Prediction by a modified severity factor in FeCl₃-catalyzed hydrothermal fractionation of coconut husk: Enhancing hemicellulose hydrolysis and enzymatic digestibility of cellulose

Hydrothermal pretreatment (HTP) is essential for producing valuable hemicellulosic sugars and enhancing the enzymatic digestibility of lignocellulosic biomass for biofuel and biochemical production. However, conventional methods are often limited by high energy demands and low sugar yields. This study explores the catalytic role of ferric chloride (FeCl₃) as a less corrosive, high-catalytic activity, and cost-effective alternative to inorganic acids in the HTP of coconut husk (CCH), with the goal of improving hemicellulose hydrolysis and subsequent enzymatic conversion. The pretreatment was carried out at 120–180 °C with FeCl₃ concentrations ranging from 20 to 100 mM. A mathematically derived Combined Hydrolysis Factor (CHF) was applied to unify pretreatment conditions and correlate them with xylan hydrolysis, hemicellulosic sugar yield, compositional changes in CCH, and improvements in enzymatic digestibility. The results showed that FeCl₃-HTP enabled substantial to near-complete hemicellulose removal, exhibited a strong correlation between xylan removal and xylose release, and significantly enhanced enzymatic digestibility, as confirmed by XRD, SEM, and FTIR analyses. Mass balance analysis identified the optimal condition at 150 °C and 60 mM FeCl₃, yielding 22.04 g of total sugar per 100 g of raw CCH, 5.34 times higher than the yield from non-catalyzed pretreatment at the same temperature. These findings confirm the effectiveness of FeCl₃ as catalytic agent and establish CHF as a reliable predictive parameter for optimizing sugar recovery in scalable biomass conversion processes, highlighting its potential applicability to other lignocellulosic biomass as transferrable parameter under similar catalytic system, such as oil palm empty fruit bunches, for high-value bioproduct production.