Hydrogenation of dimethyl malonate to 1,3-propanediol catalyzed by a Cu/SiO2 catalyst: the reaction network and the effect of Cu+/Cu0 on selectivity

Abstract

1,3-Propanediol, a vital monomer for the manufacture of commodity polytrimethylene-terephthalate, is commercially produced nowadays through either hydration of acrolein or hydroformylation of ethylene oxide. Herein, for the first time, we present the investigation of an alternative route for 1,3-propanediol production from vapor-phase catalytic hydrogenation of syngas-derived dimethyl malonate on a Cu/SiO₂ catalyst. The catalytic reaction network has been disclosed for the Cu/SiO₂ catalyst, and the reaction proceeds through sequential hydrogenation with methyl 3-hydroxypropionate as the primary product, which can be further converted into 1,3-propanediol or methyl propionate. Excessive hydrogenation of 1,3-propanediol or methyl propionate leads to the formation of n-propanol. Meanwhile, a small amount of dimethyl malonate cracks into methyl acetate. The structural and textural properties of Cu/SiO₂ catalysts with varied copper loadings were extensively characterized by X-ray diffraction, Fourier transform infrared spectroscopy, H₂-temperature programmed reduction, X-ray photoelectron spectroscopy, N₂ physisorption, CO chemisorption, N₂O titration, and transmission electron microscopy. A correlation of the areal activity to copper components suggests that metallic copper modified by the co-present Cu⁺ species may be the active site for the hydrogenation. The highest 1,3-propanediol selectivity was achieved on a catalyst with a maximum Cu⁺/(Cu⁰ + Cu⁺) ratio of 0.41. The revelation of catalytic networks and insights into the active species can provide guidance for future rational design of catalysts for regioselective hydrogenation of CO bonds in dimethyl malonate.