Title : Aqueous/ EtOH: p-TsOH deep eutectic solvent assisted low-temperature deconstruction of miscanthus x giganteus biomass for production of bioethanol
Abstract:
Technological innovations, global climate concerns, and the overuse of fossil-based feedstocks for energy and chemicals have driven the demand for viable, sustainable alternatives for value-added chemicals and fuels production. Recently developed industrially relevant bioenergy crops such as miscanthus (Mxg) have been reported to produce higher carbohydrates in the plant cell and have the potential to produce renewable fuels, chemicals, and materials altering the entire fossil-based economy 1. However, the complete valorization of Mxg is highly limited due to its natural recalcitrance2,3. Almost $6 billion has been spent on trying to develop and start viable lignocellulosic biorefineries for ethanol production. However, many plants shut down due to the enormous costs of the pretreatment, significant chemical and energy inputs, and negative environmental impacts caused by the use of harsh chemicals.
Therefore, developing efficient deconstruction methods with economic and environmental benefits is highly encouraged for deploying sustainable biorefinery. Deep eutectic solvents (DES) are a type of eutectic solvent made from a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA) and are considered greener alternatives to fossil-based solvents 4. DES enables the selective extraction of plant cell components, solubilizes lignin and hemicellulose, and exposes the cellulose surface area for enzymatic activity. DES falls under the category of green chemistry because of its low toxicity, recyclable nature, and natural origin, meets green solvent criteria, and is more cost-efficient than its traditional counterparts. Thus, herein, we aimed to develop a promising biorefinery technology to valorize bioenergy crops using an aqueous/ethanol-DES-based solvent system. The p-Toluenesulfonic acid (p-TsOH) was chosen as HBD in association with ChCl (HBA), serves as a cost-effective brønsted acid, and has unique solvatochromic properties to cleavage β-O-4 linkage benzyl ether, and benzyl ester bonds in linkage polymeric lignin units, and prevent lignin condensation under mild reaction conditions 5. The ChCl was chosen to control the H+ concentration of the p–TsOH/ChCl solvent system6. Further, the pretreatment efficacy of ChCl-p-TsOH DES was investigated by varying the addition of ethanol and water at a moderate temperature (e.g., 60 to 90 °C. Pretreatment effects were correlated with enzymatic sugar yield, regenerated lignin, and structural characteristics have been comprehensively investigated by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), 2D NMR, 31P NMR to fulfill the effective separation and depolymerization of plant components. Interestingly, ChCl:p-TsOH pretreatment at 80 °C, using 30% water, resulted in maximum glucan content of 61.7% with high xylan and lignin solubilization. The lowest weight percentage of glucan collected was found for 10% aqueous ChCl:p-TsOH solution while pretreating the biomass at 100 °C. The results showed DES pretreatment using 20% (w/w) and 30% (w/w) ethanol-ChCl:p-TsOH at 80 °C condition resulted in high (96%) glucose yield within 24 h of enzymatic hydrolysis. The findings further concluded that ethanol addition during DES pretreatment could reduce the recondensation of lignin molecules while solubilizing a higher amount of lignin in the liquor stream, thus increasing the ease of cellulose hydrolysis of pretreated pulp during enzymatic hydrolysis.
The yeast fermentation of sugar-rich hydrolysate enabled the achievement of 40 to 42 g/L of ethanol after 48 hours. Scanning electron imaging estimation revealed a very smooth, loose, and porous structure after ethanol-ChCl-p-TsOH DES pretreatment, leading to a 95% enzymatic hydrolysis yield. XRD analysis of pretreated biomass demonstrated an increased CrI value from 18.6% (raw) to 23.8%, 27.2%, and 38.2%, respectively, for aqu.-DES pretreated biomass, varying pretreatment temperatures of 60, 80, and 100 °C. This was due to the disruption of inter/intramolecular linkages in the carbohydrate-lignin cell wall matrix, leading to higher cellulose content in the pretreated pulp. In short, this work demonstrated an efficient, tailored DES system to process bioenergy crops and may offer an advanced strategy for the production of high sugar syrup and pure lignin for downstream biorefinery units for co-production of value-added chemicals and biofuels.
