Concurrent formation of furan-2,5- and furan-2,4-dicarboxylic acid: Unexpected aspects of the Henkel reaction

Article abstract of DOI:10.1039/c3ra42457j

The concurrent formation of furan-2,5- and furan-2,4-dicarboxylic acid under solvent free conditions via a disproportionation reaction is described. By reacting potassium-2-furoate at 260 °C in the presence of 22 mol% of (Lewis acidic) catalysts like CdI₂ or ZnCl₂, potassium-2-furoate is disproportionated to furan and furandicarboxylic acids. Besides furan and furan-2,5-dicarboxylic acid (2,5-FDCA) as the main products, furan-2,4-dicarboxylic acid (2,4-FDCA) is also formed as a by-product. Experimental evidence has been obtained that, under the reaction conditions applied, 2,5-FDCA and 2,4-FDCA are formed by separate reaction pathways. Selectivity towards the different FDCA isomers is affected by the type of catalyst used. Single-crystal X-ray analysis shows that 2,4-FDCA has a more 'linear' character compared to 2,5-FDCA and hence is structurally more comparable to terephthalic acid (TA), making it an interesting monomer for synthetic polyesters.

Full text of DOI:10.1039/c3ra42457j

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Concurrent formation of furan-2,5- and furan-2,4-
dicarboxylic acid: unexpected aspects of the Henkel
reaction3
Cite this: RSC Advances, 2013, 3,
15678
Shanmugam Thiyagarajan,a Aliaksei Pukin,a Jacco van Haveren,a Martin Lutzb
and Daan S. van Es*a
The concurrent formation of furan-2,5- and furan-2,4-dicarboxylic acid under solvent free conditions via a
disproportionation reaction is described. By reacting potassium-2-furoate at 260 uC in the presence of 22
mol% of (Lewis acidic) catalysts like CdI2 or ZnCl2, potassium-2-furoate is disproportionated to furan and
furandicarboxylic acids. Besides furan and furan-2,5-dicarboxylic acid (2,5-FDCA) as the main products,
furan-2,4-dicarboxylic acid (2,4-FDCA) is also formed as a by-product. Experimental evidence has been
obtained that, under the reaction conditions applied, 2,5-FDCA and 2,4-FDCA are formed by separate
reaction pathways. Selectivity towards the different FDCA isomers is affected by the type of catalyst used.
Single-crystal X-ray analysis shows that 2,4-FDCA has a more ‘linear’ character compared to 2,5-FDCA and
hence is structurally more comparable to terephthalic acid (TA), making it an interesting monomer for
synthetic polyesters.
Received 17th May 2013,
Accepted 24th June 2013
DOI: 10.1039/c3ra42457j
www.rsc.org/advances
Currently, the majority of academic and industrial efforts in
the development of 2,5-FDCA are concentrated on routes
Introduction
In recent years, the potential replacement of fossil resources
by inexpensive and renewable starting materials such as
starch, cellulose, proteins and vegetable oils is increasingly
being explored, with the aim to develop a more sustainable
bio-based economy.1 The application of renewable feedstocks
for the synthesis of a variety of bio-based building blocks for
performance polymers is also a growing subject of interest.2
2,5-FDCA is one among the top twelve bio-based building
blocks derived from biomass,3 and is widely advocated as a
potential substitute for terephthalic acid (TA), the main
constituent of synthetic polyesters such as polyethylene
terephthalate (PET) and polybutylene terephthalate (PBT).
Recent reports shows that, polyethylene furanoate (PEF) and
polybutylene furanoate (PBF) derived from 2,5-FDCA have
properties comparable to their terephthalate analogues.4 For
instance, Avantium reported superior gas barrier properties of
PEF towards CO2 and O2 compared to PET.5
starting from food-grade carbohydrates, in particular fructose,
via the 5-(hydroxymethyl)-2-furaldehyde (HMF) route.6 Since
competition with food production is undesirable for ethical as
well as economical reasons, we are focusing on the use of non-
edible feed stocks, such as lignocellulose, agro residues, and
aqueous biomass (e.g. algae) for the production of chemicals
and building blocks.
The first reported synthesis of 2,5-FDCA goes back almost
one and a half centuries to a paper by Fittig and Heinzelman
in 1876 who obtained 2,5-FDCA by reacting mucic (galactaric)
acid with hydrobromic acid (Scheme 1, route 2).7 Since
isolated yields are moderate and galactaric acid is not (yet)
readily available, thus far little attention has been paid to this
route. This may change if aldaric acids become more widely
available, resulting from pectin or alginate streams produced
by bio-refineries.8 Currently, the by-far most widely explored
synthetic route to 2,5-FDCA is based on the catalytic oxidation
of 5-(hydroxymethyl)-2-furaldehyde (HMF),9 which in turn can
be obtained from the acid-catalyzed cyclodehydration of C6-
sugars like glucose and fructose (Scheme 1, route 1).6,10
Fructose is the preferred C6 sugar, because it results in
substantially higher yields of HMF than glucose. The only
reported process that efficiently converts glucose into HMF
requires Cr-based catalysts in combination with ionic liquids,
which is undesirable from a sustainability point of view.11
Fructose can be economically obtained by acid catalyzed
hydrolysis of sucrose or inulin, or by hydrolysis of starch and
aFood & Bio-based Research, Wageningen University and Research Centre, P.O. Box
17, 6700 AA Wageningen, The Netherlands. E-mail: daan.vanes@wur.nl;
shanmugam.thiyagarajan@wur.nl; Fax: +31 317 483011; Tel: +31 317 481160
bBijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht
University, Padualaan 8, 3584, Utrecht, The Netherlands
Electronic supplementary information (ESI) available: 1H-NMR spectra of
3
potassium 2-furoate, 2,5-furandicarboxylic acid and 2,4-furandicarboxylic acid,
Relative ratio of 2,5-FDCA and 2,4-FDCA formation over time according to 1H-
NMR. CCDC 907968. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3ra42457j
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Scheme 2 Synthesis of terephthalic acid (TA) via disproportionation of
potassium benzoate.
