Highly efficient and selective CO2-adjunctive dehydration of xylose to furfural in aqueous media with THF

Article abstract of DOI:10.1039/c5gc02863a

The selective dehydration of xylose into furfural using high-pressure CO₂ as an effective and more sustainable catalyst in an H₂O/THF system is reported for the first time. The conversion of d-xylose into furfural above 83 mol% with a furfural yield of 70 mol% and a selectivity of 84% was achieved with only 50 bar of CO₂ pressure within 1 hour at 180 °C.

Full text of DOI:10.1039/c5gc02863a

View Article Online
View Journal
Green
Chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: A. R. C. Morais
and R. M. Bogel-Lukasik, Green Chem., 2016, DOI: 10.1039/C5GC02863A.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/greenchem

Page 1 of 9
Green Chemistry
View Article Online
DOI: 10.1039/C5GC02863A
Highly efficient and selective CO -adjuncted dehydration of xylose to furfural
2
in aqueous media with THF
1
,2
1*
Ana Rita C. Morais and Rafal Bogel-Lukasik
1
Unidade de Bioenergia, Laboratório Nacional de Energia e Geologia, I.P., Estrada do Paço
do Lumiar 22, 1649-038 Lisboa, Portugal. *e-mail: rafal.lukasik@lneg.pt; fax:
+
351217163636; phone: +351210924600 ext. 4224
2
LAQV/REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia,
Universidade Nova de Lisboa, 2829-516 Caparica, Portugal

Green Chemistry
Page 2 of 9
View Article Online
DOI: 10.1039/C5GC02863A
Abstract
The selective dehydration of xylose into furfural using high-pressure CO
more sustainable catalyst in H O/THF system is for the first time reported. The conversion of
D-xylose into furfural above 83 mol % with furfural yield of 70 mol % and the selectivity of 84
was achieved with only 50 bar of CO pressure within 1 hour at 180 °C.
2
as effective and
2
%
2

Page 3 of 9
Green Chemistry
View Article Online
DOI: 10.1039/C5GC02863A
1
Furfural is one of building blocks which can be used as biomass-originated substitute of
fossil-based pivot chemicals in the production of diverse chemicals (e.g. tetrahydrofuran
2
-5
(
THF)). Currently, furfural is exclusively produced by mineral acid catalysed dehydration of
C
5
-sugars present in the hemicelluloses of agricultural residues and hardwoods. It has been
reported that at industrial practice, the furfural yield originated in batch processes has
6
remained at or below 50 mol % of theoretical due to occurrence of undesired parallel
7
,8
reactions leading to formation of humins.
Up to date, the production of furfural from hemicellulose-derived sugars has been reported
5
,9,10
to be carried out in both monophasic and biphasic systems.
It is highly desired to
produce furfural via in-situ generated acid catalyst conditions, without typical drawbacks for
both homogeneous and heterogeneous catalysts. The basics of inorganic chemistry teach
2
that CO is little soluble in water, however even this negligible solubility forms unstable
carbonic acid making the aqueous phase more acidic. This weak acid promotes the medium
1
1-15
acidification helping biomass processing
and dehydration of sugars into furans as
presented in this work. The proposed approach has great environmental benefits since the
acidity of the medium can be easily turned off by simple depressurisation and removal of
1
4
CO
In this work, D-xylose was converted into furfural. The examined approach examines the
benefits from the use of high-pressure CO in the auto-generated acidic environment and
2
from environment allowing its recovery and reuse.
2
the aqueous/THF reaction phase splitting guiding to the enhancement of final furfural yield
and reaction selectivity.
Series of runs presented in Table 1 were carried out to scrutinise the role of THF (entries 1
and 2), CO
min of reaction time (entries 3 - 6). Additionally, variation of water (Vaq.) and THF (VTHF
volumetric ratios were scrutinised (entries 7 - 9) in an effort to decrease the reactor
headspace to analyse the effect of the CO amount loaded into the reactor.
2
(e.g. entries 2 and 7) and temperature in the range of 160 - 180 °C for 30 and 60
)
2
The analysis of results obtained in entries 1 and 2 confirms the expected adjunctive effect of
THF presence. This data demonstrates that albeit lower xylose concentration was examined
in entry 2, the furfural yield and selectivity of reaction raised expressively reaching values of
6
5.2 mol % and 41.4 %, respectively. This is in a good agreement with previous reports about
THF effect on furfural production. For example, Wyman and co-workers found that in acid

