Communication: Evidence of structural phase transitions in silicalite-1 by infraredspectroscopyAnthony Ballandras, Guy Weber, Christian Paulin, Jean-Pierre Bellat, and Maud Rotger

Citation: The Journal of Chemical Physics 139, 091103 (2013); doi: 10.1063/1.4819080 View online: http://dx.doi.org/10.1063/1.4819080 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/139/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Osmotic ensemble methods for predicting adsorption-induced structural transitions in nanoporous materialsusing molecular simulations J. Chem. Phys. 134, 184103 (2011); 10.1063/1.3586807 The effect of pressure on the phase transition behavior of tridecane, pentadecane, and heptadecane: A Fouriertransform infrared spectroscopy study J. Chem. Phys. 134, 144503 (2011); 10.1063/1.3573376 Formation dynamics of hexadecanethiol self-assembled monolayers on (001) GaAs observed withphotoluminescence and Fourier transform infrared spectroscopies J. Appl. Phys. 106, 083518 (2009); 10.1063/1.3248370 T dependence of vibrational dynamics of water in ion-exchanged zeolites A : A detailed Fourier transforminfrared attenuated total reflection study J. Chem. Phys. 123, 154702 (2005); 10.1063/1.2060687 Computer simulation of the adsorption of ethanethiol in silicalite of MFI and MOR J. Chem. Phys. 121, 9098 (2004); 10.1063/1.1805503

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THE JOURNAL OF CHEMICAL PHYSICS 139, 091103 (2013)

Communication: Evidence of structural phase transitions in silicalite-1by infrared spectroscopy

Anthony Ballandras,1 Guy Weber,1,a) Christian Paulin,1 Jean-Pierre Bellat,1

and Maud Rotger2

1Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS - Université de Bourgogne,9 Av. A. Savary, B.P. 47870, 21078 Dijon Cedex, France2Groupe de Spectrométrie Moléculaire et Atmosphérique, UMR 7331 CNRS - Université de ReimsChampagne-Ardenne, Moulin de la Housse, B.P. 1039 Cases 16-17, 51687 Reims Cedex 2, France

(Received 19 June 2013; accepted 8 August 2013; published online 6 September 2013)

The adsorption of trichloroethylene, perchloroethylene, and p-xylene on a MFI (Mobile-FIve) zeoliteis studied using in situ FTIR spectroscopy at 298 K. Spectra of self-supported zeolites in contact withincreasing pressures of pure gas were recorded at equilibrium in the mid-infrared domain. Analysisof the evolution of the shape and location of vibrational bands of the zeolite as a function of theamount adsorbed allowed the observation of structural modifications of the adsorbent for the firsttime by infrared spectroscopy. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4819080]

Physisorption of many adsorptives on MFI zeolitesusually gives rise to isotherms of type I at room tem-perature. However, with some adsorptives,1–7 isothermsshow a step or a kink at half loading, i.e., 4 moleculesper unit cell (molec. uc−1). Numerous studies were per-formed during the last 30 years with various experimen-tal techniques (calorimetry,8–10 X-ray diffraction,11, 12 neutrondiffraction,13 manometry,14 Raman spectroscopy,15 nuclearmagnetic resonance (NMR),16 Temperature-ProgrammedDesorption (TPD),17 etc.) and molecular simulations (Molec-ular Dynamics,18–20 Monte-Carlo methods,20–27 and Molec-ular Mechanics20, 27) to understand this difference of behav-ior. Three main explanations have been advanced in the lit-erature to elucidate the origin of the step (or the kink) inthe isotherms: (i) phase transition of the adsorbate,8, 9 (ii) ad-sorption by site,28, 29 or (iii) phase transition(s) of the hostframework.11, 12, 30, 31 Concerning the last point, it should benoted that two successive phase transitions of type MONO(monoclinic structure with space group P21/n.1.1)-ORTHO1(orthorhombic structure with space group Pnma in the caseof perchloroethylene (PER)30 and p-xylene (p-XYL)11, 12 ad-sorption or P212121 in the case of benzene31 adsorption)then, ORTHO1-ORTHO2 (orthorhombic structure with spacegroup P212121 for PER30 and p-XYL11, 12 adsorption or Pnmafor benzene31 adsorption) can be observed as a consequenceof the increasing amount adsorbed in high silica materials(Si/Al > 75). Recent numerical simulations25, 26 performedon the adsorption of PER in silicalite-1 (SIL-1) strengthenthe third interpretation and showed that the step observed at4 molec. uc−1 could be due to a symmetry change from OR-THO1 to ORTHO2 upon micropore filling.

