Uniform heterostructured MnOx/MnCO3/Fe2O3 nanocomposites assembled in an ionic liquid for highly selective oxidation of 5-hydroxymethylfurfural

Abstract

The development of non-precious metal catalysts for highly selective oxidation of the platform chemical 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) remains a challenge. Herein, we prepared uniform heterostructured MnO_x, MnCO₃, and Fe₂O₃ (MnO_x/MnCO₃/Fe₂O₃) nanocomposites assembled in the ionic liquid 1-butyl-3-methyl-imidazolium chloride ([BMim]Cl) via a hexamethylenetetramine (HMT) assisted hydrothermal process, and these nanocomposites could be used as efficient catalysts for the aerobic oxidation of HMF. Powder X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), N₂ absorption/desorption, thermogravimetric and differential thermal analysis (TG–DTA), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectrometry (ICP-AES), Fourier transform infrared (FT-IR) spectroscopy, and H₂ temperature programmed reduction (H₂-TPR) were carried out to characterize the structural properties of the nanocomposites. In the preparation process, MnCO₃/Fe₂O₃ was initially formed via hydrogen bond and π–π stacking interactions between [BMim]Cl and the metal precursor, where [BMim]Cl was used as a capping agent and HMT not only served as a carbonate source but also acted as a precipitating agent. Thus, a uniform heterostructured MnO_x/MnCO₃/Fe₂O₃ nanocomposite with a size distribution of about 60 nm was obtained through calcination at the desired temperature and it exhibited superior catalytic performance with a HMF conversion of 48% and high selectivity (100%) to DFF, which was attributed to the synergistic effect of MnO_x, MnCO₃, and Fe₂O₃ in the composite. The Mn/Fe mole ratio and HMT amount could tune the composition of MnO_x, MnCO₃, and Fe₂O₃, influence the reduction capability of the prepared nanocomposite via synergisitic effects, and thus have a definite effect on catalytic activity. Additionally, the effects of various conditions were investigated to optimize the reaction conditions. Furthermore, the uniform heterostructured MnO_x/MnCO₃/Fe₂O₃ composite could be readily recovered and reused without loss of its catalytic activity and thus is suitable for potential applications.