Catalytic hybrid solvent regeneration in membrane vacuum processes for direct air capture

Domain: chemistry
Published: 2026-02-14
Editorial outcome: minor_revision

Abstract
Direct Air Capture is a promising climate mitigation technology, but its deployment is limited by high energy demand. This study improves the energy efficiency and sustainability of liquid-based Direct Air Capture by integrating catalytic solvent regeneration and hybrid solvents with a low-temperature membrane vacuum regeneration process. Iron-sulfated zirconia catalysts supported on alumina and silica are synthesized and evaluated, with silica exhibiting superior catalytic performance. An optimal iron-sulfated zirconia to silica ratio of 1:1 reduces relative heat duty by up to 59.7% in the membrane vacuum system. To assess the impact of hybrid solvents on energy consumption, a solvent composed of 3 molar potassium taurinate and 1 molar potassium sarcosinate achieves a 69.1% improvement in CO2 desorption compared to potassium taurinate alone. Finally, combining the optimized hybrid solvent, catalyst, and membrane system at 90°C reduces thermal energy consumption by 66.8% relative to potassium glycinate, achieving an energy requirement of 2.6 GJ/tCO2.

Introduction
Direct air capture (DAC) has emerged as a key technology in addressing climate change, offering the capability to remove CO2 directly from the atmosphere and contributing to achieving net-zero emissions targets1. Despite its potential, DAC is an energy-intensive process, with significant technical and economic challenges2. Solvent-based technology, a viable and mature approach for CO2 capture (including in DAC), uses liquid as a sorbent to capture CO2. However, it requires substantial thermal energy and a high operating temperature (typically 120–140°C) to regenerate the solvent by breaking the bonds of CO2-bound species3,4.

Given these challenges, various strategies have been explored to reduce energy consumption, including phase-change solvents5, crystallization-based regeneration6, hybrid solvents7, and catalytic solvent regeneration8. In the context of DAC, improving energy efficiency is even more critical due to the ultra-low CO2 concentration (~0.04%) and partial pressure in ambient air, which results in significantly lower equilibrium loadings compared to flue gas systems9. Therefore, selecting an appropriate solvent and combining multiple energy-efficient regeneration techniques would be a promising approach to reduce energy input while enhancing the durability, reliability, and sustainability of liquid-based DAC systems, ultimately paving the way for the large-scale deployment.

Hybrid solvents (mixtures of two or more CO2-capturing absorbents) have emerged as a viable strategy for carbon capture, particularly in reducing the energy required for solvent regeneration10. By mixing various solvents with distinct properties, the blends can outperform single-solvent systems, enhancing CO2 absorption efficiency while lowering regeneration energy demands11. While extensive research has been conducted on amine-based solvent blends for point-source CO2 capture, their application in DAC remains largely unexplored. This gap is especially notable for amino acid salt solutions, which are suitable for capturing CO2 from ambient air due to their unique properties of low volatility, oxidation resistance, and environmental compatibility12,13. In this regard, investigating the performance of suitable amino acid solvent blends could offer a practical and energy-efficient pathway for advancing solvent-based DAC.

In addition to hybrid solvents, catalytic solvent regeneration has emerged as another promising approach to reducing the energy penalty in liquid-based CO2 capture technologies14. By incorporating solid acid catalysts during low-temperature regeneration (typically 80–100°C), CO2 desorption can be significantly enhanced, lowering the thermal energy required for solvent regeneration15. A wide range of solid acid catalysts, including zeolites16, metal oxides17, mesoporous silica18, and carbon-based materials19, have been introduced and fabricated for this purpose, each offering distinct physicochemical properties and have been tested with different solvent classes, including conventional amines, solvent blends, and amino acid-based solutions20 with few under DAC conditions21. Among these catalysts, sulfated zirconia (SZ) has received considerable attention due to its strong acidity, environmentally benign nature, ease of synthesis, and remarkable enhancement when supported on a mesoporous structure (e.g., SiO2, Al2O3, MCM-41), benefiting from a high density of Lewis and Brønsted acid sites, as well as a large accessible area of the super acid sites22,23. The catalytic activity of SZ can be further enhanced by metal doping, which introduces basic sites and strengthens both Brønsted and Lewis acid sites14. Among various metals, Fe-modified catalysts have demonstrated considerable catalytic effectiveness, attributed to iron’s ability to enhance both acidic and basic sites24,25. Therefore, the fabrication of Fe-modified sulfated zirconia (Fe-SZ) on mesoporous SiO2 or Al2O3 supports is considered in this work to evaluate its performance in a membrane vacuum regeneration (MVR) system.

Hence, assessing catalytic enhancement with hybrid solvents within an MVR unit, which is a practical, low-temperature, and sustainable process, can provide highly impactful and valuable insights into the energy efficiency of scalable DAC systems, which have not been investigated yet. In this study, a series of Fe-promoted SZ (Fe-SZ) catalysts supported by mesoporous silica and alumina (Fe-SZ/SiO2 and Fe-SZ/Al2O3) were synthesized and integrated into a fixed-bed system coupled with a low-temperature MVR DAC unit. The impact of the fabricated catalysts on CO2 desorption performance and energy reduction was evaluated using single amino acid solvents (potassium glycinate (GlyK), potassium sarcosinate (SarK), potassium taurinate (TauK)), as well as hybrid blends (TauK+SarK) at 90°C.

To start experiments, the CO2 and water separation performance of three hollow fiber membrane contactors (HFMCs) was assessed. Catalytic MVR trials were conducted using Fe-SZ/Al2O3 and Fe-SZ/SiO2 with varying SZ/SiO2 mass ratios (1/1, 1/2, and 2/1), followed by an evaluation of the impact of varying catalyst concentrations. Next, the solvent regeneration performance of various single and hybrid amino acid solvents was investigated, leading to the selection of the most effective blend, giving the highest CO2 flux and lowest energy consumption. In the final stage, to further enhance the energy efficiency of the MVR unit, catalytic trials were conducted using both single solvents and the selected optimal blend. The findings of this study provide valuable insights into energy efficiency, feasibility, and benefits of integrating multiple energy-efficient technologies, including low-temperature MVR, hybrid solvents, and catalytic solvent regeneration, into DAC processes.

Results
Membrane module performance
To evaluate the CO2 stripping and water flux performance of various HFMCs, 3M GlyK was used as the benchmark solvent26–28 in the MVR experimental setup shown in Fig. 1a. The CO2 desorption flux profiles for each HFMC are shown in Fig. S1. Figure 1b–d summarizes the maximum CO2 flux achieved during the operation, along with the corresponding water flux and the water to CO2 flux ratio. As shown, HFMC1, with reported specification in Table S1, exhibited the highest CO2 flux (19.7×10−5 mol/m2s) as well as the highest water flux (925×10−5 mol/m2s). In contrast, HFMC2, which had a similar pore size to HFMC1 but included an additional thin dense coating (3–5 µm), demonstrated a comparable CO2 flux while significantly reduced water flux (from 925×10−5 to 738×10−5 mol/m2s). This behavior is attributed to the pervaporation mechanism in HFMC2, facilitated by a thin non-porous PDMS layer. This coating acts as a barrier against pore wetting and convective water transport, while still permitting selective CO2 diffusion via the solution-diffusion mechanism29.

Alternatively, in membranes without a coating layer, pore wetting can be mitigated by narrowing the pore size distribution. This increases the breakthrough pressure, reducing the possibility of liquid breakthrough into the gas-filled pores30. In this regard, the performance of HFMC3, which featured a narrower pore size distribution, was examined. As shown in Fig. 1b, c, this led to a considerable reduction in water flux, achieving equal performance to HFMC2. However, the smaller pore size also introduced additional mass transfer resistance, resulting in a lower CO2 flux compared to both HFMC1 and HFMC2 (~16% reduction). This highlights the crucial importance of optimizing membrane pore structure in membrane distillation systems to achieve an effective balance between CO2 and water transport31,32. To compare overall performance, the water to CO2 flux ratio was reported for all modules (Fig. 1d). HFMC2 demonstrated the most favorable ratio of 38.2, confirming the effective role of the thin PDMS layer and pervaporation mechanism in enhancing CO2 selectivity in the MVR system in liquid-based DAC applications. Therefore, HFMC2 was selected as a suitable module for conducting the remainder of the experiments. To assess the repeatability and accuracy of the experimental setup, desorption tests with 3M GlyK and HFMC2 were repeated three times. Fig. S2 presents the results, showing minimal variation and confirming the high consistency of the data.

Composite solid acid catalyst
Catalytic support (SiO2 vs. Al2O3). Catalysts were synthesized by impregnating two distinct catalytic supports, SiO2 and Al2O3, following the procedure described and schematically illustrated in Fig. 2a. The resulting catalysts, Fe(10%)-SZ/SiO2 (1/1) and Fe(10%)-SZ/Al2O3 (1/1), were placed in a fixed-bed chamber and loaded to 6 wt% concentration to evaluate their effectiveness. MVR trials were conducted using 3M GlyK at 90°C.

Under the mentioned conditions, the two catalysts exhibited notably different performance. The Al2O3-supported catalyst showed only a minimal improvement in CO2 desorption flux, with an enhancement below 2%. In contrast, the SiO2-supported catalyst demonstrated an enhancement of 25.6%. Such superior performance of SiO2 over Al2O3 in catalytic regeneration has been observed in previous studies, both as a standalone catalyst and as a catalytic support22,23,33. Although the performance difference in supported catalyst configurations was relatively moderate, SiO2-supported catalysts generally outperform their Al2O3-based counterparts due to enhancements in Brønsted acidity and textural properties14,22. Furthermore, in this study, based on the BET analysis results (Table S2), the SiO2 support exhibited a considerably larger average pore size (~40 nm) compared to Al2O3 (~7 nm), which can facilitate a more stable integration of Fe-SZ into its mesoporous structure and improve accessibility of active sites during regeneration.

