Catalytic conversion of furfural into a 2,5-furandicarboxylic acid-based polyester with total carbon utilization

Article abstract of DOI:10.1002/cssc.201200652

One divided into two combined into one: The catalytic conversion of furfural into a 2,5-furandicarboxylic acid-based polyester, linked by the disproportionation of furoate to furan and 2,5-furandicarboxylate, is reported. In this manner, all carbons are utilized, demonstrating the success of combining a platform molecule from C₅ sugars (furfural) to one from C ₆ sugars (2,5-FDCA). Copyright

Full text of DOI:10.1002/cssc.201200652

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DOI: 10.1002/cssc.201200652
Catalytic Conversion of Furfural into a 2,5-
Furandicarboxylic Acid-Based Polyester with Total Carbon
Utilization
Tao Pan, Jin Deng, Qing Xu, Yong Zuo, Qing-Xiang Guo, and Yao Fu*[a]
The conversion of renewable biomass resources into fuels,
polymers, and fine chemicals provides solutions for the grow-
ing shortage of fossil resources, environmental pollution, and
a possible crisis in energy supply.[1] In particular, the C6 and C5
sugars in lignocellulosic biomass (e.g., d-glucose, d-fructose, d-
xylose) are promising precursors for the production of synthet-
ic polymers. Recently, a number of examples have demonstrat-
ed that sugars and related platform molecules obtained from
sugars can be converted into a series of polymer monomers
and polymers. For example, Holm and co-workers have report-
ed the preparation of lactic acid (the monomer for polylactic
acids) from fructose and glucose.[2] Heeres and co-workers
have reported the conversion of a biomass-derived platform
molecule 5-hydroxymethylfurfu-
studied,[11] the large-scale commercial production of 5-HMF has
been hampered by several difficulties (e.g., the cost of the
process,[8a] the low efficiency of the glucose-to-fructose trans-
formation,[12,13] and the difficulty encountered in the isolation
of 5-HMF[14]).
On the other hand, furfural, which is a bulk biomass-based
chemical with a global annual production of 280000 tonnes,[15]
is readily produced from d-xylose in xylan or hemicellulose.[16]
It can be converted into a variety of derivatives with potential
applications as fuels, polymers, pharmaceutical intermediates,
and pesticides.[17] It is therefore of interest to develop a route
to produce of 2,5-furandicarboxylic acid-based polyester from
furfural, and thereby avoid the short supply of 5-HMF.
ral (5-HMF, the dehydration
product of C6 sugars) into the
nylon-6 monomer caprolac-
tam.[3] Furthermore, Thomas and
co-workers have reported the
synthesis of alternating polyes-
ters from dicarboxylic acids (the
products of carbohydrate fer-
mentation).[4]
Furan-based polymers, which
may be produced from saccha-
rides, have the potential to re-
place some petroleum-based
polymers.[5] In particular, 2,5-fur-
andicarboxylic acid-based poly-
esters represent one of the most
Scheme 1. Multistep conversion of furfural into the 2,5-furandicarboxylic acid-based polyester PBF.
important types of furan-based polymers.[6,7] For example,
poly(ethylene 2,5-furandicarboxylate) (PEF) is considered to be
a potential substitute for poly(ethylene terephthalate) (PET).[8]
The key monomer 2,5-furandicarboxylic acid (2,5-FDCA) is
a promising biobased platform molecule.[1c,9] Until now, the
only developed route for the production of 2,5-FDCA involves
the selective oxidation of 5-HMF.[10] Although the preparation
of 5-HMF from glucose and cellulose has been extensively
Herein, we report a multistep catalytic conversion of furfural
into poly(butylene 2,5-furandicarboxylate) (PBF) by dispropor-
tionation of furoate to furan and 2,5-furandicarboxylate
(Scheme 1). The route includes four steps: (1) catalytic oxida-
tion of furfural into furoate; (2) catalytic disproportionation of
furoate to furan and 2,5-furandicarboxylate; (3) catalytic hydro-
genation and hydrolysis of furan to 1,4-butanediol (1,4-BDO);
and (4) polymerization of 1,4-BDO with 2,5-FDCA to PBF. The
key step of the route is the selective conversion of furoate into
furan and 2,5-furandicarboxylate, which provides an important
linkage between commercialized biobased furfural and the
furan-based polymer monomer 2,5-FDCA. Compared to the ox-
idation method for the production of 2,5-FDCA from 5-HMF,
the furfural route is more practical due to the abundant supply
of the feedstock. Furthermore, the key monomer 1,4-BDO can
be produced from furfural, leading to total carbon utilization
of furfural.
