Synthesis of ultra-high-silica ZSM-5 zeolites with tunable crystal sizes

Using silicalite-1 suspension as seed crystals and a small amount of tetraallkylammonium bromide as additional structure-directing agent, highly crystallized ZSM-5 with SiO ₂/Al ₂O ₃ ratios higher than 100 was obtained. The crystal sizes could be easily tuned from tens of nanometers to about several micronmeters by merely adjusting the amount of the seeding suspension. Seeding suspension synthesized from different silica sources and the additional structure-directing agent may also affect the crystal sizes. The synthesized samples were characterized by powder X-ray diffraction, scanning electron microscopy, N ₂ adsorption-desorption and inductively coupled plasma. When tetraethylammonuim bromide was used as additional structure-directing agent, crystals with silica-rich cores were observed, while additional nucleation could be found when tetrapropylammonium bromide was used as auxiliary template.