Nitrogen-doped Porous Carbon Materials For Green Catalytic Oxidation

The green catalytic reaction process is crucial to achieving sustainable development.As the most basic chemical reaction,oxidation reaction has a wide range of applications.In order to achieve the green catalytic oxidation process,the key is to use environmentally friendly solvents,low-toxic and low-pollution oxidants and recyclable heterogeneous catalysts.The traditional oxidations of alcohols and ketones require the use of strong acid oxidants such as sodium hypochlorite,（precious）metal catalysts,toluene,DMSO and other highly polluting and toxic organic solvents.In this work,a series of metal-free porous carboncatalysts and porous-carbon-supported metal oxide nanoparticles have been designed and prepared for oxidation.The larger specific surface area of porous carbon can solve the mass transfer problem in heterogeneous reactions.In order to solve the traditional environmental pollution problem,environmentally friendly and inexpensive solvents such as water are used as the solvent,and air or hydrogen peroxide is used as a non-toxic and pollution-free oxidant.（1）Heteroatom-doped meso-porous carbon materials are conventionally produced by harsh carbonization under an inert atmosphere involving specific precursors,hard/soft templates,and heteroatom-containing agents.Herein,we report a facile synthesis of N and O co-doped meso/micro-porous carbon（NOMC）by template-free carbonization of a small-molecule precursor in a semi-closed system.The semi-closed carbonization process yields hydrophilic NOMCs with large surface area in a high yield.The porous structure as well as the elemental composition of NOMCs can be modulated by changing the holding time at a particular temperature.NOMCs as metal-free heterogeneous catalysts can selectively oxidize benzyl alcohol and its derivatives into aldehydes/ketones with>85%conversion in aqueous solution,which is much higher than that of the control sample obtained in tube furnace（21%conversion）,mainly due to their high N content,high percentage of pyridinic N,and large surface area.The presence of O-containing moieties also helps to improve the hydrophilicity and dispersion ability of catalysts and thus facilitates the mass transfer process during aqueous oxidation.The NOMC catalysts also displayed excellent substrate universality,product selectivity and stability.（2）The selective oxidation of alcohols is traditionally catalyzed by precious metals,where the high cost,low selectivity,poor stability,and environmental concerns have hindered their wide application.Here we report a nitrogen and phosphorus co-doped microporous carbon（NPMC）as an efficient catalyst towards the selective oxidation of benzyl alcohol.NPMC is fabricated from one-step pyrolysis of an aerogel derived from the polymerization of p-phenylenediamine with phytic acid.The optimized NPMC exhibits high efficiency and universality for the selective oxidation of alcohols in aqueous environment using air as the oxidant,due to its large surface area(⁶76 m² g^-1),good hydrophilicity,suitable microporous structure（1-2 nm）,as well as the concerted effects between N and P doping.50 mg of the catalyst is sufficient for affording almost quantitative benzyl alcohol oxidized to benzaldehyde without organic solvents and co-catalysts,and the catalytic performance is among the best of the metal-free catalysts reported.The heterogeneous catalyst is also perfectly stable,which could be reused several times without significant loss of activity and selectivity.（3）The SnO₂ nanoparticles supported on mesoporous carbon（MC）material were prepared by hydrothermal method.By adjusting the amount of added SnCl₄·5H2O,catalysts with different Sn O₂ contents were prepared.A series of characterizations of the catalysts revealed that the catalytic active center Sn O₂ was uniformly dispersed on MC.The Sn O₂/MC-2 catalyst exhibits excellent catalytic activity in the Baeyer-Villiger oxidation reaction of 2-adamantane with hydrogen peroxide as an oxidant.In addition,Sn O₂/MC-2 shows high activity and selectivity to a variety of ketones in Baeyer-Villiger oxidation.The properties of Sn O₂/MC catalysts are stable,and the catalytic activity remains at a high level after reuse for several times.（4）The traditional methods for preparing porous carbon supported metal oxide nanoparticles include chemical vapor deposition（CVD）,sol-gel method,hydrothermal,and microemulsion methods.It is necessary to prepare carbon materials first,and then load metal oxide nanoparticles.Inspired by the semi-closed method for preparing carbon materials,we used a one-step method to in situ synthesize the precursor,and then combined the carbonization and oxidation process into“one-step”preparation in the muffle furnace.The as-prepared porous carbon supported metal oxide nanoparticles show small particle size,uniform distribution,stable structure,and the method are applicable to various metal oxides.The Mo O₃/MC catalysts prepared by this method exhibits very good catalytic performance for the selective oxidation of benzyl alcohol.5 mg of Mo O₃/MC-2 can be used to quantitatively convert benzyl alcohol into benzaldehyde.Furthermore,the Mo O₃/MC catalysts exhibits high selectivity,wide substrate universality and excellent stability.