In-situ transesterification of microalgae using carbon-based catalyst under pulsed microwave irradiation

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In the production of biofuels, microalgae represent an emerging class of renewable feedstock that can address the problems associated with the use of the traditional land-based lignocellulosic biomass. The high lipid content of microalgae makes them ideal in producing fatty acid methyl esters (FAME), the main components of biodiesel. In this study, the transesterification of the lipids in Chlorella vulgaris with methanol was performed in-situ using graphene oxide (GO) under microwave irradiation. From FTIR measurements and titration experiments, GO was shown to possess oxygen functional groups that can serve as catalysts in transesterification. Moreover, the catalytic performance of GO in terms of FAME yield was found to be better than conventional metal-based catalysts. Microwave irradiation, on the other hand, was found to offer a more efficient heating than conventional methods by taking advantage of the excellent microwave absorptivity of methanol and the local heating induced on the surface of GO. Furthermore, irradiation of microwave in pulses rather than in a continuous mode was shown to be more cost-effective. It is proposed that the high energy introduced into the biomass at short time intervals facilitated the release of more lipids by more effectively disrupting the algal cell wall. Lastly, operating with a methanol reflux allowed the microwave-irradiated system to be operated at the boiling point of methanol (Tb = 64.7 °C) while providing a higher FAME yield than an operation at 160 °C without reflux. This study presents graphene oxide under microwave irradiation as a green, carbon-based, and sustainable catalyst in the production of biodiesel from microalgae.