On the Gate-Opening Mechanism of Hydrophilic-Hydrophobic Metal

Article Cite This: Chem. Mater. 2018, 30, 5116−5127

pubs.acs.org/cm

Gate-Opening Mechanism of Hydrophilic−Hydrophobic Metal− Organic Frameworks: Molecular Simulations and Quasi-Equilibrated Desorption Andrzej Sławek,† José Manuel Vicent-Luna,‡ Bartosz Marszałek,† Barbara Gil,† Russell E. Morris,§ Wacław Makowski,*,† and Sofía Calero*,‡ †

Jagiellonian University, Faculty of Chemistry, Gronostajowa 2, 30-387 Kraków, Poland Universidad Pablo de Olavide, Department of Physical, Chemical and Natural Systems, Ctra. Utrera Km. 1, Seville ES-41013, Spain § EaStCHEM School of Chemistry, University of St. Andrews, Purdie Building, St. Andrews KY16 9ST, United Kingdom

Chem. Mater. 2018.30:5116-5127. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/07/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Adsorption of polar and nonpolar molecules in hydrophilic−hydrophobic STAM-1 metal−organic framework was studied by means of quasi-equilibrated temperatureprogrammed desorption and adsorption (QE-TPDA) experimental technique and molecular simulation. The QE-TPDA measurements revealed that thermal stability of the studied material in the presence of water may be lower than determined from thermogravimetric analysis. Molecular dynamics showed evident impact of diffusion on the adsorption mechanism in STAM-1. The QE-TPDA profiles recorded for adsorption of nalkanes, water, and alcohols indicate the gate-opening effect occurring only upon adsorption of polar molecules, which was confirmed by in situ IR spectroscopy. Monte Carlo molecular simulations agree with experimental data revealing preferable adsorption sites for the molecules of alcohols in STAM-1. Simulations also showed that the molecular mechanism of the gateopening is dependent on the size of polar guest molecules.

■

displacement, swelling, or linker rotation.23−26 Some of these changes lead to the gate-opening effect27 whenat certain conditionsguest molecules induce specific change in the host structure which allows for the entrance of the adsorbate. Better understanding of this phenomenon is therefore important since it can be exploited in gas separations.28−30 STAM-1 is a novel MOF synthesized for the first time by Mohideen et al.31 Subtle flexibility of the structure combined with the presence of hydrophilic/hydrophobic one-dimensional channel system and open metal sites (Figure 1) results in interesting adsorption properties of this material. According to the authors only one type of channelsno information on which oneis accessible to nonpolar molecules such as CO2 and N2, while polar molecules such as H2O and methanol adsorb in the two types of channels. Kinetics of reaching adsorption equilibrium of methanol indicated occurrence of a phase change of the STAM-1 structure during adsorption which may be connected to the gate-opening effect. However, there was neither unambiguous evidence of this phenomena nor detailed explanation in the original work.31 In another

INTRODUCTION Metal−organic frameworks (MOFs) or porous coordination polymers (PCPs) are a relatively new class of porous materials being of great interest of researchers in recent years.1−4 Due to their structural and functional customizability, the area of MOFs is one of the fastest growing fields in materials chemistry as they may be useful in numerous potential applications including hydrogen storage,5−7 carbon dioxide capture,8,9 drug delivery,10−12 catalysis,13−15 and gas separations.16,17 One of the most extraordinary properties of PCPs is that they can undergo reversible structural transformation upon external stimuli, e.g., adsorbing/desorbing guest molecules. Flexible MOFs exhibiting such changes of the pore systems are also called soft porous crystals.18 In the latest review work on these materials from 2014, Schneemann et al.19 reported that among ca. 20 000 porous coordination polymers available in the Cambridge Structural Database only less than 100 structures reveal considerable breathing transitions between structures with narrow and large pores. However, there are few systematic studies on the flexibility of these materials so far. One of the most acknowledged examples of soft porous crystals is MIL-53 exhibiting very large breathing effect.20−22 Besides breathing, MOFs can exhibit other modes of flexibility including intraframework dynamics, subnetwork © 2018 American Chemical Society

Received: April 17, 2018 Revised: July 16, 2018 Published: July 16, 2018 5116

DOI: 10.1021/acs.chemmater.8b01603 Chem. Mater. 2018, 30, 5116−5127

Article

Chemistry of Materials

(without a temperature change) to remove the excess of the vapor. All spectra were normalized to the intensity of the 720 cm−1 band, which remained unchanged upon adsorption. Adsorption isobars of H2O (distilled), methanol (anhydrous, 99.8%, Sigma-Aldrich), ethanol (anhydrous, 99.8%, POCh), and 1butanol (99.5%, POCh) on STAM-1 were determined with the use of the QE-TPDA experimental technique.36,37 Samples were either activated before experiments by heating up to 150 °C (5 °C ramp) in pure helium (Air Products, purity 5.0) or activated during the following desorption−adsorption cycles in the presence of intended adsorbate. Actual measurements with the QE-TPDA are performed in the presence of adsorptive admixed with helium (flow of 7 cm3· min−1), while its concentration is constantly monitored by the TCD detector. Desorption is imposed by heating of the sample while adsorption by cooling it. A plateau of the TCD detector indicating the end of the adsorption was observed either directly after cooling of the sample for H2O, methanol, hexane, and nonane or after ca. 1 h for ethanol and 1-butanol. For this reason, after each heating/cooling cycle a room temperature isothermal adsorption was performed for at least 2 h for H2O, methanol, and nonpolar adsorbates or 4−6 h for ethanol and butanol. Stability of porosity upon adsorption of water was also investigated with the use of the QE-TPDA technique. During such experiments three cycles of heating up to a certain temperature and cooling down to room temperature were performed. The maximum temperature was changing from 110 to 310 °C with 20 °C interval, while the heating/cooling rate was always equal to 5 °C/min. In order to obtain the amount of water adsorbed in each cycle, desorption maxima were integrated within the 30−120 °C range, and obtained areas were recalculated according to proper calibration constants. More details concerning QE-TPDA methodology and formalism of data reduction can be found in previous works.36−38

Figure 1. Gate-closed (left) and gate-opened (right) structures of STAM-1 viewed parallel to the crystallographic c-axis. Parts of the structures corresponding to the hydrophilic channels are enlarged, and the distances between the nearest atom of hydrogen in these channels are provided. Atoms of copper, carbon, oxygen, and hydrogen are marked in orange, gray, red, and white, respectively.

work, McKellar et al.32 reported a single-crystal to single-crystal phase transition of the STAM-1 framework upon ligand exchange. However, the ligand exchange reported in this experimental work was carried out at very high pressures (0.2 GPa or higher); thus, it is difficult to evaluate if reported changes of the STAM-1 structure were pressure or guestinduced. Beside these publications only three others refer to this material so far.33−35 The aim of this work is to identify the molecular mechanism of the gate opening in hydrophilic− hydrophobic STAM-1 MOF upon adsorption of polar and nonpolar molecules.

