Carboxyl-containing microporous organic nanotube networks as a platform for Pd catalysts

Article abstract of DOI:10.1039/c6ra05753e

In this work, we present a novel synthesis of carboxyl-containing microporous organic nanotube networks (COOH-MONNs) by combination of hyper-cross-linking and molecular templating of core-shell bottlebrush copolymers. Highly dispersed palladium nanoparticles (Pd NPs) anchored on the COOH-MONNs (Pd@MONNs) have been prepared by in situ thermal decomposition of Pd(OAc)₂. The Pd@MONNs composites were characterized by XRD, N₂ adsorption, TEM, ICP-AES and XPS. The results show that bulk production of highly dispersed palladium nanoparticles can be achieved by a thermal decomposition method. Moreover, the obtained Pd@MONNs exhibit high activities for the Suzuki-Miyaura cross-coupling reaction and can be easily recovered and reused. This approach of using MONNs as a platform for catalysts is expected to open doors for new types of catalytic support for practical applications.

Full text of DOI:10.1039/c6ra05753e

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Carboxyl-containing microporous organic
nanotube networks as a platform for Pd catalysts†
Cite this: RSC Adv., 2016, 6, 39933
*
Yang Xu, Tianqi Wang, Zidong He, Aiqing Zhong and Kun Huang
In this work, we present a novel synthesis of carboxyl-containing microporous organic nanotube networks
(COOH-MONNs) by combination of hyper-cross-linking and molecular templating of core–shell
bottlebrush copolymers. Highly dispersed palladium nanoparticles (Pd NPs) anchored on the COOH-
MONNs (Pd@MONNs) have been prepared by in situ thermal decomposition of Pd(OAc)2. The
Pd@MONNs composites were characterized by XRD, N2 adsorption, TEM, ICP-AES and XPS. The results
show that bulk production of highly dispersed palladium nanoparticles can be achieved by a thermal
decomposition method. Moreover, the obtained Pd@MONNs exhibit high activities for the Suzuki–
Miyaura cross-coupling reaction and can be easily recovered and reused. This approach of using MONNs
as a platform for catalysts is expected to open doors for new types of catalytic support for practical
applications.
Received 4th March 2016
Accepted 14th April 2016
DOI: 10.1039/c6ra05753e
www.rsc.org/advances
a method through thermally decomposing metal salts to deco-
rate palladium nanoparticles onto carboxylic acid functional-
1. Introduction
ized single walled carbon nanotubes (SWNTs). Recently, Guo
and co-workers22 also reported a one-pot thermal decomposi-
tion method without addition of reductant or surfactant to
synthesize multiwalled carbon nanotube (MWNT)–COOH–
stabilized Pd nanocatalysts (Pd–MWNT–COOH). These studies
have shown that the COOH-modied materials had a high
affinity for Pd NPs and exhibited a potential universality to
fabricate different supported metal catalysts by thermal
decomposition of metal salts.
In the past decade, porous materials have drawn ever-
increasing interest and have provided great new opportunities
for heterogeneous catalysis. The fusion of porous materials and
metal NPs technology has been a fruitful area in catalysis. The
traditional porous materials such as silicas, zeolites and acti-
vated carbon have been extensively used as heterogeneous
catalysts supports. Recently, microporous organic polymers
(MOPs) as an emerging class of porous materials have received
much attention as catalysts candidates owing to their high
porosity. In addition, the bottom-up or post-synthetic modi-
cation approach for MOPs synthesis provides their excellent
capability to incorporate targeted or multiple chemical func-
tionalities into the porous framework for use as catalysts. For
example, MOPs have been efficiently utilized as supports of
metal NPs in photocatalysis, oxidation reactions, and Suzuki–
Miyaura cross-coupling reactions.23,24 However, most of these
MOPs mainly contain a sole micropore, which may limit the
diffusion of the reactants and products, leading to undesirable
catalytic activity. A potential method to solve this problem was
to introduce meso/macropores into the sole microporous
materials to form a structure with hierarchically pores.25
The Suzuki–Miyaura cross-coupling reaction catalyzed by
palladium nanoparticles (Pd NPs) is one of the most effective
synthetic reactions in organic synthesis and pharmaceutical
industries.1–3 Due to the large surface-to-volume ratio and ultra-
small size, Pd NPs exhibit remarkable high catalytic activities
toward this reaction, a property not displayed in bulk palla-
dium.4,5 However, Pd NPs tend to agglomerate to form large
particles, driven by high surface energy, which results in a loss
of their catalytic activities.4 Considerable efforts and great
achievements have been made to overcome this issue by
immobilizing Pd NPs in or onto appropriate solid supports,
such as silica,6 zeolites,7 activated carbon,8 carbon nano-
tubes,9,10 polymers11 and some composite materials.12,13
Although these supported Pd catalysts have superior catalysis
activities, the development of efficient and stable heteroge-
neous catalysts is still highly desirable.
