Sonochemistry: a simple and efficient route for biodiesel production from neat and blended vegetable oils using LDH-derived base catalysts

Abstract

Different layered double hydroxides were synthesized by coprecipitation under low supersaturation and calcined to obtain mixed metal oxides that were tested as heterogeneous catalysts for ultrasound assisted transesterification of oils with methanol for the preparation of fatty acid methyl esters (or) biodiesel using <6:1 methanol:oil molar ratio. Among them, mixed oxides derived by calcining CaAl2-LDH at 700 °C (CaAl2700-CLDH) was the most active catalyst as inferred from its maximum basicity, elucidated by Hammett studies. Various vegetable oils (edible, non-edible and used cooking oils) were successfully transesterified with 80–90 % yield of FAME under ultrasound at 40 °C within 90 min. Under equivalent conditions, the yield of FAME (for sunflower oil) through conventional heating was only 9 % while under ultrasound was 63 %. A single step process was developed for biodiesel production from high free fatty acids (FFA) containing oils such as Jatropha oil by blending it with sunflower oil or used cooking oil (60/40 weight ratio) that gave 89 % yield of FAME in both cases under the optimized reaction conditions. CaAl2700-CLDH showed moderate reusability up to two cycles and decreased thereafter while its activity could be restored by recalcining the recovered catalyst at 700 °C.