Comparative effect of ionic liquids pretreatment on thermogravimetric kinetics of crude oil palm biomass for possible sustainable exploitation

Crude oil palm biomass (OPB) residue is an important source of lignocellulose and could be transformed into biofuels through thermochemical conversion for heat and power in CHP plants or act as sustainable reinforcement in the biocomposite industry. Effective pretreatment is an essential task for the utilization of waste biomass for bioenergy and biomaterials. The comparative effect of two ionic liquids (ILs), 1-butyl-3-methylimidazolium chloride ([bmim][Cl]) and 1-ethyl-3-methylimidazolium diethylphosphate ([emim[dep]), on OPB pyrolysis kinetic and thermodynamic parameters was studied using thermogravimetric analysis (TGA) coupled with Coats-Redfern integral method. The OPB residue was pretreated with ILs [bmim][Cl] and [emim][dep] under mild processing conditions of 90 °C for 3 h and was also subjected to three other conventional pretreatment methods called dilute acid, alkaline, and hot compressed water to better understand and compare the performance of ILs for pretreatment of lignocellulosic OPB. Results reveal that ILs pretreatment was superior in terms of delignification of OPB and produced the cellulose-rich fiber with 48.2% and 41.2% cellulose compared to cellulose fraction of 26.4% for the untreated sample. The values of apparent activation energies were found to be 81.8, 90.9, 92.0, 84.9, 83.8, and 93.5 kJ/mol for untreated, [bmim][Cl], [emim][dep], dilute acid, base, and hot compressed water pretreatments respectively. Further calculation of thermodynamic parameters led to the change in Gibb's free energy (ΔG°) values of 188.9, 208.0, 210.9, 219.1, 217.4 and 217.1, kJ/mol for untreated, [bmim][Cl], [emim][dep], dilute acid, base, and hot compressed water pretreatments respectively. The higher change in entropy (ΔS°) values of ILs-pretreated fibers compared to the untreated one showed that dissolution and regeneration from ILs solution had unlocked the complex 3D structure of lignocellulose, resulting in increased disorder. The results plainly revealed that ILs-based pretreatment could be a promising and environmentally benign technology for efficient utilization of agricultural residues in the fields of biofuels and biocomposite materials.