2-furaldehyde or furfural. Whereas HMF is still in a develop-
mental stage, furfural is already produced commercially on a
400–500 000 ton scale, using agro-residues like bagasse as
feedstock.16 (Catalytic) Oxidation of furfural gives 2-furoic acid
(FA), or the corresponding ester, depending on the reaction
conditions in high yields, however, subsequent introduction of
a carbon substituent at C5 is non-trivial (Scheme 1, route 3b).
Older reports on hydroxymethylation using p-formaldehyde in
sulphuric acid or chloromethylation via the Blanc reaction
gave only low to moderate yields of 5-hydroxymethyl- or
5-chloromethyl-2-furoic acid respectively.17 (N.B. Another
drawback of the Blanc reaction is that the product is an
extremely potent allergen.)18 Subsequently, oxidation is
required in order to transform this intermediate into product
2,5-FDCA.19
Scheme 1 Reported routes to 2,5-FDCA starting from different carbohydrate
sources: Sacch = saccharification, CDH = cyclodehydration, Iso = isomerization,
Ox = oxidation, XMe = chloromethylation (X = Cl) or hydroxymethylation (X =
OH).
A more attractive synthetic approach for the conversion of
2-furoic acid into 2,5-FDCA is based on the well-known Henkel
reaction (also called Raecke/Henkel process) (Scheme 1, route
3a). This process is based on the thermal disproportionation
of alkaline salts of aromatic carboxylates to give the
symmetrical aromatic dicarboxylate and the unsubstituted
aromatic compound. Raecke and co-workers were the first to
examine this type of reaction already in 1952 to Henkel & Co.20
This resulted in a commercial process for the production of TA
and benzene as by-product starting from potassium benzoate
(Scheme 2).21 In general, Henkel-type reactions are carried out
in the presence of cadmium or other metal (e.g. zinc) salts as
catalysts, under CO2 or N2 atmosphere at temperatures
between 350 uC and 500 uC along with high static pres-
sures.20,22
The Henkel-type disproportionation of aromatic carboxy-
late salts is however not limited to benzene derivatives but was
also shown to proceed with heteroaromatic compounds like
the carboxylate salts of furan, thiophene, pyrrole, and pyridine
(Scheme 3).20
Raecke et al. also studied the influence of catalysts. When
the reaction was carried out in the absence of catalyst the yield
was very low, while in the presence of Lewis acid such as CdF2,
CdCl2, CdI2, or ZnCl2, both conversion and selectivity were
found to be significantly improved.20 Later, Andrisano et al.22
reported a detailed investigation on the reaction conditions
with respect to type of catalyst, catalyst concentration,
potentially cellulose followed by a selective partial isomeriza-
tion of glucose to fructose (Scheme 1, route 1).12 The use of
sucrose or starch is however highly undesirable given their
primary use as food component.
Over decades, considerable attention has been paid to
develop technology to produce HMF in high yields with
efficient and cost-effective methods,6,10c,13 as HMF is not only
a potentially low cost precursor to 2,5-FDCA, but can also serve
as a renewable platform chemical. Furthermore, HMF is a
precursor of next generation biofuels such as 2,5-dimethyl-
furan (DMF), 5-ethoxymethylfurfural (EMF), ethyl levulinate
(EL), and x-valerolactone (gVL).13
Till now no large scale industrial commercial process for
the production of HMF is operational, although various
companies have announced plans for pilot-scale facilities.
The relatively poor stability of HMF under acidic conditions
(required for cyclodehydration reactions) often leads to the
formation of undesired by-products like levulinic acid (LA),
formic acid (FA), and insolubles such as furan resins and
humins. These by-products severely complicate down-stream
processing and reduce the yield of purified HMF.6,14 In order
to address these issues, companies like Avantium are devel-
oping alternative fructose and glucose dehydration technolo-
gies in organic solvent like methanol yielding more stable
HMF-ethers and HMF-esters as products.15
In addition to routes 1 and 2, a third route based on the use
of hemicellulose and its C5 sugar components is also possible
(Scheme 1, route 3). Given that hemicellulose is a non-edible
(and not easily fermentable) residue from lignocellulosic
biorefineries as well as the agro-food industry, it is an
attractive feedstock for the production of chemicals.
