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

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

JOURNAL OF MOLECULAR CATALYSIS A-CHEMICAL

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 1999 vol. 148 p. 203 - 214

1381-1169

Revista: Original Report ISSN: 1381-1169 - JOURNAL OF MOLECULAR CATALYSIS A-CHEMICAL Formerly part of Journal of Molecular Catalysis; Continued as Molecular Catalysis. The hydrogenation of limonene, using 5% Pd/C, 5% Pd/Al2 O3 and 0.5% Pd /Al 2O3 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, g-terpinene and p-cymene, were formed besides p-menthene and p-menthane. A significant presence of terpinolene and g-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 a-phellandrene, b-phellandrene and a-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 p-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 p-allylic mechanism, whereas terpinolene and terpinene isomerize via the carbonium ion mechanism.