Construction Of New Nano-copper-based Catalytic Materials And Their Application In Gas-phase Hydrogenation Of Dimethyl Oxalate

Ethylene glycol (EG), due to its water solubility with arbitrary ratio, high boiling point and low freezing point, is extensively used as antifreeze, lubricant, plasticizer, surfactant and so on. Besides, it has also been greatly applied in synthesizing polymer and polyester. Nowadays, ethylene oxidation is a universal industrial approach to produce EG. However, as crude oil resource shrinks, synthesis of EG from syngas attracts more and more interest. Alternative methods that proceed under mild reaction condition with stable and easy-to-handle heterogeneous catalysts are therefore economically and environmentally benign. In this dissertation, the copper catalysts were nanocasting to catalyze the gas ester hydrogenation reaction, which is a clean route in accordance with the demand of green chemistry and sustainable development.Systematic investigations on the size effect, support effect, textural structure effect, promoter effect and composite support effect of the copper based catalysts were carried out by taking the gas dimethyloxalate hydrogenation reaction as a probe reaction via series of physical and chemical characterizations. A series of highly active, selective and stable Cu-based catalysts were developed on the basis of the profound understanding of the reactions. In addition, nano silver catalysts were also developed based on the similarity of the electronic structure with Cu. High DMO hydrogenation activity and EG selectivity were obtained under the optimized reaction conditions. At the same time, the active sites and of the copper and silver catalysts, as well as the interaction between metal and support effects were also discussed and the following results were obtained.1. Support effect of Cu-based catalysts and their catalytic performances in the hydrogenation of DMOThe transition metal oxide and main group metal or nonmetal oxides were used as supports for copper catalysts, and most of them exhibited a certain activity under high reaction temperature and low liquid hour space velocity (LHSV).Among the main group nonmetal oxide supported copper catalysts, Cu/SiO2 catalyst exhibited the best hydrogenation activity and EG selectivity.100% conversion and up 95% selectivity could be obtained under the optimized reaction conditions. Carbon materials (active carbon, carbon nano-tube) supported copper catalysts showed lower activity due the weak interaction between copper species and carbon support.All the main group metal oxide supported Cu catalysts including Cu/MgO?Cu/Al2O3 and Cu/SnO2 catalysts exhibited lower hydrogenation activity, which were only half of that of Cu/SiO2 catalyst. Besides, the products composed of lots of byproducts such as ethers.Among the transition metal oxide supported Cu catalyst, Cu/TiO2?Cu/ZrO2 and Cu/CeO2 catalysts exhibited higher hydrogenation activity and EG selectivity, compared with Cu/ZnO?Cu/La2O3?Cu/Fe2O3?Cu/Co3O4?Cu/MnO2 and Cu/Y2O3 catalysts.Based on the above results, it could be concluded that copper particle size must be small enough to obtain high activity. However, the copper particle size was not the decisive factor and the type of the support was also important.2. Textural structure effect of Cu/SiO2 catalysts and their catalytic performance in the hydrogenation of DMOSeries of mesoporous SiO2 with different textural structures (HMS, SBA-15, MCM-41, MCM-48, and MCF) were used as supports to synthesize copper catalysts. It is showed that Cu/SBA-15 catalyst exhibited the best hydrogenation activity and EG selectivity. Up to 95% yield of EG could be obtained at LHSV=0.83 h-1, which might could be resulted from the highly copper dispersion, larger exposed metallic copper surface area and pore confinement effect. The sequence of the catalytic performance was Cu/SBA-15>Cu/HMS> Cu/MCM-41> Cu/MCM-48> Cu/MCF, among which, almost the same catalytic performance could be obtained on the Cu/SBA-15 and Cu/HMS catalysts.3. Quantum size effect of copper particles in the Cu/HMS catalysts and their influence on the catalytic performance of hydrogenation of DMOCu/HMS catalyst was synthesized via the ammonia evaporation method. Catalysts with different Cu particle sizes from 4.4 mn to 8.1 nm were obtained via tuning the calcinations temperature of the catalysts. The theoretical copper loading is 5 wt%. This kind of preparation method could synthesize very stable copper particles, which would not segregate even at 1073 K. Taking the weak interaction between Cu and silica into the consideration, it is convenient to research the copper size effect. It was showed that the activity is inversely