Analogous to the cyclodehydration of C6 sugars to HMF,
cyclodehydration of C5 sugars (e.g. xylose and arabinose) yields
Scheme 3 Synthesis of heteroaromatic dicarboxylic acid via disproportionation
reaction.
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temperature, time and atmosphere (N2/CO2). These authors
reported yields of up to 70–80% of dipotassium 2,5-furandi-
carboxylate by reacting potassium 2-furoate (K-2F) at 250–300
uC in the presence of CdI2 under a nitrogen atmosphere.22
Industrial production of TA via the Henkel reaction was later
on abandoned in favor of the more cost effective catalytic
oxidation of xylene (since the commercial value of benzoic acid
is higher than that of p-xylene, while the reverse is true for
benzene vs. TA).
Scheme 4 Schematic representation of the synthesis of 2,5-FDCA 3a, 2,4-FDCA
3b and its corresponding dimethyl esters 4a,b.
However, the economics in the case of 2-furoic acid differ
from that of benzoic acid. While the price of 2-furoic acid can
be comparable with that of HMF, the co-product furan can be
the chemical intermediate for high value products like
tetrahydrofuran (THF) and 1,4-butanediol, whose prices
exceed those of TA.23
initially reported by Andrisano, and subsequently investigate
alternative catalysts systems. The starting material K-2F can be
efficiently synthesized by catalytic oxidation of furfural.26
When the oxidation is performed under aqueous conditions
using KOH, in the presence of e.g. supported Pt or Au catalysts,
K-2F can be obtained directly.23,27 The Henkel-type reactions
are usually carried out between 300 to 500 uC. For benzene
derived aromatics, the preferred temperature range is between
400 uC to 500 uC. Due to the lower thermal stability
heteroaromatic rings, K-2F should be treated at lower
temperatures (,400 uC) as previously suggested by Raecke
and Andrisano.20,22
In a first set of experiments K-2F was mixed with the
catalyst CdI2 and ground well under nitrogen atmosphere. The
finely powdered mixture was then transferred into a round-
bottom flask and inserted into a Kugelrohr glass oven and was
slowly rotated for 5.5 h at elevated temperatures. As the
reaction progresses, furan is formed as co-product, which was
collected in a cooled (acetone/dry-ice) receiver bulb. After
completion of the reaction the crude reaction mixture was
dissolved in water, whereupon after acidification with conc.
HCl, 2,5-FDCA precipitates out in high purity (.98% from
NMR and GC analyses) (Scheme 4). Temperature variation
(200–280 uC) (Table 1, entries 1–4) showed that while moderate
conversion (32%) was obtained at 235 uC, high conversion
(92%) could be obtained by increasing the temperature to 260
uC. Further increasing the temperature is detrimental, since at
280 uC significant decarboxylation of the products occurs,
resulting in very poor FDCA yields.
Since the use of Cd salts is undesirable from an ecological
and toxicological point of view, alternative catalysts for the
furan–Henkel reaction are required for this process to become
industrially feasible. Pan et al. showed that ZnCl2 was the only
effective dichloride metal salt of the extensive range of metal
chloride salts they tested. Exchanging CdI2 for ZnI2 (on a 1 : 1
molar basis) at 260 uC results in a significant drop in K-2F
conversion. Subsequent exchange of iodide for chlorine results
in an apparent further drop (Table 1, entries 5–6).
Nevertheless, in all the cases 2,5-FDCA can be effectively
isolated from the reaction mixture with high purity (98% from
GC and NMR analyses). Moreover, we also screened a wide
range of catalysts like FeCl2, FeCl3, FeI2, MgI2, CoCl2, MnCl2
and ZrCl4, which were not reported by Pan et al. However,
these catalysts were not effective for the Henkel reaction under
the typical conditions.
Since, the development of economically viable routes to
polymer building blocks like 2,5-FDCA from non-edible
biomass is one of our main research interests,2a,2b,4d,24 we
decided to reinvestigate the Henkel route in order to evaluate
its industrial viability. Till recently, the last report on this
reaction was by Ratusky et al. in 1971 who investigated the
transcarboxylation reactions of heterocyclic compounds like
pyridine, thiophene, furan and pyrrole carboxylates.25 Very
recently, however, in parallel to our own developments, Pan
et al. reported the use of the Henkel reaction as part of the
strategy to obtain fully biobased polybutylene 2,5-furanoate
(PBF).23 In contrast to all the reports mentioned here, we
found that the Henkel reaction of potassium 2-furoate (K-2F) is
much less selective than previously assumed. Here, we report
the novel co-production of 2,5-FDCA and 2,4-FDCA by thermal
disproportionation of potassium 2-furoate(K-2F).
Results and discussion
In their 1963 paper, Andrisano and Angeloni reported the
disproportionation of K-2F into 2,5-FDCA using cadmium
(mostly CdI2) and zinc (mostly ZnCl2) based catalysts.22 After a
reaction time of 4 h at 250 uC typical yields of 20–67% of 2,5-
FDCA were reported. The article describes a general procedure
for the isolation of the 2,5-FDCA but does not give any detail
whether the conversion of K-2F is incomplete or other
byproducts besides furan and 2,5-FDCA are being formed.
