Hybrid mesoporous organosilicas with molecularly imprinted cavities: Towards extended exposure of active amino groups in the framework wall

Article abstract of DOI:10.1039/c7dt04832g

Hybrid molecularly imprinted mesoporous silicas were synthesized by co-condensation of tetraethoxysilane and functional organosilica precursors of HQP and BPAP, in which hydroquinone (HQ) and bisphenol A (BPA) were linked as imprinting molecules. Owing to

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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Hybrid mesoporous organosilicas with molecularly imprinted
cavities: towards extended exposure of active amino groups in
the framework wall
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
a,b
DOI: 10.1039/x0xx00000x
Hang Huo , Xianzhu Xu , Tingting Zhao , Yudong Li , Yanqiu Jiang* and Kaifeng Lin*
www.rsc.org/
Hybrid molecularly imprinted mesoporous silica were synthesized by co-condensation of tetraethoxysilane
and functional organosilica precursors of HQP and BPAP, in which hydroquinone (HQ) and bisphenol A (BPA)
were linked as imprinting molecules. Owing to the existence of thermally reversible covalent bond of
carbamate (-NH-COO-), the imprinting molecules could be eliminated under thermal treatment and
molecularly imprinted cavities were formed in framework wall. All of these materials were used to catalyze
heterogeneous Knoevenagel reactions and proved to exhibit higher catalytic conversion and turnover
frequency (TOF) number compared with the materials with imprinting molecules, which is attributed to the
presence of amino groups with higher basicity and molecularly imprinted cavities. Importantly, compared
with amino functionalized SBA-15 materials, the decisive role of molecularly imprinted cavities on the
enhanced accessibility of amino groups in mesoporous framework was further confirmed. Moreover, the size
of imprinting molecules has an influence on the catalytic conversion and TOF values: imprinting molecules
with relatively bulkier size tend to lead higher accessibility to the active sites. The amino groups in the
framework are extremely stable during the reaction procedure and recycle process.
combination of robust inorganic framework and active organic
species tremendously enriches the diverse application of the
Introduction
Silica oxide based mesoporous materials as a branch of inorganic
molecular sieves have potential industrial application in the fields of
2
4
6
materials, for instance, enzyme immobilization , chiral separation ,
2
5, 26
13, 27, 28
photochemistry
and acid/base catalysis
.
In the field of catalysis, PMO materials show perfect catalytic
performance as their large surface area, ordered porous structure,
thermal stability and abundant active species. However, it should
be taken into account that traditional PMO may suffer from a kind
of shortcoming that the access of reactants to the active species is
limited to a certain extent because some of the active species are
buried in the framework of the material. This kind of limitation can
be amplified when the bonding of active species and the framework
1
-4
5
6
catalysis
, drug or gene delivery , chromatography and
7
adsorption due to their biocompatibility, large specific surface
area, tunable pore size and thermal stability. In the year of 1999,
periodic ordered mesoporous organosilicas (PMOs) were first
synthesized via the co-condensation of inorganic silica species and
8
, 9
bridged organosilica precursors [(RO)
3
Si-R-Si(OR)
3
]
, which were
homogeneously distributed in the three dimensional framework. As
a novel concept, the development of PMOs was promising and
2
9
wall is rigid and the pore distribution is narrow . An alternative
way to overcome this problem is post-grafting method, by which
organic active species are anchored to outer surface or internal
1
0
rapid . A range of organic functional groups were successfully
incorporated into the framework of the materials, including ionic
1
1, 12
13
liquid
, alkane, aromatic benzene derivatives , cyclohexane-
3
0-32
channels of pre-synthesized silica matrix
however, some of the pores may be more or less blocked by organic
. In certain cases,
1
4
15
diamine ，binol or even a metallic center chelating to an organic
1
6, 17
ligand, as salon skeleton
. In addition to adjusting the linkage of
3
3-35
species when they are tethered in the channels
, which makes
1
8
silica blocks, decorating the organosilica precursors with alcohols ,
those pores inaccessible to reactants due to the diffusion limitation.
So it is still necessary to pay more effort to explore an effective
method to overcome these weaknesses.
1
9
20
21
22, 23
thiols , sulfonic , chiral groups and acid or base species
also
makes it possible to the successful synthesis of PMOs. The
Molecular imprinting is known as a technology that can
contribute molecularly imprinted cavities in a solid polymer matrix
to recognize template molecules or their analogues by
covalence/non-covalence interaction or hydrogen bond. Up to now,
molecularly imprinted materials have been widely used in the field
aSchool of Chemistry and Chemical Engineering, Harbin Institute of Technology,
Harbin, Heilongjiang, 150001, P. R. China.
Key laboratory of Advanced Energy Materials Chemistry (Ministry of Education),
b
Nankai University, Tianjin, 30071, P. R. China.
E-mail: jiangyanqiu@hit.edu.cn; linkaifeng@hit.edu.cn.
