Functional Porous Organic Polymers Comprising a Triaminotriphenylazobenzene Subunit as a Platform for Copper-Catalyzed Aerobic C-H Oxidation

Article abstract of DOI:10.1021/acs.chemmater.9b00590

Developing functional porous materials as a platform for the heterogeneous catalytic oxidation reaction provides a good way to solve the high environmental issues resulted by traditional oxidation processes in the industry. This article reported a design and facile synthesis of N-rich functional porous organic polymers with mesopores such as Azo-POP-4, Azo-POP-5, and Azo-POP-6 based on the triaminotriphenylazobenzene subunit for the first time. The nitrogen-rich POPs with the triaminotriphenylazobenzene subunit was found to remove 85% of copper ions from water within 30 min. The as-synthesized Cu@Azo-POP-4 demonstrated high catalytic reactivity and selectivity in aerobic C-H bond oxidation to afford the desired ketones in high yield. In addition, the catalyst could be reused easily five times without decreasing the reactivity, which will help to design catalysts reducing environmental pollution and advance the chemical technology.

Full text of DOI:10.1021/acs.chemmater.9b00590

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Article
Functional Porous Organic Polymers Comprising
Triaminotriphenylazobenzene Subunit as a Platform
for Copper Catalyzed Aerobic C-H Oxidation
Xiangxiang Liu, Xian-Sheng Luo, Han-Lin Deng, Wenhao Fan, Shuifeng
Wang, Chaojie Yang, Xiao-Yu Sun, Shi-Lu Chen, and Mu-Hua Huang
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00590 • Publication Date (Web): 17 Jul 2019
Downloaded from pubs.acs.org on July 18, 2019
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Chemistry of Materials
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Functional
Porous
Organic
Polymers
Comprising
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Triaminotriphenylazobenzene Subunit as a Platform for Copper
Catalyzed Aerobic C-H Oxidation
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Xiangxiang Liu†‡⊥, Xian-Sheng Luo†⊥, Han-Lin Deng†⊥, Wenhao Fan#, Shuifeng Wang§,
Chaojie Yang#, Xiao-Yu Sun†, Shi-Lu Chen‡ and Mu-Hua Huang†*
†School of Materials Science and Engineering, Experimental Center of Advanced Materials, Beijing Institute of
Technology, Beijing, 100081, China. Email: mhhuang@bit.edu.cn (M.-H. Huang)
‡School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, 100081, China
§Analytical and Testing Center, Beijing Normal University, Beijing 100875, China
#Beijing Center for Physical & Chemical Analysis, Beijing, 100089, China
ABSTRACT: To develop functional porous materials as a platform for heterogeneous catalytic oxidation reaction
provides good way to solve the high environmental issues resulted by traditional oxidation processes in industry. This
article reported a design and facile synthesis of N-rich functional porous organic polymers with mesopores such as Azo-
POP-4, Azo-POP-5 and Azo-POP-6 based on triaminotriphenylazobenzene subunit for the first time. The nitrogen-rich
POPs with triaminotriphenylazobenzene subunit was found to remove 85% of copper ions from water within 30 mins.
The as-synthesized Cu@Azo-POP-4 demonstrated high catalytic reactivity and selectivity in aerobic C-H bond oxidation
to afford the desired ketones in high yield. In addition, the catalyst could be reused easily for 5 times without decreasing
the reactivity, which will help design catalysts reducing environmental pollution and advance the chemical technology.
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The polyamines were usually synthesized by reduction of
1. INTRODUCTION
polynitro compounds, which were made from nitration of
aromatics. The conventional post-functionalization strategy
was unsuitable for Functional POPs comprising
triaminotriphenylazobenzene subunit.
In this regard, a facile synthesis of polyamine-functionalized
POPs (Azo-POP-4, Azo-POP-5 and Azo-POP-6, Figure 1)
was designed and carried out based on a triple azo-coupling
polymerization. In addition, their applications in fast
removal of Cu2+ from water, as well as Cu@Azo-POP-4 for
the efficient aerobic C-H bond oxidation will be discussed in
detail.
Chemical industry created lots of products beneficial to
human being, although it also produced pollution to the
society, which need seek the solution from advanced
functional materials. Oxidation reaction is one of the most
important chemical processes in industry and plays a central
role in the manufacture of high-value-added products1. Many
traditional oxidants based on Mn or Cr2 typically required
stoichiometric quantities and generated toxic and hazardous
waste chemicals. In order to achieve high atomic economic
efficiency3 and low environmental factor4 for the oxidation
reactions, heterogeneous catalysis for aerobic C-H bond
oxidation with high selectivity has been considered as an
important strategy5. Plenty of nanomaterials including Pd,
Pt, Cu, Co, Ru, and Rh have been designed to catalyze
organic oxidation reactions6. Functional carbon materials
including N-doped graphene7 and N-doped nanotube8 has
been considered as good supports to immobilize metal for
heterogeneous catalysis. As a new generation of functional
porous materials, porous organic polymers (POPs) showed
great potential for immobilization of metals9, due to their
structural designability, rich functionality and good chemical
stability. Actually, functional POPs could merge multiple
functions into one catalyst10. They incorporated coordination
sites into the polymer backbone, provided location of the
high specific surface area to promote the catalytic reactions,
and grasped metal species tightly to avoid the metal loss and
product contamination. Cu has attracted much attention in
the heterogeneous catalysis11, due to its low cost and easy
availability. And polymeric ionic liquid complex (PILC) with
mesopores containing nitrogen-rich pendant imidazolium
salt had been developed as a platform for Cu catalyst in C-H
bond oxidation reaction by Yuan group12. The ortho-
hydroxyl-aminobenzene moiety in HAzo-POP-1 with
mesopores was reported by Liu group13 to capture Cu2+
efficiently. The above mentioned examples as well as N-
doped carbons14 showed clearly that the mesopores did
contribute to the excellent performance. Considering amino
group was more Cu2+–philic than hydroxyl group, we
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2. EXPERIMENTAL SECTION
2.1. Chemicals and Materials
1,3,5-triaminobenzene was prepared by modifying a known
procedure19 (see ESI). Aniline, p-methyl aniline, p-methoxy
aniline, 4,4'-diaminobiphenyl, 4,4'-diaminobiphenylmethane,
4,4'-diaminobiphenylether, concentrated HCl, sodium nitrite,
sodium carbonate, copper acetate, methanol, ethanol, DMF,
de-ionized water were purchased from Sinopharm chemical
reagent Beijing Co. Ltd. Diphenylmethane, fluorene,
xanthene,
phthalan,
benzyl
methyl
ether,
difluorodiphenylmethane, deteurated solvents such as D2O,
CD3CN and DMSO-D6 were ordered from Energy chemical
company in China. The chemicals were used as received.
2.2. Instrumentation
Solution NMR spectra were measured in D2O (with TMS as
internal standard) CD3CN or DMSO-D6 on a Varian INOVA-
400M (1H at 400 MHz, 13C at 100 MHz) magnetic resonance
spectrometer. Solid-state 13C cross-polarization magic angle
spinning (CP-MAS) NMR spectra of solid samples were
obtained using a Bruker Avance III 600 MHz Wide Bore
spectrometer (14.2 T). A 3.2 mm MAS probe and ZrO2 motor
were used, and spin rates at 12 kHz. Chemical shifts (δ) are
reported in ppm, and coupling constants (J) are in Hz. The
following abbreviations were used to explain the
multiplicities: s = singlet, d = doublet, t = triplet, m =
multiplet. Fourier Transform Infrared (FT-IR) spectra were
recorded on a PerkinElmer Spectrum Two Fourier transform
spectrometer from 500-4000 cm−1. High-resolution EI mass
designed
the
porous
Azo-POP-4
containing
triaminotriphenylazobenzene subunit, by replacing all
hydroxyl groups in HAzo-POP-1 with amino groups.
Furthermore, polyamine-functionalized porous materials
have been interesting targets for application-oriented
spectra (HR-EI-MS) were recorded on
a GCT CA127
Micronass UK mass spectrometer. Elemental analysis was
measured in the analytical instrumentation center of Beijing
University, Beijing, China, by using Elementar Vario EL
CUBE. Thermogravimetric analysis (TGA) was carried out on
a Q600 SDT (TA, US) thermogravimetric analyzer, heated
from 25 °C to 800 °C at a rate of 20 °C/minute under N2
atmosphere. The specific surface areas of as-prepared Azo-
POPs were screened using a BEL-SORP MAX II analyzer by
nitrogen adsorption and desorption at 77 K, BET surface area
were analyzed by at-least 5-point measurement between the
pressure range of 0.05-0.35 P/P0. The Barrett-Joyner-Halenda
(BJH) method was used to determine the distribution of pore
sizes. Samples were degassed at 100℃ for over 12 h under
vacuum before all gas analysis. The Scanning Electron
Microscopy (SEM) images were taken by a Hitachi SU-70
Scanning Electron Microscope. A Tecnai G2 F30 transmission
synthesis15-18
.
X
N
N
Cu
HN
N
N
NH2
H2N
NH2
N
N
N
N
N
N
N
N
NH2
NH2
X
X
Azo-POP-4, X = none
Azo-POP-5, X = CH2
Azo-POP-6, X = O
Cu@Azo-POP-4
Figure 1 The chemical structure of Azo-POP-4, Azo-POP-5,
Azo-POP-6 and Cu@Azo-POP-4
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electron microscope (FEI) equipped with an energy-
product and the corresponding substrate.
For the
2
dispersive X-ray spectrometer system and HAADF detector
was used at 300 kV for high-resolution electron microscopy
imaging and HAADF imaging. Inductively coupled plasma-
atomic emission spectroscopy (ICP-AES) was performed on a
SPECTRO ARCOS EOP Axial View Inductively Coupled
Plasma Spectrometer after acidic digestion or dilution.
Quantitative and qualitative analyses of oxidized products
were performed by GC-MS QP2010 ultra (Shimadzu, Kyoto,
Japan) equipped with DB-5MS capillary column (Agilent,
J&W Scientific, 30 m × 0.25 mm i.d.; 0.25 μm film thickness)
using helium as the carrier gas (1 mL/min) and the split ratio
of 10:1. The conditions were as follows: column initial
experiments using recycled catalyst, Cu@Azo-POP-4 was
filtered, washed with ethanol for 3 times, dried in a vacuum
oven, and used directly in the next run’s C-H oxidation
reaction.
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3. RESULTS AND DISCUSSIONS
Functionalization of POPs has been an important research
topic since it is crucial to their performances. In fact, the
introduction of sulfonate20, hydroxyl group21, amino group22,
nitro group23, tetrazole24 and amidoxime25 into the polymer
backbone were all reported to enhance their performances of
POPs such as adsorption capacity and selectivity, by the
conventional post-functionalization strategy (Scheme S1).
However, it became challenging in the cases of densely-
functionalized POPs with well-defined structure, owing to
the reaction efficiency issue of polymer. As an alternative,
the pre-functionalization26, and in-situ-functionalization27-30
strategy possessed some advantages, including making use of
functional group31 as directing group26. In this paper, we
would like show the powerful pre- and in-situ-
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temperature (40 C, 2 min); temperature rate (10 ℃ /min);
column final temperature (320 ℃ ,
5 min); injector
temperature (250 ℃). Electron paramagnetic resonance (EPR)
spectra were recorded at ambient (~293 K) temperature with
a Bruker EMX-plus spectrometer working in the X-band:
microwave power was 5 microwatts; the field was swept 800
G in 150 s and modulated at 100 kHz with 5 G amplitude. X-
ray photoelectron spectroscopy (XPS) was performed on a
PHI Quantera II spectrometer at a pressure of 1.9×10-9 mtorr
(1 torr = 133.329 Pa) using Al Ka as the excitation source (hn =
1486.6 eV) and operated at 15 kV and 25 W.
functionalization
triaminotriphenylazobenzene-functionalized POPs, which
could serve as a support for heterogeneous catalysis.
strategy
to
make
2.3. Synthesis of Azo-POP-4 and Cu@Azo-POP-4
3.1. Design principles for synthesis of Azo-POP-4, Azo-
POP-5 and Azo-POP-6
A suspension of 4,4'-diaminobiphenyl (3.317 g, 18 mmol) in
water (50 ml) was treated with con. HCl (8 ml) at 0 oC. After
stirring for 15 min, aqueous sodium nitrite (36.1 mmol) was
added and stirred for 30 min to afford the dual-diazonium
reagent 2. Subsequently, the mixture was neutralized with
aqueous Na2CO3 solution, and then mixed with aqueous
Although metal catalyzed C-H oxidation has been studied
broadly, the high selectivity and high conversion remained as
an important issue32, 33. As far as heterogeneous Cu catalyzed
C-H bond oxidation is concerned, POPs with high N%
content and well-defined chelating sites will help, basically12.
Knowing HAzo-POP-1 was good for Cu2+ capture, we wonder
if it will further improve the performance the copper catalyst
by replacing the three hydroxyl groups in HAzo-POP-1 with
three amino groups. Thus, polyamine-functionalized POPs
such as Azo-POP-4, Azo-POP-5 and Azo-POP-6 came into
our mind (Figure 1). Amino group was usually protected
owing to its poor tolerance with polymerization conditions34,
though the protecting group-free strategy will be ideal in
materials development in the atomic economy’ point of
view3. Interestingly, the specific situation of three amino
groups of 1,3,5-triaminobenzene (1) made it very attractive
for the direct C-modification35 possible at 2,4,6-positions,
including C-azocoupling, C-thiocyanation. In addition, the
well-developed polyimide industry offered aryldiamines
widely36, and the aryldiamine-derived dual-diazonium
reagent will be easily available37. We would like to develop
nitrogen-rich functional POPs based on the azo-coupling
polymerization between 1,3,5-triaminobenzene (1) and
aryldiamine-derived dual-diazonium reagent 6-8 (Scheme 1).
o
solution of 1,3,5-triaminobenzene (1.478 g, 12 mmol) at 0 C.
After 12 h, the resulted solid was filtered, washed successively
with water, methanol, DMF, methanol, and water, then
freeze-dried to give the title porous polymer Azo-POP-4
(4.57 g, yield 88%).
FT-IR (cm-1): 3429 (w), 3377 (w), 3028 (w),1571 (s), 1522 (m),
1481 (m), 1403(w), 1342(vs), 1301 (m) ,1201 (m), 1115 (m), 1001
(m), 963 (w), 823 (s), 779 (w), 763 (w), 721 (w), 696 (w), 506
(m), 479 (m).
13C CP/MS NMR, δ (ppm): 151.93, 140.35, 127.46, 115.67.
Elemental Analysis: found N24.21%, C67.44%, H4.56%.
The preparation of Azo-POP-4 loaded with copper (II) salt
(Cu@Azo-POP-4) was carried out in ethanol. Azo-POP-4 (1
g) was loaded into flask charged with 100 ml of absolute
ethanol and Cu(OAc)2 (230 mg; 8 wt.%). After refluxing for
12 h, the Cu@Azo-POP-4 was obtained by filtration, washed
using ethanol three times and dried in vacuum till constant
weight.
2.4. Cu@Azo-POP-4 catalyzed aerobic CH oxidation
3.2. Synthesis and characterization of Azo-POP-4, Azo-
POP-5 and Azo-POP-6
A typical C-H oxidation reaction was carried out with
substrate (2.5 mmol) in CD3CN (5 ml) in a 25 ml of round
bottom flask using O2 balloon. The reaction was performed
Azo-POP-4, Azo-POP-5 and Azo-POP-6 with the cross-
linking polymer structure were supposed to have very poor
solubility, which made the characterization difficult. And the
model compounds with the same subunit would be very
helpful for the structure elucidation of Azo-POPs by simple
comparison of their spectra.
o
at 80 C under magnetic stirring for special period of time.
Then, the catalyst of Cu@Azo-POP-4 was filtered, the filtrate
was analyzed by 1H-NMR directly. The conversion was
calculated based on the integration of the peak of desired
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R
N
1
2
3
4
5
6
7
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(Ph-N=N-) group, as well as peaks at 129.3, 128.1 and 121.5
N2
R
ppm for other carbons of phenyl (Figure S14). From the FT-
IR spectra (Figure S10-11), the characteristic adsorption
peaks were seen at 3331 and 3400 cm-1 for the primary amino
group in aniline, which shifted to higher wave numbers of
3383 and 3427 cm-1 in 5a (Figure S11). This could be the
evidence for the formation of intramolecular hydrogen
bonding between H and nitrogen of its neighboring –N=N-
bond, being consistent with the 1H-NMR data. Compound 5b
and 5c were synthesized and well characterized (Figure S12-
19) by using diazonium salt 3 and 4. In order to get further
understanding on the high reactivity of the triply azo-
coupling reaction,, we looked at the electrostatic potential
NH2
2, R=H
N
5a, R=H, 97% yield
3, R=CH3
4, R=OCH3
NH2
H2
N
5b, R=CH3, 91% yield
5c, R=OCH3, 64% yield
N
N
H2
N
NH2
N
N
NH2
R
1
H2O
R
NH2
N2
NH2
N
NH2
N
N
N
N
H2
N
NH2
NaNO2, HCl
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NH2
1
N
N
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H2
O
H2
O
96%
N2
NH2
Azo-POP-4
Benzidine
6
(ESP) mapping of 1,3,5-triaminobenzene
1 and model
compound 5a (Figure S61). It showed much more electro-rich
aromatic ring in 1 compared 5a, which favored the aromatic
electronic substitution. With the clear idea of how the FT-IR
and 13C-NMR spectra looked like for the core structure of 5,
we moved to the synthesis of the Azo-POP-4 immediately.
N
N
NH2
N
NH2
N
N2
NH2
N
H2
N
NH2
NaNO2, HCl
N
H2
N
NH2
1
H2
NH2
O
H2O
Azo-POP-5
N2
86%
3.2.2. Optimization on the Synthesis of Azo-POP-4
7
4,4-Diaminodiphenyl methane
O
By taking BET surface area, isolated yield and N% content as
key parameters, we carried out a careful optimization of the
azo-coupling polymerization (Table S1-3 and Figure S27-28).
Considering the pH value, the molar ratio of two
components and addition ways might affect the property of
the resulted porous polymer, we firstly tried the coupling
NH2
N2
NH2
NH2
N
N
N
N
O
N
H2
N
NaNO2, HCl
H2
N
NH2
NH2
1
N
O
O
H2
NH2
O
H2
O
O
N2
83%
Azo-POP-6
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4,4'-oxydianiline
polymerization at pH of 7.0 using
1 mmol of 1,3,5-
Scheme 1 The synthesis of model compounds 5a, 5b and 5c
and Azo-POP-4, Azo-POP-5 and Azo-POP-6
triaminobenzene, by varying the quantities of dual-
diazonium reagent 6a derived from 4,4'-benzidine. In order
to convert 1,3,5-triaminobenzene fully, the excess of 6 (6
mmol) was used initially. However, the polymer with BET SA
of 57 m2.g-1 was obtained (Entry 1, Table S1). With the
decrease of 6 from 6 mmol to 2 mmol, the BET SA increased,
as well as the yield (Entry 2-3, Table S1). However, when 1
mmol of 6 was used, the yield dropped to 72%, although the
BET surface area remained high (Entry 4, Table S1). The try
with the ideal ratio of 6 to 1 of 1.5:1 gave the polymer in 96%
yield, showing BET surface area as high as 368 m2.g-1, and the
N% content as high as 24.21% (Entry 5, Table S1). By adding
6 into the solution of 1 also afforded the polymer in low yield
(Entry 6, Table S1). It is also found that the polymerization
at pH 6 gave low yield and N% content (Entry 7, Table S1).
Using the condition listed in entry 5 as best condition, 4.57
grams of Azo-POP-4 were easily synthesized in one batch
(Figure S29).
