Organic ligands incorporated hypercrosslinked microporous organic nanotube frameworks for accelerating mass transfer in efficient heterogeneous catalysis

Article abstract of DOI:10.1016/j.apcata.2017.05.005

Microporous organic polymers (MOP) usually have dominated micropores smaller than 2?nm, which may restrict their performance in the mass transfer processes. Adding mesopores into microporous materials to form hierarchical structure has been recognized as a promising route to eliminate their transport limitations and further improve their value in applications. Here we report a straightforward method for the synthesis of organic ligands incorporated hypercrosslinked microporous organic nanotube frameworks (O-HMONFs) via Friedel-Crafts hyper-crosslinking reaction of small aromatic organic ligands with core-shell bottlebrush copolymers as platforms for heterogeneous catalysts. In particular, because of the mesopores produced by the core degradation, O-HMONFs-based catalysts showed more comparable activities than the corresponding disordered MOP-based catalyst and homogeneous molecular catalyst under similar conditions. More importantly, this method might be suitable for various aromatic organic ligands and could be used as a general approach to produce a variety of functional microporous organic nanotube frameworks.

Full text of DOI:10.1016/j.apcata.2017.05.005

Accepted Manuscript
Title: Organic ligands incorporated hypercrosslinked
microporous organic nanotube frameworks for accelerating
mass transfer in efficient heterogeneous catalysis
Authors: Yang Xu, Tianqi Wang, Zidong He, Minghong Zhou,
Wei Yu, Buying Shi, Kun Huang
PII:
S0926-860X(17)30195-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.05.005
APCATA 16227
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
24-2-2017
1-5-2017
6-5-2017
Please cite this article as: Yang Xu, Tianqi Wang, Zidong He, Minghong Zhou, Wei Yu,
Buying Shi, Kun Huang, Organic ligands incorporated hypercrosslinked microporous
organic nanotube frameworks for accelerating mass transfer in efficient heterogeneous
catalysis, Applied Catalysis A, Generalhttp://dx.doi.org/10.1016/j.apcata.2017.05.005
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Organic ligands incorporated hypercrosslinked microporous
organic nanotube frameworks for accelerating mass transfer
in efficient heterogeneous catalysis
Yang Xu, Tianqi Wang, Zidong He, Minghong Zhou, Wei Yu, Buying Shi and Kun Huang*
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
Graphical Abstract
A straightforward method for the synthesis of organic ligands incorporated
hypercrosslinked microporous organic nanotube frameworks (O-HMONFs)
via Friedel-Crafts hypercrosslinking reaction of small organic ligands with
core-shell bottlebrush copolymers as platforms for heterogeneous catalysts
is reported for the first time.
1

Abstract
Microporous organic polymers (MOP) usually have dominated micropores
smaller than 2 nm, which may restrict their performance in the mass transfer processes.
Adding mesopores into microporous materials to form hierarchical structure has been
recognized as a promising route to eliminate their transport limitations and further
improve their value in applications. Here we report a straightforward method for the
synthesis of organic ligands incorporated hypercrosslinked microporous organic
nanotube frameworks (O-HMONFs) via Friedel-Crafts hyper-crosslinking reaction of
small aromatic organic ligands with core-shell bottlebrush copolymers as platforms for
heterogeneous catalysts. In particular, because of the mesopores produced by the core
degradation, O-HMONFs-based catalysts showed more comparable activities than the
corresponding disordered MOP-based catalyst and homogeneous molecular catalyst
under similar conditions. More importantly, this method might be suitable for various
aromatic organic ligands and could be used as a general approach to produce a variety
of functional microporous organic nanotube frameworks.
Keywords: Microporous organic nanotube frameworks • Hyper-crosslinking •
Aromatic organic ligands • Hierarchical structure • Heterogeneous catalysts
Introduction
Small organic ligands such as benzyl amine, providing excellent coordination and
stabilization for metal-catalysts, are of critical importance in homogeneous catalysis.[1-
4
] Despite fruitful advancements have been made in homogeneous catalytic study, the
difficulties in separation and recycling still restrict their more extensive applications.
To solve this problem, the heterogenization of metal complexes has been expected,
where zeolites and silicas are the most commonly used supports owe to their high
surface areas and robust porous structure.[5-7] However, the drawback of their
2

