Aromatization of short chain alkanes on zeolite catalysts

On MFI catalysts, low cost liquefied petroleum gas (LPG ) can be transformed into valuable aromatics (mainly C6-C8 benzenics) and into hydrogen. Unfortunately methane and ethane are also produced in significant amounts. A reduction of the production of these unwanted compounds would render the aromatization process economically more attractive. The reaction pathways of propane aromatization were established on H-ZSM-5 pure or loaded with platinum or with gallium. On H-ZSM-5 the first step is the dehydrogenation and the cracking of the reactant through carbonium ion intermediates. The resulting alkenes (propene and ethylene) undergo rapid successive reactions via carbenium ion intermediates: oligomerization, cyclization, hydrogen transfer. The selectivity to aromatics is limited because of the formation of methane by propane cracking and of alkanes by hydrogen transfer. Platinum increases the rate of propane transformation significantly but a higher production of methane and ethane is found on PtH-ZSM-5 catalysts, owing to the hydrogenolysis of alkanes and of alkylaromatics and to the hydrogenation of ethylene on the platinum sites. Gallium improves both the rate and the selectivity of propane aromatization. The aromatization occurs, like on PtH-ZSM-5, through a bifunctional pathway, gallium catalyzing the dehydrogenation of the alkane reactant to alkenes and of naphthenic intermediates to aromatics, and the acid sites catalyzing the oligomerization of light alkenes and the cyclization of C6-C8 alkenes. The better selectivity to aromatics is obtained for GaMFI catalysts with gallium species well dispersed within the zeolite and being very active for dehydrogenation (e.g. with gallosilicates or galloaluminosilicates steamed under mild conditions). At high temperature, in presence of hydrogen (hence during aromatization) GaMFI catalysts would undergo various modifications: reduction of gallium species, migration within the zeolite, reaction of these species with the protonic sites. The nature of the gallium active species and their role in the various steps are discussed.