The selective oxidation of n-butane to maleic anhydride: comparison of bulk and supported V-P-O catalysts

V-P-O catalysts supported on the surface of silica and titania particles we re studied and compared with bulk V-P-O. The catalytic performance was test ed in the n-butane oxidation reaction to maleic anhydride, and the structur e of the equilibrated catalysts was characterised with X-ray absorption spe ctroscopy (EXAFS) and (low-temperature) ESR spectroscopy. Our results show considerable differences in catalytic performance between VPO/TiO2 on the o ne hand, and VPO/SiO2 and VPO/bulk on the other hand, the yield to maleic a nhydride being comparable for VPO/bulk and VPO/SiO2. The differences in cat alytic behaviour are attributed to differences in the local structure aroun d vanadium (EXAFS). Furthermore, different spin exchange interactions betwe en vanadium atoms in the three samples have been observed (ESR). The combin ation of characterisation methods suggests that the structure of the suppor ted V-P-O phase is amorphous and differs considerably from that of bulk cry stalline vanadylpyrophosphate. We therefore propose that the oxidation of n -butane to maleic anhydride takes place over an amorphous surface V-P-O pha se. This finding has high relevance for our understanding of the catalytic activity of bulk crystalline V-P-O catalysts as well.