Condensation of acetophenone to α,β-unsaturated ketone (dypnone) over solid acid catalysts

Article abstract of DOI:10.1016/S1381-1169(01)00363-6

The self-condensation of acetophenone (ACP) to α,β-unsaturated ketone in the liquid phase has been studied using sulfated zirconia (SZ), sulfated titania (ST) and H-beta zeolite as catalysts in the temperature range of 403-433 K. It is found that SZ and H-beta exhibit similar activity (rate of ACP conversion), which is higher than that of ST. From the TPD of NH₃ it is seen that both SZ and H-beta are highly acidic and possess strong acid sites, as seen from NH₃ desorption in the temperature range of 513-873 K, which are responsible for the reaction. The conversion of ACP increases with reaction time, the catalyst concentration and reaction temperature. The SZ is recycled two times to show that there is only a marginal decrease in activity on recycled catalyst with no loss in the selectivity for dypnone.

Full text of DOI:10.1016/S1381-1169(01)00363-6

Journal of Molecular Catalysis A: Chemical 181 (2002) 179–187
Condensation of acetophenone to ␣,␤-unsaturated ketone
(dypnone) over solid acid catalysts
∗
C. Venkatesan, A.P. Singh
National Chemical Laboratory, Catalysis Division, Pune 411 008, India
Received 23 January 2001; received in revised form 7 June 2001; accepted 19 June 2001
Abstract
The self-condensation of acetophenone (ACP) to ␣,␤-unsaturated ketone in the liquid phase has been studied using sulfated
zirconia (SZ), sulfated titania (ST) and H-beta zeolite as catalysts in the temperature range of 403–433 K. It is found that
SZ and H-beta exhibit similar activity (rate of ACP conversion), which is higher than that of ST. From the TPD of NH3 it is
seen that both SZ and H-beta are highly acidic and possess strong acid sites, as seen from NH3 desorption in the temperature
range of 513–873 K, which are responsible for the reaction. The conversion of ACP increases with reaction time, the catalyst
concentration and reaction temperature. The SZ is recycled two times to show that there is only a marginal decrease in activity
on recycled catalyst with no loss in the selectivity for dypnone. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Condensation of acetophenone; Preparation of dypnone; Sulfated zirconia; H-beta; Solid acid catalysts
1
. Introduction
acid catalysts have several disadvantages if applied
to industrial processes such as wasting large amounts
of catalysts, corrosion of reactor, water pollution by
acidic wastewater, and difficulty of catalyst recovery.
In addition, AlCl3 being a strong Lewis acid also cat-
alyzes other undesirable side reactions such as poly-
condensation of ACP to a mixture of other products. In
view of the above, it was of interest to develop a new
solid catalyst for the selective synthesis of dypnone.
Modified zeolites are known to catalyze selectively
aldol condensation of ACP to dypnone [5–8]. The
objective of the present work is to replace the con-
ventional Lewis acids catalyst with environmentally
friendly solid acid catalysts such as sulfated zirconia
(SZ) and sulfated titania (ST), which are open structure
and H-beta zeolite is closed three-dimensional porous
structure. We have chosen the later among other zeo-
lites because it has been synthesized and characterized
in our lab shows a good activity towards a variety of
acylation reactions [9–11] and halogenation reactions
Dypnone is a useful intermediate for the produc-
tion of a large range of compounds. It has been
used as a softening agent, plastisizer and perfumery
base. Dypnone has been prepared conventionally by
the action of sodium ethoxide, aluminum bromide,
phosphorous pentachloride, aluminum triphenyl, zinc
diethyl, calcium hydroxide, anhydrous hydrogen chlo-
ride, anhydrous hydrogen bromide, aluminum chlo-
ride, aluminum tert-butoxide or hydrogen fluoride
on acetophenone (ACP) as described in the earlier
literature [1–4]. In addition, it has been obtained by
the action of aniline hydrochloride on ACP, followed
by treatment with hydrochloric acid. Unfortunately,
its synthesis is not always an easy task and Lewis
∗
Corresponding author. Tel.: +91-20-5893761;
fax: +91-20-5893761.
E-mail address: apsingh@cata.ncl.res.in (A.P. Singh).
1
381-1169/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(01)00363-6

