The role of vanadium(V) in the catalysed oxidation of hydrocarbons

Monomeric and oligomeric vanadium(V) species (the latter obtained by acid-i nduced aggregation) are mostly anionic, and can be solubilized in non-polar solvents by cationic phase-transfer catalysts. Monomeric and cyclic tetra- and pentavanadate form 1:1 ion-pair complexes with Aliquat 336 (ALCl). The ion-pairs of decavanadates have the compositions [ALCl]/[V(V)](org) 1:0.3- 0.5, due to the formation of differently protonated species at lower pHs, B oth ALCl and the ion-pair complex AL-V(V) influence the rate of decompositi on of t-butylhydroperoxide and the rates of oxidation of hydrocarbons by O- 2. Pure ALCl is more active at pH > 7 than at pH < 7, in consequence of the transformation of ALCl into ALOH in alkaline medium. In contrast, the ion- pair complex AL-V(V) is hardly active at pH > 7, due to the low or zero act ivity of monomeric V(V). The activities of ion-pairs containing oligomeric V(V) species exceed that of ALCl alone. It was observed that the activities of both ALCl and the ion-pair complex AL-V(V) depend almost linearly on th eir concentration, but when an upper limit (about 0.1 M) is exceeded, the c atalytic activity suddenly drops to a low value. (C) 1999 Elsevier Science B.V. All rights reserved.