Ruthenium supported on high-surface-area zirconia as an efficient catalyst for the base-free oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid

Article abstract of DOI:10.1002/cssc.201800448

Several ZrO₂-supported ruthenium catalysts were prepared and utilized in the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) under base-free conditions. Full conversion of HMF and almost perfect selectivity towards FDCA (97 %) were achieved after 16 h by using pure O₂ as an oxidant and water as a solvent. The catalytic tests show that the size of the Ru particles is crucial for the catalytic performance and that the utilization of high-surface-area ZrO₂ leads to formation of very small Ru particles. Superior activity was obtained for catalysts based on ZrO₂ that had been synthesized by a surface-casting method and has high surface areas up to 256 m² g¹. In addition to good activity and selectivity, these catalysts show also high stability and constant activity upon recycling, confirming the suitability of Ru/ZrO₂ in the base-free oxidation of HMF.

Full text of DOI:10.1002/cssc.201800448

Accepted Article
Title: Ruthenium supported on High Surface Area Zirconia as
an Efficient Catalyst for the Base-Free Oxidation of 5-
Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid
Authors: Christian M Pichler, Mohammad Ghith Al-Shaal, Dong Gu,
Hrishikesh Joshi, Wirawan Ciptonugroho, and Ferdi Schüth
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To be cited as: ChemSusChem 10.1002/cssc.201800448
Link to VoR: http://dx.doi.org/10.1002/cssc.201800448
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ChemSusChem
10.1002/cssc.201800448
FULL PAPER
Ruthenium supported on High Surface Area Zirconia as an
Efficient Catalyst for the Base-Free Oxidation of 5-
Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid
†
[a]
†[a]
[b]
[a]
Christian M. Pichler, Mohammad G. Al-Shaal, Dong Gu, Hrishikesh Joshi, Wirawan
[
c] [d]
[a]
Ciptonugroho
and Ferdi Schüth*
†
These authors contributed equally to this work
Abstract: Different ZrO
prepared and utilized in the oxidation of 5-hydroxymethylfurfural
HMF) to 2,5-furandicarboxylic acid (FDCA) under base-free
conditions. Full conversion of HMF and almost perfect selectivity
towards FDCA (97%) were achieved after 16 h, using pure O as an
2
supported ruthenium catalysts were
polymer generated by reacting FDCA with ethane-1,2-diol,
known as polyethylene furanoate (PEF), shows not only similar
mechanical and physical properties as the conventional
polyethylene terephthalate, but also superior barrier properties
(
2
against H
2 2 2
O, CO , and O , which are crucial parameters for
[
10]
oxidant and water as a solvent. The catalytic tests show that the size
of the Ru particles is crucial for the catalytic performance and that
further application of PEF in grocery and beverage packaging.
the utilization of high surface area ZrO
small Ru particles. Superior activity was obtained for catalysts based
on ZrO that had been synthesized by a surface casting method and
2
leads to formation of very
Hydrolysis
Oxidation
Polymerization
2
2
-1
5-Hydroxymethylfurfural
(HMF)
2,5-furandicarboxylicꢀacid
(FDCA)
possesses high surface areas up to 256 m g . In addition to good
activity and selectivity, these catalysts show also high stability and
Figure 1. Schematic representation of polymer production starting from
constant activity upon recycling, confirming the suitability of Ru/ZrO
in the base-free oxidation of HMF.
2
biomass via HMF and FDCA as intermediates.
A recent collaboration of the Coca Cola Company together with
Avantium, Danone and ALPLA for commercialization of PEF
based bottles underlines the high commercial potential of PEF
Introduction
[
11]
and thereby FDCA.
Furthermore, PEF can be enzymatically
degraded to yield the starting materials, which offers a novel
The intensive research on biomass valorization over the last
decade shows the importance of biomass as sustainable
feedstock not only in the energy sector but also in the production
[
12]
recycling pathway.
The already existing commercial interest
and the possibility to use biomass as feedstock make FDCA an
attractive target molecule. Biomass-derived FDCA can be
prepared through catalytic oxidation of HMF (Figure 1). Different
oxidizing agents have been reported for this process, such as
[
1]
of chemicals. Numerous bulk and fine chemicals, which are
finding wide applications, can be derived from biomass and are
[
2]
used as alternatives for those produced from fossil resources.
[
13]
[14,15]
KMnO
4
,
peroxides,
and molecular oxygen (pure or in
One of the most promising chemicals for implementation in the
chemical industry is 2,5-furandicarboxylic acid (FDCA), which is
derived from the key platform molecule 5-hydroxymethylfurfural
[
16–19]
air).
Oxygen is the best studied oxidant, as it offers a clean,
cheap, and efficient pathway for HMF oxidation. Numerous
catalytic systems were developed and applied to enable the
[
3–5]
(
HMF).
The high interest in FDCA results from the fact that it
[
11]
transformation
photocatalysis,
of
16,20,21]
HMF
to
FDCA.
Electro-
and
can be used for the synthesis of polymers. Due to its molecular
similarity to terephthalic acid, FDCA can be used as a monomer
[
homogeneous as well as heterogeneous
[22,23]
[15]
[
6–9]
catalysis
and also biocatalysis were utilized for this task.
in the production of bio-based polymers and composites.
