Catalytic decomposition of phenethyl phenyl ether to aromatics over Pd-Fe bimetallic catalysts supported on ordered mesoporous carbon

Article abstract of DOI:10.1016/j.molcata.2015.09.023

A series of bimetallic Pd-Fe catalysts supported on ordered mesoporous carbon (denoted as Pd₁-Fe_X/OMC) were prepared with a variation Fe/Pd molar ratio (X), and they were applied to the catalytic decomposition of phenethyl phenyl eth

Full text of DOI:10.1016/j.molcata.2015.09.023

Journal of Molecular Catalysis A: Chemical 410 (2015) 184–192
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Catalytic decomposition of phenethyl phenyl ether to aromatics over
Pd–Fe bimetallic catalysts supported on ordered mesoporous carbon
∗
Jeong Kwon Kim, Jong Kwon Lee, Ki Hyuk Kang, Jong Won 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:
Received 11 August 2015
Received in revised form
A
series of bimetallic Pd–Fe catalysts supported on ordered mesoporous carbon (denoted as
Pd1–FeX /OMC) were prepared with a variation Fe/Pd molar ratio (X), and they were applied to the catalytic
decomposition of phenethyl phenyl ether to aromatics. Phenethyl phenyl ether was used as a lignin model
compound for representing ␤-O-4 linkage in lignin. The effect of Fe/Pd molar ratio on the catalytic activ-
ities and physicochemical properties of bimetallic Pd1–FeX /OMC catalysts was investigated. It was found
that crystalline phase, reducibility, chemical state, and electronic property of Pd1–FeX /OMC catalysts
were strongly influenced by Fe/Pd molar ratio. In particular, modified electronic property derived from the
interaction between Pd and Fe significantly changed the hydrogen adsorption ability and bimetallic struc-
ture of the catalysts. Conversion of phenethyl phenyl ether continuously decreased with increasing Fe/Pd
molar ratio, whereas selectivity for aromatics increased and then became almost constant with increasing
Fe/Pd molar ratio. As a consequence, yield for aromatics showed a volcano-shaped trend with respect to
Fe/Pd molar ratio. Catalytic performance of Pd1–FeX /OMC catalysts was closely related to the hydrogen
adsorption ability and bimetallic structure of the catalysts. Among the catalysts tested, Pd1–Fe0.7/OMC
catalyst with moderate hydrogen adsorption ability and with bimetallic structure of Pd1Fe0.7 composi-
tion showed the highest yield for aromatics. Thus, an optimal Fe/Pd molar ratio was required to achieve
maximum production of aromatics through selective cleavage of C O bond in phenethyl phenyl ether
over Pd1–FeX /OMC catalysts.
1
7 September 2015
Accepted 27 September 2015
Keywords:
Lignin
Bimetallic catalyst
Phenethyl phenyl ether
Pd–Fe catalyst
Aromatics
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
4 (3–5%), and 4-O-5 (4–7%) is much larger than the amount of CC
bonds in lignin such as ␤-1 (7–9%), ˇ–ˇ (2–4%), and 5-5 (19–22%)
[7,8]. Therefore, selective cleavage of C O bond in lignin has been
considered as a key strategy for the decomposition of lignin to
aromatics.
With increasing concerns about environmental problems and
depletion of fossil fuels, lignocellulosic biomass has gained much
attention as an alternative energy resource, because biofuels
derived from biomass can reduce the dependence on fossil fuels
and the emission of greenhouse gases [1–4]. However, it has been
reported that only 2% of lignin by-products from pulp industries is
used for commercial products such as binders in asphalt, cement
or polymer [5–7]. Nevertheless, lignin is regarded as a promising
source for sustainable production of fuels and platform chemicals,
because lignin is an amorphous polymer produced by polymer-
ization of coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol
Many attempts have been made on the lignin transformation
into useful chemicals such as pyrolysis and catalytic and/or enzy-
matic depolymerization [7–10]. Among these methods, catalysis
has attracted much attention as a promising process for selec-
tive conversion of lignin into value-added chemicals, because the
reaction pathways can be controlled by appropriate selection of
catalysts [7,11–13]. Noble metal catalysts such as Pt, Pd, and Rh
are known to be effective for decomposition of C O bond in lignin
under mild conditions. Unfortunately, however, most of these cata-
lysts tend to hydrogenate aromatic ring in lignin [7,14,15]. In order
to improve aromatic production by selective cleavage of C O bond
in lignin without aromatic ring-saturation, therefore, noble metal-
based bimetallic catalysts have been investigated [16–18]. The
bimetallic systems can change the structural and electronic prop-
erties of noble metal catalysts, which induces a desired reaction
[
7,8]. Aromatic compounds in lignin are mainly linked by C O and
CC bonds such as ␤-O-4, ␣-O-4, 4-O-5, ␤-1, ˇ–ˇ, and 5-5 bonds.
The amount of C O bonds in lignin such as ␤-O-4 (40–60%), ␣-O-
∗ Corresponding author. Fax: +82 2 889 7415.
