Effect of basic properties of MgO on the heterogeneous synthesis of flavanone
Transcript of Effect of basic properties of MgO on the heterogeneous synthesis of flavanone
Effect of basic properties of MgO on the heterogeneous
synthesis of flavanone
Zheng Liu, Jose A. Cortes-Concepcion, Michael Mustian, Michael D. Amiridis *
Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208, United States
Received 12 July 2005; received in revised form 24 December 2005; accepted 13 January 2006
Available online 17 February 2006
Abstract
The effect of the surface basicity of MgO on the heterogeneous synthesis of flavanone from benzaldehyde and 20-hydroxyacetophenone was
examined through a series of MgO samples modified with different anions. CO2 temperature programmed desorption (TPD) was used to
characterize the basic properties of these samples. The results indicate that basic sites with different strengths exist on the MgO surface.
Introduction of different anions completely eliminates the weak basic sites (i.e., those desorbing CO2 in the 300–420 K range) and reduces
substantially the number of medium strength sites (i.e., those desorbing CO2 in the 420–650 K range). In contrast, no substantial effect was
observed – with the exception of the chloride-treated sample – on the stronger basic sites (i.e., those desorbing CO2 above 650 K). A strong
correlation was observed between the number of basic sites of ‘‘medium’’ strength and the catalytic activity of these samples for the heterogeneous
synthesis of flavanone. These sites are most likely involved in the activation of 20-hydroxyacetophenone for the Claisen–Schmidt condensation
with benzaldehyde, which represents the first step in the synthesis of flavanone.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Fine chemicals and pharmaceuticals; Flavanone; Benzaldehyde; 20-Hydroxyacetophenone; MgO; Basicity
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Applied Catalysis A: General 302 (2006) 232–236
1. Introduction
Flavanone represents a significant intermediate in many
pharmaceutical syntheses, and members of the flavanoid family
are attracting increased attention due to recent studies
documenting their anticancer [1], anti-inflammatory [2], anti-
bacterial [3], and anti-AIDS [4] pharamacological activity. The
synthesis of flavanone is carried out homogeneously via the
Claisen–Schmidt condensation of benzaldehyde and 20-hydro-
xyactophenone [5,6], and the subsequent isomerization of the 20-hydroxychalcone intermediate formed to flavanone (Scheme 1).
The feasibility of utilizing the same reaction scheme to produce
flavanone heterogeneously has been previously demonstrated by
different laboratories, including our own [6–13].
The kinetics of the flavanone synthesis scheme have been
studied in detail in our group [7,8,10]. We have also
investigated the reaction mechanism [9,11,13] and have
developed a fairly good understanding of the steps involved
and the nature of surface intermediates formed. Little is known
* Corresponding author. Tel.: +1 803 777 7294; fax: +1 803 777 8265.
E-mail address: [email protected] (M.D. Amiridis).
0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2006.01.007
however, about the nature of the active sites involved in this
reaction, beyond some general notion of attributing the activity
of MgO to its basicity [6].
In the current study we are addressing the nature of the MgO
sites involved in the flavanone synthesis scheme. In order to
modify the MgO basicity and create materials with different
basic strengths, we have treated MgO with different anions (i.e.,
PO43�, SO4
2�, F� and Cl�) known to selectively poison surface
basicity. The resulting catalysts were characterized by CO2
temperature programmed desorption (TPD) measurements and
provided a set of materials with varying basic strength.
Subsequently, these samples were tested for their catalytic
activity in the heterogeneous synthesis of flavanone and the
results were correlated to those of the characterization
measurements.
2. Experimental
2.1. Sample preparation
Pure MgO (Aldrich, 99% purity; SA = 65 m2/g) was
calcined at 500 8C for 4 h prior to its use. Aqueous solutions
Z. Liu et al. / Applied Catalysis A: General 302 (2006) 232–236 233
Scheme 1. Synthesis of flavanone via a two-step process.
of the precursors (H2SO4: 98%; H3PO4: 89%; HCl: 37%; NH4F,
95%; Fisher Scient.) used for the modification of MgO were
diluted to 0.1 M and impregnated onto the MgO. The loadings
of the different anions used are shown in Table 1. These were
chosen so that the total charge loading (in mmol per g of
catalyst) was the same for all modified samples. Following
impregnation, water was evaporated under vacuum and the
samples were further dried overnight at 373 K and calcined in
flowing air at 773 K for 4 h. X-ray diffraction (XRD) patterns
collected for all the samples involved in this study did not reveal
the formation of any new crystalline phases and indicated that
the crystalline structure of MgO remained intact following the
treatments with the different anions. Furthermore, this anion
modification process did not affect the morphology of MgO, as
indicated by the unaffected BET surface area measurements
shown in Table 1.
