Synthesis of 5-bromomethylfurfural from cellulose as a potential intermediate for biofuel

Article abstract of DOI:10.1002/ejoc.201001539

Cellulose can be converted into 5-bromomethylfurfural (BMF), a brand new biofuel or biofuel intermediate, in 80 yield by treatment with HBr and LiBr and continuous extraction with toluene. From other carbohydrates and straw, BMF was also obtained in high yields. The mechanism of BMF formation was investigated, and the results indicate that this furfural is formed during the depolymerization of cellulose and that glucose is not an intermediate.

Full text of DOI:10.1002/ejoc.201001539

FULL PAPER
DOI: 10.1002/ejoc.201001539
Synthesis of 5-Bromomethylfurfural from Cellulose as a Potential Intermediate
for Biofuel
Nitee Kumari,[a] Jens Kruse Olesen,[a] Christian M. Pedersen,[a] and Mikael Bols*[a]
Keywords: Biofuels / Oxygen heterocycles / Sustainable chemistry / Reaction mechanisms / Kinetics
Cellulose can be converted into 5-bromomethylfurfural
(BMF), a brand new biofuel or biofuel intermediate, in 80%
yield by treatment with HBr and LiBr and continuous extrac-
tion with toluene. From other carbohydrates and straw, BMF
was also obtained in high yields. The mechanism of BMF
formation was investigated, and the results indicate that this
furfural is formed during the depolymerization of cellulose
and that glucose is not an intermediate.
of obtaining a biofuel from cellulose. Inspired by this work
and Zhang’s report that Cr ions could improve furfural for-
mation,[6] we wondered if the formation of 1 could be im-
proved by addition of CrCl2. However, the information that
bromide is a poorer ligand for Cr than chloride led us to
investigate the reaction of carbohydrates with HBr. In this
paper we report the results of these investigation and show
that HBr is an excellent reagent for the conversion of bio-
mass carbohydrates into 5-bromomethylfurfural (3, Fig-
ure 1). Compound 3 is formed in higher yield than 1 and
can be converted into 2 or dimethylfuran allowing HBr to
be recycled.
Introduction
With the worrying growth in global CO2 levels and the
potential climate changes that may be the result of this de-
velopment, it is increasingly clear to most people that mod-
ern society’s dependence on fossil fuels is a problem that
has to be solved. While solar, wind, and nuclear power may
be relied upon to provide much of the future energy supply,
they will not be practical for transportation fuels. Combus-
tible, liquid organic compounds are much more practical,
but these will unavoidably lead to CO2 emission. The solu-
tion to this problem may be to obtain these fuels from car-
bon sources already present in the biosphere, so-called bio-
fuels, thereby avoiding increasing the amount of carbon and
CO2 in circulation. Ethanol obtained by fermentation is the
biofuel that has received most attention, but there are alter-
natives that indeed may prove to be better solutions. Etha-
nol as a fuel has the drawbacks of corrosiveness, a compar-
atively low energy content, and a costly and low-yielding
production method (when produced by fermentation). Al-
ternatives that have been suggested to solve some or all of
these problems are butanol,[1] methanol, dimethylfuran,[2]
5-ethoxymethylfurfural,[3] γ-valerolactone, and alkanes pro-
duced from biomass.[4,5] Many of these alternatives rely on
conversion of biomass carbohydrates into furfural deriva-
tives, and the methods of furfural synthesis are rapidly pro-
gressing.[6–9] One of the promising new methods of doing
that was reported by Mascal who showed that cellulose
could be converted into 5-chloromethylfurfural (1) in 71%
yield by treatment with HCl/LiCl and continuous extrac-
tion with dichloroethane.[3] From 1, potential biofuel 2
could be obtained in quantitative yield by reaction with eth-
anol. This method has been further improved and simpli-
fied,[10] and 1 has been converted into biodiesel prod-
ucts.[11,12] It is one of the most direct and high-yielding ways
Figure 1. 5-Ethoxymethylfurfural, 5-chloromethylfurfural, and 5-
bromomethylfurfural (BMF).
