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Unusual Hysteresis Loop in the Adsorption-Desorption of Water in NaY Zeolite at Very Low Pressure Jean-Pierre Bellat,*,† Christian Paulin,† Marie Jeffroy,‡ Anne Boutin,‡ Jean-Louis Paillaud,§ Joel Patarin,§ Angela Di Lella,| and Alain Fuchs| Institut Carnot de Bourgogne, UMR 5209 CNRS, UniVersite´ de Bourgogne, 9 alle´e A. SaVary, BP47870, 21078 Dijon Cedex, France, Laboratoire de Chimie Physique, UMR 8000 CNRS, UniVersite´ Paris-Sud, 91405 Orsay, France, Laboratoire de Mate´riaux a` Porosite´ Controˆle´e, UMR 7016 CNRS, UniVersite´ de Haute Alsace, ENSCMu, 3 rue Alfred Werner, 68093 Mulhouse, France, and Ecole Nationale Supe´rieure de Chimie de Paris (Chimie ParisTech) and UniVersite´ Pierre et Marie Curie, 75005 Paris, France ReceiVed: NoVember 20, 2008; ReVised Manuscript ReceiVed: February 11, 2009
We report the observation of an unusual hysteresis loop in the water adsorption-desorption isotherm in NaY in the very low pressure range. Vacuum equilibrium thermodesorption and n-pentane adsorption experiments combined with grand canonical Monte Carlo simulations show that this hysteresis phenomenon is consecutive to the trapping of water in sodalite cages. We propose that the adsorption-desorption of water in NaY occurs as follows. (i) Initial adsorption of a few water molecules on the most accessible cations located on sites II in the supercages. The solvation of these cations makes the sodalite cages accessible to water. (ii) Complete filling of supercages and sodalite cages associated with a migration of nonframework cations from D6R prisms to sodalite cages. (iii) Desorption of water from supercages with water still trapped in the sodalite cages. (iiii) Thermally activated desorption of water from sodalite cages. Thus, according to the hydration procedure used, the residual water can be located either in supercages or in sodalite cages. This provides a way to prepare zeolite samples with the same amount of residual water but with different nonframework cation and water molecule distributions. These two samples would presumably behave differently with regard to selective adsorption of hydrocarbon mixtures, a feature that could be of interest for separation technologies. Introduction The FAU type zeolites (NaY, NaX, and their exchanged cationic forms) are powerful adsorbents that are used in many industrial applications such as, for example, gas separation, dehumidification, and purification of fluids. They are crystalline and well-characterized materials, displaying very narrow pore size distributions, which makes them very selective. Faujasites have a high adsorption capacity (about 0.3 cm3 · g-1), thanks to their porosity composed of supercages (internal diameter of 1.3 nm) interconnected via dodecagonal windows (diameter of 0.74 nm) and sodalite cages (internal diameter of 0.66 nm and free aperture of 0.22 nm) linked by D6R prisms. FAU type zeolites are very hydrophilic, due to the presence of extra-framework compensation cations which are located in well-defined positions: sites SI inside D6R prisms, sites SI′ inside sodalite cages, and sites SII and/or SIII inside the supercages according to the value of the silicon/aluminum ratio (Figure 1). The cation distribution depends also on the hydration state of the material. Indeed, X-ray diffraction experiments show that a cation redistribution occurs from sites I to sites I′ during the rehydration of FAU type zeolites.1-4 It is known that the presence of a small amount of water in the microporosity influences the selective adsorption properties of faujasites. For instance, the presence of 3 wt % of preadsorbed * To whom correspondence should be addressed. E-mail:
[email protected]. † Universite´ de Bourgogne. ‡ Universite´ Paris-Sud. § Universite´ de Haute Alsace. | Ecole Nationale Supe´rieure de Chimie de Paris (Chimie ParisTech) and Universite´ Pierre et Marie Curie.
Figure 1. Schematic view of the framework structure of faujasite with the site I, I′, II′, II, and III locations.
