Catalytic Reaction of Para-Isopropylbenzaldehyde with ...

Catalytic Reaction of Para-Isopropylbenzaldehyde with Propionaldehyde
over Solid Base Catalysts
Shin Yi Shiau ( ) and An-Nan Ko* ( )
Department of Chemistry, Tunghai University, Taichung, Taiwan, R.O.C.
3-(4-Isopropyl-phenyl)-2-methyl-acrylaldehyde (4IP2MA) was synthesized from propionaldehyde
(PA) and para-isopropylbenzaldehyde (IPB) via cross aldol condensation over alkaline earth metal oxides
or Al2O3-MgO mixed oxides (AM) at 120-190 C by using a batch reactor. The catalytic activity is enhanced
with the base amounts of both alkaline earth metal oxides and the mixed oxides. The reaction system
is free of diffusional limitation at a stirring speed larger than 600 rpm. Increasing the catalyst weight,
reaction temperature, reaction time and IPB/PA mol ratio enhances the PA conversion, whereas the
4IP2MA selectivity remains small change except for IPB/PA ratio. Favorable results occur on the PA conversion
and the 4IP2MA selectivity for the reaction in nonpolar solvents of cyclohexane and benzene.
Reused catalysts of MgO and AM exhibit no catalyst decay. For the formation of 4IP2MA, the reaction
mechanism is proposed and the apparent activation energy is calculated.
Keywords: 3-(4-Isopropyl-phenyl)-2-methyl-acryaldehyde; Hydrotalcite; Alkaline earth metal oxide;
Para-Isopropylbenzaldehyde; Propionaldehyde.
INTRODUCTION
Although extensive studies have focused on heterogeneous
acid catalysis, the application of base catalysts has
received much less attention. However, an increasing interest
in this field has become important during the past two
decades. Many types of reactions are base-catalyzed, viz.
alkylation, dehydrogenation, hydrogenation, isomerization,
and aldol condensation. 1-4 Aldol condensation may
occur via self-condensation between two of the same aldehyde
or ketone molecules or via cross-condensation between
two molecules of different aldehydes or ketones. 5-11
-Pentyl cinnamaldehyde was synthesized from benzaldehyde
and n-heptaldehyde using solid base catalysts such as
anionic exchange resins, 12 solid-liquid phase transfer catalysts,
13 organic-inorganic hybride catalysts and MCM-41
supported metal oxides. 14,15 With the MCM-41 material
containing strong hindered amine base, catalyst decay occurred
due to loss of the base at higher reaction temperatures
and poisoning of strong basic sites. For this reaction
over magnesium oxide supported on Al-MCM-41, increasing
the deposited amount of MgO on Al-MCM-41 enhances
the catalyst base amount but decreases the surface area.
Journal of the Chinese Chemical Society, 2006, 53, 1539-1545 1539
Dedicated to the memory of the late Professor Ho Tong-Ing.
* Corresponding author. Fax: +886-4-23590426; E-mail: anko@thu.edu.tw
The catalytic activity of Al-MCM-41 is greatly enhanced
by the deposition of MgO. 15
Hydrotalcite-like compounds, being the precursors
for mixed oxides have recently drawn much attention due
to their potential applications in the field of adsorbants, anion
exchangers and catalysts. Hydrotalcites are layered
II III X
double hydroxides of the general formula[ M1XMX( OH)
2]
[AX/n] n- where M II and M III are di- and trivalent cations, respectively,
in the octahedral positions within the hydroxide
layers, and A n is the anion. 16 Hydrotalcites and their derived
mixed metal oxides exhibit characteristic structure
and surface properties. Accordingly, they have been used in
catalytic reactions such as dehydration, alkylation and
aldol condensation. 17-22
Cyclamen aldehyde is a raw material for the manufacture
of perfumes, flavors and fine chemicals. 23 This
compound is produced by hydrogenation of 3-(4-isopropyl-phenyl)-2-methyl-acryaldehyde
(4IP2MA) and is traditionally
made from propionaldehyde (PA) and para-isopropylbenzaldehyde
(IPB) in the liquid phase by using sodium
or potassium hydroxide as catalysts. However, such a
process suffers from separation procedures, corrosion hazards,
and environmental problems due to the use of alkali

1540 J. Chin. Chem. Soc., Vol. 53, No. 6, 2006 Shiau and Ko
metal hydroxide. In this study, solid bases such as alkaline
earth metal oxides and Al2O3-MgO mixed oxides were utilized
as catalysts for this synthetic reaction. The effects of
various factors, viz. the stirring speed, the solvent, catalyst
weight, catalyst type, para-isopropylbenzaldehyde/propionaldehyde
molar ratio (IPB/PA), reaction temperature,
and reaction time on the catalytic results were investigated.
