Evolution of active catalysts for the selective oxidative dehydrogenation of methanol on Fe2O3 surface doped with Mo oxide

Abstract

Iron molybdate catalysts are used for the selective oxidation of methanol to formaldehyde. In this paper we have attempted to understand what determines high selectivity in this reaction system by doping haematite with surface layers of Mo by incipient wetness impregnation. This works well and the Mo appears to form finely dispersed layers. Even very low loadings of Mo have a marked effect on improving the selectivity to formaldehyde. Haematite itself is a very poor catalyst with high selectivity to combustion products, whereas, when only 0.25 monolayers of Mo are deposited on the surface, formaldehyde and CO selectivities are greatly enhanced and CO2 production is greatly diminished. However, even with as much as seven monolayers of Mo dosed on to the surface, these materials achieve much less selectivity to formaldehyde at high conversion than do the industrial catalysts. The reason for this is that the Mo forms a 'skin' of ferric molybdate on a core of iron oxide, but does not produce a pure Mo oxide monolayer on the surface, a situation which is essential for very high yields of formaldehyde.

VOx/Fe2O3 Shell–Core Catalysts for the Selective Oxidation of Methanol to Formaldehyde

“…4). Similar mechanisms have been proposed for different catalyst systems, corroborating our suggested VO x spreading mechanism [18][19][20].…”

“…This shell-core approach has previously been studied in MoO 3 /Fe 2 O 3 catalysts for methanol oxidation, where it was seen that high selectivity to formaldehyde can be maintained at high methanol conversions [16][17][18][19][20]. The validity of the shell-core model when applied to molybdena-based catalysts was clearly established, in addition to its applicability to the methanol oxidation reaction [18][19][20][21].…”

“…The validity of the shell-core model when applied to molybdena-based catalysts was clearly established, in addition to its applicability to the methanol oxidation reaction [18][19][20][21]. Mo-based catalysts were investigated since current industrial catalysts for methanol oxidation include iron molybdate, but also because of the high formaldehyde selectivity (though poor activity) of MoO 3 .…”

Selectivity determinants for dual function catalysts: applied to methanol selective oxidation on iron molybdate

“…9,13,14 To summarise, the reactor was a tubular reactor, 1/4 00 OD, operating in TPD (temperature programmed desorption) mode with a continuous flow of helium at a flowrate of 30 mL min…”

“…We must also remember that XPS measures the surface region (, 5 layers), and does not give the top surface layer concentration. Studies with doping Mo onto the surface 9,10 shows that it tends to remain there and does not diffuse into the bulk. Indeed, if the reverse is attempted (Fe doping onto the surface of MoO 3 ), then Fe diffuses away from the surface into the bulk.…”