CATALYSIS BY HETEROPOLY COMPOUNDS PART 39 - THE STRUCTURE AND REDOX BEHAVIOR OF VANADIUM SPECIES IN MOLYBDOVANADOPHOSPHORIC ACID CATALYSTS DURING PARTIAL OXIDATION OF ISOBUTANE

The states and roles of vanadium of 11-molybdo-1-vanadophosphoric acid (H4PMo11VO40, PMo11V) catalyst in the partial oxidation of isobutane (2-methylpropane) were analysed by EPR, V-51 and P-31 NMB, IR spectros copy, and redox titration, and compared with dodecamolybdophosphoric a cid (H3PMo12O40, PMo12) catalyst. Thermal treatment of PMo 11V at 623 K in O-2 caused the elimination of V from the Keggin anion and formed undefined polymeric and a monomeric V species. It was shown based on t he redox titration and EPR spectra that the average valency of V in us ed catalysts significantly changed with the oxygen partial pressure, w hile that of Mo remained almost unchanged for both PMo11V and PMo12. T he selectivities to methacrylic acid (MAA) and methacrolein (MAL) extr apolated to zero conversion were similar for both catalysts, but PMo11 V showed a significantly higher selectivity as the conversion increase d. This is because the secondary reaction, that is, oxidative decompos ition of MAL and MAA, was slower over PMo11V than over PMo12. This was consistent with the marked difference in the reaction order in oxygen pressure, and corresponded to the differences in the above redox prop erties.