Effect Of The Crystallite Size Of Cu Species On The Structure And Performance Of CuZnAl Catalysts

In the 21st century,with the rapid economic development,energy and environment have become increasingly concerned issues.Therefore,the world has begun to seek new alternative energy sources.Among them,the non-petroleum route to obtain liquid fuels and bulk chemicals has attracted widespread attention from the world.As an excellent liquid fuel,gasoline additive and basic chemical raw material,higher alcohols have broad application prospects.The clean conversion and high-efficiency utilization of coal have led to the comprehensive utilization of downstream products of coal.The synthesis of higher alcohols（HAS）from syngas（CO+H₂）has a simple synthesis path,a wide range of raw materials and lower cost,which is considered to be of great industrial application value.In this thesis,based on the previous research of our group,the crystallite size of Cu species in CuZnAl catalyst was adjusted by different methods,and its influence on the catalyst structure and the catalytic performance of CO hydrogenation was explored.Firstly,the co-precipitation method adopting different precipitants and solvents was used to control the Cu species crystallite size of CuZnAl catalyst.Secondly,Cu and Zn Al nitrate was precipitated and hydrothermally combined separately,and the crystallite size of Cu species was adjusted by different alcohol solvents in the alcohol-ammonia system.Finally,different hydrothermal temperatures and PEG amounts were selected to separately prepare CuO with a large crystallite distinction,and then combined with Zn Al to prepare the catalyst.The catalysts were characterized by XRD,H₂-TPR,BET,NH₃/CO₂-TPD-MS,XPS and TEM characterization.The effect of Cu species crystallite size on the other structure and catalytic performance was revealed.Furthermore,linear regression analysis on the characterization and activity data of all catalysts was performed,and the main conclusions were drawn as follows:（1）When the catalyst was prepared by the one-step co-precipitation method,different precipitating agents and solvents showed little effect on the CuO crystallite size.The catalyst using sodium carbonate as precipitant formed Zn O crystallites during the reaction process,which increased the dispersion of Cu species,showing the optimal C₂₊OH ratio in the total alcohol（18.32%）.The CuO crystallite size prepared with ethanol as solvent was the smallest.So-prepared CuZnAl catalyst contained more easily reducible Cu species and the smaller variation of specific surface area during the reaction,exhibiting the best CO conversion rate（22.91%）and the proportion of C₂₊OH in the total alcohols（18.96%）.（2）In the alcohol-ammonia system,the CuO crystallite size was smaller in the catalyst prepared by the coordination precipitation method to replace deionized water with alcohol solvent,but the simple mechanical combination of Cu and Zn Al precursors led to weaker interaction,resulting in Cu crystallite size increased after reaction.Adopting ethanol as solvent,the catalyst had the smallest CuO crystallite size,best dispersibility,and weak bases and medium strong bases with suitable proportion,showing the best catalytic performance.（3）When CuO was prepared separately by hydrothermal method,CuO crystallite grew with the raising of hydrothermal temperature,which affected interaction of copper and zinc aluminum,the dispersion of Cu,and the electron transfer between copper and zinc oxide.The CO conversion decreased with the increased of CuO crystallite size.When the hydrothermal temperature was 130℃,the catalyst exhibited the stable crystallite size of Cu species,good copper dispersibility,high Cu/Zn molar ratio,and strong interaction between Cu and zinc oxide on the catalyst surfaces,so the selectivity of total alcohol could reach up to 71.39%.（4）Linear regression of the crystallite size and the variation of the copper species with the activity data showed that the smaller CuO crystallite size and the variation of the copper species during the reaction,the better the CO conversion rate and C₂₊OH selectivity are.The smaller Cu crystallite size,the more beneficial to increase the proportion of C₂₊OH in the total alcohol.（5）The structure characterization data of all catalysts and their corresponding C₂₊OH and C_nH_m selectivity were analyzed by multi-factor linear regression analysis.The results showed that CuO crystallite size before reaction,the Cu crystallite size after reaction,and the amount of surface acid had a highly significant effect on C₂₊OH selectivity,and they were all negatively correlated.The amount of medium and strong base had a significant influence on the C₂₊OH selectivity and it was positively correlated,while the selectivity of C_nH_m was affected by CuO crystallite size before reaction and pore size after reaction,and both were positively correlated.