Effects of cations, anions and H+ concentration of acidic ionic liquids on the valorization of polysaccharides into furfural

Article abstract of DOI:10.1039/c7nj00342k

The valorization of hemicellulose into valuable chemicals, such as C5 sugars and furfural, in a one-pot fashion is crucial. In this work, acidic ionic liquids in the presence of water showed high yields of C5 sugars (>80%) with >99% conversion of hemicelluloses at 160 °C. With a water + toluene biphasic solvent system, within 4 h, an 85% furfural yield was obtained directly from hemicellulose in a one-pot fashion using a catalytic amount of 1-methyl-3-(3-sulfopropyl)-imidazolium hydrogen sulfate. It was seen that Br?nsted acidic ionic liquids (BAILs) perform better than solid acid [Faujasite and Mordenite zeolites; ion exchange resin, Amberlyst-15] and mineral catalysts [HCl and H₂SO₄]. The higher activity of BAILs compared to solid acids and mineral acids was correlated to the Hammett acidity function (Ho) and ion-dipole type of interactions. The catalysts were characterized using NMR (¹H and ¹³C), elemental analysis and TGA to confirm that they were stable under reaction conditions and were thus recyclable.

Full text of DOI:10.1039/c7nj00342k

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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DOI: 10.1039/C7NJ00342K
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ARTICLE
Effects of cations, anions and H+ concentration of acidic ionic
liquids in the valorization of polysaccharides into furfural
Babasaheb M. Matsagar,ab Paresh L. Dhepe*ab
Received 00th January 20xx,
Accepted 00th January 20xx
Valorization of hemicellulose to valuable chemicals such as C5 sugars and furfural in the one‐pot fashion is crucial. In this
work, acidic ionic liquids in presence of water showed high yields of C5 sugars (>80%) with >99% conversion of
hemicelluloses at 160 °C. With water+toluene biphasic solvent system, within 4 h, 85% furfural yield was obtained directly
from hemicellulose in a one‐pot fashion using the catalytic amount of 1‐methyl‐3‐(3‐sulfopropyl)‐imidazolium hydrogen
sulfate. It is seen that BAILs perform better than solid acid [Faujasite and Mordenite zeolites; ion exchange resin,
Amberlyst‐15] and mineral catalysts [HCl and H2SO4]. The higher activity of BAILs compared to solid acids and mineral acid
was correlated to Hammett acidity function (Ho) and ion‐dipole type of interaction. The catalysts were characterized using
NMR (1H and 13C), elemental analysis and TGA to confirm that those are stable under reaction conditions and thus were
recyclable.
DOI: 10.1039/x0xx00000x
www.rsc.org/
9
sugars depending on the type of hemicelluloses used.8,
Introduction
Although, these methods are efficient but due to obvious
drawbacks (handling, separation, neutralization waste, pH
Biomass is a renewable, sustainable, abundant, CO2 neutral
and inexpensive alternating feedstock to fossil feedstock for
the synthesis of chemicals and fuels.1 Lignocellulosic biomass,
the main constituent of cell walls of plants is non‐edible and is
made up of cellulose (40‐50%), hemicelluloses (20‐30%) and
lignin (15‐25%). While cellulose is a polysaccharide made up of
dependent etc.) related to the utilization of acids and
enzymes, solid acid (Amberlyst‐70, sulfonated silica gel, HUSY
zeolite, etc.) catalyzed hydrolysis processes are developed.10, 11
However, it is seen that under the reaction conditions
employed, many of these solid acid catalysts undergo
morphological changes and thus it becomes obligatory to
propose a new catalytic system for the efficient valorization of
hemicelluloses into sugars. Although, upon hydrolysis, it is
easy to obtain xylose (C5 sugar) from xylan compared to
formation of glucose from cellulose, but due to the high
reactivity of C5 sugars many undesired products are formed
and hence it is challenging to achieve better yields of C5
sugars.12 To accomplish this, ionic liquids (ILs) with acidity were
shown to hydrolyze xylan type hemicellulose to yield xylose
with high yields (>80%).13
glucose units linked together via
hemicellulose is either homo‐ or hetero‐polysaccharide made
(1→4) glycosidic bonds,
up of C5 sugars, C6 sugars and sugar acids linked together via

(1→4), (1→3) and 
(1→6) glycosidic linkages.2 On the other
hand, lignin is a 3‐D polymer made up of different substituted
aromatic units linked together via ether and C‐C bonds.3
Typically, hemicellulose is rich in C5 sugars4 but, in certain
types of hemicelluloses (mannans, galactan), C6 sugars are
found to be major constituents.5 In particular, C5 sugars based
xylan type (with xylose backbone) hemicellulose is found in
most of the plants. The complex structure of biomass makes it
difficult to convert it into various chemicals and fuels in a very
selective and effective way.6, 7
In the biorefinery concept sugars obtained upon hydrolysis of
polysaccharides are considered as primary platform chemicals.