water in the BaX zeolite increases the adsorption selectivity of p-xylene with respect to m-xylene by more than 50%, at high loading of the supercages.5,6 This selectivity enhancement has been tentatively explained by entropic effects, consecutive to steric hindrance created by the presence of water molecules in the supercages, combined with a migration of compensation cations. However, no firm conclusion was drawn yet on this subject. It is thus of great interest to study the water adsorption features in these materials and, more specifically, the distribution of water molecules in the porosity. The adsorption of water in NaY and NaX FAU zeolites has been extensively studied at the macroscopic scale by thermogravimetry and calorimetry. A brief review can be found in ref 7. Since the 1960s, it is thought that the water is adsorbed in FAU type zeolites in three steps: (i) water is adsorbed on compensation cations and forms a cluster around them, (ii) water is then adsorbed as a monolayer on the aluminosilicate walls, and (iii) the filling of cavities is completed according to a process
10.1021/jp810209t CCC: $40.75 2009 American Chemical Society Published on Web 04/20/2009
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similar to a condensation. The adsorption-desorption process of water in zeolites is known as perfectly reversible. No hysteresis loop is observed during desorption as long as the zeolite does not undergo a modification like a dealumination, for example.8 In the last two decades, the development of experimental techniques of investigation such as NMR, XRD, FTIR, and neutron scattering has allowed researchers to obtain information on the water adsorption properties of zeolites at a molecular level. Thus, it is possible to imagine, with more or less certitude, where the water molecules are located in the porosity, what is their mobility, what is the physical state of the matter confined in the micropores, or how the nonframework cations can migrate during the adsorption process. Moreover, the significant advances realized in molecular modeling these last 10 years, especially by Monte Carlo simulations, has allowed the determination of a lot of information on the phenomena that take place during water adsorption (location of water molecules, interaction energies, cation distributions,...) and which are very fastidious or even impossible to determine experimentally.9,10 Thus, the following points are now recognized. Water can enter both the supercages and the sodalite cages, one sodalite cage accommodating a maximum of four water molecules. The first water molecules are located near compensation cations.11-13 However, it is possible at low filling that water molecules interact with oxygen atoms of the zeolitic framework by hydrogen bonding. During the filling of cavities, an important cationic redistribution can take place, depending on the Si/Al ratio.14,15 At intermediate loading, water molecules interact with each other via hydrogen bonding to form a cyclic hexamer in the dodecagonal windows.16,17 Nevertheless, the adsorptiondesorption mechanism of water in FAU is not completely elucidated, especially at low loading. Basically, physisorption thermodynamics predicts that water should first condense in the smallest pores. In that case, for the FAU type zeolites, the sodalite cages would be filled before the supercages. This is what Boddenberg et al.18 concluded from a very nice calorimetric study of water in the NaY zeolite. However, as outlined by these authors themselves, the observation that the first water molecules introduced in the NaY zeolite exclusively occupy the sodalite cages is at variance with literature reports.19,20 Indeed, according to these latter authors, water can be found in both the sodalite and the supercages. One possible reason for this finding is that, in order for water to enter the sodalite cages, it must pass through the supercages. The size of a water molecule is too large to enter directly into the sodalite cages via the D6R prisms. Water molecules will thus necessarily interact with compensation cations located in sites II in the supercages. On the other hand, the dehydration process is clear. The supercages are evacuated first, and the last molecules leaving the zeolite come exclusively from sodalite cages. This raises the question of the actual stable equilibrium state of NaY at low water loading, and this is precisely the subject of the present investigation. This point is important to elucidate since the distribution of residual water in the porosity has a large influence on the selective adsorption of hydrocarbons, which can only adsorb in the supercages. The aim of this work is to improve the knowledge of the residual water distribution in the microporosity. This is the reason why we have focused the study on the adsorption and desorption of water at very low filling and room temperature. This requires the investigation of the very low pressure range, using thermogravimetry under pure vapor pressure. Besides, the adsorption measurements are complemented with grand canoni-
