Ionic liquid pretreatment of lignocellulosic biomass with ionic liquid
water mixtures
Agnieszka Brandt,a,b Michael J. Ray,b,c Trang Q. To,a David J. Leak,b,c Richard J. Murphyb,c
and Tom Weltona*
Abstract
Ground lignocellulosic biomass (Miscanthus giganteus, Pine (Pinus sylvestris) or Willow
(Salix viminalis)) was pretreated with ionic liquid-water mixtures of 1-butyl-3methylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium hydrogen sulfate. A solid
fraction enriched in cellulose was recovered, which was subjected to enzymatic hydrolysis.
Up to 90% of the glucose and 25% of the hemicellulose contained in the original biomass
were released by the combined ionic liquid pretreatment and the enzymatic hydrolysis. After
the pretreatment, the ionic liquid liquor contained the majority of the lignin and the
hemicellulose. The lignin portion was partially precipitated from the liquor upon dilution with
water. The amount of hemicellulose monomers and their conversion into furfurals was also
examined. The performance of ionic liquid water mixtures containing 1,3-dialkylimidazolium
ionic liquids with acetate, methanesulfonate, trifluoromethanesulfonate and chloride anions
was investigated. The applicability of the ionic liquid 1-butylimidazolium hydrogensulfate for
lignocellulose pretreatment was also examined. It was found that ionic liquid liquors
containing methyl sulfate, hydrogen sulfate and methanesulfonate anions were most
effective in terms of lignin/cellulose fractionation and enhancement of cellulose digestibility.
Introduction
The rising demand for liquid transportation fuels is placing increasing demands on finite oil
reserves, raising prices and encouraging the search for oil in more remote locations, often in
fragile ecosystems. In addition, the planet’s climate is affected by carbon dioxide emitted
from the use of fossilised carbon as an energy source. The production of many chemicals
and materials is also reliant on fossil fuel resources.
Lignocellulose, essentially the cell wall material of woody plants, is a porous micro-structured
composite mainly consisting of cellulose, hemicellulose and lignin. It has been projected that
lignocellulosic biomass has the potential to be a large-scale, low-cost and sustainable
feedstock for renewable fuels and chemicals.1 Compared to starch or vegetable oil
a
Department of Chemistry, Imperial College London, London SW7 2AZ, UK
b
The Porter Alliance, Imperial College London, London SW7 2AZ, UK
c
Division of Biology, Imperial College London, London SW7 2AZ, UK
1
substrates that are currently used as biofuel and biomaterial feedstocks, significantly higher
biomass yields per unit area of land are expected, while requiring less energy and material
input for its production.2 Various plant species have been suggested as being suitable
dedicated biofuel crops, with properties such as rapid growth, low fertiliser input, and short
harvest cycles. Currently favoured crops include grasses (miscanthus, switchgrass),
hardwoods (willow, poplar, eucalyptus) and softwoods (pine, fir, spruce). Careful
implementation of lignocellulose based technology as a substitute for fossil resources could
help reduce man-made carbon dioxide emissions in a sustainable way.3
Proposed routes for transforming lignocellulosic biomass into useful products are the
microbial fermentation of the glucose and other carbohydrates contained in the biomass and
the thermo-chemical conversion of the lignocellulose via pyrolysis or gasification. For the
fermentation route, deconstruction of the lignocellulosic matrix is necessary before the
carbohydrates are released. A typical deconstruction sequence producing fermentable
carbohydrates is: size reduction to chips, a pretreatment that solubilises the hemicellulose
and alters/removes lignin,4 followed by detoxification and neutralisation. The pretreated
biomass is subsequently processed using hydrolytic enzymes (saccharification) to produce
sugar monomers.
The pretreatment step is responsible for a significant portion of the energy consumption and
cost of the biofuel production process and improvements are required.5 A large number of
pretreatment options are defined in the literature, such as dilute acid, concentrated acid,
ammonia fibre expansion (AFEX), lime and organolv pretreatment. Different plant groups
exhibit distinct tissue structures and varying cell wall composition, which leads to a variable
resistance to deconstruction.
This paper explores the potential of certain ionic liquids as pretreatment solvents, in
particular their mixtures with water. Ionic liquids are a diverse group of salts that are liquid at
ambient temperatures or melt at slightly elevated temperatures. In the last two decades,
ionic liquids containing organic cations with quaternised ammonium, phosphonium and
sulfonium cores have enjoyed increasing popularity in many fields of research.6
Many ionic liquids have negligible vapour pressures under process-relevant conditions and
the nature and combination of cation and anion can be tuned to suit a particular application.
Cellulose and lignocellulose processing are only two out of many recently explored
applications for these alternative solvents.7 Ionic liquids are polar solvents with varying
degrees of hydrogen-bonding ability.8 It has been found that the ionic liquid needs to contain
anions with high hydrogen-bond basicity such as chloride, phosphates, phosphonates and
carboxylates in order to be able solubilise cellulose.9 The hydrogen-bond acidity also plays a
role. If a hydrogen-bond acidic functionality is incorporated into the ionic liquid structure, it
will compete for the hydrogen-bond basic site on the anion and reduce cellulose
2
solubilisation.10, 11 Water also decreases the solubility of cellulose,12 probably for a similar
reason. The empirical Kamlet-Taft solvent descriptors can be used to predict cellulose
solubility.13 Cellulose can be reconstituted by adding a protic antisolvent, such as water or
alcohols, and spun into fibres or films. A variety of homogenous derivatisations of cellulose
dissolved in ionic liquids can be accomplished.14
Ionic liquids were initially used in cellulose processing15 before their application was
extended to lignocellulose processing. The solubility of lignocellulose in ionic liquids has
been reported in various hydrogen-bond basic ionic liquids, suggesting that ionic liquids
which are cellulose solvents are also suitable for lignocellulose processing.16, 17 Reduced
crystallinity of the cellulose contained in lignocellulose was observed upon precipitation with
an antisolvent.18 A correlation between the hydrogen-bond basicity of the anion and the ionic
liquid’s ability to swell and partially dissolve wood chips has been observed.19 The solubility
of lignin in ionic liquids also seems to depend on the anion.17, 20 It has been shown that Kraft
pulp lignin has a very high solubility in the ionic liquids 1,3-dimethylimidazolium methyl
sulfate, [C1C1im][MeSO4], and 1-butyl-3-methylimidazolium methyl sulfate,
[C4C1im][MeSO4].20
Enhanced glucose release from ionic liquid pretreated wood has also been observed, mainly
with dialkylimidazolium ionic liquids containing acetate, chloride and dimethyl phosphate
anions.21-23 However, the sugar release by hydrolytic enzymes was often less than 80-90%
(which is expected for an effective pretreatment operation). Recently, the impact of ionic
liquid pretreatment on biomass composition has received attention. It was noted that lignin
and hemicellulose are partially removed during pretreatment with 1-ethyl-3methylimidazolium acetate, [C2C1im][MeCO2].17, 24-26 A correlation between lignin removal
and cellulose digestibility was suggested.17, 21
Various publications concluded that application of methyl sulfate containing ionic liquids in
lignocellulose pretreatment did not enhance cellulose digestibility,17, 21, 24, 27 despite their
ability to dissolve large amounts of lignin.
