Aerobic oxidation of 5-(Hydroxymethyl)furfural in ionic liquids with solid ruthenium hydroxide catalysts

Article abstract of DOI:10.1007/s10562-012-0858-5

The aerobic oxidation of 5-(hydroxymethyl)furfural was investigated over solid ruthenium hydroxide catalysts in ionic liquids at elevated temperatures and pressures. Several different catalyst supports were tested in combination with various ionic liquids. The best result was obtained in [EMIm][OAc] at 100°C with 30 bar of oxygen over Ru(OH) _x /La₂O ₃ which afforded 48 % of 2,5-furandicarboxylic acid and 12 % of 5-hydroxymethyl-2-furancarboxylic acid. During use the catalysts were found to leach some of the metal inventory. However, the leached Ru species did not contribute to the catalytic activity, thus verifying that the catalytic oxidation proceeded heterogeneously under the applied reaction conditions. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-012-0858-5

Catal Lett (2012) 142:1089–1097
DOI 10.1007/s10562-012-0858-5
Aerobic Oxidation of 5-(Hydroxymethyl)furfural in Ionic Liquids
with Solid Ruthenium Hydroxide Catalysts
•
Tim Stahlberg Ester Eyjolfsdottir
•
˚
Yury Y. Gorbanev Irantzu Sadaba
´
´
•
•
´
Anders Riisager
Received: 6 January 2012 / Accepted: 13 June 2012 / Published online: 6 July 2012
Ó Springer Science+Business Media, LLC 2012
Abstract The aerobic oxidation of 5-(hydroxy-
methyl)furfural was investigated over solid ruthenium
hydroxide catalysts in ionic liquids at elevated tempera-
tures and pressures. Several different catalyst supports
were tested in combination with various ionic liquids. The
best result was obtained in [EMIm][OAc] at 100 °C with
30 bar of oxygen over Ru(OH)x/La2O3 which afforded
48 % of 2,5-furandicarboxylic acid and 12 % of 5-hydro-
xymethyl-2-furancarboxylic acid. During use the catalysts
were found to leach some of the metal inventory. However,
the leached Ru species did not contribute to the catalytic
activity, thus verifying that the catalytic oxidation pro-
ceeded heterogeneously under the applied reaction
conditions.
been carried out with stoichiometric reagents such as
chromium(VI) compounds, permanganate, iodate com-
pounds and peroxy acids which have toxicity issues and
produce a large amount of waste compared to the amount
of product [1, 2]. In order to relinquish the use of these
reagents, the chemical industry, and the scientific com-
munity are pursuing oxidations with environmentally
benign oxidants such as hydrogen peroxide and molecular
oxygen together with efficient and selective catalysts [3, 4].
Recently, Yamaguchi and Mizuno [5] introduced sup-
ported ruthenium hydroxides as versatile and highly active
catalysts for aerobic oxidations. These catalysts work
exceedingly well in connection with common functional
group transformations such as oxidations of alcohols [6]
and the oxygenation of primary amines to amides [7].
Ionic liquids (ILs) are interesting alternatives to con-
ventional molecular solvents due to their negligible vapor
pressure, redox stability, non-flammability and unique dis-
solving abilities for polar compounds [8]. The redox stability
has allowed oxidations to be widely studied in ILs for several
applications such as the oxidative Glaser coupling [9], oxi-
dations of alcohols to aldehydes or ketones [10–12] and the
oxidation of alkanes [13, 14]. Several examples of epoxi-
dations of alkenes [15–19] and other miscellaneous func-
tional group transformations [20–24] in ILs are also found in
the literature. In most cases the oxidant is H2O2, but also
stoichiometric reagents such as NaOCl, Dess-Martin perio-
date, MnO2 and meta-chloroperoxybenzoic acid (mCPBA)
have been reported. The most commonly employed ILs are
1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4])
and 1-butyl-3-methylimidazolium hexafluorophosphate
([BMIm][PF6]). The ideal oxidant for green chemical pro-
cesses is molecular oxygen and aerobic oxidations with
homogeneous catalysts in ILs have been reported by
numerous groups [25–39].
Keywords Oxidation Á Supported ruthenium hydroxide Á
5-(Hydroxymethyl)furfural Á 2,5-Furandicarboxylic acid Á
Ionic liquids
1 Introduction
In all segments of the chemical industry oxidations of
functional groups play a vital role in the derivatization of
chemical compounds. These reactions have traditionally
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-012-0858-5) contains supplementary
material, which is available to authorized users.
