Hydrotalcite reconstructed by in situ rehydration as a highly active solid base catalyst and its application in aldol condensations

Article abstract of DOI:10.1039/c2ra21762g

Reconstructed hydrotalcite is a highly active heterogeneous base catalyst for a wide variety of reactions. Herein, we report a procedure to effectively prepare the reconstructed hydrotalcite. Mg-Al mixed oxide, which originates from hydrotalcite, is added directly to the aqueous-phase reaction system of the aldol condensation without the protection of an inert gas. Under the reaction conditions, the reconstructed hydrotalcite is in-situ generated, and is used as a catalyst for then aldol condensation as soon as it forms, resulting in the elimination of the deactivation of reconstructed hydrotalcite. The experimental results showed that the in-situ reconstructed hydrotalcite had higher catalytic activity and water tolerance. The work provides a simple and effective method for reconstructed hydrotalcite.

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Hydrotalcite reconstructed by in situ rehydration as a
highly active solid base catalyst and its application in
aldol condensations3
Cite this: RSC Advances, 2013, 3, 793
Chunli Xu,*ab Yuan Gao,ab Xihong Liu,ab Ruirui Xinab and Zhen Wangab
Reconstructed hydrotalcite is a highly active heterogeneous base catalyst for a wide variety of reactions.
Herein, we report a procedure to effectively prepare the reconstructed hydrotalcite. Mg–Al mixed oxide,
which originates from hydrotalcite, is added directly to the aqueous-phase reaction system of the aldol
condensation without the protection of an inert gas. Under the reaction conditions, the reconstructed
hydrotalcite is in-situ generated, and is used as a catalyst for then aldol condensation as soon as it forms,
resulting in the elimination of the deactivation of reconstructed hydrotalcite. The experimental results
showed that the in-situ reconstructed hydrotalcite had higher catalytic activity and water tolerance. The
work provides a simple and effective method for reconstructed hydrotalcite.
Received 10th August 2012,
Accepted 5th November 2012
DOI: 10.1039/c2ra21762g
www.rsc.org/advances
rehydration, the brucite-like layers are reformed and the
charge-compensating carbonate anions are replaced by hydro-
Introduction
Hydrotalcite,
with
a
general
formula
of
xyl anions, thus forming Brønsted base sites. The recon-
structed Mg–Al hydrotalcite with Brønsted base sites exhibits
higher catalytic activity than the decomposed Mg–Al hydro-
talcite with Lewis base sites in a variety of reactions, such as
self- and cross-aldol condensations of aldehydes and
ketones,7–10 Knoevenagel and Claisen–Schmidt condensa-
tions,11–13 Michael additions,14 and the transesterification of
tributyrin with methanol.15
The catalytic performance of reconstructed hydrotalcite is
closely related to the rehydration procedure.2,16 There are two
rehydration procedures, gas-phase rehydration and liquid-
phase rehydration, for reconstructed hydrotalcite. Gas-phase
rehydration was carried out by treating the calcined sample in
an N2 or argon flow saturated with water at room temperature
for 24 h to yield rehydrated hydrotalcite.17 Liquid-phase
rehydration was carried out by directly immersing the calcined
hydrotalcite in decarbonated water at room temperature for
about 5 h. After the rehydration process, the sample was
filtered, washed with ethanol, and dried under argon to yield
the rehydrated hydrotalcite.2 The rehydration processes
performed to date are complex and time-consuming. On the
[Mg(12x)Alx(OH)2](CO3)x/222?nH2O, is a class of double layered
anionic clay having brucite-like Mg(OH)2 layers, where
magnesium cations are octahedrally-coordinated with hydro-
xyl ions and share edges to form brucite-like layers. When a
magnesium cation is replaced by an aluminum cation, a
positive charge is generated in the layer, which is balanced by
an anion, such as carbonate or hydroxyl, located between the
layers. Water molecules can also be present in the interlayer
space. In recent years, hydrotalcite-like compounds have been
used in numerous fields.1,2 Hydrotalcite-like compounds are
solid bases, and the basicity of hydrotalcite-like compounds
can be easily tuned by changing the nature and the ratios of
the M2+/M3+ metals or by introducing a suitable anion in the
interlayer region.3 Based on their unique basic properties,
hydrotalcite-like compounds have been widely used as
catalysts or catalyst precursors in organic reactions.4 The
thermal decomposition of Mg–Al hydrotalcite yields a high
surface area Mg–Al mixed oxide [hereafter indicated as
Mg(Al)O], which presumably exposes strong Lewis base sites.5
Interestingly, the calcined hydrotalcites (mixed oxides) can be
reconstructed back to the original lamellar structure upon
contact with water, due to the memory effect.4,6 During the
´
other hand, Abello et al. found that the poisoning of the active
Brønsted basic centers in the rehydrated hydrotalcite by CO2
was very fast (50% activity loss after 1 h exposure to ambient
conditions).17 This means that the rehydrated hydrotalcite,
which was prepared through a complex and time-consuming
process, could sharply lose its activity when exposure to
ambient conditions. Therefore, extreme care was taken to
avoid contact of the rehydrated hydrotalcite with air using an
inert gas atmosphere in all operations during preparation,
aKey Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal
University), Ministry of Education, Xi’an 710062, PR China
bSchool of Chemistry and Chemical Engineering, Shaanxi Normal University,
Chang’an South Road 199, Xi’an 710062, PR China. E-mail: xuchunli@snnu.edu.cn;
Fax: +86-29-85307774; Tel: +86-29-85300970
Electronic Supplementary Information (ESI) available. See DOI: 10.1039/
3
c2ra21762g
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processing, storage, and use of the solid.11–17 The disadvan-
tage in terms of the rehydration method may limit the
implementation of the rehydrated hydrotalcite catalyst in
organic reactions.17
heavy slurry, and the mixture was aged at 60 uC for 18 h with
stirring, to enhance the selective formation of the precipitated
hydrotalcite phase. The slurry was then cooled to 25 uC,
filtered, and washed with water until the pH value of the
filtrate was near 7. The precipitate was dried at 90 uC for 16 h.
