Dynamic guanidinium sulfate salt for selective carbon dioxide adsorption with negative pressure inflexion

Domain: neuroscience
Published: 2026-02-11
Editorial outcome: major_revision

Abstract
Hydrogen bonds’ flexible distances and moderate strength entitle compounds to dynamic properties under external stimuli. Here we report multiple phase transitions and counter-intuitive CO2 adsorption behavior of dynamic guanidinium sulfate (GS) salt assembled via hydrogen-bonds. Exploration based on the energy landscape generated by crystal structure prediction (CSP) reveals three porous GS phases with stability of α > β > γ and the inverse order of porosity, agreeing with experimental results. Transformations among polymorphs via heating or compressing involve ion rearrangement. Adsorption isotherms of β-GS indicate that CO2 firstly enters the isolated cavities at a low gating pressure, and further increasing CO2 pressure leads to the continuous gas uptake but reduced pressure at a critical point and thus an unexpected negative pressure inflexion (NPI), followed by the final adsorption saturation. Theoretical calculations demonstrate that the NPI behavior stemmed from the GS structural transition from β to more porous γ-phase, with the γ-GS phase becoming more energy-favorable as CO2 uptake increases. Specific supramolecular interactions ensure CO2 selectivity and easy regeneration. With a CO2 uptake of 4.2 mmol g-1 (273 K, 100 kPa), GS salt exhibits great promise for CO2 capture and transport, demonstrating the potential of simple hydrogen-bonded salts as adaptive materials.

Introduction
Gas adsorption isotherms have long served as essential tools for characterizing porous materials. Their shapes are traditionally interpreted based on the static structural features of rigid adsorbents, including pore size distribution, specific surface area, and surface chemistry. Microporous materials bearing rigid pores display a typical type I adsorption isotherm1 (Fig. 1a). However, this conventional understanding becomes inadequate when applied to dynamic sorbents that undergo structural transformations upon guest molecule uptake. In such systems, adsorption behavior reflects not only pre-existing porosity but is also significantly governed by kinetic processes driven by adsorption-induced structural changes. These processes frequently lead to distinctive isotherm features such as multi-step profiles, pronounced hysteresis, and unusual inflection points, which often indicate selective gas uptake mediated through framework flexibility or phase transitions. Adsorbents having flexible, adaptive, or dynamic pores/channels or discrete cavities exhibit stepped or S-shaped adsorption isotherms (Fig. 1b), providing significant advantages for practical adsorption and separation owing to the increased working capacity2,3. The abrupt change in gas uptake reflected by a steep profile in the isotherm is a signature of a gating effect, which involves a structural transition of the adsorbent driven by guest adsorption. A counter-intuitive phenomenon of negative gas adsorption in a stepped isotherm on DUT-49 (Dresden University of Technology No. 49) has been identified4. The process involves pressure-induced structural deformation and pore contraction with gas desorption during pressure increase, resulting in pressure amplification in a microsystem (Fig. 1c). In contrast to Negative Gas Adsorption (NGA), we identify and define a distinct phenomenon, which we term Negative Pressure Intrusion (NPI), observed in a dynamic porous framework (Fig. 1d). The NPI event is initiated by a guest-induced structural transition that triggers a substantial and abrupt increase in the material’s gas adsorption capacity. Within a constant-volume system, this surge in uptake causes a net depletion of gas molecules from the free gas phase, resulting in a characteristic pressure decrease. Consequently, although both NGA and NPI produce anomalous steps in adsorption isotherms driven by structural transitions, they represent fundamentally opposite mechanisms. While the dynamic sorption properties of flexible metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) have been extensively studied, hydrogen-bonded frameworks (HOFs) are emerging as a promising alternative5–7. Certain HOFs exhibit permanent porosity, enabling applications in gas adsorption and separation8–10. However, the construction of robust HOFs remains challenging due to the weak nature of hydrogen bonds5,11. A key strategy to overcome this limitation involves incorporating electrostatic interactions through charge-assisted hydrogen bonds, leading to a subclass known as hydrogen–bonded ionic frameworks (HIFs). Comprising ionic building units, HIFs benefit from enhanced structural stability imparted by these stronger interactions, without compromising the intrinsic flexibility derived from the versatile geometry of hydrogen bonds12. This flexibility allows HIFs to undergo reversible structural reorganization in response to external stimuli such as pressure, heat, or solvent13–16. The dynamic behavior of HIFs has been demonstrated in several systems. For example, one HIF constructed from guanidinium cations and methyl borate anions undergoes a reversible collapse and reconstruction process to capture and release methanol molecules17. Similarly, another HIF based on [Co(NH3)6]3+ and SO42− ions shows reversible framework disassembly and reassembly, facilitating low-energy CO2 capture and release15.

Here, we report three phases of guanidinium sulfate (GS) as a model HIF system, designated α-, β-, and γ-GS based on their thermodynamic stability as revealed by the complete energy landscape from crystal structure prediction (CSP). Under mild conditions, GS shows great potential for CO2 capture, storage and transport, with an uptake capability of up to 4.2 mmol g−1 at 273 K and 100 kPa, an uptake that is competitive with leading MOFs and HOFs and other leading porous materials. Impressively, we observed that this exceptional performance is attributed to a multi-stepped CO2 adsorption process exhibited by β-GS, which displays an abnormal negative pressure inflection (NPI) phenomenon under near-ambient conditions. As demonstrated in Fig. 1d, β-GS is inactive toward CO2 adsorption at low pressures. With increasing CO2 pressure, the gas overcomes an energy barrier and infiltrates the isolated cavities of the GS structure until a critical pressure is reached. Subsequent CO2 dosing leads to continuous gas uptake accompanied by a pressure decrease in the test tube, resulting in a negative pressure inflection in the adsorption isotherm, followed by complete saturation and a phase transformation from β-GS to γ-GS (Fig. 1e). We note that apparent pressure inflections in the adsorption isotherms of some MOFs have been reported and mechanistically investigated. Building upon these fundamental insights, the present study provides a more comprehensive mechanistic interpretation6,18–23. The NPI adsorption in H-bonded salt and HOF materials has not been reported, to the best of our knowledge. Note that the CO2-induced crystallization of CO2@GS clathrate from GS aqueous solution in our previous work is a typical supramolecular assembly process that is similar to the formation of natural gas hydrates and hence inherently distinct from CO2 adsorption and diffusion in the solid salt24.

Results
Structure and porosity of α-GS
α-GS is directly prepared by neutralization reaction between commercially available guanidinium carboxylate and sulfuric acid and recrystallized from aqueous solution24. As the parent compound of guanidinium organosulfonate compounds25, α-GS is assembled through N–H···O H-bonds between Gua+ ions and SO42− ions26 (Supplementary Fig. 1a–c). Detailed structural analyses disclose that α-GS contains isolated cavities, which are situated in the ionic cages composed of SO42− ions and Gua+ ions connected by H-bonds (Supplementary Fig. 1d). With a probe radius of 1.4 Å, the cavity volume within a single unit cell was found to be 750.22 Å3, accounting for 13.4% of the unit cell volume. Thermogravimetric analysis (TGA) of α-GS manifests that there is no weight loss before its decomposition temperature at 545 K, indicating there is no crystalline water in α-GS (Supplementary Fig. 2). α-GS activated at 353 K displays negligible N2 adsorption at 77 K (Supplementary Fig. 3), but a reasonable CO2 adsorption of 0.58 mmol g−1 at 100 kPa and 273 K (Fig. 2a, equilibration time of 600 s), suggesting that α-GS can selectively adsorb carbon dioxide. The scanning electron microscope (SEM) image shows that the sizes of GS particles used for adsorption measurement are on the micrometer scale (Supplementary Fig. 4), ensuring that surface adsorption effects are excluded.

Abnormal CO2 adsorption isotherm of GS
Abnormal CO2 adsorption behavior with a pronounced hysteresis phenomenon was observed when α-GS is activated at 453 K under vacuum (Fig. 2a–c). For typical isotherm measurements of porous materials, equilibration conditions for each data point are set as pressure fluctuation lower than ±1% within 150 s. Upon extending the equilibrium time to 600 s, the CO2 adsorption amount is increased to 3.8 mmol g−1 (Fig. 2a). Unexpectedly, the amount of adsorbed CO2 continues to rise during the desorption stage, which suggests that equilibrium had not been reached by the end of adsorption at 100 kPa and the CO2 adsorption continues even at the lowering pressure during the desorption process. Specifically, during the initial desorption stage in the pressure range of 70–100 kPa, the adsorbed amount exceeds the value recorded at the end of the adsorption branch at the same pressure. This phenomenon was not observed in α-GS activated at 353 K under identical measurement conditions with a 600 s equilibrium time (Fig. 2a). We attribute this behavior to the sluggish kinetics of the structural transformation in the 453 K-activated sample. Subsequently, we further prolonged the equilibrium time to 1800–3600 s to ensure adsorption equilibrium (Fig. 2b, see experimental details in “Methods”). Then an odd multi-stepped adsorption isotherm with a negative pressure inflexion (NPI) was observed (Fig. 2b). We repeated measurements to exclude the experimental errors, and the phenomenon was reproducible (Supplementary Figs. 5, 6). After carefully analyzing the adsorption isotherm in the low-pressure range, we found that CO2 adsorption below 1 kPa was negligible, indicating a gating effect (gating point) (Supplementary Fig. 8b). Subsequently, CO2 uptake displayed an approximately linear relationship with pressure increasing up to 63 kPa at 273 K (critical point). Below the critical pressure, the adsorption data obtained with 600 s and 1800 s equilibration times were coincident (Supplementary Fig. 7). With continuous pressure increase, the NPI phenomenon emerged, accompanied by a sudden increase in CO2 uptake but a sharp pressure decrease inside the sample cell, showing an anomalous pressure drop to 45 kPa (NPI point), until final adsorption saturation is reached. The isotherm shows an obvious hysteresis loop, where desorption starts when the pressure drops below 50 kPa at 273 K (Supplementary Fig. 8a).

