Study On The Preparation And Performance Of Cu-based Catalyst For Ethanol Synthesis In Slurry Reactor

Ethanol is not only the fine chemicals and raw chemical materials,but also for clean energy.The consumption of ethanol increase by 10%in our country in recent years.CO hydrogenation conversion into ethanol has become a hot topic in the C1 chemistry.At present,supported rhodium catalysts haves been known for decades to have the ability to produce ethanol,but the limited availability and high cost of Rh,and the insufficient ethanol yield,can make these catalysts unattractive for commercial application.With an aim at the characterization of liquid-phase reaction,the complete liquid-phase method was put forward by our group.It was used to prepare the Cu-base catalyst for production of methanol from CO hydrogenation.During this process,we found that the selectivity of ethanol was high on the Cu-base catalysts,but it had poor reproducibility.Under this background,the preparation and performance of Cu-base catalyst for ethanol synthesis from CO hydrogenation in a slurry reactor were investigated by our group.The Cu-base catalysts for ethanol were prepared by a complete liquid phase method and was evaluated in a slurry reactor.The influences of these factors on catalytic performance were investigated as follows:Cu/Zn/Al(mol)ratio,H2O/Al(mol)ratio,the feeding way,pH of sol,heat treatment atmosphere,heat treatment temperature and thermal decomposition way during the catalyst preparation.On those basis,the effects on the catalytic performance of ethanol synthesis in complexing agent,the adding of alkali metal and transition metal.The catalyst reducibility,bulk phase,crystal size,surface acid and basic,and surface state were characterized by H2-TPR,XRD,NH3-TPD,CO2-TPD,XPS and BET.By comparing their properties,the mechanism of ethanol synthesis was proposed through analysis and speculation.The experiment results give rise to the following conclusions:1.The slurry Cu-base catalysts prepared by the complete liquid-phase method show better selectivity towards ethanol.The highest selectivity is up to 24.5%(C-atom%),accounting for 49.3%in the mixed alcohol.2.Through the analysis of characterizations of the catalysts which exhibits high selectivity of ethanol and the comparison and summarization of the common characters of those catalysts,the features of the rapid screening catalyst for high ethanol selectivity are obtained.Specific conditions are as follows:the H2-TPR have two reduction peaks;The high temperature reduction peak which shows copper oxides anti-reduction under the reducing atmosphere and higher temperature,is the existence when the reduction temperature is over 320 ?;the high alkali/acid ratio which is more than 3.5;active sites that are close to each other and evenly distributed,at least,the crystal size of Cu0 is less than 23 nm;the size of Cu20 crystal particles is less than 13 nm,the specific surface area of catalyst is larger than 80 m/g.3.The coexistence of CO insertion mechanism and the aldol condensation mechanism as the mechanism of synthesis of ethanol from CO hydrogenation is proposed.The CO insertion mechanism:the synergetic effect between Cu0 and Cu+ leads to ethanol formation,Cu adsorbs and activates CO molecules;Cu+acts as the stabilizer of the methoxy and acyl species,which are intermediates in the ethanol hydrogenation;Cu+ can stabilize the intermediate carbon species CHx and facilitate the CHxCO conversion into ethanol.Cu0 and Cu+ are close to each other,which ensures the production rate of CHx,CHxCO and the CHxCO hydrogenation are in a dynamic balance,and if the balance is broken,the selectivity of ethanol will reduce rapidly.The Aldol mechanism:the methanol and formaldehyde condensation reaction occurs in alkaline condition,which results in the carbon chain growth;therefore,high basicity can promote the reaction and suppress the generation of hydrocarbon and dimethyl ether.4.The above two mechanisms coexist in the Cu-base catalyst for the ethanol synthesis from the CO hydrogenation.The aldol condensation mechanism is prior to the CO insertion mechanism when the alkaline/acid ratio is larger than 3.5 and the product distribution(including the C1 product)follows the Anderson-Schulz-Flory equation.Otherwise,the CO insertion mechanism will take the priority and the product(except C1)distribution follows the Anderson-Schulz-Flory equation.However,C1 products deviates from the straight line.The mechanism of ethanol formation is the same as the latter on the additive modified CuZnAl catalyst.Hydrocarbon and alcohol come from the same intermediate species,because the carbon chain growth factor of both hydrocarbon and alcohol is the same.The increase of hydrocarbon is at the expense of the alcohols and always alter or replace each other.5.The key control points which are preparation of sol and heat treatment were determined in the catalyst prepared by the completely liquid phase method.The optimum condition:Cu/Zn = 2/1(atomic ratio),sol pH = 4.0,CuZn solution adding the alumina sol,heat treatment:220 ?,H20/A1 = 50/1(mol).6.CuZnAl catalysts prepared by a complexing process are achieved with a high dispersion,a high alkaline acid ratio,and anti-reducibility copper oxides,because different coordination atoms and central atom(active metal atoms)form coordination compounds with different bond lengths and bond angles.The high ethanol selectivity has attained on the C-2NMP1TEA catalyst.7.The ethanol selectivity reaches 11.7%on the Cs modified CuZnAl catalyst and 12.9%on the Fe modified CuZnAl.This result verifies that the slurry bed is superior to the fixed bed in the synthesis of ethanol from the synthesis gas.8.Comparison of the catalytic performance and the characterization of catalysts gives a conclusion that dimethyl ether comes from the weak acid,hydrocarbon derives from medium and strong acid;Hydrocarbon and ethanol derive from the same intermediate;the particle size of Cu have effect on the product distribution,and the selectivity of hydrocarbon increases with increasing the size of Cu particles.To improve the ethanol selectivity,the Cu grain size must be controlled,which is consistent with the second conclusion.9.The complete liquid phase method can be applied to prepare the CuZnAl catalyst for synthesizing ethanol from the CO hydrogenation.The catalysts have a good stability under the experimental conditions,running for 120 hours without apparent deactivation.