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.

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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.

Deoxygenation is an important reaction in the conversion of biomass-derived oxygenates to fuels and chemicals. A key route for biomass refining involves the production of pyrolysis oil through rapid heating of the raw biomass feedstock. Pyrolysis oil as produced is highly oxygenated, so the feasibility of this approach depends in large part on the ability to selectively deoxygenate pyrolysis oil components to create a stream of high-value finished products. Identification of catalytic materials that are active and selective for deoxygenation of pyrolysis oil components has therefore represented a major research area. One catalyst is rarely capable of performing the different types of elementary reaction steps required to deoxygenate biomass-derived compounds. For this reason, considerable attention has been placed on bifunctional catalysts, where two different active materials are used to provide catalytic sites for diverse reaction steps. Here, we review recent trends in the development of catalysts, with a focus on catalysts for which a bifunctional effect has been proposed. We summarize recent studies of hydrodeoxygenation (HDO) of pyrolysis oil and model compounds for a range of materials, including supported metal and bimetallic catalysts as well as transition metal oxides, sulfides, carbides, nitrides, and phosphides. Particular emphasis is placed on how catalyst structure can be related to performance via molecular-level mechanisms. These studies demonstrate the importance of catalyst bifunctionality, with each class of materials requiring hydrogenation and C-O scission sites to perform HDO at reasonable rates. OH R' RO Phenolics O OH R Acids O R Ketones and Aldehydes R" R'

Rationally designing and producing suitable catalysts to promote specific reaction pathways remains a major objective in heterogeneous catalysis. One approach involves using traditional catalytic materials modified with self-assembled monolayers (SAMs) to create a more favorable surface environment for specific product formation. A major advantage of SAM-based modifiers is their tendency to form consistent, highly ordered assembly structures on metal surfaces. In addition, both the attachment chemistry and tail structures can easily be tuned to facilitate specific interactions between reactants and the catalyst. In this Account, we summarize our recent modification approaches for tuning monolayer structure to improve catalytic performance for hydrogenation reactions on palladium and platinum catalysts. Each approach serves to direct selectivity by tuning a particular aspect of the system including the availability of specific active sites (active-site selection), intermolecular interactions between the reactants and modifiers (molecular recognition), and general steric or crowding effects. We have demonstrated that the tail moiety can be tuned to control the density of SAM modifiers on the surface. Infrared spectra of adsorbed CO probe molecules reveal that increasing the density of the thiols restricts the availability of contiguous active sites on catalyst terraces while maintaining accessibility to sites located at particle edges and steps. This technique was utilized to direct selectivity for the hydrogenation of furfural. Results obtained from SAM coatings with different surface densities indicated that, for this reaction, formation of the desirable products occurs primarily at particle edges and steps, whereas the undesired pathway occurs on particle terrace sites. As an alternative approach, the tail structure of the SAM precursor can be tuned to promote specific intermolecular interactions between the modifier and reactant in order to position reactant molecules in a desired orientation. This technique was utilized for the hydrogenation of cinnamaldehyde, which contains an aromatic phenyl moiety. By using a phenyl-containing SAM modifier with an appropriate tether length, > 90% selectivity toward reaction of the aldehyde group was achieved. In contrast, employing a modifier where the phenyl moiety was closer to the catalyst surface biased selectivity toward the hydrogenation of the C═C bond due to reorienting the molecule to a more "lying down" conformation. In addition to approaches that target specific interactions between the reactant and modified catalyst, we have demonstrated the use of SAMs to impose a steric or blocking effect, for example, during the hydrogenation of polyunsaturated fatty acids. The SAMs facilitated hydrogenation of polyunsaturated to monounsaturated fatty acids but inhibited further hydrogenation to the completely saturated species due to the sterically hindered, single "kink" shape of the monounsaturated product. The recent contributions discussed in this Account demonstrate the...

Surface chemical reactions of highly functional biomass derivatives such as furans with oxygenated ligands are often considered in terms of the chemistry of their individual functional groups, with little focus on how multifunctionality affects surface chemistry. To probe these effects on functionalized furans, temperature-programmed desorption (TPD) experiments and density functional theory (DFT) calculations were used to study the thermal chemistry of furfural, C4H3(CHO)O, and furfuryl alcohol, C4H3(CH2OH)O on Pd(111). The TPD results indicate that furfural undergoes decomposition to produce furan, propylene, carbon monoxide, and hydrogen. Furfuryl alcohol forms the same products but also undergoes an unexpected C–O scission process that yields methylfuran and water. Together with DFT calculations, these results indicate that furfuryl alcohol can decompose through a surface furfural intermediate, similar to the reaction pathway observed for simple alcohols such as ethanol. The additional methylfuran pathway, however, is not observed for simple alcohols. In addition, the production of propylene suggests that substitution of the furan ring strongly affects the available reaction pathways, since TPD of furan does not show any propylene evolution. TPD experiments conducted with coadsorbed deuterium provide additional information on the reaction mechanism and suggest that methylfuran formation may be assisted by interactions between adsorbates. Furthermore, observed trends in the isotopic product distribution together with a thermochemical reaction pathway constructed using DFT indicate that the presence of oxygenated pendant groups on the furan ring strongly influences the chemistry of the ring. The importance of these mechanisms for catalytic reactions of sugar derivatives such as 5-hydroxymethylfurfural are discussed.

Catalyst characterization and performance Figure S1. Infrared spectra of the hydrocarbon stretching region for C18 and BDT monolayers on 5 wt% Pd/Al 2 O 3 catalysts. The feature found at 2923 cm -1 for the C18 catalyst was attributed to a CH 2 methylene stretch. The feature at 3050 cm -1 for BDT was attributed to a CH stretch of the phenyl moiety.