Decomposition of 4-phenoxyphenol to aromatics over palladium catalyst supported on activated carbon aerogel

Article abstract of DOI:10.1016/j.apcata.2011.09.043

Carbon aerogel (CA) was prepared by a sol-gel polymerization of resorcinol and formaldehyde, and a series of activated carbon aerogels (ACA-H ₃PO₄-X, X = 0, 0.5, 1.0, 2.0, and 3.0) were prepared by a chemical activation using different amount of phosphoric acid (X represented weight ratio of H₃PO₄ with respect to CA). Palladium catalysts were then supported on activated carbon aerogels (Pd/ACA-H ₃PO₄-X, X = 0, 0.5, 1.0, 2.0, and 3.0) by an incipient wetness impregnation method for use in the decomposition of 4-phenoxyphenol to aromatics. 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. Conversion of 4-phenoxyphenol and total yield for main products (cyclohexanol, benzene, and phenol) were closely related to the average palladium particle size of Pd/ACA-H₃PO ₄-X. Conversion of 4-phenoxyphenol and total yield for main products increased with decreasing average palladium particle size of Pd/ACA-H ₃PO₄-X. Among the catalysts tested, Pd/ACA-H ₃PO₄-1.0 with the smallest average palladium particle size 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/ACA-H₃PO₄-1.0 were much higher than those over palladium catalyst supported on commercial activated carbon (Pd/AC).

Full text of DOI:10.1016/j.apcata.2011.09.043

Applied Catalysis A: General 409–410 (2011) 167–173
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Decomposition of 4-phenoxyphenol to aromatics over palladium catalyst
supported on activated carbon aerogel
Hai Woong Park, Ung Gi Hong, Yoon Jae Lee, 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:
Carbon aerogel (CA) was prepared by a sol–gel polymerization of resorcinol and formaldehyde, and a
series of activated carbon aerogels (ACA-H3PO4-X, X = 0, 0.5, 1.0, 2.0, and 3.0) were prepared by a chem-
ical activation using different amount of phosphoric acid (X represented weight ratio of H3PO4 with
respect to CA). Palladium catalysts were then supported on activated carbon aerogels (Pd/ACA-H3PO4-X,
X = 0, 0.5, 1.0, 2.0, and 3.0) by an incipient wetness impregnation method for use in the decomposition of
4-phenoxyphenol to aromatics. 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. Conversion of 4-phenoxyphenol and total yield for main products (cyclohexanol,
benzene, and phenol) were closely related to the average palladium particle size of Pd/ACA-H3PO4-X.
Conversion of 4-phenoxyphenol and total yield for main products increased with decreasing average pal-
ladium particle size of Pd/ACA-H3PO4-X. Among the catalysts tested, Pd/ACA-H3PO4-1.0 with the smallest
average palladium particle size 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/ACA-H3PO4-
1.0 were much higher than those over palladium catalyst supported on commercial activated carbon
(Pd/AC).
Received 23 June 2011
Received in revised form
28 September 2011
Accepted 29 September 2011
Available online 6 October 2011
Keywords:
Activated carbon aerogel
Pd catalyst
Lignin
C–O bond
4-Phenoxyphenol
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
cleavage of C–O bond in lignin such as ␤-O-4, ␣-O-4, and 4-O-
5 bonds are known to be an advantageous technology for the
Biomass has attracted attention as a renewable energy source
due to the depletion of fossil fuel [1–4]. Lignocellulose is typically
composed of three main components; cellulose, hemicellulose, and
lignin. Lignin produced by delignification process in the pulp indus-
tries is burned as a low value fuel for power generation [5–7].
Lignin is an amorphous polymer produced by polymerization of
coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. Aromatic
compounds in the lignin are mainly linked by C–O and C–C bonds.
Decomposition of lignin to aromatic compounds has been con-
ducted by thermal cracking and catalytic decomposition processes
[8,9].
Many attempts have been made on the catalytic decomposition
of lignin to aromatics, because thermal cracking of lignin requires
high temperature (over 450 ◦C) and pressure (over 100 atm) for
considerable performance [10–13]. Hydrocracking of lignin over
zeolite, Co–Mo/Al2O3, and Ni–Mo/Al2O3 cleaves C–O bond and
relatively weak C–C bond in lignin inner units [14,15]. Selective
decomposition of lignin to aromatics [16]. Cesium-exchanged het-
eropolyacid (CsxH3.0−xPW12O40, X = 2.0–3.0) is known to be an
efficient catalyst for selective decomposition of C–O bond in lignin
model compound containing ␤-O-4 and ␣-O-4 bonds [17,18]. Novel
metal catalysts supported on activated carbon (Pd/C, Pt/C, and Rh/C)
also showed a considerable performance for selective decomposi-
tion of C–O bond in lignin. [19,20].
