Layered double hydroxide anchored ionic liquids as amphiphilic heterogeneous catalysts for the Knoevenagel condensation reaction

Article abstract of DOI:10.1039/c7dt03665e

In recent years, great attention has been dedicated to the development of heterogeneous base catalysts providing a green and sustainable process in benign aqueous media. Herein, the ionic liquid modified layered double hydroxide (LDH) based catalysts of L

Full text of DOI:10.1039/c7dt03665e

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Layered double hydroxides anchored ionic liquids as amphiphilic
heterogeneous catalysts for Knoevenagel condensation reaction
Tengfei Li,a Wei Zhang,a Wei Chen,a Haralampos N. Mirasb* and Yu-Fei Songac*
Received 00th January 20xx,
Accepted 00th January 20xx
In recent years, great attention has been dedicated to the development of heterogeneous base catalysts rendering green
and sustainable process in benign aqueous media. Herein, the ionic liquids modified layered double hydroxides (LDHs)
based catalysts of LDH-ILs-Cn (n = 4, 8, 12) were prepared by adopting an exfoliation/assembly strategy, in which the ionic
liquids (ILs) was convalently anchored on the surface of LDHs layers. The resultant LDH-ILs-Cn were charaterized by FT-IR,
the solid-state 13C NMR, 29Si CP/MAS NMR, 27Al-MAS NMR, XRD, TG-DTA, BET, XPS, SEM, CO2-TPD and contact angle
experiments etc. The catalytic performance of LDH-ILs-C12 for Knoevenagle condensation of a variety of aldehydes with
ethyl cyanoacetate/malononitrile in H2O at room temperature showed excellent yields and selectivity. Moreover, the base
catalyst of LDH-ILs-C12 can be easily recycled and reused for at least 5 times without decrease of its catalytic efficiency.
The scaled-up experiments revealed that the catalyst retained its efficiency and robustness.
DOI: 10.1039/x0xx00000x
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recyclable, sustainable and eco-friendly heterogeneous base
catalytic system for the Knoevenagel reactions.
1. Introduction
Layered double hydroxides (LDHs) are composed of
Knoevenagel reaction between an aldehyde/ketone and active
methylene-containing compounds is one of the most
significant reactions for the C-C bonds formation,1-3 which has
been applied to synthesize important intermediates or
products for perfumes, pharmaceuticals and calcium
brucite-like two-dimensional sheets and interlayer anions. The
chemical formula is generally represented as [M1-
n-
]
2+Mx3+(OH)2]x+[Ax/n
—
mH2O,14-16 where M2+ and M3+ are di-
x
and trivalent metal cations, An- is the counterion, and x=0.17-
0.33 is defined by the M3+/(M2++M3+) ratio.17 Each hydroxyl
group in the LDHs layers is oriented toward the interlayer
region and may be hydrogen bonded to the interlayer anions
and water molecules. LDHs are very attractive systems due to
its fascinating properties that can be finely tuned by
appropriate choice of the composition metal ions, functional
guest anions and so on.18 Therefore, LDHs materials have been
successfully applied in various fields, including as adsorption
materials, additives in polymers, precursors for functional
materials, and in pharmaceutics, photo-chemistry and
electrochemistry.19 In addition, by virtue of the abundance of
hydroxyl groups, LDHs are potential solid base catalysts for a
wide variety of organic transformations.14 As solid base
catalyst, the base properties of LDHs can be tuned by the
composition of metal hydroxide and type of interlayer anions.
Moreover, LDHs can be transformed to Lewis acidic and basic
bifunctional metal composite oxides by calcination.13 LDHs are
cost-effective, environmentally benign and easy to scaled-up in
practice. As a result, LDHs are attracting increasing attention
from the viewpoint of environmental and economic
concerns.20
antagonists.4-6 Generally, this reaction employs
a
homogeneous base catalyst such as ammonia, ammonium
salts, urea, pyridine, primary and secondary amines.7-9
Unfortunately, these homogeneous catalysts are difficult to be
separated and recycled, which restrict their application in
practice. To overcome these problems, development of novel
heterogeneous base catalysts have attracted great interest
and wide attention during past decades, such as aminopropyl-
functionalized SBA-15,10 nitride zeolites,11 metal-organic
frameworks,12 SiO2@LDH.13 However, these catalytic systems
still suffer from the following problems: 1) toxic organic
solvents or additives, such as toluene, cyclohexane used are
highly corrosive, carcinogenic and require frequent laborious
workup treatments; 2) partial blockage of the pores, sintering
or fusion into a bulk phase and the possible leaching of the
basic components into the reaction mixture; 3) poor
accessibility of the reactant and catalytic active sites.
