Amino-TEMPO Grafted on Magnetic Multi-Walled Nanotubes: An Efficient and Recyclable Heterogeneous Oxidation Catalyst

Article abstract of DOI:10.1002/ejoc.201801706

An efficient and easy recyclable heterogeneous oxidation catalyst was prepared by grafting TEMPO–NH₂ moieties on the surface of magnetic multi-walled carbon nanotubes (MWCNT), first by a radical reaction introducing butyric acid moieties on carbon nanotube surface. Subsequently, carboxylic acid moieties were submitted for amidation using TEMPO–NH₂. The functionalized nanotubes [MWCNT-{(CH₂)₃-CO-NH-TEMPO}_n] were investigated as a (pre-)catalyst for the oxidation of primary and secondary alcohols for the production of aldehydes and ketones in a Montanari-type catalytic oxidation using the cheap and readily available 1,3-dichloro-5,5-dimethylhydantoin as the terminal oxidant.

Full text of DOI:10.1002/ejoc.201801706

A Journal of
Accepted Article
Title: Amino-TEMPO grafted on magnetic multi walled nanotubes: An
efficient and recyclable heterogeneous oxidation catalyst
Authors: Cristian Gambarotti and Hans-René Bjørsvik
This manuscript has been accepted after peer review and appears as an
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201801706
Link to VoR: http://dx.doi.org/10.1002/ejoc.201801706
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10.1002/ejoc.201801706
European Journal of Organic Chemistry
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Amino-TEMPO grafted on magnetic multi walled nanotubes:
An efficient and recyclable heterogeneous oxidation catalyst†
Dr. Cristian Gambarotti[b] and Prof. Dr. Hans-René Bjørsvik[a]*
[a] Department of Chemistry, University of Bergen, Allégaten 41, N-5007 Bergen, Norway
[b] Department of Chemistry, Materials, and Chemical Engineering, Politecnico di Milano
Pzza Leonardo Da Vinci 32, I-20131 Milano, Italy.
eMail: hans.bjorsvik@kj.uib.no, Telephone +47 55 58 34 52, web: https://folk.uib.no/nkjhn/
Dedication: This work is dedicated to the memory of our instructor and mentor Prof. Francesco Minisci
Graphical abstract:
O
O
N
N
H
Cl
O
N
O
N
Cl
R’
R’
DCH
30 min., 50 oC, CH3CN
R
OH
R
O
Magnetic MWCNT with surface grafted TEMPO moieties
• Recyclable with excellent stability
• Isolated by magnet or by filtration
• Operates with both primary and secondary alcohols
• Provide excellent yields of aldehydes and ketones
• Very good functional group tolerance
TOC text: A process for grafting TEMPO–NH2 moieties on magnetic multi walled carbon nanotubes was
developed. The modified nanotubes was utilized as pre-catalyst in a Montanari type catalytic oxidation
using the cheap and readily available 1,3-dichloro-5,5-dimethylhydantoin as the terminal oxidant. Primary
and secondary alcohols were converted to their corresponding aldehydes and ketones in high yields.
Key Topic: TEMPO grafted nanotubes.
Abstract: An efficient and easy recyclable heterogeneous oxidation catalyst was prepared by grafting
TEMPO–NH2 moieties on the surface of magnetic multi walled carbon nanotubes (MWCNT), first by a radical
reaction introducing butyric acid moieties on carbon nanotube surface. Subsequently, carboxylic acid
moieties were submitted for amidation using TEMPO–NH2. The functionalized nanotubes [ MWCNT-{(CH2)3-
CO-NH-TEMPO}n ] were investigated as a (pre-)catalyst for the oxidation of primary and secondary alcohols
for the production of aldehydes and ketones in a Montanari type catalytic oxidation using the cheap and
readily available 1,3-dichloro-5,5-dimethylhydantoin as the terminal oxidant.
† Electronic supplementary information available
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Introduction
Contemporary organic synthesis comprise commonly reaction steps that that involving some sorts of
catalyst, that can be transition metals complexes, but also organocatalyst such as TEMPO. A common
challenge is that the catalysts are difficult or even impossible recycle. In this context, we have searched for
possibilities for attaching catalysts to the surface of magnetic nanoparticles in order to approach a simple
recycling the catalyst.
In this context, earlier we have produced according to Landfester and collaborators[1] nanoparticles that
contain a magnetite core and a polystyrene-acrylic acid co-polymer shell. These polymer encapsulated
magnetic nanoparticles was investigated after treated with various typical solvents or solvent systems
commonly used in organic synthesis. Unfortunately, in generally, the polymer shell didn’t stand the organic
solvents treatment, with the result that the polymer shell swelled and then collapsed.
Oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones, respectively, are
extremely important synthetic transformations in organic synthesis and the organic process industry. In fact,
oxidation processes all together are one of the major cash generators of the chemical industry.[2] This is
reflected by the wealth of synthetic methods and processes that exist for such transformations.[3]
Montanari and collaborators[4] developed more than three decades ago a highly selective and efficient
oxidation method based on TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl free radical) as a pre-catalyst with
the cheap and readily available sodium hypochlorite as the terminal oxidant. Since then several oxidative
systems based on transition metal salts or such salts as co-catalyst to persistent[5] and non-persistent[6]
nitroxyl free radicals have been used. A vast number of various terminal oxidant together with various
nitroxyl radicals as pre-catalyst have been investigated,[7,8] in the most cases in homogeneous phase.
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Persistent O-centered free radicals
R
R
R'
N
O
O
O
TEMPO
R=R'=H
R=Me, X=H
R=H, R'=F 5-F-AZADO
AZADO
1-Me-AZADO
R=H2 ABNO
keto-ABNO
R=O
Non-persistent O-centered free radicals
O
O
CH3
N
N O
O
PINOH
O
NMBHA
O
N
O
N
N
N
N
HN
N
N
N
N
N
N
N
NH
N
N
O
O
Tetranitroxyl radical - Chimassorb 966
Even though this widespread use of Montanari type oxidation involving TEMPO as pre-catalyst, the method
is saddled with a serious drawback, namely what concern the isolation and recycling of the organocatalyst
(originated from TEMPO). In this context, various approaches have been used in order to improve the
recycling issue. Bolm and collaborators[9] developed and utilized silica-supported TEMPO catalyst for the
Montanari oxidation.[4] Few years later, Ciba Specialty Chemicals developed a stable and persistent tetra-
nitroxyl free radical, Chimasorb 966 as a stabilizer,[10] that also was tested as a recyclable oxidation
catalyst.[11] Minisci and collaborators[12] accomplished a study based on Chimassorb 966 and various
transition metal ions as co-catalyst.
The discovery[13] and preparation[14] of single walled carbon nanotubes (SWCNT) and multi-walled carbon
nanotubes (MWCNT) have created a huge interest in the scientific and industrial communities due to the
unique mechanical, electrical, and chemical properties of the MWCNT.[15] Moreover, a 85% grade carbon
nanotubes are commercially available at a reasonable cost ( <1000 USD kg-1).
The delocalized π-bond of the nanotube permits straightforward modification of their surface by grafting
various functional groups by covalent bonds. Such modification might be conducted by means of strong
acidic oxidations and free-radical coupling reactions.[16]
Few years after the discovery of the carbon nanotubes, it was observed that magnetic fields had strong
effects on the electronic structure of carbon nanotubes.[17] This behaviour was found to be related to the
metal-based catalyst, that comprises Fe, Co and Ni, which is utelized in the nanotube production process.
During the production process, the transition metal catalyst end up trapped inside the nanotubes and cannot
be removed throughout consecutive purification steps. Later, these properties have been exploited to
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10.1002/ejoc.201801706
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prepare magnetic nanoparticles for bio-applications, drug delivery, catalysts for energy conversion etc.[18]
During the recent years, various groups have reported graphene[19] and fullerenes[20] as the carrier for TEMPO
that is grafted on various nanomaterials as heterogeneous catalysts. Moreover, TEMPO grafted on soluble
MWCNTs has been utilized in the homogeneous catalysed oxidation of alcohols.[21] Magnetic multiwalled
nanotubes have been used as polymer stabilizers where hindered phenols were grafted by covalent bonds
on the nanotube surface.[22] Likewise, hindered amine[23] and polyhedral oligomeric silsesquioxane[24]
functionalized carbon nanotubes were revealed to increase the photo-oxidation stability of polyethylene-
based nanocomposites. However, to the best of our knowledge, magnetic nanoparticles for the purpose to
simplify the separation and thus optimize the recycling of catalysts carrier after their use in chemical
processing have not been investigated.
