Elucidation of basic properties of mesoporous alumina through the temperature-programmed desorption of carbon dioxide and heterogeneous basic catalysis of mesoporous alumina for the Knoevenagel reaction in supercritical CO2

Article abstract of DOI:10.1016/j.molcata.2006.08.049

The temperature-programmed desorption (TPD) of CO₂ has been performed to elucidate the strength and the number of base sites on mesoporous alumina (mesoAl₂O₃). It was demonstrated that the surface of mesoAl₂O₃ adsorbs a much smaller amount of CO₂ than those of MgO and conventional γ-Al₂O₃ with irregular pores, but that there exist a small number of strong base sites on mesoAl₂O₃ that adsorb CO₂ very strongly. The CO₂-TPD results clearly account for the high activity of mesoAl₂O₃, the low activity of γ-Al₂O₃, and no activity of MgO for a typical base-catalyzed reaction of the Knoevenagel reaction in supercritical CO₂.

Full text of DOI:10.1016/j.molcata.2006.08.049

Journal of Molecular Catalysis A: Chemical 263 (2007) 115–120
Elucidation of basic properties of mesoporous alumina through the
temperature-programmed desorption of carbon dioxide and
heterogeneous basic catalysis of mesoporous alumina for the
Knoevenagel reaction in supercritical CO2
Tsunetake Seki1, Makoto Onaka∗
Department of Chemistry, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-Ku, Tokyo 153-8902, Japan
Received 15 July 2006; received in revised form 10 August 2006; accepted 18 August 2006
Available online 24 August 2006
Abstract
The temperature-programmed desorption (TPD) of CO2 has been performed to elucidate the strength and the number of base sites on mesoporous
alumina(mesoAl2O3). ItwasdemonstratedthatthesurfaceofmesoAl2O3 adsorbsamuchsmalleramountofCO2 thanthoseofMgOandconventional
␥-Al2O3 with irregular pores, but that there exist a small number of strong base sites on mesoAl2O3 that adsorb CO2 very strongly. The CO2-TPD
results clearly account for the high activity of mesoAl2O3, the low activity of ␥-Al2O3, and no activity of MgO for a typical base-catalyzed reaction
of the Knoevenagel reaction in supercritical CO2.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Mesoporous alumina; Solid base catalyst; CO2-TPD; Base catalysis; Supercritical CO2; Knoevenagel reaction
1. Introduction
tralization by the compressed Lewis acidic molecules of CO2
[9]. However, the interaction between mesoAl2O3 surface and
CO2 molecules was not elucidated entirely.
Mesoporous alumina (mesoAl2O3) is an interesting mate-
rial with a high surface area and a narrow pore-size distribution
in a mesopore region, and has been synthesized using various
templates such as polyethylene oxides, carboxylic acids, and
tetraalkylammonium salts [1]. While mesoAl2O3 has found its
successful application as a catalytic material in a number of reac-
tions involving epoxidation of olefins [2], hydrodechlorination
[3], hydrodesulfurization [4], hydrogenation [5], olefin metathe-
sis [6], oxidation of methane to syngas [7], and oxidative dehy-
drogenation of ethane [8], the utility of mesoAl2O3 as a solid
base catalyst and its surface basic property have been scarcely
elucidated. We have recently found out that mesoAl2O3 exhibits
a highly efficient strong base catalysis for the intramolecular
Tishchenko reaction of phthalaldehyde to phthalide even in the
acid medium of supercritical CO2 (scCO2), while conventional
␥-Al2O3 and a representative solid strong base catalyst of CaO
showed a low and no activity, respectively, due to the rapid neu-
Herein, we wish to discuss the properties of base sites
on mesoAl2O3 by comparing the profile of temperature-
programmed desorption (TPD) of CO2 and the activity for the
Knoevenagel reaction of benzaldehyde (1) and ethyl cyanoac-
etate (2) (Scheme 1) with those of MgO and conventional ␥-
Al2O3. The CO2-TPD analyses gave us novel information about
the interaction of CO2 with mesoAl2O3 surface as well as the
strength and the number of base sites on mesoAl2O3, revealing
that mesoAl2O3 is an essentially suitable solid base catalyst for
the use in scCO2.
