Water Vapor Adsorption on CAU-10-X: Effect of Functional Groups on Adsorption Equilibrium and Mechanisms.

Metal-organic frameworks (MOFs) possess unique flexibility of structure and properties, which drives them toward applications as water adsorbents in many emerging technologies, such as adsorptive heat transformation, water harvesting from the air, dehumidification, and desalination. A deep understanding of the surface phenomena is a prerequisite for the target-oriented design of MOFs with the required adsorption properties. In this work, we comprehensively study the effect of functional groups on water adsorption on a series CAU-10-X substituted with both hydrophilic (X = NH2) and hydrophobic (X = NO2) groups in the linker. The adsorption equilibrium is measured at P = 7.6-42 mbar and T = 5-100 °C. The study of water adsorption by a set of mutually complementary physicochemical methods (TG, XRD in situ, FTIR, and 1H NMR relaxometry) elucidates the nature of primary adsorption sites and water adsorption mechanisms.

Introduction

During the last decades, adsorptiopan class="Chemical">n has been attracting considerable interest for various energy and environment-related technologies such as water harvesting from the air,[1−3] thermal energy conversion and storage,[4−6] adsorptive cooling desalination,[7] etc. An adsorbent is a vital part of the energy-related applications as their performance strongly depends on the equilibrium in the system “adsorbent–adsorptive”.[6,8,9]

It was shown that apan class="Chemical">n adsorbent with a stepwise adsorption isotherm (Figure S1, top anpan>d bottom), which exchanges a large amounpan>t of adsorptive (e.g., water) in a narrow range of relative pressure, is advantageous for many adsorption applications such as heat conversion and storage[10−13] as well as a desiccant in air conditioning systems.[14] On the other hand, adsorbents with energy-different adsorption sites and gradual uptake rise in a certain range of relative humidity (Figure S1, middle) are beneficial for adsorption water harvesting from the atmosphere,[3] moisture recuperation in ventilation,[15] etc. As soon as the optimal isotherm shape is theoretically determined, the target-oriented design of the proper adsorbent with the desired adsorption equilibrium is the next, practical step (Figure S1).

In recepan class="Chemical">nt years, metal–organic frameworks (npan> class="Chemical">MOFs) have attracted researchers’ interest as water adsorbents for various adsorption technologies.[16] Due to their large porosity, specific surface area, and remarkable structural flexibility with innumerable possible structures, MOFs can demonstrate unique adsorption properties.[17] Particularly, extra-large water adsorption capacity up to 1.4 g/g of MIL-100, MIL-101, and Co2Cl2(BTDD)[3,18,19] and unique stepwise water adsorption isotherms for CAU-10-H,[20] NH2-MIL-125,[4] MOF-841,[21] Al-fumarate,[22] MOF-801,[23] and UiO-66[24] can open exciting prospects for adsorption applications.

Reticular synthesis of papan class="Chemical">n class="Chemical">MOFs, or the assembly of target structure by a modular way from selected inpan>organic and organic molecular building unpan>its linked together, is a powerful way to design a required MOF structure.[25,26] However, the atomic-level understanding of the influence of structural and chemical factors on water adsorption properties of MOFs has yet to be achieved. In this regard, a deeper understanding of the surface phenomena and “structure–properties” relationship is a prerequisite for designing the MOFs with desirable adsorption properties.

There are three waterpan> adsorption mechanisms distinguished for MOFs:[16,27] (1) adsorption on coordination-unsaturated metal sites (CUS), which are the strongest adsorption sites that retain water molecules at low relative pressure P/P0 (e.g., HKUST-1,[28] MOF-74,[29] etc.). (2) Layer (or cluster) adsorption or continuous filling of pores, with the size less than a critical diameter Dc = 4σTc/(Tc – T) equal to ∼2 nm for water. The weaker adsorption sites become filled at a higher P/P0, values leading to gradual adsorption isotherms of I or III types according to IUPAC classification for hydrophilic (e.g., HKUST-1[28] and MOF-74[29]) and hydrophobic MOFs (ZIF-8[30]), respectively. (3) Capillary diameter for mesoporous MOFs with pore size larger than the critical Dc (MIL-101-Cr,[31] MIL-100-Cr,[32] and Cr-soc-MOF-1[33]). The condensation in pores is described by adsorption isotherms of IV or V types with a hysteresis loop, which is undesirable for applications.

