Supported gold- and silver-based catalysts for the selective aerobic oxidation of 5-(hydroxymethyl)furfural to 2,5-furandicarboxylic acid and 5-hydroxymethyl-2-furancarboxylic acid

Article abstract of DOI:10.1039/c8gc01340c

The sustainable synthesis of two important intermediates relevant for the production of bio-based polymers, 2,5-furandicarboxylic acid (FDCA) and 5-hydroxymethyl-2-furancarboxylic acid (HFCA), via oxidation of 5-(hydroxymethyl)furfural (HMF) was investigated using supported gold- and silver-based catalysts in water with air as the oxidant. High yields and selectivities for the production of FDCA (89%) and HFCA (≥98%) were achieved under the optimized reaction conditions with Au/ZrO₂ and Ag/ZrO₂ catalysts, respectively. While FDCA was mainly formed in the presence of gold catalysts at a maximum productivity of 67 mol_FDCA h^-1 mol_Au^-1, silver catalysts showed a remarkably high activity in aldehyde oxidation producing HFCA in almost quantitative yields with a maximum productivity of 400 mol_HFCA h^-1 mol_Ag^-1. By variation of the reaction parameters, the Au/ZrO₂ catalyst could be tuned to produce also HFCA, whereas the Ag/ZrO₂ catalyst exclusively produced HFCA in a wide range of reaction parameters. The observed differences in catalyst selectivities can be taken as a starting point for further mechanistic investigation on the oxidation of HMF, contributing to a fundamental understanding of this reaction which is particularly important for establishing the production of bio-based polymers.

Full text of DOI:10.1039/c8gc01340c

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Supported gold- and silver-based catalysts for the
selective aerobic oxidation of 5-(hydroxymethyl)-
furfural to 2,5-furandicarboxylic acid and
Cite this: DOI: 10.1039/c8gc01340c
5-hydroxymethyl-2-furancarboxylic acid†
Oliver R. Schade,a,b Kai F. Kalz,a,b Dominik Neukum,b Wolfgang Kleist c and
a,b
Jan-Dierk Grunwaldt
*
The sustainable synthesis of two important intermediates relevant for the production of bio-based poly-
mers, 2,5-furandicarboxylic acid (FDCA) and 5-hydroxymethyl-2-furancarboxylic acid (HFCA), via oxi-
dation of 5-(hydroxymethyl)furfural (HMF) was investigated using supported gold- and silver-based cata-
lysts in water with air as the oxidant. High yields and selectivities for the production of FDCA (89%) and
HFCA (≥98%) were achieved under the optimized reaction conditions with Au/ZrO2 and Ag/ZrO2 catalysts,
respectively. While FDCA was mainly formed in the presence of gold catalysts at a maximum productivity
of 67 molFDCA h−1 molAu−1, silver catalysts showed a remarkably high activity in aldehyde oxidation produ-
cing HFCA in almost quantitative yields with a maximum productivity of 400 molHFCA h−1 molAg−1. By vari-
ation of the reaction parameters, the Au/ZrO2 catalyst could be tuned to produce also HFCA, whereas the
Ag/ZrO2 catalyst exclusively produced HFCA in a wide range of reaction parameters. The observed differ-
ences in catalyst selectivities can be taken as a starting point for further mechanistic investigation on the
oxidation of HMF, contributing to a fundamental understanding of this reaction which is particularly
important for establishing the production of bio-based polymers.
Received 28th April 2018,
Accepted 20th June 2018
DOI: 10.1039/c8gc01340c
rsc.li/greenchem
furan (DFF), 5-hydroxymethyl-2-furancarboxylic acid (HFCA)
and 2,5-furandicarboxylic acid (FDCA). Among these, FDCA
Introduction
With diminishing fossil resources, efforts have to be taken for and HFCA are considered as very important secondary pro-
the future synthesis of chemicals. In this context, biomass can ducts from HMF, since they might substitute fossil monomers
serve as an attractive renewable feedstock for chemical synth- in bio-based polymers.7–10
eses.1 One of the most versatile molecules that can be syn-
Besides the use of stoichiometric oxidants like KMnO4,11
thesized from C6 carbohydrates, which are e.g. present in numerous pathways for the synthesis of FDCA have been
biomass feedstocks containing cellulose and hemicellulose, is reported to produce FDCA in a sustainable way. These
5-(hydroxymethyl)furfural (HMF).2,3 HMF can be produced methods include bio-12,13 or electrocatalytic14,15 reactions as
from hexoses by acid-catalyzed dehydration and can sub- well as homogeneous and heterogeneous metal catalysis. A
sequently be converted into various other products.2,3 In method which is industrially applied is the homogeneously
numerous studies, the conversion of HMF into valuable sec- catalyzed AMOCO mid-century process, which uses Co(OAc)2
ondary products has been investigated including e.g. via hydro- and Mn(OAc)2 as catalysts in acetic acid solvent at 125 °C and
genation,4
dehydrogenation5
or
hydrodeoxygenation.6 70 bar air pressure.16,17 More recent studies focused on the
Products originating from HMF oxidation are 2,5-diformyl- heterogeneously catalyzed oxidation of HMF for a more sus-
tainable process. In these studies, mainly nanoparticles of
noble metals like Pt,18–24 Pd,18,25–27 Ru28–31 or Au32–36 on
aInstitute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of
Technology (KIT), 76131 Karlsruhe, Germany. E-mail: grunwaldt@kit.edu
bInstitute of Catalysis Research and Technology, KIT, 76344 Eggenstein-
Leopoldshafen, Germany
various supports have been used for the oxidation of HMF to
FDCA, often giving almost quantitative yields. For example,
FDCA was produced with 96% yield after 5 h reaction time
using a Au/CeO2 catalyst at 130 °C.32 If the reaction is carried
out in basic aqueous solution, the oxidation proceeds via the
oxidation of the aldehyde group with the oxidation of the
alcohol functionality being the rate-limiting step.32,37
cIndustrial Chemistry – Nanostructured Catalyst Materials, Ruhr-University Bochum,
44801 Bochum, Germany
†Electronic supplementary information (ESI) available: Characterization results
and further experiments. See DOI: 10.1039/c8gc01340c
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Therefore, the partially oxidized HFCA is an intermediate in using air as the oxidant. Different preparation methods for
the oxidation reaction from HMF to FDCA. Although a number silver-based catalysts have been compared and the reaction
of active catalysts have been reported in the literature, stability conditions for Ag and Au catalysts have been optimized for the
and, consequently, recyclability are still challenging.32 Further targeted synthesis of HFCA and FDCA, respectively. In
strategies to increase the activity and stability of the catalysts addition, first studies towards the identification of the active
utilize alloy formation of different noble metals,38–41 innova- sites of the silver catalysts have been carried out.