[a] T. Pan, J. Deng, Q. Xu, Y. Zuo, Prof. Q.-X. Guo, Prof. Dr. Y. Fu
Anhui Province Key Laboratory of Biomass Clean Energy
Department of Chemistry
University of Science and Technology of China
Hefei 230026 (PR China)
Fax: (+86)-551-360-6689
E-mail: fuyao@ustc.edu.cn
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200652.
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The first step of our route is the catalytic oxidation of furfu-
ral into furoate. Although a number of oxidants (e.g., KMnO4)
could be used for this oxidation, catalytic aerobic oxidation is
more suitable for biomass conversion and therefore explored
here. This catalytic aerobic oxidation of furfural to furoate has
previously been reported, employing supported metal as cata-
lyst.[18] The oxidation process was performed in aqueous phase
under alkaline conditions, using air or oxygen as oxidant with
a series of heterogeneous catalysts, including nanometer-scale
particles of CuO, CuO-Ag2O, Pt/C, Pd/C, Pt/C/CuO-Ag2O, and
Pd/C/CuO-Ag2O.[18] In our experiments, we found that nano-
scale CuO (furfural/KOH=1:1, 40 wt% KOH, 30 wt% of catalyst
intake, 608C, 30 min, air) gave 93% selectivity to furoate at
100% furfural conversion.
Table 1. Disproportionation of furoate to furan and 2,5-furandicarboxyla-
te.[a]
Entry
Cat
T
[8C]
t
FCAM
conv. [%]
Selectivity to
2,5-FDCA [%]
[min]
1
2
3
4
5
6
7
8
9
10[b]
11[c]
12[d]
13[e]
14
15
16
17
18
19
20
21
22
23
24
25
26
–
280
280
280
280
280
280
280
280
280
280
280
280
250
280
270
260
250
240
230
220
250
250
250
250
250
250
180
30
30
30
30
30
30
30
30
1
56
23
18
3
15
6
2
0
46
0
0
0
11
0
0
ZnCl2
CdCl2
CuCl2
LaCl3
SnCl2
NiCl2
ZnO
Zn(OAc)2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
14
8
0
The second step of the route is the catalytic disproportiona-
tion of furoate to furan and 2,5-furandicarboxylate. Baecke and
other groups have reported the preparation of dipotassium
terephthalate in the presence of catalysts such as cadmium or
zinc salts by reacting dipotassium phthalate or potassium ben-
zoate at 400–4508C with the carboxyl transposition reaction,[19]
known as the Henkel reaction. The Henkel reaction has been
used for the industrial production of terephthalic acid and 2,6-
naphthalene dicarboxylic acid.[20] A similar reaction using furo-
ate as the substrate could be envisaged to generate furan and
2,5-furandicarboxylate. After the first step of the catalytic oxi-
dation of furfural to furoate, the solid furoate was obtained by
filtering the catalyst from the solution and removing water
without acidification. In a subsequent step, the catalyst was
added to the solid furoate for solid-phase disproportionation
reaction to produce furan and 2,5-furandicarboxylate.
We screened different catalysts and studied the effects of the
reaction temperature and reaction time (Table 1).
30
30
30
17
29
19
73
15
44
67
86
76
65
46
7
71
77
77
76
76
48
27
21
85
75
67
61
55
35
18
9
29
37
38
45
69
180
180
180
180
180
180
180
180
30
60
90
120
150
240
[a] The potassium furoate (FCAK) 2 g, cat 0.2 g, dry ice 1.0 g, the conver-
sion of furoate and the yield of 2,5-FDCA were determined by HPLC.