˚
´
´
´
T. Stahlberg Á E. Eyjolfsdottir Á Y. Y. Gorbanev Á I. Sadaba Á
A. Riisager (&)
Centre for Catalysis and Sustainable Chemistry,
Department of Chemistry, Technical University
of Denmark, 2800 Kgs. Lyngby, Denmark
e-mail: ar@kemi.dtu.dk
123

˚
T. Stahlberg et al.
1090
Deriving chemicals from biomass has received signifi-
water and other conventional solvents makes an oxidation
process of HMF in ILs by the use of heterogeneous cata-
lysts an interesting option [56]. In such a process the cat-
alyst would be filtered off after complete reaction, followed
by the addition of water to the resulting IL/FDA mixture
making FDA precipitate. The aqueous IL would then be
stripped of water and recycled.
cant attention among chemists in later years. In particular
5-(hydroxymethyl)furfural (HMF), formed from the dehy-
dration of hexose sugars, has been in the spotlight since it
is believed to become an essential platform chemical in the
future biopetrochemical industry [40, 41]. An important
derivative of HMF is 2,5-furandicarboxylic acid (FDA),
which is formed by oxidation and is intended to replace
terephthalic acid as a monomer in polyesters [42]. As a
matter of fact, Avantium has recently opened a pilot plant
at the Chemelot site in Geleen (The Netherlands) using this
technology to produce Polyethylene Furanoate (PEF) from
HMF [43, 44]. The catalytic oxidation of HMF was ini-
tially studied by the group of van Bekkum using hetero-
geneous platinum and palladium catalysts [45, 46]. This
study has been succeeded in later years with studies on
Co(acac)3 [47], gold nanoparticles [48–50] and a more
detailed study of platinum catalysts on different supports
[51]. The oxidation of HMF proceeds via diformylfuran
(DFF) or 5-hydroxymethyl-2-furancarboxylic acid (HMFCA)
to 5-formyl-2-furancarboxylic acid (FFCA) which is rap-
idly converted to FDA [45]. HMF can also be oxidized to
DFF and maleic anhydride selectively using vanadium
catalysts, resulting in the formation of formic acid (FA) as
a by-product [52–54]. The different oxidation products of
HMF are depicted in Scheme 1.
To the best of our knowledge no investigation of het-
erogeneous catalysts for the aerobic oxidation of HMF in
ILs has been reported. Our objective in this study was to
investigate the viability of the utilization of ILs as solvent
for this reaction. In this particular case, heterogeneous
Ru(OH)x catalysts on different supports in ILs were used in
the aerobic oxidation of HMF to FDA. The reaction was
studied at different reaction conditions and leaching tests of
ruthenium were made to investigate the stability of the
catalyst in ILs. However, further experiments are required
to optimize the yield to FDA.
2 Experimental
2.1 Materials
5-(Hydroxymethyl)furfural (HMF) ([99 %), 2-furoic acid
(98 %), levulinic acid (LA) (98 %), formic acid (FA)
(98 %), ruthenium(III) chloride(purum), Mg6Al2CO3(OH)16Á
4H2O (HT), magnetite ([98 %), Ca5(PO4)3(OH) (HAp)
([97 %), aluminium oxide ([99.9 %), zirconium oxide
(99 %), lanthanum(III) nitrate hexahydrate (99.99 %) and
The synthesis of HMF from sugars benefits particularly
from using certain ILs as solvents, and thus an overall
process from HMF to FDA in ILs would be advantageous
[55]. In addition, the exceedingly low solubility of FDA in
Scheme 1 Different oxidation
products of HMF
123

Aerobic Oxidation of 5-(Hydroxymethyl)furfural
1091
sodium hydroxide ([98 %) were acquired from Sigma-
Aldrich. Ruthenium(III) nitrate hexahydrate (99.9 %) and
magnesium nitrate hexahydrate (p.a.) were obtained from
Merck. Cerium oxide (99.5 %) and lanthanum(III) oxide
(99.9 %) were purchased from Alfa Aesar. Magnesium
(12.0 g) were mixed in a Parr autoclave, pressurized with
dioxygen (10–30 bar) and stirred at 100–140 °C for 5 h.
The reaction mixture was cooled down to room tempera-
ture and diluted to 100 mL with 0.1 M NaOH. Finally, the
catalyst was filtered off and the resultant solution analyzed
by HPLC.
¨
oxide (p.a.) was purchased from Riedel-de Haen AG. 2,5-
Diformylfuran (DFF) (98 %) was obtained from ABCR
GmbH & Co. 2,5-Furandicarboxylic acid (FDA) ([99 %)
and 5-hydroxymethyl-2-furancarboxylic acid (HMFCA)
([99 %) were purchased from Toronto Research Chemicals
Inc. and dioxygen (99.5 %) from Air Liquide Denmark. The
ionic liquids 1-ethyl-3-methylimidazolium dicyanamide
([EMIm][N(CN)2]) (98 %) and 1,3-dimethylimidazolium
dimethyl phosphate ([MMIm][dmp]) (98 %) were pur-
chased from Solvent Innovation, while all other ionic liquids
([95 %) were obtained from BASF. All chemicals were
used as received. All Ru(OH)x/support catalysts were syn-
thesized according to literature procedures [57].
