Nonporous hydrophobic organic crystals for carbon dioxide capture via chain-melting phase transition

Domain: neuroscience
Published: 2026-02-03
Editorial outcome: major_revision

Abstract
An efficient carbon capture and release system necessitates rapid CO2 transport to and from active sites, a property typically associated with permanently porous materials featuring large surface areas. Here, we present hydrophobic organic crystals of alkylated monoethanolamine that, despite their nonporous nature, undergo a rapid and reversible solid-to-solid phase transition upon CO2 uptake and release. Exposure to CO2 triggers a thermodynamically favored structural rearrangement, enabling quantitative CO2 capture and forming a stable carbamate, aided by intermolecular interactions involving the long side chains. This process is fully reversible under practical flue-gas CO2 capture conditions (>0.6% CO2, 0−100% relative humidity) and enables low-temperature desorption using CO2 itself as a stripping gas (65 °C at 1 atm CO2). Structural analysis through in situ XRPD, solid-state NMR spectroscopy, electron diffraction, and Raman analysis confirms that these hydrophobic absorbents selectively uptake CO2 to form an anhydrous ammonium carbamate pair in the solid state. The non-hygroscopic nature of these organic crystals is exemplified by a representative C10-MEA in the presence of CO2, resulting in a desorption process with a minimal temperature swing (ΔT abs-des = 30 °C), offering an energy-efficient (>1.2 GJ/t of captured CO2) and economically viable alternative for carbon capture applications.

Introduction
Carbon dioxide capture is a critical technology for mitigating anthropogenic carbon emissions, enabling sustainable societal growth1–4. Reducing the cost of carbon capture processes to a level competitive with renewable energy is essential for widespread adoption5,6. Solid-state adsorbents7 offer a promising approach for cost-effective CO2 capture by avoiding the high energy penalties8,9 associated with traditional aqueous amine-based solvents10,11, which are prone to evaporative losses and oxidative degradation. Advances in solid-state CO2-capture materials have focused on enhancing stability to water and oxidation12,13 and molecular fine-tuning at the active sites14,15 in highly porous materials, such as metal-organic frameworks16–21, covalent-organic frameworks19,22, and porous organic polymers23–25.

Crystalline solid-state sorbents enable us to gain detailed insights into the molecular structures and host-guest interactions at the atomic level. Some of these materials, such as soft porous crystals26, undergo guest-responsive structural changes during reversible adsorption and desorption processes (breathing mechanism), often driven by entropic gains, enhancing their working capacity for gas separation27–29. For instance, a squaraine-derived soft porous organic crystal exhibited a CO2-triggered gate-opening mechanism30. Similar responsive behaviors have been observed with small aromatic guest molecules, mediated by weak π-π interactions, hydrogen bonding, or van der Waals forces, facilitating guest diffusion within porous structures31,32. A nonporous organic solid, p-tBu-calix[4]arene, demonstrated transportation of vinyl bromide, inducing a spontaneous solid-to-solid phase transformation33. An adaptation of this behavior is, in principle, advantageous for cost-effective gas separation with high working capacity when they are operative in practical conditions34.

For CO2 capture, a few nonporous, hydrogen-bonded organic absorbents have been reported. For example, an aqueous solution of m-xylylenediamine crystallizes upon exposure to CO2 from air, forming a highly ordered structure with hydrophilic carbamic acid layers sandwiched between hydrophobic aromatic moieties35,36. Similarly, solid tribenzylamine exhibits high CO2 capacity from humid air, forming an ammonium-carbamate framework37. However, both systems rely on volatile organic CO2 absorbents, and desorption requires high temperatures (>100 °C) to release CO2 from the stable carbamates.

Herein, we report a solid-state chemisorption process with hydrophobic organic crystals of monoalkylated monoethanolamines (Cn-MEAs) that undergo complete and reversible structural rearrangement upon CO2 uptake and desorption under practical and mild conditions. Chain melting—thermally induced reversible hydrocarbon chain disordering in solid-state—and recrystallization of the nonporous organic crystals at ambient temperatures enable rapid CO2 uptake and spontaneous solid-to-solid transformation (Fig. 1). These organic crystals exhibit high thermal and oxidation stability, a CO2 capture capacity of 2.5 mmol/g, recyclability (>200 cycles), and low volatility, making them ideal for carbon capture applications. Combined with the simplicity and accessibility of MEAs, this approach enables a significantly lower operating cost for post-combustion carbon capture processes.

Results
CO2 induced solid-to-solid crystal transformation
We previously reported a water-lean CO2 capture system with amphiphilic absorbents of the Cn-MEAs driven by entropic advantages in saline solutions38. These absorbents were prepared via a one-step modification of the parent monoethanolamine (MEA). The scalable synthesis was optimized to afford a crystalline product of high yield and purity (up to 88%) after a single recrystallization step (Fig. 2A), with a representative absorbent 2-(decylamino)-ethanol (C10-MEA). Qualitatively, C10-MEA appears as soft, plastic, colorless, needle-like crystals with a vitreous luster. These crystals rapidly transition to a chalky-white fibrous form when exposed to pure CO2 at room temperature in solid state (Fig. 2B). Thermogravimetric analysis (TGA) revealed a maximum theoretical uptake of 0.5 molar equivalents and a high gravimetric uptake of 2.5 mmol/g within 7 min (Fig. S18). Scanning electron microscopy (SEM) disclosed a substantial morphological transformation of the C10-MEA absorbent upon CO2 exposure. Initially, the material exhibits well-defined, elongated microstructures with smooth surfaces and distinct boundaries (Fig. 2C, red). After CO2 treatment, the crystals exhibit extensive fragmentation and the emergence of a highly intricate, fibrous network (Fig. 2C, blue). This qualitative change of the crystals implies a crystalline phase transition upon the exposure to CO2.

To investigate the structural changes of hydrophobic absorbents, time-dependent in situ X-ray powder diffraction (XRPD) analysis was performed on pristine C10-MEA. We observed a fully reversible transition between two crystalline phases at room temperature under a CO2 atmosphere (Fig. 3A, left) and for the CO2-loaded C10-MEA sample (denoted C10-MEA-CO2) under N2 atmosphere (Fig. 3A, right). This set of experiments led us to propose a continuous and reversible solid-to-solid transformation induced by the presence and the absence of CO2 as a stimulus. We tested a series of other alkylated absorbents (C11-MEA, C12-MEA, C14-MEA, and C16-MEA), all of which exhibited similar reversible behavior with CO2 under XRPD analysis conditions (Fig. S10A–E).

Attenuated total reflectance infrared (ATR-IR) spectroscopic analysis of C10-MEA and C10-MEA-CO2, at ambient conditions, enabled us to identify a characteristic carbamate carbonyl stretch at 1628 cm−1, along with N-H stretching and bending frequencies observed at 3200 cm−1 (broad) and 1540 cm−1, respectively (Fig. 3B, blue and Fig. S11). The band at 1409 cm−1 was attributed to the C-N stretching frequency.

Crystallographic investigations of C10-MEA and C10-MEA-CO2 proved the CO2-induced phase transition (Table S1). These analyses39 revealed molecular rearrangement and the formation of an ordered organic crystal that accommodates CO2 through a complex interplay of covalent and non-covalent interactions. The pristine C10-MEA exhibits a layered herringbone arrangement stabilized by hydrogen bonding and van der Waals interactions (Fig. 3C, left, Table S2, and Fig. S22). Specifically, the OH proton interacts with the N atom of an adjacent molecule in the same plane (O···N=2.792(2) Å), while the oxygen weakly interacts with the NH proton (N···O=3.088(2) Å) from the molecule in the plane above. In contrast, the CO2-loaded organic crystal, i.e., C10-MEA-CO2 is characterized by a motif of four distinct molecules, i.e., two crystallographically independent ammonium-carbamate pairs, arranged head-to-tail in a basket weave pattern (Fig. 3C, right, and Fig. S24). Each carbamate group acts as an acceptor of 4 H-bonds from OH and NH2+ groups, forming an extensive network of hydrogen bonds, with D···A distances ranging from 2.69(2) to 2.88(2) Å that stabilize the crystal structure (Table S6 and Fig. S23). This structural reorganization demonstrates the material’s dynamic and spontaneous response to CO2 under ambient conditions, facilitated by significant non-covalent molecular cooperativity and hydrogen bonding interactions, enabling CO2 inclusion within the nonporous solid crystal. All examined Cn-MEAs crystal structures (C12-MEA, C14-MEA, C16-MEA, Table S1, Fig. S39) with and without CO2 share the same H-bonding networks found for C10-MEA and C10-MEA-CO2, respectively, even though the packing of their alkyl chains may differ (Tables S2–S8).

