Patricia D. Zgolicz

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

Abstract The hydrogenation of limonene, using 5% Pd/C, 5% Pd/Al 2 O 3 and 0.5% Pd/Al 2 O 3 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.

Selective Hydrogenation of Citral with Carbon Nanotubes Supported Bimetallic Catalysts Prepared by Deposition–Reduction in Liquid Phase and Conventional Impregnation Methods

This paper studies the preparation of Pt-based bimetallic (PtFe, PtSn, PtIn and PtGa) catalysts supported on carbon nanotubes for the citral hydrogenation by deposition–reduction in liquid phase and their comparison with the conventional impregnation method (CI). When conventionally impregnated, the selectivity to unsaturated alcohols (UA) highly increased from the monometallic to the bimetallic samples, whereas for liquid phase reduced ones, the selectivity to UA was rather similar for both Pt and bimetallic samples except for PtFe ones and for the PtSn catalyst treated treated with N2. The results were explained taken into account the intimate interaction achieved between the metals according to the preparation method and the formation of Pt–promotor complexes in the impregnating solution.Graphical Abstract