Selective hydrogenation of phenol to cyclohexanone in water over Pd catalysts supported on Amberlyst-45

Article abstract of DOI:10.1016/S1872-2067(15)60997-4

A series of Pd catalysts were prepared on different supports (Fe₂O₃, SiO₂, ZnO, MgO, Al₂O₃, carbon, and Amberlyst-45) and used in the selective hydrogenation of phenol to cyclohexanone in water. The A

Full text of DOI:10.1016/S1872-2067(15)60997-4

Chinese Journal of Catalysis 37 (2016) 234–239
ava i l a b l e a t w w w. s c i e n c ed i r e c t . c o m
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e /c h n j c
Article
Selective hydrogenation of phenol to cyclohexanone in water over
Pd catalysts supported on Amberlyst‐45
Mengsi Zhao, Juanjuan Shi, Zhaoyin Hou *
Institute of Catalysis, Department of Chemistry, Zhejiang University, Hangzhou 310028, Zhejiang, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A series of Pd catalysts were prepared on different supports (Fe2O3, SiO2, ZnO, MgO, Al2O3, carbon,
and Amberlyst‐45) and used in the selective hydrogenation of phenol to cyclohexanone in water.
The Amberlyst‐45 supported Pd catalyst (Pd/A‐45)was highly active and selective under mild con‐
ditions (40–100 °C, 0.2–1 MPa), giving a selectivity of cyclohexanone higher than 89% even at com‐
plete conversion of phenol. Experiments with different Pd loadings (or different particle sizes) con‐
firmed that the formation of cyclohexanone was a structure sensitive reaction, and Pd particles of
12–14 nm on Amberlyst‐45 gave better selectivity and stability.
Received 3 September 2015
Accepted 29 September 2015
Published 5 February 2016
Keywords:
Phenol
Hydrogenation
Cyclohexanone
Palladium
© 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Amberlyst‐45 resin
Supported catalyst
1. Introduction
frameworks (MOFs) [23,24], SiO2, γ‐Al2O3 [25,26], and
mpg‐C3N4 [5,27], have been used as the support of Pd. The
Cyclohexanone is an important organic compound in the
manufacture of nylon. Cyclohexanone is produced by the selec‐
tive oxidation of cyclohexane in air over cobalt catalysts [1–4].
Alternatively, cyclohexanone can also be synthesized by the
selective hydrogenation of phenol, but this requires high tem‐
perature and gives several byproducts such as cyclohexanol
and cyclohexane [2,3]. Ways to increase the selectivity to cy‐
clohexanone have been much studied in recent years [5–7].
Several catalysts including Pd [8–12], Pt [13,14], Rh [15], and
Ni [16,17] catalysts were reported frequently, and Pd‐based
catalysts are favored for this reaction for its high activity and
selectivity. At the same time, many supports, including NaY
zeolite [1], carbon [18], hydrophilic carbon [19], MgO [20],
Fe2O3 [20], mesoporous CeO2 [21], HZSM‐5 [22], metal organic
product distribution depends on both the active metal and
support, and a co‐added acid or acidity of the catalyst can im‐
prove the conversion of phenol [1,28]. However, achieving a
high selectivity of cyclohexanone (> 90%) at high phenol con‐
version (> 80%) with a single catalyst remains challenging.
An ion exchange resin is a porous, insoluble matrix with a
high surface area. It is widely used in catalysis for its strong
acidity. The Amberlyst‐45 resin (A–45) is a new macroporous
polymer designed for use at high temperature. It is made of
polystyrene sulfonate and its concentration of acid sites can
reach 2.95 eq/kg. In this work, Pd nanoparticles (NPs) were
loaded on A‐45 by a facile routine, and this Pd/A‐45 catalyst
was used in the selective hydrogenation of phenol to cyclohex‐
anone in water.
* Corresponding author. Tel/Fax: +86‐571‐88273283; E‐mail: zyhou@zju.edu.cn
This work was supported by the National Natural Science Foundation of China (21473155, 21273198, 21073159) and the Natural Science Founda‐
tion of Zhejiang Province (LZ12B03001).
