Synthesis, Characterization And Catalytic Application Of Hierarchical Porous IM-5and TNU-9Zeolites

The known reserves of natural gas (mostly methane) are enormous and thereserves are increasing more rapidly than those of liquid petroleum. It is anticipatedthat this trend will extend well into the21st century and the effective utilization ofmethane is becoming more important than ever before. Since the first active catalystMo/ZSM-5was found in1993, methane non-oxidative aromatization has been avaluable and challengeable research subject in both academy and industry.Moreover, it has been found that materials with both micropores and mesoporescan remarkably enhance their activities in the catalytic reactions. Micropores inzeolite provide size and shape-selectivity for guest molecules, while mesopores leadto easier access to the active sites for reactants and better diffuse of the products.High-silica zeolite IM-5with an unusual2D10-MR channel system has beenreported by Benazzi et al. in1998. Due to its excellent physico-chemical properties,the zeolite IM-5could be applied in the petrochemical and refining industry ascommercial catalyst. For instance, IM-5is very active in hydrocarbon cracking andNO reduction due to its high thermal and hydrothermal stabilities which are evenbetter than those of ZSM-5. Although the general features of its pore system werededuced from a variety of catalytic reactions in2000, its detailed crystal structureinformation was not fully elucidated until2007. As one of the most complicatedzeolites hitherto solved, IM-5shows a special framework structure containing24topologically distinct Si atoms and an unusually large unit cell (C-centeredorthorhombic with a=14.2088, b=57.2368, c=19.9940) with nearly triplevolume than ZSM-5. IM-5consists of different2D channels with complex channelintersections connected by10-MR apertures. The central2D channel systemconnects to another2D channel system through10-MR along [010] and forms anapproximate2.5nm thick cavity. The structure of IM-5can be described asconsisting in a2D10-MR channel system with the presence of3D cavity, which isdifferent from the early zeolites, ZSM-5, ZSM-11, and MCM-22etc. The3D channel system with complex channel intersections gives IM-5a distinctive porestructure which can accommodate bulky intermediates in a catalytic reaction andIM-5also retains the long-range diffusion property of2D channel system.TNU-9, a new high-silica zeolite with an3D10-ring channel system, haspreviously been synthesized by Suk Bong Hong et al. Recently, the frameworkstructure of this zeolite has been solved through the combined use of powderdiffraction and electron microscopy. More recently, a complete account of thesynthesis, characterization and catalytic properties of TNU-9has been given in thework. As one of the most complex zeolites hitherto solved, the framework structureof TNU-9is that it contains24crystallographically and topologically distinct Siatoms in its large monoclinic unit cell (almost double that of ZSM-5) and possessesan unique3D10-ring channel system. Similar with MCM-22, TNU-9also ownslarge12-ring cavities around7.2that are accessible only through10-ring windows.Moreover, due to the unique pore structure and excellent hydrothermal stability,TNU-9exhibits superior shape selectivity for catalytic reaction such as theisomerization of m-xylene.1. Synthesis of zeolite IM-5and TNU-9under rotating and static conditions and thecatalytic performance in methane non-oxidative aromatizationThe hydrothermal crystallization of zeolite IM-5and TNU-9were investigatedunder rotating and static synthesis conditions. Mo-modified catalysts were preparedfor the methane non-oxidative aromatization. The physical properties and acidities ofthe samples were characterized by XRD, SEM, BET and IR spectroscopy. Comparedwith catalysts synthesized by static condition, catalyst synthesized by rotatingcondition showed both a higher conversion of methane and higher selectivity tobenzene in methane aromatization. We supposed that the higher catalytic activity maybe attributed to the preferable textural properties and acidities of zeolite. Moreover,the catalyst prepared by the physical mixing method exhibited lower initial activity,but better stability for methane aromatization than that prepared by the impregnationmethod. 2. Synthesis of mesoporous IM-5and TNU-9materials using three different methodsand the catalytic performance in methane non-oxidative aromatizationFirstly, the mesoporous IM-5and TNU-9samples were synthesized by usingSBA-15and MCM-48as the silica source.Secondly, the mesoporous IM-5and TNU-9samples were synthesized by usingglucose as the hard template.The third, the mesoporous IM-5and TNU-9samples were synthesized by addingordered mesoporous carbon into the synthesise system.Mesoporous IM-5and TNU-9exhibited larger geometrical shape as conventionalmaterials. Moreover, Mo-modified catalysts were prepared for non-oxidativearomatization of methane. For comparison, conventional catalysts (without addition ofcarbon template) were synthesized for the same reaction. The physical property andacidity of the samples were characterized by XRD, SEM, TEM, BET and IRspectroscopy. Compared with conventional catalysts, mesoporous Mo-modified IM-5and TNU-9catalysts showed higher yields of aromatics. In addition, the stabilities ofmesoporous Mo-modified IM-5and TNU-9catalysts were better than that ofconventional catalysts. It is considered that the catalytic behavior of Mo-modifiedmesoporous IM-5and TNU-9catalysts may be attributed to the generation ofsecondary mesoporous systems within zeolite crystal, which could improve theaccessibility of reactants to the active sites and promote the diffusion of productsformed in the microporous channels.3. Synthesis of new mesoporous IM-5and TNU-9materials using aging method andthe catalytic performance in methane non-oxidative aromatizationThe new mesoporous IM-5and TNU-9materials were prepared by agingmethod. The physical property and acidity of the samples were characterized byXRD, SEM, TEM, BET and IR spectroscopy. Compared with conventional catalysts,mesoporous Mo-modified IM-5and TNU-9catalysts showed higher yields ofaromatics. In addition, the stabilities of mesoporous Mo-modified IM-5and TNU-9catalysts synthesized by aging method were better than that of conventional catalysts. We supposed that the higher catalytic activity may be attributed to the preferabletextural properties. Mesoporous catalysts showed better performance than that ofconventional catalysts.