# Abnormal adsorption properties of the MOF CALF-20 as revealed by water and methanol vapor sorption

Julia Duplessis-Kergomard, Meryem Saidi, Olinda Gimello, Bernard Fraisse, Fabrice Salles, Philippe Trens

► To cite this version:

Julia Duplessis-Kergomard, Meryem Saidi, Olinda Gimello, Bernard Fraisse, Fabrice Salles, et al. Abnormal adsorption properties of the MOF CALF-20 as revealed by water and methanol vapor sorption. Microporous and Mesoporous Materials, 2025, 387, pp.113525. 10.1016/j.micromeso.2025.113525. hal-05321697

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Submitted on 19 Oct 2025

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# Abnormal adsorption properties of the MOF CALF-20 as revealed by water and methanol vapor sorption

Julia Duplessis-Kergomard, Meryem Saidi, Olinda Gimello, Bernard Fraisse, Fabrice Salles, Philippe Trens

ICGM, UMR5253, Univ. Montpellier, CNRS, ENSCM, Montpellier, France

<figure><img src="image_3.png" /></figure>

## ABSTRACT

MOF CALF-20 has been employed in industry due to its hydrophobic properties, which are conducive to CO₂ capture. Nevertheless, additional characteristics of this porous material are worthy of investigation, including its mechanical properties and flexibility. When CALF-20 is subjected to water vapor, it exhibits a slight contraction, which may be attributed to its hydrophobic nature. Conversely, when CALF-20 adsorbs methanol, it exhibits a 30 % increase in volume. Furthermore, our findings indicate that the interactions between MOFs and water or methanol are dependent on the crystalline phase. The aforementioned interactions exhibit a range of values between -37.8 and -54 kJ mol⁻¹ in the case of water, whereas they consistently approach -53 kJ mol⁻¹ in the case of ethanol, irrespective of the crystalline phase. This has significant implications for methanol adsorption in water-saturated CALF-20. Our findings suggest that the water confined in the pores can be pushed out by the methanol flow, whereas this phenomenon does not occur in the case of water adsorption in CALF-20 that has already been filled with methanol. These findings pave the way to improvements in Sorption-Enhanced Reaction Process technology for pure methanol production from carbon dioxide, where selective sorbents are required for shifting the methanol production equilibrium.

## 1. Introduction

In light of the pressing necessity to transition to an economy based on renewable energies, it is imperative to develop efficient energy storage processes [1]. The utilization of carbon dioxide hydrogenation technologies presents a potential avenue for the generation of high-energy chemical compounds from renewable electricity, thereby facilitating efficient energy storage. Methanol represents a chemical energy carrier that can be produced through the catalytic reduction of CO₂ [2]. It has a multitude of applications as a precursor in chemical synthesis and possesses a relatively high volumetric energy density [3]. Nevertheless, the synthesis of methanol by hydrogenation of pure CO₂ is subject to significant equilibrium limitations, necessitating the recirculation of unreacted gases and, consequently, an increase in the size and power consumption of downstream equipment [4]. To enhance the efficiency of the conversion process, techniques for the selective removal of reaction products have been developed. These include the absorption of methanol between reactors and the utilization of appropriate solvents such as ionic liquids for the absorption of methanol and water from the reaction zone. Sorption-enhanced reaction production (SERP) technology represents a viable solution for intensifying the methanol synthesis process via selective product removal in a single-bed column filled with a mixture of catalyst and selective sorbent [5,6]. In this context, the

selective capture of one product through physical adsorption appears to be the most promising technology due to its simplicity and its adaptability, provided an appropriate sorbent has been identified [7,10]. In fact, the latter is the key point in sorption technologies. In addition to activated carbons and zeolites, which are currently used worldwide for sorption purposes, MOFs are now being considered as possible alternatives due to their unique physicochemical properties [11].

MOFs are hybrid materials consisting of metallic nodes linked by organic linkers forming stable and well-defined crystalline structures [7, 11]. These are highly porous and usually have large specific surface areas which make them very suitable for gas adsorption [12]. Additionally, they can be used at quite high temperatures, as their thermal stability can reach 450 °C [13,14]. Most importantly, they can be designed for specific sorbates by varying either the metal or the ligand. This is a unique feature compared to traditional adsorbents such as zeolites or activated carbons. This tunability enables easy optimization of pore volumes and of the surface functions to selectively capture CO₂ in humid environments [15,16]. More specifically, Liu et al. have grafted hydrophobic functions such as fluorine or alkyl/aromatic groups on the ligands for enhancing the hydrophobicity of the MOF [8]. It enables to disfavor the water adsorption, which is ubiquitous in air, therefore the enhancement of their hydrothermal stability as there are less prone to ligand displacement and hydrolysis reactions [17]. The increase of their

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: Philippe.trens@enscm.fr (P. Trens).</footnote>
lipophilic properties is also of great interest in hydrocarbon capture and separation due to the specific interactions and their high affinity with lipophilic materials. Zarate et al. incorporated amino functions in MIL-53(Al) for a better adsorption of CO₂, illustrating the usefulness of synthetic modifications of the ligands in MOFs structures [18]. Moreover, Li et al. enhanced the toluene adsorption on UiO-66 by substituting 50 % of the benzene-1,4-dicarboxylate with a naphthyl ligand, increasing the hydrophobicity of the MOF and enhancing the π-π interactions with toluene [19]. They proved that at 50 % of relative humidity (RH), the toluene adsorption capacity was increased by 69 % compared to the pristine UiO-66.

