(5c) Elucidating the Essential Role of Hydrogen Bonding and Direct H-Transfer in Transfer Hydrogenation of Biomass Oxygenates on Transition Metal Catalysts
2024 AIChE Annual Meeting
(5c) Elucidating the Essential Role of Hydrogen Bonding and Direct H-Transfer in Transfer Hydrogenation of Biomass Oxygenates on Transition Metal Catalysts
Rangarajan, S., Lehigh University - Dept of Chem & Biomolecular
Hydrogenation and hydrogenolysis processes, pivotal in biomass conversion and otherwise, traditionally employ molecular hydrogen as hydrogen source under significant pressures. Catalytic transfer hydrogenation (CTH) presents an alternative using organic hydrogen donors like alcohols and formic acid (FA), offering a safer and milder approach suitable for small-scale distributed processing. Mechanistically, CTH may involve indirect H-transfer via metal hydrides, or direct H-transfer akin to MPV reductions on lewis acid catalysts. While evidence for direct H-transfer on transition metal catalysts is limited, several studies demonstrate higher rates and conversions using formic acid as H source compared to molecular hydrogen under identical conditions. Here, we show that direct hydrogen transfer between a donor and acceptor (or derived intermediates) is kinetically feasible on transition metal catalysts, especially facilitated by hydrogen bonding interaction. This mechanism opens up new hydrogenation pathways inaccessible to molecular hydrogen and could explain higher CTH rates vis-a-vis conventional hydrogenation.
In this work, we combine density functional theory (DFT) with coverage-cognizant microkinetic modeling to conceptually explain the mechanism of catalytic hydrogen transfer between a common donor, viz. HCOOH, and a model acceptor, viz. HCHO (the smallest carbonyl compound), on Cu(111) to explicate the role of indirect and direct hydrogenation routes and the effect of surface coverages and concomitant destabilization. Our results show that (1), hydrogen bonded complexes are formed when HCOOH and HCHO are both present, they enable direct hydrogen transfer which is kinetically relevant, resulting in three times higher reaction rate (compared to using molecular H2 under the same conditions). (2), hydrogen bonded complexes arise in a number of other CTH systems (furfural and lignin hydrogenolysis, reduction of nitrates, nitriles, etc.) and transition metal catalysts, potentially indicating the generality of our results to more practical chemistries in biomass conversion and beyond.
