Catalytic decomposition of 1,3-diphenoxybenzene to monomeric cyclic compounds over palladium catalysts supported on acidic activated carbon aerogels

Article abstract of DOI:10.1016/j.apcata.2013.02.020

Activated carbon aerogel (ACA) was prepared by a chemical activation of carbon aerogel using phosphoric acid (H₃PO₄). Activated carbon aerogel bearing sulfonic acid (ACA-SO₃H), Cs _2.5H_0.5PW₁₂O₄₀-impregnated activated carbon aerogel (Cs_2.5H_0.5PW₁₂O ₄₀/ACA), and Cs_2.5H_0.5PW₁₂O ₄₀-impregnated activated carbon aerogel bearing sulfonic acid (Cs_2.5H_0.5PW₁₂O₄₀/ACA-SO ₃H) were prepared in order to provide acid sites to ACA. Palladium catalysts were then supported on ACA, ACA-SO₃H, Cs _2.5H_0.5PW₁₂O₄₀/ACA, and Cs _2.5H_0.5PW₁₂O₄₀/ACA-SO₃H by an incipient wetness impregnation method. The prepared Pd/ACA, Pd/ACA-SO ₃H, Pd/Cs_2.5H_0.5PW₁₂O ₄₀/ACA, and Pd/Cs_2.5H_0.5PW₁₂O ₄₀/ACA-SO₃H catalysts were applied to the decomposition of 1,3-diphenoxybenzene. 1,3-Diphenoxybenzene was used as a trimeric lignin model compound for representing C-O bond in lignin. Cyclohexanol, benzene, and phenol were mainly produced by the decomposition of 1,3-diphenoxybenzene. 4-Phenoxyphenol was also produced as an intermediate by the decomposition of 1,3-diphenoxybenzene. Conversion of 1,3-diphenoxybenzene and total yield for main products (cyclohexanol, benzene, and phenol) increased with increasing acidity of the catalysts. Among the catalysts tested, Pd/Cs_2.5H _0.5PW₁₂O₄₀/ACA-SO₃H with the largest acidity showed the highest conversion of 1,3-diphenoxybenzene and total yield for main products. Pd/Cs_2.5H_0.5PW₁₂O ₄₀/ACA-SO₃H also served as a stable and reusable catalyst in the decomposition of 1,3-diphenoxybenzene.

Full text of DOI:10.1016/j.apcata.2013.02.020

Applied Catalysis A: General 456 (2013) 59–66
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Catalytic decomposition of 1,3-diphenoxybenzene to monomeric
cyclic compounds over palladium catalysts supported on acidic
activated carbon aerogels
Hai Woong Park, Jeong Kwon Kim, Ung Gi Hong, Yoon Jae Lee, Jung Ho Choi,
Yongju Bang, In Kyu Song∗
School of Chemical and Biological Engineering, Institute of Chemical Processes, 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) was prepared by a chemical activation of carbon aerogel using phosphoric
acid (H3PO4). Activated carbon aerogel bearing sulfonic acid (ACA-SO3H), Cs2.5H0.5PW12O40-impregnated
activated carbon aerogel (Cs2.5H0.5PW12O40/ACA), and Cs2.5H0.5PW12O40-impregnated activated car-
bon aerogel bearing sulfonic acid (Cs2.5H0.5PW12O40/ACA-SO3H) were prepared in order to provide
acid sites to ACA. Palladium catalysts were then supported on ACA, ACA-SO3H, Cs2.5H0.5PW12O40/ACA,
and Cs2.5H0.5PW12O40/ACA-SO3H by an incipient wetness impregnation method. The prepared Pd/ACA,
Pd/ACA-SO3H, Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H catalysts were applied to
the decomposition of 1,3-diphenoxybenzene. 1,3-Diphenoxybenzene was used as a trimeric lignin model
compound for representing C-O bond in lignin. Cyclohexanol, benzene, and phenol were mainly produced
by the decomposition of 1,3-diphenoxybenzene. 4-Phenoxyphenol was also produced as an intermediate
by the decomposition of 1,3-diphenoxybenzene. Conversion of 1,3-diphenoxybenzene and total yield for
main products (cyclohexanol, benzene, and phenol) increased with increasing acidity of the catalysts.
Among the catalysts tested, Pd/Cs2.5H0.5PW12O40/ACA-SO3H with the largest acidity showed the highest
conversion of 1,3-diphenoxybenzene and total yield for main products. Pd/Cs2.5H0.5PW12O40/ACA-SO3H
also served as a stable and reusable catalyst in the decomposition of 1,3-diphenoxybenzene.
© 2013 Elsevier B.V. All rights reserved.
Received 8 January 2013
Received in revised form 16 February 2013
Accepted 18 February 2013
Available online xxx
Keywords:
Acidic activated carbon aerogel
Heteropolyacid
Pd catalyst
Lignin decomposition
1,3-Diphenoxybenzene
1. Introduction
high temperature (over 450 ◦C) and pressure (over 100 atm) for
considerable performance [6,7]. On the other hand, two-step
First generation biofuels derived from corn sugar and vegetable
oil are environmentally friendly, but feedstocks for first genera-
tion biofuels are limited and compete with food resource [1,2].