Bellat et al. cal Monte Carlo simulations in order to shed some light on the adsorption process at a molecular level. Experimental Section The NaY zeolite sample was synthesized following the recipe published by the Synthesis Commission of the International Zeolite Association.21 The powder X-ray diffraction data (PXRD) pattern of the as-synthesized NaY sample was collected at 295 K on a PANalytical MPD X’Pert Pro diffractometer in the Bragg-Brentano geometry (Cu KR1 radiation, λ ) 0.15406 nm) in the range 4 < °(2θ) < 90, step ) 0.017°(2θ), time/ step ) 300 s. Elemental analysis of Na, Si, and Al was performed by atomic absorption spectroscopy (AAS) on a Varian Techtron AA6 apparatus after dissolution of the samples in HF. The size and the morphology of the crystals were determined by scanning electron microscopy (SEM) using a Philips XL 30 FEG microscope. Nitrogen adsorption-desorption isotherm studies were performed using a Micromeritics ASAP 2010 apparatus. Prior to the adsorption measurements, the sample was outgassed at 623 K overnight under vacuum. Adsorption-desorption isotherms of water and n-pentane on NaY were measured at 298 K with a homemade McBain thermobalance, using the ultrahigh vacuum technology. The vapor pressure ranged from 10-5 hPa up to 30 hPa for water and 100 hPa for n-pentane. The sample weight was about 15 mg. Prior to each adsorption the sample was outgassed at 673 K under vacuum (10-6 hPa) overnight. Vacuum equilibrium thermodesorption experiments were performed by using a static method. The hydrated sample was placed under dynamic vacuum at 298 K. Once a plateau of mass was reached, a subsequent desorption step was achieved with a small temperature increment. The heating rate was lower than 0.5 K · min-1 in order to avoid a possible deterioration of the material by steaming. Thus, each point of the thermodesorption curve corresponds to an equilibrium state of the material under the vacuum limit pressure (10-6 hPa). The n-pentane was supplied by Sigma Aldrich (>99%) and was stored on activated zeolite 4A in order to prevent any trace of water in the adsorptive gas. Experiments were carried out at least three times in order to check the reproducibility of the measurements. For clarity, we only plot average experimental values in the figures, the experimental error being estimated at about 10-3 g · g-1 for the adsorbed amount. This corresponds to 0.5 and 0.2 molecule of water and n-pentane per unit cell (molec · uc-1), respectively. The accuracy on the pressure data is 1%, and the temperature is measured with a precision of 1 K. Simulation Details Adsorption isotherms were computed using bias grand canonical Monte Carlo (GCMC) simulations.22,23 The average number of water molecules was calculated for several values of the chemical potential of the (fictious) ideal vapor reservoir at 300 K. Three different statistical bias moves were used to accelerate the simulations convergence: orientational,24,25 preinsertion,26 and “jump move” bias.17 For water molecules this bias jump move was associated to a rotational bias. Each GCMC run lasted for some 20 million steps in order to equilibrate the system, followed by 40 million steps for the data acquisition. The simulation box consisted of one unit cell of faujasite with periodic boundary conditions. The zeolite framework, taken from the experimental neutron diffraction studies of Fitch et al.,27 was considered rigid as in our previous studies.17 We have
Adsorption-Desorption of Water in NaY Zeolite
Figure 2. PXRD pattern of as-synthesized NaY at 298 K. The arrows indicate the strongest reflexions of the impurity (Na-P1 zeolite).
simulated a faujasite system with 52 cations per unit cell, Na52Y, corresponding to a Si/Al ratio of 2.69. The force field model was fully described earlier.17 This model enabled the reproduction of both the nonframework cation distributions and the water adsorption thermodynamics in faujasites of various Si/Al ratios. The force field takes into account both electrostatic and repulsion-dispersion interactions. The partial charges of the zeolite atoms (qO ) -0.8252; qT ) 1.3797; qNa ) +1) have been derived from the work of Mortier et al.28 The cation-framework force field is a Buckingham exp-6 type term adapted from the work of Jaramillo et al.29 Water molecules were modeled by the well-known TIP4P model30 and pentane molecules by the AUA model.31 Pentaneframework interaction parameters are derived through LorentzBerthelot combination rules as done for water. All the details of the force field used can be found in ref 17. Results and Discussion Characterization of the NaY Sample. The PXRD pattern of the NaY sample reported in Figure 2 is typical of a zeolite with the FAU topology. However, additional weak reflexions due to a small amount (about 2 wt %) of zeolite Na-P1 (GIS) are also present (reflections at 12.46, 17.66, and 28.10 °(2θ)). The PXRD pattern was indexed with the cubic unit cell parameter (after refinement) in space group Fd3jm (no. 228): a ) 2.4651(2) nm by the Werner indexing routine32 of the STOE WinXPow program package.33 The figure of merits of this indexation was F(30) ) 140.7. According to the chemical analysis, the Si/Al molar ratio of the NaY sample is equal to 2.67 and the unit cell formula for the anhydrous sample is Na52.3[Al52.3Si139.7O384]. From the SEM micrograph (Figure 3), more or less truncated octahedra with a crystal size close to 0.5 µm are observable, and it is rather difficult to distinguish the presence of Na-P1 impurity. This morphology is typical of well-crystallized faujasite having a good microporosity and a small external surface without important defect. The microporous volume given by the Dubinin-Radushkevich model from the N2 adsorption-desorption isotherms (not reported) is equal to 0.331 cm3 · g-1. This value is in good agreement with the values of the literature8,34-39 which lie between 0.320 and 0.340 cm3 · g-1. This confirms that our sample of NaY has a good microporosity. It may be pointed out that the volume probed by nitrogen is higher than the crystallographic
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Figure 3. SEM picture of the NaY sample.
volume of the supercages VR which was calculated from the unit cell parameter, considering that the porosity represents 48% of the cell volume and by taking a free volume of 0.15 nm3 per sodalite cage.40 This is rather surprising because, with a kinetic diameter of 0.36 nm, nitrogen molecules cannot enter the sodalite cages (free aperture of 0.22 nm). No explanation about this anomaly is given in detail in the literature, but this overestimation could be due to the value of the density used to calculate the volume of adsorbed nitrogen as outlined by Rouquerol et al.41 Indeed, studies performed on carbon nanotubes42 and mesoporous silicas43 showed that the density of nitrogen adsorbed in the pores can be quite different from the bulk liquid. Because of confinement effects, the adsorbed nitrogen does not adopt the normal liquid structure. So, it is possible that its density in NaY is higher than that of the liquid nitrogen at 77 K (Fliq ) 0.8081 g · cm-3). The physical and chemical characteristics of the studied NaY sample are summarized in Table 1. Adsorption-Desorption Isotherm of Water Vapor in NaY at Room Temperature. As expected, the adsorption-desorption isotherm of water vapor in NaY displays the type I shape, according to the IUPAC classification44 (Figure 4a). It is reversible in the pressure range 0.1-30 hPa and in good agreement with the earlier published data of Boddenberg et al.18 However, in the very low pressure range (Figure 4b), an unexpected hysteresis loop is observed, which consists of a first branch obtained by adsorption on the anhydrous zeolite and a second one obtained by desorption from the fully hydrated zeolite. Such a phenomenon has already observed in our laboratory in a study of water adsorption in BaY and BaX zeolites,7 but it was not investigated in detail. This kind of low pressure hysteresis was mentioned earlier on other materials such as activated carbons,45-47 laminar silicates,48-50 and polymers.51,52 It is interpreted as the consequence of the presence of ultramicropores with constrictions blocking the evacuation of molecules or the structural modification like a swelling, for example, which is well-known in clay minerals and polymers. However, it is the first time that this low pressure hysteresis is evidenced in the NaY zeolite. Interestingly enough, the desorption process is not complete. Some residual water remains trapped in the porous sample even when pumping under vacuum for several days at room temperature. After such a first adsorption-desorption cycle, the following cycles look reversible and follow the second (desorption) branch observed during the first cycle, with no hysteresis. All the experimental points in the first adsorption
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TABLE 1: Physicochemical Properties of the NaY Zeolitea chemical formula
a (nm)
VR (cm3 · g-1)
Vβ (cm3 · g-1)
VR + Vβ (cm3 · g-1)
VN2 (cm3 · g-1)
Na52.3[Al52.3Si139.7O384]
2.4651(2)
0.284
0.057
0.341
0.331
a
a: unit cell parameter for the hydrated state. VR and Vβ: crystallographic volumes of supercages and sodalite cages, respectively. VN2: microporous volume accessible to liquid nitrogen (FN2 ) 0.8081 g · cm-3 at 77 K).
Figure 5. Linear transformation of the Dubinin-Radushkevich model for adsorption of water on NaY at 298 K: (O) adsorption; (b) desorption and readsorption.
TABLE 2: Characteristic Parameters of the DR Model Used for the Adsorption of Water Vapor on the NaY Zeolitea adsorption desorption
VSa (cm3 · g-1)
D (K-2)
Ea (kJ · mol-1)
0.347 -
7.45 10-7 3.02 10-7
14.6 22.9
a a VS is determined with the DR model assuming the adsorbate as a liquid (FH2O ) 0.99707 g · cm-3 at 298 K).
Figure 4. (a) Adsorption-desorption of water in NaY at 298 K: (O) adsorption; (b) desorption and readsorption. (b) Adsorption-desorption of water on NaY at 298 K in the low pressure range: (O) adsorption; (b) desorption and readsorption.
adsorbate/adsorbent system. It is related to the characteristic energy of adsorption Ea by the relation
Ea ) branch apparently correspond to equilibrium states. In each case, the mass plateau is quickly reached, and the hydration state can be maintained for several days without detectable mass variation. The amounts adsorbed in this low pressure range are a little bit lower than those found by Boddenberg et al.18 This deviation is presumably due to a difference in the technique used (manometry) and sample weight (150 mg previously compacted and then crushed). It may be noted that even at very low pressure a Henry’s law regime is not observed. This could be accounted for by the existence of a heterogeneous adsorption energy landscape. Dubinin-Radushkevich Model. Adsorption-desorption data are analyzed using the empirical Dubinin-Radushkevich (DR) model53,54 which was successfully used to describe the adsorption of water in FAU type zeolites.7,8 In its linear form, the DR equation is expressed by
(
log Va ) log Vsa - D T log
ps p
)
2
(1)
where VSa is the total volume available to the adsorbate, ps is the saturated vapor pressure at temperature T, p is the water vapor pressure, and D is a constant. The adsorbate volume Va is calculated from the mass of adsorbate measured by thermogravimetry, using the density of liquid water at 298 K. The constant D is a parameter dependent on the nature of the
2.3R2 D
(2)
with R the ideal gas constant. The DR curve is plotted with the adsorption and desorption data in Figure 5, and the characteristic parameters of the DR model are given in Table 2. The micropore volume accessible to water (VSa), given by the intercept of the DR plot, is equal to 0.347 cm3 · g-1. This value is in good agreement with the volumes calculated from the crystallographic parameters (Table 1). As expected, water entirely probes supercages and sodalite cages. In the range of high x-values, corresponding to a pressure lower than 0.1 hPa, where the adsorption isotherm exhibits a hysteresis loop, the DR plot presents two straight lines. The former, with the higher slope, corresponds to the first adsorption branch of the isotherm, and the latter to the second adsorption-desorption branch. It may be noticed that the characteristic energy is very low for the first adsorption compared to the second one (Table 2). This means that the first water molecules adsorbed experience a weaker interaction with the zeolite framework than the last water molecules desorbed. Nevertheless, this interaction remains largely higher than those observed on a hydrophobic material having no strong specific adsorption sites such as activated carbons55 or dealuminated faujasites,8 for which the energy is around 2-6 kJ · mol-1. So, during adsorption and desorption, we can consider that the water molecules are in interaction with sodium cations. But, as their characteristic energies are quite different in each case, they
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Figure 6. Vacuum equilibrium thermodesorption profiles of water in NaYD18 (O) and NaYA18 (b).
probably interact with cations located on different sites. For lower x-values, corresponding to a pressure higher than 0.1 hPa, where the adsorption-desorption isotherm is reversible, the DR plot presents only one straight line. The characteristic energy is then equal to 14.6 kJ · mol-1. This value is in very good agreement with those reported in other works on NaY8 and BaY.7 This adsorption range would correspond to the complete filling of cavities under the effect of weaker interactions than at low filling (noncationic interactions). Therefore, at low filling, the adsorption energy of water appears lower than the desorption one (Table 2). This result demonstrates that for the same (low) loading, it is possible to differentiate two kinds of water which interact differently with the zeolitic walls, depending on the introduction mode of the water in the porous volume. Vacuum Equilibrium Thermodesorption. In order to characterize these two types of water differently bonded to the zeolite, vacuum equilibrium thermodesorption experiments have been performed on two NaY samples displaying the same amount of adsorbed water (18 molec · uc-1) but hydrated at 298 K following different procedures. The sample NaYA18 is hydrated through adsorption from the anhydrous NaY, while the sample NaYD18 is obtained by desorption from the fully hydrated NaY. These hydration states are reached by precisely controlling the vapor pressure. The thermodesorption curves are shown in Figure 6. For the NaYA18 sample, more than half of the confined water (10 molec · uc-1) is evacuated at 298 K by simple pumping under vacuum. This departure corresponds to water that is only slightly physisorbed in the zeolite. The other eight molecules are progressively removed by heating up to 673 K. On the contrary, for the NaYD18 sample, no water is eliminated at 298 K even after pumping under vacuum for several days. Water is desorbed only by heating up to 673 K. Water obviously experiences a stronger interaction with zeolitic framework in the NaYD18 case than in the NaYA18 one. These thermodesorption experiments confirm what we have already deduced from the DR analysis. In NaYA18 and NaYD18, water is differently bound to the zeolite. Distribution of Water Molecules in the Microporosity. Thermodesorption results suggest that for NaYA18 water would be adsorbed in supercages, whereas for NaYD18 it would be adsorbed in sodalite cages. In order to locate the water molecules in these two hydrated samples, adsorption of n-pentane has been carried out at 298 K. This molecule has been chosen because it only adsorbs in the supercages for steric reasons, through the so-called “nonspecific” interaction in contrast with water. Indeed, the adsorption affinity of NaY for this hydrocarbon is lower than for water, so that we may consider that n-pentane is not able to displace water (this is confirmed by molecular
Figure 7. (a) Adsorption isotherm of n-pentane in (O) NaY, (0) NaYD18, and (9) NaYA18 at 298 K. (b) Adsorption isotherm of n-pentane in (O) NaY, (0) NaYD18, and (9) NaYA18 at 298 K in the Henry law region. Dotted lines are only guides for eyes.
