Structural Changes in Nanoporous MFI Zeolites Induced by

ARTICLE pubs.acs.org/JPCC

Structural Changes in Nanoporous MFI Zeolites Induced by Tetrachloroethene Adsorption: A Joint Experimental and Simulation Study Marie Jeffroy,†,^ Guy Weber,‡ Sarah Hostachy,† Jean-Pierre Bellat,‡ Alain H. Fuchs,§ and Anne Boutin*,† †

Ecole Normale Superieure, CNRS and UPMC, 24 rue Lhomond, 75231 Paris, Cedex 05, France Laboratoire Interdisciplinaire Carnot de Bourgogne, CNRS-Universite de Bourgogne, 9 Avenue Alain Savary, B.P. 47 870, 21078 Dijon Cedex, France § Chimie ParisTech and CNRS, 11 rue Pierre et Marie Curie, 75005 Paris, France ^ Laboratoire de chimie physique, CNRS and Universite Paris-Sud 11, 91405 Orsay Cedex, France ‡

ABSTRACT: A joint experimental and molecular simulation study was performed to investigate the adsorption of tetrachloroethene on two MFI zeolites, ZSM-5 with a Si/Al ratio of 26.5, and silicalite-1, which is the pure silica form of ZSM-5. Adsorption isotherms and isosteric heats of adsorption of tetrachloroethene were measured and compared to molecular simulation results. Experimental curves for both MFI zeolites show a step at loading of 4 molecules 3 uc-1 that was interpreted in terms of a structural change of the host framework. A thermodynamic analysis based on the osmotic ensemble scheme allowed attributing this step to a symmetry change from ORTHO (orthorhombic form with Pnma symmetry) to PARA (orthorhombic form with P212121 symmetry) of both adsorbents upon micropore filling. The adsorption mechanism was also characterized. The first tetrachloroethene molecules adsorbs in the channel intersections of the MONO (monoclinic form with P21/n11 symmetry) and ORTHO structures of silicalite-1 and ZSM-5, respectively. Then, the above-mentioned structural change from ORTHO to PARA occurs, leading to a filling of sinusoidal channels until saturation was reached, i.e. ∼8 molecules.uc-1 for silicalite-1 and 6 molecules 3 uc-1 for ZSM-5. In addition, simulation results indicate that the sodium cations in the ZSM-5 zeolite are located at the channel intersections during the whole adsorption process.

’ INTRODUCTION Nanoporous materials are widely used in many industrial processes, such as ion exchange, molecular sieving and catalysis. Open framework inorganic materials such as zeolites and more recent nanoporous metal-organic frameworks (MOF) are gaining increasing importance in such applications.1,2 Adsorption properties of these materials play a crucial role in those processes. Molecular simulations can help in understanding adsorption, diffusion in zeolitic materials,3,4 and can also contribute to the design of new materials.5 Thanks to the advances in computer technology and the development of simulation techniques, it is now possible to study systems that were computationally unreachable to simulation before. Fifteen years ago, Monte Carlo simulations were used to study the adsorption of noble gases or small alkanes in pure siliceous zeolites. Now, more complex adsorbent/adsorbate systems such as large chain alkanes,6 aromatics7 or halocarbon8 molecules or water,9,10 in a diversity of cationic zeolites or other open framework materials11 can be simulated. In most cases, a rigid framework is assumed for the zeolitic adsorbent. Indeed, the framework flexibility generally plays a role in r 2011 American Chemical Society

transport properties,12 but not so much on thermodynamic properties. An extensive study about the sensitivity of the adsorption isotherms to the forcefield parameters of the framework shows that their influence is small in the case of alkane chains in silicalite-1.13 A few examples of “stepped” adsorption isotherms were reported, instead of the usual continuous Langmuir (or type I) isotherms. In MFI zeolites, this is the case for various adsorbates. For long alkanes, the stepped behavior of the adsorption isotherm has been attributed to entropic or packing effect in the microporous network.14,15 For other adsorbates such as argon, krypton, dioxygen, dinitrogen or carbon monoxide,16,17 but also for larger ones like benzene, toluene, ethylbenzene, p-xylene, and bromobenzene,18-30 the origin of these stepped isotherms was largely debated in the literature. One of the explanations is a structural transition of the host framework taking place upon fluid adsorption. This framework-adsorbate coupling is relatively weak in zeolite materials. However, it has been shown that the existence of floppy modes in zeolitic framework can give rise to local deformation without significant energy cost.31 In other porous Received: October 1, 2010 Revised: January 27, 2011 Published: February 22, 2011 3854

dx.doi.org/10.1021/jp109447n | J. Phys. Chem. C 2011, 115, 3854–3865

The Journal of Physical Chemistry C

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Table 1. Some Characteristics of the MFI Zeolite Samples

a

sample

shape

crystal size (μm)

Si/Al ratio

crystallographic structure at room conditions

Silicalite-1

coffin

60 25 15

¥

monoclinica

Na-ZSM-5

sphere

0.2

26.5

orthorhombic

After calcination at 873 K in air.

