Chemoselective oxidation of primary alcohols catalysed by Ce(iii)-complex intercalated LDH using molecular oxygen at room temperature

Article abstract of DOI:10.1039/c1dt11140j

Herein we describe the preparation of an efficient heterogeneous catalyst consisting of an anionic Ce(iii)-complex immobilized Zn/Al-layered double hydroxide (LDH) and its use in the catalytic liquid phase oxidation of primary alcohols using molecular O₂ at room temperature. Various primary alcohols could be transformed to their corresponding aldehydes in good to excellent yields using the set of optimal conditions. The heterogeneous catalytic system can also be recovered and reused for several cycles without a significant loss of catalytic activity. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt11140j

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PAPER
Chemoselective oxidation of primary alcohols catalysed by Ce(III)-complex
intercalated LDH using molecular oxygen at room temperature†
Sudarshan Singha, Mitarani Sahoo and K. M. Parida*
Received 17th June 2011, Accepted 19th August 2011
DOI: 10.1039/c1dt11140j
Herein we describe the preparation of an efficient heterogeneous catalyst consisting of an anionic
Ce(III)-complex immobilized Zn/Al-layered double hydroxide (LDH) and its use in the catalytic liquid
phase oxidation of primary alcohols using molecular O2 at room temperature. Various primary alcohols
could be transformed to their corresponding aldehydes in good to excellent yields using the set of
optimal conditions. The heterogeneous catalytic system can also be recovered and reused for several
cycles without a significant loss of catalytic activity.
and adsorption capacity.8 The immobilizations of different metal
complexes within mesoporous supports have been extensively
Introduction
studied.9 However studies on the intercalation of anionic tran-
sition metal complexes into LDH layers are rather limited.10
It has been demonstrated that LDH meets the require-
ment of an inorganic support for immobilizing anionic metal
complexes11 and may provide an alternative for homogeneous
metal complexes. Recently main group element12 and lanthanide13
complexes with SALEN-type ligands were studied. Moreover,
a heterogeneous catalyst in the liquid-phase offers several ad-
vantages over homogeneous ones, such as an ease of recovery
and recycling, atom utility, and enhanced stability in the oxi-
dation reactions. Noble metals, such as a styrene-divinylbenzene
copolymer anchored Ru(III) complex,14 a Pd(II) acetate–pyridine
complex supported by hydrotalcite,15 Ru hydrotalcites,16 and a
trimetallic Ru/CeO2/CoO(OH) catalyst17 were reported to be
active as heterogeneous catalysts for the oxidation of alcohols
with molecular oxygen. V. R. Choudhary et.al. have reported
solvent free oxidation of benzyl alcohol to benzaldehye using
tert. butyl hydroperoxide as the oxidant using a transition metal
containing LDH.18 Kawabata et al. reported a nickel containing
Mg–Al hydrotalcite-type anionic clay catalyst for the oxidation of
alcohols with molecular oxygen, where they found a maximum
of 69% conversion by using different Ni wt.%.19 The oxidation
of alcohols to aldehydes and ketones using Pd(II)-hydrotalcite in
the presence of air has been reported by N. Kakiuchi et al.,15
where they found a high conversion rate, but selectivity towards
the aldehyde is less than 100% and the reaction was performed
at 80 ◦C. Recently Bielawski et al. have reported the oxidation
of alcohols to carbonyl compounds using inexpensive graphene
oxide as a heterogeneous catalyst.20 Aerobic oxidation of alcohols
using metal–organic framework supported gold nanoparticles as
a heterogeneous catalyst has been reported by Liu et al.21
Selective methods allowing for the oxidation of primary alcohols
to aldehydes without over oxidation to carboxylic acids remain
a challenging task from both laboratory and synthetic industrial
chemistry points of view.1 The desired carbonyl compounds serve
as important and versatile intermediates for the synthesis of
fine chemicals.2 Many oxidations of these types are carried out
using a stoichiometric amount of oxidising agent (i.e. KMnO4,
MnO2, CrO3, Br2 etc) with considerable drawbacks such as high
cost, by-products and serious environmental issues.3 From the
standpoint of green and sustainable chemistry, the development of
catalytic systems that use molecular oxygen as the only oxidant has
attracted great interest.4 This has many remarkable advantages,
including abundance, low cost, and the by-products are benign.
