Preparation Of ZIF-67 Derived Cobalt-based Catalysts And Their Catalytic Performance Of Toluene

The emission of Volatile Organic Compounds（VOCs）can not only cause environmental pollution problems such as ozone,photochemical smog,and fine particulate matter（PM2.5）,but also pose a serious threat to people’s health.Catalytic oxidation technology has the advantages of high processing efficiency,low energy consumption,and no secondary pollutants among many VOCs end treatment technologies,and has become one of the most commonly used technologies for VOCs end treatment.Co₃O₄ material has the excellent characteristics of spinel structure and transition metal,and is commonly used as a catalyst for the catalytic oxidation of VOCs.In this paper,toluene was used as the target pollutant,and metal-organic framework Co-MOFs（ZIF-67）was used as a sacrificial template,the effects of the calcination temperature of the sacrificial template,the ratio of precursors,in-situ element doping and the amount of element doping on the catalytic oxidation of toluene were investigated.The catalytic process of the catalyst for toluene was discussed.1.ZIF-67 material was prepared as a sacrificial template by solvent method,and Co₃O₄-x（x represents calcination temperature）catalyst was prepared by pyrolysis method.By adjusting the calcination temperature to change the morphology,particle size and crystallinity of Co₃O₄,the effect of catalysts at different pyrolysis temperature on the catalytic activity of toluene was studied.The evaluation results of toluene activity showed that the activity of Co₃O₄-x to toluene was better(T₁₀₀=240℃)when the calcination temperature was between 300℃and 400℃.According to the analysis of template and catalyst characterization,the Co₃O₄ catalyst prepared at 400℃has the advantages of complete decomposition of organic ligands in the template,high crystallinity of Co₃O₄,and the prepared catalyst shows that the adsorbed oxygen content is high and has good low-temperature reduction performance.After the catalyst was operated under 5%water vapor for 2 h,the degradation rate of toluene could still be maintained above 90%;meanwhile,the catalyst had on obvious change in the degradation rate of toluene during continuous 80 h operation at 240℃,which indicates that the catalyst has good stability for the catalytic oxidation of toluene.2.Based on the ZIF-67 template,the activity of Co₃O₄（1）,Co₃O₄（4）,Co₃O₄（10）and Co₃O₄（20）catalysts with the ratio of precursor 2-methylimidazole to metal Co ion of1:1,4:1,10:1 and 20:1 was investigated,By in-situ doping of ZIF-67,the additives with a molar content of 1%Mn,Ce,Fe and Cu were added to the catalyst to prepare M-Co₃O₄ catalyst for the catalytic oxidation of toluene;then the content of additives was increased to 2%,4%and 8%.The experimental results show that when the ratio of2-methylimidazole to metal Co ions is 4:1 and Mn is 1%,the temperature of complete conversion of prepared Mn-Co₃O₄ to toluene is reduced by 5℃.The high content of additives and poor effects may be due to the limited binding ability of manganese nitrate and 2-methylimidazole,which can not completely enter the ZIF-67 lattice.The service life test of the Mn-Co₃O₄ catalyst shows that the degradation rate of the catalyst to toluene can be maintained at about 90%in continuous 80 h operation.3.The process of Co₃O₄-400 catalytic oxidation of toluene was studied by transient response experiment.Toluene is mainly adsorbed on the catalyst surface,migrates to the active oxygen vacancy on the catalyst surface,and then transforms into intermediates to be completely degraded.Oxygen in the gas phase is first adsorbed by the catalyst,then transferred to the oxygen vacancy and activated to participate in the catalytic oxidation of toluene.The reaction process follows the Mars van Krevelen（MVK）mechanism.