In this frame, the discovery of CALF-20 in 2021 appears as very promising [20]. This Zn-based MOF already proved outstanding for CO₂ capture and storage, especially in humid conditions. This is due to its hydrophobicity which results in a very high CO₂/water selectivity. These results were further assessed using experimental measurements and molecular simulations [16,21,22]. Upon water vapor sorption, a α to β CALF-20 phase transition was evidenced, inducing different pores volumes, and therefore different confinement environments [23]. Based on these findings, evaluating the sorption selectivity in the case of VOCs/water vapor system appeared promising.

We demonstrate here that methanol can be adsorbed by CALF-20 from water/methanol vapor mixtures with a very high selectivity, due to their different sorption enthalpies at low coverage. Monte Carlo simulations evidenced that the manner in which the adsorbed phase was organized was contingent upon the partial pressure of the mixture. The oxalate functions of the MOF were identified as the primary interaction sites.

## 2. Experimental section

### 2.1. Materials

CALF-20 was synthesized according to the route published by Lin et al. without any modification [20]. In brief, 3.3 g of zinc oxalate were stirred in 33 mL of methanol for 10 min 2.5 g of 1,2,4-triazole were added and stirred for 10 min. The mixture was then poured into a Teflon-lined autoclave. The autoclave was heated in an oven for 2 days at 453 K under autogenous pressure. The product was washed and centrifugated 5 times with 30 mL of methanol. The final product was dried in a desiccator under primary vacuum for 2 days. Ultrapure water (R = 18MΩ) and anhydrous methanol were used as solvent and as model VOC (>99.99 %, Carlo Erba) respectively.

### 2.2. Methods

Characterization. The crystalline phases of CALF-20 were determined by X-ray powder diffraction using a Bruker SAS AXS D8 Advance (France) diffractometer with a monochromatic Cu K-α1 radiation (λα = 1.5406 Å, 40 kV, 40 mA) and equipped with a Lynxeyes detector 1D with a 0.02° step (2θ) from 4 to 50°.

Thermodiffraction was performed on CALF-20 saturated with water or methanol using a Malvern-Panalytical Empyrean series 2 in Bragg-Brentano geometry. It is equipped with a Theta-Theta goniometer, a copper anticathode tube, a PIXcel3D detector and an Anton-Paar HTK16 chamber with a Pt-Rh10 % blade. Measurements were carried out under dynamic air using an average flow rate of 0.3 L per minute, between 25 °C and 100 °C every 5 °C. Diffractograms were recorded between 5 and 35°(2θ) with a step size of 0.026°(2θ) over a period of 5 min. A final diffractogram was measured once temperature was back down to 25 °C to distinguish between the influence of temperature and that of the sorbed species.

Scanning Electron Microscopy (SEM) analyses were conducted on a Hitachi S-4800 microscope (Japan), whereas transmission electron microscopy analyses were conducted on a JEOL JEM-1400 Plus microscope using an acceleration field of 40 kV. In both cases, the sample as a

powder was placed on carbon-coated copper grids. Thermogravimetric analyses (TGA) were performed using a PerkinElmer apparatus from ambient to 800 °C with a ramp of 10 °C/min under static air. The textural properties of the different samples were assessed by nitrogen sorption at 77 K using a 3Flex device (Micromeritics, USA). The samples were first degassed at 473 K for 12 h before analysis. The equivalent surface area was determined using the BET transform of the sorption isotherm in the range 0.05 < p/p° < 0.25 where the Rouquerol criterion applies. The cross-sectional area for nitrogen was taken as 0.162 nm². The pore size distribution was obtained by NLDFT, taking cylindrical pores as geometry.

Sorption experiments. Gravimetric vapor mixtures sorption measurements were performed using a Dynamic Vapor Sorption device (DVS resolution, SMS, UK). In such a system, the partial pressure of the sorbate is generated by saturation of a nitrogen flow which was used as a carrier gas. In the case of mixtures, the vapors are generated separately before being mixed together and contacted with the sample. This allows for different mixtures compositions to be studied [24]. The samples were degassed in the DVS at 200 °C under a 200 mL/min dry nitrogen flow for 8 h before each sorption isotherm. The sorption isotherms were recorded at 298 K with pure water or pure methanol vapors, and with four water: methanol vapor mixtures of different molar compositions: 33:67, 50:50, 67:33.

