Effect Of Precursor Precipitation Environment On Performance Of CuZnAl Catalysts For DME Synthesis

With the development of the national economy,the problems of energy reserves reducing and the environmental pollution caused by traditional fuels are becoming more and more serious.It is crucial to develop alternative energy sources.As source of“green energy”,dimethyl ether?DME?has a similar physicochemical property with liquefied petroleum gas.DME is suitable to be alternative fuels because of its high cetane value,low pollution,and non-toxicity.One-step synthesis of DME from syngas consists of two processes:methanol synthesis and methanol dehydration.Therefore,the composite bifunctional catalysts should be used for DME synthesis.In the early research in our group,the preparation environment of DME was improved,the guiding principle of the complete-liquid-phase method was proposed and the problem that the catalyst was easily deactivated in the slurry bed reactor was solved.However,the activity of catalyst in the reaction process needs to be further improved.In this paper,the activity improvement of catalysts in the precipitation environment of methanol was studied,adopting a unique combination method of coprecipitation and sol-gel.First,the effects of residual Na⁺on the structural properties of the catalyst were investigated by washing different times of Na₂CO₃.Second,the precipitants of Na₂CO₃,?NH₄?₂CO₃ and NH₄HCO₃ were applied to study the relationship between the microstructure and the performance of the catalysts.At last,?NH₄?₂CO₃ was selected as precipitation to further study the effects of different aging modes and precipitation pH on the structures and properties relationship of the catalysts.The catalysts were characterized by XRD,XRF,SEM,N₂-physisorption,H₂-TPR,ICP-AES,NH₃-TPD-MS and XPS.Combined with the structure and the activity of the catalysts,a comprehensive study on the precipitation environment of the catalysts synthesis was carried out,and the conclusions are as follows:1.Reducing the concentration of residual Na⁺can improve the performance of the catalyst effectively.The results show that increasing the number of washing times can effectively reduce the relative content of Na⁺in the surface and bulk of the catalyst,forming more smaller Cu grain size with more reducible species and surface acidity.Among them,the catalyst washed 4 times showed the best performance,the CO conversion rate was 71.41%and the DME selectivity was 85.36%in the activity evaluation period of 120 h.2.Catalysts prepared with different precipitants have a significant effect on the activity of their catalytic reactions.The precipitant of ammonium salt can form a more favorable microstructure.However,affected by the complexation,the performance of the catalyst is poor.3.Different anaerobic ammonia aging processes can greatly improve the performance of the catalyst by weakening the complexation of the active components with NH₃·H₂O molecules.The activity of the catalysts after atmospheric pressure steaming treatment is better than that of negative pressure evaporation ammonia treatment.However,from the aspect of process conditions,the methods of negative pressure evaporation ammonia can save a lot of aging time,and further weaken the complexation.4.Different pH value of the precipitate has a great influence on the structure of the catalyst.The near-neutral pH condition can enhance the complexation of the active components,but since the precipitation itself is relatively complete,the precipitation is completely detected by the interaction of the two factors.The performance improvement is more obvious,so changing the pH condition to weaken the active compon...