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

RICARDO GRAU; PATRICIA D. ZGOLICZ; CAROLINA GUTIERREZ; HUGO TAHER

JOURNAL OF MOLECULAR CATALYSIS A-CHEMICAL

ELSEVIER SCIENCE BV

Año: 1999 vol. 148 p. 203 - 214

1381-1169

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, γ-terpinene and p-cymene, were formed besides p-menthene and p-menthane. A significant presence of terpinolene and γ-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 α-phellandrene, β-phellandrene and α-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 π-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 π-allylic mechanism, whereas terpinolene and terpinene isomerize via the carbonium ion mechanism.