Stability Study On Catalytic Oxidation Of VOCs Over Sodium Modified Copper-based Zeolites

The elimination of volatile organic compounds(VOCs)from industrial source is one of the main concerns in the field of air pollution control.Nowadays,catalytic combustion process has become a widely-used terminal treatment technology of VOCs.However,the poor stability of catalysts is still a main obstacle for their real application,and the deactivation process is influenced by the nature of catalysts and different reaction conditions.In this thesis,the catalytic oxidation behaviors of VOCs(chlorinated alkanes,olefins,aromatics)over HZSM-5 supported Cu catalysts were investigated and the causes of deactivation were analyzed.Moreover,the effects of alkali metal Na doping on Cu/HZSM-5 catalysts were also studied.Firstly,a series of Cui/HZSM-5 with different loadings(1-10 wt.%)were prepared for catalytic combustion of dichloromethane.It was found that all these catalysts suffered deactivations in varying degrees.The deactivation of catalysts with low loadings was mainly due to the coke deposition,while the high Cu loading one was mainly deactivated by chlorine poisoning.The chlorine poisoning could be attributed to the formation of Cu-OH-Cl species,and the excessive surface acidity and insufficient redox ability of the catalysts were the causes of the coke deposition.Based on this,certain amount of Mn,Ce,and Na were added to Cu2/HZSM-5 catalyst to adjust the acidity and redox ability,which effectively inhibited the coke deposition.However,high loading content of Na would aggravate the chlorine poisoning of the catalyst.The catalytic oxidation of propene was then performed to disclose the influences of Na on the deactivation caused by carbon deposition.The results revealed that Na addition could somewhat lower the propene conversion rate,but effectively improve the mineralization rate and stability of the catalyst.The loading of Na could increase the content of isolated Cu2+ and active oxygen species,thus enhancing the deep oxidation ability of the catalyst and reducing the accumulation of intermediate products.Besides,the alkali metal could alter the surface acidity of the catalysts,balance the adsorption and oxidation procedure,thereby reducing the carbon deposition rate and improving the stability of the catalysts.In addition,the loading of Na could inhibit the fluctuation of catalytic activity during the oxidation of toluene.This phenomenon was derived from the formation and removal of reactive coke on the acidic HZSM-5 based catalyst.If not oxidized in time,the intermediate products(reactive coke)would be retained on the catalyst surface and further converted into unreactive coke,resulting in the deactivation of the catalyst.An appropriate amount of Na could tune the acidity of the catalyst and promote the deep oxidation process,reduce the production and adsorption of intermediate products,hence inhibiting the accumulation of active coke,which was confirmed by the concentration pulse tests.