Crystallization-assisted water adsorption in amorphous molecular adsorbents

Domain: chemistry
Published: 2026-02-23
Editorial outcome: minor_revision

Abstract
Efficient and stable water adsorbents are essential for removing moisture from natural gas and olefins during their production. Industrial desiccants such as alumina and zeolites require high regeneration temperatures, while metal-organic frameworks often suffer from limited long-term stability and reusability. Here, we introduce a new type of molecular desiccants (M-PyC) that, in principle, can be reused indefinitely. These simple molecular coordination complexes undergo fully reversible phase transitions between crystalline and amorphous states through the decoordination (bond breaking) and recoordination (bond reforming) of water molecules. They exhibit high water uptake (30 wt%) and superb selectivity, excluding hydrocarbons entirely. Their effectiveness for dehydration, combined with low regeneration temperature, fast desorption kinetics, low-cost and green synthesis, easy scalability to kilogram quantities, and essentially unlimited recyclability, make them truly competitive dehydration agents for industrial separations. The use of amorphous molecular adsorbents, where crystallinity and porosity are no longer stringent requirements, and the adsorption-desorption process entirely under ambient air, offers a conceptually different approach to adsorption-based separation science and potentially realistic solution to the long-standing challenge of sustained stability in coordinate-bonded adsorbents.

Introduction
Water adsorption plays a crucial role in numerous industrial processes, particularly in dehydration of gases. Efficient removal of water from gas streams is crucial to prevent corrosion, equipment malfunction, and the formation of gas hydrate. The petrochemical industry, a cornerstone of global energy supply and chemical production, processes critical gases such as natural gas (NG), olefins, and paraffins. During production, NG is typically saturated with water vapor from underground reservoirs, while olefin/paraffin steams often contain water vapor introduced during methanol-based conversion processes. In both vapor and liquid forms, water presents significant challenges—causing corrosion (especially in the presence of acid gases) and forming methane hydrates that can clog pipelines. Therefore, effective dehydration is essential to ensure compliance with industrial standards and specifications.

Adsorptive dehydration using solid adsorbents offers a desirable, low-cost, and energy-efficient solution, with the potential to achieve dehydration and/or purification in a single operation. An ideal water sorbent should: (i) exhibit high water sorption capacity, (ii) maintain long-term stability and recyclability across multiple water adsorption-desorption cycles, (iii) regenerate at low temperatures, and (iv) possess highly competitive water sorption over other gases. In industrial settings, dehydration is typically conducted via temperature swing adsorption, with water adsorption occurring at ambient temperatures (20–35 °C) and regeneration at elevated temperatures. Conventional adsorbents such as molecular sieves (e.g., zeolites 4A, 5A, and 13X) are widely used in these processes. However, they require energy-intensive regeneration (e.g., heating above 200 °C) to maintain sufficient working capacity between cycles. Metal-organic frameworks (MOFs), as a new type of crystalline sorbents, have shown promise for water-adsorption-related applications. Nevertheless, due to the nature of their inherent coordination bonds, most MOFs exhibit only moderate or poor hydrothermal stability. Even the most stable MOFs reported to date will inevitably suffer from slow decomposition or degradation over time, especially under repeated exposure to elevated temperatures (typically >100 °C) during reactivation cycles for a long duration (months to years). This deficiency severely impedes their practical use in real-world applications requiring thousands or even millions of adsorption-desorption cycles. Moreover, MOF synthesis often requires expensive ligands and toxic organic solvents, which severely limit their large-scale and cost-effective production at industrial scale. In addition, some MOFs, despite of high water uptake, also adsorb other molecular species, hampering their selectivity.

In conventional porous adsorbents, dehydration of industry gas mixtures is typically achieved by physisorption and/or weak chemisorption on pore surfaces. In contrast, water adsorption can also occur in nonporous or nearly nonporous compounds. A notable example is hygroscopic salts, such as CaCl2, which absorb large amounts of water into its bulk via hydration. However, these materials tend to deliquesce over time, leading to desiccant leakage and subsequent corrosion of the dehydration unit. They also require relatively high temperatures (>175 °C) for full regeneration.

Herein, we present a unique type of zero-dimensional (0D) molecular compounds, 0D-M(PyC)2(H2O)4 (abbreviated as M-PyC, where M = Mn, Co, Ni, Zn; PyC = 4-pyridinecarboxylic acid), that can efficiently address these limitations of existing desiccants. These molecular complexes undergo fully reversible and rapid water decoordination (desorption or activation) and recoordination (adsorption) processes, accompanied with a structural transformation between their crystalline and amorphous states. This transformation fundamentally differs from the “breathing” or “gating” transitions typically observed in flexible MOFs, such as MIL-53 and DUT-8(Ni), during water-induced sorption which involve two crystalline phases with the same framework connectivity. In addition, the activated amorphous a-M(PyC)2 compounds (denoted as a-M(PyC)2, where "a" stands for amorphous) do not rely on pre-existing pores, reflecting their essentially nonporous nature. All M-PyC compounds feature a three-dimensional (3D) hydrogen-bonded network consisting of M(PyC)2(H2O)4 moieties interconnected by hydrogen bonds. Upon heating at 90–120 °C in ambient air, they lose terminal water molecules and transition into an amorphous state. Remarkably, upon exposure to water (liquid or vapor), they regain crystallinity and adsorb large quantities of water (27–30 wt%). Due to their nearly nonporous nature, the a-M(PyC)2 compounds do not adsorb NG or any hydrocarbon (HC) gases, effectively acting as ideal molecular sieves for these species. Furthermore, since water desorption involves a crystalline-to-amorphous phase transition, it can be achieved simply by rapid heating in air, without the need for any additional treatment or reactivation steps under inert atmosphere—as required for most other adsorbents—making the process economically highly viable. Most significantly, the amorphous nature of these molecular sorbents eliminates the stringent need to maintain crystallinity, which constrains the longevity of crystalline adsorbents such as MOFs and zeolites. As a result, such water adsorbents offer potentially unlimited reusability, paving the way for their practical deployment in real-world dehydration processes.

Results
Synthesis and crystal structure
Conventional synthesis of sorbent materials is usually conducted under demanding conditions, such as the use of organic templates, high temperatures, agitation and calcination for zeolites, expensive, high boiling-point and often toxic organic solvents, and sometimes large quantities of modulators for MOFs. These processes are often complex and environmentally hazardous, making scale-up production challenging and requiring multistep activation procedures. In contrast, in this study, we designed a simple, one-step, easily scalable reaction in aqueous solution at room temperature and ambient pressure, using inexpensive metal salts and ligand to yield a series of pure and highly crystalline M-PyC molecular compounds.

