Recyclable pd-incorporated perovskite-titanate catalysts synthesized in molten salts for the liquid-phase oxidation of alcohols with molecular oxygen

Article abstract of DOI:10.1246/bcsj.20120215

A series of Pd-incorporated titanate catalysts (Pd/KSTO) were synthesized by the intercalation of Pd(NO3)2 into layered potassium titanate (KTO), which proceeded via a cation-exchange reaction in molten salts. Perovskite phases of Pd/KSTO for catalysis were synthesized at 600 °C and above, whereas a lepidocrocite-type layered titanate structure, similar to that of the parent KTO, was retained when a lower temperature of 400 °C was used during synthesis. In both cases, the Pd species exists as divalent Pd. The synthesized Pd/KSTO catalysts were then applied to the liquid-phase oxidation of alcohols using molecular oxygen without any additives. The perovskite-type Pd/KSTO catalyst displayed superior activity when compared to that of the lepidocrocite-type, yielding a high turnover number of 800 in the aerobic oxidation of 1-phenylethanol with no loss of catalytic activity after three runs in catalyst recycling experiments.

Full text of DOI:10.1246/bcsj.20120215

146
Bull. Chem. Soc. Jpn. Vol. 86, No. 1, 146-152 (2013)
© 2013 The Chemical Society of Japan
Recyclable Pd-Incorporated Perovskite-Titanate Catalysts Synthesized
in Molten Salts for the Liquid-Phase Oxidation
of Alcohols with Molecular Oxygen
Indri Badria Adilina,1,2 Takayoshi Hara,1 Nobuyuki Ichikuni,1
Nobuhiro Kumada,3 and Shogo Shimazu*1
1Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522
2Research Centre for Chemistry, Indonesian Institute of Sciences, Kawasan Puspiptek Serpong,
Tangerang 15314, Indonesia
3Graduate School of Medicine and Engineering, University of Yamanashi, 4-4-37 Takeda, Kofu, Yamanashi 400-8510
Received August 9, 2012; E-mail: shimazu@faculty.chiba-u.jp
A series of Pd-incorporated titanate catalysts (Pd/KSTO) were synthesized by the intercalation of Pd(NO3)2 into
layered potassium titanate (KTO), which proceeded via a cation-exchange reaction in molten salts. Perovskite phases of
Pd/KSTO for catalysis were synthesized at 600 °C and above, whereas a lepidocrocite-type layered titanate structure,
similar to that of the parent KTO, was retained when a lower temperature of 400 °C was used during synthesis. In both
cases, the Pd species exists as divalent Pd. The synthesized Pd/KSTO catalysts were then applied to the liquid-phase
oxidation of alcohols using molecular oxygen without any additives. The perovskite-type Pd/KSTO catalyst displayed
superior activity when compared to that of the lepidocrocite-type, yielding a high turnover number of 800 in the aerobic
oxidation of 1-phenylethanol with no loss of catalytic activity after three runs in catalyst recycling experiments.
The selective oxidation of alcohols to carbonyl compounds
in the aqueous phase.11,12 These Pd-intercalation catalysts have
highly stable divalent Pd species and present remarkable
activities for the aerobic oxidation of various alcohols. Herein,
we explore the use of a different type of layered material,
layered titanates, as a new catalyst support for the stable incor-
poration of Pd species toward the green oxidation of alcohols.
Layered titanates have attracted great interest due to their use
in interlayer chemistry,25,26 photooxidation catalysis,27,28 and
vapor-phase reactions, such as CO oxidation.29 However, their
application as a catalyst for liquid-phase oxidation reactions is
rarely studied. Layered titanates consist of titanium-oxygen
octahedral nanosheets, in which the interlayer regions are
occupied by alkali metals.30 This feature produces negatively
charged titanate layers that provide the ability for cation
exchange under molten salt conditions,31,32 offering a new
technique for the immobilization of Pd catalysts. This tech-
nique differs from other common immobilization techniques,
for example, impregnation, grafting, sol-gel process, atom-
planting, or ion exchange.33-36 Under molten salt conditions,
the catalyst precursors are heated to a temperature high above
the melting point of the selected salts, at which point a
homogeneous liquid phase is formed. This liquid acts as
the solvent for the dissolution and diffusion of the reactants.