However, second generation biofuels derived from lignocellulosic
feedstocks do not compete with food resource. Lignocellulosic is
mainly composed of three components; cellulose, hemicellulose,
and lignin. Among these components, lignin is a biopolymer syn-
thesized by polymerization of coumaryl alcohol, coniferyl alcohol,
and sinapyl alcohol. Lignin produced by delignification process in
the pulp industries is burned as a low value fuel for power genera-
tion [3,4]. Therefore, decomposition of lignin to aromatics becomes
as an advantageous technology for producing aromatics as biofuels
and chemicals, because lignin is the richest source of aromatics in
nature [5].
decomposition of lignin has attracted as a promising technology for
producing monomeric aromatics, because yields for aromatics via
two-step decomposition of lignin are higher than those via one-step
decomposition of lignin at low temperature and pressure [8,9]. In
the two-step decomposition, lignin is partially depolymerized over
base catalysts such as NaOH for producing oligomeric lignin units,
and subsequently, monomeric aromatics are produced by catalytic
cleavage of C-O bond in oligomeric lignin units.
Catalytic decomposition of lignin to aromatics has received
much attention as a key technology for valorization of lignocellu-
losic biomass [9–17]. 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 to aromatics [12–15]. Novel metal catalysts supported
on carbon (Pd/C, Pt/C, and Rh/C) [9] and novel metal catalysts sup-
ported on acidic activated carbon aerogel (ACA) (Pd/ACA-SO3H [16]
and Pd/Cs2.5H0.5PW12O40/ACA [17]) also showed considerable per-
formance for selective decomposition of C-O bond in lignin.
Lignin model compounds for representing C O and C C bonds
One-step decomposition of lignin such as thermal cracking and
hydrogenolysis of lignin over Ni-Mo and zeolite catalysts requires
∗
Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415.
E-mail address: inksong@snu.ac.kr (I.K. Song).
in lignin have been used as
a lignin feedstock due to the
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.02.020

60
H.W. Park et al. / Applied Catalysis A: General 456 (2013) 59–66
complex structure of lignin [18–20]. In particular, chemical com-
pounds containing aromatics linked by C O bonds 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, phenethylphenyl ether, ben-
zylphenyl ether, and 4-phenoxyphenol have been widely employed
as a lignin model compound for representing ␤-O-4, ␣-O-5, and
4-O-5 bonds in lignin, respectively [21,22]. In addition, chemical
of phosphoric acid. After stirring the mixture for 1 h, the solid was
dried at 110 ◦C for 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).
compounds containing more than three aromatics linked by C
O
2.2. Preparation of ACA-SO3H
bond have been used as an oligomeric lignin model compound [23].
Carbon aerogel (CA) [24–28] and activated carbon aerogel
(ACA) [29–34] have been employed as catalyst supports, electro-
chemical capacitors, and adsorbents due to their porous nature.
Pore structure of activated carbon aerogel can be controlled by
changing activation conditions such as activation agent, activa-
tion temperature, and activation time. Heteropolyacids (HPAs) are
inorganic acids. Acid strength of HPAs is stronger than that of con-
In order to provide acid sites to ACA, activated carbon aerogel
bearing sulfonic acid group (ACA-SO3H) was prepared by sul-
fonation of activated carbon aerogel (ACA) [16]. Fig. 1 shows the
schematic procedure for the preparation of ACA-SO3H. 1 g of acti-
vated 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 resid-
ual solid was dried at 110 ◦C for 5 h to obtain activated carbon
aerogel bearing sulfonic acid group (ACA-SO3H).
ventional solid acids [35–38]. HPA salts with K+, Cs+, and NH4
+
cations have high surface area and porous structure by forming
a tertiary structure [36,37]. It is known that acidity of cesium-
exchanged insoluble HPAs is different depending on cesium
content [35,36].
2.3. Preparation of Cs2.5H0.5PW12O40/ACA
In this work, carbon aerogel (CA) was prepared by a R-F
(resorcinol-formaldehyde) method [26,35], and subsequently,
CA was activated by H3PO4 to obtain an activated carbon aero-
gel. Activated carbon aerogel (ACA) containing acid sites were
prepared by sulfonation of ACA (ACA-SO3H) and by impregna-
tion of Cs2.5H0.5PW12O40 onto ACA (Cs2.5H0.5PW12O40/ACA).
Cs2.5H0.5PW12O40 was also impregnated on ACA-SO3H to
form Cs2.5H0.5PW12O40/ACA-SO3H. Palladium catalysts sup-
ported on ACA, ACA-SO3H, Cs2.5H0.5PW12O40/ACA, and
Cs2.5H0.5PW12O40/ACA-SO3H were then prepared by an incip-
ient wetness impregnation method, and they were applied to the
decomposition of 1,3-diphenoxybenzene. 1,3-Diphenoxybenzene
was chosen as a trimeric lignin model compound for representing
C-O bond in lignin. Three aromatics are linked by two C O bonds
in 1,3-diphenoxybenzene.
