High-performance gold-promoted palladium catalyst towards the hydrogenation of phenol with mesoporous hollow spheres as support

Article abstract of DOI:10.1016/j.catcom.2011.10.006

A high-performance, gold-promoted Pd catalyst with mesoporous hollow silica spheres as support, PdAu/MHSS, was prepared using an impregnation-reduction approach. The catalyst showed 10 times higher activity than commercial Pd/C catalyst and 6 times higher activity than Pd/MHSS catalyst. The conversion of phenol was 100% within 30 min of reaction at 80 °C. The catalyst was characterized by XRD, XPS, and TEM, which revealed that its high performance may result from both the high dispersion of active components on the MHSS, caused by the addition of gold, and the interaction between palladium and gold.

Full text of DOI:10.1016/j.catcom.2011.10.006

Catalysis Communications 17 (2012) 29–33
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
High-performance gold-promoted palladium catalyst towards the hydrogenation of
phenol with mesoporous hollow spheres as support
⁎
⁎
Xu Yang, Li Du, Shijun Liao , Yuexia Li, Huiyu Song
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
Key Lab for Fuel Cell Technology of Guangdong Province & Key Lab of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 July 2011
Received in revised form 29 September 2011
Accepted 3 October 2011
Available online 13 October 2011
A high-performance, gold-promoted Pd catalyst with mesoporous hollow silica spheres as support, PdAu/
MHSS, was prepared using an impregnation–reduction approach. The catalyst showed 10 times higher activ-
ity than commercial Pd/C catalyst and 6 times higher activity than Pd/MHSS catalyst. The conversion of phe-
nol was 100% within 30 min of reaction at 80 °C. The catalyst was characterized by XRD, XPS, and TEM, which
revealed that its high performance may result from both the high dispersion of active components on the
MHSS, caused by the addition of gold, and the interaction between palladium and gold.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Gold
Hollow silica sphere
Mesoporous
Palladium
Phenol hydrogenation
1. Introduction
Pd atomic ratio of 1:10, the activity is 10 and 6 times higher than that
of Pd/C and Pd/MHSS catalysts, respectively, at a reaction tempera-
Catalytic hydrogenation of phenol is of commercial and environ-
mental significance because cyclohexanone is an important interme-
diate for the manufacture of dyestuffs, pharmaceutical products, etc.,
and is especially significant as the key raw material to produce both
caprolactam for Nylon 6 and adipic acid for Nylon 66 [1]. Therefore,
selective hydrogenation of phenol for cyclohexanone is an important
chemical process that has been studied extensively to date [2–13]. A
broad range of catalysts, such as raney Ni [7], Pd-based film/chamber
[8,9], amorphous alloy [10], bimetal catalysts [11] and supported cat-
alysts [12,13], have been developed for this purpose. According to the
literatures, issues arising from relatively low catalytic efficiency and
severe conditions (>150 °C in the vapor phase [4,5,11,14] or
>10 MPa in sc-CO2 medium [15,16]) may be the main obstacles to
achieving practical application. Notably, an excellent study reported
by Han [17] disclosed complete phenol conversion with >99.9% se-
lectivity to cyclohexanone under mild conditions based on a dual-
supported Pd–Lewis acid catalyst. This may be the most exciting
result for phenol hydrogenation up to now.
ture of 50 °C and 1 MPa of hydrogen pressure.
2. Experimental
2.1. Preparation of catalyst
Mesoporous hollow silica spheres (MHSS) were synthesized by a
one-pot route previously reported by our group [18].
The PdAu/MHSS catalyst was prepared by a co-impregnation
method, in which lab-synthesized MHSS were impregnated with an
ethanol solution containing PdCl2 and HAuCl4 (the molar ratio of Pd/
Au was 10/1), then were dried and reduced with hydrogen at 200 °C.
The palladium and gold loadings of the catalyst were 2 wt.% and
0.37 wt.%, respectively. For comparison, a commercial Pd/C catalyst
with Pd content of 5 wt.% was purchased from ZhongKe Kaidi Catalyst
(Lanzhou, China), and Pd/MHSS and Au/MHSS catalysts were
prepared using the same procedures as for the PdAu/MHSS catalyst.
The general properties of the catalysts are given in Table 1.
Inspired by this previously reported work, we attempted to pre-
pare gold promoted Pd catalyst with mesoporous hollow silica
spheres as support (PdAu/MHSS) and obtained exciting results. The
catalyst shows excellent activity and selectivity towards the hydroge-
nation of phenol under mild conditions. For PdAu/MHSS with an Au:
2.2. Evaluation of catalysts
Hydrogenation of phenol was used to evaluate the catalyst; the re-
action was carried out in a stainless-steel autoclave equipped with a
magnified stirrer and thermostat. A reaction mixture containing
1.0 g of phenol, 0.5–1.25 g of catalyst (typically containing 0.025 g
of Pd), and 10 ml of CH2Cl2 as solvent was introduced into the auto-
clave. Before the reaction, the system was purged with pure hydrogen
⁎
Corresponding authors. Tel./fax: +86 20 87113586.
E-mail addresses: chsjliao@scut.edu.cn (S. Liao), hysong@scut.edu.cn (H. Song).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.10.006

