Theoretical Study On The Reactivity Of Surface Methoxy Species Over Acidic Zeolite Catalysts

Methanol is an important raw material, which can be viewed as a basic C1building block for chemical industry. Surface methoxy species were experimentally observed upon a large variety of reactions involving methanol on acidic zeolites. Since understanding of the reaction mechanisms is vital to the design of new and efficient catalysts for industrial processes, the research on the reactivity of surface methoxy species and their potential roles in the reaction processes is of great academic and practical importance.In this thesis, the reaction mechanisms of methylamines synthesis and propane activation involving surface methoxy species on acidic zeolites have been studied by quantum chemical calculations. The reactivity and the possible intermediate effect of surface methoxy species in the reactions have been discussed. Accordingly, the main contents can be divided into the following two parts:(1) The mechanism of methylamines synthesis on acidic zeolites has been studied by the DFT approach with a14T cluster model of H-Y zeolite. The computational results show that the reaction of methanol and ammonia can form methylamines via consecutive or stepwise reaction mechanisms, among which the trimolecular reaction pathway of consecutive mechanisms is most favorable. The surface methoxy species, which show high activity as methylating reagents when reacting with ammonia, may act as reactive intermediates in the process. The produced methylamines are adsorbed on zeolite surface, and the direct desorption of the products is difficult. However, the desorption with the assistance of ammonia becomes much easier. Meanwhile, disproportionation reactions of nitrogen-containing species, accompanied with scavenging desorptions of methylamine, also take place in the reaction system, and the disproportionation reactions favor the formation of highly substituted amines. (2) The reactivity of surface methoxy species for propane activation has been studied by ONIOM method with a46T cluster model of H-ZSM-5. The computational results show that the hydride transfer reaction between surface methoxy species and propane is much more favorable for the methylene group rather than the methyl group of propane, and the produced propene would continue to react with surface methoxy species to generate methylcyclopropane intermediate. In addition, hydrogen transfer is the rate-determining step of the whole process.