The CO2 adsorption isotherms for the sample of α-GS activated at 453 K at various temperatures were collected with an equilibrium time of 1800 s (Fig. 2c, Supplementary Figs. 8–12). Multi-stepped adsorption isotherms with NPI phenomena are observed at temperature ranging from 268 to 278 K (Fig. 2c). Once the temperature exceeds 278 K, prolonged equilibrium time setting does not change the sorption behavior of GS in the pressure range of 0 to 100 kPa and the desorption closely overlap the adsorption process, without hysteresis loop (Supplementary Fig. 13). We note that the shape of the adsorption isotherm before critical point at 273 K is similar to that at 298 K (Supplementary Fig. 14). To exclude the influence of adsorption equilibrium time, we further performed adsorption measurements on α-GS activated under 353 K with an extended equilibration period (Supplementary Fig. 15), which displayed a normal adsorption isotherm without gating effect and NPI phenomenon at pressures below 100 kPa at 273 K.

To elucidate the unconventional CO2 adsorption behavior, we conducted PXRD analyses for α-GS sample activated at 453 K and found a different crystalline phase (Supplementary Fig. 16). The in situ PXRD patterns during heating α-GS sample from 373 to 473 K also disclosed the phase transition starting from 393 K (Fig. 2d). The results are consistent with differential scanning calorimetry (DSC) data, which indicate a structural transformation initiating at 393 K and reverting at 261 K (Supplementary Fig. 17). Fortunately, the single crystals were obtained by dissolving α-GS powder in methanol and allowing the solution to stand undisturbed at room temperature for 7 days. The crystal structure was subsequently determined by single-crystal X-ray diffraction (Supplementary Table 2). The simulated PXRD patterns exactly overlap with the patterns obtained from thermal transformation of α-GS (Fig. 2e, Supplementary Fig. 18). This confirms that the same β-GS structure is produced via both solid-state thermal transformation and solution-phase recrystallization. Herein, we denote the new structure as the β-GS phase. In the β-GS structure, the SO42− anions are disordered over two equivalent positions, forming charge-assisted hydrogen bonds (Supplementary Fig. 19a, b) with surrounding Gua+ cations. The connectivity pattern generates a three-dimensional framework as illustrated in Supplementary Fig. 19c, d. Similar to α-GS, isolated cavities are also present in β-GS. The cavity volume within a single unit cell was determined to be 433.90 Å3 using a probe radius of 1.4 Å, accounting for 18.7% of the unit cell volume.

To further understand the adsorption and the associated structural transition mechanism, we collected the PXRD patterns before and after critical pressure (Supplementary Fig. 20) and identified another phase after the critical pressure, while the β-GS structure persisted below this threshold. We further performed in situ PXRD measurements over β-GS sample under CO2 pressure of 100 kPa at 278 K and observed the same new phase (Fig. 2f). Notably, the PXRD patterns of the new phase exhibited complete consistency with the one simulated from CO2@GS structure that was reported in our previous work24 (Supplementary Fig. 21). Thereafter, we named the new phase as γ-GS. However, efforts to grow CO2-free γ-GS single crystals were unsuccessful, both through optimizing crystallization conditions and activating CO2@γ-GS. Though we obtained the structural information of GS under different conditions, it is still challenging to understand the phase transition events among the GS phases, especially during the CO2 adsorption process; therefore, we resort to computational calculations to disclose the transition mechanism, as discussed as follows.

Exploration of the crystal structure energy landscape of GS
Multistep adsorption isotherms may arise from guest-induced polymorphic transitions27,28 (although other mechanisms, such as stepwise site filling, can produce similar features). Accordingly, we performed crystal structure prediction (CSP) to systematically map the high-dimensional potential energy surface and establish the relative stabilities of polymorphs, thereby providing mechanistic insight into the system. During the CSP process, hundreds of thousands of trial structures were generated and subsequently optimized geometrically using a force field (Supplementary Fig. 22). Duplicates were then removed, and the low-energy structures were selected for further optimization at the DFT level, alongside the single crystal structures obtained from experiments (Fig. 3, see details in “Methods”).

The CSP landscape for GS is markedly different from typical organic systems, because the global minimum (α-GS) is a porous crystal (Fig. 3a–c), even though the GS building unit itself does not possess an inherent intramolecular cavity, unlike classic cage-like or macrocyclic porous molecules. This landscape indicates that the α-GS crystal is the most thermodynamically stable polymorph. β-GS, which is 6.76 kJ mol−1 higher in energy than the global minimum, falls within the typical energy window for the appearance of polymorphs in experiments29. In the lower-density region of the energy landscape, γ-GS was identified with a relative energy of 18.82 kJ mol−1, and its backbone perfectly matched that of CO2@γ-GS (Fig. 3d, e). The high relative energy for such a small molecule system suggests that this crystal structure may be challenging to be accessed under normal experimental conditions. This may explain why pure γ-GS crystals have not yet been obtained in experiments. The high-dimensional energy surface, projected onto the structural parameter of surface area, is also shown in Fig. 3b. In this plot, it is clear that α-GS, β-GS, and γ-GS are all situated on the leading edge and appear on separate spikes, indicating that they reside in distinct energy basins30,31. All of them exhibit porous properties, as illustrated in Fig. 3c. Moreover, a densely packed structure with a density of 1.55 g cm−3 and a relative energy of 3.19 kJ mol−1 was also identified in the landscape, suggesting that this system may possess additional potential structures yet to be observed experimentally (Supplementary Fig. 23). CSP results reveal three GS phases with stability of α > β > γ and the inverse order of porosity, agreeing with our experimental data. However, CO2 adsorption can alter the relative energy ranking and induce a structural transition, which is associated with the abnormal NPI phenomenon observed in this system.

GS structural transformation
We further performed the density functional theory calculations to investigate the phase transition during the CO2 adsorption process. To eliminate the structural disorder in β-GS, several non-disordered crystal models were manually constructed, and the one with the lowest lattice energy after structural relaxation was selected for further analysis. For a meaningful comparison, the symmetry of γ-GS was constrained to the C4 1 22 1, giving it the same pore count and the same number of molecules per unit cell as β-GS (Supplementary Table 3). Models of both polymorphs were then constructed at a series of CO2 loadings, and their relaxed lattice energies were compared (Fig. 4a). In the guest-free state, β-GS is more stable than γ-GS, in agreement with the CSP result. However, the energy difference between CO2@β-GS and CO2@γ-GS diminishes rapidly with increasing CO2 uptake and inverts. The energy difference rises to 1.44 eV per unit cell when the pores of both polymorphs are fully saturated, providing the thermodynamic driving force for the observed phase transition during the CO2 adsorption process. These computational results support that γ-GS is a distinct thermodynamic phase, not merely a configurational variant of β-GS. γ-GS is more porous than β-GS as illustrated by CSP (cf. Specific surface area: 787.6 versus 585.4 m2 g−1, and void volume: 0.02145 versus 0.01595 mm3 g−1), therefore, the transition from β- to γ-GS at critical pressure point gives rise to the sharp CO2 adsorption and sudden pressure drop in the test tube, namely NPI (negative pressure inflexion) event in adsorption isotherm as observed in Fig. 2b, c.

As shown in Fig. 3c, void space analysis indicates that β-GS contains isolated, unconnected pores. While CO2 adsorption appears limited from a static perspective due to restricted guest diffusion, a dynamic view is required to capture the full behavior. Modeling diffusion-driven polymorphic transitions remains challenging, as such rare events occur on timescales beyond those accessible to conventional Molecular dynamics (MD) simulations. To explore potential pore-breathing behavior, MD simulations were performed on pure β-GS. Although no continuous channels were observed using a probe radius of 1.65 Å, transient channels appeared between occluded sites when a smaller probe radius of 1.2 Å was used, temporarily connecting adjacent voids (Fig. 4b). These pore-breathing events are attributed to the flexibility of the hydrogen-bond network32 (Supplementary Fig. 24). The complete CO2-induced phase transformation from β-GS to γ-GS was experimentally observed to require over 5 h (Supplementary Fig. 25)—a time scale notably longer than the 1800 s equilibration criterion employed in our adsorption measurements. Importantly, this extended transformation time does not conflict with the established equilibrium setting, where the system was considered to reach equilibrium only when the adsorbed amount varied within ±1% over 1800 s. The observed slow kinetics originates from the coupled effects of restricted diffusion through transient channels and the rate-limiting solid-state structural rearrangement, with the phase transformation representing the primary kinetic bottleneck.