Severalpan> MOFs exhibit unusual hybrids of the isotherm types. Particularly, some microporous MOFs can display the isotherms of types III or II that transforms to type I at a higher P/P0. The hybrid stepwise adsorption isotherms are usually attributed to the structural transformation of the framework induced by adsorbed molecules.[34] This behavior was observed for MOFs with a flexible structure, such as well-known MIL-53, which demonstrates a large breathing effect on the response to guest molecules inclusion.[35] Two kinds of structural changes were observed for 3D pillared {[Cd2(pzdc)2L(H2O)2]·5(H2O)·(CH3CH2OH)} coordination polymer upon water adsorption, namely (a) the gate locking/unlocking due to ligand rotation, and (b) the framework expansion due to layers slippage, which results in stepped water adsorption isotherm.[36] Nonporous (CH3)2NH2)2[Li2Zr(C2O4)4] structure transforms to another dense structure upon topotactical hydration to H2O/Zr = 0.5 mol/mol, which further reversibly transforms to a hydrated structure H2O/Zr = 4 mol/mol through the lattice expansion.[37] The stepwise water vapor adsorption was observed for Mg(HCO2)2 and Co(HCO2)2 associated with reversible amorphous-to-crystalline structure transformation trigged by initial adsorption sites of water molecules on the MOFs surface.[38] Thus, the detailed study of the structural transformations on MOFs upon hydration is crucial for understanding water adsorption mechanisms on MOFs and prediction of their adsorption equilibrium. Other reasons of the S-shaped adsorption isotherms could be the so-called cooperative adsorption mechanism with a strong adsorbate–adsorbate interaction[4] and the existence of different energy sites on the MOFs surface at a certain percentage.[39] Increasing the number of high energy adsorption by doping metal ions is a powerful tool for shifting the adsorption isotherm to a low pressure range.[39] Thus, revealing the primary water adsorption sites of MOFs and the mechanisms of adsorption interactions being responsible for the stepwise and gradual adsorption isotherms on MOFs is of importance for designing the MOFs with required properties.

A familpan>y of microporous MOFs, namely, CAU-10-X (X = H, NO2, NH2: functional group in linker), possessing similar framework topology with the basic formula unit [Al(OH)(C8H3O4X)], is characterized by different types of adsorption equilibrium with water vapor.[40] Particularly, CAU-10-H possesses the stepwise water adsorption isotherm with a sharp rise of uptake at the relative pressure P/P0 ≈ 0.18. CAU-10-NH2 is characterized by a stronger affinity to water vapor with adsorption isotherm approaching type I. CAU-10-NO2 with a weaker affinity to water vapor possesses the S-shaped water adsorption isotherm with a gradual increase of water uptake at P/P0 = 0.2–0.5. Thus, the CAU-10-X family is of great interest for studying the effect of functional groups on the mechanism of adsorption interactions and types of adsorption isotherms associated with them.

This work is dedicated to the comprehensive study of the papan class="Chemical">n class="Chemical">water vapor adsorptionpan> on the CAU-10-X (X = H, NO2, NH2). Complementary physicochemical methods (TG, XRD in situ, FTIR spectroscopy, and H1 NMR relaxometry) are used to reveal the adsorption mechanisms leading to the different types of the adsorption isotherms for these iso-structure MOFs. The results obtained will help to rationalize the design of MOFs with desired adsorption equilibrium, specialized for various applications.

Experimental Section

Materials Preparation

The three materin class="Chemical">alpan>s CAU-10-X (X = H, NO2, NH2) were synthesized by the solvothermal method following a slightly modified procedure described in refs (17 and 40).

Benzene-1,3-dicarboxylic acidpan> (Acros Organics, 99%), 5-aminobenzene-1,3-dicarboxylic acid (Acros Organics, 99%), 5-nitrobenzene-1,3-dicarboxylic acid (Aldrich, 98%), N,N-dimethylformamide (Aldrich, ≥99.8%), Al2(SO4)3·18H2O (Acros Organics, 98+%) and AlCl3·6H2O (Aldrich, 99%) were used as supplied without further purification.

For preparation of papan class="Chemical">n class="Chemical">CAU-10-H, npan> class="Chemical">Al2(SO4)3·18H2O (0.8 g, 1.2 mmol) and benzene-1,3-dicarboxylic acid (0.2 g, 1.2 mmol) were dissolved in 4 mL of H2O and 1 mL of N,N-dimethylformamide (DMF). The obtained mixture was sealed in a Teflon-lined autoclave and placed in a preheated oven at 135 °C for 12 h. After cooling down to 25 °C, the mixture was filtered and washed with 50 mL of hot water thrice. The obtained powder was dried at 100 °C, which was followed by activation at 150 °C for 24 h under evacuation.

The samples of CAU-10-NH2pan> and CAU-10-NO2 were synthesized by dissolving 5-aminobenzene-1,3-dicarboxylic acid (1.2 g, 6.63 mmol) and 5-nitrobenzene-1,3-dicarboxylic acid (1.44 g, 6.82 mmol), respectively, in DMF (4 mL) and water (12.7 mL). Then a 2 M solution of AlCl3·6H2O (3.3 and 3.4 mL for CAU-10-NH2 and CAU-10-NO2, respectively) was added to the mixtures. The mixtures were heated in the autoclaves at 120 °C for 12 h. After cooling to 25 °C, the obtained powders were processed in the same way as CAU-10-H.

Characterization

Powder X-ray Diffraction (PXRD)

Patterns were recorded opan class="Chemical">n a Rigaku Miniflex 600 X-ray diffractometer (Θ −2Θ geometry). It was equipped with an X-ray generator (Cu Kα radiation filtered with Be monochromator, λ = 1.5419 Å, 40 kV, 15 mA), D/tex Ultra2 1D detector, and an Anton Paar BTS 500 chamber for temperature and humidity control. The measurements were carried out in the scan mode over a 2Θ range from 5° to 40° with a step size of 2Θ = 0.005° and an accumulation time of 0.1 s at each point.