We assessed the reversibility of C10-MEA using magic-angle spinning (MAS) solid-state nuclear magnetic resonance (ssNMR) spectroscopy to elucidate the capture mechanism. Single-pulse MAS 13C-NMR experiments of C10-MEA and C10-MEA-CO2 in the solid phase (Fig. 3D, left, recorded at 13 °C) display high resolution, indicative of highly ordered states, and significant differences between C10-MEA without or with CO2 captured. The former shows one set of peaks for each carbon (red), (tentatively assigned to the structures shown as an insert above the middle spectrum), which, upon CO2 binding, splits into multiple resonances as exemplified by four peaks for the CH3 group consistent with the lower symmetry of the crystal structure (blue). The splitting of the signal of terminal methyl groups of C10-MEA-CO2 corresponds to the four distinct chain environments based on the tetrameric packing of the ammonium carbamates, allowing for the additional structural change information from the ssNMR (Fig. 3D). With a closer look at the crystal structures, we identified a single methyl group environment in C10-MEA (Figs. S36 and S37, left) with a 8.0 Å distance between neighboring methyl groups. C10-MEA-CO2 instead exhibit two distinct ammonium-carbamate pairs with methyl-methyl distances of ca. 6.0 Å and 7.4 Å (Figs. S36 and S37, right), producing four distinct methyl group environments of the alkyl chains. Single-pulse MAS 13C-NMR for C10-MEA and C10-MEA-13CO2 (Fig. S25) reveals significantly different chemical shifts in the presence of CO2, both in solid and melted phases, with both samples showing comparable chemical shifts in the melted phase. A single peak in the carbonyl region (164.5 ppm) for C10-MEA-13CO2 was assigned to the CO2 adduct as a carbamate. The phase changes for C10-MEA and C10-MEA-13CO2 were analyzed using 13C cross-polarization MAS (CP/MAS) NMR (Figs. S26 and S27), exploiting that 1H to 13C cross-polarization only occurs for immobile (solid) components, revealing phase transitions at 27 and 39 °C, respectively. 1H Single-pulse MAS spectra (Fig. 3D, middle) reveal sharp peaks for both C10-MEA (red) and C10-MEA-CO2 (blue), with an indication of higher structural complexity of the latter sample. To confirm the nature of nitrogen groups, quantitative 15N CP/MAS NMR analysis (Fig. 3D, right) was conducted to show a single nitrogen peak (39.1 ppm) in pristine C10-MEA (red), which disappeared upon CO2 loading. After CO2 uptake (blue), two distinct sets of nitrogen peaks appeared at 44.7 and 86.9 ppm, which are assigned as ammonium and carbamate species, respectively. These results confirm a 2:1 amine-to-CO2 ratio and the formation of an ammonium-carbamate pair in the solid state, demonstrating that CO2 induces a quantitative conversion of C10-MEA into an ordered crystalline structure in C10-MEA-CO2. We note that the appearance of two resonances around 86.9 ppm for the C10-MEA-CO2 sample is consistent with the change in the crystal structure (Fig. 3C, right), with two different carbamate environments.

We further investigated CO2-induced structural transformation with time-dependent Raman analysis. Experiments at 0 and 25 °C (Figs. S32 and S33, respectively) illustrate conformational change exemplified by peak changes in a 1400−1500 cm−1 region, corresponding to C-H and N-H bending vibrations, substantial peak changes at 2750 cm−1 corresponding to methyl and methylene stretching vibrations, and a change in the amine group peak at 3280 cm−1. Furthermore, a temperature-dependent Raman analysis of the pristine C10-MEA sample reveals changes in the spectrum at 34 °C (note that the m.p. of C10-MEA is 38 °C), attributed to the increased chain mobility and the initiation of the melting process (Fig. S34). The low-frequency Raman analysis further corroborates the conformational changes in the chains with C10-MEA-CO2 exhibiting broader low-frequency vibrations than pristine C10-MEA (Fig. S35).

Thermal properties of Cn-MEAs were investigated by differential scanning calorimetry (DSC), which was conducted for Cn-MEAs and their CO2 adducts. Analysis of the melting process revealed that C10-MEA-CO2 exhibits lower latent heat and heat capacity than pristine C10-MEA (Fig. 4D, Table S9), explaining efficient heat utilization of the CO2 adduct. Notably, CO2 desorption from C10-MEA occurs with a lower endothermic heat flow than that observed for longer alkyl chain lengths, corroborating desorption experiments (Fig. 5B). This is attributed to the exothermicity of the CO2 uptake process (~50 kJ/mol) that, together with the thermal properties of the absorbent, induces chain melting and recrystallization to C10-MEA-CO2. Van’t Hoff analysis of CO2 capture processes with C10-MEA at a various temperature range (Fig. 4B) reveals a positive entropy in the 35–45 °C region, associated with the melting phase transition (Fig. S20). The crystal phase change qualitatively occurs without visible liquefaction, resulting in a solid-to-solid transition to form a crystalline CO2 adduct under CO2 capture conditions at room temperature. Consequently, the recrystallization-assisted desorption is an energetically mild process with desorption enthalpy (ΔHdes) of 58.4 kJ/mol, a value typically associated with physisorbents40,41. This thermal hysteresis between chemisorption and desorption energies underpins the reversibility of C10-MEA for carbon capture via a selective carbamate formation (chemisorption) and chain-melting mechanism (low temperature desorption).

CO2 capture performances of C10-MEA
The C10-MEA organic crystal reversibly captures CO2 as an ammonium/carbamate pair at ambient temperatures. Isothermal CO2 uptake measurements of C10-MEA at 0, 15, and 25 °C (Fig. 6A) showed a step-shaped isotherm with a sharp increase in uptake capacity at low partial pressures (0.65 mbar), supporting a chemisorption mechanism. Notably, an inverse temperature relationship was observed, with capacity increasing with increasing temperature, attributed to a gradual increase in chain mobility. This is in stark contrast to traditional solid sorbents and amine-appended porous materials operating via physisorption or chemisorption17, where capacity decreases with temperature increase. At temperatures over 30 °C, however, the uptake capacity decreased with increasing temperature (Fig. 4B). A peak capacity of 2.3 mmol/g (2.5 mmol/g by TGA) was achieved at 25 °C and 4 mbar, which can be attributed to the spontaneous formation of carbamate with CO2 and corresponds to the structural transition to C10-MEA-CO2. It is important to note here that the performance of C10-MEA is independent of the initial crystallinity of C10-MEA from the synthesis.

Next, we tested isothermal absorption and desorption conditions for C10-MEA at 30 °C in TGA, demonstrating an energetically mild desorption process (Fig. 4A). This mild regeneration is critical for lower energy consumption and maintaining sorbent durability by minimizing thermal degradation and reducing operating costs. Such performance is unexpected for chemisorption materials involving a carbamate mechanism, as the N-C bond with CO2 typically requires significant thermal energy for dissociation and desorption40. To further evaluate the performance of C10-MEA, TGA experiments were conducted at low CO2 concentrations with N2 as the balance gas. At 15% CO2 (in 85% N2), C10-MEA retained a high capture rate and capacity (Fig. 6B). Notably, higher temperatures and longer regeneration times were required to desorb CO2 from C10-MEA-CO2 generated from the exposure to lower concentration CO2 feeds (0.58–15%, Fig. S18). The resulting ammonium carbamate has a different structure when the formation of the adduct occurs slowly (see the comparison of XRPD patterns in Fig. S16). The slower crystallization of the C10-MEA-CO2 adduct afforded a thermodynamically more stable crystal, thus requiring higher heat energy for desorption. The absorption profile of C10-MEA outperforms other alkylated variants (e.g., C12-MEA, C14-MEA, and C16-MEA), highlighting the distinct role of the C10-chain in the carbon capture and release process (Fig. 5).

To elucidate the facile desorption process of C10-MEA, TGA experiments were conducted at elevated temperatures under a 100% CO2 atmosphere (1 atm). C10-MEA was saturated with CO2 at 60 °C, where C10-MEA-CO2 remained stable over a prolonged time (60 min), as evident by isothermal TGA curve (Fig. 4C). A slow increase of the temperature revealed that a 1 °C increment from 60 to 61 °C abruptly triggered CO2 desorption from C10-MEA-CO2 under CO2 atmosphere (Fig. 6C). The desorption rate increased with further temperature rises, and C10-MEA exhibited thermal and oxidative stability in this range, as demonstrated in a 10% CO2 capture process (air balance) at 30−60 °C with 100% air at 60 °C desorption, showing minimal performance loss over 10 capture cycles (Fig. 6D). This desorption behavior can lead to a high working capacity (>2.3 mmol/g CO2) with only 30 °C of temperature swing, enabling the use of pure CO2 as a stripping gas to provide undiluted CO2 as a product.

