Synthesis of furfural from xylose and xylan

Article abstract of DOI:10.1002/cssc.201000181

Furans from pentoses: Pentose-derived furfural is a commodity chemical produced from lignocellulose. A new route to furfural from renewable carbohydrates uses chromium catalysts under mild conditions to avail a chemical mechanism distinct from the usual acid catalysis. Applicable to both xylose and xylan, this new method offers an advantageous route to biomass-derived chemicals.

Full text of DOI:10.1002/cssc.201000181

DOI: 10.1002/cssc.201000181
Synthesis of Furfural from Xylose and Xylan
Joseph B. Binder,[a, b] Jacqueline J. Blank,[c] Anthony V. Cefali,[d] and Ronald T. Raines*[a, c]
Rising fossil energy prices and environmental preservation call
for alternative sources of energy, such as renewable fuels
based on biomass.[1] Biofuels could reduce carbon dioxide
emissions and decrease fuel prices, particularly if derived from
nonfood biomass resources such as agricultural, forest, and
landfill wastes. Hemicellulose, a mixture of polysaccharides
containing xylose, arabinose, glucose, galactose, mannose, and
other sugars, is typically the second-most-abundant compo-
nent of biomass, after cellulose.[2] In grasses and hardwoods,
xylan, a polymer of xylose, is often the primary hemicellulose.
As a result, xylan conversion is critical for utilization of impor-
tant biomass feedstocks such as bagasse, corn stover, Miscan-
thus, switchgrass, and poplar.
from the manufacture of oatmeal (at one point, Quaker Oats
had amassed so much of this byproduct that the leftover oat
hulls were stored in a circus tent). Though they could be used
as livestock feed, the hulls were only partially digestible.
Quaker Oats tested a variety of processes to valorize the hulls
and found that treating them with dilute sulfuric acid yielded
useful amounts of furfural.
Since this initial discovery, many others have examined the
conversion of pentoses into furfural.[6] The process used by
Quaker Oats employs a dilute sulfuric acid catalyst and steam
pressure, achieving 50% molar yields of furfural from xylan.[3a]
Most industrial processes achieve similar yields, likely limited
by side reactions such as homopolymerization and condensa-
tion with unreacted xylose. Moreau and co-workers have con-
verted xylose into furfural in about 50% yield by using acidic
dealuminated zeolites in water along with toluene as an ex-
tracting solvent.[6b] Sulfated zirconia has also been used as a
catalyst for xylose dehydration, producing yields of around
50%.[6d] In all of these cases, Brønsted acid catalysts were used
in aqueous solution at temperatures greater than 1508C. Al-
though questions remain about the mechanism for furfural for-
mation from xylose under these conditions, recent computa-
tional work by Nimlos and co-workers[7] supports a mechanism
proposed by Antal and co-workers (Figure 1).[8] In this mecha-
nism, the C-2 hydroxyl group is displaced to form a xylose-2,5-
anhydride. Subsequent dehydration steps produce furfural.
Other evidence supports alternative mechanisms involving acy-
clic intermediates, and different mechanisms might operate
under different conditions of temperature and pH.[9]
Both xylan and xylose can be dehydrated into furfural, a bio-
fuel precursor and industrial chemical (Figure 1).[3] Indeed, fur-
fural is perhaps the most common industrial chemical derived
from lignocellulosic biomass, with an annual production
volume of more than 200000 t.[4] The commercial utility of fur-
fural was first discovered at the Quaker Oats Company in
1921.[5] The company had produced vast quantities of oat hulls
Recently, we and others reported on the conversion of
sugars[10] and biomass[11] into 5-hydroxymethylfurfural (HMF) in
ionic liquids and N,N-dimethylacetamide containing lithium
chloride (DMA-LiCl). In these initial studies, we found that the
pentosans in biomass were converted into furfural in moderate
yields by using a combination of chromium(II) or chromium(III)
salts and HCl. Unlike CrVI, which is highly toxic, CrIII is essential
for the normal metablism of carbohydrates, lipids, and fats in
humans, and is thus an essential dietary element.[12] CrII oxidiz-
es in air to CrIII. Our reaction conditions—CrII or CrIII as a cata-
lyst in a nonaqueous solvent—contrast markedly with the
aqueous Brønsted acid catalysis typical for furfural production.
