Enhancement of Molybdenum/ZSM-5 Catalysts in Methane Aromatization by the Addition of Iron Promoters and by Reduction/Carburization Pretreatment

The influence of an Fe additive and different types of pretreatment were studied on HZSM-5-supported molybdenum oxide (MoO_x) catalysts for methane aromatization. The catalytic behavior of catalysts that consist of 6 wt % Mo/ZSM-5 with 0, 0.2, and 1 wt % Fe was tested with two types of pretreatment: 1) heating under He flow and 2) reduction in H₂/CH₄ and carburization in CH₄. Under He pretreatment, the addition of 0.2 wt % Fe improved the benzene yield, but the addition of 1 % Fe decreased it slightly. Precarburization of the catalysts resulted in enhanced catalytic properties for all Fe loadings and improved the catalyst stability. The precarburized 6 wt % Mo–0.2 wt %Fe catalyst presented the highest benzene yield (6.9 %), which was almost stable in the subsequent 10 h test. The fresh and spent catalysts were characterized by using XRD, N₂ adsorption, temperature-programmed reduction, SEM, temperature-programmed oxidation, and thermogravimetric analysis. The results show that the precarburized catalysts are more stable because of the formation of smaller amounts of carbon deposits and, consequently, less pore blockage. The addition of Fe causes the carbon deposits to be more reactive and easier to burn off. Higher Fe loadings are linked to the formation of carbon nanotubes.