Effects of water and alcohols on the polymerization of furan during its acid-catalyzed conversion into benzofuran

Article abstract of DOI:10.1039/c6ra04745a

Furan, an important product from catalytic pyrolysis of biomass, has the potential to be further converted into value-added chemicals or biofuels. This study investigated the conversion of furan into benzofuran over a Br?nsted acid catalyst (Amberlyst 70) at 140-190°C in various solvents. With water as the solvent, furan could barely make its way to benzofuran as its polymerization dominated. With methanol as the solvent, the polymerization of furan was suppressed and benzofuran formation was enhanced substantially. This is because in methanol, the reactive intermediates (i.e., aldehydes) were stabilized and their involvement in polymerization reactions was suppressed. Other alcohols showed similar effects on suppressing polymerization. In dimethyl sulfoxide (DMSO), the polymerization of furan was also effectively suppressed. However, furan was not converted to benzofuran but to levulinic acid via a distinct reaction route.

Full text of DOI:10.1039/c6ra04745a

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Effects of water and alcohols on the polymerization
of furan during its acid-catalyzed conversion into
benzofuran†
Cite this: RSC Adv., 2016, 6, 40489
*
Xun Hu, Shengjuan Jiang, Sri Kadarwati, Dehua Dong and Chun-Zhu Li
Furan, an important product from catalytic pyrolysis of biomass, has the potential to be further converted
into value-added chemicals or biofuels. This study investigated the conversion of furan into benzofuran
over a Brønsted acid catalyst (Amberlyst 70) at 140–190 ꢀC in various solvents. With water as the solvent,
furan could barely make its way to benzofuran as its polymerization dominated. With methanol as the
solvent, the polymerization of furan was suppressed and benzofuran formation was enhanced
substantially. This is because in methanol, the reactive intermediates (i.e., aldehydes) were stabilized and
their involvement in polymerization reactions was suppressed. Other alcohols showed similar effects on
suppressing polymerization. In dimethyl sulfoxide (DMSO), the polymerization of furan was also
effectively suppressed. However, furan was not converted to benzofuran but to levulinic acid via
a distinct reaction route.
Received 23rd February 2016
Accepted 5th April 2016
DOI: 10.1039/c6ra04745a
www.rsc.org/advances
polymerization tendency of furan under the conditions of bio-
oil esterication. However, the unexpected conversion of
Introduction
furan into benzofuran was observed. It has been reported that
furan could be converted into benzofuran, an intermediate to
make aromatic range biofuels, but via gas-phase reactions over
ZSM-5 or HZSM-5 at 300–700 ꢀC.18–21
Bio-oil, a crude liquid produced from pyrolysis of biomass, is
a sustainable feedstock for the production of biofuels and ne
chemicals. However, bio-oil tends to be polymerized, especially
at elevated temperatures,1 which leads to the deactivation of
catalyst2 or blockage of the reactor.3 Understanding how the
main components of bio-oil (i.e. furans, sugars and phenolics)
contribute to the polymerization reactions is essential to
develop the methods for the stabilization of bio-oil.
The furans in bio-oil present in forms such as 5-(hydrox-
ymethyl)furfural (HMF), furfural, 5-methyl-2-furancarbo-
xaldehyde, acetylfuran and furfuryl alcohol.4 The functional-
ities of these furans are different though they all have the furan
ring. Thus, we started our investigation from the simplest of
furans—furan itself—to understand how furan polymerizes and
how to stabilize it. Furan is an important product in the catalytic
pyrolysis of biomass.5–7 However, unlike furfural8–12 or 5-
(hydroxymethyl)furfural (HMF),13–16 the catalytic conversion of
furan (i.e. via acid catalysis) has received much less attention.
Esterication is an important method for upgrading bio-oil
by transforming the carboxylic acids and other reactive
components. This is generally performed in the presence of an
acid catalyst at elevated reaction temperatures (ca. 170 ꢀC).17 In
this study, we initially intended to investigate the
In this study, we have developed a new approach for the
conversion of furan into benzofuran. In the liquid medium
(methanol as the solvent/reactant), with the aid of a Brønsted
acid (Amberlyst 70), furan was converted into benzofuran at
ꢀ
a temperature as low as 150 C. To the best of our knowledge,
this is the rst time the conversion of furan into benzofuran has
been reported in a liquid phase over a Brønsted acid at such
a low reaction temperature. In addition, we found that solvents
signicantly affect the reaction network of furan. For example,
in dimethyl sulfoxide (DMSO), furan was converted into levu-
linic acid, not benzofuran, representing a new route for the
production of levulinic acid from furan.
In the initial trials for the conversion of furan into benzo-
furan, we found that, in water, furan had an extremely high
tendency towards polymerization with almost no chance to
form benzofuran. Our method to prevent the polymerization
used alcohols to stabilize the reactive aldehyde intermediates by
converting them into acetals. In this way, the polymerization of
furan could be suppressed to a signicant extent, while the
formation of benzofuran was effectively enhanced.
In our previous studies,13,22–24 we found that the polymeri-
zation of sugars and HMF could be effectively suppressed in
methanol medium. In this study, we found that the polymeri-
zation of furan could also be suppressed in methanol. However,
there are several major differences in terms of the type of
Fuels and Energy Technology Institute, Curtin University of Technology, GPO Box
U1987, Perth, WA 6845, Australia. E-mail: chun-zhu.li@curtin.edu.au; Fax: +61 8
9266 1131; Tel: +61 8 9266 1138
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra04745a
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reactions and intermediates involved in the conversion of ca. 1 or 2 bar and the released valve was then turned off
sugars and the conversion of furan. First, the conversion of the immediately. The same procedure was repeated three times. It
C6 sugars into levulinic acid involves the dehydration of sugars should be noted that furan is very volatile and is not dissolved in
to form HMF and the following decomposition of HMF to form water. During the purging process, some furan might be carried
levulinic and formic acids.13,22 In comparison, the conversion of away with the nitrogen. To conrm this, blank experiments
furan into benzofuran involves the opening of the furan ring were performed to measure the loss of furan with water and
and electrophilic substitution reactions. We found that meth- methanol as the solvents during the purging process. The
anol could suppress polymerization during the opening of the results showed that in water aer the pressurization and
furan ring and the electrophilic substitutions reactions. This is depressurization processes, around 17 wt% of furan was lost.