materials such as “soft” hybrid adsorbents like MOF’s, it is likely that this coupling will become more important.32-36 We report here a joint experimental and simulation study of the adsorption of tetrachloroethene in two MFI-type zeolites: (i) silicalite-1, which is the pure silica form of MFI, and (ii) NaZSM5 zeolite with a Si/Al ratio of 26.5. Three different crystalline structures have been observed for silicalite-1. At room temperature the empty zeolite is known to be monoclinic (MONO).37,38 At higher temperatures or at intermediate loadings an orthorhombic (ORTHO) structure is observed.38-42 The PARA structure, also orthorhombic, was only characterized at high loadings.19,43 In the case of the adsorption of aromatic compounds such as benzene, o-xylene and p-xylene, on silicalite-1, the ORTHO-PARA structural change of the silicalite-1 framework is thought to be at the origin of the step in the adsorption isotherms.18,21 For MFI zeolites of Si/Al ratio lower than 75-80, the crystalline structure of the bare zeolite is not monoclinic but orthorhombic (ORTHO) at room temperature25,44 and is able to undergo an ORTHO to PARA change with increasing loading.25 In the past decade, a stepped adsorption isotherm was reported in the case of the adsorption of tetrachloroethene45-48 in MFI-type zeolites. Structural changes of the host framework were characterized but their influence on the shape of the adsorption isotherm was still unclear. Some considerations based on (i) a structural transition of the adsorbent, (ii) a phase transition of the adsorbate and (iii) a cooperative reorganization of the adsorbed molecules within the zeolite porosity, were made to account for the existence of the step. Therefore, neutron diffraction,45 calorimetric,45 NMR49,50 and infrared51 experiments were conducted to investigate the adsorption process. Recently, simulation studies allowed to get insights in the experimental findings. The ORTHO-PARA structural transition of silicalite-1 appeared to be responsible of the step in the isotherm.52 Both MONO-ORTHO and ORTHO-PARA structural transitions were predicted by thermodynamic potential calculations. Such transitions have also been predicted in the case of p-xylene adsorption in MFI zeolite30 by fitting the experimental adsorption isotherms using a thermodynamic model that contains some of the basic ideas that we fully developed in the present osmotic model. The first part of the article reports a detailed study of the adsorption process of tetrachloroethene in silicalite-1 by molecular simulation. The influence of the crystalline structural differences was investigated. A thermodynamic analysis of the frameworkadsorbate coupling, based on the so-called “osmotic” statistical ensemble was then conducted. The adsorption mechanism is also described. To get further understanding on the adsorption process of tetrachloroethene in MFI-type zeolites and on the host-adsorbate coupling phenomenon, additional experiments and simulations were performed on ZSM-5 zeolite with a Si/Al ratio of 26.5. The influence of extraframework cations on the adsorption phenomenon was also investigated.

’ EXPERIMENTAL METHOD

Figure 1. Schematic representation of the three-dimensional pore system of MFI zeolites consisting of intersecting straight and sinusoidal channels.

was synthesized by the “Laboratoire de Materiaux a Porosite Contrôlee” (UMR CNRS 7016) in Mulhouse, France, from a gel containing a source of silica, an aqueous solution of tetrapropylammonium bromide and methylamine. Prior to adsorption measurements the sample was calcined at 873 K under air to remove the template molecules and open the channels. The template-free Na-ZSM-5 sample was provided by Chemie Uetikon. For sake of confidentiality, the company did not provide information on the sample preparation conditions. X-ray diffraction (XRD) patterns performed at room conditions revealed that the samples were highly crystalline and that template-free silicalite-1 and Na-ZSM-5 exhibited monoclinic and orthorhombic crystallographic structures, respectively. In addition, scanning electron microscope micrographs showed that both zeolites contained crystals of uniform size. Their microporous network consists of straight channels (SC) in the [010] direction and sinusoidal channels (ZC) in the [100] direction. These channels cross each other at intersections (I) (Figure 1). Tetrachloroethene supplied by Prolabo (R. P. Product) was of purity greater than 99% and contain a water amount of around 0.01%. Before use, the chemical was degassed under high vacuum then stored in an evacuated vessel containing a 3A hydrophilic zeolite to trap any residual water. Methods. A gravimetric balance (McBain) was used to measure the adsorption-desorption isotherms of tetrachloroethene in ∼20 mg of template-free zeolite, which was degassed under a vacuum of 10-5 hPa at 673 K for 12 h prior to the sorption measurements at 298 K. This device is well suited to impose vapor pressures controlled by a “cold point”.53 The sorption isotherms were measured by increasing (for adsorption) or decreasing (for desorption) the equilibrium pressure in small increments ranging from 10-5 to 24.7 hPa, the saturated vapor pressure of tetrachloroethene at 298 K. Additional sorption isotherms48 were also investigated at 318, 356, and 389 K for silicalite-1 in order to determine isosteric heats of adsorption. These quantities were calculated from a set of isotherms by drawing the corresponding isosteres at various loadings Nads

Materials. Two MFI zeolites were used in this study. Some characteristics of these samples are reported in Table 1. Silicalite-1

lnðp=p0 Þ ¼ ½f ð1=TÞNads 3855

dx.doi.org/10.1021/jp109447n |J. Phys. Chem. C 2011, 115, 3854–3865


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Table 2. Lattice Parameters and Pore Dimensions of MFI Zeolites for Different Crystal Symmetries crystal symmetry a (nm)

MONO37 2.0107

ORTHO54 2.0022

PARA43 2.0102

b (nm)

1.9879

1.9899

1.9797

c (nm)

1.3369

1.3363

1.3436

R, β, γ (deg) space group

90.67, 90, 90 P21/n11

90, 90, 90 Pnma

90, 90, 90 P212 121

0.583-0.606a

0.575b

0.606-0.618a

0.518-0.527

a

b

0.589-0.578

a

0.501-0.535

a

pore opening (nm) straight channel

maximal size (nm) minimum size (nm)

sinusoidal channel

maximal size (nm) minimum size (nm)

a

0.522

0.507-0.480a a

0.637-0.615a

a

0.476-0.458a

0.555-0.560 0.528-0.529

b

Elliptical pore opening. Circular pore opening.