We chose to investigate the reactions of xylose and xylan in
DMA-LiCl and related solvents, and report our findings herein.
Others have reported on the acid-catalyzed production of fur-
fural from xylose in ionic liquids.[13]
Figure 1. Route for the synthesis of furfural from xylan. Gray box: Putative
mechanism for the acid-catalyzed dehydration of xylose, in which displace-
ment of the protonated C-2 hydroxyl group leads to a 2,5-anhydride inter-
mediate that dehydrates to furfural.[8,7]
[a] Dr. J. B. Binder, Prof. R. T. Raines
Department of Chemistry, University of Wisconsin-Madison
1101 University Avenue, Madison, WI 53706 (USA)
Fax: (+1)608-890-2583
E-mail: rtraines@wisc.edu
[b] Dr. J. B. Binder
Energy Biosciences Institute
University of California, Berkeley, CA 94720 (USA)
We began by investigating the reactivity of xylose in DMA
with acid and chromium catalysts and halide additives at
1008C (Table 1). Only low yields of furfural and moderate con-
versions of xylose were observed in DMA, both alone and with
HCl. For example, 12 mol% HCl in DMA accomplished 47%
conversion of xylose with only a 6% yield of furfural. Although
[c] J. J. Blank, Prof. R. T. Raines
Department of Biochemistry, University of Wisconsin-Madison
433 Babcock Drive, Madison, WI 53706 (USA)
[d] A. V. Cefali
College of Agricultural and Life Sciences, University of Wisconsin-Madison
1450 Linden Drive, Madison, WI 53706 (USA)
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Table 1. Synthesis of furfural from xylose at 1008C.[a]
Solvent
Catalyst Amount Additive Amount Time Yield
[mol%]
[wt%]
[h]
[%]
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
–
–
6
6
6
3
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
5
5
5
5
5
5
2
2
4
6
2
2
2
2
2
2
2
2
4
6
2
4
6
2
2
2
2
2
2
2
2
2
2
4
4
4
4
4
<5
37
38
37
3
1
6
8
1
CrCl3
CrCl3
CrCl3
HCl
HCl
HCl
HCl
HCl
HCl
HCl
CrCl3
CrCl3
CrCl3
CrCl3
CrCl3
CrCl3
CrCl3
CrCl2
CrCl2
CrCl2
6
12
24
3
6
12
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl 10
[EMIM]Cl 10
[EMIM]Cl 10
[EMIM]Cl 20
[EMIM]Cl
[EMIM]Cl 10
[EMIM]Cl 20
[EMIM]Cl
[EMIM]Cl 10
[EMIM]Cl 20
[EMIM]Cl
[EMIM]Cl 10
[EMIM]Cl 20
LiBr
LiBr
NaBr
[BMIM]Br 20
LiBr 10
2
3
37
37
34
37
37
38
40
45
38
41
40
30
37
24
33
36
47
50
54
55
56
5
Figure 2. Log–log plots of initial rates of furfural formation vs. the concen-
tration of xylose and CrCl2. a) Xylose; linear regression analysis gives
slope=1.0, which is consistent with a first-order dependence of the rate of
furfural formation on [xylose]. b) CrCl2; linear regression analysis gives
slope=0.4, which is consistent with a complex dependence of the rate of
furfural formation on [CrCl2].