E-mail address: inksong@snu.ac.kr (I.K. Song).
http://dx.doi.org/10.1016/j.molcata.2015.09.023
1
381-1169/© 2015 Elsevier B.V. All rights reserved.

J.K. Kim et al. / Journal of Molecular Catalysis A: Chemical 410 (2015) 184–192
185
of lignin. In particular, dimeric lignin model compounds containing
O bonds such as ␣-O-4, ␤-O-4, and ␤-5 bonds are generally used
C
as lignin model compounds, because C O bonds are abundant link-
age type in lignin [7,8]. Among various lignin model compounds,
phenethyl phenyl ether has been used as a lignin model compound
for representing ␤-O-4 bond in lignin [7].
Porous carbon materials have been used for long time as a cat-
alyst support [23,24]. In particular, ordered mesoporous carbon
(OMC) can be potentially available as a supporting material because
of uniform pore size distribution, efficient mass transfer of reac-
tant molecules, and controllable textural properties [25–28]. In the
selective cleavage of C O bond in phenethyl phentyl ether, fur-
thermore, carbon support can minimize the support effect due to
its non-polar and hydrophobic nature [25,26]. These unique prop-
erties make OMC well suited as a potential candidate material for
catalyst support.
In this work, a series of Pd–Fe bimetallic catalysts with differ-
ent Fe/Pd molar ratio (X) were supported on ordered mesoporous
carbon (Pd –FeX /OMC), and they were applied to the catalytic
1
decomposition of phenethyl phenyl ether to aromatics. The effect
of Fe/Pd molar ratio on the catalytic activities and physicochemi-
cal properties of the catalysts was investigated. The catalysts were
characterized by nitrogen adsorption-desorption, TPR, XRD, STEM-
EDX, TEM, H2-TPD, and XPS analyses.
Fig. 1. Representative structure of a fragment of lignin.
pathway selectively [16–18]. For example, several bimetallic cata-
lysts containing oxophilic metals such as Sn, Re, and Fe remarkably
changed the product selectivity due to the improved interaction
with the oxygenated group [19–22].
2. Experimental
2.1. Preparation of catalysts
Fig. 1 shows the representative structure of a fragment of lignin.
Due to the structural complexity of lignin, dimeric model com-
pounds for representing C O and CC bonds in lignin have been
studied as a lignin feedstock for depoloymerization of lignin [7].
Lignin model compounds, which possess chemical linkages similar
to lignin, can provide insight into the decomposition and reaction
A series of Pd–Fe bimetallic catalysts supported on ordered
mesoporous carbon were prepared by a surfactant-templating
method and a subsequent incipient wetness impregnation method.
Ordered mesoporous carbon was prepared according to the method
reported in the literature [28]. PEO-PPO-PEO tri-block copoly-
mer (P123, Sigma–Aldrich) was dissolved in 1.5 M HCl solution
Fig. 2. HR-TEM images of reduced Pd1–FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4) catalysts.

1
86
J.K. Kim et al. / Journal of Molecular Catalysis A: Chemical 410 (2015) 184–192
◦
at 40 C for 3 h. Sucrose (carbon precursor, TCI) and H SO solu-
H2 temperature-programmed desorption (H -TPD) analyses of
2
4
2
tion were then added into the solution for 1 h under stirring. After
tetraethoxysilane (TEOS, Sigma–Aldrich) was slowly added into the
the reduced catalysts were conducted using a BELCAT-B instru-
ment (BEL Japan). 10 mg of each calcined catalyst was preliminarily
◦
◦
solution, the resulting mixture was stirred at 40 C for 24 h and then
reduced at 450 C for 4 h under 5% H /Ar flow (50 mL/min), and then
2
◦
it was maintained at 100 C for 20 h under static condition for self-
it was purged with Ar flow (50 mL/min) for 10 min. After cooling the
reduced catalyst to room temperature under Ar flow (50 mL/min),
◦
assembly of micelle structure. The resultant was dried at 100 C for
4
◦
8 h, and it was carbonized at 800 C for 4 h to obtain a silica-carbon
5% H /Ar mixed gas (50 mL/min) was injected for 60 min at 250
2
◦
composite. The silica-carbon composite was treated with 10 wt% HF
solution to remove silica template, and it was finally filtered and
dried. The resulting ordered mesoporous carbon was denoted as
OMC.
C. The sample was purged under Ar flow (50 mL/min) to remove
physisorbed hydrogen, and subsequently, furnace temperature was
◦
increased from room temperature to 700 C at a heating rate of
◦
5 C/min under Ar flow (50 mL/min). The desorbed hydrogen was
For co-impregnation of palladium and iron metals onto
detected using a TCD (thermal conductivity detector).
OMC, palladium chloride (PdCl , Sigma–Aldrich) and iron nitrate
X-ray photoelectron spectroscopy (XPS) analyses (ThermoVG,
Sigma probe) were carried out to measure binding energies of
metallic palladium and iron in the reduced catalysts. For the XPS
2
(
Fe(NO ) , Junsei) were dissolved in acetone containing 0.1 M HCl.
