Cu/SiO2 Catalysts Prepared by the Sol-Gel Method for Hydrogenation of Dimethyl Oxalate to Ethylene Glycol

Alkaline earth modified activated carbon supported Cu catalysts with enhanced selectivity in the hydrogenation of dimethyl oxalate to methyl glycolate

In this work, the effect of alkaline earth metal modification on the catalytic performance of activated carbon supported Cu was investigated. The experimental results showed that the introduction of Ca and Sr improved the selectivity of methyl glycolate (MG) during hydrogenation of dimethyl oxalate (DMO) in gas phase. The optimal loading amount of Ca was 0.1 wt%, and under the optimal conditions (temperature 240 °C, pressure 2 MPa, hydrogen-ester ratio of 80, feedstock 15% DMO methanol solution, and WLHSVDMO = 0.9 h-1) the selectivity of MG was as high as 94%, and the conversion of DMO was 97%. The optimal loading of Sr was 0.2 wt% with MG selectivity up to 89% and DMO conversion of 98%. The results of catalyst characterization showed that both Ca and Sr modifications were beneficial to further reduce the particle size of Cu, improve the dispersion of Cu, increase the basicity of the catalyst, and improve the stable presence of Cu+ during the reaction process. Cu+ was beneficial to the stabilization of the MG species, in which Cu+ accounted for more in the Ca-modified catalysts Cu+/(Cu+ + Cu0) = 0.65, and in the Sr-modified ones Cu+/(Cu+ + Cu0) = 0.51. Therefore, both Ca and Sr modified catalysts showed improvement in the selectivity of MG.

Hydrogenation of Carboxylic Acids, Esters, and Related Compounds over Heterogeneous Catalysts: A Step toward Sustainable and Carbon-Neutral Processes

The catalytic hydrogenation of esters and carboxylic acids represents a fundamental and important class of organic transformations, which is widely applied in energy, environmental, agricultural, and pharmaceutical industries. Due to the low reactivity of the carbonyl group in carboxylic acids and esters, this type of reaction is, however, rather challenging. Hence, specifically active catalysts are required to achieve a satisfactory yield. Nevertheless, in recent years, remarkable progress has been made on the development of catalysts for this type of reaction, especially heterogeneous catalysts, which are generally dominating in industry. Here in this review, we discuss the recent breakthroughs as well as milestone achievements for the hydrogenation of industrially important carboxylic acids and esters utilizing heterogeneous catalysts. In addition, related catalytic hydrogenations that are considered of importance for the development of cleaner energy technologies and a circular chemical industry will be discussed in detail. Special attention is paid to the insights into the structure-activity relationship, which will help the readers to develop rational design strategies for the synthesis of more efficient heterogeneous catalysts.

Impact of preparation method on nickel speciation and methane dry reforming performance of Ni/SiO2 catalysts

The methane dry reforming reaction can simultaneously convert two greenhouse gases (CH₄ and CO₂), which has significantly environmental and economic benefits. Nickel-based catalysts have been widely used in methane dry reforming in past decade due to their low cost and high activity. However, the sintering and coke deposition of catalysts severely limit their industrial applications. In this paper, three Ni/SiO₂ catalysts prepared by different methods were systematically studied, and the samples obtained by the ammonia evaporation method exhibited excellent catalytic performance. The characterization results such as H₂-TPR, XPS and TEM confirmed that the excellent performance was mainly attributed to the catalyst with smaller Ni particles, stronger metal-support interactions, and abundant Ni-O-Si units on the catalyst surface. The anti-sintering/-coking properties of the catalyst were significantly improved. However, the Ni/SiO₂-IM catalyst prepared by impregnation method had uneven distribution of nickel species and large particles, and weak metal-support interactions, showing poor catalytic performance in methane dry reforming. Since the nickel species were encapsulated by the SiO₄ tetrahedral network, the Ni/SiO₂-SG catalyst prepared by sol-gel method could not expose more effective active sites even if the nickel species were uniformly dispersed, resulting in poor dry reforming performance. This study provides guidance for the preparation of novel anti-sintering/-coking nickel-based catalysts.

Enhanced metal-support interaction between Pd and hierarchical Nb2O5via oxygen defect induction to promote CO oxidative coupling to dimethyl oxalate

Production of ethylene glycol from coal is a particularly interesting route as it is an economic alternative to the petrochemical-based route. In this process, effectively generating dimethyl oxalate (DMO) is a crucial step by CO oxidative coupling reaction under Pd-based catalysts. However, the aggregation of Pd species over the support is still an issue that relates to the deterioration of catalytic activity and stability. To this end, enhancing the metal-support interaction is urgently required. In this work, hierarchical Nb₂O₅ (H-Nb₂O₅) microspheres with abundant oxygen defects were synthesized to anchor the Pd species thus promoting the electron transfer between Pd species and Nb species associated with the generation of interfacial Pd-NbO_x sites. Besides, the thinned electron density of Pd species resulting from the electron-withdrawing effect of Nb species is beneficial for activating the adsorbed CO molecules, leading to superior catalytic activity. The Pd/H-Nb₂O₅ catalyst exhibited 63.1% of CO conversion (theoretical maximum conversion: 64.3%) and 92.9% of DMO selectivity, with a DMO weight time yield of 1297.9 g kg_cat.^-1 h^-1, and remained robust even after 50 h of time on stream evaluation. Current work provides a deep insight into the CO activation mechanism and helps improve the catalytic stability by boosting interfacial electron interaction via oxygen defects induction, and also sheds light on the design and synthesis of high-performance catalysts in other heterogeneous catalysis fields.

