NATURE AND ACTIVITY OF SOME VANADIUM CATALYSTS

A series of vanadium-containing catalysts, where the vanadium was introduced either during synthesis (VAPO-5) or by postsynthesis treatment of a precursor solid (V-silicalite, V-ZSM5, or V-Aerosil 90), were prepared and characterized. ESR examination of these catalysts after treatment with refluxing toluene indicated that only VAPO-5 contained all of its ESR-detectable vanadium as isolated V(IV) vanadyl. The other catalysts contained varying proportions of interacting/isolated V(IV) vanadyls, and this could be varied with treatment conditions. In spite of the differences in the state of the introduced vanadium, gas-phase oxidation (O2) of toluene over VAPO-5, V-silicalite, V-ZSM5, and V-Aerosil 90 at 573-823 K gave similar reaction behavior, with activation energies falling in the range 85 +/- 10-105 +/- 10 kJ mol-1, and with total oxidation being the dominant reaction (CO(x) greater-than-or-equal-to 93 mol % of product). V2O5/quartz gave a slightly higher activation energy for this reaction (120 +/- 10 kJ mol-1), with only a slightly higher yield of partial oxidation products (9-10 mol % of products). These results are compared with those published from other vanadium catalysts, and the oxidation is discussed in terms of ring and methyl side-chain oxidation for which plausible mechanims are proposed. The catalyzed liquid-phase oxidation of toluene by H2O2 was examined. Of the catalysts examined, only VAPO-5 showed any resistance to vanadium leaching from the catalyst by refluxing with 30% w/v H2O2, with ca. 40% of the original VAPO-5 vanadium being leached into solution. This solution vanadium showed some activity as a catalyst for toluene partial oxidation by H2O2, although the toluene conversion was very low (less-than-or-equal-to 1%). The vanadium which remained in the VAPO-5 and was resistant to H2O2 leaching had negligible catalytic activity for toluene oxidation by H2O2.