(166b) Control of Furfural Catalytic Hydrogenation Selectivity Using Alkanethiolate Self-Assembled Monolayers

Furfurals represent an important class of platform chemicals in the conversion of biomass to fuels and other valuable products. These highly reactive molecules present a challenge for selectivity control of heterogeneous catalytic hydrogenation due to the presence of multiple functional groups, all of which can interact with the catalyst surface. This multifunctionality results in a diverse range of products under hydrogenation conditions. In particular for group 10 metals, the furan ring adsorbs strongly to the surface, resulting in undesirable furfural decarbonylation and ring hydrogenation, rather than aldehyde hydrogenation and hydrodeoxygenation. This contribution focuses on experimental and computational investigations into the control and promotion of the hydrodeoxygenation pathway using alkanethiolate self-assembled monolayers (SAMs).
We have previously shown using a combination of surface science techniques on a Pd(111) single crystal that when furfural is allowed to adopt a flat-lying geometry, such as the case for low surface coverage of adsorbate, the decarbonylation pathway is strongly preferred. As coverage increases, some molecules adsorb through an oxygen lone pair with the ring extending into vacuum and the deoxygenation pathway becomes feasible. The degree of molecule-surface interaction can be tailored through the use of SAMs, which restricts the surface ensemble size by strongly bound sulfur head groups. We demonstrate that using a sparsely packed SAM results in a modest increase in selectivity to aldehyde hydrogenation and hydrodeoxygenation products. Furthermore, use of a dense SAM results in a large increase in selectivity to these products via the restriction of specific surface sites that are necessary for the undesired reactions. These results have implications for other reactions where ensemble size plays an important role in determining reaction selectivity.