The resulting materials were hydrotalcites. The hydrotalcites
were labeled as HT-X, where X corresponds to the Mg/Al molar
ratio. For example, HT-2 corresponds to hydrotalcite with Mg/
Al molar ratio of 2.
The development of more efficient, safer, and environmen-
tally friendly chemical technologies is a major requirement of
humanity. The aim of this work was to seek one method which
could avoid the disadvantage of rehydrated hydrotacite result-
ing from the conventional preparation method. In this work,
Mg(Al)O, originating from hydrotalcite, was directly added into
the aqueous-phase reaction system of an aldol condensation
without the protection of an inert gas (Scheme 1). The structure
of the catalyst before and after reaction was determined. The
results showed that the reconstructed hydrotalcite was in-situ
generated under the reaction conditions. The aldol condensa-
tion of benzaldehyde with acetone was carried out to study the
catalytic activity of the in-situ rehydrated hydrotalcite. On the
basis of the structure and activity of the catalyst, we explored the
possibility of in-situ reconstructing hydrotalcite in the presence
of a reaction solution. The experimental results demonstrated
that the reconstructed hydrotalcite, formed by in situ rehydra-
tion, exhibited a higher catalytic activity than the reconstructed
hydrotalcite formed by the conventional preparation method.
Furthermore, the in-situ rehydration method for preparing
reconstructed hydrotalcite was simple and time-saving, and the
obtained solid base catalyst was water tolerant.
Preparation of mixed oxides and single oxide
The Mg–Al hydrotalcite as-prepared, was calcined at 500 uC for
5 h in static air to generate Mg(Al)O. The generated Mg(Al)O
samples were denoted Mg(Al)O-X, where X corresponds to the
Mg/Al molar ratio.
A reference MgO catalyst was prepared by the thermal
decomposition of Mg(OH)2 using the same decomposition
procedure as Mg(Al)O samples. The Mg(OH)2 precursors were
obtained by precipitation using aqueous NaOH from the
corresponding aqueous nitrate salt solutions. The c-Al2O3 used
was a commercial sample (Alfa Aesar, SBET = 220 m2 g21). It
was activated at 500 uC for 5 h in static air before the reaction
test.
Preparation of rehydrated hydrotalcite by the conventional
method
The structure of rehydrated hydrotalcite was affected by the
rehydration conditions, such as the rehydration reagent,
stirring speed, washing reagent.2,11,19–21 To compare the
activities of the rehydrated hydrotalcite catalyst prepared by
the in-situ rehydration method with that by the conventional
method, typical conventional methods of preparing rehydrated
hydrotalcite were employed according to the procedure
described in the literature.2,11,19–21 In this work, four rehydra-
tion methods were used. Their difference is in the property of
the rehydration reagent and washing reagent, details are
shown below. The Mg–Al hydrotalcite was calcined at 500 uC
for 5 h in static air to form the corresponding Mg(Al)O. The
Mg(Al)O was immersed in decarbonated water at room
temperature for 1 h. After the rehydration process, the sample
was filtered, and dried at 60 uC under N2 to yield rehydrated
hydrotalcite by the first method. The obtained rehydrated
hydrotalcite was denoted HT-R. The second method and the
third method were similar to that of the first method, with a
difference in rehydration reagent. The rehydration reagent of
the second method was ethanol solution (v/v, 1:1), while the
rehydration reagent of the third method was acetone solution
(v/v, 1 : 1). The rehydrated hydrotalcite by the second and
third method were denoted HT-RE and HT-RA, respectively.