The authors mention one example in which, in the presence of
100 atm of CO2, 80% yield of 2,5-FDCA is obtained. Very
recently, Pan et al. described the Henkel disproportionation of
K-2F to furan and 2,5-FDCA with a selectivity of 86% at 61%
conversion using ZnCl2 as catalyst.23 While these authors
reported the conversion and formation of 2,5-FDCA based on
HPLC analysis, information on isolated yields and the
purification of 2,5-FDCA are lacking. Monomer purity is a
crucial parameter in step-growth polymer synthesis, and
therefore non-trivial.
In order to explore the versatility and limitations of the
Henkel reaction for the synthesis of polymer-grade furanic
building blocks, our primary aim was to reproduce the results
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Table 1 Disproportionation of potassium 2-furoate (K-2F) by varying temperature and catalysts
Selectivityd
Entry
Catalysta
Time (h)
Temp.b (uC)
K-2F conversion (%)
FDCAs formationc (%)
2,5-FDCA (%)
2,4-FDCA (%)
1
2
3
4
5
6
CdI2
CdI2
CdI2
CdI2
ZnI2
ZnCl2
5.5
5.5
5.5
5.5
5.5
5.5
200
235
260
280
260
260
0
0
0
0
32
92
88
48
39
30
91
60
46
37
80
70
71
86
84
20
30
29
14
16
a
b
22 mol% catalyst (10 mol% of catalyst gave poor yields, increasing the catalyst loading to two-fold, 22 mol% gave sufficient yields). All
c
d
reactions were performed in a Kugelrohr glass oven. Total percentage of furandicarboxylic acids formed, calculated from NMR. Individual
selectivity of furandicarboxylic acids.
NMR analysis of the crude product from the CdI2 catalyzed
reactions showed, besides the expected product (the di-
potassium salt of 2,5-FDCA 2a) and residual starting material
K-2F, an additional unknown aromatic compound. The 1H
NMR spectrum shows one proton signal corresponding to 2,5-
FDCA 2a and 2 proton signals corresponding to the unknown
compound (Fig. 1). GC-MS and HRMS analysis indicated
comparable molecular ion mass values and fragmentation
patterns for both compounds. Based on these observations we
concluded that the unknown compound should be an
asymmetrical isomer of 2,5-FDCA 2a. The only possibilities
are either 2,4-FDCA or 2,3-FDCA. However, the lack of coupling
between both proton signals (singlets) is a strong indication
for the 2,4-isomer. Furthermore, 2,4-FDCA synthesized accord-
ing to a known literature procedure28 gave an identical NMR
spectra, confirming our assumptions.
Since 2,4-FDCA has never been reported (and identified) as
a by-product in the Henkel reaction of K-2F, including the
recent paper by Pan et al., this came as a surprise. Although
Ratusky reported the formation of furan dicarboxylic acids at
380 uC starting from K-2F and CdI2 as catalyst, no evidence is
given on the isomeric composition.25 Furthermore, the
reported reaction temperatures are over 100 uC higher
compared to our case. Ratusky also observed partial isomer-
ization of 2,5-FDCA to other isomers at the same high
temperatures, comparable to benzene dicarboxylic acid
salts.20,29
The presence of significant amounts of 2,4-FDCA (up to
30% of total FDCA yield) is intriguing, and raises the question
why this has not been observed by others before. The answer
could lie in the work-up procedure, which is based on
dissolution of the crude reaction mixture in water, followed
by (selective) precipitation of 2,5-FDCA by acidification. NMR
analysis of the acidic water layer indicated the presence of 2,4-
FDCA 3b and trace amounts of 2,5-FDCA 3a. The aqueous
solubility of 3b is apparently significantly higher than that of
the scarcely soluble 2,5-isomer 3a, which could prevented
previous authors from observing the 2,4-isomer 3b.20,22,23
Evaporation of the aqueous layer afforded the majority of
2,4-FDCA 3b and 2,5-FDCA 3a. Residual 3a was removed from
this mixture by soxhlet extraction using acetone followed by
chloroform affording 3b in 85% purity. In order to improve the
separation/purification procedure, a crude reaction mixture
was refluxed in methanolic HCl, giving the corresponding
dimethyl esters. GC-MS and NMR analysis showed beside the
presence of dimethyl 2,5- and 2,4-FDCA also small amounts of
dimethyl 3,4-FDCA and 2,3-FDCA (,5% based on 1H NMR and
GC-MS analyses). This mixture was subsequently separated by
column chromatography (silicagel, ethylacetate/petroleum
ether, 6/94), giving dimethyl 2,5-FDCA 4a and dimethyl 2,4-
FDCA 4b in high purity (¢ 99 + % from GC and NMR analyses)
(Scheme 4, Fig. 2).
