Multi-walled carbon nanotube bound nickel Schiff-base complexes as reusable catalysts for oxidation of alcohols

Article abstract of DOI:10.1080/00958972.2012.699046

Nickel salen and salophen complexes have been covalently anchored on multi-walled carbon nanotubes (MWNTs). The MWNT-supported nickel complexes have been characterized by inductive coupled plasma spectroscopy, FT-IR spectroscopy, UV-Vis spectrophotometry, transmission electron microscopy, and X-ray diffraction. The catalytic performance for the oxidation of primary and secondary alcohols was evaluated using periodic acid as oxidant. Reaction conditions have been optimized for MWNT-supported salen and salophen complexes by considering the effect of parameters such as solvent, reaction time, concentration of catalyst, amount of oxidant, etc. The catalytic activity was higher for supported catalysts than similar homogeneous ones. These supported catalysts were highly stable and reused several times without the loss of catalytic activity.

Full text of DOI:10.1080/00958972.2012.699046

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Multi-walled carbon nanotube bound
nickel Schiff-base complexes as
reusable catalysts for oxidation of
alcohols
a
a
b
Ravindra Rajarao , T.H. Kim & Badekai Ramachandra Bhat
a
Department of Chemistry, Catalysis and Materials Laboratory,
National Institute of Technology Karnataka, Surathkal, Mangalore –
5
75 025, Karnataka, India
b
Energy Material Center, Korea Institute of Energy Research, 71-2
Jang-dong, Yuseong-gu, Daejeon 305-343, South Korea
Accepted author version posted online: 01 Jun 2012. Version of
record first published: 20 Jun 2012
To cite this article: Ravindra Rajarao, T.H. Kim & Badekai Ramachandra Bhat (2012): Multi-walled
carbon nanotube bound nickel Schiff-base complexes as reusable catalysts for oxidation of alcohols,
Journal of Coordination Chemistry, 65:15, 2671-2682
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Journal of Coordination Chemistry
Vol. 65, No. 15, 10 August 2012, 2671–2682
Multi-walled carbon nanotube bound nickel Schiff-base
complexes as reusable catalysts for oxidation of alcohols
RAVINDRA RAJARAOy, T.H. KIMz and
BADEKAI RAMACHANDRA BHAT*y
yDepartment of Chemistry, Catalysis and Materials Laboratory, National Institute of
Technology Karnataka, Surathkal, Mangalore – 575 025, Karnataka, India
zEnergy Material Center, Korea Institute of Energy Research, 71-2 Jang-dong,
Yuseong-gu, Daejeon 305-343, South Korea
(
Received 28 December 2011; in final form 9 April 2012)
Nickel salen and salophen complexes have been covalently anchored on multi-walled carbon
nanotubes (MWNTs). The MWNT-supported nickel complexes have been characterized by
inductive coupled plasma spectroscopy, FT-IR spectroscopy, UV-Vis spectrophotometry,
transmission electron microscopy, and X-ray diffraction. The catalytic performance for the
oxidation of primary and secondary alcohols was evaluated using periodic acid as oxidant.
Reaction conditions have been optimized for MWNT-supported salen and salophen complexes
by considering the effect of parameters such as solvent, reaction time, concentration of catalyst,
amount of oxidant, etc. The catalytic activity was higher for supported catalysts than similar
homogeneous ones. These supported catalysts were highly stable and reused several times
without the loss of catalytic activity.
Keywords: Carbon nanotubes; Nickel salen complex; Nickel salophen complex; Oxidation of
alcohols; Reusability
1. Introduction
Schiff-base transition metal complexes have been used as efficient and selective
homogeneous catalysts in reactions such as epoxidation of olefins [1, 2], oxidation of
alcohols [3], oxidation of phenols [4], reduction of ketones [5], polymerization [6, 7],
decomposition of hydrogen peroxides [8], isomerization [9], and aldol condensation
[
10]. However, the main drawback of these catalysts is separation from the reaction
mixture at the end of the reaction; design and synthesis of catalytically active supported
metal complexes have received considerable interest. Supported catalysts offer
advantages such as high catalytic activity and selectivity besides their easy recycling
and metal recovery methodologies [11–13]. Different approaches have been used for the
immobilization of complexes on inert supports such as alumina [14], polymers [15],
silicates [16], MCM-41 [17], carbon [18], and zeolites [19].
*
Corresponding author. Email: chandpoorna@yahoo.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.699046

