Liquid-phase Oxidation And Hydrogenation Reactions Catalyzed By Metal-free Nitrogen-doped Carbon Nanomaterials And Boron Nitrides

Nanocarbon materials and hexagonal boron nitride have been regarded as new metal-free catalysts for various oxidation and hydrogenation reactions.Catalytic active sites of nanocarbon materials and hexagonal boron nitride can be introduced through controllable dopants,structural distortion/defects,and/or three-dimensional structure,allowing for tunable and efficient activity for various applications.In this work,we focused on identification of the active sites of metal-free nanocarbon materials and hexagonal boron nitride to lay the foundation for the rational design of the catalysts in the liquid-phase oxidation and hydrogenation reactions.The main contents are as follows:Nitrogen dopants on carbon materials play vital roles in metal-free carbon catalysis,while the role of pyridinic nitrogen?N_P?or graphitic nitrogen?N_G?is still on debate.In this work,N-doped carbon nanotubes were employed to catalyze the oxidation of styrene?SOR?with tert-butyl hydroperoxide as oxidant.We observed a significant enhancement of SOR activity caused by the incorporation of nitrogen dopants,but the catalytic activity increased first and then decreased.Different catalytic functions of N_P and N_G were revealed.Independent N_P and N_G as active sites may synergistically complete this catalytic reaction.A synergistic effect between N_P and N_G was demonstrated when they are quite close,which is unbeneficial for the catalytic activity due to the very high energy barriers.The finding may pave a way to the rational design of high-performance metal-free carbon catalysts.Nitrogen-doped carbon is an effective metal-free catalyst for the hydrogenation of nitrobenzene to aniline with H₂ molecule as reductant,but its catalytic mechanism is controversial.In aprotic solvent,nitrogen-doped carbon nanotubes can not directly dissociate H₂ molecules,in other words,nitrobenzene hydrogenation reaction can not be accomplished by Horiuti-Polanyi mechanism.By experimental and theoretical studies,we proposed that H₂molecules are activated on the surface of nitrogen-doped carbon nanotubes with adsorbed nitrobenzene by Eley-Rideal mechanism.The nitrobenzene hydrogenation catalyzed by nitro-doped carbon nanotubes in protic solvents has either transfer hydrogenation or direct hydrogenation of nitrobenzene.The N_G is the major active site.However,the synergistic effect between N_P and N_G at high nitrogen contents is unbeneficial for the catalytic activity.Porous hexagonal boron nitride?h-BN?was synthesized with boric acid and melamine as starting materials by a pyrolysis method.It was demonstrated that h-BN an effective catalyst for the epoxidation of styrene with tert-butyl hydroperoxide as oxidant.The active sites of h-BN were identified by turning the functional groups by heat treatment.Combining the experimental and theoretical studies,we proposed that B-OH is the major active center,while B-O₂ formed by heat treatment at 800 ^oC in oxygen atmosphere has similar catalytic activity with B-OH site.But the B-O-B sites and N-H sites have no catalytic activity.