Highly efficient formic acid-mediated oxidation of renewable furfural to maleic acid with H2O2

Article abstract of DOI:10.1039/c6gc03020c

Maleic acid and its anhydride are important intermediates in the chemical industry produced on a multimillion tonne-scale annually. The synthesis of maleic acid/anhydride from renewable biomass resources such as furfural and 5-hydroxymethylfurfural is highly desirable for the sustainability of human society. Most of the previously reported processes for maleic acid/anhydride synthesis from biomass suffer from low efficiency, complicated conditions and poor catalyst recyclability. Herein, we demonstrate a highly efficient and simple system for the synthesis of maleic acid from furfural. An excellent yield (95%) of maleic acid was achieved under mild conditions in this very simple system which requires only H₂O₂ as an oxidant in formic acid solvent. Under similar conditions, an 89% yield of maleic acid was achieved from biomass-derived 5-hydroxymethylfurfural. This study presents a novel synthetic method and a promising process for maleic acid production from renewable biomass resources.

Full text of DOI:10.1039/c6gc03020c

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DOI: 10.1039/C6GC03020C
Journal Name
ARTICLE
Highly Efficient Formic Acid-Mediated Oxidation of Renewable
Furfural to Maleic Acid with H O
2
2
Received 00th January 20xx,
Accepted 00th January 20xx
*
Xiukai Li, Ben Ho, Diane S. W. Lim and Yugen Zhang
DOI: 10.1039/x0xx00000x
Maleic acid and its anhydride are important intermediates in chemical industry that represent millions of tons of market
demands. The synthesis of maleic acid/anhydride from renewable biomass resources such as furfural and 5-
hydroxymethylfurfural is highly desirable for the sustainability of human society. Most of the previously reported
processes for maleic acid/anhydride synthesis from biomass suffer from low efficiency, complicated conditions and poor
catalyst recyclability. Herein, we demonstrate a highly efficient and simple system for the synthesis of maleic acid from
furfural. An excellent yield (95%) of maleic acid was achieved under mild conditions in this very simple system which
www.rsc.org/
2 2
requires only H O as an oxidant in formic acid solvent. Under similar conditions, an 89% yield of maleic acid was achieved
from biomass-derived 5-hydroxymethylfurfural. This study presents a novel synthetic method and a promising process for
maleic acid production from renewable biomass resources.
The unsustainable strain placed on non-renewable resources developed for furfural oxidation to maleic anhydride in acetic acid
caused by the rapidly growing global economy has triggered a solvent under 20 bar of O , the leaching of the vanadium species
2
desperate need to find substitutes for the world’s limited reserve of into the solution resulted in the MA yield decreasing from 65% to
[3g]
fossil fuels. The wide availability and enormous potential of biomass 23% in the fifth reaction with recycled catalyst. The highest yield
has made it the ideal alternative source of hydrocarbon needed in of MA (78%) from furfural to date in an aqueous phase oxidation
[1]
the chemical industry. Maleic acid (MA), fumaric acid (FumA), and was obtained over a titanium silicate-1 (TS-1) catalyst with H O as
2
2
their anhydride (MAnh) are critical chemical intermediates in the an oxidant, and significant Ti leaching was also observed in the
[3h]
manufacture of unsaturated polyester resins, vinyl copolymers, reaction. The problems of low efficiency, complicated conditions,
surface coatings, plasticizers, and pharmaceuticals. As such, their or poor catalyst recyclability facing these processes severely limit
synthesis from renewable biomass resources is highly sought their potential for industrial application.
[2]
after. 5-Hydroxymethylfurfural (HMF) and furfural, which are
derived from cellulosic biomass, are the most extensively studied
platform molecules for the green synthesis of maleic
[3]
acid/anhydride and other organic acids. In terms of cost, furfural
is more attractive as it can be produced on a large scale from
abundant agriculture waste while HMF is obtained on small scales
[
3a, 3g, 4]
from the dehydration of sugars.
Various homogeneous and heterogeneous catalytic systems
with molecular oxygen or hydrogen peroxide as oxidants have been
developed for the conversion of furfural to MA or MAnh. Currently,
liquid phase oxidations of furfural with O2 or H O using
2
2
homogeneous or heterogeneous acid catalysts typically offer MA in
[3a-c, 5]
<
50% yield.
Up to 70% MA yield can be achieved in the gas
phase oxidation of furfural over vanadium-based catalysts at low
[6]
Scheme 1. Oxidation of furfural and HMF to maleic acid with H O .
Reaction conditions: furfural or HMF (1 mmol), formic acid (4 ml),
feed concentrations (as low as 1 vol%). Heterogeneous catalytic
conversion of furfural in the liquid phase is also challenged by the
potential for catalyst leaching. In the case of a Mo-V-O catalyst
2 2
3
1% H O (1 ml), 98% H SO (d only, 1 mol% to H O ).
2 2 2 4 2 2
To date, all transformations of furfural or HMF to MA utilizing
[2c]
H O or O require a single or combination catalyst.
A metal
2
2
2
Dr. X. Li, Mr. B. Ho, Dr. Diane S. W. Lim and Dr. Y.G. Zhang.
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos,
Singapore 138669, Singapore. E-mail: ygzhang@ibn.a-star.edu.sg
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
catalyst-free system would simplify product isolation and avoid the
issues of catalyst deactivation, offering an attractive alternative to
catalyzed furfural-to-MA methods provided that the reaction
efficiency is comparable to or better than those in the catalytic
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J. Name, 2016, 00, 1-3 | 1
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Pleas eG dr eo e nn o Ct ha de jmu si st tmr yargins
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ARTICLE
Journal Name
cases. To the best of our knowledge, there has been no report of butyric acid was a modest 37% while conducting thVeiewreAraticctleioOnnlinine
metal catalyst-free transformation of furfural or HMF to MA. Herein, formic acid returned an unprecedented D9O1I:%10.y1i0e3ld9/Co6fGCM0A30, 2f0aCr
we demonstrate a highly efficient and simple system for the exceeding those (< 78%) from the aforementioned catalytic
[3a-c, 3g, h, 5-6]
conversion of furfural and HMF to MA. Using H O as the oxidant processes.
Organic acid solvents may play multiple roles
2
2
and formic acid as the solvent, MA was obtained in an in the oxidation of furfural with H O . Organic acids are readily
2
2
[8]
unprecedented 95% yield from furfural and 89% yield from HMF oxidized to organic peracids by H O even at room temperature.
2
2
(
Scheme 1).
Due to the electron-withdrawing effect of the carbonyl group, the -
In our study, furfural oxidation with H O was initially performed O-O- bond in organic peracids should be stronger than that in H O .
2
2
2
2
in acetic acid as it is a good solvent for the homogeneous and Moreover, in the acidic environment (lower pH value) of organic
heterogeneous catalytic oxidation of furfural and HMF to maleic acid solvents, the oxidation potential should increase according to
[
3g, 3i, 7]
ø
anhydride.
Complete furfural conversion and 65% yield of MA the Nernst equation (E = E – 0.059 pH). Thus, H O and organic
2
2
was observed in acetic acid solvent under conditions of peracids should be stronger oxidants in the organic acid solvents
H O /furfural = 10 (mol/mol) at 100 °C over 4 h (Table S1). Formic than in H O or other less acidic solvents. The acidities of carboxylic
2
2
2
acid was observed as the only side product (Figure S1). Using pure acids decrease with the length of the carbon chain (pKa values:
water as a solvent without any catalyst afforded maleic acid in a formic acid, 3.75; acetic acid, 4.76; propionic acid, 4.87; butyric acid,
yield of 15% (Table S1). Other solvents showed no selectivity 4.82), and this is in good agreement with the observed MA yields in
towards MA (Table S1) but 2-furoic acid and some unidentified the four solvents.
products were observed.
The amount of formic acid is also important for high MA yields.
From Figure 1B, MA yields increased with increasing amount of
formic acid and > 90% yield of MA was achieved at formic acid to
H O volume ratios greater than or equal to three. The H O
A
100
2
2
2
2
oxidation of formic acid to performic acid is a reversible reaction
and a high concentration of formic acid favors the peracid at
80
60
40
20
0
Furfural conv.
