Catalytic decomposition of 4-phenoxyphenol over Pd/XCs2.5H 0.5PW12O40/ACA (activated carbon aerogel)-SO3H (X = 10-30 wt%) catalysts

Article abstract of DOI:10.1016/j.apcata.2012.12.037

Activated carbon aerogel (ACA) bearing sulfonic acid group (ACA-SO ₃H) was prepared by a sulfonation of activated carbon aerogel, and subsequently, Cs_2.5H_0.5PW₁₂O₄₀ was impregnated on ACA-SO₃

Full text of DOI:10.1016/j.apcata.2012.12.037

Applied Catalysis A: General 453 (2013) 287–294
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Catalytic decomposition of 4-phenoxyphenol over
Pd/XCs2.5H0.5PW12O40/ACA (activated carbon aerogel)-SO3H
(X = 10–30 wt%) catalysts
Hai Woong Park, Jeong Kwon Kim, Ung Gi Hong, Yoon Jae Lee, Ji Hwan Song, In Kyu Song∗
School of Chemical and Biological Engineering, Institute of Chemical Processes, World Class University Program of Chemical Convergence for Energy &
Environment, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-744, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Activated carbon aerogel (ACA) bearing sulfonic acid group (ACA-SO3H) was prepared by a sulfona-
tion of activated carbon aerogel, and subsequently, Cs2.5H0.5PW12O40 was impregnated on ACA-SO3H
to form XCs2.5H0.5PW12O40/ACA-SO3H with a variation of Cs2.5H0.5PW12O40 content (X = 10, 15, 20, 25,
and 30 wt%). Palladium catalysts were then supported on XCs2.5H0.5PW12O40/ACA-SO3H by an incipient
wetness impregnation method. The prepared Pd/XCs2.5H0.5PW12O40/ACA-SO3H catalysts were applied
to the decomposition of 4-phenoxyphenol. 4-Phenoxyphenol was used as a lignin model compound
for representing 4-O-5 linkage of lignin. Cyclohexanol, benzene, and phenol were mainly produced by
the decomposition of 4-phenoxyphenol. Acidity of Pd/XCs2.5H0.5PW12O40/ACA-SO3H catalysts played
an important role in the decomposition of 4-phenoxyphenol. Conversion of 4-phenoxyphenol and
total yield for main products (cyclohexanol, benzene, and phenol) increased with increasing acidity of
Pd/XCs2.5H0.5PW12O40/ACA-SO3H. Among the catalysts tested, Pd/15Cs2.5H0.5PW12O40/ACA-SO3H with
the largest acidity showed the highest conversion of 4-phenoxyphenol and total yield for main products.
Conversion of 4-phenoxyphenol and total yield for main products over Pd/15Cs2.5H0.5PW12O40/ACA-SO3H
were much higher than those over palladium catalyst supported on activated carbon aerogel (Pd/ACA)
and palladium catalyst supported on activated carbon aerogel bearing sulfonic acid group (Pd/ACA-SO3H).
Furthermore, Pd/15Cs2.5H0.5PW12O40/ACA-SO3H catalyst was stable and reusable in the decomposition
of 4-phenoxyphenol.
Received 28 November 2012
Received in revised form
17 December 2012
Accepted 18 December 2012
Available online xxx
Keywords:
Heteropolyacid
Activated carbon aerogel bearing sulfonic
group
Pd catalyst
4-Phenoxyphenol
Decomposition
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
alcohol, coniferyl alcohol, and sinapyl alcohol in lignin are mainly
linked by C O and C C bonds. Therefore, decomposition of lignin to
Fossil fuels such as coal and crude oil are not sustainable. Fur-
thermore, carbon dioxide from burning of fossil fuels has been
considered as a contributor to global warming [1,2]. Therefore, con-
version of biomass to liquid fuels and chemicals has attracted much
attention as an environmentally benign process, because biomass
can reduce the dependence on fossil fuel and the emission of carbon
dioxide [3].
First generation biofuels derived from corn sugar and vegetable
oil are environmentally friendly, but feedstocks for first generation
biofuels are limited and compete with food resource [4,5]. On the
other hand, second generation biofuels derived from lignocellulosic
feedstocks do not compete with food resource. Lignin produced by
delignification process in the pulp industries is burned as a low
value fuel for power generation [6,7]. Aromatics such as coumaryl
aromatics becomes as an advantageous technology for producing
aromatics as biofuels and chemicals, because lignin is the richest
source of aromatics in nature [8,9].
Catalytic decomposition of lignin has received much attention
as a key technology for valorization of lignocellulosic biomass,
because thermal cracking of lignin requires high temperature
(over 450 ◦C) and pressure (over 100 atm) for considerable per-
formance [10–12]. Liquid acids such as H3PO4 and solid acids
such as cesium-exchanged heteropolyacid (CsxH3.0−xPW12O40
,
X = 2.0–3.0) are known as promising catalysts for the decomposi-
tion of lignin [13–15]. Novel metal catalysts supported on carbon
(Pd/C, Pt/C, and Rh/C) [16,17] and novel metal catalysts supported
on acidic activated carbon aerogel (ACA) (Pd/ACA-SO3H [18] and
Pd/Cs2.5H0.5PW12O40/ACA [19]) also showed considerable perfor-
mance for selective decomposition of C O bond in lignin.