The
Heterogeneous catalysis is the most commonly used method in
HMF oxidation due to its cost effectiveness and the facile
separation of product from catalyst. Noble metals such as
[
17,24–27]
[28–31]
[32–35]
[33,34]
[a]
C. M. Pichler, Dr. M. G. Al-Shaal, H. Joshi, Prof. Dr. F. Schüth
Max-Planck-Institut für Kohlenforschung
Pt,
Pd,
Au,
Ru,
and alloys thereof
However, a major
[39,40]
performed best in terms of activity.
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany)
E-mail: schueth@kofo.mpg.de
drawback of many of the aforementioned systems is the need of
several equivalents of base in the reaction mixture. This requires
subsequent neutralization of the reaction solution and separation
of the formed salts, which both have a negative effect on the
economy of the process and make it less eco-friendly. This fact
[
b]
c]
Dr. D. Gu
The Institute for Advanced Studies, Wuhan University, Wuhan,
Hubei 430072, P.R. China
[
Dr. W. Ciptonugroho
Group of Inorganic Chemistry and Catalysis
University Utrecht
[
41–45]
triggered the development of base-free catalytic systems.
Universiteitsweg 99, 3584 CG Utrecht (Netherlands)
Dr. W. Ciptonugroho
Chemical Engineering Department of Sebelas Maret University,
Jalan Ir. Sutami 36, A 57126 Surakarta, Indonesia
Ruthenium catalysts have been repeatedly reported as efficient
[d]
catalysts for the base-free oxidation of HMF into DFF and
[46–48]
FDCA.
The materials that have been used so far as
supports for Ru can be mainly classified into carbon based
materials and metal oxides.
Supporting information for this article is given via a link at the end of
the document.
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ChemSusChem
10.1002/cssc.201800448
FULL PAPER
Ruthenium supported on activated carbon exhibited activity for
uses different kinds of silica as hard templates, namely, silanol-
group-rich SBA-15 and aerogel. Different to conventional hard-
templating pathways using silica templates, the silanol group-
HMF oxidation and yielded 88% of FDCA (2 bar O
0 h). However, the molar ratio of Ru to HMF was found to be
lower than in comparable studies (ratio of 10 vs. 30-40 in
2
, 120 °C, and
1
rich silica creates strong interaction between the ZrO
a
2
[
46]
comparable studies).
Polymer based materials featuring a
precursor, ZrOCl ·8H O, and the silica surface. This interaction
2
2
triazine backbone structure were tested. In these materials the
Ru nanoparticles are stabilized by interactions between the
then leads to the formation of a thin layer of oxide on the silica
pore surface analogous to the synthesis of CMK-5 carbon. The
preparation of the material is straightforward and employs a dry
[
47]
metal and the nitrogen containing moieties in the polymer.
The achieved yield of FDCA was 77% (at 20 bar O 140 °C and
h). The issue observed in this study was the incomplete carbon
2
impregnation of the silica template with
ZrOCl ·8H
template removal, as we reported previously.
material is denoted as ZrO2 H-SBA
Characterization of the resulting ZrO
tubular array structure (Figure S1) with a surface area of 321 m
a
solution of
1
2
2
O, followed by sequential heat treatments and
[
49]
balance, due to the strong interaction between the support and
the reaction components. As an alternative various metal oxides
can be applied as support, which also exhibited a strong
The resulting
.
2
material shows a hollow
[
48]
2
influence on the resulting FDCA yields.
utilized oxide FDCA yields varied between 20 and 100% (at 2.5
bar O , 140 °C and 6h). The best results were obtained for Ru
supported on several magnesium containing oxides, which acted
Depending on the
-
1
g
(Figure S2).
2
In addition to ZrO
2
H-SBA, a second high surface area ZrO
aerogel as
In fact, the use of
aerogels as template in the preparation of ZrO is
2
2
+
as a solid base. However, Mg ions were found in the reaction
solution, indicating that leaching occurs during the reaction and
that the stability of the catalyst under the reaction conditions
appears to be limited.
support was prepared by using silanol-rich SiO
2
[
50]
template in the surface casting process.
SiO
2
2
advantageous, as the synthesis of aerogel does not require
surfactants, which have to be removed later, like in the case of
The previous examples clearly demonstrate that Ru is a highly
active metal for the oxidation of HMF. However, there is a need
to develop new supports that show high stability as well as low
interaction with the reaction components, while simultaneously
enabling high catalytic activity in combination with Ru. By taking
SBA-15. The resulting calcined ZrO
2
(denoted as ZrO
a surface area of 375 m g . This is considerably higher
compared to other ZrO materials obtained via the conventional
2
H-aero) has
2
-1
2
2
-1 [51]
hard templating methods (220 m g ).
these factors in consideration, ZrO
attractive option. The high mechanical stability and low chemical
reactivity make ZrO a frequently used support in chemical
2
appears to be a highly
As an alternative to the surface casting approach, a soft
2
templating technique was used to prepare a third type of ZrO .
2
For the so called EISA (evaporation-induced self-assembly)
method, the surfactant F127 served as structure directing agent
industry. Increasing the surface area of this support would be
another advantage that could potentially enhance the catalytic
4
and Zr(OBu) as a zirconia precursor. The synthesis route starts
activity. Nevertheless, the preparation of high surface area ZrO
is still challenging, as mechanisms related to sintering can lead
to remarkable loss of surface area, especially during
calcination and other necessary heat treatments. The first goal
of this study was the synthesis of ZrO supports with high
surface areas by following our recently developed surface
2
with a hydrolysis/condensation step, and after several drying
and calcination steps, which are necessary to remove the
a
2
surfactant, the final soft templated ZrO support (denoted as
2
-1
ZrO2-soft) was obtained, with a surface area of 41 m g , which is
significantly lower than the surface areas of the prepared hard
2
templated ZrO
2 2
. Finally, two commercial ZrO materials that
[
49]
2
-
casting method.
activity with established Ru/ZrO
ZrO have been used as supports for Ru catalysts. Tetragonal
and monoclinic commercial ZrO , in addition to a soft templated
one, were used as supports for Ru catalysts and tested
alongside Ru-supported on surface casted ZrO in the aqueous
To enable the comparison of the catalytic
differed in their crystal structure (monoclinic with SBET of 90 m g
1
2
-1
2
catalysts, also other types of
and tetragonal with SBET of 133 m g ) were also chosen as
supports for Ru catalysts.
2
2
2 2
The final Ru/ZrO catalysts based on different ZrO materials as
2
supports, were prepared via a wet impregnation method. The
oxidation of HMF to FDCA. Finally, the most active catalyst was
further tested in order to determine its stability and recyclability.
aforementioned supports were dispersed in an ethanolic solution
of RuCl
catalysts were reduced under H
catalysts were labelled Ru/ZrO2 H-aero, Ru/ZrO2 H-SBA, Ru/ZrO2 soft
Ru/ZrO2 C-mono and Ru/ZrO2 C-tet
3
·xH
2
O prior to the removal of the solvent. Finally, the
2
atmosphere. The prepared
,
.
Results and Discussion
Catalysts preparation
Different kinds of ZrO
for Ru based catalysts. Synthetic details are reported in the
experimental section. Two different high surface area ZrO
supports were prepared via a surface casting method which
2
were synthesized and used as supports
2
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ChemSusChem
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FULL PAPER
Characterization of catalysts
colored, can provide additional information on the distribution of
the metal on the surface of the support. Ru/ZrO2 H-aero shows a
uniform allocation of Ru, excluding the formation of single big Ru
particles (Figure 3) and confirming the distribution of Ru over the
Table 1. Specific surface area, total pore volume and Ru content of the
2
metal loaded Ru/ZrO catalysts
2
-1 [a]
[b]
[c]
Entry
Catalyst
S
BET [m ·g ]
V
P(total)
0.60
0.25
0.05
0.25
0.14
Ru[wt.%]
4.9
whole surface of ZrO
2
. A similar result is found for the other
H-SBA, where the Ru is also
surface casted catalyst Ru/ZrO
2
1
2
3
4
5
Ru/ZrO2 H-aero
Ru/ZrO2 H-SBA
Ru/ZrO2 soft
239
exhibiting a uniform distribution over the ZrO
S7).
2
support. (Figure
256
5.2
37
4.8
Ru/ZrO2 C-mono
Ru/ZrO2 C-tet
86
5.4
103
5.0
[
a] Surface area determined by the BET method. [b] Total pore volume
determined at p/p =0.98. [c] Determined by ICP-OES.
o
Textural properties of the catalysts were characterized by N
physisorption. Comparison of the Brunauer-Emmett-Teller
BET)-surface areas of the prepared Ru-catalysts (Table 1),
shows that the catalysts based on the surface casted ZrO
2
(
2
exhibit significantly higher SBET than the catalysts with soft
templated and commercial supports (Table 1, entries 1,2 vs. 3-5).
The Ru contents of the samples listed in Table 1 were
determined by ICP-OES elemental analysis and were always in
the range between 4 and 5% (Table 1, entries 1-5).
Figure 3. SEM-EDX element mapping of Ru/ZrO2 H-aero (Ru is colored yellow).
To gain further insight into the structure of the catalysts, they
were analysed by TEM. Figure 2 shows the micrographs of the
surface casted Ru/ZrO
2
catalysts (Ru/ZrO
2
H-aero (Figure 2, A)
XRD analysis confirms the conclusions that were drawn from the
TEM and SEM-EDX micrographs. The XRD patterns for both
and Ru/ZrO2 H-SBA (Figure 2, B)). Almost no Ru nanoparticles are
observed on the surface of the support. This is most likely due to
the fine distribution of very small particles and/or clusters over
Ru/ZrO
intensity reflections (Figure S10). The two broad reflections at
20-38° and 40-70° can be attributed to ZrO . This indicates that
ZrO consists of very small crystalline domains.
2 2
H-aero and Ru/ZrO H-SBA show overall broad and low
the surface of ZrO
2
. Furthermore, the unfavorable contrast
2
[
49]
between the metal and the support impedes the detection of
2
In fact, the
[
52,53]
small metal nanoparticles.
TEM analysis for Ru/ZrO2 C-mono
broadness of these features prevents any unambiguous crystal
phase assignment. An additional small feature with low intensity
can be recognized in the range of 40-45°. This can be attributed
and Ru/ZrO2 C-tet reveals the presence of Ru nanoparticles in the
range of 2-3 nm (Figure S4 and S5). Finally, a TEM analysis of
0
[36]
Ru/ZrO2 soft gives similar results to those obtained from Ru/ZrO
H-aero and Ru/ZrO H-SBA, where no Ru nanoparticles can be
observed on the surface of the support (Figure S6).
2
to the Ru nanoparticles. Nevertheless, the broadness of this
reflection indicates the presence of very small Ru nanoparticles.
2
In the case of the XRD patterns of Ru/ZrO2 soft, Ru/ZrO2 C-mono
and Ru/ZrO C-tet, strong reflections related to different ZrO
crystalline phases were observed. Reflections at 24, 28, 31, 34,
0, 50, 54 and 63°, which belong to the monoclinic phase, were
,
2
2
A
B
4
observed for Ru/ZrO2 C-mono, whereas reflections at 30, 35, 50, 60
and 82°, which identify the tetragonal phase, were found for
Ru/ZrO2 soft and Ru/ZrO2 C-tet. The sharpness and high intensity of
the ZrO
of crystallinity and rather large particles of ZrO
high intensity of ZrO reflections makes the observation of any
2
reflections of all three materials indicate a high degree
2
. However, the
50 nm
200 nm
2
reflections related to Ru nanoparticles very difficult.
Figure 2. TEM images of: A) Ru/ZrO2 H-aero; B) Ru/ZrO2 H-SBA
;
Due to the fact that it was not possible to directly observe Ru
2 2 2 soft
nanoparticles for Ru/ZrO H-aero, Ru/ZrO H-SBA, and Ru/ZrO
To overcome the difficulties caused by the low contrast in TEM
analysis, Ru/ZrO H-aero was further investigated using energy
using TEM, and because furthermore the size estimation of Ru
nanoparticles using SEM-EDX or XRD would be very inaccurate,
hydrogen temperature programmed desorption (TPD) was
performed in order to estimate the particle size of Ru in these
2
dispersive X-ray (SEM-EDX) analysis. Although SEM-EDX
analysis can only provide a rough estimation of the size of Ru
nanoparticles, element mapping, by which Ru is artificially
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ChemSusChem
10.1002/cssc.201800448
FULL PAPER
[
a]
between 4 and 12 h over Ru/ZrO2 H-SBA (Figure 5b). After 12 h, a
drop in the yield of FFCA occurs and FDCA becomes the main
product in the reaction mixture. 84% yield of FDCA is achieved
over Ru/ZrO2 H- SBA while Ru/ZrO2 H-aero enables a full conversion
of HMF with 97% selectivity towards FDCA.
Table 2. HMF oxidation over different Ru/ZrO
2
catalysts.
Selectivity [%]
Carbon
balance
HMF
Entry
Catalyst
conv. [%]
[b]
DFF
FFCA
FDCA
[%]
1
2
3
4
5
6
7
8
ZrO
2
H-aero
3
3
0
0
0
99
99
99
98
99
99
99
93
ꢀ
Ru/ZrO
2
H-aero
100
97
3
97
87
19
60
54
28
71
Ru /ZrO2 H-SBA
Ru/ZrO2 soft
0
13
45
36
40
47
29
95
26
4
Ru/ZrO2 C-mono
Ru/ZrO2 C-tet
99
100
97
6
1 % Ru/ZrO
2
H-aero
22
0
Ru/ZrO2 H-aero 450
100
[
a] Reaction conditions: HMF (63 mg, 0.5 mmol); Cat. (33 mg, 5% Ru loading
equals 0.016 mmol Ru); 10 bar O ; 10 mL H O; 120 °C; 500 rpm; and 16 h
2
2
Figure 4. Reaction pathway of the base-free HMF oxidation to FDCA through
reaction time). [b] Carbon balance [%]=100-(HMF conversion-∑yields of DFF,
DFF and FFCA as intermediates.
FFCA, and FDCA).
materials. The calculated size of the Ru particles in Ru/ZrO2 H-aero
Ru/ZrO2 H-SBA was determined to be in the range of 0.8-1nm. In
order to confirm the reliability of these findings, also Ru/ZrO2 C-
mono was analyzed by means of TPD as a reference material.
The Ru particle size for this material was determined to be about
,
As both catalysts possess similar textural properties (Table 1,
entries 1,2), the difference in the catalytic activity may be
attributed to the morphology of the supports in both catalysts.
a
Ru/ZrO2 H-aero
b
Ru/ZrO2 H-SBA
1
.5 nm. This is close to the particle size determined by TEM
100
100
analysis (Figure S4 and S5), which confirms the validity of the
results obtained by TPD (Figure S8 and Table S9). The
calculated particle size for Ru/ZrO2 soft is significantly higher, at
80
60
40
20
0
80
60
40
20
0
3
-4 nm.
Catalytic oxidation of HMF to FDCA
The prepared catalysts were tested in the base-free oxidation of
HMF to FDCA. Conditions were chosen to correspond to those
0
4
8
12
16
0
4
8
12
16
[
11,47]
reported in literature.
2
Neat O was used as an oxidant (10
Time [h]
Time [h]
bar) and 120 °C was applied to enable the conversion of HMF to
FDCA. Water was used as a solvent and the HMF/metal-ratio in
all tests was 36.5:1. Under these conditions and in the absence
of any base, HMF is oxidized firstly to 2,5-diformylfuran (DFF),
which undergoes a subsequent oxidation reaction to yield 5-
formyl-2-furan carboxylic acid (FFCA) and ultimately FDCA
HMF conv.
DFF yield
FFCA yield
FDCA yield
Figure 5. Oxidation of HMF over Ru/ZrO
Reaction conditions: HMF (63 mg, 0.5 mmol); Cat. (33 mg, 0.016 mmol Ru);
10 bar O ; 10 mL H O; 120 °C; 500 rpm; Yields are given in mol%).
2 2
H-aero (left) and Ru/ZrO H-SBA (right).
2
2
[
11,43]
(
Figure 4).
In the first test, HMF oxidation was investigated
2 soft
For the Ru-impregnated on soft-templated support, Ru/ZrO ,
over a blank ZrO2 H-aero support. After 16 h, only 3% conversion
of HMF to DFF was obtained, highlighting the need for Ru in the
oxidation reactions (Table 2, entry 1). Oxidation of HMF with
lower activity in comparison to Ru/ZrO2 H-aero and Ru/ZrO2 H-SBA
catalysts was observed (Table 2, entries 2,3 vs. 4). Although
almost full conversion of HMF (95%) can be obtained after 16 h,
the yield of FDCA does not exceed 19%, whereas the main
product was FFCA with a yield of 45% (Figure S11). These
2 2 H-aero
both Ru on surface casted ZrO catalysts, namely, Ru/ZrO
and Ru/ZrO2 H-SBA, shows high activity and yields FDCA with high
selectivity (Table 2, entries 2,3 and Figure 5). Although full
conversion of HMF was observed over both catalysts,
findings prove the poor activity of Ru/ZrO
2
soft in the oxidation of
HMF. It is reasonable to argue that the lower catalytic activity of
Ru/ZrO
2
H-aero seems to be more active than Ru/ZrO2 H-SBA. This
Ru/ZrO
2
soft in comparison to Ru/ZrO2 H-aero and Ru/ZrO2 H-SBA is
2
can be inferred from the significant drop in the concentration of
DFF after 2 h and FFCA after 4 h over Ru/ZrO2 H-aero (Figure 5a).
In contrast, the yield of DFF decreases slowly after 4 h, while the
yield of FFCA remains more or less constant over the time
2
due to the significantly lower surface area of Ru/ZrO soft (37 m
-
1
g ), leading to a bigger size of the Ru nanoparticles (0.8 nm and
1.0 nm vs. 3.4 nm) (Table S9).
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ChemSusChem
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FULL PAPER
range, emphasizing the significance of small Ru particles. The
formation of small Ru nanoparticles is facilitated by high BET
surface areas of the ZrO support. The surface casted catalysts
2
a
Ru/ZrO2 C-mono
Ru/ZrO2 C-tet
b
Ru/ZrO H-aero and Ru/ZrO H-SBA, show the highest BET surface
2
2
1
00
100
areas, resulting in the highest catalytic activity among the tested
materials in our study. The influence of Ru particle size is also
8
6
4
2
0
0
0
0
0
80
60
2
found for materials with the same ZrO support, but differently
sized Ru particles. If the Ru impregnated support is reduced at
higher temperatures, bigger Ru particles are formed (reduction
at 450 °C leads to formation of Ru particles with an average size
4
0
20
0
2
of 1.7 nm on ZrO H-aero). The catalytic tests of this material
yielded significantly less FDCA compared with the material
reduced at 250 °C (Table 2, Entry 8 and Supporting Information
Figure S18).
0
4
8
12
16
0
4
8
12
16
Time [h]
Time [h]
It was also tried to reduce the Ru loading of the most promising
HMF conv.
DFF yield
FFCA yield
FDCA yield
catalyst material, which is Ru/ZrO
2
H-aero. A catalyst with 1 wt.%
Ru on ZrO H-aero was prepared and tested for the HMF oxidation.
2
Figure 6. Oxidation of HMF over Ru/ZrO
2
C-mono (left) and Ru/ZrO
2
C-tet (right).
The results (Table 2, Entry 7) show that full HMF conversion can
be achieved, the yield of FDCA is, however, significantly lower.
To obtain full yield of FDCA in an acceptable reaction time, 5
wt.% Ru loading seem to be appropriate.
Reaction conditions: HMF (63 mg, 0.5 mmol); Cat. (33 mg, 5% Ru loading
equals 0.016 mmol Ru); 10 bar O
given in mol%).
2 2
; 10 mL H O; 120 °C; 500 rpm, Yields are
In order to investigate the influence of the crystalline phase of
the ZrO support on the catalyst performance, HMF oxidation
was investigated over Ru supported on commercially available
monoclinic and tetragonal ZrO (Figure 6a and b). Interestingly,
both catalysts, Ru/ZrO enable
As next step, recyclability tests were carried out to provide more
insight into the reusability and the stability of these materials.
2
2
The recyclability of Ru/ZrO H-aero has been investigated over five
2
consecutive runs where the reaction conditions were the same
(HMF (63 mg, 0.5 mmol); catalyst (33 mg, 0.016 mmol Ru); 10
2
C-mono and Ru/ZrO
2
C-tet
,
comparable activities and selectivity towards the intermediates
and FDCA. 59% and 54% yields of FDCA were achieved after
2 2
bar O ; 10 mL H O; 120 °C; 500 rpm; and 1 h reaction time).
Conducting the oxidation reaction over 1 h period affords
incomplete conversion of HMF enabling better comparability
between the different tests. As depicted in Figure 7, only a slight
drop in the catalytic activity can be observed over the recycling
runs. Most importantly, no loss in the carbon balance was
observed upon recycling. 5% increase in the yield of DFF in the
1
2 2
6 h over Ru/ZrO C-mono and Ru/ZrO C-tet, respectively. The
selectivity towards DFF and FDCA were also comparable in both
cases. Evidently, these findings exclude any direct influence of
the crystalline structure of the support on the catalytic activity in
HMF oxidation. However, it can be seen that Ru/ZrO2 C-mono and
Ru/ZrO2 C-tet have a considerably lower activity in comparison to
fifth cycle indicates a slight deactivation of Ru/ZrO
2
H-aero. An
the high surface materials, Ru/ZrO
2
H-aero and Ru/ZrO
2
H-SBA
ICP-OES analysis of the reaction solutions after each test
together with the catalyst after the fifth cycle reveal only a very
limited loss of the Ru content (1 ppm).
(
Figure 5 vs. Figure 6). This can be related to the influence of
the Ru nanoparticle size on the catalytic activity. Both catalysts,
2
-
Ru/ZrO2 H-aero and Ru/ZrO2 H-SBA, possess high SBET (> 200 m g
1
00%
1
)
which allows the distribution of Ru over a larger area and
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
leads to the formation of small Ru nanoparticles (0.8 nm for
Ru/ZrO2 H-aero and 1.0 nm for Ru/ZrO2 H-SBA, Table S9). This fact
enables a quantitative conversion of HMF to FDCA. In contrast,
8
0%
Ru/ZrO
2
C-mono and Ru/ZrO
2
C-tet show relatively lower catalytic
60%
activity, corresponding to the larger size of Ru particles of 1.8
nm (Ru/ZrO
comparison of the catalytic activity between Ru/ZrO
Ru/ZrO
catalytic activity. Although both catalysts share the tetragonal
crystalline structure of ZrO , the commercial catalyst shows
better catalytic performance than Ru/ZrO2 soft. While the yield of
FDCA reaches 54% after 16 h over Ru/ZrO
2
C-mono
)
and 1.5 nm (Ru/ZrO
2
C-tet).
A
further
4
0%
0%
0%
2
soft and
2
C-tet confirms the effect of the Ru particle size on the
2
2
HMF conv.
DFFyield
FFCA yield
FDCA yield
2
C-tet with a Ru
particle size of 1.5 nm, only 19% is achieved over Ru/ZrO2 soft
with a Ru particle size of 3.4 nm. Rate constants were also
calculated for the particular catalysts and normalized to the Ru
metal surface area (see Supporting Information, Table S16). The
normalized rates (for all three reaction steps) are in a similar
Figure 7. Recycling test over Ru/ZrO2 H-aero. Reaction conditions: HMF (63 mg,
0.5 mmol); Cat. (33 mg, 0.016 mmol Ru); 10 bar O ; 10 mL H O; 120 °C; 500
rpm; and 1 h reaction time, Yields are given in mol%).
2
2
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ChemSusChem
10.1002/cssc.201800448
FULL PAPER
th
The used catalysts after the
5
recycle were further Conclusions
characterized by XRD analysis (Figure S12). A clear change in
the reflections attributed to the support can be seen for the
recycled material. This is most obvious for the reflections at 30
2
In this study, we have introduced Ru supported on ZrO as a
catalyst with high potential in the oxidation of 5-
hydroxymethylfurfural to 2,5-furandicarboxylic acid under base-
2
and 50° and is explained by the increased crystallinity of ZrO .
The features attributed to Ru, in the range of 40-45°, are very
broad with low intensity. This indicates no growth in Ru particles
upon recycling. TEM analysis of the recycled catalysts confirms
the conclusions drawn from the XRD measurements. The
free conditions. Different Ru/ZrO
2
catalysts, based on
commercial ZrO as well as ZrO supports that were obtained via
2
2
surface casting and soft templating, were prepared and tested in
the oxidation of HMF. The small particle size of Ru and its fine
dispersion over the support proved to be a key factor for high
activity in the oxidation reaction. Small Ru nanoparticles can be
crystallinity of the ZrO
whereas the Ru clusters are still not visible on the high surface
ZrO support. (Figure S14).
2
has increased during the recycling tests,
2
obtained by using a high surface area ZrO
2
support material.
Therefore, the surface casted ZrO materials are especially well
2
suited supports, as they exhibit a very high surface area. The Ru
loaded catalysts based on these materials show, as expected,
very small metal nanoparticles (0.8-1 nm), which proved to be
highly beneficial for the catalytic performance. This
demonstrates the benefits of using these novel surface casted
Cycle 1
Cycle 2
4
2
0%
0%
2
materials as catalyst supports. Ru/ZrO H-aero was identified as
the most active catalyst in the oxidation of HMF and showed
clearly better catalytic performance than catalysts based on
commercial or soft templated ZrO
Ru/ZrO H-aero enables full conversion of 5-hydroxymethylfurfural
with a selectivity of 97% towards 2,5-furandicarboxylic acid at
120 °C under 10 bar of O . Ru/ZrO H-aero exhibits also high
2
.
The utilization of
2
0%
2
2
HMF conv.
DFF yield
FFCA yield
FDCA yield
stability upon recycling. Furthermore, this catalyst does not show
signs of leaching and also the carbon balances are closed,
which eliminates significant issues that were observed with other
Figure 8. Recycling test over Ru/ZrO
.5 mmol); Cat. (33 mg, 0.016 mmol Ru); 10 bar O
rpm; and 1 h reaction time, Yields are given in mol%).
2
H-SBA. Reaction conditions: HMF (63 mg,
[
47,48]
0
2
; 10 mL H O; 120 °C; 500
2
Ru based catalyst systems.
Overall, the high activity
together with high stability make Ru/ZrO
2
H-aero an excellent
candidate for a sustainable production of 2,5-furandicarboxylic
acid.
In contrast to the catalyst based on the aerogel casted ZrO
2
, a
recyclability test of Ru/ZrO H-SBA reveals low structural stability,
2
which is probably the reason for the lower catalytic activity upon
recycling. Figure 8 shows that the conversion of HMF dropped
Experimental Section
2
from 40% over the fresh Ru/ZrO H-SBA to 19% over the recycled
one even after the first recycling run. This decrease in the
conversion was accompanied with a drop in the yield of the
intermediates and FDCA but with no loss in the carbon balance.
