Rj. Grau et al., Liquid phase hydrogenation, isomerization and dehydrogenation of limonene and derivatives with supported palladium catalysts, J MOL CAT A, 148(1-2), 1999, pp. 203-214
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 ra
nge of 273-323 K, at 106.65 kPa of hydrogen pressure. In the absence of ext
raparticle and intraparticle mass transfer effects, side products, such as
terpinolene, gamma-terpinene and p-cymene, were formed besides p-menthene a
nd p-menthane. A significant presence of terpinolene and gamma-terpinene in
dicates that an extensive isomerization precedes hydrogenation, while the p
resence of p-cymene shows that a dehydrogenation occurs simultaneously. The
virtual absence of intermediates, such as alpha-phellandrene, beta-phellan
drene and alpha-terpinene, can be attributed to the fact that conjugated cy
clic dienes are too strongly adsorbed to desorb before reacting further. Po
ssible mechanisms of reaction are discussed: the involvement of pi-allyl-ad
sorbed species explains the isomerization process over palladium, but great
er isomerization activity observed for alumina-supported catalysts is expla
ined 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 th
at, over alumina, limonene isomerizes via the pi-allylic mechanism, whereas
terpinolene and terpinene isomerize via the carbonium ion mechanism. (C) 1
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