Solvent-Free Synthesis of Zeolite Crystals Encapsulating Gold-Palladium Nanoparticles for the Selective Oxidation of Bioethanol

Article abstract of DOI:10.1002/cssc.201500261

The conversion of bioethanol into valuable products is an important area in the conversion of biomass. We demonstrate the successful synthesis of bimetallic gold-palladium (Au-Pd) nanoparticles encapsulated within S-1 zeolite crystals (AuPd@S-1) by a solvent-free strategy. This strategy allows highly efficient use of the noble metals, with more than 96% of the gold and palladium being loaded into the final samples. Electron microscopy characterization and investigations with probe molecules confirm that the Au-Pd nanoparticles are encapsulated inside the S-1 crystals. The AuPd@S-1 catalyst is very active for the aerobic oxidation of bioethanol, giving 100% conversion and 99% selectivity to acetic acid. Even in the presence of 90% water, the catalyst still gives a conversion higher than 80% and a selectivity of 95%. More importantly, the AuPd@S-1 catalyst exhibits excellent stability in the oxidation of bioethanol. These features are important for future practical applications of the AuPd@S-1 catalyst. Inner strength: The conversion of bioethanol into valuable products is an important topic in biomass conversion. Bimetallic gold-palladium nanoparticles encapsulated in zeolite crystals synthesized by a solvent-free strategy exhibit superior catalytic activity, unique selectivity, and high stability in aerobic oxidation of bioethanol. These features are important for future practical applications of the AuPd@S-1 catalyst.

Full text of DOI:10.1002/cssc.201500261

DOI: 10.1002/cssc.201500261
Communications
Solvent-Free Synthesis of Zeolite Crystals Encapsulating
Gold–Palladium Nanoparticles for the Selective Oxidation
of Bioethanol
Jian Zhang, Liang Wang,* Longfeng Zhu, Qinming Wu, Chunyu Chen, Xiong Wang,
[
a]
Yanyan Ji, Xiangju Meng, and Feng-Shou Xiao*
The conversion of bioethanol into valuable products is an im-
portant area in the conversion of biomass. We demonstrate
the successful synthesis of bimetallic gold–palladium (Au–Pd)
nanoparticles encapsulated within S-1 zeolite crystals
Palladium-, copper-, and gold-based catalysts are active in
[
3–15]
the oxidation of bioethanol.
In this oxidation reaction, cata-
lyst stability is the biggest challenge, since the metal nanopar-
[
4,5]
ticles easily aggregate into bigger nanoparticles.
To over-
(
AuPd@S-1) by a solvent-free strategy. This strategy allows
come this problem, metal catalysts encapsulated in zeolites
[
18–23]
highly efficient use of the noble metals, with more than 96%
of the gold and palladium being loaded into the final samples.
Electron microscopy characterization and investigations with
probe molecules confirm that the Au–Pd nanoparticles are en-
capsulated inside the S-1 crystals. The AuPd@S-1 catalyst is
very active for the aerobic oxidation of bioethanol, giving
(M@zeolite) have been designed and prepared.
Particular-
ly, S-1 zeolite encapsulating gold nanoparticles was found to
be active, stable, and selective for the production of acetalde-
[
5]
hyde. Normally, zeolites encapsulating metal nanoparticles
are synthesized under hydrothermal conditions; a process in
which utilization of the metal is relatively low (about 34%) and
in which polluted water is produced. Recently, a solvent-free
1
00% conversion and 99% selectivity to acetic acid. Even in
[
24–27]
the presence of 90% water, the catalyst still gives a conversion
higher than 80% and a selectivity of 95%. More importantly,
the AuPd@S-1 catalyst exhibits excellent stability in the oxida-
tion of bioethanol. These features are important for future
practical applications of the AuPd@S-1 catalyst.
strategy for synthesizing various zeolites was reported.
Because the solvent (water) is absent in this strategy the pro-
duction of polluted water is avoided, and hence this strategy is
[
28]
regarded as a green process.
Herein, we demonstrate the solvent-free synthesis of an S-
zeolite encapsulating gold–palladium bimetallic nanoparti-
1
cles (AuPd@S-1), combining the advantages of both M@zeolite
catalysts (stability) and a greener synthesis strategy (solvent-
free). More than 96% of the gold and palladium in the starting
gel are transferred into the AuPd@S-1 product; the catalyst
also exhibits superior catalytic activity, unique selectivity, and
high stability towards the aerobic oxidation of bioethanol in
mixtures with water.
Because biomass is regarded as a renewable source of energy
to reduce the need for fossil sources of oil, the conversion of
biomass-derived feedstocks into valuable products is of great
[
1,2]
importance.
