Effect of reductive and oxidative atmospheres on the propane aromatisation activity and selectivity of Ga/H-ZSM-5 catalysts

The aromatisation of propane was investigated at 823 K on two Ga/H-ZSM-5 catalysts differing by their aluminum content (Si/Al = 18 and 128, respectively), i.e. their Bronsted acid site concentration, and having similar gallium content (1.6 wt%) as a function of successive pretreatments by hydrogen and oxygen. Hydrogen treatment increases activity and selectivity to aromatics while decreasing the formation of methane. Subsequent oxygen treatment enhances further the activity and aromatics selectivity and has little effect on methane selectivity. These improvements of catalytic performance are explained by gallium migration in reducing conditions. The concentration of highly dispersed gallium species is thereby enhanced. These species, in association with Bronsted centers, were reported previously to constitute the dual catalytic sites responsible for alkane activation. Hydrogen-oxygen pretreatment of Ga/H-ZSM-5 catalysts is thus a means to ensure rapid catalyst preactivation and stabilisation in large scale light alkane aromatisation units.