Pretreatment of empty fruit bunch using various choline chloride-based acidic deep eutectic solvents

Abstract

Research on renewable resources has been extensively conducted, especially on lignocellulosic biomass as it is cheap and abundant. Lignocellulosic biomass has complex polymer arrangements, making the conversion to various products difficult. The conversion requires pretreatment to improve the derivation of biomass components. Deep eutectic solvent (DES) pretreatment is widely used due to its effectiveness and simple preparation step. In this work, various acidic-based DESs were prepared through the combination of choline chloride (ChCl) as hydrogen bonding acceptors (HBA) and organic acids as hydrogen bonding donors (HBD), which include ChCl/lactic acid, ChCl/levulinic acid, ChCl/oxalic acid, and ChCl/sulfanilic acid for empty fruit bunch (EFB) pretreatment. The DESs were characterised using Fourier transform infrared spectroscopy (FTIR) and their density was determined. The acidic-based DESs with different ChCl-to-acid molar ratios of 1:1–1:5, except for ChCl/sulfanilic acid with molar ratios of 2:1 and 3:1, were utilised for the pretreatment of EFB at fixed treatment conditions to observe the effect on biomass structure and lignin degradation. The degradation of lignin was determined based on the data from UV-Vis spectrometry, while the characteristics of raw EFB and selected pretreated EFB samples were determined using FTIR, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The FTIR analysis was also conducted for selected DES before and after pretreatment to observe the changes in chemical functional groups. The results show that the density of the DESs increased with the increasing amount of HBA used. The FTIR, TGA, and SEM results support the characteristics of the pretreated EFB and explain the possible biomass components involved in the pretreatment process. The degradation of lignin was found to be between 15.0% and 81.7%, and high lignin degradation was obtained using ChCl/lactic acid and ChCl/sulfanilic acid. This result shows that ChCl/sulfanilic acid has the potential for biomass pretreatment based on the high lignin degradation and the characteristics of the pretreated samples.