Water reduces not only cellulose solubility in ionic liquids,12 but also the effectiveness of
ionic liquid pretreatment with [C2C1im][MeCO2].21, 28 Biomass contains significant quantities
of water, 2-300% relative to the oven-dried weight. In addition, ionic liquids are hygroscopic
and will absorb significant quantities of moisture when exposed to air.29 The drying of ionic
liquids requires heat and vacuum, particularly when the ionic liquids are strongly hydrogen
bond basic, like [C2C1im][MeCO2]. Therefore, an ionic liquid pretreatment that tolerates
moisture would be beneficial for the over-all energy and cost balance of a lignocellulose
processing system using ionic liquids.
An advantage of ionic liquid pretreatment could be the recovery of a separate lignin fraction
which could be converted to aromatic, value-added chemicals. Lignin recovery from ionic
3
liquids has been achieved after treatment of sugar cane bagasse with 1-butyl-3methylimidazolium alkylbenzenesulfonate, [C2C1im][ABS], an ionic liquid mixture containing
aromatic sulfonate anions, mainly xylenesulfonate.30 Lignin recovery has also been observed
after pretreatment with [C2C1im][MeCO2], when the regeneration solvent was a mixture of
water and acetone.20, 26
This study investigates the influence of water on the effectiveness of ionic liquid
pretreatment. We have devised a notation to indicate the amount of the ionic liquid contained
in the pretreatment solvent/liquor. This involves a subscript being added to the usual ionic
liquid notation indicating the ionic liquid content in volume percent (vol%), with the remainder
being water. An example is [C4C1im][MeSO4]80%, which is a mixture of 80 vol%
[C4C1im][MeSO4] and 20 vol% water. Conversions of vol% into weight percent (wt%) and
mole percent (mol%) were calculated and are listed in Table 1. When allowing
[C4C1im][MeSO4] to equilibrate with the moisture in the laboratory air a water content of
70,400 ppm or 7.0 wt% was measured (last entry of Table 1). Although the moisture content
of air is variable, the measurement demonstrates the highly hygroscopic nature of this ionic
liquid.
Table 1: Ionic liquid concentration in aqueous pretreatment liquors.
Mixture
[C4C1im][MeSO4]98%
[C4C1im][HSO4]95%
[C4C1im][MeSO4]90%
[C4C1im][HSO4]90%
[C4C1im][MeSO4]80%
[C4C1im][HSO4]80%
[C4C1im][MeSO3]80%*
[C2C1im][MeCO2]80%
[C4C1im]Cl80%*
[C4C1im][OTf]80%
[C4C1im][MeSO4]60%
[C4C1im][HSO4]60%
[C4C1im][MeSO4]40%
[C4C1im][HSO4]40%
[C4C1im][MeSO4]20%
[C4C1im][HSO4]20%
[C4C1im][MeSO4]wet
Volume percent
(vol%)
98
95
Weight percent
(wt%)
98
96
90
92
80
83
83
82
82
81
84
Molar percent
(mol%)
81
64
44
46
26
27
26
32
30
24
60
65
12
40
45
6
20
23
2
n.a.
93
49
*These ionic liquids are solid at room temperature. Therefore vol% and wt% were calculated using the
density at 80°C.
The aim of this work is to investigate the effect of the composition of the ionic liquid liquor on
the pretreatment. Solid recovery, pulp composition, its enzymatic digestibility, the
precipitation of a lignin-containing fraction and the production of furfurals in the liquor were
investigated. The application of an ionic liquid with a monoalkylated imidazolium cation was
4
also examined. Pretreatment of different feedstocks was carried out to assess their
recalcitrance towards pretreatment with ionic liquid water mixtures.
Results and Discussion
Tissue softening of Miscanthus chips
In preliminary experiments, we observed substantial disintegration of Miscanthus cross
sections immersed in the ionic liquid 1-butyl-3-methylimidazolium methyl
sulfate,[C4C1im][MeSO4], when heated above 80°C. This encouraged us to investigate the
application of this ionic liquid for biomass pretreatment. The use of [C4C1im][MeSO4], dried to
a water content below 0.3 wt%, resulted in formation of a degraded biomass-ionic liquid
composite that was not enzymatically digestible. In contrast, using a mixture of 80 vol% ionic
liquid and 20 vol% water yielded a solid fraction that was separable from the (intensely
coloured) ionic liquid fraction and highly digestible. It was concluded that a certain amount of
water was necessary for successful pretreatment with [C4C1im][MeSO4]. In the “dry” sample,
0.3 wt% water was contained in the ionic liquid as residual moisture and 0.7 wt% was
introduced with the air-dried biomass containing 8 wt% moisture, supplying 1.1 wt% or 15
mol% water in total. This was apparently not sufficient to obtain an enzymatically digestible
pulp.
Influence of the water content on the saccharification yield after ionic liquid
pretreatment with [C4C1im][MeSO4]
A range of ionic liquid water mixtures were used for pretreatment of Miscanthus to explore
the effect of the water content in more detail. The effect of water on the enzymatic release of
glucose and hemicellulose is shown in Figure 1. The yields are calculated based on the
glucose and hemicellulose content found in the untreated Miscanthus feedstock (on an
oven-dry basis), which were 43.6 wt% and 24.3 wt%, respectively. In preliminary
experiments, it was shown that the only detectable hemicellulose sugar released during
saccharification was xylose.
5
Saccharification yield, % of max. possible
100
90
80
70
60
50
40
30
20
10
0
Glucose [MeSO4]Glucose [HSO4]Xylose [MeSO4]Xylose [HSO4]-
0%
20%
40%
60%
80%
Ionic liquid content (vol%)
100%
Figure 1: Sugar yields obtained from Miscanthus pulp after pretreatment with [C4C1im][MeSO4] or [C4C1im][HSO4]
water mixtures at 120°C. The [C4C1im][MeSO4] pretreatment was carried out for 22 h, while [C4C1im][HSO4]
pretreatment lasted 13 h, and the saccharification 96 h.
