Hydrogenation of Dimethyl Oxalate over Copper-Based Catalysts: Acid–Base Properties and Reaction Paths

Hydrogenation of dimethyl oxalate (DMO) is a potentially important process in C1 chemistry, which produces ethylene glycol (EG) around 473 K and ethanol with higher alcohols (propanol, butanol, etc.) around 553 K. However, the detailed inter-relationship of formation paths for these products has not yet been discussed properly. In this study, we found that the formation paths of higher alcohols from DMO were inhibited around 473 K. On the basis of these results, a two-reactor system with different reaction temperatures was suggested to obtain less of the higher alcohols and more ethanol. Besides, it is found that a higher density of basic sites in the catalyst favors the formation of higher alcohols. An aluminum dopant was applied to decrease the basic sites in the catalysts accompanied by an increment of ethanol selectivity. When the aluminum-doped catalysts were introduced into the two-reactor system, a further improvement in ethanol selectivity was achieved

A High-Performance Nanoreactor for Carbon–Oxygen Bond Hydrogenation Reactions Achieved by the Morphology of Nanotube-Assembled Hollow Spheres

Hydrogenation of carbon–oxygen bonds is extensively used in organic synthesis. However, a high partial pressure of hydrogen or the presence of excess hydrogen is usually essential to achieve favorable conversions. In addition, because most hydrogenations are consecutive reactions, the selectivity is difficult to manipulate, leading to an unsatisfactory distribution of products. Herein, a copper silicate nanoreactor with a nanotube-assembled hollow sphere (NAHS) hierarchical structure is proposed as a solution to these problems. In the case of dimethyl oxalate (DMO) hydrogenation, the NAHS nanoreactor achieves remarkable catalytic activity (the yield of ethylene glycol is 95%) and stability (>300 h) when the H₂/DMO molar ratio is as low as 20 (in comparison to typical values of 80–200). For further investigation, nanotubes and lamellar-shaped Cu/SiO₂ catalysts with similar surface areas of active sites of NAHSs were investigated as contrasts. By a combination of high-pressure hydrogen adsorption and Monte Carlo simulation, it is demonstrated that hydrogen can be enriched on the concave surface of nanotubes and hollow spheres, leading to a favorable activity in such a low H₂ proportion. Furthermore, because of the spatial restriction effect of reactants, adjusting the diffusion path is an effective route for manipulating the selectivity and product distribution of the hydrogenation reactions. By variation in the length of nanotubes on NAHS, the yields of methyl glycolate and ethylene glycol are easily controlled. The NAHS nanoreactor, with insights into the effect of morphology on hydrogen enrichment and spatial restriction of reactant diffusion, offers inspiring possibilities in the rational design of catalysts for hydrogenation reactions

Insight into the Balancing Effect of Active Cu Species for Hydrogenation of Carbon–Oxygen Bonds

Hydrogenation of carbon–oxygen (C–O) bonds plays a significant role in organic synthesis. Cu-based catalysts have been extensively investigated because of their high selectivity in C–O hydrogenation. However, no consensus has been reached on the precise roles of Cu⁰ and Cu⁺ species for C–O hydrogenation reactions. Here we resolve this long-term dispute with a series of highly comparable Cu/SiO₂ catalysts. All catalysts represent the full-range distribution of the Cu species and have similar general morphologies, which are detected and mutually corroborated by multiple characterizations. The results demonstrate that, when the accessible metallic Cu surface area is below a certain value, the catalytic activity of hydrogenation linearly increases with increasing Cu⁰ surface area, whereas it is primarily affected by the Cu⁺ surface area. Furthermore, the balancing effect of these two active Cu sites on enhancing the catalytic performance is demonstrated: the Cu⁺ sites adsorb the methoxy and acyl species, while the Cu⁰ facilitates the H₂ decomposition. This insight into the precise roles of active species can lead to new possibilities in the rational design of catalysts for hydrogenation of C–O bonds

Modification of Y Zeolite with Alkaline Treatment: Textural Properties and Catalytic Activity for Diethyl Carbonate Synthesis