TABLE 3: Henry Constants and Adsorption Capacities of Anhydrous and Partially Hydrated NaY Zeolites for n-Pentanea -1
-1
KH (molec · uc · hPa ) VSa (cm3 · g-1)
NaY
NaYA18
NaYD18
103 0.300
51 0.284
103 0.300
a a VS is determined with the DR model assuming the adsorbate as a liquid (FC5H12 ) 0.62482 g · cm-3 at 298 K).
simulations). Moreover, G. Weber et al.56 have shown that linear paraffins like n-hexane, for example, are very good molecular probes for determining the supercage volume of FAU zeolites with the DR model. Adsorption isotherms of n-pentane in anhydrous NaY and partially hydrated NaYA18 and NaYD18 are shown on Figure 7a and b. In the low pressure range (p < 0.2 hPa) corresponding to the Henry law region, the adsorption isotherms of n-pentane on anhydrous NaY and NaYD18 are superimposed. The Henry constants are identical (Table 3). The presence of water in the microporosity of NaYD18 does not affect the adsorption affinity for n-pentane. On the contrary, the Henry constant on NaYA18 is divided by a factor of 2. This decrease in the adsorption affinity for n-pentane can be explained by the presence of water molecules inside the supercages which weaken the interactions between the hydrocarbon and the zeolitic framework. In the higher pressure range (p > 0.2 hPa), adsorption isotherms of NaY and NaYD18 are again superimposed. On the other hand, the adsorption isotherm of NaYA18 lies slightly below the two others. The presence of water in the NaYA18 seems to decrease the adsorption capacity for n-pentane. The deviation is small but significant and reproducible. The maximal volumes available to the n-pentane molecules in anhydrous and partially hydrated NaY, NaYA18, and NaYD18 zeolites have been determined by using the DR model. Results
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Figure 8. GCMC simulations of adsorption isotherms of water in NaY at 298 K: (O) experiments (equilibrium adsorption); (0) simulations with mobile cations; (9) water adsorbed in sodalite cages.
are collected in Table 3. For NaY, the maximal volume of n-pentane adsorbed is equal to 0.300 cm3 · g-1. While slightly higher, this value is of the same order of magnitude as the crystallographic volume of supercages. The slight deviation observed between these two values could be due, as for nitrogen, to the fact that n-pentane adsorbed in supercages does not behave exactly as the normal liquid. Therefore, we can consider that the n-pentane molecules entirely probe the volume of supercages. The accessible volumes to n-pentane in NaY and NaYD18 are the same. This means that the 18 water molecules per unit cell present in the microporosity are rather located in the sodalite cages. The latter molecules are those that require heating up to 673 K to be eliminated. For the NaYA18 zeolite, the volume accessible to n-pentane is lower than for the anhydrous zeolite. The difference between the two volumes is equal to 0.016 cm3 · g-1 and corresponds to 11 water molecules per unit cell. The major part of these water molecules is then located in the supercages. Among the 18 water molecules per unit cell, 11 are positioned in the supercages and 7 in the sodalite cages. The adsorbed molecules in the supercages are those evacuated at room temperature by simply pumping under vacuum in the thermodesorption experiments, while the molecules adsorbed in the sodalite cages are those eliminated by heating up to 673 K. GCMC Simulations. The adsorption isotherm of water vapor in Na52Y zeolite computed by using bias grand canonical Monte Carlo simulations has already been published in ref 17. We recall it in Figure 8 in addition to the adsorption isotherm of water in sodalite cages. The simulations are performed by considering a rigid zeolite framework and mobile Na+ cations. The simulation starts with the known cation distribution at equilibrium of the anhydrous zeolite:17 12 cations in site SI, 8 cations in site SI′, and 32 cations in site SII (12SI 8SI′ 32SII). The agreement between simulation and experimental data is reasonably good. The simulations tend to underestimate the adsorbed amounts of water for pressure ranging from 0.01 to 1 hPa, while the full loading quantity is slightly overestimated. This slight deviation between experiments and simulations is attributed to the heterogeneity of the adsorption process. The adsorbed amount measured by thermogravimetry represents the statistical average distribution of water molecules among all the cavities in zeolite particles. In fact, this distribution is less homogeneous, and the redistribution from one site to the other one occurs more gradually than in molecular simulations. The low pressure region is very well reproduced. Above 0.001 hPa, the amount of water adsorbed in the total porous volume becomes higher than the amount adsorbed in sodalite cages. This means that at least from
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Figure 9. Simulated cation distribution in NaY as a function of the water content at 298 K.
