Insights into a New Formation Mechanism of Robust Cu/SiO2 Catalysts for Low-Temperature Dimethyl Oxalate Hydrogenation Induced by a Chelating Ligand of EDTA

The Cu/SiO2 catalyst has been widely used in dimethyl oxalate (DMO) hydrogenation due to its low cost and high efficiency. However, the reaction temperature of DMO hydrogenation is higher than the Hüttig temperature of Cu, and the smaller Cu particles are easier to agglomerate. Therefore, there is much interest in constructing a catalyst with a small particle size and strong stability. In the present work, the effect of introducing EDTA on Cu/SiO2 catalysts is systematically investigated. It not only was beneficial to form smaller copper nanoparticles (CuNPs) but also to enhance the stability of Cu species by introducing a suitable amount of EDTA. Furthermore, the surface Cu species were more evenly dispersed, and the number of active sites was increased with the introduction of EDTA; subsequently, the synergistic effect between Cu+ and Cu0 was enhanced. The best performance of 0.08E-Cu/SiO2 had been achieved in the DMO hydrogenation to ethylene glycol (EG), and the DMO conversion and EG selectivity reached 99.9% and 97.7%, respectively. Above all, the 0.08E-Cu/SiO2 catalyst exhibited a high level of stability during the 1200 h life test at 180 °C.

Ti3+ Tuning the Ratio of Cu+ /Cu0 in the Ultrafine Cu Nanoparticles for Boosting the Hydrogenation Reaction

Hydrogenation of diesters to diols is a vital process for chemical industry. The inexpensive Cu⁺ /Cu⁰ -based catalysts are highly active for the hydrogenation of esters, however, how to efficiently tune the ratio of Cu⁺ /Cu⁰ and stabilize the Cu⁺ is a great challenge. In this work, it is demonstrated that doped Ti ions can tune the ratio of Cu⁺ /Cu⁰ and stabilize the Cu⁺ by the TiOCu bonds in Ti-doped SiO₂ supported Cu nanoparticle (Cu/Ti-SiO₂ ) catalysts for the high conversion of dimethyl adipate to 1,6-hexanediol. In the synthesis of the catalysts, the Ti⁴⁺ OCu²⁺ bonds promote the reduction of Cu²⁺ to Cu⁺ by forming Ti³⁺ O_V Cu⁺ (O_V : oxygen vacancy) bonds and the amount of Ti doping can tune the ratio of Cu⁺ /Cu⁰ . In the catalytic reaction, the O vacancy activates CO in the ester by forming new Ti^{3+ δ} O_R Cu^{1+ δ} bonds (O_R : reactant oxygen), and Cu⁰ activates hydrogen. After the products are desorbed, the Ti^{3+ δ} O_R Cu^{1+ δ} bonds return to the initial state of Ti³⁺ O_V Cu⁺ bonds. The reversible TiOCu bonds greatly improve the activity and stability of the Cu/Ti-SiO₂ catalysts. When the content of Ti is controlled at 0.4 wt%, the conversion and selectivity can reach 100% and 98.8%, respectively, and remain stable for 260 h without performance degradation.

Recent progress in hydrogenation of esters on heterogeneous bimetallic catalysts

The development and research of highly effective heterogeneous catalysts for the hydrogenation of esters, providing high activity and selectivity of the formation of the corresponding alcohols, is an urgent task of modern heterogeneous catalysis.

Effects of Basic Promoters on the Catalytic Performance of Cu/SiO2 in the Hydrogenation of Dimethyl Maleate

Continuous hydrogenation of dimethyl maleate (DMM) toγ-butyrolactone (GBL), 1,4-butanediol (BDO) and tetrahydrofuran (THF) is a promising process in industry. In this study, Cu-M/SiO2 catalysts modified by basic promoters (M = Mg, Ca, Sr, Ba, La) were prepared, and characterized by physical adsorption of N2, in situ XRD, H2-TPR, CO2-TPD. With the addition of basic promoters, the basicity of Cu-M/SiO2 catalysts was improved. The particle size of CuO on Cu-M/SiO2 catalyst was increased after modified by Mg, Ca, Sr, Ba. However, the CuO particle was decreased on the Cu-La/SiO2 catalyst. The series of Cu-M/SiO2 catalyst was applied to the hydrogenation of DMM. The addition of basic promoters increased the selectivity of GBL during the hydrogenation for the basic promoters improved the dehydrogenation of BDO to GBL in alkaline sites. Furthermore, Cu-La/SiO2 presented a higher activity in the hydrogenation of DMM, due to its higher dispersion of Cu.

