New Tricks for an Old Dog: Grubbs Catalysts Enable Efficient Hydrogen Production from Aqueous-Phase Methanol Reforming

Abstract

Herein, we report a new application of the prize-winning Grubbs catalysts, which have been widely applied for olefin metathesis, for hydrogen production from aqueous-phase methanol reforming under easily achievable conditions (1 atm, <100 °C) with negligible CO formation. Out of the catalysts tested, the best turnover frequency (158 h–1) and turnover number (11424, 72 h) were both achieved with a third-generation Grubbs catalyst (G-III). The best TOF was achieved with G-III and is competitive when compared with some of the best results reported (Chem. Rev.2018118372433). Also, G-III is found to be a versatile catalyst for the dehydrogenation of ethanol and formic acid. Mechanistic studies and DFT calculations shed light on the reaction mechanism, which involves an unusual substrate (solvent)-assisted six-membered-ring (σ-bond) metathesis pathway. This work should open up new opportunities in catalyst design in connection with the hydrogen economy and, more generally, with the development of clean and renewable energies.

Hydrogen production via the aqueous-phase reforming of methanol catalyzed by Ru(ii) complexes of PNNP ligands

“…High-resolution electrospray ionization mass spectrometric (HRESI-MS) analysis of the reaction mixture with 2 as the catalyst was conducted to identify any species involved in the process. 33,77,78 The reaction mixture of entry 2 in Table 1 provides plentiful information on the nature of the Ru-containing species, depicting excellent matches between the experimental and simulated data with a deviation of <5 ppm (Fig. 6a over the course of the reaction (Fig.…”

“…Every H 2 molecule represents one catalyst turnover number (TON), and the TON value for a particular reaction was calculated using eqn (1): 28,33 …”

“…30 We in this vein have explored prize-winning Grubbs catalysts for APRM under basic conditions, producing an average TOF of 158 h −1 in three days. 33…”

“…28,29 Their research inspired a series of experimental and theoretical works in the search for better APRM catalysts that are either more stable, more efficient, and less expensive, or possess a combination of these traits. Complexes of a variety of transition metals, including those of Ru, [30][31][32][33][34][35][36][37][38][39][40][41][42] Ir, [43][44][45][46] Rh, 47 Re, 48 Fe, 49,50 and Mn, 48,51,52 have been reported in the literature. Out of these works the impressive catalytic performance of an acridine-based ruthenium pincer complex (Fig.…”

“…30 We in this vein have explored prize-winning Grubbs catalysts for APRM under basic conditions, producing an average TOF of 158 h −1 in three days. 33 Most of the reported catalysts for the APRM operate in the socalled metal-ligand cooperativity (MLC) manner 29,35,[49][50][51] in which a coordinatively unsaturated metal center and a ligand coordinating atom act respectively as electrophilic and nucleophilic sites for binding and activating CH 3 OH and other possible intermediate substrates via an outer-sphere mechanism. The known MLC systems include metal-amine/amido (M-N) complexes 29,49,51 originally reported by Noyori et al, 53 the dual-mode complex system via ligand de-aromatization and metal amido bond formation reported by Milstein and coworkers, 30,35,54,55 and the systems based on the metal-O (bipyridonato ligand) mode.…”