Amino- and sulfo-bifunctionalized hyper-crosslinked organic nanotube frameworks as efficient catalysts for one-pot cascade reactions

Article abstract of DOI:10.1039/c8nj04514c

In this paper, novel amino- and sulfo-bifunctionalized hyper-crosslinked organic nanotube frameworks (H-ONTFs) were directly prepared by templating core-shell bottlebrush copolymers via a Friedel-Crafts (F-C) alkylation reaction, in which the acidic sites were anchored into the wall of nanotubes and basic sites were introduced onto the inner layer wall of nanotubes through a rational molecular design strategy. The resulting acid-base bifunctionalized H-ONTFs showed excellent catalytic activities for one-pot deacetalization-Knoevenagel cascade reactions due to their open-ended structure, large surface area and good multi-porosity interconnectivity.

Full text of DOI:10.1039/c8nj04514c

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Amino- and sulfo-bifunctionalized hyper-
crosslinked organic nanotube frameworks as
Cite this:DOI: 10.1039/c8nj04514c
efficient catalysts for one-pot cascade reactions†
Guojie Meng, Shengguang Gao, Ying Liu, Li Zhang, Chunmei Song* and
Kun Huang
*
In this paper, novel amino- and sulfo-bifunctionalized hyper-crosslinked organic nanotube frameworks
(H-ONTFs) were directly prepared by templating core–shell bottlebrush copolymers via a Friedel–Crafts
(F–C) alkylation reaction, in which the acidic sites were anchored into the wall of nanotubes and basic
sites were introduced onto the inner layer wall of nanotubes through a rational molecular design strategy.
The resulting acid–base bifunctionalized H-ONTFs showed excellent catalytic activities for one-pot
deacetalization–Knoevenagel cascade reactions due to their open-ended structure, large surface area and
good multi-porosity interconnectivity.
Received 5th September 2018,
Accepted 23rd December 2018
DOI: 10.1039/c8nj04514c
rsc.li/njc
of monofunctionalized acid- and base-PAFs by
functionalization method, which show great catalytic efficiency
a post-
Introduction
Cascade reactions, which involve several reactions to give the for one-pot reactions.16,17 Ma et al. also developed a similar
final product in one pot, have been actively used in modern strategy to incorporate two antagonistic sites of strong acid and
organic synthesis and sustainable chemistry in the last few strong base into highly porous and highly robust PAFs via
years. This kind of reaction has a lot of advantages such as a stepwise post-synthetic modification.18 Recently, Tan et al.
simplified purification process, improved overall efficiency and prepared a new acid–base bifunctional hollow hyper-cross-
decreased volume of the solvent.1–6 At present, the most widely linked nanosphere (HCP) catalyst (HCP–A–B) by an easily
applied and attractive approach used for the realization of cascade accessible Friedel–Crafts reaction-based approach, in which
reactions is spatially site-isolating two antagonist functions in the the hollow spherical structure not only offers a combination
catalyst system. For example, many works have recently demon- of two antagonistic microenvironments but also enables rapid
strated the combination of acid and base antagonist functions in diffusion of the substrate.19 In addition, Shilun Qiu et al. also
soluble polymers or solid supports for homogeneous or hetero- developed a new strategy for the construction of 3D covalent
geneous cascade reactions owing to their wide range of applications organic frameworks (COFs) comprising both acidic and basic
in various catalysis reactions.6–10 Among these methods, hosting sites for acid–base catalyzed one-pot cascade reactions.20
acid and base catalysts in a solid matrix, such as silica, clay, and Although great progress has been achieved in recent years in
polymer, etc.11–13, has received much attention due to easy recycling the development of organic porous materials with antagonist
of the catalyst and simple operation procedures.
functional groups, it is still highly desirable to develop a novel
Porous organic polymers (POPs), as a solid matrix, are widely approach for the preparation of acid–base functionalized cata-
used to host catalysts due to their high surface area, strong stability lysts with enhanced reactivity and stability.