Dimeric lignin model compounds for representing C O and
C
C bonds in lignin have been used as a lignin feedstock due to the
∗
Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415.
E-mail address: inksong@snu.ac.kr (I.K. Song).
complex structure of lignin [20–22]. In particular, dimeric chemical
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.12.037

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H.W. Park et al. / Applied Catalysis A: General 453 (2013) 287–294
compounds containing C O bond such as ␤-O-4, ␣-O-5, and 4-O-5
have been used as lignin model compounds, because C O bond is
abundant linkage type in the lignin. Among various lignin model
compounds, 4-phenoxyphenol has been widely employed as a
lignin model compound for representing 4-O-5 bond in lignin
[23,24].
3 h. The mixture of phosphoric acid and carbon aerogel was heated
to react at 800 ◦C for 1 h under nitrogen stream to obtain activated
carbon aerogel. Phosphoric acid in the activated carbon aerogel was
washed off using DI water till the pH value of solution reached ca.
7. The residual solid was finally dried at 110 ◦C for 5 h to obtain
activated carbon aerogel (ACA).
Carbon aerogel (CA) [25–29] and activated carbon aerogel (ACA)
[30–35] have been employed as catalyst support, electrochemical
capacitor, and adsorbent due to its porous nature. Pore structure
of activated carbon aerogel can be controlled by changing activa-
tion conditions such as activation agent, activation temperature,
and activation time. Palladium catalyst supported on carbon aero-
gel (Pd/CA) and palladium catalyst supported on activated carbon
2.2. Preparation of ACA-SO3H
In order to provide acid sites to ACA, activated carbon aerogel
bearing sulfonic acid group (ACA-SO3H) was prepared by sulfona-
tion of activated carbon aerogel (ACA). Fig. 1 shows the schematic
procedure for the preparation of activated carbon aerogel bear-
ing sulfonic acid group (ACA-SO3H). 1 g of activated carbon aerogel
was dispersed into 100 ml of H2SO4 solution (10 M). After stirring
the mixture for 10 h under nitrogen stream, the solid was filtered
and washed with DI water at 70 ◦C. The residual solid was dried at
110 ◦C for 5 h to obtain activated carbon aerogel bearing sulfonic
acid group (ACA-SO3H)
aerogel (Pd/ACA) [36] have been used for the decomposition of C
O
bond in the lignin to take advantage of mesoporous nature of CA
and ACA.
Heteropolyacids (HPAs) are inorganic acids. Acid strength of
HPAs is stronger than that of conventional solid acids [37–39].
HPA salts with K+, Cs+, and NH4 cations have high surface area
+
and porous structure by forming a tertiary structure [38,39]. It is
known that acidity of cesium-exchanged insoluble HPAs is different
depending on cesium content [37,38].
2.3. Preparation of XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20,
25, and 30 wt%)
In this work, activated carbon aerogel (ACA) bearing sulfonic
acid group (ACA-SO3H) was prepared by a sulfonation of ACA
with H2SO4, and subsequently, Cs2.5H0.5PW12O40 was impregnated
on ACA-SO3H to form XCs2.5H0.5PW12O40/ACA-SO3H with a varia-
tion of Cs2.5H0.5PW12O40 content (X = 10, 15, 20, 25, and 30 wt%).
Palladium catalysts supported on XCs2.5H0.5PW12O40/ACA-SO3H
(Pd/XCs2.5H0.5PW12O40/ACA-SO3H) were then prepared by an
incipient wetness impregnation method, and they were applied
to the decomposition of 4-phenoxyphenol. 4-Phenoxyphenol was
chosen as a lignin model compound for representing 4-O-5 bond in
lignin. For comparison, palladium catalyst supported on activated
carbon aerogel (Pd/ACA) and palladium catalyst supported on acti-
vated carbon aerogel bearing sulfonic acid group (Pd/ACA-SO3H)
were also employed for the decomposition of 4-phenoxyphenol.
Cesium-exchanged Cs2.5H0.5PW12O40 heteropolyacid was
impregnated on ACA-SO3H to form XCs2.5H0.5PW12O40/ACA-SO3H
with a variation of Cs2.5H0.5PW12O40 content (X = 10, 15, 20,
25, and 30 wt%). Fig. 1 also shows the schematic procedures
for the preparation of XCs2.5H0.5PW12O40/ACA-SO3H. A known
amount of H3PW12O40 (Sigma–Aldrich) was impregnated onto
1 g of ACA-SO3H (XH3PW12O40/ACA-SO3H, X = 10, 15, 20, 25, and
30 wt%) by an incipient wetness impregnation method, and then
it was dried overnight at 80 ◦C. A known amount of cesium nitrate
(CsNO3, Sigma–Aldrich) was separately dissolved in 10 ml of DI
water. 1 g of XH3PW12O40/ACA-SO3H was then dispersed into the
cesium-containing solution with constant stirring for 3 h. After
filtering and washing a solid product with DI water, the solid was
dried overnight at 80 ◦C. The solid was finally calcined at 300 ◦C for
3 h to obtain XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25, and
30 wt%).
