Selective Aerobic Oxidation of 5-(Hydroxymethyl)furfural over Heterogeneous Silver-Gold Nanoparticle Catalysts

Article abstract of DOI:10.1002/adsc.202001003

Bimetallic silver-gold alloy nanoparticles on zirconia with varying Ag/Au ratios were designed by a rational approach and tested as catalysts for the selective oxidation of the promising biomass platform molecule 5-(hydroxymethyl)furfural (HMF). For this purpose, colloidal Ag_xAu_10-x particles with molar compositions x=1/3/5/7/9 were prepared by laser ablation in liquids, a surfactant-free method for the preparation of highly pure nanoparticles, before adsorption on zirconia. In-depth characterization of the supported catalysts evidenced alloyed nanoparticles with distinct trends of the surface and bulk composition depending on the overall Ag/Au molar ratio as determined by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), respectively. To uncover the synergistic effect of the Ag/Au ratio, the catalysts were further studied in terms of the catalytic activity and selectivity in HMF oxidation. Either the aldehyde moiety or both functional groups of HMF were selectively oxidized depending on the Ag/Au composition resulting in 5-hydroxymethyl-2-furan-carboxylic acid (HFCA) or 2,5-furandicarboxylic acid (FDCA), respectively. Optimization of the reaction conditions allowed the quantitative production of HFCA over most catalysts, also after re-use. Only gold rich catalysts Ag₁Au₉/ZrO₂ and particularly Ag₃Au₇/ZrO₂ were highly active in FDCA synthesis. While Ag₃Au₇/ZrO₂ deactivated upon re-use due to sintering, no structural changes were observed for the other catalysts and all catalysts were stable against metal leaching. The present work thus provides fundamental insights into the synergistic effect of Ag and Au in alloyed nanoparticles as active and stable catalysts for the oxidation of HMF. (Figure presented.).

Full text of DOI:10.1002/adsc.202001003

Title: Selective aerobic oxidation of 5-(hydroxymethyl)furfural over
heterogeneous silver-gold nanoparticle catalysts
Authors: Oliver Schade, Frederic Stein, Sven Reichenberger, Abhijeet
Gaur, Erisa Saraci, Stephan Barcikowski, and Jan-Dierk
Grunwaldt
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Adv. Synth. Catal. 10.1002/adsc.202001003
Link to VoR: https://doi.org/10.1002/adsc.202001003

10.1002/adsc.202001003
Advanced Synthesis & Catalysis
Very Important Publication
FULL PAPER
DOI: 10.1002/adsc.202((will be filled in by the editorial staff))
Selective Aerobic Oxidation of 5-(Hydroxymethyl)furfural over
Heterogeneous Silver-Gold Nanoparticle Catalysts
Oliver R. Schade,a,b Frederic Stein,c,d Sven Reichenberger,c,d Abhijeet Gaur,a,b Erisa
Saraҫi,a,b Stephan Barcikowskic,d and Jan-Dierk Grunwaldta,b*
a
Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe,
Germany
Phone: (+49) 721 608 42120; fax: (+49) 721 608 44820; e-mail: grunwaldt@kit.edu
Institute of Catalysis Research and Technology, Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-
b
Leopoldshafen, Germany
Technical Chemistry I, University of Duisburg-Essen, 45141 Essen, Germany
Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, 47057 Duisburg, Germany
c
d
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######
Abstract. Bimetallic silver-gold alloy nanoparticles on Ag/Au composition resulting in 5-hydroxymethyl-2-furan-
zirconia with varying Ag/Au ratios were designed by a carboxylic acid (HFCA) or 2,5-furandicarboxylic acid
rational approach and tested as catalysts for the selective (FDCA), respectively. Optimization of the reaction
oxidation of the promising biomass platform molecule conditions allowed the quantitative production of HFCA
5-(hydroxymethyl)furfural (HMF). For this purpose, colloidal over most catalysts, also after re-use. Only gold rich catalysts
AgxAu10-x particles with molar compositions x=1/3/5/7/9 were Ag1Au9/ZrO2 and particularly Ag3Au7/ZrO2 were highly
prepared by laser ablation in liquids, a surfactant-free method active in FDCA synthesis. While Ag3Au7/ZrO2 deactivated
for the preparation of highly pure nanoparticles, before upon re-use due to sintering, no structural changes were
adsorption on zirconia. In-depth characterization of the observed for the other catalysts and all catalysts were stable
supported catalysts evidenced alloyed nanoparticles with against metal leaching. The present work thus provides
distinct trends of the surface and bulk composition depending fundamental insights into the synergistic effect of Ag and Au
on the overall Ag/Au molar ratio as determined by X-ray in alloyed nanoparticles as active and stable catalysts for the
photoelectron spectroscopy (XPS) and X-ray absorption oxidation of HMF.
spectroscopy (XAS), respectively. To uncover the synergistic
effect of the Ag/Au ratio, the catalysts were further studied in
terms of the catalytic activity and selectivity in HMF
oxidation. Either the aldehyde moiety or both functional
groups of HMF were selectively oxidized depending on the
Keywords: Bimetallic catalysts, FDCA, gold,
heterogeneous catalysis, hydroxymethylfurfural, laser
ablation, selective oxidation, silver, X-ray absorption
spectroscopy
8]
(FDCA) to terephthalic acid.[1c,
Because of its
Introduction
significance as a bio-derived monomer, FDCA was
even listed as one of twelve important molecules that
could be produced in an industrial scale by the U.S.
Department of Energy.[9] Selectively oxidizing the
aldehyde moiety of HMF yields 5-hydroxymethyl-2-
furancarboxylic acid (HFCA), which also has possible
applications as a monomer[1c, 8] as well as a building
The production of chemicals from renewable raw
materials is becoming increasingly important as the
growing demand for chemicals meets dwindling fossil
resources, which currently account for the largest
share of raw materials in the chemical industry by
far.[1] One promising approach for the future synthesis
block in high-value fine chemicals, like drugs[10]
.
of chemicals is the use of platform molecules from
The synthesis of FDCA starting from HMF has been
studied using stoichiometric oxidants like KMnO4.[11]
As the production of stoichiometric waste should be
circumvented,[12] catalytic routes have been studied
using homogeneous, heterogeneous, electro- and
biocatalysis. [1c, 7] Currently, the AMOCO process is
used for the large-scale industrial synthesis of
FDCA.[13] FDCA is produced in 61 % yield over a
homogeneous Co/Mn/Br catalyst at 125 °C and 70 bar
air in acetic acid as solvent.[13a] Advantages such as
inedible biomass like lignocellulose.[1b,
Partial
2]
fragmentation of biomass to sugars followed by acid-
catalyzed dehydration results, for example, in
5-(hydroxymethyl)furfural (HMF), one of the most
promising biomass platform molecules.[2a, 3] Among
other
reactions
like
hydrogenation[4],
dehydrogenation[5] or hydrodeoxygenation[6] the
selective oxidation of HMF has gained increasing
interest in recent years,[1c, 7] particularly due to the
structural similarity of 2,5-furandicarboxylic acid
1
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10.1002/adsc.202001003
Advanced Synthesis & Catalysis
simple separation of the catalyst have now shifted the
focus more on the more demanding heterogeneously
catalyzed oxidation. High FDCA yields are mostly
obtained over noble metal catalysts like Ru, Pd, Pt or
Au (Figure 1),[1c, 7, 14] which often require the addition
of a homogeneous base to enhance FDCA solubility
and accelerate the reaction.[15]
As it is mostly considered an intermediate in FDCA
synthesis, a small number of studies focus on the
targeted synthesis of HFCA. Besides a chemical
synthesis using the Cannizzaro reaction[16], which is
limited to a maximum HFCA yield of 50 %,
biocatalytic reactions were employed for HFCA
synthesis[17]. Also, heterogeneous catalysts gave high
yields in organic solvents, which however affects the
sustainability of the reaction.[18] In contrast, supported
Ag-based catalysts allow the quantitative production
of HFCA in water under mild reaction conditions
(Figure 1).[19]
pyrolysis[26], or colloidal preparation methods with
chemical reduction.[27]
The most commonly used precursors HAuCl4 and
AgNO3 have several disadvantages, such as reduction
of Au before Ag leading to the formation of a
composition gradient throughout the bimetallic
nanoparticle with a Au-rich core and a Ag-rich
shell.[28] Further, AgCl may precipitate parallel to the
reduction and high chloride concentrations can lower
the catalytic activity of Au-based catalysts.[29]
Working below the solubility limit of AgCl is
impractical for the preparation of supported catalysts
as these high dilutions complicate subsequent
deposition.[30] Finally, capping agents or ligands might
also affect the catalytic activity by blocking the active
sites.[31] This may alter oxygen activation, catalytic
activity, and selectivity in addition to the nanoparticle
composition.
To overcome such shortcomings, a rational catalyst
design is needed.[32] Pulsed laser ablation in liquids
(LAL) has shown to be a promising alternative for the
preparation of metal, oxide, and alloy nanoparticle
catalysts.[33] In this method, a pulsed laser beam is
focused onto the target immersed in a liquid, leading
to rapid heating combined with the formation of a
plasma plume. The superheated and evaporating
material subsequently forms a jet of superheated
matter which gets injected into the overlying liquid
phase on a timescale of 10th to 100th nanoseconds,
inducing rapid cooling with up to 1012 K s-1.[34] Due to
the heat transfer into the liquid, a cavitation bubble
emerges, containing the nucleating and growing
nanoparticles. The bubble collapses about 200 µs after
the laser hits the target thereby releasing the
nanoparticles.[35] Because of the uniform and rapid
evaporation and nucleation kinetics, LAL is an
efficient method to generate surfactant-free Ag-Au
nanoparticles with a homogeneous as well as average
elemental distribution and high purity, rendering them
as attractive systems to study the selectivity of Ag and
Au for HMF oxidation to FDCA and HFCA.
Within this study, (1) AgxAu10-x/ZrO2 catalysts were
prepared by supporting on ZrO2 nanoparticles
generated via LAL with systematically varying the
molar composition (Ag/Au ratio x=1/3/5/7/9) with
comparable size (2) and applied in the selective
oxidation of HMF to HFCA and FDCA. The overall
strategy is shown in Figure 2 a. After thorough catalyst
characterization concerning structure and composition,
the effect of alloy formation on the catalytic activity
and selectivity in aldehyde and alcohol oxidation was
explored. The catalytic reactions were carried out in an
aqueous medium using air as the oxidant.
Bimetallic catalysts are of particular interest in HMF
oxidation as they were reported to be more active and
stable upon re-use.[1c, 7, 20] The previously mentioned
high selectivity of Ag towards HFCA and Au towards
FDCA formation suggests that tuning the oxidation
catalyst by alloy formation of Ag with Au appears to
be especially attractive. However, to the best of our
knowledge, this has not yet been conducted in HMF
oxidation.
For other reactions it has been shown that bimetallic
Ag-Au catalysts were more active in selective[21] as
well as total oxidation[22] reactions, e.g. the superior
activity of bimetallic Ag-Au catalysts in glucose
oxidation was attributed to the activation of oxygen on
Ag. [21a] It is well known that oxygen activation is often
challenging on Au-based oxidation catalysts,[23] which
on the other hand is favored on Ag. Hence, synergistic
effects can be expected. In addition, both metals are
fully miscible in every molar ratio.
To study how Ag modifies the Au catalyst for different
molar ratios, catalysts with alloyed and comparable
particle sizes are required. Despite their miscibility,
the synthesis of alloyed Ag-Au nanoparticles is not
straightforward. Preparation methods include
sequential
impregnation,[25] flame-spray
deposition-precipitation,[24]
co-
Results and Discussion
Rational design and characterization of the catalysts
Figure 1. Schematic reaction mechanism of HMF oxidation
in alkaline aqueous solution over gold- (bottom) and silver-
based (top) catalysts. The activity, selectivity and
performance of bimetallic Ag-Au catalysts (middle) is the
focus of this study.
The general approach of this study is depicted in
Figure 2 a. Nanoparticles were synthesized by LAL
with a monomodal size distribution with average
2
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10.1002/adsc.202001003
Advanced Synthesis & Catalysis
hydrodynamic diameters of 6-7 nm except for Ag3Au7
observed both by the significant color change of the
(12 nm) based on analytical disc centrifugation (ADC). initially white ZrO2 powder (Figure 2 a) and the
The synthesis is described in detail in the experimental
part and a scheme (Figure 2 a) and photograph of the
setup is given in the supporting information (Figure
S1).
depletion of AgAu NPs in the colloidal suspensions
(Figure 2 b) during the deposition process. After
removing the aqueous supernatant, AgxAu10-x/ZrO2
catalysts were dried and characterized by DR-UV-Vis
confirming the successful adsorption of AgAu NPs on
the support (Figure S4 a). In accordance with UV-Vis
measured of the initial colloids, a composition-
dependent linear shift of the SPR peak was found
(Figure S4 b). The presence of the nanoparticles on the
ZrO2 support was further evidenced by STEM analysis.
Particle size distributions, exemplarily shown for
Ag3Au7/ZrO2 in Figure 2 d (for all other catalysts cf.
Figures S14-S15), and mean particle sizes were
derived from STEM images of the supported catalysts
(Table 1). From the comparison to the initial size
distributions measured for the colloids by ADC, the
nanoparticle size of colloidal AgAu NPs were
maintained after the deposition process for all catalysts
(see Figure S15). In accordance with ADC
measurements conducted before colloidal deposition,
AgAu NP sizes of 7-11 nm were also found by STEM
on the ZrO2 support after deposition. Consequently,
particle sizes were maintained during the deposition
reported previously in other studies for laser-generated
catalysts.[33b, 43] EDX line scans of representative
Respective particle size distributions are shown in
Figure S3. From UV-Vis spectra of the resulting
colloidal suspensions, shown in Figure 2 b, single
absorbance peaks that are shifted linearly with the
composition of the particles can be observed
(Figure 2 c), indicating successful alloy formation.[36]
Although the optical properties of the nanoparticles
also depend on their size and shape,[37] the respective
shifts are in line with literature where comparable
particle sizes and shapes were investigated.[38] Slight
differences arise from the presence of stabilizing
ligands during biological or chemical syntheses,[39]
whereas laser-generated alloy nanoparticles give the
lowest surface plasmon resonance (SPR) wavelengths
due to the absence of such agents.[36a, 40] Hence, LAL
proves to be an ideal method for the synthesis of
alloyed Ag-Au nanoparticles (AgAu NPs) compared
to
wet-chemical
preparation.[41]
Successful
electrostatic adsorption of the nanoparticles on ZrO2
was achieved by colloidal deposition under careful pH
adjustment as previously described[42] and was
Figure 2. (a) Schematic approach of the study presented here and (b) UV-Vis spectra of the alloyed colloids before and after
electrostatic adsorption on ZrO2, (c) linear shift of the SPR with increasing Au content and comparison to literature values
from Link et al.[38], (d) representative particle size distribution of Ag3Au7/ZrO2 based on TEM and (e) representative EDX
line scan of Ag3Au7 particles.