2.2. Basicity measurements
The basic properties of pure and modified MgO were
probed by CO2-TPD measurements. Prior to their use, samples
(150 mg) were treated in situ in He (UHP) at 773 K for 1 h to
remove any adsorbed impurities. Subsequently, the samples
were cooled down to room temperature in He, and exposed
to CO2 (45% CO2/He) for 30 min, followed by purging with
He for 30 min. The temperature was then increased at a
Table 1
Results of characterization measurements for anion-modified and pure MgO samp
Samples BET SA (m2/g) Anion loading
(wt.%) Charge (mm
MgO 65
PO43�/MgO 63 0.9 0.30
SO42�/MgO 59 1.3 0.27
F�/MgO 65 1.1 0.31
Cl�/MgO 61 0.7 0.32
rate of 10 K/min from 298 to 1050 K. The CO2 evolved was
converted to methane by means of a methanation catalyst
operating at 673 K and monitored using a flame ionization
detector.
2.3. Activity measurements for the synthesis of flavanone
Activity measurements were conducted in a home-made
batch reactor system described in more detail elsewhere [8,9].
The reactor was initially charged with 150 ml of a mixture
containing 1.5 M benzaldehyde (Aldrich, 99%) and 1.5 M 20-hydroxyacetophenone (Aldrich, 99%) in DMSO (Alfa, 99.9%).
After the reactor was charged, nitrogen was continuously
bubbled through the system. The reactor was then heated and
the catalyst (60–80 mesh) was added when the desired reaction
temperature was reached (t = 0). Following this point the
reactor was operated under total reflux. Small samples
(approximately 0.5 ml) were removed from the reactor
periodically during the course of the reaction. After each
sample was diluted in acetone and separated from the solid
catalyst, it was analyzed off-line by gas chromatography (SRI
Instruments 8610C GC; 5% phenyl methyl siloxane capillary
column; FID detector). As we have shown previously [7], under
these conditions the reactor is operating in the kinetic regime,
and intrinsic rates of the reaction can be calculated from the
slope of the conversion versus time curve.
les
Basicity (mmol of adsorbed CO2/g)
ol/g) Total Weak Medium Strong
0.440 0.119 0.181 0.140
0.289 0.012 0.141 0.136
0.232 0.003 0.095 0.134
0.198 0.013 0.065 0.120
0.027 0.006 0.012 0.009
Z. Liu et al. / Applied Catalysis A: General 302 (2006) 232–236234
3. Results and discussion
3.1. Temperature programmed desorption studies of CO2
TPD profiles of CO2 adsorbed on pure MgO, as well as MgO
modified with various anions are shown in Fig. 1. The TPD
profile of pure MgO contains several CO2 desorption peaks,
indicating that a variety of basic sites with different strengths
are present on this surface. The strength of such sites increases
as the peaks in the TPD profile appear at higher temperatures.
To facilitate discussion, we have divided the basic sites of MgO
into three large groups exhibiting ‘‘weak’’ (CO2 desorption
between 300 and 420 K), ‘‘medium’’ (CO2 desorption between
420 and 650 K), and ‘‘strong’’ (CO2 desorption above 650 K)
basicity. Following anion modification of MgO, several
changes can be observed in the TPD profiles. First, the peak
corresponding to the ‘‘weak’’ basic sites – which most likely are
associated with hydroxyl groups – almost completely
disappears from the profiles of all anion-modified samples.