In order to gain insight into the reaction mechanism and
the factors governing selectivity, a number of reference
reactions as well as reactions with alternatively substituted
furoate salts were performed under typical reaction condi-
tions, using CdI2 as catalyst. Surprisingly, changing the
position of the carboxylic acid group from the 2- to the
3-position resulted in a complete lack of reactivity (Table 2,
entry 1). Introduction of a methyl substituent on either the 3-
or the 5 position of K-2F also resulted in a complete lack of
reactivity (Table 2, entry 2 & 3). These results indicate a strong
influence of electronic factors in the Henkel reaction of furans.
The previously suggested carbanion character of the reactive
intermediate will be destabilised by the presence of electron
releasing substituents like methyl groups. Steric factors
Fig. 1 The 1H NMR spectrum of disproportionated crude reaction mixture.
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The selectivity of the reaction towards 2,5-FDCA or 2,4-
FDCA formation appears to depend on both the reaction
temperature and the type of catalyst (Table 1).
Whereas comparison of selectivity is only valid at compar-
able levels of conversion, time variation experiments (vide
infra) showed that maximum conversion is attained relatively
quickly, making it next to impossible to meet this require-
ment. With the CdI2 catalyst selectivity towards 2,4-FDCA 2b is
increased by 50% when the reaction temperature is increased
from 235 uC to 250 uC (and upward) indicating that formation
of the 2,5-isomer has a lower activation energy. The metal
component of the catalyst also has a pronounced effect on
selectivity. Both zinc salts showed a 50% drop in selectivity
towards 2,4-FDCA 2b compared CdI2 at the same temperature,
albeit at lower conversion.
Fig. 2 1H and 13C NMR spectra of isolated (a) dimethyl 2,5-furandicarboxylate
4a and (b) dimethyl 2,4-furandicarboxylate 4b in CDCl3.
The effect of reaction time on conversion and product
distribution was investigated by performing multiple reactions
with varying time intervals ranging from 10 min. to 12 h. NMR
analysis of the crude reaction mixtures indicates that conver-
sion starts after approx. 20 min. at 260 uC, and reaches a
maximum within less than 1 h yielding 90% of the theoretical
yield of mixtures of potassium salts of 2,5-FDCA 2a and 2,4-
FDCA 2b (see graph in supporting information).
This is in contrast to the results obtained by Andrisano,22
and Pan et al.23 where the reaction apparently took 3–4 h to
reach completion, yielding only 2,5-FDCA. NMR spectra of the
reactions performed from 2 h till 12 h showed no character-
istic changes in the spectrum or the 2,4-FDCA/2,5-FDCA ratio
(Fig. 3), indicating that the disproportionation is a very rapid
reaction.
Recrystallization of purified dimethyl 2,4-FDCA 4b yielded
crystals that allowed for structural characterisation by single
crystal X-ray diffraction (Fig. 4).
When comparing the structural parameters of 2,4-FDCA
with those of 2,5-FDCA and TA several critical aspects with
regard to their (potential) application as polymer building
block come to mind.27 Although 2,5-FDCA is widely advocated
appears less prominent, given the fact that both the 3- and
5-methyl 2-furoate were found to be unreactive. High negative
charge density is also more likely to be stabilised at C2 due to
the neighbouring oxygen group, than at C3.
The thermal stabilities of the dipotassium salts of the three
FDCA isomers (2,5-, 2,4- and 3,4-FDCA) were also investigated
under typical reaction conditions. No significant weight loss
was found after reaction, indicating that decarboxylation did
not occur to any significant extent. 1H NMR analysis of the
crude mixtures after reaction showed that no detectable
isomerisation had occurred at 260 uC (Table 2, entry 4–6).
Furthermore, the TGA analyses show that these FDCA isomers
(2,5-, 2,4- and 3,4-FDCA) were stable under the typical reaction
temperatures. In contrast, others have reported dynamic
interconversion of (hetero)aromatic diacids under Henkel
reaction conditions at much higher temperatures.25 Our
results suggest that under more mild conditions the observed
FDCA isomers are formed via independent reaction pathways
instead of by interconversion.
Table 2 Effect of substrates variation and thermal stability measurements
Entrya
1
Substrateb
Conversionc (%)
—
Resultd (NMR)
Entrya
4
Substrateb
Stabilityf
Resultd (NMR)
no changeg
e
—
stable/no weight loss
e
2
3
—
—
—
5
6
stable/no weight loss
stable/no weight loss
no changeg
no changeg
e
—
a
b
All reactions were performed in a Kugelrohr glass oven, using CdI2 as catalyst (22 mol %) at 260 uC for 5.5 h. The potassium salts of the
corresponding (di) acid were prepared by drop-wise addition of KOH in ethanol to a solution of the free acid in ethanol at room temperature.
c
The suspension was stirred further for 12 h, filtered and dried in vacuum oven at 50 uC. The conversion was determined by mass balance
after the end of reaction and also followed by 1H NMR analyses. The crude product was analysed by 1H NMR. No product was observed in
d
e
1H NMR and the starting materials were recovered. The dipotassium salts of the furandicarboxylic acids were mixed with the catalyst and
f
heated at 260 uC for 5.5 h and subsequently analysed by 1H NMR. The starting material was recovered.
g
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Fig. 5 Atom labelling scheme for 2,4-FDCA-Me, 2,5-FDCA and TA.
packing and rate of crystallisation. Investigations on the effect
of incorporating 2,4-FDCA in step-growth polymers are
currently in progress.