2. Experimental
2.1. Preparation of carbon aerogel (CA) and activated carbon
aerogel (ACA)
2.4. Preparation of Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15,
20, 25, and 30 wt%)
Carbon aerogel (CA) and activated carbon aerogel (ACA) were
prepared according to the method in the literatures [27,36]. Carbon
aerogel was prepared by a sol–gel polymerization of resorcinol and
formaldehyde. 25.9 g of resorcinol (C6H6O2, Sigma–Aldrich) was
dissolved in 60 ml of DI water. Aqueous resorcinol was mixed with
sodium carbonate (0.05 g) (a base catalyst) to accelerate dehydro-
genation of resorcinol. After stirring the solution for a few minutes,
14.1 g of formaldehyde (H2CO, Sigma–Aldrich) was slowly added
into the solution to form a sol. Molar ratio of resorcinol (R) with
respect to formaldehyde (F) was fixed at 1:2 (R/F = 1/2). R/C (resor-
cinol/catalyst) ratio was fixed at 500. The resulting sol was cured
in a vial at 80 ◦C for 24 h to produce resorcinol–formaldehyde (RF)
gel. Solvent exchange was performed with acetone at 50 ◦C for 2
days. Residual solvent was replaced with fresh acetone every 3 h
to remove water thoroughly from the pore of RF wet gel. Ambient
drying was then done at room temperature and 50 ◦C for 1 day. Car-
bon aerogel (CA) was finally obtained by carbonization of RF gel at
500 ◦C for 1 h.
The schematic procedures for the preparation of
Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25, and 30 wt%)
catalysts are also shown in Fig. 1. 5 wt% of palladium cata-
lyst was supported on XCs2.5H0.5PW12O40/ACA-SO3H (X = 10,
15, 20, 25, and 30 wt%) by an incipient wetness impregna-
tion method. The supported palladium catalysts were dried at
80 ◦C for 3 h, and they were calcined at 250 ◦C for 5 h. The sup-
ported catalysts were reduced with a mixed stream of hydrogen
(5 cm3/min) and nitrogen (30 cm3/min) at 120 ◦C for 6 h to obtain
Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25, and 30 wt%).
For comparison, palladium catalyst supported on activated car-
bon aerogel (Pd/ACA) [16] and palladium catalyst supported on
activated carbon aerogel bearing sulfonic acid (Pd/ACA-SO3H) [18]
were also prepared by an incipient wetness impregnation method.
2.5. Characterization
Activated carbon aerogel was prepared by a chemical activation
of carbon aerogel with phosphoric acid (H3PO4). 2 g of phosphoric
acid (H3PO4) was dissolved in 10 ml of DI water and 2 g of carbon
aerogel was dispersed into an aqueous solution of phosphoric acid.
After stirring the solution for 1 h, the solid was dried at 110 ◦C for
Textural properties of Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10,
15, 20, 25, and 30 wt%) catalysts were determined by nitrogen
adsorption–desorption isotherm measurements (Micromeritic,
ASAP 2010). Crystalline states of Pd/XCs2.5H0.5PW12O40/ACA-SO3H
were determined by XRD (Rigaku, D-MA2500-PC) measurements

H.W. Park et al. / Applied Catalysis A: General 453 (2013) 287–294
289
H2SO4 (10M)
H3PW12O40
CsNO3
ACA
ACA-SO3H + DI water
H3PW12O40/ACA-SO3H
drying/
calcination
reaction/
washing/
drying
drying/
washing/
calcination
Pd(NO3)2
in DI water
Pd/XCs2.5H0.5PW12O40/
ACA-SO3H
(X=10, 15, 20, 25, and 30
wt%)
PdO/XCs2.5H0.5PW12O40/
XCs2.5H0.5PW12O40/
ACA-SO3H
ACA-SO3H
drying/
calcination
(X=10, 15, 20, 25, and 30 wt%)
reduction (X=10, 15, 20, 25, and 30 wt%)
Fig. 1. Schematic procedures for the preparation of Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25, and 30 wt%).
using Cu-K␣ radiation operated at 50 kV and 100 mA. Surface mor-
phology and palladium dispersion of Pd/XCs2.5H0.5PW12O40/ACA-
SO3H catalysts were investigated by TEM (Jeol, JXA-8900R).
Chemical compositions of the catalysts were measured by ICP-AES
(Shimadzu, ICPS-7500) and CHNS (LECO, US/CHNS-932) analyses.
NH3-TPD experiment (BEL Japan, BELCAT-B) was carried out in
order to measure the acidity of the catalyst. 0.04 g of each catalyst
charged into the TPD apparatus was pretreated at 350 ◦C for 1 h
with a stream of helium (50 ml/min). A mixed stream of ammonia
(2.5 ml/min) and helium (47.5 ml/min) was then introduced into
the reactor at 35 ◦C for 30 min in order to saturate acid sites of
the catalyst with ammonia. Physisorbed ammonia was removed at
100 ◦C for 1 h under a flow of helium (50 ml/min). After cooling the
catalyst, furnace temperature was increased from 35 ◦C to 700 ◦C at
a heating rate of 5 ◦C/min under a flow of helium (30 ml/min). The
desorbed ammonia was detected using a TCD.
