The effect of oxidant species on direct, non-syngas conversion of methane to methanol over an FePO(4) catalyst material

The effect of the phase transformation of a FePO(4) catalyst material from the tridymite-like (tdm) FePO(4) to the α-domain (α-Fe(3)(P(2)O(7))(2)) during the direct selective oxidation of methane to methanol was studied using oxidant species O(2), H(2)O and N(2)O. The main reaction products were CH(3)OH, carbon dioxide and carbon monoxide, whereas formaldehyde was produced in rather minute amounts. Results showed that the single-step non-syngas activation of CH(4) to oxygenate(s) on a solid FePO(4) phase-specific catalyst was influenced by the nature of the oxidizer used for the CH(4) turnover. Fresh and activated FePO(4) powder samples and their modified physicochemical surface and bulk properties, which affected the conversion and selectivity in the partial oxidation (POX) mechanism of CH(4), 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 FePO(4) 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, FePO(4) (tdm), to the divalent metal form, iron(ii) pyrophosphate (Fe(2)P(2)O(7)); thereafter, reactivity was governed by the interaction (strength) with individual oxidizing agents. The Fe(3+) ↔ Fe(2+) 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 FePO(4)-tdm phase catalysts showed in this work exhibited a high activity towards methanol i.e., 12.3 × 10(−3) μmol(MeOH) g(cat) h(−1) using N(2)O as an oxidant. This catalyst also showed a high activity with O(2) as an oxidant (5.3 × 10(−3) μmol(MeOH) g(cat) h(−1)). Further investigations will include continuous reactor unit engineering optimisation.