Amine-functionalized hollow mesoporous nano-bowl with bulky acid-imprinted free space around base sites and DMF-annealed mesoporous channels as an efficient solid base catalyst

Article abstract of DOI:10.1016/j.apcata.2020.117560

Tailoring free spaces around catalytic sites and constructing inter-connected channels between them are highly attractive for achieving easy accessibility of reactants to catalytic sites. In this study, a strategy for constructing free space around base s

Full text of DOI:10.1016/j.apcata.2020.117560

Journal Pre-proof
Amine-functionalized hollow mesoporous nano-bowl with bulky
acid-imprinted free space around base sites and DMF-annealed
mesoporous channels as an efficient solid base catalyst
Li Zhang (Data curation) (Visualization) (Investigation), Jianing
Zhang (Formal analysis) (Software), Shuai Wei (Validation)
(Resources), Shan Li (Resources), Xuebing Ma (Conceptualization)
(Methodology) (Writing - review and editing) (Project administration)
(Funding acquisition)
PII:
S0926-860X(20)30153-8
DOI:
https://doi.org/10.1016/j.apcata.2020.117560
APCATA 117560
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
2 February 2020
1 April 2020
9 April 2020
Please cite this article as: Zhang L, Zhang J, Wei S, Li S, Ma X, Amine-functionalized hollow
mesoporous nano-bowl with bulky acid-imprinted free space around base sites and
DMF-annealed mesoporous channels as an efficient solid base catalyst, Applied Catalysis A,
General (2020), doi: https://doi.org/10.1016/j.apcata.2020.117560

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Amine-functionalized hollow mesoporous nano-bowl with bulky acid-imprinted
free space around base sites and DMF-annealed mesoporous channels as an
efficient solid base catalyst
Li Zhang, Jianing Zhang, Shuai Wei, Shan Li and Xuebing Ma* zcj123@swu.edu.cn
College of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, P. R. China
* Corresponding author.
Graphical Abstract
Well-shaped nanobowl-like solid amine base with a hollow interior, mesoporous shell, free space around amine
sites, inter-connected channels, and thin shell displayed high catalytic activity in the Knoevenagel condensations of
aromatic aldehydes with malononitrile.
Highlights
•
•
•
•
Fabrication of hollow mesoporous nano-bowl solid base with high mass transfer
Imprint of free space around base site by etching bulky organic acid template
Construction of mesoporous channel by solvent annealing
High catalytic activity in Knoevenagel condensation at room temperature

•
Good reusability and restoration of solid amine base
Abstract
Tailoring free spaces around catalytic sites and constructing inter-connected channels between them are highly
attractive for achieving easy accessibility of reactants to catalytic sites. In this study, a strategy for constructing free
space around base sites through molecular imprinting and interconnected channel between them by solvent
annealing was developed to achieve high reactant accessibility. The as-fabricated nanobowls with particle sizes of
345 nm were efficient solid bases with fast mass transfer owing to the characteristic architectural features, such as
a thin shell thickness (30 nm), free space around the base centered at 4.0 nm, mesoporous channels (5-50 nm), and
a hollow interior. The influence of porosity on catalytic behavior was investigated using Knoevenagel condensations,
and was found to promote various aromatic aldehydes to afford products at room temperature in 84–98% yield
within 2h. Furthermore, the catalyst reused ten times was able to be restored to obtain original high catalytic activity.
Keywords: Solid base; Knoevenagel condensation; Hollow mesoporous nanobowl; Molecular imprinting; Solvent
annealing.
1. Introduction
From the perspective of green and sustainable chemistry, solid base catalysts play an important role in
catalyzing various reactions such as isomerization, hydrogenation, Tishchenko, Henry, Knoevenagel, and aldol
reactions [1]. Various materials are used as supports for solid bases, ranging from inorganic materials, such as
alumina, zeolites, and mesoporous silicas [2–4], to porous organic polymers and metal–organic frameworks [5].
With the development of synthetic methods and accurate characterization for solid catalysts, their applications to

industrial organic transformations for fine chemical production [1–4] and biomass conversion for biodiesel
production [6–9] have attracted increasing attention.
Organic amines are attractive and readily available bases widely applied in base catalysis [10–12], metal
coordination chemistry [13–15], and carbon capture in environmental science [16–18]. Owing to the increasing
focus on green and sustainable chemistry, various types of solid materials, including organic, inorganic, and
inorganic-organic hybrid materials, have been used as catalyst carriers to achieve the recovery and reuse of organic
amines in heterogeneous base catalysis [19–27], and as solid supports to accomplish carbon capture and storage
[28–30]. Inevitably, the question of how to improve the mass transfer of reactants [31–33] or adsorbates [34–36] in
the frameworks of solid materials is a problem faced by all solid amines. To overcome the diffusion barrier of
reactants inside catalyst carriers, high and tunable porosity is highly desirable for solid amines to achieve high
catalytic performance in heterogeneous catalysis. Accordingly, organic amines have been grafted on ordered
mesoporous silicas [37–40] and MOFs with uniform mesopores [5, 41–45] to enhance the in-pore molecule
diffusion of reactants. Furthermore, the in-pore molecular diffusion of reactants in the solid skeleton is closely
related to the hydrophilicity/hydrophobicity of the pore wall [46–49], seepage distance of the reactants [50–54] and
hollow interior of the catalyst [55–61]. The empty space around a catalytic site is also an important contributor to
the in-pore molecular diffusion of reactants inside solid materials, providing enough free space for reactants to
access the catalytic site surface [62, 63]. To date, constructing free space around base sites to improve the
accessibility of reactants/adsorbates to the base site surfaces in heterogeneous base catalysis and environmental
science has been seldom explored.
In this study, based on the chemical reactivity of amino group for salt formation with organic acids, and the
solubility and swellability of cross-linked and linear polystyrene, novel solid amine bases #PS-NH2 HMOPBs (PS,
polystyrene; HMOPBs, hollow mesoporous organic polymeric nanobowls) with a well-shaped polystyrene

