Catalytic decomposition of 4-phenoxyphenol to aromatics over Pd/CsXH3.0-XPW12O40/activated carbon aerogel (X = 2.0-3.0)

Article abstract of DOI:10.1166/jnn.2013.8127

Cesium-exchanged heteropolyacid (Cs_XH_3.0-XPW₁₂O₄₀) was impregnated onto activated carbon aerogel (ACA) with a variation of cesium content (X = 2.0, 2.3, 2.5, 2.7, and 3.0) in order to provide acid sites to ACA. Palladium catalysts were then supported on Cs_XH_3.0-XPW₁₂O₄₀- impregnated activated carbon aerogel (Pd/Cs_XH_3.0-XPW₁₂O₄₀/ACA, X = 2.0-3.0) by an incipient wetness impregnation method for use in the decomposition of lignin model compound to aromatics. 4-Phenoxyphenol was used as a lignin model compound for representing 4-O-5 linkage of lignin. In the catalytic decomposition of 4-phenoxyphenol over Pd/Cs_XH_3.0-XPW₁₂O₄₀/ACA, cyclohexanol, benzene, and phenol were mainly produced. Conversion of 4-phenoxyphenol and total yield for main products (cyclohexanol, benzene, and phenol) were closely related to the acidity of Pd/Cs_XH_3.0-XPW₁₂O₄₀/ACA. Conversion of 4-phenoxyphenol and total yield for main products increased with increasing acidity of Pd/Cs_XH_3.0-XPW₁₂O₄₀/ACA. Among the catalysts tested, Pd/Cs_2.5H_0.5PW₁₂O₄₀/ACA catalyst with the largest acidity showed the highest conversion of 4-phenoxyphenol and total yield for main products. Therefore, it is concluded that acidity of catalysts would be an important factor determining the catalytic performance in the decomposition of 4-phenoxyphenol. Copyright

Full text of DOI:10.1166/jnn.2013.8127

Copyright © 2013 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 13, 7963–7968, 2013
Catalytic Decomposition of 4-Phenoxyphenol to
Aromatics Over Pd/CsXH3ꢀ0-XPW12O40/Activated
Carbon Aerogel (X = 2ꢀ0–3.0)
Hai Woong Park, Ung Gi Hong, Yoon Jae Lee, Jung Ho Choi, and In Kyu Song∗
School of Chemical and Biological Engineering, Institute of Chemical Processes, World Class University Program of
Chemical Convergence for Energy and Environment, Seoul National University, Shinlim-dong,
Kwanak-Ku, Seoul 151-744, South Korea
Cesium-exchanged heteropolyacid (CsX H3ꢀ0-X PW12O40) was impregnated onto activated carbon
aerogel (ACA) with a variation of cesium content (X = 2ꢀ0, 2.3, 2.5, 2.7, and 3.0) in order
to provide acid sites to ACA. Palladium catalysts were then supported on CsX H3ꢀ0-X PW12O40
-
impregnated activated carbon aerogel (Pd/CsX H3ꢀ0-X PW12O40/ACA, X = 2ꢀ0–3.0) by an incipient
wetness impregnation method for use in the decomposition of lignin model compound to aro-
matics. 4-Phenoxyphenol was used as a lignin model compound for representing 4-O-5 link-
age of lignin. In the catalytic decomposition of 4-phenoxyphenol over Pd/CsX H3ꢀ0-X PW12O40/ACA,
cyclohexanol, benzene, and phenol were mainly produced. Conversion of 4-phenoxyphenol and
total yield for main products (cyclohexanol, benzene, and phenol) were closely related to the
acidity of Pd/CsX H3ꢀ0-X PW12O40/ACA. Conversion of 4-phenoxyphenol and total yield for main
Delivered by Ingenta to: Nanyang Technological University
products increased with increasing acidity of Pd/Cs H3ꢀ0-X PW12O40/ACA. Among the catalysts
IP: 185.46.86.96 On: Sat, 11XJun 2016 20:53:42
tested, Pd/Cs2ꢀ5H0ꢀ5PW12O40/ACA catalyst with the largest acidity showed the highest conversion
Copyright: American Scientific Publishers
of 4-phenoxyphenol and total yield for main products. Therefore, it is concluded that acidity of cat-
alysts would be an important factor determining the catalytic performance in the decomposition of
4-phenoxyphenol.