The best saccharification yields were obtained after pretreatment with mixtures containing
60-90 vol% ionic liquid. Pretreatment with [C4C1im][MeSO4]90%, resulted in the release of
92% of the glucose originally contained in the biomass. Pretreatment with
[C4C1im][MeSO4]80% and [C4C1im][MeSO4]60%,resulted in the release of 89% and 87% based
on the original glucan content. Glucose yields decreased when the ionic liquid content was
higher or lower. The hemicellulose yield was significantly lower than the glucose yield,
regardless of the mixture composition; 24% of the hemicellulose sugars (based on the initial
hemicellulose content) were released after [C4C1im][MeSO4]60% pretreatment. Similar yields
were obtained with mixtures containing 40-90 vol% [C4C1im][MeSO4].
Water sensitivity of [C4C1im][MeSO4]
When attempting to recycle [C4C1im][MeSO4], we found that the ionic liquid anion was
partially hydrolysed. After recording a mass spectrum of the recovered ionic liquid, a high
abundance of a negatively charged species at m/z=97 was detected, which was ascribed to
the hydrogen sulfate, [HSO4]-, anion. This led to the conclusion that the ester bonds in
methyl sulfate anions are hydrolytically unstable under the conditions of the pretreatment
and mixtures of the ester and the hydrolysed form are produced.
The extent of anion hydrolysis depended on the water content of the liquor (Figure 2). The
more water was present in the mixture, the greater the anion hydrolysis, with exception of
mixtures where the water content was higher than 90 mol%. These results suggest that
6
without extreme precautions to protect [MeSO4]- containing ionic liquids, [HSO4]- will be
present and other studies using these ionic liquids should be interpreted in this light.20
0.8
[MeSO4]/[C4C1im]
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
20
40
60
80
100
Water content (mol%)
Figure 2: Ratio of [MeSO4]- anions to ionic liquid cations in the recycled ionic liquid after pretreatment of
Miscanthus (detected by 1H-NMR), the remaining anions being [HSO4] -.
Influence of the water content on the enzymatic saccharification of [C4C1im][HSO4]
treated miscanthus
With the knowledge that the binary 1-butyl-3-methylimidazolium methyl sulfate water
mixtures turned into quaternary mixtures of two ionic liquids plus two molecular solvents
(water and methanol) we set out to identify the active component(s). Miscanthus was
pretreated with aqueous mixtures of [C4C1im][HSO4], which allowed us to exclude methyl
sulfate and methanol. The saccharification yields obtained from the pulps pretreated with
various [C4C1im][HSO4] water mixtures are shown in Figure 1. The glucose yields were
almost identical to the glucose yields obtained with the quaternary mixtures. The pattern of
hemicellulose release was also similar, however, after [C4C1im][HSO4]40%-80% pretreatment,
less hemicellulose was recovered than after treatment with the equivalent methyl sulfate
containing mixtures.
A glucose recovery of 90% after ionic liquid pretreatment is a substantial improvement
compared with the saccharification yields reported after pretreatment with other ionic liquids.
It has been reported that 74% glucose was enzymatically released from ground maple wood
after [C4C1im][MeCO2] treatment at 90°C for 24 h.21 70% glucose was released from maple
wood after [C2C1im][MeCO2] treatment at 90°C for 24 h. Li et al. reported only 15% glucose
release from ground Eucalyptus, pretreated with 1-allyl-3-methylimidazolum chloride,
[C=C2C1im]Cl, at 120°C for 5 h,22 while 55% of the glucose was released after 1-ethyl-3methylimidazolium diethyl phosphate, [C2C1im][Et2PO4], pretreatment of ground wheat straw
at 130°C for 30 min.23 It should be noted that saccharification yields obtained from ball-milled
7
lignocellulose samples were not considered for this listing because fine milling can have a
considerable effect on cellulose digestibility.22 The use of ground material reduces the
economic viability,31 so using fine powders obtained by ball-milling is of very little relevance
for an industrial process. Studies using 3,5-dinitrosalicylic acid (DNS) for the determination
of glucose yield were also not considered. The test is not specific for glucose and therefore
glucose yields from lignocellulose are often overestimated.
Effect of pretreatment time on the enzymatic saccharification
Next, we were interested in the optimisation of the pretreatment time. Figure 3 shows the
saccharification yields for both [C4C1im][MeSO4]80% and [C4C1im][HSO4]80% pretreatment after
various lengths of time. It can be seen that the enhancement of the cellulose and
hemicellulose digestibility mainly occurred within the first 4 h. This was also the period of
where the mass loss increased significantly (data shown in ESI). The pretreatment was
practically complete after 8 h, achieving around 80% glucose and 30% hemicellulose
release. When prolonging the pretreatment, the glucose yield slightly increased to above
85%, but the hemicellulose yield decreased to just over 20%. This experiment shows that
the presence or absence of methyl sulfate in the pretreatment mixture does not significantly
influence the speed of the pretreatment. It is anticipated that the pretreatment time can be
shortened by the application of higher temperatures, but it must be balanced with the ionic
liquid stability and potential side reactions.30
Saccharification yield, % of max. possible
100
90
80
Glucose [MeSO4]-
70
60
Glucose [HSO4]-
50
Xylose [MeSO4]-
40
30
Xylose [HSO4]-
20
10
0
0
10
20
30
Time (h)
Figure 3: Glucose and hemicellulose yields after enzymatic hydrolysis of Miscanthus pretreated with
[C4C1im][MeSO4]80% and [C4C1im][HSO4]80% at 120°C.
8
The effect of [C4C1im][MeSO4]80% and [C4C1im][HSO4]80% pretreatment on biomass
composition
The composition of untreated Miscanthus and pretreated pulp is shown in Table 2 and
Figure 4. The untreated biomass contained 43.6% glucose, 24.3% hemicellulose and 26.5%
lignin. After pretreatment with [C4C1im][MeSO4]80% for 2 h, the main effect was a reduction of
the lignin content. Treatment with [C4C1im][HSO4]80% for 2 h resulted in the removal of lignin
and hemicellulose. After an extended pretreatment for 22 h, most of the lignin and
hemicellulose was solubilised and the glucan content increased from 44% in the untreated
biomass to 85% in the pretreated biomass. 91% of the original glucan was still present in the
pulp. The biomass recovery after 22 h was less than 46%, showing that more than half of the
wood had been solubilised in the ionic liquid. Tan et al. reported a mass recovery between
46% and 55% after pretreatment with [C2C1im][ABS] at 170-190°C, indicating that this ionic
liquid mixture might be capable of similar biomass fractionation.30 The simultaneous removal
of lignin and hemicellulose has also been reported for [C2C1im][MeCO2],24, 25 albeit less
complete than seen in this study with [C4C1im][HSO4]80%.
