Synthesis Of Hierarchical Pentasil Zeolites Using Dual Templates System

Mesoporous zeolites, which combine shape selectivity and hydrothermal stability of crystalline micropores with the improved diffusion properties of mesopores, are extremely desirable for catalysis and adsorption of bulky molecules. Dual templates method is one of effective routes to synthesize mesoporous zeolites. However, two different templats for micropore and mesopore worked in a competitive manner rather than a cooperative manner, which easily results in the formation of physical mixtures of amorphous mesoporous material and bulky zeolite. Sometimes the complexity of the template preparation has limited the industrial application. The dissertation is aiming at preparing hierarchical pentasil zeolites using dual template system, where three methodologies were adopted herein in order to minimize the mismatch of nanocrystals and mesophase and thus to inhibit the competition between template for zeolite and meso-template and to avoid the formation of physical mixtures. The structure and physicochemical properties are discussed in detail.Firstly, mesoporous MFI zeolites were synthesized using cetyltrimethylammonium tosylate (CTATos) and TPAOH as dual templates.(1) The physical mixtures of bulky zeolite (～15μm) and mesophase were obtained by one-step hydrothermal crystallization due to strong directing ability of TPAOH for MFI zeolitic structure.(2) Instead of hydrothermal method, steam-assisted recrystallization of TPA+-exchanged MCM-41was chosen to synthesize MCM-41/MFI composite. During dry-gel conversion (DGC) process, less water contacted with silica walls, inhibiting the easy transportation of silica species from and to, better maintaining the mesostructure and thus avoiding the phase separation of mesoporous materials and bulky zeolite. However, physical mixtures of bulky silicalite-1and mesophase were obtained at175℃when pure silica MCM-41was used as parent material, indicating the fast growth of zeolite at high temperature. The crystallization process was inhibited at150℃, as a result, MCM-41/MFI composite without phase separation was successfully fabricated. Moreover, the introduction of titanium obviously inhibited the crystallization of MFI phase, avoiding the formation of physical mixture of micro-and mesophase. The MCM-41/MFI composites with microporocity in range of0.03-0.05cm3/g were synthesized.(3) Using CTATos as mesostructure template, hexyltrimethylammonium (C6TMA+), a weak structure directing agent for MFI zeolite, was used meanwhile in order to slow the crystallization process and thus to avoid the formation of physical mixtures of amorphous mesoporous material and bulky zeolite.(a) Highly crystallized ZSM-5without ordered mesostructure was obtained at175℃for10d, showing the mixture of bulky zeolite and mesophase. And layered ZSM-5with a few of zeolite aggregates was observed at lower temperature (150℃), implying lower temperature is also beneficial to keep the mesostructure through reducing the growth rate of MFI zeolite during the crystallization process.(b) MCM-41/MFI composite, showing similar size and morphology with that of MCM-41synthesized at high temperature using CTATos as single template, was obtained in titanosilicate system at175℃because the introduction of titanium inhibited the crystallization of MFI phase.(c) Unlike aluminosilicate or titanosilicate, highly crystallized MFI zeolite aggregates of primary particles in size of50nm were synthesized when both titanium and aluminum were incorporated into zeolitic framework, and no obvious phase separation was observed. In a word, by choosing appropriate crystallization method and structure directing agents, decreasing the temperature and altering the composition of zeolitic framework, it is possible to find the balance between the formation of zeolite and the maintenance of mesostructure, possibly leading to full utilization of both mesoporosity and microporosity in certain applications.Secondly, mesoporous zeolites with MEL structure were obtained through direct hydrothermal method using TBAOH and CTATos as dual templates, taking the advantages of MEL structure zeolite that tends to form nano-sized primary crystallites preferentially. The mismatch of nanocrystals and mesophase was minimized by decreasing the crystal size of zeolite, and thus mesoporous MEL structured aggregates of nanosized primary particles were synthesized without phase separation, due to the improved interactions between zeolite precursors and mesotemplate micelles. Mesoporous ZSM-11, TS-2and Zr-MEL were fabricated through the dual templates method and the influences of temperature on structure and morphology were investigated. Mesoporous MEL zeolites were successfully synthesized in a wide range of temperature (120-175℃), where high crystallization temperature led to an increase in Vmicro but a decrease in Vmeso. Moreover, hetero atoms played an important role in the formation of mesoporous zeolites with different structure and morphology. Those aggregates combine the considerable microporous and mesoporous structure without any physical mixtures formed, which is beneficial to show higher catalytic properties than that of pure mesoporous materials, zeolites and a mechanical mixture of mesoporous materials and zeolites during the catalytic processes involving large molecules. Mesoporous ZSM-11showed better catalytic activity in LDPE pyrolysis by distinctly decreasing degradation temperature; Mesoporous TS-2in size of～2μm played comparable conversion for hydroxylation of phenol to that of nanosized TS-1and TS-2, meanwhile, curtailing the difficulties for the separation and recovery; For epoxidation of cyclohexene, mesoporous TS-2were more active than conventional TS-1and TS-2and showed comparable catalytic properties with that of Ti-MCM-41, indicating Ti sites located in the mesoporous were available to the reaction. In addition, the zeolitic framework was helpful to show higher thermal/hydrothermal stability than that of Ti-MCM-41; For mesoporous Zr-MEL, due to a certain amount of silanol groups on the surface, as well as its zeolitic framework and improved diffusion ability, mesoporous Zr-MEL showed higher catalytic properties for MPV reduction of cyclohexanone than bulky Zr-MFI zeolite.Finally, hierarchical MgO/Silicalite-1composites with high hydrothermal stability were hydrothermally synthesized using MgO as hard template of mesopores and TPAOH as zeolitic template. MgO species were uniformly distributed into porous Silica via solid-state grinding and subsequent calcination, which can enhance the cooperative manner between microtemplates and mesotemplates and thus avoid the formation of phase separation of micro-and mesophase. MgO could be partially removed by acid, leading to more mesopores into zeolite. And the introduction of MgO species could increase the hydrothermal stability of MgO/Silicalite-1at800℃in100%steam. The improved hydrothermal stability and the generation of mesopores in these MgO/Silicalite-1samples may play important roles in enhancing coke tolerance, slowing deactivation, and extending catalyst life during high-temperature reaction. Solid-state grinding can be used to introduce different mesotemplates or to prepare hierarchical porous zeolite with different topology structure.