WaterâHydrophobic Zeolite Systems - The Journal of Physical

Article pubs.acs.org/JPCC

Water−Hydrophobic Zeolite Systems Yuriy G. Bushuev,*,† German Sastre,‡ J. Vicente de Julián-Ortiz,§ and Jorge Gálvez∥ †

Ivanovo State University of Chemistry and Technology, Engelsa, 7, Ivanovo, Russia, 153000 Instituto de Tecnologia Quimica, UPV − CSIC, Avenida de los Naranjos s/n, 46022 Valencia, Spain § Departamento Química Analítica, Facultad de Química, Universitat de Valencia, Doctor Moliner 50, 46100 Burjasot, Valencia, Spain ∥ Departamento Química Física, Facultad de Farmacia, Universitat de Valencia, Avenida V. Andrés Estelles s/n, 46100 Valencia, Spain ‡

S Supporting Information *

ABSTRACT: Water intrusion−extrusion in hydrophobic microporous AFI, IFR, MTW and TON pure silica zeolites (zeosils) has been investigated through molecular dynamics (MD) simulations. It was found that intruded water volumes correlate with the free volume of the zeosil unit cells. Calculated adsorption isotherms allowed us to estimate the amounts of water intruded, and deviations from experiments (lower experimental with respect to calculated intrusion pressures) have been explained in terms of connectivity defects in the synthesized materials. Water phase transitions in defectless zeosils occur in a narrow range at high pressure. On the basis of a simple model, we derived a thermodynamic equation that allows one to estimate the intrusion pressure with few parameters, which are easy to obtain, such as fractional free volume of zeosil and the intrusion pressure of a reference system. The structural properties of water clusters inside the zeosil micropores have been interpreted from the analysis of the MD simulations. Compact “bulk-like” clusters form in large channels such as those in AFI and IFR zeosils. The smaller channels of MTW and TON promote the formation of chain-like clusters, which, interestingly, are commensurate with the zeolite channel topology due to a coincidence between the distances of the crystallographic parameter, along the channel, and a maximum in the O−O radial distribution function of bulk water.

1. INTRODUCTION Zeolites are microporous crystalline materials with an inorganic, three-dimensional host structure comprised of fully linked, corner-sharing tetrahedra. Chemically they are mostly based on aluminosilicates with a formula of the type Mn+x/n[AlO2]−x[SiO2]y[H2O]z, where the tetrahedrally coordinated atoms (T = Al, Si) can be substituted by elements such as, for instance, B, Ge, Ti, Fe, Cu, Zn, and many others. Cations (Mn+), with the exception of protons, unlike the other framework elements, are located inside the micropore voids, and they compensate the charge of the trivalent elements. The name “zeolite”, from the Greek ‘zeos lithos’ (boiling stone), reflects the high water adsorption of most natural zeolites, which is due to their high content of Al and cations. In the other end of chemical composition, with no cations, nor trivalent atoms, pure silica zeolites (zeosils) are hydrophobic materials, and water fills the micropore channels and cavities only at high hydrostatic pressure. According to the theory of hydrophobicity developed by Lum, Chandler, and Weeks,1 confined liquid water is less thermodynamically stable than its vapor. If we immerse two parallel hydrophobic plates into liquid water and approach the plates, water will be expelled from interplates volume below some critical distance ca. 50 Å.1 This effect must be present in hydrophobic zeolites, whose channels sizes usually lie in the range 4−30 Å. Upon increasing pressure in an intrusion experiment, water fills zeosil channels. © 2012 American Chemical Society

A phase transition from vapor to liquid water (from empty to filled channels) occurs in a narrow pressure range, but intrusion pressure depends on the topology of the zeolite framework and the quality of the synthesized material. Defects of the framework connectivity in the form of internal silanol groups increase the hydrophilicity of material, which decreases the intrusion pressure. In most investigated cases (silicalite-1, beta,2 chabazite,3 SSZ-23,4 SSZ-24, ZSM-12, and ZSM-225), after pressure release, the system returns to its initial state, and adsorption−desorption isotherms overlap. This is called “spring” behavior. Some zeosils behave as “bumpers” or “shock absorbers”, and they dissipate mechanical energy. For example, RUB-41 (RRO-type topology) shows a reversible phenomenon with a hysteresis at the extrusion stage (shock absorber behavior).6 The pressure−volume diagram of the water−ITQ-4 system indicates an irreversible phenomenon, with water molecules remaining confined in ITQ-4 micropores after release of pressure and hence behaving as a bumper.7 Bearing in mind that pure silica zeolites are polymorphs, the reasons to explain such different behaviors must be found in the specific properties of water upon confinement and hence in the role of defects, surface and microporous topology. In some Received: June 23, 2012 Revised: October 23, 2012 Published: November 7, 2012 24916

dx.doi.org/10.1021/jp306188m | J. Phys. Chem. C 2012, 116, 24916−24929

The Journal of Physical Chemistry C

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negative charge of −1.05 au. The parameters of the BS force field were previously published, and they can also be obtained as the Supporting Information. We have used a twofold strategy of simulations. For the first strategy (calculation of structural and thermodynamic properties), we have used as models the cells included in the upper part of Table 1. For the second strategy (calculation of isotherms), the models used are the cells in the lower part of Table 1. We now explain both strategies more in detail.

cases, zeosil materials interact chemically with water due to the presence of defects, showing a localized hydrophilic character. Defects may be present in the original material, but they can also be produced upon intrusion−extrusion cycles, by breaking Si−O−Si bonds into Si−OH (silanol) groups, as is the case in RUB-41 and ITQ-4. The scanning electron micrographs show that RUB-41 and ITQ-4 crystals become defective after intrusion−extrusion of water.6,7 Computer simulations of water−zeosils systems are mainly based on atomistic force fields, and they allow one to calculate adsorption isotherms, as discussed in the review by Smit and Maesen.8 Water adsorption, as well as structural and thermodynamic properties of confined water have been thoroughly investigated in silicalite-1 zeosil.9−13 Due to the lack of experimental data, other water−zeosil systems have deserved less attention from the computational viewpoint. Water−LTA zeosil and aluminosilica systems were investigated14 by grand canonical Monte Carlo (GCMC) and firstprinciples Car−Parrinello molecular dynamic (MD) simulations. It was shown that water in the hydrophobic zeolite behaves as a nanodroplet trying to close its hydrogen bonds onto itself, with a few short-lived dangling OH groups, while water in the hydrophilic zeolites opens up to form weak hydrogen bonds with the oxygen atoms of the framework. Dynamical properties of water at low temperature were investigated15,16 for water−AFI-type systems. Properties of water are not fully investigated, especially at low temperature.17−19 Under confinement conditions, water has specific properties.15,16,20 The hydrophobic/hydrophilic character of biological membrane pores determines their permeability for water and aqueous solutions.21 Carbon nanotubes have straight channels, whose diameters are varied in a wide range. In the CNT channels, water forms very specific structures.22 We have discussed the structure of water in TON and IFR-type zeosils.23 Water may play a structural directing role in zeolite synthesis. It was shown24 that the stability of beta zeosil polymorphs depends on water loading. Using our results, a new aging−drying method was applied for beta zeosil synthesis, and a material enriched by polymorph A was obtained.25 Thus, an investigation of confined water properties is valuable for solution of problems in a wide range of applications. Some of the open questions we would like to address in the present study, using MD simulations and making use of experimental published data, are (1) calculate adsorption isotherms for four pure silica zeolites with unidimensional channels; (2) elucidate regularities of water adsorption by hydrophobic microporous materials; (3) rationalize how and why different zeosils react energetically when water is intruded−extruded; (4) elucidate structural properties of water depending on the shape and size of the zeolite channels; and (5) explain the experimental behavior of zeosils in water intrusion−extrusion processes.