In situ pan class="Chemical">PXRD experiments were performed in the air flow saturated with water vapor at 30 °C in the reaction chamber and at a conpan>trolled RH. Before the scanning, the dry samples were powdered by using agate mortar and pestle, placed in a Ni sample holder and dried in situ at 120 °C for 2 h in the flow of dry air (800 mL/min). The humidity control was carried out by a homemade humidity generator. The relative humidity was controlled by mixing the flow of dry air (RH < 0.5%) and fully saturated air at 20 °C (RH > 98%). The humidity in the Anton Paar chamber was calibrated by using deliquescence points of the selected salts/hydrates (LiCl, MgCl2·6H2O, Mg(NO3)2·6H2O). The RH was increased in a step-wise manner, 3.5 h per step. After 3 h at a certain RH, the XRD pattern was recorded for 30 min. The RH sensor was situated at the outlet of the chamber to measure both temperature and water vapor pressure. To determine the structure parameters, the Le Bail method was used with the software package TOPAS-4.2. A Thompson-Cox-Hastings function was used for modeling the peak shape. The number of background parameters was 7.

IR Spectra

IR spectra were recorded using apan class="Chemical">n Agilent Cary 660 FTIR spectrometer at 25 °C in the 600 to 4000 cm–1 range. For studying the effect of the water adsorption, the dry samples of CAU-10-X were preliminarily saturated with water up to the uptake w = 0.20 g/g. The spectra were recorded using an attenuated total reflection (ATR) attachment, 025–2018 MIRacle ZnSe Perf Crystal Plate.

Nitrogen Sorption Isotherms

pan class="Chemical">Nitrogen sorptionpan> isotherms were measured by using a Quantachrome Nova 1200e gas sorption analyzer at 77 K, after vacuum degassing at 150 °C for 3 h. The BET surface area SBET was calculated using the BET analysis of the adsorption branch of the isotherm in the relative pressure range of 0.01–0.025. Total pore volume Vp was derived based on the amount of N2 adsorbed at a relative pressure P/P0 = 0.99. The micropore volume Vμ was calculated using the statistical thickness analysis of the isotherm adsorption branch and de Boer’s t-method.

1H NMR Relaxometry Experiments

n class="Chemical">1Hpan> NMR relaxometry experiments were performed by using a home-built GARField setup (1.5T). The NMR setup incorporates an electromagnet generating a magnetic field of 1.4 T at the position of the sample. The magnetic field gradient 36.4 T/m is maintained by using specially shaped magnetic pole tips. Other details of the setup are described elsewhere.[41]

The n class="Chemical">MOFpan>s were dried at 160 °C in air overnight and placed in open glass cells (thicknpan>ess of the glass <100 μm). Each cell was then placed in one of the desiccators with saturated solutions of LiCl (P/P0 = 0.11), CH3COOK (P/P0 = 0.22), MgCl2 (P/P0 = 0.33), K2CO3 (P/P0 = 0.43), and NaNO2 (P/P0 = 0.65) for 24 h to be saturated with water vapor. After the saturation, the cell of interest was picked from a desiccator, immediately closed by a cork and placed in the NMR cell thermostated at T = 20 °C. It is noteworthy that due to equilibration the measured parameters (signal intensity and relaxation times) did not depend on the position of the chosen slice (5–10 μm) in the whole sampling region (∼0.5 mm).

Spin–spipan class="Chemical">n relaxation time T2 was determined by using the Carr–Purcell–Meiboom–Gill pulse sequence.[42] The 90° pulse was followed by 180° pulses with the interval 2τ (Figure ). The dependence of the echo intensity on the time t elapsed from the moment of the 90° pulse was described by an exponent law exp(−t/T2), where T2 is the spin–spin relaxation time. The echo time used for this study was 50 μs with a window width of 40 μs. The duration of both 90° and 180° pulses (pulse time, Figure ) was 1 μs. The long delay was set at 0.5 s, and the number of echoes was 512. The sequence was carried out 1024 times with one equilibrated sample to improve the signal/noise ratio. Before the measurements, tests with solutions of CuSO4 of various concentrations (0.01–0.05 M) were performed to verify the T2 measurements.

Figure 1

Illustration of the CPMG pulse sequence used for measurement of T2.

Illustration of the Cpan class="Chemical">Pn class="Chemical">MG pulse sequenpan>ce used for measuremenclass="Chemical">pan>t of T2.

Water Adsorption

The waterpan> sorption isobars were measured by a thermogravimetric (TG) method using a TA Instruments Q5000SA thermobalance intended for dynamic vapor sorption measurements with the accuracy of ±0.00001 g. The relative humidity in the apparatus was controlled by mixing wet and dry N2 fluxes (200 mL/min in total) and calibrated by using deliquescence points of LiCl (>99.0% purity) and NaBr (>99.5% purity).

For a measurement, 10–15 papan class="Chemical">n class="Chemical">mg of a sample was inpan>troduced into a measuring cell, which was dried ex situ at 160 °C and then in situ in a dry nitrogen flow at 80 °C until a constant weight was reached. After that, the water vapor was introduced, and the weight change was registered until equilibrium. Then the temperature was reduced step-by-step while keeping water vapor pressure constant in flow. The water vapor pressures were 7.6, 12.1, 23.4, and 42.0 mbar, which corresponds to the water saturated pressure at 3, 10, 20, and 30 °C, respectively.