proportional to the copper size. By linear fit between the normalized relative activity and 1/D (1/D2), it is found that the relative activity of the linear fit with 1/D is better than that of 1/D2, which indicated that the interface between the copper particles and the support also played an important role in the hydrogenation reaction. The active sites are composted of exposed copper particles and the interface between the surface copper species and silica.The atmosphere treatment has great influences on the copper species transfer between the bulk and the surface. Under the treatment of air atmosphere, as the increase of the treatment temperature, the CuO species transfer from the surface to the bulk; while under the treatment of H2(5%Vol.)/Ar, as the increase of the treatment temperature, the copper species transfer from the bulk to the surface until surface saturation, which indicated that lower calcinations temperature and higher reduction temperature would be helpful to obtain higher surface copper.4. Synthetic approach effect of Cu/HMS catalysts and their influences on the catalytic performance of hydrogenation of DMOCu loading effects on the compositions, structures and performances of catalysts were investigated via Cu/HMS catalyst synthesized by the traditional impregnation method (IM).100% DMO conversion and 92% EG selectivity could be obtained under the Cu/HMS catalyst with 5% copper loading. Combining the catalyst structures and catalytic performances, it is found that the larger exposed metallic copper surface area and smaller copper particle size would great improve the catalytic activity and selectivity. Although the catalyst prepared by IM has advantages of simple operation and easy repeat, the segregation would happen as the increase of the copper loading. The catalytic performance was also limited by its maximum copper loading.Ion adsorption method (IA) was adopted to synthesize Cu/HMS catalyst for investigation of the surface copper species adsorbed on the surface of the HMS. The ion adsorption temperature and copper ion precursor were found to have great influence on the structure evolution and catalytic performance of Cu/HMS catalyst. Compared with the catalyst prepared by IM, the catalytic performance increased remarkably under the same reaction conditions. The general performance is in the following sequence:IA-333=IA-323>IA-343>IA-353>IA-303>IA-313>IM-333.The Cu precursor has great influence on the adsorption amount on the surface, which main depends on the interaction between the copper ions and the surface hydroxyl of HMS under the solution conditions. Under the base condition, it is helpful for Cu(NH3)42+ to react the surface hydroxyl of HMS. The Cu2+ complex has been grafted on the surface of silica forming ((?)SiO)2 Cu(NH3)2(H2O)4 at 298 K. When exposing to the air atmosphere again at 373 K, the NH3 and H2O molecular on the surface would be eliminated and would form three kinds of new species:(?SiO)2Cu2+(H2O)4-x, (?SiO)3Cu2+ and octahedral copper species. After exposing to the air condition, the three species would be transformed into a single copper species (?SiO)2Cu2+(H2O)4. Higher content and dispersion of copper species would be obtained by using CuNH as copper source.For the first time, Cu/HMS catalyst was synthesized via one pot synthesis method. The copper content could be controlled via tuning the ratio between copper precursor and surfactant. By optimizing preparation parameters, near 19% copper could be incorporated into the framework of the HMS. Systematic characterizations showed that the synergism of metallic Cu and Cu+ should be responsible for the high activity and selectivity. The surfactant acted as structure direction agent to form homogeneous mesoporous framework on the one hand, on the other hand, it could be used as dispersive agent of copper species.By using the HMS as hard template and silica sol as silica precursor, series of catalysts with multi-level channel structure were synthesized via a sol-gel and ammonia evaporation joint method. The pore distribution of the catalysts exhibited bimodal distribution function, which played an important role in mass transportation of substrate and products. The mol ratio of silica sol to HMS was found to have great influence on the textural structures of the catalysts and thus affect the catalytic performance.5. Promoter effect of CuP/HMS catalysts and their influence on the catalytic behaviors of DMO hydrogenation reactionMain group metal, main group nonmetal and transition group metal modified Cu/HMS catalysts were synthesized via the ammonia evaporation method. Among those catalysts, nickel modified Cu/HMS catalyst with 3% nickel content exhibited remarkably