Mechanistic Role of the Proton–Hydride Pair in Heteroarene Catalytic Hydrogenation

Kinetic and density functional theory studies probe the catalytic involvements of proton–hydride pairs in breaking the strong aromaticity of N-containing heteroarenes (pyridine and pyrrole) on sulfided Ru cluster surfaces. Under the sulfur chemical potentials relevant to hydrodenitrogenation catalysis, Ru clusters remain covered with a layer of sulfur-deficient RuS_x on which a variety of reactive hydrogen species, which bind to Ru⁴⁺, S^2–, or Ru⁴⁺–S as Ru⁴⁺–(H^δ−), S^2––(H^δ+), and Ru⁴⁺–(SH₂), respectively, coexist. These reactive hydrogen species exhibit either proton or hydride character, depending on the electronegativity of their ligands (ruthenium and sulfur). For this reason, they participate in different hydrogen addition steps during the hydrogenation of heterocyclic-N compounds. Pyridine as the basic and pyrrole as the nonbasic heterocyclic model compounds undergo hydrogenation via distinctly different kinetically relevant steps because of their different proton affinities, which influence their interactions with the various reactive hydrogen species and in turn adsorption configurations. The hydrogenation of pyridine initiates from an initial, quasi-equilibrated proton attack onto the N atom, followed by a second hydridic hydrogen addition as the kinetically relevant step. In the contrasting case of pyrrole, the hydrogenation initiates via a kinetically relevant proton attack to its β-carbon that breaks its aromaticity before a hydride addition onto its α-carbon. Both reactions require a proton attack followed by a hydride attack, but their mechanistic differences lead to contrasting rate dependences with H₂S pressure because the H₂S pressure, together with H₂ pressure, gives the H₂S:H₂ ratio that dictates the sulfur chemical potentials, the relative abundance of S anions and Ru cations coordinating to the hydrogen species, and in turn the surface concentrations of proton and hydride intermediates on Ru cluster surfaces. The catalytic involvements of proton–hydride pairs described here are general for hydrogenation reactions and, in particular, heteroarene hydrogenation in hydrotreatment processes.