Porous Alkaline-Earth Doped Multiwall Carbon Nanotubes with Base Catalytic Properties

Abstract

Alkaline-earth doped multiwall carbon nanotubes, M-CNT (M = Mg, Ca, Sr, Ba) have been prepared by a combined method of ionic exchange and precipitation. The wide characterization of the solids by nitrogen adsorption, ATR–FTIR, thermal analysis, XRD, scanning electron microscopy, transmission electron microscopy, point of zero charge (PZC), and X-ray photoelectron spectroscopy shows that the incorporation of M to the CNTs has been successfully produced. The doping with the alkaline-earth cations causes a decrease in the SBET value of the raw material, mainly due to the blockage of mesopores by the metal carbonate phase formed in most of cases. This metallic phase also contributes to the destabilization of the nanotubes by promoting their oxidation. According to PZC values, the acid character of oxidized CNTs changes to basic for the M-CNT series, Mg-CNT showing the highest PZC value. The basic properties of the catalysts have been tested in the C–C bond forming reaction of Knoevenagel, by carrying out the condensation of ethyl cyanoacetate with benzaldehyde or 4-methoxybenzaldehyde.