tive support materials, such as carbonized MgO,23 carbon
nanotubes24 or N-doped carbon42 or a combination of both
approaches.43 In addition, the base-free conversion of HMF to
Experimental
produce FDCA has attracted attention, often using basic hydro-
Materials
talcites (HT) as the catalyst support.27,36,44 For example, a Au/
HT catalyst tested by Gupta et al.36 was able to produce FDCA
All chemicals were of analytical grade and have been used
without further purification: HMF, FDCA, AgNO3, ZrO(NO3)2
(Sigma-Aldrich), HFCA, NaOH, silver benzoate (Merck),
5-formyl-2-furoic acid, DFF (TCI Chemicals), zirconium(IV)
2-ethylhexanoate, HAuCl4·3H2O, ZrO2 1/8″ pellets, TiO2 1/8″
pellets, MgO (Alfa Aesar), CeO2 (MEL chemicals) and synthetic
air (Air Liquide).
in almost quantitative yield at 95 °C. However, when HT is
used as the catalyst support, leaching of Mg remains a key
issue.27,36,44
Besides its use as a monomer,8–10 HFCA shows antitumor
activity and can be used in the synthesis of an interleukin
inhibitor.45,46 In contrast to the synthesis of FDCA, the tar-
geted synthesis of HFCA has only rarely been pursued. For
example, biocatalytic oxidation reactions using Serratia liquefa-
ciens LF14 whole cells47 or xanthine oxidase48 were reported.
Kang et al.49 used a Cannizzaro reaction in ionic liquids for
the synthesis of HFCA with a yield of 46%. In this reaction, the
dialcohol was produced in equimolar amounts limiting the
maximum yield of HFCA to 50%. As mentioned above (cf.
Scheme 1), HFCA can also be generated as an intermediate in
the FDCA synthesis using noble metal catalysts, but in most
cases reaction conditions are optimized for FDCA production.
For example, HFCA was produced in almost quantitative yield
over a Au/CeO2 catalyst at 65 °C and 10 bar air pressure in less
than one hour and subsequently oxidized to FDCA.32 One of
the first methods for the targeted synthesis of HFCA was
reported by Reichstein50 in the 1920s using stoichiometric
amounts of silver oxide as the oxidant in basic solution. The
pure product was separated with 84% yield. Also supported
silver oxide on copper oxide was used for the oxidation of HMF
to HFCA.51 Zhang et al.52 studied the synthesis of HFCA using
a molybdenum acetylacetonate complex immobilized on mon-
tmorillonite K-10 clay as a heterogeneous catalyst in HMF oxi-
dation. This catalyst was able to produce HFCA in 87% yield in
toluene at 110 °C within three hours.52 However, for the sus-
tainable production of HFCA, the oxidation reaction should
preferably be performed in aqueous medium. In a recent
review article, Ventura et al.9 pointed out that the search for
highly active systems to produce HFCA as an alternative green
monomer is still highly desired.
Catalyst preparation
Deposition–precipitation (denoted as “dp”). The supported
gold and silver catalysts were prepared by conventional depo-
sition–precipitation. For this purpose, the support was sus-
pended in water and stirred for 30 min. Then, an aqueous
solution of the precursor was added and the suspension was
stirred for another 60 min. The suspension was brought to a
pH of 9 using 0.05 M NaOH and subsequently heated to 80 °C
for 2 h. After 60 min at 80 °C, the pH was checked again and
eventually more NaOH was added. After heating, the suspen-
sion was stirred at room temperature (rt) overnight, then fil-
tered off, washed and dried at 110 °C. The catalysts were cal-
cined (350 °C, 4 h) and finally reduced in a H2 stream (350 °C,
2 h, 3 L min−1, 10% H2 in N2).
In a variation of the dp method, the support was suspended
in water, the pH was adjusted to a value of 9 and the metal pre-
cursor (HAuCl4·3H2O or AgNO3) was added dropwise at con-
stant pH. No differences in activity were observed for the cata-
lysts obtained from the two different methods.‡
Impregnation (denoted as “imp”). In addition, impregnation
was conducted by dissolution of AgNO3 in ethanol and the
subsequent addition to the ZrO2 support. The suspension was
stirred at rt for 60 min and the solvent was removed under
reduced pressure. The catalyst was dried and treated in the
same way as in the case of deposition–precipitation.
Co-precipitation (denoted as “cp”). For the preparation of a
Ag/ZrO2 catalyst, additionally co-precipitation was applied
using a slightly modified method originally reported by
Grabowski et al.53 AgNO3 was dissolved in water and added to
a solution of ZrO(NO3)2 in HNO3. This solution was then
added dropwise to an aqueous solution of NaOH at a pH of 10
while the pH was kept constant by controlled addition of 0.5
M NaOH. After the addition of the precursor solution, the sus-
Herein, we report on supported gold and silver catalysts,
which are able to produce FDCA and HFCA, respectively, in
high yields from HMF in basic aqueous solution (Scheme 1).
The reactions have been performed in water as the solvent and
‡Due to the high basicity of MgO, the pH of the suspension was already at a
value of over 10. Therefore, the deposition–precipitation method 2 was modified
by addition of the noble metal precursor solution without additional NaOH.
Scheme 1 Oxidation products of HMF using gold and silver catalysts.
