Multi-scale Design And Structure Effect Of Cu-based Catalysts For Dimethyl Oxalate Hydrogenation

Ethylene glycol(EG),as an important monomer for synthesizing polyesters,becomes a promising product in the global market.Based on the resource structure and requirement in China,the process of syngas to EG via dimethyl oxalate(DMO)hydrogenation is indispensable.Meanwhile,methyl glycolate(MG),as the preliminary product in DMO hydrogenation,is an important chemical for producing degradable glycolic acid.Efficiently producing MG could increase the technical economy and risk-resistance capacity of this process.Developing high-performance and stable Cu-based catalysts for selectively producing EG or MG is the key point in this process.However,a high H₂/DMO ratio(80?120)is usually essential to achieve high activity,and the yield is difficult to be optimized in producing MG.To solve these problems,we designed the Cu-based catalysts and researched the structure-performance relationship in multi-scale,then finally illuminated the adsorption-reaction-diffusion relationship of Cu-based catalysts in DMO hydrogenation.To decrease the high H₂/DMO ratio during reaction,we here propose a nanotube-assembled hollow sphere(NAHS).The H₂/DMO decreases from 80 to 20 but still maintain the 100%DMO conversion and 95%EG selectivity,further reduces the energy consumption and devices cost for recycling hydrogen in industry.The nanotube and lamellar structure catalysts with same quantity of surface Cu and Cu⁰/Cu⁺ratio were also fabricated as comparation.Combining adsorption experiment and kinetic simulation,we demonstrate that the enrichment of hydrogen inside nanotube and hollow sphere is the key point to enhance the catalytic performance,resulting in the decreased the H₂/DMO.Based on the NAHS formation process,we fabricated a series of NAHSs with different hollow-sphere size and nanotube length,while the quantity of surface Cu and Cu⁰/Cu⁺ratio were kept same.Because of the spatial restriction effect of reactants,increasing the nanotube length could prolong the diffusion path and further increase the yield of EG.Meanwhile,combining DFT simulation and adsorption experiment,we elaborate the effect of concave surface on adsorption.With the increased hollow sphere size,the hydrogen concentration inside hollow sphere is decreased but the diffusion flux is increased,resulting in a balancing effect between adsorption and diffusion inside hollow chamber.The mid-sized hollow chamber could perform the highest activity.This balancing effect provide a unique strategy to design efficient and selective hollow nanostructured catalysts.To investigate the reconstruction of Cu species,we restrained the Cu nanoparticle size to enhance the mobility of surface Cu atoms.The surface Cu atoms are easy to be emitted in the mild condition and anchored on the CeO₂ surface.Supported Cu catalysts with Cu single atoms or Cu clusters with different size could be obtained by this atomic diffusion process.Combining the characterization of Cu species structure and DFT study,with the addition of the catalytic performance during atomic diffusion,we further illustrate the orderly diffusion process of Cu atoms to single atoms or clusters.This atomic diffusion process,which could obtain catalysts with different Cu species dimension in large-scale,demonstrates a huge potential in industry.Based on the aforementioned atomic diffusion process,we fabricated the supported Cu₃ cluster catalysts.This Cu₃ catalyst achieves 100%DMO conversion and95%MG yield with a long stability.Meanwhile,by atomically tailoring the dimension of Cu species,we obtained the catalysts with the Cu species dimension from single atom Cu,Cu₃ cluster,Cu₃₀ cluster and Cu nanoparticles.Combining the experimental methods and DFT study,the size-effect of Cu species is further demonstrated:with the increased Cu species dimension,both the H₂ activation and MG adsorption increase,both appearing as balancing effects for producing MG but synergy effects for producing EG.The design strategy of supported Cu catalysts for selectively producing EG and MG in DMO hydrogenation is well established.