Keywords: 4-Phenoxyphenol, Aromatics, Activated Carbon Aerogel, Cesium-Exchanged
Heteropolyacid, Pd Catalyst.
1. INTRODUCTION
aromatics as biofuels, because lignin is the richest source
of aromatics in nature.6ꢀ7
Conversion of biomass to liquid fuels has attracted much
attention as an environmetally friendly and renewable
energy process, because biofuels can reduce the depen-
dence of fossil fuel and the emission of carbon dioxide.1
First generation biofuels derived from corn sugar and veg-
etable oil are environmetally friendly, but feedstocks for
first generation biofuels are limited and compete with food
source.2ꢀ3 However, second generation biofuels derived
from lignocellulosic feedstocks do not compete with food
source.4ꢀ5 Lignocellulose is mainly composed of cellu-
lose, hemicellulose, and lignin. Lignin is an amorphous
polymer produced by polymerization of coumaryl alco-
hol, coniferyl alcohol, and sinapyl alcohol. Aromatic com-
Many attempts have been made on the catalytic decom-
position of lignin to aromatics, because thermal crack-
ꢀ
ing of lignin requires high temperature (over 450 C) and
pressure (over 100 atm) for considerable performance.8–10
Acid catalysts such as H3PO4 and cesium-exchanged het-
eropolyacid (CsXH3ꢁ0-XPW12O40, X = 2ꢁ0–3.0) are known
as promising catalysts for the decomposition of lignin to
aromatics.11–13 Novel metal catalysts supported on carbon
(Pd/C, Pt/C, and Rh/C) also showed a considerable per-
formance for selective decomposition of C O bond in
lignin.14ꢀ15
Functional lignin model compounds have been used
as a lignin feedstock due to the complex structure of
lignin.16ꢀ17 In particular, dimeric chemical compounds con-
taining C O bonds such as ꢂ-O-4, ꢃ-O-5, and 4-O-5
pounds in lignin are mainly linked by C O and C
bonds. Therefore, decomposition of lignin to aromatics
becomes as an advantageous technology for producing
C
have been used as lignin model compounds, because C
O
bond is abundant linkage type in lignin. Among vari-
ous lignin model compounds, 4-phenoxyphenol has been
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 12
1533-4880/2013/13/7963/006
doi:10.1166/jnn.2013.8127
7963

Catalytic Decomposition of 4-Phenoxyphenol to Aromatics Over Pd/CsXH3ꢁ0-XPW12O40/ACA (X = 2ꢁ0–3.0)
Park et al.
widely employed as a lignin model compound for repre-
senting 4-O-5 bond in lignin.18ꢀ19
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
(resorcinol/catalyst) ratio was fixed at 500. The resulting
sol was cured in ꢀa vial to produce resorcinol-formaldehyde
(RF ) gel at 80 C for 24 h. Solvent exchange was per-
Carbon aerogel (CA)20–22 and activated carbon aerogel
(ACA)23 have been employed as a catalyst support, electro-
chemical capacitor, and adsorbent, because CA and ACA
provide both micropores and mesopores. Palladium cata-
lyst supported on carbon aerogel (Pd/CA)24 and palladium
catalyst supported on activated carbon aerogel (Pd/ACA)25
have been used for the decomposition of C O bond in
lignin to take advantage of mesoporous nature of CA
and ACA.
ꢀ
formed with acetone at 50 C for two days. Residual sol-
vent 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 aerogel (CA) was finally obtained by car-
ꢀ
bonization of RF gel at 500 C for 1 h.
