The Preparation Of Mo-based Catalyst And The Catalyst For Methane Non-oxidative Aromatization

With the increasingly scarce oil resource in recent years, natural gas as a kind of high quality, efficient, clean energy, is widely attracted reasercher’s attention around the world. Methane is the main component of natural gas, and it has been widely used in ceramics, glass, steel, chemical and other fields. Methane non-oxidative aromatization reaction is one of the few important projects with independent intellectual property rights in our country. A lot of work has been carried out and made a certain progress in terms of chemical conversion of methane. The specific research focused on the selection and modification of catalyst carrier, active center Mo species and reaction mechanism, etc. At present, Among Mo-based catalysts prepared by different carrier, three materials（Mo/HZSM-5, Mo/HMCM-22, Mo/HTNU-9） show better catalytic activity and stability, but the strong acid center of the carrier will promote the formation of naphthalene and carbon deposition, can block the channel of zeolite, leading to deactivation of the catalyst. Therefore, methane non-oxidative aromatization reaction did not achieve the ideal effect so far. Moreover, nanoparticles have been applied to multiple catalysis and obtained excellent catalytic effect, but the catalysts prepared by nanometer Mo O₃ have not been applied in methane non-oxidative aromatization reaction. This thesis mainly studied the carrier selection and the way of introducing Mo species. We synthesized TNU-9 with hierarchical pore structure, by which we prepared the catalyst which can effectively improve catalytic activity and stability in the methane non-oxidative aromatization. In addition, we synthesized Mo O₃ particles with nano structure, and by which we prepared different catalyst which can obviously improve the catalyst’s performance, at the same time can improve the catalyst’s stability. The specific analysis of the results are as follows: 1. The catalyst Mo/TNU-9 prepared by Mo O₃ for methane non-oxidative aromatizationWe prepared TNU-9 by hydrothermal synthesis method, with silica as the silica source, 1, 4-2（N-methyl pyrrole） butane as the template agent, introduced phenyl triethoxy silane as mesoporous template agent in the process of synthesis, and fabricated a series of micropore-mesopore composite materials TNU-9. The physical properties of the synthesized samples was analyzed by different characterization methods. Characterization results suggested that the intensity and the acidity of zeolite would decrease with incresing number of phenyl triethoxy silane, meanwhile mesoporous structure of zeolite would increase. Which can verify that the acidity of zeolite can be rational adjusted by introducing a proper of organic silane. In addition, We have prepared different catalysts with loading 6 wt.% content of phenyl triethoxy silane for metane non-oxidative aromatization. Catalytic tests indicated that the introduction of phenyl triethoxy silane can effectively improved catalytic performance. when the catalyst Mo/TNU-9 contained twenty percent of phenyl triethoxy silane, it showed 14.9% of methane conversion and 9.9% of aromatics yield. The results can perform that the generation of a secondary mesoporous system within TNU-9 make Mo species effectively migrate into the channel of the zeolite, thus decrasing the catalysts’ acidity. Moreover, the existence of mesopore is benefit for spreading of the product with time on stream. But when the content of phenyl triethoxy silane reached 25%, the catalytic performance would decline, which may be attributed to excessive organic silane would result in the collapse of the zeolite. So we think the concentration of the organic silane should be reasonably controlled, thereby ensure that the catalyst show excellent catalytic performance in methane non-oxidative aromatization reaction. 2. The catalyst Mo/HZSM-5 prepared by nano-Mo O₃ for methane non-oxidative aromatizationWe have synthesized different sizes of Mo O₃（I） with 0.1-1 um and Mo O₃（II） with 30-150 nm which were used to modify ZSM-5, the particle size are far less than the size of traditional Mo O₃（10-20 um）. At the same time we prepared two series of catalyst Mo/HZSM-5 with loading different concentration of nano-Mo O₃, The effects of nano-Mo O₃ concentration on structure, morphology and the acidity of samples were investigated by different characterization. Test results found that the crystallinity of HZSM-5 decreased after loading Mo O₃. Moreover, the crystallinity and the acdity of catalyst would decrease with the increasing content of nano-Mo O₃, Compared with catalyst Mo/HZSM-5 prepared with loading the same content of traditional Mo O₃, the intensity and the acidity of the catalyst decrase more obviously. Two series of catalysts have been respecively applied for methane non-oxidative aromatization, the test results classified that they can effectively improve the catalytic performance, and the content of loading nano-Mo O₃ should be controlled at 6 wt.%. In addition, the catalytic test demonstrated that the smallest particles of Mo O₃（II） showed methane conversion（14.1%） and aromatics yield（9.5%）. The results showed that the smaller particle of Mo O₃, Mo species more easily sublime and disperse into the channel of zeolite, and reduce the catalysts’ acidity, at the same time improve the catalytic activity of the catalyst. In addition, the results confirmed that the catalyst prepared by nano Mo O₃ may produce more active center of α-Mo C1-x, which not only can improve the activity of the catalyst in the reaction but also can improve the stability of the catalyst. 3. The synthesis of Mo/MCM-22 and MCM-49 catalyst by loading nano Mo O₃ and the catalytic performance in methane non-oxidative aromatizationWe respectively prepared a series of Mo/MCM-22 and Mo/MCM-49 catalysts by loading different content of nano Mo O₃（30-150 nm）, and at this time we have also synthesized Mo/MCM catalyst by loading conventional Mo O₃, which were analyzed using different characterization methods. The results performed that the intensity and the acidity of Mo/MCM-22 and Mo/MCM-49 catalysts would decline with increasing concentration of nano Mo O₃. The test results indicated that Mo species with small size would sublimate easily and migrate into the channel of zeolite, then interact well with the internal Br?nsted acid sites to form Mo-O-Al active species, thus decreasing the acidity of the zeolite. In addition, we have also made investigations on the catalytic activity related to prepared catalyst. Catalytic results confirmed that the catalytic performance of Mo/MCM-22 and Mo/MCM-49 catalysts can be gradually improved with increasing nano-Mo O₃ loading, and when the content of nano-Mo O₃ reached at 6 wt.%, Mo/MCM-22 showed methane conversion（13.1%） and aromatics yield（8.9%）, while Mo/MCM-49 showed methane conversion（13.2%） and aromatics yield（9.1%）. when the content of nano-Mo O₃ exceeded 6 wt.%, the catalytic activity of prepared catalyst would decrease. Thereby we think that the concentration of the nano Mo O₃ should be reasonabley adjusted during the process of preparing Mo/MCM-22 and Mo/MCM-49, too much nano Mo O₃ loaded in zeolite would reduce their catalytic properties in the reaction. Meanwhile we have also compared nano Mo O₃ modified MCM-22 and MCM-49 with nano Mo O₃ modified ZSM-5 zeolite, we found that the catalyst Mo/MCM-22 and Mo/MCM-49 exihibied better selectivity to bezene, which can be attributed to the existence of supercage structure which contributed to the spread of the benzene molecule.