In view of this, conventionally, hydrolysis of hemicelluloses is
achieved using mineral acids and enzymes to obtain variety of
Xylose is formed by the hydrolysis of xylan, there has been
much of an interest in its dehydration into furfural by various
catalytic methods.14‐16 This is because furfural is used as a
solvent, in the manufacturing of phenolic resins as starting
compounds and for obtaining a variety of chemicals (furfuryl
alcohol, cyclopentanone, methyl furan, pentane diols, furoic
17
acid, aromatics etc.) in several industries.15,
To obtain
furfural from xylose, heterogeneous acid catalysts such as
sulfonic acid decorated porous silicas,18 heteropolyacids,19
Lewis acid and Brønsted acid catalysts (CrCl3∙6H2O and HCl) are
known.20 Use of ILs, such as [BMIM][Cl] like a solvent in
presence of mineral acid (H2SO4) is also reported for obtaining
furfural from xylose.21 While this method is efficient but the
use of IL in large quantity is detrimental in commercializing this
method.
a. Catalysis and Inorganic Chemistry Division CSIR‐National Chemical Laboratory.
Pune 411008, India, Email: pl.dhepe@ncl.res.in; Fax +91‐2025902633; Tel: +91‐
2025902024
b. Academy of Scientific and Innovative Research (AcSIR), New Delhi (India).
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Although these two‐pot reactions (xylan to xylose and xylose speed was increased (800 rpm) and this time, is considered as
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to furfural) to yield furfural are known but, it would be better starting time of a reaction. Once the reDacOtIi:o1n0.1i0s3f9i/nCi7sNhJe0d0,3t4h2eK
to convert xylan type of hemicellulose directly into furfural in a reactor is cooled to room temperature and the reaction
one‐pot fashion. In view of this, few efforts are put towards mixture was analyzed using high performance liquid
the direct valorization of hemicelluloses into furfural over solid chromatography (HPLC). For hemicellulose to furfural reaction
acid catalysts.10 Nonetheless, as mentioned above in many of also similar reactor is used as mentioned above. In this
these reactions, catalyst’s stability is an issue.
reaction, 0.6 g hemicellulose (beechwood) was charged along
with catalyst 0.12 g (substrate/catalyst 5 wt/wt) and solvent
water+MIBK (1:5 v/v). The charged reactor is then heated to
the desired temperature and time. Here also before achieving
desired temperature 200 rpm stirring speed was maintained
and after achieving the desired temperature, stirring was
increased to 800 rpm. Upon completion of the reaction, the
biphasic reaction mixture (aqueous and organic) is separated
using a separating funnel. The aqueous reaction mixture was
analyzed using HPLC and the organic reaction mixture was
analyzed using GC.
Scheme 1. Valorization of hemicellulose (xylan).
Analysis
The reaction mixture was analyzed using HPLC, GC, and GC‐
MS. The aqueous reaction mixture is analyzed using HPLC
(Agilent 1200 series) equipped with Pb2+ column (Rezex RPM‐
The conversion of xylan type of hemicellulose into furfural
(63% yield) is also reported using CrCl3 as a catalyst in presence
of IL under microwave‐assisted heating.22 Yet, again the use of
IL on large scale is a drawback of this method. Thus, in spite of
few efforts, it is a challenge to develop a proficient method for
obtaining furfural from hemicellulose.
Since acidic ILs are known to catalyze the hydrolysis of
hemicelluloses to yield xylose,13 it was believed that similar
catalytic system can be employed for the production of
furfural from xylan in a one‐pot fashion after optimizing the
reaction conditions (Scheme 1). By doing so, drawbacks
associated with reported conventional catalytic methods like
mineral acids, enzymes and use of huge amount of ILs along
with mineral acids or metal chlorides, etc. for the valorization
of hemicellulose into monosaccharides (C5 sugars) and their
further dehydration into furfural can be avoided. Moreover,
with the use of acidic ILs, the issue of stability of solid acids
would be avoided.
Herein we report valorization of hemicellulose to furfural in a
direct fashion using the catalytic amount of Brønsted acidic ILs
(BAILs) as a catalyst in aqueous as well as biphasic solvent
systems. Moreover, the effects of cations and anions on the
catalytic performance are studied in detail. Additionally, to
overcome the problems associated with dilute substrate
systems typically employed in most of the literature reports,
studies on the higher hemicellulose loading are performed (6
wt%).
Monosaccharide, 300 x 7.8 mm; particle size 8
m). The
analysis was carried out at 80 °C of oven temperature.
Degassed HPLC grade water was used with 0.5 mL/min flow
rate. A RID (G1362A 1260RID) with 40 °C cell temperature of
was used for analysis. Prior to the analysis, the samples were
filtered through 0.22 m syringe filter and then 10 L of the
sample was injected for the analysis. Before analyzing the
reaction mixture, a calibration curve of different standards like
xylose, arabinose, furfural, were done.
The organic phase of the reaction was analyzed using GC
(Agilent 7890B) instrument equipped with HP‐5 capillary
column (30 m x 0.23
m ID) and flame ionization detector (FID)
operated at 280
the analysis: 100
°C. The following oven program was used for
°C  7 °  250 °C (hold time: 3 min).