Results and Discussion
The multiwalled carbon nanotubes (MWCNT) were produced by means of an in-house (Politecnico di
Milano) designed reactor that is based on the chemical vapor deposition (CVD) protocol.[25] The obtained
MWCNT were characterized using TEM micrography, thermo gravimetric analysis (TGA), inductively
coupled plasma (ICP) analyses, and X-ray photoelectron spectroscopy (XPS). The produced nanotubes
possessed an outer diameters of 14–20 nm, and with an inner diameters in the range of 2–5 nm. The length
of the nanotubes were in the range of 1-10 μm. TGA analysis revealed a graphitic carbon purity of >95%.
The residual mass constituted the catalyst particles trapped inside the nanotubes, which was revealed by
ICP analysis to be Fe (1.36%) and Al (1.15%), both elements constituents of the catalyst used in the CVD
nanotube production process.
Preparation of MWCNT–{(CH2)3–CO–NH–TEMPO}n. The production of the functionalized multi walled
carbon nanotubes was carried through via a three steps synthetic process, Scheme 1. Step  is an hydrogen
peroxide oxidation of glutaric anhydride 1, which produce glutaryl peroxide 2.[26] The peroxide bond of 2
(RCO–O–O–COR) is in the following step homolytically cleaved via thermolysis.[27] The formed O-centred
radicals (RCO–O•) is immediately undergone decarboxylation under the formation •CH2(CH2)2COOH free
radical. The radical reactions of step ‚, are detailed outlined in Scheme 1. The formed free radical
•CH2(CH2)2COOH was in a subsequent radical reaction grafted onto the surface of the nanotubes to afford
MWCNT–{(CH2)3–COOH}n 4.
XPS spectroscopy of the functionalized MWCNT 4 revealed a carboxyl group population grafted on the
MWCNT surface of about 4.4%, which corresponding to one –COOH group per 100 carbon atoms. TGA
analysis revealed a loss of weight of about 5.3%, DT = 200 ® 400 °C, ascribed to the release of the
functional groups on the nanotube surface. Step ƒ of Scheme 1 comprise a reaction that involves treatment
of the functionalized MWCNT 4 with NH2-TEMPO derivative 5 that undergo an amidation to produce MWCNT–
{(CH2)3–CO–NH–TEMPO}n 6.[28]
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O
O
O
O
O
H2O2 30%
O
O
OH
OH
O
H2N
N O
N
O
20-0°C
1
5
O
N
H
1
O
O
2
OH
chemical vapor
deposition
(CVD) process
H2O, 90 o
2
C
3
3
4
6
2
Detalis, step
O
O
O
O
Δ
HO
O
O
OH
HO
O
O
OH
O
O
O
O
O
2
O C O
H
H
H
+
OH
HO
H
O
O
H
H
OH
further reactions
with another radical,
atmospheric oxygen, ...
Scheme 1. Functionalization of MWCNT to approach MWCNT–{(CH2)3–CO–NH–TEMPO}n 6.  Hydrogen peroxide
oxidation, ‚ Grafting C4 moieties on surface of the MWCNT the in a radical decarboxylation reaction, ƒ Amidation of
the C4 functionalized MWCNT 4 using NH2-TEMPO derivative 5 to produce MWCNT–{(CH2)3–CO–NH–TEMPO}n 6.
With the goal to elucidate the number of TEMPO moieties grafted on the MWCNT surface, samples of
MWCNT–{(CH2)3–COOH}n 4 and MWCNT–{(CH2)3–CO–NH–TEMPO}n 6 were submitted for TGA. The
recorded TGA data for 3 and 6 are displayed in Figure 1, which reveals a relative mass difference of Dm
= 1.27% measured over the temperature range DT = 20 ® 350 °C. Considering that both MWCNT 3 and the
TEMPO functionalized nanotubes 6 were dried at 80 °C for an identical period of time, that mass difference
can be assigned to the presence of the NH-TEMPO units covalently bonded to the carboxylic group of the
pendant arm of the nanotube. That quantity of TEMPO units covalently bonded to the surface of the nanotube
correspond to an equivalent weight for the MWCNT–{(CH2)3–CO–NH–TEMPO}n of about 13000 u.m.a.,
which resemble 2% molar of TEMPO in our experimental screening.