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pension was stirred overnight at room temperature. The result-
N2 physisorption was used to determine the specific surface
ing solid was filtered off, washed and dried at 110 °C. The cata- areas using the Brunauer–Emmet–Teller method on
a
lyst was calcined at 350 °C for 4 h and 14 h (“cp_350”), respect- Rubotherm BELSORP-mini II instrument. Catalysts were
ively. Additionally, calcination at 500 °C for 4 h (“cp_500”) was heated to 300 °C for 2 h under reduced pressure prior to the
performed. After calcination, the catalyst was reduced under experiment.
the previously described conditions.
The metal loading of the catalysts and the concentrations
Flame-spray pyrolysis (denoted as “fsp”). Finally, for the of metals in the reaction solutions were determined by
preparation of a Ag/ZrO2 catalyst, flame-spray pyrolysis was ICP-OES measurements (Agilent 720/725-ES instrument) and
used. First, a precursor solution was prepared by dissolving XRF of powder catalysts (Bruker S4 Pioneer spectrometer). For
silver benozoate and zirconium(IV) 2-ethylhexanoate in xylene the analysis of the solid catalysts via ICP-OES, the samples
using an ultrasonic bath. The concentration was 3.7 mmol L−1 were digested in a mixture of sulfuric and nitric acid.
for silver and 159.1 mmol L−1 for zirconium. A syringe pump
Electron microscopy was performed on carbon covered
was used to feed the precursor solution through a capillary copper grids using reduced catalyst samples. For TEM measure-
into the combustion flame (750 mL min−1 CH4, 1.6 L min−1 ments, the catalyst powders were mounted onto a carbon coated
O2), where the solution was dispersed with O2. The particles Cu TEM grid covered with a holey carbon film in the dry state.
were collected on a filter placed in a cylindrical vessel above Subsequently, the samples were examined under an FEI Titan
the flame that was connected to a vacuum pump. The prepa- 80-300 aberration corrected electron microscope operated at
ration was performed using a setup that was built at Karlsruhe 300 kV. High angle annular dark field (HAADF) scanning trans-
Institute of Technology (KIT) and is described elsewhere.54
After preparation, the catalyst was calcined and reduced as a Fischione model 3000 HAADF-STEM detector.
mission electron microscopy (STEM) images were acquired with
described above.
The separated products, HFCA and FDCA, were character-
ized using NMR spectroscopy. 1H and 13C NMR spectra
were obtained in DMSO-d6 at room temperature with a
Selective oxidation of HMF
The catalytic tests were performed in home-built autoclaves Bruker Avance 250 and a Bruker Avance 400 spectrometer,
(max. temperature 350 °C at a max. pressure of 200 bar, 52 mL respectively.
with Teflon® inlets). In a typical reaction, 1 mmol (126 mg)
HMF was dissolved in water and the desired amount of NaOH
was added using a 2.5 M solution resulting in a total volume
of 10 mL. Then, the catalyst was added, the reactor was closed,
purged three times with synthetic air and the desired pressure
Results and discussion
Catalyst preparation and characterization
was adjusted. The autoclaves were heated with heating sleeves
and heating plates and stirred magnetically. The time when
the desired reaction temperature was first reached was defined
as the starting point (t = 0). After the reaction, the reactors
were cooled to room temperature using an ice bath, depressur-
ized and the solutions were separated from the catalysts by
The metal loading and the specific surface area of the catalysts
and the corresponding support materials are listed in Table 1.
No change in the specific surface area of the supports was
observed upon deposition–precipitation or impregnation of
decantation. The samples were taken, filtered with a 0.45 µm Table 1 Specific surface areas of catalysts and the corresponding
support materials and noble metal content of supported gold and silver
catalysts prepared by different methods
Pall Teflon filter and diluted for HPLC analysis (Hitachi
Primaide, Bio-Rad Aminex HPX-87H column, solvent 5 mM
H2SO4, temperature 50 °C, 50 bar). After the reactions, HFCA
Support/
catalyst
Metal
loading/wt%
Specific surface
area/m2 g−1
and FDCA could be separated from the reaction mixtures, in
case the reactions gave the desired products in high yields.
FDCA was separated by acidification with HCl, whereas HFCA
was separated by acidification with HCl and subsequent extrac-
tion using EtOAc and evaporation of the solvent. The amount
of catalyst needed per reaction was calculated using the
nominal metal loading of 2 wt%.
Entry
1
2
3
4
5
6
7
8
ZrO2
TiO2
CeO2
MgO
—
—
—
—
88
42
40
6
90
89
38
39
40
40
5
Au/ZrO2_dp
Ag/ZrO2_dp
Au/TiO2_dp
Ag/TiO2_dp
Au/CeO2_dp
Ag/CeO2_dp
Au/MgO_dp
Ag/MgO_dp
Ag/ZrO2_imp
Ag/ZrO2_cp_350
Ag/ZrO2_cp_500
Ag/ZrO2_fsp
1.6a
1.0b
1.1b
1.7b
1.7b
1.7b
2.0b
2.0b
2.0b
1.9b
1.9b
1.7b
9
Characterization
10
11
12
13
14
15
16
XRD measurements of the powder catalysts were performed on
a PANalytical X’Pert Pro instrument using Cu Kα radiation
(wavelength = 1.54060 Å) at diffraction angles 2θ from 20° to
80° and a step size of 0.017° (0.53 s acquisition time). Support
materials were measured in the calcined state, and catalysts in
the calcined and reduced state. Measurements were performed
on rotating sample holders.
4
92
147
82
108
a Determination of metal loading by ICP-OES. b Determination of
metal loading by XRF.
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the noble metals (Table 1, entries 5 to 13). The ZrO2 support
used for the preparation of catalysts via deposition–precipi-
tation and impregnation had a specific surface area of ca.
90 m2 g−1. For the silver catalysts prepared by co-precipitation
and flame-spray pyrolysis (Table 1, entries 14–16), higher
specific surface areas were obtained. A maximum specific
surface area of 147 m2 g−1 was observed for the Ag/ZrO2_cp
catalyst calcined at 350 °C (Table 1, entry 14), and the specific
surface area decreased upon calcination at 500 °C (Table 1,
entry 15).
The efficiency of noble metal deposition was strongly
dependent on the support material and the respective metal.