nanobowl as the catalyst carrier with a hollow interior, free spaces around the amine sites and mesoporous channels
in the shell were developed. These #PS-NH2 HMOPBs were prepared by coating the surface of spherical PS with
the ethylene glycol dimethylacrylate (EGDMA)-cross-linked copolymer of styrene and amine salts of 4-
aminomethyl styrenes (1a-c), and then etching the PS core and bulky organic acid template (1a´-c´) to fabricate
nanobowls with a hollow interior and free space around the amine base. This was followed by swelling in DMF at
80 °C, annealing in liquid N2, and removing frozen DMF using MeOH [64] to construct inter-connected mesoporous
channels in the shell (Scheme 1). Owing to the hollow interior, thin shell, free space around amine sites, and
mesoporous channel, the as-prepared solid amine #PS-NH2 HMOPBs had ideal architectural features for reactants
to access the base catalytic site surfaces and showed significantly enhanced catalytic activities in Knoevenagel
condensations of aromatic aldehydes and malononitrile.
Scheme 1. Preparation of amine-functionalized hollow mesoporous organic polymeric nano-bowls #PS-
NH2 HMOPBs.
2. Experimental
2.1. Materials and characterization

Styrene was purified by distillation under reduced pressure before use. Polystyrene nanospheres (PS, d = 285
± 15 nm, n = 100) were prepared as cores according to the reference [61], whose SEM image and particle size
distribution were shown in Fig. S1. 4-Vinyl benzylamine was synthesized according to the procedure shown in ESI#.
The contrast solid amines, organic polymeric nanospheres (PS-NH2 OPNs), and hollow organic polymeric
nanospheres (PS-NH2 HOPNs), were prepared according to the procedure (Scheme S1) in ESI#. The other chemicals
were used as received without any further purification.
The chemical structures of products in Knoevenagel condensations were confirmed by 1H NMR spectra on a
Bruker av-600 NMR instrument, in which all chemical shifts were reported down-field in ppm relative to the
hydrogen resonance of TMS. Elemental analysis was carried out on a vario Micro cube elemental analyzer. The
morphology of sample was observed using SM-7800F scanning electron microscopy (SEM) and Tecnai G2 F20
transmission electron microscope (TEM), operated at 10 kV and 200 kV, respectively. N2 adsorption–desorption
isotherms were performed at 77.4 K on Autosorb-1 apparatus, in which samples were degassed at 105 °C for 12 h
before measurement. The BET surface area was calculated from adsorption data in relative pressures P/P0
(0.05−0.30), and the pore diameter distributions and pore volumes of samples were obtained from desorption
branches using BJH method. FT-IR spectroscopy was performed on a Perkin-Elmer model GX spectrometer using
KBr pellet. Knoevenagel condensation was monitored throughout by Agilent LC-1200 HPLC with a 210 nm UV-
vis detector using Daicel Chiralpak chiral OD-H column (4.6 mm × 25 cm), eluting with n-hexane/iso-propanol
(85/15) at a flow rate of 0.8 mL min1 at 20 °C.
2.2. Synthesis of 4-vinyl benzylamine salts
In round-bottom flask (100 mL), 4-vinyl benzylamine (0.80 g, 6.3 mmol) was dissolved in 20 mL of methanol,
and then 15 mL of iso-propanol solution containing β-naphthalene sulfonic acid 1a´ (1.31 g, 6.3 mmol) was added
dropwise (about 1.5 h) at 0 °C. The precipitate was filtered and dried to afford white 4-vinyl benzylamine

naphthalene sulfonate 1a (2.0 g, 95%). 1H NMR (600.0 MHz, DMSO-d6): δ 8.17 (s, 4H), 7.97 (dd, J = 5.8, 3.4 Hz,
1H), 7.91 (dd, J = 5.8, 3.4 Hz, 1H), 7.88 (d, J = 6 Hz, 1H), 7.74 (d, J = 6 Hz, 1H), 7.54-7.52 (m, 2H), 7.50 (d, J =
12 Hz, 2H), 6.76-6.71 (m, 1H), 5.87(d, J =18 Hz, 1H), 5.29 (d, J =12 Hz, 1H), 4.03 (s, 2H).
When sulfonic acid 1a´ was replaced by camphorsulfonic acid 1b´ and adamantane formic acid 1c´, yellow
sulfonic amine salt 1b (2.1 g, 92%) and white carboxylic acid amine salt 1c (1.6 g, 80%) were also prepared
according to the above-mentioned procedure. 1b: 1H NMR (600 MHz, DMSO-d6): δ 8.36 (s, 3H), 7.64 (d, J = 6 Hz,
2H), 7.58 (d, J = 12 Hz, 2H), 6.89 - 6.85 (m, 1H), 6.00 (d, J = 18 Hz, 1H), 5.42 (d, J = 18 Hz, 1H), 4.17-4.14 (m,
2H), 2.77 (m, 2H), 2.74 (s, 1H), 2.63 (s, 1H), 2.61 (s, 1H), 2.38 (d, J = 18 Hz, 1H), 1.49-1.39 (m, 4H), 1.30-1.24
(m, 1H). 1c: 1H NMR (600 MHz, DMSO-d6): δ 7.42 (d, J = 6 Hz, 2H), 7.33 (d, J = 6 Hz, 2H), 6.73-6.69 (m, 1H),
5.80 (d, J = 18 Hz, 1H), 5.22 (d, J = 6 Hz, 1H), 3.75 (s, 2H), 1.68-1.62 (m, 13H).
2.3. Preparation of hollow mesoporous organic polystyrene nano-bowls (#PS-NH2 HMOPBs)
In N2-filled three-necked round-bottom flask, PS nanospheres (0.25 g) were dispersed in 28 mL of 0.5 wt%
PVA-containing mixed H2O/DMSO (1:1, v/v), stirred at room temperature for 12 h, and added 4 mL of DMSO
solution contained St (104 mg, 1.0 mmol), 1a (133 mg, 0.4 mmol), EGDMA (40 mg，0.2 mmol), and 3 mL of
aqueous potassium peroxydisulfate solution (KPS, 40 mg, 0.15 mmol) at 55 °C. After being stirred for 40 min, the
well-dispersed emulsion was heated to 85 °C and stirred for 24 h. The cooled mixture was added 70 mL of ethanol,
stirred for 30 min, and separated by centrifugation to afford core-shelled nanospheres PS@PS-NH3/OR OPNs (365.0
mg). The obtained PS@PS-NH3/OR OPNs (150.0 mg) were dispersed in 9 mL of THF, and stirred at room
temperature for 12 h to remove the PS cores. The procedure was repeated two more times until the turbid
phenomenon was not observed upon adding THF solution into water, indicating the complete removal of the PS
cores. The isolated hollow nanospheres PS-NH3/OR HOPNs was dealt with 9 mL of aqueous NaOH solution (2.0
mol L1) with stirring for 12 h to remove the imprinted molecule. The solids were further dealt with aqueous NaOH

