Structure and reactivity of copper iron pyrophosphate catalysts for selective oxidation of methane to formaldehyde and methanol

The structure of copper iron pyrophosphate catalysts prepared with various atomic ratios of metals and different pretreatment conditions was studied by using XRD, FT-IR, DR UV–vis and H2-TPR techniques. The preparation methods modify the phase composition, oxidation state and reducibility of the patterns. The catalytic properties in the selective oxidation of methane to formaldehyde and methanol were examined in both, pulse and flow reactors, using O2 and/or N2O oxidizing agents. The oxidants used impact the onset of methane conversion, the primary oxidation products formation and the methane conversion. The highest single-pass yield of useful oxygenates (1.8%) was achieved with N2O over the Cu/Fe ≅ 1:2 pyrophosphate catalyst which consists mainly of the crystalline FeIIFeIII2(P2O7)2 and copper containing nanodomains. Enhanced interaction and cooperation between these phases leading to a synergic effect of Cu and Fe has been emphasized. The CuII and FeIII species of the above structure can be reduced more easily but at similar temperatures than those in the crystalline CuFe2(P2O7)2. Pulse reaction studies indicate that the lattice oxygen of the catalyst could react with methane molecules producing formaldehyde and methanol and replenishment of the lattice oxygen by N2O takes place rather readily and rapidly. The nature of active species, methane conversion and oxygenates selectivity remained almost unchanged during the time on stream.

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► Hybrid phase Cu–Fe–pyrophosphate is efficient for methane selective oxidation.
► Single-pass yield of formaldehyde and methanol depends on structural characteristics.
► The structural modifications depend on pretreatment conditions and atomic ratio of metals.
► Enhanced interaction and cooperation between crystalline phase and nanodomains lead to a synergic effect of Fe and Cu.