KF on γ-alumina: An efficient catalyst for the aldol condensation to pseudoionones

Article abstract of DOI:10.1016/j.cattod.2010.10.043

KF/γ-alumina is an effective catalyst for the aldol condensation of citral with acetone. Even at a relatively low acetone/citral ratio of 1-10, the selectivity to pseudoionones was high, ranging between 82% and 97%. X-ray diffraction shows the presence of KF and K₃AlF₆ crystallographic phases on the supported catalyst. At KF loadings of 3-6.75 mmol g^-1, a pretreatment of the catalyst by heating to 450 °C was necessary to obtain an active catalyst. However, samples with KF loadings of 8.5 mmol g^-1 and higher were active even without pretreatment, facilitating handling and use of the catalysts. The development of (1 1 1) planes revealed by in situ XRD during the thermal activation process correlates with the activity for aldol condensation.

Full text of DOI:10.1016/j.cattod.2010.10.043

Catalysis Today 164 (2011) 139–142
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
KF on ␥-alumina: An efficient catalyst for the aldol condensation to
pseudoionones
∗
Vadikukarasi Raju, Rajitha Radhakrishnan, Stephan Jaenicke, Gaik Khuan Chuah
Department of Chemistry, National University of Singapore, Singapore 117543, Republic of Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 13 November 2010
KF/␥-alumina is an effective catalyst for the aldol condensation of citral with acetone. Even at a relatively
low acetone/citral ratio of 1–10, the selectivity to pseudoionones was high, ranging between 82% and 97%.
X-ray diffraction shows the presence of KF and K3AlF6 crystallographic phases on the supported catalyst.
Keywords:
Base catalyst
Alumina-supported KF
Aldol condensation
Pseudoionone formation
At KF loadings of 3–6.75 mmol g−1, a pretreatment of the catalyst by heating to 450 ◦C was necessary
−1
to obtain an active catalyst. However, samples with KF loadings of 8.5 mmol g and higher were active
even without pretreatment, facilitating handling and use of the catalysts. The development of (1 1 1)
planes revealed by in situ XRD during the thermal activation process correlates with the activity for aldol
condensation.
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
These sites are proposed to be responsible for the abstraction of an
-proton from the acetone molecule, forming a carbanion interme-
␣
The aldol condensation of citral with acetone gives pseu-
diate in the rate-determining step.
doionones which are important as precursors for vitamin A and
fragrances (Scheme 1). Industrially, this reaction is catalyzed by
sodium hydroxide with typical pseudoionone yields of 60–80% [1].
The yield is reduced by various side reactions arising from the
self-condensation of acetone and of citral and by secondary reac-
tions of the pseudoionones, so that further purification becomes
necessary. The substitution of homogeneous bases by solid cata-
lysts is highly desirable as the use of aqueous alkali leads to large
amounts of wastes and fast corrosion of the reactor. Solid base
catalysts such as alkaline earth metal oxides, modified alumina,
clays and hydrotalcites have been studied for aldol condensation
reactions [2–4]. Climent et al. [4] reported good activity over pure
Alumina-supported KF has been extensively used as a base
catalyst for many organic reactions such as aldol and Michael con-
densations, carbon–carbon and carbon–oxygen bond formation,
and elimination reactions [5–8]. It is easily prepared and is also
available commercially. Despite its wide applications, the nature
of the active sites on the catalyst is still unknown. The interac-
tion of alumina with KF leads to the formation of K AlF6 and KOH
3
according to the following equation:
1
2KF + Al O + 3H O → 2K AlF + 6KOH
2
3
2
3
6
Using 19F MAS NMR, Kabashima et al. [9] identified several F-
containing species including KF, KF associated with water, K AlF6
3
◦
MgO for the liquid phase condensation at 60 C, but the selectiv-
and AlF . The dominant species was K AlF but its concentration
3
3
6
ity to pseudoionones was only about 68% at an acetone/citral ratio
of 2.7. Calcined hydrotalcites containing aluminium and magne-
sium were more active and showed a higher selectivity than MgO.
By raising the acetone/citral ratio from 2.7 to 19, the selectivity to
pseudoionones could be increased from 79% to 99%. The controlled
addition of water to the calcined hydrotalcite further improved the
rate of reaction. Magnesium oxide doped with 0.5 wt% Li formed an
did not correlate with the catalytic activity for the Tishchenko
reaction of pivaldehyde or the double bond isomerization of 1-
pentene. Instead, the activities were linked to a fluoride-containing
species with a peak at −150 ppm. The hydroxide ion was ruled out
as the active species because the activity of KOH/alumina for the
Tishchenko reaction was lower than that of KF/alumina. Clacens
et al. [10] found that KF supported on ␣-alumina is a stronger base
than KF/␥-alumina. Despite the much lower surface areas of the ␣-
◦
active catalyst which yielded 93% pseudoionones at 80 C; however,
a rather high acetone:citral ratio of 49 was used [3]. The presence
of lithium has been correlated with an increase in the density of
strongly basic sites in the form of low-coordination oxygen anions.