Overall, reports on the synthesis of 2,4-FDCA 3b are very
scarce in the literature. Several authors,17c,31 described the
synthesis of 2,4-FDCA from methyl coumalate. Although in
principle also bio-based, this compound is more expensive
when compared to the present precursor 2-furoic acid. Pearl
et al.32 described the use of vanillin to synthesize 2,4-FDCA 3b,
however in very low isolated yield (,3%). During the
preparation of this manuscript, we came across the recent
report by Citron et al.33 describing the synthesis of dimethyl
2,4-furandicarboxylate in 3-steps starting from furan-3-carbal-
dehyde. The strategy involves low temperature lithiation
followed by carboxymethylation using methyl chloroformate,
and subsequent oxidation using sodium chlorite giving the
target compound in an overall yield of 16%. Hence, we can
conclude that our current methodology for obtaining high
purity 2,4-FDCA by disproportionation of K-2F is one of the
most straightforward and effective routes to date.
Fig. 3 1H NMR spectra (in D2O) of crude products recorded at different time
intervals.
Fig. 4 Molecular structure of 4b in the crystal. Displacement ellipsoids are
drawn at the 50% probability level.
as potential alternative for TA in e.g. polyesters, their structural
differences can have
a profound influence on polymer
Conclusions
properties. The higher order of symmetry of TA, together with
the linearity, generally benefit chain stiffness and rate of
crystallisation. At first glance 2,5-FDCA (c 129u) is more
comparable to isophthalic acid (IPA, c 120u) than to TA (c
180u) (Table 3). Yet on average 2,5-FDCA polyesters are more
comparable to the TA analogues.4e,30 The distance between the
carboxylic acid groups in 2,4-FDCA is close to that in the 2,5-
isomer, yet this parameter is more crucial for application in
polyamides (for lining up hydrogen bonding) than in
polyesters. The angle c (projected angle between carboxylic
acid groups) is however closer to that of TA, making the
molecule more linear (Table 3, Fig. 5). The lack of symmetry on
the other hand can also have a profound effect on crystal
The Henkel reaction of potassium-2-furoate (based on agri-
cultural residues) is an efficient alternative method for
obtaining furan dicarboxylic acids (FDCA), in total (unopti-
mised) isolated yields of up to 90%. In contrast to the scarce
literature reports, besides the 2,5-isomer 3a the 2,4-isomer 3b
is also formed. Selectivity towards either of the major FDCA
isomers can be tuned by reaction temperature and type of
catalyst used. Both isomers can be efficiently separated and
purified after esterification of the crude product. The structure
of 2,4-FDCA dimethyl ester 4b was confirmed by single crystal
X-ray diffraction which showed that 2,4-FDCA has unique
structural properties. The effect of incorporation of the 2,4-
FDCA isomer in step-growth polymers is currently under
investigation.
Table 3 Comparison of structural parameters of 2,4-FDCA-Me, 2,5-FDCA and TA
derived from single crystal X-ray diffraction studies
Experimental section
Name
Symmetry Da (Å) ab (u)
cc (u)
Materials
2,4-FDCA-Me C2/c
5.075
4.830
5.731
171.80 (15)/161.35 (15) 150
2,5-FDCA40
TA41
Cs
Ci
154.7
179.2
129.4
180
2-furoic acid (98% Sigma-Aldrich), 3-furoic acid (98% Sigma-
Aldrich), 5-methyl 2-furoic acid (97% Sigma-Aldrich), 3-methyl
2-furoic aicd (97% Sigma-Aldrich), dimethyl 3,4-furandicar-
boxylate (98% Sigma-Aldrich), cadmium iodide (99% Sigma-
Aldrich), zinc chloride (99.99% Sigma-Aldrich), zinc iodide
a
b
Interatomic distance between C1–C6 (carboxylic acid groups). C1–
c
C2–C5/C6–C5–C2 angle. Projected angle between the C1–C2 bond
and the C5–C6 bond.
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(99.99% Sigma-Aldrich), hydrochloric acid (reagent grade,
37%, Sigma-Aldrich), potassium hydroxide (reagent grade,
90%, flakes, Sigma-Aldrich), acetone (Merck, p.a.), methanolic
HCl (3N, Sigma-Aldrich), chloroform (Merck, p.a.), methanol
(Merck, p.a.), petroleum ether (Acros Organics, 40–60 uC), ethyl
acetate (Acros Organics, 99 + %), silicagel (Alfa Aesar, 230–400
mesh), chloroform-D (99.8 atom % D Aldrich), deuterium
oxide (99.9 atom % D Aldrich), magnesium sulfate (Acros
Organics, 99% extra pure, dried, contains 3 to 4 moles of water
were used as received, unless stated otherwise.
temperature of 150(2) K up to a resolution of (sin h/l)max =
0.61 Å21. Intensity data were integrated with the SAINT
software.34 Absorption correction and scaling was performed
based on multiple measured reflections with SADABS (correc-
tion range 0.58–0.75).35 1563 Reflections were unique (Rint
=
0.044), of which 1315 were observed [I . 2s(I)]. The structure
was solved by Direct Methods using the program SIR-97.36
Least-squares refinement was performed with SHELXL-97
against F2 of all reflections.37 Non-hydrogen atoms were
refined freely with anisotropic displacement parameters.
