Anisotropic Xe Chemical Shifts in Zeolites. The Role of Intra- and Intercrystallite Diffusion Cynthia J. Jameson* Department of Chemistry, UniVersity of Illinois at Chicago, 845 West Taylor, Chicago, Illinois 60607 A. Keith Jameson Department of Chemistry, Loyola UniVersity, Chicago, Illinois 60626 Rex E. Gerald, II Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439 Hyung-Mi Lim Department of Chemistry, UniVersity of Illinois at Chicago, 845 West Taylor, Chicago, Illinois 60607 ReceiVed: March 20, 1997; In Final Form: July 11, 1997 X We provide observations and interpretations of 129 Xe chemical shifts in open zeolite networks in which the Xe is in fast exchange among a large number of cavities and channels, in two zeolite types different from the A types that we have used before, and provide quantitative comparisons of adsorption isotherms and average 129 Xe chemical shifts using grand canonical Monte Carlo (GCMC) simulations employing the same methods and shielding functions used previously. We examine the temperature dependence of the Xe chemical shift at a fixed loading in NaY and silicalite and compare the predicted behavior with that of Xe n in the R cages of NaA. Furthermore, we consider the most general case where the chemical shift reported by a Xe atom includes sampling environments in several crystallites and intercrystalline gas during acquisition of the NMR signal. I. Introduction The extremely high sensitivity of the 129 Xe NMR chemical shift to its environment has made the Xe atom a widely used probe in the characterization of microporous materials such as zeolites, 1-3 polymers, graphite, coals, and other materials. In zeolites the 129 Xe chemical shift is known empirically to depend on zeolite pore and channel dimensions, 4-8 on its Al/Si ratio, 9,10 cation distribution, 11 location of cations, 12 coadsorbed molecules, dispersed metal atoms, and paramagnetic ions, blockage of pores by coking, and domains of different composition or crystallinity. An understanding of the sensitivity of the chemical shift to these parameters is crucial to the quantitative application of the empirical observations. The relationship of the 129 Xe chemical shift to cavity size and shape, average Xe loading, temperature, and type of cations in zeolites is still not completely understood. If we can understand these relationships quantitatively, we should be able to make a clearer connection between the average chemical shift of the Xe signal under fast exchange with various parameters of the zeolite structure, although it may not be possible to predict a unique structure just from the average chemical shift. The difficulties associated with establishing these relationships arise from the fact that the observed chemical shift in these open zeolite networks is the result of averaging over various environ- ments. The established techniques of molecular dynamics and Monte Carlo computer simulations at the grand canonical level can provide details of distributions and dynamics of sorbate molecules. However, we had deferred using simulations to model the average chemical shift of the single Xe peak under fast exchange and its dependence on the temperature and loading until we have tested our methods with more detailed observa- tions. To establish the connection between the observed chemical shift and specific attributes of the environment (such as cavity size and siting of cations), we have adopted the following strategy: (a) First make measurements in well-defined environments, propose a model for chemical shifts in these well- defined environments, and then test our ability to simulate the chemical shifts in these by using some statistical averaging method with our chemical shift model. We start with a sufficiently well-defined environment, a single zeolite cavity in which the locations of the atoms and ions are known independently, where the averaging of the chemical shift for a nucleus in a probe molecule trapped inside the cavity can be carried out using the chemical shift model, provided that a reliable description of the interaction between the probe molecule and the atoms and ions of the zeolite cavity is available. The averaging in this case is within one cavity. By experimentally changing the type and siting of the ions in this cavity while keeping the structure of the zeolite framework essentially unchanged, the chemical shift model and the descrip- tion of the interaction can be tested against the experimentally observed chemical shifts. The chemical shifts in this case are for a fixed occupancy (n Xe atoms/cage is the occupancy associated with the 129 Xe chemical shift of Xe n ). Thus, chemical shifts in cages with identical occupancies and differing only in the type and siting of cations can be compared directly. Variable temperature measurements of the chemical shift averaged over a single cavity for a fixed occupancy (Xe n ) provide additional stringent tests. The average chemical shift in this case contains information about the one-body and two-body distributions of the probe molecules within a single cavity. How such a distribution is affected by type and siting of ions and by X Abstract published in AdVance ACS Abstracts, September 15, 1997. 8418 J. Phys. Chem. B 1997, 101, 8418-8437 S1089-5647(97)01013-4 CCC: $14.00 © 1997 American Chemical Society