The n class="Chemical">waterpan> uptake was calculated as the mass w of water adsorbed by 1 g of the samplewhere m(P, T) is the equilibrium sample mass and m0 is the dry sample mass. The isosteric heat of water adsorption was calculated by the Clausius–Clapeyron equation.

Results and Discussion

Structure Characterization of As-Prepared CAU-10-X

The genuipan class="Chemical">ne structure of the synthesized CAU-10-X is confirmed by PXRD annpan> class="Chemical">alysis, FTIR spectroscopy, and low-temperature N2 adsorption. The PXRD patterns (Figure S2a) of CAU-10-X match well with those presented in the literature.[40] The structure of CAU-10-H is based on inorganic chains of AlO6-octahedra and isophthalate linker. The octahedral surroundings of the Al ion are formed from the four oxygen atoms, being parts of four different coordinating carboxylate groups, and two bridging μ-OH groups that induce the helical shape of the inorganic chain (Figure a). Wherein the inorganic chains have an inversion axis, which is parallel to the c axis (Figure b–d). Thus, each helical chain connects to four adjacent inorganic units, forming the tetragonal crystal system (Figure b–d) with a square base (parameter a = b) and height (c ≠ a).[40] Square-shaped sinusoidal channels are formed with a maximum size of about 7 Å (Figure b–d). In CAU-10-H, the distance between carbon atoms in 5-position of aromatic rings equals 6.8 Å. For CAU-10-X (X = NH2, NO2) the exact structure has not been determined yet due to their low crystallinity.[40]

Figure 2

Inorganic building unit (a helical chain of AlO6 octahedra) in the structure of CAU-10-X (a). The framework cells of CAU-10-H (b), CAU-10-NO2 (c), and CAU-10-NH2 (d). Atoms C (brown), Al (large blue), O (red), N (small blue), and H (pink).

Inorgapan class="Chemical">nic building unit (a helical chain of AlO6 octahedra) in the structure of CAU-10-X (a). The framework cells of CAU-10-H (b), CAU-10-NO2 (c), and CAU-10-NH2 (d). Atoms C (brown), Al (large blue), O (red), N (small blue), and H (pink).

The PXRD patterns of papan class="Chemical">n class="Chemical">CAU-10-NH2 anpan>d CAU-10-NO2 exhibit reflexes similar to CAU-10-H (Figure S2a in the Supporting Information), which suggests conservation of a tetragonal space group of the frameworks upon introduction of the functional groups. However, almost all reflexes of functionalized MOFs are somewhat shifted with respect to those of CAU-10-H (Figure S1a, inset). For the NO2-modified material, the shift toward higher angles is observed, suggesting shrinkage in the unit cell due to extra interactions introduced by the functional groups. In contrast, the reflexes of the NH2-modified material are shifted toward lower 2θ angles indicating some expansion of the unit cell, probably due to partial formylation of −NH2 groups, which was observed in ref (40).

The isotherms of N2pan> adsorption (Figure S3 in the Supporting Information) belong to type I (IUPAC classification) typical of microporous solids. The porous structure of CAU-10-X was characterized by the BET analysis, by the Horvath–Kawazoe method, as well as by using the Dubinin-Astakhov equation (Table , Table S1, and Figure S4). All CAU-10-X samples possess an extensive surface. The total pore volume of CAU-10-NO2Vp = 0.28 cm3/g is equal to the one for CAU-10-H. On the contrary, for CAU-10-NH2, the pore volume is reduced (Vp = 0.24 cm3/g) that is likely caused by the inclusion of large −CHO groups during partial formylation of the amino groups and the formation of high-defect structure.

Table 1

Specific Surface Area SBET, Total Vp, and Micropore Vμ Volumes of CAU-10-X

sample	SBET  ± 20 [m2/g]	Vp  ± 0.01 [cm3/g]	Vμ  ± 0.01 [cm3/g]
CAU-10-H	645	0.28	0.19
CAU-10-NO2	433	0.28	0.17
CAU-10-NH2	409	0.24	0.15

The FTIR spectra of dry CAU-10-pan>X materials (Figure S2b) show strong characteristic bands attributed to asymmetric stretching vibrations of carboxylate groups at νas(COO) = 1564, 1577, and 1572 cm–1 for CAU-10-H, −NH2, and −NO2, respectively. The appropriate bands corresponding to symmetric vibrations are observed at νs(COO) = 1413, 1410, and 1400 cm–1 for CAU-10-H, −NH2, and −NO2, respectively. The bands at νs(NH) = 3354 cm–1 and νas(NH) = 3436 cm–1 are observed for the CAU-10-NH2, which are characteristic of symmetric and asymmetric (NH)-vibrations, respectively. Furthermore, the bands at ν = 2879 and 1698 cm–1 could be assigned to (C–H) and (C–O) vibrations of amides, respectively. These groups probably arise due to partial formylation of the NH2-group during the synthesis.

Thus, the results obtained by papan class="Chemical">n class="Chemical">N2 adsorptionpan>, PXRD, and FTIR methods conpan>firm the genpan>uinpan>e structure of the prepared n class="Chemical">CAU-10-X samples.