For gold catalysts, a precipitation at lower pH values led to
higher loadings, however larger gold particles were obtained.55
An optimal pH value for deposition–precipitation of gold
seemed to be in the range of pH = 8–9.55–57 A low silver
content of 1 wt% was observed for the Ag/ZrO2_dp catalyst
(Table 1, entry 6), whereas higher silver loadings were achieved
on the other support materials. Changing the preparation
method allowed to receive an increased silver content on ZrO2
(Table 1, entries 13–16).
Most catalysts did not show reflections of noble metals or
noble metal oxides in the XRD patterns besides the reflections
of the corresponding support materials. This can be explained
by the low noble metal loadings and the desired presence of
small particles with a size below the detection limit of XRD
(<5 nm).58 The presence of small particles with a mean size of
3.7 nm was confirmed for the Au/ZrO2_dp catalyst by TEM
(Fig. S1†). No distinct Ag particles were found in the TEM
images of the Ag/ZrO2_dp catalyst probably due to the low
metal loading and the low contrast between silver and the
support material. However, HRTEM showed the presence of
some elliptical silver particles with a size of about 5 nm
(Fig. S2b†). In addition, there were no detectable changes in
the XRD patterns of commercial support materials before and
after the deposition–precipitation or impregnation of noble
metals. The only differences were those observed for the cata-
lysts supported on MgO. For the Au/MgO_dp catalyst, reflec-
tions of Mg(OH)2 were observed after calcination, which dis-
appeared after reduction (Fig. S4†). The calcined Ag/MgO_dp
catalyst showed reflections of reduced silver and Ag2O
(Fig. S5†). However, the reflections of Ag2O disappeared upon
reduction. Due to the high basicity of MgO, the deposition–
precipitation method was modified for MgO-supported cata-
lysts (cf. catalyst preparation). This high pH value might have
led to a rapid precipitation of large metal particles. Also note
the low surface area of the MgO support (Table 1), which most
likely also led to the formation of larger particles, as indicated
Fig. 1 XRD patterns of (a) the Ag/ZrO2_cp and (b) the Ag/ZrO2_fsp cat-
alysts. Solid lines correspond to calcined catalysts and dotted lines rep-
resent reduced catalysts (350 °C, 2 h, 3 L min−1, 10% H2 in N2); *cubic
ZrO2, +monoclinic ZrO2.
nation procedure at 350 °C for 4 h. This phase was also
obtained after increasing the calcination time; however, a
cubic ZrO2 phase was formed when the calcination tempera-
ture was increased to 500 °C for 4 h. The cubic ZrO2 phase was
also present in the Ag/ZrO2_fsp catalyst (Fig. 1). In contrast to
the other catalysts, a phase transition occurred upon the
reduction of the Ag/ZrO2_fsp catalyst. After reduction at 350 °C
in hydrogen (10% in N2), reflections of monoclinic ZrO2 were
observed for the Ag/ZrO2_fsp catalyst and the XRD pattern
became similar to that of the commercial monoclinic ZrO2
support, which was used for the preparation of the other
catalysts.
Oxidation of HMF over gold- and silver-based catalysts
by reflections from metallic phases for the Ag/MgO_dp cata- First, a catalyst screening was performed in basic aqueous
lyst. Although no reflections of Au were observed in the cal- solution using air as the oxidant in the presence of Au- and Ag-
cined or in the reduced Au/MgO_dp catalyst, large particle based catalysts prepared by deposition–precipitation. It is
sizes are expected for this catalyst36 as stated above.
known from the literature that HMF is unstable in basic solu-
Different XRD patterns were observed for the Ag/ZrO2_cp tion, as it undergoes a polymerization to dark polymeric pro-
and Ag/ZrO2_fsp catalysts, where the ZrO2 support was pre- ducts (humins).14 These products were not characterized and
pared together with the silver particles (Fig. 1). The Ag/ quantified but their presence explains the difference between
ZrO2_cp catalyst was X-ray amorphous after the standard calci- product yields and HMF conversion in Table 2 (i.e. a carbon
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Table 2 Catalyst screening of different Au- and Ag-based catalysts in
the oxidation of HMF. Reaction conditions: 100 °C, 10 bar air pressure, 4
equivalents of NaOH, 5 h reaction time, 1 mmol HMF in 10 mL H2O,
98 mg catalyst mass for Au-based catalysts, 54 mg catalyst mass for Ag-
based catalysts
an additional base giving higher overall yields of oxidation pro-
ducts, however a higher substrate-to-metal molar ratio was
used in that study.
The oxidation of the hydroxymethyl group usually proceeds
via an aldehyde intermediate, 5-formyl-2-furancarboxylic acid
(FFCA).37 However, FFCA has not been observed throughout
the catalytic tests presented in this study. This indicates that
the oxidation of the aldehyde group proceeds very rapidly over
all reported catalysts active in FDCA synthesis.
Optimization of reaction parameters. Since the best per-
formance towards HFCA and FDCA was observed using Ag/
ZrO2_dp and Au/ZrO2_dp, respectively (Table 2), further optim-
ization was conducted using ZrO2 as the support. The influ-
ence of the reaction temperature, the amount of NaOH added,
and the air pressure was investigated. First, the dependency on
the reaction temperature was determined by keeping the other
reaction parameters constant (Fig. 2).
Yield/%
Productivitya/
HMF
molproduct h−1
−1
Entry Catalyst
conversion/% HFCA FDCA molmetal
1
2
3
4
5
6
7
8
Au/ZrO2_dp 100
Ag/ZrO2_dp 100
Au/TiO2_dp 100
Ag/TiO2_dp 100
Au/CeO2_dp 100
Ag/CeO2_dp 100
Au/MgO_dp 100
Ag/MgO_dp 100
0
92
4
14
0
74
3
60
75
5
1
3
46
3
19
37
2
3
11
17
1
2
1
12
a Productivity is given as moles of product formed per mole of noble
metal and time. For gold- and silver-based catalysts, the productivity is
given for FDCA and HFCA production, respectively.
