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

Citation
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
Citations number
20
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF MOLECULAR CATALYSIS A-CHEMICAL
ISSN journal
13811169 → ACNP
Volume
148
Issue
1-2
Year of publication
1999
Pages
203 - 214
Database
ISI
SICI code
1381-1169(199912)148:1-2<203:LPHIAD>2.0.ZU;2-0
Abstract
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) 1 999 Elsevier Science B.V. All rights reserved.