To provide acid sites to ACA, cesium-exchanged
Cs2.5H0.5PW12O40 heteropolyacid was impregnated on ACA
[17]. Fig. 1 also shows the schematic procedure for the preparation
of Cs2.5H0.5PW12O40/ACA. 15 wt% of H3PW12O40 (Sigma–Aldrich,
99.995%) was impregnated onto 1 g of activated carbon aerogel
(H3PW12O40/ACA) by an incipient wetness impregnation method,
and it was dried overnight at 80 ◦C. A known amount of cesium
nitrate (CsNO3, Sigma–Aldrich, 99%) was dissolved in 10 ml of DI
water. 1 g of H3PW12O40/ACA was then dispersed into the aqueous
solution containing cesium salt 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 Cs2.5H0.5PW12O40/ACA.
2.4. Preparation of Cs2.5H0.5PW12O40/ACA-SO3H
2. Experimental
Cesium-exchanged Cs2.5H0.5PW12O40 heteropolyacid was
impregnated on ACA-SO3H to form Cs2.5H0.5PW12O40/ACA-SO3H.
Fig. 1 also shows the schematic procedure for the prepara-
tion of Cs2.5H0.5PW12O40/ACA-SO3H. 15 wt% of H3PW12O40
(Sigma–Aldrich) was impregnated onto 1 g of ACA-SO3H
(H3PW12O40/ACA-SO3H) by an incipient wetness impregna-
tion 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 H3PW12O40/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 Cs2.5H0.5PW12O40/ACA-SO3H.
2.1. Preparation of carbon aerogel (CA) and activated carbon
aerogel (ACA)
Carbon aerogel (CA) and activated carbon aerogel (ACA) were
prepared according to the method in the literatures [26,35]. 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 two
days. Residual solvent was replaced with fresh acetone every 3 h
to remove water from the pore of RF wet gel. Ambient drying was
then done at room temperature and 50 ◦C for one day. Carbon aero-
gel (CA) was finally obtained by carbonization of RF gel at 500 ◦C
for 1 h.
2.5. Preparation of palladium catalysts supported on ACA,
ACA-SO3H, Cs2.5H0.5PW12O40/ACA, and
Cs2.5H0.5PW12O40/ACA-SO3H
5 wt% of palladium catalyst was supported on ACA, ACA-SO3H,
Cs2.5H0.5PW12O40/ACA, and Cs2.5H0.5PW12O40/ACA-SO3H by an
incipient wetness impregnation method. The supported palladium
catalysts were dried at 80 ◦C for 3 h, and they were calcined at
250 ◦C for 5 h. The supported catalysts were reduced with a mixed
stream of hydrogen (5 cc/min) and nitrogen (30 cc/min) at 120 ◦C
for 6 h to obtain Pd/ACA, Pd/ACA-SO3H, Pd/Cs2.5H0.5PW12O40/ACA,
and Pd/Cs2.5H0.5PW12O40/ACA-SO3H.
Activated carbon aerogel was prepared by a chemical activa-
tion of carbon aerogel with phosphoric acid (H3PO4) [35]. 2 g of
phosphoric acid (H3PO4) was dissolved in 10 ml of DI water, and
2 g of carbon aerogel was then dispersed into an aqueous solution

H.W. Park et al. / Applied Catalysis A: General 456 (2013) 59–66
61
Pd/ACA-SO3H
Pd/Cs2.5H0.5PW12O40/ACA-SO3H
Pd impregnation
Pd impregnation
Sulfonation
ACA
ACA-SO3H
Cs2.5H0.5PW12O40/ACA-SO3H
Cs2.5H0.5PW12O40 impregnation
Cs2.5H0.5PW12O40 impregnation
Cs2.5H0.5PW12O40/ACA
Pd impregnation
Pd/ACA
Pd/Cs2.5H0.5PW12O40/ACA
Pd impregnation
Fig. 1. Schematic procedures for the preparation of Pd/ACA, Pd/ACA-SO3H, Pd/Cs2.5H0.5PW12O40/ACA-SO3H, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H.
2.6. Characterization
Textural properties
were identified by a GC-MS (Agilent, 6890N GC) equipped with
a mass selective detector (Agilent, 5975 MSD). Conversion of
1,3-diphenoxybenzene and selectivity for product (cyclohexanol,
benzene, and phenol) were calculated by the following equations.
Yield for product (cyclohexanol, benzene, and phenol) was cal-
culated by multiplying conversion of 1,3-diphenoxybenzene and
corresponding product selectivity.
of
Pd/ACA,
Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
catalysts were examined by nitrogen adsorption–desorption
isotherm measurements (Micromeritics, ASAP 2010). Crystalline
states of Pd/ACA, Pd/ACA-SO3H, Pd/Cs2.5H0.5PW12O40/ACA, and
Pd/Cs2.5H0.5PW12O40/ACA-SO3H were determined by XRD (Rigaku,
D-MA2500-PC) measurements using Cu-K␣ radiation operated at
50 kV and 100 mA. Surface morphology and palladium dispersion
of the catalysts were investigated by TEM (Jeol, JXA-8900R).
