Strong base catalysis of sulfated mesoporous alumina for the Tishchenko reaction in supercritical carbon dioxide

Article abstract of DOI:10.1246/Cl.2005.262

Heterogeneous strong base catalysis for the Tishchenko reaction in acidic scCO₂ solvent has been realized with mesoporous alumina modified with SO₄^2-, while a conventional solid base like CaO showed almost no catalytic performance in scCO₂. Copyright

Full text of DOI:10.1246/Cl.2005.262

2
62
Chemistry Letters Vol.34, No.2 (2005)
Strong Base Catalysis of Sulfated Mesoporous Alumina for the Tishchenko Reaction
in Supercritical Carbon Dioxide
ꢀ
Tsunetake Seki and Makoto Onaka
Department of Chemistry, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902
(Received November 22, 2004; CL-041410)
Heterogeneous strong base catalysis for the Tishchenko re-
Table 1. Catalytic activities of CaO, aluminas, and sulfated alu-
minas for the intramolecular Tishchenko reaction of o-phthalal-
dehyde (1) to phthalide (2) in supercritical carbon dioxide
action in acidic scCO2 solvent has been realized with mesopo-
rous alumina modified with SO42 , while a conventional solid
base like CaO showed almost no catalytic performance in
scCO2.
ꢁ
a
Surface
d
Pore
b
Catalyst
f
Entry
area
2
diametere Yield /%
Al/SO42
ꢁ c
)
(
ꢁ1
/m g
/nm
1
2
CaOg
48
312
129
173
474
532
567
570
—
—
1
62 (64h)
<1
The supercritical fluid (SCF) medium has recently attracted
much attention as the fourth phase for performing chemical syn-
thesis besides gas, liquid, and solid phases. The physical proper-
ties of an SCF can be continuously varied from those analogous
to gases to those to liquids by manipulating its pressure and tem-
perature, hence SCFs are expected to comprise a unique hetero-
geneous catalytic system that enhances reaction rate and product
ALO-2
ALO-3
ALO-4
3
—
—
h
4
12 (15 )
h
5
mesoAl O
2
3
ꢁ
ꢁ
2ꢁ
2.7
2.9
2.4
3.4
51 (59 )
66
mesoAl O /SO -I (89)c
2
6
2
3
4
7
mesoAl2O3/SO42 -II (45)c
62
73 (81h)
1
selectivity in comparison with conventional organic solvents.
mesoAl2O3/SO4 -III (6)c
8
Among the SCFs, supercritical carbon dioxide (scCO2) is the
most attractive as a reaction medium because of the low critical
temperature (304 K) and pressure (7.4 MPa), as well as the non-
toxic and nonflammable nature and low cost. The following
characteristics of scCO2 are advantageous especially for hetero-
geneous catalytic reactions that proceed in the nano-sized pores
of zeolites and mesoporous materials: 1) the high ability to dis-
solve or extract a variety of organic compounds, 2) the high dif-
fusivity and the low viscosity that improve mass transfer of re-
actants and products, and 3) the high thermal conductivity that
can efficiently eliminate the reaction heat.
a
In a 10-mL stainless steel autoclave, an activated solid base cata-
lyst (0.050 g) and 1 (1.00 mmol) were stirred in scCO2 at 8:0 ꢂ
:2 MPa at 313 K for 2 h.4 Each catalyst was pretreated at 773 K
b
0
for 2 h under vacuum (10 Pa) prior to use. Initial mixing molar
ꢁ2
c
2
ꢁ
d
ratios of Al(O-sec-Bu)3/SO4 in preparation. Determined by
BET method. eDetermined by DH method. Determined by
f
1
g
H NMR using Ph3CH as an internal standard. Prepared from
h
ꢁ2
Ca(OH)2 at 873 K for 2 h under vacuum (10 Pa). Reaction time
was 4 h.
However, the reaction in scCO2 hardly proceeded presumably
due to the prompt formation of the inert material of CaCO3 on
the CaO surface. This result indicated that solid strong bases like
alkaline earth oxides were inadequate for base-catalyzed reac-
tions in acidic scCO2. We next tested three types of ꢀ-aluminas,
ALO-2, -3, and -4 in the reaction: ALO-2 is contaminated with
a few weight percent of SO4 ions; ALO-3 includes Na ions;
and ALO-4 is almost pure ꢀ-alumina. The three aluminas
showed noticeable differences in the catalytic activity, depend-
ing on the kind of impurities: ALO-2 showed quite high activity
in scCO2 (Entry 2), while more basic aluminas, ALO-3 and -4,
gave poor catalysis (Entries 3 and 4).
It seems that SO4 ions in ALO-2 decrease the basic nature
of the alumina surface through the electron-withdrawing induc-
tive effects,
acidic CO2 onto the surface. The nature of ALO-2 has guided
our investigation to develop a new solid strong base catalyst that
would efficiently function in acidic scCO2.
