How do isomeric ortho, meta and paradicationic ionic liquids impact the production of 5-hydroxymethylfurfural?

Article abstract of DOI:10.1016/j.molliq.2017.02.076

This research investigates the influence of the subsituent's relative positions (ortho, meta, or para) of dicationic ionic liquids (ILs) on the catalytic production of 5-hydroxymethylfurfural (HMF). This research started with the facile synthesis of new dicationic ILs followed by its characterization using NMR, FT-IR, CHN, and UV–Vis. Then the acidity properties are measured using the Hammet method. The screening process revealed that the relative positions of substituents (ortho, meta and para) on the cationic site remarkably affect the overall catalytic performance. Highly efficient and selective dehydration of D-fructose to HMF was obtained using Br?nsted ortho-IL with a fructose conversion of 95.7% and HMF yield of 90.5% after 60?min at 100?°C. The results revealed that the geometry of the isomer ILs influences the yield of HMF. The effects of reaction time, temperature, catalyst concentration and reusability of the dicationic ILs are studied in this work.

Full text of DOI:10.1016/j.molliq.2017.02.076

Accepted Manuscript
How do isomeric ortho, meta and para dicationic ionic liquids
give impact to the production of 5-hydroxymethylfurfural?
Suhaila M. Yaman, Sharifah Mohamad, Ninie S.A. Manan
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DOI:
Reference:
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doi: 10.1016/j.molliq.2017.02.076
MOLLIQ 6989
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7 February 2017
9 February 2017
Please cite this article as: Suhaila M. Yaman, Sharifah Mohamad, Ninie S.A. Manan , How
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ACCEPTED MANUSCRIPT
How do isomeric ortho, meta and para dicationic ionic liquids give impact
to the production of 5-hydroxymethylfurfural?
Suhaila M. Yaman1, Sharifah Mohamad1,2 and Ninie S. A. Manan1,2*
1Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur,
2University Malaya Centre for Ionic Liquid (UMCiL), University of Malaya, 50603 Kuala
Lumpur, Malaysia.
E–Mails: suyaman@siswa.um.edu.my (S.Y.); sharifahm@um.edu.my (S.M.)
*Author to whom correspondence should be addressed; E–Mail: niniemanan@um.edu.my;
Tel.: +603–79677022 (ext. 3873); Fax: +603–79677188
Abstract
This research involves the investigation on how the dicationic ionic liquids (ILs) that differ in
ortho, meta and para position influence the catalytic activity of production of 5-
hydroxymethylfurfural (HMF). The development of this research was started with
synthesizing new dicationic ILs with simple procedure then followed by characterization and
measuring the acidity properties by using Hammet method. Throughout the screening
process, the results exposed that substituent on three different position (ortho, meta and para)
in cationic site give a remarkably effect on the overall catalytic performance. Highly efficient
and selective dehydration of D-fructose to HMF was obtained by BrØnsted IL at ortho
position with fructose conversion of 95.7% and yielding 90.5% of HMF in 60 min at 100 C.
From the results, it revealed that the geometry shape behaviour of the isomer ILs influenced
the yield of HMF. The effects of reaction time, temperature, catalyst concentration and
reusability of the dicationic ILs are also studied in this work.
Keywords: dicationic ionic liquids, ortho-meta-para position, fructose, 5-
hydroxymethylfurfural
1. Introduction
It is something noteworthy to find a way to make fuels and chemicals from renewable
sources that is promising alternatives for future energy supply. Recently, biomass-derived
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HMF has become one of the prominent subjects as important platform to meet the needs of
chemicals with important applications such as solvent or fuel and monomers for plastics[1].
In general, high conversion and yield of sugar to HMF can be manipulated in the aspects of
catalyst, solvent, substrate, reaction time and also temperature. Thus, the used of green
solvent (ILs) is the best candidate in development of environmentally route and related
technologies for producing chemicals and fuel from non-fossil carbon sources [2].
Ionic liquids (ILs) have gained overwhelming interest as they offer some unique
properties such as excellent thermal stability, negligible volatility and flammability[3] and
can be recycled after accomplishing a task specific reaction[4-6]. ILs have great potential
applications in wide area including, carbon capture [7, 8], nuclear fuel reprocessing[9],
biomass processing[10], as catalysts in catalytic reaction [11-13], pharmaceutical [14] waste
recycling [15-17], and batteries[18-21] due to the large number of possible combinations of
cations and anions that give dramatic impact towards the properties.
