Co/MgO-Al₂O₃ Catalyst For C₂H₆/CO₂ Co-conversion Reaction

The conversion of C₂H₆ and CO₂ to value-added chemicals is a great way to utilize CO₂resource which meets the green and sustainable development requirements.The utilization of cheap,non-toxic and abundant CO₂ as an oxidant with C₂H₆ from released automobile exhaust and shale gas to produce high value-added chemicals C₂H₄（oxidative dehydrogenation of ethane in the presence of CO₂,ODEC）and syngas（dry reforming of ethane,DRE）.This is of important significance because not only could it contribute to CO₂ emission reductions,but also to alleviate environmental issues and energy problems.C₂H₆/CO₂ co-conversion is a complex reaction system,involving the activation of C₂H₆and CO₂,as well as the selective cleavage of C–C and C–H bonds,which needs multi-functional catalysts with multiple active sites.The Co-based catalyst is harmless,inexpensive and rich in resource;the rapid conversion between Co³⁺/Co²⁺is conducive to the activation of C₂H₆ and CO₂,which has a broad promising application.In this thesis,a series Co/Mg O-Al₂O₃ catalyst were prepared and applied to ODEC and DRE reaction.We investigated the influences of Mg O-Al₂O₃ support structure on cobalt species,the activation of C₂H₆ and CO₂ and consequently to reaction path;On this basis,we focus on the influences of Mg/Al molar ratio on the properties of Co/Mg（Al）O-x catalysts for DRE to reveal the active structure.1)Al₂O₃,Mg O（25）-Al₂O₃（75）,Mg O（75）-Al₂O₃（25）and Mg O support were prepared by co-precipitation method,followed by loading Co through impregnation method and the obtained catalysts were tested for ODEC and DRE reactions after different pretreatment.The reaction evaluation was conducted in feed gas containing C₂H₆/CO₂/N₂=1:2:17 with WHSV=9000 m L g^-1·h^-1.The prepared Co/M₃A₁-Solid catalyst exhibited stable catalytic activity for both ODEC and DRE reactions;after calcined 41%ethylene yield was obtained at 700℃and83%CO yield was obtained at 600℃,respectively.The reversible conversion between Co³⁺?Co²⁺can be detected under C₂H₆/CO₂ atmosphere which can promote the activation of C₂H₆ and CO₂;the properties of supports significantly affected the location,structure and reducibility of Co species,Co species are more reducible in spinel than those in solid;Co⁰reduced from Co species on the surface of Co/M₁A₃-Spinel at high temperature in ODEC reaction condition,which shows synergetic effects with Co³⁺/Co²⁺and contributes to the cleavage of C–C and/or C=C bonds,resulting in the reaction routs transforming from ODEC to DRE.2)On the basis of the above research,Mg O-Al₂O₃ supported Co catalysts（CMA-x）with Mg（Co,Al）O solid solution structure in different Mg/Al ratios were prepared and tested for DRE reaction in order to understand the high DRE activity of Co/M₃A₁-Solid catalysts and further investigated the active site structure for DRE reaction.The optimal C₂H₆ conversion（58.34%）,CO₂ conversion（61.75%）under C₂H₆:CO₂:N₂=1:2:1,T=600℃,WHSV=60,000 m L g^-1·h^-1 reaction condition obtained on CMA-3.Co⁰/Co³⁺/Co²⁺co-exist within Co⁰NP-Mg（Al）O interface;synergetic effects with Co³⁺/Co²⁺contribute to the activation of C₂H₆and CO₂,while Co⁰ are important to C–C and/or C=C band cleavage.Mg/Al molar ratio significantly affected the distribution of acid/base sites;Moderate acid/base sites mitigate carbon deposition,while strong acid/base sites both result in severe carbon deposition.Furthermore,disordered carbon on strong base sites could react as active carbon species to produce CO and graphite carbon on strong acid sites result in irreversible deactivation of catalysts.This study provides inspiration for the design of highly stable and efficient catalysts for both ODEC and DRE reactions.