After cooling to room temperature, the liquid becomes a well-
dispersed and stable material.37 This phenomenon may lend
the Pd catalysts stability at higher reaction temperatures and
prevent the Pd from leaching during the oxidation reactions.
In the present work, a series of Pd-incorporated titanate
catalysts (Pd/KSTO) were synthesized from layered potassium
is one of the most important synthetic steps for the formation
of valuable intermediates in the production of fine chemicals.1
Under the current stringent environmental regulations, the
combination of a green oxidant, such as molecular oxygen,
with a high-performance heterogeneous catalyst is believed to
be a step toward the design of an environmentally benign and
sustainable oxidation process.2,3 Several transition-metal-based
heterogeneous catalysts that possess the ability to activate
molecular oxygen have been developed.4-7 Among them,
supported Pd catalysts in many cases have continuously shown
the highest performance under mild reaction conditions.8-13
A
number of unresolved issues remain; however, they primarily
concern the necessity of an added base for reaction initia-
tion14,15 or the leaching of Pd species from the catalyst support,
which may cause difficulty for catalyst recycling.16 The choice
of a good catalyst support and the use of a suitable immo-
bilization technique are important factors to achieve a stable
heterogeneous Pd catalyst.
Layered materials have unique properties, including strong
electrostatic interactions between nanosheets and guests, order-
ly stacking, and the ability to intercalate various guest species
displaying different functionalities.17,18 These propitious prop-
erties of layered compounds allow the compounds to be cata-
lytic supports that can stabilize active species via strong electro-
static interactions, order reaction space, and control catalytic
functionality.19-24 We have previously reported intercalation
catalysts prepared by the incorporation of Pd complexes into a
clay mineral interlayer, using a simple anion-exchange method
Published on the web January 10, 2013; doi:10.1246/bcsj.20120215

I. B. Adilina et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
147
titanate KTO (K0.6Ti1.85O4, cation-exchange capacity = 340
mequiv/100 g) via cation exchange under molten salt condi-
tions. The synthesized Pd/KSTO catalysts were characterized
by powder X-ray diffraction (XRD) and X-ray absorption fine
structure (XAFS). Subsequently, their catalytic activity for the
oxidation of primary and secondary alcohols was evaluated
under an atmosphere of molecular oxygen.
sonicated at room temperature for 30 min and then heated to
400, 600, or 700 °C for 3 h. Following the reaction, the mixture
was cooled to room temperature, washed with distilled water
to remove excess Sr, and dried at 50 °C. Three different
Pd/KSTO catalysts were obtained, denoted as Pd/KSTO(400),
Pd/KSTO(600), and Pd/KSTO(700), where the values in
parentheses represent the catalysts synthesized at 400, 600,
and 700 °C, respectively. The chemical formula of Pd/KSTO
catalysts were found as Pd0.002K0.17Ti1.86Sr0.20O4 for the
lepidocrocite-type catalyst and Pd0.003K0.03Ti1.47Sr1.02O4 for
the perovskite-type (Table S1). The surface areas estimated
Experimental
General. All alcohols used as substrates in the oxidation
reactions were purchased from either Wako Pure Chemical Ind.
Ltd. or Tokyo Chemical Industry Co., Ltd. and were distilled
under vacuum before use. The solvents®toluene, acetonitrile,
and trifluorotoluene®were purified before use by literature
procedures.38 Solid support layered potassium titanate KTO
(K0.6Ti1.85O4) was obtained from Otsuka Chemical Co., Ltd.
