Synthesis of sulphur-modified bifunctional hydrotalcites and study of their surface characteristics by inverse gas chromatography

Article abstract of DOI:10.1039/c5cy00765h

In this study, various sulphur-modified hydrotalcite catalysts were prepared, and the influence of calcination temperature on their acid-base properties was investigated. Structural characterization of the catalysts was studied using X-ray powder diffraction, scanning electron microscopy, N₂ physisorption, elemental analysis and Fourier transform infrared spectroscopy. The structural characterization indicated that the layer structure of all catalysts was retained but the specific surface areas were enlarged. Inverse gas chromatography was carried out to quantitatively determine the catalysts' acid-base properties by calculating the thermodynamic parameters, including dispersive surface free energy, adsorption free energy, adsorption enthalpy, and acid-base interaction constants. The results showed that the strength and content of acidic and alkaline sites were enhanced with increasing calcination temperature. Moreover, several typical aldol condensation reactions were selected to study the catalytic activity of the developed catalysts. The results showed that the sulphur-modified hydrotalcite catalysts possess high activity and good regenerability for typical aldol condensation reactions.

Full text of DOI:10.1039/c5cy00765h

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Synthesis of sulphur-modified bifunctional
hydrotalcites and study of their surface
Cite this: DOI: 10.1039/c5cy00765h
characteristics by inverse gas chromatography†
Xiaoqian Ren, a Xi Hu,a Feng Zhang,a Junge Wang,a Jinhua Liang,b Wenliang Wu,a
*
Min Jiangb and Jun Wanga
In this study, various sulphur-modified hydrotalcite catalysts were prepared, and the influence of calcina-
tion temperature on their acid–base properties was investigated. Structural characterization of the catalysts
was studied using X-ray powder diffraction, scanning electron microscopy, N2 physisorption, elemental
analysis and Fourier transform infrared spectroscopy. The structural characterization indicated that the layer
structure of all catalysts was retained but the specific surface areas were enlarged. Inverse gas chromatog-
raphy was carried out to quantitatively determine the catalysts' acid–base properties by calculating the ther-
modynamic parameters, including dispersive surface free energy, adsorption free energy, adsorption
enthalpy, and acid–base interaction constants. The results showed that the strength and content of acidic
and alkaline sites were enhanced with increasing calcination temperature. Moreover, several typical aldol
condensation reactions were selected to study the catalytic activity of the developed catalysts. The results
showed that the sulphur-modified hydrotalcite catalysts possess high activity and good regenerability for
typical aldol condensation reactions.
Received 24th May 2015,
Accepted 17th July 2015
DOI: 10.1039/c5cy00765h
www.rsc.org/catalysis
could be destroyed and form Mg–Al complex oxides (LDOs)
when the calcination temperature is over 473 K. LDOs and
1. Introduction
The aldol condensation reaction is one of the most powerful
and popular C–C bond-forming methods.1 In the past
decades, many homogeneous and heterogeneous catalysts
have been developed to catalyze this reaction. Because of the
great advantages of heterogeneous catalysts, there has been
much interest in the design and development of highly reac-
tive multifunctional heterogeneous catalysts for aldol conden-
sation reactions.2,3 In some studies, these catalysts showed
superior catalytic activity than traditional monofunctional cat-
alysts, which contain only acidic sites or alkaline sites.4
Multifunctional heterogeneous catalysts, such as hydrotalcite-
like compounds (HTLcs) and their derivatives, can provide
more than two types of active centers in a reaction system,
which can co-activate the substrates and promote the
reactions.
their reconstructed products have superior catalytic activity
for various reactions.9,10 Yamaguchi et al. prepared LDHs
with Mg/Al ratios from 3 to 8 and calcined them at various
temperatures from 673 K to 1273 K.11 At different calcination
temperatures, the formed catalysts may have various acid–
base properties and can provide diverse catalytic perfor-
mance. The above alkaline materials can also combine with
other functional materials, i.e. Al-containing mesoporous sil-
ica,12 Amberlyst,13 and rubber nanocomposites,14 to create
new multifunctional catalysts. In many studies, sulfate and
persulfate are applied to enhance the acidity of catalysts.