■

■

MOLECULAR SIMULATIONS To describe the molecules of methanol, ethanol, and 1-butanol, we used united atom models that merge the CH3 and CH2 alkyl groups into single interaction centers (pseudoatoms) with their own effective potential.39,40 Intramolecular interactions within these moleculesi.e. bonding, bending, and torsions were described by TraPPE force field.41 Within −OH groups only atoms of oxygen were considered as additional van der Waals interaction centers. For the molecules of alcohols, point charges were placed on the hydrogen, oxygen, and alkyl pseudoatoms adjacent to the atom of oxygen. The molecules of carbon dioxide, nitrogen, and oxygen were described by full atom models. In order to reproduce the quadrupole moment of the molecule of CO2, charges were placed on each atom. For the quadrupole moments of N2 and O2, we put negative charge on the atoms and double positive charge in the center of mass, which compensates the total charge of the molecules to zero.42,43 The molecules of water were described by Tip5pEw model parametrized for the Ewald summation method.44,45 In this model the molecule of water has a single van der Waals interaction center located at the atom of oxygen, while the atoms of hydrogen and the dummy free electron pairs have equal positive and negative charges, respectively. Guest−host interactions between the adsorbates and the MOF were defined with van der Waals and Coulombic interactions. van der Waals forces were calculated with the use of mixed Lennard-Jones (L-J) potentials, which were designated by Lorentz−Berthelot mixing rules (eq S1, Supporting Information). Effective potentials were truncated and shifted to zero at 12 Å. The L-J parameters for all framework atoms of STAM-1 were taken from the DREIDING46 force field, apart from that for Cu taken from

EXPERIMENTAL SECTION

The sample of STAM-1 used in this study was synthesized according to the original recipe31 doubling the amount of reagents. A sample of 1.9870 g (8.2 mmol) of copper(II) nitrate trihydrate (reagent grade, Sigma-Aldrich) was mixed with 23 mL of distilled water. In parallel, 1.7245 g (8.2 mmol) of benzene-1,3,5-tricarboxylic acid (reagent grade, Sigma-Aldrich) was dissolved in 23 mL of methanol. Reagents were weighted on analytical balance with an accuracy of 0.1 mg. Both solutions were combined in Teflon-lined autoclave (100 mL volume) and placed in an oven preheated to 110 °C for 7 days. The obtained sample was separated by Büchner filtration and rinsed profusely with water and methanol. To confirm the crystal structure of obtained material, powder X-ray diffraction was carried out using Rigaku Miniflex powder diffractometer (Cu Kα radiation at 10 mA and 10 kV, 2θ step scans of 0.02°, counting time of 1 s per step). Morphology of the material was confirmed with scanning electron microscopy (SEM) technique, using Tescan Vega3 LMU microscope with a LaB6 emitter (voltage of 20 kV). Sample was coated with gold before imaging to reduce charging of the crystals. FT-IR spectra were recorded on a Bruker Tensor 27 spectrometer equipped with an MCT detector, in the absorption mode, with a spectral resolution of 2 cm−1. STAM-1 was deposited as a thin layer on a silicon wafer prepared by evaporating a few drops of sample suspension in water. The wafer was placed in an IR cell closed with KBr windows. The cell was designed to allow outgassing under vacuum and adsorption of vapors at different temperatures and controlled atmosphere. Prior to experiments, samples were outgassed for 30 min at 100 °C under vacuum to remove adsorbed water and then cooled down to ambient temperature. Sorbates were loaded into the sample in excess, and then the cell was outgassed for 5 min 5117


Article

Chemistry of Materials Table 1. Characteristics of STAM-1 Structures Used in This Worka STAM-1 GC 3

PV (cm /g) HVF SA (m2/g) LCD (Å) PLD (Å) cell lengths (Å) cell angles (°) framework mass (g/mol) framework density (kg/m3) framework density (cm3/g) unit cell volume (Å3)

STAM-1 GO

total

H-philic

H-phobic

total

H-philic

H-phobic

0.231 0.318 826 5.5 4 a 18.6781 α 90

0.155 0.213 545 5.5 3.1 b 18.6781 β 90 27427 1378 0.726 2065

0.077 0.106 281 5.5 4 c 6.8358 γ 120

0.332 0.394 1305 6.3 4.4 a 18.6303 α 90

0.211 0.251 788 5.5 3.25 b 18.7526 β 90 27427 1187 0.842 2398

0.134 0.159 533 6.3 4.4 c 7.9257 γ 120

a

Where PV is pore volume, HVF is helium void fraction, SA is surface area, LCD is largest cavity diameter, and PLD is pore limiting diameter.

the UFF47 force field. To calculate Coulombic interactions, a set of partial charges of the framework atoms was determined with the EQeq method.48 A full set of charges and parameters for the L-J potentials for guests and host is available in Table S1 of the Supporting Information. The structure of STAM-1 is dynamic as it changes upon adsorption of the guest molecules. For some calculations we modeled the framework as flexible with the use of force field parameters for bonded interactions developed by Zhao et al.49 for Cu-BTC MOF. We used the OPLS-AA50,51 force fields to model the bonded interactions of the methyl ester group. However, the computation of adsorption isotherms in fully flexible structures is very expensive in computational time and cannot be done routinely. For most calculations the phase transition in the STAM-1 structure was represented with two rigid forms: gate-closed (GC) and gate-opened (GO). The GC form is the structure reported by Mohideen et al.31 where the molecules of water were removed from the hydrophilic cavities. GO form was obtained by a geometric optimization of GC structure in the presence of one molecule of methanol per Cu site using an energy minimization calculation. This imposed loading matches with the maximum amount of methanol adsorbed in the hydrophilic cages, reported by Mohideen et al.31 and confirmed by our simulations. All calculations were performed in a 2 × 2 × 4 unit cells simulation box. To better represent crystal lattice, periodic boundary conditions were applied.52 Crystallography information files (CIFs) of the two structures are available in the Supporting Information. The most important properties of GC and GO structures for adsorption are collated in Table 1. Helium void fractions (HVF) and heats of adsorption (Table S2) were calculated with the Widom particle-insertion method.53 Pore volumes were calculated by multiplying HVF by 103 and dividing the result by framework density expressed in kg·m−3. Pore limiting diameters (PLD) and largest cavity diameters (LCD) were estimated on the basis of geometries of the structure with poreblazer 3.0 tool.54 The geometric surface areas were computed with RASPA55 by rolling one molecule of helium over the surface.56 The calculations of adsorption isotherms and isobars were conducted with the Grand Canonical Monte Carlo (GCMC) method. In the GC ensemble (μVT) the chemical potential, volume, and temperature are fixed. The pressure is related to the chemical potential by fugacity. Each point on the

adsorption isotherm or isobar was computed by running at least 104 initialization cycles and 2 × 105 production cycles of equally probable trial moves: translations, rotations, regrow, and swap between the framework and reservoir. The calculations of adsorption in flexible STAM-1 were performed in the osmotic ensemble (μσT), where the atoms of the structure can move, and the volume of the unit cell can change by independent deformations in each direction. This is done with two additional MC trial moves. The first move incorporates a short Molecular Dynamics simulation (MD) in the NVE ensemble to allow framework flexibility (hybrid MCMD move). The second move allows to change the volume of the cell by modifying independently the box lengths. To calculate diffusion of methanol at infinite dilution, we used MD simulations. The initial configurations of the guest molecule inside the host structure were generated by random insertion of a single molecule in a hydrophilic cage with MC. Then we computed 109 or 107 production cycles for rigid or flexible STAM-1, respectively, with a step interval of 2 fs. The simulations were performed in the NVT ensemble, where the temperature was set to 573 K and fixed using the Nose− Hoover thermostat.57,58For determination of distribution of the guest molecules in the STAM-1 framework as well as for blocking part of it from adsorption, we defined specific adsorption sites for hydrophilic and hydrophobic channels as cylinders crossing the structure in the [001] direction with the base radii of 5 Å. The centers were located in (0, 0, 0) for hydrophobic or (1/3, 2/3, 0) and (2/3, 1/3, 0) for hydrophilic channels. All MC and MD simulations were performed with RASPA 2.0 simulation software.59,55 Pore size distributions were calculated geometrically, also with RASPA, using the method of Gelb and Gubbins.54,60 Hydrogen bonding was calculated from the MC configurations using geometric criteria described in the literature.61

