Studies on the functionalization of MWNTs and their application as a recyclable catalyst for CC bond coupling reactions

Article abstract of DOI:10.1016/j.catcom.2013.11.028

Functionalization of multi-walled carbon nanotubes (MWNTs) was studied using various anchoring methods. Among them, nitrene chemistry was employed as the method of choice to preserve desirable physical properties of MWNTs. Using this method, synthesis of carboxyl-functionalized carbon nanotubes (CNTs) was achieved, and then transition metals were incorporated on the linker-MWNT via reductive methods using hydrazine monohydrate. Among various reducing reagents, the use of hydrazine hydrate allowed us to load palladium uniformly. After immobilization of palladium on CNT, its role as a catalyst for CC bond coupling reaction (Suzuki reaction) was examined. The catalysts could be retrieved upon completion of the reaction by filtration and drying; the recycled catalysts could then be used in further reactions up to seven times before any loss in catalytic activity was observed. Further studies revealed that the Pd leached out of the MWNT may be responsible for the reactivity.

Full text of DOI:10.1016/j.catcom.2013.11.028

Catalysis Communications 46 (2014) 71–74
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Studies on the functionalization of MWNTs and their application as a
recyclable catalyst for C\C bond coupling reactions
⁎
Eunsuk Kim, Han Saem Jeong, B. Moon Kim
Department of Chemistry, Seoul National University, Seoul 151-747, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 July 2013
Received in revised form 16 November 2013
Accepted 27 November 2013
Available online 4 December 2013
Functionalization of multi-walled carbon nanotubes (MWNTs) was studied using various anchoring methods.
Among them, nitrene chemistry was employed as the method of choice to preserve desirable physical properties
of MWNTs. Using this method, synthesis of carboxyl-functionalized carbon nanotubes (CNTs) was achieved, and
then transition metals were incorporated on the linker-MWNT via reductive methods using hydrazine
monohydrate. Among various reducing reagents, the use of hydrazine hydrate allowed us to load palladium uni-
formly. After immobilization of palladium on CNT, its role as a catalyst for C\C bond coupling reaction (Suzuki
reaction) was examined. The catalysts could be retrieved upon completion of the reaction by filtration and dry-
ing; the recycled catalysts could then be used in further reactions up to seven times before any loss in catalytic
activity was observed. Further studies revealed that the Pd leached out of the MWNT may be responsible for
the reactivity.
Keywords:
Multi-walled carbon nanotube
Heterogeneous catalyst
Suzuki reaction
Recycling
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Among various functional groups, carboxylates are the most com-
monly employed for the immobilization of enzymes, ligands and sen-
Carbon nanotubes (CNTs) have attracted much attention due to
their high specific surface areas, chemical stability and electrical con-
ductivity. Modified CNTs are utilized in many applications in molecular
electronics, biological sensors, catalyst support, sorbent materials and
polymer composites [1–3]. In this regard, the unique properties make
CNTs very useful for supporting metal nanoparticles. Consequently,
since the first discovery of multi-walled carbon nanotubes (MWNTs)
by Iijima in 1991, method for chemical modifications of CNTs have
been actively studied [4,5]. Common methods include both covalent
or non-covalent interactions, including wrapping of the nanotubes
with surfactants, oxidation with strong acids and direct addition of func-
tional groups with highly reactive intermediates [6,7]. Covalent
functionalization of CNTs may allow the materials to better disperse in
solvents, however, covalent interactions with CNT surfaces may cause
undesired defects because of chemical functionalization. Common
methodologies of direct covalent modification include radical chemis-
try, Prato and Bingel reactions, as well as dichlorocarbene and nitrene
chemistry [8]. We chose to further explore the reactive nitrene chemis-
try as a functionalization method, since only desired functional groups
of the reactive species can be anchored to the CNT wall without signifi-
cantly damaging CNTs. Generation of nitrene is performed in situ from
an azide moiety to minimize unwanted additions such as insertions or
rearrangement reactions. Aziridine rings can be formed as reported in
studies examining nitrene chemistry [9–13].
sors on CNT [14–21]. Since carboxylated CNT's can stabilize metal
nanoparticles, these materials have also been used for the incorporation
of transition-metals on CNT (Pd, Pt, Ru, Rh, Au, Ni, Cu) [22–27].
The Suzuki reaction is a very useful Pd-catalyzed coupling reaction
for the synthesis of unsymmetrical biaryl compounds and has witnessed
wide use in organic synthesis [28]. Though examples on the use of Pd-
CNT catalyst systems abound in the literature, reports on the use of
the heterogeneous Pd-CNT systems for recyclable and highly reactive
catalyst systems are rare [29–35]. In this work, a novel Pd-CNT catalyst
was prepared through the incorporation of a new ligand system and its
performance as a recyclable catalyst in the Suzuki reaction was
investigated.
To prepare functionalized CNTs, we have used several function-
alization methods to modify the sidewalls of MWNTs. We utilized pre-
viously reported nitrene chemistry using an amino alkyl azide for CNT
functionalization. Various functional groups were attached to the
azido linker and transition metals were loaded on MWNTs for use in
catalytic systems.
2. Experimental
2.1. Catalyst preparation
2.1.1. Preparation of amine-CNT using nitrene chemistry
All experiments on CNTs were carried out using MWNTs prepared
by a chemical vapor deposition (CVD) method. Functionalized
CNT (CNT-ED-OH) was confirmed by Fourier-transform infrared
(FTIR) spectrometry, elemental analysis (EA) and Thermal Gravimetric
⁎
Corresponding author at: Department of Chemistry, College of Natural Sciences, Seoul
National University, Seoul, 151-747, Republic of Korea. Fax: +82 2 875 7505.
E-mail address: kimbm@snu.ac.kr (B.M. Kim).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.11.028

