Hierarchical Zeolite: Synthesis, Characterization And Catalytic Application

Hierarchical zeolite has advantages of zeolites (high thermal, hydrothermal stability and ordered microposity) and ordered mesoporous materials (large mesopore structure) simultaneity, is a proven strategy to combine shape selectivity with efficient mass transport. These features make it highly desirable in catalysis and adsorption, especially in bulk molecule processes. Its synthesis, characterization and performance in catalytic reaction have gathered a lot of research interests. However, some fundamental questions in hierarchical zeolite synthesis such as the zeolite crystallization and mesopore formation mechanism are still unresolved. Also, recently mesoporous zeolites are generally synthesized based on a nanocasting route with carbon materials as secondary template. This nanocasting route, though very effective, is industrially unfavorable because of its high cost and complex process. Moreover, the structure order in mesopore scale is need further improve. In the doctor thesis, with the recently development in nanotechnology, inorganic synthesis chemistry and zeolite crystallization mechanism, we hope to overcome above motioned question in hierarchical zeolite synthesis, develop cost effective mesoporous zeolite synthesis method, improve the properties of zeolite products and test typical zeolites in model catalytic reaction.In chapter 1, detailed literature review on hierarchical zeolite synthesis, characterization and catalytic application is given. The detailed experimental methods are given in chapter 2. In chapter 3 with the purpose to understand the influence of carbon template structure on zeolite crystallization, typical carbon materials with difference structure properties was used as template for zeolite synthesis. The final product and materials obtained with different crystallization time were carefully characterized. The results show that the structure of carbon materials has large influence on the zeolite crystallization and the structure of zeolite product can be controlled by varying structure of carbon template. Mesoporous zeolite single crystal can be synthesized with ambient drying carbon aerogel as template without any control on crystallization conditions. The structure of so called mesoporous zeolite single crystal successfully cast the nanostructure of carbon aerogel template. With carbon materials as template, mesoporous aggregate of zeolite nanocrystals is often formed as product or intermediate phase for mesoporous zeolite single crystal synthesis, determining on the structure of carbon template, the structure of zeolite nanocrystals have no obvious relationship with carbon template thus the carbon template is not strictly needed in the synthesis of this type mesoporous zeolite. In chapter 4, we show by changing the structure of carbon aerogel through controlling the catalyst concentration during synthesis, zeolite with tunable intracrystal nanoporsity over larger range (10 nm-100 nm) can be synthesized. A method based on nanocasting concept for test the interconnectivity of mesopore channel in mesoporous zeolite single crystal was developed in this chapter. The result indicate that mesoporous zeolite synthesized with different carbon template have different mesopore interconnectivity. In chapter 5, ordered mesoporous zeolite was synthesized with hard or soft template. It was found that the pore wall of SBA-15 can be recrystallized into ZSM-5 zeolite with in-situ formed carbon material (CMK-5) as template. Zeolite with intracrystal wormhole like mesopore channel can be synthesized with mixture of common cationic surfactant and its silylated analogue as template. Ordered mesoporous zeolite synthesized with both two method have very high thermal and hydrothermal stability, which is stable after treatment in steam at 850℃for 4 h or refluxing in boiling water for 120 h. The formation process of ordered mesoporous zeolite in the two synthesis methods is similar, which is, the formation and dissolve of initial mesopore phase, zeolite nucleation and the formation of ordered mesoporous zeolite. In both synthesis routes, the confine effect of secondary template on zeolite growth is needed and should be controlled in a certain degree. Ordered mesoporous zeolite can only be synthesized when a good match between zeolite growth kinetic and ordered mesopore structure formation is reached. In our synthesis method, ordered mesoporous zeolite can only be synthesized with CMK-5 have moderate thickness or mixture of common cationic surfactant and its silylated analogue as template. In chapter 6, we test the catalytic performance of mesoporous aggregate of zeolite nanocrystals in bulk molecular containing reaction and the possibility to synthesis of mesoporous aggregate of zeolite nanocrystals without secondary template. It was found that mesoporous zeolite aggregate actually show better performance in bulky molecular containing reaction, for instance, catalytic oxidative desulfurization. Hierarchical TS-1 synthesized with CMK-3 as template show improved catalytic performance for thiophene removing compare with common TS-1 and is able to catalytic oxidation remove of large sulfur containing molecular such as DBT. Mesoporous aggregate of ZSM-12 nanocrystals and ZSM-5 nanocrystals was successfully synthesized without secondary through in-situ assembly of zeolite nanocrystals into mesoporous aggregate of zeolite nanocrystals with single crystal like morphology. We further compare the synthesis method and formation process of mesoporous zeolite in different route and found that the hierarchical zeolite generally crystallization through a nanoparticles based aggregation formation mechanism. Stabilize of zeolite nanoparticles with different size and its further assembly leading to the formation of different type of mesoporous zeolite. Hence the hierarchical zeolite is generally kinetically favored product at the given condition. A general crystallization map of hierarchical was given in this chapter. In chapter 7, we test the performance of OMZ-1 in methylation of 2-methylnaphthalene. In 2,6-dimethylnaphthalene synthesis, based on quantum chemical calculation, 2,6-DMN is slightly bulkier and chemical reactivity more favored product than 2,7-DMN. OMZ-1 show better catalytic performance on this reaction than ZSM-5. With Zr/OMZ-1 as catalyst, after 8 h reaction at 400℃, the conversion of 2-methylnaphthalene, selectivity ofβ,β-DMN, ratio of 2,6- and 2,7-DMN and selectivity of 2,6-DMN is 44%, 78%, 2.7,49% and17%, respectively.