2−
Tanabe reported that the addition of SO4 anions to ZrO2
could enhance the acidity of the catalyst, and the calcination
temperature may affect the catalytic behavior.15 Yang et al.
prepared a SO42−/TiO2–SiO2 catalyst by impregnation and cal-
cination.16,17 Meanwhile, acidity measurements showed that
the acid strength of the catalyst could reach up to −12.70.
Thus, using sulfate and persulfate to modify hydrotalcites
may enhance the catalysts' acidity and various acid–base
bifunctional catalysts can be obtained. By altering the calci-
nation temperature, the acid–base properties of catalysts
could be optimized and provide better catalytic activity. As
mentioned above, HTLcs are materials with a typical layer
structure, and high temperature may destroy the layer struc-
ture (Fig. S1†), which may influence the materials'
Layered double hydroxides (LDHs) or hydrotalcite-like
compounds are widely applied as clay materials with a typical
2D nanostructure.5–8 The specific layered structure of LDHs
a College of Chemistry and Chemical Engineering, Nanjing Tech University,
210009 Nanjing, Jiangsu, China. E-mail: xqren@njtech.edu.cn;
Tel: +8613951645412
b College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech
University, 210009 Nanjing, Jiangsu, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy00765h
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fundamental acid–base properties. An appropriate testing
technique, such as inverse gas chromatography (IGC), is cru-
cial in characterizing the developed catalysts by determining
their surface properties.
Finally, the suspension was dried and the obtained powder
was named S/LDH-673 (or S/LDH-723, S/LDH-773, S/LDH-823
and S/LDH-873) catalyst.
IGC is a popular tool with the advantages of convenience,
high efficiency and mild conditions. This technique has been
applied to characterization of many materials with heat sensi-
tivity or weak acid–base properties.18,19 The IGC technique
could determine the properties of HTLc materials under mild
conditions and it just needs a small amount of experimental
data and simple thermodynamic calculation. Therefore, we
used the IGC technique as an effective tool to analyze the
acid–base properties of the prepared catalysts.
In this study, sulfate was used to adjust the acidity and
alkalinity of the catalysts. A series of acid–base bifunctional
catalysts were developed after calcination and self-assembly
processes. The IGC technique, pyridine-IR (Py-IR) analysis
and Hammett indicators were used to investigate the acid–
base properties of these developed catalysts. The catalytic
activity of the catalysts for typical aldol reactions and the
interaction mechanism between their acidity, alkalinity and
catalytic activity are also investigated.
2.4 Catalyst characterization
XRD analysis was conducted on a Bruker D8 Advance powder
X-ray diffractometer using Cu Kα radiation (λ = 0.154 nm) at
40 kV and 200 mA, a scanning rate of 20° min−1, and a 2θ
angle ranging from 5° to 80°. N2 physical adsorption–desorp-
tion measurements were carried out on an ASAP 2000 surface
analyser (Micromeritics Co., USA) at 77 K. Before measure-
ment, the catalysts were degassed at 473 K for 4 h. The tex-
tural properties of the samples were calculated by the BET
method. The contents of Mg and Al in the samples were
determined using inductively coupled plasma atomic emis-
sion spectroscopy on a Perkin Elmer ICP-OES Optima 3000
instrument using solutions prepared by dissolving the sam-
ples in dilute HNO3. The contents of non-metallic elements
in the samples were measured using an elemental analysis
instrument (Elementar, vario Micro cube, Germany). The
morphologies of the catalysts were observed by SEM using
Hitachi S-4800 instruments. X-ray photoelectron spectra
(XPS) were measured with a Kratos Axis HSi photoelectron
spectrometer equipped with Al Kα radiation (hν = 1486.6 eV).
