Halloysite nanotube functionalized with La-Ca bimetallic oxides as novel transesterification catalyst for biodiesel production with molecular simulation

Abstract

Calcium oxide is the promising transesterification catalyst for biodiesel production, but its stability is poor for the active sites leaching. Also, the free fatty acids (FFAs) in the transesterification source deactivates the basic active sites due to saponification. In this study, the fumed silica, γ-Al2O3, MCM-41 and halloysite nanotubes (HNTs) are, respectively, functionalized with the La-Ca bimetallic oxide through the hydrothermal method to prepare the transesterification catalyst with the acid-base bicharacteristic and strong stability in catalyzing transesterification for biodiesel production for the first time. The HNTs-La/Ca catalyst with the La2O3 to CaO mass ratio of 1.5 shows the best performances and the efficiency of 97.5% is achieved if transesterification is conducted with the catalyst amount of 7 wt% and methanol to oil molar ratio of 18 at 150 °C for 2 h. Also, this catalyst gains the strong stability and the efficiency of 88.7% could be obtained for the fifth reused cycle. The HNTs-La/Ca catalyst is strong to resist FFAs and the efficiency is 94.9% even FFAs in the transesterification source is high to 5 wt%. To reveal the catalytic performances, the HNTs-La/Ca catalyst is comprehensively characterized by XRD, nitrogen adsorption-desorption, SEM-EDX, XPS and CO2/NH3-TPD. The molecular simulation shows the adsorption energy of methanol at the lanthanum site is greater than at the calcium site, but the opposite is true for acetic acid, which explains the reason for the resistivity of the HNTs-La/Ca catalyst to FFAs in transesterification source.