To further demonstrate the application potential of C10-MEA, tandem ATR-IR and thermal degradation analysis were conducted to assess potential competitive water co-absorption under humid conditions7,42. Samples loaded with 100% CO2 at 100% relative humidity (RH) (Fig. 6E, orange curve) showed no difference from the adduct obtained using dry CO2 (blue curve), indicating that water has a negligible influence on adduct formation at ambient and high RH conditions (see Fig. S19 for detailed analysis). Thermal degradation profiles exhibited a single-step mass loss corresponding to the release of CO2, confirmed by TGA-FTIR-MS (Fig. S12). In a model flue gas study (15% CO2, 85% N2) at 100% RH (green curve), we observed characteristic carbamate peaks, with slight broadening in the fingerprint (400–800 cm−1) and hydrogen bonding (3000–3600 cm−1) regions in the IR spectrum, likely due to surface-level water, while the crystal remained solid. In contrast, pristine C10-MEA under 100% N2 at 100% RH (gray curve) exhibited water inclusion, as evidenced by the broadening of the IR signal and the formation of hydrogel, which showed significant mass loss due to water evaporation in TGA analysis (Fig 6E, gray). TGA-FTIR-MS analysis further confirmed the prominent CO2 evolution from the C10-MEA-CO2 (Fig. S12) and the water resistance of the hydrophobic C10-MEA crystal, which can reduce energy costs associated with humidity control and water condensation for carbon capture.

Finally, we demonstrated recyclability by performing 500 consecutive capture and release cycles with the C10-MEA crystal at isothermal conditions (40 °C, under 100% CO2 and N2, respectively, Fig. S38). We detected a minimal capacity loss of approximately 1 wt% for the 200 cycles shown in Fig. 4F, underscoring the potential of C10-MEA as a solid-state capture system driven by a crystal phase transition and chain melting mechanism.

In conclusion, we presented a class of CO2-responsive, nonporous organic crystals that, through non-covalent, intermolecular side-chain cooperativity, undergo reversible solid-to-solid transformation upon CO2 capture under practically relevant conditions. These materials, exemplified by C10-MEA, demonstrate a high CO2 capture capacity as a single organic compound, surpassing many state-of-the-art materials43,44, while requiring mild desorption conditions to yield a high-purity CO2 stream at as low as 65 °C owing to the temperature-sensitive, flexible, and hydrophobic alkyl chains45,46. By offering operational efficiency, scalability, and low energy demands, these materials will pave the way for a paradigm shift towards sustainable, cost-effective CO2 capture solutions, with profound implications for designing flexible and responsive absorbents and advancing industrial decarbonization.

Methods
Chemicals
All chemicals, unless explicitly specified, were sourced from commercial suppliers and used as received. Carbon dioxide (CO2) was employed directly from a cylinder with a purity of 99.7%, without any further treatment for reactions. The solvents utilized were HPLC (high-performance liquid chromatography) grade.

Synthesis procedure for Cn-MEAs and other derivatives
All Cn-MEAs were synthesized following the general procedure for the synthesis of C10-MEA by using an appropriate alkyl bromide. N-(2-methoxyethyl)decan-1-amine and 2-(decyl(methyl)amino)ethan-1-ol were synthesized using 2-methoxyethan-1-amine and 2-(methylamino)ethan-1-ol respectively, instead of monoethanolamine.

General procedure. Monoethanolamine (0.992 mol, 60 mL, 4 eq.) was added to 1.5 L of absolute EtOH in a 3 L three-necked round-bottomed flask. The power output of the heating jacket was adjusted to maintain the solution at 70 °C. When the desired temperature was reached, a 60 mL syringe filled with 1-bromodecane (0.243 mol, 51.5 mL, 1 eq.) was attached to a syringe pump with a long needle going through the septum of the three-necked flask. After the setup was complete, slow addition was initiated at the rate of 1 mL/h over 60 h. After completing the addition of alkyl bromide, the mixture was left to react for 24 h.

The reaction mixture was then transferred to a round-bottomed flask, and all EtOH was evaporated. To the resulting residue, 500 mL of H2O was added. The crude compound was extracted with 3 × 300 mL CHCl3. The combined organic layers were washed with 300 mL H2O, dried over Na2SO4, filtered, and concentrated under reduced pressure. To the resulting light brown solids, 100 mL of ethyl acetate was added, and the mixture was heated to 78 °C with stirring. The light brown solution was slowly cooled to room temperature and left overnight under a flow of N2, resulting in the formation of long, white, needle-shaped crystals, which were collected by filtration using a Büchner funnel and dried under reduced pressure.

All Cn-MEAs were recrystallized following the general procedure. N-(2-Methoxyethyl)decan-1-amine and 2-(decyl(methyl)amino)ethan-1-ol were distilled.

NMR spectroscopy
Liquid-state 1H and 13C NMR spectra were recorded at 500 and 126 MHz, respectively, on a Bruker Avance III spectrometer equipped with a Broad Band Fluorine–Observe (BBFO) probe. Chemical shifts (δ) are reported in parts per million (ppm), and coupling constants (J) are expressed in Hertz (Hz). The notation for NMR multiplicity includes s (singlet), bs (broad singlet), d (doublet), t (triplet), q (quartet), and m (multiplet).

All solid-state NMR analyses were performed at 16.4 T (700 MHz for 1H) on a Bruker Avance III HD spectrometer equipped with a 4 mm HXY magic-angle-spinning (MAS) probe operating at 12 kHz MAS rate. The 1H MAS NMR spectra were obtained by a rotor-synchronized echo experiment using 82.5 kHz radio-frequency (rf) pulses and 1 scan. The single-pulse 13C MAS NMR spectra in Figs. 3C and S28 were recorded with a relaxation delay of 300 s and 256 scans, while the single-pulse 13C MAS NMR spectra in Fig. S25 were recorded with a relaxation delay of 30 s and 16 scans. All single-pulse 13C MAS NMR spectra were acquired by an excitation pulse length of 4.48 μs (55.8 kHz rf field strength) and Spinal-64 decoupling with 83.3 kHz rf field strength during acquisition. 13C cross-polarization magic-angle spinning (CP/MAS) experiments (Figs. S26 and S27) were recorded using an initial 1H pulse of duration 2.92 μs (corresponding to 85.6 kHz rf field strength) followed by a CP period with 80–100% ramp on the 1H channel, a contact time of 1.2 ms, rf field strengths of 48.0 kHz on 1H and 55.8 kHz on 13C, 3 s repetition delay, Spinal-64 (93.6 kHz) decoupling during acquisition, and 128 scans. Quantitative 1H-15N multiple CP/MAS experiments (Fig. 3C, right) were recorded using an initial 1H pulse of duration 2.87 ms (corresponding to 87.1 kHz rf field strength) followed by 6 CP blocks with 0.4 s 1H recovery time in between. The CP blocks were setup with 80–100% ramp on the 1H channel, a contact time of 700 ms, and rf field strengths of 32.0 kHz on 1H and 20.0 kHz on 15N. In total, 85036 scans were acquired with an acquisition time of 25 ms and a 3 s recycle delay. Chemical shifts were calibrated indirectly through the adamantane peak observed to low frequency (29.5 ppm relative to tetramethylsilane).

Attenuated total reflection Infrared Spectroscopy (ATR-IR)
FT-Infrared spectra were recorded using a Bruker ALPHA-P FTIR spectrometer with a single reflection ATR module.

In situ Raman spectroscopy
In situ Raman experiments were conducted using a Renishaw InVia Raman spectrometer with a 514 nm laser with a power of 300 mW, used at 33% of the laser power. The grating for the in situ experiments was 1200 l/mm, while for the low-frequency Raman, the grating was switched to 2400 l/mm. The general spectral resolution of the instrument is 0.3 cm−1. 50× objective was used for the instruments. All experiments were performed in a Linkam THMS600 stage to control the temperature in the range of −196 to 600 °C. The signal acquisition time of each spectrum was ~10 s (10 accumulations with 1 s exposure). Samples were purged with Ar overnight before experiments to ensure a pristine state. All experiments were conducted under a controlled gas flow of 50 mL min−1 (CO2 or Ar).