and tunable structure.14,15 In particular, a variety of porous organic
Recently, our group developed a new route for the prepara-
polymers with acid–base sites have been reported as bifunctional tion of microporous organic nanotube networks (MONNs) as
catalysts for one-pot cascade reactions. For example, Corma heterogeneous catalyst supports by hyper-crosslinking core–
and Felix Sanchez et al. synthesized acid–base bifunctional shell bottlebrush copolymers21,22 in which the functionaliza-
porous polymeric aromatic frameworks (PAFs) and a mixture tion and hierarchically porous structures of MONNs endow
them with significant potential for creating tailored environ-
´
´
ments for catalysis. For example, the outer hyper cross-linked
wall can provide an external shield against catalyst poisoning or
leaching and the inner cavity in the nanotubes can facilitate the
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, cmsong@chem.ecnu.edu.cn
accessibility of the active sites and the diffusion of the mole-
cules to improve the catalytic efficiency. We have previously
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8nj04514c
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reported the synthesis of monofunctionalized acid catalyst
SO3H-MONNs and base catalyst NH2-MONNs, which can be
employed as heterogeneous catalysts for one-pot reactions.23
More recently, acid–base bifunctional microporous organic
nanotube networks (MONNs-SO3H-NH2) were also successfully
prepared by a combination of hyper-crosslinking core–shell
bottlebrush copolymers and a post-functionalization strategy.24
However, the post-functionalization strategy has the disadvantages
of a low functional level and tedious procedures. Herein, we report
an improved synthetic route for the preparation of acid–base hyper-
crosslinked organic nanotube frameworks (H-ONTFs) through
grafting two different functional monomers into bottlebrush
copolymer precursors, in which the basic monomer is positioned
into the inner layer of the brush and the acidic monomer is
introduced onto the outer layer of the brush through a rational
molecular design strategy. Subsequently, hyper cross-linking the
outer styrene (St) shell layer in the molecular bottlebrushes and
removing the PLA core retained in the pores in this method lead
to the appearance of an organic nanotube framework structure
with a microporous shell and a mesoporous core. And the acidic
sites were anchored into the wall of the nanotubes and the basic
sites were introduced onto the inner layer wall of the nanotubes.
Compared to the post-polymerization strategy mentioned before,
this synthesis method possesses the advantage of excellent
controllability of the amino- and sulfo- groups grafted to the
polymer, which may have great influence on the catalytic per-
formance of catalysts. This catalytic system can promote the
accessibility of the active sites and improve the diffusion of the
molecules, which will result in the enhancement of the catalytic
efficiency due to the high specific surface area, open-ended and
multi-porosity structure, and robust organic frameworks.
Scheme 1 Preparation of amino- and sulfo-bifunctionalized hyper-
crosslinked organic nanotube frameworks (NH2-SO3H-H-ONTFs).
great influence on the catalytic activity of catalysts. In addition, gel
permeation chromatography (GPC) analysis (Fig. S4, ESI†) was used
to characterize the sulfonic and amino-protected bottlebrush
copolymer precursors. Their monomodal molecular weight
distribution indicates efficient polymerization and the prepara-
tion of well-defined copolymers. Subsequently, the obtained
core–shell bottlebrush copolymers were hyper-crosslinked via a
Friedel–Crafts alkylation reaction and a subsequent deprotec-
tion process to produce H-ONTFs with acid and base-catalytic
properties, named NH2-SO3H-H-ONTFs. (Scheme 1).
The formation of functional H-ONTFs was confirmed by FT-IR.
The characteristic band of PLA at 1758 cmÀ1 disappeared, which
confirmed the complete removal of the PLA core during the hyper-
crosslinking process (Fig. 1). The successful deprotection of the acid
functional group (PPVBs) can be verified by the appearance of new
bands at 1059 cmÀ1 and 1176 cmÀ1, which is associated with the
stretching of the –SQO bond. Another new band at 1226 cmÀ1 can
be assigned to the emergence of the sulfonic acid group16 (Fig. 1c).
In the process of core-removal, tert-butyl ester was also deprotected
to produce poly(amino) on the pore interior. The enhancement of
the peak at 1550 cmÀ1 indicated the transformation from secondary
amines to primary amines, which demonstrates the appearance of
amino groups with the deprotection of PBOCa.29
Results and discussion
The novel catalysts were obtained from highly branched bottle-
brush copolymers with a comb-like architecture25,26 as the
precursor, which are synthesized by a combination of controlled
radical and ring-opening polymerizations (Scheme S1, ESI†).
Firstly, P(GM-g-PLA-TC) was prepared according to the previously
reported method,27,28 and then Boc-aminoethyl acrylamide
(BOCa) as an amino-protected monomer was introduced to the
bottlebrush copolymer precursors. The 1H NMR analysis (Fig. S1,
ESI†) showed that the appearance of the peak at 3.2 ppm
corresponds to the pendant methylene groups of BOCa, which
demonstrated the successful synthesis of P(GM-g-PLA-g-PBOCa).
