5-11: Biodiesel synthesis from a freshwater cyanobacterium (Synechococcus sp. PCC7942) by heterogeneous catalysis

Cyanobacteria have some advantages as feedstock for biodiesel production compared to microalgae, such as: genetic manipulation facility to induce a lipid content increase, because they are prokaryotic organisms; highest growth rate, nitrogen-fixing and simpler cell wall, facilitating lipid extraction. Moreover, they have capability of converting solar energy into biomass two times greater than microalgae and ten times greater than corn and sugar cane. Thus, the aim of this work was to evaluate lipid contents from Synechococcus sp. PCC7942 to generate biodiesel, employing heterogeneous catalysts (chemical and biochemical) and ethanol as acyl acceptor. The strain was found to be a potential source of lipid when cultivated in BG-11 medium at 25 ± 1°C under constant white fluorescent illumination (150 µmol photon m⁻² s⁻¹). In these conditions, biomass productivity of 124.0 ± 3.2 mg.L⁻¹.day⁻¹, lipid content of 29.5 ± 2.8% and lipid productivity of 36.5 ± 0.3 mg.L⁻¹day⁻¹ were obtained. Qualitative analysis of the fatty acid methyl esters demonstrated high proportion of unsaturated fatty acids (69%). Lipid biomass was examined in terms of kinematic viscosity, iodine value (g I₂/100 g of oil), saponification value (mg KOH/g of oil) and degree of unsaturation. Then, the oil was converted to biodiesel by transesterification reactions, carried out in batch reactors under appropriated conditions for each catalyst. Both catalysts were efficient in converting all fatty acids present in the lipidic feedstock into the corresponding ethyl esters. Even though the catalysts showed similar performances, the chemical route gave higher yield than that obtained by biochemical route.