The synthesized Pd/KSTO catalysts were characterized by
powder X-ray diffraction (XRD) acquired at room temperature
from 2 to 60° on a MAC SCIENCE MXP3V system. The
diffractometer was operated at 35 kV and 15.0 mA, utilizing
Cu K¡ radiation. Structural refinement was performed by Le
Bail intensity extraction, using the Rietica Ver.1.7.7 program.39
The perovskite-type Pd/KSTO catalyst crystallizes using a
nonlayered, perovskite-type SrTiO3 as the reference, in the
space group I4/mcm, with cell constants of a = 5.511 ¡ and
c = 7.796 ¡ and a cell formula of Z = 4. The final R factors
obtained were Rp = 11.930, Rwp = 12.300, Rexp = 23.990, and
X2 = 0.263. N2 adsorption-desorption isotherms and BET
specific surface areas were measured at ¹196 °C with a
volumetric adsorption analyzer BELSORP-max manufactured
by BEL Japan, Inc. and all samples were heated at 200 °C for
2 h prior to the analysis. The amount of Pd incorporated into the
KTO (0.1 wt %, 0.01 mmol g¹1) was determined by inductively
coupled plasma atomic emission spectroscopy (ICP-AES),
using a Seiko Instruments Inc. SPS 1700 HVR spectrometer
after dissolving the sample in 0.1 M HNO3 (35%). X-ray
absorption spectra around the Pd K-edge were recorded in
fluorescence mode using a Ge-19 solid state detector at the
BL01B1 beamline of the SPring-8 (8 GeV, 100 mA) at the
Japan Synchrotron Radiation Research Institute (Proposal
No. 2012A1316). A Si(311) double crystal monochromator
and an ion chamber filled with Ar (100%), for I0 detection,
were used for the experiment. Energy calibration was carried
out using Pd foil (25 ¯m thickness). Pd K-edge XANES and
EXAFS spectra of all samples were recorded in the quick scan
mode, and data reduction was performed using the REX2000
Ver.2.3.3 program (Rigaku). The spectra were extracted
utilizing the cubic spline method and normalized to the edge
height. The k3-weighted EXAFS oscillation was then Fourier
transformed into r space, with the Fourier transformation range
between 0.35 and 1.35 nm.
¹1
by BET method were 9, 11, and 9 m2 g for Pd/KSTO cata-
lysts synthesized at 400, 600, and 700 °C, respectively. After
the catalytic reaction using Pd/KSTO(600) catalyst, the BET
specific surface area of the recovered catalyst was found as
¹1
10 m2 g
.
Typical Procedure for the Aerobic Alcohol Oxidation.11,12
Pd/KSTO (Pd: 0.125 mol %) was placed in a Schlenk flask
equipped with a reflux condenser and mechanical stirrer.
A quantity of 1-phenylethanol (0.4 mmol) and toluene (3 mL,
as the solvent) was then added to the flask, and the solution
was kept under an O2 atmosphere (0.1 MPa) at 120 °C for
24 h. The product composition was confirmed by GC and GC-
MS analysis with either biphenyl or p-dichlorobenzene as an
internal standard. GC analysis was performed on a Shimadzu
Gas Chromatograph GC-17A packed with either a Thermon
3000 or KOCL-3000T column. The mass spectra were col-
lected using a Shimadzu GC-MS 2010 equipped with an Rtx-
5MS column. Substrate scope studies of various alcohols were
carried out using the same procedure.
Catalyst Recycling Experiments. After the first catalytic
reaction, the catalyst was separated by centrifugation, washed
with toluene (3 © 10 mL), dried in vacuo, and then heated to
200 °C before reuse. The recovered catalyst was then charac-
terized by XRD, and the next cycle of oxidation was performed
as previously described above.