The saturation pressures for the mixed solvents were calculated under the assumption that methanol/water mixtures behave ideally across all molar fractions explored [25]. In this context, the total pressure of the mixture can be estimated following Dalton's law:

$$x_{\text{methanol}} = \frac{P_{\text{methanol}}}{P_{\text{methanol}} + P_{\text{water}}} \qquad (2)$$

According to the Raoult law, $p_{\text{methanol}} = x_{\text{methanol}} p^*_{\text{methanol}}$, in which $p^*_{\text{methanol}}$ refers to the vapor pressure of pure methanol whereas $p_{\text{methanol}}$ refers to the vapor pressure of methanol in the water/methanol mixture. In a symmetric fashion, the same equation can be written for water.

Assuming that methanol and water vapors are perfect gases, the vapor pressure of mixtures can therefore be estimated as:

$$p_{\text{mix}} = p_{\text{methanol}} + p_{\text{water}} \qquad (3)$$

At saturation, the saturation pressure of the mixture can therefore be written:

$$p_{\text{mix}}^{\text{sat}} = p_{\text{methanol}}^{\text{sat}} + p_{\text{water}}^{\text{sat}} \qquad (4)$$

where $p_{\text{methanol}}^{\text{sat}}$ is the saturation pressure of methanol in the water/methanol mixture and $p_{\text{water}}^{\text{sat}}$ is the saturation pressure of water in the same mixture. It can be concluded that:

$$p_{\text{mix}}^{\text{sat}} = p_{\text{methanol}}^{*\text{sat}} + x_{\text{water}} (P_{\text{water}}^{*\text{sat}} - P_{\text{methanol}}^{*\text{sat}}) \qquad (5)$$

The partial pressure of the mixture can therefore be written as:

$$\frac{p_{\text{mix}}}{p_{\text{mix}}^{\text{sat}}} = \frac{x_{\text{methanol}} p_{\text{methanol}}^{*} + x_{\text{water}} p_{\text{water}}^{*}}{p_{\text{methanol}}^{*\text{sat}} + x_{\text{water}} (P_{\text{water}}^{*\text{sat}} - P_{\text{methanol}}^{*\text{sat}})} \qquad (6)$$

The saturation pressures of the pure sorbates were measured at 25 °C in a sorption device described elsewhere [25]. They were found to be 3200 Pa and 17053 Pa for water and methanol respectively. The saturation pressure of mixtures were also measured experimentally and they were found to be consistent with the calculated saturation pressures (difference of less than 2 %)

Each sorption isotherm was obtained by increasing the partial pressure of the mixture by 0.05 steps from 0 and up to 0.9 $p_{\text{mix}}/p_{\text{sat, mix}}$ prior to decreasing down to 0 using the same steps. Pure vapor sorption isotherms were also obtained using a 3Flex (Micromeritics, USA) apparatus. The activation procedure was the same as that used for mixtures sorption experiments.

Microcalorimetric measurements were performed using a C80 (Setaram, France) instrument connected to a built-in vapor dosing
system [26]. The sample cell and the reference cell were placed in the microcalorimeter after degassing at 200 °C for 8h under secondary vacuum. The cells were connected to the vapor dosing system which was placed in a climatic chamber at a constant temperature of 298 K. In order to obtain the interaction at very low coverage, a small amount of pure vapor sorbate was introduced in the sample cell and let to adsorb on CALF-20. The equilibrium was obtained in around 180 min for all the sorbates tested.

Molecular simulations. In order to simulate the adsorption enthalpy and sorption isotherms and to investigate the adsorption sites, Grand Canonical Monte Carlo (GCMC) simulations have been conducted. To achieve this, DFT geometry optimization of CALF-20 was prior performed to extract the partial charges for each atom using DMol3 considering GGA/PW91 as a functional and DNP basis set. The partial charges for CALF-20 are reported in the SI file. To complete the force field for the solid, UFF force field has been used for the Lennard Jones parameters [27]. For water and methanol, TIP4P-2005 and TraPPE force fields respectively were considered for both partial charges and Lennard Jones parameters [28,29]. Thus, during GCMC simulations, the electrostatic part of the energy can be calculated by the Ewald summation, while the Lennard Jones parameters were combined using the Lorentz-Berthelot rules to evaluate the van der Waals interactions. In order to take into account the flexibility of CALF-20, different multi-cell structures have been built. The multi-cell considered in the GCMC simulations was depending on the cell parameters and was 18 (3 × 3 × 2) unit cells for the saturated form for methanol, 27 (3 × 3 × 3) and 36 (3 × 4 × 3) unit cells for the initial/empty and saturated forms for water, in order to have a cut-off for Lennard Jones close to 12 Å.

The GCMC simulations were performed at 300 K (to be consistent with experiments), using SORPTION (BIOVIA) [30]. The simulations were performed using 30 × 10⁶ Monte Carlo steps for both equilibration and production steps and both the framework and vapor molecules were kept rigid during the whole adsorption process. Using the Metropolis algorithm, the ratio between for monogas adsorption for rotation/-translation/regrowth was 1/1/0.1 and in the case of mixtures for exchange/rotation/translation/regrowth was 2/1/0.1. In the case of mixtures, similarly to the experiments, saturation pressures were calculated using Raoult and Dalton laws and calculations were conducted similarly to the case of pure vapors. From these calculations, it was possible to extract the adsorption enthalpy at low coverage for the different vapors, calculate the adsorption isotherm and distinguish the adsorbed molecules in the case of mixtures, and probe the adsorption sites of CALF-20 as a function of the vapor molecules studied.