In addition, the catalytic results were correlated to the catalyst
properties. The reaction mechanism and the activation
energy for the formation of 4IP2MA was proposed and calculated.
EXPERIMENTAL SECTION
Catalyst Preparation
Both samples of MgO (Strem, 99.95%) and CaO
(R.D.H., 96%) were calcined at 600 C for 6 h prior to the
catalytic reactions. SrO and BaO were prepared, respectively,
from SrCO3 (AJX, 96%) and BaCO3 (R.D.H., 99%)
via calcinations at 900 C for6h.Al2O3-MgO mixed oxides
AMXY with Al to Mg molar ratio X:Y were prepared
by co-precipitation of two mixtures of aqueous solutions.
24,25 To prepare AM21, as an example, 64.0 g
Al(NO)39H2O (R.D.H., 99%) and 21.8 g Mg(NO3)26H2O
(R.D.H., 99%) were dissolved in 500 mL de-ionized water
to form solution A. Solution B was prepared by dissolving
40.0 g NaOH and 21.2 g Na2CO3 in 500 mL de-ionized water.
Solutions A and B were added dropwise into a beaker
containing 300 mL de-ionized water. The pH value of the
resulting solution was 9~10. The solution was kept at 60 C
for 18 h and then was filtered. The precipitate was purified,
dried and calcined at 450 Cfor8h.
Catalyst Characterization
The powder X-ray diffraction patterns of various catalysts
were measured with an X-ray diffractometer using
CuK radiation in the angle range 2 =2~80 (Shimadzu
XD-5). The BET surface areas of various samples were determined
with a gas sorption analyzer (Quantachrome
Quantasorb). The catalyst basicity was obtained by temperature-programmed
desorption of carbon dioxide. 26 Prior to
adsorption of carbon dioxide, 0.1 g of catalyst was heated
under helium flow (40 mL/min) with a rate of 10 C /min
from 110 to 500 C to remove water from the sample and
then cooled to room temperature. Measured pulses of carbon
dioxide (pulse volume, 0.5 mL) were injected into he-
lium gas and carried through the catalyst sample until saturated
adsorption. Then the temperature was increased to
110 C and kept there for 2 h to remove the physisorbed
carbon dioxide. Finally the system was heated from 110 to
500 C at10C per minute and kept at 500 C for 30 min.
The TPD diagrams were obtained by monitoring the desorbed
gas with a thermal conductivity detector at 100 C
and an electric current 160 mA. The base amounts were obtained
from the peak areas of various samples using a given
amount of CO2 as the calibration standard.
Catalytic Reaction
The catalyst (0.1~0.3 g) was added into a mixture of
PA (0.015~0.183 mol; Aldrich, 98%) and IPB (0.012~
0.092 mol; Aldrich, 98%). The solvents (35 mL) were benzene,
cyclohexane, tetrahydrofuran, and ethyl acetate. All
solvents were obtained from commercial sources with high
purity. The reaction was carried out at 120-190 C for a period
of 4hinastirred batch autoclave reactor (100 mL
EZE-Seal, Autoclave Engineering, Inc.). The reaction
products were collected periodically. Then they were identified
with a GC-MS (Hitachi M-52) and analyzed by a GC
(HP 5890 series II) using a flame-ionization detector and a
50 m × 0.2 mm PONA capillary column (Supelco).
RESULTS AND DISCUSSION
Fig. 1 shows the powder XRD spectra of various alkaline
earth metal oxides. The diffraction patterns reveal
characteristic structures of MgO, CaO, SrO and BaO. According
to our previous report, 25 the powder XRD spectra
of various AMXY mixed oxides are consistent with other
studies. 21,27 The spectrum of AM15 exhibits a structure
similar to that of MgO. As the Al/Mg mol ratio increased
from 1/5 to 1/2, the peaks characteristic of calcined hydrotalcites
appear at 2 =43 and 62. In addition, the peak intensities
are enhanced remarkably, indicating much better
crystallinity of AM12. Continuous increase of the Al/Mg
mol ratio results in the decrease of crystallinity as well as
the appearance of Al2O3 structure (2 =37, 45 and 66).