Table 1. Structural Characteristics of Zeosil Simulation Cells IZA code

Na (SiO2/ OH)

Mb (u.c.)

a × b × c, (Ǻ ); α, β, γ (o)c

Small Cell, Flexible Framework, Flexible Water, 3D Crystalse IFR 64/0 1×1×2 18.63 × 13.49 × 15.26; 90, 102.3, 90 AFI 72/0 1×1×3 13.61 × 13.61 × 24.87; 90, 90, 120 MTW 224/0 1×4×1 24.87 × 20.04 × 24.31; 90, 107.7, 90 TON 96/0 1×1×4 13.83 × 17.39 × 20.1; 90, 90, 90 Large Cell with Water Reservoir, Rigid Framework, Rigid Waterf IFR 256/16 1×1×8 18.63 × 13.49 × 61.0; 90, 101.4, 90 AFI 672/48 2×2×7 27.21 × 27.21 × 58.03 90, 90, 120 MTW 336/28 1 × 12 × 1 24.87 × 60.1 × 12.15 90, 107.7, 90 TON 228/24 1 × 1 × 12 13.83 × 17.39 × 60.4 90, 90, 90

Ld (ch.) 2 1 4 2

2 4 2 2

a Number of SiO2 units in the cell and number of silanol groups on surfaces, which are terminated broken bonds. bNumber of crystallographic unit cells in each direction. cGeometrical parameters of cells. d Number of zeosil channels in the cell. eSimulation cells using GULP software for MD simulations. fZeosil slab is inserted into simulation cell (Figure 1). The largest side of the cell (b-axis for MTW and c-axis for IFR, AFI, and TON) is 120 Ǻ .

For an investigation of structural and thermodynamic properties of water−zeolite systems, we have applied a method, which was described in details in our previous paper.23 Simulation cells were constructed from several crystallographic unit cells of AFI, IFR, MTW, and TON, all of them onedimensional channel zeosiles. The cell parameters of the systems simulated are presented in Table 1 (upper part). The channels run along the [001] crystallographic direction for all zeosils except MTW, where channels run along the [010] direction. One channel of each zeosil in simulation cell was progressively loaded by water molecules starting from an empty zeosil framework. Periodical boundary conditions were applied to all crystallographic axes of the simulation cell. Zeosils were relaxed to P1 symmetry, and the frameworks were considered flexible. In the BS forcefield, the energy of water−water interactions was calculated with the flexible simple point charge (SPC) water potential,29 and combinational rules were employed for the water−zeosil parameters, using water−water and zeosil−zeosil parameters. The used force field was tested23,26−28 for a wide range of zeolite and water−zeolite systems. It showed good reproduction of the experimental thermodynamic and structural properties. It was shown23 that calculated heats of adsorption for the IFR and TON zeosils are very close to the experimental results obtained for pure silica sodalite. For the test of sensitivity of adsorption on strength of water−zeolite

2. METHODOLOGY We have employed classical MD simulations to investigate the water−zeolite systems. Knowing the cruciality of a good force field to obtain accurate results, we previously parametrized the BS force field where the specific interactions between water and zeosils were taken into account.23,24,26−28 This unpolarizable force field contains three terms: electrostatic, van der Waals, and three-body interactions. A partial electric charge on Si atoms of SiO2 units is equal to 2.1 au, meanwhile O has a 24917



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energies of the water−water interactions were calculated through the SPC potential.31 For an investigation of the influence of the framework rigidity, we have made MD simulations of the MTW zeosil− water systems with flexible zeosil framework. The results of the simulation are presented as SI (see Figure S2). They show that the adsorption isotherm has the same shape, but it is shifted to lower pressure by 35−40 MPa. The intrusion pressure in the case of the rigid framework was 180 MPa. Using a flexible zeolite model improves the results, which are closer to the experimental data, although the physics of the water phase transition is the same in the both cases and such systematic shifts of adsorption isotherms are only a small quantitative correction. The simulation cell, considering periodic boundary conditions, contains a zeosil moiety including two external surfaces, and a water reservoir. The length of the longest axis in the simulation cell was chosen parallel to the zeosil channels, with a length of 120 Å. The zeosil slab occupied approximately half of the volume of the simulation cell. The simulation cells are presented in Table 1 (lower part). Broken Si−O−Si bonds on the zeolite surface were terminated as silanol groups (SiOH). The number of silanol groups is also included in Table 1. To calculate the adsorption isotherms, each point was calculated through a long MD simulation run using version 2.20 of DL_POLY,32 within the NVT ensemble. Equilibration periods of the simulation lasted from 1 to 10 ns, followed by production runs lasting 3−8 ns. The integration algorithms were based on the Verlet leapfrog and the Nosė−Hoover with thermostat relaxation times of 0.5 ps. To save computational time, a rigid-body model was assumed for zeolite and water, and a time step of 1 or 2 fs was considered. Hence only the electrostatic and van der Waals terms of the BS force field were needed. The Ewald method has been used for the summation of the long-range Coulombic interactions, while a direct summation was considered for the short-range interactions with a cutoff distance of half of the shortest axis length. Mimicking the experimental setup, the increasing pressure is simulated by adding water molecules. The corresponding pressure is calculated when the system reaches equilibrium from the number of water molecules in the bulk phase and their corresponding volume. From the simulation, at equilibrium, we calculated the density of bulk water in the water reservoir as well as the number of water molecules that penetrated inside the inner part of the zeosil channels. These quantities were calculated in small additional slabs, whose borders were subsequently farther from the water−zeolite interface. For calibration, separate NPT runs of bulk SPC water (with N = 500, T = 300 K and an MD production period of 0.3 ns) allowed us to calculate water P−V diagrams at different electrostatic interaction cutoff distances. The P−V diagrams allowed us to obtain the pressure in the water reservoir.