# 3. Results and discussion

## 3.1. Preliminary considerations

CALF-20 is a rather new MOF, discovered in 2021 [20]. Since, there have been quite a few studies which have praised the properties of this material for applications, primarily related to capturing CO₂ in humid environments [20,21,23,31,32]. The presence of water in the pores of CALF-20 has been highlighted as a promotor for CO₂ capture, despite its hydrophobic character [23]. However, because CALF-20 is flexible and can therefore exist as different polymorphs [16], it is important to carefully characterize it after activation in order to better understand its sorption properties. Another prime interest is the possible correlation of our experimental studies with GCMC modelling, which requires defined crystal structures.

## 3.2. Characterization

The powder x-ray diffractogram of the synthesized CALF-20 sample is presented in Fig. 1 along with the simulated diffractogram. The diffractograms obtained are very similar to those already published [33], thus confirming the successful production of CALF-20.

<figure><img src="image_4.png" /><figcaption>Fig. 1. Powder x-ray diffraction pattern of the obtained material.</figcaption></figure>

Nitrogen sorption and thermal analyses performed on our material were also consistent with these previous results (Figs. S1-S2). The former revealed a type I sorption isotherm with a flat saturation plateau obtained at $p/p° = 0.1$ while the latter confirmed both the thermal stability of CALF-20 up to 375 °C and an appropriate temperature range for degassing stages between 200 and 300 °C. An equivalent specific surface area of 515 $m^2 g^{-1}$ was obtained with micropores ranging between 6 Å and 7 Å in size. Thermodiffraction was also performed in order to confirm the phase transitions occurring upon vapor sorption and check if similar transition occur in presence of methanol. The corresponding thermo-diffractograms are presented in Fig. 2 in the case of water-saturated CALF-20 (left) and methanol-saturated CALF-20 (right). In the case of water, a clear phase transition can be seen below 40 °C, with the (100) hkl plane shifting to lower 2θ° and the (011) hkl plane shifting to higher 2θ°. In the case of methanol, a large shift of the hkl planes can be also observed which is consistent with the cell volume change (See Table S1). The crystal data and structure refinement of CALF-20 in its different states are reported in Table S1 and Table S2. These results confirm the expansion of the crystal in the former orientation and a contraction in the latter, confirming the findings of Chen et al. and Oktavian et al. [23,33].

A stronger structure modification is observed for methanol with a unit cell volume increase close to 220 Å³ from empty structure (730 Å³) to methanol-saturated structure (950 Å³), whereas a slight contraction is observed in the case of water, with a unit cell volume going from 725 Å³ down to 695 Å³ upon saturation.

## 3.3. Pure vapor sorption

All the sorption experiments, using pure vapors or vapor mixtures, were replicated three times in order to confirm our findings.

Fig. 3 presents the sorption isotherm of water and methanol by CALF-20. The obtained sorption isotherms are markedly disparate. In the case of methanol, a high affinity shape is obtained, with a flat saturation plateau reached at 0.15 $p/p_{sat}$. In contrast, the isotherm for water exhibits a sigmoidal shape, which is characteristic of cooperative sorption processes. The enthalpies of sorption at very low coverage were measured and simulated for both systems. In the case of the low loading structures, -33.4 kJ mol⁻¹ and -63.8 kJ mol⁻¹ were obtained experimentally for water and methanol, respectively.

These results are in line with the shape of the sorption isotherms, a high affinity for methanol inducing a high sorption enthalpy, while the
<figure><img src="image_5.png" /><figcaption>Fig. 2. Powder x-ray diffraction pattern of the synthesized material saturated at low temperature and heated at 100 °C with (left) water; (right) methanol.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Vapor sorption isotherms by CALF-20 at 25 °C; (Squares) Water, (Circles) Methanol. Open symbols correspond to desorption branches. Inset: methanol adsorption isotherm at low $p/p_{sat}$.</figcaption></figure>

hydrophobic character of CALF-20 leads to weak interaction with water, even below the enthalpy of liquefaction of water. The theoretical values obtained from Monte Carlo simulations (for a loading equal to 1 molecule of solvent per multicell) were in good agreement with the experimental values (−41 kJ mol⁻¹ for water and −54 kJ mol⁻¹ for methanol) in empty CALF-20 structures, confirming the hydrophobicity and the stronger affinity for methanol for the empty CALF-20. These results are consistent with those published by Oktavian et al. [33], thus confirming the consistency of the force fields used in our study. However, these interactions varied when the saturated CALF-20 structure was examined. Indeed, the water-saturated structure (extracted from thermodiffraction) became hydrophilic, since the water adsorption enthalpy at low coverage reached −53 kJ mol⁻¹, while the methanol adsorption enthalpy at low coverage in the empty methanol-saturated structure remained similar to the value calculated in the empty structure (−54 kJ mol⁻¹).

