The selective oxidation of n-butane to maleic anhydride (MA) over the vanadium phosphorus oxide (VPO) catalyst stands as a prominent example of industrial selective alkane functionalization. Despite its commercial maturity, the ambiguous relationship between the catalytic performance and vanadium valence states (Vox) or surface lattice oxygen (OS-L) persists in generating debate and uncertainty regarding the identity of the active sites in VPO catalysts. Herein, we reconcile this debate by demonstrating that the catalytic efficiency is governed by valence-state-dependent OS-L rather than by merely the OS-L content. Using time-resolved H2-TPR experiments, we successfully distinguished two distinct types of valence-state-dependent OS-L: V5+-OS-L (OS-L related to the V5+ phase) and V4+-OS-L (OS-L related to the V4+ phase). Their specific functions were then elucidated through a combination of in situ reactant-specific H2-TPR experiments (utilizing n-butane, 1,3-butadiene, or propylene as probes) and 18O-isotopic labeling studies. The results clearly demonstrate that V5+-OS-L is identified as the site for the initial hydrogen abstraction from n-butane, while V4+-OS-L facilitates the crucial epoxidation step to form MA. Furthermore, by employing CeO2 as a modulator, we precisely tune the V5+-OS-L/V4+-OS-L ratio through the formation of Ce–O–V bonds, as evidenced by the X-ray absorption near edge (XANES) and extended X-ray absorption fine structure (EXAFS) analysis. An optimal V5+-OS-L/V4+-OS-L ratio for MA synthesis is 2.22–2.82; otherwise, it would be more conducive to a nonselective reaction. This work introduces a novel design paradigm for the development of high-performance VPO catalysts, offering a comprehensive framework for the rational design of heterogeneous oxidation catalysts via precise control of valence-specific oxygen chemistry.
Li et al. (2026) studied this question.