30
X. Yang et al. / Catalysis Communications 17 (2012) 29–33
Table 1
a
6000
Specifications of several catalysts with various compositions.
Silica (200)
Pd/MHSS
Pd (220)
Catalyst
Metal loading
(wt.%)
Metal particle size
(nm)/a
Surface area
Pore size
(nm)
5000
4000
3000
2000
1000
0
(m2 g−1
)
Pd/C
5.0 (4.9)b
2.0 (1.9)
2.0 (1.8)
–
252
872
–
Pd (111)
Pd (200)
Au/MHSS
Pd/MHSS
PdAu/MHSS
–
3.2
3.8
4.0
10–20
2–3
854 (41)c
766 (166)
Pd, 2.0 (1.8)
Au, 0.32 (0.30)
10
20
30
40
50
60
70
80
a
2 theta (o)
Observed from TEM images.
b
The recipe metal content and the content measured by atomic absorption spectroscopy
(in brackets).
c
The BET surface area of the catalyst and the surface area of the metal components
(calculated from TEM images, in brackets).
1
2
3
4
5
6
7
8
9
20 30 40 50 60 70 80
2 theta (o)
(99.99%) for 5 min to remove air. The evaluation was conducted at
50 °C and 1.0 MPa of hydrogen pressure for all catalysts, unless other-
wise mentioned.
b
6000
5000
4000
3000
2000
1000
0
2.14o
Silica (200)
PdAu/MHSS
The products were analyzed using a gas chromatograph (Agilent
6840N, USA) equipped with a flame ionization detector. A GC–MS
(Shimadzu GCMS-QP5050A, Japan) equipped with a 0.25 mm×30 m
DB-WAX capillary column was used to identify cyclohexanone and
cyclohexanol. The column oven temperature was programmed from
PdAu (111)
PdAu (200)
PdAu (220)
120 °C (held for 1 min) to 280 °C at a rate of 20 °C min−1
.
10
20
30
40
50
60
70
80
2.3. Catalyst characterizations
2 theta (o)
X-ray diffraction (XRD) patterns were obtained with a D/MAX-IIIA
X-ray diffractometer (Rigaku, Japan) using Cu Kα radiation. Atomic
absorption analysis was performed with a SpectrAA-220Z (Varian,
USA). Transmission electron microscopy (TEM) was carried out on a
JEOL JEM2010 microscope (JEOL, Japan) using an accelerating voltage
of 200 kV. N2 adsorption–desorption isotherms were measured with
a Tristar 3010 isothermal nitrogen sorption analyzer (Micromeritics,
USA) using a continuous adsorption procedure. X-ray photoelectron
spectroscopy (XPS) with an AXIS Ultra DLD (Kratos, Britain) was
used to examine the electronic properties of the catalysts. The binding
energy was calibrated using a C 1s binding energy of 284.8 eV. Hydrogen
temperature-programmed reduction (H2-TPR) was performed on a
FineSorb 3010 adsorption system (Finetec, China).
1
2
3
4
5
6
7
8
9
20 30 40 50 60 70 80
2 theta (o)
Fig. 1. XRD patterns of (a) Pd/MHSS and (b) PdAu/MHSS catalysts. Inset shows the
corresponding magnified, wide-angle XRD.
The average particle sizes of Pd nanoparticles in Pd/MHSS and
PdAu nanoparticles in PdAu/MHSS were 12 nm and 3.5 nm, respec-
tively, calculated from the Scherrer equation. It appears that the addi-
tion of gold enhanced the dispersion of the active components in the
catalyst.
3. Results and discussion
