Controlling of the Active Site Environment of Pd Catalysts for Selective Deoxygenation

摘要

In this dissertation, the main active sites for deoxygenation of biomass-derived chemicals and thiol modification for selectively poisoning Pd specific sites are investigated for the catalyst selectivity improvement.;Because the high oxygen content in biomass-derived chemicals results in high viscosity, high acidity, low stability and low heating value, it is necessary to upgrade biomass-derived compounds through catalytic deoxygenation (DO). Unfortunately, during the deoxygenation process, the multiple functional groups present in biomass-derived feedstocks create the possibility for multiple reactions to occur on catalyst surfaces, resulting in low selectivity and high separations costs.;In our study, furfuryl alcohol and benzyl alcohol were employed as model oxygenates. We found that deoxygenation reactions of aromatic compounds on Pd are structure-sensitive, occurring preferentially on undercoordinated sites on supported Pd catalysts, such as the interfacial atoms on Pd/Al2O3; in contrast, undesired reactions such as C-C bond dissociation tend to occur on Pd contiguous sites. This insight provides important information for design of improved catalysts.;Density functional theory (DFT) calculation and diffuse reflectance infrared fourier transform spectra (DRIFTS) after CO adsorption were used to study the adsorption preference of thiol molecule on Pd, which indicated the thiol with longer carbon tail has stronger preference to occupy contiguous sites because of stronger van der waals interaction between the carbon tails. Therefore, through variable thiolate carbon tail length, Pd/Al2O3 active sites were freely tailored, resulting in changing catalyst DO selectivity for furfuryl alcohol.;In addition to thiolate chain length, through use of thiol tail with different steric bulks, the coverage of thiolate monolayers was variable on Pd as a method to tune catalyst performance. The bulky and low coverage adamantanethiol (AT) monolayers on Pd slightly improved the catalyst DO selectivity and activity due to less CO from benzyl alcohol decarbonylation and weaker adsorption strength of benzyl alcohol-derived products on Pd while octadecanethiol (C18)-modified catalyst has the highest DO selectivity because the dense monolayers completely occupied Pd contiguous sites. After C18 modification, the selectivity to toluene from benzyl alcohol increased from 20% to 98% at nearly full conversion. The mixed monolayers of AT and C18 provided a method to further fine-tune thiolate monolayers coverage, catalyst surface active sites environment and catalyst performance.;The previous mechanistic investigations revealed that benzene was derived from benzyl alcohol that adsorbed on the surface in a "flat-lying" structure, while toluene was produced from upright benzyl alcohol on the Pd surface. DRIFTS after CO adsorption showed that thiolate modification decreased the availability of contiguous active sites compared to unsaturated sites. Because decarbonylation to benzene required contiguous sites for flat-lying conformation, the reduced amount of contiguous Pd sites by thiol modification shut down DC of benzyl alcohol and improved the selectivity to toluene from benzyl alcohol. This work demonstrates that active site availability, and thus catalyst selectivity, can be easily tuned by Pd surface coordination environment and thiol modification.