Liquid phase hydrogenation, isomerization and dehydrogenation of limonene and derivatives with supported palladium catalysts

The hydrogenation of limonene, using 5% Pd/C, 5% Pd/Al2O3 and 0.5% Pd/Al2O3 catalysts, was studied in a stirred slurry minireactor in a temperature range of 273-323 K, at 106.65 kPa of hydrogen pressure. In the absence of extraparticle and intraparticle mass transfer effects, side products, such as terpinolene, gamma-terpinene and p-cymene, were formed besides p-menthene and p-menthane. A significant presence of terpinolene and gamma-terpinene indicates that an extensive isomerization precedes hydrogenation, while the presence of p-cymene shows that a dehydrogenation occurs simultaneously. The virtual absence of intermediates, such as alpha-phellandrene, beta-phellandrene and alpha-terpinene, can be attributed to the fact that conjugated cyclic dienes are too strongly adsorbed to desorb before reacting further. Possible mechanisms of reaction are discussed: the involvement of pi-allyl-adsorbed species explains the isomerization process over palladium, but greater isomerization activity observed for alumina-supported catalysts is explained by mechanisms involving Lewis acid sites provided by the alumina. Two possible pathways for isomerization on alumina are recognized: one of them involves the formation of carbonium ions and the other may comprise double-bond migration via allyl route including carbanions. The results suggest that, over alumina, limonene isomerizes via the pi-allylic mechanism, whereas terpinolene and terpinene isomerize via the carbonium ion mechanism. (C) 1999 Elsevier Science B.V. All rights reserved.