Combination of in situ multi-edge X-ray absorption spectroscopy at the Mo K- and Fe K-edges in combination with X-ray diffraction successfully uncovered structural dynamics and phase transformations of an iron molybdate catalyst during redox cycling.

The selective oxidation of methanol to formaldehyde is a growing million-dollar industry, and has been commercial for close to a century. The Formox process, which is the largest production process today, utilizes an iron molybdate catalyst, which is highly selective, but has a short lifetime of 6 months due to volatilization of the active molybdenum oxide. Improvements of the process’s lifetime is, thus, desirable. This paper provides an overview of the efforts reported in the scientific literature to find alternative catalysts for the Formox process and critically assess these alternatives for their industrial potential. The catalysts can be grouped into three main categories: Mo containing, V containing, and those not containing Mo or V. Furthermore, selected interesting catalysts were synthesized, tested for their performance in the title reaction, and the results critically compared with previously published results. Lastly, an outlook on the progress for finding new catalytic materials is provided as well as suggestions for the future focus of Formox catalyst research.

Alkali earth metal molybdates (MMoO₄, M = Mg, Ca, Sr, and Ba) were investigated as catalysts for the selective oxidation of methanol to formaldehyde in the search for more stable alternatives to the current industrial iron molybdate catalyst. The catalysts were prepared by either sol-gel synthesis or co-precipitation with both stoichiometric ratio (Mo:M = 1.0) and 10 mol% to 20 mol% excess Mo (Mo:M = 1.1 to 1.2). The catalysts were characterized by X-ray diffraction (XRD), nitrogen physisorption, Raman spectroscopy, temperature programmed desorption of CO₂ (CO₂-TPD), and inductively coupled plasma (ICP). The catalytic performance of the catalysts was measured in a lab-scale, packed bed reactor setup by continuous operation for up to 100 h on stream at 400 °C. Initial selectivities towards formaldehyde of above 97% were achieved for all samples with excess molybdenum oxide at MeOH conversions between 5% and 75%. Dimethyl ether (DME) and dimethoxymethane (DMM) were the main byproducts, but CO (0.1%–2.1%) and CO₂ (0.1%–0.4%) were also detected. It was found that excess molybdenum oxide evaporated from all the catalysts under operating conditions within 10 to 100 h on stream. No molybdenum evaporation past the point of stoichiometry was detected.

In this study, 10 wt% MoO₃ supported on hydroxyapatite (HAP, Ca₅(OH)(PO₄)₃) pellets were investigated as catalysts for the selective oxidation of methanol to formaldehyde. HAP was synthesized by co-precipitation and pressed into industrial sized pellets with different densities (1.18–1.76 g cm⁻³) to investigate the influence of pellet density and porosity on the catalytic performance. The pellets were wet impregnated to obtain 10 wt% MoO₃. The catalysts were characterized by X-ray diffraction (XRD), N₂ physisorption, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM–EDX), inductively coupled plasma optical emission spectroscopy (ICP-OES) and Hg-porosimetry. The catalytic performance was evaluated at 250–400 °C for up to 100 h continuous operation in a single pellet reactor with 300 NmL min⁻¹, 5 vol% MeOH, and 10 vol% O₂ in N₂. The MoO₃/HAP pellets lost between 10% and 23% of their initial activity while an industrial MoO₃/Fe₂(MoO₄)₃ reference catalyst lost ∼11% within 100 h on stream at 350 °C. Despite the MoO₃/HAP catalysts not outperforming the FeMo reference, it was still deemed of interest for further study and optimization as the difference was relatively small. Especially since the pellets weight loss due to Mo volatilization was >70% lower compared with the FeMo catalysts within a 118.5 h test. A random pore model, together with overall effectiveness factor calculations for both powder and pellets, was used to calculate the pellet activity from measured powder activity, providing reasonable agreement between the calculated and the observed rate constants for the pellets. An increase in both activity and selectivity to formaldehyde was obtained with decreasing pellet density, due to decreasing pore diffusion limitations of both methanol into and formaldehyde out of the pellet. At 350 °C the lightest (and weakest) pellet achieved an initial selectivity to formaldehyde of 87.5% at 25.1% conversion. The primary by-products were dimethoxymethane (DMM) (C based selectivity of 6%) and CO (C based selectivity of 4.7%). The catalysts are promising for application at the front-end of an industrial formaldehyde synthesis reactor to limit the loss of MoO₃ due to evaporation with accompanied increase in reactor pressure drop.