Figure 2b shows the morphological and compositional characterizations of the impregnated SiO2 support using scanning electron microscopy (SEM) and TEM-EDX analyses. SEM images display that the catalyst support exhibited a relatively smooth surface morphology, suggesting uniform surface coverage and interaction between the support and loaded Fe-SZ species. At higher magnifications, the SEM images also highlighted the preserved porous network of the support, which is essential for maintaining accessibility to active sites. Furthermore, TEM-EDX confirmed the uniform dispersion of zirconium, sulfur, and iron across the support matrix, indicating successful anchoring of the Fe-SZ component into the porous framework. High-resolution TEM images of the Fe-SZ/SiO2 (1/1) sample provide close-up views at different magnification revealing the nanostructured morphology of the synthesized catalyst along with the amorphous structure of SiO2 (Fig. S3).

Following the morphological and elemental analysis of Fe-SZ/SiO2 (1/1), X-ray photoelectron spectroscopy (XPS) was used to examine the surface chemical states and bonding environment of the synthesized catalyst (Fig. 2c–e). According to Fig. 2c, the S2p spectrum exhibited a characteristic doublet at approximately 169–170.5 eV, corresponding to the S2p3/2 and S2p1/2 binding energies of sulfate species (SO42−)34, which play an important role in catalytic activity through their high electronegativity and generation of Brønsted acid sites35,36. The Zr 3d spectrum showed two main peaks at around 183.5 eV (Zr 3d5/2) and 185.9 eV (Zr 3d3/2), which are attributed to Zr(IV), likely ones coordinated within the sulfated framework due to higher bonding energies compared to ZrO236,37. The Fe 2p spectrum displayed a prominent peak near 711 eV (Fe 2p3/2), and 724 (Fe 2p1/2), accompanied by a distinct satellite feature, which is characteristic of Fe3+ species38. This confirms that iron is predominantly present in a trivalent state, mainly as Fe2O3-like species25,39. Additionally, the O 1s spectrum (Fig. S4) was deconvoluted into two components: a peak near 529.8 eV, attributed to metal-oxygen bonds (Zr–O, Fe–O), and a main one near 532.4 eV, related to the surface sulfate groups and hydroxyl groups (-OH) and Si–O–Si37,40. These results collectively indicate the successful loading of SO42-, Zr4+, and Fe3+ species into the porous SiO2 matrix, supporting the formation of a well-dispersed, catalytically active Fe-SZ structure.

The CO2 desorption profiles of 3M GlyK regenerated at 90°C, with and without the Fe-SZ/SiO2 (1/1) catalyst, are shown in Fig. 2f. As illustrated, upon heating the solvent, the CO2 desorption flux initially increased due to the high CO2 loading and the accelerated breakdown of less stable CO2-containing species in GlyK41. As the solvent temperature approached the set point and stabilized, the CO2 flux reached a peak followed by a decline. This behavior, observed in previous studies, is attributed to the regeneration of more stable CO2 species and the progressive depletion of CO2 from the solvent41. The latter leads to a reduction in CO2 partial pressure on the solvent side of the membrane, thereby decreasing the driving force for mass transfer and resulting in a drop in CO2 flux. Compared to the blank trial, the presence of the SiO2 promoted with Fe-SZ catalyst led to an earlier onset of CO2 desorption and a higher peak flux (19.3×10−5 to 21.0×10−5 mol/m2s). This enhancement is indicative of accelerated CO2 desorption kinetics, enabled by the acidic and basic active sites of the catalyst that facilitate the breakdown of carbamate and bicarbonate species through proton transfer mechanisms14,15. This can lead to a higher partial pressure of CO2 on the solvent side and increase flux. Moreover, the gradual CO2 flux reduction once the temperature stabilized implies continued catalytic activity on the breakdown CO2-containing species under reduced solvent CO2 loading conditions, which contributes to overall higher CO2 moles desorption and efficient regeneration.

The overall catalytic effect was quantified using Eqs. 2 and 4, as shown in Fig. 2g. The SiO2-supported catalyst enabled a 25.6% increase in CO2 desorption over 30 min of operation, raising the total amount of desorbed CO2 from 244 to 307 mmol for 3M GlyK. These results highlight the potential of Fe-SZ/SiO2 as an effective and suitable catalytic material for energy-efficient solvent regeneration in MVR processes.

SZ to SiO2 ratio. To evaluate the effect of different mass ratios of SZ on the SiO2 support, catalysts were synthesized at 1/2 and 2/1 ratios, and their catalytic effectiveness was measured and compared with the baseline 1/1. Figures 3a and S5a present the CO2 desorption profiles using 6 wt% concentration of 1/2 and 2/1 ratios, respectively, in 3M GlyK. As shown in Fig. 3a, the Fe-SZ/SiO2 (1/2) exhibited a noticeable increase in CO2 desorption during the initial heating stage. The earlier onset of desorption compared to the blank solvent suggests that the catalyst promotes CO2 desorption during the heating ramp. Furthermore, the CO2 flux remained consistently higher throughout the stabilized temperature zone, indicating the catalyst activity in that region. According to Fig. 3a, the Fe-SZ/SiO2 (1/2) increased the total CO2 desorption amount from 244 to 278 mmol, corresponding to a 13.7% improvement. Notably, this value is approximately 46% lower than the one achieved with the 1/1 ratio of SZ to SiO2. Increasing the Zr and S loading on SiO2, Fe-SZ/SiO2 (2/1), not only failed to further improve performance but also resulted in a decline relative to both the 1/1 and 1/2 ratios, achieving only a ~4.0% enhancement (Fig. S5a). This decline is attributed to oversulfation of the catalytic support, resulting from the doubled sulfur content in Fe-SZ/SiO2 (2/1). Such excessive SZ loading on the support can lead to non-uniform dispersion on the surface of the support and block mesopores42,43, as clearly observed on the catalyst surface through SEM and SEM/EDX analyses (Fig. S5b). This condition can hinder the accessibility to acidic active sites of ZrO2 and Fe2O3 which are major contributors for creating Brønsted and Lewis acid sites for catalytic performance36,44.

To further investigate the acid site distribution within the SiO2-supported catalysts, NH3-TPD analysis was conducted on all synthesized samples. The desorption profiles are presented in Fig. 3b, and the corresponding quantified acidity values are listed in Table S3. In this technique, low-temperature desorption peaks (~100–200°C) are typically attributed to weak acid sites, whilst peaks in the 200–400°C range correspond to moderate acid sites, and high-temperature peaks above 400°C indicate strong acid sites8. According to the results, all Fe-SZ/SiO2 catalysts demonstrated a pronounced total acidity, predominantly in the strong acid region (>400°C), with negligible desorption in the weak to moderate regions. Additionally, the featureless profile of pristine SiO2 confirms its lack of acidic functionality, emphasizing the critical role of Fe-SZ in introducing and enhancing catalyst acidity. This observed increase in acidity can be further quantified by examining the total acid sites (TAS). The trend in TAS across the catalysts followed the order of 2/1 > 1/1 > 1/2 > SiO2 (Table S3). The high TAS, combined with the mesoporous structure of the support, which improves accessibility to carbamate species, is a key factor contributing to the enhanced catalytic performance45. As previously discussed, despite the 2/1 sample having the highest acidity and acid strength, its relatively poor performance is attributed to excessive sulfate deposition and pore blockage. Comparing the 1/1 and 1/2 ratios, the 1/1 catalyst exhibited higher TAS than the 1/2 sample, along with a distinct high-temperature desorption peak at 678°C, indicating the presence of stronger acid sites. This may reflect a more favorable ratio of Zr sites coordinated to sulfate groups and Zr–OH groups interacting with SO42−, both of which are known to contribute to strong acidity in SZ catalysts43,46. Therefore, the superior catalytic performance of the 1/1 sample can be attributed to its combination of stronger acidity and greater TAS density compared to 1/2, as well as more accessible and uniformly dispersed acid sites compared to the oversulfated 2/1 sample.

To further evaluate the role of basic sites in catalyst effectiveness, CO2-TPD analysis was conducted on all synthesized samples. The corresponding data are presented in Fig. 3d and Table S3. Results confirmed that the 1/1 sample possessed a higher basic sites density compared to 1/2 and the strongest basic sites (desorption peak at 681°C) among all, which play a critical role in the deprotonation of protonated amino acid species24,25.

