Synthesis of copper nanoparticles supported on a microporous covalent triazine polymer: An efficient and reusable catalyst for O-arylation reaction

Article abstract of DOI:10.1039/c5cy01590a

Copper nanoparticles were supported on a microporous covalent triazine polymer prepared by the Friedel-Crafts reaction (Cu@MCTP-1). The resulting material was characterized by powder X-ray diffraction, thermogravimetric analysis, N₂ adsorption-desorption isotherms at 77 K, transmission electron microscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma optical emission spectroscopy, and Cu particles with an average size of 3.0 nm and a BET total surface area of ca. 1002 m² g^-1 were obtained. Cu@MCTP-1 was evaluated as a heterogeneous catalyst for the Ullmann coupling of O-arylation over a series of aryl halides and phenols without employing expensive ligands or inert atmosphere, which produced an excellent yield of the corresponding diaryl ethers. The catalyst could be recovered by simple centrifugation and was reusable at least five times with only a slight decrease in catalytic activity.

Full text of DOI:10.1039/c5cy01590a

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Synthesis of copper nanoparticles supported on a microporous covalent
triazine polymer: an efficient and reusable catalyst for O-arylation reaction
Pillaiyar Puthiaraj and Wha-Seung Ahn*
Department of Chemistry and Chemical Engineering, Inha University,
Incheon 402-751, South Korea
*E-mail: whasahn@inha.ac.kr
Abstract
Copper nanoparticles were supported on a microporous covalent triazine polymer
prepared by the Friedel-Crafts reaction (Cu@MCTP-1). The resulting material was characterized
by powder X-ray diffraction, thermogravimetric analysis, N2-adsorption-desorption isotherms at
77 K, transmission electron microscopy, X-ray photoelectron spectroscopy, and inductively
coupled plasma optical emission spectroscopy, and Cu particles with an average size of 3.0 nm
and a BET total surface area of ca. 1002 m2g-1 were obtained. Cu@MCTP-1 was evaluated as a
heterogeneous catalyst for the Ullmann coupling of O-arylation over a series of aryl halides and
phenols without employing expensive ligands or inert atmosphere, which produced an excellent
yield of the corresponding diaryl ethers. The catalyst could be recovered by simple centrifugation
and was reusable at least five times with only a slight decrease in catalytic activity.
Keywords: copper nanoparticles, covalent triazine polymer, Ullmann coupling, O-arylation,
heterogeneous catalyst.
Introduction
Aryl-O bond (diaryl ether linkage) formation is an important and challenging task in both
synthetic and industrial chemistry. The reaction produces important chemical intermediates of
the biological, pharmaceutical, life science, and materials interest.1 Traditionally, diaryl ethers
are prepared via nucleophilic substitution of phenols with aryl halides. This method, however,
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o
suffers from drawbacks such as high reaction temperature (>200 C), necessity for a
stoichiometric amount of catalyst, and low to moderate product yield with a long reaction time.2
To overcome these problems, palladium catalysts with different ligand groups have been tested
for the formation of C-O bond. However, these Pd catalyst-based homogeneous coupling
methods have limitations, such as the use of air and moisture-sensitive phosphine ligands, high
cost of Pd salts, contamination of the products, and tedious multistep protocols necessary for the
ligand synthesis and work-up process.3 Concurrently, researchers have also been examining the
use of Cu-catalyzed Ullmann coupling using N-donors, N- and O- donors and P-based ligands,
such as neocuproinem,4 1,10-phenanthroline,5 N,N-dimethylglycine,6 tripod ligands,7
pyrrolidine-2-phosphonic acid phenyl monoester,8 β-keto esters,9 di-imine derivatives,10 nicotinic
acid,11
β-diamide-based
hybrid
silica,12
metformin,13
and
2-carbomethoxy-3-
hydroxyquinoxaline-di-N-oxide.14 These protocols proceeded homogeneously, and again the
high cost of ligands, product contamination and need for catalyst recovery are problems to
overcome. Thus the development of active, ligand-free, inexpensive, moisture-stable, and
recyclable heterogeneous catalytic systems for the clean synthesis of C-O bonds is highly
desirable.