C min‐1
The flow rates of H2, air and N2 were 30, 300 and 20 mL min‐1,
respectively. For GC analysis all the samples were filtered
through 0.22
was injected.
m syringe filter before analysis and 1 µL sample
RESULTS AND DISCUSSION
For carrying out reactions, several ILs were synthesized
(Section 1, ESI) and characterized (Sections 2‐4, ESI) for their
purity, stability, and acidity.
Valorization of hemicellulose into sugars:
Evaluation of various catalysts
Experimental
The results of catalysts evaluation study for the valorization of
hardwood hemicellulose (xylan from birchwood) into
monosaccharides such as C5 sugars (xylose+arabinose (X+A))
at 160 °C for 1 h are summarized in Fig. 1. As observed, the
[C3SO3HMIM][HSO4] BAIL catalyst could give the best yield of
C5 sugars (87% yield). Under similar conditions,
[C3SO3HMIM][PTS] BAIL (76%), [C3SO3HMIM][Cl] BAIL (75%),
and [C3SO3HBenzMIM][PTS] BAIL (73%) also showed better
activity for C5 sugars (X+A). The [C3SO3HNEt3][PTS] (39%) and
Catalytic Experiments
Reactions were performed in Parr autoclave (300 mL) having
temperature and stirring controller unit. In
hemicellulose to sugar reaction, 0.6 hemicellulose
(birchwood) along with 0.24 g catalyst (substrate/catalyst 2.5
wt/wt) and 60 mL water were charged and then reaction
temperature was set to desired value under stirring of 200
rpm. After reaching the set reaction temperature stirring
a typical
g
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[C3SO3HPPh3][PTS] (25%) BAILs showed lower activity towards interaction of BAILs with the hemicellulose (ion‐dipole type
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13
DOI: 10.1039/C7NJ00342K
C5 sugars formation compared to other BAIL catalysts used in interaction) and its homogenous nature. It is evident that
this study.
compared to HMOR (Si/Al=10) (Ho=1.31), HUSY (Si/Al=15)
(Ho=0.97) has higher strength and therefore, it showed better
activity (Fig. 1). The solid acid catalysts form the
heterogeneous phase in reaction solutions and hence showed
lower activity compared to BAILs catalyst.
Effect of H+ concentration
The effect of H+ concentration in acidic ILs and mineral acid
catalyzed reactions was studied at optimized reaction
condition (160 °C, 1 h). Fig. 1 shows that 50% C5 sugars (X+A)
yield can be obtained if 0.24 g of H2SO4 is used (4.9 mmol of
H+) for the reaction (Fig. 1; Section 5, Table S3, ESI). The
inferior yield of C5 sugars is because when 0.24 g of H2SO4 was
used, very high concentration of H+ (4.9 mmol) was present in
the reaction solution. This high concentration of H+ may
eventually initiate many side reactions, which otherwise would
not occur when lower concentrations of H+ are available in
reaction. The higher concentration of H+ (4.9 mmol) present
Figure 1. Catalyst evaluation studies for the valorization of hemicellulose into sugars.
Reaction condition: substrate 0.6 g, catalyst 0.24 g, water 60 mL, 160 °C, 1 h.
when H2SO4 is used, compared to when 0.24
g of
The [BMIM][Cl] IL which does not have any acidity under
similar reaction conditions showed only 59% of C5 sugars (X+A)
[C3SO3HMIM][PTS] BAIL (0.64 mmol H+) was used in the
reaction solution may thus hamper the activity. To cancel the
effect of the H+ concentration in H2SO4 on the formation of
products, the reaction was performed with H2SO4 using the
similar H+ concentration (0.64 mmol) for the reaction as that
observed with [C3SO3HMIM][PTS] catalyst. In this reaction, a
slight improvement in the yield (65%) was observed, but still,
this yield was lower than the yield obtained with
[C3SO3HMIM][PTS] catalyst (76%). The effect of H+
concentration was studied in detail by charging varying
quantity of H2SO4 (Table S3, ESI) in the reaction. The best yield
of C5 sugars (76%) was achieved when 2.4 mmol of H+
concentration is used when the effect of H+ concentration was
studied in detail with H2SO4. Further by decreasing the H+
concentration of H2SO4 (1.26 mmol) lower yield of C5 sugar
was seen (66%).
yield. On the other hand,
a similar type of BAIL
([C3SO3HMIM][Cl]) having Brønsted acidity showed 75% of C5
sugars yield. This suggests that the Brønsted acidity of BAIL is
important for obtaining higher C5 sugars yield. Further, the
catalytic performance of BAILs was evaluated against solid acid
catalysts such as Faujasite and Mordenite zeolites. The results
show lower C5 sugars yield with these solid acid catalysts (37%
and 22%) compared to BAILs (Fig. 1). Additionally, with mineral
acid (H2SO4) only 50% yield of C5 sugars was possible,
compared to [C3SO3HMIM][HSO4] BAIL (87%). Thus it is implied
that [C3SO3HMIM][HSO4] BAIL is the best performing catalyst
in this reaction.