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C. Venkatesan, A.P. Singh / Journal of Molecular Catalysis A: Chemical 181 (2002) 179–187
Scheme 1.
[
12]. It has also been reported that a comparative study
of SZ with H-beta zeolite for alkyaltion of isobutane
13]. Another objective is to enhance the selectivity
solution (28%) was added into the aqueous solution
with stirring until the pH of the solution reached 8.
The precipitate was kept for 3 h and filtered. After
filtration, the precipitate was washed and dried. The
dried sample was then subjected to sulfation as above
and calcined as described earlier.
[
for dypnone and to minimize the formation of con-
secutive products in the condensation of ACP using
solid acid catalysts. This paper presents the results of
a study comparing the performance of sulfated metal
oxides with H-beta catalyst in the condensation of
ACP (Scheme 1). The condensation of ACP to dyp-
none has not been reported yet using such calysts.
The influence of catalyst concentration and reaction
temperature is also reported using SZ as catalyst.
The synthesized zeolite beta was calcined at 823 K
+
in an air stream and changed into its H -form. The
crystallinity and phase purity of the catalysts were
known by X-ray diffractograms (Rigaku D MAX III
VC with Cu K␣ radiation source). The catalysts were
further characterized by SEM JEOL (JSM-5200) and
N2 adsorption (NOVA) techniques. Acidity of the cat-
alysts was evaluated by the temperature programmed
desorption of ammonia from 303 to 873 K (in several
steps) using the method described in the literature [14].
The physico-chemical properties of the samples are
given in Table 1. Before reaction, the catalysts were
dehydrated at 573 K for 3 h.
2
. Experimental
2
2
.1. Catalyst preparation
.1.1. Preparation of SZ
Zirconium hydroxide was obtained by hydrolyzing
ZrOCl2·8H2O with aqueous ammonia in the pH range
2.2. Catalytic reaction
◦
of 8–9 at a temperature of 60–70 C; aqueous ammonia
solution (28%) was added drop-wise with stirring into
Reactions were performed in the liquid phase
under batch conditions. In a typical reaction, the acti-
vated catalyst (0.5 g) was added to ACP (0.042 mol)
and the resulting suspension was magnetically stirred
at 433 K. The course of the reaction was followed by
analyzing the reaction mixture periodically using a
gas chromatograph (HP 6890) equipped with a flame
ionization detector (FID) and a capillary column (HP
5% silicone gum). The products were also identified
by GC/MS (Shimadzu, QP 2000 A) analysis. The
isolation of pure products was accomplished by silica
gel (60–120-mesh) column chromatography with 5%
ethyl acetate in petroleum ether as eluting solvent.
TLC and GC confirmed the formation of pure com-
pound. The major products obtained in this study are
5
0 g of ZrOCl2·8H2O dissolved in 1 l of distilled hot
◦
water. The precipitated solution was kept at 60–70 C
for 3 h. The precipitate was washed several times with
hot water until the filtrate was negative to silver nitrate
◦
solution and finally dried at 110 C. Dried sample was
then stirred with 1 N sulfuric acid (2 g/30 ml) for 2 h
and filtered. After drying, the sample was calcined at
8
23 K for 4 h with the flow of air.
2
.1.2. Preparation of ST
Titanium isopropoxide solution of 150 ml was
added into 1 l of distilled water with stirring. Addi-
tion of 30% nitric acid solution dissolved the white
precipitate, which formed. Ammonium hydroxide

