A Cu-based nano-catalyst has been widely used in the ethynylation of formaldehyde; however, the effects of the presence of Cu on the reaction have not yet been reported. CuO/SiO₂ catalysts with different Cu species were prepared by impregnation (IM), deposition–precipitation (DP), and ammonia evaporation (AE). The structural evolution of the Cu species in different states of the ethynylation reaction and the structure–activity relationship between the existence state of the Cu species and the catalytic properties of the ethynylation reaction were studied. The results show that the Cu species in the CuO/SiO₂ (IM), prepared using the impregnation method, are in the form of bulk CuO, with large particles and no interactions with the support. The bulk CuO species are transformed into Cu⁺ with a low exposure surface at the beginning of the reaction, which is easily lost. Thus, this approach shows the lowest initial activity and poor cycle stability. A high dispersion of CuO and copper phyllosilicate exists in CuO/SiO₂ (DP). The former makes the catalyst have the best initial activity, while the latter slows release, maintaining the stability of the catalyst. There is mainly copper phyllosilicate in CuO/SiO₂ (AE), which is slowly transformed into a highly dispersed and stable Cu⁺ center in the in situ reaction. Although the initial activity of the catalyst is not optimal, it has the optimal service stability.

Catalytic systems consisting of copper oxide and bismuth oxide are commonly employed for the industrial production of 1,4-butynediol (BD) through ethynylation. However, few studies have investigated the influence mechanism of Bi for these Cu-based catalysts. Herein, a series of nanostructured CuO-Bi₂O₃ catalysts were prepared by co-precipitation followed by calcination at different temperatures. The obtained catalysts were applied to the ethynylation reaction. The textural and crystal properties of the catalysts, their reduction behavior, and the interactions between copper and bismuth species, were found to strongly depend on temperature. When calcined at 600 °C, strong interactions between Cu and Bi in the CuO phase facilitated the formation of highly dispersed active cuprous sites and stabilized the Cu⁺ valency, resulting in the highest BD yield. Bi₂O₃ was completely absent when calcined at 700 °C, having been converted into the spinel CuBi₂O₄ phase. Spinel Cu²⁺ was released gradually to form active Cu⁺ species over eight catalytic cycles, which continuously replenished the decreasing activity resulting from the formation of metallic Cu and enhanced catalytic stability. Moreover, the positive correlation between the in-situ-formed surface Cu⁺ ions and BD yield suggests that the amount of Cu⁺ ions is the key factor for ethynylation of formaldehyde to BD on the as prepared CuO-Bi₂O₃ catalysts. Based on these results and the literature, we propose an ethynylation reaction mechanism for CuO-based catalysts and provide a simple design strategy for highly efficient catalytic CuO-Bi₂O₃ systems, which has considerable potential for industrial applications.

The highly effective catalytic synthesis of 1,4-butynediol (BD) from the Reppe process is a fascinating technology in modern chemical industry. In this work, we reported the effects of the existential states of Mg species in the CuO/silica-magnesia catalysts for the ethynylation of formaldehyde in a simulative slurry reactor. The physichemical properties of the supports and the corresponding catalysts were extensively characterized by various techniques. The experimental results indicated that the introduced Mg species in the form of MgO particles, MgO microcrystals, or Si-O-Mg structures effectively resulted in an abundance of medium-strong basic sites, which can synergize with the active Cu⁺ species, facilitate the activation of acetylene, and improve the ethynylation activity. For the CuO/MgO-SiO₂ catalyst, the existence of Si-O-Mg structures strengthened the Cu–support interaction, which were beneficial to improving the dispersion and the valence stability of the active Cu⁺ species. The highly dispersed Cu⁺ species, its stable valence state, and the abundant medium-strong basic sites enhanced the synergistic effect significantly, leading to the superior activity and stability of CuO/MgO-SiO₂. The insights into the role of the existential states of Mg species and the revelation of the synergistic effect between active Cu⁺ species and basic sites can provide theoretic guidance for future rational design of catalysts for the ethynylation reation.

Most studies on the Cu-based catalysts in the ethynylation of formaldehyde are merely focused on the tuning of electronic configuration and dispersion of the Cu⁺ species. So far, little attention has been paid to the synergy between Cu species and promoters. Herein, binary nano-CuO-MO_x catalysts (M = Si, Al, and Mg) were synthesized and the effects of the promoter on the surface basicity/acidity were systematically studied as well as the ethynylation performance of the nano-CuO-based catalysts. The results show that the introduction of MgO provided a large number of basic sites, which could coordinate with the active Cu⁺ species and facilitate the dissociation of acetylene as HC≡C^δ− and H^δ+. The strongly nucleophilic acetylenic carbon (HC≡C^δ−) is favorable to the attack at the electropositive carbonyl C^δ+ of formaldehyde. The MgO-promoted CuO catalyst showed the highest yield of BD (94%) and the highest stability (the BD yield decreased only from 94% to 82% after eight reaction cycles). SiO₂ effectively dispersed Cu species, which improved catalytic activity and stability. However, the introduction of Al₂O₃ resulted in a large number of acidic sites on the catalyst’s surface. This led to the polymerization of acetylene, which covered the active sites and decreased the catalyst’s activity.

The in situ formed Cu⁺ species serve as active sites in the ethynylation of formaldehyde. The key problem that needs to be solved in this process is how to avoid excessive reduction of Cu²⁺ to inactive metallic Cu, which tends to decrease the catalytic activity. In this work, Cl⁻-modified Cu₂O catalysts with different Cl content were prepared by co-precipitation. The characterization results demonstrated that Cl⁻ remained in the lattice structure of Cu₂O, inducing the expansion of the Cu₂O lattice and the enhancement of the Cu–O bond strength. Consequently, the reduction of Cu⁺ to Cu⁰ was effectively prevented in reductive media. Moreover, the activity and stability of Cu₂O were significantly improved. The Cl⁻ modification increased the yield of 1,4-butynediol (BD) from 73% to 94% at a reaction temperature of 90 °C. More importantly, the BD yield of Cl⁻ modified Cu₂O was still as high as 86% during the ten-cycle experiment, whereas the BD yield of Cu₂O in the absence of Cl⁻ decreased sharply to 17% at the same reaction conditions. This work provides a simple strategy to stabilize Cu⁺ in reductive media.