Synthesis and high-throughput testing of multilayered supported ionic liquid catalysts for the conversion of CO2 and epoxides into cyclic carbonates

Article abstract of DOI:10.1039/c3cy01000g

Multilayered covalently supported ionic liquid phase (mlc-SILP) materials were synthesised by grafting different bis-vinylimidazolium salts on thiol-functionalised silica. These materials, which contain a cross-linked oligomeric network of imidazolium units, were characterised and tested as catalysts for the reaction of carbon dioxide with various epoxides to produce cyclic carbonates. The materials prepared by supporting a bis-imidazolium iodide salt with xylene or octane as a linker between the imidazolium units were identified as the most active catalysts and displayed high turnover numbers and improved productivity compared to known supported ionic liquid catalysts. The most promising mlc-SILP catalysts were further studied to tune the reaction conditions towards optimum catalytic performance and to investigate their versatility with different substrates and their reusability. The rapid and parallel screening of the catalysts was efficiently carried out by means of high-throughput (HT) experimentation. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3cy01000g

Catalysis
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Synthesis and high-throughput testing of
multilayered supported ionic liquid catalysts
for the conversion of CO2 and epoxides into
cyclic carbonates†
Cite this: Catal. Sci. Technol., 2014,
4, 1598
Paola Agrigento,ab Syed M. Al-Amsyar,a Benjamin Sorée,c Masoumeh Taherimehr,a
b
c
a
*
*
*
Michelangelo Gruttadauria, Carmela Aprile and Paolo P. Pescarmona
Multilayered covalently supported ionic liquid phase (mlc-SILP) materials were synthesised by grafting
different bis-vinylimidazolium salts on thiol-functionalised silica. These materials, which contain a cross-
linked oligomeric network of imidazolium units, were characterised and tested as catalysts for the reaction
of carbon dioxide with various epoxides to produce cyclic carbonates. The materials prepared by
supporting a bis-imidazolium iodide salt with xylene or octane as a linker between the imidazolium
units were identified as the most active catalysts and displayed high turnover numbers and improved
productivity compared to known supported ionic liquid catalysts. The most promising mlc-SILP catalysts
were further studied to tune the reaction conditions towards optimum catalytic performance and to
investigate their versatility with different substrates and their reusability. The rapid and parallel screening
of the catalysts was efficiently carried out by means of high-throughput (HT) experimentation.
Received 2nd December 2013,
Accepted 15th January 2014
DOI: 10.1039/c3cy01000g
www.rsc.org/catalysis
reacting carbon dioxide with high free energy molecules,
among which epoxides have been widely studied.3 The reac-
Introduction
Carbon dioxide is generated in large amounts as the end
product of fuel combustion related to human activities and is
considered an environmental threat being one of the main
greenhouse gases.1 The widespread availability of CO2 stimu-
lated its use as a renewable, inexpensive and non-toxic carbon
source for the synthesis of useful chemical products.2 For
these features, carbon dioxide represents an attractive green
alternative to toxic reactants such as phosgene, isocyanates
or carbon monoxide. However, the use of CO2 as feedstock
requires scientific and technological progress concerning its
separation and conversion. Particularly, the challenge in the
conversion of CO2 stems from the high thermodynamic stabil-
ity of this molecule. This limitation can be overcome by
tion of CO2 with epoxides can generate two types of products:
cyclic carbonates and polycarbonates, both of which can find
relevant applications. This is an attractive reaction in terms of
green chemistry and atom efficiency, because CO2 reacts with
epoxides without generating any side-product. Various homo-
geneous and heterogeneous catalysts have been developed
to promote this reaction.3–5 Typically, the catalyst contains a
nucleophilic species that is able to attack and open the epox-
ide ring, thus facilitating the addition of CO2 (Scheme 1).
State-of-the-art homogeneous catalytic systems for this reac-
tion also include a Lewis acid species that activates the epox-
ide towards ring opening by the nucleophile.3
With the final aim of an industrial application of the reac-
tion of CO2 with epoxides, research effort should be dedi-
cated to the development of suitable heterogeneous catalysts,
which can be easily separated from the reaction products and
reused in consecutive runs. Supported ionic liquids with
halide counterions acting as nucleophiles have been reported
as a promising class of metal-free heterogeneous catalysts for
this reaction.6,7
a Centre for Surface Chemistry Catalysis, University of Leuven (KU Leuven), Kasteelpark
Arenberg 23, 3001 Heverlee, Belgium. E-mail: paolo.pescarmona@biw.kuleuven.be;
Tel: +32 16 321592
b Dipartimento di Scienze e Tecnologie Biologiche Chimiche e Farmaceutiche
(STEBICEF), Sezione di Chimica, University of Palermo, Viale delle Scienze s/n,
Ed. 17, I-90128 Palermo, Italy. E-mail: michelangelo.gruttadauria@unipa.it
c Unit of Nanomaterial Chemistry (CNano), Department of Chemistry,
University of Namur (UNAMUR), Rue de Bruxelles 61, 5000 Namur, Belgium.