In recent years, it has been targeted for use in various oxidation
reactions because it is cheap, readily available and gives no co-
product as pollutant.5
The creation of a well-defined, active metal site on a solid
surface not only opens up an avenue to materials that boost
catalytic performance but also aids an understanding of the
molecular basis of heterogeneous catalysis.6 Layered double
hydroxides (LDHs) are a class of naturally occurring and syn-
thetic materials, which can be described by the general formula
III
[MII
M
x(OH)2]z+(An-)z/n·yH2O. MII and MIII are divalent and
1-x
trivalent metals respectively and An- is the anion that compensates
for the positive charges of the metal hydroxide layers.7 These
hydrotalcite materials have received much attention as materials
for advanced heterogeneous catalysts due to some promising
avenues such as cation-exchange ability of brucite layers, the anion-
exchange ability of the interlayer, its adjustable surface basicity,
Colloids and Materials Chemistry Department, Institute of Minerals
& Materials Technology, Bhubaneswar, 751013, Orissa, India. E-mail:
paridakulamani@yahoo.com; Fax: 91674-2581637
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1dt11140j
The oxidation of alcohols using a heterogeneous catalytic
system at room temperature would be more feasible from an
industrialpointofview. Herewedescribeanewprotocolthatoffers
11838 | Dalton Trans., 2011, 40, 11838–11844
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Scheme 1 The increase in gallery height due to intercalation of the Ce-complex.
˚
height was found to be 7.06 A when the thickness of the brucite
a strategy for the design of a heterogeneous alcohol-oxidation
catalyst i.e. anionic Ce(III)-complex intercalated LDH, for use
in conjunction with molecular oxygen as the oxidant at room
temperature.
˚
layer was subtracted (4.8 A) from the basal spacing (Scheme
1). The (110) reflection at about 60◦ showed no obvious shift
after intercalation, indicating that no significant change occurred
in the LDH host layers.23 In the case of the recycled catalyst
(Fig. 1 (C) insert), all the peaks are retained in their respective
position with respect to LDH/Ce-complex, suggesting the prop-
erties of the recycled catalyst remain unaltered.
Results and discussion
Catalyst characterization results
FTIR spectroscopy is very helpful in the study of LDHs,
especially those containing interlayer organic anions, as it is
very sensitive to the symmetry of the organic anion and to the
interactions of the anion with its environment such as hydrogen
bonding and coordinate bonding. For the LDH nitrate precursor
(Fig. 2(a)) the intense and broad absorption band centred at
3445 cm-1 corresponds to the stretching vibrations of the hydroxyl
groups of both the layer hydroxide moieties and interlayer water.24
The broadening of this band is due to hydrogen-bond formation.
The band close to 1640 cm-1 corresponds to the deformation mode
(d H2O) of water molecules. The band at 1384 cm-1 is assigned to
Fig. 1 shows the XRD patterns of (a) Zn/Al-LDH and (b)
LDH/Ce-complex. The XRD pattern of Zn/Al-LDH shows
sharp, symmetrical peaks at low angles corresponding to basal
and higher order reflections. The basal spacing value (d003) of
22
˚
Zn/Al–LDH is 8.67 A, which agrees well with the literature.
In comparison with the Zn/Al-LDH, the (00l) reflections of
LDH/Ce-complex moved to a lower 2q angle, i.e., from 10.2◦
to 7.8◦ (003) and from 20.3◦ to 15.4◦ (006). This corresponds to
˚
an increase in interlayer distance from 8.67 to 11.86 A, which is
indicative of the intercalation of the Ce-complex into the interlayer
-
-
galleries of the LDH by anion exchange with NO3 . The gallery
the stretching vibration of interlayer NO3 . The bands around
Fig. 1 XRD patterns of (a) Zn/Al-LDH (b) LDH/Ce-complex (c)
Fig.
2
FT-IR spectra of (a) Zn/Al-LDH, (b) Ce-complex, (c)
recycled catalyst (insert).
LDH/Ce-complex and (d) recycled catalyst.