In recent years, synthesis of metallic nanoparticles (NPs) supported on a porous material
and their applications in catalytic reactions have attracted considerable attention owing to the
high surface-to-volume ratio in NPs that can enhance the interaction between the reactants and
catalyst, which can lead to increase the product yields compared to homogeneous catalysts.15 In
addition, catalysis by metal NPs has advantages such as easy catalyst separation from the
reaction mixture, fast catalytic reaction rates, and reuse with better retention of the catalytic
activity than their bulk counterparts. Supported Cu NPs are currently attracting more interest
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than other transition metal NPs of Au, Ag and Pd, because of their broad applications and lower
cost.16 Catalysts based on Cu2O, CuO and Cu NPs have also been considered for the synthesis of
diaryl ethers. Benyahya et al. developed a sol-gel immobilized CuI-catalyst for the arylation of
phenols with aryl iodides/bromides.17 Park et al. developed the Cu2O nanocube catalytic system
for the diaryl ethers synthesis from aryl halides and phenols at 150 oC.18 Sun et al. used magnetic
o
CuFe2O4 NPs for the Ullmann coupling synthesis of diaryl ethers at 135 C.19 Wang et al.
reported the CuO/γ-Al2O3 catalyzed C-O coupling reaction of iodobenzene with m-cresol at 150
oC.20 Heydari et al. reported the α-Fe2O3@CuO nanocomposite catalyzed the same coupling
reactions with an external diketone ligand in an argon atmosphere.21 Recently, Nagarkar et al.
prepared Cu NPs on nafion-graphene nano-ribbons by an electrochemical method and used them
as a catalyst for Ullmann coupling synthesis of ethers.22 Li et al. showed that carbon nanofiber-
o
supported Cu NPs catalyzed the Ullmann coupling of O-arylation at 140 C.23 However, almost
all the methods attempted thus far involved high cost catalytic systems and had a limited scope
of applications. The development of simple, active and low-cost heterogeneous catalysts for the
Ullmann coupling of O-arylation reactions are still highly desirable.
Porous covalent triazine polymers (CTPs) have emerged as a novel platform for
heterogeneous catalysts and support material. They have excellent textural properties with high
surface area and pore volume, tunable composition, and exhibit high thermal and chemical
stability to most organic solvents, and can be prepared via easy synthesis protocols at low cost.24
They have been considered for gas storage and separation, drug delivery, super capacitors,
photo-degradation of organic compounds and catalysis.25 CTPs with high nitrogen contents in
the framework enable the effective immobilization of metallic NPs, which leads to a uniform
particle size distribution, because the particles are immobilized permanently inside their
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cavities.26 Bhaumik et al. reported that Pd NPs were immobilized by the triazine units in
mesoporous polymers and the covalent imine networks catalyzed the C-C coupling reactions.27
Liu et al. reported the preparation of Pd NPs supported on the triazine moiety in covalent organic
polymers, and used them for CO oxidation reactions.28 Edwards et al. developed porous
melamine-formaldehyde polymers to stabilize Pd, which served as a catalyst for Suzuki coupling
reactions.29 Recently, Pitchumani et al. reported that Pd and Cu(II) ions immobilized on the
triazine units in porous materials as efficient catalysts for the Sonogashira, Heck and Chan-Lam
coupling reactions.30 Chen et al. developed Pd NPs supported on a similar porous organic
material CTF-1 for the catalytic hydrogenation of N-heterocycles, and Ru NPs for NH3
decomposition.31 To the best of the authors’ knowledge, however, there are no reports on the
utilization of a nitrogen-rich CTPs for stabilizing Cu NPs and their applications in catalysis.
Recently, we reported an inexpensive Friedel-Crafts synthesis of microporous CTPs (MCTPs)
based on readily available cyanuric chloride with tri-/dual site-reactive 1,3,5-triphenylbenzene
(MCTP-1) / trans-stilbene (MCTP-2).32 In a continuation of that work, MCTP-1 was applied as a
solid support to immobilize the Cu NPs (Scheme 1), which was tested as a catalyst towards
environmentally benign and efficient Ullmann cross coupling of O-arylation reaction (Scheme 2).
