- Email: [email protected]
Deep eutectic solvent for lignocellulosic biomass fractionation and the subsequent conversion to bio-based products – A review
Journal Pre-proofs Review Deep eutectic solvent for lignocellulosic biomass fractionation and the subsequent conversion to bio-based products – A review Yee Tong Tan, Adeline Seak May Chua, Gek Cheng Ngoh PII: DOI: Reference:
S0960-8524(19)31752-3 https://doi.org/10.1016/j.biortech.2019.122522 BITE 122522
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
4 October 2019 26 November 2019 27 November 2019
Please cite this article as: Tan, Y.T., Chua, A.S.M., Ngoh, G.C., Deep eutectic solvent for lignocellulosic biomass fractionation and the subsequent conversion to bio-based products – A review, Bioresource Technology (2019), doi: https://doi.org/10.1016/j.biortech.2019.122522
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Elsevier Ltd. All rights reserved.
Title: Deep eutectic solvent for lignocellulosic biomass fractionation and the subsequent conversion to bio-based products – A review
Yee Tong Tan, Adeline Seak May Chua, Gek Cheng Ngoh*
Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia.
*Corresponding author Tel.: +60 3 79675301; fax: +60 3 79675371 Email: [email protected] (Gek Cheng Ngoh)
Other authors’ e-mail address: [email protected] [email protected]
Present and permanent address: Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia.
1
Abstract
2
Since the introduction of deep eutectic solvent (DES) in biomass processing field, the
3
efficiency of DES in lignocellulosic biopolymer model compounds’ (cellulose,
4
hemicellulose and lignin) solubilisation and conversion was widely recognized.
5
Nevertheless, DES’s potential for biorefinery application can be reflected more
6
accurately through their performance in raw lignocellulosic biomass processing rather
7
than model compound conversion. Therefore, this review examines the studies on raw
8
lignocellulosic biomass fractionation using DES and the subsequent conversion of DES-
9
fractionated products into bio-based products. The review stresses on three key parts:
10
performance of varying types of DESs and pretreatment schemes for biopolymer
11
fractionation, properties and conversion of fractionated saccharides as well as DES-
12
extracted lignin. The prospects and challenges of DES implementation in biomass
13
processing will also be discussed. This review provides a front-to-end view on the
14
DES’s performance, starting from pretreatment to DES-fractionated products
15
conversion, which would be helpful in devising a comprehensive biomass utilization
16
process.
17
Keywords: biopolymer fractionation; carbohydrate conversion; lignin conversion;
18
biorefinery; green technology
19 20 21
1
Introduction It is inevitable to apply solvents in industrial processes due to their required role
22
in dissolving solutes, promoting mass and heat transfer, facilitating separation and
23
purification processes and so on (Cvjetko Bubalo et al., 2015). Solvents are usually
24
utilized in large quantities particularly when different types of solvents are required to
1
25
accommodate various processes. Hence, the choice of solvent greatly influences the cost,
26
safety and environmental performance of a process.
27
In tandem with the twelve principles of Green Chemistry as introduced by
28
Anastas and Eghbali (2010), innovative designer solvents have been proposed as
29
alternatives to conventional molecular solvent. For instance, deep eutectic solvent
30
(DES), a neoteric green solvent, was introduced in the last decade by Abbott et al.
31
(2003). This solvent is derived from the hydrogen bonding formation between two or
32
more constituents. The key properties of DES are their low vapour pressure, high
33
tunability of their physicochemical properties and biodegradability based on their
34
starting constituents. Ever since its introduction, DES has attracted increasing attention
35
in various fields such as electrochemistry, separation, catalysis or extraction as
36
described in other reviews (García et al., 2015; Smith et al., 2014; Zhang et al., 2012).
37
Ability of DES in solubilizing lignocellulosic biomass was first demonstrated by
38
Francisco et al. (2012). DESs have displayed their versatility on biomass processing
39
such as biopolymer dissolution, phenolic compound extraction, carbohydrate product
40
conversion and biodiesel purification. Lignocellulosic biomass are generally more
41
recalcitrant towards modification than sugar-based biomass. Thus, the technological
42
advances of the lignocellulosic biomass processing are comparatively less developed.
43
Commercial production of lignocellulosic biomass-derived products is very limited and
44
an effective biorefinery scheme is yet to be realized. It is thus very important to keep
45
track of the current research and technological development in this field, which includes
46
the application of DES in lignocellulosic biomass fractionation and conversion.
47 48
To date, many literatures have investigated the conversion of lignocellulosic biopolymer model compounds into their building blocks or bio-based products using
2
49
DES (Vigier et al., 2015; Zdanowicz et al., 2018). Nevertheless, the performance of
50
DES on the biopolymer compound conversion cannot truly reflect their efficiency on
51
the actual lignocellulosic biopolymer treatment. In biorefinery, these biopolymers have
52
to be first isolated from biomass matrix in order to be processed separately. Thus, this
53
review paper will focus on case studies investigating the pretreatment of entire biomass
54
using DES, the products conversion following DES pretreatment and the challenges and
55
outlooks towards DES utilization in industrial scale biomass processing. In this paper, DESs will be categorized based on their varying hydrogen bond
56 57
donors (HBD) and hydrogen bond acceptors (HBA) constituents, as well as ternary DES.
58
The effect of these diverse DES constituents on raw lignocellulosic biomass
59
fractionation efficiency will be reviewed. In addition, the research efforts in improving
60
DES’s efficiency through combining DES pretreatment with other existing process
61
intensification technologies are also included, namely microwave, ultrasonication,
62
sequential pretreatment and co-solvent. Subsequently, the application of DES-
63
fractionated biopolymers (saccharides and lignin) for product conversion are discussed.
64
Complete utilization of biomass is always desired for generation of fuels and chemicals
65
in biorefinery. This review will provide an insight into the current lignocellulosic
66
biomass processing and utilization strategies using DES as pretreatment medium.
67
1.1
68
What is deep eutectic solvent (DES)? Eutectic mixture refers to a homogeneous mixture of two or more constituents at
69
certain composition ratio, which melts and freezes at a temperature lower than the
70
melting points of its constituents. The introduction of molten salts (or fused salts)
71
eutectic mixtures, such as ammonium salts and metal salts, can be dated way back in
72
1914 (Abbott et al., 2003; Welton, 1999). Molten salts that present as liquids at or
3
73
below room temperature are later more commonly known as room temperature ionic
74
liquids (Welton, 1999). Recently, Abbott et al. (2003) introduced another variation of
75
room temperature eutectic mixture, comprising of choline chloride and urea at a
76
freezing temperature of 12°C. Other than the first experimented pair of constituents, this
77
type of solvent is found synthesizable using the combinations of various quaternary
78
ammonium salt (quaternary ammonium cation pairing with monovalent anion) with
79
amides (Abbott et al., 2003). The formation of eutectic mixture is attributed to the
80
charge delocalization between halide anion and amide through hydrogen bonding
81
(Abbott, Andrew P. et al., 2004a). To differentiate them from ionic liquids which
82
consist only of ionic species, this class of solvent is named as deep eutectic solvent
83
(DES) (Abbott, Andrew P. et al., 2004a). The synthesis of DES can be described by a
84
general formula developed by Smith et al. (2014):
85
Cat + X ― zY
86
Where Cat+ = ammonium, phosphonium, sulfonium cation; X– = Lewis base anion; Y =
87
Lewis or Bronsted acid at z number of molecules interacting with anion
88
As the formation of DES is built on hydrogen bonding, a wide range of
89
constituents capable of donating or accepting protons or electrons are suitable for DES
90
synthesis. Generally, DES can be categorized into four main types as shown in Table 1
91
with their respective application. Type 1 to 3 DESs are differentiated according to the
92
Lewis/Bronsted acid (Y) used to complex the Lewis base (X–, normally halide anion).
93
The anionic species X– selected must have the ability to act as hydrogen bond acceptor
94
(Abbott, Andrew P. et al., 2004b). Type 1 and 2 DESs are those with non-hydrated and
95
hydrated metal halides as Lewis acid, respectively; whereas type 3 DESs are formed by
96
using hydrogen bond donor (HBD), such as amides, amines, alcohols, sugar alcohols
4
97
and acids. Type 4 DESs are synthesized from metal salt and HBDs such as amide and
98
polyol, for instance ZnCl2 with urea. Not every HBD can form eutectic with metal salt
99
as they do with quaternary ammonium salt in type 3 DES, for example carboxylic acid
100
was unable to interact with ZnCl2 (Abbott et al., 2007).
101
Type 3 DES, which is the earliest introduced DES and arguably the greenest
102
among all, receives the most attention. Most in-depth properties studies usually revolve
103
around this type of DES (Florindo et al., 2014; García et al., 2015; Teles et al., 2017;
104
Zhang et al., 2012). The use of type 3 DES is especially encouraging, as the DES
105
constituents are mostly organic, biodegradable and some are even of food grade! The
106
synthesis process of this DES involves that combination of at least one hydrogen bond
107
donor (HBD) and hydrogen bond acceptor (HBA) to form homogeneous liquid at
108
ambient condition. Huge array of naturally existing HBD constituents such as acids,
109
alcohol, amides and others are suitable to synthesize this type of DES. Many of these
110
components are also metabolites in living organisms. Choi et al. (2011) postulated that
111
these metabolites form a third type of liquids in cells apart from lipids and water, which
112
they named as natural deep eutectic solvent (NADES). Synthesizing DES using
113
naturally occurring components is highly encouraged, as many conventional solvents
114
would pose severe hazard concern towards living organisms. Notably the application of
115
DES are well received in pharmaceutical and medical fields (Aroso et al., 2015; Mota-
116
Morales et al., 2013; Zainal-Abidin et al., 2019).
117
The pursuit of sustainability industry is a pressing matter considering the fast-
118
deteriorating environment and dwindling non-renewable resource reserves. The use of
119
green technology in renewable non-food feedstock (lignocellulosic biomass) conversion
120
to bio-based products is a constant research effort towards sustainable living. In this
5
121
regard, the application of DES in lignocellulosic biomass processing was introduced in
122
a timely matter. To date, type 3 DESs are preferred over other types of DESs in biomass
123
processing due to the availability and environmental friendliness of the constituents.
124
Hereinafter, all DES terms refer to type 3 DES, unless mentioned otherwise.
125
2
126
Lignocellulosic biomass fractionation using deep eutectic solvents (DES) Lignocellulosic biomass is the most abundant plant material on Earth (Brandt et
127
al., 2013). It is a composite material consisting mainly of three types of biopolymers:
128
cellulose, hemicellulose and lignin. Composition of these biopolymers varies with the
129
biomass species. Lignocellulosic biomass can be found in numerous forms such as
130
forest residue, agricultural waste, dedicated lignocellulosic crops and municipal wastes
131
like food and paper waste. These biomass can be used as feedstock for various bio-
132
products as well as second generation biofuel production (Rastogi and Shrivastava,
133
2017). Second generation biofuels are viewed as the solution to address many
134
sustainable issues, attributing to the non-food nature of the feedstock and their surplus
135
amount available on Earth. Food and fuel sources can be cultivated simultaneously by
136
applying agricultural and food waste (e.g. rice and wheat straw, oil palm solid waste or
137
fruit peels) as biorefinery feedstock (Brandt et al., 2013).
138
Research into biomass processing using DES was inspired by the successful
139
application of ionic liquids (IL) in biomass processing (Francisco et al., 2012). The
140
analogues of DES, ILs were found to be effective at milder conditions as opposed to the
141
effectiveness of conventional solvents at more extreme conditions (Sun et al., 2011).
142
However, ILs are costly due to the demanding synthesis process (Cvjetko Bubalo et al.,
143
2015). DESs are adopted as a cheaper alternative to ILs. Majority of DESs display high
144
solubility for lignin and poor to negligible solubility for cellulose and hemicellulose
6
145
(Francisco et al., 2012). Nevetheless for certain DESs, for example glycine:malic acid
146
and choline chloride:malic acid, considerably high cellulose solubility was achieved at
147
7.7% and 5.9%, respectively. This indicates that the selected DES constituents might
148
affect the dissolution tendency of DES to a great extent. This early work by Francisco et
149
al. (2012) sparks immediate research interest in the application of DES in biomass
150
processing due to the high selectivity for biopolymer solubilisation. In this section, performance of various DES pretreatment for lignocellulosic
151 152
biomass fractionation into their individual biopolymers (i.e. cellulose, hemicellulose
153
and lignin) are reviewed. The performance of DES will be evaluated from two main
154
perspectives, namely effect of DES constituents and process integration with other
155
intensification technologies.
156
2.1
Variation of DES The pretreatment conditions such as temperature, time or solid to liquid ratio
157 158
could affect the solvent performance, with DES type usually having the most influence
159
over the DES performance. As mentioned previously, there are a wide range of
160
constituents suitable for DES synthesis. The constituents used would determine the
161
nature of the solvent synthesized, and their behaviour in biomass pretreatment. The
162
biopolymer fractionation performance of different types of DESs will be discussed
163
based on their hydrogen bond donor (HBD), hydrogen bond acceptor (HBA) and
164
subsequently the performance of ternary DES with three constituents.
165
2.1.1
166
Categories of hydrogen bond donor (HBD) In lignocellulosic biomass pretreatment using DESs, HBDs that have been
167
investigated include carbohydrate, acid, polyalcohol, amide and phenolic compound.
168
Table 2 tabulates the DES pretreatment conditions and their performance from selected
7
169
reported works using different types of HBDs. The performance of DESs vary with the
170
categories of HBDs, as well as different types of compound within the same category.
171
a)
Carbohydrate-based DES Carbohydrate-based DES is constituted of sugar compound as HBD such as
172 173
glucose, fructose, xylitol and so on. This type of DES usually exhibits near-neutral pH
174
condition when pairing with the most common HBA, choline chloride (ChCl). In spite
175
of its mild nature, ChCl:fructose DES was able to dissolve rice straw biomass powder
176
up to 0.65 wt% (6.5 mg per g of DES) (Florindo et al., 2017). In comparison with
177
aldoses (i.e. ribose, glucose, xylose and mannose), the fructose-based DES (ketose-
178
based sugar) with less viscosity demonstrated higher dissolution ability. Even though
179
carbohydrate-based DESs can dissolve biomass, their dissolution efficiencies are far
180
lower than that of the other categories of DESs (Francisco et al., 2012). Therefore, these
181
sugar-based DESs are not usually applied in lignocellulosic biomass processing. As
182
these DESs are constructed from environmentally benign constituents, they found
183
application in other fields such as food or pharmaceutical processing (Dai et al., 2014;
184
Liew et al., 2018).
185
b)
186
Polyalcohol-based DES Application of polyalcohol-based DES in biomass pretreatment has been widely
187
investigated. Most works reported using this type of DES was targeted to increase
188
enzyme accessibility in polysaccharides conversion due to high enzyme stability in
189
polyalcohol-based DES, further details of which will be discussed in the next section.
190
The most popular polyalcohol HBDs include glycerol and ethylene glycol. Multiple
191
works reported that ChCl:glycerol was inefficient in biomass dissolution and
192
fractionation, even under elevated temperature condition (Alvarez-Vasco et al., 2016;
8
193
Chen, Z. et al., 2018; Tan et al., 2018; Xia et al., 2018). It was deduced that its extensive
194
hydrogen bonding network between HBD and HBA weakened the ability of HBA to
195
compete with the intra-molecular bonding in biopolymer matrix (Xia et al., 2018). Some
196
vast difference in findings on the performance of polyalcohol-based DES was reported.
197
When Zulkefli et al. (2017) reported that ethylene glycol-DES and glycerol-DES
198
achieved 36% and 49% of lignin removal from oil palm trunk, nearly twice the
199
extraction amount at 71% and 88% were obtained by Zhang et al. (2016) from corncob
200
biomass using the same DESs. Factors contributing to the discrepancy in the results
201
could be the varying recalcitrance degree in different biomass types, effect of different
202
HBAs or pretreatment conditions employed (Table 2). Higher degree of hydrophobicity in polyalcohol-based DES seems to facilitate
203 204
biopolymer fractionation. When comparing between different polyalcohol HBDs in
205
DESs, lignin removal increased in the order of ethylene glycol > 1,2-propanediol > 1,3-
206
propanediol (Hou, X.D. et al., 2018). The authors attributed this trend to increasing
207
hydrophobicity that facilitated the extraction of lignin with hydrophobic
208
phenylpropanoid structures. In another work, the dissolution capacity of ethylene
209
glycol-DES was higher than that of glycerol-DES in oil palm trunk dissolution (Zulkefli
210
et al., 2017). Ethylene glycol was reported to exhibit hydrophobic effect unlike that of
211
glycerol (Koga, 2003). Dedicated studies regarding the effect of hydrophobicity of DES
212
solvent on biopolymer fractionation is essential.
213
c)
214
Acid-based DES Other than polyalcohol-based DES, acid-based DES is also widely investigated.
215
The acid-based DESs applied in biomass processing are often composed of organic acid,
216
taking advantage of their biodegradable nature and availability in natural products. By
9
217
associating with a compatible HBA, the acid-based DESs perform better in biomass
218
pretreatment than their respective acids. As evident in the literatures, lignin and xylan
219
removal were enhanced by 30% when acid-based DESs were utilized instead of the
220
respective organic acids (Tan et al., 2019; Yu et al., 2018).
221
Acid-based DESs are generally more efficient in lignin and xylan removal than
222
other groups of DESs (Table 2). Most acid-based DESs had excellent performance in
223
xylan hydrolysis when pretreatment temperature was over 100°C (Alvarez-Vasco et al.,
224
2016; Yu et al., 2018). While lignin and xylose were solubilized into DES, the major
225
portion of cellulose was usually retained in the DES-pretreated biomass solid, as shown
226
in multiple studies (Table 2). Formic acid was reported to be able to recover up to 98%
227
(Yu et al., 2018) and 86% (Tan et al., 2019) of glucose in the pretreated herbal residue
228
and oil palm biomass, respectively. Even though large percentage of cellulose is
229
retained in biomass solid, DES pretreatment can certainly bring some changes to the
230
structure and properties of the pretreated biomass. These changes can affect the
231
efficiency of the downstream processing, which will be explained later in this review.
232
Some acid-based DES perform better than the others and thus investigation on
233
the effect of functional groups in the acids is important to determine the most suitable
234
acid HBD for biopolymer fractionation. The presence of different functional groups at
235
varying amount have significant impact on the DES fractionation efficiency. Acid HBD
236
with more than one carboxyl groups (diacid and triacid) could lower DES fractionation
237
efficiency as compared with that of monoacid HBD (Tan et al., 2019; Zhang et al.,
238
2016). Aside from the main functional groups in acid (i.e. carboxyl group), other
239
chemical groups present in acid influence the DES performance as well, for example the
240
presence of electron-donating or electron withdrawing groups.
10
241
Lignin extraction efficiency decreased in the order of ChCl:formic acid (62%) >
242
ChCl:acetic acid (27%) > ChCl:propionic acid (20%) > ChCl:butyric acid (14%) (Tan et
243
al., 2019). The authors attributed the trend to the electron-donating properties of alkyl
244
group to oxygen, which intensified the hydrogen bond strength in the hydroxyl group of
245
acid. This constituted to weaker acid with reduced acid ionization strength. Thus, the
246
interaction between DES solvent and biopolymer solute would be weakened when there
247
was a longer alkyl chain in HBD. In addition, levulinic acid-DES, a keto acid showed
248
poorer performance than other acid based-DESs such as lactic acid-, acetic acid-, formic
249
acid-, and glycolic acid-DES (Alvarez-Vasco et al., 2016; Yu et al., 2018). The
250
collective results indicated the negative effect of alkyl group that imposed on biomass
251
fractionation. On the other hand, another research group demonstrated that the presence
252
of a strong electron-withdrawing group i.e. chloride Cl– in acid was favourable for
253
hemicellulose hydrolysis (Hou, X.D. et al., 2018). As reported, a chlorine-containing
254
acid, 2-chloropropionic acid-DES had much greater xylan removal efficiency when
255
compared with OH-containing lactic acid-DES (54% and 19%) at 80°C.
256
Furthermore, the presence of hydroxyl group in alpha hydroxyl acid (AHA)
257
DES is also preferred for biopolymer fractionation. When compared to linear saturated
258
acid-DES with similar aliphatic chain length, AHA-DES performed better in lignin
259
extraction (Tan et al., 2019; Yu et al., 2018). The examples are; 24% higher lignin yield
260
difference in the case of ChCl:glycolic acid pretreatment than that of ChCl:acetic acid
261
(C2 acid) (Yu et al., 2018), 13% more by ChCl:lactic acid than by ChCl:propionic acid
262
(C3 acid) as well as 12% greater by ChCl:malic acid as compared to using
263
ChCl:succinic acid (C4 acid) (Tan et al., 2019). OH group in AHA-DES could increase
264
the polarity of DES, hence facilitating the hydrogen bond interaction with the biomass
11
265
matrix. However, the performance of glycerol-DES with three OH groups is less ideal
266
as previously discussed (Section 2.1.1b). Despite both bearing OH groups, polyalcohol-
267
DES and AHA-DES have other functional groups such as alkyl or carboxyl group that
268
would bring different properties to the solvents. More investigation on the synergistic
269
effect of the different functional groups with biopolymer solutes is therefore crucial.
270
d)
271
Amine- and amide-based DES Amine- and amide-based DESs are less investigated as compared to
272
polyalcohol- and acid-based DES although they were one of the first introduced DES
273
types. These basic DESs have moderate biopolymer fractionation efficiency. It was
274
demonstrated that lignin and xylan removal by ChCl:urea from different biomass
275
sources (rice straw and oil palm empty fruit bunch) were respectively around 30% and
276
20% (Pan et al., 2017; Tan et al., 2018). Even when pairing with lactic acid as another
277
DES constituent, urea-DES was ineffective in fractionation (Hou, X.D. et al., 2018),
278
probably due to the modest pH value, as described in the next case study.
279
Zhao et al. (2018) evaluated the wheat straw fractionation efficiency of various
280
amine- (monoethanolamine, diethanolamine and methyldiethanolamine) and amide-
281
based (acetamide and urea) DES. All three ethanolamines outperformed acetamide and
282
urea in biopolymer fractionation. The authors attributed the good performance of amine-
283
based DESs to their strong basicity (pH 10.4, 10.5 and 10.9) than amide-based DES (pH
284
7.3 and 8.2). Among the amine-based DES, monoethanolamine-DES topped at 81% and
285
47% of lignin and xylan removal, respectively. Additional hydroxyethyl group in
286
diethanolamine and methyldiethanolamine could have increased steric hindrance in the
287
solvent, hence reducing the fractionation efficiency. Notably, similar to acid-based DES,
12
288
methyldiethanolamine which has an alkyl group (methyl) substitution had lower
289
fractionation efficiency than diethanolamine.
290
e)
Phenolic compound-based DES
291
A few studies on the performance of phenolic compound-based DES was
292
reported. Hence, insufficient data is available to draw any hypothesis to the pretreatment
293
tendency of this DES type. Ten different phenolic compounds which could be derived
294
from lignin were screened for DES formation (Kim et al., 2018). Among them, four
295
compounds formed homogeneous solvent with ChCl, namely 4-hydroxybenzyl alcohol,
296
catechol, vanillin, p-coumaric acid (PCA). ChCl:PCA achieved the highest lignin and
297
xylan removal from switchgrass at 61% and 71%, followed by ChCl:vanillin at 53%
298
and 50%. Acidic properties arising from the presence of COOH group in PCA could
299
contribute to its high fractionation efficiency. In another work, three types of phenolic
300
compounds, namely phenol, alpha-naphthol and resorcinol were evaluated as HBD in
301
DES (Malaeke et al., 2018). With the aid of ultrasonication, resorcinol-DES achieved
302
33wt% solubility in wheat straw dissolution, which was close to the lignin composition
303
in the biomass. The researchers also hypothesized that the phenyl group in HBD was
304
desirable for lignin dissolution and the solubilisation performance decreased with the
305
presence of two phenyl groups.
306
f)
307
Cross comparison between different HBD categories Some research groups have performed cross comparison on different categories
308
of HBD for biomass fractionation. In general, DES with acid HBD has best
309
fractionation efficiency among other HBD groups. In a study in which DES with
310
various pH were compared, the acidic DES (ChCl:lactic acid and lactic acid:glucose)
311
demonstrated the greatest fractionation efficiency, followed by the basic DES
13
312
(ChCl:urea and potassium carbonate:glycerol) then the near-neutral DES (ChCl:glycerol
313
and ChCl:glucose) (Tan et al., 2018). Liu, Y. et al. (2017) reported that oxalic acid with
314
higher hydrogen bond acidity than urea and glycerol, could break down the lignin-
315
carbohydrate complex in biomass matrix more efficiently when it was used as HBD in
316
DES. Besides, basic DESs (ChCl:urea and ChCl:imidazole) also had the tendency to
317
dissolve more biopolymer than glycerol-DES (Procentese et al., 2015). In regards to
318
polyalcohol-based DES’s performance, different researchers have reached the
319
consensus in which the multiple OH in polyalcohol could restrict the extraction
320
performance (Guo et al., 2018; Hou, X.D. et al., 2018). Subsequent efforts have been
321
put in by some to increase the performance of polyalcohol-based DES via modifying the
322
constitution of the solvent by acidifying the solvent or adding third DES constituent.
323
Relevant elaboration will be presented in the Section 2.1.3.
324
2.1.2
325
Categories of hydrogen bond acceptor (HBA) Apart from HBD, HBA constituent can also impact on DES’s performance. The
326
most commonly used HBA is choline chloride (ChCl) (C5H14ClNO), a quaternary
327
ammonium salt with choline cation and chloride anion. Most studies adopted ChCl as
328
HBA in their investigation due to the benign properties of this compound as well as its
329
affordable pricing. The effect of HBA on DES biopolymer fractionation efficiency is
330
less investigated as compared to HBD. Choline, also known as vitamin B4, is
331
synthesized through metabolism process in our body and obtained through diets. It is a
332
nutrient necessary for the formation of metabolites and thus it is widely used as animal
333
feed additive for growth promotion, or sometimes as human’s vitamin supplement.