Consequently, it is of great urgency to design efficient,
Ionic liquids (ILs) have been attracted considerable
attention owing to their unique properties. For example,
nonvolatility, remarkable dissolution, high thermal stability,
good catalytic activity, electrophilic/nucleophilic dual
activation et al.21-23 Therefore, ILs has been emerged as good
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alternatives environmental benign solvents and media to
In a typical Knoevenagel reaction experiment, a mixture of
exhibit a rate acceleration effect on catalytic reactions. 1 mmol benzaldehyde (or various aldehyde), 1.5 mmol
However, the bottleneck associated with ILs is the difficulty in methylene compound (ethyl cyanoacetate or malononitrile),
product isolation and catalyst separation in many 500 μL H2O and 30 mg catalyst LDH-ILs-C12 were added into a
circumstances. Moreover, high viscosity of ILs exhibits serious 10 mL glass bottle at room temperature and the reaction
limitation in their practice of industrial catalysts due to the mixture was kept under vigorous stirring. The resulting
controlled mass transport, which is one of the significant products were extracted with diethyl ether, analysed by GC
properties in the heterogeneous catalytic system.24 Thus, the and identified by 1H-NMR and 13C-NMR spectroscopy. The
concept of heterogenization of ILs is being established, which yields were calculated by applying reference standards. After
combine the advantages of ILs and appropriate support.24
completion of the reaction, the catalyst was recovered by
In this paper, we demonstrate the successful preparation centrifugation, washed with acetone, and dried in vacuum.
of novel heterogeneous base catalysts of LDH-ILs-Cn (n=4,8,12)
by covalently grafting onto LDHs layers. Catalytic tests for
3. Results and discussion
Knoevenagel condensation in water under room temperature
show that among three amphiphilic solid base catalysts, LDH-
ILs-C12 exhibits high activity and selectivity, coupled with easy
recovery and steady reuse. The sustainable performance and
stability during the scaled-up catalytic experiments as well as
the cost-effective advantages demonstrate the potential of
LDHs-ILs-C12 for industrial applications.
3.1. Structural characterization
As shown in Fig 1A, FT-IR spectra of LDH-CO3 and LDH-ILs-
Cn (n=4, 8, 12) show the broad band at 3400-3700 cm-1 that
can be attributed to the Mg/Al-OH/H2O vibration. Additionally,
the broad peaks from 788 to 449 cm-1 can be assigned to the
O-M-O (M = Mg or Al) vibration in the brucite-like layers of the
LDHs.27 The strong peak at 1370 cm-1 is attributed to C-O anti-
symmetric stretching of the interlayer CO3 anion.28 Taking LDH-
ILs-C12 as an example, it exhibits the C-H stretching bands at
2929 and 2835 cm-1 due to the alkyl chain -CH2 and -CH3
2. Experimental
2.1 Chemical Materials
All chemicals were of analytical grade and were used as groups, and a C=N stretching band at 1668cm-1 by the
received without any further purification. Mg(NO3)2·6H2O, imidazole moieties of ionic liquids.26 Moreover, it shows the
Al(NO3)3·9H2O, and hexamethylenetetramine were obtained characteristic peak at 1060 cm-1, which is assigned to the
from Energy Chemical in Shanghai. Ethanol and diethyl ether vibration of the Si-O-M bond (M=Al or Mg).29 The results
were purchased from Beijing Chemical Company. Ethyl indicate that the ionic liquids have been successfully grafted
cyanoacetate (98%), malononitrile (97%), benzaldehyde (98%) onto the LDHs layers.
and other aldehydes were purchased from Alfa Aesar. LDH-
25
As shown in Fig. 1B, the 29Si CP/MAS NMR spectra have
CO3, LDH-NO3,
dihydro-
1-butyl-3-(3-triethoxy-silylpropyl)-4,5- been used to investigate the local environment of Si species.
For example, the LDHs-ILs-C12 displays three resonance peaks
imidazolium
silylpropyl)
bromide
(ILs-C4),
1-octyl-3-(3-triethoxy- at -65, -59 and -53 ppm that correspond to tridentate,
bidentate and monodentate ligation, respectively. The
-4,5-dihydroimidazolium bromide (ILs-C8), 1-dodelyl-3-(3- tridentate form implies that all three of the ethoxyl group of
triethoxy-silylpropyl)-4,5-dihydroimidazolium bromide (ILs- ILs-C12 are hydrolysed and condensed with the surface
C12)26 were synthesized and characterized in accordance with hydroxyl group of LDHs layer. Observation of the bidentate
the literature.
and monodentate form suggest that some ILs-C12 species
possess one or two ethoxyl/hydroxyl groups that do not
2.2. Synthesis of LDH-ILs-Cn (n=4, 8, 12)
In a typical experiment, LDH-NO3 (0.3 g) was added to
formamide (300 mL) in a three necked flask, which was purged
with N2 to avoid carbonate contamination. The mixture was
vigorously stirred for 2 days. The mixture was then centrifuged
for 10 min and the suspension was isolated by filtration. ILs-Cn
(2.6 mmol, n=4, 8, 12) was dissolved in CH3CN (5 mL) and
added dropwise to the above suspension. The reaction mixture
was stirred under N2 for 24 h. 2 g Na2CO3 was dissolved in 5 ml
H2O and added dropwise to the reaction mixture. After stirring
for 30 min, the resultant precipitate of LDH-ILs-Cn (n=4, 8, 12)
was collected by centrifugation and washed with ethanol and
water, and dried under vacuum overnight.