The isosteric heat at constant loading Nads is therefore: Qsta ¼ - RT 2 ½D½lnðp=p0 Þ=DTNads where p0 is the standard pressure. A differential heat flow Setaram C80 calorimeter coupled to a conventional manometric apparatus was used to measure adsorption heats of tetrachloroethene on the ZSM-5 zeolite. The amounts of tetrachloroethene adsorbed at increasing constant pressure and at constant volume, were calculated from a mass balance of the gas phase before and after each adsorption measurement, considering tetrachloroethene was an ideal gas. In regards to the operating conditions, measured adsorption heats were similar to the differential molar adsorption enthalpies and isosteric heats. Experiments were performed at 298 K in a pressure range from 10-4 to 24.7 hPa, with ∼500 mg of ZSM-5 zeolite activated under vacuum at 673 K for a night.

’ SIMULATION METHOD Silicalite-1 and ZSM-5 Models. Adsorption isotherms of tetrachloroethene on MFI zeolites were calculated for silicalite1 in all three structures (MONO,37 ORTHO,54 and PARA43) and for ZSM-5(26.5) in the two known structures (ORTHO and PARA). They were assumed to be rigid and their characteristics were those obtained by Van Koningsveld et al.,37,43,54 from experimental X-ray diffraction data. The unit cell and the porous volume of the structures are close to each other (Table 2). In order to check whether two experimental structures of the same symmetry group have the same adsorption properties, we calculated the corresponding adsorption isotherms. Therefore, in addition to these experimental data we considered those obtained by Olson et al.39 for the ORTHO structure and those determined by Van Koningsveld et al.19 for the PARA structure. Comparison between the two adsorption isotherms in the two experimental ORTHO structures showed only slight differences, compared to the effect of different structural phases. The same similarity is observed between the two adsorption isotherms obtained in the two PARA structures. The adsorption phenomenon described in the next section is thus not dependent on the small structure variation obtained experimentally between two structures of the same phase. The unit cell formula for silicalite-1 is Si96O192. The one for ZSM-5 is taken as Na4Si92Al4O192 for calculations, i.e., a Si/Al ratio of 23, closed to the experimental value of 26.5. We simulated a box of eight unit cells with periodic boundary

conditions for the calculation of the isotherms by GCMC simulation in the different structures. Forcefield. All the intermolecular interactions (adsorbate/ adsorbate and adsorbent/adsorbate) were modeled by a sum of electrostatic and dispersion-repulsion terms. The atomic charges of the two zeolitic materials were issued from previous studies.9,55 Silicalite-1 is the pure silica zeolite. Electrostatic charges were set to 1.4|e| for silicon atoms and -0.7|e| for oxygen atoms. These values have already been used successfully to describe the adsorption of water55,56 in silicalite-1. The ZSM-5(26.5) zeolite is composed of silicon, aluminum, oxygen and sodium atoms. We chose not to describe the electrostatic atomic charge difference between silicon and aluminum in the zeolitic framework. An average tetrahedrally bonded (T) atom was used for reasons detailed in an earlier paper.26 Atomic charges are issued from the works of Mortier57 and linearly extrapolated to a Si/Al ratio of 23. Atomic charges were set to 1.5593|e| for T atoms and to 0.8005|e| for oxygen atoms. Sodium cations bear a charge of þ1| e|. Atomic charges of the tetrachloroethene molecules are taken from “ab-initio” calculations.52 All the parameters can be found in Table 3 and Figure 2. Ewald summation technique has been used to compute the electrostatic energy. Dispersion-repulsion parameters between adsorbates, cations and the zeolitic framework were computed using the Kiselev approximation.3 Sodium-framework interaction parameters were taken from a previous work.9 Those parameters have been successfully used to describe water adsorption and cation localization in faujasite zeolite. More details can be found in ref 9. Interaction parameters between the adsorbate and the T atoms (Si or Al) were optimized to reproduce adsorption properties of tetrachloroethene in silicalite-1.52 The same parameters were used for silicalite-1 and ZSM-5. It must be stressed that the forcefield used for ZSM-5 is solely based on a combination of (i) a forcefield developed to study extraframework sodium cation location in faujasite and (ii) a forcefield adjusted to reproduce adsorption data for tetrachloroethene in silicalite-1. Adsorbatecation interaction parameters were obtained using LorentzBerthelot mixing rules, without readjustment. All interaction parameters are specified in Table 3. Unspecified parameters can be obtained with Lorentz-Berthelot mixing rules. Simulation Details. Adsorption isotherms and adsorption heats were computed using bias Monte Carlo simulations to compute the average number of adsorbed molecules for several values of the chemical potential of the fictitious vapor reservoir at 300 K. The ideal gas law was used to link the reservoir gas phase chemical potential to the vapor pressure of the gas in the reservoir. To ensure convergence, preinsertion58 and 3856



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Table 3. Forcefield Parameters Used for Modeling Repulsion and Dispersion Interactions with Lennard-Jones and Buckingham Potentials Lennard-Jones potential: -ε[((σ6)/(r6)) - ((σ12)/(r12))]

σ (nm)

ε (K)

Cl-Cl

0.350

193.8

C-C Cl-Ohost

0.335 0.3275

64.0 107.8

C-Ohost

0.3175

64.0

Na-Na

0.2584

50.27

Buckingham potential: R exp(-βr) - (γ)/(r )

R (K)

β (nm)

γ (K 3 A6)

Na-O

61.1 106

0.405

76.52 104

6

Taking into account that Ω(P = 0) = 0, this leads to the simplified expression: Z P N ads i ðPÞV m i ðPÞ dP ð5Þ ΩGC i ¼ 0

where Vm is the molar volume of the pure fluid. The host free enthalpy can be rewritten as a function of the free energy of the adsorbent at zero pressure: Figure 2. Atomic charges (in electron units) borne by the atoms of the tetrachloroethene molecule in Monte Carlo simulations.

