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23Site-Hopping Dynamics

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Site-Hopping Dynamics of Benzene Adsorbed on Ca-LSXZeolite Studied by Solid-State Exchange13C NMRD. J. Schaefer,†D. E. Favre,†M. Wilhelm,§S. J. Weigel,‡,|and B. F. Chmelka*,†Contribution from the Departments of Chemical Engineering and Chemistry,UniVersity of California, Santa Barbara, California 93106ReceiVed May 14, 1997XAbstract: First-time applications of solid-state exchange13C NMR techniques to the study of the reorientation dynamicsof hydrocarbon molecules adsorbed on zeolites have enabled the geometry and time scales of molecular hoppingprocesses between adjacent adsorption sites to be characterized directly and model free. Two-dimensional exchange13C NMR on static samples establishes the geometry of the site-hopping dynamics, while one-dimensional magic-angle spinning (MAS) exchange-induced-sidebands (EIS)13C NMR permits motional correlation times on the orderof milliseconds to seconds to be extracted directly from the experimental data. Variable-temperature experimentsperformed on Ca-LSX zeolite samples with average bulk loadings of 0.5, 1, and 2 benzene molecules per supercageyield apparent Arrhenius activation energies of about 66 ( 6kJmol-1for the discrete, localized reorientation dynamicsof benzene molecules among different Ca2+cation adsorption sites (∼0.5 nm apart). Arrhenius preexponential factorswere established to be on the order of 1 × 1012s-1, consistent with elementary hopping processes. Motional correlationtimes exhibit only minor variations upon changes in benzene loading over the range studied.IntroductionZeolites are nanoporous aluminosilicates comprised of an ex-tended network of TO4tetrahedra (T ) Si, Al) that are arrangedinto three-dimensional crystalline frameworks with molecule-sized channels or cavities and large internal surface areas.1-3Zeolites are used in a wide range of commercially importantapplications, including separations, adsorption, and catalysis,where the transport behaviors of molecular guest species insidethe host matrices play central roles in material and processfunctions.4,5Understanding the molecular origins of the mac-roscopic transport properties of these complicated heterogeneoussystems is crucial to establishing and controlling key materialsand process parameters, with the goal of improving adsorption,diffusion, and reaction performance of nanoporous solids.Ca-LSX, (AlO2)96(SiO2)96Ca48, a faujasite-type zeolite,6isclosely related to commercial nanoporous molecular sieves usedwidely as adsorbents and catalysts in hydrocarbon processingand in air separation. The selection of the benzene/Ca-LSXsystem for this first comprehensive investigation was based onseveral factors, including the industrial importance of faujasite-type zeolites and the ordered framework of the LSX structure,which reduces the effects of structural disorder and cationdistributions on adsorbate dynamics. In addition, the temper-ature dependence of the spin-echo2H NMR line shape ofperdeuterated benzene, C62H6, adsorbed on Ca-LSX indicatedthat the site-hopping motions of benzene possessed correlationtimes at room temperature suitable for exchange NMR, namely10-3s < τc< 102s.7The faujasite structure is one of the numerous known zeoliteframework topologies and has a unit cell with 192 tetrahedralunits, a portion of which is represented by the single supercageshown in Figure 1a.1A purely siliceous faujasite structure isrepresented by the formula (SiO2)192and has an electricallyneutral framework; each aluminum substitution, up to anapparent maximum of 50% of the tetrahedral framework sites(governed by Loewenstein’s rule),8gives rise to a negativecharge on the framework that must be balanced by extraframe-work cations. The zeolite compositions obtained by thesubstitution of aluminum for silicon in the faujasite frameworkhave been given different names according to their Si/Al ratio:zeolite Y for Si/Al > 1.5, zeolite X for 1 < Si/Al e 1.5, andLSX (low-silica X) for Si/Al ) 1. The number and location ofAlO4tetrahedra affect the number and placement of such cations,contributing to the structural heterogeneity of the zeolite. ByLoewenstein’s rule,8the LSX framework represents the faujasitestructure with the maximum framework aluminum content,consisting of highly ordered alternating SiO4and AlO4tetra-hedra. Cation siting in dehydrated Ca-LSX has been determinedfrom powder X-ray and neutron diffraction.7The Ca2+cationsare located preferentially in sites SI and sites SII, with noevidence found for the presence of cations in SIII positions,see Figure 1a. The four SII sites within each supercage arearranged tetrahedrally; inverting this tetrahedron gives thearrangement of the four windows connecting adjacent super-cages, two of which are shown in Figure 1b. Because thesupercages are arranged on a diamond lattice, the correspondingSII and window sites within a LSX crystallite are at angles of0°, 70.5°, 109.5°,or180°relative to each other.The extraframework cations occupy distinct positions insidethe dehydrated zeolite cavities and represent energeticallypreferred sites for adsorption of molecular guest species on the†Department of Chemical Engineering.‡Department of Chemistry.§Present address: Max-Planck-Institut fu¨r Polymerforschung, D-55021Mainz, Germany.|Present address: Air Products and Chemicals, Inc., Allentown, PA18195.XAbstract published in AdVance ACS Abstracts, September 1, 1997.(1) Breck, D. W. Zeolite Molecular SieVes; Krieger: Malabar, 1974.(2) Jansen, J. C., Sto¨cker, M., Karge, H. G., Weitkamp, J., Eds. AdVancedZeolite Science and Applications; Elsevier Science B.V.: Amsterdam, 1994.(3) Weitkamp, J., Karge, H. G., Pfeifer, H., Ho¨lderich, W., Eds. Zeolitesand Related Microporous Materials: State of the Art 1994; Elsevier ScienceB.V.: Amsterdam, 1994.(4) Chen, N. Y.; Degnan, T. F., Jr.; Smith, C. M. Molecular Transportand Reaction in Zeolites; VCH Publishers, Inc.: New York, 1994.(5) Ka¨rger, J.; Ruthven, D. M. Diffusion in Zeolites and Other Mi-croporous Solids; Wiley: New York, 1992.(6) Meier, W. M.; Olson, D. H.; Baerlocher, C. Atlas of Zeolite StructureTypes, 4th ed.; Butterworth-Heinemann: Boston, 1996.(7) Vitale, G.; Bull, L. M.; Morris, R. E.; Cheetham, A. K.; Toby, B.H.; Coe, C. G.; MacDougall, J. E. J. Phys. Chem. 1995, 99, 16087-16092.(8) Loewenstein, W. Am Mineral. 1954, 39,92-96.9252 J. Am. Chem. Soc. 1997, 119, 9252-9267S0002-7863(97)01563-1 CCC: $14.00 © 1997 American Chemical Societyheterogeneous internal zeolite


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