2.3 Catalytic test
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interlayer spacing after silylation, which may be reflected by a
shift in d003 to lower 2θ values.32 The retention of
characteristic peak (110) around 2θ= 60o 33 indicates that the
structure of the layer has been retained after exfoliation and
assembly synthesis.
Thermogravimetric differential thermal analysis (TG-DTA)
of LDH-ILs-C12 shows two weight-loss stages as the
o
temperature increasing from 30 to 800 C (Fig. 2 B). The first
weight loss of 8.86 % between 30 oC and 200 oC can be
ascribed to the removal of water molecules absorbed on the
surface and the interlayer space. The second weight loss of
47.1% between 200 oC and 650 oC corresponds to the
decomposition of ILs and subsequent disintegration of the
layered structure. Base on the TG-DTA, ICP-AES and elemental
analysis (Mg= 18.28 wt %, Al=6.80 wt %, N=1.03 wt %, ILs-C12=
13.03 wt. %), the molecular formula the can be identified as:
Mg0.75Al0.25[O3SiC18H36N2]0.036[Br]0.036 (OH)1.92[CO3]0.175—0.83H2O.
The molecular formula of LDH-ILs-Cn (n = 4, 8, 12) is
summarized in Table S1, respectively. The thermal stability of
Fig. 1. A) FT-IR spectra of : a) LDH-CO3, b) LDH-ILs-C4, c) LDH-ILs-C8, d) LDH-ILs-
C12; B) Solid-state 29Si CP/MAS NMR spectra of a) LDHs-IL-C4; b) LDHs-IL-C8; c)
LDH-ILs-C12; C) Solid-state 13C CP/MAS NMR spectrum of LDH-ILs-C12.
Fig. 3. a) XPS spectrum of the LDH-ILs-C12 sample and deconvolution of the peaks for
b) Mg1s, c) Al2p, d) C1s, e) Si2p, f) N1s.
LDH-ILs-C4 and LDH-ILs-C8 are similar to that of the LDH-ILs-
C12 (Fig. S1-2). The TG analysis plots of LDH-CO3 are shown in
the Figure S3 for comparison.
Fig. 2. A) XRD patterns of a) LDH-CO3, b) LDH-ILs-C4, c) LDH-ILs-C8, d) LDH-ILs-C12; B)
TG-DTA profile of LDH-ILs-C12; C) The 27Al-MAS-NMR spectra of LDH-CO3 and LDH-ILs-
C12. D) The N2 adsorption-desorption isotherm and the pore size distribution (inset
picture) of LDH-ILs-C12.
Solid-state 27Al-NMR spectroscopy has been used to probe
the local environments of Al3+ centres in LDH-ILs-C12. As is
known that the 27Al resonance line positions are very sensitive
to the coordination number and are expected to appear within
the range of δ= -5 to 15 ppm for octahedral geometry of AlO6
sites.34 As shown in Fig. 2C, the 27Al-NMR spectrum of LDH-CO3
shows only one peak at δ=10.75 ppm, which clearly
demonstrate that all of the Al3+ cations in the brucite-like layer
adopt octahedral geometry. In the case of LDHs-ILs-C12, the
27Al-MAS-NMR signal shows only one peak with a slight shift to
low field chemical shift compared with that of LDH-CO3
(Mg3Al-CO3). This observation indicates that the brucite-like
layered structure retains its integrity upon modification of ILs-
C12.
hydrolyze or condense.30 These results confirm that the ILs-C12
is covalently tethered onto the LDHs layers, resulting in the
formation of Al-O-Si and/or Mg-O-Si bonds in the LDHs layers.
Fig. 1C shows the solid-state 13C CP/MAS NMR spectrum of
LDH-ILs-C12. The spectrum exhibits signals at 154.7, 54.8, 43.1-
50.5, 19.9-30.9, 14.7-16.4, and 11.5 ppm that can be assigned
to the NCHN, OCH2, -NCH2-, methylene, methyl and Si-C,
respectively.30,31 These signal positions are similar to that the
ionic liquids covalently anchored onto SiO2.31 As a result, the
condensation reaction between the hydroxyl groups of LDH
layer and ethoxy groups of ionic liquids leads to the formation
of covalent Si-O-M (M=Al or Mg) linkage and the maintenance
of the dihydroimidazolium skeleton of ionic liquids at the same
time.
BET measurements have been performed on the LDH-ILs-
Cn (n=4, 8, 12) samples to obtain more detailed information on
the structures of the as-prepared material. As shown in Fig.
