Preparation,Characterization And Performance Of Mesoporous Zeolite

Zeolite molecular sieves are crystalline aluminosilicate materials with large surface areas, intricate pore and channel systems, tunable surface property, strong acidic property, well thermal stability and hydrothermal stability, immensely used in catalysis, separations, and ion exchange industry. But the innate small micropores (usually 0.3-0.7nm) of zeolite strongly limit the diffusion of reagents and reaction products, especially once the adsorption or reaction process refers to relative large molecules. The limited pore size of zeolite imposes large diffusion and spatial resistance, greatly reduces the accessibility of active sites, resulting in the maximal availiablity of zeolite difficult to be implemented in industry.Therefore, it’s of great importance to increase the transport efficiency and the availability of active sites in zeolites. Hierarchical zeolite, including mesoporous zeolite and nanosized zeolite, can be used to solve the problems of zeolite. Hierarchical zeolite, not only retains the microporous properties of zeolite, but also introduces mesopores or macropores in the materials, which largely reduce the diffusion residence, shorten the diffusion path, speed up the diffusion of molecular, enhance the accessibility and availability of active sites. The appearance of hierarchical zeolite widely broadens the field of application.Herein we show a generic route for preparing mesoporous zeolites based on the bond blocking effect of hydrophobic group linked on zeolite framework, which blocks the normal growth of zeolite crystal. Mesoporous zeolite A and ZSM-5 was synthesized through this bond blocking effect. 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, thus the design and control synthesis of mesoporous zeolite were achieved.Mesoporous zeolite A was synthesized from three different silica source organic functioned by different organosilane. The experimental results show that the mesoporous size depends on the dimension of defect sites which are determined by the hydrophobic moiety of organosilane. More organosilane introduced into the synthesis system results in larger mesoporous volumes.Mesoporous zeolite A synthesized from silica source organic functioned by phenylaminopropyl-trimethoxysilane(Y-5669), which has a nano-cage with a neck diameter of about 0.79-1.2nm and the body size centered at about 3nm, owns high Hierarchical Factor value (0.16), indicating that mesopore was created without severe penalization of the microporous system.The research results indicate that this synthesis method is based on bond blocking effect. During organic function of fumed silica, the hydrophilic moiety of organosilane is hydrolyzed into hydroxyl and dehydrated condensation with the hydroxyl on the surface of fumed silica, then the hydrophobic moiety of organosilane is linked on the surface of fumed silica through Si-C covalent bond which is stable enough under synthesis condition. During the synthesis process, the fumed silica enters into the zeolite’s framework 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 bond. Existence of the Si-C covalent bond blocks the growth of zeolite crystal in that direction. Thus the defects are generated inside zeolite crystal, and after calcination, these defects are turned into meso-channels and intracrystalline nano-cages.Mg2+ is hard exchanged by traditional zeolite A because of large hydrate ionic radius of Mg2+. Owing to the existence of mesopores, mesoporous zeolite A largely enhances the Mg2+ exchange rate. t50 and t100 were greatly shortened along with the increase of mesoporous volume of the mesoporous zeolite A, while D/r2 showed a exponential growth along with the increase of mesoporous volume. The D/r2 of hydrated Mg2+ in the synthesized mesoporous zeolite is 170 times higher than in traditional zeolite at 308K. The rapid exchange rate of Mg2+ make the mesoporous zeolite A can adsorb more Mg2+ in the Ca-Mg coexistence system, indicating that this is a perfect material for softening hard water and an ideal agent for detergent additive.The same as synthesis principle of mesoporous zeolite A, mesoporous zeolite ZSM-5 microspheres was synthesized using organic functioned fumed silica as silica source and tetrapropylammonium bromine(TPABr) as microporous SDA.The ZSM-5 microspheres are consisted by nanosized zeolite particles and there are intracrystalline mesopores inside the nanosized particles. Thus 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 inside the nano-crystal.The organic function of Si source and the existence of NaBr in the synthesis system are two preconditions for formation of ZSM-5 microspheres with intracrystalline mesopores. During the synthesis process, the organic moiety linked on the surface of fumed silica integrated into the zeolite framework through Si-C covalent bond, which blocks the growth of zeolite crystal, thus formation the nanosized zeolite crystal and intracrystalline defects sites inside the nano-crystal. Secondly, the NaBr in the synthesis system aggregates the nanosized zeolite crystal into sphere-like shape, forming 4-6μm microspheres. After calcination, the defects sites inside the nano-crystal turned into intracrystalline mesopores, and the cavity among the nano-crystal was intercrystalline mesopores.The zeolite ZSM-5 microsphere integrates the advantages of nanosized zeolite with that of mesoporous zeolite, offers a shorter diffusion path, makes the active sites more accessible and available. For HZSM-5(0), only 5% of Br(?)nsted acidic sites can be achieved by 2,6-dimethylpyridine (DMPy) with kinetic diameter of 0.67nm, whereas almost all Br(?)nsted acidic sites can be achieved by 2,6-dimethylpyridine for mesoporous HZSM-5(5) microsphere.The zeolite ZSM-5 microsphere shows enhanced catalytic effect. In benzylation of benzene by benzyl alcohol (BA), BA conversion was ten times higher for HZSM-5(5) than HZSM-5(0) after a reaction time of 10 hours; In cumene cracking reaction, the conversion of cumene almost keeps constant for 4 days on HZSM-5(5) sample whereas the conversion of cumene on HZSM-5(0) decreased to lower than 50% after 2.5 hours; In the convention of methanol into gasoline reaction, the methanol conversion on HZSM-5(0) declines to 60% within 10 hours, whereas HZSM-5(5) keeps high methanol conversion (above 90%) for more than 50 hours and the HZSM-5(5) shows a high selectivity to the carbohydrate with long carbon chains.The high catalytic performance of HZSM-5(5) microsphere can be explained as follows:the ZSM-5 microsphere exhibits a fast diffusion rate compared to solely microporous zeolites and the hierarchical pore system makes the reactants access to acid sites easy.The existence of mesopores could accelerate the elution of cracked products from the catalysts. Thus the catalyst life was prolonged for ZSM-5 microsphere samples.