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 (29)Si MAS NMR signals of Q(2) (Si(2Si,2OH)) and Q(3) (Si(3Si,1OH)) silicon species, which indicates coverage of the mesoporous support by the guest compounds. The (51)V MAS NNR spectra of the activated VPO/SBA-15 catalysts consist of patterns typical for the alpha(II)- and beta-phases of vanadyl orthophosphate. In the (31)P MAS NMR spectra of the activated VPO/SBA-15 catalysts, signals of beta-, delta-, and alpha(II)-VOPO(4) phases could be identified. Upon conversion of n-butane-(13)C(4), a strong decrease of the (31)P MAS NMR signals characteristic for the delta-VOPO(4) phase occurred, while by (13)C MAS NMR spectroscopy the formation of maleic anhydride, carbon monoxide, and carbon dioxide was observed. This finding supports the active role of the delta-VOPO(4) phase in the selective oxidation of n-butane on VPO/SBA-15 catalysts.