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.