Figure 10. GCMC simulations of adsorption isotherms of water in NaY at 298 K: (0) mobile cations; (9) fixed cations in (4SI 8SI′ 32SII) initial distribution; (O) adsorption experiments; (b) desorption experiments.
this pressure, water is simultaneously adsorbed in sodalite cages and supercages. The sodalite cages complete loading of ∼32 molec · uc-1 corresponds to 4 water molecules per sodalite cage. This result is in line with what was experimentally observed.18,20 We report in Figure 9 the cation distribution as the function of water loading. Up to 90 molec · uc-1, the cation distribution does not change. Above this loading, we observe an important redistribution of Na+ cations from SI to SI′, while no migration is observed for the SII cations. This result agrees with the X-ray diffraction experiments of the literature.3 At full loading, the cation distribution is (4SI 16SI′ 32SII). It may be noticed that this cation redistribution takes place in the loading range where the hysteresis loop is observed in the adsorption isotherm. The hysteresis phenomenon seems then to be linked with the migration of cations from the D6R prisms to the sodalite cages. It is tempting to suggest that adsorption of water takes place in NaY with the initial (12SI 8SI′ 32SII) cation distribution while desorption takes place in a NaY with the (4SI 16SI′ 32SII) cation distribution of the fully hydrated structure. Indeed, water molecules coordinated to site I′ cations inside the sodalite cages have a very strong adsorption energy,11 and one can expect that the cations in site I′ trap the water molecules during the desorption process. To check this hypothesis, a GCMC simulation has been performed in NaY with fixed cations having the distribution (4SI, 16SI′, 32SII). Thus we simulated the desorption process at very low pressure (10-2-10-3 hPa), from a zeolite having 16 cations in sodalite cages interacting with water. The desorption isotherm is shown in Figure 10 and is compared with
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Figure 11. Snapshot of the framework of partially hydrated NaY in a plane perpendicular to the [111] direction: (a) NaYA18 with the (12SI 8SI′ 32SII) cation distribution; (b) NaYD18 with the (4SI 16SI′ 32SII) cation distribution. [Cation in site I, green; cation in site I′, blue; cation in site II, cyan; oxygen atom, red; hydrogen atom, gray.]
Figure 12. Simulated amount of water adsorbed in NaY sodalite cages at 298 K: (0) mobile cations; (9) cations fixed in the (4SI 16SI′ 32SII) distribution.
the adsorption isotherm in NaY with the initial distribution (12SI 8SI′ 32SII), and also with the experimental data. The agreement between simulations and experiment is rather good. The adsorption-desorption hysteresis loop is well reproduced. The adsorption branch corresponds to the most stable thermodynamic equilibrium states and the desorption branch to a metastable regime. We show in Figure 11 a snapshot of the supercage in a plane perpendicular to the [111] direction for the partially hydrated NaYA18 with the (12SI 8SI′ 32SII) cation distribution and NaYD18 with the (4SI 16SI′ 32SII) cation distribution. As we can see, the occupancy of the supercage by water molecules is higher in NaYA18 than in NaYD18. Molecular simulations show clearly that the hysteresis loop is associated with the migration of cations from sites SI to sites SI′ during adsorption and to the blocking of cations in sites SI′ during desorption. We report in Figure 12 the amount of water adsorbed in sodalite cages computed from simulations performed in NaY with mobile and fixed cations. More water molecules are adsorbed in sodalites cages when the nonframework cations are fixed with 16 cations in sites SI′, even at very low pressure. This result confirms what we deduced above from the experimental data. During the desorption process, while the water molecules adsorbed in the supercages can be easily evacuated
under vacuum, those in interaction with cations located on sites SI′ are trapped in the sodalite cages and require much more energy to be evacuated. We can wonder whether the classical FAU zeolite Na56Y would behave in a similar way at low pressure. Indeed, as GCMC simulations17,57 show that no SI to SI′ redistribution can occur in this zeolite because exactly half of SI′ and half of SI sites are occupied by sodium cations in the anhydrous state, we could thus expect to observe no hysteresis loop on its adsorption isotherm. We have not checked this experimentally, but when we compare our data with those of Boddenberg et al.,18 performed on Na56Y, we find that our second adsorptiondesorption branch is rather close to their adsorption isotherm. In this case, both zeolites have the same cation distribution on sites SI′ and SII: (4SI 16SI′ 32SII) and (8SI 16SI′ 32SII) for Na52Y and Na56Y respectively. So, we can imagine that there will probably be no hysteresis loop with Na56Y. However, this must be taken with caution as long as it has not been experimentally proved. In order to confirm