Abstract
Ordered mesostructured hexagonal, cubic and lamellar vanadium-phosphorus-oxide phases displaying improved thermal stability, desirable bulk compositions, and vanadium oxidation states (3.8–4.3) for the partial oxidation of lower alkanes were synthesized using cationic (alkyl trimethyl ammonium salts), anionic (alkyl sulfonates and phosphates) and primary alkylamines as structure-directing agents. It was found that hexagonal mesophases were favored at short carbon chain lengths (C 12–C 16), while cubic phases were favored for longer chains (C 18). Structure-directing agents were removed by calcination or Soxhlet extraction in ethanol. When structure-directing agents were removed by extraction, phase transformations from hexagonal to cubic to lamellar structures were observed. The improved thermal stability of these mesophases was attributed to the acidic self-assembly conditions in the vicinity of the isoelectric point of V(V) oxocations.
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