E-mail: carmela.aprile@unamur.be; Tel: +32 81 724514
The most efficient approach for the immobilisation of
ionic liquids on a support is based on covalent grafting, which
grants high stability and thus good recyclability of the
catalyst. Usually, these catalysts are tested in a solvent-free
medium and often with CO2 under supercritical conditions
† Electronic supplementary information (ESI) available: GC and NMR data and
a table with comparison between the catalytic performance of the mlc-SILP
materials and other supported ionic liquids. See DOI: 10.1039/c3cy01000g
1598 | Catal. Sci. Technol., 2014, 4, 1598–1607
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the double bonds; this allows creation of the desired material
presenting a cross-linked network of imidazolium units cova-
lently anchored on the support (Fig. 2). The mlc-SILP mate-
rial prepared by supporting a bromide bis-imidazolium salt
on high surface area silica (either amorphous or ordered
mesoporous SBA-15) was identified as the best catalyst for
the synthesis of cyclic carbonates, displaying very good activ-
ity and improved productivity compared to known supported
ionic liquid catalysts.9
In this work, the potential of mlc-SILP as heterogeneous
catalysts for the reaction of CO2 with epoxides is further
explored. A series of mlc-SILP materials was prepared using
bis-vinylimidazolium salts with different organic linkers
between the two imidazolium units (Fig. 2) and two types of
halides as counterions (X = Br or I). Thiol-functionalised
silica was employed as the support.9 These materials were
characterised and tested as catalysts for the reaction of car-
bon dioxide with various epoxides. A rapid and efficient par-
allel screening of the series of mlc-SILP catalysts was carried
out using a tailored high-throughput (HT) unit for the study
of reactions under supercritical CO2.9–11
Scheme 1 Proposed mechanism for the synthesis of cyclic carbonates
via reaction of CO2 with epoxides catalysed by supported imidazolium-
based ionic liquids. X− is a halide that acts as a nucleophile.
(scCO2), which displays high solubility in various epoxides.
Since these supported ionic liquid catalysts are devoid of
metal centres that could act as Lewis acids, relatively high
temperature (typically ≥100 °C) is required in order to achieve
good conversions in the reaction between CO2 and epoxides.
This promotes the formation of cyclic carbonates, which are
the thermodynamically most favourable products.3 Cyclic car-
bonates are a class of versatile compounds, finding a broad
range of applications as aprotic polar solvents, as electrolytes
in secondary batteries, and as intermediates in the production
of polymers and fine chemicals.8
Results and discussion
Multilayered covalently supported ionic liquid phase
(mlc-SILP) materials are a recently developed class of mate-
rials that is finding promising applications in heterogeneous
catalysis.9,12 A previous study from our groups showed that
Recently, we reported the synthesis of a new class of mul-
tilayered covalently supported ionic liquid phase (mlc-SILP)
catalysts.9 These materials present a much higher loading of
the catalytically active imidazolium salt compared to conven-
tional supported ionic liquids consisting of a covalently
anchored monolayer (Fig. 1). The multilayered ionic liquid
phase can be generated by grafting a bis-vinylimidazolium
halide salt on a suitable support, typically a high surface area
silica or a polymer functionalised with groups that allow
anchoring of the ionic liquid (e.g. thiol groups, –SH). The two
terminal vinyl groups of the bis-vinylimidazolium salt can
react not only with the functionalised surface of the support
but also with each other through a self-addition reaction of
Fig. 1 Schematic representation of: (a) covalently anchored monolayer
of ionic liquid; (b) adsorbed multilayer of ionic liquid; (c) covalently
anchored multilayer of ionic liquid. The latter material combines the
asset of a stable, covalent grafting (as in a) with the high loading of
ionic liquid of a multilayered material (as in b).
Fig.
2 Schematic representation of the synthesis of multilayered
supported ionic liquid phase materials from bis-vinylimidazolium salts
with different organic linkers (a–g). The wavy lines in the scheme
symbolise further extensions of the supported ionic liquid phase.
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mlc-SILPs prepared from bis-imidazolium chloride or bro-
mide with xylene as a linker between the imidazolium units
are active catalysts for the reaction of CO2 with epoxides,
reaching higher productivity compared to monolayered
supported ionic liquids. Here, we present the synthesis, char-
acterisation and catalytic application of a series of new mlc-
SILP materials. The multilayered covalently supported ionic
liquids were prepared using a series of bis-imidazolium salts,
having different lengths and bulkiness of the organic linker
connecting the two imidazolium units (Fig. 2). The distance
between the two imidazolium units is expected to have a rele-
vant influence on the catalytic performance of the materials,
as the attack on the epoxide by the nucleophile is likely to
occur in the proximity of the imidazolium units (Scheme 1).
Too short a distance between two connected imidazolium
units might prevent simultaneous reaction at both of them,
while too large a distance would imply a lower loading of
active sites per gram of catalyst. Moreover, the chemical
nature of the linker can influence the approach of CO2 and of
the epoxide to the active sites. The second parameter that was
varied in the bis-imidazolium salts is the nature of the halide
counterion. This is a crucial element, because the halide is
the nucleophile that initiates the catalytic reaction (Scheme 1).
The catalytic performance is determined by a combination of
the nucleophilicity of the halide towards attack on the epox-
ide, its coordination ability to the imidazolium unit and its
leaving ability from the carbonate intermediate.3 A previous
study on mlc-SILPs showed that higher catalytic activity was
obtained with Br− compared to Cl−.9 This result stimulated us
to investigate mlc-SILPs prepared using Br− and I− as halides.
Iodide has been often reported to lead to higher catalytic
activity compared to bromide,10 though the complexity of the
balance between nucleophilicity, coordination ability and
leaving ability implies that different catalytic systems show
different orders of activity between the halides.3
silica and the double bond of the bis-vinylimidazolium salts,
in the presence of 2,2′-azobisisobutyronitrile (AIBN) as a radi-
cal initiator (Fig. 2). This reaction offers all of the desirable
features of a ‘click reaction’, being highly efficient and sim-
ple to execute with no side-products, and proceeding rela-
tively rapidly with high yield. Since the bis-vinylimidazolium
salt was used in excess relative to the amount of thiol groups
(3.62 mol of salt per mol of thiol group), the formation of a
cross-linked oligomeric network of imidazolium units
through self-addition reaction of the double bonds is also
expected. Therefore, this protocol aims at the formation of a
multilayered ionic liquid phase and its covalent grafting on
the functionalised silica support.
