Effect of a Thermomechanical Pretreatment Coupled to Sulfuric Acid or Sodium Hydroxide Catalysis on Physicochemical Properties and Enzymatic Hydrolysis of Alfa Fibers (Stipa tenacissima L.)

The efficacy of biomass pretreatment methods is critical for enhancing the yield of fermentable sugars, essential for bioethanol production. In this study, a pretreatment approach utilizing impregnation of alfa) fibers (Stipa tenacissima in sulfuric acid or sodium hydroxide diluted catalyst solutions with concentrations ranging from 0.5 to 3% (w/w). This step was coupled to a thermomechanical treatment using “Intensive Vacuum associated with Heat Moisture Treatment” (IV-HMT) technique, developed in our laboratory. The target was to optimize the solubilization of hemicelluloses, removal of lignin, and enhancement of enzymatic hydrolysis. Chemical composition and microstructure alterations were assessed through A_BET specific surface area, SEM observations, XRD, and FTIR analysis. Enzymatic hydrolysis efficacy was evaluated by quantifying reducing sugar yields initially (R_0h) and after 24 hours (R_24h) using DNS method, with glucose and xylose yields quantified by HPLC. Results demonstrated significant disruption of the recalcitrant structure of alfa fibers, with hemicelluloses removal reaching 76.6% and 73.4%, and lignin removal reaching 28.8% and 71.8% for H₂SO₄ and NaOH, respectively. A_BET increased around 7-fold with H₂SO₄ but decreased with alkali, most probably due to calcium carbonate precipitation. The overall reducing sugars yield increased from 31% (raw) to 89.9% and 82.7% for pretreated samples with H₂SO₄ (1%) or NaOH (3%), respectively, combined with IV-HMT processing. Maximum glucose production was 66.3 g/L and 58.8 g/L with H₂SO₄ (1%) and NaOH (3%), respectively. IV-HMT can be regarded as an environmentally friendly approach allowing for a reduced energy consumption and a minimal utilization of chemicals.