Having developed a facile synthesis of 1,3,5-triaminobenzene
(1) recently38 (See ESI), we synthesized the model
compounds 5a-5c by coupling 1 with diazonium salt 2-4 in
64-97% yield (Scheme 1 and Figure S7-24).
3.2.1. Synthesis and careful characterization of model
compounds 5a-5c
The structure of compound 5a was well characterized, by
showing a peak of 436.1996 for [M+H]+ (Figure S9) in the
high-resolution Mass Spectrum (HR-MS) and 28.47 N %,
65.57 C % and 4.80 H% in the elemental analysis (EA)
(Figure S10). Furthermore, compound 1 showed one sharp
singlet peak at 5.15 ppm for the aromatic CH, and broad
1
singlet peak at 4.31 ppm for NH2 in its H-NMR spectra
(Figure S4 and S11). The triply coupling of 1 with diazonium
salt 2 afforded the triaminotriphenylazobenzene (5a, abbr.
TATPAB). It showed a broad peak at 9.62 ppm for NH2 in its
1H-NMR spectra, which was in much lower field compared
with 1, possibly owing to the intramolecular hydrogen
bonding between the NH2 and neighboring -N=N- bond. The
phenyl ring attached to azo group showed doublet, triplet
and triplet peaks at 7.88, 7.50, 7.34 ppm respectively, which
moved to high field compared with diazonium salt derived
from aniline (8.66, 8.26, 7.98 ppm) (Figure S7 and S11). The
peaks at 149.3 and 90.9 ppm for C-NH2 and C-H of 1 were
observed in its 13C-NMR spectra (Figure S13), while the peak
of 149.1 and 114.2 ppm was found for C-NH2 and C-N=N- in 5a
(Figure S13-14), after 1 triply coupled with diazonium salt.
The peak at 153.5 ppm was observed for C-N=N- in phenylazo
3.2.3 Characterization and basic properties of Azo-POP-4,
Azo-POP-5 and Azo-POP-6
Similar to the synthesis of Azo-POP-4, Azo-POP-5 and Azo-
POP-6 were obtained in 86% and 83% yield respectively, by
using the dual-diazonium reagent 7 and 8 derived from 4,4'-
diaminodiphenylmethylene and 4,4'-diaminodiphenylether
(Scheme 1). Azo-POP-5 and Azo-POP-6 showed BET SA of
196 and 210 m2.g-1, as well as 22.54 N% and 22.35N% (Table
S3) in elemental analysis. The chemical structure of Azo-
POP-4 and Azo-POP-5 were characterized based on
comparisons between their CP-MS-13C-NMR and the solution
13C-NMR spectra of model compound 5b (Figure 2a). The
common chemical shifts of c.a.150 and 115 ppm the CPh-N=N-
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and CTATPAB-N=N- were observed in their NMR, as well as
2
c.a.140 ppm for CTATPAB-NH2. The chemical shifts for CTATPAB
-
NH2 varied a little bit with compound 5c and Azo-POP-6 due
to the presence of ether linkage (Figure 2a and S20-21).
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Figure 2 (a) 13C-NMR of model compound 5b in DMF-D7 and
13C-CP/MAS NMR of Azo-POPs. (b) FT-IR spectra of model
compound 5b and Azo-POPs.
The primary NH2 groups were observed in all the Azo-POPs
based on the peaks in the region of 3500-3300 cm-1 in their
FT-IR spectra (Figure 2b), although sharper peaks were seen
in model compound 5b. Otherwise, they had similar FT-IR
spectra in the region of 1300-1700 cm-1 owing to the common
subunit structure. The strong peak at 1232 cm-1 for ether bond
was shown for Azo-POP-6.
The adsorption-desorption isotherms of aforementioned
Azo-POPs were recorded at 77 K under N2 (Figure 3a), which
showed a composite of the type I and type II adsorption
isotherms, indicative of mixed micropores and mesopores in
the polymers. They had high pore volume of 1.8 cm3.g-1
mainly from mesopores, only <2.4% pore volume contributed
from micropores (Table S4). BJH model showed the average
pore width (4V/A) of 25.7 nm, 38.2 nm and 35.0 nm for Azo-
POP-4, Azo-POP-5 and Azo-POP-6 respectively (Figure 3b).
Figure 4 Typical images for Azo-POP-4 (a-c), Azo-POP-5 (d-
f) and Azo-POP-6 (g-i): SEM image(a, d, g), TEM image(b, e,
h), and HR-TEM image(c, f, i).
The typical SEM images of Azo-POP-4 (Figure 4a) showed
that the as-prepared Azo-POPs were composed of irregularly
tiny particles (<50 nm). And it had massive irregular holes
among the particles (Figure 4b). The enlarged TEM images
revealed the amorphous structure of polymers cross-linked
with each other to form mesopores with different pore
shapes and sizes. The similar situation was seen in Azo-POP-
5 (Figure 4d-4f) and Azo-POP-6 (Figure 4g-4i). The stacked
particle sizes of Azo-POP-5 and Azo-POP-6 were a little
larger than Azo-POP-4, and the percent of micro-pore
decreased, so did the specific surface area and the pore
volume. But the pore sizes of them increased, which were
consistent with the porosity analysis in Table S3 and Figure
3. However, the amorphous structure of Azo-POP-4, Azo-
POP-5 and Azo-POP-6 was confirmed by the broad peaks in
the Powder-XRD measurement (Figure S30).
