STRUCTURE AND CATALYTIC PROPERTIES OF SILICA-SUPPORTED POLYOXOMOLYBDATES .2. THERMAL-BEHAVIOR OF UNSUPPORTED AND SILICA-SUPPORTED 12-MOLYBDOSILIC ACID CATALYSTS FROM IR AND CATALYTIC REACTIVITY STUDIES

Thermal behavior of unsupported and silica-supported 12-molybdosilicic acid (with Mo loadings of 17.6 and 9 wt%) are studied by IR spectrometry and catalytic reactivity in the methanol oxidation reaction. In the case of the unsupported acid (SiMo12H), IR and catalytic reactivity measurements show that the destruction of the Keggin unit is rather abrupt in the temperature range 300-320°C (collapse of the acidic character in catalysis, formation of Mo03 evidenced by IR). The thermal stability of SiMo12H is reduced by ∼20-30°C when supported on silica, and the destruction of the Keggin unit extends over a larger temperature range. The nature of the Mo oxo species responsible for the redox activity after thermal treatments at temperatures higher than 330°C is discussed. © 1990.