The inset in Fig. 3 shows a sub-step which can be attributed to the phase transition between the α and the β CALF-20 structure when methanol is adsorbed. In Table S1, the cell volumes have been calculated, based on the XRD results, in the case of dehydrated CALF-20 and CALF-20 saturated with methanol. The calculated cell volumes are 730 Å³ and 950 Å³ respectively.

This swelling is consistent with the results published by Chen et al., even though water adsorption was studied instead of methanol [23]. These authors have demonstrated that upon sorption of water, CALF-20 undergoes a reversible phase transition, which can be therefore observed for methanol adsorption. In the case of water, the CALF-20 structure undergoes a small contraction from 730 Å³ down to 695 Å³ (See Table S2). This slight contraction is not particularly visible on the water sorption isotherm. However, the important sorption from 0.02 $p/p_{sat}$ upwards suggests a densification of the adsorbed phase, despite this limited contraction, likely due to the H-bonding between water molecules. This could be explained by the affinity transition of the structures when passing from the empty (hydrophobic) to the water-saturated (hydrophilic) forms. Following this hypothesis, the structural transition probably occurs around 0.02 $p/p_{sat}$.

The simulated sorption isotherms are presented in Fig. S3. They are consistent with those shown in Fig. 3 in terms of sorbate/CALF-20 affinity even though the saturation capacities slightly differ, probably due to the fact that the experimental material has defects as it is not a perfect crystal. In the case of methanol, the empty form and the methanol-saturated form have similar shape and saturation reaches 80 mg/g and 120 mg/g respectively. The shapes of the theoretical sorption isotherms were in good agreement with the experimental ones, confirming that the force fields used in the calculations were consistent to reproduce the interactions between vapors and CALF-20. However, the substep shown in Fig. 3 could not be simulated using our forcefields developed for rigid structures. More importantly, the sorption processes have been also simulated at different partial pressures. The corresponding density of presence (DOP) are shown in Fig. 4.

At low $p/p_{sat}$ water is mainly adsorbed on oxalate groups interacting with the solid through H-bonding (see also Figure S5 (a)), while as partial pressure increases, the cooperative sorption of water leads to the full occupation of the pores, regardless of the surface sites (see Figure S5 (b)). In the case of methanol, the privileged sorption sites are the oxalate groups as well, and close to saturation, the main occupancy remains located on the oxalate functions. This means that the methanol molecules are more localized in the structure than water molecules, probably due to the H-bonds accessible for water molecules and not for methanol. However, the stronger interactions seem to involve methanol, as confirmed by the experimental and theoretical adsorption enthalpy values at low coverage.

This is consistent with the fact that methanol can interact only with oxalate ligands by strong H-bonding while water can interact by many weaker H-bonds with oxalate ligands and other water molecules. Such a behavior can be observed from the snapshots (see Figure S5, c and d) in which distances for interaction range between 2.0 and 2.4 Å for both water and methanol. However, many interaction between water
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Density Of Presence for sorbate molecules in CALF-20 at 25 °C. (Left) Water; (Right) Methanol. (Top) Low $p/p_{sat}$ ($p/p_{sat}$ = 0.1 for water and 0.007 for methanol) in the empty structures for both methanol and water; (Bottom) High $p/p_{sat}$ ($p/p_{sat}$ = 0.8 for water and 0.9 for methanol) in the saturated structures for both methanol and water.</figcaption></figure>

molecules can be observed while only very few interactions between methanol can be noted. The hydrophobicity of CALF-20 shown in Fig. 4 can be accounted for by the different shapes of the saturation plateaus of the sorption isotherms. In the case of methanol, a flat plateau is obtained, consistent with a structured adsorbed phase which is not modified by increasing methanol vapor pressure. In the case of water, molecules are quantitatively adsorbed in the micropores up to 0.95 $p/p_{sat}$ which can be interpreted by a poorly stable adsorbed phase which becomes denser as $p/p_{sat}$ increases.

### 3.4. Vapors mixtures sorption

The sorption isotherms of the vapor mixtures obtained by DVS are shown in Fig. 5. Note that, compared to Fig. 3, the uptakes have been reported as masses adsorbed instead of adsorbed amounts. Indeed using our simple DVS system, in the case of vapors mixtures it is not possible experimentally to distinguish between water and methanol in the adsorbed phase.

However, by assuming that water and methanol are perfect gases at such low pressures, the adsorbed amounts can be easily converted to mg. $g^{-1}$. Doing so, the saturation plateaus of the pure sorbates are located at ~170 $mg\ g^{-1}$, superimposed with those of the water-methanol mixtures, which makes the analysis somewhat more difficult. The partial pressures of water-methanol mixtures were determined by assuming the Raoult and Dalton laws to be applicable (see the experimental section) [27]. It is difficult to have a clear understanding about the very first stages of the sorption processes, as the DVS does not allow to measure partial pressures below 0.05. However, the similarity between the mixtures and pure methanol appears clearly. Indeed, the curves are superimposed from the low partial pressure; It can be deduced that methanol facilitates