2
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R. Rajarao et al.
High-surface-area carbons have been the catalyst supports of choice for many metals
to convert a homogeneous into a heterogeneous process. A special type of carbon that is
attracting current interest is carbon nanotubes (CNTs) due to their unique mechanical,
structural, electronic, and thermal properties. High surface area and insolubility in most
organic solvents indicate CNTs as promising catalyst support [20].
Salen and salophen have received much attention with extensive applications in
catalysis [21, 22]. Oxidation of primary and secondary alcohols into carbonyl
compounds is a pivotal functional group transformation in organic synthesis. In
particular, oxidation of benzyl alcohol to benzaldehyde has applications in pharma-
ceutical and agrochemical industries.
Recently, Niasari et al. [23] used MWNT-supported cobalt Schiff-base complex as
catalyst for the epoxidation of cyclohexene by air and Zakeri et al. [24] used MWNT-
supported manganese porphyrin complex as catalyst for the epoxidation of alkenes by
sodium periodate.
In this work, we have prepared homogeneous and immobilized MWNT-anchored
nickel salen and salophen base complexes. The catalytic efficiency of the neat and
MWNT-supported salen and salophen complexes were tested for the oxidation of
alcohols under mild conditions using periodic acid as oxidant. MWNT-supported
nickel catalysts showed excellent catalytic activity and reusability.
2
2
. Experimental
.1. Materials
All reagents were chemically pure and of analytical reagent grade. The solvents were
dried and distilled before use following standard procedures [25]. All alcohols, periodic
acid, and other chemicals were purchased from Sigma Aldrich and Merck.
2.2. Physical measurements
Melting points were determined by the open capillary method. Elemental analyses were
performed on a Flash EA 1112 series CHNS–O Analyzer. IR spectra (in KBr pellets)
1
were recorded on a Thermo Nicolet Avatar 330-FT-IR spectrophotometer. H-NMR
spectra were recorded (DMSO-d ) on a Bruker (400 MHz) spectrometer using TMS as
6
an internal standard. Transmission electron microscopy (TEM) was performed on a
CM200, Philips. Scanning electron microscopy (SEM) was performed on a SUPRA
4
0VP, Carl Zeiss. X-ray diffraction (XRD) patterns were obtained on a JEOL JDX 8P
˚
diffractometer with Cu-Ka radiation, ꢀ ¼ 1.5418 A. The amount of metal in supported
catalysts was determined using ACROS Inductively Coupled Plasma-Atomic Emission
Spectrometry (ICP-AES). UV-Vis spectra were recorded on a SD 2000, Ocean optics.
2
2
.3. Synthesis of salen (L1) and salophen (L2)
0
.3.1. Synthesis of N,N -bis(4-hydroxysalicylidene)ethylene-1,2-diamine (L1). Reaction
mixture containing 4-hydroxysalicylaldehyde (0.02 mol) and 1,2-diaminoethane

