Mechanistic studies through H-D exchange reactions: Propane aromatization in HZSM5 and Ga/HZSM5 catalysts

- HZSM5 shows both stepwise and multiple H-D exchange processes.
- Multiple exchange process in HZSM5 is responsible for propylene production.
- An α,β-diadsorbed intermediate is responsible for multiple exchange in Ga/HZSM5.
- Methane and ethane are formed by propane hydrogenolysis in Ga/HZSM5.
- Aromatics are formed after many reaction steps and are equilibrated with gas phase deuterium.

HZSM5 and Ga/HZSM5 catalysts were studied in propane H-D isotopic exchange at various temperatures and D2 gas partial pressures. Stepwise and multiple exchange were observed on both the pure zeolite and the Ga/HZSM5 catalyst. Initial product distribution simulations by a Monte Carlo algorithm showed that three processes were necessary to explain the observed patterns in all catalysts: one stepwise and two multiple exchange processes, where one of the multiple exchange processes was observed only on Ga/HZSM5. Almost all processes were negatively influenced by D2 partial pressure, indicating competition with propane for the active sites. Reaction mechanisms and intermediate species were proposed for all processes discussed. In the simultaneous H-D exchange and aromatization/hydrogenolysis reactions, it was observed that aromatics were produced after several dehydrogenation-oligomerization-cracking cycles, while methane and ethane were formed mainly by direct propane hydrogenolysis on the Ga+/GaH2+ pair.

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