Preparation Of Modified ZSM-5 Supported Catalyst And Its Catalytic Performance For Toluene Oxidation

Volatile organic compounds（VOCs）were important precursors of ozone（O₃）and PM_2.5,which not only affected the quality of atmospheric environment,but also had the"three causes"effect.Catalytic combustion was the most commonly used destruction method for VOCs with low cost,high efficiency and less pollution,especially for industrial waste gas with complex components.The key point of catalytic combustion method was the selection of catalysts.Precious metal catalysts had high catalytic efficiency but were expensive,so their practical application was limited.Compared with precious metals,composite metal oxide catalysts had attracted much attention in VOCs treatment because of their low cost,good catalytic effect and stable structure.In this study,ZSM-5 molecular sieve was prepared by using urea as modified material,and the effect of urea modifier on the pore structure and adsorption performance of ZSM-5 molecular sieve was discussed,in order to obtain a modified ZSM-5molecular sieve with large specific surface area,rich pore structure and good adsorption performance for toluene,which was used as a carrier for supporting metal oxide catalysts.On this basis,a series of low-loading composite metal oxide/ZSM-5 catalysts were prepared by sol-gel method,and the catalytic oxidation performance of toluene by active components and loading was discussed.Innovation had been achieved in the preparation of modified ZSM-5zeolite catalyst,and the main research conclusions were as follows:（1）A series of modified ZSM-5 molecular sieve carriers（S-0.05,S-0.1,S-0.25,S-0.5 and S-0.6）were prepared by adding urea modifier.The BET characterization results showed that a small amount of urea modifier was beneficial to improve the pore structure of ZSM-5molecular sieve.The S-0.1 molecular sieve modified by adding 0.1 g urea had the largest total specific surface area,larger micropore surface area and micropore volume,and rich mesopore volume,which was beneficial to the adsorption and mass transfer of toluene.The dynamic adsorption test results of toluene showed that S-0.1 molecular sieve had the best adsorption performance,and the saturated adsorption capacity of toluene reaches 83.02 mg/g,which was1.11 times that of unmodified molecular sieve.（2）A series of supported ZSM-5 catalysts with low loading of CoMn,CoCu and CuMn were prepared by sol-gel method.XRD results showed that the supported catalysts still maintained the structure of ZSM-5 molecular sieve and the composite metal oxides were highly dispersed on the carrier.FTIR characterization results showed that the prepared catalysts had complete framework characteristics of ZSM-5 molecular sieve;BET characterization results showed that the catalyst had a multi-stage pore structure with more micro-mesopores.XRF characterization results showed that the prepared catalyst had a high silicon-aluminum ratio,and the contents of CoMn,CoCu and CuMn are consistent with the loading ratio.The results of SEM and TEM showed that the catalyst components were successfully loaded on the carrier,and the prepared series of composite metal oxide catalysts and modified ZSM-5 carrier were granular.（3）The catalytic oxidation performance of a series of catalysts for toluene was evaluated by a fixed-bed dynamic catalytic device.The results showed that among the three series of composite metal catalysts in this study,Co_0.75Cu_0.25/S-0.1 had the best catalytic oxidation performance for toluene,and T₉₀was 238℃.The XPS results showed that Co³⁺/Co²⁺ratio,Cu⁺/Cu²⁺ratio and O_ads/O_latt ratio of Co_0.75Cu_0.25/S-0.1 were all high,and there were abundant active oxygen vacancies on the catalyst surface,which was beneficial to the low-temperature catalytic oxidation of toluene.The best T₉₀ of Co/S-0.1 in CoMn series is 248℃,and the lowest apparent activation energy is 50.88 k J/mol.With the increase of Mn content,the catalytic efficiency showed a downward trend,and the doping of Mn inhibits the catalytic activity of Co/S-0.1.The best T₉₀ of Cu_0.5Mn_0.5/S-0.1 in CuMn series is 256℃,but it is obviously lower than CoCu and CoMn series catalysts.XPS results showed that high valence Co,low valence Mn,low valence Cu and O_ads with more catalysts are beneficial to the low temperature catalytic oxidation of toluene.In this study,a composite metal oxide catalyst with simple preparation method,low cost and good catalytic performance for toluene oxidation was developed.Among them,the catalytic performance of low-loading CoCu series catalysts for toluene was close to that of noble metal catalysts,which was obviously superior to the traditional pure composite metal oxide doped catalysts,and had great application prospects in VOCs catalytic oxidation.