According to the above experimental data, we could delineate the phase transformation map of guanidinium sulfate under external stimuli. As shown in Fig. 4c, α-GS is considered to be the thermostable phase, as revealed from the theocratical energy landscape. β-GS can be prepared via dissolving α-GS powder in methanol or obtained by heating α-GS to a temperature higher than 393 K, and β-GS is reversibly converted into α-GS via cooling at a temperature lower than 263 K. γ-GS, as a high-energy phase, can’t be produced in its pure phase under ambient conditions. Instead, γ-GS could exist as CO2 clathrate (CO2@γ-GS), via CO2 adsorption in β-GS beyond a critical pressure or directly CO2-induced crystallization from GS aqueous solution, because the specific supramolecular interaction between CO2 and guanidinium ions stabilize the γ-GS phase24.

GS for CO2 capture and storage
Although CO2@γ-GS readily releases CO2 at ambient conditions, the decomposition process is retarded by the presence of decomposed GS on the surface. The TG analysis of CO2@γ-GS showed a 17% weight loss prior to 353 K, ascribed to CO2 release (Supplementary Fig. 26). This aligns with the theoretical weight densities of CO2 in CO2@γ-GS (17 wt%), and agrees with the saturated CO2 adsorption amount (4.2 mmol g−1). In contrast to desorbing CO2 from a 5–10 wt% organic-amine aqueous solution—a process plagued by the high thermal energy demand of heating water (due to its high heat capacity)—direct heating of the GS solid proves a significantly more energy-efficient way for the adsorbent’s regeneration33. When a sealed reactor containing β-GS solid is charged with CO2 to 100 kPa at 273 K, a pressure decrease from CO2 adsorption is observed immediately, with an approximate equilibrium time of 60 min. Upon heating the reactor at 373 K, CO2 is released, and the pressure recovers. The pressure exceeds the initial value owing to the heating effect (Supplementary Fig. 27a). In the control experiment using NaCl replacing the β-GS solid, there is no observed pressure drop and heating at 373 K results in a pressure increase at the same scale raised from the heating effect. The results reveal that CO2 uptake and release are highly reversible over multiple cycles. Ten cycles of uptake/release display no performance decay (Supplementary Fig. 27b) and PXRD patterns of the β-GS adsorbent after the cycling tests align with those of the pristine β-GS (Supplementary Fig. 27c), confirming the stability of the β-GS.

A challenge associated with conventional gating effect in porous materials is the difficulty in preventing other gas species to enter the pores following the pore-opening triggered by the target molecule33,34. β-GS exhibits a unique selectivity for CO2 even in the presence of high-concentration N2. When we subjected GS to a mixture gas of 70% N2 and 30% CO2 at 330 kPa (CO2 partial pressure of 100 kPa) and 273 K, there was negligible N2 adsorption under the experimental conditions detected by gas chromatography (GC) (See details in “Methods”). It is worth noting that CO2 adsorption by GS differs from physisorption in porous materials, including flexible MOFs, as physisorption typically doesn’t change the connections between the frameworks35,36. And the adsorption is clearly not a chemisorption process, because there is no formation of new chemical bonds. Instead, it follows a supramolecular adsorption mechanism via the formation of clathrate compounds. In the process, CO2 uptake needs to overcome the energy barrier driven by pressure stimuli for lattice rearrangement of the GS crystal.

Methods
Materials
All chemicals and reagents were purchased from commercial suppliers and used without further purification. Guanidinium carbonate (Gua2CO3) was purchased from Shanghai Adamas Reagent Co., Ltd. Ethanol and sulfuric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. Nitrogen gas (99.999%), carbon dioxide (99.9%) and carbon dioxide-nitrogen mixture gas (30 mol% CO2 + 70 mol% N2) were purchased from Nanjing Special Gas Plant Co., Ltd.

Characterizations
Thermogravimetric analyses (TGA) were performed in N2 atmosphere on a TGA Q500 integration thermal analyzer. Powder X-ray diffraction (PXRD) measurement was carried out on a Rigaku MiniFlex 600 X-ray diffractometer using Cu Kα radiation (λ=1.54178 Å). Differential Scanning Calorimetry (DSC) tests were conducted with DSC Q2000 V24.10 (Module: DSC Standard Cell RC). The gas chromatography (GC) was conducted on a Shimadzu GC-2014 gas chromatograph equipped with a CBP-PSN column and a TCD detector. Ar gas (99.999%) was used as the carrier gas. Scanning electron microscopy was carried out with a field emission scanning electron microanalyzer (Gemini SEM 500).

Synthesis of α-guanidinium sulfate (α-GS)
α-GS was synthesized according to the procedures as reported in previous work24. 18.0 g Gua2CO3 (0.1 mol) was dissolved in 25 mL deionized water, and then dropwise added 5.4 mL concentrated H2SO4 (98%, 0.1 mol) under stirring. The pH of the resulting solution was then adjusted to 7 with dilute H2SO4. In order to precipitate the product, absolute ethanol was added as an anti-solvent under constant stirring. Then the colorless powder was collected by vacuum filtration, washed with absolute ethanol, and dried at 373 K in air. Yield was calculated to be 99% based on Gua2CO3.

Synthesis of β-GS single crystals (high-temperature stable phase of guanidine sulfate)
Colorless crystals were obtained through methanol diffusion (5 mL) into α-GS powder (0.18 g, 1 mmol) with subsequent static crystallization for 7 days at room temperature.

CO2 sorption measurements
Prior to adsorption measurements, the bulk powder was thoroughly ground and ball-milled to achieve a submicron particle size, thereby minimizing the impact of intracrystalline diffusion limitations on the adsorption kinetics. An activation procedure was then conducted, wherein α-GS was activated at 353 K for 8 h, while β-GS was treated at 453 K for 8 h. To avoid exposure to air during sample treatment, an additional in situ vacuum activation at a set temperature for another 2 h was performed at the site of the sample port in the adsorption instrument. Gas sorption isotherms were measured on a BEL Sorp-max instrument (BEL, Japan; detailed apparatus and mechanism illustrated in Supplementary Fig. 28). N2 sorption isotherm was measured at 77 K, the equilibration conditions for each data point were set at 1% pressure fluctuance within 150 s. It was observed that the conventional equilibrium conditions did not reach equilibrium for CO2 adsorption. Hence, the equilibrium time was extended for 600–3600 s. CO2 sorption isotherms were measured at 298 K, with the equilibration conditions for each point set at 1% pressure fluctuance within 1800 s. Furthermore, our experiments indicate that for adsorption behavior prior to the inflection point, variations in equilibrium time have a negligible effect. Therefore, different equilibrium conditions can be applied across distinct pressure ranges to optimize data collection time, as adsorption-induced structural transitions require longer equilibration time. A preliminary study was conducted to approximate the inflection pressure range before the final experiments.

For CO2 sorption isotherms at 268 K, the equilibration conditions in the range of 0.001–40 kPa were set at 1% pressure change within 600 s, while in the range of 40–100 kPa the equilibration time was set for 1800 s. For CO2 sorption isotherms at 270 K, the equilibration conditions in the range of 0.001–40 kPa were set at 1% pressure change within 600 s, while in the range of 40–100 kPa the equilibration time was set for 1800 s. For CO2 sorption isotherms at 273 K, the equilibration conditions in the range of 0.001–55 kPa were set at 1% pressure change within 600 s, while in the range of 55–100 kPa the equilibration time was set for 1800 s. For CO2 sorption isotherms at 275 K, the equilibration conditions in the range of 0.001-60 kPa were set at 1% pressure change within 600 s, while in the range of 60–100 kPa the equilibration time was set for 1800 s. For CO2 sorption isotherms at 278 K, the equilibration conditions in the range of 0.001–75 kPa were set at 1% pressure change within 600 s, while in the range of 75–100 kPa the equilibration time was set for 1800 s.

Single-crystal X-ray diffraction (SCXRD) measurements
Single-crystal X-ray diffraction data were collected by a Rigaku Oxford Diffraction Super Nova diffractometer using Cu-Kα radiation (λ=1.54178 Å) at 298 K. The data collection and processing were carried out with CrysAlisPro software. The crystal structure was solved by direct methods and refined by full-matrix least squares based on F2 using a SHELXTL 14 XL program package. Hydrogen atoms were fixed geometrically at their positions and allowed to ride on parent atoms. Quantification of electron count within pores was accomplished through the SQUEEZE method37 on the refinement model of the vacant framework. Details of structural analyses were summarized in Table S2. The cavity size of GS was determined using the Mercury software with a probe radius of 1.4 Å.

