Li-Al layered double hydroxides as catalysts for the synthesis of flavanone

Article abstract of DOI:10.1016/j.catcom.2010.08.021

Flavanone was synthesized via a series reaction scheme involving the Claisen-Schmidt condensation between 2′-hydroxyacetophenone and benzaldehyde to form 2′-hydroxychalcone and the subsequent isomerization of 2′-hydroxychalcone to flavanone. Reactions were carried out in the presence of Li-Al layered double hydroxide solid catalyst. The results showed that surface basicity varies with calcination temperature and rehydration. These variations in basicity correlate with the observed catalytic behavior. It is believed that Li⁺-O^2- groups contribute to the surface basicity and are instrumental in the abstraction of a proton from the 2′-hydroxyacetophenone in what is believed to be the first step in the reaction mechanism.

Full text of DOI:10.1016/j.catcom.2010.08.021

Catalysis Communications 12 (2010) 92–94
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Li–Al layered double hydroxides as catalysts for the synthesis of flavanone
a
a
b
a,
Dustin French , Paul Schifano , José Cortés-Concepción , Sirena Hargrove-Leak ⁎
a
Elon University, Dual Degree Engineering Program, 2625 Campus Box, Elon, NC 27244, United States
University of South Carolina, Department of Chemical Engineering, 301 Main Street, Columbia, SC 29208, United States
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 August 2010
Received in revised form 29 August 2010
Accepted 30 August 2010
Available online 25 September 2010
Flavanone was synthesized via a series reaction scheme involving the Claisen–Schmidt condensation between
2
2
′-hydroxyacetophenone and benzaldehyde to form 2′-hydroxychalcone and the subsequent isomerization of
′-hydroxychalcone to flavanone. Reactions were carried out in the presence of Li–Al layered double
hydroxide solid catalyst. The results showed that surface basicity varies with calcination temperature
and rehydration. These variations in basicity correlate with the observed catalytic behavior. It is believed that
+
2−
Li –O groups contribute to the surface basicity and are instrumental in the abstraction of a proton from the
′-hydroxyacetophenone in what is believed to be the first step in the reaction mechanism.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
2
Li–Al layered double hydroxide as catalyst
Claisen–Schmidt condensation
Flavanone synthesis
Heterogeneous catalysis
1
. Introduction
The pharmacological benefits of flavonoids continue to generate
specifically, we synthesized calcined and rehydrated [LiAl
CO 0.5·nH O. The resulting catalysts were characterized by temper-
ature programmed desorption of CO (CO -TPD) and XRD measure-
2 6
(OH) ]
(
3
)
2
2
2
interest. Therefore, the significant economic and environmental
benefits of converting the homogeneous catalytic processes, tradi-
tionally used to synthesize them, to heterogeneous ones are also of
interest. The synthesis of flavanone is one reaction scheme that has
been explored in recent years. It is the product of a 2-step reaction
scheme (Fig. 1) that involves a Claisen–Schmidt condensation
reaction of 2′-hydroxyacetophenone and benzaldehyde to form 2′-
hydroxychalcone, which then undergoes isomerization to flavanone.
Previous work has included kinetic, mechanistic, and catalytic studies
ments. Subsequently, these samples were tested for their catalytic
activity in the heterogeneous synthesis of flavanone. Finally, we
sought correlations in the results of activity studies and characteriza-
tion measurements to further elucidate the heterogeneous reaction
mechanism.
2. Experimental
[1–6]. Of key importance to this study, a recent investigation of Li-
2.1. Catalyst preparation
modified MgO indicates low Li loading has a promotional effect on the
Claisen–Schmidt condensation reaction [7].
[LiAl
reported by Shumaker et al. [9]. A 250 mL volume of 0.4 M Al
(NO ·9 H O was added dropwise to a 600 mL volume of 1.5 M
LiOH·H O and 0.08 M Na CO (prepared using 10.6 g of Na CO and
the balance DI water). The mixture was stirred vigorously until
precipitate formed. The resulting precipitate was left to age in the
reaction mixture overnight at 75 °C under gentle stirring. The
precipitate was washed with DI water to an effluent pH of 7 and
isolated by vacuum filtration. The filtrate was separated into roughly
equal quantities in preparation for a five hour calcination period.
One quantity was calcined at 300 °C and the other at 450 °C. Last,
the catalyst was sieved to 60 to 80 mesh particle size and stored in
a desiccator. When needed, catalyst samples were rehydrated to
2 6 3 2
(OH) ](CO )0.5·nH O was prepared using the method
Layered double hydroxides (LDH), both specifically and as a class
of compounds, have shown promise of excellent catalytic use in fine
chemical and pharmaceutical syntheses in recent years. They are
currently used in biodiesel, removal of agrochemicals, and medication
controlled release systems such as ibuprofen. Climent et al. [8] is most
widely noted for their use in Claisen–Schmidt condensations,
specifically Mg–Al hydrotalcites to synthesize Vesidryl from acet-
ophenone and benzaldehyde.
3
)
3
2
2
2
3
2
3
This work explores a marriage of concepts in an investigation of
flavanone synthesis over Li–Al LDH as part of an overarching
continued quest to fully understand the reaction mechanism. More
3
5 wt.% via direct dropwise application of DI water, which has been
identified as an optimum weight percentage in previous rehydration
studies [10]. The names and descriptions of the catalyst samples used
in this study are given in Table 1.
⁎
Corresponding author. Tel.: +1 336 278 6224.
E-mail address: sleak@elon.edu (S. Hargrove-Leak).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.08.021

