MnOx supported on a TiO2@SBA-15 nanoreactor used as an efficient catalyst for one-pot synthesis of imine by oxidative coupling of benzyl alcohol and aniline under atmospheric air

Abstract

In the present study, a mesoporous silica (SBA-15) encapsulated TiO₂ nanoreactor is used as a support for MnO_x and this MnO_x/TiO₂@SBA-15 acts as a catalyst for the one-pot synthesis of imine by oxidative coupling between benzyl alcohol and aniline in the presence of atmospheric air. To understand the properties, the catalysts were characterized by several analytical techniques, namely, N₂ adsorption–desorption isotherm, small angle X-ray scattering (SAXS), wide angle X-ray diffraction, high resolution transmission electron microscopy (HRTEM), H₂-temperature programmed reduction (H₂-TPR), O₂-temperature programmed oxidation (O₂-TPO) and NH₃-temperature programmed desorption (NH₃-TPD). The pore encapsulation process by SBA-15 causes TiO₂ to be in a highly dispersed state, and this highly dispersed TiO₂ makes maximum contact with the MnO_x species as well as the reactant molecules. The reaction was carried out at atmospheric pressure with equimolar amounts of substrates without additives in the presence of atmospheric air. The yield and selectivity of imines vary with the MnO_x and TiO₂ loading. The 7.5 wt% MnO_x loaded TiO₂@SBA-15 (5 wt% TiO₂) nanoreactor showed the highest catalytic activity. With the increase in weak acid sites and the oxygen activation ability of the prepared catalyst, the conversion and selectivity of the desired product reached 96% and 97%, respectively. The investigation of the reaction mechanism suggests that there is a synergistic effect between highly dispersed TiO₂ and MnO_x, which improves the reactant conversion and the selectivity of the desired product (N-benzylideneaniline) and also the prepared catalyst shows excellent recyclability up to the 10^th cycle. The recyclability and hot filtration study confirms the true heterogeneity of the prepared catalyst during imine synthesis. The heterogeneity of the prepared catalyst, the avoidance of any noble metal and the utilization of air as an oxidizing agent represent an efficient, green reaction pathway for imine synthesis.