Electron diffraction
Structural characterization of C12-MEA, C14-MEA, C16-MEA, C10-MEA-CO2, C12-MEA-CO2 and C14-MEA-CO2 was performed with electron diffraction (3D ED). The dry powders were deposited on 300-mesh copper TEM grids coated with a continuous film of ultrathin amorphous carbon (from EMS). 3D ED measurements were performed at ca. 100 K and high vacuum using a Gatan Elsa Cryoholder, on a Rigaku XtaLAB Synergy-ED, equipped with a LaB6 source operating at 200 kV (λ=0.0251 Å). A series of diffraction patterns were recorded using a Rigaku HyPix-ED detector during continuous rotation of the crystals over ~120°, with a width of 0.25° and an exposure time of 0.5 s per frame. A selected area aperture with an apparent diameter of 2 μm was used to define the diffracting area. The program CrysAlisPro was used to control data collection and to process the diffraction data for indexing, data reduction, and space group determination.

The crystal quality was generally quite low and very variable from particle to particle, likely due to the softness and tendency to disorder of these materials. Whenever possible, the datasets of two or three crystals were merged to improve data completeness. Structure solution and refinement were performed in Olex2. The crystal structures were solved ab initio with SHELXT, and least-squares refinement in kinematic approximation was performed with SHELXL using scattering factors for electrons. An extinction parameter was refined to mitigate the effects of multiple diffraction. Soft restraints were applied to the 1,2 and 1,3 distances of the aliphatic chains. H-atoms for -NH2+ and OH groups were assigned based on the protonation state and H-bonds pattern deduced from the interpretation of solid-state NMR data, and in a few cases, the OH or NH protons were visible in the difference-Fourier map. All other H-atoms were placed in geometrically calculated positions and refined with a riding model, using tabulated distances from neutron diffraction. Anisotropic atomic displacement parameters were refined for non-hydrogen atoms with the aid of similarity and rigid-body restraints. For H-atoms, isotropic atomic displacement parameters were derived from the parent atom. Further details and results are reported in Table S1.

Thermogravimetric analysis (TGA)
Thermogravimetric analysis (TGA) was conducted using a Discovery TGA550 instrument from TA Instruments in New Castle, DE, USA. The samples were subjected to a constant nitrogen flow of 90 mL/min and heated in platinum TGA pans from room temperature to 300 °C at a heating rate of 10 °C/min. TGA results were analyzed to determine the thermal decomposition, the first derivative of TGA (dTGA) temperatures of the samples, and CO2 gravimetric uptakes using Trios v5.1.1.46572 software from TA Instruments. CO2 absorption experiments were conducted with a flow rate of 90 ml/min using a Discovery TGA550 instrument equipped with a Blending Gas Delivery Module. Samples were activated under an N2 flow for 15 min at 60 °C before reaching the desired experiment temperature and switching the gas stream to CO2 mixtures.

Differential scanning calorimetry (DSC)
Differential scanning calorimetry (DSC) analysis was carried out using a Discovery DSC instrument from TA Instruments in New Castle, USA. Samples weighing 4−5 mg were placed in Tzero aluminum pans with a perforated lid. Analyses were conducted under a nitrogen flow of 50 mL/min, with a linear heating rate of 10 °C/min from 30 to 300 °C. The melting point temperatures (Tm) and melting enthalpy (ΔHm) were determined using Trios v5.1.1.46572 software (TA Instruments, New Castle, USA).

Note: The C10-MEA thermodynamic parameters presented in the main text were extracted from the closed pan DSC analysis shown in Fig. 4D.

X-ray powder diffraction (XRPD)
X-ray powder diffraction patterns were recorded in Bragg-Brentano geometry for 1 h using a Bruker D8 Advance powder diffractometer with λCuKα=1.5406 Å (40 kV, 40 mA) and a LYNXEYE detector, in the 2θ range from 4 to 50°.

In situ X-ray powder diffraction
The in situ XRPD experiment was performed at the DanMAX beamline at MAX IV in Lund, Sweden. The samples were packed in a polyimide capillary with 1.0 mm diameter. The X-ray radiation was set to a 20 keV energy (λ=0.6199 Å), and the diffraction patterns were captured with a DECTRIS PILATUS 3X 2M CdTe area detector. The CO2 and N2 flow were controlled by a mass flow controller (SFC5500-2slm-Sensirion Co., Ltd., Switzerland).

Single-crystal X-ray diffraction (SC-XRD)
SC-XRD datasets were collected on the BioMAX beamline at MAX IV (Lund, Sweden), equipped with an Arinax MD3 Microdiffractometer and a DECTRIS EIGER2 16M CdTe detector. The X-ray radiation was set to 20 keV (corresponding to a wavelength of 0.61992 Å). The diffraction environment was controlled with an Oxford Instruments Cryojet set to 100 K. The crystals were mounted at ambient temperature on MiTeGen loops using MiTeGen NVH oil and cooled down directly in the cold N2 stream. Data collection was controlled via the software MXCube3. For each crystal, 3 series of 720 frames (0.5° oscillation steps and 0.01 s exposure time) were recorded, at 3 different κ angles, to maximize completeness. All datasets were converted with convert2sfrm and processed with the program APEX5 for indexing, data reduction, scaling, and empirical absorption correction. The structures were solved with SHELXT and refined with SHELXL using the interface ShelxLe. N-H and O-H hydrogen atoms were located in the difference-Fourier map, and their coordinates were freely refined, while the aliphatic H-atoms were placed in geometrically idealized positions and refined with a riding model. Anisotropic atomic displacement parameters were refined for non-hydrogen atoms, while for H-atoms, isotropic atomic displacement parameters were derived from the parent atom. Further details and results are reported in Table S1.

Brunauer-Emmett-Teller (BET) analysis
Carbon dioxide adsorption isotherms were collected using a Micromeritics 3Flex analyzer instrument (Micromeritics Instrument Corp., Norcross, GA, USA). The CO2 gas purity was 99.998%. The temperature was controlled using a water/ethylene glycol bath during the collection of CO2 adsorption isotherm data. The compound (0.1−0.2 g) was transferred to a glass adsorption tube. Samples were cleaned under vacuum at 25 °C for 24 h using the SmartVac Prep. Isotherm data points were considered equilibrated if the pressure change was <0.01% after time intervals of 10 s.

Scanning electron microscopy (SEM)
Specimens were mounted on stubs using double adhesive carbon tape (Ted Pella) as an adhesive and sputter-coated with 6 nm gold (Leica ACE200). Specimens were examined with a FEI Quanta 3D scanning electron microscope operated at an accelerating voltage of 2 kV. The experiment is performed at room temperature.