rotational7,59 biases were used. The calculation of the isotherms in each rigid structure of silicalite-1 was done in the grandcanonical ensemble. Each GCMC run lasted for 150 million steps. GCMC simulations were run using the GIBBS Code.60 Grand Potential Calculations. We showed in previous papers34,35,52 that the most appropriate ensemble to describe the adsorption process in a flexible porous material is the osmotic ensemble (Nhost, μads, σ, T) where Nhost is the number of molecules of the host framework, μads the chemical potential of the adsorbate, σ a mechanical constraint applied on the system, and T the temperature. In our case, the mechanical constrain is the pressure of the gas phase P. This osmotic ensemble can be viewed as an extension of the grand canonical ensemble that allows volume variation of the system. The osmotic thermodynamic potential Ωos can be written as: Ωos ¼ U - TS - μads N ads þ PV

ð1Þ

In the case of a limiting number of existing phases, we can used a subensemble of the osmotic ensemble where the number of degrees of freedom of the host material is limited to a set of defined structures.29,34,52 The osmotic potential Ωosi for a specific phase i of the host material is: Ωos i ¼ Ghost i þ ΩGC i

ð2Þ

where ΩGCi is the grand canonical potential of the fluid adsorbed in the phase i and Ghosti the free enthalpy of the host material in the phase i. The grand canonical potential can be extracted directly from the experimental or calculated adsorption isotherms:61,62 ðDΩGC i =DμÞV , T ¼ - N ads i The grand potential can thus be rewritten as: Z P N ads i ðPÞðDμ=DPÞT , N dP þ Ωi ðP ¼ 0Þ ΩGC i ¼ 0

Ghost i ¼ Ghost i ðP ¼ 0Þ þ PV i ¼ F host i þ PV i ð6Þ Finally, this leads to a complete expression of the osmotic potential which involves only three key parameters: the free energy of the solid phase, the adsorption isotherm of fluid inside phase i (Nadsi(T, P)) and the molar volume of the pure fluid as a function of pressure: Z P Ωos i ðT, PÞ ¼ F host i ðTÞ þ PV i N ads i ðPÞV m ðPÞ dP ð7Þ 0

The method proposed here consists of calculating the fluid adsorption isotherms in the different rigid host structures through molecular simulation in the Grand Canonical ensemble, and to extract the corresponding osmotic grand potential, using eq 7. The knowledge of Ωos of the different guest/host structures in the same osmotic ensemble allows the prediction of the relative stability of each structure at every pressure. The main difficulty of this approach is the experimental measurement or calculation by simulation of the free energy differences between the different crystalline structures of the adsorbent. Calorimetric measurement allows to access to relative enthalpy between two structures. To our knowledge, two different calorimetric studies measured the relative stability of MONO and ORTHO phases for MFI zeolites with different Si/ Al ratios.40,63 These enthalpies vary from 1.8 to 18.3 kJ 3 mol-l, depending on the Si/Al ratio. However, no experimental value is known for the relative entropy of the different structures. Starting from the experimental values of the relative enthalpies and the transition temperature between the MONO and ORTHO phases one can obtain an estimation of the relative entropy of the system. At the structural transition temperature Ttrans, the equilibrium is reached between the MONO and ORTHO phases. Thus, we can write the relation:

ð3Þ

ð4Þ

H MONO - T trans SMONO ¼ H ORTHO - T trans SORTHO

ð8Þ

Mentzen et al.63 proposed the values of 19 kJ 3 mol-l for ΔH (HORTHO - HMONO) and 370 K for the transition temperature. Using these values, ΔS = SORTHO - SMONO is estimated to be around 50 J 3 K-1 3 mol-1. We assume that in the temperature 3857


The Journal of Physical Chemistry C range 300-370 K the relative entropy and enthalpy are constant for MFI zeolites. This assumption is a good approximation as long as the degrees of freedom of the system are the same in the whole temperature range. The free enthalpy difference (ΔG = ΔH - TΔS) at room temperature is thus estimated to be around 3.7 kJ 3 mol-l. Even if this value was obtained with large approximations, this allows us to evaluate the order of magnitude of the relative free energy of the different structures. No value of the relative enthalpy or the temperature of transition exists for the ORTHO-PARA transition. We assumed here that this free energy difference is the same that the one estimated for the MONO-ORTHO transition. It is worth mentioning that for both structural transitions, we have tested different values of the relative free energy ranging from 2 to 15 kJ 3 mol-l and the same behavior was predicted. Only the predicted transition pressures are sensitive to this value. For the whole range of tested values we predict two transitions of the adsorbent: the first one from a MONO to an ORTHO structure occurs between 4.45 and 127 Pa (depending on the free energy difference) and the second one from an ORTHO phase to a PARA one between 16 and 370 Pa. To our knowledge, no similar value exists for the ORTHOPARA transition of ZSM5. We choose to take zero for the free energy difference between the two structures in our calculation. As in the case of silicalite-1, the free energy difference at zero pressure value only slightly influences our results. This thermodynamic method is powerful to predict structural transition in a porous material. Only the adsorption isotherms in the different structures are needed as an input. With the help of simulation, it can be applied to any (host-guest) system in which the guest has a finite number of structures during the adsorption process. Knowing the adsorption isotherms in every crystalline structure of the adsorbent, we can predict the structural transition pressures. Coudert et al. proposed to extract thermodynamic information on adsorption phenomenon by fitting the experimental adsorption isotherms with the Langmuir model.34 However, in the case of tetrachloroethene in silicalite-1, this model does not fit well the experimental isotherm (see below). Information about the hypothetic adsorption isotherms in each rigid structure of MFI can only be calculated by GCMC simulation.

