Nanoplatelet-based reconstructed hydrotalcites: Towards more efficient solid base catalysts in aldol condensations

Article abstract of DOI:10.1039/b417322h

Rehydration of Mg-Al hydrotalcite in the liquid phase using ultrasounds or a high stirring speed leads to nanoplatelets with surface areas of 400 m ² g^-1, displaying catalytic activities in aldol condensations up to 8 times higher than the best catalytic system reported in the literature. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b417322h

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Nanoplatelet-based reconstructed hydrotalcites: towards more efficient
solid base catalysts in aldol condensations{
S. Abello´,ab F. Medina,*a D. Tichit,b J. Pe´rez-Ram´ırez,c Y. Cesteros,a P. Salagrea and J. E. Sueirasa
Received (in Cambridge, UK) 16th November 2004, Accepted 5th January 2005
First published as an Advance Article on the web 25th January 2005
DOI: 10.1039/b417322h
hydrotalcites leads to outstanding activities, up to 8 times higher
than the best catalytic system reported to date.
Rehydration of Mg–Al hydrotalcite in the liquid phase using
ultrasounds or a high stirring speed leads to nanoplatelets with
surface areas of 400 m2 g21, displaying catalytic activities in
aldol condensations up to 8 times higher than the best catalytic
system reported in the literature.
As described below{ and shown in Table 1, the synthesised Mg–
Al hydrotalcite (Mg/Al 5 3/1) was calcined and rehydrated in gas
(HTRG) or liquid phases (HTRL). Liquid-phase rehydration was
carried out under stirring in water at different conditions or in 1 : 1
mixtures of water and ethanol (HTRL-ET). Besides, ultrasound
irradiation was applied as an alternative to stirring, leading to
HTRL-US. Table 1 compares the BET surface areas of the
differently rehydrated samples. Rehydration in liquid phase at
100 rpm leads to a surface area of 203 m2 g21, remarkably higher
than in the gas-phase rehydrated sample (15 m2 g21). The surface
area of HTRL is practically doubled upon increasing the stirring
speed from 100 to 700 rpm, reaching 374 m2 g21. A slight increase
in surface area was observed when the rehydration time was
increased from 1 to 6 h or using ethanol, reaching surface areas
Activated Mg–Al hydrotalcites (HTs) are efficient heterogeneous
catalysts in a number of base-catalysed reactions, among others
the self and cross-aldol condensation of aldehydes and ketones,
Knoevenagel and Claisen-Schmidt condensation, and Michael
addition.1 These processes are important in the pharmaceutical
and fragrance industries where HTs are progressively replacing the
classically applied and less environmental friendly liquid bases such
as NaOH.2 The activity of these layered materials originates from
the presence of basic hydroxyl ions of Brønsted character in the
interlayer space.3 The as-synthesised Mg–Al hydrotalcite is
typically activated by thermal decomposition into a high surface
area mixed oxide and then reconstructed by rehydration in contact
with water vapour or immersed in liquid water under decarbo-
nated atmosphere. This ultimately converts the starting hydro-
xycarbonate (Mg6Al2CO3(OH)16?4H2O) into the active topotactic
hydroxide hydrate (Mg6Al2(OH)18?4.5H2O).
around 400 m2 g21
.
The FE-SEM micrographs of two representative rehydrated
samples are shown in Fig. 1. HTRG is built of thicker aggregates,
while HTRL700-A presents a more exfoliated morphology. The
degree of exfoliation of the HTRL samples increases with the
stirring speed or conducting longer treatments. Sonication causes
cavitation in aqueous media, where the formation, growth, and
collapse of microbubbles occurs.6 So, the use of ultrasounds
should stimulate exfoliation of hydrotalcite particles during
rehydration, as confirmed by the surface area of HTRL-US in
Table 1 (440 m2 g21).
The rehydration conditions of calcined HTs strongly influence
the properties and performance of the final catalyst in aldol
condensations. Roelofs et al.4 showed that rehydration in vapour
phase leads to a lower crystallinity and surface area than
rehydration by immersion in liquid water (57 vs. 200 m2 g21).
Figueras et al.5 observed a similar effect, and the surface area of
the mixed oxide decreased to 20 m2 g21 after rehydration in
vapour phase. This originated a decreased activity in isophorone
isomerisation, which was attributed to the lower accessibility of the
basic sites in meixnerite. Despite numerous studies, conclusive
relationships between the rehydration procedure and (i) the surface
area, (ii) the number and nature of the active OH-groups, and (iii)
the activity of the activated HTs in aldol condensations have not
been derived. This fundamental understanding is essential for
obtaining more efficient catalysts.
Strikingly, the surface areas of the reconstructed HTRL samples
are significantly higher than that of HTC, and to the best of our
knowledge, the highest reported in the literature for rehydrated
HT.
