An efficient and recyclable acid-base bifunctional core-shell nano-catalyst for the one-pot deacetalization-Knoevenagel tandem reaction

Article abstract of DOI:10.1039/c8nj01934g

A novel magnetic nanoparticle supported acid-base bifunctional catalyst was synthesized by anchoring [3-(2-aminoethyl)aminopropyl]triethoxysilane and sulfonic acid successively onto the surface of silica-coated Fe₃O₄ nanoparticles. The as-prepared nanoparticle catalyst was used as a quasi-homogeneous and recyclable catalyst for the one-pot deacetalization-Knoevenagel tandem reaction of benzaldehyde dimethylacetals and malononitrile. It showed high catalytic activity because the reaction proceeded smoothly and afforded benzylidenemalononitriles in excellent yields. Besides, it showed synergistic effect and facilitated the mass transfer process during the tandem reaction because the two catalytic moieties were connected together on the same MNP. Moreover, the catalyst can be readily recovered under an external magnetic field and reused without any significant loss of the catalytic activity after six runs.

Full text of DOI:10.1039/c8nj01934g

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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An efficient and recyclable acid-base bifunctional core-shell nano-
catalyst for the one-pot deacetalization-Knoevenagel tandem
reaction
Received 00th April 2018,
Accepted 00th January 20xx
Zitao Wang‡, Xiaofeng Yuan‡, Qi’an Cheng, Tichun Zhang and Jun Luo*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
novel magnetic nanoparticle supported acid-base bifunctional catalyst was synthesized by anchoring [3-(2-
aminoethyl)aminopropyl]triethoxysilane and sulfonic acid successively onto the surface of silica-coated Fe3O4
nanoparticles. The as-prepared nanoparticle was used as a “quasi-homogeneous” and recyclable catalyst for the one-pot
deacetalization-Knoevenagel tandem reaction of benzaldehyde dimethylacetals and malononitrile. It shows high catalytic
activity because the reaction proceeded smoothly and afforded benzylidenemalononitriles with excellent yields. Besides, it
shows synergistic effect and faciliates the mass transfer process during the tandem reaction because the two catalytic
moieties were connected together on the same MNP. Moreover, the catalyst can be readily recovered under the external
magnetic field and reused without any significant loss of catalytic activity after six runs.
to catalyse deacetalization-Henry tandem reactions of
benzaldehyde derived acetals[19]
.
Rana reported the
Introduction
deacetalization-Knoevenagel tandem reaction catalysed by a
silica gel supported sulfonic acid and amine bifunctional
catalyst[20]. Zhao et al. used ortho aminophene terephthalic
acid and Zinc nitrate to prepare a metal organic framework
encapsulated palladium nanoparticle ad applied it as a primary
The separation and purification of organic intermediates from
reaction mixtures containing homogenerous catalysts
generally are confronted with tedious workup procedure and
more or less loss of total yield, especially in multistep
synthesis[1]. The application of multifunctional catalysts in
multistep one-pot reactions can overcome the above problems
because this protocol can reduce the separation step of
intermediate products or show synergistic effect[2-8]. It is
generally known that more than 85% organic reactions involve
catalysis, and the use of acid or base as catalysts is pretty
common in the catalytic reactions[9-12]. For a large number of
multistep reactions, both acid and base are simultaneously
needed. However, it is not easy to synthesize hostile groups on
solid catalysts[13]. Moreover, the examples of acid and base co-
supported catalysts are limited.
amine and palladium bifunctional catalyst[21]
. Li et al.
deposited palladium nanoparticles on the surface of
aminopropyl functionalized nano iron oxide and applied it to
catalyse the Knoevenagel condensation and hydrogenation
cascade reactions of aromatic aldehydes and malononitrile in
one pot with almost quantitative yield[22]
.
Magnetic nanoparticle (MNP) supported catalysts, as the ideal
alternative of homogeneous and classical heterogeneous
catalysts, have attracted broad attention.[23-30] MNPs generally
have high specific surface areas so that high loading ratio of
catalyst is predictable. In comparison with common solid
carriers, MNPs can disperse thoroughly in the reaction mixture
Bifunctional catalysts have attracted intensive interests in
recent years[14-17]
. For example, Wang and co-workers
so as to show “quasi-homogeneous” property. At the same
prepared a porous palladium-acid bifunctional nano-catalyst
and used it in the one-pot synthesis of methyl isobutyl ketone
from acetone[18]. Rothenberg and co-workers added different
amounts of heteropoly acid to the aminopropylsilica to
prepare acid-base bifunctional supported catalyst and used it
time, MNPs also has the advantages of easy recovery by
external magnetic field and diverse surface decoration.[31-35]
Compared with other magnetic nanoparticles, nano-Fe3O4 has
some attractive advantages including very low price, extra
fineness and easy functionalization, so it is widely applied in
the fields of nanotechnology, biological applications[36-37]
,
pollution control[38] and catalysis[39-43]
.
For the catalysts used in one-pot tandem reactions, the
catalytic moieties of bifunctional catalysts are basically on the
solid surface, however, this kind of nanoscale catalysts are few
in literatures. That is, most of the supported bifunctional
catalysts belong to the conventional heterogeneous catalysis.
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Compared with the reported stepwise catalytic activity of the Fig.1, the relatively high intensity of a band atVie5w9A4rticclme O-n1linies
DOI: 10.1039/C8NJ01934G
dual functional catalysts and the single functional catalysts, the characteristic of the Fe-O stretching vibrations, which indicates
bifunctional catalyst has no obvious promoting effect on the the high content of Fe3O4 nanoparticles. Meanwhile, the Si-O-
catalytic activity between different catalytic centers, and each Si stretching modes of the silica shell can be observed as a
plays independent catalytic roles in the tandem reactions.
strong peak at around 1090 cm-1, which shows the existence of
Herein, we report an efficient and magnetically recoverable silicon film. The signal at about 1,100 cm-1 is due to S=O
sulfonic acid and secondary amine bifunctional core-shell stretching vibration of sulfonic acid motif. The small peak at
nano-catalyst, Scheme 1, and used it as
homogeneous
and recyclable catalyst for the one-pot band at 1413 cm-1 is assigned to the asymmetric stretching of
deacetalization-Knoevenagel tandem reaction of benzaldehyde SO2 moiety and a band at 1623 cm-1 belongs to sulfonic OH
a “
quasi- the 2940 cm-1 band proves the presence of the CH2 group. The
”
dimethylacetals and malononitrile.
deformation vibration, which can confirm the existence of
sulfonic acid. Meanwhile, the peaks between 1400 cm-1 and
1600 cm-1 correspond to the deformation mode of extracted
R2NH, hydrogen-bonded R2NH and R2NH2+ [44]. Therefore, the
extra peak at 1457 cm-1 could be considered as an amine
group, which is also confirmed by the NH2 peak near 3300 cm-
1. These characteristic peaks in the FTIR could prove that the
SiO2@Fe3O4 nanoparticles were successfully immobilized with
sulfonic acid and secondary amine moieties conjointly.
Scheme 1. Synthetic route to acid and base monofunctional and bifunctional
NMPs.
Results and Discussion
The acid-base bifunctional catalyst SO3H-AA@MNP was prepared
via the two-step procedure demonstrated in Scheme 1. Firstly, a
condensation
reaction
of
SiO2@Fe3O4
with
3-(2-
aminoethyl)aminopropyltrimethoxysilane
afforded
amine-
functionalized MNP (AA@MNP). Subsequently, AA@MNP was
reacted with chlorosulfonic acid in chloroform at room temperature
for 3 h to afford the target bifunctional catalyst SO3H-AA@MNP.
The loading amount of the organic moiety was determined to be
0.75 mmol·g-1 by elemental analysis, which was also supported by
TG analysis (see Supplementary information).
Figure 2. Room-temperature magnetization curves of (a) SiO2@Fe3O4 (green line),
(b) AA@MNP (blue line) and (c) SO3H-AA@MNP (red line).