Table 1. Metal loading and specific surface area of the supported catalysts and the ZrO2 support material.
Entry Catalyst / Support
Metal loading
/ wt.%
-
1.5
1.3
1.3
0.7
1.4
Specific surface
Mean particle
size /nma)
-
9.4 ± 1.4
6.9 ± 1.6
8.1 ± 2.2
9.9 ± 2.1
11.4 ± 1.9
Ag:Au molar ratiob)
area / m2 g-1
1
2
3
4
5
6
--/ZrO2
99
99
98
100
100
98
Ag1Au9/ZrO2
Ag3Au7/ZrO2
Ag5Au5/ZrO2
Ag7Au3/ZrO2
Ag9Au1/ZrO2
11:89 (10:90)
32:68 (30:70)
53:47 (50:50)
74:26 (70:30)
90:10 (90:10)
a) Mean particle size based on TEM. b) values from ICP-OES with theoretical values given in brackets for comparison.
3
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10.1002/adsc.202001003
Advanced Synthesis & Catalysis
Ag1Au9/ZrO2 and Ag3Au7/ZrO2 catalysts showed a
homogeneous distribution of both metals along the
particle diameters (Figure 2 e, Figures S16-S17). ICP-
OES verified the metal loading to be close to 1.5 wt.%
in all but one cases (Table 1). Additionally, the Ag/Au
molar ratios are well in line with the nominal
compositions of the target alloys. It can be observed
from N2 physisorption, that the specific surface area
was maintained during the colloidal AgAu NP
deposition process (Table 1). Since the deviations are
within the margin of error, we can conclude that the
support was not changed during the adsorption process
and no pores were blocked by the metal particles,
further confirmed by similar pore sizes of the pure
ZrO2 and the supported catalysts.
All catalysts were examined via XAS to further
unravel the local electronic structure (oxidation state),
charge transfer phenomena, and alloy formation. The
former two can be derived from the X-ray absorption
near edge structure (XANES) while the extended X-
ray absorption fine structure (EXAFS) provides
information about alloy formation, as it is sensitive to
the different backscattering of Ag and Au
neighbors.[44] The overall oxidation state of Au can be
estimated from the XANES region by the area of white
line features in the Au L3 spectra, which is related to
the density of d states of the absorbing Au atom.[45]
This area can be correlated to the d-electronic structure
by using the difference spectrum obtained from the Au
rich composition. On the other hand, a high Ag
fraction in Ag7Au3 and Ag9Au1 seems to lead to
Figure 3. Proof of alloy formation and insights into the Au
oxidation state using XAS. (a) Au L3 XANES spectra, (b)
difference curves obtained by subtracting the Au L2-edge
metal foil spectrum from the Au L3-edge spectra of samples
and Au foil, (c) k2 weighted χ(k) spectra and (d) Fourier
transformation of the k2-weighted EXAFS spectra of the
different AgxAu10-x/ZrO2 (x =1/3/5/7/9) catalysts. Changes
observed in χ(k) spectra (c) due to alloy formation are
highlighted by corresponding symbols and the grey area.
L3- and the L2-edge XANES spectra, which gives a
single distinct peak.[44a,
Figure 3 a shows the
45-46]
diffusion of Ag atoms into the center as evidenced by
the increase in the corresponding areas (Table 2,
entries 5-6). The k2 weighted χ(k) EXAFS spectra for
the AgxAu10-x catalysts are shown in Figure 3 c in
comparison to Au foil and corresponding Fourier
transformed (FT) spectra are given in Figure 3 d (fits
given in Figures S12-S13). EXAFS oscillations in the
k region from 5-8 Å-1 change with increasing Ag
content, which correspond to the increase of Ag
neighbors around the absorbing Au atom with different
backscattering phase shifts at higher k.[47] In the FT
spectra, the metal-metal scattering doublet peaks shift
towards lower R with increasing Ag content, which
also points towards replacement of Au atoms by Ag
atoms i.e. an increase in Au-Ag coordination. The first
shell contribution which appears as a double peak can
be attributed to the interference between Au−Au and
Au−Ag backscattering with different phase shift and
amplitude. EXAFS analysis was performed to derive
the degree of alloying by comparing the fitted
coordination number (CN) with the composition
(Table 3).
normalized Au L3 XANES spectra of the catalysts
along with Au foil for comparison. From the XANES
features, Au is in the metallic state in all catalysts.
However, the spectra differ in the white line region
(11920-11940 eV) and in the post-edge region
(11960-11980 eV) as shown in the inset of Figure 3 a.
The variation in the white line feature can be attributed
to electron transfer between Au and Ag. Figure 3 b
shows the corresponding difference spectra obtained
by subtracting the Au L2-edge spectrum of Au foil
from the samples where a single peak with varying
width and area is obtained (Table 2, detailed procedure
in Supporting Information and Figure S2). In general,
the calculated areas for AgAu catalysts are larger as
compared to pure Au foil, thereby indicating Ag-Au
interaction, i.e. due to alloy formation. The larger area
of Ag1Au9 compared to Au foil (Table 2, entries 1-2)
indicates an increase in the d-hole density.[45] Hence,
even at a low Ag fraction an electron transfer between
Au and Ag is evident confirming strong Ag-Au
interaction, e.g. by alloy formation. From Ag1Au9 to
Ag5Au5 (Table 2, entries 2-4), the areas decrease with
increasing Ag content. This reflects a decreasing
number of d-holes, but all areas remain larger as
compared to pure Au foil, thereby indicating Ag-Au
interaction. Based on the variation in the d-hole
density, the interaction between Au and Ag appears to
vary with the molar ratios. From this trend, more Ag
atoms appear present in the outer layer while higher
Au atom concentration is located in the center for Ag-
Table 2. Area under the marked region in Figure 3 b.
Entry
Sample
Au foil
Ag1Au9
Ag3Au7
Ag5Au5
Ag7Au3
Ag9Au1
∆A3 / eV cm-1
1.67 ± 0.03
1.92 ± 0.02
1.80 ± 0.03
1.75 ± 0.03
1.89 ± 0.05
2.44 ± 0.11
1
2
3
4
5
6
4
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10.1002/adsc.202001003
Advanced Synthesis & Catalysis
For Ag1Au9, the CNAu-Au of 12.1 (Au-Au 2.85 Å) is
close to its value in Au foil and no Au-Ag scattering
was observed. On the other hand, a lower CNAu-Au of
10.5 was found for Ag3Au7. Here Au-Ag scattering
occurred with a CNAu-Ag of 2.2 at 2.86 Å. For a
homogeneously mixed alloy, the ratio of CNAu-Au to
CNAu-Ag should be close to the molar ratio of Au:Ag.[48]
Since XAS is a volume-sensitive method, spatially
resolved information e.g. on the composition of the
surface can only be derived indirectly from
comparison of the ratio of Au-Au and Au-Ag CNs as
well as their deviation from the expected values for
homogeneous composition. The ratio of CNs is shown
in the last column of Table 3. Evidently, the ratio of
CNs is higher in gold while ICP verified that the
overall composition agrees well with the nominal
composition (Table 1). Hence, a small composition
gradient with Ag slightly enriched in the surface-near
volume is expected for some compositions.[49] For
Ag7Au3, the ratio of CNs is closest to the molar ratio
(Table 3, entry 5), hence a homogeneous alloy is more
probable in this case.
atom fraction on the surface. In summary, apart from
the successful alloy formation, slight compositional
gradients of the alloy nanoparticles are indicated by
the EXAFS and XANES analysis.
To further investigate the oxidation state and the
surface-near alloy composition, a complementary XPS
study has been conducted with the as-prepared
AgxAux-10/ZrO2 catalysts and the results are given in
Figure 4. The molar fraction of Au in the outer surface
layers of AgxAux-10/ZrO2 catalysts gained from XPS is
given in Figure 4 a, where the as-prepared, gold-rich
Ag1Au9/ZrO2 and Ag3Au7/ZrO2 catalysts in Figure 4 a
show an enrichment of the surface with Ag. This is in
line with EXAFS analysis. On the other hand, the
surface composition is closest to the nominal
composition for Ag5Au5. At higher Ag content, more
Au appears to be present on the surface of the catalysts
before the reaction. Note upfront that after the catalytic
reaction, the compositional gradient disappeared for
low gold contents (shift toward nominal composition),
while it was still present after reaction at gold molar
fractions above 50 %. From XPS peak deconvolution
summarized in Figures S6-S9, the surface oxidation of
the Au 4f and Ag 3d peaks has been extracted and
summarized in Figure 4 b. Therein, a correlation of the
amount of oxidized atomic species of gold and silver
For Ag9Au1, no Au-Au scattering was observed, but a
Au-Ag CN of 10.1 was found. Since the total CNs for
Ag7Au3 and Ag9Au1 are less than its maximum value
of 12, this indicates the presence of an increased Au
Table 3. EXAFS fitting results obtained from Au L3-edge absorption spectra of AgxAu10-x/ZrO2; EXAFS spectra were fitted
in the range of R = 1.0-3.4 Å and k = 2.4-12.7 Å-1 using kw= 2.
Entry Sample CNAu-Au R / Å
σ2 / Å2
CNAu-Ag
R / Å
σ2 / Å2
CNTotal
12.0
12.1
12.7
11.0
10.6
10.1
Ag:Au
ratioa),b)
-
E0
/eV
4.8 ±
0.5
4.6 ±
0.5
1
2
3
4
5
6
Au foil 12.0c)
2.86 ±
0.02
2.85 ±
0.03
2.85 ±
0.03
2.85 ±
0.03
0.0077 ±
0.0004
0.0092 ±
0.0005
0.0099 ±
0.0015
0.0075 ±
0.0020
0.0054 ±
0.0028
-
-
-
-
-
-
-
Ag1Au9 12.1 ±
- (10:90)
0.8
Ag3Au7 10.5 ±
1.4
Ag5Au5 7.1 ±
1.7
2.2 ± 0.8 2.86 ±
0.10
3.9 ± 1.2 2.84 ±
0.07
6.7 ± 1.4 2.84 ±
0.08
10.1 ±
2.6
0.0079 ± 4.6 ±
0.0028 0.8
0.0079 ± 4.2 ±
0.0024 1.2
0.0078 ± 4.3 ±
0.0017 1.2
0.0097 ± 4.8 ±
0.0020 1.7
17:83
(30:70)
35:65
(50:50)
63:37
(70:30)
- (90:10)
Ag7Au3 3.9 ±
1.6
2.85 ±
0.04
-
Ag9Au1
-
2.88 ±
0.12
a) based on CNs from EXAFS fitting. b) theoretical values are given in brackets. c) fixed during the fitting.
Figure 4. (a) Molar fraction of Au in the outer surface layers of AgAu/ZrO2 catalysts gained from XPS. Molar fraction is
balanced with the sum of Au and Ag; (b) Molar fraction of oxidized Au and Ag gained from XPS (cf. Figures S6-S9), shown
for the as-prepared catalyst before reaction as well as the same catalysts after reaction.
5
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10.1002/adsc.202001003
Advanced Synthesis & Catalysis
is evident, respectively. In this context, XRD patterns
of the bulk catalyst showed that the (111) reflections
of Ag and Au at 38.2° decreased with higher Ag
content (Figure S10). Taking the large extent of
oxidized Ag surface atoms found by XPS into account,
the decrease of the (111) reflection with increasing Ag
content appears to indicate the presence of small Au
crystallites and amorphous oxidized Ag phases on the
surface. On the other hand, for gold-rich compositions
a higher crystallinity is evident in XRD, which is in
line with the slight silver enrichment in the surface
near volume observed in XPS for Ag3Au7 and Ag1Au9
(compare Figure 4 a). Additionally, for gold-rich
compositions, less oxidized gold atoms are present in
the outer nanoparticle region while the percentage of
oxidized silver atoms rises to a limited extent. In turn,
a high silver content leads to a higher content of gold
atoms present in oxidized form, such as silver renders
gold to become less noble as previously discussed in
the literature.[50] Consequently, an electronic
interaction of Au and Ag atoms is evident and in line
with the results from XANES. In summary, by
combining different X-ray based, optical and element-
specific techniques, a successful alloy nanoparticle
preparation of the desired composition has been
proven. Slight element-enrichment at the nanoparticle
surface has been observed with about 10 % more Ag
present at the surface in case of high Au content or
about 10 % enrichment of Au at the surface, in case of
high Ag content. Compared to wet-chemical
nanoparticles, which always show strong enrichment
of Ag in the surface-near volume due to preferential
reduction of more noble Au-based precursors, the
laser-generated catalysts show a gradual elemental
composition of the surface-near volume. In addition,
they hit the right size regime reported to be active for
Au-based catalysts for HMF to FDCA oxidation.[51]
Consequently, these laser-generated catalysts are ideal
candidates for studying the influence of the Ag content
on the catalytic activity and selectivity, presented in
the following section.
presence of 4 equivalents of NaOH, 10 bar air pressure
and a constant HMF-to-metal molar ratio (HMF:M;
M=sum of Ag and Au content Ag and Au) of 100:1
(Figure 5).
All catalysts were found to be active in the selective
oxidation of HMF even at room temperature, where
HFCA was produced as the main product. The highest
HFCA yield of 95 % under these conditions was
achieved over Ag1Au9/ZrO2. Increasing the reaction
temperature caused the HFCA yield to pass through a
maximum for Ag3Au7, Ag5Au5, and Ag7Au3 at 50 °C,
75 °C, and 75 °C, respectively, where HFCA was
quantitatively produced over Ag5Au5/ZrO2. Increasing
the reaction temperature led to decreasing HFCA
yields for all catalysts. Yet, FDCA production started
at 100 °C over Ag1Au9, Ag3Au7, and Ag5Au5, reaching
up to 31 %, 74 %, and 29 % yield at 125 °C,
respectively. Carbon balances of less than 100 % in
some reactions can be attributed to the formation of
unquantified so-called humins by multiple side-
reactions of HMF in alkaline solution.[52]
The formation of HFCA as the primary oxidation
product at room temperature confirms that the
oxidation of the aldehyde moiety of HMF proceeds
readily and that the oxidation of the hydroxymethyl
group is the rate-limiting step in alkaline conditions.[53]
The fact that the reaction temperature barely affected
the activity and HFCA yield of the Ag9Au1 catalyst up
to 100 °C can be attributed to the high Ag content
(Table 1, entry 6), in line with literature on pure silver-
based catalysts.[19]
The oxidation of the hydroxymethyl group to finally
form FDCA proceeded both at higher temperatures
and increasing Au content, probably because Ag is
inactive in this reaction.[19a] Surprisingly, Ag1Au9 with
the highest Au content gave a lower FDCA yield of
31 % at 125 °C compared to Ag3Au7 (74 %), while
Ag5Au5 gave a similar yield of 29 %. The optimum of
FDCA production over Ag3Au7, may be attributed to
two effects: particle size or alloy composition. In
general, the particle size is an important factor in pure
Au catalysis, although Au catalysts in the given size
range (≈7-11 nm, Table 1) were all found active in
FDCA production.[51, 54] Note, that for CO oxidation, a
reduced effect of particle size was found for alloy
formation with Ag.[55] Thus, the particle composition
seems to have a stronger effect on FDCA production,
with an optimal Ag/Au composition for both aldehyde
and alcohol oxidation.
Selective oxidation of HMF by the supported AgxAu10-x particles
After thorough characterization, the catalysts were
tested in the selective oxidation of HMF. First, the
oxidation reactions were performed at different
temperatures to investigate its influence on the activity
and selectivity. The reactions were performed in the
Figure 5. Variation of the reaction temperature using the different AgxAu10-x/ZrO2 catalysts. Reaction conditions: 10 bar air,
HMF:NaOH 1:4, HMF:M 100:1, 5 h reaction time, 1 mmol HMF in a total volume of 10 mL.
6
This article is protected by copyright. All rights reserved.

10.1002/adsc.202001003
Advanced Synthesis & Catalysis
Generally, the catalytic tests uncovered a pronounced
effect of the catalyst composition on the product
distribution at different temperatures.
Next, the influence of added NaOH on the product
distributions was studied (Figure 6). Temperatures
that gave the maximum production of HFCA were
chosen for all catalysts except Ag3Au7, which was
highly active in FDCA production at 125 °C. The
respective temperatures are indicated for each
composition in Figure 6. Lowering the amount of
added base is relevant both ecologically and
economically, as increasing the pH of a HMF solution
prior to acidic separation of FDCA leads to the
production of large quantities of salt.[12]
As can be seen in Figure 6 (discussed from right to left),
decreasing the amount of added NaOH by half had no
effect on the production of HFCA over Ag1Au9,
Ag5Au5, Ag7Au3, and Ag9Au1, all of which gave a
constant HFCA yield. Further decreasing the added
NaOH to one equivalent regarding HMF led to a
slightly lower yield of HFCA; however, it had no
effect for silver-rich alloys such as Ag7Au3 and
Ag9Au1, which both produce HFCA with a relatively
constant yield of 80-85 %. Hence, these catalysts are
active also under conditions which are less alkaline
and more environmentally favorable.
In the absence of NaOH, negligible or low HMF
conversion and HFCA formation was observed. The
lowest HMF conversion of 3 % was observed for
Ag7Au3, where no oxidation products were detected.
On the other hand, for gold-rich catalysts ≈20-25 % of
HMF was converted, while the yield of HFCA and
FDCA was below 5 % (selectivity below 15 %). A
maximum HFCA yield of 9 % was gained from
Ag5Au5 and Ag9Au1 catalysts at 13 % and 15 % HMF
conversion, representing high selectivities of 68 % and
62 %, respectively.
As for Ag3Au7, for which a temperature of 125 °C was
chosen for FDCA production, reduction of the added
NaOH to two equivalents even resulted in an increase
of FDCA yield to 95 %. At one equivalent of added
NaOH, HFCA was produced as the main product in
74 % yield (2 % FDCA yield). Under base-free
conditions, HMF conversion dropped to 24 % while no
oxidation products were formed.
These results show that for the present catalysts one
equivalent of hydroxide is required to achieve high
HMF conversion and HFCA yield. In addition, FDCA
production was observed over Ag3Au7 only when two
or more equivalents of hydroxide were present. Hence,
the reaction mechanism seems to involve a
nucleophilic attack of a hydroxide ion on the aldehyde
moiety of HMF with subsequent dehydrogenation of
the formed geminal diol.[15, 56] Another role of the
homogeneous base may be to prevent catalyst
deactivation by precipitation of poorly soluble FDCA
onto the catalyst.
To compare the performances of all catalysts with
literature (see Table S2), the productivity, i.e. moles of
product formed normalized to the noble metal content
and reaction time, was calculated. The highest
productivity for HFCA of 19 molHFCA h-1 molmetal-1 was
Figure 6. Variation of the added base using the different
AgxAu10-x/ZrO2 catalysts at different temperatures that were
previously optimized (Figure 5). Reaction conditions:
10 bar air, HMF:NaOH:M 1:0/1/2/4:100, 5 h reaction time,
1 mmol HMF in a total volume of 10 mL.
obtained over Ag5Au5 in the presence of two
equivalents of NaOH. A similar FDCA productivity of
19 molFDCA h-1 molmetal-1 was observed over Ag3Au7 in
the presence of two equivalents of NaOH.
In general, the variation in the amount of the added
base revealed that the catalyst are active even in the
presence of less base. Also, some activity was
observed in the base-free oxidation of HMF over some
catalysts; however, longer reaction times may aid to
achieve higher yields of HFCA. Alternatively, solid
bases could be used as support materials in future
studies as reported recently.[57]
In a next set of reactions, the influence of oxygen
partial pressure was studied. An air pressure of 10 bar
was chosen in the previous experiments. Oxygen
participates indirectly in the reaction mechanism by
removing hydrogen adsorbed on the catalyst surface.[15,
56] The hydrogen is formed during the dehydrogenation
of the intermediately formed geminal diol from
nucleophilic addition of hydroxide ions to the
aldehyde group.[15] Thus, despite gas phase oxygen not
being directly incorporated into the oxidation product,
its presence is crucial. Using air enhances the
sustainability of the reaction,[12] but the low
concentration of oxygen makes its use more
challenging compared to pure oxygen. To further
elucidate the role of oxygen, reactions were performed
at ambient pressure under previously optimized
conditions (Table 4).
7
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10.1002/adsc.202001003
Advanced Synthesis & Catalysis
Table 4. Variation of the applied air pressure using the different AgxAu10-x/ZrO2 (x=1/3/5/7/9) catalysts at different
temperatures and amounts of added NaOH that were previously optimized. Reaction conditions: HMF:M 100:1, 5 h reaction
time, 1 mmol HMF in a total volume of 10 mL.
Pressure
Entry
Catalyst
1 bar
10 bar
X(HMF)
100
Y(HFCA)
Y(FDCA)
X(HMF)
100
Y(HFCA)
Y(FDCA)
a),c)
1
2
3
4
5
Ag1Au9
77
17
25
18
80
0
3
3
2
0
95
5
97
95
83
0
95
1
0
0
a),d)
a),e)
a),e)
b),c)
Ag3Au7
Ag5Au5
Ag7Au3
Ag9Au1
100
100
100
100
100
100
100
100
Individual reaction conditions: a) two equivalents of NaOH. b) one equivalent of NaOH. c) room temperature. d) 125 °C.
e) 75 °C.
In general, the lower air pressure resulted in lower
product yields over all catalysts except of Ag9Au1,
which gave a constant HFCA yield of 80% (Table 4,
entry 5). The HFCA yield over Ag1Au9 (Table 4, entry
In a final step after optimizing reaction temperature,
added NaOH and air pressure, the temporal
progression of the reactions was studied (Figure 7).
For Ag1Au9 and Ag9Au1, which were tested at room
temperature, a classical batch process conversion
profile starting from no HMF conversion at t=0 was
observed. HMF was very rapidly and selectively
converted to HFCA over Ag1Au9, which gave HFCA
in 64 % yield (76 % selectivity) after 30 min.
Quantitative HMF conversion was observed after 1 h
1)
decreased
from
95 %
at
10 bar
(19 molHFCA h-1 molmetal-1) to 77 % at 1 bar air
(15 molHFCA h-1 molmetal-1). HFCA yields over Ag5Au5
and Ag7Au3 decreased more strongly to 25 % and
18 %, respectively. Also, the FDCA yield over Ag3Au7
dropped from 98 % (19 molFDCA h-1 molmetal-1, 2 %
-1
HFCA
yield)
at
10 bar
to
just
3 %
giving 83 % HFCA yield at 83 molHFCA h-1 molmetal
(1 molFDCA h-1 molmetal-1, 17 % HFCA yield) at ambient
pressure (Table 4, entry 2).
productivity. Prolonged reaction time led to an
increase in HFCA yield up to 95 % after 5 h, despite
These results underline differences in activity and
selectivity depending on the composition of Ag-Au
catalysts and demonstrate the crucial role of oxygen in
the selective oxidation of HMF. Depending on the
Ag/Au ratio, the effect of oxygen partial pressure
differed. Previous studies on HFCA synthesis over
pure Ag catalysts revealed a minor influence of air
pressure,[19a] which also seems to be the case for
bimetallic Ag-Au catalysts rich in Ag like Ag9Au1.
That might be attributed to favored adsorption of
oxygen on Ag as compared to Au with the reaction
proceeding on neighboring Au active sites.[21a] This is
further confirmed by comparison of the Ag-rich
Ag9Au1 to Au-rich Ag1Au9, both of which were tested
at room temperature. The more pronounced decrease
in HFCA yield over silver-depleted Ag1Au9 confirms
that Au-based or in this case Au-rich catalysts are more
influenced by air pressure due to limited oxygen
adsorption. Hence, catalysts rich in gold generally
require higher air pressure to achieve high product
yields.
Neither Au nor Ag were found in the reaction solutions
of all catalysts by ICP-OES, showing that the catalysts
are stable against metal leaching in solution.
The productivity of Ag3Au7 towards FDCA of
-1
19 molFDCA h-1 molmetal is in the range or higher as
compared to other alloys like AuPd[20a, 58] or AuCu[59]
in literature (Table S2), which underlines the potential
of Ag-Au alloys as highly active catalysts for HMF
oxidation. However, note that comparability should be
taken with caution due to different reaction conditions
like the addition of a base.
Figure 7. Temporal evolution of HMF oxidation using the
different AgxAu10-x/ZrO2 catalysts under different reaction
conditions that were previously optimized. Reaction
conditions: 1 mmol HMF in a total volume of 10 mL,
HMF:M 100:1.
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10.1002/adsc.202001003
Advanced Synthesis & Catalysis
the quantitative conversion of HMF after just 1 h,
which might be attributed to adsorbed intermediates or
the product itself on the catalyst surface. On the other
hand, HMF conversion and HFCA yield steadily
increased with time over Ag9Au1, which can be
attributed to the milder reaction conditions. However,
longer reaction times are required to achieve high
HMF conversion and HFCA yield under ambient
conditions.
liquid interface, thus better gas-liquid mass transfer. It
was found that a high HFCA yield of 82 % could be
reached within 2 h (41 molHFCA h-1 molmetal-1) with
optimal mixing of the different phases of the reaction.
The profiles are similar but with a higher productivity,
evidencing that with careful considerations, a future
scale-up is indeed possible.
Catalyst stability and reuse
In case of the intermediate compositions Ag3Au7,
Ag5Au5, and Ag7Au3, conversions of HMF in Figure 7
were found to be high already at t=0. This observation
is linked to the definition of the starting time t=0,
which was set to reaching the desired reaction
temperature for the first time. For example, reaching
125 °C for Ag3Au7 took about 15 min. Consequently,
for Ag3Au7, high product yields of 85 % HFCA and
10 % FDCA were already achieved at the nominal start
of the reaction. This shows Ag3Au7 is extremely active
in aldehyde oxidation, which is already quantitatively
converted during heating up. With longer reaction
times, HFCA yield decreased linearly with a linear
increase in FDCA yield, pointing out alcohol oxidation
as the rate-limiting step.[53] Therefore, a reaction time
of 5 h is required to produce FDCA in 95 % yield. The
fact that no 5-formyl-2-furancarboxylic acid (FFCA)
is found, which is the intermediate of HFCA oxidation
to FDCA,[15] underlines the high activity in aldehyde
oxidation. Note that for all catalysts not tested under
ambient conditions, the aldehyde oxidation activity is
even higher, since high yields are obtained despite
harsher reaction conditions in terms of HMF stability.
The influence of the HMF:M ratio was studied by
decreasing the amount of catalyst added to the reaction
solution, (Figure S19) resulting in increased
Catalysts were separated from the reaction mixture and
dried overnight before testing in a subsequent reaction
under identical conditions for reusability studies
(Figure 8 a-e). A slight loss of catalyst mass during
this recycling process was taken into account by
adjusting the amount of reactant and total volume to
the recovered catalyst mass. Ag1Au9, Ag5Au5, and
Ag7Au3 proved very stable upon reuse, and the product
yield and productivity rate remained constant for five
reuse cycles. Particle sizes did not severely change for
these catalysts (Figure 8 f) and as HFCA synthesis is
not highly affected by particle size, this explains their
constant activity upon reuse. Interestingly, the surface
composition of Ag5Au5 and Ag7Au3 changed slightly
(Figure 4 a) but maintained their activity. For the
silver-rich Ag9Au1, a slight decrease in activity was
observed after the third catalytic cycle, but the particle
size was unaffected by recycling (Figure 8 f,
Figure S15). Thus, the decreasing activity of Ag9Au1
might be related to the deposition of impurities from
HMF degradation,[53] because the carbon balance was
less than 100 % and no drastic changes in the surface
composition were found. Since monometallic Ag-
based catalysts deactivate more rapidly under
comparable reaction conditions,[19] alloy formation
with less than 90 % of Ag greatly enhances the catalyst
stability.
-1
productivity rates up to 254 molHFCA h-1 molmetal for
Ag5Au5 at a HMF:M ratio of 300. Decreasing HMF
conversion or product yields upon the addition of less
catalyst showed that the reactions performed so far
were not limited by solid-liquid mass transfer. If
reactions are scaled up at a later stage, however, mass
transfer limitations have to be considered. To study
possible mass transfer effects, the oxidation of HMF
over Ag9Au1/ZrO2 was scaled up to 30 mL by a factor
of three. As in the previous reactions, the catalyst was
used as a fine powder to minimize possible internal
mass transport limitations. With increasing reaction
volume, good mixing becomes increasingly important
to maintain a slurry, which minimizes solid-liquid
transfer limitations. To study the effect of mixing at
the increased volume of 30 mL, the stirring speed was
varied by almost an order of magnitude, (150-
1400 rpm, Figure S20) and the HFCA yield was
increased accordingly (5-30 %), revealing that the
reaction was limited to some extent by solid-liquid
mass transfer in the bigger reaction volume. Next, the
reaction was performed in a special reactor equipped
with a mechanical gas entrainment impeller for better
mixing of the liquid phase and enhancement of the gas-
Ag3Au7, which was tested at an elevated reaction
temperature of 125 °C for FDCA synthesis, decreased
in activity after the first catalytic reaction, giving
HFCA as the main product with a yield of 38 % in the
second reaction. For this catalyst, the particle size
increased from 6.9 to 10.2 nm after the first catalytic
reaction (Figure 8 f, Figure S15) probably due to the
higher reaction temperature applied for this catalyst.