The basic properties of the rehydrated samples were studied by
temperature-programmed desorption (TPD) using different probe
molecules, including CO2, CH3CN, and CH3NO2.{
The CO2 TPD profiles for HTRG and HTRL500 samples (see
Fig. 2) are very similar (see supplementary information for other
Herein, we have identified relevant activity-directing factors
associated with the rehydration method of Mg–Al hydrotalcite in
the condensation between citral and acetone or MEK, to yield
pseudoionones (PS). Novel approaches to maximise the accessi-
bility of active sites by synthesis of nanoplatelet-based activated
Table 1 BET surface area of differently rehydrated hydrotalcites
Catalyst
Time/h
Stirring speed/rpm
SBET/m2 g21
HTC
HTRG
—
15
—
—
210
15
HTRL100
HTRL500
HTRL700-A
HTRL700-B
HTRL-ET
HTRL-US
1
1
1
6
100
500
700
700
700
—
203
270
374
401
404
440
{ Electronic Supplementary Information (ESI) available: XRD and TPD
of CH3CN and CH3NO2 for rehydrated Mg–Al mixed oxides. See http://
www.rsc.org/suppdata/cc/b4/b417322h/
1
0.083
*francesc.medina@urv.net
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The latter observation already evidences that a large fraction of
OH2 groups in the HTs are unreachable by CO2, due to
diffusion limitations in the interlayer space. Obviously, the
results with CO2 should be dramatically aggravated in the
condensation of ketones, which are considerably bulkier.
Accordingly, a decisive factor to maximise the catalytic perfor-
mance of reconstructed HT in aldol condensations should be
mainly directed towards securing an improved accessibility of
the basic groups to the reactants. Then, the formation of smaller
platelets in the liquid-phase rehydrated samples using ultrasounds
or vigorous mechanical stirring, increases the number of
external OH2 groups near the edges of the layers, which are
obviously the most accessible. Additionally, the crystallite size
(expressed as average thickness) of the different rehydrated
samples has been calculated from the integral breadth (b) of the
(001) reflection in the XRD patterns, according to the
Scherrer equation. The results are summarised in Table 2 where
it can be seen that vigorous stirring and ultrasounds produce the
exfoliation of the sample, obtaining small crystallites. In
concordance with this reasoning, a linear correlation has been
obtained between the surface area and the initial reaction rate in
the aldol reactions between citral–acetone or citral–MEK (Fig. 3).
The lower activity of HTRG can be attributed to the low surface
area, resulting from the collapse of the platelets into larger particles
upon gas-phase rehydration. Contrarily, the higher surface areas
and thinner platelets of the HTRL samples automatically derive in
a larger number of exposed OH2, and particularly those located at
the edges of the platelets are readily accessible. The limited
diffusion of reactants in the interlayer space of reconstructed
hydrotalcites was originally proposed by Roelofs et al.,4 who
suggested that only 5% of the hydroxyl ions of the rehydrated
samples were active enough to carry out the aldol condensation
of citral with acetone. This was concluded from the preparation
of samples with different platelet size using different ageing
conditions.
Fig. 1 FE-SEM image of (A) HTRG, (B) HTRL700-A
.
Fig. 2 CO2 uptake during temperature-programmed desorption over
HTRG and HTRL500. CO2 adsorption at 353 K for 1 h.
probe molecules{), indicating that the nature of the resulting basic
hydroxyl groups does not depend on the rehydration method.
First peak, which decomposes at temperatures around 700 K, is
attributed to the contribution of mainly bicarbonate species, on the
catalyst surface, while the smaller peak above 800 K is due to
carbonate species.
The total number of hydroxyls in the reconstructed hydrotalcites
can be determined from thermogravimetric analysis (see Table 2)
and depends on the rehydration process. The HTRG sample shows
the highest amount of total OH2 groups (3251 mmol gcat21), while
the HTRL-US sample shows the lowest amount (2250 mmol gcat21 ).
This could be explained by the different rehydration time (15 h and
5 min, respectively) that produces different reconstruction degree
(see supplementary data).{ As concluded from the uptake of CO2
during TPD experiments (see Table 2), the number of probed
OH2 groups in the samples is in the range of 438 up to 972 mmol
gcat21, rather similar in contrast with the BET surface values
(between 15 and 440 m2 g21). Comparison of the TGA with the
CO2 uptake enables to conclude that the number of basic sites
probed by CO2 is estimated at only 13, 28.5, 31 and 32%, for
HTRG, HTRL500, HTRL700-B and HTRL-US, respectively.
