Dehydroaromatization Pathway of Propane on PtZn/SiO₂ + ZSM‑5 Bifunctional Catalyst

The Cyclar process was previously developed to convert propane and butane into aromatics using gallium-loaded ZSM-5 catalysts (Ga/ZSM-5). However, the BTX (benzene, toluene, xylenes) yield is limited by light gas formation, primarily methane and ethane. In this study, relative rates and selectivity for propane conversion on two catalytic components, gallium (Ga/Al₂O₃) and acid ZSM-5 (H-ZSM-5), were investigated, and the results suggest that light gas was produced by propane monomolecular cracking on ZSM-5 due to the imbalance of alkane dehydrogenation and olefin conversion rates on two catalytic functions. A PtZn alloy catalyst, which has >99% propene selectivity and 100 times higher rate than Ga, was used for the dehydrogenation function. The bifunctional PtZn/SiO₂ + H-ZSM-5 catalyst has high yields of aromatics with low selectivity to methane (<5%) at ∼70% propane conversion. The results suggest that a light gas yield can be minimized by utilizing the PtZn alloy and lowering the monomolecular cracking rate by reducing the amount of ZSM-5. In addition, at 5 kPa and ∼65% propane conversion, the rate and selectivity of aromatics formation is around 10 times and 30% higher, respectively, on this bifunctional catalyst than ZSM-5. The aromatics formation pathway was investigated by studying the rate and selectivity of a model intermediate (cyclohexene) on ZSM-5, PtZn/SiO₂, and Ga/Al₂O₃. Benzene is formed at similar rates on Ga/Al₂O₃ and ZSM-5, but cracking of cyclohexene on the latter is 2 orders of magnitude higher than the benzene formation rate, indicating cracking of cyclic hydrocarbons leads to a low aromatization rate on Ga/ZSM-5. The benzene formation rate on PtZn/SiO₂ is 200 times higher than that on ZSM-5, suggesting aromatics are formed by the metal pathway on PtZn/SiO₂ + ZSM-5.