different from the suppression of polymerization reactions The loss of furan in methanol is negligible (<1 wt%). The data in
during the dehydration of sugars and the decomposition of the gures/tables has been calibrated by considering the loss of
HMF. Second, comparing with HMF, furan does not have either furan during the purging process.
the hydroxyl group or the carbonyl group, while methanol as the
Aer the purging with nitrogen, the reactor was heated to the
reactant/solvent could still suppress the polymerization of furan required reaction temperatures (120–190 ꢀC) over 10 to 14 min
during the opening of the furan ring. This is new when with a stirring rate of 350 rpm under the vapour pressure
compared with the previous studies.13,22–24 Third, in this study generated from the reactants. For some experiments, sampling
we found that methanol could stabilize and retain the soluble was performed at a certain time interval to understand the
polymers in the reaction medium, slowing down the further progress of the reactions. A sampling tube with one outlet
conversion of soluble polymer into insoluble polymer. This extending to the bottom of the reactor vessel was used to take
constitutes new knowledge about the mechanism for the liquid samples. A stirrer was used to stir the reactant mixture.
suppression of polymerization reactions in methanol.
When water was used as the solvent, the samples taken from the
reactor were rst weighed and then mixed with THF of a similar
volume and weighed again. Such sampling procedure was not
applied when methanol was used as the solvent as methanol
can effectively dissolve furan. For the experiments aimed to
measure the formation of insoluble polymer, no sampling was
performed. Aer cooling the reactor by a cooling coil inside the
reactor vessel, the liquid and the solid were collected and
separated by ltering. The solid residue was then dried at 105 ꢀC
for ca. 4 hours to measure the amount of the insoluble polymer
formed. Because the solid residue included the catalyst, the
weight of the catalyst (on a moisture-free basis) was subtracted
to obtain the weight of the insoluble polymer (on a moisture-
free basis).
Experimental
Materials
The chemicals used in this study were obtained commercially
and were of analytical grade. Amberlyst 70 is a commercially
available solid acidic resin catalyst (Dow Chemical). The acid
density is >2.55 mmol gꢁ1 and the maximum operating
ꢀ
temperature is 190 C. The catalyst was used directly without
any pretreatment. This study involves the use of furan and
benzofuran. Researchers need to pay particular attention in
handling these chemicals due to their potential health
hazards. Furan is an extremely ammable liquid with a low
boiling point (31.3 ꢀC). It can cause skin irritation and damage
to organs through prolonged or repeated exposure. It also may
cause cancer and is suspected of causing genetic defects.
Personal protective equipment, such as lab coat, gloves, and
masks, are required for dealing with this chemical. The
operations with furan have to be undertaken in a fume
cupboard or in a well ventilated area. Benzofuran is ammable
and is also suspected of causing cancer. Personal protective
equipment and a well ventilated area are also required for
dealing with benzofuran.
Analytical methods
The liquid samples were analyzed with an Agilent GC/MS (6890
series GC with a 5973 MS detector) equipped with a HP-
INNOWax capillary column. The analysis procedures are
detailed eleswhere.26 Synchronous spectra of the soluble
polymers in the liquid samples were acquired with a Perkin-
Elmer LS50B spectrometer. The energy difference used was
ꢁ2800 cmꢁ1 and the scan speed was 200 nm minꢁ1. The UV
uorescence was mainly used to characterize the p-conjugated
structures in the soluble polymers. The p-conjugated struc-
tures can be a single or a fused aromatic ring system or a long-
Experimental procedure
The acid-catalyzed conversion of furan was performed in chain alkene with p-conjugation. The emission spectrum is
a Hastelloy autoclave reactor. The experimental procedure related to the size of the p-conjugated structures and also their
was similar to that in our previous studies.22,25 In brief, abundance in the solution. In general, the bigger the p-
furan, solvent(s) and Amberlyst 70 were mixed and loaded conjugated structure, the longer the wavelength of the
into the reactor at room temperature. The reactor was spectrum.
assembled and the residual air inside was replaced with
nitrogen by purging the reactor. The detailed purging terized with a Perkin-Elmer Spectrum GX FT-IR/Raman spec-
procedure was as follows. trometer to probe their functionalities. Because the polymer
The polymers formed inside the catalyst were also charac-
The reactor was pressurized to 30 bar with nitrogen at room cannot be separated from the catalyst, the polymer and catalyst
temperature. Subsequently, the pressure was released slowly to were thus analyzed together. The fresh catalyst was also
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analyzed for comparison. The spectrometer has a spectral
resolution of 4 cmꢁ1. The samples were milled together with
KBr with a concentration of 1 wt%. The pressed pellet was
scanned 20 times and the spectrum represents the average.
Elemental composition of the polymer plus catalyst was
determined with an elemental analyzer. Because it is almost
impossible to separate the polymer from the catalyst, they were
analyzed together. The fresh catalyst was also analyzed with the
element analyzer and the C, H, S, and O contents in the fresh
catalyst were obtained directly. The elemental contents of the
polymer could not be obtained directly as from the element
analyzer, only the total C, H, S, and O contents were obtained.