2. Experimental
2.1. Catalyst preparations
MgO. To a 3-L beaker containing an excess amount of dis-
tilled water (ca. 1 L) was slowly added commercially available
high-grade MgO powder (100 g, Merck). The resultant solu-
tion was stirred with a mechanical stirrer overnight at room
temperature, and boiled in the open air for several hours with
∗
Corresponding author.
E-mail address: conaka@mail.ecc.u-tokyo.ac.jp (M. Onaka).
Research Fellow of the Japan Society for the Promotion of Science.
1
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.08.049

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T. Seki, M. Onaka / Journal of Molecular Catalysis A: Chemical 263 (2007) 115–120
apparatus. The vials were placed under very slow N2 flow to
avoid their contacts with humidity as well as the air oxidation of
benzaldehyde; nevertheless, benzaldehyde was frequently puri-
fied by distillation.
Scheme 1. General reaction equation for the Knoevenagel reaction.
2.4. Temperature-programmed desorption (TPD) of CO2
stirring to evaporate water almost completely. The resultant
slurry was dried overnight in a furnace to give Mg(OH)2 as white
cakes, which was broken into pieces and powdered through a
mesh (355–600 ␮m). Thermal decomposition of the Mg(OH)2
at 773 K under a vacuum (10−4 Torr) afforded MgO catalyst.
mesoAl2O3. Pure mesoporous alumina (mesoAl2O3) was
synthesized according to Davis’ report [10]. Into a polypropy-
lene vessel containing a stirring bar were added Al(O-sec-Bu)3
(21.9 g, 89.0 mmol, Tokyo Kasei), 1-propanol (120 g, 2.00 mol,
Kanto Kagaku), and deionized water (5.15 g, 286 mmol), fol-
lowed by vigorous stirring at room temperature. After 1 h, a
solution of lauric acid (5.40 g, 27.0 mmol, Kanto Kagaku) in 1-
propanol (17.5 g, 290 mmol, Kanto Kagaku) was added under
vigorous stirring, and the mixture was further stirred vigorously
at room temperature for 24 h. The resulting white liquid was
placed in a 300-mL autoclave (Taiatsu Techno), and heated at
383 K for 48 h without stirring to afford a white precipitate. The
precipitate was washed on a filter paper with a large amount of
ethanol, and dried at room temperature under an N2 flow. The
white solid thus obtained was heated from room temperature to
873 K at a ramping rate of 10 K min−1 under an N2 flow, and
then calcined under an air flow at 873 K for 5 h to burn out the
organic species to give pure mesoAl2O3.
The CO2-TPD was performed in a vacuum-line apparatus
equipped with a quadrupole mass spectrometer (QMS 200 F,
Pfeiffer Vacuum). Each sample was pretreated at 773 K for 2 h
under vacuum. The activated sample (10 mg) was subsequently
exposed to 60 Torr of CO2 at room temperature for 10 min, fol-
lowed by evacuation at room temperature for 30 min. The TPD
was run from room temperature to 1273 K at a heating rate of
10 K min−1 under vacuum with the quadrupole mass spectrome-
ter. For ␥-Al2O3 and mesoAl2O3, the CO2-TPD was also carried
out with an increased amount (50 mg) of the samples, then the
pre-evacuation of CO2 was conducted at 673 K for 5 min just
before the TPD-run.