DMA-LiCl(10%) CrCl3
DMA-LiCl(10%) CrCl3
DMA-LiCl(10%) CrCl3
DMA-LiCl(10%) CrCl2
DMA-LiCl(10%) CrCl2
DMA-LiCl(10%) CrCl2
DMA
DMA
DMA
DMA
DMA
5
5
CrCl3
CrBr3
CrCl2
CrCl2
CrCl2
10
10
10
concentration and a half-order dependence on CrII concentra-
tion. These results indicate that chromium is directly involved
in the mechanism of conversion prior to the rate-determining
step, perhaps in a complex manner.
Based on the kinetic analyses (Figure 2) and on the different
efficacies of chromium and HCl as catalysts (Table 1), we pro-
pose a mechanism by which chromium could effect the trans-
formation of xylose into furfural when the acid-catalyzed
mechanism is sluggish. In this mechanism (Figure 3), which is
analogous to that of the chromium-catalyzed conversion of
glucose into HMF,[14] chromium catalyzes xylose isomerization
through a 1,2-hydride shift. Acting as a Lewis acid, chromium
then converts the resulting xylulose into an oxocarbenium ion.
Deprotonation of this species produces an enol, which loses
two molecules of water to form furfural. This mechanism is
consistent with our observations of carbohydrate reactivity. Al-
dohexoses likely form HMF through this isomerization mecha-
nism, implying that xylose might do so under similar condi-
tions.[14] In addition, xylulose is far more predisposed to form
furfural than is xylose at 1008C,[9b] suggesting that transforma-
tion of xylose into xylulose would facilitate efficient furfural for-
mation.
[a] Xylose was reacted at a concentration of 10 wt% relative to the total
mass of the reaction mixture. The solvent composition is indicated by the
wt% of LiCl relative to DMA with additive concentrations relative to the
total mass of the reaction mixture. Catalyst loading is relative to xylose.
Yields are based on HPLC analysis.
Brønsted acids such as HCl are typically used to produce furfu-
ral, these reactions are commonly carried out at temperatures
>1508C. Under our more mild reaction conditions, acids alone
were less effective for rapid xylose conversion.
Conversely, xylose was converted into furfural in 30–40%
yield in DMA using CrCl2 and CrCl3 (Table 1). With these cata-
lysts, we observed no consistent effect of 1-ethyl-3-methylimi-
dazolium ([EMIM]) chloride or LiCl additives on the furfural
yield. Bromide additives such as 1-butyl-3-methylimidazolium
([BMIM]) bromide and LiBr improved furfural yields up to 56%.
Yields in these reactions are probably reduced by the reaction
of furfural with itself and with xylose to form oligomeric spe-
cies.[3a] The contrast between furfural yields using acid and
chromium catalysts suggests that an alternative mechanism of
xylose dehydration might be occurring in the presence of
chromium salts.
To test the validity of our hydride-shift mechanism, we per-
formed two deuterium-labeling experiments.[14] In the first, un-
labeled xylose was dehydrated in the presence of D2O;[15]
<5% deuterium incorporation was observed in the furfural
1
product by HNMR spectroscopy. This result is compatible with
To illuminate further the mechanism of this reaction, we ex-
amined the initial rates of furfural appearance in DMA-LiCl con-
taining CrCl2 (Figure 2). These analyses revealed that the rate
of furfural formation has a first-order dependence on xylose
the proposed hydride-shift mechanism (as well as the 2,5-anhy-
dride mechanism proposed for acid catalysis; Figure 1), but in-
consistent with a mechanism that avails an enediol intermedi-
ate (Figure 3). In the second experiment, xylose-2-d was con-
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Table 2. Synthesis of furfural from xylan at 1008C.[a]
Solvent
Catalyst
Amount
[mol%]
Additive
Amount
[wt%]
Yield
[%]
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
HCl
HCl
HCl
CrCl2
CrCl2
CrCl2
CrCl3
CrCl3
CrCl3
HCl
3
6
12
6
6
6
6
6
6
–
–
–
–
–
–
0
1
0
0
0
0
0
1
0
0
0
1
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
5
10
20
5
10
20
10
10
10
3
6
12
HCl
HCl
[a] Birch xylan was reacted for 4 h at a concentration of 5 wt% relative to
the total mass of the reaction mixture. The solvent composition is indicat-
ed by the wt% of LiCl relative to DMA with additive concentrations rela-
tive to the mass of the reaction mixture. Catalyst loading and yield are
relative to moles of xylose monomers contained in xylan. Yields are
based on HPLC analysis.
