Elucidating the essential role of hydrogen bonding and direct H-transfer in transfer hydrogenation on transition metal catalysts

Abstract

Catalytic transfer hydrogenation (CTH) employs molecular hydrogen donors such as isopropanol and formic acid as H source. Using periodic density functional theory (DFT) and microkinetic modeling, here we show that direct hydrogen atom transfer between a donor and acceptor is kinetically feasible on transition metal catalysts especially if the donor and the acceptor (or intermediates derived from them) can interact via hydrogen bonding. This direct hydrogen transfer opens up new hydrogenation pathways not available in conventional hydrogenation with H₂. The mechanism of catalytic hydrogen transfer between formic acid, and a model acceptor, viz. formaldehyde (HCHO, the smallest carbonyl compound), on Cu(111) is first studied to conceptually explain the role of indirect and direct hydrogenation routes and the effect of surface coverages and concomitant destabilization. Our results show that (1), when HCOOH and HCHO are both present, hydrogen bonded complexes may form that enable direct hydrogen transfer which can then be kinetically relevant; (2), the direct hydrogen transfer with formic acid results in three times higher reaction rate (compared to using molecular H₂ under the same conditions). We finally show that hydrogen bonded complexes arise in a number of other CTH reactions on transition metal catalysts (furfural and lignin hydrogenolysis, reduction of nitrates, nitriles, etc.), potentially indicating the generality of our results to more practical chemistries.