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Synthesis and Characterization of Mesoporous Carbons and Mesoporous Zeolites

Author: XingLiHong
Tutor: ZuoQiuBin
School: Jilin University
Course: Physical and chemical
Keywords: Template Mesoporous Carbon Tunable pore size Mesoporous zeolite Multi-level hole
CLC: O611.3
Type: Master's thesis
Year: 2006
Downloads: 732
Quote: 2
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Abstract

The conventional zeolite moleculars sieves have strong acidity and uniquecrystal structures. However, the pore sizes of these zeolite materials are normallyless than 2 nm, consequently restricting their potential applications foracid-catalyzed processes, such as petroleum refining, fine chemicals synthesis, etc.The introduction of mesoporous molecular sieves with MCM-41 and SBA-15structures has developed new possibilities for adsorption and catalytic reactions oflarge molecules. These novelty nano-materials were firstly reported in 1990s, withlarge and tunable pore diameters, large SBET surface areas and uniform porosity.Based on such excellent properties, they have shown superior catalyticperformance than those conventional zeolite moleculars sieves.With the development of science and technology, the mesoporous materials arenow not only simply used as acid or alkali catalysts, but also used to prepare newmaterials. The traditional methods of the preparation of mesoporous materials are“Cooperative formation Mechanism” and “Liquid Crystal TemplatingMechanism”. And recent years, a new technique named “Nanocasting templatemethod” is also proved to be effective to synthesize mesoporous materials. Thistechnique was firstly introduced by Ryoo et al, the main idea of the templateprocedure is to create cross-like carbon framework by infilling and pyrolyzing thecarbon precursors within the confined pore system of mesoporous silicas.Templated on the mesoporous carbons, inverse mesoporous silicas, mesoporousmetal oxides, mesoporous macromolecule materials can easily been synthesized.Also, such compendious template strategy can be used to prepare mesoporouszeolites, which are relatively difficult to synthesize directly.In this paper, ordered mesoporous carbons with alterable pore sizes have beenprepared by adjusting the ageing temperature of Ia3d silica templates to controltheir pore dimensions, according to the procedure described in the reference. XRDand TEM proved that the mesoporous carbons prepared here are of inverse Ia3dstructure from their Ia3d silica templates. In our previous research, we found thatthe pore dimension and wall thickness of Ia3d mesoporous silica could be easilytuned by changing the ageing temperature during the synthesis procedure. Allmesoporous carbons have typical IV isotherms and narrow pore size distributionsas their silica templates. With the aging temperature increasing from 80 to 130 0C,the pore sizes of Ia3d silicas increase distinctly from 6.1 nm to 8.9 nm. And thepore sizes of of their carbon replicas decrease from 4.7nm to 3.2nm. If we ignorethe influence of structural shrinkage due to carbonization at high temperature, thepore diameters of carbon replicas can reflect the pore wall thickness of their silicatemplates. In addition, if the ageing temperature was as low as 50 oC, a structuraltransition from the Ia3d structure of silica template to the CMK-1-like structure ofmesoporous carbon could be observed. Such novelty structural transformation canbe explained by displacement model. Thermogravimetric weight analysisindicated that the purity of the mesoporous carbons is as high as 99.3%. All thecarbon materials prepared have large pore volume and high BET surface area (upto 1308 cm3g-1).Templated on the mesoporous carbons C-Ia3d (80-130), we have successfullyprepared mesoporous zeolite ZSM-5 with multiple pore size distributions ofmicropore, mesopore or macropore, by the method of impregnation with ZSM-5precursor. It is evidenced that the meso-ZSM-5 materials are of typical MFI-typecrystal by XRD and FTIR spectras. N2 adsorption-desorption experiment and poresize distribution show the appearance of mesopore or macropore in meso-ZSM-5materials. This analysis can also be conformed by TEM technique. Allmeso-ZSM-5 materials have similar SBET surface areas to conventional zeolites.However, their total volumes are much higher, which can be attributed to theappearance of mesopore. NH3-TPD profiles of mesoporous zeolites show thepresence of stronger acidic centers besides weaker acid sites on the framework.Here, we also prepared a series of mesoporous zeolites materials namedmeso-silicate, by combined use of impregnation with carbon and crystallizationmethods. The materials synthesized are characterized by means of XRD and FTIR.The existence of mesopore is confirmed by XRD. FTIR and XRD proved that allmeso-silicate samples are pure zeolites. In order to understand more informationabout the structural and physicochemical properties of the samples, morecharacterization techniques such as N2 adsorption-desorption experiment, SEMand TEM will be applied.

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