Green Chemistry
Page 4 of 9
View Article Online
DOI: 10.1039/C5GC02863A
catalysed reaction with THF as co-solvent, furfural yield was as high as 86 mol % due to THF
1
6
“
protecting” effect of furfural from degradation. However, it is important to state that
Wyman and co-workers did not clarify how THF, as co-solvent in a monophasic system, is
able to protect furfural from further degradation reactions. Beyond THF, another equally
important agent in the formation of furfural is CO
entries 2 and 7 demonstrates that the presence of CO
indicates that in the presence of water, high-pressure CO
leading to generation of acidic environment and consequently to more favourable xylose
2
. The comparison of results obtained in
enhances the furfural yield by 2/3. It
produces unstable carbonic acid
2
2
dehydration boosting the furfural yields in comparison to reactions without CO
and 7).
2
(entries 2
a
Table 1. Furfural production by dehydration of D-xylose using high-pressure CO
2
as catalyst.
T
t
[Xylose]
(g/L)
V
aq.
V
THF
X
Xylose
Y
furfural
S
furfural
Entry
b
b
b
(
°C) (min)
mL
(mol %)
(mol %)
(%)
c
1
180
180
160
160
180
180
180
180
180
60
60
30
60
30
60
60
60
60
18.8
12.5
18.8
18.8
18.8
18.8
12.5
12.5
12.5
15
10
25
25
25
25
10
20
40
0
5
0
0
0
0
5
54.0
65.2
42.1
44.8
73.1
88.0
82.9
81.9
68.3
24.5
41.4
21.0
25.1
31.2
42.7
69.4
66.9
17.9
45.5
63.5
49.9
56.0
42.7
48.6
83.7
81.7
26.3
c
2
3
4
5
6
7
8
9
10
20
a
b
All CO -assisted dehydration experiments were carried out at 50 bar of initial CO pressure.
2
2
For xylose conversion (Xxylose), furfural yield (Yfurfural) and reaction selectivity to furfural
c
(
Sfurfural) definitions please refer to Electronic Supplementary Information (ESI). Reactions
performed without CO2.
Also the selectivity of xylose dehydration is driven by the presence of CO . A maximum 83.7
2
%
selectivity was achieved when CO and THF were used together (entry 7). This result again
2
confirms the dual adjunctive character of CO and THF on dehydration of xylose to furfural.
2
The formed carbonic acid accelerated the conversion of sugars and THF acting as extracting
solvent reduces the occurrence of secondary loss reactions (e.g. products of condensation –

Page 5 of 9
Green Chemistry
View Article Online
DOI: 10.1039/C5GC02863A
1
7
2
humins). It is possible because as little as 30 bar of CO makes a phase splitting separating
1
8
2
furfural from water to THF-expanded CO -rich phase. This, due to an advantageous
partition of furfural between THF- and water-phases, helps to extract furfural from aqueous
to organic gaseous phase protecting furfural from further conversion and degradation
reactions in aqueous phase.
The preliminary tests showed that the reaction temperature is another important factor
influencing the furfural production. Considering the results from entries 3-6, the xylose
conversion doubled and furfural yield increased from 25.1 mol % to 42.7 mol % with an
increase of temperature by only 20 °C from 160 °C (entry 4) to 180 °C (entry 6) for 1 h
reaction. One the other hand, the increase of furfural yield was accompanied by a decrease
of reaction selectivity to furfural from 56.0 % to 48.6 %. The obtained results indicate that
higher reaction temperature increased the xylose conversion, either to furfural or other
1
7
products (i.e. formic acid and/or humins) often reported in literature.
Entries 7-9 show that either furfural yield or selectivity of the reaction is sensitive on the
amount of feed charged in the reactor. It seems to be obvious, because the headspace of the
reactor diminishes by increasing the amount of feed. Hence, at fixed initial temperature and
pressure, less CO
could be generated. The highest furfural yield was obtained for the lowest feed charged
69.4 mol % and 83.7 % of furfural yield and reaction selectivity, respectively). Increasing 4
2
was charged into the reactor and consequently less acidic environment
(
times the amount of feed charged into reactor either furfural yield or selectivity decreased
to only 17.9 mol % and 26.3 %, respectively.
The effect of volumetric aqueous to organic ratio on the CO
2
-assisted dehydration of D-
O and
xylose to furfural was investigated as is shown in Table 2. Varying the amount of H
2
THF notable changes in composition of system occur, which influences the composition of
both gaseous and liquid phases. The analysis of these changes and the knowledge of phase
behaviour of H
obtained following the modelling of the reaction mixture according to procedure given in the
ESI) confirm that the increase of VTHF in the reactive system favours CO solubility in aqueous
2 2
O:THF:CO system (the phase envelopes for examined systems were
2
phase. This promotes more favourable conditions of acid-catalysed dehydration of xylose
into furfural. For the highest Vaq./VTHF ratio (entry 11), the xylose conversion was relatively
high (73.8 mol %) but the furfural yield was only 45 mol %. Decreasing the Vaq./VTHF ratio