2D, taking LDH-ILs-C12 as example, it shows typical type-IV
adsorption isotherms, and H3 type hysteresis loops at higher
As shown in Fig. 2A, XRD patterns show the characteristic
reflection of LDH-CO3. For LDH-ILs-Cn (n=4, 8, 12), upon
modification of ILs, no distinct changes in the refection are
observed. In addition, we do not observe a change in the
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relative pressure (P/P0>0.5), indicating the presence of both sites ranging from 500 to 630 oC. The desorption peak of CO2 in
interlayer and inter-particle mesopore structures. The pore the range of 640-900 oC, which can be assigned to the
size distribution calculated by the Barrett-Joyner-Halenda decomposition of internal standard CaCO3. The data in Table 1
(BJH) method shows a peak centered at around 4.78 nm (Inset is on the basis of deduction of the CO2 contribution by the
2-
Fig. 2D). The specific surface area, pore volume and average decomposition of the intercalated CO3 anions. The results
pore diameter estimated from N2 adsorption-desorption show that the total basic sites remain approximately equal for
isotherms of LDH-ILs-Cn (n = 4, 8, 12) are reported in Table S2. LDH-ILs-Cn (n = 4, 8, 12).
SEM images of the as-prepared LDH-ILs-Cn (n=4, 8, 12) show
To have a better understanding of the interactions
the porous stacking of sheet-like crystallites (Fig. S4). For LDH- between the catalyst and reactants under the experimental
ILs-C12, we can see the particle size of these crystallites fall in conditions, the amphiphilic property of the LDH-ILs-Cn (n=4, 8,
the range of 1~2 μm.
12) catalysts has been investigated. As shown in Fig. 4a, when
As shown in Fig. 3, the initial survey of the XPS spectrum a droplet of water contact the surface of LDH precursor, it
shows that the LDH-ILs-C12 sample is composed of Mg, Al C, O, forms a contact angle of approximately 29.1o, indicating that
N, Si et al (Fig. 3a). In Fig. 3b-c, the relevant binding energies of the LDH precursor is highly hydrophilic. However, when a
Mg 1s and Al 2p can be observed at 1303.6 eV and 77.2 eV, water droplet is in contact with the LDH-ILs-Cn (n=4, 8, 12), the
respectively. And Fig. 3d-f exhibit the relevant binding energies contact angles increase with the increase of the alkyl chain
of C1s, Si2p, N1s resulting from the modification of the ILs-C12 length of the modified ILs (Fig. 4b-d). And the hydrophobicity
in the LDH-ILs-C12. In addition, the characteristic peaks for C1s increases according to the following sequence: LDH-ILs-C4 <
(Fig. 3d) are fitted, in which the three peaks at 284.8 eV (C-C
Table 1. Temperature-programmed desorption of CO2.
specific basicity [CO2 μmol g-1][a]
Sample
total
668
672
666
w
m
s
LDH-ILs-C4
LDH-ILs-C8
LDH-ILs-C12
324
217
243
200
248
137
144
207
286
[a]w: weak basic sites; m: moderate basic sites; s: strong basic sites. The calculation of
the amount of specific basicity using CaCO3 as an internal standard substance and the
data was deducted from the decomposition of interlayer of carbonate.
Fig. 4. Water droplets on the surface of the catalyst a) LDH-CO3, b) LDH-ILs-C4, c) LDH-
ILs-C8, d) LDH-ILs-C12. Photographs of LDH-ILs-C12 appeared at the H2O/organic
substrates interface.
bond) of the alkyl chain, 286.7 eV (C-O bond), and 288.6 eV (O-
C=O bond) can be seen.29 Moreover, in order to detect the
electronic state of Si atoms, the Si2p spectra of LDH-ILs-C12
are fitted to two main peaks as shown in Fig. 3e. The Si-O bond
at ca.101.7eV can be assigned to the reacted Si-O-M (M=Al or
Mg); The Si-C bond at ca. 102.1 eV can be attributed to the
silane moieties of ILs-C12.29 In addition, the Si atoms in Si-OH
can be distinguished by fitting program as represented with a
weak intensity peak at 103.1 eV. It may be due to the
unreacted Si-OH (hydrolysis of the triethoxysilyl group) in the
region where is geometrically unavailable.30,35 This is
consistent with the characterization of solid-state 29Si CP/MAS
NMR spectrum that shows bidentate and monodentate of Si
species. Furthermore, the characteristic peak associated with
N1s (Fig. 3f) is centered at 399.7 eV, which can be assigned to
the C-N bond of the imidazole rings in ILs-C12.36 The results
provide further support for the covalently grafting ILs-C12 onto
the LDH successfully.
LDH-ILs-C8< LDH-ILs-C12. The enhanced hydrophobicity for
LDH-ILs-Cn is attributed to the hydrophobic nature of the ILs-
Cn. For example, LDH-ILs-C12, exhibits a CA (contact angle) of
ca.128o
, which is indicative of its increased hydrophobic
character compared with LDH precursor. Moreover, the
increased hydrophobicity helps the accessibility of the
reactants to the catalytic site. For example, the LDH-ILs-C12
can be well dispersed at the interface of the H2O and organic
phases (Fig. 4), which further confirms the amphiphilic nature
of LDH-ILs-C12.