Results and Discussion
In a paper from 1922, 5-bromomethylfurfural (BMF, 3)
was prepared in 28–48% yield by treatment of cellulose
with HBr in chloroform.[13] Curiously, glucose and cello-
biose gave significantly lower yields (10–23%). Otherwise, 3
was prepared from 5-hydroxymethylfurfural (HMF, 4) by
substitution of the hydroxy group, which is a facile reac-
tion.[14] However, when a procedure equivalent to Mascal’s
was employed, replacing concentrated HCl with concen-
trated HBr and LiCl with LiBr, excellent yields of 3
(Scheme 1) were obtained. These experiments were done in
an apparatus equipped for continuous extraction either for
solvents denser or lighter than water. Fructose gave a yield
of 68% of 3 after chromatography, when dichloroethane
was used as the extraction solvent. Variation in the solvent
[a] Department of Chemistry, University of Copenhagen,
Universitetsparken 5, 2100 Copenhagen, Denmark
E-mail: bols@kemi.ku.dk
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Synthesis of 5-Bromomethylfurfural as a Potential Intermediate for Biofuel
showed that toluene improved the yield (82%), whereas
tetrahydrofuran gave no product. A decrease in the amount
of LiBr used (Table 1, Entry 4) gave a lower yield; obviously
a high bromide concentration is important.
Scheme 1. Synthesis of 3.
Table 1. Yields of purified 5-bromomethylfurfural (3) obtained
from carbohydrates and biomass.
Entry Substrate Solvent[a] Temp.[b]
[°C]
LiBr
[equiv.]
Time Yield
Scheme 2. Classical mechanism of 5-hydroxymethylfurfural forma-
tion adapted to BMF. The mechanism supposes formation of glu-
cose and isomerization to fructose.
[h]
[%]
1
2
3
4
5
6
7
fructose
fructose
fructose
fructose
glucose
cellulose
straw
DCE
MePh
THF
MePh
MePh
MePh
MePh
25
25
25
25
65
65
65
20
20
20
2
20
20
20
48
48
48
48
48
28
48
68
82
0
70
50
80
68
resulted in a rate enhancement, especially for cellulose. This
suggest that CrII/CrIII catalyze both the transformation of
glucose and cellulose. Finally, it was observed that the reac-
tion of fructose is faster than the process of continuous ex-
traction: When manual extraction was performed on reac-
tion samples, a higher rate is obtained. The above rates were
determined simply by evaporating the extraction solvent
and weighing the residue. NMR spectroscopy was used to
verify that the product was essentially 1.
[a] DCE = 1,2-dichloroethane, MePh = toluene, THF = tetra-
hydrofuran. [b] Temperature applied to the reaction chamber.
When the reaction is applied to other carbohydrates the
following results were obtained: glucose gave a 50% yield
of 3, whereas cellulose (microcrystalline) surprisingly (but
in accordance with ref.[13]) gave a 80% yield of 3 (Table 1).
Finally 68% of 3 can be obtained from straw, showing that
the method is easily applied to biomass. This yield is that
after chromatography and does not include some furfural
coming from hemicellulose. In these reactions, the rate
is slower and heating was therefore (unlike for fructose)
applied to the reaction chamber.
Formation of 3 would normally be expected to follow
the mechanism shown in Scheme 2: Cellulose is degraded
to glucose by the strong acid, glucose isomerizes to fructose
by acid-catalyzed enolization, fructose undergoes several
dehydrations to give 4, which has a very reactive alcohol
that reacts with HBr to form 3. An alternative mechanism,
where 3-deoxy-2-ketoglucose, and not fructose is an inter-
mediate, is also possible.[15] However, the fact that both
fructose and cellulose give a much higher yield than glucose
is not consistent with these mechanisms.
To gain better insight into what is happening in this reac-
tion, we followed the rate of formation of both BMF (3)
and CMF (1). The CMF experiments were essentially per-
formed as described above and should be identical to those
by Mascal[3] subject to possible differences associated with
equipment and setup (see the Experimental Section). Pre-
Figure 2. Yield of 1 obtained after different times by treatment of
carbohydrates with conc. HCl/LiCl and continuous extraction with
DCE. Fructose (manual extraction, ♦), fructose (᭿), cellulose (w.
Cr, ᭹), glucose (w. Cr, ᭛), glucose (ᮀ), cellulose (᭺).
The rate of formation of 3 from different starting materi-
paratively, we obtained 90–95% yield of 1 from fructose, als is shown in Figure 3. These rates were determined by
62% of 1 from glucose, 63% of 1 from microcrystalline HPLC analysis of the extraction solvent – manual extrac-
cellulose, and 56% of 1 from cotton. The rate of these reac- tion was performed to avoid potential problems of con-
tions is shown in Figure 2. It is seen that formation of 1 tinuous extraction being rate limiting. Formation of BMF
from fructose was very fast, the formation of 1 from glucose (3) from fructose was again very fast, but the reaction from
is slower, and formation of cellulose even more so. This is cellulose was faster than that from glucose. Even the reac-
basically in accordance with the mechanism in Scheme 2. tion from straw was faster than the reaction from glucose.
