Catalytic and biocatalytic cascade conversion of giant reed and cardoon residues to glucose, levulinic acid, and long-chain fatty acid methyl esters

Giant reed was hydrolysed by the green salt FeCl3 under microwave irradiation. Differently, cardoon residues were pretreated by steam explosion and hydrolysed by the same catalyst. Each pretreatment showed efficient biomass fractionation, with xylan hydrolysis reaching 99 mol% for giant reed and 70 mol% for cardoon. Then, glucose yields of 55 and 30 mol% were obtained for giant reed and cardoon, respectively, under mild reaction conditions (34 min, 150 °C, biomass loading 9 wt%, FeCl3 2.7 wt%). Sugars-rich hydrolysates were fermented by the yeasts L. starkeyi and L. tetrasporus to produce triglycerides. L. starkeyi achieved a lipid yield of 13.5 wt% from giant reed hydrolysate and 14.2 wt% from cardoon hydrolysate. L. tetrasporus reached a lipid yield of 16.0 and 17.6 wt% from giant reed and cardoon hydrolysate, respectively. Moreover, both yeasts were able to convert cardoon hemicellulose into triglycerides, reaching at pH 5.5 a lipid yield of 13.9 wt% (L. starkeyi) and 19.4 wt% (L. tetrasporus). These oils were converted to long-chain fatty acids methyl esters. The final valorisation of unconverted glucan in solid residues resulted in the FeCl3-catalysed production of levulinic acid and formic acid to close the biorefinery cycle of both biomasses.