Catalytic Oxidation Process Of Chlorobenzenes Over Manganese-based Oxide Catalysts And The Inhibition Of By-products

Volatile organic compounds(VOCs)are considered to be one of the major gaseous pollutants affecting air quality in China.Catalytic combustion is a mainstream technology for VOCs elimination.However,catalytic combustion of chlorobenzenes,which are a typical kind of VOCs,encounters numerous problems,including chlorine poisoning to catalysts,poor products selectivity and formation of toxic by-products.In this thesis,the catalytic activity and stability of manganese-based metal oxides for chlorobenzene oxidation are systematically studied.The adsorption and desorption behaviors of chlorine species from catalysts surface are explored.Besides,the types and formation mechanism of toxic by-products during the reaction process are investigated whilst the effects of water vapor and alkaline earth metals on reaction process and the formation of by-products are analyzed.Firstly,toluene was selected as the represented phenyl intermediate for the investigation of catalytic performance and reaction mechanism over manganese perovskites.The isotopic experiment results revealed that toluene oxidation over LaMnO3 followed both L-H and MvK reaction mechanism,and L-H mechanism was dominant.Herein,oxygen vacancies were found to serve as active sites for both activating gas-phase oxygen species and transporting surface lattice oxygen.The doping of Sr2+ and Fe3+ increased the amount of oxygen vacancies and surface lattice oxygen,and the modified catalysts could achieve 90%toluene conversion at 236℃.Secondly,the oxidation efficiency and chlorine resistance of manganese perovskites for chlorobenzene oxidation were investigated.It was found that LaMnO3 catalyst was prone to deactivation and the by-products in exhausts were found to involve certain polychlorinated hydrocarbons,including C2HCl3 and C2Cl4,etc.After CeO2 doping and HF treatment,nearly 90%chlorobenzene could be converted to CO2 and H2O at 210℃,and the stability could be maintained at 320℃.A linearity between reaction rate constant and the concentration of surface adsorbed oxygen was built in kinetic measurements,indicating that surface adsorbed oxygen was active species.Additionally,CB-TPSR analyses verified that chlorine desorption from catalyst surface was closely related to the mobility of surface lattice oxygen whilst the process mainly followed Deacon reaction mechanism.To evaluate the effect of water activation on chlorine desorption process,Octahedral molecular sieve(OMS-2)catalyst with abundant water activation sites was studied.The results showed that catalytic performance and stability of OMS-2 catalyst were improved by K+ doping,which could convert 90%chlorobenzene into CO2 and H2O within 230℃.Characterization results showed that K+ doping increased the number of active hydroxyl groups,which could promote desorption of chlorine species from catalysts surface in the form of HCl.In addition,active hydroxyl groups can be supplemented by H2O dissociation in both K+ ions and oxygen vacancies.Finally,the effects of alkaline-earth metal on the catalytic activity and productselectivity over OMS-2 catalysts were investigated.Though the redox ability and mobility of lattice oxygen species were declined after Ca2+ deposition,the catalytic activity was effectively retained and more than 90%chlorobenzene could be converted within 220℃.The deposited Ca2+ species were mainly in the form of CaCO3,which could react with dissociative chlorine species to inhibit chlorine poisoning,thus preventing the formation of polychlorinated by-products.