Electronic Supplementary Information (ESI) available: Fig. S1-S8, Scheme S1 and
Table S1. See DOI: 10.1039/x0xx00000x
3
6, 37
38, 39
of heterogeneous catalysis
, sustained release drugs
,
chemical bionic sensor , exclusive adsorption and separation41,42. It
40
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has been reported that the active sites buried in solid polymers are
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DOI: 10.1039/C7DT04832G
3
6
inactive , whereas molecularly imprinted cavities in the materials
make these active sites accessible to the reactants. Therefore,
introducing some molecularly imprinted cavities nearby active sites
in framework of PMO is reasonably expected to make an effort
towards extended exposure of the active sites located in the
framework, thus resulting in the enhanced accessibility. It is worth
noting that such molecularly imprinted mesoporous organosilicas
can be prepared using a semi-covalent imprinting technique. On
such materials, a study has also been performed to compare
materials imprinted with a range of molecularly imprinted cavities
of varying size in mesoporous silicas and their recognizing
properties for different imprinting molecules. Although such
molecularly imprinted materials have not been devoted to apply as
heterogeneous catalysts, the study inspired us a useful technique to
prepare molecularly imprinted mesoporous organosilicas.
In our study, we prepared a series of molecularly imprinted
hybrid mesoporous materials using hydroquinone (HQ) and
bisphenol A (BPA) as imprinting molecules via a triblock copolymer
templated sol-gel approach. Removing the imprinting molecules by
thermal treatment in DMSO yields novel materials with molecularly Scheme 1 Synthesis of HQP/BPAP with HQ/BPA and ICPTES in stoichiometric
ratio
imprinted cavities and functional amino groups in the framework. It
is worth noting that based on the fact that most of the amino
groups can be exposed in the molecularly imprinted cavities and no
organic groups occupy the mesopore channels, such hybrid
materials showed enhanced catalytic performance in
heterogeneous Knoevenagel reactions compared with the materials
containing imprinting molecules in the framework wall and the
traditional amino functionalized one-pot synthesized or post-
grafted SBA-15 materials.
methanol. Then, removing the solvent in vacuum gets the product
as a white powder, and the yield is 91%. The H NMR spectrum is
shown in Fig. S1A. Bisphenol A-based organosilica precursor (BPAP)
was synthesized in a similar way with bisphenol A (BPA, 15 mmol)
and 3-Isocyanatopropyltriethoxysilane (ICPTES, 30 mmol) as starting
materials, and the reaction was carried out in THF and refluxed for
1
2
4 h under an inert atmosphere. The BPAP was obtained with yield
1
of 87% as a white powder and its H NMR spectra is shown in Fig.
S1B.
Experimental Section
Chemical reagents
Preparation of molecularly imprinting mesoporous
(
3-Isocyanatopropyl) triethoxysilane (ICPTES), hydroquinone and silica of MIMS-NHQ and MIMS-NBPA
bisphenol A were purchased from Aladdin Industrial Corporation
and used without further purification. tetraethoxysilane (TEOS) was
obtained from Sinopharm Chemical Reagent Beijing Co., Ltd.
PluronicP123 (P123, EO20PO70EO20) was purchased from Aldrich. All
chemical reagents were used as received.
In the molecularly imprinted mesoporous silica materials, the
imprinting molecules of HQ and BPA are covalent bonded with the –
NH-CO- group, and they are denoted as MIMS-NHQ/NBPA (x) (x
represents the molar ratio of TEOS and HQP or BPAP, and x= 7, 10,
14 for HQP or x= 14 for BPAP). Here the characters of "NHQ" (or
"
NBPA") mean that the imprinting molecules of HQ (or BPA) are
Preparation of organosilica precursors of HQP and contained in the materials and linked to thecarbamate (-NH-COO-)
BPAP
bond. These materials are synthesized by the co-condensation of
organosilica precursor and TEOS in the presence of triblock
copolymer P123 as template under acidic condition. In a typical
preparation method for MIMS-NHQ (14), 0.8g of P123 was
The organosilica precursors were synthesized according to
4
3
previously reported method and the synthesis process is
illustrated in Scheme 1. Typically, the hydroquinone-based
organosilica precursor (HQP) was synthesized as follows:
hydroquinone (HQ, 15 mmol) and (3-Isocyanatopropyl)
triethoxysilane (ICPTES, alkoxysilane monomer; 30 mmol) were
added to 20 mL of absolute tetrahydrofuran (THF) in a round-
bottomed flask with a magnetic stirrer, and the reaction proceeded
under an inert atmosphere at the temperature of 65 C for 24 h.