Catalyst concentration and stability. To investigate the effect of catalyst loading on system performance, energy reduction, and to determine its optimal value, the catalyst concentration was increased from 6 to 9 wt% and 12 wt%. Figure 3c displays the CO2 desorption profiles for 3M GlyK with 9 wt% catalyst in the MVR unit. As shown, the presence of an increasing amount of catalyst had a remarkable impact on CO2 desorption rates, evident in the regions where the solvent temperature rises, at the peak flux, and in the stabilized temperature zone; Fig. 3e, f highlight this impact. Upon increasing the catalyst concentration to 9 wt%, the maximum CO2 flux improved from 19.3×10−5 to 28.2×10−5 mol/m2s, and the total desorbed CO2 increased from 244 to 377 mmol, corresponding to a 54.3% catalytic enhancement. This improvement is more than two-fold compared to that observed with 6 wt% catalyst. This enhancement results mainly from the greater number of accessible acidic and basic sites that facilitate the deprotonation of H+GlyK and the breakdown of carbamate and bicarbonate species. These effects promote greater CO2 release in the solvent, raise the partial pressure on the solvent side of the membrane, and ultimately contribute to the observed increase in flux28,47. To determine if this positive trend would continue with the same intensity, a 12 wt% fixed-bed catalyst trial was conducted (Fig. S6a). The outcome showed a clear improvement, with the peak CO2 flux reaching 30.2×10−5 mol/m2s, and the enhancement rising to 67.5%, as shown in Fig. 3e, f. Notably, the magnitude of improvement from 6 to 9 wt% (112%) was greater than that from 9 to 12 wt% (24.3%), suggesting a reduced catalytic effect beyond a certain concentration, indicating the presence of an optimum. This can be primarily due to the solvent approaching its CO2 equilibrium as the catalyst amount increases in the system, considering the presence of the catalyst does not alter the thermodynamic equilibrium22,48. Under these conditions, the system shifts from kinetically limited to one constrained by thermodynamics49. Hence, to identify the optimum catalyst concentration, the enhancement factor was calculated using Eq. 5, and the corresponding values are reported in Fig. 3f. As shown, the use of 9 wt% catalyst resulted in the highest enhancement factor of 6.0 and was therefore selected as the optimal concentration for further evaluations.

For relative heat duty and thermal energy consumption, both parameters were calculated using Eqs. 6 and 7 and are reported in Fig. 3g. Accordingly, at 9 wt% and 12 wt%, the heat duty decreased by 35.2 and 40.3% as compared to the blank, respectively, reducing the thermal energy requirement from 7.8 to 5.1 and 4.7 GJ/tCO2. These reductions clearly demonstrate the importance of catalyst presence in the MVR system and its significant role in reducing energy demand under DAC conditions.

Since the 9 wt% Fe-SZ/SiO2 (1/1) was identified as the optimal catalyst concentration and support ratio (SZ/SiO2), its stability was evaluated over six extended regeneration cycles using 3M GlyK at 90°C (Fig. S6b). As shown, the RH% increased from 64.8% in cycle 1 to 68.8% in cycle 2, representing an increase of approximately 6.2%. From cycle 2 to cycle 6, RH% rose more gradually from 68.8 to 71.8%, a change of about 4.3% over the next four cycles. This trend suggests that the catalyst experiences a decline in effectiveness after the first cycle, likely due to surface chemical changes, but begins to stabilize in subsequent cycles. The results highlight the catalyst recyclability and its ability to retain performance across repeated regeneration cycles15.

Furthermore, the selected catalyst’s chemical stability was evaluated by ICP-OES analysis to measure the amount of metals leached into the solvent after MVR trials (Table S4). The results showed negligible metal leaching, demonstrating the high chemical stability of the catalyst and its minimal impact on solvent durability.

Hybrid amino acid solvents
By evaluating the performance of 3M GlyK in previous sections, the regeneration performance of two additional single amino acid solvents, 3M SarK and 3M TauK, was also examined. To analyze and optimize any possible improvements using hybrid solvents in MVR, a series of mixtures were prepared using 3M TauK as the main solvent, combined with varying concentrations of SarK (0.5 to 1.5 M). The selection of TauK as the main component was primarily based on its superior desorption performance as well as its high thermal stability, as measured among the tested solvents (Fig. S7). Figure 4a presents the CO2 DAC absorption equilibrium points for both the single solvents, previously reported in our study21,28, and the hybrid solvents, measured at 25°C. According to Fig. 4a, increasing the SarK concentration in the 3M TauK solution up to 1.5 M resulted in an increase in DAC equilibrium CO2 uptake from 0.29 to 0.37 mol/mol. This improvement is primarily attributed to the higher CO2 uptake capacity of SarK compared to TauK, which shifts the equilibrium upward upon its addition.

Figure 4b shows the CO2 desorption profiles of the prepared solvents over a 30-min run, while Fig. 4c, d displays the maximum achieved CO2 flux and total desorbed CO2, respectively. As shown, SarK exhibited the poorest regeneration performance among the tested solvents, consistent with previous observations28,50, achieving a maximum CO2 flux of 19.2×10−5 mol/m2s and a total desorbed CO2 amount of 224 mmol. This can be attributed to the stronger basic nature of SarK (higher pKa value) compared to the other evaluated solvents, despite its superior CO2 absorption capacity50,51. In contrast, 3M TauK demonstrated considerably better desorption performance than both GlyK and SarK. It achieved a maximum CO2 flux of 32.5×10−5 mol/m2s and a total desorbed amount of 348 mmol of CO2, 55.3% higher than SarK. This superior performance is linked to the unique chemical structure of taurine, specifically the presence of the sulfonic acid group with high electronegativity52. This group induces a strong inductive effect that lowers the pKa53 compared to GlyK and SarK, facilitating the deprotonation of protonated taurine (H+TauK) and accelerating CO2 desorption rates.

For the hybrid solvents, blending 3M TauK with 0.5 to 1.5 M SarK significantly enhanced CO2 desorption performance in all cases (Fig. 4b–d). With the addition of only 0.5 M SarK, the maximum CO2 flux reached 44.3×10−5 mol/m2s with 526 mmol total desorbed CO2, whilst the addition of 1 M SarK further improved this to 53.9×10−5 mol/m2s and 589 mmol, corresponding to 51.1 and 69.2% improvements over 3M TauK alone, respectively. One of the main reasons for this observed behavior is the higher total molarity of the hybrid solvent, which can lead to increased CO2 absorption capacity (Fig. 4a), resulting in the formation of a greater total number of CO2-containing species. Breaking down a larger quantity of CO2-containing compounds during regeneration allows for enhanced CO2 liberation and improved performance26,54. This was confirmed by 13C-NMR analysis of CO2-loaded 3M TauK, and hybrid solvents with varying SarK concentrations (Fig. S8), used to evaluate the quantity of CO2-containing species present. Based on the spectral results and the intensity of the peak and area around 164.2 ppm, the quantity of carbamate species increased steadily (up to 30%) with the addition of 1.5 M SarK. Considering the 3M TauK as major content in the hybrid solution, it seems that the coupling of H+TauK deprotonation (a better proton provider than H+SarK) with the increased carbamate species is the reason for the considerable CO2 desorption enhancement observed. However, with continued MVR trials, an interesting trend emerged when the SarK concentration increased beyond 1 M. The CO2 desorption performance declined, having a lower total desorbed CO2 amount compared to the 3M TauK+0.5 M SarK (506 versus 526 mmol). This can be attributed to both the physical and chemical properties of the solvent. Increasing the SarK content raises the overall molarity, which in turn increases the viscosity of the solution and can reduce regeneration efficiency55. On the chemical side, the strongly basic nature of SarK (due to its high pKa) can also begin to negatively affect regeneration beyond a certain concentration. In other words, the amount of SarK added to TauK introduces a trade-off between beneficial and adverse effects. Based on this balance, 3M TauK+1 M SarK was identified as the optimum hybrid solvent formulation.

The water transfer rate in the membrane module for all prepared solutions was also evaluated. As shown in Fig. 4c, among the single solvents, SarK exhibited the lowest water flux, followed by TauK and GlyK. This trend was consistent with previous findings and can be attributed to differences in molecular weight, viscosity, and degree of hydrophilicity properties of the amino acids28. For the hybrid solvents, increasing the total molarity further reduces water transfer, due to the higher molecular weight, viscosity, and greater amino acid concentration in the solution, highlighting their suitability for use in MVR systems.

Finally, Fig. 4d presents the thermal energy consumption required for the regeneration of all single and hybrid solvents. As shown, adding 1 M SarK to 3M TauK reduced the regeneration energy from 5.5 GJ/tCO2 to a minimum energy value of 3.2 GJ/tCO2, demonstrating the effectiveness of hybrid solvent application on improving energy efficiency in DAC processes.

Catalytic hybrid amino acid solvents in MVR
To further evaluate the performance of a system combining hybrid solvents with catalytic solvent regeneration, a series of catalytic trials were conducted on SarK, TauK, and the selected hybrid solvent, 3M TauK+1 M SarK. Figure 5a–c presents the catalytic CO2 desorption profiles of the mentioned solvents compared to their blank performance. As shown in the profiles, all solvents exhibited an improvement in cumulative CO2 desorption by using 9 wt% Fe-SZ/SiO2 (1/1) catalyst. The quantified values for these performances are presented in Fig. 5d, e, highlighting the catalytic enhancements achieved up to 54.3%. As discussed earlier in this work and observed in previous studies21,28, among the evaluated short-chain single amino acids, 3M GlyK exhibited the highest catalytic effectiveness. For 3M SarK, due to its much higher pKa relative to GlyK and TauK, the addition of 9 wt% catalyst resulted in a relatively minor improvement, increasing the total desorbed CO2 from 224 to 271 mmol, corresponding to a 21.0% enhancement. This limited effect is consistent with the earlier discussed negative impact of SarK’s strong basic nature on efficient regeneration when used at high concentrations; Similar catalytic behavior for SarK has been observed under DAC conditions with various catalysts21,28. For TauK, a 25.9% improvement was recorded, which is lower than that observed with GlyK. This behavior is mainly due to the sulfonic acid groups in TauK, which provide readily available protons by H+TauK, thereby reducing its catalytic effect28.