One typical example is bioethanol, which has
already been used as a fuel additive and has great potential
[3–17]
for producing valuable products such as acetic acid.
Con-
ventionally, acetic acid is synthesized by carbonylation of
methanol. This process is not considered green because it in-
volves the use of CO and iodocompounds as catalytic media-
The AuPd@S-1 samples were synthesized by grinding the
solid raw materials [bimetallic gold–palladium nanoparticles
encapsulated with amorphous SiO (AuPd@SiO ) and tetrapro-
2
2
[
8]
tors. Enzymatic oxidation of bioethanol could produce acetic
acid, but enzymatic catalysts are not popular in chemical in-
dustry. Considering the very high output of bioethanol (more
than 100 billion liters every year), there is a strong case for the
development of efficient catalysts for the direct oxidation of
pylammonium hydroxide (TPAOH)], followed by thermal treat-
ment at 1808C for 2–4 days (Scheme 1). The gold and palladi-
um contents could be adjusted, and herein we designate our
samples as Au Pd @S-1, where x and y stand for the molar
x
y
ratio of gold and palladium, respectively.
[1,3–17]
the bioethanol to acetic acid.
Figure 1 shows X-ray diffraction (XRD) patterns of the
AuPd@S-1 samples. All of the samples exhibit XRD peaks typi-
[
29,30]
cal for the MFI zeolite structure.
It is difficult to observe
[
a] J. Zhang, Dr. L. Wang, Dr. L. Zhu, Q. Wu, C. Chen, X. Wang, Y. Ji,
Dr. X. Meng, Prof. F.-S. Xiao
Key Laboratory of Applied Chemistry of Zhejiang Province
Zhejiang University
the peaks related to the Au–Pd nanoparticles, indicating that
these Au–Pd nanoparticles are well-dispersed within the S-
1
crystals. Table 1 lists the textural parameters and metal load-
Hangzhou 310028 (PR China)
E-mail: liangwang@zju.edu.cn
ings of the AuPd@S-1 samples. These samples have high sur-
2
À1
face areas (349–413 m g ) and pore volumes (0.16–
fsxiao@zju.edu.cn
3
À1
0
.18 cm g ) (entries 1–6). The total metal loadings in the sam-
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201500261.
ples are about 1.5%, and their gold and palladium contents
could be effectively adjusted. Through our use of the solvent-
free strategy, the utilization of gold and palladium in the sam-
ples is above 96%. These results confirm that most of the gold
This publication is part of a Special Issue on the “Future Energy” confer-
ence in Sydney, Australia. To view the complete issue, visit:
http://onlinelibrary.wiley.com/doi/10.1002/cssc.v8.17/issuetoc.
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Scheme 1. Solvent-free synthesis of zeolite crystals encapsulating bimetallic
Au–Pd nanoparticles. TPAOH=tetrapropyl ammonium hydroxide.
Figure 2. Tomogram section TEM images of (A,B) Au0.4Pd0.6@S-1, and
(C,D) Au0.4Pd0.6/S-1. Inserts in (A) and (C): metal particle size distribution.
Insert in (B): HRTEM image of Au0.4Pd0.6 {111} plane.
Figure 2 shows transmission electron microscopy (TEM) to-
mography images of the Au Pd @S-1 and Au Pd /S-1 sam-
0.4
0.6
0.4
0.6
ples. This technique completely avoids the influence of metal
nanoparticles on the external surface. In Au Pd @S-1 (Fig-
0.4
0.6
ure 2A,B), Au–Pd nanoparticles can be directly observed, con-
firming that these particles are indeed encapsulated within the
S-1 crystals. STEM images show that the size distribution of the
Au–Pd nanoparticles mainly falls in the range 2.5–6.5 nm (Fig-
ure 2A, insert and Supporting Information, Figure S1). In
Au Pd /S-1, the Au–Pd nanoparticles are found only on the
Figure 1. XRD patterns of (a) Au@S-1, (b) Au0.6Pd0.4@S-1, (c) Au0.5Pd0.5@S-1,
(
d) Au0.4Pd0.6@S-1, (e) Au0.2Pd0.8@S-1, and (f) Pd@S-1.