The crystal structures of AM61 and AM15 resemble those
of Al2O3 and MgO, respectively. Fig. 2 shows the temperature-programmed
desorption of carbon dioxide from various
AMXY samples. The base amount increases with the
mol ratio of Mg to Al.
Table 1 lists the physical properties and catalytic re-

Aldolization over Solid Base Catalysts J. Chin. Chem. Soc., Vol. 53, No. 6, 2006 1541
sults of various samples. The surface area of Al2O3 is apparently
larger than that of MgO. Consequently, AM61 and
AM15 exhibit, respectively, the largest and the smallest
surface areas among the AM samples. The base amount of
Fig. 1. Powder XRD patterns of alkaline earth metal
oxides. (a) MgO; (b) CaO; (c) SrO; (d) BaO.
Fig. 2. TPD of CO2 from various AMXY samples. XY:
(a) 15; (b) 12; (c) 21; (d) 31; (e) 61.
AMXY catalysts increases with decreasing the Al2O3/MgO
mol ratio. For alkaline earth metal oxides, the surface areas
follow the order of MgO CaO SrO BaO. The base
amount was reported to show a similar trend, whereas the
basic strength exhibited the reverse order. 28
In the reaction of IPB with PA, the products include
4IP2MA and 2-methyl-2-pentenal (2M2P). The chemical
equations are shown as follows:
The conversion, product selectivity and product yield are
defined according to the mol of PA converted. Therefore i
is 1 for 4IP2MA and 2 for 2M2P.
mol of PA converted
Conversion X (mol %) = 100%
mol of PA in the feed
i × mol of product j
Selectivity Sj of product j (mol %) = 100%
mol of PA converted
i × mol of product j
Yield Yj of product j (mol %) = 100%
mol of PA in the feed
The reaction system was shown to be free of diffusional
limitation at a stirring speed larger than 600 rpm. Accordingly,
the following experiments were performed at 700
rpm. Increasing the catalyst weight enhances significantly
both the PA conversion and the 4IP2MA selectivity. Fig. 3
shows the catalytic results over alkaline earth metal oxides.
The catalytic activity is consistent with both the catalyst
surface area and the base amount. However, the 4IP2MA
selectivity shows no regular trend. Fig. 4 shows the catalytic
results over various AMXY catalysts. The catalytic
activities diminish as follow: AM15 > AM12 > AM21 >
AM31 > AM61. Apparently, both the PA reaction rate and
the 4IP2MA formation rate are parallel to the catalyst base
amount as shown in Fig. 5. Similarly, the 4IP2MA selectivity
exhibits no apparent order. The catalytic results are
summarized in Table 1.
Fig. 6 shows the influence of reaction temperatures
on the catalytic results. The conversion and 4IP2MA selec-

1542 J. Chin. Chem. Soc., Vol. 53, No. 6, 2006 Shiau and Ko
Table 1. Catalyst physical properties and catalytic results a
Catalyst
Surface area
(m 2 /g)
tivity increase from 36.9% to 98.2% and 36.8% to 59.5%,
respectively, in the reaction over MgO for 2 h. As there are
two competitive reaction paths, i.e. self-aldolization of PA
and cross aldolization between PA and IPB, these results
Fig. 3. Catalytic results of PA conversion and 4IP2MA
selectivity over alkaline earth metal oxides.
Conditions: Catalyst, 0.3 g; 160 C; PA, 0.015
mol; IPB, 0.092 mol; solvent, 35 mL C6H6;()
MgO; () CaO; () SrO; () BaO.
Base amount PA conversion (mol%) 4IP2MA selectivity (mol%)
(mmol CO2/g) 1h 4h 1h 4h
MgO 56 n. d. 49.6 89.5 43.5 53.7
CaO 12 n. d. 29.9 75.3 23.6 30.1
SrO 1.2 n. d. 8.0 22.6 28.3 31.8
BaO < 1.0 n. d. 5.5 19.3 30.4 41.3
AM15 186 0.173 64.9 98.1 47.8 58.3
AM12 203 0.167 47.8 93.1 39.7 51.9
AM21 236 0.124 28.7 76.5 46.9 55.5
AM31 258 0.105 13.7 58.3 39.3 49.3
AM61 264 0.094 6.6 31.9 46.4 54.3
a
Reaction conditions: PA 0.015 mol; IPB 0.092 mol; catalyst 0.3 g; solvent benzene 35 mL; 160 C; ambient
pressure.
imply favorable formation of 4IP2MA at higher temperatures
and thus a higher activation energy for cross aldolization.
