Abstract
The effect of the phase transformation of a FePO4 catalyst material from the tridymite-like (tdm) FePO4 to the α-domain (α-Fe3(P2O7)2) during the direct selective oxidation of methane to methanol was studied using oxidant species O2, H2O and N2O. The main reaction products were CH3OH, carbon dioxide and carbon monoxide, whereas formaldehyde was produced in rather minute amounts. Results showed that the single-step non-syngas activation of CH4 to oxygenate(s) on a solid FePO4 phase-specific catalyst was influenced by the nature of the oxidizer used for the CH4 turnover. Fresh and activated FePO4 powder samples and their modified physicochemical surface and bulk properties, which affected the conversion and selectivity in the partial oxidation (POX) mechanism of CH4, were investigated. Temperature-programmed re-oxidation (TPRO) profiles indicated that the type of moieties utilised in the procedures, determined the re-oxidizing pathway of the reduced multiphase FePO4 system. Mössbauer spectroscopy measurements along with X-ray diffraction (XRD) examination of neat, hydrogenated and spent catalytic compounds, demonstrated a variation of the phosphate into a mixture of crystallites, which depended on operating process conditions (for example time-on-stream). The Mössbauer spectra revealed the change of the initial ferric orthophosphate, FePO4 (tdm), to the divalent metal form, iron(ii) pyrophosphate (Fe2P2O7); thereafter, reactivity was governed by the interaction (strength) with individual oxidizing agents. The Fe3+ ↔ Fe2+ chemical redox cycle can play a key mechanistic role in tailored multistep design, while the advantage of iron-based heterogeneous catalysis primarily lies in being inexpensive and comprising non-critical raw resources. When compared to the other catalysts reported in the literature, the FePO4-tdm phase catalysts showed in this work exhibited a high activity towards methanol i.e., 12.3 × 10-3 μmolMeOH gcat h-1 using N2O as an oxidant. This catalyst also showed a high activity with O2 as an oxidant (5.3 × 10-3 μmolMeOH gcat h-1). Further investigations will include continuous reactor unit engineering optimisation.