Panoramic Mechanistic Insights into Hydrogen Production via Aqueous-Phase Reforming of Methanol Catalyzed by Ruthenium Complexes of Bis-<i>N</i>-Heterocyclic Carbene Pincer Ligands

Abstract

Hydrogen gas has been actively pursued as a renewable alternative energy carrier to fossil fuels. However, the liquefaction, storage, and transportation of preprepared hydrogen gas present daunting challenges for its widespread use in the future energy landscape. Herein, toward the ultimate goal of on-demand hydrogen production whenever and wherever, two series of Ru(II) complexes with, respectively, lutidine- and pyridine-linked bis- N -heterocyclic carbene pincer ligands were synthesized and evaluated for catalytic hydrogen production by aqueous-phase reforming of methanol (APRM). All 11 complexes were capable of catalyzing the acceptorless dehydrogenation of methanol, with the best-performing complex C7 ([Ru(L mes )Cl 2 (CO)], where L mes is the bis-NHC ligand with a mesityl functional group) producing a maximum TON of 14,564 with an average TOF of 89 h –1 over a period of 164 h at 94 °C. This performance places C7 in the best-performing group of noble-metal-based catalysts for APRM. Importantly, hydrogen generation occurs efficiently only during the first and second stages, yielding two molecules of H 2, while HCOOK emerges as the byproduct. Studies by high-resolution electrospray ionization-mass spectrometry revealed abundant information on the possible intermediates involved in the catalytic APRM. Augmented with the evidence from NMR and kinetic isotope effect experiments, a mechanism possibly responsible for the observed catalysis was proposed. Supported by DFT computations of the free-energy profiles of the reaction, substrate activation in the three-step dehydrogenation process is believed to involve the operation of both outer-sphere (for CH 3 OH and HOCH 2 OH) and inner-sphere (for formate) schemes. The panoramic mechanistic understanding thus achieved is helpful in bettering the APRM catalyst design through the tuning of the chemical and electronic structures of the complexes.

Topics & Concepts