Microporous organic nanotube network supported acid and base catalyst system for one-pot cascade reactions

Article abstract of DOI:10.1039/c6nj01457g

In this work, we reported a novel synthesis of acid or base functionalized microporous organic nanotube networks (MONNs) by hyper-crosslinking core-shell bottlebrush copolymers. With a high specific surface area, excellent hierarchical porosity and site-i

Full text of DOI:10.1039/c6nj01457g

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Microporous organic nanotube network
supported acid and base catalyst system
Cite this:DOI: 10.1039/c6nj01457g
for one-pot cascade reactions†
Received (in Nottingham, UK)
9th May 2016,
Accepted 7th July 2016
Hui Zhang, Linfeng Xiong, Zidong He, Aiqing Zhong, Tianqi Wang, Yang Xu and
Kun Huang*
DOI: 10.1039/c6nj01457g
www.rsc.org/njc
In this work, we reported a novel synthesis of acid or base functionalized one-pot strategy that mimics nature’s reactions, developing a
microporous organic nanotube networks (MONNs) by hyper- sequential cascade reaction using multi-catalysts with designable
crosslinking core–shell bottlebrush copolymers. With a high specific morphology, well-controlled site distribution and high catalytic
surface area, excellent hierarchical porosity and site-isolation features, efficiency is still desirable.
the resultant SO3H– and NH2–MONNs catalyst systems present an
Hierarchically porous materials have drawn increasing interest
excellent catalytic performance for deacetalization-Knoevenagel and provided great new opportunities for catalytic applications
cascade reactions.
owing to the high surface area and enhanced diffusive properties
obtained by introducing meso/macropores into the sole micropore
Over the past decades, green chemistry has been of considerable catalysts. Up to now, various hierarchically porous materials have
interest owing to environmental considerations.1,2 As a green been reported as catalyst supports and been well used in organic
synthetic method, one-pot cascade reactions have attracted much chemistry, such as porous carbon,15–18 zeolites19,20 and silica.21–23
attention and provide an intriguing new direction for modern Recently, hierarchically porous polymers as an emerging class of
organic synthesis.3–6 The multi-step tandem process can save porous materials have received much attention as catalyst
reaction time and simplify synthetic procedures, which could candidates owing to their hierarchical porosity, robust organic
spare manufacturing costs and decrease chemical waste that frameworks and excellent chemical modifiability.24–27 However,
results from the reaction, improving the atom efficiency and reports on hierarchically porous polymers as catalyst supports
biomimetic properties. Recently, many efforts have been focused for one-pot cascade reactions are relatively rare.
on acid–base one-pot catalysis to realize cascade reactions.3,7–9 The
Herein, we described a novel method for the synthesis of
key to cascade reactions is the spatial site-isolated acid and base acid- and base-functionalized microporous organic nanotube
groups of multiple catalysts which prevent their deactivation. networks (MONNs) by hyper-crosslinking core–shell bottlebrush
To date, site-isolation in acid–base multiple catalysts has been copolymers. Sulfonic acid groups (–SO3H) and basic amino groups
achieved in soluble polymers for homogeneous catalysis10–12 (–NH2) were successfully anchored into the cavity of the nanotubes
and solid supports for heterogeneous catalysis.13,14 Our previous by a reasonable molecular design strategy. The resultant SO3H–
work demonstrated an acid- and base-functionalized soluble and NH2–MONNs catalyst systems possess a unique hierarchically
copolymer catalyst for a simple two-step sequential reaction in porous structure, which not only can facilitate the accessibility
one flask.9 Recently, Jian Liu and co-workers also reported a of the active sites and the diffusion of the molecules to improve
cascade reaction using dual-functionalized mesoporous silica the catalytic efficiency, but can effectively offer site-isolation
nanospheres with basic amino groups in the pores and carboxylic effects for incompatible acid/base groups. Therefore, the acid/
acid on the surface.8 However, the relatively intricate recovery of base-functionalized MONNs present excellent catalytic perfor-
homogeneous catalysis and the inferior transport efficiency of mance for deacetalization-Knoevenagel cascade reactions.
traditional solid catalysts may limit further application in organic
As is shown in Scheme 1, the MONNs were prepared from
synthesis. Although there has been significant progress in a core–shell bottlebrush copolymer precursors by a combination
