Sequential pretreatment strategies under mild conditions for efficient enzymatic hydrolysis of wheat straw

This work studies the sequential execution of dilute sulfuric acid (DAP) (0.1-0.75 %, v/v) and dilute sodium hydroxide (AKP) (0.25-3 %, w/v) [i.e., DAP followed by AKP (DAP+AKP) and vice versa (AKP+DAP)] at low temperatures (<121 °C) and short reaction times (5-60 min) for maximizing sugar recovery in the enzymatic hydrolysis of wheat straw with low enzyme dosage. The pretreatment effectiveness was measured by the sum of the severity factors of both pretreatments and the saccharification yield achieved in the subsequent stage of enzymatic hydrolysis. Degradation compounds were quantified and mass balance calculations were carried out for each pretreatment sequence to verify the correct account of the input biomass and output products. Results show that sequential pretreatments (AKP+DAP and DAP+AKP) had a positive effect in enzyme accessibility thus improving monosaccharide yields compared to single DAP and AKP pretreatments. DAP+AKP achieved a high xylose yield (ca. 0.867 of theoretical) at the DAP stage, while no xylose monosaccharides were detected in the subsequent AKP. After enzyme saccharification of double-pretreated solids, the total monosaccharide yield was 0.786 (of theoretical). For AKP+DAP sequence, lower results were obtained (total monosaccharide yield 0.783 of theoretical). Sequential pretreatments total yields increased by 21 % compared to single pretreatments. AKP removed more than half of the lignin from the wheat straw in all cases. Acid and alkali concentrations played a relevant role in all pretreatment sequences, while reaction time and temperature were less important with an almost-linear effect on the total monosaccharide yields.