All of the mlc-SILP materials were characterised by means
of elemental analysis to evaluate the loading of the ionic
liquid phase and the molar ratio between the supported
ionic liquid and the original amount of –SH groups (Table 1).
The employed method allows reaching a high loading of
anchored bis-vinylimidazolium salt (51 to 67 wt.%) and high
IL/-SH molar ratios. These results demonstrate the general
validity of this synthesis method, in which the use of excess
bis-vinylimidazolium salt with respect to the thiol groups
leads to the formation of the desired multilayered covalently
supported ionic liquid phase. The successful formation of
the oligomeric network of supported ionic liquids was con-
firmed by solid-state 13C NMR spectroscopy (Fig. 3); the
absence of the signal corresponding to the vinyl group of the
bis-vinylimidazolium salts (at δ = 110 ppm) is an indication
that the terminal double bonds reacted completely, either
with the thiol groups of the support, or with the vinyl groups
of other bis-vinylimidazolium salts. The characteristic signals
of the carbon atoms of the imidazolium ring are observed at
δ = 121–124 and 136 ppm, whereas the aliphatic carbon
atoms resonate in the range 27–60 ppm. These data indicate
that the main structure of the bis-vinylimidazolium salts is not
affected by the grafting process. The weak signal at δ = 12 ppm
is due to the carbon atoms connected to the silicon atoms
of the functionalised silica.
High surface area amorphous silica (entry 1 in Table 1)
functionalised with thiol groups was employed as a support
for the new mlc-SILP materials. The bis-vinylimidazolium
salts were grafted on the silica support through a thiol–ene
coupling reaction between the thiol groups of the modified
The textural properties of the mlc-SILP materials were
studied by N2 adsorption/desorption measurements (Table 1).
Table 1 Loading of the ionic liquid phase and textural properties of the mlc-SILP materials as a function of the type of bis-vinylimidazolium salt
employed in the synthesisa
Imidazolium unitsb
Loading of
supported IL (wt.%)
Surface area
Cumulative pore
Entry
mlc-SILP
(mmol g−1
)
IL/–SH molar ratioc
(m2 g−1
)
volume (cm3 g−1
)
1
2
3
4
5
6
7
8
SiO2
—
—
—
750
129
146
134
89
121
124d
133
0.7
0.2
0.2
0.3
0.2
0.1
0.1d
0.3
SiO2–ethane–Br
SiO2–butane–Br
SiO2–butane–I
SiO2–octane–Br
SiO2–octane–I
SiO2–p-xylene–Br
SiO2–p-xylene–I
2.7
2.9
2.7
2.5
2.2
2.8d
2.0
2.3
2.8
3.4
2.5
2.4
3.5d
1.8
51
58
67
58
61
64d
53
a
Synthesis of mlc-SILP materials: bis-vinylimidazolium salt (3.62 mol mol-SH−1, 130 mM), thiol-functionalised silica (1.2 mmol-SH g−1), ethanol,
b
c
AIBN, 78 °C, 20 h. Each bis-vinylimidazolium salt contains two imidazolium units. Molar ratio between the amount of bis-vinylimidazolium
salt in the obtained mlc-SILP and the amount of potential anchoring sites on the support (i.e. the original amount of –SH groups). d Data taken
from ref. 9.
1600 | Catal. Sci. Technol., 2014, 4, 1598–1607
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in the conversion of the two epoxides (Table 2), with virtually
complete selectivity (>99%) towards the cyclic carbonate prod-
uct in all cases.13 The lack of activity of the thiol-functionalised
silica (entry 9, Table 2) confirms that the observed activity
originates from the supported ionic liquid phases.
This set of results shows some clear trends that deserve to
be discussed. The mlc-SILP materials with iodide as the
counterion display higher activity in terms of conversion,
productivity (defined as gcarbonate gcatalyst−1) and turnover
Fig. 3 Solid state MAS 13C NMR spectra of SiO2–octane–Br (a) and
SiO2–octane–I (b).
number (TON = molepoxide
molhalide−1) compared to
converted
their counterparts with bromide as the counterion (Table 2).
This trend is an indication that the leaving ability of the
halide plays a role in determining the reaction rate.3 It
should be noted that the extent of this effect changes with
the nature of the organic linker between the two imidazolium
units (for example, compare entries 4 and 5 to entries 6 and
7 in Table 2). A second trend can be observed by comparing
the catalytic results as a function of the type of organic
linker: the activity gradually increases with the length of the
chain passing from ethane to octane, with the trend being
more pronounced with styrene oxide as the substrate (com-
pare entries 1, 2 and 4 in Table 2). These results suggest that
a better separation between the imidazolium units can prove
favourable for simultaneous reaction at both of them.
However, the final catalytic performance of each of the mlc-SILP
materials is likely to be influenced also by other physico-
chemical features of the material, which determine how read-
ily the epoxide and the CO2 can access the ionic liquid phase.
In this context, the lengthening of the organic linker can
increase the affinity of the mlc-SILP for CO2, as suggested by
previous studies that evidenced the increase in the solubility
of supercritical CO2 in ionic liquids with increasing length of
the alkyl chain on the imidazolium cation (from C2 to C8).14
Moreover, the features of the bis-vinylimidazolium salt used
as the precursor are expected to influence the way in which
the multilayered ionic liquid phase is organised, as reflected
In line with previous reports, a substantial decrease in surface
area and cumulative pore volume was observed after grafting
of the ionic liquid phase.9 This dramatic change in the tex-
tural properties compared to those of the parent silica further
confirms the successful grafting of the bis-vinylimidazolium
salts on the support.
Although the characterisation data clearly prove the for-
mation of the desired mlc-SILP with all of the employed bis-
vinylimidazolium salts, they do not allow identifying the
exact structure of the oligomeric network of supported ionic
liquids, which is likely to be disordered (see Fig. 2 for a possi-
ble representation).
