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

www.elsevier.com/locate/apcata

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: amiridis@engr.sc.edu (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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