Vanadium phosphates on mesoporous supports: model catalysts for solid-state NMR studies of the selective oxidation of n-butane.

SBA-15 was utilized as mesoporous support for the dispersion of vanadium phosphate (VPO) compounds. Loading of SBA-15 with VPO compounds was found to be accompanied by decreasing 29Si MAS NMR signals of Q2 (Si(2Si,2OH)) and Q3 (Si(3Si,1OH)) silicon species, which indicates coverage of the mesoporous support by the guest compounds. The 51V MAS NNR spectra of the activated VPO/SBA-15 catalysts consist of patterns typical for the αII- and β-phases of vanadyl orthophosphate. In the 31P MAS NMR spectra of the activated VPO/SBA-15 catalysts, signals of β-, δ-, and αII-VOPO4 phases could be identified. Upon conversion of n-butane-13C4, a strong decrease of the 31P MAS NMR signals characteristic for the δ-VOPO4 phase occurred, while by 13C MAS NMR spectroscopy the formation of maleic anhydride, carbon monoxide, and carbon dioxide was observed. This finding supports the active role of the δ-VOPO4 phase in the selective oxidation of n-butane on VPO/SBA-15 catalysts.