The magnetic behaviour of the sample was measured at room
temperature within the range of -15000 to 15000 Oe magnetic
field by using a vibrating sample magnetometer (VSM). As
shown in Figure 2, the magnetic properties of the three
samples reach saturation in 1.5 T magnetic field, and the
maximum magnetic saturation strengths of Fe3O4, AA@MNP
and SO3H-AA@MNP are 43.55, 32.97 and 13.93 emu·
g-1
respectively. The decrease of the saturation magnetization is
due to the decrease of the iron oxide content in the sample,
which proves the existence of silica in the core-shell structured
materials. Meanwhile, the magnetization intensity of
SiO2@Fe3O4 immobilized with SO3H group is much lower than
that of bare SiO2@Fe3O4 nanoparticles, and the decrease of
saturation magnetization may be related to the presence of
sulfonic group. In addition, all the magnetization curves pass
through the zero point, so there is no hysteresis, indicating
that all the three nanoparticles are superparamagnetic. Once
the external magnetic field is removed, the as-prepared
catalyst has no residual magnetism, which facilitates the
recovery of the catalyst. As shown in Figure 3(a), the catalyst
Figure 1. FTIR spectra for (a) AA@MNP (blue line) and (b) SO3H-AA@MNP (red
line).
The FTIR spectroscopy was used to characterize the
composition of SO3H-AA@MNP nanoparticles. As shown in
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can be uniform dispersed in the solution by shaking or respectively (Fig. 6(c))[46]. SO3H-AA@MNP displaVyisewaArtpiceleaOknlianet
[47]
ultrasonic irradiation, which results in the “quasi- 168.6 eV associated with a S-O boDnOdI: 1(0F.i1g0.396/C(d8)N)J01.934AGs
homogeneous” phenomenon. Meanwhile, SO3H-AA@MNP can mentioned previously, IR spectroscopy shows that as-prepared
be quickly collected in a few seconds in the external magnetic catalyst has S=O bond. The XPS test further confirms the
field to make the solution clear (Figure 3(b)).
presence of S element in SO3H-AA@MNP.
(a)
(c)
(b)
(d)
Figure 3. SO3H-AA@MNP is (a) dispersed well in the reaction system and (b)
separated simply from the reaction system using an external magnet.
The morphology of the catalyst was analysed by TEM. As
shown in Figure 4(a), the dark nano-Fe3O4 cores are
surrounded by grey silica shell of about 3-5 nm thick and the
size of the obtained catalyst falls in the range of 20-30 nm.
Moreover, the nanoparticles are well dispersed. Figure 4(b)
Figure 5. XPS spectra of SO3H-AA@MNP. (a) Survey scan, (b) O 1s, (c) N 1s and (d)
S 2p.
shows the recovered catalyst after reused in model reaction X-ray diffraction (XRD) is an effective method for the analysis
for six times. There is almost no apparent change in its of the composition and structure of solid materials. Figure 6
morphology can be observed. In other words, the as-prepared shows the high-angle XRD patterns of SiO2@Fe3O4, AA@MNP,
supported bifunctional nano-catalyst has excellent chemical and SO3H-AA@MNP. The wide peak at 2θ=20~30o can be
stability for the deacetalization-Knoevenagel tandem reaction. attributed to the amorphous silica layer of the outer layer. The
diffraction peaks (220), (311), (400), (422), (511) and (440) are
attributed to the standard Fe3O4 sample (JCPDS file no. 19-
0629). It can be concluded that the surface modification of
Fe3O4 does not lead to phase transition.
(b)
(a)
Figure 4. TEM images of fresh catalyst (a) recycled catalyst after six runs (b).
