(782f) Using Non-Covalent Interactions to Control Availability of Different Catalyst Sites

Furfuryl oxygenates are among the main oxygenates produced from biomass deconstruction, and it is desirable to upgrade components such as furfural and furfuryl alcohol to value-added fuels and chemicals. Furfuryl alcohol can undergo hydrodeoxygenation (HDO) to methyl furan, dehydrogenation to furfural, and decarbonylation (DC) to furan, among other reactions. It has been proposed that DC and HDO are carried out on different types of Pd sites. Thus, controlling the relative amount of terrace and edge sites is a potential method to manipulate catalyst performance. Recently, thiolate self-assembled monolayers (SAMs) on catalysts have been considered as selectivity modifiers. In this study, experimental and theoretical techniques were combined to study the effect of thiolate chain length on Pd surface active site distribution and furfuryl alcohol HDO/DC selectivity.

Hexanethiol (C6), decanethiol (C10), tetradecanethiol (C14) and octadecanthiol (C18)-coated Pd/Al₂O₃ were used to investigate trends in selectivity with chain length. Addition of the C6 thiol modifiers was found to slightly improve methyl furan selectivity from 5% to 15%. As the tail length was increased from C6 to C18, the selectivity to methyl furan improved dramatically because the active sites for DC were selectively blocked by the SAMs. The nature of these sites was investigated by infrared spectroscopy during CO adsorption. We found that the longer thiolate tails strongly decreased the availability of contiguous active sites was decreased much more dramatically than step and edge sites. DFT calculations indicated that short thiols prefer to occupy Pd edge sites because of a stronger covalent bond between low-coordination Pd atoms and the thiol group. However, through the stronger van der Waals interaction between thiolate chains on terrace sites, longer thiol molecules were found to preferentially bind on (111) surfaces, selectively poisoning those sites. DFT models of the surface reaction chemistry revealed that reaction at steps between thiolate-covered terraces could serve as active sites for HDO. These results show that the self-organizing properties of alkanethiols can be used to selectively poison specific sites on supported metal catalysts, leading to improved reaction selectivity.