’ RESULTS AND DISCUSSION Silicalite-1. Adsorption Isotherms. The adsorption isotherms of tetrachloroethene computed in each of the three crystalline structures of silicalite-1 by grand-canonical Monte Carlo simulation are represented in Figure 3. The adsorption isotherm experimentally obtained45,46,48 is shown for comparison. None of the calculated isotherms reproduces the step observed experimentally. The adsorption isotherms of tetrachloroethene in the MONO and ORTHO structures are very similar. In both cases, an inflection point can be observed at a loading of 4 molecules 3 uc-1. The experimental maximum loading of 8 molecules 3 uc-1 is only reached at very high pressure in the ORTHO structure, and never in the MONO structure. The isotherm calculated in the PARA structure is very different. It belongs to the very common group of type I isotherms with a maximum loading of 8 molecules 3 uc-1. This difference supports the hypothesis that the ORTHO-PARA transition is responsible for the step in the isotherm. Osmotic Grand Potential. The grand potential corresponding to the osmotic ensemble was calculated from the isotherms

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Figure 3. Adsorption isotherms of tetrachloroethene in the MONO (green squares), ORTHO (blue diamonds), and PARA (red triangles) silicalite-1, calculated by GCMC simulations at 298 K. Experimental data are shown in black dots for comparison.

Figure 4. Osmotic ensemble grand potential as a function of pressure calculated for tetrachloroethene adsorption in the MONO (green squares), ORTHO (blue diamonds), and PARA (red triangles) silicalite-1 at 298 K.

calculated in the Grand Canonical ensemble, using the method described in the Simulation Method section. It is reported for each structure of silicalite-1 in Figure 4. At a given pressure, all the three host-guest systems belong to the same osmotic ensemble (Nhost, μads, P, T) where P is the total pressure in the reservoir. The comparison of grand potential values allows the prediction of the relative stability of the (guest/host) couple whatever the pressure. An initial structural transition is observed in silicalite-1 at P = 8 Pa followed by a second one at 22 Pa. Since the adsorbed amount at 22 Pa is higher in PARA than in ORTHO (around 6 molecules 3 uc-1 compared to 3.5 molecules 3 uc-1), this second structural change induces a jump in the adsorption isotherm. At this transition pressure, we have the equilibrium between two stable phases: an ORTHO phase containing 3.5 tetrachloroethene molecules 3 uc-1 and a PARA one containing 6 tetrachloroethene molecules 3 uc-1. Experimentally, the three structures are observed during the adsorption process of tetrachloroethene. Two studies report the structural changes of silicalite-1 during tetrachloroethene adsorption. The first one characterizes structural changes by measuring the cell parameter variations.45 A first transition have been 3858


The Journal of Physical Chemistry C detected at 4 molecule 3 uc-1 (P = 40 Pa) and a second one at ∼6.5 molecule 3 uc-1 (60 Pa).45 The second one performed by X-ray diffraction and NMR spectroscopy49,50 characterizes a structural transition from MONO to ORTHO between 0 and 2 molecules 3 uc-1 and a second one at 4 molecules 3 uc-1. Both results are in good agreement with our simulation results. During the desorption process, the structural change from ORTHO to MONO occurs at very low pressure (∼0 Pa).48 A hysteresis is observed experimentally in the structural transition induced by fluid adsorption. The thermodynamic transition pressure lies in between the adsorption and desorption transition pressures observed experimentally. Our simulations give a value of 22 Pa for the thermodynamic transition pressure, which agrees with the experimental observations. Location of the Molecules and Adsorption Mechanism. Three different adsorption sites were characterized in silicalite1: the first one is located at the intersections between straight and sinusoidal channels, the two other ones are located in each of the channels between two intersections (see Figure 1). In order to

Figure 5. Adsorption isotherms (obtained from GCMC simulations) of tetrachloroethene in the MONO (green squares), ORTHO (blue diamonds) and PARA (red triangles) silicalite-1 at 298 K, for the different parts of the zeolite porous volume (intersections: continuous lines, sinusoidal channels: dotted lines). No tetrachloroethene molecules were observed in the straight channels.

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investigate the repartition of the adsorbed molecules in these sites during the adsorption process, the porous volume of the zeolite was divided into three sub volumes as explained in ref.55 A portion of a straight channel is defined as the volume between two intersections. These two subvolumes are physically separated by ten-oxygen atom ring window. The separation between two sinusoidal channels and one intersection is not so clearly defined. We defined an arbitrary plane located at 0.32 nm from the center of the straight channel to separate the intersections and the sinusoidal channels. The molecules were sorted in the different adsorption sites according to the localization of their center of mass in the different porous volumes. The number of molecules filling the different adsorption sites is reported in Figure 5. In all the three structures, tetrachloroethene only occupy intersections and sinusoidal channels. No molecules were observed in the straight channels. In the MONO and ORTHO structures, the filling of the porous material occurs in two steps: the molecules first occupy all the intersections, and then, sinusoidal channels. In the PARA structure, the most favorable adsorption site is also the intersection one though the filling of the different porous volumes (intersections and sinusoidal channels) happens simultaneously. This difference between the MONO and ORTHO structures in the one hand, and the PARA structure in the other hand, comes from the geometry of the sinusoidal channels (see Table 2). The sinusoidal channels of the MONO and ORTHO structure have a maximum pore diameter of around 0.59 and 0.56 nm, respectively, whereas in the PARA structure it reaches around 0.64 nm. This favors the adsorption of large molecules like tetrachloroethene in this site. Two experimental studies report the location of tetrachloroethene molecules as a function of the loadings.45,49,50 Both of them show that at low loadings tetrachloroethene molecules only occupy intersection sites, in perfect agreement with our simulation results. At higher loadings, Mentzen et al.49,50 report that tetrachloroethene molecules fill all intersections and sinusoidal channels. Floquet et al.45 report that tetrachloroethene molecules fill all intersections and indifferently half sinusoidal and straight channel sites. Our simulation results show that straight channel sites are not occupied by tetrachloroethene molecules.