Up to now, a variety of synthetic methods have been devel-
oped towards the fabrication of metal-loaded solid catalysts,
such as chemical reduction,14,15 supercritical uid deposition
method,16 self-assembly,17 electrochemical decoration18 and
thermal decomposition of metal salts.19–22 Among them,
thermal decomposition or pyrolysis of metal salts was consid-
ered as a straightforward and easy handling method to anchor
metal nanoparticles on the appropriate solid supports. For
example, Swadhin K. Mandal and co-workers21 have developed
School of Chemistry and Molecular Engineering, East China Normal University, 500 N,
Dongchuan Road, Shanghai, 200241, P. R. China. E-mail: khuang@chem.ecnu.edu.cn
† Electronic supplementary information (ESI) available: Experimental data of
Fig. S1–S6. See DOI: 10.1039/c6ra05753e
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 39933–39939 | 39933

View Article Online
RSC Advances
Paper
were prepared by dip-coating a 400 mesh carbon-coated
copper grid from the dilute sample solution allowing the
solvent to evaporate. A Quantachrome Autosorb IQ surface
area and porosity analyzer was utilized to study the pore
structure of the samples. Before measurements, the polymer
samples were degassed for more than 10 h at 120 ꢁC. The
Brunauer–Emmett–Teller surface area and the micropore
surface area were determined by the BET equation and the t-
plot equation, respectively. The pore size distribution was
analyzed by original density functional theory (DFT). X-ray
photoelectron spectroscopy (XPS) spectra were obtained on
an ESCALAB 250Xi spectrometer. The powder X-ray diffrac-
tion (XRD) pattern of the sample was collected using a D8
Advance X-ray diffractometer (Bruker AXS, Germany) with Cu
K radiation. The infrared (IR) spectra were recorded using
Thermo NICOLET is 50. TG analyses were carried out with
a NETZSCH STA449F3 simultaneous thermal analyzer at
Scheme 1 Fabrication of Pd NPs catalysts supported on carboxyl-
containing microporous organic nanotube networks.
Herein, we present a novel synthesis of carboxyl-containing
microporous organic nanotube networks (COOH-MONNs) by
combination of hyper-cross-linking and molecular templating
of core–shell bottlebrush copolymers (Scheme 1). The well-
dispersed Pd NPs could be anchored into the COOH-MONNs
by in situ thermal decomposition of Pd(OAc)2, in which the
carboxyl groups served as anchors were generated and tuned
from the controllable synthesis of the materials. To the best of
our knowledge, direct thermal decomposition of Pd(OAc)2 in
the presence of carboxyl-containing porous organic polymers
without the addition of surfactants or reducing reagents has
rarely been reported. On this basis, the as-prepared novel Pd
NPs catalysts (Pd@MONNs) may be promising heterogeneous
catalysts towards Suzuki–Miyaura cross-coupling reaction
under mild conditions.
a heating rate of 10 K minꢀ1 from 30 to 800 C in a nitrogen
ꢁ
atmosphere.
2.3. Synthetic procedures
2.3.1. Synthesis of PGM. GM (2 ml), CPD (18 mg), AIBN (2.4
mg) and benzene (2 ml) were mixed in a reaction vessel and
degassed by 3 freeze–pump–thaw cycles. The polymerization
ꢁ
was conducted at 60 C for 16 h. The mixture was diluted with
DCM, precipitated into methanol 3 times and dried under
vacuum at room temperature for 24 h. Yield ¼ 1.77 g (89%). GPC
(PS standards): Mn ¼ 1.5 ꢂ 104 g molꢀ1, Mw/Mn ¼ 1.07. 1H NMR:
n(GM) ¼ 200.
2.3.2. Hydrolysis of PGM. PGM (600 mg), THF (12 ml) and
acetic acid (24 ml) were mixed in a 250 ml two-necked round-
bottom ask. The reaction mixture was stirred and placed in
an oil bath at 60 ꢁC, followed by the slow addition of 35 ml water
over the course of 1 h. Aer stirring for 24 h, the solvent was
removed on a rotary evaporator. The isolated polymer was
precipitated from methanol into diethyl ether 3 times and dried
under vacuum at 25 ꢁC for 24 h. Yield ¼ 0.52 g (95%). 1H NMR:
conversion ¼ 95+%.
2. Experimental
2.1. Materials
All reagents were used as received unless stated otherwise.
Glycidyl methacrylate (GM, Acros 97%) was distilled before use.
Dichloromethane (DCM) and N,N-dimethylformamide (DMF)
were dried using CaH2 and distilled. Styrene (Aldrich 99%) was
puried by passing over basic alumina. 2,2-Azoisobutyronitrile
(AIBN) and D,L-lactide were puried by recrystallization from
methanol and ethyl acetate, respectively. S-1-Dodecyl-S0-(a,a0-
dimethyl-a00-acetic acid) trithiocarbonate26 and 2-cyanoprop-2-
yl-4-cyanodithiobenzoate (CPD)27 were synthesized according
to literature procedures.