A powder of 250 W and a pass energy of 37.5 eV were used
during the experiment. The base pressure of the chamber
was higher than 3 × 10−9 Torr. FT-IR spectra were obtained
using a Nexus 870 FTIR spectrometer in the range of 4000 to
400 cm−1. Py-IR spectra were recorded using a Perkin Elmer
Spectrum One FT-IR spectrometer. The solid sample was
pretreated under vacuum at 423 K for 24 h. Then, the solid
sample (about 20 mg) was pressed into a self-supporting
wafer of 13 mm diameter and treated under vacuum at 423 K
for 2 h. The background spectrum was recorded first after
the sample was cooled to room temperature. Pyridine was
introduced into the sample for 5 min, and then consequently
desorbed at 423 K. Finally, the spectrum was recorded with a
4 cm−1 resolution in the range of 1600–1400 cm−1. The base
strength and alkali number were determined by using Ham-
mett indicators. The number of alkaline sites was estimated
by titration using benzoic acid. IGC was carried out to mea-
sure the acid–base interaction constants Ka and Kb. A conven-
tional gas chromatograph (Kechuang, GC-910, China) with a
thermal conductivity detector was used for the measure-
ments. The catalysts with a loading between 0.5 and 0.6 g
were placed into a 304 stainless steel column 15 cm in length
with an inner diameter of 3 mm. All columns were packed
with mechanical vibration, and the two ends were plugged
with deactivated glass wool. The columns were pretreated in
the GC system at 403 K overnight with a nitrogen flow rate of
20 mL min−1. Measurements were carried out in the tempera-
ture range from 333 to 363 K. High purity nitrogen was used
as the carrier gas, and flow rates were measured using a soap
bubble flow meter. In order to meet the requirement of
adsorption at infinite dilution, the probes injected were in
2. Experimental section
2.1 Materials
All chemicals were purchased from Sinopharm Reagent Co.
(Shanghai, China). Magnesium nitrate IJMgIJNO3)2·6H2O), alu-
minum nitrate IJAlIJNO3)3·9H2O), urea IJCOIJNH2)2), ammonium
persulfate IJ(NH4)2S2O8) and other reagents were used as-
obtained without further purification.
2.2 Hydrotalcite preparation
A certain weight of MgIJNO3)2·6H2O, AlIJNO3)3·9H2O and urea
was dissolved in 250 mL of deionized water to yield a solu-
tion containing 0.15 mol of MgIJNO3)2·6H2O, 0.05 mol of
AlIJNO3)3·9H2O and 0.9 mol of urea. Then, this solution was
transferred into an autoclave placed in an oven at a constant
temperature of 453 K to react for 24 h. After cooling to room
temperature, the precipitate formed was filtered and washed
with deionized water until pH = 7. The solid was dried at 373
K for 24 h to yield the as-synthesized layered double hydrox-
ide (LDH).
2.3 Modification of hydrotalcites
LDH was calcined at 723 K for 8 h in a muffle to obtain LDO.
The calcination temperature was raised at a heating rate of 5
K min−1. Then, 1.0 g of LDO was suspended in 100 mL of an
aqueous solution containing 0.05 g of ammonium persulfate
and stirred at 298 K for 24 h. The suspension was evaporated
at 343 K for 1 h using a rotary evaporator and the obtained
solid was ground into powder. The following treatment con-
ditions are similar to those of the process described above;
the powder was calcined at 673 K (or 723 K, 773 K, 823 K and
873 K) for 8 h and treated with 100 mL of water again.
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the range from 0.2 to 0.3 μL. The procedures and thermody-
namic concepts which link the net retention volume of the
IGC experiment with the dispersive surface free energy of the
catalysts, γds , and their acid and base interaction constants, Ka
and Kb have been fully described elsewhere.20–24 The proper-
ties of the probes used for IGC are shown in Table S1.† All
calculations and results of the IGC measurements are sum-
marized in Table S2–S23.†
However, the morphology of S/LDH-823 displays irregular
particles (Fig. S3b†) and an obvious layer structure can be
observed (Fig. S3c†). The generation of these smaller particles
may be due to vigorous mechanical stirring in the reconstruc-
tion procedure. The generated small plates may agglomerate
into larger particles with the presence of a small amount of
absorbed water. The SEM-EDS spectrum graph in Fig. S3d†
demonstrates the existence of the sulfur element on the sur-
face of S/LDH-823. The molar ratio of Mg to Al is 3.07 IJ14.53/
4.73), which is consistent with the molar ratio of MgIJNO3)2
·6H2O to AlIJNO3)3·9H2O in the experimental section.
