Selective Hydrogenation of Polyunsaturated Fatty Acids Using Alkanethiol Self-Assembled Monolayer-Coated Pd/Al₂O₃ Catalysts

Pd/Al2O3 catalysts coated with various thiolate self-assembled monolayers (SAMs) were used to direct the partial hydrogenation of 18-carbon polyunsaturated fatty acids, yielding a product stream enriched in monounsaturated fatty acids (with low saturated fatty acid content), a favorable result for increasing the oxidative stability of biodiesel. The uncoated Pd/Al2O3 catalyst quickly saturated all fatty acid reactants under hydrogenation conditions, but the addition of alkanethiol SAMs markedly increased the reaction selectivity to the monounsaturated product oleic acid to a level of 80–90%, even at conversions >70%. This effect, which is attributed to steric effects between the SAMs and reactants, was consistent with the relative consumption rates of linoleic and oleic acid using alkanethiol-coated and uncoated Pd/Al2O3 catalysts. With an uncoated Pd/Al2O3 catalyst, each fatty acid, regardless of its degree of saturation had a reaction rate of ∼0.2 mol reactant consumed per mole of surface palladium per second. Using alkanethiol-coated Pd/Al2O3 catalysts, the activity was reduced by a factor of 4 for polyunsaturated reactants and by a factor of 100 for the monounsaturated reactants. In contrast to the hydrophobic alkanethiol modifiers, hydrophilic thioglycerol SAM modifiers were found to strongly inhibit reaction kinetics.