Studies On Catalytic Performance Of Carbon Material Supported Copper Catalysts For Oxidative Carbonylation Of Methanol

Activated carbon?AC?supported Cu and its oxide catalysts show good initial activity for oxidative carbonylation of methanol to dimethyl carbonate?DMC?.However,Cu species are difficult to enter into the pores of AC and generally located on the outer surface of AC due to the dominated micropores structure of AC.Cu species are easily sintered and agglomerated during the calcination of the precursor and reaction process of oxidative carbonylation of methanol,resulting in the decreased dispersion of the Cu species and the deactivation of catalyst.Ordered mesoporous carbon?OMC?and carbon nanotube?CNT?,with rich mesoporous structure,are widely used in the preparation of catalysts.The uniform mesoporous structure can facilitate the dispersion of active species in mesopores and inhibits agglomeration of active species because of the confinement effects of mesoporous channel,so as to stabilize the dispersion of active species and improve the catalytic activity and stability.In addition,the surface chemistry of the carbon material is easily modified.The surface oxygen and nitrogen-containing groups can be introduced by the oxidizing or nitrogen-doping treatment,which can provide more additional anchoring site for active species and enhance the interaction between active species and support.This is aim to improve the dispersion of active species and stabilize of the particle size and valence state of the active species.All this will be contributed to further improve the catalytic activity and stability of catalyst.In this paper,firstly,three carbon materials?AC,OMC,and CNT?with different pore structures were used as supports treated with HNO₃ for preparing a series of supported Cu-based catalysts.The effects of pore structure and surface oxygen-containing groups of carbon supports on the composition,dispersion of Cu species and their catalytic performance for oxidative carbonylation of methanol were investigated.Secondly,the efficient nitrogen-doped CNT?NCNTx?were prepared by procedures of hydrothermal oxidation of 0.5M HNO₃ solution followed by high-temperature nitrogen doping and then Cu based catalysts Cu/NCNTx were prepared.The effects of nitrogen-containong groups on the composition,dispersion of Cu species,as well as the catalytic activity,selectivity,and stability for oxidative carbonylation of methanol were investigated.Combining characterization methods such as BET,TPR,FTIR,TEM,XPS and ICP,the main research results are as follows:?1?Cu species on Cu/AC catalyst was mainly dispersed on the external surface of AC because of its micropore structure.For Cu/OMC,some Cu species highly dispersed inside the mesoporous,while the others located outside the pores of the OMC.Cu species on Cu/CNT mainly confined inside the mesoporous channels of CNTs.?2?The average grain size of the supported Cu particles depended on the structural characteristics of carbon supports.And the particle size of Cu species increased following the order:Cu/OMC<Cu/CNT<Cu/AC.?3?The content of active Cu species?Cu₂O+Cu?increased following the sequence:Cu/CNT<Cu/OMC<Cu/AC,which was due to that the increased SOG enhanced the interaction between supports and Cu species and thus promoted the autoreduction of CuO to the active Cu₂O and Cu.?4?Cu/OMC catalyst performed better in terms of the initial activity than Cu/AC and Cu/CNT catalyst because of the smaller Cu particle size and moderate amount of active species.While the Cu/CNT catalyst exhibited better stability than Cu/AC and Cu/OMC during the reaction process owing to the sufficient confinement effect of the mesoporous channels.The factor that caused Cu/CNT catalyst was only the oxidation of active Cu species.Unlike Cu/CNT,agglomeration and oxidation of active Cu species jointly led to the deactivation of Cu/OMC and Cu/AC.?5?The pyridinic and pyrrolic nitrogen groups was introduced on the surface of NCNT prepared by direct high-temperature nitrogen doping.While the pyridinic,pyrrolic and pyridinic-N-oxide nitrogen groups was generated on the surface of NCNTx prepared by hydrothermal oxidation of 0.5M HNO₃solution followed high-temperature nitrogen doping.In addition,the content of nitrogen of NCNTx is much higher than that of NCNT and the increase with the elevated hydrothermal temperature.Hydrothermal oxidation of CNT was conducive to the doping of nitrogen atoms.?6?The introduced nitrogen-containing groups of NCNTx could provide more additional anchoring site for Cu species,especially the pyridinic nitrogen groups,were considered as the main anchoring site,which could promote the dispersion of Cu species and the formation of more active site,and thus improved the catalytic activity.Futhermore,the introduced nitrogen-containing groups had enhanced the interaction between Cu species and support,which was beneficial to stabilize the particle size and valence state of Cu species during the reaction process,and suppressed the agglomeration and oxidation of Cu species,and at the same time,the loss of Cu species was retarded.Hence,the catalytic stability of the catalysts was improved.