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Effect of T(II1) Zoning in MFI-Type Zeolites on the Adsorption Isotherm and Differential Enthalpies of Adsorption at 77 K P. L. Llewellyn,* Y. Grillet, and J. Rouquerol Centre de Thermodynamique et de Microcalorimetrie, Centre National de la Recherche Scientifique, 26 Rue du 141eR.I.A., 13003 Marseille, France Received July 21,1993. I n Final Form: November 1,1993” The characterizationof MFI-typesampleswithcrystals exhibitingeither a homogeneous or heterogeneous distribution of the trivalent metal (aluminum or iron, labeled T(1II)) is carried out using isothermal gas adsorption microcalorimetry (Ar, Nz, and CO) coupled with quasi-equilibrium adsorption volumetry at 77 K. It is found that, especially during the “fluid-like”to “solid-like”phasq transition exhibited on these samples, carbon monoxide can distinguish those with a distinct T(I1I) zoning (whether it is toward the interior or exterior of the crystals). Argon however, is not greatly affected by the T(II1) content (whatever the distribution) and provides a complementary textural examination of these samples.
Introduction The unique catalytic properties of zeolites result from both their porous structure giving shape selectivity and their trivalent metal content (labeled T(II1) and usually aluminum) giving the catalytic centers (Bronsted or Lewis acid sites). To estimate the properties of potential catalysts, physisorption studies on large crystals, allowing phenomena due to the external surface to be virtually ignored, provides interesting information about the textural and chemical nature of these materials. In the present work, the MFI-type zeolites’ have been chosen. Adsorption studies at around 77 K with the pure silica end member of the MFI-type zeolites, silicalite-I, have shown the existence of adsorbate phase transitions. For nonpolar molecules, such as argon” and krypton? one transition is observed of the type
(Le. ZSM-5) has the effect of rendering these transitions more diffuse in nature and also leads to their shift to lower relative pressures.e The effect of introducing iron(II1) into the MFI-type structure on the adsorption properties has not been reported yet in the literature. Many of the zeolite catalysts employed today have a certain degree of T(II1) zoning at the scale of the individual crystals. The extent of this zoning results from the synthesis procedure.’ Although MFI-type zeolites with distinct zoning have previously been employed in low temperature adsorption studies,e no distinct effects have been observed which correspond to this localization of specific adsorption sites. This study firstly presents the adsorption on two (Fe)MFI-type zeolites with about the same Si/Fe ratio (-70): one with a homogeneous distribution of iron across the crystals and the second exhibiting distinct heterogeneous zoning (with a high Si/Fe ratio in the center of the crystals disordered fluid-like phase A ordered solid-like phase and a low Si/Fe ratio at the exterior of the crystals). For specific molecules, such as nitrogen (quadrupoSecondly, the adsorption on two (A1)MFI-type zeolites lar)2J15v6and carbon monoxide (dipolar)? two transitions with distinct zoning (one with most of the aluminum may be observed of the type toward the exterior and the second with most of it toward the interior of the crystal) is compared with the results disordered fluid-like phase A localized fluid-like obtained for the (Fe)MFI-type zeolites. Finally a 50:50 B mixture of pure silicalite-I and (A1)MFI (Si/Al = 16) is phase ordered solid-like phase taken and the adsorption behavior is compared with that These transitions are marked by distinct steps in the of the former materials. The adsorption of a nonspecific relative adsorption sharp changes in the molecule (argon) and a specific molecule (carbon mondifferential enthalpy of adsorption,u*6 and the appearance oxide) is contrasted, allowing both a textural and chemical of sharp peaks in the neutron diffraction ~ p e c t r a . ~ ~study ~ ~ ~of the samples to be attained. Introduction of aluminum(II1) into the MFI-type zeolites
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* Author to whom correspondence should be addressed. e Abstract
published in Advance ACS Abstracts, December 15,
1993. (1)Meier, W. M.; Olson, D. H. Atlas of Zeolite Structure Types, 3rd ed.; Butterworth-Heinemann: London, 1987. (2) Miiller, U.; Reichert, H.; Robens, E.; Unger, K. K.; Grillet, Y.; Rouquerol, F.; Rouquerol, J.; Pan, D.; Mersmann, A. Fresenius 2 . Anal. Chem. 1989,39, 433. ( 3 ) Reichert, H.; Muller, U.; Unger, K. K.; Grillet, Y.; Rouquerol, F.; Rouquerol, 3.; Coulomb, J.-P. In Characterisation of Porous Solids II; Rodriguez-Reinoso, F., Rouquerol, J., Sing, K. S. W., Unger, K. K., Eds.; Elsevier: Amsterdam, 1991; p 535. (4) Llewellyn, P. L.; Coulomb, J.-P.;Grillet, Y.; Patarin, J.; Reichert, H.; Andre, G.; Rouquerol, J. Langmuir 1993, 9, 1846. (5) Muller, U.;Unger, K. K. In Characterisation of Porous Solids; Unger, K. K., Rouquerol, J., Sing, K. S. W., Kral, H., Eds.; Elsevier: Amsterdam, 1988; p 101. (6) Llewellyn, P. L.; Coulomb, J.-P.;Grillet, Y.; Patarin, J.; Lauter, H.; Rouquerol, J. Langmuir 1993,9, 1852.