Heteropolyacids (HPAs) are inorganic acids. Acid
strength of HPAs is stronger than that of conventional
solid acids.26–28 HPA salts with K+, Cs+, and NH4+
cations have high surface area and porous structure by
forming a tertiary structure.27 It is known that surface
acidity of cation-exchanged insoluble HPAs is differ-
ent depending on cation content.29ꢀ30 For example, sur-
face acidity of CsxH3ꢁ0-xPW12O40 (X = 2ꢁ0–3.0) shows a
volcano-shaped trend with respect to cesium content, and
shows maximum value when X is 2.5.26–28
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
the solution of phosphoric acid. After stirring the mixture
ꢀ
for 1 h, the solid was dried at 110 C for 3 h. The mix-
ture of phospꢀhoric acid and carbon aerogel was heated to
react at 800 C for 1 h under N2 stream to obtain acti-
vated carbon aerogel. Phosphoric acid in activated carbon
aerogel was washed off using DI water till the pH of wash-
ing solution reached ca. 7. The residual solid was finally
dried at 110 ꢀC for 5 h to obtain activated carbon aerogel
(ACA).
In this work, activated carbon aerogel (ACA) was
prepared by a chemical activation of carbon aero-
gel using phosphoric acid (H3PO4).
A series of
CsXH3ꢁ0-XPW12O40-impregnated activated carbon aerogels
(CsXH3ꢁ0-XPW12O40/ACA) were prepared with a variation
of cesium content (X = 2ꢁ0, 2.3, 2.5, 2.7, and 3.0) in order
A series of CsXH3ꢁ0-XPW12O40-impregnated activated
carbon aerogels (CsXH3ꢁ0-XPW12O40/ACA) were prepared
with a variation of cesium content (X = 2ꢁ0, 2.3, 2.5, 2.7,
Delivered by Ingenta to: Nanyang Technological University
to provide acid sites to ACA. Palladium catalysts were
IP: 185.46.86.96 On: Sat, 11 Jun 2016 20:53:42
and 3.0) in order to provide acid sites to ACA. 0.2 g
of H3PW12O40 (Sigma-Aldrich) was impregnated onto
1 g of activated carbon aerogel (H3PW12O40/ACA) by an
incipient wetness impregnation method, and it was dried
then supported on CsXH3ꢁ0-XPW O -impregnated acti-
Copyright: American Scientific Publishers
12 40
vated carbon aerogel (Pd/CsXH3ꢁ0-XPW12O40/ACA, X =
2ꢁ0, 2.3, 2.5, 2.7, and 3.0) by an incipient wet-
ness impregnation method for use in the decomposi-
tion of 4-phenoxyphenol to aromatics. 4-Phenoxyphenol
was used as a lignin model compound for represent-
ing 4-O-5 linkage of lignin. Cyclohexanol, benzene,
and phenol were mainly produced by the decompo-
sition of 4-phenoxyphenol over Pd/CsXH3ꢁ0-XPW12O40/
ACA. The effect of acidity of Pd/CsXH3ꢁ0-XPW12O40/ACA
on the catalytic performance in the decomposition of
4-phenoxyphenol was investigated.
ꢀ
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 was then dispersed
into the solution containing cesium nitrate with constant
stirring, and the mixture was stirred 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 CsXH3ꢁ0-XPW12O40/ACA (X = 2ꢁ0,
2.3, 2.5, 2.7, and 3.0).
2. EXPERIMENTAL DETAILS
2.2. Preparation of Palladium Catalyst Supported on
CsXH3ꢁ0-XPW12O40/ACA (X = 2ꢁ0, 2.3,
2.5, 2.7, and 3.0)
2.1. Preparation of CsXH3ꢁ0-XPW12O40-Impregnated
Activated Carbon Aerogel
(CsXH3ꢁ0-XPW12O40/ACA)
5 wt% of palladium was supported on CsXH3ꢁ0-XPW12O40/
ACA. 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 CsXH3ꢁ0-XPW12O40/ACA
(0.5 g). 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/CsXH3ꢁ0-XPW12O40/ACA (X = 2ꢁ0, 2.3,
2.5, 2.7, and 3.0).
Carbon aerogel (CA) was prepared according to the
method in the literatures.20ꢀ22 Carbon aerogel was prepared
by a sol–gel polymerization of resorcinol and formalde-
hyde. 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 stirring
the solution for a few minutes, 14.1 g of formaldehyde
(H2CO, Sigma-Aldrich) was added slowly into the solution
ꢀ
ꢀ
7964
J. Nanosci. Nanotechnol. 13, 7963–7968, 2013

Park et al.