Addition of CrCl2 to the reaction of glucose and cellulose Addition of CrBr3 led to a slight decrease in the formation
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N. Kumari, J. K. Olesen, C. M. Pedersen, M. Bols
FULL PAPER
of BMF (3) from cellulose and a large increase in formation intermediate (Scheme 4). The reaction occurs by transfor-
of BMF (3) from glucose. Altogether, these rate results are mation of cellulose from the reducing end. The terminal
not consistent with the mechanism in Scheme 2, because residue undergoes acid-catalyzed rearrangement to a fruc-
the reaction from glucose is clearly slower than that from tofuranoside, which subsequently undergoes dehydration.
cellulose.
However, depolymerization does not occur by glycoside
cleavage but by elimination of the 4-OR groups as part of
the dehydration.
Scheme 4. Revised mechanism for the formation of 3 from cellulose
not involving the formation of glucose (top). Suggested mechanism
for CrIII catalysis for the formation of 3 from glucose (bottom).
Remaining is the question of the influence of Cr catalysis
on these reactions, because a significant rate acceleration
was observed for the transformation of glucose into BMF
(3) and for cellulose into CMF (1). It has been suggested
that the effect of Cr in furfural synthesis is to catalyze isom-
erization of glucose into fructose.[6,7] However, it has been
shown by Mønsted that various pentoses upon complex-
ation with CrIII undergo elimination of the 3-OH group,
whereas 2-epimerization, which is a sign of 1,2-enolization,
does not occur.[16] It therefore seems more plausible that
glucose undergoes elimination of the 3-OH group in the
presence of chromium catalysis (Scheme 4, bottom). This
leads to the 3-deoxy-2-ketoglucose that then rearranges into
2,5-furanose and undergoes further elimination to give 3.[15]
The bromine in BMF (3) was found to be very reactive;
thus, 3 could readily be converted into 2 by gentle reflux in
ethanol or into HMF (4) by simple dissolution in water
(Scheme 5). Both reactions gave a quantitative yield and
potentially allow recovery of the HBr formed in the reac-
tion.
Figure 3. Yield of 3 obtained after different times by treatment of
carbohydrates with conc. HBr/LiBr and continuous extraction with
toluene. fructose (♦), cellulose (᭿), cellulose (w. Cr, ᭹), glucose (w.
Cr, ᭛), straw (ᮀ), glucose (᭺).
A faster reaction from cellulose, compared to that from
glucose, suggests that the 4-O-acetal link somehow is an
advantage in the dehydration reaction. To confirm this, we
prepared 4-O-methylglucose (8) as outlined in Scheme 3:
From methyl 2,3,6-tri-O-benzyl-d-glucopyranoside (5)
methylation with MeI/NaH gave an excellent yield of 6, hy-
drolysis with H2SO4 gave 40% 7, and finally hydrogenolysis
led to 8 in 80% yield. Treatment of 8 with HBr/LiBr and
continuous extraction with toluene gave an 80% yield of
BMF (3). That means that 8 is as good a substrate as cellu-
lose and much better than glucose.
Scheme 5. Conversion of BMF (3) into 2 and 5-hydroxymethylfur-
fural (HMF, 4).
Conclusions
Scheme 3. Conversion of 4-O-methylglucose into BMF (3).
We have shown that 5-bromomethylfurfural (BMF, 3)
On the basis of the above, we suggest a revised mecha- can be made very efficiently from cellulose or straw by treat-
nism for the formation BMF (3), in which glucose is not an ment with concentrated aqueous HBr. The mechanism does
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Synthesis of 5-Bromomethylfurfural as a Potential Intermediate for Biofuel
100 min, the reaction was stopped and the extracts were concen-
trated, affording 95% of crude 1 (0.93 g).
not involve the intermediate formation of glucose, and the
reaction is suggested to occur from the reducing end of the
polysaccharide. BMF (3) can be readily converted into po-
tential biofuel 4 and HMF (2) by ethanolysis and hydroly-
sis, respectively.
Chromium-Catalyzed Experiments: The chromium-catalyzed reac-
tions were carried out by using the procedures above, except that
chromium(III) bromide (1.12 g) was added for the preparation of
3 or chromium(II) chloride (0.25 g) was added for the preparation
of 1.
