Vanadium phosphate

Vanadium phosphates are inorganic compounds with the formula VOxPO4 as well related hydrates with the formula VOxPO4(H2O)n. Some of these compounds are used commercially as catalysts for oxidation reactions.

Vanadium phosphate
Identifiers
Properties
H4O7PV (dihydrate)
Molar mass 197.94 (dihydrate)
Appearance yellow solid (dihydrate)
insoluble
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Vanadium(V) phosphates

A common vanadium phosphate is VOPO4•2H2O.

Seven polymorphs are known for anhydrous VOPO4, denoted αI, αII, β, γ, δ, ω, and ε.[lower-alpha 1][1][2][3] These materials are composed of the vanadyl group (VO) and phosphate (PO43−). They are yellow, diamagnetic solids, although when contaminated with vanadium(IV) derivatives, samples exhibit EPR signals and have bluish cast. For these materials, vanadyl refers to both vanadium(V) oxo and vanadium(IV) oxo centers, although conventionally vanadyl is reserved for derivatives of VO2+.

Structure of the V-O-P framework of VOPO4(H2O)2 (one lattice water and all hydrogen atoms are omitted).

Preparation, reactions, and applications of VOPO4•2H2O

Heating a suspension of vanadium pentoxide and phosphoric acid gives VOPO4•2H2O, isolated as a bright yellow solid.[4] According to X-ray crystallography, the V(V) centers are octahedral, with long, weak bonds to aquo ligands.[5]

Reduction of this compound with alcohols gives the vanadium(IV) phosphates.

These compounds are catalysts for the oxidation of butane to maleic anhydride. A key step in the activation of these catalysts is the conversion of VO(HPO4)•0.5H2O to the pyrophosphate (VO)2(P2O7). This material (CAS#58834-75-6) is called vanadyl pyrophosphate as well as vanadium oxide pyrophosphate.[6]

Vanadium(IV) phosphates

Several vanadium(IV) phosphates are known. These materials are typically blue. In these species, the phosphate anion is singly or doubly protonated. Examples include the hydrogenphosphates, VOHPO4.4H2O and VO(HPO4).0.5H2O, as well as the dihydrogen phosphate VO(H2PO4)2.

Portion of the crystal structure of VO(HPO4).0.5H2O. The vanadium is octahedrally coordinated and phosphorus is tetrahedral. Water is a bridging ligand. Color code: red = O; gray = V, H; ochre = P.[7]

Vanadium(III) phosphates

Vanadium(III) phosphates lacking the oxo ligand have the formula VPO4•H2O and VPO4•2H2O. The monohydrate is isostructural with MgSO4•H2O[8] It adopts the structure of the corresponding hydrated aluminium phosphate. Oxidation of VPO4•H2O yields the two-electron electroactive material ε-VOPO4[9]

Notes
  1. The αI and αII polymorphs are sometimes called α1 and α2 instead.

References
  1. Gautier, Romain; Gautier, Régis; Hernandez, Olivier; Audebrand, Nathalie; Bataille, Thierry; Roiland, Claire; Elkaïm, Erik; Pollès, Laurent Le; Furet, Eric; Fur, Eric Le (2013-05-15). "DFT-assisted structure determination of α1- and α2-VOPO4: new insights into the understanding of the catalytic performances of vanadium phosphates". Dalton Transactions. 42 (22): 8124–8131. doi:10.1039/C3DT50217A. ISSN 1477-9234. PMID 23584576.
  2. "New Contributions to the Structural Chemistry of Vanadyl Orthophosphate VOPO4". edoc.mpg.de. Retrieved 2019-09-13.
  3. Girgsdies, Frank; Schneider, Matthias; Brückner, Angelika; Ressler, Thorsten; Schlögl, Robert (2009-07-01). "The crystal structure of δ-VOPO4 and its relationship to ω-VOPO4". Solid State Sciences. 11 (7): 1258–1264. Bibcode:2009SSSci..11.1258G. doi:10.1016/j.solidstatesciences.2009.03.017. hdl:11858/00-001M-0000-0010-FAAA-9. ISSN 1293-2558.
  4. Brody, John F.; Johnson, Jack W.; Vaughey, Jack (2007-01-05), Murphy, Donald W.; Interrante, Leonard V. (eds.), "Vanadyl Phosphates and Organylphosphonates", Inorganic Syntheses, John Wiley & Sons, Inc., pp. 241–248, doi:10.1002/9780470132616.ch45, ISBN 9780470132616
  5. Tietze, Hr (1981). "The crystal and molecular structure of oxovanadium(V) orthophosphate dihydrate, VOPO4(H2O)2". Australian Journal of Chemistry. 34 (10): 2035. doi:10.1071/CH9812035. ISSN 0004-9425.
  6. Dummer, N.F.; Bartley, J.K.; Hutchings, G.J. (2011), "Vanadium Phosphate Materials as Selective Oxidation Catalysts", Advances in Catalysis, Elsevier, 54, pp. 189–247, doi:10.1016/b978-0-12-387772-7.00004-6, ISBN 9780123877727
  7. Koo, Hyun-Joo; Whangbo, Myung-Hwan; VerNooy, Paul D.; Torardi, Charlie C.; Marshall, William J. (2002). "Flux Growth of Vanadyl Pyrophosphate, (VO)2P2O7, and Spin Dimer Analysis of the Spin Exchange Interactions of (VO)2P2O7 and Vanadyl Hydrogen Phosphate, VO(HPO4)·0.5H2O". Inorganic Chemistry. 41 (18): 4664–4672. doi:10.1021/ic020249c. ISSN 0020-1669. PMID 12206689.
  8. Vaughey, J. T.; Harrison, William T. A.; Jacobson, Allan J.; Goshorn, David P.; Johnson, Jack W. (1994). "Synthesis, structure, and properties of two vanadium(III) phosphates: V(PO4)•H2O and V1.23(PO4)(OH)0.69(H2O)0.31•0.33H2O". Inorganic Chemistry. 33 (11): 2481–2487. doi:10.1021/ic00089a027. ISSN 0020-1669.
  9. Lim, S; Vaughey, J. T.; Harrison, W. T. A.; Dussack, L. L.; Jacobson, A. J.; Johnson, J. W. (1996). "Redox transformations of simple vanadium phosphates: the synthesis of ε-VOPO4". Solid State Ionics. 84 (3–4): 219–226. doi:10.1016/0167-2738(96)00007-0.
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