Materials
All chemicals were purchased from commercial suppliers and used as
received: tetraethylorthosilicate (Sigma-Aldrich, reagent grade 98%)
2
TEM investigation of Ru/ZrO H-SBA, before and after the reaction
shows clearly a collapse of the tubular structure of the support,
although no drastic change in the SBET was found for the fresh
and the used catalyst (Figure S14 and Table S15). These
findings confirm that the morphology of the supporting material
has also a major impact on the catalytic activity. The changes in
the structure of the support during the reaction can lead to a
change in the catalytic activity. It can be considered, that a part
of the Ru particles becomes inaccessible for the reactants
through this major change in morphology. A growth of the Ru
Pluronic P123 (Sigma-Aldrich, average M
n 2 2
≈ 5800) ZrOCl ·8H O (Sigma-
Aldrich, reagent grade 98%), 5-hydroxymethylfurfural (Sigma-Aldrich,
®
9
9%), RuCl
Aesar, catalyst support). The tetragonal ZrO
Gobain NorPro (type SZ6*152 with an impurity of 3.3 % SiO
3
·xH
2
O (Sigma-Aldrich, ReagentPlus ), monoclinic ZrO
was supplied by Saint-
). ZrO was
2
(Alfa
2
2
2
crushed and milled with a mortar and subsequently sieved. Only particles
sized < 50 µm were used.
Preparation of Si-OH rich SBA-15
particles was not observed, for the used Ru/ZrO
as the H -TPD measurement of the used catalyst gave similar
results as for the fresh material (Table S9). Thus, the superiority
2
H-SBA catalyst,
The preparation of silanol rich SBA-15 was performed according to
[
49]
2
2
literature. Briefly, 20.0 g Pluronic P123 were dissolved in 650 mL H O
together with 37 wt.% HCl and heated under stirring at 38 °C. Following
addition of 41.6 g tetraethylorthosilicate (TEOS) the solution was kept at
of the relatively stable Ru/ZrO
2
H-aero in comparison to Ru/ZrO
2
H-
3
8 °C and stirred for 24 h. The resulting white suspension was
SBA can be related to the collapse of the structure of Ru/ZrO
SBA that takes place during the oxidation reaction.
2
H-
hydrothermally treated at 110 °C for another 24 h. Filtration of the
suspension yielded a white precipitate, which was dried at 80 °C for at
least 12 h. To remove the P123 template 8.0 g of the solid was dispersed
3 2 2
in 120 mL 65 wt.% HNO and 40 wt.% H O (careful, highly corrosive) in
This article is protected by copyright. All rights reserved.

ChemSusChem
10.1002/cssc.201800448
FULL PAPER
a 1 L round bottom flask and heated under stirring with a magnetic
was monochromatized with a secondary graphite monochromator. The
nitrogen adsorption-desorption measurements were performed on a
NOVA 3200e instrument at -195.8 °C. The transmission electron
microscopy (TEM) images were taken with a Hitachi H-7100 and HF-
2000 at an acceleration voltage of 100 or 200 kV. STEM images were
stirring bar at 80 °C. (The highly corrosive mixture releases brown NO
x
gases after a temperature of 80 °C was reached. The gases were
absorbed in a 4 M NaOH solution). After 3 h the product was collected by
filtration and thoroughly washed with water and ethanol and finally dried
overnight at 50 °C.
recorded on
a Hitachi HD-2700 dedicated Scanning Transmission
Electron Microscope operated at 200 kV equipped with an EDAX Octane
T Ultra W EDX detector. The ICP-OES measurements were conducted at
micro analytic laboratory Kolbe (Mülheim an der Ruhr, Germany).
2
Preparation of SiO aerogel
[
50]
The SiO
beaker 20.8 g TEOS was dissolved under stirring with a magnetic stirring
bar in 99 mL ethanol. Then 7.2 g H O was added and the pH value of the
2
aerogel was prepared according to literature.
In a glass
Temperature programmed desorption of hydrogen (TPD) was conducted
on
beginning 50 mL/min pure Ar stream was used to preheat approx. 50 mg
catalyst at 300 °C for 15 min. After cooling down to 50°C, 10% H in Ar
was passed through the sample and held for 10 min before heating to
50 °C with a heating rate of 1 °C/min. This temperature was held for 180
a Micromeritics AutoChem II (Chemisorption Analyzer). In the
2
solution was adjusted to pH 3 by addition of 15 wt.% HCl, while
monitoring the pH value with a pH electrode. The solution was stirred for
2
5
h 30 min. Afterwards the pH of the solution was adjusted to pH 7 by
-
1
2
addition of 1 mol·L NH
solution was aged by letting it stand for 14-16 h. After this time a gel had
formed. This gel was subjected to drying with supercritical CO with a
CO flow setup at 200 bar and 50 °C. The resulting product was a fine
3
solution and the stirring was stopped. The
min to reduce the sample. Afterwards the gas flow was switched back to
pure Ar while heating to 300 °C (3 °C/min). To remove all hydrogen this
temperature was held for 60 min. Then the sample was cooled down to
2
2
5
2
0 °C and the gas was switched back to 10% H in Ar and held at this
white powder.
