Impact of microporous zeolite-coated macroporous supports on the catalytic pyrolysis of methylcyclohexane under supercritical conditions

Under supercritical conditions, the effects of the microporous zeolite-coated macroporous supports (i.e., ceramic-supported ZSM-5 catalysts and metal-supported ZSM-5 catalysts) on the endothermic pyrolysis process of methylcyclohexane (MCH, CH₃C₆H₁₁) were examined. The micro-macroporous composite materials were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and NH₃ temperature-programmed desorption (NH₃-TPD) analysis. ZSM-5-coated Al₂O₃ hollow fiber and ZSM-5-coated NiCrFeAl metal foam were both tested using an isothermal constant-pressure continuous-flow heterogeneous catalytic reactor at high temperature (550 °C) and pressure (50 bar) that simulate supersonic jet engines. The MCH pyrolysis conversions and post-pyrolysis products were determined by gas chromatography, while the deposited coke on the microporous zeolite catalysts was quantified using thermogravimetric (TG) analysis. ZSM-5-coated NiCrFeAl metal foam catalyst showed a slightly higher MCH pyrolysis conversion (%) than ZSM-5-coated Al₂O₃ hollow fiber catalyst. On the other hand, the ZSM-5-coated Al₂O₃ hollow fiber produces fewer aromatic hydrocarbons (benzene, toluene, and xylene, BTX), as well as cyclic or polycyclic hydrocarbons (naphthalene). Additionally, the coke deposition was approximately 2 times lower on the ZSM-5-coated Al₂O₃ hollow fiber catalyst compared to the ZSM-5-coated NiCrFeAl metal foam. Furthermore, due to the endothermic nature of the catalytic pyrolysis of MCH under supercritical jet conditions over the studied microporous zeolite-coated macroporous supports, a temperature decrease of approximately 15–20 °C was observed.