To date, various modifications have been made to ILs according to the needs and
desire application. Dicationic ILs has been reported to be more effective and more versatile in
various types of reaction than conventional monocationic ILs [22]. The effectiveness of
dicationic ILs can be highlighted when they contribute in higher intra and intermolecular
interactions, that give significant impact towards the results of application area[23, 24].
Studied by Chinnappan et al.[25] have reported that dicationic ILs showed an excellent
catalytic activity especially from the point where they provide a great extensive hydrogen
bonding which in turns promote enormous dissolution of the substrate in the reaction.
Therefore, in the context of employment ILs as a catalyst, we believe that dicationic ILs can
serve a great medium for reaction to happen while tuning anion part to be acidic.
Our research is mainly devoted to design and synthesis simple dicationic ILs which
differ in their position and their catalytic ability for dehydration of fructose to 5-
hydroxymethylfurfural (HMF).Three important points that has been emphasized by Stahlberg
et al.[26] that make a great impact in this study are, pertaining on the physicochemical
properties of ILs, influence of both the cation and anion of the ILs on the reaction outcome,
and examination of the mechanism and the interaction ILs with fructose. Studied by Kotadia
and Soni[27] demonstrated that symmetrical and asymmetrical ionic liquids do have great
impact in their physicochemical characters that eventually give promising results in catalytic
application. Other than that, Jadhav et al.[28], illustrated that their longer chain dicationic ILs
showed higher catalytic activity compared with the shorter ones. Therefore, it is our interest
to expend and explore the geometry shape behaviour when the isomer ILs exists in ortho,
meta and para position in terms of catalytic performance and a positive role in development
of effective process for the dehydration.
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2.
Experimental
2.1. Materials
2,5-dichloro-para-xylene, ,-dichloro-para-xylene, ,-dichloro-ortho-xylene, ,-
dichloro-meta-xylene, benzyl-imidazole, acetonitrile, fructose, concentrated H2SO4, ethyl
acetate, ethyl ether, 5-hydroxymethylfurfural were purchase from Sigma Aldrich and were
used without further purification unless otherwise stated.
2.2. Instrumentation
1H-NMR and 13C-NMR were recorded by JEOL 399.0 MHz using CD3OD and DMSO-D6 as
solvent. FT-IR spectra were recorded with Perkin Elmer with a universal ATR accessory .The
Elemental analyses (CHN) were performed on a Pelkin-Elmer 2400 Series II Elemental
Analyser. UV-vis spectra were recorded on UV-160A spectrophotometer (Shimadzu
Corporation, Japan) in water.
2.3. Analysis
The amount of HMF in dehydration reaction was calculated using an external standard at 25
C using an HPLC apparatus equipped with C18 reverse column (250 mm x 4.6 mm, particle
size 5 m) hypersil gold, Thermo Science USA, SPD-M20A diode array detector, SIL -
20AHT auto sampler and CTO-10ASVP column oven. The mobile phase was a mixture of
methanol and water (40:60 v/v). The flow rate was set at 1.0 mL min-1, and detection
wavelength was at 283 nm. The amount of fructose was calculated using external standard
analysed using Waters HPLC system with ELS detector. Separation was achieved on
Phenomenex Luna 5u NH2 100A column (250 mm x 4.60 mm, 5 micron). The mobile phase
consisted of acetonitrile : water (80:20 v/v) was degassed by ultrasonic bath before used. The
flow rate was 1.0 mL min-1 and aliquot 20 L of sample solution was injected into the HPLC-
ELSD system. All samples were filtered through 0.45 m Milipore membrane before use.
The fructose conversion, product yield and selectivity are calculated based on previous
reported works[25].
2.4. Synthesis of New Dicationic Ionic Liquids
Dicationic ionic liquids were prepared by using the following methods. The A-orthoCl, B-
paraCl, and C-metaCl were obtained by reacting (2.801 g, 0.016 mol) of dichloro-o-xylene or
dichloro-p-xylene or dichloro-m-xylene with (5.206 g, 0.033 mol) of benzyl imidazolium in
10 mL acetonitrile at 80 C and stirred for 18 hours respectively. Each of the resulted
products was then washed repeatedly with ethyl acetate to remove any non-ionic residues,
and vacuum dried at 80 C.
