Mechanism Of Cu/Mo Co-catalyzed Synthesis Of Ethanol From Syngas

Using coal,natural gas and biomass as raw materials to synthesize ethanol through syngas is a new method for clean and efficient utilization of carbon-based resources,which is an important research topic in carbonization science.Cu-base catalyst is an important catalyst for the conversion of syngas.However,Cu-base catalyst has weak ability to activate C–O bond,which leads to easy methanol generation of by-product and low concentration of CH_x,thus affecting the C–C bond formation ability.How to solve the scientific problem of C–O bond breaking and C–C bond forming on Cu-based catalyst becomes the key.The Cu/Mo synergistic catalytic syngas synthesis of ethanol were studied systematically by the quantum chemistry density functional theory calculation method.The structure-activity relationship of Cu/Mo catalyst is used to analyze the micro-control of the catalytic performance of Cu/Mo catalyst surface active sites by factors such as the different forms,proportions,crystals and surface structure of Cu/Mo alloys at the electron-molecule level.Which gives a target structure with good performance of the catalyst and provides a definite theoretical clue for the design of high activity catalyst for synthesis of ethanol from syngas.The main conclusions are as follows:1.The reaction mechanism of synthesis of ethanol from syngas on Cu/Mo S₂（100）surface is defined:（1）CH₂ is the main intermediate of CH_x on Cu/Mo S₂（100）surface,which is mainly formed by the formation of CH₂OH→CH₂+OH;（2）the formation of CH₂ and CH₃OH forms a competitive reaction.Compared with Cu（111）and Mo S₂（100）surfaces,Cu/Mo S₂（100）surface promotes the formation of CH_x and inhibits the formation of CH₃OH;（3）regarding the formation of C–C bond,it is easiest for CO to insert into CH₂ to generate CH₂CO,and the Cu/Mo S₂（100）surface has a higher activity and selectivity for the formation of C₂ oxygenate;（4）the analysis of electronic properties shows that the Cu/Mo d-band center is closer to the Fermi level and the catalytic activity is stronger,which proves that the synergistic effect of Cu-Mo-S active site promotes the C–O bond breaking and C–C bond formation.2.The mechanism of ethanol synthesis from syngas on Mo₆S₈/Cu（111）surface was clarified:（1）CO+H→CHO is the main way of CO activation on the Mo₆S₈/Cu（111）surface;（2）regarding the generation of CH_x,CO+H→CHO+H→CH₂O+H→CH₂OH→CH₂+OH is the most advantageous path for CH_x intermediate CH₂ generated;（3）in the process of C–C bond formation,the insertion of CHO into CH₂ is the most favorable reaction of CH₂-related reactions,and the generated CH₂CHO is subsequently through continuous hydrogenation to produce ethanol;（4）the interfacial interaction of Mo₆S₈/Cu（111）not only enhanced the ability of C–O bond breaking,but also promoted the formation of C–C bond,which effectively improved the activity and selectivity of ethanol generation.3.The mechanism of synthesis of ethanol from syngas on the surface of MoCu（211）is elucidated,and it is clear that the modulation of the electronic properties of Cu by Mo on the Cu-based catalyst can effectively solve the two key problems of C–O bond breaking and C–C bond forming:（1）compared with Cu（211）surface,MoCu（211）surface has two improvements.One is,CH₃ is the most advantageous monomer on the MoCu（211）surface,which provides abundant CH₃ intermediate for syngas to ethanol.The other is,the C-C bond formation is mainly by the insertion of CHO into abundant CH₃,then the generated CH₃CHO undergoes multi-step hydrogenations to generate C₂H₅OH;（2）the key of the promoter Mo on the MoCu（211）surface also has been verified by the analysis of its electronic properties,that is,it promotes the formation of the most favorable monomer CH₃ and the precursor of ethanol C₂ oxygenate CH₃CHO,and inhibits the formation of by-product methanol.4.The sulfur tolerance of MoCu catalyst was confirmed:（1）the Cu/Mo S₂（100）,Mo₆S₈/Cu（111）and MoCu（211）surfaces have sulfur tolerance due to the resistance of Mo to sulfur poisoning;（2）regarding the formation of CH_x,the Cu-Mo-S active site formed by dissociated sulfur adsorbed on the MoCu（211）surface promotes the formation of CH_x（x=2,3）and inhibites the formation of CH₃OH,which improves the activity and selectivity of CH_x（x=2,3）formation;（4）sulfur atoms are not always poisons for surface reactions.