Study On The Catalysts For Ethane Oxychlorination To Vinyl Chloride

Vinyl chloride monomer (VCM) is an important monomer in the manufacture of polyvinyl chloride (PVC). The cost and quality of VCM determines the ones of PVC. Nowadays, VCM is commercially produced by three processes: acetylene process, ethylene oxychlorination and ethane oxychlorination. Acetylene process of production of vinyl chloride involves reacting hydrogen chloride and acetylene in the presence of a mercuric chloride catalyst. The method is outdated in the world for its high energy consumption and pollution, but it is still prevalent in China. In the ethylene oxychlorination process, vinyl chloride is produced by the cracking of ethylene dichloride (EDC), which is produced by the direct chlorination of ethylene or oxychlorination of ethylene in the presence of oxygen and hydrogen chloride. Almost 90% percent of vinyl chloride is produced by ethylene oxychlorination process in the world. There are two disadvantages of this process. One is the high cost of ethylene due to the decrease in storage of petroleum, the other is its complicated technique. The oxychlorination reaction of ethane is a promising and economic route to produce vinyl chloride, because ethane as a raw material is cheaper than ethylene. Many foreign corporations have made great efforts to study production of vinyl chloride by ethane oxychlorination process, but only Europen Vinyls Corporation succeed to set up the equipment. There was no report about this technology in our country heretofore. Study of preparation of vinyl chloride by ethane oxychlorination process is of great importance.Ethane oxychlorination process involves two independent catalytic steps: the oxidation of HCl and dehydrochlorination of dichlroethane. The commonly used catalysts are copper based catalysts, Cu²⁺ mainly catalyze the conversion of hydrogen chloride to free chlorine and the activity of dehydrochlorination of dichlroethane is determined by surface properties of the catalyst. Since the first report of oxychlorination of ethane in the 1970s, many kinds of catalysts have been investigated for the reaction. However, further work is still required in order to improve the catalytic performance and stability of the catalysts, and to study the catalyst structure, interaction between copper and support, the nature of active sites.In this work, a series of supported copper-base catalysts were prepared by impregnation method, and the catalytic activity for ethane oxychlorination reaction was investigated for the purpose to search the new copper-base catalysts system, and to study the influence of various factors on the catalytic performance for oxychlorination of ethane. The catalysts were characterized by means of XRD, BET, TPR, TEM and UV-Vis etc. to establish a relationship between the structure, surface acidity, redox properties of the catalysts and the catalytic performance. Meanwhile, the nature of active sites,the reaction mechanism and the reason for deactivation were also discussed. The main experimental results and conclusions are as follows:1. Oxychlorination of ethane overγ-Al₂O₃ supported CuCl₂ catalyst CuCl₂-KCl/γ-Al₂O₃ supported catalysts were prepared by an impregnation method, and the influence of Cu content and K/Cu molar ratio on the structure and catalytic performance of ethane oxychlorination was investigated. It has been found that there are two Cu compounds in the CuCl₂-KCl/γ-Al₂O₃ catalyst: surface copper aluminate (insoluble copper), which formed by the interaction between Cu(â…¡) and alumina, and excessive CuCl₂ (soluble copper), which does not interact with the support. TPR, UV and soluble test of acetone indicated that interaction between Cu(â…¡) and support promotes the Cu(â…¡)→Cu(â… ) reduction step. Additionally, combining with some related references and activity test results, the authors found that ethane conversion did not increased at specific Cu content, and CuCl₂ could cover and poison the alumina surface active sites, which catalyzes the dehydrochlorination of dichlroethane. Addition of KCl decreased the interaction between active species Cu(â…¡) andγ-Al₂O₃, and made the active species more dispersed. An appropriate K/Cu molar ratio made the salt mixture molten at the reaction temperature, favored the chlorine release from the salt melts and increased the vinyl chloride selectivity. The reaction conditions have considerable effect on the catalytic performance. The relatively low reaction temperature is in favor of the ethylene selectivity, and the relatively high HCl/C₂H₆ ratio can increase the vinyl chloride selectivity. The preferable reaction condition is : T=500℃,GHSV=3200h-1,VC₂H₆/VHCl/Vair=1/2/5.2. Modification of CuCl₂-KCl/γ-Al₂O₃ catalyst by MgAl₂O₄ for Ethane OxychlorinationImpregnation ofγ-Al₂O₃ with Mg and Al nitrates formed MgAl₂O₄ on the support. Effect of MgAl₂O₄ on the structure, acidity as well as catalytic activity of CuCl₂-KCl/γ-Al₂O₃ catalyst in ethane oxychlorination was studied. With modification by MgAl₂O₄ ethane conversion increases because Cu(â…¡) is easy reduced to Cu(I).CuCl₂-KCl-LaCl3/MgAl₂O₄-Al₂O₃ catalyst was chosen for its better catalytic activity and stability than the modified one. At reaction conditions: T=500℃, GHSV=3200h-1, VC₂H₆/VHCl/Vair=1/2/5, the ethane conversion and vinyl chloride selectivity reached 95.3% and 46.8% over the CuCl₂-KCl-LaCl3/MgAl₂O₄-Al₂O₃ catalyst, respectively, and it was still high after 210 h reaction. Much larger quantities of coke were deposited on the CuCl₂-KCl-LaCl3/γ-Al₂O₃ than on the modified catalyst, which was attributed to its more strong acid sites.Impregnation ofγ-Al₂O₃ with Mg nitrates also formed MgAl₂O₄ on the support. The catalytic evaluation demonstrated that modification of support with MgAl₂O₄ by this method resulted in the increase in vinyl chloride selectivity from 37.3% to 49.1% at MgO loadings of 10%. The adjustment of surface acidity and increase of weak acid sites are the main reason for increase of selectivity of vinyl chloride.3. Study of ethane oxychlorination over CuCl₂ catalysts supported on Mg-Al-O mixed oxidesA series of Mg-Al-O mixed oxides were prepared by co-precipitation method. The influence of with various Al/Mg molar ratios on the composition and surface acidity was studied by XRD, BET and NH3-TPD technique. It was found that the number of strong acid sites decreased and that of weak acid sites increased firstly and then decreased with the decrease of Al/Mg molar ratio. Catalytic activity test showed that the CuCl₂-KCl/MgAlO-3 catalyst exhibits highest catalytic performance than CuCl₂-KCl/γ-Al₂O₃, which indicated that the weak acid sites favor the formation of vinyl chloride.Based above results it is suggested that acceleration of CuII→CuI→CuII reduction-oxidation step and more surface weak acid sites favored the increase of the catalytic performance for ethane oxychlorination.