Catalytic reactions of n-pentylbenzene and 2-phenylpentane

Abstract

n-Pentylbenzene and 2-phenylpentane were reacted over a series of catalysts covering a wide range of acidic and dehydrogenation activities. Acid-catalyzed cyclization favors the formation of six-membered rings provided that the same type of carbonium ion intermediate is involved in both six- and five-membered cyclization processes. Side-chain branching increases the rate of acid-catalyzed cyclization. Dehydrogenation of the alkylbenzene could be the rate-controlling step in acid-catalyzed cyclizations. Pt-catalyzed cyclization of n-alkylbenzenes forms about equal amounts of five- and six-membered rings. Five-membered ring cyclization is favored with secondary alkylbenzenes. The Pt does not function as an acid when catalyzing cyclization of alkylbenzenes. Isomerization may be catalyzed by acids or by the Pt metal. Pt-catalyzed isomerizations proceed either via cyclic intermediates or through 1,3-diadsorbed intermediates. In certain Pt-catalyzed isomerizations, a 1,4-diadsorbed intermediate might also be involved. At 371 °C, acid-catalyzed cracking rates of a secondary side chain are about an order of magnitude higher than that of a normal side chain. Pt-catalyzed hydrogenolysis affects the side-chain bonds of n-alkylbenzenes at an approximately equal rate. In secondary alkylbenzenes, cleavage of the bond between the tertiary carbon atom and the ring is preferred. This work completely confirmed the reaction mechanisms suggested by our earlier n-butylbenzene work; namely, (a) in addition to its dehydrogenation activity, Pt has considerable cyclization, isomerization, and hydrogenolysis activities; (b) cyclization occurs on both acid and metal sites; and (c) in the acid-catalyzed cyclization process, the stability of the carbonium ion intermediate determines whether cyclization forms five- or six-membered ring products.