3.2. Effect of catalysts on the Knoevenagel condensation
To evaluate the catalytic performance of LDH-ILs-C12, we
select the Knoevenagel condensation of benzaldehyde with
ethyl cyanoacetate as
a
model reaction under the
Fig. S5a-c presents the Temperature-programmed
desorption of CO2 (CO2-TPD) curves of LDH-ILs-C4, LDH-ILs-C8
and LDH-ILs-C12, while Fig. S5d is the CO2-TPD curves of LDH-
CO3 as control experiment. Note that the TCD signals in Fig.
S5a-c include the contribution of decomposition of the
intercalated carbonate anions. It can be seen that these
composites display three kinds of CO2 desorption peaks, which
experimental conditions (Scheme 1). The reaction proceeds
efficiently in water in the presence of a catalytic amount of
LDH-ILs-C12 at room temperature and the corresponding
product can be obtained in an excellent yield of 99.9 % (entry
o
can be attributed to weak basic sites centered at 90-250 C,
o
medium basic sites centered at 300-430 C; and strong basic
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ethyl cyanoacetate can be complete with a yield of > 99% in
3h. Thus, the catalyst LDH-ILs-C12 exhibits a high catalytic
Scheme 1. The Knoevenagel condensation of benzaldehyde with ethyl
cyanoacetate.
Table 2 Effect of different catalysts on Knoevenagel condensation of
benzaldehyde with ethyl cyanoacetate at room temperature in aqueous
solution.[a]
Entry
Catalysts
Yield (%)
1
2
3
4
5
LDH-ILs-C4
LDH-ILs-C8
LDH-ILs-C12
LDH-CO3
ILs-C4
85.1
84.2
99.9
19.0
30.7
31.4
34.6
45.2
<1
6
ILs-C8
ILs-C12
LDH-CO3+ILs-C12
None
Fig. 5. A) Kinetic profiles of the Knoevenagel condensation of benzaldehyde with
ethyl cyanoacetate catalyzed by LDH-ILs-C12-1 (red line: yield of corresponding
products; blue line: ln(Ct/C0), data were obtained by carrying out parallel
experiments under the same experimental conditions); B) Experiment to prove
the nature of heterogeneous catalyst.
7
8b
9
[a]Reaction conditions: 1 mmol benzaldehyde, 1.5 mmol ethyl cyanoacetate, 30 mg
catalyst, 0.5 mL H2O, 3 h, rt. [b]The catalytic amount of the physical mixture control
experiments was used its corresponding individual components load in the composite
of LDH-ILs-C12. Yields were determined by GC analysis using reference standards.
Assignments of corresponding products were analysed by 1H NMR and 13C NMR
spectroscopy in Fig S6.
⁄
ꢀꢁꢂꢃ ꢁꢄ ꢅ ꢆꢂꢃ
(1)
(2)
lnꢇꢂꢃ/ꢂꢈꢉ ꢅ ꢀꢆꢄ
3). Notably, the excellent catalytic activity of LDH-ILs-C12 may
originate from stronger basic sites of LDH-ILs-C12 than those
of LDH-ILs-C4 and LDH-ILs-C8. However, almost no yield is
obtained in the absence of catalyst (entry 9). In another words,
the reaction cannot proceed without catalyst. Application of
ILs-C4, ILs-C8 and ILs-C12 to catalyse the reaction at same
experiment conditions gives the yields of 30.7%, 31.4%, 34.6%,
respectively (entry 5-7). If the unmodified LDH-CO3 is used as a
catalyst, the reaction proceeds sluggish with a very low yield of
19.0% (entry 4). Moreover, the reaction shows poor activity
using a physical mixture of the individual components of the
composite (entry 8). Control experiments (entries 1-2) using
LDH-ILs-C4 and LDH-ILs-C8 exhibit inferior catalytic activities to
that of LDH-ILs-C12. All the above experimental results in Table
1 indicate the LDH-ILs-C12 is a highly effective catalyst for
Knoevenagel reactions. Taking into account the previously
reported results and our experimental data discussed herein,
the synergistic effect between the LDH layers and the ILs-C12
in the LDH-ILs-C12 is crucial for the efficient promotion of the
Knoevenagel reaction.
efficiency for the Knoevenagel condensations, and the catalytic
reaction obeys pseudo-first-order kinetics with 99% selectivity.
In order to compare the kinetic parameters with different
composite materials for Knoevenagel condensation of
benzaldehyde with ethyl cyanoacetate, we also investigate the
kinetic parameters of LDH-ILs-C4 and LDH-ILs-C8, and they
show lower reaction rate compared with LDH-ILs-C12. (Fig. S7-
8).