Carbon nanotubebound nickel Schiff-base
2673
ꢀ
(0.01 mol) in methanol was refluxed at 60 C for 4 h. The reaction mixture after cooling
produced yellow crystals, which were filtered and recrystallized in methanol. Yield:
ꢀ
ꢂ1
9
0%; yellowish orange solid; the ligand decomposes at 230 ꢁ 2 C; IR (KBr, cm ):
1
ꢁ 1604.8; H-NMR (DMSO, ppm) ꢂ 8.78 (s, 2H, CH¼N); CHN found (Calcd) for
C H N O , C: 63.84 (63.99), H: 5.22 (5.37), N: 9.42 (9.33), C/N: 6.77% (6.86%); UV-
C¼N
1
6
16
2
4
Vis: ꢀmax (nm) intraligand transitions: 320, 370.
0
2.3.2. Synthesis of N,N -bis(4-hydroxysalicylidene)phenylene-1,2-diamine (L2). Reaction
mixture containing 4-hydroxysalicylaldehyde (0.02 mol) and phenylene-1,2-diamine
ꢀ
(0.01 mol) in methanol was refluxed at 60 C for 4 h. The reaction mixture after cooling
produced yellow crystals, which were filtered and recrystallized in methanol. Yield:
ꢀ
ꢂ1
9
0%; yellowish orange solid; the ligand decomposes at 240 ꢁ 2 C; IR (KBr, cm ):
1
ꢁ 1611; H-NMR (DMSO, ppm) ꢂ 8.52 (s, 2H, CH¼N); CHN found (Calcd) for
C H N O , C: 68.86 (68.96), H: 4.52 (4.63), N: 8.08 (8.04), C/N: 8.25% (8.57%); UV-
C¼N
2
0
16
2
4
Vis: ꢀmax (nm) intraligand transitions: 280, 300.
2.4. Synthesis of NiL1 and NiL2
L1 and L2 (0.01 mol) in the minimum quantity of methanol : acetone (3 : 1) was mixed
with a methanolic solution of nickel(II) acetate tetrahydrate (0.01 mol). The resulting
ꢀ
solutions were refluxed with stirring on a magnetic stirrer equipped at 80 C for 2 h. The
bright red NiL1 and NiL2 separated out, were filtered, washed, and dried under
vacuum [26].
ꢀ
ꢂ1
NiL1 yield: 70%; red solid; the complex decomposes at 260 ꢁ 2 C; IR (KBr, cm ):
ꢁ
7
1599; CHN found (Calcd) for C H N NiO , C: 53.87 (53.83), H: 3.96 (3.95), N:
16 14
C¼N
2
4
.87 (7.85); UV-Vis: ꢀmax (nm) intraligand transitions: 320, 370, d ! d: 440.
ꢀ
ꢂ1
NiL2 yield: 70%; red solid; the complex decomposes at 270 ꢁ 2 C; IR (KBr, cm ):
ꢁ
1606; C H N NiO , C: 59.25 (59.31), H: 3.45 (3.48), N: 6.90 (6.92); UV-Vis:
20 14
C¼N
2
4
ꢀmax (nm) intraligand transitions: 280, 310, d ! d: 430.
2
.5. Multi-walled carbon nanotubes synthesis and purification
The multi-walled carbon nanotubes (MWNTs) prepared by the Ni/Co catalyst over
CaCO support using chemical vapor deposition method were used in this study. The
3
diameter of MWNTs obtained was 20–40 nm. The raw product was first immersed in an
aqueous solution of HCl to remove CaCO , then filtered, washed with distilled water,
3
and dried in an oven. The purity was up to 99%. The SEM micrograph of purified
MWNT is shown in figure 1.
2
.6. Oxidation and chlorination of MWNTs
The purified MWNTs were refluxed in dilute HNO3 for 4 h. The resulting solid
MWNT-COOH) was then separated by filtration and washed with water and dried.
The MWNT-COOH (5 g) previously dried was suspended in a solution of SOCl2
(

2
674
R. Rajarao et al.
Figure 1. SEM image of purified MWNT.
ꢀ
(100 mL). The suspension was stirred at 65 C for 24 h under nitrogen. After that, the
reaction mixture was cooled and SOCl was evaporated [27]. The resulting precipitate
was chlorinated multi-walled carbon nanotubes (MWNT-COCl) (scheme 1).
2
2
.7. Supporting nickel complexes on functionalized MWNT
The MWNT-COCl was added (5 g) to a solution of nickel complex, i.e., NiL1 or NiL2
ꢀ
(1 g) in CHCl , and the suspension was stirred for 40 h under N at 70 C (scheme 1).
The solid was then separated by filtration and washed with THF, CH Cl , and dried in
3
2
2
2
vacuum.
2
.8. General procedure for the oxidation of alcohols
Catalytic oxidation of primary and secondary alcohols to corresponding aldehydes and
ketones by MWNT-supported nickel complexes was studied in acetonitrile (CH CN)–
3
periodic acid (H IO ). The reaction conditions have also been optimized for the
5
6
maximum oxidation of benzyl alcohol by varying different parameters. The reaction
mixture was filtered and then analyzed by gas chromatography.
2
.9. Catalyst reuse and stability
The reusability of supported catalysts was investigated in the multiple sequential
oxidation of benzyl alcohol. At the end of each reaction, the catalyst was separated
from the reaction mixture by simple filtration, washed with diethyl ether, and dried
before using in the next run.

