Synthesis of a sulfonato-salen-nickel(ii) complex immobilized in LDH for tetralin oxidation

Article abstract of DOI:10.1039/b9nj00314b

A novel sulfonato-salen-nickel(ii) complex has been immobilized on a Zn(ii)-Al(iii) layered double hydroxide (LDH) host. XRD, FT-IR, TGA and UV-vis spectroscopy, as well as chemical analysis, confirmed the successful incorporation of the nickel-salen complex within the LDH structure. BET surface area measurements, SEM and TEM were also used to characterize the heterogenized catalyst. The sulfonato-salen-nickel(ii) complex-immobilized material, LDH-[nickel-salen], was found to be effective in the oxidation of tetralin, where a combination of trimethylacetaldehyde and dioxygen at atmospheric pressure was employed as the oxidant. At 72.3% conversion, tetralin was converted to 1-tetralone with 72.2% selectivity at 70 °C after 7 h. Tetralin oxidation using tert-butyl hydroperoxide afforded a lower conversion and selectivity of 1-tetralone than with trimethylacetaldehyde and dioxygen as the oxidant. The effect of various reaction parameters on catalytic performance was also investigated. A hot filtration experiment coupled with a blank test revealed that oxidation proceeded mostly on nickel-salen sites in LDH-[nickel-salen]. A reaction mechanism is proposed based on the experimental results.

Full text of DOI:10.1039/b9nj00314b

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Synthesis of a sulfonato-salen-nickel(II) complex immobilized in LDH
for tetralin oxidation
Samiran Bhattacharjee,a Kwang-Eun Jeong,b Soon-Yong Jeongb and
Wha-Seung Ahn*a
Received (in Victoria, Australia) 7th July 2009, Accepted 28th September 2009
First published as an Advance Article on the web 5th November 2009
DOI: 10.1039/b9nj00314b
A novel sulfonato-salen-nickel(II) complex has been immobilized on a Zn(II)-Al(III) layered double
hydroxide (LDH) host. XRD, FT-IR, TGA and UV-vis spectroscopy, as well as chemical
analysis, confirmed the successful incorporation of the nickel-salen complex within the LDH
structure. BET surface area measurements, SEM and TEM were also used to characterize
the heterogenized catalyst. The sulfonato-salen-nickel(II) complex-immobilized material,
LDH-[nickel-salen], was found to be effective in the oxidation of tetralin, where a combination of
trimethylacetaldehyde and dioxygen at atmospheric pressure was employed as the oxidant.
At 72.3% conversion, tetralin was converted to 1-tetralone with 72.2% selectivity at 70 1C after
7 h. Tetralin oxidation using tert-butyl hydroperoxide afforded a lower conversion and selectivity
of 1-tetralone than with trimethylacetaldehyde and dioxygen as the oxidant. The effect of various
reaction parameters on catalytic performance was also investigated. A hot filtration experiment
coupled with a blank test revealed that oxidation proceeded mostly on nickel-salen sites in
LDH-[nickel-salen]. A reaction mechanism is proposed based on the experimental results.
problem in many heterogeneous catalysts.16,17,19 Further
Introduction
efforts to improve catalytic methods for benzylic oxidation
Tetralin (1,2,3,4-tetrahydronaphthalene) oxidation produces
with improved stability are highly desired.