interaction, we made MD simulations of the water−MTW zeosil system with a large deviation from the combinational rule applied for van der Waals interactions and additional simulations with increased charges on silicon and oxygen atoms. The results of the simulation are presented and discussed in the Supporting Information (SI) (see Figures S3, S4). The total energy is determined from the evaluation of the appropriate energy terms for every atom−atom interaction in the system. The Ewald method has been used for the summation of the long-range Coulombic interactions and direct summation of short-range interactions with a cutoff distance of 12 Å. The MD simulations were carried out with the General Utility Lattice Program (GULP).30 MD simulations were done with the NVE and NPT ensembles at T = 300 K and P = 0 Pa with a time step of 1 fs. Equations of motion were integrated using the leapfrog Verlet algorithm. The production time was 100 ps after a 10 ps equilibration of each system with the energy convergence evaluated and ensured within all production data. Unit cells of AFI, IFR, MTW, and TON zeosils were used to calculate Connolly surfaces and their enclosed volumes using a probe particle radius of 2 Å, using Materials Studio v.5.0 Accelrys Software, Inc. The second method of simulation was applied to the calculation of isotherms, and this required larger simulation cells containing zeosil surfaces and an empty volume where water molecules are initially introduced. The simulation cell is presented in Figure1. The simulation of adsorption isotherms

Figure 1. The cell is for MD simulation of water−zeolite systems. The shadowed blocks are the regions where adsorbed water (left block) and density of bulk water (right block) were calculated.

demands considerable computational resources due to large simulation cells containing thousands of atoms and the fact that each point of the isotherm requires an MD run lasting several nanoseconds. Because of this, some simplifications in the model were assumed: The zeosil framework and the water molecules were considered rigid bodies. The zeolite atoms and unit cell parameters were fixed at their crystallographic positions, and only electrostatic and van der Waals terms of the BS force field were used for calculations of water−zeosil interactions. The Table 2. Volumetric Characteristics of Zeosils

a

IZA code

VCa (Å3/SiO2)

VC, (mL/g)

φb

Vexpt (N2), (mL/g)

Vcalc (H2O), (mL/g)

Vexpt (H2O), (mL/g)

IFR AFI MTW TON

20.15 15.02 11.39 10.32

0.201 0.150 0.114 0.103

0.349 0.272 0.221 0.206

0.23c d 0.05 0.14−0.16e; 0.16f 0.15d 0.112g; 0.1−0.12i;0.1j 0.02d

0.155 0.133 0.098 0.08

0.136c 0.102d 0.114d 0.075d

Connolly free volume at radius of probe particle of 2 Å. bFractional free volume: Connolly free volume per u.c. volume (VC/Vuc). cReference 7. Reference 5. eReference 41. fReference 42. gReference 44. iReference 45. jReference 46.

d

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Figure 2. (a) Experimental and calculated intrusion−extrusion isotherms of TON zeosil. The shadowed block is the region of the experimental isotherms hysteresis and the calculated data scattering. (b) Channel shape of TON-type zeosil.

3. RESULTS AND DISCUSSION 3.1. Adsorption Isotherms. From a general point of view, the volume of any substance adsorbed by a zeolite must depend on the available free volume of zeolite. The free volume was estimated by the Connolly method as indicated above, where the probe hard sphere particle creates a surface rolling on the framework atoms represented by hard spheres too. For these calculations we used a probe particle radius of 2 Å, higher than the usually accepted 1.4 Å for the water molecule, this being justified in order to account for hydrophobic repulsion. With this, we obtain the micropore volume accessible to water molecules, which, in the zeosils simulated, belong to channels, and not to other zeosil cavities. Another valuable property of the zeolite structure is the free volume fraction (φ), which we calculated as the Connolly free volume divided per unit cell (u.c.) volume. The corresponding data for IFR, AFI, MTW, and TON-type zeosils are presented in Table 2, together with estimations of free volume from N2 adsorption experiments. Estimations from water adsorption, both from our MD simulations of intrusion−extrusion isotherms and experiments are also included. Before discussing these data, a general consideration must be taken into account. Even considering that the simulation cells explicitly contain the external surface terminated by silanol groups, these cells do correspond to perfect (defectless) crystals. Real synthesized materials taken for water intrusion− extrusion experiments do contain structural defects, most of them being internal (hydrophilic) silanol groups and silanol nests. Moreover, real crystals may adsorb ions on the external and/or internal surfaces, and they may also contain mesopores. It was shown4,13,33,34 experimentally and theoretically that defects, extra framework ions, and hence the zeolite chemical composition, are significantly changed, and this affects the adsorption isotherms shifting them toward a lower pressure region with respect to the isotherm of a perfect (defectless) crystal, such as those simulated. TON-Type Zeosil. This zeosil contains a 10-ring unidimensional system of sinusoidal channels with elliptical openings (4.6 × 5.7 Å). This channel window opening is the smallest among the discussed structures, and this is reflected in the smallest Connolly free volume, 10.3 Å3/SiO2, equivalent to 0.103 mL/g (Table 2). The experimental intrusion−extrusion isotherm5 is presented in Figure 2a together with our calculated data. The synthesized ZSM-22 (TON) material shows a

hysteresis loop in the water intrusion−extrusion diagram. In the phase transition region, our calculated data are scattered. Increasing the number of simulation runs and starting from different initial configurations slightly helped to obtain less noise, but the quality of the results in this region could not be improved. However, the fit to the experimental results outside the shaded region (Figure 2a) is considerably good and very close to the experimental results. The reason for this good agreement between experimental and calculated results is the fact that the ZSM-22 material has been reported to be very stable upon intrusion/extrusion, and the synthesized crystals are virtually defectless5 as in our simulated system. It must be noted that the estimated water density in the channels at saturation is 0.8 g/mL, significantly lower than the density of bulk water, this reflecting the hydrophobic nature of the zeosil. Finally, the experimental adsorbed water volume (0.075 mL/g) and the calculated (0.08 mL/g) are very close, and the Connolly volume is qualitatively close (0.103 mL/g) (Table 2). For these reasons, this material will be taken as a reference system in a further section. AFI-Type Zeolites. These zeolites are the most investigated regarding water adsorption, both through experimental35−37 and theoretical methods.15,16,38−40These materials have been synthesized not only as pure silica (SSZ-24) but also with less hydrophobic AlPO4 composition, called AlPO-5. This will allow us to discuss the role of hydrophobicity/hydrophilicity on water adsorption. AlPO-5 has larger polarity and slightly larger u.c. volume. The AFI-type framework has straight channels running along the crystallographic c-axis (Figure 3). There are 4-, 6-, and 12-ring channels (7.3 × 7.3 Å), all of them parallel and non intersecting. The Connolly volume of AFI (0.150 mL/g) is