Figure 1. TGA analysis of MWCNT–{(CH2)3–COOH}n 3 (red line) and MWCNT–{(CH2)3–CO–NH–TEMPO}n 6 (blue line)
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Montanari type oxidation using the pre-catalyst (6). Montanari and collaborators,[4] disclosed originally
an oxidation method involving TEMPO free radical as the pre-catalyst and sodium hypochlorite as the
terminal oxidant. In previous works in our laboratory (not published), we have experienced that calcium
hypochlorite to be more better terminal oxidant compared with sodium hypochlorite.
Table 1. Investigation of MWCNT–{(CH2)3–CO–NH–TEMPO}n 6 as pre-catalyst and calcium hypochlorite as the terminal
oxidant.
O
O
N
N
H
6
OH
7d
HO
O
O
Ca(OCl)2
+
[a]
8d
9d
Cl
Cl
Cl
Quantity of Ca(ClO)2
S
T
[°C]
t
#
Cat. 6
C
Ratio
m [g]
n [mmol]
[min.]
–CHO
–COOH
Ox:S
1[b]
2 : 1
0.750
0.750
0.750
0.072
0.143
0.143
1.000
1.000
1.000
0.105
0.500
0.500
–
+
+
+
+
+
50
50
50
50
50
30
240
30
5
57
100
93
96
96
96
96
0
7
4
6
4
4
2
2 : 1
100
88
64
88
88
3
4[c]
2 : 1
1 : 1
5
5
1.5 : 1
1.5 : 1
10
10
6
[a] Reaction conditions: Catalyst 6 (0.150 g), (4-chlorophenyl)methanol (0.072 g, 0.5 mmol), Ca(OCl)2 see the tabel,
acetic acid (3 mL), and water (3 mL). Reaction temperature and time are provided in the table. Ox=oxidant Ca(ClO)2,
S=alcohol substrate, C = %-conversion, and S = %-selectivity.
[b] Similar results after a reaction time of t = 18 h.
[c] Similar results after a reaction time of t = 40 min.
We initiated to investigated the MWCNT–{(CH2)3–CO–NH–TEMPO}n 6 as an pre-catalyst for a Monanari
type oxidation with calcium hypochlorite as the terminal oxidant and 4-chlorophenyl)methanol 7d as the
substrate, see Table 1. (4-Chlorophenyl)methanol 7d was readily oxidized by calcium hypochlorite even
without the presence of the nitroxyl free radical 6. A conversion of 57% was achieved, but a lengthy reaction
time was required (240 min.) when an oxidant : alcohol substrate ratio of 2:1 (entry #1 of Table 1) was
used.
A similar experiment (entry #2), but with the presence of the pre-catalyst 6, afforded quantitative
conversion with a selectivity of 93% to the aldehyde 8d. The remaining 7% was the over-oxidation product
4-chlorobenzoic acid 9d. Attempts to improve the selectivity by shortening the reaction time and lower the
quantity of terminal oxidant resulted only in a lower conversion of the alcohol substrate 7d and
correspondingly less quantity of target molecule 8d (see entries #3-6).
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At the outset of the investigation of the pre-catalyst 6, we believed that a Montanari type oxidation
following a mechanism as outlined in Scheme 2 could be conducted with 1,3-dichloro-5,5-
dimethylhydantoin (DCH) as the terminal oxidant. This because we recently developed an efficient and
high yielding method based on TEMPO with DCH that was successfully adapted and used in telescoped
processes for the preparation of amides from amines and alcohols[29] and nitriles from alcohols.[30] Based
on the disclosed processes, we embarked on to exploit similar reaction conditions for novel applications
using the MWCNT–{(CH2)3–CO–NH–TEMPO}n 6 as the pre-catalyst.
O
OH
O
H
O
Cl
Cl
HN
N
N
R
R’
R
R’
MWCNT
H
O
DCH
N
O
O
O
O
HN
HN
HN
Cl
N
MWCNT
O
MWCNT
6
N
N
O
O
R1
O
H
H
O
O
HN
HN
O
NH
O
MWCNT
N
R’
R
HO
DMH
Scheme 2. Mechanism of the catalytic oxidation reaction involving MWCNT–{(CH2)3–CO–NH–TEMPO}n 6 as pre-catalyst
and 1,3-dichloro-5,5-dimethylhydantoin (DCH) as the terminal oxidant.