temperature for 80 min so hydrogen could chemisorb on all metal
particles. To remove the physisorbed hydrogen the sample was flushed
with pure Ar for 60 min. The measurement itself was then performed by
gradually heating the sample with a ramping rate of 10 °C/min up to
Preparation of high surface area ZrO
2
The preparation of high surface area ZrO
2
was performed according to
5
00 °C that all chemisorbed hydrogen will be removed from the metal
surface. To quantify the amount of hydrogen desorbed during the heating
of the samples, defined volumes of H were injected in the TCD detector
[
49]
literature. At first 2.0 g ZrOCl ·8H O was dissolved by heating to 50 °C
2 2
-
1
in 1.5 g of 1.07 mol·L HCl. 0.5 g of silica template (SBA-15 or aerogel)
2
was impregnated with the calculated amount of the above solution (15.6
and plotted against the areas of the obtained peaks. The Ru particle size
was determined by using the relation l=5/S*d, where l is the particle size,
S is the metal surface area (that can be derived from the amount of
wt.% calculated for ZrO
for SiO aerogel). The impregnated material was sealed in a glass vial
and aged at 50 °C for 24 h and 90 °C for 48 h, followed by calcination at
2 2
for SBA-15 and 14.5 wt.% calculated for ZrO
2
3
[55,56]
desorbed hydrogen) and d is the density of ruthenium (12.3 g/cm ).
-
1
4
50 °C for 5 h (heating ramp 1 °C·min ). To remove the SiO
2
template
the calcined material was treated with 35 mL of 2 mol·L NaOH solution
at 70 °C two times and finally washed with H O and ethanol.
-
1
Catalytic tests
2
The catalytic tests were performed in stainless-steel autoclaves (50 mL)
with Teflon inlet and temperature and pressure sensing elements. The
autoclave was charged with 10 mL of an aqueous HMF solution (0.05
Preparation of soft templated ZrO
2
The preparation of soft templated ZrO
2
was performed according to the
-1
mol·L ) and 33 mg catalyst (molar ratio Ru:HMF = 31:1 for 5% Ru
[
54]
literature-known EISA method. In a glass beaker 2.44 g F127 and 1.54
g citric acid were dissolved in 74 mL ethanol while stirring with a
magnetic stirring bar. Then 3.67 g HCl (37%) was added and 5.85 g
zirconium(IV)butoxide was added dropwise into the homogeneous
solution and stirred for 3 h until the solution became turbid. The obtained
dispersion was poured into a petri dish to allow evaporation of the solvent
for 48 h at room temperature. Afterwards the sample was dried at 100 °C
and the obtained solid was calcined at 550 °C for 5 h (heating ramp
containing catalysts) and a magnetic stirring bar. Then the autoclave was
sealed, flushed three times with O
2
and then pressurized to 10 bar with
O
2
, heated to 120 °C with a metallic heating jacket and stirred at 500 rpm.
After the designated time the autoclave was cooled in an ice bath, the
pressure was released and the catalyst was removed by filtration. The
filtrated catalyst was washed with methanol to remove also solid reaction
products. Samples were taken from the aqueous reaction solution as well
as from the methanolic washing solution and analyzed by HPLC on a
Shimadzu LC20-Prominence chromatograph equipped with a 100 mm
organic acid resin column with 8.0 mm i.d. and a precolumn (40 mm
organic acid resin with 8.0 mm i.d.). As mobile phase an aqueous
-
1
1
°C·min ).
Preparation of impregnated ruthenium loaded catalyst
-
1
solution of trifluoracetic acid (2 mmol·L ) was used with a flow rate of 1
-
1
In a round bottom flask the calculated amount of RuCl
3
·xH
2
O was
mL·min at a temperature of 40 °C. For detection an UV detector was
used and external one point calibration was applied to quantify HMF,
DFF, FFCA and FDCA.
2 2
dissolved in ethanol (25 mL per 0.25 g ZrO ) and the desired ZrO
support was added and the suspension was stirred for 16 h. Then the
solvent was removed by evaporation on the rotary evaporator and the
material was dried at 95 °C for 48 h. Finally, the dry material was
The recyclability of the catalysts was tested in five consecutive runs.
These tests were carried out under the same reaction conditions as
stated above only with the reaction time fixed at 1 h. In the first
recyclability run the reaction was carried out five times in parallel and the
calculated conversions and yields were averaged. After the reaction the
catalyst was filtered, washed with methanol and dried under vacuum at
50 °C and the materials from the single reactions were merged together
before forwarding the material for the next run. In the subsequent runs
the number of parallel reactions had to be reduced successively to
compensate the loss of catalyst during recovery of the material. At the
reduced in a tube furnace with pure H
2
at 250 °C for 3 h (heating ramp
-
1
-1
1
°C·min and H flow 33 mL·min ). For the synthesis of samples with
2
bigger Ru particles, the impregnated materials were reduced at 450 °C
for 3 h.
Characterization
The powder X-ray diffraction (XRD) patterns were recorded on a Stoe Ɵ-
Ɵ diffractometer operating in reflection mode with Cu Kα1,2 radiation that
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ChemSusChem
10.1002/cssc.201800448
FULL PAPER
fifth recyclability run only one reaction could be performed. Nevertheless
this system assures that the conditions are the same for all of the
reactions and no effects of down scaling or similar issues can influence
the results.
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C. V Nguyen, Y.-T. Liao, T.-C. Kang, J. E. Chen, T. Yoshikawa, Y.
C. M. Pichler gratefully acknowledges financial support from the
Marietta Blau-scholarship of the OeAD funded by the Austrian
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STEM-EDX measurements, Laila Sarahoui for the skillful
assistance in the performance of reaction tests and Prof. Rolf
Breinbauer for helpful remarks and comments on the manuscript.
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