The effect of hydrochloric acid on the conversion of glucose to 5-hydroxymethylfurfural in AlCl3-H2O/THF biphasic medium

Article abstract of DOI:10.1016/j.molcata.2013.04.016

The effect of hydrochloric acid (HCl) on the conversion of glucose to 5-hydroxymethylfurfural (HMF) in AlCl₃-H₂O/THF biphasic system was investigated. The route via isomerization to fructose followed by tandem dehydration to HMF is the dominant pathway. Lewis acid sites of Al ³⁺ aqua/hydroxo complexes play the leading role for the dehydration step. The introduction of a suitable amount to HCl increases the amount of Br?nsted acid sites and improves the selectivity for HMF. The maximum HMF yield from glucose is enhanced from 51% to 62% in the presence of 0.1 M HCl. The introduction of HCl lowers the reaction rate of glucose-to-fructose isomerization but enhances the reaction rate of fructose dehydration. Higher loading of HCl (1.0 M) leads to increased direct dehydration of glucose to humins and results in lower selectivity to HMF.

Full text of DOI:10.1016/j.molcata.2013.04.016

Accepted Manuscript
Title: The effect of hydrochloric acid on the conversion of
glucose to 5-hydroxymethylfurfural in AlCl3-H2O/THF
biphasic medium
Author: Yu Yang Changwei Hu Mahdi M.
Abu-Omar<ce:footnote id="fn1"><ce:note-para
id="npar0005">Fax: +011 86 8541
1105.</ce:note-para></ce:footnote>
PII:
S1381-1169(13)00154-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2013.04.016
MOLCAA 8735
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
4-6-2012
8-4-2013
17-4-2013
Please cite this article as: Y. Yang, C. Hu, M.M. Abu-Omar, The effect of
hydrochloric acid on the conversion of glucose to 5-hydroxymethylfurfural in AlCl3-
H2O/THF biphasic medium, Journal of Molecular Catalysis A: Chemical (2013),
http://dx.doi.org/10.1016/j.molcata.2013.04.016
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Graphical abstract
k1
HCl
HO
OH
O
k2
HCl
HO
OH
OH
Fructose
AlCl3
AlCl3
isomerization
dehydration
HO
O
OH
O
AlCl3
HO
HO
dehydration
HCl
O
OH
k3
OH
Glucose
HMF
The introduction of HCl decreases k1 and increases k2 and k3.
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Highlights
The route via fructose is the dominant pathway for the conversion of glucose to HMF in
AlCl3-H2O/THF.
The Lewis acidic sites are responsible for isomerization as well as dehydration.
The introduction of a suitable amount of HCl improved the selectivity for HMF.
An optimal HMF yield of 62% can be obtained with 0.1 M HCl added to AlCl3-H2O/THF.
The introduction of HCl improved the direct dehydration route of glucose to HMF.
2
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The effect of hydrochloric acid on the conversion of glucose to 5-hydroxymethylfurfural in
AlCl3-H2O/THF biphasic medium
Yu Yang,a, b Changwei Hu,a* and Mahdi M. Abu-Omarb*
a Key Laboratory of Green Chemistry and Technology, Ministry of Education,
College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, PR China
b Department of Chemistry and the Center for direct Catalytic Conversion of Biomass
to Biofuels (C3Bio), Purdue University, 560 Oval Drive, West Lafayette, Indiana
47907, U.S.A
* To whom correspondence should be addressed: Fax: +011 86 85411105 (C. w. Hu),
tel.: +1 765 4945302; fax: +1 765 4940239 (M. M. Abu-Omar). E-mail addresses:
gchem@scu.edu.cn (C. w. Hu) , mabuomar@purdue.edu (M.M. Abu-Omar).
3
Page 3 of 24

Abstract: The effect of hydrochloric acid (HCl) on the conversion of glucose to
5-hydroxymethylfurfural (HMF) in AlCl3-H2O/THF biphasic system was investigated.
The route via isomerization to fructose followed by tandem dehydration to HMF is
the dominant pathway. Lewis acid sites of Al3+ aqua/hydroxo complexes play the
leading role for the dehydration step. The introduction of a suitable amount to HCl
increases the amount of Brønsted acid sites and improves the selectivity for HMF.
The maximum HMF yield from glucose is enhanced from 51% to 62% in the presence
of 0.1 M HCl. The introduction of HCl lowers the reaction rate of glucose-to-fructose
isomerization but enhances the reaction rate of fructose dehydration. Higher loading
of HCl (1.0 M) leads to increased direct dehydration of glucose to humins and results
in lower selectivity to HMF.