2.3.3. Synthesis of PGM-g-PLA. Hydrolyzed PGM (50 mg)
and D,L-lactide (1.80 g) were added to a dried 50 ml round-
bottom ask. Dried DMF (4.5 ml) was then added and the
mixture was stirred until all polymer dissolved. 1,8-Diazbicyclo
[5.4.0] undec-7-ene (DBU, 54 ml) was then injected into the ask.
Aer stirring at room temperature for 1.5 h, the reaction was
quenched by adding 162 mg benzoic acid. The resulting poly-
mer was precipitated from THF into methanol/water (1 : 1) 3
2.2. Instrumentations
All 1H NMR spectra were recorded on a Bruker AVANCE III™ times and dried under vacuum at 25 ꢁC for 24 h. Yield ¼ 1.25 g
500 spectrometer (500 MHz) by using CDCl3 as a solvent. GPC (75%). GPC (PS standards): Mn ¼ 1.9 ꢂ 105 g molꢀ1, Mw/Mn ¼
1
data were obtained from Waters GPC system equipped with 1.11. H NMR: n(PLA) ¼ 25.
a Waters 1515 isocratic HPLC pump, a 2414 refractive index
2.3.4. Synthesis of PGM-g-(PLA-b-PS)/PGM-g-(PLA-b-tBA-b-
(RI) detector, and two Waters' HPLC columns. Tetrahydro- PS). Hydroxyl end groups of the PLA brush were then modied
furan was used as the solvent for polyꢁmers and eluent for GPC to install trithiocarbonate functionalities as described in the
with a ow rate of 1 ml minꢀ1 at 30 C. The GPC instrument literature.29 PGM-g-PLA-TC (30 mg) was mixed with AIBN (0.24
was calibrated with narrowly dispersed linear polystyrene mg), styrene (1.98 ml) and toluene (1.98 ml) in a reaction vessel
standards. Transmission electron microscopy (TEM) images and degassed by 3 freeze–pump–thaw cycles. The reaction was
were obtained using a JEM-2100F TEM instrument. Samples then conducted at 50 ꢁC for 15 h and stopped by cooling to room
39934 | RSC Adv., 2016, 6, 39933–39939
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
temperature and opening the ask to air. The resulting reaction
mixture was then precipitated from DCM to methanol 3 times
and dried under vacuum at 25 ꢁC for 24 h. Yield ¼ 125 mg. GPC
(PS standards): Mn ¼ 4.8 ꢂ 105 g molꢀ1, Mw/Mn ¼ 1.21. 1H NMR:
n(styrene) ¼ 67.
2.4. Catalysis of the Suzuki–Miyaura coupling reaction
The Suzuki–Miyaura coupling reactions were carried out under
the catalysis of Pd@MONNs. 5 mg Pd@MONNs catalyst, K2CO3
(1.5 mmol), and borophenylic acid (0.75 mmol) were added into
a round-bottom ask under nitrogen atmosphere. Halobenzene
(if halobenzene was liquid) (0.5 mmol), H2O (0.8 ml), and
ethanol (1.2 ml) were successively added using syringes, andꢁthe
PGM-g-(PLA-b-tBA-b-PS) was prepared as same as the above
procedures. The synthesis of PGM-g-PLA-b-tBA was as follow.
PGM-g-PLA-TC (80 mg) was mixed with AIBN (0.42 mg), tert-
butyl acrylate (tBA 1.10 ml) and 1,4-dioxane (3.20 ml) in a reac-
tion vessel and degassed by 3 freeze–pump–thaw cycles. The
reaction was then conducted at 50 ꢁC for 22 minutes and
stopped by cooling to room temperature and opening the ask
to air. The resulting reaction mixture was then precipitated
from THF to the solution methanol/water (1 : 2) 3 times and
dried under vacuum at 25 ꢁC for 24 h. Yield ¼ 125 mg. GPC (PS
ꢁ
mixture was stirred at 50 C (aryl chlorides reactions at 80 C).
Aer the reaction nished, the catalyst solid was separated from
the reaction mixture by centrifuging. The product was extracted
with ethyl acetate and the liquid was concentrated. The
conversion of product was quantied by gas chromatograph
test. If it is necessary, the product could be puried by column
chromatography. Then the separated catalyst was washed
several times with distilled water (5 ml ꢂ 2), methanol (5 ml ꢂ
2) and dried under vacuum at 60 ꢁC for next run. The content of
Pd species leached off from the catalyst was determined by ICP
analysis.
1
standards): Mn ¼ 9.3 ꢂ 105 g molꢀ1, Mw/Mn ¼ 1.25. H NMR:
n(tBA) ¼ 3.