Variable-temperature powder X-ray diffraction measurement on α-GS
Variable-temperature powder X-ray diffraction (VT-PXRD) measurements were performed on finely ground and dried α-GS samples using a Rigaku MiniFlex 600 X-ray diffractometer equipped with an in-situ temperature control stage. The temperature protocol was set as follows: heating from 298 K to 473 K through incremental steps (373, 383, 403, 423, 448 and 473 K) at 10 °C min−1, with 3 min isothermal holds before data collection at each temperature.

In situ powder X-ray diffraction (PXRD) patterns monitoring the phase transition from β-GS to CO2@γ-GS under 100 kPa CO2 at varying exposure times
The α-GS powder was heated at 453 K under dynamic vacuum for 1 h to induce a complete phase transition to β-GS, as confirmed by PXRD. The sample was then cooled to 278 K. Under a static CO2 pressure of 100 kPa at 278 K, time-dependent PXRD patterns were collected at 0, 5, 10, 20, 30, 60, 110, 140 min to monitor the structure transformation, using a Rigaku MiniFlex 600 X-ray diffractometer equipped with a pressure control system.

PXRD of β-GS before and after the inflection point of the adsorption isotherm at 273 K
When the adsorption pressure reaches 40 kPa (or 70 kPa), encapsulate the adsorption tube under the protection of an inert atmosphere. Then, prepare the test samples in a glove box. After that, encapsulate the silicon wafer with a transparent plastic bag that does not affect the diffraction intensity and conduct the test immediately.

Isochoric adsorption-desorption experiments
An autoclave (30 mL) was charged with β-GS powder (1.0 g). The autoclave temperature was kept at 273 K using a cooling jacket. After the temperature was steady, the gas within the autoclave is evacuated using a vacuum pump and filled with 100 kPa CO2. The pressure inside the autoclave was monitored using a digital pressure meter (5 s per data point). After the adsorption was finished, as indicated by the inner pressure getting steady. The autoclave was placed in a 373 K oil bath for CO2 release. To illustrate that CO2 was successfully adsorbed and released, the same experiment was carried out with the same volume of NaCl instead of β-GS powder as a control experiment. Ten runs were conducted to evaluate the cyclic performance of CO2 adsorption. After the final cycle, β-GS powder was heated at 373 K for 10 min, and the PXRD patterns were collected to confirm the phase stability.

Selective adsorption of CO2 in mixture gas
The selective adsorption measurement was conducted in a 250 mL autoclave charged with 0.50 g of pre-activated β-GS powder and maintained at 273 K using a cooling jacket. Prior to measurements, the system dead volume was calibrated using helium expansion. After temperature stabilization, the autoclave was evacuated and pressurized to 330.0 kPa with a certified gas mixture containing 70.0 mol% N2 and 30.0 mol% CO2, corresponding to a CO2 partial pressure of 99.0 kPa. The adsorption process continued for 24 h, with blank experiments confirming negligible background adsorption or system leakage.

Following adsorption, the solid adsorbent was transferred under argon atmosphere to a quartz bottle, where the atmosphere was replaced through five argon flush-purge cycles. The adsorbed gases were liberated by adding 5.0 mL deionized water to the solid sample under argon flow. Gas analysis was performed using a Shimadzu GC-2014 chromatograph equipped with a 5Å molecular sieve column (2 m × 1/8 inch) and a thermal conductivity detector, using argon as carrier gas. The system calibration established a detection limit of 200 ppm for N2. Analysis of the released gas from β-GS samples showed N2 concentrations below this detection limit, confirming the highly selective adsorption of CO2 over N2.

Validation of TGA mass loss
The theoretical mass loss is calculated as:
ωCO2 = n × MCO2 / (Mhost + n × MCO2)

host = γ-GS, n = the number of CO2 molecules per structural unit (formula unit) (here, n = 4).

Computational method
Force field level CSP. The CSP process based on the force field was performed using the open source code mol-CSPY38. All geometries were optimized using Gaussian 16 software at B3LYP/6-31G(d,p) level of theory39, followed by frequency calculations to ensure the true local minima of the optimized structure. These molecular geometries were held rigid throughout force field level crystal structure generation and lattice energy minimization. Trial crystal structures were generated with one guanidinium sulfate salt (CN3H6+·CN3H6+·SO42−) in the asymmetric unit for the following space group and generation number of trial structure: P1 (10000), P-1 (10000), P21 (20000), C2 (20000), Cc (20000), P21/c (50000), C2/c (50000), P212121 (10000), Pna21 (20000). In the mol-CSPY CSP, a quasi-random sampling method was used that all structural variables within each space group were generated from a Sobol vector: unit cell lengths and angles, and molecular positions and orientations within the asymmetric unit. Space-group symmetry was then applied, and a geometric test was performed for overlap between molecules, which was removed by lattice expansion. DMACRYS40 software was used to calculate the lattice energy with an anisotropic atom–atom potential. The atomic multipole of the molecular charge distribution41 based on B3LYP/6-31G(d,p) level charge density was used to model the electrostatic interactions. Atom–atom repulsion and dispersion interactions were modeled using a revised Williams42 intermolecular potential. Charge–charge, charge–dipole and dipole–dipole interactions were calculated using Ewald summation. Removal of duplicate structures was performed with the following method: all structures within a lattice energy window of 1.0 kJ mol−1 and within a density window of ±0.05 g cm−3 were compared using PXRD patterns generated by Platon43 using a constrained dynamic time-warping method as implemented in the mol-CSPY.

DFT level calculations. After the force-field-level geometry optimization, structures within an energy window of 100 kJ mol−1 above the global minimum were taken out. Duplicate structures were removed again when a 15/15 packing similarity was found within a 0.2 Å distance tolerance and a 20° angle tolerance using the CCDC API. Finally, the 2015 crystal structures, together with the two known polymorphs α-GS and β-GS, were recalculated with the DFT method in VASP44. Projector augmented-wave (PAW) method45 was applied to describe the electron–ion interactions. Generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) exchange correlation functional was adopted to treat electron interaction energy46. Grimme’s semi-empirical DFT-D3 scheme with Becke–Johnson damping functions was used here to give a better description of interactions47. A kinetic-energy cut-off of 600 eV was used to define the plane-wave basis set. Electronic Brillouin zone was integrated with the smallest allowed spacing between k-points (KSPACING) being 0.3/Å, and the generated grid is centered at the Gamma-point. Convergence threshold of self-consistency was set to 10−6 eV during total energy and force calculation, and the Hellmann–Feynman force convergence criterion on each atom was set to smaller than 0.01 eV Å−1 when geometry optimizations are needed.

Pore-geometry analysis. Topological analysis of the pore space within a crystal structure was performed using the void analysis tool zeo++48. The pore volume as well as pore dimension were used in this research. Probe radius of both 1.2 Å as well as 1.65 Å was used in calculations as described in the manuscript.

Molecular dynamic method. MD simulations were performed using the Atomic Simulation Environment (ASE) package49. The machine learning force field SevenNet-050 was used to evaluate the energy and force during the MD process. An NPT MD simulation at 273 K with a 1 fs timestep was applied, using a Nosé–Hoover thermostat51 with a damping parameter of 50 timesteps and a Parrinello–Rahman barostat with a damping parameter of 75 timesteps.