To understand the differences in the activities between all
these catalysts, the strength of the acid in all the catalysts was
measured using Hammett acidity function (Ho). From the data
of Hammett acidity function (Section 4, ESI) the order of acid
strength was found as,
The highest C5 sugar yield (76%) was obtained when 2.4 mmol
of H+ concentration of H2SO4 was used. By maintaining similar
H+ concentration (0.64 mmol of H+) HCl (39%) and organic acid
(p‐toluenesulfonic acid monohydrate (PTSA) 44%) were also
used in the reactions; however, those showed lower activity
(Table S3, ESI) than BAIL. This phenomenon of BAIL showing
higher yields than conventional acids is because of the ion‐
dipole type of interaction as shown in earlier work.13
The solid acid catalysts showed 37% (Faujasite) and 22%
(Mordenite) of C5 sugars yield when 0.24 g catalyst loading
was used (Section 5, Table S3, ESI). When reactions were
carried using similar H+ concentration (0.64 mmol, similar like
BAIL [C3SO3HMIM][PTS] by altering the catalyst loading (Table
S3, ESI), with HUSY catalyst, 61% yield and with HMOR catalyst
48% C5 sugars yield was observed. It is anticipated that due to
diffusivity problem in solid acids, their activity is lower than
BAILs.
HUSY (Si/Al=15; (0.97)) > HMOR (Si/Al=10; (1.31)) > H2SO4
(1.67) > [C3SO3HMIM][HSO4] (2.08) > [C3SO3HMIM][PTS] (2.33)
> [C3SO3HMIM][Cl] (2.47).
The acid strength is higher for the catalyst if its Ho value is
lower. Among all the BAILs evaluated for hemicellulose
valorization, [C3SO3HMIM][HSO4] BAIL has a Ho of 2.08, which
is lowest than all other BAILs ([C3SO3HMIM][PTS] (2.33) and
[C3SO3HMIM][Cl] (2.47)). This trend is in line with the Ho, as
[C3SO3HMIM][HSO4] BAIL showed the maximum C5 sugars
yield (87%), and [C3SO3HMIM][PTS], [C3SO3HMIM][Cl] BAILs
showed lower (76 and 75%) C5 sugars yields. The [BMIM][Cl] IL
is neutral and hence, its activity is the lowest (59% C5 sugar
yield) compared to all other ILs evaluated present work.
Moreover, mineral acid (H2SO4) has a lower Ho (Ho=1.67) than
the BAILs; yet, the activity of [C3SO3HMIM][HSO4] is higher
than H2SO4. This implies that even if BAIL has lower acid
strength than H2SO4 catalyst, it has shown higher activity for
C5 sugars formation. The higher activity of BAILs is due to the
The reactions were performed with different substrates using
[C3SO3HMIM][PTS] BAIL catalyst and it was seen that all other
substrates could also give higher yields of C5 sugars (Fig. S18,
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ESI). This conveys that the catalyst is capable of valorizing
almost all types hemicellulose substrates.
Role of cations on activity
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DOI: 10.1039/C7NJ00342K
To comprehend the effect of cations of BAILs on the
valorization of hemicellulose, BAILs with different types of
cations were used by keeping similar anion p‐toluenesulfonate
(PTS) in all the BAILs. Additionally, in all the catalysts propyl
sulfonic acid group attached to cation was kept constant which
actually is responsible for giving acidity. The reactions were
carried out at optimized reaction condition (1 h, 160 °C) in the
water medium. The results presented in Fig. 2, shows that
BAILs with imidazolium‐based cations shows the best catalytic
activity (76% C5 sugar yield) compared to BAILs with
quaternary ammonium (39% C5 sugar yield) and
triphenylphosphonium (25% C5 sugar yield) based cations.
The discrepancy in the C5 sugars yield among all the catalysts
is believed to be due to the structure of cations because as
mentioned above anion part was kept similar in all the BAILs. It
is understood that the BAILs with imidazolium based cation
shows the better interaction (ion‐dipole type) with the
substrate.13 While, the structure of imidazolium cation is
planar which can help for this type of interaction, in the case of
BAIL with quaternary ammonium based cation, the structure
of cation is not planar and hence there might not be sufficient
interaction between the IL and substrate molecule (Section 6,
Table S4, ESI). This effect eventually would show lower C5
sugar yield (38%). The possibility of varying H+ concentration
on the yields was nullified by considering that quaternary
ammonium based IL has almost similar H+ concentration (0.6
mmol g‐1) in reaction system compared with imidazolium
based BAIL, [C3SO3HMIM][PTS] (0.64 mmol g‐1). The BAIL with
triphenylphosphonium based cation has bulky phenyl groups
attached to the phosphorus atom and also the structure of
cations is non‐planar and hence it is suggested that this factor
is responsible for achieving poor interaction between BAIL and
polysaccharide. Therefore, triphenylphosphonium based BAIL
also shows a lower yield of C5 sugars (25%). On the contrary,
since benzimidazolium and pyridinium cations have planar
structures those showed nearly same activity as that was
observed for imidazolium‐based BAIL. It is particularly
interesting because, in the case of benzimidazolium based IL,
slightly lower H+ concentration (0.56 mmol g‐1) is present
compared with imidazolium based BAIL (0.64 mmol g‐1).
