The Study On The Complete Catalytic Oxidation Of O-Xylene Over Transition Metal Oxides

Benzene hydrocarbon (benzene, toluene and xylene) are the major composition of volatile organic compounds (VOCs). They are mainly vented from a variety of industrial and commercial processes, such as chemical production, printing and mobile emission and cause serious harm to human health and environment. Therefore, the removal of these pollutants has been one of the important research topics in environmental treatment. Catalytic deep oxidation technique which can convert VOCs into carbon dioxide and water is regarded as one of the most effective and promising way to remove the low concentration VOCs. In this paper, a series of Mn-Ce mixed oxides, CoAl2O4 spinel,γ-Al₂O₃ and CeO₂-Al₂O₃ supported Cu, Co, CuCo composite oxide catalysts were prepared by redox-precipitation, sol-gel method and solid state reaction, respectively. The catalysts were applied in the deep catalytic oxidation of o-xylene. The research took into account the catalytic behavior of the catalysts in term of both the conversion of o-xylene and the yield into CO2. Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), X-ray photoelectron spectroseopy (XPS), X-ray fluorescence (XRF), hydrogen temperature-programmed reduction (H2-TPR) etc were also applied to research the microstructure of the catalysts. The detail contents are as follows:First, a series of Mn-Ce (denoted as RP-MnCe) mixed oxides catalysts were prepared by"redox- precipitation"method using KMnO₄, Mn(NO₃)₂ and Ce(NO₃)₃ as precursors. The influence of Mn /Ce atomic ratios and calcination temperature on the activity of catalysts were investigated. The results showed that the catalyst exhibited excellent catalytic performance with Mnat/Ceat ratio of 1.5 calcined at 400 oC, Which could converse 0.08vol% of o-xylene into CO2 at 240 oC. Structure analysis by XRD, XPS, XRF and H2-TPR revealed that amorphous MnO2 is the active center of RP-MnCe catalyst and microcrystal state of CeO2 promot the mobilityof oxylene species. The synergistic effects between MnO2 and CeO2 improved the redox property and further affected the activity of the catalysts.Second, 20Co/γ-Al2O3, 20Cu20Co/γ-Al2O3, 20Cu20Co/CeO2-Al2O3 and other catalysts were synthesized by solid-state reaction at room temperature. Catalytic activity test and BET, SEM, XRD, TPR researchs showed that the catalyst prepared by this method had better catalytic activities as well as higher specific surface area, lower degree of crytallinity, smaller crystals grain size and so on. Among them 20Cu20Co/CeO2-Al2O3 catalyst conversed o-xylene into CO2 and H2O at 275 oC due to the synergistic effects between copper and cobalt.The third, a series of CoAl2O4 catalysts were prepared by sol-gel method using aluminium isopropoxide and cobalt nitrate as precursors. The catalysts were used to the deep catalytic oxidation of o-xylene after reduced in hydrogen flow at 500 oC. The influence of Co/Al molecular ratio, reduction temperatures and space velocity on the catalytic performance were investigated. BET, XRD characterization results showed that the reduction by hydrogen caused lattice distortion of the CoAl2O4 and then lead to the reduce of their crystallization degree and crystals grain size. XPS study revealed that the binding energy of Co 2p, Al 2p and O 1s shifted to lower bond energy after 500oC hydrogen reduction and more lattice oxygen species increased the catalytic activity. The CoAl₂O₄ reduced at 500 oC conversed 0.08 vol% of o-xylene into CO2 and H2O at 275 oC. Moreover the catalyst showed good stability in oxidative atmosphere after 60 h test at 275 oC.