Green Chemistry
Page 6 of 9
View Article Online
DOI: 10.1039/C5GC02863A
from 12:3 to 10:5 (entry 11 and 7), both xylose conversion and furfural yield increased up to
2.9 mol % and 69.4 mol %, respectively. Conversion of xylose reached its maximum of 100
8
mol % when Vaq./VTHF ratio was 3:12 (entry 14) but simultaneously only 45.4 mol % of
furfural yield was achieved. Hence, higher VTHF in reactive system adjuncts to achieve higher
xylose conversion, however about certain value the excessive amount of THF has negative
effect on furfural yield and reaction selectivity as both decreases. Similar observation was
made by Li et al. who found that when the amount of organic phase was higher than the
1
9
aqueous phase, the production of humins was increased.
2
Table 2. Furfural production by dehydration of D-xylose using high-pressure CO as catalyst
a
at various volumetric aqueous to organic solvent ratios.
V
aq.
V
THF
XXylose
(mol %)
60.6
Yfurfural
(mol %)
28.4
Sfurfural
Entry
b
b
b
mL
(%)
1
1
0
1
15.0 0.0
12.0 3.0
10.0 5.0
46.9
60.9
83.7
63.6
58.1
45.4
73.8
45.0
7
82.9
69.4
1
1
1
2
3
4
7.5
7.5
86.2
54.8
5.0 10.0
3.0 12.0
87.8
51.0
100.0
45.4
a
All experiments were performed at the following conditions: 50 bar of initial CO
2
pressure,
b
1
2.5 g/L of xylose concentration in the feed, at 180 °C during 60 min of reaction time. For
xylose conversion (Xxylose), furfural yield (Yfurfural) and reaction selectivity to furfural (Sfurfural
)
definitions please refer to Electronic Supplementary Information (ESI).
The behaviours found for 12.5 g/L of xylose concentration in the feed were similar also for
two other studied (9.4 and 6.3 g/L) xylose concentrations as can be seen in Figure 1.

Page 7 of 9
Green Chemistry
View Article Online
DOI: 10.1039/C5GC02863A
1
00
80
60
40
20
0
0.0
2.5
5.0
7.5
10.0
12.5
V /mL
THF
Figure 1. The evolution of xylose conversion (bars) and furfural yield (symbols) for various
THF as a function of initial xylose concentration in the feed (black – 12.5 g/L, white – 9.4 g/L,
grey – 6.3 g/L). All experiments were performed at the following conditions: 50 bar of initial
CO pressure, at 180 °C during 60 min of reaction time and at total volume (Vaq.+VTHF) of 15
V
2
mL. Lines are given as guide for the eye.
Besides the above discussed parameters, the furfural yield also depends on reaction time
and initial xylose concentration in the feed as it is depicted in Figure 2. The obtained results
show that at the initial stage of reaction, the furfural production was the fastest for the
lowest xylose concentration; however the differences between concentrations especially for
longer reaction times (e.g. 90 min) are negligible. However, prolonged reaction time had a
negative effect on the furfural yield, especially for more concentrated solutions, because for
1
20 min of the reaction time, a decrease of furfural yield was observed mostly due to
7
possible aforementioned side reactions.

Green Chemistry
Page 8 of 9
View Article Online
DOI: 10.1039/C5GC02863A
8
6
4
2
0
0
0
0
0
0
20
40
60
80
100
120
140
Reaction time/min
Figure 2. The evolution of furfural yield for various initial xylose concentrations (
– 9.4 g/L, – 6.3 g/L) achieved over time in reactions performed at 50 bar of initial CO2
■
-12.5 g/L,
○
△
pressure, 180 °C and Vaq./VTHF ratio of 10:5 mL/mL. Lines merely demonstrate the difference
in the initial furfural production rate for various xylose concentrations.
The furfural yields presented in this work are in the similar range to those reported for
2
0
xylose dehydration using mineral acid (HCl) as catalyst. Gairola and Smirnova used
different approach to investigate the hydrothermal dehydration of D-xylose to furfural with
2
1
simultaneous furfural extraction with supercritical CO
2
.
The maximal furfural yield from
xylose was similar to those obtained in this work (68 mol % vs. 69 mol %) but it was achieved
at severer reaction conditions (230 °C and 120 bar of CO pressure) than those presented in
2
this work.
Concluding, the efficient production of furfural in selective way using high-pressure CO as
2
catalyst in H O/THF system was reported. The combined adjunctive character of CO in
2
2
aqueous media and THF as extracting solvent enabled a simple operational procedure for
xylose dehydration into furfural. This methodology led to obtain a xylose conversion above
8
3 mol % resulting in a final furfural yield of 70 mol %. The used approach does not require

