Nitroxyl radical based conjugated microporous polymers as heterogeneous catalysts for selective aerobic alcohol oxidation

Article abstract of DOI:10.1039/c8ta02664e

Covalent anchoring of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) on the scaffold of conjugated microporous polymers (CMPs) was achieved via cost-effective FeCl₃-catalyzed oxidative polymerization of dicarbazole functionalized TEMPO. These TEMPO-CMPs exhibit excellent catalytic performance for the aerobic oxidation of a wide variety of alcohols under metal-free conditions. In addition, the TEMPO-CMPs could be recycled at least 20 times with good retention of high catalytic activities.

Full text of DOI:10.1039/c8ta02664e

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Kun Chang, Zhaorong Chang et al.
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JPolueransael odfoMnaotetraiadljsuCshtemmairsgtriynsA
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DOI: 10.1039/C8TA02664E
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Nitroxyl Radical Based Conjugated Microporous Polymers as
Heterogeneous Catalysts for Selective Aerobic Alcohol Oxidation
Mingxuan Liu, Baolong Zhou, Lei Zhou, Zhen Xie, Shen Li,* Long Chen*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Covalent anchoring of 2,2,6,6-tetramethylpiperidine-1-oxyl performance fading due to the losing of catalysts during cycling and
(TEMPO) on the scaffold of conjugated microporous polymers the instability issues, etc. Prior to this work, immobilizing TEMPO on
(CMPs) was achieved via cost-effective FeCl3-catalyzed oxidative insoluble supports is the main approach to develop TEMPO based
polymerization of dicarbazole functionalized TEMPO. These heterogeneous catalysts.9 For example, Karimi and co-workers
TEMPO-CMPs exhibit excellent catalytic performance for the utilized the mesoporous silica SBA-15 as a support for TEMPO.9a
aerobic oxidation of a wide variety of alcohols under metal-free Reiser’s team developed a highly reactive supported catalyst using
conditions. In addition, TEMPO-CMPs could be recycled at least 20 click reaction to connect TEMPO to multiple perfluoroalkyl chains
times with good retention of high catalytic activities.
and recovered the catalysts by liquid/emulsion filtration.9b Although
these methods can overcome many shortcomings of homogeneous
catalysis. Nevertheless, some severe problems still exist such as low
immobilization capacity, poor catalytic activity and selectivity, etc.10
In contrast to this conventional approach, we are particularly
interested in the covalent construction of a built-in catalyst system
using the bottom-up strategy to directly incorporate catalytic
moieties into conjugated microporous polymer (CMP) scaffolds.
CMPs were first reported by Cooper and coworkers,11 exhibiting high
surface area, large pore volume, tunable porosity, good chemical and
thermal stabilities, and facile post-functionalization.12 Because of
these merits, CMPs have drawn much attentions in a wide range of
applications including gas storage,13 catalysis,14 supercapacitors,15
light harvesting16 and so on. However, the synthesis of most reported
CMPs involves the use of noble metal based catalyst for the C-C
coupling or C-N coupling reactions. Recently, an alternative approach
of using cost effective FeCl3-catalyzed oxidative coupling
polymerization for knitting electron rich building blocks (e.g.
thiophene, carbazole, etc.) into CMP networks has been
developed.17 Such strategy exhibits several advantages such as low
cost, mild reaction conditions, and high polymerization efficiency. In
addition, only one single kind of monomer is needed for this type of
oxidative polymerization. All these advantages are essential for the
scale-up of porous materials for real applications.17 Thus, we are
committed to design adaptive catalytic TEMPO-based monomers for
efficient construction of TEMPO built-in catalytic CMPs. Such built-in
CMP based catalysts are intriguing because they are “self-supported
catalysts”,18a which means that the porous polymer scaffolds both
serve as catalytic centers and porous supports. The CMP networks
usually bear inherent open porous structures accessible for both
The selective oxidation of alcohols to the corresponding
carbonyl compounds represents one of the fundamental
transformation in organic synthesis and industrial chemical
process.1 The increasing demand for “green” chemistry has
stimulated the development of efficient metal-free oxidation
protocols.2 Using 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)
as the catalyst in combination with O2 as the terminal oxidant is
a highly attractive method because of its readily available
materials, environmental friendly conditions, high catalytic
efficiency and chemoselectivity.3 In the past decades, a number
of TEMPO-catalyzed aerobic alcohol oxidation protocols have
been developed with notable achievements.4 However, the
considerable difficulties involved in the separation of the
catalyst from products has restricted its applications in
industrial processes.