2
alumina supported KF (4.8–9.4 m /g) as compared to the ␥-alumina
samples, the samples were more active for Michael reactions. Acti-
vation of the catalysts could be carried out by simply drying at oven
temperature instead of high temperature calcination. Using XRD
and NMR results, the authors proposed that on ␣-alumina, KF can
◦
∗ Corresponding author. Tel.: +65 6516 2839; fax: +65 6779 1691.
E-mail address: chmcgk@nus.edu.sg (G.K. Chuah).
be formed at low temperatures (120 C) by reaction of a liquid-
−
+
like F -species which then reacts with adsorbed K . However, on
0
920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.10.043

1
40
V. Raju et al. / Catalysis Today 164 (2011) 139–142
OH
O
O
O
base
-H O
2
+
CHO
pseudoionone (cis and trans)
Scheme 1. Cross-aldol condensation between citral and acetone to pseudoionones.
Table 1
␥
-alumina, KF reacts completely with the support forming K AlF
3 6
◦
Textural properties of alumina-supported KF.
and activation at 450 C is necessary to decompose K AlF to the
active KF phase.
3
6
Sample (mmol KF g−1)
Surface area (m /g)
2
Pore volume
3
(
cm /g)
In this work, we investigated the use of KF/␥-alumina in the
aldol condensation of citral with acetone. In situ powder X-ray
diffractometry was used to follow any phase changes on heating
in vacuo.
␥-Aluminaa
3 KF/␥-alumina
111
0.21
0.14
0.11
0.09
0.08
0.08
0.07
0.07
0.05
0.06
67.1
45.7
33.2
27.9
28.8
26.6
26.4
19.8
16.1
6
8
8
.75 KF/␥-alumina
.5 KF/␥-alumina
.5 KF/␥-alumina
a
2
. Experimental
10 KF/␥-alumina
a
1
1
1
0 KF/␥-alumina
2.5 KF/␥-alumina
a
2.1. Preparation of supported KF catalyst
a
5 KF/␥-alumina
a
Fluka
Supported KF on ␥-alumina with loadings of 3–15 mmol KF/g
was prepared by the wet impregnation method. ␥-Alumina was
a
◦
◦
Degassed at 300 C under N2 flow for 4 h; rest degassed at 100 C.
added to a solution of KF in water and the suspension was stirred
at room temperature for 2 h. Water was evaporated at 60 C under
◦
The surface area also decreased slightly when the sample was pre-
treated to higher temperatures. For example, the surface area of
reduced pressure using a rotary evaporator. Teflon apparatus was
used throughout the preparation as KF attacks laboratory glass. The
− −1
1
2
8
.5 mmol g KF/␥-alumina, was 33.2 m g when the sample was
◦
2
−
1
◦
degassed at 100 C but decreased to 26.6 m g after degassing at
3
catalyst was dried at 100 C overnight, and pretreated in vacuum
◦
◦
00 C. It appears that degassing at 300 C can already cause some
at the desired temperature before testing for any catalytic activity.
A commercial sample KF/alumina (Fluka, 5.5 mmol KF/g) was also
tested for comparison. Nitrogen adsorption–desorption isotherms
were measured using a Micromeritics Tristar 3000. X-ray diffrac-
tion measurements were carried out using a Siemens D5005 as
well as a Bruker D8 equipped with a high temperature stage. In
both instruments, a Cu anode was used. In situ X-ray patterns were
sintering of the material.
3.2. Aldol condensation of citral and acetone
A commercial KF/␥-alumina (Fluka, 5.5 mmol g 1) was initially
−
used as catalyst for the formation of pseudoionones. The catalyst
◦
◦
◦
was pretreated in vacuum at 450 C. The influence of the reaction
recorded as the sample was heated from 100 C to 700 C under
evacuation.
temperature was investigated using an acetone/citral molar ratio
◦
of 2.5 (Table 2). At 25 C, a conversion of 77% with a selectivity
of 93% to pseudoionones was obtained after 6 h. With increase of
the reaction temperature to 50 C, the selectivity to pseudoionones
2.2. Catalytic measurements
◦
decreased slightly to 89%. Other products were due to the self-
condensation of citral and of acetone. Despite the excess acetone
used, mesityl oxide constituted only 0.8% of the reaction mixture.
