Properties of Cs2.5 salts of transition metal M substituted Keggin-type M1−xPV1MxMo11−xO40 heteropolyoxometallates in propane oxidation Original Research Article

Keggin-type heteropolyoxometallate compounds were prepared with introducing a redox transition metal M (M=CoII, FeIII, GaIII, NiII, SbIII or ZnII) in the solution before precipitating the Cs2.5 salts The transition metal M was partly substituting Mo in the Keggin anion and partly replacing the charge balancing protons, as represented by the general formula: Cs2.5H6x−yM1−xPV1MxMo11−xO40. In some experiments, the transition metal cation was simply replacing the charge balancing protons as in Cs2.5H1.5−3xMxPV1Mo11O40 samples with M=Fe3+ or Ga3+. TGA measurements, IR of lattice vibration bands and 31P MAS-NMR data allowed us to estimate the relative amount of exchanged and substituted transition metal cations and the Keggin anion structure stability. It appeared that introducing a transition metal M in the Keggin anion led in propane oxidation reaction near 340 °C to much more propylene formation (60–80% selectivity) and lower acetic acid and COx formation than for Cs2.5H1.5PV1Mo11O40 sample, while Keggin anion was observed not being destroyed during the reaction. When the transition metal cation was exchanged for protons, it was observed that the effect of redox transition metal was much less pronounced while the number of acid sites was decreased. At last, water introduced in the reaction feed led for all samples to a sharp increase in acetic acid and decrease in COx and propylene. This allowed us to propose a two-pathway reaction mechanism for the reaction with one pathway via isopropyl alcohol and then acetone being favoured by water and the strong Brønsted acidity of the heteropolyoxometallate compounds.