However, because the amount of polymer on/in the catalyst is
known, the elemental composition of the polymer can then be
calculated based on the mass balance. For example, the carbon
content of the polymer can be calculated by the following mass
balance equation:
Results and discussion
Conversion of furan into polymer in water
Water is commonly used as a medium for the hydrolysis of
biomass, sugars and furans.27–29 Therefore, we initially tried to
convert furan in water over Amberlyst 70 (A70) (Fig. 1). Although
furan was converted completely (Fig. 1a), only a small amount of
benzofuran was produced (Fig. 1b). Succinaldehyde was
another detectable product with a negligible concentration
(Fig. S1†). The mass balance based on the detectable products
was extremely low. Therefore, we suspected that a signicant
amount of polymer was formed. The following experiment
conrmed our hypothesis (Fig. 2). The yield of the insoluble
polymer from the conversion of furan in water was around 90%
(weight basis) and benzofuran was produced in a negligible
level (<1%). Clearly, the polymerization was preferred and
dominated the reaction pathway for the acid-catalyzed conver-
sion of furan in water.
cpolymer ꢂ mpolymer + ccatalyst ꢂ mpolymer ¼ mcatalyst and polymer
ꢂ ctotal
In water, the furan ring can be opened to form succinalde-
hyde (Scheme 1) under the acidic conditions. The carbonyl
group of aldehydes has been known to play essential roles in
polymerization reactions.30,31 Succinaldehyde has carbonyl
groups and two active a-H's, making it a very reactive electro-
phile in the Aldol condensation and in electrophilic
substitution/addition reactions. The electron density of the
furan ring is even higher than that of the benzene ring due to
conjugation of the isolated electron pair on oxygen (in the furan
ring) with the carbon–carbon double bonds, which makes it
a good target for electrophiles such as succinaldehyde.
It was likely that furan polymerized with succinaldehyde,
decreasing its further conversion into benzofuran. Succinalde-
hyde was possibly one key intermediate inducing the polymer-
ization reactions. The high reactivity of succinaldehyde towards
polymerization originates from its carbonyl groups. If the
carbonyl groups were transformed into acetals via reaction with
methanol, the involvement of succinaldehyde in the polymeri-
zation might be suppressed. This concept was conrmed by the
following experiments with methanol as the solvent.
where c is the carbon content. Hydrogen content was also
calculated in a similar way. The content of oxygen was calcu-
lated by difference. The element content was based on a dry ash-
free basis.
Calculation of the carbon mass balance was based on the
results of the elemental analysis. The carbon balance for the
acid-catalyzed conversion of furan in water was typically >95%.
The yield of the insoluble polymer (%, on a dry weight basis)
was calculated by the equation:
Yield ð%Þ ¼
ðweight of the solid collected ꢁ weight of the fresh catalystÞ
weight of furan loaded
ꢂ 100
The yield of benzofuran (molar basis) was dened by the
equation:
moles of benzofuran produced ꢂ 2
Yield ðmol%Þ ¼
ꢂ 100
moles of furan loaded
Conversion of furan into benzofuran in methanol
In methanol, benzofuran was formed even at temperatures as
low as 150 C (Fig. 1b). Increasing the reaction temperature to
The selectivity of benzofuran was dened by the following
equation:
ꢀ
170 ꢀC further promoted its formation. Succinaldehyde, the
intermediate detected in water, was not detected in methanol.
Instead, 1,1,4,4-tetramethoxybutane, the acetal of succinalde-
hyde, was formed as the main product (Fig. S2† and Scheme 1).
1,1,4,4-Tetramethoxybutane could maintain its presence in the
acidic medium for a prolonged reaction time (Fig. 1c), indi-
cating its improved stability. This was the important reason for
the suppression of the polymerization reactions in methanol.
The identication of 1,1,4,4-tetramethoxybutane was the key
to understand the roles of methanol in the suppression of the
polymerization reactions. Its mass spectrum and the proposed
dissociation routes in the mass spectrometer are shown in
Fig. S3 and S4,† respectively. Further to this, methanol also
reacted with furan via electrophilic addition to form
moles of benzofuran produced ꢂ 2
Selectivity ð%Þ ¼
ꢂ 100
moles of furan converted
It should be noted herein that formation of 1 mol benzo-
furan consumes 2 mol of furan and therefore the amount of
benzofuran produced (molar basis) has to be multiplied by 2 to
calculate the yield.
The yield of levulinic acid was dened by the equation:
moles of levulinic acid produced
Yield ðmol%Þ ¼
ꢂ 100
moles of furan loaded
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Fig. 1 The conversion of furan in methanol and water. Reaction conditions: furan, 3 g; solvent, 100 ml; catalyst, A70, 3 g; stirring rate, 350 rpm;
reaction time, 90 min; pressure, autogenous vapor pressure. (a) The conversion of furan. The conversion of furan in water was measured by
mixing the residual products with THF as furan does not dissolve in water. (b) The yields of benzofuran; (c) the abundance of 1,1,4,4-tetra-
methoxybutane; (d) the yields of dibenzofuran. In the experiment at 190 ꢀC in methanol, no sampling was performed. “0 min” in the x-axis means
the reaction temperature just reached the required temperature.
Fig. 2 Constant energy (ꢁ2800 cmꢁ1) synchronous spectra for the soluble polymers from the acid-treatment of furan in the various solvents. (a)
Spectra of the soluble polymers versus the reaction time in methanol; (b) spectra of the soluble polymers versus the reaction time in water; (c)
spectra of the soluble polymers versus the reaction time in methanol/water; (d) spectra of the soluble polymers versus solvents. The
concentration of the samples prepared for UV fluorescence analysis was 1600 ppm in Fig. 3c, and others were 400 ppm. The fluorescence peaks
at ca. 271 and 278 nm are attributed to benzofuran and dibenzofuran, respectively. The fluorescence peaks at ca. 317 to 329 nm are roughly
attributed to the compounds with three fused benzene rings. The fluorescence peaks above 360 nm are roughly attributed to the compounds
with four or more fused benzene rings.
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ꢀ
to 190 C did not yield signicantly more benzofuran (Fig. 1b)
but much more dibenzofuran (Fig. 1d). In fact, the formation of
dibenzofuran occurred almost simultaneously with the forma-
tion of benzofuran (Fig. 1b and d).
In water, the furan ring could be opened to form succi-
naldehyde. In methanol, the furan ring could also be opened to
form 1,1,4,4-tetramethoxybutane. It is likely that the formation
of benzofuran in methanol proceeded via the ring-opening
route with 1,1,4,4-tetramethoxybutane as the intermediate
product. The proposed reaction pathways involved three main
steps, as depicted in Scheme 3. The initial step was the attack on
C2 or C5 of the furan ring by a proton to form the carbon cation.