Catalytic Decomposition of 4-Phenoxyphenol to Aromatics Over Pd/CsXH3ꢁ0-XPW12O40/ACA (X = 2ꢁ0–3.0)
2.3. Characterization
Total selectivity for main products
Total moles of cyclohexanol, benzene, and phenol formed
Textural properties of Pd/CsXH3ꢁ0-XPW12O40/ACA (X =
2ꢁ0, 2.3, 2.5, 2.7, and 3.0) catalysts were determined
by nitrogen adsorption–desorption isotherm measure-
ments (ASAP 2010, Micromeritics). Crystalline states
of Pd/CsXH3ꢁ0-XPW12O40/ACA were determined by
XRD (D-MA2500-PC, Rigaku) measurements using
Cu-Kꢃ radiation operated at 50 kV and 100 mA.
Surface morphology and palladium dispersion of
Pd/CsXH3ꢁ0-XPW12O40/ACA catalysts were investigated
by transmission electron microscopy (TEM, Jeol, JXA-
8900R). NH3-TPD (temperature-programmed desorption)
experiments (BEL Japan, BELCAT-B) were carried out in
order to measure the acidity of the catalysts. 0.04 g of each
catalyst charged into the TPD apparatus was pretreated
at 400 ꢀ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 cata-
lyst with ammonia. Physisorbed ammonia was removed at
=
(3)
moles of 4-phenoxyphenol reacted
Total yield for main products
= (Conversion of 4-phenoxyphenol)
×(Total selectivity for main products) (4)
3. RESULTS AND DISCUSSION
3.1. Characterization of Pd/CsXH3ꢁ0−XPW12O40/ACA
(X = 2ꢁ0–3.0)
Textural properties of Pd/CsXH3ꢁ0-XPW12O40/ACA (X =
2ꢁ0, 2.3, 2.5, 2.7, and 3.0) catalysts were examined by
nitrogen adsorption–desorption isotherm measurements.
Figure 1 shows the N2 adsorption–desorption isotherms
of Pd/CsXH3ꢁ0-XPW12O40/ACA catalysts. All the sam-
ples showed type-IV isotherm with type-H2 hysteresis
loop. This indicates that Pd/CsXH3ꢁ0-XPW12O40/ACA still
retained a porous structure even after the activation of
CA with H3PO4 and the subsequent impregnation of
CsXH3ꢁ0-XPW12O40 onto ACA. Surface area and average
pore size of Pd/CsXH3ꢁ0-XPW12O40/ACA are summarized
in Table I. Surface area of Pd/CsXH3ꢁ0-XPW12O40/ACA
was in the range of 875–932 m2/g. Average pore size of
ꢀ
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 750 C at a heating rate of 5 C/min under
a flow of helium (30 ml/min). Desorbed ammonia was
detected using a thermal conductivity detector.
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2.4. Catalytic Decomposition of 4-Phenoxyphenol
IP: 185.46.86.96 On: Sat, P1d1/CJusXnH2016PW20:5O3:4/A2CA calculated by the BJH desorp-
3ꢁ0-X
12 40
Decomposition of 4-phenoxyphenoCl o(pCyrHighOt:CAHmeOriHca, n Scientific Publishers
tion branch was in the range of 3.9–4.7 nm.
6
5
6
4
Sigma-Aldrich) over Pd/CsXH3ꢁ0-XPW12O40/ACA (X =
2ꢁ0–3.0) 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 reactor was purged with hydrogen
several times to remove residual air. Reactor temperature
Figure 2 shows the XRD patterns of Pd/CsXH3ꢁ0-X
PW12O40/ACA and Cs2ꢁ5H0ꢁ5PW12O40. No diffraction
peaks for CsXH3ꢁ0-XPW12O40 were observed in the
Pd/CsXH3ꢁ0-XPW12O40/ACA (X = 2ꢁ0, 2.3, 2.5, 2.7, and
3.0) catalysts due to fine dispersion of CsXH3ꢁ0-XPW12O40
onto ACA. Successful impregnation of palladium metal
on CsXH3ꢁ0-XPW12O40/ACA (X = 2ꢁ0, 2.3, 2.5, 2.7,
ꢀ
was increased to 200 C and hydrogen was then charged
into the reactor (10 atm). The catalytic reaction was per-
ꢀ
formed at 200 C and 10 atm (H2) for 1 h with agitation
Pd/Cs2.0H1.0PW12O40/ACA
Pd/Cs2.3H0.7PW12O40/ACA
Pd/Cs2.5H0.5PW12O40/ACA
speed of 100 rpm. Reaction products were analyzed with
a gas chromatograph (Younglin, ACME 6100) equipped
with DB-5 column and flame ionization detector. Con-
version of 4-phenoxyphenol and selectivity for product
(cyclohexanol, benzene, and phenol) were calculated by
the following equations. Yield for product (cyclohexanol,
benzene, and phenol) was calculated by multiplying con-
version of 4-phenoxyphenol and corresponding product
selectivity.