From a practical point of view, heterogeneous catalysts exhibit
many advantages over their homogeneous counterparts regarding to
the catalyst/product separation and recycling of the catalysts from
the reaction system.5 Thus, many organocatalysts have been
immobilized on different platforms like mesoporous silica,6
polymers7 and metal organic frameworks (MOFs)8 as composites and
applied into heterogeneous catalysis. However, this immobilization
approach usually suffers from low catalyst loading capacity,
* Tianjin Key Laboratory of Molecular Optoelectronic Science, School of Science,
Tianjin University, Tianjin 300072, P.R. China. E-mail: shenli@tju.edu.cn (S. Li);
long.chen@tju.edu.cn (L. Chen)
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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tetramethylpiperidine (TEMP) based TEMP-CViMewPAsrticlewOnelirnee
DOI: 10.1039/C8TA02664E
synthesized following the same procedures (Scheme S2) as
control samples for NMR measurements to validate the
formation of covalent linkage in these TEMPO based CMPs. As
shown in Figure S2, all of the chemical shifts could be properly
assigned to the polymer backbones. Take TEMP-CMP-4 as an
example to illustrate the assignment of the solid state NMR
spectra (Figure S2d). The three intensive peaks at 139.8, 124.8
and 110.3 ppm correspond to the aromatic carbons. The first
peak at 139.8 ppm can be assigned to the substituted phenyl
carbons binding with nitrogen atom (i.e. C8, C10) or the phenyl
carbon (C6) close to the carbonyl group. The most intensive peak
at 124.8 ppm can be attributed to the four carbons (C12-15) of
the carbazole unit, while the peaks around 110.3 ppm are
correspoding to the signal of the unsubstituted phenyl carbon
(C7, C9) and the 2-position carbon of the carbazole segment (C11).
The four sharp signals at the upfield around 20~60 ppm could
be assigned to the four kinds of carbons (C1-4) from the
piperidine ring. In addition, the weak broad peak at downfield
(ca.165.6 ppm) is attributed to the carbonyl carbon (C5) of the
amide linkage in TEMP-CMP-4.
Scheme 1. Synthetic routes of the four TEMPO-CMPs.
substrates and reagents, and their large specific surface areas can
also facilitate the mass transfer process in transformation reaction.
In this work, four TEMPO-based CMPs (Scheme 1) with
different linkages (i.e. ether, amine, ester, and amide) between
TEMPO and carbazole units were designed and synthesized.
Their catalytic properties were tested to compare the influence
of the different linkages on their performance for selective
catalytic oxidation of alcohols. TEMPO-CMP-4 with amide
linkages exhibits the best catalytic efficiency among the four
CMPs, which was comparable with most of the nonmetal
aerobic alcohol oxidation catalytic systems reported18b and
FT-IR spectra analysis was applied to further demonstrate the
successful incorporation of TEMPO and the covalent
crosslinking between carbazole units in these CMPs. As
displayed in Figure S4, FT-IR spectra of TEMPO based CMPs
superior to the recent developed TEMPO based porous show characteristic bands at near 2970 cm−1, 2930 cm−1 and
polymerized organocatalyst (PPO-TEMPO).5a In addition, the
optimized conditions using molecular oxygen as clean oxidant
at room temperature and well adaptable scope of substrates
enables the promising application in large scale industrial
process.