0743-7463/94/2410-0570$04.50/0
Experimental Section Adsorbents. The MFI-type samples’ (Table 1) employed in this study come from two sources. The (Fe)MFI-typezeolites (samples ZFH and ZFI) as well as the silicalite-I and (AI)MFI sample (Si/Al= 16) were kindly supplied by Dr.J. Patarin of the Ecole Nationale Superieure de Chimie in Mulhouse (France). These samples were prepared in a fluoride medium via synthesis routes described elsewhere.* Both iron containing samples have an overall silicon to iron ratio of 70 obtained by atomic adsorption spectroscopy.9 Sample ZFH has a homogeneousiron distribution across the crystals as seen by Fe K a X-ray mapping (Figurela,b). Sample ZFI however, has a distinct iron population toward the (7) Jacobs, P. A.; Martens, J. A. Synthesis of High Silica Aluminophosphate Zeolites; Elsevier: Amsterdam, 1987. (8)Patarin, J.; Kessler, H.; Guth, J.-L. Zeolites 1990, 10, 674.
0 1994 American Chemical Society
Characterization of MFI-Type Zeolites
Langmuir, VoZ. 10, No. 2, 1994 571
, k
Figure 1. SEM images and corresponding Fe Ka X-ray images of the iron-containing samples ZFH (a (top left) and b (top right)) and ZFI (c (bottom left) and d (bottom right)). Table 1. MFI-Type Zeolites Employed during the Present Study Zeolite
source
ZFH ZFI silicalite-I* ZSM-56 ZAE ZAI
ENSC Mulhouse ENSC Mulhouse ENSC Mulhouse ENSC Mulhouse Uni Mainz Uni Mainz
T(II1) Si/T(III) atom ratio Fe Fe
Al A1
T(II1) locationa
70 70
Homogeneous Interior of crystals
>10 OOO 16 46 42
homogeneous Exterior of crystals Interior of crystals
Al From X-ray mapping. Samples used for the 5050 mixture employed in this study. 0
interior of the crystals (Figure lc,d). For both samples, a small amount of extraframework iron impurities were also p r e ~ e n t . ~ The (A1)MFI-typezeolites (samplesZAE and ZEI) were kindly supplied by Professor K. K. Unger’s group a t the JohannesGutenbergUniversitiit in Mainz (Germany). These sampleswere prepared via the alkaline-free synthesis route.1° Sample ZAE has an overall silicon to aluminum ratio of 46 with the aluminum concentrated toward the exterior of the crystal. Sample ZAI, on the other hand, has an overall silicon to aluminum ratio of 42 with the aluminum concentrated toward the interior of the crystals. In both cases, no extraframework aluminum was found to be present. All the samples were calcined to 823K to eliminate the organic template. Before each adsorption experiment the samples are pretreated to a temperature of 473 K via the procedure of controlled rate thermal analysisll which proffers a predetermined rate of outgassing (most often chosen as constant) thus ensuring (9) Patarin, J.; Tuilier, M. H.; Durr, J.; Kessler, H. Zeolites 1992,12, 70. (10) Miiller, U.; Brenner, A,; Reich, A.; Unger, K. K. In Zeolite Synthesis; Occelli, M. L., Robson, H. E., Eds.; American Chemical Society: Washington, DC, 1989; p 346. (11) Rouquerol, J. Thermochim. Acta 1989,144,209.
that the samples are prepared in a reproducible manner. The final pressure above the sample after pretreatment is inferior to 10-1Pa. Adsorptives. The adsorptives, argon and carbon monoxide, used in this study are of high-purity grade (>99.9995% and >99.998 % purity, respectively)and obtained from Alphagaz (Air Liquide, France). The argon molecules, which are spherical and nonpolar, interact in a nonspecific manner with a surface, thus allowinga textural study of the adsorbent. The carbon monoxide molecule is a nonspherical and polar molecule, with both a dipole (0.39 X 103 C m) and quadrupole moment (-12.3 X 1VC m2). It interacts in a specific manner with a surface, allowing the influence of the adsorbent field gradient to be investigated. Carbon monoxide is preferred to nitrogen (quadrupole moment of -5 X 1VC m2)as a “specificprobe molecule” as it gives rise to stronger specific interactions due to its greater cumulative moment. Experimental Techniques. The adsorption isotherms at 77 K are obtained via a volumetric technique employing a quasiequilibrium (extremely slow and constant) method of adsorptive introduction.12 This latter technique may be coupled with isothermal microcalorimetry13allowing a high-resolution determination of the differential enthalpies of adsorption (or net differential enthalpies, if the enthalpy of vaporization at the experimental temperature is deducted from the previous ones).
Results and Discussion Argon on the (Fe)MFI-TypeZeolites ZFH and ZFI. From Figures 2 and 3 it may be seen that the general shape of the isotherms and net differential enthalpy of adsorption curves are similar for the two iron(II1)(12) Rouquerol,J.; Rouquerol,F.; Grillet,Y.; Ward, R. J. In Characterisation of Porous Solids; Unger, K. K., Rouquerol,J., Sing,K. S. W., Kral, H., Eds.; Elsevier: Amsterdam, 1988; p 67. (13) Rouquerol, J. Thermochim. Acta 1985,96,377.
572 Langmuir, Vol. 10, No. 2, 1994
Llewellyn et al.
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containing samples ZFH and ZFI. The net differential enthalpy of adsorption in the primary micropore filling region of the isotherm (0
