Reaction pathways for the aromatization of paraffins in the presence of H-ZSM-5 and Zn/H-ZSM-5

Abstract

The effect of the dehydrogenating component of H-ZSM-5 and Zn/H-ZSM-5 catalysts on n-heptane aromatization reaction was studied. An increase in the yield of aromatics observed over Zn/H-ZSM-5 is explained by the enhanced production of olefins, facilitating the aromatization reaction by the effective dehydrogenating action of zinc on paraffins. A significant change in the selectivities of toluene and C₉₊ aromatics was observed after addition of zinc. The increase in the yield of toluene observed over Zn/H-ZSM-5 from both n-heptane and Bombay High (BH) light naphtha indicates the possible occurrence of direct dehydrocyclization of paraffins in the presence of zinc. Zinc also facilitates the effective conversion of C₆-C₈ oligomers to the corresponding aromatics, an in that way suppresses the further oligomerization of C₆-C₈ oligomers on one hand, and cracking of C₆-C₈ oligomers on the other; which is reflected in the decreased yields of fuel gas and C₉₊ aromatics in the product. The product pattern of the BH light naphtha aromatization suggests that the presence of zinc also suppresses the acid catalyzed alkyl transfer reactions which generally take place over H-ZSM-5. Reaction pathways have been established to explain the selectivity changes that occurred in presence of zinc.