity decreased from 1660 to 661, and the He/N2 separation selectivity decreased from 664 to 358. This decline in selectivity is primarily due to the formation of non-selective regions at high filler loading. Although more Cyclen molecules can form more selective gas transport channels during the process of membrane formation, excessive Cyclen concentration uniformly dispersed in the polymer chains can also cause an increase in polymer chain spacing, leading to the formation of non-selective regions larger than the kinetic diameters of N2 and CH4, which diminishes the molecular sieving capacity, reduces the overall selectivity of the membrane. At the same time, the formation of non-selective regions with larger sizes increases the permeability of all gases.

Effects of operating temperatures, feed pressures and aging behavior
As shown in Fig. 4f and Fig. S8, the effect of operating temperature on the separation performance of the Matrimid-Cyclen-5% membrane at 1 bar for He/N2 and He/CH4 was investigated (Table S3). As the operating temperature increased from 0 °C to 100 °C, the permeability of He, N2 and CH4 increased, while the selectivity of He/N2 and He/CH4 decreased. This could be attributed to the intense thermal motion movement of polymer chains at higher temperatures, which increased the percentages of larger size transient gaps in the membrane. N2 and CH4 molecules benefit more from increased percentage of larger size transient gaps than He molecules with smaller kinetic diameters, and the increased permeability of N2 and CH4 molecules is more dramatic, weakening the membrane’s selectivity for gas pairs. The selectivity of He/N2 and He/CH4 decreased rapidly as the operating temperature increased from 40 to 60 °C. This can be explained by the accelerated movement frequency of polymer chains at high temperature, rendering fast transport of gas molecules through the matrix, the permeability of larger molecules (N2 and CH4) increases more than that of He, weakening the molecular sieving effect of the membrane. After cooling from 20 to 0 °C, the He permeability decreased from 63 to 53 Barrer, while the He/N2 selectivity increased from 631 to 7534, and the He/CH4 selectivity increased from 2104 to 17580, corresponding to 12-fold and 9-fold increases, respectively. The ultra-high selectivity of the Matrimid-Cyclen-5% membrane at 0 °C for He/N2 and He/CH4 facilitates the extraction of high-purity He from natural gas at low temperatures. In industrial applications, improving gas purity is often more challenging than enhancing transport efficiency, especially for He, which is critical for industries such as aerospace, electronics, medical equipment and scientific research. Notably, the gas separation performance of the membranes fully recovered as the temperature changed from 100 °C and 0 °C to 20 °C, indicating that the observed temperature-dependent performance changes are related to the characteristics of the membrane rather than any structure damage.

To explore the practical applicability of the Matrimid-Cyclen membranes at different feed pressures, the gas separation performance of the Matrimid-Cyclen-5% membrane was evaluated at pressures up to 30 bar, and the data are summarized in Table S4. As presented in Fig. 4g, as the pressure increases from 1 bar to 10 bar, the permeability of He decreases slightly, and the pressure dependence of N2 and CH4 was almost unrecognizable, with only small fluctuations in the selectivity for He/N2 and He/CH4. These observations suggest that the membranes remain structurally stable under moderate pressure increases. This may be attributed to the presence of Cyclen molecules, which increase the packing density of the polymer chains, thereby reducing their mobility, increasing chain stiffness and improving the high-pressure resistance of the membranes. As the operating pressure increases from 10 bar to 20 bar, the permeability of all gases rises, and the selectivity for He/N2 and He/CH4 decreases. This behavior is typical of glassy polymers, such as Matrimid, under plasticization pressure, where increased feed pressure leads to higher polymer segment mobility and larger chain spacing, resulting in higher permeability and lower selectivity. This phenomenon aligns with the dual-mode sorption theory. As the pressure increases from 20 bar to 30 bar, the permeability of He remains almost constant, while the permeability of N2 and CH4 gradually decreases, and the selectivity of He/N2 and He/CH4 tends to increase, which is ascribed to the existence of unrelaxed volume within the polymer chain structure. An increase in feed pressure results in greater compactness of the polymer matrix, which reduces the gas permeability in the membrane by reducing fractional free volume.

The membrane was stored at room temperature without any protective gases, and its performance was periodically tested to evaluate the physical aging performance (Table S5). As shown in Fig. 4h, after 110 days of aging, the He permeability of the Matrimid-Cyclen-5% membrane decreased from 66 Barrer to 54 Barrer, a decrease of 18%. However, the He/N2 selectivity increased from 664 to 1086, and the He/CH4 selectivity increased from 1660 to 6788, corresponding to increases of 64% and 310%, respectively. After physical aging for 480 days, the He permeability of Matrimid-Cyclen-5% membrane decreased from 66 Barrer to 43 Barrer, a decrease of 36%. The selectivity showed an order of magnitude improvement due to the fact that the polymer chain segments in the non-equilibrium state within the membrane gradually adjust to an equilibrium state by optimizing the chain stacking during the physical aging process. The shrunk fractional free volume reduces the gas permeability to a certain extent, while greatly improves the molecular sieving capacity of the membrane. The XRD pattern of Matrimid-Cyclen-5% membrane showed a significant decrease in chain spacing after aging 480 days, further demonstrating that the adjustment of chain stacking during the aging process has improved the molecular sieving capacity of the membrane (Fig. S9).

Figure 5 compares the He/N2, He/CH4, H2/N2, H2/CH4, CO2/N2 and CO2/CH4 separation performance of the Matrimid-Cyclen membranes with those of the Matrimid-based membranes and other membranes, and the data are presented in Tables S6–S8. It is observed that the Matrimid-Cyclen-5% membranes exhibited optimized comprehensive separation performance, with He permeability of 66 Barrer, He/N2 and He/CH4 selectivities up to 664 and 1660, respectively (Fig. 5a, b). The Matrimid-Cyclen membranes exhibit ultra-high He selectivity compared to the other membranes reported in the literature, which breaks the 2008 upper bound limit. After physical aging for 110 days, the separation selectivities of He/N2 and He/CH4 reach up to 1086 and 6788, an increase of 18-fold and 110-fold over pure Matrimid membrane, respectively. The selectivities of Matrimid-Cyclen membrane even exceeded those of some carbon molecular sieve (CMS) membranes, which is one of the highest values reported for He gas separation membrane (Fig. S10). Owing to the strong hydrogen bonding between Cyclen and Matrimid, the Matrimid polymer chains aligned more tightly, thus achieving significantly higher He/N2 and He/CH4 selectivities. Other notable gas pairs including H2/N2, H2/CH4, CO2/N2 and CO2/CH4 also show considerably high gas selectivities at an optimum Cyclen loading of 5% (Fig. 5c–f). The gas separation performance far exceeds the latest gas separation upper bound line. Figure 5g–h show the improvement in both He permeability and He/CH4 selectivity of MMMs formed in Matrimid with four different fillers, the data are summarized in Table S9. The increase in He/CH4 selectivity of MMMs prepared with Cyclen and Cyclodecane, which have the same intra-ring dimensions as fillers is 427% and -18%, respectively, as compared to the pristine Matrimid membrane. This demonstrates that tightening the polymer chain stacking by hydrogen bonding can effectively enhance the separation performance of MMMs. Compared to Piperazine without an annular cavity, the He permeability of MMMs relative to the Matrimid membrane increased by 143 and 108%, respectively. This indicates that the piperazine without macrocyclic cavities also can enhance He/CH4 selectivity by narrowing the polymer interchain spacing via hydrogen bonding. Compared to Hexacyclen with a larger cavity size, the increase in He/CH4 selectivity of MMMs over the Matrimid membrane was 428% and 210%, respectively. This indicates that the filler with suitable cavity size can improve the gas transport efficiency and separation performance of MMMs. The three sets of comparison experiments confirmed the hydrogen bonding interaction between Cyclen and Matrimid, and the suitable gas molecule recognition window size of Cyclen as the reason for improving the molecular sieving performance of the membranes, respectively. The possible gas-transport mechanisms of Matrimid-Cyclen membranes are presented in Fig. 5i. The strong hydrogen bonding between Cyclen and Matrimid chains reduces the d-spacing of the chains, and the synergistic interaction with Cyclen with a suitable inner cavity size (3.2–3.4 Å) increases the transport resistance of N2 and CH4, which significantly improves the selectivity of Matrimid-Cyclen membranes.

Molecular dynamics simulations were performed to determine the mechanism of gas separation in the membranes. The membrane structure model is generated from MD simulation based on PACKMOL software (Fig. 6a). A comparison of molecular modeling for the pristine Matrimid membrane and Matrimid-Cyclen membranes reveals efficient chain packing with increasing of Cyclen loading. This hydrogen bond-induced tightening effect aligns with the simulation for free volume and density (Table S10). He and N2 were used as the probe gases for the membrane model to analyze the gas accessible volume, the average gas transmembrane path and the inter-connectivity of void space. Figure 6b, c shows the accessible volumes of He and N2 within Matrimid-Cyclen-n% membranes (n = 0, 5, 10). It was found that some channels allow He transport but block N2. As the Cyclen content increased in the membrane, the difference in accessible volume for He and N2 significantly increased and then slightly decreased (Fig. 6e). This suggests that the presence of Cyclen enables the membrane to accommodate more He and less N2 than the pristine Matrimid membrane, thus greatly improving the molecular sieving ability of the membrane. Figure 6d and Fig. S11 present the connectivity of He with N2-passable void space within Matrimid-Cyclen-n% membranes (n = 0, 5, 10). The increase in Cyclen content within the membranes resulted in a continuous enhancement in the connectivity of the He passable pore space within the membranes, and the average He transmembrane path calculated on the basis of the He-passable pore space increased from 635 Å to 922 Å (Fig. 6f), thus significantly improving the He permeability. In addition, the average transmembrane path length difference between He and N2 increased from 23 Å to 331 Å and then decreased to 322 Å, indicating that the presence of Cyclen could create a special transport channel for He, increasing its transport paths and transmission path length in the membrane, effectively enhancing the transport capacity of He and the separation of gas pairs in the membrane. To gain further insight into microstructures, the pore size distribution of membranes was characterized using H2 adsorption experiments combined with molecular dynamic simulation. As shown in Fig. 6g and Fig. S12, the micropore sizes in Matrimid membrane is mainly located at 6.9 Å, accompanied by a small proportion of ultra-micropores with a size of 3.9 Å. In contrast, the Matrimid-Cyclen membranes show smaller micropore sizes at around 6.3 and 6.5 Å, respectively. The Matrimid-Cyclen membranes exhibited significantly reduced micropore sizes, indicating that hydrogen bonding interaction between Cyclen and Matrimid effectively modulate the membrane pore structure. This pore size modulation enhances the molecular sieving properties of Matrimid-Cyclen membranes. The content of ultramicropores with a size of about 3.9 Å in Matrimid-Cyclen membranes gradually increased with increasing Cyclen content. Many studies have demonstrated that an increase in the content of ultra-microporous pores in the membrane often significantly enhances the membrane’s gas molecular sieving ability. Therefore, as the Cyclen content increases, the membrane’s selectivity for He/CH4 significantly enhanced. Furthermore, the ultra-microporous pores create selective diffusion pathways that preferentially promote the transport of small gas molecules through the membrane. Consequently, the permeability of small gases gradually increases with increasing Cyclen content. The addition of Cyclen resulted in the appearance of new pores in the membrane with a size of about 3.2 Å, which agrees well with the theoretical value of the molecular window of Cyclen55,56. This indicates that Cyclen creates ultra-micropores with the ability to recognize gas molecules, further enhancing molecular sieving properties of the membranes. Their presence creates more and longer transport channels for He, which significantly improves the transport capacity of He. This gives the membrane satisfactory sieving properties while obtaining good permeability.

Thin-film composite (TFC) membranes, composed of a highly selective thin layer supported by a mechanically robust porous support, have been employed for practical gas separation60,61. Matrimid-Cyclen TFC membranes were fabricated on AAO supports using a scalable bar-coating method62. Scanning electron microscopy (SEM) images (Fig. 7a, b) showed no significant aggregation or particulate formation. As the membrane thickness increased, the membrane structure evolved to become denser and smoother (Fig. 7a). For the calculation of gas permeance, the thickness of the Matrimid-Cyclen TFC membrane’s selective layer was determined from cross-sectional SEM images, with values of ~110 nm, 1.8 µm, and 5.6 µm. Figure 7c illustrates the gas permeance and selectivity of the Matrimid-Cyclen TFC membranes with average thicknesses ranging from 110 nm to 5.6 µm (Table S11). The He permeance decreased and the He/CH4 selectivity increased as the membrane thickness increased. Reproducibility was confirmed by testing six randomly selected Matrimid-Cyclen TFC membranes with an average thickness of 1.8 µm, which exhibited consistent He permeance and He/CH4 selectivity (Fig. S13). The average He permeance was 102 ± 6 GPU, with a corresponding He/CH4 selectivity of 728 ± 59. Figure 7d illustrates the He/CH4 selectivity and He permeance for Matrimid-Cyclen TFC membranes and other types of membranes reported in the literature (please see the detailed comparison in Table S12). Although the membrane’s gas permeance is not as high as that of some polymer membranes, Matrimid-Cyclen-1.8 µm exhibits higher He/CH4 selectivity than most reported membranes. Moreover, its performance still exceeds the 2008 Robeson upper bound. In addition, the gas permeance of the AAO supports, pure Matrimid TFC membranes (without filler), and Matrimid-Cyclen TFC membranes (with filler) is summarized in Table S13. Notably, the Matrimid-Cyclen TFC membrane exhibits significantly higher He/CH4 selectivity than the pure Matrimid TFC membrane, the critical role of the cyclen filler in enhancing gas selectivity.

Methods
Materials
1,4,7,10-Tetraazacyclododecane (Cyclen, >98%, C8H20N4, 172.27 g/mol), Cyclododecane (Cyclodecane, >99%, C12H24, 168.32 g/mol), and 1, 4, 7, 10, 13, 16-Hexaazacyclooctadecane (Hexacyclen, >98%, C12H30N6, 258.41 g/mol) from Shanghai BeiDi Pharmaceutical Technology Co. were supplied as fine solid powder. Matrimid was supplied by Huntsman Advanced Materials. ReagentPlus®-grade N, N-dimethylformamide (DMF) solvent (≥99.5%) purchased from China National Pharmaceutical Group was used directly for making polymer solutions without further purification. Matrimid 5218 was dried under vacuum at 120 °C for 12 h each time before use. Methanol solvent (≥99.5%) from China National Pharmaceutical Group was directly utilized as received. Purified He gas (99.9%) was supplied by Air Liquide Shanghai Co., H2 gas (99.999%) and CH4 gas (99.9%) were supplied in compressed gas cylinders from Nanjing Nanyuan Gas company with limited liability, CO2 gas (99.9%) was supplied by Nanjing Shangyuan Industrial Gas Plant, and N2 gas (99.999%) was supplied by Nanjing Special Gas Plant.

Fabrication of pristine Matrimid membranes
A raw Matrimid dope solution was prepared by dissolving pure Matrimid powder (0.1 g) in DMF (4.0 g) solvent, stirred for 10 h and sonicated for 2 h. The clear Matrimid dope solution was obtained by filtering the raw Matrimid dope solution through a 1.2 μm PTFE syringe. The clear dope solution was then directly poured into a clean Petri-dish by a syringe and placed in a vacuum oven to heat from room temperature to 120 °C for 8 h to evaporate the solvent. The original membrane, which still contained a small amount of DMF solvent, was peeled off, and a 14 mm diameter circular membrane was cut from the original membrane, which was submerged in methanol for 12 h to remove the residual DMF from the membrane by solvent exchange. Then, the membranes were placed in a Petri-dish and dried in a vacuum oven at 120 °C for 8 h to obtain membrane samples that could be tested directly with a thickness of 20 ± 5 μm.

Fabrication of the Matrimid-Cyclen mixed matrix membranes
A raw Matrimid dope solution was prepared by dissolving 0.1 g pure Matrimid powder in 4.0 g DMF solvent, stirred for 10 h and sonicated for 2 h. The clear Matrimid dope solution was obtained by filtering the raw Matrimid dope solution through a 1.2 μm PTFE syringe. Cyclen powder (1.0, 3.0, 5.0, 10.0 or 15.0 mg) were added to 6.0 g DMF solvent, and the mixture was stirred for 10 h and sonicated for 2 h to obtain clear and transparent solutions.
The clear Matrimid dope solution was added to the Cyclen/DMF solution, stirred for 24 h and sonicated for 1 h to obtain homogeneous Matrimid/Cyclen solutions, which were then injected into a clean Petri-dish by a syringe and then transferred into a vacuum oven at 120 °C for 8 h solvent evaporation (Fig. S14). The residual DMF was similarly removed from the Matrimid-Cyclen membranes by solvent exchange with methanol. The as-fabricated samples were named as Matrimid-Cyclen-n% (n% is the Cyclen mass loading and was varied from 1 to 15 wt%). These Cyclen-incorporated mixed matrix membranes had an average thickness of 25 ± 5 μm.

Fabrication of the Matrimid-Hexacyclen membrane, Matrimid-Piperazine membrane and Matrimid-Cyclodecane membrane
A raw Matrimid dope solution was prepared by dissolving 0.1 g pure Matrimid powder in 4.0 g DMF solvent, stirred for 10 h and sonicated for 2 h. The clear Matrimid dope solution was obtained by filtering the raw Matrimid dope solution through a 1.2 μm PTFE syringe. Hexacyclen/Piperazine/Cyclodecane powder (1.0 mg) were added to 6.0 g DMF solvent, and the mixture was stirred for 10 h and sonicated for 2 h to obtain clear and transparent solutions. The clear Matrimid dope solution was added to the Hexacyclen/Piperazine/Cyclodecane solution, stirred for 24 h and sonicated for 1 h to obtain homogeneous Matrimid/Hexacyclen solutions, which was then injected into a clean Petri-dish by a syringe and was then transferred into the 120 °C vacuum oven at ambient temperature for 8-h solvent evaporation. The residual DMF was similarly removed from the Matrimid-Hexacyclen/Matrimid-Piperazine/Matrimid-Cyclodecane membranes by solvent exchange with methanol. The as-fabricated samples were named as Matrimid-Hexacyclen-1% membrane, Matrimid-Piperazine-1% membrane and Matrimid-Cyclodecane-1% membrane.

Fabrication of TFC membranes
A raw Matrimid dope solution was prepared by dissolving 0.4 g pure Matrimid powder in 16.0 g DMF solvent stirred for 10 h and sonicated for 2 h. The clear Matrimid dope solution was obtained by filtering the raw Matrimid dope solution through a 1.2 μm PTFE syringe. Cyclen powder (20.0 mg) were added to 24.0 g DMF solvent, and the mixture was stirred for 10 h and sonicated for 2 h to obtain clear and transparent solutions. The clear Matrimid dope solution was added to the Cyclen/DMF solution, stirred for 24 h and sonicated for 1 h to obtain homogeneous Matrimid/Cyclen solutions. The solution was then cast onto the anodic aluminum oxide (AAO) support using a bar-coater with a uniform speed and then put into a vacuum oven at 120 °C for 2 h solvent evaporation. The as-fabricated samples were named as Matrimid-Cyclen-n (where n represents the membrane thickness). The membrane thickness was measured using cross-sectional SEM imaging.

Characterizations
The surface and cross-sectional morphologies of the pristine Matrimid and Matrimid-Cyclen-n% membranes were observed by scanning electron microscope (SEM, GeminiSEM 450). The chain packing structure and d-spacing of the Matrimid and Matrimid-Cyclen-n% membranes were examined by a Bruker D8 Advance X-ray diffractometer (XRD) with the X-ray radiation source being Cu Kα (1.54 Å) and the diffraction angles ranged from 10˚ to 25˚ with a scan speed of 5°/min. The average d-spacing was evaluated based on the Bragg’s Law (nλ=2dsinθ), where n is an integer (1, 2, 3), λ represents the wavelength of the X-ray radiation source, d means the average interchain/intersegmental distance between adjacent polymer chains in semi-crystalline regions (d-spacing) and θ denotes the X-ray diffraction angle. Fourier transform infrared spectroscopy (FTIR) (Bruker TENSOR II) was used to analyze the chemical structures of pristine Matrimid and Matrimid-Cyclen-n% membranes through attenuated total reflection (ATR) mode with the spectra over the wavenumber range of 800–4000 cm−1. Thermal stability of the pristine Matrimid and Matrimid-Cyclen-n% membranes was measured by a thermogravimetric analyzer (TGA, Q5000IR) in the temperature range of 25–800 °C with a heating rate of 10 °C/min under a flowing N2 atmosphere (50 mL min−1). X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) equipped with a monochromatic Al Kα X-ray source (hν= 1486.6 eV) of 12 kV and 150 W was used to detect the binding signals of key functionalities and interactions in the membrane samples. The dynamic mechanical thermal analysis (DMTA) was conducted for Matrimid-Cyclen samples (20 mm long, and 5 mm wide) on a Mettler Toledo DMTA at a heating rate of 3 °C/min from 30 to 350 °C with a load frequency of 1 Hz in air. The Brunauer-Emmett-Teller (Quantachrome Autosorb IQ) technique was used to measure the pore structure of the prepared membrane. The membrane has been freeze-dried for 10 h before degassing. Membrane samples were degassed under a high vacuum at 120 °C for 8 h before analysis and Cyclen was degassed under a high vacuum at 80 °C for 8 h before analysis.

Molecular dynamics (MD) simulation
MD simulation was performed using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS23) software together with the pysimm program63. The amorphous 3D membrane models were constructed using the PACKMOL package64. The models were used with a periodic boundary condition and the cut-off radius was set to 12 Å, and the Ewald method was used to describe the long-range interaction force65. Before the MD simulation, the NPT simulation was performed to form a stable system. In detail, the system was first annealed under the NPT (P=101 KPa) ensemble for 20 ps at circulating temperatures ranging from 300 to 500 K. The MD simulation of the system was performed under the NVT ensemble (T=300 K) for 500 ps to obtain the final structures. All initial models were firstly energy-minimized by using steepest descent algorithm, and the temperature and pressure were controlled by Nose-Hoover thermostat and Parrinello-Rahman barostat, respectively. Visualization and image rendering were performed using OVITO software66.

Gas permeation tests of membranes
Gas permeation tests of the membranes were conducted on a variable-pressure constant-volume gas permeation cell. The dense membrane was mounted onto the permeation cell and vacuumed at 25 °C for 12 h before the gas permeation test was carried out. Gas permeability was tested following the order of He, H2, CO2, N2 and CH4 corresponding with the order of gas kinetic diameter (He: 2.60 Å, H2: 2.89 Å, CO2: 3.30 Å, N2: 3.64 Å and CH4: 3.80 Å) from smallest to largest where the pressure was 1 bar. The cell temperature was kept constant at 25 °C. The gas permeability coefficient through the membrane was calculated according to the steady state pressure increment (dp/dt) via the following Eq. (1):

P = (273 × 10^10) * (Vl / (AT * p * 76)) * (dp / dt) (1)

where P denotes the gas permeability coefficient in Barrer (1 Barrer = 1 × 10^−10 cm3 (STP) · cm · cm−2 · s−1 · cmHg−1), V refers to the volume of the downstream reservoir (cm3), A is the effective membrane area (cm2), l represents the membrane thickness (cm), T is the testing temperature (K) and p is defined as the upstream pressure of the system (atm). The permeability tests were repeated at least three times and the average deviation obtained was less than 5%.

The ideal selectivity between two different gases across a polymeric membrane is the ratio of their single gas permeability coefficient as described in the following Eq. (2):

α A/B = P_A / P_B (2)

where P_A and P_B refer to the permeability coefficients of gases A and B, respectively.

For TFC membrane, the gas separation parameters, including gas permeance (J) and separation selectivity, were calculated according to the following Eq. (3):

J = P / l (3)

Where J denotes the gas permeance of the TFC membrane in GPU (1 GPU = 1 × 10^−6 cm3 (STP) ∙ cm−2 ∙ s−1 ∙ cmHg−1), P denotes the gas permeability coefficient in Barrer (1 Barrer = 1 × 10^−10 cm3 (STP) ∙ cm ∙ cm−2 ∙ s−1 ∙ cmHg−1), l represents the thickness of membrane selective-layer (μm).

The gas permeation ideal selectivity (α A/B) is obtained by the following Eq. (4):

α A/B = J_A / J_B (4)