The high-resolution TEM images in Fig. 2 reveal the dispersion of
the loading metal on the silica support. For Pd/MHSS (Fig. 2a and b),
relatively dark coloration was found at the silica shell; energy-
dispersive X-ray spectroscopy confirmed this cloud-like aggregation
to be Pd, which was in agreement with the sharp reflection peaks
observed in the XRD pattern. Notably, the PdAu/MHSS catalyst
exhibited high dispersion of metal nanoparticles that had a narrow
size range of 2.0–4.0 nm (Fig. 2c and d), further confirming that the
addition of gold promoted the dispersion of palladium. The data for
several catalysts, shown in Table 1, also clearly reveal the superiority
of the MHSS support and the promotion of gold for the dispersion of
active components. For the PdAu/MHSS catalyst, the high dispersion
of its active components as observed by TEM is quite consistent with
the results from XRD and the Scherrer equation. But for the Pd/
MHSS, the situation is different: the particle size obtained by XRD is
smaller than that observed by TEM, indicating aggregation of the
primary particles of palladium in Pd/MHSS, because the particle
size given by XRD and the Scherrer equation is the average particle
size of primary particles of palladium.
For both the Pd/MHSS and the PdAu/MHSS, a well-defined diffrac-
tion peak occurred at 2–3° (see XRD patterns in Fig. 1a), demonstrating
the well-preserved, wormhole-like mesoporous structure of the MHSS
support after metal loading. As shown in the magnified, wide-angle
XRD pattern inset in Fig. 1a, the supported palladium catalysts, including
Pd/C (not shown, for simplicity) and Pd/MHSS exhibit three obvious,
sharp diffraction peaks corresponding to the (111), (200), and (220)
planes of the face-centered cubic structure of palladium, confirming
metallic palladium loading on the support after hydrogen reduction.
The PdAu/MHSS catalyst exhibited comparable broad peaks, as featured
in the (111) plane (see Fig. 1b). Generally, the broadened peaks may be
caused by high dispersion and small particle size, or the amorphous
structure of the sample. For our PdAu/MHSS catalyst, however,
we attributed the broadened peaks to the high dispersion and
small particle size of PdAu on the PdAu/MHSS catalyst, caused by
the addition of gold; this is further proved by the TEM images. It
should be pointed out that the diffraction peaks of gold cannot
be observed in the XRD pattern, while the peak (111) of palladium
is shifted slightly to the left. We suggest that traces of gold may
have entered the palladium lattice, or were outside the detection
limits of XRD.
Actually, some researchers have previously recognized and
reported the difficulty of achieving high dispersion of pure palladium,
and the fact that dispersion can be promoted by additives [19–21].

