Vanadyl(FV) Phosphonates, VOC_nH_2n+1PO₃•xH₂O (n = 0-4, x = 1 or 1.5), as Precursors of Vanadyl(IV) Pyrophosphate, (VO)₂P₂O₇

Vanadyl(IV) phosphonates with the formula VOC_nH_2n+1PO₃•xH₂O (n = 0-4, x = 1 or 1.5) have been synthesized in organic medium by reaction of V₂O₅ with the corresponding phosphonic acid. The low-temperature magnetic susceptibility of vanadyl(IV) phosphite (n = 0) and n-butylphosphonate (n = 4) was indicative of antiferromagnetically exchangecoupled V2O9 vanadyl dimers similar to previously reported results for n = 1-3 phosphonates and vanadyl(IV) hydrogen phosphate hemihydrate (VOHPO₄•0.5H₂O). Raman spectroscopy of vanadyl(IV) phosphonates (n = 0-4) also suggested the similar local crystalline order in these solids. The close structural relationship between these phosphonates and VOHPO₄•0.5H₂O is further substantiated by the thermal transformation of the former phases in the presence of oxygen into vanadyl(IV) pyrophosphate. Kinetic measurements of the n-butane selective oxidation to maleic anhydride demonstrated that vanadyl methylphosphonate (n = 1) and phoshite (n = 0) produced vanadyl pyrophosphate catalysts with comparable (the methylphosphonate) and even superior performance (vanadyl phosphite) compared to the conventional unpromoted vanadium - phosphorus - oxide (VPO) catalyst.