Figure 6. Views perpendicular to the y (a) and x (b) axes of two tetrachloroethene molecules located in an intersection (in blue) and in a sinusoidal channel (in cyan) of silicalite-1. 3859



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Figure 7. Isosteric heats of adsorption of tetrachloroethene in the MONO (green squares), ORTHO (blue diamonds), and PARA (red triangles) silicalite-1 at 298 K, calculated by GCMC simulations. Experimental isosteric data are shown in black dots for comparison.

An adsorption mechanism can be proposed based on our simulation results. Tetrachloroethene molecules first adsorb in intersections in the MONO structure. This adsorption process induces a structural transition of the adsorbent toward the ORTHO structure. Intersection sites are then filled completely in this ORTHO structure. Adsorption in the first sinusoidal channel site induces a second structural transition toward the PARA structure, which favors adsorption in sinusoidal channels. Then, a rearrangement occurs, one molecule in the intersection moves to a sinusoidal channel site freeing the intersection site where a new molecule adsorbs, until the maximum of loading is reached. Geometry of the adsorbed molecules in the different sites is very similar in the different structures of silicalite-1. Snapshots of tetrachloroethene molecules located in different adsorption sites can be found in Figure 6. Molecules in the intersections are slightly shifted at low loadings from the straight channel central axis (about 0.10 nm) toward the sinusoidal channel entrance. This shift decreases when sinusoidal channel sites are filled to reach about 0.08 nm at maximum loading. Energetic of the Adsorption. Isosteric heats of tetrachloroethene adsorption in the three structures of silicalite-1 are reported in Figure 7 together with the experimental results.47 A jump in the experimental curve can be observed at a loading of 4 molecules 3 uc-1, due to a change of the adsorption sites inside the porous volume in conjunction with a structural transition of the adsorbent. The amplitude of the experimental jump is around 10 kJ 3 mol-1. A jump is observed in the calculated isosteric heats in the MONO and ORTHO structures, even though it is weaker than the experimental curve’s one. This jump is due to a change in the adsorption sites, from intersections to vicinal sites located in the sinusoidal channels. For the PARA structure a smooth curve is observed, which reflects a more homogeneous filling of the porous volume, in agreement with the location of the molecules described before. This is also related to the type-I isotherm calculated for this system. The distribution of interaction energy between the molecules and the zeolitic framework for the different crystalline structures of silicalite-1 at different loadings is shown in Figure 8. In the MONO structure, adsorption sites in the channel intersections and sinusoidal channels have similar energy (around -56 kJ 3 mol-1). The difference observed in GCMC at high pressure

Figure 8. Adsorbent/adsorbate potential energy distribution function for tetrachloroethene adsorption in the MONO, ORTHO, and PARA silicalite-1 (from top to bottom), at 10, 100, and 1000 Pa.

in the occupancy of those two sites mainly comes from entropic differences. In the ORTHO structure, one can clearly discriminate between molecules located in the intersections and molecules located in the sinusoidal channels. The sinusoidal channel sites are highly unfavorable in this structure (∼-60 kJ 3 mol-1 for a molecule located in the intersections, and ∼-50 kJ 3 mol-1 for a molecule located in the sinusoidal channels). Adsorption of molecules in the intersections in the ORTHO structure is more favorable than in the MONO structure, which may be part of the reason why the structural change occurs (there are of course also entropic effects). In the PARA structure, we are not able to differentiate energetically channel intersection and sinusoidal channel sites. This is in complete agreement with the shape of the isotherm and adsorption heats. Adsorption sites in the channel intersections in the ORTHO and PARA structures have the same energy, whereas those in the sinusoidal sites are more favorable in the PARA structure than in the ORTHO structure. These results are in complete agreement with the thermodynamic analysis that predicts a structural transition from ORTHO to PARA when the sinusoidal channels start to be filled. A plateau is observed experimentally in the isosteric heat curve in the filling domain where the jump occurs. In this domain, the system is biphasic, composed of some crystallites in an ORTHO phase containing 3.5 molecules 3 uc-1 and others in PARA one containing 6 molecules 3 uc-1. The existence of phase mixtures at pressures close to the phase equilibrium pressure has been explained within the context of the thermomechanical model proposed for adsorption in multistable porous materials.63 This can explain the plateau observed in the experimental curve. The isosteric heat measured for the increase of one molecule 3 uc-1 corresponds to the sum of the heat given by the structural transition from ORTHO to PARA of 1/4 of the unit cell of the sample and the heat given by the filling of the sinusoidal sites of this 1/4 unit cell (Figure 9). This heat is the same whatever the average loading in the biphasic system of the plateau region. 3860



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Figure 9. Biphasic system during the ORTHO-PARA structure transition (from 4 to 8 molecules 3 uc-1). The adsorption of 1 molecule 3 uc-1 induces the transition of 1/4 of the unit cells from an ORTHO phase to a PARA phase and the filling of the sinusoidal channels of these unit cells. The isosteric heat of adsorption measured in this part of the isotherm corresponds to the sum of: (i) the heat of transition from ORTHO to PARA of 1/4 of the unit cells, (ii) the energy change of 1/4 of the adsorbed molecules in the channel intersections when the structure undergoes the structural change from ORTHO to PARA, and (iii) the adsorption energy of molecules filling the 1/4 of the sinusoidal channel sites of the sample located in the PARA phase.