2.3.5. Hyper-cross-linking and core etching (MONNs).
Here, we choose PGM-g-(PLA-b-PS) as an example to demon-
strate the general procedure for the hyper-cross-linking and
core etching. PGM-g-(PLA-b-PS) (110 mg) was dissolved into 11
ml dried 1,2-dichloroethane (DCE) under nitrogen in a 100 ml
round-bottom ask. Aer purging with N2 for 10 minutes,
formaldehyde dimethyl acetal (FDA) (134 ml) and anhydrous
FeCl3 (340 mg) were added. The reaction mixture was le under
a N2 ow for a further 10 minutes. The reaction was then sealed
3. Results and discussion
Scheme 1 describes the synthetic strategy of Pd NPs catalysts
supported on carboxyl-containing microporous organic nano-
tube networks (Pd@MONNs). First, the core–shell bottlebrush
copolymer precursors were synthesized via a “graing-from”
approach with the help of ring-opening and reversible addition–
fragmentation chain transfer (RAFT) polymerizations. Next, the
outer polystyrene (PS) side chains in molecular bottlebrush
were then hyper-cross-linked via a Friedel–Cras alkylation
reaction.28,30 By removal of the polylactide28 core retained in the
pores and further acidication, the COOH-MONNs were ob-
tained and abundant inherent carboxyl groups were produced
on the pore walls. The number of carboxyl groups could be
tuned by introduced a functional block between the PLA core
and PS shell layer, such as poly(tert-butyl acrylate) (PtBA).
Scheme 2 presents the synthesis routes of well-dened
core–shell bottlebrush copolymers, as previously described
ꢁ
and heated under reux at 80 C for 24 h. The solid products
were washed 3 times with methanol. To assure complete
removal of any PLA-related residues (and deprotection of tBA to
produce more carboxyl), the product was then heated in
hydrochloric acid (5 M) at 60 ꢁC for 24 h. The nal product was
collected and ꢁwashed with methanol 3 times and dried under
vacuum at 60 C for 24 h. Yield ¼ 108 mg.
2.3.6. Preparation of the Pd catalyst (Pd@MONNs).
MONNs (60 mg) and palladium acetate (21 mg) (COOH : Pd ¼
1 : 2) were uniformly dispersed into 30 ml dry DMF under
nitrogen atmosphere in a 100 ml round-bottom ask with the
aid of stirring. To carry out the thermal decomposition of
palladium acetate, the mixed system was heated at 95 ꢁC for 4 h,
and then cooled to 25 ꢁC under nitrogen atmosphere. Aer the
reaction nished, the solid product was centrifuged and washed
several times with ethanol (10 ml ꢂ 3), acetone (10 ml ꢂ 2) until
the ltrate was clear and dried under vacuum at 60 ꢁC for 24 h.
Yield ¼ 62 mg.
2.3.7. Preparation of the comparative Pd catalyst
(Pd@MOP). PGM-g-(PLA-b-PS) (70 mg) was dissolved in 16 ml
THF, and 1 M NaOH methanol solution (2 ml) was then added
to the mixture. The reaction solution was stirred at room
temperature for 12 h. The solvent was evaporated and the
polymer was precipitated from THF to methanol 2 times and
dried under vacuum at 25 ꢁC for 24 h. GPC data (Fig. S8D, ESI†)
shown that the core of PGM-g-(PLA-b-PS) has been etched with
the PS–COONa block fragment remained. The corresponding
microporous organic polymer (MOP) was constructed via hyper-
cross-linking of the PS–COONa block fragment and the Pd
catalyst (Pd@MOP) was prepared at the same condition as
Pd@HMONNs.
Scheme 2 Synthesis routes of the bottlebrush copolymer precursors
of (A) MONNs-1 and (B) MONNs-4.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 39933–39939 | 39935

View Article Online
RSC Advances
Paper
method.31,32 From the 1H NMR spectra (Fig. S1, ESI†), the nm, whereas the internal cavity average diameter was about 5
backbone of poly(glycidyl methacrylate) (PGM) with an average nm. The Brunauer–Emmet–Teller (BET) surface area and total
degree of polymerization of 200 and a polydispersity index (PDI) pore volume of MONNs-1 determined by nitrogen adsorption–
of 1.07 was obtained. For PGM-g-(PLA-b-PS), every branch was desorption isotherms were 996 m2 gꢀ1 and 1.74 cm3 gꢀ1
,
composed of a PLA block with an average of 25 repeat units and respectively (Fig. 2). The presence of microporous structures
a PS block with an average of 67 units. As a regulation tech- was conrmed by an adsorption uptake at low relative pressure
nique, poly(tert-butyl acrylate) (PtBA) was selected as a middle (P/P0 < 0.001) in the N2 adsorption–desorption. A sharp rise at
functional block to introduce the core–shell bottlebrush medium and high relative pressure (P/P0 ¼ 0.8–1.0) indicated
copolymers. From the 1H NMR spectrum of PGM-g-(PLA-b-PtBA- the presence of macropores in the support. However,
b-PS), the characteristic signal of tBA at 2.26 ppm was observed a pronounced hysteresis loop observed in the desorption
and the number of PtBA and PS groups was 3 and 70, respec- isotherm indicated the existence of accessible mesopores in the
tively. GPC analysis of all polymers exhibited low poly- polymeric matrices (Fig. 2A). The micropore was within the
dispersities (Mw/Mn < 1.30), which revealed the formation of hyper-cross-linking PS shell layer, mesopore was produced by
well-dened copolymer precursors (Fig. S2, ESI†). Subsequently, the selectively removed PLA core layer, while the meso/
the resulting precursors were hyper-cross-linked via a Friedel– macropore resulted from a 3D continuous bottlebrush poly-
Cras alkylation. In this work, the pendant phenyl groups from meric cross-linked network. The pore-size distribution was also
PS shell layer were hyper-cross-linked by anhydrous FeCl3 with calculated by the nonlocal density functional theory (NLDFT)
the extra cross-linker of formaldehyde dimethyl ether (FDA). model. Two dominating pore-size distributions about 1.4–1.8
Aer that, the hyper cross-linked polymer networks were then and 5.0 nm were observed, further conrming that uniform
subjected to hydrochloric acid to remove the PLA core. Under mesopores existed in hollow microporous organic nanotubes,
this circumstance, the one-fold carboxyl groups will be which agreed with the results of TEM (Fig. 2B). Because of the
produced aer the PLA core removed (MONNs-1). In addition, same degrees of gra of PLA and PS, the BET data of MONNs-4
the functional block of PtBA can be deprotected to yield poly- are similar shown in the above results (Table 1). Based on their
(acrylic acid) (PAA) layer, where extra threefold carboxyl groups rigid network structures, high surface areas and excellent
were added into the above system (MONNs-4). These trans- hierarchical porosity, the obtained supports can be expected to
formations were evidenced by FTIR spectra, where the PLA be superior platforms for the heterogeneous catalysts.
