Controllable and SDA-free synthesis of sub-micrometer sized zeolite ZSM-5. Part 1: Influence of alkalinity on the structural, particulate and chemical properties of the products

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 483 K for 2 h) of the TPA-free reaction mixture: 1.0Al2O3/100SiO2/xNa2O/4000H2O seeded by silicalite-1 nanocrystals (260 nm, 4 wt.% 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 260 nm was used as seed, fully crystalline product (silicalite-1/ZSM-5) having the size in the range of 400–600 nm and Si/Al ratio of 10–18 can be obtained for 0.006 ⩽ A ⩽ 0.01, in less than 2 h. 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.

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► Low alkalinity, amorphous impurity, high Si/Al ratio with irregular particles of ZSM-5 zeolites.
► Medium alkalinity, pure ZSM-5 phase, decreasing Si/Al ratio with cubic particles.
► High alkalinity, phillipsite and amorphous impurity, low Si/Al ratio with spherical and irregular particles.