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

In this study, natural diatomaceous earth was utilized for the preparation of efficient lithium metasilicate (Li2SiO3) 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 Li2SiO3 catalysts were characterized by using XRD, 29Si 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 Li2SiO3 catalyst was 61.65 kJ/mol. The durability test of the Li2SiO3 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 Li2SiO3 catalyst was mainly attributed to the loss of surface base sites coincident with more Q4 and less Q2 Si atoms in catalyst after reaction, as found by 29Si NMR.