where J_A and J_B refer to the permeance of gases A and B, respectively.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript reports on the combination of cyclen and Matrimid into a mixed matrix membrane that has high selectivity for gas separation. Both cyclen and matrimid have been studied separately, and there exists a lot of literature on both materials. Hence, the novelty being presented is the combination of the two into a membrane. The resulting selectivities are very high, which gives me concern, as generally for mixed matrix membrane systems, selectivity is aligned with either of the membrane or additive, and not an enhancement. I can understand the argument for the strong binding between cyclen and Matrimid, but that does not necessarily translate into the high selectivities reported. There is no reported error for the experimental results and no discussion on reproducibility, so this raises questions about the provided data.\nI am also concerned about the high cyclen loading in the membrane, up to 15%. There is no discussion about mechanical properties in the manuscript, but the literature on Matrimid is very clear that high levels of additives result in a very brittle membrane. I see no reason why this is not the case here. This has issues with the ability to translate the performance into a viable membrane.\nThe operating temperature, feed pressure and aging behaviors follows the expected trend of mixed matrix membranes, with the presented discussion valid.\nThe application for helium extraction is important, and it is good that the authors considered both separation from methane and nitrogen.\nI recommend that the manuscript be rejected, and the authors submit to a more appropriate journal.", "reviewer_2": "• What are the noteworthy results?\nThe work seems to be looking at high selectivity membranes for He/CH4 separation, by altering the properties of a commonly used polyimide in gas separation, Matrimid, adding different concentrations of Cycle, which is presented as a macrocycle (particle, molecule? Please clarify) with a wide opening providing H-bonding with the Matrimid polymer chains. Both the porosity of this filler and the crosslinking thereof seems to be having a significant influence in increasing the selectivity of Matrimid above the state-of-the-art of such membrane materials without decreasing the permeability of the smaller gas molecules.\nAll statements in the scientific paper should be justified, either by results presented in the work, or by references to previous reports in state-of-the-art literature. See for instance lines 193-196, “The ultrahigh gas separation performance of mixed matrix membrane attributed to enhanced interfacial compatibility due to dense hydrogen bonding between Cyclen and Matrimid chains”, before showing any gas separation results.\nThe authors often do not comment on the hypotheses until they are about to introduce the results (see lines 307-311 introducing the effect of operating temperature, pressure and “aging”.\nAlready in the Highlights, why do the authors talk of cyclen as a “window” and not an amine molecule with such and such properties that give the potential to provide the Matrimid polyimide matrix with the desired transport/separation properties in the He/CH4 separation? Please, be more precise.\n• Will the work be of significance to the field and related fields? How does it compare to the established literature? If the work is not original, please provide relevant references.\nGiven the low quantity of He in the feed stream it is reasonable that the authors look at Matrimid because of its low permeability and high selectivity properties and try this hybridization to increase the pore size and therefore the permeability of the Matrimid polymer. The state-of-the-art does very vaguely support this justification, and only reading between lines with prior knowledge of mixed matrix membrane technology and challenges allows the reader to withdraw the novelty of the work. This is probably due to the fact that the bibliographic review of the authors is not mentioned until Figure 5 where the data on Matrimid mixed matrix membranes collected in the supplementary information is used to build the Robeson upper-bound trade-off figures for the different gas pairs considered in this work. Please revise.\n• Does the work support the conclusions and claims, or is additional evidence needed?\nBesides, but no other information on the morphology and porosity characteristics justifying the authors’selection as filler in Matrimid polyimide are presented, so the reader cannot judge for himself the reasons why the crosslinking only appear at loadings above 15%, or the presence or absence of crosslinking with the polymer matrix that leads to higher selectivity and usually decreases permeability, or the “torn lamellar structures” formation, as well as the evidence of the strong H-bonding mentioned in the Results section.\nHave authors envisaged the measuring of BET surface area of Cyclen and the MMM for verification? I guess they have done, and the Cyclen is a sacrificial filler given the statement in page This could visualize the statement of the authors “ in lines 214 – 216, “the decomposition of Cyclen leaves many micropores in the Matrimid-Cyclen membranes”, but this porosity has not been shown in the paper, or is not revealed in the cross-sectional images in SEM pictures. Please clarify, this is important to show evidence on the trade-off between crosslinking and additional porosity effects on the mixed matrix membrane performance observed in the results section.\n• Is the methodology sound? Does the work meet the expected standards in your field?\nThe membrane materials were characterized by the common range of analytical techniques in this kind of work: SEM (surface and cross section), XRD, ATR-FTIR, NMR, TGA, XPS, as well as single gas permeation at room temperature by time-lag experiments at 25ºC. This are adequate in membrane materials characterization. A but lays in the fact that the authors claim to evaluate the effect of temperature and temperature in permeation in the range 25-45ºC and 1-6 bar. These are very narrow in gas permeation membranes. Although it is shown an impact in selectivity in this narrow temperature range, the value of 25ºC cannot be considered as “low temperature” standard (line 327). Likewise, 6 bar is the regular operating pressure in gas separation. Please consult previous works such as that by R. Baker, studying the optimal pressure for gas separation as 5 bar. The pressure of 6 bar in a strong polyimide as Matrimid is not usually sufficient enough to see any observable packing density of the polymer chains (line 336). The authors should give evidence of that, or appropriately check literature on polyimide membrane materials characterization. The same can be said of the effect of aging, which in this work is attributed to the measurements performed with the membranes stored away from the gas permeation plant up to 110 days after the synthesis. Have the membranes been taken out of the module in this intervals? The damage caused by the joints may be more significant that any physical or chemistry change due to a slight variation of the polymer packing. Please revise.\n• Are there any flaws in the data analysis, interpretation and conclusions? Do these prohibit publication or require revision?\nThe commentary, though, seems to be mingling these characterization results since the authors speak of diffraction peaks regarding the absorption bands in the ATR -FTIR spectra (line 191). Please correct. Also, I think for amorphous polymers such as Matrimid, it is more correct to talk about diffraction ands as well in the XRD, since the polymer diffracts in broad bands whose width is given information regarding the morphology and opening and crosslinking of the polymer chains that influence the membrane performance afterwards. Please revise.\nBesides, other comments, according to the TGA curves presented, Cyclen does not provide thermal stability to the Matrimid membrane materials, since it is degraded at a low temperature. Therefore, the statement where the second stage of the TGA of MMM at 300-450ºC attributed to Cyclen degradation should be revised for clarification.\nLines 257, please provide evidence to such statements.\nLine 267, please specify to which membrane composition corresponds to these three % values.\nLine 292, again, it is not clear whether is selectivity enhancement the property the authors are seeking for or permeability?\nWhy is a macrocycle with additional porosity chosen as fillers then, and Matrimid, a polyimide known for its selectivity and low gas permeability? Please clarify.\nPlease revise the experimental gas permeation values given in Tables S2-S5, since the values for permeation at 25ºC and 1 atm in Tables S2 and S3, and tables S4 and S5, do not agree. Have the authors measured the same membrane materials several times? The information on reproducibility is very important. Please comment.\nLine 299, the authors attribute now that too many Cyclen molecules tend to agglomerate at high loading, but no such agglomeration was shown in the SEM images above. Please provide evidence to these conclusions.\nLine 353, the plots are the 2008 Robeson upper-bounds for the target gas pair separations. Please be specific.\nLine 382, the uahtors compare suddenly the Cyclen -Matrimid membranes of this work with additional Cycldecane-hybrid membranes, whose synthesis and characterization has not been elsewhere described in this manuscript. Please revise and include, either experimental evidence or reported reference.\nLines 384-394, again these statements need additional evidence to verify that the supposedly hypothesis that Cyclen hybridization of Matrimid is providing porosity and crosslinking thus inducing remarkable increase in selectivity in Matrimid towards He gas separation, is fulfilled or not.\nEnglish revised by professional native translator recommended. There are several wrong verb tenses and other grammar or semantic mistakes that could easily be avoided. For example, consider the use of the adjective “residual” instead of “resident in line 201, comparisons wrongly used, such as “with a higher filler content increased” (line 293), “shrined” (line 348) instead of “shrinked”, and so on.\nSmall notations to be corrected, for instance: Figure 4c, the permeability changes are not shown in figure 4c, but the single gas permeability in Barrer.\nLine 249, the gas permeability measurement is a time-lag variable pressure- constant volume setup, I guess, since the starting pressure is 1 bar, what is the vacuum pressure?\nThe selectivity in Table S2 is the intrinsic selectivity in the table below are obtained as the ratio between two single gas permeabilities from the table above.\n• Is there enough detail provided in the methods for the work to be reproduced?\nI have included my comments through the review according to the journal requests. The aim of the paper and the gas permeation improvement of Matrimid membranes are significant and worthwhile but the manuscript must be improved.", "reviewer_3": "Response to the Reviewers' Comments:\nReviewer #1 (Remarks to the Author):\nThe manuscript reports on the combination of cyclen and Matrimid into a mixed matrix\nmembrane that has high selectivity for gas separation. Both cyclen and matrimid have\nbeen studied separately, and there exists a lot of literature on both materials. Hence, the\nnovelty being presented is the combination of the two into a membrane. The resulting\nselectivities are very high, which gives me concern, as generally for mixed matrix\nmembrane systems, selectivity is aligned with either of the membrane or additive, and\nnot an enhancement. I can understand the argument for the strong binding between\ncyclen and Matrimid, but that does not necessarily translate into the high selectivities\nreported.\nReply: Many thanks for your valuable comments. While it is true that both Cyclen and\nMatrimid have been extensively studied separately, their combination into a mixed\nmatrix membrane represents a new approach to achieve high gas separation\nperformance, especially for He/CH₄ selectivity. The Matrimid membrane is widely\nstudied for gas separation, while its low permeability (33 Barrer) and moderate\nselectivity (86) still present a great challenge to extract Helium(He) from natural gas\n(ultra-low helium concentration). The cyclen (1, 4, 7, 10-tetraazacyclododecane) is one\nof the most extensively studied ligands in coordination chemistry (Shinoda, S.\nDynamic cyclen-metal complexes for molecular sensing and chirality signaling. Chem.\nSoc. Rev. 2013, 42, 1825-1835.), while its potential for He extraction from natural gas\nhas not been explored until now. The ultra-low concentration of He makes it very\ndifficult to extract from natural gas. Therefore, it’s of great significance to develop the\nMatrimid-Cyclen membranes with high He/CH selectivity (>1000). In this work, the\n4\nthe combination of Matrimid and Cyclen (Matrimid-Cyclen-5%) membranes\ndemonstrates high He permeability (66Barrer) and He/CH selectivity (>1660).\n4\nWe fully understand your concern regarding the relationship between selectivity and\nthe materials involved. The enhancement in selectivity is not simply additive, but from\nthe strong hydrogen bonding interactions between Cyclen and Matrimid, which\nmodifies the polymer chain packing and results in a tighter interchain structure. This\nleads to a more effective sieving mechanism. Usually, polymer membranes have good\nprocessability, high stability, and low cost, but they often suffer from plasticization\nproblems, physical aging issues, and intrinsic permeability-selectivity trade-off\nlimitations, which makes it a great challenge to obtain high permeability together with\nsufficient selectivity [1-4]. Molecular sieve materials have attractive transport properties\nwell above the polymer upper bound, but they tend to have poor membrane-forming\nand mechanical property [5]. Mixed matrix membranes incorporate molecular sieve\nmaterials with certain pore sizes into a processable polymer matrix to improve the gas\nselectivity and permeability of polymer membranes by integrating the advantages of\nboth [6-8], which is the significance of preparing hybrid matrix membranes. Therefore,\nthe mixed matrix membranes (Matrimid-Cyclen) combine the advantages of easy\nprocessibility from Matrimid polymer and high gas permeability/selectivity from\nmolecular sieve material (Cyclen).\nTable 1 summarizes the He separation performance of Matrimid-based membranes.\nThe Matrimid-Cyclen-5% membranes exhibit high He permeability (66 Barrer) and\nHe/CH selectivity (>1660), which far exceed most of the other Matrimid-based\n4\nmembranes.\nTable 1. Summary of literature He/N and He/CH separation performance.\n2 4\nHe He/N He/CH\n2 4\nOperating Gas Refer\nMembrane Name Permeabili Selectivit Selectivit\nconditions Type ence\nty (Barrer) y y\nMatrimid-Cu-BTC-R-40 66.40 265.80 369.10 5 atm, 35 °C Pure 1\nMatrimid-Cu-BTC-30 51.80 193.40 257.90 5 atm, 35 °C Pure 2\nMatrimid-Cu-BDC-15% 20.50 341.70 410.00 7 atm, 35 °C Pure 3\nMatrimid-MgO-40% 45.00 86.1 118.4 10 atm, 35 °C Pure 4\nMatrimid-CNFs-10% 16.10 89.50 179.50 20 atm, 35 °C Pure 5\nMatrimid-PIM-EA(H )- \n2\n197.00 28.84 21.55 1 atm, 25 °C Pure 6\nTB-50%\nMatrimid-p-xylenediami\n21.70 112.00 155.00 10 atm, 35 °C Pure 7\nne-CL-14\nMatrimid-C60-2.5% 21.00 91.70 148.80 10 atm, 35 °C Pure 8\nMatrimid-Cyclen-3% 57.57±0.2 428.35±2. 959.50±4. This\n1 atm, 25 °C Pure\n1 15 67 work\nMatrimid-Cyclen-5% 66.43±0.2 621.70±3. 1660.75± This\n1 atm, 25 °C Pure\n2 20 5.25 work\nMatrimid-Cyclen-10% 78.63±0.2 543.43±4 1310.50± This\n1 atm, 25 °C Pure\n8 0.34 4.67 work\nSupplementary References\n[1] A. Akbari, J. Karimi-Sabet, S.M. Ghoreishi, Intensiffcation of helium separation\nfrom CH and N by size-reduced Cu-BTC particles in Matrimid matrix, Separation and\n4 2\nPuriffcation Technology, 251, 117317 (2020).\n[2] A. Akbari, J. Karimi-Sabet, S.M. Ghoreishi, Matrimid® 5218 based mixed matrix\nmembranes containing metal organic frameworks (MOFs) for helium separation,\nChemical Engineering & Processing: Process Intensification, 148, 107804 (2020).\n[3] A. Ali, J. Karimi-Sabet, S.M. Ghoreishi, Polyimide based mixed matrix membranes\nincorporating Cu-BDC nanosheets for impressive helium separation, Separation and\nPurification Technology, 253, 117430 (2020).\n[4] S.S. Hosseini, Y. Li, T.S. Chung, Y. Liu, Enhanced gas separation performance of\nnanocomposite membranes using MgO nanoparticles, Journal of Membrane Science,\n302 (1-2), 207-217 (2007).\n[5] M. Dohade, Incorporation of carbon nanofibers into a Matrimid polymer matrix:\nEffects on the gas permeability and selectivity properties, Journal of Applied Polymer\nScience, 135 (12) 46019 (2018).\n[6] E. Esposito, I. Mazzei, M.Monteleone, A. Fuoco, M. Carta, N.B. Makeown, R.\nMalpass-Evans, J.C. Jansen, Highly permeable Matrimid®/PIM-EA (H )-TB blend\n2\nmembrane for gas separation, Polymers, 11 (1), 46 (2018).\n[7] P.S. Tin, T.S. Chung, Y. Liu, R. Wang, S.L. Liu, K.P. Pramoda, Effects of\ncross-linking modification on gas separation performance of Matrimid membranes,\nJournal of Membrane Science, 225(1-2), 77-90 (2003).\n[8] T.S. Chung, S.S. Chan, R. Wang, Z.H. Lu, C.B. He, Characterization of\npermeability and sorption in Matrimid/C60 mixed matrix membranes, Journal of\nMembrane Science, 211 (1), 91-99 (2003).\nThere is no reported error for the experimental results and no discussion on\nreproducibility, so this raises questions about the provided data.\nReply: Many thanks for your comments. Repeated experiments were conducted and\nthe error bars have been included in the revised manuscript, The gas separation\nperformance and fabrication of mixed matrix membrane with gas molecule recognition\nwindow (cyclen) was found to be reproducible. The specific data has been listed as\nfollows.\nTable 1. Summary of Gas transport properties of the Matrimid and\nMatrimid-Cyclen-n% membranes at 1.0 atm and 25 ℃ (n = 1-15% indicated the degree\nof Cyclen mass loading).\nGas permeability (Barrer)\nMembrane\nHe H CO N CH\n2 2 2 4\nMatrimid 33.59±0.41 27.45±0.08 15.03±0.06 0.42±0.03 0.39±0.03\nMatrimid-Cyclen-1% 47.87±0.24 43.04±0.16 16.90±0.06 0.30±0.02 0.13±0.01\nMatrimid-Cyclen-3% 57.57±0.21 55.69±0.23 17.66±0.12 0.13±0.00 0.06±0.00\nMatrimid-Cyclen-5% 66.43±0.22 62.17±0.32 20.07±0.08 0.10±0.00 0.04±0.00\nMatrimid-Cyclen-10% 78.63±0.28 76.08±0.12 22.17±0.15 0.14±0.01 0.06±0.00\nMatrimid-Cyclen-15% 85.93±0.80 80.85±0.30 24.76±0.07 0.24±0.02 0.13±0.01\nIdea Selectivity\nMembrane\nHe/N He/CH H /N H /CH CO /N CO /CH\n2 4 2 2 2 4 2 2 2 4\n79.97±6. 86.13±2. 65.00±4 70.38±1 35.79±2. 38.54±0.\nMatrimid\n05 25 .82 .98 88 99\n145.06±1 368.23±2 143.47± 331.08± 56.33±3. 130.00±1\nMatrimid-Cyclen-1%\n0.29 8.10 9.71 26.34 81 0.33\n575.70±2 959.50±4 428.35± 928.17± 135.85± 294.33±2\nMatrimid-Cyclen-3%\n.15 .67 2.15 4.67 0.92 .00\n664.30±2 1660.75± 621.70± 1554.25 200.70± 501.75±3\nMatrimid-Cyclen-5%\n.10 5.25 3.20 ±8.00 0.80 .75\n561.64±4 1310.50± 543.43± 1268.00 158.36± 369.50±2\nMatrimid-Cyclen-10%\n1.36 4.67 40.34 ±3.17 11.10 .33\n358.04±1 661.00±4 336.88± 621.92± 103.17± 190.46±1\nMatrimid-Cyclen-15%\n2.28 8.42 11.59 49.33 3.64 4.71\nTable 2. Summary of gas transport permeability and selectivity of the\nMatrimid-Cyclen-5% membrane at 1.0 atm and different temperatures.\nGas permeability (Barrer) Selectivity\nTemperat\nure (℃)\nHe N CH He/N He/CH\n2 4 2 4\nReturn to\n63.97±0.46 0.09±0.00 0.03±0.00 710.78±2.33 2132.33±7.00\n20 ℃\n0 ℃ 52.74±0.46 0.007±0.00 0.003±0.00 7534.29±24.28 17580.00±56.67\n20 ℃ 63.13±0.46 0.10±0.00 0.03±0.00 631.30±2.60 2104.33±8.67\n40 ℃ 80.14±0.46 0.32±0.02 0.10±0.01 250.44±15.13 801.40±83.82\n60 ℃ 96.91±0.46 1.43±0.46 1.05±0.00 67.77±2.90 92.30±1.73\n80 ℃ 125.12±0.46 3.45±0.46 3.05±0.46 36.27±0.36 41.02±0.87\n100 ℃ 202.55±0.46 9.76±0.46 9.42±0.46 20.75±0.40 21.73±0.58\nReturn to\n68.69±0.27 0.12±0.01 0.03±0.00 572.42±49.85 2289.67±8.00\n20 ℃\nTable 3. Summary of gas transport permeability and selectivity of the\nMatrimid-Cyclen-5% membrane at different pressures and 25℃.\nGas permeability (Barrer) Selectivity\nPressure\n(Bar)\nHe N CH He/N He/CH\n2 4 2 4\n1 Bar 66.43±0.22 0.10±0.00 0.04±0.00 664.30±2.10 1660.75±5.25\n5 Bar 66.12±0.20 0.09±0.00 0.04±0.00 734.67±2.22 1652.00±6.00\n10 Bar 65.48±0.13 0.10±0.01 0.04±0.00 654.80±1.30 1637.00±3.25\n15 Bar 67.08±0.32 0.11±0.01 0.05±0.00 609.82±132.85 1341.60±6.40\n20 Bar 68.34±0.57 0.13±0.01 0.07±0.00 525.69±39.06 976.29±8.15\n25 Bar 68.28±0.34 0.12±0.02 0.06±0.00 569.00±73.14 1137.67±19.33\n30 Bar 72.21±1.33 0.10±0.00 0.01±0.005 722.10±13.70 1444.20±130.02\nReturn to 1\n66.84±0.42 0.10±0.00 0.04±0.00 668.40±4.20 1671.00±10.50\nBar\nTable 4. Summary of gas transport properties of the Matrimid and\nMatrimid-Cyclen-5% membrane at 1.0 atm and 25 ℃ during the aging process.\nGas permeability (Barrer)\nMembrane\nHe H CO N CH\n2 2 2 4\nFresh 66.43±0.22 62.17±0.32 20.07±0.08 0.10±0.00 0.04±0.00\nAging for 7 days 64.22±.34 59.90±0.23 18.30±0.07 0.09±0.01 0.04±0.00\nAging for 20 days 59.96±0.53 54.68±0.14 16.67±0.17 0.07±0.005 0.02±0.00\nAging for 40 days 59.73±0.09 53.39±0.33 15.96±0.09 0.06±0.00 0.01±0.00\nAging for 110 days 54.31±0.10 50.76±0.10 14.12±0.03 0.05±0.00 0.008±0.00\nAging for 480 days 42.87±0.21 31.43±0.13 7.82±0.04 0.009±0.00 0.001±0.00\nSelectivity\nMembrane\nHe/N He/CH H /N H /CH CO /N CO /CH\n2 4 2 2 2 4 2 2 2 4\n664.30±2.1 1660.75± 621.70± 1554.25± 200.70 501.75±3\nFresh\n0 5.25 3.20 8.00 ±0.80 .75\n713.56±67. 1605.50± 665.56± 1497.50± 203.33 457.50±1\nAging for 7 days\n960 8.50 68.04 21.22 ±21.79 .42\n856.57±57. 2998.00± 781.14± 2734.00± 238.14 833.50±9\nAging for 20 days\n74 26.50 54.20 6.76 ±18.02 .66\n995.50±1.5 5973.00± 889.83± 5339.00± 266.00 1596.00±\nAging for 40 days\n0 9.00 5.07 30.45 ±1.33 7.94\n1086.20±2. 6788.80± 1015.20 6345.00± 282.40 1765.00±\nAging for 110 days\n00 12.45 ±1.74 10.90 ±0.53 3.31\n4763.33±2 42870.00 3496.22 31430.00 868.89 7820.00±\nAging for 480 days\n3.34 ±200.00 ±12.62 ±113.58 ±4.01 36.06\nFigure 1 shows the preparation process of the Matrimid-Cyclen membrane, the\ndispersion of cyclen in Matrimid solutions is homogeneous. There is no Tyndall\nphenomenon in the clear and transparent Matrimid/Cyclen solutions, indicating that\nCyclen is uniformly dispersed in the mixed solution. The reproducibility of a mixed\nmatrix membrane depends on the dispersibility of the filler, the higher the solubility of\nthe filler and the better the dispersion in the mixed solution, the smaller the error in the\nperformance of the membrane material.\nFigure 1. Preparation process of the Matrimid-Cyclen membranes. (a) Cyclen powder\n(1.0, 3.0, 5.0, 10.0 or 15.0 mg) were added to 6.0 g DMF solvent to obtain clear and\ntransparent solutions. (b) The clean Matrimid dope solution was added to the\nCyclen/DMF solution to obtain homogeneous Matrimid/Cyclen solutions. (c) The\nMatrimid/Cyclen solutions was injected into clean petri dishes and transferred to a\nvacuum oven at an ambient temperature of 120 °C for 8-hour solvent evaporation to\nobtain Matrimid-Cyclen membranes.\nI am also concerned about the high cyclen loading in the membrane, up to 15%. There\nis no discussion about mechanical properties in the manuscript, but the literature on\nMatrimid is very clear that high levels of additives result in a very brittle membrane. I\nsee no reason why this is not the case here. This has issues with the ability to translate\nthe performance into a viable membrane.\nReply: Thanks for your comments. We understand your concern about the potential\nbrittleness of the Matrimid mixed matrix membranes with higher Cyclen loadings. To\naddress this, the dynamic mechanical thermal analysis (DMTA) was conducted on\nMatrimid-Cyclen samples to assess the mechanical properties across a temperature\nrange of 50 to 350 °C. As shown in Figure 3 (g-i), the storage modulus (E′) of the\nMatrimid-Cyclen membranes (1800 MPa) was almost completely maintained before\nreaching the corresponding T . This indicated that the membrane retains its mechanical\ng\nintegrity even with higher Cyclen content. Furthermore, we observed that higher\nCyclen loadings did increase in the brittleness of the membranes, as seen in the\ndecreasing peak intensity of damping (Tan δ) with increasing Cyclen content (Figure\n3i). These data demonstrated that this kind of gas separation membrane was endowed\nwith good mechanical property and thermal stability, even with the higher Cyclen\nloading.\nFigure 3. Characterization of Matrimid-Cyclen membranes. (g) The dynamic\nmechanical thermal analysis (DMTA) spectra of Matrimid-Cyclen membranes at\ntemperature range from 50 to 350 ℃. (h) The storage modulus (E′) and (i) Tan δ of\nMatrimid-Cyclen membranes.\nThe operating temperature, feed pressure and aging behaviors follows the expected\ntrend of mixed matrix membranes, with the presented discussion valid. The application\nfor helium extraction is important, and it is good that the authors considered both\nseparation from methane and nitrogen. I recommend that the manuscript be rejected,\nand the authors submit to a more appropriate journal.\nReply: Thank you very much for your professional comments and suggestions. We\nsincerely apologize for some inadequate in data analysis and descriptions in the original\nmanuscript. As you pointed out, several issues required further attention, and we have\nmade substantial revisions to address the concerns. Below is a detailed list of our\nresponses to your comments. In our work, we designed the mixed matrix membranes\nbased on several key considerations derived from numerous studies.\nFirstly, Cyclen was selected as filler due to its size-sieving effect, which is beneficial\nfor He separation. Based on the molecular dynamic diameters of He (2.60 Å), H (2.89\n2\nÅ), CO (3.30 Å), N (3.64 Å) and CH (3.80 Å), the Cyclen with cavity size (3.2-3.4 Å)\n2 2 4\nbetween the molecular dynamic diameters of He and N was selected, which makes it\n2\nan ideal candidate for He separation, such as He/N and He/CH . Furthermore, the\n2 4\nCyclen with a certain cavity size creates a special transport channel for small gases, like\nHe, which increases their transport paths in the membrane, counteracting the\nnegative effect of the increased chain stacking density and enhancing the permeability\nof small gas molecules.\nSecondly, the chosen filler must have good solubility and dispersion in solvents. To\nensure uniform dispersion of the filler in the membrane and to minimize the formation\nof non-selective voids caused by filler agglomeration, we selected Cyclen for its high\nsolubility and good dispersion in organic solvents. Cyclen was fully dissolved in DMF,\nand then the clean Matrimid dope solution was added to the Cyclen/DMF solution. The\nresulting homogeneous Matrimid/Cyclen solution, which was stirred for 24 hours and\nsonicated for 1 hour, remained clear and transparent, even with up to 15% Cyclen\ncontent, as shown in Figure 1.\nFigure 1. (a) Cyclen powder (1.0, 3.0, 5.0, 10.0 or 15.0 mg) were added to 6.0 g DMF\nsolvent to obtain clear and transparent solutions. (b) Matrimid dope solution. (c) The\nclean Matrimid dope solution was added to the Cyclen/DMF solution to obtain clear and\ntransparent mixed Matrimid/Cyclen solutions.\nThirdly, compatibility between filler and polymer matrix. It is crucial to create mixed\nmatrix membranes with fair matching between filler and polymer matrix to gain\nattractive performance. This is why we chose Cyclen as a filler. Hydrogen bonding\nbetween Cyclen and the Matrimid polymer chains immobilizes Cyclen molecules in the\ninterchain gaps of the Matrimid membrane, resulting in homogeneously mixed matrix\nmembranes. The SEM image (Figure 2) shows the surface and interface of the\nmembrane, confirming the good interfacial compatibility between Cyclen and\nMatrimid.\nFigure 2. SEM images of the cross section of the Matrimid-Cyclen-n% membranes up\nto ×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nFourthly, Helium (He) purification requirements. Helium is an extremely scarce,\nnon-renewable gas with a strong safety profile that is an essential resource for the\ndevelopment of many high-tech industries, such as the aerospace industry, electronics,\nmedical equipment, and scientific research. On Earth, He occurs in very low\nconcentrations, mainly in the atmosphere and in natural gas. Helium is commonly\nextracted industrially from high-pressure natural gas. However, the initial\nconcentration of He in most natural gas reservoirs is still as low as 0.3%, and to extract\nhelium from natural gas with ultra-low He content, the gas separation membranes must\npossess He/CH selectivity greater than 1000. Highly permeable polymers rarely\n4\nprovide sufficient selectivity for high-purity He extraction, and Matrimid was chosen as\nthe polymer matrix due to its dense stacking of polymer chains. XRD characterization\nrevealed that the strong hydrogen bonding between Cyclen and Matrimid modulated\nthe chain stacking of the mixed matrix membranes, which reduce the pore size, narrow\nthe pore size distribution and enhances the selectivity of the membrane by modulating\nthe chain spacing.\nWe have verified the success of the hybrid matrix membrane design with ultra-high\nhelium selectivity to achieve excellent separation performance.\nReviewer #2 (Remarks to the Author):\n• What are the noteworthy results?\nThe work seems to be looking at high selectivity membranes for He/CH4 separation, by\naltering the properties of a commonly used polyimide in gas separation, Matrimid,\nadding different concentrations of Cycle, which is presented as a molecule (particle,\nmolecule? Please clarify) with a wide opening providing H-bonding with the Matrimid\npolymer chains. Both the porosity of this filler and the crosslinking thereof seems to be\nhaving a significant influence in increasing the selectivity of Matrimid above the\nstate-of-the-art of such membrane materials without decreasing the permeability of the\nsmaller gas molecules.