Na-ZSM-5 Zeolite. Adsorption Isotherms. The experimental isotherm of tetrachloroethene in ZSM-5(26.5) is reported in Figure 10. This isotherm is a stepped isotherm similar to the one of tetrachloroethene in silicalite-1. The step occurs at a pressure of 140 Pa, which is a higher pressure than the one observed for silicalite-1. The maximum loading is 6.5 molecules 3 uc-1, but the constant increase of adsorbed quantity after micropore filling suggests the existence of defects inside the porosity of the adsorbent. Adsorption isotherms have been calculated in the ORTHO and PARA structures only because the ZSM-5 zeolite with a Si/Al ratio of 26.5 does not display MONO structure at room temperature. No forcefield adjustment was done for this particular system. Adsorption isotherm calculated in the ORTHO structure is of type I with a maximum loading of around 4 molecules 3 uc-1. The introduction of cations decreases the maximum loading of tetrachloroethene in MFI zeolites. The comparison of this isotherm with the one calculated in the ORTHO structure of silicalite-1 shows that adsorption of the first molecules is more favorable in ZSM-5 than in silicalite-1. The isotherm calculated in PARA is also of type I with a maximum loading of around 7.5 molecules 3 uc-1. This value is higher than the one observed experimentally. This can be attributed to the forcefield but also to the existence of a high number of defects in the experimental sample, which is not the case in simulation. It is worth mentioning that the presence of sodium cations in ZSM-5 induces a decrease of the maximum adsorption capacity of the zeolite in the ORTHO structure only. Osmotic Grand Potential. As for silicalite-1, grand potential calculations in the osmotic ensemble were performed to predict structural transition during tetrachloroethene adsorption. Grand potential was extracted from the calculated isotherms in the ORTHO and PARA structures. Results are reported in Figure 11.

Figure 10. Adsorption isotherms of tetrachloroethene in the ORTHO (blue diamonds) and PARA (red triangles) ZSM-5 (Si/Al = 26.5), calculated by GCMC simulations at 298 K. Experimental data are shown in black dots for comparison.

Comparison between grand potentials calculated in the ORTHO and PARA structures shows that the ORTHO crystalline structure is the most favorable one until 800 Pa, where a transition can be predicted toward the PARA structure. Like for silicalite-1, an ORTHO-PARA phase transition is induced by fluid adsorption. This transition is predicted at a higher pressure than the experimental pressure where the step is observed. However, in agreement with experimental results, this ORTHO-PARA transition is predicted at a higher pressure than the one occurring in silicalite-1. Introduction of cations in MFI-zeolites appears to increase the pressure where structural transition of the material occurs. At the transition pressure, the amount adsorbed in the ORTHO structure is around 4 molecules 3 uc-1 whereas it is 3861



Figure 11. Osmotic ensemble grand potential as a function of pressure, calculated for tetrachloroethene adsorption in the ORTHO (blue diamonds) and PARA (red triangles) ZSM-5 at 298 K.

almost 7.5 molecules 3 uc-1 in the PARA structure. This difference in loading at the transition pressure induces a step in the adsorption isotherm. Location of the Molecules and Cations. The porosity of the ZSM-5 zeolite was divided into three sub volumes is the same way as in silicalite-1. Sodium cations and tetrachloroethene molecules were classified according to the location of their center of mass. In the empty structure, cations are located at the intersections, in both ORTHO and PARA structures. They are shifted from the center of these cavities toward the sinusoidal channel entrance. They lay at the intersections during the whole adsorption process, and only slightly move under the influence of the adsorbed molecules. The adsorbate location dependence on pressure in the different adsorption sites for the ORTHO and PARA structures of ZSM-5 is shown in Figure 12. Again, no tetrachloroethene molecules adsorb into the straight channels. In the ORTHO structure, the tetrachloroethene molecules only adsorb in intersection sites whereas in the PARA structure they can access to both intersections and sinusoidal channels. Surprisingly, in the PARA structure, adsorption of tetrachloroethene molecules seems to be slightly more favorable in sinusoidal channels than in intersections, contrary to what was observed for silicalite-1. The filling of the two sites is almost simultaneous in the PARA structure. Figure 13 shows projections of the centers of mass of tetrachloroethene molecules and cations in the ORTHO and PARA structures for the empty structures and at the maximum loading of micropores (respectively 3.5 and 7.5 molecules 3 uc-1). In the empty ORTHO structure, cations are located in the channel intersections, close to the sinusoidal channels. Tetrachloroethene molecules only fill the intersection sites in this structure. No effect of tetrachloroethene adsorption can be seen on the cation location in the ORTHO structure. In the empty PARA structure, cations are also located in the intersections. They are less localized in this structure than in the ORTHO one. Tetrachloroethene molecules can adsorb in both intersections and sinusoidal channels. The presence of adsorbate constrains the cations to occupy a smaller volume space in the PARA structure. The adsorption mechanism in ZSM-5 is very similar to the one in silicalite-1. The empty structure of ZSM-5 is ORTHO. The filling of the tetrachloroethene molecules starts in the intersections until all these adsorption sites are filled, which corresponds

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Figure 12. Adsorption isotherms (obtained from GCMC simulations) of tetrachloroethene in the ORTHO (blue diamonds) and PARA (red triangles) ZSM-5 at 298 K, for the different parts of the zeolite porous volume (intersections, continuous lines; sinusoidal channels, dotted line). No tetrachloroethene molecules were observed in the straight channels.