chemical nature limits these traditional porous materials extensive applications via
chemical modification processes. Recently, many kinds of microporous organic
polymers (MOPs) with large surface area, low skeletal density and high chemical and
physical stabilities have been developed based on several types of reactions such as
condensation reaction, ‘Click’ chemistry and Friedel-Crafts reaction.[8-10] More
importantly, these MOPs can be used as versatile platforms for incorporating metal-
organic ligands into the highly stable and recyclable heterogeneous catalysts by taking
advantage of their porosity and the ability to tune their compositions and properties at
the molecular level. Among these MOPs, hypercrosslinked microporous organic
polymers via Friedel-Crafts reaction have received considerable interest due to their
ease of preparation, low cost and high stability, which possess potential advantage for
large industrial applications.[11] In recent years, Han[12] and Tan[8, 13] groups have
demonstrated that Friedel-Crafts reaction is a straightforward method to prepare
hypercrosslinked porous polymers with a variety of useful applications in catalysis and
gas adsorption. It is worth noting that most research is focused on amorphous
hypercrosslinked microporous materials with dominated microporous structure.
Although, the existence of strong confinements giving rise to the prosperous material
properties, microporous structure also simultaneously severely affects their
performance in the mass transfer processes.[14, 15] It has been recognized that adding
mesopore into microporous materials to form hierarchical structure is a promising route
to eliminate their transport limitations and further improve their value in catalysis
applications.[15] However, exploring economical and straightforward methods for
synthesizing MOPs with hierarchically porous structure are still full of challenges.
Bottlebrush copolymers are a class of shape-persistent macromolecules with highly
branched comblike architecture that adopt a cylindrical shape in solutions.[16] As we
and others have demonstrated that cylindrical shape and dimensions of bottlebrush
copolymers in solution are preserved during organic nanotube fabrication, which is
achieved by selective shell cross-linking and core degradation.[16-18] Recently, our
group have developed a novel method to prepare a kind of microporous organic
nanotube frameworks for catalyst supports from bottlebrush copolymers templating.
3

Such materials possess a hierarchically porous structure that micropore was within
cross-linking shell layer, while the mesopore was produced by selectively degraded
polylactide (PLA) core layer.[18-20] However, the design and synthesis of functional
monomers to introduce the required materials still inconvenient and expensive. To
render this method more straightforward and economical, herein, we report the
synthesis of organic ligands incorporated hypercrosslinked microporous organic
nanotube frameworks (O-HMONFs) via one-step Friedel-Crafts hyper-crosslinking
reaction of small aromatic organic ligands with core-shell bottlebrush copolymers as
platforms for heterogeneous catalysts. Benzyl amine, 2,2’-bipyridine and
triphenylphosphine as typical aromatic organic ligands are selected to be incorporated
into the materials to support Ru, Cu and Pd metal catalysts, respectively. Moreover, this
work demonstrates that O-HMONFs-based catalyst exhibited higher catalytic activity
than the corresponding disordered MOP-based catalyst and homogeneous molecular
catalysts under similar conditions. To the best of our knowledge, this is the first time to
report a straightforward and economical way to prepare an organic ligands incorporated
hypercrosslinked microporous organic nanotube frameworks with hierarchically
porous structure.
Results and discussion
The synthetic strategy of organic ligands incorporated hypercrosslinked microporous
organic nanotube frameworks (O-HMONFs) as platforms for heterogeneous catalyst
was depicted in Scheme 1. First, the core-shell bottlebrush polymer precursor
[
poly(GM-g-LA-g-S)] was synthesized via a “grafting-from” approach with the help of
ring-opening and reversible addition-fragmentation chain transfer (RAFT)
polymerizations as described previously.[18, 20] Next, the bottlebrush copolymer
precursors and organic ligands were dissolved in dry dichloroethane (DCE) under
constant stirring and subsequently treated through Friedel-Crafts hypercrosslinking
reaction of aromatic groups by using formaldehyde dimethyl acetal (FDA) as a cross-
4

