Selective Aerobic Oxidation of 5-(Hydroxymethyl)furfural to 5-Formyl-2-furancarboxylic Acid in Water

Article abstract of DOI:10.1002/cssc.201600060

A simple, cheap, and selective catalyst based on copper/cerium oxides is described for the oxidation of 5-(hydroxymethyl)furfural (5-HMF) in water. An almost quantitative conversion (99 %) with excellent (90 %) selectivity towards the formation of 5-formyl-2-furancarboxylic acid, a platform molecule for other high value chemicals, is observed. The catalyst does not require any pretreatment or additives, such as bases, to obtain high yield and selectivity in water as solvent and using oxygen as oxidant. When a physical mixture of the oxides is used, low conversion and selectivity are observed. Air can be used instead of oxygen, but a lower conversion rate is observed if the same overall pressure is used, and the selectivity remains high. The catalyst can be recovered almost quantitatively and reused. Deactivation of the catalyst, observed in repeated runs, is due to the deposition of humins on its surface. Upon calcination the catalyst almost completely recovers its activity and selectivity, proving that the catalyst is robust. Aerobic workout: A mixed oxide of copper and cerium (CuOCeO₂) is prepared by a milling method. The catalyst shows high activity towards the selective conversion of 5-(hydroxymethyl)furfural (5-HMF) in water, using O₂ as oxidant and without any external additives. Deposition of humins during reaction causes deactivation, but upon calcination the original activity is mostly restored.

Full text of DOI:10.1002/cssc.201600060

DOI: 10.1002/cssc.201600060
Communications
Selective Aerobic Oxidation of 5-(Hydroxymethyl)furfural
to 5-Formyl-2-furancarboxylic Acid in Water
Maria Ventura,[a] Michele Aresta,[b, c] and Angela Dibenedetto*[a, d]
A simple, cheap, and selective catalyst based on copper/
cerium oxides is described for the oxidation of 5-(hydroxyme-
thyl)furfural (5-HMF) in water. An almost quantitative conver-
sion (99%) with excellent (90%) selectivity towards the forma-
tion of 5-formyl-2-furancarboxylic acid, a platform molecule for
other high value chemicals, is observed. The catalyst does not
require any pretreatment or additives, such as bases, to obtain
high yield and selectivity in water as solvent and using oxygen
as oxidant. When a physical mixture of the oxides is used, low
conversion and selectivity are observed. Air can be used in-
stead of oxygen, but a lower conversion rate is observed if the
Scheme 1. Molecules derived from 5-HMF and their use.
same overall pressure is used, and the selectivity remains high.
The catalyst can be recovered almost quantitatively and
reused. Deactivation of the catalyst, observed in repeated runs,
is due to the deposition of humins on its surface. Upon calci-
nation the catalyst almost completely recovers its activity and
selectivity, proving that the catalyst is robust.