DOI: 10.1016/S1872‐2067(15)60997‐4 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 37, No. 2, February 2016

Mengsi Zhao et al. / Chinese Journal of Catalysis 37 (2016) 234–239
235
2. Experimental
200 kV (TEM, JEOL‐2010F).
2.1. Catalyst preparation
2.3. Hydrogenation of phenol
Commercial resin Amberlyst‐45 (A‐45, Dow Chemical Com‐
pany) was purchased from Sigma Co. (China Branch). It was
washed with distilled water 3 times and dried in vacuum at 50
°C for 12 h before use. First, 1.0 g of A‐45 was immersed in an
aqueous solution of PdCl2 (30 mL, containing a controlled
amount of Pd) under stirring at 50 °C for 24 h. Then, the solid
particles were separated by filtration, washed with a mixed
solution of ethanol/water (1:1 by volume) until free of Cl–, and
further dried under vacuum at 50 °C for 12 h. Finally, the dried
solid was pretreated in a flow of H2 at 120 °C for 1.0 h before
the catalytic reaction. These catalysts were denoted as
Pd(x)/A‐45, where x wt% is the loading of Pd.
As references, Fe2O3, SiO2, ZnO, MgO, Al2O3, and active car‐
bon (AC) supported Pd catalysts were also prepared by an im‐
pregnation method described elsewhere [29]. Before impreg‐
nation, the support was first calcined at 500 °C in N2 flow for 4
h and then impregnated with an aqueous solution of PdCl2 with
equal weight ratio of Pd and support. The precursor was dried
in N2 at 110 °C overnight followed by calcination in N2 at 500 °C
for 4 h. Before the catalytic reaction, the catalyst was reduced
in H2 at 120 °C for 1 h.
The hydrogenation of phenol was carried out in a 50 mL
custom designed stainless steel autoclave with a Teflon inner
layer. In a typical reaction, a controlled amount of catalyst was
dispersed in 20 mL aqueous solution of phenol. Then, the reac‐
tor was sealed, purged with purified hydrogen 5 times, and
pressurized to the desired pressure. The reactor was heated in
an oil bath and stirred with a magnetic stirrer (MAG‐NEO,
RV‐06M, Japan). After reaction, the solid catalyst was separated
by centrifugation. The liquid reaction mixture was analyzed by
a gas chromatograph (HP 5890, USA) with a 30 m capillary
column (HP‐5) using a flame ionization detector. All products
were confirmed by GC‐MS (Agilent 6890‐5973N). For each
successive use, the catalyst was washed with water three times
and dried under vacuum at 40 °C for 6 h. The conversion of
phenol and selectivity for cyclohexanone (and cyclohexanol)
were calculated as:
Conversion = (phenoladded‐mol – phenolremain‐mol
)
/phenoladded‐mol × 100%
Selectivity = cyclohexanoneformed‐mol/(phenoladded‐mol
– phenolremain‐mol) × 100%
The loading of Pd on the above catalysts was checked by
inductively coupled plasma atomic emission spectroscopy (ICP,
Plasma‐Spec‐II spectrometer). The results are summarized in
Table 1.
3. Results and discussion
3.1. Characterization of the catalysts
Figure 1 shows the XRD patterns of the pristine A‐45 resin
and Pd/A‐45 catalysts with different loadings of Pd. Only a
broad peak of amorphous carbon appeared with A‐45. Beside
the peak of amorphous carbon, four characteristic diffraction
peaks of Pd were detected at 40.0°, 46.5°, 68.1°, and 82.1° with
all the Pd/A‐45 catalysts, which corresponded to the (111),
(200), (220), and (311) crystalline planes of face centered cubic
of Pd (JCPDS 46‐1043). The crystalline size of Pd was calculated
from the half‐width of the Pd(111) peak using the Scherrer
equation. The results are summarized in Table 1. The particle
size of Pd on A‐45 increased from 9.0 nm (in Pd(0.9)/A‐45) to
2.2. Catalyst characterization
X‐ray diffraction (XRD) patterns were performed on a
Rigaku D/MAX 2550/PC diffractometer (18 kW) at 40 kV and
100 mA with Cu Kα radiation (λ = 1.5406 Å) in the range of
5°–80°. The surface area of the catalysts was measured by N2
adsorption using an ASAP 2010 analyzer (Micromeritics) after
pretreatment at 100 °C for 4 h under vacuum. X‐ray photoelec‐
tron spectra (XPS) were recorded on a Perkin‐Elmer PHI ESCA
System. The X‐ray source was an Mg standard anode (1253.6
eV) at 12 kV and 300 W. Transmission electron microscopy
(TEM) images were obtained using an accelerating voltage of
Table 1
Physical properties of the catalysts.
