Tuning the Concentration of Acid Sites on Zsm-5 Zeolite for Improving Light Olefins Production in the Catalytic Pyrolysis of Paraffin

To investigate the influence of different Brønsted acid sites concentration and the ratio of Lewis/Brønsted on the cracking pathway and product distribution in hexane cracking reactions as a model component of naphtha. The different sodium loading modified ZSM-5 was prepared through the wet impregnation method, the Si-OH-Al groups were disrupted in the post-treatment process, resulting in a decrease in the number of strong B acid sites (SBAS) and an increase in the amount of strong L acid sites (SLAS). The n-hexane catalytic cracking reaction pathway was established, the reduced SBAS inhibits secondary reactions of light olefins such as hydrogen transfer, oligomerization, cyclization, and aromatization, and the increased SLAS and the ratio of SLAS/SBAS promoted the β scission reaction that significantly improved the yield 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 4.44-6.69 μmol/g and 9.18-11.86 for maximum ethylene and propylene yields of 16.19% and 17.76% in hexane catalytic pyrolysis reaction, respectively. Furthermore, the optimally prepared catalysts illustrated promising reactivity and catalytic stability in naphtha catalytic pyrolysis reactions.