72
E. Kim et al. / Catalysis Communications 46 (2014) 71–74
Scheme 1. Synthesis of Pd-CNT-ED-OH-.
Analysis (TGA, see Supporting Information Fig. S1). Pd-CNT-ED-OH cat-
alyst (Scheme 1) was prepared using various reducing agents. Detailed
procedure and data are included in the Supporting Information.
been the partial loading of palladium on MWNTs surfaces, with subse-
quent leaching of the metal after reaction. We examined various reduc-
ing agents such as hydrogen gas, sodium borohydride and hydrazine, in
an effort to efficiently immobilize palladium on the CNTs surface. Palla-
dium loading with hydrogen gas and sodium borohydride resulted in
irregularly fixed metal on MWNTs, as shown in the TEM data. Fortu-
nately, the use of the weaker reducing agent, hydrazine monohydrate
for immobilization allowed loading of uniform-sized palladium
(4–5 nm) as shown in Fig. 2. Further TEM images and XRD data are
supplied in the Supporting Information (Fig. S2 and S3, respectively).
2.2. Characterization
The infrared spectrum of CNT-COCl (Fig. 1) exhibited a peak at
1711 cm−1, indicative of the C_O stretching vibration of the COCl
group. The C_O stretching frequency corresponding to the amide
bond appeared at 1640 cm−1 (Fig. 1). The band at 1384 cm−1 was
assigned to the C\N bond stretch.
The degree of functionalization on the MWNT could be calculated
from the weight loss between 200 °C and 800 °C on TGA (Table 1).
Comparison of the data from EA and TGA showed no significant differ-
ences, confirming successful installation of the linker. Palladium content
in the synthesized catalyst was 0.98 wt.% as measured by Inductively
Coupled Plasma Mass Spectrometry (ICP-MS). The shape and size of
the palladium nanoparticles on Pd-CNT-ED-OH could be assessed from
the TEM image (Fig. 2). Until now, a disadvantage of CNT catalysis has
2.3. Activity tests on the Suzuki reaction
The Pd-CNT-ED-OH catalyst was chosen for the reactivity test on
the Suzuki reaction due to its excellent dispersion properties. A 25 mL
two-necked reaction flask was charged with Pd-CNT-ED-OH catalyst
(20 mg, 0.3 mol%) in degassed DMF (5.0 mL). Bromobenzene (52 μL,
0.50 mmol), phenylboronic acid (91 mg, 0.75 mmol) and anhydrous
sodium carbonate (106 mg, 1.0 mmol) were added and the mixture
Fig. 1. FT-IR of functionalized CNT: 1) CNT-pristine, 2) CNT-COOH, 3) CNT-COCl, and 4) CNT-ED-OH.