To study the effect of reactant ratios, a decrease of the
mol ratio PA/IPB from 15 to 1/6 increases the conversion
significantly and enhances the 4IP2MA selectivity dramat-
Fig. 4. Catalytic results of PA conversion and 4IP2MA
selectivity over various AMXY catalysts. Conditions:
the same as in Fig. 3. XY: () 15; ()
12; () 21; (+) 31; (×) 61.

Aldolization over Solid Base Catalysts J. Chin. Chem. Soc., Vol. 53, No. 6, 2006 1543
ically, because a lesser amount of PA enhances its extent of
contact with IPB molecules and suppresses its self-aldol
condensation.
Fig. 7 shows the effect of solvents. The nonpolar solvents
of benzene and cyclohexane exhibit better results in
both conversion and 4IP2MA selectivity. The reasons
might be explained as follows: (1) polar reactant molecules
of PA and IPB can diffuse more easily to the catalyst surface
among nonpolar solvents as compared to polar solvents;
(2) the polar solvent interacts more strongly with the
polar reactant that retards the aldol condensation; (3) the
solvation of the activated complex could also end in the
Fig. 5. Rate (first hour) as a function of the base
amount of AMXY catalysts. () PA reaction;
() 4IP2MA formation.
Fig. 6. Influence of the reaction temperature on catalytic
results. () PA conversion; () 4IP2MA
selectivity.
same results. As these effects are more pronounced with
PA than IPB reactant, the resulting selectivity trends are
observed. In order to investigate the performance of reused
catalysts, both MgO and AM15 were tested and no catalyst
decay after one run was found. Fig. 8 illustrates the proposed
reaction mechanism for the formation of 4IP2MA.
The catalyst basic site abstracts a proton from the -carbon
of PA to form carbanion (A), followed by attacking the carbon
of CO in IPB (step I) and then by dehydration to produce
4IP2MA (step II). A similar mechanism was reported
on the aldol condensation of acetaldehyde and heptanal. 29
Fig. 9 shows the Arrhenius plot of the formation rate of
4IP2MA. The calculated apparent activation energy Ea is
31.7 kJ/mol for the MgO catalyst.
CONCLUSIONS
The property of Al2O3-MgO catalysts (AMXY) depends
on the Al/Mg mol ratio (X/Y). For the AM15 and
Fig. 7. The effect of solvents on catalytic results. Conditions:
MgO, 0.3 g; others the same as in Fig.
3. () cyclohexane; () benzene; (+) tetrahydrofuran;
(O) ethyl acetate.

1544 J. Chin. Chem. Soc., Vol. 53, No. 6, 2006 Shiau and Ko
Fig. 8. The proposed reaction mechanism for the formation of 4IP2MA.
Fig. 9. The Arrhenius plot for the formation of 4IP2MA
from PA and IPB on the MgO catalyst. r, the
-1 1
formation rate of 4IP2MA (mmol h).
AM12 samples, the XRD patterns show the characteristic
structure of calcined hydrotalcites. In addition, AM12 exhibits
much better crystallinity. Further increase of the X/Y
ratio of AMXY results in the decrease of crystallinity as
well as the appearance of Al2O3 structure. The base amount
of AMXY materials decreases with increasing the Al/Mg
mol ratio, whereas the surface area exhibits the reverse
trend.
In the reaction of IPB with PA, 2M2P and 4IP2MA
were produced via self aldol condensation of PA and cross
aldol condensation between the two reactants. With alkaline
earth metal oxides, the catalytic activity is enhanced
with an increase of both the catalyst base amount and the
surface area. For catalytic reactions over AMXY catalysts,
the PA reaction rate is parallel to the catalyst amount. The
calculated apparent activation energy Ea is 31.7 kJ/mol for
the formation of 4IP2MA over the MgO catalyst.
g cat.
ACKNOWLEDGEMENTS
We gratefully acknowledge financial support by the
National Science Council of the Republic of China.
Received August 11, 2006.
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