Figure 3. (a) Structure of AFI-type zeolite. (b) 12-ring channel of AFI. 24919



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the AlPO-5 structure. Newalkar et al.36 proposed that the initial water sorption occurs in the 6-ring channels, and then water fills the 12-ring channels. This explanation is difficult to accept taking into account the energetics of water in 6-ring pores, where density functional theory (DFT) calculations show repulsive rather than attractive interactions.39 On an entropic basis, the adsorption in 6-rings should not be preferential due to the larger entropic decrease when water fills 6-rings compared to 12-rings.8,34 A different interpretation has been proposed on the basis of a rigorous investigation using incoherent quasi-elastic neutron scattering spectra,37 which shows that at p/ps = 0.35 (where ps is the vapor saturation pressure), the growth of two helices of water is driven by structural commensurability with the AlPO-5 channel structure. A high density ice-like phase is then formed in the 12-ring channels above p/ps = 0.35, and this justifies the large water saturation capacity of this material. In the slightly smaller unit cell of SSZ-24, this commensurability is lost, and this is why a significantly lower amount of water is adsorbed at saturation (see Figure 4a). Our simulation shows that for SSZ-24 zeosil, water adsorption reaches a maximum ca. 11 molecules/u.c. (with the u.c. consisting of 24 SiO2 units), which is clearly less than the above value of 16 molecules/u.c. in AlPO-5, but quite similar (and slightly less) than the value of 12 molecules/u.c. for AlPO-5 based on free volume estimations.37 The agreement with the value based on free volume estimations is because such computation does not contemplate the possibility of forming high density ice-like structures within the 12-ring channels. Without high density water, AlPO-5 would be expected to have a saturation capacity of about 12 molecules/u.c.; and a slightly lower value for SSZ-24 can be perfectly understood on account of a slightly smaller cell volume (see Table 3). These estimations are in agreement with our Connolly calculated value of 0.15 mL/g (Table 2), and with the experimental values obtained from several N2 adsorption measurements such as those by Kawagoe et al.,41 who investigated [B]-SSZ-24 and reported a free volume of 0.16 mL/g (with SiO2/B2O3 = 53) and of 0.14 mL/g (with SiO2/B2O3 = 361). Also Kubota et al.42 reported the value of 0.16 mL/g for [Al]-SSZ-24 (with SiO2/ Al2O3 = 100−200). However, a value of 0.05 mL/g, reported by Tzanis et al.5 for SSZ-24, is 3 times smaller than all the other estimations, and it will not be taken into consideration, due to

considerably larger than that of TON zeosil (see Table 2). For the discussion below, and taking into account the literature cited, it will be useful to consider that several structures corresponding to different symmetry groups were proposed for these zeolites (see Table 3). Table 3. Structural Characteristics of Zeolites with AFI Topology and Maximum Water Loading reference ref 15

symmetry space group

ref 37

orthorhombic Pcc2 hexagonal P6cc

ref 35.

hexagonal

ref 5

hexagonal P6/ mcc hexagonal P6/ mcc orthorhombic Pcc2

this work ref 15

size of u.c. AlPO-5 13.79 × 23.9 × 8.42 Å3 13.77× 13.77 × 8.38 Å3 13.72× 13.72 × 8.47 Å3 SSZ-24 13.61× 13.61 × 8.29 Å3 13.61× 13.61 × 8.29 Å3 13.27× 23.0 × 8.32 Å3

number of H2O/number of T-atoms 18/48 18/24 16/24

8.2/24 11/24 16.7/48

A comparison between the two AFI materials with aluminophosphate (AlPO-5) and pure silica (SSZ-24) compositions shows that the calcined AlPO-5 material adsorbs water from the vapor phase,35,36 meanwhile a high hydrostatic pressure has to be applied to see water penetration into SSZ-24 crystals, with an intrusion pressure of 132 MPa.5 The experimental curve for AlPO-5 shown in Figure 4a contains several regions corresponding to different adsorption mechanisms. The first region, observed at low pressure, corresponds to water adsorption by internal defects within the AlPO-5 crystal presented in the form of P−OH groups, which interact strongly with water. This interpretation has been confirmed from heat of water adsorption measurements35 presented in Figure 4b, where very large heats are observed at low loading. If we now look at the water saturation experimentally observed in AlPO-5, a large value of ca. 16 water molecules/u.c. is observed (with the u.c. consisting of 24 tetrahedral atoms) in Figure 4a. Some controversy in the literature has been seen regarding the location of the corresponding water molecules in

Figure 4. (a) Adsorption isotherms for AlPO-535 and SSZ-24 AFI-type zeolites. (b) Differential heats of adsorption of water vapor on AlPO-535 and SSZ-24 zeolites and heat of condensation of water. 24920



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fills the channels of MTW zeosil at zero pressure when the εOO parameter is twice higher than that obtained by the combinatorial rule. Our results show that a higher polarity of the framework (or strong intercomponent interactions) contributes to increase the hydrophilicity of the material and may explain experimentally observed shifts of adsorption isotherms toward lower pressures in the case of aluminophosphates or in the presence of hydrophilic framework defects. While in this case, the hydrophobicity has been compared in two defectless materials (AlPO-5 and SSZ-24) with the same topology (AFI), in the following we will study how the presence of defects will affect the hydrophobicity. MTW-Type Zeosil. The zeosil corresponding to the MTW topology is called ZSM-12, which has a 12-ring channel whose window opening (5.6 × 6.0 Å) is much smaller than that of SSZ-24 (7.3 × 7.3 Å). The main channel runs along the crystallographic [010] axis and shows a corrugated shape (Figure 5a). The calculated Connolly free volume of ZSM-12 is 0.114 g/mL (Table 2).

the particular synthetic route and the characteristics of the resulting material. The authors5 explained that the low value of the SSZ-24 free volume was due to several reasons: incommensurate structural modulation along the c-axis as described by Liu et al.;43 stacking of hexagonal platelets as observed in scanning electron microscopy (SEM) microphotographs; and stacking defects along channel axis. The latter two reasons might explain locking of some channels and they correspond to specific properties of the synthesized material, but it is doubtful that a small structural modulation can lock channels for N2. A recent study40 showed that the computed structural data agree very well with the experiments in AlPO-5, reproducing the presence of helicoidal chains of adsorbed water molecules, but predict a different arrangement of water molecules for SSZ24, namely linear chains running along the channel axis. High density water, therefore, only forms on AlPO-5 and not on SSZ-24, and below we suggest an explanation for the different behavior of these two materials containing the AFI topology. Our calculated data in Figure 4b show heats of water adsorption for SSZ-24. Calculated values for AFI zeosil are close to recent calculated results23 for IFR and TON zeosils, and also close to the experimental heat of water adsorption in sodalite. At all loadings it can be seen that a stronger interaction does appear in the case of AlPO-5 with respect to SSZ-24. These differences are more marked at low loading, when the water−AFI interactions are dominant. As the loading increases, the water−water interactions become dominant and both heats of adsorption tend to the same limiting value corresponding to the sublimation heat of bulk water. The more hydrophobic AFI zeosil (SSZ-24) weakly interacts with water in the channels, and water is less energetically stable than in the bulk phase. The less hydrophobic AFI aluminophosphate interacts more strongly with water molecules mainly due to the more important electrostatic interactions of water with the larger charge separation in AlPO-5 (formally trivalent Al and pentavalent P) with respect to SSZ-24 (formally tetravalent Si, all equally charged). This difference of interactions with respect to bulk water explains the horizontal shift (across the pressure axis) of the respective adsorption isotherms (Figure 4a). AlPO-5 adsorbs water from vapor, but SSZ-24 adsorbs water only at high hydrostatic pressure. We may expect a strong dependence of the intrusion pressure with the hydrophobicity of materials. For an investigation of the influence of water−zeolite interactions on the position of the adsorption isotherm, we made specific calculations with slightly stronger water−MTW zeolite electrostatic interactions. Electrostatic charges on silicon and oxygen atoms were increased with respect to the original BS force field (2.1 and −1.05) by 0.2 and 0.1 au, respectively. The resulting adsorption isotherm is presented as Supporting Information (Figure S2). This change of interactions shifts the curve toward lower pressure by 25 MPa with respect to the curve calculated for flexible MTW framework but does not change the amount of adsorbed water. Higher polarity of the framework improves calculated results. They become more close to experimental, but the BS force field was calibrated to reproduce thermodynamic properties of zeosils, and this change of charges will significantly affect the relative energetic stability of zeosils26,28 and will contradict experimental data. The adsorption isotherm is sensitive to water−zeolite van der Waals interactions. Additional calculations show that with an increasing of intermolecular attraction, the adsorption isotherm shifts toward lower pressure (see Figure S4 of SI), and water

Figure 5. Channels in MTW (a) and IFR (b) zeosils.