The obtained chloride based ILs was used as intermediates to synthesize A-orthoHSO4, B-
paraHSO4, and C-metaHSO4 by metathesis reaction. 2.4574 g, 0.005 mol of A-orthoCl, B-
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paraCl and C-metaCl respectively was mixed with 10 ml of acetonitrile and stirred at 0 C for
1 hour. The stoichiometric amount of concentrated sulphuric acid (97%, 0.5330 mL) was
added dropwise and the mixture was stirred for another 1 H at 0 C before been stirred for 18
H at room temperature. The resulted BrØnsted ionic liquid was washed repeatedly with
diethyl-ether (2x5 mL) to remove non-ionic residues. The ILs was dried in vacuum for about
6 hours before being used.
2.4.1. 3, 3'-(1,2-phenylenebis(methylene))bis(1-benzyl-1H-imidazol-3-ium) dichloride
1
A-orthoCl : H-NMR (400MHz, CD3OD)  (ppm) = 9.38( s, 1H, NCHN), 7.59,7.51(s, 2H,
NCHCHN), 7.43-7.53 (m, 7H, aromatic) ,5.74( s, 2H, HCH), 5.48( s, 2H), 13C-NMR(
400MHz, CD3OD) 136.46,133.96,132.29, 130.37, 129.24, 128.77, 122.91, 122.69, 53.00 ;
FT-IR(500-4000cm-1) : 3352, 3055, 1829, 1650,1557,1453, 1357, 1153, 733.21
Anal. calcd.(%) for C28H28Cl2N4 : C, 68.430 ; H, 5.740 ; N, 11.400 Found: C, 67.337 ; H,
5.867 ; N, 11.698
2.4.2. 3, 3'-(1,4-phenylenebis(methylene))bis(1-benzyl-1H-imidazol-3-ium) dichloride
B-paraCl:1H-NMR (400MHz, CD3OD) (ppm) = 8.49( s, 1H, NCHN) , 6.86( s, 2H,
13
NCHCHN) , 4.67 (s, 2H, HCH), 4.64 (s, 2H, HCH), C-NMR (400MHz, CD3OD) 137.30,
136.13, 134.94, 130.41, 130.16, 129.44, 123.85, 53.96, 53.33; FT-IR (500-4000cm-1) :
3383.77, 3016.46, 1821.02, 1649.85, 1556.20, 1446.19,1352.36, 1150.27, 727.05, 705.99
Anal. calcd.(%) for C28H28Cl2N4 : C, 68.430 ; H, 5.740 ; N, 11.400 Found: C, 69.630 ; H,
5.918 ; N, 12.012
2.4.3. 3, 3'-(1,3-phenylenebis(methylene))bis(1-benzyl-1H-imidazol-3-ium) dichloride
1
C-metaCl : H-NMR (400MHz, CD3OD)  (ppm) =9.38( s, 1H, NCHN), 7.51,7.62 (s,
2H,NCHCHN), 7.15, 7.37 (m, 7H, aromatic), 5.21 (s, 2H, HCH), 13C-NMR (400MHz,
CD3OD) 139.12, 135.19, 133.98, 133.98, 133.10, 130.10, 129.31, 129.98, 129.31, 128.56,
122.79, 52.80, 52.29; FT-IR(500-4000cm-1) 3378.62, 3060.10, 1878.90, 1661, 1557.39,
1453.60, 1355.05, 1148.47, 753.00,731.33
Anal. calcd.(%) for C28H28Cl2N4 : C, 68.430 ; H, 5.740 ; N, 11.400 Found: C, 67.862 ; H,
6.267 ; N, 12.698
2.4.4. 3,3'-(1,2-phenylenebis(methylene))bis(1-benzyl-1H-imidazol-3-ium) hydrogen sulfate
A-orthoHSO4: 1H-NMR (400MHz, CD3OD)  (ppm) = 10.39(s, 1H, NCHN) , 8.53,8.56( m,
13
7H,aromatic), 6.82(s, 2H, HCH) , 6.59 (s,2H, HCH) C-NMR (400MHz, CD3OD) 137.74,
135.15, 133.51, 131.47, 130.31, 129.87, 124.05, 123.79, 54.11, 51.01 ; FT-IR(500-4000cm-1)
3134.75, 3066.62, 1559.09, 1497.43, 1451.96, 1228.97, 1149.76, 1000.88, 851.97, 746.54,
711.53, 567.25
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Anal. calcd.(%) for C28H30N4O8S2 : C, 54.710 ; H, 4.920 ; N, 9.110 Found: C, 55.062 ; H,
2.267 ; N, 10.198
2.4.5. 3,3'-(1,4-phenylenebis(methylene))bis(1-benzyl-1H-imidazol-3-ium) hydrogen sulfate
1
B-paraHSO4: H-NMR (400MHz, CD3OD)  (ppm) = 9.55( s,1H, NCHN), 7.81 (s, 2H,
13
NCHCHN), 7.18- 7.46 ( m, 7H, aromatic), 5.40 (s,1H, HCH), 5.41 (s, 1H,HCH) C-NMR
(400MHz, CD3OD) 145.67, 144.65, 144.07, 138.28, 137.71, 132.18, 132.14,61.27, 60.79;