Table 1 shows the physicochemical properties of the cata-
lysts. The surface areas of the modified catalysts are larger
than that of LDH, which may be caused by vigorous mechani-
cal stirring.25 The stirring process could exfoliate the smaller
particles to form very thin plates. The thinner plates and
higher surface area may provide more active sites, such as
more surface hydroxyl groups and defects of the layer.27
Under the same treatment conditions (room temperature,
700 rpm and 24 h), it is observed that the pore volume of S/
LDH-823 is smaller than that of LDH-823. This phenomenon
may be attributed to blockage of the pores of S/LDH catalysts
by internal sulfate. Many research studies demonstrated that
LDHs could generate strong alkaline sites at an appropriate
calcination temperature.28–30 Similarly, the results of Ham-
mett indicators indicated that the alkaline content of the cat-
alysts increased with calcination temperature. S/LDH-773, S/
LDH-823 and S/LDH-873 catalysts exhibited similar total alka-
line content. It could be concluded that when the calcination
temperature is over 773 K, the alkalinity of the catalysts reach
the highest level. The results of ICP characterization are simi-
lar to those of SEM-EDS. The Mg/Al molar ratios of the cata-
lysts increase slightly after modification, which may be attrib-
uted to several times of calcination and reconstruction.
These processes may result in the removal of the aluminum
element.
2.5 Catalytic testing
In a typical experiment, 1.0 g of p-nitrobenzaldehyde, 5.0 g of
acetone, and 0.05 g of the catalyst were mixed in a 50 ml
round-bottomed flask. The reaction was carried out with
reflux and nitrogen protection at 333 K for a required time.
The conversion of p-nitrobenzaldehyde was determined using
a
high performance liquid chromatograph (HPLC,
SHIMADZU, Japan) equipped with a C-18 column (4.6 × 250
mm, 5 μm). The UV detection wavelength and the volume
ratio of the mobile phase were based on the reaction.
3. Results and discussion
3.1 Structural characterization
There are seven typical hydrotalcite diffraction peaks ([003],
[006], [009], [015], [018], [110] and [113]) on the XRD patterns
(Fig. S2†), which showed that all modified catalysts retain the
layer structure. The XRD patterns of S/LDH and LDH-823
revealed broader and lower peaks than that of LDH, indicat-
ing lower crystallinity, which may be attributed to vigorous
mechanical stirring during the rehydration process.25,26 This
process can break and exfoliate the platelets of LDH to
increase the surface area of the catalysts.25 When the calcina-
tion temperature is 973 K, the patterns exhibited three spinel
peaks, which revealed that this part of the metal oxides is
unrecoverable.26 However, when the calcination temperature
was below 873 K, the patterns did not exhibit any obvious
spinel peaks. Thus, 873 K was selected as the highest calcina-
tion temperature in this research.
The FT-IR spectra of the catalysts are presented in Fig. 1a.
The absorption bands at 3470 cm−1 and 1640 cm−1 observed
in the spectra of these samples are attributed to the
stretching modes of absorbed water and hydroxyl groups.31
The absorption band at 870 cm−1 is characteristic of the out-
Fig. S3† shows the SEM images of LDH and S/LDH-823.
The morphology of LDH is obviously a layered solid (Fig.
S3a†), which indicates
a well-developed layer structure.
Table 1 Physicochemical properties of the catalysts
Alkali numberc
Mass fraction/%
ICP
a
b
Catalyst
ABET
VT
≥7.2
≥9.3
Total
H
S
nIJMg) : nIJAl)
LDH
LDH-823
77
0.13
0.65
0.38
0.42
0.49
0.54
0.52
—
0.32
2.74
1.69
1.82
2.13
2.21
2.05
—
0.00
1.89
1.28
1.39
1.73
1.80
1.67
—
0.32
2.74
1.69
1.82
2.13
2.21
2.05
—
1.00
4.28
3.81
3.87
3.87
3.86
3.83
3.86
0.00
0.00
0.39
0.37
0.36
0.36
0.35
0.36
2.98 : 1
3.17 : 1
3.13 : 1
3.17 : 1
3.16 : 1
3.13 : 1
3.15 : 1
—
197
128
144
158
165
155
—
S/LDH-673
S/LDH-723
S/LDH-773
S/LDH-823
S/LDH-873
S/LDH-873-5
a
−1
b
c
BET method, m2
g
.
Total pore volume IJP/P0 = 0.990), cm3 g−1
.
Hammett titration method, bromothymol blue and phenolphthalein as
indicators, mmol g−1
.
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Fig. 1 (a) FT-IR spectra of the seven samples, (b) XPS spectrum of S 2p for S/LDH-823 catalyst and (c) Py-IR analysis of LDH-823 and S/LDH-823
catalysts.
of-plane deformation of carbonate, whereas the band for in-
plane bending is located at 667 cm−1.25 The vibration band of
carbonates appearing at 1370 cm−1 could be assigned to
interlayer carbonates. The absorption bands at 450 cm−1 and
554 cm−1 are attributed to the vibrations of MgO and Al2O3.
The bands observed at 1120 cm−1 and 955 cm−1 correspond
to the asymmetric and symmetric stretching frequencies of
SO and S–O bonds, respectively, which indicate that the
configuration of the acidic sites is chelating bidentate.32 The
spectrum of S 2p for S/LDH-823 is shown in Fig. 1b. The
binding energy of S 2p is 169.6 eV, which suggest the pres-
ence of S6+ IJSO42−).33,34 Fig. 1c shows the Py-IR spectra of
LDH-823 and S/LDH-823 catalysts. It is observed that the two
catalysts exhibit bands at 1544 cm−1 assigned to pyridine
bound to Brönsted acidic sites and at 1448 cm−1 assigned to
pyridine bound to Lewis acidic sites. In terms of the area of
the absorption bands, the acidic content of the S/LDH-823
catalyst is higher than that of the LDH-823 catalyst. Mean-
while, the content of S/LDH-823 catalyst's Lewis acidic sites is
increased greatly, which may be attributed to the interaction
of metal oxides with sulfate. As a result of the strong attrac-
tion of electrons from metal ions by way of two covalent
SO bonds, the center metal atom lacks electrons severely,
thus strong Lewis acidic sites are generated.32 When a Lewis
acidic site absorbs water, it may transform into a Brönsted
acidic site.
of surface free energy (γds ) can be obtained from a plot of the
logarithm of retention volume of a series of alkane probe
molecules versus the product of liquid tension and molecular
area. The value for γds can be calculated from the slope
according to eqn (2):22,23
RT ln(Vn) = (γd )0.5 × 2aN(γd )0.5 + C
(2)
s
l
The plots of γds values versus column temperature are shown
in Fig. 2a, which showed a good linear fit. The γds values
decreased with increasing column temperature, which indi-
cated that the interaction of the catalysts with the probes
decreased. When the calcination temperature is above 823 K,
the γds values for S/LDH-823 and S/LDH-873 are similar, which
may be attributed to these two samples having similar energy
spots on the surface.
The contribution of Lewis acid–base interactions to the
adsorption free energy of polar probes (− ΔGsa) is marked in
Fig. 2b. The straight line shows the dispersive interactions of
the stationary phase, while the polar probes lie above this
line, indicating that acid–base interactions are present. The −
ΔGsa values for all acidic, alkaline, and amphoteric probes are
relatively high. It can be concluded that all types of active
sites are present on the surface of the S/LDH-823 catalyst.
When − ΔGsa is available, the adsorption enthalpy values for
polar probes can be obtained from a plot of ΔGsa/Tc versus 1/
Tc; a straight line should be obtained with its slope, − ΔHsa,
reflecting the acid–base interactions, as shown in Fig. 2c. The
results of the calculation are summarized in Table 2.
