Cotton fiber-anchored Nb single-site catalyst for selective oxidation of anilines to azoxybenzenes

This work demonstrated a facile methodology to construct single Nb site catalysts with merely 0.63 wt% Nb by employing alpha-hydroxy carboxylic acid-functionalized cotton fiber as green and sustainable supporting material. The as-synthesized catalysts have been characterized thoroughly and exhibited a superior catalytic activity for selective oxidation of aniline into azoxybenzene with nearly 100% utilization efficiency of H2O2 under mild and green conditions. Notably, it demonstrated the highest TOF (4806 h-1) among the currently reported hetero-geneous catalysts. Moreover, the catalyst has steadily been operated under continuous-flow conditions, which afforded significant benefits compared to batch processes as reported in these transformations. The activity test and catalyst characterization indicated that the atomically dispersed Nb-peroxo coordination centers were actually catalytic sites. Especially, the alpha-hydroxy carboxylic acid groups on cotton fiber played an important role in coordinating/stabilizing single Nb site. Moreover, 1H MAS NMR spectra verified that there exists an acid-base interaction between arylamines and free alpha-hydroxy carboxylic acid groups covalently grafted on cotton fiber, which was favorable for adsorption and activation of the substrates. Mechanism studies indicated that the azoxy derivatives were produced through condensation step between N-phenylhydroxylamine and nitrosobenzene rather than direct oxidation of azobenzene. The work provides an economical and environmentally friendly approach to construct single-site catalysts for the oxidation catalysis.