2.5. Reaction procedures
Safety Warning! Operators handling high-pressure equip-
ments for reactions in scCO2 should take proper precautions
to minimize the risk of personal injury. The Knoevenagel reac-
tion was performed with an scCO2 reactor system of SCF-Get,
SCF-Bpg, and SCF-Sro (Jasco). The detailed reaction proce-
dures were as follows. Into a 10-mL stainless steel autoclave
(SUS316, Taiatsu Techno) containing a stirring bar were added
freshly distilled benzaldehyde (1, 0.1 mL; 0.98 mmol), ethyl
cyanoacetate (2, 0.1 mL; 0.94 mmol), and a catalyst (30 mg),
which had been preactivated at 773 K under vacuum (10−4 Torr)
for 2 h, in a nitrogen-filled glove bag. Liquid CO2 cooled to
263 K was introduced into the autoclave, which had been heated
to 313 K, using an HPLC pump and the pressure was adjusted
to 8.0 0.2 MPa. The resultant mixture was stirred at 313 K
at the constant CO2 pressure of 8.0 0.2 MPa. The autoclave
was cooled in an ethanol-dry ice bath, and the pressurized CO2
was gradually released. The resulting mixture was washed out
from the autoclave with CH2Cl2. The CH2Cl2 slurry was fil-
tered through a Celite pad. Evaporation of the CH2Cl2 filtrate
gaveacrudeproduct, whichwasdissolvedinCDCl3 withPh3CH
(internal standard substance) and applied to NMR analysis. The
amount of the product (3) was determined with 1H NMR (270
or 500 MHz) by comparison between the peak area of Ph3CH
and that of the vinyl proton in 3 [12].
γ-Al2O3. JRC-ALO-4, which had been supplied from the
Catalysis Society of Japan, was used without any modification.
The contents of impurities are as follows: 0.01% Fe2O3, 0.01%
SiO2, and 0.01% Na2O.
2.2. Characterization of the catalysts
N2 adsorption–desorption measurements were performed at
77 K with a BELSORP 28SA (Bel Japan) using static adsorp-
tion procedures. The samples were pretreated under the same
condition as employed for the reactions (773 K, 10−4 Torr,
2 h) prior to analysis. Adsorption data were analyzed by the
Brunauer–Emmett–Teller (BET) method for surface areas and
the Dollimore–Heal (DH) method [11] to obtain a pore-size dis-
tribution curve.
Powder X-ray diffraction measurement was performed with
a MultiFlex (Rigaku) under ambient atmosphere. The sample
was pretreated at 773 K at 10−4 Torr for 2 h, mounted on an
XRD sample cell as fine powder under ambient atmosphere,
and analyzed at a current of 40 mA and a voltage of 40 kV in the
The reaction procedures described above gave experimen-
tally reproducible data for several runs.
For comparative purpose, the reaction with MgO was also
performed under solvent-free condition in a 10-mL glass flask
under N2 atmosphere.
2θ range 1–80◦ with a scanning rate of 1.0◦ min−1
.
2.3. Organic reagents
3. Results and discussion
Benzaldehyde (1, Aldrich) and ethyl cyanoacetate (2, Kanto
Kagaku) were purified by distillation under vacuum, and stored
in N2-filled vials with MS 4A (Wako), which had been activated
at 773 K for 2 h under vacuum (10−4 Torr) in a vacuum-line
3.1. Surface area and mesoporosity of the catalysts
TheN2 adsorption–desorptionmeasurementsshowedthatthe
BET surface areas of MgO, ␥-Al2O3, and mesoAl2O3 were 267,

T. Seki, M. Onaka / Journal of Molecular Catalysis A: Chemical 263 (2007) 115–120
117
Fig. 2. TPD plots of CO2 desorbed from 10 mg of MgO (thick black line), ␥-
Al2O3 (thin gray line), and mesoAl2O3 (thin black line) in the temperature range
303–1273 K.
Fig. 1. DH pore-size distribution curve of mesoAl2O3 with XRD patterns of
␥-Al2O3 and mesoAl2O3.
which indicates that the main CO2-desorption took place at a
higher temperature for mesoAl2O3 than for ␥-Al2O3 in this tem-
perature region. To further clarify the difference, we performed
the TPD experiment with an increased amount of the samples.