NH3-TPD experiments (BEL Japan, BELCAT-B) were carried out
in order to measure the acidity of the catalysts. 0.04 g of each cata-
lyst 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.
conversion of 1, 3-diphenoxybenzene
moles of 1, 3-dipenoxybenzene reacted
=
(1)
(2)
moles of 1, 3-diphenoxybenzene supplied
selectivity for product (cyclohexanol, benzene, or phenol)
moles of cyclohexanol, benzene, or phenol formed
=
moles of 1, 3-diphenoxybenzene reacted
total selectivity for main products
total moles of cyclohexanol, benzene, and phenol formed
=
moles of 1, 3-diphenoxybenzene reacted
(3)
(4)
total yield for main products = (conversion of 1, 3-diphenoxybenzene) × (total selectivity for main products)
total moles of cyclohexanol, benzene, and phenol formed
=
moles of 1, 3-diphenoxybenzene supplied
2.7. Decomposition of 1,3-diphenoxybenzene
3. Results and discussion
Decomposition of 1,3-diphenoxybenzene (C18H14O2, Alfa Aesar,
99%) over Pd/ACA, Pd/ACA-SO3H, Pd/Cs2.5H0.5PW12O40/ACA, and
Pd/Cs2.5H0.5PW12O40/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 catalyst were charged into
the reactor. 0.2 g of 1,3-diphenoxybenzene was then added into the
reactor. The catalytic reaction was performed at 260 ◦C and 10 atm
(H2) for 5 h with agitation speed of 100 rpm. Reaction products
were analyzed with a gas chromatograph (Younglin, ACME 6100)
equipped with a DB-5 column and a FID. Main products and by-
products formed by the decomposition of 1,3-diphenoxybenzene
3.1. Characterization of Pd/ACA, Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
Textural
properties
of
Pd/ACA,
Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
catalysts were examined by nitrogen adsorption–desorption
isotherm
measurements.
Fig.
2
shows
the
N2
adsorption–desorption isotherms of Pd/ACA, Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
catalysts. All the samples showed type-IV isotherm with type-
H2 hysteresis loop. This result indicates that Pd/ACA-SO3H,

62
H.W. Park et al. / Applied Catalysis A: General 456 (2013) 59–66
Pd
Pd/ACA
Pd/ACA-SO3H
800
700
600
500
400
300
200
100
Pd/Cs2.5H0.5PW12O40 /ACA-SO3H
Pd/Cs2.5H0.5PW12O40 /ACA
Pd/Cs2.5
H
0.5PW12O40/ACA
Pd/Cs2.5
H
0.5PW12O40/ACA-SO3H
Pd/ACA-SO3H
Pd/ /ACA
20
30
40
50
60
10
70
80
0
0.2
0.4
0.6
0.8
1.0
2 Theta (Degree)
Relative pressure (P/P0)
Fig. 3. XRD patterns of Pd/ACA, Pd/ACA-SO3H, Pd/Cs2.5H0.5PW12O40/ACA, and
Pd/Cs2.5H0.5PW12O40/ACA-SO3H.
Fig. 2. N2 adsorption–desorption isotherms of Pd/ACA, Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA-SO3H, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H.
Diffraction peaks for Cs2.5H0.5PW12O40 were not observed in the
Pd/Cs2.5H0.5PW12O40/ACA and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
catalysts due to fine dispersion of Cs2.5H0.5PW12O40 onto ACA and
ACA-SO3H [17].
Table 1
Surface area, average palladium particle size, and acidity of Pd/ACA, Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H.
Catalyst
Surface
area (m2/g) particle sizea (nm)
Average palladium
Acidityb
(␮mol-NH3/g)
Fig.
4
shows the TEM images of Pd/ACA, Pd/ACA-SO3H,
and Pd/Cs2.5H0.5PW12O40/ACA-
Pd/Cs2.5H0.5PW12O40/ACA,
SO3H catalysts. Palladium metal on ACA, ACA-SO3H,
Cs2.5H0.5PW12O40/ACA, and Cs2.5H0.5PW12O40/ACA-SO3H was
Pd/ACA
Pd/ACA-SO3H
Pd/Cs2.5H0.5PW12O40/ACA
Pd/Cs2.5H0.5PW12O40/ACA- 1002
SO3H
824
743
1057
11.5
10.7
10.4
10.3
34
79
227
258
observed by TEM images as
a dark spot. Palladium metal
size of Pd/ACA, Pd/ACA-SO3H, Pd/Cs2.5H0.5PW12O40/ACA, and
Pd/Cs2.5H0.5PW12O40/ACA-SO3H measured by TEM images was in
the range of 10–13 nm, in good agreement with XRD result (Fig. 3
and Table 1). Cs2.5H0.5PW12O40 heteropolyacid was not observed
in the Pd/Cs2.5H0.5PW12O40/ACA and Pd/Cs2.5H0.5PW12O40/ACA-
SO3H catalysts due to fine dispersion of Cs2.5H0.5PW12O40 onto ACA
and ACA-SO3H. This result was well consistent with the previous
study [17] reporting that finely dispersed Cs2.5H0.5PW12O40 on ACA
and ACA-SO3H was not observed by XRD and TEM measurements.
a
Calculated by the Debye–Scherrer equation.