Then, phenyl 4-vinylbenzene sulfonate (PVBs) as a sulfonic acid
protected monomer was introduced to P(GM-g-PLA-PBOCa)
through copolymerization with a styrene monomer. The resulting
The BET surface area and the pore volume of the NH2-SO3H-
H-ONTFs were characterized by nitrogen adsorption–desorption
1
P(GM-g-PLA-g-PBOCa-g-PPVBs/PS) could be confirmed by H NMR
analysis (Fig. S2, ESI†), in which the resonance peaks at 7.15, 7.25,
7.3 and 7.6 ppm are ascribed to the protons in the benzene ring of
PVBs. Besides, 13C NMR analysis (Fig. S3, ESI†) was also used to
confirm the successful synthesis of P(GM-g-PLA-g-PBOCa-g-PPVBs/
PS). As we can see, one of the advantages of the design of the catalyst
is the controllability of the position of functional groups, which has
Fig. 1 FT-IR spectra of (a) deprotected NH2-SO3H-H-ONTFs, (b) after
hyper-crosslinking of P(GM-g-PLA-g-PBOCa-g-PPVBs/PS), (c) P(GM-g-
PLA-g-PBOCa-g-PPVBs/PS) precursor.
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Table 1 One-pot deacetalization–Knoevenagel cascade reaction cata-
lyzed by different catalystsa
Conv. of Yield of Yield of
Entry Catalyst
1b [%]
2b [%]
3b [%]
1
2
3
4
5
6
None
H-ONTFs
0
10
499
6.1
0
9
499
3.1
499
0
0
1
0
3.0
58
499
SO3H-H-ONTFs
NH2-H-ONTFs
NH2-H-ONTFs + NH2-H-ONTFs 499
NH2-SO3H-H-ONTFs 499
a
Reaction conditions: 10 mol% acid/base bifunctional catalyst, 75 mM
Fig. 2 N2 adsorption–desorption isotherms and pore size distributions
(inset) of the NH2-SO3H-H-ONTFs.
of benzaldehyde dimethylacetal, 300 mM of ethyl-cyanoacetate, 0.4 mL of
toluene, 80 1C, and 1 h. Conversion and yield are determined by GC.
b
isotherms, which are 845 m2 gÀ1 and 1.39 cm3 gÀ1, respectively Knoevenagel reaction as the model reaction to examine the catalytic
(Table S1, ESI†). A steep uptake at low relative pressures (P/P0 o activity of the NH2-SO3H-H-ONTFs. For comparison, various
0.001) in the nitrogen sorption isotherms confirmed the presence of catalysts such as H-ONTFs, monofunctionalized H-ONTFs with
the microporous nanostructures in the materials (Fig. 2 and Fig. S5, only acid or base sites were used in control experiments under
ESI†). Besides, a hysteresis loop at relatively high pressure confirmed similar reaction conditions. The different synthesis routes for
the presence of a mesoporous structure.30 The pore-size distribution NH2-H-ONTFs (base catalyst), SO3H-H-ONTFs (acid catalyst) and
was then calculated by using the nonlocal density functional theory NH2-SO3H-H-ONTFs (acid–base bifunctional catalyst) can be
(NLDFT) model. The population ranges from 1 nm to 35 nm, which found in Scheme S2 (ESI†). As shown in Table 1, product 3
implies the hierarchically porous structure of the resulting NH2- (ethyl a-cyanocinnamate) was the only obtained product and the
SO3H-H-ONTFs. Therefore, the NH2-SO3H-H-ONTFs were expected to conversion of the one-pot reaction can reach 99% after 1 h when
exhibit excellent catalytic activity in one-pot cascade reactions due to the NH2-SO3H-H-ONTFs are used as the catalyst. However, when
their high surface area, robust organic frameworks, hierarchical the one-pot cascade reaction was only catalyzed by SO3H-H-
porosity and site-isolation properties. The morphology of the NH2- ONTFs or NH2-H-ONTFs, the yield of desired product 3 is low
SO3H-H-ONTF catalyst was further characterized by transmission because of the lack of a base catalytic second step reaction or
electron microscopy (TEM). As shown in Fig. 3, the unique hollow acid catalytic first step reaction, respectively. Similarly, when
tubular networks can be distinctly observed. The network units catalyzed by the H-ONTFs without any acid and base function-
contain tubular channels of 3–5 nm in diameter and shell thick- alized groups, both the conversion and yield of the final product
nesses of 2–3 nm, whereas the average length of the cylindrical unit were low. Furthermore, in order to further confirm the acid–base
was 50 Æ 5 nm. According to the previous reports,31–33 the hollow bifunctionalized cooperative effect on the catalytic activity, a
tubular network with an open-ended structure can also facilitate the
accessibility of the active sites and molecular diffusion.