In Fig. 3 are shown the TPD plots of CO2 desorbed from 50 mg
of ␥-Al2O3 and mesoAl2O3, where the pre-evacuation of CO2
was performed at 673 K for 5 min just before the TPD-run; the
amounts of desorbed CO2 in this temperature range were 0.092
and 0.26 ␮mol for ␥-Al2O3 and mesoAl2O3, respectively. In
this case with an increased amount of the samples is also seen
the tendency that the desorption occurred at a higher tempera-
ture for mesoAl2O3 than for ␥-Al2O3. Although both ␥-Al2O3
and mesoAl2O3 gave the peak in the high temperature region
800–1000 K, only mesoAl2O3 showed the peak centered at a
higher temperature of ca. 1100 K. These results clearly indicate
173, and 471 m2 g−1, respectively, after they had been activated
at 773 K for 2 h under vacuum (10−4 Torr) prior to the analysis.
The measurement also gave the DH pore-size distribution curve
of mesoAl2O3, which is shown in Fig. 1. It can be seen from the
figure that the mesoAl2O3 possessed a narrow pore size distri-
bution centered at 1.6 nm. The pore volume was 478 mm3 g−1
.
In Fig. 1 are also shown XRD patterns of ␥-Al2O3 and
mesoAl2O3 in the 2θ region 1–10◦. The peak at 1–2◦ for
mesoAl2O3 is due to the mesoporous structure, while the similar
peak did not appear for ␥-Al2O3 with irregular pores.
3.2. TPD of CO2
Fig. 2 shows the TPD plots of CO2 desorbed from 10 mg of
MgO, ␥-Al2O3, andmesoAl2O3. Thepre-evacuationofCO2 was
performed at room temperature for 30 min. The amount of CO2
desorbed from MgO (4.9 ␮mol) was far greater than those from
␥-Al2O3 (0.84 ␮mol) and mesoAl2O3 (0.83 ␮mol). The amount
of desorbed CO2 per surface area of MgO was also the highest;
the surface area of MgO was 1.5-fold larger than that of ␥-Al2O3
and 1.8-fold smaller than that of mesoAl2O3. Thus it can be
stated that MgO possesses the most CO2-philic character among
the catalysts tested, where we evaluate the “CO2-philicity” of a
material on the basis of the CO2-adsorbing capacity per surface
area.
The amounts of CO2 desorbed from ␥-Al2O3 and mesoAl2O3
were almost equal, and far smaller than that of MgO. Note, how-
ever, that the amount of desorbed CO2 per surface area was
much smaller for mesoAl2O3 than for ␥-Al2O3; the surface area
of mesoAl2O3 was 2.7-fold larger than that of ␥-Al2O3. There-
fore, it can be concluded that the CO2-philic character decreases
in the order of MgO ꢀ ␥-Al2O3 > mesoAl2O3.
We confirmed a slight difference in the TPD plots in the tem-
perature range 800–1273 K between ␥-Al2O3 and mesoAl2O3,
Fig. 3. TPD plots of CO2 desorbed from 50 mg of ␥-Al2O3 (gray line) and
mesoAl2O3 (black line) in the temperature range 800–1273 K.

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T. Seki, M. Onaka / Journal of Molecular Catalysis A: Chemical 263 (2007) 115–120
Scheme 2. Migration of adsorbed CO2 over alumina surface. The electronic structures in III were quoted from Ref. [16].
that mesoAl2O3 possesses the base sites with supreme strength
that are even stronger than the base sites on ␥-Al2O3, although
there is a possibility that neighboring Lewis acid sites (Al3+)
would more or less affect the difference in the TPD profiles,
because CO2 is usually adsorbed on a metal oxide surface in
bidentate forms [13].
To sum up, the surface of mesoAl2O3 is essentially less CO2-
philic, but it has a small number of very strong base sites that
adsorb acidic CO2 molecules strongly. We consider that even
such strong base sites can easily replace the adsorbed CO2
we are unaware of any published reports using scCO2 solvent
so far. It is undoubtedly a novel and challenging approach to
use acidic scCO2 medium in the base-catalyzed Knoevenagel
reaction.
Fig. 4 shows the comparison of the activities of MgO, ␥-
Al2O3, and mesoAl2O3 for the Knoevenagel reaction of 1 with
2 to 3. Although MgO yielded 0.78 mmol of 3 in 30 min under
solvent-free condition, the amount of 3 did not increase in scCO2
from that of 3 formed before starting the reaction under scCO2
condition (i.e., 0.33 mmol) [31]. This was due to the rapid for-
mation of the inactive species of MgCO3 on the surface, and
clearly indicates that the representative solid base catalyst of
MgO cannot be utilized for base-catalyzed reactions in scCO2.
