The autocatalytic hydrolysis kinetics of dimethyl oxalate (DMO) was investigated in an isothermal batch reactor at 328.15–358.15 K. It was observed that DMO hydrolysis involved two reactions in series, with monomethyl oxalate (MMO) as an intermediate product. The results showed that water dominates the initial hydrolysis rate of DMO. In later stages, oxalic acid plays a major role in catalyzing DMO hydrolysis and MMO further hydrolysis. Based on these observations, a mechanism involving the ionization of water and oxalic acid was developed that assumes nucleophilic substitution to be the rate-determining step. Concentration-based rate equations were further deduced including the contribution of water startup and the catalytic action of oxalic acid. In addition, the kinetic and equilibrium parameters were estimated from the experimental data through regression analysis.

•High efficient copper-based catalyst was prepared with ordered mesoporous silica as support precursor.•Appropriate pH value of the solution can ensure the high dispersion of copper species.•The presence of mesoporous structure enhanced the formation of copper phyllosilicate.•20Cu/OMS presented excellent catalytic performance in dimethyl oxalate hydrogenation.A modified ammonia evaporation method with an ordered mesoporous silica as the precursor of the support was applied to prepare the well dispersed copper-based catalysts. Appropriate amount of ammonia was used during the aging stage to prevent the destruction of the ordered mesoporous structure, which can ensure the homogeneous pre-distribution of the copper precursor ([Cu(NH3)4]2+) in the mesopores. Then the formation of copper phyllosilicate or surface Cu–O–Si species can be prompted during the ammonia evaporation stage, resulting in large surface areas of both Cu0 and Cu+ species in the final catalysts. It was also revealed that the formation of copper phyllosilicate led to the destruction of mesoporous silica structure in the ammonia evaporation stage especially at the higher copper loading. The catalysts with various copper loading were systematically characterized and applied in the hydrogenation of dimethyl oxalate to ethylene glycol (EG). An excellent low-temperature catalytic performance and stability were achieved on 20Cu/OMS with EG selectivity of 98.2% at 453 K, due to the superior surface areas of both Cu0 and Cu+, as well as the highest ratio of Cu+/(Cu0 + Cu+).Download high-res image (173KB)Download full-size image

We found that the standard B3LYP and dispersion-corrected B3LYP-D3 methods predicted completely opposite energy order for the Lewis acid–base adducts formed by Ih-C60 or D5h-C70 with normal C2-bound (nNHC) and abnormal C5-bound (aNHC) N-heterocyclic carbenes. By using the validated B3LYP-D3 method and taking the solvent effects into account, the exclusive formations of the nNHC-C60/70 (Li et al. J. Am. Chem. Soc., 2011, 133, 12410–12413) and aNHC-Sc3N@Ih-C80 complexes (Chen et al. Chem. Sci., 2016, 7, 2331–2334) in two recent experiments were suggested to be thermodynamically and kinetically controlled, respectively. In contrast to the reported reactions of endohedral metallofullerenes, aNHC-Sc3N@Ih-C80 hardly isomerizes to the low-energy normal adducts under heat treatment probably due to the substantial energy barrier and excess NHC reagent used in the experiment. Furthermore, the highly regioselective addition of aNHC to the triple-hexagon-junction carbon atom of Sc3N@Ih-C80 was rationalized by using the frontier molecular orbital theory.

A Fe-based catalyst exhibits extremely high selectivity (89.6%) besides excellent catalytic activity in gas-phase dimethyl oxalate hydrogenation. The ethanol formation occurs via hydrogenation of methyl acetate instead of ethylene glycol over the active species Fe5C2.

The novel fibrous nano-silica (KCC-1) based silver nanocatalyst exhibits excellent catalytic activity with a high TOF value (53.2 h−1) in the gas-phase hydrogenation of DMO to MG. Compared with the traditional mesoporous silica materials, KCC-1 remarkably enhances the accessibility of the silver active sites due to its three dimensional hierarchical channel structure.

This work reported a well-fabricated PtSn/SiO2 catalyst prepared by a modified two-step sol–gel (MTSG) method. The adoption of strong electrostatic adsorption (SEA) and sol–gel (SG) method could give both a significantly high Pt dispersion and a large amount of Lewis acid sites. The homogenous distribution of Sn species could not only provide more opportunity for the Pt precursor to disperse on the support by forming Pt–(O–Sn)y2−y analogous species, but also enhance the synergy between Pt and Sn species. Consequently, an excellent activity was achieved in the hydrogenation of acetic acid (AcOH) with a conversion of 100% and ethanol (EtOH) selectivity of 93%. Investigations on the effect of Sn/Pt molar ratio showed that the dispersion of Pt decreases obviously with the increasing Sn/Pt ratio due to the geometric or electronic effects caused by SnOx species. A balancing effect between Pt active sites and Lewis acid sites was found to be responsible for the superior catalytic performance in AcOH hydrogenation. Moreover, a parallel reaction path model was proposed for the hydrogenation of AcOH over PtSn/SiO2 catalyst, in which ethanol and ethyl acetate (AcOEt) are formed competitively through the adsorbed ethoxy intermediate.

The effect of thermal pretreatment on the active sites and catalytic performances of PtSn/SiO2 catalyst in acetic acid (AcOH) hydrogenation was investigated in this article. The catalysts were characterized by N2 physical adsorption, X-ray diffraction, transmission electron microscopy, pyridine Fourier-transform infrared spectra, and H2-O2 titration on its physicochemical properties. The results showed that Pt species were formed primarily in crystalline structure and no PtSnx alloy was observed. Meanwhile, with the increment of thermal pretreatment temperature, Pt dispersion showed a decreasing trend due to the aggregation of Pt particles. Simultaneously, the amount of Lewis acid sites was remarkably influenced by such thermal pretreatment owning to the consequent physicochemical property variation of Sn species. Interestingly, the catalytic activity showed the similar variation trend with that of Lewis acid sites, confirming the important roles of Lewis acid sites in AcOH hydrogenation. Moreover, a balancing effect between exposed Pt and Lewis acid sites was obtained, resulting in the superior catalytic performance in AcOH hydrogenation.

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.

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 Fe-based catalyst exhibits extremely high selectivity (89.6%) besides excellent catalytic activity in gas-phase dimethyl oxalate hydrogenation. The ethanol formation occurs via hydrogenation of methyl acetate instead of ethylene glycol over the active species Fe5C2.

We found that the standard B3LYP and dispersion-corrected B3LYP-D3 methods predicted completely opposite energy order for the Lewis acid–base adducts formed by Ih-C60 or D5h-C70 with normal C2-bound (nNHC) and abnormal C5-bound (aNHC) N-heterocyclic carbenes. By using the validated B3LYP-D3 method and taking the solvent effects into account, the exclusive formations of the nNHC-C60/70 (Li et al. J. Am. Chem. Soc., 2011, 133, 12410–12413) and aNHC-Sc3N@Ih-C80 complexes (Chen et al. Chem. Sci., 2016, 7, 2331–2334) in two recent experiments were suggested to be thermodynamically and kinetically controlled, respectively. In contrast to the reported reactions of endohedral metallofullerenes, aNHC-Sc3N@Ih-C80 hardly isomerizes to the low-energy normal adducts under heat treatment probably due to the substantial energy barrier and excess NHC reagent used in the experiment. Furthermore, the highly regioselective addition of aNHC to the triple-hexagon-junction carbon atom of Sc3N@Ih-C80 was rationalized by using the frontier molecular orbital theory.