Design and stabilisation of a high area iron molybdate surface for the selective oxidation of methanol to formaldehyde

Abstract

The performance of Mo-enriched, bulk ferric molybdate, employed commercially for the industrially important reaction of the selective oxidation of methanol to formaldehyde, is limited by a low surface area, typically 5-8 m(2) g(-1). Recent advances in the understanding of the iron molybdate catalyst have focused on the study of MoOx@Fe2O3 (MoOx shell, Fe2O3 core) systems, where only a few overlayers of Mo are present on the surface. This method of preparing MoOx@Fe2O3 catalysts was shown to support an iron molybdate surface of higher surface area than the industrially-favoured bulk phase. In this research, a MoOx@Fe2O3 catalyst of even higher surface area was stabilised by modifying a haematite support containing 5 wt% Al dopant. The addition of Al was an important factor for stabilising the haematite surface area and resulted in an iron molybdate surface area of ∼35 m(2) g(-1), around a 5 fold increase on the bulk catalyst. XPS confirmed Mo surface-enrichment, whilst Mo XANES resolved an amorphous MoOx surface monolayer supported on a sublayer of Fe2(MoO4)3 that became increasingly extensive with initial Mo surface loading. The high surface area MoOx@Fe2O3 catalyst proved amenable to bulk characterisation techniques; contributions from Fe2(MoO4)3 were detectable by Raman, XAFS, ATR-IR and XRD spectroscopies. The temperature-programmed pulsed flow reaction of methanol showed that this novel, high surface area catalyst (3ML-HSA) outperformed the undoped analogue (3ML-ISA), and a peak yield of 94% formaldehyde was obtained at ∼40 °C below that for the bulk Fe2(MoO4)3 phase. This work demonstrates how core-shell, multi-component oxides offer new routes for improving catalytic performance and understanding catalytic activity.

Al-doped Fe2O3 as a support for molybdenum oxide methanol oxidation catalysts

“…2. The minor changes were a shifting of the peak to higher temperature by B20 1C, and a broadening of the peaks, similar to those previously reported by Chapman et al 6 TPD for the four Al loadings are shown in Fig. S5 (ESI †).…”

“…[14][15][16][17][18] Initial studies of Al-doped haematite have been conducted as support materials in shell-core selective oxidation catalysts. 6 It was found that the incorporation of 5 wt% Al into Fe 2 O 3 doubled the surface area of the post-calcination material, while essentially retaining the corundum structure. Subsequently, after adding MoO x shell layers to the Al-doped Fe 2 O 3 and assessing the reactivity of the resulting catalysts, it was determined that the greater surface area of the catalyst reduced the requisite operating temperature of the catalyst by approximately 20 1C.…”

“…The effects of doping haematite with small quantities of aluminium have been explored previously, primarily from a mineralogical perspective, although more recently from the viewpoint of MoO x /Fe 2 O 3 catalysts also. [1][2][3][4][5][6] Al-substitution into Fe 2 O 3 is ubiquitous in terrestrial sources of haematite, but importantly the Al incorporates without significantly disrupting the overall haematite structure. 7 There are, however, small changes in structural properties arising from the presence of Al throughout the haematite structure: typically, structural lattice parameters decrease as a function of the lower atomic displacement of Al in comparison to Fe (0.535 A and 0.645 A respectively).…”

“…This reduction in lattice spacing and the concomitant altering of electrostatic interactions induces a degree of lattice strain throughout the structure, encouraging the growth of smaller crystallites: this in turn affords a greater value of specific surface area. 6 Since catalytic activity is dependent on surface area, increases in surface area are hugely attractive to the catalysis chemist. In the present case the reaction of interest is the selective oxidation of methanol to yield formaldehyde, which is carried out industrially 8 at the scale of B6 M tonnes per y.…”

Carbidisation of Pd Nanoparticles by Ethene Decomposition with Methane Production

“…Reaction measurements were carried out in a Hiden Catlab using its pulsed flow reactor capability . 0.22 g of catalyst was loaded into the quartz reactor tube and heated in‐situ prior to reaction to 250 °C in 10 % H 2 in He, followed a pure He flow for one hour prior to the pulsing experiments.…”