3 3
During this process, Pd:Fe molar ratio was adjusted to be 1:0,
.8:0.2, 0.6:0.4, 0.4:0.6, and 0.2:0.8, while the total loading of two
0
analyses, each calcined catalyst was reduced using an ex-situ reduc-
◦
metals was fixed at 0.5 mol% in all samples to maintain the same
number of active sites. The precursor solution was then intro-
duced into the pores of OMC by an incipient wetness impregnation
tion system at 450 C for 4 h under 5% H /Ar flow (50 mL/min), and
2
the catalyst was then transported to glass jar with sample holder
in argon atmosphere glove box to minimize air exposure. After
outgassing the glass jar in a vacuum oven, the sample holder was
transferred to the XPS chamber as quickly as possible. All the XPS
spectra were calibrated using C 1s peak (284.5 eV) as a reference.
◦
method. After drying the impregnated catalyst at 50 C overnight,
◦
it was calcined at 450 C for 3 h in a nitrogen stream. The sup-
ported catalyst was then reduced with a mixed stream of hydrogen
◦
(
2.5 mL/min) and nitrogen (47.5 mL/min) at 450 C for 4 h prior
to the catalytic reaction. The prepared catalysts were denoted as
Pd –FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4), where X represented the
1
2.3. Catalytic decomposition of phenethyl phenyl ether
Fe/Pd molar ratio.
Catalytic decomposition of phenethyl phenyl ether (PPE) to aro-
matics was carried out in a stainless steel autoclave reactor (25 mL)
under hydrogen atmosphere. Prior to the reaction, each catalyst
was reduced using an ex-situ reduction system at 450 C for 4 h
2.2. Characterization
◦
under 5% H /N flow (50 mL/min). 50 mg of reduced catalyst, 0.25 g
2
2
For HR-TEM, nitrogen adsorption-desorption, ICP-AES, and XRD
of phenethyl phenyl ether (Frinton Laboratory, a reactant), and 9 mL
of hexadecane (Sigma–Aldrich, a solvent) were charged into the
reactor at room temperature. The reactor was purged with nitro-
analyses of the reduced Pd –FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4)
1
catalysts, ex-situ reduction of the catalysts was carried out under
◦
5
% H /Ar flow (50 mL/min) at 450 C for 4 h. Pore structure and
2
gen several times in order to remove air. The catalytic reaction was
pore size of the reduced catalysts were examined by HR-TEM (JEM-
100, JEOL) analyses. In order to see the distribution of palladium
◦
performed at 250 C and 10 bar (H ) for 3 h with agitation speed of
2
3
2
50 rpm. After the reaction, reaction products were analyzed using
and iron in the reduced catalysts, scanning transmission electron
microscopy (STEM) analyses (JEM-2100F, JEOL) were conducted
with energy dispersive X-ray spectroscopy (EDX) mapping.
Nitrogen adsorption–desorption measurements were con-
ducted to investigate textural properties of the reduced catalysts
using an ASAP-2010 (Micromeritics) instrument. Surface area of
the reduced catalysts was measured by Brunaur–Emmett–Teller
a gas chromatograph (Younglin, YL6100) equipped with a DB-1 col-
umn and a flame ionization detector (FID). Conversion of phenethyl
phenyl ether and selectivity for aromatic product (benzene, phe-
nol, or ethylbenzene) were calculated according to the following
equations on the basis of mole balance. Yield for aromatic product
(benzene, phenol, or ethylbenzene) was calculated by multiplying
conversion of phenethyl phenyl ether and corresponding product
selectivity.
(
BET) method [29]. Pore volume and average pore diameter were
moles of phenethyl phenyl ether reacted
moles of phenethyl phenyl ether supplied
Conversion of phenethyl phenyl ether (%) =
× 100
(1)
(2)
(3)
molesofbenzene,phenol,orethylbenzeneformed
molesofphenethylphenyletherreacted
Selectivityforaromaticproduct% =
Yield for aromatic product (%) =
× 100
(
Conversion of phenethyl phenyl ether) × (Selectivity for aromatic)
1
00
determined by the Barrett–Joyner–Halenda (BJH) method applied
to the desorption branch of the N2 isotherm [30].
3. Results and discussion
Chemical compositions of the reduced catalysts were deter-
mined by ICP-AES analyses (Optima-4300 DV, Perkin-Elmer).
Crystalline states of the reduced catalysts were examined by XRD
3.1. Characterization of catalysts
Fig. 2 shows the HR-TEM images of the reduced Pd –FeX /OMC
1
(
D-MAX-2500-PC, Rigaku) measurements using Cu–K␣ radiation
(X = 0, 0.25, 0.7, 1.5, and 4) catalysts. All the catalysts had an ordered
mesoporous carbon structure with pores in the range of 4–6 nm,
and they retained well-dispersed metal particles. Small metal par-
ticles with an average size of ca. 4 nm were observed in the TEM
images of the catalysts, indicating that metal species were uni-
formly dispersed in the channels or on the walls of OMC. There
operated at 40 kV and 100 mA.