Dimeric lignin model compounds for representing C–O and C–C
bonds in lignin have been used as a lignin feedstock due to the
complex structure of lignin [21–23]. Therefore, dimeric chemi-
cal 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
[24,25].
Carbon aerogel has received much attention as catalyst support,
electrochemical capacitor, and adsorbent, because of its porous
nature [26–30]. Carbon aerogel has a network structure of primary
carbon particles, providing micropore and mesopore. Micropore is
related to the intra-particle structure, while mesopore and macro-
pore are produced by the inter-particle structure. Pore structure of
∗
Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415.
E-mail address: inksong@snu.ac.kr (I.K. Song).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.09.043

168
H.W. Park et al. / Applied Catalysis A: General 409–410 (2011) 167–173
Activated
carbon aerogel
(ACA)
Carbonization
Pyrolysis at 500 oC
Activation
800 oC and N2
Mixing with
H3PO4
Carbon
aerogel
RF wet
gel
Fig. 1. Preparation of carbon aerogel and activated carbon aerogel.
carbon aerogel can be controlled by activation treatment of carbon
aerogel. Activation of carbon aerogel by KOH, ZnCl2, steam, and CO2
increases surface area and pore volume of carbon aerogel due to the
formation of micropore and mesopore on carbon aerogel [31–37].
In this work, carbon aerogel (CA) was prepared by a sol–gel poly-
merization of resorcinol and formaldehyde, and subsequently, it
was activated using different amount of H3PO4 (ACA-H3PO4-X, X
(weight ratio of H3PO4 with respected to CA) = 0, 0.5, 1.0, 2.0, and
3.0) for use as a catalyst support. Palladium catalysts supported on
activated carbon aerogel (Pd/ACA-H3PO4-X, X = 0, 0.5, 1.0, 2.0, and
3.0) were prepared by an incipient wetness impregnation method,
and were applied to the decomposition of 4-phenoxyphenol to aro-
matics. 4-Phenoxyphenol was chosen as a lignin model compound
for representing 4-O-5 bond in lignin. For comparison, palladium
catalyst supported on commercial activated carbon (Pd/AC) was
also employed for the decomposition of 4-phenoxyphenol to aro-
matics. This is the first example reporting the catalytic performance
of Pd/ACA-H3PO4-X for the decomposition of lignin model com-
pound to aromatics.
2.2. Preparation of palladium catalyst supported on activated
carbon aerogel (Pd/ACA-H3PO4-X)
5 wt% of palladium catalyst was supported on activated carbon
aerogel (ACA-H3PO4-X, X = 0, 0.5, 1.0, 2.0, and 3.0) by an incipi-
ent wetness impregnation method. Palladium nitrate (Pd(NO3)2,
Sigma–Aldrich) as a palladium precursor was dissolved in DI water,
and it was then introduced into the pores of ACA-H3PO4-X (X = 0,
0.5, 1.0, 2.0, and 3.0). 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 hydro-
gen (5 cc/min) and nitrogen (30 cc/min) at 120 ◦C for 6 h to obtain
Pd/ACA-H3PO4-X (X = 0, 0.5, 1.0, 2.0, and 3.0). For comparison,
5 wt% palladium catalysts supported on AC-H3PO4-1.0 (Pd/AC-
H3PO4-1.0) and commercial activated carbon (Pd/AC, AC: MSP20,
Kanshai) were also prepared by an incipient wetness impregnation
method.
2.3. Characterization
2. Experimental
Textural properties of Pd/ACA-H3PO4-X (X = 0, 0.5, 1.0, 2.0, and
3.0) catalysts were determined by nitrogen adsorption–desorption
isotherm measurements (ASAP 2010, Micromeritics). Surface mor-
phology and palladium dispersion of Pd/ACA-H3PO4-X catalysts
were investigated by transmission electron microscopy (TEM, Jeol,
JXA-8900R). Crystalline states of Pd/ACA-H3PO4-X were deter-
mined by XRD (D-MA2500-PC, Rigaku) measurements using Cu K␣
radiation operated at 50 kV and 100 mA. Average palladium par-
ticle size of Pd/ACA-H3PO4-X catalysts was calculated using the
Debye–Scherrer equation.
2.1. Preparation of carbon aerogel and activated carbon aerogel
(ACA-H3PO4-X)
Carbon aerogel and activated carbon aerogel (ACA) were pre-
pared as presented in Fig. 1. Carbon aerogel was prepared by a
sol–gel polymerization of resorcinol and formaldehyde accord-
ing to the method in literature [30]. 25.9 g of resorcinol (C6H6O2,
Sigma–Aldrich) was dissolved in 60.0 ml of DI water. Aqueous
resorcinol was mixed with sodium carbonate (0.05 g) (a base
catalyst) to accelerate dehydrogenation of resorcinol. After stir-
ring the solution for a few minutes, 14.1 g of formaldehyde
(H2CO, Sigma–Aldrich) was added slowly into the solution to
form a sol. Molar ratio of resorcinol with respect to formalde-
hyde was fixed at 1:2 (R/F = 1/2). R/C (resorcinol/catalyst) ratio
was fixed at 500. The resulting sol was cured in a vial to pro-
duce resorcinol–formaldehyde (RF) gel at 80 ◦C for 24 h. Solvent
exchange was performed with acetone at 50 ◦C for two days. Resid-
ual 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 one day. Carbon aero-
gel was finally obtained by carbonization of RF gel at 500 ◦C for
1 h.
Activated carbon aerogel was prepared by a chemical activation
of carbon aerogel with phosphoric acid (H3PO4). The weight ratio
(X) of H3PO4 as an activation agent with respect to carbon aero-
gel was fixed at 0, 0.5, 1.0, 2.0, and 3.0. A known amount of H3PO4
was dissolved in 10 ml of DI water, and subsequently, carbon aero-
gel (1 g) was dispersed into an aqueous solution of H3PO4. After
stirring the mixture for 1 h, the solid was recovered and dried at
110 ◦C for 3 h. The resulting solid was allowed to react at 800 ◦C for
1 h under N2 stream. The activated carbon aerogel containing phos-
phoric acid was washed with DI water till the pH value of solution
reached ca. 7. The resulting material was then dried at 110 ◦C for 5 h
to obtain activated carbon aerogel (ACA-H3PO4-X, X = 0, 0.5, 1.0, 2.0,
and 3.0). For comparison, commercial activated carbon activated by
phosphoric acid (AC-H3PO4-1.0) was prepared by a chemical acti-
vation of commercial activated carbon (AC: MSP20, Kanshai) with
phosphoric acid.
2.4. Decomposition of 4-phenoxyphenol
Decomposition
of
4-phenoxyphenol
(C6H5OC6H4OH,
Sigma–Aldrich) over Pd/ACA-H3PO4-X (X = 0, 0.5, 1.0, 2.0, and
3.0) and Pd/AC 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 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 flame ionization detector. Conversion of
4-phenoxyphenol and selectivity for product (cyclohexanol, ben-
zene, and phenol) were calculated by following equations. Yield
for product (cyclohexanol, benzene, and phenol) was calculated
by multiplying conversion of 4-phenoxyphenol and corresponding
product selectivity.
Conversion of 4-phenoxyphenol
moles of 4-phenoxyphenol reacted
moles of 4-phenoxyphenol supplied
=
(1)
Selectivity for product (cyclohexanol, benzene, or phenol)
moles of cyclohexanol, benzene, or phenol formed
=
(2)
moles of 4-phenoxyphenol reacted

H.W. Park et al. / Applied Catalysis A: General 409–410 (2011) 167–173
169
800
700
600
500
400
300
200
100
Pd/ACA-H3PO4-0
Pd/ACA-H3PO4-0.5
Pd/ACA-H3PO4-1.0
Pd/ACA-H3PO4-2.0
Pd/ACA-H3PO4-3.0
Pd/ACA-H3PO4-3.0
Pd/ACA-H3PO4-2.0
Pd/ACA-H3PO4-1.0
Pd/ACA-H3PO4-0.5
Pd/ACA-H3PO4-0
0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/Po)
70
80
10
20
30
40
50
60
Fig. 2. N2 adsorption–desorption isotherms of Pd/ACA-H3PO4-X.