Results and Discussion
Characterization of Catalysts. The XRD patterns of the
parent KTO and of all of the Pd/KSTO catalysts are shown
in Figure 1. As observed for the Pd/KSTO(400) catalyst,
characteristic phases of a lepidocrocite-type layered titanate41,42
were observed at diffraction angles of 11.8 (020), 24.1 (040),
and 29.6° (130). The diffraction pattern of the Pd/KSTO(400)
catalyst is similar to that of the parent KTO, which also
displays the (020), (040), and (130) planes found at diffraction
angles of 11.3, 23.9, and 28.6°, respectively. This result shows
that the lepidocrocite-type layered structure was retained, even
after intercalation, when the temperature during synthesis was
400 °C. A slight shift in the peaks to higher diffraction angles
shows that the cation exchange occurred within the unit cell
of the Pd/KSTO(400) catalyst and that the unit cell is slightly
smaller when compared to the starting KTO. When a higher
synthesis temperature of 600 or 700 °C was used, a different
diffraction pattern was observed. New peaks corresponding
to the (100), (110), (111), and (200) planes were detected in
both the Pd/KSTO(600) and the Pd/KSTO(700) catalysts at
angles of 22.6, 32.3, 39.9, and 46.4°, respectively, and these
peaks are assignable to a typical perovskite structure.43,44 The
Synthesis of Pd/KSTO Catalysts. Pd/KSTO was syn-
thesized using the molten salts method described previously.40
To an Al2O3 crucible, KTO (1.00 g) was inserted. This step was
followed by the addition of a solution of Pd(NO3)2 (8 mL, 0.08
mmol), Sr(NO3)2 (1.20 g, 5.7 mmol), SrCl2¢6H2O (1.52 g, 5.7
mmol), and H2O (7 mL). A combination of Sr precursors was
required for the synthesis to form a eutectic system, which
decreases the melting point of the components. The mixture was

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Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
Recyclable Pd-Incorporated Perovskite-Titanate Catalysts
5
(e)
(f)
4
(d)
(e)
(d)
(c)
(b)
(a)
3
2
1
0
(c)
(b)
(a)
10
20
30
40
50
24320 24340 24360 24380 24400
Photon energy/eV
2θ/ degree
Figure 1. Powder X-ray diffraction patterns of the catalyst;
(a) KTO, (b) KSTO(400), (c) KSTO(600), (d) Pd/
KSTO(400), (e) Pd/KSTO(600), and (f) Pd/KSTO(700).
Figure 2. Pd K-edge XANES spectra of (a) Pd foil,
(b) PdO, (c) Pd/KSTO(400), (d) Pd/KSTO(600), and
(e) recovered Pd/KSTO(600).
Pd-free catalyst precursors, KSTO(400) and KSTO(600), were
synthesized without the addition of Pd(NO3)2 solution and
displayed similar structure with the corresponding Pd/KSTO
catalysts, giving a lepidocrocite-type structure at synthesis
temperature of 400 °C and a perovskite-type structure at 600
and 700 °C.
nm (Pd-O bond) was observed for both catalysts, whereas in
the second coordination sphere of the Pd foil, a peak at 0.25 nm
(Pd-Pd bond) was difficult to detect. A peak at 0.30 nm (Pd-O-
Pd bond) was found only for the Pd/KSTO(400) catalyst and
not for the Pd/KSTO(600) catalyst. This difference suggests
that oligomeric Pd sites were formed for the lepidocrocite-
type Pd/KSTO(400) catalyst, whereas almost all of the Pd sites
present are monomeric in the perovskite-type Pd/KSTO(600)
catalyst.
The absence of peaks at lower diffraction angles showed that
the layered titanate structure changed to a nonlayered perov-
skite network when heated to temperatures of 600 °C or higher.