Figure 3. Putative mechanisms for the chromium(III)-catalyzed conversion of
xylose into furfural. Hydrogens replaced with deuteriums in isotopic labeling
studies are depicted in boldface type.
Table 3. Synthesis of furfural from xylan at 1208C.[a]
verted into furfural in the presence of H2O;[16] 48% deuterium
Solvent
Catalyst
Amount
[mol%]
Additive
Amount
[wt%]
Yield
[%]
1
incorporation was observed at C-1 of furfural by HNMR spec-
troscopy. This transfer of the deuterium label from C-2 to C-1
is consistent only with the hydride-shift mechanism (Figure 3).
The conversion of xylan into furfural was significantly more
challenging than was the dehydration of xylose. Birch xylan,
the typical material used in these studies, was not readily solu-
ble in DMA. Instead, it was solubilized by heating and stirring
in DMA with LiCl or LiBr at 80–1208C for several hours. Xylan
dissolved in DMA-LiCl precipitated by addition of water, indi-
cating that it was not fully depolymerized into xylose under
these conditions. Initially, we attempted the conversion of
xylan at 1008C using conditions similar to those used success-
fully with xylose (Table 2). With xylan, however, only trace
yields of furfural were obtained. Slightly higher yields were
possible from reactions per-
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl (10%)
CrCl2
CrCl3
HCl
CrCl2
CrCl2
CrCl3
CrCl3
6
6
6
6
6
6
6
–
–
–
–
–
–
10
20
10
20
0
4
3
0
3
3
0
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[a] Birch xylan was reacted for 4 h at a concentration of 5 wt% relative to
the total mass of the reaction mixture. The solvent composition is indicat-
ed by the wt% of LiCl relative to DMA with additive concentrations rela-
tive to the mass of the reaction mixture. Catalyst loading and yield are
relative to moles of xylose monomers contained in xylan. Yields are
based on HPLC analysis.
formed at 1208C (Table 3), sug-
Table 4. Synthesis of furfural from xylan at 1408C.[a]
gesting that these conditions
were too mild to accomplish the
depolymerization of xylan into
xylose. By increasing the reaction
temperature to 1408C, 7–8%
yields of furfural were obtained
when using CrCl2, although CrCl3
was less effective (Table 4). In
similar studies that emphasize
the role of temperature in the
conversion of xylan to furfural,
Zhao and co-workers obtained a
furfural yield of up to 63% in
ionic liquids heated to ~2008C
by microwave irradiation.[17]
Adding HCl to the reaction mix-
Biomass
Solvent
Catalyst
Amount
[mol%]
Additive
Amount
[wt%]
Yield
[%]
xylan
xylan
xylan
xylan
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
[EMIM]Cl
CrCl2
CrCl2
CrCl3
HCl
6
6
6
6
6
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
–
–
–
–
–
–
5
10
20
–
–
–
–
–
–
8
7
1
1
2
18
25
11
22
xylan
HCl
xylan
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
CrCl2,HCl
CrCl2,HCl
CrCl2,HCl
CrCl2,HCl
10,10
10,10
10,10
10,10
xylan (oat)
xylan (beech)
corn stover
[a] Xylan was reacted for 2 h at a concentration of 5 wt% relative to the total mass of the reaction mixture.
Birch xylan was used except where noted. The solvent composition is indicated by the wt% of LiCl relative to
DMA with additive concentrations relative to the mass of the reaction mixture. Catalyst loading and yield are
relative to moles of xylose monomers contained in xylan. Yield from corn stover is based on a xylan content of
22.8%. Yields are based on HPLC analysis.