1-tetralone, an important source of synthetic precursors and a
Schiff base ligands derived from salicylaldehyde and amines
reactive intermediate for a wide range of products, including
are known as salen ligands, and their transition metal complexes
pharmaceuticals, dyes and agrochemicals.1,2 In addition,
have been widely applied to the synthesis of fine chemicals by
1-tetralone and the by-product 1-tetralol can be used as
oxidation, showing high catalytic activity under both
additives to enhance the cetane number of diesel fuels for
homogeneous and heterogeneous conditions.20–23 However,
cleaner combustion.3 Selective oxidation of such benzylic
among these, nickel-salen-based complexes have not been
hydrocarbons in the liquid phase has been investigated in
extensively studied as catalysts compared to their Mn, Cr or
conjunction with a variety of catalytic systems over recent
Co analogues. While few studies of Ni complexes with salen
decades, viz., using soluble metal (Ni, Mn and Cu) catalysts,4,5
ligands have been reported, in terms of their utility as
2,4-dimethylpentane-2,4-diol cyclic chromate6 and Ni complexes
catalysts, excellent catalytic activities in olefin epoxidation
with surface-active ligands.7 Transition metal-containing
and phenol oxidation reactions have been reported.24–28 Our
molecular sieves such as MAPO-5, MAPO-11 and MAPSO-34
earlier work with surface-active Ni complex catalysts7 for
(M = Cr, Co, Mn and V)8,9 can immobilize transition metals
tetralin oxidation in biphasic reactions also established the
(viz., Cr, Ni, Cu, Fe, Co, Mn, La and Ru) on MCM-41
high potential of Ni-based catalytic systems and led us to test
mesoporous silica;10–14 chromium-exchanged zeolite Y15 and
ZSM-516 have also been studied. A recent catalyst test of
a nickel-salen complex immobilized on an inorganic solid
support as a heterogeneous catalyst for tetralin oxidation in
tetralin oxidation over various transition metal-containing
the liquid phase.
molecular sieve catalysts by us has shown that Cr-incorporated
AlPO4-5 is more active than Fe- or Co-substituted catalysts.17
Layered double hydroxides (LDHs) are a family of anionic
clays consisting of hydroxides of common metals, such as
As a consequence, very recently, we reported that a mesoporous
Mg and Al, and can be synthesized at ambient temperature
Cr(III) terephthalate metal–organic framework, MIL-101,
affords excellent catalytic activity with high catalyst stability.18
and pressure from aqueous solutions. They have received
increasing attention in recent years as catalysts and catalyst
However, despite the high activity and selectivity offered by
supports.29–35 More importantly, it has been claimed that
liquid phase oxidation, leaching of the active metal species
LDH-hosted catalysts do not suffer from leaching of the active
metal in epoxidation reactions, and that they can be reused
several times without loss of activity.33,34 These findings
prompted us to test the efficacy of an LDH-hosted nickel-
salen complex as a catalyst for the oxidation of bicyclic arenes
where dioxygen serves as an oxidant.
from the matrix during the catalytic reaction usually poses a
a Department of Chemical Engineering, Inha University,
Incheon 402-751, Korea. E-mail: whasahn@inha.ac.kr;
Fax: +82 32-872 0959; Tel: +82 32-866 0143
b Applied Chemistry and Engineering Division, KRICT,
Yuseong 305-600, Korea
ꢀc
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In this work, we describe the synthesis of a new catalyst, in
which a nickel-salen complex, [NiL] [L = sulfonato-salen], is
immobilized on a Zn-Al LDH by the anion exchange method.
To gain insight into the structure of the nickel-salen complex
intercalated into the LDH host, we performed detailed
characterization, including X-ray diffraction (XRD), BET
surface area measurements, scanning electron microscopy
(SEM) and transmission electron microscopy (TEM). The
oxidation of tetralin was studied using in situ-generated
acylperoxy radicals by dioxygen at atmospheric pressure in
the presence of sacrificial aldehyde, as well as tert-butyl
hydroperoxide. The influence of various reaction parameters,
such as temperature and the amount of catalyst, on tetralin
conversion was investigated; a plausible reaction mechanism is
proposed based on the experimental results. To the best of our
knowledge, the application of an intercalated system containing
a salen-nickel(II) complex for the oxidation of tetralin has not
been previously reported.
absorption spectrometer (model: AAnalyst 400); samples for
the measurements were dissolved in a mixture of HCl, HNO3
and hydrogen peroxide (1 : 2 : 1). SEM micrographs were
obtained on a Hitachi S-4200 instrument. TEM micrographs
were obtained on a JEOL JEM-2100F instrument. Samples
were dispersed in ethanol in an ultrasonic bath, and a drop of
the supernatant suspension was placed onto a holey carbon
coated grid and dried at 60 1C.
