Synthesis And Catalytic Performance Of Highly Dispersed Copper Based Catalysts For Dimethyl Oxalate To Ethylene Glycol

Numerous works have been devoted to the hydrogenation of oxalates toethylene glycol （EG）, which is an important chemical widely used in finechemical industry. The dimethyl oxalate （DMO） to EG process was developedand the application on industrial scale was planned for the nineties of the lastcentury. The commercial catalysts used in the hydrogenation of dialkyl oxalatesare Cr₂O₃promoted copper-based heterogeneous catalysts. The toxicity of suchtype of Cr-containing catalysts, however, can cause severe environmentalpollution and thus limits their practical applications, in spite of high yield ofEG. Recently, Cu-based catalysts supported on high-surface-area oxidematerials, such as SiO₂and mesoporous materials, have been reported to showgood catalytic activity towards oxalates hydrogenation to EG. In these cases, it is found that the weak acidic and basic property of supports is responsible forthe high selectivity to EG in hydrogenation. The copper-containing catalystshave been of great interest due to their good selectivities and activities in widerange of reactions of various organic compounds. The most widely usedpreparation routes for above supported Cu-based catalysts aredeposition-precipitation and incipient wetness impregnation, which easily leadto the formation of copper particles with large diameters and low metalloadings.Layered double hydroxides （LDHs） are known as a family of highlyordered two-dimensional anionic clay materials. Because different M2+and M3+metal cations with tunable compositions can uniformly distribute and orderlyprearrange at an atomic level within the layers or in the interlayer space in theform of metal complexes, well-dispersed metal catalysts can be obtained byreducing calcined LDHs with desired metal species. LDH-derived supportedmetal catalysts have two advantages:（ⅰ） active components with adjustablecontent can be uniformly integrated into the LDH structure;（ⅱ） metalnanoparticles with tunable particle size can be formed in a controllable manner.At present, the design of efficient, nontoxic and cheap supported catalysts hasbecome an important issue in terms of economic and environmental aspects.（1）New environmentally friendly Cu-based catalysts with high dispersionwere synthesized through a facile CuZnAl-LDH precursor route. The catalyticperformance of the catalysts for gas phase hydrogenation of DMO to EG was evaluated, and the effect of calcination temperature for CuZnAl-LDH precursoron the composition, texture, structure and catalytic properties of the resultingCu-based catalysts was preliminarily investigated. Powder X-ray diffraction（XRD）, transmission electron microscopy （TEM）, X-ray photoelectronspectroscopy （XPS）, N₂adsorption-desorption, H₂temperature programmedreduction （H₂-TPR） and H₂-N₂O titration indicated that the composition,texture, and structure of resulting copper-based catalysts were profoundlyaffected by the calcination temperature of LDH precursor. Moreover, theas-synthesized catalyst calcined at600°C was found to exhibit superiorcatalytic hydrogenation performance with an EG yield of94.7%to the othercatalysts calcined at500and700oC, which should be attributed to the presenceof the highly-dispersed active metallic copper species over metal oxide matrix.（2）we have prepared a variety of Cu/ZnO/ZrO₂catalysts and examinedthe catalytic behavior in the hydrogenation of DMO to EG. The XPScharacterization was carried out to study the changes of surface copper speciesinduced by the addition of Zr. This work was undertaken to provide furtherinsight into the nature of active sites of Cu/ZnO/ZrO₂catalysts.Among variousCu/ZnO/ZrO₂catalysts with the Cu/Zn/Zr ratio, the one ratio1/1/1ofCu/Zn/Zr obtains the best activity for EG synthesis by hydrogenation of DMO.The TPR and XPS analyses reveal that a new copper oxide phase is formed inthe calcined Cu/ZnO/ZrO₂catalysts by the dissolution of zirconium ions incopper oxide. In addition, the Cu/ZnO/ZrO₂catalyst with Cu/Zn/Zr mol ratio 1/1/1turns out to contain the appropriate amount of the new copper oxidephase. When the Cu/ZnO/ZrO₂catalysts is reduced, the Cu²⁺species present inthe ZrO₂lattice is transformed to Cu⁺species. This leads to the speculationthat the addition of ZrO₂to Cu/ZnO catalysts gives rise to the formation ofCu⁺species, which is related to activity of Cu/ZnO/ZrO₂catalyst in addition toCu metal particles. Consequently, the ratio of Cu⁺/Cu⁰is an important factorfor the specific activity of Cu/ZnO/ZrO₂catalyst for the hydrogenation ofDMO to EG.（3）High-surface area microcellular structure CuO/ZnO/ZrO₂catalystswere prepared by a surfactant-assisted method of nanocrystalline particleassembly, and characterized by XRD, N₂adsorption, SEM, H₂-TPR, TG-DTAand XPS techniques. The catalytic properties of the CuO/ZnO/ZrO₂nanocatalysts were evaluated by the hydrogenation of DMO to EG using amicroreactor-GC system. XRD and SEM analysis indicated that the catalystparticles were nanoscaled with rhombic structure. N₂adsorption–desorptionisotherms revealed a microcellular structure system with high-surface area anduniform pore-size distribution. The results of catalytic activity measurementsshowed that these microcellular nanostructured CuO/ZnO/ZrO catalysts werevery active for the hydrogenation of DMO to EG. The catalytic behaviordepended on the dispersion degree, the surface area and the particle size of thecatalyst.The as-synthesized catalyst was found to exhibit superior catalytichydrogenation performance with an EG yield of91%, which should be attributed to the presence of the highly-dispersed active metallic copperspecies over metal oxide matrix.