zation to fructose, which is dehydrated to afford 5-HMF,[3] or
even directly from cellulose.[4] It is the platform for a large vari-
ety of high value chemicals,[5] such as 2,5-diformylfuran (DFF),
5-hydroxymethyl-2-furancarboxylic acid (HMFCA), 5-formyl-2-
furancarboxylic acid (FFCA), 2,5-furandicarboxylic acid (FDCA),
levulinic acid (LA), and formic acid (FA) (Scheme 1). Currently,
several research groups are making efforts towards optimizing
the synthesis of 5-HMF[6] and investigating the reactions in
which it is involved.[7] 5-HMF has two functional groups,
namely an alcoholic and an aldehydic moiety, which allows the
production of a variety of chemicals via oxidation, hydrogena-
tion, condensation, and reduction.[8]
The increasing demand for energy is causing a rapid depletion
of fossil carbon sources (coal, oil, gas) while increasing the at-
mospheric level of CO2, necessitating the exploration of alter-
native carbon sources for chemicals and fuels, such as renewa-
ble carbon. Biomass, which is cheap and abundant in nature, is
considered the most promising alternative to fossil carbons for
the production of chemicals and, in some cases, fuels.[1]
Because naturally available terrestrial biomass contains at least
75% carbohydrates, much emphasis has been placed on devel-
oping efficient approaches to transform biomass-sourced car-
bohydrates (C6 and C5) into value-added chemicals. Among
the numerous chemical building blocks derived from renewa-
ble resources, 5-hydroxymethylfurfural (5-HMF)[2] is one of the
most investigated (Scheme 1). 5-HMF can be obtained either
from second-generation (cellulose-derived) glucose via isomeri-
Selective oxidation of 5-HMF to obtain DFF or FDCA has
been much investigated owing to the versatility of these prod-
ucts as monomers, intermediates for pharmaceuticals, ligands,
and others applications.[9] Because of difficulties with isolating
the pure compound, the selective oxidation to FFCA has not
attracted much attention despite its great potential as an inter-
mediate. Oxidation of 5-HMF, due to the presence of two dif-
ferent reactive functionalities, can generate several derivatives
that can find different applications.[10] A key objective of re-
search, for a possible industrial application of processes,[11] is to
find catalytic systems that are cheap, selective, and do not
generate waste. In recent years, the use of oxygen or air as oxi-
dant has been extensively investigated. Concerning the oxida-
tion to DFF, metal oxides have been used with good yields of
DFF. Using ruthenium over carbon nanotubes and N,N-dime-
thylformamide (DMF) as solvent,[12] yields of DFF of more than
90% have been reported. Yields of 80% have been reported
using Au/MnO2 in DMF as solvent.[13] Ce1ÀxBixO2 can produce
yields higher than 64% towards HMFCA in basic media.[14]
Nitrogen-based compounds[12b,15] play an important role when
copper[16] catalysts are used, because of the capacity of this
metal to coordinate to nitrogen ligands that enhance its reac-
tivity. Gold[17] is the most-often used metal, because of its
great activity and selectivity towards either DFF or FDCA. Sev-
[a] M. Ventura, A. Dibenedetto
CIRCC
Via Celso Ulpiani, 27, 70126 Bari (Italy)
[b] M. Aresta
Department of Chemical and Biomolecular Engineering
National University Singapore
Engineering Drive 4, 117585 Singapore (Singapore)
[c] M. Aresta
University of Bath
Bath BA2 7AY (United Kingdom)
[d] A. Dibenedetto
Department of Chemistry
University of Bari
Campus Universitario, 70126 Bari (Italy)
The ORCID identification numbers for the authors of this article can be
found under http://dx.doi.org/10.1002/cssc.201600060.
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eral studies using this metal have been published, employing
different kinds of solvents and in the presence or absence of
bases. A 70:28 ratio of FFCA/FDCA was obtained when using
a mixture of Pd/C and bismuth.[18] Yields of 98% toward FDCA
were obtained with Pd/C[19] in a basic medium. Some studies
suggest that the catalyst may undergo modification by action
of the formed acids, causing poor selectivity; therefore, a basic
medium is often used. However, bases may promote other ad-
verse reactions that reduce the availability of the starting
polyol. In a recent publication,[20] an excellent yield of FDCA
(99%) from 5-HMF using hydrotalcite-supported gold nanopar-
ticles, in water at 368 K, under atmospheric oxygen pressure
without addition of base was reported. Nevertheless, there are
only very few reports on the use of water as solvent, in ab-
sence of bases, despite its environmental friendliness. Reac-
tions in water are difficult to control, because of the reactivity
of the aldehydes that can be hydrated and produce oxidation
or decomposition products. Considering this state of affairs,
a selective oxidation of 5-HMF to FFCA, working in water and
using dioxygen as oxidant, is missing.
of O2, using the mixed oxide as catalyst, the reaction rate of
oxidation of 5-HMF decreased (conversion=13% after 3 h),
but FFCA was formed as the sole product (selectivity=99%).
Interestingly, when a physical mixture of the two oxides in
a 1:1 molar ratio is used (Table 1, entry 6), the conversion and
yield are lower and the selectivity towards FFCA is much de-
creased. This implies that the mixed oxides form a new entity
and not a simple mixture of oxides. Entry 7 shows that if the
reaction is carried out in the absence of catalyst and in the
same conditions (O2 pressure and temperature) as used in
entry 5, formic acid is obtained as major product after 24 h (se-
lectivity 64%).
These results prompted us to make a comparative study of
the acid–base properties of the single oxides and mixed
oxides. Literature studies on the mechanism of 5-HMF oxida-
tion using precious metal-catalysts demonstrate that the sup-
port must have a high basicity, in order to avoid the addition
of external bases, and a moderate surface area.[22] To under-
stand the importance of these parameters in our case, a surface
characterization was carried out. Table 2 shows the Brunauer–
We investigate catalysts built on mixed oxides targeting the
selective oxidation of 5-HMF to any of the products from DFF
to FDCA in Scheme 1. Herein, we present results on the oxida-
tion of 5-HMF to FFCA using water as solvent. The oxidation is
performed by cheap, earth-abundant metal oxides (copper/
cerium) without addition of bases under mild conditions, using
molecular oxygen or air as oxidant.[21]
Table 2. BET surface area, basicity, and acidity of metal oxides.
Entry
Solid
CO2 adsorbed
NH3 adsorbed
BETsurface
area [m2 gÀ1
]
[mLgÀ1
]
[mLgÀ1
]
1
2
3
4
5
CuO[a]
0.25
2.35
0.053
2.23
3.15
0.23
3.66
0.23
3.84
2.24
9.53
66.13
10.67
51.54
28.49
[a]
CeO2
CuO[b]
We first performed the oxidation of 5-HMF with single
oxides, CuO or CeO2, either obtained commercially or synthe-
sized in our laboratory. As shown in Table 1 the reaction with
CuO (entry 1) achieves good selectivity towards FFCA (40%)
with a moderate conversion (33%). CeO2 (entry 2) shows
a higher selectivity towards FFCA (76.6%), but a lower conver-
sion. CuO is more active than CeO2 and it also forms several
other products, such as DFF, HMFCA and FDCA, which are not
formed when CeO2 is used. Performing the oxidation with the
mixed oxide CuO·CeO2 leads to a quantitative conversion of 5-
HMF (>99%, entry 5), high yield (90%), and good selectivity
towards FFCA, without other oxidation or decomposition prod-
ucts, although H2O was used as solvent and no external base
was added to the reaction medium. When air is used instead
[b]
CeO2
CuO·CeO2
[a] Commercial oxide. [b] Synthesized in our laboratory.
Emmett–Teller (BET) surface area and acid and basic sites, ex-
pressed through the volume of NH3 and CO2 uptake and re-
lease, respectively. A comparison of entries 1 and 3 reveals that
commercial and synthetized CuO have substantial different ba-
sicities, corresponding to markedly different catalytic activities
(Table 1, entries 1 and 3). The lower basicity of the synthesized
CuO (entry 3) as compared to the commercial one causes
a lower activity towards the formation of FFCA. The properties
shown by CeO2 are more uniform when the commercial and
synthesized oxides are compared (entries 2 and 4). The mixed
oxide CuO·CeO2 has a higher basicity than the single oxides
and a lower acidity than CeO2. Hence, this simple mixture of
the two oxides does not have reproducible values owing to
the heterogeneity of the mixture. This indicates that the mixed
oxide is not a simple mixture of oxides but really a new entity.
Its BET area (28.49 m2 gÀ1) is intermediate between those of
CeO2 (51–66 m2 gÀ1) and CuO (ca. 10 m2 gÀ1).
