Application of Density Functional Theory to Al Distribution in Mordenite
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APPLICATION OF DENSITY FUNCTIONAL THEORY TO AI DISTRIBUTION IN MORDENITE
G. FITZGERALD*, C.G. COE**, H.C. KLOTZ**, and J.E. MACDOUGALL** *Cray Research, Inc. 655-E, Lone Oak Drive, Eagan, MN 55121 **Air Products and Chemicals, 7201 Hamilton Boulevard, Allentown, PA 18195
ABSTRACT Mordenite zeolites are well known for their acid cracking abilities, but are often hindered by coke formation which limits their life. The dealumination and generation of Bronsted acid sites in the MOR structure will have significant implications as to their catalytic activity. Density functional theory (DFT) has been applied to determine preferred aluminum siting in the Namordenite zeolite structure. DFT correctly reproduces the 6-31IG* Hartree-Fock predictions for the relative stabilities of the four T sites of mordenite in a T(OH) 4 cluster. Larger cluster sizes influence the preferred Al siting with DFT' predicting T2 as the preferred tetrahedral site. Since DFT computational requirements are considerably smaller than Hartree-Fock theory, it is an attractive alternative for studying zeolites. The calculations performed were based on fixed geometries obtained from crystallographic data. Further work employing local geometry optimization in the vicinity of the substituted Al is underway.
Introduction There are many recent examples of the use of molecular modelling to investigate properties of inorganic crystalline systems [ 1, 2]. These calculation methods range from ab initio and semiempirical calculations of model clusters [3 - 5] to molecular mechanics, monte carlo, and molecular dynamics calculations of periodic crystal structures [1, 6 ,7]. Inorganic non-metallic microporous crystals display a rich range of compositions and have practical utility in both adsorption and catalysis. One of the most interesting classes of microporous oxides ,are the tectosilicates, and more particularly the zeolites, which contain framework aluminum and associated extra framework cations. Ab initio calculations based on the Hartree-Fock approximation have been used by Deroune et. al. [3, 8] to analyze the relative energetics of aluminum sitting in various T sites for a number of different zeolite frameworks. Although Hartree-Fock methods allow the prediction of many chemical properties, calculations are limited to relatively few atoms because of computational power limitations. To overcome the computational limitations, semi-empirical molecular orbital calculations have also been applied to analyze preferred aluminum sitting in H-ZSM-5 [5]; however, these methods require parameterization and their accuracy for zeolite systems is not that well tested. Density functional theory is a third alternative quantum method that has possible advantages over both Hartree-Fock and semi-empirical methods. Since it is an ab initio method which includes electron correlation, it requires no parameterization. Also, the computational requirements of density functional theory are low so large clusters (up to about 100 atoms) can be simulated. In this paper, we report ou
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