Further attack on oxygen in the furan ring resulted in the
opening of the furan ring and the formation of 1,1,4,4-tetra-
methoxybutane via the acetalization. The methoxy group in the
acetal could then be removed via the formation of methanol
(Scheme 3), producing the carbocation. The carbocation could
react with the furan with its high electron density via electro-
philic substitution. The further removal of the methoxy group
followed by the subsequent electrophilic substitution eventually
formed benzofuran. The electrophilic substitution reaction was
the key step for the formation of the benzene ring in benzo-
furan, which was also reported as the main reaction pathway for
benzofuran formation via the gas-phase condensation of
furan.20
Scheme 1 The distribution of the main products during the conver-
sion of furan in water and in methanol. All the compounds listed above
except the polymer were detected with GC-MS. Tetrahydrofuran-2,5-
diol, succinaldehyde, 2,5-dimethoxytetrahydrofuran and 1,1,4,4-tet-
ramethoxybutane are the intermediates. The opening of the furan ring,
the acetalization, the formation of benzofuran and the polymer were
believed to be the acid-catalyzed reactions.
Benzofuran continued to react with the intermediate derived
from furan, forming dibenzofuran. The formation of benzo-
furan and dibenzofuran possibly involved similar intermedi-
ate(s) derived from the opening of the furan ring. Dibenzofuran,
like benzofuran, was also not stable. It was further hydrolyzed to
2,60-biphenyldiol (Scheme 2). The means to control specic
selectivity in the acid-catalyzed conversion of furan is another
challenge apart from the polymerization. Corma et al. devel-
oped a process for the production of alkanes (C14H30 and
higher, the diesel range fuel) via the acid-catalysed alkylations
of 2-methylfuran and the following hydrogenation.32,33 In this
2,5-dimethoxytetrahydrofuran with the cis-/trans-isomers
(Scheme 2), which apparently proceeded in a stepwise manner.
The formation of 2,5-dimethoxytetrahydrofuran eliminated the
carbon–carbon double bonds in the furan ring and minimized
the involvement of furan in the polymerization.
Benzofuran was the main product in methanol and its
formation was promoted at elevated temperatures (Fig. 1b).
However, benzofuran was not stable in the acidic environment,
as the “furan ring” was still preserved in benzofuran. It
continued to condense with furan to form dibenzofuran
(Fig. 1d). The further increase of reaction temperature from 170
Scheme 2 A proposed reaction network for the conversion of furan in methanol. All the compounds listed above except the polymer were
detected with GC-MS.
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Scheme 3 Proposed reaction pathways for the formation of benzofuran from furan via the ring-opening route in methanol. The intermediate,
1,1,4,4-tetramethoxybutane, was detected with GC-MS.
study, benzofuran was produced from furan via two alkylation The concentration of water in the acid treatment of furan in
reactions in the presence of an acid catalyst. Without following methanol at 170 ꢀC reached ca. 14 wt% at the end. The presence
a hydrogenation step, the rings in benzofuran were retained. It of water in methanol had signicant effects on the formation of
is believed that if hydrogen and a hydrogenation catalyst were benzofuran, which was conrmed by the following experiments
also present in the reaction medium, C8 alkanes, instead of
benzofuran, would be produced.
Polymerization of furan in methanol
The polymerization could be signicantly suppressed in
methanol, but not be eliminated completely, even at temper-
ꢀ
atures as low as 150 C, as evidenced by the formation of the
soluble polymers (Fig. S5†). This might be explained in three
ways. First, benzofuran and dibenzofuran were still reactive.
Benzofuran could multiply to form dibenzofuran, whereas
dibenzofuran could hydrolyze to 2,60-biphenyldiol, which
could not be prevented by methanol. These products might
continue their way to form bigger molecules such as soluble
polymers (Fig. 2a).
Second, methanol could not stabilize all the compounds
containing carbonyl groups. In addition to the p-conjugated
structures in the soluble polymer (Fig. 2), aromatic ketone
functionalities were also identied by the characterization of
the soluble polymer with FT-IR (Fig. 3). The IR absorption at
1670–1650 cmꢁ1 was assigned typically to aromatic ketones,34
but not to aliphatic aldehydes with a typical absorption at 1740–
1720 cmꢁ1 34 The abundance of aromatic ketones was increased
.
monotonically with the progress of the reaction (Fig. 3b). This
result was in line with the monotonic increase in the abundance
of the p-conjugated structures with the reaction time (Fig. 2a).
The aromatic ketone groups were possibly assigned to the
functionalities of the heavy products (i.e. soluble polymers) as
the detectable light products did not contain the aromatic
ketone functionality. Methanol could transform the aliphatic
aldehydes such as succinaldehyde into acetals, but was not able
to transform the aromatic ketones effectively. With the progress
of the reaction, these compounds continue to polymerize.
Third, a signicant amount of water was formed during the
acid-catalyzed conversion of furan in methanol. In the presence
of the acid catalyst, a part of the methanol underwent inter-
molecular etherication, forming dimethyl ether and water.
Fig. 3 The FT-IR characterization of the liquid samples from the acid-
treatment of furan in methanol at (a) T ¼ 190 ꢀC and (b) T ¼ 170 ꢀC. The
sample in Fig. 4a was analyzed directly without dilution. The samples in
Fig. 4b were diluted to 10 wt% in chloroform before the analysis.
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on the acid treatment of furan in the mixed methanol/water methanol/water (Fig. 4c), due to the equilibrium being shied
solvent.
to the aldehyde side. The acetal was a relatively stable inter-
mediate, formation of which created the chance and the time
for the formation of benzofuran. It played essential roles in the
suppression of the polymerization reactions. As was evidenced
in methanol/water, without the signicant formation of 1,1,4,4-
tetramethoxybutane, the polymerization dominated.
Conversion of furan in methanol/water
Fig. 4a shows that the conversion of furan was very quick in
methanol/water. This, however, was not accompanied with
a proportional increase in benzofuran formation (Fig. 4b).