The crystal structures of M-PyC were determined by single-crystal X-ray diffraction (SCXRD) analysis. M-PyC (M = Mn, Ni, Zn) were found to crystallize in the triclinic crystal system, space group P-1 (Supplementary Tables 1–3) with the molecular formula M(PyC)2(H2O)4. As shown in Fig. 2a, each metal center (e.g., Ni) is coordinated to two PyC ligands at the axial position via nitrogen atoms and to four water molecules at the equatorial positions. The hydrogen atoms in the coordinated water molecules further form hydrogen bonds with the oxygen atoms of the carboxylate group from adjacent PyC ligands, generating a 3D hydrogen-bonded network with an average hydrogen bond distance of 1.85 Å (Fig. 2b–c and Supplementary Figs. 1 and 2). The calculated hydrogen bond energies are 0.865 eV (highest) for Ni-PyC, 0.859 eV (slightly lower) for Mn-PyC, and 0.832 eV (lowest) for Zn-PyC (Supplementary Notes), which are consistent with their corresponding decomposition temperatures: 115 °C (highest) for Ni-PyC, 100 °C (slightly lower) for Mn-PyC, and 90 °C (lowest) for Zn-PyC (Supplementary Figs. 3–6). Furthermore, the cohesive energies per M-PyC molecule were determined to be 7.480 eV (highest) for Ni-PyC, 7.433 eV (slightly lower) for Mn-PyC, and 7.210 eV (lowest) for Zn-PyC (Supplementary Notes), reflecting contributions from the combined effects of strong hydrogen bonding and weak van der Waals interactions between the stacked ligand (PyC) rings.

Water-induced reversible structure transformation and competitive water adsorption
Upon activation, coordinated water molecules were removed from M-PyC, accompanied by the complete loss of hydrogen bonds. As a result, the 3D hydrogen-bonded network collapsed, and the structure decomposed into an amorphous form. Powder X-ray diffraction (PXRD) analysis confirmed that the activated samples, a-M(PyC)2, lost their crystallinity entirely (Fig. 3a and Supplementary Figs. 7–9), indicating a crystalline-to-amorphous transformation during activation. On the other hand, upon re-adsorption of water, all a-M(PyC)2 samples fully reverted to their crystalline phases, yielding PXRD patterns that match perfectly with the as-made M-PyC structures. This structural transformation, associated with water removal (dehydration) and re-coordination (rehydration), is not only fully reversible but can in principle be repeated indefinitely. Thermogravimetric analysis (TGA) confirmed that the weight loss around 90–120 °C corresponds to the release of four equivalents of water molecules (Supplementary Figs. 3–6). Additionally, porosity analysis revealed that the M-PyC compounds are nearly inaccessible to nitrogen gas. The Brunauer-Emmett-Teller (BET) surface areas, estimated from nitrogen adsorption data collected at 77 K on activated a-M(PyC)2 samples, are all below 5 m2/g (Supplementary Fig. 10), verifying their essentially nonporous nature.

We compared the water adsorption performance of a-M(PyC)2 with several commercial benchmark water adsorbents (alumina, zeolite 4A, 5A, and 13X) and two top-performing MOF-based water adsorbents, MOF-801 and CAU-10 (CAU, Christian-Albrechts-University). The water adsorption isotherm of a-Ni(PyC)2 exhibits a gradual increase at low pressures, followed by a pronounced inflection as the pressure reaches ~16 mbar (50% of RH), boosting the water uptake to its saturated capacity of nearly 300 mg g−1 (30 wt%) at 25 °C (Fig. 3b). The desorption branch of the isotherms clearly shows that water molecules remain undesorbed during the desorption process, suggesting their chemisorptive nature. Other isostructural a-M(PyC)2 compounds exhibit very similar water adsorption behavior (Supplementary Fig. 11). The water uptake capacity of a-M(PyC)2 is well comparable to benchmark water adsorbents at room temperature (25 °C) (Supplementary Table 4 and Supplementary Figs. 12–14): 318 ± 5 mg g−1 (alumina), 273 ± 7 mg g−1 (zeolite 4A), 206 ± 12 mg g−1 (zeolite 5A), 334 ± 2 mg g−1 (zeolite 13X), 359 ± 10 mg g−1 (MOF-801) and 368 ± 5 mg g−1 (CAU-10) but will well exceed all of them at higher temperatures as its uptake amount will remain more or less a constant while all other sorbents will inevitably undergo a significant decrease in water uptake due to their physisorption nature. For example, at 35 °C, a-Ni(PyC)2 takes up 290 mg g−1 of water (a decrease of ~3% compared to 25 °C), while the water uptake amounts for zeolite 4A and zeolite 13X are 253 mg g−1 and 300 mg g−1, respectively, counting a decrease of ~10% and ~24%. The adsorption kinetics study on a-Ni(PyC)2 further confirmed its maximum H2O adsorption rate of 0.32 mmol g−1 min−1: the saturation is reached (~16 mmol g−1) within 50 min at 95% RH and 60 min at 75% RH (Fig. 3c). Although its adsorption kinetics is slower than zeolites (Supplementary Fig. 16) due to its distinctly different adsorption mechanism involving chemical sorption of water via crystallization-assisted reconstruction of its hydrogen-bonded network, rather than through rapid physisorption as in zeolites, a-Ni(PyC)2 exhibits nearly identical adsorption kinetics under ambient air and completely excludes other gases and maintains its full capacity under these conditions, confirming that the process can be carried out with the same efficiency and performance without the need of protective atmosphere (Supplementary Fig. 15). On the contrary, zeolites suffer from partial pre-adsorption of ambient moisture and/or atmospheric impurities, resulting in up to a 30% loss in water uptake when operated in open air (Supplementary Figs. 24–25). We further investigated the effect of a-Ni(PyC)2 particle size on adsorption rate. We synthesized samples of smaller particle sizes by varying the precursor concentration and performing differential centrifugation. Samples with smaller particles exhibit enhanced adsorption rate under different relative humidities (Supplementary Fig. 17).

To investigate desorption kinetics, we measured desorption curves at various temperatures in open air (Fig. 3d and Supplementary Fig. 18). Complete water desorption required ~60 min at 90 °C. Raising the temperature to 120 °C reduced desorption time significantly and achieved full desorption within 12 min, corresponding to a maximum desorption rate of 1.34 mmol g−1 min−1. These regeneration temperatures (90–120 °C) are significantly lower than those required for zeolites (~200–300 °C, Supplementary Figs. 19–21). Moreover, Ni-PyC can be fully regenerated within a very short time (30 min) in open air without a protective atmosphere (Supplementary Figs. 22 and 23), highlighting its unique capability for facile and energy-efficient regeneration. In contrast, conventional adsorbents such as zeolites 4A and 13X require not only much higher temperatures and longer times for complete regeneration but also suffer from substantial losses in their uptake capacity (up to 30%) when exposed to open air. This reduction arises from their rapid adsorption of ambient moisture or atmospheric impurities, leading to partial pre-adsorption after regeneration in open air (Supplementary Figs. 24 and 25), thereby limiting their practicality under open-air conditions and necessitating a controlled or protective atmosphere.