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N
N
N
N
N
N
Cl
N
N
N
(i) Cu(NO3)2·3H2O, 4 hrs
Cu NPs
(ii) NH4OH, NH2NH2,
100 oC, 4 hrs
anhydrous AlCl3
Cl
Cl
DCM, 70 oC, 16 hrs
N
N
N
N
N
N
N
N
N
N
N
N
MCTP-1
Cu@MCTP-1
Scheme 1. Synthesis of the Cu NPs supported on MCTP-1 (Cu@MCTP-1)
X
OH
O
+
R1
R2
X = I & Br
R2
R1
Scheme 2. Ullmann cross coupling of O-arylation
Experimental section
Synthesis of Cu NPs supported on MCTP-1 (Cu@MCTP-1)
Unless specified otherwise, all chemicals were purchased from commercial suppliers and
used as received. Cu(NO3)2·3H2O (70 mg) was dissolved in 4 mL of methanol and then MCTP-1
material (500 mg) was added with stirring at room temperature over a 2 h period. The solid was
then filtered, and washed with methanol to remove the unreacted Cu(NO3)2·3H2O. The freshly
prepared Cu2+ incorporated MCTP-1 was added to 5 mL of water, and NH4OH (2 mL; 28%) and
hydrazine hydrate (2 mL) were then added with vigorous stirring. Finally, the mixture was
transferred to a 15 mL capacity Teflon-lined stainless steel autoclave and heated to 100 oC for 4
h in an oven. After heating, the autoclave was cooled to room temperature and the solid
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Cu@MCTP-1 was collected by filtration, washed with methanol, and kept under a nitrogen
atmosphere. MCTP-1 was prepared using the procedure reported elsewhere.32
Characterization
The powder X-ray diffraction (PXRD, Rigaku) patterns of the catalysts were recorded
using CuKα (λ=1.54 Å) with 0.5o min-1 over the scanning range of 4-80o. The N2 adsorption-
desorption isotherms were measured in a BELsorp-Max (BEL, Japan) at 77K. The specific
surface areas and pore sizes of the samples were calculated using the Brunauer–Emmett–Teller
(BET) and nonlocal density functional theory (NL-DFT) model with slit pore geometry,
respectively. The metal loading was measured by inductively coupled plasma – optical emission
spectrometry (ICP-OES, Optima 7300DV, USA). The thermal stability of the samples was
evaluated by thermogravimetric analysis (TGA, SCINCO thermal gravimeter S-1000, Japan)
o
under an Argon (Ar) atmosphere over the temperature range, 30–700 C in flowing argon at a
o
heating rate of 5 C min-1. Transmission electron microscopy (TEM) was performed using a
CM200 UT (Philips, Germany) instrument. The catalytic reaction conversions were analyzed gas
chromatography (GC, Agilent Technologies, 7890A, USA) fitted with a HP-5 capillary column
and a flame ionization detector. The nuclear magnetic resonance (NMR, VARIAN
UNITYINOVA (USA) and JEOL 400 MHz instruments (USA)) spectra were recorded in
tetramethylsilane as the internal standard and CDCl3 as a solvent. High resolution X-ray
photoelectron spectroscopy (XPS, Thermo scientific, USA) was performed using a
monochromatic Al Kα X-ray source and a hemispherical analyzer.
General procedure for the Ullmann coupling of O-arylation reaction
In a typical procedure, a mixture of aryl halide (1 mmol), phenol (1.2 mmol), Cs2CO3 (2
mmol), Cu@MCTP-1 (100 mg, which contains 2.68 wt.% of Cu as determined by ICP-OES) in
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DMF (3 mL) was placed in a 25 mL round bottom flask. The reaction was conducted at 130 °C
in an oil bath with magnetic stirring for various reaction times. After the reaction, the reaction
mixture was cooled to room temperature, and diluted with ethyl acetate (10 mL). The catalyst
was then centrifuged, the supernatant was extracted with distilled water, and the organic layer
was dried over anhydrous sodium sulfate. The organic solvent was removed under reduced
pressure and the resulting residue was purified by column chromatography to provide the desired
pure product.