C. Venkatesan, A.P. Singh / Journal of Molecular Catalysis A: Chemical 181 (2002) 179–187
181

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C. Venkatesan, A.P. Singh / Journal of Molecular Catalysis A: Chemical 181 (2002) 179–187
−
−1
and 86, 86 and
in agreement with the spectroscopic data of the pure
compound.
Spectroscopic data of dypnone: H NMR (200 MHz)
40.8 wt.%, 6.7, 4.8, 6.8 mmol g 1
h
85 wt.%, respectively. AlCl3 catalyst is found to be
1
−1 −1
) with comparable
more active (180.5 mmol g
h
(
(
(
CDCl3) δ: 2.48 (s, 3H); 7.05 (m, 1H); 7.05–7.87
selectivity (84 wt.%) to dypnone. The higher conver-
sion of ACP over SZ and H-beta seems to be due
to the fact that these catalysts exhibit stronger acid
sites (Table 1). These results indicate that strong acid
sites are the most important factor to enhance both
the protonation and the dehydration and consequently
to force the aldol condensation of ACP to dypnone.
Optimization of the reaction condition was done with
catalyst SZ and other catalysts were compared under
optimized conditions.
−
1
m, 10H). GC–FT-IR (vapor phase): 1610 cm
−
1
−C=C−); 1675 cm (−C=O).
The conversion is defined as the percentage of ACP
transformed. The rate of ACP conversion is given
as the amount of ACP (mmol) converted per gram
of the catalyst per hour during the initial period. The
selectivity (wt.%) for a product is expressed as the
amount of the product divided by the amount of total
products and multiplied by 100.
3.2. Duration of the run
3
. Results and discussion
Fig. 1 shows the time dependence of the conversion
3
.1. Effect of various catalysts
of ACP (wt.%) over SZ, ST, H-beta and AlCl3 under
similar conditions. H-beta showed a lower initial activ-
The catalytic activity, rate of ACP conversion and
ity compared to SZ and AlCl , because of diffusional
3
selectivity to dypnone formation with different cata-
lysts, under similar reaction conditions, are given in
Table 2. The results with AlCl3 are also included for
comparison. As seen from Table 2, trans and cis dyp-
none together with ketene are formed in the condensa-
tion of ACP. The aldol condensation of ACP produces
dypnone as a major and primary product while ketene
is a secondary product.
As can be seen from Table 2, SZ and zeolite H-beta
exhibit similar and higher activity compared to the
ST. However, the selectivity for dypnone over all cata-
lysts remains nearly constant. The conversion of ACP,
rate of ACP conversion and selectivity for dypnone
over SZ, ST and H-beta are found to be 40.1, 28.5,
resistance in H-beta. However, after 2 h of reaction
time H-beta performed better than AlCl . The lower
3
activity of AlCl after 2 h may be attributed to the
3
amount of AlCl (in relation to ACP) used in the re-
3
action, which was less than the stoichiometric amount
normally required. The ST catalyst was considerably
less active than H-beta and SZ due to its lower acidity
and weaker acid sites (Table 1). Based on this conver-
sion of ACP after 1 h of reaction time, positive trend
in activities for the catalyst SZ was studied varying
other reaction parameters. Fig. 2 shows the conversion
of ACP and product distribution over SZ with the in-
crease in reaction time up to 10 h. Reaction conditions
were detailed in Table 2. It can also be seen from the
Table 2
Condensation of ACP
a
Catalyst
Conversion of
ACP (wt.%)
Rate of ACP conversion
(mmol g
Product distribution (wt.%)
b
−1 −1 c
h )
Dypnone
Othersd
2
−
−
SO4 /ZrO2
40.1
28.5
40.8
36.1
6.7
4.8
6.8
86
86
85
84
14
14
15
16
2
SO4 /TiO2
H-beta
AlCl3
e
180.5
a
Reaction conditions: catalyst (g) = 0.5; acetophenone (mol) = 0.042; reaction temperature (K) = 433; reaction time (h) = 5.
b
c
d
e
Conversion of ACP is defined as the percentage of ACP transformed into product; ACP: acetophenone.
−1 −1
) is expressed as the ACP converted per hour per gram of the catalyst.
Rate of ACP conversion (mmol g
cis form of dypnone and ketenes.
Reaction time (h) = 0.16.
h

C. Venkatesan, A.P. Singh / Journal of Molecular Catalysis A: Chemical 181 (2002) 179–187
183
Fig. 1. Conversion of ACP (wt.%) vs reaction time (h) over various catalysts (see footnotes of Table 2 for the reaction conditions).
results that there is a steady increase in the conversion
of ACP and the yield of dypnone, which level off after
3.3. Influence of catalyst/ACP ratio on ACP
conversion vs reaction time
3
h of reaction time. The result shows that the reaction
time influenced the conversion of ACP and product
yields in the condensation of ACP.
Fig. 3 compares the conversion of ACP as a func-
tion of reaction time using different concentrations of
Fig. 2. Conversion of ACP (wt.%) and product distribution vs reaction time (h) over SZ catalyst.

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C. Venkatesan, A.P. Singh / Journal of Molecular Catalysis A: Chemical 181 (2002) 179–187
Fig. 3. Influence of catalyst (SZ) concentration (SZ/ACP ratio by wt.) in the conversion of ACP (wt.%) with reaction time (h) (see footnotes
of Table 2 for the reaction conditions).
catalyst under similar conditions as those depicted in
Table 2. The conversion of ACP is found to increase
markedly up to 2 h of reaction time with the increase
in SZ/ACP ratio from 0.05 to 0.15. However, for
longer reaction time (5 h), the conversion of ACP is
similar at all catalyst concentrations. A negligible,
lower conversion of ACP (wt.%) is observed in the
absence of catalyst. Fig. 4 shows the effect of SZ
(catalyst)/ACP ratio (wt./wt.) on the conversion of
ACP, rate of ACP conversion and product distribution.
Fig. 4. Effect of SZ/ACP ratio (wt./wt.) on the ACP conversion, rate and product distribution; reaction time (h) = 1 (see footnotes of
Table 2 for the other conditions).

