Tuning the Concentrations of Acid Sites on ZSM-5 Zeolite for Improving Light Olefin Production in Catalytic Pyrolysis of Paraffin

The influences of different B (Brønsted) acid site concentrations and the ratio of L (Lewis)/B (Brønsted) on the cracking pathways and product distributions in hexane cracking reactions as a model component of naphtha were investigated. The sodium-modified ZSM-5 catalysts were prepared by the wet impregnation method, and the Si–OH–Al groups of ZSM-5 were broken in the post-treatment process, resulting in a decrease in the number of strong B acid sites (SBAS) and an increase in the number of strong L acid sites (SLAS). The correlations between the n-hexane catalytic cracking reaction pathways and the catalyst acid properties were established. The reduced SBAS inhibits secondary reactions of light olefins such as hydrogen transfer, oligomerization, cyclization, and aromatization, while the increased SLAS and the ratio of SLAS/SBAS promoted the β scission reactions that significantly improved the yields of light olefins. Consequently, the appropriate SBAS and SLAS/SBAS ratios are desirable for the production of ethylene and propylene. The optimal SBAS concentration and SLAS/SBAS ratio ranged from 4.44 to 6.69 μmol/g and 9.18–11.86 for maximum ethylene and propylene yields of 16.14% and 14.63% in hexane catalytic pyrolysis reactions, respectively. Furthermore, the optimally prepared catalysts illustrated superior reactivities and catalytic stabilities in naphtha catalytic pyrolysis reactions under industrial conditions.