2 Theta (Degree)
Fig. 3. XRD patterns of Pd/ACA-H3PO4-X reduced at 120 ◦C for 6 h.
Total selectivity for main products
Total moles of cyclohexanol, benzene, and phenol formed
Table 2
Average palladium particle size of Pd/ACA-H3PO4-X (X = 0, 0.5, 1.0, 2.0, and 3.0).
=
moles of 4-phenoxyphenol reacted
(3)
Catalysts
TEMa (nm)
XRDb (nm)
Total yield for main products
Pd/ACA-H3PO4-0
Pd/ACA-H3PO4-0.5
Pd/ACA-H3PO4-1.0
Pd/ACA-H3PO4-2.0
Pd/ACA-H3PO4-3.0
17.8
16.9
9.4
13.4
16.1
20.2
19.7
11.5
14.8
18.2
= (Conversion of 4-phenoxyphenol)
×(Total selectivity for main products)
(4)
a
Measured from TEM image.
Calulated by the Debye–Scherrer equation.
b
3. Results and discussion
3.1. Characterization of Pd/ACA-H3PO4-X (X = 0, 0.5, 1.0, 2.0, and
3.0)
mesopore surface area (Smeso) and ratio of mesopore surface area
with respect to micropore surface area (Smeso/Smicro) showed
maximum values (492 m2/g and 1.48, respectively) at X = 1.0.
Average pore size calculated by the BJH desorption curve increased
in the order of Pd/ACA-H3PO4-3.0 (3.3 nm) < Pd/ACA-H3PO4-
Textural
properties
of
Pd/ACA-H3PO4-X
(X = 0,
0.5,
1.0, 2.0, and 3.0) catalysts were examined by nitrogen
adsorption–desorption isotherm measurements. Fig. shows
2
2.0
(3.7 nm) < Pd/ACA-H3PO4-0
(3.9 nm) < Pd/ACA-H3PO4-0.5
the N2 adsorption–desorption isotherms of Pd/ACA-H3PO4-X
catalysts. All the samples showed type-IV isotherm with type-
H2 hysteresis loop. This indicates that Pd/ACA-H3PO4-X still
retained porous structure even after the activation of carbon
aerogel with H3PO4. Surface area (Stotal, Smeso, and Smicro), ratio
of mesopore surface area with respect to micropore surface area
(Smeso/Smicro), and average pore size of Pd/ACA-H3PO4-X (X = 0, 0.5,
(4.5 nm) < Pd/ACA-H3PO4-1.0 (6.7 nm).
Successful impregnation of palladium metal on ACA-H3PO4-X
(X = 0, 0.5, 1.0, 2.0, and 3.0) was confirmed by XRD analysis, as
shown Fig. 3. Pd/ACA-H3PO4-X exhibited characteristic XRD peaks
at 39.8◦, 46.3◦, and 68.1◦. This result indicates that metallic palla-
dium was well formed after reduction process employed in this
work. Characteristic carbon aerogel peak was also observed at
23.5◦. This result indicates that the structure of carbon aerogel was
still maintained even after the activation of carbon aerogel with
H3PO4.
1.0, 2.0, and 3.0) are summarized in Table 1. Surface area (Stotal
)
of Pd/ACA-H3PO4-X increased from 640 m2/g (X = 0) to 1169 m2/g
(X = 3.0) with increasing X (weight ratio of H3PO4 with respect
to CA), because micro surface area (Smicro) of Pd/ACA-H3PO4-X
increased with increasing X from 284 m2/g (X = 0) to 778 m2/g
(X = 3.0). This result implies that textural structure of carbon
aerogel was changed upon chemical activation with phosphoric
acid, because carbon aerogel was partially burned off in the
presence of phosphoric acid. As a result, micropore and meso-
pore were created during the activation process. Micropore was
increased with increasing the amount of phosphoric acid. However,
Fig.
4 shows the TEM images of Pd/ACA-H3PO4-X (X = 0,
0.5, 1.0, 2.0, and 3.0) catalysts. Carbon particles in Pd/ACA-
H3PO4-X were observed to form an interconnecting network
structure with non-uniform textural porosity. Average palla-
dium particle sizes of Pd/ACA-H3PO4-X calculated using the
Debye–Scherrer equation and measured from TEM images are
listed in Table 2. Average palladium particle size calculated using
the Debye–Scherrer equation decreased in order of Pd/ACA-H3
Table 1
Textural properties of Pd/ACA-H3PO4-X (X = 0, 0.5, 1.0, 2.0, and 3.0).
