1. INTRODUCTION
Finding a renewable alternative to petroleum fuels is a significant issue for mankind’s sustainability. Lignocellulosic biomass, a promising renewable and abundant material, can produce currently petroleum-derived fuels and platform chemicals by the conversion of its three major components: cellulose, hemicellulose, and lignin.1Cellulose, the most abundant carbohydrate component in biomass, can be chemically or biologically converted into biofuels and platform chemicals.2,3 Hemicellulose also finds value-added applications, as it can be converted to value-added chemicals like furfural and xylitol or applied in bioplastics.4-6Lignin is a biopolymer of aromatic compounds and has great potential for future industry as a source of bio-based chemicals.7-9
As a green solvent, deep eutectic solvents (DESs) have been investigated to overcome the challenges of conventional pretreatment solvents and have great potential to achieve high productivity of fermentable sugars as well as minimal condensation of lignin, which are crucial factors in a successful biorefinery process. DESs can be prepared via the simple mixing and heating of hydrogen bond acceptors (HBA) and donors (HBD). Various compounds including amines, carboxylic acids, polyols, and aromatics have been applied for the formation of DESs.10-12 Recently, DESs prepared from biomass metabolites such as lactic acid, oxalic acid, and lignin-derived phenolic compounds like p -coumaric acid, p -hydroxybenzoic acid, and guaiacol have been reported to effectively pretreat the biomass.13-16 Enhanced fermentable sugar yield and lignin quality can be achieved by DES pretreatment compared to hydrothermal, dilute acid, or conventional organosolv pretreatment due to its mild processing conditions.17-19 Additionally, biomass-derived DESs possess the potential to facilitate a circular process, as fractionated biomass components can be reused as the processing solvent, decreasing the overall cost.20Also, recent DES studies showed that introducing a third constituent like water, acids, or organic solvents into the DES improved the removal of lignin and hemicellulose from the biomass, further enhancing fermentable sugar yield.21,22 However, studies on lignin-based three-component DESs are scarce.
3,4-Dihydroxybenzoic acid (DHBA), a metabolite found in several plant species, is a promising platform chemical that can be used as a precursor to industrially important chemicals including muconate, beta-ketoadipate, 2-pyrone-4,6-dicarboxylate, gallates, and vanillin.23-25 It has also seen applications in the pharmaceutical and food packaging industries. In a recent study, Tian et al. reported the accumulation of DHBA in engineered sorghum by expressing a bacterial dehydroshikimate dehydratase (QsuB) gene, converting 3-dehydroshikimate to protocatechuate.26,27A study by Unda et al. showed increased saccharification yield from poplar expressing QsuB, which led to the reduction of lignin and the incorporation of DHBA into lignin. Additionally, our study finds that DHBA can act as an HBD to form DESs and can be directly applied in biomass pretreatment.
Herein, we report the influence of the third component in the DHBA-based DES system on the fractionation of the DHBA-enriched engineered sorghum stover in a biorefinery approach. Biomass from the DHBA-enriched engineered sorghum was pretreated by binary DES, composed of choline chloride (ChCl) and DHBA, and ternary DESs with additional third constituents like water and ethylene glycol. Chemical compositions and enzymatic digestibility of the processed sorghum stover were investigated. The structural properties of lignins recovered from each processing method were also analyzed to evaluate the quality of lignin for accomplishing a sustainable biorefinery strategy.