Figure 2. Effect of cations of BAILs on the valorization of hemicelluloses. Reaction
condition: substrate 0.6 g, catalyst 0.24 g, water 60 mL, 160 °C, 1 h
Valorization of hemicellulose into furfural:
Effect of solvent system and the ratio of biphasic solvent system
From the above studies it is understood that when
hemicellulose
conversion
is
carried
out
using
[C3SO3HMIM][HSO4] BAIL at 160 °C for 1 h in only water as a
solvent, 10% furfural yield along with 87% C5 sugars (X+A)
yield could be obtained (Fig. 1). To improve the yield of
furfural which is a dehydration product of C5 sugars (X+A), the
time of reaction was increased, and maximum furfural yield i.e.
21% was achieved after 4 h (Fig. 3). Subsequently, to improve
the furfural yield, the reaction was performed for 6 h, but a
decrease in furfural yield (17%) was seen. This is because in
water many undesired side reactions are prominent which
decreases the furfural yield when reactions are carried out for
a longer time. Since sugars, furfural and BAILs are soluble in
water, it is possible that sugars and furfural may react together
in water to yield some undesired products in presence of
catalyst, which may hamper the yield of furfural.23 From the
earlier studies it is evident that with the introduction of water
immiscible organic solvent (biphasic system), improvement in
furfural yield is possible.
The employment of polar aprotic solvents is reported in the
literature for obtaining furfural.24 It is reported that the
furfural yield of 37% can be achieved from xylose in N, N‐
dimethylformamide (DMF) solvent in presence of Amberlyst‐
15 along with hydrotalcite (HT) as a catalyst. When the
reaction was performed in DMSO over Nafion 117, furfural
yield of 60% was obtained. While these solvents could yield
furfural in good quantity but their high boiling points are
detrimental in using them on a larger scale and in the
25
separation of products from them.23,
To surmount the
problems related to earlier systems, use of the biphasic
solvent system was proposed. It is successfully shown that
with water either addition of toluene or MIBK in the presence
of zeolitic catalysts (HY, HMOR) can efficiently conversion of
xylose into furfural.26, 27 Considering this, in this work, various
biphasic solvent systems were studied to improve the furfural
yield.
As seen from Fig. 3, along with only water, 3 biphasic solvent
systems were evaluated for their effect on improvement in
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furfural yield using [C3SO3HMIM][HSO4] BAIL as a catalyst at was studied since depending on the ratio VieowfArtsicolelvOennlinte,
170 °C for 4 h. While carrying out this study, reactions were concentration of substrate in water andDeOxI:t1r0a.c1t0i3o9n/Cp7oNsJs0i0b3i4li2tKy
done at 170 °C for 4 h instead of at 160 °C because with an of furfural in toluene will increase which may affect the
increase in temperature improvement in yield was observed. furfural yield. The reactions were carried out with various
As seen, with water+toluene solvent system maximum furfural water+toluene ratios such as 1:1 v/v, 1:3 v/v, 1:5 v/v and 1:7
yield (62%) was achieved compared to water+p‐xylene (49%) v/v using beechwood hemicellulose and [C3SO3HMIM][HSO4]
and water+MIBK (52%). When the hemicellulose valorization BAIL. As observed from Fig. S20 (ESI), when 1:1 v/v and 1:3 v/v
into furfural was carried out using [C3SO3HMIM][HSO4] BAIL ratios were used, 45 and 47% furfural yields were observed,
under similar reaction condition with only water as a solvent, it respectively. The best result (62% furfural yield) was obtained
showed a lower yield of furfural (21%) (Fig. 3). The effect of with a water+toluene biphasic solvent system having 1:5 v/v
solvent study on the valorization of hemicellulose into furfural ratio. Further, increase in toluene concentration in the
shows that to achieve maximum furfural yield, the addition of water+toluene biphasic solvent system (1:7 v/v) showed a
toluene to water is beneficial.
decrease in the yield of desired products (60% furfural yield
along with 10% C5 sugars yield). This is due to the fact that
with a decrease in water in the system, the solution becomes
very concentrated with respect to catalyst and substrate and
this enhances the probability of occurrence of side reactions
(almost similar phenomenon observed with higher H+
concentration of H2SO4). The decreased yield of furfural with
1:7 v/v ratio, in turn, showed a poor mass balance (for
water+toluene (1:5 v/v) 81% mass balance and for
water+toluene (1:7 v/v) 73% mass balance was observed).
Effect of reaction temperature & time
As it was believed that with alteration of reaction
temperature, improvement in furfural yield is possible using
[C3SO3HMIM][HSO4] catalyst and water+toluene biphasic
solvent system. When reactions were carried out at 160 °C,
Figure 3. Valorization of hemicellulose to furfural in different solvents. Reaction
condition: substrate 0.6 g, [C3SO3HMIM][HSO4] 0.12 g, water+organic solvent 60 mL
(1:5 v/v), 170 °C, 4 h
170
41% (160 °C), 14% (170
yield was seen (Fig. S21, ESI). Although, similar furfural yield
was achieved at 170 and 180 C, but after the reaction was
°C and 180 °C, 35, 62 and 64% furfural yield along with
°
C) and 5% (180 C) C5 sugars (X+A)
°
°
The observance of maximum furfural yield with water+toluene
solvent system can be explained on the basis of the partition
coefficient (Table S5, and Fig. S19, ESI). It was observed that
furfural has a maximum solubility in toluene compared to p‐
xylene and therefore in the water+toluene biphasic solvent
system effective extraction of furfural is possible in toluene
during the course of reaction compared to p‐xylene in the
water+p‐xylene system. Likewise, furfural has a slightly higher
solubility in MIBK compared to toluene, this can be understood
from the partition coefficient of furfural in MIBK/water (1.38)
and toluene/water (1.32) solvent system (Table S5, and Fig.