MA yield
[8a, 8c]
equilibrium.
The presence of additional water in the system is
detrimental to the reaction. A volume ratio of water/formic acid 1:1
yielded 27% of MA, while a water/formic acid ratio of 3:1 yielded 13%
of MA (Table S2). Additional water changes the pH value of the
solvent and decreases the rate of performic acid formation. In the
cases where formic acid was absent or present in low
concentrations (5.5 vol%), 2(5H)-furanone was the major product
Formi
Acetic
Propion
Butyric acid
[9]
c ac
aci
ic a
and the yield for maleic acid was < 17%. As formic acid can also be
id
d
cid
[10]
produced from biomass, the H O oxidation of furfural to MA in
2
2
formic acid is essentially a green synthetic method. Additionally, the
side products of this reaction are formic acid and water, which
enables facile product isolation from the reaction system. After the
reaction, the solvent could be removed by evaporation to give
crude MA as a white solid (Figures S2 and S3).
B
1
00
9
8
7
6
5
4
3
0
0
0
0
0
0
0
100
90
80
70
100 °C
100
80
0
.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
60 °C
Formic acid / H O (v/v)
60
50
60
2
2
2
5 °C
4
0
0
Figure 1. (A) Furfural oxidation to maleic acid in organic acids with
H O as oxidant. Reaction conditions: furfural (1 mmol), organic acid
4 ml), 31% H O (1 ml), 100 °C, 4 h. (B) Furfural oxidation to maleic
acid with H O in different amounts of formic acid. Reaction
2
2
2
4
0
0
0
2
4
6
8
10 12 14 16 18 20
(
2 2
Time / h
40 50
3
2
2
conditions: furfural (1 mmol), 31% H O (1 ml), 100 °C, 1 h.
0
10
20
30
60
70
2
2
Time / min
It was therefore presumed that carboxylic acids play a critical
role in furfural oxidation to MA with H O . Thus, C to C carboxylic
acids were tested as solvents with the results shown in Figure 1A.
Complete furfural conversions were achieved in all carboxylic acid
solvents but the yields of MA decreased linearly with increasing
length of the carboxylic acid carbon chain. The yield of MA in
Figure 2. Kinetics of furfural oxidation to maleic acid at different
reaction temperatures. Reaction conditions: furfural (1 mmol),
formic acid (4 ml), 31% H O (1 ml).
2
2
1
4
2
2
2
| J. Name., 2016, 00, 1-3
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The kinetics of furfural oxidation in formic acid at different 95% was achieved at a H
ARTICLE
O
2 2
/furfural ratio of 6 at 60 °C. It should be
2
2
View Article Online
noted that the optimal H O /furfural ratioDoOfI:610i.s10lo39w/Cer6GthCa0n30t2h0aCt
reaction temperatures are presented in Figure 2. The reaction was
very fast when performed at 100 °C, requiring just 5 minutes to
achieve a 72% yield of MA. The maximum yield of MA (93%) was
achieved in 40 min, while a 91% of MA yield was still retained on
prolonged reaction times of one to four hours, indicating that MA is
stable in this system. The reaction rate was slower when the
experiment was performed at 60 °C, with the maximum yield of MA
[3h]
(
7.5) employed in the TS-1 catalyzed system.
The observed
relationship between MA yield and H O /furfural ratio at different
2
2
reaction temperature can be rationalized as a balance of furfural
oxidation and peroxide decomposition. There were more gas
bubbles in the reactor at a reaction temperature of 100 °C, less
bubbles at 60 °C, and almost no bubbles at 25 °C, indicating faster
decomposition of the peroxides at higher reaction temperatures. At
(
95%) achieved in 4 h. The reaction even proceeds at room
1
00 °C, the oxidation of furfural is fast; however, rapid peroxide
temperature (25 °C) and a MA yield of 80% can be obtained within decomposition leads to a high consumption of H O . At room
2
2
2
0 h. The high efficiency of the current H O oxidation of furfural to temperature, the decomposition of the peroxide oxidants was
2 2
MA is remarkable compared with previously reported catalytic slower; however, the rate of furfural oxidation was also slower and
[
3g, h, 5a, 5c]
therefore larger amounts of H O were required to obtain high MA
processes.
