Effects of Zeolite Type and Acidic Properties on the Catalytic Cracking Performance of Dodecane
Catalytic cracking of gasoline and diesel to light olefins is a pivotal route for high-value utilization of surplus fuels, typically employing zeolite catalysts. This study systematically investigates the effects of zeolite type and acidic properties on the catalytic cracking of dodecane, a diesel model compound, using SAPO-34, ZSM-5 with SiO2/Al2O3 ratios of 38, 85, and 200, and USY. Catalysts were characterized by XRD, SEM, N2 physisorption, NH3-TPD, and pyridine-FTIR, and evaluated in a fixed-bed reactor. Results demonstrate that zeolite type is the primary determinant of conversion and product distribution. SAPO-34, with 0.38 nm pores, achieved only 24.33% conversion and negligible BTX yield, with severe external coking. ZSM-5-38 and USY, with larger pores, achieved near-complete conversion; however, ZSM-5-38, possessing moderate acidity, yielded the highest light olefins (18.40%) and minimal coke (0.18%), while USY, with higher acidity, promoted hydrogen transfer and coking (12.90% coke). Within ZSM-5 series, lower acid site density (ZSM-5-200) proved optimal, achieving 97.79% conversion and a total light olefin yield of 41.93% (ethylene 11.11%, propylene 20.33%, butenes 10.49%) with low coke (0.43%). The study proposes reaction pathways and regulatory mechanisms, highlighting that zeolite type and acidity govern the relative rates of cracking, hydrogen transfer, oligomerization, aromatization, and coking, thereby dictating performance. These findings provide a rational basis for optimizing zeolite catalysts in commercial gasoline/diesel cracking processes.