Study On Selective Oxidation Of Diethylamine To N₂ Over MnO_x/ZSM-5 And MnO₂-CeO₂/CH Catalysts

Volatile organic compounds containing groups such as-NO_x,-NH₂,-CONH-,-CN,etc.as are very hazardous to human health and environment,which are widely used in petrochemical,pharmaceutical,and leather production industries.Up to date,there is still a lack of treatment to eliminate nitrogen-containing volatile organic compounds?NVOCs?.Although direct combustion is a convenient method to eliminate NVOCs effectively,the N element in NVOCs will be transformed to NO_x which is also harmful.Hence,the addition of subsequent SCR denitration process to eliminate NO_x is necessary.Catalytic combustion is considered as a promising method to control the generation of NO_x effectively in the NVOCs oxidation process.However,it still remains challenging to develop catalyst to inhibit the generation of NO_x and improve the N₂ selectivity.In this work,a series of MnO_x/ZSM-5 catalysts and MnO₂-CeO₂/CH catalysts were prepared and tested for catalytic oxidation of diethylamine.The effects of MnO₂-CeO₂ mass ratio,SiO₂/Al₂O₃ ratio in ZSM-5,Mn loading and calcination temperature on catalytic performance were investigated.Moreover,the possible reaction mechanisms of catalytic diethylamine oxidation over catalysts were discussed.Detailed contents of this work are shown as following:1.The selective catalytic oxidation diethylamine to N₂ over MnO_x/ZSM-5catalysts were demonstrated,which were prepared by traditional impregnation on ZSM-5?with SiO₂/Al₂O₃ of 18 or 360?.The physicochemical properties of catalysts were investigated by N₂ adsorption/desorption,powder X-ray diffraction?XRD?,temperature-programmed reduction by hydrogen?H₂-TPR?,temperature-programmed desorption of NH₃?NH₃-TPD?and X-ray photoelectron spectroscopy?XPS?.It was found that the oxidation activity of diethylamine over MnO_x/ZSM-5 increased when the loading of Mn increased,but the N₂ selectivity decreased at 420 ^oC.The MnO_x/ZSM-5?360?catalysts showed higher oxidation activity of diethylamine than MnO_x/ZSM-5?18?catalysts with equivalent Mn loading while the latter gave significantly higher selectivity of N₂.20MnO_x/ZSM-5?18?had relatively higher oxidation activity of diethylamine and N₂ selectivity,of which the lowest reaction temperature for diethylamine conversion at 98%(T₉₈)is 240 ^oC and the N₂ selectivity is exceeding 95.5%at 420 ^oC.Furthermore,20MnO_x/ZSM-5?18?catalyst showed wider temperature window??T=180 ^oC?with N₂ selectivity over 95%than20MnO_x/ZSM-5?360?catalyst??T=80 ^oC?.The reduction temperature of MnO_x on the surface of MnO_x/ZSM-5?360?catalyst is obviously lower than that of MnO_x/ZSM-5?18?catalyst,and the MnO_x/ZSM-5?360?catalyst surface had a higher Mn atom concentration,indicating that MnO_x could be easily reduced and enriched on ZSM-5?360?support surface.The surface of MnO_x/ZSM-5?360?catalyst possessed only weak acid centers,while the surface of the MnO_x/ZSM-5?18?catalyst contained moderate and strong acidic sites in addition to the weak acidic sites.Thus,it could be inferred that the easily reduced and surface enriched MnO_x species are favorable to the oxidation of diethylamine,the moderate and strong acidic sites are beneficial to the generation of N₂.2.A series of MnO_x/ZSM-5 catalysts calcined at different temperatures were prepared by impregnation on ZSM-5 with SiO₂/Al₂O₃ of 360 and uesed for selective catalytic oxidation diethylamine to N_2.It was found that the catalytic oxidation activity of diethylamine over MnO_x/ZSM-5 catalysts decreased when the calcination temperature of the catalyst increased.And the N₂ selectivity at the T₉₈+80 ^oC also decreased.The reducibility of MnO_x species and the amount of Mn⁴⁺species on MnO_x/ZSM-5 catalysts decreased due the increase of the calcination temperature,which resulted in decrease of the catalytic oxidation activity of diethylamine.The decrease of acid sites on MnO_x/ZSM-5 catalysts was not favorable for the formation of N₂.3.A series of MnO₂-CeO₂ monolithic catalysts were prepared by the impregnation method and their activities for catalytic oxidation of diethylamine were investigated.It was revealed that composite oxides monolithic catalysts showed much higher catalytic activity than single-component catalysts for diethylamine catalytic oxidation reaction.The MnO₂-CeO₂ catalyst with mass ratio 8:1 showed the highest activity and selectivity of N₂?exceeding 85%?,which gave the lowest temperature of T₉₈8 for diethylamine oxidation.The MnO₂-CeO₂ catalysts mainly contained MnO₂ and CeO₂ phases according to X-ray powder diffraction?XRD?characterization.As shown in hydrogen temperature programmed reduction?H₂-TPR?and X-ray photoelectron spectroscopy?XPS?results,MnO₂ on the composite oxides catalysts gave more hydrogen consumption than single component catalysts,especially for MnO₂-CeO₂mass ratio 4:1 and 8:1 with the largest hydrogen consumption.The addition of proper amount of CeO₂ was beneficial for the presence of Mn⁴⁺and Ce³⁺species in the catalyst and the increase of its reduciablility,which resulted in the enhancement of catalytic activity.The Mn⁴⁺specie content on the catalyst surface increased after oxidation reaction due to its formation during the reaction process,which could be beneficial for the increase of catalytic activity and stability of the catalyst.