1000
800
600
400
200
(e)
(d)
(c)
(b)
(a)
0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
2.6. Decomposition of 4-phenoxyphenol
Fig. 2. N2 adsorption–desorption isotherms of Pd/XCs2.5H0.5PW12O40/ACA-SO3H:
(a) Pd/10Cs2.5H0.5PW12O40/ACA-SO3H, (b) Pd/15Cs2.5H0.5PW12O40/ACA-SO3H, (c)
Pd/20Cs2.5H0.5PW12O40/ACA-SO3H, (d) Pd/25Cs2.5H0.5PW12O40/ACA-SO3H, and (e)
Pd/30Cs2.5H0.5PW12O40/ACA-SO3H. The adsorption data for (a)–(e) were offset ver-
tically by 0, 150, 300, 450, and 600 (STP)/g, respectively.
Decomposition
of
4-phenoxyphenol
(C6H5OC6H4OH,
Sigma–Aldrich) over Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10,
15, 20, 25, and 30 wt%), Pd/ACA, and Pd/ACA-SO3H catalysts was
carried out in an autoclave reactor under hydrogen atmosphere.
9 ml of hexadecane (Sigma–Aldrich) (a solvent) and 0.01 g of cata-
lyst were charged into the reactor. 0.2 g of 4-phenoxyphenol was
then added into the reactor. The catalytic reaction was performed
at 200 ◦C and 10 atm (H2) for 1 h with agitation speed of 100 rpm.
Reaction products were analyzed with a gas chromatograph
(Younglin, ACME 6100) equipped with DB-5 column and FID.
Main products and by-products produced by the decomposi-
tion of 4-phenoxyphenol over Pd/XCs2.5H0.5PW12O40/ACA-SO3H
were identified by GC-MS (Agilent, 6890 N GC) equipped with
mass selective detector (Agilent, 5975 MSD). Conversion of
4-phenoxyphenol and selectivity for product (cyclohexanol, ben-
zene, and phenol) were calculated by the following equations. Yield
for product (cyclohexanol, benzene, and phenol) was calculated
by multiplying conversion of 4-phenoxyphenol and corresponding
product selectivity.
3. Results and discussion
3.1. Characterization of Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10,
15, 20, 25, and 30 wt%)
Textural properties of Pd/XCs2.5H0.5PW12O40/ACA-SO3H
(X = 10, 15, 20, 25, and 30 wt%) catalysts were examined
by nitrogen adsorption–desorption isotherm measurements.
Fig.
2 shows the N2 adsorption–desorption isotherms of
Pd/XCs2.5H0.5PW12O40/ACA-SO3H catalysts. All the samples
showed type-IV isotherm with type H2 hysteresis loop. This result
indicates that Pd/XCs2.5H0.5PW12O40/ACA-SO3H still retained
a porous structure even after the sulfonation of ACA and the
moles of 4-phenoxyphenol reacted
Conversion of 4-phenoxyphenol =
(1)
moles of 4-phenoxyphenol supplied
moles of cyclohexanol, benzene, or phenol formed
Selectivity for product (cyclohexanol, benzene, or phenol) =
(2)
moles of 4-phenoxyphenol reacted
Total moles of cyclohexanol, benzene, and phenol formed
Total selectivity for main products =
Total yield for main products = (Conversion of 4-phenoxyphenol) × (Total selectivity for main products)
(3)
(4)
moles of 4-phenoxyphenol reacted

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H.W. Park et al. / Applied Catalysis A: General 453 (2013) 287–294
Pd/30Cs2.5H0.5PW12O40/ACA-SO3H
Pd/25Cs2.5H0.5PW12O40/ACA-SO3H
Pd/20Cs2.5H0.5PW12O40/ACA-SO3H
Pd/15Cs2.5H0.5PW12O40/ACA-SO3H
Pd/10Cs2.5
100
H0.5PW12O40/ACA-SO3H
200
300
400
500
600
700
Temperature (oC)
20
30
40
50
60
10
70
80
2 Theta (Degree)
Fig. 5. NH3-TPD profiles of Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25, and
Fig. 3. XRD patterns of Cs2.5H0.5PW12O40 and Pd/XCs2.5H0.5PW12O40/ACA-SO3H
(X = 10, 15, 20, 25, and 30 wt%).
30 wt%).
catalysts calculated by the Debye–Scherrer equation was in the
range of 10.2–10.7 nm, as listed in Table 1.
subsequent impregnation of Cs2.5H0.5PW12O40 onto ACA-SO3H.
Surface area and average pore size of Pd/XCs2.5H0.5PW12O40/ACA-
SO3H are summarized in Table 1. Surface area of
Pd/XCs2.5H0.5PW12O40/ACA-SO3H decreased from 1075 m2/g
Fig. 4 shows the TEM images of Pd/XCs2.5H0.5PW12O40/ACA-
SO3H (X = 10, 15, 20, 25, and 30 wt%) catalysts. Palladium metal
on XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25, and 30 wt%)
was observed by TEM image as a dark spot. Palladium metal
size of Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25,
and 30 wt%) measured by TEM images was in the range of
5–15 nm, in good agreement with XRD result (Fig. 3 and Table 1).