Furthermore, the presence of these anions also leads to a
substantial reduction of ‘‘medium’’ strength basic sites in the
following order: MgO > PO43�/MgO > SO4
2�/MgO > F�/
MgO > Cl�/MgO, with the chloride-modified catalyst having
almost no basic sites of ‘‘medium’’ strength. Finally, no
significant changes are observed in the region of strong basicity,
with the exception of the chloride-modified sample, in which
case, a strong decrease is once again observed.
The TPD results shown in Fig. 1 are quantified in Table 1.
The ‘‘total’’ basicity refers to the total amount of CO2 desorbed
in the temperature range between 298 and 1050 K. Unmodified
MgO exhibits a total basicity of 0.44 mmol/g distributed as
27% in ‘‘weak’’, 41% in ‘‘medium’’, and 32% in ‘‘strong’’
basic sites. A decrease is observed in the total basicity of all
anion-modified samples. In these cases, the total basicities fall
in the range of 0.03–0.29 mmol/g. This decrease however is
Fig. 1. CO2-TPD profiles of the anion-modified and pure MgO.
more substantial among sites of lower strength, while the
‘‘strong’’ basic sites are mostly retained, with the exception of
the chloride-modified sample.
Overall, these CO2-TPD results allow us to classify the
samples studied according to the distribution of basic sites.
They indicate that the incorporation of anions in MgO leads to a
decrease of the total basicity in the following order:
MgO > PO43�/MgO > SO4
2�/MgO > F�/MgO > Cl�/MgO.
In all cases complete elimination of the weak basicity of MgO
was observed, but no significant effect on ‘‘strong’’ basicity,
with the exception of chloride. Finally, the effect of these
anions on sites of ‘‘medium’’ basic strength is markedly
different, with the changes observed following a gradual
decrease, similar to that of the total basicity.
3.2. Activity measurements for the synthesis of flavanone
Benzaldehyde conversions and flavanone yields obtained
over time with the pure, as well as the anion-modified MgO
samples are shown in Figs. 2 and 3. The results are quantified in
Table 2, where the calculated initial rates for the Claisen–
Schmidt condensation reaction are presented. Selectivities for
flavanone as functions of benzaldehyde conversion are shown
in Fig. 4. Selectivities of benzaldehyde after 20 min of reaction
time are also included in Table 2. The results of Fig. 2 and
Table 1 demonstrate the inhibiting effects of the different
anions used on the Claisen–Schmidt condensation. In
particular, the catalytic activity for this reaction decreases
in the following order: MgO > PO43�/MgO > SO4
2�/
MgO > F�/MgO > Cl�/MgO, in agreement with the observed
decrease in basicity for the same samples. A similar trend is
also observed in the flavanone yields shown in Fig. 3. Finally,
the results shown in Fig. 4 and Table 2 indicate that no
significant differences in selectivity to flavanone can be
observed among the different samples, including pure MgO,
Fig. 2. Benzaldehyde conversions vs. time obtained at 160 8C (initial con-
centrations: 1.5 M benzaldehyde, 1.5 M 2-hydroxyactophenone; 0.1 wt.% cat-
alysts). (*) MgO; (^) PO43�/MgO; (~) SO4
2�/MgO; (&) F�/MgO; (*) Cl�/
MgO.
Z. Liu et al. / Applied Catalysis A: General 302 (2006) 232–236 235
Fig. 3. Flavanone yields vs. time obtained at 160 8C (initial concentrations:
1.5 M benzaldehyde, 1.5 M 2-hydroxyactophenone; 0.1 wt.% catalysts). (*)
MgO; (^) PO43�/MgO; (~) SO4
2�/MgO; (&) F�/MgO; (*) Cl�/MgO.Fig. 4. Selectivity to flavanone vs. conversion obtained at 160 8C (initial
concentrations: 1.5 M benzaldehyde, 1.5 M 2-hydroxyactophenone; 0.1 wt.%
catalysts). (*) MgO; (^) PO43�/MgO; (~) SO4
2�/MgO; (&) F�/MgO; (*)
Cl�/MgO.
although the catalytic activities for these samples are
substantially different. This result appears to imply that the
isomerization of chalcone to flavanone over these samples is
much faster than the Claisen–Schmidt condensation and is not
affected by the observed changes in basicity.
