Synthesis Of Ethylene Glycol From CO Via Diethyl Oxalate Hydrogenation

Ethylene glycol is a valuable commercial chemical and the import for it increased year by year due to strong demand and limited production compacity in China. Currently the major method widely used for the industrial production of ethylene glycol is based on the direct hydrolysis of ethylene oxide obtained from oil as raw material. However this method is plagued by some problems such as the increased price of crude oil as a result of the long-term of shortage and high energy consumption for the distillation of the large amount of excess water etc. To deal with these problems, it is good choice to prepare ethylene glycol by alternative processes based on non-oil routes taking into account the advantages of sources of raw materials and the characteristics of oil-poor and coal-rich country. Synthesis of ethylene glycol from CO via oxalate ester is two-step process. The first one is oxalate ester synthesis by CO coupling and the second one is synthesis of ethylene glycol by hydrogenation of oxalate ester. In this thesis, some Pd/Al2O3 catalysts prepared for oxalate ester synthesis and Cu/SiO2 catalysts prepared for hydrogenation of oxalate ester were studied and characterized systematically. The catalytic performance was investigated under different preparation conditions and reaction conditions. The specific research work is as follows:(1) Pd/Al2O3 catalysts for diethyl oxalate (DEO) synthesisA series of Pd/Al2O3 catalysts were prepared by the impregnation method. Some supports and catalyst samples were characterized by nitrogen physisorption, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Temperature programmed reduction(TPR), Scanning electron microscopy (SEM) and Transmission electron microscopy(TEM). The results show that, after calcinations at high temperature, the corresponding specific surface area, pore structure, crystal type and surface acidity of y-alumina were changed. For pure Pd/a-Al2O3 catalysts, the active species for the reaction was zero valent palladium with egg-shelled distribution in the support. For the bimetallic Pd-Fe/a-Al2O3 catalyst, the promoter existed in the form of FeO, resulting in the formation of a "sandwich" typed catalyst which could improve the catalytic activity and stability. Some catalysts in terms of their activity and selectivity were evaluated in a laboratory fixed bed reactor. Superior catalytic performance of Pd/α-Al2O3 catalyst over Pd/y-Al2O3 catalyst was ascribed to the support effect. It was found that the effect of [Cl-] content in the palladium based catalyst on the catalytic selectivity and the reseaon for the formation of diethyl carbonate as by-product was disscussed. The Pd-Fe/α-Al2O3 sample for diethyl oxalate synthesis showed good performance and stability with CO conversion 35% and DEO selectivity 95% over Pd/y-Al2O3 catalyst under the optimal reaction conditions with reaction temperature ranged froml 15～125℃, molar ratio of reactants CO to DEO ranged from 1.2 to 1.6 and gas space velocity ranged from 2800-3600h-1(2) Characterization of Cu/SiO2 catalysts for diethyl oxalate hydrogenationA series of Cu/SiO2 catalysts for diethyl oxalate hydrogenation were prepared by an improved sol-gel method with tetraethoxysilane(TEOS) used as silica source and copper nitrate as copper source. Some catalyst samples before and after reduction were characterized systematically. It was found that the copper content and pH value of ammonia solution had a great impact on the specific surface area, pore structure, phase composition, copper oxidation state, reducibility and dispersion. In a certain range of copper content, the copper species in catalyst precursor appeared mainly in the form of copper phyllosilicate with finely dispersed in the support even at elevated copper loading, resulting in the catalyst with higher specific surface area. Copper phyllosilicate in the Cu/SiO2 catalyst precursor undergoes partial decomposition during calcination, resulting in well dispersed CuO particles and intact copper phyllosilicate. In the calcined sol-gel-derived catalyst samples with a copper loading lower than 37.8 wt%, there were two copper species:copper phyllosilicate and well-dispersed CuO whereas in the high copper content (> 37.8 wt%) catalysts the copper species were composed of copper phyllosilicate, well-dispersed CuO, and some of copper oxide agglomerates. Reduction of calcined Cu/SiO2 catalyst sample could lead to Cu+ and Cu0 species, originating from reduction of copper phyllosilicate and CuO respectively. As for the impact of pH value of the ammonia solution, it was found that higher pH value favored the formation of copper phyllosilicate in the catalyst because the silica sol formed during the hydrolysis of TEOS became very reactive at elevated pH value of ammonia soulution. For comparison purpose, some Cu/SiO2 catalyst samples prepared by the deposition precipitation method were characterized, in which the copper species appeared in the form of CuO without copper phyllosilicate, leading to the lower specific surface area and presence of Cu0 in the catalyst sample after reduction process.(3) Evaluation of Cu/SiO2 catalyst for diethyl oxalate hydrogenation to ethylene glycolThe Cu/SiO2 samples prepared by sol-gel method and deposition precipitation method were evaluated focusing on the effect of preparation method on the catalytic properties. The experiment results show that the Cu/SiO2 catalyst prepared by the sol-gel method was superior in catalytic performance to the Cu/SiO2 catalyst prepared by the deposition precipitation method, which could be explained by the existence of finely dispersed copper phyllosilicate in the sol-gel derived catalyst precursor resulting in the reduced catalyst with higher copper surface area and the suitable Cu+/Cu0 ratio. At the same time, the effect of preparation conditions and reaction conditions on catalytic performance was investigated in hydrogenation of diethyl oxalate to ethylene glycol. It is found that the catalytic performance was heavily dependent on copper loading and the pH value of ammonia solution, which was understandable considering that the effect of copper loading and pH value on the catalyst structure. With the rise in copper loading, the diethyl oxalate conversion and ethylene glycol selectivity increased, reaching a maximum at a copper loading of 37.8 wt%, followed by a drop with further rise in copper loading. The same trend was found for the effect of the pH value on the catalytic performance. In addition, the calcination temperature and reduction temperature also played an important role in the catalytic performance. As for the effect of reaction conditions on the catalytic performance, it was found that under optimum reaction conditions[temperature=220℃, pressure=2.0MPa, GHSV=7000h-1, n (H2) :n(DEO)= 70], conversion of diethyl oxalate was 96%, and selectivity of ethylene glycol was 88%. On the other hand, it was suggested that the main reasons for catalyst deactivation were coke polymer formation and the sintering of the Cu0 phase. Catalytic mechanism for hydrogenation of diethyl oxalate was analyzed. In hydrogenation of diethyl oxalate to ethylene glycol, the catalytic performance is related with synergy effect between Cu0 and Cu+...