(664d) Interaction of Furan and Benzene Derivatives with Palladium Nanoparticle Catalysts and the Mechanism of Conversion into Biofuels
Noble metals and their nanocrystals are promising catalysts with high activity and selectivity for a broad variety of reactions including the conversion of biomass-derived compounds into biofuels. Self-assembled monolayers (SAMs) on the particle surface can be used to control the availability of contiguous surface sites and increase the catalytic selectivity. This work provides detailed understanding of the binding mechanism of the reactants and products and of the role of thiols in controlling activity using molecular dynamics simulations with accurate interatomic potentials and DFT calculations. The binding geometry and energy of a series of furan and benzene derivatives is analyzed on palladium (111) and (100) surfaces, as well as on cuboctahedral Pd nanocrystal surfaces with and without octadecane thiolate surfactants. The aromatic molecules exhibit predominantly strong flat-on binding to the bare metal surfaces with -20 to -45 kcal/mol adsorption energy. The presence of thiol ligands changes the orientation of the molecules towards tilted and upright binding configurations on the metal surface, reduces the binding energy to approximately 2/3, and stabilizes the molecules in the corona through van-der-Waals interactions proportional to molecule size and polarizability. Diffusion through the surfactant corona is faster for oxygenated molecules compared to hydrocarbon compounds. Mechanistic details for competing hydro-deoxygenation reactions and dehydrogenation-decarbonylation reactions of furfuryl alcohol and benzyl alcohol are elucidated. Thiol ligands support hydro-deoxygenation reactions in upright orientation and poison particle terrace sites necessary for flat-on binding and undesired decarbonylation reactions, enabling an excess of desirable methylfuran and toluene products. Rate predictions for specific nanocrystals, thiol coverage, and reaction conditions are feasible in comparison to measurements to design more effective catalysts for biofuel conversion.