In this work, we modified NaY zeolite (Si/Al = 5) with NaOH solutions of different concentrations followed by ion exchange with NH₄NO₃ to the H form of the zeolite. Treated NaY was used as a catalyst support for the preparation of CuY for diethyl carbonate (DEC) synthesis through the oxidative carbonylation of ethanol. The textural and acidic properties of NaY and the catalytic performance of the corresponding CuY materials were investigated. Compared with the untreated sample, CuY catalysts using modified NaY samples as supports exhibited higher conversions of ethanol and similar selectivities to DEC. Inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), N₂ adsorption, Fourier transform infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM) were used to explore the origin of the improvement in activity. The experimental results showed that alkaline treatment induced defects in the zeolite framework and greatly promoted dealumination through ion exchange assisted by microwave radiation, which caused the generation of meso- and macropores in zeolite Y and contributed to the catalytic performance. Furthermore, the increased amount of hydroxyl species in supercages and extraframework aluminum species resulted in an increase in the number of Cu active sites and further DEC production

Insight into the Tunable CuY Catalyst for Diethyl Carbonate by Oxycarbonylation: Preparation Methods and Precursors

Three different methods were used to prepare CuY catalysts, which play an important role in Cu loading and ion-exchange level during the oxidative carbonylation of ethanol to synthesize diethyl carbonate. Of the prepared CuY catalysts, those synthesized by the ammonia evaporation method exhibited a significantly enhanced activity compared to those obtained by the other two methods. In addition, under optimized conditions, four different copper precursors were applied to adjust the textural properties and chemical states of the CuY catalysts. To obtain a deep understanding of their structure–performance relationship, XRD, XPS, CO adsorption, DRIFTS, and NH₃ TPD were conducted to characterize the CuY catalysts. The experimental results indicated that the catalytic performances were in line with the proportions of Cu⁺ in CuY catalysts, which can be regulated by cupric precursors. In addition, the textural structures of the catalysts and the acidity and type of Cu⁺ species influenced by the precursors were all responsible for the activity and product distribution

Deactivation Kinetics for the Carbonylation of Dimethyl Ether to Methyl Acetate on H‑MOR

The carbonylation of dimethyl ether (DME) to methyl acetate (MA) is one of the crucial steps in an indirect synthesis route of ethanol from syngas (CO+H₂). The H-MOR zeolite exhibits excellent activity and selectivity at mild conditions. However, the catalyst suffers rapid deactivation due to the carbonaceous deposits on Brønsted acid sites. In this study, the deactivation kinetics for the carbonylation of DME to MA on the H-MOR zeolite was investigated. Based on the fitting results and <i>in situ</i> FTIR analysis, a model taking into account the composition concentration was established. This deactivation kinetic model allows simulating the concentration of different compounds in the reaction medium with time on stream under different experimental conditions. In this model, coke is considered to be derived from DME and CO. Moreover, CO remarkably accelerates the coke formation, and the effect of its concentration on the deactivation rate is quantified. The establishment of deactivation kinetics will be conductive to elucidate the coke formation mechanism and optimize the process conditions

Hydrogen Production via Glycerol Steam Reforming over Ni/Al₂O₃: Influence of Nickel Precursors

This paper describes an investigation regarding the influence of Ni precursors on catalytic performances of Ni/Al₂O₃ catalysts in glycerol steam reforming. A series of Ni/Al₂O₃ is synthesized using four different precursors, nickel nitrate, nickel chloride, nickel acetate, and nickel acetylacetonate. Characterization results based on N₂ adsorption–desorption, X-ray diffraction, H₂ temperature-programmed reduction, H₂ chemisorption, transmission electron microscopy, and thermogravimetric analysis show that reduction degrees of nickel, nickel dispersion, and particle sizes of Ni/Al₂O₃ catalysts are closely dependent on the anion size and nature of the nickel precursors. Ni/Al₂O₃ prepared by nickel acetate possesses the moderate Ni reduction degree, high Ni dispersion, and small nickel particle size, which possesses the highest H₂ yield. Reaction parameters are also examined, and 550 °C and a steam-to-carbon ratio of 3 are optimized. Moreover, coke deposition, mainly graphite species, leads to the deactivation of Ni/Al₂O₃ catalysts in glycerol steam reforming. Nickel chloride-derived Ni/Al₂O₃ catalysts suffer from severe coke deposition and low reaction activity due to large Ni particle size, low Ni dispersion, and residual chloride