[b] Without dry ice. [c] 0.1 g HZSM-5 was added. [d] 0.01 g NaOH was
added. [e] The substrate is sodium furoate (FCANa).
To avoid the decomposition of furoate and 2,5-furandicar-
boxylate under the reaction conditions, the decomposition
temperatures of potassium furoate and dipotassium 2,5-furan-
dicarboxylate were determined by thermogravimetric analysis
(TGA). The TGA curves showed that the substrate and product
are stable below 3308C. First, a control experiment showed
that almost no conversion of furoate took place without cata-
lyst (Table 1, entry 1). Then the disproportionation of furoate
was tried with Henkel reaction catalysts of zinc salts and cad-
mium salts (entries 2 and 3). Cadmium chloride showed almost
no catalytic activity at 2808C, while zinc chloride showed a sig-
nificant catalytic activity. Copper salts have been shown to cat-
alyze the decarboxylation of aromatic carboxylate under ho-
mogeneous conditions,[21] but showed no selectivity to 2,5-
FDCA here (entry 4). Two Lewis-acid catalysts and a transition
metal catalyst (i.e., nickel salt) were tested, also, but did not
give good results (entries 5–7). According to previous studies
of the Henkel reaction, the anion of the catalyst has a pro-
nounced impact on the conversion.[19] Zinc oxide and zinc ace-
tate (entries 8 and 9) were used as catalysts, but no products
could be detected (zinc acetate was in a molten state at the
reaction temperature).
carbon dioxide may be involved in the reaction. To elucidate
the effect of CO2, an additional experiment was performed
without addition of dry ice (Table 1, entry 10). The result
showed that CO2 has a beneficial effect on reactivity and selec-
tivity. Furthermore, solid acid and base were added as addi-
tives, but their effect on the results was found to be detrimen-
tal (entries 11 and 12). Finally, sodium furoate was tried at
2508C for 180 min, and similar results were obtained
(entry 13).
The reaction temperature was further optimized (220~
2808C, Table 1, entries 14–20) while extending the reaction
time to 180 min. With the decrease of the reaction tempera-
ture, the conversion of furoate gradually decreased. Mean-
while, the selectivity to 2,5-FDCA increased at first and then
dropped. The highest selectivity to 2,5-FDCA (86%) was ach-
ieved at 2508C with a conversion of 61% (entry 17). A longer
reaction time led to an increase in furoate conversion at 2508C
with an unchanged selectivity to 2,5-FDCA (30~240 min, en-
tries 21–26). The other product furan was in the gas state
under the reaction conditions and gaseous products were col-
lected and analyzed by GC after the reaction. GC analysis re-
vealed no gaseous products other than CO2 and furan.
Considering the possible mechanism of the Henkel reac-
tion,[22] the disproportionation reaction comprises an initial de-
carboxylation and
a subsequent carboxylation. Therefore,
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lytic disproportionation of furoate to furan and 2,5-furandicar-
boxylate are obtained with a selectivity to 2,5-FDCA of 86%, at
61% furoate conversion. This demonstrates a new pathway of
converting furfural, a bulk biomass-based chemical, into 2,5-
FDCA, a key monomer for 2,5-furandicarboxylic acid-based
polyesters, with a good overall selectivity (80% for the two-
step process). Moreover, furan is converted into 1,4-BDO at
a very high selectivity of 70%, at 99% furan conversion. The
furan-based polyester PBF is thus prepared from 2,5-FDCA and
1,4-BDO with total carbon utilization of furfural. This route
demonstrates an important, yet so far neglected strategy of
linking a platform molecule from C5 sugars (furfural) to one
from C6 sugars (2,5-FDCA). Further studies should enable to
open up a new pathway for biorefineries.
Scheme 2. Routes for the conversion of furan into 1,4-butanediol.
For the third step of the conversion of furan to 1,4-butane-
diol (1,4-BDO), we propose two routes (Scheme 2). One route
comprises three steps. First, furan is catalytically oxidized to
maleic anhydride (MA), which is then hydrogenated to 1,4-
BDO in two steps [hydrogenation to butyrolactone (GBL) and
then hydrogenation to 1,4-BDO]. The two-step hydrogenation
of MA to 1,4-BDO is currently a mature technology in indus-
try.[23] Thus, the only step missing from our proposed route is
the catalytic oxidation of furan to MA. Based on the industrial
catalytic oxidation of n-butane to MA, vanadium-containing
catalysts (VOPO4, VOHPO4, and VPO) were selected in our ex-
periments, with air as oxidant. The highest selectivity to MA
(48%) was obtained when using VPO as catalyst at 74% furan
conversion, at 4008C.
Experimental Section
Catalytic Oxidation of Furfural to Furoate: A 40% KOH solution and
6 g of furfural (distilled before use) were added dropwise to a reac-
tion flask containing 2 g of copper oxide nanoparticles at 608C
(furfural/KOH=1:1). Air was passed into the reaction mixture at
the same time (40 mLminÀ1). The mixture was stirred for 10 min
after adding KOH and furfural. After reaction, the mixture was fil-
trated to remove the catalyst and the filtrate was analyzed by
HPLC. The filtrate was concentrated in vacuo to remove water and
the crude material was dried in vacuo at 508C.
Because the selectivity to the product 1,4-BDO was not high
in the first route, another route was tested (Scheme 2). This
route involves direct hydrogenation of furan into 1,4-BDO,[24]
and Scheme 2 shows the possible pathway of catalytic hydro-
genation and hydrolysis of furan to the 1,4-BDO. The main by-
products were tetrahydrofuran (THF) and n-butanol. A range of
catalysts (Pt/C, Ru/C, Pd/C, Raney-Ni, Ru-Ni/C and Ru-Ni/Si-Al)
was explored in a batch autoclave at 1608C and 30 bar hydro-
gen pressure, with water as solvent. The main product was
THF and the yield of target product 1,4-BDO was less than 5%.
A small quantity of Brønsted acid (H2SO4) as catalytic additive
was added to promote the ring-opening hydrolysis of 2,3-dihy-
dro-furan, but the results were unsatisfactory (less than 2%
yield of 1,4-BDO). Recently, a number of reports on rhenium-
containing catalysts applied to the hydrogenolytic ring-open-
ing of furan and THF have appeared.[25] Thus, the hydrogena-
tion of furan was carried out by using bimetallic catalysts con-
taining Re. The best result was obtained when using Re–Ru/C
(4% Re and 1% Ru) at 1608C under a hydrogen pressure of
3 MPa for 3 h, giving a maximum selectivity to 1,4-BDO of 70%
at 99% furan conversion. The remaining byproducts included
25% of THF and a small amount of n-butanol. The final step
was polycondensation of 2,5-FDCA and 1,4-BDO to PBF. The
preparation and characterization of furan-based polyester from
2,5-FDCA and diol (e.g., ethylene glycol, 1,3-propylene glycol,
and 1,4-BDO) have been reported.[26,27]
Catalytic Disproportionation of Furoate: 2 g of furoate, 2 g of cata-
lyst, and 1 g of dry ice were mixed and added into a 25 mL stain-
less steel autoclave. The reactor was stirred and heated to a certain
temperature for a setting time (see Table 1). After cooling, the gas
product was collected for GC analysis, and the solid product was
poured into water and analyzed by HPLC.
Hydrogenation of Furan to 1,4-BDO: 1 g of furan, 0.5 g of catalyst,
a certain quantity of additive and 5 mL of water were added into
a 25 mL stainless steel autoclave (Parr). The reactor was flushed
three times with hydrogen and pressurized to 3 MPa. The reaction
mixture was stirred and heated to 1608C for 3 h. After cooling to
room temperature, the pressure was released carefully and the
products were analyzed by GC.
Acknowledgements
This work was supported by the National Basic Research Pro-
gram of China (2012CB215305), the National Natural Science
Foundation of China (21172209), and the Chinese Academy of
Science (KJCX2-EW-J02).
Keywords: biomass
· furfurals · polymers · renewable
resources · synthetic methods
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