In the present work, we have minimised diffusion limitations by
the synthesis and stabilisation of nanoplatelets. For the aldol
condensation of citral–acetone, rehydration in liquid phase
provides different well-exfoliated catalysts, either by increasing
the stirring speed or the time of rehydration. Initial reaction
21
rates between 5 6 1023 and 8 6 1023 mol min21 gcat are
obtained, which are higher than the best activity reported so far
(1 6 1023 mol min21 gcat21).7 This is represented in Fig. 3 by a
black square, which corresponds to a Mg–Al hydrotalcite calcined
at 723 K and rehydrated by direct addition of decarbonated water
at room temperature.7 Indeed, the surface area of this rehydrated
sample fits well to the obtained correlation. The aldol condensa-
tion of citral–MEK is somewhat slower due to the different acidic
character of the a-H in the MEK molecule and its bigger size.
However, an enhanced catalytic activity is observed when
increasing the stirring speed or using ultrasounds in the
rehydration step. In all cases, the selectivity to pseudoionone
products is higher than 95% and no loss of activity is detected
upon three consecutive runs at 1 h of reaction (conversion higher
than 93%).
Table 2 Characterisation measurements of rehydrated hydrotalcites
Crystallite
˚
size (00l)/A
Total OH2/a
mmol gcat
Probed OH2/b
mmol gcat
21
21
Catalyst
HTRG
141
71
—
49
3251
2826
3137
2250
438
804
972
720
HTRL500
HTRL700-B
HTRL-US
a
In summary, our results confirm that only hydroxyl groups in
the edges of the platelets, i.e. at the entrances of the interlayer space
are mainly responsible for the observed activity. This leads to a
dramatic underutilization of the active sites through the interlayer
b
By TGA. By TPD of CO2.
1454 | Chem. Commun., 2005, 1453–1455
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Fig. 3 Initial reaction rate vs. surface area of differently rehydrated hydrotalcites in the aldol condensation of citral–acetone and citral–MEK.
space. Liquid-phase rehydration generally leads to catalysts
with higher surface areas and thinner platelets as compared to
gas-phase rehydration. The nature of the basic sites does not
depend on the rehydration conditions, as evidenced by TPD
with different probe molecules. A relatively simple and cheap
method to further overcome the limited accessibility of OH2
groups in the interlayer of liquid-phase reconstructed hydrotalcites
has been established, by using a higher stirring speed or
ultrasounds during rehydration. These protocols lead to materials
with surface areas up to 440 m2 g21, due to the high degree of
exfoliation of the HT platelets. The linear correlation between the
surface area of the rehydrated hydrotalcite and the catalytic
performance in Fig. 3 is useful to predict initial activities of
rehydrated hydrotalcites by knowing their surface area. Some of
our materials display nearly one order of magnitude higher
activities than the most active systems reported to date in the
literature.
Notes and references
{ Mg–Al (Mg/Al 5 3/1) hydrotalcite was prepared by coprecipitation at
constant pH, thermally decomposed in air at 723 K for 15 h (HTC), and
rehydrated both in gas and liquid phases. A gas-phase sample was obtained
under an argon flow saturated with water (15 h at 40 ml min21).
Rehydration in liquid phase was performed in decarbonated water (1 g in
100 ml of water) at RT using (i) mechanical stirring at different times (1 or
6 h) and stirring speed (100, 500, and 700 rpm) or (ii) sonication (5 min).
After that, the samples were filtered, washed with ethanol and dried under
Ar. The samples were characterized by N2 adsorption, SEM, and TPD
with different probe molecules (CO2, CH3CN, CH3NO2). The condensa-
tion reactions were carried out at 333 K with a ratio ketone/citral 5 4.4.
Samples were taken at regular time periods and analysed by gas
chromatography. Tetradecane was used as internal standard.
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This work was financially supported by the Ministerio de
Ciencia y Tecnolog´ıa of Spain (REN2002-04464-CO2-01 and
PETRI 95-0801.OP) and Destilaciones Bordas S.A.
S. Abello´,ab F. Medina,*a D. Tichit,b J. Pe´rez-Ram´ırez,c Y. Cesteros,a
P. Salagrea and J. E. Sueirasa
aDept. de Qu´ımica i Enginyeria Qu´ımica, Universitat Rovira i Virgili,
43007, Tarragona, Spain. E-mail: francesc.medina@urv.net;
Fax: 34 977559621; Tel: 34 977559787
5 F. Figueras, J. Lopez, J. Sanchez-Valente, T. T. H. Vu, J.-M. Clacens and
J. Palomeque, J. Catal., 2002, 211, 144.
6 F. Cataldo, Ultrason. Sonochem., 2000, 7, 35.
7 M. J. Climent, A. Corma, S. Iborra, K. Epping and A. Velty, J. Catal.,
2004, 225, 316; M. J. Climent, A. Corma, S. Iborra and A. Velty, J.
Catal., 2004, 221, 474.
bLaboratoire de Mate´riaux Catalytiques et Catalyse en Chimie
Organique, UMR 5618 CNRS-ENSCM, 34296, Montpellier Cedex 5,
France
cYara Technology Centre Porsgrunn, Catalysis and Nitric Acid
Technology, P.O. Box 2560, N-3908, Porsgrunn, Norway
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