Investigation On Catalytic Combustion Performance Of N-butane Over Co-ZSM-5 Catalysts

In recent years,with the continuous growth and massive emission of volatile organic compounds(VOCs),they have harmful effects on the atmospheric environment and human health.Therefore,it's great of importance and urgency to develop appropriate and effective treatment technology for the abatement of VOCs.At present,the catalytic combustion technology has been considered as one of the most attractive technologies with high efficiency,low energy consumption,environmental protection and no secondary pollution.Obviously,the development and design of high efficiency and low cost catalyst is the core of this technology.At present,the application of noble metal catalysts has been greatly limited due to their high cost and easy sintering.However,transition metal oxides have been widely used because of their low price,easy availability,relatively stable and efficient catalytic activity.Among them,cobalt oxides have higher reducibility and stability,which exhibit excellent catalytic activity in the field of catalytic combustion.Therefore,in this paper,Co-ZSM-5 was taken as research object and n-butane was selected as the probe molecule of VOCs to further understand the performance of such catalysts for n-butane catalytic oxidation.The impacts of gas hourly space velocity(GHSV)and the equivalence ratio(?)on the catalytic combustion performance of n-butane were investigated.In addition,we obtained the initial reaction path of n-butane by quantum chemistry calculation.The oxidation mechanism of n-butane on Co-ZSM-5 catalyst was further investigated combined with experimental results.In this paper,cobalt modified ZSM-5 zeolite catalysts(3-18 wt%Co)were prepared by impregnation to investigate the combustion of n-butane.The experimental results showed that compared with ZSM-5 catalyst,the reaction temperature of n-butane oxidation decomposition was reduced significantly and a significant enhancement of the catalytic activity was demonstrated in terms of the cobalt supported over ZSM-5.Besides,the loading of Co has a significant influence on catalytic activity for oxidation of n-butane.The catalytic activity was the highest with an optimal cobalt loading of 7 wt%.And T90(the reaction temperature corresponding to 90%n-butane conversion)is 374?.While when the loading of cobalt is 3 wt%,the activity was the lowest and T90 is 442?.Moreover,with the increase of gas hourly space velocity(GHSV),the catalytic activity of the catalyst was decreased.In a certain range,the catalytic c ombustion performance of n-butane can be significantly improved by reducing the equivalence ratio(?).The catalyst characterization tests including XRD,FT-IR,BET and XPS were carried out to explore the relationship between the difference of catalytic activity and the physical and chemical properties of the catalysts.The XRD and FT-IR spectra showed that the cobalt element was uniformly dispersed in ZSM-5 and mainly existed in the form of Co3O4.According to the BET test results,when the loading of cobalt is 7 wt%,the specific surface area of the catalyst was up to 253 m2 g-1.Moreover,it had a rich pore size distribution.This enhanced the adsorption and diffusion capacity to n-butane,which was conducive to the further oxidation of n-butane.Combined with the activity test and XPS characterization results,the conversion of n-butane increased with the increasing Co³⁺/Co²⁺ ratio on the catalyst surface.Therefore,it can be seen that Co³⁺ is the active ion in the catalyst,and the higher the concentration of Co³⁺ on the catalyst surface,the more conducive to the catalytic combustion of n-butane.In the alkane catalytic oxidation,the activation of the initial C-H bond is regarded as a rate-determining step.It is of great significance to understand the activation reaction mechanism of n-butane C-H bond over Co-ZSM-5 for exploring the catalytic combustion reaction of n-butane.Therefore,based on DFT theory,we used B3LYP method to calculate the initial reaction path of n-butane over Co-ZSM-5 catalyst.The calculated results indicated that Co-ZSM-5 catalyst has a high catalytic activity because the reaction barrier of C-H bond is significantly reduced.In addition,it was also found that the formation of n-butyl and sec-butyl is the most important reaction path in the initial oxidation dehydrogenation reaction path of n-butane,which are basically consistent with the experimental data.