Here, we can estimate the acid–base properties of the cata-
lysts based on the adsorption enthalpy values. From Table 2,
the − ΔHsa values for LDH, LDH-823, S/LDH-673 and S/LDH-
723 are in the order of acid probes > neutral probes > base
probes, which indicated that these four samples have stron-
ger interaction with acid probes and the acid–base properties
of these catalysts are more alkaline. Moreover, upon increas-
ing the calcination temperature, the base probes' enthalpy
values for the S/LDH catalysts increased obviously, which
indicated that the acidity of these catalysts has been further
enhanced. Compared to those of the LDH-823 catalyst, the S/
LDH-823 catalyst's enthalpy values for THF, DEE and Acet are
obviously greater, but the enthalpy values for DCM and TCM
3.2 The inverse gas chromatography study
In general, the retention time is converted into net retention
volume, Vn, which can be obtained by using eqn (1):22,23
(1)
Here, F is the carrier gas flow rate in mL s−1. Pw is the vapor
pressure of water in the flow rate. P0 is the atmospheric pres-
sure at room temperature. tr and t0 are the retention times of
the probe and methane, respectively. J is the well-known
James–Martin correction factor for gas compressibility. Tc
and T are the temperatures at the column and flow meter.
When retention volume is available, the disperse component
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Fig. 2 (a) Plot of γsd vs. column temperature, (b) plot of energy of adsorption RT ln IJVn) vs. 2aN(γds)0.5 of n-alkanes and polar probes on the surface
of S/LDH-823 at T = 333 K, (c) plot of −ΔGsp/T as a function of 1/T for DCM on S/LDH-823 and (d) plot of ΔHsa/AN* as a function of AN*/DN on S/
LDH-823.
Table 2 Specific components of adsorption enthalpy of polar probes on the surface of the catalysts (kJ mol−1)
Probe
LDH
LDH-823
S/LDH-673
S/LDH-723
S/LDH-773
S/LDH-823
S/LDH-873
DCM
TCM
DEE
Acet
THF
7.9726
9.3247
2.9666
4.7289
3.0660
37.4520
43.9359
16.0085
17.7204
8.6862
22.1723
28.1398
15.7577
18.4871
12.7577
26.6000
32.4984
20.6286
24.2871
17.4809
29.8810
39.5418
30.7334
37.3122
28.8744
32.3101
40.4524
36.1386
37.7271
35.6832
30.8445
39.7836
33.4512
40.8128
32.2674
are lower. This indicates that the modification process could
introduce acidic sites into hydrotalcite materials, but some
alkaline sites would be quenched. Here, the S/LDH-823 cata-
lyst has the maximum − ΔHsa value for THF which means that
it may have the strongest acidity.
When enthalpy data are available, the empirical relation-
ship given by eqn (3) (ref. 22 and 23) can be used to establish
the acid–base parameters of the catalysts. As shown in
Fig. 2d, when − ΔHsa/AN* is plotted versus DN/AN* of a set of
polar probes, a straight line could be regressed. The slope,
Ka, designates the acidity of a sample, and the intercept, Kb,
reflects its electron donor ability. According to the electronic
theory of acid and alkali, determination of the Ka and Kb
values provides a quantitative basis to correlate the acid–base
character of the surface to their composite strength.21
LDH catalyst is the least active catalyst. The huge difference
between the LDH and LDH-823 catalysts may be attributed to
the rehydration process. This process could enhance the sur-
face of the catalysts and expose more Brönsted alkaline sites
(surface hydroxyl groups and defects of layer) in the interlayer
spaces.35 The acidity of the sulphur-modified catalysts
increased with increasing calcination temperature. When the
calcination temperature increases up to 823 K, the acidity of
the S/LDH catalyst reaches the highest value. The Ka value for
Table 3 Acid–base properties of the catalysts
Sample
Ka
Kb
Ka + Kb
Ka/Kb
LDH
0.0255
0.0508
0.1195
0.1696
0.3107
0.3774
0.3381
0.4955
2.1227
1.3123
1.5590
1.7619
1.8747
1.8519
0.5210
2.1735
1.4318
1.7286
2.0726
2.2521
2.1900
0.0515
0.0239
0.0911
0.1088
0.1734
0.2013
0.1826
LDH-823
S/LDH-673
S/LDH-723
S/LDH-773
S/LDH-823
S/LDH-873
− ΔHsa = Ka × DN + Kb × AN*
(3)
The results in Table 3 show that the S/LDH-823 catalyst is
the most active catalyst (maximum Ka + Kb value) and the
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the S/LDH-823 catalyst (0.3774) is greater than that for the
LDH-823 catalyst (0.0508), which means that the acidity of
the former is stronger than that of the latter. The Kb value for
the S/LDH-823 catalyst is lower than that for the LDH-823 cat-
alyst. Moreover, the LDH-823 catalyst has the highest Kb
value (2.1227) which indicates the strongest alkalinity. These
results are consistent with the results of Py-IR analysis and
Hammett indicators. The values of Ka/Kb are all greater than
1, which indicated that the alkalinity of all catalysts is stron-
ger than their acidity. Thus, it can be concluded that the S/
LDH catalysts are amphoteric and predominantly alkaline
materials.
(Table 4) showed that the S/LDH-823 catalyst has higher cat-
alytic activities than the LDH-823 catalyst, especially in the
self-condensation reaction of cyclohexanone. Moreover,
when the aldehyde contains an aromatic ring, the S/LDH-
823 catalyst displays excellent performance for the reaction.
The aldol condensation reaction of acetone and
p-nitrobenzaldehyde was selected as a probe to check the
catalytic performance of the catalysts, as shown in Fig. 3a.
When the calcination temperature is over 773 K, the catalysts
showed high activity with greater than 90% conversion of
p-nitrobenzaldehyde after 2 h. However, the LDH-823 catalyst
showed 55.3% conversion of p-nitrobenzaldehyde under the
same reaction conditions. The experimental results indicated
that the S/LDH-823 catalyst showed higher catalytic activities
than LDH-823 and other catalysts.
3.3 Catalytic performance
Various typical aldol condensation reactions were selected to
compare the catalytic activity of the S/LDH-823 catalyst with
that of the LDH-823 catalyst. The experimental results
The used S/LDH-823 catalyst was washed with acetone sev-
eral times, followed by the same calcination and reconstruc-
tion processes described previously. Then, the aldol
Table 4 Direct asymmetric aldol condensation reactions of aldehydes with ketones catalyzed by S/LDH-823 catalyst
LDH-823
S/LDH-823
Yield/%
Entry
R1
R2
R3
R4
Yield/%
Selectivity/%
Selectivity/%
1a,d
2a,d
3a,e
4-NO2Ph
Ph
2-furanyl
IJCH3)2CH
4-NO2Ph
Ph
2-Furanyl
IJCH3)2CH
Ph
2-furanyl
IJCH3)2CH
–IJCH2)5–
–IJCH2)4–
H
H
H
H
H
H
H
H
H
H
H
–IJCH2)3–
–IJCH2)3–
–IJCH2)3–
–IJCH2)3–
–IJCH2)2–
–IJCH2)2–
–IJCH2)2–
–IJCH2)2–
H
69.5
34.2
44.9
11.9
75.8
55.1
41.7
10.9
82.5
45.1
3.8
98.2
97.5
95.8
99.3
99.5
96.2
95.4
99.5
98.6
94.2
98.8
—
95.5
69.2
78.4
11.9
98.7
83.0
90.2
23.1
95.7
72.5
14.4
5.7
96.7
95.2
93.6
99.3
94.5
96.2
95.8
97.8
95.3
98.1
94.7
97.1
98.5
4a,e
5b,d
6b,d
7b,e
8b,e
9c,d
H
H
H
10c,e
11c,e
12a,e
13a,e
H
H
–IJCH2)3–
–IJCH2)2–
<0.1%
<0.1%
—
8.4
a
b
c
d
Reaction conditions: 373 K, 12 h. Reaction conditions: 353 K, 8 h. Reaction conditions: 333 K, 4 h. Yield and selectivity were based on
e
the aldehyde and were determined by HPLC using the area normalization method. Yield and selectivity were based on the aldehyde and were
determined by gas chromatography using the area normalization method.