S19, ESI). Hence, the water+MIBK biphasic solvent system
should have shown higher furfural yield, but experimentally
water+toluene solvent system showed better furfural yield.
This is because MIBK has higher solubility in water (19.1 g L‐1 at
20 °C) compared to the solubility of toluene in water (0.52 g L‐1
at 20 °C).10 Because of higher miscibility of MIBK in water,
furfural extracted in MIBK can again come in contact with
water and eventually with catalyst and thus it can undergo
further side reactions. This phenomenon is less prominent in
the case of water+toluene biphasic solvent system and thus
the maximum yield of furfural was obtained with this solvent
system.
carried out at 180 °C lower C5 sugars yield (5%) compared to
the reaction performed at 170 °C (14%) for a similar time (4 h)
was obtained. These results suggest that at 170 °C, 81% mass
balance could be achieved while at 180 °C, 72% mass balance
could be obtained. Considering this, evaluation of other
catalysts was done at 170
°C for 4 h in presence of
water+toluene (1:5 v/v).
The influence of reaction time on the valorization of
hemicellulose into furfural was also checked and the results
are presented in Fig. S22 (ESI). When the reactions were
carried out using [C3SO3HMIM][HSO4] BAIL at 170 °C for 4 and
5 h in water+toluene (1:5 v/v) biphasic solvent system, the
similar yield of furfural (62%) could be obtained. However,
higher C5 sugars (X+A) yield was obtained in 4 h (14%)
compared to 5 h (11 %). The mass balance calculations show
that when the reaction is carried out for 4 h, higher mass
balance is possible (81%) compared with 5 h reaction (73%).
This is obvious since, with longer reaction time, degradation
reactions become predominant.
Evaluation of catalysts
Conversion of hemicellulose into furfural is much challenging
compared to the conversion of xylose into furfural. This is
because in the conversion of hemicellulose into furfural two
reactions are happening i.e. hydrolysis of hemicellulose into
sugars and further dehydration of these sugars into furfural.
While converting the hemicellulose into furfural the catalyst
After identifying the best biphasic solvent system,
water+toluene for the valorization of hemicellulose into
furfural, the effect of the ratio of the biphasic solvent system
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should first convert hemicellulose into sugars in a short compared with solid acid catalyst can be explained as; the
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reaction time compared to the conversion of xylose into BAILs homogeneous reaction phase but DsoOlIi:d10a.1c0id39c/Cat7aNlJy0s0ts34d2oK
furfural. Because if catalyst and furfural along with sugar are not. It is very important to note that the concentrated
present in the solution for a longer time then it becomes hemicellulose solution (6 wt%; with respect to water) was used
difficult to obtain higher selectivity for furfural due to the for the reaction and 62% yield of furfural could be achieved.
formation of humins through side reactions. As BAIL However, in previous work on hemicellulose conversion into
([C3SO3HMIM][HSO4]) is observed to convert hemicellulose furfural, 1wt% hemicellulose solutions were used.28
into sugars in a very short time (< 1 h) compared to many It is expected that the catalyst with Brønsted acidity is active
other solid acid catalysts. Therefore it is anticipated that for hydrolysis of hemicellulose to yield xylose and for
[C3SO3HMIM][HSO4] BAIL can covert hemicellulose into dehydration of this xylose to yield furfural. Considering this
furfural efficiently.13
then H2SO4 should show better furfural yield from
Since with [C3SO3HMIM][HSO4] BAIL catalyst, maximum C5 hemicellulose compared to BAIL because as shown in our
sugars (X+A) yield (87 %) was achieved in hemicellulose earlier work in a two‐pot reaction, it can efficiently convert
hydrolysis reaction, the same catalyst was also employed in hemicellulose into sugars (76% C5 sugar yield; 0.117 g H2SO4)
earlier discussed works on furfural synthesis. After optimizing and sugars into furfural (60% furfural yield; 0.08 g H2SO4).13,15
the reaction parameters (t, T, solvent ratio etc.) with this But in reality, when direct one‐pot conversion of
catalyst, subsequently, all other catalysts were evaluated for hemicelluloses is performed (this work), BAIL performs better
their performance in yielding furfural under optimized reaction compared to H2SO4 ([C3SO3HMIM][HSO4] 62% furfural and 14%
conditions. Fig. 4 illustrates that the solid acid catalysts like C5 sugar yield; H2SO4 44% furfural and 3% C5 sugar yield from
HUSY (Si/Al=15) and Amberlyst‐15 could yield 34 and 42% of hemicellulose). It is also reported in the literature that the
furfural, respectively. Although as seen from results, solid acid catalytic system which is efficient in the conversion of xylose
catalysts showed comparable results as that observed with a into furfural is not efficient for the conversion of xylan
mineral acid and BAILs but due to their lower thermal stability (hemicellulose) into furfural.29 The CrCl3 catalyst in a DMA‐LiCl
those could not be reused. It is reported in the literature that solvent and [EMIM][Cl] IL as additive showed 40% furfural
the solid acid catalysts often undergo morphological changes yield from xylose while the same catalyst showed very less
during the course of reaction10 and similar results were also furfural yield (3%) from xylan. This shows that conversion of
observed in this work (data not shown). In case of all the BAILs, hemicellulose into furfural is much difficult compared to the
following trend of activity was observed,
conversion of xylose into furfural. The catalyst investigated in
the present study is capable of converting hemicellulose into
furfural successfully in the one‐pot fashion.