and palladium species in the raw AuPd@SiO materials are ef-
2
0.4
0.6
fectively transferred into the AuPd@S-1 product. In contrast,
the utilization of gold and palladium reaches only 34–36% in
the synthesis of Au Pd @S-1 by a conventional hydrothermal
side of the S-1 crystal (Figure 2C,D), indicating that the metal
nanoparticles are only located on the external surface of the
crystal (Supporting Information, Scheme S1). Similarly, the
STEM images show that the Au–Pd nanoparticles in the
Au Pd @S-1, Au Pd @S-1, Au Pd @S-1, Au Pd @S-1,
0.4
0.6
route. This might be due to dissolution of amorphous SiO in
2
the AuPd@SiO2 under hydrothermal conditions, resulting in
0.4
0.6
0.5
0.5
0.6
0.4
0.2
0.8
leaching of Au–Pd species into the liquor. By contrast, SiO dis-
and Pd@S-1 samples have a similar metal size distribution (2.5–
6.5 nm; Supporting Information, Figure S2–S5). In comparison,
monometallic Au nanoparticles on Au@S-1 had a larger size
distribution, in the range 3–9 nm
2
solution is negligible under solvent-free conditions, thus giving
high metal utilization in the AuPd@S-1 samples.
(
Supporting Information, Fig-
Table 1. The textual parameters and metal loadings of AuPd@S-1.
ure S6), which agrees well with
the fact that addition of palladi-
um to gold nanoparticles signifi-
cantly reduces the particle
Entry Sample
S
BET
2
V
P
Metal loading
Metal loading
in M@S-1 [wt%]
Metal
utilization [%]
Pd
À1
3
À1
[a]
[a]
[b]
[
m g
]
[cm g
]
in M@SiO
Au
2
[wt%]
Pd
Au
Au
Au
1
2
3
4
5
6
7
8
Au@S-1
360
349
413
349
362
378
376
0.17
0.16
0.17
0.16
0.17
0.18
0.18
0.18
1.50
1.10
0.98
0.81
0.47
–
–
1.50
1.10
0.96
0.79
0.46
–
–
>99
>99
98
97
98
–
–
97
[31]
size.
Au0.6Pd0.4@S-1
Au0.5Pd0.5@S-1
Au0.4Pd0.6@S-1
Au0.2Pd0.8@S-1
Pd@S-1
0.42
0.51
0.69
1.01
1.50
–
0.41
0.50
0.66
1.01
1.50
0.70
0.71
Energy dispersive X-ray (EDX)
analysis of the Au Pd @S-1 indi-
98
96
>99
>99
–
0.4
0.6
cates the presence of both gold
and palladium species, and the
ratios of gold to palladium are
almost unchanged for the ran-
domly selected regions (Sup-
Au0.4Pd0.6/S-1
–
0.80
0.81
–
34
Au0.4Pd0.6@S-1-HT 376
2.40
2.01
36
[
2
a] By ICP analysis. [b] Calculated from metal content in AuPd@S-1/metal content in raw AuPd@SiO .
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porting Information, Figure S7). Furthermore, the HRTEM
image shows that the Au–Pd nanoparticle has a lattice spacing
of 0.230 nm for the {111} plane, which falls between the values
of 0.239 nm for {111} Au and 0.223 nm for {111} Pd. These re-
sults indicate the formation of a bimetallic gold–palladium
[32–34]
alloy, rather than separate gold and palladium phases.
Noble-metal nanoparticles usually have a relatively low sta-
[18,19]
bility at high temperature (>5008C).
However,
Au Pd @S-1 synthesized by the solvent-free strategy shows
0.4
0.6
extraordinary thermal stability. For example, after calcination at
508C for 2 h in air, it is very difficult to observe any change in
the size of the nanoparticles in the Au Pd @S-1 samples
6
0
.4
0.6
(
Supporting Information, Figure S8). In comparison, Au–Pd
nanoparticles on the external surface of S-1 (Au Pd /S-1)
0
.4
0.6
became larger after the same treatment (4–20 nm; Supporting
Information, Figure S9A). This phenomenon can be attributed
to confinement of Au–Pd nanoparticles within the S-1 frame-
work, preventing the aggregation of Au Pd @S-1 at high
0
.4
0.6
temperature.
To further understand the structure of Au Pd @S-1, we em-
0
.4
0.6
ployed aerobic oxidation of probe molecules of different sizes:
benzyl alcohol (BA) and 3,5-dimethyl benzyl alcohol (DMBA).