1312 cm−1 for each CMPs, which are assigned to the −CH3 and
N−O stretching vibration of TEMPO units.18b,19 The C=O
stretching bands appear at 1725 cm−1 for TEMPO-CMP-3 and
1665 cm−1 for TEMPO-CMP-4, respectively. The FT-IR spectra
clearly demonstrated the linkages between the TEMPO groups
and the carbazole substituted phenyl groups were survived
after the oxidative polymerization. The high polymerization
efficiency of the FeCl3-oxidative coupling could also be validated
by their high thermal stabilities. As displayed in Figure S5,
thermogravimetric analysis (TGA) of the polymer networks
demonstrates that all of the TEMPO-CMPs show good thermal
Four dicarbazole functionalized TEMPO (DCT) monomers (
) were synthesized from carbazole and 1,3-dibromo-5-
methylbenzene. As shown in Scheme S1, the key intermediate
9,9'-(5-methyl-1,3-phenylene)bis(9H-carbazole) was
1-
4
(5)
synthesized by Buchwald-Hartwig coupling of 1,3-dibromo-5-
methylbenzene and carbazole in 76% yield. Further
bromination or oxidation at the benzyl group afforded the
o
stability up to 260 C and the weight content could maintain
bromobenzyl functionalized biscarbazole
acid terminated biscarbazole intermediate
Nucleophilic substitution of with 4-hydroxy-2,2,6,6-
6
or the carboxylic
o
above 50% even at 800 C. PXRD measurement indicated that
7
, respectively.
the four polymer frameworks are amorphous in nature (Figure
S6), similar to previously reported CMP networks.20
6
tetramethylpiperidine-1-oxyl (4-hydroxy-TEMPO) or 4-amino-
2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) gave
We further performed the electron paramagnetic resonance
(EPR) spectroscopy of these four TEMPO-CMPs to validate the
rentention of the TEMPO active sites after the oxidative
polymerization (Figure S3). Pronounced paramagnetic
resonance signals for all the four TEMPO-CMPs exhibit very
similar g values (2.0072, 2.0061, 2.0071, and 2.0070 for TEMPO-
CMP-1~4, respectively), which is very similar to that of
conventional TEMPO monomer (2.0056)21 and corresponding
polymers.5a,19c These results again confirm the successful
construction of these TEMPO-CMPs. The elementary analysis
results of these four TEMPO-CMPs are summarized in Table S1.
The nitrogen cotents are 4.60%, 3.79%, 4.34%, and 7.10% for
TEMPO-CMP-1~4, respectively. Based on these results and the
molecular structure of the repeating units in CMP networks, we
estimated that the concentration of the nitroxyl radicals22 was
about 1.09 mmol/g, 0.68 mmol/g, 1.03 mmol/g, and 1.27
the dicarbazole functionalized TEMPO monomer
ether and amine linkage. On the other hand, acylation of
1
or
2
7
with
with
4-hydroxy-TEMPO or 4-amino-TEMPO led to the other two
dicarbazole functionalized TEMPO monomers and with ester
3
4
or amide linkage. Finally, the four TEMPO based CMP networks
were directly obtained by FeCl3-catalyzed oxidative
polymerization of these four TEMPO monomers 1-4 (Scheme 1).
The detailed reaction conditions were summarized in the ESI.
The resulting polymer networks are insoluble in common
organic solvents due to the highly extended rigid framework
structures.
Owning to the high-spin paramagnetic effect of TEMPO
radicals, the solid-state 1H-13C CP-MAS NMR spectra of TEMPO-
CMPs gave only weak and broad peaks. Thus, the 2,2,6,6-
2 | J. Name., 2012, 00, 1-3
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mmol/g, respectively. The unexpected low TEMPO content in
TEMPO-CMP-2 is probably due to the relatively low
polymerization degree compared with the other three CMPs,
which is consistent with the lowest specific surface area of
TEMPO-CMP-2 as discussed below. The residue Fe contents
were determined by inductively coupled plasma mass
spectrometry (ICP-MS) to eliminate the interference of iron
species on the oxidation catalysis. The iron contents were as low
as 0.23%, 0.26%, 0.15% and 0.15% for TEMPO-CMP-1~4,
respectively, which indicate that the iron species is negligible in
the polymer networks.
