Highly Stable Apatite Supported Molybdenum Oxide Catalysts for Selective Oxidation of Methanol to Formaldehyde: Structure, Activity and Stability

Abstract

Molybdenum oxide (5 to 20 wt.%) supported on calcium or strontium hydroxyapatite were investigated as catalysts for selective oxidation of methanol to formaldehyde. These catalysts were both active and selective, with a maximum yield achieved at 95 % conversion and 96 % selectivity. The main byproducts were CO (3.2 %) and dimethyl ether (DME, 0.7 %). The catalytic performance of the catalysts was measured for up to 600 h at 350 °C. Compared to an industrial iron molybdate catalyst, the hydroxyapatite based catalysts deactivated slower.The active species were found to be a surface layer of MoO x on the hydroxyapatite support, while excess molybdenum formed crystalline (Ca/Sr)MoO 4 , acting as a reservoir replenishing surface MoO x lost by volatilization with methanol. The excess molybdenum in the form of (Ca/Sr)MoO 4 was found to volatilize significantly slower than the excess MoO 3 in iron molybdate catalysts. The combination of high activity and selectivity with low rate of Mo volatilization makes this class of catalysts interesting for industrial production of formaldehyde.

Ensemble Effects on Methanol Oxidation to Formaldehyde on Ferric Molybdate Catalysts

“…Others have suggested that there could be better catalysts. For instance, Thrane et al [35] consider that apatite may be a more stable and active support medium for molybdenum oxide, and Bowker et al [36] report that higher area, more active materials may be produced by Al doping in the bulk of the catalyst.…”

A Review and Experimental Revisit of Alternative Catalysts for Selective Oxidation of Methanol to Formaldehyde

“…At this point, no catalyst that has the potential to fully exchange the FeMo catalyst in the industrial (Formox) process has been reported. However, some catalysts showed higher stability/lower molybdenum loss and similar activity and reasonable, but slightly lower, selectivity compared to the FeMo catalyst, including a recently developed MoO 3 /HAP catalyst [110,112]. Such catalysts could possibly be used in the inlet section of the reactor, providing moderate conversion over this alternative catalyst, where selectivity is rather high.…”

“…However, no Mo volatilization past the point of stoichiometry for the bulk alkali earth metal molybdates was observed by Raman spectroscopy [107]. However, in the later study of MoO 3 supported on HAP and the Sr analogue, Thrane et al [110] found that the amount of CaMoO 4 and SrMoO 4 quantified by XRD decreased with TOS for samples with nominal loadings of 10 and 15 wt% MoO 3 . Thus, CaMoO 4 loading went from 8.1 to 3.1 wt% after 408 h on stream for the 10 wt% MoO 3 /HAP.…”

“…The active phase of the catalyst was suggested to be calcium molybdate formed under calcination [108,109]. Thrane et al [110] investigated MoO 3 supported on hydroxyapatite and the Sr analogue in a feed of 150 NmL/min, 5 vol.% MeOH, and 10 vol.% O 2 in N 2 . MoO 3 /CaHAP and MoO 3 /SrHAP were found to be more stable than an industrial reference FeMo catalyst when measured as powder catalysts for up to 600 h on stream at 350 • C (Figure 20).…”

“…The active phase was proposed to be amorphous MoO x on the surface of the HAP support, which was observed with STEM, substantiated by XAS (XANES and EXAFS) and X-ray PDF, as previous studies found stoichiometric CaMoO 4 by itself was not very active or selective. The phosphate was proposed to decrease selectivity towards CO and CO 2 compared to stoichiometric CaMoO 4 and SrMoO 4 [110,111]. Thrane et al [112] further investigated samples of 10 wt% MoO 3 /HAP pressed as industrial sized Mo/HAP pellets with different pellet densities, measuring the catalytic performance in a single pellet reactor with 300 NmL/min, 5 vol.% MeOH, and 10 vol.% O 2 in N 2 .…”