X in Pd/ACA-H3PO4-X
X = 0
X = 0.5
X = 1.0
X = 2.0
X = 3.0
Surface area (Stotal) (m2/g)
Micro surface area (Smicro) (m2/g)
Meso surface area (Smeso) (m2/g)
Smeso/Smicro
640
284
356
1.25
3.9
682
301
381
1.27
4.5
824
332
492
1.48
6.7
972
593
379
0.72
3.7
1169
778
391
0.50
3.3
Average pore size (nm)

170
H.W. Park et al. / Applied Catalysis A: General 409–410 (2011) 167–173
Fig. 4. TEM images of Pd/ACA-H3PO4-X: (a) Pd/ACA-H3PO4-0, (b) Pd/ACA-H3PO4-0.5, (c) Pd/ACA-H3PO4-1.0, (d) Pd/ACA-H3PO4-2.0, and (e) Pd/ACA-H3PO4-3.0.
PO4-0 (20.2 nm) > Pd/ACA-H3PO4-0.5 (19.7 nm) > Pd/ACA-H3PO4-
3.0 (18.2 nm) > Pd/ACA-H3PO4-2.0 (14.8 nm) > Pd/ACA-H3PO4-1.0
(11.5 nm). This trend was well consistent with the trend of aver-
age palladium particle size measured from TEM image. Among
the palladium catalysts supported on ACA-H3PO4-X (X = 0, 0.5, 1.0,
2.0, and 3.0), palladium supported on ACA-H3PO4-1.0 was most
finely dispersed on activated carbon aerogel. Average particle size
of supported metal catalysts is affected by many factors such as
preparation condition, the amount of metal loading, pore structure,
and physical–chemical properties of support [38]. It is expected
that pore structure may affect the formation of different metal
size on carbon, because the same preparation method and metal
loading were imposed in all the catalysts. It has been accepted
that finely dispersed metal particle supported on carbon can be
easily obtained with mesoporous carbon material, because meso-
pore of porous carbon serves as an individual nanoscale reactor
or a barrier for metal aggregation [38]. On the contrary, aggre-
gated metal particle will be formed with microporous carbon
material, because metal cannot be easily introduced into micro-
pore of carbon. Therefore, average pore size of carbon and ratio
of mesopore surface area with respect to micropore surface area
(Smeso/Smicro) play key roles in determining metal dispersion [38].
In our supported catalyst system, micropore of palladium catalyst
supported on activated carbon aerogel (Pd/ACA-H3PO4-X, X = 0, 0.5,
1.0. 2.0, and 3.0) increased with increasing the amount of phospho-
ric acid as listed Table 1. Formation of mesopore of Pd/ACA-H3PO4-X
showed the maximum at X = 1.0, because mesopore of activated
carbon aerogel was converted into micropore when X > 2.0. There-
fore, Pd/ACA-H3PO4-1.0 with the largest average pore size (6.7 nm)
and the highest Smeso/Smicro (1.48) retained most finely dispersed
palladium, because mesoporosity of ACA-H3PO4-1.0 served as an
individual nanoscale reactor or a barrier for metal aggregation. On
the contrary, microporosity of ACA-H3PO4-2.0 and ACA-H3PO4-3.0
may affect the formation of agglomerated metal on carbon, because
metal cannot be easily introduced into micropore of carbon.
ICP-AES analysis (Shimadz, ICP-1000 IV) revealed that phos-
phoric acid did not remain in the final catalyst (Pd/ACA-H3PO4-X),
because phosphoric acid used as a chemical activation agent was
removed during the activation and washing steps.