Temperature-programmed reduction (TPR) analyses of the cal-
cined catalysts were conducted in a flow reactor system equipped
with a thermal conductivity detector (TCD). 10 mg of each catalyst
was reduced with a mixed stream of 5% H (2 mL/min) and N flow
2
2
(
5
20 mL/min) at temperatures ranging from room temperature to
00 C with a heating rate of 10 C/min.
was no noticeable difference in metal particle size of Pd –FeX /OMC
(X = 0, 0.25, 0.7, 1.5, and 4) catalysts.
1
◦
◦

J.K. Kim et al. / Journal of Molecular Catalysis A: Chemical 410 (2015) 184–192
187
Fig. 3. STEM and EDX mapping images of Pd1–FeX /OMC (X = 0.25, 0.7, and 4) catalysts.
increasing Fe/Pd molar ratio. This trend can be understood by the
fact that the atomic radius of palladium (145 pm) is larger than that
of iron (126 pm), which means that pore blockage by palladium is
more severe than that by iron. However, average pore diameter of
the catalysts was almost identical, suggesting that all the catalysts
still retained pore characteristics of OMC. Chemical compositions
of the reduced catalysts determined by ICP-AES analyses are also
listed in Table 1. Actual Pd:Fe molar ratios of the catalysts were in
good agreement with the designed values.
Pd -Fe /OMC
1
0
Pd -Fe /OMC
1
0.25
Pd -Fe /OMC
1
0.7
Pd -Fe /OMC
1
1.5
Pd -Fe /OMC
1
4
Crystalline phases of the reduced catalysts were examined by
XRD measurements as shown in Fig. 5. All the catalysts showed
◦
a characteristic diffraction peak for graphitic carbon at 2ꢀ = 23.5
[
33]. The reduced Pd - Fe /OMC catalyst showed diffraction peaks
1
0
◦
◦
◦
at 40.1 , 46.6 , and 68.3 , corresponding to metallic palladium
(
(
closed circle in Fig. 5(a)) [34,35]. Fig. 5(b) shows the enlarged Pd
1 1 1) XRD patterns of Pd –FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4)
1
catalysts. It was found that the characteristic diffraction peaks for
palladium slightly but consistently shifted to higher angle in the
Pd –FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4) catalysts with increasing
1
Fe/Pd molar ratio. The Pd (1 1 1) peaks for Pd –FeX /OMC (X = 0, 0.25,
1
◦
◦
◦
◦
0
4
.7, 1.5, and 4) catalysts appeared at 40.1 , 40.2 , 40.4 , 40.6 , and
0.7 , respectively (Fig. 5(b)). This consistent shift is an evidence
◦
0
0.2
0.4
0.6
0.8
1.0
for the formation of miscible phase between Pd and Fe through
incorporation Fe atom into the lattice of Pd, as reported in the liter-
atures [34,35]. On the other hand, diffraction peaks for metallic iron
Relative pressure (P/P0)
was not observed in the Pd –FeX /OMC (X = 0.25 and 0.7) catalysts
due to its small crystalline size or low concentration (opened cir-
1
Fig. 4. Nitrogen adsorption–desorption isotherms of reduced Pd1–FeX /OMC (X = 0,
0
.25, 0.7, 1.5, and 4) catalysts.
cle in Fig. 5(a) and (b)) [36,37]. However, Pd –FeX /OMC (X = 1.5 and
1
4
) catalysts exhibited a diffraction peak for metallic iron (opened
In order to see the distribution of palladium and iron species,
STEM-EDX analyses were conducted. Fig. 3 shows the STEM and
circle in Fig. 5(a) and (b)), and the peak intensity increased with
increasing iron content. This is because both Pd –Fe1.5/OMC and
Pd –Fe /OMC catalysts had Fe enriched-surface by segregating Fe
on the Pd–Fe surface, especially for the catalyst with higher Fe con-
1
EDX mapping images of Pd –FeX /OMC (X = 0.25, 0.7, and 4) cata-
1
1
4
lysts. It is noteworthy that particles of palladium and iron species
were homogeneously distributed in the bimetallic Pd–FeX /OMC
catalysts. Moreover, population density of Fe increased with
increasing Fe/Pd molar ratio. On the other hand, aggregation of
Fe was observed when a large amount of Fe was added in the
Pd–FeX /OMC catalyst (Pd –Fe /OMC). Therefore, it is inferred that
tent (Fe/Pd ≥ 1.5) [18,38]. From XRD results, it was revealed that
bimetallic phase of Pd –FeX /OMC catalysts were greatly affected
1
by the Fe/Pd molar ratio.
It is known that H -TPR measurement is a powerful tool for
2
1
4
examining reduction behavior and interaction between two metal
species of the catalyst. Fig. 6 shows the TPR profiles of the cal-
cined Pd –FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4) catalysts. TPR
a Pd–Fe miscible phase was partially formed in the bimetallic cat-
alysts; the rest of iron was presented as metallic iron.
