Iron-catalyzed furfural production in biobased biphasic systems: From pure sugars to direct use of crude xylose effluents as feedstock

Article abstract of DOI:10.1002/cssc.201100259

Working with real samples: Aqueous solutions of FeCl₃-NaCl (or seawater) dehydrate xylose to afford furfural, which can extracted insitu into 2-MTHF as second phase. Furfural is successfully obtained when aqueous nonpurified xylose effluents directly from lignocellulose fractionation are tested. Copyright

Full text of DOI:10.1002/cssc.201100259

DOI: 10.1002/cssc.201100259
Iron-Catalyzed Furfural Production in Biobased Biphasic Systems: From
Pure Sugars to Direct Use of Crude Xylose Effluents as Feedstock
Thorsten vom Stein,[a] Philipp M. Grande,[a] Walter Leitner,*[a, b] and Pablo Domꢀnguez de Marꢀa*[a]
Selective catalytic routes for processing the carbohydrate frac-
tions of lignocellulose to deliver valuable platform chemicals
are important and challenging paths for biomass valorization.[1]
A key step in this value chain is the dehydration of monomeric
sugars to afford furan derivatives as valuable materials for nu-
merous applications.[1–3] Furfural can be derived from the C5-
sugar xylose, which is the most abundant sugar of the hemi-
cellulose fraction in lignocellulose.[4–7] Chemical approaches for
xylose dehydration usually involve acidic conditions, using
either mineral acids[4,5,8] or acidic heterogeneous catalysts such
as zeolites,[9] MCM-41 materials,[10] and heteropolyacids.[11] To
overcome humin formation in furfural dehydration,[12,13] the ap-
plication of aqueous biphasic systems (using organic solvents
such as methyl isobutyl ketone or toluene) for the in situ ex-
traction of furfural has recently been proposed.[7,14–16]
is also assessed by directly using the aqueous, nonpurified
xylose effluent obtained from pretreatment of lignocellulose
with oxalic acid.[20]
In preliminary experiments, aqueous solutions of xylose
were treated with catalytic amounts of different catalysts [i.e.,
Fe(acac)3, FeCl3·6H2O, FeSO4·7H2O, FeCl2·4H2O, MnCl2,
Cu(OAc)2, and CuCl2·2H2O] and subsequently layered with 2-
MTHF as organic phase. Among the tested catalysts,
FeCl3·6H2O displayed superior results and hence was selected
for further assessments. After conducting the reaction at
1408C for up to 6 h, the resulting furfural concentration in the
2-MTHF phase was determined by gas chromatography (GC).
Initial kinetic measurements were taken with FeCl3·6H2O load-
ings of 40 mol%. The furfural yield increased linearly up to
40% furfural yield after 6 h. Hence the furfural production rate
kfurfural was determined, based on the slope of the data from ki-
netic experiments conducted on 1 mmol scale. Further studies
were done to optimize the efficiency. Thus, different amounts
of NaCl were added to the aqueous phase (Table 1).
For sugar dehydration, different catalysts (e.g., CrCl2, ZnCl2,
FeCl3) have been assessed in non-aqueous deep-eutectic sol-
vents such as choline chloride fructose mixtures[17] as well as in
monophasic aqueous media.[18,19] In this Communication a bi-
phasic approach for xylose dehydration to afford furfural is re-
ported. The approach is based on aqueous solutions of
FeCl3·6H2O and NaCl, combined with a second 2-methyltetra-
hydrofuran (2-MTHF) phase as biomass-derived solvent (Fig-
ure 1).[1a] After proof-of-concept experiments using pure com-
mercially available crystalline xylose, the dehydration strategy
The furfural production rate kfurfural improved considerably
with increasing NaCl loading (Table 1, entries 1, 3–6). The rate
could be increased by a factor of more than two by adding
20 wt% NaCl (entries 1 and 5). The effect of salt has been sug-
gested to enhance the partitioning coefficient of furfural to or-
ganic phase.[21] Consequently, running the reaction with
20 wt% NaCl (entry 5) for 4 h afforded a 70% yield of furfural.
However, the yield did not increase at longer reaction times
(6 h) due to humin formation, which was avoided by applying
shorter residence times. Increasing the amount of catalyst (up
to 0.6 mmol) at 20 wt% NaCl loading afforded high furfural
yields, of 65 to 70%, after 2 h reaction time at 1408C. A further
increase of the NaCl loading to 30 wt% did not result in a
better furfural production rate (entry 6), presumably due to fur-
fural degradation. Finally, previous studies on biomass process-
ing showed the potential of using seawater as solvent.[22,23]
Gratifyingly, in this case the direct use of seawater (comprising
different salts[23]) with FeCl3·6H2O also resulted in an improved
furfural production rate (entry 8).
Aqueous solutions of FeCl3 (0.08m) are acidic (pH 1.4). To
assess whether or not the sugar dehydration in these solutions
was dominated by Brønsted acidity, we ran control experi-
ments in aqueous HCl with an identical proton concentration,
c(H+)=0.04m. Table 1, entries 1 and 2 show that the dehydra-
tion rate with FeCl3·6H2O is significantly higher than that with
HCl at the same pH. Consistently, the addition of NaCl im-
proves the performance of both, HCl and FeCl3·6H2O, but with
largely superior outcomes in the case of FeCl3·6H2O (entries 6
and 7). This demonstrates that the activity of FeCl3·6H2O in
xylose dehydration is not solely governed by its Brønsted acidi-
ty.
Figure 1. Iron-catalyzed xylose dehydration. 98% of the furfural was extract-
ed into the 2-MTHF phase.
[a] T. vom Stein, P. M. Grande, Prof. Dr. W. Leitner, Dr. P. Domꢀnguez de Marꢀa
Institut fꢁr Technische und Makromolekulare Chemie (ITMC)
RWTH Aachen University
Worringerweg 1, 52074 Aachen (Germany)
Fax: (+49)2418022177
E-mail: leitner@itmc.rwth-aachen.de
dominguez@itmc.rwth-aachen.de
[b] Prof. Dr. W. Leitner
Max-Planck-Institut fꢁr Kohlenforschung
45470 Mꢁlheim an der Ruhr (Germany)
1592
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2011, 4, 1592 – 1594