2.4 Procedure for Leaching Test
Catalyst (100 mg, 0.025 mmol Ru) and [EMIm][OAc]
(1.0 g) were mixed in a 40 mL tube and stirred at 100 °C
for 1 h. The reaction solution was diluted in 5 mL of water
and the catalyst was filtered off. The water was evaporated
by heating the ionic liquid at 120 °C overnight. HMF
(70 mg, 0.56 mmol) was added to the liquid and the mix-
ture was stirred in an open flask at 100 °C for 6 h. The
mixture was cooled down to room temperature, diluted to
10 mL with 0.1 M NaOH, filtered and analyzed by HPLC.
2.2 Apparatus and Analysis
3 Results and Discussion
Catalytic oxidation experiments at ambient pressure were
performed using a Radley Carousel 12 Plus Basic System,
while high pressure oxidation reactions were carried out in
stirred Parr autoclaves equipped with internal thermocon-
trol (T316 steel, TeflonTM beaker insert, 100 mL). All
reaction samples were analyzed by HPLC (Agilent 1200
series, Bio-Rad Aminex HPX-87H, 300 mm 9 7.8 mm
pre-packed column, 0.005 M H2SO4 mobile phase, 60 °C,
0.6 mL/min). The yields and selectivities were based on
conversion of HMF and confirmed by calibration of stan-
dard solutions of the products and reactants.
3.1 HMF Oxidation at Ambient Pressure
The ILs best suited for the synthesis of HMF from fructose
or glucose have proven to be 1-ethyl-3-methylimidazolium
chloride ([EMIm]Cl) and 1-butyl-3-methylimidazolium
chloride ([BMIm]Cl) [55]. Consequently, [EMIm]Cl was
applied for the initial screening of catalysts with Ru(OH)x
supported on different carriers for the HMF oxidation at
ambient pressure. A relatively high temperature was cho-
sen in order to reduce the impact of viscosity of the mixture
formed between the catalyst and IL. Since the screening
was performed to allow comparison of the influence
induced by the different types of catalyst supports, the
reduced solubility of oxygen at elevated temperature was at
this stage disregarded. The experiments were performed in
open tubes and the results are shown in Table 1.
Surface areas were determined by nitrogen sorption
measurements at liquid nitrogen temperature on a Mi-
cromeritics ASAP 2020. The samples were outgassed in
vacuum at 150 °C for 4 h prior to the measurements. The
total surface areas were calculated according to the BET
method. X-ray fluorescence (XRF) analyses were recorded
on a Minipal 4 X-ray spectrometer (PANalytical).
All catalysts were found to exhibit activity in
[EMIm]Cl. Even though the yields of FDA in all cases
were low, an indication of which catalysts were the most
promising candidates for further study could be derived
from the yields of HMFCA. The best catalysts (Table 1,
entries 3 and 7–9) were Ru(OH)x/Fe2O3, Ru(OH)x/HT,
Ru(OH)x/MgO and Ru(OH)x/La2O3, which gave yields of
HMFCA in the range of 14–25 %. No DFF was observed in
any of these experiments.
2.3 Oxidation Procedures
In the catalytic screening experiments performed at ambi-
ent air pressure, catalyst (100 mg, 0.025 mmol Ru) and IL
(1.0 g) were mixed in a 40 mL tube and stirred at
100–140 °C for 10 min. HMF (70 mg, 0.56 mmol) was
added and the mixture was stirred in an open flask for 24 h.
The reaction was subsequently cooled down to room
temperature and diluted to 10 mL with 0.1 M NaOH.
Finally, the catalyst was filtered off and the resultant
solution analyzed by HPLC.
After oxidation activity had been proven, albeit with low
yields, the study was expanded to include other ILs.
Notably, the study did not include [BMIm][BF4] and
[BMIm][PF6] which have been commonly employed in
earlier oxidation studies in ILs. These tend to form HF
upon contact with moisture at elevated temperature which
In the high pressure oxidation reactions, catalyst (0.5 g,
0.125 mmol Ru), HMF (350 mg, 2.78 mmol) and IL
123