Carbon nanotubebound nickel Schiff-base
2675
Scheme 1. Synthesis of MWNT-supported nickel complexes.
3
3
. Results and discussion
.1. Characterization of MWNT-supported nickel salen complex
The synthesized MWNTs were functionalized by COOH groups by oxidation using
nitric acid. In order to increase the reactivity, it was further converted to COCl by
thionyl chloride. The MWNT-COCl was reacted with the nickel salen complex to
obtain MWNT-supported catalyst. The prepared supported catalyst was characterized
by ICP-AES, FT-IR, UV-Vis, TEM, and XRD. The nickel content in the sup-
ꢂ1
ported catalysts NiL1 and NIL2 was 0.099 and 0.102 mmol g , respectively, measured
by ICP-AES.
More information about anchoring of NiL1 and NiL2 to MWNT was obtained by
FT-IR spectroscopy. The intensity of the IR spectrum of MWNT-supported nickel

2
676
R. Rajarao et al.
Figure 2. Electronic spectra of (a) NiL1 and (b) MWNT-NiL1.
complexes was weak due to the low concentration of the complex. The IR spectra of
supported complexes are similar to those of neat complexes. The azomethine band
ꢂ1
(
C¼N) is observed at ꢃ1600 cm in supported catalysts when compared to its neat
ꢂ1
complexes at ꢃ1605 cm (Supplementary material). This observation clearly confirms
the anchoring of NiL1 and NiL2 to MWNT.
The electronic spectra of neat and supported complexes were recorded. The electronic
spectra of the neat and supported complexes were identical, indicative of no changes in
the structural features before and after anchoring to MWNT (figures 2 and 3). The
spectrum of MWNT did not show any peak, therefore, it is confirmed that complexes
have been supported on MWNTs.
TEM images of clean MWNT and complexes supported on MWNT show further
evidence for attachment to MWNTs. CNTs are neat, straight and long (figure 4a).
Complexes (black dots) attached on the surfaces and tips of MWNTs were clearly
observed from the supported MWNT TEM image (figure 4b and c).
X-ray diffraction studies were performed for all MWNT-anchored complexes. The
diffraction pattern of MWNT-COOH was the same as raw CNT, clearly indicating that
structural perfection was not altered after oxidation (figure 5). The XRD patterns of
CNTs anchored by complexes also show similar patterns as raw MWNT, indicating
that crystallinity and morphology were not altered after anchoring of complexes.
3.2. Catalytic activity
The prepared catalysts were used for the oxidation of primary and secondary alcohols
using periodic acid as oxidant. The oxidation was studied in more detail to optimize
reaction variables such as solvent, length of reaction time, alcohol/oxidant, and alcohol/
catalyst molar ratio. We checked the activity of supported catalysts in different solvents
(methanol, acetonitrile, and acetone) with periodic acid as the oxidant and the best
conversion was observed in acetonitrile. The optimizations of other reaction conditions

Carbon nanotubebound nickel Schiff-base
2677
Figure 3. Electronic spectra of (a) NiL2 and (b) MWNT-NiL2.
were studied by taking benzyl alcohol as a substrate in the acetonitrile–H IO system.
5
6
The benzaldehyde formed was quantified by GC. The product analysis was done at
regular intervals of time, under similar reaction conditions for supported catalyst. In the
CH CN–H IO system, the time required for maximum conversion of benzaldehyde to
3
5
6
benzyl alcohol was 90 min.
In order to study the effect of the concentration of catalysts with respect to substrate,
the reaction was studied at different substrate to catalyst ratios for supported catalysts.
The reaction was also studied in the absence of catalyst and no conversion was observed
(
table 1). A 400 mg sample (0.04 mmol) of catalyst was sufficient for effective
transformation of benzyl alcohol to benzaldehyde for both supported complexes. The
reactions were studied at various substrate-to-oxidant ratios. A minimum quantity of
1 mmol of oxidant was sufficient for effective oxidation of benzyl alcohol to
benzaldehyde by supported catalysts.
The supported catalysts exhibited high catalytic activity for the oxidation of various
alcohols (table 2). Allylic alcohols, benzylic alcohols, and secondary alcohols gave high
yields of the corresponding carbonyls. All benzylic primary and secondary alcohols
studied were oxidized smoothly to give aldehydes and ketones, respectively. It is clear
from the GC analysis that no side reaction took place during catalytic oxidation. The
over oxidation to carboxylic acid was ruled out by a derivative test. All the experiments
were carried out in air since there is no change in conversion if the reaction is carried
out under argon. This indicates that air is not involved in the oxidation.
3.3. Influence of support
The anchoring of complexes on MWNT could result in a favorable combination of
heterogeneous and homogeneous catalysis. The MWNT support can provide a
stabilizing effect, since it can prevent catalyst deactivation by hindering the formation
of peroxo- and ꢃ-oxo-bridged species. The ꢃ-oxo dimers can be formed easily with neat
complexes during oxidation [28]. In order to demonstrate the effect of MWNT
supporting on the catalytic activity of complexes in the oxidation of alcohols, we