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445 and 675 cm-1 are due to Al–O and Zn–O lattice vibrations
respectively. In Fig. (2(b) & (c)) the bands around 3300 cm-1
and 1577 cm-1 are due to N–H stretching and N–H bending
for the secondary amide, respectively. The strongest and most
characteristic absorption band for the carboxylate group is due
to the antisymmetric COO- vibration (nas) and appears in the
region of 1570–1660 cm-1 for both the Ce-complex and LDH/Ce-
complex. In case of LDH/Ce-complex, the bands are merged in
the region of 3250–3450 cm-1, which may be due to overlap of the
N–H band of the amide and the O–H band of the brucite layer.
All the above results indicate the successful intercalation of the
Ce-complex in the interlayer space of LDH. All the vibrations
do not change significantly in the recycled catalyst (Fig. 2(d))
compared to LDH/Ce-complex, which suggests the existence of
all the properties.
The UV-vis DRS spectra of (a) LDH/Ce-complex (b) Ce-
complex (c) Zn/Al-LDH are shown in Fig. 3. The spectra for the
Ce-complex and complex intercalated LDH are very similar. Peaks
around 250, 310 and 360 nm are due to intra-ligand transition.
A broad peak around 480 nm is due to 4f–5d transition of the
metal. But the peaks for the complex intercalated LDH slightly
shifted to a higher wavelength compared to the metal complex.
This is a direct indication for intercalation of the metal complex
in between LDH layers. The heterogeneous catalyst was also well
characterized by SEM, TGA and solid state NMR study (see
ESI†).
different modified hydrotalcites (RuMnMn on hydrotalcite surface
and nickel in Ni–Al hydrotalcite) have been reported by Ebitani
et al.26 and Choudhary et al.,27 respectively, where the reactions
were performed at higher temperatures. Compared to these
previous studies, the most important feature in this investigation
is that the reaction has been carried out at room temperature. In
our present study, primary alcohols are quantitatively oxidized to
aldehydes with good to excellent results, without any noticeable
oxidation to carboxylic acid under optimal reaction conditions.
In this context, the electronic influence of substitution on the
phenyl ring has also been studied (Table 1, entries 5–7). An electron
withdrawing group accelerates the reaction rate, while a donating
group retards the reaction rate. Cinnamyl alcohol (Table 1, entry
8) is oxidized to give cinnamaldehyde and primary diol (Table 1,
entry 9) is converted to dialdehyde. The oxidation of straight chain
aliphatic alcohols (Table 1, entry 2 and 3) under standard reaction
conditions took place effectively over this catalyst. The process
reported here allows a convenient method for the oxidation of
primary alcohols to aldehydes.
The possible mechanistic pathway for the oxidation of primary
alcohols proceeds through the formation of a Ce-alcohol complex
(ESI†). The subsequent step involves the reductive elimination to
afford the aldehyde product.28 Direct contact of gaseous oxygen
with the catalytic active site is necessary for oxidation of alcohols,
as only a trace amount of product was obtained when oxygen
was not bubbled through the reaction mixture.29 The LDH/Ce-
complex catalytic system can be used for the selective oxidation
of primary alcohols compared to secondary alcohols. Secondary
alcohols are often oxidized faster than primary alcohols owing to
the increase of electron density on the former. However, in our
experiment we found primary alcohols are selectively oxidized in
the presence of secondary ones (Table 2). An equimolar mixture
of benzyl alcohol and 1-phenyl ethanol (Table 2, entry 1) gave
a mixture of benzaldehyde and acetophenone in 96 and 37%
conversion, respectively. Similarly, the competitive oxidation of
1-octanol and 4-octanol (Table 2, entry 2) showed 82 and 25%
conversions. Furthermore, the process exhibits an unprecedented
degree of chemoselectivity for the oxidation of primary hydroxyl
groups in the presence of secondary ones. A similar type of results
was observed by Kaneda et al.16
The effect of organic solvents on the oxidation was examined
by choosing toluene as the model solvent, as polarity plays an
important role in alcohol oxidation. Very little oxidation product
was obtained with polar solvents such as acetonitrile and methanol
(29 and 7% conversion, respectively). Non polar solvents such as
benzene, hexane, cyclohexane and toluene lead to 76, 73, 79 and
96% conversion, respectively, in 8 h at room temperature. These
results are in good agreement with those obtained by Choudhary
et al.27,30 It may presumably be due to the reason that polar solvents
can be more strongly adsorbed than non polar solvents on the
active sites of the catalyst surface and prevent the adsorption of
alcohols, resulting in a net lowering in the reaction rate. To examine
the effect of reaction time on the conversion percentage we have
conducted the reaction up to 24 h. As expected, the conversion
rate increases with time up to 8 h (Fig. 4) and maintained a steady
rate up to 24 h, i.e. no appreciable change in conversion is observed
with further increase in reaction time.