The current reaction can also be performed in
2
2
yields. Thus, the reaction temperature of 60 °C allows the minimum
amount of H O to be employed. To further verify this deduction,
open system with a slightly lower MA yield (Table S3).
2
2
the reaction was studied at different temperatures with the
H O /furfural ratio fixed at 6. The results are shown in Figure 3B and
confirm the optimal MA yield was achieved at 60 °C.
A
2
2
100
90
80
70
60
50
40
30
9
0
0
H SO
2
4
8
Amberlyst 15
No additive
2
6
1
5 C, 20 h
0 C, 8 h
00 C, 1 h
70
60
50
40
30
20
2
4
6
8
10
12
14
H O / furfural
2
2
B
100
10
1
2
3
4
5
6
7
9
8
7
6
5
4
3
0
0
0
0
0
0
0
Time / h
Figure 4. Oxidation of furfural to maleic acid with H O in formic
acid at room temperature. Reaction conditions: furfural (1 mmol),
formic acid (4 ml), 31% H O (1 ml), 98% H SO (5.3 µl, 1 mol% to
2
2
2
2
2
4
+
H
O
2
), Amberlyst 15 (42.5 mg, 2 mol% of H to H
O ), 25 °C.
2
2 2
One attractive feature of this method is that the reaction
proceeds even at room temperature, albeit at a much slower rate of
MA formation. As a reaction performed at room temperature is
more energy efficient and cost effective for industrial application,
we tried to improve the reaction rate at room temperature. Based
on the hypothesis that acid catalysts can promote the oxidation of
formic acid to performic acid and enhance the acidity of the
system, we added soluble and solid acid additives into the system
in order to promote the reaction. It can be seen from Figure 4 that
both H SO and Amberlyst 15 had a positive impact on MA
1
0
20 30 40 50 60 70 80 90 100 110
Temperature / °C
Figure 3. Oxidation of furfural to maleic acid with H O in formic
acid at (A) different H O /furfural ratios. Reaction conditions:
furfural (1 mmol), total volume of formic acid and H O , 4 ml
formic acid 2.8−3.7 ml; 31% H O 0.3−1.2 ml); (B) different reaction
temperatures with H O /furfural = 6. Reaction conditions: furfural
1 mmol), formic acid (4.4 ml), 31% H O (0.6 ml), 60 °C, 8 h.
2
2
[
8]
2
2
2
2
(
2
4
2
2
formation. The addition of H SO had a more pronounced effect,
2
4
2
2
where the addition of 1 mol% afforded an 83% yield of MA within 7
h at room temperature, as compared to the 80% MA yield achieved
in 20 h in the absence of acid additives (Figure 2). The wide range of
reaction temperatures which can be employed for the current
synthetic method allows flexibility in practical application.
(
2 2
As H O is consumed in the transformation of furfural to MA,
2
2
the amount of H O input is crucial for the economy of this
synthetic route. Thus, the effect of varying H O to furfural ratio
2
2
2
2
In order to probe the influence that substrate structure has on
the current reaction, various furan compounds were tested. All of
the furan compounds shown in Scheme 2 can be derived from
biomass platform molecules except unsubstituted furan. 2,5-
Diformylfuran (DFF), 2,5-formylfurancarboxylic acid (FFCA), 2,5-
furandicarboxylic acid (FDCA), and 2,5-dimethylfuran (DMF) can be
was studied under various conditions (Figure 3). Longer reaction
times were used at lower reaction temperatures according to the
kinetic studies shown in Figure 2. The optimal H O /furfural ratios
were 10, 6, and ≥ 12, respectively, at 100 °C, 60 °C, and 25 °C. The
yield of MA when the reaction was conducted at 60 °C is generally
higher than that achieved at 100 °C and 25 °C. The best MA yield of
2
2
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[
11]
derived from the oxidation or hydrogenation of HMF, while 2- using H O as the oxidant and formic acid as the solvent. Reaction
2
2
View Article Online
DOI: 10.1039/C6GC03020C
[
12]
furoic acid is obtained from the oxidation of furfural.