For a mixture of TauK and SarK solution, catalytic regeneration resulted in a 24.8% improvement, promoting cumulative CO2 desorption from 589 to 735 mmol. The latter is the highest recorded cumulative CO2 desorption value in this study. It is noteworthy that, although the relative catalytic enhancements for TauK and TauK+SarK are quite similar, the absolute enhancement (the difference in total desorbed CO2 between the catalytic trial and the blank trial) achieved by the hybrid solution was the highest among all the solvents. As shown in Fig. 5e, catalytic regeneration of the TauK+SarK mixture increased the total desorbed CO2 by 146 mmol compared to the other solvents, demonstrating a greater net improvement in regeneration performance. The main factor contributing to this improvement is the higher overall molarity and the greater availability of the carbamate species in the solution. However, the higher proportion of TauK in the hybrid solvent, which provides available protons, along with the presence of more stable carbamate species formed from strongly basic SarK, may partially limit further catalytic enhancement.

To better illustrate the impact of hybrid solvent integration with catalytic solvent regeneration on system energy efficiency, the relative heat duty using different solutions with and without 9 wt% Fe-SZ/SiO2 (1/1) is shown in Fig. 5f. Considering GlyK as a benchmark, the catalyst reduced the RH of GlyK, SarK, and TauK to 64.8%, 90.1%, and 55.8%, respectively. Notably, the combination of a hybrid solvent with the catalyst further reduced the RH to 33.2%, corresponding to a 66.8% reduction in energy consumption. According to Fig. 5g, which presents catalytic energy consumption for different solutions, the minimum recorded energy consumption belongs to the catalyst-hybrid solvent combination, reaching 2.6 GJ/tCO2. This result, along with the observed improved performance, highlight the importance of integrating energy-efficient technologies and demonstrates the significant potential of this approach for developing a sustainable and energy-efficient DAC process.

It is noted that the reported energy values here represent only the sensible heat component, calculated using Eq. 6. To estimate the total thermal energy consumption, additional factors such as the heat of reaction and heat loss associated with water transfer through fibers should also be considered. Due to the lack of available data for the used hybrid solvent, the heat of reaction was assumed to be equivalent to that of 3M TauK (2.0 GJ/tCO2)56. The heat loss through water vapor transfer for 3M TauK+1 M SarK was estimated to be 1.9 GJ/tCO2, assuming 90% heat recovery efficiency via a heat pump57–59. When combined with the calculated sensible heat of 2.6 GJ/tCO2, the total energy consumption is estimated to be 6.5 GJ/tCO2, which is comparable to that of well-established DAC technologies that are shown in Table S528. It is also noteworthy that the cost and energy of the DAC unit are expected to decrease for larger scale due to the economies of scale (for capital cost) and enhanced process optimization (for thermal energy efficiency)60,61.

Methods
Chemicals
Glycine (Gly) and sarcosine (Sar) (>98% purity) were purchased from Sigma-Aldrich. Taurine (Tau, 99.5% purity) was supplied by Consolidated Chemical Co., Australia. Potassium hydroxide (KOH, >85.0% purity) was purchased from Chem-Supply, Australia. Zirconium sulfate tetrahydrate (Zr(SO4)2·4H2O) was obtained from Shanghai Minstar Chemical Co., Ltd., China. Iron (III) nitrate nonahydrate (Fe(NO3)3·9H2O, >98%) was provided by Sigma-Aldrich. Silica (SiO2) and alumina (Al2O3) spherical particles were purchased from Fuji Silysia Chemical Ltd., Japan, and Hunan Joseph Technology Co., Ltd., China, respectively.

Solutions of GlyK, TauK, and SarK were freshly prepared by dissolving the corresponding amino acids in Milli-Q water containing an equimolar amount of KOH. Milli-Q water (Elix Millipore, resistivity >18.2 MΩ cm) was used in all experiments.

HFMCs
Polypropylene (PP)-based modules are well known for their low cost, robustness, and chemical resistance, and their stability has been repeatedly evaluated and demonstrated in the literature62,63. Moreover, previous studies have shown that adding a coating layer, such as PDMS, can significantly improve membrane performance and durability64,65. Hence, three commercial polypropylene (PP) HFMCs were utilized, and their CO2 stripping performance was evaluated in the MVR DAC unit. The modules differ in pore size distribution, structural configuration, and surface modifications (coated PDMS layer), all of which can influence mass transfer and wetting resistance. Detailed specifications are provided in Table S1.

Catalyst synthesis procedure
A wet impregnation method, adapted from a previously reported procedure25, with minor modifications, was used to synthesize the composite solid acid catalysts (Fe-SZ/SiO2 and Fe-SZ/Al2O3); a schematic synthesis procedure is shown in Fig. 2a. An aqueous solution of metal salts with a total concentration of 2.0 mol/L was first prepared by dissolving the desired amounts of Fe(NO3)3·9H2O and Zr(SO4)2·4H2O in deionized water. The solution was vigorously stirred and maintained at 60°C for 2 h. Afterward, the solution was cooled to room temperature (~25°C). Mesoporous supports (either SiO2 or Al2O3) were then added to Schott bottles containing the metal salt solution and mixed for 14 h. Subsequently, the mixture was treated with ultrasonication for 30 min. The solid phase was separated by filtration, dried at 120°C overnight, and then calcined in air at 500°C for 5 h. The final catalysts were labeled as Fe-SZ/SiO2 or Fe-SZ/Al2O3. Depending on the SZ/support weight ratio (1/2, 1/1, or 2/1), these ratios were used as postfixes in the labeling, resulting in labels such as Fe-SZ/SiO2 (1/2), Fe-SZ/SiO2 (1/1), and Fe-SZ/SiO2 (2/1).

Characterization methods
XPS analysis was performed using a Thermo Scientific K-Alpha instrument equipped with a monochromated Al Kα source (1486.7 eV). SEM was conducted using an FEI Nova NanoSEM 200, an ultra-high-resolution SEM equipped with a field emission gun. High-resolution transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) mapping were carried out on a JEOL JEM-F200 equipped with an Oxford EDX detector.

NH3-TPD and CO2-TPD analyses were conducted using a Micromeritics AutoChem II 2920 chemisorption analyzer equipped with a thermal conductivity detector to evaluate the basicity and acidity of the material, respectively. In each case, a specific amount of the sample was heated to 100°C for 1 h under a helium flow of 30 mL/min to remove surface impurities. After cooling to 50°C, the sample was exposed to either 10 vol% CO2/He or 5 vol% NH3/He (both at 30 mL/min) for 1 h to allow gas adsorption. Subsequently, pure helium was flowed for 60 min to remove physically adsorbed species. The desorption profiles were then recorded by heating the sample from 50 to 900°C at a rate of 10°C/min.

The chemical species formed in the liquid phase after CO2 absorption were identified using 13C-NMR spectroscopy. Spectra were collected on a Bruker Avance NEO 500 MHz NMR spectrometer with SampleJet. For sample preparation, D2O was mixed with the liquid samples, with each NMR tube containing 600 μL of sample and 100 μL of D2O. The 13C-NMR analysis was performed with a delay time of 20 s and 256 scans. Peak analysis was conducted using Bruker TopSpin software, and the concentration of chemical species was quantified by integrating the corresponding 13C-NMR peak areas.

The BET analyses were conducted using a Micromeritics 3Flex system equipped for nitrogen, argon, and krypton gas porosimetry at 77 K, enabling accurate determination of surface area (BET and Langmuir), pore volume, and pore size distribution using NLDFT and Rouquerol-corrected methods.

ICP-OES analysis was performed using a Varian 720-ES to measure metal leaching from the catalyst into the solvent. The instrument consists of an axial-view plasma torch, a solid-state charge-coupled device detector, a peristaltic pump, and a “SeaSpray” nebulizer coupled to a cyclonic spray chamber.

MVR apparatus
An upscaled version of a previously developed MVR DAC system was employed in this study26,28. Design improvements were made to make the system compatible with larger commercial membrane modules and to enhance overall system performance, particularly in terms of CO2 stripping, heating, water condensation, and hydraulics. A schematic of the system configurations is shown in Fig. 1a. The DAC CO2 saturated solvents21,28 were loaded into a 1 L stainless steel vessel and circulated through the system at 1.4 L/min using a chemically resistant magnetic drive pump (Micropump). The solvent was heated to a fixed regeneration temperature of 90°C using an upgraded external oil bath. To monitor the pressure on the solvent side, a pressure gauge was installed at the inlet of the membrane module. A thermometer was also placed at the same location to monitor the solvent temperature at the membrane inlet. On the permeate side, a vacuum pump applied a low pressure (~2 kPa(a)), and a gauge was installed to monitor vacuum pressure levels during operation. As the solvent heated and CO2 and water vapor started transferring to the permeate side, the pressure gradually increased, reaching ~20 kPa(a). Two acetone-ice baths were used to condense water vapor on the permeate side, enabling accurate measurement of the high-purity CO2 stream. A CO2 analyzer and a flow meter were installed at the outlet of the vacuum pump for detailed analysis. Notably, in all tests, the CO2 stream purity was >99%.

For catalytic trials, a fixed bed was placed upstream of the HFMC. After heating the solvent, it passed through the catalyst bed before entering the membrane module. All experimental runs were conducted for a duration of 30 min.

MVR performance analysis
To calculate the CO2 flux passing through the fibers in the membrane modules, Eq. 1 was used:
J CO2 = M° × xCO2 / A (1)
where J CO2 (mol/m2s) is the CO2 desorption flux, M° (mol/s) is the molar flow rate of the high-purity CO2 stream at the outlet of the vacuum pump, and x is the CO2 mole fraction. A (m2) is the membrane surface area.

