Highly Efficient Catalysts Based on Divanadium-Substituted Polyoxometalate and N-Doped Carbon Nanotubes for Selective Oxidation of Alkylphenols

Alkyl-substituted benzoquinones serve as versatile building blocks for a variety of biologically active compounds. In this work, we present an approach for the environmentally benign synthesis of alkyl-p-benzoquinones, in particular trimethyl-p-benzoquinone (TMBQ, vitamin E precursor), which employs aqueous hydrogen peroxide as oxidant and a divanadium-substituted gamma-Keggin polyoxotungstate, [gamma-PW10O38V2(mu-O)(mu-OH)](4-) (V-2-POM), immobilized on nitrogen-doped carbon nanotubes (N-CNTs) as heterogeneous catalyst. A series of supported catalysts V-2-POM/N-CNTs containing 5-25 wt % of V-2-POM and 0-4.8 atom % of N has been prepared and characterized by elemental analysis, N-2 adsorption, SEM, TEM, XPS, and FTIR techniques. The catalytic performance of V-2-POM/N-CNTs was assessed in the selective oxidation of 2,3,6-trimethylphenol (TMP) with H2O2 under mild reaction conditions (60 degrees C, MeCN). The presence of nitrogen in the support ensures strong adsorption and molecular dispersion of V-2-POM on the carbon surface, leading to highly active and selective heterogeneous catalysts, which do not suffer from metal leaching and can be used repeatedly without loss of the catalytic performance. By application of the optimal catalyst V-2-POM/N-CNTs enclosing 15 wt % of V-2-POM and 1.8 atom % of N, TMBQ could be obtained with 99% yield and 80% oxidant utilization efficiency. The catalyst demonstrated the truly heterogeneous nature of the catalysis and high turnover frequencies (500 h(-1)) and space-time yield (450 g L-1 h(-1)). FTIR and XPS techniques confirmed the stability of V-2-POM and N-CNT support under the turnover conditions.