Metasilicate-based catalyst prepared from natural diatomaceous earth for biodiesel production

Abstract In this study, natural diatomaceous earth was utilized for the preparation of efficient lithium metasilicate (Li 2 SiO 3 ) catalyst for biodiesel production via the transesterification of soybean oil and waste cooking oil. Lithium metasilicate was successfully synthesized from the hydrothermal reaction of natural diatomite in LiOH solutions at 150 °C for 24 h and, subsequently, by calcination at 500 °C for 6 h. Both fresh and spent Li 2 SiO 3 catalysts were characterized by using XRD, 29 Si NMR, BET, ICP-OES and acid-base titration. The acidity of the soybean oil and waste cooking oil used in this study was 0.54 mg KOH/g oil and 1.67 mg KOH/g oil, respectively. The yields of biodiesel from soybean oil reached ca. 80.3% in 1 h, respectively, in presence of catalyst at 3 wt% of oil used. Generally, more catalyst present would lead to a higher yield of biodiesel. With the mass loading of catalyst-to-oil at 0.05, the activation energy of the catalyzed transesterification using Li 2 SiO 3 catalyst was 61.65 kJ/mol. The durability test of the Li 2 SiO 3 catalysts was conducted in the same system. A high catalytic activity after being reused for 19 times was observed with a yield of biodiesel near ca. 85% after 4 h of the transesterification reaction at 60 °C. The deactivation mechanism of the Li 2 SiO 3 catalyst was mainly attributed to the loss of surface base sites coincident with more Q 4 and less Q 2 Si atoms in catalyst after reaction, as found by 29 Si NMR.