View Article Online
DOI: 10.1039/C8TA02664E
Figure 2. SEM images of a) TEMPO-CMP-1, b) TEMPO-CMP-2, c) TEMPO-
CMP-3 and d) TEMPO-CMP-4 (The scale bars are 1 μm).
We further tested the chemical stabilities of these TEMPO-
CMPs in harsh conditions. As shown in Figure S10, the FT-IR
spectra of these TEMPO-CMPs after treatment with strong acid,
caustic base and boiling water keep unchanged with their
pristine samples (Figure S4). Specifically, the characteristic IR
stretching modes around 1312 cm-1 representing the N−O
stretching vibration of TEMPO radicals all remained after the
treatments, which demonstrated that the active TEMPO center
could be survived in these harsh conditions. These results
proved that all these four TEMPO-CMPs showed good stability
in acidic, alkaline and boiling water conditions. Therefore, these
features would facilitate their applications in heterogeneous
catalysis with good tolerance and adaptability.
Figure 1. a) Nitrogen adsorption (solid) and desorption (open) isotherms at
77 K and b) Pore size distribution based on NLDFT calculation for TEMPO-
CMP-1 (olive), TEMPO-CMP-2 (red), TEMPO-CMP-3 (blue) and TEMPO-CMP-4
(magenta).
The porosity of the polymers was assessed by nitrogen sorption
measurements at 77 K (Figure 1a). The adsorption isotherms of the
four CMPs exhibit type-I sorption isotherms with fast uptake of N2 at
low relative pressure (P/P0 < 0.05) and also type-IV sorption features
with adsorption/desorption hysteresis at higher relative pressure.23
Significant desorption hysteresis loops were observed for all four
TEMPO-CMPs, which may be explained by the swelling effect as a
result of gas sorption.23b The Brunauer–Emmett–Teller (BET) surface
areas were calculated to be 495 m2 g−1 for TEMPO-CMP-1, 357 m2 g−1
for TEMPO-CMP-2, 603 m2 g−1 for TEMPO-CMP-3 and 836 m2 g−1 for
TEMPO-CMP-4, respectively. The BET surface areas are somewhat
lower compared with previously reported polycarbazole based
CMPs.17b,c The reason might be attributed to the appended TEMPO
side groups on the polycarbazole scaffolds tend to block the porous
channel to some extent, thus decreasing the surface area.24 Pore size
distribution (PSD) of the four polymers was calculated by the
nonlocal density functional theory (NLDFT) method based on the
adsorption isotherm branches. The calculated pore size distribution
(Figure 1b) is mainly centered at 1.62, 2.33, 1.40 and 1.72 nm for
TEMPO-CMP-1~4, respectively. And TEMPO-CMP-1~3 also exhibit
side mesopores distribution peaks around 4.16-4.80 nm.
The morphologies of these four CMPs particles are visualized
by scanning electron microscopy (SEM) and transmission
electron microscopy (TEM). As shown in Figure 2, the FE-SEM
images showed that TEMPO-CMP-1, TEMPO-CMP-2 and
TEMPO-CMP-4 all adopt sphere shaped nanoparticles with size
ranging from 100 to 200 nm. In contrast, TEMPO-CMP-3
exhibited flower like morphologies (see also Figure S7). The TEM
images are consistent with the FE-SEM results. High-resolution
TEM revealed the presence of nanoporous cavities (Figure S9).
We chose the aerobic oxidation of 2-octanol to evaluate the
catalytic performances of these four TEMPO-CMPs. The
reaction was initially conducted with a catalytic amount of
25
NaNO2 as the dioxygen activator in acetic acid with an O2
balloon at room temperature. As shown in Table 1, all CMPs
could convert 2-octanol into 2-octanone in moderate
conversions with good selectivity within 6 h (entries 1−4).