Experimental Section
HPLC Experiments: Carbohydrate (500 mg) and LiBr (4.8 g) were
mixed with HBr (55%, 4 mL). At different intervals, a 0.5-mL ali-
quot was extracted with toluene (2 mL). A 0.5-mL sample of this
mixture was diluted with toluene to 8 mL, and the concentration
of 3 in this solution was determined by HPLC. The HPLC column
Conversion of Cellulose into 5-Bromomethylfurfural (BMF, 3): Tolu-
ene was introduced into the extraction chamber of a standard ap-
paratus for continuous extraction with a solvent lighter than water.
A homogeneous suspension of microcrystalline cellulose (2.05 g,
5% water by mass) was prepared in a solution of lithium bromide
(10 g) in concentrated hydrobromic acid (150 mL), and this was
added to the extraction chamber. A boiling flask containing tolu-
ene (150 mL) and anhydrous sodium sulfate was attached to the
apparatus, and the solvent was heated to reflux. The aqueous slurry
was kept at 65 °C. During the extraction, the boiling flask was emp-
tied every 6 h and replaced with fresh toluene (150 mL). The com-
bined organic extracts were distilled to recover the solvent, and
the residual oil (1.69 g) was purified by chromatography (silica gel;
petroleum ether/ethyl acetate, 2:1) to give 3 (80%). 1H NMR
(300 MHz, CDCl3): δ = 4.46 (s, 2 H), 6.55 (d, J = 3.6 Hz, 1 H),
7.16 (d, J = 3.3 Hz, 1 H), 9.59 (s, 1 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 21.8, 112.3, 122.2, 153.0, 156.4, 177.9 ppm. HRMS:
calcd. for C6H5BrO2 188.9551; found 188.9551.
used was ACE
5 C18 and the solvent MeOH/H2O (3:2,
0.5 mLmin). BMF was monitored by UV at 254 nm and gave a
retention time of 5.6 min.
Methyl 2,3,6-Tri-O-benzyloxy-4-O-methyl-α-D-glucopyranoside (6):
To a suspension of NaH (80 mg, 3.3 mmol) in dry DMF (7 mL)
was added dropwise a solution of methyl 2,3,6-tri-O-benzyloxy-α-
d-glucopyranoside (5; 700 mg, 1.508 mmol) in DMF (3 mL) at
0 °C. The reaction mixture was stirred for 1 h at 0 °C and methyl
iodide (0.14 mL, 2.2 mmol) was added to the reaction mixture. The
reaction mixture was stirred for another 2 h to complete the reac-
tion. The reaction was quenched by adding the reaction mixture
into ice-cold water, which was then extracted with ethyl acetate
(2ϫ25 mL). The collected organic layers were washed with brine,
dried with Na2SO4, and concentrated to obtain alkylated product
6, which was purified by column chromatography. Yield: 95%, vis-
cous liquid Rf = 0.50 (petroleum ether/ethyl acetate, 4:1), 1H NMR
(300 MHz, CDCl3) as reported.[17]
Conversion of Glucose into BMF (3): Using the general procedure
described above, glucose (2.01 g) gave a crude product that was
purified by chromatography to give 3 (1.0 g, 50%).
Conversion of Fructose into BMF (3): Using the general procedure
described above, fructose (2.00 g) gave 3 (1.75 g, 82%).
2,3,6-Tri-O-benzyloxy-4-O-methyl-D-glucopyranose (7): Compound
6 (500 mg, 1.1 mmol) in a solution of 1 n H2SO4/CH3COOH (1:2)
was heated at reflux for 7 h to get product 7 (200 mg, 40%). Vis-
cous liquid Rf = 0.50 (petroleum ether/ethyl acetate, 6:4), 1H NMR
(300 MHz, CDCl3) as reported.[18] 13C NMR (75 MHz, CDCl3): δ
= 138.9, 138.8, 138.2, (Cquat, Ph) 128.7, 128.6, 128.57, 128.4, 128.3,
128.2, 128.1, 127.9, 127.8, (Ph), 97.7 (C-1β), 91.5 (C-1α), 84.6
(Bnβ), 83.1 (Bnβ), 81.9 (Bnα), 80.1 (Bnβ), 80.0 (Bnα), 79.8 (Bnα),
75.8, 75.7, 74.9, 73.7, 73.7 (2 C), 73.4, 70.5, 69.3, 68.9 (C-2, C-3,
C-4, C-5, C-6), 60.9 (2 C, OMe) ppm.
