Preparation And Performance Of N-doped Carbons-Supported Catalysts For Selective Transfomation Of Alcohols

In recent years,the design of emerging N-doped carbon-supported catalysts has attracted the attention of researchers due to the fact that nitrogen doping can change the surface properties of porous carbon materials.Great progress has been made over the past few decades.Compared to pristine carbon supported catalysts,N-doped carbon supported catalysts generally exhibit excellent catalytic performance in many heterogeneous reactions because the introduction of N species can alter the metal-support interaction.For this type of catalytic materials,it can be found from the existing research reports that the controllable preparation of nitrogen species,the determination of active sites,and the establishment of regularity between catalyst structure and activity have always been difficult to study,and further exploration and research are needed.In order to address these issues,we chose the alcohol selective transfomation reaction system as research goals,including(1)selective oxidation of alcohols to prepare corresponding aldehydes,(2)oxidative carbonylation of methanol to prepare dimethyl carbonate(DMC).It is important to explore the structure regulation,active center and relationship with catalytic activity.For the alcohol selective oxidation reaction system,we choose the supported metal nitride as the catalyst.The transition metal nitride supported on N-doped carbon material can exhibit the catalytic performance comparable to that of the noble metal catalyst.However,research on the correlation between catalyst structure,performance and stability is still highly desirable.Here,a series of metal nitrides were prepared and the selective oxidation of alcohol by molecular oxygen was evaluated.Among them,FeNx/C-T(T stands for pyrolysis temperature)catalysts exhibit high selectivity(95%)for the corresponding aldehyde.The optimized FeNx/C-900 catalyst could conver 5-hydroxymethylfurfural(HMF)to 2,5-diformylfuran(DFF)with a TOF of 7.0 h-1.A combined study of characterizations and experiments indicates that the Fe-N4 species are the major active sites.In addition,we investigated the nature behind the catalyst deactivation and provided an effective method to regenerate the catalyst.The results show that the deactivated catalyst can be regenerated by heat treatment after each inactivation in the NH3/N2 atmosphere.Based on these studies,a reasonable reaction mechanism is proposed.over the FeNx/C-900 catalyst in the selective oxidation of alcohol to aldehyde.For the reaction of methanol oxidative carbonylation to prepare DMC,we chose N-doped carbon material supported Cu-based catalyst.On the one hand,we report the preparation of hierarchically porous NCs derived from the direct pyrolysis of ZIFs(ZIF-7 and ZIF-8)and poly-m-phenylenediamine-covered carbon black(PmPDA-C)for the stabilization of Cu nanoparticles.By changing the ligand and pyrolysis atmosphere of ZIFs,the configuration of the N species can be effectively regulated.In the reaction of oxidative carbonylation of methanol to DMC under molecular oxygen conditions,the effect of N species on Cu nanoparticles and catalytic activity was observed.The results show that the TOF values of N-doped carbon carrier-supported Cu nanoparticles from ZIF-8 and ZIF-7 are as high as 17.9 h-1 and 28.5 h-1,respectively.A series of characterization and experiment results show that the catalyst exhibits good catalytic performance when the nitrogen content of the catalyst is 2-5 wt.%and the pyrrole-N/pyridine-N molar ratio is 2-3.This work shows that the rational design of the precursor can effectively regulate the N species configuration in the catalyst,achieve the stability of the active Cu species on the N-doped carbon carrier and effectively improve the catalytic performance.Based on the experiments that discussed the effect of N species on the oxidative carbonylation of methanol,we further chose NCs-1000 prepared by calcining ZIF-8 at 1000? for 4 h in an Ar atmosphere as supports.A series of Cu/NCs-1000 catalysts were prepared by the impregnation method.It was investigated that the effects of different treatment atmospheres,treatment temperature,loading amount,valence state and particle size on the catalytic activity.The experiments showed that when the Cu loading was less than 4 wt.%,with the increase of the calcination temperature,the average particle size of Cu species is less than 3 nm during the Ar atmosphere,and the catalytic activity changed in a volcanic type.In the H2/Ar atmosphere,with the increase of the calcination temperature,Cu species were easily aggregated,and the catalytic activity decreased.When the Cu loading was higher than 4 wt.%,the average particle size of Cu species gradually increased with the increase of the loading amount,but the activity of the catalyst remained basically unchanged during the Ar atmosphere.Combining XPS analysis,we speculate that the molar ratio between Cu species and N and O species(Cu/(N+O))has a greater effect on the activity of the catalyst than the effect of Cu size on the activity and the relatively low coordination number of Cu/(N+O)x(x?5-6)species are active centers.At the same time,it is speculated that the valence of Cu species also affects the activity when the catalysts contain low-coordinate Cu/(N+O)x structures.