■

RESULTS AND DISCUSSION The structure of the material under this study was confirmed by powder X-ray diffraction (Figure 2). The diffractogram is almost identical to the modeled one and also very similar to that reported by Mohideen et al.31 in the original work. The SEM images (Figure 3) show that STAM-1 crystallizes in hexagonal-plate crystals with maximum dimensions of ca. 10 μm. The morphology of the obtained sample is similar to that reported in the original work.31 5118


Article


that STAM-1 structure may change upon adsorption of polar molecules, but the phase transition does not occur in contact with nonpolar (or practically nonpolar) guests. Thermal stability is an essential feature for MOFs,62 and it is also a crucial aspect of measurements with the use of temperature-dependent QE-TPDA technique. The results from thermogravimetry coupled with mass spectroscopy (TG-MS) showed that water comes out of the STAM-1 structure below 150 °C, while the temperature of material decomposition is about 400 °C.63 However, the results from the QE-TPDA indicate that STAM-1 starts to lose its sorption properties at 270−290 °C in the presence of water steam (Figure 5). This is manifested by sudden decrease of sorption Figure 2. P-XRD diffractogram recorded for the STAM-1 sample under this study (top, black), reported by Mohideen et al.31 (middle, purple), and calculated from the .cif file provided by them (bottom, red).

Figure 3. SEM images of the STAM-1 MOF under this study.

To see the structural changes of the STAM-1 material upon adsorption we recorded in situ IR spectra with several probe molecules (Figure 4). The spectrum of the activated sample

Figure 5. Stability of porosity of the STAM-1 upon adsorption of H2O (top). Corresponding temperature program for the first seven cycles of the QE-TPDA experiment (bottom).

capacity for desorption−adsorption cycles for these maximum temperatures. The QE-TPDA profiles corresponding to this experiment are presented in Figure S1 of the Supporting Information. This finding indicates that thermal stability of MOFs can be significantly lower in the presence of adsorbing and desorbing molecules than in inert atmosphere. It is worth noting that HKUSTMOF having very similar chemistry to STAM-1is much less stable as it falls apart at room temperature in the presence of water.64 In Figure 6 the experimental QE-TPDA profiles are presented. Above the background line we observe desorption maxima resulting from increased amount of adsorbate in the carrier gas when heating the sample. On the other hand, adsorption takes place when the sample is being cooled; thus, the concentration of adsorbate in the carrier gas is lowered, and the QE-TPDA adsorption minima are below the background line. High-temperature parts of the QE-TPDA maxima correspond to adsorption in the internal micropores of the adsorbent. For butanol, water, and nonane we applied relatively high partial pressures p/ps of ca. 0.5, 1, and 1, respectively. For this reason, we believe that low-temperature parts of maxima recorded for these adsorbates correspond to adsorption in the interparticle mesopores or on the external surface. A well-developed external surface is also confirmed by the morphology of the obtained sample as we observe small crystals (2−10 μm) on the SEM images (Figure 3). The QETPDA profiles of ethanol were measured at low relative pressure (p/ps < 0.1), and most likely the low-temperature maxima are related to adsorption in the interparticle

Figure 4. From bottom to top: IR spectra of activated STAM-1 sample (black) and with adsorbed molecules of CO (orange), hexane (red), H2O (blue), and ethanol (green). Spectra are normalized to the intensity of the 720 cm−1 band, not changing upon adsorption.

exhibits bands corresponding to the symmetric (1370, 1450 cm−1) and asymmetric (1590, 1645 cm−1) vibrations of the carboxylate groups of the MOF linker as well as out of plane vibrations (below 1300 cm−1). Upon exposure to polar molecules, noticeable changes are observed in the region 1100−1800 cm−1 indicating a modification of the carboxylate environment. Adsorption of water causes decrease of the 1510 cm−1 band, related to the change in geometry and therefore the dipole moment of the CC bond in the carboxylate group of the linker. On the other hand, one can hardly find any differences between spectra of activated STAM-1 and with molecules of CO or n-hexane adsorbed. These findings show 5119


Article


Figure 6. QE-TPDA profiles of methanol, ethanol, butanol, water, hexane, and nonane on STAM-1 represented as plots of temperature-dependent specific sorption rate (ssr). Pressures were ca. 1260, 700, 460, 2700, 980, and 410 Pa, respectively. Path of the experiment is marked with green arrows.

of Mohideen et al.31 showing that the adsorption kinetics for methanol adsorption is slower at the gate-opening conditions. Strong adsorption−desorption hysteresis is not unusual for adsorption of polar molecules in metal−organic frameworks as it was reported in numerous works.68−70 Molecular simulations were used to better understand the gate-opening mechanism upon adsorption of guest molecules. For most of our calculations the flexibility of the STAM-1 structure was mimicked by using the two rigid formsgateclosed (GC) and gate-opened (GO). Pore size distributions presented in Figure 7 reveal the differences between these

mesopores only. Relative pressures for methanol and hexane were so low (p/ps < 0.05) that almost no low-temperature adsorption can be observed. In the QE-TPDA experiments good control of adsorption− desorption equilibrium is often manifested by symmetrical adsorption and desorption profiles; i.e., desorption maxima are mirror images of adsorption minima. This means that adsorption and desorption processes run in similar ways, as observed for numerous systems.36,65,66 However, even though the experiments were performed with extremely low heating and cooling rates of 1 °C/min and amount of samples of 3−5 mg, we find that desorption maxima recorded for adsorption of polar molecules on STAM-1 are either heavily shifted to high temperatures or do not have counterparts in adsorption branches. Not only desorption and adsorption occur at different conditions, but the parts of the QE-TPDA profiles of methanol, ethanol, and H2O measured with different heating/cooling rates overlap. This could indicate occurrence of vapor-condensed phase transition that is controlled only by partial pressure of the adsorptive and does not depend on the rate of temperature change. In other words, if phase transition of the STAM-1 structure is caused by the presence of the polar guest molecules, there is an equilibrium partial pressure for these adsorbates. This pressure also signifies the equilibrium constant, as the other phases coexisting in the system, i.e., gateopened and gate-closed STAM-1, have constant activity. Similar effects were observed for desorption of nonane from liquidlike phase filling the mesopores upon capillary condensation.67 On the other hand, the profiles recorded for hexane and nonane consist of almost symmetrical desorption and adsorption branches. The lack of phase transition of the host structure confirms that the adsorption mechanism is different for nonpolar molecules than for polar molecules. Our results also indicate that the gate-opening effect hinders the rate at which the adsorption equilibrium of polar molecules in STAM-1 is reached. This is in agreement with the findings