Determined by NH3-TPD.
b
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
still retained a porous structure even after either sulfonation of
ACA or impregnation of Cs2.5H0.5PW12O40 onto ACA. Surface
areas of Pd/ACA, Pd/ACA-SO3H, Pd/Cs2.5H0.5PW12O40/ACA,
and Pd/Cs2.5H0.5PW12O40/ACA-SO3H are summarized in
Table 1. Surface area of Pd/ACA (824 m2/g) was higher
than that of Pd/ACA-SO3H (743 m2/g). In addition, sur-
face area of Pd/Cs2.5H0.5PW12O40/ACA (1057 m2/g) and
Pd/Cs2.5H0.5PW12O40/ACA-SO3H (1002 m2/g) was much higher
than that of Pd/ACA (824 m2/g) due to impregnation of
Cs2.5H0.5PW12O40 onto ACA and ACA-SO3H.
Palladium
content
of
Pd/ACA,
Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
was in good agreement with the designed value, as listed in Table 2.
Sulfur content of Pd/ACA-SO3H and Pd/Cs2.5H0.5PW12O40/ACA-
SO3H determined by CHNS analysis was in the range of
0.87–0.94 wt% (Table 2). Cs2.5H0.5PW12O40 content in the
Pd/Cs2.5H0.5PW12O40/ACA and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
catalysts determined by ICP-AES analysis was in the range of
13.4–13.8 wt% (Table 2). This result indicates that Cs2.5H0.5PW12O40
was successfully impregnated on ACA and ACA-SO3H. The mea-
sured Cs2.5H0.5PW12O40 content in the Pd/Cs2.5H0.5PW12O40/ACA
and Pd/Cs2.5H0.5PW12O40/ACA-SO3H was slightly smaller than the
theoretical value. This is because Cs2.5H0.5PW12O40 located on
the outer surface of ACA and ACA-SO3H was removed during the
washing step.
XRD
patterns
of
Pd/ACA,
Pd/Cs2.5H0.5PW12O40/ACA-
Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA,
and
SO3H catalysts are shown in Fig. 3. Successful impregnation
of palladium metal on ACA, ACA-SO3H, Cs2.5H0.5PW12O40/ACA,
and Cs2.5H0.5PW12O40/ACA-SO3H was confirmed by XRD pat-
terns. Pd/ACA, Pd/ACA-SO3H, Pd/Cs2.5H0.5PW12O40/ACA, and
Pd/Cs2.5H0.5PW12O40/ACA-SO3H catalysts exhibited characteristic
XRD peaks for palladium at 39.8◦, 46.3◦, and 68.1◦. Average palla-
dium particle size of the catalysts calculated by the Debye–Scherrer
equation was in the range of 10.3–11.5 nm, as listed in Table 1.
Fig. 5 shows the NH3-TPD profiles of Pd/ACA, Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
Table 2
Pd content, sulfur content, and Cs2.5H0.5PW12O40 content of Pd/ACA, Pd/ACA-SO3H, Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H.
Catalyst
Pd content (wt%)a
Sulfur content (wt%)b
Cs2.5H0.5PW12O40 content (wt%)a
Theoretical
Measureda
Pd/ACA
Pd/ACA-SO3H
Pd/Cs2.5H0.5PW12O40/ACA
Pd/Cs2.5
4.9
4.8
4.9
4.9
–
0.94
–
–
–
15
15
13.4
13.8
H
0.5PW12O40/ACA-SO3H
0.87
a
Determined by ICP-AES analysis.
Determined by CHNS analysis.
b

H.W. Park et al. / Applied Catalysis A: General 456 (2013) 59–66
63
Fig. 4. TEM images of (a) Pd/ACA, (b) Pd/ACA-SO3H, (c) Pd/Cs2.5H0.5PW12O40/ACA, and (d) Pd/Cs2.5H0.5PW12O40/ACA-SO3H.
catalysts. Pd/Cs2.5H0.5PW12O40/ACA and Pd/Cs2.5H0.5PW12O40
/
containing both sulfonic acid and Cs2.5H0.5PW12O40 showed the
largest acidity.
