Generating Self-Assembled ZSM-5 Nanozeolite from Natural Diatomite to Promote Propylene Production in Catalytic Cracking of Plastic Pyrolysis Oil

Nanozeolite aggregates (NZAs) have emerged as a class of promising catalysts for waste plastic recycling and industrial applications, owing to the coexistence of nanozeolite domains for short diffusion lengths and micron-scale bulk size for the ease of industrial processing. However, it is challenging to synthesize NZAs in a facile and green routine. Conventional hydrothermal routes often rely on commercial silicon reagents and costly secondary templates to create mesopores, rendering them difficult for practical applications. Herein, a facile approach for NZAs synthesis is developed using natural silicon-rich diatomite as the sole silicon source, and a continuous three-step process on diatomite surface is streamlined, including (1) Si dissolution, (2) surface-involved nucleation, and (3) Na+-assisted rapid in situ crystallization. As characterized by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), Brunauer–Emmett–Teller (BET), Hg intrusion, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and electron probe microanalysis (EPMA), the obtained NZAs exhibit micron-scale particle sizes (1–5 μm) with hierarchically porous structures (SBET = 336 m2/g). The surfaces of the obtained NZAs are fully covered with self-assembled ZSM-5 nanocrystals, meanwhile mesopores that run through the nanocrystal domains are spontaneously created without the need for additional secondary templates. Compared to commercial microsized zeolites with higher crystallinity, NZAs exhibit better activity and higher propylene yields in catalytic cracking of n-octane and polypropylene pyrolysis oil. The enhanced performances may be linked to the existence of highly accessible acid sites in NZAs, which help suppress secondary reactions of propylene. This work offers an economically viable and environmentally benign for novel NZA catalyst synthesis aiming at circular economy.