Tailoring single site VO₄ on flame-made V/Al₂O₃ catalysts for selective oxidation of n-butane

Selective aerobic oxidation of n-butane is one of the most complex selective aerobic oxidation reactions but is of great importance in producing maleic anhydride (MA) as a versatile chemical intermediate/platform in the chemical industry. Supported VO_x catalysts are extensively used in the selective oxidation of alkanes. However, the identification of active sites is quite challenging since various VO_x structures (single sites, clusters, nanoparticles, and crystals) and the related surface functional groups (V = O, V-O-V, and V-OH) can be formed on the surface. By simply tuning the V/Al ratios of V/Al₂O₃ catalysts, we prepared various VO_x structures using flame-spray pyrolysis and investigated their activities in the selective oxidation of n-butane to MA. The presence of single-site VO_x (SSV), polymeric VO₄ species, crystalline AlVO₄ clusters, and vanadium oxide is probed by systematic studies using ⁵¹V and ²⁷Al solid-state NMR spectroscopy and confirmed by XRD, HRTEM, Raman, and UV–vis spectroscopies. VO_x single-sites interacting with surface penta-coordinated Al (Al^V) sites are identified as active sites promoting the conversion of n-butane to MA. V/Al₂O₃ with a high population of SSV-Al^V pairs in the amorphous Al₂O₃ network provides the best performance compared to other V/Al₂O₃ catalysts and those reported in the literature. Finally, a reaction mechanism based on the concerted action of SSV-Al^V pairs is proposed.