\nReply: Thanks for your recognition and comments. In this work, the cyclic Cyclen with\nspecific intra-ring dimensions was incorporated into Matrimid as a pore structure\nmodifier to improve the He/CH selectivity. We explored both inorganic or organic\n4\nfillers for fabricating the MMMs to enhance the gas separation performance. While, the\ninorganic fillers (e.g., MOFs, ZIFs, and Mxene) often suffer from poor compatibility\nwith polymer matrix, and the agglomeration results in defects in the membrane.\nNotably, Cyclen, a purely organic additive has better compatibility with Matrimid\npolymer matrices than inorganic fillers.\nAs shown in Figure 1, even with Cyclen loading increased from 1% to 15%, the tyndall\neffect never appeared in the Cyclen solution and the Cyclen/Matrimid mixture,\nindicating that Cyclen was bound to Matrimid at the molecular level.\nFigure 1. (a) Cyclen powder (1.0, 3.0, 5.0, 10.0 or 15.0 mg) were added to 6.0 g DMF\nsolvent to obtain clear and transparent solutions. (b) Matrimid dope solution. (c) The\nMatrimid dope solution was added to the Cyclen/DMF solution to obtain clear and\ntransparent mixed Matrimid/Cyclen solutions.\nThis molecular dispersion of Cyclen within the membrane was further confirmed by\nSEM and EDS analysis. The cross-section morphology of Matrimid-Cyclen-n% (n = 1,\n3, 5, 10, 15) membranes shown in Figure 2, at different magnifications , revealed an\nincrease in torn lamellar structures as Cyclen loading increases. This morphology is\nattributed to the enhanced interaction between the polymer chains and filler, which\nresults in interfacial stress during the fracturing of MMMs in liquid nitrogen for SEM\nanalysis. This stress can deform the polymer segments, leading to increased plastic\ndeformation. The Matrimid-Cyclen-15% membrane material remained homogeneous\nat 50,000x magnification, without any significant aggregation or discrete particulate\nphases observed. The elemental mapping (Figure 3) further supports the homogeneous\ndistribution of Cyclen within the membrane, showing a gradual increase in nitrogen (N)\nelemental distribution as Cyclen content rises, without any noticeable aggregation.\nThis is further evidence that Cyclen is dispersed as a molecule in polymer membrane.\nFigure 2. SEM images of the cross section of the Matrimid-Cyclen-n% membranes up\nto ×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nFigure 3. SEM images and EDS with nitrogen (N) mapping of planar scans of the cross\nsection of the Matrimid-Cyclen-n% membranes (n% is the Cyclen mass loading and\nwas varied from 1 to 15 wt%, n = 0, 1, 3, 5, 10, 15).\nIn this work, Cyclen was selected as the filler for several key reasons to prepare the\nMatrimid matrix, which can lead to ultra-high He permeability and ultra-high He/CH\n4\nseparation selectivity for several main reasons:\nFirstly, Cyclen is an organic macrocyclic compound with a flexible structure and its\ncavity size is approximately 3.2-3.4 Å. This cavity size lies between the molecular\ndynamic diameters of He (2.60 Å) and N (3.64 Å), producing a strong molecular\n2\nsieving effect, particularly for He/N and He/CH .\n2 4\nSecondly, the strong hydrogen bonds between Cyclen and Matrimid modulate the chain\nspacing increasing the chain stacking density of the mixed matrix membranes. This\nmodification enhances the resistance to mass transfer for larger gases, such as N₂ and\nCH₄, improving the overall selectivity of the membrane.\nThirdly, Cyclen with a certain cavity size creates a special transport channel for small\ngases, like He, which increases their transport paths in the membrane, counteracting the\nnegative effect of the increased chain stacking density and enhancing the permeability\nof small gas molecules.\nHerein MMMs with ultra-high He/CH separation selectivity, exceeding the\n4\nperformance of most existing membrane, were prepared using a simple solution casting\nmethod. The He/CH selectivity reached up to an impressive value of 6788 after\n4\nphysical aging for 110 days, which outperformed almost all reported polymer-based\nmembranes and was even comparable to that of some advanced carbon molecular sieve\n(CMS) membranes. In addition, compared with CMS membrane,MOF membrane and\nCOF membrane with high He/CH selectivity , the MMMs have good solution\n4\nprocessability and stable performance, which is easy to achieve industrial production.\nAll statements in the scientific paper should be justified, either by results presented in\nthe work, or by references to previous reports in state-of-the-art literature. See for\ninstance lines 193-196, “The ultrahigh gas separation performance of mixed matrix\nmembrane attributed to enhanced interfacial compatibility due to dense hydrogen\nbonding between Cyclen and Matrimid chains”, before showing any gas separation\nresults.\nReply: Many thanks for your valuable comments. We have revised the statements\naccording to your suggestion by adding the references after the expression. Please refer\nto the revised manuscript for the updated text. The sentence has been revised as “The\ndense hydrogen bonding between Cyclen and Matrimid enhances their interfacial\ncompatibility, reduces non-selective interfacial gaps and significantly improves the\nhigh gas separation performance of the mixed matrix membranes [21, 32]”.\nThe authors often do not comment on the hypotheses until they are about to introduce\nthe results (see lines 307-311 introducing the effect of operating temperature, pressure\nand “aging”.\nReply: Thanks for your comments. The hypotheses were deleted according to your\nsuggestion. Please refer to the revised manuscript.\nAlready in the Highlights, why do the authors talk of Cyclen as a “window” and not an\namine molecule with such and such properties that give the potential to provide the\nMatrimid polyimide matrix with the desired transport/separation properties in the\nHe/CH4 separation? Please, be more precise.\nReply: Thanks for your comments. We agree with your description that the Cyclen is\nan amine molecule from the perspective of its chemical structure. We would like to\nclarify our use of the term \"window\" in the context of its role in the membrane.\nFirstly, as an amine molecule, Cyclen forms the strong hydrogen bonding with the\nMatrimid polymer, which tightens the d-spacing of Matrimid polymer chains and\nminimizes defects within the membrane. This arrangement leads to longer and more\ntortuous transport pathways for the kinetically larger diameters of N and CH in the\n2 4\nmembrane.\nSecondly, and more importantly, Cyclen acts as pore structure modifier with a cavity\nsize of approximately 3.2-3.4 Å, which allows Cyclen to create dedicated transport\nchannels for He molecules providing a “window” for smaller gas molecules, such as He,\nto pass through more easily. The size of this “window” induces a strong molecular\nsieving effect, reducing the permeability of N and CH and increasing the selectivity of\n2 4\nmultiple important gas pairs.\nTherefore, the presence of Cyclen plays a dual role in the MMM, i.e., as a “window”\nand an amine molecule to construct the molecular sieve membrane for He/N and\n2\nHe/CH separation.\n4\n• Will the work be of significance to the field and related fields? How does it compare to\nthe established literature? If the work is not original, please provide relevant references.\nReply: Thank you for recognizing the significance of this work in He purification from\nnatural gas and related fields.\nThe Matrimid membrane has been widely studied for He separation, while its low He\npermeability (33 Barrer) and He/CH selectivity (86) still meet a great challenge to\n4\nextract He from natural gas (ultra-low helium concentration). The Cyclen (1, 4, 7,\n10-tetraazacyclododecane) is one of the most extensively studied ligands in\ncoordination chemistry (Shinoda, S. Dynamic Cyclen-metal complexes for molecular\nsensing and chirality signaling. Chem. Soc. Rev. 2013, 42, 1825–1835.), but it has not\nbeen explored for He extraction from natural gas till now. The ultra-low concentration\nof He makes it very difficult to be extracted from natural gas, highlighting the\nsignificance of developing Matrimid-Cyclen membranes with high He/CH selectivity\n4\n(>1000). In this work, the combination of Matrimid and Cyclen (Matrimid-Cyclen-5%)\nmembranes demonstrated high He permeability (66 Barrer) and He/CH selectivity\n4\n(>1660).\nThe Matrimid-based membranes and related works for He separation were summarized\nin Table 1. Compared with the existing literature, Matrimid-Cyclen membranes offer\nmoderate He permeability and ultra-high He/CH selectivity, surpassing the\n4\nperformance of all other Matrimid-based membranes (Figure 1). Notably, this is the\nfirst report in where the cyclic Cyclen molecules are incorporated into Matrimid as\nmolecular windows to prepare hybrid matrix membranes for He purification.\nFigure 1. Plots of (a) He/N , (b) He/CH separation performance of Matrimid-Cyclen\n2 4\nmembranes compared with other Matrimid-based membranes.\nTable 1. Summary of literature He/N and He/CH separation performance.\n2 4\nHe He/N He/CH\n2 4\nOperating Gas Refer\nMembrane Name Permeabili Selectivit Selectivit\nconditions Type ence\nty (Barrer) y y\nMatrimid-Cu-BTC-R-40 66.40 265.80 369.10 5 atm, 35 °C Pure 1\nMatrimid-Cu-BTC-30 51.80 193.40 257.90 5 atm, 35 °C Pure 2\nMatrimid-Cu-BDC-15% 20.50 341.70 410.00 7 atm, 35 °C Pure 3\nMatrimid-MgO-40% 45.00 86.1 118.4 10 atm, 35 °C Pure 4\nMatrimid-CNFs-10% 16.10 89.50 179.50 20 atm, 35 °C Pure 5\nMatrimid-PIM-EA(H )- \n2\n197.00 28.84 21.55 1 atm, 25 °C Pure 6\nTB-50%\nMatrimid-p-xylenediami\n21.70 112.00 155.00 10 atm, 35 °C Pure 7\nne-CL-14\nMatrimid-C60-2.5% 21.00 91.70 148.80 10 atm, 35 °C Pure 8\nMatrimid-Cyclen-3% 57.57±0.2 428.35±2. 959.50±4. This\n1 atm, 25 °C Pure\n1 15 67 work\nMatrimid-Cyclen-5% 66.43±0.2 621.70±3. 1660.75± This\n1 atm, 25 °C Pure\n2 20 5.25 work\nMatrimid-Cyclen-10% 78.63±0.2 543.43±4 1310.50± This\n1 atm, 25 °C Pure\n8 0.34 4.67 work\nSupplementary References\n[1] A. Akbari, J. Karimi-Sabet, S.M. Ghoreishi, Intensiffcation of helium separation\nfrom CH and N by size-reduced Cu-BTC particles in Matrimid matrix, Separation and\n4 2\nPuriffcation Technology, 251, 117317 (2020).\n[2] A. Akbari, J. Karimi-Sabet, S.M. Ghoreishi, Matrimid® 5218 based mixed matrix\nmembranes containing metal organic frameworks (MOFs) for helium separation,\nChemical Engineering & Processing: Process Intensification, 148, 107804 (2020).\n[3] A. Ali, J. Karimi-Sabet, S.M. Ghoreishi, Polyimide based mixed matrix membranes\nincorporating Cu-BDC nanosheets for impressive helium separation, Separation and\nPurification Technology, 253, 117430 (2020).\n[4] S.S. Hosseini, Y. Li, T.S. Chung, Y. Liu, Enhanced gas separation performance of\nnanocomposite membranes using MgO nanoparticles, Journal of Membrane Science,\n302 (1-2), 207-217 (2007).\n[5] M. Dohade, Incorporation of carbon nanofibers into a Matrimid polymer matrix:\nEffects on the gas permeability and selectivity properties, Journal of Applied Polymer\nScience, 135 (12) 46019 (2018).\n[6] E. Esposito, I. Mazzei, M.Monteleone, A. Fuoco, M. Carta, N.B. Makeown, R.\nMalpass-Evans, J.C. Jansen, Highly permeable Matrimid®/PIM-EA (H )-TB blend\n2\nmembrane for gas separation, Polymers, 11 (1), 46 (2018).\n[7] P.S. Tin, T.S. Chung, Y. Liu, R. Wang, S.L. Liu, K.P. Pramoda, Effects of\ncross-linking modification on gas separation performance of Matrimid membranes,\nJournal of Membrane Science, 225(1-2), 77-90 (2003).\n[8] T.S. Chung, S.S. Chan, R. Wang, Z.H. Lu, C.B. He, Characterization of\npermeability and sorption in Matrimid/C60 mixed matrix membranes, Journal of\nMembrane Science, 211 (1), 91-99 (2003).\nGiven the low quantity of He in the feed stream it is reasonable that the authors look at\nMatrimid because of its low permeability and high selectivity properties and try this\nhybridization to increase the pore size and therefore the permeability of the Matrimid\npolymer. The state-of-the-art does very vaguely support this justification, and only\nreading between lines with prior knowledge of mixed matrix membrane technology and\nchallenges allows the reader to withdraw the novelty of the work. This is probably due\nto the fact that the bibliographic review of the authors is not mentioned until Figure 5\nwhere the data on Matrimid mixed matrix membranes collected in the supplementary\ninformation is used to build the Robeson upper-bound trade-off figures for the different\ngas pairs considered in this work. Please revise.\nReply: Many thanks for your comments. Matrimid and Matrimid-based mixed matrix\nmembranes have been extensively studied for gas separation and pervaporation\nprocesses due to the combination of relatively high gas permeability coefficients and\nseparation factors as well as excellent mechanical properties, high solubility in\nnon-hazard organic solvents, and high glass transition temperature in comparison to\npolycarbonate, polysulfone, and other materials. Due to the excellent properties of the\nshown by Matrimid membrane, which is desirable for the gas separation process, it is\nworthwhile to explore modifications to further enhance their performance for He\nseparation.. In response to your suggestion, we have added the rationale for choosing\nMatrimid as the polymer matrix in the “Introduction” section of the manuscript. We\nhave also summarized the relevant studies on Matrimid-based membrane materials for\nHe separations, discussed the challenges associated with mixed-matrix membrane\ntechnology, and clarified the idea and novelty and purpose of our design. These\nrevisions aim to better explain the basis and significance of our work. Please refer to the\nintroduction in the revised manuscript.\nDoes the work support the conclusions and claims, or is additional evidence needed?\nBesides, but no other information on the morphology and porosity characteristics\njustifying the authors’selection as filler in Matrimid polyimide are presented, so the\nreader cannot judge for himself the reasons why the crosslinking only appear at\nloadings above 15%, or the presence or absence of crosslinking with the polymer\nmatrix that leads to higher selectivity and usually decreases permeability, or the “torn\nlamellar structures” formation, as well as the evidence of the strong H-bonding\nmentioned in the Results section.\nReply: We apologize for any inadequate data analysis and descriptions in the\nmanuscript, which may have made some of this evidence unclear. We have tried our\nbest to resolve the concerns and made a proper revision of the manuscript.\nAdditional experiments and evidence related to morphology and porosity\ncharacteristics have been supplemented. The SEM images in Figure 1 shows that the\ncross-section morphology of the MMMs is dense and homogeneous, demonstrating\nexcellent interfacial compatibility between Cyclen and Matrimid.\nAs Cyclen loading increases, the torn lamellar structures in the membranes also\nincrease. This morphology is attributed to the enhanced interaction between the\nMatrimid and Cyclen, which results in the formation of interfacial stress during the\nfracturing of MMMs in liquid nitrogen. This stress causes deformation of the polymer\nsegments, resulting in elongated polymer segments with increased plastic deformation.\nThere is no significant aggregation or discrete particulate phases observed in the\nMatrimid-Cyclen-15% membrane from 1,000x to 50,000x electron microscopy. The\nimproved compatibility between organic fillers and polymer matrices is conducive to\nobtain simultaneous improvements in gas permeability and selectivity.\nFigure 1. SEM images of the cross section of the Matrimid-Cyclen-n% membranes up\nto ×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nThe pore size distribution of the Matrimid-Cyclen membranes in Figure 2 shows that\nthe addition of Cyclen significantly reduced the micropore size of the Matrimid-Cyclen\nmembrane. Additionally, the content of ultramicropores with a size of about 3.9 Å,\ngradually increased compared to the original Matrimid membrane. The introduction of\nCyclen also resulted in the formation of new pores with a size of about 3.2 Å.\nThis modification of the pore structure is attributed to the hydrogen bonding interaction\nbetween Cyclen and Matrimid which enhances the molecular sieving properties of\nMatrimid-Cyclen membranes by narrowing the pore size distribution. Cyclen creates\nadditional and longer transport channels for He, which significantly improves the\ntransport capacity of He.\nMatrimid-Cyclen-10%\nMatrimid-Cyclen-5%\nMatrimid\n0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1\nPore Width (nm)\n)g/³mc(\nWd/Vd\nFigure 2. Pore size distribution of the Matrimid-Cyclen membranes. The pore size\ndistribution was calculated from the corresponding H adsorption isotherm using the\n2\nDFT method.\nThe presence of strong hydrogen bonding interactions was further confirmed by FTIR\nspectra, which shows that the hydrogen bonding between Cyclen and Matrimid\npolymer chains immobilizes Cyclen molecules within the interchain gaps of the\nMatrimid matrix, resulting in homogeneous mixed matrix membranes.\nHave authors envisaged the measuring of BET surface area of Cyclen and the MMM\nfor verification? I guess they have done, and the Cyclen is a sacrificial filler given the\nstatement in page This could visualize the statement of the authors “ in lines 214 – 216,\n“the decomposition of Cyclen leaves many micropores in the Matrimid-Cyclen\nmembranes”, but this porosity has not been shown in the paper, or is not revealed in the\ncross-sectional images in SEM pictures. Please clarify, this is important to show\nevidence on the trade-off between crosslinking and additional porosity effects on the\nmixed matrix membrane performance observed in the results section.\nReply: Thanks for your comments. The pore size distribution of the Matrimid-Cyclen\nmembranes in Figure 1 shows that the addition of Cyclen significantly reduced the\nmicropore size of the Matrimid-Cyclen membrane. The content of ultramicropores\nwith a size of about 3.9 Å, gradually increased compared to the original Matrimid\nmembrane. This modification of the pore structure is attributed to the hydrogen\nbonding interaction between Cyclen and Matrimid which enhances the molecular\nsieving properties of Matrimid-Cyclen membranes by narrowing the pore size\ndistribution. Additionally, Cyclen with pore size (3.2 Å) creates additional and longer\ntransport channels for He in the membranes, which significantly improves the\ntransport capacity of He. Carbon molecule sieving (CMS) membrane after thermal\ntreament will be evaluated in the future.\nFigure 1. BET surface area and pore size analysis for Cyclen and Matrimid-Cyclen\nmembranes. (a) H sorption isotherm curves collected at 77K and (b) Corresponding\n2\npore size distribution profiles. The pore size distribution was calculated from the\ncorresponding H adsorption isotherm using the DFT method.\n2\nIs the methodology sound? Does the work meet the expected standards in your field?\nThe membrane materials were characterized by the common range of analytical\ntechniques in this kind of work: SEM (surface and cross section), XRD, ATR-FTIR,\nNMR, TGA, XPS, as well as single gas permeation at room temperature by time-lag\nexperiments at 25ºC. This are adequate in membrane materials characterization. A but\nlays in the fact that the authors claim to evaluate the effect of temperature and\ntemperature in permeation in the range 25-45ºC and 1-6 bar. These are very narrow in\ngas permeation membranes. Although it is shown an impact in selectivity in this narrow\ntemperature range, the value of 25ºC cannot be considered as “low temperature”\nstandard (line 327). Likewise, 6 bar is the regular operating pressure in gas separation.\nPlease consult previous works such as that by R. Baker, studying the optimal pressure\nfor gas separation as 5 bar. The pressure of 6 bar in a strong polyimide as Matrimid is\nnot usually sufficient enough to see any observable packing density of the polymer\nchains (line 336). The authors should give evidence of that, or appropriately check\nliterature on polyimide membrane materials characterization. The same can be said of\nthe effect of aging, which in this work is attributed to the measurements performed with\nthe membranes stored away from the gas permeation plant up to 110 days after the\nsynthesis. Have the membranes been taken out of the module in this intervals? The\ndamage caused by the joints may be more significant that any physical or chemistry\nchange due to a slight variation of the polymer packing. Please revise.\nReply: Thanks for your comments. Following your suggestion, the extended gas\nseparation test of the Matrimid-Cyclen membranes was carried out including a wider\nrange from 0 ℃ to 100 ℃ to better assess the impact of operating temperature on the\ngas separation performance. The results of the gas separation performance of the\nMatrimid-Cyclen-5% membrane are shown in Figure 1, with the data summarized in\nTable 1.\nAs the operating temperature increased from 0 ℃ to 100 ℃, the permeability of He, N\n2\nand CH increased,while the selectivity of He/N and He/CH decreased.\n4 2 4\nThis behavior\ncould be attributed to the increased gas fugacity at elevated temperatures. The\nselectivity of He/N and He/CH decreased rapidly as the operating temperature\n2 4\nincreased from 40 ℃ to 60 ℃. This phenomenon was likely due to the accelerated\nmovement frequency of polymer chains at high temperatures, rendering transport of\ngas molecules through the matrix and weakening the molecular sieving effect of the\nmembrane. Notably, after cooling the membrane from 25 ℃ to 0 ℃, the He\npermeability decreased from 63.13 to 52.74 Barrer, while the He/N selectivity\n2\nincreased from 631 to 8790, and the He/CH selectivity increased from 2104 to 17580,\n4\ncorresponding to 14-fold and 9-fold increases, respectively. This dramatic\nimprovement in selectivity at low temperatures significantly enhances the potential for\nhigh-purity He extraction from natural gas.\nThe ultra-high selectivity of the Matrimid-Cyclen-5% membrane at 0 ℃ for He/N and\n2\nHe/CH facilitates the extraction of high-purity He from natural gas at low\n4\ntemperatures. Although the He permeability decreased by 16% at low temperatures, the\nincrease in He selectivity indicates that improving gas purity is more challenging than\nenhancing transport efficiency. High purity is particularly important for He, given its\ncritical applications in the aerospace industry, electronics, medical equipment, and\nscientific research. Furthermore, the gas separation performance of the membranes\nfully when the temperature was returned to 20 ℃, both from 100 ℃ and from 0 ℃,\nindicating that the changes in membrane performance with temperature are inherent to\nthe membrane properties and do not result in structural damage.\nFigure 1. Effect of operating temperature on the separation performance of\nMatrimid-Cyclen-5% membrane.\nTable 1. Summary of gas transport permeability and selectivity of the\nMatrimid-Cyclen-5% membrane at 1.0 Bar and different temperatures.\nGas permeability (Barrer) Selectivity\nTemperat\nure (℃)\nHe N CH He/N He/CH\n2 4 2 4\nReturn to\n63.97±0.46 0.09±0.00 0.03±0.00 710.78±2.33 2132.33±7.00\n20 ℃\n0 ℃ 52.74±0.46 0.007±0.00 0.003±0.00 7534.29±24.28 17580.00±56.67\n20 ℃ 63.13±0.46 0.10±0.00 0.03±0.00 631.30±2.60 2104.33±8.67\n40 ℃ 80.14±0.46 0.32±0.02 0.10±0.01 250.44±15.13 801.40±83.82\n60 ℃ 96.91±0.46 1.43±0.46 1.05±0.00 67.77±2.90 92.30±1.73\n80 ℃ 125.12±0.46 3.45±0.46 3.05±0.46 36.27±0.36 41.02±0.87\n100 ℃ 202.55±0.46 9.76±0.46 9.42±0.46 20.75±0.40 21.73±0.58\nReturn to\n68.69±0.27 0.12±0.01 0.03±0.00 572.42±49.85 2289.67±8.00\n20 ℃\nTo explore the practical applicability of the Matrimid-Cyclen membranes at different\nfeed pressures, the gas separation performance of the Matrimid-Cyclen-5% membrane\nwas re-evaluated at pressures up to 30 bar, and the data were summarized in Table 2. As\npresented in Figure 2, as the pressure increased from 1 bar to 10 bar, the permeability of\nHe decreased slightly, and the pressure dependence of N and CH was almost\n2 4\nunrecognizable, with only small fluctuations in the selectivity for He/N and He/CH .\n2 4\nThese observations suggest that the membranes remain structurally stable under\nmoderate pressure increases, which may be attributed to the presence of Cyclen\nmolecules, which increase the packing density of the polymer chains, thereby reducing\ntheir mobility, increasing chain stiffness and improving the high-pressure resistance of\nthe membranes. As the operating pressure increased from 10 bar to 20 bar, the\npermeability of all gases rose, while the selectivity for He/N and He/CH decreased.\n2 4\nThis behavior is typical of glassy polymers, such as Matrimid, under plasticization\npressure, where increased feed pressure leads to higher polymer segment mobility and\nlarger chain spacing, resulting in higher permeability and lower selectivity [1-5]. As the\npressure increases from 20 bar to 30 bar, the permeability of He remained almost\nconstant, while the permeability of N and CH gradually decreased, and the selectivity\n2 4\nof He/N and He/CH tends to increase, which was ascribed to the existence of\n2 4\nunrelaxed volume within the polymer chain structure. An increase in feed pressure\nresults in greater compactness of the polymer matrix, which reduces the gas\npermeability in the membrane by reducing fractional free volume [6-7].\nSupplementary References\n[1] H. Rajati, A.H. Navarchian, D,. Rodrigue, S. Tangestaninejad, Improved CO\n2\ntransport properties of Matrimid membranes by adding amine-functionalized PVDF\nand MIL-101 (Cr), Separation and Purification Technology, 235 116149 (2020).\n[2] S. Salman, K. Nijmeijer, High pressure gas separation performance of\nmixed-matrix polymer membranes containing mesoporous Fe (BTC), Journal of\nmembrane science, 459, 33-44 (2014).\n[3] S. Salman, K. Nijmeijer, Performance and plasticization behavior of polymer-MOF\nmembranes for gas separation at elevated pressures, Journal of membrane science, 470,\n166-177 (2014).\n[4] A.E. Amooghin, M. Omidkhah, H. Sanaeepur, A. Kargari, Preparation and\ncharacterization of Ag+ ion-exchanged zeolite-Matrimid® 5218 mixed matrix\nmembrane for CO /CH separation, Journal of Energy Chemistry, 25 (3) 450-462\n2 4\n(2016).\n[5] H. Asghar, A. Ilyas, Z. Tahir, X.F. Li, A.L. Khan, Fluorinated and sulfonated poly\n(ether ether ketone) and Matrimid blend membranes for CO separation, Separation and\n2\nPurification Technology, 203, 233-241 (2018).\n[6] A.E. Amooghin, H. Sanaeepur, M. Omidkhah, A. Kargari, “Ship-in-a-bottle”, a\nnew synthesis strategy for preparing novel hybrid host-guest nanocomposites for\nhighly selective membrane gas separation, Journal of Materials Chemistry A, 6 (4)\n1751-1771 (2018).\n[7] A. Mirzaei, A.H. Navarchian, S. Tangestaninejad, Mixed matrix membranes on the\nbasis of Matrimid and palladium-zeolitic imidazolate framework for hydrogen\nseparation, Iranian Polymer Journal, 29 (6), 479-491 (2020).\nFigure 2. Effect of feed gas pressures on the separation performance of\nMatrimid-Cyclen-5% membrane.\nTable 2. Summary of gas transport permeability and selectivity of the\nMatrimid-Cyclen-5% membrane at different pressures and 25℃.\nGas permeability (Barrer) Selectivity\nPressure\n(Bar)\nHe N CH He/N He/CH\n2 4 2 4\n1 Bar 66.43±0.22 0.10±0.00 0.04±0.00 664.30±2.10 1660.75±5.25\n5 Bar 66.12±0.20 0.09±0.00 0.04±0.00 734.67±2.22 1652.00±6.00\n10 Bar 65.48±0.13 0.10±0.01 0.04±0.00 654.80±1.30 1637.00±3.25\n15 Bar 67.08±0.32 0.11±0.01 0.05±0.00 609.82±132.85 1341.60±6.40\n20 Bar 68.34±0.57 0.13±0.01 0.07±0.00 525.69±39.06 976.29±8.15\n25 Bar 68.28±0.34 0.12±0.02 0.06±0.00 569.00±73.14 1137.67±19.33\n30 Bar 72.21±1.33 0.10±0.00 0.01±0.005 722.10±13.70 1444.20±130.02\nReturn to 1\n66.84±0.42 0.10±0.00 0.04±0.00 668.40±4.20 1671.00±10.50\nBar\nAs shown in the following Figure 3, the aging test was performed by taking samples\n(labled A , B , C , D and E , where n is the number of samples taken) from the same\nn n n n n\nbatch of the Matrimid-Cyclen-5% membrane at different aging times (0, 7, 20, 40, 110,\n480 days). No damage was caused by the joints since the membrane samples removed\nat each interval will not be used for retesting.\nFigure 3. Multiple small membranes are removed from the intact membrane to be used\nfor testing.\nRegarding the aging performance, we continued to test the gas separation performance\nof the membrane material after physical aging for 480 days, the data are summarized in\nTable 3. As shown in Figure 4, compared to the fresh membranes, the He permeability\nof the Matrimid-Cyclen-5% membrane decreased from 66.34 Barrer to 42.87 Barrer\nafter 480 days of physical aging, a decrease of 35%. Meanwhile, the He/N selectivity\n2\nincreased from 663.4 to 4763.3, and the He/CH selectivity increased from 1660.75 to\n4\\"}, "author_response": "Response to the Reviewers' Comments:\nReviewer #1 (Remarks to the Author):\nThe manuscript reports on the combination of cyclen and Matrimid into a mixed matrix\nmembrane that has high selectivity for gas separation. Both cyclen and matrimid have\nbeen studied separately, and there exists a lot of literature on both materials. Hence, the\nnovelty being presented is the combination of the two into a membrane. The resulting\nselectivities are very high, which gives me concern, as generally for mixed matrix\nmembrane systems, selectivity is aligned with either of the membrane or additive, and\nnot an enhancement. I can understand the argument for the strong binding between\ncyclen and Matrimid, but that does not necessarily translate into the high selectivities\nreported.\nReply: Many thanks for your valuable comments. While it is true that both Cyclen and\nMatrimid have been extensively studied separately, their combination into a mixed\nmatrix membrane represents a new approach to achieve high gas separation\nperformance, especially for He/CH₄ selectivity. The Matrimid membrane is widely\nstudied for gas separation, while its low permeability (33 Barrer) and moderate\nselectivity (86) still present a great challenge to extract Helium(He) from natural gas\n(ultra-low helium concentration). The cyclen (1, 4, 7, 10-tetraazacyclododecane) is one\nof the most extensively studied ligands in coordination chemistry (Shinoda, S.