linker and anhydrous FeCl
3
as a promoter. Interestingly, FeCl here not only acts as a
3
catalyst for the Friedel-Crafts alkylation reaction, but also supplies a Lewis acidic
hydrolysis environment for the efficient removal of the polylactide (PLA) core during
the preparation. The resulting catalyst supports were denoted as Ba-HMONFs (benzyl
amine), Bp-HMONFs (2,2’-bipyridine) and Tp-HMONFs (triphenylphosphine),
according to the organic ligands used during the synthesis. Notably, the incorporated
organic ligands play the roles as both supports and ligands simultaneously. Finally, Ru,
Cu and Pd metal species could be successfully loaded into Ba-HMONFs, Bp-HMONFs
and Tp-HMONFs to form Ru-HMONFs, Cu-HMONFs and Pd-HMONFs catalysts,
respectively. More experimental details can be found in the experimental section.
For a typical example of O-HMONFs supports, Ba-HMONFs was selected for
subsequent studies in detail. The FT-IR spectra of poly(GM-g-LA-g-S), Ba-HMONFs
and Ru-HMONFs were shown in Figure 1A. The absorption bands centered at 3420
-1
-1
cm and 1299 cm can be ascribed for the N-H and C-N stretching vibrations present
in the Ba-HMONFs and Ru-HMONFs.[8, 21] The stretching vibrations of the aromatic
-1
-1
C=C bonds at 1606 cm and 1502 cm enhanced obviously, suggesting to the extra
addition of benzyl amine ligands. The PLA carbonyl stretch peak of poly(GM-g-LA-g-
-1
S) (1758 cm ) completely disappeared after Friedel-Crafts hyper-crosslinking reaction,
indicating that completely removal of the PLA core layer.[18] The TEM image of Ru-
HMONFs revealed that the cylindrical network morphology was maintained and the
hollow tubular units also could be clearly observed, which was in good agreement with
that of the FT-IR spectra (Figure 1B). The average length of the obtained tubular units
was measured to be 35 ± 5 nm, whereas the internal cavity average diameter was about
5
nm. These parameters can be tuned during core-shell bottlebrush copolymer synthesis
at the molecular level.[18] The inserted HR-TEM image also revealed that the Ru NPs
of Ru-HMONFs have a spherical morphology and average dimension of 2.5 nm. In the
o
XRD pattern of Ru-HMONFs, only a weak peak at 42.5 corresponding to Ru (002)
planes was detected, further confirming the formation of very small Ru nanoparticles
5

(NPs) (Figure 2d).[21] In this case, the Ru NPs size is small enough, so that they can
be easily packaged in the shell layer interior of nanotubes. Figure 1C shows N sorption
2
isotherms of the Ba-HMONFs, which give the curve of type-I plus type-IV. Asharp rise
at relative pressure less than 0.01 is due to the filling of micropores, while a pronounced
hysteresis loop observed in the desorption isotherm indicated the existence of accessible
mesopores in the support. Correspondingly, the pore sizes are distributed at 1.38, 2.61
and 4.90 nm (Figure 1C, inset). These results suggest that Ba-HMONFs has hierarchical
porosity structure with a uniform mesopores within hypercrosslinked materials. The
BET surface area and total pore volume of the Ba-HMONFs are as high as 1193 m2g-1
3
-1
and 2.42 cm g , which are favorable for the dispersion of catalytically active sites
Table 1). After loading Ru metals, the BET surface area and pore volume of Ru-
(
2
-1
3 -1
HMONFs (1101 m g and 2.29 cm g , respectively) were smaller than those of Ba-
HMONFs, but the pore size distribution of Ru-HMONFs remained almost unchanged
after supporting the Ru NPs, indicating that Ba-HMONFs were stable during the
reduction process (Figure S1). The decrease in specific surface area can be attributed
to the partial pore filling after Ru loading. Furthermore, TG analysis also shown that
Ba-HMONFs had superior thermal stability, giving the decomposition temperature
o
starting at 400 C (Figure 1D).
XPS spectroscopy was employed to elucidate the oxidation state of Ru NPs and to
show that the Ru NPs were coordinated by the amine groups of Ru-HMONFs. The XPS
survey spectra reveals that, in addition to binding energy peaks corresponding to N 1s
and C 1s, a new obvious peak is generated that is attributable to the Ru 3p binding
energy (Figure 2a). The binding energy peak of N 1s centered at 399.0 and 402.0 eV
signifies the presence of amine groups in the Ru-HMONFs (Figure. 2b). In comparison
with the free amine groups (399.0 eV), the positive shift in the binding energy for N 1s
to higher energy (402.0 eV) can be attributed to the Ru NPs are coordinated strongly
by the nitrogen atoms of the amino groups of Ba-HMONFs.[21] In Figure 2c, the
binding energy for Ru 3p1/2 and Ru 3p3/2 in Ru-HMONFs show peaks at 484.8 and 463.2
6