Results and discussion
The synthesis procedure and physic-chemical properties of the MCTP-1 is given in our
previous publication.32 The synthetic strategy of Cu@MCTP-1 in this study involves the simple
preparation of a mixture of MCTP-1 and Cu(II) ions from Cu(NO3)2·3H2O, and the subsequent
chemical reduction of the immobilized Cu(II) ions to Cu(0) NPs. A uniform distribution of nano-
sized Cu NPs in Cu@MCTP-1 was confirmed by PXRD, TGA, XPS, N2 adsorption-desorption
isotherms, TEM, and ICP-OES analyses.
The crystallinity of the Cu@MCTP-1 was examined by PXRD in the range, 4 to 80° 2θ
(Figure 1). The two broad diffraction peaks at 10.3° and 21.8° 2θ suggest the potential stacking
of triazine linkage (001) plane.24a The strong Bragg reflection peaks are observed at 43.38°,
50.48° and 74.13° 2θ, which were assigned to the (111), (200) and (220) sets of lattice planes of
the face centered cubic (fcc) structures of Cu, respectively. The oxide forms of the Cu phase
peaks were not observed in PXRD, which clearly pointed out that the produced Cu NPs are
present in the fully-reduced Cu(0) state. TGA of Cu@MCTP-1 exhibited good thermal stability
up to 260 °C (Figure 2).
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Figure 1. Powder XRD pattern of the fresh and reused Cu@MCTP-1
Figure 2. TGA curves of MCTP-1 and fresh Cu@MCTP-1
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The oxidation state of Cu and the strong affinity between Cu and N in triazine in MCTP-
1 were confirmed by XPS (Figure 3). In the fresh Cu@MCTP-1, two strong binding energy
peaks at 952.3 and 932.5 eV were assigned to Cu 2p1/2 and Cu 2p3/2 level of Cu(0), respectively.
In addition, the difference between the binding energy peaks of 19.8 eV (between Cu 2p1/2 and
Cu 2p3/2 level) and the absence of a Cu 2p3/2 satellite peak around 942 eV also confirmed the
formation of Cu NPs and that all the particles were present in the pure Cu(0) state.33 The triazine-
unit N 1s core level was previously reported at 398.8 eV.34 The observed nitrogen 1s core level
binding energy at 400.1 and 400.0 eV, respectively, of the fresh and reused (Figure 3b) catalyst
clearly showed that Cu metals are strongly bound to triazine nitrogen species in MCTP-1.
Figure 3. XPS of fresh and reused Cu@MCTP-1 catalyst (a) Cu 2p and (b) N 1s core levels
The N2 adsorption-desorption isotherm of the fresh and reused Cu@MCTP-1 catalyst at
77 K showed a type I isotherm with steep N2 uptakes in the low pressure region (Figure 4),
which indicated a microporous structure. Table 1 lists the resulting textural properties of
Cu@MCTP-1. The fresh and reused catalyst BET surface areas were 1002 and 1034 m2g-1,
respectively, which were estimated from the 0.01 - 0.05 relative P/P0 pressure region of the
adsorption isotherms. The hysteresis in the isotherms in Figure 4 is reported elsewhere.24a,35 This
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is probably due to the flexibility, amorphous nature of the polymers with soft pore walls and pore
shape effect, which lead to irreversible desorption.
Figure 4. (a) N2 adsorption (solid) – desorption (empty) isotherm of MCTP-1 (black), fresh
Cu@MCTP-1 (blue) and reused Cu@MCTP-1 (red) catalyst, and (b) pore size distribution of
fresh and reused Cu@MCTP-1 catalyst
Table 1. N2 isotherm properties of MCTP-1, fresh and reused Cu@MCTP-1
Sample
SBET (m2g-1) Total Vpore (cm3g-1)
Pore width (nm)
MCTP-1
1459
1002
0.73
0.58
0.60
1.51
1.34
1.40
Fresh Cu@MCTP-1
Reused Cu@MCTP-1 1034
The morphology and particle size of Cu NPs supported on MCTP-1 was confirmed by
TEM (Figure 5), which revealed a narrow particle size distribution ranging from 2.0 to 6.0 nm
with a mean particle size of 3.0 nm, and the Cu particles were highly dispersed on MCTP-1.
From ICP-OES, the fresh Cu@MCTP-1 was found to contain ca. 2.68 wt.% of Cu NPs.
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Figure 5. TEM image of the fresh Cu@MCTP-1 (a & b), the recovered Cu@MCTP-1 after the
fifth runs (c & d) and particle size distribution plot of fresh (e) and reused Cu@MCTP-1 (f)
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The catalytic property of Cu@MCTP-1 was examined in the Ullmann coupling of O-
arylation using phenols with aryl iodides and bromides (Scheme 2). In the initial study to
establish the optimized reaction conditions, the experiments were conducted with the cross
coupling of phenol and iodobenzene as model substrates with different solvents and bases. The
detailed results of these experiments are listed in Table 2. Without a catalyst or with MCTP-1
alone, the reaction did not proceed in 24 h, clearly establishing the active role of Cu in this
Ullmann coupling reaction (Table 2, entries 1-2). When the reaction was carried out with the
homogeneous Cu salt catalysts, the reaction proceeded very slowly and only resulted in poor
yields (Table 2, entries 3-5). The reaction with Cu@MCTP-1 in DMF, however, produced the
diaryl ether product in 94% yield (Table 2, entry 6). Apparently, the efficacy of the base is
critical for achieving a good product yield, because the reaction did not proceed without a base
(Table 2, entry 7). The effect of different bases was screened in this reaction system (Table 2,
entries 8-11). After testing five different bases (Cs2CO3, K2CO3, Na2CO3, NaHCO3 and
NaOAc), Cs2CO3 gave superior results and the corresponding diaryl ether product was observed
in 94% yield, due to the higher basicity and rapid dissociation of the intermediate cesium
phenoxide. Polar and nonpolar solvents (dioxane and toluene) were screened to determine the
solvent effects in this reaction. When polar solvents such as DMF, DMSO, DMAc and ACN,
were used, the products were observed in 94%, 77%, 89%, and 58% yields (Table 2, entries 12-
14), respectively. The nonpolar dioxane and toluene solvents were less active in this reaction
(Table 2, entries 15-16). The reaction temperature also plays an important role in the yield of the
o
reaction (Table 2, entry 17). As the reaction temperature was decreased from 130 to 100 C, the
product yield also decreased gradually. In addition, a higher product yield was achieved with 100
mg of catalyst than with 60 or 80 mg of catalyst (Table 2, entry 18). Finally, the cross coupling
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reaction was examined at reaction different times (0, 4, 8, 12, 16, 20, and 24 h), and the yield
was 0, 25, 47, 63, 75, 85, and 94%, respectively (Figure 6a). For further investigation of the
Cu@MCTP-1 catalysis, the following set of reaction conditions was selected: Cs2CO3 as a base
and Cu@MCTP-1 (100 mg) as a catalyst in DMF at 130 oC for 24 h.
Table 2. Optimization of the reaction condition for the Ullmann coupling of O-arylationa
Entry
Base
Solvent
Conversion (%)b
1c
2d
3e
4f
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
-
DMF
DMF
0
0
DMF
22
DMF
13
5g
6
DMF
11
DMF
94
7
DMF
0
8
K2CO3
Na2CO3
NaHCO3
NaOAc
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
DMF
53
9
DMF
26
10
11
12
13
14
15
16
17
18
DMF
12
DMF
17
DMSO
DMAc
ACN
77
89
58
8
Dioxane
Toluene
DMF
19
52h, 81i
83j, 65k, 33l
DMF
aReaction Conditions: Iodobenzene (1 mmol), phenol (1.2 mmol), base
(2 mmol), solvent (3 mL), Cu@MCTP-1 (100 mg), 130 oC, 24 h; bGC
conversion based on iodobenzene; cWithout Cu@MCTP-1; dMCTP-1
o
o
e
f
g
h
i
j
alone; CuCl2; Cu(NO3)2.3H2O; CuSO4.5H2O; 100 C; 120 C; 80
mg of catalyst; k60 mg of catalyst; l40 mg of catalyst.