\nDynamic cyclen-metal complexes for molecular sensing and chirality signaling. Chem.\nSoc. Rev. 2013, 42, 1825-1835.), while its potential for He extraction from natural gas\nhas not been explored until now. The ultra-low concentration of He makes it very\ndifficult to extract from natural gas. Therefore, it’s of great significance to develop the\nMatrimid-Cyclen membranes with high He/CH selectivity (>1000). In this work, the\n4\nthe combination of Matrimid and Cyclen (Matrimid-Cyclen-5%) membranes\ndemonstrates high He permeability (66Barrer) and He/CH selectivity (>1660).\nthe materials involved. The enhancement in selectivity is not simply additive, but from\nthe strong hydrogen bonding interactions between Cyclen and Matrimid, which\nmodifies the polymer chain packing and results in a tighter interchain structure. This\nleads to a more effective sieving mechanism. Usually, polymer membranes have good\nprocessability, high stability, and low cost, but they often suffer from plasticization\nproblems, physical aging issues, and intrinsic permeability-selectivity trade-off\nlimitations, which makes it a great challenge to obtain high permeability together with\nsufficient selectivity [1-4]. Molecular sieve materials have attractive transport properties\nwell above the polymer upper bound, but they tend to have poor membrane-forming\nand mechanical property [5]. Mixed matrix membranes incorporate molecular sieve\nmaterials with certain pore sizes into a processable polymer matrix to improve the gas\nselectivity and permeability of polymer membranes by integrating the advantages of\nboth [6-8], which is the significance of preparing hybrid matrix membranes. Therefore,\nthe mixed matrix membranes (Matrimid-Cyclen) combine the advantages of easy\nprocessibility from Matrimid polymer and high gas permeability/selectivity from\nmolecular sieve material (Cyclen).\nTable 1 summarizes the He separation performance of Matrimid-based membranes.\nThe Matrimid-Cyclen-5% membranes exhibit high He permeability (66 Barrer) and\nHe/CH selectivity (>1660), which far exceed most of the other Matrimid-based\n4\nmembranes.\nTable 1. Summary of literature He/N and He/CH separation performance.\n2 4\nHe He/N He/CH\n2 4\nOperating Gas Refer\nMembrane Name Permeabili Selectivit Selectivit\nconditions Type ence\nty (Barrer) y y\nMatrimid-Cu-BTC-R-40 66.40 265.80 369.10 5 atm, 35 °C Pure 1\nMatrimid-Cu-BTC-30 51.80 193.40 257.90 5 atm, 35 °C Pure 2\nMatrimid-Cu-BDC-15% 20.50 341.70 410.00 7 atm, 35 °C Pure 3\nMatrimid-MgO-40% 45.00 86.1 118.4 10 atm, 35 °C Pure 4\nMatrimid-CNFs-10% 16.10 89.50 179.50 20 atm, 35 °C Pure 5\nMatrimid-PIM-EA(H )- \n2\n197.00 28.84 21.55 1 atm, 25 °C Pure 6\nTB-50%\nMatrimid-p-xylenediami\n21.70 112.00 155.00 10 atm, 35 °C Pure 7\nne-CL-14\nMatrimid-C60-2.5% 21.00 91.70 148.80 10 atm, 35 °C Pure 8\nMatrimid-Cyclen-3% 57.57±0.2 428.35±2. 959.50±4. This\n1 atm, 25 °C Pure\n1 15 67 work\nMatrimid-Cyclen-5% 66.43±0.2 621.70±3. 1660.75± This\n1 atm, 25 °C Pure\n2 20 5.25 work\nMatrimid-Cyclen-10% 78.63±0.2 543.43±4 1310.50± This\n1 atm, 25 °C Pure\n8 0.34 4.67 work\nSupplementary References\n[1] A. Akbari, J. Karimi-Sabet, S.M. Ghoreishi, Intensiffcation of helium separation\nfrom CH and N by size-reduced Cu-BTC particles in Matrimid matrix, Separation and\n4 2\nPuriffcation Technology, 251, 117317 (2020).\n[2] A. Akbari, J. Karimi-Sabet, S.M. Ghoreishi, Matrimid® 5218 based mixed matrix\nmembranes containing metal organic frameworks (MOFs) for helium separation,\nChemical Engineering & Processing: Process Intensification, 148, 107804 (2020).\n[3] A. Ali, J. Karimi-Sabet, S.M. Ghoreishi, Polyimide based mixed matrix membranes\nincorporating Cu-BDC nanosheets for impressive helium separation, Separation and\nPurification Technology, 253, 117430 (2020).\n[4] S.S. Hosseini, Y. Li, T.S. Chung, Y. Liu, Enhanced gas separation performance of\nnanocomposite membranes using MgO nanoparticles, Journal of Membrane Science,\n302 (1-2), 207-217 (2007).\n[5] M. Dohade, Incorporation of carbon nanofibers into a Matrimid polymer matrix:\nEffects on the gas permeability and selectivity properties, Journal of Applied Polymer\nScience, 135 (12) 46019 (2018).\n[6] E. Esposito, I. Mazzei, M.Monteleone, A. Fuoco, M. Carta, N.B. Makeown, R.\nMalpass-Evans, J.C. Jansen, Highly permeable Matrimid®/PIM-EA (H )-TB blend\n2\nmembrane for gas separation, Polymers, 11 (1), 46 (2018).\n[7] P.S. Tin, T.S. Chung, Y. Liu, R. Wang, S.L. Liu, K.P. Pramoda, Effects of\ncross-linking modification on gas separation performance of Matrimid membranes,\nJournal of Membrane Science, 225(1-2), 77-90 (2003).\n[8] T.S. Chung, S.S. Chan, R. Wang, Z.H. Lu, C.B. He, Characterization of\npermeability and sorption in Matrimid/C60 mixed matrix membranes, Journal of\nMembrane Science, 211 (1), 91-99 (2003).\nThere is no reported error for the experimental results and no discussion on\nreproducibility, so this raises questions about the provided data.\nReply: Many thanks for your comments. Repeated experiments were conducted and\nthe error bars have been included in the revised manuscript, The gas separation\nperformance and fabrication of mixed matrix membrane with gas molecule recognition\nwindow (cyclen) was found to be reproducible. The specific data has been listed as\nfollows.\nTable 1. Summary of Gas transport properties of the Matrimid and\nMatrimid-Cyclen-n% membranes at 1.0 atm and 25 ℃ (n = 1-15% indicated the degree\nof Cyclen mass loading).\nGas permeability (Barrer)\nMembrane\nHe H CO N CH\n2 2 2 4\nMatrimid 33.59±0.41 27.45±0.08 15.03±0.06 0.42±0.03 0.39±0.03\nMatrimid-Cyclen-1% 47.87±0.24 43.04±0.16 16.90±0.06 0.30±0.02 0.13±0.01\nMatrimid-Cyclen-3% 57.57±0.21 55.69±0.23 17.66±0.12 0.13±0.00 0.06±0.00\nMatrimid-Cyclen-5% 66.43±0.22 62.17±0.32 20.07±0.08 0.10±0.00 0.04±0.00\nMatrimid-Cyclen-10% 78.63±0.28 76.08±0.12 22.17±0.15 0.14±0.01 0.06±0.00\nMatrimid-Cyclen-15% 85.93±0.80 80.85±0.30 24.76±0.07 0.24±0.02 0.13±0.01\nIdea Selectivity\nMembrane\nHe/N He/CH H /N H /CH CO /N CO /CH\n2 4 2 2 2 4 2 2 2 4\n79.97±6. 86.13±2. 65.00±4 70.38±1 35.79±2. 38.54±0.\nMatrimid\n05 25 .82 .98 88 99\n145.06±1 368.23±2 143.47± 331.08± 56.33±3. 130.00±1\nMatrimid-Cyclen-1%\n0.29 8.10 9.71 26.34 81 0.33\n575.70±2 959.50±4 428.35± 928.17± 135.85± 294.33±2\nMatrimid-Cyclen-3%\n.15 .67 2.15 4.67 0.92 .00\n664.30±2 1660.75± 621.70± 1554.25 200.70± 501.75±3\nMatrimid-Cyclen-5%\n.10 5.25 3.20 ±8.00 0.80 .75\n561.64±4 1310.50± 543.43± 1268.00 158.36± 369.50±2\nMatrimid-Cyclen-10%\n1.36 4.67 40.34 ±3.17 11.10 .33\n358.04±1 661.00±4 336.88± 621.92± 103.17± 190.46±1\nMatrimid-Cyclen-15%\n2.28 8.42 11.59 49.33 3.64 4.71\nTable 2. Summary of gas transport permeability and selectivity of the\nMatrimid-Cyclen-5% membrane at 1.0 atm and different temperatures.\nGas permeability (Barrer) Selectivity\nTemperat\nure (℃)\nHe N CH He/N He/CH\n2 4 2 4\nReturn to\n63.97±0.46 0.09±0.00 0.03±0.00 710.78±2.33 2132.33±7.00\n20 ℃\n0 ℃ 52.74±0.46 0.007±0.00 0.003±0.00 7534.29±24.28 17580.00±56.67\n20 ℃ 63.13±0.46 0.10±0.00 0.03±0.00 631.30±2.60 2104.33±8.67\n40 ℃ 80.14±0.46 0.32±0.02 0.10±0.01 250.44±15.13 801.40±83.82\n60 ℃ 96.91±0.46 1.43±0.46 1.05±0.00 67.77±2.90 92.30±1.73\n80 ℃ 125.12±0.46 3.45±0.46 3.05±0.46 36.27±0.36 41.02±0.87\n100 ℃ 202.55±0.46 9.76±0.46 9.42±0.46 20.75±0.40 21.73±0.58\nReturn to\n68.69±0.27 0.12±0.01 0.03±0.00 572.42±49.85 2289.67±8.00\n20 ℃\nTable 3. Summary of gas transport permeability and selectivity of the\nMatrimid-Cyclen-5% membrane at different pressures and 25℃.\nGas permeability (Barrer) Selectivity\nPressure\n(Bar)\nHe N CH He/N He/CH\n2 4 2 4\nReturn to 1\n66.84±0.42 0.10±0.00 0.04±0.00 668.40±4.20 1671.00±10.50\nBar\nTable 4. Summary of gas transport properties of the Matrimid and\nMatrimid-Cyclen-5% membrane at 1.0 atm and 25 ℃ during the aging process.\nGas permeability (Barrer)\nMembrane\nHe H CO N CH\n2 2 2 4\nFresh 66.43±0.22 62.17±0.32 20.07±0.08 0.10±0.00 0.04±0.00\nAging for 7 days 64.22±.34 59.90±0.23 18.30±0.07 0.09±0.01 0.04±0.00\nAging for 20 days 59.96±0.53 54.68±0.14 16.67±0.17 0.07±0.005 0.02±0.00\nAging for 40 days 59.73±0.09 53.39±0.33 15.96±0.09 0.06±0.00 0.01±0.00\nAging for 110 days 54.31±0.10 50.76±0.10 14.12±0.03 0.05±0.00 0.008±0.00\nAging for 480 days 42.87±0.21 31.43±0.13 7.82±0.04 0.009±0.00 0.001±0.00\nSelectivity\nMembrane\nHe/N He/CH H /N H /CH CO /N CO /CH\n2 4 2 2 2 4 2 2 2 4\n664.30±2.1 1660.75± 621.70± 1554.25± 200.70 501.75±3\nFresh\n0 5.25 3.20 8.00 ±0.80 .75\n713.56±67. 1605.50± 665.56± 1497.50± 203.33 457.50±1\nAging for 7 days\n960 8.50 68.04 21.22 ±21.79 .42\n856.57±57. 2998.00± 781.14± 2734.00± 238.14 833.50±9\nAging for 20 days\n74 26.50 54.20 6.76 ±18.02 .66\n995.50±1.5 5973.00± 889.83± 5339.00± 266.00 1596.00±\nAging for 40 days\n0 9.00 5.07 30.45 ±1.33 7.94\n1086.20±2. 6788.80± 1015.20 6345.00± 282.40 1765.00±\nAging for 110 days\n00 12.45 ±1.74 10.90 ±0.53 3.31\n4763.33±2 42870.00 3496.22 31430.00 868.89 7820.00±\nAging for 480 days\n3.34 ±200.00 ±12.62 ±113.58 ±4.01 36.06\nFigure 1 shows the preparation process of the Matrimid-Cyclen membrane, the\ndispersion of cyclen in Matrimid solutions is homogeneous. There is no Tyndall\nphenomenon in the clear and transparent Matrimid/Cyclen solutions, indicating that\nCyclen is uniformly dispersed in the mixed solution. The reproducibility of a mixed\nmatrix membrane depends on the dispersibility of the filler, the higher the solubility of\nthe filler and the better the dispersion in the mixed solution, the smaller the error in the\nperformance of the membrane material.\nFigure 1. Preparation process of the Matrimid-Cyclen membranes. (a) Cyclen powder\n(1.0, 3.0, 5.0, 10.0 or 15.0 mg) were added to 6.0 g DMF solvent to obtain clear and\ntransparent solutions. (b) The clean Matrimid dope solution was added to the\nCyclen/DMF solution to obtain homogeneous Matrimid/Cyclen solutions. (c) The\nMatrimid/Cyclen solutions was injected into clean petri dishes and transferred to a\nvacuum oven at an ambient temperature of 120 °C for 8-hour solvent evaporation to\nobtain Matrimid-Cyclen membranes.\nI am also concerned about the high cyclen loading in the membrane, up to 15%. There\nis no discussion about mechanical properties in the manuscript, but the literature on\nMatrimid is very clear that high levels of additives result in a very brittle membrane. I\nsee no reason why this is not the case here. This has issues with the ability to translate\nthe performance into a viable membrane.\nReply: Thanks for your comments. We understand your concern about the potential\nbrittleness of the Matrimid mixed matrix membranes with higher Cyclen loadings. To\naddress this, the dynamic mechanical thermal analysis (DMTA) was conducted on\nMatrimid-Cyclen samples to assess the mechanical properties across a temperature\nrange of 50 to 350 °C. As shown in Figure 3 (g-i), the storage modulus (E′) of the\nMatrimid-Cyclen membranes (1800 MPa) was almost completely maintained before\nreaching the corresponding T . This indicated that the membrane retains its mechanical\ng\nintegrity even with higher Cyclen content. Furthermore, we observed that higher\nCyclen loadings did increase in the brittleness of the membranes, as seen in the\ndecreasing peak intensity of damping (Tan δ) with increasing Cyclen content (Figure\n3i). These data demonstrated that this kind of gas separation membrane was endowed\nwith good mechanical property and thermal stability, even with the higher Cyclen\nloading.\nFigure 3. Characterization of Matrimid-Cyclen membranes. (g) The dynamic\nmechanical thermal analysis (DMTA) spectra of Matrimid-Cyclen membranes at\ntemperature range from 50 to 350 ℃. (h) The storage modulus (E′) and (i) Tan δ of\nMatrimid-Cyclen membranes.\nThe operating temperature, feed pressure and aging behaviors follows the expected\ntrend of mixed matrix membranes, with the presented discussion valid. The application\nfor helium extraction is important, and it is good that the authors considered both\nseparation from methane and nitrogen. I recommend that the manuscript be rejected,\nand the authors submit to a more appropriate journal.\nReply: Thank you very much for your professional comments and suggestions. We\nsincerely apologize for some inadequate in data analysis and descriptions in the original\nmanuscript. As you pointed out, several issues required further attention, and we have\nmade substantial revisions to address the concerns. Below is a detailed list of our\nresponses to your comments. In our work, we designed the mixed matrix membranes\nbased on several key considerations derived from numerous studies.\nFirstly, Cyclen was selected as filler due to its size-sieving effect, which is beneficial\nfor He separation. Based on the molecular dynamic diameters of He (2.60 Å), H (2.89\n2\nÅ), CO (3.30 Å), N (3.64 Å) and CH (3.80 Å), the Cyclen with cavity size (3.2-3.4 Å)\n2 2 4\nbetween the molecular dynamic diameters of He and N was selected, which makes it\n2\nan ideal candidate for He separation, such as He/N and He/CH . Furthermore, the\n2 4\nCyclen with a certain cavity size creates a special transport channel for small gases, like\nHe, which increases their transport paths in the membrane, counteracting the negative\neffect of the increased chain stacking density and enhancing the permeability of small\ngas molecules.\nSecondly, the chosen filler must have good solubility and dispersion in solvents. To\nensure uniform dispersion of the filler in the membrane and to minimize the formation\nof non-selective voids caused by filler agglomeration, we selected Cyclen for its high\nsolubility and good dispersion in organic solvents. Cyclen was fully dissolved in DMF,\nand then the clean Matrimid dope solution was added to the Cyclen/DMF solution. The\nresulting homogeneous Matrimid/Cyclen solution, which was stirred for 24 hours and\nsonicated for 1 hour, remained clear and transparent, even with up to 15% Cyclen\ncontent, as shown in Figure 1.\nFigure 1. (a) Cyclen powder (1.0, 3.0, 5.0, 10.0 or 15.0 mg) were added to 6.0 g DMF\nsolvent to obtain clear and transparent solutions. (b) Matrimid dope solution. (c) The\nclean Matrimid dope solution was added to the Cyclen/DMF solution to obtain clear\nand transparent mixed Matrimid/Cyclen solutions.\nThirdly, compatibility between filler and polymer matrix. It is crucial to create mixed\nmatrix membranes with fair matching between filler and polymer matrix to gain\nattractive performance. This is why we chose Cyclen as a filler. Hydrogen bonding\nbetween Cyclen and the Matrimid polymer chains immobilizes Cyclen molecules in the\ninterchain gaps of the Matrimid membrane, resulting in homogeneously mixed matrix\nmembranes. The SEM image (Figure 2) shows the surface and interface of the\nmembrane, confirming the good interfacial compatibility between Cyclen and\nMatrimid.\nFigure 2. SEM images of the cross section of the Matrimid-Cyclen-n% membranes up\nto ×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nFourthly, Helium (He) purification requirements. Helium is an extremely scarce,\nnon-renewable gas with a strong safety profile that is an essential resource for the\ndevelopment of many high-tech industries, such as the aerospace industry, electronics,\nmedical equipment, and scientific research. On Earth, He occurs in very low\nconcentrations, mainly in the atmosphere and in natural gas. Helium is commonly\nextracted industrially from high-pressure natural gas. However, the initial\nconcentration of He in most natural gas reservoirs is still as low as 0.3%, and to extract\nhelium from natural gas with ultra-low He content, the gas separation membranes must\npossess He/CH selectivity greater than 1000. Highly permeable polymers rarely\n4\nprovide sufficient selectivity for high-purity He extraction, and Matrimid was chosen as\nthe polymer matrix due to its dense stacking of polymer chains. XRD characterization\nrevealed that the strong hydrogen bonding between Cyclen and Matrimid modulated\nthe chain stacking of the mixed matrix membranes, which reduce the pore size, narrow\nthe pore size distribution and enhances the selectivity of the membrane by modulating\nthe chain spacing.\nWe have verified the success of the hybrid matrix membrane design with ultra-high\nhelium selectivity to achieve excellent separation performance.\nReviewer #2 (Remarks to the Author):\n• What are the noteworthy results?\nThe work seems to be looking at high selectivity membranes for He/CH4 separation, by\naltering the properties of a commonly used polyimide in gas separation, Matrimid,\nadding different concentrations of Cycle, which is presented as a molecule (particle,\nmolecule? Please clarify) with a wide opening providing H-bonding with the Matrimid\npolymer chains. Both the porosity of this filler and the crosslinking thereof seems to be\nhaving a significant influence in increasing the selectivity of Matrimid above the\nstate-of-the-art of such membrane materials without decreasing the permeability of the\nsmaller gas molecules.\nReply: Thanks for your recognition and comments. In this work, the cyclic Cyclen with\nspecific intra-ring dimensions was incorporated into Matrimid as a pore structure\nmodifier to improve the He/CH selectivity. We explored both inorganic or organic\n4\nfillers for fabricating the MMMs to enhance the gas separation performance. While, the\ninorganic fillers (e.g., MOFs, ZIFs, and Mxene) often suffer from poor compatibility\nwith polymer matrix, and the agglomeration results in defects in the membrane.\nNotably, Cyclen, a purely organic additive has better compatibility with Matrimid\npolymer matrices than inorganic fillers.\nAs shown in Figure 1, even with Cyclen loading increased from 1% to 15%, the tyndall\neffect never appeared in the Cyclen solution and the Cyclen/Matrimid mixture,\nindicating that Cyclen was bound to Matrimid at the molecular level.\nFigure 1. (a) Cyclen powder (1.0, 3.0, 5.0, 10.0 or 15.0 mg) were added to 6.0 g DMF\nsolvent to obtain clear and transparent solutions. (b) Matrimid dope solution. (c) The\nMatrimid dope solution was added to the Cyclen/DMF solution to obtain clear and\ntransparent mixed Matrimid/Cyclen solutions.\nThis molecular dispersion of Cyclen within the membrane was further confirmed by\nSEM and EDS analysis. The cross-section morphology of Matrimid-Cyclen-n% (n = 1,\n3, 5, 10, 15) membranes shown in Figure 2, at different magnifications , revealed an\nincrease in torn lamellar structures as Cyclen loading increases. This morphology is\nattributed to the enhanced interaction between the polymer chains and filler, which\nresults in interfacial stress during the fracturing of MMMs in liquid nitrogen for SEM\nanalysis. This stress can deform the polymer segments, leading to increased plastic\ndeformation. The Matrimid-Cyclen-15% membrane material remained homogeneous\nat 50,000x magnification, without any significant aggregation or discrete particulate\nphases observed. The elemental mapping (Figure 3) further supports the homogeneous\ndistribution of Cyclen within the membrane, showing a gradual increase in nitrogen (N)\nelemental distribution as Cyclen content rises, without any noticeable aggregation.\nThis is further evidence that Cyclen is dispersed as a molecule in polymer membrane.\nFigure 2. SEM images of the cross section of the Matrimid-Cyclen-n% membranes up\nto ×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nFigure 3. SEM images and EDS with nitrogen (N) mapping of planar scans of the cross\nsection of the Matrimid-Cyclen-n% membranes (n% is the Cyclen mass loading and\nwas varied from 1 to 15 wt%, n = 0, 1, 3, 5, 10, 15).\nIn this work, Cyclen was selected as the filler for several key reasons to prepare the\nMatrimid matrix, which can lead to ultra-high He permeability and ultra-high He/CH\n4\nseparation selectivity for several main reasons:\nFirstly, Cyclen is an organic macrocyclic compound with a flexible structure and its\ncavity size is approximately 3.2-3.4 Å. This cavity size lies between the molecular\ndynamic diameters of He (2.60 Å) and N (3.64 Å), producing a strong molecular\n2\nsieving effect, particularly for He/N and He/CH .\n2 4\nSecondly, the strong hydrogen bonds between Cyclen and Matrimid modulate the chain\nspacing increasing the chain stacking density of the mixed matrix membranes. This\nmodification enhances the resistance to mass transfer for larger gases, such as N₂ and\nCH₄, improving the overall selectivity of the membrane.\nThirdly, Cyclen with a certain cavity size creates a special transport channel for small\ngases, like He, which increases their transport paths in the membrane, counteracting the\nnegative effect of the increased chain stacking density and enhancing the permeability\nof small gas molecules.\nHerein MMMs with ultra-high He/CH separation selectivity, exceeding the\n4\nperformance of most existing membrane, were prepared using a simple solution casting\nmethod. The He/CH selectivity reached up to an impressive value of 6788 after\n4\nphysical aging for 110 days, which outperformed almost all reported polymer-based\nmembranes and was even comparable to that of some advanced carbon molecular sieve\n(CMS) membranes. In addition, compared with CMS membrane,MOF membrane and\nCOF membrane with high He/CH selectivity , the MMMs have good solution\n4\nprocessability and stable performance, which is easy to achieve industrial production.\nAll statements in the scientific paper should be justified, either by results presented in\nthe work, or by references to previous reports in state-of-the-art literature. See for\ninstance lines 193-196, “The ultrahigh gas separation performance of mixed\nmatrix membrane attributed to enhanced interfacial compatibility due to dense hydrogen\nbonding between Cyclen and Matrimid chains”, before showing any gas separation\nresults.\nReply: Many thanks for your valuable comments. We have revised the statements\naccording to your suggestion by adding the references after the expression. Please refer\nto the revised manuscript for the updated text. The sentence has been revised as “The\ndense hydrogen bonding between Cyclen and Matrimid enhances their interfacial\ncompatibility, reduces non-selective interfacial gaps and significantly improves the\nhigh gas separation performance of the mixed matrix membranes [21, 32]”.\nThe authors often do not comment on the hypotheses until they are about to introduce\nthe results (see lines 307-311 introducing the effect of operating temperature, pressure\nand “aging”.\nReply: Thanks for your comments. The hypotheses were deleted according to your\nsuggestion. Please refer to the revised manuscript.\nAlready in the Highlights, why do the authors talk of Cyclen as a “window” and not an\namine molecule with such and such properties that give the potential to provide the\nMatrimid polyimide matrix with the desired transport/separation properties in the\nHe/CH4 separation? Please, be more precise.\nReply: Thanks for your comments. We agree with your description that the Cyclen is\nan amine molecule from the perspective of its chemical structure. We would like to\nclarify our use of the term \"window\" in the context of its role in the membrane.\nFirstly, as an amine molecule, Cyclen forms the strong hydrogen bonding with the\nMatrimid polymer, which tightens the d-spacing of Matrimid polymer chains and\nminimizes defects within the membrane. This arrangement leads to longer and more\ntortuous transport pathways for the kinetically larger diameters of N and CH in the\n2 4\nmembrane.\nSecondly, and more importantly, Cyclen acts as pore structure modifier with a cavity\nsize of approximately 3.2-3.4 Å, which allows Cyclen to create dedicated transport\nchannels for He molecules providing a “window” for smaller gas molecules, such as He,\nto pass through more easily. The size of this “window” induces a strong molecular\nsieving effect, reducing the permeability of N and CH and increasing the selectivity of\n2 4\nmultiple important gas pairs.\nTherefore, the presence of Cyclen plays a dual role in the MMM, i.e., as a “window”\nand an amine molecule to construct the molecular sieve membrane for He/N and\n4\n• Will the work be of significance to the field and related fields? How does it compare to\nthe established literature? If the work is not original, please provide relevant references.\nReply: Thank you for recognizing the significance of this work in He purification from\nnatural gas and related fields.\nThe Matrimid membrane has been widely studied for He separation, while its low He\npermeability (33 Barrer) and He/CH selectivity (86) still meet a great challenge to\n4\nextract He from natural gas (ultra-low helium concentration). The Cyclen (1, 4, 7,\n10-tetraazacyclododecane) is one of the most extensively studied ligands in\ncoordination chemistry (Shinoda, S. Dynamic Cyclen-metal complexes for molecular\nsensing and chirality signaling. Chem. Soc. Rev. 2013, 42, 1825–1835.), but it has not\nbeen explored for He extraction from natural gas till now. The ultra-low concentration\nof He makes it very difficult to be extracted from natural gas, highlighting the\nsignificance of developing Matrimid-Cyclen membranes with high He/CH selectivity\n4\n(>1000). In this work, the combination of Matrimid and Cyclen (Matrimid-Cyclen-5%)\nmembranes demonstrated high He permeability (66 Barrer) and He/CH selectivity\n4\n(>1660).\nThe Matrimid-based membranes and related works for He separation were summarized\nin Table 1. Compared with the existing literature, Matrimid-Cyclen membranes offer\nmoderate He permeability and ultra-high He/CH selectivity, surpassing the\n4\nperformance of all other Matrimid-based membranes (Figure 1). Notably, this is the\nfirst report in where the cyclic Cyclen molecules are incorporated into Matrimid as\nmolecular windows to prepare hybrid matrix membranes for He purification.\nFigure 1. Plots of (a) He/N , (b) He/CH separation performance of Matrimid-Cyclen\n2 4\nmembranes compared with other Matrimid-based membranes.\nTable 1. Summary of literature He/N and He/CH separation performance.\n2 4\nHe He/N He/CH\n2 4\nOperating Gas Refer\nMembrane Name Permeabili Selectivit Selectivit\nconditions Type ence\nty (Barrer) y y\nMatrimid-Cu-BTC-R-40 66.40 265.80 369.10 5 atm, 35 °C Pure 1\nMatrimid-Cu-BTC-30 51.80 193.40 257.90 5 atm, 35 °C Pure 2\nMatrimid-Cu-BDC-15% 20.50 341.70 410.00 7 atm, 35 °C Pure 3\nMatrimid-MgO-40% 45.00 86.1 118.4 10 atm, 35 °C Pure 4\nMatrimid-CNFs-10% 16.10 89.50 179.50 20 atm, 35 °C Pure 5\nMatrimid-PIM-EA(H )- \n2\n197.00 28.84 21.55 1 atm, 25 °C Pure 6\nTB-50%\nMatrimid-p-xylenediami\n21.70 112.00 155.00 10 atm, 35 °C Pure 7\nne-CL-14\nMatrimid-C60-2.5% 21.00 91.70 148.80 10 atm, 35 °C Pure 8\nMatrimid-Cyclen-3% 57.57±0.2 428.35±2. 959.50±4. This\n1 atm, 25 °C Pure\n1 15 67 work\nMatrimid-Cyclen-5% 66.43±0.2 621.70±3. 1660.75± This\n1 atm, 25 °C Pure\n2 20 5.25 work\nMatrimid-Cyclen-10% 78.63±0.2 543.43±4 1310.50± This\n1 atm, 25 °C Pure\n8 0.34 4.67 work\nSupplementary References\n[1] A. Akbari, J. Karimi-Sabet, S.M. Ghoreishi, Intensiffcation of helium separation\nfrom CH and N by size-reduced Cu-BTC particles in Matrimid matrix, Separation and\n4 2\nPuriffcation Technology, 251, 117317 (2020).\n[2] A. Akbari, J. Karimi-Sabet, S.M. Ghoreishi, Matrimid® 5218 based mixed matrix\nmembranes containing metal organic frameworks (MOFs) for helium separation,\nChemical Engineering & Processing: Process Intensification, 148, 107804 (2020).\n[3] A. Ali, J. Karimi-Sabet, S.M. Ghoreishi, Polyimide based mixed matrix membranes\nincorporating Cu-BDC nanosheets for impressive helium separation, Separation and\nPurification Technology, 253, 117430 (2020).\n[4] S.S. Hosseini, Y. Li, T.S. Chung, Y. Liu, Enhanced gas separation performance of\nnanocomposite membranes using MgO nanoparticles, Journal of Membrane Science,\n302 (1-2), 207-217 (2007).\n[5] M. Dohade, Incorporation of carbon nanofibers into a Matrimid polymer matrix:\nEffects on the gas permeability and selectivity properties, Journal of Applied Polymer\nScience, 135 (12) 46019 (2018).\n[6] E. Esposito, I. Mazzei, M.Monteleone, A. Fuoco, M. Carta, N.B. Makeown, R.\nMalpass-Evans, J.C. Jansen, Highly permeable Matrimid®/PIM-EA (H )-TB blend\n2\nmembrane for gas separation, Polymers, 11 (1), 46 (2018).\n[7] P.S. Tin, T.S. Chung, Y. Liu, R. Wang, S.L. Liu, K.P. Pramoda, Effects of\ncross-linking modification on gas separation performance of Matrimid membranes,\nJournal of Membrane Science, 225(1-2), 77-90 (2003).\n[8] T.S. Chung, S.S. Chan, R. Wang, Z.H. Lu, C.B. He, Characterization of\npermeability and sorption in Matrimid/C60 mixed matrix membranes, Journal of\nMembrane Science, 211 (1), 91-99 (2003).\nGiven the low quantity of He in the feed stream it is reasonable that the authors look at\nMatrimid because of its low permeability and high selectivity properties and try this\nhybridization to increase the pore size and therefore the permeability of the Matrimid\npolymer. The state-of-the-art does very vaguely support this justification, and only\nreading between lines with prior knowledge of mixed matrix membrane technology and\nchallenges allows the reader to withdraw the novelty of the work. This is probably due\nto the fact that the bibliographic review of the authors is not mentioned until Figure 5\nwhere the data on Matrimid mixed matrix membranes collected in the supplementary\ninformation is used to build the Robeson upper-bound trade-off figures for the different\ngas pairs considered in this work. Please revise.