Fig. 3 UV-vis DRS spectra of (a) LDH/Ce-complex, (b) Ce-complex and
(c) Zn/Al-LDH.
The oxidation of alcohols using different metal supported
hydrotalcite hosts has been widely studied. M. L. Kantam
et al. have reported a Cu–Al hydrotalcite/rac-BINOL system,
where the aerobic oxidation of alcohols was performed at room
temperature using K2CO3 as the base to promote the conversion.25
In this present investigation, the oxidation of benzyl alcohol was
examined as a standard substrate in the presence of a catalytic
amount of LDH/Ce-complex, taking toluene as the solvent in
the presence of molecular O2. Generally, the possible oxidation
products are benzaldehyde, benzoic acid and benzyl benzoate
depending upon the reaction conditions and mechanistic pathway.
It has been found in our experiment that benzaldehyde is the
sole oxidation product with 100% selectivity (Table 1, entry
1). The oxidation of alcohols by activating molecular O2 using
We believe that the employment of a chelating ligand has
provided the major driving force behind the active metal catalyst,
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Table 1 The results of oxidations of various alcohols catalyzed by LDH/Ce-complex with molecular oxygena
Entry
1
Substrate
Product
Timeb/h
8
Conv./%
96
Selectivity/%
100
TOFc/h-1
133
2
3
4
12
12
10
82
80
91
100
100
98
75
74
101
5
10
87
100
96
6
7
10
8
85
91
100
100
94
126
8
9
8
8
92
92
98
127
127
100
10
8
74
87
102
a Reaction conditions: alcohol (1 mmol), catalyst (50 mg, 0.09 mol% Ce), reaction performed at room temperature, molecular O2 as oxidant, toluene
as solvent (5 mL), conversion and selectivity were determined by gas chromatography. b Time required for maximum conversion. c Turnover frequency
(TOF) = Turnover number (TON) per hour, TON calculated as (mol% of product)/(mol% of metal catalyst).
which in turn activates the molecular O2 in the oxidation process.
We have also tested our reaction by using the homogeneous Ce-
complex, but it showed a maximum of 72% conversion for benzyl
alcohol in similar reaction conditions in 12 h (Table 3). The
lowering of the conversion rate in the case of the homogeneous
complex compared to the heterogeneous catalyst may be attributed
to the effect of immobilization. Again, this may presumably be
due to the formation of a catalytic inactive m-oxo dimer by the
metal complex in the absence of the LDH support. For example,
the iron(III) porphyrins and phthalocyanines under oxidising
conditions form a dimer containing LFe-O-FeL linkages (where L
is a macrocyclic ligand), and in this form they are unable to bind
oxygen. But the enhanced catalytic activity could be attributed
to the isolated catalytic sites in case of the complex immobilized
LDH, which could prevent the formation of inactive dimers and
other polymeric species.31 Reutilization is one of the greatest
advantages of a heterogeneous catalyst, which can also provide
useful information about the intercalation process and catalytic
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Table 2 The selective oxidation of primary alcoholsa
Entry
1
Alcohol
Product
Timeb/h
8
Conv./%
96
37
2
3
12
82
25
8
96
33
a Reaction conditions: alcohol (1 mmol), catalyst (50 mg, 0.09 mol% Ce), reaction performed at room temperature, molecular O2 as oxidant, toluene as
solvent (5 mL), conversion was determined by gas chromatography. b Time required for maximum conversion.
Table 3 The oxidation of benzyl alcohol by different catalystsa
Catalyst
Timeb/h
Conv./%
Selectivityc/%
LDH
24
12
12
8
7
81
87
100
100
Ce(NO3)3·6H2O
Ce-amide complex
LDH/Ce-complex
41
72
96
a Reaction conditions: benzyl alcohol (1 mmol), catalyst (50 mg), reaction
performed at room temperature, molecular O2 as oxidant, toluene as
solvent (5 mL), conversion and selectivity were determined by gas
chromatography. b Time required to get maximum conversion. c Selectivity
for benzaldehyde.
