Synthesis, Characterization And Performance Of Nanozeolite Beta Self-aggregates With Intracrystal Mesoporous

Zeolite Beta is the only high silica zeolite with a three dimensional net work of 12 membered ring pores channel system. Owing to the unique topology BEA, it exhibits sigular properties in terms of the hydrothermal stability, molecular shape selectivity, stabilized active, anti-coking, and hydrophobicity. Compared with zeolite Y, Zeolite Beta has greater range ratios of Si/AI which can be easily modulated from a few dozen to pure silica, resulted in well hydrothermal stability and adjustable acid. Compared with ZSM-5, it consists two different types of channels and larger pores that can give some advantages in the diffusion of reagents and reaction products, especially in some huge molecules.Therefore, it should be noted that zeolite Beta has a more promising future in catalysis, adsorption and separation, ion exchange areas.However, like other microporous zeolite catalyst, zeolite Beta application are also limited by its innate small micropores,thus seriously affecting the catalytic activity and selectivity.Hierarchical zeolite, retains the microporous properties of zeolite, as well as introduces the supermicropores, mesopores or macropores in the materials, which largely shorten the diffusion path, reduce the diffusion residence, speed up the diffusion of molecular, enhance the accessibility and availability of active sites. The appearance of hierarchical zeolite widely broadens the applied scope of zeolite.Hence we show a generic route for preparing mesoporous zeolite Beta in hydrothermal system based on silanizing of silica source in advance, introducing hydrophobic group into zeolite framework. This synthesis method is simple and easy to control. The sizes and the volumes of mesopores can be controlled by choosing different kinds of organosilane and adding different amount of organosilane.In this paper, detailed examination of the impact of product factors were studied and determine preparing nanozeolite Beta self-aggregates with intracrystal mesoporous optimum conditions:Y-5669 as organosilane;Y-5669 /SiO2 (mol%)=0.15 obtained organic functionalized SiO2 as the silicon source; first dissolved aluminum dissolution and then addition of silicon, synthesis mixture with TEA+/SiO2=0.16;H2O/SiO2=14;Na2O/SiO2=0.039; olding time was 40℃; crystallization temperature was 140℃, crystallization time longer than 16 days;The material is found a nearly spherical self-aggregates formed 1.5-3μm in size. The self-aggregates are consisted by nanosized zeolite particles and there are intracrystalline mesopores inside the 60-120nm nanoparticles. Hierarchical pore system exists in these materials:The first one is the innate micropore of zeolite ZSM-5, the second one is the intercrystalline mesopores formed owing to the aggregation of nano-crystal, and the third one is the intracrystalline mesopores (about 2-3nm) inside the nano-crystal. Meanwhile, owns high Hierarchical Factor value (0.166), indicating that mesopore was created without severe penalization of the microporous system.The introduction of the mesopores and the nanoparticles creats a large number of vacancies which results in Si-O-Al, and Si-O-Si bond rupturing, thereby affecting the change of the surface acidity. Compared with conventional zeolites, Br(?)nsted acid changes were not significant, but the amount of Lewis acid has greatly improved. Bronsted acid accessibility of 2,6-ditertbutylpyridine (0.79nm) increased from 50% to 100%.As a solid acid catalyst in benzylation of aromatic hydrocarbon and transeterification, the product exhibits excellent catalytic performance. The reason is mainly attributed to two factors:On one hand, it exhibits a higher accessibility which makes huge molecules participate in the reaction and makes more acidic centers involved.On the other hand, the secondary pores increase in the rate intracrystalline diffusion, thereby avoiding secondary reactions and coke deactivation,at last, extending the life of the zeolite catalyst.Recently, our group have demonstrated a synthesis method using organosilane to create intracrystal mesopores by bond blocking effects. The key of the method is fumed silica Silylated by organosilane, the hydrophobic moiety of organosilane is linked on the surface of fumed silica through Si-C covalent bond. During synthesis process, the silica enters into the framework of zeolite through covalent bonds of Si-O-Si or Si-O-Al, whereas the hydrophobic moiety still links with the Si atoms through Si-C covalent bonds. The Si-C covalent bonds hinder the growth of zeolite crystal in the corresponding direction. Thus crystal defects in the zeolite are generated, and after calcination, these defects turn into mesopores.