Promoted copper catalysts are emerging as catalysts for the hydrogenation of aldehyde and ketone compounds, a key step in biomass conversions. Their superior selectivity and low material cost make them an attractive alternative to the conventional platinum and palladium catalysts in the hydrogenation of carbonyl functionalities. In this presentation, I will discuss our recent work on the development of a comprehensive coverage-dependent microkinetic model for this reaction. Hydrogenation of alkoxy intermediates is often rate-limiting in the hydrogenation of carbonyl groups over copper catalysts, leading to high alkoxy coverages. In a first study, [1] we showed that the addition of small amounts of water shifts the rate-limiting step from alkoxy hydrogenation to the regeneration of the surface hydroxyl groups, by opening up proton-transfer pathways. This finding also provides an alternative interpretation for the role of promotors in Cu-catalysis. Alternatively, when the stability difference between the ketone and enol tautomer is below about 30 kJmol-1, e.g., in the case of alpha-hydroxy ketones such as acetol, hydrogenation of the stable alkoxy intermediate can be avoided via the enol pathway.[2] In this pathway, the C=C double bond is hydrogenated, rather than the C=O bond, in agreement with isotope labeling studies. Finally, the hydrogenation of the alkoxy-intermediate can be facilitated by the introduction of a sacrificial alcohol. [3] Again, proton transfer between the sacrificial alcohol and the alkoxy species enhances the reaction rate. Since hydrogenation of the carbon atom in C=O still requires surface hydrogen, we find that the addition of gas phase hydrogen has a surprising catalytic effect – it speeds up the overall reaction without itself being consumed in the hydrogenation reaction. The detailed microkinetic simulations illustrate the complexity of this seemingly simple reaction, and show the importance of self-consistent, coverage-dependent microkinetic simulations to discover the different kinetic regimes.

Coverage-dependent ab initio microkinetic models for the copper-catalyzed hydrogenation of carbonyl groups: Role of hydrogen, water, and sacrificial alcohols

2019 Fall National Meeting

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