Adsorption and Reaction of Furfural and Furfuryl Alcohol on Pd(111): Unique Reaction Pathways for Multifunctional Reagents

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, C 4 H 3 (CHO)O, and furfuryl alcohol, C 4 H 3 (CH 2 OH)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.