Structural Regulation Of Nano Cu₂O Catalyst Supported On Porous Carbon Materials For Catalyzing The Methanol Oxidative Carbonylation Reaction

The synthesis of dimethyl carbonate（DMC）by oxidative carbonylation of methanol is an environment-friendly coal chemical route that integrates green raw materials,green process,green product applications.The Cu Cl catalysts currently used in industry are prone to deactivation and equipment corrosion due to the formation of dissociative HCl during the reaction,which greatly restricts the industrialization.The supported nano Cu₂O/C catalysts,prepared by impregnating chlorine-free copper precursors on porous carbon supports subsequently with carbon thermal auto-reduction,avoid the equipment corrosion and show good catalytic performance in the methanol oxidative carbonylation.Studies reveals that the surface oxygen-containing groups and pore structure in the carbon supports play a critical role in the dispersion,valence state and electronic state of the Cu nanoparticles,further influencing the catalytic performance in the methanol oxidative carbonylation.Therefore,in this paper,ordered mesoporous carbon supports rich with surface hydroxy and carbonyl groups are prepared to investigate the effect of surface oxygen-containing groups on the catalyst structure and performance.Further,micropore-rich mesoporous carbon supports are constructed to investigate the effect of micropores on the catalyst structure and performance,as well as the effect of Cu₂O electronic state on the intrinsic activity.The method to prepare high-purity Cu₂O catalysts is thus obtained.DFT theoretical calculation is conducted to investigate the surface properties of the micropore-rich carbon support,and to expound the formation mechanism of the nano Cu₂O/C catalyst.By comparing the catalytic performance of the Cu₂O and Cu⁰ active species,the catalytic mechanism of the reaction is discussed.The specific findings of the study are as follows.（1）In the process of ordered mesoporous carbon（OMC）synthesis by the hard-template method,H₃BO₃ is added during the step of filling carbon source.Accompanying with the subsequent template removal,boron species are released and hydroxyl dominated surface oxygen-containing groups are formed.With the further increase of H₃BO₃ addition,the relative content of carbonyl group is enhanced.The surface content reaches up to 8.14%.The oxygen-containing groups improve the interaction between Cu and support,which obviously facilitates the dispersion of copper nanoparticles,but restrains the reduction from Cu O to Cu₂O resulting in slightly reducing the Cu₂O content in the catalyst.The optimized catalyst shows obviously enhanced activity in the methanol oxidative carbonylation.Among theses,the hydroxyl is favorable for improving the intrinsic activity of the reaction.（2）A micropore-rich mesoporous carbon（MMC）support is prepared by KOH etching the pristine OMC.The ordered mesoporous structure is favorable for the entrance and dispersion of the Cu²⁺precursors,while the micropores at the mesopore wall are responsible for immobilizing the Cu nanoparticles.Increasing the microporosity of the carbon supports facilitates the reduction from Cu O to Cu₂O active species,forming a high-purity Cu₂O catalyst.The micropores in the carbon supports reduce electronic cloud density of the Cu₂O active species,which is beneficial for improving the intrinsic activity of the methanol oxidative carbonylation over each Cu⁺active site.（3）Results of the in-situ experiments and DFT theoretical calculation suggest that there are numerous under-coordinated carbon atoms in the micropores of carbon support.The unpaired electrons at these atoms assemble as a local negative potential region on the carbon support,which is susceptible to electrophilic attack by copper ion precursors during the impregnation process,further resulting in a high dispersion of the Cu species.During the carbon thermal auto-reduction,increasing the number of the under-coordinated carbon atoms enhances the initial auto-reducing rate from Cu O to Cu₂O at surface.It further promotes the diffusion of lattice oxygen during the drastic crystalline phase transition,achieving reduction of bulk Cu O and obtaining the high-purity Cu₂O catalyst.The under-coordinated carbon atoms facilitate the charge transfer from Cu₂O active species to the carbon supports,resulting in Cu₂O electronic cloud density reducing to form an electron-deficient Cu⁺active site.（4）Cu₂O/AC and Cu⁰/AC catalysts are prepared by in-situ calcinating the catalyst precursor at 350℃under N₂ and H₂ atmosphere,respectively.Comparative study finds that the supported Cu₂O species catalyze the methanol oxidative carbonylation more efficiently than Cu⁰ species.It is mainly because the co-adsorbed CH₃O and CO at Cu₂O surface are reacted via Langmuir-Hinshelwood（L-H）mechanism to form CH₃OCO intermediate,while the CH₃OCO is formed from single-adsorbed CH₃O and gaseous CO molecule via Eley-Rideal（E-R）mechanism.In the reaction process catalyzed by Cu₂O,inletting the feed gases simultaneously is beneficial to the formation of co-adsorbed CH₃O and CO,so as to improve the catalytic activity.