˚
T. Stahlberg et al.
1092
Table 1 HMF oxidation in [EMIm]Cl with various Ru(OH)x/support catalysts
Entry
Catalyst
BET surface
area (m2/g)
Conversion
(%)
Yield (%)
DFF
HMFCA
FDA
1
2
3
4
5
6
7
8
9
Ru(OH)x/TiO2
Ru(OH)x/spinel
Ru(OH)x/Fe2O3
Ru(OH)x/ZrO2
Ru(OH)x/CeO2
Ru(OH)x/HAp
Ru(OH)x/HT
128
54
45
97
8
92
89
0
0
0
0
0
0
0
0
0
1
7
3
3
5
5
4
4
5
2
1
99
14
3
84
86
7
25
6
81
4
[99
[99
[99
20
20
25
Ru(OH)x/MgO
Ru(OH)x/La2O3
27
5
Reaction conditions 1.0 g [EMIm]Cl, 68 mg (0.54 mmol) HMF, 100 mg catalyst (2.5 wt% Ru, 0.025 mmol), 140 °C, 24 h, ambient air pressure
could be detrimental to reaction intermediates as well as
catalyst [8, 58]. The ions of the ILs chosen are depicted in
Scheme 2.
IL and spinel resulted in species that had a negative effect
on HMF stability (entries 11–13).
In our previous work at high temperature [64] we have
demonstrated that [EMIm][OAc] has a detrimental effect
on HMF stability, lowering the carbon balance. Accord-
ingly, we investigated the effect of lowering the reaction
temperature. Both [EMIm][HSO4] and [Bu3MeN][MeOSO3]
proved to be very viscous in combination with the catalyst
at 100 °C, which was why experiments with these liquids
were only performed at lower temperature in autoclave
with mechanical stirring in the later high-pressure experi-
ments (vide infra). In Fig. 1 the product distribution for the
oxidation of HMF in [EMIm][OAc] with Ru(OH)x/spinel at
100 °C is shown as a function of time. The final yield of
FDA was lower than in the high-temperature experiments
and no DFF was detected, whereas the yield of HMFCA
was higher and almost amounted to 50 %. The conversion
of HMF remained high, accentuating the need of faster
oxidation kinetics to avoid degradation. We did not detect
any by-products on our HPLC system and assumed there-
fore that the degradation of HMF in [EMIm][OAc] resulted
in polymeric condensation products, as has been previously
reported [65]. Neither levulinic acid nor formic acid,
common degradation products of HMF [66], were formed
at ambient pressure. According to these results, we can
propose a scheme of the formation of by-products during
the aerobic oxidation of HMF, represented in Scheme 3. In
this case, under high temperature conditions, the HMF, as
well as the intermediates or the final FDA product can form
higher molecular weight compounds via different mecha-
nisms, like ketonization, condensation or even Diels-alder
condensation [67–69]. As shown later, higher pressures and
conversion values are required to form degradation prod-
ucts such as formic acid. Other oxidation products such as
maleic anhydride are also possible, as presented in
Scheme 1. These three types of side reactions lead to a
The most promising catalysts from the first screening
were tested together with Ru(OH)x/CeO2 which had shown
superior activity in water [57]. Additionally, the catalyst
precursor RuCl3 was included for comparison. Most of the
reactions afforded very low yields for the different oxida-
tion products and only a few gave interesting results for
further studies. The results for the most interesting ILs are
summarized in Table 2 while the complete screening
results of all ILs can be found in supporting information
(Table S1).
For 1-ethyl-3-methylimidazolium acetate ([EMIm][OAc])
formation of FDA was observed with Ru(OH)x/La2O3 and
Ru(OH)x/spinel (Table 2, entries 7 and 10). Surprisingly,
6 % of FDA was also formed when using the homogeneous
catalyst RuCl3 (entry 9). Formation of HMFCA was also
observed for all other catalysts in [EMIm][OAc] indicating
that the oxygen solubility was higher compared to other
ILs. No solubility data for dioxygen in [EMIm][OAc] was
available in literature, but the solubility of O2 is generally
low in comparison to other gases in ILs [59–63]. Another
IL that showed promising results was 1-ethyl-3-methyl-
imidazolium hydrogen sulfate ([EMIm][HSO4]) which
afforded 19 % FDA using Ru(OH)x/HT (entry 1). When
employing Ru(OH)x/CeO2 in the same IL 9 % of DFF was
formed. No other oxidation products were observed along
with a slightly lower conversion of HMF compared to
Ru(OH)x/HT (entry 3). The catalyst precursor RuCl3 had
no activity in [EMIm][HSO4] (entry 4). Tributylmethy-
lammonium methyl sulfate ([Bu3MeN][MeOSO3]) and
1,2,4-trimethylpyrazolium methyl sulfate ([MMMPz]
[MeOSO3]) gave DFF formation in combination with
Ru(OH)x/spinel. The HMF conversion was much higher in
[MMMPz][MeOSO3] with Ru(OH)x/spinel compared to
Ru(OH)x/La2O3 suggesting that the interaction between the
123

Aerobic Oxidation of 5-(Hydroxymethyl)furfural
1093
Scheme 2 The cations and
anions of the ILs used in the
HMF oxidation study
clear decrease in the carbon balance closures in the oxi-
dation experiments.
leads to the assumption that pressures significantly higher
than ambient were required to reach full conversion of
HMFCA to FDA. In addition to the oxidation products
obtained in the reactions at atmospheric pressure, formic
acid (FA) was formed to various extent at higher oxygen
pressure, as also depicted in Scheme 3. A summary of the
results from the high pressure experiments is shown in
Table 3.