On the contrary, ␥-Al2O3 and mesoAl2O3 promoted the reac-
tion even under scCO2 condition. Since the pKa value of 2 is 9,
¨
with another Bronsted or Lewis acidic molecule (e.g., a reac-
tant in base-catalyzed reactions) that approaches to the base
sites, through the action of adjacent Lewis acid sites as shown
in Scheme 2.
The similar dynamic behavior of adsorbed CO2 has been
confirmed by Tsuji et al. even over MgO and CaO [14], of which
Lewis acid sites (metal cations) are far weaker in acidity than
those on alumina [15]. The confirmation of adsorbed species
III on alumina through infrared spectroscopy by Parkyns [16]
also supports the speculation that the same dynamic behavior
of adsorbed CO2 can take place over mesoAl2O3. Moreover,
the high activity of mesoAl2O3 for the Tishchenko reaction,
the base sites stronger than H = 9 functioned in the compressed
−
CO2 medium to promote the reaction [15]. It is noteworthy that
the activity of mesoAl2O3 was much higher than that of ␥-Al2O3
at the initial stage of the reaction, demonstrating that mesoAl2O3
is the most suitable solid base catalyst for the Knoevenagel reac-
tion in scCO2 medium; the activity increases in the order of
MgO ꢁ ␥-Al2O3 < mesoAl2O3.
which proceeds only by the action of strong base sites (O2−
)
It should be noted that there is a good correlation between
the activity order for the Knoevenagel reaction in scCO2 and the
on metal oxides [17], under scCO2 condition strongly indicates
that the strong base sites on mesoAl2O3 are able to release the
strongly adsorbed CO2 by the migration, leading to the constant
regeneration of the strong base sites in scCO2 [9].
3.3. The catalytic performances for the Knoevenagel
reaction under scCO2 condition
The Knoevenagel reaction is a condensation of an aldehyde
or a ketone with an active methylene compound to give the corre-
spondingalkeneproducttogetherwithH2O[18]. Thecommonly
used base catalysts for Knoevenagel reactions involve secondary
amines such as piperidine and Et2NH, pyridine or pyridine with
piperidine (Doebner modification), and metal alkoxides such as
NaOEt. These homogeneous catalysts, however, possess several
disadvantages in the separation process after the reaction; sep-
aration of a product and a catalyst from the resulting solution
causes loss of the catalyst and reduction of the product yield.
To solve the problems, several researchers have developed solid
base catalysts utilizable for Knoevenagel reactions, which, in
some cases, are also superior to homogeneous ones in activity
and selectivity [19–30].
Both the homogeneous and heterogeneous catalytic reactions
have been performed under solvent-free conditions or in con-
ventional organic solvents such as toluene and methanol, and
Fig. 4. Variation of the amount of 3 formed over MgO (filled circle), ␥-Al2O3
(triangle), and mesoAl2O3 (unfilled circle) under scCO2 condition as a function
of stirring time (see, Ref. [31]).

T. Seki, M. Onaka / Journal of Molecular Catalysis A: Chemical 263 (2007) 115–120
119
Scheme 3. Plausible mechanism of the Knoevenagel reaction over alumina in scCO2.
results of CO2-TPD; the less CO2-philic catalyst with stronger
base sites exhibited a higher activity.
ature range can promote the Knoevenagel reaction, because the
Knoevenagel reaction proceeds by the action of both weak and
strong base catalysts [32]. Consequently, even the CO2 adsorbed
on active strong base sites smoothly diffuse over the surface of
mesoAl2O3 by the mechanism shown in Scheme 2, due to the
large “vacancy” around the sites, when 2 approaches the sites.
On the other hand, the CO2 tightly adsorbed on the active strong
base sites of γ-Al2O3 are “comparatively jam-packed” because
of the high density of active strong base sites, retarding the sur-
face diffusion with each other. As a result, the activation energy
for the adsorption of 2 onto the active base sites becomes higher
for ␥-Al2O3 than for mesoAl2O3, which should cause the dif-
ference in the activity for the Knoevenagel reaction between
␥-Al2O3 and mesoAl2O3.
3.4. Mechanistic consideration
A plausible mechanism for the Knoevenagel reaction was
depicted in Scheme 3 [22]. It can be deduced that the reac-
tion mainly took place through the states I → II → III → IV,
though some weakly adsorbed H2O in IV might dissolve into
scCO2 phase to regenerate I. The IV is undoubtedly a deac-
tivated state of the active sites, and it can no longer promote
the reaction. We consider that the deactivation observed for
mesoAl2O3 and ␥-Al2O3 (Fig. 4) is attributed to the increment
of IV with the progress of the reaction. Since the equal amount
of 3 was finally formed over both the aluminas at 1440 min,
almost the same number of active base sites would promote the
reaction.
4. Conclusions
The surface area of mesoAl2O3 was 2.7-fold larger than that
of ␥-Al2O3, nevertheless almost the same amount of CO2 was
desorbed from 10 mg of each alumina in the CO2-TPD profiles
(Fig. 2). Thisindicatesthatmostofthebasesitesonthesurfaceof
mesoAl2O3 were very weak and released CO2 molecules by the
evacuation at room temperature that was conducted just before
carrying out the TPD measurement, and also implies that the
numbers of the base sites on mesoAl2O3 and ␥-Al2O3 that were
strong enough to promote the Knoevenagel reaction were almost
equal.
Thus the higher activity of mesoAl2O3 at the initial stage
of the Knoevenagel reaction in Fig. 4 was not attributed to
the higher surface area, but to the nature of active base sites.
Although the numbers of active base sites per gram were almost
equal for both mesoAl2O3 and ␥-Al2O3, the density of active
base sites, of which strength is enough to promote the Kno-
evenagel reaction, must be far lower for mesoAl2O3 than for
␥-Al2O3, considering the much higher surface area of the
mesoAl2O3. From the CO2-TPD results in the temperature range
303–1273 K (Fig. 2), the densities of ␥-Al2O3 and mesoAl2O3
were estimated as 0.29 and 0.11 nm−2, respectively, where we
assumethatalmostallthebasesitesreleasingCO2 atthistemper-
It was, for the first time, revealed through the CO2-TPD
experiment that most of the base sites on mesoAl2O3 are weak
and do not bond with Lewis acidic molecule of CO2 strongly,
though there are a slight number of very strong base sites that
adsorb CO2 very strongly. There is a good correlation between
the CO2-philic nature of a catalyst and its activity for a typical
base-catalyzed reaction of the Knoevenagel reaction in scCO2;
a catalyst with the less CO2-philic character exhibited the higher
activity. The CO2-TPD result also shows the existence of highly
strong base sites on the least CO2-philic material of mesoAl2O3,
which adsorb acidic CO2 molecules very strongly. It must be
emphasized that the desorption peak with mesoAl2O3 appeared
at a higher temperature than that of conventional ␥-Al2O3 in
the temperature range 800–1273 K. However, it is apparent that
those strongest base sites on mesoAl2O3 function even under
scCO2 condition, because mesoAl2O3 exhibited high activities
for a typical strong base-catalyzed reaction of the Tishchenko
reaction [9] as well as the Knoevenagel reaction in scCO2
medium.
Our present results clearly demonstrate that mesoAl2O3 is
an essentially appropriate solid base catalyst for various base-
catalyzed reactions in scCO2, which is one of the promising

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T. Seki, M. Onaka / Journal of Molecular Catalysis A: Chemical 263 (2007) 115–120
alternatives to conventional organic solvents in view of green
chemistry [33].
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Acknowledgements
We are grateful to Prof. Dr. Keiichi Tomishige, Dr. Shigeru
Kado, and Dr. Takeshi Nobukawa (University of Tsukuba) for
the CO2-TPD measurements.
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