Water Adsorption Equilibrium

Waterpan> adsorption isobars of CAU-10-X were measured to investigate the effect of the functional groups (Figure S5). The sorption isobars of CAU-10-H, presented as a function of relative pressure P/P0, are stepwise curves (Figure a), which agree well with the data reported in ref (43). At P/P0 < 0.18, the uptake w does not exceed 0.01 g/g. At P/P0 = 0.19 to 0.22, the jump in uptake up to w = 0.25 g/g is observed. Upon further increase in relative pressure, a smooth rise of uptake occurs up to w = 0.32–0.33 g/g at P/P0 = 0.8.

Figure 3

Isobars of water adsorption on CAU-10-H, −NH2 and −NO2 as a function of P/P0 at P(H2O) = 23.4 mbar (a) and isosteres of sorption for CAU-10-H (b).

n class="Chemical">Isobarspan> of water adsorption on CAU-10-H, −NH2 and −NO2 as a function of P/P0 at P(H2O) = 23.4 mbar (a) and isosteres of sorption for CAU-10-H (b).

n class="Chemical">CAU-10-NO2pan> possesses the S-shaped isobars of type IV (Figure a). At P/P0 < 0.2, the uptake is quite small w ≤ 0.05 g/g, which, however, is somewhat higher than for CAU-10-H. At the P/P0-range from 0.2 to 0.4, the inflection is observed on the isobar; the uptake rises up to w = 0.15 g/g. At further increase in P/P0, the gradual growth up to w = 0.22–0.25 g/g at P/P0 = 0.7–0.8 is detected. The net uptake, achieved at P/P0 = 0.8 is somewhat lower than the pore volume Vp = 0.28 cm3/g of this MOF, which can be attributed to its lowest hydrophilicity.

CAU-10-pan class="Chemical">NH2 is characterized by a higher affinpan>ity toward H2O and demonstrates a rapid increase in uptake at small P/P0, achieving w = 0.1 g/g at P/P0 = 0.2, with the adsorption isotherm approaching to I type (Figure a). At increasing P/P0 > 0.3, a gentle rise in the uptake w is detected from 0.17 to 0.23 g/g at P/P0 = 0.8, which is in agreement with the adsorbent specific pore volume (Vp = 0.24 cm3/g). Such isotherms can be attributed to the presence of highly polar hydrophilic −NH2 groups, as well as formylated −NH–CHO groups.

The isosteric heat Qis of waterpan> adsorption was calculated from the adsorption isobars by linearization of isosteres (Figure b) according to the Clausius–Clapeyron equation:The curves Qis(w) are essentially nonmonotonous for all CAU-10-X materials (Figure ). For CAU-10-H, the first water molecules (w < 0.02 g/g) adsorb on strong surface sites with Qis = 52 kJ/mol. At increasing uptake, the isosteric heat diminishes to 48 kJ/mol at w = 0.025 g/g. Then, the gradual growth of the adsorption heat up to 61 ± 3 kJ/mol is observed at w = 0.25 g/g; this uptake corresponds to the step on the adsorption isobar. At further increase in uptake, the heat decreases to 52 ± 2 kJ/mol. The similar trend is observed for CAU-10-NO2; however, the swing of the adsorption heat variation is smaller. On the contrary, for CAU-10-NH2, a gradual increase in the isosteric adsorption heat is observed in the uptake range w = 0.03–0.15 g/g.

Figure 4

Isosteric enthalpy of water adsorption on CAU-10-H (a), CAU-10-NO2 (b), and CAU-10-NH2 (c).

Isosteric enthpapan class="Chemical">n class="Chemical">alpy of class="Chemical">pan> class="Chemical">water adsorption on CAU-10-H (a), CAU-10-NO2 (b), and CAU-10-NH2 (c).

Such a nopan class="Chemical">nmonotonic behavior of the isosteric adsorption heat is typical of water adsorption on MOFs[4,24,29] and quite different from that for the common water adsorbents, like silica gels and zeolites, on which at first water molecules adsorb rapidly onto sites of high energy and then onto sites of decreasing energy as adsorption progresses.[44] It probably stems from the ambivalent nature of the MOF surface, constituted from both hydrophilic and hydrophobic moieties. The first water molecules are adsorbed on strong primary adsorption sites of the CAU-10-X (X = −H, −NO2) with large adsorption heat. As the water uptake increases, the weaker adsorption sites react with water molecules and adsorption heat goes down. For CAU-10-NH2, no decrease in the adsorption heat was detected, which probably can be attributed to the fact that it possesses the lowest pore volume of 0.24 cm3/g and specific surface of 409 m2/g and most hydrophilic groups −NH2 and formylated amine group −NHCOH, which results in higher density of the adsorption sites and agrees with the shape of adsorption isotherm, approaching type I. The growth of the adsorption heat at uptake ranges of 0.03–0.25, 0.07–0.17, and 0.03–0.10 g/g for CAU-10-H, CAU-10-NO2, and CAU-10-NH2, respectively, corresponds to the step (or, more precisely, the segment with sharp slope) on the adsorption isobar, which is indicative of a strong adsorbate–adsorbate interaction. It is probably caused by the cooperative interactions of water molecules with both hydrophilic surface centers and preadsorbed water molecules, which result in the formation of adsorbed water clusters.[4,30,45] At the largest uptake close to the pore saturation, the adsorption heat decreases and for CAU-10-X (X = H, NO2) still remains essentially higher than the condensation heat of water Qcon = 43.8 kJ/mol at 30 °C. It indicates a quite strong interaction between the water molecules and MOF surface. It is noteworthy that the adsorption heat variation with uptake is more gradual for the case of functionalized CAU-10-X (Figure ) in accordance with the smoother adsorption isobars.