Keywords: Glucose; 5-Hydroxymethylfurfural; AlCl3; Fructose; HCl.
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1. Introduction
Glucose, which is the monomer unit of cellulose in biomass, will be one of the
most important raw materials of the future as society moves towards biomass
utilization for renewable fuels and chemicals [1,2]. Among the products derived from
glucose, 5-hydroxymethylfurfural (HMF) is of high value. HMF can be transformed
into liquid fuel and other important chemicals [3,4]. It serves as a precursor for
levulinic acid (LA), 2,5-furandicarboxylic acid, 2,5-diformylfuran, and
dihydroxymethylfuran, to name a few. Liquid alkanes [5] and dimethylfuran [6],
which could directly serve as a high heating value fuel, can also be produced from
HMF. Hence, efficient and environmentally responsible production of HMF from
abundant and cheap glucose has received considerable attention recently [7-15].
The conversion of glucose to HMF may proceed by two routes as illustrated in Fig.
1 [3,16,17], via fructose or direct dehydration of glucose. Brønsted acids such as HCl,
H2SO4 and H3PO4 are typically used for the conversion of glucose to HMF via the
direct route because Brønsted acids do not affect glucose isomerization [7]. However,
mineral acids give only low HMF yield and produce additional byproducts [7]. The
route via fructose involves two main steps: (1) isomerization of glucose to fructose; (2)
dehydration of fructose to HMF. Using a bifunctional catalyst that can affect
glucose-to-fructose isomerization and subsequent dehydration should give higher
HMF yield/selectivity [10-14]. Some Lewis acids such as CrCl2 [10], SnCl4 [11], and
AlCl3 [12] can be used for glucose conversion giving reasonable HMF yield. Among
these catalysts, AlCl3 is the cheapest with the lowest toxicity. Unlike CrCl2 and SnCl4,
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which require the use of expensive ionic liquid as solvent, AlCl3 displays high activity
in cheap H2O/THF biphasic medium, which can inhibit the further reaction of HMF
and lead to a high HMF yield [12]. Dumesic, Shanks, and coworkers have reported
recently on the combination of Lewis and Brønsted acids for the conversion of
glucose in biphasic water and alkyl phenols [15]. The effect of Brønsted acid and its
mechanistic implication on the conversion of glucose to HMF in the AlCl3-H2O/THF
biphasic system is the subject matter of this study. In order to avoid the effect of
anions with the introduction of Brønsted acid, HCl was used to adjust the
concentration of Brønsted acid in the AlCl3-H2O/THF biphasic system.
2. Experimental
2.1 Materials
Glucose, fructose, AlCl3·6H2O, HCl (37 wt%), THF, HMF, all of analytical grade,
were purchased from Sigma Aldrich, and used as received.
2.2. General procedure for the reaction
All reactions were performed under microwave heating in a Discover TM System
(CEM Corporation) using a 10 mL batch reaction vessel. Reaction solutions were
mixed using a magnetic stir bar. The reactor temperature was measured by a fiber
optic sensor. In a typical dehydration experiment, 10 mL reaction tube was charged
with glucose or fructose (0.25 mmol), catalyst (HCl or/and AlCl3·6H2O (0.1 mmol)),
o
nanopure water (1 mL) and THF (3 mL); the reaction was heated to 160 C for the
desired time. In a typical isomerization of glucose-to-fructose experiment, 10 mL
reaction tube was charged with glucose (0.25 mmol), catalyst (HCl and AlCl3·6H2O
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(0.1 mmol)) and nanopure water (1 mL); the reaction was heated to 120 oC for 30 min.
The reaction was stopped by nitrogen flow cooling. Samples were filtered with a 0.2
μm syringe filter prior to analysis.
2.3 Analytic methods
Quantitative analysis of the aqueous phase was performed by HPLC using a
Waters 1525 pump, an aminex column HPX-87 column (Agilent) and Waters 2412
Refractive Index detector. 0.005 M H2SO4 solution was used as the mobile phase at a
flow rate of 0.6 mL/min, and the column temperature was maintained at 338 K. The
concentration of HMF in the organic phase was determined by HPLC using a Waters
1525 pump, a XDB-C18 column, (Agilent) and Waters 2487 UV detector. 0.5 wt%
formic acid solution was used as the mobile phase at a flow rate of 0.4 mL/min, and
the column temperature was maintained at 303 K. All concentrations of carbohydrates
in the aqueous phase and HMF in the aqueous phase and organic (THF) phase were
determined by comparison to standards calibration curves.