Page 9 of 9
Green Chemistry
View Article Online
DOI: 10.1039/C5GC02863A
post-reaction neutralisation typical for acids and uses the most environmentally friendly
solvents such as CO and H O to obtain furfural yields similar to those with mineral acid and
2
2
combined extraction systems.
Electronic Supporting Information
The electronic supporting information contains the experimental section, details of phase
equilibrium prediction and reaction (heating and holding time) profiles.
Acknowledgments
This work was supported by the Fundação para a Ciência e Tecnologia (FCT, Portugal)
through strategic project UId/QUI/5006/2013 and grants SFRH/BD/94297/2013 and
IF/00424/2013. Authors wish also thank to Scientific Computing & Modelling for ADF 2014
trial licence to use the software for phase equilibria modelling.
References
1
2
3
4
5
.
.
.
.
.
J. J. Bozell and G. R. Petersen, Green Chem., 2010, 12, 539-554.
M. Bicker, J. Hirth and H. Vogel, Green Chem., 2003, 5, 280-284.
M. E. Zakrzewska, E. Bogel-Lukasik and R. Bogel-Lukasik, Chem. Rev., 2011, 111, 397-417.
R. Xing, W. Qi and G. W. Huber, Energ. Environ. Sci., 2011, 4, 2193-2205.
S. Peleteiro, A. M. da Costa Lopes, G. Garrote, J. C. Parajó and R. Bogel-Łukasik, Ind. Eng.
Chem. Res., 2015, 54, 8368-8373.
6
.
A. S. Mamman, J.-M. Lee, Y.-C. Kim, I. T. Hwang, N.-J. Park, Y. K. Hwang, J.-S. Chang and J.-S.
Hwang, Biofuel. Bioprod. Bior., 2008, 2, 438-454.
7
8
9
1
1
.
.
.
0.
1.
D. L. Williams and A. P. Dunlop, Ind. Eng. Chem., 1948, 40, 239-241.
I. C. Rose, N. Epstein and A. P. Watkinson, Ind. Eng. Chem. Res., 2000, 39, 843-845.
R. Weingarten, J. Cho, W. C. Conner and G. W. Huber, Green Chem., 2010, 12, 1423-1429.
A. S. Dias, M. Pillinger and A. A. Valente, J. Catal., 2005, 229, 414-423.
S. P. Magalhães da Silva, A. R. C. Morais and R. Bogel-Lukasik, Green Chem., 2014, 16, 238-
2
46.
1
1
1
1
2.
3.
4.
5.
A. R. C. Morais, A. C. Mata and R. Bogel-Lukasik, Green Chem., 2014, 16, 4312-4322.
F. M. Relvas, A. R. C. Morais and R. Bogel-Lukasik, J. Supercrit. Fluid., 2015, 99, 95-102.
A. R. C. Morais, A. M. da Costa Lopes and R. Bogel-Lukasik, Chem. Rev., 2015, 115, 3-27.
M. H. L. Silveira, A. R. C. Morais, A. M. da Costa Lopes, D. N. Olekszyszen, R. Bogel-Łukasik, J.
Andreaus and L. Pereira Ramos, ChemSusChem, 2015, 8, 3366-3390.
1
1
6.
7.
C. M. Cai, T. Y. Zhang, R. Kumar and C. E. Wyman, Green Chem., 2013, 15, 3140-3145.
K. J. Zeitsch, The chemistry and technology of furfural and its many by-products, Elsevier,
2
000.
P. Pollet, E. A. Davey, E. E. Urena-Benavides, C. A. Eckert and C. L. Liotta, Green Chem., 2014,
6, 1034-1055.
H. L. Li, A. J. Deng, J. L. Ren, C. Y. Liu, W. J. Wang, F. Peng and R. C. Sun, Catal. Today, 2014,
34, 251-256.
1
1
8.
9.
1
2
2
2
0.
1.
E. I. Gurbuz, S. G. Wettstein and J. A. Dumesic, Chemsuschem, 2012, 5, 383-387.
K. Gairola and I. Smirnova, Bioresource Technol., 2012, 123, 592-598.