To revealpan> the primary adsorption sites on the CAU-10-X surface and elucidate mechanisms of water adsorption, associated with the different types of adsorption equilibrium on various CAU-10-X, the samples saturated with water were studied by FTIR and XRD in situ methods. The spin–spin relaxation time T2 of the adsorbed water molecules was measured by 1H NMR relaxometry.

FTIR Spectrum of the Wet and Dry Materials

The FTIR spectra of alpan>l CAU-10-X materials saturated with water show a broad band in the region of 3360 cm–1, corresponding to the valence vibration of O–H groups in the adsorbed water molecules (Figure ). The peak corresponding to bridging group μ-OH vibrations in the aluminum oxide {AlO6} clusters is shifted from 3685, 3687, and 3673 cm–1 to 3622, 3633, and 3611 cm–1 for X = −H, −NO2 and −NH2, respectively (Figure ). It can be caused by water adsorption and clearly indicates this group to be a primary adsorption site for all the materials. The characteristic signals of the asymmetric [νas(COO) = 1564, 1572, and 1577–1579 cm–1] and symmetric [νs(COO) = 1413, 1400, and 1410–1412 cm–1] stretching vibration of the coordinating carboxylate groups are shifted to lower frequency [νas(COO) = 1552, 1558, and 1556 cm–1 and νs(COO) = 1402, 1408, and 1396 cm–1] for CAU-10-H, −NH2, and −NO2, respectively. It shows that −COO group is another primary adsorption site for all CAU-10-X samples.

Figure 5

FTIR-spectra of the dry (black) and wet (w = 0.20 g/g) (blue) samples of CAU-10-H (a), −NO2 (b), and −NH2 (c).

FTIR-spectra of the dry (black) and wet (w = 0.20 g/g) (blue) samples of papan class="Chemical">n class="Chemical">CAU-10-H (a), −class="Chemical">pan> class="Chemical">NO2 (b), and −NH2 (c).

For n class="Chemical">CAU-10-pan class="Chemical">NH2 saturated with npan> class="Chemical">water, the band at 1697 cm–1, assigned to the vibrations of the −C=O group of formylated amine group (−NHCOH) shifts to the lower frequency (ν = 1682 cm–1), which may indicate water adsorption on this group. The highly hydrophilic amino-group (the hydrophobic parameter π = −1.23[46]) could also be a primary adsorption site. However, the bands at 3436 and 3354 cm–1 corresponding to ν(NH)-vibrations are overridden by the wide band at 3360 cm–1 assigned to the vibrations of the −OH group of the adsorbed water molecules in the wet sample.

The signpapan class="Chemical">n class="Chemical">al at νas(NO) = 1547 cm–1 related to the asymmetric stretchinpan>g of −NO2 for CAU-10-NO2 is not shifted (Figure b), while the signal at νs(NO) = 1350 cm–1, corresponding to the symmetric stretching of −NO2 demonstrates a slight shift to νs(NO) = 1352–1354 cm–1. That can be attributed to the weak interaction of the water molecules with the −NO2 group through the formation of hydrogen bonds with Oδ− bearing a partial negative charge. It agrees with the data reported in ref (46), where this group is identified as weakly hydrophilic (the hydrophobic parameter π = −0.28). Interestingly, despite weak hydrophilicity of the NO2-group, its insertion in the structure of MIL-101(Cr) results in the reducing of the MOF affinity to water vapor.[31] On the contrary, Zr-based UiO-66, functionalized by the NO2-group demonstrated a higher affinity to water than the virgin UiO-66.[47]

In situ PXRD Structural Investigation upon Water Adsorption

To identify the structurpapan class="Chemical">n class="Chemical">al features associated with the effect of guest class="Chemical">pan> class="Chemical">water molecules on the crystal structure, PXRD patterns were collected in situ during water adsorption on CAU-10-X (Figures S6–S11).

In accordapan class="Chemical">nce with PXRD data for CAU-10-NO2, the additional peaks appear during water adsorption (Figure S6). It indicates a change in the lattice symmetry. For CAU-10-NO2 at P/P0 ≥ 0.2, XRD patterns are fitted by the P41 space group (Figure S7b) as reported in ref (40) for the as-synthesized sample. However, at lower water pressure, the P41 space group was not suitable for the accurate description. The indexing of the PXRD patterns suggests that the possible space group for this phase is I41 (Figure S7a). The lattice parameter c increases from 10.257 (2) to 10.368 (1) Å upon hydration at the range of relative pressure P/P0 = 0–0.5 (Figure a). The parameters a and b significantly decrease from 21.566 (3) to 21.308 (1) Å when the symmetry changes at P/P0 ≈ 0.2 (Figure a). Such change can be associated with the displacement of atoms from their crystallographic positions and the construction of a new unit cell in the P41 symmetry group with different lattice parameters. An increase in P/P0 above 0.2 leads to an increase in the parameters a and b, which is associated with the incorporation of water molecules into the lattice.

Figure 6

Cell parameters a = b (blue ■) and c (red ●) for CAU-10-H (a), CAU-10-NH2 (b), and CAU-10-NO2 (c).

Cell parameters a = b (blue ■) and c (red ●) for papan class="Chemical">n class="Chemical">CAU-10-H (a), class="Chemical">pan> class="Chemical">CAU-10-NH2 (b), and CAU-10-NO2 (c).

For dry n class="Chemical">CAU-10-Hpan>, the phase having an I41 space group was observed in accordance with[40] (Figure S6a), with the parameters a = b = 21.572 (2) Å and c = 10.443 (1) Å. The PXRD data indicate a change in symmetry at P/P0 ≥ 0.1 (Figure S8). Indexing results show that the most suitable is the P4 space group (Figure S9b), where the parameters a and b are 21.523(1) Å and c = 10.403 (1) Å at P/P0 = 0.12 (Figure b). Again, the drastic change of the cell parameters can be associated with the construction of a new unpan>it cell in the P4 symmetry group with different lattice parameters. At further increase in P/P0, the gradual growth of the a, b, and c parameters is observed. It is worth noting that structure transformation arisen from the guest water molecules effect was also reported for CAU-10-H.[48] However, the observed set of reflections on PXRD patterns is not comparable with that observed for the CAU-10-H in our work. The authors pointed out that the dry CAU-10-H sample has the I41/amd space group and the sample saturated with water, the I41 space group.

During papan class="Chemical">n class="Chemical">water adsorptionpan> on CAU-10-NH2, no structural changes are observed (Figure S10). The space group of symmetry P4̅n2 of CAU-10-NH2 does not change during water adsorption (Figure S11). When water molecules integrate into the lattice, an increase in the c parameter from 10.641 (3) to 10.947 (2) Å is observed. On the contrary, the parameters a and b gradually decrease during adsorption at P/P0 ≤ 0.3 and then remain almost constant and equal to 21.415 (2) Å (Figure c). Despite the fact that the parameters a and b decrease, the cell volume of CAU-10-NH2 increases due to a significant increase in the parameter c (Figure S12).

Thus, for CAU-10-pan>X with X = H and NO2, the observed change of the unit cell parameters indicates a conformational change of the crystal lattice caused by adsorbed water molecules. This change can probably be attributed to a “flapping” motion or slight rotation of the benzene rings around bonds connecting them to the carboxylate group in the organic linkers.[49] The structure change along the c axis is associated with a small change in the slope of the inorganic helical chain of AlO6 octahedra (Figure a) around their symmetry axis,[48] induced by the water adsorption on the closely spaced μ-OH groups and coordinated carboxyl groups. The centrosymmetric structure with the space group I41 is observed for CAU-10-H and CAU-10-NO2 in the absence of water molecules and at low vapor pressure. Adsorbed water molecules cause a noncentrosymmetric structure organization of CAU-10-H and CAU-10-NO2 at the relative pressure P/P0 ≥ 0.1 and P/P0 ≥ 0.2, respectively. The structure of CAU-10-NH2 remains noncentrosymmetric without changing the symmetry group (P4̅n2), although the gradual change in the cell parameters occurs during adsorption as well. Apparently, it may be due to steric hindrance of the movement of the organic linker due to the presence and larger size of formylated amino-groups formed during the synthesis.

1H NMR Relaxometry

The spin–spipan class="Chemical">n relaxation time T2 for CAU-10-H increases with water uptake (Figure ). The greatest growth of T2 is observed at w > 0.25 g/g corresponding to the step on the adsorption isobar. Probably, at large pore filling, a significant part of the adsorbed water molecules is quite far from the pore walls, which increases the field homogeneity, and hence the average duration of their magnetization. This is in accordance with decreasing isosteric adsorption heat at w > 0.25 g/g (Fig. 4 a), which indicates a lower adsorbent - adsorbate interaction.At the maximum saturation w = 0.32 g/g, the value T2 = 1.1 ms is rather long and may indicate the volume filling of micropores, taking into account a jump of the uptake to 0.25 g/g (Figure a). In contrast, the T2-values for CAU-10-NO2 and CAU-10-NH2 almost do not depend on the saturation degree within the experimental error (±150 μs). Such behavior suggests that the environment of the adsorbed water molecules changes only slightly with w, being in accordance with the gradual uptake change at the large uptake (Figure a). The lower values of T2 in comparison with CAU-10-H indicate the higher field inhomogenity in the vicinity of the protons of adsorbed water. It could be associated with higher electron density in the pores, arisen from the presence of functional groups −NO2, −NH2, and −NHCOH formed during formylation of −NH2 for the functionalized CAU-10-X.

Figure 7

Dependence of the spin–spin relaxation time T2 on the water uptake for the studied CAU-10-X.

Dependepan class="Chemical">nce of the spin–spin relaxation time T2 on the n class="Chemical">water uptake for the studied n class="Chemical">CAU-10-X.