Disorder structure treat. During the theoretical calculation, the disordered crystal structure β-GS was treated to remove the disordered part by the following method. Several different initial structures, each removing a different disordered part, were constructed and kept at the highest symmetry possible. Then a DFT-level geometry optimization was performed on these structures. The crystal structure with the lowest relative lattice energy was selected to represent the non-disordered β-GS for the remaining calculations.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "1. Figure 1 presents the negative pressure inflection point adsorption isotherm and structural changes in the adsorbent with\nincreasing gas pressure. Please provide corresponding characterization data to substantiate these findings.\n2. The setting of adsorption equilibrium time lacks a unified standard: Different equilibrium times (600 s vs. 1800 s) were\nused across distinct pressure ranges without adequate justification, potentially compromising the comparability and\nreproducibility of the isotherms.\n3. Insufficient verification of NPI phenomenon reproducibility: Although repeated experiments are mentioned, statistical error\nor standard deviation from multiple independent trials is not provided, making reliability assessment difficult.\n4. Incomplete characterization of the β-GS to γ-GS structural transition: The “CO₂-free” γ-GS structure was not experimentally\nobtained, relying solely on computational models without structural validation.\n5. Inadequate experimental design for selective adsorption: When using GC to detect N₂ adsorption, considerations for dead\nvolume, background adsorption, or detection limits were not specified. The conclusion of “no N₂ adsorption” requires more\nrigorous data support.\n6. Neglect of solvation effects in CSP energy landscapes: Calculations did not account for the influence of methanol or water\nmolecules on structural stability, potentially compromising the accuracy of polymorphic energy ordering.\n7. Unclear force field applicability in MD simulations: The machine learning force field (SevenNet-0) used has not been\nvalidated for this system.\n8. Absence of CO₂ adsorption or binding energy calculations: Lack of quantitative analysis of interaction energies between\nCO₂ and GS hinders explanation of adsorption selectivity and mechanisms.\n9. Ambiguous explanation of NPI mechanism: While attributed to the β→γ phase transition, the reason for decreasing rather\nthan increasing pressure remains unclear, with insufficient comparison to the negative adsorption mechanism of DUT-49.\n10. The concept of “supramolecular adsorption” is poorly defined: It is neither physical nor chemical adsorption, yet its\nthermodynamic or kinetic characteristics remain unclear, lacking a basis for classification.\n11. The effect of initial water saturation on phase transitions is not discussed: The differing preparation methods for α-GS\nand β-GS (aqueous solution vs. methanol) may influence their adsorption behavior.\n12. No discussion on particle size effects on kinetics: While nanoscale particles accelerate adsorption, no quantitative\nanalysis addresses diffusion limitations in existing micron-scale particles.\n13. Insufficient PXRD pattern comparisons: Key diffraction peaks remain unlabeled in experimental vs. simulated pattern\ncomparisons (Fig. 2d, 2e).\n14. Capacity decay data for CO₂ adsorption–desorption cycles is not provided: While 10 cycles without decay are\nmentioned, no specific data or graphs support this claim.\n15. TGA data is not directly correlated with adsorption capacity: The relationship between the 17% weight loss of CO₂@γ-\nGS and the corresponding 4.2 mmol/g adsorption capacity is not explicitly calculated or presented.\n16. The concepts of “negative pressure inflection point” and “negative adsorption” are easily confused: Clearly distinguish\nbetween NPI (pressure drop) and NGA (negative gas adsorption) to avoid misleading readers.\n17. Some conclusions are overly absolute: Expressions like “fully reversible” and “no N₂ adsorption” should be qualified with\nconditions or supported by data. Additionally, pressure drop diagrams and computational processes corresponding to the\ngas chromatography detection results mentioned in the text during nitrogen–carbon dioxide separation should be provided.\n18. Insufficient discussion of comparisons with existing literature: Inadequate comparisons with DUT-49, flexible MOFs, etc.,\nlacking positioning within the class of materials.", "reviewer_2": "Authors report a CO2 inclusion complex based on guanidinium sulfate (GS) and investigate the variation in crystal phases to\nprovide new insights into the mechanism of selective CO2 capture using GS. The manuscript is carefully prepared and\ncontains substantial detail. I recommend the following revisions before it can be considered for publication:\n1. The introduction would benefit from a concise perspective on the relevance of the GS–CO₂ complex to carbon capture\nand storage (CCS) applications. The authors should also clearly delineate how the present findings advance beyond their\nprevious work on CO₂@GS clathrates. In addition, I recommend minimizing subjective descriptors (e.g., “impressive,”\n“remarkable”) so that readers may form their own assessments of the results.\n2. The statement “We note that during the desorption process, the adsorbed amount is even higher than that at the point of p/p₀ = 1” is confusing. Do the authors mean that more CO₂ uptake was recorded during desorption than during adsorption at\nequivalent pressures? If so, a mechanistic explanation is required, as this appears counterintuitive.\n3. The isotherm data suggest that the system may not reach equilibrium within the 10-minute equilibration period used. For\nreliable hysteretic measurements, the equilibration time should be extended to at least 10 minutes per point across the full\npressure range, as also implied in the Supplementary Information (Fig. S14). Otherwise, the reported negative pressure\ninflection (NPI) may reflect non-equilibrium effects rather than intrinsic material behavior. The discussion should address this\nissue more explicitly.\n4. The formation of the β-GS phase is crucial for interpreting the clathrate-like adsorption mechanism. The authors should\nelaborate on how hydrogen bonding rearrangements influence phase selection. Furthermore, it is not clear that γ-GS\nrepresents a distinct crystalline phase. It could instead be interpreted as a CO₂-disordered β-GS, stabilized only under CO₂\nloading conditions. Clarification of this point, supported by structural evidence, would strengthen the conclusions.", "reviewer_3": "In „Dynamic guanidinium sulfate salt for selective carbon dioxide adsorption with negative pressure inflexion” Zhao et al.\ndemonstrate a molecular H-bonded solid-state material capable of taking up CO2 with an uncommon kinetically controlled\nadsorption profile the authors term negative inflexion point. They investigate the phase transitions caused by temperature\nchanges and CO2 uptake in detail and discuss the origin of the underlying adsorption-induced structural transitions.\nThe phenomenon of a “negative inflexion point” has to date been reported in several papers (some of which not yet cited by\nthe authors) but to date lacks little investigation, in particular with respect to the kinetic origin. When first examining the work\nand read the abstract I was hopeful that the authors would address this underexamined phenomenon more fundamentally\nand I remain at least partly excited by the extend of the work in its current state. However, the adsorption analysis in\nparticular with respect to a more extended view on the underlying kinetics is limited. I also think the paper could be\ndiscussed a bit more by comparing the molecular nature of the GS against true framework materials such as MOFs or COFs.\nThat being said I think the work describes an exciting phenomenon in a not-so-typical molecular material on a critical\nprocess (CO2 capture) and also already contains a large body of experimental work. In addition, I find the ESF maps and\nrelated analysis quite interesting and a nice addition to the work. Also, the DFT and MD data helps to explain the ongoing\nthermodynamic and kinetic processes to a certain degree. In its current state I find the wording and in parts description and\ndiscussion difficult to understand (see comments below) and think this manuscript needs heavy editing of language before\nthe nice experimental results can be well represented and understood. As such I think the paper is generally suitable for\npublication in Nature Communications after major revisions and the authors should consider the following\ncomments to make improve this work.\nThe authors start the paper by “The shape of the adsorption isotherm is closely related to the structural feature of the\nadsorbents.” – Following with 3 paragraphs of how this is not the case in dynamic sorbents. This seems like a strange way\nto start the paper. In addition, I think its both structural and “chemical” features (adsorption strength, preferred adsorption\nsites) as well as adsorbate-adsorbent interactions that dictate the isotherm shape. I suggest the authors introduce in the area\nof kinetically dominated adsorption processes and demonstrate that exotic isotherm shapes can be generated if the\nmaterials exhibit adsorption-induced transitions. I think the kinetic nature of these phenomena should be better elaborated in\nthe intro and set in more context to related materials and the mechanisms derived for them.\nIn general language needs to be more precise. E.g. “The abrupt change of isotherms caused by guest uptake…” should read\nmore like: The abrupt change in gas uptake reflected by a steep profile in the isotherm.\nThe authors write one paragraph on introducing H-bonded frameworks and defining acronyms such as (DHA) without ever\nmentioning them again in the manuscript. At least in this part of the intro the connection to GS should be established.\n“We note that during the desorption process, the adsorbed amount is even higher than that at the point of p/p0 = 1“ – I am not\nsure what is meant with p/p0 = 1 given that all isotherms in Fig. 2a-c are labeled in absolute pressure scale and that p/p0 = 1\nfor CO2 at 273 K would be 3471 kPa which is clearly beyond the pressure range investigated in this work. When looking at\nFigure 2a I think the authors might refer to an increase in uptake during desorption in the range of 100-70 kPa. The authors\nassign the to “sluggish adsorption dynamics” – which to me is an unclear description and might be related to “sluggish\nadsorption kinetics”? In fact this sentence is picked up at a later stage so the above statement is likely redundant.