Preparation And Characterization Of Microfiber Composite ZSM-5 Zeolite Membrane Catalysts For VOCs Combustion

The rapid development of science and technology has brought great convenience to human daily life.At the same time,it has caused serious damage to the environment on which human beings depend.It is mainly caused by emissions of gas pollution,water pollution and waste residue pollution.Among them,volatile organic compounds?such as toluene and trichloroethylene?have the characteristics of easy diffusion,unpredictability and high toxicity,and it is extremely urgent to effectively control it.At present,the common processing technologies mainly include recycling technology and destruction technology.Among them,the catalytic oxidation method is widely used in the treatment of VOCs due to its low light-off temperature,high removal efficiency,and no secondary pollution.The key to the catalytic oxidation process is to find a suitable catalyst.At present,the granular catalyst with Cu and Cr as active components exhibits excellent catalytic activity for toluene or trichloroethylene.However,the active components of the catalyst prepared by the impregnation method are easy to agglomerate,and the larger particles tend to block the pore structure of the porous material,which affects the further improvement of the catalytic activity of the catalyst.In addition,the shortcomings of poor mass/heat transfer,low contact efficiency and high bed pressure exist in the fixed bed catalyst reactor fixed with the granular catalyst.Therefore,developing a high catalytic performance zeolite membrane catalyst with small active particles and uniform dispersion is a meanful work,and the design of catalytic oxidation VOCs based on the catalyst is a frontier topic of theoretical significance and practical value.In this paper,the synthesis process of catalyst prepared by chemical vapor deposition?CVD?method was designed based on the tube furnace,and then a series of microfiber composite zeolite membrane catalysts for VOCs treatment were developed,and the fixed bed reaction based on the catalytic material was investigated.Catalytic oxidation performance,stability deactivation mechanism and catalytic oxidation kinetics of toluene or trichloroethylene are investigated.Firstly,the catalytic combustion performances of toluene in fixed bed reactor based on granular Cu/ZSM-5 zeolite catalyst were investigated.The Cu/ZSM-5 catalysts were characterized by XRD,N₂ adsorption/desorption,SEM,EDS,H₂-TPD and XPS,and then,the catalytic performance of granular Cu/ZSM-5 zeolite catalyst prepared by impregnation method and chemical vapor deposition method were studied.The characterization results showed that the active component particles of the catalyst prepared by CVD method are small?23.84 nm?and uniformly dispersed compared with the catalyst prepared by the impregnation method.The catalytic results showed that the catalyst prepared by the CVD method displayed high catalytic activity and high stability,demonstrating by a lower T₉₀?the temperature when toluene conversion reaches 90%?of 293 ? and it decreased about 20 ? compared with Cu/ZSM-5 prepared by impregnation.Besides,the conversion of toluene maintained at about90%after continuously run for 63 hour.Secondly,the catalytic combustion performance of trichloroethylene over granular Cr-Cu/ZSM-5 zeolite catalyst in fixed bed reactor was studied.Mono-metallic Cu/ZSM-5,Cr/ZSM-5 and bimetallic Cr-Cu/ZSM-5 zeolite catalysts with a total metal loading of 3 wt%were prepared by chemical vapor deposition.The catalysts were characterized by N₂adsorption/desorption,XRD,SEM,XPS,NH₃-TPD and H₂-TPR,and the effect of active components,inlet concentration,bed height and space velocity on the catalytic combustion of trichloroethylene were investigated.The characterization results showed that the active component was uniformly dispersed on the surface of ZSM-5 zeolite,and there was a synergistic effect between the Cu and Cr two-component metals,which improved the catalytic combustion performance of the catalyst,domenstrated by a T₉₀0 of 348?,and its T90 reduced 23 and 38 ? compared with Cu/ZSM-5 and Cr/ZSM-5,respectively.Besides,only trace amounts of by-products C₂Cl₄ were detected.In addition,the results of catalytic experiments showed that the catalytic activity of Cr-Cu/ZSM-5 zeolite catalyst for TCE combustion decreased with the increase of GHSV as well as TCE inlet concentration,and increased with the increase of bed height.Thirdly,the catalytic performance of TCE in the fixed bed membrane reactor based on microfiber composite Cu or Cr/ZSM-5 zeolite membrane catalyst was studied.A series of Cu or Cr modified microfiber composite zeolite membrane catalysts were successfully prepared by chemical vapor deposition.The characterization results showed that the thickness of ZSM-5 zeolite membrane was about 3.5?m,and its BET surface area was 198 m²/g.Besides,the Cu or Cr component was uniformly dispersed on the surface of the ZSM-5 zeolite membrane and its crystallite diameter was 0.53 nm.The catalytic results showed that 1%Cr/ZSM-5/PSSF catalyst showeded better catalytic activity than 7%Cu/ZSM-5/PSSF catalyst,demonstrated by T₉₀0 of 440? and 490?,respectively,and the Cr/ZSM-5/PSSF catalyst exhibited lower concentration of by-product and higher catalytic stability after continuously run for 40 h.Meanwhile,the T₉₀ of the Cr?1?/ZP catalyst was reduced by 62 ? compared with the 1%Cr/ZSM-5 guarnular zeolite catalyst,and the bed pressure drop of reactor fixed with Cr?1?/ZP catalyst reduced 221%compared with 1%Cr/ZSM-5 catalyst.The characterization results showed that the partial inactivation resulted from the loss of the active component,and Cl poisoning by the adsorption of the Cl component and the blockage of the pores by the coke formation.Finally,the modeling studies of the catalytic combustion reaction of trichloroethylene on a zeolite membrane reactor were studied by Power-rata Law and Mars-van Krevelen kinetic models.The influence of internal and external diffusion on the catalytic reaction was eliminated by investigating the influence of zeolite membrane thickness,grain size and space velocity on trichloroethylene combustion.The results showed that the Mars-van Krevelen model was more suitable for describing the catalytic combustion performance of trichloroethylene on the zeolite membrane fixed bed reactor.The reaction mechanism was the combustion-reduction mechanism,and the surface combustion reaction activation energy and surface reduction activation energy were 122.25 and 64.57 kJ/mol,respectively.