There are several reports about free volumes of ZSM-12 with the following values: 0.112 mL/g (SiO2/Al2O3 = 116);44 0.1− 0.12 mL/g (SiO2/Al2O3 = 40−100);45 and 0.1 mL/g (SiO2/ Al2O3 = 40).46 These experimental results are very close to our calculated Connolly volume. The value of 0.15 mL/g, reported by Tzanis et al.5 for ZSM12, again contradicts all other calculated and measured data and will not be taken into consideration. As in the case of the AFI topology, we believe the disagreement between this source5 and the other data (Table 2) is due to the particular synthetic route employed and the possible presence of macropores. Coming back to the mainstream results, real materials may have significant deviations from the idealized structures that we are using as computational models, due to the presence in the former of defects of framework connectivity and the possible presence of impurities, such as cations ionically stabilized by the subsequent polar character of these defects. Due to these and other defects coming throughout the stages of nucleation and crystal growth, the presence of defects, even if not many, are believed to significantly affect the adsorption isotherms. A recent breakthrough in this area has been achieved by Kärger et al.47,48 showing that a large number of outer channels in the external surface of a zeolite crystal are not available for diffusion of incoming molecules. Computational studies have only recently included the effect of surfaces to study water adsorption,49 as is the case in the present study; however, a computational challenge remains to simulate more accurately 24921



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hydrophilicity of the material, the larger the amount of water adsorbed, and the smaller the pressure at which adsorption starts. IFR-Type Zeosil. This zeosil has the largest calculated Connolly free volume, 20.2 Å3/SiO2 (0.201 mL/g), among the zeosils investigated in the present work, although the 12ring channels (6.2 × 7.2 Å) are smaller than those of AFI (7.3 × 7.3 Å). Free volume is close to the pore volume determined from nitrogen adsorption of ∼0.23 mL/g (see Table 2). The experimental and calculated isotherms are presented in Figure 7. A synthesized ITQ-4 (IFR) material7 adsorbs 0.136 mL/g of

the real shape of the external surface as well as its real chemical composition. On the other hand, the experimental data are still scarce, and for ZSM-12 we can only rely on the results previously discussed,5 which we plot in Figure 6 together with the present

Figure 6. Experimental5 and calculated water adsorption isotherms for SSZ-24 (AFI) and ZSM-12 (MTW) zeosils. Horizontal lines show estimated free volumes for AFI and MTW zeolites.

simulated isotherms. For the reasons outlined above, we include here both SSZ-24 and ZSM-12 zeosils, where it is somehow obvious that there is a considerable disagreement between theory and experiments which, nevertheless, we will try to explain on the basis of the above-referenced particularities of the real samples and the limited accuracy of our models and methods. Formally, our calculated data for SSZ-24(AFI) do seem to fit better the experimental curve of ZSM-12 (MTW), and the experimental data of SSZ-24 give a very low water adsorption saturation capacity. Regarding this, it is difficult to explain why ref 5 gives 2 times larger adsorption of water (0.102 mL/g) than N2 (0.05 mL/g) for SSZ-24 (Table 2). However, our calculated values of maximum water sorption are 0.133 mL/g for AFI and 0.098 mL/g for MTW zeosils (Table 2). Such calculated values are in reasonably good agreement with the free volumes obtained from most N2 sorption experiments, ca. 0.15 mL/g and ca. 0.11 mL/g, respectively (see Table 2). For these reasons, we trust that the computed data of the saturation water capacity in SSZ-24 and ZSM-12 zeosils are in agreement with most experiments, in spite of the particular disagreement shown in Figure 6. Regarding the small calculated values (compared to experiments in Figure 5) of water adsorption at low pressure, we again invoke the hydrophobic nature of the modeled systems with respect to the real crystals. SSZ-24 (AFI) material, used for the water intrusion−extrusion experiment, had 2.7% of Si atoms in a form of hydrophilic silanol groups.5 The number of silanol groups is slightly less (2.5%) in the case of ZSM-12 (MTW) material. Most of these groups are in framework channels and can strongly interact with water molecules and play a role of nucleation centers during a water condensation in zeolite channels.23 In the present work, the results were obtained for defectless zeosils. We may conclude that the strength of water−zeolite interactions, which defines the hydrophobic/hydrophilic properties of the materials, significantly influences the amount of adsorbed water, especially at low pressure. The larger the

Figure 7. Experimental and calculated adsorption isotherms of ITQ-4 (IFR) zeosil.

intruded water. If we take into account the amount of physisorbed water (∼0.015 mL/g) from vapor,7 we may estimate a total amount of adsorbed water of 0.151 mL/g. In our simulation we have reached the value of 0.155 mL/g at 200 MPa (see Figure 7), in close agreement with the experimental results. From our results, a water density in the zeolite channels of 0.8−0.9 g/mL is obtained. The synthesized material7 contains internal silanol defects whose amount were estimated by 1.8 defects/u.c. (with a u.c. containing 32 SiO2 units). This means that 5.6% of the Si atoms form silanol groups. In a previous study23 we discussed water−IFR and water−TON systems in detail. ITQ-4 (IFR) material has low chemical stability, and the crystals are destructed during the water intrusion process.7 After water extrusion, the number of internal silanol defects significantly increases and reaches a value of 3 defects/u.c., which means that 9.5% of Si atoms have silanol groups. In our simulated system, no internal silanols have been considered in the computational models. Synthesized ITQ-4 material showed a “bumper” behavior. Water stayed inside the channels after pressure was released. For water removal from the zeolite, one must apply low pressure and high temperature. During the intrusion, this material is changing its properties from hydrophobic to hydrophilic. This is how we explain the considerable shift of the intrusion part of the adsorption isotherm with respect to the calculated data (see Figure 7), due to the lower hydrophobicity of the ITQ-4 samples with respect to the simulation. 3.2. Structure of Water in Zeolites. The degree of hydrophobicity of any material is determined by the relative strength of the water−material and water−water interactions. All zeosils considered in this study are simulated with the same 24922



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Figure 8. Low energy water clusters in the AFI channels: (a) n = 8, (b) n = 18 molecules in the simulation cell (see Table1, upper part); (c) n = 720, (d) n = 750 (p = 140 MPa, MD configurations in the see Table1, lower part). Only fragments of zeosil are shown for clarity.