Conversion of carbohydrates and selectivity of products are defined as follows:
Glucose conversion = moles of glucose reacted/moles of starting glucose
Fructose conversion = moles of fructose reacted/moles of starting fructose
Fructose selectivity = moles of fructose produced/ moles of glucose reacted
Glucose selectivity = moles of glucose produced/ moles of fructose reacted
HMF selectivity = moles of HMF produced/(moles of starting carbohydrate- moles of
carbohydrate unreacted)
HMF yield = moles of HMF produced/moles of starting carbohydrate
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The pH value was measured on an Accumet AB15/15+ pH meter ( 0.01 pH units)
calibrated with standard buffer solutions. The temperature of the solution was
controlled by a thermostated water bath (± 0.1 oC).
3. Results and Discussion
Although AlCl3 can be used as a Lewis acid in water [18-19], it is
moisture-sensitive and hydrolyzes to form Brønsted acid (H3O+). At 25 oC, the pH of
0.1 M AlCl3 aqueous solution is 3.03. At higher temperature, the hydrolysis of AlCl3
in H2O is even more significant. For example, at 80 oC the pH of 0.1 M AlCl3 aqueous
solution is 2.63. However, it is difficult to reliably measure the pH at the reaction
o
conditions (120-160 C), and therefore, one cannot use HCl to emulate exactly the
same [H3O+] of 0.1 M AlCl3 aqueous solution at these high temperatures.
Alternatively, one can use InCl3, which bears similar character to AlCl3, but higher
Brønsted acidity and lower Lewis acidity [19], to decipher which kind of acid in
AlCl3 system play the major role for the conversion of glucose to HMF. We tested
InCl3 for the conversion of glucose. While the AlCl3-H2O/THF biphasic system
affords 98% conversion of glucose with 48% yield of HMF after 10 min of reaction at
160 oC, the InCl3-H2O/THF system under the same conditions affords 86%
conversion of glucose with HMF yield of only 7%. However, the indium system gives
fructose as the major product (59% yield). In summary, InCl3 shows high activity for
glucose-to-fructose isomerization, but much lower activity than AlCl3 for the
dehydration of fructose to HMF. Since the Brønsted acidity of InCl3 in water is higher
than that of AlCl3, a plausible interpretation of these results is that Lewis acid sites in
8
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AlCl3 system are crucial in the dehydration step.
As discussed above the AlCl3-H2O/THF system provides both Lewis and Brønsted
acid sites. Brønsted acids, however, do not catalyze the isomerization of
glucose-to-fructose [20]. On the contrary, the dehydration of fructose can be catalyzed
by both Lewis and Brønsted acids [21]. Thus, the fructose dehydration reaction was
used as a probe reaction to provide insight into the acidic strength of AlCl3 under our
reaction conditions.
The effect of pH on the dehydration of fructose is shown in Fig. 2. At pH = 2 the
conversion of fructose and HMF yield are quite low. As the pH is decreased, fructose
conversion and HMF yield increase, reaching a plateau at pH 1.0. Increasing the
Brønsted acidity to [H3O+] = 1.0 M results in a decline in HMF yield due to
rehydration to make levulinic acid (LA) [22]. In comparison with AlCl3 as a catalyst,
fructose conversion of 67% and HMF yield of 38% are observed (filled triangles in
Figure 2). Accordingly, the acidic strength of AlCl3 aqueous solution is equal to HCl
solutions pH values in the range of 1.35-1.61 (Figure 2). Next we investigated HCl at
different pH values for the conversion of glucose to HMF. The results are displayed in
Fig. 3. Fructose was not observed over the investigated pH range. Kuster and
Temmink [22] had investigated the influence of pH on the conversion of glucose; they
observed no glucose-to-fructose isomerization at pH < 4.5. Thus, with HCl in the pH
range of 0-2, the conversion of glucose to HMF most likely proceeds through direct
dehydration of glucose. However, HMF yield of only ca. 2% is observed in the pH
range of 1.61-1.35 (which is comparable to the acidic strength of AlCl3, vide supra).
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In comparison the use of AlCl3 gives HMF yield of 43%. So far the results indicate
that direct dehydration of glucose to HMF has little contribution to HMF formation
from the AlCl3-catalyzed reaction.