3.3. Relationship between catalytic performance and
basicity of MgO samples
The catalytic behavior of the anion-modified MgO samples
can be correlated with their surface basicity. Two types of basic
sites are known to exist on the MgO surface, i.e.: (1) lattice-
bound and isolated hydroxyl groups exhibiting Bronsted
basicity and (2) surface O2�sites with different coordinations
exhibiting Lewis basicity. These O2� sites exhibit a five-fold-
coordination on a flat surface, but can also be found onto
defect, edge or corner positions with lower coordinations (e.g.
three-fold for a corner O2� site and four-fold for an edge O2�
site). As a result, the basic strength of these sites varies and a
distribution is expected. As indicated by the CO2 TPD data, the
basicity of MgO can be divided into three broad categories
(i.e., ‘‘weak’’, ‘‘medium’’, and ‘‘strong’’ basic sites). The
‘‘weak’’ basic sites are probably associated with Bronsted
Table 2
Catalytic activity of anion-modified and pure MgO samples for the synthesis of
flavanone
Samples Claisen–Schmidt
condensation rate
� 104 (mol/g/s)
Flavanone
selectivity
(after 20 min) (%)
MgO 6.5 63.5
PO43�/MgO 5.5 66.1
SO42�/MgO 3.2 61.5
F�/MgO 2.9 61.7
Cl�/MgO 1.8 60.6
basicity and mostly likely with lattice-bound OH groups
present under our experimental conditions. The ‘‘medium’’
and ‘‘strong’’ sites are probably associated with Lewis basicity,
with the three- and four-fold-coordinated O2� anions
representing the stronger among these sites.
Almost all ‘‘weak’’ basic sites disappear upon treatment of
MgO with the different anions used in this study. In contrast, no
significant differences are observed in the ‘‘strong’’ basicity
with the exception of the chloride-treated sample, while a
gradual effect is observed on the sites of ‘‘medium’’ strength
depending on the nature of the anion used. The order of
‘‘medium’’ basicity among these anion-modified MgO samples
correlates very well with the observed catalytic activity for the
Claisen–Schmidt condensation reaction, which represents the
first step for the synthesis of flavanone. In contrast, no
correlation can be found between catalytic activity and either
‘‘weak’’ or ‘‘strong’’ basic sites. Therefore, these results appear
to suggest that surface O2� ions (mostly likely five-fold-
coordinated) with medium basic strength are the main active
sites for the Claisen–Schmidt condensation reaction on MgO.
In combination with the results of our FTIR studies of the
adsorption and reaction of benzaldehyde and 20-hydroxyace-
tophone on MgO [13], these results suggest that a conjugate
Lewis acid–base pair (Mg2+–O2�) is involved in the activation
of 20-hydroxyacetophenone. During this process the surface
O2� ion abstracts an H+ from the hydroxyl group of 20-hydroxyacetophone to produce an anionic intermediate, which
is subsequently stabilized through additional bond formation
between the carbonyl group and the conjugate Mg2+ ion.
4. Conclusions
The results of CO2-TPD measurements conducted in this
study indicate that the basic properties of MgO can be modified
by the introduction of different anions (i.e., PO43�, SO4
2�, F�
Z. Liu et al. / Applied Catalysis A: General 302 (2006) 232–236236
and Cl�). The presence of these anions completely eliminates the
weak basic sites of MgO (i.e., sites that desorb CO2 below
420 K), most probably associated with surface hydroxyl groups.
Furthermore, it reduces the number of sites of medium strength
(i.e., sites desorbing CO2 between 420 and 650 K) with the
magnitude of this effect depending on the nature of the anion, but
has no effect on the strong basic sites of MgO (i.e., sites desorbing
CO2 above 650 K). A strong correlation was observed between
the number of basic sites of medium strength and the initial rate
of the Claisen–Schmidt condensation reaction, which represents
the first step in the heterogeneous synthesis of flavanone. Based
on the results of these and previous studies, it is proposed that
basic sites of medium strength, most probably five-fold-
coordinated surface oxygen anions, are involved in the activation
of 20-hydroxyacetophenone on MgO.
Acknowledgements
We gratefully acknowledge the financial support of the
NSF-REU program (DMR-0353840 and EEC-0097695) for
Michael Mustian.
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