eV, respectively, indicated that Ru NPs are present in zero oxidation state. The positive
shift in the binding energy of Ru 3p compared with RuCl also can be attributed to the
3
strong interaction between the Ru NPs and Ba-HMONFs, which suggests electron
transfer from the metal to organic ligands. These results further confirmed that Ru NPs
are successfully immobilized on the Ba-HMONFs by coordination to amino groups
rather than physical adsorption of RuCl on the surface.
3
To test the catalytic property of the synthesized Ru-HMONFs catalyst, standard p-
nitrophenol (p-NP) reduction by NaBH was conducted at first. The reduction of p-NP
in the presence of Ru-HMONFs has been monitored by UV-Vis spectroscopy (Figure
a). During the reduction reaction, the yellowish colored solution of p-NP disappeared
4
3
with time and the decoloration process can be attributed to the formation p-
aminophenol (p-AP) as a result of the reduction reaction between p-NP and the Ru-
HMONFs catalyst. As shown in Figure 3a, the characteristic absorption peak of p-NP
at 400 nm gradually decreased, while a new absorption band which appeared at 300 nm
corresponding to the p-aminophenol (p-AP) product. In order to check out the catalytic
nature of Ru-HMONFs catalyst, the reduction of p-NP only in the presence of Ba-
HMONFs was performed. It is obvious to see in Figure S2 that p-NP characteristic
absorption peak intensity was unchanged even after 600 s. From a practical point of
view, the reusability of the catalyst is an important factor for heterogeneous systems.
As shown in Figure 3c, the catalytic activity of Ru-HMONFs remains almost
unchanged even after 13 runs. Although the porous properties (surface area, pore size
distribution and pore volume) of Ru- HMONFs after 13 runs are quite smaller than the
fresh one (Table 1), the TEM image of Ru-HMONFs does not exhibit observable
-3
change (Figure S3). The Ru content of the reused Ru-HMONFs is 1.6×10 wt.%,
indicating nearly no Ru leaching occurred for the Ru-HMONFs. These results indicated
that Ru-HMONFs had a good stability, which can be contributed to the highly cross-
linked microporous nanotube structure that confines Ru inside each micropore alone
and prevents the aggregation of Ru NPs, as well as loss of Ru.
7

