C 6-alkane conversion over γ-alumina supported palladium and platinum catalysts

Abstract

A comparable study of n-hexane and 2,2-dimethylbutane reactions in excess hydrogen carried out on differently loaded Pd/Al 2O 3 and Pt/Al 2O 3 catalysts at ≤290 °C furnished information which is useful in assessing the role of activated alumina in catalyzing C 6-alkane conversion. In particular, it was confirmed that in effect of high temperature reduction (at 600 °C) Al 2O 3 gains a considerable acidity which must play an important role in transformation of C 6-alkanes. This effect is more clearly seen for palladium than for platinum catalysts, the role of palladium is even overshadowed by the action of activated alumina. Platinum exhibits higher intrinsic activity than palladium, so the respective changes associated with the presence of activated alumina are not so pronounced for Pt/Al 2O 3. Highly reduced catalysts showed completely different performance than lowly reduced samples. In the case of palladium, the high temperature reduction leads to a big enhancement of the catalytic activity and also vastly increases the isomerization selectivity. These variations accompanied by a big decrease of activation energy (from ∼50 to 20 kcal/mol) manifest an important modification of the reaction mechanism which changes from “metallic” to “acidic” one. The 2-methylpentane/3-methylpentane product ratio in n-hexane conversion represents another useful diagnostic parameter in assessing the role of activated alumina. “Regeneration” of highly reduced catalysts realized by oxidation of highly reduced samples with subsequent low temperature reduction (at 3000 °C) brings about a partial recovery of metal functioning. Temperature-programmed desorption of pyridine from variously pretreated catalysts appeared a promising method in diagnosing changes in surface acidity and metal state. A drastic decrease of chemisorption capability of palladium upon high temperature reduction of Pd/Al 2O 3, in combination with a positive correlation of catalytic activity with the amount of alumina in the sample, suggests that a dominant part of C 6-alkane isomerization takes place on acid sites of alumina, whereas the role of palladium is largely limited to supplying active hydrogen via spillover.