Study On Rare Earth Element Modified Cu-based Catalysts For Ethane Oxychlorination To Vinyl Chloride

Vinyl chloride monomer (VCM) is an important monomer in the manufacture ofpolyvinyl chloride (PVC). Nowadays, VCM is commercially produced by threeprocesses: acetylene process, ethylene oxychlorination and ethane oxychlorination.Acetylene process of production of vinyl chloride involves reacting hydrogen chlorideand acetylene in the presence of a mercuric chloride catalyst. The method has beenoutdated all over the world for the high energy consumption and serious environmentpollution, it is yet predominant in China. The ethylene oxychlorination processincludes the cracking of ethylene dichloride (EDC) and the direct chlorination ofethylene or oxychlorination of ethylene with oxygen and hydrogen chloride. Ethyleneoxychlorination process is the principal way to produce vinyl chloride all over theworld. But the expense of ethylene as the raw material is usually high due to thedecrease in storage of petroleum. Furthermore, the technique of the process iscomplicated. The oxychlorination reaction of ethane is a promising and economicroute to produce vinyl chloride, because ethane as a raw material is much cheaper andwidely distributed. Many foreign institution have made great efforts to researchproduction of vinyl chloride by ethane oxychlorination process. By contrast there hasbeen little report about this technology in our country. Therefore, Study of preparationof vinyl chloride by ethane oxychlorination process is of great scientific value. Ethaneoxychlorination process includes two independent catalytic reactions: the oxidation ofHCl with the copper based catalysts and dehydrochlorination of dichlroethanedepended on the surface nature of the catalyst. Many kinds of catalysts have beeninvestigated for the reaction.In this work, a series of rare earth element modified copper-base catalysts wereprepared by conventional impregnation method and the catalytic activity for ethaneoxychlorination reaction was also investigated. The catalysts were characterized bymeans of XRD, BET, H2-TPR, NH3-TPD, SEM, TEM, ICP, UV-Vis and XPS to discuss the structure, surface acidity, redox properties of the catalysts and the catalyticperformance. The main experimental results and conclusions are as follows:1. Ethane oxychlorination over Pr modified CuCl2-KCl/MgO-γ-Al2O3catalystPr modified CuCl2-KCl/MgO-γ-Al2O3catalysts were prepared by animpregnation method and the influence of Pr additive content on the structure surfaceacidity, redox properties of the catalysts and catalytic performance of ethaneoxychlorination was investigated. It was found that the Cu species must be anamorphous phase, or in the form of nanoclusters, which cannot be detected by XRD.Addition of Pr decreased the interaction between active species CuCl2and γ-Al2O3and made the active species more dispersed. The experiment results showed theelectronic transformation from Pr to Cu species, which accelerated the conversion ofredox couple (Cu+/Cu2+). The latter will promote the conversion of C2H6andselectivity to C2H3Cl. More amorphous CuCl2species were present on the surface ofsupport due to the presence of Pr species in the samples. At the same time, theaddition of Pr can reduce the particle sizes of the copper species and improve thedispersion in the catalysts. The ethane conversion and selectivity of vinyl chloridereached97.5%and52.0%respectively for Pr6O11-CuCl2-KCl/MgO-γ-Al2O3catalystwhile praseodymium content was5wt%with the reaction condition: T=500℃，GHSV＝3200h-1and VC2H6/VHCl/Vair=1/2/5.2. Y2O3modified CuCl2catalyst for Ethane OxychlorinationA series of Y2O3promoter modified CuCl2-based catalysts were prepared by theincipient impregnation method. Effect of Y2O3promoter on the structure, redoxproperties, acidity as well as catalytic performance of CuCl2-based catalyst for ethaneoxychlorination was discussed. With modification by Y2O3promoter, there were moresurface oxygen species on the surface of CuCl2-based catalysts, which increased theselectivity of the vinyl chloride. The Y2O3promoter adjusted surface acidity andincreased weak acid sites of the support, which contributed to decrease the amount ofdeposited coke. Y2O3promoter modified Pr-Cu/Yxcatalysts showed the better activityand stability than the unmodified ones. The ethane conversion and the selectivity ofvinyl chloride and ethylene were98.5%,55.8%and34.7%, respectively. Moreoverthe catalytic activity for Pr-Cu/Y3catalysts was still high after120h reaction, theconversion of ethane and selectivity of vinyl chloride was more than93.8%and43.0%, respectively. 3. Study of the nature of Cu-based catalysts supported on rare earth element(La, Ce, Pr) modified TiO2supportThe different TiO2support were prepared using hydrothermal treatment method.The structure and physico-chemical properties of the rare earth element La, Ce, Pr,motified TiO2support were searched by BET, XPS, XRD, SEM, NH3-TPD andH2-TPR. Furthermore, the catalytic activity of La, Ce, Pr, motified TiO2supportedCuCl2and KCl catalysts were evaluated. It was found that the addition of La, Ce andPr promoter to TiO2support increased the surface area and reduced the particle size ofthe support. Because the radius of La, Ce and Pr atom was much bigger than that of Tiatom, La, Ce and Pr additives were adsorbed on the external surface of the supportand inhibited the growth of the crystallite size. The SEM result indicated that thereexisted a great deal of pore, which improved the dispersion of the active species. TheLa, Ce and Pr promoter decreased the reduction temperature of CuCl2-based catalystsuppored on TiO2support, which accelerated the reduction-oxidation step forCuII�'CuI�'CuIIand was responsible for the excellent catalytic performance. The La,Ce and Pr species modified TiO2support showed that the number of strong acid sitesdecreased and that of weak acid sites increased, which increased of the catalyticactivity for ethane oxychlorination.