Molecular understanding of enyne hydrogenation over palladium and copper catalysts

Partial hydrogenation of unsaturated hydrocarbons comprises an important family of reactions that are applied in many industrial sectors. Understanding the hydrogenation of polyunsatured and polyunsaturated compounds on heterogeneous catalysts at its molecular level remains a challenging milestone to fine-tune the product distribution. Our study presents a detailed mechanistic analysis of the gas phase hydrogenation of vinylacetylene (1-butene-3-yne) and valylene (2-methyl-1-butene-3-yne) over palladium and copper catalysts. These two metals are selected owing to their pronounced differences in continuous flow catalytic tests at ambient pressure. The chemoselectivity, regioselectivity, isomerization, and oligomerization patterns measured in a broad range of feed hydrogen/hydrocarbon ratios are rationalized by density functional theory simulations. An extended Brønsted-Evans-Polanyi relationship for both substrates and active metals is found for the hydrogenation processes. The identified factors that govern selectivity are similar to those identified for smaller C₂ and C₃ compounds and, thus, a means of systematization to other polyunsaturated compounds is opened up.