Among the screened catalysts, TEMPO-CMP-4 exhibited the
highest catalytic activity (entry 4), probably benefits from
higher density of TEMPO active centers and the amide
substituent in the 4-position of TEMPO.26 In addition, it may also
be correlated with the highest midpoint potential (Emp) value of
the nitroxyl in DCT-4 bearing an electron-withdrawing amide
substituent and less susceptive to hydrolysis compared with the
ester group in DCT-3.27a On the other hand, it is reported that
the surface area of TEMPO contained CMPs has little effect on
their catalytic activity.27b Therefore, TEMPO-CMP-4 is
considered to be the best candidate in catalysis regarding these
two aspects. To further improve the oxidation efficiency,
various bromo-containing compounds, such as KBr, Bu4NBr, N-
Bromosuccinimide
(NBS)
and
1,3-Dibromo-5,5-
dimehtylhydantoin (DBDMH), were tested as co-catalyst
(entrie5−8).26a,28 To our delight, DBDMH cooperated efficiently
with TEMPO-CMP-4 and NaNO2, attained a conversion of more
than 95% within the identical time (entry 8). To verify the
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a
Table 1. Condition optimization of aerobic oxidation of 2-octanol.a
Table 2. Aerobic oxidation of alcohols using TEMPO-CMP-4.
DOI: 10.1039/C8TA02664E
Entry
1
R1
R2
H
time (h)
0.5
Conv.b (%)
99
Sele.b (%)
98
C6H5
Entry
1
Cat.
“Br” (5 mol%)
Conv.b (%)
Sele.b (%)
98
2
3
4-CH3O-C6H4
4-CH3-C6H4
4-Cl-C6H4
H
H
1
1
99
99
99
99
99
99
99
99
99
99
99
99
99
99
97
97
98
99
99
99
98
98
97
97
99
99
99
98
TEMPO-CMP-1
TEMPO-CMP-2
TEMPO-CMP-3
TEMPO-CMP-4
TEMPO-CMP-4
TEMPO-CMP-4
TEMPO-CMP-4
TEMPO-CMP-4
TEMPO-CMP-4
TEMPO-CMP-4
DCT-4
-
49
12
37
64
58
49
71
95
9
2
-
97
4
H
1
3
-
99
5
2-Pyridyl
H
2
4
-
99
6
2-Furyl
H
2
5
KBr
98
7
2-Thienyl
H
2
6
Bu4NBr
NBS
99
8
Ph(CH2)2CH2
CH3(CH2)5CH2
CH3(CH2)9CH2
(CH3)2C=CH
C6H5
H
1.5
2
7
99
9
H
8
DBDMH
DBDMH
DBDMH
DBDMH
99
10
11
12
13
14
15c
H
3
9c
10d
11e
99
H
5
99
76
99
Me
Me
1
99
CH3(CH2)4CH2
−CH2(CH2)3CH2−
C6H5
8
aReaction conditions: 0.3 mmol of 2-octanol, TEMPO-CMPs (5 mol% of
nitroxide radicals), 4.2 mg of NaNO2 (20 mol%) in 0.4 mL of CH3COOH with
an O2 balloon at room temperature for 6 h. bDetermined by GC analysis.
cWithout NaNO2. dUnder air atmosphere, at room temperature for 24 h, the
conversion yield was 49%, 64%, 80% and 99% monitored at 6, 9, 12, and 24
h, respectively. eReplace TEMPO-CMP-4 with DCT-4, 8 h.
4
H
3
aReaction conditions: 0.3 mmol of alcohol, 12 mg of TEMPO-CMP-4 (5
mol% of nitroxide radical), 4.2 mg of NaNO2 (20 mol%), 4.3 mg of DBDMH
(5 mol%) in 0.4 mL of CH3COOH with an O2 balloon at room temperature.
bConversion and selectivity were determined by GC. cReaction conditions:
6 mmol of alcohol, 4.8 mg of TEMPO-CMP-4 (0.1 mol% of nitroxide radical),
84 mg of NaNO2 (20 mol%), 172 mg of DBDMH (10 mol%) in 10 mL of
importance of NaNO2 in the catalytic system, the oxidation of 2-
octanol was carried out in the absence of NaNO2. A very low
conversion was observed (entry 9). It is noteworthy that when
pure O2 was replaced with air, the oxidation could also happen
smoothly with high conversion and selectivity when the
reaction time was extended to 24 h (entry 10).
o
CH3COOH with an O2 balloon at 50 C.