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Figure 6. (a) Time interval plot for the coupling of iodobenzene with phenol over Cu@MCTP-1
and (b) after removing the catalyst at 47% product formation
The reaction of phenol with various substituted aryl iodides also produced to excellent
product yields (Table 3, entries 1-12). Furthermore, the electron-withdrawing groups in the para-
position of the iodo substrates reacted more readily with phenols to give slightly higher yields of
the corresponding products (Table 3, entries 4-9) compared to the para- or meta-substituted
electron-donating iodo substrates (Table 3, entries 2, 3, 10 & 11), but the ortho- substituted 2-
nitroiodobenezene gave somewhat lower yield due, probably to steric hindrance (Table 3, entry
12). On the substrates containing either chloro- or fluoro- group in iodobenzene, phenol coupled
selectively only with the iodo-sides (Table 3, entries 6-7).
The catalyst efficiency was examined further with less reactive aryl bromides under the
same reaction conditions. Here, the same type of electronic effect was observed as in aryl iodides,
and gave the products in 77-86% yield (Table 3, entries 13-17). In a further set of experiments,
the reaction between substituted phenols and various aryl iodides were examined. In this case,
the electron donating groups on the phenol reacted more readily with iodobenzene than the
electron withdrawing groups on the phenol (Table 3, entries 18-27). Interestingly, heterocyclic 4-
iodopyridine and 1,4-diiodobenzene coupled easily with phenol, which gave the corresponding
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products in 89 and 83% yield, respectively (Table 3, entries 28-29). The coupling products of
diaryl ethers were characterized by NMR spectroscopy and the corresponding data are presented
in the supporting information.
Table 3. Diversity of Cu@MCTP-1 catalyzed O-arylation of aryl halides with various phenolsa
Entry Phenols (1)
Aryl halides (2)
C6H5-I (2a)
Products (3) Yield of 3 (%)b
1
C6H5-OH (1a)
3a
3b
3c
3d
3e
3f
94
89
87
96
94
97
95
93
91
88
85
71
82
79
77
85
86
95
93
78
90
89
88
84
82
84
86
89
83
2
1a
4-CH3-C6H4-I (2b)
4-CH3O-C6H4-I (2c)
4-NO2-C6H4-I (2d)
4-NC-C6H4-I (2e)
4-F-C6H4-I (2f)
4-Cl-C6H4-I (2g)
4-CH3CO-C6H4-I (2h)
4-CF3-C6H4-I (2i)
3-CH3-C6H4-I (2j)
3-CH3O-C6H4-I (2k)
2-NO2-C6H4-I (2l)
C6H5-Br (2m)
4-CH3-C6H4-Br (2n)
4-CH3O-C6H4-Br (2o)
4-NO2-C6H4-Br (2p)
4-NC-C6H4-Br (2q)
2a
3
1a
4
1a
5
1a
6
1a
7
1a
3g
3h
3i
3j
3k
3l
8
1a
9
1a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29c
1a
1a
1a
1a
1a
1a
1a
1a
3a
3b
3c
3d
3e
3b
3c
3d
3m
3n
3o
3p
3q
3r
4-CH3-C6H4-OH (1b)
4-CH3O-C6H4-OH (1c) 2a
4-NO2-C6H4-OH (1d)
1b
1b
1c
1d
1d
4-F-C6H4-OH (1e)
4-NC-C6H4-OH (1f)
1a
1a
2a
2b
2j
2h
2d
2h
2f
2f
3s
3t
3u
4-iodopyridine (2r)
1,4- I-C6H4-I (2s)
aReaction Conditions: Aryl halides (1 mmol), phenols (1.2 mmol), Cs2CO3 (2 mmol), DMF
(3 mL), Cu@MCTP-1 (100 mg), 130 oC, 24 h; bIsolated yields; c200 mg of Cu@MCTP-1.