TGA-FTIR-MS
TGA-MS analyses were conducted employing a NETZSCH TG 209 F1 Libra® thermal analyzer, operating under nitrogen flow at a heating rate of 10 °C/min. The temperature range spanned from 28 to 140 °C in open Al2O3 crucibles. Mass spectrometric (MS) data were collected using a QMS 403 Aëolos® Quadro mass spectrometer within the 1–50 m/z range.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript presents an interesting tackling of the important and timely problem of CO2 capture using responsive organic solids. Simple monoalkylated monoethanolamines (Cn-MEAs), with the best performing of them being already reported by some of the authors in ref. 37, allow an efficient, scalable, and cheap strategy for the capture of CO2 at ambient conditions and desorption at mild and accessible temperatures. The work could potentially be interesting for the broad audience of Nature Communications, however several aspects of the work need to be revised before being published. The major concern is the validity of the “chain melt” hypothesis.\nHere below I have listed the issues to be addressed before considering the manuscript further for publication.\n1) “Chain melting” simply refers to order-disorder phase transitions involving alkyl side chains/fragments. See some literature examples: (1) Nat. Commun. 2015, 6, 8810; (2) Nat. Commun. 2023, 14, 1304; (3) J. Am. Chem. Soc. 2024, 146, 46, 31911–31919.\nIf the authors want to use this qualitative definition, as it is already used in the literature (see ref 45 for example), I suggest to clearly define what “chain melting” means because it could be confusing for the readership of Nature Communications.\nIn ref 45, “chain melting” refers to order-disorder transitions in asymmetric resulting in disorder phases that were not fully structurally characterized because of the side-chain disorder and twinning of the crystalline samples. In the present manuscript, the authors seem to refer to the “chain melting” as a qualitative mechanistic understanding of the phase change, see Scheme 1. In particular, the authors write: “This is attributed to the exothermicity of the CO2 uptake process (~50 kJ/mol) that is sufficient to induce chain melting (m.p. of C10-MEA = 38.1 °C) and initiate recrystallization to C10-MEA-CO2. The crystal phase change qualitatively occurs without liquefaction, resulting in a solid-to-solid transition to form a crystalline CO2 adduct.” Indeed, they were able to solve both phases (also thanks to the great facilities at their disposal), and while the CO2- loaded crystals show indeed disorder, they do not appear to be melted in any way. To corroborate the fully crystalline nature of the loaded compound (and somehow contradicting their mechanistic model), the authors write: “Single-pulse MAS 13C NMR experiments highlight the high mobility of C10-MEA, while C10-MEA-CO2 was characterized by broader, although still relatively sharp and well-defined 13C resonances, being indicative of increased rigidity and formation of a more crystalline structure. A single peak in the carbonyl region (158 ppm) was assigned to the CO2 adduct as a carbamate. A single-pulse 1H MAS NMR experiment revealed high mobility in pristine C10-MEA, characterized by the sharp peaks in the spectrum, whereas significant broadening of the spectrum of C10-MEA-CO2 indicated markedly increased rigidity, confirming a structural distinction from the parent compound.” To obtain a more informative picture of the order-disorder mechanism, the authors should turn their attention to different experimental techniques such as low-frequency Raman spectroscopy and/or THz spectroscopy, see for example the works of Omer Yaffe (Weizmann Institute of Science) and Michael Ruggiero (University of Rochester).\n2) Given the substantial discussion of porous and guest-responsive crystals, please engage more fully with foundational work from Susumu Kitagawa and co-workers on soft, porous crystals/flexible MOFs and related gate-opening/breathing phenomena, to situate the present contribution within that context. A brief paragraph in the Introduction would help readers outside the subfield.\n3) Figure 2 panel C the molecular structures are barely readable. Increase their size and/or change their color. Additionally, it is not particularly clear when the desorption process starts and why it is happening at different times for the various molecules.\n4) The authors use both ton and tonne in the manuscript. I would suggest using the standard symbol for tonne/ton that is “t”.\n5) Figure S8 looks like standard %Transmittance vs Frequency FT-IR spectra. Correct the axis label “absorbance”.\n6) “…squarine-derived soft porous organic crystal exhibited…” should be “…squaraine-derived soft porous organic crystals exhibited…”\n7) “This transformation indicates a fundamental structural alteration of C10-MEA in the solid state.” Does the change of morphology always imply a “fundamental structural alteration”?\n8) Even though qualitative, the PXRD comparison in Figure 3A clearly shows a structural change. On the other hand, Figure S7 is less clear, and I would suggest to stack pure absorbents with the CO2-loaded counterparts to better showcase the structural rearrangement.\n9) The Supporting Information file has a table of contents but no page numbering. Please add it.\n10) The font size of Table S6 is too small.\n11) Extended data Figure 1A, it is not clear when the desorption process starts.\n12) The authors should expand the discussion of the effect of the length of the alky chains on the performance of the absorbents.\n13) Few references have typos to be fixed. Here are some examples: (1) Ref. 22 Polym. Chem.; (2) Ref. 38 J. Synchrotron Radiat.; (3) Ref. 40 Sep. Purif. Technol.", "reviewer_2": "The manuscript presents a study on nonporous hydrophobic organic crystals (alkylated monoethanolamines, e.g., C10- MEA) for CO₂ capture via a reversible solid-to-solid phase transition, which is relevant to advancing carbon capture technologies. Below are specific comments to strengthen the work.\n1.The article emphasizes \"nonporous but fast absorption\" but lacks a detailed explanation of the underlying mechanism. Regarding regeneration, it is stated that thermal triggering is required for structural rearrangement (chain melting), while Figure 4F shows desorption via atmosphere change (isothermal desorption), leading to reasonable doubts about the desorption cycle performance under CO₂ atmosphere. Additionally, the application scenarios are insufficient: the article has verified that it is not applicable to direct air capture, and industrial carbon capture may not meet the required temperature variation range.\n2.A reviewer read a recent paper by this team (Adv. Energy Sustainability, 2024, 5, 2400204), which reported a liquid amine system using this solid material, indicating that the system is not entirely new.\n3.The use of nonporous materials for CO₂ capture via chain-melting phase transition is an interesting approach, challenging the dominance of porous frameworks (e.g., MOFs, COFs). The demonstration of reversible structural rearrangement driven by CO₂ uptake/release, supported by in situ XRPD, solid-state NMR, and electron diffraction, provides robust evidence for the proposed mechanism.\n4.The claim that exothermic CO₂ uptake induces chain melting (m.p. of C10-MEA = 38.1°C) requires further elaboration. While DSC data show lower latent heat for C10-MEA-CO₂, direct evidence linking the exothermicity (ΔH_abs ≈ 50 kJ/mol) to localized melting (e.g., in situ calorimetry or temperature-dependent XRPD during uptake) would strengthen this argument.\n5.While 100 cycles show minimal capacity loss, characterization (e.g., XRPD, SEM) of the material after extended cycling (>500 cycles) would confirm structural integrity and rule out gradual degradation.", "reviewer_3": "In the manuscript, the authors report the fabrication of nonporous hydrophobic organic crystals and their application in carbon capture and release systems. Taking C10-MEA as an exemplar, these materials exhibit exceptional CO2 capture efficiency and capacity as single organic compounds. The results show that through non-covalent intermolecular side-chain interactions, a reversible solid-solid transformation occurs after capturing carbon dioxide under actual flue gas conditions. In situ XRPD, solid-state NMR spectroscopy, and electron diffraction analyses confirm that these hydrophobic absorbents selectively uptake CO2 in the solid state to form carbamic acid species. This work demonstrates remarkable novelty and comprehensiveness. I would like to recommend its acceptance for publication in Nature Communications after the following revisions.\n1.The author concludes from Fig. 3B that the vibration peaks of carbonyl, N-H bond and C-N bond can be observed in the FTIR spectrum. This phenomenon lacks basis. The author is requested to provide a detailed explanation.\n2.In a single-pulse 1H MAS NMR experiment (Fig. 3C), the spectral peaks of C10-MEA-CO2 were not observed to be significantly broadened. The peak width might be caused by unstable baseline. The author concludes in the manuscript that ‘whereas significant broadening of the spectrum of C10-MEA-CO2’, Please provide a clear explanation for this conclusion.\n3.In page 4, ‘in stark contrast to traditional solid sorbents and amine-appended porous materials operating via physisorption or chemisorption’. It is suggested that the author create relevant tables and cite relevant literature to enhance the novelty of the work.\n4.In page 4, ‘It is important to note here that the performance of C10-MEA is independent of the initial crystallinity of C10-MEA from the synthesis’. Why? Please provide a detailed explanation\n5.The author repeatedly uses the word \"hypothesize\" in the manuscript. Is this \"hypothesize\" a conclusion drawn from experimental phenomena?