Synthesis And Characterization Of Mesoporous LTA Zeolite

In recent years, synthesis of mesoporous zeolites with mesoporous-microporous hierarchical structures has been the hot topic of research due to the combination of the advantages of the high acidity and hydrothermal stability of the crystalline zeolites and the facile diffusion of bulky molecules in a mesoporous material. The generation of mesopores in zeolite crystals offers the possibility of reducing the intracrystalline diffusion path lengths of reagents and reaction products to overcome the diffusion limitation of zeolites (03-1.5nm in pore diameter) on the reaction rate and improving product selectivity. Mesoporous zeolites have attracted much attention due to improved mass transportation rate and catalytic performance and have been widely accepted as a kind of catalyzed material with potential industrial application prospect.Mesoporous zeolites with highly crystalline zeolite walls have been successfully synthesized by hard or soft templating methods. However, the resultant mesopores are usually too large (>10nm) and the corresponding pore size distributions are too broad (>10nm widths at half maximum) for conventional mesoporous zeolites, so that the tunability of the mesopore structure is limited and the high acidity and hydrothermal stability is insufficiency.Herein, mesoporous LTA zeolite with small and uniform intracrystal mesopores and narrow pore size distributions was hydrothermally synthesized with the bridged silsesquioxane as the mesoporous structure-directing agent, with the aim of solving the technique problem for conventional mesoporous zeolites that acidity and hydrothermal stability is insufficiency because of too large mesopores diameters and too broad pore size distributions. The mesoporous LTA zeolite was characterized by XRD, N2adsorption, SEM, and TEM characterization techniques, and the results were given in following.(1) Obviously, the wide-angle XRD shows a gradual decrease in the diffraction peak intensities and a gradual increase in the diffraction peak widths as the bridged silsesquioxane quantity increased, indicating that the size of the crystalline domain decreased because of the bridged silsesquioxane addition. Moreover, the samples exhibit well-defined small-angle XRD peaks at2θ=1.3°and a progressive increase in intensity of the XRD peaks as the amount of the bridged silsesquioxane increases. These changes well support the formation of the mesoporous structure and the outstanding mesostructure regularity.(2) The samples all show the type-IV N2adsorption-desorption isotherm, which is characteristic for mesoporous materials. Correspondingly, the pore size analysis by the Barret-Joyner-Halenda (BJH) algorithm for all samples give a narrow pore size distribution centered at around2.0nm with the peak width at half maximum being1.5nm, confirming the presence of uniform intracrystal mesopores in samples. The mesopores diameters are approximately consistent with the molecular size of the bridged silsesquioxane, confirming that the uniform intracrystal mesopores are structured by the bridged silsesquioxane moleculars instead of polymer or micelles. It is worth noting that the samples show a progressive increase in mesopore volume and also BET special surface area as the amount of the mesoporogen increases. The mesopore volume and BET special surface area are increased to0.162cm3g-1and257.498m2g-1, respectively.(3) Outstandingly, from the low-resolution SEM images of the mesoporous LTA zeolite, it can be observed that uniform spherical particles with a rough surface and the particle size distribution is in the same range of700-800nm. Furthermore, SEM images in high magnification clearly reveal the presence of mesopores in the mesoporous LTA zeolite particles and partial connections between these intracrystal mesopores.(4) The high-resolution TEM images reveal that the mesoporous LTA zeolite particles have a highly mesoporous structure, and these partially interconnected mesopores are distributed throughout the particles in a disordered way. In addition, it is noted that the intracrystal mesopores are possessed of nearly the same size around2nm, which is in good agreement with the relatively well-defined small-angle XRD peaks and the narrow pore size distribution determined by N2adsorption.In conclusion, the mesoporous LTA zeolite with small and uniform intracrystal mesopores (average pore size around2.0nm) and narrow pore size distributions (ca.1.5nm width at half maximum) has been synthesized using the bridged silsesquioxane as a mesoporous structure-directing agent. On the basis of the attractive structural properties, such as large BET special surface area, large mesopore volume, uniform interconnected intracrystal mesopores and narrow pore size distributions, such mesoporous LTA zeolite may find potential applications as advanced materials in various fields such as catalysis, adsorption and separation.