In this work, we chose a tandem reaction involving the acid-
catalyzed hydrolysis of an acetal and the subsequent base-catalyzed
Fig. 4 Conversion (%) vs. time (min) for the one-pot reaction of ethyl
cyanoacetate with benzaldehyde dimethyl acetal in toluene catalyzed by
the NH2-SO3H-H-ONTFs and a simple mixture of NH2-H-ONTFs and
Fig. 3 TEM image of the NH2-SO3H-H-ONTFs.
SO3H-H-ONTFs.
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simple mixture of NH2-H-ONTFs and SO3H-H-ONTFs was used
as the catalyst to perform the one-pot reaction. As shown in
Table 1 and Fig. 4, the yield of product 3 can only reach 58%
under the same conditions after 1 h. The above results showed
that the catalytic activity of the NH2-SO3H-H-ONTFs was higher
(B2 times) than that of the physical mixtures of NH2-H-ONTFs
and SO3H-H-ONTFs, which might be resulted from a shortened
mass transfer path in the NH2-SO3H-H-ONTFs. These results
demonstrated that the NH2-SO3H-H-ONTF catalyst system can
not only prevent the acid and base functionalized groups from
mutual deactivation, but also shows the best catalytic perfor-
mance due to their open-ended structure, large surface area,
hierarchical porosity and cooperative effect for the one-pot
cascade reaction between benzaldehyde dimethylacetal and ethyl
cyanoacetate compared to various reported catalysts as showed
in Table S2 (ESI†). In addition, the generality of the NH2-SO3H-H-
ONTF catalyst was also evaluated by using ethyl cyanoacetate,
malononitrile, dimethyl malonate and various acetals as sub-
strates and by performing the reaction under the same condi-
tions. All the products of the reactions were characterized by
1H-NMR and 13C-NMR and are listed in the ESI.† The results
of the catalysis, which are shown in Table 2, indicate that the
Fig. 5 Catalytic performance of the recycled NH2-SO3H-H-ONTFs.
NH2-SO3H-H-ONTF catalyst exhibit similar excellent catalytic
activity for various substrates.
The stability and reusability of the NH2-SO3H-H-ONTF catalyst
were also tested by performing a recycling experiment. The catalyst
can be isolated from the reaction medium through simple filtration
and washing it with methanol. The conversion of benzaldehyde
dimethyl acetal and the yield of ethyl a-cyanocinnamate were still
99% after at least seven runs of experiments (Fig. 5). Besides, the
atomic percentage of nitrogen (N) and sulfur (S) of the organic
nanotubes before and after the catalyzed reaction is 0.67% (N) and
1.57% (S) and 0.66% (N) and 1.55% (S) as measured by XPS (Fig. S6,
ESI†), which proves the non-destruction of the active sites. Although
the destruction of the hollow tubular structure may contribute to the
slight decrease in the conversion in the eighth catalytic test, there is
no obvious difference in the morphology of the catalyst and the
reused catalyst under TEM observation. The hollow tubular structure
of the network units was still maintained in the network units, which
confirmed the excellent stability of the NH2-SO3H-H-ONTF catalyst
(Fig. S7, ESI†). As is shown in Fig. S5 and Table S1 (ESI†), the porous
structure and high specific surface area were still preserved after
cycle tests as the further evidence for the excellent recyclability and
stability of the NH2-SO3H-H-ONTF catalyst. In addition, the TEM
images of the monofunctionalized ONTFs (Fig. S8, ESI†) before and
after use also demonstrate good preservation of the structures after
the control catalytic reaction.
Table 2 One-pot deacetalization–Knoevenagel cascade reactions with
various substratesa
Entry Reagent A
2a
Reagent B
Product C
Yield of Cb [%]
499
2b
2c
2d
2e
499
499
499
499
Experimental
For more details please see the ESI.†
2f
96
Conclusions
We have successfully prepared novel amino- and sulfo-
bifunctionalized hyper-crosslinked organic nanotube frame-
works NH2-SO3H-H-ONTFs via the combination of templating
the core–shell bottlebrush copolymer and a building-in strat-
egy. The large surface area and the presence of microporous
and mesoporous structures through a Friedel–Crafts hyper-
crosslinking reaction enable the obtained NH2-SO3H-H-ONTFs
2g
499
2h
82
a
Reaction conditions: 10 mol% acid/base bifunctional catalyst, 75 mM
of reagent A, 300 mM of reagent B, 0.4 mL of toluene, 80 1C, and 1 h.
b
Yield is determined by GC.
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