Fabrication of SiO2@silicalite-1 and its use as a catalyst support

SiO2@silicalite-1, using silica sol (pH = 9.47, SiO2 approximate to 30 wt%) as the silica source, was directly synthesized in a eutectic mixture where silicalite-1 grains were formed in the three-dimensional net structure of silica gel, grown in situ by transforming amorphous SiO2 into an MFI-type structure and coated with amorphous SiO2. The alkalinity, template agent, and crystallization time strongly affect the physicochemical properties of SiO2@silicalite-1. The physicochemical properties of these samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectra and nitrogen adsorption. The results show that the SiO2@silicalite-1 is synthesized in a eutectic mixture and its physicochemical properties can be tuned by controlling the content of sodium hydroxide and tetrapropyl ammonium bromide (TPABr). A time-dependent study reveals that the formation process obeys an in situ epitaxial growth and phase transformation mechanism. Finally, SiO2@silicalite-1 was used as the support to prepare TiO2-loaded SiO2@silicalite-1 (TiO2@SiO2@silicalite-1). After five loading procedures, it could load 0.44% TiO2 nanoparticles, which is higher than the TiO2 nanoparticle loading in traditional silicalite-1 (0.13%). When the catalysts were used as a catalyst for the degradation of rhodamine B (RhB) aqueous solution under UV light, the photocatalytic efficiency of TiO2@SiO2@silicalite-1 (89.2%) is higher than TiO2@silicalite-1 (only 34.6%). The rate of degradation using TiO2@SiO2@silicalite-1 is 4.3 times faster than that using TiO2@silicalite-1. Furthermore, SiO2@TiO2@silicalite-1 exhibits high stability of photocatalytic performance. After five repeated cycles, the photocatalytic efficiency of TiO2@SiO2@silicalite-1 is 88.09%, which reduces only by 1.1%.