\nReply: Many thanks for your comments. Matrimid and Matrimid-based mixed matrix\nmembranes have been extensively studied for gas separation and pervaporation\nprocesses due to the combination of relatively high gas permeability coefficients and\nseparation factors as well as excellent mechanical properties, high solubility in\nnon-hazard organic solvents, and high glass transition temperature in comparison to\npolycarbonate, polysulfone, and other materials. Due to the excellent properties of the\nshown by Matrimid membrane, which is desirable for the gas separation process, it is\nworthwhile to explore modifications to further enhance their performance for He\nseparation.. In response to your suggestion, we have added the rationale for choosing\nMatrimid as the polymer matrix in the “Introduction” section of the manuscript. We\nhave also summarized the relevant studies on Matrimid-based membrane materials for\nHe separations, discussed the challenges associated with mixed-matrix membrane\ntechnology, and clarified the idea and novelty and purpose of our design. These\nrevisions aim to better explain the basis and significance of our work. Please refer to the\nintroduction in the revised manuscript.\nDoes the work support the conclusions and claims, or is additional evidence needed?\nBesides, but no other information on the morphology and porosity characteristics\njustifying the authors’selection as filler in Matrimid polyimide are presented, so the\nreader cannot judge for himself the reasons why the crosslinking only appear at\nloadings above 15%, or the presence or absence of crosslinking with the polymer\nmatrix that leads to higher selectivity and usually decreases permeability, or the “torn\nlamellar structures” formation, as well as the evidence of the strong H-bonding\nmentioned in the Results section.\nReply: We apologize for any inadequate data analysis and descriptions in the\nmanuscript, which may have made some of this evidence unclear. We have tried our\nbest to resolve the concerns and made a proper revision of the manuscript. A detailed\nlist of responses to the comments is provided below.\nAdditional experiments and evidence related to morphology and porosity\ncharacteristics have been supplemented. The SEM images in Figure 1 shows that the\ncross-section morphology of the MMMs is dense and homogeneous, demonstrating\nexcellent interfacial compatibility between Cyclen and Matrimid.\nAs Cyclen loading increases, the torn lamellar structures in the membranes also\nincrease. This morphology is attributed to the enhanced interaction between the\nMatrimid and Cyclen, which results in the formation of interfacial stress during the\nfracturing of MMMs in liquid nitrogen. This stress causes deformation of the polymer\nsegments, resulting in elongated polymer segments with increased plastic deformation.\nThere is no significant aggregation or discrete particulate phases observed in the\nMatrimid-Cyclen-15% membrane from 1,000x to 50,000x electron microscopy. The\nimproved compatibility between organic fillers and polymer matrices is conducive to\nobtain simultaneous improvements in gas permeability and selectivity.\nFigure 1. SEM images of the cross section of the Matrimid-Cyclen-n% membranes up\nto ×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nThe pore size distribution of the Matrimid-Cyclen membranes in Figure 2 shows that\nthe addition of Cyclen significantly reduced the micropore size of the Matrimid-Cyclen\nmembrane. Additionally, the content of ultramicropores with a size of about 3.9 Å,\ngradually increased compared to the original Matrimid membrane. The introduction of\nCyclen also resulted in the formation of new pores with a size of about 3.2 Å.\nThis modification of the pore structure is attributed to the hydrogen bonding interaction\nbetween Cyclen and Matrimid which enhances the molecular sieving properties of\nMatrimid-Cyclen membranes by narrowing the pore size distribution. Cyclen creates\nadditional and longer transport channels for He, which significantly improves the\ntransport capacity of He.\nMatrimid-Cyclen-10%\nMatrimid-Cyclen-5%\nMatrimid\n0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1\nPore Width (nm)\n)g/³mc(\nWd/Vd\nFigure 2. Pore size distribution of the Matrimid-Cyclen membranes. The pore size\ndistribution was calculated from the corresponding H adsorption isotherm using the\nThe presence of strong hydrogen bonding interactions was further confirmed by FTIR\nspectra, which shows that the hydrogen bonding between Cyclen and Matrimid\npolymer chains immobilizes Cyclen molecules within the interchain gaps of the\nMatrimid matrix, resulting in homogeneous mixed matrix membranes.\nHave authors envisaged the measuring of BET surface area of Cyclen and the MMM\nfor verification? I guess they have done, and the Cyclen is a sacrificial filler given the\nstatement in page This could visualize the statement of the authors “ in lines 214 – 216,\n“the decomposition of Cyclen leaves many micropores in the Matrimid-Cyclen\nmembranes”, but this porosity has not been shown in the paper, or is not revealed in the\ncross-sectional images in SEM pictures. Please clarify, this is important to show\nevidence on the trade-off between crosslinking and additional porosity effects on the\nmixed matrix membrane performance observed in the results section.\nReply: Thanks for your comments. The pore size distribution of the Matrimid-Cyclen\nmembranes in Figure 1 shows that the addition of Cyclen significantly reduced the\nmicropore size of the Matrimid-Cyclen membrane. The content of ultramicropores\nwith a size of about 3.9 Å, gradually increased compared to the original Matrimid\nmembrane. This modification of the pore structure is attributed to the hydrogen\nbonding interaction between Cyclen and Matrimid which enhances the molecular\nsieving properties of Matrimid-Cyclen membranes by narrowing the pore size\ndistribution. Additionally, Cyclen with pore size (3.2 Å) creates additional and longer\ntransport channels for He in the membranes, which significantly improves the\ntransport capacity of He. Carbon molecule sieving (CMS) membrane after thermal\ntreament will be evaluated in the future.\nFigure 1. BET surface area and pore size analysis for Cyclen and Matrimid-Cyclen\nmembranes. (a) H sorption isotherm curves collected at 77K and (b) Corresponding\n2\npore size distribution profiles. The pore size distribution was calculated from the\ncorresponding H adsorption isotherm using the DFT method.\nThe membrane materials were characterized by the common range of analytical\ntechniques in this kind of work: SEM (surface and cross section), XRD, ATR-FTIR,\nNMR, TGA, XPS, as well as single gas permeation at room temperature by time-lag\nexperiments at 25ºC. This are adequate in membrane materials characterization. A\nbut lays in the fact that the authors claim to evaluate the effect of temperature and\ntemperature in permeation in the range 25-45ºC and 1-6 bar. These are very narrow in\ngas permeation membranes. Although it is shown an impact in selectivity in this narrow\ntemperature range, the value of 25ºC cannot be considered as “low temperature”\nstandard (line 327). Likewise, 6 bar is the regular operating pressure in gas separation.\nPlease consult previous works such as that by R. Baker, studying the optimal pressure\nfor gas separation as 5 bar. The pressure of 6 bar in a strong polyimide as Matrimid is\nnot usually sufficient enough to see any observable packing density of the polymer\nchains (line 336). The authors should give evidence of that, or appropriately check\nliterature on polyimide membrane materials characterization. The same can be said of\nthe effect of aging, which in this work is attributed to the measurements performed with\nthe membranes stored away from the gas permeation plant up to 110 days after the\nsynthesis. Have the membranes been taken out of the module in this intervals? The\ndamage caused by the joints may be more significant that any physical or chemistry\nchange due to a slight variation of the polymer packing. Please revise.\nReply: Thanks for your comments. Following your suggestion, the extended gas\nseparation test of the Matrimid-Cyclen membranes was carried out including a wider\nrange from 0 ℃ to 100 ℃ to better assess the impact of operating temperature on the\ngas separation performance. The results of the gas separation performance of the\nMatrimid-Cyclen-5% membrane are shown in Figure 1, with the data summarized in\nTable 1.\nAs the operating temperature increased from 0 ℃ to 100 ℃, the permeability of He, N\n2\nand CH increased,while the selectivity of He/N and He/CH decreased. This behavior\n4 2 4\ncould be attributed to the increased gas fugacity at elevated temperatures. The\nselectivity of He/N and He/CH decreased rapidly as the operating temperature\n2 4\nincreased from 40 ℃ to 60 ℃. This phenomenon was likely due to the accelerated\nmovement frequency of polymer chains at high temperatures, rendering transport of\ngas molecules through the matrix and weakening the molecular sieving effect of the\nmembrane. Notably, after cooling the membrane from 25 ℃ to 0 ℃, the He\npermeability decreased from 63.13 to 52.74 Barrer, while the He/N selectivity\n2\nincreased from 631 to 8790, and the He/CH selectivity increased from 2104 to 17580,\n4\ncorresponding to 14-fold and 9-fold increases, respectively. This dramatic\nimprovement in selectivity at low temperatures significantly enhances the potential for\nhigh-purity He extraction from natural gas.\nThe ultra-high selectivity of the Matrimid-Cyclen-5% membrane at 0 ℃ for He/N and\n4\ntemperatures. Although the He permeability decreased by 16% at low temperatures, the\nincrease in He selectivity indicates that improving gas purity is more challenging than\nenhancing transport efficiency. High purity is particularly important for He, given its\ncritical applications in the aerospace industry, electronics, medical equipment, and\nscientific research. Furthermore, the gas separation performance of the membranes\nfully when the temperature was returned to 20 ℃, both from 100 ℃ and from 0 ℃,\nindicating that the changes in membrane performance with temperature are inherent to\nthe membrane properties and do not result in structural damage.\nFigure 1. Effect of operating temperature on the separation performance of\nMatrimid-Cyclen-5% membrane.\nTable 1. Summary of gas transport permeability and selectivity of the\nMatrimid-Cyclen-5% membrane at 1.0 Bar and different temperatures.\nGas permeability (Barrer) Selectivity\nTemperat\nure (℃)\nHe N CH He/N He/CH\n2 4 2 4\nReturn to\n63.97±0.46 0.09±0.00 0.03±0.00 710.78±2.33 2132.33±7.00\n20 ℃\n0 ℃ 52.74±0.46 0.007±0.00 0.003±0.00 7534.29±24.28 17580.00±56.67\n20 ℃ 63.13±0.46 0.10±0.00 0.03±0.00 631.30±2.60 2104.33±8.67\n40 ℃ 80.14±0.46 0.32±0.02 0.10±0.01 250.44±15.13 801.40±83.82\n60 ℃ 96.91±0.46 1.43±0.46 1.05±0.00 67.77±2.90 92.30±1.73\n80 ℃ 125.12±0.46 3.45±0.46 3.05±0.46 36.27±0.36 41.02±0.87\n100 ℃ 202.55±0.46 9.76±0.46 9.42±0.46 20.75±0.40 21.73±0.58\nReturn to\n68.69±0.27 0.12±0.01 0.03±0.00 572.42±49.85 2289.67±8.00\n20 ℃\nTo explore the practical applicability of the Matrimid-Cyclen membranes at different\nfeed pressures, the gas separation performance of the Matrimid-Cyclen-5% membrane\nwas re-evaluated at pressures up to 30 bar, and the data were summarized in Table 2. As\npresented in Figure 2, as the pressure increased from 1 bar to 10 bar, the permeability of\nHe decreased slightly, and the pressure dependence of N and CH was almost\n2 4\nunrecognizable, with only small fluctuations in the selectivity for He/N and He/CH .\n2 4\nThese observations suggest that the membranes remain structurally stable under\nmoderate pressure increases, which may be attributed to the presence of Cyclen\nmolecules, which increase the packing density of the polymer chains, thereby reducing\ntheir mobility, increasing chain stiffness and improving the high-pressure resistance of\nthe membranes. As the operating pressure increased from 10 bar to 20 bar, the\npermeability of all gases rose, while the selectivity for He/N and He/CH decreased.\n2 4\nThis behavior is typical of glassy polymers, such as Matrimid, under plasticization\npressure, where increased feed pressure leads to higher polymer segment mobility and\nlarger chain spacing, resulting in higher permeability and lower selectivity [1-5]. As the\npressure increases from 20 bar to 30 bar, the permeability of He remained almost\nconstant, while the permeability of N and CH gradually decreased, and the selectivity\n2 4\nof He/N and He/CH tends to increase, which was ascribed to the existence of\n2 4\nunrelaxed volume within the polymer chain structure. An increase in feed pressure\nresults in greater compactness of the polymer matrix, which reduces the gas\npermeability in the membrane by reducing fractional free volume [6-7].\nSupplementary References\n[1] H. Rajati, A.H. Navarchian, D,. Rodrigue, S. Tangestaninejad, Improved CO\n2\ntransport properties of Matrimid membranes by adding amine-functionalized PVDF\nand MIL-101 (Cr), Separation and Purification Technology, 235 116149 (2020).\n[2] S. Salman, K. Nijmeijer, High pressure gas separation performance of\nmixed-matrix polymer membranes containing mesoporous Fe (BTC), Journal of\nmembrane science, 459, 33-44 (2014).\n[3] S. Salman, K. Nijmeijer, Performance and plasticization behavior of polymer-MOF\nmembranes for gas separation at elevated pressures, Journal of membrane science, 470,\n166-177 (2014).\n[4] A.E. Amooghin, M. Omidkhah, H. Sanaeepur, A. Kargari, Preparation and\ncharacterization of Ag+ ion-exchanged zeolite-Matrimid® 5218 mixed matrix\nmembrane for CO /CH separation, Journal of Energy Chemistry, 25 (3) 450-462\n2 4\n(2016).\n[5] H. Asghar, A. Ilyas, Z. Tahir, X.F. Li, A.L. Khan, Fluorinated and sulfonated poly\n(ether ether ketone) and Matrimid blend membranes for CO separation, Separation and\n[6] A.E. Amooghin, H. Sanaeepur, M. Omidkhah, A. Kargari, “Ship-in-a-bottle”, a\nnew synthesis strategy for preparing novel hybrid host-guest nanocomposites for\nhighly selective membrane gas separation, Journal of Materials Chemistry A, 6 (4)\n1751-1771 (2018).\n[7] A. Mirzaei, A.H. Navarchian, S. Tangestaninejad, Mixed matrix membranes on the\nbasis of Matrimid and palladium-zeolitic imidazolate framework for hydrogen\nseparation, Iranian Polymer Journal, 29 (6), 479-491 (2020).\nFigure 2. Effect of feed gas pressures on the separation performance of\nMatrimid-Cyclen-5% membrane.\nTable 2. Summary of gas transport permeability and selectivity of the\nMatrimid-Cyclen-5% membrane at different pressures and 25℃.\nGas permeability (Barrer) Selectivity\nPressure\n(Bar)\nHe N CH He/N He/CH\n2 4 2 4\nReturn to 1\n66.84±0.42 0.10±0.00 0.04±0.00 668.40±4.20 1671.00±10.50\nBar\nAs shown in the following Figure 3, the aging test was performed by taking samples\n(labled A , B , C , D and E , where n is the number of samples taken) from the same\nn n n n n\nbatch of the Matrimid-Cyclen-5% membrane at different aging times (0, 7, 20, 40,\n110, 480 days). No damage was caused by the joints since the membrane samples removed\nat each interval will not be used for retesting.\nFigure 3. Multiple small membranes are removed from the intact membrane to be used\nfor testing.\nRegarding the aging performance, we continued to test the gas separation performance\nof the membrane material after physical aging for 480 days, the data are summarized in\nTable 3. As shown in Figure 4, compared to the fresh membranes, the He permeability\nof the Matrimid-Cyclen-5% membrane decreased from 66.34 Barrer to 42.87 Barrer\nafter 480 days of physical aging, a decrease of 35%. Meanwhile, the He/N selectivity\n2\nincreased from 663.4 to 4763.3, and the He/CH selectivity increased from 1660.75 to\n4\n42870.0, corresponding to increases of 718% and 2581%, respectively.\nFigure 4. Long-term stability of the Matrimid-Cyclen-5% membrane (the whole\nmembrane was stored in an ambient environment without the introduction of any\nprotective gas at room temperature, small pieces of membrane were extracted from the\nwhole membrane at different aging times for a gas separation performance test).\nTable 3. Summary of gas transport properties of the Matrimid and Matrimid-Cyclen-\n5% membrane at 1.0 atm and 25 ℃ during the aging process.\nMembrane Gas permeability (Barrer)\nAging\nHe H CO N CH\n(Days) 2 2 2 4\n0 66.43±0.22 62.17±0.32 20.07±0.08 0.10±0.00 0.04±0.00\n7 64.22±.34 59.90±0.23 18.30±0.07 0.09±0.01 0.04±0.00\n20 59.96±0.53 54.68±0.14 16.67±0.17 0.07±0.005 0.02±0.00\n40 59.73±0.09 53.39±0.33 15.96±0.09 0.06±0.00 0.01±0.00\n110 54.31±0.10 50.76±0.10 14.12±0.03 0.05±0.00 0.008±0.00\n480 42.87±0.21 31.43±0.13 7.82±0.04 0.009±0.00 0.001±0.00\nMembrane Selectivity\nAging\nHe/N He/CH H /N H /CH CO /N CO /CH\n(Days) 2 4 2 2 2 4 2 2 2 4\n664.30±2. 1660.75± 621.70±3. 1554.25± 200.70 501.75±3\n0\n10 5.25 20 8.00 0.80 .75\n713.56±67. 1605.50± 665.56±6 1497.50± 203.33 457.50±1\n7\n960 8.50 8.04 21.22 ±21.79 .42\n856.57±57. 2998.00± 781.14±5 2734.00± 238.14 833.50±9\n20\n74 26.50 4.20 6.76 ±18.02 .66\n995.50±1. 5973.00± 889.83±5. 5339.00± 266.00± 1596.00\n40\n50 9.00 07 30.45 1.33 ±7.94\n1086.20±2. 6788.80± 1015.20± 6345.00± 282.40± 1765.00\n110\n00 12.45 1.74 10.90 0.53 ±3.31\n4763.33±2 42870.00 3496.22± 31430.00 868.89± 7820.00\n480\n3.34 ±200.00 12.62 ±113.58 4.01 ±36.06\nAre there any flaws in the data analysis, interpretation and conclusions? Do these\nprohibit publication or require revision?\nReply: Thanks for your comments. Generally speaking, the permeability and\nselectivity should be increased with higher Cyclen loading. However, it was found that\nthe CO /N selectivity of the Matrimid-Cyclen-15% membrane decreased when the\n2 2\nCyclen loading increased from 10% to 15%. This phenomenon may be attributed to the\nfact that the excess Cyclen is uniformly distributed between the Matrimid chains, and\nas a result, the Cyclen increases the chain spacing by its own volume, leading to an\nincrease in interchain distance. Importantly, SEM and EDS analyses confirm that\nCyclen is uniformly dispersed in the membrane, ruling out the possibility of\nagglomerating to form non-selective defects. This behavior suggests a balance between\nCyclen concentration and membrane performance that requires further optimization.\nThe commentary, though, seems to be mingling these characterization results since the\nauthors speak of diffraction peaks regarding the absorption bands in the ATR -FTIR\nspectra (line 191). Please correct. Also, I think for amorphous polymers such as\nMatrimid, it is more correct to talk about diffraction ands as well in the XRD, since the\npolymer diffracts in broad bands whose width is given information regarding the\nmorphology and opening and crosslinking of the polymer chains that influence the\nmembrane performance afterwards. Please revise.\nReply: Thanks for your comments. We have corrected this error according to your\nsuggestion, changed “diffraction peaks” to “characteristic peaks” for FTIR and\nchanged “diffraction bands” to “diffraction peaks” for XRD. Please refer to the revised\nmanuscript.\nBesides, other comments, according to the TGA curves presented, Cyclen does not\nprovide thermal stability to the Matrimid membrane materials, since it is degraded at a\nlow temperature. Therefore, the statement where the second stage of the TGA of MMM\nat 300-450ºC attributed to Cyclen degradation should be revised for clarification.\nReply: Thanks for your comments. The TGA curves were revised for clarification in\nresponse to your suggestion.\nIn the first stage of the TGA, which occurs between 100-400 °C, the weight loss is\nattributed to the evaporation of residual organic solvent and the Cyclen degradation in\nthe MMMs. The weight loss gradually increases with increasing Cyclen mass loading,\nreflecting the higher content of Cyclen in the membrane. The second weight loss,\noccurring in the range of 400 to 550 °C, is associated with the degradation of lateral\nmethyl of polymer. The third stage is caused by the decomposition of the polymer main\nchains, which begins at approximately 550 °C.\nLines 257, please provide evidence to such statements.\nReply: Thanks for your comments. The evidence for our findings is supported by BET\ncharacterization of the membranes.\nThe pore size distribution of the Matrimid-Cyclen membranes shows that the addition\nof Cyclen resulted in the formation of new pores in the membrane, approximately 3.2 Å\nin size (Figure 1), which agrees well with the theoretical value of the molecular window\nof Cyclen. This indicates that Cyclen creates ultra-micropores capable of selectively\nrecognizing gas molecules. The presence of these ultra-micropores creates more and\nlonger transport channels for He, which significantly improves the transport capacity of\nHe.\nMatrimid-Cyclen-10%\nMatrimid-Cyclen-5%\nMatrimid\n0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1\nPore Width (nm)\n)g/³mc(\nWd/Vd\nFigure 1. Pore size distribution of the Matrimid-Cyclen membranes. The pore size\ndistribution was calculated from the corresponding H adsorption isotherm using the\nLine 267, please specify to which membrane composition corresponds to these three %\nvalues.\nReply: Thanks for your comments. Compared with the Matrimid membrane, the He,\nH and CO permeability of the Matrimid-Cyclen-5% membrane increased by\n2 2\napproximately 108%, 99% and 20%, respectively. Please refer to the revised\nmanuscript.\nLine 292, again, it is not clear whether is selectivity enhancement the property the\nauthors are seeking for or permeability?\nWhy is a macrocycle with additional porosity chosen as fillers then, and Matrimid, a\npolyimide known for its selectivity and low gas permeability? Please clarify.\nReply: Thanks for your comments. What we are aiming for is a simultaneous\nimprovement in both permeability and selectivity, with priority given to improving\nselectivity. To achieve this, the macrocycle with additional porosity, specifically\nCyclen, was chosen as filler for the following reasons:\nFirstly, Cyclen was selected as filler due to its size-sieving effect, which is beneficial\nfor He separation. Based on the molecular dynamic diameters of He (2.60 Å), H (2.89\n2\nÅ), CO (3.30 Å), N (3.64 Å) and CH (3.80 Å), the Cyclen with cavity size (3.2-3.4 Å)\n2 2 4\nbetween the molecular dynamic diameters of He and N was selected, which makes it\n2\nan ideal candidate for He separation, such as He/N and He/CH . Furthermore, the\n2 4\nCyclen with a certain cavity size creates a special transport channel for small gases, like\nHe, which increases their transport paths in the membrane, counteracting the negative\neffect of the increased chain stacking density and enhancing the permeability of small\ngas molecules.\nSecondly, the chosen filler must have good solubility and dispersion in solvents. To\nensure uniform dispersion of the filler in the membrane and to minimize the formation\nof non-selective voids caused by filler agglomeration, we selected Cyclen for its high\nsolubility and good dispersion in organic solvents. Cyclen was fully dissolved in DMF,\nand then the clean Matrimid dope solution was added to the Cyclen/DMF solution. The\nresulting homogeneous Matrimid/Cyclen solution, which was stirred for 24 hours and\nsonicated for 1 hour, remained clear and transparent, even with up to 15% Cyclen\ncontent, as shown in Figure 1.\nFigure 1. (a-b) Cyclen powder (1.0, 3.0, 5.0, 10.0 or 15.0 mg) were added to 6.0 g DMF\nsolvent to obtain clear and transparent solutions. (c-d) The clean Matrimid dope\nsolution was added to the Cyclen/DMF solution to obtain clear and transparent mixed\nMatrimid/Cyclen solutions.\nThirdly, compatibility between filler and polymer matrix. It is crucial to create mixed\nmatrix membranes with fair matching between filler and polymer matrix to gain\nattractive performance. This is why we chose Cyclen, as a filler. Hydrogen bonding\nbetween Cyclen and the Matrimid polymer chains immobilizes Cyclen molecules in the\ninterchain gaps of the Matrimid membrane, resulting in homogeneously mixed matrix\nmembranes. The SEM image (Figure 2) shows the surface and interface of the\nmembrane, confirming the good interfacial compatibility between Cyclen and\nMatrimid.\nFigure 2. SEM images of the cross section of the Matrimid-Cyclen-n% membranes up\nto ×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nFourthly, Helium (He) purification requirements. Helium is an extremely scarce,\nnon-renewable gas with a strong safety profile that is an essential resource for the\ndevelopment of many high-tech industries, such as the aerospace industry, electronics,\nmedical equipment, and scientific research. On Earth, He occurs in very low\nconcentrations, mainly in the atmosphere and in natural gas. Helium is commonly\nextracted industrially from high-pressure natural gas. However, the initial\nconcentration of He in most natural gas reservoirs is still as low as 0.3%, and to extract\nhelium from natural gas with ultra-low He content, the gas separation membranes must\npossess He/CH selectivity greater than 1000. Highly permeable polymers rarely\n4\nprovide sufficient selectivity for high-purity He extraction, and Matrimid was chosen as\nthe polymer matrix due to its dense stacking of polymer chains. XRD characterization\nrevealed that the strong hydrogen bonding between Cyclen and Matrimid modulated\nthe chain stacking of the mixed matrix membranes, which reduced the pore size,\nnarrowd the pore size distribution and enhances the selectivity of the membrane by\nmodulating the chain spacing..\nIn summary, it’s a great significance to develop the Matrimid-Cyclen membrane with\nenhanced He permeability (66 Barrer) and high He/CH selectivity (>1660).\nvalues for permeation at 25ºC and 1 atm in Tables S2 and S3, and tables S4 and S5, do\nnot agree. Have the authors measured the same membrane materials several times?\nThe information on reproducibility is very important. Please comment.\nReply: Thanks for your comments. According to your suggestion, the membranes were\nre-prepared to test the performance under different temperature and pressure conditions.\nThe updated gas separation performance is summarized in S3-S4, and the data in\nTables S2-S5 is consistent now. Please refer to the revised manuscript.\nThe discrepancy in the permeation values at 25ºC and 1 atm between Tables S2 and S3,\nTables S4 and S5, because the aging data for the Matrimid-Cyclen-5% membranes in\nTable S2 were averaged values, as summarized in Table S5. Additionally, the changes\nin gas separation performance for the measured membranes as a function of\ntemperature and pressure after 40 days of aging are summarized in Tables S3 and S4,\nrespectively. Multiple measurements were not performed on the same piece of tested\nmembrane, as the experimental procedure involved sampling different locations from\ndifferent batches of Matrimid-Cyclen-5% membranes.\nSampling Procedure: Three small membrane sheets (named A1, B1 and C1) from five\ndifferent preparation batches of Matrimid-Cyclen-5% membranes were taken for gas\nseparation performance testing. Subsequently, these different preparation batches of\nMatrimid-Cyclen-5% membranes were re-sampled for different aging times, operating\npressures, and operating temperature conditions for performance testing. The gas\ntransport properties of the membrane materials were averaged over these different\nconditions.\nLine 299, the authors attribute now that too many Cyclen molecules tend to\nagglomerate at high loading, but no such agglomeration was shown in the SEM images\nabove. Please provide evidence to these conclusions.\nReply: Thanks for your comments. We have conducted additional SEM (higher\nmagnification) and EDS analysis of the membrane materials to confirm whether the\nCyclen molecules in the highly loaded Mtrimid-Cyclen-n% membrane were\nagglomerated. The results showed that all the Matrimid-Cyclen membranes display\nhomogeneous cross-sections and surfaces at 50,000x electron magnification, without\nany significant aggregation or discrete particulate phases observed (Figure 1 and Figure\n2), confirming the presence of only a continuous single phase within these membranes.\nAdditionally, energy dispersive X-ray spectroscopy (EDS) mapping of the\ncross-sections of the Matrimid-Cyclen membranes further verifies the uniform\ndistribution of Cyclen as evidenced by the detection of characteristic nitrogen (N)\nsignals (Figure 3).\nFigure 1. SEM images of the surface of Matrimid-Cyclen-n% membranes up to\n×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nFigure 2. SEM images of the cross section of the Matrimid-Cyclen-n% membranes up\nto ×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nFigure 3. EDS cross-section with nitrogen (N), carbon(C) and oxygen (O) mapping of\nplanar scans of Matrimid-Cyclen membranes (n% is the Cyclen mass loading and was\nvaried from 1 to 15 wt%, n = 0, 1, 3, 5, 10, 15).\nThe XRD characterization showed a gradual increase in polymer chain spacing after\n≥10%. We speculate that this may be due to the fact that the excess Cyclen is uniformly\ndistributed between the Matrimid chains, and the Cyclen increases the chain spacing by\nits volume, rather than agglomerating to form non-selective defects between the\npolymer chains. This hypothesis is supported by SEM and EDS analyses, which show\nthat Cyclen is uniformly dispersed in the membrane.\nWe have corrected these questions according to your suggestion. Please refer to the\nrevised manuscript.\nLine 382, the uahtors compare suddenly the Cyclen -Matrimid membranes of this work\nwith additional Cycldecane-hybrid membranes, whose synthesis and characterization\nhas not been elsewhere described in this manuscript. Please revise and include, either\nexperimental evidence or reported reference.\nReply: Thanks for your comments. The Cyclen, Hexacyclen, and Cycldecane are\nderivatives of the organic macrocyclic. We have added the preparation procedure of the\nMatrimid-Hexacyclen membrane and Matrimid-Cyclodecane membrane in the\nMethods section according to your suggestion. Please refer to the revised manuscript.\nLines 384-394, again these statements need additional evidence to verify that the\nsupposedly hypothesis that Cyclen hybridization of Matrimid is providing porosity and\ncrosslinking thus inducing remarkable increase in selectivity in Matrimid towards He\ngas separation, is fulfilled or not.