2
678
R. Rajarao et al.
Figure 4. TEM images of (a) neat MWNT, (b) MWNT-NiL1, and (c) MWNT-NiL2.
repeated all reactions under the same reaction conditions with neat complexes.
The catalytic activity was higher for supported catalysts than homogeneous. The
higher activity is due to site isolation effect, suppressing the formation of peroxo- and
ꢃ-oxo-bridged complexes.
3
.4. Recycling and stability of catalyst
The recycling of catalysts is very important for industrial applications. After
filtration and washing with diethyl ether, we supported catalysts for the oxidation of

Carbon nanotubebound nickel Schiff-base
2679
Figure 5. XRD pattern of MWNT, MWNT-COOH, MWNT-NiL1, and MWNT-NiL2.
Table 1. Optimization of concentration of oxidant and catalyst for oxidizing
benzyl alcohol to benzaldehyde by supported NiL1 and NiL2 complexes.
a
b
% Yield
Amount of Amount of
Sl. No. catalyst (mmol) oxidant (mmol) MWNT-NiL1 MWNT-NiL2
1
2
3
4
5
6
7
8
9
0
1
2
0
1.0
1.0
1.0
1.0
1.0
1.0
0
0.25
0.50
0.75
1.0
1.25
2.0
58.1
72.4
80.3
80.4
80.3
3.6
26.5
52.2
73.1
80.3
80.1
2.2
64.7
73.0
85.1
95.2
95.0
2.7
26.4
50.2
72.8
95.1
95.2
0.01
0.02
0.03
0.04
0.05
0.04
0.04
0.04
0.04
0.04
0.04
1
1
1
a
ꢀ
1
GC yield, average of three trials.
mmol alcohol, 15 mL CH
3
CN, 90 min, stirring at 80 C.
b

2
680
R. Rajarao et al.
a
Table 2. Oxidation of alcohols catalyzed by neat and supported nickel complexes in CH
3
CN–H
5
6
IO .
b
%
Yield of carbonyl compound
Entries
1
Alcohols
Product
NiL1 MWNT-NiL1 NiL2 MWNT-NiL2
O
OH
8
0.2
80.2
93.1
95.2
H
O
OH
H
H
2
3
7
7
6.9
7.5
76.9
77.5
86.5
90.3
87.1
91.0
H C
3
H C
3
O
OH
Cl
Cl
Cl
Cl
O
OH
H
4
7
8.3
78.3
89.2
90.1
NO2
NO2
O
OH
5
6
CH3
CH3
8
8
2.4
1.7
82.4
81.7
94.7
94.1
95.1
94.6
OH
O
OH
O
7
8
9
6
6
6
7
1.5
3.0
4.1
0.5
61.5
63.0
64.1
70.5
70.3
72.2
74.5
79.8
71.2
73.0
75.2
80.5
H C
O
3
H C
3
H
O
OH
H C
3
H C
3
H
H
H C
1
0
3
OH
H C
3
O
a
ꢀ
5 6 3
IO , 0.04 mmol complex, 15 mL CH CN, 90 min, stirring at 80 C.
1 mmol alcohol, 1 mmol H
Average of three trials.
b

Carbon nanotubebound nickel Schiff-base
2681
benzyl alcohol. The oxidation was carried out five times under identical reaction
conditions with the recycled MWNT-supported nickel salen and salophen catalysts. The
yield of product was 80.2% in the first run. No significant decrease was observed in the
yield of products. The catalyst stability and yield were good, indicating excellent
reusability of the catalysts.
4. Conclusions
We have successfully synthesized MWNT-supported nickel salen and salophen
complexes. The heterogeneous catalysts show high catalytic activity in the oxidation
of primary and secondary alcohols. The catalytic activity was evaluated in the presence
ꢀ
of periodic acid as oxidant in acetonitrile at 80 C. The MWNT-supported nickel
complexes were catalytically more active than unsupported complexes. The advantages
of supported catalysts are high activity, stability, reusability, and avoidance of the
deactivation of catalyst by site isolation.
Acknowledgments
The authors would like to thank Defense Research and Development Organization
(DRDO), Government of India for financial assistance and also SAIF, IITB, and DST
Unit on Nanoscience, IITK, for providing analytical services.
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