These three methods did not contain a washing step, while the
fourth method was subjected to an extra washing step using
acetone. The procedure of the fourth method was similar to
that of the third method. The rehydrated hydrotalcite by the
fourth method was denoted HT-RAA.
Experimental section
Preparation of Mg–Al hydrotalcites
Mg–Al hydrotalcite with Mg2+/Al3+atomic ratios of 2, 3 and 4
were prepared using a standard aqueous co-precipitation
method at constant pH and temperature.18 An aqueous
solution (166 ml) of the metal nitrates in the desired Mg2+
/
Al3+ molar ratio, with a total concentration of 1.5 M, was mixed
slowly with an alkaline solution of Na2CO3/NaOH with
continuous stirring. The molar quantity of Na2CO3 employed
was twice that of Al3+. The pH value of the mixture was kept
constant, typically at values between 9 and 10, by adjusting the
rate of addition of the alkaline solution. The temperature was
maintained at 25 uC, which resulted in the formation of a
Catalyst characterization
X-ray diffraction patterns were recorded using a D/Max-3C
X-ray powder diffractometer (Rigaku Co., Japan), using a Cu Ka
source fitted with an Inel CPS 120 hemispherical detector. The
Scheme 1 The aldol condensation of benzaldehyde with acetone catalyzed by
in-situ redydrated hydrotalcite.
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elemental compositions of the catalysts were determined
using energy dispersive X-ray spectroscopy (EDS). The results
hydrotalcite samples. The surface area of HT-R was 23 m2 g21
(Table 2, entry 1), which was lower than other rehydrated
hydrotalcites. The surface area of HT-RE (Table 2, entry 2) and
HT-RA (Table 2, entry 3) was circa 80 m2 g21, which was similar
to that of in-situ rehydrated HT-3 (Table 2, entry 5). HT-RAA
of the elemental analyses are shown in Table S1 (ESI ).
3
Infrared transmission spectra were recorded in the range of
400–4000 cm21 by a FTIR spectrometer (Bruker Tensor 27,
Germany). The KBr pellet technique was applied for determin-
ing the IR spectra of the samples.
(Table 2, entry 4) had the largest surface area (130 m2 21).
g
Besides, the pore diameter of the samples were in the range of
10–42 nm, suggesting that all the samples are mesoporous
materials irrespective of the nature of the sample (i.e., whether
it was hydrotalcite or oxide).
The surface area and pore characteristics of the catalysts
were determined using a Micromeritics ASAP 2020 instrument.
The sample was degassed at different temperatures (100 uC for
hydrotalcite, 250 uC for oxide) for 4 h in N2 prior to surface
area measurements. Nitrogen adsorption and desorption
isotherms were measured at 2196 uC, and the specific surface
areas of the catalysts were determined by applying the BET
(Brunauer–Emmett–Teller) method to the nitrogen adsorption
data obtained in the relative pressure range from 0.06 to 0.30.
Total pore volumes were estimated from the amount of
nitrogen adsorbed at a relative pressure of 0.995. Pore volume
and pore size distributions were obtained from analysis of the
desorption branches of the nitrogen isotherms using the BJH
(Barrett–Joyner–Halenda) method.
Effect of water and acetone amount
In the tested system, water may work as both the rehydrating
agent to in-situ reconstruct hydrotalcite and the solvent of the
aldol condensation between benzaldehyde and acetone;
acetone was both solvent and reactant. Therefore, their
amount may affect the in-situ formation and activity of
rehydrated hydrotalcite. For the three variables (amount of
water, acetone as well as benzaldehyde), we studied the effect
of one variable by keeping the ratio of the other two variables
constant. The weights of Mg(Al)O and benzaldehyde were kept
constant during the whole process.
Reaction procedure
Effect of water amount when the ratio of B/A was constant
The reaction was carried out in a round bottom glass flask
equipped with a vertical condenser under vigorous stirring.
Typical reactions were performed with 0.02 mol of benzalde-
hyde, 0.1 mol of acetone and 0.1 mol of de-ionized and de-
carbonated water [mole ratio of benzaldehyde/acetone/water
(hereafter denoted B/A/W) was 1 : 5 : 5] using 0.4245 g of
Mg(Al)O catalyst at 65 uC for the specified time (3 h). The
reaction products were analyzed by gas chromatography
(Shimadzu GC-2014) using a flame ionization (FID) detector
and an Rtx-wax column (30 m 6 0.32 mm 6 0.5 um). Decane
was used as the internal standard. The reaction products were
characterized by GC-MS (Shimadzu GCMS-QP2010UItra).
The effect of the water amount was studied when the ratio of
B/A was set constantly as 1 : 5 (Fig. 1). The results showed that
benzaldehyde conversion and benzalacetone yield increased
with water amount and reached a maximum when the ratio of
B/A/W was 1 : 5 : 5. Above a 1 : 5 : 5 ratio of B/A/W, the
conversion and yield remained constant at the maximum and
did not change with further increasing the amount of water.