solution two more times till no white solid was observed upon adding aqueous BaCl2 solution. The solids were dried
naturally to afford hollow mesoporous organic polymeric nanobowls PS-NH2 HMOPBs (70.2 mg). Subsequently,
PS-NH2 HMOPBs (100 mg) was dispersed in 10 mL of DMF at 80 °C for 24 h, and transferred to liquid N2 to freeze
the reaction mixture. The frozen mixture was transferred to a refrigerator with −80 °C, and added 25 mL of CH3OH
to extract the frozen DMF molecules. The solids were dried under vacuum at room temperature to yield nanobowls
#PS-NH2 HMOPBs(80) (98.2 mg).
At DMF swelling temperature of 115 °C and 145 °C, other solid amines, designated as #PS-NH2 HMOPBs(115)
and #PS-NH2 HMOPBs(145), were also prepared according to the above-mentioned procedure.
2.4. General procedure of Knoevenagel condensation
In a dried vial, benzaldehyde (106.0 mg, 1.0 mmol), malononitrile (80.0 mg, 1.2 mmol), #PS-NH2 HMOPBs
(61.2 mg, 0.05 mmol of amine) and ethanol (2 mL) were charged. Then, the reaction mixture was stirred at room
temperature for 2 h. After the benzaldehyde was completely consumed, #PS-NH2 HMOPBs was recovered by
centrifugal separation, washed by ethanol, and directly reused in the following catalytic cycles. The centrifugate
was purified by column chromatography on silica gel eluted with petroleum ether/diethyl ether (=10:1, v/v) to afford
the white product (150.9 mg, 98%).
2.5. Restoration of #PS-NH2 HMOPBs reused ten times
The 10th-reused #PS-NH2 HMOPBs(80) (100 mg) was allowed to stir in 6 mL of aqueous HCl (1.0 mol L1)
ethanol solution (1:1, v/v) at room temperature for 12 h and separated by centrifugation. The isolated #PS-NH2
HMOPBs(80) was neutralized in 6 mL of aqueous NaOH (2.0 mol L1) ethanol solution (1:1, v/v) at room
temperature for 12 h, separated by centrifugation, washed by distilled water to pH = 7, and dried naturally to afford
restored #PS-NH2 HMOPBs.

3. Results and discussion
3.1. Fabrication of well-shaped hollow mesoporous nanobowl
Theoretically, the mass transport of reactants in a porous spherical catalyst has been discussed in depth [65–
67]. For first-order kinetics, the effectiveness factor (η) is given as a function of the Thiele modulus (ϕs) with
constant concentration at the catalyst surface, as shown in equations (1) and (2).
3
1
1
휂 =
(
− )
휙
푠
(1)
(2)
휙
푠
푡푎푛ℎ휙
푠
퐾
1
ϕ = R
s
√
퐷
푒
where K1 is the first-order kinetic constant (s-1), R is the sphere radius (cm), and De is the effective diffusion
coefficient (cm2 s-1). Based on equations (1) and (2), a higher De and shorter R can achieve a better η value. The
effective diffusion co-efficient (De), based on Fick's law, is mainly related to geometrical pore properties including
void fraction (θ) and tortuosity factor (τ) [68, 69]. Furthermore, the hollow structure can provide a driving force due
to concentration differences at internal and external surfaces, improving the permeation rate of reactants into solid
catalysts [55–61]. Therefore, the desired architectural features, such as a hollow interior, mesoporous channels, free
space around catalytic sites, short seepage distance, and well-shaped morphology, are essential for achieving a high
effectiveness factor (η) for the catalyst.
Accordingly, solid amines with characteristic architectural features suitable for a high effectiveness factor (η)
were prepared through four steps, as shown in Scheme 1. First, the EGDMA-cross-linked copolymers of styrene
and monomers 1a‒c were coated on the surface of PS nanospheres to form core-shelled PS@PS-NH3/OR OPNs.
After the PS core was etched by THF, hollow nanospheres with a thin shell, denoted as PS-NH3/OR HOPNs, were
fabricated. Mesopore-forming acid templates 1a‒c attachedto the shell were then eluted with aqueous NaOH
solution to imprint free space around the amine site in hollow mesoporous nanobowls PS-NH2 HMOPBs. Finally,

