Urea-assisted morphological engineering of MFI nanosheets with tunable b-thickness

Engineering of crystal morphology affects the catalytic and adsorption properties of zeolitic materials. Considering the anisotropic diffusion of molecules derived from its topological features, MFI zeolite nanosheets with short b-axis thickness are highly desired materials to reduce diffusion resistance. However, the design and development of eco-friendly synthesis protocols with reasonable cost and high efficiency remain elusive. Herein, we reported a systematic study on the synthesis of MFI nanosheets using urea as an additive. Both silicalite-1 and ZSM-5 zeolites (MFI type framework structure) with controllable b-thicknesses ranging from 50–200 nm were achieved by optimizing the synthetic parameters including water content, urea and SDA concentrations. The concentration of hydroxide anions was found to dominate the crystallization kinetics compared with the counterpart tetrapropylammonium cations (TPA⁺). To facilitate the crystal growth of MFI zeolites in the presence of urea, the ratio OH⁻/SiO₂ has to be higher than 0.2, independent of the TPA⁺ concentration. The role of urea in the assistance of plate-like crystal formation through the inhibition of (010) facet growth was revealed by electron microscopy and infrared (IR) spectroscopy analyses. The developed strategy for morphological engineering is not limited to the MFI-type zeolite and can be applied to other frameworks depending on the intrinsic properties of additive molecules and the interactions between them.