Carbon nanofibers decorated with oxo-rhenium complexes: Highly efficient heterogeneous catalyst for oxidation of amines with hydrogen peroxide

Article abstract of DOI:10.1016/j.molcata.2015.03.007

Pyridine functionalized carbon nanofibers (Py-CNFs) were synthesized, characterized and used for the grafting of oxo-rhenium complex i.e., methyltrioxorhenium (MTO) via acid-base ionic interaction. The grafting of MTO to Py-CNFs was confirmed by FTIR, UV-vis, SEM, TEM and TGA analyses. The determination of rhenium content by ICP-AES further suggested the successful immobilization of MTO to the support. The developed heterogeneous material i.e., MTO@Py-CNFs was found to be an efficient catalyst for the oxidation of various secondary as well as tertiary amines to corresponding nitrones or N-oxides, respectively by using hydrogen peroxide. The developed catalyst was readily recovered by centrifugation and reused for subsequent ten runs without any significant loss in catalytic activity.

Full text of DOI:10.1016/j.molcata.2015.03.007

Accepted Manuscript
Title: Carbon nanofibers decorated with oxo-rhenium
complexes: highly efficient heterogeneous catalyst for
oxidation of amines with hydrogen peroxide
Author: Sanny Verma Subodh Kumar Efrat Shawat Gilbert
Daniel Nessim Suman L. Jain
PII:
S1381-1169(15)00100-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.03.007
MOLCAA 9446
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
30-11-2014
19-2-2015
10-3-2015
Please cite this article as: Sanny Verma, Subodh Kumar, Efrat Shawat,
Gilbert Daniel Nessim, Suman L.Jain, Carbon nanofibers decorated with oxo-
rhenium complexes: highly efficient heterogeneous catalyst for oxidation of
amines with hydrogen peroxide, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2015.03.007
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Carbon nanofibers decorated with oxo-rhenium complexes: highly efficient heterogeneous
catalyst for oxidation of amines with hydrogen peroxide
Sanny Verma,a Subodh Kumar,a Efrat Shawat,b Gilbert Daniel Nessim,b and Suman L. Jaina*
aChemical Sciences Division, CSIR-Indian Institute of Petroleum, Dehradun 248005, India
bDepartment of Chemistry, Bar Ilan Institute for Nanotechnology and Advanced materials (BINA), Bar
Highlights
►
Ilan University, Ramat Gan, 52900, Israel
*Email : suman@iip.res.in
Highlights ► of the research ► Efficient and improved heterogeneous oxo-rhenium catalyst for oxidation of
amines with hydrogen peroxide ► Chemically functionalized carbon nanofibres as support for immobilization
of homogeneous metal complex ► Green oxidation methodology for oxidation of amines using hydrogen
peroxide as oxidant ► Easily recoverable and efficiently recyclable metal catalyst
Graphical abstractCarbon nanofibers decorated with oxo-rhenium complexes:
highly efficient heterogeneous catalyst for oxidation of amines with hydrogen peroxide
Sanny Verma, Subodh Kumar, Efrat Shawat, Gilbert Daniel Nessim, and Suman L. Jain
Abstract
Pyridine functionalized carbon nanofibers (Py-CNFs) were synthesized, characterized and used
for the grafting of oxo-rhenium complex i.e. methyltrioxorhenium (MTO) via acid-base ionic
interaction. The grafting of MTO to Py-CNFs was confirmed by FTIR, UV–VIS, SEM, TEM
and TGA analyses. The determination of rhenium content by ICP-AES further suggested the
successful immobilization of MTO to the support. The developed heterogeneous material i.e.
MTO@Py-CNFs was found to be an efficient catalyst for the oxidation of various secondary as
well as tertiary amines to corresponding nitrones or N-oxides, respectively by using hydrogen

peroxide. The developed catalyst was readily recovered by centrifugation and reused for
subsequent ten runs without any significant loss in catalytic activity.
Keywords: Carbon nanofibrs,heterogeneous catalyst,oxidation,amine,methyltrioxorhenium.
1.
2.
3.
4. Introduction
The carbon cluster-based materials such as carbon nanotubes (CNTs) and carbon nanofibers
(CNFs) have received considerable interest in recent years owing to their unique structure
and inherent physical, chemical, electronic and mechanical properties [1-2]. Since 1991 [3],
carbon nanotubes (CNTs) have been established as a new class of nanomaterials and are
being used for various applications including in batteries, hydrogen storage, gas sensors,
optoelectronic devices and catalytic applications [4-6]. Carbon nanofibers have also
gained significant attention given that they are easier to synthesize and have shown
potential for many applications [6]. In recent years, great interest is being paid towards the
development of versatile chemical modification methodologies to obtain carbon
nanostructure derivatives with more attractive features [7-8]. In this regard, a number of
derivatives have been prepared and fully characterized that exhibit promising applications in
nano-biotechnology and catalysis [9-10].
Owing to the growing environmental and economical considerations, the recycling of
catalysts has become a prime need in the area of catalysis. One logical approach to address
this concern is the immobilization of catalysts onto solid supports like organic polymers,

nanoparticles or porous inorganic materials in order to recycle the catalyst [11-12]. Owing to
their high surface area, extreme thermal stability and the tailorable shape and surface
properties, carbon nanostructures can be served as supports for catalysts [13-14]. Most of the
applications aim for carbon materials as a support for metallic nanoparticles, particularly
palladium on charcoal [15]. Nevertheless, in the last decade, an increasing interest is grown
towards the chemical functionalization and the immobilization of molecular catalysts onto
carbon supports [16-19]. CNTs or CNFs can be modified by two ways: 1) covalent
modification, 2) organic non covalent modification [20]. Covalent modification is an
important tool for introducing new functional groups and this can be accomplished by the
oxidation of defect sites and subsequent conversion in to derivatives such as amide, aryl
diazonium salt addition, cycloaddition etc [21-25]. After covalent modification, the modified
carbon nanostructures are subjected to the metal complexes, forming heterogeneous high
performance nanocatalytic materials for various applications including catalysis.
Methyltrioxorhenium (VII) (MTO) described by Hermann et al. about 23 years ago as
the first organometallic rhenium oxide, is a homogeneous catalyst which has extensively
been used for various organic transformations such as in oxidation reactions (epoxidation of
olefins, Bayer-Villiger oxidation, oxidation of alcohols, oxidation of amines), in Diels-
Alders reactions and in the metathesis of olefins [26-28]. However the difficult recovery and
non-recycling ability of the catalyst restricts its utility from industrial viewpoints.
Subsequently, heterogenization of MTO to various solid supports has come out to be a
promising approach in order to make it easily recoverable and recyclable [29-32]. In 2007 a
review on MTO heterogenization on different supports was carried out, and since then only
few reports are published in this area [33]. Very recently, we have reported the

immobilization of MTO to chemically functionalized graphene oxide and its subsequent use
for oxidation of secondary amines to nitrones [34]. However the multi-step synthetic
procedure of immobilization, higher reaction temperature and longer reaction times were the
main drawbacks of the reported method. In continuation to our efforts towards improving the
methodology, herein we report the immobilization of oxo-rhenium complex i.e methyl
trioxo-rhenium to chemically functionalized carbon nanofibers and their subsequent use for
the oxidation of secondary and tertiary amines with hydrogen peroxide (Scheme 1). The
carbon nanofibers were extracted from carbon nanofiber mats. The synthesis of the CNF
mats is described in our recent literature report [35].
+
H
R1
R
N
R1
N
R2
R2
MTO@Py-CNF
_
O
H2O2, MeOH
RT
R
+
N
O
N
-
Scheme 1: MTO@Py-CNF-catalyzed oxidation of amines
2.0 Experimental
2.1 Techniques used
The XRD pattern was collected using a Bruker D8 Advance diffractometer at 40 kV and 40
mA with Cu Kα radiation (λ= 0.15418 nm). Sample was evenly spread on glass slide and
analyzed after drying well. Fourier Transform Infrared Spectroscopy (FT-IR) analysis of
sample in KBr pallets was conducted by Perkin–Elmer Spectrum RX-1 IR
spectrophotometer. Absorption spectra(UV-Vis) of Py-CNFs and MTO@ Py-CNFs were
collected in water with a Perkin Elmer lambda-19 UV-VIS-NIR spectrophotometer using a

10 mm quartz cell. Morphology and EDX of catalyst was determined using Scanning
electron microscopy (SEM) Jeol Model JSM-6340F. For SEM analysis aqueous dispersion of
samples were deposited on glass slide. Transmission electron microscopy (HR-TEM) images
with EDX data of MTO@Py-CNFs on carbon coated copper grid was extracted using FEI-
TecnaiG2 Twin TEM operating at an acceleration voltage of 200 kV. Thermo gravimetric
analyses (DT-TGA) curve for determining thermal stability of catalyst was plotted using a
thermal analyzer TA-SDT Q-600 from 30 to 550°C with heating rate of 10°C/min under the
nitrogen flow. Rhenium wt% in catalyst was calculated using Inductively coupled plasma
Atomic emission spectrometer (ICP-AES, DRE, PS-3000UV, Leeman Labs Inc, USA). 0.01
g of sample was digested in minimum amount of conc. HNO3 with heating, and volume made
up to 10 ml. The conversions and selectivity of the products were determined by high
resolution GCMSD, EI, quadrapole mass analyzer, EM detector.
2.2 Synthesis of Py-CNFs
The synthesis of pyridine-functionalized carbon nanofibers (Py-CNFs) was carried
out by following the literature procedure [36]. In a typical experiment, 0 .5 g of NaNO2 was
dissolved in 1 ml H2O, and then cooled to 0 °C by ice bath. 0.7 g of 4-aminopyridine (4-AP) was
dissolved in 0.5 ml 4 M HCl, and then cooled to 0°C by ice bath. The solution of NaNO2 was
added drop wise to 4-AP solution. The resulting yellow solution was maintained at 0°C in ice
bath and stirred for 30 min. CNFs were purified before use by being immersed in conc. HCl
solution for 5 days and then refluxed for 5 h, followed by thoroughly washing with water and
acetone. The purified CNFs were dried at 80 °C under vacuum for 12 h. 10 mg of purified CNFs
were dispersed in a 15 ml N,N-dimethylformamide (DMF) using a sonicator bath for 2 h. The
CNF solution was cooled to 0°C by ice bath, and then the yellow solution of NaNO2 and 4-AP

was added drop wise, maintained at 0°C to prevent the decomposition of diazonium salts. The
resulting mixture was kept at room temperature under stirring for 20 h. The resulting pyridyl
functionalized CNFs (Py-CNFs) were collected by filtration and then thoroughly washed with
2M HCl, 2M NaOH, water, acetone and then dried at 80 °C under vacuum for 12 h.
2.3 Immobilization of MTO to Py-CNFs [31, 38]
Py-CNFs (10 mg) were dispersed in 50 ml dry tetrahydrofuran (THF) using a bath
sonicator for 2 h. A solution of methyltrioxorhenium (MTO; 2.5 mg) in THF (5 ml) was added to
the above prepared suspension of CNFs and stirred the resulting mixture for 3h under argon
atmosphere. The resulting product of MTO@Py-CNF was collected by centrifugation,
thoroughly washed with THF, methanol and acetone and finally dried under vacuum for 6 h at 50
°C.
2.4 Typical experimental procedure for oxidation of amines
A 25 mL round bottomed flask was charged sequentially with catalyst (1.0 mol %),
CH3OH (2 mL), 50 wt% aq. H2O2 (1.5 mmol) and then stirred at room temperature for 10 min.
Diethylamine (1mmol) was added to the reaction mixture and then stirred the resulting mixture at
room temperature. The progress of the reaction with time was monitored by thin layer
chromatography (SiO2). At the end of the reaction the catalyst was filtered over membrane filter
paper and reused for subsequent runs. The filtrate was evaporated under reduced pressure.
Organic layer was extracted with CH2Cl2 and dried over anhydrous sodium sulfate. The product
was purified by column chromatography over silica gel. Conversion of the product was
determined by the GC-MS analysis.
3. Results and discussion
3.1 Synthesis and characterization of the catalyst

To achieve the objective of the present work, chemical functionalization of the carbon
nanofibers was carried out by using the diazonium reaction to graft the pyridyl moieties at
the surface of carbon nanofibers. Thus obtained Py–CNFs were dispersed in dry THF and
then treated with MTO in dry THF to obtain heterogeneous catalyst i.e. MTO supported to
pyridyl-functionalized carbon nanofibers (MTO@Py-CNFs) through the ionic interaction
between rhenium centre of MTO and nitrogen of pyridine in CNF’s as shown in Scheme 2.
O
O
Re
O
CH3
N
N
O
O
Re
O
CH3
4-aminopyridine
CNF-Py
CNF-Py-MTO
CNF
NaNO2
THF
Scheme 2: Synthesis of MTO@Py-CNF catalyst
The rhenium metal content in the synthesized heterogeneous material was determined by
ICP-AES analysis and was found to be 1.4 wt% (0.07 mmol/g).
FTIR
The chemical changes occurred during the immobilization of MTO on CNFs were monitored by
FTIR (Fig. 1). As shown in Fig. 1a, the vibrational peaks with strong intensity at 3428 and 1632
cm-1 attributed to the stretching modes of O-H and C=C bond, respectively in carbon nanofibers.
Subsequent, attachment of 4-amino pyridine with CNFs (Py-CNF) via diazotization reaction was
confirmed by appearance of new vibrational bands i.e. a) at 1442 cm-1 corresponding to N=N and
b) in the region of 1650-1570 cm-1 related to the C=C and C=N of pyridine ring (Fig. 1b). In case

of MTO@Py-CNF (Fig. 1c) a characteristic band at 906 cm-1 corresponding to the Re=O
asymmetric stretching, confirmed the successful immobilization of MTO to the support.
Fig. 1: FTIR spectra of (a) CNF, (b) Py-CNF, (c) MTO@Py-CNF
XRD
The XRD pattern of the synthesized MTO@Py-CNF as shown in Fig. 2 clearly indicated that the
synthesized material was crystalline in nature. The diffraction peaks at 2θ=27.79o, 41.98o, 54.86o
and 77.12o are indicating the hexagonal graphite structure of the CNFs.
250
200
150
100
50
0
20
40
60
80
2-theta

Figure 2: MTO@Py-CNF
TEM
The TEM analysis of synthesized MTO@Py-CNF shows a homogeneous distribution of MTO
throughout the surface of CNF (Fig. 3). Appearance of black spots in Fig 3a,c suggesting the
successful grafting of MTO molecules to the chemically functionalized CNF support.
Furthermore, EDX analysis clearly indicated the presence of rhenium in the synthesized material
(Fig. 3d).
(a)
(b)
(c)
(d)
Re
Figure 3: TEM analysis of MTO-Py-CNF (a) and (c) TEM images of MTO-Py-CNF; b) SAED

image; d) EDX
UV-Vis
Furthermore, we analyzed the synthesized pyridyl functionalized carbon nanofibers (Py-CNF),
and MTO supported CNF (MTO@Py-CNF) by UV-Vis spectroscopy (Fig. 4). The pyridine
functionalized CNFs (Py-CNF) show characteristic absorption at ~ 220 nm owing to the discrete
units of sp2 bonded carbons (π-π*) (Fig. 4a). However, this peak at 220 nm in Py-CNF was
shifted to 226 nm in MTO@Py-CNF revealing the attachment of MTO molecules to support
matrix. Furthermore, the presence of n-π* also support the loading of MTO catalyst on the MTO-
Py-CNF (Fig. 4b).
Fig. 4: UV-Vis of a) Py-CNF (b) MTO@Py-CNF
TGA
The thermal decomposition behaviour of Py-CNF and MTO@Py-CNF catalyst was examined
by thermogravimetric analysis (Fig. 5). In case of Py-CNFs, the major weight loss is
observed in the range of 170–260 °C, due to thermal decomposition of functionalized

pyridine moieties (Fig. 5a). However, MTO@Py-CNF catalyst shows a different weight loss
pattern than the pyridyl functionalized CNFs. It is well reported that the MTO sublimes
below 100 °C, thus we presumed that the initial weight loss (3-4%) in MTO@Py-CNF up to
100 °C is owing to the decomposition/sublimation of MTO. Further weight loss is in the
range of 160–270 °C attributed to the decomposition of the pyridyl moieties. Then, the major
mass loss in the range of 350-550 °C is attributed to the decomposition of the carbon
backbone. Furthermore, the ICP-AES analysis of the synthesized heterogeneous MTO@Py-
CNF revealed 1.4 wt % rhenium (0.07 mmol/g) in the prepared catalyst.
Fig. 5: Thermogram of: a) Py-CNF; b) MTO@Py-CNF (TGA analysis was performed under
nitrogen atmosphere)
3.2 Catalytic activity
The catalytic activity of the synthesized heterogeneous catalyst i.e. MTO@Py-CNF was
tested for the oxidation of various secondary and tertiary amines including aliphatic,
aromatic and heterocyclic to the corresponding nitrones or N-oxides respectively by using
hydrogen peroxide as oxidant and methanol as solvent at room temperature (Scheme 1).

At first, the oxidation of diethylamine was carried out in the absence of catalyst. The reaction
did not take place even after prolonged reaction time (24 h) in the absence of catalyst (Table
1, entry 1). However, in the presence of catalyst (1 mol %), the oxidation of diethyl amine
occurred efficiently and gave 90 % conversion within 3h. Further, the effect of various
solvents was investigated for the oxidation of diethylamine under described experimental
conditions. Among the various solvents such as methanol, dichloromethane, benzene and
acetonitrile studied, methanol was found to be best and provided the highest product yield.
To evaluate the effect of catalyst amount, the oxidation of dibenzylamine was performed by
varying the amount of catalyst from 0.5 to 2.0 mol %. The reaction was found to increase
initially with increasing the catalyst amount (Table 1, entry 1). However, further increase in
catalyst concentration from 1 to 1.5 and 2.0 mol % did not give any significant improvement
in the yield of the desired product (Table 1, entry 1). Thus, we have considered the 1.0 mol
% as the optimum catalyst amount for this transformation. The reaction was found to occur
efficiently at room temperature and higher reaction temperatures (50 °C) did not influence
the reaction to any significant extent. Thus, the optimized reaction conditions for the present
reaction are: substrate (1 mmol), hydrogen peroxide (1.5 mmol), catalyst (1.0 mol %), and
methanol (2 ml) at room temperature.
The reaction was extended to a variety of secondary and tertiary amines under the optimized
reaction conditions. The results of these experiments are summarized in Table 1. In general,
secondary amines (Table 1, entries 1-11) were found to be less reactive than tertiary amines
(Table 1, entries 10-13).
The recycling of the MTO@Py-CNF catalyst was examined by selecting the oxidation of

diethylamine as a model reaction under optimized conditions. After completion of the
reaction as monitored by TLC (SiO2), the catalyst was easily separated by simple filtration,
washed with methanol and dried under vacuum for 8 h. The recovered catalyst was
subsequently used for the recycling experiments. The recovered catalyst showed consistent
activity up to six runs which was marginally decreased for the subsequent runs as shown in
Fig. 6. The possible reason for decrease in activity after sixth run might be due to the slower
decomposition of the oxo-rhenium to inactive perrhenate species in the presence of aqueous
hydrogen peroxide [41]. The catalyst did not show any detectable leaching of metal as
confirmed by ICP-AES. The rhenium content in the recovered catalyst after tenth run was
found to be 1.36 wt%, which was nearly similar to the fresh catalyst (1.4 wt%).
92
90
88
86
84
82
80
1
2
3
4
5
6
7
8
9
10
Run
Fig. 6: Recycling of the catalyst
The developed heterogeneous catalyst was found to be superior to the existing supported
catalysts. For example oxidation of diethylamine with H2O2 in the presence of tungstate-
exchanged Mg-Al layered double hydroxide provided 60% yield of nitrone in 5h [39]. The
oxidation of benzaldehyde and benzylamine using UHP as oxidant using Nafion immobilized

