Study On Rare Earth Element (La, Ce) Modified CuCl₂-Based Catalysts For Ethane Oxychlorination To Vinyl Chloride

Vinyl chloride monomer (VCM) is an important monomer in the manufactureof polyvinyl chloride (PVC). Nowadays, VCM is commercially produced by threeprocesses: acetylene process, ethylene oxychlorination and ethane oxychlorination.We often select mercuric chloride as a main catalyst to produce vinyl chloride inprocess of acetylene oxychlorination. The method is easy to ultilize due to its simpleand mature, but it brings some disadvantage including high energy consumption andserious pollution to environment.Ethylene oxychlorination is main route to produce vinyl chloride. This processincludes two reactions: the process of manufacture of ethane chlorides and thecracking of ethylene dichloride (EDC). This method is advantageous to theimprovement of quality of products, but the technics is complex to produce vinylchloride. In addition, its cost fluctuates along with the change of price of oil.The oxychlorination reaction of ethane is a promising and economic route toproduce vinyl chloride, because ethane as a raw material is cheaper than ethylene.Europe Vinyl Corporation succeeds to set up two equipments at the Bay of Mexicoin1999s, and make it come true in2003s. The total capacity of production of vinylchloride reaches up0.3million ton per year. By contrast, there is no report about thistechnique in our county. Therefore, study of preparation of vinyl chloride by ethaneoxychlorination is of great importance.In this work, copper-base catalysts modified with other components wereprepared by conventional impregnation method, and the catalytic activity for ethaneoxychlorination reaction was investigated simultaneously. The catalysts werecharacterized by means of XRD, BET, H₂-TPR, SEM, TEM, UV-Vis and XPS etc. toinvestigate the relationship between the structure, surface acidity, redox properties,existence state of components and the catalytic performance. The main experimentalresults and conclusion are as follows:1. Oxychlorination of ethane over CeO₂modified CuCl₂-KCl/MgO^-Al₂O₃catalystCeO₂modified CuCl₂-KCl/MgO^-γ-Al₂O₃catalysts were prepared by an impregnation method, and the influence of Cerium content on the structure, redox,acidity and existence state of active components was investigated. It is found thatthree types Cerium species exist on the surface of catalysts: dispersed ceria, smallaggregated crystalline ceria species and large ceria particles.XRD, BET and H₂-TPR tests indicated that ceria is highly dispersed on thesupport at cerium content x <1.4wt.%, and large ceria particles begin to form on thesurface of catalysts at cerium loading x≥9wt.%; while for catalysts with ceriumcontent1.4wt%≤x <9wt%is responsible for the formation of small aggregatedcrystalline ceria species on the catalysts.Cerium promoter plays a key role for the formation of weak acid center on thesurface of catalysts. CeO₂-CuCl₂-KCl/MgO^-γ-Al₂O₃catalysts exhibit mainly Lewisacid center, and the amount of weak acid sites reach a maximum for catalyst withcerium content x=5wt%. It is known that weak acid center is advantageous tode-hydrochloric from ethane dichloride, which is responsible for the improvement ofselectivity of ethylene and vinyl chloride.CuCl₂-based catalysts with small aggregated crystalline ceria possess largeamount of surface capping oxygen species (O^-, O₂^-) on the surface and reach amaximum with cerium content x=5wt%. As a result, Cu⁺species is hardlyreduced to Cu⁰, which is responsible for the high stability of CuCl₂-based catalyst.The experiment results show that both ethane conversion and selectivity ofvinyl chloride reach a maximum (98.6%and54.6%) for CuCl₂-based catalyst withcerium content x=5wt%, while the selectivity of ethylene reach a minimum(₂6.₂%) simultaneously. Additionally, the best reaction condition is that: T=500℃,GHSV＝₃₂00h-1and VC₂H6/VHCl/Vair=1/₂/5.2. Oxychlorination of ethane over CeO₂/La₂O₃bi-promoter modifiedCuCl₂-KCl/MgO^-Al₂O₃catalystCeO₂/La₂O₃bi-promoter modified CuCl₂-KCl/MgO^-Al₂O₃catalysts wereprepared by conventional impregnation method. The experiment results show thatimpregnation order of La₂O₃precursor can affect the structure, redox, acidity andexistence state of active component of CuCl₂-based catalyst notably.MgO^-γ-Al₂O₃support impregnated with La₂O₃precursor firstly improves thedispersion capacity of CeO₂and KCl promoters on the support. In addition, itbenefits the deposition of active CuCl₂species on the surface of support. As a result, the conversion of Cu⁺to Cu²⁺is accelerated and more active Cu²⁺can participate inthe reaction, which is responsible for the high activity of ethane oxychlorination.Lanthanum content can affect the acidity of CuCl₂-based catalysts notably. Afew amount of La species benefit the formation of weak acidic centers, which isattributed to the coverage of uncovered Al³⁺by La³⁺species. By contrast, largeamount of La promoter is responsible for the decrease of acidic amount, which isattributed to the big radius of La³⁺ion and the decrease of interaction between La³⁺and OH-species.La₂O₃promoter improves the stability of Ce-Cu/Laxcatalysts notably, whichis attributed to the higher ability of prohibiting the coke species deposited on thesurface. After reaction for180hours, conversion of ethane and selectivity of vinylchloride is97%and₃8.₂%, respectively; and the selectivity of ethylene is44%,simultaneously.TEM images show that coke species deposited on the Ce-Cu/La₃are easilyeliminated and form large amount of circle coke layer on the edge of used catalyst,which is responsible for the high stability of CuCl₂-based catalysts.₃. Study of ethane oxychlorination over CuCl₂catalysts supported on La, Ce,Zr atoms modified TiO₂supportPure TiO₂and La, Ce, Zr atoms modified TiO₂support were prepared usinghydrothermal treatment method. It is found that the introduction of La, Ce and Zratom to TiO₂support plays a key role for the formation of physico-chemicalproperties of support.The surface area of La, Ce, Zr atoms modified TiO₂increases apparentlycompared to pure TiO₂support; and these atoms exist in the different position ofTiO₂support. Zr⁴⁺can insert in to Ti⁴⁺lattice or interstitially position of TiO₂due tothe closer radius; while La³⁺or Ce³⁺ion can insert into interstitially position or existon the surface of TiO₂attributed to the bigger radius.The introduction of La, Ce and Zr atom to TiO₂support affects the acidity ofsupport notably. It is worth to point out that Ce³⁺ion benefits the formation of weakacidic center; and both weak and middle acidic amount increase for La³⁺modifiedTiO₂support. By contrast, the amount of middle acidic center increases apparently onZr^4-ions modified TiO₂support.H₂-TPR tests show that the reduction temperature of active Cu²⁺species decreases with an order of CuK/TiO₂<CuK/La-TiO₂<CuK/Ce-TiO₂≈CuK/Zr-TiO₂.This fact suggests that higher dispersion of active component benefits the reductionof active copper species on the support. However, Ce-TiO₂support displays thelower surface area, which suggests the strong interaction existed between activeCu₂+and Ce₃+species. As a result, more Cl₂intermediate can be formed during thereaction, which is advantageous to the conversion of ethane and selectivity of vinylchloride.XPS results show that both Cu⁺and Cu²⁺ions exist on the surface ofCuCl₂-based catalyst, simultaneously. Additionally, Ce-TiO₂support exhibits thestronger electro-negativity compared to other support, which is advantageous to thereduction of Cu²⁺to Cu+and is responsible for the excellent activity.