The multilayered covalently supported ionic liquid mate-
rials were tested as catalysts for the synthesis of cyclic car-
bonates via the reaction of epoxides with carbon dioxide
under solvent-free conditions and with CO2 under supercriti-
cal conditions (i.e. at a pressure above 73.8 bar and a temper-
ature above 31 °C). All of the reactions were carried out in a
high-throughput unit that enables a rapid and reliable paral-
lel testing of the series of catalysts in a block consisting of 24
parallel batch reactors.9–11 In order to investigate the versatil-
ity of our catalysts, two different epoxides were selected as
substrates for the reaction with CO2: an aliphatic terminal
epoxide (1,2-epoxyhexane) and an aromatic terminal epoxide
(styrene oxide). All of the mlc-SILP catalysts display activity
Table 2 Catalytic test of the mlc-SILP materials in the cycloaddition of CO2 to styrene oxide and 1,2-epoxyhexanea
Styrene oxide
Conv.c (%)
1,2-Epoxyhexane
Conv.c (%)
Entry
mlc-SILP
Prod.d
TONe
Prod.d
TONe
1
2
3
4
5
6
7
8
9
SiO2–ethane–Br
SiO2–butane–Br
SiO2–butane–I
SiO2–octane–Br
SiO2–octane–I
SiO2–p-xylene–Br
SiO2–p-xylene–I
SiO2–p-xylene–Ib
SiO2–SH
48
56
66
79
85
51
99
37
0
36
43
51
60
65
39
76
28
0
82
92
116
146
180
84
237
88
0
20
20
29
27
37
12
45
21
0
13
14
19
18
25
8
30
14
0
34
33
50
50
78
20
108
51
0
a
b
Reaction conditions: epoxide (14.0 mmol), mesitylene (1.40 mmol), 30 mg of catalyst, CO2 (80 bar), 150 °C, 3 h. Reaction conditions:
c
epoxide (46.7 mmol), mesitylene (4.67 mmol), 100 mg of catalyst, CO2 (80 bar), 125 °C, 3 h. Conversion and product yields determined by GC,
using mesitylene a−s1the internal standard. For selected samples the conversion−a1nd yields were confirmed by 1H NMR. Productivity, defined
d
e
as gcarbonate gcatalyst
.
Turnover number, defined as molepoxide converted molhalide .
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by the observed differences in the pore volume of the various
mlc-SILPs (Table 1), and this can affect the catalytic perfor-
mance of the material.
With all of the studied mlc-SILP catalysts, higher epoxide
conversions were observed with styrene oxide as compared to
1,2-epoxyhexane (Table 2). This behaviour is different from the
generally observed lower conversion with styrene oxide6d,15 and
is ascribed to the higher polarity of this substrate, which can
favour the interaction with the polar supported ionic liquid
phase in which the catalytic reaction takes place.
amounts due to the high loading of the catalytically active phase
(Table 1).
The pressure of carbon dioxide at which the reaction is
performed is known to influence the degree of conversion of
the epoxide.9,16 Two fluid phases are present under the reac-
tion conditions employed in the first set of tests reported
here, and only a fraction of the CO2 present in the reactor
dissolves in the bottom phase containing the epoxide.9 Ide-
ally, the CO2 pressure should be high enough to reach a suffi-
ciently high concentration of CO2 dissolved in the phase
where the reaction occurs, but not too high to avoid excessive
dilution of the epoxide. A previous study with mlc-SILP
catalysts showed that higher carbonate yields are obtained
with a CO2 pressure of 80 bar compared to 100 bar.9 Here,
this parameter was tuned by investigating the catalytic activ-
ity of SiO2–p-xylene–I in the reaction of 1,2-epoxyhexane with
CO2 at 40, 60 and 80 bar. An intermediate pressure of 60 bar
of CO2 (i.e. below the supercritical point) provided the
highest epoxide conversion with the employed amount of
epoxide and catalyst (Fig. 4), though the differences in the
conversion obtained in the three experiments are not major.
The versatility of the mlc-SILP catalysts in the synthesis of
cyclic carbonates from different epoxides and CO2 was tested
using SiO2–octane–I (Table 4). Both terminal and internal
epoxides were selected as substrates. Internal epoxides are
generally more difficult to convert to cyclic carbonates due to
the higher steric congestion around the epoxide ring, which
can hinder the nucleophilic attack by the halide.3 Moreover,
the formation of cyclic carbonates from cyclic epoxides such
as cyclohexene oxide and the seldom investigated cyclo-
pentene oxide is further limited by the geometric strain of
the five-membered cyclic carbonate ring imposed by the adja-
cent cyclohexane or cyclopentane ring. In line with these
argumentations, higher carbonate yields were observed with
the terminal epoxides, while cyclohexene oxide and cyclo-
pentene oxide showed lower reactivity, leading to only low
carbonate yields (Table 4). The selectivity towards the cyclic
carbonate product was excellent with all substrates. Only the
cis-isomer of the cyclic carbonate was observed using cyclo-
hexene oxide and cyclopentene oxide as substrates.11,18
The best catalysts identified in this set of experiments are
SiO2–octane–Br, SiO2–octane–I and SiO2–p-xylene–I, with the
latter reaching full conversion of styrene oxide to styrene
carbonate under the employed reaction conditions, with a
−1
productivity of 76 gcarbonate gcatalyst and TON = 237 (Table 2).
The results with these catalysts are markedly superior compared
to the previous optimum mlc-SILP catalyst (SiO2–p-xylene–Br),9
which was used in this work as a reference. SiO2–p-xylene–I
was also tested at lower temperature (125 °C), achieving lower
but still appreciable conversion of the two epoxides with the
same complete selectivity towards the cyclic carbonate prod-
uct (entry 8 in Table 2). The three most promising mlc-SILP
catalysts identified in the first set of tests were further
employed to study the effects of the amount of the catalyst
and the CO2 pressure employed in the reaction, their versatil-
ity with different substrates and their reusability.
The effect of the amount of the catalyst employed in the
reaction of styrene oxide with CO2 was studied using the two
mlc-SILPs prepared with octane as the linker, SiO2–octane–Br
and SiO2–octane–I. The higher activity of the catalyst with
iodide was confirmed with each of the three tested amounts
of the catalyst (Table 3). Remarkably, the degree of conversion
of styrene oxide over SiO2–octane–I decreases only moderately
upon halving the amount of the catalyst, and upon decreasing
it to one quarter of the original loading, an appreciable con-
version of 45% is achieved, with excellent productivity
−1
(138 gcarbonate gcatalyst−1) and TON (381 molepoxide converted molhalide
)
after 3 h of reaction. Complete selectivity towards styrene carbon-
ate was observed in all cases. These results underline the assets of
the mlc-SILP catalysts, which can be employed in rather low
Table 3 Effect of the amount of the SiO2–octane–X catalyst (X = Br or I) in the reaction of CO2 with styrene oxidea
Entry
Catalyst amount (g)
Halide (X)
Conv.b (%)
Prod.c
TONd
Sel.cyclic carbonate (%)
1
2
3
4
5
6
0.030
0.016
0.008
0.030
0.016
0.008
I
I
I
Br
Br
Br
85
72
45
79
50
24
65
103
138
60
72
72
180
286
381
145
174
173
>99
>99
>99
>99
>99
>99
a
b
Reaction conditions: epoxide (14.0 mmol), mesitylene (1.40 mmol), catalyst, CO2 (80 bar), 150 °C, 3 h. Conversion and product yields
c
−1
d
determined by GC, using m−e1sitylene as the internal standard. Productivity, defined as gcarbonate gcatalyst
.
Turnover number, defined
as molepoxide converted molhalide
.
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powder. The samples typically contain a fraction of polymer-
looking slabs with size in the millimetre-range. These flat
particles probably originate from a fraction of the bis-
vinylimidazolium salt that polymerised without reacting with
the support, thus generating unanchored polymeric species.
The formation of these slabs was particularly evident for
sample SiO2–octane–I. The possible effect of these slabs on
the catalytic activity of the material was studied by separating
them using a 200 μm analytical sieve and by testing the
remaining fine powder as a catalyst in the reaction of CO2 with
1,2-epoxyhexane. The sieved catalyst displays much higher
catalytic activity compared to the as-synthesised SiO2–octane–I
(entries 1 and 2 in Table 5), indicating that the presence of
the large particles is detrimental for the catalytic activity of
the material. This suggests that the support plays an impor-
tant role in the catalytic performance of the multilayered ionic
liquid phase. In order to investigate further this hypothesis,
a material was prepared using the same conditions as in the
synthesis of SiO2–octane–I, but without the silica support.
The polymerisation of the bis-vinylimidazolium salt was com-
plete, as indicated by the absence of the signal at 110 ppm
in the 13C NMR spectrum (Fig. S1 in the ESI†). The obtained
material (octane–I) displays significantly lower catalytic
activity compared to SiO2–octane–I in the reaction of CO2 with
1,2-epoxyhexane (Table 5 – the activity difference is particu-
larly evident when comparing the TONs), confirming the
importance of the support in creating an accessible and thus
catalytically active ionic liquid phase. However, it should be
noted that the polymeric octane–I material does not consist
exclusively of large particles, and the finer material obtained
by sieving octane–I with a 200 μm analytical sieve displays
slightly higher activity (entries 3 and 4 in Table 5), though still
clearly lower compared to SiO2–octane–I. This set of experi-
ments underlines the complexity of the prepared mlc-SILP
materials, which mainly consist of the desired, highly active
supported ionic liquid phase, but also contain a fraction of
unsupported polymeric ionic liquid phase, which displays
decreasing activity as the size of the particles increases. Future
Fig. 4 Effect of CO2 pressure on the cycloaddition to 1,2-epoxyhexane
over SiO2–p-xylene–I. Reaction conditions: epoxide (23.4 mmol), mesitylene
(2.34 mmol), catalyst (50 mg), CO2 (80 bar), 150 °C, 3 h. Conversion and
product yields were determined by GC, using mesitylene as the internal
standard. These experiments were performed in a batch reactor with
two borosilicate windows that enable the direct visualisation of the
phases during the reaction.17
Table 4 Catalytic tests of SiO2–octane–I in the cycloaddition of CO2
to different epoxidesa
Entry Epoxide Carbonate
Yieldcarbonate (%) Prod.d
TONe
69
1
2
3
32b
67b
5b
18
36
3
143
10
4
10c
6
20
a
Reaction conditions: epoxide (14.02 mmol), mesitylene (1.40 mmol),
30 mg of catalyst, CO2 (80 bar), 150 °C, 3 h. b Conversion and product
yields determined by 1H NMR, using mesitylene as the internal
c
standard. Conversion determined by GC, using mesitylene as the
d
−1
internal standard.
Productivity, defined as gcarbonate gcatalyst
.
e
−1
Turnover number, defined as molepoxide converted molhalide
.
The reusability of a selected mlc-SILP catalyst, SiO2–octane–Br,
was investigated in consecutive runs of the reaction of styrene
oxide with CO2. The tests were carried out with a much lower
amount of catalyst relative to the epoxide as compared to the
experiments in Table 2, in order to achieve intermediate con-
version. These conditions are suitable for monitoring possible
changes in the catalytic activity. Mlc-SILP catalysts are not
prone to leaching, but have been reported to present a varia-
tion in activity upon recycling.9 In line with previous reports,
the catalytic activity increased until the third run and then
gradually decreased to achieve a similar activity in the fifth as
in the first run (Fig. 5). This behaviour has been ascribed to
modifications (plasticisation) in the oligomeric network of
supported ionic liquids induced by CO2.9
Fig.