MoOCl2 as catalyst afforded 65% yield of the phenyl benzyl nitrone in 4.5 h [40]. Similarly,
the oxidation of diethyl amine using graphene oxide immobilized MTO as catalyst afforded
85 % yield of corresponding nitrone in 8h [34].
In order to prove the identity of MTO in grafted catalyst and formation of active peroxo-
species during the reaction we performed the blank experiment by using excess of oxidant
hydrogen peroxide (5 eq) and catalyst (1 eq). The resulting mixture was stirred at room
temperature for 2h and then the catalyst was recovered and analyzed by Raman spectroscopy
(Fig. 7). Raman spectrum clearly shows the characteristic bands of a Re-peroxo moiety,
found at 509, 452 and 977 cm-1 (Fig. 7). In addition, the Re(=O)2 vibrational bands at 917
and 885 cm-1 are still visible, indicating the formation of a mono-peroxo complex.
Furthermore, absence of characteristic OH bands of perrhenate anion (Re–OH bands) at 998,
995, 556 and 538 cm-1 in Raman spectra of MTO@Py-CNF/H2O2 catalyst was in analogy to
the existing literature [41] that confirm the formation of active mono peroxo-intermediate
species during the reaction. Furthermore, these results indicated that MTO was stable during
the immobilization and the grafted moiety was MTO not dissociated perrhenate anion.
Fig. 7: Raman spectra of a) MTO@Py-CNFs; b) H2O2 treated MTO@Py-CNFs

4.0 Conclusion
In summary, we have reported for the first time the use of chemically functionalized carbon
nanofibers as the support for the grafting of methyltrioxorhenium catalyst. Pyridine ligands
on CNFs led to the enduring and uniform loading of MTO on the support. The developed
heterogeneous material was found to be an efficient and highly reactive catalyst for the
oxidation of various secondary and tertiary amines to the nitrones and N-oxides respectively
by using hydrogen peroxide as oxidant at room temperature. The catalyst was easily
recovered at the end of the reaction and recycled for several runs without any remarkable
loss in catalytic activity.
Acknowledgement
We kindly acknowledge Director, CSIR-IIP for his kind permission to publish these results.
SV and SK are thanksful to CSIR, New Delhi for providing research fellowships. The
Analytical Sciences Division of the Institute is kindly acknowledged for providing analysis
of samples.
5.0
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Table 1: MTO@Py-CNF catalyzed oxidation of amines[a]
Entry
1
Reactant
Product
Time/
Conv.
(%)[b]
Yield
(%)c
-
h
24
3
d
-[d], 94.5[e]
90
+
N
N
H
89
O
3
82,[f] 91,[g]
93[h]
2
3
85
83
+
N
N
H
O
+
N
3
4
3
90
88
89
86
N
H
O
3.5
+
N
N
H
O
_
5
6
3.5
3.5
88
88
87
85
O N+
HN
_
HN
O N+
_
O
H
N
7
4
86
83
N
+
8
9
3
3
92
83
90
82
+
N
H
N
_
O
+
N
N
H
_
O
10
2.0
88
86
+
N
N
_
O
+
11
12
2.0
2.0
87
87
92
85
N
N
_
O
+
N
N
_
O

O-
13
2.5
88
85
N
N
+
[a]Reaction conditions: amine (1 mmol), 50% aq. H2O2 (1.5 mmol), catalyst (1 mol%), methanol (2
ml) at room temperature, [b]determines by GC-MS, [c]isolated yield; [d]in the absence of the catalyst;
[e]using homogeneous MTO as catalyst; [f]using 0.5 mol % catalyst, [g]using 1.5 mol % catalyst;
[h]using 2.0 mol % catalyst.