The best result at 100 °C was obtained with La2O3 as
catalyst support (Fig. 2). Here the HMF conversion
reached a maximum already after 6 h which coincided with
an HMFCA yield of 58 %. The amount of HMFCA then
declined slowly to form FDA which reached a yield of
around 10 % after 30 h.
The first series of autoclave experiments were conducted
with 10 bar of dioxygen at 140 °C with a reaction time of
5 h. Using Ru(OH)x/spinel in [EMIm][HSO4] (Table 3,
entry 1) a very similar result to the open flask experiment at
ambient pressure was obtained (Table 2, entry 2). Evi-
dently, the increased pressure was in this case insufficient
3.2 HMF Oxidation at High Pressure
The open-flask experiments performed at ambient air
pressure showed potential for oxidations at higher pressure.
The generally low solubility of molecular oxygen in ILs
123

˚
T. Stahlberg et al.
1094
Table 2 HMF oxidation in different ILs with various Ru(OH)x/support catalysts
Entry
Ionic liquid
Catalyst
Conversion (%)
Yield (%)
DFF
HMFCA
FDA
1
[EMIm][HSO4]
[EMIm][HSO4]
[EMIm][HSO4]
[EMIm][HSO4]
[EMIm][HSO4]
[EMIm][OAc]
Ru(OH)x/HT
Ru(OH)x/spinel
Ru(OH)x/CeO2
RuCl3
99
55
1
3
0
0
19
0
2
3
77
9
1
0
4
84
0
0
0
5
Ru(OH)x/La2O3
Ru(OH)x/HT
Ru(OH)x/spinel
Ru(OH)x/CeO2
RuCl3
71
3
0
0
6
93
0
13
27
2
0
7
[EMIm][OAc]
99
0
13
0
8
[EMIm][OAc]
55
1
9
[EMIm][OAc]
[99
[99
64
0
27
30
0
6
10
11
12
13
[EMIm][OAc]
Ru(OH)x/La2O3
Ru(OH)x/spinel
Ru(OH)x/spinel
Ru(OH)x/La2O3
0
10
0
[Bu3MeN][MeOSO3]
[MMMPz][MeOSO3]
[MMMPz][MeOSO3]
12
6
95
0
0
46
10
0
0
Reaction conditions 1.0 g IL, 68 mg (0.54 mmol) HMF, 100 mg catalyst (2.5 wt% Ru, 0.025 mmol), 140 °C, 24 h, ambient air pressure
was most likely a consequence of higher viscosity which
made mixing more difficult, resulting in slower gas diffu-
sion and accompanying mass transfer limitations. The same
phenomenon appeared in the case of Ru(OH)x/spinel in
[Bu3MeN][MeOSO3] where the FDA yield was lowered
drastically from 10 to 1 %. Improvement was observed at
100 °C for Ru(OH)x/La2O3 in [EMIm][OAc] where 23 %
of FDA was formed along with 34 % of HMFCA (entry 4).
Clearly, the detrimental effect of [EMIm][OAc] on HMF
stability was reduced and conversion to the desired product
was favored when decreasing the temperature.
The final adjustment made was to increase the dioxygen
pressure even further to 30 bars. This gave a slight increase
in HMF conversion from 32 to 52 % for Ru(OH)x/HT in
[EMIm][HSO4], but still with no FDA being formed (entry
7). The best result was obtained with Ru(OH)x/La2O3 in
[EMIm][OAc] which afforded an FDA yield of 48 % and
an HMFCA yield of 12 % (entry 8). In addition to the
desired products, 30 % of FA was formed, suggesting that
some oxidative degradation was favored at elevated pres-
sure in [EMIm][OAc], as suggested in Scheme 3.
Fig. 1 Oxidation of HMF with Ru(OH)x/spinel in [EMIm][OAc] at
100 °C. Reaction conditions 1.0 g IL, 68 mg (0.54 mmol) HMF,
100 mg catalyst (2.5 wt% Ru, 0.025 mmol), 100 °C, ambient air
pressure
to impose an effect on the slow gas mass transfer. The use
of Ru(OH)x/spinel in [EMIm][OAc] resulted in a decrease
in HMFCA yield from 27 to 4 %, while the yield of FDA
only increased from 13 to 14 % (entry 2). The high tem-
perature in combination with an increased pressure almost
certainly accelerated degradation in [EMIm][OAc], leading
to lower yields than at ambient pressure.
