Metal modified graphene oxide composite catalyst for the production of biodiesel via pre-esterification of Calophyllum inophyllum oil

• A graphene oxide-metal composite as a novel heterogeneous acid catalyst to obtain biodiesel from bio-waste.
• A catalyst for performing >90% esterification of stearic acid and free fatty acid in Calophyllum inophyllum oil.
• A method for avoiding undesirable soap formation in biodiesel production.

Biodiesel production from high free fatty acid (FFA) containing feedstock through direct base catalyzed transesterification is not desirable due to the yield reduction caused by saponification. As a remedy, acid catalyzed pre-esterification of FFA is performed to bring the FFA content into a negligible level. The major drawbacks encountered due to the use of such homogeneous acid catalysts are the product separation and raw material oxidation. The work reported herein describes the application of a metal (Al3+, Fe3+) modified graphene oxide composite (GO/metal composite) as a novel heterogeneous acid catalyst that can be employed for two related processes; the esterification of stearic acid (SA) and the reduction of FFA level of Calophyllum inophyllum oil. The catalyst characterization was performed by spectroscopic (FTIR and AAS), X-ray diffractive (XRD) and microscopic (SEM-EDX) methods. Hammett indicator method was used to determine its surface acidity. The results revealed 92.72% of SA conversion and 95.37% of FFA level reduction in Calophyllum inophyllum oil. Optimum conditions were achieved when methanol to FFA molar ratio was 10:1 for 3 h in the presence of 8% of catalytic dose at 65 °C. More importantly, the catalyst could be effectively used for more than four cycles in order to convert high FFA containing oil into biodiesel without encountering undesirable waste formation. The calculated activation energy for the esterification in the presence of the catalyst is significantly lower (23.67 kJ mol−1) than that for similar esterification reactions reported.