Study on the preparation, characterization of a novel solid Lewis acid Al3+-SO42-/MWCNTs catalyst and its catalytic performance for the synthesis of biodiesel via esterification reaction of oleic acid and methanol

A novel solid Lewis acid catalyst Al3+-SO42-/MWCNTs was prepared by impregnating multi walled carbon nanotubes (MWCNTs) with concentrated sulfuric acid and Al3+. The physical and chemical properties of Al3+-SO42-/MWCNTs was characterized by X-ray diffraction, transmission electron microscopy, Raman spectroscopy, X-ray fluorescence spectroscopy, NH3 temperature programmed desorption, X-ray photoelectron spectroscopy and pyridine adsorption IR spectra. The catalytic activities of SO42-/MWCNTs, Al3+-SO42-/MWCNTs and Al3+-SO42-/SWCNTs, Al3+-SO42-/C were compared when they were respectively applied for the catalytic synthesis of biodiesel via esterification reaction of oleic acid and methanol. Results showed that Al3+-SO42-/MWCNTs had the highest catalytic activity: the conversion ratio of oleic acid reached 95% when reaction temperature was 65 degrees C, mass ratio of catalyst to reactants was 0.9 wt% and molar ratio of methanol to oleic acid was 12: 1 after 7 h. The high catalytic activity of Al3+-SO42-/MWCNTs can be ascribed to the flow and escape of electronic is very easily in the tubular structure of MWCNTs since it has low C1s binding energy. It will stimulate a strong interaction between Al3+ and SO42-, accompanied with the formation of a stable coordination bond, which increased the crystallization degree of Al3+-SO42-/MWCNTs catalyst and improved the combination stability of SO(4)(2-)and MWCNTs. In addition, S=O band can exert strong electron induced effect, which leads to the increasing of the valence electron layer density of Al element, reduces the shielding effect of outer electrons on core electrons and increases the binding energy of core electrons. It directly affects the electronic state of S and Al and changes the chemical state of SO42-, enhances the absorption capacity of Al3+ and increases the imbalance of the system. Eventually, it changed the original acidity of the Al3+-SO42-/MWCNTs catalyst and enhanced its Lewis acidity. Hence, the acid site of Al3+-SO42-/MWCNTs is mainly comprised by Lewis acid, which is favorable for the avoiding of the occurrence of hydration of Brnsted acid active sites.