334 335
Florindo et al. (2017) investigated on the biomass dissolution capacity of three different HBAs in carbohydrate-based DESs, namely ChCl, acetylcholine chloride
14
336
(C7H16ClNO2, AC) and benzyldimethyl(2-hydroxyethyl)ammonium chloride
337
(C11H18ClNO, BAC). The dissolution percentage of DES decreased in the order of
338
ChCl-DES > AC-DES > BAC-DES. Regardless of the HBA used, DESs with fructose
339
as HBD could dissolve the highest amount of biomass (50-65 mg per g of DES). Even
340
though the use of suitable HBA used could increase the dissolution efficiency, HBD
341
was the deciding factor of the DES performance in this case study. In another work on
342
dissolution of oil palm trunk biomass, ethylammonium chloride (C₂H₈ClN, EAC)
343
performed better than ChCl as HBA (Zulkefli et al., 2017). Furthermore, the selectivity
344
of both HBAs was different in biopolymer fractionation such that ChCl was more
345
effective in delignification while EAC was more suitable for xylan removal. The
346
authors stated that EAC as a primary ammonium salt is less bulky in structure than
347
ChCl with quaternary structure. This resulted in the former HBA solubilizes biopolymer
348
better due to less steric hindrance. The same theory can also be applied to the previous
349
case involving the AC and BAC HBAs as their larger structure as compared to that of
350
ChCl led to poorer performance in biomass dissolution.
351
Since chemical compound consists of various functional groups (both electron-
352
withdrawing and electron-donating), some HBDs were used interchangeably as HBA in
353
some reported works; for instance lactic acid (Hou, X.D. et al., 2018) and sucrose (Yiin
354
et al., 2016). When lactic acid was used as HBA, the DES’s dissolution efficiency was
355
greater than that of ChCl-based DES, regardless of the HBD employed. This implies
356
versatile combinations of different DESs can be synthesized for various applications. It
357
should be noted that the first step towards DES utilization is to identify the most
358
suitable combinations for any intended purpose.
15
359
2.1.3
Ternary DES
360
The construction of a DES is not limited to using only two constituents. Ternary
361
DES can be synthesized using the same methodology as in regular two-component DES
362
synthesis by mixing three compatible constituents (Dai et al., 2013; Liu et al., 2014). It
363
was reported that addition of alcohol as the third component to ChCl:oxalic acid DES
364
could enhance the delignification of different biomasses (rice husk, wheat straw, rice
365
straw) (Kandanelli et al., 2018). ChCl:oxalic acid:butanol and ChCl:oxalic
366
acid:propanol achieved 49% and 41% lignin removal as compared with the regular
367
ChCl:oxalic acid DES performance at 23-31%. The authors also exhibited that not every
368
ternary DES had better performance. DES containing ethyl acetate as the ternary
369
component showed a 15% decreament in delignification efficiency. The ratio of DES to
370
alcohol in the ternary DES is also an important determining factor of the pretreatment
371
efficiency. While DES:butanol with the ratio of 2:1 achieved 49% delignification,
372
ternary DES with the ratio of 1:2 could only achieve 18%.
373
As mentioned previously, ChCl:glycerol was not effective in biopolymer
374
fractionation. Inspired by type 4 DES, Xia et al. (2018) synthesized a ternary DES
375
system by combining ChCl:glycerol with seven different types of metal chlorides
376
(hydrates). Upon addition of AlCl3·6H2O, the α and β solvatochromic parameters of
377
DES increased from 0.77 to 1.99 and from 0.48 to 0.68, respectively, which indicated
378
improved H-bond donating and accepting capability. Based on density functional theory
379
calculation, the inefficiency of ChCl:glycerol in biomass fractionation was due to weak
380
HBA which had less ability to compete with intra-molecular H-bond in biomass. The
381
Cl– ion was held tightly by glycerol and insufficient protons were available to cleave the
382
H-bonds. In response to that, the metal chloride in the ternary DES would function as
16
383
anion donor and acidic site holder to break the lignocellulosic bond structure. After
384
pretreatment at 120°C, the biomass residue recovery and lignin extraction using
385
ChCl:glycerol was 90% and 0.04%. The results had been improved to 42% and 96%,
386
respectively with the use of ternary DES. Comparatively the lignin extraction efficiency
387
of the corresponding type 4 DES (AlCl3·6H2O and glycerol) was at a modest 64%. Up till now, there is no definite answer as to which DES is best for biopolymer
388 389
fractionation. Every DES constituent affects the solvent’s performance and the
390
synergistic effect of the constituents has added further impact. The most exciting
391
finding in our opinion is, by changing the HBD or HBA constituent, the fractionation
392
selectivity of DES could be controlled. To date, the research scopes are widely-
393
dispersed in regards of the types of DES used. More rigorous and in-depth studies in
394
determining the suitable DES constituent are needed to realize the intended purposes,
395
such as lignin extraction or polysaccharides depolymerisation.
396
2.2
397
Integration of DES pretreatment with other technologies A number of different biomass pretreatment methods have been adopted to
398
increase the accessibility to the biopolymers such as chemical, thermal, mechanical,
399
biological pretreatment and so on (Hendriks and Zeeman, 2009). In the conventional
400
pretreatment, different biomass processing technologies are often combined to boost the
401
process efficiency (Menon and Rao, 2012). Likewise, biomass processing schemes
402
involving integration of DES pretreatment with other existing process intensification
403
technologies such as microwave, ultrasound and sequential pretreatment were
404
developed. Also, works have been reported on the addition of acid co-solvents in DES
405
pretreatment processes. In this section, impact of the combined pretreatment strategies
406
and their performance in comparison with sole DES pretreatment will be reviewed.
17
407
2.2.1
Microwave-assisted DES pretreatment
408
Microwave heating, a non-conventional heating method, can increase the
409
cellular pressure in plant through non-ionizing radiation to rupture plant tissue (Liew et
410
al., 2016). Integrating microwave heating into DES pretreatment does not impose
411
chemical changes in biopolymers, but facilitate structural changes in biomass for
412
solvent to interact more efficiently with biopolymers. Microwave irradiation can
413
maximize the ionic character of DESs and increase their molecular polarity, enabling
414
the use of lower pretreatment temperature and duration (Liu, Y. et al., 2017). This
415
integrated technology was first applied by Liu, Y. et al. (2017) for lignin extraction
416
from wood biomass. After subjecting to microwave-assisted ChCl:oxalic acid DES
417
pretreatment for 3 mins at 800 W, 80% of the initial lignin was extracted and only 40%
418
of pretreated solid biomass was recovered. However, pretreatment using conventional
419
heating in oil bath for 9 h at 110°C achieved 90% lignin extraction and 64% pretreated
420
biomass recovery. Evidently, microwave heating required drastically shorter
421
pretreatment time; at mere 3 mins as compared with 9 h by conventional heating while
422
achieving the similar efficiency level.
423
The efficiency of microwave-assisted DES pretreatment was also reported by
424
another group of researchers who combined acidic ChCl:lactic acid pretreatment with
425
ultrafast 45 second microwave heating at 800 W on three different types of biomass
426
namely switchgrass, corn stover, miscanthus (Chen and Wan, 2018). Xylan removal of
427
the pretreatments ranged between 77-90%, lignin removal 65-80%, and glucan removal
428
4-25% were obtained. In both the works reviewed, the microwave-assisted DES
429
pretreatment were able to produce extracted lignin with high purity at 94-96% in Liu, Y.
430
et al. (2017) and 85-87% in Chen and Wan (2018).
18
431
2.2.2
Ultrasonication-assisted DES pretreatment Other than microwave heating, ultrasonication is another commonly applied
432 433
process intensification technology. Sonication creates cavitation in the reaction mixture.
434
Pressure and temperature increase when the cavitation bubbles collapse near biomass
435
surface, which breaks down the cell wall more efficiently. Similar to microwave,
436
ultrasonication-assisted process improves the efficiency and shorten the treatment
437
duration. When DES pretreatment was integrated with ultrasonication at 90°C for 20
438
mins, 48% of lignin solubility was achieved (Malaeke et al., 2018). The study displayed
439
high lignin selectivity of DES based on the low cellulose and hemicellulose solubility at
440
0.9% to 6.1% even when ultrasonication-assisted pretreatment was applied.
441
2.2.3
Sequential pretreatment Other than microwave- and ultrasonication-assisted processes, DES pretreatment
442 443
has been applied in sequence with other pretreatment modes namely hydrothermal,
444
biological and inorganic salt pretreatment to enhance the biopolymer fractionation
445
efficiency. In addition, sequential DES pretreatment scheme was also developed by a
446
group of researchers, which will also be discussed in this section.
447
a)
448
Hydrothermal+DES pretreatment Hydrothermal pretreatment involves the use of water solvent under high
449
temperature and pressure condition to break down the biomass cell wall (Saha et al.,
450
2013). Fang et al. (2017) employed hydrothermal pretreatment to reduce the
451
recalcitrance of date palm residue prior to DES treatment. In the study, ChCl:glycerol
452
pretreatment was ineffective in removing lignin and xylan, even when the pretreatment
453
time (6 h to 15 h) and molar ratio of HBD to HBA (1:2, 1:3, 1:6) were adjusted. After
454
sequential hydrothermal (200°C, 10 mins) and DES pretreatment (70°C, 6 h), 22% and
19
455
25% of lignin and xylan were removed, respectively. Regardless of the operation modes,
456
cellulose was well reserved in the biomass solid at >90%. The authors reported that
457
hydrothermal pretreatment disrupted the cell wall and broke down lignin and xylan to
458
small fragments. The fragments which might inhibit enzymatic action were
459
subsequently removed by DES, leading to enhancement of hydrolysis performance.
460
b)
Biological+DES pretreatment Application of sequential biological and DES pretreatment can also enhance
461 462
enzymatic accessibility to polysaccharides. Biological pretreatment using Galactomyces
463
sp. CCZU11-1 was carried out on bamboo shoot shell at 30°C for 3 days prior to DES
464
pretreatment (Dai et al., 2017). The authors achieved 77% and 20% of xylan and lignin
465
removal through biological pretreatment and following that, DES pretreatment further
466
reduced the xylan content from 7.4% to 6.1% and lignin from 12.6% to 11.6%. After
467
enzymatic hydrolysis, the total reducing sugar production from the sequential pretreated
468
biomass reached 90%, whereas the biological-pretreated and untreated biomass were
469
respectively at 73% and 42%. The recovered hydrolysate from the enzymatic hydrolysis
470
treatment was successfully employed for microbe lipid production, which will be
471
explained further in the upcoming section (Section 3.2.2).
472
c)
473
DES+inorganic salt pretreatment Inorganic salt pretreatment was found effective in recovering xylose from
474
biomass when applied with oxidative agent (Loow et al., 2017). With the hypothesis
475
that DES could first extract lignin from biomass (based on DES’s high lignin
476
selectivity), which then facilitate selective xylose hydrolysis using inorganic salt, the
477
sequential DES and inorganic salt treatment scheme was introduced (Loow et al., 2018).
478
In this work, ChCl:urea pretreatment (120°C, 4 h) was carried out, followed by CuCl2
20
479
pretreatment (0.4 mol/L concentration, 120°C, 30 mins) to maximize xylose recovery
480
from oil palm frond. 74% xylose recovery was obtained through the sequential
481
pretreatment, as compared with 59% from CuCl2 pretreatment alone.
482
d)
483
DES+DES pretreatment Taking advantage of the varying properties displayed in DESs synthesized from
484
different constituents, Hou et al. (2017) designed a two-stage sequential DES
485
pretreatment scheme to enhance the enzymatic accessibility of rice straw. The xylan and
486
lignin removal of the process varied with the pretreatment order. In malic acid:proline
487
followed by ChCl:urea pretreatment, the total removal rate were 50% for xylan and 73%
488
for lignin. In the reverse sequence, the removal rates were lower at 30% and 60%,
489
respectively. In another combination whereby ChCl:oxalic acid followed by ChCl:urea
490
pretreatment were applied, greater biopolymer removal rates (i.e. 92%, 60%) than the
491
reverse sequence (i.e. 90%, 45%) were imposed. From the two DES combinations,
492
apparently the fractionation efficiency was higher when acidic DES pretreatment was
493
applied prior to basic ChCl:urea. The authors related the trend to the ability of acidic
494
DES to swell and loosen the biomass structural linkages, which promoted the high
495
biopolymer solubility in ChCl:urea. In the subsequent enzymatic hydrolysis process of
496
the sequential pretreated rice straw, an optimum glucose yield of 90% was achieved
497
whereas in the single DES-pretreated biomass only obtained 47-73% yield. Interestingly,
498
when both the acidic and basic DESs were combined in a single step operation, the
499
pretreatment was reported to be ineffective as the biomass composition remained
500
similar to that of untreated biomass.
21
501
2.2.4
Acidified DES pretreatment Other than ternary components as described in Section 2.1.3, researchers had
502 503
devised a novel way of increasing the efficiency of ChCl:glycerol by acidifying the
504
solvent using H2SO4 (Chen, Z. et al., 2018). When 0.9 wt% of acid was added to the
505
DES, solid recovery of the pretreated biomass reached 47.2%, which marked a huge
506
decrement to 89.4% in neat ChCl:glycerol pretreatment. Xylan and lignin removal from
507
acidified DES pretreatment were close to 80% whereas in neat DES pretreatment the
508
removal rates were only 7% and 18%, respectively. Due to high biopolymer removal
509
efficiency, glucan content in acidified DES-pretreated biomass was enriched to 64% and
510
the glucan to glucose conversion reached nearly 100% after enzymatic hydrolysis. In
511
comparison, the neat DES pretreated biomass achieved only 11.8% sugar yield. The
512
authors later adopted the pretreatment liquid stream as substrate for lipid production, as
513
illustrated in the coming section (Section 3.3).
514
3
515
DES-extracted polysaccharides upgrading The properties of the DES-fractionated products are altered from their respective
516
native state in the biomass. The remaining DES pretreated biomass solid fraction (SF)
517
would normally be enriched with cellulose content while in most cases, the labile
518
hemicellulose would be extracted into the pretreatment liquid fraction (LF) (Table 2). In
519
this section, the discussion focuses on the properties of DES-fractionated saccharide
520
streams and their successful conversion to carbohydrate-based products. Three main
521
carbohydrate substrate streams namely DES-pretreated solid fraction (SF), enzymatic
522
hydrolysate produced from enzymatic hydrolysis of SF and lastly DES pretreatment
523
liquid fraction (LF) will be discussed. Some research groups had even developed full
524
biomass utilization schemes by transforming each fractionated biopolymers into
22
525
intermediate bio-materials and bio-products (Chen, Zhu et al., 2018; Chen, Z. et al.,
526
2018; Liu, Y. et al., 2017).
527
3.1
DES-pretreated solid fraction (SF) conversion
528
Understanding on the SF’s properties is essential to identify the possible
529
downstream products. The properties of SF vary depending on the pretreatment
530
condition. Cellulose has low solubility in DES and therefore the DES-pretreated
531
biomass solid fraction (SF) is usually comprised largely of cellulose. So far, many DES
532
pretreatment studies on lignocellulosic biomass processing focus on the fermentation
533
sugar production from SF. Table 3 compiles that sugar production yield from enzymatic
534
hydrolysis process of both untreated and DES-pretreated biomass for comparison
535
purpose. The effect of the structural, surface and thermal properties, and also the
536
application of SF for bio-products conversion will be presented accordingly.
537
3.1.1
538
a)
539
Properties of SF Crystal structure and morphology Crystallinity index (CrI) can be measured using X-ray diffraction (XRD) to
540
reveal the solid structure of SF (Table 3). High CrI indicates higher crystal content in
541
the biomass solid. Majority of the studies reported on increased CrI in the SF after DES
542
pretreatment (Chen, Z. et al., 2018; Dai et al., 2017; Fang et al., 2017; Loow et al., 2018;
543
Yiin et al., 2017). The increment is due to removal of amorphous content in biomass
544
such as hemicellose and lignin, which in turn increasing the relative cellulose content in
545
SF. Liu, Y. et al. (2017) reported that SF remained as crystal type I which had high
546
crystallinity after DES pretreatment. Hou, X.-D. et al. (2018) discovered that CrI was
547
closely related to the severity of pretreatment conditions. Using ChCl:oxalic acid at
548
molar ratio of 2:1, the CrI increased from 55.8% to 59.6%. When the similar DES with
23
549
1:2 ratio was used, the CrI dropped to 55.0%, which was lower than that of the
550
untreated biomass. Using confocal laser scanning microscopy, xylan removal was
551
observed at vascular bundle of the biomass in the latter case. In contrast, removal of
552
recalcitrant lignin at secondary cell wall was not observed in the biomass pretreated by
553
ChCl:oxalic acid 2:1. The work verified that when acid content in DES was increased,
554
deconstruction ability of DES was greater to the extent of depolymerizing biopolymers
555
at the more recalcitrant layer that were normally unaffected under mild condition.
556
b)
Structural analysis Molecular structure of SF is not usually investigated, probably due to the known
557 558
knowledge that it is composed mainly of cellulose, a homogeneous polymer. Kim et al.
559
(2018) confirmed that the biopolymer composition can reflect on the structural changes
560
of SF. ChCl:4-Hydroxybenzyl alcohol DES which had limited fractionation efficiency
561
produced SF with unaltered carbohydrates signals in NMR spectrum when compared
562
with that of untreated biomass. Also, SF with more compact structure can be produced
563
from DES pretreatment. DES-pretreated pine biomass (0.39 g/cm3) was 35% denser
564
than the untreated biomass (0.29 g/cm3) (Lynam et al., 2017). The author elaborated that
565
this structural change is beneficial for future biorefinery operation as biomass with
566
higher density and lower bulk volume is easier to be transported.
567
3.1.2
568
Enzymatic hydrolysis for fermentable sugar production Conversion of biomass-derived saccharides to fermentable sugar for fuel
569
production is arguably one of the most developed bio-products conversion. To date,
570
most studies on the SF application focus on fermentable sugar production. Table 3
571
compiles the reported results on sugar yield from enzymatic hydrolysis processing of SF.
572
Many researchers concluded that the improved hydrolysis rate after DES pretreatment
24
573
was due to extensive hemicellulose or lignin removal or the combination of both (Hou
574
et al., 2017; Procentese et al., 2015). Guo et al. (2018) attributed the high sugar yield
575
from betaine:lactic acid pretreated xylose residue to high delignification efficiency at
576
81.6%. Also, a linear correlation between xylan removal and cellulose digestibility was
577
established with high R2 value at 0.86 (Hou, X.D. et al., 2018).
578
Literature reported that sugar yield resulted from enzymatic hydrolysis was
579
highly dependent on the pretreatment conditions such as biomass type, temperature,
580
duration, as well as solvent type (Table 3). Wahlström et al. (2016) compared the
581
hydrolysis performance of different biomass types under similar mild pretreatment
582
conditions (90% aqueous DES solution, 24 h, 80°C). DES-pretreated eucalyptus
583
dissolving pulp and microcrystalline cellulose achieved 100% and 70% hydrolysis,
584
respectively. However, lignocellulosic wheat straw and sawdust SFs showed very
585
minimal improvement from the untreated biomass in hydrolysis performance.
586
High temperature and large solid-solvent ratio could increase sugar yield
587
effectively, however the biopolymer recovery would be compromised due to the harsh
588
environment (Procentese et al., 2018). Li et al. (2018) demonstrated that by increasing
589
pretreatment temperature from 90°C to 120°C, SF digestibility improved significantly
590
from 40% to 80%. Nevertheless, xylose yield decreased by around 2-3% due to sugar
591
loss under high temperature. The group also explored the effect of HBA to HBD molar
592
ratio on hydrolysis performance. Under low temperature condition at 90°C, increasing
593
molar ratio of ChCl:lactic acid from 1:1 to 1:3 enhanced biomass digestibility and
594
glucose yield. Further increase of the ratio to 1:5 led to glucose yield reduction. Under
595
elevated temperature at 120°C, sugar yield decreased even when molar ratio was
596
increased from 1:1 to 1:3, which was hypothesized to be due to the combined effect of
25
597
high temperature and acid content. Types of DES employed also have great impact over
598
the SF hydrolysis performance (Table 3). Xing et al. (2018) reported that ternary acidic
599
DES with two acidic HBD constituents performed better than DES with single acidic
600
HBD in hydrolysis rate enhancement. SF from ChCl:formic acid:acetic acid DES
601
pretreatment achieved 21.5g/L sugar concentration after enzymatic hydrolysis, 1.14 and
602
1.37 times higher than that of ChCl:formic acid and ChCl:acetic acid, respectively. Interestingly, Pan et al. (2017) reported that ChCl:urea DES pretreatment failed
603 604
to enhance hydrolysis performance of SF. Sugar yield from rice straw SF (1.3%) was 10
605
times lesser than that of untreated biomass (13.0%). The author explained that cellulase
606
enzyme might have attacked the high CrI rice straw residue instead of the regenerated
607
rice straw. However, it is unclear about the difference between rice straw residue and
608
regenerated rice straw. It was suggested that ChCl:urea might have certain inhibiting
609
influence over the enzyme. Further study regarding this hypothesis is suggested.
610
3.1.3
611
Production of nanocrystals and nanofibrills cellulose Production of fermentable sugar from cellulose-rich SF is achieved by
612
depolymerisation of polysaccharides into monomers. Apart from that, cellulose can also
613
be utilized in its polymeric form for advanced material production. Several successful
614
attempts have been made in producing nanocrystals and nanofibrills cellulose from pure
615
cellulose source such as cotton fibres, microcrystalline cellulose or cellulose pulp using
616
DES as pretreatment agent (Liu, Yongzhuang et al., 2017; Sirvio et al., 2016;
617
Suopajarvi et al., 2017). Using SF as cellulose source, Liu, Y. et al. (2017) produced a
618
homogenous solution of nanofibrillated cellulose with uniform diameter of 8-14 nm by
619
conducting a 1000 W ultrasonication treatment for 20 mins. This study marked the first
620
report on the application of SF for cellulosic material production. Nanotechnology has
26
621
brought significant evolution to many industries (Brinchi et al., 2013), particularly
622
material engineering. Further advancement of nanocellulose, a renewable and non-toxic
623
material development using green solvents and lignocellulosic biomass would be
624
beneficial to the relevant industries.
625
3.2
Enzymatic hydrolysate conversion As reviewed in Section 3.1.2, SFs are frequently used as substrate in enzymatic
626 627
hydrolysis for the production of fermentable sugar (Table 3). In this section, case
628
studies of the carbohydrate-based products conversion using hydrolysate from SF
629
hydrolysis through fermentation process are discussed. Table 4 records the works
630
reporting fermentation of the sugar-rich enzymatic hydrolysate using different strains of
631
microorganisms to various carbohydrate-based products such as alcohols and lipids.
632
3.2.1
633
Alcohol production Commercialized ethanol production from sugar-based crops has been developed
634
since 1980s (Sun and Cheng, 2002). Production of this biofuel from second generation
635
biomass is still actively under investigation. Similar to sugar production, the
636
fermentation efficiency differs according to types of DES applied in the pretreatment
637
process (Table 4). Kumar et al. (2016) first tested on the fermentation efficiency of
638
Clavispora (NRRL Y-50464) strain on 5% glucose solution in the presence of DES.
639
Glucose was completely consumed in 18 h of fermentation when 5% and 10% of
640
ChCl:glycerol or 5% ChCl:ethylene glycol was present in the medium. However, when
641
concentration of ChCl:ethylene glycol increased to 10%, 16 g/L of glucose was still
642
remained after 24 h. Lowest ethanol production at 7.5 g/L was found in 10%
643
ChCl:propanediol medium after 24 h. When using actual hydrolysate from rice straw as
644
fermentation substrate, 89.5% ethanol conversion efficiency was successfully achieved.
27
Several groups of researchers reported positive results on producing butanol
645 646
using different bacteria strain. Procentese et al. (2017) successfully produced butanol
647
through simultaneous acetone, butanol, ethanol (ABE) fermentation using Clostridium
648
acetobutylicum (DSMZ 792) bacteria strain. The hydrolysate produced from
649
ChCl:glycerol pretreated SF contained 10 g/L glucose and 1.5 g/L xylose. The sugar
650
was completely consumed after 60 h of fermentation, resulting in 0.5 g/L of butanol
651
(0.04 g/gsugar) and total ABE yield of 0.1 g/gsugar, as shown in Table 4. In two other
652
works using Clostridium saccharobutylicum (DSM 13864) strain, total ABE yield of
653
0.36 g/gsugar (Xing et al., 2018) and 0.21 g/g sugar (Xu et al., 2016) were achieved. After
654
72 h, 93% of initial sugar (40 g/L) was consumed to produce 0.25 g/gsugar of butanol
655
(Xing et al., 2018). Xu et al. (2016) achieved 0.17 g/gsugar butanol yield from
656
hydrolysate that contained 48.2 g/L sugar. The result was comparable to the 51.7 g/L
657
glucose controlled medium with 0.21 g/gsugar butanol yield. Furthermore, Chen, Zhu et al. (2018) successfully produced 2,3-butanediol using
658 659
Bacillus vallismortis (NRRL B-14891). With 226.3 g/L of sugar substrate, the diol
660
production yield reached 89.7%. However, the total xylose conversion was very low at
661
18.4% after 72 h. The author suggested that the bacteria performance on xylose
662
conversion was less ideal in the mixed medium of C5 and C6 sugars. The collective
663
results of the above works show that hydrolysate from DES-pretreated SF can be used
664
as fermentation substrate without apparent inhibitory effect.