The yield reached a maximum value for the different
oxidation products for both catalysts and decreased due to
the thermal degradation of HMF at higher temperatures.
balance of less than 100%). From the screening results it was However, different selectivities of Au/ZrO2_dp and Ag/ZrO2_dp
observed that Au and Ag showed superior activity if they were towards oxidation products are visible. Using the Au/ZrO2_dp
supported on ZrO2 compared to the other support materials catalyst in basic solution (Fig. 2a), the oxidation of the hydroxy-
(Table 2, entries 1 and 2). This is reflected both by the product methyl group became the rate-limiting step. While mainly
yields and the catalyst productivity, giving an absolute value of HFCA was produced at 50 °C, HMF was further oxidized to
the product formed per noble metal content and reaction
time. The trend of a higher activity of ZrO2 supported metals
was previously reported by Sahu et al.22 for platinum-based cat-
alysts. A comparison of Ag/ZrO2_dp and Au/ZrO2_dp revealed
that while Au/ZrO2_dp mainly produced FDCA, the Ag/ZrO2_dp
catalyst did not produce FDCA in a yield higher than in blank
experiments. However, Ag/ZrO2_dp produced HFCA with a
high yield of 92%. Although silver catalysts have been used in
the oxidation of furfural,59 there is, to the best of our knowl-
edge, no report yet on silver catalysts used for the oxidation of
HMF.
The different selectivity of gold- and silver-based catalysts
can also be observed by comparing different support
materials. Astonishingly, TiO2 supported catalysts (Table 2,
entries 3 and 4) showed poor activity, although Au/TiO2 cata-
lysts have been reported to be highly active in the literature,
even with a rather low metal loading of 1 wt%.32,33 In the case
of CeO2, the lower activity can be explained by the type of
support used. Casanova et al.32 described a lower activity of
CeO2-supported gold catalysts if ceria was not of nanoparticu-
late nature. The low activity for the Au catalyst supported on
MgO might be explained by the presence of larger Au particles
on MgO (cf. catalyst preparation).36 The relatively high activity
of the Ag/MgO catalyst might be explained by the fact that
larger Ag particles are known to be catalytically active, for
example in ethylene epoxidation.60 A reason for the generally
lower activity of the MgO-supported catalysts might be the low
surface area and the high basicity of MgO. This might cause
HMF degradation if this catalyst is used with additional homo-
Fig. 2 Influence of the reaction temperature on product distributions
using (a) Au/ZrO2_dp and (b) Ag/ZrO2_dp. Reaction conditions: 10 bar
geneous bases resulting in the formation of humins. A Au/
air pressure, 4 equivalents of NaOH, 5 h reaction time, 1 mmol HMF in
MgO catalyst investigated by Gupta et al.36 was used without 10 mL H2O, 98 mg Au/ZrO2_dp catalyst, 54 mg Ag/ZrO2_dp catalyst.
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FDCA by increasing the reaction temperature to 125 °C. In
addition, the productivity reached a maximum value of
20 molFDCA h−1 molAu−1. Regarding the FDCA yield and pro-
ductivity, 125 °C seemed to be optimal for the production of
FDCA, while being a compromise between a short reaction
time and sufficient FDCA yields. This trend is similar to the
one observed by Casanova et al.32 who described 130 °C as the
upper limit of the reaction. In general, increasing the tempera-
ture accelerated the reaction, however, HMF suffered from
thermal degradation in basic aqueous solution,38 which was
confirmed in blank experiments (Table S1†). This is also rep-
resented in the carbon balance with the degradation of HMF
leading to a worse carbon balance with increasing tempera-
ture. Since HMF was quantitatively converted in almost all
reactions, highly active catalysts are needed to convert HMF
before its thermal polymerization in basic solution, as stated
above.
Comparing these results with the Ag/ZrO2_dp catalyst at
varying reaction temperatures (Fig. 2b), it was observed that
the synthesis of HFCA with Ag/ZrO2_dp proceeded at signifi-
cantly lower temperatures. The HFCA yield passed through a
maximum at 50 °C, where HFCA was produced in an almost
−1
quantitative yield of 98% with a productivity of 39 mol
h
HFCA
molAg−1. It seems that medium temperatures are beneficial for
the production of HFCA, in line with other catalysts.32,36,38–40
Interestingly, at room temperature the HFCA yield was lower
although HMF was fully converted. This might be due to the
polymerization of HMF under basic conditions at room temp-
erature where the catalyst was not as active as it was at slightly
elevated temperatures. A further increase of the temperature
lowered the HFCA yield while the FDCA yield remained at
about 2%. This together with the fact that FDCA production
Fig. 3 Influence of NaOH addition on the product distribution using (a)
the Au/ZrO2_dp and (b) the Ag/ZrO2_dp catalyst. Reaction conditions:
(a) 125 °C, 10 bar air pressure, 5 h reaction time, 1 mmol HMF in 10 mL
H2O, 98 mg Au/ZrO2_dp catalyst. (b) 50 °C, 10 bar air pressure, 1 h reac-
tion time, 1 mmol HMF in 10 mL H2O, 54 mg Ag/ZrO2_dp catalyst.
was in the range of blank experiments in all cases showed that the amount of base to two equivalents, the selectivity shifted to
silver-based catalysts are not capable of oxidizing the hydroxy- FDCA, which indicates that at least one hydroxide ion per side-
methyl group under the applied conditions, although silver chain is necessary for the oxidation of HMF. With an excess of
catalysts have generally been reported to oxidize alcohols in base, the FDCA yield increased further, probably by accelerat-
organic solvents.61 Consequently, the selective synthesis of ing the reaction, and then reached a plateau. Fig. 3b shows the
HFCA was possible in the presence of silver-based catalysts influence of the amount of added base on the yield of HFCA
leaving the alcohol group untouched, which is the main differ- using the Ag/ZrO2_dp catalyst. HMF remained unreacted in
ence from other catalyst systems where also the alcohol was the absence of a base contrary to the results of the Au/ZrO2_dp
converted to some extent.
catalyst. However, no conversion of HMF was observed in a
To prevent a possible degradation of HMF under highly control experiment at 50 °C in the presence of pure ZrO2
basic conditions, the amount of NaOH added to the solution within one hour (not shown). When no or only one equivalent
was decreased (Fig. 3). Both catalysts required the presence of of base was applied using Ag/ZrO2_dp, the HFCA yield
a base for the oxidation of HMF. However, the influence of the increased linearly with HMF conversion. A pH of 7 after those
added base and the observed selectivities differed when the reactions indicated a total consumption of the added OH−
Au/ZrO2_dp and the Ag/ZrO2_dp catalysts are compared.
