Catalytic Oxidation Of CVOCs Over HZSM-5 Zeolite Supported Rare-earth Mixed Oxide Catalysts

Halogenated hydrocarbons, especially chlorinated volatile organic compounds (CVOCs), constitute a significant fraction of toxic air containments. They not only can destroy the ozone layer (one free radical of Cl consumes a maximum of 106 molecules of O3), but also cause long-lasting and cumulative effects on human health and the ecological system. Chlorinated hydrocarbons have been widely used as industrial chemicals and organic solvents because of their relative inertness in chemical processes and their ability to dissolve many compounds. Various disposal methods have been investigated for the abatement of CVOCs world-wide, mainly are thermal incineration, biological degradation, photocatalytic degradation, hydrodechlorination et. al. Catalytic oxidation has gained much attention as the most economic and efficient technique for CVOCs destruction due to its lower conversion temperature and higher selectivity. The difference between catalytic oxidation of chlorinated hydrocarbons and non-chlorinated hydrocarbons is the replacement of C-H bond breaking by C-Cl bond breaking. It is known that the cleavage of C-Cl bond is more easily through thermal analysis. However, the strong adsorption of Cl species on the catalyst surface not only causes the deactivation of catalyst, but also results in the production of polychlorinated hydrocarbons, which will lead to the secondary pollution.In this paper, HZSM-5 zeolite was chosen as the support, the influences of different SiO2/Al2O3 ratios and the introduction of CeO2 to HZSM-5 on the catalytic performance for DCE destruction were systematically studied. Based on the above results, further investigation of the effect of transition metal oxides MOX (M=Cr, Mn, Fe, Co, Ni and Cu) added to CeO2/HZSM-5 catalysts on the catalytic performance for DCE destruction was done. Moreover, the mechanisms of catalytic decomposition of DCE, DCM and TCE over CrOx-CeO2/HZSM-5 catalysts were also studied. The texture-structure, surface acidity distribution and redox properties of these catalysts were characterized by XRD, N2 adsorption/desorption, NH3-TPD, H2-TPR and DCE-TPSR techniques. Some specific conclusions from this work are drawn as follows:1. Catalytic performances for DCE destruction over HZSM-5 zeolites with different SiO2/Al2O3 ratios and CeO2/HZSM-5 catalysts were studied. The results show that HZSM-5(22) and CeO2/HZSM-5(22) exhibit the highest catalytic activity. The interactions between CeO2 and HZSM-5 improve the migration of oxygen species and increase the ratio of strong acid concentration to weak acid concentration, which promote the dehydrochlorination of DCE to form C2H3Cl and the deep oxidation of the intermediates.2. The catalytic performances for DCE destruction over 9%M/HZSM-5(22) and 9%M-12%CeO2/HZSM-5(22) catalysts were investigated. It is found that the addition of CeO2 to 9%Ni/HZSM-5(22) and 9%Cu/HZSM-5(22) catalysts causes a decline in catalytic activity, whereas the catalytic performance of 9%Cr-12%CeO2/HZSM-5(22) is significantly prompted compared with that of 9%Cr/HZSM-5(22). The sequence of the catalytic activity of 9%M-12%CeO2/HZSM-5(22) catalysts for DCE destruction is as follows:9%Ni-12%CeO2/HZSM-5(22)> 9%Co-12%CeO2/HZSM-5(22), 9%Mn-12%CeO2/HZSM-5(22)> 9%Cu-12%CeO2/HZSM-5(22),9%Cr-12%CeO2/ HZSM-5(22)> 9%Fe-12%CeO2/HZSM-5(22)> 12%CeO2/HZSM-5(22). With regard to the production of intermediates, the increasing concentration of weak and medium strong acid sites promotes the formation of C2H3Cl from dehydrochlorination of DCE. Due to the strong acidity of metal cations, large amount of C2HCl3 and C2Cl4 are generated over Cu, Co and Mn impregnated catalysts, and more CH3Cl is formed over Ni contained catalysts. The co-existence of transition metal oxides and CeO2 improves the mobility of active oxygen species in the catalysts, which is in favor of the deeper oxidation of DCE. The doping of Cr significantly improves the further dehydrochlorination of C2H3C1 and inhibits the formation of polychlorinated by-products.3. The influence of the synergy between active phases on the catalytic performance and the stability of 9%Cr-12%CeO2/HZSM-5(22),9%Cr/HZSM-5(22),12%CeO2/ HZSM-5(22) and HZSM-5(22) catalysts were studied. The results indicate that the interactions between CrOx and CeO2 avoid coke deposition and loss of active components, since they stabilize the active components and prompt the mobility of active oxygen species. Consequently,9%Cr-12%CeO2/HZSM-5(22) shows the best catalytic activity and stability. The catalytic activity for the oxidation of different CVOCs over 9%Cr-12%CeO2/HZSM-5(22) is displayed as follows:DCE> TCE> DCM.