Synthesis of fatty acids methyl esters (FAMEs) from Nannochloropsisgaditana microalga using heterogeneous acid catalysts

•The oil fraction (42 wt%) of Nannochloropsis gaditana was extracted with methanol.•Oil fraction contains 86.7 wt% of saponifiable lipids mainly polar lipids and FFAs.•The conversion of lipids with ion-exchange resins gave FAMEs yields above 90 mol%.•In comparison, the direct conversion of microalga gave lower FAMEs yield (80 mol%).•Ion-exchange resin catalysts were regenerated by washing with methanol and HCl.

Oleaginous microorganisms like microalgae have emerged as a promising alternative feedstock in the production of fatty acid methyl esters (FAMEs) since they can accumulate high levels of lipids without competing with food production and having oil productivity values higher than oilseed crops. The lipids of Nannochloropsisgaditana microalga were extracted with methanol and analysed to determine its chemical composition. Since typical homogenous catalysis requires additional purification units and extracted oil presented many free fatty acids (FFAs) (∼22 wt%), FAMEs were synthesized using solid acid catalysts like ion-exchange resins (Amberlite-15, CT-275, CT-269), KSF clay and silica–alumina. Despite their high surface area, the lower acidity of silica–alumina led to a FAME yield lower than the ones obtained using KSF clay and ion-exchange resins. The good results obtained with these catalysts discard diffusion limitations when resins or KSF clay are used as catalysts. FAME synthesis through an indirect method with a previous lipid extraction was compared with the direct reaction of dry microalga biomass. Better results (FAME yields above 90 mol%) were obtained in the two-step method using ion-exchange resins. However, these catalysts lost their activity, so they were regenerated by washing with methanol and HCl.