Figure 3 (a) Adsorption (filled symbols) and desorption
(empty symbols) isotherms recorded at 77 K under N2 and (b)
pore-size-distribution of Azo-POP-4, Azo-POP-5 and Azo-
POP-6 based on BJH model.
The high thermal stability of Azo-POP-4, Azo-POP-5 and
Azo-POP-6 was investigated by TGA (Figure S31), which
showed decomposition (5% weight loss) started at 284, 289
and 309 oC respectively, and maximum decomposition
temperature occurred at c. a. 350 oC to give the char in
higher than 40% yield. Compared with the very popular
imine-based COFs, the azo-linked POPs possessed higher
chemical stability towards acid and aqueous media, by
replacing the CH=N bond with N=N bond. It was found that
all the three Azo-POPs were stable upon treating with 1%
aqueous HCl for 24 h (Figure S32), and no mass decrease of
the materials was observed. This is really important when the
It is worth mentioning that the preparation of mesoporous
organic polymer usually required template such as silica and
surfactants40. To our delight, the porous Azo-POPs with
mesopores containing densely-functionality were obtained
without any template. The mesoporosity and morphology of
Azo-POPs were also investigated by SEM and HR-TEM
(Figure 4).
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reported adsorbent materials. (d) Removal efficiency of Cu2+ over
adsorption-elution-regeneration cycles.
real application of the porous materials is concerned,
especially in the cases requiring exposure to water or
moisture.
The adsorption isotherms of Azo-POP-4 toward Cu2+ (Figure
S35, Table S6) showed the adsorption fitted Langmuir model
well, and the adsorption capacity increased along with the
temperature increased, indicating that the adsorption
process was an endothermic adsorption reaction. To examine
3.3. Fast removal of Cu2+ from water by Azo-POP-4
It is well known that Cu2+ in drinking water has been
considered a very important health-related issue for human
the
reusability
of
Azo-POP-4,
adsorption-elution-
body. Thus the fast removal of Cu2+ from water with high
9
regeneration cycles were evaluated and the data are shown in
Figure 5d. The EDTA-2Na solution (0.1 M) followed by water
was used for successful elution and simultaneous
regeneration. It was found that the removal efficiency of Cu2+
by Azo-POP-4 remained 95% after 5 cycles. This made the
Azo-POPs very promising for the purification of drinking
water in the future.
42
efficiency became a hot research topic in recent years41,
,
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where high-performance porous materials were highly
desirable. Realizing the densely-functionalized POPs with
triaminotriphenylazobenzene subunit structure could be
helpful to chelate with Cu2+, we evaluated the removal of
Cu2+ from water using Azo-POP-4. The removal efficiency of
Cu2+ was found to be influenced by the pH value of the
medium (Figure S32), and pH 5.0 was set for the
experiments. A removal efficiency of 61% for Cu2+ was
observed within only 3 min (initial Cu2+ concentration = 10
ppm) in the removal kinetics experiments (Figure 5a),
comparable with highly efficient adsorbents reported very
recently such as N-doped graphene nanosheets43 and
mesoporous silica44. The curve fitted very well with pseudo-
second-order (PSO) model (Figure 5b, Table S5), indicating
the rate-limiting step of the chemo-sorption process owing
to the chelating of Cu2+ with amino-azo group. It was
confirmed by comparison between FT-IR spectra of Azo-
POP-4 before and after adsorption (Figure S34). An obvious
shift from 1574 cm-1 to 1596 cm-1 was seen for aromatic ring
vibration, as well as from 1196 cm-1 to 1265 cm-1 for C-N bond
after Cu2+ was loaded onto Azo-POP-4. Besides, the removal
efficiency reached 85% within the first 30 min, which is
much faster than some reported adsorbent materials,
including biocomposite hydrogel (TP-PS4 Hydrogel, 60
min)45, sugar cane bagasse cellulose and gelatin based
composite hydrogels (CPGTCB Hydrogel, 80 min)46, drinking
3.4. Aerobic C-H bond oxidation by Cu@Azo-POP-4
Realizing the fast removal of Cu2+ could be due to chelation
between Cu2+ and the ortho-amino-azo moiety, we tried
loading more of Cu2+ onto the backbone of the Azo-POPs.
The resulted catalyst could serve as efficient solid catalyst for
aerobic C-H bond oxidation.