3.3 Leaching Study at Ambient Pressure
Heterogeneous catalysts can be sensitive to leaching of the
active catalytic specie from the support which can be
deceptive when interpreting experimental results. The
reaction conditions for the performed oxidation experi-
ments were such that leaching was not unlikely, and the
experiments using the catalyst precursor RuCl3 at ambient
pressure had already confirmed that HMF oxidation could
also proceed homogeneously. Accordingly, the two most
promising catalysts (Ru(OH)x/La2O3 and Ru(OH)x/spinel)
In light of the improved results obtained when lowering
the reaction temperature for the open flask experiments,
high pressure reactions were also conducted at 100 °C. For
Ru(OH)x/HT in [EMIm][HSO4] this resulted in a signifi-
cant reduction of HMF conversion from 99 to 32 % and no
formation of FDA, whereas HMFCA and DFF were
obtained in 3 and 18 % yield, respectively (entry 3). This
123

Aerobic Oxidation of 5-(Hydroxymethyl)furfural
1095
Scheme 3 Possible degradation
products obtained during HMF
oxidation at high temperature
and/or pressures in the presence
of IL and supported catalyst
purposes, 6 h reactions were also carried out with both
catalysts, and the Ru content in the liquid phase (expressed
as wt.% of the initial Ru loading in the fresh catalyst)
calculated.
As summarized in Table 4, 3–5 wt% of the total initial
Ru was leached within the first hour of reaction (entries 1b
and 2b) and 5–10 wt% of the Ru was detected after 6 h of
reaction (entries 1a and 2a). Nevertheless, these leached Ru
species did apparently not present any significant catalytic
activity, since identical product yields of 36–38 %
HMFCA and 4 % FDA were obtained in a comparative
blank experiment without added catalyst (entry 3). These
experiments thus verified that the catalytic oxidation pro-
ceeded heterogeneously under these conditions, with the
active sites on the surface of the catalyst being responsible
for the catalytic activity. However, during the contact of
the catalyst with the reaction medium part of the supported
Ru species are leached into the solution, causing a metal
loss and making the long term reutilization of these cata-
lysts difficult.
Fig. 2 Oxidation of HMF with Ru(OH)x/La2O3 in [EMIm][OAc] at
100 °C. Reaction conditions 1.0 g IL, 68 mg (0.54 mmol) HMF,
100 mg catalyst (2.5 wt% Ru, 0.025 mmol), 100 °C, ambient air
pressure
were tested towards leaching. This was made by stirring the
catalysts in [EMIm][OAc] at 100 °C for 1 h, after which
they were removed by filtration. A dilution with water was
required to fully recover the catalyst, which was evaporated
afterwards. In both cases the resultant liquid was used as
solvent for the oxidation of HMF without addition of fur-
ther catalyst. The recovered liquid after reaction was also
analyzed by XRF to measure the amount of Ru leached into
the solution by lixiviation of the catalysts. For comparison
4 Conclusions
The aerobic oxidation of HMF was investigated in various
ILs using solid ruthenium hydroxide catalysts supported on
different carrier materials. The IL best suited for the oxi-
dation was [EMIm][OAc] which afforded an FDA yield of
123

˚
T. Stahlberg et al.
1096
Table 3 HMF oxidation in different ILs with Ru(OH)x/support catalysts
Entry
Ionic liquid
Catalyst
PO2 (bar)
T (°C)
Conversion (%)
Yield (%)
FAa
DFF
HMFCA
FDA
1
2
3
4
5
6
7
8
[EMIm][HSO4]
[EMIm][OAc]
Ru(OH)x/spinel
Ru(OH)x/spinel
Ru(OH)x/HT
10
10
10
10
30
30
30
30
140
140
100
100
100
100
100
100
58
[99
32
0
0
3
0
1
4
0
14
0
[EMIm][HSO4]
[EMIm][OAc]
0
18
0
3
Ru(OH)x/La2O3
Ru(OH)x/HT
97
31
0
34
16
26
8
23
1
[Bu3MeN][MeOSO3]
[Bu3MeN][MeOSO3]
[EMIm][HSO4]
[EMIm][OAc]
60
26
18
25
0
Ru(OH)x/spinel
Ru(OH)x/HT
62
0
3
52
0
0
Ru(OH)x/La2O3
98
30
12
48
Reaction conditions 12 g IL, 350 mg (2.78 mmol) HMF, 0.5 g catalyst (2.5 wt% Ru, 0.125 mmol), 5 h
a
Calculated as 1 mol of FA produced per mol of initial HMF
Table 4 Leaching study for HMF oxidation in [EMIm][OAc] with Ru(OH)x/support catalysts
Entry
Catalyst
Ru in fresh
catalysta (wt%)
Ru in solutiona
(wt% of the initial Ru loading)
Conversion (%)
Yield (%)
HMFCA
FDA
1a
1b
2a
2b
3
Ru(OH)x/spinel
Lixiviated
2.5
–
4.7
2.7b
9.9
4.7b
–
99
99
99
97
99
43
36
56
37
38
6
3
6
4
4
Ru(OH)x/La2O3
Lixiviated
5.0
–
No catalyst
–
Reaction conditions 1.0 g [EMIm][OAc], 68 mg (0.54 mmol) HMF, 100 mg catalyst (2.5 wt% Ru, 0.025 mmol), 100 °C, 6 h, ambient air
pressure
a
Measured by XRF analysis
b
Amount leached after 1 h in contact with the IL
48 % using 30 bar of O2 at 100 °C with Ru(OH)x on La2O3
support.