\nThe authors state: We note a few metal-organic frameworks (MOFs) also show specious pressure inflexion in multi-stepped\nadsorption isotherms, in despite of lack of interpretation6,15-17 – I am not sure what they mean with “despite lack of\ninterpretation” as some of these works (in particular ref 16) provide a very detailed and fundamental analysis at least on par\nwith the currently presented study. In addition a review article from 2020 specifically illustrates the thermodynamic and\nkinetic aspects of this phenomenon in flexible MOFs https://onlinelibrary.wiley.com/doi/full/10.1002/anie.202004535\nI also don’t understand what is meant with “specious pressure inflexion in multi-stepped adsorption isotherms” some of these\nisotherms are single stepped and I don’t see how the steps reported in these references are “specious” or does this relate to\nporosity? There are also a few papers that feature such NPI slopes, in particular earlier work that could be cited if deemed\nrelevant:\nKaskel and co-workers https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201702357\nKitagawa and coworkers https://pubs.acs.org/doi/10.1021/ja0561439\nIt is not a surprise that the authors don’t see the effect at 298 K simply because they do not probe a high enough pressure\nregion. At 273 K the critical point is at p/p0 of ca. 0.018. The P0 of CO2 at 298 K is ca 6400 kPa which means that the critical\npoint (if occurring close to the same relative pressure point which is quite common for such dynamic systems) is to be\nexpected at 116 kPa which is out of the range of the low pressure adsorption analysis. I suggest the authors should probe\nthis effect at higher pressures to truly see if there is a thermal effect involved or if they simply do not record the relevant\npressure region with their method applied.\nCan the authors explain their rational why the temperature-induced structural transition is comparable to the recrystallization\nin methanol - “… new single crystals were obtained by dissolving α-GS powder in methanol and allowing the solution to\nstand undisturbed at room temperature for 7 days” – How is the recrystallization in methanol related to a thermally induced\nchange in structure?\n“The simulated PXRD patterns exactly overlaps with the patterns obtained from thermal transformation of α-GS” – I suggest\nthe authors perform at least Pawley if not even Riteveld analysis of the PXRD patterns to match against the MeOH\nrecrystallized structure β-GS. A pure visual comparison seems not appropriate here.\nIf the authors have access to in situ PXRD during CO2 adsorption (Figure 2f) why do they perform the complex ex situ PXRD\nanalysis described in ESI and FigS 18? I don’t fully understand the nature and origin of this experiment.\nI am missing a discussion on the kinetics of the structural transformation in figure 2f. Clearly even after 140 min PXRD\nmeasurement time the sample is still changing its structure, illustrating the extremely long equilibration time. These 8400 s\nare at least twice as long as the longest equilibration time in the adsorption analysis (3600 s) so it comes to no surprise that\nthe isotherms suffer from kinetic phenomena. I recommend to refine the obtained PXRD patterns and include a plot that\nillustrates the evolution of unit cell dimensions with time.\nI do not understand this statement: “As multiple adsorption isotherms are typically associated with polymorphism and\nstructural transformations21,22“ – should this read multiple steps in adsorption isotherms? Also polymorphism does not\ndescribe structural transformations but rather the presence of a certain chemical composition in different phases.\n“The CSP landscape for GS shows markedly different to typical organic system as the energy global\nminimal structure of α-GS shows a porous property (Fig. 3a-c), while the component is obvious non-porous.” – but it is porous? Again, I can not\nfollow what this statement means. Further up the text the authors state “Detailed structural analyses disclose α-GS contains\nisolated cavities, which are situated in the ionic cages composed of SO42- ions and Gua+ ions connected by H-bonds\n(Supplementary Fig. 1d). With a probe radius of 1.4 Å, the cavity volume within a single unit cell was found to be 750.22 Å3,\naccounting for 13.4% of the unit cell volume.“ – So why state that it is non-porous or does this refer to accessible porosity\nprobed by CO2 adsorption?\n“It is worth noting that CO2 adsorption by GS differs from physisorption in porous materials, including flexible MOFs, as\nphysisorption typically doesn't change the connections between the frameworks29,30“ – This statement is generally true\nalthough there are examples bond rearrangements or changes on coordination sites in MOFs upon CO2 adsorption.\nHowever, in the current state one could also argue – The authors further argue: “And the adsorption is clearly not a\nchemisorption process, because there is no formation of new chemical bonds. Instead, it resembles as a supramolecular\nadsorption process, as new clathrate compound is formed. In the process, CO2 uptake need overcome the energy barrier\ndriven by pressure stimuli for lattice rearrangement of GS crystal.” – I think this is a good summary of the process observed\nand reported here. It would be great to see the authors describe how the different processes could be usefull in different\nscenarios. E.g. the slow kinetics might also be usefull in certain CO2 capture scenarios.\nI suggest that the computational analysis (input files, structural data, etc) are made available so other can replicate analysis\nof these materials. Can the structure file or at least cartesian coordiantes of γ-GS (CSP) be added as ESI?\nI currently fail to see a data availability statement for this and strongly recommend that raw data be made available. In\naddition, adsorption isotherms might be added in form of the adsorption information file for others to compare.", "reviewer_author_response": "Response to Reviewers\nManuscript ID:NCOMMS-25-66884-T\nTitle:Dynamic guanidinium sulfate salt for selective carbon dioxide adsorption with negative\npressure inflexion\nReviewer #1\nWe thank the Reviewer for their thorough review and valuable suggestions, which have helped us\nto significantly improve the manuscript. We have addressed all comments point-by-point below.\nComment 1:\n“Figure 1 presents the negative pressure inflection point adsorption isotherm and structural\nchanges... Please provide corresponding characterization data to substantiate these findings.”"}, "author_response": ""}, {"round": 2, "reviewer_comments": {"reviewer_1": "The manuscript presents a comprehensive set of data, simulations, and analysis supporting the concept of a guanidinium\nsulfate salt for selective carbon dioxide adsorption featuring a negative pressure inflexion. This work represents a significant\ntechnological contribution to the field of carbon management. It is recommended for acceptance after minor linguistic and\ngraphical optimizations.\nThe following points should be addressed by the authors to enhance the clarity of the presentation and potentially contribute\nto the interpretation of the observations:\n1.Further improve the resolution of the figures and the font size/readability of labels within the figures.\n2.Provide more explicit annotations in Figures 1d and 1e to clarify the structural states corresponding to each stage of the\nprocess, facilitating reader comprehension.\n3.In Figure 2, the data points for adsorption isotherms under different activation temperatures and equilibration times are\ndensely clustered. It is suggested to use sub-figures or inset magnified views to highlight the details of pressure and uptake\nchanges around the NPI inflection point, thereby improving the readability of the graphs.\n4.Although MD simulations indicate \"pore-breathing\" behavior in β-GS, the kinetic pathway of the CO₂-induced structural\ntransformation could be further discussed. A brief commentary in the main text on whether potential intermediate states or\ncooperative rearrangement processes exist is recommended.\n5.Compare the CO₂ adsorption performance (e.g., uptake capacity, selectivity, regeneration conditions) of the GS material\nwith existing technologies such as MOFs, HOFs, and hydrates. This would better highlight the advantages and limitations of\nthe GS material and clarify its positioning for practical applications. Furthermore, discuss the specific mechanisms and\nanticipated effects of the proposed nano-sizing or confinement strategies (mentioned in the text) on enhancing adsorption\nkinetics.\n6.The text notes that the NPI phenomenon could enable pressure reduction in microsystems. It is suggested to briefly\nexplore potential application scenarios for this characteristic, such as in micro-scale gas separation devices or pressure-\nregulation components, thereby broadening the applied dimensions of the research.", "reviewer_2": "the revisions adequately addresses all my comments", "reviewer_3": "I thank the authors for their extensive improvements of the manuscript. Most of my comments have been addressed\nadequately and I think the manuscript is in a much better state now. There are a few aspects I would like to further discuss:\nThe temperature-dependent appearance of the NPI effect is not fully clarified in my opinion.\nAs the authors outline and as it is common understanding the NPI effect is of pure kinetic origin. The authors state:\n\"We appreciate the reviewer's valuable perspective. Our analysis confirms that the absence of NPI at\nthe temperature exceeds 278 K, prolonged equilibrium time setting does not change the\nsorption behavior... without hysteresis loop (Supplementary Fig. 13).\" - Now this statement implies that NPI is expected for\nextended equilibration times. It would be interesting to probe the appearance of NPI at higher temperatures with SHORTER\nequilibration times.\nThe Pawley fit presented now only sows a small section of the PXRD pattern. was this pattern only recorded up to 35°\n2Theta?\nI am glad that the authors \"We agree that the adsorption isotherms in this region are\ninfluenced by these kinetic phenomena.\" - I would argue its the whole point of the phenomenon described. As such there is\nstill to me a lack of kinetic data provided from the adsorption analysis. The excel sheets provided as additional data lack any\nlabelling of units or axis. Furthermore No metadata or isotherm info is provided which would be easy to do by using the AIF\nplatform: https://adsorptioninformationformat.com/\nI also fail to see any kinetic data represented in ESI or manuscript that goes beyond the sinde NPI feature. As far as I can tell\nthe ESI contains no raw data but depicted isotherms and for a true analysis of this underlying effect - which is the critical\naspect of the paper - such data would be required.