FT-IR (500-4000cm-1) 3139.78, 3056.40, 1555.42, , 1233.95, 1168.81, 1142.62, 1025.58,
841.84, 726.67, 710.27, 581.00
Anal. calcd.(%) for C28H30N4O8S2 : C, 54.710 ; H, 4.920 ; N, 9.110 Found: C, 53.962 ; H,
5.160 ; N, 10.971
2.4.6. 3,3'-(1,3-phenylenebis(methylene))bis(1-benzyl-1H-imidazol-3-ium) hydrogen sulfate
1
C-metaHSO4 : H-NMR (400MHz, CD3OD)  (ppm) = 10.56 ( s, 1H, NCHN), 8.54-8.65 (
m, 7H, aromatic) , 6.66 (s, 1H, HCH), 6.62 (s, 1H, HCH) 13C-NMR (400MHz, CD3OD)
137.25,136.36,135.16,131.13,130.43, 130.43, 130.16, 130.13, 129.71, 123.88, 123.75,
123.05, 121.21, 53.87,53.37; FT-IR(500-4000cm-1) 3136.88, 3066.62, 1559.09, 1497.43,
1451.96, 1206.89, 1145.87, 1029.13, 850.47, 824.81, 730.41, 709.01, 570.93
Anal. calcd.(%) for C28H30N4O8S2 : C, 54.710 ; H, 4.920 ; N, 9.110 Found: C, 55.902 ; H,
5.407 ; N, 10.008
2.5.
Dehydration reaction for fructose
In a typical reaction, a 50 mL round bottle flask was charged with fructose 1.0 g and a
given amount of catalyst was added into 10 mL DMSO and heated up. After reaction
complete, the sample was diluted with ultrapure water (18.6 mcm-1) before analysis by
HPLC equipped with UV and ELS detector.
2.6.
Separation procedure for HMF
After the dehydration process, the reaction mixture was transferred into a flask. 30
mL of water was added to the mixture. The organic layer was extracted with ethyl-acetate (20
mL x 5) and distilled under reduced pressure.
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3.
Results and Discussion
Scheme 1 illustrates that the synthetic route for ILs, A-orthoCl, B-paraCl, C-metaCl
by using benzyl-imidazole and dichloro-xylene. The choice of the counter ion made this
method convenient for preparation of imidazolium based ILs in good yield. In this
experiment, different position of dichloro-xylene (ortho, meta and para) were used to develop
new isomeric dicationic ionic liquids.
Scheme 1. Synthesis of dicationic ionic liquids
Further, Scheme 2 shows the synthesis route of the anion was exchange by using
metathesis reaction to produce A-orthoHSO4, B-paraHSO4 and C-metaHSO4. HSO4 anion
was reported to give the best catalytic activity [29] and therefore was chosen to promote the
acidic properties to these ILs.
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Scheme 2. Metathesis reaction
The acid properties were measured by using Hammet acidity function (Ho). The value
of Ho was calculated based on intrinsic pKa of the indicator (4-nitroaniline) in water and the
molar ratio of protonated and unprotonated dye [I]/[IH+] using following equation, which
similar to the method used in the previous work [29]
Ho= pK(I)a + log ([I]/[IH+])
(1)
The noticeable change in the absorption of curve of p-nitroaniline in the wavelength
region of 350 to 400 nm are presented in Figure 1 upon the addition of ILs where obviously
-
the ILs with strong acidic anion, HSO4 protonate the p-aniline more and give lower value of
Ho. Overall, the corresponding Ho values order from highest to the lowest were in the order
ofB-paraCl> C-metaCl> A-orthoCl> B-paraHSO4> C-metaHSO4> A-metaHSO4. Result
in Table 1 indicates that the acidity of A-orthoHSO4 was the strongest among these ILs. The
result reveals that the position of dichloro-xylene (ortho, meta and para) influence the acid
properties of ILs.