1
Fig. 4 shows the nitrogen adsorption-desorption isotherms of
◦
profile of Pd –Fe /OMC catalyst showed a negative peak at 60 C
1
0
the reduced Pd –FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4) catalysts. All
1
◦
and a positive peak at 170 C; the former was attributed to the
decomposition of palladium hydride phase (PdHX ) [39,40], while
the latter was attributed to the reduction of palladium species
the catalysts exhibited type-IV isotherms with well-defined type-
H3 hysteresis loops, indicating the formation of well-developed
mesoporous structure [31,32]. Detailed textural properties of
the reduced catalysts are summarized in Table 1. All the cata-
[
39,40]. Interestingly, the reduction behaviors of Pd –FeX /OMC
1
2
(X = 0.25, 0.7, 1.5, and 4) catalysts were quite different depend-
ing on Fe/Pd molar ratio. For precise investigation, TPR profiles
of Pd –FeX /OMC (X = 0.25, 0.7, 1.5, and 4) catalysts were decon-
lysts retained high surface area (>732 m /g), large pore volume
3
(
>0.88 cm /g), and large average pore diameter (> 4.0 nm). Sur-
1
face area and pore volume of the catalysts slightly increased with

1
88
J.K. Kim et al. / Journal of Molecular Catalysis A: Chemical 410 (2015) 184–192
Table 1
Textural properties of reduced Pd1–FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4) catalysts.
Pd: Fe molar ratioa
Surface area (m /g)
2
b
Pore volume (cm /g)c
3
Pore diameter (nm)d
Catalyst
Pd1–Fe0/OMC
Pd1–Fe0.25/OMC
Pd1–Fe0.7/OMC
Pd1–Fe1.5/OMC
Pd1–Fe4/OMC
–
732
750
762
772
782
0.88
0.90
0.91
0.92
0.94
4.1
4.2
4.0
4.1
4.3
0.21: 0.79
0.40: 0.60
0.61: 0.39
0.81: 0.19
a
Determined by ICP-AES measurement.
Calculated by the BET equation.
Total pore volume at P/P0 = 0.99.
Average pore diameter.
b
c
d
Graphitic carbon
Pd (111)
Pd
Fe
(a)
Pd
Fe
(b)
Pd -Fe /OMC
1
0
Pd -Fe /OMC
1
0
Pd -Fe0.25/OMC
1
Pd -Fe0.25/OMC
1
Pd -Fe0.7/OMC
1
Pd -Fe0.7/OMC
1
Pd -Fe1.5/OMC
1
Pd -Fe1.5/OMC
1
Pd -Fe /OMC
1
4
Pd -Fe /OMC
1
4
2
0
30
40
2
50
60
70
80
34
36
38
40
42
44
46
Theta (degree)
2
Theta (degree)
Fig. 5. XRD patterns of (a) reduced Pd1–FeX /OMC catalysts and (b) enlarged Pd (1 1 1) diffraction peaks for Pd1–FeX /OMC catalysts.
voluted. As shown in Fig. 6, bimetallic Pd –FeX /OMC (X = 0.25, 0.7,
that Pd and Fe fractions were co-reduced by partially forming a
Pd–Fe bimetallic phase. From XRD and TPR results, it can be inferred
that bimetallic phase could be controlled by changing Fe/Pd ratio,
which might strongly affect the catalytic performance.
1
1
.5, and 4) catalysts showed three deconvoluted reduction peaks.
In our previous work [38], it was reported that the reduction of
Fe catalyst supported on ordered mesoporous carbon (Fe/OMC)
◦
occurred through three steps. Fe O was reduced to Fe O (335 C)
2
3
3
4
in the first step. The second step involved the reduction of Fe O
3
4
◦
to FeO (496 C), which was subsequently reduced to metallic Fe
3.2. Catalytic performance in the decomposition of phenethyl
◦
(
596 C). As shown in Fig. 6, Pd –FeX /OMC (X = 0.25, 0.7, 1.5, and
1
phenyl ether
4
) catalysts also showed three distinct peaks, but the peaks moved
toward low temperature compared to those of Fe/OMC catalyst. It
is well known that reducibility of bimetallic catalyst was strongly
affected by the interaction between two metal species [40,41]. In
particular, bimetallic catalysts containing noble metal facilitate the
reduction of other transition metal due to hydrogen transfer from
their reduced species [41,42]. Therefore, it can be inferred that
activated hydrogen on the reduced palladium species was trans-
ferred to unreduced Fe species during the reduction process, which
promoted the reduction of Fe species. In addition, the negative
peak corresponding to palladium hydride (PdHX ) disappeared upon
addition of Fe, indicating that iron species suppressed the formation
of PdHX phases due to the strong interaction between metal species
Fig. 7 shows the reaction pathways for decomposition of
phenethyl phenyl ether (PPE). According to the literatures [43,44],
the cleavage of ␤-O-4 linkage follows parallel-consecutive reac-
tion pathways. Aromatic compounds are produced by the cleavage
of C␤ O bond in phenethyl phenyl ether, and consecutive hydro-
genation of aromatic ring leads to the formation of aromatic
ring-saturated products. On the other hand, hydrogenation of aro-
matic ring in phenethyl phenyl ether occurs prior to C␤ O bond
cleavage, which also produces aromatic ring-saturated products.