The surface composition of SO3H-AA@MNP was analysed with
XPS. As shown in Fig. 5(a), the peaks with binding energy at
25.9, 103.0, 154.8, 169.0, 232.1, 285.4, 402.2 and 532.1 eV are
distributed to O 2s, Si 2p, Si 2s, S 2p, S 2s, C 1s, N 1s and O 1s,
which affirms that the catalyst is made up of C, O, Si, S and N in
the survey scan of SO3H-AA@MNP. The peaks with binding
energy at 711 and 724 eV are distributed to Fe 2p1/2 and Fe
2p3/2, which also affirms that the catalyst contains Fe. The
peaks with binding energy at 711 and 724 eV are weak,
attributing to the fact that the Fe3O4 nanoparticles are
surrounded by silica shell, which is in accordance with the FTIR
and TEM results discussed previously. The O 1s components
centered at 529.8, 531.6 and 532.7 eV, which correspond to
Fe-O, S=O and Si-O, respectively (Fig. 6(b))[45]. In the N 1s
spectrum of SO3H-AA@MNP, the component peaks at 401.6
eV and 399.6 eV are attributed to the C-N and C-N-C bonds,
Figure 6. XRD patterns of (a) SiO2@Fe3O4 (green line), (b) AA@MNP (blue line)
and (c) SO3H-AA@MNP (red line).
The activity of the novel catalyst was tested using the one-pot
deacetalization–Knoevenagel
tandem
reaction
of
benzaldehyde dimethylacetal (2 mmol) and malononitrile (2.1
mmol) as model substrates under nitrogen atmosphere and
the results are listed in Table 1.
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functionalized catalyst SO3H@MNP was used but only the
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DOI: 10.1039/C8NJ01934G
deacetalization product benzaldehyde was obtained with 97% yield,
and no Knoevenagel condensation product was detected (Table 1,
entry 14). As a contrast, the amine functionalized catalyst AA@MNP
could efficiently catalysed the Knoevenagel condensation of
benzaldehyde and malononitrile under the standard reaction
conditions and afford 2-benzylidenemalononitrile in 90% yield,
which is slightly lower than the SO3H-AA@MNP catalysed one-pot
deacetalization–Knoevenagel tandem reaction (Table 1, entry 15). It
is notable that the combined use of the two solely functionalized
catalysts AA@MNP and SO3H@MNP in the tandem reaction
resulted in 88% yield, which is obviously lower than the bifunctional
catalyst. The reason was assigned to the more efficient mass
transfer process when the MNP supported bifunctional catalyst, in
which two catalytic moieties were connected together, was used.
Table 1 Optimization of the reaction conditionsa
Temp.
(oC)
reflux
reflux
90
Time
(h)
2
Yield
(%)b
87
Entry
Catalyst (mg)
Solvent
1
2
SO3H-AA@MNP (20)
SO3H-AA@MNP (20)
SO3H-AA@MNP (20)
SO3H-AA@MNP (20)
―
EtOH
H2O
2
54
3
DMF
2
91
4
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
CDCl3
90
2
94
5
90
2
trace
81
6
SO3H-AA@MNP (5)
SO3H-AA@MNP (10)
SO3H-AA@MNP (30)
SO3H-AA@MNP (20)
SO3H-AA@MNP (20)
90
2
7
90
2
89
8
90
2
94
9
70
2
86
10
11
12
13
14
15
50
2
83
[6]
AMSA-MIL-101-NH2
DL-COF-1[2]
90
10
20
2
>99c
98d
trace
97e
90f
RT
SiO2@Fe3O4 (20)
SO3H@MNP (20)
AA@MNP (20)
Toluene
Toluene
Toluene
90
90
2
90
2
SO3H@MNP (20) &
AA@MNP (20)
16
Toluene
90
2
88
a Reaction conditions: benzaldehyde dimethylacetal (2 mmol), malononitrile (2.1
b
c
mmol), SO3H-AA@MNP, H2O (0.2 mmol) and solvent (20 mL). GC yields.
A
mixture of benzaldehyde dimethylacetal (0.76 g, 5 mmol), malononitrile (0.40 g, 6
mmol), AMSA-MIL-101-NH2 (0.08 g, 10 wt%), anhydrous toluene (10 mL), 363 K,
d
10 h. benzaldehyde dimethylaceta(1 mmol), malononitrile (1 mmol), DL-COF-1
e
(10 mol%), CDCl3(3 mL), RT, 20 h.
the yield of deacetalization product
f
benzaldehyde; the yield of Knoevenagel condensation of benzaldehyde and
malononitrile.