more depth whether or not the
catalyst was still active, another
set of cycles, this time without
addition of fresh xylose among
cycles, was conducted. Thus,
after addition of xylose in the
first cycle, the furfural yield de-
creased each cycle, resulting in a
xylose conversion of ca. 75%.
Once fresh xylose was added,
higher furfural yields were again
achieved, clearly demonstrating
that the catalyst remained active
during all of the cycles (Fig-
ure 2B).
Table 1. Influence of NaCl loading on furfural production rate.[a]
Entry
Catalyst
NaCl
[wt%]
kfurfural
[mmolhÀ1
R2
Furfural yield(4h) [%]
expected (kinetics)
]
actual (GC)
1[b]
2[b]
3
4
5
6
7
8
FeCl3·6H2O
HCl[c]
0
0
5
10
20
30
30
0.0784
0.0111
0.0949
0.1272
0.1778
0.169
0.97
0.95
0.97
0.95
0.95
0.96
0.93
0.99
31
4
27
5
FeCl3·6H2O
FeCl3·6H2O
FeCl3·6H2O
FeCl3·6H2O
HCl[c]
37
51
71
68
21
41
35
54
71
64
20
40
0.0533
0.1022
FeCl3·6H2O
seawater
[a] Reaction conditions: 1 mmol xylose, 0.4 mmol FeCl3·6H2O, 2.5 mL H2O, 2.5 mL 2-MTHF, 1408C. A blank reac-
tion with seawater gave 4.7% furfural (2 h), and a blank reaction in brine gave 1.9% furfural (2 h). [b] pH 1.4 ap-
plied in both cases. [c] c(HCl)=0.04 m. NaCl loading with respect to aqueous phase. Furfural concentration of
the 2-MTHF phase determined by GC. kfurfural determined in kinetic experiments (<6 h).
Novel catalytic systems devel-
oped for biomass-processing
Subsequently, several consecutive batch experiments were
conducted re-using the same aqueous FeCl3·6H2O–NaCl solu-
tion, while replacing the organic phase and adding new sub-
strate (1 mmol xylose, corresponding to a concentration of
0.4m in the aqueous phase). After the first two experiments
the furfural concentration in 2-MTHF reached a steady value of
0.33m for each reaction–separation cycle. No loss of activity
was apparent when the aqueous phase was re-used over 10
consecutive cycles (Figure 2A). After furfural recovery, 2-MTHF
can easily be recycled in the setup (b.p.~808C). To study in
must always be assessed with “real” samples, because cost re-
strictions will hardly afford the working-up of lignocellulosic
derivatives or effluents, to use pure or crystalline raw materials
for subsequent reaction steps. Therefore, the performance of
FeCl3·6H2O in the dehydration of nonpurified aqueous xylose
effluents, directly obtained from the fractionation of lignocellu-
lose, was tested. Beech wood was used as prototypical raw
material, and particles (particle size: 0.5–0.1 mm) were heated
in a biphasic water/2-MTHF system using oxalic acid as catalyst
according to a recently reported procedure.[20] The suspended
cellulose pulp and the 2-MTHF phase mainly containing the
lignin were separated from the aqueous phase, and this was
used for further processing. After removal of the oxalic acid for
potential re-use, analysis of the remaining aqueous solution
(2.5 mL) showed the presence of xylose in a concentration of
about 30 gLÀ1, together with small amounts of other carbohy-