that the presence of water molecules in the supercages influences the adsorption capacity of n-pentane, molecular simulations have been performed on two samples of partially hydrated NaY, as in the thermodesorption experiments: NaYD18 (18 molec · uc-1 in sodalite cages) and NaYA18 (7 molec · uc-1 in sodalite cages and 11 molec · uc-1 in supercages). The adsorption capacities of n-pentane under the pressure of 30 hPa are 45 molec · uc-1 for NaYA18 and 48 molec · uc-1 for NaYD18. These values are higher than those measured by thermogravimetry. Despite this, the difference between these two adsorption capacities corresponds to a decrease in volume equal to 0.027 cm3 · g-1. This value is of the same order of magnitude as the one experimentally measured. This confirms that the presence of water molecules in the supercages can be evidenced by the decrease in the adsorption capacity of n-pentane. Conclusion In this work, we report the existence of a hysteresis loop in the water adsorption-desorption isotherm in NaY at 298 K in the very low pressure range. Vacuum equilibrium thermodesorption and n-pentane adsorption experiments combined with
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grand canonical Monte Carlo simulations show that this hysteresis phenomenon is consecutive to the trapping of water molecules in sodalite cages and associated with a migration of nonframework cations from D6R prisms to sodalite cages during water adsorption. We propose that the adsorption-desorption of water in NaY at 298 K occurs as follows. (i) At first, water molecules are adsorbed on the most accessible cations located on sites II in the supercages. The solvation of these cations makes the sodalite cages accessible to water. (ii) Then, the filling of sodalite cages occurs with a migration of nonframework cations from D6R prisms (site SI) to sodalite cages (site SI′). These steps could correspond to the pressure range in which is observed the hysteresis loop. The adsorption of water continues in the supercages through an adsorption process in the porous volume which is well described by the empirical DR model. (iii) Starting from a full loading, the desorption occurs by evacuation of water from supercages first. Then half of the four water molecules adsorbed in sodalite cages are eliminated. Two water molecules still remain trapped in these cavities even after pumping under vacuum. (iiii) Finally, these last molecules which are in strong interaction with two cations located on sites SI′ are evacuated by heating up to 673 K under dynamic vacuum according to a thermally activated desorption process. Thus, the distribution of the residual water molecules in the microporosity of NaY zeolite depends on the hydration procedure used. When it is introduced by adsorption in the anhydrous material, it is essentially located in the supercages, while it remains trapped in the sodalite cages when introduced by desorption from the saturated material. This provides a way to prepare zeolite samples with different nonframework cations and water distributions and the same amount of residual water. These two samples would presumably behave differently with regard to selective adsorption of hydrocarbon mixtures, a feature that might be of interest for separation technologies. These results are consistent with the conclusions drawn from our studies performed on the competitive adsorption of p-xylene and m-xylene on partially hydrated BaY and BaX zeolites.5,6,14 We showed in these works that the presence of a few percent of water introduced by adsorption in the anhydrous zeolites increased significantly the p-xylene/m-xylene selectivity at high filling. At this time we already suggested that the migration of barium cations consecutive to the hydration of the zeolite played an important role in the adsorption process. In this case, water was localized in the supercages and created a decrease of the adsorption capacity by steric hindrance and an increase of the p-xylene/m-xylene selectivity by an entropic effect. The presence of water molecules in the supercages added to a migration of barium cations reduced the free space available to xylene isomers. This was in favor of the adsorption of p-xylene which is more symmetric and smaller than m-xylene. Thus, we can expect to have a different behavior if water is introduced by desorption from the saturated barium-exchanged zeolites. Indeed, water being adsorbed in sodalite cages should not influence the selective adsorption of xylenes. Such coadsorption experiments are in progress. We also intend to perform GCMC simulations using a flexible framework in order to study the effect of cell parameter fluctuations on the observed hysteresis phenomenon. Acknowledgment. This work was supported by the French “Agence Nationale de la Recherche”, under contract no. BLAN06-3_144027. References and Notes (1) Pichon, C.; Palancher, H.; Hodeau, J. L.; Be´rar, J. F. Oil Gas Sci. Technol.sReV. IFP 2005, 60, 831.
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