structurally similar ILs to increase the yield of FDA and
reduce the amount of by-products.
An apparent improvement of the yield was found when
lowering the temperature and increasing the pressure. Even
though [EMIm][OAc] is notorious in degrading HMF [64]
and naturally not ideal as solvent for HMF oxidation, it
appears to have a higher solubility for oxygen compared to
other ILs, which made it the best solvent under the applied
reaction conditions. In several ILs the stirring was ham-
pered because of high viscosity which most likely also had
an impact on gas mass transfer and consequently on con-
version and yield.
Acknowledgments The work was supported by the Danish National
Advanced Technology Foundation in cooperation with Novozymes
A/S. Special gratitude to BASF for providing some of the ionic
liquids.
References
1. Smith MB, March J (2000) March’s advanced organic chemistry:
reactions, mechanisms, and structure, 5th edn. Wiley, New York
2. Sheldon RA (1996) J Mol Catal A Chem 107:75
3. Zhan BZ, Thompson A (2004) Tetrahedron 60:2917
4. Sheldon RA, Wallau M, Arends IWCE, Schuchardt U (1998) Acc
Chem Res 31:485
5. Yamaguchi K, Mizuno N (2010) Synlett 2382
6. Kotani M, Koike T, Yamaguchi K, Mizuno N (2006) Green
Chem 8:735
7. Kim J, Yamaguchi K, Mizuno N (2008) Angew Chem Int Ed
47:9249
8. Wasserscheid P, Welton T (2008) Ionic Liquids in Synthesis.
Wiley-VCH Verlag GmbH & Co, KGaA
We believe that this work provides valuable insights
about the scope and limitations of aerobic oxidations in ILs
using solid ruthenium hydroxide catalysts. The study also
shows that the IL [EMIm][OAc] worked well as solvent for
aerobic oxidations making it attractive as reaction media
considering its unique dissolving properties. Future work
will focus on synthesizing catalysts with higher chemical
resistance by alternative preparation methods and investi-
gating other oxidation reactions in [EMIm][OAc] or
123

Aerobic Oxidation of 5-(Hydroxymethyl)furfural
1097
9. Yadav J, Reddy B, Reddy K, Gayathri K, Prasad A (2003) Tet-
rahedron Lett 44:6493
10. Yadav J, Reddy B, Basak A, Venkat Narsaiah A (2004) Tetra-
hedron 60:2131
41. Bozell JJ, Petersen GR (2010) Green Chem 12:539
42. Gomes M, Gandini A, Silvestre AJD, Reis B (2011) J Polym Sci
Part A Polym Chem 49:3759
43. Tullo AH (2011) Chem Eng News 89(38):10
44. http://www.yxy.com Accessed 2 July 2012
45. Vinke P, van Dam H, van Bekkum H (1990) Stud Surf Sci Catal
55:147
46. Vinke P, van der Poel W, van Bekkum H (1991) Stud Surf Sci
Catal 59:385
47. Ribeiro ML, Schuchardt U (2003) Catal Commun 4:83
48. Taarning E, Nielsen I, Egeblad K, Madsen R, Christensen C
(2008) ChemSusChem 1:75
11. Walker AJ, Bruce NC (2004) Tetrahedron 60:561
12. Wu XE, Ma L, Ding MX, Gao LX (2005) Synlett 607
13. Peng J, Shi F, Gu Y, Deng Y (2003) Green Chem 5:224
14. Li Z, Xia CG (2004) J Mol Catal A Chem 214:95
15. Owens GS, Abu-Omar MM (2000) Chem Commun 1165
16. Owens GS, Durazo A, Abu-Omar MM (2002) Chem Eur J 8:3053
17. Tong KH, Wong KY, Chan TH (2003) Org Lett 5:3423
18. Li Z, Xia CG (2003) Tetrahedron Lett 44:2069