The total amount of CO2 desorbed during operation, M total_CO2 (mol), was determined by integrating the flux profile over the duration of the process (Eq. 2), where t is the operation time.
M total_CO2 = ∫ J CO2 A dt (2)

To determine the water flux over the 30 min of operation, the collected water was accurately measured before being returned to the solvent container. Based on this measurement, Eq. 3 was used to calculate the water flux.
J H2O = N / AΔt (3)
where J H2O (mol/m2s) is the water desorption flux, N (mol) is the amount of water collected, A (m2) is the surface area of the membrane, and Δt (s) is the duration of the experiment.

To evaluate the enhancement of the synthesized catalysts and the enhancement factor, Eqs. 4 and 5 were employed, respectively.
CE = (M total_CO2_cat - M total_CO2) / M total_CO2 × 100 (4)
Enhancement Factor = CE / Wcat (5)
where CE (%) is the catalyst enhancement, M total_CO2_cat and M total_CO2 (mol) are the total CO2 desorbed in 30 min with and without the presence of synthesized catalysts, respectively. Wcat is the catalyst concentration (wt.%).

To estimate the thermal energy consumption (GJ/tCO2) of the system, Eq. 6 was used:
Q = mc p ΔT / M total_CO2 Mw (6)
Q (KJ/grCO2 or GJ/tCO2) is the thermal energy consumption, m (kg) represents the total mass of the solvent in the system, while cp (kJ/kg°C), and ΔT (°C) refer to specific heat capacity and the temperature change of the solvent, respectively. Mw (g/mol) is the molecular weight of CO2. It is also important to note that a thermal approach temperature of 20 K was considered due to the integration of energy in a rich/lean heat exchanger47,66.