Pd/Cs2.7H0.3PW12O40/ACA
Pd/Cs3.0H0PW12O40/ACA
Conversion of 4-phenoxyphenol
moles of 4-phenoxyphenol reacted
=
(1)
moles of 4-phenoxyphenol supplied
0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Selectivity for product (cyclohexanol, benzene, or phenol)
moles of cyclohexanol, benzene, or phenol formed
Fig. 1. N2 adsorption–desorption isotherms of Pd/CsX H3ꢁ0-X PW12O40/
=
(2)
ACA.
moles of 4-phenoxyphenol reacted
J. Nanosci. Nanotechnol. 13, 7963–7968, 2013
7965

Catalytic Decomposition of 4-Phenoxyphenol to Aromatics Over Pd/CsXH3ꢁ0-XPW12O40/ACA (X = 2ꢁ0–3.0)
Park et al.
Table I. Surface area, average pore size, average palladium particle
size, acidity, and catalytic performance of Pd/CsX H3ꢁ0-X PW12O40/ACA in
the decomposition of 4-phenoxyphenol.
Catalyst
X in Pd/CsX H3ꢁ0-X PW12O40/ACA
(a)
(b)
(c)
X = 2ꢁ0 X = 2ꢁ3 X = 2ꢁ5 X = 2ꢁ7 X = 3ꢁ0
SBET (m2/g)a
875
3ꢁ9
10ꢁ7
913
4ꢁ1
10ꢁ2
932
4ꢁ7
10ꢁ4
927
4ꢁ3
10ꢁ5
919
4ꢁ4
10ꢁ7
Average pore size (nm)b
Average palladium
particle size (nm)c
Acidity (ꢄmol-NH3/g)d
94
154
225
181
71
Conversion (%)e
69ꢁ8
71ꢁ7
73ꢁ1
71ꢁ5
93ꢁ7
71ꢁ0
77ꢁ6
72ꢁ5
66ꢁ0
71ꢁ7
(d)
(e)
Total selectivity for
main products (%)e
Cyclohexanol (%)
Benzene (%)
Fig. 3. TEM images of Pd/CsX H3ꢁ0-X PW12O40/ACA: (a) Pd/Cs2ꢁ0H1ꢁ0
PW12O40/ACA, (b) Pd/Cs2ꢁ3H0ꢁ7PW12O40/ACA, (c) Pd/Cs2ꢁ5H0ꢁ5PW12O40/
ACA, (d) Pd/Cs2ꢁ7H0ꢁ3PW12O40/ACA, and (e) Pd/Cs3ꢁ0H0PW12O40/ACA.
29ꢁ6
37ꢁ1
5ꢁ0
29ꢁ8
37ꢁ5
4ꢁ2
30ꢁ5
37ꢁ5
3ꢁ3
30ꢁ6
37ꢁ9
3ꢁ9
30ꢁ1
36ꢁ9
4ꢁ6
Phenol (%)
determined from TEM image was in the range of 5–15 nm,
in good agreement with XRD result (Fig. 2 and Table I).
On the other hand, particles of CsXH3ꢁ0-XPW12O40 were
not observed in the Pd/CsXH3ꢁ0-XPW12O40/ACA catalysts
due to fine dispersion of CsXH3ꢁ0-XPW12O40 onto ACA.