\nlastly, I encourage the authors to upload inpout files for the computaional analysis beyond the pure\ncoordinates of the relaxed structures."}, "author_response": "Response to Reviewers\nManuscript ID:NCOMMS-25-66884-A\nTitle:Dynamic guanidinium sulfate salt for selective carbon dioxide adsorption with negative\npressure inflexion\nReviewer #1\nWe sincerely thank the reviewer for their positive assessment of our work and for recognizing its\nsignificance in the field of carbon management. We are particularly grateful for the endorsement of\nour comprehensive dataset and the validation of the negative pressure inflection concept. Following\nthe reviewer's recommendation, we have carefully reviewed the manuscript to implement linguistic\nrefinements and have optimized all figures to ensure maximum clarity. We believe these final\nimprovements further strengthen the presentation of our work and appreciate the reviewer's\nsupportive comments throughout the review process.\nComment 1:\nFurther improve the resolution of the figures and the font size/readability of labels within the figures.\nResponse:\nWe thank the reviewer for this valuable suggestion. In response, we have thoroughly revised all\nfigures in the main text and Supplementary Information to significantly improve their quality and\nclarity.\nComment 2:\nProvide more explicit annotations in Figures 1d and 1e to clarify the structural states corresponding\nto each stage of the process, facilitating reader comprehension.\nResponse:\nWe thank the reviewer for this constructive suggestion. In response, we have revised Figures 1d and\n1e to include more explicit.\nComment 3:\nIn Figure 2, the data points for adsorption isotherms under different activation temperatures and equilibration times are\ndensely clustered. It is suggested to use sub-figures or inset magnified views to highlight the details of pressure and uptake\nchanges around the NPI inflection point, thereby improving the readability of the graphs.\nResponse:\nWe thank the reviewer for this suggestion. We appreciate the reviewer's attention to the detailed\npresentation of the isotherm data. Indeed, to ensure the clarity of the key features from the beginning,\nwe had previously provided, in the Supplementary Information, dedicated and enlarged figures that\nspecifically focus on the NPI critical region and the low-pressure gate-opening area. These figures\noffer a clear and magnified view of the subtle changes in pressure and uptake around these critical\npoints, complementing the comprehensive view presented in the main text Figure 2. We are grateful\nthat the reviewer has highlighted this point, confirming the importance of such detailed\nvisualizations for reader comprehension.\nFigure S8. (a) CO adsorption-desorption isotherm of α-GS (activated at 453 K) measured at 273\nAmplification of low-pressure area of adsorption isotherms (cell pressure in logarithmic\ncoordinates), with a red star denoting the “Gating point”.\nComment 3:\nAlthough MD simulations indicate \"pore-breathing\" behavior in β-GS, the kinetic pathway of the\nCO₂-induced structural transformation could be further discussed. A brief commentary in the main\ntext on whether potential intermediate states or cooperative rearrangement processes exist is\nrecommended.\nResponse:\nWe thank the reviewer for this insightful suggestion regarding the kinetic pathway of the CO₂-\ninduced structural transformation.\nThe structural evidence clearly demonstrates that γ-GS represents a distinct polymorph from β-GS,\ncharacterized by different crystal systems (orthorhombic versus tetragonal), unit cell parameters,\nand hydrogen-bonding configurations (Supplementary Fig. 25, Table S2). The absence of structural\nsimilarity between these phases, confirmed by packing-similarity analysis even after removal of\nCO₂ guests, indicates that the transformation necessitates substantial reconstruction of the hydrogen-\nbond network.\nThe remarkably slow transformation kinetics observed experimentally, suggests that this hydrogen-\nbond rearrangement occurs through a complex pathway. While our current data cannot identify\nspecific intermediate states, the extended transformation timescale implies the existence of\nsignificant energy barriers associated with the cooperative reorganization of the framework. The\nmolecular dynamics simulations revealing pore-breathing behavior through transient channels (Fig.\n4b) further support the dynamic nature of the hydrogen-bond network that enables such structural\nrearrangement.\nFigure. S25. H-bond modes of polymorph of GS: (a) α-GS, (b) β-GS and (c) γ-GS.\nComment 5:\nCompare the CO adsorption performance (e.g., uptake capacity, selectivity, regeneration conditions)\n2\nof the GS material with existing technologies such as MOFs, HOFs, and hydrates. This would better\nhighlight the advantages and limitations of the GS material and clarify its positioning for practical\napplications. Furthermore, discuss the specific mechanisms and anticipated effects of the proposed\nnano-sizing or confinement strategies (mentioned in the text) on enhancing adsorption kinetics.\nResponse:\nWe sincerely thank the reviewer for these valuable suggestions regarding performance comparison\nand kinetic enhancement strategies. In response to these comments, we have revised our manuscript\nto provide a more comprehensive contextualization of our work.\nSpecifically, in the performance evaluation section, we have now explicitly stated: \"GS shows great\npotential for CO capture, storage and transport, with an uptake capability of up to 4.2 mmol\n2\ng⁻¹ at 273 K and 100 kPa, an uptake that is competitive with leading MOFs and HOFs and\nother leading porous materials.\" This statement is further supported by a new comparative table\nin the Supplementary Information that systematically benchmarks the CO uptake capacity of GS\n2\nagainst representative HOF materials.\nFurthermore, in the Conclusion section, we have strengthened the positioning of GS as a hydrogen-\nbonded framework salt by stating: \"Owing to its high CO uptake, facile regeneration under\n2\nmild conditions, combined with the intrinsic advantages of a solid salt such as abundance, low\ncost, and high stability, GS demonstrates great potential for practical CO capture, storage,\n2\nand transport.\" It clearly highlights GS's competitive adsorption performance under mild\nconditions, complemented by its low-energy regeneration requirements and cycling stability.\nRegarding the adsorption kinetics, the observed slow CO adsorption kinetics in GS primarily stems\n2\nfrom the rate-limiting solid-state phase transformation process, complemented by restricted gas\ndiffusion through transient channels and we plan to systematically validate the efficacy of these\nstrategies in enhancing adsorption kinetics in future work. We have removed the concluding\nsentence regarding nanosizing and confinement strategies from the main text. While we maintain\nthat these approaches represent promising directions for future research, we recognize that their\ninclusion in the conclusion without sufficient experimental validation could be perceived as\nspeculative. The revised conclusion now focuses more appropriately on the key experimental\nfindings and fundamental mechanistic insights established in this work, while leaving the\nexploration of kinetic enhancement strategies for future investigations. This modification ensures\nour claims remain well-supported by the presented data and maintains the appropriate scope of the\ncurrent study.\nWe believe these revisions have significantly strengthened our manuscript by providing a clearer\npositioning of GS among existing adsorbents and offering a mechanistic perspective on future\nkinetic optimization, directly addressing the reviewer's insightful comments.\nFigure S29. CO uptake performance of GS at 273 K and 1 bar for this work and previously reported\n2\na structural transition from β to γ-phase.\nTable S4. CO uptake (273 K, 1 bar) performance comparison of some previous reported\nCompound CO uptake (mmol g-1) Reference\nHOF-FJU-8a (278 K) 1.65 19\nHOF-NBDA (278 K) 2.99 19\nPFC-12 1.67 19\nHOF-BTB-NH 3.17 19\nCo-HOF 2.60 20\nNi-HOF 2.60 20\nHOF - FJU - 88 3.06 21\nHOF-6a 1.09 22\nSOF-1a 4.05 23\nTTBI 3.62 24\nJZU-SOF2 4.25 25\nHOF-3a 1.47 26\nHOF-5a 5.22 27\nZJU-HOF-1 4.91 28\nGS 4.20 This work\nComment 6:\nThe text notes that the NPI phenomenon could enable pressure reduction in microsystems. It is\nsuggested to briefly explore potential application scenarios for this characteristic, such as in micro-\nscale gas separation devices or pressure-regulation components, thereby broadening the applied\ndimensions of the research.\nResponse:\nWe thank the reviewer for this excellent suggestion to broaden the applied dimensions of our\nresearch. In response, we have incorporated a discussion of potential application scenarios into the\nmain text. The relevant section now reads: \"Distinct from negative gas adsorption (NGA) which\namplifies pressure, the NPI phenomenon enables active pressure reduction, suggesting\npromising applications in microscale gas regulation, such as in integratedmicro-gas\nseparatorsorself-regulating microfluidic components.\" We believe this addition significantly\nstrengthens the impact of our work by highlighting the practical utility of the NPI effect.\nReviewer #2\nWe sincerely thank the reviewer for their thoughtful comments and valuable suggestions throughout\nthe review process. We are pleased to hear that our revisions have adequately addressed all the raised\nconcerns. We appreciate the reviewer's expertise and insights that have helped strengthen the\nscientific rigor and clarity of our work.\nReviewer #3\nWe sincerely thank the reviewer for their positive assessment of our manuscript revisions and for\nacknowledging the substantial improvements made. We appreciate the reviewer's constructive\nengagement throughout this process, which has undoubtedly strengthened our work. We have\ncarefully considered the remaining points raised by the reviewer and have provided detailed\nresponses below. We are committed to addressing these final aspects thoroughly and believe these\nadditional clarifications will further enhance the quality and impact of the manuscript.\nComment 1:\nThe temperature-dependent appearance of the NPI effect is not fully clarified in my opinion.\nAs the authors outline and as it is common understanding the NPI effect is of pure kinetic origin. The\nauthors state:\n\"We appreciate the reviewer's valuable perspective. Our analysis confirms that the absence of NPI\nat 298 K is a true thermal effect, not a pressure range limitation. As we noted in the manuscript,\n\"once the temperature exceeds 278 K, prolonged equilibrium time setting does not change the\nsorption behavior... without hysteresis loop (Supplementary Fig. 13).\" - Now this statement implies\nthat NPI is expected for extended equilibration times. It would be interesting to probe the appearance\nof NPI at higher temperatures with SHORTER equilibration times.\nResponse:\nWe appreciate the reviewer's valuable perspective. Our analysis confirms that the absence of the\nNegative Pressure Inflection (NPI) at 298 K reflects a fundamental change in the phase transition\nmechanism under different thermodynamic conditions.\nAs established in our manuscript, the NPI phenomenon observed at lower temperatures results from\na guest-induced structural transition that abruptly increases the material's adsorption capacity within\na constant-volume system, causing a characteristic pressure decrease. At 298 K, the transformation\nmechanism shifts to what resembles a pressure-gated phase transition. While this transition\nmaintains the same β- to γ-GS structural change, it occurs through a more gradual cooperative\nrearrangement that prevents the abrupt uptake surge necessary for NPI manifestation.\nNotably, the adsorption kinetics at 298 K remain remarkably slow, with the complete stepwise\ntransition requiring up to 10 hours to reach equilibrium at the transition pressure of approximately\n3 bar. This extended transformation time, observed even in the absence of NPI, confirms that the\nsolid-state phase transformation itself represents the rate-determining step.\nFurthermore, the significant increase in transition pressure with temperature from tens of kilopascals\nat lower temperatures to about 3 bar at 298 K aligns with thermodynamic expectations for this\nexothermic adsorption-induced phase transition, consistent with Clausius-Clapeyron relationships.\nIn summary, temperature governs the transition pathway: at lower temperatures, it proceeds through\na discontinuous transformation exhibiting NPI; at 298 K, it occurs via a pressure-gated mechanism\nthat maintains the slow transformation kinetics but without the characteristic pressure decrease.\nThe CO adsorption isotherm measured at 298 K with 1800 s equilibrium time from 0-6 bar.\nPressure (bar) Uptake (cm³/g STP) Uptake (wt%)\nduration (H:m)\n0.08608269 1.9285 0.378574201 0:40\n0.41985897 8.867 1.740636474 0:45\n0.93853558 17.962 3.52603049 0:48\n1.43888685 25.3715 4.98055242 0:48\n1.96129811 32.2453 6.32991376 0:48\n2.61426 40.0515 7.862309885 0:55\n2.90468177 79.5746 15.62089221 10:00\n3.05360413 88.5285 17.37858759 4:08\n3.07675437 89.5254 17.57428404 0:37\n3.18095362 92.0312 18.06618512 1:30\n3.28209092 93.9768 18.44811614 1:07\n3.39591677 95.8952 18.82470766 1:10\n3.51194631 97.0122 19.04398035 0:40\nComment 2:\nThe Pawley fit presented now only sows a small section of the PXRD pattern. was this pattern only\nrecorded up to 35° 2Theta?\nResponse:\nWe thank the reviewer for this comment. The full PXRD patterns was indeed collected from\n5° to 40° 2θ. The Pawley refinement plot in the manuscript presents the range up to 35° 2θ, as\nthis region contains characteristic diffraction peaks of the GS structure.\nFigure. Experimental PXRD patterns of β-GS with the simulated patterns.\nComment 3:\nI am glad that the authors \"We agree that the adsorption isotherms in this region are\ninfluenced by these kinetic phenomena.\" - I would argue its the whole point of the phenomenon\ndescribed. As such there is still to me a lack of kinetic data provided from the adsorption analysis.\nThe excel sheets provided as additional data lack any labelling of units or axis. Furthermore No\nmetadata or isotherm info is provided which would be easy to do by using the AIF\nplatform: https://adsorptioninformationformat.com/\nI also fail to see any kinetic data represented in ESI or manuscript that goes beyond the sinde NPI feature. As far as I can tell\nthe ESI contains no raw data but depicted isotherms and for a true analysis of this underlying effect - which is the critical\naspect of the paper - such data would be required.\nlastly, I encourage the authors to upload inpout files for the computaional analysis beyond the pure\ncoordinates of the relaxed structures.\nResponse:\nWe sincerely thank the reviewer for these critical comments, which have helped us substantially\nimprove the quality and reproducibility of our work. We fully agree that kinetic analysis is\nfundamental to understanding the NPI phenomenon, and we have now addressed this aspect.\nIn response to the reviewer's valid concerns about kinetic data, we have now included detailed time-\ndependent adsorption measurements in the Supplementary Information, which directly capture the\nprogression of CO uptake over the >5-hour phase transformation period. The experimentally\n2\nobserved complete phase transformation time exceeding 5 hours does not conflict with the 1800 s\nequilibrium setting used in the adsorption measurements. where the system was considered to reach\nequilibrium only when the adsorbed amount varied within ±1% over 1800 s. These two distinct time\nscales reflect different aspects of the adsorption process: the 1800-second equilibrium criterion\nensures reliable data collection at each pressure point, while the transformation period exceeding 5\nhours characterizes the complete kinetic profile of the solid-state phase transition. Our experimental\nkinetic data, combined with molecular dynamics simulations, demonstrate that the remarkably slow\nadsorption kinetics primarily originates from the solid-state phase transformation process. This\nprocess involves initial restricted diffusion through transient channels, followed by the phase\ntransformation itself acting as the rate-determining step. This multi-stage kinetic pathway provides\na coherent explanation for the NPI phenomenon: the extended induction period corresponds to the\nslow initial stage of phase transformation, while the subsequent pressure drop marks the rapid\npropagation of the phase transition once initiated. Further investigation of adsorption kinetics and\nits correlation with the phase transformation mechanism represents an important direction for future\nresearch. \"The complete CO₂-induced phase transformation from β-GS to γ-GS was\nexperimentally observed to require over 5 hours (Supplementary Fig. 24)—a time scale\nnotably longer than the 1800 s equilibration criterion employed in our adsorption\nmeasurements. Importantly, this extended transformation time does not conflict with the\nestablished equilibrium setting, where the system was considered to reach equilibrium only\nwhen the adsorbed amount varied within ±1% over 1800 s. The observed slow kinetics\noriginates from the coupled effects of restricted diffusion through transient channels and the\nrate-limiting solid-state structural rearrangement, with the phase transformation\nrepresenting the primary kinetic bottleneck.\"\nRegarding data presentation, we have now standardized and uploaded source data (adsorption\nsource data in excel and raw aif data).\nFigure S24. Cell pressure vs. equilibrium time during CO adsorption at 273 K, with corresponding\n2\nsegments of the adsorption isotherm. Time-dependent pressure profiles are shown for two discrete\nequilibrium points within the isotherm region."}, {"round": 3, "reviewer_comments": {"reviewer_1": "1 You should carefully check. for exmple,(e) in Fig 1 Should be（f）. Figure 1. The relationship between the adsorption\nisotherm and the adsorbent structure. (e) Negative pressure inflexion adsorption isotherm and structural changes of\nadsorbent with increase gas pressure.", "reviewer_3": "I thank the authors for addressing the points raised and further improving the manuscript, but also substantially providing raw\ndata. I think the manuscript is in a good state and can be published in its current state. However, I would like to take the\nchance to further address the aspect of equilibration times: the authors discuss the mechanism as predominantly caused by\nthe phase transition which without a doubt is the origin for the hysteretic isotherm. An aspect that to date finds little attention\nin this work and the community in general is both the aspect of molecular transport and the effect of crystal/particle/domain\nsize. Both will have intrinsic impact on the isotherm and its underlying kinetic profile. I furthermore suggest to move towards\ndynamic sorption systems in the future given that such analysis provides a better control over transport processes. On the\npositive side: there is lots of more aspects to explore in this and related systems and I wish the authors all the best in their\nendeavour."}, "author_response": "Response to Reviewers\nManuscript ID:NCOMMS-25-66884-B\nTitle:Dynamic guanidinium sulfate salt for selective carbon dioxide adsorption with negative\npressure inflexion\nReviewer #1\nComment: You should carefully check. for example, (e) in Fig 1 Should be (f). Figure 1. The\nrelationship between the adsorption isotherm and the adsorbent structure. (e) Negative pressure\ninflexion adsorption isotherm and structural changes of adsorbent with increase gas pressure.\nResponse: We sincerely apologize for this labeling error. We have checked Figure 1 and corrected\nthe label from (e) to (f) as suggested, and carefully re-examined the whole main text. The\ncorresponding caption and the text in the manuscript have also been updated to ensure consistency.\nReviewer #3\nComment: I think the manuscript is in a good state and can be published in its current state.\nHowever, I would like to take the chance to further address the aspect of equilibration times... An\naspect that to date finds little attention in this work and the community in general is both the aspect\nof molecular transport and the effect of crystal/particle/domain\nsize. Both will have intrinsic impact on the isotherm and its underlying kinetic profile. I furthermore suggest to move towards\ndynamic sorption systems in the future given that such analysis provides a better control over transport processes.\nResponse: We would like to express our sincere appreciation to the reviewer for the positive\nevaluation and the insightful comments regarding the adsorption kinetics. We agree that the\nequilibration time, molecular transport, and the influence of crystal/particle/domain size are\npotential factors that deserve deeper investigation. While the current study focuses primarily on the\nphase transition mechanism and the resulting hysteretic isotherms, we are also very interested in the\nunderlying kinetic processes. Your suggestion to move towards dynamic sorption systems and\ninvestigate transport processes is highly valuable. We plan to systematically explore these kinetic\nfactors and their impact on the adsorption profiles in our future research. Thank you again for these\nconstructive suggestions, which provide a clear and important direction for our next steps."}]

Decision letter