The relative heat duty, RH (%), was calculated using Eq. 714:
RH = Q Cat / Q Blank × 100 (7)
Q Cat and Q Blank are thermal energy consumption (GJ/tCO2) with and without a catalyst, respectively.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The experimental work was well-designed to demonstrate the catalytic effect and the hybrid solvent's role in reducing regeneration energy. The reduction in desorption energy by approximately 67% is impressive and potentially addresses the challenge of high regeneration costs in solvent-based Direct Air Capture (DAC) operations.\nHowever, considering the high standards required by Nature Communications, this work must include a high-level technoeconomic analysis (TEA) based on the proposed process—membrane vacuum regeneration. While the observed reduction in desorption energy is noteworthy, it is crucial to evaluate the overall economic feasibility of the proposed system comparatively with other state-of-the-art DAC technologies.\nIn fact, many novel approaches have been explored for reducing regeneration energy, including the use of solid sorbents, moisture swing, electrodialysis membrane separation, direct steam regeneration, and direct conversion of carbon dioxide to value-added materials. However, these processes often incur other significant costs, such as higher CAPEX (Capital Expenditure), increased O&M (Operational and Maintenance) expenses, and material costs, making the overall capture cost prohibitively high.\nSimilarly, the proposed process is not a simple system; it requires heat, vacuum, novel catalysts in a packed bed system, PDMS-coated membrane systems, and a hybrid amino acid solvent. These added complexities bring associated operational, material, and maintenance costs (e.g., potential solvent and catalyst degradation) that must be addressed and quantified. Without this assessment, it is difficult to see the clear economic advantage of low desorption energy compared to other DAC systems, such as those developed by Carbon Engineering, Climeworks, Heirloom, and others.\nIn conclusion, while the reduction in desorption energy is significant from a scientific perspective, the manuscript will require a detailed and rigorous technoeconomic analysis to comprehensively evaluate the proposed process's feasibility and advantages relative to existing DAC systems. Providing this analysis will be critical to ensuring that the work meets the standards of novelty and practicality expected by Nature Communications.", "reviewer_2": "What are the noteworthy results?\nThis is an experimental study at a laboratory scale of CO2 vacuum desorption from some solvents using hollow fiber membrane devices. The authors claim a reduction of the energy demand in the CO2 desorption process of DAC, but it is not easy to identify the reference desorption.\nWill the work be of significance to the field and related fields?\nThe work supply some experimental results which low significance in DAC.\nHow does it compare to the established literature? If the work is not original, please provide relevant references.\nThe work agrees well with the established literature. Te novelty lies in the catalytic step.\nDoes the work support the conclusions and claims, or is additional evidence needed?\nTHis is a laboratory scale which demands a deeper comparison with other alternatives.\nAre there any flaws in the data analysis, interpretation and conclusions? Do these prohibit publication or require revision?\nNo\nIs the methodology sound? Does the work meet the expected standards in your field?\nYES\nIs there enough detail provided in the methods for the work to be reproduced?\nYES", "reviewer_3": "This manuscript presents a highly innovative integration of catalytic regeneration, hybrid solvents, and membrane vacuum regeneration (MVR) for DAC applications. The reported energy consumption 2.6 GJ/tCO2 demonstrates transformative potential for decarbonization. While the study exhibits exceptional novelty in solvent-catalyst-membrane synergy, critical aspects regarding industrial-scale viability and fundamental mechanistic depth require strengthening to solidify its impact.\n1. It would be valuable to supplement experimental evidence to clarify the physicochemical driving factors that underlie the superior performance of SiO2 over Al2O3. Additionally, quantifying the correlation between these factors and the enhanced CO2 desorption performance would further strengthen the analysis.\n2. For the optimized catalyst Fe-SZ/SiO2, it is recommended to conduct quantitative analysis of metal leaching risk under MVR conditions. Assessing the impact of leached ions (e.g., Fe3+, Zr4+) on solvent degradation would also help validate the robustness of MVR system.\n3. While the integrated system demonstrates impressive short-term performance in reducing regeneration energy. It is recommended that evaluating the physicochemical stability of key components (e.g., catalyst, membrane fouling/corrosion and solvent degradation) before and after extended MVR cycling would provide valuable insights into the system's operational durability.\n4. Based on the information in the manuscript and supporting information, the thermal energy consumption mentioned in the manuscript appears to be sensible heat. In lines 455-458, the manuscript states that the total energy consumption calculation (6.5 GJ/tCO2) includes assumed reaction heats and estimated water vaporization heat losses. It is recommended to clearly state the basis for these critical assumptions—are they cited, measured, or is there literature supporting the achievability of such efficiency at scale? And the thermal energy consumption calculated according to the formula in the supporting information is not GJ/t CO2, please further confirm whether there is a mistake.\n5. Lab results (1L solvent) significant energy reduction are promising. Discussion lacks assessment of industrial deployment feasibility: catalyst long-term stability/regeneration under real conditions, membrane fouling/lifetime in air (for example, during long-term operation in real air, what’s the impact of altcrystallization, degradation products and pore blockage on membrane mass transfer performance and lifespan?), and scaled-up energy/cost estimates vs. benchmarks. Essential for applicability.\n6. Figure 1d is how to calculate, the percentage of it? The value seems to be expanded by 100 times.In addition, there are some details of the small problem, such as the unit of CO2 flux is “mol/m2·s” rather than “mol/m2.s”."}, "author_response": "Point to point Responses Letter to Reviewers’ Comments\nResearch Article: Nature Communications manuscript NCOMMS-25-58689\nTitle: Catalytic Hybrid Solvent Regeneration in Membrane Vacuum Processes for Direct\nAir Capture\nReviewer #1:\nThe experimental work was well-designed to demonstrate the catalytic effect and the hybrid\nsolvent's role in reducing regeneration energy. The reduction in desorption energy by\napproximately 67% is impressive and potentially addresses the challenge of high regeneration\ncosts in solvent-based Direct Air Capture (DAC) operations.\nHowever, considering the high standards required by Nature Communications, this work must\ninclude a high-level technoeconomic analysis (TEA) based on the proposed process-membrane\nvacuum regeneration. While the observed reduction in desorption energy is noteworthy, it is\ncrucial to evaluate the overall economic feasibility of the proposed system comparatively with\nother state-of-the-art DAC technologies.\nIn fact, many novel approaches have been explored for reducing regeneration energy, including\nthe use of solid sorbents, moisture swing, electrodialysis membrane separation, direct steam\nregeneration, and direct conversion of carbon dioxide to value-added materials. However, these\nprocesses often incur other significant costs, such as higher CAPEX (Capital Expenditure),\nincreased O&M (Operational and Maintenance) expenses, and material costs, making the\noverall capture cost prohibitively high.\nSimilarly, the proposed process is not a simple system; it requires heat, vacuum, novel catalysts\nin a packed bed system, PDMS-coated membrane systems, and a hybrid amino acid solvent.\nThese added complexities bring associated operational, material, and maintenance costs (e.g.,\npotential solvent and catalyst degradation) that must be addressed and quantified. Without this\nassessment, it is difficult to see the clear economic advantage of low desorption energy\ncompared to other DAC systems, such as those developed by Carbon Engineering,\nClimeworks, Heirloom, and others.\nIn conclusion, while the reduction in desorption energy is significant from a scientific\nperspective, the manuscript will require a detailed and rigorous technoeconomic analysis to\ncomprehensively evaluate the proposed process's feasibility and advantages relative to existing\nDAC systems. Providing this analysis will be critical to ensuring that the work meets the\nstandards of novelty and practicality expected by Nature Communications.\nAuthors’ response:\nWe highly appreciate the positive feedback from the reviewer for our manuscript.\nWe completely agree that a techno-economic analysis (TEA) is an essential requirement for\nevaluating MVR-based DAC systems. A rigorous TEA must account for membrane lifetime,\ncatalyst durability, solvent stability, and all relevant capital and operating costs. Furthermore,\na complete and accurate TEA for liquid-based DAC systems requires simultaneous evaluation\nof both the absorption and desorption sections, as overall performance depends on the optimal\nintegration of these two units. Parameters such as CO capture rate, air temperature and\n2\nhumidity, solvent temperature, rich/lean loadings, solvent circulation rate, and regeneration\ntemperature all strongly influence equipment sizing (CAPEX) and operating costs (OPEX).\nFor example, in this study, for our bench-scale unit, the regeneration temperature was limited\nto 90 °C due to rig constraints. Operating at higher temperatures (e.g., ~100 °C) would lead to\nconsiderable improved performance1, or using two membrane modules in series would increase\nCAPEX but also enhance productivity. These types of experimental analysis were not\naccomplished in the current work for the current rig. As the reviewer correctly pointed out,\nwell-established DAC developers report TEA values after full system optimization on scale.\nTherefore, accomplishing a rigorous TEA for a bench-scale system with the fully optimized\ncommercial large-scale DAC systems of companies such as Carbon Engineering or\nClimeworks would not yield a fair or meaningful comparison.\nHowever, the major and common challenge in such systems remains the high thermal energy\ndemand of absorbent/adsorbent regeneration, an issue further amplified in DAC due to the\nextremely low CO concentration in ambient air. Therefore, in this work our primary objective\n2\nwas to investigate this most energy-intensive step and explore pathways to significantly reduce\nits regeneration energy requirement. Our focus was to quantify how much the thermal energy\ndemand can be reduced by lowering the regeneration temperature and enabling operation under\nmembrane vacuum regeneration (MVR), while further enhancing performance through\ncatalytic solvent regeneration and the use of a hybrid amino-acid solvent. We focused on the\noperability and compatibility of these combined technologies, assessed their stability, evaluated\ncatalyst performance across different loadings, identified suitable membrane modules and\nsolvent formulations, and quantified the achievable reduction in regeneration energy. Total\nthermal energy consumption for the best-performing was calculated, and we added Table S5 to\nthe Supplementary Information comparing thermal energy consumption with other well-\nstablished DAC technologies along with the related description.\nReviewer #2:\nWhat are the noteworthy results?\n1. This is an experimental study at a laboratory scale of CO2 vacuum desorption from some\nsolvents using hollow fiber membrane devices. The authors claim a reduction of the energy\ndemand in the CO2 desorption process of DAC, but it is not easy to identify the reference\ndesorption.\nAuthors’ response:\nWe are thankful for reviewer’s time in reviewing and providing feedback. In response, the\nnegative emission technologies (NETs), particularly Direct Air Capture (DAC), have recently\nattracted significant global attention due to their strong potential to mitigate climate change.\nAmong the two main DAC approaches, adsorption and absorption, the latter has received\ncomparatively less research focus, despite being a well-established and mature technology\nwidely used in industrial CO capture processes. One of the main challenges of absorption-\n2\nbased DAC processes is the high operating temperature (120-140 °C) and the significant energy\nrequired for solvent regeneration, both of which lead to elevated operating costs, especially\nunder the ultra-dilute CO concentrations typical of DAC applications. In addition, high\n2\noperating temperatures can accelerate solvent degradation, further increasing costs and\nreducing the overall sustainability of the process.\nTo address these limitations, we implemented advanced regeneration strategies by integrating\nthree advanced techniques to reduce energy demand and enhance process compatibility with\nrenewable energy sources. Specifically, we focused on lowering the regeneration temperature\nto prevent solvent boiling and enable coupling with low-grade waste heat or solar energy\n(≤90°C). This was achieved using the Membrane Vacuum Regeneration (MVR) process.