On the other hand, Cs2.5H0.5PW12O40 heteropolyacid (gray
area) was not observed in the Pd/10Cs2.5H0.5PW12O40/ACA-
SO3H and Pd/15Cs2.5H0.5PW12O40/ACA-SO3H catalysts due to
fine dispersion of Cs2.5H0.5PW12O40 onto ACA-SO3H. How-
ever, Cs2.5H0.5PW12O40 heteropolyacid (gray area) was clearly
observed in the Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 20,
25, and 30 wt%) catalysts due to excess impregnation of
(X = 10 wt%) to 948 m2/g (X = 30 wt%) with increasing
X
(Cs2.5H0.5PW12O40 content) due to the impregnation of
Cs2.5H0.5PW12O40 onto ACA-SO3H. Average pore size of
Pd/XCs2.5H0.5PW12O40/ACA-SO3H was in the range of 4.6–4.8 nm.
XRD
patterns
of
Cs2.5H0.5PW12O40
and
Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25, and 30 wt%)
are shown in Fig. 3. Diffraction peaks for Cs2.5H0.5PW12O40
were not observed in the Pd/10Cs2.5H0.5PW12O40/ACA-SO3H
and Pd/15Cs2.5H0.5PW12O40/ACA-SO3H catalysts due to fine
dispersion of Cs2.5H0.5PW12O40 onto ACA-SO3H. However,
Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 20, 25, and 30 wt%) cat-
alysts exhibited diffraction peaks for Cs2.5H0.5PW12O40 due to
excess impregnation of Cs2.5H0.5PW12O40 onto ACA-SO3H. This
was well supported by the fact that diffraction peak inten-
sity for Cs2.5H0.5PW12O40 increased with increasing X in the
Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 20–30 wt%). Successful
impregnation of palladium metal on XCs2.5H0.5PW12O40/ACA-
SO3H (X = 10, 15, 20, 25, and 30 wt%) was also confirmed by XRD
patterns, as shown in Fig. 3. Average palladium particle size of the
Cs2.5H0.5PW12O40
patterns of Pd/XCs2.5H0.5PW12O40/ACA-SO3H (Fig. 3).
Fig. shows the NH3-TPD profiles
. This result was well consistent with XRD
5
of
Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25, and
30 wt%) catalysts. Two desorption peaks were observed at
around 170 ◦C and 560 ◦C. Acidity of the catalysts measured
from NH3-TPD peak area is summarized in Table 1. Acidity of
Pd/XCs2.5H0.5PW12O40/ACA-SO3H catalysts was in the range of
Fig. 4. TEM images of Pd/XCs2.5H0.5PW12O40/ACA-SO3H: (a) Pd/10Cs2.5H0.5PW12O40/ACA-SO3H, (b) Pd/15Cs2.5H0.5PW12O40/ACA-SO3H, (c) Pd/20Cs2.5H0.5PW12O40/ACA-SO3H,
(d) Pd/25Cs2.5H0.5PW12O40/ACA-SO3H, and (e) Pd/30Cs2.5H0.5PW12O40/ACA-SO3H.

H.W. Park et al. / Applied Catalysis A: General 453 (2013) 287–294
291
Table 1
Surface area, average pore size, average palladium particle size, and acidity of Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25, and 30 wt%).
Pd/XCs2.5H0.5PW12O40/ACA-SO3H
X = 10
X = 15
X = 20
X = 25
X = 30
Surface area (m2/g)
1075
1002
988
4.6
10.2
246
965
4.6
10.7
211
948
4.7
10.4
194
Average pore sizea (nm)
Average palladium particle sizeb (nm)
Acidityc (␮mol-NH3/g)
4.7
10.7
189
4.8
10.3
258
a
Calculated by the BJH desorption branch.
Calculated by the Debye–Scherrer equation.
Determined by NH3-TPD.
b
c
175–258 ␮mol-NH3/g. Acidity of Pd/XCs2.5H0.5PW12O40/ACA-
(X = 10, 15, 20, 25, and 30 wt%) served as an efficient catalyst in the
SO3H catalysts showed a volcano-shaped trend with respect
decomposition of 4-phenoxyphenol.
to X (Cs2.5H0.5PW12O40 contents). Among the catalyst tested,
Pd/15Cs2.5H0.5PW12O40/ACA-SO3H catalyst showed the largest
acidity due to fine dispersion of Cs2.5H0.5PW12O40 onto ACA-SO3H,
as evidenced by XRD and TEM analyses.
3.3. Effect of X on the catalytic performance of
Pd/XCs2.5H0.5PW12O40/ACA-SO3H
Sulfur content of Pd/XCs2.5H0.5PW12O40/ACA-SO3H deter-
mined by CHNS analysis was in the range of 0.88–0.92 wt%,
as listed in Table 2. Cs2.5H0.5PW12O40 content in the
Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25, and
30 wt%) catalysts determined by ICP-AES analysis is also listed
in Table 2. The measured Cs2.5H0.5PW12O40 content in the
Pd/XCs2.5H0.5PW12O40/ACA-SO3H was smaller than the theoretical
value. This result indicates that Cs2.5H0.5PW12O40 located on the
outer surface of ACA-SO3H was removed during the washing step.