\nReply: Thanks for your comments. As shown in Figure 1, the micropore sizes in\nMatrimid, Matrimid-Cyclen-1% and Matrimid-Cyclodecane-1% membranes were\naround 6.9Å, 6.3Å and 7.1 Å, respectively. This suggests that the hydrogen bonding\ninteraction between Cyclen and Matrimid indeed modulates the pore structure in the\nmembranes to enhance the molecular sieving properties of Matrimid-Cyclen\nmembranes by narrowing pore sizes. Compared with Matrimid-Cyclen-1% membranes,\nthe Matrimid-Hexacyclen-1% membranes prepared with fillers with larger window\nsizes had larger micropore sizes, and their separation of He is weaker, demonstrating\nthat the filler with suitable pore size is more effective in improving the separation\nperformance of MMMs.\nFigure 1. Pore size distribution of the mixed matrix membranes (Matrimid-Cyclen-1%\nmembrane, Matrimid-Hexacyclen-1% membrane and Matrimid-Cyclodecane-1%\nmembrane). The pore size distribution was calculated from the corresponding H\n2\nadsorption isotherm using the DFT method.\nHere, the possible gas transport mechanism of Matrimid-Cyclen membranes is derived\nfrom two sets of comparison experiments. Then, molecular dynamics simulations and\nBET characterization of the membranes were used to determine the gas separation\nmechanism of the membranes.\nThe membrane structure model generated from MD simulation is based on PACKMOL\nsoftware (Figure 6a). He and N were used as the probe gases for the membrane model\n2\nto analyze the gas accessible volume, the average gas transmembrane path and the\ninterconnectivity of void space. Figure 6b-c shows that some channels allow He\ntransport but block N . The increase in Cyclen content within the membranes resulted\n2\nin a continuous enhancement in the connectivity of the He passable pore space within\nthe membranes (Figure 6d), and the average He transmembrane path calculated on the\nbasis of the passable pore space increased from 635 Å to 922 Å (Figure 6f), thus\nsignificantly improving the He permeability. In addition, the average transmembrane\npath length difference between He and N increased from 22.58 Å to 331.34 Å and then\n2\ndecreased to 321.90 Å, indicating that the presence of Cyclen could create a special\ntransport channel for He, increasing its transport paths and transmission path length in\nthe membrane, effectively enhancing the transport capacity of He and the separation of\ngas pairs in the membrane.\nThe pore size distribution of the Matrimid-Cyclen membranes shows that the\nmicropore size of the Matrimid-Cyclen membrane is significantly reduced, and the\ncontent of ultra-micropores with a size of about 3.9 Å is gradually increased compared\nto the original Matrimid membrane (Figure 6g). In addition, the addition of Cyclen\nresulted in the appearance of new pores in the membrane with a size of about 3.2 Å,\nwhich agrees well with the theoretical value of the molecular window of Cyclen.\nFigure 6. Structural analysis of Matrimid-Cyclen membrane models. (a) 3D view of\nMatrimid-Cyclen-n% membrane models (n% is the Cyclen mass loading, n = 0, 5, 10).\nThe gray and blue shades indicate the free volume detected by a probe with a radius of\n2.6 Å. (b-c) Accessible void spaces for He (left) and N (right) in Matrimid-Cyclen-n%\n2\nmembranes models (n% is the Cyclen mass loading, n = 0, 5, 10); The red circles in the\nleft picture indicate additional accessible void spaces for He, compared with that of N .\n2\n(d) Interconnected (green) and disconnected (red) voids in Matrimid-Cyclen-n%\nmembrane models (n% is the Cyclen mass loading, n = 0,5,10) with respect to a probe\nof 1.3 Å radius. (e) The accessible volume of He and N as a percentage of the\n2\nmembrane pore volume and the simulated density of the Matrimid-Cyclen membranes.\n(f) The average He and N transmembrane path calculated based on accessible void\n2\nspace. (g) Pore size distribution of the Matrimid-Cyclen membranes. The pore size\ndistribution was calculated from the corresponding H adsorption isotherm using the\nEnglish revised by professional native translator recommended. There are several\nwrong verb tenses and other grammar or semantic mistakes that could easily be avoided.\nFor example, consider the use of the adjective “residual” instead of “resident in line 201,\ncomparisons wrongly used, such as “with a higher filler content increased” (line 293),\n“shrined” (line 348) instead of “shrinked”, and so on.\nReply: Many thanks for your comments. The English was carefully revised by\nprofessional native specialist.\nSmall notations to be corrected, for instance: Figure 4c, the permeability changes are\nnot shown in figure 4c, but the single gas permeability in Barrer.\nReply: Thanks for your comments. We have corrected this question according to your\nsuggestion. The legend for Figure 4 (c-e) has been revised to the following:\nFigure 4. Gas separation performance of Matrimid-Cycien membranes. (c) He, N and\n4 2 4\nloading. (d) H , N and CH permeability and H /N and H /CH selectivity with\n2 2 4 2 2 2 4\ndifferent Cyclen mass loading. (e) CO , N and CH permeability and CO /N and\n2 2 4 2 2\nCO /CH selectivity with different Cyclen mass loading. Please refer to the revised\n2 4\nmanuscript.\nLine 249, the gas permeability measurement is a time-lag variable pressure- constant\nvolume setup, I guess, since the starting pressure is 1 bar, what is the vacuum pressure?\nThe selectivity in Table S2 is the intrinsic selectivity in the table below are obtained as\nthe ratio between two single gas permeabilities from the table above.\nReply: Thanks for your comments. The vacuum pressure value is 6.579 × 10-3 Torr.\nWe have corrected this question according to your suggestion by changing the\nselectivity in Table S2 to ideal selectivity. Please refer to the revised manuscript.\n• Is there enough detail provided in the methods for the work to be reproduced?\nReply: Thanks for your comments. Details of the membrane fabrication method are\nprovided in the manuscript, followed by the gas transport performance test procedure of\nthe membrane materials.\nThe dense membrane was applied to the permeation cell using aluminium tape, and the\ncell was sealed using rubber rings and screws. The gas permeation unit was evacuated\nat 25 ℃ for 12 hours before starting the gas permeation test, after which all valves of\nthe gas permeation unit were closed. Helium was first introduced into the gas\npermeation unit first to initiate the test. After that, the gas permeation unit was\nevacuated at 25 ℃ for 8 hours to remove any residual gas, and all the valves were\nclosed. The transport properties of the next gas were then tested in the following order\nof He, H , CO , N and CH , which also corresponds to the order of gas kinetic diameter\n2 2 2 4\n(He: 2.60 Å, H : 2.89 Å, CO : 3.30 Å, N : 3.64 Å and CH : 3.80 Å) from smallest to\n2 2 2 4\nlargest. The pressure was 1 bar and the cell temperature was kept constant at 25 ℃.\nI have included my comments through the review according to the journal requests. The\naim of the paper and the gas permeation improvement of Matrimid membranes are\nsignificant and worthwhile but the manuscript must be improved.\nReply: Thank you for your valuable comments. The manuscript has been revised based\non your suggestions to improve its clarity and quality.\nReviewer #3 (Remarks to the Author):\nIn my opinion, the paper is a relevant contribution to the Membrane Science and\nTechnology Field. The title is somehow limiting the scope of the paper as far as, for\nexample, the CO /CH separation features of the material produced are as much\n2 4\nrelevant or even more than those concerning the He/CH pair announced in the title and\n4\nabstract. Nevertheless, there are some questions I would like the authors to correct or\nanswer.\n1) English needs a careful revision as far as there are many errors concerning for\nexample conjugation and concordance. Some strange election of words as, for example,\n“scalloped vein structure” or “fan-shaped vein structures” should be reviewed.\nReply: We appreciate your feedback regarding the English language. The manuscript\nhas been carefully revised by a professional native language specialist to correct errors\nrelated to conjugation, concordance, and other language issues. We have also reviewed\nthe use of terms such as “scalloped vein structure” and “fan-shaped vein structures” and\nmade adjustments to ensure clarity and accuracy. The descriptions “scalloped vein\nstructure” and “fan-shaped vein structures” have been revised as “vein structures”.\n2) Figure 1 is nice and stunning but useless. Remove it.\nReply: Thanks for your comments. We acknowledge your concerns and have revised\nFigure 1 in the manuscript, as illustrated below (Figure 1), to more clearly convey the\ndesign and purpose of this work.\nFigure 1. Schematic of the Matrimid mixed-matrix membrane (MMM) for helium\nextraction from natural gas via gas molecule recognition window.\n3) In Figure 2, only the transversal cuts give some information. A higher magnification\nshould be shown anyway. How were the transversal sections performed? Some, let say,\ninhomogeneity appears, probably forming paths through the membrane, appear when\nCyclen load increases. These areas should be studied in some detail. By the way, I\nwould assume that the first file of pictures corresponds to the zero Cyclen filling. This\nshould be clarified. Please select only the transversal cuts with a higher magnification\nand include labels with the Cyclen content on the pictures.\nReply: Thanks for your comments. The cross-section of the membrane was prepared\nby fracturing the membrane in liquid nitrogen. Based on your suggestion, SEM (higher\nmagnification) and EDS analysis of the membrane have been conducted to investigate\nthe dispersion of Cyclen within the membranes. The morphology of the Matrimid\n-Cyclen-n% (n = 1, 3, 5, 10, 15) membranes in cross section at different magnifications\nis shown in Figure 1. The fissures in the membranes increased with increasing Cyclen\nloading. This morphology was attributed to the enhanced interaction between the\npolymer chains and filler, which increases with Cyclen loading. The formation of\ninterfacial stress during the fracturing of MMMs samples in liquid nitrogen caused\ndeformation of the polymer segments, resulting in elongated segments with increased\nplastic deformation. The Matrimid-Cyclen-15% membrane remained homogeneous at\n50,000x magnification without any significant aggregation or discrete particulate\nphases observed. The EDS mapping of the cross-sections of the matrimid-Cyclen\nmembranes further confirmed the uniform distribution of Cyclen, as indicated by the\ndetection of characteristic nitrogen (N) (Figure 2). In addition, we have labelled the\nCyclen content in Figure 2 of the manuscript, as shown in Figure 3 below.\nFigure 1. SEM images of the surface of Matrimid-Cyclen-n% membranes up to\n×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nFigure 2. SEM images of the cross section of the Matrimid-Cyclen-n% membranes up\nto ×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nFigure 3.SEM images of the Matrimid-Cyclen membranes. (a) Surface morphology, (b)\nphotograph, and (c,d) cross section of the Matrimid and Matrimid-Cyclen-n%\nmembranes (n% is the Cyclen mass loading and was varied from 1 to 15 wt%, n = 0, 1,\n3, 5, 10, 15). (e) EDS cross-section with nitrogen (N) mapping of planar scans of\nMatrimid-Cyclen membranes.\n4) How do you get the very same thickness with or without Cyclen? Authors mention\n“injected into a clean Petri-dish by a syringe” How was this controlled as to get errors\nof only 25% in thickness? Were there variations depending on the Cyclen load? How\nwere these errors evaluated?\nReply: Thanks for your comments. Figure 1 shows photographs of the Matrimid and\nCyclen materials used in the membranes. To achieve consistent membrane thickness,\nwe used the same mass of polymer, the same area of the evaporation dish, and a filler\nloading of up to 15% of the polymer mass. These preparations were designed to\nproduce in essentially the same thickness for all Matrimid-Cyclen membranes. Figure 2\nshows the cross-section images of the membrane materials with the original scales\nretained, and all the images are magnified by 1000x, showing slight variations in\nmembrane thickness. These measurements were consistent, with only minor\ndifferences observed depending on the Cyclen load.\nFigure 1. Photographs of the Matrimid and Cyclen materials used in the membranes.\nFigure 2. SEM images of the cross-section of Matrimid-Cyclen-n% membranes.\n5) Authors write, “However, the self-aggregation of Cyclen tends to happen when the\nCyclen loading was too high, and the hydrogen bonding between excess Cyclen leads\nto the formation of unobservable discrete particulates in the membrane”. This statement\nconstitutes an authors’ guess, or what does it say? I think that if aggregation clusters\nwere unobservable, in absolute terms, they should be essentially not existing instances.\nAvoid this kind of statements deleting the phrase or argue on the probability and signs\nof the existence of such aggregation that cannot be seen at the magnifications or with\nthe procedures of analysis used.\nReply: Thanks for your comments. The statement “However, the self-aggregation of\nCyclen tends to happen when the Cyclen loading was too high, and the hydrogen\nbonding between excess Cyclen leads to the formation of unobservable discrete\nparticulates in the membrane”is indeed a conjecture, which stems from the general\nunderstanding that in mixed matrix membranes, there is usually an optimal\nfiller-to-polymer ratio, below which the polymer matrix wraps around the filler and the\ninteraction between the two phases helps to create a defect-free interface [1-3]. When the\nfiller content exceeds this threshold, the excess filler tends to agglomerate into clusters,\nforming interfacial gaps with the polymer matrix that are larger than the size of the\nseparated gases, thereby resulting in a decrease in the selectivity of the membrane for\nthe separation of gas pairs [4-6].\nIn this work, a new XRD diffraction peak attributed to Cyclen was observed when the\nCyclen content in the membrane was ≥10%. The gas separation performance tests\nshowed a reduction in He/N and He/CH gas pairs as Cyclen content increased.\n2 4\nHowever, no aggregation clusters were found in the electron microscopy detection.\nTherefore, we hypothesize that there are aggregation clusters in the membrane at\nCyclen content ≥10%, but they are not visible under the electron microscopy conditions\nused, and those potential interfacial defects may explain the observed decrease in the\nseparation selectivity.\nFollowing your suggestion, we conducted SEM (at higher magnification) and EDS\nanalysis of the membrane materials to further investigate the dispersion of Cyclen\nwithin the membranes. All the Matrimid -Cyclen-n% (n = 1,3,5,10,15) membranes\nremained homogeneous in cross-section and surface at 50,000x magnification, without\nany significant aggregation or discrete particulate phases observed (Figure 1 and Figure\n2), indicating that Cyclen was uniformly dispersed in the membranes. The EDS\nmapping of the cross-sections of the Matrimid-Cyclen membranes further confirmed\nthe uniform distribution of Cyclen via the detection of characteristic nitrogen (N)\n(Figure 3).\nMoreover, the XRD characterization showed a gradual increase in polymer chain\nspacing after ≥10% Cyclen loading. We speculate that this may be due to the fact that\nthe excess Cyclen is uniformly distributed between the Matrimid chains, and the\nCyclen increased the chain spacing by its volume, leading to an increase in chain\nspacing, rather than agglomerating to form non-selective defects between the polymer\nchains. This is consistent with the SEM and EDS analyses, which demonstrate that\nCyclen is uniformly dispersed in the membrane.\nWe have corrected this questions according to your suggestion. Please refer to the\nrevised manuscript.\nSupplementary References\n[1] Y. Liu, G.P. Liu, C. Zhang, W.L. Qiu, S.L. Yi, V. Chernikova, Z.J. Chen, Y.\nBelmabkhout, O. Shekhah, M. Eddaoudi, W. Koros, Enhanced CO /CH separation\n2 4\nperformance of a mixed matrix membrane based on tailored MOF-polymer\nformulations, Advanced Science, 5 (9), 1800982 (2018).\n[2] C.H. Ma, J.J. Urban, Hydrogen‐bonded polyimide/metal‐organic framework hybrid\nmembranes for ultrafast separations of multiple gas pairs, Advanced Functional\nMaterials, 29 (32), 1903243(2019).\n[3] H.Z. Dou, M. Xu, B.Y. Wang, Z. Zhang, D. Luo, B.B. Shi, G.B. Wen, M. Mousavi,\nA.P. Yu, Z.Y. Bai, Z.Y. Jiang, Z.W. Chen, Analogous mixed matrix membranes with\nself-assembled interface pathways, Angewandte Chemie International Edition, 60 (11),\n5864-5870 (2021).\n[4] X.Y. Tan, S. Robijns, R.Thür, Q.L. Ke, N.D. Witte, A. Lamaire, Y. Li, I. Aslam,\nD.V. Havere, T. Donckels, T.V. Assche, V.V. Speybroeck, M. Dusselier, I.\nVankelecom, Truly combining the advantages of polymeric and zeolite membranes for\ngas separations, Science, 378 (6625), 1189-1194 (2022).\n[5] B. Zhu, S.S. He, Y. Yang, S.W. Li, C.H. Lau, S.M. Liu, L. Shao, Boosting\nmembrane carbon capture via multifaceted polyphenol-mediated soldering, Nature\nCommunications, 14 (1), 1697 (2023).\n[6] T.H. Lee, B.K. Lee, S.Y. Yoo, H. Lee, W.N. Wu, Z.P. Smith, H.B. Park, PolyMOF\nnanoparticles constructed from intrinsically microporous polymer ligand towards\nscalable composite membranes for CO separation, Nature Communications, 14 (1),\n2\n8330 (2023).\nFigure 1. SEM images of the cross section of the Matrimid-Cyclen-n% membranes up\nto ×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nFigure 2. SEM images of the surface of Matrimid-Cyclen-n% membranes up to\n×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nFigure 3. EDS cross-section with nitrogen (N), carbon(C) and oxygen (O) mapping of\nplanar scans of Matrimid-Cyclen membranes (n% is the Cyclen mass loading and was\nvaried from 1 to 15 wt%, n = 0, 1, 3, 5, 10, 15).\n6) In the Figure 3 caption, authors do not mention plot “e” referring to element analysis\nperformed by an XPS analysis. Authors should mention that their scheme in the “c”\nsection refers to the 5 % Cyclen membrane. Both the smallest (4.09 Angstrom) and\nhigher (5.93 Angstrom) d_spacing decrease only until 5 % Cyclen and then increase\nmonotonously until the highest Cyclen and eventually surpass the pure matrimid ones.\nCan it be correlated with the appearance of inhomogeneities detected in Figure 2? How\nis it possible, given that both the figures refer to very different scales?\nReply: Thanks for your comments. We have corrected these issues according to your\nsuggestion. The legend of Figure 3 (c-e) has been revised as follows: Structure\ncharacterization of Matrimid-Cyclen membranes. (a) XRD patterns of the\nMatrimid-Cyclen membranes. (b) ATR-FTIR spectra of the Matrimid-Cyclen\nmembranes. (c) Schematic illustration of the effect of intramembrane hydrogen\nbonding on polymer chain stacking as the Cyclen content is increased from 0% to 5%.\n(d) TGA plots of the Matrimid-Cyclen membranes. (e) XPS broad scan spectra of the\nMatrimid-Cyclen membranes. (f) X-ray photoelectron spectroscopy (XPS) data for the\nO 1 s peaks of the Matrimid-Cyclen membranes. (g) The dynamic mechanical thermal\nanalysis (DMTA) spectra of Matrimid-Cyclen membranes at a temperature range from\n50 to 350 °C. (h) The storage modulus (E′) and (i) Tan δ of Matrimid-Cyclen\nmembranes.\nAt Cyclen contents ≤5%, the decrease in d-spacing is due to inter-chain hydrogen\nbonding [1-3]. The inhomogeneous distribution of the Cyclen in the membrane could\nlead to small changes in the d-spacing [4]. However, the SEM (at higher magnification)\nand EDS indicating that Cyclen is uniformly dispersed in the membranes. XRD\ncharacterization showed that the polymer chain spacing gradually increased after\nCyclen content ≥5%. We speculate that this may be due to the fact that the excess\nCyclen is uniformly distributed between the Matrimid chains, and that Cyclen increases\nthe chain spacing by its own volume, leading to an increase in chain spacing.\nSupplementary References\n[1] F. Feng, J. Wu, C.Z. Liang, M. Weber, S. Zhang, T.S. Chung, Synergistic\ndual-polymer blend membranes with molecularly mixed macrocyclic cavitands for\nefficient pre-combustion CO capture, Chemical Engineering Journal, 470, 144073\n2\n(2023).\n[2] J. Wu, C.Z. Liang, A. Naderi, T.S. Chung, Tunable supramolecular cavities\nmolecularly homogenized in polymer membranes for ultraefficient precombustion\nCO2 capture, Advanced Materials, 34 (3), 2105156 (2022).\n[3] J. Wu, T.S. Chung, Supramolecular Polymer network membranes with\nmolecular-sieving nanocavities for efficient pre-combustion CO2 capture, Small\nMethods, 6 (1), 2101288 (2022).\n[4] W. He, X.Z. Wang, J. Guan, Q.S. Liang, J. Ma, Y. Liu, W.W. Lim, C.W. Zhang,\nS.U. Hassan, H.J. Zhang, J.T. Liu, Membranes with Molecular Gatekeepers for\nEfficient CO Capture and H Purification, ACS Applied Materials & Interfaces 16 (16)\n2 2\n21222-21232, (2024).\n7) In reference to the higher d-spacing (out of the two detected) what do the authors\nthink is the cause of this interchain segment distance and their changes? Have you\ntested the evolution of d-spacing for the aging membranes? If decreases in d_spacing\nare associated with hidrogen bonding, what is causing the high load increase in\nd-spacing?\nReply: Thanks for your comments. We observed a decrease and then an increase in\nd-spacing with increasing Cyclen content. At low Cyclen content (1-5% Cyclen) the\nhydrogen bonding interaction between Cyclen and Matrimid chains reduces the\ninter-chain spacing. However, after further increasing in Cyclen content (10-15%\nCyclen mass loading), we notice a gradual increase in polymer chain spacing. We\nspeculate that this may be due to the fact that the excess Cyclen is uniformly distributed\nbetween the Matrimid chains, with the volume of Cyclen increasing the chain spacing\nrather than agglomerating into non-selective defects between the polymer chains. This\nis supported by SEM and EDS analyses, which show that Cyclen is uniformly dispersed\nin the membrane.\nFollowing your suggestion, we also tested the change in the d-spacing of membranes\naged for 480 days. As shown in Figure 1, after 480 days of aging, the diffraction peaks\nwith chain spacing >5Å disappeared at Cyclen content >1%. With increasing Cyclen\ncontent, the diffraction peaks with chain spacing <5Å first weakened and then\nstrengthened.while the chain spacing gradually decreased.\nFigure 1. XRD patterns of the Matrimid-Cyclen membranes after aging for 480 days.\nWe have corrected these issues according to your suggestion. Please refer to the revised\nmanuscript.\n8) Authors say that “which adjusted the submicroporous structure of the MMM\nmembrane, increased the stacking density of Matrimid polymer chains”. This could\nonly be true until 5% Cyclen, and should be stated so. Moreover, it is important to\nmention that small decreases in d-spacing do not automatically translate into a\nsignificant change in packing density.\nReply: Thanks for your comments. We have made a proper revision of the manuscript,\n“which adjusted the submicroporous structure of the MMMs, increased the stacking\ndensity of Matrimid polymer chains” has been changed to “which adjusted the\nsubmicroporous structure of the MMM membrane, tightened the chain spacing”.\n9) Authors say that “Cyclen begins to degrade at temperatures slightly greater than\n100 °C”; why do the membranes containing Cyclen only decompose at 300 to 450 ºC?\nHave you performed mechanical or, better, thermomechanical investigations on the\nmembranes? Any idea on the mechanical behavior that could be expected?\nReply: Thanks for your comments. The statement that “Cyclen begins to degrade at\ntemperatures slightly greater than 100°C” refers to the degradation behavior of Cyclen\nitself, whereas the decomposition of the Matrimid-Cyclen membranes occurs at higher\ntemperatures (300-450°C). This is because the membrane is a composite material, and\nthe thermal decomposition of the polymer matrix and filler materials typically happens\nat different temperatures. The dynamic mechanical thermal analysis (DMTA) was\nconducted for Matrimid-Cyclen samples to evaluate the mechanical properties of the\nmembranes at a temperature range from 50 to 350 °C. As shown in Figure 3 (g-i), the\nstorage modulus (E′) (1800 MPa) of these Matrimid-Cyclen membranes remained\nalmost constant until reaching the corresponding glass transition temperature Tg. It was\nobserved that the higher Cyclen loadings resulted in more brittle membranes. The effect\nof Cyclen content on the peak intensity of damping (Tan δ) is shown in Figure 3 (i). The\npeak intensity of Tan δ (damping value) decreased as the Cyclen content increased,\nindicating an overall increase in stiffness and a decrease in flexibility. These data\ndemonstrated that this kind of gas separation membrane is endowed with good\nmechanical properties and thermal stability.\nFigure 3. Characterization of Matrimid-Cyclen membranes. (g) The dynamic\nmechanical thermal analysis (DMTA) spectra of Matrimid-Cyclen membranes at\ntemperature range from 50 to 350 ℃. (h) The storage modulus (E′) and (i) Tan δ of\nMatrimid-Cyclen membranes.\n10) Could the authors explain how Table S-1 was obtained? What are the respective\nroles of N-H…O and C=O…H in Table S1. Do they refer to the percentage of NH\nlinked by hydrogen bridges (relevance of bridges for Cyclen) and the percentage of\nC=O linked by hydrogen bridges (relevance of bridges for matrimid)? Therefore they\nshould be correlated, are they?\nReply: Thank you for raising a thought-provoking question. As is well known, XPS is a\nsemi quantitative detection method that uses the area of the bonding peak. We\ncalculated the area of peaks corresponding to different bonds in XPS (shown in Figure\n3(f) and Figure S7) and introduced the sensitivity factor of the instrument. The relative\ncontents of each key were determined based on the final ratio. For clarity, we have\nadded the title of Table S1 to “Normalized relative intensities (%) of nitrogen\nfunctionalities N 1s and Oxygen functionalities O 1s XPS spectra for\nMatrimid-Cyclen-n% membranes (The relevant contents were calculated from the XPS\nresults in Figure 3 (f) and Figure S7)”. Table S1 shows the change in the percentage of\nN-H and C=O linked by hydrogen bonding as the amount of Cyclen added increases,\nand they are correlated.\n11) It is declared that “gas permeation tests for He (2.60 Å), H2 (2.89 Å), CO2 (3.3 Å),\nN2 (3.64 Å) and CH4 (3.80 Å)…” These Kinetic (?) diametres should be described and\nreferenced. What were they used for?\nReply: Thanks for your comments. The molecular kinetic diameters mentioned (He\n(2.60 Å), H (2.89 Å), CO (3.3 Å), N (3.64 Å), CH (3.80 Å) refer to the effective\n2 2 2 4\nmolecular sizes that influence the permeation of gases through the membrane. These\nvalues are commonly used in gas separation membrane researches to explain the\nseparation mechanisms based on size sieving [1-2]. The kinetic diameter is a crucial\nfactor in determining how well the membrane can separate gases based on their\nmolecular sizes, as smaller molecules are able to permeate through the membrane more\neasily than larger molecules.\nThe molecular dynamic diameters of the gases to be separated in this work are as\nfollows : He (2.60 Å)＜H (2.89 Å)＜CO (3.3 Å)＜N (3.64 Å)＜CH (3.80 Å). This\n2 2 2 4\nsize-based distinction supports the selection of Cyclen as the filler, as it exhibits a high\nsize-sieving effect. Cyclen, an organic macrocyclic compound with a flexible structure,\nhas a cavity size of approximately 3.2-3.4 Å (between the molecular dynamic diameters\nof He and N ), which makes Cyclen an effective molecular sievefor gases like He/N\n2 2\nand He/CH .\n[1] R.S.K. Valappil, N. Ghasem, M. Al-Marzouqi,Current and future trends in polymer\nmembrane-based gas separation technology: A comprehensive review, Journal of\nIndustrial and Engineering Chemistry, 98, 103-129 (2021).\n[2] Y.H. Wang, H.F. Jiang, Z.Y. Guo, H.Z. Ma, S.Y. Wang, H.J. Wang, S.Q. Song, J.F.\nZhang, Y. Yin, H. Wu, Z.Y. Jiang, M.D. Guiver, Advances in organic microporous\nmembranes for CO separation, Energy & Environmental Science, 16 (1), 53-75\n2\n(2023).\n12) Referring to aging, it mainly reflects lability of the structure than collapses to some\nextent with as time passes; this reduces the size of the sites making up the gas paths.\nInitially it leads to an increase of selectivity until the small gas molecules are also\nhindered with a decrease in selectivity and a substantial decrease in permeability.\nFigure 5 shows how selectivity increases with a reduction in permeability with aging.\nUntil what time is it so? Have you tested plastification? Have you changed the applied\npressure over 6 bar pressure drop?\nReply: Thanks for your comments. The gas separation performance of the membrane\nafter aging for 480 days was evaluated. As shown in Figure 1, compared to fresh\nmembranes, the He permeability decreased from 66.34 to 42.87 Barrer, corresponding\nto a decrease of 35.38%. The N permeability decreased from 0.10 to 0.009 Barrer, and\n2\nthe CH permeability decreased from 0.04 to 0.001 Barrer, corresponding to decreases\n4\nof 91% and 97.5%, respectively. However, there was an approximately 6-fold and\n25-fold increment in the He/N and He/CH selectivity, respectively.\n2 4\nFigure 1. Long-term stability of the Matrimid-Cyclen-5% membrane (the whole\nmembrane was stored in an ambient environment without the introduction of any\nprotective gas at room temperature, small pieces of membrane were extracted from the\nwhole membrane at different aging times for a gas separation performance test).\nWe also tested the change in the d-spacing of membranes after aging for 480 days. As\nshown in Figure 2, after aging for 420 days, the diffraction peaks with chain\nspacing >5Å disappeared at Cyclen content >1%, and the diffraction peaks with chain\nspacing <5Å first became weaker and then stronger with increasing Cyclen content.\nWith increasing Cyclen content, the chain spacing gradually decreased.\nFigure 2. XRD patterns of the Matrimid-Cyclen membranes after aging for 480 days.