Study Of Catalysts For Chemoselective Hydrogenation Of Dimethyl Oxalate To Methyl Glycolate

Energy is still a hot and important strategic issue in nowadays.The C1 chemistry route well caters to the distribution of resource structure in some countries such as China and effectively weakens the dependence on petroleum resources.The hydrogenation of dimethyl oxalate?DMO?,as a key part of the coal-to-ethylene glycol?CTEG?process,has attracted extensive attention in academic and industrial areas.Methyl glycolate?MG?,because of its special chemical structure?containing both hydroxyl and ester groups?,can undergo various chemical reactions.Therefore,MG is usually used as an important fine chemical intermediate to synthesize a series of downstream products with higher value-added.To date,the synthesis of MG by hydrogenation of DMO is a more economical and environmentally-friendly route compared with other catalytic procedures.However,the poor stability of the catalyst,low catalytic activity and selectivity of the target product are the key problems to be solved in this synthesis process.Hence,in order to satisfy industrial demands and practical application,it is essential to develop a novel and efficient catalyst.In this paper,the application of copper-based catalysts and silver-based catalysts in the selective hydrogenation of DMO to MG was investigated.The effects of Ag and Ni additives on the catalytic properties of copper-based catalysts,and the influence of B and Ni contents on the catalytic performance of silver-based catalysts were studied.The main results are as follows:1.Application of copper-based catalyst in the synthesis of MG by DMO hydrogenationA series of Cu-Ni-Ag/SiO₂ catalysts with different metal contents were prepared by ammonia-evaporation deposition-precipition?AE?method.The effects of metal contents and reaction conditions on the catalytic performance of Cu-based catalysts were investigated.The20%Cu-1%Ni-3%Ag/SiO₂ catalyst exhibited the best catalytic performance at 220?,2 MPa,H₂/DMO molar ratio of 80 and LHSV of 0.7 h^-1.Combined with the characterization results,it was found that the change of the amounts of metallic Ag and Ni had influence on the dispersion of Cu species and the distribution of Cu⁰ and Cu⁺species on the surface of the catalysts.In addition,the catalytic performance of Cu-based catalysts supported on different silica materials?SiO₂?HMS?SBA-15?KIT-6 and MCF?was compared.The results showed that the Cu-based catalyst supported on SiO₂ presented the best catalytic activity.2.Application of the Ag/SiO₂ catalyst in the synthesis of MG by DMO hydrogenationThe Ag/SiO₂ catalysts with different Ag loadings were synthesized by AE method.The catalytic performance of these catalysts was determined on the DMO selective hydrogenation to MG.The 10%Ag/SiO₂ catalyst had the best catalytic activity with the highest surface Ag concentration.In addition,the catalytic performance of 10%Ag-1%X/SiO₂?X=Ni,B,La,Ce,Mo?catalysts in the DMO hydrogenation to MG was investigated.The results showed that the conversion of DMO was improved by the introduction of B and the selectivity of MG was improved by the addition of Ni.In general,the desired results were not achieved by adding additives,but it provided some useful information for the design of a novel Ag-based catalyst.3.Effect of boria on the catalytic performance of the Ag/SiO₂ catalystA series of Ag-B/SiO₂ catalysts with different boron contents were fabricated by two steps,i.e.ammonia evaporation followed by impregnation,and showed excellent catalytic performance and stability in the synthesis of MG by DMO hydrogenation.It was found that the addition of B increased the dispersion of active components and inhibited the aggregation and growth of silver nanoparticles,thus improving the catalytic activity and stability of Ag/SiO₂ catalyst.The Ag-1%B/SiO₂ catalyst exhibited the best catalytic performance.The conversion of DMO was 100%,the yield of MG was 88.3%,and the catalyst could run stably for more than 300 h.4.Effect of nickel on the catalytic performance of the Ag/SiO₂ catalystA series of Ag-Ni/SiO₂ catalysts with different Ni contents were prepared by similar produces as that used for the Ag-B/SiO₂ catalysts.The results showed that the doping of Ni significantly improved the catalytic performance of Ag/SiO₂ catalyst.When 0.5 wt.%Ni was added,the catalyst had the best catalytic activity.The conversion of DMO was 100%,the yield of target product was 92.5%,and the catalyst could run stably and efficiently for more than 300 h.The experimental results indicated that the introduction of Ni reduced the grain size of Ag nanoparticles and changed the chemical environment on the surface of the catalyst,thus affecting the catalytic activity of Ag-based catalysts.Additionally,the deactivation of Ag-based catalysts was investigated by XRD characterization,which could be attributed to the aggregation and growth of silver nanoparticles,the loss of active sites and the decrease of dispersion of active components.