Seed-assisted synthesis of ZSM-5 nanozeolites with high catalytic activities for cracking oleic acid to light olefins and light aromatics

Abstract

A series of nanosized ZSM-5 (SNZSM-5) zeolites has been synthesized by the seed-assisted method using silicalite-1 as seeds. Catalytic cracking of oleic acid to produce light olefins and light aromatics using SNZSM-5 as the catalyst was performed using a laboratory-scale fixed-bed reactor. The effects of the crystallization time and Si/Al molar ratio on the morphology and multilamellar structure of SNZSM-5 were studied to reveal how these properties affect the catalytic performance. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, temperature-programmed desorption of ammonia, and N₂ physisorption and desorption were performed to characterize the morphological features, acidities, and pore structures of the SNZSM-5 nanozeolites. Compared with conventional ZSM-5 zeolite, the SNZSM-5 nanozeolites possessed smaller particle diameters (90–450 nm), and they thus contained more intercrystalline voids. The Brunauer–Emmett–Teller surface areas (211–309 m² g⁻¹) and mesopore volumes (0.08–0.27 cm³ g⁻¹) were both higher than those of ZSM-5 zeolite. Moreover, owing to the larger specific surface areas of the nanocrystals, more acidic sites can be exposed. Therefore, the SNZSM-5 nanozeolites showed higher catalytic activity and stability than conventional ZSM-5. The catalytic activity could be sustained for up to 35 h. Under the conditions of a reaction temperature of 500 °C, an oleic acid flow rate of 0.04 mL min⁻¹, and a nitrogen flow rate of 30 mL min⁻¹, the SNZSM-5 nanozeolite prepared with a Si/Al molar ratio of 16 showed the highest catalytic activity, producing up to 67 % yield of light olefins and showing 14.5 % selectivity for light aromatics.