carbonyl stretch peak (1758 cmꢀ1) completely disappeared aer
With the as-prepared supports in hand, the connement of
hydrolysis (Fig. S3, ESI†). The carbonyl stretch peak of PtBA at MONNs and coordination ability of carboxyl groups with Pd NPs
1728 cmꢀ1 was replaced by a broad signal from 1716 to 1732 were investigated (Pd@MONNs). As shown in Scheme 1, a facile
cmꢀ1, which was attributed to free and H-bonded carbonyl method, in situ thermal decomposition of Pd(OAc)2 in a solu-
vibrations of carboxyl groups on the nanotube interior.33
tion of MONNs dispersed in N,N-dimethylformamide (DMF),
Transmission electron microscopy (TEM) analysis revealed was used. TEM images showed that Pd NPs were well dispersed
that the cylindrical morphology of bottlebrush copolymer in the MONNs-1 and MONNs-4, and their average diameters are
precursors was maintained aer the core removal process. 3.98 ꢃ 0.5 and 4.16 ꢃ 0.6 nm, respectively, which are small
Especially, the hollow tubular network structure could be clearly enough to anchored into the interior cavities of MONNs-1 and
observed (Fig. 1A and C). The average length of the obtained MONNs-4 (Fig. 1B and D). In the powder X-ray diffraction (XRD)
tubular units in polymer networks was measured to be 38 ꢃ 5 pattern of Pd@MONNs-1, only a weak peak about (111) plane of
the Pd NPs was detected (Fig. S4, ESI†), revealing the Pd NPs are
Fig. 1 TEM images of (A) MONNs-1; (B) Pd@MONNs-1; (C) MONNs-4
and (D) Pd@MONNs-4. The size distributions of Pd NPs were calcu-
lated from TEM images and were illustrated in the inset histograms ((B): Fig. 2 Nitrogen adsorption–desorption isotherms and NLDFT pore
(3.98 ꢃ 0.5) nm; and (D): (4.16 ꢃ 0.6) nm).
size distributions of MONNs-1 (A, B) and Pd@MONNs-1 (C, D).
39936 | RSC Adv., 2016, 6, 39933–39939
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Table 1 Surface area and pore volume of the samples
with MONNs-1, the intensity of oxygen-containing groups
(C]O) in the Pd@MONNs-1 decreased obviously, which can be
attributed to the strong binding of oxygen-containing groups
with Pd NPs.22 These results conrmed that Pd NPs were
successfully anchored into the MONNs-1 by coordination to
carboxyl groups, and that the bonding interaction appeared
between Pd NPs and the C]O.
SBETa m2 gꢀ1
VMicrob cm3 gꢀ1
VTotal cm3 gꢀ1
c
Samples
MONNs-1
MONNs-4
Pd@MONNs-1
996
1036
850
0.11
0.13
0.08
1.74
1.83
1.31
a
BET specic surface area calculated from N2 adsorption isotherm.
Microporous volume obtained from t-plots method. Total pore
b
c
To evaluate the catalytic efficiency and stability of
Pd@MONNs, we selected the Suzuki–Miyaura coupling of p-
iodoanisole and phenylboronic acid as a model reaction (Fig. 4).