E. Kim et al. / Catalysis Communications 46 (2014) 71–74
73
Table 1
Table 2
Comparsion of TGA and elemental analysis data.
Optimization of the Suzuki coupling reaction of phenyl boronic acid with bromobenzene.a
Sample
Weight loss between The amount of functional Elemental (C,N,S)
.
200–480 °C (%)
groups per gram of
MWNT (mmol)
analysis result
(mmol)
CNT-COOH
CNT-ED-OH
34.78
30.96
6.20
1.66
4.00
1.80
Entry
Pd-CNT-ED-OH
amount (mg)
Solvent
Temperature (°C)
GC conversion (%)b
1
2
3
4
20
20
10
20
DME
1,4-Dioxane
DMF
80
80
120
120
47
4
60
90
was heated to 120 °C. After 24 h, the reaction mixture was cooled to
room temperature; solids formed during the reaction were collected
by filtration. Pd-CNT-ED-OH was sequentially washed with DCM,
water and ethanol, and dried under vacuum for 2 h. Dried Pd-CNT-
ED-OH was transferred to a 25-mL two-necked reaction flask, which
was subsequently used for the next reaction cycle.
DMF
a
All reactions were carried out with 0.5 mmol bromobenzene in 5.0 mL of solvent with
0.3 mol% catalyst.
b
Conversions were determined through GC analyses.
concentration of 0.98 wt.%. To study the leaching of Pd into the reaction
solution, we investigated the amount of Pd after the first reaction and
also carried out a test reaction using 2-mercaptoethylamine resin for
Pd capture. After the first run, 0.65 ppm of palladium was detected in
the solution. It was also found that the coupling reaction of
bromobenzene and phenylboronic acid was considerably slower than
the original rate when 1 mol% 2-mercaptoethylamine resin was added
at 5 h. When the reaction was run in the presence of the resin at the be-
ginning, there was almost no reaction. This indicates that the leached Pd
species from the Pd-CNT-ED-OH catalyst is at least partially responsible
for the Suzuki reaction. Detailed data are included in the Supporting
Information (Fig. S4). Fig. 3 shows the TEM images of the Pd-CNT-ED-
OH catalyst after the seventh run. Although a slight aggregation of nano-
particles was observed, the structure did not change significantly in size
and shape.
We then carried out a series of Suzuki coupling reactions to test the
scope of the substrates on the Pd-CNT-ED-OH catalyzed reaction
(Table 4). The Pd-CNT-ED-OH catalyst showed a broad substrate scope
for the reactions of various arylboronic acids with iodobenzene (entries
1–6). Under the optimized reaction conditions, a maximum yield of 94%
could be obtained (entries 1 and 2). Although the reactions of aryl bro-
mides proceeded smoothly (entries 10–12), low yield was observed for
the reaction of sterically hindered substrate (entry 12). Reaction of chlo-
robenzene (entry 13) gave a poor yield of the coupling product. The Pd-
CNT-ED-OH catalyst also showed compatibility with various substituted
aryl iodides in CNT-catalyzed reactions with phenylboronic acid, with
moderate yields (entries 7–9).
3. Results and discussion
First, the catalytic activity of Pd-CNT-ED-OH in various solvents
was tested for the Suzuki-coupling reaction of bromobenzene and
phenylboronic acid. As shown in Table 2, the reaction in DME gave
47% yield based on GC measurement; in 1,4-dioxane, however, much
lower yields were obtained (entries 1 and 2). Then a higher-boiling
solvent, DMF, was investigated. As a result, the coupling reaction
proceeded smoothly at 120 °C in DMF, yielding biphenyl in 60% yield
upon the use of 10 mg (0.15 mol%) of Pd-CNT-ED-OH (entry 3). When
a higher catalyst loading (20 mg, 0.3 mol%) of Pd-CNT-ED-OH was
employed, excellent yield was observed (entry 4).
With the optimized reaction conditions in hand, we then carried out
recycling tests on the Pd-CNT-ED-OH catalyst. After completion of the
reaction, the catalyst was filtered off and the reaction products were
directly analyzed with GC. The remaining catalyst was then washed
with ethanol and water, dried under vacuum, and reused for the next
reaction; the yields obtained for eight reaction cycles using the recycled
catalyst are summarized in Table 3.
To examine the extent of Pd leaching from the CNT catalyst, the con-
centration of the CNT-bound palladium was measured through ICP-MS
analysis. The palladium in the catalyst after seven reaction cycles was
0.70 wt.%, corresponding to slight reduction from the initial
4. Conclusion
The synthesis and characterization of MWNTs modified by surface
functionalization using nitrene chemistry and the utility of the resulting
material as a new support for Pd catalysts were reported. Various reduc-
tion methods were scanned to immobilize Pd on the MWNT surfaces
and use of hydrazine hydrate was found to be the most optimal. These
new catalytic systems exhibited good distribution of the metal on
Table 3
Recycling test of Pd-CNT-ED-OH catalyst of Suzuki reaction.a
Run
Time (h)
24 h
GC yield (%)b
1
2
3
4
5
6
7
8
71
70
76
77
85
77
70
16
a
All reactions were carried out with 0.5 mmol bromobenzene in 5.0 mL of solvent with
0.3 mol% of Pd-CNT-ED-OH catalyst.
b
Fig. 2. TEM image of Pd-CNT-ED-OH catalyst.
Dodecane was used as an internal standard.

74
E. Kim et al. / Catalysis Communications 46 (2014) 71–74
the functionalized MWNT. The new Pd-MWNT catalyst showed good
reactivity in Suzuki coupling reactions and the new linker incorporating
N-(2-hydroxyethyl)ethylenediamine was effective in holding the metal
more effectively than simple amine linkers. As a result, the catalyst was
recycled over seven reactions before any loss of catalytic activity was
observed, with only minimal catalyst loading (0.3 mol%). To develop
more effective CNT-based transition metal catalysts, we are examining
additional organic reactions in which these catalytic systems can be
applied.
Acknowledgments
This work was supported by the Nano Material Development
Program through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology
(2012M3A7B4049637) and the Mid-career Researcher Program
through National Research Foundation (NRF) grant funded by the
Ministry of Education, Science and Technology (MEST) (No. 2011-
0016187).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2013.11.028.
Fig. 3. TEM image of Pd-CNT-ED-OH catalyst after seven reaction cycles.
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a
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