The effect of zirconium in metal/bromide catalysts on the autoxidation of p-xylene

Abstract

Abstract Metals, such as Fe, V, Ti, Hf, have the same properties as Zr(IV), i.e. the ability to activate a Co/Mn/Br catalyst and to enhance the rate of benzaldehyde formation. There is a perfect correlation with the pK of hydrolysis, a measure of Lewis acidity, and the degree of activation of the metals. The data is consistent with the activation mechanism being a Lewis acid catalyzed dehydration of the benzyl peroxides. Certain metals, including zirconium, inhibit the formation of the undesirable black, insoluble MnO2, during Co/Mn/Br catalyzed autoxidations. In general, the increasing ability of a metal to avoid MnO2 formation is correlated to increasing Z2/r values (Z is the formal valence state and r the ionic radius). It is proposed that the metal can avoid MnO2 formation by decreasing the precursor concentration of Mn–OH2. The solid MnO2 produced during an autoxidation can produce radicals derived from p-xylene which further react to form undesirable dimeric by-products. The presence of Zr(IV) or Hf(IV) kinetically and thermodynamically inhibit the precipitation of the Co(II) and Mn(II) pyromellitic acid (PMA) complexes. The phenomena of activation, prevention of Mn(IV) formation, and solubilization in the presence of select carboxylic acids all occur when the metal has high Lewis acidity. This is consistent with the ability of Zr and Hf to preferentially complex with the acid. Mechanisms are suggested for the above phenomena which illustrates the intimate role that the solvent, acetic acid, performs in these autoxidations.