For practical dehydration applications, an optimal adsorbent must simultaneously achieve high water uptake, rapid release/regeneration at low temperatures, and highly competitive water adsorption against HC molecules—specifically the main components of natural gas like methane (CH4) and important chemical feedstocks such as ethylene (C2H4) and propylene (C3H6). To assess competitive adsorption, we measured the adsorption isotherms of CH4, C2H4, C2H6, C3H6, and C3H8 on a-M(PyC)2. Water vapor, interacting strongly via M-OH2 bonds and, as described in detail above, can be readily accommodated, while all HC gases are effectively sieved out, showing negligible adsorption (Fig. 3e and Supplementary Fig. 26). Notably, activated a-M(PyC)2 compounds exhibit remarkably high uptake ratios of water vapor over HC gases, e.g., 138.7 for H2O/CH4, 51.1 for H2O/C2H4 and 40.4 for H2O/C3H6 at 25 °C, demonstrating distinct advantages to zeolites and alumina currently used in industry, as well as some of the best performing MOFs (Supplementary Figs. 27–35 and Supplementary Table 4). Differential scanning calorimeter (DSC) measurements confirmed the binding affinities, with adsorption heats of 49.40, 3.92, 6.30, and 7.03 kJ mol−1 for H2O, CH4, C2H4, and C3H6 on a-Ni(PyC)2, respectively (Fig. 3f), fully consistent with the observed selective adsorption behavior.

In situ probing of water adsorption-assisted recrystallization
To elucidate the relationship between the unique structure formation-deformation behavior associated with water adsorption-desorption in M-PyC compounds, we performed detailed structural analysis of a-Ni(PyC)2 by probing its recrystallization process as a function of time and humidity using humidity-dependent PXRD. As shown in Fig. 4a, at a high RH (90%), adsorption took place immediately as soon as exposing the activated sample to water vapor. The compound began to regain its crystallinity within a few minutes and completed water recoordination within 30 min. Decreasing the RH increased the time required to reach complete recrystallization (Fig. 4a and Supplementary Figs. 37–39). The recrystallization process was highly sensitive to the changes in humidity: an immediate response was observed when a sample initially placed under a low humidity (35% RH) was subsequently exposed to high humidity (90% RH) (Supplementary Fig. 40). Furthermore, in situ PXRD measurements using a humidity- and temperature-controlled cell were performed to monitor the reversible structure transformation between crystalline and amorphous states during water-induced sorption (Fig. 4b). Starting from the amorphous form (activated a-Ni(PyC)2), exposure to water vapor restored its crystalline hydrogen-bonded network, as indicated by the reappearance of characteristic peaks in the PXRD patterns collected at various time intervals. During the subsequent regeneration, the hydrogen-bonded network gradually decomposed upon heating, eventually losing all terminal water molecules and transforming into the amorphous phase, as evident by the disappearance of characteristic diffraction peaks in the PXRD patterns over a short time period. This recrystallization-decrystallization process was fully reversible and reproducible, as demonstrated by multiple consecutive cycles of water vapor adsorption-desorption (Supplementary Fig. 41).

To gain a deeper insight into the structural evolution of Ni-PyC during the amorphous-to-crystalline transformation, we conducted in situ IR spectroscopy on the as-made sample during activation and rehydration processes to investigate structures lacking long-range order. Vibrational bands were assigned by comparison with the PyC ligand (Supplementary Fig. 42). The results show that key pyridine vibrations (CC/CN stretches and CH deformations) were retained, while new bands at 1750–1300 cm−1 appeared in as-made Ni-PyC (Fig. 5, blue curve), indicating the presence of carboxylate (COO-) groups. A strong band at 1382 cm−1 corresponded to the symmetric COO- stretching mode, while the counterpart νas(COO-) band typically appeared above 1500 cm−1. Broad OH stretching bands at ~3405, 3265, and 2905 cm−1 reflected hydrogen-bonded water; notably, the largely red-shifted 2905 cm−1 band with respect to gas phase value (νas, 3756 and νs, 3657 cm−1) suggested strong hydrogen bonding. A wagging mode of H2O at 938 cm−1 further indicated its coordination of water molecules to Ni2+ via oxygen atoms.

Upon activation at 120 °C, these water-related bands disappeared, and the νs(COO-) band blue-shifted and broadened (Fig. 5, brown curve), overlapping with pyridine CC stretching at 1420 cm−1, reflecting a crystalline-to-amorphous transition. Rehydration by exposure to ~20 Torr water vapor gradually restored the broad water stretching bands above 2800 cm−1, along with the wagging band at 938 cm−1 (Fig. 5, dark-to-light-red curves), similar to the as-made structure. This suggests water insertion and binding to Ni2+ as bridging units. The νs(COO-) band gradually shifted back to 1382 cm-1, indicating that COO- groups now interacts with H2O through forming hydrogen-bond. The formation of hydrogen-bond is also clearly evidenced by the evolution of water stretching bands, e.g., red-shifting of the 3380 cm−1 band to 3374 cm−1 and gradual appearance of 2905 cm−1 band within 2 h. Spectral sharpening confirmed the restoration of crystallinity upon H2O insertion. Furthermore, the νas(COO-) band at 1630 cm−1 vanishes upon water adsorption, replaced by a new band at 1590 cm−1 (difference spectrum), suggesting a red shift of ~40 cm−1 upon rehydration. A final dehydration at 120 °C removed the inserted water again, reverting the spectrum to the amorphous state (Fig. 5, purple curve).

Dehydration under conditions mimicking industrial settings
To validate the practical use of M-PyC compounds for industrial dehydration, dynamic column breakthrough experiments were carried out on Ni-PyC for different HC gases under different humidity levels. The results are in excellent agreement with the adsorption strength trends determined from single-component gas (vapor) adsorption isotherms. In all cases, water was preferentially adsorbed onto the column packed with a-Ni(PyC)2, while HC gases including CH4, C2H4, and C3H6 were immediately eluted from the humid mixtures (Fig. 6). The H2O retention times for different HC gases were similar (19 h g−1 at 25% RH, 14 h g−1 at 50% RH, and 12 h g−1 at 75% RH). Regardless of the humidity level, CH4, C2H4, and C3H6 were excluded (no retention in the column) immediately, revealing a negligible adsorption of these gases and extremely competitive adsorption for water vapor. Remarkably, at RH values of 75%, 50%, and 25%, the experiment yielded 2.97 L g−1, 3.97 L g−1, and 5.92 L g−1 of methane, respectively, with water content below 1 ppm (0.0001%) in all three cases (Supplementary Figs. 43–45), well below the industrial standard for deep dehydration (<147 ppm). Similar productivities were also achieved for ethylene and propylene during dehydration (Supplementary Figs. 46–51). Additionally, humidity-dependent methane breakthrough experiments confirmed the immediate elution of methane from the a-Ni(PyC)2 column (Supplementary Fig. 52), in contrast to the varying retention behaviors observed for other zeolites (e.g., 0.5 h g−1 of methane retention for zeolite 4A and 1.5 h g−1 for zeolite 13X), even though zeolite 4A exhibits a longer water elution time (Supplementary Fig. 53). This further highlights the highly competitive water adsorption performance of the M-PyC compounds. To evaluate its recyclability for real-world applications, we also conducted multiple consecutive breakthrough experiments on the same a-Ni(PyC) sample. As shown in Supplementary Fig. 54, the dynamic breakthrough curves remained nearly identical across cycles, demonstrating the excellent dehydration reusability of a-Ni(PyC)2.

