Catalyst performance of novel Pt/Mg(Ga)(Al)O catalysts for alkane dehydrogenation

The dehydrogenation of ethane and propane using a Pt catalyst supported on a novel Mg(Ga)(Al)O mixed oxide support was investigated. Catalyst performance is strongly dependent on Ga content in the support, a peak in activity for both ethane and propane dehydrogenation occurs at Ga/Pt = 1.4–5.4, and selectivity is a monotonic function of Ga/Pt, reaching nearly 100% at Ga/Pt = 5.4. The addition of hydrogen to the feed resulted in a peak in activity with respect to H2/alkane. The increase in dehydrogenation rate with H2 addition is attributed to H-atom-assisted dehydrogenation of alkyl species formed upon dissociative adsorption of the reactant alkane. Beyond the peak in activity with H2 addition, a further increase in H2 feed concentration contribute to alkene hydrogenation, thereby reducing the net rate of dehydrogenation. Hydrogen addition to the feed, however, had relatively little effect on alkene selectivity, which remained near 100%. The presence of Ga also suppressed coke formation. Interestingly, less coke was formed during propane dehydrogenation than ethane dehydrogenation, and no correlation was found between coke formation and catalyst deactivation. Thus, the extent of deactivation was lower for ethane than propane dehydrogenation, whereas the amount of coke deposited was higher in the former case. Since the amount of carbon deposited as coke is higher than the amount of exposed Pt, it is concluded that most of the coke resides on the support, and that only a small amount resides on the Pt particles. The higher level of deactivation seen during propane versus ethane dehydrogenation is attributed to a higher coverage of Pt by coke precursors derived from propane than ethane.