drates. Without further purification, FeCl3·6H2O (0.12m relating
to the total volume) was added, together with 30 wt% NaCl
(relating to the aqueous phase). After adding 2.5 mL of 2-
MTHF, the resulting biphasic mixture was heated at 1408C for
2 h. Gratifyingly, analysis of the 2-MTHF phase revealed a furfu-
ral concentration of ca. 7 gLÀ1, indicating a rate of formation
of ca. 3.5 gLÀ1 hÀ1 and corresponding to an initial yield of 37%
at this stage (Figure 3).
Figure 2. Recycling of the catalytically active aqueous phase for the forma-
tion and extraction of furfural in the biphasic H2O/2-MTHF system.
A) Adding fresh xylose each time. B) Without addition of fresh xylose during
several cycles. Reaction conditions: 1 mmol xylose (corresponding to a 0.4m
concentration in the aqueous phase), 0.12m FeCl3·6H2O, 20 wt% NaCl, 5 mL
2-MTHF/H2O, 1408C, 2 h.
Figure 3. Conversion of xylose into furfural mediated by FeCl3·6H2O, using
aqueous nonpurified xylose effluents obtained from beech wood fractiona-
tion.[20] Reaction conditions: xylose~30 gLÀ1, 0.12m FeCl3·6H2O, 30 wt%
NaCl with respect to aqueous phase, 2.5 mL 2-MTHF, 2.5 mL of aqueous
phase produced in fractionation system, 1408C.
ChemSusChem 2011, 4, 1592 – 1594
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In summary, the biphasic system H2O/2-MTHF provides an
interesting approach to the conversion of xylose to furfural. By
using FeCl3·6H2O as catalyst and NaCl as additive, effective de-
hydration in the aqueous phase is combined with in situ ex-
traction of furfural into the 2-MTHF phase, thus reducing
humin formation. Likewise, concentrated seawater can be used
directly as the H2O/NaCl phase. The aqueous phase can be re-
cycled without apparent loss of activity, opening the possibility
for continuous operation of the biphasic catalytic system. An
overall process scheme for the production of furfural from
wooden biomass has been developed and experimentally veri-
fied by coupling of this system with lignocellulose fractiona-
tion in a similar biphasic reaction medium. This further sup-
ports the potential of biphasic aqueous phase catalysis for
future biorefinery concepts, in particular in H2O/2-MTHF sys-
tems.
lence Initiative of the German Research Foundation to promote
science and research at German universities. Mrs. Hannelore Es-
chmann and Ms. Julia Wurlitzer are acknowledged for support in
analytical measurements (GC). We also thank Henning Kayser,
Frank Geilen, and Barthel Engendahl for fruitful and stimulating
discussions.
Keywords: acidity · biomass · carbohydrates · homogeneous
catalysis · iron
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Acknowledgements.
Received: May 22, 2011
Revised: July 22, 2011
Published online on October 12, 2011
This work was performed as part of the Cluster of Excellence
“Tailor-Made Fuels from Biomass”, which is funded by the Excel-
1594
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ChemSusChem 2011, 4, 1592 – 1594