Hydrogenation activity and selectivity behavior of supported palladium nanoparticles

Efficient selective hydrogenation of 2-butyne-1,4-diol to 2-butene-1,4-diol by silicon carbide supported platinum catalyst

The reduced platinum particles (2–3 nm) supported on silicon carbide are the active species for the selective hydrogenation of 2-butyne-1,4-diol to 2-butene-1,4-diol.

Application of CuxO-FeyOz Nanocatalysts in Ethynylation of Formaldehyde

Composite nanomaterials have been widely used in catalysis because of their attractive properties and various functions. Among them, the preparation of composite nanomaterials by redox has attracted much attention. In this work, pure Cu₂O was prepared by liquid phase reduction with Cu(NO₃)₂ as the copper source, NaOH as a precipitator, and sodium ascorbate as the reductant. With Fe(NO₃)₃ as the iron source and solid-state phase reaction between Fe³⁺ and Cu₂O, Cu_xO-Fe_yO_z nanocatalysts with different Fe/Cu ratios were prepared. The effects of the Fe/Cu ratio on the structure of Cu_xO-Fe_yO_z nanocatalysts were studied by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet confocal Raman (Raman), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS, XAES), and hydrogen temperature-programmed reduction (H₂-TPR). Furthermore, the structure–activity relationship between the structure of Cu_xO-Fe_yO_z nanocatalysts and the performance of formaldehyde ethynylation was discussed. The results show that Fe³⁺ deposited preferentially on the edges and corners of the Cu₂O surface, and a redox reaction between Fe³⁺ and Cu⁺ occurred, forming Cu_xO-Fe_yO_z nanoparticles containing Cu⁺, Cu²⁺, Fe²⁺, and Fe³⁺. With the increase of the Fe/Cu ratio, the content of Cu_xO-Fe_yO_z increased. When the Fe/Cu ratio reached 0.8, a core–shell structure with Cu₂O inside and a Cu_xO-Fe_yO_z coating on the outside was formed. Because of the large physical surface area and the heterogeneous structure formed by Cu_xO-Fe_yO_z, the formation of nonactive Cu metal is inhibited, and the most active species of Cu⁺ are exposed on the surface, showing the best formaldehyde ethynylation activity.

Ethynylation of Formaldehyde over CuO/SiO2 Catalysts Modified by Mg Species: Effects of the Existential States of Mg Species

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.

Ethynylation of Formaldehyde over Binary Cu-Based Catalysts: Study on Synergistic Effect between Cu+ Species and Acid/Base Sites

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.

High-Performance Chlorine-Doped Cu2O Catalysts for the Ethynylation of Formaldehyde

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.

Enhancing the Ethynylation Performance of CuO-Bi2O3 Nanocatalysts by Tuning Cu-Bi Interactions and Phase Structures

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.

Sonochemical preparation of alumina-spheres loaded with Pd nanoparticles for 2-butyne-1,4-diol semi-hydrogenation in a continuous flow microwave reactor

A novel protocol for microwave-assisted alkyne semi-hydrogenation under heterogeneous catalysis in a continuous flow reactor is reported herein. This challenging task has been accomplished using a multifaceted strategy which includes the ultrasound-assisted preparation of Pd nanoparticles (average Ø 3.0 ± 0.5 nm) that were synthesized on the μ-metric pores of sintered alumina spheres (Ø 0.8 mm) and a continuous flow reaction under H₂ (flow rate 7.5 mL min⁻¹) in a microwave reactor (counter-pressure 4.5 bar). The semi-hydrogenation of 2-butyne-1,4-diol in ethanol was chosen as a model reaction for the purposes of optimization. The high catalyst efficiency of the process, in spite of the low Pd loading (Pd content 111.15 mg kg⁻¹ from ICP-MS), is due to the pivotal role of ultrasound in generating a regular distribution of Pd nanoparticles across the entire support surface. Ultrasound promotes the nucleation, rather than the growth, of crystalline Pd nanoparticles and does so within a particularly narrow Gaussian size distribution. High conversion (>90.5%) and selectivity to (Z)-2-butene-1,4-diol (95.20%) have been achieved at an alkyne solution flow rate of 10 mL min⁻¹. The lead-free, alumina-stabilized Pd catalyst was fully characterized by TEM, HR-TEM, EDX, IR, XRPD and AAS. Highly dispersed Pd nanoparticles have proven themselves to be stable under the reaction conditions employed. The application of the method is subject to the dielectric properties of substrates and solvents, and is therefore hardly applicable to apolar alkynes. Considering the small volume of the reaction chamber, microwave-assisted flow hydrogenation has proven itself to be a safe procedure and one that is suitable for further scaling up to industrial application.

A novel protocol for microwave-assisted alkyne semi-hydrogenation under heterogeneous catalysis in a continuous flow reactor is reported herein.

Selective gas phase hydrogenation of nitroarenes over Mo2C-supported Au–Pd

The first reported synthesis of Au–Pd/Mo₂C from colloidal nanoparticles with enhanced selective catalytic hydrogenation of functionalised nitroarenes.

Insights into the hydrogenation mechanism of nitrobenzene to aniline on Pd3/Pt(111): a density functional theory study

The hydrogenation mechanism of nitrobenzene to aniline on Pd₃/Pt(111) surface preferentially follows the direct route and fits best the Jackson reaction mechanism (mechanism B).