Investigation On The Optimization Of Performance And Component Roles Of Cu-Zn-Al Catalyst In CO Hydrogenation For Ethanol Synthesis

Ethanol is an important chemical raw material and a recognized pollution-free high-octane vehicle fuel or additive,and it has huge market prospects.Given the characteristics of China’s energy structure,direct synthesis of ethanol from coal-derived synthesis gas is of great significance for promoting the clean and diversified development of China’s energy and ensuring the secure supply of liquid fuels.However,the key and difficult point that hinders the industrialization of this technology lies in the development of catalysts.Our research group unexpectedly discovered that the CuZn Al catalyst,synthesized using our proprietary completely liquid-phase method,exhibits a high capacity for ethanol production without alkali metals or F-T elements.This finding overturns the traditional understanding of CuZn Al catalysts held by scholars in the field,and has significant scientific and practical implications.However,due to the complexity of the CuZn Al catalyst system and the reaction process,the mechanism of carbon chain growth and the role of each component in the ethanol synthesis process remain unclear,leading to the unresolved trade-off problem between ethanol selectivity and total alcohol selectivity.In this study,we simplified the complex catalytic system,and combined catalyst structural characterization with catalytic performance evaluation,so as to systematically investigate and identify the role of each component,and the following main conclusions were drawn:（1）Simplified catalyst compositions of binary CuZn,Zn Al,and Cu Al catalysts were prepared and compared with the performance of the ternary CuZn Al catalyst.The binary Cu Al catalyst primarily produced methanol,while the alcohol product in the Zn-containing catalyst was mainly ethanol.The Zn component played a more important role in the CO hydrogenation process for ethanol synthesis.（2）Catalysts with different zinc-aluminum ratios（0.5,1.25,2.5,3.5）were prepared to explore the roles of each component and their influence on CO hydrogenation performance.The results showed that aluminum species primarily facilitated the hydrogenation and dissociation of methoxy groups or methanol to form methyl species.The oxygen vacancies promoted non-dissociative adsorption of CO,leading to the formation of formate and subsequent alcohol production.Among them,the catalyst with a zinc-aluminum ratio of 1.25exhibited the best performance,with the highest product selectivity and a relatively high CO conversion rate（5.0%）and ethanol proportion in the total alcohol（60.0%）.The addition of different molecular sieves as modifiers to the optimal zinc-aluminum ratio catalyst did not significantly improve the product selectivity of the Zn Al catalyst.（3）Different additives were introduced to modify the surface hydroxyl and oxygen vacancy content of the CuZn catalyst（with the optimal copper-zinc ratio）in order to improve the overall alcohol selectivity and optimize catalyst performance.The addition of alkali metal additives（K and Cs）to the CuZn catalyst promoted the formation of surface hydroxyl groups and facilitatedβ-nucleophilic attack reactions,thereby increasing the overall alcohol selectivity and the proportion of branched alcohols in the total alcohols.Compared to Cs,K additives were more effective in reducing alkane formation,and the catalyst exhibited the best performance when the K additive amount was 1.64%.The addition of different amounts of Ce additives was used to regulate the surface oxygen vacancy content of the catalyst.The introduction of Ce significantly increased the surface content of Cu and defect species Ce O_2-x,enhanced non-dissociative adsorption of CO,and improved the selectivity of overall alcohols and methanol.However,as the Ce content increased,the content of defect species Zn O_2-xdecreased,which was unfavorable for achieving carbon chain growth.（4）Different zinc sources were used to optimize CuZn Al catalyst and further confirm the role of zinc component in ethanol synthesis.The catalyst prepared with zinc oxide as zinc source had the highest CO conversion rate（6.9%）and the lowest selectivity towards byproducts（11.9%）,with total alcohols（47.3%）and dimethyl ether selectivity（35.2%）dominating,but ethanol selectivity in total alcohols was only 35.0%,indicating that the chain growth ability of the catalyst needs further improvement.Three different morphologies of Zn O（rod-shaped,granular,and flower-shaped）were then prepared using a hydrothermal method as zinc sources.Among them,the flower-shaped Zn O had more polar surfaces（002）,which can produce more donor-deficient structures(Ov and Zn^（2-δ）+),promoting CO non-dissociative adsorption to form formate species and carbon chain growth.There is a strong interaction between flower-shaped Zn O and Cu,and there is a suitable ratio of Cu⁰/Cu⁺（0.57）in the catalyst,all of which promoting the generation of ethanol.Ultimately,the catalyst prepared with flower-shaped Zn O as the zinc source had a high CO conversion rate（4.5%）and total alcohol selectivity（45.0%）,as well as the highest proportion of ethanol in total alcohols（50.0%）.This approach partially resolved the trade-off between ethanol selectivity and total alcohol selectivity.（5）The ethanol synthesis mechanism of the catalyst containing zinc was proposed and investigated using in-situ FT-IR characterization,along with the roles of each component.For the CuZn catalyst,Cu and oxygen vacancies served as non-dissociative adsorption sites for CO,inserting the adsorbed CO into the surface hydroxyls of the catalyst to form formate species.Then,with the assistance of hydrogen,formate species directly dissociated to form methoxy groups.Finally,with the assistance of Zn^（2-δ）+and Cu⁺,CO was inserted into the methoxy groups to achieve chain growth.For the Zn Al catalyst,oxygen vacancies acted as non-dissociative adsorption sites for CO,further forming formate species.Subsequently,with the assistance of Zn/Al composite oxides,formate species directly dissociated to generate methoxy groups,which further underwent hydrogenation and dissociation to form methyl species.Finally,on the Zn^（2-δ）+sites,methyl species interacted with formate species to form acetyl ester groups,which were further hydrogenated to ethanol.For the CuZn Al catalyst,there are multiple mechanisms involved in ethanol production.