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. •OPF pretreatment using Type II DES and CuCl2 led to xylose recovery up to 15.78 g/L.•Dissolved lignin increased to 1.98 g/L when H2O was added into DES and CuCl2 system.•By using H2O, DES and CuCl2 system, high glucose conversion of 64.24% was obtained.