Pd loading a
(wt%)
2.5
Particle size of Pd b
Surface area c
(m2/g)
9.2
(4)
(3)
Catalyst
(nm)
5.5
7.4
10.1
8.0
4.5
6.6
9.0
12.0
14.5
Pd/Fe3O4
Pd/SiO2
Pd/ZnO
Pd/MgO
Pd/Al2O3
2.7
2.8
2.8
2.6
2.9
0.9
2.7
289.3
14.7
39.4
156.2
1396.0
40.3
(2)
(1)
Pd/AC
Pd(0.9)/A‐45
Pd(2.7)/A‐45
Pd(4.5)/A‐45
39.8
38.0
10
20
30
40
50
60
70
80
4.5
2/( o )
a Measured by ICP.
b Calculated using Scherrer's equation.
c Measured by N2 adsorption.
Fig. 1. XRD patterns of A‐45 (1), Pd(0.9)/A‐45 (2), Pd(2.7)/A‐45 (3),
and Pd(4.5)/A‐45 (4).

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Mengsi Zhao et al. / Chinese Journal of Catalysis 37 (2016) 234–239
14.5 nm (in Pd(4.5)/A‐45) with increasing Pd loading.
Figure 2 shows typical TEM images and the histogram of the
Pd particle size distribution of the Pd(2.7)/A‐45 catalyst. Pd
particles were dispersed mainly in the macropore channels of
A‐45, and the particles of Pd were in the range of 5–20 nm. The
calculated mean particle size of the counted particles was 12.2
nm, which agreed with the XRD results.
Table 2
Surface composition of the Pd(2.7)/A‐45 catalyst.
Binding energy (eV)
Surface content (wt%)
Sample
Pd
C
S
Pd
C
S
O
A‐45
Pd(2.7)/A‐45
—
284.40 168.89
—
79.38 5.13 15.49
335.74 284.32 168.45
340.81
3.98 76.30 4.92 14.80
Figure 3 presents the XPS spectra of pristine A‐45 and the
Pd(2.7)/A‐45 catalyst. The binding energy and surface content
of C, S, O, and Pd are summarized in Table 2. The binding ener‐
gy confirmed that plentiful –SO3H (168.9 eV) groups existed on
the surface of pristine A‐45 [30,31]. When Pd was loaded, the
surface composition of A‐45 remained stable, and mainly me‐
tallic Pd was formed after reduction [32].
none was performed over the different catalysts in water at
100 °C for 3 h (Table 3). All the tested catalysts were active for
this reaction. Cyclohexanone and cyclohexanol were the only
reaction products observed over the entire range of conditions
studied. The conversion of phenol increased in the order of
Pd/Fe3O4, Pd/SiO2, Pd/ZnO, Pd/MgO, and Pd/Al2O3. Only
mainly the byproduct cyclohexanol was formed over Pd/MgO,
Pd/ZnO, and Pd/Fe3O4. On the other hand, Pd/AC and
Pd(2.7)/A‐45 were more active than the oxide supported Pd
catalysts. Phenol was converted completely over Pd(2.7)/A‐45
3.2. Catalytic reaction
First, the selective hydrogenation of phenol to cyclohexa‐
(a)
(b)
(d)
30
(c)
25
20
15
10
5
d1 = 0.238 nm
d2 = 0.260 nm
0
7
10
12
15
17
20
Diameter (nm)
Fig. 2. (a, b) TEM images of Pd(2.7)/A‐45; (c) Histogram of Pd particle size distribution in Pd(2.7)/A‐45; (d) High resolution TEM image of Pd nano‐
particles in Pd(2.7)/A‐45.
