(307a) Quantum Chemical Methods for the Simulation of Reactions of Organic Molecules in Zeolites

Zeolites are used extensively to promote the cracking and reforming of hydrocarbons and the synthesis of various organic products. Given the large number of zeolite architectures and compositions, it would be highly desirable to develop a methodology for screening the effects of these variables on the zeolite activity for chosen chemical reactions. This talk will illustrate efforts devoted to this end. It will be shown that a hybrid quantum-mechanical/molecular mechanics (QM/MM) model can be used to assess the enthalpies of species adsorption and activation. Parameters for the MM portion of the model have been optimized and transportable from one zeolite to another. Using the QM/MM model, adsorption and activation enthalpies can be achieve routinely that are within a few kcal/mol of those found experimentally. More recently, a method has been developed for estimating entropies of adsorption and activation that are accurate to within a few cal/mol K of measured values. This method uses a quasi-rigid-rotor harmonic oscillator approximation and takes into account the symmetry factor for a given reaction. We will also show how these methods can be used to explain the experimentally observed effects of zeolite structure on alkane cracking and dehydrogenation. A brief discussion of future advancements will also be presented.