Benzofuran yields barely exceeded 5%. In addition, the abun-
dance of the soluble polymer formed was also remarkably less
in methanol/water than in methanol (Fig. 2a and c). Thus, the
polymerization to the insoluble polymer was supposed to be the
main reaction pathway for the conversion of furan in methanol/
water. The methanol/water volumetric ratio of 1 : 1 or 2 : 1
made no marked difference.
It is believed that water and methanol have very different
solubility towards the soluble polymer. It can be found that in
water, the abundance of the soluble polymer was also signi-
cantly less than that in methanol (Fig. 2a and b), but the amount
of the insoluble polymer was signicantly more (ca. 90% in
water at 170 ꢀC versus ca. 42% in methanol at 170 ꢀC). This
result indicated that in water, the transformation of the soluble
polymer into the insoluble polymer proceeded quickly, which
was probably related to the low solubility of the polymer in
water. In contrast, in methanol, a signicant portion of the
soluble polymer was retained in the reaction medium. Meth-
anol probably helped to stabilize the soluble polymers.
The conversion of furan over a higher loading of A70 was also
investigated (Fig. 4). With the availability of more acidic sites,
the conversion of furan (Fig. 4a) was relatively quicker. As
mentioned above, 1,1,4,4-tetramethoxybutane (Fig. 4c) was the
reaction intermediate. It was produced initially and soon it was
further converted into benzofuran and dibenzofuran. Corre-
spondingly, the formation of benzofuran and dibenzofuran
(Fig. 4b and d), as well as the soluble polymer (Fig. S6†), was also
promoted. This was understandable as the conversion of furan
into benzofuran via the electrophilic substitution was catalyzed
by the acid catalyst. Higher catalyst loading would accelerate the
acid-catalyzed reactions. However, one drawback with the high
A70 loading was that the formation of water via the ether-
ication of methanol was accelerated (water content at the end
was 24.7 wt%), which consumed methanol and suppressed the
formation of 1,1,4,4-tetramethoxybutane.
Characterization of the insoluble polymer and the soluble
polymer
The presence of water in methanol also affected the forma- From the abovementioned results, it is concluded that the
tion of intermediates such as 1,1,4,4-tetramethoxybutane. The formation of polymer was the main reason for the low yield of
abundance of 1,1,4,4-tetramethoxybutane was quite small in benzofuran from furan, especially in water. To understand the
Fig. 4 The conversion of furan in methanol and in methanol/water. (a) The conversion of furan; (b) the yields of benzofuran; (c) the abundance of
1,1,4,4-tetramethoxybutane; (d) the yields of dibenzofuran. Reaction conditions: furan, 3 g; T ¼ 170 ꢀC; catalyst loading and solvent used are
specified in Fig. 4a; stirring rate, 350 rpm; reaction time, 90 min; pressure, autogenous vapor pressure. The ratio of methanol/water referred is the
volumetric ratio. “0 min” in the x-axis means the reaction temperature just reached the required temperature.
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intermediates involved in the polymerization reactions, the
polymer formed in methanol (Run 5# and 6#), in water (Run 1#,
2# and 3#) and in methanol/water (Run 4#) with a different
loading of A70 (Run 3# versus the others) and different reaction
temperatures (150 versus 170 ꢀC) were characterized. Fig. 5
shows the yields of the insoluble polymer and of benzofuran.
The results were in line with those in Fig. 1. In water, furan
was mainly polymerized, with little formation of benzofuran.
Interestingly, with a lower A70 loading (Run 3#), there was less
polymer formed and furan was not fully converted. The poly-
merization of furan mainly occurred inside the pores of the
catalyst, which probably deactivates the catalyst. Deactivation of
A70 due to polymerization was also observed in our previous
study.23 Aer deactivation of the acid catalyst, further conver-
sion of furan would be slowed down. Although the yield of the
polymer in Run 3# was lower than that in Run 1# and 2#, the
selectivity for the polymer, which was dened by the amount of
the polymer formed divided by the amount of furan converted,
was close to 90%. Obviously, with a lower or higher loading of
the acid catalyst, the main reaction pathways of furan in water
remained the same. Furan has a low solubility in water, which
might also contribute to the polymer formation in Run 1#, 2#
and 3#. In Run 4# with the presence of 50% (by volume) of
water, the polymerization was still signicant, but the produc-
tion of benzofuran was increased due to the presence of
methanol (50%). With methanol as the solvent, the formation of
the insoluble polymer was suppressed to a signicant extent.
Furan, with no other functionalities, is mainly polymerized
in water in the presence of the acid catalyst. Understanding the
functionality of the insoluble polymer will help to understand
Fig. 6 FT-IR spectra of the fresh Amberlyst 7 and the used catalysts
(polymer plus catalyst) obtained at (a) 150 ꢀC and (b) 170 ꢀC. The typical
reaction conditions for the catalysts experienced were described
inside figures. The detailed conditions can refer to that specified in the
caption of Fig. 5.
the reaction intermediates involved in the polymerization
reactions. The insoluble polymer formed under the conditions
specied in Fig. 5 was characterized with FT-IR, as shown in
Fig. 6. The polymer shows a strong absorption for carbonyls,
especially for the ones formed in water or in methanol/water.
Evidently, the polymerization of furan involved the opening of
the furan ring to form aldehydes or ketones. In fact, the
formation of succinaldehyde has been detected with water as
the solvent. This further conrms the important roles of
aldehydes/ketones in the polymerization of furan. The absorp-
tion of the carbonyl group in the polymer formed in methanol
was relatively weaker than that in the polymer formed in water.
The aldehydes such as succinaldehyde could be converted into
1,1,4,4-tetramethoxybutane via acetalization, which protected
these reactive functionalities and suppressed their involvement
in the polymerization reactions.
Fig. 5 Formation of insoluble polymer and benzofuran during the
acid-treatment of furan. Typical reaction conditions: furan, 10 g; water
or methanol, 80 mL; catalyst, Amberlyst 70 (A70); stirring rate, 350
rpm, reaction time, 90 min; pressure, autogenous vapor pressure. No
sampling was performed during the experiments. Typical reaction
conditions for individual experiments: 1#: water, T ¼ 150 ꢀC; A70, 10 g.
2#: water, T ¼ 170 ꢀC; A70, 10 g. 3#: water, T ¼ 170 ꢀC; A70, 5 g. 4#:
water/methanol (volume/volume ¼ 1 : 1), T ¼ 170 ꢀC; A70, 10 g. 5#:
methanol, T ¼ 150 ꢀC; A70, 10 g. 6#: methanol, T ¼ 170 ꢀC; A70, 10 g.
The conversion of furan in Run 1#, 2#, and 4# was close to 100%. In
Run 3# the conversion was ca. 75%, while that in Run 5# and 6# were
ca. 50% and 83%, respectively.
Soluble polymer was the precursor for the formation of the
insoluble polymer, characterization of which may also obtain
important information about the reactive intermediates
involved in the polymerization of furan. FT-IR characterizations
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Fig. 7 (a) FT-IR spectra of the soluble polymers; (b) deconvolutions of
the FT-IR spectra in Fig. 7a. The liquid products were diluted to 50%
(by volume) with methanol before the characterization with FT-IR.
of the soluble polymer in the liquid samples were carried out
with the focus on the absorptions of carbonyls. Deconvolution
of the spectra was also performed, as shown in Fig. 7. The
absorption at 1696 cmꢁ1 represents the C]O stretching vibra-
tion of unsaturated aldehydes/ketones. The absorption at 1654
cmꢁ1 represent the C]O stretching vibration of the hydroxyl
unsaturated aldehydes/ketones, whereas the absorption at 1606
cmꢁ1 represents the aromatic C]C ring breathing in aromatics
with various types of substitution. Evidently, relatively more
aldehydes/ketones and more aromatics with varied substitu-
tions were observed in the polymer formed in water than in that
formed in methanol. This result was in line with the charac-
terization of the insoluble polymer with FT-IR (Fig. 6), wherein
the polymer formed in water tended to have more aldehydes/
ketone groups on its surface. The presence of aromatics with
fused-ring structures were also conrmed from the character-
ization of the soluble polymer with UV uorescence, as shown
in Fig. 8. The uorescence spectra extended to ca. 425 nm,
indicating that the aromatics have 3- to 5-fused ring structures.
Based on the above characterization, it was obvious that the
polymerization of furan formed the polymer with aromatic ring
structures and aldehyde/ketone functionalities.
Fig. 9 TGA and DTG curves for the used catalyst (polymer plus
catalyst) and fresh Amberlyst 70 catalyst. The fresh catalyst and the
used catalysts were dried at 100 ꢀC in nitrogen for 60 min before
further heating to 700 ꢀC in nitrogen and 600 ꢀC in air at 10 ꢀC minꢁ1
.
The gas flow rate was 100 mL minꢁ1. Typical reaction conditions
experienced by the catalysts are described inside the figures. Detailed
conditions are specified in the caption of Fig. 5.
produced in the two different solvents was conducted using
TGA and elemental analysis. Pyrolysis and combustion behav-
iours of the polymer plus catalyst were performed with TGA in
nitrogen and in air, respectively. As shown in Fig. 9, the weight
loss was not signicantly different for the fresh catalyst and for
the polymer (plus catalyst) formed in water or in methanol.
However, the formation of polymer inside the catalyst did
change the pyrolysis behaviours of the catalyst and the used
catalyst (polymer plus catalyst). The weight loss became more
signicant at ca. 420 ꢀC for the used catalyst, while for the fresh
catalyst the weight loss was also signicant at 308 and 504 ꢀC. It
was possible that the pyrolysis of the polymer mainly occurred
at ca. 420 ꢀC. The combustion of the polymer (plus catalyst)
formed in water or in methanol showed almost the exact same
trends. From the TGA results, the differences in the polymer
formed in water or in methanol in terms of pyrolysis or
combustion behaviours were insignicant. The polymer was
further characterized with an elemental analyzer to measure
potential differences in their elemental composition.
Because polymer can be formed from furan in either meth-
anol or water, further study on the difference of the polymer
Because the polymer was formed on/in the catalyst, the
polymer plus catalyst was thus characterized as one entity. The
carbon, hydrogen and oxygen contents in the polymer were
calculated based on the mass ratio of the polymer to catalyst. As
shown in Table 1, compared to that in the fresh catalyst, the
content of oxygen and sulphur were decreased, while the
content of carbon and hydrogen were increased correspond-
ingly. The abundance of oxygen and sulphur were clearly
diluted with the presence of the polymer on/in the catalyst as
the polymer has no sulphur and lower oxygen content when
compared with the catalyst.
Fig. 8 Constant energy (ꢁ2800 cmꢁ1) synchronous spectra for the
soluble polymers in the liquid samples. The typical reaction conditions
are described inside the figure. Detailed conditions are specified in the
caption of Fig. 5.