To further investigate and compare the catalytic activities of Ru-HMONFs with
different structure, an amorphous hypercrosslinked microporous material supported Ru
NPs (Ru-MOP) through Friedel-Crafts alkylation of benzene and benzyl amine by
using the similar method as Ru-HMONFs was prepared. The BET surface areas and
pore size distributions of the pure Ba-MOP and Ru-MOP were depicted in the table 1
(also shown in Figure S4). It is worth noting that such material also possesses high
surface area as Ru-HMONFs, however, this amorphous hypercrosslinked microporous
material consists of dominated microporous structure. Under the similar condition of
reduction, it is observed that all the synthesised Ru catalysts catalysed the reduction of
p-NP efficiently; however, the rate of the reduction significantly varies between the
different catalysts (Figure. 3b). The Ru-HMONFs have shown better activity than the
corresponding Ru-MOP indicating the importance of the structure design in the
catalysis system. The rate constants (k) for the reduction of p-NP by using different Ru
catalysts were obtained from the slope of the plot between ln(A
time according to the pseudo-first order kinetics rate constant equation: ln(A
where A is the absorbance at time t and A is the absorbance at time zero. The reaction
t
/A ) versus reaction
0
t
/A ) = -kt,
0
t
0
-1
-1
rate constant was calculated to be 0.09788 s (Ru-HMONFs) and 0.04481 s (Ru-
MOP), which revealed that the Ru-HMONFs exhibit superior catalytic performance
than Ru-MOP for the reduction reaction. This might be attributed to the internal
mesoporous channels in the nanotube frameworks, which will accelerate the mass
transfer in Ru-HMONFs and enhance the catalytic rate towards the reduction of p-NP.
In particular, Ru-HMONFs catalyst also showed more comparable activities than the
corresponding homogeneous molecular catalyst (RuCl
3
-
) under similar conditions
1
(Figure. S5), which has a lower rate constant of 0.01478 s .
To further demonstrate the importance of pore structure in support for heterogeneous
catalysis, we next extend the catalytic activity of different Ru catalysts towards the
reduction of Crystal violet (CV) dye, which possesses larger molecular size than p-NP.
8

At the same time, the removal of CV from wastewater is also a major environmental
problem and poses a health risk to humans. Figure 4a represents the reduction of CV
dyes at 581 nm with the formation of leuco crystal violet (LCV) at 260 nm in the
presence of Ru-HMONFs catalyst. During the reduction reaction, the solution color
changed from dark violet of CV to colorless of LCV. The rate constants for the reduction
-1
-1
of CV were calculated to be 0.4740 min (Ru-HMONFs) and 0.1437 min (Ru-MOP),
which also revealed that Ru-HMONFs show a better catalytic performance than Ru-
MOP under the similar condition (Figure 4b). On the basis of the above results, the
faster reaction rate of Ru-HMONFs can be owed to the internal mesoporous channels
within the microporous nanotube frameworks, which made the reactants more
accessible on the catalyst surface and the products easy to leave. According to the
2
2
previous literature reports, a cartoon description of the plausible reaction pathway on
the Ru-HMONFs for the reduction is shown in Scheme 2. For materials containing
dominating micropores that sizes are comparable to the molecular sizes of guest species,
the diffusion of guest molecular is predominantly determined by the guest-host
interactions. Obviously, the tighter the confinement of micropores, the slower is the
motion of reactants. At the same time, the diffusivities may also be determined by the
guest concentration in the pore spaces. However, in this work, the reduction rate of the
Ru-HMONFs is obviously better than that of Ru-MOP under the similar catalytic
conditions (same guest concentration, etc). These results proved that the transport
enhancement through networks of Ru-HMONFs might be benefited from mesopore’
accessibility for mass transfer.
Similarly, Cu-HMONFs and Pd-HMONFs materials from Bp-HMONFs and Tp-
HMONFs supports are also carefully characterized, as shown in Figure S6-S7. They
also possess hierarchical pore structures, high surface areas and large pore volume,
which can be served as efficient catalysts. Herein, the Cu-HMONFs exhibits
comparable catalytic performance with the corresponding Cu/bpy homogeneous
catalyst in selective oxidations of primary alcohols to aldehydes. In addition, the Cu-
HMONFs can be easily reused at least 8 times without significant loss in catalytic
9

activities (Table 2). The Pd-HMONFs also proved to be a superb catalyst for the model
Suzuki cross-coupling reaction of chlorobenzene with phenylboronic acid, which
shows much better activity than its homogeneous counterparts such as PdCl
Pd(PPh under similar conditions (Table 3).
2
and
)
3 4
Conclusions
Experimental Section
αα
1
0

α
2
20
O
2
20
2
20
) H O+
S
SR
O
O
O
1
3
1) Cl
O
O
O
O
S
2) DBU / D, L -lactide
2)
O
O
O
O
O
O
O
O
O
O
O
O
O
24
24
24
24
O
H
H
S
S
C12H25
X
=
S
9
6
96
X
X
11



