Preparation, characterization and catalytic performance of polyoxometalate immobilized on the surface of halloysite

Polyoxometalates (POMs) are well known to have excellent catalytic performances for oxidation of organic substrates. In this paper, we immobilized POM, Cs3(NH4)[{Ru4O6(H2O)9}2Sb2W20O68(OH)2]·9H2O (SbWRu), on the surface of halloysite nanotubes (HNTs) functionalized by 3-aminopropyltriethoxysilane (Apts) to prepare a novel heterogeneous catalyst, HNTs/Apts/SbWRu, which is never reported in the literature to our knowledge. The catalyst HNTs/Apts/SbWRu was characterized by elemental analysis, IR spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and N2 adsorption measurements to determine its composition, structure and morphology. The catalytic performance of this catalyst was tested in green oxidation system of n-tetradecane using air as oxidant under the mild reaction condition with normal atmospheric pressure and low temperature and without adding any solvents and additives. Furthermore, in order to find the optimum catalytic reaction condition, we prepared five catalysts containing different amounts of SbWRu, 0.97%, 1.94%, 2.80%, 4.23% and 5.87%. The results of the controlled experiments confirmed that the catalyst containing SbWRu of 1.94% exhibited high activity with a conversion (53.30%) and turnover frequency (TOF: 52396 h−1) at the optimal reaction condition. Moreover, this catalyst can be recovered and reused by filtration without significant loss of its catalytic performance for at least five times. The final conversion of n-tetradecane runs up to 87.97% after five consecutive cycles without the separation of the catalyst HNTs/Apts/SbWRu (1.94%).