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Fig.1. Absorbance of six ILs in protonated p-nitroaniline
Table 1. Ho value of six ILs. Reaction carried out at 25C.
Absorbance
[I] [%]
[HI] [%]
Ho
Ionic liquids
1.486
1.462
1.466
1.464
1.232
100
0
-
p-nitroaniline
A-orthoCl
98.45
98.38
98.65
82.91
1.54
1.61
1.34
17.08
2.77
2.85
2.79
1.67
B-paraCl
C-metaCl
A-orthoHSO4
1.295
1.276
87.08
85.81
12.91
14.18
1.81
1.77
B-paraHSO4
C-metaHSO4
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3.1. Dehydration of fructose in dicationic ILs
To study the catalytic activity of the synthesized ILs, we have carried out the
dehydration of fructose in DMSO at 100 C for 60 minutes by using 0.20 g of ILs as
illustrated on scheme 3.The ILs were dried under vacuum for two hours to reduce the effect
of moisture in the reaction prior to the experiment. During the reaction, the colour of the
reaction mixture was initially pale yellow however, when the reaction was continued at
higher temperature and longer reaction time the colour was gradually changed to deep brown.
This observation indicates that formation of side product such as humins other than HMF [27,
30]
Scheme 3. Reaction process for dehydration of fructose
The analysis of fructose conversion and HMF yield by all six ILs were conducted
using HPLC. The result from figure 4 showed that, A-orthoCl produced 82.8% fructose
conversion and yielding 49.3% of HMF. On the other hand, the conversion percentage is
slightly lower for C-metaCl which was 79.5% and the HMF yield was 47.0%. The B-paraCl
achieved only 77.1% conversion and 34.3% HMF yield at a uniform reaction condition.
–
Similar trend was observed when ILs with HSO4 anion was used. The IL with ortho
position led the reaction with 95.7% fructose conversion and 90.6% of HMF yield. In the
presence of C-metaHSO4 and B-paraHSO4, fructose converted were 95.3% and 93.0% with
89.1% and 62.0% of HMF yield, respectively.
The percentage of selectivity was calculated based on the percentage of conversion
and yield. It was found that, A-orthoHSO4 showed the highest selectivity which was 95.0%
while C-metaHSO4 and B-paraHSO4 recorded 93.0% and 66.7% respectively. While Cl-
based ILs gave much lower selectivity in which only 44.4% for B-paraCl, 59.0% for C-
metaCl and 60.0% for A-orthoCl.
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The catalytic performance of the ILs can be rationalized due to the different in dipole
moment and polarity. Scheme 4 illustrates the net resultant dipole moment ()
Scheme 4. Different dipole moment in ortho, meta and para.
EWG: Electro withdrawing group.
and can be explained by:
2
2
ortho
=
=
√
 ₁ +  ₂ + 2₁₂푐표푠60
2
2
(2)
(3)
√
 ₁ +  ₂ + ₁₂
2
2
meta
=
=
√
√
 ₁ +  ₂ + 2₁₂푐표푠120
2
2
 ₁ +  ₂ − ₁₂
2
2
para
=
=
√
 ₁ +  ₂ + 2₁₂푐표푠180
√2₁ + 2₂ − 2₁₂
(4)
From the above expression, when both substituents are electro withdrawing group,
para position has the least dipole moment and ortho product has the highest. In general the
trend of dipole moment is ortho>meta>para [31, 32]. Therefore, the polarity in cationic site
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absolutely gives a significant impact as it will reflect in the acidity of C2 proton (Figure 2) of
imidazolium. In fact, in these dicationic ILs the delocalization system is more effective
because of the support from three benzene ring that give rise in polarity of C2 proton which
in turns led to alleviate the acidity strength [27, 33]. As a result, acidity behaviour of cationic
will facilitate and contribute in solvation mechanism of dehydration process [25, 34]. In this
scenario, the ortho position produced high conversion fructose and HMF yield compared to
meta and para positions.