Figure 7. Pore size distribution of the gate-closed (GC) and gateopened (GO) structures of STAM-1.

structures. For GC we observe a single maximum at ca. 5.4 Å corresponding to both hydrophilic and hydrophobic cavities. For GO a shift of the size of hydrophobic pores from 5.4 to 6.3 Å is observed. The remaining two maxima at 5.2 and 5.5 Å stand for hydrophilic channels and indicate their different geometry than in GC form. The results are in agreement with the values of pore limiting diameters (PLD) and largest cage diameters (LCD) from Table 1. Pore size distributions show 5120


Article


framework. In the first stage (1) up to 10−1 ps, the molecule moves freely inside the channel until it “hits” the pore wall. (2). Afterward we observe subdiffusion (3) and diffusion (4) of the molecule through GO structure or its confinement in the cavity of GC structure (5). On the other hand, we found molecular diffusion with the flexible model starting from both GC and GO initial configurations. These findings denote that kinetic effects hinder adsorption of methanol in gate-closed structure of STAM-1, and the presence of the polar molecules is a key stimulus for GC−GO phase transition. It is worth noting that MSD of the methane molecule using rigid GC structure of STAM-1 is 2 orders of magnitude greater than using flexible models. In the case of rigid GO framework, the gate is always opened, which does not cause diffusion limitations and results in higher values of diffusion coefficients. Otherwise, the fluctuations of the flexible framework cause more collisions with the molecule which hinder diffusion. Figure 9 compares the calculated adsorption isotherms of CO2, N2, and O2 with the experimental ones reported by Mohideen.63 Experimental data are in agreement with the calculation results obtained for the rigid gate-closed form of the MOF structure when the hydrophilic channels are artificially blocked from adsorption. On the other hand, adsorption calculated for the gate-opened form is overestimated for CO2 and N2 and underestimated for O2 (Figure S2 in the Supporting Information). The adsorption of nonpolar molecules occurs only in hydrophobic channels and does not cause gate opening. This may be due to weak guest− host interactions since the calculated heats of adsorption for O2, N2, and CO2 are 16, 17, and 32 kJ/mol, respectively (Table S2). Unlike for nonpolar molecules, adsorption of methanol (MeOH) affects the STAM-1 structure. In Figure 10 the experimental adsorption isotherm of methanol63 is compared with the values calculated in this work. Experimental isotherm indicates that adsorption of methanol undergoes the gateopening mechanism as it may be divided in four different stages. At pressures lower than ca. 0.005 kPa no adsorption can be found experimentally (1). Then, between 0.005 and 0.02 kPa gate-opening takes place, which is reflected in the isotherm as a sudden increase of the adsorbed amount (2). After complete filling of the hydrophilic micropores (3), the molecules start to adsorb in hydrophobic cavities (4). The agreement between experimental and calculated values is better for the GC structure in the middle pressure range, up to 1 kPa. We believe that at these conditions the structure still

that the size of the hydrophilic cavities is almost unaffected when the structure goes from GC to GO. Diffusion of guest molecules is essential in the gate-opening mechanism. Figure 8 shows the mean square displacements

Figure 8. Time-dependent mean square displacement (MSD) of a single molecule of methanol in the hydrophilic channel of STAM-1 at 573 K. The calculations were performed using rigid (top) and flexible (bottom) MOF frameworks. For clarification the MSDs were divided into stages: (1) ballistic regime, (2) regime dominated by collisions between host and guest, (3) and (4) subdiffusive and diffusive regime, respectively, and (5) confinement of the molecule in the pore.

(MSD) of a single molecule of methanol through the hydrophilic channels of STAM-1 calculated using rigid and flexible framework models. For the calculations with the rigid model we used the gate-opened (GO) and gate-closed (GC) forms of the STAM-1 structure. These forms were also used as initial configurations for the calculations with flexible STAM-1

Figure 9. Experimental63 and calculated (GCMC) adsorption isotherms of CO2, N2, and O2 in STAM-1 at 195, 77, and 195 K, respectively. Vertical dotted lines correspond to the pressures of condensation of pure adsorbates obtained from the Antoine equation.71 5121


Article


Figure 10. Experimental63 and calculated (GCMC) adsorption isotherms of methanol in STAM-1 at 298 K. Vertical dotted lines correspond to the pressure of condensation of pure adsorbate obtained from the Antoine equation.71

prefers to maintain the GC form, but local changes allow for diffusion of the molecules. Similar effects can be found for short n-alkanes in LTA zeolite, where the molecules can diffuse through the narrow windows but mainly remain inside bigger cavities.72 One should also notice that the low-pressure regime of the adsorption isotherm cannot be reproduced by Monte Carlo simulations. This computational technique neglects diffusion limitations that prevent the adsorbate from entering the structure at low pressure. For pressures higher than 1 kPa the computed values of total adsorption are underestimated for GC and overestimated for GO structure. On the other hand, the values calculated with the flexible model fit the experimental data very well for all pressures higher than the gate-opening value. This confirms that during adsorption of methanol the actual structure oscillates between GC and GO forms. Figure S3 shows the evolution of the unit cell of flexible STAM-1 model for the adsorption of methanol at 10 kPa and room temperature. In addition, we compared it with the unit cell volume of the rigid models of STAM-1 and experimental data from McKellar et al.31 The volume change predicted with simulation is in line with the experimental observations and confirms the mentioned oscillation of the structure between GC and GO models. It is worth mentioning that the experimental results also showed that the structure of STAM-1 may change mostly in the z direction which is in concordance with the phase transition discussed in this work. Average occupation profiles of methanol in STAM-1 (Figure 11) reveal that the molecules are adsorbed close to the Cu atoms for loadings corresponding to saturation of hydrophilic sites (six molecules per unit cell). At high pressures, when the hydrophilic channels are fully filled, the molecules of methanol adsorb in the hydrophobic cavities as well (Figure 11, bottom) but they concentrate in the middle of the cavities. This is a good illustration of the behavior of polar molecules in hydrophilic and hydrophobic neighborhoods. In addition to the analysis of isothermal adsorption data for methanol, the adsorption behavior of selected aliphatic alcohols was investigated with the QE-TPDA experimental technique. Although the QE-TPDA profiles (Figure 6) provide a large amount of information, they cannot be directly compared with the results obtained from molecular simulations. For this reason, the profiles were integrated to obtain desorption and adsorption isobars. In Figure 12 we present adsorption isobars of methanol (MeOH), ethanol (EtOH), and 1-butanol (BuOH) in STAM-1 obtained with the use of QE-TPDA method and molecular simulations. Hysteresis loops between experimental adsorption and desorption isobars

Figure 11. Average occupation profiles of methanol in flexible STAM1 framework (xy plane) calculated at 0.1 (top) or 10 kPa (bottom) and 298 K. The schemes of the structure and color scale are also included.