to a loading of around 3.5 molecules 3 uc-1. The mechanical constraint created by the adsorbed tetrachloroethene molecules on the zeolitic framework induces a structural transition of the zeolite from the ORTHO to the PARA structure. The filling of sinusoidal channels can then happen in the PARA structure until the maximum loading is reached. Energetic of the Adsorption. Experimental calorimetric heats of adsorption are reported in Figure 14. Between 0 and 4 molecules 3 uc-1, the isosteric heat slightly increases from around 63 to 70 kJ 3 mol-1. At 4 molecules 3 uc-1 a clear jump of around 6 kJ 3 mol-1 is observed, consecutive to a change of the adsorption sites of tetrachloroethene molecules and a structural change of the framework. Then, a plateau is observed at 76 kJ 3 mol-1 between 4 and 5.5 molecules 3 uc-1 followed by a strong decrease probably due to adsorption of molecules on the external surface of the material and inside the porosity as long as saturation is not reached. As for silicalite-1, the plateau suggests the existence of a biphasic system. Comparison with the case of silicalite-1 shows that adsorption of tetrachloroethene is more favorable in ZSM-5 than in silicalite-1 whatever the loading. This was already suggested by comparing adsorption isotherms and is confirmed by isosteric heats of adsorption. Calculated isosteric heats of adsorption in the ORTHO and PARA structures are shown in Figure 14. Although the agreement is not quantitative, the simulation reproduces the order of magnitude of the isosteric heats. Isosteric heat of adsorption in the ORTHO structure increases regularly from 70 to 75 kJ 3 mol-1 until a loading of 3.5 molecules 3 uc-1, which corresponds to the maximum loading of micropores. This is in perfect agreement with the type I isotherm calculated by GCMC method. Isosteric heat of adsorption in the PARA structure slightly increases from 67 up to 77 kJ 3 mol-1 at 7.5 molecules 3 uc-1. Again, this shape is perfectly consistent with the type I isotherm presented before. In our simulation, isosteric heats are higher in the ORTHO structure than in the PARA one at low loading whereas a reverse behavior is observed at high loading. The calculated isosteric heats are higher in ZSM-5 than in silicalite-1, in agreement with the experimental results. The same difference of around 3862



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Figure 13. Projection of the center of mass of tetrachloroethene molecules (red color for molecules in channel intersections; orange color for molecules in sinusoidal channels) and sodium cations (blue color) in the ORTHO (upper panel: a, b) and PARA (lower panel: c, d) ZSM-5, before adsorption θ = 0 (a, c) and at saturation θ = 1 (b, d).

Figure 14. Isosteric heats of adsorption of tetrachloroethene in the ORTHO (blue diamonds) and PARA (red triangles) ZSM-5(26.5) at 298 K, calculated by GCMC simulations. Experimental isosteric data are shown in black dots for comparison.

10 kJ 3 mol-1 is found. Some potential adjustments would help to reproduce more quantitatively the energetics of the adsorption. Energy distributions at different loadings are shown in Figure 15 for the ORTHO and PARA structures. In this representation, the interaction energy between each tetrachloroethene molecule and the framework and extraframework cations is calculated for each configuration. Then, we plotted the energy distribution as a function of energy. The calculated energy does not include intermolecular interactions between tetrachloroethene molecules. This explains why the calculated energy is different from the adsorption heats. In the ORTHO structure, molecules only adsorb in intersection sites, and therefore, only one peak at -67 kJ 3 mol-1 is observed in the corresponding energy distribution curve. In the PARA structure two peaks are observed, the first one at -53 kJ 3 mol-1 corresponding to molecules located at the intersections and the second one at

Figure 15. Adsorbent/adsorbate potential energy distribution function for tetrachloroethene adsorption in the ORTHO and PARA ZSM-5 (from top to bottom), at 1, 20, and 100 Pa.

-68 kJ 3 mol-1 corresponding to molecules in the sinusoidal channels. The presence of cations at the intersections leads to a very high decrease of the adsorption energy of the molecules in these sites due to steric hindrance in such confined space. Sinusoidal channels become energetically more favorable than intersections when sodium cations are present. Because of entropic reasons, the filling of the two sites is still almost simultaneous. We have tried to increase the size of the cation in our model (σNa-Na = 0.35 nm). In that case, molecules first adsorb in the sinusoidal channels and when all the sinusoidal channels are filled, molecules start to fill the intersections.

’ CONCLUSION The adsorption process of tetrachloroethene in two MFI type zeolites was studied by experiments and Monte Carlo simulations. Two structures were studied: the pure silica MFI analogue, 3863


The Journal of Physical Chemistry C silicalite-1 and ZSM-5 with a Si/Al ratio of 26.5. The influence of the structural changes of the adsorbent was characterized by grand canonical ensemble calculations in each of the three experimental structures of MFI zeolites. Structural changes were predicted by grand potential calculations, and the influence of the Si/Al ratio on the thermodynamic aspects of the adsorption process was studied. The influence of the zeolitic crystalline structure on the adsorption process of tetrachloroethene has been shown for silicalite-1 and ZSM-5. For both zeolites, the step observed in the experimental isotherm is due to an ORTHO-PARA structural transition of the adsorbent. The step occurs at a higher pressure in ZSM-5 than in silicalite-1. This behavior is well reproduced by simulation. Adsorption mechanism has been also studied by simulation. Molecules first adsorb at the intersections in the MONO structure for silicalite-1 and in the ORTHO structure for ZSM-5. Then, when all intersections are filled, an ORTHOPARA transition occurs, and sinusoidal channels start to be filled. No tetrachloroethene molecule is adsorbed in straight channels. The microporosity is never completely filled even at saturation. This adsorption mechanism is similar for both zeolites. Joint experimental and simulation studies allowed shedding some light on the adsorption process of a chloroalkene in MFI zeolites and on the thermodynamics aspects of stepped isotherms. The framework flexibility appears to be an important feature of the adsorption process. The osmotic ensemble provides a framework for modeling adsorption in flexible porous media. Reproducing framework flexibility during the adsorption process is one of the actual challenges if one wants a complete description by simulation of adsorption phenomena. Simulation with fully flexible zeolitic framework needs complete forcefield description and efficient algorithms. Work is in progress in that direction.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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Structural Changes in Nanoporous MFI Zeolites Induced by

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