Importance of Zinc Oxide in Cu-Based Catalysts for the Ethynylation of Formaldehyde to 1,4-Butynediol

As more technologies have been developed to produce formaldehyde and acetylene from biomass, the condensation–addition reaction between formaldehyde and acetylene provides a green and atom-economic method to synthesize 1,4-butynediol (BD). It is important to design and construct a new efficient copper-based catalyst for formaldehyde ethynylation with multi-center synergy to improve the reaction efficiency. In this work, a CuO–xZnO composite catalyst with an interface structure was constructed. Compared with a CuO-based catalyst, the yield of BD was increased from 19.07 to 70.9% by introducing an appropriate amount of ZnO, and the reaction activation energy decreased from 28.94 to 16.35 kJ/mol, which improved the catalytic reaction efficiency. The results showed that ZnO in the CuO–xZnO catalyst dispersed the active Cu species and also promoted the conversion of CuO to copper acetylide by electron transfer in the interface structure. Moreover, Zn²⁺ in ZnO acted as a Lewis acid and adsorbed the carbonyl oxygen of formaldehyde, causing the carbonyl electrons to flow to Zn²⁺, which enhanced the electropositivity of C⁺ in the formaldehyde molecule. This resulted in a nucleophilic addition reaction between formaldehyde activated by ZnO and acetylene activated by copper acetylide at the interface, which efficiently generated the target product BD.