Selective Liquid Phase Oxidation Of Cyclohexane, Cyclohexene And α-pinene Catalyzed By Nanocarbons

Nanocarbon materials as a kind of new metal-free catalysts have attracted much attentionin the field of catalysis, due to their excellent physical and chemical properties, such as highspecific surface area, high stability, corrosion resistance etc. At present, breakthrough studiesfocusing on oxidative dehydrogenation of hydrocarbons in gas-phase carbon materials havebeen carried out. However, there are few pulications on selective liquid phase oxidationreactions catalyzed by nanocarbon materials, which is at the fledgling stage. The role ofnanocarbon materials in the liquid phase oxidation reaction is still unclear. Therefore, thestudy on exploring the application of nanocarbon materials in the liquid phase oxidationreaction and understanding the reaction mechanism is of great theoretical and practicalsignificance. Selective oxidation of cyclohexane and selective allylic oxidation or epoxidationof cycloolefin to high-valued products are important chemical industry. In this work, selectiveoxidation of cyclohexane, cyclohexene and α-pinene, catalyzed by carbon materials weresystematically investigated. The relationship between the surface chemistry of nanocarbonmaterials and the catalytic performance were studied in detail, and the plausible catalyticmechanisms were proposed. The main contents are presented as follows.(1) The catalytic properties of sp2-and sp3-hybridized carbons, represented by grapheneand diamond, were investigated to understand the structure dependence of carbon materials inthe selective oxidation of cyclohexane. sp2carbons showed the higher activity than sp3carbons. The highest activity was obtained over a mesoporous graphene, a representativesp2-hybridized carbon, yielding a weight-normalized activity of162.6mmol g-1h-1. Theexcellent performance of sp2carbons was attributed to the activity of catalytic decompositionto peroxide intermediates, such as cyclohexyl hydroperoxide, which put the oxidative reactionforward.(2) Carbon nanotubes (CNTs) and nitrogen-doped CNTs (NCNTs) were systematicallyinvestigated as metal-free catalysts in the selective allylic oxidation of cyclohexene usingmolecular oxygen as oxidant in the liquid-phase. High up to59.0%cyclohexene conversionand mass-normalized activity of620.1mmol g-1h-1were obtained over NCNTs, competingwith the state-of-the-art metal catalysts. The doping of nitrogen in CNTs is favorable to theactivity enhancing and the selectivity for2-cyclohexen-1-one increasing. The unique catalyticrole of NCNTs was attributed to their capability to promote the radical chain propagation viastabilizing peroxyl and cycloxyl radicals, which boosted further the conversion ofcyclohexene. (3) The catalytic performance and probable reaction mechanism in the selectiveepoxidation of α-pinene to α-pinene oxide (PO) were studied using the CNTs and NCNTs ascatalysts.54.5%α-pinene conversion and37.8%PO selectivity were obtained on NCNTscatalyst. It was proven that R(a)-OO-(b)R(c) and R(a)-O-(b)R(c) radicals are the most importantintermediate and the main chain propagator in the oxidation of α-pinene toward producing PO.NCNTs showed outstanding recyclability and the conversion of α-pinene was not changedafter five consecutive usages, showing excellent potential for industrial application.