Kinetics Study of Hydrogenation of Dimethyl Oxalate over Cu/SiO₂ Catalyst

Gas-phase hydrogenation of dimethyl oxalate (DMO) on a copper-based catalyst is one of the crucial technologies in the production of ethylene glycol (EG) from syngas. Even though Cu/SiO₂ catalyst is widely used in ester hydrogenation reactions, a kinetics study considering multiple active sites has not yet been reported. In this study, a series of experiments were carried out to investigate the heterogeneous catalytic reaction kinetics of the hydrogenation of DMO over Cu/SiO₂ catalyst. Considering different situations of ester adsorption, H₂ adsorption, and active sites, 34 possible kinetics models were proposed and screened to identify the one most appropriate to describe the hydrogenation of DMO over Cu/SiO₂ catalyst. With the help of relevant thermodynamic theories and statistical evaluations, the optimal model was found to fit well to our experimental data. This model proved that the hydrogenation of DMO depends on the synergistic effect of two active sites, wherein hydrogen and the ester were adsorbed on two different active sites with dissociative states. The dissociative adsorption of the ester was found to be the rate-controlling step in the hydrogenation of DMO over Cu/SiO₂ catalyst prepared by an ammonia-evaporation method

Hydrogen Production via Steam Reforming of Ethanol on Phyllosilicate-Derived Ni/SiO₂: Enhanced Metal–Support Interaction and Catalytic Stability

This paper describes the design of Ni/SiO₂ catalysts obtained from a phyllosilicate precursor that possess high activity and stability for bioethanol steam reforming to sustainably produce hydrogen. Sintering of metal particles and carbon deposition are two major issues of nickel-based catalysts for reforming processes, particularly at high temperatures; strong metal–support interaction could be a possible solution. We have successfully synthesized Ni-containing phyllosilicates by an ammonia evaporation method. Temperature programmed reduction results indicate that the metal–support interaction of Ni/SiO₂ catalyst prepared by ammonia evaporation method (Ni/SiO_2P) is stronger due to the unique layered structure compared to that prepared by conventional impregnation (Ni/SiO_2I). With the phyllosilicate precursor nickel particles highly disperse on the surface, remaining OH groups in the unreduced phyllosilicates promote nickel dispersion and carbon elimination. We also show that high dispersion of Ni and strong metal–support interaction of Ni/SiO_2P significantly promote ethanol conversion and H₂ production in ethanol steam reforming. Ni/SiO_2P produces less carbon deposition compared to Ni/SiO_2I; for the latter, a surface layer of Ni₃C formed during the deactivation

Hollow Copper Nanocubes Promoting CO₂ Electroreduction to Multicarbon Products

Electrochemical carbon dioxide reduction reaction (CO2RR) to multicarbon (C2+) compounds holds great potential for achieving carbon neutrality and storing intermittent renewable energy. The formation of carbon–carbon (C–C) bonds, affected by the concentration of *CO intermediates on the surface of catalysts, is critical to facilitate the production of C2 species. Here, a novel method to prepare uniform hollow oxide-derived copper crystals is reported, reducing CO2 to C2 products (ethylene and ethanol) with an outstanding Faradaic efficiency of 71.1% in 0.1 M KHCO3. The degree of hollowness shows a positive tendency to C2 selectivity but negative to H2 and C1 selectivity. In situ surface-enhanced infrared absorption spectroscopy indicates that hollow structures enhance localized *CO concentration, boosting C–C coupling for producing C2 products. This provides a feasible strategy to enrich important intermediates to deeper reduction products through catalyst structure engineering