Solvent effects. For the present oxidation experiments, we used previously reported conditions for a small
solvent screening. As reported in Figure 2, both in the presence and in the absence of catalyst, in polar
solvents (AcOH and MeCN) the reaction is faster than in less low polar solvents such as EtOAc and MTBE.
The catalytic activity of the MWCNT–{(CH2)3–CO–NH–TEMPO}n 6 is particularly noticeable in the lowest
polar MTBE, where the reaction in the absence of catalyst seems to be about five time slower than the
catalysed one. It is known that the oxidation of alcohols by oxoammonium cations (the effective oxidant
specie in the TEMPO catalysed processes) are slower in acidic media than neutrals,[31] and for this reason in
our opinion, despite the beneficial effect of the polarity, the efficiency of the catalysed reaction in acetic
acid resulted less significant in comparison with the not catalysed one.
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Figure 2. The solvents influence on the course of the oxidation reaction.
Effect of the reaction temperature. The similar reaction protocol as for the solvent screening was utilized
to investigate the effect of the reaction temperature. Figure 3, shows the results form experiments performed
at reaction temperatures in the range 20–50 oC. The graph shows evidently a significant influence on the
outcome of the reaction. The screening revealed 50 oC as an optimal reaction temperature.
Figure 3. The effect of the reaction temperature on the oxidation reaction using MWCNT–{(CH2)3CO–NH–TEMPO}n 6 as
the pre-catalyst with DCH as the terminal oxidant.
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Evaluating the oxidant versus substrate ratio. 1,3-Dichloro-5,5-dimethylhydantoin (DCH) is a cheap
and readily available even for large scale use. DCH bearing two chlorine that both can be used, which
contribute to an improved oxidant atom economy. Based on the screening of reaction medium and reaction
temperature above, a series of experiments varying the molar ratio of oxidant DCH to substrate was carried
out, the achieved results are displayed in Figure 4.
Figure 4. Effect of the molar ratio of the oxidant (DCH) versus the substrate (menthol 7a) using MWCNT–{(CH2)3–CO–
NH–TEMPO}n 6 as the pre-catalyst producing menthone 8a
Comparison of homogeneous and heterogeneous TEMPO catalyst. The oxidation of menthol to menthone
was used a model reaction for comparison of our heterogeneous pre-catalyst MWCNT–{(CH2)3–CO–NH–
TEMPO}n 6 and TEMPO dissolved in the reaction medium as a pre-catalysts, respectively. TEMPO is one of
the more frequently used nitroxyl radicals used as the pre-catalyst in homogeneous catalysed Montanari
type oxidation processes,[4] Scheme 3, pathway .
Cl
O
N
1
O
N
O
N
Cl
TEMPO
DCH
O
O
N
OH
O
N
H
6
Cl
7a
8a
O
N
O
N
2
Cl
Scheme 3. Comparison of Montanari type oxidation in  homogeneous phase using TEMPO as pre-catalyst and
‚ heterogeneous phase using MWCNT–{(CH2)3–CO–NH–TEMPO}n 6 as the pre-catalyst for the oxidation of the secondary
alcohol menthol 7a to menthone 8a. In both cases, the cheap and readily available 1,3-dichloro-5,5-dimethylhydantoin
(DCH) were used as the terminal oxidant.
For the trials involving homogeneous catalysis, a typical quantity of 5 mol-% TEMPO pre-catalyst was
utilized. Figure 5, shows results from sampling during the course of the oxidation reaction for (a) the
homogeneous catalysed reaction using TEMPO, and (b) the heterogeneous catalysed reaction catalysed using
MWCNT–{(CH2)3–CO–NH–TEMPO}n 6.
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Figure 5. Results of oxidation experiments using (a) 5% TEMPO in homogeneous phase as the oxidation (pre-)catalyst,
and (b) the heterogeneous MWCNT–{(CH2)3–CO–NH–TEMPO}n 6 as the oxidation (pre-)catalyst. In both cases 1,3-
dichloro-5,5-dimethylhydantoin (DCH) were used as the terminal oxidant.
Evidently, the oxidation using the heterogeneous catalyst 6 (Scheme 3, pathway ‚) afford a rate that is
twice the rate of the homogeneously catalysed (TEMPO) reaction. The heterogeneous catalysed reaction
(using 6) is completed after a reaction time of 30 min., while the process involving TEMPO in homogeneous
solution request 1 h to achieve a quantitative conversion. The achieved selectivity for the heterogeneous
phase oxidation catalyst was 97% and for the homogeneous phase oxidation catalyst was 93%, respectively.
These somewhat surprising result might be explained by the fact that carbon nanomaterial such as carbon
nanotubes both single and multi-walled adsorb organic substrates. The quantity of TEMPO units grafted on
the nanotubes correspond to 2% compared to the experiment conducted in homogeneous phase where 5%
of TEMPO was used. The higher efficiency observed for the catalyst grafted on the surface of the nanotubes
might be ascribed to the influence of the absorption of the alcohol substrate on the surface of the carbon
nanotubes 6, which ultimately facilitates the substrate for the desired oxidative transformation as outlined
in Scheme 2.
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Recycling of the heterogeneous pre-catalyst (6). The nanotube catalyst MWCNT–{(CH2)3–CO–NH–
TEMPO}n 6 was repeatedly investigated in a small series of experiments, where menthol 7a was oxidized to
menthone 8a (pathway ‚ of Scheme 2). The pre-catalyst 6 was used and recovered in total five times. The
obtained results are displayed in the bar graph of Figure 6. A mean yield !"=96.6% was acheived, which
demonstrate an excellent efficiency of the catalyst 6. Moreover, the upper part (the line graph) of Figure 6,
shows the cummulative sum (CuSum) for the yield over the five experiments. This line graph reveals a very
weak falling trend over the experiments (i = 0, …, 4) and thus also an outstanding chemical stability of the
catalyst 6.
Figure 6. A series of five experiments where the catalyst MWCNT–{(CH2)3–CO–NH–TEMPO}n 6 were used five times for
Montanari type oxidation experiments. The five experiments afforded a mean yield !" = $% ')* !' = 96.6%. The CuSum
(
∑
(
∑
=
')*(!' − !") graph reveals a very weak falling trend over the five experiments, which demonstrates an excellent
efficiency and stability of the recycled TEMPO modified nanotube catalyst 6, which moreover is demonstrated by the high
mean yield (!" = 96.6%.) and the small standard deviation, Std=0.548.
Scope and Limitation
A series of primary and secondary alcohols were submitted for the established oxidation process using the
MWCNT–{(CH2)3–CO–NH–TEMPO}n 6 as pre-catalyst with DCH as the terminal oxidant. The conversions
were evaluated by means of 1H NMR analysis of filtered samples of the post reaction mixture. The various
products were identified by comparison with data previously reported in the literature.
In general, the oxidation performed using the nanotube catalyst 6 with DCH as the terminal oxidant revealed
high efficiency regarding conversion and selectivity. In the experiment using p-methoxybenzyl alcohol as
substrate, the conversion was quantitative, but the reaction products were mainly chlorinated by-products
and only 3% of target aldehydes was observed. Similarly, oxidation of cinnamic alcohol afforded various
oxidation product together with 38% of target aldehyde. Mandelic acid afforded benzaldehyde in a yield of
57% and target phenyl glyoxylic acid as a secondary product together with benzoic acid in 16% and 26%
yields, respectively. The relative high selectivity toward benzaldehyde can be related to the oxidation of
mandelic acid.[32]
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Table 2. Scope and limitation of the Montanari type oxidation using the heterogenious MWCNT–{(CH2)3–CO–NH–TEMPO}n 6
pre-catalyst with 1,3-dichloro-5,5-dimethylhydantoin as the terminal oxidant (DCH).
O
O
N
N
H
Cl
N
O
6
N
Cl
O
O
8
HO
7
DCH
[a]
Y
R
R
O
NO2
8h
Y
C
S
C
S
O
O
100 97 97
76 100 76
100 100 100
8a
8b
O
O
100 100 100
8i
70 100 70
100 100 100
100 100 100
O
8j
O
8c
8d
8e
O
41[d] 100 41
Cl
8k
8l
O
100[b]
0 nd
O
100[e] 38 38
OH
O
O2N
O
100
O
97 97
88 100 88
8m
8n
8f
O
O
O
100[f]
3
3
33[c]
0 nd
O
8g
[a] Conditions: MWCNT based catalyst 6 (0.150 g), substrate (alcohol) (0.50 mmol), terminal oxidant (DCH) (1.5 mmol),
o
solvent (acetonitrile) (6 mL), reaction temperature 50 C, and reaction time t = 30 min. C = %-conversion, S = %-
selectivity, Y = %-yield, nd = not detected.
[b] Major product benzaldehyde (57%)
[c] Major product benzoic acid (18%)
[d] The experiment was conducted at a temperature of 75 oC.
[e] Reaction time of 10 min. Three side products were formed; 11%, 48% and 3%, respectively.
[f] Major products appeared to be 1,2-bis(4-methoxyphenyl)-ethane-1,2-dione (50%) and 3-chloro-4-methoxy-
benzaldehyde (45%). The experiment was conducted under, air, molecular oxygen, and inert atmosphere using argon.
The three experiments afforded similar product profile and outcomes.
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Conclusions
We have developed a three steps process for grafting TEMPO moieties on the surface on the surface of
magnetic multiwalled carbon nanotubes. In this process amino- TEMPO is used in an amidation reaction to
obtain chemical stable grafted moieties (compared to previously prepared ester links usually used in
stabilizers) on the surface of magnetic multiwalled carbon nanotubes. The functionalized nanotubes
operated superbly as heterogeneous catalysts in Montanari type oxidations with 1,3-dichloro-5,5-
dimethylhydantoin as the terminal oxidant, but also with calcium hypochlorite as the terminal oxidant.
Recycling of the functionalized magnetic nanotubes 6 displayed excellent activity and afforded excellent
product yields after each recovery and recycling, which also shows high chemical stability of the MWCNT–
{(CH2)3–CO–NH–TEMPO}n 6. Furthermore, the oxidation using the nanotubes 6 demonstrated an higher
reaction rate when compared with TEMPO in homogeneous phase. We believe that this might be ascribed to
a surface effect which can efficiently promote the interaction between the catalytic centre on the nanotube
surface and the substrate.
Experimental Section
General methods. The NMR spectra of the screening experiments were acquired by means of a 500 MHz
instrument. The acquired spectra were compared with literature data.
Preparation of the multi-walled nanotubes MWCNT. The multiwall carbon nanotubes (MWCNT) were
prepared by means of the chemical vapor deposition (CVD) protocol.25 The produced MWCNT were
analyzed and characterized by TEM, XPS, TGA, and ICP.
Instrumental investigation of MWCNT.
TEM micrography were performed with a Philips CM200 field emission transmission electron microscope.
The sample of the nanotubes was dispersed in a 4% aqueous SDS, and deposited an a holey carbon films
on 200 mesh copper grids and observed under accelerated voltage of 200 kV.
XPS spectra were obtained by using an M-probe apparatus (Surface Science Instruments). The source was
monochromatic Al Kα radiation (1486.6 eV). A spot size of 200 × 750 μm and a pass energy of 5 eV were
used. Fits were performed using pure Gaussian peaks, Shirley’s baseline, and without any constraints.
The outer diameters was in the range do = 14 ® 20 nm, the inner diameters was in the range di = 2 ® 5 nm
and the nanotube length was in the range L = 1 ® 10 μm.
TGA profile curves were recorded by using a Perkin Elmer STA 6000. The analyses were performed in air
over a temperature range of DT = 35 ® 900°C with a continuous heating rate of 10°C min.-1 The analyses
revealed a graphitic carbon purity >95%. The remaining mass was represented by residual catalyst trapped
inside the nanotubes.
ICP analyses were performed on a Perkin Elmer Optima 8300. The analysis revealed the presence of 1.36%
of Fe and 1.15% of Al, both constituents of the catalyst utilized in the CVD process.
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Synthesis of TEMPO functionalized MWCNT.[33] In order to introduce the carboxylic acid functions, the
nanotubes were functionalized to the corresponding MWCNT–{(CH2)3–COOH}n 3 by in situ thermal
decomposition of glutaryl peroxide 2,[ 34 ] which was previously prepared from glutaric anhydride 1,
following the procedure reported in literature (Scheme 1).[35] The XPS spectroscopy of the functionalized
MWCNT 3 showed an amount of carboxyl groups on the surface of about 4% (in the suppl. info),
corresponding to about one carboxyl group for every 100-carbon atoms. The further reaction with 4-amino-
TEMPO 4 was performed as previously reported elsewhere,[36] afforded MWCNT–{(CH2)3CO–NH–TEMPO}n
6, Scheme 1.
Glutaryl peroxide.[33a] Glutaric anhydride (12.5g, 2.74 mmol) was suspended in distilled water (20 mL).
H2O2 (30%, 15 mL) was then dropwise added under vigorous stirring. Few minutes after the complete
addition of the hydrogen peroxide, the anhydride become complete dissolved. The resulting mixture was
cooled at 0-5°C at an ice bath and the peroxide precipitated as a white solid. The peroxide was filtered off,
washed with cold water and dried under reduced pressure to achieve an isolated yield of 10.5 g, which
correspond to a yield of 73%. 1H NMR (400 MHz, DMSO, d6): δ = 1.82 (quintet, J = 7.35 Hz, 4 H, 2 CH2),
2.32 (t, J = 7.35 Hz, 4 H, 2 CH2), 2.54 (t, J = 7.35 Hz, 4 H, 2 CH2) ppm, 13C NMR (100 MHz, DMSO, d6):
δ = 20.77, 29.41, 33.20, 169.72, 174.51 ppm.
MWCNT–{(CH2)3COOH}n. Pristine multi-walled carbon nanotubes (MWCNT) (1.0 g) was dispersed in a
mixture of glutaryl peroxide 2 (3.8 mmol, 1.0 g) in H2O (250 mL). The resulting mixture was sonicated
using an ultrasound bath (240 W, 2.5 L) for 10 min. then heated and stirred for 7 h at 90 °C. After that time,
the suspension was filtered on a sintered glass filter, and the obtained MWCNT–{(CH2)3COOH}n were
washed using hot water (100 mL ´ 10), filtered, and dried overnight at a temperature of 90 °C.
MWCNT–{(CH2)3CO–NH–TEMPO}n (amidation).[33b] The obtained MWCNT–{(CH2)3COOH}n (1.0 g, 1.25
mmol COOH, based on the XPS data) were mixed with NH2–TEMPO (0.343 g, 2.0 mmol), DCC (0.413 g,
2.0 mmol), DMAP (0.122 g, 1.0 mmol), and anhydrous dichloromethane (30 mL). The resulting suspension
was sonicated (1 min.) in an ultrasonic bath and then vigorously stirred for 4 days at room temperature. The
solid was then filtered and washed with hot DMF (50 mL ´ 3), hot methanol (50 mL ´ 3), and finally hot
acetone (50 mL ´ 3) with the goal to remove adsorbed by-products. Finally, the washed MWCNT–{(CH2)3–
CO–NH–TEMPO}n was dried under reduced pressure. TGA analysis of dry functionalized nanotubes shows
6.5766 % decrease in the mass between 50 and 250°C due to the loss of the functional groups covalently
bonded to the nanotube surface, which correspond to » 0.08 mmol TEMPO moieties per gram of the
functionalized nanotube MWCNT–{(CH2)3–CO–NH–TEMPO}n 6.
Oxidation of alcohols using MWCNT–{(CH2)3–CO–NH–TEMPO}n, as pre-catalyst. This procedure was
utilized for the entries of the scope and limitation investigation, Scheme 4. The alcohol (0.25 mmol) was
dissolved in acetonitrile (3 mL) whereupon the oxidant 1,3-dichloro-5,5-dimethylhydantoin (DCH, 0.148
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g, 0.75 mmol) was added. Then the pre-catalyst MWCNT–{(CH2)3–CO–NH–TEMPO}n (0.075 g) was added.
The resulting suspension was sonicated (1 min.) using an ultrasonic bath, and then stirred and heated at
50°C for 30 min. After this time, the reaction mixture was filtered and added CH2Cl2 (10 mL). The organic
phase was washed with aqueous Na2S2O3 (10%, 10 mL) and H2O (10 mL ´ 2), dried over Na2SO4 and the
solvent was finally removed under reduced pressure using a rotary evaporator.
Supporting Information
Supporting information and chemical compound information are available in the online version of the paper.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
CG acknowledges financial support from Politecnico di Milano for funding a sabbatical stay (2017) at the
University of Bergen.
Key words.
Synthetic methods, Oxidation, Nanotubes, TEMPO, 1,3-dichloro-5,5-dimethylhydantoin
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