Table 2: Composition of untreated Miscanthus and Miscanthus pretreated with [C4C1im][MeSO4] and
[C4C1im][HSO4].
Xyl
Ara
Man
Gal
Lignin
Ash
43.6
45.4
44.5
39.5
18.3
18.3
8.6
3.3
3.4
2.1
0
0
1.1
1.3
0
0
2.4
2.2
3.4
1.1
26.5
19.3
14.9
1.9
1.3
1.1
0.6
0.6
wt%
untreated
[MeSO4] 2 h
[HSO4] 2 h
[HSO4] 22 h
Glu
100
90
80
70
60
50
40
30
20
10
0
Extra
ctives
4.7
-
Mass
loss
0
10
28
56
Mass loss
Extracts
Ash
Lignin
Arabinan
Galactan
Mannan
Xylan
Glucan
HSO4 22h
HSO4 2h
MeSO4 2h untreated
Figure 4: Composition of Miscanthus before and after pretreatment with [C 4C1im][HSO4]80% and
[C4C1im][MeSO4]80% at 120°C for 2 h or 22 h.
Production of solubilised sugars and furfurals
As seen above, the hemicellulose was removed from the biomass during treatment with
[C4C1im][HSO4] and [C4C1im][MeSO4] water mixtures. It is likely that under the conditions of
the pretreatment, (partial) hydrolysis of solubilised hemicellulose occurred. Therefore the
9
concentration of monomeric carbohydrates in the pretreatment liquor was investigated.
Figure 5 shows the relative amount of hemicellulose sugars and glucose in
[C4C1im][HSO4]80% and [C4C1im][MeSO4]80% liquors at different time points.
The amount of hemicellulose monomers in the liquor increased within the first 4 h. The
increase was more pronounced in the [C4C1im][HSO4]80% liquor. The maximum amount of
hemicellulose monomers was detected around 4-8 h. This coincided with a major increase of
cellulose digestibility after 4-8 h of treatment (Figure 3). Subsequently, the hemicellulose
concentration in the pretreatment liquor decreased, suggesting that conversion of
carbohydrate monomers into furfurals was occurring.
18
% of maximum possible
16
14
Hemicellulose
[HSO4]Hemicellulose
[MeSO4]-
12
10
8
Glucose [HSO4]-
6
4
Glucose
[MeSO4]-
2
0
0
10
20
30
Time (h)
Figure 5: Amount of glucose and hemicellulose monomers found in [C4C1im][HSO4]80% and [C4C1im][MeSO4]80%
liquors during pretreatment at 120°C.
Furfural was detected in the ionic liquid liquors and quantified for selected mixtures (Figure
6).
The glucose content was significantly lower than the hemicellulose sugar content and hardly
changed over time. The smaller amount of solubilised glucose is ascribed to the slow
hydrolysis of cellulose under the conditions of the pretreatment and the decomposition of
glucose to HMF. The small amount of HMF might be due to its decomposition to other
degradation products in the presence of water.
10
35
% of maximum possible
30
25
Glucose
20
Hemicellulose
15
Furfural
10
HMF
5
0
[HSO4]-, 22h
[HSO4]-, 2h
[MeSO4]-, 2h
Figure 6: Solubilised carbohydrates (monomers only) and the fraction converted to furfural after pretreatment with
[C4C1im][HSO4]80% and [C4C1im][MeSO4]80% liquors.
Lignin recovery
We attempted to recover lignin from the liquor (Figure 7), as this has been successfully
demonstrated for other ionic liquids.26, 30 It was found that diluting the ionic liquid liquor with
water precipitated a fine powder. The powder was characterised by IR spectroscopy and
comparison with a spectrum of a reference lignin (alkaline lignin) showed that the precipitate
is likely to be mostly lignin (see ESI). When methanol was used for washing the pulp, instead
of water, the majority of the precipitate remained in solution and a 15-20% improvement of
precipitate recovery was observed. Therefore washing the pulp with methanol was preferred.
The final protocol consisted of washing the pulp with methanol, drying the combined ionic
liquid fractions by evaporating the methanol, and precipitating the lignin by diluting the dried
ionic liquid liquor with water. The precipitate was washed with copious amounts of water and
dried before the yield was determined. The data (Figure 7) show that the yield of precipitate
was up to 50% of the Klason lignin content of the untreated biomass. More precipitate was
obtained when the ionic liquid content in the pretreatment liquor was high.
11
60
Lignin yeld (%)
50
40
30
20
10
0
0%
20%
40%
60%
80%
100%
Ionic liquid content
Figure 7: Yield of precipitate (relative to Klason-lignin content of the untreated biomass) after pretreatment of
Miscanthus with [C4C1im][HSO4] water mixtures at 120°C for 13 h.
We also examined the time dependency of the precipitate yield and observed that the yield
of precipitate plateaued within 8 h (Figure 8). The yield was slightly higher from
[C4C1im][HSO4]80% compared to [C4C1im][MeSO4]80%..
Yield, % of maximum possible
60
50
40
30
[C4C1im][MeSO4]80%
20
[C4C1im][HSO4]80%
10
0
0
5
10
15
Time (h)
20
25
30
Figure 8: Time course of lignin recovery after pretreatment of Miscanthus with [C4C1im][MeSO4]80% and
[C4C1im][HSO4]80% at 120°C. The lignin was isolated from the liquor by precipitation with water.
The effect of the ionic liquid cation
The use of ionic liquids with mono-alkylated imidazolium cations (1-alkylimidazolium,
[CnHim]+) is advantageous from an industrial point of view, as the ionic liquids are easier to
synthesise and thus cheaper to produce.7 Therefore an exemplary pretreatment of
Miscanthus with 1-butylimidazolium hydrogen sulfate, [C4Him][HSO4], was carried out. The
sugar yields after treatment with [C4Him][HSO4]80% and a subsequent enzymatic
saccharification are shown in Figure 9. After 4 h pretreatment, 69% of the original glucose
12
and 10% of the original hemicellulose were enzymatically released. The yield was somewhat
improved by prolonging the treatment to 20 h, when 75% of the glucose was recovered.
However, the xylose yield was reduced to only 3%. Pretreatment with [C4Him][HSO4]95%
resulted in significantly reduced glucose yields (44%).
