Kinetic Study of the Hydrogenation of Unsaturated Aldehydes Promoted by CuPt_<i>x</i>/SBA-15 Single-Atom Alloy (SAA) Catalysts

Abstract

A comprehensive study of the kinetics of the catalytic hydrogenation of unsaturated aldehydes, in particular of cinnamaldehyde, promoted by CuPtx/SBA-15 single-atom alloy catalysts was carried out in order to identify trends as a function of the composition of the bimetallic nanoparticles, that is, of the value of x. The optimum performance reported by us recently [ ACS Catal. 2019, 9, 9150−9157] in terms of selectivity toward the formation of cinnamyl alcohol, the desired product, by the catalyst with x = 0.005 was corroborated. A rapid decrease was seen in catalytic activity in batch reactors with all catalysts, in particular with pure Cu/SBA-15. This was ascribed, based on DFT calculations and microkinetic simulations, to the relative weak adsorption of the reactant compared to that of the products, which leads to the blocking of catalytic sites by the latter early on in the catalytic runs. It was determined that selectivity is controlled by the relative values of the initial rate constants for hydrogenation to the unsaturated alcohol versus the saturated aldehyde. Those were found to vary by up to an order of magnitude as a function of Pt content in the catalyst, in spite of the fact that the hydrogenation steps are presumed to occur on Cu, not Pt, sites. In general, significant changes in equilibrium and kinetic parameters were seen across the series of catalysts tested (versus x), indicating that the addition of even small amounts of Pt to these Cu single-atom alloy (SAA) catalysts affects the intrinsic performance of the hydrogenation catalytic sites.