Hydrogen atoms were located in difference Fourier maps
and refined with a riding model. 120 Parameters were refined
with no restraints. R1/wR2 [I . 2s(I)]: 0.0411/0.1096. R1/wR2
[all refl.]: 0.0516/0.1200. S = 1.048. Residual electron density
between 0.24 and 0.22 e Å23. Geometry calculations and
checking for higher symmetry was performed with the
PLATON program.38
Methods
Fourier transform infrared (FT-IR) spectra were obtained on a
Varian Scimitar 1000 FT-IR spectrometer equipped with a Pike
MIRacle ATR Diamond/ZnSe single reflection plate and a
DTSG-detector. The measurement resolution was set at 4
cm21, and the spectra were collected in the range 4000–650
cm21 with 32 co-added scans. NMR spectra were recorded on a
Bruker Avance III spectrometer operating at 400.17 MHz (1H)
and 100.62 MHz (13C). Gas chromatography was performed on
an Interscience Focus GC equipped with an AS 3000 series
auto sampler. Injection volume 1 mL. Injector temperature 275
uC. Split ratio 1 : 33. Column flow (at 275 uC) 50 mL min21
helium. GC column: Restek Rxi-5 ms, 30 m 6 0.25 mm 6 0.25
mm. GC program (2,5-FDA.mth); hold 2 min at 70 uC, ramp 10
uC min21, final temperature 300 uC, hold 2 min. Total run time
27 min. Detector; FID at 300 uC. GC-MS analysis was
performed on an Interscience TraceGC Ultra GC with AS3000
II autosampler (He carrier gas, flow 1 ml min21, split flow 20
ml min21; Restek GC column Rxi-5 ms 30 m 6 0.25 mm 6
Synthesis of furan-2,5-dicarboxylic acid 3a and furan-2,4-
dicarboxylic acid 3b
Potassium 2-furoate 1 (10.0 g, 0.066 mol) and CdI2 (5.3 g, 0.014
mol) were manually ground well in a mortar and pestle under
nitrogen atmosphere and charged into a round-bottom flask
and inserted into the Kugelrohr glass oven. The setup was
slowly rotated at 260 uC under a continuous (low) flow of
nitrogen. During the course of the reaction, the furan formed
was collected in the cooling flask which was pre-cooled at 278
uC using a dry-ice/acetone bath, yielding furan quantitatively
(95% of the theoretical amount). After 5.5 h, the mixture had
turned into black colored solid lumps. The reaction was
stopped and allowed to cool down to room temperature over 1
h. The black mass was subsequently dissolved in deminer-
alized water (100 mL). Residual insoluble black material was
filtered off and the pale yellow filtrate was acidified using 12 N
HCl until the pH attains 1. Precipitated 2,5-FDCA was filtered
off using a G-3 glass filter. The product was subsequently dried
in vacuum oven at 40 uC over Sicapent for 12 h affording an
off-white powdered material.
0.25 mm; GC program hold 3 min at 50 uC, ramp 7.5 uC min21
final temperature 330 uC) connected to Interscience
,
a
TraceDSQ II XL quadrupole mass selective detector (EI, mass
range 35–500 Dalton, 150 ms sample speed). Electrospray
Ionisation (ESI) mass spectrometry was carried out using a
Bruker microTOF-Q instrument in positive ion mode (capillary
potential of 4500 V). Melting points were measured on a
Thermal Fisher Scientific IA 9000 Series digital melting point
apparatus. Differential Scanning Calorimetry (DSC) measure-
ments were conducted on a Perkin Elmer Diamond series DSC.
The temperature range used was 20 uC up to 250 uC at a
heating rate of 10 uC min21. Short path distillations and
disproportionation reactions were carried out using a Buchi
Glass Oven B-585 (Kugelrohr). The thermal stability of
dimethyl furan-2,5- and 2,4-dicarboxylate was determined by
Yield 3a: 6.47 g, 62.2%; purity: .95% (from GC and NMR
analyses); m.p. . 250 uC (lit.39 m.p. 342 uC); 1H NMR (400
MHz, DMSO-d6, d (ppm)): 7.27 (s, 2H); 13C NMR (100 MHz,
DMSO-d6, d (ppm)): 158.94, 147.07, 118.43; IR (neat, u cm21):
3147 (C–H), 3125 (C–H), 2878 (br, OH), 1667 (CLO), 1571, 1523,
1417, 1268, 1224, 1187, 1163 and 1041; GCMS (EI): found
156.0513. C6H4O5 requires 156.0059.
thermogravimetric analysis (TGA) with
a
STA 6000
The pale yellow aqueous filtrate was evaporated to dryness
under reduced pressure using a rotary film evaporator. The
crude product of 2.9 g, was subjected to soxhlet extraction
using acetone for 6 h at reflux temperature. The residue was
evaporated to dryness under reduced pressure affording a
mixture of predominantly 2,4-FDCA 3b and trace amounts of
2,5-FDCA 3a. The crude mixtures of diacids were subjected
again to soxhlet extraction using chloroform for 3 h at reflux
temperature. The residue was evaporated to dryness under
reduced pressure using a rotatory film evaporator affording
2,4-FDCA 3b with 85% purity.
