Reforming of ethanol to hydrogen and acetic acid catalyzed by pincer-ruthenium complexes

Abstract

Ethanol is feed-agnostic and is hence currently one of the most preferred renewable resources for manufacturing essential industrial commodities globally. Herein, we present the catalytic aqueous-phase reforming of ethanol employing a series of NNN pincer-ruthenium complexes based on bis(imino)pyridine and 2,6-bis(benzimidazole-2-yl) ligands. These ruthenium complexes have been studied for hydrogen production from ethanol in water in the presence of a base at 120 °C. Among the complexes considered, the best results were obtained using 0.2 mol% of (^Cy2NNN)RuCl₂(PPh₃), which gave a yield of up to 70% of H₂ and 73% of acetic acid in 100% selectivity from a mixture of ethanol and water in a 2 : 1 ratio in the presence of 1.5 equivalents of KO^tBu. Labelling studies provide key evidence for the involvement of C–H activation in the catalytic ethanol reforming reaction with an average KIE of 5.23. Kinetic studies indicate the first order dependence of rate on the concentration of both the pincer-ruthenium catalyst and ethanol, which also validates the homogeneity of the reaction. HRMS and NMR studies provide conclusive evidence not only for the initiation of the reaction via the release of PPh₃ but also for the generation of (^Cy2NNN)RuCl(H) species which is likely to be the resting state of the catalytic aqueous ethanol reforming cycle. DFT studies are in agreement with the experiments, and indicate the alcoholysis step (ΔG^‡₁₂₀ = 23.19 kcal mol⁻¹) that results in the release of the first molecule of hydrogen (dehydrogenolysis) along with the concomitant formation of (^Cy2NNN)RuCl(H) to be the rate-determining step (RDS). This work promises to open up exciting avenues in the aqueous reforming of feed-agnostic ethanol in the context of production of clean-burning H₂ and synthetically versatile acetic acid.