Catalytic aspects in the transformation of pinenes to p-cymene

The reaction investigated in this work is the dehydrogenation of alpha -pinene to p-cymene over carriers impregnated with Pd. An optimal acid strength is required to cleave selectively the C-C bond in the cyclobutane ring of oc-pinene. Too strong acid sites such as in zeolites favor side reactions like oligomerization and cracking. Too weak acid sites fail to cleave the aforementioned C-C bond and rapid hydrogenation of the alpha -pinene is a consequence. Hydrogenolysis is also a major side reaction leading to tetramethylcyclohexanes. A reaction mechanism is proposed in which first isomerization is involved followed by hydrogenation/dehydrogenation to stabilize the components. The catalyst has a dual-functionality with the acid sites in charge of isomerization and the metallic sites responsible of hydrogenation/dehydrogenation. The use of crude sulfate turpentine (CST) as raw material shows that beta -pinene has a similar reactivity as alpha -pinene and high yields of p-cymene can be obtained from this cheap starting material. The sulfur remains however a major drawback. (C) 2001 Elsevier Science B.V. All rights reserved.