Oxidation of methanol to formaldehyde over a series of Fe1−xAlx-V-oxide catalysts

A series of triclinic Fe1−xAlxVO4 phases with 0⩽x⩽10⩽x⩽1 were prepared and used in the oxidation of methanol to formaldehyde. The activity measurements revealed that both the activity and especially the selectivity to formaldehyde increased with time of operation for at least 16 h, indicating restructuring of the catalysts. Characterisation of the catalysts with XRD, XANES, and electron microscopy after use in methanol oxidation showed that the stability of the bulk phases improved when Al was substituted for Fe in the structure. XRD and XANES of the used FeVO4 showed that it partly transformed into a cation-vacant spinel-type Fe1.5V1.5O4 phase, whereas the AlVO4 phase showed no change in the bulk structure. HRTEM imaging of used catalysts confirmed that structural changes, including in the surface, occurred during catalysis. Quantitative surface analysis by XPS of the catalysts before and after use in methanol oxidation revealed no significant change in the metal composition, in good agreement with the corresponding bulk values, except for a lower Fe value. Steady-state activity data showed a modest increase in specific activity with the Al content, whereas the selectivity to formaldehyde was about 90% for all samples at high methanol conversion. The similar catalytic behaviour of the vanadates irrespective of the differences in the bulk structure indicates that the surface structure differed from the bulk structure. Compared with pure vanadia, the vanadates had lower activity per V atom and slightly greater selectivity to formaldehyde. Consequently, for methanol oxidation, the role of Al and Fe on the catalyst surface can be described as that of a spacer, decreasing the surface concentration of active V sites and the number of less selective VOV ensembles.