Reactivities of C6–8 hydrocarbons and effect of coexistence of another hydrocarbon in cracking on H-ZSM-5 catalyst at high temperatures

• At high reaction temperature, the cracking proceeds through monomolecular mechanism.
• In cycloparaffin cracking, the selectivity for the corresponding aromatics was high.
• Deactivation of H-ZSM-5 in cycloparaffin cracking was faster than that in n-paraffin.
• 1-Hexene did not affect n-heptane, cyclohexane, or methylcyclohexane cracking.
• Cycloparaffin affected n-heptane cracking due to slow diffusion in the zeolite pore.

The cracking of C6–8 hydrocarbons on H-ZSM-5 (Si/Al = 51) at high temperatures was carried out as a model reaction of naphtha cracking to produce light olefins. The reactivity of n-paraffin increased with an increase in carbon number, while the selectivities for light olefins were almost constant. In the cracking of cycloparaffin, the selectivity for the corresponding aromatics was very high, indicating that the dehydrogenation occurred directly without C-C bond cleavage. In the case of cycloparaffin cracking, the deactivation of H-ZSM-5 with time-on-stream (TOS) was fast due to high BTX (benzene, toluene, and xylene) selectivity. The deactivation of H-ZSM-5 in the cracking of methylcyclohexane was especially fast due to very high toluene selectivity. Coexistent 1-hexene did not affect the cracking rate of n-heptane, cyclohexane, or methylcyclohexane, indicating that monomolecular cracking was predominant at as high a temperature as 923 K. In contrast, coexistent cyclohexane and methylcyclohexane considerably affected the cracking rate of n-heptane because of their slow diffusion in the pores of H-ZSM-5.

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