665
3.2.2
666
Lipid production Other than alcohol, a group of researchers successfully converted the hydrolysate
667
into lipids (Dai et al., 2017). Triacylglycerol (TAG) was converted from sugar
668
hydrolysate through fermentation using Bacillus Galactomyces sp. CCZU11-1. The
28
669
optimum condition of fermentation was determined to be 20 g/L glucose and carbon
670
source/nitrogen source at 30/1 ratio. 97% glucose was converted into 0.22 g lipid after
671
72 h of fermentation. According to the study, the lipid produced was comprised mainly
672
of C16 and C18 fatty acid chain, namely palmitic acid, palmitoleic acid, stearic acid and
673
oleic acid. The lipid obtained was recommended to be the alternative feedstock for
674
biodiesel or fatty-acid derived chemicals production due to its similar fatty acid content
675
to that of vegetable oil.
676
3.3
677
Pretreatment liquid fraction (LF) conversion Table 4 tabulates the studies reporting carbohydrate-based products conversion
678
of pretreatment liquid fraction (LF) from DES pretreatment. During biopolymer
679
fractionation, certain amount of depolymerized polysaccharides will dissolve into the
680
LF. In a study, 2.0% and 5.1% of glucose and xylose were recovered in LF, which
681
amounted to 8.6% and 23.6% of initial glucan and xylan in wood biomass, respectively
682
(Xia et al., 2018). 1.9% and 8.3% of fructose and hydroxymethylfurfural (HMF) were
683
also detected, which implied the occurrence of glucose isomerization and dehydration
684
process during the pretreatment. In the report by Liu, Y. et al. (2017), DES fractionation
685
resulted in 5.1% of glucose and 7.0% of xylose content in LF, as well as 1.1% of HMF.
686
The author demonstrated through TEM image that the dissolved saccharides can be
687
transformed directly into carbon materials using hydrothermal treatment without being
688
separated from the solvent mixture.
689
One of the major concerns in using LF as fermentation substrate is the possible
690
inhibiting effect of DES solvent residue on microbial activity. However, Chen, Z. et al.
691
(2018) showed that the solvent residue can actually act as extra carbon source for yeast
692
fermentation. The collected LF after 5 recycling cycles of ChCl:glycerol pretreatment
29
693
contained 3.14 g/L glucose, 17.49 g/L xylose and 143.23 g/L glycerol. Using oleaginous
694
yeast Rhodosporidium toruloides (NRRL Y-1091), the 144 h (6-day)-fermentation
695
process produced 8.1 g/L lipids and 15.0 mg/L carotenoid. Glucose and residual
696
glycerol in the LF were completely consumed after the process. LF containing solvent
697
residue can still be a good substrate for R.toruloides strain as glycerol and glucose can
698
both act as the carbon sources for fermentation. Carotenoid which is a natural pigment
699
and antioxidant is a very valuable chemical compound. The work on direct utilization of
700
LF can increase the economic return and reduce the operating cost of the process by
701
eliminating the purification process. Generally, LF is rich in C5 hemicellulose sugar due to the labile nature of the
702 703
sugar. Using aluminium chloride as catalyst, furfural was produced from xylose-rich LF
704
(Chen, Zhu et al., 2018). The furfural production yield reached 92% of the recovered
705
xylose content in the LF from the first DES pretreatment cycle and 85% in the recycled
706
DES pretreatment LF. However, the xylose recovery was much lower in the LF from
707
recycled DES (56.8%) as compared with first cycle LF (94.4%). Thus on the basis of
708
1000 g raw rice straw biomass, 101 g of furfural was produced from first cycle LF and
709
57 g from recycled LF. The drastic reduction was linked to accumulated impurities in
710
the LF which reduced xylose conversion in the recycled DES pretreatment. Hence, the
711
authors recommended more research efforts on minimizing xylose degradation during
712
the recycling process to increase the feasibility of the production scheme.
713
4
714
DES-extracted lignin upgrading Currently, most lignin extraction studies involve the use of acidic DES solvent
715
made of organic acid HBD. Studies showed that the lignin extraction yields of the DESs
716
were higher than their respective organic acid (Tan et al., 2019; Yu et al., 2018). Tan et
30
717
al. (2018) reported that similar to conventional acid pretreatment, dissolved lignin can
718
be precipitated from the acidic DES pretreatment mixture by using a simple water anti-
719
solvent rinsing. However, dissolved lignin in near-neutral and basic DES pretreatment
720
mixture could not be precipitated using the same methodology. More steps such as pH
721
adjustment might be needed to achieve lignin precipitation.
722
To evaluate the potential of DES for lignin processing more closely, Soares et al.
723
(2017) performed an extensive solubility study of lignin monomers (syringaldehyde,
724
vanillic acid, syringic acid and ferulic acid) and technical lignin (kraft and organosolv)
725
in different DESs. Effect of HBD and HBA type, HBA to HBD molar ratio, water
726
content and temperature on DES’s solubilisation efficiency were investigated. The
727
optimum conditions were reported to be 40-50°C using urea:propionic acid DES with
728
the ratio of 2:1 in 50-75 wt% aqueous condition.
729
The lignin solubilisation trend of DES differs from that of lignin extraction
730
performance from whole lignocellulosic biomass. DES with acid HBD which contained
731
longer alkyl chain (i.e. propionic acid) had greater solubilisation performance compared
732
to those with shorter alkyl chain (i.e. formic acid and acetic acid) (Soares et al., 2017).
733
The author associated the higher solubilisation of acid HBD with longer alkyl chain to
734
the dispersive interaction of alkyl chain with the lignin monomers. Contradicting result
735
was observed in lignin extraction from whole biomass (Tan et al., 2019; Yu et al., 2018).
736
Formic acid-DES was found to have higher lignin extraction efficiency compared to
737
acetic acid- and propionic acid-DES. The degree of lignin solubilisation can indeed
738
provide the basic information of whether a DES is suitable for lignin extraction.
739
Nevertheless, the efficiency of DES for lignin processing can be more correctly
740
assessed using the actual bulk biomass.
31
741
4.1
Properties of lignin
742
As a heterogeneous polymer, lignin constitutes of three main phenylpropanoid
743
structures namely syringyl (S), guaiacyl (G) and p-hydroxyphenyl (H) unit. Properties
744
of lignin polymer vary according to the type of solvent used, processing conditions and
745
biomass species. Investigation on the lignin’s properties is essential to identify the
746
suitable downstream conversion processes.
747
4.1.1
Purity
748
Extracted lignin usually contains solvent contaminants and polysaccharides
749
residues. Purification process such as extensive washing or dissolution followed by
750
precipitation can increase the lignin’s purity, but risk the loss of yield or properties
751
alteration. This has prompted researchers to seek for an optimized lignin extraction
752
schemes for high purity lignin production. A research work reported that anti-solvent
753
used had great effect over the quality of lignin produced (Alvarez-Vasco et al., 2016).
754
Water-ethanol mixture at volume ratio of 9:1 as anti-solvent for lignin precipitation
755
produced lignin with 88% purity. Comparatively the purity of lignin precipitated using
756
deionized water was <60%. With additional two washing cycles using the similar water-
757
ethanol mixture, lignin at 95% purity was obtained with negligible trace of carbohydrate
758
and DES residual. The effectiveness of the same washing solvent achieving >90%
759
lignin purity was also reported by Li et al. (2017) and Lyu et al. (2018).
760
Other than the washing procedure, pretreatment condition was reported to affect
761
lignin’s purity. By increasing the pretreatment time from 6 h to 24 h, the purity
762
increased from 90% to 95.4% while the carbohydrate content in lignin decreased
763
gradually from 1.12 to 0.11% (Lyu et al., 2018). Another attempt to increase the lignin’s
764
purity is by using DES in aqueous condition for extraction. Chen, Zhu et al. (2018)
32
765
found that when water content of DES solution was increased from 0-20%, lignin purity
766
improved from 83.3-86.1% and 82.4-85.0% at 20 wt% and 27 wt% biomass solid
767
loading, respectively. Another interesting finding reported that the yield of lignin
768
extracted at 130°C exceeded the initial lignin content in raw poplar wood biomass (Xia
769
et al., 2018). The excess weight of lignin produced was attributed to the generation of
770
pseudolignin from carbohydrate, as observed in SEM.
771
4.1.2
Molecular structure
772
Lignin’s reactivity for downstream modification can be reflected in their
773
phenolic hydroxyl (PhOH) group content as PhOH is the most reactive functional group
774
in lignin (Lai, 1992). After confirming the presence of strong PhOH signal in FTIR,
775
Tan et al. (2019) quantified the PhOH content in lignin extracted using various acidic
776
DES solvents. Among the nine types of acidic HBDs (formic acid, acetic acid,
777
propionic acid, butyric acid, lactic acid, malic acid, citric acid, succinic acid and maleic
778
acid) investigated, lignin extracted using acidic DES with lactic acid HBD contained the
779
highest PhOH content at 3.33-3.72 mmol/g, depending on the HBA:HBD ratio. The
780
PhOH was higher than that of pure lactic acid-extracted lignin at 2.85 mmol/g. The
781
authors associated the finding with the condensation of lignin fragments in pure acid
782
environment which decreased the PhOH moiety in the lignin structure. Some works also
783
identified appreciable PhOH content in DES-extracted lignin through NMR
784
spectroscopy (Alvarez-Vasco et al., 2016; Li et al., 2017; Xia et al., 2018).
785
To study the lignin’s structure more in-depth, NMR is usually performed. Most
786
studies focused on determining the presence of carbohydrate impurities in lignin, as
787
well as the condition of the major linkages in the structure, namely β-O-4 and C-C
788
bonds. β-O-4 ether bond is recognized as the most abundant and cleavable linkage in
33
789
lignin (Constant et al., 2016). Some studies reported on the absence or the negligible
790
amount of β-O-4 in the structure once lignin was extracted from biomass using DES,
791
leaving predominant amount of C-C bond, such as resinols β-β or phenylcoumarans β-5
792
in the structure (Alvarez-Vasco et al., 2016; Liu, Y. et al., 2017; Xia et al., 2018).
793
Alvarez-Vasco et al. (2016) further clarified that DES depolymerized lignin by selective
794
cleavage of the labile ether bond, hence lignin with better polymer stability can be
795
produced. Contrarily, several works detected appreciable amount of β-O-4 remained in
796
the lignin. Hiltunen et al. (2016) reported that in the wood lignin, main linkage detected
797
was β-O-4, followed by β-β and β-5. The result deviation could be due to different
798
choice of solvent which caused varying degree of depolymerisation in lignin. For
799
example, acidified ChChl:glycerol used in Chen, Z. et al. (2018) might be able to retain
800
the ether bond unlike the commonly used acidic DES such as ChCl:lactic acid.
801
Nevertheless, the ether bond albeit existed (1.53% and 4.98%), was significantly lower
802
than that of milled wood lignin (46.7%). Some catalytic conversions target on β-O-4
803
cleavage in the product formation schemes (Zakzeski et al., 2010). Low value of β-O-4
804
in lignin might deter the application of DES-extracted lignin in certain lignin utilization
805
process. To cater to this requirement, research on DES type which can preserve as much
806
β-O-4 linkages as possible will be an interesting subject to pursue. Investigation on
807
suitable processing schemes for lignin with low β-O-4 linkage structure is also
808
necessary to create more variety of lignin-derived products.
809
4.2
810
DES-extracted lignin conversion Lignin is always known as a very useful macromolecule in material industries
811
(Laurichesse and Avérous, 2014; Northey, 1992). Due to lignin polymer’s insolubility
812
in many solvents, the application of lignin nanoparticles for bio-based functional
34
813
materials has been researched by many (Lievonen et al., 2016; Roopan, 2017). Lyu et al.
814
(2018) successfully produced lignin nanoparticle nanoscale lignin particle with uniform
815
diameter of 200-420 nm and smooth topographic surface by dissolving the DES-
816
extracted lignin in acetone at 0.1 g/L concentration. DES-extracted lignin represents a
817
new class of extracted lignin and its potential is still largely untapped. To fully explore
818
the possibility of applying DES-extracted lignin as fuel and aromatic feedstock, it is
819
promptly to initiate more comprehensive structural and properties studies of this lignin
820
type. Subsequently, the utilization of DES-extracted lignin in current existing fuel,
821
macromolecular and fine chemicals conversion schemes are highly encouraged. While
822
the DES-fractionated carbohydrate products have gained a lot of attention in
823
downstream conversion, the application study of DES-extracted lignin is comparably
824
scarce. To move forward in biorefinery development, the conversion scheme of
825
different biopolymers should be designed concurrently.
826
5
827
Challenges and outlook for DES utilization in biomass processing In view of the huge varieties of DES available, the research focus on the
828
selection of DES constituent across the DES pretreatment field is rather scattered.
829
Despite various categories of DES constituents have been investigated, the most
830
suitable DES type for biomass processing application is non-conclusive. Researchers
831
need to define the pretreatment goal, for instance enzymatic hydrolysis enhancement or
832
lignin extraction, in order to select the best performing solvent. Establishment on the
833
fundamental knowledge on how DES interacts with different biopolymer solutes would
834
greatly help in the selection process. In addition, it is well known that high viscosity of
835
DES solvent is a major obstacle in its application. Despite one can opt for constituents
836
that will lead to less viscous DES, the synthesized solvent might not be suitable for the
35
837
intended application. Water addition to DES not only can decrease the solvent’s
838
viscosity, but also reduce the amount of neat solvent needed, which translates to lower
839
operating cost. Nevertheless, there is a limit to how much water can be added to DES
840
without disrupting the solvent’s intermolecular bonding structure. Data collection on
841
how water addition affects the DES molecular framework and their interaction with
842
targeted solutes would provide insightful information to facilitate application of
843
aqueous DES.
844
Multiple types of carbohydrate-derived product such as bioethanol, biobutanol,
845
2,3-butanediol, lipids, 5-HMF have been successfully produced from the DES-
846
fractionated products in several works. However, the gathered information showed that
847
the selectivity and yield of these conversion processes varied greatly in different works
848
(Table 3 and Table 4). Strategies to improve the productivity of intended products,
849
therefore establishing the process feasibility and accuracy, will be a great research
850
direction in future works. Aside from the commonly reported biofuels, it will also be
851
worthwhile to explore more varieties of higher value saccharides-derived commodity
852
chemicals such as organic acids or sugar alcohols to be converted from DES-
853
fractionated saccharides to increase the process revenue.
854
It has been demonstrated in the previous sections that different categories of
855
DESs were investigated for enzymatic hydrolysis enhancement. For lignin extraction,
856
vast majority of the studies involved the use of acidic DES (DES with acid HBD or
857
acidified DES) while other types of DES such as basic DES can also extract an
858
extensive amount of lignin. It is recommended to diversify the type of DES adopted in
859
lignin extraction study to fully explore the potential of DES in this application. The
860
extracted lignin might have different properties to acidic DES-extracted lignin. The two
36
861
might be applicable for different sorts of downstream processing and this can create
862
more variety of lignin-derived aromatic products as alternatives to petrochemicals.
863
Additionally, comparison study on DES-extracted lignin’s properties with other
864
technical lignin is necessary to evaluate the possibility of this new type of lignin to be
865
applied in existing lignin conversion schemes, as well as to explore the possible
866
applications which are yet to be accomplished by the existing technical lignin.
867
Extended techno-economic assessment is also a pressing research topic to further
868
establish the feasibility of this alternative technology. However, this research aspect has
869
been neglected. Recently, Ma et al. (2018) conducted an extensive techno-economic
870
analysis of biogas upgrading process to compare the performance of DES with the other
871
conventional solvents. The energy utilization, amount of solvent and dimension of
872
equipment columns were simulated using ASPEN PLUS, which showed DES had
873
similarly promising efficiency as the conventional solvent. Meticulous investigation
874
using the aid of mathematical models or simulation software is highly encouraged.
875
The question that always arises when a new technology being introduced is “Can
876
the same level of performance transfer from research laboratory scale to a larger scale
877
setup?” Despite we are at the starting point of DES exploration, consideration has to be
878
taken on the prospect of applying the technology in large scale and eventually achieving
879
the commercial biomass utilization process. Researchers are to constantly evaluate the
880
progress of the developed technology and scale up the process when deemed suitable.
881
Conclusion
882
DES has manifested promising performance in raw lignocellulosic biomass
883
fractionation. This enables the independent biopolymer upgrading process according to
884
their inherent properties. However, the ideal DES type for biomass conversion is still
37
885
uncertain. Also, there is a lack of product variety in the downstream conversion of DES-
886
fractionated products. Rigorous investigations in solvent selection, bio-products
887
diversification, as well as techno-economic analysis are needed to advance the DES
888
fractionation technology. The employment of DES in achieving a complete biomass
889
utilization process will be a significant approach to reduce the dependency on non-
890
renewable resources and move towards sustainable economy.
891
Acknowledgements
892
This work was supported by the Fundamental Research Grant Scheme (FRGS) by
893
Ministry of Education Malaysia [FP047-2017A and FP128-2019A].
894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918
References 1. Abbott, A.P., Barron, J.C., Ryder, K.S., Wilson, D., 2007. Eutectic-based ionic liquids with metal-containing anions and cations. Chemistry 13(22), 6495-6501. 2. Abbott, A.P., Boothby, D., Capper, G., Davies, D.L., Rasheed, R.K., 2004a. Deep Eutectic Solvents Formed between Choline Chloride and Carboxylic Acids: Versatile Alternatives to Ionic Liquids. J. Am. Chem. Soc. 126(29), 9142-9147. 3. Abbott, A.P., Capper, G., Davies, D.L., Rasheed, R., 2004b. Ionic Liquids Based upon Metal Halide/Substituted Quaternary Ammonium Salt Mixtures. Inorg. Chem. 43(11), 3447-3452. 4. Abbott, A.P., Capper, G., Davies, D.L., Rasheed, R.K., 2004. Ionic liquid analogues formed from hydrated metal salts. Chemistry 10(15), 3769-3774. 5. Abbott, A.P., Capper, G., Davies, D.L., Rasheed, R.K., Tambyrajah, V., 2003. Novel solvent properties of choline chloride/urea mixtures. Chem. Commun.(1), 7071. 6. Abbott, A.P., Harris, R.C., Ryder, K.S., D'Agostino, C., Gladden, L.F., Mantle, M.D., 2011. Glycerol eutectics as sustainable solvent systems. Green Chem. 13(1), 82-90. 7. Alvarez-Vasco, C., Ma, R.S., Quintero, M., Guo, M., Geleynse, S., Ramasamy, K.K., Wolcott, M., Zhang, X., 2016. Unique low-molecular-weight lignin with high purity extracted from wood by deep eutectic solvents (DES): a source of lignin for valorization. Green Chem. 18(19), 5133-5141. 8. Anastas, P., Eghbali, N., 2010. Green chemistry: principles and practice. Chem. Soc. Rev. 39(1), 301-312. 9. Aroso, I.M., Craveiro, R., Rocha, A., Dionisio, M., Barreiros, S., Reis, R.L., Paiva, A., Duarte, A.R.C., 2015. Design of controlled release systems for THEDES-
38
919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959
Therapeutic deep eutectic solvents, using supercritical fluid technology. Int. J. Pharm. 492(1-2), 73-79. 10. Brandt, A., Gräsvik, J., Hallett, J.P., Welton, T., 2013. Deconstruction of lignocellulosic biomass with ionic liquids. Green Chem. 15(3), 550. 11. Brinchi, L., Cotana, F., Fortunati, E., Kenny, J.M., 2013. Production of nanocrystalline cellulose from lignocellulosic biomass: technology and applications. Carbohydr. Polym. 94(1), 154-169. 12. Chen, Z., Bai, X., A, L., Wan, C., 2018. High-Solid Lignocellulose Processing Enabled by Natural Deep Eutectic Solvent for Lignin Extraction and Industrially Relevant Production of Renewable Chemicals. ACS Sustain. Chem. Eng. 6(9), 12205-12216. 13. Chen, Z., Reznicek, W.D., Wan, C., 2018. Deep eutectic solvent pretreatment enabling full utilization of switchgrass. Bioresour. Technol. 263, 40-48. 14. Chen, Z., Wan, C., 2018. Ultrafast fractionation of lignocellulosic biomass by microwave-assisted deep eutectic solvent pretreatment. Bioresour. Technol. 250, 532-537. 15. Choi, Y.H., van Spronsen, J., Dai, Y., Verberne, M., Hollmann, F., Arends, I.W., Witkamp, G.J., Verpoorte, R., 2011. Are natural deep eutectic solvents the missing link in understanding cellular metabolism and physiology? Plant Physiol 156(4), 1701-1705. 16. Constant, S., Wienk, H.L.J., Frissen, A.E., Peinder, P.d., Boelens, R., van Es, D.S., Grisel, R.J.H., Weckhuysen, B.M., Huijgen, W.J.J., Gosselink, R.J.A., Bruijnincx, P.C.A., 2016. New insights into the structure and composition of technical lignins: a comparative characterisation study. Green Chem. 18(9), 2651-2665. 17. Cvjetko Bubalo, M., Vidović, S., Radojčić Redovniković, I., Jokić, S., 2015. Green solvents for green technologies. Journal of Chemical Technology & Biotechnology 90(9), 1631-1639. 18. Dai, Y., van Spronsen, J., Witkamp, G.J., Verpoorte, R., Choi, Y.H., 2013. Natural deep eutectic solvents as new potential media for green technology. Anal. Chim. Acta 766, 61-68. 19. Dai, Y., Zhang, H.S., Huan, B., He, Y., 2017. Enhancing the enzymatic saccharification of bamboo shoot shell by sequential biological pretreatment with Galactomyces sp. CCZU11-1 and deep eutectic solvent extraction. Bioprocess Biosyst Eng 40(9), 1427-1436. 20. Dai, Y.T., Verpoorte, R., Choi, Y.H., 2014. Natural deep eutectic solvents providing enhanced stability of natural colorants from safflower (Carthamus tinctorius). Food Chem. 159, 116-121. 21. Fang, C., Thomsen, M.H., Frankær, C.G., Brudecki, G.P., Schmidt, J.E., AlNashef, I.M., 2017. Reviving pretreatment effectiveness of deep eutectic solvents on lignocellulosic date palm residues by prior recalcitrance reduction. Industrial & Engineering Chemistry Research 56(12), 3167-3174.
39
960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999
22. Florindo, C., Oliveira, F.S., Rebelo, L.P.N., Fernandes, A.M., Marrucho, I.M., 2014. Insights into the Synthesis and Properties of Deep Eutectic Solvents Based on Cholinium Chloride and Carboxylic Acids. ACS Sustain. Chem. Eng. 2(10), 24162425. 23. Florindo, C., Oliveira, M.M., Branco, L.C., Marrucho, I.M., 2017. Carbohydratesbased deep eutectic solvents: Thermophysical properties and rice straw dissolution. J. Mol. Liq. 247, 441-447. 24. Francisco, M., van den Bruinhorst, A., Kroon, M.C., 2012. New natural and renewable low transition temperature mixtures (LTTMs): screening as solvents for lignocellulosic biomass processing. Green Chem. 14(8), 2153. 25. García, G., Aparicio, S., Ullah, R., Atilhan, M., 2015. Deep Eutectic Solvents: Physicochemical Properties and Gas Separation Applications. Energy & Fuels 29(4), 2616-2644. 26. Guo, Z., Ling, Z., Wang, C., Zhang, X., Xu, F., 2018. Integration of facile deep eutectic solvents pretreatment for enhanced enzymatic hydrolysis and lignin valorization from industrial xylose residue. Bioresour. Technol. 265, 334-339. 27. Hendriks, A.T., Zeeman, G., 2009. Pretreatments to enhance the digestibility of lignocellulosic biomass. Bioresour. Technol. 100(1), 10-18. 28. Hiltunen, J., Kuutti, L., Rovio, S., Puhakka, E., Virtanen, T., Ohra-Aho, T., Vuoti, S., 2016. Using a low melting solvent mixture to extract value from wood biomass. Sci Rep 6, 32420. 29. Hou, X.-D., Lin, K.-P., Li, A.-L., Yang, L.-M., Fu, M.-H., 2018. Effect of constituents molar ratios of deep eutectic solvents on rice straw fractionation efficiency and the micro-mechanism investigation. Ind. Crops. Prod. 120, 322-329. 30. Hou, X.D., Feng, G.J., Ye, M., Huang, C.M., Zhang, Y., 2017. Significantly enhanced enzymatic hydrolysis of rice straw via a high-performance two-stage deep eutectic solvents synergistic pretreatment. Bioresour. Technol. 238, 139-146. 31. Hou, X.D., Li, A.L., Lin, K.P., Wang, Y.Y., Kuang, Z.Y., Cao, S.L., 2018. Insight into the structure-function relationships of deep eutectic solvents during rice straw pretreatment. Bioresour. Technol. 249, 261-267. 32. Kandanelli, R., Thulluri, C., Mangala, R., Rao, P.V.C., Gandham, S., Velankar, H.R., 2018. A novel ternary combination of deep eutectic solvent-alcohol (DES-OL) system for synergistic and efficient delignification of biomass. Bioresour. Technol. 265, 573-576. 33. Kim, K.H., Dutta, T., Sun, J., Simmons, B., Singh, S., 2018. Biomass pretreatment using deep eutectic solvents from lignin derived phenols. Green Chem. 20(4), 809815. 34. Koga, Y.J.J.o.S.C., 2003. Effect of Ethylene Glycol on the Molecular Organization of H2O in Comparison with Methanol and Glycerol: A Calorimetric Study. 32(9), 803-818.