(Simultaneous Thermal Analyser) from PerkinElmer
Instrument. The samples were heated from 30 to 600 uC at a
heating rate of 10 uC min21 under a nitrogen flow of 40 mL
min21
.
X-ray crystal structure determination of dimethyl furan 2,4-
dicarboxylate 4b
C8H8O5, Fw = 184.14, colourless needle, 0.75 6 0.26 6 0.14
mm3, monoclinic, C2/c (no. 15),
a = 15.2706(19), b =
11.0750(13), c = 11.6658(14) Å, b = 122.294(3)u, V = 1667.8(3)
Å3, Z = 8, Dx = 1.467 g cm23, m = 0.12 mm-1. 14085 Reflections
were measured on a Bruker Kappa ApexII diffractometer with
sealed tube and Triumph monochromator (l = 0.71073 Å) at a
Yield 3b: 2.8 g, 27%; m.p. 266 uC (lit.32 m.p. 264 uC); 1H NMR
(400 MHz, DMSO-d6, d (ppm)): 8.43 (s, 1H), 7.26 (s, 1H); 13C
NMR (100 MHz, DMSO-d6, d (ppm)): 170.91, 166.11, 149.77,
15684 | RSC Adv., 2013, 3, 15678–15686
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147.58, 125.67, 114.32; IR (neat, u cm21): 3150 (C–H), 3120 (C–
H), 2880 (br, OH), 1683 (CLO), 1580, 1520, 1418, 1262, 1191,
1165, 1036; GCMS (EI): found 156.0674. C6H4O5 requires
156.0059.
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Synthesis of dimethyl furan-2,5-dicarboxylate 4a and dimethyl
furan-2,4-dicarboxylate 4b
5.0 g of a crude reaction mixture containing potassium salts of
2-furoic acid, 2,5-FDCA, 2,4-FDCA and CdI2, were refluxed in
methanolic HCl (1.2 M) (50 mL) at 75 uC for 6 h. After
completion, the reaction mixture was cooled down to room
temperature and the insoluble metal salts were filtered off.
The deep yellow colored residue was evaporated under
reduced pressure using a rotatory film evaporator. The
resulting yellow viscous oily product was dissolved in chloro-
form (100 mL) and washed with water (3 6 50 mL), dried over
magnesium sulfate, filtered and evaporated to dryness afford-
ing the dimethyl esters of 2,5-FDCA and 2,4-FDCA and
2-methyl furoate. The crude products were separated by
column chromatography over silica gel using a mixture of
ethyl acetate and petroleum ether (6 : 94). The dimethyl 2,5-
furandicarboxylate 4a and dimethyl 2,4-furandicarboxlyate 4b
were obtained in high purity (.95%, from NMR and GC
analyses). The diesters were further purified by recrystalliza-
tion from methanol.
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7 R. Fittig and H. Heinzelman, Ber. Dtsch. Chem. Ges., 1876,
9, 1198.
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Engelen, Europ. Pat., EP1263081, 2012; (b) F. van der Klis,
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Dimethyl furan-2,5-dicarboxylate 4a
1
Yield, 3.7 g, 67%; mp 110 uC (lit.25 mp 107–111 uC); H NMR
(400 MHz, CDCl3, d (ppm)): 7.21 (s, 2H), 3.93 (s, 6H); 13C NMR
(100 MHz, CDCl3, d (ppm)): 158.34 (CLO), 146.61, 118.38,
52.29; IR (neat, u cm21): 3118 (C–H), 3021 (C–H), 2963 (C–H),
1721 (CLO), 1583, 1516, 1432, 1379, 1272, 1237, 1191, 1161,
1133, 1030; GCMS (EI): found 184.0488. C8H8O5 requires
184.0372.
Dimethyl furan-2,4-dicarboxylate 4b
1
Yield, 1.6 g, 28%; mp 109 uC (lit.31 mp 109–110 uC); H NMR
(400 MHz, CDCl3, d (ppm)): 8.10 (s, 1H), 7.46 (s, 1H), 3.91 (s,
3H), 3.86 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3, d (ppm)):
162.28 (CLO), 158.47 (CLO), 150.08, 145.48, 120.97, 117.02,
52.23, 51.92 ppm; IR (neat, u cm21): 3138 (C–H), 3015 (C–H),
2962 (C–H), 1719 (CLO), 1585, 1519, 1436, 1394, 1270, 1195,
1138, 1081; HRMS (ESI): MH+, found 185.0442. C8H9O5
requires 185.0450.
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Acknowledgements
Dr G. Frissen, W. Teunissen and Elwin Janssen are gratefully
acknowledged for NMR, GC-MS and HR-MS measurements
respectively. This work was (partly) financed by Braskem S/A.
Dr A.T. Morita, Dr R. Coimbra and Dr P.A. Coutinho are
thanked for fruitful discussions.
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