Study On Novel Indium Oxide Based Catalysts For Oxidative Dehydrogenation Of Ethylbenzene With CO₂

Styrene is one of the most important monomers in the chemical industry,which is widely used in the production of resins,plastics and synthetic rubber.In industry,90%of styrene is made from ethylbenzene dehydrogenation at high temperature（600～700℃）,and requires a large amount of superheated steam,resulting in high energy consumption.Using CO₂ as a mild oxidant instead of superheated steam for dehydrogenation of ethylbenzene to styrene can not only reduce the reaction temperature and energy consumption of styrene production,but also improve the conversion of ethylbenzene and the selectivity of styrene,and realize the resource utilization of CO₂.Therefore,CO₂ oxidative dehydrogenation of ethylbenzene to styrene as an energy-saving,efficient and environmentally friendly green process has been highly concerned by researchers at home and abroad.At present,the main problem of the new green process is the serious deactivation of the catalyst,and the key is to design and develop a new type of efficient and stable catalyst.Indium-based catalysts show excellent catalytic performance in selective catalytic reduction of nitrogen oxides,CO₂ hydrogenation and dehydrogenation of light alkanes,but their application in ethylbenzene dehydrogenation has not been reported.In this paper,indium-based catalyst was used for the dehydrogenation of ethylbenzene by CO₂ for the first time.The catalytic performance of supported In₂O₃ and In₂O₃-Al₂O₃ composite oxides was studied by adjusting the support,preparation method and component ratio of the catalyst.Through detailed physical and chemical characterization,the structure-activity relationship of the catalyst and the active center of the reaction were discussed.The specific research contents are as follows:（1）Supported In₂O₃ catalysts（In₂O₃/MO_x）were prepared by isovolumetric impregnation method using Mg O,Si O₂ andγ-Al₂O₃ as supports.The effects of different supports on the catalytic performance of In₂O₃/MO_x for oxidative dehydrogenation of ethylbenzene by CO₂were studied,and the catalysts were characterized by N₂ adsorption-desorption,XRD,TEM,NH₃-TPD,CO₂-TPD,H₂-TPR and XPS.The experimental results show that the support has a significant effect on the performance of In₂O₃/MO_x catalyst.In₂O₃/γ-Al₂O₃ catalyst showed the best catalytic performance.After 5 h reaction induction period,the highest conversion of ethylbenzene reached 51.6%.After 37 h reaction,the catalyst still had high catalytic activity.The conversion of ethylbenzene over In₂O₃/γ-Al₂O₃ catalyst in CO₂ atmosphere was twice as high as that in inert N₂ atmosphere,indicating that CO₂ significantly promoted the dehydrogenation of ethylbenzene.The characterization results show that theγ-Al₂O₃ support not only improves the dispersibility and reducibility of In₂O₃,but also increases the acid site and medium-strong basic site of the catalyst,which is beneficial to the adsorption and activation of ethylbenzene and CO₂,thus significantly improving the catalytic activity and stability of the catalyst for the oxidative dehydrogenation of ethylbenzene by CO₂.After 12 h of reaction,the In₂O₃/γ-Al₂O₃ catalyst produced a large number of In⁰ species,and the reaction induction period of the pre-reduced In₂O₃/γ-Al₂O₃ catalyst disappeared,and the catalytic activity increased obviously,indicating that the metal In formed after reduction of highly dispersed In₂O₃ may be the active component of CO₂ oxidative dehydrogenation of ethylbenzene.（2）In₂O₃-Al₂O₃ complex oxides of 10 wt%In₂O₃ were prepared by coprecipitation method,hydrothermal synthesis method and sol-gel method,labeled In-Al（CO）,In-Al（HS）and In-Al（SG）,respectively.The effects of different preparation methods on the performance of CO₂ catalyst for oxidative dehydrogenation of ethylbenzene were studied by XRD,N₂adsorption-desorption,SEM,NH₃-TPD,H₂-TPR,TGA characterization and catalyst performance evaluation.The results showed that the In-Al（SG）catalyst prepared by sol-gel method showed the highest catalytic activity and better stability,and the order of ethylbenzene conversion was In-Al（SG）>In-Al（HS）>In-Al（CO）.In-Al（SG）catalyst has a rod-like morphology,with larger specific surface area,mesoporous pore size and pore volume,which is conducive to the formation of highly dispersed In₂O₃ species,which is easy to be reduced at low temperature.In-Al（SG）has more medium-strong acid sites and weak strong acid sites,which is conducive to the adsorption of ethylbenzene and the desorption of styrene,which slows down the rate of carbon deposition.Therefore,the sol-gel method significantly improved the catalytic activity and stability of In₂O₃-Al₂O₃ complex oxides.（3）In₂O₃-Al₂O₃ composite oxides with In₂O₃ mass fraction of 5%,7%,10%,15%and20%were prepared by sol-gel method,labeled In-Al-5,In-Al-7,In-Al-10,In-Al-15 and In-Al-20,respectively.The experimental results show that at 550℃,the catalytic performance of In-Al-10 is the best,the conversion of ethylbenzene can reach 65%,and the selectivity of styrene is higher than 98%.After 30 h of reaction,In-Al-7 and In-Al-10 still have high catalytic activity,but the deactivation of In-Al-15 catalyst is serious.The ethylbenzene conversion of In-Al-10 catalyst in CO₂ atmosphere is more than 3 times higher than that in inert N₂ or Ar atmosphere,indicating that CO₂ greatly improves the dehydrogenation efficiency of ethylbenzene.The characterization results show that when In₂O₃≤10 wt%,the composite oxides have high specific surface area and pore volume and large mesoporous pore size,and In₂O₃ exists in highly dispersed state;when the content of In₂O₃ is 15 wt%and 20wt%,the specific surface area and pore diameter of composite oxides decrease greatly,and In₂O₃ exists in cubic form.The high dispersion of In₂O₃ species is beneficial to improve the catalytic performance,especially the stability of In₂O₃-Al₂O₃ composite oxides.