\nFurthermore, to enhance CO desorption at lower operating temperatures and reduce the\n2\nspecific energy consumption per unit of CO released, the MVR process was integrated with\n2\ncatalytic solvent regeneration and hybrid amino acid solvents. A thorough experimental\nsensitivity analysis was conducted to evaluate this integration.\nThis integration resulted in a notable reduction in thermal energy demand, reaching\napproximately 2.6 GJ tCO -1 for sensible heat and 6.5 GJ tCO -1 for total thermal energy\n2 2\nconsumption. These values are reported and discussed in the final paragraph before Fig. 5 in\nthe manuscript.\nWe appreciate the reviewer for bringing up this important point for identifying the reference\ndesorption. To address the reviewer’s comment regarding the reference desorption process, we\nprepared a comparative table included in the Supplementary Information (SI), Table S5 along\nwith related discussion. This table summarizes and compares the technology type, operating\ntemperature, total thermal energy consumption, and energy source of our system with those of\nwell-established DAC technologies. The table has also been provided here for the reviewer’s\nkind consideration.\nTable R1. Thermal energy consumption of this work and well-established DAC technologies\nThermal energy\nThermal\nTechnology System type Thermal energy source Reference\nconsumption\nenergy needs\n(GJ.tCO2 -1)\nThis work Liquid solvent 90 °C 6.5 Renewable energy sources -\nCarbon Engineering Liquid solvent 900 °C 5.3-8.81 Natural gas with CCS 2, 3\nClimeworks Solid sorbent 80-120 °C 5.8-14.5 Non-fossil energy resources 4, 5, 6\n(geothermal, waste heat)\nGlobal Thermostat\nSolid sorbent 105-120 °C 4.4 Energy resource agnostic 3, 5\nCompany\nAs shown in Table R1, a range of energy consumption values are reported for various DAC\ntechnologies. Compared with Carbon Engineering’s process which is solvent-based, it is\ncompelling to note that the system developed in this work operates at considerably lower\ntemperature ranges and also is compatible with renewable heat sources (also sits well in lower\nranges of energy consumption of Carbon Engineering technology). Furthermore, when\ncompared with two leading adsorption-based DAC technologies, Climeworks and Global\nThermostat, its energy use also lies within a comparable range. Given that both adsorption\nsystems are compatible with renewable energy sources and operatable at low ranges of\ntemperature, the comparable energy efficiency and mild operating conditions of the present\nsystem highlight its potential as a practical and scalable liquid-based DAC technology.\n2. Will the work be of significance to the field and related fields? The work supply some\nexperimental results which low significance in DAC.\nAuthors’ response:\nWe are thankful for reviewer’s comments. As discussed in our response to Comment 1, the\ndetailed analysis and comparison presented in this work regarding the energy consumption of\nsolvent-based DAC technology address one of the most critical challenges in this process.\nAlthough most of the analyses conducted so far, including our previously published work, have\nbeen performed at the laboratory scale, the technology has successfully passed the proof-of-\nconcept stage based on all recorded measurements.\n3. How does it compare to the established literature? If the work is not original, please\nprovide relevant references. The work agrees well with the established literature. The novelty\nlies in the catalytic step. Does the work support the conclusions and claims, or is additional\nevidence needed? This is a laboratory scale which demands a deeper comparison with other\nalternatives.\nAuthors’ response:\nWe agree that a broader comparison enhances the context of the work. Accordingly, we\nexpanded the discussion in the revised manuscript and SI as mentioned in response to comment\n1 to include a detailed comparison with well-established DAC technologies, such as Carbon\nEngineering, Climeworks, and Global Thermostat, focusing on energy consumption and\noperating temperature and energy sources. This addition highlights the advantages of the\ndeveloped system under low-temperature, energy-efficient conditions.\n4. Are there any flaws in the data analysis, interpretation and conclusions? Do these prohibit\npublication or require revision? No\nIs the methodology sound? Does the work meet the expected standards in your field? YES\nIs there enough detail provided in the methods for the work to be reproduced? YES\nAuthors’ response:\nWe appreciate reviewer’s positive feedback.\n5. In order to compare complex processes the results should be based in the required KPIs. In\nthis paper the novel catalyst combined to a vacuum regeneration is claimed as an alternative\nprocess based on the reduction of the desorption energy.\nA Process Systems Analysis is recommended taking the alternatives as references in order to\nidentify pros y cons of this process.\nAccording to the supplied information the novelty relies in the selected catalyst an vacuum\ndesorption. The lack of novelty is the main weakness of the paper\nAuthors’ response:\nWe thank the reviewer for the valuable comments. The comparison based on relevant KPIs,\nparticularly thermal energy consumption for solvent regeneration, has been addressed and\ndiscussed in response to Comment 1, where a comparative analysis with well-established DAC\ntechnologies has been included in the revised manuscript and Supplementary Information\n(Table S5).\nAs described in the previous responses, the primary scope of the present work is a laboratory-\nscale experimental investigation focusing on solvent regeneration performance. Within this\nscope, a systematic experimental analysis was conducted, including optimization of catalyst\ncharacteristics and concentration, and assessment of hybrid solvent formulations. This analysis\nenabled identification of operating conditions leading to a considerable reduction in solvent\nregeneration energy demand. In addition, our previous studies provide further analysis of\nprocess conditions in membrane vacuum regeneration systems1, 7.\nRegarding novelty, as discussed in responses to Comments 1-3, the contribution of this work\nlies in demonstrating how the integration of multiple advanced energy-efficient regeneration\nstrategies (membrane vacuum regeneration, catalytic solvent regeneration, and hybrid\nsolvents) can collectively improve the energy performance of an inherently energy-intensive\ndirect air capture process.\nReviewer #3:\nThis manuscript presents a highly innovative integration of catalytic regeneration, hybrid\nsolvents, and membrane vacuum regeneration (MVR) for DAC applications. The reported\nenergy consumption 2.6 GJ/tCO2 demonstrates transformative potential for decarbonization.\nWhile the study exhibits exceptional novelty in solvent-catalyst-membrane synergy, critical\naspects regarding industrial-scale viability and fundamental mechanistic depth require\nstrengthening to solidify its impact.\nAuthors’ response:\nWe sincerely thank the reviewer for giving positive feedback and for the valuable comments.\n1. It would be valuable to supplement experimental evidence to clarify the physicochemical\ndriving factors that underlie the superior performance of SiO2 over Al2O3. Additionally,\nquantifying the correlation between these factors and the enhanced CO2 desorption\nperformance would further strengthen the analysis.\nAuthors’ response:\nWe are thankful for reviewer’s comment. One of the most critical aspects of evaluating catalyst\nactivity is understanding the influence of multiple parameters, including total acid sites\n(Brønsted and Lewis acid sites), acid strength (strong, medium, weak), acid site distribution,\nand key physical characteristics such as total surface area, mesoporous surface area, average\npore size, and pore size distribution. Based on these factors, studies have investigated how\nthese physicochemical properties collectively affect catalytic performance 8. In fact, recently\none study employed a machine-learning approach and identified a clear and robust relationship\nbetween catalyst features and performance 9. According to that analysis, one of the primary\nparameters governing catalytic activity is the combined contribution of total acid site density\nand mesoporous surface area, which serves as a strong predictor of catalytic behavior.\nIn our manuscript, we addressed this point and noted that previous studies have consistently\ndemonstrated that SiO exhibits stronger acidic characteristics than Al O , primarily due to its\n2 2 3\nhigher density of Brønsted acid sites (Section Catalytic Support (SiO vs. Al O )) 10, 11, 12.\n2 2 3\nUnfortunately, we were unable to perform pyridine-TPD analysis to directly compare Brønsted\nand Lewis acidity in our samples. This technique could not be conducted because pyridine is\nclassified as a hazardous substance and its use is strongly restricted at our university. It is\nnotable also that we have already conducted the NH -TPD and CO -TPD analysis for all SiO\n3 2 2\nsynthesized samples (Fig. 3b and 3d and Table S3).\nFor the physical property section, we had previously provided general descriptions and\napproximate pore size values for the materials (Section Catalytic Support (SiO vs. Al O )). To\n2 2 3\nmore comprehensively elaborate on the effect of this parameter and address the reviewer’s\ncomment, we have now conducted detailed BET analyses to quantitatively evaluate the pristine\nSiO and Al O supports. The results and their implications for catalyst performance are\n2 2 3\nincluded in SI along with a table (Table S2) summarizing the key BET parameters, description\nand references. Also, we provided the table here as well for reviewer’s kind consideration:\nTable R2. BET analysis results of SiO and Al O\n2 2 3\nSamples Surface Area (m2/g) Average Pore Size (nm)\nSiO 61.5 37.7\n2 3\nIn Table R2, the BET results are reported. As shown, there are substantial differences in\nphysical properties between the two samples. A particularly noteworthy finding is that the\naverage pore size and pore-size distribution provide a clear explanation for the superior\nperformance of the SiO -based catalyst relative to the Al O -based catalyst, despite the latter\n2 2 3\nexhibiting a substantially higher surface area. This highlights the dominant influence of pore\nsize on the effective integration and dispersion of Fe-SZ within the catalyst structure, as well\nas on the accessibility of larger carbamate species during regeneration. This behavior is\nconsistent with previous studies demonstrating that mesoporous characteristics play a decisive\nrole in determining catalyst performance along with acid strength 8, 9.\nOverall, these results and discussions demonstrate how both the chemical and physical\nproperties of SiO outperform those of Al O , further validating the trends observed in our\n2 2 3\nstudy.\nNotably, the specifications of the BET instrument used for these measurements were also added\nto the SI in section 4 Characterization Methods.\n2. For the optimized catalyst Fe-SZ/SiO2, it is recommended to conduct quantitative analysis\nof metal leaching risk under MVR conditions. Assessing the impact of leached ions (e.g., Fe3+,\nZr4+) on solvent degradation would also help validate the robustness of MVR system.\nWe appreciate the reviewer’s insightful comment regarding catalyst stability. To assess this,\nleaching analysis of Fe and Zr was performed using ICP-OES in three replicates after the MVR\ncatalytic trials. The concentrations of leached metals in 1 L of solvent were extremely low, with\nFe at 0.936 ppm and Zr at 8.537 ppm. These values correspond to less than 0.0072% and\n0.0192% of the total Fe and Zr content in the catalyst, respectively, indicating negligible metal\nleaching.\nA detailed table presenting the ICP-OES measurements for Fe and Zr, along with the\ncorresponding discussion, is provided in the supplementary Information (Table S4) and in the\nmanuscript (Section Catalyst Concentration and Stability). For the reviewer’s kind\nconsideration, we also include the table here.\nTable R3. ICP-OES Analysis of Fe and Zr Leaching from Catalyst after MVR Operation\nSpecies ICP-OES (mg·L-1) Mass leached Metal leached (%)\n(mg, 1 L)\nFe 0.9357 0.9357 0.0072\nZr 8.5371 8.5371 0.0192\nSuch negligible leaching confirms that the catalyst remains chemically stable under the applied\nMVR conditions and ensures minimal degradation of the solvent, supporting its long-term use.\n3. While the integrated system demonstrates impressive short-term performance in reducing\nregeneration energy. It is recommended that evaluating the physicochemical stability of key\ncomponents (e.g., catalyst, membrane fouling/corrosion and solvent degradation) before and\nafter extended MVR cycling would provide valuable insights into the system's operational\ndurability.\nAuthors’ response:\nWe appreciate reviewer comments on this important matter. We acknowledge that the\noperational durability of the system is one of the most critical factors influencing its overall\nperformance.\nFor the catalyst section, we have already presented catalytic performance stability data for four\ncycles in Fig. S6b. To further validate long-term performance, the experiments were extended\nbeyond four cycles. As stable performance was consistently observed, two additional cycles\nwere conducted. The updated results, presented in Fig. S6b (also included here for the\nreviewer’s kind consideration Fig. R1), show that the catalyst maintained stable activity with\nonly minor variations throughout the extended trials. This confirms the high recyclability and\ndurability of the catalyst, indicating that the system had reached a stable performance region.