Palladium content of Pd/XCs2.5H0.5PW12O40/ACA-SO3H was in
good agreement with the designed value (Table 2).
Fig. 7 shows the conversion of 4-phenoxyphenol, total selec-
tivity for main products, and total yield for main products plotted
as a function of X in the Pd/XCs2.5H0.5PW12O40/ACA-SO3H. Con-
version of 4-phenoxyphenol showed a volcano-shaped curve with
respect to X. Total yield for main products also showed a volcano-
shaped curve with respect to X. Among the catalysts tested,
Pd/15Cs2.5H0.5PW12O40/ACA-SO3H showed the best catalytic per-
formance in terms of conversion of 4-phenoxyphel (94.8%) and total
yield for main products (67.5%).
Yields for cyclohexanol, benzene, and phenol plotted
as
are shown in Fig. 8. Yields for cyclohexanol and benzene
showed volcano-shaped curves with respect to in the
a function of X in the Pd/XCs2.5H0.5PW12O40/ACA-SO3H
X
3.2. Catalytic performance of Pd/XCs2.5
H0.5PW12O40/ACA-SO3H
Pd/XCs2.5H0.5PW12O40/ACA-SO3H. However, yield for phenol
(1.3–2.2%) was almost constant with regard to X. Among the
catalyst tested, Pd/15Cs2.5H0.5PW12O40/ACA-SO3H showed the
highest yields for cyclohexanol (31.2%) and benzene (35.0%).
in the decomposition of 4-phenoxyphenol
Fig.
6 shows the scheme for the decomposition of 4-
phenoxyphenol. Phenol is produced by the cleavage of C O bond
in the 4-phenoxyphenol [25]. According to the previous study
[18,19], phenol is then hydrogenated to 2-cyclohexen-1-ol as an
intermediate, and it is further hydrogenated to cyclohexanol over
palladium catalyst. Phenol is also directly converted to benzene
by hydrogenolysis. In our reaction system, phenol, benzene, and
cyclohexanol were mainly produced by the decomposition of 4-
phenoxypheol over Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15,
20, 25, and 30 wt%). Light hydrocarbons (C2–C6) and alcohols were
produced as by-products in the decomposition of 4-phenoxyphenol
(32.5–56.2%).
3.4. Correlation between acidity of
Pd/XCs2.5H0.5PW12O40/ACA-SO3H and catalytic performance in
the decomposition of 4-phenoxyphenol
Fig.
9
shows the correlation between conversion
over Pd/XCs2.5H0.5PW12O40/ACA-
of
4-phenoxyphenol
SO3H (X = 10, 15, 20, 25, and 30 wt%) and acidity of the
100
Catalytic performance of Pd/XCs2.5H0.5PW12O40/ACA-SO3H
(X = 10, 15, 20, 25, and 30 wt%) in the decomposition of
4-phenoxyphenol performed at 200 ◦C and 10 atm for 1 h
is listed in Table 3. Conversion of 4-phenoxyphenol over
Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25, and 30 wt%)
was in the range 82.1–94.8%. Total selectivity for main products
(cyclohexanol, benzene, and phenol) and total yield for main prod-
ucts were in the range of 53.3–71.2% and 43.8–67.5%, respectively.
The above results indicate that Pd/XCs2.5H0.5PW12O40/ACA-SO3H
80
60
40
Conversion of 4-phenoxyphenol
Total selectivity for main products
Total yield for main products
20
O
H2
2H2
H2
H2
2
HO
0
HO
HO
OH
10
15
20
25
30
X in Pd/XCs2.5H0.5PW12O40/ACA-SO3H
+ H2O
Fig. 7. Catalytic performance of Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25,
and 30 wt%) in the decomposition of 4-phenoxyphenol plotted as a function of X.
Reaction conditions: temperature = 200 ◦C, pressure = 10 atm (H2), time = 1 h.
Fig. 6. Scheme for the cleavage of 4-O-5 bond in 4-phenoxyphenol.

292
H.W. Park et al. / Applied Catalysis A: General 453 (2013) 287–294
Table 2
Sulfur, Cs2.5H0.5PW12O40, and Pd contents of Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25, and 30 wt%).
Catalyst
Sulfur content (wt%)a
Cs2.5H0.5PW12O40 content (wt%)
Pd content (wt%)
Theoretical
Theoretical
Measuredb
Measuredb
Pd/10Cs2.5H0.5PW12O40/ACA-SO3H
Pd/15Cs2.5H0.5PW12O40/ACA-SO3H
Pd/20Cs2.5H0.5PW12O40/ACA-SO3H
Pd/25Cs2.5H0.5PW12O40/ACA-SO3H
Pd/30Cs2.5H0.5PW12O40/ACA-SO3H
0.88
0.91
0.87
0.92
0.89
10
15
20
25
30
8.2
13.8
17.4
22.3
26.8
5
5
5
5
5
4.9
4.8
4.9
5.0
4.8
a
Determined by CHNS analysis.