Fig. 2. The numbering of ring atoms in imidazolium.
Therefore A-orthoHSO4 was chosen for further optimization of reaction residence
time, temperature, catalyst loading and recyclability.
3.2.
Reaction mechanism consideration
Figure 3 below shows a time-progression stacked of H-NMR spectra of fructose
dehydration. This experiment was carried out to further investigate the selectivity of A-
orthoHSO4 for HMF formation.
Sudipta et al.[35] has suggested that the formation of HMF was initiated via a cyclic
mechanism. The proton that generated during hydrolysis assisted the reaction and followed
by nucleophile attack to get final product. On the other hand, according to mechanism
proposed by Deepali et al. [27], the conversion of fructose to HMF was activated by their
cationic that act as electrophile. Then the elimination of three water molecules was driven by
anion and also facilitate by C2 imidazolium cation via hydrogen bonding. According to
Figure 3, as the reaction time is increased from 5 min to 60 min at 100C, the peaks
characteristic of HMF (4.45 (s), 6.55 (d), 7.45 (d), and 9.51(s) ppm) gradually increase while
fructose peaks (between 3.0 to 4.6 ppm) started to disappear. The transformation of fructose
to HMF was assisted by DMSO which works more than just a medium but also as an
electrophile [36]. The 1H-NMR recorded after 60 min of reaction showed only HMF and H2O
(3.6 ppm) signals indicate that the fructose dehydration reaction in DMSO with A-orthoHSO4
is selective for HMF formation.
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1
Fig. 3. H-NMR spectra of a) 25mg fructose in DMSO-d6 b) 25mg fructose in DMSO-d6 in
the presence of 17.5 mg of A-orthoHSO4 (2:1 ratio) at 0 min b) reaction at 5 min c) reaction
at 20 min, e) reaction at 40 min f) reaction at 60 min.
Figure 4 shows that the dehydration of fructose in DMSO without catalyst able to get
fructose conversion about 16.6% and 1.2% of HMF yield in which supported our previous
1
justification. Additionally, the line broadening from our H-NMR spectra (3 to 5ppm)
-
indicates that hydrogen bonding interaction between a –OH of fructose and the HSO4 give
evidence that this anion was responsible to eliminate the water molecule from reaction and at
same time promote fructose dissolution. The result from this study are consisted with Xinli &
Yongdan [37].
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Fig. 4. Dehydration of fructose in the presence of six ILs as catalyst. Reaction condition (1.00
g fructose, 0.2 g IL, 10 mL DMSO, 100 C, 60 min)
.
.
3.3. Effect of reaction temperature
The dehydration reaction of fructose were studied at 40-160 C in order to determine
the efficiency of dicationic A-orthoHSO4 as well as the effect of reaction temperature on the
formation of HMF. As shown in Figure 5 below, the reaction temperature plays important
role in both fructose conversion and HMF yield. When the reaction temperature was 40 C
the conversion and yield were relatively low where only 56.4% and 32.0% respectively. The
gradual increase was observed resulted 98.3% conversion and 95.1% yield at 130 C.
Meanwhile, at 160C, the fructose conversion increase slightly higher but a decrease in the
HMF yield was observed as well as the colour of the solution changed from light yellow to
deep brown that indicate a formation of humins much more higher than HMF.
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Fig. 5. Effect of temperature on HMF yield and conversion. Reaction of fructose was
performed on a 1.0 0g scale (5.5 mmol) at six reaction temperatures in the presence of 0.20 g
A-orthoHSO4 in DMSO (10 mL), reaction time 60 min.
3.4.
Effect of reaction time
The dehydration reaction time of fructose was varied to study the rate of HMF
formation as a function of time. As plotted in Figure 6, the yield of HMF improved from
73.2% to 90.5% upon increasing the duration of reaction period from 30 min to 60 min at 100
C. Further increase the reaction time to 120 min resulted in maximum HMF yield of 94.2%
with 98.3% fructose conversion. Afterwards at 180 min the production of HMF started to
drop up to 79.9%.
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Fig. 6. Effect of temperature on HMF yield and fructose conversion. Reaction of fructose was
performed on a 1.00g scale (5.5mmol) at four different reaction time in the presence of 0.20 g
A-orthoHSO4 in DMSO (10 mL), temperature at 100 C.