Discussion

On the basis of the obtaipan class="Chemical">ned data, the following adsorption mechanisms are suggested for CAU-10-X materials with different types of water adsorption equilibrium. CAU-10-H with stepped adsorption isotherm w(P/P0) is characterized by a small uptake at P/P0 < 0.2 (Figure ). The first water molecules adsorb with a large adsorption heat equal to 52 kJ/mol on the strongest adsorption centers, which could be μ-OH and −COO groups. The small initial uptake may be caused by some steric inaccessibility of these adsorption centers located on the inorganic moieties and the close proximity of hydrophobic benzene rings (Figure ).[40,48] Actually, if we assume that each μ-OH group adsorbs one water molecule, the uptake equal to w = 0.087 g/g can be calculated, which is 4 times larger than the experimentally measured w = 0.02 at P/P0 = 0.2. The low accessibility of the μ-OH group was also theoretically shown by GCMC methods in ref (49) where were shown that for the rigid CAU-10-H structure at small P/P0, water molecules are adsorbed predominantly by the −COO groups. At increasing P/P0, the water–water interactions arise due to forming H-bonds with preadsorbed water molecules, thus creating water clusters. The adsorbed water molecules induce the structure transformation of CAU-10-H, leading to the change of space group at P/P0 ≈ 0.1. This change can probably be attributed to a slight rotation of the benzene rings around bonds connecting them to the carboxylate group in the organic linkers[49] that makes the hydrophilic μ-OH groups to become unblocked and causes a jump of the uptake from 0.02 to 0.25 g/g. The cooperative interactions of water molecules with both hydrophilic surface sites and preadsorbed water molecules, as well as opening the strong μ-OH groups, result in the growth of the isosteric adsorption heat up to 61 kJ/mol at w = 0.25 g/g. At further increase in P/P0 > 0.2, the volume pore filling occurs up to w = 0.33 g/g with gradual lowering of the isosteric adsorption heat to 52 kJ/mol. The final uptake w = 0.33 g/g exceeds the total pore volume Vp = 0.28 cm3/g of as-prepared CAU-10-H (Table ) that can be attributed to the structural changes triggered by water adsorption. The pore filling is accompanied by the sharp increase in the spin–spin relaxation time T2 from 500 to 700–1030 μs at w = 0.26–0.32 g/g due to higher field homogeneity at a larger distance from the pore walls.

Quite similpan>ar behavior was observed for CAU-10-NO2 with S-shaped adsorption isotherm. At P/P0 < 0.2, the uptake is only slightly higher than for CAU-10-H, probably due to the presence of weakly hydrophilic but highly accessible −NO2 groups (Figure c). These groups, along with −COO, can be additional adsorption sites due to the formation of hydrogen bonds with water protons at small P/P0 values. At increasing P/P0 ≈ 0.2, CAU-10-NO2 demonstrates the structural transformation with the change of the space group that probably increases the accessibility of highly hydrophilic μ-OH groups and results in the inflection on the adsorption isotherm w(P/P0) followed by the increasing slope (Figure a). The similar dependence of the isosteric heat of adsorption Qis(w) is observed, showing the important contribution of the adsorbate–adsorbate interactions. A smaller range of the adsorption heat variation of 49–54 kJ/mol is consistent with the smoother adsorption isotherm.

Among the studied samples, papan class="Chemical">n class="Chemical">CAU-10-NH2 possesses a higher affinity to npan> class="Chemical">H2O with the adsorption isotherm approaching to type I. The largest uptake at small values P/P0 < 0.19 can be attributed to the presence of highly hydrophilic −NH2 groups and formylated −NHCHO groups, being the strongest and spatially most accessible primary adsorption centers (Figure d). No change in the space group is observed, although the cell parameters gradually change during water adsorption, which is consistent with the smooth adsorption isotherm. The relaxation time T2 for CAU-10-NO2 and CAU-10-NH2 almost does not depend on the saturation degree. This suggests that the environment of the adsorbed water molecules changes only slightly in the course of adsorption, being in accordance with a gradual uptake change in the adsorption isotherms w(P/P0).

Conclusions

In this work, the effect of a fupan class="Chemical">nctional group on water adsorption on a series of CAU-10-X (X = H, NH2, and NO2) was comprehensively studied by mutually complementary physicochemical methods. It allows elucidation of the nature of adsorptive centers and water adsorption mechanisms. For CAU-10-H, the water sorption isotherm is a stepwise curve, which transforms to the smoother S-shaped curve (IV type) for CAU-10-NO2, and further approaches to I type for CAU-10-NH2. According to the FTIR spectra, the primary adsorptive centers for all the materials are μ-OH and −COO groups, being partially inaccessible for guest molecules. The strong hydrophilic −NH2 and formylated −NH–CHO groups and weak hydrophilic −NO2 groups can also serve as adsorption centers, which results in a larger water uptake for CAU-10-NH2 and CAU-10-NO2 at low P/P0 values. The adsorbed water molecules induce the change of the symmetry group for CAU-10-H and CAU-10-NO2, which increase the accessibility of the hydrophilic μ-OH group and result in the growth of the uptake. Accordingly, stepwise and S-shaped adsorption isotherms are observed for CAU-10-H and CAU-10-NO2, respectively. For CAU-10-NH2, with the strongest affinity to water and adsorption isotherm approaching to type I, no transformation of the symmetry group occurs, although the cell parameters gradually change during water adsorption. The 1H NMR relaxometry shows a sharp increase in the T2 time for the wet CAU-10-H. It is observed at the water uptake corresponding to the step on the adsorption isobar, which may indicate a volume filling of the CAU-10-H micropores. For the −NH2 and −NO2 substituted CAU-10, the relaxation time is shorter than for CAU-10-H and only slightly varies in the course of adsorption. This suggests that the vicinity of H2O molecules remains virtually unchanged in this process.