After the reaction, the volatile solvent of THF was evaporated by a
rotary evaporator. The crude product was purified by washing with
o
dissolved in 25 mL pH= 1.5 HCl solution at the temperature of 40 C
for 12 h. Afterwards, 1.5 mL of TEOS and 0.29 g of HQP precursor
dissolving in 1.0 mL of ethanol were added to the solution in
o
dropwise and stirred at 40 C for 24 h. Finally, the suspension was
o
aged at 100 C for 48 h in a sealed PTFE autoclave. The surfactant
was removed by refluxing with 85 mL of a mixture (ethanol/HCl;
o
8
0:5, v/v) for 24 h. After filtration and being washed thoroughly
with deionized water, a white powder was obtained. MIMS-NHQ
10), MIMS-NHQ (7) and MIMS-NBPA (14), were synthesized in the
(
2
0 mL of hexane and recrystalliz ed in solvent of
same waywith 0.41 g HQP, 0.54 g HQP, 0.35 g BPAP, respectively. In
2
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these cases, the imprinting molecules (HQ and BPA) were
incorporated in the mesoporous framework wall.
The carbon dioxide chemisorption was conducted on
Micromeritics AutoChem 2920 instrument with a TCD detector. The
a
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DOI: 10.1039/C7DT04832G
typical CO
2
chemisorption was carried out as follows: about 100 mg
o
of sample was heated to 100 C under helium atmosphere and kept
for 20 minutes. Then the temperature was cooled to 75 C, CO
Preparation of molecularly imprinting mesoporous
silica of MIMS-HQ and MIMS-BPA
o
2
chemisorption was performed with a flow of 10 ml/min for 30
minutes. When the temperature was reduced to 30 C, the
The materials with molecularly imprinted cavities in framework
were synthesized with thermal treatment of MIMS-NHQ/NBPA (x)
by refluxing in DMSO to remove imprinting molecule HQ or BPA,
and the materials are designated as MIMS-HQ/BPA (x) (x represents
the molar ratio of TEOS and HQ/BPA, and x=7, 10, 14 respectively).
In a typical procedure, to a round-bottomed flask, MIMS-NHQ (14)
was suspended in a solution of 60 mL dimethylsulfoxide (DMSO)
and 5 mL deionized water. Then the mixture was heated with
o
desorption was executed by purging helium gas at rate of 50
o
mL/min and ramping the temperature from 30 to 100 C at a rate of
o
o
5
C/min and holding at 100 C for 30 minutes. The CO
2
uptake was
calculated on the basis of the desorption curve.
Adsorption of imprinting molecules with the material
as a solid absorbent
o
stirring for 6 hours at 165 C. After such thermal treatment, the
imprinting molecules (HQ and BPA) were removed and the terminal The static adsorption experiments were carried out as following:
amino groups and molecularly imprinted cavities were formed in 20.0 mg of MIMS-HQ (14) was mixed with 3.0 mL HQ solution at
-
1
the mesoporous framework wall. The thermal-treated mesoporous concentration of 1.5 mmol L in n-hexane/n-propanol (95:5, v/v).
o
materials were isolated by filtration, washed with deionized water The mixtures were stirred at 150 rpm for 20 h at 25 C. After
o
and ethanol and dried in vacuum oven at 60 C for 6 h, leading to binding, the mixtures were filtered through a 0.22 μm membrane,
the formation of the resultant materials of MIMS-HQ (7), MIMS-HQ and the concentration of the filtrates were determined by UV- vis
(
10), MIMS-HQ (14) and MIMS-BPA (14).
characterization. MIMS-BPA (14) was conducted in the same
treatment for the adsorption experiment except with a solution of
BPA. At the same time, we also used a sample that was synthesized
only with inorganic precursor TEOS (the sample was designated as
SBA-15) for comparison. The adsorption capacity was calculated
according to the following equation:
Characterization
Solution 1H NMR spectrums of HQP and BPAP in CDCl
obtained at 25 C on Bruker-400 MHz spectrometer using CDCl
solvent and TMS as internal standard. Si (79.4 MHz) CP-MAS NMR
was obtained on Bruker-AVANCE III-400 WB spectrometer with
samples packed in ZrO
X channel of the 4 mm standard bore CP MAS probehead was tuned where C
to 79.5 MHz for Si and the other channel was tuned to 400.18
MHz for broad band H decoupling, at a magnetic field of 9.39T of
3
were
as
o
3
2
9
(
C − C ) * ν
0
f
Q =
rotator with KeI-F cap spinning at 8 kHz. The
m
2
-
1
-1
0
f
(mmol L ) and C (mmol L ) are the initial and final
2
9
concentration of template molecules, ν (L) is the total volume of the
solution, m (g) is the mass of the sorbent, Q is the amount of
template molecules adsorbed, respectively.
1
2
9
2
97 K with the contact time of 2 ms. All Si CP-MAS chemical shifts
are referenced to the resonances of 3-(trimethylsilyl)-1-
propanesulfonic acid salt standard. FT-IR spectrum was obtained,
using the KBr buffer technique, on Perkin-Elmer Spectrum 100
Catalytic procedure of Knoevenagel reaction
In a typical procedure, the reaction mixture consisted of 40 mg of
catalyst, 2 mL of absolute toluene, 1 mmol of ethyl cyanoacetate
and 1 mmol of p-Anisaldehyde was allowed to reflux at 110 C for 3
hours. After cooling to room temperature, the catalyst was
separated from the reaction mixture via centrifugation and the
liquid was analyzed on a gas chromatograph instrument (7890A
with FID detector) with n-Butanol as internal standard substance.