PS-NH2 HMOPNs underwent swelling in DMF at different temperatures, followed by annealing in liquid N2 and
extraction with methanol to produce inter-connected channels between spaces around amines in the shell. The as-
prepared #PS-NH2 HMOPBNs possessed a well-shaped bowl-like morphology, hollow interior, mesoporous
channel, free space around amines and a thin shell, which provided the characteristic architectural features to achieve
the fast mass transfer of reactants in solid base catalysis.
3.1.1. Well-shaped nanobowl with thin shell and hollow interior
Fig.1. SEM images of PS-NH3/OR HOPNs in DMSO/H2O (a: 2.5:1;c: 1:1), PS-NH2 HMOPBs in DMSO/H2O
(b: 1:2.5; d: 1:1), DVB-cross-linked PS-NH3/OR HOPNs (e), and PS-NH2 HMOPBs (f).
The effect of the mixed solvent volume ratio and different cross-linkers on the shell thickness and morphology
of solid amines was investigated in detail. The SEM images and particle size distributions of core-shelled PS@PS-
NH3/OR OPNs are shown in Fig. S2. When the DMSO/H2O volume ratio was 2.5:1.0, the obtained core-shelled
PS@PS-NH3/OR OPNs had a very thin outer shell, only about several nanometers in thickness (Fig. S2d). With
decreasing DMSO/H2O volume ratio from 2.5:1 to 1:1 and 1:2.5, the outer shell thickness increased to 30 nm (Fig.

S2e) and 29 nm (Fig. S2f), respectively. This indicated that the emulsion model with more monomers around the
PS core was favored at a lower volume ratio of DMSO to H2O, with the thicker outer shell coated on the PS core
surface [59]. However, fewer monomers of 1a were loaded on the PS surface, as determined by elemental analysis
of nitrogen and sulfur contents, owing to the poorer dispersion of monomer 1a in the DMSO/H2O (1:2.5, v/v) mixed
solvent. The mixed DMSO/H2O solvent at 1:1 (v/v) was the best solvent for the formation of a thicker polymeric
shell with a higher content of 1a on the PS core. After the PS cores were removed by THF, the obtained thin shell
of the hollow PS-NH3/OR HOPNs could not provide sufficient support to balance the combined actions of
centrifugal force (FC) and osmotic pressure (OP) toward the interior. Therefore,the original spherical morphology
(Fig. S2a) was deformed into a bowl-like morphology with no void owing to the overlap of two semi-shells (Fig.
1a). Owing to the higher shell thicknesses of 30 and 29 nm, hollow PS-NH3/OR HOPNs retained the original
spherical morphology (Figs. 1c and S3). After imprinted molecule 1a' in the shell was etched by aqueous NaOH
solution, the released free spaces weakened the support force of the shell. Therefore, spherical PS-NH3/OR HOPNs
were observed to deform into well-shaped bowl-like PS-NH2 HMOPBs (Figs. 1b and d). To strengthen the support
force of the shell, divinylbenzene (DVB) was used instead of EGDMA as a cross-linker to enhance the shell rigidity.
As expected, before and after the removal of 1a', the same well-shaped spherical morphology remained (Figs. 1e
and 1f). Furthermore, hollow interiors were clearly observed in both bowl-like PS-NH2 HMOPBs and DVB-cross-
linked nanospheres (Fig. 2). Therefore, the as-fabricated solid base with architectural features, including a thin shell,
hollow interior, and well-shaped morphology, provided a short transmission distance (30 nm) for reactants,
concentration difference-driven force, and textural uniformity to achieve a high effectiveness factor (η).

Fig.2. TEM images of (a, b) EGDMA-cross-linked PS-NH2 HMOPBs, and (c, d) DVB-cross-linked nanospheres.
3.1.2. Free space around amine site.
Fig.3. N2 adsorption-desorption isotherms (upper) and pore diameter distributions (bottom) of (a) PS-NH2
HMOPBs, (b) PS@PS-NH3/OR OPNs, (c) PS-NH3/OR HOPNs, and (d) DVB-cross-linked PS-NH2 HMOPBs.
Generally, catalytic sites embedded in the solid support framework result in the poor accessibility of reactants

to the catalytic site surface. Imprinting free space around the catalytic site could effectively improve the catalytic
activity in acid catalysis [63]. Herein, bulky organic acids 1a‒c were used as templates to imprint free spaces around
amine sites. As shown in Fig. 3 and Table S1, core-shelled PS@PS-NH3/OR OPNs in the absence of mesopore (Fig.
3b) were found to possess a very low surface area (9.7 m2 g-1) and low pore volume (0.10 cm3 g-1). After removing
PS, the surface area and pore volume increased to 26.9 m2 g-1 and 0.15 cm3 g-1, respectively, owing to the formation
of a hollow interior and slight increase in mesopores with pore sizes in the range of 15–50 nm (Fig. 3c). Interestingly,
when imprinted molecules 1a', attached to the framework via acid–base reaction, were etched by aqueous NaOH
solution, new emerging mesopores with pore sizes in the range of 3.5–10 nm, centered at 4.0 nm were obtained with
the release 1a' (Fig. 3a). Meanwhile, the specific surface area and pore volume of PS-NH2 HMOPBs increased to
41.3 m2 g-1 and 0.21 cm3 g-1, respectively. When replacing cross-linker EGDMA with DVB, the as-prepared PS-
NH2 HMOPBs showed a similar trend in pore size distributions (Fig. S7). Unfortunately, owing to the skeleton
rigidity of DVB, the DVB-cross-linked PS-NH2 HMOPBs displayed a lower surface area (21.9 m2 g-1) and pore
volume (0.09 cm3 g-1) compared with EGDMA-cross-linked PS-NH2 HMOPBs. These results indicated that the
etching of acid 1a' could imprint free spaces around amine sites.
To enhance the imprinted free spaces around amine sites, acids 1b' and 1c' with three-dimensional structures,
instead of two-dimensional acid 1a', were used as bulky templates to imprint larger free spaces. Unfortunately,
owing to the difficulty in eluting bulky 1b' and 1c' from the shells completely, the as-fabricated spherical PS-NH2
HMOPBs (Fig. S5) failed to achieve the desired surface areas (24.2 m2 g-1 and 18.8 m2 g-1, respectively), pore
volume (0.17 cm3 g-1 and 0.10 cm3 g-1, respectively), and imprinted free space around the amines (Fig. S8). In
particular, after the salts of etched molecules1a'-c' in NaOH solution were acidified and purified by column
chromatography, organic acids 1a'-c' could be reused to prepare amine salts 1a-c.
3.1.3. Mesoporous channels

