Controllable synthesis of novel nanoporous manganese oxide catalysts for the direct synthesis of imines from alcohols and amines

A novel template-free oxalate route was applied to synthesize different mesoporous manganese oxides (amorphous manganese oxide (AMO), Mn5O8, Mn3O4, MnO2) in the narrow temperature range from 350 °C to 400 °C by controlling the calcination conditions, which were employed as the efficient catalysts for the oxidative coupling of imines from alcohols and amines. The chemical and structural properties of the manganese oxides were characterized by the methods of thermal analysis and heat flow (TG-DSC), X-ray diffraction (XRD), nitrogen sorption, scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR), and inductively coupled plasma optical emission spectrometry (ICP-OES) techniques. The structures of different manganese oxides were confirmed by characterization. The M-350 (AMO) presented the maximum surface area, amorphous nature, the lowest reduction temperature, the higher (Mn3+ + Mn4+)/Mn2+ ratio, and the higher adsorbed oxygen species compared to other samples. Among the catalysts, M-350 showed the best catalytic performance using air as an oxidant, and the conversion of benzyl alcohol (BA) and the selectivity of N-benzylideneaniline (NBA) reached as high as 100% and 97.1% respectively at the lower reaction temperature (80 °C) for 1 h. M-350 had also the highest TOF value (0.0100 mmol·mg−1·h−1) compared to the other manganese oxide catalysts. The catalyst was reusable and gave 95.8% conversion after 5 reuse tests, the XRD pattern of the reactivated M-350 did not show any obvious change. Lattice oxygen mobility and (Mn3+ + Mn4+)/Mn2+ ratio were found to play the important roles in the catalytic activity of aerobic reactions.