ions during the reaction. In the previous mechanistic studies
HFCA was produced in 11% yield (26% selectivity) over Au/ on HMF oxidation over Pt and Au catalysts, oxidation of HMF
ZrO2_dp in the absence of a base (Fig. 3a). A control experi- via germinal diols formed by the addition of hydroxide ions to
ment using pure ZrO2 revealed that 39% of HMF was converted carbonyl groups has been proposed.37 Their formation seems
without the addition of a base at 125 °C (not shown). However, to be crucial for the oxidation of HMF over both catalysts used
only degradation products of HMF were formed. This showed in this study. In contrast to the Au/ZrO2_dp catalyst, no shift in
that Au/ZrO2 was active to a certain extent in HMF oxidation in the selectivity was observed using the Ag/ZrO2_dp catalyst,
the absence of a base. An increasing NaOH addition led to a which exclusively produced HFCA.
more selective conversion of HMF. The HFCA yield increased to
To establish a truly sustainable process, air was used as the
74% at one equivalent of NaOH related to HMF. By increasing oxidant in all reactions, which is, due to the lower oxygen con-
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centration, more demanding than using pure oxygen. Since pressure from 1 bar to 50 bar led to an increase in HFCA yield.
the reactions were performed at the previously optimized HFCA was produced quantitatively at an air pressure of 50 bar
temperature, high yields of HFCA and FDCA were already with the maximum productivity as high as 400 molHFCA h−1
−1
achieved at 10 bar air pressure. Therefore, the amount of cata- molAg
lyst was reduced compared to the standard amount of catalyst
.
Recycling of both catalysts was tested by separating the cata-
used in most reactions. As expected, using less catalyst at a lysts after the reactions, drying them at 110 °C and using them
pressure of 10 bar led to a lower amount of detected oxidation in subsequent runs (Fig. 5). Since a slight loss of catalyst
products for both catalysts (Fig. 4). Interestingly, the influence occurred during this process, the amount of HMF in the sub-
of air pressure on the product yields for Au/ZrO2_dp differed sequent runs was adjusted to the recovered catalyst mass. Both
from the Ag/ZrO2_dp catalyst.
catalysts remained active for a second cycle with a slight loss
The total yield of oxidation products remained almost con- of activity in the third run. The FDCA yield decreased to 63%
stant upon increasing the air pressure to 20 bar in the pres- using the Au/ZrO2_dp catalyst in a third reaction where also
ence of the Au/ZrO2_dp catalyst, where FDCA was produced 2% of HFCA were produced (Fig. 5a). Further re-use led to an
with 15% yield (Fig. 4a). Further increase of the air pressure increase in HFCA yield (13%) and a further decrease in FDCA
led to increased FDCA yields with a maximum yield of 89% yield. Since the carbon balance did not reach 100%, degra-
−1
and a maximum productivity of 67 molFDCA h−1 molAu at dation products of HMF might block the active sites during
50 bar. This productivity is in the upper range compared to consecutive reactions. For the Ag/ZrO2_dp catalyst, the loss in
other monometallic Au catalysts described in the literature activity was less pronounced and HFCA was produced with a
(Table S3†). In contrast to the gold catalyst and other reaction yield of 86% in a third reaction. The activity of both catalysts
parameters mentioned above, air pressure seemed to have a could not be restored by calcination and reduction of the cata-
significantly smaller influence on HFCA yield using the Ag/ lysts, which has also been reported earlier.32 Further aspects
ZrO2_dp catalyst (Fig. 4b). Already at ambient air pressure, that might be the reasons for a limited recyclability of the cata-
HFCA was produced with 68% yield. Increasing the air lysts might be sintering of particles or leaching of the active
Fig. 4 Influence of the air pressure on product distribution using
(a) Au/ZrO2_dp and (b) Ag/ZrO2_dp. Reaction conditions: (a) 125 °C,
4 equivalents of NaOH, 5 h reaction time, 1 mmol HMF in 10 mL
H2O, 33 mg Au/ZrO2_dp catalyst. (b) 50 °C, 4 equivalents of NaOH,
1 h reaction time, 1 mmol HMF in 10 mL H2O, 28 mg Ag/ZrO2_dp
catalyst.
Fig. 5 Stability tests of (a) Au/ZrO2_dp and (b) Ag/ZrO2_dp. Reaction
conditions: (a) 100 °C, 10 bar air pressure, 4 equivalents of NaOH, 5 h
reaction time, HMF and H2O adjusted to recovered catalyst. (b) 50 °C, 10
bar air pressure, 4 equivalents of NaOH, 5 h reaction time, HMF and H2O
adjusted to recovered catalyst.
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metal. However, neither detectable amounts of gold nor of that either soluble silver ions or in situ formed nanoparticles
silver were found in the solutions after the reaction by means might be a catalytically active species in the oxidation of HMF.
of ICP-OES performed under standard conditions (<0.5 ppm). Directly after the addition of AgNO3 to the basic HMF solution,
In contrast, reflections of reduced metals appeared both for a black precipitate was formed, which indicates the reduction
the Au/ZrO2_dp and the Ag/ZrO2_dp catalysts in XRD of the silver ions, most probably by the aldehyde functionality
(Fig. S6†). This indicates a growth of the metal particles for of HMF.64 In addition, a mechanism for HMF oxidation cata-
both catalysts, which might be the reason for a decreased lyzed by gold and platinum nanoparticles proposed by Davis
activity with an increasing number of runs. Especially for the et al.37 claims that the electrons of HMF oxidation are taken
Au/ZrO2_dp catalyst, this might play a significant role since it up by the metal particles, further keeping the metal in the
is known that particle size is a crucial parameter for Au-based reduced state. Next, AgNO3 was added to a basic suspension
catalysts.62
containing HMF and ZrO2. The reaction was performed analo-
In none of the studies using silver catalysts, a significant gously to the previous experiment at 10 bar air pressure and
amount of FDCA was produced, which is the main difference 50 °C. In this reaction, 61% of HFCA was produced and, as
between the commonly used noble metal catalysts and the before, no silver was detected by ICP-OES in solution after the
silver catalysts described in the present study. To investigate reaction. However, XRD and XRF confirmed the presence of
whether silver is capable of oxidizing the alcohol group of metallic silver particles on the separated solid (see Fig. S8†).