5 Recycling tests of SiO2–octane–Br as the catalyst for the
synthesis of styrene carbonate from CO2 and styrene oxide. Reaction
conditions: styrene oxide (174 mmol), mesitylene (17.4 mmol), 75 mg of
the catalyst, CO2 (80 bar), 150 °C, 3 h. Conversion and product yields
were determined by 1H NMR, using mesitylene as the internal standard.
The mlc-SILP materials prepared by grafting and cross-
linking the bis-vinylimidazolium salts on a thiol-functionalised
silica support are obtained in the form of a light-yellow
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Table 5 Comparison between the mlc-SILP SiO2–octane–I and the corresponding unsupported polymeric material octane–I as catalysts for the
cycloaddition of CO2 to 1,2-epoxyhexanea
Entry
Catalyst
Conv.b (%)
Prod.c
TONd
Sel.cyclic carbonate (%)
1
2
3
4
SiO2–octane–I
SiO2–octane–I (sieved)
Octane–I
37
61
13
17
25
41
9
78
n.d.
17
>99
>99
>99
>99
Octane–I (sieved)
12
23
a
b
Reaction conditions: 1,2-epoxyhexane (14.0 mmol), mesitylene (1.40 mmol), catalyst (30 mg), CO2 (80 bar), 150 °C, −31 h. Conversion and
c
d
product yields determined by GC, usin−1g mesitylene as the internal standard. Productivity, defined as gcarbonate gcatalyst
.
Turnover number,
defined as molepoxide converted molhalide
.
studies should aim at optimising the synthesis method to
minimise the formation of the polymeric slabs, thus leading
to further improvements in the catalytic performance of the
mlc-SILP materials.
using a Shimadzu FTIR 8300 infrared spectrophotometer.
Melting points were determined using a Kofler hot plate. The
specific surface areas (BET method) and the cumulative pore
volumes (BJH method) of the silica-based materials, before
and after supporting the ionic liquid phase, were determined
from the N2 adsorption/desorption isotherms measured at
−196 °C using a Micromeritics Tristar 3000 instrument.9
Analysis of the reaction mixture at the end of the catalytic
test was performed by gas chromatography (GC) on a Trace
GC Ultra from Interscience (RTX-5 column, 5 m, 0.1 mm).
The temperature profile during the analysis was: 50 °C for
0.50 min, from 50 to 250 °C at 150 °C min−1, 0.5 min at
250 °C. When necessary, the identity of the products was deter-
mined by gas chromatography-mass spectrometry (GC-MS) on
an Agilent 6890N gas chromatograph (WCOT fused silica col-
umn, 30 m, 0.25 mm) coupled to an Agilent 5973 MSD mass
spectrometer. Additionally (or alternatively), the conversion
Conclusions
Multilayered covalently supported ionic liquid phase (mlc-SILP)
materials were synthesised using a protocol involving covalent
grafting of bis-vinylimidazolium salts on a thiol-functionalised
silica support and cross-linking between the bis-vinylimidazolium
units. The method proved versatile and allowed the prepara-
tion of a series of mlc-SILP materials with different organic
linkers between the imidazolium units and with two types of
halides as counterions. The mlc-SILP materials are active
heterogeneous catalysts for the reaction of carbon dioxide
with various epoxides to yield cyclic carbonates. The most
promising catalysts achieved excellent turnover number and
productivity compared to the state-of-the-art supported ionic
liquid catalysts for the conversion of CO2 and epoxides into
cyclic carbonates (see Table S1 in the ESI† for a comparison
with selected supported ionic liquid catalysts).6,9,16,19
1
and product yields were determined by H NMR spectroscopy
carried out on a 300 MHz spectrometer (7.0 T). 32 scans were
accumulated with a recycle delay of 1 s. The pulse length was
8.0 ms and the power level was 0.0 dB. The samples were pre-
pared by adding around 1 mL of CDCl3 to an aliquot of the
reaction solution at the end of the catalytic test. Cyclic carbon-
ates are known compounds and showed spectroscopic data in
agreement with their structures.10,11,18
Experimental section
Spectroscopic and analytical methods
1H NMR and 13C NMR spectra of the bis-vinylimidazolium
salts were recorded on a Bruker 300 MHz spectrometer. The
chemical shifts for 1H and 13C are given in ppm relative to
the residual signal of the solvent (CD3OD). The following
abbreviations are used to indicate the multiplicity: s (singlet);
d (doublet); t (triplet); q (quartet); m (multiplet); bs (broad
signal). Solid-state 13C CP MAS NMR spectra of the mlc-SILP
materials were recorded on an Agilent 400 MHz spectrome-
ter. The samples were packed in zirconia rotors spinning at a
frequency of 15 kHz. Tetramethylsilane was employed as a
chemical shift reference. Carbon, nitrogen and sulphur con-
tents were determined by combustion analysis using a
Thermo Finnigan Flash EA 1112. FT-IR spectra were obtained
Synthesis of the bis-vinylimidazolium salts
1,4-Bis(iodomethyl)benzene (1g) was prepared according to a
procedure reported in the literature.20 Bis-vinylimidazolium
salts 2b and 2f were prepared following previously reported
methods and showed spectroscopic and analytical data in
agreement with those in the literature.9,12c All of the other
bis-vinylimidazolium salts are reported here for the first time.
Their synthesis procedures were similar to those previously
reported (Scheme 2).9,12c In a typical synthesis, a solution of
the selected compound 1 (0.01 mol) and 1-vinylimidazole
(0.021 mol) in toluene (10 mL, in the case of compounds 2f–g)
or chloroform (5 mL, in the case of compound 2a–e) was
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46.9; H, 6.1; Br, 34.7; N, 12.2; C18H28Br2N4 requires C, 46.8; H,
6.2; Br, 34.5; N, 12.1].