In our catalytic reaction, aromatic compounds such as benzene,
phenol, and ethylbenzene were produced by the cleavage of C
O
bond in phenenthyl phenyl ether. Aromatic ring-saturated products
including cyclohexane derivatives (cyclohexane, cyclohexanol, and
ethylcyclohexane) and fully hydrogenated PPE were also produced
by the hydrogenation of aromatic rings. Light hydrocarbon (C –C )
[
39]. With increasing Fe content (Fe/Pd ≥ 1.5), furthermore, inten-
◦
sity of reduction peak above 350 C was remarkably increased. This
might be because a fraction of Fe remained as metallic species on
the carbon support, as evidenced by XRD results. Thus, it is expected
1
3
and phenylethyl benzene were produced as by- products.

J.K. Kim et al. / Journal of Molecular Catalysis A: Chemical 410 (2015) 184–192
189
Table 2
Catalytic performance of reduced Pd1–FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4) catalysts in the catalytic decomposition of phenethyl phenyl ether (PPE) performed at 250 C and
◦
1
0 bar for 3 h.
Catalyst
Conversion of
PPE (%)
Selectivity (%)
Yield for aromatics
Aromatics
Total selectivity
for aromatics
Aromatic ring-saturated product
By-products
Benzene Phenol Ethylbenzene
Cyclohexane
derivatives
Fully hydrogenated
PPE
No catalyst
Pd1–Fe0/OMC
Pd1–Fe0.25/OMC 88.3
Pd1–Fe0.7/OMC 75.6
Pd1–Fe1.5/OMC 63.2
3.2
94.1
2.7
7.2
13.6
26.5
22.8
21.4
40.3
5.4
6.2
21.3
23.3
25.6
34.5
3.4
12.3
27.2
27.6
28.4
77.5
16.0
32.1
75.0
73.7
75.4
2.8
2.5
15.6
9.8
2.6
6.1
6.2
17.2
9.1
9.6
10.0
9.4
9.9
2.5
15.1
28.3
56.7
46.6
31.1
59.3
48.5
12.4
10.8
8.5
Pd1–Fe4/OMC
41.2
Table 3
H2-TPD results of reduced Pd1–FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4) catalysts.
Catalyst
Amount of desorbed hydrogen (␮mol-H2/g-cat.)a
Pd1–Fe0/OMC
Pd1–Fe0.25/OMC
Pd1–Fe0.7/OMC
Pd1–Fe1.5/OMC
Pd1–Fe4/OMC
128.2
92.9
79.1
52.8
37.8
Pd -Fe /OMC
1
0
a
◦
Calculated from peak area of H2-TPD profile (<600 C) in Fig. 7.
Pd -Fe0.25/OMC
1
a function of Fe/Pd molar ratio (X) as shown in Fig. 8. Conversion
of phenethyl phenyl ether decreased with increasing Fe/Pd molar
ratio. On the other hand, selectivity for aromatics increased with
increasing Fe/Pd molar ratio up to 0.7 (Fe/Pd < 0.7), but it was almost
constant at high Fe/Pd molar ratio (Fe/Pd ≥ 0.7). The compensation
between conversion of phenethyl phenyl ether and selectivity for
aromatics led to a volcano-shaped curve of aromatic yield with
respect to Fe/Pd molar ratio. Thus, an optimal Fe/Pd molar ratio was
required for achieving maximum catalytic performance toward
Pd -Fe0.7/OMC
1
Pd -Fe1.5/OMC
1
aromatic production. Among the catalysts tested, Pd –Fe0.7/OMC
1
catalyst showed the highest yield for aromatics.
3
.3. Hydrogen adsorption study on the reduced Pd –FeX /OMC
1
catalysts
Pd -Fe /OMC
1
4
0
100
200
300
400
500
Catalytic decomposition of lignin through hydrogenation
including hydrogenolysis) on metal sites occurs via following
steps; (1) gas phase hydrogen is dissociatively adsorbed and acti-
vated on the surface of metal sites, and (2) adsorbed hydrogen on
the surface of metal sites reacts with oxygen groups of reactant or
o
Temperature ( C)
(
Fig. 6. TPR profiles of calcined Pd1–FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4) catalysts.
Catalytic performance of the reduced Pd –FeX /OMC (X = 0, 0.25,
C C bond in lignin, leading to the cleavage of C O or C C bond
1
0
.7, 1.5, and 4) catalysts in the decomposition of phenethyl phenyl
in reactant, respectively. Therefore, hydrogen adsorption ability on
the surface of metal sites plays an important role in the catalytic
decomposition of lignin model compounds [45,46]. For this reason,
◦
ether performed at 250 C and 10 bar for 3 h is summarized in
Table 2. In the absence of catalyst, conversion of phenethyl phenyl
ether was very low although selectivity for aromatics was high,
resulting in low yield for aromatics. This result indicates that cat-
alytic decomposition of phenethyl phenyl ether does not proceed
in the absence of catalyst because C O and CC bonds are ther-
mally stable. All the catalysts showed an enhanced conversion
of phenethyl phenyl ether compared to the case of no catalyst.
Pd –Fe /OMC catalyst was very active for hydrogenation of aro-
H -TPD measurement was carried out in order to investigate the
2
hydrogen adsorption ability of the reduced catalysts.
