Studies On The Preparation Of Methanol And Dimethyl Ether By CO₂ Catalytic Hydrogenation

Due to the excessive emission of carbon dioxide,the carbon cycle is out of balance,so carbon neutrality has now become one of the most important goals of environmental governance.The conversion of carbon dioxide into high value-added chemicals can not only reduce carbon dioxide in the atmosphere,but also provide chemicals that meet the needs of industrial development.Methanol is an extremely important chemical raw material in chemical production.Methanol can be used to further prepare olefins,aromatics and other chemicals,or it can be used directly as a fuel.Dimethyl ether is known as the "clean fuel of the 21st century",has excellent combustion performance,and is easy to liquefy and store.Catalytic hydrogenation of carbon dioxide to methanol and dimethyl ether is a very promising carbon neutral strategy.CO2+3H2(?)CH3OH+H2O ? H298K=-49.5 kJ·mol-1(1)2CH3OH(?)CH3OCH3+H2O ? H298K=-23.4 KJ·mol-1(2)2CO2+6 H2(?)CH3OCH3(DME)+3H2O ? H298K=-122.4 KJ mol-1(3)Cu based catalyst is widely used as a common catalyst for the preparation of methanol and dimethyl ether by catalytic hydrogenation of carbon dioxide.The modification and composition optimization research are carried out on the basis of Cu based catalyst,and its catalytic performance can be further optimized on the basis of maintaining the relatively low price of the catalyst.The hydrogenation of carbon dioxide to methanol and dimethyl ether is a dynamic chemical equilibrium process,thus the yield of methanol and dimethyl ether is greatly affected,and the by-product water will significantly reduce the activity of the catalyst.Therefore,the focus of this paper is to couple the reaction process of high-efficiency catalytic conversion of CO2 and the separation process of products in the same membrane reactor,and selectively separate the by-product steam from the reaction system.Firstly,starting with the common Cu-ZnO catalyst in industry,a series of performance optimization experiments were carried out.At the same time,LTA,FAU and ZSM-5 molecular sieve were used as the selective dehydration and separation membrane to carry out the design and application of membrane catalytic reactor for carbon dioxide catalytic hydrogenation to methanol and dimethyl ether.The main research contents are as follows:(1)The performance of Cu-ZnO catalyst,which is widely used in industry,was optimized in detail.Firstly,the performance of Cu-ZnO and Cu-ZnO-Al2O3 catalysts were compared and studied.It was found that the addition of Al2O3 was conducive to improve the performance of carbon dioxide catalytic hydrogenation to methanol.The effect of Cu/Zn ratio on the performance of the catalyst was further explored.It was found that the performance of the catalyst was the best when the Cu/Zn ratio was 1.In addition,the addition of ZrO2 can effectively improve the catalytic performance of Cu-ZnO-Al2O3,while the strong hydrophilic LTA molecular sieve with low silicon aluminum ratio has better catalytic performance.(2)Membrane catalytic reaction is a new catalytic reaction technology developed in recent years with broad application prospects.It can complete the two processes of catalytic reaction and product separation at the same time,so it can overcome many shortcomings of traditional fixed bed reaction.Firstly,a dense LTA molecular sieve membrane was synthesized in the porous Al2O3 tube,and then the Cu-ZnO-Al2O3 precursor was supported on the surface of the LTA molecular sieve membrane by the pull-up impregnation method.After sintering at a high temperature,Cu-ZnO-Al2O3-ZrO2@LTA membrane reactor was prepared and used for catalytic hydrogenation of carbon dioxide to methanol.The results show that the hydrophilic LTA molecular sieve membrane can remove the reaction by-product water,thus breaking through the thermodynamic equilibrium limitation of the reaction,and realing the efficient catalytic hydrogenation of CO2 to methanol.Under the reaction conditions of 260? and 3.0 MPa,the CO2 conversion increased from 21.9%in the fixed bed reaction to 36.1%in the membrane catalytic reaction,and the methanol selectivity increased from 67.3%in the fixed bed reaction to 100%in the membrane catalytic reaction.(3)As a very promising alternative to clean diesel,dimethyl ether has the advantages of the high cetane number and low emission pollutants.In addition,dimethyl ether is also an important raw material for the production of many high-value chemicals.Therefore,the preparation of dimethyl ether has attracted more and more attention.The one-step synthesis of dimethyl ether from carbon dioxide by using bifunctional catalyst is conducive to the simultaneous realization of methanol synthesis and methanol dehydration,which is beneficial in economy and thermodynamics.Comparing with the Cu-ZnO catalyst prepared by traditional precipitation method,the Cu-ZnO catalyst prepared by pyrolysis of metal organic framework Cu/Zn-MOF has a better morphology and crystallinity.At the same time,the addition of promoter Pt can further improve the catalysis of the catalyst.HZSM-5 molecular sieve is more conducive to the catalytic dehydration of methanol to dimethyl ether than LTA and FAU zeolite.(4)The catalytic hydrogenation of carbon dioxide to dimethyl ether is also a reversible equilibrium reaction.Due to the limitation of thermodynamic equilibrium and catalyst deactivation caused by by-product water,the yield of dimethyl ether is usually relatively low.Based on the reaction separation coupling strategy,Cu-ZnO-Pt@HZSM-5 catalytic membrane reactor was prepared and used for the preparation of dimethyl ether by catalytic hydrogenation of carbon dioxide,realizing the two processes of CO2 hydrogenation to dimethyl ether and water removal at the same time.First,hydrophilic HZSM-5(Si/Al ratio about 13.5)membrane with catalytic and separation functions was synthesized on the porous ?-Al2O3 tube for methanol catalytic dehydration and the separation of by-product water.Then,the Cu-ZnO-Pt catalytic layer was coated on the HZSM-5 film by dip coating,and then calcined and reduced to obtain Cu-ZnO-Pt@HZSM-5 catalytic membrane reactor.The results show that Cu-ZnO-Pt@HZSM-5 membrane catalytic reaction is conducive to breaking the thermodynamic equilibrium limitation of the reaction and realizing the efficient catalytic hydrogenation of CO2 to dimethyl ether.Under the reaction conditions of 280? and 3.25 MPa,the CO2 conversion is 41.1%and the dimethyl ether selectivity is 100%,which are better than those obtained in traditional fixed bed reaction.