Comparison of homogeneous and heterogeneous palladium hydrogenation catalysts

Abstract

AbstractMechanistic and kinetic studies of Pd‐catalyzed hydrogenation at atmospheric pressure and 30–100 C were carried out with methyl sorbate, methyl linoleate and conjugated linoleate. Homogeneous Pd catalysts and particularly Pd‐acetylacetonate [Pd(acac)2] were significantly more selective than Pd/C in the hydrogenation of sorbate to hexanoates, mainlytrans‐2‐hexenoate. Relative rate constants for the different parallel and consecutive reactions, determined by computer simulation, indicated that the low diene selectivity of Pd/C can be dattributed to a significant direct reduction of sorbate to hexanoate. The similar behavior of PdCl2 to that of Pd/C suggests that Pd(II) was initially reduced to Pd(O). Valence stabilization of PbCl2 by adding DMF or a mixture of Ph3P and SnCl2 increased the diene selectivity but decreased the activity. Stabilization of Pd(acac)2 with triethylaluminum (Ziegler catalyst) resulted in increased activity but decreased selectivity. The kinetics of methyl linoleate hydrogenation showed that although Pd(acac)2 was only half as active as Pd/C, their respective diene selectivity was similar (10.4 and 9.6). The much greater reactivity of conjugated compared with unconjugated linoleate toward Pd(acac)2 suggests the possible formation of conjugated dienes as intermediates that are rapidly reduced and not detected in the lipid phase during hydrogenation.