From the temperature dependence of the reaction rate, an apparent
Commercial citral consisting of a mixture of cis and trans isomers
35:65) was used without purification. Typically, citral (7.6 mmol),
acetone (45 mmol) and nitrobenzene (10 ␮l) as internal standard
(
were added to a two-necked round-bottomed flask equipped with
−
1
activation energy of 32 kJ mol was determined. This is compara-
◦
a condenser system. The solution was heated under stirring to 45 C
−
1
ble to the 25 kJ mol
reported for hydrotalcites, where the low
in nitrogen before addition of the catalyst (0.25 g). Aliquots were
taken at regular time intervals and analyzed by gas chromatog-
raphy using a FID detector and HP-5 column. The products were
identified by GC–MS (Shimadzu). The effective carbon number was
calculated for the individual compounds and taken into account
for calculating conversion and selectivity. The effect of varying
acetone/citral was carried out by keeping the amount of citral at
7
.6 mmol and varying the amount of acetone.
3
. Results and discussion
3.1. Textural properties
2
−1
␥
-Alumina, has a surface area of 111 m g (Table 1). Increas-
−1
ing the KF loading from 3 to 15 mmol g alumina resulted in lower
surface areas. The pore size distribution curve for ␥-alumina falls
in the range of 2.7–10 nm (Fig. 1). After loading with KF, there was
a shift to bigger pores along with a decreased pore volume. This
shows that the smaller pores become blocked upon impregnation.
−1
−1
Fig. 1. Pore size distribution of (ꢀ) ␥-alumina (ꢁ) 3 mmol g (᭹) 8.5 mmol g and
(ꢀ) 10 mmol g KF/␥-alumina.
−
1

V. Raju et al. / Catalysis Today 164 (2011) 139–142
141
Table 2
Influence of reaction temperature on the formation of pseudoionones.
Table 4
Effect of pretreatment on activity for pseudoionone formation.
a
Temperature
Time (h)
Conversion
(%)
Selectivity
(%)
Initial rate
KF loading
Time (h)
Conversion
(%)
Selectivity
(%)
Initial rate
(mmol g min
◦
−1
−1
−1
−1
−1
(
C)
(mmol g min
)
(mmol g
)
)
2
3
4
5
5
6
6
6
6
77
83
90
94
93
93
91
89
0.41
0.40
0.58
1.26
6.75a
6.75
1
1
0.5
0.5
0.5
0.5
53
80
81
83
82
85
94
94
94
95
93
94
0.36
0.53
1.70
1.75
2.2
b
0
0
0
a
8.5
b
8.5
a
1
1
0
0
Reaction conditions: citral (7.6 mmol), acetone (19.1 mmol), Fluka–KF/alumina
0.22 g).
a
b
2.1
(
◦
With respect to citral consumed.
Reaction conditions: citral 7.6 mmol, acetone (45 mmol), catalyst (0.25 g), 45 C.
a
◦
Dried at 100 C for 4 h.
Pretreatment at 450 C under vacuum for 2 h.
b
◦
value was attributed to strong adsorption of the reagents on the
catalyst surface [11].
of 8.5 and 10 mmol g 1, as-synthesized samples showed already
−
To assess the influence of the reagent concentrations on the
selectivity to pseudoionones, the acetone/citral ratio was varied
◦
comparable activity to those pretreated to 450 C. The selectiv-
ity to pseudoionones was ∼94–95%, with little difference between
◦
from 1 to 10 at a reaction temperature of 40 C (Table 3). The rate
untreated and pretreated catalysts. In contrast, hydrotalcites need
of reaction was lowest for an acetone/citral ratio of 1. After 6 h, the
conversion was 62% and the selectivity to pseudoionone was 82%.
At this high citral concentration, self-condensation of citral is sig-
nificant. The reaction was faster when the acetone/citral ratio was
increased. Using a ratio of 6–10, the selectivity to pseudoionone was
to be carefully rehydrated under CO -free conditions or to have
2
an optimum amount of water added to the reaction medium for
better activity [11,13]. Climent et al. [4] reported that the ini-
tial rate of pseudoionone formation over Al–Mg hydrotalcite was
.87–0.92 mmol g min for acetone/citral ratio of 5–20. These
values are lower than those found for KF/␥-alumina in this work
Table 3).
The leaching of KF was investigated by hot filtration of a
.5 mmol g KF/␥-alumina catalyst from the reaction mixture after
0 min. No further conversion was observed in the catalyst-free
reaction mixture. Furthermore, the catalyst could be reused for
subsequent batch reactions after recalcining at 450 C.
