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yields of glucose (45–50 wt%, based on substrate, Table 4,
entries 4, 7, 9, 11). This is an efficient process for the conversion
of a range of non-pretreated cellulosic substrates into glucose,
involving environmentally benign solvent-catalyst media. The
results compare favourably with outcomes in the literature
where mineral acids or zeolites are employed as catalysts for
cellulose hydrolysis in [C4mim]Cl (Table 4, entries 21–23).[11,15]
The conversion of native cellulosic biomass is a significant
challenge, and we applied our hydrolysis protocols to biomass
derived from terrestrial (chips obtained from softwood or
corncob) and marine sources (macroalgae Ulva lactuca and
microalgae Porphyridium cruentum), respectively. Optimal con-
ditions for each source of cellulose or biomass and the reaction
outcomes are given in Table 4, entries 12–20. The direct
processing of biomass is inherently difficult because native
cellulose is usually entangled into plant cell walls with other
polysaccharides (e.g., hemicellulose, polymannosides, glycopro-
teins, etc.) and aromatic polymers (e.g., lignin) forming a rigid
polymer system.[3,6] In our hands, the transformation of soft-
wood chips (Table 4, entry 13) in the mixed solvent [C4mim]Cl/
ChCl/oxalic acid yielded low molecular weight carbohydrates
(glucose+glucosyl oligosaccharides) in a respectable 38 wt%
yield based on the glucan content in the biomass. This required
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Figure 1. Acid-catalysed conversion of MCC in [C4mim]Cl/ChCl/oxalic acid
with gradually added water. * conversion, * glucose yield (wt% based on
MCC), × HMF yield (mol% based on anhydroglucose units present). [a]
°
Dissolution of MCC (50 mg), [C4mim]Cl (1.000 g), DES (0.100 g), 100 C, 2 h.
°
Reaction of cellulose: T=120 C; addition of water in two steps (step 1:
0.220 mL, water content 20 wt%, based on IL, t=0.5 h; step 2: 0.110 mL,
total water content 30 wt%, based on IL, t=1 h).
respectively (Table 1, entry 12). In contrast, in the presence of
water, glucose was the major product (maximum at 4 h, 49 wt
%, Figure 1), with only little accumulation of oligosaccharides
(maximum at 2 h, 8, 9, 7 wt% of cellobiose, cellotriose and
cellotetraose, respectively). These results suggest that added
water improved the rates of hydrolysis of glucans. Other studies
also note that water suppresses the conversion of glucose into
HMF in ILs, most likely related to the reduced acidity of the
diluted media.[14,15] Longer reaction times nonetheless led to
diminished yields of glucose by the formation of HMF and
humins (Figure 1).
All of the reactions performed to this stage had been
conducted using MCC as a substrate. MCC is a polysaccharide
obtained after the acid-catalysed depolymerisation of native
cellulose and is a commodity product for many industries.[35]
However, the conversion of non-pretreated substrates is
desirable. A subsequent set of reactions was performed employ-
ing non-pretreated cellulose of various origins (cotton linter,
cellulose extracted from eucalyptus and Pinus, microalgal
biomass, macroalgal biomass). Processing of non-pretreated
bulk cellulose in the co-solvent system [C4mim]Cl / ChCl/oxalic
°
processing of wood chip biomass at 120 C for 6 h, followed by
further processing for 4 h after the addition of water (30 wt% of
water, based on solvent, added as detailed in Table 4, entry 13).
Under these conditions, the xylans (part of the hemicellulose)
were hydrolysed into monomer xylose in 25 wt% yield (based
on xylan content in the biomass), demonstrating the hydrolysis
of both linear and branched polysaccharides (Table 4, entry 13).
Most likely, the lower yields of low molecular weight carbohy-
drates are caused by complicated depolymerisation of wood
biomass in ILs. Nevertheless, higher yields of glucose (25 wt%)
and xylose (30 wt%, Table 4, entry 14) were attainable after
°
somewhat extended processing of the wood chips (120 C,
12 h) before addition of water and further heating. In distinct
contrast to softwood, the conversion of corncob provided
excellent yields of glucose (54 wt%) and xylose (35 wt%) under
°
milder processing conditions before the dilution (100 C, 2 h,
Table 4, entry 16), likely due to the less rigid molecular structure
and larger amount of structurally branched polysaccharides
present (e.g., hemicellulose and starch).[37] For the same
reason,[38–40] marine cell walls were more amenable to hydrol-
ysis. For example, the conversion of the seaweed Ulva lactuca in
°
acid at 120 C for 6 h afforded lower yields of low molecular
weight reducing sugars compared to MCC (Table 4, entries 1, 3,
5, 8, 10), showing the difficulties experienced when working
with native biomass, and the need to improve and modify
reaction conditions. Depolymerisation of bulk cellulose requires
more forcing conditions compared with MCC, and therefore the
overall rate of hydrolysis is lower.[36] The addition of La(OTf)3
slightly improved the transformation of cellulose extracted from
eucalyptus to glucose (yield 20 wt%, Table 4, entry 6), but the
effect of the Lewis acid is less prominent, when compared to
the conversion of MCC under identical reaction conditions
(glucose yield 35 wt%, Table 3). Similar to the processing of
MCC, longer reactions were accompanied by the formation of
humins. Pleasingly, the addition of water in two steps as before,
improved the hydrolysis of polysaccharides, affording excellent
°
[C4mim]Cl / ChCl/oxalic acid at 120 C for 6 h, followed by
addition of water (30 wt% of water) and further heating at
°
120 C for 4 h, provided glucose in 40 wt% yield (based on the
glucan content, Table 4, entry 17). This yield can reach 43 wt%
when adding water at the 4 h mark (instead of at 6 h, Table 4,
entry 18). The experimental data confirm that the branched
saccharides which are predominant in marine plant cell walls
require less forcing reaction conditions for selective conversion
into desirable low molecular weight saccharides. In a remark-
able example, the direct processing of microalgae P. cruentum,
as raw biomass, in [C4mim]Cl / ChCl/oxalic acid, yielded 55 wt%
of glucose and 40 wt% of xylose (based on the content of
glucans and xylans in biomass, respectively, Table 4, entry 20),
ChemistryOpen 2019, 8, 1316–1324
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© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA