Efficient Aerobic Oxidation of 5-Hydroxymethylfurfural in Aqueous Media with Au–Pd Supported on Zinc Hydroxycarbonate

Article abstract of DOI:10.1002/cctc.201600852

Bimetallic catalysts with Au–Pd supported on zinc hydroxycarbonate (ZOC) were synthesized by a simple deposition–precipitation method and analyzed by transmission electron microscopy to have a narrow-size distribution of predominantly 1–2 nm. The prepared Au–Pd/ZOC catalysts exhibited excellent activity towards 5-hydoxymethylfurfural (HMF) oxidation in water in the presence of the base NaHCO₃ at benign conditions of 80 °C and 3 bar O₂, resulting in quantitative yield of 2,5-furandicarboxylic acid (FDCA). The addition of base not only enhanced the yield of FDCA but also stabilized the support ZOC by preventing ZOC from the reaction with formed carboxylic acid intermediates/products, thus allowing Au–Pd/ZOC to be recycled for at least six times without significant loss of activity. The basicity of ZOC could play an important role in obtaining the improved yield of FDCA as compared to other supports.

Full text of DOI:10.1002/cctc.201600852

Accepted Article
Title: Efficient Aerobic Oxidation of 5-Hydroxymethylfurfural in Aqueous
Media with Au-Pd Supported on Zinc Hydroxycarbonate
Authors: Zhenyou Gui, Wenrong Cao, Shunmugavel
Saravanamurugan, Anders Riisager, Lifang Chen, and
Zhiwen Qi
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Efficient Aerobic Oxidation of 5-Hydroxymethylfurfural in
Aqueous Media with Au-Pd Supported on Zinc Hydroxycarbonate
Zhenyou Gui,[a,b] Wenrong Cao,[a] Shunmugavel Saravanamurugan,[b,c] Anders Riisager,[b]
Lifang Chen,*[a] Zhiwen Qi*[a]
Abstract: Bimetallic catalysts with Au-Pd supported on zinc
hydroxycarbonate (ZOC) were synthesized via a simple deposition-
precipitation method and analyzed by transmission electron
microscope (TEM) to have a narrow-size distribution of predominantly
1-2 nm. The prepared Au-Pd/ZOC catalysts exhibited excellent
activity towards 5-hydoxymethyl furfural (HMF) oxidation in water in
the presence of the base NaHCO3 at benign conditions of 80 °C and
3 bar O2, resulting in quantitative yield of 2,5-furandicarboxylic acid
(FDCA). The addition of base not only enhanced the yield of FDCA
but also stabilized the support ZOC via preventing ZOC from the
reaction with formed carboxylic acid intermediates/products, thus
allowing Au-Pd/ZOC to be recycled for at least six times without
significant loss of activity. It was speculated that the basicity of ZOC
could play an important role in obtaining the improved yield of FDCA
as compared to other supports.
Energy.[11] Moreover, FDCA can potentially substitute petroleum-
derived terephthalic acid as a precursor for the production of PET
(polyethylene terephthalate) plastic, due to the similar structural
properties of the two compounds. It was recently reported that
PEF (polyethylene furanoate), polymers of FDCA and ethylene
glycol, showed many similar or improved properties compared to
PET plastic.[13,45] Accordingly, the exploration of efficient and
reusable catalytic systems for the oxidation of HMF into FDCA
has recently spurred a great deal of interest in the field of biomass
transformations.[14-18]
Noble metal-containing supported catalysts have been widely
investigated for the oxidation of HMF into FDCA with molecular
O2 as the oxidant.[19-24] Particularly, Au-supported catalysts have
shown high catalytic performances for HMF oxidation into FDCA
in aqueous medium, as Au has relatively high resistance to water
and O2 compared to Pt and Pd.[25,26] However, Au can quite easily
deactivate by sintering of Au nanoparticles and adsorption of
formed products or intermediates during the oxidation process,
especially in the absence of base, which hampers the lifetime of
the catalyst and limits the industrial application.[27-29] Thus, stable
and efficient supported Au catalysts have to be developed to
lower costs for HMF oxidation in order to be industrially viable.
In general, Au catalysts showed high catalytic activities for
many oxidation reactions and their catalytic activity and stability
could be enhanced by alloying with other metals, such as Cu and
Pd.[30-34] In connection to this, Hutchings and coworkers have
discovered that Au-Pd alloy supported on TiO2 catalysts showed
higher catalytic activity and stability than corresponding Au or Pd-
based catalysts for the selective oxidation of benzyl alcohol to
benzyl aldehyde and toluene to benzyl benzoate.[33,34] However,
understanding of the observed synergic effect between the two
metals is still lacking.
Introduction
Over the past few years, a great deal of attention has been
devoted to the production of alternative sustainable energy and
chemical feedstocks, mainly due to the rising cost, diminishing
supply and environmental impact of fossil fuels.[1-4] Therefore, the
chemical industry and academia are striving to produce fuels and
chemicals from renewable resources which have potential to
replace fossil-based chemicals and fuels. The biomass is one of
the potential renewable and sustainable sources which can
efficiently be transformed into high-value chemicals and fuels,
such as 5-hydoxymethyl furfural (HMF), levulinic acid, lactic acid,
and their derivatives, and thus reduce the heavy dependence on
unsustainable petroleum-based resources.[5-7]
The catalytic oxidation of HMF - a dehydrated product of
glucose/fructose - to 2,5-furandicarboxylic acid (FDCA) has been
well studied among various routes of biomass transformation.[8-10]
HMF is considered as an important biomass-based platform
chemical, and the product FDCA is identified as one of 12
potentially useful building blocks by the U.S. Department of
Typically, the support for noble metal nanoparticles is an
important factor to determine the catalytic activity.[35-37] For
example, Corma and coworkers have found that Au/CeO2 and
Au/TiO2 catalysts afforded over 99 % yield of FDCA after 8 h of
reaction time, while only 44 and 15 % yield of FDCA were
obtained with Au/C and Au/Fe2O3 catalysts under the same
reaction conditions, respectively.[24] However, the role of support
on the catalytic activity is still unclear for noble metal catalysts. On
the other hand, H-Y zeolite, hydrotalcite and Mg(OH)2 have been
employed as support for Au.[20,38] Although they showed
significant catalytic activities towards HMF oxidation, the stability
of these catalysts was poor as they were prone to deteriorate in
water, leading to Au nanoparticle sintering and leaching of Au
nanoparticles.[20,38,39] These previous studies indicate that
choosing a suitable support for Au is important to enhance the
catalytic activity and stability during the reaction. In accordance
with this, Yang and coworkers have recently reported that water
insoluble zinc hydroxycarbonate (Zn5(CO3)2(OH)6, ZOC) as a
good support for Au catalysts in the oxidation of benzyl alcohol to
[a]
Max Planck Partner Group at the State Key Laboratory of Chemical
Engineering, East China University of Science and Technology,
Shanghai 200237, China
Fax: +86 21 64253528.
E-mail address: lchen@ecust.edu.cn (L. Chen); zwqi@ecust.edu.cn
(Z. Qi).
[b]
[c]
Centre for Catalysis and Sustainable Chemistry, Department of
Chemistry, Technical University of Denmark, DK-2800 Kgs. Lyngby,
Denmark
Center of Innovative and Applied Bioprocessing, Mohali 160 071,
Punjab, India
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201600852
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benzaldehyde. Interestingly, it was shown that the basicity of the
support played an essential role to promote the oxidation of
benzyl.[40] Moreover, the basicity of the support might influence
the dispersion and average size of the Au nanoparticles.[41] These
studies encouraged us to explore the catalytic activity and stability
of Au and Pd on ZOC towards HMF oxidation in aqueous solution.
To the best of our knowledge, the aerobic oxidation of HMF to
FDCA in water with noble metal supported on ZOC has not been
reported previously.
In the present work, we have synthesized Au-Pd/ZOC,
catalysts using a simple deposition-precipitation method and
characterized the materials with transmission electron
microscope (TEM) and X-ray photoelectron spectroscopy (XPS)
and X-ray diffraction (XRD). Considering the superior catalytic
performance of Au-Pd/ZOC catalyst, we have systematically
investigated the effects of various supports on catalytic activity of
Au-Pd alloy towards HMF oxidation to FDCA. Additionally, various
reaction parameters correlated with the yield of FDCA such as
influence of metal loading, reaction time, reaction temperature,
and the addition amount of NaHCO3 have been discussed
detailedly in the study. The stability of Au-Pd/ZOC was also
studied by performing recyclability test for seven runs.
peak corresponding to Au or Pd was observed, mainly due to
adequate small metallic particles on ZOC. The microstructures of
the Au-Pd/ZOC catalyst was examined by TEM images, as shown
in Figure 1. In Figure 1a, Au-Pd nanoparticles were
homogeneously dispersed on the surface of the ZOC support.
The metal particle size distribution of 2.0 wt% Au-Pd/ZOC was
determined by measuring the mean diameter of over 400 particles
from different areas and the results were illustrated in Figure 1b.
The particle size distribution indicated that the diameter of the Au-
Pd nanoparticles were predominantly in the range of 1-2 nm with
an average particle size of 1.6 nm. As reported, possessing
particle sizes smaller than 2 nm in noble metal-based catalysts
would greatly improve the catalytic activity of HMF oxidation to
FDCA.[44,45] Comparatively, the particle sizes of Au-Pd
nanoparticles loaded on hydrotalcite (MG20), CeO2, TiO2 and
activated carbon (AC) were also measured by TEM and found to
be 2.1, 1.8, 1.7 and 2.2 nm, respectively (Figure S3).
The composition of the Au-Pd nanoparticles was determined
by energy-dispersive X-ray spectroscopy (EDS) elemental
mapping of randomly chosen nanoparticles and the images were
given in Figure 1c-1f. The elemental maps of Au, Pd and Au-Pd
overlay signals for the same nanoparticle were homogeneously
distributed in each particle (Figure 1d-1f). The relative difference
of atomic radii of Au (0.144 nm) and Pd (0.138 nm) was 4.17%
based on the Hume-Rothery rule,[51] which revealed that Pd atoms
might be incorporated into the Au lattice, and thus Au and Pd were
alloyed in the lattice of the corresponding nanoparticle.[52] In
addition, the EDS showed that the atomic ratio of Au to Pd is close
to 1 (Figure S2), which was the exact molar ratio of Au:Pd used
during the synthesis.
Results and Discussion
The XRD patterns were performed to investigate the crystal
structure of Au-Pd/ZOC catalyst. As seen in Figure S1, all the
major peaks corresponded to the reflections of Zn5(CO3)2(OH)6
(ZOC) (JCPDS card No.19-1458). In addition, no characteristic
Figure 1. a) TEM images of 2.0 wt% Au-Pd/ZOC (Au:Pd = 1:1). b) Particle size distribution of 2.0 wt% Au-Pd/ZOC. c) High-angle annular dark-field (HAADF)
image of 2.0 wt% Au-Pd/ZOC. d), e) and f) EDS mapping images corresponding to Au, Pd and Au-Pd overlay signals for the selected nanoparticle in the white
square c) of 2.0 wt% Au-Pd/ZOC, respectively.
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Figure 2. XPS spectra of Au 4f7/2 and Pd 3d5/2 of 2.0 wt% Au-Pd/ZOC (Au:Pd = 1:1) and 2.0 wt% Au/ZOC or 2.0 wt% Pd/ZOC catalysts.
To further investigate the chemical state and composition of
the sample surface, the Au-Pd/ZOC catalysts were subjected to
XPS measurements and the results are shown in Figure 2. The
binding energy (BE) of the Au 4f7/2 for the Au-Pd/ZOC catalyst
was found to be 83.6 eV, which was shifted slightly towards higher
BE value of 84.0 eV for the Au/ZOC catalyst. Similarly, the BE of
the Pd 3d5/2 for Au-Pd/ZOC was 335.3 eV, which was slightly
shifted towards lower BE value of 334.9 eV for the Pd/ZOC
catalyst. The BE differences of Au and Pd further substantiated
the formation of Au-Pd alloy nanoparticles and the electron
transfer between Pd and Au. As in line with the previous report,
the activity of Au-Pd alloy could be significantly enhanced via the
electron transfer between Au and Pd.[46]
The HMF oxidation experiments were carried out with Au-Pd
supported on various supports, such as MG20, CeO2, TiO2, AC
and ZOC under identical reaction conditions and the results were
summarized in Table 1. A full conversion of HMF with 77% and
55% selectivity to FDCA was obtained after 4 h, when Au-Pd
supported on CeO2 and TiO2 were employed as catalysts,
respectively. When the support was changed to AC, the
conversion of HMF and selectivity to FDCA decreased to 75% and
35%, respectively. Interestingly, over 99% yield of FDCA was
achieved when the support switched to ZOC. The order of activity
of Au-Pd on different supports was as follows: Au-Pd/ZOC > Au-
Pd/MG20 > Au-Pd/CeO2 > Au-Pd/TiO2 > Au-Pd/AC.
As there was no significant difference in the particle sizes of
Au-Pd regardless of the support, it was speculated that the high
catalytic activity of Au-Pd/ZOC could also be in part from the
surface property of the support. In order to understand the role of
support, the basicity/acidity of the support was evaluated by
measuring the pH of an equilibrated suspension of different
supports in Milli-Q water. It was found that each support indeed
exhibited differences in their acidic/basic properties. The pH value
of the Milli-Q water (6.95) was taken into account to calculate the
pH value of each support. The order of pH value of the support in
water was: ZOC > MG20 > CeO2 > TiO2 > AC. Compared with
corresponding support, the pH value of Au-Pd/ZOC, Au-Pd/MG20,
Au-Pd/CeO2, Au-Pd/TiO2 and Au-Pd/AC was about 8.16, 7.20,
6.25, 5.84 and 5.25, respectively. The slight differences indicated
that Au-Pd alloys hardly had an effect on the pH of the supported
catalysts due to their low loading (2.0 wt%). The activity order of
Au-Pd with different support ascended gradually with the
increased pH values of the supports/catalysts, which revealed
that the basicity of the support (ZOC) could significantly improve
the FDCA yield. This is in accordance with previous reports
showing that the strongly basic supports can promote the
Table 1. Aerobic oxidation of HMF with Au-Pd supported on various supports.[a]
Selectivity / %
Catalyst
Support pH in water
(Au+Pd) / wt%
Au/Pd molar ratio
Conv. / %
HMFCA[b]
FFCA[c]
FDCA
>99
90
Others
Au-Pd/ZOC
Au-Pd/MG20
Au-Pd/CeO2
Au-Pd/TiO2
Au-Pd/AC
8.05
7.11
6.13
5.79
5.20
2.01
2.00
1.98
2.00
1.96
1.00
1.00
0.99
1.00
0.98
>99
>99
>99
>99
75
-
-
-
1
8
1
2
1
4
3
18
35
44
77
14
17
50
35
[a] Au:Pd=1:1, 0.5 mmol HMF, HMF/(Au+Pd)=100, 0.1679 g NaHCO3, 10 mL H2O, 353 K, 3 bar O2, 4 h. [b] 5-hydroxymethyl-2-furancarboxylic acid. [c] 5-formyl-
2-furancarboxylic acid.
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Table 2. Catalytic performance of Au-Pd/ZOC catalysts with different Au:Pd molar ratios for the aerobic oxidation of HMF.[a]
Selectivity / %
Catalyst
Au/Pd molar ratio
Conv. / %
HMFCA
FFCA
7
FDCA
Others
Au/ZOC
-
>99
>99
>99
>99
>99
>99
>99
19
13
2
73
79
90
>99
87
69
50
2
0
2
0
0
1
1
Au3-Pd1/ZOC
Au2-Pd1/ZOC
Au1-Pd1/ZOC
Au1-Pd2/ZOC
Au1-Pd3/ZOC
Pd/ZOC
3.00
1.99
1.01
0.50
0.33
-
8
6
0
0
3
10
24
35
6
14
[a] 38 mg catalyst, 0.5 mmol HMF, 0.1679 g NaHCO3, 10 mL H2O, 353 K, 3 bar O2, 4 h. The molar ratios of the metals were adjusted to 2.0 wt% with respect to
ZOC in all catalysts
oxidation of benzyl alcohol or aldehyde.[53,54]
Table 3. The ICP-AES results showed that almost all Au and Pd
metals were immobilized onto ZOC and the molar ratios between
Au and Pd in all the Au-Pd/ZOC catalysts were close to 1. As
expected, no yield of FDCA was observed when using the ZOC
support alone as catalyst. 28% conversion of HMF to the reaction
intermediates, 5-hydroxymethyl-2-furancarboxylic acid (HMFCA)
and 5-formyl-2-furancarboxylic acid (FFCA) were found as the
major products, implying that only the oxidation of HMF to FFCA
took place in the absence of Au-Pd alloy nanoparticles. As the
loading of Au-Pd on ZOC increased from 0.5 to above 2.0 wt%,
the FDCA selectivity increased from 37% to > 99%. Based on
these results, one can expect that the incorporation of active
metals into the suitable support would improve the oxidation
further from FFCA to FDCA. These observations revealed that an
appropriate amount of Au and Pd with a molar ratio of 1:1 was
essential to achieve a quantitative yield of FDCA in water.
It was reported that the addition of base, especially strong
base (e.g. NaOH and KOH) could accelerate the HMF oxidation
via the promotion of O-H and C-H bond activation from OH-.
[27]
The main role of weak base was to maintain the activity of catalyst
by dissolving the formed products, especially FDCA, due to its low
[18]
solubility in water.
The formed FDCA or other intermediates
would block the active sites of the catalysts without addition of
base, thus resulting in the decrease of the catalytic activity. In
addition, the yields of FDCA increased dramatically with the
increase of support acidity/basicity even under the optimal
NaHCO3/HMF molar ratio of 4. These results indicated that the
support basicity could be the predominant factor to improve the
catalytic activity rather than the addition of NaHCO3.
To gain insights into the alloying effect between Au and Pd on
the HMF oxidation, the influence of Au:Pd molar ratio in catalysts
were examined. The inductively coupled plasma-atomic emission
spectrometry (ICP-AES) measurements for all of these catalysts
substantiated that the molar ratios between Au and Pd on the
ZOC support were close to the actual molar ratio during the
preparation and the catalytic results were summarized in Table 2.
Au/ZOC and Pd/ZOC were also tested and the yields of FDCA
were 73% and 50%, respectively. The yields of FDCA increased
to > 99% when Pd was introduced to the Au-Pd/ZOC catalyst with
an Au:Pd molar ratio of 1:1. These results evidently indicated a
synergistic effect between Au and Pd on enhancing the HMF
conversion and FDCA selectivity. However, the yield of FDCA
significantly varied by changing the ratio between Au and Pd.
Hence, lowering the Au/Pd ratio from 1 to 0.33, the yield of FDCA
decreased from 99% to 69%, whereas the FFCA yield was
increased from 0 to 24%, suggesting that Pd demoted the
catalytic activity. A similar descending trend on the yield of FDCA
was also observed when the Au content increased with respect to
Pd in Au-Pd/ZOC catalysts. It clearly demonstrated the
importance of having an appropriate Au:Pd molar ratio, especially
1:1 on ZOC, in order to achieve the desired catalytic activity.
The catalytic performance of Au-Pd/ZOC catalysts with different
metal loadings was also tested and the results were compiled in
Table 3. The influence of metal contents (Au+Pd) on ZOC on the yield of
FDCA.[a]
Selectivity / %
Conv.
Catalyst
/ %
HMFCA
FFCA
28
37
27
13
0
FDCA
0
Others
ZOC
28
62
25
12
0
0
1
0
2
0
0
0.5% Au-Pd/ZOC
1.0% Au-Pd/ZOC
1.5% Au-Pd/ZOC
2.0% Au-Pd/ZOC
3.0% Au-Pd/ZOC
>99
>99
>99
>99
>99
37
61
82
0
>99
>99
0
0
[a] 38 mg catalyst, Au:Pd=1:1, 0.5 mmol HMF, 0.1679 g NaHCO3, 10 mL H2O,
353 K, 3 bar O2, 4 h.
The influence of reaction temperature (60 to 90 °C) on the
yield of FDCA was also been investigated with 2.0 wt% of Au-
Pd/ZOC (Au:Pd = 1:1). As shown in Figure 3a, the obtained
FDCA yields were 32% and 54% at 60 and 70°C, respectively.
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When the temperature further increased to 80 °C, the yield of
FDCA was enhanced to over 99%. The yield of FDCA remained
unchanged even if the temperature was raised further to 90 °C,
implying that FDCA was quite stable under these conditions. This
indicated that the reaction temperature had a positive influence
on the catalytic activity of Au-Pd/ZOC catalyst. Based on the
obtained results at various temperatures, the optimal reaction
temperature for further studies was chosen to be 80 °C.
Catalytic experiments were also performed by varying the
molar ratio of NaHCO3/HMF and the results were illustrated in
Figure 3b. A significant influence of the NaHCO3 to HMF ratio was
observed on the conversion of HMF and product distribution. A
poor yield of FDCA (20%) was observed at a HMF conversion of
73% with Au-Pd/ZOC when no NaHCO3 was added in the
reaction system, leading to an unselective reaction pathway.
Moreover, the catalyst was deactivated presumable due to
adsorption of the formed carboxylic intermediates/products and/or
humins on the surface of the noble metal particles. [14,17] When the
NaHCO3/HMF molar ratio increased to 2, a total conversion of
HMF with FDCA yield of 88% was obtained. As the NaHCO3/HMF
molar ratio increased further to 4 or 6, a quantitative yield of FDCA
(>99%) was attained. It was reported that the main role of weak
base NaHCO3 was to act as a promotor by reacting with the
formed mono and/or dicarboxylic acid products/intermediates to
make the corresponding salts, whereby the catalyst activity was
retained.[18,19] Moreover, the pKa1 of FDCA was 2.60, which was
much lower than the pKa1 of carbonic acid (3.60).[50] Accordingly,
it could be understood that the formed FDCA could react with the
support if no base was added, thereby the role of NaHCO3 in the
oxidation process of HMF was not only to neutralize the formed
products or intermediates, but also to stabilize the ZOC support.
In order to get insight into the reaction pathway for the
oxidation of HMF into FDCA over Au-Pd/ZOC, a time course study
was also performed, and the results were shown in Figure 4a. The
conversion of HMF gradually increased with the prolonged
reaction time. The HMF conversion was about 73% after 30 min
while nearly quantitative conversion of HMF (> 99%) was
obtained after one hour. The yield of FDCA steadily increased at
the expense of HMFCA and FFCA intermediates as the reaction
time increased, and a FDCA yield of 99% was achieved within 4
h. As seen in Figure 4a, moderate yields of HMFCA (ca. 25%) and
FFCA (55%) were obtained after 30 min and then the
intermediates gradually transformed into FDCA. In contrast, the
yield of 2.5-diformylfuran (DFF) remained below 2% during the
entire reaction process. These results confirmed that HMFCA and
FFCA were the main intermediates during the consecutive
oxidation of HMF into FDCA. Moreover, the results revealed that
the aldehyde functionality in HMF was preferentially oxidized
compared to the hydroxyl group in HMF. Therefore, the oxidation
of HMF to HMFCA, followed by oxidation to FFCA, and further to
FDCA, rather than via DFF, was the fastest and thus predominant
reaction pathway using Au-Pd/ZOC as catalyst (Scheme 1). The
obtained reaction pathway was in good agreement with the
pathway previous reported for the oxidation of HMF into FDCA
with Au-Cu/CeO2 as catalyst.[24]
Figure 3. Aerobic oxidation of HMF at a) different temperatures and b) with
different NaHCO3/HMF molar ratios. Reaction conditions: 38 mg 2.0 wt% Au-
Pd/ZOC catalyst, Au:Pd=1:1, 0.5 mmol HMF, 10 mL H2O, 3 bar O2, 4 h. For the
different NaHCO3/HMF ratio experiments, the reaction temperature was 80 °C.
Thus, the catalytic activity of 2.0 wt% Au-Cu/ZOC catalyst was
also tested under the same conditions. Unfortunately, only around
18% yield of FDCA along with 93% conversion of HMF were
obtained after 4 h reaction time, which were much lower than
those of 2.0 wt% Au-Pd/ZOC catalyst. The difference could be
ascribed to the different role of Pd and Cu in the corresponding
Au-based bimetallic catalysts. For Au-Pd alloys catalyst, Au as an
efficient promoter of Pd prevented the overoxidation and
poisoning of Pd by formed intermediates or products.[33] In
contrast, Au was the active part in Au-Cu alloys while CuOx as a
promotor would be formed in Au-Cu alloys by the oxidation of Cu,
which could enhance oxidation performance via the activation of
oxygen. [53,54] Generally, Pd was more active than Au, Cu or CuOx
for oxidation reactions, which resulted in the high activity of Au-
Pd/ZOC. In order to substantiate the influence of the formed
HMFCA intermediate, it was also used as starting substrate. As
shown in Figure 4b, > 99% conversion of HMFCA was achieved
only after 4 h, establishing that the oxidaion of HMFCA to FFCA
was slower than the oxidation of HMF to HMFCA. This was also
in accordance with previous results obtained for aqueous HMF
oxidation using Au catalysts.[25]
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thereby revealing that the crystalline structure of the ZOC support
is quite stable under the chosen reaction conditions. The TEM
image of the used 2.0 wt% Au-Pd/ZOC catalyst displayed that Au-
Pd nanoparticles were still well dispersed on the surface of ZOC
and the average diameter of Au-Pd nanoparticles was around 1.8
nm, indicating the good stability of Au-Pd alloys on the support
ZOC. These results evidently demonstrated that Au-Pd/ZOC was
a highly stable and efficient catalyst for the conversion of HMF to
FDCA under the chosen reaction conditions.
Figure 5. Recyclability tests for the Au-Pd/ZOC catalyst. Reaction conditions:
38 mg catalyst (2.0 wt% Au-Pd/ZOC), molar ratio of Au:Pd=1:1, HMF (63 mg),
H2O (10 mL), NaHCO3 (0.1679 g), 3 bar O2, 353 K, 4 h, molar ratio of (Au +
Pd)/HMF = 1:100).
Figure 4. Time course of the oxidative conversion of a) HMF and b) HMFCA.
Reaction conditions: 38 mg 2.0 wt% Au-Pd/ZOC catalyst, Au:Pd=1:1, 0.5 mmol
substrate, 0.1679 g NaHCO3, 10 mL H2O, 353 K, 3 bar O2.
Conclusions
In summary, a series of Au-Pd bimetallic nanoclusters supported
on ZOC, MG20, CeO2, TiO2 and AC have been synthesized. TEM
images showed that all of the Au-Pd supported catalysts
contained particle size distributions of around 1.6 nm. Among the
employed supports, ZOC was found to be the best support on
improving the yield of FDCA because the basicity of ZOC support
was an important factor on the enhancement of catalytic activity.
In addition, Au-Pd/ZOC with an Au:Pd ratio of 1:1 showed the
highest catalytic activity towards HMF oxidation in water at
relative mild conditions, achieving > 99% yield of FDCA. A
synergistic effect between Au and Pd played an important role on
enhancing the catalytic activity towards the formation of FDCA. It
was also found that the presence of NaHCO3 in the catalytic
system could improve the yield of FDCA and stabilize the ZOC
support, thus avoiding deterioration of ZOC by the formed
carboxylic acid intermediates/products. Recyclability of Au-
Pd/ZOC with respect to the catalytic conversion of HMF to FDCA
revealed no significant change in seven consecutive runs.
Moreover, XRD and TEM analysis of the Au-Pd/ZOC after the
seven reaction runs indicated no change in the XRD patterns and
nanoparticle size distributions compared to the parent catalyst,
implying that Au-Pd/ZOC was a highly stable and efficient catalyst
for the conversion of HMF to FDCA under the applied reaction
conditions.
Scheme 1. The possible reaction pathway in HMF oxidation to FDCA. The blue
arrow is the predominant pathway.
The recyclability of Au-Pd/ZOC was also examined and the
results were given in Figure 5. After each run, the catalyst was
simply reused by centrifugation, washed with water and ethanol,
and drying at 60 °C overnight. HMF was completely converted for
each run, and the FDCA yield was generally stable at around 96%.
Meanwhile, the turnover frequencies (TOF) for each run were all
kept at around 24 h-1, indicating that no obvious deactivation
occurred even after 7 runs. In addition, the filtrates of seven runs
were subjected to ICP-AES analysis to detect possible leaching
of metals. The amount of leached zinc for each run were less than
0.12% of total zinc amount in the corresponding used catalyst.
Notably, no Au or Pd leaching was detected in the filtrates and it
strongly demonstrated the good stability of the Au-Pd/ZOC
catalyst. The 2.0 wt% Au-Pd/ZOC catalyst was further subjected
to XRD and TEM analysis after being used for seven reaction runs
(Figure S1, S4). No significant changes were observed in the
major peaks of the XRD patterns before and after reaction,
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10.1002/cctc.201600852
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HMF (≥ 97% purity) was purchased from Adamas Reagent Co., Ltd. The
aerobic oxidation of HMF was performed in a batch stainless-steel
autoclave (20 mL). Typically, 63 mg HMF and an appropriate amount of
catalyst corresponding to a HMF:(Au + Pd) molar ratio = 100:1) were
added into the reactor precharged with 10 mL of an alkaline aqueous
solution containing 0.1679 g NaHCO3 (molar ratio NaHCO3:HMF = 4:1).
The reactor was then sealed and purged with O2 for four times. The mixture
was heated to the reaction temperature of 80 °C. Once the system reached
the desired temperature, the reactor was pressurized with 3 bar O2 and
stirred vigorously with a magnetic stirrer at a speed of 600 rpm. The
autoclave was cooled down to room temperature by quenching with cold
water after a specific reaction time, and the catalyst was filtered off prior to
the analysis.
Experimental Section
Catalyst preparation
The ZOC support was prepared via a hydrothermal method previously
reported.[42] Typically, 1.10 g zinc acetate dihydrate and 1.50 g urea were
dissolved in 80 mL deionized water. After stirring for 10 min, the solution
was transferred to a Teflon-lined stainless steel autoclave and treated at
120 ºC for 5 h and then naturally cooled down to room temperature. The
precipitate was separated by centrifugation, washed with copious amount
of deionized water and ethanol, and finally dried at 60 ºC for 6 h in static
air to get the ZOC support.
The other supports (TiO2, CeO2 and activated carbon) were purchased
from Aladdin Industrial Corporation and used as received. The hydrotalcite
PURALOX ® MG 20 was purchased from Sasol Performance Chemicals.
Prior to use, activated carbon (AC) was pretreated with 100 mL aqueous
solution of 7 M HNO3 at 80 ºC for 48 h. AC was then separated by
centrifugation, washed with deionized water and ethanol for several times
and then dried at 80 ºC overnight in static air.
Analysis of the reaction mixture
The reaction mixtures were analyzed by high-performance liquid
chromatography (HPLC, Shimazu LC-20A) equipped with
detector (206 nm) and an ODS-SP column (8 mm
thermostatted at 50 °C. An aqueous buffer solution of KH2PO4/K2HPO4
(pH = 3.50) was used as a mobile phase at a flow rate of 0.5 mL/min. To
a
UV/vis
×
250 mm)
analyse the amount of 2.5-diformylfuran (DFF),
a HPLC (Agilent
The Au-Pd/ZOC bimetallic catalysts were prepared via a deposition-
precipitation method.[43] Appropriate amounts of HAuCl4 and K2PdCl4
aqueous solution were added into the mixture containing 0.15 g of ZOC
dispersed in 30 mL distilled water. For example, to prepare a catalyst
containing a molar ratio of Au:Pd = 1:1, 198 μL aqueous solution of 50
mmol/L HAuCl4 and 85.5 μL aqueous solution of 115.7 mmol/L K2PdCl4
were used for synthesis, corresponding to 2 wt% with respect to ZOC. The
mixture was stirred for a short while and heated to 80 ºC, and an
appropriate amount of urea (molar ratio: urea:metal = 100:1) was then
added to the mixture and stirred for 12 h. The final pH value of the mixture
was around 8.50 due to the decomposition of the urea. The obtained
precipitate was collected by centrifugation, washed with plenty of distilled
water and ethanol, and dried at 60 °C overnight. The catalyst was then
reduced at 100 oC under H2 atmosphere (1 atm) with a flow rate of 60
mL/min for 6 h. Other catalysts were synthesized by adopting a similar
procedure except Au-Cu/ZOC. The Au-Cu/ZOC catalyst was prepared via
a similar procedure while the reduction temperature was increased to 200
ºC since Cu could be reduced at high temperature.
Technologies 1200) equipped with an Aminex HPX-87H column (7.8 mm
× 300 mm) thermostated at 60 °C was employed. Here a 0.005 M aqueous
solution of H2SO4 was used as the mobile phase. The conversion of HMF,
the yield of FDCA and other intermediates were quantified by making
series of individual standards with propandioic acid as the internal
standard. All the samples were neutralized with 1% H3PO4 solution prior
to the analysis.
Acknowledgements
We gratefully acknowledge the support of the National Natural
Science Foundation of China (NSFC 21476084 and 21576081),
the Fundamental Research Funds for the Central Universities,
PetroChina Innovation Foundation and the Open Project of
Shanghai Key Laboratory of Molecular Catalysis and Innovative
Materials (2014MCIMKF02). Zhenyou Gui acknowledges the
financial support of China Scholarship Council (CSC) for his joint
PhD program (No. 201506740037).
Catalysts characterization
The crystal phase of the catalyst was examined by powder X-ray diffraction
(XRD, Bruker D8 Advance diffractometer) with Cu-Kα radiation (λ = 1.5406
Å ) in the range of 2θ = 10 - 80°. Transmission electron microscopy (TEM)
coupled with energy-dispersive X-ray spectroscopy (EDS) were recorded
by a FEI Tecnai G2 F20 S-Twin (operated at 200 kV) to determine the
particle sizes and metal compositions of the obtained materials. X-ray
photoelectron spectroscopy (XPS) measurements were recorded on a
Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer with
Al-Kα X-ray as the excitation source. All binding energies were referred to
the C 1s peak at 284.8 eV of the surface adventitious carbon. The metal
contents in the prepared catalysts and the leaching of metals to the
solution during the reaction were determined by inductively coupled
plasma-atomic emission spectrometry (ICP-AES) on an Agilent 725ES
instrument. The basicity of the ZOC support was measured by using
Mettler Toledo FE-20 pH meter. To measure pH, 0.05 g of ZOC was
suspended in 5 mL Milli-Q water and stirred for 30 min prior to the
measurement.
Keywords: 5-hydroxymethylfurfural • 2,5-furandicarboxylic acid
• aerobic oxidation • bimetallic catalyst • zinc hydroxycarbonate
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Table of Contents
FULL PAPER
Au-Pd
supported
on
zinc
Z.Y. Gui, W.R. Cao, S.
hydroxycarbonate (ZOC) catalysts
showed high activity for HMF oxidation.
The basicity of ZOC could play a
crucial role for enhancing the yield of
FDCA. Moreover, Au-Pd/ZOC was
prone to recycling for at least six times
without significant loss of its activity.
Saravanamurugan, A. Riisager, L.F.
Chen*, Z.W. Qi*
Efficient Aerobic Oxidation of 5-
Hydroxymethylfurfural in Aqueous
Media with Au-Pd Supported on Zinc
Hydroxycarbonate
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