Fig.4. SEM and TEM images of #PS-NH2 HMOPBs prepared at DMF swelling temperatures of (a, c) 80 °C,
and (b, d) 115 °C.
Fig.5. N2 adsorption-desorption isotherms (upper) and pore diameter distributions (bottom) of (d) PS-NH2
HMOPBs and #PS-NH2 HMOPBs at different swollen temperatures of (a) 145 °C, (b)115 °C, and (c) 80 °C.
We attempted to construct mesoporous channels in the shells of PS-NH2 HMOPBs to connect free spaces
around bases using a Co(OAc)2-templated method [50, 63]. Unfortunately, no improvement in surface area (22.2
m2 g-1) and pore volume (0.14 cm3 g-1) was observed. Based on the known swelling behavior of polystyrene in DMF

at high temperature, and the formation of a mesoporous network after removal of crystalline DMF by MeOH at
cryogenic temperatures [64], hollow mesoporous #PS-NH2 HMOPBs with a well-shaped hollow morphology were
fabricated, and the specific surface areas and pore volumes were improved. From the SEM (Fig. 4a) and TEM (Fig.
4c) images, #PS-NH2 HMOPBs(80), prepared using a DMF swelling temperature of 80 °C, possessed the original
hollow interior and well-shaped bowl-like morphology. The specific surface area and pore volume increased from
41.3 m2 g-1 and 0.21 cm3 g-1 to 56.5 m2 g-1 and 0.41 cm3 g-1, respectively. In particular, the desorption D(r) of
mesopores in the range of 5–50 nm was significantly enhanced (Fig.5c), indicating that the mesoporous channels
had newly emerged in the shell. At a DMF swelling temperature of 115 °C, some #PS-NH2 HMOPBs(115) were
observed to collapse and lose the original well-shaped morphology and hollow interior (Figs. 4b and 4d). However,
the specific surface area and pore volume increased to 66.3 m2 g-1 and 0.45 cm3 g-1, and mesopores in the range of
3–50 nm were also significantly increased (Fig. 5b). Upon further increasing the DMF swelling temperature to
145 °C, the obtained #PS-NH2 HMOPBs(145) exhibited the highest specific surface area (84.1 m2 g-1) and pore
volume (0.51 cm3 g-1). Frustratingly, all nanoparticles of #PS-NH2 HMOPBs(115) collapsed and lost their well-
shaped morphology and hollow structure (Fig. S6). Furthermore, mesopores with pore diameters of 3–10 nm were
significantly reduced, while mesopores with pore diameters of 10–50 nm further increased (Fig.5a), indicating that
the mesoporous structure was destroyed owing to excessive spinodal decomposition of the polymeric shell [64].
Based on these results, N2 annealing was considered as a method to effectively improve the specific surface area
and pore volume, and produce mesoporous channels to connect the free spaces around amine sites. Considering
these integrated factors, such as morphology and porosity, the optimal DMF swelling temperature for PS-NH2
HMOPBs was concluded to be 80 °C, at which the mesoporous channels (5–50 nm) in the shell of #PS-NH2
HMOPBs(80) were constructed and could connect free spaces around base sites.
3.2. Architectural features of various solid amines

Fig.6. Architectural features of various catalysts: (a) PS-NH2 OPNs, (b) PS-NH2 HOPNs, (c) PS-NH2
HMOPBs, (d) #PS-NH2 HMOPBs(80), (e) #PS-NH2 HMOPBs(115), and (f) #PS-NH2 HMOPBs(145).
To clarify the vital functions of architectural features, such as the hollow interior, free spaces around base sites,
and inter-connected mesoporous channels, in catalysis, solid amines, solid PS-NH2 OPNs and hollow PS-NH2
HOPNs, were prepared as contrast catalysts according to the procedures shown in Scheme S1. The main differences
among these catalysts were the architectural features, including morphology and porosity (Fig. 6). PS-NH2 OPNs
with a mean particle size of 79 nm were solid, while PS-NH2 HOPNs with a mean particle size of 314 nm were
hollow. Both solid amines had low specific areas and pore volumes. By imprinting free spaces around the amine
sites in the shell of hollow PS-NH2 HOPNs, more amine sites in PS-NH2 HMOPBs were exposed to reactants due
to imprinted free spaces around base sites centered at 4.0 nm. Interestingly, after PS-NH2 HMOPBs were swollen
in DMF, annealed in liquid N2, and extracted by methanol to remove frozen DMF, more mesopores were constructed
in #PS-NH2 HMOPBs at DMF swelling temperatures of 80, 115, and 145 °C. The porosity in the shell of #PS-NH2
HMOPBs(80), obtained at 80 °C, comprised the original free spaces around amine sites, centered at 4.0 nm, and
newly emerged mesoporous channels (5–50 nm). Upon increasing the DMF swelling temperature to 115 °C, all
mesopores (3–50 nm) in #PS-NH2 HMOPBs(115) were significantly increased. At DMF swelling temperature of
145 °C, mesopores (3–10 nm) were significantly reduced, while mesopores (10–50 nm) were further increased (Fig.