enhanced catalytic behaviors. Introduction of other promoters such as Ag, Co, La, Ce into the Cu/HMS, the catalytic performance could also be improved, but not better than that of Ni-modified catalyst.Introduction of B onto the surface of the Cu/HMS catalyst greatly improved the hydrogenation activity, EG selectivity and the catalyst stability. The source of B (B2O3?KBH4) and the introduction method (deposition-precipitation method and post impregnation method) were found to have great influence on the surface composition, dispersion of active copper species and electron structure of the catalyst. XPS characterization showed that B2O3 species existed on the surface of the catalyst. The high affinity of B for electrons tended to lower the reducibility of the Cu+ species thus enhanced the stability of the catalysts.A novel family of heterogeneous Cu-Ni/SiO2 catalysts with appropriate metal ratios displayed outstanding selectivity to methyl glycolate (96%) and to ethylene glycol (98%) in the chemoselective gas-phase hydrogenation of dimethyloxalate. The chemical states of nickel species were found to have a strong influence on the structural evolution of the catalysts and correspondent catalytic behaviors. The selectivity to the two products could be tuned by modulating the chemical states of nickel species. It is shown that oxidative nickel species are helpful in improving the dispersion of copper species because of the enrichment of copper on the surface of the nickel species, thus enhancing the catalytic activity and selectivity to ethylene glycol. The selectivity to methyl glycolate could be greatly improved by the Cu-Ni bimetallic catalyst. An 83% yield of methyl glycolate and a 98% yield of ethylene glycol could be obtained over the bimetallic Cu-Ni catalyst and the NiO-modified catalyst, respectively.6. Composite support effect of Cu/SiMOx catalysts and their influences on the catalytic performance of hydrogenation of DMOCopper-containing mesoporous Al-HMS catalysts prepared via the deposition-precipitation method have been found to be highly efficient in the catalytic hydrogenation of dimethyl oxalate (DMO) to ethylene glycohol (EG). Besides the Al chemical environment, the Si/Al ratios of the mesoporous support show remarkable effect on the catalytic performance. The DMO hydrogenation activity increased with the increasing of Al content in the support, and the highest catalytic activity was obtained when the Si/Al ratio of the support reached 25.27 Al NMR shows that the tetrahedral coordination Al species that exists in the framework of the support could enhance the catalytic performance while the extraframework Al species would decrease the catalytic properties. The structural defects produced in the Al-containing mesoporous support play an important role in improving the dispersion of active copper species and enhancing the interaction between the copper species and support. On the basis of the characterizations, the copper species on calcined CuO/Al-HMS samples and reduced Cu/Al-HMS samples were assigned. The improvement of the catalytic performance with proper Si/Al ratio may be ascribed to the increasing defect sites associated with Al cations and the electronic promotion.Highly dispersed and strong interacted copper species could be achieved by combination of titanium sol and silica sol as combination support. The Ti-Si interface might play a significant role in enhancing the catalytic performance of hydrogenation of DMO to EG. Electron transfer happened between the copper species and the titanium. Under the exciting of reaction heat, the electron in the valence band of TiO2 transfer to the Fermi energy level of metal copper.7. Synthesis of Ag/SiO2 catalysts and their influences on the catalytic behaviors of DMOAg/SiO2 prepared by a sol-gel process is highly effective for selective gas-phase hydrogenation of dimethyl oxalate to corresponding alcohols. The catalysts are of great potential as industrially viable and novel catalysts for the production of MG and EG. Different from the Cu-based catalysts, the selectivity of products could be easily tuned by adjusting the reaction temperature. Up to 95% yield of MG could be achieved at 473 K, while 99% yield of EG could be obtained at 553 K. In addition, after 260 h's long time running, the catalyst could still keep stable under the reaction conditions.Silver loading, amount of surface hydroxyl, calcinations temperature and treatment atmosphere were found to have great influences on the catalytic behaviors of the silver-based catalysts. Ag/MCM-41 catalyst with 10% silver loading, calcined at 673 K, pretreated with H2(5%Vol.)/Ar exhibited good catalytic performance in hydrogenation of DMO to MG and EG.