To confirm that the catalyst is true heterogeneous catalyst,
the Knoevenagel reaction is performed using LDH-ILs-C12 as
the catalyst in water at room temperature. When the yield
reaches about 65 % after 30 min, the solid catalyst can be
removed from the reaction mixture. The reaction is allowed to
proceed under the same conditions. As a result, it shows that
no additional conversion of reactant to product can be
observed (Fig. 5B). Addition of the catalyst back to the reaction
mixture, the reaction continues. Furthermore, elemental
analysis of the removed catalyst and the filtrate by ICP-AES
shows that the leaching of magnesium and aluminium are
about of 0.15 wt.% of the total composition metal ions. And
the slight amount of the leached Mg and Al species has not
taken a role of catalysis during the hot-filtration test. This
result strongly verifies the heterogeneous nature of LDH-ILs-
C12.
3.3 Kinetic study of Knoevenagel condensation of benzaldehyde
with ethyl cyanoacetate
In an effort to obtain the kinetic parameters for the
Knoevenagel condensation of benzaldehyde with ethyl
cyanoacetate, control experiments was performed with 1
mmol benzaldehyde, 1.5 mmol ethyl cyanoacetate, and 30 mg
LDH-ILs-C12, at room temperature in 0.5 ml H2O conditions.
The yield and ln(Ct/C0) are plotted against the reaction time in
Fig. 5A, where C0 and Ct are the initial corresponding product
concentration and corresponding product concentration at
time “t”, respectively. LDH-ILs-C12 composite shows excellent
catalytic activity and the yield can reach 95.6% over 2 h. The
linear fit of the data reveals that the catalytic reaction exhibits
pseudo-first-order kinetics for the Knoevenagel condensation
(R2=0.994). The rate constant k for the Knoevenagel reaction
can be determined to be 0.023 min-1 on the basis of Equations
(1) and (2). The Knoevenagel reaction of benzaldehyde with
To demonstrate the wide applicability of the catalytic
system, the Knoevenagel reaction of various aldehydes with
ethyl cyanoacetate has been carried out in aqueous solution at
room temperature. As shown in Table 3, the corresponding
products can be obtained in high yields using LDH-ILs-C12 as a
heterogeneous catalyst. For example, the aromatic aldehydes
(benzaldehyde) can be converted to the corresponding
product in good yield of 95.4 (2h) and 99.9 % (3h), respectively
(entries 1-2). For methyl-substituted aromatic aldehydes, such
as 2-methylbenzaldehyde, 3-methyl-benzaldehyde, and 4-
methylbenzaldehyde can all be converted to the
corresponding products in high yields of >99.9% (entries 3-5) in
2h. As for the methoxy-substituted aromatic aldehydes
including (2-methoxylbenzaldehyde, 3-methoxybenzaldehyde,
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and 4-methoxybenzaldehyde), the corresponding products can The Knoevenagel condensation of 2-thenaldehyde and 2,4-
be obtained in high yield of 99.2%, 98.2% and 91.5% (entries 6- difluorobenzaldehyde can give excellent yields of > 99%
8). The above results indicate that the electron-donating group and97.3%, respectively (entries 17-18). In terms of the
substituted aromatic aldehydes achieve high yields. In the case aliphatic aldehydes, the Knoevenagel condensation of
of the electron-withdrawing group substituted aromatic heptanal, nonanal and decanal obtain the yield of 92.5 %, 87.1
aldehydes,
such
as
2-chlorobenzaldehyde,
2-bromobenzaldehyde,
4-bromobenzaldehyde,
3- % and 82.0 %, respectively (entries 19-21). The catalytic
3- activities decrease slightly with the increase of length of the
2- alkyl chains. This may be due to the increased hydrophobicity
4- of the substrate. Furthermore, all the resulting products
chlorobenzaldehyde,
bromobenzaldehyde,
nitrobenzaldehyde,
3-nitrobenzaledhyde,
and
nitrobenzaldehyde, high yields can be achieved (entries 9-16). exhibit over
Table 3 The Knoevenagel condensation of various aldehydes with ethyl cyanoacetate catalysed by LDH-ILs-C12 in aqueous solution at RT[a]
Entry
Donor
Acceptor
Product
Time (h)
Yield[b] (%)
E/Z[c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
3
2
2
2
2
2
2
3
2
2
2
2
2
2
2
2
99.9
95.4
99.9
99.9
99.9
99.2
98.2
91.5
98.9
99.9
96.3
99.9
99.9
99.9
99.9
99.9
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
99:1
X=H
X=H
X=2-Me
X=3-Me
X=4-Me
X=2-MeO
X=3-MeO
X=4-MeO
X=2-Cl
X=3-Cl
X=2-Br
X=3-Br
X=4-Br
X=2-Me
X=3-Me
X=4-Me
X=2-MeO
X=3-MeO
X=4-MeO
X=2-Cl
X=3-Cl
X=2-Br
X=3-Br
X=4-Br
X=2-NO2
X=3-NO2
X=4-NO2
X=2-NO2
X=3-NO2
X=4-NO2
17
18
2
2
99.9
97.3
99:1
99:1
19
20
21
R=C7H15
R=C9H19
R=C10H21
R=C7H15
R=C9H19
R=C10H21
2
2
2
92.5
87.1
82.0
99:1
99:1
99:1
[a]Reaction conditions: 1 mmol Substrate, 1.5 mmol ethyl cyanoacetate, 30 mg catalyst LDH-ILs-C12, 0.5 mL H2O, rt. [b]Yields were determined by GC analysis using reference
standards. Assignments of corresponding products were analyzed by 1H NMR and 13C NMR. [c]E/Z = E isomers of corresponding Knoevenagel condensation products/Z isomers of
corresponding Knoevenagel condensation products
E/Z>99:1. In contrast, the LDH-CO3 precursor shows inferior high stability and recyclability with a five-run recycling test. In
activity for these aforementioned substrates under same contrast, some other reported catalysts either use toxic and
conditions (Table S3).