The FTIR spectroscopy technique is largely used to char-acterize the surface acidity of zeolite catalysts (Lewis andBrönsted acidities) by means of suitable molecular probes.32

a)Electronic mail: guy.weber@u-bourgogne.fr

However, studies dedicated to the understanding of physisorp-tion in zeolites are scarce. In most cases, the spectra of ad-sorbed molecules are analyzed upon adsorption either to esti-mate the electric field inside zeolite trapping sites33, 34 or tocharacterize the binding state and the conformation of ad-sorbed molecules.35–37 In a few cases only, the evolution ofthe zeolite spectrum has been analyzed in a restricted domainof wavenumber, mainly in the hydroxyl vibration region.38 Toour knowledge, this spectroscopy technique has never beenused, to characterize, a structural change of a zeolite fromthe evolution of structural framework vibrational bands of thematerial.

The target of the present work is to determine from well-chosen and well-known adsorbate/adsorbent systems largelystudied in the literature whether FTIR spectroscopy is usefulto characterize phase transitions of a material during an ad-sorption phenomenon.

In this communication, we focus on the adsorption of per-chloroethylene (PER), p-xylene (p-XYL), and trichloroethy-lene (TRI) on silicalite-1 (SIL-1) which is a pure silicazeolite with a three-dimensional pore structure of straight andsinusoidal intersecting channels. The adsorptives (PER, TRI,p-XYL) are better than 99.0% pure. Before use, they are de-gassed under high vacuum, then stored in an evacuated ves-sel containing a 3A hydrophilic zeolite to trap residual water.The zeolite synthesized in fluorine media39 is monoclinicat room temperature. Infrared spectroscopic experiments areconducted on self-supported samples of SIL-1 compacted un-der the lowest possible pressure, at around 0.2 GPa, in orderto prevent any change in the physical properties of the startingpowdered zeolite. Infrared absorption measurements are per-formed in an optical cell specially built to study the interac-tion of a gas on a nanoporous material under controlled vaporpressures.40 The cell is a stainless six-way cube equipped withtwo KBr windows distant from one another of about 10 mm inorder to optimize the signal-to-noise ratio and two view portsfor high vacuum, each of them being placed on opposite sides

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091103-2 Ballandras et al. J. Chem. Phys. 139, 091103 (2013)

of the cube. The low part of the cube is closed by means ofa fixed solid flange and the upper part is connected to a heatchamber and to both a vacuum line and a gas inlet. The sam-ple holder on which is deposited the self-supported samplecan be moved up and down from the heat chamber to the op-tical cell by using a magnetic sample transporter. The FTIRspectrometer is a Bruker Equinox 55 instrument equippedwith a globar source, a DTGS (Deuterated TriGlycine Sulfate)detector and a KBr beamsplitter. All spectra were recordedat a resolution of 2 cm−1 by coadding 100 scans in trans-mission mode. The procedure used for adsorption measure-ments consists first in activating the self-supported sampleto eliminate all molecules preadsorbed at room conditions.This activation is performed in the heat treatment chamberunder a flow rate of dry nitrogen for 5 h at around 573 K.Then, the chamber is purged under vacuum and the sam-ple is exposed to the gaseous adsorptive. Adsorption mea-surements are performed at equilibrium, by increasing thepressure step by step, to describe the adsorption isotherm at298 K. The amount of gas adsorbed at equilibrium is deter-mined from the corresponding adsorption isotherm obtainedby thermogravimetry. Each spectrum of the self-supportedzeolite is recorded under equilibrium conditions at constantpressure from P = 0 hPa (unloaded zeolite) to the saturationvapor pressure of the adsorptive at 298 K. Each spectrum iscorrected from the background spectrum recorded in the sameconditions on the gas phase without sample.

The vibrational modes of the framework of SIL-1 arevisible in the spectral range 400–1280 cm−1. However, thestrongest bands at around 1000 cm−1 cannot be analyzeddue to the total absorption of the infrared beam by the self-supported sample. On the other hand, the bands located inthe wavenumber domain 580–760 cm−1 are of lower intensityand can be analyzed during the adsorption process (Figure 1).They are assigned to O-T-O and T-O-T symmetric stretchingand double ring vibrations according to the classification ofFlanigen et al.41

In a first experiment, we investigated the interaction ofPER on SIL-1. Figures 1 and 2(a) show the evolution of the

750 700 650 600

0.5

Wavenumber (cm-1

)

Absorbance (a.u.)