of alcohols originate from the shifts on the QE-TPDA profiles (Figure 6) which is explained in detail in the Supporting Information (Figure S4, S5). One may notice that the shape of the desorption isobar of MeOH resembles a mirror reflection of the adsorption isotherm at 298 K (Figure 10), which validates the QE-TPDA experimental technique. In the cyclic QE-TPDA experiments adsorption occurs directly after desorption. Desorption branch allows for determination of the gate-closing temperature and adsorption branch for the gate-opening temperature. For all alcohols temperature for gate-closing is significantly higher than for gate-opening. Adsorption isobars calculated for MeOH fit the experimental data at temperatures between 300 and 400 K, for EtOH between 300 and 350 K, and for BuOH between 350 and 400 K. These regions correspond to adsorption predominantly occurring in hydrophilic channels of the STAM-1 structure. Similarly, as for adsorption isotherms of methanol, the calculated values are overestimated at conditions at which the structure is closed, i.e., for temperatures higher 5122


Article


than gate-closing temperature. This is due to the fact that MC simulations are “immune” to diffusion limitations. One can also notice that for BuOH the experimental adsorption exceeds the calculated values at temperature between 300 and 350 K. This is because the QE-TPDA experiments for BuOH were carried out at conditions close to the saturation, and, at low temperatures, the adsorbate may condensate in the interparticle mesopores or be adsorbed on the external surface due to strong lateral interactions. We observe such behavior also for adsorption of EtOH but in a lesser extent, evidenced by a break on the adsorption isobar at 310 K. The calculation results obtained with the GC model of the structure (Figure S6) are in worse agreement than for the gate-opened form, but differences for ethanol and butanol are not as substantial as for methanol. Similarly, as for methanol, the molecules of ethanol and butanol tend to be adsorbed in the hydrophilic and the hydrophobic channels (Figure S7). However, as the length of the alcohol chain increases, the molecules are more likely to be adsorbed in the hydrophobic channels. At the gate-opening conditions almost no methanol (at 353 K), only 3% of ethanol (at 373 K), and 12% of butanol (at 413 K) are located in the hydrophobic cavities. In Figure 13 the experimental and calculated adsorption isotherms31 and isobars of water are presented. For adsorption isotherms the gate-opening mechanism appears at ca. 0.14−0.3 kPa (298 K) and 0.7−1.4 kPa (313 K). It is also visible on the desorption isobar between 360 and 370 K. As for methanol, the shape of desorption isobar recorded with the QE-TPDA method (Figure 13c) looks like a mirror reflection of the adsorption isotherm at 298 K (Figure 13a). The GCMC simulations agree with experimental results at intermediate loadings of ca. 5−8 mol kg−1. Calculations indicate that adsorption takes place only in the hydrophilic cages of the GO structure. As for adsorption of methanol, the calculated values are overestimated at the conditions in which the real structure is gate-closed, i.e., low pressure or high temperature. Unlike for alcohols, the molecules of water are located only in the hydrophilic cavities of the STAM-1 structure (Figure 14). Moreover, these guest molecules are small enough to occupy voids between copper clusters (in z direction), which were also ascribed to the hydrophilic channels. Even at saturation (3 kPa) the molecules do not enter the hydrophobic cavities.

Figure 12. Experimental (this work) and calculated (GCMC) adsorption isobars of methanol, ethanol, and 1-butanol in STAM-1 at 1260, 700, and 460 Pa, respectively. Adsorption isobars were integrated from the QE-TPDA profiles recorded with heating/cooling rates of 1 °C/min. Vertical dotted lines correspond to the temperature of condensation of pure adsorbates obtained from the Antoine equation.71

Figure 13. Experimental31 and calculated (GCMC) adsorption isotherms of water in STAM-1 at 298 K (a) and 313 K (b) compared with experimental (this work) and calculated (GCMC) adsorption isobars at 2700 Pa. Adsorption isobar was integrated from the QE-TPDA profiles recorded with heating/cooling rate of 1 °C/min. Vertical dotted lines correspond to the pressure/temperature of condensation of pure adsorbate obtained from the Antoine equation.71 5123


Article


create hydrogen-bonded clusters. Water cannot be adsorbed in the hydrophobic cavities because of lack of attractive interactions there. For these reasons, once the gate is opened by water it does not close again. The gate opens in the presence of five molecules of ethanol per unit cell, and also with three molecules of butanol per unit cell. The behavior of these adsorbates in gate-opened (Figure 12) and gate-closed structures (Figure S6) is similar since they are too large to occupy only one open-metal site.

■

CONCLUSIONS

The gate-opening mechanism of adsorption of methanol and water in STAM-1 reported by Mohideen et al.31 was explained in detail with the use of molecular simulations supported by extensive experimental research. In situ infrared spectroscopy indicated that the vibrations of the STAM-1 structure change only upon adsorption of polar probe molecules and evidenced that the gate-opening mechanism may not occur for nonpolar adsorbates. Calculations of the diffusion of the molecule of methanol in flexible STAM-1 framework confirmed that the presence of guest molecules is the key stimulus for the phase transition from gate-closed to gate-opened form. The QE-TPDA experimental technique showed that the studied material starts to lose its sorption properties at 270− 290 °C in the presence of water steam, which disagrees with the results from TGA indicating decomposition of the studied material at 400 °C. The QE-TPDA profiles also revealed different adsorption behavior for polar and nonpolar molecules. For molecules of water and alcohol, we observed a huge hysteresis between adsorption and desorption isobars as well as overlapping of parts of the QE-TPDA profiles. These findings indicate that adsorption−desorption equilibrium is disturbed by phase transition taking place upon adsorption of polar molecules under specific conditions of temperature and pressure. None of these effects has been observed for adsorption of n-hexane and n-nonane. The QE-TPDA method also allowed for determination of adsorption and desorption isobars of water, methanol, ethanol, and butanol in STAM-1. Experimental data obtained in this work for polar molecules as well as previously reported adsorption isotherms were successfully reproduced by Monte Carlo simulations for conditions at which diffusion is allowed. Calculation showed that nonpolar molecules such as CO2, N2, and O2 are being adsorbed only in the hydrophobic channels of this material. Conversely, polar adsorbates prefer the hydrophilic channels. Water occupies only the hydrophilic cavities, while alcohols can be adsorbed in both types of channels. The longer the aliphatic chain of the alcohol is, the more willingly the molecule locates in the hydrophobic channels. The gate-opening mechanism is affected by the size of the polar guest molecules. Methanol fits the hydrophilic cavities perfectly, one molecule per open metal site. This allows for reversible opening and closing the host structure upon adsorbing of the following guests. Smaller molecules of water enter the structure and create hydrogen bond network close to the open metal sites in hydrophilic cavities, while some of them can locate between copper clusters. For this reason, the presence of water prevents the closing of the structure once this is being opened.

Figure 14. Average occupation profiles of water in gate-opened STAM-1 framework (xy plane) calculated at 0.3 (top) or 3 kPa (bottom) and 298 K. The schemes of the structure are also included.