Saccharification yield, % of max. possible
90
80
70
60
50
Glucose
40
Xylose
30
20
10
0
80%, 4 h
80%, 20 h
95%, 20 h
Water
Figure 9: Enzymatic saccharification yields obtained from Miscanthus after pretreatment with [C4Him][HSO4]95%
and [C4Him][HSO4]80%. Saccharification was carried out for 96 h.
The results of the compositional analysis and the mass loss of [C4Him][HSO4] treated
Miscanthus are presented in Table 3 and Figure 10. 80-93% of the lignin and more than 95%
of the hemicellulose wereremoved. The thorough removal of hemicellulose is reflected by
the low xylose yields obtained during saccharification. Treatment with [C4Him][HSO4]95% not
only resulted in the solubilisation of lignin and hemicellulose, but also in a substantial
removal of the cellulose fraction (51% of the glucan), explaining the reduced glucose yield
shown in Figure 9. The results indicate that pretreatment with [C4Him][HSO4] was harsher
than with [C4C1im][HSO4] under comparable conditions, potentially due to the increased
acidity of the [C4Him][HSO4] compared to [C4C1im][HSO4].
Table 3: Composition of Miscanthus pretreated with [C4Him][HSO4]80% and [C4Him][HSO4]95% at 120°C.
Values are given in %; Glu=glucan, Xyl=xylan, Man=mannan, Gal=galactan, Ara=arabinan.
IL content,
treatment time
80%, 4 h
80%, 20 h
99%, 20 h
Glu
Xyl
Man
Gal
Ara
Lignin
Ash
40.9
37.7
22.4
2.9
1.0
0.6
0
0
0
0.7
1.0
0.6
0.2
0
0
5.0
5.4
1.9
0.8
0.6
0.4
13
Mass
loss
49.5
54.2
74.2
%
100
90
Mass loss
80
Extracts
70
Ash
60
Lignin
50
Arabinan
40
Galactan
30
20
Mannan
10
Xylan
0
Glucan
80%, 4 h
80%, 20 h
95%, 20 h
Figure 10: Composition of Miscanthus after pretreatment with [C4Him][HSO4] water mixtures at 120°C.
It was also possible to obtain a precipitate upon dilution of the ionic liquid liquor. For the
[C4Him][HSO4]80% liquor, the yield was nearly 100% of the lignin content. For the 95% liquor,
the amount of precipitate was almost double the amount of the lignin content. We explain the
unusually high precipitate yield with the formation of pseudo-lignin. The formation of waterinsoluble carbohydrate degradation products has been observed during biomass
pretreatment under severe acidic conditions and found to obscure the Klason lignin yield.
Therefore it has been termed pseudo-lignin.32, 33 The formation of such degradation products
% of maximum possible
is undesirable and optimisation of the pretreatment conditions is required to minimise this
200
180
160
140
120
100
80
60
40
20
0
Lignin removed (%)
Recovered precipitate
(% of lignin)
80%, 4 h
80%, 20 h
95%, 20 h
Figure 11: Lignin removal and precipitate yield after pretreatment of Miscanthus with [C4Him][HSO4] water
mixtures at 120°C.
The effect of the ionic liquid anion on the composition of ionic liquid treated Miscanthus
The effect of [C4C1im][HSO4]80% was compared with the effect 20/80 vol% water
dialkylimidazolium ionic liquid mixtures. The anions that we examined were
trifluoromethanesulfonate, [OTf]-, methanesulfonate, [MeSO3]-, chloride, Cl-, and acetate,
14
[MeCO2]-. It should be noted that the acetate containing ionic liquid, [C2C1im][MeCO2], was
of commercial quality.
Table 4: Composition of pretreated Miscanthus after treatment with 80/20% ionic liquid water mixtures at
120°C for 22 h. Values are given in %; Glu=glucan, Xyl=xylan, Man=mannan, Gal=galactan, Ara=arabinan.
%
Ionic liquid
anion
[MeCO2]Cl[MeSO3][HSO4][OTf]-
Glu
Xyl
Man
Gal
Ara
Lignin
Ash
41.9
44.5
37.1
39.5
43.6
7.9
17.8
4.3
3.3
13.7
0
0
0
0
0
4.0
2.3
2.3
1.1
5.1
3.4
2.7
0
0
4
11.6
22.5
8.5
1.9
24.3
0.5
0.7
1.0
0.6
1.0
100
90
80
70
60
50
40
30
20
10
0
Mass
loss
30.6
9.5
46.8
53.6
8.3
Mass loss
Extractives
Ash
Lignin
Arabinose
Galactose
Mannose
Xylose
Glucose
[MeCO2]
Cl
[MeSO3] [HSO4]
[OTf] untreated
Figure 12: Effect of the ionic liquid anion on the mass loss and the composition of the recovered pulp after
pretreatment of Miscanthus with 80% ionic liquid water mixtures at 120°C for 22 h. The data are ordered (left to
right) according to the hydrogen-bond basicity of the ionic liquid, which is, in case of 1,3-dialkylimidazolium ionic
liquids, a property of the anion.
Figure 12 and Table 4 show that the nature of the anion has a profound effect on mass loss
and pulp composition. [C4C1im][HSO4]80% removed lignin and hemicellulose most thoroughly,
followed by [C4C1im][MeSO3]80% and then by [C2C1im][MeCO2]80%. Hardly any change of the
composition was observed when the biomass was treated with [C4C1im]Cl80% and
[C4C1im][OTf]80%, despite the fact that high solubility of Kraft lignin has been reported for both
ionic liquids(in anhydrous form).17, 20 The contradiction could be resolved if lignin
solubilisation and lignin extraction (which usually involves chemical modifications) were
regarded as different properties.
The effect of the anion on the saccharification yield
Enzymatic saccharification of Miscanthus treated with the ionic liquid liquors was also carried
out (Figure 13). In general, the enzymatic glucose release appeared to reflect the extent of
compositional change/mass loss achieved during ionic liquid pretreatment. The highest
glucose yield was observed after [C4C1im][MeSO3]80% and [C4C1im][HSO4]80% pretreatment.