0

8

FIG. 1. Dependence of the shape of the infrared spectrum of silicalite-1(SIL-1) on the number of perchloroethylene (PER) molecules adsorbed (frombottom to top: 0, 1, 2, 3, 4, 5, 6, 7, and 8 molec. uc−1) at 298 K, over thewavenumber domain 580–760 cm−1.

FIG. 2. Dependence of the shape of the infrared spectrum of silicalite-1(SIL-1) on the number of (a) perchloroethylene (PER) and (b) p-xylene(p-XYL) molecules adsorbed (from bottom to top: 0, 1, 2, 3, 4, 5, 6, 7, and 8molec. uc−1) at 298 K, over the wavenumber domain 675–705 cm−1.

shape of the infrared spectra of the zeolite during the adsorp-tion process. The zeolite spectrum does not change signifi-cantly with loading, below 2 molec. uc−1. On the contrary,strong modifications appear at 3 molec. uc−1. We observe (i)a change of the relative intensity of the two bands locatedat around 700 cm−1, (ii) the appearance of a new band atabout 622 cm−1, and (iii) the disappearance of a band at731 cm−1. Upon further loading, the spectrum of the zeolitedoes not change anymore up to 6 molec. uc−1 but changesagain at 7 molec. uc−1. Above this loading, the double bandat 700 cm−1 displays a major contribution at 692 cm−1 andthe two other bands located at around 625 cm−1 show asplitting. Such evolution of zeolite spectrum which happensby successive steps can be accounted for by large modifi-cations of the zeolite framework consecutive to structuralchanges. Note that this type of structural modification hasbeen already characterized by other experimental and simula-tion techniques.13, 25, 26, 30, 42 Despite some small discrepanciesbetween the values of micropore loadings where adsorbentphase transitions were characterized, FTIR spectroscopic dataare in overall good agreement with published data.13, 25, 26, 30, 42

Such evolution of the infrared spectra of SIL-1 is consis-tent with the adsorption process of PER in micropores assuggested in the following words by Jeffroy et al.:26 “tétra-chloroéthylène molecules first adsorb in intersections in theMONO structure inducing a structural transition of the adsor-bent toward the ORTHO1 structure; intersection sites are thencompletely filled in this ORTHO1 structure and the adsorp-tion in the first channel site induces a second structural tran-sition toward the ORTHO2 structure, which favors adsorp-tion in sinusoidal channels; then a rearrangement occurs, onemolecule in the intersection moves to a sinusoidal channel sitefreeing the intersection site where a new molecule adsorbs,until the maximum of loading is reached.” On the basis of thisadsorption mechanism we assume that the modifications bysteps of the spectrum observed between 2 and 3 molec. uc−1

and between 6 and 7 molec. uc−1 are the signatures of the

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091103-3 Ballandras et al. J. Chem. Phys. 139, 091103 (2013)

MONO-ORTHO125, 26, 30 and ORTHO1-ORTHO213, 42 struc-tural changes of SIL-1, respectively. As the overall shape ofthe infrared spectrum does not change significantly before andafter these steps, we can expect that the structure of the adsor-bent does not change within a given micropore filling domainand that in these domains the spectroscopic signatures are notrepresentative of the adsorption of molecules in a specific site,either a sinusoidal channel or an intersection.

By comparison, Figures 2(b) and 3 show the evolution ofthe spectrum of SIL-1 during the adsorption of p-XYL andTRI, respectively, in the restricted domain 675–705 cm−1.These two systems are other case studies largely discussedin the literature.3, 4, 11–14, 25, 40, 42

For p-XYL adsorption (Figure 2(b)), sudden spectralmodifications also occur at the same loadings than thoseobserved for PER adsorption. These modifications can becorrelated again to the successive MONO-ORTHO1, thenORTHO1-ORTHO2 structural modifications of the zeolitecharacterized by X-ray diffraction studies.11, 12 We can, there-fore, make the assumption that the adsorption processes of p-XYL and PER which are both non-polar molecules tight fittedin the channels of the zeolite are very similar. Such assump-tion agrees with published data.12, 13

For TRI adsorption (Figure 3), the shape of the spectrachanges between 4 and 5 molec. uc−1. These steps can beassociated again to the MONO-ORTHO1 structural change ofthe adsorbent, in agreement with previous published data.13, 42

