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

• Excellent performance in converting n -butane to maleic anhydride achieved. • Single-site VO x interacting with penta-coordinated Al (Al V ) are active sites. • A reaction mechanism based on single-site VO x -Al V pairs is proposed. 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/Al 2 O 3 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 VO 4 species, crystalline AlVO 4 clusters, and vanadium oxide is probed by systematic studies using 51 V and 27 Al 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/Al 2 O 3 with a high population of SSV-Al V pairs in the amorphous Al 2 O 3 network provides the best performance compared to other V/Al 2 O 3 catalysts and those reported in the literature. Finally, a reaction mechanism based on the concerted action of SSV-Al V pairs is proposed.