Hydrogenation of arenes and carbonyls require several H-attack events; their reactivities depend on the charge and chemical identity of H species involved in each H-attack, its kinetic relevance, and the overall H-addition sequence. Under H2S and H2 gas mixture, sulfided Ru nanoparticles are covered by Run+-(Hδ-)-S2--(Hδ+) proton-hydride pairs from H2 heterolytic dissociation, and Run+-(SH2)-S2- protons from H2S molecularly adsorption.[1] These proton-hydride pairs can effectively attack polar C=X (X = N, O, S) bonds in arenes and carbonyls during hydrogenation catalysis. The first-order rate coefficients of arene hydrogenation at 573 K increase with increasing reactant proton affinities. Pyridine, which has the highest proton affinity, can readily accept a proton, before accepting a hydride in the second hydrogen addition as a kinetically relevant step. Since its protonation is a fast and quasi-equilibrated step, it exhibits the highest reactivity. Five-membered heteroarenes (pyrrole, thiophene, and furan) have lower proton affinities, and thus their initial protonation limits rates. We found that the difference in the heats of initial protonation of a five-membered heteroarene and a reference heteroarene depends inversely on the difference in proton affinities, and thus the one with a higher proton affinity shows a lower activation energy for hydrogenation and a higher reactivity, according to the Bell–Evans–Polanyi principle. Conversely, the first-order rate coefficient ratios of hydrogenation of a carbonyl to a reference carbonyl at 503 K increase with increasing reactant hydride affinities. Their hydrogenation occurs via a kinetically relevant attack of a Run+-(Hδ-) hydride at the carbonyl C atom, followed by a S2--(Hδ+) attack at the O atom, as confirmed by DFT calculations and isotopic exchange studies. We revealed that the difference in the heats of initial hydride addition of a carbonyl and a reference carbonyl depends inversely on the difference in their hydride affinities. Therefore, hydrogenation of a carbonyl with a higher hydride affinity has a lower activation barrier and thus is more reactive. In conclusion, this study provides a mechanistic interpretation on the catalytic roles of proton-hydride pairs in contrasting reactivity trends of arene and carbonyl hydrogenation, which can shed light on improving hydrotreating efficiency.

Contrasting reactivity trends of hydrogenation of aromatic and carbonyl compounds on sulfided Ru catalysts

ACS Fall 2021

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