3.2. Catalytic performance in the decomposition of
4-phenoxyphenol to aromatics
As preliminary experiments, cesium-exchanged heteropoly-
acid catalyst (CsxH3.0−xPW12O40
catalyst supported on cesium-exchanged heteropolyacid
(Pd/CsxH3.0−xPW12O40 X = 2.0–3.0), which were highly active
, X = 2.0–3.0) and palladium
,
for the decomposition of benzyl phenyl ether (a model compound
containing ␣-O-4 bond in lignin) and phenethyl phenyl ether
(a model compound containing ␤-O-4 bond in lignin) [17,18],
were tested in the decomposition of 4-phenoxyphenol. However,
these catalysts showed very low catalytic performance in the
decomposition of 4-phenoxyphenol (a compound containing
4-O-5 bond in lignin). These results are well consistent with the
previous study [39] reporting that activation energy for the decom-
position of 4-O-5 bond in lignin inner unit is much higher than
that for the decomposition of ␤-O-4 and ␣-O-4 bonds. Therefore,
many attempts have been made to find suitable catalysts for the
decomposition of 4-phenoxyphenol, and finally, we found that
novel metal catalyst supported on carbon showed a considerable
catalytic performance in the decomposition of 4-phenoxyphenol.
Fig.
5 shows the scheme for the decomposition of 4-
phenoxyphenol to aromatics. Phenol is produced by the cleavage
O
H2
2H2
H2
H2
2
HO
HO
HO
OH
+ H2O
Fig. 5. Scheme for cleavage of 4-O-5 bond in 4-phenoxyphenol.

H.W. Park et al. / Applied Catalysis A: General 409–410 (2011) 167–173
171
Table 3
Catalytic performance of Pd/ACA-H3PO4-X in the decomposition of 4-phenoxyphenol.
X in Pd/ACA-H3PO4-X
X = 0
X = 0.5
X = 1.0
X = 2.0
X = 3.0
Conversion (%)
37.4
76.7
40.6
24.4
11.7
28.7
15.2
9.1
42.1
78.6
39.7
26.5
12.4
33.1
16.7
11.2
5.2
62.4
80.0
38.7
30.1
11.2
49.9
24.1
18.8
7.0
49.5
87.4
43.3
31.7
12.4
43.2
21.4
15.7
6.1
42.9
85.6
40.9
28.4
16.3
36.7
17.5
12.2
7.0
Toal selectivity for main productsa (%)
Cyclohexanol (%)
Benzene (%)
Phenol (%)
Total yield for main productsb (%)
Cyclohexanol (%)
Benzene (%)
Phenol (%)
4.4
a
Calculated by Eq. (3).
Calculated by Eq. (4).
b
of C–O bond in the 4-phenoxyphenol [25]. According to the pre-
vious study [40], phenol is then hydrogenated to cyclohexenol
as an intermediate, and it is further hydrogenated to cyclohex-
anol over palladium catalyst. Phenol is also directly converted to
benzene by hydrogenolysis. In our reaction system, aromatic com-
pounds such as phenol, benzene, and cyclohexanol were mainly
produced by the decomposition of 4-phenoxypheol over Pd/ACA-
H3PO4-X (X = 0, 0.5, 1.0, 2.0, and 3.0). Light hydrocarbons (C2–C6)
and alcohols were produced as by-products in the decomposi-
tion of 4-phenoxyphenol. Main products and by-products produced
by the decomposition of 4-phenoxyphenol over Pd/ACA-H3PO4-X
were identified by GC–MS (6890N GC, Agilent) equipped with mass
selective detector (5975 MSD, Agilent).
Catalytic performance of Pd/ACA-H3PO4-X (X = 0, 0.5, 1.0, 2.0,
and 3.0) 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/ACA-H3PO4-X (X = 0, 0.5, 1.0, 2.0, and 3.0)
was in the range 37.4–62.4%. Total selectivity for main products
(cyclohexanol, benzene, and phenol) and total yield for main prod-
ucts were in the range of 76.7–87.4% and 28.7–49.9%, respectively.
Yield for cyclohexanol (15.2–24.1%) was higher than that for ben-
zene (9.1–18.8%) and phenol (4.4–7.0%). The above results indicate
that palladium catalyst supported on activated carbon aerogel
(Pd/ACA-H3PO4-X, X = 0, 0.5, 1.0, 2.0, and 3.0) served as an efficient
catalyst in the decomposition of 4-phenoxyphenol to aromatics.
with respect to X. Among the catalysts tested, Pd/ACA-H3PO4-1.0
showed the best catalytic performance in terms of conversion of
4-phenoxyphel (62.4%) and total yield for main products (49.9%).
Fig. 6(b) shows the yield for cyclohexanol, benzene, and phenol
plotted as a function of X in Pd/ACA-H3PO4-X. Yields for cyclohex-
anol and benzene showed volcano-shaped curves with respect to X
in Pd/ACA-H3PO4-X. However, yield for phenol was almost constant
with regard to X. Among the catalyst tested, Pd/ACA-H3PO4-1.0
showed the highest yield for cyclohexanol (24.1%) and benzene
(18.8%).
