Carbon-Based Nanocage As Metal-Free Catalysts For Selective Hydrocarbon Oxidation

Selective oxidation reaction of hydrocarbon is an important chemical engineering process to produce high-valued oxygen containing organic products via transforming saturated alkanes and phenyl alkanes by autoxidation or catalytic oxidation,such as ethylbenzene oxidation to acetophenone,cyclohexane oxidation to adipic acid.Traditional oxidations using stoichiometric strong oxidant suffer from high cost and environmental unfriendliness.Catalytic oxidation using metal-based catalysts has the advantages of high activation and selectivity of major products but have the disadvantages of expensive noble metal,easy-poisoning of catalysts,loss of active components and metallic residual in products as well.Thus,it is an attractive strategy to develop metal-free hydrocarbon oxidation catalysts with high catalytic activity and selectivity.Compared to metal-based catalysts,novel carbon-based metal-free catalysts are attracting attention owing to the advantages of wide source,good chemical stability,large specific surface area,abundant surface defects,and easy-doping.Recently,our group has developed hierarchical carbon-based nanocages featured unique mesostructures,high specific surface area,coexisting micro-meso-macro-pores,tunable content and status of doped nitrogen,rich defects.In this thesis,we investigate the catalytic performances and structure-activity relationship of carbon-based nanocages as a metal-free catalyst for catalytic a-H oxidation of ethylbenzene and cyclohexane oxidation.Some progresses have been made.(1)Controllable preparation of carbon-based nanocages with unique structures and components:The mesostructured carbon nanocages(hCNC)and nitrogen-doped carbon nanocages(hNCNC)were prepared using benzene and pyridine as carbon source and in-situ MgO as template,which possess multi-scale hierarchical porous structure,large specific surface area,rich defects and good graphitization degree.Thereinto,the specific surface area of hNCNC is up to 2018 m2g-1 and its nitrogen content reaches 9.7 at.%.By controlling the annealing temperature,the content and status of nitrogen and number of defects as well were successfully tuned without changing the morphology and structure.(2)Carbon-based nanocages as an efficiently metal-free catalyst for selective oxidation ?-H of ethylbenzene to acetophenone:Using tert-butyl hydroperoxide as oxidant,the hNCNC catalyst delivers excellent metal-free catalytic performances of?-H selective oxidation of ethylbenzene to acetophenone at 80? for 24 h,e.g.more than 98%of ethylbenzene conversion and acetophenone selectivity,far superior to the counterparts of hCNC and carbon nanotubes(CNTs).Under mass ratio of 100 for ethylbenzene and hNCNC,yield of acetophenone still keeps at the high level of 95.6%.Namely,just 1 wt.%hNCNC is needed to achieve efficient conversion.It is the state-of-the-art for metal-free carbon-catalyzed ?-H selective oxidation of ethylbenzene.In addition,hNCNC catalyst exhibits good recyclability.The outstanding catalytic performances of hNCNC could be mainly ascribed to graphitic nitrogen rather than defects.(3)Carbon-based nanocages as metal-free catalyst for cyclohexane oxidation to adipic acid:Using molecular oxygen as oxidant,to a certain extent,carbon-based nanocages exhibits metal-free catalytic performance of cyclohexane oxidation at 125? with 3 MPa O2 and acetone solvent.After 8 h,hCNC delivers 66.6%of cyclohexane conversion and 14.3%of adipic acid yield which is the same as CNTs but superior to hNCNC.In addition,cyclohexane is unable to be oxidized with ethanol,acetic acid and acetonitrile as solvent and without solvent.Acetone plays an important role in metal-free cyclohexane oxidation.These results provide a promising strategy to develop the novel metal-free carbocatalysts with high performance for the selective oxidation of hydrocarbons.