The affinity of different guest molecules to hydrophilic and hydrophobic channels of STAM-1 was predicted by calculations of heat of adsorption (Table S2). In general, the heat of adsorption is much higher for polar molecules (112−149 kJ mol−1) than for the nonpolar molecules (16−32 kJ mol−1). Clearly, polar molecules have higher affinity to hydrophilic channels as the heats of adsorption in the hydrophilic channels (112−149 kJ mol−1) are higher than in the hydrophobic channels (32−73 kJ mol−1). Nonpolar molecules exhibit similar trend, but because these molecules are located only in hydrophobic cavities (Figure 9), the heats of adsorption obtained in hydrophilic channels can be treated only as hypothetical values. Polar molecules with different sizes affect the STAM-1 structure differently, which is consistent with previous work of McKellar et al.32 For methanol the gate-opening effect takes place for loading of six molecules per unit cell, i.e., one molecule per open metal site. Once each subsequent molecule is adsorbed the gate can be closed again. This is the reason why near gate-opening pressure methanol is adsorbed only in the hydrophilic cavities. In fact, for adsorption of methanol the structure is more stable in the GC form, while GO is only transition state during diffusion through the gate. Because the molecules of methanol locate close to the Cu open metal sites they do not form hydrogen bonds in the hydrophilic channels. However, these molecules are able to do so in the hydrophobic cavities, where up to 70% of them are involved in formation of hydrogen-bonded aggregates (Figure S8). The molecules of water are smaller, so they can fit between Cu nodes (in z direction). This is reflected on radial distribution functions showing that water can be located closer to the open metal sites than alcohols (Figure S9). The hydrophilic channel can allocate two or three molecules of water per open metal site, and the adsorbate may form hydrogen bonds network (Figure S8). At loading corresponding to one molecule per open metal site, only 10−15% of the molecules form hydrogen-bonded clusters. This percentage increases significantly with loading, and for saturation almost all adsorbed molecules of water 5124


Article


■

(7) Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D. W. Hydrogen Storage in Metal-Organic Frameworks. Chem. Rev. 2012, 112, 782− 835. (8) Millward, A. R.; Yaghi, O. M. Metal-organic frameworks with exceptionally high capacity for storage of carbon dioxide at room temperature. J. Am. Chem. Soc. 2005, 127, 17998−17999. (9) Sumida, K.; Rogow, D. L.; Mason, J. A.; McDonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T. H.; Long, J. R. Carbon Dioxide Capture in Metal-Organic Frameworks. Chem. Rev. 2012, 112, 724− 781. (10) Horcajada, P.; Serre, C.; Vallet-Regi, M.; Sebban, M.; Taulelle, F.; Ferey, G. Metal-organic frameworks as efficient materials for drug delivery. Angew. Chem., Int. Ed. 2006, 45, 5974−5978. (11) Horcajada, P.; Serre, C.; Maurin, G.; Ramsahye, N. A.; Balas, F.; Vallet-Regi, M.; Sebban, M.; Taulelle, F.; Ferey, G. Flexible porous metal-organic frameworks for a controlled drug delivery. J. Am. Chem. Soc. 2008, 130, 6774−6780. (12) Horcajada, P.; Gref, R.; Baati, T.; Allan, P. K.; Maurin, G.; Couvreur, P.; Ferey, G.; Morris, R. E.; Serre, C. Metal-Organic Frameworks in Biomedicine. Chem. Rev. 2012, 112, 1232−1268. (13) Ma, L. Q.; Abney, C.; Lin, W. B. Enantioselective catalysis with homochiral metal-organic frameworks. Chem. Soc. Rev. 2009, 38, 1248−1256. (14) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Metal-organic framework materials as catalysts. Chem. Soc. Rev. 2009, 38, 1450−1459. (15) Corma, A.; Garcia, H.; Xamena, F. Engineering Metal Organic Frameworks for Heterogeneous Catalysis. Chem. Rev. 2010, 110, 4606−4655. (16) Li, J. R.; Kuppler, R. J.; Zhou, H. C. Selective gas adsorption and separation in metal-organic frameworks. Chem. Soc. Rev. 2009, 38, 1477−1504. (17) Li, J. R.; Sculley, J.; Zhou, H. C. Metal-Organic Frameworks for Separations. Chem. Rev. 2012, 112, 869−932. (18) Horike, S.; Shimomura, S.; Kitagawa, S. Soft porous crystals. Nat. Chem. 2009, 1, 695−704. (19) Schneemann, A.; Bon, V.; Schwedler, I.; Senkovska, I.; Kaskel, S.; Fischer, R. A. Flexible metal-organic frameworks. Chem. Soc. Rev. 2014, 43, 6062−6096. (20) Serre, C.; Millange, F.; Thouvenot, C.; Nogues, M.; Marsolier, G.; Louer, D.; Ferey, G. Very large breathing effect in the first nanoporous chromium(III)-based solids: MIL-53 or Cr-III(OH)center dot{O2C-C6H4-CO2}center dot{HO2C-C6H4-CO2H}(x)center dot H2Oy. J. Am. Chem. Soc. 2002, 124, 13519−13526. (21) Loiseau, T.; Serre, C.; Huguenard, C.; Fink, G.; Taulelle, F.; Henry, M.; Bataille, T.; Ferey, G. A rationale for the large breathing of the porous aluminum terephthalate (MIL-53) upon hydration. Chem. - Eur. J. 2004, 10, 1373−1382. (22) Serre, C.; Bourrelly, S.; Vimont, A.; Ramsahye, N. A.; Maurin, G.; Llewellyn, P. L.; Daturi, M.; Filinchuk, Y.; Leynaud, O.; Barnes, P.; Ferey, G. An explanation for the very large breathing effect of a metal-organic framework during CO2 adsorption. Adv. Mater. 2007, 19, 2246−2251. (23) Bousquet, D.; Coudert, F. X.; Fossati, A. G. J.; Neimark, A. V.; Fuchs, A. H.; Boutin, A. Adsorption induced transitions in soft porous crystals: An osmotic potential approach to multistability and intermediate structures. J. Chem. Phys. 2013, 138, 174706 (9 pages). (24) Coudert, F. X.; Boutin, A.; Fuchs, A. H.; Neimark, A. V. Adsorption Deformation and Structural Transitions in Metal-Organic Frameworks: From the Unit Cell to the Crystal. J. Phys. Chem. Lett. 2013, 4, 3198−3205. (25) Gonzalez, M. I.; Kapelewski, M. T.; Bloch, E. D.; Milner, P. J.; Reed, D. A.; Hudson, M. R.; Mason, J. A.; Barin, G.; Brown, C. M.; Long, J. R. Separation of Xylene Isomers through Multiple Metal Site Interactions in Metal-Organic Frameworks. J. Am. Chem. Soc. 2018, 140, 3412−3422. (26) Luna-Triguero, A.; Vicent-Luna, J. M.; Calero, S. Phase Transition Induced by Gas Adsorption in Metal-Organic Frameworks. Chem. - Eur. J. 2018, 24, 8530−8534.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01603. Set of parameters used in molecular simulations for guest−guest and guest−host interactions; heats of adsorption calculated using Widom particle-insertion method; QE-TPDA profiles corresponding to the experiment for the stability; calculated adsorption isotherms of CO2, N2, and O2 in gate-opened structure and of ethanol and butanol in gate-closed structure; comparison between unit cell sizes using different models of the STAM-1 structure and experimental data; explanation of the hysteresis phenomenon between adsorption and desorption of methanol observed in the QE-TPDA experiments; average occupation profiles of ethanol and butanol in STAM-1; analysis of hydrogen bonding between the molecules of methanol and water inside the micropores of STAM-1; radial distribution functions between the atoms of oxygen of the guest molecules and the framework Cu atoms (PDF) Crystallography information files of the gate-closed and gate-opened STAM-1 structures (CIF) Crystallography information files of the gate-closed and gate-opened STAM-1 structures (CIF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel. + 48 12 686 2471. *E-mail: [email protected]. Tel.: + 34 954 977594. ORCID

