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Synthesis, Characterization and Application of Micro-Mesoporous Composite Aluminosilicate Molecular Sieves

Author: SongChunMin
Tutor: YanZiFeng
School: China University of Petroleum
Course: Chemical Engineering and Technology
Keywords: Mesoporous molecular sieves ZSM-5 zeolite alkaline treatment assembly hydrothermal stability
CLC: TQ426
Type: PhD thesis
Year: 2007
Downloads: 1314
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Abstract


Mesoporous molecular sieves have potential applications in industrially important reactions such as catalytic cracking, alkylation and isomerization. However, the acidity and hydrothermal stability of mesostructured aluminosilicates MCM-41 are less than required for many catalytic applications, which are the main obstacles to applying mesoporous molecular sieves as catalysts or catalyst supports. Recently, the acidity and the hydrothermal stability of mesostructured aluminosilicate are significantly improved by direct assembly of the zeolite precursors. Based on the previous results, new routes to synthesize mesoporous molecular sieves from commercial ZSM-5 zeolite are developed, and the assembly behaviours of zeolite precursors in different routes were investigated in the present work.The commercial ZSM-5 zeolites were treated by the alkaline solutions to obtain slurry or filtrate and filter cake of alkaline dissolution of ZSM-5 zeolite. The mechanism of alkaline dissolution of ZSM-5 zeolite was proposed. And then the assembly behaviours of slurry, filtrate and filter cake of alkali-treated ZSM-5 zeolite as silica-alumina source were studied. The slurry and filtrate of alkaline dissolution of ZSM-5 zeolite, which contain soluble aluminosilicate species with fragments of ZSM-5 zeolite structure, were used for the synthesis of an MCM-41-type mesoporous material. And filter cake of alkali-treated ZSM-5 zeolite contain many small grains, and the surfaces of these grains are very rough. It is feasible to overgrow mesoporous MCM-41 on the rough surfaces of the grains. The obtained samples possess the ordered hexagonal mesoporous structure of MCM-41-type molecular sieves.Mesoporous molecular sieves (denoted as MZ-B) with ordered hexagonal structure have been successfully synthesized by the assembly of the precursors from filtrate of alkali-treated ZSM-5 zeolite with CTMABr surfactant micelles in alkaline media based on S+I– static electricity route. The samples were characterized by XRD, N2 adsorption, IR, SEM, TEM and so on. The synthesized samples exhibit highly hydrothermal stability and strongly acidity compared with conventional MCM-41. It’s indicated that the mesoporous walls of MZ-B samples contain the secondary building units similar to the units in microporous crystal of ZSM-5 zeolite. Furthermore, the mechanism of self-assembly using filtrate of alkali-treated ZSM-5 zeolite as the silica-alumina source was proposed. The composite molecular sieves (denoted as MZ-A) were hydrothermally synthesized using the slurry of alkaline dissolution of ZSM-5 zeolite. The obtained MZ-A samples were composed of the mesoporous molecular sieves with pore walls of secondary building units of ZSM-5 microporous zeolite, mesoporous MCM-41 overgrowth on the rough surfaces of the microporous grains, and normal MCM-41. The structure, morphology, acidity and hydrothermal stability of the composite materials were thoroughly characterized by XRD, N2 adsorption, IR, SEM, TEM, etc. The structures of obtained samples are similar to these from filtrate with the acrid degree of alkaline treatment increased. The mechanism of self-assembly from the slurry of alkali-treated ZSM-5 zeolite was put forward. The properties of obtained composite samples are related with the condition of alkaline treatment of ZSM-5 zeolite.In this work, it was investigated the possibility of the coassembly of the slurry or filtrate of alkali-treated ZSM-5 zeolite and monomeric silicate and aluminate species into mesostructured aluminosilicates. The behaviours of the soluble aluminosilicate species and monomeric silicate-aluminate in current coassembly were elucidated. The results show that the obtained samples possess more ordered hexagonal mesoporous structure as Si/Al ratio increased. The performance of the conversion reactions of n-C12 on composite molecular sieves shows high isomerization selectivity for non-hydrogen process, and good hydrocracking selectivity for catalytic hydrogenation process.

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CLC: > Industrial Technology > Chemical Industry > Reagents and the production of pure chemicals > Catalyst ( catalyst )
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