CO2 is notorious as a poison for heterogeneous base-cata-
2
lyzed reactions: upon exposure of a solid base to CO2, the acidic
2
CO2 molecules are promptly adsorbed on basic sites of the solid
base, and hinder its base catalysis. Therefore, an attempt to per-
form base-catalyzed reactions in scCO2 seems reckless, but is
certainly a challenging theme in catalysis chemistry.
5
2ꢁ
þ
Here we wish to report that sulfated mesoporous alumina
designated as mesoAl2O3/SO42ꢁ) can act as a solid strong base
(
2ꢁ
catalyst even in acidic scCO2; mesoAl2O3/SO4 exhibits the
highest catalytic performance in scCO2 for the intramolecular
Tishchenko reaction of aromatic dialdehydes, which is a typical
solid base-catalyzed reaction and normally performed by strong
2
ꢁ
2
ꢁ
2,3
base sites (O ) on metal oxides (Scheme 1).
2
a,6
resulting in the prevention of the adsorption of
Firstly, we applied CaO prepared by thermal decomposition
of Ca(OH)2 to the transformation of o-phthalaldehyde (1) into
phthalide (2) in scCO2 (Table 1, Entry 1), because CaO exhibit-
ed the highest activity for this reaction in benzene solvent.3c,3d
Interestingly, we found that mesoporous alumina (meso-
Al2O3) possessing uniform nano-sized pores prepared from
Al(O-sec-Bu)3 through sol–gel process promoted the reaction
O
7
CHO
Solid Strong Base Catalyst
(Entry 5), even though the mesoAl2O3included no sulfate ions.8
The yield of 51% with mesoAl2O3 was much higher than that of
12% with pure ꢀ-alumina, ALO-4.
O
scCO2 (8 MPa)
CHO
3
13 K
1
2
This finding encouraged us to further develop mesoAl2O3-
2ꢁ
Scheme 1.
based solid base intentionally modified with SO4
ions to
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.2 (2005)
263
achieve higher catalytic performance. We introduced sulfate
ions into the framework of mesoAl2O3, not on its surface, in a
was demonstrated to transform 1 into 2 effectively in scCO2 un-
der mild reaction conditions. The addition of a small amount of
tetrahydrofuran as a co-solvent led to a remarkable increase in
the reaction rate through the enhancement of the solubility of
1 in scCO2. The present heterogeneous solid base-catalyzed
Tishchenko reaction in scCO2 will realize the clean production
9
similar way as in the preparation of sulfated zirconia aerogels,
because 1 requires close contact with a metal oxide surface in the
2
,3
Tishchenko reaction.
A mesoporous alumina containing
SO42 ions (mesoAl2O3/SO4 -I) was successfully synthesized
ꢁ
2ꢁ
1
0
by the hydrolysis of Al(O-sec-Bu)3 with deionized water con-
8
of pharmaceutically promising phthalide derivatives from the
viewpoint of green chemistry.
taining H2SO4, and it showed a higher yield of 66% than the
non-sulfated mesoAl2O3 (Entry 5 vs 6). It is noteworthy that
2
ꢁ
an increase in the amount of SO4
in mesoAl2O3 (meso-
The authors wish to express their thanks to Professor Ernest
G. Nolen of Colgate University for his helpful discussions.
2ꢁ
8
Al2O3/SO4 -II) brought about an increase in the surface area,
but a decrease in the yield, presumably owing to some reduction
of the surface basicity (Entry 7). mesoAl2O3/SO4 -III was syn-
thesized with relatively a large amount of Al2(SO4)3 in place of
H2SO4 as a sulfate source, having a higher surface area than the
other aluminas. With mesoAl2O3/SO42 -III an introduction of
sulfate ions into the alumina framework led to not only an ex-
pansion of a surface area of mesoporous alumina, but also the
highest yield of 2 (Entry 8), indicating that mesoAl2O3/
SO42 -III has the most suitable basic character for the Tishchen-
2ꢁ
References and Notes
1
a) Supercritical Fluids: Chem. Rev., ed. by R. Noyori, 99, 353
(1999). b) R. Noyori and T. Ikariya, in ‘‘Supercritical Fluids
for Organic Synthesis,’’ Vol. 13 ed. by F. V o¨ gtle, J. F.
Stoddart, and M. Shibasaki, Wiley-VCH, Weinheim
(2000). c) J.-D. Grunwaldt, R. Wandeler, and A. Baiker,
Catal. Rev., 45, 1 (2003).
8
ꢁ
8
ꢁ
2
3
a) K. Tanabe, M. Misono, Y. Ono, and H. Hattori, in ‘‘New
Solid Acids and Bases,’’ Kodansha-Elsevier, Tokyo (1989).
b) H. Hattori, Chem. Rev., 95, 537 (1995).
a) K. Tanabe and K. Saito, J. Catal., 35, 247 (1974). b) T.
Seki, K. Akutsu, and H. Hattori, Chem. Commun., 11,
1000 (2001). c) T. Seki and H. Hattori, Chem. Commun.,
23, 2510 (2001). d) T. Seki, H. Tachikawa, T. Yamada,
and H. Hattori, J. Catal., 217, 117 (2003). e) H. Tsuji and
H. Hattori, ChemPhysChem, 5, 733 (2004).
ko reaction in acidic scCO2.
Although scCO2 is an ideal reaction medium for organic re-
actions, in some cases we encounter lack of dissolving power of
scCO2 for polar organic substrates. The use of a co-solvent is one
1
of workable solutions to overcome the problem. Table 2 shows
the effect of co-solvents on the mesoAl2O3/SO42 -III-catalyzed
reaction. It should be noted that a drastic increase in the reaction
rate was observed when a small amount of benzene or tetrahy-
drofuran (THF) was added as a co-solvent (Entries 2 and 3).
Especially, the reaction in the scCO2-THF system was almost
twice as fast as that in pure scCO2. In contrast, the addition of
protic solvents brought about remarkable decreases in the reac-
tion rates (Entries 4 and 5). The poisoning effect of methanol
ꢁ
4
5
Operators of high-pressure equipment should take proper
precautions to minimize the risk of personal injury.
ALO-2, -3, and -4 are the reference alumina catalysts
supplied from the Catalysis Society of Japan, and contain
the following impurities. ALO-2: 0.03% Fe2O3, 0.22% SiO2,
0.04% Na2O, 1.72% SO4 ; ALO-3: 0.01% Fe2O3, 0.01%
SiO2, 0.3% Na2O, 0.01% TiO2; ALO-4: 0.01% Fe2O3,
0.01% SiO2, 0.01% Na2O.
2ꢁ
(
pKa ¼ 15:5) clearly demonstrated that the Tishchenko reaction
in scCO2 proceeded by the action of strong base sites on meso-
Al2O3/SO4 -III.
2ꢁ
In summary, we have designed a new strong base material of
6
7
K. Arata, Adv. Catal., 37, 165 (1990).
mesoAl2O3/SO42 capable of catalyzing the Tishchenko reac-
tion in scCO2. Two key factors were considered to realize the
strong base catalysis in acidic scCO2; 1) a metal oxide that does
not form a metal carbonate was chosen, and 2) the surface basic-
ity of the metal oxide was adjusted by the introduction of sulfate
ions into its oxide framework in order to suppress the strong CO2
ꢁ
a) F. Vaudry, S. Khodabandeh, and M. E. Davis, Chem.
Mater., 8, 1451 (1996). b) T. Oikawa, T. Ookoshi, T.
Tanaka, T. Yamamoto, and M. Onaka, Microporous
Mesoporous Mater., 74, 93 (2004).
8
Preparation methods of mesoporous alumina catalysts:
mesoAl2O3, mesoAl2O3/SO4 -I, -II, and -III were synthe-
2ꢁ
2ꢁ
7a
adsorption that quenches the reaction. The mesoAl2O3/SO4
sized in one-half the literature scale. In the preparation of
mesoAl2O3/SO4 -I and -II, Al(O-sec-Bu)3 was hydrolyzed
by deionized water containing 0.1 and 0.2 g of H2SO4, re-
2ꢁ
2ꢁ
Table 2. Effect of co-solvent on the mesoAl2O3/SO4 -III-cat-
alyzed intramolecular Tishchenko reaction of o-phthalaldehyde
2ꢁ
spectively. In the preparation of mesoAl2O3/SO4 -III,
9.5 g (79.0 mmol) of Al(O-sec-Bu)3 and 1.7 g (5.0 mmol)
a
1
(
1) to phthalide (2) in supercritical carbon dioxide
of Al2(SO4)3 were used. The XRD spectrum attributed to
mesopores of mesoAl2O3/SO4 -III appeared at a higher
Entry
Co-solvent
Yield/%
2ꢁ
1
2
3
4
5
None
Benzene
31
48
58
<1
6
d-spacing than that of pure mesoAl2O3, indicating that the
sulfur atoms in the alumina framework brought about the
7
a
Tetrahydrofuran
Acetic acid
Methanol
partial structural collapse. The absence of IR absorption
2a,6
2
ꢁ
bands assigned to surface SO4
species
implied that
2ꢁ
2ꢁ
SO4 ions in mesoAl2O3/SO4 -III were incorporated into
the alumina framework.
A. F. Bedilo and K. J. Klabunde, J. Catal., 176, 448 (1998).
aReaction conditions are as follows: a 10-mL stainless
steel autoclave; catalyst, 0.050 g (activated at 773 K
9
ꢁ
2
for 2 h under vacuum (10 Pa)); 1, 1.00 mmol; scCO2,
4
10 T. K. Devon and A. I. Scott, in ‘‘Handbook of Naturally
Occurring Compounds,’’ Academic Press, New York
(1975), Vol. 1, p 249.
8
3
:0 ꢂ 0:2 MPa; co-solvent, 0.1 mL; reaction temp.,
13 K; reaction time, 15 min.
Published on the web (Advance View) January 26, 2005; DOI 10.1246/cl.2005.262