1
Diiodide salt 2e. Pale yellow powder. Yield: 94%; H NMR
(300 MHz, CD3OD): δ (ppm)=1.42(8H,bs,CH2CH2(CH2)4CH2CH2),
1.97–1.93 [4H, m, CH2CH2(CH2)4CH2CH2)], 4.31–4.26 [4H, m,
CH2CH2(CH2)4CH2CH2)], 5.44 (2H, dd, J = 9.0 and 2.7 Hz,
cis-CHCH2), 5.93 (2H, dd, J = 15.6 and 2.4 Hz, trans-CHCH2),
7.26 (2H, dd, J = 15.6 and 8.7 Hz, CHCH2), 7.79 (2H, d,
J = 1.2 Hz, 4-H or 5-H), 8.00 (2H, d, J = 1.5 Hz, 4-H or 5-H),
9.44 (2H, s, 2-H); 13C NMR (CD3OD): δ (ppm) = 27.1, 29.8,
30.9, 51.3, 110.0, 120.8, 124.6, 129.9, 136.2. [Found: C, 39.1; H,
5.2; I, 45.8; N, 10.1; C18H28I2N4 requires C, 39.3; H, 5.4; I, 45.1;
N, 10.8].
Scheme 2 Synthesis of bis-vinylimidazolium salts.
heated under stirring for 24 h in an oil bath at 90 °C (for tolu-
ene) or at 50 °C (for chloroform). After cooling down to room
temperature, the mixture was filtered and the residue was
washed several times with diethyl ether. Finally, the solid
product was dried at 40 °C.
Dibromide salt 2a. White powder. Yield: 75%; mp >300 °C;
1H NMR (300 MHz, CD3OD): δ (ppm) = 4.96 (4H, m, CH2CH2),
5.49 (2H, dd, J = 10.4 and 3.0 Hz, cis-CHCH2), 5.98 (2H, dd,
J = 18.9 and 3.6 Hz, trans-CHCH2), 7.31 (2H, dd, J = 10.4
and 18.8 Hz, CHCH2), 7.85 (2H, d, J = 2.2 Hz, 4-H or 5-H),
8.09 (2H, d, J = 2.3 Hz, 4-H or 5-H), 9.54 (2H, s, 2-H). 13C
NMR (CD3OD): δ (ppm) = 50.0, 110.8, 121.4, 124.6, 129.8,
130.9, 137.4. [Found: C, 38.3; H, 4.3; Br, 42.5; N, 14.9;
C12H16Br2N4 requires C, 38.3; H, 4.4; Br, 42.5; N, 14.9].
Diiodide salt 2g. Pale yellow powder. Yield: 88%; mp 232 °C;
1H NMR (300 MHz, CD3OD): δ (ppm) = 5.46 (2H, dd, J = 8.7
and 2.7 Hz, cis-CHCH2), 5.52 (4H, s, CH2Ph), 5.95 (2H, dd,
J = 15.6 and 2.7 Hz, trans-CHCH2), 7.29 (2H, dd, J = 15.6 and
8.7 Hz, CHCH2), 7.60 (4H, s, Ph), 7.77 (2H, d, J = 1.8 Hz, 4-H
or 5-H), 8.05 (2H, d, J = 1.8 Hz, 4-H or 5-H), 9.46 (2H, s, 2-H).
13C NMR (CD3OD): δ (ppm) = 53.9, 110.3, 121.1, 124.4, 129.8,
128.8, 130.9, 136.1. [Found: C, 39.6; H, 3.7; I, 46.5; N, 10.3;
C18H20I2N4 requires C, 39.4; H, 3.5; I, 46.4; N, 10.2].
Diiodide salt 2c. Pale yellow powder. Yield: 95%; mp
160 °C. 1H NMR (300 MHz, CD3OD): δ (ppm) = 2.12–2.07 [4H, m,
CH2(CH2)2CH2)], 4.42 [4H, m, CH2(CH2)2CH2)], 5.46 (2H, dd,
J = 8.7 and 2.7 Hz, cis-CHCH2), 5.97 (2H, dd, J = 15.6 and
2.7 Hz, trans-CHCH2), 7.30 (2H, dd, J = 15.6 and 8.7 Hz,
CHCH2), 7.86 (2H, d, J = 1.5 Hz, 4-H or 5-H), 8.02 (2H, d,
J = 1.5 Hz, 4-H or 5-H), 9.48 (2H, s, 2-H); 13C NMR
(CD3OD): δ (ppm) = 27.7, 50.4, 110.1, 120.9, 124.5, 129.8,
136.5; IR (nujol) max (cm−1): 921, 1154, 1377, 1458, 1550,
2854, 2922. [Found: C, 33.8; H, 4.1; I, 50.9; N, 11.3;
C14H20I2N4 requires C, 33.5; H, 4.5; I, 50.3; N, 11.2].
Synthesis of thiol-functionalised silica
Thiol-functionalised silica was prepared using Sigma–Aldrich
SiO2 (403563) as the silica source. 1.5 g of silica gel was heated
overnight at 150 °C in a vacuum line, in order to remove phys-
ically adsorbed water. Then, the solid was reacted with
3-(mercaptopropyl)-trimethoxysilane (2 mL) in 10 mL toluene.
The mixture was heated under reflux in an argon atmosphere
for 24 h. After cooling to room temperature, the material was
isolated by filtration and washed with methanol (50 mL) and
diethyl ether (50 mL). The solid was dried under reduced pres-
sure to give the final material (1.9 g). The loading of thiol
groups was determined by means of elemental analysis of sul-
phur (1.2 mmol g−1).
