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

摘要

As the commercial Fe-Mo-oxide catalyst that is used for oxidation of methanol to formaldehyde suffers from deactivation by Mo volatilization, alternative catalysts are of interest. Therefore, TiO_2-, α-Al_2O_3- and SiO_2-supported (Fe)-V-O catalysts were prepared with a loading of up to 30 μmol of each metal per m~2 surface area of the support. The samples were tested for activity 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 TiO_2 > Al_2O_3 > SiO_2. 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 FeVO_4 > (Fe)VO_x/TiO_2 ≈ (Fe)VO_x/Al_2O_3 > FeVO_x/SiO_2 > VO_x/SiO_2. Iron has only a small effect on the catalytic performance, whereas it has a stabilizing effect on vanadium decreasing its volatility. However, volatilization experiments reveal that the volatilization of V from the supported (Fe)-V-oxide is much severer than that from bulk FeVO_4 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 a catalyst in the industrial scale production of formaldehyde by methanol oxidation.