\nComparable stability behavior was also observed in our previously published work using a\ndifferent catalyst type 13.\nFig. R1. Catalyst stability in six cycles for 9wt% Fe-SZ/SiO2(1/1). All MVR trials accomplished at 90°C using 3M GlyK. No\ncatalyst is used for the blank trial.\nFor the long-term stability of the membrane modules, our experiments did not show any\nnoticeable decline in performance due to the number of runs or total operating time.\nAdditionally, because the current system is not fully automated and is not designed for\ncontinuous long-term operation under elevated temperature and vacuum, we were unable to\nconduct extended durability tests on the membrane modules with heated CO -loaded amino\n2\nacids solution under vacuum pressure. However, it is important to highlight that the membrane\nmodules used in this work are commercial polypropylene (PP) membranes. PP-based modules\nare well known for their low cost, robustness, and chemical resistance, and their stability has\nbeen repeatedly evaluated and demonstrated in the literature 14, 15. Moreover, previous studies\nhave shown that adding a coating layer, such as PDMS, can significantly improve membrane\nperformance and durability and prevent pore blockage 16. Taking together, these factors make\nPDMS-PP membranes a suitable choice for our application using aqueous solvent-membrane\ncontactors. To address the reviewer’s comment, we added further discussion in SI in section\n“2. HFMCs” to elaborate on the reasons for selecting PDMS-PP membrane modules and\nincluded references that report their high stability and long-term durability.\nRegarding solvent thermal stability, we further evaluated the thermal degradation of each\nsolvent used in this study, TauK, SarK, and GlyK according to reviewer’s comments. Each\nsolvent sample was placed in a vial and maintained at 98 °C for over 25 days. After this period,\nthe CO absorption capacity of each solvent was re-measured. Among the tested solvents, TauK\n2\nexhibited the highest thermal stability, with only a 1.2 % decrease in CO capture capacity,\n2\nfollowed by GlyK with a 13.1 % reduction, and SarK with a 17 % reduction.\nThese results clearly demonstrate that TauK possesses superior thermal stability, likely related\nto the presence of its sulfonic functional group. This observation supports our selection of TauK\nas a key component in the hybrid solvent formulation presented in this study. The results, along\nwith the related discussion, are added into the manuscript (Section Hybrid Amino Acid\nSolvents) and SI (Fig. S7). Also, we provided the figure here as well for reviewer’s kind\nconsideration (Fig. R2).\nFig. R2. Thermal degradation of solvents for over 25 days of heating under 98 °C temperature\n4. Based on the information in the manuscript and supporting information, the thermal energy\nconsumption mentioned in the manuscript appears to be sensible heat. In lines 455-458, the\nmanuscript states that the total energy consumption calculation (6.5 GJ/tCO ) includes assumed\n2\nreaction heats and estimated water vaporization heat losses. It is recommended to clearly state\nthe basis for these critical assumptions—are they cited, measured, or is there literature\nsupporting the achievability of such efficiency at scale? And the thermal energy consumption\ncalculated according to the formula in the supporting information is not GJ/t CO2, please\nfurther confirm whether there is a mistake.\nAuthors’ response:\nWe appreciated reviewer’s comment. In response, as the reviewer noted, the heating values\ncome from various factors and assumptions. The first key assumption involves a 20 K\ntemperature approach across the rich/lean heat exchanger. In many MVR processes, this\ntemperature difference is reported to be as low as 10 K, as reported in several studies 17, 18.\nBased on our observations, this value can reach approximately 16 K when using a high-\nefficiency plate heat exchanger, as confirmed during our operations. Therefore, we chose 20 K\nin our analysis to consider the worst-case scenario. Also, in line with the reviewers’ comments,\nwe also added the two relevant references for this matter to the SI document after Equation S6\nand highlighted them 17, 18.\nFor the heat of reaction, due to the use of our selected hybrid solvent with a specific formulation\nand limited data availability, we relied on reported values for 3 M TauK at around 80 °C, as\nalready cited in the manuscripts.\nRegarding the heat pump heat recovery, several studies have reported achievable heat recovery\nefficiencies exceeding 80-90 % in desalination and distillation systems, supporting the\nfeasibility of this assumption 19, 20, 21. We added these references in the section mentioned by\nthe reviewer in the manuscript (final paragraph of section Catalytic Hybrid Amino Acid\nSolvents in MVR) and highlighted them.\nWe thank the reviewer for pointing out the energy unit. Indeed, the formula in SI gives thermal\nenergy in [kJ/gCO ]. As we double-checked since the GJ/tCO is equivalent to kJ/gCO , no\n2 2 2\nadditional conversion is required. To avoid any confusion, we mentioned and added both units\nafter Equation S6. We also removed the extra term A from the formula.\n5. Lab results (1L solvent) significant energy reduction are promising. Discussion lacks\nassessment of industrial deployment feasibility: catalyst long-term stability/regeneration under\nreal conditions, membrane fouling/lifetime in air (for example, during long-term operation in\nreal air, what’s the impact of salt crystallization, degradation products and pore blockage on\nmembrane mass transfer performance and lifespan?), and scaled-up energy/cost estimates vs.\nbenchmarks. Essential for applicability.\nAuthors’ response:\nWe appreciate the reviewer’s comments regarding catalyst long-term stability and potential\nimpacts such as salt crystallization, degradation products, and pore blockage on membrane\nmass-transfer performance. As outlined in our response to Comment 3, we conducted\nadditional catalyst stability trials, extended the number of cycles, and confirmed that the\ncatalyst performance reached a stable region, demonstrating good recyclability and durability.\nFurthermore, regarding membrane stability, the selected membrane module used in our work\nincorporates a coated micro-layer design that substantially extends membrane lifetime. As\ndiscussed in our response to Comment 3, the dense structure considerably reduces the risk of\npore blockage compared with typical porous membranes.\nNonetheless, we acknowledge that fouling or scaling cannot be entirely ruled out during\nprolonged operation. However, employing common membrane-cleaning procedures, such as\nbackwashing and Clean-In-Place (CIP), can restore flux and extend membrane lifetime if\nperformance decline is observed.\nRegarding the reviewer’s comment on energy and cost scaling which is an important matter,\nincreasing the system size generally leads to lower cost per unit CO captured, which can be\n2\nattributed to several factor such as improved capital efficiency through economies of scale and\nspecialized supply chains and enhanced process optimization that enables sophisticated energy\nand heat integration 22, 23. To clarify this point, we added a statement in the revised manuscript\nhighlighting that, in general, energy and cost per unit CO are expected to decrease at larger\n2\nscale due to scale effects and improved operational conditions along with related refences at\nthe end of section Catalytic Hybrid Amino Acid Solvents in MVR.\n6. Figure 1d is how to calculate, the percentage of it? The value seems to be expanded by 100\ntimes.In addition, there are some details of the small problem, such as the unit of CO2 flux is\n“mol/m2·s” rather than “mol/m2.s”.\nAuthors’ response:\nIn response, as we double-checked, the values in Fig. 1d were calculated as the ratio of the two\nfluxes reported in Fig. 1b and Fig. 1c. The water flux was multiplied by 10 to bring it to the\nsame scale as the CO flux before calculating the ratio. We thank the reviewer for this comment,\n2\nas it helped us recognize that the original graphs were confusing. Accordingly, we revised the\nwater-flux bar chart values (Fig. 1c). Specifically, we adjusted the scale of the water-flux bar\nchart (Fig. 1c), multiplying them by 10 to match that of the CO flux. Now both of CO2 and\n2\nwater fluxes are in the same scale. Since the water flux is higher than the CO flux, the resulting\n2\nratio (water flux/ CO flux) is greater than 1.\nReference:\n1. Momeni A, et al. Catalytic Membrane Vacuum Regeneration: Enhancing Energy\nEfficiency and Renewable Compatibility in Direct Air Capture. Small 21, 2503023.\n2. Keith DW, Holmes G, St. Angelo D, Heidel K. A Process for Capturing CO2 from the\nAtmosphere. Joule 2, 1573-1594 (2018).\n3. Katie Lebling, Haley Leslie-Bole, Zach Byrum, Bridgwater L. 6 Things to Know\nAbout Direct Air Capture.).\n4. McQueen N, Gomes KV, McCormick C, Blumanthal K, Pisciotta M, Wilcox J. A\nreview of direct air capture (DAC): scaling up commercial technologies and\ninnovating for the future. Progress in Energy 3, 032001 (2021).\n5. David Sandalow JF, Colin McCormick. Direct Air Capture of Carbon Dioxide. In:\nICEF Roadmap) (2018).\n6. Akaya H, Lamnini S, Sehaqui H, Jacquemin J. Amine-Functionalized Cellulose as\nPromising Materials for Direct CO2 Capture: A Review. ACS Applied Materials &\nInterfaces 17, 16380-16395 (2025).\n7. Momeni A, McQuillan RV, Alivand MS, Zavabeti A, Stevens GW, Mumford KA.\nDirect air capture of CO2 using green amino acid salts. Chemical Engineering\nJournal 480, 147934 (2024).\n8. Alivand MS, Mazaheri O, Wu Y, Stevens GW, Scholes CA, Mumford KA. Catalytic\nSolvent Regeneration for Energy-Efficient CO2 Capture. ACS Sustainable Chemistry\n& Engineering 8, 18755-18788 (2020).\n9. Zhang R, et al. A green approach to CO2 capture using fly ash-based catalysts:\nPerformance and mechanistic insights. Fuel 394, 135098 (2025).\n10. Wang T, Yu W, Liu F, Fang M, Farooq M, Luo Z. Enhanced CO2 Absorption and\nDesorption by Monoethanolamine (MEA)-Based Nanoparticle Suspensions.\nIndustrial & Engineering Chemistry Research 55, 7830-7838 (2016).\n11. Ali Saleh Bairq Z, Gao H, Huang Y, Zhang H, Liang Z. Enhancing CO2 desorption\nperformance in rich MEA solution by addition of SO42−/ZrO2/SiO2 bifunctional\ncatalyst. Applied Energy 252, 113440 (2019).\n12. Zhang X, et al. SO42−/ZrO2 supported on γ-Al2O3 as a catalyst for CO2 desorption\nfrom CO2-loaded monoethanolamine solutions. AIChE Journal 64, 3988-4001\n(2018).\n13. Alivand MS, et al. Engineered assembly of water-dispersible nanocatalysts enables\nlow-cost and green CO2 capture. Nature Communications 13, 1249 (2022).\n14. Kim K, Lee H, Park HS, Song H, Kim S. Surface modification of polypropylene\nhollow fiber membranes using fluorosilane for CO(2) absorption in a gas-liquid\nmembrane contactor. Heliyon 9, e19829 (2023).\n15. Ramezani R, Di Felice L, Gallucci F. A Review on Hollow Fiber Membrane\nContactors for Carbon Capture: Recent Advances and Future Challenges. Processes\n10, 2103 (2022).\n16. Chen Y, Jia Z. Preparation of polydimethylsiloxane/polypropylene composite\nmembranes for degassing of water with high running stability. Desalination and\nWater Treatment 215, 43-49 (2021).\n17. Guo Y, Zhang H, Fu K, Chen X, Qiu M, Fan Y. Integration of solid acid catalyst and\nceramic membrane to boost amine-based CO2 desorption. Energy 274, 127329\n(2023).\n18. Wang Z, Fang M, Ma Q, Zhao Z, Wang T, Luo Z. Membrane Stripping Technology\nfor CO2 Desorption from CO2-rich Absorbents with Low Energy Consumption.\nEnergy Procedia 63, 765-772 (2014).\n19. Soumbati Y, Bouatou I, Abushaban A, Belmabkhout Y, Necibi MC. Review of\nmembrane distillation for desalination applications: Advanced modeling, specific\nenergy consumption, and water production cost. Journal of Water Process\nEngineering 71, 107296 (2025).\n20. Najib A, et al. An experimental investigation of a solar-driven desalination system\nbased on multi-effect membrane distillation. Desalination and Water Treatment 198,\n1-18 (2020).\n21. Farquet P. Efficiency redefined: The role of heat pumps and MVR in sustainable\ndistillation. White Paperby (Sulzer), ( November 2024).\n22. Küng L, et al. A roadmap for achieving scalable, safe, and low-cost direct air carbon\ncapture and storage. Energy & Environmental Science 16, 4280-4304 (2023).\n23. van der Spek M, et al. An ecosystem of carbon dioxide removal reviews – part 1:\ndirect air CO2 capture and storage. Energy & Environmental Science 18, 9713-9785\n(2025)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Authors have been well addressed the TEA question with details. It's recommended for acceptance.", "reviewer_2": "In order to compare complex processes the results should be based in the required KPIs. In this paper the novel catalyst combiened to a vacuum regeneration is claimed as an alternative process based on the reduction of the desorption energy.\nA Process Systems Analysis is recommended taking the alternatives as references in order to identify pros y cons of this process.\nAccording to the suplied information the novelty relies in the selected catalyst an vacuum desorption.\nThe lack of novelty is the main weakness of the paper", "reviewer_3": "The returned manuscript has been carefully revised, with all comments properly addressed. The overall quality of the manuscript has improved significantly, and it is recommended for publication."}, "author_response": ""}]

Decision letter