Catalytic reaction
Tetralin oxidation reactions using t-BuOOH as the oxidant
were carried out using a Chemistation PPS-2510 fitted with a
condenser (Eyela). In a typical reaction, a mixture of 2 mmol
tetralin, 5 cm3 chlorobenzene (solvent) and 0.02 g catalyst were
placed in a Chemistation glass reactor and heated to the
desired temperature. The oxidant (t-BuOOH, 4 mmol) was
then added through a septum to the reactant mixture and
stirred at 600 rpm. The oxidation of tetralin was carried out
using dioxygen at atmospheric pressure in a twin-necked
round flask equipped with a condenser. In a typical run,
2 mmol tetralin, 4 mmol trimethylacetaldehyde, 10 cm3
acetonitrile and 0.02 g catalyst were stirred while dioxygen
was bubbled through at atmospheric pressure and kept at the
desired reaction temperature. After completion of the
reaction, the catalyst was filtered off, and the selectivity and
conversion were measured using a GC (Acme 6000, Younglin,
Korea) fitted with a high performance HP-5 capillary column
(60 m, 0.32 mm, 0.25 mm) and an FID.
Experimental methods
Materials
Salicylaldehyde (98%, Aldrich), aniline (99.5%, Aldrich), 1,2-
ethylenediamine (TCI), zinc(II) nitrate tetrahydrate (Aldrich),
aluminium(III) nitrate nonahydrate (Aldrich), nickel(II)
perchlorate hexahydrate (Aldrich), trimethylacetaldehyde
(96%, Aldrich) and tert-butyl hydroperoxide (5–6 M solution
in decane, Aldrich) were purchased and used without further
purification.
Catalyst hot filtration experiments were carried out by
separating the catalyst quickly from the reaction solution after
1 h of reaction while maintaining the reaction temperature
(50 1C). The filtrate was further stirred at 50 1C for an
additional duration of up to 7 h.
Catalyst preparation
Sodium salicylaldehyde-5-sulfonate (1) and [NiL] [L =
sulfonato-salen ligand] (2) were prepared using methods
previously reported in the literature.36,37 The immobilized
benzoate ion on the LDH host, LDH-[C6H5COO] (3), was
prepared according to a previously reported method.33 0.4 g of
2 was dissolved in de-carbonated water (300 cm3) and 1.0 g of
3 was added to the solution, which was stirred for 10 h at room
temperature under a nitrogen atmosphere to produce the
[NiL]2À-immobilized material, 4 (4). The orange solid was
filtered off and washed with water and ethanol, and dried
overnight at 60 1C.
Results and discussion
Synthesis of 4
The synthesis route to the 4 catalyst is shown in Scheme 1.
The host hydrotalcite-like material, 3, was prepared by the
co-precipitation of a solution of zinc and aluminium nitrates
with an aqueous solution of NaOH and benzoic acid.33 2 was
intercalated into 3 at room temperature from an aqueous
solution by exchange of the C6H5COOÀ ion to produce 4.
Catalyst 4 is stable in air for several months, and remains
stable in most organic solvents and water at elevated temperature.
Catalyst characterization
X-Ray powder diffraction patterns were recorded on a Rigaku
Miniflex diffractometer using Cu-Ka radiation (l = 1.54 A).
FT-IR spectra of the catalyst samples were recorded using
KBr discs on a Bruker VERTEX 80V FT-IR spectro-
photometer. Thermogravimetric analyses in the range
25–850 1C were carried out on a TGA (Scinco S-1000) system
under an air flow at a heating rate of 5.0 1C minÀ1. BET
surface areas were determined on a Micromeritics ASAP 2000
surface analyzer at 77 K. UV-vis diffuse reflectance spectra
were obtained on a Varian Cary 3E double-beam spectro-
photometer using MgO as a reference at ambient temperature.
Elemental analyses were performed using a FLASH EA 1112
elemental analyzer (Thermo Electron Corporation). Metal
Characterization of [Ni(II)-(sulfonato-salen)]2À and the
LDH-[Ni-(sulfonato-salen)] catalyst
The XRD patterns of 3 and 4 are shown in Fig. 1. The basal
spacing of 3 was increased from 15.4 to 19.5 A following the
exchange process. The gallery height of catalyst 4 is 14.8 A when
the thickness of the brucite layers (4.7 A) are subtracted. This
increase in gallery height strongly suggests successful intercalation
of the [NiL]2À ion into the LDH host. An elemental analysis
(Table 1) of the catalyst was in good agreement with the formula
unit [Zn1.87Al1.04(OH)5.81][NiL]0.41[C6H5COO]0.21Á6H2O. TGA up
to 250 1C revealed a sample weight loss of 18.0% (calculated loss
of 6H2O: 18.2%). The [Al/(Zn + Al)] ratios of 3 and 4 were
contents were determined on
a Perkin-Elmer atomic
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interlayer water molecules.41 The weight loss behavior of 3
(Fig. 3(b)) reflects a two-step weight loss in the temperature
range 340–445 1C. In this case, the first loss may be due to the
loss of the C6H5COOÀ ion and the second may result
from dehydroxylation of the LDH layers.41 Collapse of the
hydrotalcite structure in 3 occurred at 445 1C, and no further
weight loss occurred up to 700 1C. On the other hand, 4
showed a significant weight loss between 330 and 415 1C,
which may be due to decomposition of the organic part of the
chelate in the [NiL] complex, as well as dehydroxylation of the
layers. Gradual weight loss was observed up to 850 1C.
Continuous weight loss behavior has also been found in other
layered hydroxides, and was assigned to the continuous loss of
volatile gas from the micropores of the oxide residue.41 Upon
increasing the temperature in 4 from 415 1C, gradual weight
loss was observed due to sintering of the oxide, as well as the
collapse of the pores at elevated temperatures. Based on the
decomposition temperatures, thermal stability in the solid
state can be inferred in each case. Fig. 3 shows that [NiL]2À
intercalated compound 4 is thermally less stable than benzoate
intercalated LDH material 3, whereas 4 showed a higher
thermal stability than free complex 2.
Scheme 1 Synthesis route to the LDH-[NiL] catalyst, 4.
almost identical, indicating that leaching of neither Zn(II) nor
Al(III) occurred during the exchange procedure. The FT-IR
spectra of 2, 3 and 4 are shown in Fig. 2. These spectra confirm
the intercalation of the nickel-salen complex into the LDH host
via exchange of the C6H5COOÀ ion. The FT-IR spectrum of 4
shows strong bands at 1116 and 1035 cmÀ1 due to the anti-
symmetric and symmetric stretching modes of the SO32À moiety.38
This corresponds closely to the spectrum of the free sulfonato-
salen-nickel(II) (2), which registers strong bands at 1111 and
1030 cmÀ1, thus indicating the presence of the [NiL]2À ion in
the LDH host. Further support for this intercalation into the LDH
N2 adsorption–desorption isotherms at 77 K for 3 and 4 are
shown in Fig. 4. The isotherms of 3 and 4 show a type II
pattern according to the IUPAC classification,42 with a
hysteresis loop beginning at 0.40 and 0.15 P/P0 for 3 and 4,
respectively. The BET surface area of benzoate anion-pillared
hydrotalcite 3 (32.3 m2 gÀ1) is marginally lower than that of 4
(38.4 m2 gÀ1), indicating that intracrystalline gallery surfaces
have been created in the latter, and that 4 is slightly more
accessible due to pillaring of the Ni(II) complex.
host is provided by the nNi–N vibration of 4 at 465 cmÀ1 39
.