Fig. 3 (a) The catalytic performance of the five samples. (b) The regeneration performance of the S/LDH-823 catalyst. Reaction conditions:
p-nitrobenzaldehyde, 1.0 g; acetone, 5.0 g; catalyst dosage, 0.05 g; reaction temperature, 333 K; reaction time, 2 h. Product: A = 4-hydroxy-4-
IJnitrophenyl)-2-butanone, B = p-nitrobenzylidene acetone.
Catal. Sci. Technol.
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Paper
further generate product B after dehydration. In this reaction
system, the roles of Brönsted acidic sites not only strengthen
the electrophilicity of the aldehyde but also activate acetone
to generate an enol. The activated surface hydroxyl groups
play the dominant role in the process of aldol reactions.
4. Conclusion
In this study, acid–base bifunctional heterogeneous sulphur-
modified catalysts were prepared using hydrotalcite, charac-
terized, and applied successfully in aldol condensation reac-
tions. The advantages of the S/LDH catalysts are low price,
easy preparation, high catalytic activity and better regene-
rability. The IGC technique was employed successfully to ana-
lyze the acid–base properties of the catalysts. The results of
IGC are consistent with those of Hammett indicators and Py-
IR analysis. The acidity of the S/LDH catalysts was enhanced
as the calcination temperature increased. Meanwhile, the S/
LDH catalyst with the strongest acidity can be obtained when
the calcination temperature increases up to 823 K. The exper-
imental results show that the S/LDH-823 catalyst has higher
catalytic activity for aldol reactions than the LDH-823 cata-
lyst. It can be explained that the introduced acidic sites coop-
erate with alkaline sites to activate the aldehyde and acetone
molecules simultaneously, which could reduce the difficulty
of carrying out the nucleophilic addition reaction signifi-
cantly. The acid–base characteristics indicate that the S/LDH-
823 catalyst is amphoteric and predominantly an alkaline
material. This catalyst is different from many reported acid–
base bifunctional catalysts which are amphoteric and pre-
dominantly acidic materials.
Fig.
4 Possible acid–base cooperative mechanism in aldol
condensation reaction.
condensation reaction of p-nitrobenzaldehyde with acetone
was used as a probe to determine the regeneration perfor-
mance of the S/LDH-823 catalyst. The results of the regenera-
tion experiment are shown in Fig. 3b. After regeneration for 5
times, no obvious changes in the catalytic activity and prod-
uct selectivity of the S/LDH-823 catalyst can be observed.
Zeidan et al. described a possible reaction mechanism
when they studied multi-functional modified SBA-15 cata-
lysts.36 They believed that acidic sites and alkaline sites could
synergistically activate acetone, forming a stable enol. Then,
Acknowledgements
This work was supported by the National High Technology
Research
and
Development
Program
of
China
(2011AA02A203, 2014AA021205) and the Priority Academic
Program Development of Jiangsu Higher Education Institu-
tions (PAPD).
the
nucleophilic
reaction
of
the
enol
and
p-nitrobenzaldehyde occurred. Moreover, Climent et al. also
reported a potential reaction mechanism when they studied
bifunctional amorphous aluminophosphate catalysts to cata-
lyze the aldol condensation of heptaldehyde and benzalde-
hyde.37 They thought that acidic sites could activate the car-
bonyl of benzaldehyde and strengthen the polarization of the
CO double bond, which enhance the electropositivity of
the carbonyl carbon atom. This process is conducive to the
nucleophilic reaction of benzaldehyde and enol.
Considering the above two reaction mechanisms, we pro-
posed a probable reaction mechanism as shown in Fig. 4.
Firstly, a Brönsted acidic site attacks the carbonyl oxygen
atom of the aldehyde, which will cause the transfer of an
electron of the CO covalent bond to the oxygen atom and
strengthen the electrophilicity of the aldehyde. The activated
hydroxyl of LDHs and Brönsted acidic sites synergistically
activate acetone to generate an enol. Finally, the activated
aldehyde reacts with enol to obtain product A directly or
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