Synthesis, Characterization And Catalytic Applications Of Zeolitic Materials With Hierarchical Porous Structure

Zeolites are genuine shape-selective molecular sieves with molecular size pores(dmicro<2nm) which are applied to adsorption, catalytic and detergency. However, thesole presence of micropores in these materials often confines intracrystalline diffusion,resulting in low utilisation of the zeolite active volume in catalysed reactions. Thepore width of mesopores (2nm <dmeso<50nm) are bigger than micropores, thus theyinduce new properties and reveal unexpected applications. The major drawbacks inmost industrial reactions are the low thermal, hydrothermal stability and weak acidity.Generally, the macroporous materials with poor mechanical stability cannot sievemolecules, since their pore size exceeds50nm.'Hierarchical' structured zeolitesintegrate at least two levels (types) of porosity, i.e. micro-and mesopores. They offera solution to the mass transfer problem associated with conventional zeolites, sincethey couple, in a single material, the catalytic features of micropores and the improvedaccess and transport properties of the additional porosity (mesopores). Thus, it has apromising prospect in the macromolecular reaction. The alkylation of phenol andtert-butyl alcohol is a typical acid-catalyzed reaction. The synthesis of bulky organicmolecules for industrial purposes is an important research area for solid acid catalysis.tert-Butylated phenols, typically bulky molecules, are commercially importantchemical intermediates in the industry. Therefore, the investigation of the reaction notonly deserves important academic value but also has some commercial value. Basedon this background, in this paper we use different types of templates to synthesize thevarious morphologys and the nature of the hierarchical materials, simultaneouslyinvestigate the physical and chemical properties of synthetic materials, and thenexplore its applications in acid catalysis.1. Synthesis of belt-like silicalite-1nanocrystals with mesoporous structure usinghydroxide as hard template At present, most kinds of one-dimensional nanomaterials (1D) are carbonmaterials and binary oxides, but1D zeolites have been less researched. In the thirdchapter, we try to synthesize a novel belt-like Silicalite-1nanocrystal withmesoporous structure by using nanosized calcium hydroxide and copper hydroxide asa template. In the synthesis process, the first step was the preparation of nanoscaletemplate (calcium hydroxide and copper hydroxide). Then the nanosized template andthe mixture of silica sol mixed thoroughly and introduced into a teflon-lined steelautoclave for further hydrothermal treatment at130oC for3days. The hydroxide canbe removed by acid treatment method, and then obtained the Silicalite-1by roasting.To investigate the role of hydrogen bonds in the synthesis, calcium hydroxide wascalcined in a nitrogen stream to obtain the calcium oxide as comparison experiment.Characterization of the synthetic template shows that, the nanosized Ca(OH)₂particlescross link with each other due to the interaction between hydroxyls and form a richchannel, and the obtained calcium oxide after calcinations compared with the calciumhydroxide, the particle distribution is relatively loose. Nonosized copper hydroxidepresent uniform nanorods close together but no channels appeared. In addition, thesynthesized materials were characterized by XRD, FT-IR, SEM, TEM, and N2adsorption-desorption. The results show, it is a unique one-dimensional belt-likeSilicalite-1nanocrystal with microporous and mesoporous structure by using Ca(OH)₂as a template. The synthesized material with CaO as a template appeared a smallamount of belt-like morphology of Silicalite-1materials, which most of them aresugar lump morphology of Silicalite-1. As may be gathered from this, hydrogen bondsare the main factors of the synthesis of belt-like morphology. In addition, themorphology of the synthetic materials by Cu(OH)₂as the template is similar with theconventional Silicalite-1. The Cu(OH)₂are not easy removal and will not achieve therole of the template. Further illustrates that the structure of the hydroxide, especiallythe pore structure has a significant influence on the morphology of the syntheticmaterial.2. Synthesis of a new types of mesoporous materials and micro/mesoporousmaterials by using glucose as a template The surfactant is expensive, so it is not conducive to wide application in thechemical industry. In the fourth chapter, new types of mesoporous materials andmicro/mesoporous materials have been synthesized by using glucose as a template.Glucose is inexpensive, readily available and nontoxic, so the synthesis process ispollution-free.