3.4.1. Synthesis and characterization of Cu@Azo-POP-4
The solid catalyst Cu@Azo-POP-4 was prepared by refluxing
the as-synthesized Azo-POP-4 with Cu(OAc)2 in ethanol for
12 h, with slightly decrease of the surface area and pore
volume (Figure S36). The loading of Cu2+ was measured to
be as high as 61.7 mg/g by ICP-AES. The FT-IR spectra of
catalyst Cu@Azo-POP-4 was also showed in Figure S34. It
appeared the similar peak shifts for aromatic ring vibration
and C-N to the FT-IR spectra of the Cu2+ adsorption from
water by Azo-POP-4, both of which illustrated the chelation
between Cu and the ortho-amino-azo moiety on the
backbone of the Azo-POP-4. The morphology of Cu@Azo-
POP-4 was observed by TEM image (Figure 6a), the porous
structure remained unchanged.
water
treatment
solids
(DWTS,
300
min),
geopolymer/alginate hybrid spheres (GAS-4, 1600 min)47 and
magnetic beads based on modified gum tragacanth/graphene
oxide (Magnetic Beads, 5400 min)48 (Figure 5c).
Figure 6 The morphology of the catalyst Cu@Azo-POP-4. TEM
images of (a) the catalyst and (c) the catalyst after reuse for 5 times.
Dispersion of different elements in the catalyst of Cu@Azo-POP-4 (b)
and (d) after 5 times’ recycling.
XPS measurements were performed to investigate the
coordination site and oxidation state of N and Cu species
(Figure S52-53). The majority of N in the Azo-POP-4 were
stable azo nitrogen, the binding energy (BE) at 397.9 eV, and
Figure 5 Removal of Cu2+ by Azo-POP-4. (a) Effect of contact time
on the removal efficiency of Cu2+ from water. (b) Adsorption kinetics
fitted by PSO kinetics model. (c) Comparisons of Cu2+ removal time
required with 85% removal efficiency among Azo-POP-4 and
the other peak with the BE of 400.6 eV for N 1s were
49, 50
associated with aryl amino group
. The BE of N 1s in
Cu@Azo-POP-4 catalyst exhibited a slightly higher value of
398.8 eV (azo nitrogen) and 401.2 eV (aryl amino) than that
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in Azo-POP-4, while the BE of P 2p showed no difference.
combination of Cu@Azo-POP-4 and NHPI did play as an
efficient catalyst system.
2
3
4
5
This implied that Cu were coordinated at the azo and aryl
amino sites in the polymer. Cu 2p3/2 of the catalyst exhibited
a sharp peak at 934.2 ± 0.1 eV and a strong satellite peak on
the higher BE site, which indicated the presence of divalent
Table 1 NHPI/Cu@Azo-POP-4 catalyzed aerobic C-H
oxidation of organic compounds
51
Cu species . Additionally, high-angle annular dark-field
6
scanning transmission electron microscopy (HAADF-STEM)
was used to investigate the dispersion of carbon, nitrogen
and copper elements in catalyst Cu@Azo-POP-4 (Figure 6b).
It showed that all the three elements dispersed highly
uniformly within the Azo-POP-4, which corresponded to the
specific attachment sites for precisely controlled Cu2+
distribution within the frameworks. Such uniform dispersion
mode could lead to a high catalytic activity of the Cu@Azo-
POP-4.
Cu2+@Azo-POP-4
O
7
8
9
H
H
O2 (1 atm)
R1
R2
R1
R2
NHPI (10 mol%)
Acetonitrile
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Entry Substrate
1
Product
Yieldd Selectivityf
O
96%
99%
2
91%
42%
22%
0%
94
--
O
3a
4b
5c
3.4.2. Cu@Azo-POP-4 catalyzed C-H bond oxidation reaction
--
Metal-catalyzed C-H oxidation reaction has been developed
as one of the most important organic transformations in
chemical industry, owing to the serious pollution by
stoichometirc oxidants such as permanganate, pyridinium
--
O
6
97%
97%
99%
99%
O
52
chlorochromate and Dess-Martin periodinanes . Various
7
O
oxidants
such
as
hydrogen
peroxide,
tert-butyl
O
O
hydroperoxide (TBHP), oxygen and ozone have been used for
the oxidation of C-H bond. Among them, oxygen was
obviously one of the most environmentally friendly and
cheap oxidants. And N-hydroxyphthalimide (NHPI) has been
proved as efficient co-catalyst53 owing to the formation of
phthalimide-N-oxyl reactive radical species (PINO)54, which
helped abstract a hydrogen atom in the C-H oxidation
reaction55. The metal used to generate PINO included Pb56,
Co57, 58, Cu59 and so on. Catalytic tests in our work were
performed for the oxidation of various hydrocarbons with a
balloon of oxygen, using Cu@Azo-POP-4 (3 mol% based on
Cu) as catalyst and N-hydroxyphthalimide (NHPI) (10 mol%)
as co-catalyst. The reaction temperature and time were
optimized to be 80 ℃ and 12 h based on C-H oxidation on
diphenylmethane (Table S7), by monitoring the reaction
O
8
94%
95%
99%
99%
O
O
9
O
O
O
F
F
10
84%
98%
F
F
Reaction conditions: substrate (2.5 mmol), NHPI (10 mol%),
Cu@Azo-POP-4 (3 mol% based on Cu), 4mL CH3CN, O2 in
balloon, 80 ℃ for 12 h. aWithout Cu@Azo-POP-4. bWithout
NHPI. cReacted in the presence of Azo-POP-4 without
Cu@Azo-POP-4 and NHPI. dIsolated yield after column
f
chromatography. The ratio of desired product to other
oxidized side products based on GC-MS.