The effect of some commonly used solvents including ethanol,
water, N,N-dimethylformamide (DMF) and toluene was first tested
(Table 1, entries 1-4). As shown in Table 1, toluene is the best
choice since it gives the highest yield (Table1, entry 4) while water
is the worst one. So toluene was used in the subsequent
investigations. The yield ascends with the increase of catalyst
amount (Table1, entries 4-8). In the absence of catalyst, almost no
product can be detected (Table 1, entry 5). However, when the
catalyst amount was above 20 mg, the yield remains unchanged
(Table 1, entry 8). So the reaction conditions of entry 4 were
applied as the standard conditions for the subsequent study. Lower
yields were observed when the reaction temperature was reduced
to 70 ℃and 50 ℃(Table1, entries 9 and 10). Compared with the
catalysts studied by scientists in recent years[2,6], the as-prepared
catalyst shows higher catalytic activity because similar yields are
achieved but the reaction time is much shorter (Table1, entries 11
and 12). To better show why the catalyst has so high catalytic
activity, some control experiments were studied. There are some
free Si-OH remains on the surface of SiO2@Fe3O4, but it showed no
catalytic activity in this reaction (Table 1, entry 13). The acid mono-
Scheme 2. Proposed mechanism of the one-pot deacetalization-Knoevenagel
tandem reaction catalysed by SO3H-AA@MNP.
A cooperative mechanism of the one-pot deacetalization-
Knoevenagel reaction catalysed by SO3H-AA@MNP was
proposed and depicted in Scheme 2. The reaction sequences
involve two steps as follows: The first step is an acid-catalysed
deacetalization and the subsequent step is a base-catalysed C-
C
bond formation. In other words, the benzaldehyde
dimethylacetal is activated by the sulfonic acid moiety to form
oxonium ion. At the same time, active α-H of malonitrile is
captured by the secondary amine moiety. Then the protonated
oxonium ion is attacked by water and release methanol to
afford protonated benzaldehyde. Subsequently, the activated
carbonyl of aldehyde undergoes nucleophilic attack by
malonitrile carbanion. At last, a synergistic dehydration occurs
to produce the final product 2-benzylidenemalononitrile and
regenerate the catalyst.
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DOI: 10.1039/C8NJ01934G
The generality of this catalyst was evaluated by using various
acetals and malononitrile as substrates and performing the
reaction under the optimized reaction conditions. The
collected results are listed in Table 2. As shown in Table 2,
benzaldehyde dimethylacetal with electron-withdrawing or
electron-donating groups underwent efficient coupling with
malononitrile at 90°C for 1.5-2.5 h to afford the corresponding
substituted benzylidenemalononitriles in good to excellent
yields ranging from 74% to 97% (Table 2, entries 1-13).
However, electron-donating groups such as alkyl and hydroxyl
are unfavorable to the reaction in comparison to electron-
withdrawing ones including nitro, chlorine, bromine,
trifluoromethyl and cyano functionalities. In addition, steric
hindrance obviously influences the reaction, which can be seen
from a comparison of the results for methylbenzaldehyde
acetal (Table 2, entries 2-4) and bromobenzaldehyde acetal
(Table 2, entries 9 and 10). Because the vicinal steric effect is
greater than the contrapuntal one, 4-substituted
benzaldehyde acetals show higher reactivity than 2-
substituted isomers.
Figure 7. Recycling results of SO3H-AA@MNP.
The recovery and reuse of a catalyst is highly preferable for a
green process. Thus, the recyclability of SO3H-AA@MNP was
investigated by using benzaldehyde dimethylacetal and
malononitrile as model substrates. The catalyst was easily
separated by attaching an external magnet after the reaction.
The reclaimed catalyst was washed with ethyl acetate, dried
under vacuum and reused in subsequent reactions. As shown
in Figure 7, the results show that the catalytic activity of the
catalyst has no obvious loss within six runs. Furthermore, the
TEM image demonstrated in Figure 4(b) indicates the
morphology of the recovered catalyst remains unchanged. The
above results show that the catalyst is tolerable to the one-pot
deacetalization-Knoevenagel tandem reaction and exhibits
excellent chemical stability.
