High-Quality Single-Crystalline MFI-Type Nanozeolites: A Facile Synthetic Strategy and MTP Catalytic Studies

A facile strategy affording high-quality single-crystalline MFI-type nanozeolites (10-55 nm) with hexagonal prism morphology, good monodispersity, high crystallinity, and high product yield (above 97%) has been developed. This is achieved by synergistically using an l-lysine-assisted approach and a two-step crystallization process in a concentrated gel system (H₂O/Si = 9). The morphological evolution of nanosized silicalite-1 is monitored by high-resolution transmission electron microscopy (HRTEM). In this process, metastable irregular nanoparticles are initially obtained at 80 °C as the first step. Consequently, a rearrangement in morphology toward equilibrium crystal shape and without excessive growth for the metastable nanoparticles occurs at 170 °C as the second step. Throughout the whole process, l-lysine acts as an inhibitor to effectively limit the crystal growth of zeolites. Thanks to the high-quality nanosized crystals, the as-prepared ZSM-5 catalysts exhibit superior performance in methanol-to-propylene (MTP) reactions, which deliver a prolonged lifetime of 54 h with a total light olefin selectivity of 74% and a high propylene selectivity of 49% at 470 °C at a high methanol weight hourly space velocity (WHSV) of 7.2 h^-1. This synthetic route provides a general strategy for preparing other types of zeolites with good monodispersity, nanosize, high yield, and high crystallinity.