To confirm the perovskite-type structure, the crystalline lattice
of Pd/KSTO(600) was extracted by the Le Bail method
using the XRD data in the 10 to 80° range.39 The structural
refinement showed that the diffraction profiles could be pre-
cisely fitted with a nonlayered, perovskite-type SrTiO3 struc-
ture (Figure S1). Although the Pd sites are presumably located
in the interlayers of the Pd/KSTO(400) catalyst, with the
perovskite-type Pd/KSTO(600) and Pd/KSTO(700) catalysts,
they are predicted to be embedded in the B sites of the
perovskite crystalline lattice (ABO3). This prediction is based
on the nonlayered structure observed in previously reported
perovskite-based catalysts.45,46
The relationship between the Pd species and the structure
type of the Pd/KSTO catalysts was investigated by XAFS. The
Pd K-edge XANES spectra, shown in Figure 2, reveals that the
spectra of both the Pd/KSTO(400) and the Pd/KSTO(600)
catalysts do not resemble the spectra of Pd foil; instead, they
resemble the spectra of Pd oxide. This finding suggests that
the Pd species exists as divalent Pd in both types of the Pd/
KSTO catalysts, either in the lepidocrocite-type layered titanate
(Pd/KSTO(400)) or in the perovskite-type titanate catalyst
(Pd/KSTO(600)). The FT of the k3-weighted EXAFS data
(Figure S2) also supports this assessment. A peak at ca. 0.15
Catalytic Reactions. The catalytic activities of the Pd/
KSTO(400), Pd/KSTO(600), and Pd/KSTO(700) catalysts
were first tested on the catalytic oxidation of 1-phenylethanol,
as summarized in Table 1. The oxidation reaction was per-
formed at temperatures of 80, 100, and 120 °C. At a reac-
tion temperature of 80 °C, no reaction proceeded using the Pd/
KSTO(400) catalyst (Entry 1). When the temperature was
elevated to 100 °C, a 9% yield of the carbonyl product,
acetophenone, was detected (Entry 2). An increase of the
temperature to 120 °C resulted in a 91% yield (Entry 3),
however, up to a reaction temperature of 120 °C, the Pd/
KSTO(400) catalyst was unable to oxidize the 1-phenylethanol
to completion. Alternatively, in the case of the Pd/KSTO(600)
catalyst, a higher catalytic performance was observed. The
formation of acetophenone was observed even at a reaction
temperature of 80 °C, with a yield of 12% (Entry 4), and the
yield rose to 26% at 100 °C (Entry 5). When a reaction
temperature of 120 °C was used, the yield of acetophenone
increased to >99%, giving a turn over number (TON) of 800
without the addition of any bases (Entry 6). This high catalytic
activity was also demonstrated by the Pd/KSTO(700) catalyst,
which has the same perovskite-type titanate structure as the Pd/
KSTO(600) catalyst (Entry 7). From these results, it is evident

I. B. Adilina et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
149
OH
O
Pd/KSTO
H2O
+
+
1/2 O2
PhCH3, 24 h
Scheme 1. Aerobic oxidation of 1-phenylethanol using Pd/KSTO(600) catalyst.
Table 2. Effect of Radical Scavenger on the Aerobic
Oxidation of 1-Phenylethanol Using Pd/KSTO(600)
Catalysta)
Table 1. Aerobic Oxidation of 1-Phenylethanol Using
Various Pd/KSTO Catalystsa)
OH
O
OH
O
Pd/KSTO (Pd: 0.125 mol%)
O2, PhCH3, 24 h
Pd/KSTO(600) (Pd: 0.125 mol%)
O2, PhCH3, 24 h, radical scavenger
Temperature Conv.c) Yieldc)
Entry Catalystb)
TONd)
Temperature Conv.c) Yieldc)
/°C
/%
/%
Entry Radical scavengerb)
/°C
/%
/%
1
2
3
4
5
6
7
8
Pd/KSTO(400)
80
100
120
80
100
120
120
120
120
120
120
120
0
0
0
72
728
96
208
800
800
0
0
0
768
744
1
2
3
TEMPO
DBPC
Galvinoxyl
120
120
120
>99
>99
>99
>99
>99
>99
Pd/KSTO(400)
Pd/KSTO(400)
Pd/KSTO(600)
Pd/KSTO(600)
Pd/KSTO(600)
Pd/KSTO(700)
KTO
9
91
12
26
>99
>99
0
0
0
96
93
9
91
12
26
>99
>99
0
0
0
96
93
a) Reaction conditions: 1-phenylethanol, 0.4 mmol; Pd/
KSTO(600) catalyst, Pd: 0.125 mol %; O2 as oxidant, 0.1
MPa; PhCH3 as solvent, 3 mL; reaction time, 24 h. b) Addition
of radical scavenger (1 equiv relative to Pd). c) Determined by
GC using an internal standard method.