(a)
(b)
Pd(0)
337.74
Pd(0)
342.81
A-45
Pd(II)
344.85
Pd(2.7)/A-45
Pd(II)
339.69
0
200
400
600
800
1000
332
336
340
344
348
352
Binding energy (eV)
Binding energy (eV)
Fig. 3. XPS spectra of A‐45 and Pd(2.7)/A‐45 (a), and the Pd 3d in Pd(2.7)/A‐45 (b).

Mengsi Zhao et al. / Chinese Journal of Catalysis 37 (2016) 234–239
237
Table 3
Hydrogenation of phenol over the different catalysts.
Selectivity
100
80
60
40
20
0
Conversion
(%)
Selectivity (%)
Cyclohexanone Cyclohexanol
Entry
Catalyst
1
2
3
4
5
6
7 a
8 a
9
blank
Pd/Fe3O4
Pd/SiO2
Pd/ZnO
Pd/MgO
Pd/Al2O3
Pd/AC
0
—
32.2
>99.0
26.1
17.7
65.1
62.4
89.0
—
—
67.8
—
74.0
82.3
34.9
37.6
11.0
—
14.7
26.6
27.7
35.0
87.5
69.1
100.0
0
Conversion
Pd(2.7)/A‐45
A‐45
0
20 40 60 80 100 120 140 160 180
Ratio of phenol/Pd
Reaction conditions: phenol 0.5 mmol, water 20 mL, phenol/Pd ratio
8.0, 100 °C, 3 h, H2 1.0 MPa, 1000 r/min.
a 0.5 h.
Fig. 4. Performance of Pd(2.7)/A‐45 for different phenol/Pd ratios in
the feed. Reaction conditions: 100 °C, H2 1.0 MPa,0.5 h.
even for 0.5 h, and the selectivity for cyclohexanone reached
89.0%. These results suggested that the ion exchanged resin
(A‐45) was a good support for the selective hydrogenation of
phenol to cyclohexanone.
mmol/g in A‐45). At the same time, the acid sites also enrich
the electron density of Pd, enhance the desorption of phenoxy
species [28], and inhibit further hydrogenation of the interme‐
diate product [1].
The good performance of Pd(2.7)/A‐45 was confirmed un‐
der various reaction conditions (Table 4 and Fig. 4). The con‐
version of phenol increased quickly from 23.8% (at 40 °C) to
100% (at 100 °C) with rising temperature, while the selectivity
for cyclohexanone only decreased slightly from 92.4% to
89.0% (Table 4). At the same time, it was confirmed that a low
pressure was more favorable for the formation of cyclohexa‐
none and the selectivity for cyclohexanone reached 95.6% at
0.2 MPa. More importantly, the selectivity for cyclohexanone
remained higher than 89.0% in a wide range of phenol/Pd ratio
in the feed at 100 °C and 1.0 MPa (Fig. 4). The performance of
the Pd(2.7)/A‐45 catalyst was lower than that of Pd/C + AlCl3
[1] and mpg‐C3N4 supported Pd [27], but it was better than
those of active carbon, hydrophilic carbon, and Al2O3 supported
Pd catalysts [19], and the selectivity for cyclohexanone was
higher than that of Pd/C [33].
The published work [1,21–28] suggested that the selectivity
for cyclohexanone in phenol hydrogenation depended on the
adsorption mode of phenol, and the nonplanar form of ad‐
sorbed phenol was more favorable for the formation of cyclo‐
hexanone than a coplanar form of adsorbed molecule [21].
Wang et al. [27] further disclosed that the acid sites on the sur‐
face of the catalyst helped the nonplanar form adsorption of
phenol through the hydroxy group forming strong O−H···π
interactions. Therefore, we think that the good performance of
Pd(2.7)/A‐45 can be attributed to its strong acidity (2.95
The recycle experiments also indicated that Pd/A‐45
showed
a better performance in reusability (Fig. 5).
Pd(2.7)/A‐45 could be reused at least 6 times without signifi‐
cant loss of activity, maintaining at least 46.6% conversion,
which is a prerequisite for practical application. It is important
to highlight that the selectivity for cyclohexanone remained
high (about 90%) and comparable to that of the fresh catalyst.
