Studies In The Preparation Of Novel Embedded Cu-based Bifunctional Catalyst And The Catalytic Dimethyl Ether Production From Syngas

Dimethyl ether?DME?has been considered as industrially important intermediates and clean alternative fuel.To develop a highly efficient syngas-to-DME bifunctional catalyst is of great importance but remains a challenge.Therefore,it has received extensive attention in recent years.The generally used bifunctional catalyst for the one-step synthesis of DME from syngas is composed of two types of active sites:one is methanol synthesis transition metallic catalyst,and the other is methanol dehydration solid acid catalyst.However,previous studies have paid more attention to the simple mechanical mixing and loading of the two catalysts.The far distance between active sites of syngas-to-methanol and solid acidic sites cannot be avoided in one-pot transformation process over this kind of mixed hybrid catalyst,resulting in a low selectivity of dimethyl ether in the production process.In recent years,the core-shell structured bifunctional catalysts with closed structures have been reported,due to the characteristics of this implanted closed structure,their selectivity to DME significantly exceeded the physically mixed catalysts and supported catalysts.However,coating the H-ZSM-5 shell on the CuZnAl core will cause severe damage to Cu,thereby reducing the activity of the bifunctional catalyst.Therefore,it is of great academic value and practical significance to find a simple and effective method to prepare the implanted bifunctional catalyst of this copper-based methanol synthesis unit implanted into a solid acid methanol dehydration unit,and can reduce or avoid the damage of copper active components.Therefore,this work used different preparation methods to synthesize two new implantable copper-based bifunctional catalysts,and the catalytic performance and structure-activity relationship of the direct synthesis of dimethyl ether from syngas were studied,and some valuable laws were found,which provided a valuable reference for subsequent research.The specific research contents are as follows:?1?CZA-oa@H-ZSM-5 core-shell structure bifunctional catalyst was prepared by coating H-ZSM-5 shell on the millimeter-sized copper-zinc-aluminum oxalate?CZA-oa?.From characterization and reaction results,the direct use of CZA-oa to replace CZA efficiently inhibits Cu leaching in the coating process,besides no need to use rotary oven for well coating H-ZSM-5 shell on the core owing to high hydrophilic property.The prepared new core-shell bifunctional catalyst exhibits excellent catalytic performance for one-step synthesis of dimethyl ether from syngas.Compared with the bifunctional catalyst CZA-oxi@H-ZSM-5R prepared with copper,zinc and aluminum oxide as the core,the CO conversion rate was increased from 5.9%to 13.2%,and DME selectivity of 93.7%was obtained.Due to the proper copper-zinc interaction of the catalyst CZA-oa@H-ZSM-5,conversion frequency?TOF?of 1.5 min-1 was obtained,which is 5.0 times that of the CZA-oxi@H-ZSM-5 R catalyst.The use of CZA-oa@H-ZSM-5 catalyst can obtain a high DME formation rate of0.122 mmol m_Cu^-2 h^-1,while using the CZA-oxi@H-ZSM-5 R catalyst,DME formation rate can only reach 0.031 mmol m _Cu^-2 h^-1.?2?Using steam induction method,the copper-zinc oxalate particles prepared in advance was added to the precursor solution of mesoporous alumina to successfully prepare a new type of confinement bifunctional catalyst with copper-zinc nanoparticles implanted into mesoporous alumina?CuZn@m-Al₂O₃?.Using this method,copper-zinc nanoparticles are implanted into the matrix of mesoporous alumina,creating closed confined space.The mesoporous alumina was prepared by steam-induced evaporation,and its precursor solution was near-neutral,so as to avoid the damage of the copper-zinc methanol synthesis catalyst by the reaction system during the preparation of alumina.Syngas was hydrogenated on copper-zinc nanoparticles to produce methanol.Before leaving the catalyst,methanol must pass through mesoporous alumina with suitable acid properties to achieve methanol dehydration and reduce the methanation reaction caused by strong acid centers.The results show that the prepared CuZn@m-Al2O3 catalyst exhibits excellent catalytic performance for direct synthesis of dimethyl ether from syngas.A 15.5%CO conversion rate and 89.0%DME selectivity were obtained,and the DME formation rate reached 0.160 mmol m_Cu^-2 h^-1.