Angewandte
Chemie
DOI: 10.1002/anie.201203774
Heterogeneous Catalysis
Hydration of Epoxides on [CoIII(salen)] Encapsulated in Silica-Based
Nanoreactors**
Bo Li, Shiyang Bai, Xuefeng Wang, Mingmei Zhong, Qihua Yang,* and Can Li*
Monoethylene glycol (MEG) and 1,2-propylene glycol (PG)
are important raw materials for the manufacture of polyester
resins, antifreezes, cosmetics, medicines, and other products.[1]
The total global demand for MEG has been estimated to be
over 19 million metric tons per year.[2] The production of
MEG in industry predominantly involves the liquid-phase
hydration of ethylene oxide (EO), whereby a large excess of
water (20–25 mol of water/mol of EO) is required for the high
conversion of EO and high selectivity for MEG. The
concentration of MEG in the final aqueous solution is only
approximately 10 wt%, and huge energy is consumed for the
distillation of the product from the aqueous solution. As
a result, epoxide hydration is one of the most cost- and
energy-intensive processes in the chemical industry.
In this study, we developed a solid catalyst that is different
from the conventional liquid/solid acid or base catalysts for
the hydration of epoxides. We constructed the solid catalyst
by encapsulating the molecular catalyst [CoIII(salen)], with
a salen ligand derived from 3,5-di-tert-butylsalicyclaldehyde
and trans-1,2-diaminocyclohexane,[11] in the nanocage of the
mesoporous silica FDU-12. This catalyst exhibits high activity
and selectivity in the hydration of epoxides under mild
reaction conditions. Furthermore, the H2O/epoxide molar
ratio can be decreased to as low as 2:1 while maintaining the
conversion of EO above 98% and the selectivity for MEG
above 98%. This novel catalytic approach has great potential
for the green and energy-saving hydration of epoxides, as well
as many other conventional chemical reaction processes in
industry.
Possible catalytic hydration processes have been exten-
sively investigated for the environmentally friendly produc-
tion of MEG at a low energy cost. Various types of catalysts,
including liquid and solid acids or bases, have been explored,
such as sulfuric acid,[3] the salts of some acids,[4] cyclic
amines,[5] cation- and anion-exchange resins,[6] quaternary
phosphonium halides,[7] polymeric organosilane ammonium
salts,[8] macrocyclic chelating compounds,[9] and supported
metal oxides.[10] However, a high H2O/EO molar ratio (i.e.
> 10) is still required for high MEG selectivity. The MEG
selectivity is very low at low H2O/EO molar ratios owing to
the formation of diethylene glycol (DEG) and triethylene
glycol (TEG) by the self-condensation of MEG, which is
readily catalyzed by acid and base catalysts. Moreover, the
inherent corrosion and environmental problems associated
with the liquid acids/bases limit their application in industry.
The development of an efficient and environmentally benign
process for the hydration of epoxides with an H2O/epoxide
molar ratio approaching the stoichiometric value of the
chemical reaction is still a huge challenge.
The solid catalyst (denoted as FDU-12–[CoIII(salen)]) was
prepared by the encapsulation of [CoIII(salen)] molecular
catalysts into the nanocages of mesoporous silica (FDU-12);
the size of the pore entrance was then reduced by the use of
propyltrimethoxysilane as a silylation reagent to prevent the
molecular catalysts in the nanoreactor from leaching (for
synthesis details, see the Supporting Information).[12] FDU-12
is a mesoporous silica material with a cavity diameter of
18.6 nm and a pore-entrance size less than 4.0 nm.[13] These
dimensions make FDU-12 an ideal nanoreactor to host the
molecular catalyst (see Figures S1 and S2 in the Supporting
Information). According to TEM studies and its N2-sorption
isotherm, FDU-12–[CoIII(salen)] also has a cagelike porous
structure similar to that of FDU-12, with a uniform distribu-
tion of mesopores arranged in cubic Fm3m symmetry (Fig-
ure 1A,B). The apparent decrease in the BET surface area
and pore volume of the solid catalyst in comparison to FDU-
12 is due to the occupation of the nanocages by [CoIII(salen)]
molecules (see Table S1 in the Supporting Information).
FDU-12–[CoIII(salen)] has a similar UV/Vis spectrum to that
of [CoIII(salen)] dissolved in CH2Cl2, except for a slight red
shift of the UV bands, which suggests a weak host–guest
interaction between the nanocages of FDU-12 and [CoIII-
(salen)] (Figure 1C). NMR spectroscopy confirmed that
[CoIII(salen)] encapsulated in the nanocages of FDU-12
retains the molecular structure and properties of its free
form almost completely (see Figure S3). The molecular
catalyst confined in the nanoreactor maintained the behavior
of the homogeneous solution of the catalyst. Thus, FDU-12–
[CoIII(salen)] exhibits the characteristics of both homogene-
ous and heterogeneous catalysts.
[*] Dr. B. Li,[+] S. Bai,[+] Dr. X. Wang, M. Zhong, Prof. Dr. Q. Yang,
Prof. Dr. C. Li
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences
457 Zhongshan Road, Dalian 116023 (China)
E-mail: yangqh@dicp.ac.cn
[+] These authors contributed equally to this research.
[**] This research was supported financially by the National Basic
Research Program of China (2009CB623503) and the National
Natural Science Foundation of China (20921092). A salen ligand
derived from 3,5-di-tert-butylsalicyclaldehyde and trans-1,2-diami-
nocyclohexane was used.
We compared a variety of catalytic processes for epoxide
hydration at 408C with a 2:1 H2O/EO molar ratio (Table 1).
No product was detected without a catalyst or with only FDU-
12 in the absence of the molecular catalyst [CoIII(salen)]. The
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2012, 51, 11517 –11521
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11517