Conversion of Straw into BMF (3): Using the general procedure
described above, straw (2.00 g) gave a crude product (1.43 g) that
was purified by chromatography to give 3 (1.2 g, 68%).
Conversion of Cellulose into 5-Chloromethylfurfural (CMF, 1): 1,2-
Dichloroethane (DCE) was added to the reaction chamber of a
standard setup for continuous extraction with solvents denser than
water. Microcrystalline cellulose (1.24 g) was added to a solution
of lithium chloride (6.08 g) in concentrated hydrochloric acid
(90 mL). The suspension was added to the reaction chamber, which
was kept at 65 °C, whilst the collection flask was kept at 140 °C
thereby continuously refluxing DCE through the aqueous solution.
The collection flask was emptied every 30 min, and the contents
was concentrated on a rotary evaporator, noting the mass of the
resulting oily crude product. The reaction was stopped after 30 h,
yielding 1 as an oily crude product (0.63 g, 63%). 1H NMR
4-O-Methyl-D-glucose (8): Compound 7 (200 mg) was dissolved in
MeOH and subjected for debenzylation in H-cube by using
Pd(OH)2 as catalyst at 50 bar at 50 °C for 2 h. Yield 80%. 1H NMR
(300 MHz, D2O, 1:1 mixture of anomers): δ = 5.09 (d, J = 3.5 Hz,
1 H) 4.84 (s, 1 H), 4.44 (d, J = 7.8 Hz, 1 H), 3.85–3.61 (m, 7 H),
3.55 (s, 6 H), 3.47–3.19 (m, 4 H), 3.17–3.00 (m, 3 H) ppm. 13C
NMR (75 MHz, MeOD): δ = 98.2, 93.9 (C-1), 81.3, 81.1, 78.2, 77.1
(300 MHz, CDCl3): δ = 4.62 (s, 2 H), 6.60 (d, J = 3.3 Hz, 1 H), 76.5, 75.0 74.0, 72.2 (C-2, C-3, C-4, C-5), 62.5, 62.4 (C-6), 61.0,
7.22 (d, J = 3.0 Hz, 1 H), 9.65 (s, 1 H) ppm.
60.9 (OMe) ppm.
Conversion of Glucose into CMF (1): Carried out as above, except
replacing cellulose with glucose (1.37 g). The reaction was stopped
after 26 h, affording 62% of crude 1 (0.68 g).
Conversion of 4-O-Methylglucose into BMF: Using the general pro-
cedure described above for BMF formation, 4-O-methylglucose (8,
2.00 g) gave 3 in 80% yield.
Conversion of Fructose into CMF (1): Carried out as above, except
replacing cellulose with fructose (1.24 g). The reaction was stopped
after 30 h, affording 90% of crude 1 (0.89 g).
5-Ethoxymethylfurfural (2): BMF (3, 500 mg) was dissolved in eth-
anol (10 mL), and the solution was heated at reflux for 3 h. The
solution was concentrated and subjected to chromatography
(CH2Cl2/diethyl ether, 2:1). This gave 2 in quantitative yield. 1H
NMR (300 MHz, CDCl3): δ = 9.40 (s, 1 H), 7.06 (d, J = 3.1 Hz, 1
H), 6.34 (d, J = 3.5 Hz, 1 H), 4.32 (s, 2 H), 3.39 (q, 2 H), 1.04 (t,
3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 176.7, 157.9, 151.8,
121.8, 110.3, 65.3, 63.6, 14.1 ppm.
Conversion of Fructose into CMF (1) with Manual Extraction: Lith-
ium chloride (6.23 g) was dissolved in concentrated hydrochloric
acid (90 mL) and fructose (1.23 g) was added to the solution, which
was kept at 65 °C. The reaction mixture was manually extracted
with dichloromethane (100 mL) after 25, 60, and 100 min. After
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FULL PAPER
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5-Hydroxymethylfurfural (4): BMF (3, 500 mg) was dissolved in
distilled water (10 mL), and the solution was stirred for 1 h. The
solution was concentrated and subjected to chromatography
(CH2Cl2/diethyl ether, 2:1). This gave 4 in quantitative yield. 1H
NMR (300 MHz, CDCl3): δ = 9.57 (s, CHO), 7.13 (d, J = 3.6 Hz,
1 H), 6.53 (d, J = 3.3 Hz, 1 H), 4.56 (s, 2 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 178.2, 161.6, 153.4, 121.8, 111.3, 57.1 ppm.
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Received: November 15, 2010
Published Online: January 21, 2011
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