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

Abstract

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 γ-Keggin polyoxotungstate, [γ-PW10O38V2(μ-O)­(μ–OH)]4– (V2-POM), immobilized on nitrogen-doped carbon nanotubes (N-CNTs) as heterogeneous catalyst. A series of supported catalysts V2-POM/N-CNTs containing 5–25 wt % of V2-POM and 0–4.8 atom % of N has been prepared and characterized by elemental analysis, N2 adsorption, SEM, TEM, XPS, and FTIR techniques. The catalytic performance of V2-POM/N-CNTs was assessed in the selective oxidation of 2,3,6-trimethylphenol (TMP) with H2O2 under mild reaction conditions (60 °C, MeCN). The presence of nitrogen in the support ensures strong adsorption and molecular dispersion of V2-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 V2-POM/N-CNTs enclosing 15 wt % of V2-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 V2-POM and N-CNT support under the turnover conditions.

Polyoxometalates: Tailoring metal oxides in molecular dimension toward energy applications

“…4,49,89,228,[304][305][306][307][308][309][310][311] 7 | POLYOXOVANADATE Vanadium oxide enjoys high coefficient of thermal resistance and has been applied to vanadium redox batteries and various catalysts. [312][313][314] Vanadium-containing polyoxometalates have been used in dye-sensitized solar cells via as cosensitizers 84 and as precursors for solar conversion photoanode synthesis such as BiVO 4 . 315 The inclusion of vanadium in polyoxometalates can improve the battery performance.…”

Carbon‐Support‐Based Heterogeneous Nanocatalysts: Synthesis and Applications in Organic Reactions

“…al. first time reported the immobilization of V 2 ‐POM (a catalytic active polyoxometalate) on N‐doped CNT for the investigation of catalytic activity towards the oxidation of industrial relevant phenols with aqueous H 2 O 2 . Here, the nitrogen doping plays a crucial role for enhancing the activity, selectivity and recyclability of V 2 ‐POM catalysts supported over CNT.…”

“…In this work, for the synthesis of CNT and N‐doped CNT a flow‐type quartz reactor was used. Finally immobilization of V 2 ‐POM on N‐doped CNT was carried out at room temperature …”

“…Evtushok et al. reported an approach for the environment friendly synthesis of TMBQ by divanadium based Keggin polyoxometalate (γ‐PW 10 O 38 V 2 (μ‐O) (μ‐OH)] 4− (V 2 ‐POM) immobilized over nitrogen‐doped carbon nanotubes (N‐CNT) using H 2 O 2 as an oxidant . They utilized divanadium‐substituted polyoxometalate supported on N‐doped CNTs (V 2 ‐POM/N‐CNT) for oxidation of alkylphenols.…”

Epoxidation of cyclohexene with H2O2 over efficient water‐tolerant heterogeneous catalysts composed of mono‐substituted phosphotungstic acid on co‐functionalized SBA‐15

“…Heterogenization of M‐POMs/POMs can offer materials that are much easier to separate, and may show improved selectivity and activities . This concept has been demonstrated in several systems, including V 2 ‐POM/Support, GO/MNPs/PW, POM‐poly and POM‐ZrO 2 . Despite interesting features of M‐POMs‐based heterogeneous catalysts, and recent progress in design and synthesis of these efficient catalysts, clean separation and recycling still present a considerable challenge .…”