Pretreatment and sugar recovery of oil palm fronds using choline chloride:calcium chloride hexahydrate integrated with metal chloride

In this study, metal chlorides with different valences, namely potassium chloride (KCl), copper (ii) chloride (CuCl₂) and iron (iii) chloride (FeCl₃), were used as co-catalysts in a Type II deep eutectic solvent (DES) system, namely, choline chloride:calcium chloride hexahydrate (ChCl:CaCl₂·6H₂O). After determining their corresponding recommended pretreatment conditions, specific volumetric water content was integrated into the DES + co-catalyst system to aggrandize the pretreatment for oil palm fronds (OPF). At one atmospheric condition, pretreatment of OPF using ChCl:CaCl₂·6H₂O/1 wt% CuCl₂/30 vol% water (115 °C, 1.5 h) and ChCl:CaCl₂·6H₂O/1 wt% FeCl₃/20 vol% water (125 °C, 1.5 h) could produce xylose recovery up to 15.78 g/L (76.42%) and 15.02 g/L (72.74%), respectively. Additionally, both transition metal salt co-catalysts expedited the solvent systems with high lignin (≥50%), xylan (≥85%) and arabinan (∼100%) removal from the OPF. Therefore, applying the studied Type II DES containing co-catalyst while integrated with additional water content had enabled an effective OPF pretreatment for dissolving the ligno-hemicellulose components and expanded a bright horizon for biorefinery processes. A utilization of DES + co-catalyst system could lead to the reduction of energy intensity as compared to the conventional DES system.