Preparation of Hierarchically Trimodal-Porous ZSM-5 Composites through Steam-assisted Conversion of Macroporous Aluminosilica Gel with Two Different Quaternary Ammonium Hydroxides

The hierarchically structured ZSM-5 composites (HZC) with trimodal pores (macropores/mesopores/micropore) were synthesized by transforming the skeleton of the macroporous aluminosilica gel into ZSM-5 pure phase crystals through steam-assisted conversion method. The monolithic aluminosilica gel was used not only as the silicon and aluminum sources, but also as a structural support. The synthesis parameters, such as the composition of the precursor solution, the Si/Al feed ratios of the support gel, the time for impregnation of the support with the synthesis mixture and the structure-directing agents (SDAs) with different crystallization temperatures, were investigated. The textural properties of the resulting samples were characterized by means of the Fourier transform infrared (FT-IR) spectroscopy, X-ray powder diffractometry (XRD), scanning electron microscopy (SEM) and N-2 adsorption-desorption isotherms. The experimental results indicated that the HZC could be manufactured with quaternary ammonium hydroxide as SDA without seeding and additional sources of silicon or aluminum, the Si/Al feed ratio of the support and pretreatment time had a significant effect on the morphology of the products using tetrapropyl ammonium hydroxide (TPAOH) as SDA. Furthermore, the textural properties varied from the macroporous aluminosilica gel when different SDAs at different crystallization temperatures were also discussed.