40
1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041
35. Kumar, A.K., Parikh, B.S., Shah, E., Liu, L.Z., Cotta, M.A., 2016. Cellulosic ethanol production from green solvent-pretreated rice straw. Biocatalysis and Agricultural Biotechnology 7, 14-23. 36. Lai, Y.-Z., 1992. Determination of Phenolic Hydroxyl Groups, in: Lin, S.Y., Dence, C.W. (Eds.), Methods in Lignin Chemistry. Springer Berlin Heidelberg, Berlin, Heidelberg, pp. 423-434. 37. Laurichesse, S., Avérous, L., 2014. Chemical modification of lignins: Towards biobased polymers. Prog. Polym. Sci. 39(7), 1266-1290. 38. Li, A.L., Hou, X.D., Lin, K.P., Zhang, X., Fu, M.H., 2018. Rice straw pretreatment using deep eutectic solvents with different constituents molar ratios: Biomass fractionation, polysaccharides enzymatic digestion and solvent reuse. J Biosci Bioeng 126(3), 346-354. 39. Li, T., Lyu, G., Liu, Y., Lou, R., Lucia, L.A., Yang, G., Chen, J., Saeed, H.A.M., 2017. Deep Eutectic Solvents (DESs) for the Isolation of Willow Lignin (Salix matsudana cv. Zhuliu). Int J Mol Sci 18(11). 40. Lievonen, M., Valle-Delgado, J.J., Mattinen, M.-L., Hult, E.-L., Lintinen, K., Kostiainen, M.A., Paananen, A., Szilvay, G.R., Setälä, H., Österberg, M., 2016. A simple process for lignin nanoparticle preparation. Green Chem. 18(5), 1416-1422. 41. Liew, S.Q., Ngoh, G.C., Yusoff, R., Teoh, W.H., 2016. Sequential ultrasoundmicrowave assisted acid extraction (UMAE) of pectin from pomelo peels. Int. J. Biol. Macromol. 93, 426-435. 42. Liew, S.Q., Ngoh, G.C., Yusoff, R., Teoh, W.H., 2018. Acid and Deep Eutectic Solvent (DES) extraction of pectin from pomelo (Citrus grandis (L.) Osbeck) peels. Biocatalysis and Agricultural Biotechnology 13, 1-11. 43. Liu, Y.-T., Chen, Y.-A., Xing, Y.-J., 2014. Synthesis and characterization of novel ternary deep eutectic solvents. Chin. Chem. Lett. 25(1), 104-106. 44. Liu, Y., Chen, W., Xia, Q., Guo, B., Wang, Q., Liu, S., Liu, Y., Li, J., Yu, H., 2017. Efficient cleavage of lignin–carbohydrate complexes and ultrafast extraction of lignin oligomers from wood biomass by microwave-assisted treatment with deep eutectic solvent. ChemSusChem. 10(8), 1692-1700. 45. Liu, Y., Guo, B., Xia, Q., Meng, J., Chen, W., Liu, S., Wang, Q., Liu, Y., Li, J., Yu, H., 2017. Efficient Cleavage of Strong Hydrogen Bonds in Cotton by Deep Eutectic Solvents and Facile Fabrication of Cellulose Nanocrystals in High Yields. ACS Sustain. Chem. Eng. 5(9), 7623-7631. 46. Loow, Y.-L., Wu, T.Y., Lim, Y.S., Tan, K.A., Siow, L.F., Md. Jahim, J., Mohammad, A.W., 2017. Improvement of xylose recovery from the stalks of oil palm fronds using inorganic salt and oxidative agent. Energy Convers. Manage. 138, 248-260. 47. Loow, Y.L., Wu, T.Y., Yang, G.H., Ang, L.Y., New, E.K., Siow, L.F., Md Jahim, J., Mohammad, A.W., Teoh, W.H., 2018. Deep eutectic solvent and inorganic salt pretreatment of lignocellulosic biomass for improving xylose recovery. Bioresour. Technol. 249, 818-825.
41
1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081
48. Lynam, J.G., Kumar, N., Wong, M.J., 2017. Deep eutectic solvents' ability to solubilize lignin, cellulose, and hemicellulose; thermal stability; and density. Bioresour. Technol. 238, 684-689. 49. Lyu, G., Li, T., Ji, X., Yang, G., Liu, Y., Lucia, L., Chen, J., 2018. Characterization of Lignin Extracted from Willow by Deep Eutectic Solvent Treatments. Polymers 10(8), 869. 50. Ma, C., Liu, C., Lu, X., Ji, X., 2018. Techno-economic analysis and performance comparison of aqueous deep eutectic solvent and other physical absorbents for biogas upgrading. Applied Energy 225, 437-447. 51. Malaeke, H., Housaindokht, M.R., Monhemi, H., Izadyar, M., 2018. Deep eutectic solvent as an efficient molecular liquid for lignin solubilization and wood delignification. J. Mol. Liq. 263, 193-199. 52. Menon, V., Rao, M., 2012. Trends in bioconversion of lignocellulose: Biofuels, platform chemicals & biorefinery concept. Prog. Energy Combust. Sci. 38(4), 522550. 53. Mota-Morales, J.D., Gutierrez, M.C., Ferrer, M.L., Sanchez, I.C., Elizalde-Pena, E.A., Pojman, J.A., Del Monte, F., Luna-Barcenas, G., 2013. Deep eutectic solvents as both active fillers and monomers for frontal polymerization. Journal of Polymer Science Part a-Polymer Chemistry 51(8), 1767-1773. 54. Northey, R.A., 1992. Low-Cost Uses of Lignin. ACS Symp. Ser. 476, 146-175. 55. Pan, M., Zhao, G., Ding, C., Wu, B., Lian, Z., Lian, H., 2017. Physicochemical transformation of rice straw after pretreatment with a deep eutectic solvent of choline chloride/urea. Carbohydr. Polym. 176, 307-314. 56. Procentese, A., Johnson, E., Orr, V., Garruto Campanile, A., Wood, J.A., Marzocchella, A., Rehmann, L., 2015. Deep eutectic solvent pretreatment and subsequent saccharification of corncob. Bioresour. Technol. 192, 31-36. 57. Procentese, A., Raganati, F., Olivieri, G., Russo, M.E., Rehmann, L., Marzocchella, A., 2017. Low-energy biomass pretreatment with deep eutectic solvents for biobutanol production. Bioresour. Technol. 243, 464-473. 58. Procentese, A., Raganati, F., Olivieri, G., Russo, M.E., Rehmann, L., Marzocchella, A., 2018. Deep Eutectic Solvents pretreatment of agro-industrial food waste. Biotechnol. for Biofuels 11, 37. 59. Rastogi, M., Shrivastava, S., 2017. Recent advances in second generation bioethanol production: An insight to pretreatment, saccharification and fermentation processes. Renewable and Sustainable Energy Reviews 80, 330-340. 60. Roopan, S.M., 2017. An overview of natural renewable bio-polymer lignin towards nano and biotechnological applications. Int. J. Biol. Macromol. 103, 508-514. 61. Saha, B.C., Yoshida, T., Cotta, M.A., Sonomoto, K., 2013. Hydrothermal pretreatment and enzymatic saccharification of corn stover for efficient ethanol production. Ind. Crops. Prod. 44, 367-372.
42
1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123
62. Sirvio, J.A., Visanko, M., Liimatainen, H., 2016. Acidic Deep Eutectic Solvents As Hydrolytic Media for Cellulose Nanocrystal Production. Biomacromolecules 17(9), 3025-3032. 63. Smith, E.L., Abbott, A.P., Ryder, K.S., 2014. Deep eutectic solvents (DESs) and their applications. Chem. Rev. 114(21), 11060-11082. 64. Soares, B., Tavares, D.J.P., Amaral, J.L., Silvestre, A.J.D., Freire, C.S.R., Coutinho, J.A.P., 2017. Enhanced Solubility of Lignin Monomeric Model Compounds and Technical Lignins in Aqueous Solutions of Deep Eutectic Solvents. ACS Sustain. Chem. Eng. 5(5), 4056-4065. 65. Sun, N., Rodriguez, H., Rahman, M., Rogers, R.D., 2011. Where are ionic liquid strategies most suited in the pursuit of chemicals and energy from lignocellulosic biomass? Chem. Commun. 47(5), 1405-1421. 66. Sun, Y., Cheng, J., 2002. Hydrolysis of lignocellulosic materials for ethanol production: a review. Bioresour. Technol. 83(1), 1-11. 67. Suopajarvi, T., Sirvio, J.A., Liimatainen, H., 2017. Nanofibrillation of deep eutectic solvent-treated paper and board cellulose pulps. Carbohydr. Polym. 169, 167-175. 68. Tan, Y.T., Ngoh, G.C., Chua, A.S.M., 2018. Evaluation of fractionation and delignification efficiencies of deep eutectic solvents on oil palm empty fruit bunch. Ind. Crops. Prod. 123, 271-277. 69. Tan, Y.T., Ngoh, G.C., Chua, A.S.M., 2019. Effect of functional groups in acid constituent of deep eutectic solvent for extraction of reactive lignin. Bioresour. Technol. 281, 359-366. 70. Teles, A.R.R., Capela, E.V., Carmo, R.S., Coutinho, J.A.P., Silvestre, A.J.D., Freire, M.G., 2017. Solvatochromic parameters of deep eutectic solvents formed by ammonium-based salts and carboxylic acids. Fluid Phase Equilib. 448, 15-21. 71. Vigier, K.D.O., Chatel, G., Jérôme, F., 2015. Contribution of deep eutectic solvents for biomass processing: Opportunities, challenges, and limitations. ChemCatChem 7(8), 1250-1260. 72. Wahlström, R., Hiltunen, J., Pitaluga de Souza Nascente Sirkka, M., Vuoti, S., Kruus, K., 2016. Comparison of three deep eutectic solvents and 1-ethyl-3methylimidazolium acetate in the pretreatment of lignocellulose: effect on enzyme stability, lignocellulose digestibility and one-pot hydrolysis. RSC Adv. 6(72), 68100-68110. 73. Welton, T., 1999. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. Rev. 99(8), 2071-2084. 74. Xia, Q., Liu, Y., Meng, J., Cheng, W., Chen, W., Liu, S., Liu, Y., Li, J., Yu, H., 2018. Multiple hydrogen bond coordination in three-constituent deep eutectic solvents enhances lignin fractionation from biomass. Green Chem. 20(12), 27112721. 75. Xing, W., Xu, G., Dong, J., Han, R., Ni, Y., 2018. Novel dihydrogen-bonding deep eutectic solvents: Pretreatment of rice straw for butanol fermentation featuring enzyme recycling and high solvent yield. Chem. Eng. J. 333, 712-720.
43
1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156
76. Xu, G.C., Ding, J.C., Han, R.Z., Dong, J.J., Ni, Y., 2016. Enhancing cellulose accessibility of corn stover by deep eutectic solvent pretreatment for butanol fermentation. Bioresour. Technol. 203, 364-369. 77. Yiin, C.L., Quitain, A.T., Yusup, S., Sasaki, M., Uemura, Y., Kida, T., 2016. Characterization of natural low transition temperature mixtures (LTTMs): Green solvents for biomass delignification. Bioresour. Technol. 199, 258-264. 78. Yiin, C.L., Quitain, A.T., Yusup, S., Uemura, Y., Sasaki, M., Kida, T., 2017. Choline chloride (ChCl) and monosodium glutamate (MSG)-based green solvents from optimized cactus malic acid for biomass delignification. Bioresour. Technol. 244(Part 1), 941-948. 79. Yu, Q., Zhang, A., Wang, W., Chen, L., Bai, R., Zhuang, X., Wang, Q., Wang, Z., Yuan, Z., 2018. Deep eutectic solvents from hemicellulose-derived acids for the cellulosic ethanol refining of Akebia’ herbal residues. Bioresour. Technol. 247, 705710. 80. Zainal-Abidin, M.H., Hayyan, M., Ngoh, G.C., Wong, W.F., 2019. From nanoengineering to nanomedicine: A facile route to enhance biocompatibility of graphene as a potential nano-carrier for targeted drug delivery using natural deep eutectic solvents. Chem. Eng. Sci. 195, 95-106. 81. Zakzeski, J., Bruijnincx, P.C.A., Jongerius, A.L., Weckhuysen, B.M., 2010. The Catalytic Valorization of Lignin for the Production of Renewable Chemicals. Chem. Rev. 110(6), 3552-3599. 82. Zdanowicz, M., Wilpiszewska, K., Spychaj, T., 2018. Deep eutectic solvents for polysaccharides processing. A review. Carbohydr. Polym. 200, 361-380. 83. Zhang, C.W., Xia, S.Q., Ma, P.S., 2016. Facile pretreatment of lignocellulosic biomass using deep eutectic solvents. Bioresour. Technol. 219, 1-5. 84. Zhang, Q., De Oliveira Vigier, K., Royer, S., Jerome, F., 2012. Deep eutectic solvents: syntheses, properties and applications. Chem. Soc. Rev. 41(21), 7108-7146. 85. Zhao, Z., Chen, X., Ali, M.F., Abdeltawab, A.A., Yakout, S.M., Yu, G., 2018. Pretreatment of wheat straw using basic ethanolamine-based deep eutectic solvents for improving enzymatic hydrolysis. Bioresour. Technol. 263, 325-333. 86. Zulkefli, S., Abdulmalek, E., Abdul Rahman, M.B., 2017. Pretreatment of oil palm trunk in deep eutectic solvent and optimization of enzymatic hydrolysis of pretreated oil palm trunk. Renew. Energy 107, 36-41.
44
Table 1 Categorization of DESs and their respective application DES
Constituent 1 Quaternary ammonium salt Quaternary ammonium salt
Constituent 2 Application types* References Metal halides, Type 1 Electrochemical, metal plating (Abbott, Andrew P. et al., 2004b; Smith et al., 2014) Eg: ZnBr2, FeCl3 Hydrated metal halides, Type 2 Electrochemical, metal plating (Abbott, A. P. et al., 2004; Smith et al., 2014) Eg: CrCl3·6H2O, CaCl2·6H2O Hydrogen bond donor, Quaternary Metal plating, gas adsorption, (Abbott, Andrew P. et al., 2004a; Abbott et al., 2003; Type 3 Eg: urea, carboxylic acid, ammonium salt biomass processing Abbott et al., 2011; García et al., 2015; Smith et al., 2014) polyol Hydrogen bond donor, Type 4 Metal salt Electrochemical, metal plating (Abbott et al., 2007; Smith et al., 2014) Eg: amide, diol *The application types listed includes several selected common DES application area and do not represent all the application introduced up to date.
45
Table 2 DES pretreatment condition, biopolymer composition in DES-pretreated biomass and biopolymer removal percentage of DESs with varying HBDs DES pretreatment condition Work
DES
Molar ratio
Biomass type
S/L ratio (wt%)
Temperature (°C)
Time (hour)
Solid yield (%)
Biopolymer composition in pretreated biomass Glucan
Xylan
Lignin
35
20
19
Biopolymer removal Glucan
Xylan
Lignin
2
1
24
Polyalocohol-based DES (Chen, Z. et ChCl:glycerol al., 2018) (Chen, Zhu ChCl:ethylene glycol et al., 2018)
1:2
Switchgrass
10
110
1
1:2
Switchgrass
10
130
0.5
(Fang et al., 2017)
ChCl:glycerol
1:2
Date palm
5
70 70
ChCl:ethylene glycol ChCl:1,2-Propanediol
1:1 1:1
6 15 3 3
77 76 78 76
35 34 37 34
19 19 21 20
36 37 20 19
ChCl:1,3-Propanediol ChCl:glycerol ChCl:xylitol
1:1 1:1 1:1
Rice straw
5
120
3 6 6
ChCl:glycerol
1:2
Corncob
6.25
80 115
76 80 81 85 82
41 36 36 33 32
20 20 20 25 24
19 21 21 16 15
Oil palm empty fruit bunch
10
120
8
55 96
53 31
21 15
13 17
21
16
22
97
30
14
18
24
19
17
Corncob
5
90
24
(Hou, X.D. et al., 2018)
(Procentese et al., 2015)
150
(Tan et al., 2018)
ChCl:glycerol
1:2
ChCl:glucose
1:1
(Zhang et al., 2016)
ChCl:ethylene glycol ChCl:glycerol ChCl:glycerol ChCl:ethylene glycol Ethylammonium chloride:glycerol Ethylammonium chloride:ethylene glycol
1:2 1:2 1:2 1:2
(Zulkefli et al., 2017)
15
Acid-based DES (AlvarezChCh:lactic acid
1:2
Oil palm trunk
5
100
48
1:2
Poplar wood
10
90
6
89
73 71 63 68
52 57
36 31
10 10
48
53
27
10
58
82
2
12
33
13
21
88 71 49
87
36
25
46
Vasco et al., 2016)
(Hou, X.D. et al., 2018)
(Tan et al., 2018)
(Tan et al., 2019)
(Yu et al., 2018)
(Zhang et al., 2016)
ChCl:glycolic acid
1:1
ChCl:lactic acid
1:1
ChCl:2-chloropropionic acid
1:1
ChCl:oxalic acid
1:1
ChCl:malonic acid ChCl:lactic acid
1:1 1:5
Glucose:lactic acid
1:5
ChCl:lactic acid ChCl:malic acid ChCl:citric acid
1:1 1:1 1:1
ChCl:formic acid ChCl:acetic acid ChCl:propionic acid ChCl:butyric acid ChCl:salicylic acid ChCl:maleic acid ChCl:formic acid ChCl:acetic acid ChCl:glycolic acid ChCl:levulinic acid ChCl:lactic acid ChCl:lactic acid ChCl:lactic acid ChCl:lactic acid ChCl:glycolic acid ChCl:levulinic acid
1:2 1:2 1:2 1:2 2:1 1:1 1:2 1:6 1:4 1:4 1:2 1:5 1:10 1:15 1:2 1:2
120 145 180
3 9 0.5 6 3 6 3 6 3 6 3 6 8
Rice straw
5
80 120 80 120 80 120 80 120 80
Oil palm empty fruit bunch
10
120
Oil palm empty fruit bunch
10
120
8
Akebia herbal residue
10
120
8
Corncob
5
90
24
37 36 30
5 5 5
7 6 10
72 79
71 62 79 60 73 59 68 58 75 53
44 54 38 55 45 59 47 54 43 71
22 7 21 7 13 0.1 5 1 12 0
20 22 13 27 14 32 20 34 14 5
0
100
88
70
46
5
14
14
79
55
60 63 73
73 64 58
0 0 0
12 21 23
51 65 72 73 76 47 58 75 39 78 50 48 47 46 51 71
86 63 54 50 45 74 98 71 55 82
0 3 4 4 5 0
7 14 15 15 19 7 87 55 90 47
41 34 58 20 65 78 86 93 56 43
47
ChCl:malonic acid ChCl:glutaric acid ChCl:oxalic acid
1:1 1:1 1:1
ChCl:malic acid 1:1 Basic DES (amide-, amine-, imidazole-based DES) 1:1 (Hou, X.D. ChCl:formamide et al., 2018) ChCl:urea 1:1
ChCl:urea
(Procentese et al., 2015)
ChCl:urea Potassium carbonate:glycerol ChCl: monoethanolamine ChCl:diethanolamine (Zhao et al., ChCl: 2018) methyldiethanolamine ChCl:acetamide ChCl:urea Phenolic-based DES ChCl:4-hydroxybenzyl alcohol (Kim et al., ChCl:catechol 2018) ChCl:vanillin ChCl:p-coumaric acid (Tan et al., 2018)
22
81 80 78 58 58 94
15 14 9 10 10 10 8 8 15 13 11 7 4 15
24
20
34
75
38
14
14
25
36
51
1:6
61
52
19
6
9
47
81
1:8
73
47
25
7
2
15
74
84
41
23
13
1
10
45
1:2 1:2
95 90
36 37
23 23
19 15
2 4
2 6
3 28
1:1
83
41
22
31
7
29
0.4
67 63 57
53 51 48
22 20 13
20 19 18
5 13 26
43 50 71
49 53 61
1:2
Rice straw
120
5
6.25
80 115 80 115 150
10
120
1:2 Corncob
ChCl:imidazole
64 21 21 12 12 13 11 12 12 27 24 30 22 6 14
5
130
ChCl:urea
57 34 99
41 39 27 28 28 24 25 26 33 32 38 46 41 31
Rice straw
110 (Pan et al., 2017)
54 74 53
3:7 1:2 1:6
1:10
1:1 1:2 1:1
Oil palm empty fruit bunch
Wheat straw
Switchgrass
5
5
90
160
6
78 78
4 6 8 4 6 8 15 15
8
12
3
48
Table 3 Glucose and xylose yield from enzymatic hydrolysis process of untreated and DES-pretreated biomass and their respective CrI
(Chen, Z. et al., 2018) (Chen, Zhu et al., 2018) (Guo et al., 2018) (Hou, X.-D. et al., 2018) (Kim et al., 2018)
ChCl:glycerol, 0.9wt% H2SO4 ChCl:ethylene glycol, 1.0wt% H2SO4 betaine:lactic acid
Switchgrass
DES pretreatment condition (S/L ratio, temperature, time) 1:10, 110°C, 1h
Switchgrass
1:10, 130°C, 30mins
11
98
2
105
54
67
Xylose residue
1:20, 120°C, 2h
55*
96.8*
-
-
13.3
14.6
ChCl:oxalic acid
Rice straw
1:20, 120°C, 3h
18.4
52.9
3.1
0.4
55.8
55-59.6
ChCl:p-coumaric acid
Switchgrass
1:20, 160°C, 3h
-
85.7
-
28.8
-
-
-
77
-
42.4
-
-
(Kumar et al., 2016) (Li et al., 2018)
ChCl:glycerol
Rice straw
1:10
56.6*
87.1*
-
-
33.5
31.9
ChCl:lactic acid, 10wt% H 2O ChCl:urea
Rice straw
1:20, 120°C, 3h
24.2
66.8
6.7
21.7
-
-
Rice straw
1:20, 130°C, 4h
4.46
0.87
8.53
0.31
66.2
74.5
ChCl:imidazole
Corncob
1:16, 80°C, 15h
32.8
85.5
15.5
63
30.1
31.6
ChCl:glycerol
Lettuce
1:16, 150°C, 16h
-
94.9
-
75
-
-
ChCl:glycerol
Potato peel
1:16, 115°C, 3h
1
41
-
-
-
-
Apple residue
2
76
-
-
-
-
Coffee silverskin Brewer's spent grain Corn stover Corncob
3
29
-
-
-
-
2
34
-
-
-
-
1:20, 130°C, 2h 1:20, 90°C, 24h
22.1*
91.5* 96.4*
-
-
31.1 31.6
57.2 33.1
Wheat straw
1:20, 70°C, 9h
20.9
84.1
8.9
35.9
41.2
53.9
Oil palm trunk
1:19, 100°C, 48h
25*
60*
-
-
-
-
Work
(Pan et al., 2017) (Procentese et al., 2015) (Procentese et al., 2017) (Procentese et al., 2018)
DES
Biomass
ChCl:catechol
(Xu et al., 2016) ChCl:formic acid (Zhang et al., ChCl:glycerol 2016) (Zhao et al., ChCl:monoethanolamine 2018) (Zulkefli et al., Ethylammonium 2017) chloride:ethylene glycol *represents total sugar production.
Glucose yield (%)
Xylose yield (%)
CrI
Untreated biomass
DES-pretreated biomass
Untreated biomass
DES-pretreated biomass
Untreated biomass
DES-pretreated biomass
15.13
102.02
3.15
98.78
54
56-63
49
Table 4 Carbohydrate-based products produced from enzymatic hydrolysate and DES pretreatment liquid fraction Carbohydratebased products
Work
Enzymatic Hydrolysate Ethanol (Kumar et al., 2016) Butanol (Procentese et al., 2017) (Xing et al., 2018) (Xu et al., 2016) Butanediol (Chen, Zhu et al., 2018) Pretreatment liquid fraction (LF) Furfural (Chen, Zhu et al., 2018) Lipid (Dai et al., 2017)
Carotenoid
(Chen, Z. et al., 2018) (Chen, Z. et al., 2018)
DES pretreatment
ChCl:glycerol 1:1 ChCl:glycerol 1:2, 150°C, 16h NaCO3, 1h, 140°C; ChCl:formic acid:acetic acid 1:1:1, 130°C, 2h ChCl:formic acid, 130°C, 2h ChCl:ethylene glycol 1:2, acidified with 1.0wt% H2SO4, 130°C, 0.5h ChCl:ethylene glycol 1:2, acidified with 1.0wt% H2SO4, 130°C, 0.5h Biological pretreatment, 30°C, 3day; ChCl:oxalic acid 1:2, 120°C, 1.5h ChCl:glycerol 1:2, 120°C, 1h ChCl:glycerol 1:2, 120°C, 1h
Biomass source
Fermentation strain
Fermentation duration (hour)
Conversion efficiency (%)
Product concentration (g/L)
Product yielda (g/gsugar)
Rice straw Lettuce leave
NRRL Y-50464 DSMZ 792
24 60
89.5 -
22.6 0.5
0.04 (0.1)
Rice straw
DSM 13864
72
93
9.5
0.25 (0.36)
Corn stover Switchgrass
DSM 13864 NRRL B-14891
48 36
70.5 89.7
5.63 90.2
0.17 (0.21)
Switchgrass
N.A.b
N.A.
84.5
12.03
-
Bamboo shoot shell Switchgrass
CCZU11-1
72
97
2.2
-
NRRL Y-1091
144
-
8.1
-
Switchgrass
NRRL Y-1091
144
-
0.015
-
a
Figure in parenthesis are the total product yield including butanol and other types of fermentation products in the ABE fermentation which are acetone and ethanol. b Not applicable. The production does not involve fermentation process.
50
Highlights 1. Application of DES in raw lignocellulosic biomass fractionation and utilization. 2. Categorization of DES constituents used in lignocellulosic biopolymer fractionation. 3. Enhanced pretreatment efficiency through integration of DES with existing technology. 4. Downstream conversion of DES-fractionated products to a variety of bio-products. 5. Challenges and outlooks for DES utilization in biomass processing.