In comparison to HCl, AlCl3 shows higher activity for the conversion of glucose
to HMF, but lower activity for the conversion of fructose to HMF. With AlCl3 as
catalyst, HMF selectivity (based on converted fructose) is only 57%, compared to
82% for HCl. Some fructose is isomerized to glucose in the presence of AlCl3; a
glucose yield of 7% is observed. Correcting for glucose formation, HMF selectivity
affords only 64%, which is lower than the 82% observed for HCl. Huber at al. [23]
studied the effect of Lewis and Brønsted acid sites on the dehydration of xylose. They
found that a lower ratio of Brønsted acid sites versus Lewis acid sites results in
lowered furfural selectivity because Lewis acid sites are principally responsible for
the further conversion of furfurals to intractable humins. Thus Lewis acids show
lower HMF selectivity from fructose than Brønsted acids [24]. While the
AlCl3-H2O/THF biphasic system is superior in HMF selectivity from glucose in
comparison to HCl, it is inferior in the selective dehydration of fructose.
In an attempt to increase HMF selectivity, we introduced HCl as an additive to
AlCl3-H2O/THF biphasic system. The effect of HCl on AlCl3 catalyzed fructose
conversion is summarized in Fig. 4. The introduction of HCl increases the total
Brønsted acid sites, and as a result fructose conversion increases. In the absence of
HCl, fructose conversion of 67% and HMF selectivity of 63% are observed. As HCl is
added the selectivity to HMF are increased reaching a maximum at ca. 80% with [HCl]
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= 0.316 M (pH = 0.5). Further increase in HCl beyond this optimal value results in
further degradation of HMF to LA and humins [21]. Furthermore, it is worth noting
that the introduction of HCl inhibited the isomerization of fructose to glucose.
The effect of adding HCl on the AlCl3 catalyzed glucose conversion is
summarized in Fig. 5. The introduction of HCl does not show a discernible increase in
glucose conversion. On the contrary, a decrease of glucose conversion was observed
at pH values lower than 1. Considering that the dehydration reaction rate increases as
the pH decreases, the decrease in glucose conversion must be caused by the decrease
in the rate of isomerization. The effect of pH on the isomerization of
o
glucose-to-fructose was investigated at 120 C, a temperature under which the
dehydration of fructose to HMF is significantly retarded. As evident in Fig. 6, fructose
yield decreases as the pH is decreased. At pH = 0, nearly no fructose was detected.
The introduction of HCl not only increased the amount of Brønsted acid sites, but also
affected the equilibrium of Al3+ aqua/hydroxo complexes (Al3++H2O ⇋ Al3+ hydroxo
complexes + H+). These results imply that Al3+ hydroxo complexes are responsible
for the glucose-fructose ismerization reaction. When pH=0, glucose conversion with
HCl alone was 52%, and 60% with AlCl3. The introduction of HCl affected the
contribution from two routes, direct glucose conversion and via fructose. As a result, a
maximum HMF selectivity is observed over the pH range of 0.5-1.
Given that the AlCl3-HCl-H2O/THF (pH=1) biphasic system resulted in the
highest HMF selectivity, a time profile of the reaction was obtained (Fig. 7), and
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compared to the profile for the AlCl3 catalyzed reaction without HCl (Fig. 8). In the
absence of HCl, fructose builds up to 35% yield after 1 min. In the presence of HCl
(pH=1), the maximum fructose yield was only 14% and tmax = 2.5 min. The
isomerization of glucose-to-fructose is somewhat slower with added HCl. However,
the total conversion of glucose was comparable under both conditions. The
introduction of HCl increases the selectivity of HMF from 51% to 62%. The route via
fructose remains the dominant route for glucose conversion to HMF in the
AlCl3-HCl-H2O/THF (pH=1) biphasic system.
4. Conclusions
For the AlCl3 catalyzed glucose conversion to HMF in H2O/THF biphasic
medium, the route via fructose as an intermediate is the dominant pathway. The Lewis
acidic sites are responsible for isomerization as well as dehydration. The introduction
of HCl as an additive/co-catalyst to AlCl3 can improve the selectivity to HMF as long
as the pH is not low enough so that direct dehydration of glucose becomes dominant.
An optimal HMF yield of 62% can be obtained from glucose with 0.1 M HCl added
to AlCl3-H2O/THF medium. Our results are consistent with the report from Dumesic
and co-workers on the effect of HCl in conjunction with Lewis acids in alkyl phenol
aqueous biphasic medium [15].