D. French et al. / Catalysis Communications 12 (2010) 92–94
93
O
R
C
H
OH
O
OH
Acid
+
+
H O
2
H
COCH3
or Base
H
R
2'-hydroxyacetophenone
substituted benzaldehyde substituted 2'-hydroxychalcone
water
R
R
Acid
O
OH
O
or Base
H
H
O
substituted 2'-hydroxychalcone
substituted flavanone
Fig. 1. Synthesis of flavanone via a two-step process (Claisen–Schmidt condensation and isomerization).
2
.2. Characterization studies
determined from the slope of conversion versus time data within in
the first few minutes of reaction.
X-ray diffraction (XRD) measurements were conducted to exam-
Samples of the reaction slurry (approximately 0.1 mL) were taken
periodically over the course of the reaction. Samples were centrifuged
to separate any catalyst trapped in the sample, diluted in isopropyl
alcohol, and analyzed offline using a gas chromatograph (GC). The HP
5890 GC was equipped with a Restek RTX-1 capillary column and
flame ionization detector.
ine catalyst structure. The instrument used has a Rigaku D/MAX RB X-
ray powder diffractometer, graphite monochromator, and nickel-
filtered Cu Kα radiation source (λ=0.15406 nm). It was operated at
0 kV and 50 mA, in the 5° to 80° range, at a scanning rate of 1.2°/min.
The diffractograms are shown in Fig. 2.
4
The basic properties of the dry and rehydrated catalyst were
explored through CO
2
temperature programmed desorption (CO
2
-
3. Results and discussion
TPD) measurements. A Micromeritics Chemisorb 2750 instrument
with a TCD detector was used. Samples were pretreated in situ in UHP
He at the corresponding calcination temperature for 1 h, then
returned to room temperature under continuous He flow. After
3.1. Characterization studies
The XRD patterns for the dry catalyst samples prepared for this
investigation are shown in Fig. 2. The observed patterns are consistent
with the known literature on Li–Al LDH [9,12].
being exposed to 45% CO
2
/55% He mixture at a rate of 20 mL/min for
3
0 min, the sample was purged with UHP He for 30 min to remove
physisorbed CO
2
. Profiles were collected continuously as the temper-
TPD profiles of CO
calcined at the two temperatures are shown in Fig. 3. In following Liu
et al. [13], basic sites may be described as “weak” basic sites for CO
desorption occurring in the range of 27 °C and 147 °C, “medium”
basic sites for CO desorption occurring in the range of 147 °C and
77 °C, and “strong” basic sites for CO desorption occurring above
2
adsorbed on the dry and rehydrated catalyst
ature was increased to the corresponding calcination temperature at a
rate of 10 °C/min. The TPD profiles are shown in Fig. 3.
2
2
.3. Activity studies
2
3
2
Flavanone synthesis was used to explore the activity of the
catalyst. The synthesis involves series Claisen–Schmidt condensation
and isomerization reactions as shown in Fig. 1.
377 °C. Here, the focus is on “weak” and “medium” sites due to the
temperature limitations imposed by the preparation techniques
(i.e., calcination temperatures).
Reactions were carried out in a custom stirred reflux batch reactor
at 160 °C. Since benzaldehyde oxidizes easily in air, the reactor was
purged with UHP N prior to and during its use. Equimolar (1.5 M)
2
amounts of 2′-hydroxyacetophenone (Aldrich, 99.0% purity, 27 mL)
and benzaldehyde (Aldrich, 99+% purity, 23 mL) in DMSO solvent
(
Alfa Aesar, 99.9% purity, 100 mL) were charged to the reactor and
brought to the reaction temperature.
A 0.1 wt.% catalyst loading (165 mg) was used and it was added
once the reaction temperature was reached (t=0). The stirring rate
was 500 rpm. These operating conditions were previously found to
fall in the kinetic regime [11]; therefore, initial reaction rates were
450
Table 1
Names and descriptions of Li–Al LDH samples for the synthesis of flavanone.
3
00
Catalyst
Description
3
3
4
4
00
00 RE
50
Calcined at 300 °C
Calcined at 300 °C and rehydrated with 35 wt.% water
Calcined at 450 °C
0
10
20
30
40
50
60
70
80
90
2
θ/degree
50 RE
Calcined at 450 °C and rehydrated with 35 wt.% water
Fig. 2. XRD patterns of Li–Al LDH samples.

