Effect of manganese substitution on the physicochemical properties and catalytic toluene oxidation activities of Mg-Al layered double hydroxides

Abstract

A series of Mg-Mn-Al ternary hydrotalcite-like layered double hydroxides (LDHs) with (Mg+Mn)/Al atomic ratios of ~3 and Mg:Mn atomic ratios ranging from 3:0 to 0:3 were synthesized by a co-precipitation method. The incorporation of Mn in the MgAl-LDH was investigated by employing various techniques, such as powder X-ray diffraction, Fourier transform infrared spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, UV-visible diffuse-reflectance spectroscopy (UV-Vis DRS) and thermogravimetry. A single phase corresponding to LDH was obtained up to the composition Mg:Mn:Al=2.2:0.8:1. A further increase in the Mn content resulted in the formation of a mixture of LDH, Mn(OH)₂ and MnCO₃ phases. Partial oxidation of Mn²⁺ to Mn³⁺ took place during synthesis, as evidenced by UV-Vis DRS. According to the EPR and UV-Vis DRS results, the Mn²⁺/Mn³⁺ ions were present in a (distorted) octahedral environment. Calcination at 723 K resulted in the formation of a poorly crystalline MgO-like solid solution. Mn retained its +2 oxidation state (EPR active) even upon calcination at very high temperature (1473 K) in air. At a higher Mn content, in addition to the MgO-like phase, spinel phases such as Mn₃O₄ and MnAl₂O₄ were also obtained at 723 K. The catalytic activity of the as-synthesized samples was tested in the liquid-phase oxidation of toluene using tert-butyl hydroperoxide as the oxidant. Benzaldehyde and benzoic acid were the major products with minor amounts of benzyl alcohol. The compound without an LDH phase was found to be less active for the oxidation of toluene under similar reaction conditions. Solvents were found to have a profound influence on the catalytic activity and the product selectivity.