Figure 3A shows catalytic data for the aerobic oxidation of
mixed alcohols of BA and DMBA (BA/DMBA at 5) over
Au Pd @S-1 and Au Pd /S-1. Notably, both Au Pd @S-
0.4
0.6
0.4
0.6
0.4
0.6
1
and Au Pd /S-1 are active for the BA oxidation, giving
0.4 0.6
a conversion higher than 99% at the initial stage of the reac-
tion. However, for the DMBA oxidation, the Au Pd /S-1 is still
0
.4
0.6
very active, while the Au Pd @S-1 exhibits very low conver-
0
.4
0.6
sion (ca. 10%). This is because the molecular size of DMBA is
larger than the size of the micropores of the S-1 crystals,
making it difficult for DMBA molecules to diffuse towards Au–
Pd nanoparticles encapsulated within the S-1 crystals (Support-
ing Information, Figure S13). By contrast, DMBA oxidation is
not limited in the case of Au–Pd nanoparticles on the external
Figure 3. (A) Aerobic oxidation of benzyl alcohol (BA) and 3,5-dimethyl
[19,21]
benzyl alcohol (DMBA) over Au0.4Pd0.6@S-1 and Au0.4Pd0.6/S-1 at mixed alco-
surface of Au Pd /S-1.
Furthermore, after poisoning by
0.4
0.6
À1
hol inlet rate at 0.12 mLh (BA/DMBA at 5), 0.2 g of catalyst, O
5
2
flow rate at
2
,4-quinolinediol Au Pd /S-1 nearly loses its activity, while
0.4
0.6
À1
mLmin , 2208C. (a) BA and (b) DMBA conversion over Au0.4Pd0.6@S-1,
Au Pd @S-1 still exhibits full BA conversion (Figure 3B).
0.4
0.6
(c) BA and (d) DMBA conversion over Au0.4Pd0.6/S-1. (B) Aerobic oxidation of
BA in the presence of 2,4-quinolinediol over (a) Au0.4Pd0.6@S-1 and
These results support the notion that the Au–Pd nanoparticles
are encapsulated within the S-1 crystals.
(
b) Au0.4Pd0.6/S-1 catalysts. The mixture (BA/2,4-quinolinediol at 98) inlet rate
À1
À1
2
at 0.12 mLh , 0.2 g of catalyst, O flow rate at 5 mLmin , 2208C.
To investigate the stability of the Au Pd @S-1 sample, we
0
.4
0.6
investigated the dependence of the activity on reaction time
in the BA oxidation, as shown in Figure 4 and Figure S10 (Sup-
porting Information). Au Pd @S-1 and Au Pd /S-1 catalysts
the S-1 crystals (Figure S1B). In contrast, the BA conversion
and benzaldehyde selectivity over Au Pd /S-1 decreased with
0
.4
0.6
0.4
0.6
are very active, giving full BA conversion (>99.5%) to benzal-
dehyde (sole product) at the start of the reaction. After 11 h,
the Au Pd @S-1 still exhibits a high BA conversion at 96.0%,
0.4
0.6
increasing reaction time. After 6 h, Au Pd /S-1 gave a BA
0.4
0.6
conversion and a benzaldehyde selectivity of 71.5 and 82.3%,
respectively. Calcination in oxygen did not recover the catalytic
performance of Au Pd /S-1. Because metal leaching was not
0.4
0.6
with a benzaldehyde selectivity higher than 99.5%. Even after
reaction 36 h, Au Pd @S-1 still shows a BA conversion of
0.4
0.6
0.4
0.6
7
6.6% with a benzaldehyde selectivity of 95.0%. After calcina-
detected during the reaction (Supporting Information,
Table S1), the deactivation of Au Pd /S-1 should be related
tion at 5008C for 2 h in oxygen, the Au Pd @S-1 catalyst
0
.4
0.6
0.4
0.6
completely recovers its catalytic performance through removal
of cokes, showing BA conversion and benzaldehyde selectivity
values higher than 99.5%. TEM images of Au Pd @S-1 sam-
to aggregation of the Au–Pd nanoparticles during the reaction,
as evidenced by a sample TEM image (Figure S9B).
Figure 5A shows the dependence of catalytic activity on re-
action temperature in the oxidation of bioethanol over
Au Pd @S-1. When the temperature is lower than 1708C, the
0
.4
0.6
ples after 38 h of reaction show size distributions for the Au–
Pd nanoparticle similar to those of as-synthesized samples, in-
dicating the high stability of the nanoparticles encapsulated in
0.4
0.6
catalyst is nearly inactive. When the reaction temperature
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Figure 4. Dependence of BA conversion and benzaldehyde selectivity on re-
action time in aerobic oxidation of BA over Au0.4Pd0.6@S-1 and Au0.4Pd0.6/S-
À1
1
5
catalysts. BA inlet rate of 0.12 mLh , 0.2 g of catalyst, O
2
flow rate of
À1
mLmin , 2208C. (a) BA conversion and (b) benzaldehyde selectivity over
Au0.4Pd0.6@S-1, (c) BA conversion and (d) benzaldehyde selectivity over
Au0.4Pd0.6/S-1.
reaches 2008C, the ethanol conversion is 92.8%, which is
much higher than the value reported for a gold nanoparticle
[5]
catalyst (ca. 50%) at the same reaction temperature. This can
be attributed to the higher activity of bimetallic Au–Pd nano-
particles compared to monometallic Au nanoparticles in alco-
[
31–34]
hol oxidations.