of the molecular template of the bottlebrush polymers and
hyper-crosslinking via the F–C alkylation reaction. To endow
the MONNs with catalytic properties, a functional middle layer,
School of Chemistry and Molecular Engineering, East China Normal University,
500 N, Dongchuan Road, Shanghai, 200241, P. R. China.
formed by a sulfonic/amino-protected polymer, was introduced
between the polylactide (PLA) core and the polystyrene (PS)
E-mail: khuang@chem.ecnu.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nj01457g
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methanol for the deprotection of PVBS and then H2SO4 to produce
the sulfonic acid groups. Similarly, the amino-functionalized
MONNs (NH2–MONNs) could be obtained by the deprotection of
BAEAM from the hyper-crosslinked amino-protected bottlebrush
copolymer. In the IR spectrum (Fig. S4, ESI†), the vanishing of the
PLA carbonyl stretch peak at 1759 cmÀ1 demonstrated the etching
of the PLA core. New peaks at 1224 cmÀ1 and 1124 cmÀ1
corresponding to the –SO3H groups indicated the successful
deprotection of PVBS.29 In the NH2–MONNs, complete removal
of the PLA chains was also confirmed by the disappearance
of the PLA peak. Meanwhile, the core-removal process could
also deprotect the tert-butyl ester and exposed the poly(amino)
coating on the pore interior. The enhancement of the peak at
1550 cmÀ1 in the IR spectrum was ascribed to a certain extent to
the transformation from secondary amines to primary amines,
which implies the deprotection of BAEAM and the generation of
amino groups.30
The transmission electron microscopy (TEM) image of the
SO3H–MONNs showed that unique organic nanotube networks,
constructed by multi-porous architecture and robust organic
frameworks, were fabricated by the hyper-crosslinking of the
bottlebrush copolymers via the F–C reaction (Fig. 1). In the
SO3H–MONNs, the cylindrical morphology of the bottlebrush
precursors was still maintained after transformation into the
network units and clearly distinguishable channels were
observed crossing through the network units. The sulfonic acid
groups anchored on the inner cavity of the tubular units can be
protected by the outer shell to avoid undesirable interactions.
The obtained tubular units contain tubular channels of 3–5 nm in
diameter and shell thicknesses of 7–9 nm, whereas the average
length of the cylindrical unit is 48 Æ 5 nm. The Brunauer–Emmett–
Teller (BET) surface area and the pore volume of the SO3H–MONNs
detected by nitrogen adsorption–desorption isotherms were
676 m2 gÀ1 and 1.07 cm3 gÀ1 respectively (Table S1, ESI†).
The presence of the microporous nanostructures is confirmed
by a steep uptake at low relative pressures (P/P0 o 0.001) in the
nitrogen sorption isotherms (Fig. 2).31 The pronounced hyster-
esis loop in the desorption isotherm (P/P0 = 0.8–1.0) is assigned
to the presence of accessible mesopores in the SO3H–MONNs.
Scheme 1 Preparation of the acid/base-functionalized microporous
organic nanotube networks.
shell blocks. Subsequently, selective removal of the inner core
and deprotection of the functional block produce the acid/base-
functionalized MONNs, incorporating a trimodal micro, meso- and
macroporous architecture. As a result, sulfonic-functionalized acid
catalyst SO3H–MONNs and amino-functionalized base catalyst
NH2–MONNs were successfully prepared as described above.
Scheme 2 illustrates that the sulfonic/amino-protected bottle-
brush copolymer precursors with triblock terpolymer side-chains
were prepared by a ‘‘graft from’’ method via the combination of
controlled radical and ring-opening polymerizations as previously
described.28 As a functional block, phenyl 4-vinylbenzene sulfonate
(PVBS) or Boc-aminoethyl acrylamide (BAEAM) was introduced
between the PLA core and the PS shell to endow the MONNs with
1
acid/base-catalytic properties. From the H NMR (Fig. S1 and S2,
ESI†), the final sulfonic-protected bottlebrush copolymer precursors
consist of a degradable PLA block with an average of 25 repeat units,
a functional PVBS block with an average of 10 repeat units and a
cross-linkable PS block with an average of 118 repeat units. The
amino-protected bottlebrush copolymer precursors were composed
of a 25 repeat unit PLA block, a 7 repeat unit functional BAEAM
block and a 103 repeat unit PS block (Fig. S2, ESI†). As is shown
in Fig. S3 (ESI†), the gel permeation chromatography (GPC)
characterization of the sulfonic/amino-protected bottlebrush
copolymer precursors shows a monomodal molecular weight
distribution and presents efficient polymerization and the
preparation of well-defined copolymers.
To fabricate the sulfonic/amino-functionalized MONNs, the
bottlebrush copolymer precursors were hyper-crosslinked via the
F–C alkylation reaction with a catalyst, ferric chloride, and an
external cross-linker, formaldehyde dimethyl acetal. The hyper-
crosslinked bottlebrush copolymer was subjected to acidic dioxane
to etch out the PLA core. To prepare the sulfonic-functionalized
MONNs (SO3H–MONNs), the mixture was treated with basic
Scheme 2 Synthesis of the sulfonic/amino-protected bottlebrush copolymer
precursors.
Fig. 1 TEM image of the SO3H–MONNs.
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Table 1 Catalytic performances of different catalysts in the ‘‘one pot’’
reactionsa
No.