9
KSTO(400)
10
KSTO(600)
11e) Pd/KSTO(600)
12f) Pd/KSTO(600)
the catalytic reaction. The consumption of O2 gas was also
measured,12 and a ca. 1:2 ratio of O2 uptake to acetophenone
yield was observed. This finding showed that molecular
oxygen was quantitatively used as the oxidant in the oxidative
dehydrogenation (Scheme 1).
a) Reaction conditions: 1-phenylethanol, 0.4 mmol; Pd/KSTO
catalyst, Pd: 0.125 mol %; O2 as oxidant, 0.1 MPa; PhCH3
as solvent, 3 mL; reaction time, 24 h. b) Value in parentheses
defines the synthesis temperature. c) Determined by GC using
an internal standard method. d) Turnover number (TON)
based on Pd. e) Trifluorotoluene as solvent. f) Acetonitrile
as solvent.
The scope of substrates for the Pd/KSTO catalysts was
investigated using the perovskite Pd/KSTO(600) catalyst listed
in Table 3. In general, oxidations of the secondary alcohols
(Entries 1-4, 8, and 9) showed cleaner conversions and
higher selectivity toward the carbonyl product than those of
the primary alcohols (Entries 5-7). In addition to the activity
surrounding 1-phenylethanol, high activity was observed in
the oxidation of benzohydrol and benzoin, with 95% and
82% yields of the ketones, respectively (Entries 2 and 8).
The oxidation of secondary benzylic alcohols with electron-
releasing substituents, such as 1-(p-tolyl)ethanol, proceeded,
although at a lower activity, corresponding to a yield of 52%
of 1-(p-tolyl)ethanone (Entry 3). This result was also found
for secondary alcohols having an electron-withdrawing group,
such as 1-(4-fluorophenyl)ethanol, which yielded 30% of
the desired product (Entry 4). Despite the lower conversions,
the selectivity in all of the cases of secondary alcohols was
preserved. The highest activity for primary alcohols was seen
in the oxidation of benzyl alcohol, with 83% conversion and
a 60% yield of benzaldehyde (TON = 480) (Entry 5). The by-
products were estimated to be the corresponding carboxylic
acids caused by over-oxidation. In the oxidation of p-
tolylmethanol, a lower yield (40%) of the aldehyde product
was obtained (Entry 6). The formation of benzenepropanal
due to hydrogen transfer to the double bonds accompanied
the oxidation of cinnammyl alcohol, which yielded 50% of
cinnamaldehyde (Entry 7). The oxidation of cyclic aliphatic
that all of the reported Pd/KSTO catalysts give high catalytic
activity in the oxidation of 1-phenylethanol, with the perov-
skite-type Pd/KSTO catalysts displaying superior activity
compared to the lepidocrocite-type layered titanate catalyst.
In addition, control reactions under the same reaction con-
ditions were conducted using the starting material KTO and the
Pd-free catalyst precursors KSTO(400) and KSTO(600). These
catalyst precursors did not show any conversion of 1-phenyl-
ethanol (Entries 8-10), indicating that the oxidation was in
fact facilitated by the Pd/KSTO catalysts. Solvents other than
toluene, such as trifluorotoluene and acetonitrile, were also
tested at 80 and 120 °C. The reaction proceeded smoothly at
120 °C with 96% and 93% yields, respectively (Entries 11 and
12), while no reaction occurred at 80 °C.