ACA-SO3H catalysts showed two desorption peaks at around 170 ◦C
and 560 ◦C. One desorption peak of Pd/ACA-SO3H catalyst was
observed at around 220 ◦C. On the other hand, Pd/ACA showed
a very weak acid peak. Acidity of the catalysts measured from
NH3-TPD peak area is summarized in Table 1. Acidity of the cat-
alysts was in the range of 34–258 ␮mol NH3/g. Acidity of the
catalysts increased in order of Pd/ACA (34 ␮mol NH3/g) < Pd/ACA-
SO3H (79 ␮mol NH3/g) < Pd/Cs2.5H0.5PW12O40/ACA (227 ␮mol
NH3/g) < Pd/Cs2.5H0.5PW12O40/ACA-SO3H (258 ␮mol NH3/g). It
can be inferred that sulfonation of ACA and impregnation of
Cs2.5H0.5PW12O40 onto ACA were efficient to provide acid sites to
ACA. Among the catalysts tested, Pd/Cs2.5H0.5PW12O40/ACA-SO3H
3.2. Catalytic performance of Pd/ACA, Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H in
the decomposition of 1,3-diphenoxybenzene
Fig. 6 shows the scheme for the cleavage of C O bond in 1,3-
diphenoxybenzne. Monomeric cyclic compounds such as cyclohex-
anol, benzene, and phenol were mainly produced by the cleavage
of C-O bond in 1,3-diphenoxybenzene over Pd/ACA, Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H.
4-Phenoxyphenol was also produced as an intermediate (a dimeric
aromatic linked by C O bond) by the decomposition of 1,3-
diphenoxybenzene. Small amounts of light hydrocarbons (C2–C6)
and alcohols were produced as by-products in the decomposition
of 1,3-dipheoxybenzene.
Pd/Cs2.5H0.5PW12O40/ACA-SO3H
+
+
Pd/Cs2.5H0.5PW12O40/ACA
Pd/ACA-SO3H
O
OH
Cyclohexanol
OH
H2
Benzene
O
Phenol
O
Pd/ACA
+
1,3-Diphenoxybenzene
100
200
300
400
500
600
700
OH
Temperature (oC)
4-Phenoxyphenol
Fig. 5. NH3-TPD profiles of Pd/ACA, Pd/ACA-SO3H, Pd/Cs2.5H0.5PW12O40/ACA, and
Pd/Cs2.5H0.5PW12O40/ACA-SO3H.
Fig. 6. Scheme for the cleavage of C O bond in 1,3-diphenoxybenzene.

64
H.W. Park et al. / Applied Catalysis A: General 456 (2013) 59–66
Table 3
Catalytic performance of Pd/ACA, Pd/ACA-SO3H, Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H in the decomposition of 1,3-diphenoxybenzene.
Catalyst
Pd/ACA
Pd/ACA-SO3H
Pd/Cs2.5H0.5PW12O40/ACA
Pd/Cs2.5H0.5PW12O40/ACA-SO3H
Conversion (%)
17.4
66.0
9.1
46.6
10.3
27.6
25.4
66.5
9.4
51.8
5.3
78.1
85.6
12.2
68.8
4.6
95.1
89.7
9.2
75.8
4.7
Toal selectivity for main productsa (%)
Cyclohexanol (%)
Benzene (%)
Phenol (%)
Selectivity for 4-phenoxyphenol (%)
21.6
10.6
2.9
Total yield for main productsb (%)
Cyclohexanol (%)
Benzene (%)
Phenol (%)
Yield for 4-phenoxyphenol (%)
11.5
1.6
8.1
1.8
4.8
16.9
2.4
13.2
1.3
66.9
9.5
53.8
3.6
85.3
8.8
72.0
4.5
5.5
8.3
2.8
a
Calculated by Eq. (3).
Calculated by Eq. (4).
b
Catalytic
performance
of
Pd/ACA,
Pd/ACA-SO3H,
showed the best catalytic performance in terms of con-
version of 1,3-diphenoxybenzene and total yield for main
products.
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
in the decomposition of 1,3-diphenoxybenzene performed
at 260 ◦C and 10 atm for 5 h is listed in Table 3. Conver-
sion of 1,3-diphenoxybenzene over Pd/ACA, Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
catalysts was in the range 17.4–95.1%. Total selectivity for main
products (cyclohexanol, benzene, and phenol) and total yield for
main products were in the range of 66.0–89.7% and 11.5–85.3%,
respectively. Selectivity for 4-phenoxyphenol (a reaction interme-
diate) was in the range of 2.9–27.6%. Yield for benzene (8.1–72.0%)
was higher than that for cyclohexanol (1.6–9.5%) and phenol
(1.3–4.5%).
3.3. Correlations between acidity and product selectivity
Fig. 8 shows the correlations between acidity of the catalysts
and selectivity for main products (cyclohexanol, benzene, and
phenol) and 4-phenoxyphenol. Total selectivity for main prod-
ucts increased with increasing acidity of the catalysts. Among the
catalysts tested, Pd/Cs2.5H0.5PW12O40/ACA-SO3H with the largest
acidity showed the highest selectivity for main products (89.7%). On
the other hand, selectivity for 4-phenoxyphenol (a reaction inter-
mediate) decreased with increasing acidity of the catalysts. Among
the catalysts tested, Pd/Cs2.5H0.5PW12O40/ACA-SO3H with the
largest acidity showed the lowest selectivity for 4-phenoxyphenol
(2.9%). This result indicates that acidity of Pd/ACA, Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
catalysts played an important role in determining the selectivity
for main products and 4-phenoxyphenol. It can be summarized that
Pd/Cs2.5H0.5PW12O40/ACA-SO3H was the most efficient catalyst in
the decomposition of 1,3-diphenoxybenzene to monomeric cyclic
compounds (cyclohexanol, benzene, and phenol) due to its largest
acidity.
