ACIDITY OF VPO CATALYSTS - AMMONIAC ADSORPTION STUDIED BY MICROCALORIMETRY AND TPD

Three different V-P-O catalysts have been prepared. DRX, XPS and P-31-NMR have shown that (VO)(2)P2O7 is the main phase in the three catalysts which however display differences regarding for example the crystallisation and the additionnal presence in two solids of either V3+ or V5+ species. Ammonia adsorption has been used to measure the strenght of acidic sites by microcalorimetry and TPD, the number of these sites being determined by volumetry and TPD. The acidity of these solids has been discussed in relation with their catalytic performance in the reaction of n-butane oxidation. The intrinsic activity of the three catalysts for maleic anhydride formation increases in the same order as their number of relatively strong acid sites. Then the present results support the view which considers that acid sites on V-P-O catalysts participate to the reaction, likely in the initial step of butane activation as already proposed.