19. Bernini R, Mincione E, Coratti A, Fabrizi G, Battistuzzi G (2004)
Tetrahedron 60:967
49. Gorbanev YY, Klitgaard S, Woodley JM, Christensen CH, Rii-
sager A (2009) ChemSusChem 2:672
20. Singer RD, Scammells PJ (2001) Tetrahedron Lett 42:6831
21. Lo WH, Yang HY, Wei GT (2003) Green Chem 5:639
22. Yadav JS, Reddy BVS, Basak AK, Narsaiah AV (2004) Chem
Lett 33:248
23. Martiz B, Keyrouz R, Gmouh S, Vaultier M, Jouikov V (2004)
Chem Commun 6:674
24. Bianchini G, Crucianelli M, Angelis FD, Neri V, Saladino R
(2005) Tetrahedron Lett 46:2427
25. Wolfson A, Wuyts S, De Vos DE, Vankelecom IFJ, Jacobs PA
(2002) Tetrahedron Lett 43:8107
50. Gupta NK, Nishimura S, Takagaki A, Ebitani K (2011) Green
Chem 13:824
51. Lilga MA, Hallen RT, Gray M (2010) Top Catal 53:1264
52. Carlini C, Patrono P, Galletti AMR, Sbrana G, Zima V (2005)
Appl Catal A 289:197
53. Ma J, Du Z, Xu J, Chu Q, Pang Y (2011) ChemSusChem 4:51
54. Du Z, Ma J, Wang F, Liu J, Xu J (2011) Green Chem 13:554
˚
55. Stahlberg T, Fu W, Woodley JM, Riisager A (2011) ChemSu-
sChem 4:451
56. Payne SM, Kerton FM (2010) Green Chem 12:1648
57. Gorbanev YY, Kegnæs S, Riisager A (2011) Top Catal 54:1318
58. Swatloski RP, Holbrey JD, Rogers RD (2003) Green Chem 5:361
59. Anthony JL, Maginn EJ, Brennecke JF (2002) J Phys Chem B
106:7315
26. Farmer V, Welton T (2002) Green Chem 4:97
27. Ansari A, Gree R (2002) Org Lett 4:1507
28. Wang JR, Liu L, Wang YF, Zhang Y, Deng W, Guo QX (2005)
Tetrahedron Lett 46:4647
29. Sun H, Li X, Sundermeyer J (2005) J Mol Catal A Chem 240:119
30. Jiang N, Ragauskas AJ (2005) Org Lett 7:3689
31. Jiang N, Ragauskas AJ (2006) Tetrahedron Lett 48:273
32. Jiang N, Ragauskas AJ (2007) J Org Chem 72:7030
33. Shen HY, Mao HL, Ying LY, Xia QH (2007) J Mol Catal A
Chem 276:73
34. Shaabani A, Rezayan AH, Heidary M, Sarvary A (2008) Catal
Commun 10:129
35. Shaabani A, Farhangi E, Rahmati A (2008) Appl Catal A 338:14
36. Lin L, Ji L, Wei Y (2008) Catal Commun 9:1379
37. Dabiri M, Salehi P, Bahramnejad M (2010) Synth Commun
40:3214
60. Anthony JL, Anderson JL, Maginn EJ, Brennecke JF (2005) J
Phys Chem B 109:6366
61. Morgan D, Ferguson L, Scovazzo P (2005) Ind Eng Chem Res
44:4815
62. Ferguson L, Scovazzo P (2007) Ind Eng Chem Res 46:1369
´
63. Kumelan J, Perez-Salado Kamps I, Tuma D, Maurer G (2009) J
Chem Eng Data 54:966
˚
64. Stahlberg T, Sørensen MG, Riisager A (2010) Green Chem
12:321
65. Blanksma JJ, Egmond G (1946) Recl Trav Chim Pays-Bas 65:309
66. Teunissen HP (1930) Recl Trav Chim Pays-Bas 49:784
67. Gandini A, Belgacem MN (1997) Prog Polym Sci 22:1203
68. James OO, Maity S, Usman LA, Ajanaku KO, Ajani OO,
Siyanbola TO, Sahu S, Chaubey R (2010) Energy Environ Sci
3:1833
38. Basle O, Borduas N, Dubois P, Chapuzet JM, Chan TH, Lessard
J, Li CJ (2010) Chem Eur J 16:8162
39. Miao CX, Wang JQ, Yu B, Cheng WG, Sun J, Chanfreau S, He
LN, Zhang SJ (2011) Chem Commun 47:2697
69. Gandini A (2011) Green Chem 13:1061
40. Werpy T, Petersen G (2004) US DOE 2004, No. DOE/GO-
102004-1992. http://www.nrel.gov/docs/fy04osti/35523.pdf
123