The Character And Suppression Strategy Of Secondary Pollution In The Catalytic Oxidation Of Chlorobenzene

Chlorinated volatile organic compounds?Cl-VOCs?are widely used in industrial processes,which are harmful to ecological environment and human health.Catalytic combustion technology has been considered to an effective technology for the elimination of Cl-VOCs.However,catalysts deactivation by chlorine poisoning and more toxic and persistent polychlorinated by-products generation severely hinder the catalytic technology application.Taking typical Cl-VOCs/chlorobenzene as the investigate object,this paper studied the reaction performances of various metal oxide catalytic systems in the elimination of chlorobenzene,explored the characteristics of chlorine poisoning and secondary pollution,and preliminarily evaluated the application environmental risk of Cl-VOCs catalytic elimination technology.Additionally,this paper also explored the inhibition effect and influence mechanism of phosphoric acid modification on the generation of high toxic polychlorinated by-products,which provides technical support for the efficient Cl-VOCs catalytic control and reducing the risk of secondary pollution.Firstly,with MWCNT as carriers,four metal oxides?Ru,V,Ce,and Mn?that are widely utilized in the catalytic elimination of VOCs are selected to study their reaction performance and toxic by-product formation in the catalytic elimination of chlorobenzene.It was noted that Ru-CNT could promote the formation of Cl₂ through semi-Deacon reaction;V-CNT can promote the formation of HCl as a result of enriched Br?nsted acid sites on the surface.The strong dechlorination characteristics of these two catalysts kept a good stability in the catalytic elimination of chlorobenzene.In comparison,owning to the lack of dechlorination capacity of Ce-CNT and Mn-CNT catalysts,the accumulation of chlorine on the catalyst surface led to rapid deactivation,and promoted the electrophilic chlorination reaction to generate a large number of high toxic polychlorinated by-products.Among the four catalysts,Mn-CNT produced the most chlorine containing by-products,and the total concentration of polychlorobenzene in the tail gas was shown up to 41.9?g/m³.Meanwhile,a large number of short-carbon chain polychlorobenzene by-products were also detected,such as chloroform,carbon tetrachloride,trichloroethylene and tetrachloroethylene.Secondly,the risk of PCDD/Fs formation in the catalytic elimination of chlorobenzene t was evaluated.PCDD/Fs were detected both on the exhaust gas and catalyst surface for the four catalysts,most of which were adsorbed on MWCNT.The total concentration of PCDD/Fs accumulated on the surface of Mn-CNT(140.9 ng I-TEQ g^-1)and Ce-CNT(13.05 ng I-TEQ g^-1)are remarkably higher than Chinese national standard of solid residue(at 3 ng I-TEQ g^-1).Even with dichloromethane as the precursor,PCDD/Fs was still observed on the Mn-CNT catalyst.The results show that there may be a risk of dioxin secondary pollution in the use of catalytic control technology to eliminate Cl-VOCs.Finally,the surface modification of Mn O_x catalyst was carried out by introducing phosphate group to explore the effect of promoting the migration and transformation of chlorine on the catalyst surface and inhibiting the by-products of high toxic polychlorination.Through a series of characterizations,it was noted that the oxygen transport capacity of Mn O_x catalyst was decreased after phosphate modification,but the enriched Lewis and Br?nsted Acid Sites on the surface enhanced the adsorption and activation of water,thus promoting the formation of HCl.After phosphate modification,the total selectivity of CO₂ and CO in the off-gas was increased,and the type and amount of organic by-products were decreased.In particular,the formation of dichloromethane,polychlorinated benzene and other high toxic by-products were significantly inhibited,which was attributed to the improvement of dechlorination phosphate modification,thus inhibiting the electrophilic chlorination reaction.