Fig. 9 shows the H -TPD profiles of the reduced Pd –FeX /OMC
2
1
(X = 0, 0.25, 0.7, 1.5, and 4) catalysts. H -TPD profiles of the reduced
2
catalysts could be deconvoluted into two hydrogen desorption
◦
peaks. According to the literature [47], the peak above 600 C
is related to the gasification of carbon support. In this work,
1
0
◦
matic ring in phenethyl phenyl ether, leading to the formation of
aromatic ring-saturated products. On the other hand, bimetallic
catalysts showed the relatively low activity toward hydrogena-
therefore, hydrogen desorption peak below 600 C, which was
related to the desorption of hydrogen from the reduced palla-
dium and iron species, was only considered for quantification.
tion of aromatic ring. Catalytic performance of Pd –FeX /OMC (X = 0,
The amount of desorbed hydrogen was calculated from the
1
◦
0.25, 0.7, 1.5, and 4) catalysts was strongly influenced by Fe/Pd
deconvoluted peak area of H -TPD profile (<600 C), as listed in
2
molar ratio. In order to visualize the effect of Fe/Pd molar ratio on
Table 3. It was found that the amount of desorbed hydrogen was
greatly affected by Fe/Pd molar ratio. The amount of hydrogen
desorbed from the reduced catalysts decreased in the order
the catalytic performance of bimetallic Pd –FeX /OMC (X = 0, 0.25,
1
0.7, 1.5, and 4) catalysts, conversion of phenethyl phenyl ether,
selectivity for aromatics, and yield for aromatics were plotted as
of Pd –Fe /OMC > Pd –Fe
/OMC > Pd –Fe /OMC > Pd –Fe1.5/
1
0
1
0.25
1 0.7 1

1
90
J.K. Kim et al. / Journal of Molecular Catalysis A: Chemical 410 (2015) 184–192
Fig. 7. Reaction pathways for decomposition of phenethyl phenyl ether (PPE).
1
00
425
8
6
4
2
0
0
0
0
6
22
6
Pd -Fe /OMC
1
0
4
83
32
Pd -Fe0.25/OMC
1
4
94
Conversion of phenethyl phenyl ether
Selectivity for aromatics
Yield for aromatics
629
Pd -Fe0.7/OMC
1
0
0.25
X in Pd -Fe /OMC catalysts
0.7
1.5
4
5
07
5
6
34
1
X
Pd -Fe1.5/OMC
1
Fig. 8. Catalytic performance of Pd1–FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4) catalysts
in the decomposition of phenethyl phenyl ether, plotted as a function of Fe/Pd
molar ratio (X). Reaction conditions: temperature = 250 C, pressure = 10 bar (H2),
◦
34
6
30
and time = 3 h.
Pd -Fe /OMC
1
4
1
00
200
300
400
500
600
700
OMC > Pd –Fe /OMC. This trend was well consistent with the trend
o
1
4
Temperature ( C)
of conversion of phenethyl phenyl ether (Fig. 8). It is known that the
enhancement in the amount of chemisorbed hydrogen is beneficial
to improve the catalytic activity. However, it was observed that
Pd –Fe /OMC catalyst had a strong tendency to saturate aromatic
Fig. 9. H2-TPD profiles of reduced Pd1–FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4) catalysts.
1
0
Pd –FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4) catalysts were carried
ring in the reaction. This is because the surface of Pd with strong
hydrogen adsorption ability stabilizes the ␲-bonded complex
with aromatic rings and participates in hydrogenation, resulting
in cleavage of C C bond in aromatic ring [18]. With increasing
Fe/Pd molar ratio, on the other hand, conversion of phenethyl
phenyl ether was monotonically decreased (Fig. 8). This trend can
be explained by the dilution effect of inactive Fe onto Pd, which
decreases the total number of available active sites for hydrogen
adsorption. Therefore, it is believed that moderate hydrogen
adsorption ability was required for enhanced aromatic production.
However, no correlation between hydrogen adsorption ability and
aromatic selectivity was found. Therefore, our next investigation
was focused on the effect chemical state of metal species on the
aromatic selectivity. For this purpose, XPS analyses of the reduced
1
out.
3.4. XPS study of reduced Pd –FeX/OMC catalysts
1
Fig. 10 shows the XPS spectra of Pd 3d (Fig. 10(a)) and Fe
2p3/2 (Fig. 10(b)) levels of the reduced Pd –FeX /OMC (X = 0, 0.25,
0.7, 1.5, and 4) catalysts. The Pd 3d spectra were deconvoluted
into Pd5/2 (solid line) and Pd3/2 (dashed line) peaks, and then
1
0
Pd 3d spectra were divided into Pd (335.2 eV and 340.5 eV for
3d5/2 and 3d3/2, respectively) and Pd2+ (337.1 eV and 341.9 eV for
3d5/2 and 3d3/2, respectively) species [48,49]. The Fe 2p
were assigned to metallic Fe (707.6 eV), Fe (709.6 eV), and Fe
(710.9 eV) species [50]. Interestingly, the binding energy of Pd 3d
spectra
3/2
0
2+
3+

J.K. Kim et al. / Journal of Molecular Catalysis A: Chemical 410 (2015) 184–192
191
Pd2+
Pd2+
Fe0
Fe2+ Fe3+
High electronic density of Pd
Pd0
Electron-deficient Fe
(a)
(b)
Pd 3d5
Pd 3d3
/2
/2
Pd -Fe /OMC
Pd -Fe0.25/OMC
1
0
1
Pd -Fe0.25/OMC
1
Pd -Fe0.7/OMC
1
Pd -Fe0.7/OMC
1
Pd -Fe1.5/OMC
1
Pd -Fe1.5/OMC
1
Pd -Fe /OMC
1
4
Pd -Fe /OMC
1
4
3
30
332
334
336
338
340
342
344
346
700
702
704
706
708
710
712
714
716
Binding energy (eV) of Pd 3d
Binding energy (eV) of Fe 2p3/2
Fig. 10. XPS spectra of (a) Pd 3d and (b) Fe 2p3/2 levels in the reduced Pd1–FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4) catalysts.