Conclusions
In summary, secondary amine and sulfonic acid were
sequentially immobilized on the surface of silica-coated Fe3O4
nanoparticle to afford an acid-base bifunctional magnetic
nano-catalyst SO3H-AA@MNP, which was used as a recyclable
catalyst for the one-pot deacetalization-Knoevenagel reaction
of benzaldehyde dimethylacetals and malononitrile. It showed
high catalytic activity because the reaction proceeded
smoothly and afforded benzylidenemalononitriles with high to
excellent yields. Because the two catalytic moieties were
connected together on the same MNP, a possible reaction
mechanism involving a co-operative action of acid and base,
which can faciliate the mass transfer process during the
tandem reaction, was established. Besides, the catalyst can be
readily recovered under external magnetic field, and reused
without any significant loss of catalytic activity after six runs.
Table 2 SO3H-AA@MNP catalysed one-pot deacetalization-Knoevenagel tandem
reactionsa
Experimental section
General information
The IR spectra were recorded on a Nicolet spectrometer (KBr).
1H NMR and 13C NMR spectra were recorded on a Bruker
Avance-III DRX spectrometer operating at 500 MHz and 125
MHz respectively, using CDCl3 as solvent. X-ray diffraction
(XRD) images were obtained from a Bruker XRD D8 Advance
instrument with Cu Kα radiation (λ
= 0.15418 nm).
Transmission electron microscopy (TEM) images were
obtained using a JEM-2100 instrument. The magnetization
curve was obtained by a vibrating sample magnetometer
(JDM-13T, China). Elemental analysis was performed with an
Elementar Vario EL
β
recorder. X-ray photoelectron
spectroscopy (XPS) spectra were obtained from an ESCALABTM
250Xi instrument. All the solvents used were strictly dried
a Reaction conditions: benzaldehyde dimethylacetals (2 mmol), malononitrile (2.1
mmol), toluene (20 mL), SO3H-AA@MNP (20 mg), H2O (22 mmol), 90 oC.
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7
8
9
M. Mu, X. Yan, Y. Li and L. Chen. J. Nanopart. Res., 2017,
View Article Online
according to standard operation and stored on 4 Å molecular
sieves. All other chemicals (AR grade) were commercially
available and used without further purification. Silica-coated
Fe3O4 nanoparticle (SiO2@Fe3O4) was prepared according to
19(12), 1-9.
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sonication
for
0.5
h,
[3-(2-
aminoethyl)aminopropyl]triethoxysilane (1.32 g, 5 mmol) was
added and the reaction mixture was refluxed for 24 h under
nitrogen atmosphere. After being cooled to ambient
temperature, the amine-functionalized MNP was collected by
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acetone successively, then dried under vacuum overnight.
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Synthesis of acid-base bifunctional MNP (SO3H-AA@MNP)
A mixture of AA@MNP (0.2 g), ClSO3H (1 mmol 0.07 mL) and
chloroform (20 mL) was stirred at room temperature for 3 h. The
acid-base functional MNPs were collected by a permanent magnet
and washed thoroughly with dichloromethane (3 × 5 mL), and then
dried under vacuum overnight. The loading content of the organic
moiety was determined to be 0.75 mmol·g-1 by elemental
analysis.
General procedure for SO3H-AA@MNP catalysed one-pot
deacetalization–Knoevenagel tandem reaction
A mixture of benzaldehyde dimethylacetal (2 mmol), malononitrile
(2.1 mmol), SO3H-AA@MNP (20 mg), H2O (0.2 mmol) and toluene
(20 mL) was stirred at 90 oC under nitrogen atmosphere for 2 h. The
solid catalyst was recovered using a permanent magnet, and the
filtrate was dried over anhydrous sodium sulfate and evaporated
under reduced pressure to afford the product. The crude product
was purified by column chromatography on silica gel to give the
pure product.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was sponsored by the Natural Science Foundation of
Jiangsu Province of China (no. BK2010485) and Qing Lan
Project of Jiangsu Province, P. R. China.
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Notes and references
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6 | J. Name., 2012, 00, 1-3
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COMMUNICATION
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Table of Contents:
An acid-base bifunctional nano-catalyst was synthesized and applied as
efficient
and
recoverable
catalyst
in
the
one-pot
deacetalization-Knoevenagel tandem reaction.