51
Graphical abstract
Introduction of deep eutectic solvents (DES) Biomass fractionation using DES
Effect of different hydrogen bond donor (HBD)
Effect of different hydrogen bond acceptor (HBA)
Ternary DES
Integration of DES pretreatment with other technologies
Microwave
Ultrasonication
Sequential
Acidified DES
Application of DES-fractionated products
Polysaccharides
DES-pretreated solid fraction
Enzymatic hydrolysate
Lignin
Pretreatment liquid fraction
Challenges and outlook of DES utilization
52
Author Contribution Statement Yee Tong Tan – Conceptualization, Writing - Original Draft; Adeline Seak May Chua – Writing - Review & Editing, Supervision; Gek Cheng Ngoh – Writing - Review & Editing, Supervision, Funding acquisition
53
S0960-8524(19)31752-3 https://doi.org/10.1016/j.biortech.2019.122522 BITE 122522
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
4 October 2019 26 November 2019 27 November 2019
Please cite this article as: Tan, Y.T., Chua, A.S.M., Ngoh, G.C., Deep eutectic solvent for lignocellulosic biomass fractionation and the subsequent conversion to bio-based products – A review, Bioresource Technology (2019), doi: https://doi.org/10.1016/j.biortech.2019.122522
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Elsevier Ltd. All rights reserved.
Title: Deep eutectic solvent for lignocellulosic biomass fractionation and the subsequent conversion to bio-based products – A review
Yee Tong Tan, Adeline Seak May Chua, Gek Cheng Ngoh*
Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia.
*Corresponding author Tel.: +60 3 79675301; fax: +60 3 79675371 Email: [email protected] (Gek Cheng Ngoh)
Other authors’ e-mail address: [email protected] [email protected]
Present and permanent address: Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia.
1
Abstract
2
Since the introduction of deep eutectic solvent (DES) in biomass processing field, the
3
efficiency of DES in lignocellulosic biopolymer model compounds’ (cellulose,
4
hemicellulose and lignin) solubilisation and conversion was widely recognized.
5
Nevertheless, DES’s potential for biorefinery application can be reflected more
6
accurately through their performance in raw lignocellulosic biomass processing rather
7
than model compound conversion. Therefore, this review examines the studies on raw
8
lignocellulosic biomass fractionation using DES and the subsequent conversion of DES-
9
fractionated products into bio-based products. The review stresses on three key parts:
10
performance of varying types of DESs and pretreatment schemes for biopolymer
11
fractionation, properties and conversion of fractionated saccharides as well as DES-
12
extracted lignin. The prospects and challenges of DES implementation in biomass
13
processing will also be discussed. This review provides a front-to-end view on the
14
DES’s performance, starting from pretreatment to DES-fractionated products
15
conversion, which would be helpful in devising a comprehensive biomass utilization
16
process.
17
Keywords: biopolymer fractionation; carbohydrate conversion; lignin conversion;
18
biorefinery; green technology
19 20 21
1
Introduction It is inevitable to apply solvents in industrial processes due to their required role
22
in dissolving solutes, promoting mass and heat transfer, facilitating separation and
23
purification processes and so on (Cvjetko Bubalo et al., 2015). Solvents are usually
24
utilized in large quantities particularly when different types of solvents are required to
1
25
accommodate various processes. Hence, the choice of solvent greatly influences the cost,
26
safety and environmental performance of a process.
27
In tandem with the twelve principles of Green Chemistry as introduced by
28
Anastas and Eghbali (2010), innovative designer solvents have been proposed as
29
alternatives to conventional molecular solvent. For instance, deep eutectic solvent
30
(DES), a neoteric green solvent, was introduced in the last decade by Abbott et al.
31
(2003). This solvent is derived from the hydrogen bonding formation between two or
32
more constituents. The key properties of DES are their low vapour pressure, high
33
tunability of their physicochemical properties and biodegradability based on their
34
starting constituents. Ever since its introduction, DES has attracted increasing attention
35
in various fields such as electrochemistry, separation, catalysis or extraction as
36
described in other reviews (García et al., 2015; Smith et al., 2014; Zhang et al., 2012).
37
Ability of DES in solubilizing lignocellulosic biomass was first demonstrated by
38
Francisco et al. (2012). DESs have displayed their versatility on biomass processing
39
such as biopolymer dissolution, phenolic compound extraction, carbohydrate product
40
conversion and biodiesel purification. Lignocellulosic biomass are generally more
41
recalcitrant towards modification than sugar-based biomass. Thus, the technological
42
advances of the lignocellulosic biomass processing are comparatively less developed.
43
Commercial production of lignocellulosic biomass-derived products is very limited and
44
an effective biorefinery scheme is yet to be realized. It is thus very important to keep
45
track of the current research and technological development in this field, which includes
46
the application of DES in lignocellulosic biomass fractionation and conversion.
47 48
To date, many literatures have investigated the conversion of lignocellulosic biopolymer model compounds into their building blocks or bio-based products using
2
49
DES (Vigier et al., 2015; Zdanowicz et al., 2018). Nevertheless, the performance of
50
DES on the biopolymer compound conversion cannot truly reflect their efficiency on
51
the actual lignocellulosic biopolymer treatment. In biorefinery, these biopolymers have
52
to be first isolated from biomass matrix in order to be processed separately. Thus, this
53
review paper will focus on case studies investigating the pretreatment of entire biomass
54
using DES, the products conversion following DES pretreatment and the challenges and
55
outlooks towards DES utilization in industrial scale biomass processing. In this paper, DESs will be categorized based on their varying hydrogen bond
56 57
donors (HBD) and hydrogen bond acceptors (HBA) constituents, as well as ternary DES.
58
The effect of these diverse DES constituents on raw lignocellulosic biomass
59
fractionation efficiency will be reviewed. In addition, the research efforts in improving
60
DES’s efficiency through combining DES pretreatment with other existing process
61
intensification technologies are also included, namely microwave, ultrasonication,
62
sequential pretreatment and co-solvent. Subsequently, the application of DES-
63
fractionated biopolymers (saccharides and lignin) for product conversion are discussed.
64
Complete utilization of biomass is always desired for generation of fuels and chemicals
65
in biorefinery. This review will provide an insight into the current lignocellulosic
66
biomass processing and utilization strategies using DES as pretreatment medium.
67
1.1
68
What is deep eutectic solvent (DES)? Eutectic mixture refers to a homogeneous mixture of two or more constituents at
69
certain composition ratio, which melts and freezes at a temperature lower than the
70
melting points of its constituents. The introduction of molten salts (or fused salts)
71
eutectic mixtures, such as ammonium salts and metal salts, can be dated way back in
72
1914 (Abbott et al., 2003; Welton, 1999). Molten salts that present as liquids at or
3
73
below room temperature are later more commonly known as room temperature ionic
74
liquids (Welton, 1999). Recently, Abbott et al. (2003) introduced another variation of
75
room temperature eutectic mixture, comprising of choline chloride and urea at a
76
freezing temperature of 12°C. Other than the first experimented pair of constituents, this
77
type of solvent is found synthesizable using the combinations of various quaternary
78
ammonium salt (quaternary ammonium cation pairing with monovalent anion) with
79
amides (Abbott et al., 2003). The formation of eutectic mixture is attributed to the
80
charge delocalization between halide anion and amide through hydrogen bonding
81
(Abbott, Andrew P. et al., 2004a). To differentiate them from ionic liquids which
82
consist only of ionic species, this class of solvent is named as deep eutectic solvent
83
(DES) (Abbott, Andrew P. et al., 2004a). The synthesis of DES can be described by a
84
general formula developed by Smith et al. (2014):
85
Cat + X ― zY
86
Where Cat+ = ammonium, phosphonium, sulfonium cation; X– = Lewis base anion; Y =
87
Lewis or Bronsted acid at z number of molecules interacting with anion
88
As the formation of DES is built on hydrogen bonding, a wide range of
89
constituents capable of donating or accepting protons or electrons are suitable for DES
90
synthesis. Generally, DES can be categorized into four main types as shown in Table 1
91
with their respective application. Type 1 to 3 DESs are differentiated according to the
92
Lewis/Bronsted acid (Y) used to complex the Lewis base (X–, normally halide anion).
93
The anionic species X– selected must have the ability to act as hydrogen bond acceptor
94
(Abbott, Andrew P. et al., 2004b). Type 1 and 2 DESs are those with non-hydrated and
95
hydrated metal halides as Lewis acid, respectively; whereas type 3 DESs are formed by
96
using hydrogen bond donor (HBD), such as amides, amines, alcohols, sugar alcohols
4
97
and acids. Type 4 DESs are synthesized from metal salt and HBDs such as amide and
98
polyol, for instance ZnCl2 with urea. Not every HBD can form eutectic with metal salt
99
as they do with quaternary ammonium salt in type 3 DES, for example carboxylic acid
100
was unable to interact with ZnCl2 (Abbott et al., 2007).
101
Type 3 DES, which is the earliest introduced DES and arguably the greenest
102
among all, receives the most attention. Most in-depth properties studies usually revolve
103
around this type of DES (Florindo et al., 2014; García et al., 2015; Teles et al., 2017;
104
Zhang et al., 2012). The use of type 3 DES is especially encouraging, as the DES
105
constituents are mostly organic, biodegradable and some are even of food grade! The
106
synthesis process of this DES involves that combination of at least one hydrogen bond
107
donor (HBD) and hydrogen bond acceptor (HBA) to form homogeneous liquid at
108
ambient condition. Huge array of naturally existing HBD constituents such as acids,
109
alcohol, amides and others are suitable to synthesize this type of DES. Many of these
110
components are also metabolites in living organisms. Choi et al. (2011) postulated that
111
these metabolites form a third type of liquids in cells apart from lipids and water, which
112
they named as natural deep eutectic solvent (NADES). Synthesizing DES using
113
naturally occurring components is highly encouraged, as many conventional solvents
114
would pose severe hazard concern towards living organisms. Notably the application of
115
DES are well received in pharmaceutical and medical fields (Aroso et al., 2015; Mota-
116
Morales et al., 2013; Zainal-Abidin et al., 2019).
117
The pursuit of sustainability industry is a pressing matter considering the fast-
118
deteriorating environment and dwindling non-renewable resource reserves. The use of
119
green technology in renewable non-food feedstock (lignocellulosic biomass) conversion
120
to bio-based products is a constant research effort towards sustainable living. In this
5
121
regard, the application of DES in lignocellulosic biomass processing was introduced in
122
a timely matter. To date, type 3 DESs are preferred over other types of DESs in biomass
123
processing due to the availability and environmental friendliness of the constituents.
124
Hereinafter, all DES terms refer to type 3 DES, unless mentioned otherwise.
125
2
126
Lignocellulosic biomass fractionation using deep eutectic solvents (DES) Lignocellulosic biomass is the most abundant plant material on Earth (Brandt et
127
al., 2013). It is a composite material consisting mainly of three types of biopolymers:
128
cellulose, hemicellulose and lignin. Composition of these biopolymers varies with the
129
biomass species. Lignocellulosic biomass can be found in numerous forms such as
130
forest residue, agricultural waste, dedicated lignocellulosic crops and municipal wastes
131
like food and paper waste. These biomass can be used as feedstock for various bio-
132
products as well as second generation biofuel production (Rastogi and Shrivastava,
133
2017). Second generation biofuels are viewed as the solution to address many
134
sustainable issues, attributing to the non-food nature of the feedstock and their surplus
135
amount available on Earth. Food and fuel sources can be cultivated simultaneously by
136
applying agricultural and food waste (e.g. rice and wheat straw, oil palm solid waste or
137
fruit peels) as biorefinery feedstock (Brandt et al., 2013).
138
Research into biomass processing using DES was inspired by the successful
139
application of ionic liquids (IL) in biomass processing (Francisco et al., 2012). The
140
analogues of DES, ILs were found to be effective at milder conditions as opposed to the
141
effectiveness of conventional solvents at more extreme conditions (Sun et al., 2011).
142
However, ILs are costly due to the demanding synthesis process (Cvjetko Bubalo et al.,
143
2015). DESs are adopted as a cheaper alternative to ILs. Majority of DESs display high
144
solubility for lignin and poor to negligible solubility for cellulose and hemicellulose
6
145
(Francisco et al., 2012). Nevetheless for certain DESs, for example glycine:malic acid
146
and choline chloride:malic acid, considerably high cellulose solubility was achieved at
147
7.7% and 5.9%, respectively. This indicates that the selected DES constituents might
148
affect the dissolution tendency of DES to a great extent. This early work by Francisco et
149
al. (2012) sparks immediate research interest in the application of DES in biomass
150
processing due to the high selectivity for biopolymer solubilisation. In this section, performance of various DES pretreatment for lignocellulosic
151 152
biomass fractionation into their individual biopolymers (i.e. cellulose, hemicellulose
153
and lignin) are reviewed. The performance of DES will be evaluated from two main
154
perspectives, namely effect of DES constituents and process integration with other
155
intensification technologies.
156
2.1
Variation of DES The pretreatment conditions such as temperature, time or solid to liquid ratio
157 158
could affect the solvent performance, with DES type usually having the most influence
159
over the DES performance. As mentioned previously, there are a wide range of
160
constituents suitable for DES synthesis. The constituents used would determine the
161
nature of the solvent synthesized, and their behaviour in biomass pretreatment. The
162
biopolymer fractionation performance of different types of DESs will be discussed
163
based on their hydrogen bond donor (HBD), hydrogen bond acceptor (HBA) and
164
subsequently the performance of ternary DES with three constituents.
165
2.1.1
166
Categories of hydrogen bond donor (HBD) In lignocellulosic biomass pretreatment using DESs, HBDs that have been
167
investigated include carbohydrate, acid, polyalcohol, amide and phenolic compound.
168
Table 2 tabulates the DES pretreatment conditions and their performance from selected
7
169
reported works using different types of HBDs. The performance of DESs vary with the
170
categories of HBDs, as well as different types of compound within the same category.
171
a)
Carbohydrate-based DES Carbohydrate-based DES is constituted of sugar compound as HBD such as
172 173
glucose, fructose, xylitol and so on. This type of DES usually exhibits near-neutral pH
174
condition when pairing with the most common HBA, choline chloride (ChCl). In spite
175
of its mild nature, ChCl:fructose DES was able to dissolve rice straw biomass powder
176
up to 0.65 wt% (6.5 mg per g of DES) (Florindo et al., 2017). In comparison with
177
aldoses (i.e. ribose, glucose, xylose and mannose), the fructose-based DES (ketose-
178
based sugar) with less viscosity demonstrated higher dissolution ability. Even though
179
carbohydrate-based DESs can dissolve biomass, their dissolution efficiencies are far
180
lower than that of the other categories of DESs (Francisco et al., 2012). Therefore, these
181
sugar-based DESs are not usually applied in lignocellulosic biomass processing. As
182
these DESs are constructed from environmentally benign constituents, they found
183
application in other fields such as food or pharmaceutical processing (Dai et al., 2014;
184
Liew et al., 2018).
185
b)
186
Polyalcohol-based DES Application of polyalcohol-based DES in biomass pretreatment has been widely
187
investigated. Most works reported using this type of DES was targeted to increase
188
enzyme accessibility in polysaccharides conversion due to high enzyme stability in
189
polyalcohol-based DES, further details of which will be discussed in the next section.
190
The most popular polyalcohol HBDs include glycerol and ethylene glycol. Multiple
191
works reported that ChCl:glycerol was inefficient in biomass dissolution and
192
fractionation, even under elevated temperature condition (Alvarez-Vasco et al., 2016;
8
193
Chen, Z. et al., 2018; Tan et al., 2018; Xia et al., 2018). It was deduced that its extensive
194
hydrogen bonding network between HBD and HBA weakened the ability of HBA to
195
compete with the intra-molecular bonding in biopolymer matrix (Xia et al., 2018). Some
196
vast difference in findings on the performance of polyalcohol-based DES was reported.
197
When Zulkefli et al. (2017) reported that ethylene glycol-DES and glycerol-DES
198
achieved 36% and 49% of lignin removal from oil palm trunk, nearly twice the
199
extraction amount at 71% and 88% were obtained by Zhang et al. (2016) from corncob
200
biomass using the same DESs. Factors contributing to the discrepancy in the results
201
could be the varying recalcitrance degree in different biomass types, effect of different
202
HBAs or pretreatment conditions employed (Table 2). Higher degree of hydrophobicity in polyalcohol-based DES seems to facilitate
203 204
biopolymer fractionation. When comparing between different polyalcohol HBDs in
205
DESs, lignin removal increased in the order of ethylene glycol > 1,2-propanediol > 1,3-
206
propanediol (Hou, X.D. et al., 2018). The authors attributed this trend to increasing
207
hydrophobicity that facilitated the extraction of lignin with hydrophobic
208
phenylpropanoid structures. In another work, the dissolution capacity of ethylene
209
glycol-DES was higher than that of glycerol-DES in oil palm trunk dissolution (Zulkefli
210
et al., 2017). Ethylene glycol was reported to exhibit hydrophobic effect unlike that of
211
glycerol (Koga, 2003). Dedicated studies regarding the effect of hydrophobicity of DES
212
solvent on biopolymer fractionation is essential.
213
c)
214
Acid-based DES Other than polyalcohol-based DES, acid-based DES is also widely investigated.
215
The acid-based DESs applied in biomass processing are often composed of organic acid,
216
taking advantage of their biodegradable nature and availability in natural products. By
9
217
associating with a compatible HBA, the acid-based DESs perform better in biomass
218
pretreatment than their respective acids. As evident in the literatures, lignin and xylan
219
removal were enhanced by 30% when acid-based DESs were utilized instead of the
220
respective organic acids (Tan et al., 2019; Yu et al., 2018).
221
Acid-based DESs are generally more efficient in lignin and xylan removal than
222
other groups of DESs (Table 2). Most acid-based DESs had excellent performance in
223
xylan hydrolysis when pretreatment temperature was over 100°C (Alvarez-Vasco et al.,
224
2016; Yu et al., 2018). While lignin and xylose were solubilized into DES, the major
225
portion of cellulose was usually retained in the DES-pretreated biomass solid, as shown
226
in multiple studies (Table 2). Formic acid was reported to be able to recover up to 98%
227
(Yu et al., 2018) and 86% (Tan et al., 2019) of glucose in the pretreated herbal residue
228
and oil palm biomass, respectively. Even though large percentage of cellulose is
229
retained in biomass solid, DES pretreatment can certainly bring some changes to the
230
structure and properties of the pretreated biomass. These changes can affect the
231
efficiency of the downstream processing, which will be explained later in this review.
232
Some acid-based DES perform better than the others and thus investigation on
233
the effect of functional groups in the acids is important to determine the most suitable
234
acid HBD for biopolymer fractionation. The presence of different functional groups at
235
varying amount have significant impact on the DES fractionation efficiency. Acid HBD
236
with more than one carboxyl groups (diacid and triacid) could lower DES fractionation
237
efficiency as compared with that of monoacid HBD (Tan et al., 2019; Zhang et al.,
238
2016). Aside from the main functional groups in acid (i.e. carboxyl group), other
239
chemical groups present in acid influence the DES performance as well, for example the
240
presence of electron-donating or electron withdrawing groups.
10
241
Lignin extraction efficiency decreased in the order of ChCl:formic acid (62%) >
242
ChCl:acetic acid (27%) > ChCl:propionic acid (20%) > ChCl:butyric acid (14%) (Tan et
243
al., 2019). The authors attributed the trend to the electron-donating properties of alkyl
244
group to oxygen, which intensified the hydrogen bond strength in the hydroxyl group of
245
acid. This constituted to weaker acid with reduced acid ionization strength. Thus, the
246
interaction between DES solvent and biopolymer solute would be weakened when there
247
was a longer alkyl chain in HBD. In addition, levulinic acid-DES, a keto acid showed
248
poorer performance than other acid based-DESs such as lactic acid-, acetic acid-, formic
249
acid-, and glycolic acid-DES (Alvarez-Vasco et al., 2016; Yu et al., 2018). The
250
collective results indicated the negative effect of alkyl group that imposed on biomass
251
fractionation. On the other hand, another research group demonstrated that the presence
252
of a strong electron-withdrawing group i.e. chloride Cl– in acid was favourable for
253
hemicellulose hydrolysis (Hou, X.D. et al., 2018). As reported, a chlorine-containing
254
acid, 2-chloropropionic acid-DES had much greater xylan removal efficiency when
255
compared with OH-containing lactic acid-DES (54% and 19%) at 80°C.
256
Furthermore, the presence of hydroxyl group in alpha hydroxyl acid (AHA)
257
DES is also preferred for biopolymer fractionation. When compared to linear saturated
258
acid-DES with similar aliphatic chain length, AHA-DES performed better in lignin
259
extraction (Tan et al., 2019; Yu et al., 2018). The examples are; 24% higher lignin yield
260
difference in the case of ChCl:glycolic acid pretreatment than that of ChCl:acetic acid
261
(C2 acid) (Yu et al., 2018), 13% more by ChCl:lactic acid than by ChCl:propionic acid
262
(C3 acid) as well as 12% greater by ChCl:malic acid as compared to using
263
ChCl:succinic acid (C4 acid) (Tan et al., 2019). OH group in AHA-DES could increase
264
the polarity of DES, hence facilitating the hydrogen bond interaction with the biomass
11
265
matrix. However, the performance of glycerol-DES with three OH groups is less ideal
266
as previously discussed (Section 2.1.1b). Despite both bearing OH groups, polyalcohol-
267
DES and AHA-DES have other functional groups such as alkyl or carboxyl group that
268
would bring different properties to the solvents. More investigation on the synergistic
269
effect of the different functional groups with biopolymer solutes is therefore crucial.
270
d)
271
Amine- and amide-based DES Amine- and amide-based DESs are less investigated as compared to
272
polyalcohol- and acid-based DES although they were one of the first introduced DES
273
types. These basic DESs have moderate biopolymer fractionation efficiency. It was
274
demonstrated that lignin and xylan removal by ChCl:urea from different biomass
275
sources (rice straw and oil palm empty fruit bunch) were respectively around 30% and
276
20% (Pan et al., 2017; Tan et al., 2018). Even when pairing with lactic acid as another
277
DES constituent, urea-DES was ineffective in fractionation (Hou, X.D. et al., 2018),
278
probably due to the modest pH value, as described in the next case study.
279
Zhao et al. (2018) evaluated the wheat straw fractionation efficiency of various
280
amine- (monoethanolamine, diethanolamine and methyldiethanolamine) and amide-
281
based (acetamide and urea) DES. All three ethanolamines outperformed acetamide and
282
urea in biopolymer fractionation. The authors attributed the good performance of amine-
283
based DESs to their strong basicity (pH 10.4, 10.5 and 10.9) than amide-based DES (pH
284
7.3 and 8.2). Among the amine-based DES, monoethanolamine-DES topped at 81% and
285
47% of lignin and xylan removal, respectively. Additional hydroxyethyl group in
286
diethanolamine and methyldiethanolamine could have increased steric hindrance in the
287
solvent, hence reducing the fractionation efficiency. Notably, similar to acid-based DES,
12
288
methyldiethanolamine which has an alkyl group (methyl) substitution had lower
289
fractionation efficiency than diethanolamine.
290
e)
Phenolic compound-based DES
291
A few studies on the performance of phenolic compound-based DES was
292
reported. Hence, insufficient data is available to draw any hypothesis to the pretreatment
293
tendency of this DES type. Ten different phenolic compounds which could be derived
294
from lignin were screened for DES formation (Kim et al., 2018). Among them, four
295
compounds formed homogeneous solvent with ChCl, namely 4-hydroxybenzyl alcohol,
296
catechol, vanillin, p-coumaric acid (PCA). ChCl:PCA achieved the highest lignin and
297
xylan removal from switchgrass at 61% and 71%, followed by ChCl:vanillin at 53%
298
and 50%. Acidic properties arising from the presence of COOH group in PCA could
299
contribute to its high fractionation efficiency. In another work, three types of phenolic
300
compounds, namely phenol, alpha-naphthol and resorcinol were evaluated as HBD in
301
DES (Malaeke et al., 2018). With the aid of ultrasonication, resorcinol-DES achieved
302
33wt% solubility in wheat straw dissolution, which was close to the lignin composition
303
in the biomass. The researchers also hypothesized that the phenyl group in HBD was
304
desirable for lignin dissolution and the solubilisation performance decreased with the
305
presence of two phenyl groups.
306
f)
307
Cross comparison between different HBD categories Some research groups have performed cross comparison on different categories
308
of HBD for biomass fractionation. In general, DES with acid HBD has best
309
fractionation efficiency among other HBD groups. In a study in which DES with
310
various pH were compared, the acidic DES (ChCl:lactic acid and lactic acid:glucose)
311
demonstrated the greatest fractionation efficiency, followed by the basic DES
13
312
(ChCl:urea and potassium carbonate:glycerol) then the near-neutral DES (ChCl:glycerol
313
and ChCl:glucose) (Tan et al., 2018). Liu, Y. et al. (2017) reported that oxalic acid with
314
higher hydrogen bond acidity than urea and glycerol, could break down the lignin-
315
carbohydrate complex in biomass matrix more efficiently when it was used as HBD in
316
DES. Besides, basic DESs (ChCl:urea and ChCl:imidazole) also had the tendency to
317
dissolve more biopolymer than glycerol-DES (Procentese et al., 2015). In regards to
318
polyalcohol-based DES’s performance, different researchers have reached the
319
consensus in which the multiple OH in polyalcohol could restrict the extraction
320
performance (Guo et al., 2018; Hou, X.D. et al., 2018). Subsequent efforts have been
321
put in by some to increase the performance of polyalcohol-based DES via modifying the
322
constitution of the solvent by acidifying the solvent or adding third DES constituent.
323
Relevant elaboration will be presented in the Section 2.1.3.
324
2.1.2
325
Categories of hydrogen bond acceptor (HBA) Apart from HBD, HBA constituent can also impact on DES’s performance. The
326
most commonly used HBA is choline chloride (ChCl) (C5H14ClNO), a quaternary
327
ammonium salt with choline cation and chloride anion. Most studies adopted ChCl as
328
HBA in their investigation due to the benign properties of this compound as well as its
329
affordable pricing. The effect of HBA on DES biopolymer fractionation efficiency is
330
less investigated as compared to HBD. Choline, also known as vitamin B4, is
331
synthesized through metabolism process in our body and obtained through diets. It is a
332
nutrient necessary for the formation of metabolites and thus it is widely used as animal
333
feed additive for growth promotion, or sometimes as human’s vitamin supplement.