C. Venkatesan, A.P. Singh / Journal of Molecular Catalysis A: Chemical 181 (2002) 179–187
185
The catalyst/ACP ratio was varied from 0.05 to 0.15
by keeping the concentration of ACP constant. The
total surface area available for the reaction depends
on the catalyst loading. It was found that with an
increase in catalyst/ACP ratio the conversion of
ACP increases linearly from 17.2 to 30.7 wt.% for
the reaction time of 1 h, because of the increase in
the total number of acid sites available for the re-
action. The corresponding rate decreases from 28.7
ature studied. The effect of reaction temperature on
the conversion of ACP, rate of ACP conversion and
product distribution is investigated in the range of
403–443 K using the SZ as catalyst (Fig. 6). The con-
version of ACP and rate of ACP conversion increases
−
1
−1
from 17.1 to 40.1 wt.% and 2.8 to 6.7 mmol g
h ,
respectively, when the temperature is raised from 403
to 433 K. However, the selectivity for dypnone re-
mains nearly constant as shown in Fig. 5. A maximum
in dypnone selectivity (87 wt.%) over this catalyst is
observed at 403 K. The apparent activation energy for
the condensation reaction over SZ is estimated to be
−
1
−1
to 17.1 mmol g
h , respectively. No change in
the dypnone selectivity is observed with the change
in catalyst/ACP ratio. These results indicate the
need of a catalyst for decreasing the activation en-
ergy barrier in the condensation of ACP to dyp-
none.
−
1
40.8 kJ mol
.
3.5. Catalyst recycling
3
.4. Influence of reaction temperature on ACP
SZ sample used in the condensation of ACP was
recycled two times (fresh+two cycles). Fig. 7 presents
the results of these experiments. The catalyst shows
a marginal decrease of the activity after the use of
fresh catalyst. However, after first recycle of the cat-
alyst, a minor decrease in the catalytic activity was
noticed without losing its selectivity for dypnone. The
present study indicates that the catalyst can be recy-
cled a number of times without losing its activity to a
greater extent.
conversion vs reaction time
Fig. 5 shows the effect of the reaction temperature
on the conversion of ACP vs reaction time during the
condensation of ACP. The reaction temperature was
varied between 403 and 433 K. Increasing the reaction
temperature increases the catalytic activity sharply up
to 3 h of reaction time and the conversion of ACP
levels off with reaction time at all reaction temper-
Fig. 5. Influence of reaction temperature (K) on the conversion of ACP (wt.%) over SZ catalyst with reaction time (h) (see footnotes of
Table 2 for the reaction conditions).

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C. Venkatesan, A.P. Singh / Journal of Molecular Catalysis A: Chemical 181 (2002) 179–187
Fig. 6. Effect of reaction temperature (K) on the conversion of ACP (wt.%), rate of ACP conversion (mmol g 1
−
−1
h ) and product distribution,
over SZ catalyst.
Fig. 7. Effect of recycling of catalyst (SZ); reaction conditions: reaction time (h) = 1 (see footnotes of Table 2 for the other conditions).

C. Venkatesan, A.P. Singh / Journal of Molecular Catalysis A: Chemical 181 (2002) 179–187
187
3
.6. Mechanism
4. Conclusions
The formation of dypnone takes place when one
In summary, SZ and zeolite H-beta can catalyze
the condensation of ACP to dypnone efficiently and
selectively. ST is found to be less active. A higher
yield of dypnone is obtained by increasing the catalyst
(SZ) concentration and reaction temperature from 403
to 433 K.
of ACP molecule gets protonated and forms its con-
jugate acid (Eq. (1)). The electrophilic addition of
the carbonium ion (conjugate acid) to the enol form
of another molecule produces aldol (Eq. (2)). Fur-
ther protonation of the alcoholic OH of the aldol and
elimination of water molecule forms a carbonium ion
+
intermediate (Eq. (3)). Loss of H from the ␣-C atom
results in the formation of ␣,␤-unsaturated ketone
Acknowledgements
(dypnone) (Eq. (4)).
The authors thank Dr. A.V. Ramaswamy for his
useful comments and discussions. CV thanks IFCPAR
for financial assistance.
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