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Table 1 Elemental composition of polymer plus catalyst (on a dry ash-
free basis)
Elemental
composition (wt%)
Entry Experimental conditions
C
H
S
O
1
2
3
4
Furan, water, A70: 5 g, T ¼ 170 ꢀC
69.5 4.7 2.8 23.0
67.3 4.4 3.9 24.4
66.6 4.2 3.9 25.3
65.5 4.0 4.1 26.4
Furan, water, A70: 10 g, T ¼ 150 ꢀ
Furan, water, A70: 10 g, T ¼ 170 ꢀ
C
C
Furan, water/methanol, A70: 10 g, T ¼
170 ꢀ
C
5
6
7
Furan, methanol, A70: 10 g, T ¼ 150 ꢀ
Furan, methanol, A70: 10 g, T ¼ 170 ꢀ
Fresh Amberlyst 70
C
C
63.5 3.6 5.7 27.2
63.4 3.9 5.2 27.5
47.9 2.9 9.7 39.5
Table 2 Elemental composition of the polymer (on a dry ash-free
basis)
Elemental
composition
(wt%)
Entry Experimental conditions
C
H
O
1
2
3
4
5
6
7
Furan, water, A70: 5 g, T ¼ 170 ꢀC
78.2 5.4 16.4
78.8 5.2 16.0
77.9 5.1 17.0
Furan, water, A70: 10 g, T ¼ 150 ꢀ
Furan, water, A70: 10 g, T ¼ 170 ꢀC
C
Furan, water/methanol, A70: 10 g, T ¼ 170 ꢀC 77.8 4.8 17.4
Furan, methanol, A70: 10 g, T ¼ 150 ꢀ
Furan, methanol, A70: 10 g, T ¼ 170 ꢀ
Furan chemical
C
C
81.8 4.4 13.8
80.1 4.9 15.0
70.6 5.9 23.5
Fig. 10 The conversion of furan in the alcohols. (a) Furan conversion in
the alcohols; (b) the yields of benzofuran in the alcohols. Reaction
conditions: furan, 3 g; solvent, 100 ml; catalyst, A70, 3 g; stirring rate,
350 rpm; reaction time, 90 min; pressure, autogenous vapor pressure.
“0 min” in the x-axis means the reaction temperature just reached the
required temperature.
The carbon content of the polymer was in the range of ca. 78–
82%, which is ca. 10% higher than that of furan (Table 2).
Moreover, the contents of hydrogen and oxygen were lower than
that in furan. Obviously, the formation of the polymer was
accompanied by dehydration reactions. The dehydration was towards benzofuran, which eliminates the difference in furan
believed not to be signicant; however, as the polymer formed conversion in the different alcohols, were not signicantly
in water still has relatively high oxygen content (ca. 17% versus different (ranging from 20 to 30%). The alcohols investigated
23.5% in furan). The oxygen was possibly retained in the poly- had similar capacities to suppress the polymerization reactions.
mer in functionalities, such as aldehydes and ketones, or in the
The soluble polymers formed in the various alcohols were
form of the ether bond connecting aromatic ring structures. The not exactly the same (Fig. 2d and S7†). The peaks at 317 and 330
carbon content in the polymer formed in methanol was slightly nm reected soluble polymers with different p-conjugated
higher than that formed in water and correspondingly the structures. The peak at 330 nm was more intensive in methanol
oxygen content was slightly lower. Methanol could transform than in other alcohols, indicating that methanol affected the
the aldehyde functionality into acetals, which might be one formation of the soluble polymer in a way different from other
reason for this slight difference. In addition, the reaction alcohols, which was probably related to the smaller size of
intermediates involved in the polymerization reactions were not methanol.
the same in water as in methanol, which might also affect the
elemental composition of the polymer.
Conversions of furan in tetrahydrofuran (THF) and dimethyl
sulfoxide (DMSO)
Conversion of furan in ethanol, 1-propanol and 2-propanol
The distinct effects of water and methanol on the polymeriza-
The conversions of furan in the other alcohol solvents (ethanol, tion of furan encouraged us to explore the conversion of furan
1-propanol and 2-propanol) were not as signicant as that in in other solvents. Fig. 11 shows the conversion of furan in THF
methanol (Fig. 10). Benzofuran yields in the alcohols were also and in DMSO. THF is not an alcohol and is not involved much
different (Fig. 10b), which is related to the different conversions in the conversion of furan. It could not react with succinalde-
of furan in the alcohols. However, the overall selectivities hyde to form the acetal to protect the reactive aldehyde
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Fig. 12 The formation of levulinic acid and furfuryl methyl sulfide in
acid treatment of furan in DMSO.
were also negligible (Fig. 2d). Evidently, the polymerization of
furan was effectively suppressed in DMSO, which was similar to
the suppression of the polymerization of HMF or furfural in
DMSO.35,36 DMSO does not have a hydroxyl group as that in
water or alcohols, and conversion of furan in DMSO was
negligible. It seems that the hydroxyl group (either that in water
or in alcohols) is very important for the conversion of furan via
the opening of the furan ring.
In DMSO, instead of polymerization or conversion to
benzofuran, furan was oxidized by DMSO to form 2(5H)-fur-
anone or reacted with DMSO (or the derivative of DMSO), via
electrophilic substitution, forming furfuryl methyl sulde
(Scheme 4, Fig. S8 and S9†). The detailed reaction route has not
been understood yet. Furfuryl methyl sulde might further react
with furan, forming 2,20-methylenebis-furan via electrophilic
substitution. More interestingly, furfuryl methyl sulde was
also further converted into levulinic acid and methanethiol
(Scheme 4). It is believed that some reducing agent(s) are
involved in the transformation of furan in DMSO into furfuryl
methyl sulde. In our previous study on the conversion of xylose
to furfural,10 we found that formic acid, the degraded product of
furfural, was the hydrogen donor promoting the conversion of
furfural into furfuryl alcohol. Formic acid was not detected in
the conversion of furan in DMSO. Thus, some other reducing
Fig. 11 The conversion of furan in THF and in DMSO. (a) Furan
conversion in the solvents; (b) the yields of benzofuran in the solvents.
Reaction conditions: furan, 3 g; solvent, 100 ml; catalyst, A70, 3 g;
stirring rate, 350 rpm; reaction time, 90 min; pressure, autogenous
vapor pressure. “0 min” in the x-axis means the reaction temperature
just reached the required temperature.
functionality. The yield of benzofuran barely exceeded 5%
(Fig. 11b). THF was not an effective solvent for the conversion of
furan into benzofuran.
DMSO was also not effective for the conversion of furan into
benzofuran. The amount of benzofuran formed was negligible
(Fig. 11b) and the conversion of furan was small (Fig. 11a). In
addition, the soluble polymers formed from furan in DMSO
Scheme 4 A proposed reaction network for the conversion of furan in DMSO. All the compounds listed above were detected with GC-MS.
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agent must be involved in the reduction of DMSO, which is
under current further investigation.
