Sol-gel synthesized lithium orthosilicate as a reusable solid catalyst for biodiesel production

Lithium orthosilicate (Li4SiO4) is a promising solid catalyst for biodiesel synthesis. However, Li4SiO4 is traditionally prepared by a solid-state reaction, which results in the unstable activity for the reaction. In the present study, Li4SiO4 was successfully prepared using a simple sol-gel method and employed as an efficient solid alkali catalyst for biodiesel synthesis. The molar ratio of precursors and calcination temperature were optimized for the synthesis of Li4SiO4 by using the sol-gel method. The physical and chemical properties were determined using X-ray diffraction, scanning electron microscopy, laser diffraction particle size, and thermogravimetric analysis. The as-prepared Li4SiO4 catalyst had much smaller particle size, pore volume, and pore size, but higher surface area and basicity than Li4SiO4 catalyst prepared by the solid-state reaction. It was then used to transesterify methanol and soybean oil into biodiesel. The effect of reaction factors (reaction time from 1 to 3 hours, catalyst concentration from 3 to 9%; molar ratio of methanol to oil from 6:1 to 18:1, and temperature from 55°C to 75°C) on the Li4SiO4-catalyzed transesterification was systematically examined. The highest biodiesel conversion of 91% was reached under the following conditions: reaction time of 2 hours, Li4SiO4 concentration of 6%, 12:1 methanol:oil molar ratio, and temperature of 65°C. Notably, Li4SiO4 could be efficiently reused for at least 10 times without significant loss of its activity; this suggests that the sol-gel synthesized Li4SiO4 is a potential solid alkali catalyst for biodiesel synthesis.