Stability of furfural
[C3SO3HMIM][HSO4] (62%)
[C3SO3HMIM][PTS] (41%).
> [C3SO3HMIM][Cl] (44%) >
This trend can be explained on the basis of Hammett acidity
(Ho) as explained above and in ESI (Section 4, Table S2).
Careful optimization of reaction parameters was performed to
advance the furfural yield. It was seen that maximum possible
yield of furfural was only 62%. This might be due to two
reasons; first, due to the stability of furfural in the reaction
mixture and the second, possible side reactions of furfural with
C5 sugars and reactions between C5 sugars. To check the first
possibility, furfural stability under optimized reaction condition
was carried out (water+toluene (1:5 v/v) at 170 °C for 5 h with
and without BAIL catalyst ([C3SO3HMIM][HSO4]). Since in a
typical reaction, 0.436 g of furfural can be obtained from 0.6 g
of hemicelluloses charged anticipating 100% yield is achieved,
this quantity of furfural was charged in the reactor. After the
completion of a reaction, the water layer was analyzed using
HPLC while the toluene layer was analyzed using GC. The result
shows that there is no any extra peak apart from furfural
(furfural and BAIL in the case of reaction carried out with BAIL)
in both the layers in GC and HPLC. The furfural stability study
carried out with BAIL catalyst showed 7% furfural conversion
and the reaction carried out without BAIL catalyst showed 4%
furfural conversion, which means that furfural is stable under
reaction conditions. Furthermore, to validate the second
possibility of side reactions between C5 sugars and furfural,
the reactions were carried out under optimized reaction
condition [water+toluene 60 mL (1:5 v/v), 170 °C] for 5 h with
Figure 4. Catalyst evaluation for the valorization of hemicellulose into furfural. Reaction
conditions: substrate 0.6 g, catalyst 0.12 g, water+toluene 60 mL (1:5 v/v), 170 °C, 4 h
The [BMIM][Cl] IL used for the valorization of hemicellulose
showed only 30% furfural yield along with 3% C5 sugars (X+A)
yield. When the reaction was performed in absence of a
catalyst, 22% furfural yield along with 8% C5 sugars (X+A) yield
was obtained. Even if almost half the furfural yield obtained in
this reaction in comparison to catalytic reactions, but the poor
mass balance (30%) emphasizes the fact that dehydration
reactions are needed to be carried out with the catalyst. More
importantly, the [C3SO3HMIM][HSO4] BAIL showed higher
furfural yield compared to HUSY (Si/Al=15) and Amberlyst‐15.
The higher yield of furfural with [C3SO3HMIM][HSO4] BAILs
0.3
g
xylose and 0.16
g
furfural using 0.12
g
[C3SO3HMIM][HSO4] BAIL. After the reaction, the formation of
6 | J. Name., 2012, 00, 1‐3
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black colored solid along with 70% furfural yield (0.25 g) was highest possible furfural yields obtained from hemicellulose in
View Article Online
DOI: 10.1039/C7NJ00342K
seen. It was also observed that almost 90% xylose was a one‐pot method.
Recycling and stability of [C3SO3HMIM][HSO4] BAIL
converted during the course of the reaction. By considering
this, it was expected to achieve 94% (0.33 g) yield of furfural,
however, only 70% (0.25) yield was seen. This shows that in
the hemicellulose valorization into furfural, the mass loss is
observed because of side reactions between furfural and C5
sugars.
For the recycling study of a catalyst, after the first reaction, the
catalyst was recovered (Section 15, ESI) and was used in the
next reaction. In the first run, 62% furfural yield along with
14% C5 sugars yield was achieved. In a 1st recycle run, similar
furfural yield (62%) as that obtained with fresh catalyst was
seen. However, the yield of C5 sugars (X+A) was marginally
Effect of catalyst & substrate concentration
As it is always desired to obtain higher product yields with decreased in this reaction (11%). Again in 2nd (59%) and 3rd
optimum quantity of catalyst, the study on the effect of (57%) recycle runs almost similar yields were obtained. This
catalyst concentration was carried out and the results are suggests that the catalyst is recyclable.
illustrated in Fig. S23 (ESI). The reactions were performed at For understanding the stability of [C3SO3HMIM][HSO4] BAIL
170 °C in water+toluene (1:5 v/v; 60 mL) solvent system for 4 under reaction conditions, recovered catalyst was
h. The results indicate that a very less quantity (0.12 g BAIL for characterized by 1H NMR technique (Section 14, Fig. S25, ESI).