To further assess dehydration under realistic conditions, we performed breakthrough experiments on a simulated natural gas mixture (CH4/C2H6/C3H8/C4H10 = 80:10:5:5, v/v/v/v) at varying humidity levels (Supplementary Figs. 55–57). The a-Ni(PyC)2 compound effectively captured H2O vapor from the gas mixture, with H2O retention times for dehydration recorded as 17 h g−1 at 25% RH, 16 h g−1 at 50% RH, and 12 h g−1 at 75% RH. Moreover, because HC streams may contain other impurities such as CO2, we examined the CO2 tolerance of a-Ni(PyC)2. The compound exhibits negligible CO2 uptake at 25 °C (Supplementary Fig. 58). We further introduced ~2% CO2 into the simulated natural gas (under 50% RH) during breakthrough experiments (Supplementary Fig. 59) and observed no reduction in dehydration performance. To assess tolerance to sulfur-containing contaminants, the Ni-PyC molecular compound was immersed in 1 M H2S-aqueous solution. The PXRD analysis confirmed its structural integrity and high resistance to these harsh conditions (Supplementary Fig. 60). Overall, these results underscore the excellent dehydration performance and impurity tolerance of the Ni-PyC molecular adsorbent across relevant HC gas mixtures.

Recyclability and chemical stability for scalable industrial processes
Many MOFs reported for water adsorption applications lack long-term recyclability and sustained chemical stability, particularly those featuring open metal sites. Such materials often experience gradual loss of crystallinity or become amorphous after brief chemical exposure. In contrast, M-PyC compounds represent a rare group of molecular chemisorbents for which crystallinity is no longer required for their activated form, making them highly promising candidates for water-sorption-based applications. To evaluate the recyclability of Ni-PyC, we performed 100 consecutive adsorption-consecutive adsorption-desorption cycles over a time period of 2 months. The water uptake capacity remained constant throughout the entire process, confirming its superior reusability (Fig. 7a). In addition, chemical stability tests under various harsh conditions demonstrated the robustness of the as-made Ni-PyC sample over extended periods. As confirmed by PXRD analysis (Fig. 7b), Ni-PyC retained highly crystallinity after 3 months in open air, 1 day in hot water (80 °C), 3 days in boiling water (100 °C), 2 months in hydrothermal autoclave reactor (100 °C), and 1 day in highly acidic and basic aqueous solutions, highlighting its exceptional durability.

Another critical aspect is the scalability of candidate materials for industrial production. The use of inexpensive, commercially available raw chemicals and eco-friendly reaction conditions renders M-PyC compounds well suitable for large-scale synthesis. As an illustrative example, we successfully prepared 1.62 kilograms of Ni-PyC in a simple one-pot reaction at room temperature, using water as the sole solvent. The reaction was completed in just 1 h with a quantitative yield (>99%, Supplementary Fig. 61). The product was readily formulated into uniform pellets (Fig. 7c) and exhibited high crystallinity (Supplementary Fig. 62). Overall, Ni-PyC offers a compelling combination of high water uptake capacity, facile synthesis, easy scalability and unlimited recyclability. Very importantly, the entire adsorption-desorption process can be conducted in open air without the need of protection, which is impossible for most other water sorbents. These attributes position the M-PyC compounds strong candidates for industrial dehydration applications (Fig. 7d).

Methods
Materials
All chemicals and gases were purchased commercially and used without further purification. 4-pyridinecarboxylic acid (PyC, 99%), manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O, 99%), cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O, 99%), nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O, 99%), and zinc acetate tetrahydrate (Co(CH3COO)2·4H2O, 99%) were purchased from Alfa Aesar. Nitrogen, carbon dioxide, methane, ethylene, ethane, propylene, and propane gases were obtained from Praxair with a purity of 99.9+%.

Synthesis of M-PyC (M = Mn, Co, Ni, Zn) crystals
4-pyridinecarboxylic acid (PyC, 1 mmol) was first dissolved in deionized water (15 mL). An aqueous solution (5 mL) of nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O, 0.5 mmol) was then added to the PyC solution in a 25 mL glass vial and stirred for 1 h at room temperature. The resulting blue crystals of Ni-PyC were washed with water, filtered, and dried in air. Syntheses of Mn-PyC (colorless crystals), Co-PyC (orange crystals), and Zn-PyC (colorless crystals) followed the same procedure, with the metal salt replaced by manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O), cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O), or zinc acetate tetrahydrate (Zn(CH3COO)2·4H2O), respectively.

Scalable synthesis of Ni-PyC
In a typical procedure, 4-pyridinecarboxylic acid (8 mol, 0.985 kilograms) was dissolved in 18 L deionized water in a 22 L container. Ni(CH3COO)2·4H2O (6 mol, 1.490 kilograms) was then added to the aqueous solution and stirred at room temperature for 1 h. The crystalline product was collected by filtration and dried in air. A dry product weighing 1.62 kilograms was obtained, corresponding to a qualitative yield (>99%, ligand-based). For pellet molding, a portion of the Ni-PyC powder was mixed with 1 wt% of methyl cellulose as a binder material, and an appropriate amount of water was added to form a uniform paste (e.g., 5 g of Ni-PyC, 2 mL of deionized water, and 50 mg of methyl cellulose). The paste was manually formulated into pellets. Each pellet was approximately 5 mm in diameter and 50 mg in weight.

Physical characterizations
Powder X-ray diffraction (PXRD) measurements were performed on a Rigaku Ultima IV automated diffraction system using Cu-Kα radiation (λ = 1.5406 Å) over a 2 Theta range of 3–40°, with a scan rate of 3° min-1. The operating powder was 40 kV/44 mA. Thermogravimetric analysis (TGA) of the M-PyC samples was conducted using a TA Instrument Q5000IR thermogravimetric analyzer. For each run, about 10 mg of powder sample was loaded onto a platinum pan and heated under a nitrogen flux at a heating rate of 5 °C min-1.

Gas (vapor) sorption measurements
Before gas sorption experiments, fresh samples were activated at 120 °C for 2 h under dynamic vacuum conditions. BET surface area values were estimated based on nitrogen adsorption data collected at 77 K. Single-component water vapor adsorption isotherms, as well as adsorption isotherms for CH4, C2H4, C2H6, C3H6, and C3H8 gases, were recorded using a Micromeritics ASAP 3Flex adsorption analyzer (Micrometrics Instruments), with a water bath used to maintain the sorption tube at a constant temperature of 25 °C.