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ture as a co-catalyst for xylan saccharification and using
[EMIM]Cl as the solvent improved the yield of furfural to 18%.
Disparate yields under these conditions were obtained with
each xylan source, suggesting that xylan recalcitrance depends
on its origin. These results imply that depolymerization is a
major barrier for chromium-catalyzed furfural production from
xylan.
by using high temperatures and strong acid catalysts, we have
found that chromium halide catalysts enable furfural synthesis
from xylose in N,N-dimethylacetamide and ionic liquids at
moderate temperatures. Mechanistic investigations suggest
that these catalysts isomerize xylose into xylulose, a reactive
ketose intermediate that dehydrates readily into furfural. We
have also demonstrated the transformation of xylan into furfu-
ral under similar conditions. Our chromium-based process
offers an advantageous route from pentoses and pentosans to
renewable furfural-derived fuels and chemicals.
We attempted to address this problem by using acid for
xylan saccharification prior to furfural synthesis. To do so, we
treated xylan with HCl at 1408C prior to addition of the chro-
mium catalyst and reaction at 1208C (Table 5). This process en-
Experimental Section
Table 5. Synthesis of furfural from xylan at 1208C after treatment with HCl at 1408C.[a]
General considerations: Commer-
cial chemicals were of reagent
grade or better, and were used
without further purification. Reac-
tions were performed in glass ves-
sels heated in a temperature-con-
trolled oil bath with magnetic stir-
ring. [EMIM]Cl (99.5%) was from
Solvent-Innovation (Cologne, Ger-
many). Birchwood xylan (X0502,
98% xylose residues, ca. 95% dry
solids) and beechwood xylan
(X4252, 95% xylose residues, ca.
95% dry solids) were from Aldrich
Chemical (Milwaukee, WI). Oat hull
xylan (X0011, 70.2% xylose resi-
dues, ca. 95% dry solids) was from
TCI (Tokyo, Japan). Milled and
sieved corn stover (ca. 95% dry
solids) was generously provided by
B. E. Dale and co-workers (Michi-
gan State University),[19] and was
passed through a 40-mesh screen
prior to use.
Solvent
Catalyst
Amount
[mol%]
Additive
Amount
[wt%]
Yield
[%]
DMA
DMA
DMA
DMA
DMA
DMA
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(10%)
DMA-LiCl(5%)
DMA-LiCl(5%)
DMA-LiCl(5%)
DMA-LiCl(5%)
DMA-LiCl(5%)
DMA-LiCl(5%)
DMA
HCl
CrCl3
CrCl3,HCl
HCl
CrCl3
25
6
6,25
25
–
–
–
–
–
–
7
0
11
10
3
15
7
4
6
6
6
6
8
4
0
8
6
11
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
–
–
–
–
–
5
6
10
20
–
–
–
–
–
–
5
10
5
10
20
10
CrCl3,HCl
CrCl2,HCl
CrCl3,HCl
CrCl2,HCl
CrCl3,HCl
CrCl2,HCl
CrCl3,HCl
CrCl2,HCl
CrCl2,HCl
CrCl3,HCl
CrCl3,HCl
CrCl2,HCl
CrCl2,HCl
6,25
6,25
6,25
6,25
6,25
6,25
6,25
6,25
6,25
6,25
6,25
6,25
6,25
–
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[BMIM]Br
LiBr
DMA
[a] Birch xylan was reacted for 2 h at a concentration of 5wt% relative to the total mass of the reaction mixture.
The solvent composition is indicated by the wt% of LiCl relative to DMA with additive concentrations relative
to the mass of the reaction mixture. Catalyst loading and yield are relative to moles of xylose monomers con-
tained in xylan. Yields are based on HPLC analysis.
Analytical methods: All reaction
products were analyzed by HPLC and quantified using calibration
curves generated with commercially available standards. Following
a typical reaction, the product mixture was diluted with a known
mass of deionized water, centrifuged to sediment insoluble prod-
ucts, and analyzed. The concentrations of products were calculated
from HPLC-peak integrations and used to calculate molar yields.