Considering the subject of environment-friendly reaction
processes, the model reaction was conducted in a relatively mild
aqueous media, K2CO3 (3 equiv.) as a base at 50 ꢁC in EtOH/H2O
(3 : 2) solutions. Fig. 4A shows kinetic proles of the catalytic
reactions, which were monitored by Gas Chromatography-Mass
Spectrometer (GC-MS). For Pd@MONNs-1, full conversion of
this reaction afforded within 5 h, while the use of Pd@MONNs-4
under the same conditions gave obviously rise to a complete
conversion within 3 h. The higher activity of Pd@MONNs-4 than
Pd@MONNs-1 probably results from the higher Pd contents of
Pd@MONNs-4. The increase of the carboxyl groups for MONNs-
4 supports maybe offer multidentate anchor sites to Pd NPs,
which shows better immobilization ability than one-fold
carboxyl groups.36,37 This observation also demonstrated that
Pd NPs were coordinated with carboxyl groups, which can be
tuned from the controllable synthesis of the materials. Recy-
clability is a considerable quality of heterogeneous catalyst,
which is necessary for industrial applications. Pd@MONNs-1,
as a general example, was successfully reused with no obvious
loss of activity aer eight runs and recovered by simple ltration
(Fig. 4B). The XPS and TEM image of Pd@MONNs-1 aer eight
runs revealed that the oxidation state of Pd in Pd@MONNs-1
presented 0 and +2 states and still remained a well-dispersed
form with slightly increased in size, as well as the morphology
of the support was well maintained (Fig. S6 and S7, ESI†). To
further investigate the stability of the catalyst, a brief hot
volume at P/P0 ¼ 0.995.
small and well dispersed in the support.34 The BET surface area
of Pd@MONNs-1 was reduced to 850 m2 gꢀ1 (Table 1), indi-
cating the successful immobilization of Pd NPs into the support
(Fig. 2C and D).24 Thermogravimetric analysis (TGA) analyses
showed Pd contents in Pd@MONNs-1 and Pd@MONNs-4 are 10
wt% and 13 wt%, which are in good agreement with ICP anal-
yses (9.7 wt% and 12.5 wt%) respectively (Fig. S5, ESI†). The
different Pd loading in Pd@MONNs-1 and Pd@MONNs-4 may
be owing to the different number of carboxyl groups.
To further investigate the coordination ability of carboxyl
groups with Pd NPs, Pd@MONNs-1 was selected for subsequent
X-ray photoelectron spectroscopy (XPS) characterizations. As
shown in Fig. 3, their wide-scan survey spectrum possess four
peaks centered at 284.1, 336.0, 533.4, and 561.5 eV, corre-
sponding to C 1s, Pd 3d, O 1s, and Pd 3p, respectively (Fig. 3A).
In the high-resolution XPS spectrum, the peaks of Pd 3d5/2 and
Pd 3d3/2 binding energies were observed slightly higher than the
bulk Pd 3d levels (3d5/2 ¼ 334.7 eV and 3d3/2 ¼ 340.3 eV), at
336.1 and 341.7 eV (Fig. 3B). The peaks shi positively sug-
gested that the catalyst contains almost Pd metal and an
interaction between the Pd NPs with the support.21,35 High-
resolution C 1s XPS spectrum of MONNs-1 and Pd@MONNs-1
were shown in Fig. 3C and D. The deconvolution of the C 1s
curve for Pd@MONNs-1 was almost the same as the curve of
MONNs-1, indicated that the load of Pd NPs on the supports has
not inuenced its stable structure. However, in comparison
Fig. 4 (A) Kinetic profiles of Pd@MONNs-1 and Pd@MONNs-4 in the
model Suzuki–Miyaura coupling reaction of p-iodoanisole and phe-
nylboronic acid. (B) Recycle performances of the typical Pd@MONNs-
1 catalyst in the model reaction. Reaction conditions: p-iodoanisole
(0.5 mmol), phenylboronic acid (0.75 mmol), K2CO3 (1.5 mmol), EtOH/
H2O (2 ml, 3 : 2 v/v), Pd@MONNs-1 (5 mg, 9.7 wt%) or Pd@MONNs-1
Fig. 3 XPS spectra of Pd@MONNs-1: (A) wide-scan survey, (B) curve fit (5 mg, 12.5 wt%), 50 ꢁC, under a N2 atmosphere. For recycle tests,
of Pd 3d, (C) curve fit of C 1s of MONNs-1 and (D) curve fit of C 1s of the reaction time was set at 5 h. The conversion yields were monitored
Pd@MONNs-1.
by GC.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 39933–39939 | 39937

View Article Online
RSC Advances
Paper
ltration test for Pd@MONNs-1 was adopted. When the procedure is similar to that of the Pd@MONNs-1 except with
p-iodoanisole conversion reached 55% (1 h), the solid linear PS polymers to replace the PGM-g-(PLA-b-PS) bottlebrush
Pd@MONNs-1 catalyst was ltered and removed from the copolymers. The BET surface area and total pore volume of the
reaction mixture. Then the mother liquor of the mixture was resulted Pd@MOP were 0.86 m2 gꢀ1 and 706 cm3 gꢀ1, respec-
allowed to react for another 4 h under the similar conditions. tively (Fig. S8A and S8B, ESI†). However, no pronounced
No signicant change in conversion was observed, indicating hysteresis loop observed in the desorption isotherm indicated
that the catalyst active phase was not the dissolved homoge- that the comparative Pd@MOP catalyst mainly consisted of
neous Pd atoms leached from the support. ICP analyses of the a microporous structure (Fig. S8A, ESI†). TEM image also
Pd content in the ltrate demonstrated nearly no Pd leaching showed that Pd@MOP has a disordered structure and the Pd
(0.7 ppm) occurred for the Pd@MONNs-1, which agrees with the species tend to agglomeration on the MOP surface with
result of the hot ltration test.
