(521ci) A DFT-Based Microkinetic Modeling Study to Elucidate the Role of Direct Hydrogen Transfer in Transfer Hydrogenation on Transition Metal Catalysts

Hydrogenation and hydrogenolysis, two important catalytic chemistries employed in the chemical industry, are typically carried out at high pressure using gaseous molecular hydrogen as the hydrogen source. Catalytic transfer hydrogenation (CTH), is an alternative process that employs organic hydrogen donors such as alcohols and formic acid (FA) as the hydrogen source, offering a safer and milder option for small-scale and distributed processing, such as for biomass conversion. The underlying mechanism of such reactions is not fully understood. In particular, CTH may occur through an indirect “metal hydride route” where the donor dehydrogenates on a metal surface and the surface hydrogen atoms get picked by the acceptor molecule; alternatively, donors may directly transfer hydrogen to the acceptor, especially on Lewis acid catalysts.

In this talk, we use FA as a representative hydrogen donor and formaldehyde as a representative hydrogen acceptor to elucidate how hydrogen bonding affects the reaction mechanism for CTH of carbonyl bonds on Cu(111). FA is a promising hydrogen carrier as it is an environmentally friendly by-product from biomass upgrading while formaldehyde is the simplest unsaturated oxygenate. Using dispersion-corrected periodic density functional theory (DFT, PBE-dDSC) calculations and coverage self-consistent mean-field microkinetic modeling, we develop a mechanistic model of this CTH system. 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 can interact via hydrogen bonding. Such direct transfer significantly enhances the rate of catalytic hydrogenation (by at least 7 orders of magnitude) while using a hydrogen donor relative to using molecular hydrogen as the hydrogen source. This work, thus, indicates the potential tuning of activity and selectivity of hydrogenation through judicious choice of hydrogen donors.