334 335
Florindo et al. (2017) investigated on the biomass dissolution capacity of three different HBAs in carbohydrate-based DESs, namely ChCl, acetylcholine chloride
14
336
(C7H16ClNO2, AC) and benzyldimethyl(2-hydroxyethyl)ammonium chloride
337
(C11H18ClNO, BAC). The dissolution percentage of DES decreased in the order of
338
ChCl-DES > AC-DES > BAC-DES. Regardless of the HBA used, DESs with fructose
339
as HBD could dissolve the highest amount of biomass (50-65 mg per g of DES). Even
340
though the use of suitable HBA used could increase the dissolution efficiency, HBD
341
was the deciding factor of the DES performance in this case study. In another work on
342
dissolution of oil palm trunk biomass, ethylammonium chloride (C₂H₈ClN, EAC)
343
performed better than ChCl as HBA (Zulkefli et al., 2017). Furthermore, the selectivity
344
of both HBAs was different in biopolymer fractionation such that ChCl was more
345
effective in delignification while EAC was more suitable for xylan removal. The
346
authors stated that EAC as a primary ammonium salt is less bulky in structure than
347
ChCl with quaternary structure. This resulted in the former HBA solubilizes biopolymer
348
better due to less steric hindrance. The same theory can also be applied to the previous
349
case involving the AC and BAC HBAs as their larger structure as compared to that of
350
ChCl led to poorer performance in biomass dissolution.
351
Since chemical compound consists of various functional groups (both electron-
352
withdrawing and electron-donating), some HBDs were used interchangeably as HBA in
353
some reported works; for instance lactic acid (Hou, X.D. et al., 2018) and sucrose (Yiin
354
et al., 2016). When lactic acid was used as HBA, the DES’s dissolution efficiency was
355
greater than that of ChCl-based DES, regardless of the HBD employed. This implies
356
versatile combinations of different DESs can be synthesized for various applications. It
357
should be noted that the first step towards DES utilization is to identify the most
358
suitable combinations for any intended purpose.
15
359
2.1.3
Ternary DES
360
The construction of a DES is not limited to using only two constituents. Ternary
361
DES can be synthesized using the same methodology as in regular two-component DES
362
synthesis by mixing three compatible constituents (Dai et al., 2013; Liu et al., 2014). It
363
was reported that addition of alcohol as the third component to ChCl:oxalic acid DES
364
could enhance the delignification of different biomasses (rice husk, wheat straw, rice
365
straw) (Kandanelli et al., 2018). ChCl:oxalic acid:butanol and ChCl:oxalic
366
acid:propanol achieved 49% and 41% lignin removal as compared with the regular
367
ChCl:oxalic acid DES performance at 23-31%. The authors also exhibited that not every
368
ternary DES had better performance. DES containing ethyl acetate as the ternary
369
component showed a 15% decreament in delignification efficiency. The ratio of DES to
370
alcohol in the ternary DES is also an important determining factor of the pretreatment
371
efficiency. While DES:butanol with the ratio of 2:1 achieved 49% delignification,
372
ternary DES with the ratio of 1:2 could only achieve 18%.
373
As mentioned previously, ChCl:glycerol was not effective in biopolymer
374
fractionation. Inspired by type 4 DES, Xia et al. (2018) synthesized a ternary DES
375
system by combining ChCl:glycerol with seven different types of metal chlorides
376
(hydrates). Upon addition of AlCl3·6H2O, the α and β solvatochromic parameters of
377
DES increased from 0.77 to 1.99 and from 0.48 to 0.68, respectively, which indicated
378
improved H-bond donating and accepting capability. Based on density functional theory
379
calculation, the inefficiency of ChCl:glycerol in biomass fractionation was due to weak
380
HBA which had less ability to compete with intra-molecular H-bond in biomass. The
381
Cl– ion was held tightly by glycerol and insufficient protons were available to cleave the
382
H-bonds. In response to that, the metal chloride in the ternary DES would function as
16
383
anion donor and acidic site holder to break the lignocellulosic bond structure. After
384
pretreatment at 120°C, the biomass residue recovery and lignin extraction using
385
ChCl:glycerol was 90% and 0.04%. The results had been improved to 42% and 96%,
386
respectively with the use of ternary DES. Comparatively the lignin extraction efficiency
387
of the corresponding type 4 DES (AlCl3·6H2O and glycerol) was at a modest 64%. Up till now, there is no definite answer as to which DES is best for biopolymer
388 389
fractionation. Every DES constituent affects the solvent’s performance and the
390
synergistic effect of the constituents has added further impact. The most exciting
391
finding in our opinion is, by changing the HBD or HBA constituent, the fractionation
392
selectivity of DES could be controlled. To date, the research scopes are widely-
393
dispersed in regards of the types of DES used. More rigorous and in-depth studies in
394
determining the suitable DES constituent are needed to realize the intended purposes,
395
such as lignin extraction or polysaccharides depolymerisation.
396
2.2
397
Integration of DES pretreatment with other technologies A number of different biomass pretreatment methods have been adopted to
398
increase the accessibility to the biopolymers such as chemical, thermal, mechanical,
399
biological pretreatment and so on (Hendriks and Zeeman, 2009). In the conventional
400
pretreatment, different biomass processing technologies are often combined to boost the
401
process efficiency (Menon and Rao, 2012). Likewise, biomass processing schemes
402
involving integration of DES pretreatment with other existing process intensification
403
technologies such as microwave, ultrasound and sequential pretreatment were
404
developed. Also, works have been reported on the addition of acid co-solvents in DES
405
pretreatment processes. In this section, impact of the combined pretreatment strategies
406
and their performance in comparison with sole DES pretreatment will be reviewed.
17
407
2.2.1
Microwave-assisted DES pretreatment
408
Microwave heating, a non-conventional heating method, can increase the
409
cellular pressure in plant through non-ionizing radiation to rupture plant tissue (Liew et
410
al., 2016). Integrating microwave heating into DES pretreatment does not impose
411
chemical changes in biopolymers, but facilitate structural changes in biomass for
412
solvent to interact more efficiently with biopolymers. Microwave irradiation can
413
maximize the ionic character of DESs and increase their molecular polarity, enabling
414
the use of lower pretreatment temperature and duration (Liu, Y. et al., 2017). This
415
integrated technology was first applied by Liu, Y. et al. (2017) for lignin extraction
416
from wood biomass. After subjecting to microwave-assisted ChCl:oxalic acid DES
417
pretreatment for 3 mins at 800 W, 80% of the initial lignin was extracted and only 40%
418
of pretreated solid biomass was recovered. However, pretreatment using conventional
419
heating in oil bath for 9 h at 110°C achieved 90% lignin extraction and 64% pretreated
420
biomass recovery. Evidently, microwave heating required drastically shorter
421
pretreatment time; at mere 3 mins as compared with 9 h by conventional heating while
422
achieving the similar efficiency level.
423
The efficiency of microwave-assisted DES pretreatment was also reported by
424
another group of researchers who combined acidic ChCl:lactic acid pretreatment with
425
ultrafast 45 second microwave heating at 800 W on three different types of biomass
426
namely switchgrass, corn stover, miscanthus (Chen and Wan, 2018). Xylan removal of
427
the pretreatments ranged between 77-90%, lignin removal 65-80%, and glucan removal
428
4-25% were obtained. In both the works reviewed, the microwave-assisted DES
429
pretreatment were able to produce extracted lignin with high purity at 94-96% in Liu, Y.
430
et al. (2017) and 85-87% in Chen and Wan (2018).
18
431
2.2.2
Ultrasonication-assisted DES pretreatment Other than microwave heating, ultrasonication is another commonly applied
432 433
process intensification technology. Sonication creates cavitation in the reaction mixture.
434
Pressure and temperature increase when the cavitation bubbles collapse near biomass
435
surface, which breaks down the cell wall more efficiently. Similar to microwave,
436
ultrasonication-assisted process improves the efficiency and shorten the treatment
437
duration. When DES pretreatment was integrated with ultrasonication at 90°C for 20
438
mins, 48% of lignin solubility was achieved (Malaeke et al., 2018). The study displayed
439
high lignin selectivity of DES based on the low cellulose and hemicellulose solubility at
440
0.9% to 6.1% even when ultrasonication-assisted pretreatment was applied.
441
2.2.3
Sequential pretreatment Other than microwave- and ultrasonication-assisted processes, DES pretreatment
442 443
has been applied in sequence with other pretreatment modes namely hydrothermal,
444
biological and inorganic salt pretreatment to enhance the biopolymer fractionation
445
efficiency. In addition, sequential DES pretreatment scheme was also developed by a
446
group of researchers, which will also be discussed in this section.
447
a)
448
Hydrothermal+DES pretreatment Hydrothermal pretreatment involves the use of water solvent under high
449
temperature and pressure condition to break down the biomass cell wall (Saha et al.,
450
2013). Fang et al. (2017) employed hydrothermal pretreatment to reduce the
451
recalcitrance of date palm residue prior to DES treatment. In the study, ChCl:glycerol
452
pretreatment was ineffective in removing lignin and xylan, even when the pretreatment
453
time (6 h to 15 h) and molar ratio of HBD to HBA (1:2, 1:3, 1:6) were adjusted. After
454
sequential hydrothermal (200°C, 10 mins) and DES pretreatment (70°C, 6 h), 22% and
19
455
25% of lignin and xylan were removed, respectively. Regardless of the operation modes,
456
cellulose was well reserved in the biomass solid at >90%. The authors reported that
457
hydrothermal pretreatment disrupted the cell wall and broke down lignin and xylan to
458
small fragments. The fragments which might inhibit enzymatic action were
459
subsequently removed by DES, leading to enhancement of hydrolysis performance.
460
b)
Biological+DES pretreatment Application of sequential biological and DES pretreatment can also enhance
461 462
enzymatic accessibility to polysaccharides. Biological pretreatment using Galactomyces
463
sp. CCZU11-1 was carried out on bamboo shoot shell at 30°C for 3 days prior to DES
464
pretreatment (Dai et al., 2017). The authors achieved 77% and 20% of xylan and lignin
465
removal through biological pretreatment and following that, DES pretreatment further
466
reduced the xylan content from 7.4% to 6.1% and lignin from 12.6% to 11.6%. After
467
enzymatic hydrolysis, the total reducing sugar production from the sequential pretreated
468
biomass reached 90%, whereas the biological-pretreated and untreated biomass were
469
respectively at 73% and 42%. The recovered hydrolysate from the enzymatic hydrolysis
470
treatment was successfully employed for microbe lipid production, which will be
471
explained further in the upcoming section (Section 3.2.2).
472
c)
473
DES+inorganic salt pretreatment Inorganic salt pretreatment was found effective in recovering xylose from
474
biomass when applied with oxidative agent (Loow et al., 2017). With the hypothesis
475
that DES could first extract lignin from biomass (based on DES’s high lignin
476
selectivity), which then facilitate selective xylose hydrolysis using inorganic salt, the
477
sequential DES and inorganic salt treatment scheme was introduced (Loow et al., 2018).
478
In this work, ChCl:urea pretreatment (120°C, 4 h) was carried out, followed by CuCl2
20
479
pretreatment (0.4 mol/L concentration, 120°C, 30 mins) to maximize xylose recovery
480
from oil palm frond. 74% xylose recovery was obtained through the sequential
481
pretreatment, as compared with 59% from CuCl2 pretreatment alone.
482
d)
483
DES+DES pretreatment Taking advantage of the varying properties displayed in DESs synthesized from
484
different constituents, Hou et al. (2017) designed a two-stage sequential DES
485
pretreatment scheme to enhance the enzymatic accessibility of rice straw. The xylan and
486
lignin removal of the process varied with the pretreatment order. In malic acid:proline
487
followed by ChCl:urea pretreatment, the total removal rate were 50% for xylan and 73%
488
for lignin. In the reverse sequence, the removal rates were lower at 30% and 60%,
489
respectively. In another combination whereby ChCl:oxalic acid followed by ChCl:urea
490
pretreatment were applied, greater biopolymer removal rates (i.e. 92%, 60%) than the
491
reverse sequence (i.e. 90%, 45%) were imposed. From the two DES combinations,
492
apparently the fractionation efficiency was higher when acidic DES pretreatment was
493
applied prior to basic ChCl:urea. The authors related the trend to the ability of acidic
494
DES to swell and loosen the biomass structural linkages, which promoted the high
495
biopolymer solubility in ChCl:urea. In the subsequent enzymatic hydrolysis process of
496
the sequential pretreated rice straw, an optimum glucose yield of 90% was achieved
497
whereas in the single DES-pretreated biomass only obtained 47-73% yield. Interestingly,
498
when both the acidic and basic DESs were combined in a single step operation, the
499
pretreatment was reported to be ineffective as the biomass composition remained
500
similar to that of untreated biomass.
21
501
2.2.4
Acidified DES pretreatment Other than ternary components as described in Section 2.1.3, researchers had
502 503
devised a novel way of increasing the efficiency of ChCl:glycerol by acidifying the
504
solvent using H2SO4 (Chen, Z. et al., 2018). When 0.9 wt% of acid was added to the
505
DES, solid recovery of the pretreated biomass reached 47.2%, which marked a huge
506
decrement to 89.4% in neat ChCl:glycerol pretreatment. Xylan and lignin removal from
507
acidified DES pretreatment were close to 80% whereas in neat DES pretreatment the
508
removal rates were only 7% and 18%, respectively. Due to high biopolymer removal
509
efficiency, glucan content in acidified DES-pretreated biomass was enriched to 64% and
510
the glucan to glucose conversion reached nearly 100% after enzymatic hydrolysis. In
511
comparison, the neat DES pretreated biomass achieved only 11.8% sugar yield. The
512
authors later adopted the pretreatment liquid stream as substrate for lipid production, as
513
illustrated in the coming section (Section 3.3).
514
3
515
DES-extracted polysaccharides upgrading The properties of the DES-fractionated products are altered from their respective
516
native state in the biomass. The remaining DES pretreated biomass solid fraction (SF)
517
would normally be enriched with cellulose content while in most cases, the labile
518
hemicellulose would be extracted into the pretreatment liquid fraction (LF) (Table 2). In
519
this section, the discussion focuses on the properties of DES-fractionated saccharide
520
streams and their successful conversion to carbohydrate-based products. Three main
521
carbohydrate substrate streams namely DES-pretreated solid fraction (SF), enzymatic
522
hydrolysate produced from enzymatic hydrolysis of SF and lastly DES pretreatment
523
liquid fraction (LF) will be discussed. Some research groups had even developed full
524
biomass utilization schemes by transforming each fractionated biopolymers into
22
525
intermediate bio-materials and bio-products (Chen, Zhu et al., 2018; Chen, Z. et al.,
526
2018; Liu, Y. et al., 2017).
527
3.1
DES-pretreated solid fraction (SF) conversion
528
Understanding on the SF’s properties is essential to identify the possible
529
downstream products. The properties of SF vary depending on the pretreatment
530
condition. Cellulose has low solubility in DES and therefore the DES-pretreated
531
biomass solid fraction (SF) is usually comprised largely of cellulose. So far, many DES
532
pretreatment studies on lignocellulosic biomass processing focus on the fermentation
533
sugar production from SF. Table 3 compiles that sugar production yield from enzymatic
534
hydrolysis process of both untreated and DES-pretreated biomass for comparison
535
purpose. The effect of the structural, surface and thermal properties, and also the
536
application of SF for bio-products conversion will be presented accordingly.
537
3.1.1
538
a)
539
Properties of SF Crystal structure and morphology Crystallinity index (CrI) can be measured using X-ray diffraction (XRD) to
540
reveal the solid structure of SF (Table 3). High CrI indicates higher crystal content in
541
the biomass solid. Majority of the studies reported on increased CrI in the SF after DES
542
pretreatment (Chen, Z. et al., 2018; Dai et al., 2017; Fang et al., 2017; Loow et al., 2018;
543
Yiin et al., 2017). The increment is due to removal of amorphous content in biomass
544
such as hemicellose and lignin, which in turn increasing the relative cellulose content in
545
SF. Liu, Y. et al. (2017) reported that SF remained as crystal type I which had high
546
crystallinity after DES pretreatment. Hou, X.-D. et al. (2018) discovered that CrI was
547
closely related to the severity of pretreatment conditions. Using ChCl:oxalic acid at
548
molar ratio of 2:1, the CrI increased from 55.8% to 59.6%. When the similar DES with
23
549
1:2 ratio was used, the CrI dropped to 55.0%, which was lower than that of the
550
untreated biomass. Using confocal laser scanning microscopy, xylan removal was
551
observed at vascular bundle of the biomass in the latter case. In contrast, removal of
552
recalcitrant lignin at secondary cell wall was not observed in the biomass pretreated by
553
ChCl:oxalic acid 2:1. The work verified that when acid content in DES was increased,
554
deconstruction ability of DES was greater to the extent of depolymerizing biopolymers
555
at the more recalcitrant layer that were normally unaffected under mild condition.
556
b)
Structural analysis Molecular structure of SF is not usually investigated, probably due to the known
557 558
knowledge that it is composed mainly of cellulose, a homogeneous polymer. Kim et al.
559
(2018) confirmed that the biopolymer composition can reflect on the structural changes
560
of SF. ChCl:4-Hydroxybenzyl alcohol DES which had limited fractionation efficiency
561
produced SF with unaltered carbohydrates signals in NMR spectrum when compared
562
with that of untreated biomass. Also, SF with more compact structure can be produced
563
from DES pretreatment. DES-pretreated pine biomass (0.39 g/cm3) was 35% denser
564
than the untreated biomass (0.29 g/cm3) (Lynam et al., 2017). The author elaborated that
565
this structural change is beneficial for future biorefinery operation as biomass with
566
higher density and lower bulk volume is easier to be transported.
567
3.1.2
568
Enzymatic hydrolysis for fermentable sugar production Conversion of biomass-derived saccharides to fermentable sugar for fuel
569
production is arguably one of the most developed bio-products conversion. To date,
570
most studies on the SF application focus on fermentable sugar production. Table 3
571
compiles the reported results on sugar yield from enzymatic hydrolysis processing of SF.
572
Many researchers concluded that the improved hydrolysis rate after DES pretreatment
24
573
was due to extensive hemicellulose or lignin removal or the combination of both (Hou
574
et al., 2017; Procentese et al., 2015). Guo et al. (2018) attributed the high sugar yield
575
from betaine:lactic acid pretreated xylose residue to high delignification efficiency at
576
81.6%. Also, a linear correlation between xylan removal and cellulose digestibility was
577
established with high R2 value at 0.86 (Hou, X.D. et al., 2018).
578
Literature reported that sugar yield resulted from enzymatic hydrolysis was
579
highly dependent on the pretreatment conditions such as biomass type, temperature,
580
duration, as well as solvent type (Table 3). Wahlström et al. (2016) compared the
581
hydrolysis performance of different biomass types under similar mild pretreatment
582
conditions (90% aqueous DES solution, 24 h, 80°C). DES-pretreated eucalyptus
583
dissolving pulp and microcrystalline cellulose achieved 100% and 70% hydrolysis,
584
respectively. However, lignocellulosic wheat straw and sawdust SFs showed very
585
minimal improvement from the untreated biomass in hydrolysis performance.
586
High temperature and large solid-solvent ratio could increase sugar yield
587
effectively, however the biopolymer recovery would be compromised due to the harsh
588
environment (Procentese et al., 2018). Li et al. (2018) demonstrated that by increasing
589
pretreatment temperature from 90°C to 120°C, SF digestibility improved significantly
590
from 40% to 80%. Nevertheless, xylose yield decreased by around 2-3% due to sugar
591
loss under high temperature. The group also explored the effect of HBA to HBD molar
592
ratio on hydrolysis performance. Under low temperature condition at 90°C, increasing
593
molar ratio of ChCl:lactic acid from 1:1 to 1:3 enhanced biomass digestibility and
594
glucose yield. Further increase of the ratio to 1:5 led to glucose yield reduction. Under
595
elevated temperature at 120°C, sugar yield decreased even when molar ratio was
596
increased from 1:1 to 1:3, which was hypothesized to be due to the combined effect of
25
597
high temperature and acid content. Types of DES employed also have great impact over
598
the SF hydrolysis performance (Table 3). Xing et al. (2018) reported that ternary acidic
599
DES with two acidic HBD constituents performed better than DES with single acidic
600
HBD in hydrolysis rate enhancement. SF from ChCl:formic acid:acetic acid DES
601
pretreatment achieved 21.5g/L sugar concentration after enzymatic hydrolysis, 1.14 and
602
1.37 times higher than that of ChCl:formic acid and ChCl:acetic acid, respectively. Interestingly, Pan et al. (2017) reported that ChCl:urea DES pretreatment failed
603 604
to enhance hydrolysis performance of SF. Sugar yield from rice straw SF (1.3%) was 10
605
times lesser than that of untreated biomass (13.0%). The author explained that cellulase
606
enzyme might have attacked the high CrI rice straw residue instead of the regenerated
607
rice straw. However, it is unclear about the difference between rice straw residue and
608
regenerated rice straw. It was suggested that ChCl:urea might have certain inhibiting
609
influence over the enzyme. Further study regarding this hypothesis is suggested.
610
3.1.3
611
Production of nanocrystals and nanofibrills cellulose Production of fermentable sugar from cellulose-rich SF is achieved by
612
depolymerisation of polysaccharides into monomers. Apart from that, cellulose can also
613
be utilized in its polymeric form for advanced material production. Several successful
614
attempts have been made in producing nanocrystals and nanofibrills cellulose from pure
615
cellulose source such as cotton fibres, microcrystalline cellulose or cellulose pulp using
616
DES as pretreatment agent (Liu, Yongzhuang et al., 2017; Sirvio et al., 2016;
617
Suopajarvi et al., 2017). Using SF as cellulose source, Liu, Y. et al. (2017) produced a
618
homogenous solution of nanofibrillated cellulose with uniform diameter of 8-14 nm by
619
conducting a 1000 W ultrasonication treatment for 20 mins. This study marked the first
620
report on the application of SF for cellulosic material production. Nanotechnology has
26
621
brought significant evolution to many industries (Brinchi et al., 2013), particularly
622
material engineering. Further advancement of nanocellulose, a renewable and non-toxic
623
material development using green solvents and lignocellulosic biomass would be
624
beneficial to the relevant industries.
625
3.2
Enzymatic hydrolysate conversion As reviewed in Section 3.1.2, SFs are frequently used as substrate in enzymatic
626 627
hydrolysis for the production of fermentable sugar (Table 3). In this section, case
628
studies of the carbohydrate-based products conversion using hydrolysate from SF
629
hydrolysis through fermentation process are discussed. Table 4 records the works
630
reporting fermentation of the sugar-rich enzymatic hydrolysate using different strains of
631
microorganisms to various carbohydrate-based products such as alcohols and lipids.
632
3.2.1
633
Alcohol production Commercialized ethanol production from sugar-based crops has been developed
634
since 1980s (Sun and Cheng, 2002). Production of this biofuel from second generation
635
biomass is still actively under investigation. Similar to sugar production, the
636
fermentation efficiency differs according to types of DES applied in the pretreatment
637
process (Table 4). Kumar et al. (2016) first tested on the fermentation efficiency of
638
Clavispora (NRRL Y-50464) strain on 5% glucose solution in the presence of DES.
639
Glucose was completely consumed in 18 h of fermentation when 5% and 10% of
640
ChCl:glycerol or 5% ChCl:ethylene glycol was present in the medium. However, when
641
concentration of ChCl:ethylene glycol increased to 10%, 16 g/L of glucose was still
642
remained after 24 h. Lowest ethanol production at 7.5 g/L was found in 10%
643
ChCl:propanediol medium after 24 h. When using actual hydrolysate from rice straw as
644
fermentation substrate, 89.5% ethanol conversion efficiency was successfully achieved.
27
Several groups of researchers reported positive results on producing butanol
645 646
using different bacteria strain. Procentese et al. (2017) successfully produced butanol
647
through simultaneous acetone, butanol, ethanol (ABE) fermentation using Clostridium
648
acetobutylicum (DSMZ 792) bacteria strain. The hydrolysate produced from
649
ChCl:glycerol pretreated SF contained 10 g/L glucose and 1.5 g/L xylose. The sugar
650
was completely consumed after 60 h of fermentation, resulting in 0.5 g/L of butanol
651
(0.04 g/gsugar) and total ABE yield of 0.1 g/gsugar, as shown in Table 4. In two other
652
works using Clostridium saccharobutylicum (DSM 13864) strain, total ABE yield of
653
0.36 g/gsugar (Xing et al., 2018) and 0.21 g/g sugar (Xu et al., 2016) were achieved. After
654
72 h, 93% of initial sugar (40 g/L) was consumed to produce 0.25 g/gsugar of butanol
655
(Xing et al., 2018). Xu et al. (2016) achieved 0.17 g/gsugar butanol yield from
656
hydrolysate that contained 48.2 g/L sugar. The result was comparable to the 51.7 g/L
657
glucose controlled medium with 0.21 g/gsugar butanol yield. Furthermore, Chen, Zhu et al. (2018) successfully produced 2,3-butanediol using
658 659
Bacillus vallismortis (NRRL B-14891). With 226.3 g/L of sugar substrate, the diol
660
production yield reached 89.7%. However, the total xylose conversion was very low at
661
18.4% after 72 h. The author suggested that the bacteria performance on xylose
662
conversion was less ideal in the mixed medium of C5 and C6 sugars. The collective
663
results of the above works show that hydrolysate from DES-pretreated SF can be used
664
as fermentation substrate without apparent inhibitory effect.
665
3.2.2
666
Lipid production Other than alcohol, a group of researchers successfully converted the hydrolysate
667
into lipids (Dai et al., 2017). Triacylglycerol (TAG) was converted from sugar
668
hydrolysate through fermentation using Bacillus Galactomyces sp. CCZU11-1. The
28
669
optimum condition of fermentation was determined to be 20 g/L glucose and carbon
670
source/nitrogen source at 30/1 ratio. 97% glucose was converted into 0.22 g lipid after
671
72 h of fermentation. According to the study, the lipid produced was comprised mainly
672
of C16 and C18 fatty acid chain, namely palmitic acid, palmitoleic acid, stearic acid and
673
oleic acid. The lipid obtained was recommended to be the alternative feedstock for
674
biodiesel or fatty-acid derived chemicals production due to its similar fatty acid content
675
to that of vegetable oil.
