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

5 years ago

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⁺-SO4 ²⁻/MWCNTs was prepared by impregnating multi walled carbon nanotubes (MWCNTs) with concentrated sulfuric acid and Al³⁺. The physical and chemical properties of Al³⁺-SO4 ²⁻/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 SO4 ²⁻/MWCNTs, Al³⁺-SO4 ²⁻/MWCNTs and Al³⁺-SO4 ²⁻/SWCNTs, Al³⁺-SO4 ²⁻/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⁺-SO4 ²⁻/MWCNTs had the highest catalytic activity: the conversion ratio of oleic acid reached 95% when reaction temperature was 65°C, mass ratio of catalyst to reactants was 0.9wt% and molar ratio of methanol to oleic acid was 12:1 after 7h. The high catalytic activity of Al³⁺-SO4 ²⁻/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 stronginteraction between Al³⁺ and SO4 ²⁻, accompanied with the formation of a stable coordination bond, which increased the crystallization degree of Al³⁺-SO4 ²⁻/MWCNTs catalyst and improved the combination stability of SO4 ²⁻ and MWCNTs. In addition, SO 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 SO4 ²⁻, enhances the absorption capacity of Al³⁺ and increases the imbalance of the system. Eventually, it changed the original acidity of the Al³⁺-SO4 ²⁻/MWCNTs catalyst and enhanced its Lewis acidity. Hence, the acid site of Al³⁺-SO4 ²⁻/MWCNTs is mainly comprised by Lewis acid, which is favorable for the avoiding of the occurrence of hydration of Brönsted acid active sites.

Publisher URL: www.sciencedirect.com/science

DOI: S0016236117309754