HMF, further reactions were performed. No FDCA was pro- This material was then used as a catalyst in a subsequent reac-
duced by extending the reaction time from 5 to 17 h. Since the tion, where HFCA was produced in a high yield of 92%, indicat-
catalyst showed reflections of metallic silver after the reaction, ing that indeed reduced silver particles might be responsible for
a possible deactivation by sintering of silver particles was HMF oxidation. The generally lower HFCA yield using AgNO3
investigated. Therefore, a fresh catalyst was added to the HFCA might be explained by a rather slow particle growth compared to
solution produced in the first catalytic run. However, HFCA the fast HMF polymerization. A comparable pathway of metallic
was not further converted to other products. This shows that nanoparticles forming upon reduction of a homogeneous cata-
the silver catalysts were not able to oxidize the hydroxymethyl lyst by the substrate as the active species has been reported for
group of HMF under the applied conditions.
the Au-catalyzed oxidation of dibenzylamine.65
Despite the obtained evidence for the formation of metallic
nanoparticles, dissolved silver species might still be catalyti-
Stability and active sites of silver catalysts
During this study, it was observed that the Ag/ZrO2_dp catalyst cally active in the oxidation of HMF, e.g. via leaching and sub-
slightly deactivated upon storage for some days in air. Besides sequent redeposition onto the support. To gain information
a color change of the catalyst from dark grey to a lighter brown- on the degree of silver leaching, the Ag/ZrO2_dp catalyst was
ish color, the HFCA yield decreased from 98% to 84%, which stirred under reaction conditions without the addition of HMF
nearly corresponds to the yield of HFCA achievable with a fully as a reducing agent at different pH values and the silver
calcined catalyst (82%). Although a reductive pretreatment is content was analyzed via ICP-OES (Fig. 6a). The results showed
not necessarily required for noble metal catalysts as they are that significant leaching took place below pH 12, while a slight
normally reduced also at high temperatures or even under leaching was recognized above pH 12. Only at pH 12, no silver
reaction conditions, the used calcination temperature of was found in solution by means of ICP-OES. Oxidative dis-
350 °C might not have been sufficient for the reduction of oxi- solution of silver from silver nanoparticles is a subject of
dized silver species.61 This might indicate an oxidation of the current research.66–68 In general, different mechanisms for
reduced catalyst in air, which is not reversible under the silver dissolution have been proposed; however, a common
applied reaction conditions. As already stated above, no silver feature is a strong effect of pH on silver dissolution. For
species could be observed in the XRD patterns, probably due example, Sotiriou et al.66 studied the oxidative dissolution for
to the low metal loading and small particle sizes. The loss in Ag/SiO2 prepared by impregnation and flame-spray pyrolysis. It
activity of stored samples could be partially regained by calci- was found that the degree of silver dissolution was the same
nation and reduction of the catalyst for a second time (88% for both preparation methods and that small silver particles
HFCA yield). This finding raised the question of the true were more prone to dissolution. Comparing the amount of
nature of the active species in the oxidation reaction in terms silver with the particle size suggested silver dissolution from
of oxidation state and nature of active sites.
one to two monolayers of Ag2O on silver particles.66 Reduction
As has been shown in the literature, homogeneous silver of both samples in hydrogen led to a decrease in dissolution
catalysts are able to oxidize furanic compounds like furfural.63 by minimizing the fraction of oxidized silver. In support of the
However, no silver dissolution was detected by means of assumptions made by Sotiriou et al.,66 a small fraction of oxi-
ICP-OES (<0.5 ppm) in the reaction solutions. Nevertheless, dized silver after preparation was observed by X-ray absorption
control experiments were performed: first, AgNO3 was used as spectroscopy of flame-made Ag/SiO2 catalysts.69 The formation
a model system for a soluble silver catalyst at 50 °C and 10 bar of a stabilizing layer of Ag2O which is poorly soluble and there-
air pressure. AgNO3 was added at 1 mol%, which was the same fore stable at high pH might be an explanation for a decreas-
amount as for a supported 2 wt% catalyst. Under these con- ing silver content at high pH (Fig. 6a).68 After these initial
ditions, HFCA was produced with a yield of 59%. This shows leaching experiments, the reaction solutions of the experi-
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further studies have to be performed. Silver catalysts are often
employed in selective oxidation reactions and therefore, the
formation of different silver oxygen species and their appear-
ance in gas phase oxidation mechanisms have been studied
extensively.70–74 In general, their formation strongly depends
on the pretreatment conditions especially at high tempera-
tures. Three main silver oxygen species are generated upon
exposure of silver to oxygen. At lower temperatures (below
330 °C), surface adsorbed oxygen (Oα) is formed.70,72
Increasing temperatures first lead to bulk dissolved oxygen
(Oβ) before strongly bound surface oxygen species (Oγ, 630 °C)
are generated. These species are crucial in some reactions. For
example, a silver catalyst calcined at high temperature was
more active in the selective oxidation of CO in hydrogen than a
reduced catalyst, proving silver oxygen species to be active in
this case.73 Given that a reduction in hydrogen atmosphere
leads to more active catalysts in HMF oxidation (see above) and
a possible formation of a thin layer of Ag2O on silver particles
at high pH,68 it can be speculated that a surface layer oxide is
involved in the catalytic cycle. The formation of this surface
silver oxide at high pH might also explain the HFCA yield only
being moderately influenced by increasing air pressures
(Fig. 4). In a recent publication, Durndell et al.75 studied the
oxidation of cinnamaldehyde over platinum catalysts and
found PtO2 to be an active species. Because of the lack of infor-
mation on reactions in the liquid phase, further studies of
silver catalysts under HMF oxidation conditions are very
important. For this purpose, operando X-ray absorption spec-
troscopy is expected to be a powerful tool.76–78
Fig. 6 Comparison of (a) the leaching behavior and (b) catalytic activi-
ties (b) of Ag/ZrO2 catalysts prepared by different methods. Reaction
conditions: (a) 50 °C, 10 bar air pressure, adjustment of pH with NaOH,
1 h, 54 mg catalyst. (b) 50 °C, 10 bar air pressure, 4 equivalents of NaOH,
1 h reaction time, 1 mmol HMF in 10 mL H2O, 54 mg catalyst.