Interestingly, Au Pd @S-1 gave a very
0.4 0.6
high selectivity for the formation of acetic acid. At 2008C, the
selectivity for acetic acid was 96.7%; when the temperature
was higher than 2108C, the selectivity was higher than 99.0%,
which is even higher than that over Au/MgAl O , one of the
Figure 5. (A) Dependences of ethanol conversion (black line) and selectivities
for acetic acid (blue column), ethyl acetate (red column), and acetaldehyde
(green column). (B) Dependences of (a) catalytic conversion and selectivities
for (b) acetic acid, (c) ethyl acetate, and (d) acetaldehyde on the time in the
2
4
À1
bioethanol oxidation over Au0.4Pd0.6@S-1 at ethanol inlet rate at 0.12 mLh
,
most efficient catalyst for the oxidation of ethanol to acetic
À1
80 mg of Au0.4Pd0.6@S-1, O
2
rate of 8 mLmin , 2158C.
[8]
acid (Supporting Information, Table S2). This process offers
a route for the selective oxidation of bioethanol to acetic acid
with ideal atomic economy. Figure 5B shows the dependence
of conversion and selectivity on reaction time in the oxidation
of bioethanol over Au Pd @S-1. The catalyst retains its activi-
the Au–Pd nanoparticles in the zeolite’s micropores. In com-
parison, the Au Pd /S-1 catalyst is strongly influenced by the
0.4
0.6
presence of water in the bioethanol. When 20% water was
added to the feedstock, the Au Pd /S-1 catalyst showed an
0
.4
0.6
ty and selectivity even after 50 h, indicating its extraordinarily
high stability (Supporting Information, Figure S11).
0.4
0.6
ethanol conversion of 17% and an acetic acid selectivity of
32%; when 90% water was added, ethanol conversion was as
low as 2%. The excellent water tolerance of Au Pd @S-1 is
Crude bioethanol produced by fermentation is well-known
to contain water, and separating ethanol from water is an
0.4
0.6
[35]
energy-consuming process. Therefore, it is preferred that the
ethanol oxidation reaction is performed using crude bioetha-
nol (i.e, a mixture of ethanol and water) as feedstock. Figure 6
shows the dependence of product yields on reaction time in
the oxidation of bioethanol (a mixture of water and ethanol as
model substrate) containing various water contents over
Au Pd @S-1 and Au Pd /S-1. In the case of Au Pd @S-1,
important for catalytic applications involving crude bioethanols
in the future.
Figure 6 shows how after reaction for 30 h in the presence
of water, when pure ethanol is introduced as feedstock,
Au Pd @S-1 still exhibits a very high conversion (97.5%) and
0.4
0.6
selectivity (95.8%) to acetic acid, while Au Pd /S-1 shows
0.4
0.6
a much lower conversion (40.0%) and selectivity (68%). The
poor recovery of catalytic performance over Au Pd /S-1 cata-
0.4
0.6
0.4
0.6
0.4
0.6
there is little influence for the catalytic conversion of the bioe-
thanol. When 20% water is present in the feedstock,
Au Pd @S-1 gives an ethanol conversion of 85% and a selec-
0.4
0.6
lyst is assigned to the aggregation of the Au–Pd nanoparticles
in the presence of water.
0.4
0.6
tivity to acetic acid of 94%. When the water content reaches
0%, Au Pd @S-1 still exhibits an ethanol conversion of 82%
In conclusion, a solvent-free strategy is successfully used to
synthesize S-1 zeolite crystals encapsulating bimetallic gold–
palladium nanoparticles (Au Pd @S-1). Compared to the con-
9
0.4
0.6
and an acetic acid selectivity of 94%. This is due to the hydro-
phobicity of the S-1 zeolite, which enables to efficiently sepa-
0.4
0.6
ventional hydrothermal route, the solvent-free strategy allows
efficient use of the noble metals and significantly reduces the
[36,37]
rate the ethanol and water,
hindering access of water to
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(
À1
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[
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Received: February 17, 2015
Published online on June 3, 2015
ChemSusChem 2015, 8, 2867 – 2871
www.chemsuschem.org
2871
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim