Reduction of CO2 into liquid fuels is anticipated to play an important role in mankind’s efforts to reduce the devastating effects of climate change. However, the underlying chemistry required to generate liquid fuels selectively and efficiently is underdeveloped and key mechanistic steps remain poorly understood. In this work, a cascade strategy for the generation of methanol efficiently and selectively from CO2 is presented. It involves CO2 reduction to CO by one catalyst, a transformation well known in the literature, followed by further reduction to methanol by a combination between a coordination compound and benzimidazole-based organic hydrides. These organohydrides are potential key components in forming C—H bonds, are inactive towards direct CO2 reduction and hydrogen generation, and are known to be regenerated electrochemically or photochemically. In particular, reaction between [Ru(bpy)2(CO)2]2+, where bpy = 2,2'-bipyridine, and one equivalent of the organic hydride PMBIH (1,2,3,5,6-pentamethyl-2,3-dihydro-1H-benzo[d]imidazole) first yields the formyl intermediate [Ru(bpy)2(CO)(CHO)]+. The transformation occurs quantitatively with excess PMBIH in acetonitrile solution on a seconds time scale. The stability of this reactive formyl intermediate significantly increased to weeks when Lewis acids were present, such as Li+— behavior consistent with a Lewis acid-base adduct formation with the oxygen atom of the formyl group. Further reduction of [Ru(bpy)2(CO)(CHO)]+ with two equivalents of PMBIH yields quantitively the hydroxymethyl complex [Ru(bpy)2(CO)(CH2OH)]+. Methanol release from [Ru(bpy)2(CO)(CH2OH)]+ was accomplished under irradiation (LED, λmax = 420 nm) in the presence of excess PMBIH. Identification and quantification of methanol as the single product was confirmed by 1H-NMR and gas chromatography. Using NMR, IR, and mass spectrometry, together with labeling studies, chemical synthesis of authentic samples of intermediates, and theoretical calculations, the key elementary reaction steps required to selective and efficient convert CO to methanol are demonstrated and conditions suggest a viable approach for methanol production from CO, and from CO2 through cascade catalysis that capitalizes upon recent advances in the regeneration of organic hydrides.

Selective and efficient cascade strategy for CO2 reduction to methanol using renewable organic hydrides

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