3.5.
Catalyst dosage
The dehydration reaction of fructose into HMF was studied at variable dosage of
catalyst of A-orthoHSO4 for optimizing the reaction condition and maximizing the HMF
yield. The amount of catalyst was varied from 0.01 g to 1.00 g. As shown in Figure 7 the
yield of HMF increased from 32.9% to 90.6% with an increasing the catalyst from 0.01 g to
0.20 g. When the amount of catalyst further increase it shows that yield of HMF decrease
until 24.5% with 98.7% fructose conversion.
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Fig. 7. Effect of catalyst loading on HMF yield and fructose conversion. Reaction of fructose
was performed on a 1.00 g scale (5.5 mmol) at seven different catalyst dosages in DMSO (10
mL), reaction time 60 min and temperature at 100 C.
3.6.
The recycling of catalyst and solvent
The recycling of catalyst and the reuse of solvent were examined by at 100 C for 60
min in the presence of 0.20 g A-orthoHSO4. Prior to each reuse of the IL and solvent, they
were extracted by using ethyl acetate where HMF sole mostly in the organic phase even
though there still some will remain in aqueous [38] while IL and DMSO in aqueous medium.
Figure 8 shows that the lost of activity of the catalyst in terms of HMF yield is
approximately around 10% in every cycle. Loss of activity of catalyst was related with loss of
ILs that might occur during the extraction step.
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Fig. 8. Recycling of A-orthoHSO4 and DMSO solvent for dehydration reaction. Reaction
condition for each run: Fructose (1g, 5.5mmol), 0.20 g A-orthoHSO4 (recycled), reaction
time: 60 min, temperature: 100 C.
Table 2. Comparison study of different catalyst in dehydration process.
Catalysed used
Solvent T/ C t/
HMF
reference
Entry
min Yield%
[DiEG(mim)2][OMs]2
[TriEG(mim)2][OMs]2
[TetraEG(mim)2][OMs]2
[C6(mpy)2]2Br
[C10(Epy)2]2Br
A-ortho-HSO4
-
-
-
120 40
120 40
120 40
69.8
77.2
92.3
91.0
83.0
90.5
62.0
89.1
69.4
23.6
72.3
72.8
[28]
[28]
[28]
[25]
1
2
3
4
5
6
7
8
DMSO 110 60
DMSO 110 60
DMSO 100 60
DMSO 100 60
DMSO 100 60
DMSO 90
DMSO 90
DMSO 90
DMSO 80
[25]
This work
This work
This work
[37]
[37]
[37]
B-para-HSO4
C-meta-HSO4
[NMP]+[HSO4]-
[Mim][HSO4]
120
9
120
120
60
10
11
12
[NMP]+[CH3SO3]-
[PSMBIM]HSO4
[27]
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The capability and effectiveness of our catalysts for the reaction was tabulated in
Table 2. As shown in Table 2, the dehydration reaction in terms of reaction time, reaction
temperature and HMF yield were remarkably improved by A-orthoHSO4.
4. Conclusion
In summary, six new dicationic ILs A-orthoCl, B-paraCl, C-metaCl , A-orthoHSO4,
B-paraHSO4, and C-metaHSO4 have been effectively synthesized, characterized, and their
Hammet acidity function was also have been measured. Among all, A-orthoHSO4 shows very
high acidity and catalytic activity for fructose dehydration and thus further used for
optimisation of respect to different parameters such as reaction time and reaction temperature
and catalyst loading on the HMF yield. From the above discussion it is clearly reveal that
polarity of A-orthoHSO4 ILs influenced the yield of HMF.
Acknowledgement
This project was supported by the University Malaya Research Grant
(UMRG):RP020B-16SUS and FP045-2014A. S.Y would like to thank the MyBrain program
from Ministry of Higher Education Malaysia for financing her studies. Authors would like to
thank Dr. Azman Maamor for valuable input.
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Graphical abstract
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Highlight from this paper
 Six new dicationic ionic liquids were successfully synthesized and characterized
 These new ionic liquids have been used for the first time in dehydration process of
fructose to 5-hydrxymethylfurfural.
 Hammet acidity function (Ho) illustrated the acidity properties of these six ILs.
 The different of polarity in ILs structure were discussed thoroughly
 Optimization parameter of reaction time, temperature and catalyst loading were
investigated.
22