Scope of Zeolite Modeling

This subsection gives a short overview of the general themes from the literature, showing how simulations can be of help in zeolite research. 

Molecular dynamics has been used to study dyamics of zeolite frameworks, particularly fluctuations in zeolite aperture dimensions, whereby the zeolite framework can breathe substantially depending on the topology. For example, some O—O distances across the window were found to vary by as 

The catalytic properties of zeolites can be influenced by rearranging the Si and A1 atoms in the framework, modifying the Si/Al ratio, or by introducing other metal atoms into the framework (the latter is called isomorphous framework substitution). Knowledge of the location of the substitution site is desirable to better understand the catalytic behavior of substituted zeolites. This information is hard to obtain experimentally, so modeling studies have been devoted to isomorphous framework substitution. Some recent examples are given in Refs. 22-24. 

ZSM-5 (Al-MFI) is used as a catalyst in petroleum refining, in the production of synthetic fuels, and in other petrochemical processes, whereas TS-1 (Ti-MFI) is applied as a catalyst in fine chemical processes. The orthorhombic MFI structure exhibits 12 crystallographically unique tetrahedral sites. Calculations have been carried out on substitution preferences using classical as well as quantum models. In most studies 12 simulations were conducted, and in each run, one or more crystallographically equivalent sites of the subsequently crystallographically unique tetrahedral sites were substituted. Energy minimization and molecular dynamics techniques were employed to calculate 

Employing a classical model, Monte Carlo calculations were performed to investigate the A1 distribution in zeolites. Si and A1 atoms were randomly distributed over the tetrahedral sites, and in each step an attempt was made to interchange each A1 with a nearby Si. Loewenstein s rule, which forbids Al-O-Al bridging, was investigated, and results from the simulation indicate that the A1 distribution in zeolite structures is generally in accordance with this rule. 

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