Boosting Interfacial Electron Transfer between Pd and ZnTi-LDH via Defect Induction for Enhanced Metal-Support Interaction in CO Direct Esterification Reaction

Strong metal-support interaction is crucial to the stability of catalysts in heterogeneous catalysis. However, reports on boosting interfacial electron transfer between metal and support via defect induction for enhanced metal-support interaction are limited. In this work, ultrathin reducible ZnTi-layered double hydroxide (LDH) nanosheets with rich oxygen defects were synthesized to stabilize Pd clusters, and the rich oxygen defects promoted Pd cluster bonding with Zn and Ti atoms in supports, thereby forming a metal-metal bond. Electron spin resonance (ESR), X-ray absorption fine spectra (XAFS), and density functional theory (DFT) calculations demonstrate remarkable interfacial electron transfer (0.62 e). The Pd/ZnTi-LDH catalyst shows superior catalytic stability for CO direct esterification to dimethyl oxalate. By contrast, the nonreducible Pd/ZnAl-LDH catalyst with a few oxygen defects shows minimal interfacial electron transfer (0.08 e), which leads to relatively poor catalytic stability. This work provides a deep insight into promoting the stability of catalysts by boosting interfacial electron transfer via defect induction.

Zn2+ stabilized Pd clusters with enhanced covalent metal-support interaction via the formation of Pd-Zn bonds to promote catalytic thermal stability

Pd-Based heterogeneous catalysts have been demonstrated to be efficient in numerous heterogeneous reactions. However, the effect of the support resulting in covalent metal-support interaction (CMSI) has not been researched sufficiently. In this work, a Lewis base is modulated over MgAl-LDH to investigate the support effects and it is further loaded with Pd clusters to research the metal-support interactions. MgAl-LDH with ultra-low Pd loading (0.0779%) shows CO conversion (55.0%) and dimethyl oxalate (DMO) selectivity (93.7%) for CO oxidative coupling to DMO, which was gradually deactivated after evaluation for 20 h. To promote the stability of Pd/MgAl-LDH, Zn2+ ions were introduced into the MgAl-LDH support to strengthen the CMSI by forming Pd-Zn bonds, which further increased the adsorption energy of the Pd clusters on ZnMgAl-LDH, and this was verified by X-ray absorption fine structure (XAFS) measurements and density functional theory (DFT) calculations. The stability of the Pd/ZnMgAl-LDH catalyst could be maintained for at least 100 h. This work highlights that covalent metal-support interactions can be strengthened by forming new metal-metal bonds, which could be extended to other systems for the stabilization of noble metals over supports.

Applied catalysis for sustainable development of chemical industry in China

Progressing green chemical technologies is significant to the sustainable development of chemical industry in China, as the energy and environment problems increasingly became great challenges to the whole society. The scientific connotation of sustainable energy chemical engineering can be generalized as green carbon/hydrogen science which means optimization of carbon/hydrogen atom economics based on high efficient catalysis and low-carbon emission. This review illustrated recent advances in developing sustainable technologies for applied catalysis in chemical industry of China, including the fields of high efficient conversion of heavy oil, green petrochemical catalytic technologies, clean utilization of coal and natural gas, promoting sustainable resources and clean energy, etc. Moreover, from the view of industrial point, some important common scientific problems were discussed and summarized, such as the relation between molecular diffusion and catalyzing efficiency, homogeneous catalysis in heterogeneous catalysts, in situ or operando characterization of industrial catalysis, etc., aiming to supplying a forward roadmap to academia and/or industry.

Dynamic redox cycle of Cu0 and Cu+ over Cu/SiO2 catalyst in ester hydrogenation

Esters can oxidize Cu⁰ to Cu⁺ and the obtained Cu⁺ can be reduced back to Cu⁰ by H₂ under reaction conditions. Cu⁰ and Cu⁺ are in a dynamic cycle during ester hydrogenation.

Insights into the reaction mechanism of CO oxidative coupling to dimethyl oxalate over palladium: a combined DFT and IR study

Oxidative coupling of toxic pollutant CO to form the platform raw chemical material dimethyl oxalate (DMO) has been industrialized however the catalytic mechanism has been unknown so far.