Kinetics of the catalytic cracking of naphtha over ZSM-5 zeolite: effect of reduced crystal size on the reaction of naphthenes

The catalytic cracking of model naphthenes (cyclohexane and methylcyclohexane) over ZSM-5 zeolites of different crystal sizes (macro-and nano-ZSM-5) was examined at reaction temperatures ranging from 748 to 923 K under atmospheric pressure, focusing on the associated reaction rate constants and activation energies. The catalytic cracking was found to follow first-order kinetics with respect to the naphthene concentrations and the activation energies for cyclohexane and methylcyclohexane cracking over nano-ZSM- 5 were determined to be 119 and 116 kJ mol(-1), respectively. In order to elucidate the rate-limiting step in the cracking process, the Thiele modulus and the effectiveness factor obtained from cracking over the two ZSM-5 zeolites were evaluated. Cracking with nano-ZSM-5 proceeded under reaction-limiting conditions, whereas the reaction over macro-ZSM-5 at 923 K took place under transition conditions between reaction-and diffusion-limiting. Nano-ZSM-5 was applied to the catalytic cracking of a model naphtha and the results demonstrated that this catalyst was both effective and stable and generated a high yield of light olefins.