The assignment of the products was further confirmed by GC-MS
experiment. Similarly, the blank comparison experiment was carried
out in the same condition except the addition of catalyst.
-
1
spectrometer in the wavenumber range from 4000 to 400 cm .
Powder XRD was recorded on Bruker -D8 ADVANCE diffractometer
with a Ni-filtered Cu-Kα (λ= 1.5418 Å) radiation operating at 40 kV
o
o
o
and 30 mA in the 2θ range from 0.8 to 6 . The nitrogen adsorption-
desorption measurement was carried out at 77 K on ASAP 2000
system and the specific surface area and pore distribution were
calculated from the adsorption isotherm by using Brunauer-Emmet-
Teller (BET) and Barrett-Joyner-Halenda (BJH) methods,
respectively. For all of the materials, the average pore size was
estimated from the pore size distribution curve at the maximum
point. TG/DTA analysis was recorded on TA-SDT Q600 analyzer
o
o
under air condition with the temperature range from 30 C to 800 C.
C, H and N elemental analysis was recorded on PerkinElmer PE
Results and discussion
Synthesis of the materials
2
400II, and each sample was tested for twice. SEM and TEM images
were recorded on Hitachi SU8010 series and Hitachi H-7650 Preparation of MIMS-NHQ and MIMS-NBPA materials containing
electron microscope, respectively. A UV-Vis spectrophotometer HQ and BPA units as imprinting molecules was accomplished
(
BLV-GHX-V, Bilang Corporation, China) was utilized to detect the following a procedure similar to the synthesis of hybrid organic
concentration of imprinting molecules in the adsorption incorporated mesoporous SBA-15 material. At first, HQ and BPA
experiment.
molecules were covalent condensed with ICPTES to form
organosilica precursors HQP and BPAP. Subsequently, the silylated
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precursors were combined with TEOS in the synthesis of MIMS- organosilica precursors were successfully embedVdieewdAritnictleoOntlhinee
NHQ and MIMS-NBPA, using P123 as structure directing agent. framework of mesoporous materials. BeinDgOIh: 1e0a.t1e0d39/inC7DDTM04S8O32aGt
Scheme 2 Possible procedure for fabricating the hybrid mesoporous
materials with molecularly imprinted cavities
Fig. 1 FT–IR spectra of MIMS-NHQ (14), MIMS-HQ (14) (left) and MIMS-
NBPA (14), MIMS-BPA (14) (right)
Materials of MIMS-HQ and MIMS-BPA were derived from MIMS-
NHQ and MIMS-NBPA as the carbamate group in HQP and BPAP can
thermally decompose to amino group with water.
o
1
65 C, the carbamate group of HQP/BPAP was decomposed into
According to the synthesis of SBA-15 type materials, it is possible
to propose a route for fabrication of the hybrid molecularly
imprinted materials and the materials with cavities in the
mesoporous framework wall (Scheme 2). Under acidic condition,
TEOS and organosilica precursors hydrolysed and co-condensed
synergistically to construct 2D hexagonal p6mm structure with
imprinting molecules in the framework, in which P123 was used as
structure directing agent. Then, the materials with molecularly
imprinted cavities andextended exposure of amino groups in the
framework wall were obtained via crystalization and removal of
P123 followed by being heated in DMSO. During the thermal
isocyanato intermediate and HQ/BPA molecules, then the
isocyanoto intermediate was hydrolyzed into amino groups that
were anchored in the molecularly imprinted cavities.
The fact that the two organosilica precursors were incorporated
into the framework wall during the co-condensation procedure can
29
29
also be evidenced by Si MAS NMR. In the Si spectra of materials
of MIMS-HQ (14) and MIMS-BPA (14), five peaks can be observed
(
Fig. S2). The chemical shifts of siloxanes and partially condensed
siloxanes are well-known.Two obvious peaks at -60 ppm and -68
ppm for both of the materials can be assigned to T-type organosilica
2
3
species of T [RSi(OSi)
2 3 2 3 2
(OH)] and T [RSi(OSi) ] (R = -(CH ) -NH ).
o
treatment at 165 C, the covalent incorporated HQP and BPAP can
decomposed into HQ/BPA and ICPTES. In a further step, the ICPTES
Three peaks at -93 ppm, -102 ppm and -113 ppm respectively can
2
3
2 2
be assigned to Q-type silicon species of Q [Si(OSi) (OH) ], Q
moiety can hydrolysis with adventitious water into amine and CO
2
,
4
[
Si(OSi)
3 4
(OH)] and Q [Si(OSi) ].
leading to the functional organic group in the molecularly imprinted
cavities. The react procedure was illustrated in Scheme S1 in
supporting information. In sol-gel procedure, the precursor species
and ratio of organosilica precursors have an effect on the
morphology, mesopore structure and mesopore ordering of the
terminal materials.
