Characterization and catalytic activity of cobalt containing MCM-41 prepared by direct hydrothermal, grafting and immobilization methods

Cobalt containing hexagonal mesoporous molecular sieves (MCM-41) were prepared by different methods viz., direct hydrothermal synthesis, grafting and immobilization. The calcined material was characterized by various spectroscopic tools such as powder X-ray diffraction (XRD), N2 adsorption–desorption isotherms, Fourier transformed infra-red (FT-IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance UV–vis and X-ray photoelectron spectroscopy (XPS). The XRD patterns show the highly intense (100) reflection peak in the range 2–3° (2θ angle), characteristic of mesoporous material. Higher order reflection peaks suggest highly ordered mesoporous structure. N2 adsorption–desorption isotherms are of type IV according to IUPAC classification and the steep rise in the isotherms in the narrow range 3.5–4.5 of relative pressure (P/P0) is due to the condensation of N2 gas molecules in the pores (characteristic of mesoporous structure). In FT-IR increase in the intensity of band at 960 cm−1 with the increase of the cobalt content in Co-MCM-41 samples indicate the incorporation of cobalt ions in the framework of MCM-41. SEM and TEM reveal spherical morphology for the cobalt substituted MCM-41. UV–vis spectra demonstrate the characteristic features of framework and extraframework cobalt in MCM-41. Cobalt is in +2 oxidation state as evidenced from UV–vis and XPS. The catalysts were tested for the side chain oxidation of ethylbenzene using 70 wt.% tert-butyl hydrogen peroxide as an oxidant with and without the use of solvent. Solvents have effect in the catalytic activity and selectivity. In the absence of solvent, substituted cobalt [Co-MCM-41 (50)] in the framework of MCM-41 gives maximum ethylbenzene conversion while higher selectivity to acetophenone was achieved over immobilized cobalt on the MCM-41. Leaching data shows, immobilized cobalt catalyst is well hetrogenized.

Graphical abstractCobalt containing MCM-41 was prepared by direct hydrothermal, grafting and immobilization methods. The catalyst shows excellent activity in the oxidation of ethylbenzene in presence of tert-butyl hydroperoxide oxidant, under solvent free conditions. Figure optionsDownload full-size imageDownload as PowerPoint slide