Figure 9. Water clusters in the IFR channels: (a) n = 8 molecules in the simulation cell (energy minimization, Table1, upper part); (b) n = 550 (p = 110 MPa, MD configuration in the large cell, Table1, lower part). Only a fragment of zeosil is shown for clarity.

force field, and they have similar hydrophobicity. The strength of these interactions depends not only on the force field but also on the zeolite topology too. Deviations in the degree of hydrophobicity can be explained by a confinement effect. Each particular pore size, shape, and positions of oxygens may stabilize some water clusters and destabilize others. This effect of stabilization was clearly shown37,40 for the hydrophilic material AlPO-5 (AFI). The specific positions and distances between oxygens in the channels strengthen the interactions with water and direct to the formation of two helicoidal chains of water molecules. However, a different picture was observed40 for the related hydrophobic material SSZ-24 (AFI). The weaker water−zeosil interactions in this material do not facilitate the formation of helicoidal chains that are commensurate to the zeolite structure. This zeosil has a large pore opening, and our simulations show that water forms compact clusters at low

loadings instead of the formation of chain or double-chain structures. For an illustration, some configurations are presented in Figure 8. Compact clusters are typical at low water loading (Figure 8a). These tiny water drops constantly change their shape due to molecular motion. Sometimes they have a shape resembling chains. For example, eight water molecules in Figure 8a are associated with two pentagonal rings. With increasing loading, water clusters grow until a percolated structure was observed (Figure 8b), where compact bulky fragments alternate with chain-like structures. These two structures were obtained with the computational model of the upper part of Table 1, and through a lattice energy minimization method. The next two figures (Figure 8c,d) were extracted from the long simulation MD runs based in the models of the lower part of Table 1, where a tendency of water toward self-association is visible in the two snapshots presented. 24923



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Figure 10. Low energy water clusters in the MTW channel at n = 8 water molecules in the small simulation cell (see Table 1, upper part): (a) water in the channel, (b) water (blue) and some zeosil oxygens (red) highlighted.

Figure 11. Water clusters in the MTW channels (MD simulation in the large cell (see Table 1, lower part) at n = 640 (p = 200 MPa). Only fragments of zeosil are shown for clarity.

MTW zeosil also contains a straight and corrugated 12-ring channel, as AFI, but of smaller size (5.6 × 6.0 Å). The channel section is also smaller than that of the previous IFR structure and thus, following the above arguments, it is expected that this will strengthen the role of water−zeosil interactions in the water clustering. MTW zeosil channel shape and oxygen atom positions are stabilized as nearly linear chains as Figure 10 shows. This configuration, obtained by energy minimization, is a structural attractor to the formation of water chains in MTW zeosil. This linear water chain is different from the helical chain observed in the IFR channels. The most striking feature is the commensurate water−zeosil distances shown in Figure 10b. The distance between oxygen third neighbors in the zeosil is equal to the distance between oxygens second neighbors in the water chain (∼5 Å). This distance corresponds to the position of the second maximum in the O−O radial distribution function of water, goo. The repeating motif formed by three consecutive water molecules hints that we may expect a high stability and hence a large number of water triplets throughout the MD run. Two snapshots near the percolation threshold are shown in Figure 11. There are two zeosil channels in our simulation cell, and water fills both channels (Figure 12a) or one channel (Figure 12b). The number of water molecules in the channels is approximately the same, but the structures are very different. Single chains with bulky fragments are observed in the first case, and bulky double chains are seen in the second

Two bulky clusters are growing inside the zeosil channel throughout the entire zeosil framework length until percolation is observed. A different picture is observed for ITQ-4 (IFR) zeosil whose micropore topology presents differences with respect to AFI. IFR zeosil has the largest free volume of the four studied (Table 2), but a smaller 12-ring-opening (6.2 × 7.2 Å) than AFI (7.3 × 7.3 Å). IFR channels are sinusoidal, while those of AFI, MTW, and TON are straight and corrugated (Figures 2a, 3, and 5). The sinusoidal shape of the IFR channel promotes the formation of chain-like water clusters. Figure 9a shows how the water cluster formed is commensurate with the channel shape and length. This configuration was obtained through lattice energy minimization for the model described in the upper part of Table 1. Water molecules form H-bonds with framework oxygens lying on opposite sides of the channel, and water−IFR zeosil interactions stabilize the water chains. Using the model from the lower part of Table 1, and a long MD simulation, a plethora of water structures are observed in the zeosil channels. Configurations corresponding to local energy minima play the role of structural attractors. Three structural elements are shown in Figure 9b, namely: bulky clusters, chains, and isolated water drops. The formation of bulky structures is common for pre- and postpercolation periods of water condensation, but the chain structure is more common for clusters near the percolation threshold. 24924



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3.3. H-Bonds and Thermodynamics. The results of the MD simulations in small cells (upper part of Table 1) were used for the investigation of H-bonds statistics and water thermodynamics. There are a large number of definitions of Hbonds in water proposed in the literature dealing with energetic, geometric, or mixed criteria.19,50−52 We used a very weak geometric threshold criterion so that not only strong linear but also weak or bifurcated H-bonds are detected. Two water molecules are considered H-bonded if the distance between hydrogen of the first and oxygen of the second water molecule is less than 2.4 Å. The same criterion is applied for the water−zeolite H-bonds (between H-water and O-zeolite). This distance corresponds to the first minima of the experimental radial distribution function of water (gOH). Taking into account the intramolecular O−H bond length (∼1 Å), we conclude that intermolecular O···O distances should not exceed 3.4 Å among H-bonded water molecules (or water−zeosil). This distance corresponds to the position of the first minimum of the experimental radial distribution function of water (gOO). We did not apply any further restriction on H-bond angles (O−H···O), and as a result we have counted bifurcated bonds where the H atom is simultaneously close to two O atoms. According to this definition, a water molecule, playing a role of hydrogen donor, can have more than two H-bonds. The H-bonds analysis is presented in Figure 14 for the four zeosils studied. The number of water−zeolite (WZ) H-bonds is significantly less than the number of water−water (WW) Hbonds except at the lowest loading. The formation of water clusters is reflected by an increasing of water−water H-bonding as the loading increases. TON zeosil, with the smallest channels, gives the largest number of water−zeolite H-bonding, while the lowest is for AFI, with the largest channels. The opposite ordering is observed for the number of water−water interactions, the largest and smallest in AFI and TON, respectively. The summation of the total number of H-bonds (water−water and water−zeosil) is nearly constant regardless of the zeosil topology.

