(679c) Formation of Proton-Hydride Pair and Its Kinetic Significance for Heteroarene Hydrogenation in Hydrotreating and Hydrodeoxygenation Reactions
Hydrogenation of arenes and heteroarenes (benzene, quinoline, thiophene, furan, etc.) is of industrially importance for petrorefining (in hydrotreating processes), producing sustainable fuels (hydrodeoxygenation), and synthesizing fine chemicals. These diverse reaction systems and catalytic processes require the sequential addition of reactive hydrogen species that breaks the strong aromaticity of the arene reactants. We interrogate the mechanistic requirements for effective ring saturation, by modifying the active sites and reaction environment, through which we alter the charges of reactive hydrogen species and of the activated arenes on transition metal and metal sulfide surfaces. A core catalytic strategy is to tune the charge of the reactive hydrogen species, by having a base as a ligand adjacent to the metal, as this arrangement transforms the hydrogen dissociation pathway from homolytic to heterolytic cleavage and promotes the formation of proton-hydride pairs. On Ru metals, H2S dissociates and deposits on the surfaces, creating Ru cation and S anion site pairs, on which reactive hydrogen species acquire proton and hydride characters. These proton and hydride pairs both participate in hydrogenation. The catalytic requirements of the proton and hydride addition remain the same across the series of arenes, but the catalytic relevance of the two addition steps differs, depending on the adsorption configurations of the arenes and their affinity to proton. On basic pyridine, hydrogenation occurs via a rapid protonation followed by kinetically relevant hydride addition step. In contrast, on the less basic pyrrole, the initial proton transfer limits rates. The catalytic influences of the charge are similar to those observed during hydrodeoxygenation catalysis in the condensed phase, during which solvent molecules mediate the charges of the surface hydrogen species, generating surface protons and hydrides. This mechanistic knowledge established here allows us to design active site structures and environments that alter the charges of the reactive hydrogen species, their catalytic involvement, the potential energy landscape, and in turn rates.