4.2 Aromatization of Short Chain Alkanes on Ga MFI Catalysts

Abstract

Propane aromatization on GaZSM5 catalysts was reexamined in order to clarify the nature and the role of gallium species. Kinetic modeling allows to conclude that gallium species play a significant role in the dehydrogenation of propane, of C6-C9 alkenes and of cycloalkenes and also catalyze the formation of methane from propane and the hydrogenation of ethene. These two latter reactions are responsible for limiting aromatic selectivity. The effect of calcination of gallosilicates on their catalytic properties shows that framework gallium species are much less active for dehydrogenation than extraframework species and indeed are probably inactive. Model reactions show that hydrogen treatment at 873 K of a Ga2O3-HZSM5 mixture causes a significant increase in the dehydrogenating and hydrogenolysis activities of gallium but only a small decrease in the protonic activity of the zeolite. The great activity and selectivity to aromatics of hydrogen pretreated catalysts can be explained through the bifunctional scheme of propane aromatization.