SILICA-SUPPORTED 12-MOLYBDOPHOSPHORIC ACID CATALYSTS - INFLUENCE OF THE THERMAL TREATMENTS AND OF THE MO CONTENTS ON THEIR BEHAVIOR, FROM IR, RAMAN, X-RAY-DIFFRACTION STUDIES, AND CATALYTIC REACTIVITY IN THE METHANOL OXIDATION

The thermal behavior of silica-supported 12-molybdophosphoric acid cat alysts (PMoH-x series, x = Mo wt%, varying from 16.2 to 0.9) was inves tigated by using different techniques (IR and Raman spectroscopies, X- ray diffraction, and catalytic reactivity in methanol oxidation). The silica support used in the study (surface area 376 m(2) g(-1)) induces destabilization of 12-molybdophosphoric acid. The transformation into beta-MoO3 begins from 250 degrees C, and is achieved at 350 degrees C . beta-MoO3 is the main species on silica support in a wide range of t emperatures, and seems to be stabilized by the support. The role of th is molybdenum oxide is discussed in terms of active species in the cat alytic reaction, and the capability to rebuild the Keggin unit under e xposure to water vapor.