The hemicellulose yield behaved slightly differently. The xylose yield was the highest after
15
pretreatment with [C2C1im][MeCO2]80%. The yield was significantly lower after
[C4C1im][MeSO3]80% and [C4C1im][HSO4]80% pretreatment. Comparatively high hemicellulose
yields after [C4C1im][MeCO2] treatment can also be found in the literature.21 The increased
hemicellulose recovery after [C2C1im][MeCO2]80% treatment could be due to a buffering effect
exerted by the basic acetate anion. Its ability to combine with protons to form acetic acid
may limit the acid-catalysed hydrolysis of hemicellulose polymers. Inhibition of the hydrolysis
of cellobiose by [C4C1im][MeCO2] has been observed in mixtures of the ionic liquid, water
and catalytic amounts of strong acid.34 Binder et al., have also observed inhibition of
cellulose depolymerisation in [C4C1im][MeCO2], despite addition of catalytic amounts of
HCl.35 The methanesulfonate anion appears to have a less protective effect and acidcatalysts which are released from the biomass (acetic acid and hydroxycinnamic acids) can
aid xylan hydrolysis. Hydrogensulfate increases the amount of available protons, which
could explain the particularly low xylan content in the pulp. The glucose and xylose yields
obtained after treatment with [C4C1im]Cl80% and [C4C1im][OTf]80% were low, despite their
ability to dissolve cellulose and lignin preparations (in case of triflate only lignin solubility).
Saccharification yield, % of max. possible
100
90
80
70
Glucose
60
50
Hemicellulose
sugars
40
30
20
10
0
[MeCO2]
Cl
[MeSO3]
[HSO4]
[OTf]
not
treated
Figure 13: Impact of the ionic liquid anion on glucose and hemicellulose yields after enzymatic saccharification of
Miscanthus pulp pretreated with 80/20 vol% ionic liquid water mixtures at 120°C for 22 h.
The effect of the anion on delignification and precipitate recovery
The yield of precipitate seems to be related to the ability of the liquor to extract lignin (Figure
14). The best delignification and the highest precipitate yield was obtained with
[C4C1im][HSO4]80%, followed by [C4C1im][MeSO3]80% and then[C2C1im][MeCO2]80%. This
supports the notion that the precipitate comprises lignin, although it was shown in Figure 11
that pseudo-lignin also precipitate upon dilution of the ionic liquid liquor.
16
% of max. possible
100
90
80
70
60
50
40
30
20
10
0
Delignification
Lignin recovery
[MeCO2]
Cl
[HSO4]
[MeSO3]
[OTf]
Figure 14: Effect of the anion on the lignin removal and precipitate yield after pretreatment of Miscanthus with
80/20 vol% ionic liquid water mixtures. The higher yield from [HSO4]- containing liquors (compared to Figure 7
and Figure 8) is ascribed to the larger quantity of ionic liquid and biomass used in this experiment. Values are
relative to the lignin content of the untreated biomass.
The effect of the anion on the formation of soluble degradation products
The quantities of carbohydrate monomers and dehydration products solubilised in the
pretreatment liquors are shown in Figure 15. The [C4C1im][HSO4]80% and [C4C1im][MeSO3]80%
liquors contained approximately 45% of the total hemicellulose as either sugar monomers or
furfural. In [C4C1im][HSO4]80%, the majority of the largest fraction was furfural. Conversion of
pentoses into furfurals was also observed in [C4C1im][MeSO3]80%, but to a lesser extent. This
is ascribed to the non-acidic nature of this ionic liquid. Only small quantities of monomers
were detected in the acetate containing liquor, which is probably due to fact that the
solubilised carbohydrates are mostly in oligomeric form. No furfural was formed in
[C2C1im][MeCO2]80% in our experiment. It is likely that the acidity of the liquor is responsible
for the varying concentrations of sugar monomers and furfural found in the liquor. Like the
hydrolysis of glycosidic bonds, the rate of furfural formation depends on the acid
concentration.36 Since the acidity/basicity of 1,3-dialkylimidazolium ionic liquids is
determined by the anion, its nature should have a profound impact on the fate of the
solubilised hemicellulose. The amount of solubilised glucose and HMF were small in all
cases. This is ascribed to the enhanced stability of the cellulose fraction towards hydrolysis
under pretreatment conditions and the propensity of HMF to react to formic and levulinic acid
in the presence of water.
17
% of max. possible
45
40
35
30
Glucose
25
Hemicellulose
20
Furfural
15
HMF
10
5
0
[OTf]
[HSO4]
[MeSO3]
Cl
[MeCO2]
Figure 15: Sugar monomers and furfurals solubilised in liquors containing 80 vol% 1,3-dialkylimidazolium ionic
liquid with various anions after treatment of Miscanthus at 120°C for 22 h.
The effect of the biomass type: pretreatment of willow and pine
Pretreatment with [C4C1im][HSO4]80% was also performed on ground willow (a hardwood
species) and pine (a softwood species). For comparison, willow and pine were also
pretreated with [C2C1im][MeCO2]80%. The effect of the pretreatment on the biomass
composition is shown in Table 5 and Figure 16.
Table 5: Composition of untreated willow and pine and the pulps after treatment with [C4C1im][HSO4]80%
and [C4C1im][MeCO2]80%.
Willow
Willow, [MeCO2]
Willow, [HSO4]
Pine
Pine, [MeCO2]
Pine, [HSO4]
Glu
Xyl
Man
Gal
Ara
Lignin
Ash
46.7
36.3
39.1
45.8
40.4
37.9
16.8
6.4
3.4
2.5
2.5
3.2
3.6
2.9
0
12.0
16.1
4.6
1.9
2.7
0.8
2.6
3.4
0
2.5
1.9
0.9
3.4
2.7
0
24.1
19.9
3.6
25.5
21.1
8.8
0.7
0.7
0.5
1.3
0.6
0.2
18
Extra
ctives
3.7
4.3
-
Mass
loss
0
29
52
0
13
45
100
90
Mass loss
80
Extracts
70
Ash
%
60
Lignin
50
40
Arabinan
30
Galactan
20
Mannan
10
Xylan
0
Pine
Willow
[MeCO2] [HSO4] untreated [MeCO2] [HSO4]
untreated
Glucan
Figure 16: Composition of willow (3 bar graphs on the left) and pine (on the right) before and after pretreatment
with [C4C1im][HSO4]80% and [C4C1im][MeCO2]80% for 22 h at 120°C.
For both substrates, lignin and hemicellulose removal were more extensive after
[C4C1im][HSO4]80% pretreatment than after treatment with [C2C1im][MeCO2]80%. The degree of
cellulose enrichment after [C4C1im][HSO4]80% pretreatment of willow was almost as good as
the enrichment observed for Miscanthus pulp. A precipitate could be recovered from all
samples. Significantly higher yields were obtained from the [C4C1im][HSO4]80% liquors (see
ESI). The glucose yields obtained via enzymatic saccharification are shown in Figure 17.
