Stability and performance of supported Fe-V-oxide catalysts in methanol oxidation

As the commercial Fe-Mo-oxide catalyst for methanol oxidation to formaldehyde suffers from deactivation by Mo volatilization, alternative catalysts are of interest. Therefore, TiO2-, alpha-Al2O3- and SiO2-supported (Fe)-V-O catalysts were prepared with loading up to 30 μmol of each metal per msq. surface area of the support. The samples were activity tested using a high inlet concentration of methanol (10 vol.%) and were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XANES). The activity measurements show that the preparations with the highest loads of V give the best performance. With regard to the support, the activity of the supported catalysts decreases in the order TiO2 > Al2O3 > SiO2. According to XPS the surface concentration of V decreases in the same order, confirming that vanadium is an active element. At high methanol conversion, the selectivity to formaldehyde decreases from 90% to 80% in the sequence unsupported FeVO4 > (Fe)VOx/TiO2 = (Fe)VOx/Al2O3 > FeVOx/SiO2 > VOx/SiO2. Iron has only a small effect on the catalytic performance, whereas it has a stabilizing effect on vanadium decreasing its volatility. However, compared with bulk FeVO4, volatilization experiments reveal that the volatilization of V from the supported (Fe)-V-oxide is much severer due to the dispersion and the comparatively low amount of active metal. Our data demonstrate that neither supported V-oxide nor supported Fe-V-oxide is suitable as catalyst in the industrial scale production of formaldehyde by methanol oxidation.