Study of cesium or cesium-transition metal-substituted Keggin-type phosphomolybdic acid as isobutane oxidation catalysts

Cesium and cesium-transition metal-substituted Keggin-type phosphomolybdic acids have been prepared and characterized using several techniques. The re sults obtained show that all heteropoly-compounds with formulae H3-xCsxPMo1 2O40 where 0 < x < 3 were composed of two phases corresponding to the hydra ted acid and the pure cesium salt with the acid phase coating the salt part icles. For compounds with x < 2, the acid was present in large amounts and could be detected by X-ray diffraction. For compounds with x > 2 the precip itation rates vary, leading to very small cesium salt particles. In this se cond case the acid phase also coated the particles, but it was no longer de tectable by X-ray diffraction, although it could be observed by XPS or Rama n spectroscopy. All the data obtained show that the transition metal cation s are replacing the protons in the supported acid phase and not the cesium atoms in the salt. This replacement does not change the structure of the bu lk acid phase even at high substitution levels, but it decreases the hydrat ion rate of the supported acid. (C) 1999 Academic Press.