A holistic understanding of the key catalytic features of vanadyl­(IV) pyrophosphate enabling high maleic anhydride (MAN) yields in n-butane oxidation has fostered a debate which has continued since the finding of the catalyst. Under reaction conditions, vanadium­(V) orthophosphate structure fragments were detected on the surface of the catalyst. However, single-phase αII- and β-VVOPO4 reveal a much lower catalytic performance. This study shows that introducing Nb into αII-VOPO4 forming a solid solution (V1-x Nb x )OPO4 yields a bulk material with tunable catalytic properties. Selectivities of S MAN = 48% at a conversion of X n‑butane = 30% on (V0.1Nb0.9)­OPO4 are shown to be related to the isolation of surface V-sites, which surpass known VOPO4 catalysts by far. A boost in the overall n-butane consumption and MAN selectivity under alkane-rich feed conditions is shown to be another characteristic of (V1-x Nb x )­OPO4, leading to a highly increased MAN productivity. XPS studies reveal that a progressive replacement of V by Nb induces a reduction of the averaged oxidation state of near-surface V from +4.7 to +4.3, a finding that correlates linearly with an elevated MAN selectivity. This study experimentally confirms site isolation and electronic environment of the near-surface V-species as the key catalytic properties, from which catalyst design rules are derived to optimize partial oxidation reactions.

Two thermodynamically metastable polymorphs of vanadium­(III) phosphate, VIIIPO4-m1 and VPO4-m2, have been obtained via reduction of β-VVOPO4 by moist hydrogen. The XRPD pattern of VPO4-m1 can be assigned based on the crystal structure of β-VVOPO4, though with distinctly different lattice parameters (VPO4-m1/β-VOPO4: Pnma, a = 7.3453(12)/7.7863(5) Å, b = 6.4001(12)/6.1329(3) Å, c = 7.3196(13)/6.9673(5) Å). The XRPD pattern of VPO4-m2 was found to be very similar to that of Fe2(VO)­(P2O7)­(PO4) (VPO4-m2: P21/m, Z = 2, a = 8.792(4) Å, b = 5.269(2) Å, c = 10.398(6) Å, β = 112.60(4)°). The crystal structure models for VPO4-m1 and VPO4-m2 have been optimized by DFT calculations. Polymorph m1 contains the unprecedented butterfly shaped [VIIIO4] chromophore and has been further characterized by magnetic measurements, by powder reflectance spectroscopy (NIR/vis/UV), and IR spectroscopy. For six polymorphic forms of VPO4 (m1′, m1′′, m2, m3, m4, and m5), DFT calculations have been performed. For the existence of VPO4-m1′, -m1′′, and -m2, our experiments provide evidence. VPO4-m3, -m4, and -m5 were obtained by structure optimization based on reduced β-VOPO4. Their stability is predicted by the DFT calculations.

No abstract