Determined by ICP-AES analysis.
b
Table 3
Catalytic performance of Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25, and 30 wt%) in the decomposition of 4-phenoxyphenol.
Pd/XCs2.5H0.5PW12O40/ACA-SO3H
X = 10
X = 15
X = 20
X = 25
X = 30
Conversion (%)
82.1
94.8
88.3
87.9
84.0
Toal selectivity for main productsa (%)
Cyclohexanol (%)
Benzene (%)
53.3
27.2
24.1
2.0
71.2
32.9
36.9
1.4
61.6
32.6
27.0
2.0
58.9
31.0
25.4
2.5
56.2
28.3
25.3
2.6
Phenol (%)
Total yield for main productsb (%)
Cyclohexanol (%)
Benzene (%)
43.8
22.3
19.8
1.7
67.5
31.2
35.0
1.3
54.4
28.8
23.8
1.7
51.8
27.3
22.3
2.2
47.2
23.8
21.3
2.2
Phenol (%)
a
Calculated by Eq. (3).
Calculated by Eq. (4).
b
catalysts determined from NH3-TPD measurement (Table 1).
The correlation shows that conversion of 4-phenoxyphenol
over Pd/XCs2.5H0.5PW12O40/ACA-SO3H was closely related
to the acidity of Pd/XCs2.5H0.5PW12O40/ACA-SO3H. Conver-
sion of 4-phenoxyphenol increased with increasing acidity
of Pd/XCs2.5H0.5PW12O40/ACA-SO3H. Among the catalysts tested,
Pd/15Cs2.5H0.5PW12O40/ACA-SO3H with the largest acidity showed
the highest conversion of 4-phenoxyphenol.
Fig. 10 shows the correlation between total yield for main prod-
ucts over Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20, 25, and
30 wt%) and acidity of the catalysts. The correlation shows that total
yield for main products over Pd/XCs2.5H0.5PW12O40/ACA-SO3H was
also closely related to the acidity of Pd/XCs2.5H0.5PW12O40/ACA-
SO3H. Total yield for main products increased with increasing
acidity of Pd/XCs2.5H0.5PW12O40/ACA-SO3H. Among the catalysts
tested, Pd/15Cs2.5H0.5PW12O40/ACA-SO3H with the largest acid-
ity showed the highest total yield for main products. This result
was well consistent with the previous work [37] reporting that
the catalytic performance was increased with increasing acidity of
the supported catalyst. Thus, acidity of Pd/XCs2.5H0.5PW12O40/ACA-
SO3H played an important role in determining the catalytic
performance in the decomposition of 4-phenoxyphenol.
95
90
85
80
50
40
30
20
Yield for cyclohexanol
Yield for benzene
Yield for phenol
10
0
180
200
220
240
260
10
15
20
25
30
Acidity of Pd/XCs2.5H0.5PW12O40/ACA-SO3H
X in Pd/XCs2.5H0.5PW12O40/ACA-SO3H
Fig. 9. A correlation between conversion of 4-phenoxyphenol over
Pd/XCs2.5H0.5PW12O40/ACA-SO3H and acidity of the catalysts. Acidity of
Pd/XCs2.5H0.5PW12O40/ACA-SO3H was taken from Table 1. Reaction conditions:
temperature = 200 ◦C, pressure = 10 atm (H2), time = 1 h.
Fig. 8. Yields for cyclohexanol, benzene, and phenol produced by the decompo-
sition of 4-phenoxyphenol over Pd/XCs2.5H0.5PW12O40/ACA-SO3H (X = 10, 15, 20,
25, and 30 wt%). Reaction conditions: temperature = 200 ◦C, pressure = 10 atm (H2),
time = 1 h.

H.W. Park et al. / Applied Catalysis A: General 453 (2013) 287–294
293
70
60
50
40
Conversion of 4-phenoxyphenol
Total selectivity for main products
Total yield for main products
100
80
60
40
20
0
94.8
84.9
80.0
71.9
71.2
67.5
62.4
61.0
49.9
Pd/ACA
Pd/ACA-SO3H Pd/15Cs2.5
H
0.5PW12O40/ACA-SO3H
Catalyst
180
200
220
240
260
Acidity of Pd/XCs2.5H0.5PW12O40/ACA-SO3H
Fig. 11. Comparison of catalytic performance between Pd/ACA, Pd/ACA-SO3H,
and Pd/15Cs2.5H0.5PW12O40/ACA-SO3H catalysts in the decomposition of 4-
phenoxyphenol. Reaction conditions: temperature = 200 ◦C, pressure = 10 atm (H2),
time = 1 h.
Fig. 10. A correlation between total yield for main products over
Pd/XCs2.5H0.5PW12O40/ACA-SO3H and acidity of the catalysts. Acidity of
Pd/XCs2.5H0.5PW12O40/ACA-SO3H was taken from Table 1. Reaction conditions:
temperature = 200 ◦C, pressure = 10 atm (H2), time = 1 h.
