Study On The Ethanol Synthesis From Syngas Over CuZnAl Catalysts

The pressure of energy and the environment has caused a worldwide attention to obtain liquid fuels as well as bulk chemicals from the non-oil route.,ethanol can not only be used as a fuel additive,it is also a kind of excellent substitute for fuel and oil.The direct synthesis of ethanol from syngas is considered as one of the most valuable research subjects which have great industrial application value and industrial prospect.At present,several catalytic systems involved in the direct synthesis of ethanol from syngas have many shortcomings,such as lower activity and ethanol selectivity due to multi-product,the poor stability and carbon deposition etc,which have a large distance from industrial applications.It is well known that CuZnAl catalysts are usually used to synthesize methanol from syngas and successfully applied to commercialization.Nonetheless,in our previous study,it was unexpected found that the selectivity of ethanol could reach an unexpected point?reaching 40%?over CuZnAl catalyst prepared by complete liquid phase method when it was first used for methanol synthesis from syngas,and no deactivation was found during the 200 h reaction,there were only methanol and ethanol in the liquid product,which has never been reported previous by others.Up to now,our group still not completely know the essential reason for the high selectivity of ethanol,Therefore,it is difficult to reproduce the former results,the selectivity of ethanol would decreased sharply as time on stream.In this paper,the ethanol synthesis from syngas was investigated over CuZnAl catalyst without alkalis or Fischer-Tropsch elements by complete liquid-phase method in slurry bed,we hope to develop a cheap and efficient catalyst for ethanol stnthesis.Firstly,the differences of the structure and properties of the catalysts prepared by different methods were compared,and then the influence of pH,complexing agent and its content on the ethanol synthesis was also investigated.We also investigated the Cu-based catalyst structure through adding promoters and support and combined the catalyst characterization.The effects of raw materials and component ratio of catalysts on the synthesis of ethanol from syngas were also investigated,the catalytic performance and the microstructure were also clarified to obtain the best catalysts formulation.Moreover,the effect of reaction conditions?such as temperature,GHSV,and H₂/CO ratio?were investigated on the performance of CO hydrogenation,and the mechanism of ethanol synthesis over Cu based catalysts was also discussed,the main conclusions are as follows:1.The syngas could be directly catalyzed to ethanol and higher alcohols over CuZnAl catalysts without alkalis or F-T elements by complete liquid-phase method.The selectivity to ethanol and higher alcohols reached 40%and 65%,respectively,2.It could reproduce well for CuZnAl catalysts catalyze syngas to high selectivity of ethanol and it exhibited well stability,no inactivation was found in240 h reaction.3.The catalyst prepared by the conventional calcination method exhibited high activity,but the liquid products were nearly composed of methanol.It was found that the Cu species presented as Cu²⁺,while Al species presented as Al₂O₃.Meanwhile,its has more amount of reducible Cu species,lower amount of weak acid and relatively higher Cu,Zn and Al content on catalyst surface.The Cu component was relatively dispersed,the particle size of Cu was uneven,and the interaction among the metal was more tightly.The catalyst prepared by the complete liquid phase method had lower activity,but it showed higher selectivity of ethanol and higher alcohols.It was also found that the Cu species presented as Cu⁰ and or Cu⁺,while Al species presented as AlOOH.Meanwhile,its had less amount of reducible Cu species,larger amount of weak acid on catalyst surface,the Cu particles are more uniform,but the particle size is relatively larger.The Cu components of the catalysts prepared by the two methods were exposed to Cu???,Zn was in the form of ZnO.4.The preparation of Al sol environment should be controlled near the neutral.Acidic or alkaline environment would promote the formation of methanol.In order to obtain high selectivity of ethanol and C₂₊OH,the consumption of NH₃on catalyst surface should between 0.19?4.64 mmol/g,5.Al element play a key role in the formation of ethanol and C₂₊OH,Zr,La,P and a small amount of alkaline si sol was beneficial to improve the stability of catalysts,but it would promote the formation of methanol.6.The catalyst showed best when it was prepared using copper nitrate as Cu source,aluminum isopropoxide as aluminum source.The catalysts with lower Cu content showed higher activity,while higher Cu content on catalyst surface was beneficial to improve the stability of catalysts.The content of Al mainly influenced the selectivity of ethanol and C₂₊OH,and the best composition of catalysts was n?Cu?:n?Zn?:n?Al?=2:1:0.8.7.It was also found that the catalysts showed higher activity when it has more amount of reducible Cu species on catalyst surface.The high Cu/Zn,Cu⁺and large particles of Cu species favored the formation of ethanol and C₂₊OH.8.Ethanol and C₂₊OH synthesis over Cu-based catalysts mainly based on the CO insertion mechanism and accompanied by aldol condensation mechanism.Al species favored CO dissociation and chain growth,Cu⁺or Cu^?+favored CO insert CH₃^*to generate surface acyl species,and Cu⁰ mainly acted as hydrogen dissociation,the synergism of Cu⁰,Cu⁺and Al species was beneficial to the formation of ethanol and higher alcohols.