Fig.
7
compares
the
catalytic
performance
of
Pd/ACA,
Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA,
and
Pd/Cs2.5H0.5PW12O40/ACA-SO3H in the decomposition of
1,3-diphenoxybenzene to monomeric cyclic compounds. Con-
version of 1,3-diphenoxybenzene increased in order of Pd/ACA
(17.4%) < Pd/ACA-SO3H
(25.4%) < Pd/Cs2.5H0.5PW12O40/ACA
(78.1%) < Pd/Cs2.5H0.5PW12O40/ACA-SO3H (95.1%). Total selec-
tivity for main products and total yield for main products
increased in order of Pd/ACA (66.0% and 11.5%) < Pd/ACA-
SO3H (66.5% and 16.9%) < Pd/Cs2.5H0.5PW12O40/ACA (85.6%
and 66.9%) < Pd/Cs2.5H0.5PW12O40/ACA-SO3H (89.7% and 85.3%).
Among the catalysts tested, Pd/Cs2.5H0.5PW12O40/ACA-SO3H
30
20
10
0
100
90
80
70
60
Pd/ACA
Conversion of 1,3-diphenoxybenzene
Total selectivity for main products
Selectivity for 4-phenoxyphenol
100
80
60
40
20
0
95.1
89.7
Pd/Cs
PW
H PW O /ACA-SO H
85.6
85.3
Total yield for main products
78.1
Pd/ACA-SO H
Pd/Cs
H
O
/ACA
Pd/Cs
66.9
66.5
66.0
H
PW O /ACA
27.6
25.4
21.6
17.4
16.9
Pd/ACA-SO H
11.5
Pd/ACA
10.6
Pd/Cs
H PW O /ACA-SO H
2.9
Pd/Cs2.5H0.5PW12
0
50
100
150
200
250
300
O
40/ACA-SO3H
Pd/ACA
Pd/ACA-SO3H Pd/Cs2.5H0.5PW12O40/ACA
Acidity of catalyst (µmol-NH3/g)
Catalyst
Fig. 7. Comparison of catalytic performance of Pd/ACA, Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H in the decom-
position of 1,3-diphenoxybenzene. Reaction condition: temperature = 260 ◦C,
pressure = 10 atm (H2), time = 5 h.
Fig. 8. Correlations between acidity of the catalysts (Pd/ACA, Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H) and selectivity for
main products and 4-phenoxyphenol. Acidity of the catalysts was taken from Table 1.
Reaction condition: temperature = 260 ◦C, pressure = 10 atm (H2), time = 5 h.

H.W. Park et al. / Applied Catalysis A: General 456 (2013) 59–66
65
Conversion of 1,3-diphenoxybenzene
Pd/Cs2.5H0.5PW12O40/ACA-SO3H
Total selectivity for main products
Selectivity for 4-phenoxyphenol
Total yield for main products
100
80
60
40
20
100
80
60
40
20
Pd/Cs2.5H0.5PW12O40/ACA
100
80
95.1
94.4
94.5
89.7
88.3
88.9
85.3
83.4
84.0
Pd/Cs2.5H0.5PW12O40/ACA
60
40
20
0
Pd/ACA-SO3H
Pd/ACA
Pd/ACA-SO3H
Pd/ACA
50
0
0
0
100
150
200
250
300
3.0
2.9
3.1
Acidity of catalyst (µmol-NH3/g)
Run 1
Run 2
Run 3
Fig. 9. Correlations between catalytic performance and acidity of Pd/ACA, Pd/ACA-
SO3H, Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H catalysts.
Acidity of the catalysts was taken from Table 1. Reaction condition: tempera-
ture = 260 ◦C, pressure = 10 atm (H2), time = 5 h.
Recycle run
Fig. 10. Result for the decomposition of 1,3-diphenoxybenzene over
Pd/Cs2.5H0.5PW12O40/ACA-SO3H catalyst with respect to recycle run. Reaction
condition: temperature = 260 ◦C, pressure = 10 atm (H2), time = 5 h.
3.4. Correlation between acidity and catalytic performance in the
decomposition of 1,3-diphenoxybenzene
reusable catalyst in the decomposition of 1,3-diphenoxybenzene
to monomeric cyclic compounds.
Fig.
1,3-diphenoxybenzene
Pd/Cs2.5H0.5PW12 40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
9
shows the correlation between conversion
of
over Pd/ACA, Pd/ACA-SO3H,
4. Conclusions
O
and acidity of the catalysts. The correlation showed that con-
version of 1,3-diphenoxybenzene over Pd/ACA, Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
was closely related to the acidity of the catalysts. Conversion of
1,3-diphenoxybenzene increased with increasing acidity of the
catalysts. Among the catalysts tested, Pd/Cs2.5H0.5PW12O40/ACA-
SO3H with the largest acidity showed the highest conversion of
1,3-diphenoxybenzene.