Water adsorption kinetics
Dynamic water vapor adsorption kinetics measurements were performed using a homemade gravimetric adsorption unit modified from a TGA Q50 thermogravimetric analyzer (TA Instruments). Pure nitrogen or open air was used as the carrier gas. The absorbed amount was monitored by recording the weight change of the sample over time using the TGA computer system. About 20 mg of Ni-PyC sample was first activated at 120 °C under a gas flow for 1 h to remove residual water molecules. The temperature was then cooled to 25 °C, and a secondary gas stream, passed through a bubbler filled with pure liquid water, was mixed with the dry gas stream. The resulting mixed gas stream (total flow rate of 40 mL min−1) was introduced into the adsorption chamber, and the adsorption process was monitored until equilibrium (saturated capacity) was reached. The recorded weight change were converted into transient normalized uptake curves, reflecting the adsorption kinetics over time.

Multicomponent column breakthrough tests
Breakthrough experiments were carried out using an automated mixed-gas breakthrough apparatus (3P MIXSORB). About 100 mg of Ni-PyC sample was loaded into the column. The packed adsorbent was activated by heating to 120 °C for 2 h under helium purging (10 mL min−1). After stopping the helium flow, humidified feed gases (pure CH4, pure C2H4, pure C3H6, or mixed CH4/C2H6/C3H8/C4H10 at 80:10:5:5, v/v/v/v) were pass through the adsorption bed at a flow rate of 3.5 mL min−1. The outlet gas was analyzed using a mass spectrometer (MKS). The breakthrough amount (q) was calculated by integrating the outlet flow rate f(t) over time according to the following equation:
q = (1/m) * ∫0t f(t)dt (1)
where m is the mass of the adsorbent used for the test.
The purity (c) of the breakthrough gas was calculated using the following equation:
c = q_gas / (q_H2O + q_gas) (2)

Heat flow measurements
The heat flows during the adsorption process were measured for water vapor, methane, ethylene, and propylene on Ni-PyC using a differential scanning calorimetry (DSC) system (TA Instruments). Measurements were conducted by feeding a flowing stream of gas (water vapor, methane, ethylene, or propylene) over about 10 mg of activated sample at 25 °C. Prior to water vapor measurements, the baseline was recorded under dry nitrogen flow at 25 °C. Subsequently, nitrogen gas was passed through a saturator containing liquid water at 25 °C, and the resulting water vapor was introduced into the system while the DSC signal was recorded. The calorimetry curves were collected by real-time detection of the heat flow changes inside the chamber during the adsorption process. The enthalpies (ΔH) were determined by integrating the heat flow over time, using the following equation:
ΔH = S * m / (6 * 10^2 * n) (3)
where ΔH is the enthalpies (kJ mol−1), S is the integrating area (W min g−1), m is the sample mass (g), and n is the amount of adsorbed gas (mol).

Humidity-dependent powder X-ray diffraction
Humidity-dependent PXRD measurements coupled with water vapor adsorption were performed on a Bruker D8 Advance system (CuKα radiation, λ = 1.5406 Å) equipped with a temperature-humidity chamber for adjusting the humidity level. About 20 mg of as-made Ni-PyC was first activated at 120 °C for 1 h, and then placed into the humidity chamber to adsorb water vapor at different humidity levels for various time periods. PXRD patterns were collected after the sample was exposed to different humidity conditions.
In situ PXRD measurements were carried out on a Bruker D8 Advance diffractometer equipped with an XRK900 in situ cell. The Ni-PyC sample was loaded into the cell, activated under nitrogen flow, and subsequently cooled to 25 °C for water adsorption. Humid nitrogen (50% RH) was then introduced to the cell to promote water uptake accompanied by recrystallization-assisted structural changes. The sample was subsequently heated to dehydrate the water-saturated compound, with PXRD patterns recorded continuously throughout the entire process.

In situ infrared spectroscopy
In situ IR measurements coupled with water vapor adsorption were performed on a Nicolet iS50 infrared spectrometer equipped with a liquid nitrogen-cooled mercury cadmium telluride (MCT-A) detector and a temperature controller. The spectrometer was fitted with a vacuum cell placed in the main compartment, positioning the sample at the focal point of the infrared beam. To avoid direct pressing that might damage the crystalline structure, the Ni-PyC sample (5 mg) was prepared as a slurry by mixing with a small amount of water and pasted onto a KBr pellet. After drying the sample by blowing a nitrogen stream for a few minutes, it was directly attached onto the KBr pellet and transferred into the cell, which was connected to a vacuum line for evacuation and a water vapor line for controlled moisture exposure to enable adsorption. The sample was activated by heating to 120 °C under vacuum, then cooled back to room temperature for water vapor adsorption measurements. All spectra were recorded in transmission mode over a frequency range of 600–4000 cm−1 with a spectral resolution of 4 cm−1.

Recyclability tests
Recyclability tests of Ni-PyC were performed using a gravimetric adsorption analyzer over 100 consecutive water adsorption-desorption cycles. About 50 mg of Ni-PyC sample was first activated at 120 °C for 1 h prior to the experiments. After cooling to room temperature, water vapor generated from a liquid water bubbler was introduced into the adsorption chamber. Once water adsorption reached equilibrium, the water-adsorbed sample was desorbed at 120 °C for 1 h to reactivate it for the next adsorption-desorption cycle.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The work by Li et al. reported the use of a series molecular compounds in dehydrating natural gases and olefins. A phase transition mechanism between amorphous phase and crystalline phase was revealed. Such a mechanism sounds very refreshing to the reviewer. These materials fully exclude the hydrocarbons while having a relatively high water uptake. Finally, the authors could also scale up the synthesis and demonstrate their high chemical stability. Overall, I believe this work is a nice contribution to Nature Communications. I would be happy to recommend its publication after the following concerns are addressed.\n1. The advantages of this kind of materials have been fully revealed in the paper but it seems that the disadvantages of these materials are not discussed which I believe will not affect the publication. For example, the comparisons with zeolites in terms of adsorption kinetics and breakthrough experiments were not discussed. I believe it could be very hard to compete with zeolites in these two aspects. That is also probably the reason why zeolites lose 30% uptake if they are activated in open air due to zeolites’ fast adsorption.\n2. As stated in the manuscript, the deep dehydration require that the water content is below 147 ppm, which is probably corresponding to a very low RH. At this RH, the materials reported in this work could have a much lower uptake than zeolites\n3. I would suggest the authors use P/P0 or relative humidity instead of absolute pressure in water adsorption isotherms.\n4. It is clear that space group P-1 belongs to triclinic system instead of orthorhombic system.\n5. I am curious about the C2H2 uptakes of this family of materials.", "reviewer_2": "This manuscript reports molecular adsorbents (M-PyC, M = Mn, Ni, Zn; PyC=4-pyridinecarboxylic acid) that can adsorb water to form crystalline materials from amorphous nature. They exhibited high water uptake and excellent selectivity over hydrocarbons due to nonporous nature. In addition, desorption temperature is relatively lower than conventional sorbents (zeolite, alumina, silica), and low cost, green synthesis, and scalability are also fascinating properties of these adsorbents. The crystallographic evidence when adsorbed and desorbed water was provided and hydrogen bonding mediated 3D structure was also characterized. Furthermore, when they are tested in the similar hydrocarbon separation conditions, they showed excellent selectivity for water over hydrocarbons, strongly confirming the feasibility of these adsorbents for real industrial applications.\nThus, it is recommended for publication after addressing the following comments.\n1. It was suggested that excellent selectivity for water over hydrocarbons can be attributed to nonporous nature, but according to CO2 sorption measurement, the adsorbents have surface areas of 70-130 m2/g, which doesn’t’ make sense. Why they have relatively high surface areas?\n2. Because the water adsorption of amorphous adsorbents occurs through crystallization, the sorption kinetics should be strongly related to the size of amorphous adsorbents. It is better to study or show the size effect of amorphous sorbents on sorption kinetics.\n3. The reason for desorption temperatures decrease with Ni, Mn, and Zn-PyCs was suggested hydrogen boding energies for those adsorbents. Is there any correlation between metal and hydrogen bonding energy? It is better to explain the differences by comparing transition metal differences (maybe using computational analysis).\n4. 50-cycle test is not enough to show the long-term stability, so please provide longer long-term test.", "reviewer_3": "The authors a study on the synthesis and characterization of a series of amorphous molecular coordination complexes that undergo a reversible amorphous-to-crystalline phase transition upon water adsorption. This process results in high water selectivity over hydrocarbons, fast kinetics, and regeneration at relatively low temperatures.\nThis approach could represent a new design strategy for selective sorbents in separations. However, the present version requires more rigorous mechanistic evidence, stronger contextualization within existing literature, and expanded stability/performance validation to meet the novelty and rigor standards of Nature Comm.\nMajor Comments\n1. Novelty: The authors state that crystallization-assisted water adsorption in amorphous molecular adsorbents is unprecedented. However, water-induced structural transformations have been reported in other systems — notably in MOFs such as MIL-53 and DUT-8(Ni), certain coordination polymers, and even silica-based adsorbents. The authors should explicitly compare their findings with these precedents, explaining mechanistic and performance differences and making clear what is fundamentally new here.\n2. Mechanistic Validation: The hypothesis that hydrogen bonding between coordinated water molecules and carboxylate (COO⁻) groups drives crystallization is reasonable, but the evidence is largely circumstantial. PXRD, IR, and/or even DFT suggest a connection but do not confirm causality. The authors are encouraged to look into: (1) In situ PXRD or synchrotron-based diffraction during water uptake/desorption could directly capture the structural pathway. (2) In situ IR could monitor hydrogen-bond changes in real time.\n3. Selectivity: The H₂O/hydrocarbon selectivity ratios are very exciting, but no direct benchmarking is made against best-performing materials for natural gas dehydration. Without quantitative comparisons of working capacity, regeneration energy, and cycle stability, the practical significance remains unclear. For industrial uptake, cost per cycle, durability, and contaminant tolerance (e.g., CO₂, H₂S) must be addressed. Cycling data cover 50 adsorption–desorption cycles, which is insufficient to justify “unlimited recyclability” claims. At least accelerated cycling tests (over months or extended period of time) should be performed. Furthermore, stability tests under mixed-gas conditions, hydrothermal stress, and in the presence of acid gases would strengthen claims of practical viability.\nMinor Comments\n• Page 5, Line 8 – The authors are advised to rephrase “superior to all known adsorbents” with a qualified statement unless supported by a comprehensive benchmarking table.\n• Page 6, Line 20 – Provide error bars or uncertainties for reported adsorption capacities.\n• Page 8, Line 15 – Cite prior examples of water-induced crystallization in porous materials to better contextualize the novelty.\n• Supplementary Figures 40–48 – Hydrocarbon dehydration experiments should include replicate runs to establish reproducibility."}, "author_response": "We thank both editor and reviewers for their valuable comments and constructive suggestions.\nWe have carefully considered and addressed each question and revised our manuscript\naccordingly. All changes are clearly marked in red colored text in the revised manuscript and\nsupplementary information.\nReviewer #1:\nThe work by Li et al. reported the use of a series molecular compounds in dehydrating natural\ngases and olefins. A phase transition mechanism between amorphous phase and crystalline\nphase was revealed. Such a mechanism sounds very refreshing to the reviewer. These\nmaterials fully exclude the hydrocarbons while having a relatively high water uptake. Finally,\nthe authors could also scale up the synthesis and demonstrate their high chemical stability.\nOverall, I believe this work is a nice contribution to Nature Communications. I would be happy\nto recommend its publication after the following concerns are addressed.\nResponse: We sincerely thank the reviewer for the positive remarks and enthusiasm about\nthis work.\n1. The advantages of this kind of materials have been fully revealed in the paper but it seems\nthat the disadvantages of these materials are not discussed which I believe will not affect the\npublication. For example, the comparisons with zeolites in terms of adsorption kinetics and\nbreakthrough experiments were not discussed. I believe it could be very hard to compete with\nzeolites in these two aspects. That is also probably the reason why zeolites lose 30% uptake if\nthey are activated in open air due to zeolites’ fast adsorption.\nResponse: We thank the reviewer for this insightful comment. We fully agree that, compared\nwith zeolites, our molecular compounds exhibit slower water adsorption kinetics. This\ndifference arises from the crystallization-assisted chemisorption mechanism in our materials,\nin contrast to the rapid physisorption that occurrs within the micropores of zeolites. To clarify\nthis point, we have included experiments of the water adsorption kinetics of zeolite 4A at\ndifferent humidities (Supplementary Fig. 16). Furthermore, due to their faster adsorption\nkinetics, zeolites such as 4A shows longer retention of water eluent in breakthrough\nexperiments at different humidities (Supplementary Fig. 53). As the reviewer correctly\npointed out, this fast adsorption of zeolites also leads to partial pre-adsorption of ambient\nmoisture or atmospheric impurities, resulting in up to a 30% loss in water uptake when\noperated in open air (Supplementary Figs. 24-25). In contrast, our Ni-PyC compound, while\nslower in adsorption kinetics, completely excludes other gases and maintains its full capacity\nunder ambient conditions. A brief discussion acknowledging this trade-off has been added to\nthe revised manuscript (page 7, paragraph 1).\n2. As stated in the manuscript, the deep dehydration require that the water content is below\n147 ppm, which is probably corresponding to a very low RH. At this RH, the materials reported\nin this work could have a much lower uptake than zeolites.\nResponse: We thank the reviewer for bringing up this important point. The value of 147 ppm\nrefers to the required final water content of the dehydrated natural gas products. In practical\nnatural gas production, the raw gas mixtures are typically saturated with water vapor from\nunderground reservoirs, corresponding to over 70% relative humidity. The breakthrough\nexperiments conducted under different humidity levels (25-75% RH) demonstrate that our\ncompound can effectively dehydrate natural gas (and olefins) to water contents below 1 ppm,\n1\nwell below the industrial standard for deep dehydration (<147 ppm). This confirms their\npractical applicability for real gas dehydration processes, even though their equilibrium water\nuptakes are somewhat lower than those of zeolites. Importantly, our compounds also offer\nadvantages such as low regeneration temperatures, fast desorption rate, high water selectivity,\nand practically unlimited recyclability.\n3. I would suggest the authors use P/P0 or relative humidity instead of absolute pressure in\nwater adsorption isotherms.\nResponse: We thank the reviewer for this good suggestion! Relative pressure (P/P ) has been\n0\nused to replace absolute pressure in all water adsorption isotherms (Fig. 3b and\nSupplementary Figs. 11-13 & 23-25).\n4. It is clear that space group P-1 belongs to triclinic system instead of orthorhombic system.\nResponse: We apologize for this typo. The crystal system has been corrected to the triclinic\n(page 5, paragraph 3).\n5. I am curious about the C2H2 uptakes of this family of materials.\nResponse: To answer this question, we have added C H adsorption isotherms of various\n2 2\nadsorbents, including Ni-PyC, MOF-801, CAU-10, alumina, zeolites 4A and 5A, in\nSupplementary Fig. 35. Ni-PyC exhibits negligible C H uptake, whereas other adsorbents\n2 2\nshow substantial uptake capacities (e.g., 2.5 mmol/g for MOF-801, 4.3 mmol/g for CAU-10,\n2.9 mmol/g for zeolite 4A, and 3.3 mmol/g for zeolite 5A). Related discussion has been added\nto the main article (page 8, paragraph 2).\nReviewer #2:\nThis manuscript reports molecular adsorbents (M-PyC, M = Mn, Ni, Zn; PyC=4-\npyridinecarboxylic acid) that can adsorb water to form crystalline materials from amorphous\nnature. They exhibited high water uptake and excellent selectivity over hydrocarbons due to\nnonporous nature. In addition, desorption temperature is relatively lower than conventional\nsorbents (zeolite, alumina, silica), and low cost, green synthesis, and scalability are also\nfascinating properties of these adsorbents. The crystallographic evidence when adsorbed and\ndesorbed water was provided and hydrogen bonding mediated 3D structure was also\ncharacterized. Furthermore, when they are tested in the similar hydrocarbon separation\nconditions, they showed excellent selectivity for water over hydrocarbons, strongly\nconfirming the feasibility of these adsorbents for real industrial applications. Thus, it is\nrecommended for publication after addressing the following comments.\nResponse: We sincerely thank the reviewer for the positive and valuable comments.\n1. It was suggested that excellent selectivity for water over hydrocarbons can be attributed to\nnonporous nature, but according to CO2 sorption measurement, the adsorbents have surface\nareas of 70-130 m2/g, which doesn’t’ make sense. Why they have relatively high surface areas?\nResponse: We thank the reviewer for this good question. We apologize for the previous\nconfusion regarding surface area. We have retested and corrected the surface area\nmeasurements using nitrogen adsorption isotherms at 77 K for activated compounds. All\nsamples exhibit negligible surface areas (<5 m2/g), confirming their nonporous nature, as\nshown in Supplementary Fig. 10. The CO adsorption observed at 195 K arises from chemical\n2\ninteraction with open metal sites after removal of terminal water molecules from the as-made\n2\n(Supplementary Fig. 58). A brief discussion has been added to the main text (page 6,\nparagraph 2).\n2. Because the water adsorption of amorphous adsorbents occurs through crystallization, the\nsorption kinetics should be strongly related to the size of amorphous adsorbents. It is better\nto study or show the size effect of amorphous sorbents on sorption kinetics.\nResponse: Following the reviewer’s suggestion, we have carried out additional experiments\nto investigate the particle size effect on the water adsorption kinetics of the amorphous\ncompounds. We have prepared a-Ni(PyC) with smaller particle sizes. As shown in\nunder various relative humidities. A brief discussion of these results has been added to the\nrevised manuscript (page 7, paragraph 1).\n3. The reason for desorption temperatures decrease with Ni, Mn, and Zn-PyCs was suggested\nhydrogen bonding energies for those adsorbents. Is there any correlation between metal and\nhydrogen bonding energy? It is better to explain the differences by comparing transition metal\ndifferences (maybe using computational analysis).\nResponse: We thank the reviewer for this excellent question. Indeed, there is a strong\ncorrelation between the metal type and the hydrogen bonding energy. We calculated the\nhydrogen bonding energies as a function of metal type and found them to be 0.865 eV (highest)\nfor Ni-PyC, 0.859 eV (slightly lower) for Mn-PyC, and 0.832 eV (lowest) for Zn-PyC. This trend\nis consistent with their decomposition temperatures: 115 C (highest) for Ni-PyC, 100 C\n(slightly lower) for Mn-PyC, and 90 C (lowest) for Zn-PyC. Furthermore, the cohesive energies\nper M-PyC molecule were calculated to be 7.480 eV (highest) for Ni-PyC, 7.433 eV (slightly\nlower) for Mn-PyC, and 7.210 eV (lowest) for Zn-PyC. These cohesive energies result from a\ncombination of strong hydrogen bonding and weak van der Waals interactions between\nstacked PyC rings. More detailed discussion and calculations are given in the “Synthesis and\ncrystal structure” section (page 5, paragraph 3) and in the Supplementary Notes.\n4. 50-cycle test is not enough to show the long-term stability, so please provide longer long-\nterm test.\nResponse: In response to this request, we have extended our recyclability test to 100 cycles\n cycles\nover a period of two months to further confirm the long-term stability. The results have been\nadded to the revised manuscript (Fig. 7a). PXRD analysis shows that Ni-PyC retained its high\ncrystallinity after 100 consecutive sorption cycles (Supplementary Fig. 60), highlighting the\ncompound’s excellent long-term stability.