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Sorption isotherms of the vapor mixtures, along with the pure sorbates at 25 °C, where $p_{sat}$ refers to the saturation pressure of the pure sorbates or that of the mixtures, as defined in equation (6).</figcaption></figure>

the sorption of the mixtures regardless of their composition, at least in range of the compositions studied. This is confirmed by the DOP obtained from GCMC simulations, as shown in Fig. 6 in the case of a 50 mol % for each component while other compositions evidenced a similar behavior (see Fig. S6).
<figure><img src="image_9.png" /><figcaption>Fig. 6. Density Of Presence for a 50 % molar water-methanol mixture in CALF-20 at 25 °C. (Left) Low p/p<sub>sat</sub>; (Right) High p/p<sub>sat</sub> (green points for methanol and red point for water). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

Indeed, the data presented in Fig. 6 allow us to confirm that methanol molecules again interact with oxalate groups, whereas water molecules appear to interact with the other organic component (triazolate group) of CALF-20.

Compared to experimental gravimetric measurements of sorption isotherms, GCMC can distinguish which molecules are effectively adsorbed from the vapor mixture. The corresponding sorption isotherms along with the main interaction sites for each sorbate are shown in Fig. S7 while the snapshots extracted from calculations are presented in Fig. S8. Fig. S7 confirms the higher affinity of CALF-20 for methanol compared to water at low loadings, while a switching of affinity seems occur at higher pressures for all water/methanol ratios, even if the affinity for methanol is larger for 33/66 and 50/50 methanol ratios. Indeed, methanol is systematically adsorbed from low mixture partial pressures, while water is adsorbed from higher mixture partial pressures, even in the case of water-rich compositions of the mixtures. It is noteworthy that methanol is preferentially adsorbed at low partial pressure at the expense of water molecules regardless of the composition of the vapor phase. This trend is reversed at high partial pressure where water is more adsorbed than methanol. Moreover, irrespective of the H2O/MeOH ratio, the saturation plateau is observed to reach 110–120 mg/g, which is in reasonable agreement with the respective experimental plateaus observed for all mixtures. In the particular case of the methanol-rich mixture, similar amounts of both components are adsorbed at saturation which shows that CALF-20 can be used for efficient methanol adsorption, even in the presence of water.

In the snapshots from Fig. S8, it can be seen that methanol molecules interact mainly with the framework but also with water molecules, while water molecules can interact together, thus confirming the cooperative water sorption process described in the case of pure water sorption. Due to the similar interaction between water molecules or water and framework, the hydrophobic behavior of CALF-20 is therefore emphasized. Interestingly, the distances between water and framework seem shorter, which strongly suggests that the proximity of methanol species promotes water-CALF-20 interaction. As it was already found in the case of pure methanol sorption, methanol species occupy the oxalate functions while water mostly interacts with methanol through H-bonding. At high p/p<sub>sat</sub>, a phase separation appears as water species are mostly located in the centers of the pores, far from the oxalate groups. This is confirmed by measuring the distances between sorbates species and the oxalate functions as can be seen in Fig. S8. This result is quite surprising as water and methanol are fully miscible in the liquid phase, for any composition.

### 3.5. Kinetics aspects

In order to have a better understanding on the sorption mechanisms involved, we measured the duration required to reach the saturation plateaus in the case of pure water or methanol sorption at 25 °C. The kinetic profiles are shown in Fig. S9. Saturation equilibrium is reached in less than 30 min for methanol against 85 min in the case of water. This is consistent with the experimental sorption enthalpies of the two sorbates (–33.4 kJ mol<sup>-1</sup> and -63.8 kJ mol<sup>-1</sup> for water and methanol respectively), as higher interaction leads to faster siting.

Because of this enthalpic difference, we performed water or methanol sorption on CALF-20 pre-saturated with methanol or water respectively. In the first case, Fig. 7 was obtained.

In the case of CALF-20 saturated with methanol, the sorption of water proved quite long, the thermodynamic equilibrium being obtained after 6 h. This is very different to the case where water was adsorbed on pristine CALF-20. It is quite understandable that obtaining a thermodynamic equilibrium is more difficult to be reached, compared to the situation where CALF-20 is empty of molecules. Indeed, water

<figure><img src="image_10.png" /><figcaption>Fig. 7. Sorption kinetics of water on CALF-20 pre-saturated with methanol at 25 °C using a Dynamic Vapor Sorption device. The schemes, as insets, represent the sorption selectivity of CALF-20 under pure vapor flow.</figcaption></figure>
molecules must diffuse in the pores which are already filled with methanol molecules. More importantly, at the thermodynamic equilibrium (at 540 min in Fig. 7), the adsorbed amount at saturation is located above the saturation plateaus observed when water or methanol were adsorbed as single components or even as mixtures (see in Fig. 5). We have already mentioned that CALF-20 has a flexible structure whose volume can increase by 30 % upon methanol adsorption. However, the structure has been demonstrated to be open after methanol adsorption (See Fig. S4), which is precisely the case on the saturation plateau at 180 min in Fig. 7. As a consequence, the flexibility of CALF-20 cannot be evoked for explaining sorption beyond saturation. Another interpretation of this peculiar feature could be that upon mixing, the total volume decreases, compared to that of an ideal adsorbed solution, which could allow extra molecules to be adsorbed, leading to an increased adsorbed mass at saturation. However, this effect should be seen in Fig. 5 where mixtures were adsorbed. A remaining plausible explanation is a rearrangement of the adsorbed phase, imposed by the pure water vapor flow on the material. This situation is very different to the sorption experiments where mixtures were prepared at first and let to adsorb in CALF-20. In this case, water and methanol are introduced at the same time, and these two species adsorb in the pores in equilibrium with a vapor phase which has a constant composition.