The incorporation of HQP/BPAP and the removal of HQ/BPA can
also be obviously distinguished from the TG (Fig. S3) analysis. It is
shown that the mass loss increases from 18% to 25% with the
dosage of the organosilica precursors increased for molecularly
imprinted materials of MIMS-NHQ and from 16% to 22% for non-
imprinted materials of MIMS-HQ. Clearly, more molar ratio of HPQ
was used in the material, higher mass loss can be observed. At the
same time, the materials of MIMS-NHQ display a higher mass loss
than the materials of MIMS-HQ owing to the removal of the
imprinting molecules. MIMS-NBPA (14) prepared with larger size of
imprinting molecule of BPA gives higher mass loss compared with
MIMS-NHQ (14), implying larger volume of imprinted cavities in
MIMS-BPA (14) can be formed after removal of the imprinting
molecules.
The powder XRD characterization of hybrid materials MIMS-NHQ
shows three well-resolved Bragg diffraction peaks in region of 2θ =
.5~2.3º after removal of the surfactant P123 by solvent extraction
of ethanol and concentrated hydrochloric acid that can be indexed
as [100], [110], [200] reflections. These reflections are typical for a
well-ordered 2D-hexagonal (p6mm) mesostructure. (Fig. 2, left).
Meanwhile, three reflections on [100], [110], [200] plane in the
Characterization of the materials
The chemical composition of the molecularly imprinted hybrid
materials and non-imprinted materials was characterized by FT-IR
spectroscopy. The FT-IR spectrum of molecularly imprinted hybrid
-
1
material MIMS-NHQ (14) shows two peaks at 1720 cm and 1503
-
1
cm stretching vibrations of C=O bond in carbamate and C-H bond
in benzene moiety, respectively. After thermal treatment, both of
the two stretching vibrations disappear and a bending vibration
0
-
1
2
emerges at 1670 cm for NH group (Fig. 1 left). The same vibration
characters have also been observed in the case of MIMS-NBPA (14)
and the thermal treatment also works for creation of amino groups
in MIMS-BPA (14) (Fig. 1 right). All of the results indicate that the
4
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Fig. 3 TEM images of a) MIMS-HQ (7), b) MIMS-HQ (10), c)ViMewIMArSti-cHleQOn(1lin4e)
and d) MIMS-BPA (14).
range of 2θ =0.5~2.3º for materials of MIMS-HQ can be indexed as
the preservation of highly ordered mesostructure after the thermal
treatment in solvent of DMSO, as seen in Fig. 2, right. Nevertheless,
DOI: 10.1039/C7DT04832G
and MIMS-HQ exhibit noodle-like shape with rough surface,
whereas MIMS-NBPA and MIMS-BPA possess irregular particles
with random shapes. This confirms that a larger size of organosilica
precursor strongly influences co-condensation procedure with
TEOS, as observed in XRD results. Note that the HQP precursor can
be homogeneously incorporated into framework wall of the
materials with a uniform morphology even the molar ratio of TEOS
and organosilica HQP precursor was decreased to 7. The
morphology of the materials could be retained during the
procedure of eliminating the imprinting molecules by refluxing in
DMSO for 5-6 hours, demonstrating that the framework wall of the
materials is robust enough to endure high temperature. At the
same time, the energy- dispersive spectroscopy mappings (Fig. S5)
were taken on the materials of MIMS-NHQ (14), MIMS-HQ (14),
MIMS-NBPA (14) and MIMS-BPA (14). It is observed that the N
element is uniformly distributed in the framework of all materials,
which suggests that the condensation of HQP or BPAP with TEOS
randomly occurs.
Fig. 2 Powder XRD patterns of the hybrid mesoporous materials of MIMS-
NHQ/NBPA (left) and MIMS-HQ/BPA (right)
Transmission electron microscopy (TEM) images (Fig. 3) of the
only one broad diffraction peak in region of 2θ =0.5~3.0º can be hybrid mesoporous materials show the existence of mesopores
observed in the materials of MIMS-NBPA and MIMS-BPA, pointing after removing the surfactant P123. Clearly, MIMS-HQ (7), MIMS-
to a kind of less ordered mesostructure. The absence of [110], [200] HQ (10) and MIMS-HQ (14) possess ordered hexagonal mesopores,
reflections indicates limited periodicity in the long range order. One whereas the less ordered worm-like mesopore structure is
can conclude that varying the imprinting molecule from HQP to observed in MIMS-BPA (14). This result indicates that the
BPAP with a larger molecular size broadens the [100] peak and incorporation of a larger organic groups into the skeleton may
vanishes the [110] and [200] peaks, which suggests that a larger size destroy the ordered mesostructure, which is in accordance with the
oforganosilica precursor strongly influenced co-condensation XRD results. Additionally, the pore size measured from the TEM
procedure with TEOS and tended to destroy the ordered assembling images is around 7.2 nm, and this is very close to that calculated
micelles, and finally resulted in disordered mesostructure. This can
be obviously evidenced by the corresponding SEM and TEM images.
The morphology and mesoporous structure of the hybrid
Table 1 Physicochemical properties of the hybrid mesoporous materials
mesoporous materials were revealed by SEM and TEM techniques.
Pore
N contenta
%)
BET surface
Pore volume
Based on the SEM images (Fig. S4), the morphologies of MIMS-NHQ
Samples
diameterb
2
-1
3 -1
(
area (m g )
(cm g )
0.97
1.07
0.92
1.05
0.76
0.77
0.35
0.60
0.64
(nm)
MIMS-NHQ
3.3
3.4
2.4
2.5
2.0
2.2
1.3
1.6
536
6.8
(
7)
MIMS-HQ
7)
MIMS-NHQ
10)
MIMS-HQ
10)
MIMS-NHQ
14)
MIMS-HQ
14)
512
6.9
7.7
7.9
6.2
7.5
3.2
3.4
6.2
(
531
(
498
(
634
(
623
(
MIMS-
430
NBPA (14)
MIMS-BPA
730
(14)
SBA-15-
APTES
2.33
513
SBA-15-
APTES-
grafted
3.41
288
5.5
0.39
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[
[
a] Calculated from the C, H, N elemental analysis.
molecules, these materials were applied as solid-phase extraction
absorbent for template molecules hydroquinone and bisphenol A.
View Article Online
DOI: 10.1039/C7DT04832G
b] Calculated from the adsorption branch with the BJH method.
Fig. 5 Adsorption capacity of imprinting molecules HQ and BPA on SBA-15,
MIMS-HQ (10) and MIMS-BPA (14)
Based on the UV-vis absorption results, the adsorption capacities
of materials MIMS-HQ (14) and MIMS-BPA (14) were calculated as
-
1
-1
1
71.8 μmol g and 246.6 μmol g , respectively. Meanwhile, the
sample of pure SBA-15 just displayed much less adsorption of HQ
-
1
-1
Fig. 4 N
distribution (right) for materials of (a) MIMS-NHQ (7), (b) MIMS-NHQ (10),
c) MIMS-NHQ (14), (d) MIMS-NBPA (14) (A) and (a) MIMS-HQ (7), (b) MIMS-
HQ (10), (c) MIMS-HQ (14), (d) MIMS-BPA (14) (B)
2
adsorption and desorption isotherms (left) and pore size (39.2 μmol g ) and BPA (33.0 μmol g ), as shown in Fig. 5. Clearly,
the molecularly imprinting cavities were successfully formed after a
given thermal treatment.
(
Catalytic performance
from N
2
adsorption test.
The Knoevenagel reaction of aldehyde is one of the most important
The pore size and other physicochemical properties of the hybrid and widely used methodology for C-C bond formation in the
materials were further investigated by N adsorption (Fig. 4). For synthetic field of fine chemical industry as well as biological
each sample, the adsorption-desorption isotherm shows a type IV heterocyclic compounds . Usually, this kind of reaction is catalyzed
curve with H -type hysteresis loop at relative pressure P/P region by weak base chemicals like amines, piperidine and so on. In view of
higher than 0.5, which is the typical characteristic of ordered the existence of weak base species of amino groups in our
2
4
5
2
0
4
4
mesoporous materials . Table 1 displays physical parameters of materials, all of these hybrid mesoporous materials were evaluated
the hybrid materials. Interestingly, the BET surface area and pore as heterogeneous catalysts in Knoevenagel reactions and proved to
volume increase dramatically in material of MIMS-BPA than those in exhibit excellent catalytic activity and selectivity for various
MIMS-NBPA, implying the formation of molecularly imprinted aromatic aldehydes. Table 2 displays the catalytic performance of
cavities after the removal of imprinting molecules. Such Knoevenagel reactions about anisaldehyde and ethyl cyanoacetate
phenomenon is less pronounced in the cases of MIMS-HQ and with different catalysts. For the result of blank reaction, no product
MIMS-NHQ. The reason may be that the skeleton of these materials formed without catalyst under the condition (entry 1). All of the
o
can be shrunk further when heated at 165 C in DMSO, leading to samples containing N species were active and selective for the
decline of the BET surface. However, larger imprinting molecules reaction, and therefore, N species were active sites for the
facilitate to create larger volume of molecularly imprinted cavities, reactions.
which can make up for the loss of the BET surface. Obviously, the
issue of HQ type materials is inclined to the former factor.
In order to evaluate the basicity of the materials and the
accessibility of the N species, CO
chemisorption characterization of
2
all the catalysts was carried out and the results are summarized in
Table S1. It has been reported that the weak chemical interactions
Adsorption of the imprinting molecules
between CO
sorption of CO
molecularly imprinted cavities (MIMS-HQ and MIMS-BPA) possess
higher absorbed CO amount compared with the corresponding
2
and amine groups may play an important role in
To confirm the presence of molecularly imprinted cavities,
adsorption experiments on imprinting molecules were carried out.
In view of molecular imprinting polymers can recognize specific
substrate in size, shape and functional groups similar to template
2
at 60-90°C. Clearly, all the materials with
2
materials containing imprinting molecules in framework (MIMS-
6
| J. Name., 2012, 00, 1-3
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ARTICLE
NHQ and MIMS-NBPA). Higher content of CO
2
absorption points to materials are similar, the difference of TOF values relieVsieownArttihclee Ovonliidne
higher basicity of the material and easier accessibility of the N volume of molecularly imprinted cavities. DAOpIp: a10re.1n0t3l9y/,Ct7hDeT0s4iz8e32oGf
Table 2 Catalytic conversion of anisaldehyde employing different catalysts in
Knoevenagel reactiona
NC
O
O
O
Cat.
H
3
CO
CHO
+
NC
toluene
o
H
3
CO
O
1
10
C
Entry
Catalyst
Conversion (%)b
﹤ 1
TOF (h-1)c
1
2
3
4
5
6
7
8
9
Blankd
-
MIMS-NHQ (7)
MIMS-HQ (7)
89.5
16.0
23.3
15.4
24.4
29.5
30.3
15.2
35.8
94.5
MIMS-NHQ (10)
MIMS-HQ (10)
MIMS-NHQ (14)
MIMS-HQ (14)
MIMS-NBPA (14)
MIMS-BPA (14)
85.5
93.1
Fig. 6 Catalytic activities in Knoevenagel reaction as a function of reaction
time over MIMS-HQ/NHQ and MIMS-BPA/NBPA.
90.4
94.8
82.3
imprinting molecules has an influence on the catalytic TOF values:
imprinting molecules with relatively bulkier size tend to lead higher
accessibility to the active sites.
92.6
[
[
[
[
a] The reactions were carried out in 2 mL of toluene with 40 mg of catalyst
at 110oC for 3 hours.
In order to prove if the molecularly imprinted cavities facilitate
b] Conversion determined by GC analysis with n-butanol as internal the improvement of activity in the reactions, two control samples
standard substance.
c] The TOF values were calculated at the reaction time of 20 minutes based
on N content of the catalyst.
d] The blank reaction was performed in the same condition but without any
catalyst.
(SBA-15-APTES and SBA-15-APTES-grafted) were prepared. SBA-15-
APTES was prepared via co-condensation of TEOS and 3-
aminopropyltriethoxysilane (APTES) in acid aqueous and SBA-15-
APTES-grafted was synthesized through grafting APTES onto the
channel of as-prepared SBA-15 material by refluxing in absolute
toluene (the detailed synthesis procedure and characterization
were displayed in SI). It is well known that a large fraction of amino
groups might be located in the mesopores in materials of SBA-15-
APTES, which would undermine mass transfer procedure during
reactions. Because the amino contents in the materials are quite
different, it is more meaningful to compare the TOF values in order
to evaluate the superiority of the catalysts. It is remarkable that
species. Based on such results, amino groups with higher basicity
and molecularly imprinted cavities in the mesopore framework are
simultaneously created via the thermal treatment in DMSO, which
may contribute to the improved performances in Knoevenagel
reactions.
Compared with the materials of MIMS-NHQ/NBPA, MIMS-
HQ/BPA materials give higher conversion of anisaldehyde and TOF
(
-1
MIMS-BPA give higher initial reaction rate (67.5 h ) for the
Knoevenagel reaction of 1-naphthaldehyde with cyanoacetate
number of converted aldehyde molecules per active site per hour)
numbers based on the N content in the materials at the reaction
time of 20 minutes. Meanwhile, higher initial reaction rates can also
be observed on MIMS-HQ/BPA obtained from the conversions of
anisaldehyde with different reaction time (Fig.6). During the
thermal treatment in DMSO, the covalently bonded moieties of –
NH-COO- in framework of MIMS materials were decomposed into
Table 3 Catalytic conversion and TOF comparison in Knoevenagel reaction 1-
naphthaldehyde with different catalystsa
CN
CHO
O
O
Cat.
+
NC
O
toluene
o
O
110
C
naked groups of –NH
2
in molecularly imprinted cavities, thus the
latter will show a better performance in Knoevenagel reaction
owing to the increase of basicity.
The TOF value of the reaction catalyzed by MIMS-BPA (14) was
also compared with that of MIMS-HQ (14). It can be observed that
MIMS-BPA (14) prepared from imprinting molecule (BPA) with
larger size give higher TOF number than MIMS-HQ (14) prepared
with imprinting molecule HQ. Note here that imprinting molecules
with larger size finally results in larger volume of molecularly
imprinted cavities. Based on the fact that the basicity of the two
b
TOFc
Entry
Catalyst
N content (%)
Conversion (%)
MIMS-BPA
1
2
3
1.6
51.5
54.3
89.6
67.5
48.9
55.2
(14)
SBA-15-APTES
SBA-15-
APTES-grafted
2.33
3.41
[
[
a] 1mmol 1-naphthaldehyde and 1 mmol cyanoacetate were dispersed in 2
mL of toluene with 40 mg of catalyst and the reactions were proceeded
o
at 110 C for 10 minutes.
b] The conversion of aldehyde was calculated by GC analysis with n-butanol
as internal standard substrate.
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[
c] The number of TOF was calculated depending on N content of the
catalyst with the result at the reaction time of 10 minutes.
View Article Online
DOI: 10.1039/C7DT04832G
based on the activity at the reaction time of 10 minutes than SBA-
-
1
-1
1
5-APTES (48.9 h ) and SBA-15-APTES-grafted (55.2 h ), although
3
-1
SBA-15-APTES possesses slightly higher pore volume (0.64 cm g )
3
-1
compared with MIMS-BPA (0.60 cm g ). The data is displayed in
Table 3. By considering that the amino groups anchored in the
mesopores might limit the diffusion of the reactants in the
mesopore channels, especially in the cases of bulky reactants, the
higher initial reaction rate on MIMS-BPA could be ascribed to the
higher accessibility of the unblocked active amino groups in the
framework via the unblocked mesopore channels as well as the
molecularly imprinted cavities. This confirms the advantages of
such molecularly imprinted organosilicates applied in catalysis.
The mechanism of Knoevenagel condensation is generally
accepted that the reaction proceeds through the proton abstraction
Scheme 3 Catalytic procedure for Knoevenagel reaction
4
6, 47
of active methylene compounds
. The activation of methylene
group is the key step for the proceeding of the reaction. Based on
all of the above characterizations and analysis, the reaction process
in the mesopore channels of the catalysts is shown in Scheme 3. It
is assumed that the cyanoacetate and aldehyde diffuse into the
mesopore channels of the materials and reach the active amino
groups. Such base amino groups would take a proton from the
methylene group in cyanoacetate and then negative charged
cyanoacetate would attack the carbonyl group in aldehyde to form
negative oxygen type intermediate. Finally, the protonated base
Fig. 7 Catalytic activity of repeated use of MIMS-HQ (14) in the Knoevenagel
would neutralize the intermediate, so the neutralized base can be reaction of anisaldehyde and ethyl cyanoacetate by washing (Run 2&3) and
intermediate calcination (Run 4-6) approaches.
active in the next run of catalysis to boost the reaction going
fluently. Note here that the introduction of ordered mesopores into
molecule imprinting materials improves the accessibility of active
species.
In order to assess the recyclability potential of such mesoporous
evidenced by the FT-IR spectrum of the material after calcination
Fig. S8). The almost complete reactivation of the recycled MIMS-
HQ (14) was achieved by the intermediate calcination treatment
Run 4), giving comparable conversion as that in Run 1. Therefore,
(
materials and the stability of amino groups in the framework wall,
recycling experiments were carried out with MIMS-HQ (14) as a
model catalyst over the substrate anisaldehyde. Fig. 7 shows the
results of the recycled catalyst of MIMS-HQ (14) after five-times
recycling by washing with ethanol for three times (Runs 2 and 3)
and regenerating under nitrogen atmosphere (Run 4-6). In the
former cases, the conversion of anisaldehyde in Knoevenagel
reaction with ethyl cyanoacetate gradually decreased in repeated
runs, which is possibly due to the deposition of reaction residues on
the active amino groups in the framework wall or the leaching of
amino groups during the reaction. An alternative way to regenerate
the catalyst is via intermediate calcination at 200°C for 2 hours at
inert atmosphere. Under such treatment, the amino groups
exposed in the cavities of the catalyst are almost preserved, as
(
the gradual reduced catalytic conversion for anisaldehyde in the
washing cases is mainly due to deposition of residues on the active
amino groups because they may hinder the access of reagents to
the active amino groups, however, which would be vanished during
the intermediate calcination procedure. The conversion of
anisaldehyde was almost constant in Runs 4-6, suggesting that the
intermediate calcination at 200°C for 2 hours at inert atmosphere is
an effective way to regenerate MIMS-HQ (14) in the Knoevenagel
reactions and that the stability of the amino groups in the
framework wall is high.
One of the benefits of MIMS-BPA (14) is represented by the
molecularly imprinted cavities in the mesoporous framework. To
investigate if this effect is limited to anisaldehyde, MIMS-BPA (14)
was tested for the Knoevenagel reactions of other aldehydes, such
as formylofuran, benzaldehyde and 1-naphthaldehyde (as shown in
Table S2). Clearly, MIMS-BPA (14) proved to be catalytically active
in all reactions tested, which indicates that this kind of material fit
to catalyze various reactants. These results further confirmed the
advantage of the molecularly imprinted cavities in the mesopore
framework.
Conclusions
8
| J. Name., 2012, 00, 1-3
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In summary, hybrid molecularly imprinted mesoporous 11. S. Rostamnia, E. Doustkhah, R. Bulgar and BV.iewZeAyrtnicilzeaOdnelinhe,
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precursor via a thermally reversible covalent bond by linking
an imprinting molecule to a functional alkoxysilane monomer.
After a given thermal treatment, amino groups with higher
basicity and molecularly imprinted cavities were
simultaneously formed in the mesoporous framework wall.
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