Fig. 4 The effect of time on the oxidation of benzaldehyde.
Table 4 Heterogeneity test up to 4 cyclesa
Entry
Conv./%
Selectivity/%
stability along the catalytic cycles. To test if metal is leached
out from the solid catalyst during the reaction, the liquid phase
of the reaction mixture is collected by filtration at the reaction
temperature. It was confirmed by AAS technique that the Ce
content of the used LDH/Ce complex catalyst was the same as
that of the fresh catalyst and that no Ce was observed in the
filtrate. In addition, the oxidation of alcohol (benzyl alcohol) was
completely stopped by removal of the LDH/Ce complex from the
reaction solution. The recovered catalyst was then subjected to
XRD and FT-IR spectroscopy. Comparison of the XRD patterns
and IR spectra of the LDH/Ce-complex and recovered catalyst
convincingly demonstrates that the structural integrity of the
complex intercalated LDH remains unaltered after the oxidation
reaction. Notably, the recovered catalyst can be reused in oxidation
Fresh
96
96
95
94
94
100
100
100
100
100
Cycle 1
Cycle 2
Cycle 3
Cycle 4
a Reaction conditions: benzyl alcohol (1 mmol), catalyst (50 mg), reaction
performed at room temperature, molecular O2 as oxidant, toluene as
solvent (5 mL), conversion and selectivity were determined by gas
chromatography.
reactions several times without appreciable loss of activity i.e.
conversion and selectivity. (Table 4)
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the metal complex was transferred and refluxed at 60 ◦C for
24 h with constant stirring. The final product was isolated by
filtrati◦on, washed with ethanol and kept overnight in vacuum
Conclusions
In summary, we have developed a heterogeneous catalyst by
immobilizing an anionic Ce(III)-complex into the interlayer space
of LDH, which provides a chemoselective oxidation of primary
alcohols. The use of a heterogeneous reusable catalyst in the oxi-
dation of alcohols at room temperature is an important protocol
from an industrial point of view for large scale production, as it
offers several advantages over the existed conventional method.
-
at 70 C. Anal. found/calcd (%) for [Zn2Al(OH)2(NO3 )0.01](Ce-
complex)0.33·0.63H2O (Zn,Al-LDH/Ce complex): Zn 29.76/29.94,
Al 6.01/6.18, C 23.22/23.57, N 4.18/4.24, H 2.08/2.1, Ce
10.58/10.63. IR (KBr, cm-1) 3445, 3300, 3295, 1703, 1655, 1587,
1564, 1384, 675, 445.
Characterization techniques
Experimental
Powder XRD measurements were performed on a Rigaku
D/MAX2500 diffractometer, using Cu-Ka radiation at 40 kV,
30 mA, a scanning rate of 5◦ min-1, and a 2q angle ranging from
3◦ to 80◦. The Fourier transform infrared (FTIR) spectra of the
samples were recorded using a Varian 800-FTIR spectrometer.
The powered samples were mixed with KBr and pressed in
form of pellets for measurement of FTIR analysis in the range
4000–400 cm-1. The co-ordination environments of the samples
were examined by diffuse reflectance UV–vis spectroscopy. The
spectra were recorded using a Varian-100 spectrophotometer in
the wavelength range of 200–800 nm in BaSO4 phase. Scanning
electron microscopy (SEM) images were obtained using HITACHI
3400 N microscope. The samples were placed on a copper tape
and then coated with a thin layer of platinum (layer thickness
3 nm) using a sputter coater. Thermogravimetric analysis (TGA)
was performed under air with a Shimadzu TGA-50 system at
Materials preparation
Preparation of LDH. Zn/Al-LDH was prepared by a co-
precipitation method similar to that already reported in the
literature.32 A mixed metal nitrate solution containing mixtures
of Zn(NO3)2·6H2O (0.133 M) and Al(NO3)3·9H2O (0.066 M) salts
was prepared. The synthesis was carried out by the slow addition
of 200 mL of mixed metal solution and 2 M NaOH solution
simultaneously to a reaction vessel containing 50 mL of deionized
water. The reaction pH was found to be 11 during the precipitation
by the continuous addition of 2 M NaOH solution under magnetic
stirring. A nitrogen atmosphere was maintained throughout the
addition. The white precipitate obtained after 2 h was collected by
filtration, washed several times with deionized water till washings
◦
were neutral and then dried at 100 C overnight.33 Elemental
analysis showed the following composition; found/calcd (%)
◦
a heating rate of 5 C min-1. The chemical composition of the
-
for Zn0.68Al0.32(OH)2(NO3 )0.32·0.38H2O (Zn,Al-LDH-NO3): Zn
products was confirmed quantitatively and qualitatively by energy
dispersive X-ray (EDX) using a HITACHI 3400 N microscope.