\nThis change is attributed to the flexibility of the polymer chains and the fact that the\nfresh membrane is in a thermodynamic non-equilibrium state. During physical aging,\nchain collapse occurs in some regions, and the polymer chain segments in the\nnon-equilibrium state within the membrane gradually transfer into the equilibrium state\nby adjusting the chain stacking, resulting in a decrease in free volume and a decrease in\ngas permeability. As the aging time increases, a point is reached where the membrane\nmaterial is able to completely block the transmission of large gases and the\ntransmission of small gases is significantly reduced. The ideal selectivity between two\ndifferent gases across a polymeric membrane is the ratio of their single gas permeability\ncoefficient. As the gas permeability of large gas molecules (such as N and CH )\n2 4\napproaches zero, the selectivity of He/N and He/CH continues to increase even\n2 4\nthough the permeability of He decreases significantly. This work investigated the gas\nseparation properties of Matrimid-based membrane materials for up to ten years, as\nshown in Figure 3 (Petroleum Chemistry, 58, 2018, 760-769), the permeability of He\ncontinued to decrease and the selectivity of He/CH4 separation continues to increase\nduring the physical aging process. After ten years of aging, the permeability of He\ndecreased by 55.4% and the He/CH4 selectivity increased from 146 to 4460, a\n29.5-fold increase.\nFigure 3. Influence of the sample storage time on the gas separation characteristics of\nthe hollow fiber module modified with a He/F mixture (CF = 2 vol %) for 2 min.\n2 2\nTo explore the practical applicability of the Matrimid-Cyclen membranes at different\nfeed pressures, the gas separation performance of the Matrimid-Cyclen-5% membrane\nwas re-evaluated at pressures up to 30 bar, and the data were summarized in Table 1. As\npresented in Figure 4, as the pressure increased from 1 bar to 10 bar, the permeability of\nHe decreased slightly, and the pressure dependence of N and CH was almost\n2 4\nunrecognizable, with only small fluctuations in the selectivity for He/N and He/CH .\n2 4\nThese observations suggest that the membranes remain structurally stable under\nmoderate pressure increases, which may be attributed to the presence of Cyclen\nmolecules, which increase the packing density of the polymer chains, thereby reducing\ntheir mobility, increasing chain stiffness and improving the high-pressure resistance of\nthe membranes. As the operating pressure increased from 10 bar to 20 bar, the\npermeability of all gases rose, while the selectivity for He/N and He/CH decreased.\n2 4\nThis behavior is typical of glassy polymers, such as Matrimid, under plasticization\npressure, where increased feed pressure leads to higher polymer segment mobility and\nlarger chain spacing, resulting in higher permeability and lower selectivity [1-5]. As the\npressure increases from 20 bar to 30 bar, the permeability of He remained almost\nconstant, while the permeability of N and CH gradually decreased, and the selectivity\n2 4\nof He/N and He/CH tends to increase, which was ascribed to the existence of\n2 4\nunrelaxed volume within the polymer chain structure. An increase in feed pressure\nresults in greater compactness of the polymer matrix, which reduces the gas\npermeability in the membrane by reducing fractional free volume [6-7].\nSupplementary References\n[1] H. Rajati, A.H. Navarchian, D,. Rodrigue, S. Tangestaninejad, Improved CO\n2\ntransport properties of Matrimid membranes by adding amine-functionalized PVDF\nand MIL-101 (Cr), Separation and Purification Technology, 235 116149 (2020).\n[2] S. Salman, K. Nijmeijer, High pressure gas separation performance of\nmixed-matrix polymer membranes containing mesoporous Fe (BTC), Journal of\nmembrane science, 459, 33-44 (2014).\n[3] S. Salman, K. Nijmeijer, Performance and plasticization behavior of polymer-MOF\nmembranes for gas separation at elevated pressures, Journal of membrane science, 470,\n166-177 (2014).\n[4] A.E. Amooghin, M. Omidkhah, H. Sanaeepur, A. Kargari, Preparation and\ncharacterization of Ag+ ion-exchanged zeolite-Matrimid® 5218 mixed matrix\nmembrane for CO /CH separation, Journal of Energy Chemistry, 25 (3) 450-462\n2 4\n(2016).\n[5] H. Asghar, A. Ilyas, Z. Tahir, X.F. Li, A.L. Khan, Fluorinated and sulfonated poly\n(ether ether ketone) and Matrimid blend membranes for CO separation, Separation and\n[6] A.E. Amooghin, H. Sanaeepur, M. Omidkhah, A. Kargari, “Ship-in-a-bottle”, a\nnew synthesis strategy for preparing novel hybrid host-guest nanocomposites for\nhighly selective membrane gas separation, Journal of Materials Chemistry A, 6 (4)\n1751-1771 (2018).\n[7] A. Mirzaei, A.H. Navarchian, S. Tangestaninejad, Mixed matrix membranes on the\nbasis of Matrimid and palladium-zeolitic imidazolate framework for hydrogen\nseparation, Iranian Polymer Journal, 29 (6), 479-491 (2020).\nFigure 4. Effect of feed gas pressures on the separation performance of\nMatrimid-Cyclen-5% membrane.\nTable 1. Summary of gas transport permeability and selectivity of the\nMatrimid-Cyclen-5% membrane at different pressures and 25℃.\nGas permeability (Barrer) Selectivity\nPressure\n(Bar)\nHe N CH He/N He/CH\n2 4 2 4\nReturn to 1\n66.84±0.42 0.10±0.00 0.04±0.00 668.40±4.20 1671.00±10.50\nBar\n13) Authors state “which facilitated the extraction of high-purity He from natural gas at\nlow temperatures”. It would be interesting to see how selectivity changes with\ntemperature. In Figure 4f you show permeabilities evolution in a cyclic loop from 25 to\n45 ºC and back to 25 ºC but it is difficult to get an idea on the evolution of selectivity.\nAuthors say: “The gas separation performance of the membranes fully recovered as the\ntemperature decreased from 45 ℃ to 25 ℃, indicating that the changes in membrane\nperformance with temperature are related to the characteristics of the membrane”;\nwhat should be, according to authors, the membrane characteristics that would explain\nthis? Why is CO2 absent from all Figure 4?\nReply: Thanks for your comments. Following your suggestion, the extends gas\nseparation test of the Matrimid-Cyclen membranes is carried out including a wider\nrange from 0 ℃ to 100 ℃ to better assess the impact of operating temperature on the\ngas separation performance. The gas separation performance of Matrimid-Cyclen-5%\nmembrane has been shown in Figure 1, with the data summarized in Table 1.\nAs the operating temperature increased from 0 ℃ to 100 ℃, the permeability of He, N\n2\nand CH increased, while the selectivity of He/N and He/CH decreased, which could\n4 2 4\nbe attributed to the increased gas fugacity at elevated temperature. The selectivity of\nHe/N and He/CH decreased rapidly as the operating temperature increased from\n2 4\n40 ℃ to 60 ℃. This phenomenon was likely due to the accelerated movement\nfrequency of polymer chains at high temperatures, rendering transport of gas molecules\nthrough the matrix and weakening the molecular sieving effect of the membrane.\nNotably, after cooling the membrane from 25 ℃ to 0 ℃, the He permeability decreased\nfrom 63.13 to 52.74 Barrer, while the He/N selectivity increased from 631 to 8790, and\n2\nthe He/CH selectivity increased from 2104 to 17580, corresponding to 14-fold and\n4\n9-fold increases, respectively. This dramatic improvement in selectivity at low\ntemperatures significantly enhanced the potential for high-purity He extraction from\nnatural gas.\nThe ultra-high selectivity of the Matrimid-Cyclen-5% membrane at 0 ℃ for He/N and\n4\ntemperatures. Although the He permeability decreased by 16% at low temperatures, the\nincrease in He selectivity indicated that improving gas purity was more challenging\nthan enhancing transport efficiency. High purity is particularly important for He, given\nits critical applications in the aerospace industry, electronics, medical equipment, and\nscientific research. Furthermore, the gas separation performance of the membrane was\nfully recovered when the temperature was returned to 20 ℃, both from 100 ℃ and from\n0 ℃, indicating that the changes in membrane performance with temperature were\ninherent to the membrane properties and did not result in structural damage.\nFigure 1. Effect of operating temperature on the separation performance of\nMatrimid-Cyclen-5% membrane.\nTable 1. Summary of gas transport permeability and selectivity of the\nMatrimid-Cyclen-5% membrane at 1.0 Bar and different temperatures.\nGas permeability (Barrer) Selectivity\nTemperat\nure (℃)\nHe N CH He/N He/CH\n2 4 2 4\nReturn to\n63.97±0.46 0.09±0.00 0.03±0.00 710.78±2.33 2132.33±7.00\n20 ℃\n0 ℃ 52.74±0.46 0.007±0.00 0.003±0.00 7534.29±24.28 17580.00±56.67\n20 ℃ 63.13±0.46 0.10±0.00 0.03±0.00 631.30±2.60 2104.33±8.67\n40 ℃ 80.14±0.46 0.32±0.02 0.10±0.01 250.44±15.13 801.40±83.82\n60 ℃ 96.91±0.46 1.43±0.46 1.05±0.00 67.77±2.90 92.30±1.73\n80 ℃ 125.12±0.46 3.45±0.46 3.05±0.46 36.27±0.36 41.02±0.87\n100 ℃ 202.55±0.46 9.76±0.46 9.42±0.46 20.75±0.40 21.73±0.58\nReturn to\n68.69±0.27 0.12±0.01 0.03±0.00 572.42±49.85 2289.67±8.00\n20 ℃\nMembrane performance refers to the gas separation capabilities of the membrane\nmaterial itself, rather than the separation performance resulting from irreversible\nstructural damage in a high temperature environment. At higher temperatures, the\nintensity of the thermal movement of polymer chains increases, leading to larger scale\nmovements that create transient gaps at the penetrant scale, thus enhancing diffusion.\nThe increase in permeability for larger molecules, such as N and CH , is more\n2 4\npronounced than smaller molecules like He, thus diminishing the gas separation effect.\nAs the temperature drops from high to low, the intensity of the polymer chain thermal\nmotion is weakened, causing the larger transient gaps generated by the thermal motion\nto rapidly shrink. This change reduces gas diffusion and enhances the molecular sieving\nability of the membrane, restoring the membrane’s performance at lower temperatures.\nSeveral studies have examined the performance of membrane materials after high\ntemperature and pressure testing, returning them to baseline conditions to assess the\nstability of the membrane structure [1-2].\nSupplementary References\n[1] Y.P. Shi, Y. Liu, Z.G. Wang, W.K. Lai, Y.N. Liao, K. Lu, Z. Niu, J. Jin, Dual‐Wing\nLigand Constructed Metal-Organic Framework Membranes with Finely Tuned\nApertures for Natural Gas Separation, Advanced Functional Materials 2404681 (2023).\n[2] S. Zhao, Z.Y. Zhao, Z.Y. Zha, Z.H. Jiang, Z. Wang, M.D. Guiver, Amine‐Rich\nMolecular Nodule-Assembled Membrane Having 5 Angstrom Channels for CO /N\n2 2\nSeparation, Advanced Functional Materials, 2314469 (2024).\nAs shown below, Figure 4 (e) in the manuscript shows the changes in CO , N and CH\n2 2 4\npermeability and CO /N and CO /CH selectivity as a function of the Cyclen mass\n2 2 2 4\nloading. Considering that the original legend for Figure 4 was not clear enough, we\nhave revised the legend for Figure 4 as follows:\nGas separation performance of Matrimid-Cycien membranes. (a) Correlations between\nthe permeabilities and molecular kinetic diameters of various gases. (b) Schematic\nillustration of gas transport behaviors of Matrimid-Cyclen membrane. (c) He, N and\n4 2 4\nloading. (d) H , N and CH permeability and H /N and H /CH selectivity with\n2 2 4 2 2 2 4\ndifferent Cyclen mass loading. (e) CO , N and CH permeability and CO /N and\n2 2 4 2 2\nCO /CH selectivity with different Cyclen mass loading. (f) Effect of operating\n2 4\ntemperature on the separation performance of Matrimid-Cyclen-5% membrane. (g)\nEffect of feed gas pressures on the separation performance of Matrimid-Cyclen-5%\nmembrane. (h) Long-term stability of the Matrimid-Cyclen-5% membrane (complete\nmembrane was stored in an ambient environment without the introduction of any\nprotective gas at room temperature, small pieces of membrane were extracted fromthe\ncomplete membrane at different aging times for a gas separation performance test).\nFigure 4. (e) Changes in CO , N and CH permeability and CO /N and CO /CH\n2 2 4 2 2 2 4\nselectivity as a function of the Cyclen mass loading.\nWe have corrected the above questions according to your suggestion. Please refer to the\nrevised manuscript.\n14) In Figures 5g and 5f the results shown must be referenced and described including,\nfor example, to what load percentages do they correspond (maybe 1 %?). In Figure 5h,\nwhat is in ordinates? Enhancement of what? Figure 5i is useless.\nReply: Thanks for your comments. Figure 5g illustrates the gas separation performance\nof mixed matrix membranes prepared with different fillers at 1% loading (i.e.,\nMatrimid-Cyclen-1% membrane, Matrimid-Hexacyclen-1% membrane and\nMatrimid-Cyclodecane-1% membrane). The horizontal axis in Figure 5h represents the\ndifferent filler types, and the vertical axis shows the multiplicity of enhancement on\nselectivity and permeability. The purpose of this figure is to compare the magnitude of\nenhancement in permeability and selectivity of MMM using different fillers.\nThe two sets of comparison experiments confirmed the hydrogen bonding interaction\nbetween Cyclen and Matrimid, and the suitable gas molecule recognition window size\nof Cyclen as the reason for improving the molecular sieving performance of the\nmembranes, respectively. Figure 5i shows the possible gas transport mechanism of\nMatrimid-Cyclen membranes derived from two sets of comparison experiments. Then,\nmolecular dynamics simulations and BET characterisation of the membranes were used\nto determine the gas separation mechanism of the membranes.\nOriginal figure 5i has been replaced by the following figure:\nFigure 5. (i) Gas-transport mechanism in Matrimid-Cyclen membranes.\nWe have addressed these issues according to your suggestion. Please refer to the\nrevised manuscript.\nResponse to the Reviewers' Comments:\nReviewer #1 (Remarks to the Author):\nThe authors have not adequately addressed the reviewers comments and have not\nimproved the quality of their manuscript. This manuscript should not be accepted, as it\ndoes not meet the standards of the journal.\nReply: After carefully read your comments, I feel a little bit confused as your\ndecision. We understand that the misunderstanding might be caused by the unclear\ndescription in our revised manuscript,. In this work, the Cyclen (3.2-3.4 Å) creates a\nsignificant permeation cut-off between quick gases (He: 2.60 Å, H : 2.89 Å) and slow\n2\ngases (N : 3.64 Å and CH : 3.80 Å). Molecular dynamics simulations show that the\n2 2\nMatrimid-Cyclen-5% membrane exhibits greater He-accessible space and longer\ntransport paths, while N exhibits the opposite behavior, compared to the Matrimid\n2\nmembrane. Additionally, molecular dynamics simulations showed that the\nconnectivity of He transport channels in the Matrimid-Cyclen-5% membrane was\nsignificantly enhanced, while the connectivity of N channels was weakened.\nRecent work published in Nature Sustainability and Science Advances demonstrated\nthat the enhanced channel connectivity can significantly improve the membrane’s\nmass transfer and separation capabilities (Nature Sustainability, 7, 910-919 (2024),\nScience Advances, 10 (32), eado7687 (2024)).\nWe have provided comprehensive responses to all of the reviewers’ comments. The\nquality of the manuscript has been significantly improved. Your comments were\nreally addressed one by one. First, through molecular dynamics (MD) simulations,\nthe gas transport mechanism of the membrane has been studied in greater depth and detail.\nMolecular dynamics simulations show that the Matrimid-Cyclen-5% membrane\nexhibits greater He-accessible space and longer transport paths, while N exhibits the\n2\nopposite behavior, compared to the Matrimid membrane. Additionally, molecular\ndynamics simulations showed that the connectivity of He transport channels in the\nMatrimid-Cyclen-5% membrane was significantly enhanced, while the connectivity\nof N channels was weakened. Obviously, Cyclen significantly enhanced the\n2\nconnectivity of gas transport channels. Recent work published in Nature\nSustainability and Science Advances demonstrated that enhanced channel\nconnectivity can significantly improve the membrane’s mass transfer and separation\ncapabilities (Nature Sustainability, 7, 910-919 (2024), Science Advances, 10 (32),\neado7687 (2024)). Obviously, Cyclen has adjusted the microporous structure of the\nmembrane, enhancing He permeability while inhibiting N penetration. The research\n2\npublished in Nature Communications similarly analyzed gas-accessible space and\ntransport channel length to investigate membrane transport mechanisms (Nature\nCommunications, 11 (1), 1633 (2020)).\nSecond, the BET results from the H adsorption isotherm (not N adsorption isotherm)\n2 2\nshowed that new ultra-microporous pores (3.2 Å) appeared in the MMMs, and the\ncontent of two types of ultra-microporous pores (3-4 Å range) increased continuously\nwith the loading of Cyclen. Clearly, the introduction of Cyclen create new\nultra-microporous pores, regulated the membrane’s microporous structure, and\nincreased 3.8 Å pore content. This synergistic effect enhanced the membrane’s He\ntransport efficiency and selectivity. Our work provides a strong support for the\nmechanism understanding of performance enhancement through material\ncharacterization and MD simulations.\nThird, dynamic mechanical thermal analysis (DMTA) demonstrated that the\nMatrimid-Cyclen membrane exhibited excellent mechanical properties and thermal\nstability.\nFourth, the stability of membranes under harsh conditions is a critical requirement for\npractical applications (Nature Nanotechnology, 1-8 (2025)), and our work has\nexpanded the range of membrane usability. The operating temperature range was\nextended from 25-45 °C to 0-100 °C, while the feed pressure range was broadened\nfrom the 1-6 Bar employed in most He-extraction studies to 1-30 Bar. Experimental\nresults demonstrated the membrane’s high stability and strong recovery capability\nunder these harsh conditions.\nIn summary, the quality of the revised manuscript has been significantly improved.\nReviewer #1 (Remarks to the Author):\nThe manuscript reports on the combination of cyclen and Matrimid into a mixed matrix\nmembrane that has high selectivity for gas separation. Both cyclen and matrimid have\nbeen studied separately, and there exists a lot of literature on both materials. Hence, the\nnovelty being presented is the combination of the two into a membrane. The resulting\nselectivities are very high, which gives me concern, as generally for mixed matrix\nmembrane systems, selectivity is aligned with either of the membrane or additive, and\nnot an enhancement. I can understand the argument for the strong binding between\ncyclen and Matrimid, but that does not necessarily translate into the high selectivities\nreported.\nReply: Many thanks for your valuable comments. It is true that both Cyclen and\nMatrimid have been extensively studied separately, while their combination into a\nmixed matrix membrane represents a new approach to achieve high gas separation\nperformance, especially for He/CH₄ selectivity. The Matrimid membrane is widely\nstudied for gas separation, while its low permeability (33 Barrer) and moderate\nselectivity (86) still present a great challenge to extract Helium(He) from natural gas\n(ultra-low helium concentration). The cyclen (1, 4, 7, 10-tetraazacyclododecane) is one\nof the most extensively studied ligands in coordination chemistry (Shinoda, S.\nDynamic cyclen-metal complexes for molecular sensing and chirality signaling. Chem.\nSoc. Rev. 2013, 42, 1825-1835.), while its potential for He extraction from natural gas\nhas not been explored until now. The ultra-low concentration of He makes it very\ndifficult to extract from natural gas. Therefore, it’s of great significance to develop the\nMatrimid-Cyclen membranes with high He/CH selectivity (>1000). In this work, the\n4\nthe combination of Matrimid and Cyclen (Matrimid-Cyclen-5%) membranes\ndemonstrates high He permeability (66 Barrer) and He/CH selectivity (>1660).\nthe materials involved. The enhancement in selectivity is not simply additive, but from\nthe strong hydrogen bonding interactions between Cyclen and Matrimid, which\nmodifies the polymer chain packing and results in a tighter interchain structure. This\nleads to a more effective sieving mechanism. Usually, polymer membranes have good\nprocessability, high stability, and low cost, but they often suffer from plasticization\nproblems, physical aging issues, and intrinsic permeability-selectivity trade-off\nlimitations, which makes it a great challenge to obtain high permeability together with\nsufficient selectivity [1-4]. Molecular sieve materials have attractive transport properties\nwell above the polymer upper bound, but they tend to have poor membrane-forming\nand mechanical property [5]. Mixed matrix membranes incorporate molecular sieve\nmaterials with certain pore sizes into a processable polymer matrix to improve the gas\nselectivity and permeability of polymer membranes by integrating the advantages of\nboth [6-8], which is the significance of preparing hybrid matrix membranes. Therefore,\nthe mixed matrix membranes (Matrimid-Cyclen) combine the advantages of easy\nprocessibility from Matrimid polymer and high gas permeability/selectivity from\nmolecular sieve material (Cyclen).\nFigure 1 illustrates the redissolution process of equal masses of Matrimid and\nMatrimid-Cyclen membrane in equal masses of DMF solvent. The Matrimid\nmembrane dissolves completely at room temperature, whereas the Matrimid-Cyclen\nmembrane retains significant insoluble material even at 120 ℃ (the membrane-forming\ntemperature). Which demonstrates the exceptional stability of strong hydrogen-bond\ncross-linking between Matrimid and Cyclen. This strong cross-linking interaction\navoids the formation of interfacial defects while increasing chain packing density. By\nsynergizing with Cyclen’s molecular sieving capability, this cross-linking structure\nendows the membrane with high separation selectivity.\nFigure 1. The redissolution process of Matrimid membrane and Matrimid-Cyclen-5%\nmembrane.\nTable 1 summarizes the He separation performance of Matrimid-based membranes.\nThe Matrimid-Cyclen-5% membranes exhibit high He permeability (66 Barrer) and\nHe/CH selectivity (>1660), which far exceed most of the other Matrimid-based\n4\nmembranes.\nTable 1. Summary of literature He/N and He/CH separation performance.\n2 4\nHe He/N He/CH\n2 4\nOperating Gas Refer\nMembrane Name Permeabilit Selectivit Selectivit\nconditions Type ence\ny (Barrer) y y\nMatrimid-Cu-BTC-R-40 66.40 265.80 369.10 5 atm, 35 °C Pure 1\nMatrimid-Cu-BTC-30 51.80 193.40 257.90 5 atm, 35 °C Pure 2\nMatrimid-Cu-BDC-15% 20.50 341.70 410.00 7 atm, 35 °C Pure 3\nMatrimid-MgO-40% 45.00 86.1 118.4 10 atm, 35 °C Pure 4\nMatrimid-CNFs-10% 16.10 89.50 179.50 20 atm, 35 °C Pure 5\nMatrimid-PIM-EA(H )- \n2\n197.00 28.84 21.55 1 atm, 25 °C Pure 6\nTB-50%\nMatrimid-p-xylenediami\n21.70 112.00 155.00 10 atm, 35 °C Pure 7\nne-CL-14\nMatrimid-C60-2.5% 21.00 91.70 148.80 10 atm, 35 °C Pure 8\nMatrimid-Cyclen-3% 57.57±0.21 428.35±2. 959.50±4. This\n1 atm, 25 °C Pure\n15 67 work\nMatrimid-Cyclen-5% 66.43±0.22 621.70±3. 1660.75± This\n1 atm, 25 °C Pure\n20 5.25 work\nMatrimid-Cyclen-10% 78.63±0.28 543.43±4 1310.50± This\n1 atm, 25 °C Pure\n0.34 4.67 work\nSupplementary References\n[1] A. Akbari, J. Karimi-Sabet, S.M. Ghoreishi, Intensiffcation of helium separation\nfrom CH and N by size-reduced Cu-BTC particles in Matrimid matrix, Separation and\n4 2\nPuriffcation Technology, 251, 117317 (2020).\n[2] A. Akbari, J. Karimi-Sabet, S.M. Ghoreishi, Matrimid® 5218 based mixed matrix\nmembranes containing metal organic frameworks (MOFs) for helium separation,\nChemical Engineering & Processing: Process Intensification, 148, 107804 (2020).\n[3] A. Ali, J. Karimi-Sabet, S.M. Ghoreishi, Polyimide based mixed matrix membranes\nincorporating Cu-BDC nanosheets for impressive helium separation, Separation and\nPurification Technology, 253, 117430 (2020).\n[4] S.S. Hosseini, Y. Li, T.S. Chung, Y. Liu, Enhanced gas separation performance of\nnanocomposite membranes using MgO nanoparticles, Journal of Membrane Science,\n302 (1-2), 207-217 (2007).\n[5] M. Dohade, Incorporation of carbon nanofibers into a Matrimid polymer matrix:\nEffects on the gas permeability and selectivity properties, Journal of Applied Polymer\nScience, 135 (12) 46019 (2018).\n[6] E. Esposito, I. Mazzei, M.Monteleone, A. Fuoco, M. Carta, N.B. Makeown, R.\nMalpass-Evans, J.C. Jansen, Highly permeable Matrimid®/PIM-EA (H )-TB blend\n2\nmembrane for gas separation, Polymers, 11 (1), 46 (2018).\n[7] P.S. Tin, T.S. Chung, Y. Liu, R. Wang, S.L. Liu, K.P. Pramoda, Effects of\ncross-linking modification on gas separation performance of Matrimid membranes,\nJournal of Membrane Science, 225(1-2), 77-90 (2003).\n[8] T.S. Chung, S.S. Chan, R. Wang, Z.H. Lu, C.B. He, Characterization of\npermeability and sorption in Matrimid/C60 mixed matrix membranes, Journal of\nMembrane Science, 211 (1), 91-99 (2003).\nThere is no reported error for the experimental results and no discussion on\nreproducibility, so this raises questions about the provided data.\nReply: Many thanks for your comments. Repeated experiments were conducted and\nthe error bars have been included in the revised manuscript. The gas separation\nperformance and fabrication of mixed matrix membrane with gas molecule recognition\nwindow (cyclen) was found to be reproducible. The specific data has been listed as\nfollows.\nTable 1. Summary of Gas transport properties of the Matrimid and\nMatrimid-Cyclen-n% membranes at 1.0 atm and 25 ℃ (n = 1-15% indicated the degree\nof Cyclen mass loading).\nGas permeability (Barrer)\nMembrane\nHe H CO\n2 2\nMatrimid 33.59±0.41 27.45±0.08 15.03±0.06\nMatrimid-Cyclen-1% 47.87±0.24 43.04±0.16 16.90±0.06\nMatrimid-Cyclen-3% 57.57±0.21 55.69±0.23 17.66±0.12\nMatrimid-Cyclen-5% 66.43±0.22 62.17±0.32 20.07±0.08\nMatrimid-Cyclen-10% 78.63±0.28 76.08±0.12 22.17±0.15\nMatrimid-Cyclen-15% 85.93±0.80 80.85±0.30 24.76±0.07\nIdea Selectivity\nMembrane\nHe/N He/CH H /N H /CH CO /N CO /CH\n2 4 2 2 2 4 2 2 2 4\n79.97±6. 86.13±2. 65.00±4 70.38±1 35.79±2. 38.54±0.\nMatrimid\n05 25 .82 .98 88 99\n145.06±1 368.23±2 143.47± 331.08± 56.33±3. 130.00±1\nMatrimid-Cyclen-1%\n0.29 8.10 9.71 26.34 81 0.33\n575.70±2 959.50±4 428.35± 928.17± 135.85± 294.33±2\nMatrimid-Cyclen-3%\n.15 .67 2.15 4.67 0.92 .00\n664.30±2 1660.75± 621.70± 1554.25 200.70± 501.75±3\nMatrimid-Cyclen-5%\n.10 5.25 3.20 ±8.00 0.80 .75\n561.64±4 1310.50± 543.43± 1268.00 158.36± 369.50±2\nMatrimid-Cyclen-10%\n1.36 4.67 40.34 ±3.17 11.10 .33\n358.04±1 661.00±4 336.88± 621.92± 103.17± 190.46±1\nMatrimid-Cyclen-15%\n2.28 8.42 11.59 49.33 3.64 4.71\nTable 2. Summary of gas transport permeability and selectivity of the\nMatrimid-Cyclen-5% membrane at 1.0 atm and different temperatures.\nGas permeability (Barrer) Selectivity\nTemperat\nure (℃)\nHe N CH He/N He/CH\n2 4 2 4\nReturn to\n63.97±0.46 0.09±0.00 0.03±0.00 710.78±2.33 2132.33±7.00\n20 ℃\n0 ℃ 52.74±0.46 0.007±0.00 0.003±0.00 7534.29±24.28 17580.00±56.67\n20 ℃ 63.13±0.46 0.10±0.00 0.03±0.00 631.30±2.60 2104.33±8.67\n40 ℃ 80.14±0.46 0.32±0.02 0.10±0.01 250.44±15.13 801.40±83.82\n60 ℃ 96.91±0.46 1.43±0.46 1.05±0.00 67.77±2.90 92.30±1.73\n80 ℃ 125.12±0.46 3.45±0.46 3.05±0.46 36.27±0.36 41.02±0.87\n100 ℃ 202.55±0.46 9.76±0.46 9.42±0.46 20.75±0.40 21.73±0.58\nReturn to\n68.69±0.27 0.12±0.01 0.03±0.00 572.42±49.85 2289.67±8.00\n20 ℃\nTable 3. Summary of gas transport permeability and selectivity of the\nMatrimid-Cyclen-5% membrane at different pressures and 25℃.\nGas permeability (Barrer) Selectivity\nPressure\n(Bar)\nHe N CH He/N He/CH\n2 4 2 4\nReturn to 1\n66.84±0.42 0.10±0.00 0.04±0.00 668.40±4.20 1671.00±10.50\nBar\nTable 4. Summary of gas transport properties of the Matrimid and\nMatrimid-Cyclen-5% membrane at 1.0 atm and 25 ℃ during the aging process.\nGas permeability (Barrer)\nMembrane\nHe H CO\n2 2\nFresh 66.43±0.22 62.17±0.32 20.07±0.08\nAging for 7 days 64.22±.34 59.90±0.23 18.30±0.07\nAging for 20 days 59.96±0.53 54.68±0.14 16.67±0.17\nAging for 40 days 59.73±0.09 53.39±0.33 15.96±0.09\nAging for 110 days 54.31±0.10 50.76±0.10 14.12±0.03\nSelectivity\nMembrane\nHe/N He/CH H /N H /CH CO /N CO /CH\n2 4 2 2 2 4 2 2 2 4\n664.30±2.1 1660.75± 621.70± 1554.25± 200.70 501.75±3\nFresh\n0 5.25 3.20 8.00 ±0.80 .75\n713.56±67. 1605.50± 665.56± 1497.50± 203.33 457.50±1\nAging for 7 days\n960 8.50 68.04 21.22 ±21.79 .42\n856.57±57. 2998.00± 781.14± 2734.00± 238.14 833.50±9\nAging for 20 days\n74 26.50 54.20 6.76 ±18.02 .66\n995.50±1.5 5973.00± 889.83± 5339.00± 266.00 1596.00±\nAging for 40 days\n0 9.00 5.07 30.45 ±1.33 7.94\n1086.20±2. 6788.80± 1015.20 6345.00± 282.40 1765.00±\nAging for 110 days\n00 12.45 ±1.74 10.90 ±0.53 3.31\nFigure 1 shows the preparation process of the Matrimid-Cyclen membrane, the\ndispersion of cyclen in Matrimid solutions is homogeneous. There is no Tyndall\nphenomenon in the clear and transparent Matrimid/Cyclen solutions, indicating that\nCyclen is uniformly dispersed in the mixed solution. The reproducibility of a mixed\nmatrix membrane depends on the dispersibility of the filler, the higher the solubility of\nthe filler and the better the dispersion in the mixed solution, the smaller the error in the\nperformance of the membrane material.\nFigure 1. Preparation process of the Matrimid-Cyclen membranes. (a) Cyclen powder\n(1.0, 3.0, 5.0, 10.0 or 15.0 mg) were added to 6.0 g DMF solvent to obtain clear and\ntransparent solutions. (b) The clean Matrimid dope solution was added to the\nCyclen/DMF solution to obtain homogeneous Matrimid/Cyclen solutions. (c) The\nMatrimid/Cyclen solutions was injected into clean petri dishes and transferred to a\nvacuum oven at an ambient temperature of 120 °C for 8-hour solvent evaporation to\nobtain Matrimid-Cyclen membranes.\nI am also concerned about the high cyclen loading in the membrane, up to 15%. There\nis no discussion about mechanical properties in the manuscript, but the literature on\nMatrimid is very clear that high levels of additives result in a very brittle membrane. I\nsee no reason why this is not the case here. This has issues with the ability to translate\nthe performance into a viable membrane.