Dynamic redox properties of vanadium and copper in microporous supports during the selective oxidation of propene

Abstract

Vanadium(V) and copper(II) were co-deposited into the microporous neutral AlPO-5 and the acidic ZSM-5, obtaining VCu:AlPO-5 and VCu:ZSM-5. In this study we seek to investigate the effect of feed gas composition for the selective oxidation of propene applying C3H6/O2 ratios from 4 to 0.25 over VCu:AlPO-5 and VCu:ZSM-5. The results from catalytic measurements were correlated with in situ X-ray Absorption Spectroscopy (XAS) to monitor metal speciation for different feeds. VCu:AlPO-5 produces acrolein over the whole feed composition range, reaching maximum yield (1.5%) for C3H6/O2=0.5. In situ XAS reveal that when the major valence fractions present are the Cu(I)/Cu(II) and V(III)/VIV) redox pairs, the selectivity and activity is improved over VCu:AlPO-5. By comparison, VCu:ZSM-5 produce acrolein for C3H6/O2=1 and 0.5 only, but with significantly lower yields (0.1% and 0.2%, respectively). At these conditions copper is mainly present as Cu(0), whereas vanadium exists as V(III)/V(IV). We believe activity is governed by the presence of available oxygen associated with a binuclear Cu/V complex in VCu:AlPO-5, not formed in the zeolite. Also, the activity is independent of copper valence state, indicating a possible Cu0/VOx interface is present in VCu:AlPO-5.