Fast synthesis and morphology control of silicalite-1 in the presence of polyvinyl alcohol

Silicalite-1 crystals with various morphologies were prepared by addition of polyvinyl alcohol (PVA) in silicalite-1 synthesis solution at 180 °C for 6-40 h. The samples were characterized by X-ray diffraction, scanning electron microscopy, FT-IR, thermogravimetric analysis and N₂ sorption. PVA in the synthesis solution promoted the fast crystallization of silicalite-1 within 6 h due to its adsorption on the surface of silicalite-1 precursors and the hydrogen bonding effect. The addition of PVA would produce silicalite-1 without aggregation because PVA served as temporary physical barriers. However, more PVA in the system (weight ratio of PVA/SiO₂ = 1.75) slowed down the nucleation rate of silicalite-1 as the high viscosity of PAV hindered the selectivity between PVA and crystal facet. PVA also changed the surface energy of silicalite-1 precursors, and resulted in silicalite-1 crystals with sizes from 6 × 2 × 0.6 to 15 × 10 × 3 μm³. Silicalite-1 crystals exhibited coffin-, plate-, and rod-like structure when different weight ratios of PVA/SiO₂ were applied, and the silicalite-1 crystals had high BET surface areas of 485-513 m²/g.