In order to elucidate the mechanisms occurring in this case where methanol is adsorbed before water, Monte Carlo calculations were performed as follows: the most plausible (lowest energy) configuration for saturated methanol CALF-20 structure was used to adsorb water molecules at saturation pressure of water. The first adsorbed water molecule generated an adsorption enthalpy of –74 kJ mol⁻¹. This value has to be compared with the sorption enthalpy of water in the empty methanol-saturated structure, that is –47 kJ mol⁻¹ (see Table S3). It can be deduced that the sorption of water in the structure saturated by methanol is energetically stable which was confirmed by GCMC. The amount of water adsorbed in the process could be calculated, taking the cell volumes of the saturated phases into account. An additional water sorption of 74 mg g⁻¹ from Monte Carlo which is compatible with the mass uptake observed in Fig. 7 (from 170.6 mg g⁻¹ to 192.8 mg g⁻¹) when replacing the pure methanol flow with the pure water flow. From thermodiffraction, the cell volume of CALF-20 filled with water or methanol were found to be 695 Å³ and 950 Å³ respectively. The discrepancy observed is likely related to the fact that the calculations are based on the hypothesis that the structure does not evolve upon water adsorption (after methanol saturation). In situ XRD was not performed during sorption experiments, but it is possible to think that the material at the end of the sorption process should be a mixture of crystalline phases. Besides, the composition of the adsorbed phase was also investigated. If the nature of the adsorbed phase is clear when only methanol is adsorbed, it is questionable when the equilibrium is reached after 550 min under water vapor flow. To answer this question, CALF-20 was further removed from the DVS machine at the end of the water adsorption on methanol-saturated CALF-20, and quickly placed in a flask filled with argon to prevent water vapor from air to interfere with water adsorbed in CALF-20. Then, the closed flask was placed at 150 °C to evaporate the adsorbed phase. After 2 h of desorption at 150 °C, the composition of the vapor was assumed to be that of the adsorbed phase, apart from argon. Some vapor was taken from the closed flask through a septum using a syringe and injected in a GC-MS which was calibrated using methanol-water mixtures, both in terms of retention times, but more importantly in terms of correlation between peaks surfaces and composition (see Fig. S10). The average composition of the equilibrated adsorbed phase was found to be 97.14 wt% in water and 2.86 wt% in methanol. It can be concluded that, in this hydrophobic material, water is able to fully wash CALF-20 from methanol, simply by using a water vapor flow at ambient temperature. Having such a high water adsorbed content, it is also surprising that the saturation is higher compared to that obtained with pure water. The role of the remaining few methanol molecules in the adsorbed phase is therefore quite important, as it allows

a greater amount of sorbates to be adsorbed and probably modifies the hydrophobic/hydrophilic character of the structure by swelling as described above.

The symmetrical experiment was also examined. In this instance, CALF-20 was initially saturated with water through a water vapor flow, followed by a methanol vapor flow. The identical experiment was conducted three times to confirm our findings. Fig. 8 was generated. The adsorption of methanol on the water-saturated CALF-20 was found to be unexpected, exhibiting a striking characteristic. By switching the order of sorbates in contact with CALF-20 (methanol-water in Fig. 7, and now water-methanol), a very large mass increase appears. As the saturation of CALF-20 was already obtained with water beyond 120 min, it is indeed very surprising that CALF-20 was still able to accommodate extra molecules in its porosity. Even though upon methanol sorption, the pore volume increases by about 30 %, the uptake cannot be accounted for by this flexibility. In fact, the mass uptake at 210 min is 400 mg g⁻¹, which is even higher than the sum of the saturation plateaus using pure water and pure methanol alone. However, this situation is not stable, as the mass uptake rapidly decreases down to 175 mg g⁻¹, which precisely corresponds to the saturation isotherm using pure sorbates. The compositions of the system were investigated, in order to better interpret our findings. Two locations were of first interest: the maximum uptake (at 210 min) and the uptake at the stable equilibrium (at 600 min).

The very same procedure detailed above was followed to extract the adsorbed phase from the porosity of CALF-20, even if in this case, a larger flask was used. Indeed, at the maximum uptake, a doubled amount of vapor was produced. To ensure that the whole adsorbed species would be desorbed from CALF-20, the pressure in the flask had to be kept close to the atmospheric pressure. At the maximum, the composition of the adsorbed phase was found to be 53 wt% of methanol against 47 wt% of water. Additionally, the two chromatographic peaks surfaces corresponding to water and methanol were found to be very close to those obtained in the case of pure components kinetics analysis. It can be concluded that CALF-20 can accommodate water and methanol as if they were adsorbed alone, even though they are adsorbed at the same time in the micropores of CALF-20.

Different interpretations can be put forward to explain how CALF-20 can accommodate both water and methanol as if they were alone in the pores. A first hypothesis could be a densification of the adsorbed phase, or even a liquefaction of the adsorbed phase. This could be understood in a material in which molecules do not completely fill the cavities, as can

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Sorption kinetics of methanol on CALF-20 pre-saturated with water at 25 °C using a Dynamic Vapor Sorption device. The schemes, as insets, represent the sorption selectivity of CALF-20 under pure vapor flow.</figcaption></figure>
be seen in Figs. 4 and 6. Furthermore, the presence of pre-adsobered water molecules in the pores can favor the adsorption of methanol by H-bonding on sites different from the oxalate groups. Although the transfer of the sample from the DVS machine to the flask may introduce some uncertainty, as the system is not at equilibrium with the vapor phase, the results are nevertheless compelling. They indicate that, at maximum, the adsorbed phase is composed of equal proportions of water and methanol, which is consistent with some mixtures sorption isotherms shown in Fig. S7.

A second interpretation could be that under methanol flow, the water adsorbed phase is pushed out from the pores. This would result in an increase of the mass uptake, the pores being progressively filled with a methanol water mixture while some of the adsorbed water would be desorbed from the pores. In this case, the decrease of the curve would correspond to the vaporization of the liquid phase formed on the external surface of the pores.

At 600 min, where a stable equilibrium is obtained, the composition of the adsorbed phase was found to be 61 wt% in methanol, and therefore 39 wt% in water while the vapor flow is still 100 wt% methanol. It can be concluded that methanol is not able to fully desorb water from CALF-20 to adsorb itself, even if this MOF is demonstrated to be very hydrophobic. This sorption mechanism can be discussed, based on the interactions involved. In the first 2 h of water sorption, the structure slightly shrinks from 725 Å down to 695 Å. During this phase change, the water sorption enthalpy increases from $-41 \text{ kJ mol}^{-1}$ to $-54 \text{ kJ mol}^{-1}$. Methanol is then contacted with the water-saturated CALF-20 structure. The first methanol molecule generates $-40 \text{ kJ mol}^{-1}$ (See Table S3). It seems surprising that methanol can be adsorbed in the material in which water is already at saturation. However, methanol and water do not necessarily occupy the same sites as seen in Fig. 6. This could explain why methanol and water can be adsorbed according to the kinetic profile shown in Fig. 8. If the two different sorption mechanisms can be interpreted in terms of interactions, the reason why the maximum is not stable is yet to be rationalized. However, different suggestions can be put forward. First, it can be suggested that the interactions involved gradually change as the composition changes. At the maximum it could be possible that the mixture is no longer stable, resulting in the desorption of half of the adsorbed phase, its composition remaining very close at the end of the desorption. Second, it can be proposed that because of the interaction involved at the maximum, a phase separation occurs, as evidenced in Fig. 6(right), resulting in a destabilization of the adsorbed phase.

## 4. Conclusion

CALF-20 is a fascinating material already being used at an industrial scale for carbon dioxide capture. In this study, it is shown that beyond its CO₂ sorption capabilities, CALF-20 is also able to selectively adsorb humid VOCs such as methanol. Complementary experimental and simulation results allowed us to assess the sorption mechanisms for pure sorbates as well as for water-methanol mixtures. It is demonstrated that the interactions involved are clearly related to the crystal phase in which species sorbates are adsorbed. These interactions therefore play a major role, both in terms of thermodynamic equilibrium as well as in terms of sorption kinetics. We demonstrated that CALF-20 is capable of adsorbing comparable quantities of methanol-water mixtures, irrespective of the mixture composition. More importantly, we showed that despite CALF-20's hydrophobic nature, water can effectively remove methanol from its pores, ought to the flexibility of the MOF structure which deeply modify the sorbate-structure interactions. However, the reverse phenomenon is not possible: methanol vapor cannot displace water from the pores. In fact, its dissymmetry with regard to these sorption processes renders it even more fascinating. The dissymmetry can be observed at two different levels: The overall adsorbed amount levels and the composition of the adsorbed phase at 560 min on one hand (see Figs. 7 and 8), and the occurrence of the maximum uptake in the case of the

water followed by methanol ad-sorption on the other hand. These findings pave the way to improvements in Sorption-Enhanced Reaction Process technology for pure methanol production from carbon dioxide, where selective sorbents are required for shifting the methanol production equilibrium.

## CRediT authorship contribution statement

**Julia Duplessis-Kergomard:** Writing – original draft, Investigation, Data curation. **Meryem Saidi:** Investigation, Data curation. **Olinda Gimello:** Supervision, Conceptualization. **Bernard Fraisse:** Validation, Supervision, Formal analysis, Conceptualization. **Fabrice Salles:** Writing – review & editing, Writing – original draft, Software, Formal analysis, Conceptualization. **Philippe Trens:** Writing – original draft, Supervision, Project administration, Funding acquisition, Data curation, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.micromeso.2025.113525.

## Data availability

Data will be made available on request.

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