Andrzej Sławek: 0000-0002-1846-6883 José Manuel Vicent-Luna: 0000-0001-8712-5591 Wacław Makowski: 0000-0002-4055-9664 Sofía Calero: 0000-0001-9535-057X Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the National Science Centre, Poland Grant No. 2016/21/N/ST5/00868, from Spanish Ministerio de Economiá y Competitividad (CTQ2016-80206-P), and by the Andaluciá Region (FQM-1851). We also thank C3UPO for the HPC support.

■

REFERENCES

(1) Zhou, H. C.; Long, J. R.; Yaghi, O. M. Introduction to MetalOrganic Frameworks. Chem. Rev. 2012, 112, 673−674. (2) James, S. L. Metal-organic frameworks. Chem. Soc. Rev. 2003, 32, 276−288. (3) Wang, C.; Liu, D. M.; Lin, W. B. Metal-Organic Frameworks as A Tunable Platform for Designing Functional Molecular Materials. J. Am. Chem. Soc. 2013, 135, 13222−13234. (4) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. The Chemistry and Applications of Metal-Organic Frameworks. Science 2013, 341, 1230444 (14 pages). (5) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. Hydrogen storage in microporous metalorganic frameworks. Science 2003, 300, 1127−1129. (6) Murray, L. J.; Dinca, M.; Long, J. R. Hydrogen storage in metalorganic frameworks. Chem. Soc. Rev. 2009, 38, 1294−1314. 5125


Article

Chemistry of Materials (27) Tanaka, D.; Nakagawa, K.; Higuchi, M.; Horike, S.; Kubota, Y.; Kobayashi, L. C.; Takata, M.; Kitagawa, S. Kinetic gate-opening process in a flexible porous coordination polymer. Angew. Chem., Int. Ed. 2008, 47, 3914−3918. (28) Li, L. B.; Krishna, R.; Wang, Y.; Yang, J. F.; Wang, X. Q.; Li, J. P. Exploiting the gate opening effect in a flexible MOF for selective adsorption of propyne from C1/C2/C3 hydrocarbons. J. Mater. Chem. A 2016, 4, 751−755. (29) Li, L. B.; Wang, Y.; Yang, J. F.; Wang, X. Q.; Li, J. P. Targeted capture and pressure/temperature-responsive separation in flexible metal-organic frameworks. J. Mater. Chem. A 2015, 3, 22574−22583. (30) Rodenas, T.; van Dalen, M.; Garcia-Perez, E.; Serra-Crespo, P.; Zornoza, B.; Kapteijn, F.; Gascon, J. Visualizing MOF Mixed Matrix Membranes at the Nanoscale: Towards Structure-Performance Relationships in CO2/CH4 Separation Over NH2-MIL-53(Al)@PI. Adv. Funct. Mater. 2014, 24, 249−256. (31) Mohideen, M. I. H.; Xiao, B.; Wheatley, P. S.; McKinlay, A. C.; Li, Y.; Slawin, A. M. Z.; Aldous, D. W.; Cessford, N. F.; Duren, T.; Zhao, X. B.; Gill, R.; Thomas, K. M.; Griffin, J. M.; Ashbrook, S. E.; Morris, R. E. Protecting group and switchable pore-discriminating adsorption properties of a hydrophilic-hydrophobic metal-organic framework. Nat. Chem. 2011, 3, 304−310. (32) McKellar, S. C.; Graham, A. J.; Allan, D. R.; Mohideen, M. I. H.; Morris, R. E.; Moggach, S. A. The effect of pressure on the postsynthetic modification of a nanoporous metal-organic framework. Nanoscale 2014, 6, 4163−4173. (33) El Mkami, H.; Mohideen, M. I. H.; Pal, C.; McKinlay, A.; Scheimann, O.; Morris, R. E. EPR and magnetic studies of a novel copper metal organic framework (STAM-I). Chem. Phys. Lett. 2012, 544, 17−21. (34) Dawson, D. M.; Jamieson, L. E.; Mohideen, M. I. H.; McKinlay, A. C.; Smellie, I. A.; Cadou, R.; Keddie, N. S.; Morris, R. E.; Ashbrook, S. E. High-resolution solid-state C-13 NMR spectroscopy of the paramagnetic metal-organic frameworks, STAM-1 and HKUST-1. Phys. Chem. Chem. Phys. 2013, 15, 919−929. (35) Mohideen, M. I.; Allan, P. K.; Chapman, K. W.; Hriljac, J. A.; Morris, R. E. Ultrasound-driven preparation and pair distribution function-assisted structure solution of a copper-based layered coordination polymer. Dalton Trans. 2014, 43, 10438−10442. (36) Makowski, W.; Ogorzalek, L. Determination of the adsorption heat of n-hexane and n-heptane on zeolites beta, L, 5A, 13X, Y and ZSM-5 by means of quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA). Thermochim. Acta 2007, 465, 30−39. (37) Makowski, W. Quasi-equilibrated temperature programmed desorption and adsorption: A new method for determination of the isosteric adsorption heat. Thermochim. Acta 2007, 454, 26−32. (38) Slawek, A.; Vicent-Luna, J. M.; Marszalek, B.; Makowski, W.; Calero, S. Ordering of n-Alkanes Adsorbed in the Micropores of AIPO(4)-5: A Combined Molecular Simulations and QuasiEquilibrated Thermodesorption Study. J. Phys. Chem. C 2017, 121, 25292−25302. (39) Ryckaert, J. P.; Bellemans, A. MOLECULAR-DYNAMICS OF LIQUID ALKANES. Faraday Discuss. Chem. Soc. 1978, 66, 95−106. (40) Dubbeldam, D.; Calero, S.; Vlugt, T. J. H.; Krishna, R.; Maesen, T. L. M.; Smit, B. United atom force field for alkanes in nanoporous materials. J. Phys. Chem. B 2004, 108, 12301−12313. (41) Chen, B.; Potoff, J. J.; Siepmann, J. I. Monte Carlo calculations for alcohols and their mixtures with alkanes. Transferable potentials for phase equilibria. 5. United-atom description of primary, secondary, and tertiary alcohols. J. Phys. Chem. B 2001, 105, 3093−3104. (42) Stogryn, D. E.; Stogryn, A. P. MOLECULAR MULTIPOLE MOMENTS. Mol. Phys. 1966, 11, 371−393. (43) Murthy, C. S.; Singer, K.; Klein, M. L.; McDonald, I. R. PAIRWISE ADDITIVE EFFECTIVE POTENTIALS FOR NITROGEN. Mol. Phys. 1980, 41, 1387−1399. (44) Castillo, J. M.; Vlugt, T. J. H.; Calero, S. Understanding Water Adsorption in Cu-BTC Metal-Organic Frameworks. J. Phys. Chem. C 2008, 112, 15934−15939.

