Synthesis, characterization and heterogeneous catalytic application of copper integrated mesoporous matrices

Abstract

Ordered copper integrated mesoporous silicate catalysts (CuMSC) have been synthesized by the utilization of the amphiphilic tri-block copolymer pluronic F127 as a structure directing agent (SDA) under acidic aqueous conditions. The mesophase of the materials was investigated using small-angle powder X-ray diffraction and transmission electron microscopic (TEM) image analysis. N₂ adsorption–desorption studies show that the BET surface area of CuMSC (214–407 m² g⁻¹) is lower than that of pure silica (611 m² g⁻¹) and has smaller average pore dimensions (4.0–5.0 nm), both prepared following the same synthetic route. The reduction of pore size and surface area points to incorporation of copper within the silicate network. FEG-SEM results suggest that the materials have a plate-like morphology and are composed of very tiny nanoparticles. EDS surface chemical analysis was utilized for the detection of the distribution of Si, O and Cu in the matrix. The FT IR spectral study suggests the complete removal of the surfactants from the calcined materials and the presence of Si–O–Cu bonds for high nominal contents. X-ray photoelectron spectroscopy (XPS) and UV-vis reflectance spectra show the oxidation state of copper and coordination mode, respectively. These mesoporous materials display a good catalytic activity in the oxidation of cyclohexane to cyclohexanone and cyclohexanol in the presence of the green oxidant hydrogen peroxide. The maximum yield (cyclohexanone and cyclohexanol) was ca. 29% and the TON (turnover number) was 276 under optimal reaction conditions. The good catalytic activity could be attributed to the large surface area and the presence of a high number of active sites located at the surface of the material, as well as to its stability. The catalysts showed negligible loss of activity after five cycles.