Preparation of a novel solid Lewis acid Ce³⁺-Ti⁴⁺-SO₄^2-/MWCNTs and its application in the synthesis of biodiesel from esterification reaction

The Ce³⁺-Ti⁴⁺-SO₄^2-/MWCNTs catalyst was prepared from the modification treatment of multi walled carbon nanotubes by concentrated sulfuric acid, Ce³⁺ and Ti⁴⁺ through the employing of high temperature impregnation method. Physicochemical properties and structural characteristics of the obtained-catalysts were characterized by means of transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, pyridine adsorption FT-IR spectra, X-ray fluorescence spectroscopy, X-ray diffraction and NH₃ temperature programmed desorption. The catalytic activity of Ce³⁺-Ti⁴⁺-SO₄^2-/MWCNTs for the synthesis of biodiesel from the esterification of methanol and oleic acid was investigated. The influence of SO₄^2-/MWCNTs, which was resulted from the addition of Ce³⁺ and Ti⁴⁺, on the structure and catalytic activity was cleared based on the above structure characterization and catalytic activity investigation. The results showed that the conversion of oleic acid reached 93.4% after 5 h reaction at 65℃, when the catalyst/reactants was 1% and the molar ratio of methanol/oleic acid was 12:1. The conversion of oleic acid was 80.8% after the Ce³⁺-Ti⁴⁺-SO₄^2-/MWCNTs were cycled for eight times. Therefore, it can be concluded that this catalyst has high catalytic activity and stability. The high catalytic activity and stability can be explained as follows:the C 1s binding energy of carbon nanotube is much lower than other carbon materials, resulting in easy flow and escape of the electrons in the tubular structure. Thus, the strong interactions will occur among the active groups that have been loaded on the carbon nanotube, which impels Ce³⁺ and Ti⁴⁺ to respectively form stable coordination bonds with SO₄^2-, increases the crystallization degree of the Ce³⁺-Ti⁴⁺-SO₄^2-/MWCNTs catalyst and the active acid sites without increasing the surface defects, and the combination of SO₄^2- and MWCNTs was more stable. In addition, the chemical state of surface atom on the Ti-SO₄^2- was changed due to the strong interaction between SO₄^2- and Ce³⁺, which strengthened the electron withdrawing ability of S⁶⁺ and Ti⁴⁺ ions and enhanced the acidity strength of Lewis acid with changing of the acid type. Hence, the Ce³⁺-Ti⁴⁺-SO₄^2-/MWCNTs will be composed by Lewis acid sites mainly, which is favorable for avoiding the occurrence of hydration of acid active sites for the SO₄^2-/MWCNTs catalyst because it was composed of Brönsted acid sites mainly.