94
D. French et al. / Catalysis Communications 12 (2010) 92–94
0
.02
300 RE
Table 2
Kinetic data for the Claisen–Schmidt condensation of 2′-hydroxyacetophenone over Li–
Al LDH catalysts.
Catalyst
Initial reaction rate (104) mol/g/s
0
.015
3
4
4
3
00
50
50 RE
00 RE
2.93
3.45
3.80
4.70
0
.01
4
50 RE
4
50
specifically, recall that the CO
2
-TPD profiles indicated an increase in
3
00
0
.005
0
both “weak” and “medium” basic sites in the order of 300b450b450
REb300 RE (Fig. 3). As the number of basic sites increase, the initial
reaction rates increase in parallel order (Table 2). These sites,
−
+
2−
attributable to OH and Li –O
groups, are instrumental in the
0
50
100
150
200
250
300
350
400
450
500
deprotonation of the 2′-hydroxyacetophenone, which is believed to
be the first step in the Claisen–Schmidt condensation reaction
mechanism for benzaldehyde and 2′-hydroxyacetophenone [7,16,17].
The isomerization reaction of 2′-hydroxychalone to flavanone
appears to be unaffected by the catalyst since flavanone selectivity is
the same for all samples after 60 min of reaction time (59%). This
behavior is consistent with previous investigations, thus it further
supports the belief that the isomerization reaction occurs significantly
faster than the Claisen–Schmidt condensation reaction [7,13].
o
Temperature, C
2
Fig. 3. CO -TPD profiles of Li–Al LDH samples.
Direct dropwise water addition is known to introduce OH− anions,
which present Brönsted basicity [14]. The “weak” basic sites are likely
due to these hydroxyl groups and an increase in the number of these
sites is expected upon rehydration. Additionally, calcined LDHs have
−
been found to contain “weak” sites due to OH and “medium” sites
due to Li–O pairs [15]. Therefore, in the case of Li–Al LDH, the
4. Conclusions
+
2−
“
medium” basic sites are likely due to Li –O pairs [9].
.2. Activity studies
′-Hydroxyacetophenone conversion data is shown in Fig. 4. Initial
The results presented here signify that Li–Al layered double
hydroxide is active for the heterogeneous catalytic synthesis of flavanone.
Further, increases in basicity via calcination and rehydration directly
correlate with increases in catalytic activity. The literature supports that
3
2
−
+
2−
reaction rates for the Claisen–Schmidt condensation reaction step
were calculated from that data, normalized with respect to catalyst
weight, and are presented in Table 2.
this observed increased basicity and activity is due to OH and Li –O
+
2−
groups. The Li –O groups are of particular interest because they may
be instrumental in the abstraction of a proton from 2′-hydroxyaceto-
phenone, which is believed to be the first step in the heterogeneously
catalyzed Claisen–Schmidt condensation reaction mechanism.
First, comparing the activity of the dry Li–Al LDH samples calcined
at the two different temperatures, the sample calcined at 450 °C is
more active, as indicated by the higher initial reaction rate. Li–Al LDH
has been found to decompose upon calcination at 450 °C due to
decarboxylation and dehydroxylation and, in turn, increase its surface
area and pore volume [9]. Therefore, the observed increase in activity
with calcination temperature may be attributed to this reported
increase in surface area and pore volume.
Acknowledgements
We gratefully acknowledge the financial support of the NSF-REU
Program (Grant No. EEC-0552702) and Elon University Summer
Research Fellowship for Dustin French and Paul Schifano. We also
gratefully acknowledge Dr. Michael D. Amiridis for welcoming Sirena
Hargrove-Leak to the Department of Chemical Engineering at the
University of South Carolina as a Visiting Research Associate.
Next, the catalytic behavior of Li–Al LDH for the Claisen–Schmidt
condensation reaction can be correlated to the surface basicity. More
3
00
300 RE
450
450 RE
6
5
4
3
2
1
0
0
0
0
0
0
0
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Reaction time, min
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Fig. 4. 2′-Hydroxyacetophenone conversion as a function of time for Li–Al LDH catalysts.