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

Abstract

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 H 2-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 O 2 and/or N 2O 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 N 2O over the Cu/Fe ≅ 1:2 pyrophosphate catalyst which consists mainly of the crystalline Fe IIFe III 2(P 2O 7) 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 Cu II and Fe III species of the above structure can be reduced more easily but at similar temperatures than those in the crystalline CuFe 2(P 2O 7) 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 N 2O takes place rather readily and rapidly. The nature of active species, methane conversion and oxygenates selectivity remained almost unchanged during the time on stream.