Table 1. Catalytic oxidation of 5-HMF using various metal oxides as cata-
lyst.[a]
Entry Solid
catalyst
t
Conversion Yield/selectivity [%]
[h] [%]
DFF
HMFCA FFCA
FDCA
1
2
3
4
5
6
7
CuO[b]
15 33.05
15 19.9
15 28.7
15 21.3
1.7/5.1 2.3/6.9 13.2/40 8.4/25.4
[b]
CeO2
CuO[c]
0
0
15.3/76.6 0
2.4/8.5 1.2/4.4
0.1/0.6 0
0
5.62/17.4
[c]
The new properties of the mixed oxide with respect to the
parent oxides has a direct correlation to its catalytic activity.
The increased basicity and good acidity of the catalyst result in
an increased conversion yield (99%) and selectivity (90%) to-
wards FFCA. Figure 1 shows the correlation between basic
sites and selectivity towards FFCA, while Figure 2 correlates
the number of acid sites to the selectivity. This evidences that
CeO2
18.9/88.7 0
CuO·CeO2
CuO+CeO2 15 5.3
No catalyst 24 93
3
99
0
0
0
0
90/90
1.2/22.6 3.2/60.4 0
7.4/7.9
0
[d]
0
0
[a] Reaction conditions: [5-HMF]i =0.2m, 0.05 g of catalyst, 7 mL of water,
PO2 =0.9 MPa, temperature=383 K. [b] Commercial oxide. [c] Synthesized
in our laboratory. [d] Physical mixture of CuO and CeO2.
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working in water, with dioxygen (using air leads to more slug-
gish kinetics) as oxidant and in the absence of external bases.
The oxidation of 5-HMF may involve either the oxidation of an
aldehyde or that of the alcohol function. Figure 3 shows the
Figure 3. Kinetics of HMFCA and FFA formation and evolution of 5-HMF con-
version. Catalyst: CuO·CeO2. Reaction conditions: [5-HMF]i =0.2m, 0.05 g of
Figure 1. Selectivity toward FFCA vs VCO adsorbed.
2
catalyst, 7 mL of water, PO =0.9 MPa, T=383 K. ~: Conversion of 5-HMF;
^
:
2
&
HMFCA yield; : FFCA yield.
evolution of the reaction with time: HMFCA is first formed
(through the aldehyde moiety oxidation), and is then convert-
ed into FFCA.
Our attempts to investigate the role of the reaction parame-
ters in the oxidation to products other than FFCA, has allowed
to discover and interesting concentration effect. In fact, when
the initial concentration of 5-HMF in the reaction mixture was
lowered from 2.0m to 0.2m, as Figure 4 shows, HMFCA was
Figure 2. Selectivity toward FFCA vs VNH adsorbed.
3
what matters is not the absolute value of a single property,
namely “acidity” or “basicity”, but the balance of the two with
“basicity” prevailing on “acidity”. Moreover, the ratio of strong
basic sites to strong acid sites is important.
The maximum selectivity was obtained with the catalysts
that have the highest basicity coupled to a significant acidity.
Notably, solids with very poor acidity have poor selectivity and
activity. Additionally, a large BET area is not recommended for
a good activity
Figure 4. Conversion of 5-HMF and yield of different products with
CuO·CeO2 as catalyst. The effect of changing the initial concentration of
HMF is shown. Reaction conditions: [5-HMF]i =0.02m, [5-HMF]i =0.2m, and
Preliminary analyses by X-ray diffraction (XRD) clearly show
the presence of a single nanocrystalline phase in the mixed
oxides. Peaks relevant to tenorite-CuO and cerianite-CeO2 (ac-
cording to the database of the Joint Committee on Powder
Diffraction Standards (JCPDS)) are evident. Their shapes indi-
cate the presence of a nanocrystralline phase. Such studies are
still ongoing for an in-depth analysis of the change of XRD
spectra with the composition of mixed oxides and for a com-
parison before and after reaction.
[5-HMF]i =2m, 5 mL of H2O, 0.05 g of catalyst, PO =0.9 MPa.