a non-uniform distribution (Fig. S8C, ESI†). Compared with
To expand the scope of practical applications, the Pd@MONNs, the full conversion of the model reaction with
Pd@MONNs-1 was selected as a typical catalyst for further Pd@MOP catalyst for the rst time needs 12 h under the same
examined in Suzuki–Miyaura coupling reaction of various types reaction conditions. This might be attributed to the dominated
of aryl halide and aryboronic acids. As shown in Table 2, when micropore structure of Pd@MOP, which may limit the diffusion
the coupling reaction of p-iodoanisole and phenylboronic acid of the reactants and products, leading to undesirable catalytic
was carried out in the presence of Pd@MONNs-1 (5 mg, 9.7 activity. In addition, the poor distribution of the bulk Pd species
wt%) at 50 ꢁC, 99% GC yield of product was obtained in 5 h also reduced the recyclability of Pd@MOP, which showed an
(entry 1). No corresponding product was formed in the control obvious loss of activity aer the second runs. Therefore, the
experiment of the above reaction in the presence of MONNs-1 Pd@MONNs, with unique microporous nanotube networks
only (entry 2). When various types of aryl bromides and aryl- structure and well-dispersed Pd NPs, have shown better cata-
boronic acids were used as substrates, the corresponding lytic activity and stability than the obtained disordered micro-
products were obtained over 85% (entry 3–7). These results porous Pd@MOP catalysts.
suggest that the electronic effects of reactants have no obvious
inuence on the desired coupling products obtained under the
given reaction conditions. As we all know, the use of aryl chlo-
4. Conclusions
rides reactants is a particular challenge for the coupling reac- In conclusion, we reported an efficient way to prepare Pd NPs
tion due to their inactivated characters. It was noteworthy that catalysts anchored into the carboxyl-containing microporous
the couplings of various arylboronic acids with chlorobenzene organic nanotube networks (COOH-MONNs) by in situ thermal
easily converted with 84–89% yields at 80 ꢁC (entry 8, 9), which decomposition of Pd(OAc)2. The high surface areas (996 m2
showed a remarkable universality for the Suzuki–Miyaura gꢀ1), large pore volume (1.74 cm3 gꢀ1) and various pore sizes
coupling reactions with our new catalysts.
distributions of supports ensured the high dispersion of Pd NPs
To prove the unique advantage of MONNs as catalyst active sites as well as improved the diffusion of reactants and
support, a new Pd catalyst (Pd@MOP) with dominated micro- products. In addition, the carboxyl groups served as anchors
pore structure was synthesized for comparison. Its synthetic can be directly generated and tuned from the controllable
synthesis of the materials, which offer excellent regulation to
the contents of Pd NPs. Pd@MONNs, with unique structure,
exhibited a high catalytic activity and excellent stability during
the catalytic process of the Suzuki–Miyaura coupling reactions.
Encouraged by the excellent catalytic performance of
Table 2 Pd@MONNs-1 catalyzed Suxuki–Miyaura reactions of various
substratesa
Pd@MONNs, our further research will be devoted to the prep-
aration of high-efficiency catalysts based on MONNs for more
practical catalytic applications.
Entry
R1
X
R2
Time (h)
Yieldb (%)
Acknowledgements
1
CH3O
CH3O
CH3O
H
CH3CO
CN
H
H
H
I
I
H
H
H
H
H
H
CH3O
H
CH3O
5
5
5
5
10
5
5
5
10
>99
0
This work is supported by National Natural Science Foundation
of China grant 51273066, 21574042, Shanghai Pujiang Program
grant 13PJ1402300.
2c
3
Br
Br
Br
Br
Br
CI
CI
91
89
86
90
97
84e
89e
4
5
6
Notes and references
7
8d
9d
1 A. Balanta, C. Godard and C. Claver, Chem. Soc. Rev., 2011,
40, 4973.
a
Reaction conditions: aryl halides (0.5 mmol), arylboronic acids (0.75
mmol), K2CO3 (1.5 mmol), EtOH/H2O (2 ml, 3 : 2 v/v), Pd@MONNs-1
´
2 A. R. D. Molnar, Chem. Rev., 2011, 111, 2251–2320.
b
c
(5 mg, 9.7 wt%), 50 ꢁC, under a N2 atmosphere. GC yield. Only
3 L. Yin and J. Liebscher, Chem. Rev., 2007, 107, 133–173.
4 N. Huang, Y. Xu and D. Jiang, Sci. Rep., 2014, 4, 7228.
MONNs-1 without Pd. d 80 ꢁC. e Isolated yield.
39938 | RSC Adv., 2016, 6, 39933–39939
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
5 P. Wang, H. Zhu, M. Liu, J. Niu, B. Yuan, R. Li and J. Ma, RSC 22 Y. Wang, Q. He, J. Guo, H. Wei, K. Ding, H. Lin, S. Bhana,
Adv., 2014, 4, 28922.
