Supported C-Scorpionate Vanadium(IV) Complexes as Reusable Catalysts for Xylene Oxidation

Abstract

C-Scorpionate vanadium(IV) [VO_x Cl_3-x {κ³ -RC(pz)₃ }] [pz=pyrazol-1-yl; x=0, R=SO₃ (1); x=1, R=CH₂ OH (2) or CH₂ OSO₂ Me (3)] complexes supported on functionalized carbon nanotubes (CNTs) are the first V-scorpionate catalysts used so far for the neat oxidation of o-, m- or p-xylene, with TBHP (70 % aqueous solution), to the corresponding toluic acids (main products), tolualdehydes and methylbenzyl alcohols. Remarkably, a p-toluic acid yield of 43 % (73 % selectivity, TON=1.34×10³ ) was obtained with 2@CNT in a simple microwave-assisted mild oxidation procedure, using a very low catalyst charge (3.2×10^-2 mol ^% vs. substrate). Further, this occurred in the absence of any bromine source, what is significant towards the development of a greener and more sustainable process for oxidation of xylenes. Moreover, reuse of catalysts with preservation of their activity was found for up to six consecutive cycles. The effects of reaction parameters, such as reaction time, temperature, amount of catalyst or type of heating source, on the performance of the above catalytic systems are reported and discussed.

Keywords: C-scorpionate; carbon nanotubes; oxidation; vanadium; xylenes.