More than 80% of the original glucose was released from [C4C1im][HSO4]80% pretreated
willow pulp, approaching the saccharification yields obtained from Miscanthus pretreated
with this liquor. However, enzymatic saccharification of pine pulp only released up to 30% of
the glucose; the type of ionic liquid playing a minor role. The generally higher yields obtained
after [C4C1im][HSO4]80% pretreatment could be due to the improved lignin and hemicellulose
removal by the hydrogen sulfate containing liquor, as observed for Miscanthus.
19
Glucose yield, % of max. possible
100
90
80
Miscanthus, [HSO⊝]
70
Willow, [HSOÁ]
60
Willow, [MeCO⊇]
50
40
Pine, [HSO⊝]
30
20
Pine, [MeCO⊇]
10
0
0
50
100
Saccharifcation time (h)
Figure 17: Enzymatic saccharification of lignocellulosic feedstocks after pretreatment with [C4C1im][HSO4]80% or
[C2C1im][MeCO2]80% for 22 h at 120°C.
Ionic liquid solvent properties and biomass digestibility
We measured the Kamlet Taft polarity of [C4C1im][HSO4] and [C4C1im][MeSO3] (Table 6), as
it has not been reported in the literature. Three parameters are used to determine the
strength of solvent solute interactions. The parameter  describes the hydrogen-bond acidity
of the solvent,  the hydrogen-bond basicity and * the polarisability. Our measurements
showed that the  parameter of [C4C1im][HSO4] is the same as the value for
[C4C1im][MeSO4]. The hydrogen-bond acidity is very different, in fact, the  value cannot be
determined for [C4C1im][HSO4], because it protonates one of the dye probes.
We would like to point out that the high glucose yields were achieved without complete
solubilisation of the biomass. This is due to the relatively low  values of [C4C1im][MeSO4],
[C4C1im][HSO4] and [C4C1im][MeSO3], which do not enable cellulose solubilisation. The 
parameters are lower than the values of [C4C1im][MeCO2] (=1.20), 1-butyl-3methylimidazolium dimethyl phosphate, [C4C1im][Me2PO4], (=1.12) and [C4C1im]Cl
(=0.83).19 Although [C2C1im][MeCO2] can dissolve cellulose when it is anhydrous, the
presence of 20 vol% water prevents cellulose solubility.
Table 6: Kamlet-Taft parameters of selected ionic liquids used in this work.


0.77
[C4C1im][MeSO3]
0.44
*
1.02
[C4C1im][MeSO4]19
[C4C1im][HSO4]
0.55
0.67
1.05
-
0.67
1.09
20
We also attempted to correlate the glucose yields with the ionic liquids’ hydrogen-bond
basicity. While it is clear that the nature of the anion affects the saccharification yield, it could
not be correlated with the ionic liquid’s  value.
Conclusions
It has been demonstrated for the first time that the ionic liquids [C4C1im][HSO4],
[C4C1im][MeSO3] and the ionic liquid mixture [C4C1im][MeSO4]/[HSO4] can be used to
pretreat lignocellulosic biomass. These ionic liquids functioned effectively in the presence of
significant quantities of water, eliminating the need for anhydrous conditions during
pretreatment. Commercial [C2C1im][MeO2] was also effective in the presence of 20 vol%
water, but the saccharification yield was lower. Lignin and hemicellulose were solubilised
during pretreatment, leaving behind a solid residue that was highly enriched in cellulose. The
enzymatic saccharification of Miscanthus pulp pretreated at 120°C with liquors containing 80
vol% ionic liquid resulted in glucose yields of ca. 90%. The hemicellulose was partially
recovered with the solid and readily hydrolysable during enzymatic saccharification.
However, a significant portion of the hemicellulose remained in the pretreatment liquor as
sugar monomers and was partially converted dehydration products. The amount of furfurals
generated during ionic liquid pretreatment arises from the acidity of the ionic liquid liquors. In
the presence of 20 vol% water, treatment with [C4C1im]Cl and [C4C1im][OTf] had little effect
on the biomass, showing that the anion of 1,3-dialkylimidazolium ionic liquids plays an
important role in determining the effectiveness of ionic liquid pretreatment and the tolerance
towards water. We could not find a correlation between the pretreatment effectiveness and
the anion basicity, as previously found for cellulose solubility or wood chips swelling. While
the enzymatic sugar release from the grass and hardwood pulps was very good, yields from
softwood pulp were only moderate. Upon dilution with water a precipitate was recovered that
is likely to contain lignin as well as pseudo-lignin. This study also suggests that monoalkylated imidazolium ionic liquids, such as [C4Him][HSO4], are in principle promising,
industrially relevant alternatives to dialkylimidazolium ionic liquids. The next challenge is to
optimise these processes.
Acknowledgements
We gratefully acknowledge the help of Julian Gianuzzi with pretreatment and
saccharification experiments and funding of the studentship for Agnieszka Brandt by the
Porter Institute.
21
Experimental
Materials
The lignocellulosic feedstocks used in this study were pine sapwood (Pinus sylvestris,
variety SCOES) from East Sussex, de-barked mixed willow (Salix viminalis, variety TORA)
stems and Miscanthus giganteus whole stems. The biomass was air-dried, ground and
sieved (0.18-0.85 mm, -20+80 of US mesh scale) before use. The moisture content of
untreated lignocellulose was 8.0% (Miscanthus), 8.9% (Pine) and 7.6% (Willow) based on
oven-dry weight. The biomass was stored in plastic bags at room temperature.
1-butyl-3-methylimidazolium methyl sulfate (Basionic AC01) was purchased from SigmaAldrich. 1-ethyl-3-methylimidazolium acetate (Basionic BC01) was a gift from BASF AG,
Ludwigshafen. 1-butyl-3-methylimidazolium chloride and 1-butyl-3-methylimidazolium
trifluoromethanesulfonate were synthesised as described previously.19 The syntheses of 1butyl-3-methylimidazolium hydrogen sulfate, 1-butylimidazolium hydrogen sulfate and 1butyl-3-methylimidazolium methanesulfonate are described in the ESI. The ionic liquids were
dried to a water content <0.3 wt%, with exception of [HC4im][HSO4] which had a water
content of 1 wt%.
Measurement of Kamlet-Taft parameters
Kamlet-Taft parameter were measured as reported previously.19
Recycling of [C4C1im][MeSO4]
The ionic liquid liquor obtained after lignin precipitation was dried under vacuum at 40°C. A
sample of the dried ionic liquid was submitted to mass spectrometry. Part of the recovered
ionic liquid was dissolved in DMSO-d6 and a 1H NMR spectrum recorded. The peaks of
the methyl group at 3.40 ppm and of the C-2 ring hydrogen were used to determine the
anion to cation ratio. The pretreatment was carried out in capped vessels, so it is
reasonable to assume that the water content did not change substantially during the
pretreatment. The water introduced by the ionic liquid and the air-dried biomass was taken
into account, but not water consumed in hydrolytic reactions.