2
formed instead of FFCA, also if at moderate yield, but high se-
lectivity. We also investigated whether or not the same catalyst
would have been able to oxidize 5-HMF to FDCA. Attempts
made by using the same reaction conditions used in the oxida-
tion to FFCA and simply increasing the reaction time failed, as
FFCA was decomposed on the long term and polymerization
products were formed.
An elemental analysis of the catalyst, carried out by energy
dispersed X-ray (EDX) spectroscopy, shows that the mixed
oxide has a composition 56.1% cerium and 43.71% copper,
with respect to calculated values of 69.68% cerium and
30.32% copper for a 1:1 CeO2/CuO composition. However, the
results discussed above show that mixed oxides can be an in-
teresting solution to modulate the acid/base catalysts proper-
ties and drive the reaction in the direction of a target product
The catalyst recovery and reusability was also tested using
the mixed oxide CuO·CeO2 (Figure 5). In a typical test, the cata-
lyst was allowed to react for 3 h, and then filtered, washed
with water (7 mL) three times, and reused in a new run. The
activity of the catalyst decreased with increasing the number
of cycles (1–4 in Figure 5) most probably because humins gen-
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Experimental Section
Materials: Cerium(IV) ammonium nitrate ꢀ98% (by titration); cer-
ium(IV) oxide nanopowder, <25 nm particle size (BET), 99.95%
trace rare earth metals basis; copper(II) nitrate trihydrate puriss.,
99–104%; copper(II) oxide 99.9% trace metals basis; 2,5-furandicar-
boxaldehyde ꢀ97%; 5-formyl-2-furoic acid 99%; 5-hydroxymethyl-
2-furancarboxylic 99%, were ACS-grade reagents purchased from
Sigma–Aldrich. 5-(Hydroxymethyl)furfural ꢀ99% was prepared as
reported earlier.[6a]
Analytical methods: 5-HMF and derivatives were analyzed by
using a Jasco HPLC equipped with a UV detector at 284 nm and
a Phenomenex Rezex RHM monosaccharide H+(8%) 300”7.8 mm
column at 343 K. A 0.005 N solution of sulphuric acid was used as
the mobile phase at a 0.5–0.9 mLminÀ1. The concentration of 5-
HMF and reaction products were determined by using a RI detec-
tor. Surface characterization of the catalysts was carried out by
using a Pulse ChemiSorb 2750 Micromeritics instrument. Analyses
of the acid/basic sites was carried out using NH3 or CO2, respective-
ly, as probe-gas using 100 mg of catalyst and He as carrier gas
Figure 5. Catalyst reusability in the oxidation of 5-HMF using CuO·CeO2.
erated in the reaction may remain attached to the solid. We
noted a change of the color of the catalyst in its deactivated
form. When the deactivated catalyst was calcined at 823 K for
3 h (5c in Figure 5), the catalytic activity was almost recovered
at comparable selectivity. This proved true also in additional
runs (6c in Figure 5), also. Notably, when the catalyst is deacti-
vated, a rapid degradation of 5-HMF to FA takes place. Inter-
estingly, the catalyst was recovered at 99% and EDX analysis
of the catalyst after the runs confirmed that there was no sig-
nificant metal loss after the reaction: only traces of copper
were observed. We conclude that the catalyst is deactivated by
deposition of organic species on its surface, more than by
leaching: the activity can be recovered by calcination that
burns the organics and cleans the surface.
(30 mLminÀ1). The samples were pretreated under N2 (30 mLminÀ1
)
flow and 673 K. The Pulse Chemisorb was performed with NH3 or
CO2. BET area was determined using N2/He as a carrier gas at 273 K
followed by heating up to 923 K. X-ray powder diffraction (XRD)
patterns of the samples were determined using a Rigaku powder
diffractometer (CuKa). All XRD patterns were collected in the 2q
range 108–1208 at a scanning rate of 0.0088sÀ1. The elemental anal-
ysis of CuO·CeO2 was carried out by using a Shimadzu 720P Energy
Dispersive X-Ray spectrometer and a calibration curve made with
the standard single oxide, CuO and CeO2.