(45) Rick, S. W. A reoptimization of the five-site water potential (TIP5P) for use with Ewald sums. J. Chem. Phys. 2004, 120, 6085− 6093. (46) Mayo, S. L.; Olafson, B. D.; Goddard, W. A. DREIDING - A GENERIC FORCE-FIELD FOR MOLECULAR SIMULATIONS. J. Phys. Chem. 1990, 94, 8897−8909. (47) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. UFF, A FULL PERIODIC-TABLE FORCE-FIELD FOR MOLECULAR MECHANICS AND MOLECULAR-DYNAMICS SIMULATIONS. J. Am. Chem. Soc. 1992, 114, 10024−10035. (48) Wilmer, C. E.; Kim, K. C.; Snurr, R. Q. An Extended Charge Equilibration Method. J. Phys. Chem. Lett. 2012, 3, 2506−2511. (49) Zhao, L.; Yang, Q. Y.; Ma, Q. T.; Zhong, C. L.; Mi, J. G.; Liu, D. H. A force field for dynamic Cu-BTC metal-organic framework. J. Mol. Model. 2011, 17, 227−234. (50) Jorgensen, W. L.; Maxwell, D. S.; TiradoRives, J. Development and testing of the OPLS all-atom force field on conformational energetics and properties of organic liquids. J. Am. Chem. Soc. 1996, 118, 11225−11236. (51) Kaminski, G.; Jorgensen, W. L. Performance of the AMBER94, MMFF94, and OPLS-AA force fields for modeling organic liquids. J. Phys. Chem. 1996, 100, 18010−18013. (52) Frenkel, D.; Smit, B. Understanding Molecular Simulation; Academic Press: San Diego, CA, 1996. (53) Widom, B. SOME TOPICS IN THEORY OF FLUIDS. J. Chem. Phys. 1963, 39, 2808−2812. (54) Sarkisov, L.; Harrison, A. Computational structure characterisation tools in application to ordered and disordered porous materials. Mol. Simul. 2011, 37, 1248−1257. (55) Dubbeldam, D.; Calero, S.; Ellis, D. E.; Snurr, R. Q. RASPA: molecular simulation software for adsorption and diffusion in flexible nanoporous materials. Mol. Simul. 2016, 42, 81−101. (56) Duren, T.; Millange, F.; Ferey, G.; Walton, K. S.; Snurr, R. Q. Calculating geometric surface areas as a characterization tool for metal-organic frameworks. J. Phys. Chem. C 2007, 111, 15350−15356. (57) Nose, S. A molecular dynamics method for simulations in the canonical ensemble (Reprinted from Molecular Physics, vol. 52, pg 255, 1984). Mol. Phys. 2002, 100, 191−198. (58) Hoover, W. G. CANONICAL DYNAMICS - EQUILIBRIUM PHASE-SPACE DISTRIBUTIONS. Phys. Rev. A: At., Mol., Opt. Phys. 1985, 31, 1695−1697. (59) Dubbeldam, D.; Torres-Knoop, A.; Walton, K. S. On the inner workings of Monte Carlo codes. Mol. Simul. 2013, 39, 1253−1292. (60) Gelb, L. D.; Gubbins, K. E. Pore size distributions in porous glasses: A computer simulation study. Langmuir 1999, 15, 305−308. (61) Padro, J. A.; Saiz, L.; Guardia, E. Hydrogen bonding in liquid alcohols: a computer simulation study. J. Mol. Struct. 1997, 416, 243− 248. (62) Bosch Mathieu, M.; Zhang, M.; Zhou, H.-C. Increasing the Stability of Metal-Organic Frameworks. Adv. Chem. 2014, 2014, 1 Article ID 182327. (63) Mohideen, M. I. H. St. Andrews Research Repository. Novel metal organic frameworks: Synthesis, characterisation and functions. http://hdl.handle.net/10023/1892 (accessed July 11, 2017). (64) Todaro, M.; Buscarino, G.; Sciortino, L.; Alessi, A.; Messina, F.; Taddei, M.; Ranocchiari, M.; Cannas, M.; Gelardi, F. M. Decomposition Process of Carboxylate MOF HKUST-1 Unveiled at the Atomic Scale Level. J. Phys. Chem. C 2016, 120, 12879−12889. (65) Makowski, W.; Manko, M.; Dudek, A.; Mlekodaj, K. Application of quasi-equilibrated thermodesorption of hexane and cyclohexane for characterization of porosity of zeolites and ordered mesoporous silicas. Adsorption 2013, 19, 537−544. (66) Roth, W. J.; Gil, B.; Makowski, W.; Slawek, A.; Korzeniowska, A.; Grzybek, J.; Siwek, M.; Michorczyk, P. Framework-substituted cerium MCM-22 zeolite and its interlayer expanded derivative MWWIEZ. Catal. Sci. Technol. 2016, 6, 2742−2753. (67) Makowski, W.; Kustrowski, P. Probing pore structure of microporous and mesoporous molecular sieves by quasi-equilibrated 5126


Article

Chemistry of Materials temperature programmed desorption and adsorption of n-nonane. Microporous Mesoporous Mater. 2007, 102, 283−289. (68) Biswas, S.; Ahnfeldt, T.; Stock, N. New Functionalized Flexible Al-MIL-53-X (X = -Cl, -Br, -CH3, -NO2, -(OH)(2)) Solids: Syntheses, Characterization, Sorption, and Breathing Behavior. Inorg. Chem. 2011, 50, 9518−9526. (69) Motkuri, R. K.; Thallapally, P. K.; Annapureddy, H. V. R.; Dang, L. X.; Krishna, R.; Nune, S. K.; Fernandez, C. A.; Liu, J.; McGrail, B. P. Separation of polar compounds using a flexible metalorganic framework. Chem. Commun. 2015, 51, 8421−8424. (70) Kim, H.; Cho, H. J.; Narayanan, S.; Yang, S.; Furukawa, H.; Schiffres, S.; Li, X. S.; Zhang, Y. B.; Jiang, J. C.; Yaghi, O. M.; Wang, E. N. Characterization of Adsorption Enthalpy of Novel Water-Stable Zeolites and Metal-Organic Frameworks. Sci. Rep. 2016, 6, 19097 (8 pages). (71) NIST Standard Reference Database 69: NIST Chemistry WebBook. http://webbook.nist.gov/chemistry/ (accessed July 27, 2017). (72) Combariza, A. F.; Sastre, G.; Corma, A. Molecular Dynamics Simulations of the Diffusion of Small Chain Hydrocarbons in 8-Ring Zeolites. J. Phys. Chem. C 2011, 115, 875−884.

5127


On the Gate-Opening Mechanism of Hydrophilic-Hydrophobic Metal

Recommend Documents