Acidity of Pd/CsXH3ꢁ0-XPW12O40/ACA (X = 2ꢁ0, 2.3,
2.5, 2.7, and 3.0) catalysts determined from NH3-TPD
peak area was in the range of 71–225 ꢄmol-NH3/g,
as listed in Table I. Acidity of the catalysts showed a
volcano-shaped trend with respect to X (cesium con-
Total yield for
main products (%)e
Cyclohexanol (%)
Benzene (%)
50ꢁ0
52ꢁ3
66ꢁ5
56ꢁ2
47ꢁ3
20ꢁ6
25ꢁ9
3ꢁ5
21ꢁ8
27ꢁ4
3ꢁ1
28ꢁ6
34ꢁ9
3ꢁ0
23ꢁ8
29ꢁ4
3ꢁ1
19ꢁ9
24ꢁ3
3ꢁ1
Phenol (%)
Notes: aCalculated by N2 adsorption–desorption isotherm; bCalculated by the BJH
desorption branch; cCalculated by the Debye-Scherrer equation; dCalculated by
peak area of NH3-TPD profile; eCalculated according to the Eqs. (1)–(4).
and 3.0) was also confirmed by XRD patterns, as
shown in Figure 2. Average palladium particle size
of Pd/CsXH3ꢁ0-XPW O /ACA calculated by the Debye-
Scherrer equation was in the range of 10.2–10.7 nm, as
listed Table I.
Figure 3 shows the TEM images of Pd/CsXH3ꢁ0-X
PW12O40/ACA (X = 2ꢁ0–3.0) catalysts. Carbon particles in
the Pd/CsXH3ꢁ0-XPW12O40/ACA were observed to form an
interconnecting network structure with non-uniform textu-
ral porosity. Palladium metal on CsXH3ꢁ0-XPW12O40/ACA
was observed by TEM image as a dark spot. Palladium
metal size of Pd/CsXH3ꢁ0-XPW12O40/ACA (X = 2ꢁ0–3.0)
Delivered by Ingenta to: Nanyang Technological University
tent) of Pd/CsXH3ꢁ0-XPW12O40/ACA. Among the cata-
lyst tested, Pd/Cs2ꢁ5H0ꢁ5PW12O40/ACA catalyst showed the
largest acidity. This result was well consistent with the
previous study reporting that acidity of CsXH3ꢁ0-XPW12O40
was different depending on Cs content and showed maxi-
mum at X = 2ꢁ5.12ꢀ27
IP: 185.46.86.96 On: Sat, 11 Jun 2016 20:53:42
12 40
Copyright: American Scientific Publishers
3.2. Catalytic Performance in the Decomposition of
4-Phenoxyphenol to Aromatics
Figure 4 shows the scheme for the decomposition of
4-phenoxyphenol to aromatics. Phenol is produced by the
cleavage of C O bond in 4-phenoxyphenol.20 Accord-
ing to the previous study,28 phenol is then hydrogenated
to 2-cyclohexen-1-ol as an intermediate, and it is fur-
ther hydrogenated to cyclohexanol over palladium catalyst.
Phenol is also directly converted to benzene by hydrogenol-
ysis. In our reaction system, aromatic compounds such
as phenol, benzene, and cyclohexanol were mainly pro-
duced by the decomposition of 4-phenoxypheol over
Pd
Pd/Cs2.0H1.0PW12O40/ACA
Pd/Cs2.3H0.7PW12O40/ACA
Pd/Cs2.5H0.5PW12O40/ACA
Pd/Cs2.7H0.3PW12O40/ACA
Pd/Cs3.0H0PW12O40/ACA
Cs2.5H0.5PW12O40
O
2H2
H2
H2
H2
2
HO
HO
HO
OH
10
20
30
40
50
60
70
80
2 Theta (Degree)
+ H2O
ꢀ
Fig. 2. XRD patterns of Pd/CsX H3ꢁ0-X PW12O40/ACA reduced at 120 C
for 6 h.
Fig. 4. Scheme for the cleavage of 4-O-5 bond in 4-phenoxyphenol.
J. Nanosci. Nanotechnol. 13, 7963–7968, 2013
7966

Park et al.
Catalytic Decomposition of 4-Phenoxyphenol to Aromatics Over Pd/CsXH3ꢁ0-XPW12O40/ACA (X = 2ꢁ0–3.0)
100
Conversion of 4-phenoxyphenol
Pd/Cs2.5H0.5PW12O40/ACA
Total yield for main products
100
80
90
80
70
60
Pd/Cs2.7H0.3PW12O40/ACA
60
Pd/Cs2.3H0.7PW12O40/ACA
Pd/Cs2.0H1.0PW12O40/ACA
Pd/Cs3.0H0PW12O40/ACA
40
2.0
2.5
3.0
50
100
150
200
250
Acidity (µmol-NH3/g)
X in Pd/CsXH3.0-XPW12O40/ACA
Fig. 6. A correlation between conversion of 4-phenoxyphenol over
Pd/CsX H3ꢁ0-X PW12O40/ACA and acidity of the catalysts. Acidity of
Pd/CsX H3ꢁ0-X PW12O40/ACA catalysts was taken from Table I. Reaction
condition: temperature = 200 ꢀC, pressure = 10 atm (H2), time = 1 h.
Fig. 5. Catalytic performance of Pd/CsX H3ꢁ0-X PW12O40/ACA (X =
2.0–3.0) in the decomposition of 4-phenoxyphenol, plotted as a function
of X. Reaction condition: temperature = 200 ꢀC, pressure=10 atm (H2),
time = 1 h.
Pd/CsXH3ꢁ0-XPW12O40/ACA. Light hydrocarbons (C2–C6)
and alcohols were produced as by-products in the decom-
position of 4-phenoxyphenol.
Pd/Cs2ꢁ5H0ꢁ5PW12O40/ACA with the largest acidity showed
the highest conversion of 4-phenoxyphenol.
Figure 7 shows the correlation between total yield
for main products (cyclohexanol, benzene, and phe-
nol) over Pd/CsXH3ꢁ0-XPW12O40/ACA (X = 2ꢁ0–3.0) and
acidity of the catalysts. The correlation shows that
total yield for main products over Pd/CsXH3ꢁ0-XPW12O40/
Catalytic performance of Pd/CsXH3ꢁ0-XPW12O40/ACA
in the decomposition of 4-phenoxyphenol performed at
ꢀ
200 C and 10 atm for 1 h is listed in Table I. Conversion
of 4-phenoxyphenol over Pd/CsXH3ꢁ0-XPW12O40/ACA was
in the range 66.0–93.7%. Total selectivity for main prod-
ACA was also closely related to the acidity of
Delivered by Ingenta to: Nanyang Technological University
ucts (cyclohexanol, benzene, and phenol) and total yield
Pd/Cs H
PW O /ACA. Total yield for main products
IP: 185.46.86.96 On: Sat, 11 Jun 2016 20:53:42
X
3ꢁ0-X
12 40
for main products were in the range of 71.0–72.5% and
Copyright: American Scientific Publishers
increased with increasing acidity of the catalysts. Among
47.3–66.5%, respectively. The above results indicate that
Pd/CsXH3ꢁ0-XPW12O40/ACA served as efficient catalysts in
the decomposition of 4-phenoxyphenol to aromatics.
Conversion of 4-phenoxyphenol and total yield for
main products (cyclohexanol, benzene, and phenol)
the catalysts tested, Pd/Cs2ꢁ5H0ꢁ5PW12O40/ACA with the
largest acidity showed the highest total yield for main
products. Thus, acidity of Pd/CsXH3ꢁ0-XPW12O40/ACA
played an important role in determining the catalytic per-
formance in the decomposition of 4-phenoxyphenol to
aromatics.
plotted as
a function of X (cesium content) in
Pd/CsXH3ꢁ0-XPW12O40/ACA are shown in Figure 5. Con-
version of 4-phenoxyphenol and total yield for main prod-
ucts showed volcano-shaped curves with respect to X.
Among the catalysts tested, Pd/Cs2ꢁ5H0ꢁ5PW12O40/ACA
showed the best catalytic performance in terms of conver-
sion of 4-phenoxyphel (93.7%) and total yield for main
products (66.5%).