676
3.3
677
Pretreatment liquid fraction (LF) conversion Table 4 tabulates the studies reporting carbohydrate-based products conversion
678
of pretreatment liquid fraction (LF) from DES pretreatment. During biopolymer
679
fractionation, certain amount of depolymerized polysaccharides will dissolve into the
680
LF. In a study, 2.0% and 5.1% of glucose and xylose were recovered in LF, which
681
amounted to 8.6% and 23.6% of initial glucan and xylan in wood biomass, respectively
682
(Xia et al., 2018). 1.9% and 8.3% of fructose and hydroxymethylfurfural (HMF) were
683
also detected, which implied the occurrence of glucose isomerization and dehydration
684
process during the pretreatment. In the report by Liu, Y. et al. (2017), DES fractionation
685
resulted in 5.1% of glucose and 7.0% of xylose content in LF, as well as 1.1% of HMF.
686
The author demonstrated through TEM image that the dissolved saccharides can be
687
transformed directly into carbon materials using hydrothermal treatment without being
688
separated from the solvent mixture.
689
One of the major concerns in using LF as fermentation substrate is the possible
690
inhibiting effect of DES solvent residue on microbial activity. However, Chen, Z. et al.
691
(2018) showed that the solvent residue can actually act as extra carbon source for yeast
692
fermentation. The collected LF after 5 recycling cycles of ChCl:glycerol pretreatment
29
693
contained 3.14 g/L glucose, 17.49 g/L xylose and 143.23 g/L glycerol. Using oleaginous
694
yeast Rhodosporidium toruloides (NRRL Y-1091), the 144 h (6-day)-fermentation
695
process produced 8.1 g/L lipids and 15.0 mg/L carotenoid. Glucose and residual
696
glycerol in the LF were completely consumed after the process. LF containing solvent
697
residue can still be a good substrate for R.toruloides strain as glycerol and glucose can
698
both act as the carbon sources for fermentation. Carotenoid which is a natural pigment
699
and antioxidant is a very valuable chemical compound. The work on direct utilization of
700
LF can increase the economic return and reduce the operating cost of the process by
701
eliminating the purification process. Generally, LF is rich in C5 hemicellulose sugar due to the labile nature of the
702 703
sugar. Using aluminium chloride as catalyst, furfural was produced from xylose-rich LF
704
(Chen, Zhu et al., 2018). The furfural production yield reached 92% of the recovered
705
xylose content in the LF from the first DES pretreatment cycle and 85% in the recycled
706
DES pretreatment LF. However, the xylose recovery was much lower in the LF from
707
recycled DES (56.8%) as compared with first cycle LF (94.4%). Thus on the basis of
708
1000 g raw rice straw biomass, 101 g of furfural was produced from first cycle LF and
709
57 g from recycled LF. The drastic reduction was linked to accumulated impurities in
710
the LF which reduced xylose conversion in the recycled DES pretreatment. Hence, the
711
authors recommended more research efforts on minimizing xylose degradation during
712
the recycling process to increase the feasibility of the production scheme.
713
4
714
DES-extracted lignin upgrading Currently, most lignin extraction studies involve the use of acidic DES solvent
715
made of organic acid HBD. Studies showed that the lignin extraction yields of the DESs
716
were higher than their respective organic acid (Tan et al., 2019; Yu et al., 2018). Tan et
30
717
al. (2018) reported that similar to conventional acid pretreatment, dissolved lignin can
718
be precipitated from the acidic DES pretreatment mixture by using a simple water anti-
719
solvent rinsing. However, dissolved lignin in near-neutral and basic DES pretreatment
720
mixture could not be precipitated using the same methodology. More steps such as pH
721
adjustment might be needed to achieve lignin precipitation.
722
To evaluate the potential of DES for lignin processing more closely, Soares et al.
723
(2017) performed an extensive solubility study of lignin monomers (syringaldehyde,
724
vanillic acid, syringic acid and ferulic acid) and technical lignin (kraft and organosolv)
725
in different DESs. Effect of HBD and HBA type, HBA to HBD molar ratio, water
726
content and temperature on DES’s solubilisation efficiency were investigated. The
727
optimum conditions were reported to be 40-50°C using urea:propionic acid DES with
728
the ratio of 2:1 in 50-75 wt% aqueous condition.
729
The lignin solubilisation trend of DES differs from that of lignin extraction
730
performance from whole lignocellulosic biomass. DES with acid HBD which contained
731
longer alkyl chain (i.e. propionic acid) had greater solubilisation performance compared
732
to those with shorter alkyl chain (i.e. formic acid and acetic acid) (Soares et al., 2017).
733
The author associated the higher solubilisation of acid HBD with longer alkyl chain to
734
the dispersive interaction of alkyl chain with the lignin monomers. Contradicting result
735
was observed in lignin extraction from whole biomass (Tan et al., 2019; Yu et al., 2018).
736
Formic acid-DES was found to have higher lignin extraction efficiency compared to
737
acetic acid- and propionic acid-DES. The degree of lignin solubilisation can indeed
738
provide the basic information of whether a DES is suitable for lignin extraction.
739
Nevertheless, the efficiency of DES for lignin processing can be more correctly
740
assessed using the actual bulk biomass.
31
741
4.1
Properties of lignin
742
As a heterogeneous polymer, lignin constitutes of three main phenylpropanoid
743
structures namely syringyl (S), guaiacyl (G) and p-hydroxyphenyl (H) unit. Properties
744
of lignin polymer vary according to the type of solvent used, processing conditions and
745
biomass species. Investigation on the lignin’s properties is essential to identify the
746
suitable downstream conversion processes.
747
4.1.1
Purity
748
Extracted lignin usually contains solvent contaminants and polysaccharides
749
residues. Purification process such as extensive washing or dissolution followed by
750
precipitation can increase the lignin’s purity, but risk the loss of yield or properties
751
alteration. This has prompted researchers to seek for an optimized lignin extraction
752
schemes for high purity lignin production. A research work reported that anti-solvent
753
used had great effect over the quality of lignin produced (Alvarez-Vasco et al., 2016).
754
Water-ethanol mixture at volume ratio of 9:1 as anti-solvent for lignin precipitation
755
produced lignin with 88% purity. Comparatively the purity of lignin precipitated using
756
deionized water was <60%. With additional two washing cycles using the similar water-
757
ethanol mixture, lignin at 95% purity was obtained with negligible trace of carbohydrate
758
and DES residual. The effectiveness of the same washing solvent achieving >90%
759
lignin purity was also reported by Li et al. (2017) and Lyu et al. (2018).
760
Other than the washing procedure, pretreatment condition was reported to affect
761
lignin’s purity. By increasing the pretreatment time from 6 h to 24 h, the purity
762
increased from 90% to 95.4% while the carbohydrate content in lignin decreased
763
gradually from 1.12 to 0.11% (Lyu et al., 2018). Another attempt to increase the lignin’s
764
purity is by using DES in aqueous condition for extraction. Chen, Zhu et al. (2018)
32
765
found that when water content of DES solution was increased from 0-20%, lignin purity
766
improved from 83.3-86.1% and 82.4-85.0% at 20 wt% and 27 wt% biomass solid
767
loading, respectively. Another interesting finding reported that the yield of lignin
768
extracted at 130°C exceeded the initial lignin content in raw poplar wood biomass (Xia
769
et al., 2018). The excess weight of lignin produced was attributed to the generation of
770
pseudolignin from carbohydrate, as observed in SEM.
771
4.1.2
Molecular structure
772
Lignin’s reactivity for downstream modification can be reflected in their
773
phenolic hydroxyl (PhOH) group content as PhOH is the most reactive functional group
774
in lignin (Lai, 1992). After confirming the presence of strong PhOH signal in FTIR,
775
Tan et al. (2019) quantified the PhOH content in lignin extracted using various acidic
776
DES solvents. Among the nine types of acidic HBDs (formic acid, acetic acid,
777
propionic acid, butyric acid, lactic acid, malic acid, citric acid, succinic acid and maleic
778
acid) investigated, lignin extracted using acidic DES with lactic acid HBD contained the
779
highest PhOH content at 3.33-3.72 mmol/g, depending on the HBA:HBD ratio. The
780
PhOH was higher than that of pure lactic acid-extracted lignin at 2.85 mmol/g. The
781
authors associated the finding with the condensation of lignin fragments in pure acid
782
environment which decreased the PhOH moiety in the lignin structure. Some works also
783
identified appreciable PhOH content in DES-extracted lignin through NMR
784
spectroscopy (Alvarez-Vasco et al., 2016; Li et al., 2017; Xia et al., 2018).
785
To study the lignin’s structure more in-depth, NMR is usually performed. Most
786
studies focused on determining the presence of carbohydrate impurities in lignin, as
787
well as the condition of the major linkages in the structure, namely β-O-4 and C-C
788
bonds. β-O-4 ether bond is recognized as the most abundant and cleavable linkage in
33
789
lignin (Constant et al., 2016). Some studies reported on the absence or the negligible
790
amount of β-O-4 in the structure once lignin was extracted from biomass using DES,
791
leaving predominant amount of C-C bond, such as resinols β-β or phenylcoumarans β-5
792
in the structure (Alvarez-Vasco et al., 2016; Liu, Y. et al., 2017; Xia et al., 2018).
793
Alvarez-Vasco et al. (2016) further clarified that DES depolymerized lignin by selective
794
cleavage of the labile ether bond, hence lignin with better polymer stability can be
795
produced. Contrarily, several works detected appreciable amount of β-O-4 remained in
796
the lignin. Hiltunen et al. (2016) reported that in the wood lignin, main linkage detected
797
was β-O-4, followed by β-β and β-5. The result deviation could be due to different
798
choice of solvent which caused varying degree of depolymerisation in lignin. For
799
example, acidified ChChl:glycerol used in Chen, Z. et al. (2018) might be able to retain
800
the ether bond unlike the commonly used acidic DES such as ChCl:lactic acid.
801
Nevertheless, the ether bond albeit existed (1.53% and 4.98%), was significantly lower
802
than that of milled wood lignin (46.7%). Some catalytic conversions target on β-O-4
803
cleavage in the product formation schemes (Zakzeski et al., 2010). Low value of β-O-4
804
in lignin might deter the application of DES-extracted lignin in certain lignin utilization
805
process. To cater to this requirement, research on DES type which can preserve as much
806
β-O-4 linkages as possible will be an interesting subject to pursue. Investigation on
807
suitable processing schemes for lignin with low β-O-4 linkage structure is also
808
necessary to create more variety of lignin-derived products.
809
4.2
810
DES-extracted lignin conversion Lignin is always known as a very useful macromolecule in material industries
811
(Laurichesse and Avérous, 2014; Northey, 1992). Due to lignin polymer’s insolubility
812
in many solvents, the application of lignin nanoparticles for bio-based functional
34
813
materials has been researched by many (Lievonen et al., 2016; Roopan, 2017). Lyu et al.
814
(2018) successfully produced lignin nanoparticle nanoscale lignin particle with uniform
815
diameter of 200-420 nm and smooth topographic surface by dissolving the DES-
816
extracted lignin in acetone at 0.1 g/L concentration. DES-extracted lignin represents a
817
new class of extracted lignin and its potential is still largely untapped. To fully explore
818
the possibility of applying DES-extracted lignin as fuel and aromatic feedstock, it is
819
promptly to initiate more comprehensive structural and properties studies of this lignin
820
type. Subsequently, the utilization of DES-extracted lignin in current existing fuel,
821
macromolecular and fine chemicals conversion schemes are highly encouraged. While
822
the DES-fractionated carbohydrate products have gained a lot of attention in
823
downstream conversion, the application study of DES-extracted lignin is comparably
824
scarce. To move forward in biorefinery development, the conversion scheme of
825
different biopolymers should be designed concurrently.
826
5
827
Challenges and outlook for DES utilization in biomass processing In view of the huge varieties of DES available, the research focus on the
828
selection of DES constituent across the DES pretreatment field is rather scattered.
829
Despite various categories of DES constituents have been investigated, the most
830
suitable DES type for biomass processing application is non-conclusive. Researchers
831
need to define the pretreatment goal, for instance enzymatic hydrolysis enhancement or
832
lignin extraction, in order to select the best performing solvent. Establishment on the
833
fundamental knowledge on how DES interacts with different biopolymer solutes would
834
greatly help in the selection process. In addition, it is well known that high viscosity of
835
DES solvent is a major obstacle in its application. Despite one can opt for constituents
836
that will lead to less viscous DES, the synthesized solvent might not be suitable for the
35
837
intended application. Water addition to DES not only can decrease the solvent’s
838
viscosity, but also reduce the amount of neat solvent needed, which translates to lower
839
operating cost. Nevertheless, there is a limit to how much water can be added to DES
840
without disrupting the solvent’s intermolecular bonding structure. Data collection on
841
how water addition affects the DES molecular framework and their interaction with
842
targeted solutes would provide insightful information to facilitate application of
843
aqueous DES.
844
Multiple types of carbohydrate-derived product such as bioethanol, biobutanol,
845
2,3-butanediol, lipids, 5-HMF have been successfully produced from the DES-
846
fractionated products in several works. However, the gathered information showed that
847
the selectivity and yield of these conversion processes varied greatly in different works
848
(Table 3 and Table 4). Strategies to improve the productivity of intended products,
849
therefore establishing the process feasibility and accuracy, will be a great research
850
direction in future works. Aside from the commonly reported biofuels, it will also be
851
worthwhile to explore more varieties of higher value saccharides-derived commodity
852
chemicals such as organic acids or sugar alcohols to be converted from DES-
853
fractionated saccharides to increase the process revenue.
854
It has been demonstrated in the previous sections that different categories of
855
DESs were investigated for enzymatic hydrolysis enhancement. For lignin extraction,
856
vast majority of the studies involved the use of acidic DES (DES with acid HBD or
857
acidified DES) while other types of DES such as basic DES can also extract an
858
extensive amount of lignin. It is recommended to diversify the type of DES adopted in
859
lignin extraction study to fully explore the potential of DES in this application. The
860
extracted lignin might have different properties to acidic DES-extracted lignin. The two
36
861
might be applicable for different sorts of downstream processing and this can create
862
more variety of lignin-derived aromatic products as alternatives to petrochemicals.
863
Additionally, comparison study on DES-extracted lignin’s properties with other
864
technical lignin is necessary to evaluate the possibility of this new type of lignin to be
865
applied in existing lignin conversion schemes, as well as to explore the possible
866
applications which are yet to be accomplished by the existing technical lignin.
867
Extended techno-economic assessment is also a pressing research topic to further
868
establish the feasibility of this alternative technology. However, this research aspect has
869
been neglected. Recently, Ma et al. (2018) conducted an extensive techno-economic
870
analysis of biogas upgrading process to compare the performance of DES with the other
871
conventional solvents. The energy utilization, amount of solvent and dimension of
872
equipment columns were simulated using ASPEN PLUS, which showed DES had
873
similarly promising efficiency as the conventional solvent. Meticulous investigation
874
using the aid of mathematical models or simulation software is highly encouraged.
875
The question that always arises when a new technology being introduced is “Can
876
the same level of performance transfer from research laboratory scale to a larger scale
877
setup?” Despite we are at the starting point of DES exploration, consideration has to be
878
taken on the prospect of applying the technology in large scale and eventually achieving
879
the commercial biomass utilization process. Researchers are to constantly evaluate the
880
progress of the developed technology and scale up the process when deemed suitable.
881
Conclusion
882
DES has manifested promising performance in raw lignocellulosic biomass
883
fractionation. This enables the independent biopolymer upgrading process according to
884
their inherent properties. However, the ideal DES type for biomass conversion is still
37
885
uncertain. Also, there is a lack of product variety in the downstream conversion of DES-
886
fractionated products. Rigorous investigations in solvent selection, bio-products
887
diversification, as well as techno-economic analysis are needed to advance the DES
888
fractionation technology. The employment of DES in achieving a complete biomass
889
utilization process will be a significant approach to reduce the dependency on non-
890
renewable resources and move towards sustainable economy.
891
Acknowledgements
892
This work was supported by the Fundamental Research Grant Scheme (FRGS) by
893
Ministry of Education Malaysia [FP047-2017A and FP128-2019A].
894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918
References 1. Abbott, A.P., Barron, J.C., Ryder, K.S., Wilson, D., 2007. Eutectic-based ionic liquids with metal-containing anions and cations. Chemistry 13(22), 6495-6501. 2. Abbott, A.P., Boothby, D., Capper, G., Davies, D.L., Rasheed, R.K., 2004a. Deep Eutectic Solvents Formed between Choline Chloride and Carboxylic Acids: Versatile Alternatives to Ionic Liquids. J. Am. Chem. Soc. 126(29), 9142-9147. 3. Abbott, A.P., Capper, G., Davies, D.L., Rasheed, R., 2004b. Ionic Liquids Based upon Metal Halide/Substituted Quaternary Ammonium Salt Mixtures. Inorg. Chem. 43(11), 3447-3452. 4. Abbott, A.P., Capper, G., Davies, D.L., Rasheed, R.K., 2004. Ionic liquid analogues formed from hydrated metal salts. Chemistry 10(15), 3769-3774. 5. Abbott, A.P., Capper, G., Davies, D.L., Rasheed, R.K., Tambyrajah, V., 2003. Novel solvent properties of choline chloride/urea mixtures. Chem. Commun.(1), 7071. 6. Abbott, A.P., Harris, R.C., Ryder, K.S., D'Agostino, C., Gladden, L.F., Mantle, M.D., 2011. Glycerol eutectics as sustainable solvent systems. Green Chem. 13(1), 82-90. 7. Alvarez-Vasco, C., Ma, R.S., Quintero, M., Guo, M., Geleynse, S., Ramasamy, K.K., Wolcott, M., Zhang, X., 2016. Unique low-molecular-weight lignin with high purity extracted from wood by deep eutectic solvents (DES): a source of lignin for valorization. Green Chem. 18(19), 5133-5141. 8. Anastas, P., Eghbali, N., 2010. Green chemistry: principles and practice. Chem. Soc. Rev. 39(1), 301-312. 9. Aroso, I.M., Craveiro, R., Rocha, A., Dionisio, M., Barreiros, S., Reis, R.L., Paiva, A., Duarte, A.R.C., 2015. Design of controlled release systems for THEDES-
38
919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959
Therapeutic deep eutectic solvents, using supercritical fluid technology. Int. J. Pharm. 492(1-2), 73-79. 10. Brandt, A., Gräsvik, J., Hallett, J.P., Welton, T., 2013. Deconstruction of lignocellulosic biomass with ionic liquids. Green Chem. 15(3), 550. 11. Brinchi, L., Cotana, F., Fortunati, E., Kenny, J.M., 2013. Production of nanocrystalline cellulose from lignocellulosic biomass: technology and applications. Carbohydr. Polym. 94(1), 154-169. 12. Chen, Z., Bai, X., A, L., Wan, C., 2018. High-Solid Lignocellulose Processing Enabled by Natural Deep Eutectic Solvent for Lignin Extraction and Industrially Relevant Production of Renewable Chemicals. ACS Sustain. Chem. Eng. 6(9), 12205-12216. 13. Chen, Z., Reznicek, W.D., Wan, C., 2018. Deep eutectic solvent pretreatment enabling full utilization of switchgrass. Bioresour. Technol. 263, 40-48. 14. Chen, Z., Wan, C., 2018. Ultrafast fractionation of lignocellulosic biomass by microwave-assisted deep eutectic solvent pretreatment. Bioresour. Technol. 250, 532-537. 15. Choi, Y.H., van Spronsen, J., Dai, Y., Verberne, M., Hollmann, F., Arends, I.W., Witkamp, G.J., Verpoorte, R., 2011. Are natural deep eutectic solvents the missing link in understanding cellular metabolism and physiology? Plant Physiol 156(4), 1701-1705. 16. Constant, S., Wienk, H.L.J., Frissen, A.E., Peinder, P.d., Boelens, R., van Es, D.S., Grisel, R.J.H., Weckhuysen, B.M., Huijgen, W.J.J., Gosselink, R.J.A., Bruijnincx, P.C.A., 2016. New insights into the structure and composition of technical lignins: a comparative characterisation study. Green Chem. 18(9), 2651-2665. 17. Cvjetko Bubalo, M., Vidović, S., Radojčić Redovniković, I., Jokić, S., 2015. Green solvents for green technologies. Journal of Chemical Technology & Biotechnology 90(9), 1631-1639. 18. Dai, Y., van Spronsen, J., Witkamp, G.J., Verpoorte, R., Choi, Y.H., 2013. Natural deep eutectic solvents as new potential media for green technology. Anal. Chim. Acta 766, 61-68. 19. Dai, Y., Zhang, H.S., Huan, B., He, Y., 2017. Enhancing the enzymatic saccharification of bamboo shoot shell by sequential biological pretreatment with Galactomyces sp. CCZU11-1 and deep eutectic solvent extraction. Bioprocess Biosyst Eng 40(9), 1427-1436. 20. Dai, Y.T., Verpoorte, R., Choi, Y.H., 2014. Natural deep eutectic solvents providing enhanced stability of natural colorants from safflower (Carthamus tinctorius). Food Chem. 159, 116-121. 21. Fang, C., Thomsen, M.H., Frankær, C.G., Brudecki, G.P., Schmidt, J.E., AlNashef, I.M., 2017. Reviving pretreatment effectiveness of deep eutectic solvents on lignocellulosic date palm residues by prior recalcitrance reduction. Industrial & Engineering Chemistry Research 56(12), 3167-3174.
39
960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999
22. Florindo, C., Oliveira, F.S., Rebelo, L.P.N., Fernandes, A.M., Marrucho, I.M., 2014. Insights into the Synthesis and Properties of Deep Eutectic Solvents Based on Cholinium Chloride and Carboxylic Acids. ACS Sustain. Chem. Eng. 2(10), 24162425. 23. Florindo, C., Oliveira, M.M., Branco, L.C., Marrucho, I.M., 2017. Carbohydratesbased deep eutectic solvents: Thermophysical properties and rice straw dissolution. J. Mol. Liq. 247, 441-447. 24. Francisco, M., van den Bruinhorst, A., Kroon, M.C., 2012. New natural and renewable low transition temperature mixtures (LTTMs): screening as solvents for lignocellulosic biomass processing. Green Chem. 14(8), 2153. 25. García, G., Aparicio, S., Ullah, R., Atilhan, M., 2015. Deep Eutectic Solvents: Physicochemical Properties and Gas Separation Applications. Energy & Fuels 29(4), 2616-2644. 26. Guo, Z., Ling, Z., Wang, C., Zhang, X., Xu, F., 2018. Integration of facile deep eutectic solvents pretreatment for enhanced enzymatic hydrolysis and lignin valorization from industrial xylose residue. Bioresour. Technol. 265, 334-339. 27. Hendriks, A.T., Zeeman, G., 2009. Pretreatments to enhance the digestibility of lignocellulosic biomass. Bioresour. Technol. 100(1), 10-18. 28. Hiltunen, J., Kuutti, L., Rovio, S., Puhakka, E., Virtanen, T., Ohra-Aho, T., Vuoti, S., 2016. Using a low melting solvent mixture to extract value from wood biomass. Sci Rep 6, 32420. 29. Hou, X.-D., Lin, K.-P., Li, A.-L., Yang, L.-M., Fu, M.-H., 2018. Effect of constituents molar ratios of deep eutectic solvents on rice straw fractionation efficiency and the micro-mechanism investigation. Ind. Crops. Prod. 120, 322-329. 30. Hou, X.D., Feng, G.J., Ye, M., Huang, C.M., Zhang, Y., 2017. Significantly enhanced enzymatic hydrolysis of rice straw via a high-performance two-stage deep eutectic solvents synergistic pretreatment. Bioresour. Technol. 238, 139-146. 31. Hou, X.D., Li, A.L., Lin, K.P., Wang, Y.Y., Kuang, Z.Y., Cao, S.L., 2018. Insight into the structure-function relationships of deep eutectic solvents during rice straw pretreatment. Bioresour. Technol. 249, 261-267. 32. Kandanelli, R., Thulluri, C., Mangala, R., Rao, P.V.C., Gandham, S., Velankar, H.R., 2018. A novel ternary combination of deep eutectic solvent-alcohol (DES-OL) system for synergistic and efficient delignification of biomass. Bioresour. Technol. 265, 573-576. 33. Kim, K.H., Dutta, T., Sun, J., Simmons, B., Singh, S., 2018. Biomass pretreatment using deep eutectic solvents from lignin derived phenols. Green Chem. 20(4), 809815. 34. Koga, Y.J.J.o.S.C., 2003. Effect of Ethylene Glycol on the Molecular Organization of H2O in Comparison with Methanol and Glycerol: A Calorimetric Study. 32(9), 803-818.