This suggests that an amount of water that was equal to or
greater that of acetone, was favored to obtain a maximum yield
when the ratio of B/A was 1 : 5.
Effect of acetone amount when the ratio of B/W was constant
The effect of the amount of acetone was studied when the ratio
of B/W was set constantly as 1 : 5 (Fig. 2). As shown in Fig. 2,
the maximum conversion and yield was obtained when the
amount of acetone and water was equal. When the amount of
acetone was lower than that of water, the conversion was high
(.90%) and the yield was higher than 40%. However, both the
conversion and yield decreased sharply when the amount of
acetone was higher than that of water.
Results and discussion
BET surface area and pore size
Table 1 shows the nitrogen physisorption data of hydrotalcite
and Mg(Al)O. The surface area of hydrotalcite was in the range
of 35–78 m2 g21 (Table 1, entries 1–3). The Mg(Al)O, originaing
from hydrotalcite, showed a higher surface area, which was in
the range of 168–215 m2 g21 (Table 1, entries 4–6). Table 2
shows the nitrogen physisorption data of the rehydrated
Table 1 Nitrogen physisorption data of hydrotalcite as-synthesized and Mg(Al)O
Entry
Sample
Surface areaa (m2 g21
)
Pore diameterb (nm)
Pore volumeb (cm3 g21
)
1
2
3
4
5
6
HT-2
HT-3
HT-4
Mg(Al)O-2
Mg(Al)O-3
Mg(Al)O-4
78
67
35
199
168
215
28
19
15
27
13
10
0.66
0.47
0.22
0.87
0.64
0.53
a
b
Calculated by the BET method. Calculated by the BJH method from the desorption isotherm.
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Table 2 Nitrogen physisorption data of rehydrated hydrotalcite samples and their catalytic activities
Entry
1
2
3
4
5
Rehydrated HT
Rehydration conditions Rehydration H2O
reagent
HT-R HT-RE
HT-RA
HT-RAA
In-situ rehydrated HT-3
H2O + ethanol (1 : 1) H2O + acetone (1 : 1) H2O + acetone (1 : 1) In-situ rehydration
Washing
reagent
No
23
14
No
75
9
No
80
10
Acetone
130
Surface areaa
82
42
(m2 g21
)
Pore diameterb (nm)
Pore volumeb
12
0.52
0.18
0.38
0.24
0.14
(cm3 g21
)
Conversion of
benzaldehyde (%)
Yield of
benzalacetone (%)
Selectivity to
28
12
43
32
16
50
30
13
43
25
11
44
97
77
81
benzalacetone (%)
a
b
Calculated by the BET method. Calculated by the BJH method from the desorption isotherm.
1 : 5 : 5 B/A/W ratio, the benzalacetone yield declined.
Therefore, the optimum ratio of B/A/W was 1 : 5 : 5.
The low benzalacetone yield under the ratio of 1 : 1 : 1 may
be attributed to the formation of the by-product dibenzalace-
tone. For a ratio of 1 : 1 : 1, the amount of acetone was low. A
low amount of acetone may increase the formation of
dibenzalacetone, formed by further reaction of benzaldehyde
with benzalacetone.10,22The formation of dibenzalacetone
decreased the yield of benzalacetone.
Cooperative effect of water and acetone amount when the ratio
of acetone to water was constant
Since Fig. 1 and Fig. 2 indicate that when the amount of water
is equal to or greater than the amount of acetone both high
conversion and yield can be achieved, the cooperative effect of
the amount of water and acetone was studied under the
conditions of 1 : 1 A/W molar ratio. As shown in Fig. 3, the
conversion of benzaldehyde could reach nearly 100% when the
ratio of B/A/W was as low as 1 : 1 : 1. It was maintained near
100% as the ratio increased to 1 : 5 : 5. Above a 1 : 5 : 5 B/A/W
ratio, the conversion of benzaldehyde declined. It decreased to
70% when the ratio of B/A/W was 1 : 15 : 15. The variation of
benzalacetone yield with respect to the B/A/W ratio was
different to that of benzaldehyde conversion. The benzalace-
tone yield was 42% when the ratio of B/A/W was 1 : 1 : 1 and
went to a maximum as the ratio of B/A/W was 1 : 5 : 5. Above a
XRD patterns of in-situ rehydrated HT-3
Fig. 4 presents the X-ray diffraction patterns of as-synthesized
HT-3, Mg(Al)O-3, and in-situ rehydrated HT-3. The as-synthe-
sized HT-3 sample presented a pure hydrotalcite phase,
characterized by diffraction peaks at 11.4u, 23.0u and 34.9u
(Fig. 4a). Calcination of hydrotalcite at 500 uC gave a uniform
Fig. 1 Effect of water amount on the activity of rehydrated hydrotalcite.
Reaction conditions: benzaldehyde (0.02 mol), ratio of benzaldehyde to acetone
(1 : 5), reaction temperature (65 uC), Mg(Al)O-3 catalyst amount (0.4245 g),
reaction time (3 h).
Fig. 2 Effect of acetone amount on activity of rehydrated hydrotalcite. Reaction
conditions: benzaldehyde (0.02 mol), ratio of benzaldehyde/water (1 : 5),
reaction temperature (65 uC), Mg(Al)O-3 catalyst amount (0.4245 g), reaction
time (3 h).
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A/W ratio, where the amount of water was lower and far less
than that of acetone, the in-situ rehydrated sample still kept
the structure of Mg(Al)O and did not show the typical X-ray
diffractograms of hydrotalcite (Fig. 4f). However, under a B/A/
W ratio of 1 : 15 : 5, where the absolute amount of water was
high and far less than that of acetone, the in-situ rehydrated
sample showed the typical X-ray diffractograms of hydrotalcite
(Fig. 4g). This indicates that, even if the relative concentration
ratio of water/acetone was less than 1, HT-3 could be in-situ
reconstructed if the absolute amount of water was enough.
Combining the results of activity and structure, it was found
that the amount of water, equal to or greater than acetone,
favors the maximum yield and the formation of in-situ
reconstructed hydrotalcite. Whereas when the amount of
water was less than acetone, the maximum yield and the
formation of in-situ reconstructed hydrotalcite was disfavored.
Furthermore, when the water amount was less than acetone, it
showed low activity even if hydrotalcite was successfully in-situ
reconstructed. This indicated that the amount of water
affected both the formation of rehydrated hydrotalcite and
the activity of the in-situ reconstructed hydrotalcite.
Fig. 3 The cooperative effect of the amount of water and acetone when the
ratio of acetone to water was constantly 1 : 1. Reaction conditions: benzalde-
hyde (0.02 mol), reaction temperature (65 uC), Mg(Al)O-3 catalyst amount
(0.4245 g), reaction time (3 h).
XRD patterns of in-situ rehydrated HT-2 and HT-4
mixed oxide phase Mg(Al)O, with diffraction lines very similar
to that of MgO (Fig. 4b).2,7 Under B/A/W ratios of 1 : 5 : 5
(Fig. 4c) and 1 : 1 : 1 (Fig. 4d), where the amount of water was
equal to that of acetone, the in-situ rehydrated sample showed
the typical X-ray diffractograms of hydrotalcite. Furthermore,
under a B/A/W ratio of 1 : 1 : 5, where the amount of water was
more than that of acetone, the in-situ rehydrated sample
showed the typical X-ray diffractograms of hydrotalcite
(Fig. 4e). The results demonstrated that HT-3 could be
successfully in-situ reconstructed if the amount of water was
equal to or greater than the amount of acetone.
The Mg/Al ratio of hydrotalcite may affect its in-situ rehydra-
tion, so the XRD patterns of in-situ rehydrated hydrotalcite
with different of Mg/Al ratios were tested. Fig. 5 presents the
X-ray diffraction patterns of in-situ rehydrated HT-2 and HT-4
under a 1 : 5 : 5 B/A/W ratio. Both the in-situ rehydrated HT-2
and HT-4 showed the typical X-ray diffractograms of hydro-
talcite, which showed that HT-2 and HT-4 could also be
successfully in-situ reconstructed under a B/A/W ratio of
1 : 5 : 5.
XRD patterns of reference samples
In contrast, the structure of the in-situ rehydrated sample
was affected by the absolute amount of water when the relative
concentration ratio of W/A was less than 1. Under a 1 : 5 : 1 B/
Fig. 6 shows the X-ray diffraction patterns of the reference
samples (MgO, c-Al2O3, and a physical mixture of MgO and
c-Al2O3 before and after reaction. The c-Al2O3 before and after
Fig. 5 X-ray diffraction patterns of (a) as-synthesized HT-2; (b) as-synthesized
HT-4; (c) Mg(Al)O-2; (d) Mg(Al)O-4; (e) in-situ rehydrated HT-2; (f) in-situ
rehydrated HT-4.
Fig. 4 X-ray diffraction patterns of (a) as-synthesizedHT-3; (b) Mg(Al)O-3; and
(c–g) the in-situ rehydrated samples under different ratios of benzaldehyde/
acetone/water: (c) 1 : 5 : 5, (d) 1 : 1 : 1, (e) 1 : 1 : 5, (f) 1 : 5 : 1, (g) 1 : 15 : 5.
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Fig. 6 X-ray diffraction patterns of the reference samples before and after
reaction. (a) c-Al2O3 before reaction; (b) MgO before reaction; (c) a physical
mixture of MgO and c-Al2O3 (before reaction); (d) c-Al2O3 after reaction; (e)
MgO after reaction; (f) a physical mixture of MgO and c-Al2O3 (after reaction).
Reaction conditions: benzaldehyde (0.02 mol), molar ratio of benzaldehyde/
acetone/water (1 : 5 : 5), reaction temperature (65 uC), Mg(Al)O-3 catalyst
amount (0.4245 g), reaction time (3 h).
Fig. 7 X-ray diffraction patterns of rehydrated samples by conventional
methods (a) HT-R, (b) HT-R after reaction, (c) HT-RAA, (d) HT-RE, (e) HT-RA.
the catalytic activity of the in-situ rehydrated hydrotalcites. The
difference in the activity of in-situ rehydrated HT-2, HT-3 and
HT-4 may be ascribed to the effect of the Mg/Al ratio on the
process of their rehydration.23
However other workers have reported much smaller varia-
tions when using conventionally rehydrated hydrotalcites.6
The reason for this discrepancy is discussed as follows. For the
conventionally rehydrated methods, the catalyst was comple-
tely rehydrated before it was used for the aldol condensation.6
Therefore, the rehydration rate of hydrotalcite did not affect its
activity. However, for the in-situ rehydration method, the
process of rehydration and the reaction of the aldol condensa-
tion happened almost simultaneously. Therefore, the in-situ
rehydration rate of hydrotalcite affected its activity. It was
reported that the rehydration rate of hydrotalcite was affected
by the Mg/Al ratio.23 Therefore, the Mg/Al ratio in in-situ
reaction displayed the typical X-ray diffractograms of c-Al2O3,
which indicated that c-Al2O3 did not in-situ rehydrate to form
the rehydration product. MgO before reaction showed diffrac-
tion peaks ascribed to MgO, whereas MgO after reaction
showed X-ray diffractograms typical of Mg(OH)2. This indi-
cates that MgO was in-situ rehydrated to form Mg(OH)2 in the
aqueous-phase reaction system of the aldol condensation. For
the physical mixture of MgO and c-Al2O3, a similar phenom-
enon was observed. Before reaction, the physical mixture of
MgO and c-Al2O3 showed the diffraction peaks ascribed to
MgO and c-Al2O3, after reaction, it showed, in addition to
c-Al2O3, a series of new diffraction peaks ascribed to Mg(OH)2.
Fig. 7 shows the X-ray diffraction patterns of the rehydrated
hydrotalcite prepared by the conventional method. All the
rehydrated samples showed the typical X-ray diffractograms of
hydrotalcite with a difference in the intensity of diffraction.
HT-R showed the highest intensity of diffraction, while HT-
RAA showed the lowest intensity. HT-R after reaction was also
determined. It showed a similar diffraction pattern with the
fresh one.
Effect of Mg/Al ratio on the catalytic activity of in-situ
rehydrated hydrotalcite
The activity of in-situ rehydrated HT-2, HT-3 and HT-4 was
tested under the same reaction conditions. The molar ratio of
B/A/W was set as 1 : 5 : 5, where all three samples could be
successfully in-situ rehydrated (Fig. 4 and Fig. 5). As shown in
Table S2 (ESI ), the three rehydrated samples displayed
3
different activities. In-situ rehydrated HT-3 displayed the
highest activity, with 97% benzaldehyde conversion. Next
was in-situ rehydrated HT-2, which showed the middle activity
(benzaldehyde conversion, 56%). In-situ rehydrated HT-4
presented the lowest activity, in which the benzaldehyde
conversion was 15%. This showed that the Mg/Al ratio affected
Fig. 8 The IR spectra of (a) benzalacetone ;(b) HT-R (before reaction); (c) in-situ
rehydrated samples under a 1 : 5 : 5 B/A/W ratio; (d) in-situ rehydrated samples
under a 1 : 1 : 1 B/A/W ratio.
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rehydrated hydrotalcites had a marked effect on their catalytic
activity.
show apparent catalytic activity, with a benzaldehyde conver-
sion ,2% (Table 3, entries 3 and 4). Besides, MgO and c-Al2O3,
as well as a physical mixture of MgO and c-Al2O3, were inactive
(Table 3, entries 5–10). Mg(Al)O also displayed low activity
(benzaldehyde conversion, 7%), in which no benzalacetone
was detected (Table 3, entry 11). Rehydrated HT-3 prepared by
the conventional method showed middle activity, with a
benzaldehyde conversion of 25%–32% and a benzalacetone
yield of 11%–16% (Table 2, entries 1–4). However, high
benzaldehyde conversion (97%) and benzalacetone yield
(77%) were obtained when in-situ rehydrated HT-3 was used
as the catalyst (Table 2, entry 5). This indicates that in-situ
rehydrated hydrotalcite could work as an active catalyst for the
aldol condensation.
MgO could in-situ rehydrate to form Mg(OH)2 (Fig. 6). The
catalytic activity of the in-situ rehydrated Mg(OH)2 was very low
(Table 3, entry 5), suggesting that the hydroxyl groups
coordinated to Mg in the brucite layers of in-situ rehydrate
Mg(OH)2 have negligible catalytic activity. The as-synthesized
(mostly carbonate and hydroxyl anions, Brønsted basic sites),
activated [Mg(Al)O, Lewis basic sites] and conventional
reconstructed (mostly hydroxyl anions, Brønsted basic sites)
hydrotalcite samples did not work well. At the start of the
reaction, the material Mg(Al)O (activated hydrotalcite) has
mostly Lewis basic sites. On introducing water to the reaction
mixture, the Lewis basic sites convert to Brønsted basic sites.
So, the activity of the material may be due to either the in-situ
reconstruction (charge-balancing hydroxyl anions, the
Brønsted basic sites) or due to the combined effect of Lewis
and Brønsted basic sites.6 The active sites of in-situ rehydrated
hydrotalcite will be investigated in future work.
FTIR analysis
The infrared spectrum of catalyst after reaction was deter-
mined. Fig. 8 shows the IR of benzalacetone, HT-R (before
reaction) and in-situ rehydrated samples (after reaction). The
IR spectra of the sample benzalacetone and HT-R indicated
that they were typical benzalacetone and pure hydrotalcites,
respectively. The infrared spectra of in-situ rehydrated samples
(after reaction), in addition to two bands at 1604 cm21 and
1558 cm21, were similar with that of HT-R (before reaction).
The bands at 1604 cm21 and 1558 cm21 were due to the
vibration of CLC of benzene ring, indicating the existence of
benzalacetone on the surface of used catalyst. Furthermore,
the result also showed that intensity of those two bands for B/
A/W ratio of 1 : 1 : 1 was greater than that of 1 : 5 : 5. This
indicated that the amount of benzalacetone adsorbed in B/A/
W ratio of 1 : 1 : 1 was higher that in ratio of 1 : 5 : 5.
The XRD graphics could confirm that the materials after the
reaction were only deposited on the surface, not in the
interlayer space. If the materials after the reaction were in the
interlayer space, the interlayer space of HT would increase. If
the interlayer space of HT was increased, the original (003) and
(006) reflections would move to a lower angle of diffraction.24
However, as shown in Fig. 4, the XRD pattern of in-situ HT was
similar to that of as-synthesized HT. This result showed that
the interlayer space of in-situ HT was unchanged, and
materials after reaction were deposited on the surface, not in
the interlayer space.
Catalytic activity of in-situ rehydrated HT-3
It was reported that the OH2 species (active sites), weakly
adsorbed on the surface of rehydrated hydrotalcite, were easily
removed as H2O by the evacuation.25 The in-situ rehydration
method avoided the process of evacuation, whereas the
conventional rehydration method did go through the process.
Therefore, the amount of active sites on in-situ rehydrated HT-
3 may be higher than that of rehydrated HT-3 prepared by the
conventional method. The higher number of active sites on in-
situ rehydrated HT-3 accounted for its high activity.
The aldol condensation of benzaldehyde with acetone has
been carried out to study the catalytic activity of in-situ
rehydrated HT-3. The main product obtained is benzalacetone,
formed by dehydration of 4-hydroxy-4-phenyl butan-2-one. The
catalytic activity of in-situ rehydrated HT-3 was compared with
that of as-synthesized HT-3, rehydrated HT-3 by the conven-
tional method, and oxide (Table 2 and 3). In the absence of
catalyst, no conversion of benzaldehyde was observed irre-
spective of the presence of H2O (Table 3, entries 1 and 2). The
as-synthesized HT-3 sample, with or without water, did not
For the conventional method, the structure and activity of
reconstructed hydrotalcite was strongly affected by the CO2 in
Table 3 Catalytic activity of reference samplesa
Entry
Catalystb
Conversion of benzaldehyde (%)
Yield to benzalacetone (%)
Selectivity to benzalacetone (%)
1
2
3
4
5
6
7
Blank
H2O
0
0
2
1
13
8
0
0
0
0
1
0
0
0
0
0
0
8
0
0
As-synthesized HT-3
H2O + as-synthesized HT-3
MgO
MgO + H2O
c-Al2O3
7
8
9
10
11
12
c-Al2O3 + H2O
MgO + c-Al2O3
MgO + c-Al2O3 + H2O
Mg(Al)O-3
2
0
1
0
0
0
8
0
0
12
10
7
H2O + HT-R
25
10
40
a
b
Reaction conditions as described in the Experimental section. With or without water.
