Propane conversion over a H-ZSM5 acid catalyst: Part 1. Observed kinetics

Propane conversion over H-ZSM5 zeolite follows two parallel reaction pathways: monomolecular cracking/dehydrogenation prevailing at high temperatures and low propane pressure involving pentacoordinated carbonium ions; bimolecular classical cracking through carbenium chain carriers is enhanced at low temperatures and high propane pressures. Dehydrogenation reactions are favored at low temperatures, while at higher temperatures cracking dominates. This is the first of a series of papers concerning the transformation of propane over a H-ZSM5 catalyst comprising experimental data, a kinetic model, and molecular dynamics calculations. The aim of this work is to provide a more fundamental insight on the catalytic processes involving light alkanes activation over solid acid catalysts. Experimental data for propane cracking was collected in the temperature range 623–773 K and low propane feed partial pressures varying from 3.0 to 9.1 kPa. The results show the existence of two parallel reaction pathways: (1) two monomolecular initiation steps (protolytic cracking or dehydrogenation), characterized by a relatively high activation energy, which becomes predominant at low conversions and high temperatures. Bond rupture may occur on either a CC or CH position leading to stoichiometric amounts of methane and ethene, or hydrogen and propene, respectively, when extrapolated at zero conversion; (2) a bimolecular route (classical cracking mechanism) with lower activation energy which involves carbenium ions chain carriers, and whose relative importance grows with increasing conversion and decreasing temperature, as secondary products, mainly olefins, become important. It is also clear that dehydrogenation reactions are favored at low temperatures, while at higher temperatures cracking is the dominant reaction pathway.