80
70
Pd/Cs2.5H0.5PW12O40/ACA
60
Pd/Cs2.7H0.3PW12O40/ACA
3.3. Correlation Between Catalytic Performance and
Acidity of Pd/CsXH3ꢁ0-XPW12O40/ACA
Pd/Cs2.3H0.7PW12O40/ACA
50
Pd/Cs2.0H1.0PW12O40/ACA
Figure 6 shows the correlation between conversion of
4-phenoxyphenol over Pd/CsXH3ꢁ0-XPW12O40/ACA (X =
2ꢁ0–3.0) and acidity of the catalysts. Acidity of Pd/
CsXH3ꢁ0-XPW12O40/ACA was determined from NH3-TPD
measurement (Table I). The correlation shows that conver-
sion of 4-phenoxyphenol over Pd/CsXH3ꢁ0-XPW12O40/ACA
was closely related to the acidity of catalysts. Conversion
of 4-phenoxyphenol increased with increasing acidity of
Pd/CsXH3ꢁ0-XPW12O40/ACA. Among the catalysts tested,
Pd/Cs3.0H0PW12O40/ACA
40
50
100
150
200
250
Acidity (µmol-NH3/g)
Fig. 7. A correlation between total yield for main products over
Pd/CsX H3ꢁ0-X PW12O40/ACA and acidity of the catalysts. Acidity of
Pd/CsX H3ꢁ0-X PW12O40/ACA catalysts was taken from Table I. Reaction
condition: temperature = 200 ꢀC, pressure = 10 atm (H2), time = 1 h.
J. Nanosci. Nanotechnol. 13, 7963–7968, 2013
7967

Catalytic Decomposition of 4-Phenoxyphenol to Aromatics Over Pd/CsXH3ꢁ0-XPW12O40/ACA (X = 2ꢁ0–3.0)
Park et al.
4. CONCLUSIONS
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Activated carbon aerogel (ACA) was prepared by a chemi-
cal activation of carbon aerogel using phosphoric acid, and
CsXH3ꢁ0-XPW12O40 was then impregnated onto ACA with a
variation of cesium content (CsXH3ꢁ0-XPW12O40/ACA, X =
2ꢁ0, 2.3, 2.5, 2.7, and 3.0). Palladium catalysts supported
on CsXH3ꢁ0-XPW12O40/ACA (Pd/CsXH3ꢁ0-XPW12O40/ACA,
X = 2ꢁ0, 2.3, 2.5, 2.7, and 3.0) were prepared by an
incipient wetness impregnation method, and they were
applied to the decomposition of lignin model compound
to aromatics. 4-Phenoxyphenol was used as a dimeric
lignin model compound for representing 4-O-5 bond in
lignin. Conversion of 4-phenoxyphenol and total yield
for main products (cyclohexanol, benzene, and phenol)
over Pd/CsXH3ꢁ0-XPW12O40/ACA were in the range of
66.0–93.7% and 47.3–66.5%, respectively. Conversion
of 4-phenoxyphenol and total yield for main prod-
ucts were closely related to the acidity of Pd/CsXH3ꢁ0-X
PW12O40/ACA. Conversion of 4-phenoxyphenol and
total yield for main products increased with increas-
ing acidity of Pd/CsXH3ꢁ0-XPW12O40/ACA. Among
the catalysts tested, Pd/Cs2ꢁ5H0ꢁ5PW12O40/ACA with
the largest acidity showed the highest conversion of
4-phenoxyphenol (93.7%) and total yield for main prod-
ucts (66.5%). Thus, Pd/Cs2ꢁ5H0ꢁ5PW12O40/ACA catalyst
served as an efficient catalyst in the decomposition of
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4-phenoxyphenol to aromatics. It is concluded that acidity
Delivered by Ingenta to: Nanyang Technological University
IP: 185.46.86.96 On: Sat, 11 Jun 2016 20:53:42
of Pd/CsXH3ꢁ0-XPW12O40/ACA played an important role in
Copyright: American Scientific Publishers
determining the catalytic performance.
Acknowledgments: 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 WCU (World
Class University) program through the National Research
Foundation of Korea funded by the Ministry of Education,
Science and Technology (R31-10013).
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Received: 31 July 2012. Accepted: 6 November 2012.
7968
J. Nanosci. Nanotechnol. 13, 7963–7968, 2013