Research Of The Solid Acid Catalysts For The Synthesis Of Dimethyl Ether Process

With the rapid growth of the national economy, the demand of energy becomes a widely existed problem both in china and all over the world. Dimethyl ether (DME) is an important chemical material and also a potential clean energy which can substitute diesel and liquefied petroleum gas. Synthesis of DME plays an important role in the aspect of rational use of energy in china.At Present, DME mainly produced from syngas by the following two traditional processes: frist synthesis methanol and then dehydration to DME (so called MTD) over the solid-acid catalyst, the second way is direct synthesis of DME form syngas (so called STD) over the bifunctional catalysts consisting of methanol synthesis component and solid-acid component. For MTD process, a suitable solid catalyst is the key factor for its excellent performance.For STD process, the methanol synthesis component has been used successfully for several decades and the reaction mechanism of the catalysts has been well studied. In contrast to methanol synthesis, the methanol dehydration component has received less attention. Among these solid-acid catalysts, γ-Al2O3and HZSM-5are the most widely materials, however, the most suitable temperature for γ-Al2O3is above300℃, and undesirable byproducts in DME synthesis such as hydrocarbons and even coke formed on the HZSM-5zeolite catalyst due to the presence of strong acid sites. So it become very important to search new solid-acid catalyst and develop the post treatment techniques for the MTD and STD process.On the basis of the previous study, in this work we choose HZSM-5as research objectives and study its acidity and texture properties, and got the following conclusions:Study thermal steam modified HZSM-5zeolite and apply them in the MTD and STD processes. The surface acidity, pores structures, and Si/Al ratio has changed in the dealumination process. The result shows that HZSM-5treat at500℃for2h shows better stability than parent zeolite in MTD process, but for sample which treat at600℃for2h, amorphous fragments produced in the dealumination process blocked some micropores.Amorphous alumiums deduce the side reaction in the STD process.Thermal steam method made the total acity on the surface of HZSM-5decreased, which result in a decreased activity of methanol dehydration. So this paper tried to study fluoride solution modified HZSM-5zeolite and apply them in the MTD and STD processes. The incorporation of F-species as electron withdrawing compounds reduce the amount of acid sites but enhance the surface acid strength. NH4F modified HZSM-5zeolite shows excellent activity in MTD process, but in STD process it increase the selectivities for undesired by-products like hydrocarbons and CO2due to the new formed strong acid siteFluoride solution modified HZSM-5zeolite exhibed excellent methanol dehydration activities, but fluoride solution seriously pollute the environmental, so modify HZSM-5zeolite with solid AlF3·3H2O by the way of mechanical mixture route was tried and apply them in the MTD and STD processes. The structure, texture, and acidity of HZSM-5catalysts can be adjusted with loading of AIF3. The results show that2wt%of AIF3modification could increase the framework aluminum content and the surface area of HZSM-5, however, when the loading of AIF3came to3wt%or more, the contrary results were obtained, which could be ascribed to the dealumination of the zeolitic framework。2wt%AlF3-modified HZSM-5exhibited much higher activity and better stability than parent HZSM-5in MTD process. In the STD process, it shows relative high selectivities for undesired by-products due to the strong acid site.In order to ascertain the role of mesopores in the synthesis of dimethyl ether reaction. The application of HZSM-5with hierarchical mesopores to synthesis DME process was performed in this work. The hierarchical mesopore ZSM-5zeolites shows better stability and high DME selectivity than conventional micropore HZSM-5zeolites for MTD process. The results also show that micropore was more favorable for methanol adsorption, however, the stronger methanol adsorption on the zeolite, the easier by-products can be produced. The acidity properties on the hierarchical mesopore HZSM-5are quite different, whereas the conversion of carbon monoxide was scarcely affected.Through comparison of the MTD and STD activities on the various method modified HZSM-5zeolite, we conclude the role of texture, acidity and morphology of HZSM-5catalysts played in the synthesis of DME process. In the STD process, the acidity properties on the hierarchical mesopore HZSM-5are quite different, whereas the conversion of carbon monoxide and selectivity of DME were scarcely affected.However, on the thermal steam and AlF3modified HZSM-5, when dealumination cause acidity decrease obviously, the conversion of carbon monoxide was decrease correspondingly and selectivities for methanol and by-products were increased obviously. This indicate the texture property of solid-acid was more important than acidity property in the STD process.