New Mo-Fe-O silica supported catalysts for methanol to formaldehyde oxidation Original Research Article

Silica supported Mo-Fe and Mo-Fe-P mixed oxides (Mo/Fe = 3 atomic ratio) were prepared in the presence of citric acid as chelating agent to obtain a uniform distribution of the active phase. Loading of 23.1% (w/w) of MoO3 was used exceeding the monolayer content (around 2.2 monolayers of MoO3). The low interaction between the silica and the mixed oxide in addition to the high content of the active phase led to a three-dimensional distribution of the active phase, confirmed by SEM.Several iron (Fe3+) precursors (nitrate, chloride, sulfate, phosphate and pyrophosphate) were used in order to assess their effect on the catalysts performances.Some elements (N, Cl, S and P) from the iron precursors remained in the catalyst. Samples prepared from phosphate and pyrophosphate had phosphorus in their compositions and Mössbauer spectra revealed the presence of iron phosphate. Nevertheless, due to lack of crystallinity, these phases (iron phosphates) are probably inactive.All the prepared catalysts display high activity and selectivity towards formaldehyde, even for low and high methanol conversions. The catalyst prepared from iron pyrophosphate was the most active. The presence of heteropolymolybdates, containing phosphorus, or its Mo rich surface can perhaps explain its enhanced catalytic behavior. For temperatures lower than 625 K this sample (merely 26.2% (w/w) of active phase) presented higher formaldehyde yields than one unsupported industrial catalyst. Such a catalyst prepared from iron pyrophosphate presents promising catalytic performances for use in fluidized bed reactors.The loss of Mo during reaction was observed for all the prepared catalysts. Micrographs for post reaction samples displayed MoO3 needles (or plates) formed during the redox cycle, which are easily sublimated. Conversely, for the sample prepared from iron pyrophosphate the MoO3 needles are absent, which is probably indicative of a different reaction mechanism.