J.K. Kim et al. / Applied Catalysis A: General 498 (2015) 142–149
143
In order to investigate decomposition mechanism of lignin inner
bond, dimeric lignin model compounds have been used as a feed-
stock due to the complex structure of lignin [1,2]. Therefore, various
dimeric chemical compounds containing C O bonds such as ␣-O-4,
-O-4, and -5 bonds are used as lignin model compounds, because
570 ◦C for Fe/OMC for 3 h in a nitrogen stream. The metal loading
in the monometallic catalysts was fixed at 10 wt%.
2.2. Characterization
C
O bonds are abundant linkage type in lignin [1,2]. Among vari-
For characterization of calcined catalysts, Pd/OMC, Pd–Fe/OMC,
and Fe/OMC were calcined at 250 ◦C, 450 ◦C, and 570 ◦C,
respectively. For characterization of reduced catalysts, Pd/OMC,
Pd–Fe/OMC, and Fe/OMC were reduced at 250 ◦C, 450 ◦C, and
570 ◦C, respectively.
ous lignin model compounds, benzyl phenyl ether has been widely
used as a lignin model compound for representing ␣-O-4 bond in
lignin [1].
Carbon material has been widely used as a catalyst support
ordered mesoporous carbon (OMC) offers significantly advan-
tage over conventional carbon support due to uniform pore size
distribution, efficient mass transfer of reactant molecules, and con-
trollable textural properties [25–28]. These porous properties make
OMC well suited as a potential candidate material for catalyst sup-
port.
In this work, bimetallic Pd–Fe catalyst supported on ordered
mesoporous carbon (OMC) was prepared by a surfactant tem-
plate method and a subsequent incipient wetness impregnation
method (Pd–Fe/OMC). For comparison, monometallic Pd and Fe
catalysts supported on ordered mesoporous carbon (Pd/OMC and
Fe/OMC) were also prepared. The catalysts were applied to the
catalytic cleavage of C O bond in benzyl phenyl ether to aromat-
ics. The effect of bimetallic combination on the catalytic activities
and physicochemical properties of Pd–Fe/OMC catalyst was inves-
tigated.
Nitrogen adsorption–desorption measurements were con-
ducted to investigate textural properties of the catalysts using an
ASAP-2010 (Micromeritics) instrument. Surface areas of the cata-
lysts were calculated by BET method, and pore volume and average
pore diameter were determined by the BJH method applied to the
desorption branches of nitrogen isotherms. Chemical compositions
of the catalysts were determined by ICP-AES analyses (Optima-
4300 DV, Perkin-Elmer). Crystalline states of the catalysts were
investigated by XRD (D-MAX-2500-PC, Rigaku) measurements
using Cu-K␣ radiation operated at 40 kV and 100 mA. Average par-
ticle sizes of Pd and Fe in the catalysts were calculated using
the Debye–Scherrer equation. Pore structure, pore size, and par-
ticle size distribution of the catalysts were examined by HR-TEM
(JEM-3100, JEOL) analyses. In order to confirm detailed dispersion
of metal species, STEM analyses (JEM-2100F, JEOL) were con-
ducted with energy dispersed X-ray spectroscopy (EDX) mapping.
Temperature-programmed reduction (TPR) analyses of the cata-
lysts 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% H2 (2 ml/min) and N2 flow
(20 ml/min) at temperatures ranging from room temperature to
700 ◦C with a heating rate of 10 ◦C/min. X-ray photoelectron spec-
troscopy (XPS) analyses (AXIS-HIS, KRATOS) were carried out to
measure binding energies of metallic palladium and iron in the
reduced catalysts. For the XPS analyses, the calcined catalysts were
reduced using an ex situ reduction system at 250 ◦C for Pd/OMC,
at 450 ◦C for Pd–Fe/OMC, and at 570 ◦C for Fe/OMC for 4 h with a
heating rate of 5 ◦C/min under 5% H2/Ar flow (50 ml/min), and the
catalysts were then transported to glass jar with sample holder in
argon atmosphere glove box to minimize air exposure. After out-
gassing 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 refer-
ence. H2 temperature-programmed desorption (H2-TPD) analyses
of the reduced catalysts were conducted using a BELCAT-B instru-
ment (BEL Japan). Prior to the H2-TPD measurements, 10 mg of
calcined catalyst was reduced at different temperature (at 250 ◦C
for Pd/OMC, at 450 ◦C for Pd–Fe/OMC, and at 570 ◦C for Fe/OMC) for
4 h with a heating rate of 5 ◦C/min under 5% H2/Ar flow (50 ml/min),
and then 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), 5% H2/Ar mixed gas (50 ml/min) was injected for
60 min at 250 ◦C. The sample was purged under Ar flow (50 ml/min)
to remove physisorbed hydrogen, and subsequently, furnace tem-
perature was increased from room temperature to 750 ◦C at a
heating rate of 5 ◦C/min under Ar flow (50 ml/min). The desorbed
hydrogen was detected using a TCD (thermal conductivity detec-
tor).
2. Experimental
2.1. Preparation of catalysts
and
a subsequent incipient wetness impregnation method
(Pd–Fe/OMC). Ordered mesoporous carbon (OMC) was prepared
by a single step surfactant-templating method according to the
method reported in the literatures [28,29]. PEO-PPO-PEO tri-block
copolymer (P123) was dissolved in 1.5 M HCl solution at 40 ◦C
for 3 h. Sucrose (carbon precursor) and H2SO4 solution were then
added into the solution for 1 h under stirring. After tetraethoxysi-
lane (TEOS, silica precursor) was slowly added into the solution, the
resulting mixture was stirred at 40 ◦C for 24 h and then it was main-
tained at 100 ◦C for 20 h under static condition for self-assembly of
micelle structure. The resultant was dried at 100 ◦C for 48 h, and it
was carbonized at 800 ◦C for 4 h to obtain a silica-carbon compos-
ite. The obtained 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.
For co-impregnation of palladium and iron metals onto
OMC, palladium chloride (PdCl2, Sigma–Aldrich) and iron nitrate
(Fe(NO3)3, Junsei) were dissolved in acetone containing 0.1 M HCl.
The precursor solution was then introduced into the pores of OMC
by an incipient wetness impregnation method. The supported cat-
alyst was dried at 50 ◦C overnight and calcined at 450 ◦C for 3 h in a
nitrogen stream. The resulting catalyst was denoted as Pd–Fe/OMC.
Palladium and iron loadings in the catalyst were fixed at 5 wt%,
respectively (total metal loading = 10 wt% and Pd:Fe ratio = 1:1).
For comparison, monometallic Pd and Fe catalysts supported
on ordered mesoporous carbon (denoted as Pd/OMC and Fe/OMC,
respectively) were also prepared by an incipient wetness impreg-
nation method according to the similar method described above.
The supported catalysts were calcined at 250 ◦C for Pd/OMC and at
2.3. Catalytic reaction
Catalytic cleavage of C O bond in benzyl phenyl ether to aro-
matics was carried out in a stainless steel autoclave reactor (25 ml)
under hydrogen atmosphere. Prior to the reaction, Pd–Fe/OMC cat-
alyst was reduced using an ex situ reduction system at 450 ◦C for
4 h with a heating rate 5 ◦C/min under 5% H2/N2 flow (50 ml/min).