Theoretical Study On Syngas Conversion To Ethanol Over The CuZnAl Catalyst

As one of typical atomic economic reaction,CH₃OH synthesis via CO hydrogenation does not involve the C-O bond breakage,and only refer to C?O saturation hydrogenation.However,FT reaction needs to cleave C-O bond for CH_x formation,then finishes carbon chain growth by CH_xcoupling,the generated active O species in the process is stabilized by CO or H₂ to CO₂ or H₂O.The ratio of O/C atoms in ethanol is located between CH₃OH and hydrocarbons,and the CO activation and breakage modes also have the characteristics of both CH₃OH syntesis and FT reaction.In addition to the similar hydrogenation sites in syngas to ethanol conversion,the CO non-dissociation and dissociation sites as well as the promoting CH_xO(x=0?2)insertion sites are also necessary.The completely liquid phase CuZnAl catalyst developed independently by our group has performed good catalytic performance for ethanol synthesis with about 40%ethanol selectivity^[94],but the development of this technology is limited by the low catalyst repeatability and ambiguous reaction mechanism.At present,the research on the reaction mechanism of ethanol synthesis over complete liquid phase CuZnAl catalyst focuses on the crystal orientation and existence valence state of the active Cu component,the existence form and role of Zn promotor.It was found that^[127]the co-existence of Cu⁰ and Cu⁺active sites which interact strongly with ZnO is vital for maintainning high stability and selectivity,however,to the best of our knowledge,little work has been done on the influence of Al composition,different solvents and synergistic effect of different componenets on ethanol synthesis.By joint DFT calulation and micro-dynamic analysis,we fully study the ethanol reaction mechanism on the catalyst models which are based on the Al elment.Our results clarify the stable configuration and catalytic performance of Al components,elucidate the intrinsic role of solvents in the process and the influence on the reaction path of different solvent model,reveal structure activity relationship between catalytic performance and the electronic and surface structures of ternary catalyst active sites,establish the relationship of key species charge transfer and energy barrier of specific elemental reaction,thus the work provides new direaction for rational tailoring and designing of new catalysts.The main conclusions are as follows:(1)By constructing the?-Al₂O₃(110D)surfaces with different hydroxyl coverage,the influence of the structural evolution from?-Al₂O₃to?-Al OOH on the reaction mechanism of ethanol synthesis has been systematically studied.Although the optimal reaction routes are similar on the different hydroxyl coverage surfaces,the rate-determining steps are different.The competitive products DME,CH₄ and C₂H₆ are unfavorable kineticly.The existence of hydrogen bond on the high hydroxyl coverage?-Al₂O₃(110D)surface promotes the activation of CH₃OH,via breaking the C-O bond.The rate constant analyses show that with the increment of hydroxyl coverage,the rate constant of CH₃ formation increases,while the CH₃CO formation rate constant increases firstly and then decreases.The Microkinetic modeling discover that the ethanol productivity and selectivity could be improved by doping transition metal to enhance CO adsorption energy or reduce H₂/CO ratio.(2)Based on the?_OH?8.9 OH·nm^-2?-Al₂O₃(110D)surface model in the liquid paraffin,the reaction mechanism of syngas-to-ethanol conversion has been studied.We found that the presence of liquid paraffin solvent not only made CO become weak chemisorption from physical adsorption,but also affected the rate-determining step of ethanol synthesis,the corresponding rate-determining steps on the surface with and without solvent effect are CH₃ and C-C-O framework formation,respectively.Moreover,Al component by combining solvent effect shows stronger ability of CH₃ carbonylation.(3)The interface model of?_OH?8.9 OH·nm^-2?-Al₂O₃(110D)surface and PEG solvent was constructed for the first time,which fully considered the interaction between the solvent molecules and the Al component.Here the co-catalytic conversion of syngas and methanol into ethanol was studied and discovered the interface is unadvantageous for CH₃OH activation and CO adsorption,and it has no obvious effect on the energy barrier of CH₃ carbonylation.Due to the existence of hydrogen bonds among the PEG solvent,on the surface and the interface,the optimal reaction route of ethanol synthesis is different from that on the?-Al₂O₃(110D)surface,and the rate determining step of the whole process is CH₃OH direct dissociation with an energy barrier of 1.29 e V.As the main kinetic competitor of CH₃CO,C₂H₆ is formed by CH₃ coupling.Charge analysis showed that the ability of CO insertion into CH₃ for achieving carbon chain growth has not been affected by the PEG solvent effect,it is because the surface hydrogen bond triggers the CO polarization with the net charge change from+0.01 to-0.41|e|.It is more favorable to form C-C-O key structure by CO nucleophilic attack on CH₃.(4)Based on the stable Cu/ZnO/Al₂O₃ ternary catalyst model,the intrinsic relationship between the three components synergetic effect and the syngas conversion possible reaction routes was studied systematically.It was found that CHO species is one of common intermediate for the possible products CH₃OH,C₂H₅OH and HOH₂CCH₂OH.The synergetic interaction of three components makes the d-band center rising of the catalyst surface,which is conducive to CO adsorption and activation.The Cu-ZnO interface sites are helpful for CO hydrogenation to generate CHO species,and the three components synergy promote CH₃ formation via CH₃OH direct dissociation.CH₃ and CO are adsorbed at O site of surface Al component and fcc site of Cu component,respectively,which is profitable to the C-C-O key framework formation.The cooperation of Cu-ZnO sites facilitate the CHO coupling to form O-C-C-O structure to some extent that provides a possibility for HOH₂CCH₂OH synthesis.According to the optimal reaction pathway,We summarized that CH₃ formation is the rate control step for C₂H₅OH formation,and the generation of CHO key species is the rate determining step for the competing products CH₃OH and HOH₂CCH₂OH.The microkinetic modeling analysis shows that the reaction temperature increasing is beneficial to improve the relative selectivity of C₂H₅OH.Furthermore,the C₂H₅OH selectivity could be improved by adjusting the CHO coupling or hydrogenation barrier through rationally regulating the electronic structure of CuZnAl catalyst.