College Park, Maryland June 6 - 10 , 2004 |
M2-D1 (10:30 AM): Structural Refinement of Disordered Zeolites Using the Pair-Distribution Function Method (Invited)
I. Peral (Department of Chemical Engineering, University of Delaware, Newark, DE 19716; NIST Center for Neutron Research), M.M. Martínez-Iñesta, R..F. Lobo (Department of Chemical Engineering, University of Delaware, Newark, DE 19716), T. Proffen (Los Alamos Neutron Science Center, Los Alamos National Laboratory)
Structural disorder is a common fact in zeolite materials and the methods commonly used for the refinement of zeolite structures (Rietveld refinements and single crystal structures) are unsuitable to obtain detailed atomic information of the disordered counterparts. To address this need, we have applied the Pair Distribution Function Method (PDF), an experimental function that describes the distribution of the atomic distances of a material, to zeolite beta, an important zeolite catalyst in the petrochemical industry. Zeolite beta is a high-silica 12-ring pore zeolite containing a three-dimensional pore system. The structure of beta was conceived as an intergrowth of two crystalline (hypothetical) polytypes A and B. The pores in polytype A are arranged in an ABABAB… type configuration and in polytype B they are arranged in an ABCABC… type configuration. We were able to refine the structure of both polytypes of zeolite beta by combining refinements with the neutron and synchrotron pair distribution functions. Comparing the calculated neutron PDFs, we observe a large general improvement in the fit of the experimental neutron-PDF from the accepted structural model to our refined model as seen in the difference curves. It is, however, necessary to refine both neutron and x-ray PDFs sequentially to obtain reasonable models of the zeolite structures.
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