(1) Mesoporous materials can be prepared by using glucose as templatethrough one-step crystallization. In the synthesis process, the disordered mesoporousmaterials can be prepared by hydrothermal crystallization after fully blending glucosewith Si-Al gel, and examine the amount of glucose on the structure of the syntheticmaterial. The characterization results show that mesoporous materials have largesurface area and pore volume, and with the amount of glucose increased, surface areaincrease and the pore size become smaller. The synthetic new mesoporous materialswere used for the reaction of tert-butylation with phenol to investigate its catalyticperformance, and the material exhibits superior to Al-MCM-41in catalytic activity.The results further indicate that the pore organization is not a necessary condition forhigh catalytic performance in many catalytic reactions. Compared with the orderedmesoporous materials, disordered mesoporous materials are cheapness and easy to getin this article, therefore they have potential applications.(2) Micro/mesoporousmaterials can be synthesized by using glucose as the template through two-stepcrystallization procedure. Firstly, zeolite nonoparticles were obtained by hydrothermalcrystallization. Then these small zeolite particles and glucose were assembled intomesoporous materials during the hydrothermal treatment. Upon calcinations, glucoseand TPAOH were removed to form micro/mesoporous composites materials. In thesecondary crystallization process, the influence of crystallization time on the productwas investigated. Characterization results show that the sample possessed bothmicroporous and mesoporous structure, and with the extension of the crystallizationtime, surface area decreases gradually and the mesoporous pore size increases. Thesample was found to have both Br nsted and Lewis acidity, and the acidity becamestronger as aging time increased. The hydrothermal stability was studied by treatingmicro/mesoporous materials in boiling water. The result shows that acid sites and poresystem of the materials have been little damaged during boiling in water for160hours. Therefore, the material has a remarkable hydrothermal stability. The obtained materialwas applied to the alkylation reaction of phenol with tert-butanol, and displayedhigher conversion of phenol compared with the disordered mesoporous materials. Themain reason may be that the presence of microporous structure of themicro/mesoporous materials, so acid strength and the number of acid centers areincreased. The result further indicates that hierarchical porous materials have moreadvantages than the single-stage porous materials in the structure and nature.3. In situ synthesis of micro/mesoporous materials using urea as templateNonsurfactant organic compounds containing hydroxyl or carboxyl groups can besuccessfully employed as template or pore-forming agents for the synthesis ofmesoporous materials. To expand the kinds of organic compounds with otherfunctional groups that can act as templates or pore-forming agents and to understandfurther how the non-surfactant compounds direct the mesophase formation, herein wereport the preparation of micro/mesoporous materials using urea as the pore-formingagent by in situ crystallization in the fifth chapter. Based on optimal reactionconditions, urea concentration, fluoride ion effect and crystallization time were alsostudied. The synthesis process is simple and short, urea as non-surfactant template isversatile and low-cost. The structures of the synthetic materials have beencharacterized by XRD, FT-IR, SEM, TEM, and N₂adsorption-desorption, the resultsprove that they have microporous and mesoporous composite structure. NH₃-TPD andPyridine FT-IR spectra confirmed that the synthetic samples have a strong acidity, andBr nsted and Lewis acid at the same time. In addition, pore size of mesoporous hasbeen changed by adjusting urea concentration, and the unit of the microporousstructure can be increased by prolonging the crystallization time. Fluoride ionsdecrease nucleation rate, and prompt the crystal growth. The pore structure and acidityof the material can be effectively adjusted by controlling the synthesis conditions, andobtained micro/mesoporous materials with different pore system and the acid strength.These materials showed excellent catalytic activity in the reaction of tert-butylationwith phenol, conversion of phenol can reach up to95.6%, the correspondingselectivity to2,4-DTBP can be achieved51.6%. Thus, this series of materials have a good prospect in acid catalyzed reaction of macromolecules.