DOI: 10.1002/cssc.201500128
Communications
Recyclable Bifunctional Polystyrene and Silica Gel-
Supported Organocatalyst for the Coupling of CO2 with
Epoxides
Christina Kohrt and Thomas Werner*[a]
Dedicated to Prof. Dr. Detlef Heller on the occasion of his 60th birthday.
A bifunctional ammonium salt covalently bound to a polysty-
rene or silica support proved to be an efficient and recyclable
catalyst for the solvent-free synthesis of cyclic carbonates from
epoxides and CO2. The catalyst can be easily recovered by
simple filtration after the reaction and reused in up to 13 con-
secutive runs with retention of high activity and selectivity
even at 908C. The scope and limitations of the reaction has
been evaluated in terms of reaction conditions and substrate
scope.
Numerous catalytic systems able to mediate this reaction
have been reported.[5] However, most of them require unfavor-
able elevated temperatures. Recently, metal-based as well as
organocatalytic systems have been evolved, permitting the use
of more sustainable reaction conditions.[6] A crucial point in
the development of environmentally benign processes is the
catalyst separation and recycling. Besides many other possibili-
ties for green separation techniques, the immobilization of cat-
alysts permits both facile separation from the product avoiding
tedious purification and isolation steps as well as easy recy-
cling of the catalyst.[7] In this context, amongst others, there
have been reports on immobilized catalysts for the conversion
of epoxides with CO2 on various inorganic supports, for exam-
ple, silica[8] as well as organic supports such as polyethylene
glycol,[9] various synthetic polymers,[8a,10] and even natural oc-
curring polymers.[11] Even though there are examples on immo-
bilized metal-based catalysts,[12] the majority of those reports
are on immobilized organocatalysts. However, most of those
systems usually require harsh reaction conditions, for example,
reaction temperatures >1008C, need reactivation, or loose ac-
tivity during the recycling processes, thus hampering the over-
all sustainability.
The development and design of sustainable synthetic methods
as well as environmentally benign products and processes is
guided by the general principles of green chemistry.[1] One
major scientific challenge in this context is the efficient utiliza-
tion of CO2 for the sustainable development of our society.[2]
The principle limitation to the use of CO2 as a C1 building
block originates from its inherent kinetic and thermodynamic
stability.[3] Thus, chemical processes utilizing CO2 as a feedstock
usually require high-energy starting materials and/or elevated
operating temperatures, which hampers the overall sustainabil-
ity of those processes. However, ideally such a process should
emit less CO2 than it uses. The atom-economic addition of CO2
to epoxides yielding cyclic carbonates is an interesting and fre-
quently studied reaction as those products might be utilized
as green solvents, synthetic building blocks, or plasticizers
(Scheme 1).[4]
Earlier, we developed homogeneous two-component cata-
lyst systems operating at favorable reaction temperatures of
458C.[13] Moreover, we described the straight forward synthesis
and utilization of functionalized phosphonium and ammonium
salts as very efficient homogeneous one-component organoca-
talysts for the conversion of epoxides 1 to cyclic carbonates
2.[14] Herein, we report the synthesis of a silica- as well as poly-
styrene-supported ammonium salt as recyclable and sustaina-
ble immobilized organocatalyst that shows high activity even
at reaction temperatures <1008C. Both catalysts were pre-
pared by the conversion of readily available supported amines
3a or 3b with iodoethanol (Scheme 2). Complete alkylation
was achieved after 4 days at 608C. Scheme 2 shows the syn-
thesis of catalyst 4a and 4b, their respective SEM images are
shown in Figure 1.
Scheme 1. Atom-economic conversion of epoxides 1 to cyclic carbonates 2
with CO2.
The coupling of butylene oxide (1a) and CO2 served as
a model reaction to determine the catalytic activity of the pre-
pared catalysts 4 and was performed in a batch-wise operation
(Table 1). Initial experiments were performed under identical
conditions (908C, 5 MPa, 2 h) in the presence of 1 mol% cata-
lyst based on the nitrogen content of 4 and with respect to
1a. It is worthwhile to mention that the supporting materials
3a and 3b showed no conversion under the chosen condi-
tions (entries 1 and 2). In contrast to 3a and 3b, the supported
[a] Dr. C. Kohrt, Dr. T. Werner
Leibniz-Institut fꢀr Katalyse e. V. an der Universitꢁt Rostock
Albert-Einstein Straße 29a, 18059 Rostock (Germany)
Fax: (+49)381-1281-5132
Supporting Information for this article is available on the WWW under
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