Figure 12. Low energy water clusters in the TON channel at n = 8 water molecules in the small simulation cell (see Table 1, upper part).

case. We observed such structural diversity for zeosils with narrow channels. TON zeosil contains a smaller 10-ring, slightly elliptical, channel (4.6 × 5.7 Å) with a straight and corrugated shape (Figure 2b). Here again, as in MTW, one-parameter of the unit cell (c) is ∼5 Å, and water chains are energetically favored due to a specific match with the channel shape (Figure 12). Being the narrowest channel, water is under strong confinement, and this is the only case in which the formation of compact structures is significantly suppressed due to steric factors. As a result, we observe a single-file structure where water molecules form weak H-bonds with zeolite oxygens lying in opposite sides of the channel. This chain is a structural attractor for water in TON zeosil. Single-file structures with more bulky fragments were observed in TON channels near the percolation threshold (Figure 13). And, again, different types of channel loading are observed. Channels may be filled by water more or less evenly, as it is presented in Figure 13a. In the other case, water forms percolated clusters only in some channels, while other channels are empty. If larger simulation cells were used, we should expect a smoother distribution of channel filling. At high hydrostatic pressure, all zeosil channels are fully loaded, and water structures are determined by channel sizes and shapes. For IFR and TON zeosils, an analysis of water structures at the entire range of loading was made in our previous article.23

Figure 13. Water clusters in the TON channels (MD simulation in the large cell (see Table 1, lower part) at n = 540 molecules in the cell (p = 210 MPa). Only fragments of zeosil are shown for clarity. 24925



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Figure 14. Statistics of H-bonds. Number of bonds per water molecule versus number of molecules in small simulation cells (see Table 1, upper part): (a) H-bonds between water molecules (WW) and between water hydrogens and zeosil oxygens (WZ); (b) total number of H-bonds per water molecule.

Figure 15. (a) Energy of water molecules in IFR, AFI, MTW, and TON zeosils. For the sake of clarity, energies of zeosils have been shifted by: 10 (AFI), 20 (MTW) and 30 (TON) kJ/mol. (b) The same plot without energy shifting. Region corresponding to half-maximum loadings is shaded.

channel promote the formation of a particularly stable water structure. From a thermodynamic point of view, water is at equilibrium state if Gibbs free energies of water in reservoir (r) and in zeolite (z) are equal. With standard state parameters, we may write the following equation at thermodynamic equilibrium:

As specific features, there is a threshold loading at which the number of H-bonds is a relative minimum. These minima were observed at six water molecules in IFR, seven in TON, and eight in MTW zeosil simulation cells (Table 1, upper part). This corresponds to 2.55 Å between water molecules along the channel axis of IFR and MTW zeosils, and 2.87 Å of TON zeosil. An analysis of the MD simulations revealed that this represents a structural transformation from compact to chainlike shapes (Figures 8−13). There is no such sharp transformation for AFI zeosil, where the large channel size allows the formation of bulky compact clusters at all loadings. The calculated energies of water molecules in water−zeosil systems are presented in Figure 15. Single water molecules weakly interact with zeosils, with energies in the range 3−10 kJ/mol in contrast to ca. 40 kJ/mol in the case of bulk water. At low loading, interactions become stronger with increasing loading, which reflects water self-association. Four- and fivemember water clusters are energetically stable. In fact, above four water molecules, there is a range (roughly 5−8 water molecules, as shaded in Figure 15b) in which the energy per water molecule remains nearly constant. We will use this range to apply a thermodynamic equation whose derivation follows. The presented plots for TON and IFR zeosils were previously discussed in detail.23 Only one zeosil, IFR, shows larger energetic stability than bulk water, and this occurs only at large loading. The largest free volume and the specific shape of the

ΔUr − T ΔSr + PV r r = ΔUz − T ΔSz + PzVz

(1)

where ΔU = U − U is internal energy and ΔS = S − S is entropy, both with respect to water in the standard state. We will consider an extremely simplified situation to estimate the intrusion pressure when half of the water volume at saturation is adsorbed. Pressure inside zeosils is very small with respect to the intrusion pressure and we can neglect the PzVz terms. According to the NIST database,53 for bulk water ΔUr − TΔSr = 0.117 kJ/mol within 0 to 200 MPa and T = 300 K. This value is smaller than the maximum pressure−volume variation in the interval (0−200 MPa), ΔPrVr = 3.36 kJ/mol, and for water in the reservoir we can neglect the energy−entropy variations in the pressure interval. Then, if we take two zeosils (z1 and z2), the difference of intrusion pressures in the bulk water phase may be estimated from: 0

Pr1 − Pr2 ≈ [(Uz1 − Uz2) − T (Sz1 − Sz2)]/Vr

0

(2)

Energies of the zeosils are very close at the region corresponding to half-maximum loadings according to the data presented in Figure 15b. The energy difference is small 24926



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within Nw = 5−9 molecules in zeosil channels (near percolation threshold) and according to eq 2 the difference in intrusion pressures is governed by the entropic term. For the calculation of water entropy in zeosils, it is possible to use the Widom test particle method,54 but for the estimation of the entropy we will use another simple model. We consider a system of hard spheres interacting with potential: E(r) = 0, if r > rC ; and E(r) = ∞, if r < rC. Here rC is the radius of the water molecule. In the case of the NVT canonical ensemble the difference of Helmholtz free energies between systems with zeolite (Az,) and without zeolite (A), may be calculated as follows: ΔA = A z − A = ΔU − T ΔS = − kT ln(Q z /Q )

(3) Figure 16. Correlations of intrusion pressure with free volume fractions according to eq 7.

where Qz and Q are the configurational parts of the canonical partition function for N particles in volume V at temperature T, with both systems having the same volume V. Q=C

∫

⎛ E (r ) ⎞ (dr )N exp⎜ − ⎟ ⎝ kT ⎠

permeable for water at the pressure range considered in our derivation. For example, water does not penetrate in SOD at pressures below 400 MPa due to the small 6-ring window opening of the sodalite cage,6 and hence we cannot apply our finding to this particular case, and perhaps in other cases. The shift of adsorption isotherms to small intrusion pressures due to stronger water−zeosil interactions was observed in calculations and experiments.4,13,14,33 The slope of the straight line is −RT/Vr, (see eqs 6 and 7), which depends linearly on the temperature, and hence we may expect that intrusion pressures will decrease with increasing temperature. According to our and previously published results of calculations,13,14,33 phase transition occurs in a narrow pressure interval in which the chemical potential of water does not change significantly in the reservoir. Energy and entropy of water in zeosils decrease with loading, and an effect of enthalpy−entropy compensation is observed for water in zeosils during the phase transition. At higher loading, the loss of configurational entropy is not compensated by energy stabilization, and for additional water penetration in zeosil, one needs to apply higher external pressure to bulk water in the reservoir. Molecular Mechanisms of Water Penetration. With the results of MD simulations, we can visualize all stages of water intrusion−extrusion processes in zeosils. Some movies are presented as SI. Different mechanisms of water penetration in large and small channels have been observed. Water molecules form tiny drops in large channels at low loading because such clusters are energetically stable. They may exist for a relatively long time and move along the channel merging together and breaking into smaller drops (see Figure 9b). These processes do correspond to the features of capillary adsorption theory and nucleation theory. In the case of small channels, compact chain-like cluster transformations are observed (see Figures 11 and 13), and chains may break on parts. An interesting phenomenon is observed in MTW and especially in TON zeosils. Our simulation cell has two channels. In some simulations, we had two partially filled channels, but in others one channel was empty, and hence there are two stable states of water in the simulation cell. A similar water dynamic behavior was observed in small-diameter carbon nanotubes.22 The entrance of the first water molecules is thermodynamically disadvantaged due to lose of part of water−water hydrogen bonds. However, the filling of the whole nanotube (or zeolite channel) is more favorable because water molecules in single file clusters interact

