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.