Search

Abstract: In this work, the influence of alkalinity, A =[Na2O/H2O]b (b=batch), of the reaction mixture on structural, particulate, morphological and chemical properties of the crystalline end products obtained by hydrothermal treatment (heating at 483K for 2h) of the TPA-free reaction mixture: 1.0Al2O3/100SiO2/xNa2O/4000H2O seeded by silicalite-1 nanocrystals (260nm, 4wt.% of silica in gel mixture), was investigated by different characterization methods such as powder X-ray diffraction (XRD), scanning-electron microscopy (SEM), particle size distribution (PSD) measuring by laser light scattering (LLS) and X-ray fluorescence (XRF). The obtained results showed that, when silicalite-1 having the size of 260nm was used as seed, fully crystalline product (silicalite-1/ZSM-5) having the size in the range of 400–600nm and Si/Al ratio of 10–18 can be obtained for 0.006⩽ A ⩽0.01, in less than 2h. However, the product obtained at low (A ⩽0.005) or high (A ⩾0.011) alkalinities possess amorphous and/or phillipsite impurities, respectively. The influence of alkalinity, A, on the mentioned properties of the crystalline end product was discussed in relation to the influence of alkalinity on the relevant critical process of zeolite ZSM-crystallization in the absence of organic templates. [ABSTRACT FROM AUTHOR]

Abstract: A method based on seed surface crystallization (SSC) mechanism has been proposed for facile, rapid, controllable and low-cost synthesis of ZSM-5 zeolite with submicron-size and interesting core–shell structure. The crystallization process of the ZSM-5 zeolite with all sizes can be accomplished within 2.5h. The morphology and size of zeolite ZSM-5 products can both be facilely adjustable in the domain of 270–1100nm depending upon the characters of silicalite-1 seeds, while the thickness of aluminum-containing zeolite ZSM-5 shells can be well tuned by changing the amount of silicalite-1 seed. More interestingly, the siliceous silicalite-1 seeds within the obtained zeolite crystals could be selectively removed by alkaline treatment. By systematically tracking the crystallization process combined with mathematical analysis, the crystallization of ZSM-5 zeolites in the reaction system is revealed as SSC mechanism with a size-independent, linear growth process. Their crystal growth rate is calculated as c.a. 0.15μmh−1 regardless of the seed size and amount. [Copyright &y& Elsevier]