Fabrication Of Nanocrystalline ZSM-5Zelites And The Study Of Their Catalytic Performances In The Alkylation Of Benzen/ethylene

Catalyst materials of ZSM-5zeolites have a wide range ofapplications in the petrochemical and fine chemical industry becauseof its unique pore structure. However, the sole presence of microsporeslimits the diffusion of organic macromolecules in the zeolites. Comparedwith the traditional zeolites, nanocrystalline ZSM-5zeolites have manyadvantages, such as large surface area, strong surface acidity, andshort channel. These properties are beneficial to the catalytic reactions.Therefore, the nano-ZSM-5zeolites have a broad prospect in theindustrial applications and the synthesis of nano-zeolites has arousedthe great concern. In view of this, nano-ZSM-5zeolites prepared bypre-crystallization method are chosen for this thesis. The phase structure,morphology and pore structure of the zeolites were studied by meansof XRD, FTIR, SEM, TEM, Py-FTIR, BET, and the performances of thecatalysts have been investigated in the alkylation of benzene andethylene.In this thesis, nano-ZSM-5zeolites were synthesized bypre-crystallization method. The molar ratio of silica to alumina whichaffects the morphology and structure of the zeolites was studied. Thesynthesized molecular sieve is tablet-like crystals with the diameter of~400nm, the thickness of~90nm, and a high degree of crystallinity. Thetablet-like zeolites include lots of nanoparticles with the size of~20nm.The nanoparticles decreased with the increase of Si/Al ratio. In thecalcining process, the smaller nanoparticles are easier to berecrystallized to single crystal. So the crystal of high Si/Al ratio zeolite issingle. The nitrogen adsorption-desorption isotherms are type IVisotherms with H3hysteresis loops. The porous structures deposited bynanoparticles is hierarchical, the supermicropores (1.4nm and1.8nm)and mesopores (2.6nm). The factors, crystallization time and the amount of the organosilane, were also investigated. The results showthat the sample crystallized for5days possesses uniform crystal size,regular morphology and does not contain SiO2. With the increase oforganosilane’s amount, more crystals are linked together, reducing thezeolites surface area. So it is appropriate to add the organosilane equalto5%of the SiO2content in the gel.On the basis of the synthesis of high Si/Al ratio nano-ZSM-5zeolites,the hollow zeolites with~15nm wall thickness were prepared by alkalinetreatment with Na2CO3solution. Because the negative chargeassociated with AlO4-tetrahedra in the zeolite framework hinders theextraction of Si through hydrolysis of Si–O–Al bonds by negativelycharged hydroxyl groups and the exterior of the crystals is rich in Al andthe interior is poor in Al, the outer shell of the zeolite is more inert toalkaline desilication than the interior. However, the crystallinity of thehollow nano-ZSM-5zeolites is lower.In Chapter5, the performances of all the as-synthesized zeoliteshad been investigated in the alkylation of benzene and ethylene. Thesample with Si/Al ratio60has higher crystallinity, larger surface area,more acid sites, so the catalyst shows better catalytic activity, the singlepass conversion of ethylene65.8%and the selectivity of ethyl98.7%. Thecrystallinity of the hollow nano-ZSM-5zeolites is low, and the zeolitestructure was slightly damaged by alkali solution, so the hollownano-ZSM-5zeolite catalyst shows the worst performance, the singlepass conversion of ethylene is only40.3%.