The Ce content in the catalyst and in the leaching solution was
determined using atomic absorption spectroscopy (Perkin–Elmer
AAS 300 with acetylene (C2H2) flame). The 13C CP MAS NMR
spectrum was recorded at 100.62 MHz using a Bruker Avance
400 MHz spectrometer.
38.87/39.07, Al 7.53/7.63, N 3.86/3.93, H 2.19/2.26. IR (KBr,
cm-1) 3445, 1640, 1384, 675, 445.
Synthesis of the metal complex
2-aminonicotinic acid treated with Na2CO3 in an ethanolic
medium produces the sodium salt of 2-aminonicotinic acid.
Equimolar amounts of the sodium salt of 2-aminonicotinic acid
(0.69 g, 5 mmol) and methyl salicylate (0.65 mL, 5 mmol) were
refluxed in 20 mL of ethanolic solution to produce an amide
ligand.34 The final product was filtered, washed several times with
ethanol and recrystallised from diethyl ether. The cerium complex
was formed by refluxing 0.57 g (2 mmol) of ligand and 0.434 g (1
mmol) of Ce(NO3)3·6H2O on a water bath at 50 ◦C using ethanol
as the solvent for 4 h. The metal complex was isolated by filtration
and washed repeatedly with ethanol to get rid of the unreacted
ligand. Finally the metal complex was dried in vacuum and kept
in a desiccator. Anal. found/calcd (%) for the ligand: C 55.9/56.31,
H 3.21/3.24, N 10.05/10.10. For the Ce-complex: C 44.2/44.57,
H 2.54/2.57, N 7.9/8. For the amide ligand: IR (KBr, cm-1) 3300,
1705, 1660, 1590, 1570. For the Ce-complex: IR (KBr, cm-1) 3295,
1703, 1655, 1587, 1564. For the amide ligand: 13C CP MAS NMR
(100.62 MHz, CDCl3, d) 115.1, 118.7, 130.4, 130.9, 132.1, 134.3,
134.7, 139.4, 142.1, 148.7, 158.4, 167.4, 169.8. For the Ce-complex:
13C CP MAS NMR (100.62 MHz, CDCl3, d) 117.1, 120.2,
129.7, 131.6, 133.0, 133.7, 134.3, 137.8, 143.7, 147.9, 157.2, 165.8,
167.9.
Catalytic oxidation reaction
The oxidation of alcohols (Scheme 2) was typically carried out
as follows: A suspension of LDH/Ce complex (50 mg) in toluene
(5 mL) was stirred for 10 min. 1.0 mmol of alcohol was added
to the reaction mixture and the suspension was purged with
molecular oxygen. The concentration of the molecular oxygen
(oxygen stream) in toluene is 10 mL min-1. The reaction was
started by bubbling O2 gas through the reactor under vigorous
stirring at room temperature for 8 h. After completion of the
reaction, the catalyst was separated by filtration/centrifugation.
The solid residue was washed with EtOAc to make the catalyst
free from organic matter for further reuse. The product was ana-
lyzed by GC-2010 (Shimadzu) equipped with a capillary column
(ZB-WAX, 30 m, 0.25 mm I.D) and flame ionization detector
(FID).
LDH/Ce complex
Zn/Al-LDH was dried under reduced pressure at 100 ◦C. To
an ethanolic suspension of 1 g of dried LDH, 0.5 mmol of
Scheme 2
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The authors are extremely thankful to Prof. B. K. Mishra,
Director, IMMT, Bhubaneswar 751013, Orissa, India, for his
constant encouragement and permission to publish the paper. One
of the authors S. Singha is obliged to CSIR for a senior research
fellowship.
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