The TGA curves of 2, 3 and 4 are shown in Fig. 3. The TGA
diagram of free complex 2 shows three weight loss steps
(Fig. 3(a)). The first step, up to 150 1C, is due to the removal
of water molecules. The second step, which is observed in the
temperature range 270–360 1C, may be due to the loss of a
sulfonato group.40 The decomposition of the organic part of
the chelate occurred in the temperature range 360–385 1C
and resulted in the formation of nickel oxide at the final
temperature.40 The weight loss behaviors of 3 and 4 revealed
that the weight loss of both compounds occurred below
250 1C, which can be attributed to the loss of adsorbed and
SEM images of 3 and 4 are shown in Fig. 5. Similar
agglomerates with a granular morphology and individual
particle sizes as small as 100 nm or less are observed in the
SEM images in both cases (Fig. 5(a) and (b)). These results
indicate that the morphology of the host hydrotalcite-like
material, 3, is not significantly influenced by the intercalation
of the nickel-salen complex into the LDH host by partial
substitution of benzoate ions.
Fig. 6(a) and (b) present TEM images of 3 and 4. Both
images show agglomerates of small, smooth, rounded particles
of variable shape and size. Particle sizes typically fall in the
10–100 nm range for both materials. The parallel, linear, dark
features seen in both images are evidence of a layered structure
in each, indicating that the LDH structure remains intact
during the ion-exchange process. Interestingly, one of the
TEM images of 4 (Fig. 6(c)) appears to show single-layer
scroll- or nanotube-like structures (lower right) and multi-
layer nanotubes (top right), both viewed end-on and in a large
crystal (centre), where the layer structure is clear (parallel dark
lines).
The UV-vis diffuse reflectance spectra of 2, 3 and 4 are shown
in Fig. 7. The free Ni(II) complex, 2, shows two absorption
maxima above 400 nm (Table 1), where that at 535 nm may
represent the lowest energy d–d transition of Ni(II). Earlier
studies37 reported similar band spectra in this region for Ni(II)-
salen complexes in solution. The UV-vis spectrum of 4 showed
identical features to the free complex, indicating that no change
at the Ni(II) coordination centre took place during intercalation.
Fig. 1 X-Ray powder patterns of (a) 3 and (b) 4.
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Table 1 Characterization data for 2, 3 and 4
Analytical data (wt%)
S
l
max/nmb
C
H
N
Ni
Zn
Al
Compound
2a
3
34.42 (34.02)
14.4
16.3
2.77 (2.83)
3.7
2.3
4.62 (4.96)
—
2.0
—
—
4.4
10.28 (10.39)
—
3.64
—
21.85
20.24
—
5.04
4.56
535, 440, 397
—
530, 445, 400
4
a
b
Calculated values are shown in parentheses. UV-vis diffuse reflectance spectra.
Fig. 5 Scanning electron micrographs of (a) 3 and (b) 4.
Fig. 2 FT-IR spectra of (a) free 2, (b) 3 and (c) 4.
Fig. 6 Transmission electron micrographs of (a) 3, (b) and (c) 4.
The electronic spectrum of 3 did not show any band maxima in
Fig. 3 TGA of (a) free 2, (b) 3 and (c) 4.
the 300–800 nm region (Fig. 7(c)).
Tetralin oxidation over the LDH-[Ni-(sulfonato-salen)] catalyst
Benzylic oxidation reactions are commonly carried out using
t-BuOOH as the oxidant.43 The liquid phase oxidation of
tetralin over heterogeneous catalysts affords 1-tetralone and
1-tetralol the as major products, and 1-naphthol and naphthalene
as by-products.17 Initially, tetralin oxidation using t-BuOOH
as the oxidant was carried out under varying reaction
temperatures from 70 to 100 1C in a chlorobenzene medium.
As shown in Table 2, tetralin conversion gradually increased
from 36 to 45% as the temperature was increased from 70 to
100 1C. The selectivity for 1-tetralone reached 66% at 70 1C
and then slightly dropped to 64% at 100 1C. The reaction was
then carried out with a varying amount of catalyst from 5.0 to
40.0 mg with a tetralin : t-BuOOH molar ratio of 1 : 2 at 70 1C.
Fig. 4 N2 adsorption–desorption isotherms at 77 K of (a) 3 and (b) 4.
ꢀc
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Based on the experimental results and evidence from the
reported data, a plausible reaction mechanism is suggested in
Scheme 2. The mechanism involves the initial activation of
dioxygen in the presence of the aldehyde by the catalyst to
produce the corresponding peroxy acid; a similar pathway was
reported by Mukaiyama et al.45 using Mn(III)-salen complexes
under homogeneous conditions. The oxidation of alkenes
and alkanes using the Ni(II) complex as a catalyst under
homogeneous conditions suggests the formation of either
oxo-Ni(IV) or Ni(III)–Oꢁ as intermediate species, the formation
of these species being dependent on the nature of the ligand
field and the ability of the oxygen atom donor.46,47 In case of
the LDH hosted nickel-salen system, we propose that
trimethylacetaldehyde is initially converted to highly active
acylperoxy radical I, leading to peroxy acid II, and then
forms a higher valent nickel-oxo speices III. This species
reacts with tetralin to form radical IV and nickel(III)-hydroxy
complex V.46 In addition to hydrogen atom abstraction
from C–H bonds by the high valence nickel(III)-oxo species
(path A), the reaction may simultaneously produce the
Fig. 7 Diffuse reflectance UV-vis spectra of (a) free 2, (b) 4 and (c) 3.
Tetralin conversion gradually increased with increasing amounts
of catalyst, while the selectivity for 1-tetralone and 1-tetralol
remained virtually unaltered (Table 2). It was also found that
non-catalytic thermal oxidation takes place in the presence of
t-BuOOH, leading to 12% tetralin conversion at 70 1C after 8 h.
In situ-generated acylperoxy radicals from an aldehyde and
dioxygen were also considered as an alternative oxidant. The
LDH-[NiL] catalyst, 4, was thus tested in the oxidation of
tetralin using dioxygen (1 atm) in the presence of trimethyl-
acetaldehyde. The effect of the reaction temperature on the
oxidation of tetralin was investigated between 25–70 1C. As
presented in Table 3, the conversion of tetralin gradually
increased with increasing temperature. With an increase in
temperature from 25 to 70 1C, the total reaction time was
shortened and the selectivity for 1-tetralone decreased from
79 to 72% at 70 1C, and where tetralin conversion levelled off
at 72%. The reaction was then carried out with a varying
amount of catalyst from 5.0 to 40.0 mg at 50 1C. According to
Table 3, the conversion gradually increased from 53 to 68%
with increases in the catalyst amount, while the selectivity for
1-tetralone and 1-tetralol remained virtually unaltered.
Therefore, in situ-generated acylperoxy radicals proved to be
a more effective oxidant than t-BuOOH for tetralin oxidation.
A group of researchers have reported that the epoxidation of
styrene using in situ-generated acylperoxy radicals from a
dioxygen/isobutylraldehyde oxidant over an immobilized
Cu(II)-perchlorophthalocyamine complex on MCM-41 as the
catalyst gave a higher yield of epoxide than when t-BuOOH
was employed as the oxidant.44 Thermal oxidation was found
to be promoted by changing the oxidant from t-BuOOH to
dioxygen/trimethylacetaldehyde (see Table 3).
ꢁ
C12H11 radical by donating hydrogen atoms to acylperoxy
radicals (path B, Scheme 2) to form the corresponding
products.