On the other hand, EDX scans and the surface
composition derived by XPS did not show severe
changes (Figures S6-S9, Figure S17). Although the
particle size effect may be limited for Ag-Au alloys,
this catalyst was the only one active in FDCA
production, thus probably having active Au sites.
Together with the high Au content and smaller
particles favoring FDCA synthesis[51], sintering seems
to be the primary reason for the deactivation of this
catalyst.
In general, the reuse stability of catalysts active in
HFCA synthesis was greatly enhanced by alloy
formation of Ag and Au and the catalysts were also
structurally stable.
9
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10.1002/adsc.202001003
Advanced Synthesis & Catalysis
Figure 8. (a-e) Recycling experiments of the AgxAu10-x/ZrO2 catalysts and (f) changes in the mean particle size of each
catalyst upon re-use. Reaction conditions: 1 mmol HMF in a total volume of 10 mL, HMF:M 100:1; (a) Ag1Au9 – room
temperature, 10 bar air, HMF:NaOH 1:2; (b) Ag3Au7 – 125 °C, 10 bar air, HMF:NaOH 1:2; (c) Ag5Au5 – 75 °C, 10 bar air,
HMF:NaOH 1:2; (d) Ag7Au3 – 75 °C, 10 bar air, HMF:NaOH 1:2; (e) Ag9Au1 – room temperature, 1 bar air, HMF:NaOH
1:1.
Mechanistic outlook
During oxygen activation, hydrogen peroxide is
formed, whose decomposition has a high energy
barrier on Au but proceeds readily on Ag or Ag-Au
alloys.[64] Consequently, two different kinds of active
centers for HMF oxidation are present in Ag-Au
catalysts (Figure 9 c). The catalysts differed in bulk
and surface composition (Table 1, Figure 4 a). This
alters the oxygen activation and respective selectivity
to either only oxidize the aldehyde moiety or both
functional groups of HMF with Ag active in oxygen
activation and Au active in HMF conversion.[21a]
While catalysts with higher amounts of either one of
both metals favor alcohol oxidation (Figure 9 a-c) an
ideal catalyst for FDCA synthesis should have an
optimal Ag:Au-ratio, which may result in a favorable
amount of Ag atoms next to Au atoms. Based on in-
depth characterization and extensive catalytic testing,
Ag3Au7 was identified to have the most favorable
Ag:Au ratio of 38:62 with active centers for alcohol
oxidation i.e. FDCA synthesis.
In summary, differences in initial activity of the
prepared catalysts were observed and can be mainly
attributed to the particle composition due to similar
particle sizes. The catalytic activities at room
temperature and at 125 °C, i.e. HFCA and FDCA
synthesis conditions, respectively, are summarized in
Figure 9. An optimal composition for HMF oxidation
activity can be concluded with Ag3Au7 being active
both in aldehyde and alcohol oxidation, giving FDCA
as the main product. The alcohol oxidation activity is
lower for Ag1Au9 and Ag5Au5 and no activity in FDCA
synthesis was observed at higher Ag content (Figure 9).
Since the activation energy for aldehyde oxidation is
lower,[60] this step proceeds rapidly over all catalysts.
The rate-limiting alcohol oxidation, on the other hand,
has a higher energy barrier and only occurs on Au
catalysts, as Ag is inactive in FDCA synthesis.[19a]
Besides activation barriers, oxygen activation is a
crucial factor, which is limited on Au[61] or limited to
active Au sites possibly forming a perimeter with the
support similar to literature on selective alcohol
oxidation with Au/TiO2.[62] Since hydroxide ions form
a geminal diol in solution by nucleophilic attack on the
aldehyde moiety of HMF, which is then
dehydrogenated, oxygen is not incorporated in the
oxidized molecule and thus has an indirect role in the
mechanism by removal of electrons and adsorbed H-
species.[60, 63]
All catalysts other than Ag3Au7 gave HFCA as the
main product with enhanced stability. While pure Au
and Ag catalysts deactivate by particle growth,[19a]
sintering was significantly reduced for the bimetallic
catalysts except for Ag3Au7.
10
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10.1002/adsc.202001003
Advanced Synthesis & Catalysis
nanoparticles with comparable moderate particle sizes
without any surfactant or organic ligand. Thus,
catalytic oxidation activities can be linked directly to
the catalyst composition. Particle sizes were
maintained upon adsorption of colloids on ZrO2, as
shown by TEM. The supported catalysts still contained
alloyed nanoparticles with slight Ag-enrichment in the
surface-near volume for high Au content and about
10 % enrichment of Au in the surface-near volume at
high Ag content as shown by a combination of XAS
and XPS. Catalytic tests in HMF oxidation revealed
differences in selectivity towards HFCA or FDCA
depending on the composition. Optimization of the
reaction conditions for each catalyst allowed the
production of both HFCA and FDCA in yields above
90 % and high productivity rates. At high Ag content,
catalysts were active in HFCA synthesis, whereas the
high Au content in Ag3Au7 and Ag1Au9 featured the
FDCA synthesis, with an optimal activity for Ag3Au7.
Thus, a synergistic effect between Ag and Au was
shown in varying selectivity and enhanced catalyst
stability against sintering for most catalysts. The
observed optimal compositions for either HFCA
(Ag9Au1) or FDCA (Ag3Au7) formation might be
linked to an ideal ratio of surface Ag and Au atoms,
and respective oxygen activation to either promote
selective aldehyde oxidation or both aldehyde and
alcohol oxidation. For the future, it might be rewarding
to substitute the homogeneous base by a solid base as
catalyst support. All catalysts were stable against
metal leaching under the reaction conditions applied in
this study, and recycling experiments revealed great
catalyst stability of Ag1Au9, Ag5Au5 and Ag7Au3,
which also preserved their structure. Hence, the highly
active and stable bimetallic Ag-Au catalysts may be
suitable for future tests on the scale-up or the
continuous oxidation of HMF.
Experimental Section
General remarks
Sheets of pure Ag-Au alloys in five different compositions
(AgxAu10-x, x=1/3/5/7/9, Research Institute for Precious
Metals and Metal Chemistry, Schwäbisch Gmünd), HMF,
FDCA, HFCA, NaOH, NaCl (Sigma-Aldrich/Merck), 5-
formyl-2-furoic acid, 2,5-diformylfurane (TCI Chemicals),
ZrO2 1/8” pellets (Alfa Aesar) and synthetic air (Air
Liquide) were used in analytical grade without further
purification.
Figure 9. Schematic representation of the influence of the
catalyst composition and structure on the activity in (a)
aldehyde and (b) alcohol oxidation of HMF over
AgxAu10-x/ZrO2 catalysts as well as (c) active centers. The
respective product yields are taken from the temperature
screening at 25 °C (active conditions for aldehyde
oxidation) and 125 °C (active conditions for alcohol
oxidation). Lines are added to guide the eye. Reaction
conditions are shown in Figure 5.
Catalyst preparation
In a first step, the alloy nanoparticles (AgAu NPs) were
synthesized by laser ablation of an alloy target in flowing
micromolar aqueous saline, followed by electrostatic
adsorption of the AgAu NPs in a dispersion of the ZrO2
support material by adjusting the pH to 3.8 between the
isoelectric points (IEP) of both reactants (Figure 2 a).[42, 65]
Conclusion
The influence of alloy formation of Ag and Au on the
catalytic activity in HMF oxidation was investigated.
For this, we took a rational approach starting from the
synthesis of bimetallic nanoparticles with varying
Ag/Au ratio followed by extensive catalytic studies.