The effect of radical scavengers (1 equivalent relative to
Pd) on the reactivity is shown in Table 2. These results con-
firmed that the addition of various radical scavengers, such
as 2,2¤,6,6¤-tetramethylpiperidine-N-oxyl (TEMPO), 2,6-di-
tert-butyl-p-cresol (DBPC), or galvinoxyl, did not inhibit the
reaction, and indicated that the catalytic system did not involve
any free radical intermediates. Addition of a higher amount
of DBPC (up to 5 equivalent relative to Pd) hardly inhibited

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Recyclable Pd-Incorporated Perovskite-Titanate Catalysts
Table 3. Substrate Scope for the Aerobic Oxidation of Alcohols Using Pd/KSTO(600) Catalysta)
Conv.b)
/%
Yieldb)
/%
Entry
Substrate
Product
R2
R2
O
OH
R1
R1
1
R1 = H
R1 = H
R1 = Me
R1 = F
R1 = H
R1 = Me
R2 = Me
R2 = Ph
R2 = Me
R2 = Me
R2 = H
R1 = H
R1 = H
R1 = Me
R1 = F
R1 = H
R1 = M
R2 = Me
R2 = Ph
R2 = Me
R2 = Me
R2 = H
>99
82
52
30
83
>99
82
52
30
60
2c)
3d)
4d)
5
6c)
R2 = H
R2 = H
55
40
OH
O
7c)
80
95
10
50
95
10
OH
O
8
O
O
O
OH
9
tBu
tBu
10
11
trace
trace
trace
trace
OH
OH
O
O
a) Reaction conditions: 1-phenylethanol, 0.4 mmol; Pd/KSTO(600) catalyst, Pd: 0.125 mol %; O2
as oxidant, 0.1 MPa; PhCH3 as solvent, 3 mL; reaction time, 24 h; reaction temperature, 120 °C.
b) Determined by GC using an internal standard method. c) Pd/KSTO(600) catalyst, Pd:
0.250 mol %. d) Reaction time, 72 h.
100
80
60
40
20
0
100
80
60
40
20
0
0
10
20
30
40
50
1
2
3
Reaction time/ h
Run
Figure 4. Reaction profile ( ) and the effect of catalyst
removal by hot filtration at t = 5 h ( ).
Figure 3. Catalyst recycle test.
alcohols, such as 4-tert-butylcyclohexanol, was relatively slow
with a 10% yield of 4-tert-butylcyclohexanone (Entry 9).
Unfortunately, the oxidation of aliphatic alcohols, such as 1-
octanol and 2-octanol, did not proceed smoothly, yielding only
trace amounts of the desired carbonyl product (Entries 10 and
11). These results indicated that the oxidation of secondary
alcohols exclusively yielded the corresponding ketones, where-
as the primary alcohols were converted to aldehydes with 62-
73% selectivity.
runs (Figure 3). Reaction profiling of the oxidation of 1-
phenylethanol and leaching of Pd species during the oxida-
tion were examined by hot filtration of the catalyst (catalyst
removal) from the reaction solution at t = 5 h (Figure 4). The
oxidation did not continue after removal of the catalyst, thus
confirming the necessity of the catalyst for the reaction to
proceed and that no leaching of Pd species from the catalyst
support had occurred. Moreover, XRD patterns of the recov-
ered Pd/KSTO(600) catalyst (Figure 5b) and its Pd K-edge
XANES spectra (Figure 2e) hardly changed, which indicated
that the structure of the Pd/KSTO(600) catalyst was maintained
even after the catalytic reaction.
Catalyst Reuse. Recycling tests were conducted using
the Pd/KSTO(600) catalyst, and the results showed that the
catalyst was reusable with no loss of its activity within three

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Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
151
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(a)
50
10
20
30
40
2θ/ degree
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Conclusion
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The incorporation of Pd into layered potassium titanate
proceeded effectively by intercalation via a cation-exchange
reaction in molten salts. XRD patterns confirmed that a
lepidocrocite-type layered structure, similar to the parent
KTO, was retained using a synthesis temperature of 400 °C.
Meanwhile, at temperatures of 600 °C and above, a change
of the layered structure occurred. Perovskite phases were
then formed, enhancing the materials’ catalytic activity. XAFS
measurements showed that the Pd species existed as divalent
Pd for all of the Pd/KSTO catalysts. In the oxidation of 1-
phenylethanol by the Pd/KSTO(600) catalyst, a TON of 800
(>99% yield) was achieved using molecular oxygen as the sole
oxidant, without any additives. The catalyst was reusable, with
no loss of activity for at least three runs. Thus, a new type of
highly stable heterogeneous Pd catalyst for the green oxidation
of alcohols has been successfully synthesized and may be a
promising material for new applications of layered titanates for
liquid-phase oxidations.
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