(67.5%) over Pd/15Cs2.5H0.5PW12O40/ACA-SO3H were much higher
than those over Pd/ACA-SO3H (71.9% and 61.0%, respectively)
due to the large acidity of Pd/15Cs2.5H0.5PW12O40/ACA-SO3H
derived from the impregnation of Cs2.5H0.5PW12O40. In summary,
Pd/15Cs2.5H0.5PW12O40/ACA-SO3H catalyst was more efficient
than Pd/ACA and Pd/ACA-SO3H catalysts in the decomposition of
4-phenoxyphenol due to its large acidity.
3.5. Comparison of catalytic performance of Pd/ACA,
Pd/ACA-SO3H, and Pd/15Cs2.5H0.5PW12O40/ACA-SO3H in the
decomposition of 4-phenoxyphenol
For comparison, palladium catalyst supported on activated
carbon aerogel (Pd/ACA) and palladium catalyst supported on acti-
vated carbon aerogel bearing sulfonic acid (Pd/ACA-SO3H) were
prepared by an incipient wetness impregnation method [16,18].
Surface area of Pd/15Cs2.5H0.5PW12O40/ACA-SO3H (1002 m2/g)
was much higher than that of Pd/ACA (824 m2/g) and Pd/ACA-
SO3H (743 m2/g), as listed in Table 4. However, average pore
size of Pd/15Cs2.5H0.5PW12O40/ACA-SO3H (4.8 nm) was smaller
than that of Pd/ACA (6.7 nm) and Pd/ACA-SO3H (6.9 nm). Aver-
age palladium particle size of Pd/15Cs2.5H0.5PW12O40/ACA-SO3H,
Pd/ACA-SO3H, and Pd/ACA was in the range of 10.3–10.7 nm
with no great difference. Acidity of Pd/15Cs2.5H0.5PW12O40/ACA-
SO3H (258 ␮mol-NH3/g) measured by NH3-TPD was much larger
than that of Pd/ACA (34 ␮mol-NH3/g) and Pd/ACA-SO3H (79 ␮mol-
NH3/g) due to the impregnation of Cs2.5H0.5PW12O40 onto
ACA-SO3H.
3.6. Stability and reusability of
Pd/15Cs2.5H0.5PW12O40/ACA-SO3H catalyst
To investigate the stability and reproducibility of the cata-
lyst, recycle test for the decomposition of 4-phenoxyphenol over
Pd/15Cs2.5H0.5PW12O40/ACA-SO3H catalyst was performed three
times (Fig. 12). Fig. 12 shows that conversion of 4-phenoxyphenol
(94.8–93.6%) and total yield for main products (67.5–66.5%) of
fresh and spent catalysts were almost constant with regard
to recycle run. Furthermore, no significant Pd leaching was
detected by ICP-AES analysis after each run. This result shows that
Fig. 11 compares the catalytic performance of Pd/ACA,
Pd/ACA-SO3H, and Pd/15Cs2.5H0.5PW12O40/ACA-SO3H catalysts
in the decomposition of 4-phenoxyphenol. Conversion of 4-
phenoxyphenol (71.9%) and total yield for main products
(61.0%) over Pd/ACA-SO3H were much higher than those over
Pd/ACA (62.4% and 49.9%, respectively) due to the acidity of
Pd/ACA-SO3H derived from sulfonic acid. However, conversion
of 4-phenoxyphenol (94.8%) and total yield for main products
Conversion of 4-phenoxyphenol
Total selectivity for main products
Total yield for main products
100
94.8
94.2
93.6
80
60
40
20
0
71.1
71.2
70.8
67.5
66.7
66.5
Table 4
Surface area, average pore size, average palladium particle size, and acidity of
Pd/ACA, Pd/ACA-SO3H, and Pd/15Cs2.5H0.5PW12O40/ACA-SO3H.
Pd/ACA Pd/ACA-SO3H Pd/15Cs2.5H0.5PW12O40
ACA-SO3H
/
Surface area (m2/g)
824
6.7
743
6.9
1002
4.8
Average pore sizea (nm)
Average palladium particle 10.7
10.7
10.3
Run 1
Run 2
Run 3
sizeb (nm)
Recycle run
Acidityc (␮mol-NH3/g)
34
79
258
a
Calculated by the BJH desorption branch.
Calculated by the Debye–Scherrer equation.
Determined by NH3-TPD.
Fig.
12. Result
for
the
decomposition
of
4-phenoxyphenol
over
b
c
Pd/15Cs2.5H0.5PW12O40/ACA-SO3H catalyst with respect to recycle run. Reaction
conditions: temperature = 200 ◦C, pressure = 10 atm (H2), time = 1 h.

294
H.W. Park et al. / Applied Catalysis A: General 453 (2013) 287–294
Pd/15Cs2.5H0.5PW12O40/ACA-SO3H catalyst served as a stable and
reusable catalyst in the decomposition of 4-phenoxyphenol.
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(X = 10, 15, 20, 25, and 30 wt%) were in the range of 82.1–94.8%
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with the largest acidity showed the highest conversion of 4-
phenoxyphenol (94.8%) and total yield for main products (67.5%).
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phenoxyphenol. It is concluded that acidity of the catalysts played
an important role in determining the catalytic performance in the
decomposition of 4-phenoxyphenol.
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