6 M. S. Mettler, S. H. Mushrif, A. D. Paulsen, A. D. Javadekar,
D. G. Vlachos and P. J. Dauenhauer, Energy Environ. Sci.,
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7 Q. Lu, W.-M. Xiong, W.-Z. Li, Q.-X. Guo and X.-F. Zhu,
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8 M. J. Climent, A. Corma and S. Iborra, Green Chem., 2014, 16,
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9 D. M. Alonso, S. G. Wettstein, M. A. Mellmer, E. I. Gurbuz
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The conversion of furfuryl methyl sulde into levulinic acid
was very similar to the conversion of furfuryl alcohol into lev-
ulinic acid,37,38 due to the similar molecular conguration of the
two furans. To the best of our knowledge, this is the rst time
for this reaction to be reported. The yields of levulinic acid and
its precursor, furfuryl methyl sulde, increased monotonically
with reaction time (Fig. 12), which was a favorable reaction
route. This result revealed the possibility for the production of 10 X. Hu, C. Lievens and C.-Z. Li, ChemSusChem, 2012, 5, 1427–
levulinic acid from furan via modifying its molecular structures. 1434.
The reaction conditions need to be further optimized to make 11 J. Guo, G. Xu, Z. Han, Y. Zhang, Y. Fu and Q. Guo, ACS
furan the building block for chemical diversity and biofuels Sustainable Chem. Eng., 2014, 2, 2259–2266.
production, in addition to HMF and furfural, as another 12 X. Hu, Y. Song, L. Wu, M. Gholizadeh and C.-Z. Li, ACS
precursor for the production of levulinic acid.
Sustainable Chem. Eng., 2013, 1, 1593–1599.
13 X. Hu and C.-Z. Li, Green Chem., 2011, 13, 1676–1679.
14 A. A. Rosatella, S. P. Simeonov, R. F. M. Frade and
C. A. M. Afonso, Green Chem., 2011, 13, 754–793.
Conclusions
Polymerization is the key challenge in the conversion of furan 15 T. Wang, M. W. Nolte and B. H. Shanks, Green Chem., 2014,
into benzofuran. Our results demonstrated that the polymeri- 16, 548–572.
zation of furan or its derivatives could be suppressed effectively 16 S. Eminov, J. D. E. T. Wilton-Ely and J. P. Hallett, ACS
using methanol as the solvent/reactant. The transformation of Sustainable Chem. Eng., 2014, 2, 978–981.
the aldehyde intermediate into the acetal with methanol was 17 X. Hu, D. Mourant, Y. Wang, L. Wu, W. Chaiwat,
the key prerequisite to tackle the polymerization of furan. In
water, without the conversion of succinaldehyde into the acetal,
R. Gunawan, M. Gholizadeh, C. Lievens, M. Garcia-Perez
and C.-Z. Li, Fuel Process. Technol., 2013, 106, 569–576.
furan was mainly polymerized. In addition, the conversion of 18 C. J. Gilbert, J. S. Espindola, W. C. Conner Jr, J. O. Trierweiler
furan was sensitive to the molecular size of the alcohols. and G. W. Huber, ChemCatChem, 2014, 6, 2497–2500.
Methanol presented much higher capacity than higher alcohols 19 Y.-T. Cheng and G. W. Huber, ACS Catal., 2011, 1, 611–628.
such as ethanol and 1-propanol for the conversion of furan. 20 S. Vaitheeswaran, S. K. Green, P. Dauenhauer and
Adjusting solvents could also modify the reaction pathways of
furan during the acid-catalyzed conversion. In DMSO, instead of 21 J.-L. Grandmaison, P. D. Chantal and S. C. Kaliaguine, Fuel,
benzofuran, levulinic acid was produced, which diversies the 1990, 69, 1058–1061.
application of furan as a chemical feedstock. The conversion of 22 X. Hu, C. Lievens, A. Larcher and C.-Z. Li, Bioresour. Technol.,
furan in a wider range of solvents and acid catalysts needs to be
2011, 102, 10104–10113.
explored to effectively upgrade the furan range of biofuels to the 23 X. Hu, R. J. M. Westerhof, L. Wu, D. Dong and C.-Z. Li, Green
S. M. Auerbach, ACS Catal., 2013, 3, 2012–2019.
aromatic range of biofuels.
Chem., 2015, 17, 219–224.
24 X. Hu, S. Kadarwati, Y. Song and C.-Z. Li, RSC Adv., 2016, 6,
4647–4656.
25 X. Hu, R. Westerhof, D. Dong, L. Wu and C.-Z. Li, ACS
Sustainable Chem. Eng., 2014, 11, 2562–2575.
Acknowledgements
This study is supported by the Commonwealth of Australia
under the Australia–China Science and Research Fund, as well 26 X. Hu, Y. Wang, D. Mourant, R. Gunawan, C. Lievens,
as Curtin University of Technology through the Curtin Research
Fellowship Scheme. This project received funding from ARENA
as part of ARENA's Emerging Renewables Program.
W. Chaiwat, M. Gholizadeh, L. Wu, X. Li and C.-Z. Li,
AIChE J., 2013, 59, 888–900.
27 B. Saha and M. M. Abu-Omar, Green Chem., 2014, 16, 24–38.
28 C. M. Cai, N. Nagane, R. Kumar and C. E. Wyman, Green
Chem., 2014, 16, 3819–3829.
29 N. Shi, Q. Liu, T. Wang, L. Ma, Q. Zhang and Q. Zhang, ACS
Sustainable Chem. Eng., 2014, 2, 637–642.
30 S. K. R. Patil and C. R. F. Lund, Energy Fuels, 2011, 25, 4745–
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31 X. Hu, L. Wu, Y. Wang, D. Mourant, C. Lievens, R. Gunawan
and C.-Z. Li, Green Chem., 2012, 14, 3087–3098.
32 A. Corma, O. Torre and M. Renz, ChemSusChem, 2011, 4,
1574–1577.
33 A. Corma, O. Torre, M. Renz and N. Villandier, Angew. Chem.,
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