LAL allowed the production of highly pure bimetallic
In brief, the experimental laser ablation setup to synthesize
the AgAu NPs consists of a flow chamber containing the
alloy target, a piston pump (Ismatec ISM321C) and a
magnetically stirred beaker that was filled with an aqueous
(Milli-Q water, resistivity 18.2 MΩ·cm) 200 µM NaCl
solution.[34a] The respective alloy target foil (dimensions:
11
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10.1002/adsc.202001003
Advanced Synthesis & Catalysis
50 mm*20 mm*0.5 mm) was fixed inside the ablation
chamber. The liquid layer in front of the target was set by
the chamber window and fixed to 4 mm.[34a] The piston
pump was set to a flow rate of 50 mL min-1 and the liquid
was circulated through PTFE tubes. For the ablation, a
nanosecond laser from Edgewave (model: IS160-1-T)
operating with a pulse duration of 8 ns, a repetition rate of
5 kHz, a pulse energy of 8 mJ, and a wavelength of 532 nm
was focused onto the target. The second harmonic laser
wavelength was chosen to additionally reduce particle size
by laser fragmentation of already produced colloids when
passing through the ablation zone during cycling.[34a] During
ablation, the laser pulses were directed in a rectangular
pattern using a scanner (Raylase SS-IIE-12) at a scan rate of
2000 mm s-1 to minimize cavitation bubble shielding. The
presence of the 200 µM NaCl solution allowed an in situ
size quenching and to prevent agglomeration as well as
particle growth.[66] The ablation was conducted between
30 min (Ag1Au9) up to 120 min (Ag9Au1) depending on the
target composition, as the ablation rate scales with the target
composition.[67] The mass concentration of colloids was
determined gravimetrically by differential weighting of the
target with a microbalance (ABT 120-5DM from KERN)
before and after LAL, as well as UV-Vis extinction
spectroscopy of the colloids (Figure 2 b).
catalyst powders were added at a constant HMF-to-metal
molar ratio (HMF:M; M=sum of metal content Ag and Au)
based on the metal loading of each catalyst, and the reactors
were sealed and purged three times with synthetic air before
the desired pressure was adjusted. The reaction temperature
was set in a controller and the autoclaves were heated with
heating sleeves based on the thermocouple inside the
reaction solution. The temperature was kept constant with a
maximum variation of 1 °C and the starting point of each
reaction was set after the solution first reached the set
reaction temperature. Additional reactions were performed
in a batch reactor with the same specifications that was
equipped with a mechanical gas entrainment impeller. After
the reactions, the reactors were quenched in an ice bath,
depressurized, and the catalysts were separated by
decantation.
Samples for analysis by high-performance liquid
chromatography (HPLC) were taken before and after the
catalytic tests, filtered with 0.45 μm Teflon filters and
diluted according to the calibration range. HPLC was
measured on a Hitachi Primaide at 50 °C and 25 °C (Bio-
Rad Aminex HPX-87H column, solvent 5 mM H2SO4). The
compounds were detected by a refractive index (Hitachi
Chromaster 5450) and a diode array (Hitachi 1430) detector.
Reference solutions of HMF, HFCA, FDCA, DFF and
FFCA were used for calibration as external standards and
concentrations of reaction solutions were derived from the
peak areas. HMF conversion (eq 1), product yield (eq 2) and
selectivity (eq 3), as well as the carbon balance (eq 4), were
calculated based on the concentrations determined by HPLC.
For supporting the nanoparticles, 5 g of ZrO2 was suspended
in 200 ml of deionized water (Milli-Q water, resistivity
18.2 MΩ·cm) in individual, continuously stirred batches.
0.1 g of AgAu NPs were added as colloids in respective
volumes based on the measured concentrations after
ablation to reach a mass loading of 2 wt.%. To improve the
electrostatic adsorption of the AgAu NP to the ZrO2
microparticles, the pH was adjusted to pH 3.8.[42] After that,
the suspension was allowed to sediment. The deposition
efficiency was derived from the supernatant phase via UV-
Vis extinction spectroscopy. The sedimented catalyst was
separated by decantation and freeze-drying (Christ Alpha 1-
4 LSCplus).
n(HMF)
initial
−n(HMF)
final
(
)
X HMF =
(1)
(2)
(3)
(4)
n(HMF)
initial
n(x)
final
( )
Y x =
n(HMF)
initial
n(x)
final
( )
S x =
n(HMF)
−n(HMF)
initial
final
∑
n(x)
final
C − balance =
n(HMF)
initial
Catalyst characterization
Error bars were derived from Student’s t-distribution with
95 % confidence interval (two tailed t-test), a degree of
freedom of 4 (5 recycling cycles) and the standard deviation
of the respective recycling experiment. The standard
deviation was calculated from samples shown in Figure 8 c)
as the sample showed good cycle stability. Consequently, as
slight degradation might add to the error evaluation, error
bars can be considered a worst-case estimation.
The catalysts were thoroughly characterized using multiple
methods. Analytical disc centrifugation (ADC) was
performed to determine AgAu NP sizes of the freshly
prepared particles. UV-Vis and diffuse reflectance (DR)
UV-Vis spectra were recorded to study alloy formation of
the freshly prepared and the supported AgAu NPs,
respectively. The particle size of supported AgAu NPs was
determined by transmission electron microscopy (TEM).
Inductively coupled plasma optical emission spectroscopy
(ICP-OES) was measured to verify metal loadings and
molar compositions of the supported catalysts and N2
physisorption using the Brunauer–Emmet–Teller method
was employed to determine the specific surface area of the
solid catalysts. Powder X-ray diffraction (XRD) patterns
were measured to study crystalline phases of supported
catalysts. The surface composition and oxidation states of
both metals were investigated using X-ray photoelectron
spectroscopy (XPS). The degree of alloy formation and the
local structure in the bulk AgAu NPs was investigated by X-
ray absorption spectroscopy (XAS), which was recorded at
the CAT-ACT beamline of the KIT synchrotron[68] and
Athena and Artemis software from the IFEFFIT/Demeter
package (version 0.9.25)[69] were used for data analysis.
FDCA could be extracted by acidification of the reaction
solutions with high FDCA yield to a pH value of 1 with HCl.
For HFCA, the acidified solution was extracted with ethyl
acetate and the solvent was removed under reduced pressure.
1
The purity of both products was confirmed by H and 13C
NMR spectroscopy, which was measured at room
temperature in DMSO-d6 on a Bruker Avance 250 and a
Bruker Avance 400 spectrometer (Figure S21).
Acknowledgements
We acknowledge the KIT light source for provision of beamtime at
the CAT-ACT beamline and the Institute of Beam Physics and
Technology (IBPT) for the operation of the storage ring, the
Karlsruhe Research Accelerator (KARA). We are grateful to Dr.
Anna Zimina and Dr. Tim Prüßmann (IKFT/KIT) for providing
assistance and technical support during the beamtime as well as
Angela Deutsch (ITCP/KIT, N2 physisorption), Armin Lautenbach
(IKFT/KIT, ICP-OES) and Dr. Thomas Bergfeld (IAM-AMP/KIT,
ICP-OES).
For further details and experimental procedures, consult the
Supporting Information of this article.
Catalytic tests
Selective oxidation of HMF was carried out in Teflon®
inlets (52 mL), which were placed in home-built
magnetically stirred stainless steel autoclaves equipped with
pressure inlet and outlet, manometer, thermocouple, and a
relief valve. The reactors were charged with appropriate
amounts of 0.2 M HMF solution, 2.5 M NaOH and distilled
water to give a total volume of 10 mL in each reaction. The
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