Using the
temperature significantly affects the reaction rate and dictates the
optimal H O to furfural ratio. A H O /furfural ratio of 6 is optimal
method we developed, almost quantitative yield (99%) of MA was
achieved from furan. When the furan ring was substituted with
aldehyde group(s), as was the case with furfural, HMF, and DFF,
good to excellent yields of MA (77−91%) were still achieved. When
the furan ring was substituted with carboxylic acid group(s) - in
FFCA, 2-furoic acid, and FDCA - the yield of MA decreased notably
to lower than 33%. With methyl substitution (DMF), the yield of MA
was only 5%. Thus, carboxylic acid and methyl-substituents on the
furan ring have a deleterious effect on MA formation compared to
aldehyde substituents.
2
2
2 2
for achieving the highest yield of MA (95%) at 60 °C. Remarkably,
acid additives promote the reaction rate even at room temperature.
This method is also effective for the synthesis of MA from other
biorenewable platform molecules such as HMF, from which an
unoptimized 89% yield of MA was achieved.
Acknowledgements
This work was supported by the Institute of Bioengineering and
Nanotechnology (Biomedical Research Council, Agency for Science,
Technology and Research (A*STAR), Singapore), Biomass-to-
Chemicals Program (Science and Engineering Research Council,
A*STAR, Singapore).
Notes and references
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Scheme 2. Oxidation of various substituted furan compounds to MA
in formic acid with H O . Reaction conditions: substrate (1 mmol),
2
2
[2] a) X. K. Li, W. J. Ji, J. Zhao, Z. B. Zhang and C. T. Au, J. Catal. 2006,
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formic acid (4 ml), 31% H O (1 ml), 100 °C, 4 h.
2
2
The mechanism for acid-catalyzed H O oxidation of
2
2
furfural/HMF to MA is still under debate with two main reaction
[
2c, d, 3a, 3c-i, 5a, 5c, 6-7]
pathways that have been proposed.
pathway is the 2-furoic acid route, that is, furfural is first oxidized to
-furoic acid, which is then converted to furan by decarboxylation.
Further oxidation of furan by H O affords maleic acid (Scheme
The first
[
3] a) J. Lan, Z. Chen, J. Lin and G. Yin, Green Chem. 2014, 16, 4351-
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7516–17522; d) Z. Du, J. Ma, F. Wang, J. Liu and J. Xu, Green
2
4
2
2
[3h, 13]
S1).
In our reaction system, 2-furoic acid was not observed and
1
the reaction using 2-furoic acid as the starting material yielded a
much lower yield of maleic acid (15% yield). Therefore, the 2-furoic
acid route could be discarded. The second pathway starts with
Baeyer-Villiger oxidation of furfural to furanol formate ester, which
is hydrolyzed to formic acid and 2-hydroxyfuran. 2-Hydroxyfuran
isomerizes to furan-2(5H)-one which is oxidized to maleic acid
Chem. 2011, 13, 554-557; e) S. Li, K. Su, Z. Li and B. Cheng,
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[2c, 3h]
(
Scheme S2).
Because both formic acid and the key
intermediate furan-2(5H)-one were observed in our control
reactions (Figures S1 and S4), it is likely that the present reaction
proceeds through the Baeyer-Villiger oxidation route. Fully
elucidating the reaction mechanism requires further efforts and this
will be the focus of our future research.
2
013, 6, 826-830; k) X. Li, X. Lan and T. Wang, Catal. Today 2016,
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We have demonstrated a highly efficient and simple system for
the conversion of furfural to maleic acid with H O as oxidant and
formic acid as the solvent. The nature of the solvent was crucial for
the high efficiency of MA formation and formic acid was found to
be the best simple organic acid. Under mild reaction conditions, an
unprecedented 95% yield of maleic acid was achieved from furfural
2
2
[
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