XRD analysis of a spent Pd(2.7)/A‐45 catalyst indicated that the
particle size of Pd was increased slightly from 12.0 to 13.6 nm
(Fig. 6), and the decreased conversion of phenol can be at‐
tributed to the sintering of Pd. Another possible reason for ac‐
tivity loss may be due to the swelling of the polymer resin in
the polar solvent and/or reactants [34], or because the resin
beads were gradually mechanically damaged by being vigor‐
ously stirred with a magnetic stirrer bar during the reaction
[35].
As Pd(2.7)/A‐45 with a similar loading amount of Pd exhib‐
ited the best performance among the tested catalysts (Table 3),
Conversion
Selectivity
100
80
60
40
20
0
Table 4
Performance of Pd(2.7)/A‐45 at different temperatures and pressures.
Temperature H2 pressure Conversion
(°C)
40
60
Selectivity (%)
Cyclohexanone Cyclohexanol
(MPa)
1.0
1.0
1.0
1.0
0.5
0.2
(%)
23.8
60.2
84.5
100.0
52.3
21.7
92.4
91.0
90.0
89.0
91.8
95.6
7.6
9.0
10.0
11.0
8.2
80
1
2
3
4
5
6
100
100
100
Run
4.4
Fig. 5. Recycling of Pd(2.7)/A‐45 for phenol hydrogenation. Reaction
conditions: 0.24 g Pd(2.7)/A‐45 in the first run (no further addition),
0.5 mmol phenol in each run, 60 °C, H2 1.0 MPa, 0.5 h.
Reaction conditions: phenol 0.5 mmol, water 20 mL, phenol/Pd ratio
8.0, 1000 r/min, 0.5 h.

238
Mengsi Zhao et al. / Chinese Journal of Catalysis 37 (2016) 234–239
hexanone in water under mild conditions (40–100 °C, 0.2–1
MPa). The selectivity for cyclohexanone remained higher than
89% even at complete conversion of phenol. A low pressure is
more favorable for the formation of cyclohexanone. These re‐
sults showed the potential of Pd/A‐45 for practical application.
(2)
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Table 5
Performance of Pd/A‐45 with different Pd loadings.
Selectivity (%)
Cyclohexa‐ Cyclohexa‐
Pd
Size Conversion
D a TOF b
loading
(wt%)
0.1
0.3
0.6
0.9
1.8
2.7
3.6
(nm)
(%)
(%) (h–1)
none
6.3
nol
93.7
78.5
57.8
34.3
22.4
9.0
5.0
6.3
7.7
100.0
95.5
90.3
82.5
67.1
60.2
50.0
44.5
19.4
15.4
80
96
21.5
42.2
65.7
77.6
91.0
90.2
89.8
12.6 111
10.8 118
8.9 116
8.1 116
7.2 108
6.7 103
9.0
10.9
12.0
13.6
14.5
10.8
10.2
4.5
Reaction conditions: phenol 0.5 mmol, water 20 mL, phenol/Pd ratio
8.0, 60 °C, 0.5 h, H2 1.0 MPa, 1000 r/min.
a Dispersion of Pd calculated as D = 0.97/particle size in nm.
b
Turnover frequency defined as (converted phenol molecules at 0.5
h)/((number of surface Pd atoms)  (reaction time, h)).
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Mengsi Zhao et al. / Chinese Journal of Catalysis 37 (2016) 234–239
239
Graphical Abstract
Chin. J. Catal., 2016, 37: 234–239 doi: 10.1016/S1872‐2067(15)60997‐4
Selective hydrogenation of phenol to cyclohexanone in water over Pd
catalysts supported on Amberlyst‐45
H2
Mengsi Zhao, Juanjuan Shi, Zhaoyin Hou*
Zhejiang University
phenol
cyclohexanone
Pd
Ion‐exchange resin Amberlyst‐45 supported Pd nanoparticles was highly active
and selective for the selective hydrogenation of phenol to cyclohexanone in
water under mild conditions (40–100 °C, 0.2–1 MPa).
HSO3-
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