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.