The dependence of the catalytic activity on the pretreatment
conditions has been reported previously. Hattori and co-workers
7,9] observed that the catalytic activity of KF/alumina for the iso-
merisation of pent-1-ene, Michael addition of nitromethane to
buten-2-one and Tischenko reaction of pivalaldehyde reached a
−
1
−1
0
9
5–97% and the self-condensation of citral was greatly reduced.
These results show that KF/␥-alumina is an effective catalyst for
the aldol condensation of citral and acetone. The yield of pseu-
doionones is high even when using a moderate excess of acetone.
In comparison, Roelofs et al. [12,13] required an extremely high
acetone/citral mole ratio close to 250 to obtain a pseudoionone
selectivity of 90% over activated hydrotalcite catalysts. In this case,
(
−
1
8
1
◦
◦
the reaction was conducted at 0 C using 1 wt% of citral. After
2
4 h, the citral conversion reached 65%. However, no reaction was
observed when the acetone/citral ratio was reduced to 20. The
authors postulated that the inhibition of the reaction could be due
to strong adsorption of citral on the catalyst surface. This trend is
also observed in our data: the initial rate increases 10-fold when
the concentration of citral is reduced by a factor of 3.3 (Table 3).
The use of KF/␥-alumina for aldol condensation of methylpseu-
doionones from citral and butanone has been reported previously
[
◦
maximum when the catalyst had been pretreated at 450 C. The
authors proposed that the increase in activity with pretreatment
temperature up to 450 C is caused by the removal of adsorbates
such as water and carbon dioxide which initially cover the basic
◦
[
14]. However, the selectivity was only 50% as oligomers of cit-
◦
sites, while the decrease after heating above 450 C may be due
ral were formed. It was postulated that despite excess butanone,
citral could not be effectively desorbed from the surface of the cat-
alyst. In this study, the high selectivity to pseudoionones obtained
may be attributed to the higher polarity of acetone as compared
to butanone. Hence, the surface concentration of citral is kept low,
thus minimizing self-condensation of citral.
to structural changes in the surface leading to elimination of basic
sites. Some hints towards the active species came from 19F NMR,
where a signal at about −150 ppm was observed whose inten-
−
1
At KF loadings of 3–5 mmol g
KF, the conversion was <10%
◦
even after pretreatment at 450 C in vacuum. The activity for cross-
aldol condensation of citral and acetone increased with KF loading.
The conversion was 53% after 1 h over a 6.75 mmol g
alumina sample that had dried at 100 C (Table 4). Pretreatment at
a higher temperature, 450 C, produced a more active catalyst that
−
1
KF/␥-
◦
◦
was able to convert 80% citral after the same time. At KF loadings
Table 3
Influence of acetone/citral ratio on the aldol condensation of citral and acetone.
Acetone/citral
ratio
Time (h)
Conversion
(%)
Selectivity
(%)
Initial rate
(mmol g min )
−
1
−1
1
6
6
6
6
6
2
2
62
85
87
88
98
98
99
82
87
89
90
91
95
97
0.20
0.48
0.52
0.61
1.27
1.81
2.00
1
2
3
4
6
0
.5
.3
1
Reaction conditions: citral (7.6 mmol), acetone (7.6–76 mmol), Fluka–KF/alumina
−1
◦
◦
Fig. 2. X-ray diffractograms of 6.75 mmol g KF/␥-alumina at (a) 100 C, (b) 200 C,
(
◦
(
0.22 g), 40 C.
◦
◦
◦
◦
◦
c) 300 C, (d) 400 C, (e) 450 C, (f) 500 C and (g) 600 C. (ꢁ) KF (ꢀ) K3AlF6 (ꢂ) Al2O3.

1
42
V. Raju et al. / Catalysis Today 164 (2011) 139–142
◦
◦
the most intense peaks of KF (2Â ∼33.6 and ∼48.1 ) and K AlF
3
6
◦
◦
◦
(
2Â ∼29.7 , ∼36.6 and ∼42.5 ) can still be observed. These peaks
◦
were no longer observable on further heating to 600 C. As sam-
◦
ples treated to 500 C were no longer catalytically active, neither
K AlF nor the exposed (2 0 0) and (2 2 0) planes of KF can be the
3
6
−
1
active phases. For the 5 mmol g sample, the (1 1 1) peak was very
small at 100 C and not observed after heating to 450 C (Fig. 4).
