Catalytic conversion of C₆-alkanes over Pd/Al₂O₃ catalysts: The effect of support acidity

A comparable study of catalytic conversion of two C₆-alkanes (n-hexane and 2,2-dimethybutane) has shown several analogies in the catalytic behavior of a series of differently loaded, chlorine-free Pd/Al₂O₃ catalysts. Different sample pretreatments have been used to modify the catalytic behavior. The catalytic performance of lowly reduced (at 573K) samples of Pd/Al₂O₃ catalysts is similar to that exhibited by unsupported or silica-supported palladium, i.e. the predominance of metal-catalyzed reactions is observed with high hydrogenolysis selectivity and a bond-shift mode of isomerization. Highly reduced (at 873K) samples experience a serious increase in overall activity, and simultaneously, big enhancement of isomerization selectivity (at ≤563K), in agreement with previous reports. At the same time, the activity is strongly correlated with the amount of activated alumina in the catalyst and only insignificantly dependent on Pd content in the catalyst. TPD of ammonia has been employed to confirm expected changes in the acidity generated by various catalyst pretreatments. It is considered that Lewis acidity of alumina, increased upon high temperature reduction (HTR), is a source of enhanced isomerization activity exhibited by Pd/Al₂O₃ catalysts. However, a palladium-free alumina cannot serve as efficient catalyst in alkane hydroconversion. Metallic function is needed to provide active hydrogen for a hydrogenative desorption of rearranged hydrocarbon species from acid sites of γ-Al₂O₃.