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Research Of The Solid Acid Catalysts For The One-step Synthesis Of Dimethyl Ether From Syngas

Posted on:2007-04-29Degree:DoctorType:Dissertation
Country:ChinaCandidate:J C XiaFull Text:PDF
GTID:1101360212484662Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Dimethyl ether (DME) has received a world-wide attention as a clean alternative fuel for diesel engines since it was disclosed to have better combustion performance than the conventional diesel fuel, e.g. lower NOx emission, lesser smoke and engine noise. At present, DME is mainly from the traditional two-step process, i.e. syngas was firstly transferred into methanol over a hydrogenation catalyst and methanol was then converted into DME over a dehydration catalyst. Lately, an original technique named STD (syngas to DME) process was worked out for the direct synthesis of DME from synthesis gas in a single reactor on hybrid catalysts composed of copper-based methanol synthesis catalysts and solid acids. Besides the simplification of reactor, the combination of the two reactions into one reactor can allow the CO conversion to reach a very high value.The most common hybrid catalysts reported for STD process are the physical mixture of the methanol synthesis catalyst and the solid acid catalyst. The Cu/ZnO-based catalyst has been used successfully for several decades for the production of methanol from syngas, and the reaction mechanism and the role of each active ingredient of the catalyst have been well studied. In contrast to methanol synthesis, the catalytic dehydration of methanol to DME has received less attention. At present, the most common solid acids applied in the STD process are γ-Al2O3 and HZSM-5. However, these solid acids don't match the optimized temperature (ca. 260℃) for copper-based component. So it becomes very necessary to develop the post-treatment techniques for the amelioration of the acidity of these acid catalysts and search some other solid acid catalysts for the STD process.According to the description above, I divide my research into three sections:Section 1 Research of HZSM-5 CatalystThe optimized temperature of ZSM-5 for methanol dehydration is much lower than 260 °C, so it will produce many byproducts when the reaction temperature is at 260℃. Thus, the modification of ZSM-5 should aim at the weakening of its acidity.(1) First, the acidity of ZSM-5 was ameliorated by elevating its SiO2/Al2O3 ratio. As the acid sites of zeolites are generated by aluminum, the decrease of aluminum content would greatly decrease the acid concentration of ZSM-5 zeolite. The optimized SiO2/Al2O3 ratio of ZSM-5 for methanol dehydration in STD process was around 200.(2) The calcination, steaming and alkaline steaming treatments of ZSM-5 zeolite werealso studied. Calcination had the least effect on weakening the acidity of ZSM-5. The modification effect of alkaline steaming was better than that of steaming, but just a very little better than that of steaming, so the catalytic performance of alkaline steamed ZSM-5 in STD process is no better than that of steamed ZSM-5.(3) The ferric modification also had a positive effect on the amelioration of acidity of ZSM-5 zeolite. The Fe-ZSM-5 sample prepared by solid ion exchanging method had a better catalytic property in STD reaction than the sample prepared by directly hydrothermal synthesis.(4) The modification of ZSM-5 with MgO induced the decrease of Bro|¨nsted acid sites of ZSM-5 and the increase of its Lewis acid sites. After MgO loading, ZSM-5 catalyst possessed a better catalytic property with the increase of selectivity to DME and the decrease of byproducts. However, the content of MgO should be well controlled and be in the range of 0.5%~2.5%.(5) The application of ZSM-5 with small crystal size to STD process was also performed. Compared to the ZSM-5 zeolite with larger particle size, the small crystal ZSM-5 didn't have a longer life on steam, in spite of its lower selectivity to byproducts.Section 2 Research of Alumina CatalystAlumina has a much lower acidity than HZSM-5 zeolite and isn't active enough when applied as methanol dehydration catalyst for STD process. Thus, as followed, some modifications were adopted to enhance its acidity.(1) The influence of fluorination on the acidity of alumina was studied. The fluorinating process was performed by methods of marination under stirring. The degree of fluorination was controlled by the concentration of ammonium fluoride. It came out that 0.1 mol/l is the optimized concentration of ammonium fluoride.(2) The loading of sulfate can also enhance the acidity of alumina. In our study, among sulfate treated alumina, the best catalytic performance was attained by the sample with 10 wt% SO42- and being calcinated at 550℃.In addition, the sulfate modified alumina possessed a good stability during the STD test.Section 3 Applications of Other Solid Acid MaterialsThe seeking of new acid catalyst for STD process should focus on the materials with acidity weaker than that of HZSM-5 and simultaneously stronger than that of alumina as mentioned above. The MWW-type zeolite is just such a material that matching the two requirements above. Thus, the application of MWW-type zeolite in STD process was detailedly studied.(1) We first adopted MCM-49, belonging to MWW-type zeolite, as the dehydration catalyst for STD process. However, MCM-49 zeolite can only be synthesized in a narrow range of Si/Al ratio around 13 and its Al content is too high to achieve good performance in STD process. Hereby, we tried to decrease the aluminum content in MCM-49 sample by steaming. The zeolite was treated under several temperatures in order to attain different degree of dealumination. It came out that the dealuminated sample had a good catalytic behavior only when the treating temperature ≥ 500℃.(2) MCM-22 zeolite has the same framework topology as MCM-49 zeolite but can be synthesized in a gel with a much wider range of Si/Al ratio. Upon that, we synthesized a high silica MCM-22 zeolite with Si/Al ratio at 60 and introduced it to the STD test. As expected, it had a much better catalytic property than MCM-49 zeolite.(3) As pure phase MCM-22 zeolite with Si/Al ratio > 60 still can't be directly synthesized, we performed a dealuminating process for the further decrease of the Si/Al ratio of MCM-22 zeolite. Oxalic acid, citric acid and (NH4)2SiF6 were used as the dealuminating reagents. After characterization and catalytic test, (NH4)2SiF6 treated sample was found to have the highest Si/Al ratio with the concurrence of better catalytic performance than all of the other dehydration catalyst. The CO conversion and DME selectivity reached 95.95% and 69.26%, respectively.
Keywords/Search Tags:Dimethyl ether, one-step, methanol dehydration, solid acid, catalysis, syngas
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