Variation of the preparation method
In order to minimize the silver leaching, different preparation
methods were applied. Preferably catalysts with a stronger
interaction of the silver particles with the support should be
used. Catalysts were prepared by flame-spray pyrolysis, co-pre-
cipitation and impregnation. The catalysts were tested under
the optimized reaction conditions for silver-based catalysts. As
Fig. 6b shows, the catalysts differed in activity with the Ag/
ZrO2_fsp catalyst giving the best results. Comparison of the
Ag/ZrO2_cp catalysts calcined at different temperatures
revealed that the Ag/ZrO2_cp_500 catalyst gave 86% yield of
HFCA whereas the Ag/ZrO2_cp_350 catalyst produced HFCA in
42% yield (not shown). Most Ag/ZrO2 catalysts prepared in this
study produced HFCA in comparable yields, apart from Ag/
ZrO2_imp which was the only catalyst that produced HFCA
with a lower yield of 61% (Fig. 6b). Since the catalysts differed
in silver loadings (Table 1), the productivity toward HFCA is
another reasonable parameter for catalyst comparison. Ag/
ZrO2_imp gave the lowest productivity of 61 molHFCA h−1
ments using the Ag/ZrO2_dp catalyst with the initial pH values
of 8, 12 and 14 were tested in the oxidation of HMF
(Table S2†). No conversion of HMF was observed using the
solutions with the initial pH values of 8 and 12, which showed
that no reaction took place if either the base (pH 8) or soluble
silver species (pH 12) were missing. In contrast, HFCA was pro-
duced using the solution with an initial pH value of 14, where
obviously enough silver and hydroxide ions were present to
convert HMF into HFCA. However, only 62% of HMF was con-
verted and HFCA was produced with a low selectivity of 24%
(15% yield). These results show that silver leached from the
catalyst with no reducing agent (HMF) present in solution.
However, leaching occurred to a large extent at pH values,
where HMF could not be converted in further experiments. At
the high pH values (pH 14) that were also applied for the sup-
ported catalysts, soluble Ag species showed inferior activities
and selectivities compared to the reactions where the hetero-
geneous catalysts were used.
molAg−1. The productivity was 103 molHFCA h−1 molAg−1 for the
−1
Ag/ZrO2_fsp and 91 molHFCA h−1 molAg
for the Ag/
These experiments provided a first information on the
active species of silver catalysts in the oxidation of HMF. Our
results thus indicate that the reaction takes place on reduced
silver particles and further studies hinted at a predominantly
heterogeneous mechanism. However, concerning the mecha-
nism i.e. the role of silver particles in the catalytic reaction,
ZrO2_cp_500 catalyst, which corresponds to their performance
described in Fig. 6b. Since Ag/ZrO2_dp was the most active
catalyst (productivity of 164 molHFCA h−1 molAg at identical
−1
reaction conditions), but featured the lowest silver content, the
preparation method strongly influenced the catalytic activity.
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Deposition–precipitation seemed to be optimal for achieving a for the synthesis of HFCA to a certain extent. However, control
high activity per silver content. experiments evidenced that the reaction was mostly hetero-
In addition, the catalysts prepared by different methods geneously catalyzed. By variation of the preparation method
differed in their leaching behavior (Fig. 6a). The general trend for Ag/ZrO2 catalysts, leaching could be significantly reduced
in leaching behavior as a function of pH remained the same. demonstrating the potential for a future continuous process.
However, all catalysts were significantly less prone to leaching With the investigated silver catalysts, the synthesis of HFCA in
compared to the Ag/ZrO2_dp catalyst. For Ag/ZrO2_fsp (≤7%) high yields with water as the solvent was possible, opening up
and Ag/ZrO2_imp (≤13%), less Ag leaching was observed. an attractive route to produce a promising polymer precursor
These preparation methods might have led to a stronger based on renewable resources.
binding of silver particles or even an encapsulation of silver in
ZrO2, which might prevent the oxidation of silver and, there-
fore, promote a lower degree of leaching at low pH. As dis-
Conflicts of interest
cussed above, an oxide layer on silver particles may be the
reason for silver leaching at low pH.66 Since almost no leach-
There are no conflicts of interest to declare.
ing occurred using the Ag/ZrO2_cp_500 catalyst (3% at pH 14),
it was tested in several consecutive runs. However, a loss of
activity was recognized after two runs leading to a yield of 58%
of HFCA in the third reaction (Fig. S9†).
Acknowledgements
This work was funded and supported by KIT. The authors
thank Wu Wang, Dr. Sabrina Müller, and the Karlsruhe Nano
Micro Facility (KNMF) at KIT for providing STEM-EDX
measurements. They also thank Angela Beilmann (BET),
Hermann Köhler (XRF and ICP-OES) and Matthias Stehle
(support during the synthesis of the flame-made catalyst). The
KIT synchrotron provided beamtime at the CAT-ACT beamline
and Dr. Anna Zimina and Dr. Tim Prüßmann are gratefully
acknowledged for the XAS measurements.
Considering the limited number of studies focusing on the
targeted synthesis of HFCA, the silver-based catalysts pre-
sented in this study show a superior performance compared to
other routes, i.e. the Cannizzaro reaction49 or the synthesis of
HFCA in toluene using pure oxygen.52 Moreover, the silver-
based catalytic oxidation of HMF to HFCA reported herein
opens up an attractive alternative to the more frequently
applied noble metal-catalyzed production of FDCA. The silver-
based catalysts did not only show an excellent performance in
the production of HFCA, they also required a smaller amount
of base for almost quantitative HFCA yields (1 eq., see Fig. 3).
Ongoing work in our laboratories aims – among others – at
obtaining a deeper mechanistic understanding and at using
HFCA as a monomer for bio-based polymers.
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