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Table 4 Recycling experiments for the in-situ rehydated HT-3 catalysta
Entry
Catalytic cycles
Conversion of benzaldehyde (%)
Yield to benzalacetone (%)
Selectivity to benzalacetone (%)
1
2
3
4
1st use
97
43
4
77
2
1
81
4
18
0
2nd usea
3rd usea
2nd useb
20
0
a
The spent in-situ rehydated HT-3 catalyst was separated from the reaction solution by centrifugation. It was taken as the catalyst for the
b
repeated reactions without further treatment. The repeated reactions were run under the same reaction conditions to the last reaction. The
spent in-situ rehydated HT-3 catalyst was washed with acetone, then activated at 500 uC for 5 h.
the air. Therefore, extreme care was taken to avoid contact of
the rehydrated hydrotalcite with air using an inert gas
atmosphere in all operations during preparation, processing,
and storage.11–15 However, an inert gas atmosphere was not
used during the entire in-situ rehydration process. There may
be very strong chances to intercalate carbonate ions instead of
hydroxyl ions in the interlayer space of in-situ reconstructed
hydrotalcite. The high activity of the in-situ reconstructed
hydrotalcite indicated that the in-situ reconstructed sample
was not affected by the air.
Compared with the conventional rehydration method, the in-
situ rehydration method was simple and time-saving. In the
case of the in-situ rehydration method, the formation of
rehydrated hydrotalcite and the catalytic reaction were simulta-
neously developed. Furthermore, an inert gas atmosphere was
not used at all during the entire process. In the case of the
conventional rehydration method, the rehydrated hydrotacite
was prepared before the reaction. The preparation of rehydrated
hydrotacite went through the process of contacting mixed oxide
with water, filtration, washing, and drying. Furthermore,
extreme care was taken to avoid contact of the rehydrated
hydrotalcite with air using an inert gas atmosphere in all
operations during preparation, processing, and storage.11–15
reusability of in-situ rehydrated hydrotalcite is supposed to be
an area that can be the subject of further investigation.
Conclusions
In this work, water and Mg(Al)O were directly added to the
mixed solution of benzaldehyde and acetone without the
protection of an inert gas. Water worked as both the
rehydrating agent to in-situ reconstruct hydrotalcite, and as
the solvent of the aldol condensation between benzaldehyde
and acetone. The amount of water affected both the formation
of the rehydrated hydrotalcite and the activity of the in-situ
reconstructed hydrotalcite. It was found that the Mg/Al ratio
affected the catalytic activity of the in-situ rehydrated hydro-
talcite. Under optimum conditions, 0.02 mol of benzaldehyde,
0.1 mol of acetone and 0.1 mol of de-ionized and de-
carbonated water (mole ratio of B/A/W was 1 : 5 : 5) using
0.4245 g of Mg(Al)O catalyst at 65 uC for the specified time (3
h), Mg(Al)O was successfully in-situ rehydrated to form
meixnerite. Furthermore, the in-situ rehydrated hydrotalcite
showed higher activity (benzaldehyde conversion 97%, and
benzalacetone yield 77%) than that of rehydrated hydrotalcite
prepared by the conventional method (benzaldehyde conver-
sion 40%, and benzalacetone yield 21%). The in-situ rehy-
drated hydrotalcite also had a higher tolerance to water than
the rehydrated hydrotalcite prepared by the conventional
method. This in-situ rehydration method was simple and
time-saving, which avoided the shortcomings of the conven-
tional rehydration method.
Reutilization of in-situ rehydrated hydrotalcite
Attempts to reuse the in-situ rehydrated hydrotalcite catalyst
have as yet not been successful (Table 4), just as the reusability
of rehydrated hydrotalcite prepared by the conventional
method.17 The benzaldehyde conversion over the catalyst after
one run was 43% and the catalyst after two consecutive runs
showed extremely low activity (benzaldehyde conversion, 4%).
As shown in the previous section, the spent catalyst was
deposited with reaction products (Fig. 8 and Fig. S1 ). The
3
deactivation of the in-situ rehydrated catalyst may be due to
the adsorption of reaction products during the reaction. These
deposits likely modify the nature of the few active OH2 centers
left and/or its accessibility by reactants in subsequent
reactions.17 In order to decrease the effect of deposits, the
spent solid was rinsed with acetone at room temperature, then
activated at 500 uC for 5 h to convert washed material into
Mg(Al)O and burn the deposits. The regenerated catalyst
displayed an off-white color, not the white color of the fresh
catalyst. Its off-white color may result from the deposited
carbon, which was generated by the burning of the reaction
products. The regenerated catalyst showed low activity,
suggesting that the treatment with acetone and calcination
did not increase the catalytic activity (Table 4, entry 4). The
Acknowledgements
This work was supported by Natural Science Basic Research
Plan in Shaanxi Province of China (Program No. 2010JM2003)
and the Fundamental Research Funds for the Central
Universities,
State
Education
Ministry
(Program
No.GK200902006).
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