Freezing copper as a noble metal-like catalyst for preliminary hydrogenation

The control of product distribution in a multistep catalytic selective hydrogenation reaction is challenging. For instance, the deep hydrogenation of dimethyl oxalate (DMO) is inclined to proceed over Cu/SiO₂ catalysts because of inevitable coexistence of Cu⁺ and Cu⁰, leading to hard acquisition of the preliminary hydrogenation product, methyl glycolate (MG). Here, the oriented DMO hydrogenation into MG is achieved over the sputtering (SP) Cu/SiO₂ catalysts with a selectivity of more than 87% via freezing Cu in a zero-valence state. Our density functional theory calculation results revealed that Cu⁰ is the active site of the preliminary hydrogenation step, selectively converting DMO to MG via •H addition, while Cu⁺ is a key factor for deep hydrogenation. The prominent Coster-Kronig transition enhancement is observed over SP-Cu/SiO₂ from Auger spectra, indicating that the electron density of inner shells in Cu atoms is enhanced by high-energy argon plasma bombardment during the SP process. Thus, the "penetration effect" of outermost electrons could also be enhanced, making these Cu nanoparticles exhibit high oxidation resistance ability and present noble metal-like behaviors as Au or Ag. Therefore, the regulation of Cu chemical properties by changing the electron structure is a feasible strategy to control the hydrogenation products, inspiring the rational design of selective hydrogenation catalysts.

Status and prospects in higher alcohols synthesis from syngas

Higher alcohols are important compounds with widespread applications in the chemical, pharmaceutical and energy sectors. Currently, they are mainly produced by sugar fermentation (ethanol and isobutanol) or hydration of petroleum-derived alkenes (heavier alcohols), but their direct synthesis from syngas (CO + H₂) would comprise a more environmentally-friendly, versatile and economical alternative. Research efforts in this reaction, initiated in the 1930s, have fluctuated along with the oil price and have considerably increased in the last decade due to the interest to exploit shale gas and renewable resources to obtain the gaseous feedstock. Nevertheless, no catalytic system reported to date has performed sufficiently well to justify an industrial implementation. Since the design of an efficient catalyst would strongly benefit from the establishment of synthesis-structure-function relationships and a deeper understanding of the reaction mechanism, this review comprehensively overviews syngas-based higher alcohols synthesis in three main sections, highlighting the advances recently made and the challenges that remain open and stimulate upcoming research activities. The first part critically summarises the formulations and methods applied in the preparation of the four main classes of materials, i.e., Rh-based, Mo-based, modified Fischer-Tropsch and modified methanol synthesis catalysts. The second overviews the molecular-level insights derived from microkinetic and theoretical studies, drawing links to the mechanisms of Fischer-Tropsch and methanol syntheses. Finally, concepts proposed to improve the efficiency of reactors and separation units as well as to utilise CO₂ and recycle side-products in the process are described in the third section.

Recent progress in improving the stability of copper-based catalysts for hydrogenation of carbon–oxygen bonds

Five different strategies to enhance the stability of Cu-based catalysts for hydrogenation of C–O bonds are summarized in this review.

Effect of SiO2 support properties on the performance of Cu–SiO2 catalysts for the hydrogenation of levulinic acid to gamma valerolactone using formic acid as a hydrogen source

Vapor phase catalytic transfer hydrogenation of levulinic acid with formic acid was carried out over Cu–SiO₂ catalysts having different physicochemical properties.

Comparative study of silica-supported copper catalysts prepared by different methods: formation and transition of copper phyllosilicate

Copper phyllosilicate turned into Cu⁰ rather than Cu⁺ after calcination and reduction.