The specificity of chemical reactions conducted over solid catalysts can potentially be improved by utilizing noncovalent interactions to direct reactant binding geometry. Here we apply thiolate self-assembled monolayers (SAMs) with an appropriate structure to Pt/Al2O3 catalysts to selectively orient the reactant molecule cinnamaldehyde in a configuration associated with hydrogenation to the desired product cinnamyl alcohol. While nonspecific effects on the surface active site were shown to generally enhance selectivity, specific aromatic stacking interactions between the phenyl ring of cinnamaldehyde and phenylated SAMs allowed tuning of reaction selectivity without compromising the rate of desired product formation. Infrared spectroscopy showed that increased selectivity was a result of favorable orientation of the reactant on the catalyst surface. In contrast, hydrogenation of an unsaturated aldehyde without a phenyl ring showed a nontunable improvement in selectivity, indicating that thiol SAMs can improve reaction selectivity through a combination of nonspecific surface effects and ligand-specific near-surface effects.

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.

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Thiolate self-assembled monolayers have recently been demonstrated to be effective catalyst modifiers for selectivity control, but these studies have not extensively explored the long term stability of these modifiers or the effects of specific reaction conditions. Here we investigate how the performance of thiolate-modified Pt/Al 2 O 3 catalysts is affected by recycling and regeneration, using the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol as a probe reaction. Although modification of Pt catalysts with 3-phenyl-1-propanethiol results in high selectivity due to a ligand-specific interaction between modifier and reactant during the first use of a given catalyst, repeated recycling was shown to decrease the efficacy of this mechanism due to increasing disorder in the monolayer. However, selectivity and order could be stabilized using a thiol regeneration step or by feeding dilute concentrations of thiols in the reaction mixture. Similarly, aging in air was shown to decrease the order of the thiol and reduced the selectivity improvement of both a 3-phenyl-1-propanethiol and an octadecanethiol (C18) modified Pt/Al 2 O 3 catalyst. These studies show that the ligand-specific interactions between the SAM and the reactant are particularly sensitive to conditions that can degrade the monolayer.

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