Effects of Thiol Modifiers on the Kinetics of Furfural Hydrogenation over Pd Catalysts

Thiolate self-assembled monolayers (SAIV1s) were used to block specific active sites on Pd/Al2O3 during the hydrogenation of furfural to elucidate site requirements for each process involved in this complex reaction network. Reactions were performed on uncoated, 1-octadecanethiol (C18) coated, and benzene-1,2-dithiol (BDT) coated catalysts. Selectivity among key reaction pathways was sensitive to the SAM modifier, with increasing sulfur density strongly suppressing furfinal decarbonylation, less strongly suppressing furfural hydrogenation, and minimally affecting furfuryl alcohol hydrodeoxygenation to methylfuran. Diffuse reflectance infrared Fourier transform spectroscopy with CO was used to characterize site availability on the catalysts. The presence of a C18 modifier restricted the availability of Pd terrace sites, while accessibility to Pd edges and steps was practically unaffected with respect to the uncoated catalyst. The B DT modifier further restricted terrace accessibility but additionally restricted adsorption at particle edges and steps. Comparison between reaction rates and site availability suggested that decarbonylation occurred primarily on terrace sites, while hydrodeoxygenation occurred on particle steps and edges. Aldehyde hydrogenation, and its reverse process of alcohol dehydrogenation, was found to occur on both terrace or edge sites, with the dominant pathway dependent on surface coverage as determined by reaction conditions. The results of a detailed kinetic study indicate that in addition to changing the availability of specific sites, thiol monolayers can strongly affect reaction energetics and decrease the coverage of strongly adsorbed furfural-derived intermediates under reaction conditions. Ambient pressure X-ray photoelectron spectroscopy experiments indicated that the metal sulfur bonds were not changed appreciably under reaction conditions. The results of this work show that HDO is not appreciably affected even with drastic decreases in the density of available sites as measured by CO adsorption, providing opportunities to design isolated catalyst sites for selective reaction.