5). Furthermore, the specific surface areas and pore volumes of #PS-NH2 HMOPBs increased with increasing DMF
swelling temperature. Notably, the well-shaped nanobowls gradually collapsed into floccus solids at DMF swelling
temperatures above 115 °C, with free spaces around the amine sites disappearing due to excessive spinodal
decomposition of the shell.
3.3. Amine loading capacity
FT-IR spectra were recorded to confirm the immobilization of benzylamine in the framework of various solid
amines (Fig. S9). The adsorption peaks at 3483 and 3437 cm1 corresponded to symmetric and asymmetric N‒H
stretching vibrations in NH2 groups. The band at 1739 cm1 was assigned to the stretching vibration of C=O in
EGDMA, with the band at 1630 cm1 attributed to the flexural vibration of N‒H bonds in NH2 groups. These results
indicated that benzylamine moieties had been successfully loaded onto the solid amine frameworks. According to
the nitrogen contents obtained by elemental analysis, the amine loading capacities in PS-NH2 OPNs, PS-NH2
HOPNs, and PS-NH2 HMOPBs were calculated to be 0.70, 0.60, and 0.82 mmol g1, respectively.
3.4. Catalytic activity in Knoevenagel condensation
Based on the inherent functions of amines, the as-fabricated well-shaped solid amines with hollow interiors,
mesoporous channels and thin shells have extensive potential applications, including in base catalysis, metal
coordination chemistry, and carbon capture in environmental science. Therefore, the Knoevenagel condensation of
benzaldehyde with malononitrile was selected as a model reaction to investigate the improved activity of solid
amines in heterogeneous base catalysis.
3.4.1. Optimization of reaction parameters
Table 1
Knoevenagel condensation of benzaldehyde with malononitrile catalysed by PS-NH2 HMOPBs under different

conditions.a
Entry
Cat. (g/mol%)
61/5.0
Solvent
Toluene
Toluene
Toluene
Toluene
Acetonitrile
CH2Cl2
Ethanol
THF
Temp. (°C)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
15
25
40
60
25
25
25
25
25
25
89
92
92
93
89
80
98
91
93
86
61/5.0
61/5.0
61/5.0
61/5.0
61/5.0
61/5.0
61/5.0
37/3.0
Ethanol
Ethanol
12/1.0
a Reaction conditions: benzaldehyde (106.0 mg, 1.0 mmol), malononitrile (79.0 mg, 1.2 mmol), 2 mL of solvent, 3
h.
b Isolated yield.
Using PS-NH2 HMOPBs as the base catalyst, the effects of different parameters, such as temperature, solvent,
and amount of solid base, on the Knoevenagel condensation of benzaldehyde with malononitrile were investigated,
with the results shown in Table 1. At 25, 40, and 60 °C, similar yields were obtained. Therefore, with regard to
energy consumption, 25 °C was selected as the optimum reaction temperature. PS-NH2 HMOPBs afforded the
highest product yield (98%) in ethanol, indicating that polar protic solvents were favorable for the Knoevenagel
condensation. When the amount of solid amine used was decreased from 5 mol% to 3 and 1 mol%, the yield
decreased from 98% to 93 and 86%, respectively. According to these results, 5 mol% of PS-NH2 HMOPBs in
ethanol at 25 °C were considered the optimum reaction conditions for subsequent reactions.
3.4.2. Effects of porosity and morphology.

Fig.7. HPLC monitoring of Knoevenagel condensation promoted by #PS-NH2 HMOPBs (80) (top), and
concentrations and yields of product versus reaction times (bottom) catalyzed by various solid amines: (a) PS-NH2
OPNs, (b) PS-NH2 HOPNs, (c) PS-NH2 HMOPBs, (d) #PS-NH2 HMOPNs(80), (e) #PS-NH2 HMOPBs (115), and
(f) #PS-NH2 HMOPNs (145).
Under the optimized conditions, the Knoevenagel condensation of benzaldehyde with malononitrile was used
as a model reaction to investigate the morphology- and mesopore-dependent catalytic activities of solid amines. The
Knoevenagel condensation reactions, promoted by various solid amines, were monitored throughout by HPLC (Fig.
S10–S14). The benzaldehyde concentrations and product yields were plotted versus reaction times, as shown at the
bottom of Fig. 7. According to the benzaldehyde concentration as a function of reaction time, the relationship
between concentration (mmol L1) and reaction time (min) could be expressed as follows:
PS-NH2 OPNs (a):

ca = 14/t + 2.2 (R2=0.998)
PS-NH2 HOPNs (b):
cb = 19/t + 1.2 (R2=0.992)
PS-NH2 HMOPBs (c):
cc = 23/t + 0.40
#PS-NH2 HMOPBs(80) (d):
(R2=0.999)
cd = 25/t + 0.60
(R2=0.996)
#PS-NH2 HMOPBs(115) (e):
ce = 21/t + 0.80 (R2=0.990)
#PS-NH2 HMOPNs(145) (f):
cf = 15/t + 2.0
(R2=0.999)
The derivative of concentration versus time was considered to be the reaction rate of benzaldehyde at a certain
time. The initial reaction rate of benzaldehyde at 20 min was selected as a parameter for evaluating the catalytic
activity of solid amine. Therefore, d[ca]/dt = 14/t2, d[cb]/dt = 19/t2, d[cc]/dt = 23/t2, d[cd]/dt = 25/t2, d[ce]/dt = 21/t2
,
and d[cf]/dt = 15/t2. At 20 min, the initial reaction rates of the various solid bases were calculated to be 35.0, 47.5,
57.5, 62.5, 52.5, and 37.5 mmol L−1 min−1, respectively. The reaction rates were shown to be directly related to the
architectural features of solid amines. Solid PS-NH2 OPNs displayed the lowest catalytic activity owing to the low
specific surface area (17.1 m2 g1) and pore volume (0.15 cm3 g1) (Fig. 7a). After the PS-templated hollow interior
was fabricated, the hollow interior in PS-NH2 HOPNs produced a driving force owing to the internal–external
concentration difference, with the initial reaction rate increasing from 35.0 to 47.5 mmol L1 min1 (Fig. 7b). As
expected, upon imprinting the free spaces around –NH2 sites, the initial reaction rate of PS-NH2 HMOPNs increased
to 57.5 mmol L1 min1 (Fig. 7c), which was attributed to the easy accessibility of reactants to exposed –NH2 sites.