1
0.6 g hemicellulose, S/C=5 wt/wt) of [C3SO3HMIM][HSO4] BAIL The H NMR spectrum of the recovered catalyst is similar to
was required to achieve 62% furfural yield along with 14% C5 the fresh catalyst and thus it is believed that the catalyst does
sugars (X+A) yield (Fig. S23, ESI). In this reaction, >96% not undergo any transformation during reactions. The TGA
hemicellulose conversion was observed with 81% mass study of recovered BAILs shows that there is no mass loss up
balance, which might be due to the fact that some of the to 250 °C (Section 3, Fig. S17, ESI), which again emphasizes
products were not detected in HPLC and GC. The reaction that the [C3SO3HMIM][HSO4] BAIL catalyst which showed the
carried out with S/C=10 wt/wt (0.06 g BAIL for 0.6 g best performance in the valorization of hemicellulose into
hemicellulose) gave 30% furfural yield and 3% C5 sugars with furfural reaction is a stable catalyst.
90% hemicellulose conversion. In this reaction, besides furfural
and C5 sugars formation, oligomers and some side products
which were not soluble in reaction solvent were observed.
CONCLUSION
With higher loading of [C3SO3HMIM][HSO4] catalyst, S/C=2.5
wt/wt (0.24 g BAIL for 0.6 g hemicellulose), 60% furfural yield
and 7% C5 sugars yield was achieved. In the reaction, >99%
hemicellulose conversion was seen with 71% mass balance.
This implies that by increasing the catalyst quantity from 0.12
g to 0.24 g the mass balance decreases because of increase in
side reactions. The reaction carried out with S/C=5 wt/wt and
S/C=2.5 wt/wt showed almost similar furfural yield (60 and
62%) however, higher C5 sugars yield (14%) was achieved
when S/C=5 was used compared to S/C=2.5. This indicates that
S/C=5 wt/wt is optimized S/C ratio for the valorization of
hemicellulose. The higher catalyst quantity (0.12 g) is required
for converting hemicellulose into furfural in one‐pot fashion
compared to the conversion xylose into furfural (0.08 g).
We demonstrated herein that BAILs are a selective and
efficient catalyst for the valorization of hemicellulose into
furfural in a one‐pot fashion. Different types of hemicellulose
employed in the study and observed that BAILs can efficiently
convert a wide range of hemicellulose into sugar monomers.
The catalytic amount (0.12 g) of [C3SO3HMIM][HSO4] BAIL
(without addition of any extra catalyst like mineral acids or
metal halides) efficiently converted hemicellulose into furfural
in a one‐pot fashion with a very high yield of furfural i.e. 85%
under optimized reaction conditions (170 °C,
4
h,
water+toluene 60 mL (1:5 v/v)). The reaction carried out with
concentrated hemicellulose under similar conditions showed
62% furfural yield. The result shows that with increasing
hemicellulose concentration side reactions were increased and
hence furfural yield and selectivity was decreased. Similarly,
with increasing the reaction time more than 4 h at 170 °C and
temperature above 170 °C (4 h) favours degradation reactions.
For achieving a better yield of furfural, water+toluene solvent
system is necessary; this was explained with the help of
partition coefficient of furfural in organic/water biphasic
solvent system and the solubility of the organic phase in water.
The [C3SO3HMIM][HSO4] BAIL catalyst used for the valorization
of hemicellulose into furfural performs better compared to
solid acid and mineral acid catalysts such as HUSY (Si/Al=15),
Amberlyst‐15 and H2SO4. The [C3SO3HMIM][HSO4] BAIL
recovered from the reaction mixture is a stable and recyclable
catalyst which was confirmed by 1H NMR study.
It is vital to test the developed catalytic method with the
concentrated substrate solutions to make the system
economical. Considering this, the effect of substrate
concentration was studied. The reactions were carried out
using [C3SO3HMIM][HSO4] BAIL as
a
catalyst with
water+toluene (1:5 v/v; 60 mL) as a biphasic solvent system at
170 °C by maintaining BAIL and solvent quantity constant (Fig.
S24, ESI). The reaction carried out with 0.2 g hemicellulose (2
wt% hemicellulose solution with respect to water; S/C=1.6
wt/wt) showed 85% furfural yield, while the reaction carried
out with 0.4 g (4 wt% hemicellulose solution with respect to
water; S/C=3.3 wt/wt) and 0.6 g (6 wt% hemicellulose solution
with respect to water; S/C=5 wt/wt) of hemicellulose showed
70 and 62% furfural yield. These results suggest that lower
hemicellulose concentration (0.2 g hemicellulose in 60 mL
water+toluene (1:5 v/v)) is beneficial to achieve higher (85%)
furfural yield. To the best of our knowledge, these are the
Acknowledgements
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Author B. M. Matsagar thanks CSIR New Delhi for research
fellowship.
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DOI: 10.1039/C7NJ00342K
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Graphical Abstract: