Mg-Al mixed oxides as highly active acid-base catalysts for cycloaddition of carbon dioxide to epoxides

Full text of DOI:10.1021/ja9902165

4526
J. Am. Chem. Soc. 1999, 121, 4526-4527
addition Via a retention of the configuration of epoxides. In this
paper, we report the unique catalysis of the Mg-Al mixed oxides
for the selective formation of cyclic carbonates on the basis of
characterization of the oxide catalysts using physicochemical
methods.
Mg-Al Mixed Oxides as Highly Active Acid-Base
Catalysts for Cycloaddition of Carbon Dioxide to
Epoxides
Kazuya Yamaguchi,† Kohki Ebitani,† Tomoko Yoshida,‡
Hisao Yoshida,§ and Kiyotomi Kaneda*,†
Department of Chemical Science and Engineering
Graduate School of Engineering Science
Osaka UniVersity,1-3 Machikaneyama, Toyonaka
Osaka 560-8531, Japan
Center for Integrated Research in
To optimize Mg/Al ratios in Mg-Al mixed oxides for the
addition reaction of CO2 with styrene oxide, hydrotalcites with
Mg/Al ratios from 3 to 8 were prepared and calcined at various
temperatures between 400 and 1000 °C.2 Their catalytic activities
were evaluated from the yields of styrene carbonate normalized
to surface areas.7 We found that the Mg-Al oxide with the Mg/
Al ratio of 5 calcined at 400 °C was the most active catalyst for
the addition reaction and DMF was the best solvent.8 By using
the Mg-Al oxide with the Mg/Al ratio of 5 calcined at 400 °C,
various kinds of epoxides could be quantitatively converted to
the corresponding cyclic carbonates, as shown in Table 1. The
reaction hardly occurred in the absence of Mg-Al mixed oxides.
Interestingly, the substitution of the para hydrogen of styrene oxide
for CH3 and Cl groups did not strongly affect the reaction rates.9
This addition reaction proceeded with a retention of stereochem-
istry of epoxides; the reactions of CO2 with (R)- and (S)-benzyl
glycidyl ethers gave (R)- and (S)-4-(benzyloxymethyl)-1,3-dioxo-
lane-2-one in 78 and 76% chemical yields with >99% ee, respec-
tively.10 Moreover, the mixed oxide catalysts could be reused with-
out losing its high catalytic activity and selectivity; the yield of
styrene carbonate could be kept over 90% in the third reuse
experiment.
Science and Engineering, Nagoya UniVersity
Nagoya 464-8603, Japan
Department of Applied Chemistry
Graduate School of Engineering
Nagoya UniVersity, Nagoya 464-8603, Japan
ReceiVed January 22, 1999
The efficient transformation of harmful wastes such as CO2
into useful chemicals is an important contribution to the preserva-
tion of the earth. Chemical fixation of CO2 onto organic com-
pounds is interesting because of a synthetic merit; one carbon
atom and two oxygen atoms can be incorporated in one step with
a high atom efficiency. In this sense, the addition reaction of CO2
to epoxides is a powerful candidate for CO2 fixation to produce
five-membered cyclic carbonate which can be used as valuable
raw materials for engineering plastics and highly polar solvents.1
Various catalysts such as metal halides, onium halides, and MgO
have been continuously explored in the addition of CO2 to epox-
ides.1 A key step in the above reaction, as well as other chemical
fixation reactions of CO2, is activation of the unreactive CO2 mole-
cule with metal catalysts and light.
Hydrotalcites consist of Brucite-like layers having a positive
charge with anionic species in the interlayer, forming neutral clay
materials.2 We have already reported that hydrotalcites are
excellent catalysts for the Baeyer-Villiger oxidation,3 epoxida-
tion,4 and oxidative dehydrogenation5 with the use of H2O2 and
molecular oxygen as oxidants. Further, hydrotalcites have been
often used as precursors for active Mg-Al mixed oxide catalysts
having strong basic sites on their surface.6 Here, we found that
the Mg-Al mixed oxides obtained by calcination of the hydro-
talcites were effective catalysts for the fixation of CO2 to various
epoxides to form the corresponding five-membered cyclic carbon-
ates. Compared with other reported catalysts for this addition
reaction, the Mg-Al mixed oxides have the following advan-
tages: (1) highly catalytic actiVity eVen under an atmospheric
CO2 pressure, (2) reusable catalysts, and (3) the stereospecific
In the reaction of CO2 and styrene oxide, the yields of styrene
carbonate were proportional to the basicity of calcined hydrotal-
cites.3 Although the uncalcined hydrotalcites had high basicity,
the catalytic activities were lower than those of the above calcined
ones. These results indicate that the Lewis basic sites of the mixed
oxide catalysts would play an important role in this addition
reaction. Further, the calcined hydrotalcite catalysts had higher
catalytic activities than the MgO. From the temperature-pro-
grammed desorption (TPD) of ammonia, it became clear that the
acid sites also existed on the surface of the active Mg-Al mixed
oxides and that no acid site was observed on the MgO and the
mixed oxide with the Mg/Al ratio of 3.6b,11 Therefore, the noble
catalysis of the active Mg-Al mixed oxides could be caused by
the cooperation of the acid sites and the strong basic sites.
The XRD peak positions of the calcined hydrotalcites did not
vary with the Mg/Al ratios, which were similar to that of MgO.
* Correspondence author. Telephone and fax: +81-6-6850-6260. E-mail:
kaneda@cheng.es.osaka-u.ac.jp.
† Graduate School of Engineering Science, Osaka University.
‡ Center for Integrated Research in Science and Engineering, Nagoya
University.
(7) A typical example for the addition of CO2 to epoxides is as follows.
Into a stainless steel autoclave (100 mL) were placed styrene oxide (0.96 g,
8 mmol), calcined hydrotalcite (1.0 g), DMF (6 mL), and CO2 (5 atm). The
resulting mixture was stirred at 100 °C for 15 h. The hydrotalcite was separated
by filtration. The filtrate was subjected to column chromatography on silica
gel and yielded styrene carbonate (1.20 g, 91%). The spent Mg-Al mixed
oxide was calcined at 400 °C, and then the calcined mixed oxide could be
reused without losing its high catalytic activity and selectivity.
(8) CO2 pressure during 1-20 atm did not strongly influence the yields of
styrene carbonate under our reaction conditions. The increase of the reaction
temperature over 120 °C resulted in low selectivity to carbonate because of
the formation of styrene glycol.
§ Graduate School of Engineering, Nagoya University.
(1) (a) Peppel, W. J. Ind. Eng. Chem. 1958, 50, 767. (b) Ratzenhofer, M.;
Kisch, H. Angew. Chem., Int. Ed. Engl. 1980, 19, 317. (c) Rakicki, G.; Kuran
W.; Marciniak, B. P. Monat. Chem. 1984, 115, 205. (d) Kihara, N.; Hara, N.;
Endo, T. J. Org. Chem. 1993, 58, 6198. (e) Yano, T.; Matsui, H.; Koike, T.;
Ishiguro, H.; Fujihara, H.; Yoshihara, M.; Maeshima, T. Chem. Commun. 1997,
1129.
(2) (a) Cavani, F.; Trifiro´, F.; Vaccari, A. Catal. Today 1991, 11, 173. (b)
Miyata, S. Clays Clay Miner. 1980, 28, 50.
(3) Ueno, S.; Ebitani, K.; Ookubo, A.; Kaneda, K. Appl. Surf. Sci. 1997,
121/122, 366.
(9) The relative reactivity of para-substituted styrene oxides was examined
to give p-methylstyrene oxide (1.3), styrene oxide (1.0), and p-chlorostyrene
oxide (1.0). The values in the parentheses are relative initial rates for the
addition reaction, which are normalized to that of styrene oxide.
(4) Ueno, S.; Yamaguchi, K.; Yoshida, K.; Ebitani, K.; Kaneda, K. Chem.
Commun. 1998, 295.
(5) Kaneda, K.; Yamashita, T.; Matsushita, T.; Ebitani, K. J. Org. Chem.
1998, 63, 1750.
(10) The enantiomeric excesses of (R)- and (S)-4-(benzyloxymethyl)-1,3-
dioxolane-2-one were determined by HPLC (Daicel Chiralcel OD, n-hexane/
2-propanol (9:1 v/v), 1.0 mL/min, 254 nm). Configurations of the above
carbonates were determined by optical purities of 3-benzyloxy-1,2-propanediols
derived from the hydrolysis using NaOH, respectively; (R)-3-benzyloxy-1,2-
(6) (a) Kumbhar, P. S.; Sanchez-Valente, J.; Lopez, J.; Figueras, F. Chem.
Commun. 1998, 535. (b) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. Studies
in the Surface Science, New Solid Acids and Bases; Kodansha-Elsevier:
Tokyo-Amsterdom, 1989; Vol. 51.
propanediol, [R]20 +5.8 ° (neat).
D
10.1021/ja9902165 CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/24/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 18, 1999 4527
Table 1. Cycloaddition of CO2 to Various Epoxides Catalyzed by
the Mg-Al mixed Oxidea
Scheme 1
distance of 3.12 Å with a coordination number of about 2. These
results mean the severe disordering of the MgO-like surface
structure by the introduction of an excess Al3+ ion. Vide supra,
the calcined hydrotalcite with the Mg/Al ratio of 3 did not have
acid and strong basic sites, which would be caused by the severe
disordering of the MgO-like structure. Judging from the small
degrees of the EXAFS damping and XRD broadening observed
for the mixed oxide with Mg/Al ratio of 5, a mild disordering of
the MgO structure occurred on this oxide; Mg2+ ions are
isomorphically substituted by Al3+ ions to form Mg-O-Al bonds
to disorder the small portion of the Mg-Mg shell, which would
bring about the formation of acid and strong basic sites on the
surface of the mixed oxides.14
a Reaction conditions: Mg-Al mixed oxide (Mg/Al ) 5, calcined
at 400 °C) 0.5 g, epoxide 4 mmol, DMF 3 ml, CO2 pressure 5 atm,
100 °C, and 24 h. b Yields of carbonates were determined by GLC
analysis using internal standards, based on epoxides. Values in
parentheses are isolated yields. In the case of the product isolation
experiments, the reaction scale was twice as much as that of reaction
conditions (a). c 120 °C. d 15 h. e A physical mixture of MgO and Al2O3
(Mg/Al ) 5) was used in place of the above Mg-Al mixed oxide.
We propose a possible reaction mechanism of the addition
reaction as shown in Scheme 1. The addition reaction is initiated
by adsorption of CO2 on the Lewis basic sites to form a carbonate
species, and independently, an epoxide is coordinated on the
neighboring an acid site on the surface. The coordinated epoxide
is ring-opened by a nucleophilic attack of the carbonate species,
which leads to an oxy anion species yielding the corresponding
cyclic carbonate as a product. The activity of the Mg-Al mixed
oxide was greatly larger than that of a physical mixture of MgO
and Al2O3 (entry 5, Table 1). Therefore, it is likely that a
prominent feature of the actiVe Mg-Al mixed oxide catalysts can
be found to originate from the cooperatiVe action of both the
basic and acidic sites located in a neighbor on the surface.
Kinetics data of this addition reaction could be well accom-
modated with a rate equation based on the Langmuir-Hinshel-
wood model where the CO2 and epoxide are independently
adsorbed on the different sites, i.e., basic sites and acidic sites,
respectively.15
Figure 1. Fourier transforms of k3-weighted Mg K-edge EXAFS of (a)
MgO calcined at 400 °C and those of Mg-Al mixed oxides obtained by
calcination of hydrotalcites with Mg/Al of (b) 8, (c) 5, and (d) 3 at
400 °C.
In conclusion, the Mg-Al mixed oxides obtained by the
calcination of hydrotalcites acted as efficient catalysts for the
addition of CO2 with various epoxides under mild reaction
conditions; in particular, the mixed oxide with the Mg/Al ratio
of 5 calcined at 400 °C showed the highest catalytic activity. This
unique catalysis is due to the cooperative action of acid-base
sites derived from the formation of Mg-O-Al bonds, i.e., the
substitution of Al for Mg in MgO matrix, on the surface of mixed
oxides. The Mg-Al mixed oxide catalysts were reusable and
could be proved effective for many organic reactions as envi-
ronmentally friendly acid-base catalysts.
However, the full width at half-maximum of (200) diffraction
peaks of calcined hydrotalcites gradually increased with increasing
the Al content. The Mg K-edge XANES spectrum of the calcined
hydrotalcite with the Mg/Al of 5 at 400 °C was closely resemble
to that of the MgO, while the Al K-edge XANES showed an
existence of a six-coordinated Al3+ ion within a local structure.12
Figure 1 depicts Fourier transforms of k3-weighted Mg K-edge
EXAFS of the Mg-Al mixed oxides with different Mg/Al ratios
calcined at 400 °C. The peaks around 2.6 Å attributable to the
Mg-Mg shell of MgO with 2.97 Å distance13 varied in their
intensities and positions with an increase of the Al content,
whereas those of the peaks around 1.5 Å due to the Mg-O shell
did not change. The curve-fitting analysis of the peak around 2.6
Å observed for the oxide with the Mg/Al ratio of 3 showed the
existence of the second neighboring cations located at a longer
Acknowledgment. We thank DAISO Co., Ltd. for providing chiral
epoxides of (R)- and (S)-benzyl glycidyl ether. X-ray absorption experi-
ments were approved by the Joint Studies Program (1997) of UVSOR of
the Institute for Molecular Science. We thank Professor Tsunehiro Tanaka,
Kyoto University, for providing his program for analysis of EXAFS.
JA9902165
(11) The NH3-TPD profiles were obtained by Q-mass spectrometer after
adsorption of NH3 onto the samples at 100 °C. The peak positions of the
desorbed NH3 of ca. 240 °C for the mixed oxides with the Mg/Al ratio of 5
and 8 calcined at 400 °C were higher than that of a low-temperature desorption
peak observed for hydrogen-bonded NH3 species on the zeolite. See; Katada,
K.; Igi, H.; Kim, J.-H.; Niwa, M. J. Phys. Chem. B 1997, 101, 5969.
(12) X-ray absorption spectra were recorded at the beam line 7 A station
attached with a beryl two-crystal monochromator (d ) 7.9825 Å) at UVSOR
of the Institute for Molecular Science, Okazaki, Japan. Data were collected
in a total electron yield mode, which mainly monitors X-ray energy dependence
of Mg KLL auger electron yield.
(14) The severely disordered surface of the Mg-Al oxide with Mg/Al ratio
of 3 cannot act as acid and strong base. This is related to the existence of
extraframework Al3+ with a five or four-coordinated Al3+ as evidenced by
Al K-edge XANES.
(15) The rate (R) of the reaction of styrene oxide with CO2 could be
expressed as R ) kobs K[S]{[A]/(1 + K[S])}, where kobs is an apparent rate
constant of the surface reaction of epoxide and CO2, K is an adsorption
equilibrium constant for epoxide adsorption on the acidic site of the Mg-Al
mixed oxide, [A] is the amount of the catalyst, and [S] is the concentration of
styrene oxide. The apparent activation energy of the addition reaction of CO2
(13) Yoshida, T.; Tanaka, T.; Yoshida, H.; Funabiki, T.; Yoshida, S. J.
Phys. Chem. 1996, 100, 2302.
with styrene oxide was estimated to be 78.0 kJ mol-1
.