The catalytic efficiency of the LDH-ILs-C12 composite is
evaluated in the Knoevenagel condensation of various aldehydes
and ketones with malononitrile in aqueous solution at room
temperature. As shown in Table 4, the catalytic performance is
excellent for the aromatic aldehyde. This is due to the electron-
withdrawing ability of -CN is stronger than that of -COOEt, the
methylene group in malononitrile is more active than that in
ethyl cyanoacetate. As such, the LDH-ILs-C12 composite can be
used for highly efficient catalytic Knoevenagel reaction of
various aldehydes with active methylene compounds.
3.4 Comparison with other reported heterogeneous catalysts
The Knoevenagel condensation of ethyl cyanoacetate with
benzaldehyde by using different heterogeneous catalysts
reported so far is summarized in Table 5. The Knoevenagel
condensation reaction by using LDH-ILs-C12 as the catalyst
shows its prominent features: 1) excellent catalytic activity can
be achieved with the yield of 95.6% in 2 h; 2) the experiments
were carried out in aqueous solution at room temperature; 3)
6 | J. Name., 2012, 00, 1-3
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11
12
13
14
15
X=3-Br
X=4-Br
X=2-NO2
X=3-NO2
X=4-NO2
X=3-Br
X=4-Br
X=2-NO2
X=3-NO2
X=4-NO2
5
5
5
5
5
99.2
92.2
95.8
97.6
99.9
Table 4 The Knoevenagel condensation of various aldehydes and ketones
with malononitrile catalyzed by LDH-ILs-C12 in aqueous solution at RT.[a]
Entry
Donor
Acceptor
Product
t (min)
Yield (%)
16
5
99.9
1
2
3
4
5
6
7
8
9
X=H
X=H
5
5
5
5
5
5
5
5
5
5
99.7
99.9
89.7
99.5
89.5
98.9
99.9
91.9
91.3
89.6
X=2-Me
X=3-Me
X=4-Me
X=2-MeO
X=3-MeO
X=4-MeO
X=2-Cl
X=2-Me
X=3-Me
X=4-Me
X=2-MeO
X=3-MeO
X=4-MeO
X=2-Cl
17
18
19
R=C8H17
R=C9H19
R=C10H21
R=C8H17
R=C9H19
R=C10H21
5
5
5
77.6
87.0
81.8
[a]Reaction conditions: 1 mmol Substrate, 1.5 mmol malononitrile, 30 mg catalyst LDH-
ILs-C12, 0.5 mL H2O, rt. Yields were determined by GC analysis using reference
standards.
X=3-Cl
X=2-Br
X=3-Cl
X=2-Br
10
Table 5. Various catalytic systems for Knoevenagel condensation of ethyl cyanoacetate with benzaldehyde using different catalysts.
Entry
1
Catalyst
Ratio[a]
Amount(mg)
20
Solvent
water
T [°C]
t [h]
2
3
Yield
95.6
99.9
99
Ref
This
work
LDH-ILs-C12
1.5:1
r.t.
2
3
Aminopropylsily-MCM-41
LaHAP
(HAP: Ca10(PO4)6(OH)2)
1.3:1.25
1.5:1
100
toluene
toluene
r.t.
60
1
37
38
2mmol%
24
>99
4
5
6
7
8
LDH-F(Mg3Al)
LDH-diisopropylamide
1:1
2:2
35
50
45
250
150
1.2mol%
50
1 wt.%
DMF
DMF
60
r.t.