Then, the infrared spectra continuously change when theamount adsorbed increases from 5 to 8 molec. uc−1. Un-like to what was observed for PER and p-XYL adsorption,the evolution proceeds mainly through a small and progres-sive blue shift of vibrational bands. Such evolution, whichwas not observed for PER and p-XYL adsorption on SIL-1, may be indicative of either the coexistence of two phases(ORTHO1 and ORTHO2) or the variation of the unit cell pa-rameters of the phase ORTHO1. It may be also due to the

FIG. 3. Dependence of the shape of structural vibrational bands of silicalite-1 (SIL-1) on the amount adsorbed (from bottom to top: 0, 1, 2, 3, 4, 5, 6, 7, 8,9, and 10 molec. uc−1) for trichloroethylene (TRI) adsorption at 298 K, overthe wavenumber domain 675–705 cm−1.

adsorption process of TRI which differs from that of PERand p-XYL in that molecules indifferently fill straight andsinusoidal channels and intersections during the adsorptionprocess. Above 8 molec. uc−1, two new bands appear withinthe wavenumber range 630–640 cm−1 (not seen in Figure 3),whereas the one located at 696 cm−1 disappears. Above thisvalue, the spectrum does not change, indicating that only theORTHO2 phase remains.42 To consider again the shape ofthe framework vibration bands within the wavenumber do-main 675–705 cm−1 as representative of a given structureof the adsorbent is not unreasonable. Indeed when we car-ried out the adsorption of TRI on a Na-ZSM-5 zeolite ofSi/Al ratio of 26.5 which is orthorhombic at zero loading, theshape of the spectra recorded up to the maximum loading of 8molec. uc−1 is very close to the ones observed between 5 and8 molec. uc−1 for SIL-1.

This comparative study of the evolution of the infraredspectrum of SIL-1 during the accumulation of polar (TRI)and non-polar molecules (PER, p-XYL) tight fitted withinmicropores allows to characterize singular shapes of frame-work vibrational bands depending on the loading, the natureof the adsorbed molecule and thereby on the structure of theadsorbent. Three distinct filling domains are identified withinthe wavenumber range 675–705 cm−1. First, at low loadingsfrom 0 to 2 molec. uc−1 for PER and p-XYL and from 0 to4 molec. uc−1 for TRI, the band located at around 685 cm−1

is stronger than the one located at 700 cm−1. Second, be-tween 3 and 6 molec. uc−1 for PER and p-XYL and between5 and 8 molec. uc−1 for TRI, a modification of the relativeintensity of these two bands is observed, the one located at700 cm−1 being stronger than the other one. Finally, at highloading, i.e., above 7 molec. uc−1 for PER and p-XYL andabove 9 molec. uc−1 for TRI, these two bands change againin shape and intensity to give a strong band and a shoulder ataround 680 and 690 cm−1, respectively. Each infrared spec-troscopic signature in a defined filling domain is consideredas representative of a given structure of the adsorbent.

To conclude, this study shows that the FTIR spectroscopytechnique is a suitable and powerful technique for the char-acterization of structural changes of MFI zeolites. Structuralchanges can be suspected when the infrared spectrum of thezeolite undergoes modifications (change of the relative inten-sity and/or a shift, appearance or disappearance of frameworkvibrational bands) during the adsorption process. These mod-ifications can be progressive with loading, as in the case ofTRI adsorption (isotherm of type I). In this case, a small vari-ation of the unit cell parameters of the phase present or thecoexistence of two phases can be suspected. Such changescan also happen, as in the case of PER or p-XYL adsorption(stepped isotherms), by two successive steps in very restrictedmicropore filling domains without strong modifications of theinfrared spectrum outside these limits. In these cases, it canbe expected that the addition of molecules does not affectgreatly the structure of the zeolite before and after structuraltransitions so much so the unit cell parameters of the phasepresent does not significantly change over narrow filling do-mains. However, with other adsorptives such as ethylene theadsorption does not induce any modification of the frameworkinfrared spectrum over the entire domain of micropore filling

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up to 11 molec. uc−1; in this case we can assume that SIL-1keeps its monoclinic structure during the adsorption process.

This study shows that the infrared spectroscopy is a goodtechnique to evidence the framework flexibility of porousmaterial during the adsorption process. Further experimentalstudies are in progress to verify the application of the method,to soft porous materials such as MOFs (Metal Organic Frame-works), which exhibit breathing effects during adsorption.

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