◦
◦
−
1
In contrast, for 10 mmol g KF/␥-alumina, this peak is observed
already at room temperature. The correlation with conversion mea-
surements suggests that the (1 1 1) planes of KF is responsible for
the catalytic activity in the aldol condensation. With high KF load-
ings, more of these planes are present from the beginning, while
at low KF loading, decomposition of the K AlF phase during pre-
3
6
◦
treatment at 450 C may give rise to the formation of KF with more
1 1 1) planes exposed.
Fig. 3. Ratio of KF(1 1 1)/KF(2 0 0) peak intensity as a function of temperature.
(
4. Conclusions
␥
-Alumina-supported KF is an active catalyst in the cross-aldol
condensation between citral and acetone. The selectivity to pseu-
doionones is >93% for acetone/citral ratios above 6. For KF loadings
−
1
◦
below 6.75 mmol g , pretreatment at 450 C resulted in a signif-
icant increase of the activity compared to the untreated sample.
−
1
For KF loading above 8.5 mmol g , the activity of the pretreated
catalyst was similar to that of the untreated catalyst. The catalytic
activity was correlated with the appearance of a strong KF (1 1 1)
reflection in the X-ray diffractogram. Considering the ease of prepa-
ration, handling, good activity and selectivity coupled with the lack
of leaching, KF/␥-alumina is an effective catalyst in the synthesis of
pseudoionones.
Acknowledgement
Financial support from National University of Singapore under
grant number R-143–000–418–112 is gratefully acknowledged.
−
1
◦
◦
Fig. 4. X-ray diffractograms of 5 mmol g KF/␥-alumina at (a) 100 C, (b) 450 C and
1
References
−1
◦
◦
0 mmol g KF/␥-alumina at (c) 100 C and (d) 400 C. (ꢁ) KF (ꢀ) K3AlF6 (ꢂ) Al2O3.
[
[
1] K. Bauer, D. Garbe, H. Surburg, Common Fragrance and Flavors Materials, 5th
ed., Wiley-VCH, Weinheim, 2006.
sity correlated with the activity for double bond isomerisation and
Tishchenko reaction.
2] S. Abelló, S. Dhir, G. Colet, J. Pérez-Ramirez, Appl. Catal.
12.
[3] V.K. Diez, J.I.Di. Cosimo, C.R. Apesteguia, J. Catal. 240 (2006) 235.
[4] M.J. Climent, A. Corma, S. Iborra, A. Velty, Catal. Lett. 79 (2002) 157.
A 325 (2007)
2
In order to elucidate the nature of the activity for aldol con-
densation, we recorded the powder X-ray diffractograms in situ
during heat treatment and determined the phases present on the
alumina-supported KF samples as the temperature was raised. For
[
[
[
5] J.H. Clark, D.J. Macquarrie, Org. Process Res. Dev. 1 (1997) 149.
6] Y. Ono, T. Baba, Catal. Today 38 (1997) 321.
7] H. Hattori, J. Jpn. Petrol. Inst. 47 (2004) 67.
−1
6
.75 mol g KF/␥-alumina (Fig. 2), both KF (ICCD 00-036-1458) and
[8] B. Basu, P. Das, S. Das, Curr. Org. Chem. 12 (2008) 141.
9] H. Kabashima, H. Tsuji, S. Nakata, Y. Tanaka, H. Hattori, Appl. Catal. A 194–195
2000) 227.
[
K AlF (ICCD 00-003-0615) phases could be observed in the as-
3
6
(
synthesized sample. The presence of K AlF6 suggests that the KF
3
[10] J.-M. Clacens, D. Genuit, L. Delmotte, A. Garcia-Ruiz, G. Bergeret, R. Montiel, J.
Lopez, F. Figueras, J. Catal. 221 (2004) 483.
[11] S. Abelló, F. Medina, D. Tichit, J. Pérez-Ramírez, J.C. Groen, J.E. Sueiras, P. Salagre,
Y. Cesteros, Chem. Eur. J. 11 (2005) 728.
12] J.C.A.A. Roelofs, A.J. van Dillen, K.P. de Jong, Catal. Lett. 74 (2001) 91.
[13] J.C.A.A. Roelofs, A.J. van Dillen, K.P. de Jong, Catal. Today 60 (2000) 297.
[14] M.J. Climent, A. Corma, S. Iborra, A. Velty, Green Chem. 4 (2002) 474.
interacts strongly with the alumina support. The presence of the
◦
◦
◦
KF phase is indicated by peaks at 2Â ∼28.9 , 33.6 and 48.1 , corre-
sponding to diffraction from the (1 1 1), (2 0 0), and (2 2 0) planes,
respectively. Upon heating, the (1 1 1) peak increased in intensity
[
◦
◦
and reached a maximum at 400–450 C (Fig. 3). At 500 C, only