\n6.Some of the images in the manuscript have poor clarity, and the font and font size are not uniform. For example, Extended data Fig. 2C, Fig. S7, etc. Please have the authors carefully review and correct the entire manuscript\n7.There are formatting errors and abbreviation mistakes in the citation of references, such as (2), (6), (13), (40), etc. Please have the authors carefully check and revise the manuscript."}, "author_response": "Dear Editor,\nWe would like to share our revised manuscript, along with the supporting information and\nour detailed point-by-point responses to the constructive comments and criticisms from\nthe reviewers. Please find our response below in blue. Note that all the major changes in\nthe revised manuscript are highlighted in yellow.\nReviewer #1 (Remarks to the Author)\nThe manuscript presents an interesting tackling of the important and timely problem of\nCO2 capture using responsive organic solids. Simple monoalkylated monoethanolamines\n(Cn-MEAs), with the best performing of them being already reported by some of the\nauthors in ref. 37, allow an efficient, scalable, and cheap strategy for the capture of CO2\nat ambient conditions and desorption at mild and accessible temperatures. The work\ncould potentially be interesting for the broad audience of Nature Communications,\nhowever several aspects of the work need to be revised before being published. The\nmajor concern is the validity of the “chain melt” hypothesis.\nHere below I have listed the issues to be addressed before considering the manuscript\nfurther for publication.\nResponse: Thanks for the positive comments. Here, we will address the concern of the\nchain melting mechanism in detail.\n1) “Chain melting” simply refers to order-disorder phase transitions involving alkyl side\nchains/fragments. See some literature examples: (1) Nat. Commun. 2015, 6, 8810; (2)\nNat. Commun. 2023, 14, 1304; (3) J. Am. Chem. Soc. 2024, 146, 46, 31911–31919.\nIf the authors want to use this qualitative definition, as it is already used in the literature\n(see ref 45 for example), I suggest to clearly define what “chain melting” means because\nit could be confusing for the readership of Nature Communications.\nResponse: Thanks for the comment. The concept is further defined at the end of the\nintroduction section to clarify as follows: “Chain melting - thermally induced reversible\nhydrocarbon chain disordering in solid-state - and rapid recrystallization of the nonporous\norganic crystals at ambient temperatures may enable rapid CO uptake and spontaneous\n2\nsolid-to-solid transformation (Fig. 1).”\nIn ref 45, “chain melting” refers to order-disorder transitions in asymmetric resulting in\ndisorder phases that were not fully structurally characterized because of the side-chain\ndisorder and twinning of the crystalline samples. In the present manuscript, the authors\nseem to refer to the “chain melting” as a qualitative mechanistic understanding of the\n1\nphase change, see Scheme 1. … To obtain a more informative picture of the order-\ndisorder mechanism, the authors should turn their attention to different experimental\ntechniques such as low-frequency Raman spectroscopy and/or THz spectroscopy, see\nfor example the works of Omer Yaffe (Weizmann Institute of Science) and Michael\nRuggiero (University of Rochester).\nResponse: Thanks for the comment and suggestions. First, we used the chain melting\nmechanism to explain the observed phenomena since our molecule has a low melting\npoint, which is low enough to spontaneously melt under CO absorption conditions.\nconditions, presumably due to the rapid recrystallization to the adduct (C10-MEA+CO ).\nresponsible for the facile absorption.\nTo support this idea, we conducted more ssNMR spectroscopic analysis (Fig S23-\nS28) and Raman spectroscopy analysis as shown in the revised manuscript and the\nsupporting information (Figs S29-S32), corroborating our hypothesis.\nRight now, we don’t have access to a low-frequency Raman spectrometer or THz\nspectroscopy (which would be fantastic for future studies) to meaningfully assign the\nvibration mode of our molecule, which would be highly flexible under heated conditions.\nInstead, the ssNMR data and additional Raman experiments clearly showed the transition\nupon mild heating, indicating the molecular mobility of the C10-MEA suddenly increases\nat around 35 °C. This explains the observed facile CO capture performance of C10-MEA\n2\nowing to the liquid-like status (chain-melted, high entropy) at the interface, compensating\nfor the negative entropy of the bimolecular CO capture process. It is also beneficial to\n2\nhave this phase transition for desorption since the melted status will facilitate the diffusion\nand mobility of the sorbent and CO at mildly elevated temperatures (60-70 °C).Thanks\n2\nagain for the suggestion.\nexperiments recorded at different temperatures (normalized to the integral at 13 °C). A\nsigmoid curve is fitted for each region for C10-MEA with (blue) and without (red) 13CO ,\n2\nand the midpoint of the curve is found to be 39 and 27 °C, respectively.\n2) Given the substantial discussion of porous and guest-responsive crystals, please\nengage more fully with foundational work from Susumu Kitagawa and co-workers on soft,\nporous crystals/flexible MOFs and related gate-opening/breathing phenomena, to situate\nthe present contribution within that context. A brief paragraph in the Introduction would\nhelp readers outside the subfield.\nResponse: The review article was added as ref number 26, and the text has been\nmodified to acknowledge the foundational work from S. Kitagawa and others: Some of\nthese materials, such as soft porous crystals (26), undergo guest-responsive structural\nchanges during reversible adsorption and desorption processes (breathing mechanism),\noften driven by entropic gains, enhancing their working capacity for gas separation (27-\n29).\n(2009). https://doi.org/10.1038/nchem.444\n3) Figure 2 panel C the molecular structures are barely readable. Increase their size\nand/or change their color. Additionally, it is not particularly clear when the desorption\nprocess starts and why it is happening at different times for the various molecules.\nResponse: We assumed that this comment refers to Extended Data Figure 2. The Ext.\nFig. 2C has been corrected as follows to increase readability.\n4) The authors use both ton and tonne in the manuscript. I would suggest using the\nstandard symbol for tonne/ton that is “t”.\nResponse: Thank you for the comments. All units are corrected to ”t”.\n5) Figure S8 looks like standard %Transmittance vs Frequency FT-IR spectra. Correct\nthe axis label “absorbance”.\nResponse: Corrected accordingly in Figure S8, but also in Figure 3B, 4E, and S16.\n4\n6) “…squarine-derived soft porous organic crystal exhibited…” should be “…squaraine-\nderived soft porous organic crystals exhibited…”\nResponse: Corrected to “squaraine” on page 2, first paragraph.\n7) “This transformation indicates a fundamental structural alteration of C10-MEA in the\nsolid state.” Does the change of morphology always imply a “fundamental structural\nalteration”?\nResponse: The sentence has been modified as follows: “This qualitative change of the\ncrystals implies a crystalline phase transition upon the exposure to CO .”\n2\n5\n8) Even though qualitative, the PXRD comparison in Figure 3A clearly shows a structural\nchange. On the other hand, Figure S7 is less clear, and I would suggest to stack pure\nabsorbents with the CO2-loaded counterparts to better showcase the structural\nrearrangement.\nResponse: The figure was remade as suggested, with separate graphs stacked for each\nabsorbent, comparing the XRPD pattern before and after CO absorption as shown\n2\nbelow. The following figure was added in the Supporting Information Section, Fig.S7:\n6\nC14-MEA, and e) C16-MEA in pristine form (red) and CO -loaded form (blue). The y-\n2\naxis was normalized to 100.\n9) The Supporting Information file has a table of contents but no page numbering. Please\nadd it.\nResponse: Page numbers are added to the SI.\n10) The font size of Table S6 is too small.\nResponse: The table is Table S11 in the revision Supporting Information file. The font\nsize is increased from 10 to 12.\n8\n11) Extended data Figure 1A, it is not clear when the desorption process starts.\nResponse: Absorption and desorption are separated by the blue and red shaded regions.\nThe figure caption has also been corrected as shown below.\n9\n12) The authors should expand the discussion of the effect of the length of the alkyl chains\non the performance of the absorbents.\nResponse: The effect of alkyl chain length on CO capture rate and performance is\n2\npresented in the Extended data Fig. 2. Furthermore, the discussion of DSC profiles\nbetween the chains and their role in the chain-melting mechanism is on page 4, in the last\nparagraph in the section of “CO Induced Solid-to-Solid Crystal Transformation”. Tables\n2\nvarious alkylated MEA absorbents, while Tables S1 to S8 provide additional\ncrystallographic information of various alkylated MEAs and their CO -loaded\n2\ncounterparts.\nThe following sentence has been “modified: “Notably, the facile desorption of CO from\nlengths, corroborating desorption experiments of the absorbents (Extended data Fig. 2B).\nThis is attributed to the exothermicity of the CO uptake process (~50 kJ/mol) that,\n2\ntogether with the thermal properties of the absorbent, induces facile chain melting and\nrecrystallization to C10-MEA-CO .”\nendothermic heat flow by increasing the alkyl chain length was observed. However, C10-\nMEA and C12-MEA exhibit lower enthalpy changes for CO adducts than pristine\n2\nsamples, indicating more efficient heat utilization for CO adduct, therefore lower energy\n2\nrequirement for desorption.”