40
1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041
35. Kumar, A.K., Parikh, B.S., Shah, E., Liu, L.Z., Cotta, M.A., 2016. Cellulosic ethanol production from green solvent-pretreated rice straw. Biocatalysis and Agricultural Biotechnology 7, 14-23. 36. Lai, Y.-Z., 1992. Determination of Phenolic Hydroxyl Groups, in: Lin, S.Y., Dence, C.W. (Eds.), Methods in Lignin Chemistry. Springer Berlin Heidelberg, Berlin, Heidelberg, pp. 423-434. 37. Laurichesse, S., Avérous, L., 2014. Chemical modification of lignins: Towards biobased polymers. Prog. Polym. Sci. 39(7), 1266-1290. 38. Li, A.L., Hou, X.D., Lin, K.P., Zhang, X., Fu, M.H., 2018. Rice straw pretreatment using deep eutectic solvents with different constituents molar ratios: Biomass fractionation, polysaccharides enzymatic digestion and solvent reuse. J Biosci Bioeng 126(3), 346-354. 39. Li, T., Lyu, G., Liu, Y., Lou, R., Lucia, L.A., Yang, G., Chen, J., Saeed, H.A.M., 2017. Deep Eutectic Solvents (DESs) for the Isolation of Willow Lignin (Salix matsudana cv. Zhuliu). Int J Mol Sci 18(11). 40. Lievonen, M., Valle-Delgado, J.J., Mattinen, M.-L., Hult, E.-L., Lintinen, K., Kostiainen, M.A., Paananen, A., Szilvay, G.R., Setälä, H., Österberg, M., 2016. A simple process for lignin nanoparticle preparation. Green Chem. 18(5), 1416-1422. 41. Liew, S.Q., Ngoh, G.C., Yusoff, R., Teoh, W.H., 2016. Sequential ultrasoundmicrowave assisted acid extraction (UMAE) of pectin from pomelo peels. Int. J. Biol. Macromol. 93, 426-435. 42. Liew, S.Q., Ngoh, G.C., Yusoff, R., Teoh, W.H., 2018. Acid and Deep Eutectic Solvent (DES) extraction of pectin from pomelo (Citrus grandis (L.) Osbeck) peels. Biocatalysis and Agricultural Biotechnology 13, 1-11. 43. Liu, Y.-T., Chen, Y.-A., Xing, Y.-J., 2014. Synthesis and characterization of novel ternary deep eutectic solvents. Chin. Chem. Lett. 25(1), 104-106. 44. Liu, Y., Chen, W., Xia, Q., Guo, B., Wang, Q., Liu, S., Liu, Y., Li, J., Yu, H., 2017. Efficient cleavage of lignin–carbohydrate complexes and ultrafast extraction of lignin oligomers from wood biomass by microwave-assisted treatment with deep eutectic solvent. ChemSusChem. 10(8), 1692-1700. 45. Liu, Y., Guo, B., Xia, Q., Meng, J., Chen, W., Liu, S., Wang, Q., Liu, Y., Li, J., Yu, H., 2017. Efficient Cleavage of Strong Hydrogen Bonds in Cotton by Deep Eutectic Solvents and Facile Fabrication of Cellulose Nanocrystals in High Yields. ACS Sustain. Chem. Eng. 5(9), 7623-7631. 46. Loow, Y.-L., Wu, T.Y., Lim, Y.S., Tan, K.A., Siow, L.F., Md. Jahim, J., Mohammad, A.W., 2017. Improvement of xylose recovery from the stalks of oil palm fronds using inorganic salt and oxidative agent. Energy Convers. Manage. 138, 248-260. 47. Loow, Y.L., Wu, T.Y., Yang, G.H., Ang, L.Y., New, E.K., Siow, L.F., Md Jahim, J., Mohammad, A.W., Teoh, W.H., 2018. Deep eutectic solvent and inorganic salt pretreatment of lignocellulosic biomass for improving xylose recovery. Bioresour. Technol. 249, 818-825.
41
1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081
48. Lynam, J.G., Kumar, N., Wong, M.J., 2017. Deep eutectic solvents' ability to solubilize lignin, cellulose, and hemicellulose; thermal stability; and density. Bioresour. Technol. 238, 684-689. 49. Lyu, G., Li, T., Ji, X., Yang, G., Liu, Y., Lucia, L., Chen, J., 2018. Characterization of Lignin Extracted from Willow by Deep Eutectic Solvent Treatments. Polymers 10(8), 869. 50. Ma, C., Liu, C., Lu, X., Ji, X., 2018. Techno-economic analysis and performance comparison of aqueous deep eutectic solvent and other physical absorbents for biogas upgrading. Applied Energy 225, 437-447. 51. Malaeke, H., Housaindokht, M.R., Monhemi, H., Izadyar, M., 2018. Deep eutectic solvent as an efficient molecular liquid for lignin solubilization and wood delignification. J. Mol. Liq. 263, 193-199. 52. Menon, V., Rao, M., 2012. Trends in bioconversion of lignocellulose: Biofuels, platform chemicals & biorefinery concept. Prog. Energy Combust. Sci. 38(4), 522550. 53. Mota-Morales, J.D., Gutierrez, M.C., Ferrer, M.L., Sanchez, I.C., Elizalde-Pena, E.A., Pojman, J.A., Del Monte, F., Luna-Barcenas, G., 2013. Deep eutectic solvents as both active fillers and monomers for frontal polymerization. Journal of Polymer Science Part a-Polymer Chemistry 51(8), 1767-1773. 54. Northey, R.A., 1992. Low-Cost Uses of Lignin. ACS Symp. Ser. 476, 146-175. 55. Pan, M., Zhao, G., Ding, C., Wu, B., Lian, Z., Lian, H., 2017. Physicochemical transformation of rice straw after pretreatment with a deep eutectic solvent of choline chloride/urea. Carbohydr. Polym. 176, 307-314. 56. Procentese, A., Johnson, E., Orr, V., Garruto Campanile, A., Wood, J.A., Marzocchella, A., Rehmann, L., 2015. Deep eutectic solvent pretreatment and subsequent saccharification of corncob. Bioresour. Technol. 192, 31-36. 57. Procentese, A., Raganati, F., Olivieri, G., Russo, M.E., Rehmann, L., Marzocchella, A., 2017. Low-energy biomass pretreatment with deep eutectic solvents for biobutanol production. Bioresour. Technol. 243, 464-473. 58. Procentese, A., Raganati, F., Olivieri, G., Russo, M.E., Rehmann, L., Marzocchella, A., 2018. Deep Eutectic Solvents pretreatment of agro-industrial food waste. Biotechnol. for Biofuels 11, 37. 59. Rastogi, M., Shrivastava, S., 2017. Recent advances in second generation bioethanol production: An insight to pretreatment, saccharification and fermentation processes. Renewable and Sustainable Energy Reviews 80, 330-340. 60. Roopan, S.M., 2017. An overview of natural renewable bio-polymer lignin towards nano and biotechnological applications. Int. J. Biol. Macromol. 103, 508-514. 61. Saha, B.C., Yoshida, T., Cotta, M.A., Sonomoto, K., 2013. Hydrothermal pretreatment and enzymatic saccharification of corn stover for efficient ethanol production. Ind. Crops. Prod. 44, 367-372.
42
1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123
62. Sirvio, J.A., Visanko, M., Liimatainen, H., 2016. Acidic Deep Eutectic Solvents As Hydrolytic Media for Cellulose Nanocrystal Production. Biomacromolecules 17(9), 3025-3032. 63. Smith, E.L., Abbott, A.P., Ryder, K.S., 2014. Deep eutectic solvents (DESs) and their applications. Chem. Rev. 114(21), 11060-11082. 64. Soares, B., Tavares, D.J.P., Amaral, J.L., Silvestre, A.J.D., Freire, C.S.R., Coutinho, J.A.P., 2017. Enhanced Solubility of Lignin Monomeric Model Compounds and Technical Lignins in Aqueous Solutions of Deep Eutectic Solvents. ACS Sustain. Chem. Eng. 5(5), 4056-4065. 65. Sun, N., Rodriguez, H., Rahman, M., Rogers, R.D., 2011. Where are ionic liquid strategies most suited in the pursuit of chemicals and energy from lignocellulosic biomass? Chem. Commun. 47(5), 1405-1421. 66. Sun, Y., Cheng, J., 2002. Hydrolysis of lignocellulosic materials for ethanol production: a review. Bioresour. Technol. 83(1), 1-11. 67. Suopajarvi, T., Sirvio, J.A., Liimatainen, H., 2017. Nanofibrillation of deep eutectic solvent-treated paper and board cellulose pulps. Carbohydr. Polym. 169, 167-175. 68. Tan, Y.T., Ngoh, G.C., Chua, A.S.M., 2018. Evaluation of fractionation and delignification efficiencies of deep eutectic solvents on oil palm empty fruit bunch. Ind. Crops. Prod. 123, 271-277. 69. Tan, Y.T., Ngoh, G.C., Chua, A.S.M., 2019. Effect of functional groups in acid constituent of deep eutectic solvent for extraction of reactive lignin. Bioresour. Technol. 281, 359-366. 70. Teles, A.R.R., Capela, E.V., Carmo, R.S., Coutinho, J.A.P., Silvestre, A.J.D., Freire, M.G., 2017. Solvatochromic parameters of deep eutectic solvents formed by ammonium-based salts and carboxylic acids. Fluid Phase Equilib. 448, 15-21. 71. Vigier, K.D.O., Chatel, G., Jérôme, F., 2015. Contribution of deep eutectic solvents for biomass processing: Opportunities, challenges, and limitations. ChemCatChem 7(8), 1250-1260. 72. Wahlström, R., Hiltunen, J., Pitaluga de Souza Nascente Sirkka, M., Vuoti, S., Kruus, K., 2016. Comparison of three deep eutectic solvents and 1-ethyl-3methylimidazolium acetate in the pretreatment of lignocellulose: effect on enzyme stability, lignocellulose digestibility and one-pot hydrolysis. RSC Adv. 6(72), 68100-68110. 73. Welton, T., 1999. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. Rev. 99(8), 2071-2084. 74. Xia, Q., Liu, Y., Meng, J., Cheng, W., Chen, W., Liu, S., Liu, Y., Li, J., Yu, H., 2018. Multiple hydrogen bond coordination in three-constituent deep eutectic solvents enhances lignin fractionation from biomass. Green Chem. 20(12), 27112721. 75. Xing, W., Xu, G., Dong, J., Han, R., Ni, Y., 2018. Novel dihydrogen-bonding deep eutectic solvents: Pretreatment of rice straw for butanol fermentation featuring enzyme recycling and high solvent yield. Chem. Eng. J. 333, 712-720.
43
1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156
76. Xu, G.C., Ding, J.C., Han, R.Z., Dong, J.J., Ni, Y., 2016. Enhancing cellulose accessibility of corn stover by deep eutectic solvent pretreatment for butanol fermentation. Bioresour. Technol. 203, 364-369. 77. Yiin, C.L., Quitain, A.T., Yusup, S., Sasaki, M., Uemura, Y., Kida, T., 2016. Characterization of natural low transition temperature mixtures (LTTMs): Green solvents for biomass delignification. Bioresour. Technol. 199, 258-264. 78. Yiin, C.L., Quitain, A.T., Yusup, S., Uemura, Y., Sasaki, M., Kida, T., 2017. Choline chloride (ChCl) and monosodium glutamate (MSG)-based green solvents from optimized cactus malic acid for biomass delignification. Bioresour. Technol. 244(Part 1), 941-948. 79. Yu, Q., Zhang, A., Wang, W., Chen, L., Bai, R., Zhuang, X., Wang, Q., Wang, Z., Yuan, Z., 2018. Deep eutectic solvents from hemicellulose-derived acids for the cellulosic ethanol refining of Akebia’ herbal residues. Bioresour. Technol. 247, 705710. 80. Zainal-Abidin, M.H., Hayyan, M., Ngoh, G.C., Wong, W.F., 2019. From nanoengineering to nanomedicine: A facile route to enhance biocompatibility of graphene as a potential nano-carrier for targeted drug delivery using natural deep eutectic solvents. Chem. Eng. Sci. 195, 95-106. 81. Zakzeski, J., Bruijnincx, P.C.A., Jongerius, A.L., Weckhuysen, B.M., 2010. The Catalytic Valorization of Lignin for the Production of Renewable Chemicals. Chem. Rev. 110(6), 3552-3599. 82. Zdanowicz, M., Wilpiszewska, K., Spychaj, T., 2018. Deep eutectic solvents for polysaccharides processing. A review. Carbohydr. Polym. 200, 361-380. 83. Zhang, C.W., Xia, S.Q., Ma, P.S., 2016. Facile pretreatment of lignocellulosic biomass using deep eutectic solvents. Bioresour. Technol. 219, 1-5. 84. Zhang, Q., De Oliveira Vigier, K., Royer, S., Jerome, F., 2012. Deep eutectic solvents: syntheses, properties and applications. Chem. Soc. Rev. 41(21), 7108-7146. 85. Zhao, Z., Chen, X., Ali, M.F., Abdeltawab, A.A., Yakout, S.M., Yu, G., 2018. Pretreatment of wheat straw using basic ethanolamine-based deep eutectic solvents for improving enzymatic hydrolysis. Bioresour. Technol. 263, 325-333. 86. Zulkefli, S., Abdulmalek, E., Abdul Rahman, M.B., 2017. Pretreatment of oil palm trunk in deep eutectic solvent and optimization of enzymatic hydrolysis of pretreated oil palm trunk. Renew. Energy 107, 36-41.
44
Table 1 Categorization of DESs and their respective application DES
Constituent 1 Quaternary ammonium salt Quaternary ammonium salt
Constituent 2 Application types* References Metal halides, Type 1 Electrochemical, metal plating (Abbott, Andrew P. et al., 2004b; Smith et al., 2014) Eg: ZnBr2, FeCl3 Hydrated metal halides, Type 2 Electrochemical, metal plating (Abbott, A. P. et al., 2004; Smith et al., 2014) Eg: CrCl3·6H2O, CaCl2·6H2O Hydrogen bond donor, Quaternary Metal plating, gas adsorption, (Abbott, Andrew P. et al., 2004a; Abbott et al., 2003; Type 3 Eg: urea, carboxylic acid, ammonium salt biomass processing Abbott et al., 2011; García et al., 2015; Smith et al., 2014) polyol Hydrogen bond donor, Type 4 Metal salt Electrochemical, metal plating (Abbott et al., 2007; Smith et al., 2014) Eg: amide, diol *The application types listed includes several selected common DES application area and do not represent all the application introduced up to date.
45
Table 2 DES pretreatment condition, biopolymer composition in DES-pretreated biomass and biopolymer removal percentage of DESs with varying HBDs DES pretreatment condition Work
DES
Molar ratio
Biomass type
S/L ratio (wt%)
Temperature (°C)
Time (hour)
Solid yield (%)
Biopolymer composition in pretreated biomass Glucan
Xylan
Lignin
35
20
19
Biopolymer removal Glucan
Xylan
Lignin
2
1
24
Polyalocohol-based DES (Chen, Z. et ChCl:glycerol al., 2018) (Chen, Zhu ChCl:ethylene glycol et al., 2018)
1:2
Switchgrass
10
110
1
1:2
Switchgrass
10
130
0.5
(Fang et al., 2017)
ChCl:glycerol
1:2
Date palm
5
70 70
ChCl:ethylene glycol ChCl:1,2-Propanediol
1:1 1:1
6 15 3 3
77 76 78 76
35 34 37 34
19 19 21 20
36 37 20 19
ChCl:1,3-Propanediol ChCl:glycerol ChCl:xylitol
1:1 1:1 1:1
Rice straw
5
120
3 6 6
ChCl:glycerol
1:2
Corncob
6.25
80 115
76 80 81 85 82
41 36 36 33 32
20 20 20 25 24
19 21 21 16 15
Oil palm empty fruit bunch
10
120
8
55 96
53 31
21 15
13 17
21
16
22
97
30
14
18
24
19
17
Corncob
5
90
24
(Hou, X.D. et al., 2018)
(Procentese et al., 2015)
150
(Tan et al., 2018)
ChCl:glycerol
1:2
ChCl:glucose
1:1
(Zhang et al., 2016)
ChCl:ethylene glycol ChCl:glycerol ChCl:glycerol ChCl:ethylene glycol Ethylammonium chloride:glycerol Ethylammonium chloride:ethylene glycol
1:2 1:2 1:2 1:2
(Zulkefli et al., 2017)
15
Acid-based DES (AlvarezChCh:lactic acid
1:2
Oil palm trunk
5
100
48
1:2
Poplar wood
10
90
6
89
73 71 63 68
52 57
36 31
10 10
48
53
27
10
58
82
2
12
33
13
21
88 71 49
87
36
25
46
Vasco et al., 2016)
(Hou, X.D. et al., 2018)
(Tan et al., 2018)
(Tan et al., 2019)
(Yu et al., 2018)
(Zhang et al., 2016)
ChCl:glycolic acid
1:1
ChCl:lactic acid
1:1
ChCl:2-chloropropionic acid
1:1
ChCl:oxalic acid
1:1
ChCl:malonic acid ChCl:lactic acid
1:1 1:5
Glucose:lactic acid
1:5
ChCl:lactic acid ChCl:malic acid ChCl:citric acid
1:1 1:1 1:1
ChCl:formic acid ChCl:acetic acid ChCl:propionic acid ChCl:butyric acid ChCl:salicylic acid ChCl:maleic acid ChCl:formic acid ChCl:acetic acid ChCl:glycolic acid ChCl:levulinic acid ChCl:lactic acid ChCl:lactic acid ChCl:lactic acid ChCl:lactic acid ChCl:glycolic acid ChCl:levulinic acid
1:2 1:2 1:2 1:2 2:1 1:1 1:2 1:6 1:4 1:4 1:2 1:5 1:10 1:15 1:2 1:2
120 145 180
3 9 0.5 6 3 6 3 6 3 6 3 6 8
Rice straw
5
80 120 80 120 80 120 80 120 80
Oil palm empty fruit bunch
10
120
Oil palm empty fruit bunch
10
120
8
Akebia herbal residue
10
120
8
Corncob
5
90
24
37 36 30
5 5 5
7 6 10
72 79
71 62 79 60 73 59 68 58 75 53
44 54 38 55 45 59 47 54 43 71
22 7 21 7 13 0.1 5 1 12 0
20 22 13 27 14 32 20 34 14 5
0
100
88
70
46
5
14
14
79
55
60 63 73
73 64 58
0 0 0
12 21 23
51 65 72 73 76 47 58 75 39 78 50 48 47 46 51 71
86 63 54 50 45 74 98 71 55 82
0 3 4 4 5 0
7 14 15 15 19 7 87 55 90 47
41 34 58 20 65 78 86 93 56 43
47
ChCl:malonic acid ChCl:glutaric acid ChCl:oxalic acid
1:1 1:1 1:1
ChCl:malic acid 1:1 Basic DES (amide-, amine-, imidazole-based DES) 1:1 (Hou, X.D. ChCl:formamide et al., 2018) ChCl:urea 1:1
ChCl:urea
(Procentese et al., 2015)
ChCl:urea Potassium carbonate:glycerol ChCl: monoethanolamine ChCl:diethanolamine (Zhao et al., ChCl: 2018) methyldiethanolamine ChCl:acetamide ChCl:urea Phenolic-based DES ChCl:4-hydroxybenzyl alcohol (Kim et al., ChCl:catechol 2018) ChCl:vanillin ChCl:p-coumaric acid (Tan et al., 2018)
22
81 80 78 58 58 94
15 14 9 10 10 10 8 8 15 13 11 7 4 15
24
20
34
75
38
14
14
25
36
51
1:6
61
52
19
6
9
47
81
1:8
73
47
25
7
2
15
74
84
41
23
13
1
10
45
1:2 1:2
95 90
36 37
23 23
19 15
2 4
2 6
3 28
1:1
83
41
22
31
7
29
0.4
67 63 57
53 51 48
22 20 13
20 19 18
5 13 26
43 50 71
49 53 61
1:2
Rice straw
120
5
6.25
80 115 80 115 150
10
120
1:2 Corncob
ChCl:imidazole
64 21 21 12 12 13 11 12 12 27 24 30 22 6 14
5
130
ChCl:urea
57 34 99
41 39 27 28 28 24 25 26 33 32 38 46 41 31
Rice straw
110 (Pan et al., 2017)
54 74 53
3:7 1:2 1:6
1:10
1:1 1:2 1:1
Oil palm empty fruit bunch
Wheat straw
Switchgrass
5
5
90
160
6
78 78
4 6 8 4 6 8 15 15
8
12
3
48
Table 3 Glucose and xylose yield from enzymatic hydrolysis process of untreated and DES-pretreated biomass and their respective CrI
(Chen, Z. et al., 2018) (Chen, Zhu et al., 2018) (Guo et al., 2018) (Hou, X.-D. et al., 2018) (Kim et al., 2018)
ChCl:glycerol, 0.9wt% H2SO4 ChCl:ethylene glycol, 1.0wt% H2SO4 betaine:lactic acid
Switchgrass
DES pretreatment condition (S/L ratio, temperature, time) 1:10, 110°C, 1h
Switchgrass
1:10, 130°C, 30mins
11
98
2
105
54
67
Xylose residue
1:20, 120°C, 2h
55*
96.8*
-
-
13.3
14.6
ChCl:oxalic acid
Rice straw
1:20, 120°C, 3h
18.4
52.9
3.1
0.4
55.8
55-59.6
ChCl:p-coumaric acid
Switchgrass
1:20, 160°C, 3h
-
85.7
-
28.8
-
-
-
77
-
42.4
-
-
(Kumar et al., 2016) (Li et al., 2018)
ChCl:glycerol
Rice straw
1:10
56.6*
87.1*
-
-
33.5
31.9
ChCl:lactic acid, 10wt% H 2O ChCl:urea
Rice straw
1:20, 120°C, 3h
24.2
66.8
6.7
21.7
-
-
Rice straw
1:20, 130°C, 4h
4.46
0.87
8.53
0.31
66.2
74.5
ChCl:imidazole
Corncob
1:16, 80°C, 15h
32.8
85.5
15.5
63
30.1
31.6
ChCl:glycerol
Lettuce
1:16, 150°C, 16h
-
94.9
-
75
-
-
ChCl:glycerol
Potato peel
1:16, 115°C, 3h
1
41
-
-
-
-
Apple residue
2
76
-
-
-
-
Coffee silverskin Brewer's spent grain Corn stover Corncob
3
29
-
-
-
-
2
34
-
-
-
-
1:20, 130°C, 2h 1:20, 90°C, 24h
22.1*
91.5* 96.4*
-
-
31.1 31.6
57.2 33.1
Wheat straw
1:20, 70°C, 9h
20.9
84.1
8.9
35.9
41.2
53.9
Oil palm trunk
1:19, 100°C, 48h
25*
60*
-
-
-
-
Work
(Pan et al., 2017) (Procentese et al., 2015) (Procentese et al., 2017) (Procentese et al., 2018)
DES
Biomass
ChCl:catechol
(Xu et al., 2016) ChCl:formic acid (Zhang et al., ChCl:glycerol 2016) (Zhao et al., ChCl:monoethanolamine 2018) (Zulkefli et al., Ethylammonium 2017) chloride:ethylene glycol *represents total sugar production.
Glucose yield (%)
Xylose yield (%)
CrI
Untreated biomass
DES-pretreated biomass
Untreated biomass
DES-pretreated biomass
Untreated biomass
DES-pretreated biomass
15.13
102.02
3.15
98.78
54
56-63
49
Table 4 Carbohydrate-based products produced from enzymatic hydrolysate and DES pretreatment liquid fraction Carbohydratebased products
Work
Enzymatic Hydrolysate Ethanol (Kumar et al., 2016) Butanol (Procentese et al., 2017) (Xing et al., 2018) (Xu et al., 2016) Butanediol (Chen, Zhu et al., 2018) Pretreatment liquid fraction (LF) Furfural (Chen, Zhu et al., 2018) Lipid (Dai et al., 2017)
Carotenoid
(Chen, Z. et al., 2018) (Chen, Z. et al., 2018)
DES pretreatment
ChCl:glycerol 1:1 ChCl:glycerol 1:2, 150°C, 16h NaCO3, 1h, 140°C; ChCl:formic acid:acetic acid 1:1:1, 130°C, 2h ChCl:formic acid, 130°C, 2h ChCl:ethylene glycol 1:2, acidified with 1.0wt% H2SO4, 130°C, 0.5h ChCl:ethylene glycol 1:2, acidified with 1.0wt% H2SO4, 130°C, 0.5h Biological pretreatment, 30°C, 3day; ChCl:oxalic acid 1:2, 120°C, 1.5h ChCl:glycerol 1:2, 120°C, 1h ChCl:glycerol 1:2, 120°C, 1h
Biomass source
Fermentation strain
Fermentation duration (hour)
Conversion efficiency (%)
Product concentration (g/L)
Product yielda (g/gsugar)
Rice straw Lettuce leave
NRRL Y-50464 DSMZ 792
24 60
89.5 -
22.6 0.5
0.04 (0.1)
Rice straw
DSM 13864
72
93
9.5
0.25 (0.36)
Corn stover Switchgrass
DSM 13864 NRRL B-14891
48 36
70.5 89.7
5.63 90.2
0.17 (0.21)
Switchgrass
N.A.b
N.A.
84.5
12.03
-
Bamboo shoot shell Switchgrass
CCZU11-1
72
97
2.2
-
NRRL Y-1091
144
-
8.1
-
Switchgrass
NRRL Y-1091
144
-
0.015
-
a
Figure in parenthesis are the total product yield including butanol and other types of fermentation products in the ABE fermentation which are acetone and ethanol. b Not applicable. The production does not involve fermentation process.
50
Highlights 1. Application of DES in raw lignocellulosic biomass fractionation and utilization. 2. Categorization of DES constituents used in lignocellulosic biopolymer fractionation. 3. Enhanced pretreatment efficiency through integration of DES with existing technology. 4. Downstream conversion of DES-fractionated products to a variety of bio-products. 5. Challenges and outlooks for DES utilization in biomass processing.
51
Graphical abstract
Introduction of deep eutectic solvents (DES) Biomass fractionation using DES
Effect of different hydrogen bond donor (HBD)
Effect of different hydrogen bond acceptor (HBA)
Ternary DES
Integration of DES pretreatment with other technologies
Microwave
Ultrasonication
Sequential
Acidified DES
Application of DES-fractionated products
Polysaccharides
DES-pretreated solid fraction
Enzymatic hydrolysate
Lignin
Pretreatment liquid fraction
Challenges and outlook of DES utilization
52
Author Contribution Statement Yee Tong Tan – Conceptualization, Writing - Original Draft; Adeline Seak May Chua – Writing - Review & Editing, Supervision; Gek Cheng Ngoh – Writing - Review & Editing, Supervision, Funding acquisition
53