Biomass-derived compounds are highly functionalized, making them difficult to selectively upgrade into a desired product. This is because the functional groups interact with the surface, resulting in multiple and often uncontrolled adsorption geometries. Previous experimental studies on Pd indicated that adsorption geometry of these oxygenates dictates product selectivity, where flat-lying configurations result in higher production of undesired decarbonylation products and upright conformations favor CâO bond activation, producing the desired hydrodeoxygenation (HDO) products. Previously, our group has shown that thiolate self-assembled monolayers (SAMs) are able to block specific active sites on Pd nanoparticles during the adsorption and reaction of furfural and furfuryl alcohol, thereby reducing selectivity towards decarbonylation without affecting HDO to methylfuran. However, the effect of the thiolates is not understood fundamentally, hampering the efforts to design better catalysts. Molecular dynamics (MD) has provided insight into the molecular effects of the SAMs on the reactions and possible mechanisms. Simulations with CHARMM-Interface were conducted using the Nanoscale Molecular Dynamics (NAMD) program and also showed the same accuracy as DFT calculations for smaller systems without surfactants in vacuum.
These studies have found that many aromatic oxygenates, such as furfuryl alcohol, furan, and methylfuran adopt an upright adsorption configuration, with the aromatic ring repelled away from the surface. The results indicate that the thiol ligands affect both binding energies and configurations of these aromatic oxygenates compared to uncoated Pd. These changes impact product selectivity. For example, furfural on a SAMs covered nanoparticle adopts an upright conformation, as well as diffuses to the edge sites, which correlates to the reduced selectivity towards the decarbonylation product, furan. These aromatic oxygenates on SAMs-coated nanoparticles also desorb from the surface and can interact with the SAMs for some residence time before desorbing into vacuum.