Molybdenum substitution by copper or zinc in H-ZSM-5 zeolite for catalyzing the direct conversion of natural gas to petrochemicals under non-oxidative conditions

This work aims to investigate the influence of substituting half the Mo content in a standard 6%Mo/H-ZSM-5 catalyst with either Cu or Zn in H-ZSM-5 support, on the direct conversion of methane in a flow-type fixed-bed reactor atmospherically at 700 °C at a gas hourly space velocity 1500 cm³ h⁻¹ g⁻¹ and times-on stream (TOS) up to 240 min. The most active was 6%Mo/H-ZSM-5 catalyst while Mo-Zn/H-ZSM-5 was more active than Mo-Cu/H-ZSM-5. The XRD data showed that the crystallite (particle) sizes were found compatible with the catalytic activities. At the initial TOS (5 min), methane splitting selectivity to C and H₂ approached 100%. Higher carbon deposition on Mo-Cu catalyst caused more inhibition of ethylene further conversion to larger hydrocarbons thus leading to ethylene accumulation. TPR showed an almost complete reduction of Cu oxides at relatively lower temperatures such that the Mo-Cu catalyst acquired the highest dehydrogenation activity that enhanced markedly ethylene formation. An outstanding accomplishment is obtaining the highest benzene yield and selectivity using Mo-Zn/H-ZSM-5 catalyst, which is a prosperous challenge against the standard monometallic one. The large size of naphthalene molecule caused significant diffusion restriction on its formation in catalytic pores. The exceptional enhancement of naphthalene yield and selectivity at longer TOS using the Mo-Cu/H-ZSM-5 catalyst can be indicative that naphthalene was mostly formed on external zeolitic surface.