\nAlso note that we obtained new crystal structures of C12-, C14, and C16-MEA as pristine\nfree base, and CO adducts (C12-MEA-CO , and C14-MEA-CO ) as shown in Fig S36\n2 2 2\nand corresponding structure refinement parameters in Tables S2-S8. All new structures\nwere deposited in CCDC database.\n10\n13) Few references have typos to be fixed. Here are some examples: (1) Ref. 22 Polym.\nChem.; (2) Ref. 38 J. Synchrotron Radiat.; (3) Ref. 40 Sep. Purif. Technol.\nResponse: Thank you for the comments. References are corrected.\nThe manuscript presents a study on nonporous hydrophobic organic crystals (alkylated\nmonoethanolamines, e.g., C10-MEA) for CO₂ capture via a reversible solid-to-solid phase\ntransition, which is relevant to advancing carbon capture technologies. Below are specific\ncomments to strengthen the work.\n1. The article emphasizes \"nonporous but fast absorption\" but lacks a detailed explanation\nof the underlying mechanism. Regarding regeneration, it is stated that thermal triggering\nis required for structural rearrangement (chain melting), while Figure 4F shows desorption\nvia atmosphere change (isothermal desorption), leading to reasonable doubts about the\ndesorption cycle performance under CO₂ atmosphere. Additionally, the application\nscenarios are insufficient: the article has verified that it is not applicable to direct air\ncapture, and industrial carbon capture may not meet the required temperature variation\nrange.\nResponse: Thank you for the comments. For regeneration, based on our thermal\nanalysis (TGA and DSC analysis), it is evident that the molecule undergoes rapid and\nsignificant changes in terms of heat exchange - fusion and recrystallization. Since we\ncouldn’t record any observable surface area, we concluded that the compound is not\nporous therefore, the observed CO capture performance should be relied on the diffusion\n2\nof CO gas on the surface, adsorb, and then infuse in to the crystals to induce\n2\nrearrangement of the whole crystal, affording quantitative structural changes as observed\nby all spectroscopic analysis as shown in the manuscript.\nFigure 4F shows the isothermal carbon sorption process using a BET surface analyzer.\nWe couldn’t record desorption steps under isothermal conditions (0-25 oC), which is also\nan indication that the mild heating is necessary for efficient desorption. To make it clear,\nthis was not to confuse the regeneration method, but it is merely to accommodate a\ncommon characterization method for carbon capture materials to show the capacity.\nFor absorption and desorption cycles, the material performs better under continuous feed\nof exchangeable gas – it could be N or CO or a mixture.\n2 2,\nFor application, it is plausible to plug in our capture method to biogas carbon capture for\nbiogas upgrading, where the temperature of the flue gas is much lower. Even for a post-\ncombustion CO capture scenario, we can envision using our capture method after heat\n2\nexchange of the flue gas down to 40-60 °C, where a very narrow TSA will allow us to\nregenerate the sorbent. For this, we are currently working on the stability of C10-MEA\nand other molecules for SOx and NOx conditions.\n12\n2. A reviewer read a recent paper by this team (Adv. Energy Sustainability, 2024, 5,\n2400204), which reported a liquid amine system using this solid material, indicating that\nthe system is not entirely new.\nResponse: The previous paper discusses a water-lean system in brine solution where\nthe presence of salt affects CO capture capacity, which was explained by the increased\n2\nentropy of water at the interface. In the current work, we showcase solid-state CO\n2\ncapture without water and solvents. It is correct that the absorbent component is the same\n(C10-MEA); however, this new study shows for the first time the solid state structures of\nthe pristine sample and also the CO adduct, which are remarkable in their structures\n2\nalone.\n3.The claim that exothermic CO₂ uptake induces chain melting (m.p. of C10-MEA =\n38.1°C) requires further elaboration. While DSC data show lower latent heat for C10-\nMEA-CO₂, direct evidence linking the exothermicity (ΔH_abs ≈ 50 kJ/mol) to localized\nmelting (e.g., in situ calorimetry or temperature-dependent XRPD during uptake) would\nstrengthen this argument.\nResponse: Thank you for the constructive comments. In situ Raman spectroscopy and\ntemperature variant solid-state NMR spectroscopy were conducted to elaborate this point\n(see below). The new data showed alignment with the proposed chain melting\nmechanism in the temperature range that is relevant to our CO capture and release\n2\nexperiments. Once again, this supports our idea that efficient chemisorption is facilitated\nby the entropic gain despite the moderate heat of absorption. This question has also been\nraised by Reviewer 1, as you can find in response Q1.\nexperiments recorded at different temperatures (normalized to the integral at 13 °C). A\nsigmoid curve is fitted for each region for C10-MEA with (blue) and without (red) 13CO ,\n2\nand the midpoint of the curve is found to be 39 and 27 °C, respectively.\n2\nrange is presented (top), with expanded views of the 7501600 (bottom left) and\n32503350 cm-1 (bottom right) regions to highlight spectral changes of C10-MEA upon\nCO exposure. Shifts in aliphatic C-H bending (14001500 cm-1) and C-H stretching\n2\n(26002800 cm-1) indicate a conformational change in the C10-MEA structure induced by\nthe CO uptake.\n2\n15\n5. While 100 cycles show minimal capacity loss, characterization (e.g., XRPD, SEM) of\nthe material after extended cycling (>500 cycles) would confirm structural integrity and\nrule out gradual degradation.\nResponse: Thanks for the suggestion. We have conducted cycling experiments\nisothermal sorption/desorption cycles, more than 550 cycles (see below). Please ignore\naround 200-250 cycles, where a nitrogen tank was empty so the measurement was not\naccurate. We have included the figure only until 200 cycles in the manuscript. Overall,\nafter >500 cycles, we have observed ca. 30% reduction of capacity. This is due to the\nunoptimized absorption and desorption cycles with the high velocity of purge gas.\nThank you for the positive evaluation. Here is a more in-depth discussion for the\nreviewer’s comments.\n1. The author concludes from Fig. 3B that the vibration peaks of carbonyl, N-H bond and\nC-N bond can be observed in the FTIR spectrum. This phenomenon lacks basis. The\nauthor is requested to provide a detailed explanation.\nResponse: Reference for peak assignment was added based on the literature. Further\nelaboration of these assignments can be found in this paper, which is cited in the SI Figure\nS8, Ref 14.\nA. Danon, P. C. Stair, E. Weitz, FTIR Study of CO Adsorption on Amine-Grafted SBA-15:\n(2011).\n2. In a single-pulse 1H MAS NMR experiment (Fig. 3C), the spectral peaks of C10-MEA-\nCO2 were not observed to be significantly broadened. The peak width might be caused\nby unstable baseline. The author concludes in the manuscript that ‘whereas significant\nbroadening of the spectrum of C10-MEA-CO2’, Please provide a clear explanation for this\nconclusion.\nResponse: Thank you for the comments. We conducted more in-depth analysis with\nadditional ssNMR experiments (Supporting Information Figures S22 to S28), which is\nshown below and added as Figure S24. This analysis presents our interpretation of the\n13C-CP ssNMR, where we assign the integration of C10-MEA peaks at various\ntemperatures to elucidate the mobility of the compound under CP experiments. As\nindicated with inverse sigmoid plots, the mobility (based on the decrease of signals in 13C-\nCP experiments) is increasing with the increase in temperature for all carbon signals.\nAdditionally, it is worth noting that C10-MEA (red) exhibits a lower transition temperature\nthan the C10-MEA-CO adduct (blue), which is in good agreement with our thermal\n2\nanalysis, melting point measurements, DSC data, and CO capture experiments.\nWe assessed the reversibility of C10-MEA using magic-angle spinning (MAS)\nsolid-state nuclear magnetic resonance (NMR) spectroscopy to elucidate the capture\nmechanism. Single-pulse MAS 13C NMR experiments of C10-MEA and C10-MEA-CO in\n2\nthe solid phase (Fig. 3D, left, recorded at 13 C) display high resolution, indicative of a\nhighly ordered state, and significant differences between C10-MEA without or with CO\n2\ncaptured. The former shows one set of peaks for each carbon (tentatively assigned to the\nstructure shown as an insert in the middle spectrum), which, upon CO binding, splits into\n2\nmultiple resonances as exemplified by four peaks for the CH group consistent with the\n3\n17\nlower symmetry of the crystal structure. The splitting of the signal of terminal methyl\ngroups of C10-MEA-CO corresponds to the four distinct chain environments based on\n2\nthe tetrameric packing of the ammonium carbamates, in agreement with the additional\nstructural change information from the ssNMR (Fig. 3D). With a closer look at the crystal\nstructures, we identified a single methyl group environment in C10-MEA (Fig. S33) with\na 4.4 Å distance between neighboring methyl groups. C10-MEA-CO instead exhibits two\n2\ndistinct ammonium carbamate pairs with methyl-methyl distances of ca. 6.0 Å and 7.4 Å\n(Fig. S34), producing four distinct methyl group environments and a looser packing of the\nalkyl chains. Single-pulse MAS 13C NMR for C10-MEA and C10-MEA-13CO (Fig. S22)\n2\nreveal significantly different chemical shifts in the presence of CO both in solid and\n2,\nmelted phases, with both samples showing comparable chemical shifts in the melted\nphase. A single peak in the carbonyl region (158 ppm) for C10-MEA-13CO was assigned\n2\nto the CO adduct as a carbamate. The phase changes for C10-MEA and C10-MEA-\n2\n13CO were analyzed using 13C cross-polarization MAS (CP/MAS) NMR (Figs. S23 and\ncomponents, revealing phase transitions at 27 and 39 °C, respectively. 