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

Abstract

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 VOx catalysts are extensively used in the selective oxidation of alkanes. However, the identification of active sites is quite challenging since various VOx 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 VOx structures using flame-spray pyrolysis and investigated their activities in the selective oxidation of n-butane to MA. The presence of single-site VOx (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. VOx single-sites interacting with surface penta-coordinated Al (AlV) sites are identified as active sites promoting the conversion of n-butane to MA. V/Al2O3 with a high population of SSV-AlV 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-AlV pairs is proposed.