Design And Synthesis Of Perovskite Catalysts For Partial Oxidation Of Cyclohexane

Selective oxidation of cyclohexane is a fundamental industrial process for the manufacture of cyclohexanol and cyclohexanone（known as KA oil）which are utilized as intermediates for the production of nylon-6 and nylon-66.In the current industrial process,homogeneous formulations of cobalt or manganese-based catalysts at relatively elevated temperature and pressure are used.In spite of harsh operation conditions,the process suffers from serious shortcomings of very low cyclohexane conversion,poor useful product selectivity,high operation costs and also environmental-related concerns.Therefore,achieving a better cyclohexane conversion along with a substantial KA oil selectivity continues to be a great challenge at present.Undeniably,noble metal-based catalysts have exhibited high activities and selectivities for cyclohexane oxidation with molecular oxygen as oxidant.However,their high cost and rapid deactivation due to metal leaching/aggregation have limited their application in the industry.Transition metal-based catalysts,such as transition metal oxide or supported transition metals have also been studied for this reaction,which exhibited acceptable catalytic performance.However,the poor stability and/or metal leaching still restrict their application.Perovskite materials have gained a lot of attention as a new class of materials in low-temperature oxidation catalysis due to their interesting physicochemical properties such as flexible redox behavior,oxygen mobility and good stability.These properties are highly influenced by the preparation method,type and ratio of precursor used.In this work,transition metal perovskites La BO₃（B=Mn,Fe,Co,Ni）have been synthesized by a modified citrate method.The catalytic performance of these materials was evaluated in solvent-free liquid phase oxidation of cyclohexane with molecular oxygen as oxidant.The reported results show that the calcination temperature,type of complexing agent and its ratio to the total metal ions influenced the perovskite phase formation.Cobalt-based perovskite（LaCoO₃）reported superior catalytic activity among all materials,with a cyclohexane conversion of 8.3%and KA oil selectivity of 90%in 1 hour of reaction at 150^oC.Furthermore,non-stoichiometric cobalt perovskites were also synthesized to probe the essence of high catalytic performance of LaCoO₃.The characterization results of XRD,FTIR,SEM,TEM,and SAED confirm the single-phase formation with a small crystal size,whereas,XPS and EPR indicate the dominance of Co³⁺ions on the stoichiometric LaCoO₃ surface.DFT calculations emphasize the role of La ions in oxygen adsorption.The excellent catalytic activity was explained by the abundance of active Co³⁺ions and adsorbed oxygen species on the catalyst surface.Our work provides an insight into the nature of surface species for highly active cobalt-based catalysts.