It is important to verify the stability of the heterogeneous
catalyst under the given reaction conditions, since metal
leached from the catalyst could be responsible for the observed
catalytic behavior.16,19 The stability of the catalyst was tested
by performing repeat reaction cycles under the same reaction
conditions as described above. At the end of each reaction
cycle, the catalyst was recovered by filtration, washed
with acetonitrile, dried and reused. The results are shown in
Table 3. Tetralin conversion dropped from 65.6 to 64.8%
after the first run and remained almost identical after the
second run. Further experiments on the stability of the
catalyst during tetralin oxidation were performed by the hot
filtration technique. The catalyst was filtered off at 50 1C
after 1 h in order to avoid re-adsorption of leached nickel
onto the catalyst surface. The reaction still proceeded, but
with a substantially lower conversion than in the presence of
the catalyst (Fig. 8). This higher conversion (3.3%) after
hot filtration relative to the blank run conversion after an
8 h reaction may be contributed-to by the initially formed
active radical intermediates remaining in the filtrate,48 as well
as by a very minor homogeneous contribution. The FT-IR
spectrum of the solid catalyst after reuse was indistinguishable
from that of the fresh catalyst (Fig. 9). This observation
Table 2 The influence of temperature and the amount of catalyst on the oxidation of tetralin using t-BuOOHa
Selectivity (%)c
1-Tlone
1-Tlol
Nthol
Nlene
Temp./1C
Catalyst amount/mg
Conversion (%)
70b
5.0
24.4
36.2
39.1
39.9
45.2
66.8
66.2
66.0
65.5
64.2
25.7
25.9
26.2
26.8
24.9
6.1
6.0
5.8
5.4
6.8
1.4
1.9
2.0
2.3
4.1
20.0
40.0
20.0
20.0
80
100
a
b
Reaction conditions: tetralin (2 mmol), t-BuOOH (4 mmol), chlorobenzene (5 cm3), 8 h. Without catalyst (in the presence of t-BuOOH)
c
conversion = 12%. 1-Tetralone, 1-tetralol, 1-naphthol and naphthalene are denoted as Tlone, Tlol, Nthol and Nlene, respectively.
ꢀc
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Table 3 The influence of temperature, amount of catalyst and catalyst recycling on the oxidation of tetralin using dioxygen and trimethyl-
acetaldehydea
Selectivity (%)f
Tlone
Tlol
Temp./1C
Time/h
Catalyst amount/mg
Conversion (%)
25
50b
18
8
20.0
5.0
37.2
52.5
65.6
64.8
64.5
64.5
67.7
72.3
77.0
78.6
78.8
78.6
78.3
78.4
79.1
72.2
22.1
21.0
20.8
21.0
20.3
20.6
20.5
21.0
20.0
20.0c
20.0d
20.0e
40.0
20.0
70
7
a
b
Reaction conditions: tetralin (2 mmol), trimethylacetaldehyde (4 mmol), acetonitrile (10 cm3) and dioxygen (1 atm). Without catalyst (in the
c
d
e
f
presence of dioxygen and trimethylacetaldehyde) conversion = 15%. Second run. Third run. Fourth run. 1-Tetralone and 1-tetralol are
denoted as Tlone and Tlol, respectively.
Fig. 9 FT-IR spectra of (a) fresh 4 and the reused catalyst after the
(b) first run, (c) second run and (d) fourth run.
Scheme 2 The proposed reaction mechanism for the oxidation of
Conclusions
tetralin over 4.
A sulfonato-salen-nickel(II) compound was successfully inter-
calated into a Zn(II)-Al(III) LDH host at room temperature from
an aqueous medium and used as a catalyst for the oxidation of
indicates that the catalyst remains unchanged during the
tetralin. The in situ-generated acylperoxy radical (via a dioxygen/
oxidation reaction. Therefore, the catalytic activity seen in
trimethyl-acetaldehyde oxidant system) was found to be a more
the present catalyst must be mostly attributable to the
efficient method than t-BuOOH for transforming tetralin into
nickel-salen center in 4.
1-tetralone using LDH-[nickel-salen] as the catalyst. Recycling
and hot filtration experiments indicated that the oxidation
occurred over LDH-[nickel-salen] through mostly heterogeneous
nickel-salen centers and thermal autooxidation, but a homoge-
neous contribution could not be entirely excluded. The formation
of acylperoxy radicals and a high-valent nickel-oxygen species
was proposed as the mechanism responsible for the oxidation of
tetralin over the LDH hosted Ni-complex catalyst.
This work was supported by the Korea Energy Manage-
ment Corporation (KEMCO) through the Energy and
Resources Development Program (2008).
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Fig. 8 Tetralin oxidation at 50 1C: (a) fresh catalyst, (b) filtrate
(filtered 4 after a 1 h reaction time) and (c) without catalyst (in the
presence of trimethylacetaldehyde and dioxygen).
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