The interconnected mesoporous channels in the shell, constructed by solvent annealing of the DMF-swollen solid
base, improved the accessibility of reactants to the –NH2 sites. The as-obtained #PS-NH2 HMOPNs(80) with an
increased specific surface area (56.5 m2 g1) and pore volume (0.41 cm3 g1) showed the highest initial reaction rate
(62.5 mmol L1 min1) (Fig. 7d). When the DMF-swollen temperature was increased to 115 and 145 °C, the specific
surface areas and pore volumes of #PS-NH2 HMOPNs(115) and #PS-NH2 HMOPNs(145) further increased to 66.3
m2 g1 and 0.45 cm3 g1, and 84.1 m2 g1 and 0.51 cm3 g1, respectively. Unfortunately, both solid amines showed
inferior catalytic activities. The initial reaction rate of #PS-NH2 HMOPNs(115) decreased to 52.5 mmol L1 min1
(Fig. 7e), resulting from damage to the free spaces around –NH2 sites (Fig. 5b) and the hollow interior (Fig. 4d).
At the DMF-swollen temperature of 145 °C, the initial reaction rate of as-prepared #PS-NH2 HMOPNs(145) sharply
decreased to 37.5 mmol L1 min1 (Fig. 7f). From the FT-IR spectrum of #PS-NH2 HMOPNs(145) (Fig. S9c), the
newly emerged N–H bending vibration at 1670 cm1 indicated that some –NH2 groups reacted with the carboxylic
ester in EGDMA to form the corresponding amides. Meanwhile, a strong decrease in mesopores below 10 nm and
complete collapse of the well-shaped nanobowls into floccus solids were observed using the pore size distribution
(Fig. 5a) and SEM and TEM images (Fig. S6). These three factors unfavorable to the mass transfer of reactants and
reactivity of –NH2 sites resulted in a sharp decrease in the catalytic activity of #PS-NH2 HMOPNs(145).
3.4.3. Scope of Knoevenagel condensations
Table 2 Knoevenagel condensation reactions of aromatic aldehydes and malononitrile catalyzed by #PS-NH2
HMOPBs(80) and PS-NH2 HMOPBs.a
Entry
1
Cat.
Ad
R
Time (h)
3
Conv. (%)b
>99
Yield (%)c
82
2-NO2

Be
Bf
2
2
3
2
3
2
3
2
3
2
3
2
3
2
3
2
3
2
2
3
2
3
>99
>99
>99
>99
>99
>99
96
85
96
82
86
96
96
92
93
94
95
95
96
97
98
94
96
81
84
95
80
85
79
2
3
4
5
6
7
8
9
Ad
Be
Ad
Be
Ad
Be
Ad
Be
Ad
Be
Ad
Be
Ad
Be
Ad
Be
Bf
3-NO2
4-NO2
2-OCH3
3-OCH3
2-CH3
3-CH3
4-CH3
2-Cl
98
98
99
99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
10
11
Ad
Be
Ad
3-Cl
4-Cl

Be
Ad
Be
2
3
2
>99
>99
>99
85
98
98
12
H
a Reaction conditions: benzaldehyde (106.0 mg, 1.0 mmol), malononitrile (79.0 mg, 1.2 mmol), Cat. (61.0 mg, 5 mol%),
2 mL of ethanol, 25 °C.
b Determined by HPLC.
c Isolated yield.
d A: PS-NH2 HMOPBs.
e B: #PS-NH2 HMOPNs(80).
f Toluene as solvent.
Encouraged by the excellent catalytic activities of #PS-NH2 HMOPNs(80) at 25 °C, the scope of Knoevenagel
condensation was broadened to various aromatic aldehydes. The catalytic results are summarized in Table 2. Except
for p-NO2-C6H4CHO, the aromatic aldehydes bearing electron-withdrawing groups, such as nitro (–NO2) and chloro
(–Cl) groups attached to the o-, m- and p-positions of phenyl were less reactive in ethanol, producing the
corresponding products in only 76%–86% yields, despite high conversions of the aromatic aldehydes (>99%). These
results were inconsistent with previously reported results [70–72,20,21,23,26]. When the temperature was increased
to 60 °C, the yields were not significantly improved. Fortunately, high yields of above 95% were achieved in toluene
(Table 2, entries 1 and 9). Aromatic aldehydes bearing electron-withdrawing groups were speculated to react with
ethanol to form corresponding hemiacetals, which resulted in the decreased yields. For aromatic aldehydes
containing electron-donating groups, such as o-, m-, or p-OCH3 and –CH3, high yields (92-98%) were obtained at
room temperature. Knoevenagel condensations at room temperature that afforded high yields in short reaction times
using a low catalyst loading (5 mol%) were attributed to the comprehensive effect of thin shells, free space around

the amine sites and mesoporous channels of hollow #PS-NH2 HMOPNs(80) and PS-NH2 HMOPNs.
3.4. Catalyst reusability and restoration
Fig.8. Reusability (a) of #PS-NH2 HMOPBs (80), pore diameter distribution (b) of fresh, 10th-reused, and
restored #PS-NH2 HMOPBs(80), SEM (c) and TEM (d) images of 10th-reused #PS-NH2 HMOPBs(80).
The reusability of #PS-NH2 HMOPBs(80) and PS-NH2 HMOPBs in the Knoevenagel condensation of
benzaldehyde and malononitrile was investigated under the optimized conditions. When the Knoevenagel
condensation was determined to be complete by TLC monitoring, the catalyst was separated by centrifugation,
washed several times with ethanol, and reused directly in the next catalytic cycle. The yields are shown in Fig. 8a
and Table S3. As shown in Fig. 8a, #PS-NH2 HMOPBs(80) showed higher yields than PS-NH2 HMOPBs until the
tenth run. However, a small decrease was observed in the yield from the first run (98% and 98%) to the tenth run
(92% and 89%), for #PS-NH2 HMOPBs(80) and PS-NH2 HMOPBs, respectively. Using #PS-NH2 HMOPBs(80) as
an example, N2 adsorption–desorption isotherm, SEM, and TEM were used as tools to explain the decreased yield.
The SEM and TEM images showed that #PS-NH2 HMOPNs(80) reused ten times maintained the original bowl-like
morphology (Fig. 8c) and hollow interior (Fig. 8d). The N2 adsorption–desorption isotherm of #PS-NH2
HMOPNs(80) reused ten time showed that the D(r) values of the mesopores (3–50 nm) were significantly decreased

(Fig. 8b), and that the specific surface area and pore volume decreased to 25.6 m2·g1 and 0.08 cm3·g1, respectively.
Therefore, the decreased yields were speculated to be attributed to blockage of the mesopores, resulting in the poorer
mass transfer of reactants. The removal of plugging material was an effective and convenient strategy for restoring
the catalytic activity of #PS-NH2 HMOPNs(80). The catalyst (100 mg) reused for the tenth time was allowed to stir
in HCl-containing (1.0 mol L1) aqueous ethanol solution (1/1, v/v) at room temperature for 12 h and separated by
centrifugation. HPLC spectra indicated that the yellow centrifugate contained a large number of
ylidenemalononitriles, confirming that the bulky product clogged mesopores in the shell. After isolated #PS-NH2
HMOPNs(80) was neutralized by NaOH-containing aqueous ethanol solution at room temperature for 12 h, the
specific surface area and pore volume of #PS-NH2 HMOPNs(80) were refreshed to 41.8 m2·g–1 and 0.25 cm3·g–1,
and the mesopores (3–50 nm) were reconstructed (Fig. 8b). As expected, the catalytic activity of solid amines was
restored to achieve a high yield (96%) similar to a fresh solid amine catalyst. In the subsequent six runs, high yields
above 90% were obtained (Fig. 8a).
4. Conclusions
In heterogeneous catalysis, chemical catalytic reactions are conducted at the surface of catalytically active sites.
Pore-, size-, and morphology-dependent factors, such as mesopores, hollow interior, and transmission thickness, are
keys to the accessibility of reactants to catalytic sites. Herein, the other contributors to mass transfer, namely, free
spaces around base sites and interconnected mesoporous channels, were designed and applied to knoevenagel
condensation. Through etching the PS core in the interior and the attached organic acid in the shell to release their
occupied spaces, and construction of mesoporous channels via swelling in DMF, annealing in liquid N2, and frozen
DMF removal, nanobowls with desired architectural features, such as hollow interior, free spaces around amine
sites, and interconnected mesoporous channels, were fabricated to effectively improve the accessibility of reactants

to the surface of base sites. The as-fabricated amine-functionalized hollow nanobow-like solid base promoted
Knoevenagel condensations of various aromatic aldehydes with malononitrile at room temperature in high yields.
Furthermore, the solid base possessed good reusability, and was restored using aqueous HCl solution to achieve the
original high catalytic activity. Therefore, the construction of free spaces around the base site and interconnected
mesoporous channels between them, is an effective and facile method for fabricating permeable skeleton structures
in polymeric amine base catalysts to achieve fast mass transfer in the catalytic process. The nanobowl-like hollow
solid amine base with imprinted free spaces around catalytic sites and interconnected mesopores is believed to have
high potential in heterogeneous base catalysis. The new concept of the construction of free space around the catalytic
site and mesoporous channel to access catalytic sites is of interest in environmental sciences, such as coordination
chemistry and carbon capture To further improve this strategy, possible future directions include: (i) Accurate
control of the volume of free room around amine sites; (ii) the preparation of more rigid and stable polymeric
skeletons suitable for pore maintenance; and (iii) investigations into the applicable scope for other organic base.
CRediT author statement
Li Zhang: Data curation, Visualization, Investigation. Jianing Zhang: Formal analysis,
Software. Shuai Wei: Validation, Resources. Shan Li: Resources. Xuebing Ma:
Conceptualization, Methodology, Writing-Reviewing and Editing, Project administration,
Funding acquisition.
Declaration of interests
No.
Acknowledgements
This work was supported by National Natural Science Foundation of China [51973177], and Basic Science and
Advanced Technology Research Project of Chongqing Science&Technology commission, P. R. China (cstc
2017jcyjBX0027). We thank Simon Partridge, PhD, from Liwen Bianji, Edanz Editing China

(www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.
Supplementary data
Characterizations of solid bases such as FT-IR, SEM, TEM and N2 adsorption–desorption isotherms, data of
products such as 1H NMR and synthetic procedures of monomers and contrast solid bases can be found, in the online
version, at…...
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