Structure and reactivity of ZSM-5 supported oxalate ligand functionalized nano-Fe catalyst for low temperature direct methane conversion to methanol

Highlights: • Synthesis of ZSM-5 supported oxalate ligand functionalized Fe catalyst (FeOx/ZSM-5). • Characterization of ZSM-5 supported oxalate ligand functionalized Fe catalyst. • Evaluation of FeOx/ZSM-5 for direct methane conversion to methanol. • Characterization of used FeOx/ZSM-5 catalyst. - Abstract: The feasibility of low temperature direct methane conversion to methanol (DMCM) over ferrioxalate supported on ZSM-5 catalyst (FeOx/ZSM-5) was investigated in this study. FeOx/ZSM-5 was synthesized via functionalization of Fe with oxalate ligand and employed to generate hydroxyl radicals (HO^·) from hydrogen peroxide (HP) in a Fenton-like process to activate the methane C−H bond scission to form methoxyl radical which in turn reacted with water to form methanol. The Fourier transformed infrared (FTIR) spectroscopy result showed the presence of ν_a(C=O) from the oxalate anion (C₂O₄²⁻) at 1655 cm⁻¹ and ν₃ asymmetry stretch from CO₃²⁻ at 1343 cm⁻¹. The presence of Fe in the catalyst was elucidated by the energy dispersive X-ray, X-ray diffraction patterns and X-ray photoelectron spectroscopy (XPS). Furthermore, the XPS results showed that Fe²⁺/Fe³⁺ ratio in FeOx/ZSM-5 was 4.29. The effect of operating conditions showed that while FeOx/ZSM-5 loading and reaction time followed a simple power function (y = a · xⁿ), the effect of HP dosage followed exponential function (y = a · e^x), which signifies that the latter is more significant. The used-FeOx/ZSM-5 characterization result showed that virtually all the Fe²⁺ have been oxidized to Fe³⁺ after 15 min of reaction time. This novel result is promising for further research towards commercial low temperature DMCM.