Catalyst A
Catalyst B
Yield 1b
Yield 2b
1
2
3
4
5
6
SO3H–MONNs
No
SO3H–MONNs
No
Benzoic acid
SO3H–MONNs
NH2–MONNs
No
No
NH2–MONNs
NH2–MONNs
n-Butylamine
499%
0
499%
4.4%
66%
499%
0
0
3.8%
1.1%
3.5%
28.7%
a
Reaction conditions: 75 mM of benzaldehyde dimethylacetal, 300 mM
of ethyl cyanoacetate, 10 mol% acid and base catalysts, 0.4 mL of
b
toluene, 80 1C. Yields are based on GC.
The yield can be up to 99% when the one-pot reaction is
conducted with the SO3H–MONNs and NH2–MONNs at 80 1C
for 1.5 h (Table 1). To our best knowledge, the catalytic activity
of the mixed MONNs (SO3H/NH2) is among the highest values for
reported heterogeneous catalysts for one-pot cascade reactions
(Table S2, ESI†). The outstanding multi-catalytic properties
benefited from the unique hierarchically porous structures that
can facilitate the accessibility of the active sites and molecular
diffusion. More importantly, the acidic and basic active-sites in
the cavities were successfully isolated by the cross-linked shell
in the network, which can prevent the undesirable interaction
between incompatible acid/base sites. The MONNs supported
catalysts can be separated conveniently via high-speed centri-
fugation after the reaction, which is favourable for the recycling
of the catalysts. To further explore the recoverability of the
SO3H–MONNs and NH2–MONNs in one-pot reactions, cycle
experiments were performed under the same conditions (Fig. 3).
The catalysts could be employed more than four times in the
cascade reactions, with a slight decrease in catalytic activity. The
decrease of the catalyst activity is probably ascribed to the partially
deteriorated structure in the MONNs. However, the morphology of
the SO3H–MONNs and NH2–MONNs catalysts was only slightly
destroyed and the hollow tubular structure in the network units
could still be observed after the recycle tests. In addition, the BET
result shows that the mixed MONNs (SO3H/NH2) also preserved
the hierarchically porous structure and high specific surface area
Fig. 2 N2 adsorption–desorption isotherms and pore size distributions
(inset) of the SO3H–MONNs.
Correspondingly, the pore size distributions are centered at 1.5,
3.8, and 9.3 nm, as calculated by the nonlocal density functional
theory method (NLDFT). The micropores emerged from the free
volume by the bridge formation between aromatic rings in the
cross-linked shell, and the inner cavity in the nanotube units
contributed to the formation of mesopores in the MONNs. The
intermolecular cross-linking of the bottlebrush copolymers
facilitates the creation of the meso/macropores in the polymer
frameworks. With the innovative synthesis strategy, we also
obtained amino-functionalized microporous organic nanotube
networks (NH2–MONNs). The nitrogen adsorption–desorption
isotherms and TEM image show that the NH2–MONNs possess
a similar structure to the SO3H–MONNs, such as a high BET
surface area and pore volume (936 m2 gÀ1 and 1.67 cm3 gÀ1) as
well as a multi-porous architecture (Fig. S5, S6 and Table S1, ESI†).
Based on the high surface area, robust organic frameworks, excellent
hierarchical porosity and site-isolation properties, the SO3H–MONNs
and NH2–MONNs were expected to present great catalytic
performance in one-pot cascade reactions.
To explore the possibility of the SO3H–MONNs and
NH2–MONNs as catalysts for cascade reactions, an acid-catalyzed
deacetalization and the subsequent base-catalyzed Knoevena-
gel condensation were chosen as model reactions (Scheme 3).
The results exhibited a complete hydrolysis of acetal to benzaldehyde
and ethyl a-cyanocinnamate was obtained as the only target product.
Scheme 3 Illustration of SO3H–MONNs and NH2–MONNs catalysts for
the ‘‘one pot’’ reactions.
Fig. 3 Catalytic performance of the recycled SO3H– and NH2–MONNs.
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Cascade reactions with either free acid or free base, and no
catalysts were also performed as control experiments (Table 1).
As predicted, little or no target product was generated when
the reaction was conducted only with the SO3H–MONNs,
NH2–MONNs or without the catalysts. Similar results were also
obtained when using small-molecule catalysts benzoic acid or
n-butylamine with NH2–MONNs or SO3H–MONNs as catalysts
(entries 5 and 6) respectively, which might be related to the fact
that the small molecule catalysts could diffuse freely into the
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active-sites through the acid–base neutralization reaction. So,
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reactions. The above results proved that the MONNs can
effectively isolate the active-sites and protect from the mutual
deactivation of the incompatible catalytic groups, with high
catalytic performance in one-pot cascade reactions.
In summary, we have developed a new way to prepare
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reactions. Based on their novel hierarchically porous structure
and site-isolation effect, the MONNs supported acid and
base catalyst systems present a good catalytic performance for
one-pot cascade reactions. This work might provide a general
pathway to anchor incompatible catalyst systems into MONNs
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This work was financially supported by the National Natural
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Shanghai Pujiang Program grant (13PJ1402300).
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