X. Huang, Z. Luo, T. D. Shen, S. Wei and Z. Guo,
ChemElectroChem, 2015, 2, 559–570.
ˇ
ˇ
ˇ
ˇ
6 M. Opanasenko, P. Stepnicka and J. Cejka, RSC Adv., 2014, 4,
65137–65162.
23 Y. Zhang and S. N. Riduan, Chem. Soc. Rev., 2012, 41, 2083–
2094.
24 L. Li, H. Zhao, J. Wang and R. Wang, ACS Nano, 2014, 8,
5352–5364.
25 Q. Sun, Z. Dai, X. Meng and F.-S. Xiao, Chem. Soc. Rev., 2015,
44, 6018–6034.
26 J. T. Lai, D. Filla and R. Shea, Macromolecules, 2002, 35,
6754–6756.
27 M. Benaglia, E. Rizzardo, A. Alberti and M. Guerra,
Macromolecules, 2005, 38, 3129–3140.
28 S. Bhunia, B. Banerjee and A. Bhaumik, Chem. Commun.,
2015, 51, 5020–5023.
7 B. M. Weckhuysen and J. Yu, Chem. Soc. Rev., 2015, 44, 7022–
7024.
8 C. Zhao, Y. Yang, Z. Wu, M. Field, X.-y. Fang, N. Burke and
K. Chiang, J. Mater. Chem. A, 2014, 2, 19903–19913.
9 L. Ren, L. Yang, P. Yu, Y. Wang and L. Mao, ACS Appl. Mater.
Interfaces, 2013, 5, 11471–11478.
10 H. Yuan, H. Liu, B. Zhang, L. Zhang, H. Wang and D. S. Su,
Phys. Chem. Chem. Phys., 2014, 16, 11178.
11 S. Ogasawara and S. Kato, J. Am. Chem. Soc., 2010, 132, 4608–
4613.
12 J. Guerra and M. A. Herrero, Nanoscale, 2010, 2, 1390.
13 B. Yuan, Y. Pan, Y. Li, B. Yin and H. Jiang, Angew. Chem., Int. 29 J. Rzayev and M. A. Hillmyer, J. Am. Chem. Soc., 2005, 127,
Ed., 2010, 49, 4054–4058. 13373–13379.
14 P. Xu, X. Han, B. Zhang, Y. Du and H.-L. Wang, Chem. Soc. 30 R. T. Woodward, L. A. Stevens, R. Dawson,
Rev., 2014, 43, 1349–1360.
15 S.-I. Yamamoto, H. Kinoshita, H. Hashimoto and Y. Nishina,
Nanoscale, 2014, 6, 6501.
16 C.-Y. Chen, J.-K. Chang, W.-T. Tsai and C.-H. Hung, J. Mater.
Chem., 2011, 21, 19063.
17 J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang and C.-Y. Su, Chem.
Soc. Rev., 2014, 43, 6011–6061.
18 H. Bai, M. Han, Y. Du, J. Bao and Z. Dai, Chem. Commun.,
2010, 46, 1739.
19 Y. Lin, K. A. Watson, M. J. Fallbach, S. Ghose, J. G. Smith Jr,
M. Vijayaraghavan, T. Hasell, I. P. Silverwood, A. V. Ewing,
T. Ratvijitvech, J. D. Exley, S. Y. Chong, F. Blanc,
D. J. Adams, S. G. Kazarian, C. E. Snape, T. C. Drage and
A. I. Cooper, J. Am. Chem. Soc., 2014, 136, 9028–9035.
31 K. Huang and J. Rzayev, J. Am. Chem. Soc., 2009, 131, 6880–
6885.
32 K. Huang, M. Johnson and J. Rzayev, ACS Macro Lett., 2012,
1, 892–895.
33 K. Huang and J. Rzayev, J. Am. Chem. Soc., 2011, 133, 16726–
16729.
D. M. Delozier, W. Cao, R. E. Crooks and J. W. Connell, ACS 34 Q. L. Zhu, N. Tsumori and Q. Xu, J. Am. Chem. Soc., 2015, 137,
Nano, 2009, 3, 871–884. 11743–11748.
20 S. Santra, K. Dhara, P. Ranjan, P. Bera, J. Dash and 35 S. Santra, P. K. Hota, R. Bhattacharyya, P. Bera, P. Ghosh and
S. K. Mandal, Green Chem., 2011, 13, 3238. S. K. Mandal, ACS Catal., 2013, 3, 2776–2789.
21 S. Santra, P. Ranjan, P. Bera, P. Ghosh and S. K. Mandal, RSC 36 C. N. R. Rao, S. Natarajan and R. Vaidhyanathan, Angew.
Adv., 2012, 2, 7523.
Chem., Int. Ed., 2004, 43, 1466–1496.
37 H. Choudhary, S. Nishimura and K. Ebitani, J. Mater. Chem.
A, 2014, 2, 18687–18696.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 39933–39939 | 39939