X. Yang et al. / Catalysis Communications 17 (2012) 29–33
31
a
b
c
d
Fig. 2. TEM images of (a, b) Pd/MHSS and (c, d) PdAu/MHSS.
a
b
3d5/2 335.8 eV
Pd
Au
4f7/2 83.9 eV
4f5/2 87.6 eV
3d3/2 341.0 eV
350 348 346 344 342 340 338 336 334 332 330
Binding energy (eV)
94
92
90
88
86
84
82
80
Binding energy (eV)
c
d
3d5/2 335.1 eV
4f7/2 84.0 eV
4f5/2 87.7 eV
Pd
Au
3d3/2 340.3 eV
350 348 346 344 342 340 338 336 334 332 330
Binding energy (ev)
94
92
90
88
86
84
82
80
Binding energy (ev)
Fig. 3. XPS spectra of (a) Pd and (b) Au in PdAu/MHSS, (c) Pd in Pd/MHSS, and (d) Au in Au/MHSS.

32
X. Yang et al. / Catalysis Communications 17 (2012) 29–33
Table 2
Fig. 3 shows the XPS spectra of Pd and Au in the PdAu/MHSS cat-
Hydrogenation of phenol catalyzed with different catalysts.
alyst; the spectra were calibrated using the C 1s peak energy of
284.8 eV. As shown in Fig. 3a, the binding energies of palladium in
the catalyst were 341.0 eV (Pd 3d3/2) and 335.8 eV (Pd 3d5/2),
which can be assigned to the Pd0 species. Compared with 340.3 eV
Entry
Catalyst
Time
(h)
Conversion
(%)
Selectivity (%)
C_O
C\OH
1
2
3
Au/MHSS
Pd/C
Pd/MHSS
PdAu/MHSS
PdAu/MHSS3rd
PdAu/MHSS5th
20
Trace
9.1
15.0
97.5
95.0
93.7
–
–
(Pd 3d3/2
) and 335.1 eV (Pd 3d5/2) for the Pd/MHSS catalyst
0.75
0.75
0.75
0.75
0.75
95.2
94.6
96.6
96.5
97.0
4.8
5.4
3.4
3.5
3.0
(Fig. 3c), the shift toward a higher binding energy could be an indica-
tion of charge transfer from Pd to Au in the PdAu/MHSS catalyst,
which would suggest Pd–Au alloy formation [22,23]. No obvious
binding energy shift can be observed for Au (see Fig. 3b and d). We
also investigated the interaction between palladium and gold by
temperature-programmed reduction (TPR); after the addition of
gold, the reduction peak is shifted toward high temperature with an
increasing range of 30 °C, which supports the suggestion of a strong
interaction or the formation of an alloy between the palladium and
the gold [24,25].
4
5a
6a
Reaction conditions: mass ratio Mphenol:MPd =1:0.025; 10 ml CH2Cl2; 50 °C, 1 MPa of H2
pressure; stirring speed 1000 rpm.
a
Recycle usability test, 3rd/5th denotes number of times catalyst tested.
PdAu/MHSS
100
It is noteworthy that the addition of gold greatly enhanced the
performance of the catalyst towards the hydrogenation of phenol, as
shown in Table 2. The Au/MHSS catalyst showed almost no activity
toward the reaction, while under the same reaction conditions, Pd/C
and Pd/MHSS showed conversions of 9.1% and 15.0%, respectively.
However, PdAu/MHSS exhibited an excellent conversion of 97.5%
with a high selectivity of 96.6% for cyclohexanone, clearly indicating
that the addition of gold significantly improved the performance of
the catalyst. The exceptional promotion may be attributed to high dis-
persion of the active components and interactions between the gold
and palladium. Furthermore, the catalyst showed good stability,
with almost no degradation in activity or observable selectivity after
5 tests. We were not able to prepare a Pd/MHSS catalyst with the
same smaller particle size as PdAu/MHSS using the same metal load-
ing, in the absence of gold.
Pd/MHSS
80
60
40
20
0
30
40
50
60
70
80
Temperature (oC)
Fig. 4. Relationship of conversion to reaction temperature for catalysts Pd/MHSS and
PdAu/MHSS; mass ratio of phenol to catalyst 1:0.01, reaction time 30 min.
Fig. 4 shows the relationship of conversion to reaction temperature.
To make a clear comparison, we increased the mass ratio of Mphenol:MPd
Table 3
The catalytic activities and selectivity of PdAu/MHSS catalyst towards hydrogenation of several aromatic derivatives.
Entry
1
Substrate
Temperature (°C)
50
Conversion (%)
>99
Selectivity (%)
OH
O
OH
96;
4
2
50
50
50
50
>99
OH
OH
O
OH
OH
96;
4
3
>99
>99
>99
3
4
5
O
97;
OH
O
OH
HO
HO
HO
95;
5
O
OH
>99
50
6
7
>99
22
NH2
NO2
>99
O
100
O
OH
Reaction conditions: mass ratio: Msubstrate:MPd =1:0.025; hydrogen pressure, 1 MPa; impeller speed, 1000 rpm; and reaction time 1 h.
OH
>99

X. Yang et al. / Catalysis Communications 17 (2012) 29–33
33
to 1:0.01. For both Pd/MHSS and PdAu/MHSS, conversion increased
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Acknowledgments
The authors would like to thank the National Natural Science
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