Recent applications of various in situ techniques have dramatically improved our understanding of the self-organization process of adsorbed molecular monolayers on solid surfaces. The process involves several steps, starting with bulk solution transport and surface adsorption and continuing with the two-dimensional organization on the substrate of interest. This later process can involve passage through one or more intermediate surface phases and can often be described using two-dimensional nucleation and growth models. A rich picture has emerged that combines elements of surfactant adsorption at interfaces and epitaxial growth with the additional complication of long-chain molecules with many degrees of freedom.

The selective reaction of one part of a bifunctional molecule is a fundamental challenge in heterogeneous catalysis and for many processes including the conversion of biomass-derived intermediates. Selective hydrogenation of unsaturated epoxides to saturated epoxides is particularly difficult given the reactivity of the strained epoxide ring, and traditional platinum group catalysts show low selectivities. We describe the preparation of highly selective Pd catalysts involving the deposition of n-alkanethiol self-assembled monolayer (SAM) coatings. These coatings improve the selectivity of 1-epoxybutane formation from 1-epoxy-3-butene on palladium catalysts from 11 to 94% at equivalent reaction conditions and conversions. Although sulphur species are generally considered to be indiscriminate catalyst poisons, the reaction rate to the desired product on a catalyst coated with a thiol was 40% of the rate on an uncoated catalyst. Interestingly the activity decreased for less-ordered SAMs with shorter chains. The behaviour of SAM-coated catalysts was compared with catalysts where surface sites were modified by carbon monoxide, hydrocarbons or sulphur atoms. The results suggest that the SAMs restrict sulphur coverage to enhance selectivity without significantly poisoning the activity of the desired reaction.

One key route for controlling reaction selectivity in heterogeneous catalysis is to prepare catalysts that exhibit only specific types of sites required for desired product formation. Here we show that alkanethiolate self-assembled monolayers with varying surface densities can be used to tune selectivity to desired hydrogenation and hydrodeoxygenation products during the reaction of furfural on supported palladium catalysts. Vibrational spectroscopic studies demonstrate that the selectivity improvement is achieved by controlling the availability of specific sites for the hydrogenation of furfural on supported palladium catalysts through the selection of an appropriate alkanethiolate. Increasing self-assembled monolayer density by controlling the steric bulk of the organic tail ligand restricts adsorption on terrace sites and dramatically increases selectivity to desired products furfuryl alcohol and methylfuran. This technique of active-site selection simultaneously serves both to enhance selectivity and provide insight into the reaction mechanism.

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.

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