Biodiesel synthesis by direct transesterification of microalga Botryococcus braunii with continuous methanol reflux

•Reflux extraction reactor promotes high fatty acid methyl esters yield (FAME).•One step process combining lipid extraction and lipid conversion into FAME.•Biomass is repeatedly contacted with pure solvent enhancing FAME yield.•Hexane use as co-solvent enhances FAME yield.

Direct transesterification of Botryococcus braunii with continuous acyl acceptor reflux was evaluated. This method combines in one step lipid extraction and esterification/transesterification. Fatty acid methyl esters (FAME) synthesis by direct conversion of microalgal biomass was carried out using sulfuric acid as catalyst and methanol as acyl acceptor. In this system, once lipids are extracted, they are contacted with the catalyst and methanol reaching 82% wt of FAME yield. To optimize the reaction conditions, a factorial design using surface response methodology was applied. The effects of catalyst concentration and co-solvent concentration were studied. Hexane was used as co-solvent for increasing lipid extraction performance. The incorporation of hexane in the reaction provoked an increase in FAME yield from 82% (pure methanol) to 95% when a 47% v/v of hexane was incorporated in the reaction. However, the selectivity towards non-saponifiable lipids such as sterols was increased, negatively affecting biodiesel quality.