Fig. 9 also shows the correlation between total yield for main
products over Pd/ACA, Pd/ACA-SO3H, Pd/Cs2.5H0.5PW12O40/ACA,
and Pd/Cs2.5H0.5PW12O40/ACA-SO3H and acidity of the catalysts.
The correlation showed that total yield for main products over the
catalysts was also closely related to the acidity of the catalysts. Total
yield for main products increased with increasing acidity of the
catalysts. Among the catalysts tested, Pd/Cs2.5H0.5PW12O40/ACA-
SO3H with the largest acidity showed the highest total yield for
main products. This result was well consistent with the previ-
ous works [14–17,35] reporting that the catalytic performance
in the decomposition of lignin model compounds increased with
increasing acidity of the supported catalysts. Thus, acidity of the
catalysts played an important role in determining the catalytic
performance in the decomposition of 1,3-diphenoxybenzene to
monomeric cyclic compounds.
Carbon aerogel (CA) was prepared by
a R-F (resorcinol-
formaldehyde) method, and subsequently, carbon aerogel was
activated by H3PO4 to obtain activated carbon aerogel (ACA).
Activated carbon aerogels (ACA) containing acid sites were
prepared by either sulfonation of ACA (ACA-SO3H) or impreg-
nation of Cs2.5H0.5PW12O40 onto ACA (Cs2.5H0.5PW12O40/ACA).
Cs2.5H0.5PW12O40 was also impregnated on ACA-SO3H to form
Cs2.5H0.5PW12O40/ACA-SO3H. Palladium catalysts supported
on acidic activated carbon aerogel (Pd/ACA, Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H)
were then prepared by an incipient wetness impregnation
method, and they were applied to the decomposition of 1,3-
diphenoxybenzene. Cyclohexanol, benzene, and phenol were
mainly produced by the decomposition of 1,3-diphenoxybenzene.
4-Phenoxyphenol was also produced as a reaction intermediate
by the decomposition of 1,3-diphenoxybenzene. Conversion
of 1,3-diphenoxybenzene and total yield for main products
(cyclohexanol, benzene, and phenol) over Pd/ACA, Pd/ACA-SO3H,
Pd/Cs2.5H0.5PW12O40/ACA, and Pd/Cs2.5H0.5PW12O40/ACA-SO3H
were in the range of 17.4–95.1% and 11.5–85.3%, respectively. Con-
version of 1,3-diphenoxybenzene and total yield for main products
increased with increasing acidity of the catalysts. Among the
catalysts tested, Pd/Cs2.5H0.5PW12O40/ACA-SO3H with the largest
acidity showed the highest conversion of 1,3-diphenoxybenzene
(95.1%) and total yield for main products (85.3%). Total selectivity
for main products increased with increasing acidity of the catalysts,
while selectivity for 4-phenoxyphenol (a reaction intermediate)
decreased with increasing acidity of the catalysts. Among the
catalysts tested, Pd/Cs2.5H0.5PW12O40/ACA-SO3H catalyst with
the largest acidity showed the highest selectivity for main prod-
ucts (89.7%) and the lowest selectivity for 4-phenoxyphenol
(2.9%). It was also found that Pd/Cs2.5H0.5PW12O40/ACA SO3H
was an efficient and reusable catalyst in the decomposition of
1,3-diphenoxybenzene. It is concluded that acidity of the catalysts
played an important role in determining the catalytic performance
in the decomposition of 1,3-diphenoxybenzene to monomeric
cyclic compounds (cyclohexanol, benzene, and phenol).
3.5. Stability and reusability of Pd/Cs2.5H0.5PW12O40/ACA-SO3H
catalyst in the decomposition of 1,3-diphenoxybenzene
To
investigate
the
stability
and
reusability
of
Pd/Cs2.5H0.5PW12O40/ACA-SO3H catalyst, recycle test for the
decomposition of 1,3-diphenoxybenzene to main products
(cyclohexanol, benzene, and phenol) was performed three times
(Fig. 10). Fig. 10 showed that conversion of 1,3-diphenoxybenzene
(95.1–94.4%) and total yield for main products (85.3–83.4%) 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 indicates that
Pd/Cs2.5H0.5PW12O40/ACA-SO3H catalyst served as a stable and

66
H.W. Park et al. / Applied Catalysis A: General 456 (2013) 59–66
Acknowledgements
[16] H.W. Park, U.G. Hong, Y.J. Lee, I.K. Song, Catal. Commun. 20 (2012) 89–93.
[17] H.W. Park, U.G. Hong, Y.J. Lee, J.H. Choi, I.K. Song, Appl. Catal. A: Gen. 437–438
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This work was supported by the National Research Foundation
of Korea grant funded by the Korean government (MEST)(NRF-
2009-C1AAA001-0093292). This work was also supported by
Mid-career Researcher Program of National Research Foundation
(NRF) grant funded by the Korea government (MEST) (No. 2012-
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