Fabrication, Mass Transport And Catalytic Properties Study Of Hierarchical Zeolites

Zeolites are microporous inorganic materials with regular intracrystalline cavities and channels of molecular dimensions. Zeolites have been extensively used in heterogeneous catalysis, separation and purification, due to their proximate size with the involved guest molecules. With the development of modern industry, the market demands of propylene and other basic chemical raw materials are growing. Some conventional zeolites, e.g. ZSM-5, Beta, Y, have been unable to meet the requirements of Fluidized Catalytic Cracking(FCC) in heavy oil. Because of the diffusion limitation, the problem of catalyst deactivation and the decreasing of the conversion became more and more serious. However, the MCM-41, a typical mesoporous molecular sieve from M41 S series, can’t achieve a high conversion in many catalytic reactions because of the poor hydrothermal stability, amorphous wall with weak acid sites, insufficient catalytic activity, and low sharp selectivity. Creating mesopores in zeolites could modulate their catalytic selectivity, reduce the formation of coke, and enable the preparation of multifunctional catalysts. Hierarchical zeolites that contain both mesopores to allow for the rapid diffusion of bulky molecules and zeolitic micropores to provide catalytic activity and selectivity are therefore desirable. Based on previous works, we rationally designed and prepared two hierarchical zeolites named MLMFI and Meso-ZSM-5, and studied by means of experimental characterizations and computer simulation technology; The adsorption-diffusion behavior of the guest molecules in hierarchical zeolite was studied by using Zero Length Column(ZLC) technique, the effect of introduction of mesoporous on the mass transfer performance of microporous materials was also discussed; Also, we analyzed the difference of activities of hierarchical zeolites of different topological structures in the catalytic reactions; At the end of the study, the Catalytic Fast Pyrolysis(CFP) of furan on different zeolite catalysts were investigated, and discussed the influence of morphology and hierarchically structure on furan CFP process.Firstly, based on the previous works, we designed a kind of Bola form bifunctional surfactant with hydrophilic diquaternary ammonium group and hydrophobic long alkyl chain group which are connected with the phenol hydroxyl group. We successfully used it as the Structure Directing Agents(SDAs) in the hierarchical zeolite synthesis process, and analyzed the structural properties of the obtained material combining with a series of characterization methods, X-ray diffraction(XRD), N2 adsorption-desorption isotherm, scanning electron microscope(SEM) and transmission electron microscope(TEM), etc. The results showed that the MLMFI sample has the characteristics of the microporous zeolite and the mesoporous structure. Combined with the computer simulation technology, we studied the role of SDAs in the formation of hierarchically structure in zeolite. Compared with the conventional zeolite catalysts, the hierarchical zeolite sample has a very high activity, which is characterized by the condensation and esterification of n-butyl alcohols with benzaldehyde, the introducing of mesoporous improved the mass transfer in the pore system, which showed a very excellent catalytic activity and a longer catalytic lifetime. This provides a new way for the synthesis of ordered hierarchical porous materials by soft-template method, and it is expected to be applied and developed in the future.In this work, we report a facile method for direct, one-step synthesis of hierarchical zeolites that circumvents the need for secondary templating and yields a remarkably small diffusion length(ca. 25 nm) within sub-micrometer spherical zeolite particles. The structure of the resulting material was studied by combining with a series of characterization methods, X-ray diffraction(XRD), N2 adsorption-desorption isotherm, scanning electron microscope(SEM) and transmission electron microscope(TEM), etc. In this method, the mesoporosity in the ZSM-5 crystals derives from controlling the formation and template-free self-assembly of zeolite precursor nanoparticles, formed during the initial stage of the crystallization process. The largely reduced diffusion length endows the hierarchical ZSM-5 with superior catalytic activity for reactions involving bulky molecules. This attractive approach for synthesizing hierarchical ZSM-5 catalysts provides an alternative to more costly and complex ‘bottom-up’ and ‘top-down’ approaches, and holds potential for even broader applicability to a wide range of zeolites and even other porous materials.In this paper, based on the adsorption diffusion theory and zero length column(ZLC) apparatus, we successfully refitted and improved the ZLC device for the first time. The intracrystalline diffusion(Deff) of cyclohexane within different hierarchical zeolites, 800 nm MFI, 100 nm MFI, Meso MFI, 3DOm-i MFI, SPP MFI and MLMFI, were studied using ZLC technique. The obtained mass transport results were used to achieve the fundamental understanding of diffusion limitations in the hierarchical zeolites, and optimizing of the catalytic properties of the novel zeolitic materials. In addition, the activity and selectivity of catalyst were evaluated in aldol condensation reactions. It was concluded that the hierarchically structured zeolites combine the advantages of both mesoporous structure and microporous zeolite framework, providing reduced diffusion length and enhanced accessibilities to the active acid sites located within micropores.The catalytic performance of three-dimensionally ordered mesoporous imprinted(3DOm-i) zeolite catalysts with different frameworks(3DOm-i MFI, 3DOm-i BEA, and 3DOm-i LTA) were investigated by the liquid-phase catalytic conversion of benzyl alcohol in mesitylene and compared to that of other microporous and mesoporous catalysts with a high external surface area, which included MCM-22, 300 nm MFI, ITQ-2, and Al-MCM-41. The mesoporosity in MFI and BEA zeolites can effectively enhance the catalytic performance of the zeolite catalysts for benzyl alcohol self-etherification catalyzed by both the acid sites on the external and internal surface and the alkylation of mesitylene with benzyl alcohol catalyzed exclusively by the acid sites on the external surface. For 3DOm-i LTA, MCM-22, and Al-MCM-41, only the acid sites on the external surface can be utilized in the catalytic reactions. A distinct difference in the product selectivity was also observed for the microporous and mesoporous catalysts.ZSM-5 catalysts with different morphologies were synthesized and employed for the catalytic fast pyrolysis(CFP) of biomass-derived furan in a fixed bed reactor. The effects of mesoporosity and morphology of the ZSM-5 catalysts on the production of aromatics and olefins as well as catalyst deactivation were investigated in order to provide insights into the rational design of zeolite catalysts for the CFP of biomass. It was observed that mesoporosity in hierarchical ZSM-5 and small crystallite size not only benefit the furan conversion but also change the product selectivities due to reduced diffusion limitation. Significantly improved selectivities to benzene, toluene and xylene(BTX) and olefins was achieved over mesoporous ZSM-5 and 100 nm ZSM-5 samples. The coke formation on the zeolite catalysts over the CFP process was also largely reduced by introducing mesoporosity. It was observed that the coke is mainly formed and accumulated inside the micropores of the ZSM-5 catalyst rather than on the external surface or within the mesopores. Mass transport study on the coked zeolite samples suggested that during the catalyst deactivation process the coke instantly blocks the pore like a solid plug leading to the decrease in micropore volume but the same pore diameter.