Lignocellulose pretreatment and isolation of pulp
0.500 g biomass (on oven-dry weight basis) was placed in wide-mouthed Pyrex culture
tubes with screw cap and Teflon-lining. 5 ml dried ionic liquid or the equivalent volume of
ionic liquid plus water were added. Mixing effects were neglected. The samples were
incubated without stirring in an oven at 120°C. After the pretreatment was finished, the
samples were cooled to room temperature and mixed with 10 ml methanol. After 2 h, the
suspension was filtered through hardened cellulose filter papers (Whatman 541 or
22
equivalent). The supernatant was set aside for precipitation and analysis of the monomer
and furfural content. The pulp was washed with methanol from a wash bottle, placed in a vial
and incubated with 10 ml fresh methanol overnight. The suspension was filtered again,
rinsed with methanol and air-dried on the filter paper overnight. The air-dried weight was
recorded and the samples transferred into re-sealable air-tight sample bags. In order to
obtain enough material for compositional analysis the pretreatment experiments were scaled
up 2-3x.
Lignin recovery
The supernatant obtained after pretreatment was dried under mild vacuum at 40°C to
remove the organic wash solvent. 10 ml water was added per 5 ml of original liquor. The
suspension was centrifuged and the precipitate washed 3x with 10 ml distilled water, airdried for several days and dried under vacuum at room temperature. The precipitate yield
was calculated based on the Klason lignin content of untreated biomass using Equation 1.
Part of the precipitate may be pseudo-lignin.
𝐿𝑖𝑔𝑛𝑖𝑛 𝑦𝑖𝑒𝑙𝑑 (%) =
𝑚precipitate
𝑚Klason lignin
Eq. 1
∙ 100%
The precipitate was characterised by IR spectroscopy using a Spectrum 100 IR machine
(Perkin-Elmer) equipped with a universal ATR sampling accessory with diamond crystal.
Determination of moisture content
100-200 mg air-dried biomass was wrapped in aluminium foil of known weight and heated at
105°C overnight. The samples were transferred into a desiccator and the weight determined
after 5 min. The moisture content was calculated according to Eq. 2 and used to determine
the oven-dried weight of untreated and pretreated biomass. The moisture content of the airdried biomass was in the range 5-12%.
𝑚𝑜𝑖𝑠𝑡𝑢𝑟𝑒 (%) =
𝑚𝑎𝑖𝑟 𝑑𝑟𝑖𝑒𝑑 − 𝑚𝑜𝑣𝑒𝑛 𝑑𝑟𝑖𝑒𝑑
𝑚𝑜𝑣𝑒𝑛𝑑𝑟𝑖𝑒𝑑
∙ 100%
Eq. 2
Enzymatic saccharification
Enzymatic saccharification was performed according to LAP “Enzymatic saccharification of
lignocellulosic biomass” (NREL/TP-510-42629). 150 mg untreated or pretreated air-dried
lignocellulosic biomass was used per saccharification experiment. If a pretreatment condition
was run in duplicate or triplicate, one saccharification experiment was performed per sample.
If the pretreatment condition was not replicated, the saccharification was performed in
duplicate. The enzyme preparations used for the saccharification were Celluclast (cellulase
mix from T. reseei) and Novozyme 188 -glucosidase which also contain hemicellulolytic
23
activity and can therefore hydrolyse xylan (both from Sigma). Glucose and hemicellulose
yields were calculated based on the glucose and hemicellulose content of the untreated
biomass, respectively.
Compositional analysis
The compositional analysis (lignin, carbohydrates, ash) was performed according to
laboratory analytical procedure (LAP) “Determination of structural carbohydrates and lignin
in biomass” (NREL/TP-510-42618). 300 mg of oven-dried sample were used per
experiment. Extractives from untreated pine and willow flour were removed by a one-step
automated solvent extraction with 95% ethanol using the ASE 300 accelerated solvent
extractor (Dionex) according to the LAP “Determination of extractives” (NREL/TP-510-
42619). Extractives from untreated Miscanthus were removed by a two-step solvent
extraction using deionised water and subsequently 95% ethanol according to the same LAP.
HPLC analysis of glucose and hemicellulose sugars was performed on an Agilent 1200
system equipped with an Aminex HPX-87P column, a de-ashing column and a Carbo-P
guard column (all Biorad). The mobile phase was de-ionised water, the column temperature
80°C and the flow rate 0.6 ml/min. The content of carbohydrates, Klason lignin, ash and
extractives (where applicable) was expressed as a fraction of the sum (normalised to 100%).
Quantification of solubilised sugars and furfurals
200 μl pretreatment liquor was mixed with 600 μl deionised water in 1.5 ml plastic cup,
vortexed and centrifuged with a table-top centrifuge (Biofuge 13, Heraeus) at maximum
speed for 10 min. The supernatant was transferred into a clean cup and centrifuged for 10
min. The supernatant was transferred into HPLC sample vials and analysed on a Jasco
HPLC system equipped with an Aminex HPX-87H column (Biorad) using a 10 mM sulfuric
acid mobile phase. The column oven temperature was 55°C, the flow rate 0.6 ml/min and
the acquisition time 55 min. Standard concentrations of 2-furaldehyde (furfural) and 5(hydroxymethyl)-2-furaldehyde (HMF) standards were prepared in deionised water to
concentration of 0.01, 0.02, 0.1, 0.2 and 0.4 mg/ml. The factor fHPLC(S) was obtained from
the respective calibration curve. The relative yield of solubilised sugar monomers and
furfurals, wt%(S), was calculated using Eq. 3. The molecular mass transformation factor FT
was 1.37 for furfural, 1.28 for HMF and 0.91 for glucose and 0.88 for hemicellulose sugars.
The mass fraction factor FC was 0.243 for hemicellulose sugars and furfural and 0.436 for
glucose and HMF.
wt%(S) = F
AHPLC ∙FD ∙VPL
HPLC (S)∙mbiomass
Eq. 3
∙ FC ∙ FT
24
AHPLC: area of HPLC peak, FHPLC(S): HPLC calibration factor for substance S, FD: dilution
factor, VPL: Volume of pretreatment liquor in ml, mbiomass: biomass (oven-dried weight) in mg,
FC: fraction of glucan or hemicellulose sugars in untreated biomass as determined by
compositional analysis (in %), FT: Transformation factor accounting for molecular mass
differences between starting material and product
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