(207c) A Novel Framework for 3-Dimensional Shape-Induced Reaction Selectivity in Zeolite Structures
Zeolites are microporous crystalline materials used for catalysis and separation applications due to their selectivity to molecular shape. While there are nearly 200 known natural and synthetic zeolite frameworks , as well as millions of theoretically plausible zeolite-like structures , only a few of these are commonly used in industry. Previous work [3,4,5] has developed a systematic computational approach to identify candidate zeolites for selectivity applications by considering the shapes and sizes of zeolite windows. For pure separations, such an approach provides sufficient insight into a zeolite's ability to serve as an efficient adsorbent, as this is mainly determined by which molecules can pass through the windows and enter into the pores of the structure. However, in the context of catalysis applications, additional criteria, such as the ability of the appropriate transition states to be formed near the catalytically active site, play a significant role in determining reaction selectivity.
In this paper, we develop a new computational framework for analysis, based on geometry and graph algorithms, to fully characterize the three-dimensional porous structure of zeolites. All void spaces that can potentially accommodate a guest molecule are identified and classified into interconnected channels and cages. This complete description is then used to locate energetically favorable regions in which a transition state of interest, computed using electronic structure calculations [6,7], can form. Combined with our prior work [3,4,5] regarding the ability of the reactants and products to enter and exit the structures, we are able to determine the potential of a zeolite to catalyze a reaction of interest. Finally, by applying our methodology over a large database of structures, we can identify ones with the greatest potential to be selective catalysts of a given reaction.
We demonstrate our approach through several computational studies including the well-known reaction of toluene disproportionation [8,9,10], in which the methyl groups of two toluene molecules are rearranged to form benzene and xylene. We show that some structures exhibit different favorability toward accommodating the various competing transition states of this reaction, effectively leading to different reaction selectivity toward the three xylene isomers. Our results come in good agreement with experimental observation and support hypotheses appearing in the literature  on why certain structures, such as the well-known ZSM-5, exhibit increased selectivity toward para-xylene, which is typically the desirable product isomer.
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