Formulation of zeolite-mesoporous silica composite catalysts for light olefin production from catalytic cracking

Framework materials such as zeolites and mesoporous silicas are commonly used for many applications, especially catalysis and separation. Here zeolite-mesoporous silica composite catalysts (employing zeolite Y, ZSM-5, KIT-6, SBA-15 and MCM-41 mesoporous silica) were prepared (with different weight percent of zeolite Y and ZSM-5) and assessed for catalytic cracking (using n-heptane, as the model compound at 550 °C) with the aim to improve the selectivity/yield of light olefins of ethylene and propylene from n-heptane. Physicochemical properties of the parent zeolites and the prepared composites were characterized comprehensively using several techniques including X-ray diffraction, nitrogen physisorption, scanning electron microscopy, fourier transform infrared spectroscopy, pulsed-field gradient nuclear magnetic resonance and thermogravimetric analysis. Catalytic cracking results showed that the ZY/ZSM-5/KIT-6 composite (20:20:60 wt %) achieved a high n-heptane conversion of 85% with approximately 6% selectivity to ethylene/propylene. In contrast, the ZY/ZSM-5/SBA-15 composite achieved a higher conversion of 95% and an ethylene/propylene ratio of 8%, indicating a more efficient process in terms of both conversion and selectivity. Magnetic resonance relaxation analysis of the ZY/ZSM-5/KIT-6 (20:20:60) catalyst confirmed a micro-mesoporous environment that influences n-heptane diffusion and mass transfer. As zeolite Y and ZSM-5 have micropores, n-heptane can move and undergo hydrogen transfer reactions, whereas KIT-6 has mesopores that facilitate n-heptane’s accessibility to the active sites of zeolite Y and ZSM-5.