82
82
82
130
80
60
2
2
1
36
1
2
8
92
95
>99.9
94
>99
>99
85
20
39
40
41
10
42
43
44
45
amine-grafted PE-MCM-41[b]
aminopropyl-functionalized MCM
aminopropyl-functionalized SBA-15
20:20
20:20
10:10
1.1:1
3:2.5
50:50
50:50
cyclohexane
cyclohexane
cyclohexane
solvent-free
solvent-free
solvent-free
solvent-free
2-Propanol/
Water
9
Organic-inorganic hybrid silica material[c]
Ni-Fe MMO
10
11
12
Mg–Al-In MMO
Mg–Ga/Al-MMO
24
4
>99
82
1 wt.%
60
13
Mg3Al-PW12
1.5:1
0.25mol %
60
6
>99.9
17
[d]
14
15
DMAN/SiO2
7:8
10:10
1 mol%
150
ethanol
Water
60
60
6
10
>99.9
94
46
47
LDH-OH (Mg3Al)
Note: [a]Ratio= Ethyl cyanoacetate (mmol)/benzaldehyde (mmol); [b]PE-MCM-41=pore-expanded MCM-41; [c]Organic-inorganic hybrid silica material: containing
imidazolium and dihydroimidazolium salts [d]DMAN/SiO2=1,8-bis(dimethylamino)naphthalene-functionalized SiO2.
LDH-F (entry 4), toluene and DMF was used as solvents. In the
case of organic-inorganic hybrid silica material, a higher
o
temperature (130 C) was applied to achieve good catalytic
results (entry 9).
We have performed a scaled-up experiment by using 50
times of the original experimental conditions (50 mmol
benzaldehyde, 75 mmol ethyl cyanoacetate, 1.5 g catalyst, 25
mL H2O, 3 h, rt.). The reaction proceeds smoothly at room
temperature in aqueous solution. An excellent yield of
benzylidene ethyl cyanoacetate of 96% can be obtained with
exclusively E-diastereo isomer formed and >99% selectivity.
The above experiments demonstrate the LDH-ILs-C12 is an
ideal candidate for further application.
3.5 Recycling experiments
Catalyst stability and recyclability is a practical and vital topic
for heterogeneous catalysis. Taking the Knoevenagel
Fig. 6. A) Recycling experiments of LDH-ILs-C12 for the Knoevenagel condensation of
condensation of benzaldehyde with ethyl cyanoacetate as an
benzaldehyde with ethyl cyanoacetate at room temperature in aqueous solution; B) FT-
example, the recycling experiments having been performed by
IR spectra; C) solid-state 29Si CP/MAS NMR spectra and D) XRD patterns of the fresh and
using LDH-ILs-C12 at room temperature in aqueous solution.
After reaction, the solid catalyst was recovered by
reused LDH-ILs-C12.
centrifugation, washed with ethanol, dried and reused for the
explosive organic solvents (entry 2-8) or suffer from higher
next run without adding any fresh catalyst. The catalytic
reaction temperature (entry 3-4, 6-15). For example, the
reusability for LDH-ILs-C12 in Knoevenagel condensation
amine-grafted PE-MCM-41 (entry 6) and aminopropyl-
reaction is measured by five-run tests (Fig. 6A). The results
functionalized MCM (entry 7) catalysts require a much higher
temperature of 82 oC in cyclohexane to reach a comparable
demonstrate
a
steady reusability without obvious
deterioration of its catalytic activity for this heterogeneous
performance; For the catalytic systems of LaHAP (entry 3) and
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only the corresponding condensation product can be detected
and no other byproducts are found. Furthermore, the
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6B), solid-stare 29Si cross-polarization magic-angle spinning
(CP/MAS) NMR spectrum (Fig. 6C) and XRD analyses
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adopting an exfoliation/assembly approach. The grafting of ILs-
Cn onto LDHs not only helps to adjust the distribution of basic
sites, but also induces flexibility to the catalyst and allows easy
accessibility of the active centre by the substrates. Application
of LDH-ILs-C12 for Knoevenagel condensation of various
aldehydes with ethyl cyanoacetate/malononitrile shows an
excellent yield, high selectivity, and efficacy in aqueous
solution at room temperature. In addition, the LDH-ILs-C12
exhibits excellent structural stability and recyclability.
Therefore, this work presents the robust, sustainable, cost-
effective and highly efficient heterogeneous base catalytic
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Acknowledgements
This research was supported by the National Key Research and
Development Program of China (2017YFB0307303), the
National Basic Research Program of China (973 program,
2014CB932104), National Nature Science Foundation of China
(U1407127,
U1507102,
21521005,
21625101),
and
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Fundamental Research Funds for the Central Universities
(XK1530, XS1601, ZY1709). H. N. M. acknowledges the financial
support from the University of Glasgow.
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Graphic abstract
In this paper, three solid base catalysts of LDHs-ILs-Cn (n=4,8,12) were synthesized
by adopting an exfoliation/assembly approach, in which the ionic liquids were
covalently modified onto the layer of LDHs. The grafting of ionic liquids onto LDHs
not only helps to adjust the distribution of basic sites, but also induces flexibility to
the catalyst and allows easy accessibility of the active center to the substrates. The
as-prepared catalyst showed excellent activity and selectivity for Knoevenagel
condensation reaction at room temperature in aqueous solution.