3.4. Correlation between average palladium particle size of
Pd/ACA-H3PO4-X and catalytic performance in the decomposition
of 4-phenoxyphenol
Fig.
7 shows the correlation between conversion of 4-
phenoxyphenol over Pd/ACA-H3PO4-X and average palladium
particle size of Pd/ACA-H3PO4-X (X = 0, 0.5, 1.0, 2.0, and 3.0).
Average palladium particle sizes of Pd/ACA-H3PO4-X were calcu-
lated using the Debye–Scherrer equation (Table 2). The correlation
shows that conversion of 4-phenoxyphenol over Pd/ACA-H3PO4-
X is closely related to the average palladium particle size of
Pd/ACA-H3PO4-X. Conversion of 4-phenoxyphenol increased with
decreasing average palladium particle size of Pd/ACA-H3PO4-X.
Among the catalysts tested, Pd/ACA-H3PO4-1.0 with the smallest
average palladium particle size showed the highest conversion of 4-
phenoxyphenol. This result was well consistent with the previous
works [38,41] reporting that the catalytic performance increased
with decreasing metal particle size of supported catalysts because
small metal particle provided high metal surface area.
3.3. Effect of X in Pd/ACA-H3PO4-X on catalytic performance
Conversion of 4-phenoxyphenol and total yield for main prod-
ucts (cyclohexanol, bezene, and phenol) plotted as a function of
X in Pd/ACA-H3PO4-X are shown in Fig. 6(a). Conversion of 4-
phenoxyphenol showed a volcano-shaped curve with respect to X.
Total yield for main products also showed a volcano-shaped curve
Fig. 8 shows the correlation between total yield for main
products (cyclohexanol, benzene, and phenol) over Pd/ACA-H3PO4-
X and average palladium particle size of Pd/ACA-H3PO4-X. The
Fig. 6. Catalytic performance of Pd/ACA-H3PO4-X in the decomposition of 4-phenoxyphenol plotted as a function of X. Reaction condition: temperature = 200 ◦C, pres-
sure = 10 atm (H2), time = 1 h.

172
H.W. Park et al. / Applied Catalysis A: General 409–410 (2011) 167–173
65
60
55
50
45
40
35
30
80.0
80
70
60
50
40
30
20
10
0
Pd/ACA-H3PO4-1.0
Conversion of 4-phenoxyphenol
Total selectivity for main products
Total yield for main products
62.4
49.9
Pd/ACA-H3PO4-2.0
42.1
40.5
34.2
33.8
Pd/ACA-H3PO4-3.0
Pd/ACA-H3PO4-0.5
Pd/ACA-H3PO4-0
14.4
13.7
Pd/ACA-H3PO4-1.0
Pd/AC
Pd/AC-H3PO4-1.0
18
16
14
12
10
8
Average Pd particle size of Pd/ACA-H3PO4-X (nm)
Catalyst
Fig. 7. Correlation between conversion of 4-phenoxyphenol over Pd/ACA-H3PO4-X
and average palladium particle size of Pd/ACA-H3PO4-X. Average palladium particle
sizes were calculated by the Debye–Scherrer equation (Table 2). Reaction condition:
temperature = 200 ◦C, pressure = 10 atm (H2), time = 1 h.
Fig. 9. Comparison of catalytic performance between Pd/ACA-H3PO4-1.0, Pd/AC,
and Pd/AC-H3PO4-1.0 catalysts in the decomposition of 4-phenoxyphenol. Reaction
condition: temperature = 200 ◦C, pressure = 10 atm (H2), time = 1 h.