(4)

For hard spheres at low density (N/V → 0), neglecting of hard spheres volume, Q ≈ CVN and Qz ≈ CVzN, where Vz is free volume of system with zeolite, and C is a constant. Taking into account that ΔU = 0 for one mole of particles we have: ΔS = R ln(Vz /V )

(5)

This entropy corresponds to configurational entropy of the ideal gas at the limit of N/V → 0, which is S = R ln(V). Equation 5 describes the entropy difference for an ideal gas, not for water, but we may try to check the validity of this approximation. At the water loading half of saturation (Vz/2), neglecting nonideality, we may use the following approximation: Pr1 − Pr2 ≈ − (RT /Vr) ln(φ1/φ2)

(6)

where φi is the free volume fraction of zeolite (i = 1, 2), which may be estimated as the Connolly volume divided by the volume of the unit cell (Table 2). Vr is molar volume of bulk water in reservoir (∼17 cm3/mol at P = 100−200 MPa). If we take TON zeosil as a reference system, for other zeosils the intrusion pressures can be calculated as follows: P ≈ PTON − (RT /Vr) ln(φ /φTON)

(7)

We choose the material ZSM-22 (TON) as a reference because the reported samples were synthesized with a minimal amount of defects, and the calculated intrusion pressure is very close to the experimental value. The results of calculations according to eq 7, together with the results of our MD calculation, and results of GCMC simulations for MFI13,33 and LTA14 zeosils are presented in Figure 16. The fractions of free volumes are 0.291 for MFI and 0.462 for LTA zeosil (rC = 2 Å). The corresponding calculated intrusion pressures are 125 and 85 MPa, respectively. Even though results for MFI and LTA have been obtained using different force fields than in this study, all the results in Figure 16 correlate with the theoretical predictions of eq 7. Within these approximations, the configurational entropy controls the pressure of water intrusion in zeosils. In the case of real materials, we cannot neglect the terms dealing with the difference of internal energy, which may vary from one material to other due to the presence of connectivity defects. Equally, other limitations include that the zeosils to which this approximation is applied must be 24927



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more strongly. Free energy decreases when a channel is filled by water molecules, due to the formation of percolated clusters. The presence of two states with low energy justifies the observed pulse-like behavior. A real material contains an ensemble of channels. When water is intruded into the zeosil, chains start to grow up uniformly due to symmetry, and the number of percolated clusters is relatively small. At full loading, water molecules are in all channels and chains look like “rubber threads”. After breaking a thread, water molecules leave the channel. There is a dynamical equilibrium between empty and filled states of channels. Threads are breaking and forming dynamically in channels. During the phase transition from filled to empty state, the number of broken threads increases. This may be an explanation for the hysteresis that is observed in experiments. Such phase transitions are described by the percolation theory. At high loading, chain-like percolated clusters connect two water−zeosil interfaces. With decreasing loading, some “threads” are broken, and the number of paths connecting interfaces decreases. There is the so-called percolation threshold, which is a critical point that separates two states. Unfortunately, we cannot simulate such process using standard MD methods due to the complexity of the required system, containing thousands of channels. But we suggest that such molecular mechanism of intrusion−extrusion processes is more favorable for molecular sieves with narrow channels.

know the intrusion pressure of a reference system. With the only additional data of the fractional free volume of zeosils, which can be easily estimated from the zeosil structure, the intrusion pressures for AFI, MTW, IFR and TON zeosils were calculated according to such equation, and the results correlated accurately with the intrusion pressures calculated from the adsorption isotherms. Previously reported intrusion pressures for MFI and LTA zeosils correspond to data calculated by the theoretical equation. Hence, before making experiments and long calculations of adsorption isotherms, the use of this equation can give us a prompt estimation of the water intrusion pressure in zeosils. This prediction may be useful, but the results will not correlate when the experiments are performed in samples containing significant amounts of connectivity defects or impurities. In fact, our simulations have also allowed estimating the influence of such hydrophilic defects in the intrusion pressure by comparison of experimental data with calculated data, the latter taking into account defectless structures. Our simulations also allowed extracting useful structural and energetic information of water clustering in the channels of AFI, IFR, MTW, and TON zeosils. Compact “bulk-like” clusters form in large channels such as in AFI and IFR zeosils. The smaller channels of MTW and TON zeosils favor the formation of chain-like water clusters. Interestingly, these two zeosils promote the formation of water chains with commensurate positions of oxygen atoms in the framework. An additional finding was the observation of structural transformations at intermediate loadings, with compact clusters transforming into percolated chain-like clusters. For zeosils with large channels (AFI and IFR), phase transitions relate to the water condensation process. Tiny water drops may be associated with nucleus of the condensed phase, but the effect of confinement adds specific features that make this picture depart from the one accepted in the classical nucleation theory.

4. CONCLUSIONS We made a set of MD simulations trying to calculate water adsorption isotherms, and to elucidate the molecular mechanism of water penetration in hydrophobic AFI, MTW, IFR, and TON zeosils. Two strategies of MD simulations were used. For calculation of adsorption isotherms, we made MD simulations of heterophase systems. A simulation cell, with the longest side of 120 Å, contained a zeosil, including surfaces, and a bulk water reservoir. We made the simulations in the canonical NVT ensemble applying periodical boundary conditions to the cell. Pressure in the water reservoir was defined by water density calculated after equilibration of the system. The amount of adsorbed water was counted in a narrow slab inside the zeosil phase far from water−zeolite interfaces. In order to obtain detailed information about energetic and structure of water in zeosils, we made MD simulations with smaller simulation cells containing flexible zeosil framework and flexible water molecules in framework channels. It was found that intruded water volumes correlate to Connolly free volume (one of the main adsorption parameters) of zeosil unit cells at radius of probe particles of 2 Å. Calculated free volumes also correlate to free volumes experimentally measured by N2 adsorption. The estimated water density in zeosils at high loading is within the range of 0.8−0.9 g/mL, in agreement with the reported14 value of 0.83 g/mL for pure silica LTA-type zeolite. The second parameter defining water adsorption isotherm is the intrusion pressure. We found that a phase transition, corresponding to the filling of empty channels, occurs in a narrow pressure range. For an estimation of the intrusion pressure, we used thermodynamic considerations based on a simplified model. It was shown that the intrusion pressure strongly depends on the configurational entropy of water in zeosils, and an equation was derived that allows one to estimate the intrusion pressure for any water penetrable zeosil if we

■

ASSOCIATED CONTENT

S Supporting Information *

This includes GULP and DL_POLY input files with the used force field and movie files with visualization of MD calculations. This information is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Y.G.B. acknowledges Universitat de Valencia for a visiting professor fellowship through the program “'Atraccio de talent' VLC Campus”. J.V.d.J.-O. and J.G. acknowledge the support from the Spanish Ministerio de Ciencia e Innovación (project SAF2009-13059-C03-02). Y.G.B. and G.S. thank CPD-UPV for the use of their computational facilities.

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REFERENCES

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