1
with TLC (Figure S65) and crude H-NMR spectra (Figure
S49-50). The yield was calculated to be 96% based on the
products isolated by flash column chromatography
purification. The selectivity of the C-H oxidation were
determined to be 99% by the ratio of desired ketone product
to the corresponding secondary alcohol and other related by-
products based on GC-MS (Figure S59). Similar situation
was found for indane, when indane in CD3CN was subjected
to the aforementioned oxidation condition for 12 h. It gave
the desired indanone in a yield of 91% after column
chromatography. Based on the GC-MS, c.a. 92% selectivity
was obtained (Figure S58 and Entry 2 of Table 1). Besides,
the TOF value was as high as 479 h-1, which were measured
after the first 3 h of reaction (Table S9).
3.4.3. Recyclability of Cu@Azo-POP-4 for aerobic C-H bond
oxidation
As control experiments, the above aerobic C-H oxidation
gave much lower isolated yield without NHPI, without
Cu@Azo-POP-4, or without Cu loading and NHPI (Entries 3-
5 of Table 1 and Figure S38-40). The aerobic C-H oxidation
reaction with combination of Cu(OAc)2 (3 mol%) and NHPI
gave 86% yield, lower than 91% yield by using the same
molar amount of copper catalyst. It revealed that the
The stability and reusability of heterogeneous catalyst
systems are crucial parameters for their practical applications.
The recycling test of Cu@Azo-POP-4 for above aerobic C-H
bond oxidation was conducted and the results showed in the
Table S8. The Cu@Azo-POP-4 can be easily recovered by
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filtration and reused for the next-run of C-H oxidation
polymerization are undergoing, the relevant results will be
reported in due course.
reaction. The C-H oxidation on indane with the recycled
catalyst gave 85% isolated yield after 5 cycles, i.e. without
significant loss of reactivity. The remaining Cu species in the
crude product was measured to be 240-616 ppm by ICP-AES,
which was about 1/30 of the case when Cu(OAc)2 was used as
catalyst (Figure 7a and S43).
ASSOCIATED CONTENT
Supporting Information
Additional FT-IR, 1H-NMR, 13C-NMR, HR-MS for model
compounds 5a-5c, tables for optimization of synthetic
conditions of Azo-POP-4, EA, TGA and PXRD for Azo-POP-
4, Azo-POP-5 and Azo-POP-6, adsorption isotherms of Cu2+
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1
onto the Azo-POP-4, as well as the crude H-NMR spectra
for the key C-H oxidation reactions. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Phone: (0086)-10-68912658. Email: mhhuang@bit.edu.cn
Figure 7 The Cu leaching of Cu@Azo-POP-4 during the recycling
experiments. (a) The residual Cu2+ in crude product of C-H oxidation
on indane, catalyzed by recycled Cu@Azo-POP-4 and by Cu(OAc)2
(3 mol% based on Cu2+). (b) The change of Cu2+ loading in Cu@Azo-
POP-4 after recycle test.
Author Contributions
⊥These authors contributed equally.
Notes
The authors declare no competing financial interests.
In addition, the copper content of Cu@Azo-POP-4 catalyst
dropped 12% from 61.7 mg to 54.1 mg after 5 cycles. i.e., c.a.
2.5% copper loss for each cycle averagely. This corresponded
to the low copper contamination of the crude product, much
lower than the case of Cu(OAc)2 catalyst. This also meant
tight grasping of Cu by Azo-POP-4, which was especially
crucial for heterogeneous catalysts for continuous operation
without reactivation of recovered catalyst. TEM
characterization confirmed that uniformly elemental
distribution of Cu@Azo-POP-4 remained unchanged after 5
times’ recycling (Figure 6c). The elemental mapping image
(Figure 6d) showed the Cu still uniformly dispersed in the
catalyst. Active Cu2+ species in the Cu@Azo-POP-4 catalyst
were observed before and after catalysis based on EPR
measurements (Figure S51), which showed two (instead of
one) features were found at high field at 3470 and 3521 G60. In
addition, XPS measurements were performed to further
investigate the state of Cu (Figure S53-54). The BE of Cu on
the catalyst after catalysis showed no difference from the
freshly catalyst, indicating the stability of catalyst.
ACKNOWLEDGMENT
The authors thank the National Natural Science Foundation
of China (No. 21772013 and 21202008), Beijing Natural Science
Foundation (No. 2162039) and the Fundamental Research
Funds for the Central Universities for generous support.
Professor Yangping Liu is thanked for helping the analysis on
EPR of the Cu@Azo-POP-4 catalyst.
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This paper reported the design and facile synthesis of well-defined functional porous organic
polymers comprising triaminotriphenylazobenzene subunit. Their application for the fast
removal of copper ions from water and copper catalyzed heterogeneous aerobic C-H bond
oxidation were investigated. The Cu@Azo-POP-4 showed high catalytic reactivity, high
selectivity, good reusability and low metal contamination on the product.
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