Acknowledgement
This work was financially supported by the National Basic Research Program of
China (973 program, No. 2007CB210203), NNSFC 21072136, PCSIRT (No.
IRT0846), the Center for direct Catalytic Conversion of Biomass to Biofuels (C3Bio),
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an Energy Frontier Research Center funded by the U.S. Department of Energy, Office
of Science, and Office of Basic Energy Sciences under Award Number
DE-SC0000997. C3Bio provided funding for laboratory space, instrumentation, and
chemical supplies at Purdue University. Y.Y. was supported by a China Scholarship
grant (Grant no. 2010624099).
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6276-6268
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Caption of Figures
Fig. 1. Reaction routes from glucose to HMF
Fig. 2. The effect of pH (Brønsted acidity) on the conversion of fructose to HMF.
o
Reaction condition: fructose = 0.25 mmol in H2O/THF (1:3 mL) at 160 C under microwave
heating for 2.5 min. pH is adjusted by HCl and are the values measured at 25 C.
a Results with 0.1 mmol AlCl3·6H2O as catalyst.
Fig. 3. The effect of pH on the conversion of glucose to HMF with HCl.
o
Reaction condition: glucose = 0.25 mmol in H2O/THF (1:3 mL) at 160 C under microwave
heating for 5 min. pH is adjusted by HCl.
Fig. 4 The effect of HCl on AlCl3·6H2O catalyzed fructose conversion.
Reaction condition: fructose = 0.25 mmol, AlCl3·6H2O = 0.10 mmol in H2O/THF (1:3 mL) at 160
oC under microwave heating for 2.5 min. pH is adjusted by HCl.
Fig. 5. The effect of HCl on AlCl3·6H2O catalyzed glucose conversion.
Reaction condition: glucose = 0.25 mmol, AlCl3·6H2O = 0.10 mmol in H2O/THF (1:3 mL) at 160
oC under microwave heating for 2.5 min. pH is adjusted by HCl.
Fig. 6. The effect of HCl on AlCl3·6H2O catalyzed glucose-fructose isomerization.
o
Reaction condition: glucose = 0.25 mmol, AlCl3·6H2O = 0.10 mmol in H2O (1 mL) at 120 C
under microwave heating for 30 min. pH is adjusted by HCl.
Fig. 7. Conversion of glucose as a function of time using AlCl3·6H2O-HCl (pH = 1) in H2O/THF
biphasic medium.
Reaction condition: glucose = 0.25 mmol, AlCl3·6H2O = 0.1 mmol, HCl= 0.1 mmol in H2O/THF
(1:3 mL) at 160 oC under microwave heating.
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Fig. 8. Conversion of glucose as a function of time using AlCl3·6H2O in H2O/THF biphasic
medium.
Reaction condition: glucose = 0.25 mmol, AlCl3·6H2O = 0.1 mmol in H2O/THF (1:3 mL) at 160
oC under microwave heating.
16
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HO
OH
O
HO
OH
The route
via fructose
OH
Fructose
dehydration
isomerization
HO
HO
HO
O
OH
O
O
OH
The direct
route
OH
dehydration
Glucose
HMF
Fig. 1
17
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fructose conversion
HMF yield
100
80
60
40
20
0
fructose conversion a
HMF yield a
pH=1.35
pH=1.61
0.0
0.5
1.0
1.5
2.0
pH value
Fig 2.
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glucose conversion
HMF yield
60
40
20
0
pH=1.35
pH=1.61
0.0
0.5
1.0
1.5
2.0
pH value
Fig 3.
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Fructose conversion
glucose selectivity
HMF selectivity
100
80
60
40
20
0
No HCl is add
pH= 2.04
0.0
0.5
1.0
1.5
2.0
pH value
Fig 4.
20
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glucose conversion
fructose selectivity
HMF selectivity
100
80
60
40
20
0
No HCl is add
0.0
0.5
1.0
1.5
2.0
pH value
Fig 5.
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30
25
20
15
10
5
glucose conversion
Cfructose yield
No HCl is add
0
0.0
0.5
1.0
1.5
2.0
2.5
pH value
Fig 6.
22
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100
80
60
40
20
0
glucose conversion
fructose yield
HMF yield
0
5
10
15
20
Reaction time (min)
Fig 7.
23
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100
80
60
40
20
0
glucose conversion
fructose yield
HMF yield
0
5
10
15
20
Reaction time (min)
Fig 8.
24
Page 24 of 24