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MATERIALS RESEARCH

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Crystal defects of mordenite structures S. G. Songa) UOP LLC, Des Plaines, Illinois 60017 and United Technologies Research Center, East Hartford, Connecticut 06108 (Received 22 April 1998; accepted 1 March 1999)

Crystal defects in mordenite materials were observed and characterized by high-resolution electron microscopy (HREM). These defects are identified as line, planar, and 3D defects, which may result in pore blockage of the mordenite channel structure and be detrimental to their chemical properties for catalytic applications. The planar defects are the most severe ones affecting the structural properties of mordenite because of their density and scale. The main emphasis of this investigation is focused on the crystallographic aspects of these defects.

I. INTRODUCTION

In analogy to the mechanical significance of crystal defects of structural materials, lattice imperfections of zeolitic materials can markedly affect their chemical performance as catalytic agents. For a given chemical reaction in oil refining or petrochemical processes, a zeolite catalyst is selected because the combination of its pore structure and active sites provides the best selectivity for the desired process. Thus, changes in the structural characteristics of the zeolite materials due to the presence of defects can affect significantly the desired performance. Prior studies have shown that lattice defects such as stacking faults,1 coherent intergrowth boundaries,2 and surface amorphous layers of zeolite particles3 are consequential in determining the performance of the catalysts for the desired reactions. Thus, control over the extent of crystal defects of catalyst products is of importance in reaction design and quality improvement of a catalyst for selected applications. A review of the literature on zeolite structures indicates an apparent gap between our understanding of zeolite crystal structures and that of lattice defects. Since the first structural analysis of analcime,4 numerous zeolite structures have been studied and solved using x-ray diffraction methods. Less effort, however, has been given to the characterization of crystal defects of these materials, partly because (i) x-ray methods are insensitive to local lattice defects and (ii) the zeolitic materials are in general highly susceptible to radiation when studied by electron microscopy.5 In this study, lattice defects in mordenite structures were investigated using HREM. Various types of defects, namely dislocations, planar defects, and local intergrowth in mordenite, have been observed and chara)

Present address: United Technologies Research Center, 411 Silver Lane, East Hartford, Connecticut 06108. e-mail: [email protected]

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http://journals.cambridge.org

J. Mater. Res., Vol. 14, No. 6, Jun 1999

Downloaded: 23 Mar 2015

acterized. These defects can be categorized as one-, two-, and three-dimensional lattice defects. The main objective of the present investigation is to characterize the lattice defects and to establi