HPLC was performed with an Agilent 1200 system equipped with
refractive index and photodiode array detectors. Furfural was ana-
lyzed by ion-exclusion chromatography using a Bio-Rad Aminex
abled furfural yields similar to those obtained at 1408C. Use of
HCl during the solubilization step was essential for furfural pro-
duction, though HCl alone did not result in yields as high as
those obtained with both chromium and HCl. The effects of
other additives such as LiCl, LiBr, and ionic liquids were
modest. These data suggest that pretreatment of xylan to
form xylose improves furfural yields. We anticipate that im-
proved methods for xylan depolymerization under our reaction
conditions would provide higher furfural yields at lower tem-
peratures. Using HCl in water/ionic liquid mixtures, we have
demonstrated that 77% of birch xylan can be hydrolyzed to
xylose.[18] The major side products observed by HPLC analysis
are furfural and species with retention times characteristic of
carboyhydrate oligomers. Both xylose and xylose oligomers are
possible intermediates on the route from xylan to furfural.
Combining this hydrolysis process with chromium catalysts
could enable the efficient transformation of xylan into furfural.
In conclusion, renewable fuels and chemicals such as pen-
tose-derived furfural can reduce the dependence of humanity
on fossil resources. Although furfural is produced industrially
HPX-87H column (300ꢁ7.8 mm; 5 mm H2SO4, 0.6 or 0.9 mLminÀ1
658C).
,
Representative procedure for synthesis of furfural from xylose:
Xylose (25.5 mg, 170 mmol) was mixed with DMA (225 mg) and
CrCl2 (2.5 mg, 20 mmol), and the reaction mixture was stirred at
1008C for 4 h. For reactions of xylose using [EMIM]Cl, chromium
salts were mixed with a portion of the ionic liquid (25 mg) before
addition to the reaction mixture.
Representative procedures for synthesis of furfural from xylan:
Xylan (10.2 mg, 72 mmol) and concd HCl (1.6 mL, 19 mmol) were
stirred in DMA (240 mg) at 1408C for 2 h. Following addition of
CrCl3 (1 mg, 6 mmol), the reaction mixture was stirred at 1208C for
2 h.
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and Products (Ed. B. Kamm, P. R. Gruber, M. Kamm), Wiley-VCH, Wein-
heim 2006.
In another procedure, xylan (6.0 mg, 43.5 mmol) and [EMIM]Cl
(266.9 mg) were stirred at 858C for 24 h. Following addition of HCl
(0.33 mL, 4 mmol) and CrCl2 (0.8 mg, 6.5 mmol), the reaction mixture
was stirred at 1408C for 2 h.
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Kinetic analysis of furfural formation from xylose: Xylose (250.7 mg,
1.67 mmol) and LiCl (227.8 mg) were added to DMA (2.0427 g) and
dissolved at 1008C for 5 min. Five known weights of this stock so-
lution (~0.4 g) were combined with CrCl2 (five different weights
from 0–15 mggÀ1). The solutions were heated at 1008C for 10 min
and cooled rapidly on ice prior to HPLC analysis.
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This work was supported by the Great Lakes Bioenergy Research
Center, a DOE Bioenergy Research Center. J.B.B. was supported by
an NSF Graduate Research Fellowship. We are grateful to B. R.
Caes for advice, and B. E. Dale and B. Venkatesh for corn stover
samples.
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[15] Reaction conditions: DMA (825 mg), CrCl2 (6 mol%), D2O (100 mg),
xylose (100 mg), LiCl (100 mg), 1008C, 4 h.
[16] Reaction conditions: DMA (825 mg), CrCl2 (6 mol%), H2O (100 mg),
xylose-2-d (100 mg; Omicron Biochemicals), LiCl (100 mg), 1008C, 4 h.
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Keywords: biofuels · biomass · catalysis · carbohydrates ·
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Received: June 24, 2010
Published online on September 10, 2010
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