Table 4
XPS analyses results of reduced Pd1–FeX /OMC (X = 0, 0.25, 0.7, 1.5, and 4) catalysts.
Catalyst
Ratio of Pd speciesa
Ratio of Fe speciesa
Fe␦+/Fetotalb
Fe ␦+/Pdb
Fe␦+/(Pd0 + Fe␦+)b
Composition of bimetallic structure
0
Pd /Pdtotal
Pd1–Fe0/OMC
Pd1–Fe0.25/OMC
Pd1–Fe0.7/OMC
Pd1–Fe1.5/OMC
Pd1–Fe4/OMC
0.52
0.59
0.68
0.73
0.74
–
–
–
–
0.28
0.45
0.47
0.49
0.49
0.65
0.66
0.65
0.29
0.40
0.39
0.40
Pd1Fe0.4
Pd1Fe0.7
Pd1Fe0.7
Pd1Fe0.7
a
Calculated from deconvoluted peak area of XPS spectra in Fig. 8.
b
2
≤ ␦ ≤ 3.
iron atoms (Fe␦+) was in contact with metallic palladium atoms
(Pd ), forming a bimetallic phase on the catalyst surface [51,52] .
level in the Pd –FeX /OMC (X = 0.25, 0.7, 1.5, and 4) catalysts slightly
shifted toward low value with increasing Fe/Pd molar ratio (Fig. 10
1
0
(
a)). On the other hand, the binding energy of Fe 2p3/2 level in
In order to confirm the chemical state of bimetallic phase in the
the Pd –FeX /OMC (X = 0.25, 0.7, 1.5, and 4) catalysts shifted toward
1
X
Pd –Fe /OMC catalysts, the chemical state ratios of Pd and Fe were
1
high value with increasing Fe/Pd molar ratio (Fig 10(b)). The change
of electronic properties was attributed to charge transfer from Fe
to Pd, which was related to the relative electronegativity of each
metal (1.8 and 2.2 for Fe and Pd, respectively, according to the Paul-
ing’s scale) [34,35]. The charge transfer between two metal species
induces polarity on bimetallic phase. In the case of Pd on the Pd–Fe
surface, electron-rich palladium atoms decreased the stability of
quantified as summarized in Table 4. Chemical state of each metal
species was obtained by integrating each deconvoluted XPS peak
0
area for Pd and Fe species. It was found that Pd /Pd
increased
total
with increasing Fe/Pd molar ratio, and Fe␦ /Fetotal also increased
+
␦
+
with increasing Fe/Pd molar ratio. Interestingly, Fe /Pd showed
no significant difference at high Fe/Pd molar ratio above 0.7. This
result suggests that Pd and Fe are in contact with each other in the
same molar ratio at high Fe/Pd molar ratio above 0.7. In addition, the
␲
-bonded complex with aromatic ring, resulting in the suppressed
result of Fe␦ / (Pd + Fe ) molar ratios of Pd –Fe /OMC (X = 0.7, 1.5,
+
0
␦+
rate of hydrogenation of C C bond in aromatics. In the case of Fe on
the Pd–Fe surface, on the other hand, electron- deficient Fe atoms
acted as adsorption sites for activating oxygenated group and facil-
itating hydrogenation of C O bond in phenethyl phenyl ether. For
1
X
and 4) catalysts revealed that the bimetallic catalysts were mainly
composed of bimetallic structure of Pd Fe composition at high
1
0.7
Fe/Pd molar ratio above 0.7; the remaining fraction of metal species
existed as a metallic state on the carbon support, as evidenced
by STEM-EDX, XRD and TPR results. This result was well consis-
tent with the previous work [34], reporting that metallic Fe alone
this reason, bimetallic Pd –FeX /OMC (X = 0.25, 0.7, 1.5, and 4) cat-
1
alysts were more efficient in the selective cleavage of C O bond in
phenethyl phenyl ether to aromatics than Pd –Fe /OMC catalyst.
1
0
Judging from XPS results, it can be inferred that electron-deficient

1
92
J.K. Kim et al. / Journal of Molecular Catalysis A: Chemical 410 (2015) 184–192
was catalytically inactive for the hydrogenation C C bond in aro-
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