Syntheses And Catalytical Activities Of Beta Zeolites

This dissertation focuses on the synthesis, characterizations and catalytic properties of hierarchical Beta zeolites, as well as improving the utilization rate of the silica in organic template free synthetic system of Beta zeolite.In the first part, we added sodium phosphate monobasic (NaH2PO4) to synthetic mixture, which led to one-step synthesis of mesoporous Beta zeolite. The aggregation of nanosized Beta zeolites overcame the filtration difficulties, while the intercrystalline mesopores reduced the diffusion limitation and thus enhanced the catalytic activity. The obtained zeolite Beta samples were characterized by X-ray diffraction, scanning electron microscopy, N2 adsorption-desorption and temperature-programmed desoprtion of ammonia. It was shown that the amount of phosphate strongly influenced the mesopore volume of Beta zeolites. When the molar ratio of NaH2PO4/SiO2 reached 0.1, the sample with good textural and acidic properties exhibited the best catalytic activity in the alkylation of phenol with tert-butanol.In the second part, zeolite Beta aggregates have been synthesized by the self-assembly of Beta nanoparticles without the use of second template or additive, where the large zeolitic aggregates avoided the filtration difficulties and the presence of mesopore reduced the diffusion limitation. The samples were characterized by XRD, N2-sorption, SEM, TEM, thermogravimetric analysis, IR spectroscopy and temperature programmed desorption of ammonia. The results showed that the SiO2/Al2O3 ratio played a key role on the particle size, mesopore volume and particle-particle aggregation process of zeolite Beta. The sample with SiO2/Al2O3 ratio below 60 had improved textual and acidic properties and exhibited the best catalytic activity to the alkylation of phenol with tert-butanol.In the third part, we focus on improving the utilization of the silica in organic template free synthetic system of Beta zeolite. In this synthetic system, large NaOH/SiO2 molar ratio made 70% of silica dissolve in the mother liquid, and MOR and ZSM-5 were liable to form. In order to inhibit the generation of impure crystals, the ageing temperature was lowered to 120℃, more favorable for Beta zeolite nucleation. In order to increase the utilization rate of the silica, the amount of sodium hydroxide was decreased and the mother liquid of zeolite synthesis was recovered and reused in the synthess. However, only recovery and reusing of mother liquid could improve the utilization rate of silica, up to 80%.