PREPARATIVE EFFECTS ON THE SURFACE-AREA AND PORE STRUCTURE OF MICROPOROUS HETEROPOLY OXOMETALATES

The ammonium, potassium, and cesium salts of the heteropoly oxometalates of Keggin structure, in particular 12-molybdophosphoric and 12-tungstophosphoric acids have previously been shown to have high surface areas and microporous structures. In the present work the effect of the source of the cation, the stoichiometry of the preparation, the nature of the cation, the peripheral metal atom of the anion, and the thermal stability of a number of heteropoly oxometalates of Keggin structure has been evaluated through measurements of adsorption-desorption isotherms of nitrogen and comparisons of the calculated surface areas and pore structures. The source of the cation has relatively little effect, except where the sulfate is the anion while a deficit of ammonium carbonate employed in the preparation produces a substantial change. The surface areas and contribution of the micropores generally decrease markedly with increase in temperature but the extent of such decreases depends significantly on the nature of the cation.