\nReviewer #3:\nThe authors a study on the synthesis and characterization of a series of amorphous molecular\ncoordination complexes that undergo a reversible amorphous-to-crystalline phase transition\nupon water adsorption. This process results in high water selectivity over hydrocarbons, fast\nkinetics, and regeneration at relatively low temperatures. This approach could represent a\nnew design strategy for selective sorbents in separations. However, the present version\nrequires more rigorous mechanistic evidence, stronger contextualization within existing\nliterature, and expanded stability/performance validation to meet the novelty and rigor\n3\nstandards of Nature Comm.\nResponse: We sincerely appreciate the reviewer’s through evaluation of our manuscript, as\nwell as insightful and helpful comments.\nMajor Comments\n1. Novelty: The authors state that crystallization-assisted water adsorption in amorphous\nmolecular adsorbents is unprecedented. However, water-induced structural transformations\nhave been reported in other systems — notably in MOFs such as MIL-53 and DUT-8(Ni),\ncertain coordination polymers, and even silica-based adsorbents. The authors should\nexplicitly compare their findings with these precedents, explaining mechanistic and\nperformance differences and making clear what is fundamentally new here.\nResponse: We thank the reviewer for bringing up this point. We agree that water-induced\nstructural transformations have been reported in numerous systems, such as the “breathing”\neffect in MIL-53 (J. Mater. Chem. A, 2025, 13, 32446-32459; Chem. Eur. J. 2004, 10, 1373-1382)\nand the “gating” behavior in DUT-8 (Ni) (Chem. Sci., 2025, 16, 6402-6417; Adv. Mater. 2023,\n35, 2207741; Sci. Adv. 2022, 8, eabn7035), to name just a few. These transformations, however,\ninvolve changes between two crystal structures with the same connectivity, where both forms\nof the compounds remain crystalline. In contrast, M-PyC series, as molecular coordination\ncomplex, undergo a fully reversable amorphous-to-crystallization transition triggered by the\nchemisorptive coordination and decoordination of water molecules, a fundamentally\ndifferent phenomenon and mechanism. This reversible transformation does not rely on pre-\nexisting pores as they are essentially nonporous, distinguishing them from the previously\nreported flexible MOFs. On the other hand, silica-based adsorbents (e.g., silica gel) adsorb\nwater via surface physisorption without structural reconstruction or chemical bond\nrearrangement. We have now included a brief discussion (page 4, paragraph 3) and cited\nrepresentative references on MIL-53 and DUT-8 (Ni) (Refs. 34 & 41-43) in the revised\nmanuscript. These additions highlight the mechanistic distinctions and clarify the novelty of\nour system.\n2. Mechanistic Validation: The hypothesis that hydrogen bonding between coordinated water\nmolecules and carboxylate (COO⁻) groups drives crystallization is reasonable, but the\nevidence is largely circumstantial. PXRD, IR, and/or even DFT suggest a connection but do not\nconfirm causality. The authors are encouraged to look into: (1) In situ PXRD or synchrotron-\nbased diffraction during water uptake/desorption could directly capture the structural\npathway. (2) In situ IR could monitor hydrogen-bond changes in real time.\nResponse: We appreciate the reviewer’s thoughtful comment and suggestion regarding\nmechanistic validation. To strengthen our analysis on crystallization driven by hydrogen\nbonding, we have conducted additional in situ experiments following the reviewer’s\nsuggestion and expanded the discussion accordingly. In situ PXRD measurements using a\nhumidity- and temperature-controlled cell were performed to directly monitor the structural\nevolution between the crystalline and amorphous states during water-induced sorption (Fig.\n4b). A detailed discussion of this process has been added to the revised manuscript (page 9,\nparagraph 1). We have also revised and enhanced discussion on the in situ IR experiments by\nincluding more details of the changes occurring in the vs(COO-) and v(OH) bands during the\nwater adsorption process (page 9, paragraph 1 and page 10, paragraph 2, Fig. 5).\n4\n3. Selectivity: The H₂O/hydrocarbon selectivity ratios are very exciting, but no direct\nbenchmarking is made against best-performing materials for natural gas dehydration.\nWithout quantitative comparisons of working capacity, regeneration energy, and cycle\nstability, the practical significance remains unclear. For industrial uptake, cost per cycle,\ndurability, and contaminant tolerance (e.g., CO₂, H₂S) must be addressed. Cycling data cover\n50 adsorption–desorption cycles, which is insufficient to justify “unlimited recyclability”\nclaims. At least accelerated cycling tests (over months or extended period of time) should be\nperformed. Furthermore, stability tests under mixed-gas conditions, hydrothermal stress,\nand in the presence of acid gases would strengthen claims of practical viability.\nResponse: We fully agree with the reviewer regarding the importance of providing more\nquantitative data in evaluating the performance of the title compounds under real-world\nconditions. While it is not possible to include all such work in this paper we have made great\neffort and conducted additional experiments at our capacity to offer more quantitative data\nfor comparisons. For the recyclability test of Ni-PyC we carried out twice as many cycles (100)\nover an extended period of two months. The results confirm sample’s excellent long-term\nstability and consistent dehydration performance without capacity loss (Fig. 7a,\nSupplementary Fig. 60).\nTo assess contaminant tolerance, we performed breakthrough experiments using a simulated\ngas mixture containing ~2% CO impurity under 50% relative humidity. The presence of CO\n2 2\nhad no observable effect on the breakthrough result (Supplementary Fig. 59), consistent with\nthe negligible CO uptake measured at 25 C (Supplementary Fig. 58).\nsubjected to various harsh conditions (Supplementary Fig. 60), including: (i) hydrothermal\ntreatment in autoclave reactor (100 C, 2 months); (ii) sulfur-containing acidic environments\ncontaining 1M HS- (25 C, 7 days); and (iii) thermal cycling over 100 adsorption-desorption\ncycles. In all cases, PXRD patterns confirmed full structural integrity and crystallinity retention,\nhighlighting the compound’s robustness under real-world operational conditions.\nWe have expanded the discussion in the revised manuscript (page 12, paragraph 2).\nMinor Comments\n• Page 5, Line 8 – The authors are advised to rephrase “superior to all known adsorbents”\nwith a qualified statement unless supported by a comprehensive benchmarking table.\nResponse: We thank the reviewer for this comment and apologize for using a statement that\nappears boastful. Following the reviewer’s advice, we have modified the statement to:\n“demonstrating distinct advantages to zeolites and alumina currently used in industry, as well\nas some of the best performing MOFs” in the revised manuscript (page 8, paragraph 2).\nAdditionally, we included a comparison table (Supplementary Table 4) summarizing the water\nuptake ratios of our compounds alongside those of zeolites, alumina, MOF-801, and CAU-10,\nbased on experimental adsorption data, to provide appropriate benchmarking.\n• Page 6, Line 20 – Provide error bars or uncertainties for reported adsorption capacities.\nResponse: Error bars are added for all reported adsorption capacities of the compared zeolites\nand MOFs (page 7, paragraph 1 and Supplementary Fig. 14).\n• Page 8, Line 15 – Cite prior examples of water-induced crystallization in porous materials to\nbetter contextualize the novelty.\nadded a short paragraph to better describe the novelty of our work (page 4, paragraph\n3).\n• Supplementary Figures 40–48 – Hydrocarbon dehydration experiments should include\nreplicate runs to establish reproducibility.\nResponse: We thank the reviewer’s constructive suggestion. We have conducted four\nconsecutive dehydration experiments to confirm reproducibility and observed negligible\ndecrease in its dehydration performance (Supplementary Fig. 54).\nWe hope that these revisions meet the experctations of all three reviewers. We extent our\ngratitude again for their invaluable insights, comments, and suggestions. Their constructive\nfeedback has helped us greatly in enhancing the overall quality of this manuscript.\n6"}, {"round": 2, "reviewer_comments": {"reviewer_1": "All of my comments have been adequately addressed.", "reviewer_2": "The questions fro the reviewer were suitably revised by the authors, so that it is now recommended for publication at NC.", "reviewer_3": "Thank you for authors for addressing the comments adequately."}, "author_response": ""}]

Decision letter