Dibromide salt 2d. Pale yellow powder. Yield: 78%; mp
>178 °C; 1HNMR (300 MHz, CD3OD): δ (ppm) = 1.42 (8H, bs, CH2
CH2(CH2)4CH2CH2), 1.97–1.93 [4H, m, CH2CH2(CH2)4CH2CH2)],
4.32–4.28 [4H, m, CH2CH2(CH2)4CH2CH2)], 5.45 (2H, dd, J = 15.9
and 2.7 Hz, cis-CHCH2), 5.96 (2H, dd, J = 15.9 and 2.7 Hz,
trans-CHCH2), 7.29 (2H, dd, J = 15.6 and 8.7 Hz, CHCH2),
7.82 (2H, d, J = 1.8 Hz, 4-H or 5-H), 8.03 (2H, d, J = 1.8 Hz, 4-H
or 5-H), 9.44 (2H, s, 2-H); 13C NMR (CD3OD): δ (ppm) = 27.1,
29.7, 30.9, 51.2, 109.9, 120.7, 124.5, 129.8, 136.5. [Found: C,
Synthesis of mlc-SILP materials 1a–g
The mlc-SILP materials were synthesised using excess bis-
imidazolium salt with respect to the –SH groups: 3.62 mol of salt
per mol of –SH. In a typical synthesis, the thiol-functionalised
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silica, a bis-imidazolium salt, 2,2′-azobisisobutyronitrile (AIBN,
60 mg) and ethanol were placed in a three-necked round bot-
tom flask. The reactions were carried out employing a concen-
tration of 130 mM for all of the salts, except for the case of
SiO2–p-xylene–I in which the diiodide salt was used as a 40 mM
solution due to its lower solubility in ethanol. The suspension
was flushed by bubbling argon for 10 min. The flask was heated
to 78 °C to favour the dissolution of the bis-imidazolium salt.
The reaction mixture was magnetically stirred under argon for
20 h. After cooling down to room temperature, the solid was
filtered and washed with hot methanol and diethyl ether and
then dried in an oven at 40 °C overnight.
allowed to depressurise until the reactors were at room tem-
perature with an internal pressure <2 bar.9 Once the
depressurisation was complete, the reactor block was opened
and the glass vials were removed and centrifuged at 2000 rpm
for 3 min to separate the solid catalyst from the reaction mix-
ture. The supernatant solution was sampled and then diluted
with toluene or with ethyl acetate and analysed by GC, or
1
diluted with CDCl3 and analysed by H NMR (see Fig. S2 and
S3 in the ESI† for representative chromatograms and NMR
spectra). The assignment of GC peaks due to new products
was performed by gas chromatography-mass spectrometry
analysis (GC-MS). 1H NMR was also employed to confirm
the absence of the ionic liquid in the solution and to deter-
mine the ratio of carbonate to epoxide, which was used to cal-
culate the GC response factor for compounds that are not
available commercially.
Selected catalytic tests were performed in duplicate, and
the results did not show significant deviations between the
two tests. For these experiments, the average of the conver-
sions is reported.
Synthesis of the cross-linked polymeric ionic liquid phase
(octane–I)
The material was prepared by radical polymerisation. Bis-
imidazolium diiodide salt 2e (1.09 mmol, 130 mM), AIBN
(0.16 mmol) and ethanol were placed in a three-necked
round bottom flask. The reaction conditions were identical to
those employed for the synthesis of mlc-SILPs (vide supra).
Recycling experiments were performed by removing the
supernatant solution from the solid catalyst after the centri-
fugation step (see above). The solid was washed with toluene
(twice) and ethanol (once). After each addition of solvent, the
vial containing the suspension was placed in an ultrasonic
water bath for about 30 min to favour the removal of reaction
residues, followed by centrifugation. Finally, the catalyst was
dried in an oven at 85 °C overnight and ground before being
used for the next experiment.
Catalytic tests
The catalytic tests were performed in a high-throughput
block consisting of 24 batch reactors under individual mag-
netic stirring, except for the tests to study the effect of CO2
pressure and the recycling tests, which were carried out in a
visualisation batch reactor equipped with two borosilicate
windows to allow monitoring of the phases present in the
reaction mixture.9–11 Both the 24-reactors block and the visu-
alisation reactor are part of a custom made high-throughput
unit (Integrated Lab Solutions, ILS) for performing reactions
in (supercritical) carbon dioxide. The pressurisation of the
reactors is achieved by means of two ISCO pumps. The 24
parallel batch reactors are pressurised simultaneously with
CO2, while check valves protect against back-flow. Risks of
overpressure are avoided by automated depressurisation proto-
cols and by the presence of rupture disks. The temperature of
the 24-reactors block is regulated using a Huber Thermostat.
In a typical experiment, the epoxide, the catalyst and
mesitylene (as the GC and NMR internal standard) were intro-
duced in an open glass vial, which was then placed into the
selected batch reactor. The reaction block, either the 24-reactors
block or the visualisation reactor, was closed and the lines and
the reactor were purged with N2 for 10 min. For a typical test
carried under 80 bar of CO2, each reactor was pressurised under
40 bar of CO2 while keeping the block at room temperature.
Then, the temperature was increased to 150 °C with a rate of
10 °C min−1. During this process, the pressure inside the reac-
tor increased to 60 bar. Once a stable pressure value was
reached, the reactor was filled with CO2 to the selected pres-
sure (80 bar). The reactor was kept under the selected condi-
tions for 3 h with a stirring speed of 900 rpm. Then, the
stirring was stopped and the reactors were cooled down.
Depressurisation was started when the carbon dioxide was
not anymore under supercritical conditions.9 The system was
Acknowledgements
PA, SMA, MT and PPP acknowledge sponsorship in the frame
of a START1 research program. PA and MG acknowledge the
University of Palermo and COST Action CM0905 ORCA.
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