The most active amide functionalized TEMPO monomer DCT-4 was
selected. As shown in Table 1 (entry 11) and Table S4, the TEMPO
monomer DCT-4 exhibited equally good catalytic performance for
the oxidation of benzyl alcohols. However, for other less reactive
alcohols such as secondary alcohols and allylic primary alcohol, the
catalytic activity of DCT-4 was dramatically dropped (Table S4) when
comparing with that of TEMPO-CMP-4 (Table 2). Thus, it can be
concluded that the catalytic activity was amplified by grafting TEMPO
on the organic porous polymer scaffold.
Finally, the possibility of catalyst recycling and reuse was
investigated and benzylic alcohol was taken as the model
substrate. After the oxidation was completed, the CMP catalyst
was collected by centrifugation and reused in the subsequent
reaction without further activation. The results illustrated in
Figure 3 show that the activity of the catalyst remained
sufficient after more than 20 subsequent runs. The conversion
efficiency was slightly decreased to 90% after twenty cycles
probably due to the decrease of the porosity
Having established the optimal conditions, the generality of
the TEMPO-CMP catalyst system was explored with structurally
diverse alcohols, and the results are summarized in Table 2.
Substituted benzylic alcohols were selectively oxidized to the
corresponding aldehydes and gave full conversions within 1 h
(entries 1−4). The para-substituents on the phenyl ring have no
influence on the reaction outcome. The heteroatom-containing
aromatic methanol29 were also successfully converted into their
corresponding aldehydes with very high selectivity (entries 5−7).
By using our protocol, oxidation of aliphatic primary alcohols to
the corresponding aldehydes could be completed within 3 h
(entries 8−10). We were pleased to find that allylic primary
alcohol was also well-tolerated in our catalytic system, affording
the corresponding aldehyde without disturbing the carbon-
carbon double bond (entry 11). In addition, aerobic oxidation of
secondary alcohols went smoothly under identical conditions,
resulting in full conversions into the corresponding ketones with high
selectivity (entries 12−14).30 To our surprise, when much less catalyst
(only 0.1 mol%) was used for the oxidation, the reaction could also
be completed within 3 hours. The turnover frequency (TOF) can
reach up to 333 h-1, which is higher than most of the TEMPO-based
catalyst under similar reaction conditions (Table S3). We further
compare the catalytic activity of the TEMPO-CMP with its monomer.
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Notes and references
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DOI: 10.1039/C8TA02664E
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In summary, four TEMPO-based polycarbazole CMPs with
different linkages between TEMPO and carbazoles were
successfully constructed by one step highly efficient and cost-
effective oxidative polymerization. All these TEMPO-CMPs
exhibited excellent stability and good catalytic performance on
aerobic selective oxidation of alcohols. Among that, TEMPO-
CMP-4 (with electron-withdrawing amides linkages) showed
persistent high activity of catalytic oxidation of alcohols in acetic
acid with NaNO2 and DBDMH as co-catalyst and auxiliary,
respectively. Such environmental benign condition meets the
requirements of green chemistry and is suitable for various
alcohols. This study provides a facile approach of directly
introducing TEMPO catalytic centers to the conjugated porous
polycarbazole networks. Further investigation on utilizing both
the photoactive conjugated polymer skeleton31 and the radical
based catalytic active centers for cascade catalysis are currently
in progress in our laboratory.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science Foundation
of China (51522303, 21602154) and the National Key Research and
Development Program of China (2017YFA0207500).
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Journal of Materials Chemistry A
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Page 7 of 7
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C8TA02664E
Graphic Abstract
Porous radical polymers were constructed via cost-effective FeCl3-catalyzed oxidative
polymerization of dicarbazole functionalized TEMPO. These TEMPO-based porous
polymers exhibit high catalytic activity and excellent recycling performance for the
aerobic oxidation of a wide variety of alcohols.