Modified zeolite ZSM-5 for the methanol to aromatics reaction

Catalysts comprising zeolite ZSM-5 impregnated with precious metals including Ag, Cu, Ni, Pd, Ir and Ru, have been tested for the methanol to hydrocarbons reaction in a continuous flow fixed bed reactor. Comparison with the activity of unmodified ZSM-5 showed that Ag, Cu and Ni enhanced the selectivity to C 6-C 11 aromatic products by a factor of two or higher. Moreover, Ag/ZSM-5 showed improved selectivity for the C 6-C 7 fraction of aromatic products. Ni/ZSM-5 was found to be selective to naphthalene, while Cu/ZSM-5 was selective for C 9-C 11 aromatic products. It was ascertained that all the impregnated metals were present as metal oxides in the starting materials. It is therefore proposed that the enhanced selectivity to aromatic products is due to the interaction of the acid sites of the zeolite with the basic sites of the metal oxide at the edge of the zeolite crystals, as well as the possible coordination of propene molecules formed during the reaction, that are likely to be the building blocks for the formation of aromatics.