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To check for the possibility of Cu leaching from the Cu@MCTP-1 solid support to the
solution and confirm that the catalysis is truly heterogeneous in nature, a hot filtering experiment
proposed by Sheldon was carried out using iodobenzene and phenol as the substrates under
optimized conditions (Figure 6b). The reaction was quenched after an 8 h reaction, and the
catalyst was removed by centrifugation. The supernatant solution was further subjected to
o
reaction at 130 C for the remaining period of time (16 h). After the reaction time, the mixture
was cooled and extracted with ethyl acetate to identify the product and yield. Practically no
further conversion was detected in the supernatant solution, which clearly supported that the
catalysis is heterogeneous in nature.
Figure 7. Recycling test of Cu@MCTP-1 catalyst towards the Ullmann coupling of phenol with
iodobenzene
Easy product separation and reusability of the catalyst are highly desirable for industrial
applications. The catalyst reusability was assessed in the coupling reaction of O-arylation
between iodobenzene and phenol under the optimized reaction conditions (Figure 7). The
catalyst was recovered by centrifugation after each experiment, washed with DMF followed by
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o
water to remove the base. The catalyst was dried under vacuum at 100 C for 2 h, and the
recovered catalyst was used for the next four consecutive runs. As shown in Fig. 7, the
conversion of iodobenzene steadily decreased after each runs. Subsequently, the recovered
catalyst stability was examined by PXRD, TEM, XPS, ICP-OES, and BET surface area
measurement (N2 adsorption-desorption isotherm) analyses. The PXRD pattern (Figure 1) of the
reused catalyst showed similar set of peaks to those of the fresh one and no additional peaks
were observed, which indicates that the recovered catalyst is highly stable even after the fifth
runs. The TEM analysis of the recovered Cu@MCTP-1 catalyst after the 5th run (Figure 5)
showed that the size of the Cu particles have grown slightly to 3.0-7.0 nm with a mean particle
size of 5.0 nm reflecting mild agglomeration. Similar finding was reported for Pd on a porous
covalent triazine polymer support.28 The XPS spectra of the reused catalyst revealed two strong
binding energy peaks at 952.1 eV and 932.3 eV respectively, and no impure (Cu2+ state) satellite
peak was observed, suggesting that the Cu NPs are not oxidized in the catalytic reaction and all
the Cu particles are present in the zero valence state (Figure 3a). The reused Cu@MCTP•1
catalyst had shown a BET surface area (1034 m2g-1) slightly higher than the fresh catalyst (1002
m2g-1). According to the TEM analysis of the used Cu@MCTP•1, Cu particles have somewhat
grown in size, which could have made more pore interior exposed. In addition, The Cu content at
the fifth cycle, however, was 2.55 wt.% as measured by ICP-OES analysis, which indicates
minute leaching of Cu particle from the external surface of the material. The Cu NPs might have
detached from the Cu@MCTP-1 through attrition of the catalysts particles caused by magnetic
stirring during the long reaction time. Despite the minor limitation, the catalyst could be reused
at least five times.
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Conclusion
A simple and economical method for preparing a catalyst comprised of Cu NPs
immobilized on an organic porous polymer MCTP-1 with high nitrogen content was reported for
the first time. This Cu@MCTP-1 showed excellent catalytic activity in the Ullmann coupling of
O-arylation under optimized conditions and the system provided a wide range of diaryl ethers
from various phenols and aryl halides. The catalyst was reusable for up to five consecutive runs
with a slight decrease in product yield and the heterogeneity of the catalyst was proven by the
Sheldon test. Further applications of this catalyst to other chemical transformations are currently
underway.
Acknowledgement
This study was supported by Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant number:
2015042434).
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Graphical Abstract
Synthesis of copper nanoparticles supported on a microporous covalent
triazine polymer: an efficient and reusable catalyst for O-arylation reaction
Pillaiyar Puthiaraj and Wha-Seung Ahn*
Department of Chemistry and Chemical Engineering, Inha University,
Incheon 402-751, South Korea
*E-mail: whasahn@inha.ac.kr
Cost-effective synthesis of Cu NPs immobilized on microporous covalent triazine
polymer was evaluated as catalyst for Ullmann coupling of O-arylation.