Abstract

This dissertation presents the conception, synthesis, optimization and evaluation of a novel class of catalysts used in aromatization of short chain hydrocarbons. It was found that when an acidic H-ZSM-5 zeolite with Si/Al atomic ratio equal to about 40, was mixed with a ZnO based co-catalyst, aromatization selectivity followed the principles of synergy. For a feed made up of mostly ethylene, as generated in a bench scale propane steam-cracker, ZnO precipitate in the amount of 5wt.-% or ZnO/Al$\sb2$O$\sb3$ co-precipitate in the amounts of 5-20wt.-%, provided optimum catalytic activity. Furthermore, it was established that for the co-precipitate, a Zn/Al atomic ratio equal to or greater than 1.0 conferred the best performance. The distance between acid (aromatizing) major component and oxide minor component, was evaluated as being greater than 1$\mu$m. The increased aromatization selectivity found in hybrid systems was explained by the migration of hydrogen adsorbed species across surface boundaries. A Hydrogen Back-Spillover model was devised. Sink and scavenging actions on co-catalyst surfaces, resulting in the formation of more important amounts of molecular hydrogen or ethane, were investigated. The differences in intrinsic selectivities of ZnO precipitate and ZnO/alumina co-precipitate (Zn/Al = 1.0) were explained. The nature of the active sites was deduced from physico-chemical characterization data correlated with catalytic activity experiments. The influence of preparation techniques as well as starting reagents was studied. The disordered nature of the co-precipitate, which is favorable to surface restructuration during induction, was found to be a pre-requisite. Kinetic studies of ethylene aromatization on a pure H-ZSM-5 zeolite, yielded a negative apparent activation energy, indicative of intracrystallite diffusional limitations. Thermogravimetric studies of used catalysts indicated that soluble coke was trapped inside the zeolite pores when the catalyst was used in its pure form at high temperatures. Migration of C$\sb9\sp+$ aromatic coke precursors out of ZSM-5 channels is induced by the co-catalyst, which gives rise to synergy in hybrid systems. A model based on transport barriers was proposed to explain hydrogen and coke migration.