1H single-pulse\nMAS spectra (Fig. 3D, middle) reveal sharp peaks for both C10-MEA and C10-MEA-CO ,\n2\nwith an indication of higher structural complexity of the latter sample. To confirm the\nnature of nitrogen groups, quantitative 15N CP/MAS NMR analysis (Fig. 3D, right) was\nconducted to show a single nitrogen peak in pristine C10-MEA (38.2 ppm), which\ndisappeared upon CO loading. After CO uptake, two distinct sets of nitrogen peaks\n2 2\nappeared at 45.0 ppm and 87.5 ppm, which are assigned as ammonium and carbamate\nspecies, respectively. These results confirm a 2:1 amine-to-CO ratio and the formation\n2\nof an ammonium-carbamate pair in the solid state, demonstrating that CO induces a\n2\nquantitative conversion of C10-MEA into an ordered crystalline structure in C10-MEA-\nCO . We note that the appearance of two resonances around 87.5 ppm for the C10-MEA-\n2\ntwo different carbamate environments.\nat different temperatures (normalized to the integral at 13 °C). A sigmoid curve is fitted for\neach region for C10 MEA with (blue) and without (red) 13CO , and the midpoint of the\n2\ncurve is found to be 39 and 27 °C, respectively.\n3. In page 4, ‘in stark contrast to traditional solid sorbents and amine-appended porous\nmaterials operating via physisorption or chemisorption’. It is suggested that the author\ncreate relevant tables and cite relevant literature to enhance the novelty of the work.\nResponse: Thank you for the comment. First, a comparative table is in the supporting\ninformation Table S11, as attached also below.\nsorbents, where the higher the temperature, the lower the CO uptake capacity is (both\n2\nframework materials and aqueous systems follow this trend, ref 17 cited). While C10-MEA\nfollows the same trend at temperatures above 30 °C (Extended data Fig. 1B), the BET\nmeasurements in the temperature range 0 to 30 °C show the inverse trend where uptake\ncapacity increases with increasing temperature (Fig. 4A). We explain this observation so\nthat certain amount of thermal energy is required to induce mobility in the chains and\ntogether with CO , trigger the structural rearrangement.\ntemperature relationship was observed, with capacity increasing with increasing\ntemperature, attributed to a gradual increase in chain mobility. This is in stark contrast to\ntraditional solid sorbents and amine-appended porous materials operating via\nphysisorption or chemisorption (17), where capacity decreases with temperature\nincrease. At temperatures over 30 °C, however, the uptake capacity decreased with\nincreasing temperature (Extended data Fig. 1B). “\n20\n4. In page 4, ‘It is important to note here that the performance of C10-MEA is independent\nof the initial crystallinity of C10-MEA from the synthesis’. Why? Please provide a detailed\nexplanation.\nResponse: The performance remains identical, no matter the initial state of the sample\n(purification method, crystallinity, wetness etc.). We tested both powder and highly\ncrystalline forms of C10-MEA. We also include a pre-evacuation step (isothermal at 60\n°C, N atmosphere) in TGA before CO capturing experiments, where the sample is\n2 2\nmelted and recrystallized on the TGA pan before CO capture begins. All these control\n2\nexperiments showed identical performance in terms of CO capture capacity and\n2\ndesorption pattern, indicating that the CO sorption is not dependent on the crystalline\n2\nstructure of C10-MEA, rather it is dependent on the transient “melting” of the crystal or\npowder sample.\n5. The author repeatedly uses the word \"hypothesize\" in the manuscript. Is this\n\"hypothesize\" a conclusion drawn from experimental phenomena?\nResponse: We changed the wording to “propose”, such as the following sentence: “This\nset of experiments led us to hypothesize propose a continuous and reversible solid-to-\nsolid transformation induced by the presence and the absence of CO as a stimulus.”\n2\n6. Some of the images in the manuscript have poor clarity, and the font and font size are\nnot uniform. For example, Extended data Fig. 2C, Fig. S7, etc. Please have the authors\ncarefully review and correct the entire manuscript\nResponse: New figures are produced to accommodate the comments. Thank you for\npointing out the irregularity. This also addresses Reviewer 1 Q8. Figure 1 was revised\nsuch that ammonium-carbamate pairs in the C10-MEA-CO structure correspond to the\n2\ncrystal packing. Figure 3 fonts were increased for clarity, and figures C and D were\nexchanged. New ssNMR figures are included as shown below.\nFigure S7 was redone:\nFig. S7. XRPD patterns of C10-MEA compared with C11-MEA, C12-MEA, C14-MEA,\nand C16-MEA in pristine form (red) and CO -loaded form (blue). The y-axis was\n2\nnormalized to 100.\n2\nhydrophobic nonporous amphiphiles. The negative ΔH of the chemisorption process (> 0.6%,\nabs\nrt to 60 °C) is sufficient (H > H ) to induce localized chain melting of the nonporous crystals,\nabs m\ntriggering the rearrangement from S1 to M (red to purple, respectively). The efficient CO\n2\ntransport through a nonporous crystalline system (M to S2, purple to blue) is facilitated by\nsignificant non-covalent cooperativity of the side chains to afford a thermodynamically stable\nammonium carbamate structure (S2, blue) with high CO uptake capacity (2.5 mmol/g). This\n2\nphase-changing mechanism results in a mild –58.4 kJ/mol (by DSC) chemisorption process that\nallows for a facile, low-energy (> 60 °C, under 100% CO ) desorption (S2 to S1, blue to red).\n2\nrepresenting ammonium and carbamate species, respectively); H = melting enthalpy; H = heat\nm c\nof crystallization.\nFig. 2 Synthetic and uptake conditions with qualitative analysis. A) Synthetic scheme and\nphysical appearance of C10-MEA organic crystals (50 g scale). B) CO capture results in\n2\nammonium carbamate (blue) in the solid state to afford the maximum theoretical capacity of 2.5\nmmol/g in 2:1 amine: CO stoichiometry; crystals qualitatively transition from colorless vitreous\n2\nto chalky white appearance upon CO loading. C) SEM images of C10-MEA before (red) and after\n2\n(blue) CO exposure (100% CO balloon).\n2 2\nmoved from Figure 3C to 3D, while crystal structures were moved from 3D to 3C. New\nssNMR spectra were included in this figure. All important text fonts are increased and\nmade uniform to highlight important information in the figure.\nFig. 3 Spectroscopic analysis of C10-MEA with and without CO by A) in-situ powder X-ray\n2\ndiffraction pattern (λ = 0.6199 Å) of: C10-MEA CO absorption (left, red to blue, 100% CO , rt)\n2 2\nand C10-MEA-CO desorption (right, blue to red, 100% N , rt). B) ATR spectra of C10-MEA (red)\n2 2\nand C10-MEA-CO (blue, 100% CO balloon, rt). C) Crystal structures of C10-MEA before (left,\n2 2\nred) and after (right, blue) exposure to CO (100% CO balloon, rt). D) Solid-state 13C (left,\n2 2\naliphatic region) and 1H (middle) NMR single pulse spectra of C10-MEA with (blue) and without\n(red) 13CO at 13 °C, and 15N CP/MAS (right) NMR spectra of C10-MEA (red) and C10-MEA-\n2\nchanged to more distinguishable colors.\nFig. 4 CO capture and release performance of C10-MEA. A) CO Isotherms of C10-MEA at\n2 2\n0, 15, and 25 °C. B) CO uptake of C10-MEA at different CO concentrations (N balance); 90\n2 2 2\nmL/min flow, 30 °C. C) Desorption under 100% CO at elevated temperature (60 °C absorption\n2\nfor 90 min, 60-65 °C for desorption, both under 100 % CO , 90 mL/min flow. D) Cyclic thermal\n2\nand oxidative stability tests of C10-MEA (10% CO , 60 °C, red) and its CO capture (10% CO ,\n2 2 2\n26\n30 °C, blue) and desorption (100% air, 60 °C, yellow) performance under flue gas conditions (air\nas balance gas, 90 mL/flow), E) Tandem FTIR and TGA analysis on C10-MEA under humidity-\ncontrolled conditions at room temperature. Exposure to both dry and fully humid CO (blue and\n2\norange, respectively) affords an identical spectrum, and crystals maintain a chalky appearance,\nindicating no water inclusion. In contrast, exposure to fully humid N (grey) results in hydrogel\n2\nformation due to water uptake. F) Recyclability profile of C10-MEA at 40 °C, 90 mL/min flow in\nTGA; 200 consecutive CO capture (100% CO )/release (100% N ) cycles shown.\n2 2 2\nExtended data figure 1A:\n7. There are formatting errors and abbreviation mistakes in the citation of references,\nsuch as (2), (6), (13), (40), etc. Please have the authors carefully check and revise the\nmanuscript.\nResponse: Thank you for the comments. All typos were corrected.\n27"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors addressed the reviewer's comments/concerns, they performed a considerable number of new experiments to strengthen their claims, and revised typos and figures to improve readability and the overall clarity of the manuscript. The work can be accepted for publication in Nat. Commun.", "reviewer_2": "[Note from the editor: the reviewer also looked over the responses to reviewer 3, and felt that these had been addressed]\nThe authors have addressed all major and minor comments raised in the initial review, with substantial revisions to the manuscript, supporting information, and experimental data. The key concerns—including clarification of the \"chain melting\" mechanism, supplementation of comparative analysis, improvement of figure quality, and elaboration of structural- performance relationships—have been effectively resolved. The revised work demonstrates enhanced scientific rigor, clearer mechanistic insights, and more reliable data presentation, meeting the publication standards of the journal."}, "author_response": ""}]

Decision letter