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Preparation And Properties Of Industrial Catalysts For Dimethyl Ether Synthesis From Methanol Dehydration

Posted on:2017-05-17Degree:MasterType:Thesis
Country:ChinaCandidate:X Z FengFull Text:PDF
GTID:2271330503958339Subject:Chemical Engineering and Technology
Abstract/Summary:PDF Full Text Request
γ-alumina and zeolites are the most common solid acid catalysts in methanol dehydration to dimethyl ether(MTD)reaction, however, both of them possess defects. For example, the γ-alumina catalysts present poor activity at low temperatures, the existence of narrow and long pores in zeolites easily causes carbon deposition at high temperatures,leading to the deactivation of catalyst. Therefore, there are certain limitations to their industrial applications. Recently, the optimization of catalyst composition and structure to improve DME production and catalyst stability has been widely pursued by researchers.The purpose of this work is to exploit the catalysts suitable for the industrial synthesis of dimethyl ether from the dehydration of methanol. By optimizing and improving the production process, high efficiency and stable γ-Al2O3 and HZSM-5/MCM-41/γ-Al2O3 catalyst with special structure were successfully prepared, which reduced the energy consumption during the production of dimethyl ether and improved product quality and production efficiency.Firstly, the γ-alumina catalyst was prepared with pseudo-boehmite prepared by double-aluminum method as precursor. The effect of extruding conditions such as variety and dosage of peptizers, the ratio of water to powder, dry temperature and calcination time on the structure and catalytic activity of γ-alumina catalyst was investigated. Under the appropriate operation conditions(peptizer B dosage is 24.88 ml/kg powder, A and C as second peptizers, 120℃ drying, and 600℃ calcination for 4 h), the as-prepared catalyst presented an excellent catalytic activity with 86.5% methanol conversion at 300℃ and100% selectivity of dimethyl ether. The economic cost of the as-prepared catalyst was38000 yuan/ton.Secondly, HZSM-5/MCM-41 composites molecular sieves(ZM) were hydrothermally synthesized with commercial HZSM-5 zeolites used as silica-alumina source and cetyl trimethyl ammonium bromide(CTAB) as template. After molding with γ-alumina, the industrial HZSM-5/MCM-41/γ-Al2O3(ZMA) catalyst was successfully prepared, and the performance of three types of catalysts was investigated, respectively. HZSM-5 molecularsieve had an orderly microporous structure, and crystal shape, as well as Br?nsted acid sites and Lewis acid sites. The of total acid amount NT, Br?nsted acid amount NB, weak Br?nsted acid amount NBW, strong Br?nsted acid amount NBS, Lewis acid amount NL and the ratio of NB to NL of the catalysts all reduces with the increasing SiO2/Al2O3 ratio, while the NBW/NBS increased. Compared with the parent HZSM-5 molecular sieve, ZM catalysts possessed a uniform microporous and mesoporous structure, high mesopore volume and high specific surface area. The acid amounts of different acid sites on the ZM catalysts significantly reduced, and NB/NL reduced, while NBW/NBS increased. The activity of ZM catalysts ranked in the order as follows: ZM038 U > ZM050 U > ZM150 N. Compared with the ZM composite catalyst, the catalytic activity of ZMA catalysts showed a little decline of5% in methanol conversion at the same temperature, however, the producing temperature of by-products over ZMA was shifted 20℃ to a higher temperature.Finally, a series of silica-modified γ-alumina(AN、AS、AC) were prepared by the precipitation method with different aluminum salt, tetraethyl orthosilicate and aqueous ammonia as starting materials. The silica-modified γ-alumina catalysts presented a similar appearance with irregular solid geometry shape, and the Lewis acidity. The acid strength of the catalysts as well as the activity sequence were ordered as follows, AN > AS > AC. The amount of surface anion had an important effect on the catalytic activity of the catalysts.
Keywords/Search Tags:methanol dehydration, dimethyl ether, γ-Al2O3, zeolite, solid acid
PDF Full Text Request
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