Synthesis of single oxides CuO or CeO2: 3 mmol of copper(II) ni-
trate trihydrate (0.72 g) or cerium(IV) ammonium nitrate (1.64 g),
were calcined for 3 h at 823 K giving a dark solid (CuO) or pale-
yellow solid (CeO2). The solids were transferred into a flask and
stored under N2 to prevent uncontrolled surface deterioration prior
to catalysis.
Such results are of interest because they indicate that speci-
alized catalysts are needed for producing different oxidation
products of 5-HMF. While a copper/cerium mixed oxide is
a good catalyst for the oxidation of 5-HMF to HMFCA and
FFCA, we have isolated DFF with high selectivity (ꢀ99%)
when working with other mixed oxides.[23] Our studies towards
the discovery of the best conditions for a selective and effi-
cient conversion of 5-HMF into FDCA, an interesting substitute
of terephtalic acid, are on-going.
Synthesis of CuO·CeO2: 3 mmol of cerium(IV) ammonium nitrate
(1.64 g) and 3 mmol of copper(II) nitrate trihydrate (0.72 g) were
mixed in a HEM apparatus, pulverized at 790 rpm during 1 h with
pauses of 1 min every 15 min. The pale green mixture was calcined
for 3 h at 823 K giving a dark-brown solid that was transferred into
a flask under N2 atmosphere.
In conclusion, a cheap mixed oxide CuO·CeO2 was prepared
by high energy milling (HEM), and is highly active towards the
selective conversion of 5-hydroxymethylfurfural (5-HMF) into 5-
formyl-2-furancarboxylic acid (FFCA; 99% conversion yield,
90% selectivity) in water, using molecular oxygen as oxidant
and without external base. Oxidation in air was also effective,
but with slower kinetics. The dependence of the activity and
selectivity on the acid–base sites is demonstrated. The basicity
of the catalysts drives the activity and selectivity. High acidity
results in a poor activity, but if the acidity is too low, the cata-
lyst is not active. The correct ratio of acid/base sites can be
tuned by choosing the correct oxides, that is, by using a mixed
oxide. The results presented here are of use for the further ra-
tional design of earth-abundant mixed oxide catalysts, and can
drive the oxidation of cellulose-derived platform molecules for
the production of fine chemicals and monomers.
Typical experimental procedure: The kinetics of conversion of 5-
HMF at a fixed temperature (383 K) were studied in a 50 mL stain-
less-steel reactor equipped with a withdrawal valve and an electri-
cal heating jacket. 0.177 g of 5-HMF was dissolved in 7 mL of dis-
tilled water in a glass reactor, in which 0.05 g of the catalyst under
study and a magnetic stirrer were placed. The glass reactor was
then transferred into the reactor that was closed and purged three
times with O2. It was charged with 0.9 MPa of oxygen and heated
to the reaction temperature (383 K). At fixed intervals of time, stir-
ring was stopped; a sample was withdrawn and analyzed by HPLC.
Recovery and reuse of the catalyst as such: The mixed oxide cat-
alyst was recovered by filtration at the end of the first run, washed
with water (3”7 mL), and reused in a second run. These operations
were repeated three times on the same catalyst. Both conversion
and selectivity were reduced in each next run. (Figure 5).
Recovery and reuse of the catalyst after calcination: The catalyst
was recovered after the first run, washed with water (3”7 mL) and
calcined at 823 K for 3 h. This operation was repeated two times.
The catalyst almost recovered its activity and selectivity. (Figure 5)
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Acknowledgements
The work was funded by the MIUR Industrial Research Project
CTN01_00063_49393 “REBIOCHEM” in the frame of the Operative
National Program-Research and Competitiveness 2007–2013. The
authors thank the VALBIOR-Apulia Region Project (#33) for the
use of equipment.
Keywords: heterogeneous catalysis · metal oxides · oxidation ·
renewable resources · water chemistry
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Published online on April 21, 2016
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