1
2

1
3

Acknowledgements
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[
[
[
[
[
[
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1
5

Figure 1. (A) IR spectra of (a) poly(GM-g-LA-g-S), (b) Ba-HMONFs and (c) Ru-
HMONFs, (B) TEM image of Ru-HMONFs and HR-TEM of Ru NPs (inset), (C) N
2
sorption isotherms and pore size distribution of Ba-HMONFs, (D) TG curve of Ba-
HMONFs.
1
6

Figure 2. (a) XPS survey spectra, (b) N 1s and (c) Ru 3p spectra of Ru-HMONFs. And
d) wide-angle powder XRD pattern of Ru-HMONFs.
(
1
7

Figure 3. (a) Absorption spectra of the p-NP and NaBH
4
solution mixture after 25 s in
the presence of Ru-HMONFs. (b) Plots of ln(A /A ) vs. time (s) for the reduction of p-
t
0
NP catalyzed by Ru-HMONFs and Ru-MOP. (c) Recycling tests with Ru-HMONFs. (d)
structural illustration of p-NP molecules in reduction reactions.
1
8

Figure 4. (a) Absorption spectra of the CV and NaBH4 solution mixture after 10 min
in the presence of Ru-HMONFs. (b) Plots of ln(A /A ) vs. time (s) for the reduction of
t
0
CV catalyzed by Ru-HMONFs and Ru-MOP. (c) Structural illustration of CV dye in
reduction reactions.
1
9

Scheme 2. Acartoon description of the proposed reaction pathway on the Ru-HMONFs
Path 1) and Ru-MOP (Path 2) for the reduction reactions.
(
2
0

total m2g-1
a
S
micro m2g-1
b
S
meso m2g-1
c
V
total cm3g-1
d
V
micro cm3g-1 Metal wt.%
e
f
Samples
S
Ba-HMONFs
Ru-HMONFs
1193
1101
306
321
185
621
602
302
129
887
780
598
280
247
538
510
2.42
2.29
1.26
0.64
0.62
1.34
1.39
0.14
0.13
0.09
0.27
0.26
0.13
0.06
-
1.8×10-3
1.6×10-3
-
Ru-HMONFs g 783
Ba-MOP
901
849
840
639
Ru-MOP
1.2×10-3
1.6×10-2
5.0×10-3
Cu-HMONFs
Pd-HMONFs
a)
Surface area calculated from the nitrogen adsorption isotherm using the BET method; b) micropore surface area; c)
mesopore surface area; d) total pore volume at P/P = 0.99; e) micropore volume; f) metal (Ru, Cu or Pd) content
0
obtained from ICP analyses; g) physical properties of Ru-HMONFs after 13 times towards the reduction of p-
nitrophenol.
2
1

R
CH OH
R
CHO
2
b
Entry
R
Time (h) Yield (%)
Yield (%)
Cu-HMONFs CuBr
2
1
H
10
8
99
99
99
85
77
82
85
-
2
CH
CH
H
3
3
3
O
7
4c
10
a)
Reaction conditions: primary alcohol (0.5 mmol), catalyst (10 mg, 0.5 mol% of Cu), TEMPO (1.0 mol%), KOH
(
2.0 mol%), CH
3
CN (1.0 mL), H
2
O (0.5 mL), 1 atm of air, and RT. b) 2,2-Bipyridyl (0.5 mol%), CuBr
(0.5 mol%).
2
c)
Recycled 8 times.
CI
B(OH)2
Entry
Pd catalyst
Time (h)
Yield (%)b
1
PdCl
2
3
3
3
3
11
4
2
Pd(PPh
3 4
)
3
Pd-HMONFs
Pd-HMONFs
99
98
4c
a)
2 3 2
Reaction conditions: chlorobenzene (0.5 mmol), phenylboronic acid (0.75 mmol), K CO (1.5 mmol), EtOH/H O
-
3
o
(
2
2 ml, 3:2 v/v), Pd-HMONFs 20 mg, 5.0×10 wt%, 80 C, under a N atmosphere.
b)
Isolated yield.
c)
Recycled 5 times.
2
2