\nReply: Thanks for your comments. We understand your concern about the potential\nbrittleness of the Matrimid mixed matrix membranes with higher Cyclen loadings. To\naddress this, the dynamic mechanical thermal analysis (DMTA) was conducted on\nMatrimid-Cyclen samples to assess the mechanical properties across a temperature\nrange of 50 to 350 °C. As shown in Figure 3 (g-i), the storage modulus (E′) of the\nMatrimid-Cyclen membranes (1800 MPa) was almost completely maintained before\nreaching the corresponding T . This indicated that the membrane retains its mechanical\ng\nintegrity even with higher Cyclen content. Furthermore, we observed that higher\nCyclen loadings did increase in the brittleness of the membranes, as seen in the\ndecreasing peak intensity of damping (Tan δ) with increasing Cyclen content (Figure\n3i). These data demonstrated that this kind of gas separation membrane was endowed\nwith good mechanical property and thermal stability, even with the higher Cyclen\nloading.\nFigure 3. Characterization of Matrimid-Cyclen membranes. (g) The dynamic\nmechanical thermal analysis (DMTA) spectra of Matrimid-Cyclen membranes at\ntemperature range from 50 to 350 ℃. (h) The storage modulus (E′) and (i) Tan δ of\nMatrimid-Cyclen membranes.\nThe operating temperature, feed pressure and aging behaviors follows the expected\ntrend of mixed matrix membranes, with the presented discussion valid. The application\nfor helium extraction is important, and it is good that the authors considered both\nseparation from methane and nitrogen. I recommend that the manuscript be rejected,\nand the authors submit to a more appropriate journal.\nReply: Thank you very much for your professional comments and suggestions. We\nsincerely apologize for some inadequate in data analysis and descriptions in the original\nmanuscript. As you pointed out, several issues required further attention, and we have\nmade substantial revisions to address the concerns. Below is a detailed list of our\nresponses to your comments. In our work, we designed the mixed matrix membranes\nbased on several key considerations derived from numerous studies.\nFirstly, Cyclen was selected as filler due to its size-sieving effect, which is beneficial\nfor He separation. Based on the molecular dynamic diameters of He (2.60 Å), H (2.89\n2\nÅ), CO (3.30 Å), N (3.64 Å) and CH (3.80 Å), the Cyclen with cavity size (3.2-3.4 Å)\n2 2 4\nbetween the molecular dynamic diameters of He and N was selected, which makes it\n2\nan ideal candidate for He separation, such as He/N and He/CH . Furthermore, the\n2 4\nCyclen with a certain cavity size creates a special transport channel for small gases, like\nHe, which increases their transport paths in the membrane, counteracting the negative\neffect of the increased chain stacking density and enhancing the permeability of small\ngas molecules.\nSecondly, the chosen filler must have good solubility and dispersion in solvents. To\nensure uniform dispersion of the filler in the membrane and to minimize the formation\nof non-selective voids caused by filler agglomeration, we selected Cyclen for its high\nsolubility and good dispersion in organic solvents. Cyclen was fully dissolved in DMF,\nand then the clean Matrimid dope solution was added to the Cyclen/DMF solution. The\nresulting homogeneous Matrimid/Cyclen solution, which was stirred for 24 hours and\nsonicated for 1 hour, remained clear and transparent, even with up to 15% Cyclen\ncontent, as shown in Figure 1.\nFigure 1. (a) Cyclen powder (1.0, 3.0, 5.0, 10.0 or 15.0 mg) were added to 6.0 g DMF\nsolvent to obtain clear and transparent solutions. (b) Matrimid dope solution. (c) The\nclean Matrimid dope solution was added to the Cyclen/DMF solution to obtain clear\nand transparent mixed Matrimid/Cyclen solutions.\nThirdly, compatibility between filler and polymer matrix. It is crucial to create mixed\nmatrix membranes with fair matching between filler and polymer matrix to gain\nattractive performance. This is why we chose Cyclen as a filler. Hydrogen bonding\nbetween Cyclen and the Matrimid polymer chains immobilizes Cyclen molecules in the\ninterchain gaps of the Matrimid membrane, resulting in homogeneously mixed matrix\nmembranes. The SEM image (Figure 2) shows the surface and interface of the\nmembrane, confirming the good interfacial compatibility between Cyclen and\nMatrimid.\nFigure 2. SEM images of the cross section of the Matrimid-Cyclen-n% membranes up\nto ×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nFourthly, Helium (He) purification requirements. Helium is an extremely scarce,\nnon-renewable gas with a strong safety profile that is an essential resource for the\ndevelopment of many high-tech industries, such as the aerospace industry, electronics,\nmedical equipment, and scientific research. On Earth, He occurs in very low\nconcentrations, mainly in the atmosphere and in natural gas. Helium is commonly\nextracted industrially from high-pressure natural gas. However, the initial\nconcentration of He in most natural gas reservoirs is still as low as 0.3%, and to extract\nhelium from natural gas with ultra-low He content, the gas separation membranes must\npossess He/CH selectivity greater than 1000. Highly permeable polymers rarely\n4\nprovide sufficient selectivity for high-purity He extraction, and Matrimid was chosen as\nthe polymer matrix due to its dense stacking of polymer chains. XRD characterization\nrevealed that the strong hydrogen bonding between Cyclen and Matrimid modulated\nthe chain stacking of the mixed matrix membranes, which reduce the pore size, narrow\nthe pore size distribution and enhances the selectivity of the membrane by modulating\nthe chain spacing.\nWe have verified the success of the hybrid matrix membrane design with ultra-high\nhelium selectivity to achieve excellent separation performance."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have not adequately addressed the reviewers comments and have not improved the quality of their manuscript.\nThis manuscript should not be accepted, as it does not meet the standards of the journal.", "reviewer_4": "This study reports the preparation of mixed matrix membranes by incorporating Cyclen molecules into Matrimid polyimide, and explores their application in gas separation. The approach of membrane development is certainly innovative. I have carefully reviewed the revised manuscript and the authors’ responses to the previous reviewers' comments. The authors have provided a comprehensive set of characterization data and presented a plausible explanation for the polymer-filler interactions—primarily through hydrogen bonding—which may play a critical role in enhancing gas selectivity. Most of the previous concerns have been addressed appropriately. In my view, the revised manuscript could be considered for publication in Nature Communications, but the authors must address the remaining concerns regarding data accuracy and reproducibility, especially the membranes showed record-high gas separation selectivity.\n1. A key concern raised by previous reviewers pertains to the unusually record-high gas separation selectivity of the\nMatrimid-Cyclen mixed matrix membranes. I share Reviewer 1’s skepticism about the record-high selectivities reported. I\nstrongly recommend that the authors re-evaluate their results and, if possible, validate the membrane performance through\ncollaboration with other membrane research groups, either in China or internationally, to rule out any inadvertent errors.\n2. The authors should carefully re-examine their raw data and the time-lag profiles. Based on my understanding, Matrimid\ntypically exhibits low gas permeability for CH₄ and N₂ (<0.5 Barrer). In this study, the reported CH₄ permeability is below 0.1\nBarrer, for instance, the Matrimid-Cyclen-5% membrane shows a CH₄ permeability of only 0.04 Barrer, which further\ndeclines to 0.001 Barrer after aging. At such low permeability values, the pressure increase on the permeate side would be\nextremely slow. In my experience, accurate measurement in this regime is highly sensitive to experimental conditions,\nparticularly potential gas leakage. Notably, the reported vacuum pressure of 0.006579 Torr (equivalent to ~0.877 Pa) is\nrelatively high for reliable time-lag measurements. The addition of nanofillers can increase gas sorption, prolonging the time\nneeded to reach steady-state permeation. This can lead to underestimation of the slope and thus an artificially low gas\npermeability. Furthermore, leakage from the atmosphere could distort measurements. Did the authors account for and\nsubtract gas leakage? This could be a critical source of error. The authors should revisit their raw data and provide\nrepresentative time-lag profiles, including slope fittings, for key samples in the Supplementary Information. The authors\nshould also provide more experimental details of their time-lag rig, incuding the accuracy of the pressure sensor.\n3. Given the reported high performance, I strongly recommend that the authors fabricate thin-film composite (TFC)\nmembranes and measure gas permeance and selectivity for both single gases and gas mixtures, ideally with gas\nchromatography. The Matrimid-Cyclen solution should be suitable for casting onto porous polymeric or AAO supports. With\nselective layer thicknesses below 1 micron, or ideally down to ~100 nm, the gas permeance should be significantly higher,\nand the influence of leakage on time-lag measurements substantially reduced. Such tests would help confirm whether the\nTFC membranes maintain high gas permeance and selectivity (e.g., for He/CH₄ or CO₂/CH₄ mixtures). This is crucial, as the\nlow permeability of thick films may introduce significant measurement error.\n4. Building on the points above, the authors are encouraged to test their approach using other polymer matrices with higher\nintrinsic gas permeability. This would help determine whether similar enhancements can be achieved and reduce the\nsusceptibility to measurement error.\n5. A central hypothesis is that the Cyclen macrocycle facilitates gas transport through its cavity and narrows the polymer\ninterchain spacing via hydrogen bonding. The authors performed control experiments with Cyclododecane and Hexacyclen.\nHowever, it would strengthen their case to include additional controls using amine-containing molecules without macrocyclic\ncavities, such as piperazine.\n6. The three-dimensional plots in Figure 4 are difficult to interpret. A clearer presentation would be to convert them to two-\ndimensional plots.", "reviewer_5": "The authors have carefully addressed the comments raised form the previous review round - the manuscript now has met the standards of Nature Communications and it can be considered for publication."}, "author_response": "Response to the Reviewers' Comments:\nReviewer #1 (Remarks to the Author):\nThe manuscript reports on the combination of cyclen and Matrimid into a mixed matrix\nmembrane that has high selectivity for gas separation. Both cyclen and matrimid have\nbeen studied separately, and there exists a lot of literature on both materials. Hence, the\nnovelty being presented is the combination of the two into a membrane. The resulting\nselectivities are very high, which gives me concern, as generally for mixed matrix\nmembrane systems, selectivity is aligned with either of the membrane or additive, and\nnot an enhancement. I can understand the argument for the strong binding between\ncyclen and Matrimid, but that does not necessarily translate into the high selectivities\nreported.\nReply: Many thanks for your valuable comments. While it is true that both Cyclen and\nMatrimid have been extensively studied separately, their combination into a mixed\nmatrix membrane represents a new approach to achieve high gas separation\nperformance, especially for He/CH₄ selectivity. The Matrimid membrane is widely\nstudied for gas separation, while its low permeability (33 Barrer) and moderate\nselectivity (86) still present a great challenge to extract Helium(He) from natural gas\n(ultra-low helium concentration). The cyclen (1, 4, 7, 10-tetraazacyclododecane) is one\nof the most extensively studied ligands in coordination chemistry (Shinoda, S.\nDynamic cyclen-metal complexes for molecular sensing and chirality signaling. Chem.\nSoc. Rev. 2013, 42, 1825-1835.), while its potential for He extraction from natural gas\nhas not been explored until now. The ultra-low concentration of He makes it very\ndifficult to extract from natural gas. Therefore, it’s of great significance to develop the\nMatrimid-Cyclen membranes with high He/CH selectivity (>1000). In this work, the\n4\nthe combination of Matrimid and Cyclen (Matrimid-Cyclen-5%) membranes\ndemonstrates high He permeability (66Barrer) and He/CH selectivity (>1660).\nthe materials involved. The enhancement in selectivity is not simply additive, but from\nthe strong hydrogen bonding interactions between Cyclen and Matrimid, which\nmodifies the polymer chain packing and results in a tighter interchain structure. This\nleads to a more effective sieving mechanism. Usually, polymer membranes have good\nprocessability, high stability, and low cost, but they often suffer from plasticization\nproblems, physical aging issues, and intrinsic permeability-selectivity trade-off\nlimitations, which makes it a great challenge to obtain high permeability together with\nsufficient selectivity [1-4]. Molecular sieve materials have attractive transport properties\nwell above the polymer upper bound, but they tend to have poor membrane-forming\nand mechanical property [5]. Mixed matrix membranes incorporate molecular sieve\nmaterials with certain pore sizes into a processable polymer matrix to improve the gas\nselectivity and permeability of polymer membranes by integrating the advantages of\nboth [6-8], which is the significance of preparing hybrid matrix membranes. Therefore,\nthe mixed matrix membranes (Matrimid-Cyclen) combine the advantages of easy\nprocessibility from Matrimid polymer and high gas permeability/selectivity from\nmolecular sieve material (Cyclen).\nTable 1 summarizes the He separation performance of Matrimid-based membranes.\nThe Matrimid-Cyclen-5% membranes exhibit high He permeability (66 Barrer) and\nHe/CH selectivity (>1660), which far exceed most of the other Matrimid-based\n4\nmembranes.\nTable 1. Summary of literature He/N and He/CH separation performance.\n2 4\nHe He/N He/CH\n2 4\nOperating Gas Refer\nMembrane Name Permeabili Selectivit Selectivit\nconditions Type ence\nty (Barrer) y y\nMatrimid-Cu-BTC-R-40 66.40 265.80 369.10 5 atm, 35 °C Pure 1\nMatrimid-Cu-BTC-30 51.80 193.40 257.90 5 atm, 35 °C Pure 2\nMatrimid-Cu-BDC-15% 20.50 341.70 410.00 7 atm, 35 °C Pure 3\nMatrimid-MgO-40% 45.00 86.1 118.4 10 atm, 35 °C Pure 4\nMatrimid-CNFs-10% 16.10 89.50 179.50 20 atm, 35 °C Pure 5\nMatrimid-PIM-EA(H )- \n2\n197.00 28.84 21.55 1 atm, 25 °C Pure 6\nTB-50%\nMatrimid-p-xylenediami\n21.70 112.00 155.00 10 atm, 35 °C Pure 7\nne-CL-14\nMatrimid-C60-2.5% 21.00 91.70 148.80 10 atm, 35 °C Pure 8\nMatrimid-Cyclen-3% 57.57±0.2 428.35±2. 959.50±4. This\n1 atm, 25 °C Pure\n1 15 67 work\nMatrimid-Cyclen-5% 66.43±0.2 621.70±3. 1660.75± This\n1 atm, 25 °C Pure\n2 20 5.25 work\nMatrimid-Cyclen-10% 78.63±0.2 543.43±4 1310.50± This\n1 atm, 25 °C Pure\n8 0.34 4.67 work\nSupplementary References\n[1] A. Akbari, J. Karimi-Sabet, S.M. Ghoreishi, Intensiffcation of helium separation\nfrom CH and N by size-reduced Cu-BTC particles in Matrimid matrix, Separation and\n4 2\nPuriffcation Technology, 251, 117317 (2020).\n[2] A. Akbari, J. Karimi-Sabet, S.M. Ghoreishi, Matrimid® 5218 based mixed matrix\nmembranes containing metal organic frameworks (MOFs) for helium separation,\nChemical Engineering & Processing: Process Intensification, 148, 107804 (2020).\n[3] A. Ali, J. Karimi-Sabet, S.M. Ghoreishi, Polyimide based mixed matrix membranes\nincorporating Cu-BDC nanosheets for impressive helium separation, Separation and\nPurification Technology, 253, 117430 (2020).\n[4] S.S. Hosseini, Y. Li, T.S. Chung, Y. Liu, Enhanced gas separation performance of\nnanocomposite membranes using MgO nanoparticles, Journal of Membrane Science,\n302 (1-2), 207-217 (2007).\n[5] M. Dohade, Incorporation of carbon nanofibers into a Matrimid polymer matrix:\nEffects on the gas permeability and selectivity properties, Journal of Applied Polymer\nScience, 135 (12) 46019 (2018).\n[6] E. Esposito, I. Mazzei, M.Monteleone, A. Fuoco, M. Carta, N.B. Makeown, R.\nMalpass-Evans, J.C. Jansen, Highly permeable Matrimid®/PIM-EA (H )-TB blend\n2\nmembrane for gas separation, Polymers, 11 (1), 46 (2018).\n[7] P.S. Tin, T.S. Chung, Y. Liu, R. Wang, S.L. Liu, K.P. Pramoda, Effects of\ncross-linking modification on gas separation performance of Matrimid membranes,\nJournal of Membrane Science, 225(1-2), 77-90 (2003).\n[8] T.S. Chung, S.S. Chan, R. Wang, Z.H. Lu, C.B. He, Characterization of\npermeability and sorption in Matrimid/C60 mixed matrix membranes, Journal of\nMembrane Science, 211 (1), 91-99 (2003).\nThere is no reported error for the experimental results and no discussion on\nreproducibility, so this raises questions about the provided data.\nReply: Many thanks for your comments. Repeated experiments were conducted and\nthe error bars have been included in the revised manuscript, The gas separation\nperformance and fabrication of mixed matrix membrane with gas molecule recognition\nwindow (cyclen) was found to be reproducible. The specific data has been listed as\nfollows.\nTable 1. Summary of Gas transport properties of the Matrimid and\nMatrimid-Cyclen-n% membranes at 1.0 atm and 25 ℃ (n = 1-15% indicated the degree\nof Cyclen mass loading).\nGas permeability (Barrer)\nMembrane\nHe H CO N CH\n2 2 2 4\nMatrimid 33.59±0.41 27.45±0.08 15.03±0.06 0.42±0.03 0.39±0.03\nMatrimid-Cyclen-1% 47.87±0.24 43.04±0.16 16.90±0.06 0.30±0.02 0.13±0.01\nMatrimid-Cyclen-3% 57.57±0.21 55.69±0.23 17.66±0.12 0.13±0.00 0.06±0.00\nMatrimid-Cyclen-5% 66.43±0.22 62.17±0.32 20.07±0.08 0.10±0.00 0.04±0.00\nMatrimid-Cyclen-10% 78.63±0.28 76.08±0.12 22.17±0.15 0.14±0.01 0.06±0.00\nMatrimid-Cyclen-15% 85.93±0.80 80.85±0.30 24.76±0.07 0.24±0.02 0.13±0.01\nIdea Selectivity\nMembrane\nHe/N He/CH H /N H /CH CO /N CO /CH\n2 4 2 2 2 4 2 2 2 4\n79.97±6. 86.13±2. 65.00±4 70.38±1 35.79±2. 38.54±0.\nMatrimid\n05 25 .82 .98 88 99\n145.06±1 368.23±2 143.47± 331.08± 56.33±3. 130.00±1\nMatrimid-Cyclen-1%\n0.29 8.10 9.71 26.34 81 0.33\n575.70±2 959.50±4 428.35± 928.17± 135.85± 294.33±2\nMatrimid-Cyclen-3%\n.15 .67 2.15 4.67 0.92 .00\n664.30±2 1660.75± 621.70± 1554.25 200.70± 501.75±3\nMatrimid-Cyclen-5%\n.10 5.25 3.20 ±8.00 0.80 .75\n561.64±4 1310.50± 543.43± 1268.00 158.36± 369.50±2\nMatrimid-Cyclen-10%\n1.36 4.67 40.34 ±3.17 11.10 .33\n358.04±1 661.00±4 336.88± 621.92± 103.17± 190.46±1\nMatrimid-Cyclen-15%\n2.28 8.42 11.59 49.33 3.64 4.71\nTable 2. Summary of gas transport permeability and selectivity of the\nMatrimid-Cyclen-5% membrane at 1.0 atm and different temperatures.\nGas permeability (Barrer) Selectivity\nTemperat\nure (℃)\nHe N CH He/N He/CH\n2 4 2 4\nReturn to\n63.97±0.46 0.09±0.00 0.03±0.00 710.78±2.33 2132.33±7.00\n20 ℃\n0 ℃ 52.74±0.46 0.007±0.00 0.003±0.00 7534.29±24.28 17580.00±56.67\n20 ℃ 63.13±0.46 0.10±0.00 0.03±0.00 631.30±2.60 2104.33±8.67\n40 ℃ 80.14±0.46 0.32±0.02 0.10±0.01 250.44±15.13 801.40±83.82\n60 ℃ 96.91±0.46 1.43±0.46 1.05±0.00 67.77±2.90 92.30±1.73\n80 ℃ 125.12±0.46 3.45±0.46 3.05±0.46 36.27±0.36 41.02±0.87\n100 ℃ 202.55±0.46 9.76±0.46 9.42±0.46 20.75±0.40 21.73±0.58\nReturn to\n68.69±0.27 0.12±0.01 0.03±0.00 572.42±49.85 2289.67±8.00\n20 ℃\nTable 3. Summary of gas transport permeability and selectivity of the\nMatrimid-Cyclen-5% membrane at different pressures and 25℃.\nGas permeability (Barrer) Selectivity\nPressure\n(Bar)\nHe N CH He/N He/CH\n2 4 2 4\nReturn to 1\n66.84±0.42 0.10±0.00 0.04±0.00 668.40±4.20 1671.00±10.50\nBar\nTable 4. Summary of gas transport properties of the Matrimid and\nMatrimid-Cyclen-5% membrane at 1.0 atm and 25 ℃ during the aging process.\nGas permeability (Barrer)\nMembrane\nHe H CO N CH\n2 2 2 4\nFresh 66.43±0.22 62.17±0.32 20.07±0.08 0.10±0.00 0.04±0.00\nAging for 7 days 64.22±.34 59.90±0.23 18.30±0.07 0.09±0.01 0.04±0.00\nAging for 20 days 59.96±0.53 54.68±0.14 16.67±0.17 0.07±0.005 0.02±0.00\nAging for 40 days 59.73±0.09 53.39±0.33 15.96±0.09 0.06±0.00 0.01±0.00\nAging for 110 days 54.31±0.10 50.76±0.10 14.12±0.03 0.05±0.00 0.008±0.00\nAging for 480 days 42.87±0.21 31.43±0.13 7.82±0.04 0.009±0.00 0.001±0.00\nSelectivity\nMembrane\nHe/N He/CH H /N H /CH CO /N CO /CH\n2 4 2 2 2 4 2 2 2 4\n664.30±2.1 1660.75± 621.70± 1554.25± 200.70 501.75±3\nFresh\n0 5.25 3.20 8.00 ±0.80 .75\n713.56±67. 1605.50± 665.56± 1497.50± 203.33 457.50±1\nAging for 7 days\n960 8.50 68.04 21.22 ±21.79 .42\n856.57±57. 2998.00± 781.14± 2734.00± 238.14 833.50±9\nAging for 20 days\n74 26.50 54.20 6.76 ±18.02 .66\n995.50±1.5 5973.00± 889.83± 5339.00± 266.00 1596.00±\nAging for 40 days\n0 9.00 5.07 30.45 ±1.33 7.94\n1086.20±2. 6788.80± 1015.20 6345.00± 282.40 1765.00±\nAging for 110 days\n00 12.45 ±1.74 10.90 ±0.53 3.31\n4763.33±2 42870.00 3496.22 31430.00 868.89 7820.00±\nAging for 480 days\n3.34 ±200.00 ±12.62 ±113.58 ±4.01 36.06\nFigure 1 shows the preparation process of the Matrimid-Cyclen membrane, the\ndispersion of cyclen in Matrimid solutions is homogeneous. There is no Tyndall\nphenomenon in the clear and transparent Matrimid/Cyclen solutions, indicating that\nCyclen is uniformly dispersed in the mixed solution. The reproducibility of a mixed\nmatrix membrane depends on the dispersibility of the filler, the higher the solubility of\nthe filler and the better the dispersion in the mixed solution, the smaller the error in the\nperformance of the membrane material.\nFigure 1. Preparation process of the Matrimid-Cyclen membranes. (a) Cyclen powder\n(1.0, 3.0, 5.0, 10.0 or 15.0 mg) were added to 6.0 g DMF solvent to obtain clear and\ntransparent solutions. (b) The clean Matrimid dope solution was added to the\nCyclen/DMF solution to obtain homogeneous Matrimid/Cyclen solutions. (c) The\nMatrimid/Cyclen solutions was injected into clean petri dishes and transferred to a\nvacuum oven at an ambient temperature of 120 °C for 8-hour solvent evaporation to\nobtain Matrimid-Cyclen membranes.\nI am also concerned about the high cyclen loading in the membrane, up to 15%. There\nis no discussion about mechanical properties in the manuscript, but the literature on\nMatrimid is very clear that high levels of additives result in a very brittle membrane. I\nsee no reason why this is not the case here. This has issues with the ability to translate\nthe performance into a viable membrane.\nReply: Thanks for your comments. We understand your concern about the potential\nbrittleness of the Matrimid mixed matrix membranes with higher Cyclen loadings. To\naddress this, the dynamic mechanical thermal analysis (DMTA) was conducted on\nMatrimid-Cyclen samples to assess the mechanical properties across a temperature\nrange of 50 to 350 °C. As shown in Figure 3 (g-i), the storage modulus (E′) of the\nMatrimid-Cyclen membranes (1800 MPa) was almost completely maintained before\nreaching the corresponding T . This indicated that the membrane retains its mechanical\ng\nintegrity even with higher Cyclen content. Furthermore, we observed that higher\nCyclen loadings did increase in the brittleness of the membranes, as seen in the\ndecreasing peak intensity of damping (Tan δ) with increasing Cyclen content (Figure\n3i). These data demonstrated that this kind of gas separation membrane was endowed\nwith good mechanical property and thermal stability, even with the higher Cyclen\nloading.\nFigure 3. Characterization of Matrimid-Cyclen membranes. (g) The dynamic\nmechanical thermal analysis (DMTA) spectra of Matrimid-Cyclen membranes at\ntemperature range from 50 to 350 ℃. (h) The storage modulus (E′) and (i) Tan δ of\nMatrimid-Cyclen membranes.\nThe operating temperature, feed pressure and aging behaviors follows the expected\ntrend of mixed matrix membranes, with the presented discussion valid. The application\nfor helium extraction is important, and it is good that the authors considered both\nseparation from methane and nitrogen. I recommend that the manuscript be rejected,\nand the authors submit to a more appropriate journal.\nReply: Thank you very much for your professional comments and suggestions. We\nsincerely apologize for some inadequate in data analysis and descriptions in the original\nmanuscript. As you pointed out, several issues required further attention, and we have\nmade substantial revisions to address the concerns. Below is a detailed list of our\nresponses to your comments. In our work, we designed the mixed matrix membranes\nbased on several key considerations derived from numerous studies.\nFirstly, Cyclen was selected as filler due to its size-sieving effect, which is beneficial\nfor He separation. Based on the molecular dynamic diameters of He (2.60 Å), H (2.89\n2\nÅ), CO (3.30 Å), N (3.64 Å) and CH (3.80 Å), the Cyclen with cavity size (3.2-3.4 Å)\n2 2 4\nbetween the molecular dynamic diameters of He and N was selected, which makes it\n2\nan ideal candidate for He separation, such as He/N and He/CH . Furthermore, the\n2 4\nCyclen with a certain cavity size creates a special transport channel for small gases, like\nHe, which increases their transport paths in the membrane, counteracting the negative\neffect of the increased chain stacking density and enhancing the permeability of small\ngas molecules.\nSecondly, the chosen filler must have good solubility and dispersion in solvents. To\nensure uniform dispersion of the filler in the membrane and to minimize the formation\nof non-selective voids caused by filler agglomeration, we selected Cyclen for its high\nsolubility and good dispersion in organic solvents. Cyclen was fully dissolved in DMF,\nand then the clean Matrimid dope solution was added to the Cyclen/DMF solution. The\nresulting homogeneous Matrimid/Cyclen solution, which was stirred for 24 hours and\nsonicated for 1 hour, remained clear and transparent, even with up to 15% Cyclen\ncontent, as shown in Figure 1.\nFigure 1. (a) Cyclen powder (1.0, 3.0, 5.0, 10.0 or 15.0 mg) were added to 6.0 g DMF\nsolvent to obtain clear and transparent solutions. (b) Matrimid dope solution. (c) The\nclean Matrimid dope solution was added to the Cyclen/DMF solution to obtain clear\nand transparent mixed Matrimid/Cyclen solutions.\nThirdly, compatibility between filler and polymer matrix. It is crucial to create mixed\nmatrix membranes with fair matching between filler and polymer matrix to gain\nattractive performance. This is why we chose Cyclen as a filler. Hydrogen bonding\nbetween Cyclen and the Matrimid polymer chains immobilizes Cyclen molecules in the\ninterchain gaps of the Matrimid membrane, resulting in homogeneously mixed matrix\nmembranes. The SEM image (Figure 2) shows the surface and interface of the\nmembrane, confirming the good interfacial compatibility between Cyclen and\nMatrimid.\nFigure 2. SEM images of the cross section of the Matrimid-Cyclen-n% membranes up\nto ×50,000 magnification (n% is the Cyclen mass loading and was varied from 1 to 15\nwt%, n = 0, 1, 3, 5, 10, 15).\nFourthly, Helium (He) purification requirements. Helium is an extremely scarce,\nnon-renewable gas with a strong safety profile that is an essential resource for the\ndevelopment of many high-tech industries, such as the aerospace industry, electronics,\nmedical equipment, and scientific research. On Earth, He occurs in very low\nconcentrations, mainly in the atmosphere and in natural gas. Helium is commonly\nextracted industrially from high-pressure natural gas. However, the initial\nconcentration of He in most natural gas reservoirs is still as low as 0.3%, and to extract\nhelium from natural gas with ultra-low He content, the gas separation membranes must\npossess He/CH selectivity greater than 1000. Highly permeable polymers rarely\n4\nprovide sufficient selectivity for high-purity He extraction, and Matrimid was chosen as\nthe polymer matrix due to its dense stacking of polymer chains. XRD characterization\nrevealed that the strong hydrogen bonding between Cyclen and Matrimid modulated\nthe chain stacking of the mixed matrix membranes, which reduce the pore size, narrow\nthe pore size distribution and enhances the selectivity of the membrane by modulating\nthe chain spacing.\nWe have verified the success of the hybrid matrix membrane design with ultra-high\nhelium selectivity to achieve excellent separation performance."}, {"round": 3, "reviewer_comments": {"reviewer_4": "The authors have made considerable efforts to address the reviewer’ comments, and most of the concerns have been satisfactorily resolved. I am particularly pleased to see that the authors prepared thin-film composite membranes with varied thickness and systematically measured gas permeance and selectivity. The results are reasonable. These additional experiments significantly strengthen the manuscript and greatly improve the overall quality of the work.\nThe new results confirm that my previous concerns were valid and, importantly, they have been clearly and convincingly explained in the revised manuscript. Overall, the revisions have substantially enhanced the rigor, clarity, and impact of the study. The manuscript is now much improved and suitable for publication.\nMinor comment: The title of the manuscript could be further improved for clarity and specificity. For example, “Mixed-matrix membranes with molecular recognition windows for selective helium extraction from natural gas” would more accurately reflect the content and focus of the work."}, "author_response": "Reply: Thank the reviewer for the highly positive remarks and support of publication of this work. The title of the manuscript has been changed to “Mixed-matrix membranes with molecular recognition windows for selective helium extraction from natural gas”."}]

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