3.5. Catalytic performance of Pd/ACA-H3PO4-1.0, Pd/AC and
Pd/AC-H3PO4-1.0 in the decomposition of 4-phenoxyphenol
55
50
For comparison, palladium catalysts supported on commercial
activated carbon (Pd/AC) and commercial activated carbon acti-
vated by phosphoric acid (Pd/AC-H3PO4-1.0) were prepared by
an incipient wetness impregnation method. Textural properties
and average palladium particle size of Pd/AC and Pd/AC-H3PO4-
1.0 were almost the same, as listed in Table 4. Surface area
(824 m2/g) and micro surface area (332 m2/g) of Pd/ACA-H3PO4-
1.0 were smaller than those of Pd/AC (1832 m2/g and 1604 m2/g,
respectively) and Pd/AC-H3PO4-1.0 (1991 m2/g and 1748 m2/g,
respectively). However, mesopore surface area (492 m2/g), ratio
of mesopore surface area with respect to micropore surface area
(1.48), and average pore size (6.7 nm) of Pd/ACA-H3PO4-1.0 were
larger than those of Pd/AC (228 m2/g, 0.14, and 1.3 nm, respectively)
and Pd/AC-H3PO4-1.0 (243 m2/g, 0.14, and 1.2 nm, respectively),
because mesopore was dominant in Pd/ACA-H3PO4-1.0. Average
palladium particle size of Pd/ACA-H3PO4-1.0 (11.5 nm) was smaller
than that of Pd/AC (28.1 nm) and Pd/AC-H3PO4-1.0 (28.7 nm) due to
large average pore size and high Smeso/Smicro of Pd/ACA-H3PO4-1.0.
Fig. 9 shows the catalytic performance of Pd/ACA-H3PO4-1.0,
Pd/AC, and Pd/AC-H3PO4-1.0 catalysts in the decomposition of
4-phenoxyphenol. Catalytic performance of Pd/AC and Pd/AC-
H3PO4-1.0 catalysts was almost the same, due to similar textural
properties and palladium particle size of Pd/AC and Pd/AC-H3PO4-
1.0 catalysts. However, conversion of 4-phenoxyphenol (62.4%)
and total yield for main products (49.9%) over Pd/ACA-H3PO4-1.0
were much higher than those over Pd/AC (40.5% and 13.7%, respec-
tively) and Pd/AC-H3PO4-1.0 (42.1% and 14.4%, respectively). This
result also indicates that average palladium particle size supported
on carbon played an important role in determining the catalytic
performance in the decomposition of 4-phenoxyphenol. It can be
Pd/ACA-H3PO4-1.0
45
40
35
30
25
Pd/ACA-H3PO4-2.0
Pd/ACA-H3PO4-3.0
Pd/ACA-H3PO4-0.5
Pd/ACA-H3PO4-0
18
16
14
12
10
8
Average Pd particle size of Pd/ACA-H3PO4-X (nm)
Fig. 8. Correlation between total yield for main products over Pd/ACA-H3PO4-X
and average palladium particle size of Pd/ACA-H3PO4-X. Average palladium particle
sizes were calculated by the Debye–Scherrer equation (Table 2). Reaction condition:
temperature = 200 ◦C, pressure = 10 atm (H2), time = 1 h.
correlation shows that total yield for main products over Pd/ACA-
H3PO4-X is also closely related to the average palladium particle
size of Pd/ACA-H3PO4-X. Total yield for main products increased
with decreasing average palladium particle size of Pd/ACA-H3PO4-
X. Among the catalysts tested, Pd/ACA-H3PO4-1.0 with the smallest
average palladium particle size showed the highest total yield
for main products. Thus, the average palladium particle size of
Pd/ACA-H3PO4-X played an important role in determining the cat-
alytic performance in the decomposition of 4-phenoxyphenol to
aromatics.
Table 4
Textural properties and average palladium particle size of Pd/ACA-H3PO4-1.0, Pd/AC, and Pd/AC-H3PO4-1.0.
Pd/ACA-H3PO4-1.0
Pd/AC
Pd/AC-H3PO4-1.0
Surface area (Stotal) (m2/g)
Micro surface area (Smicro) (m2/g)
Meso surface area (Smeso) (m2/g)
Smeso/Smicro
824
332
492
1.48
6.7
1832
1604
228
0.14
1.3
28.1
1991
1748
243
0.14
1.2
28.7
Average pore size (nm)
Average palladium particle sizea (nm)
11.5
a
Calulated by the Debye–Scherrer equation.

H.W. Park et al. / Applied Catalysis A: General 409–410 (2011) 167–173
173
summarized that Pd/ACA-H3PO4-1.0 catalyst was more efficient
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4-phenoxyphenol to aromatics.
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Carbon aerogel was activated using different amount H3PO4
(ACA-H3PO4-X, X = 0, 0.5, 1.0, 2.0, and 3.0). Palladium catalyst sup-
ported on activated carbon aerogel (Pd/ACA-H3PO4-X, X = 0, 0.5,
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