Supported Polyhedral Oligomeric Silsesquioxane-Based (POSS) Materials as Highly Active Organocatalysts for the Conversion of CO2

Article abstract of DOI:10.1002/cctc.201801351

Very high turnover numbers (TON) and productivity values up to 7875 and 740 respectively have been obtained for the conversion of CO₂ into cyclic carbonates by using hybrid materials based on imidazolium modified polyhedral oligomeric silsesquioxanes (POSS-Imi) grafted on amorphous silica (SiO₂) and mesostructured SBA-15. The heterogeneous organocatalysts were easily prepared via a straightforward synthetic procedure allowing to generate high local concentration spots of imidazolium active sites surrounding the POSS core. This synthetic procedure is also a promising approach for the design of a wide library of hybrid functional materials. The materials do not possess other co-catalytic species with Lewis or Br?nsted acid functionalities which still represents a challenging aspect for the outcome of the process. The recyclability of the catalysts was successfully verified for four consecutive runs. The catalytic versatility was proved with a wide range of epoxides and with the most challenging oxetane on large scale (105–210 mmol) showing higher performances in comparison with other unmodified imidazolium-based catalytic systems. The new hybrids based on supported POSS nanostructures allowed the sustainable conversion of carbon dioxide under solvents- and metal-free reaction conditions with a full selectivity toward cyclic carbonates.

Full text of DOI:10.1002/cctc.201801351

Accepted Article
Title: Supported POSS Based Materials as Highly Active
Organocatalysts for the Conversion of CO2
Authors: Carla Calabrese, Leonarda Liotta, Francesco Giacalone,
Michelangelo Gruttadauria, and Carmela Aprile
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10.1002/cctc.201801351
ChemCatChem
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Supported POSS Based Materials as Highly Active
Organocatalysts for the Conversion of CO2
Carla Calabrese,[a,b] Leonarda F. Liotta,[c] Francesco Giacalone,[a] Michelangelo Gruttadauria,*[a] and
Carmela Aprile*[b]
Abstract: Very high turnover numbers (TON) and productivity values
up to 7875 and 740 respectively have been obtained for the
conversion of CO2 into cyclic carbonates by using hybrid materials
based on imidazolium modified polyhedral oligomeric silsesquioxanes
(POSS-Imi) grafted on amorphous silica (SiO2) and mesostructured
SBA-15. The heterogeneous organocatalysts were easily prepared
via a straightforward synthetic procedure allowing to generate high
local concentration spots of imidazolium active sites surrounding the
POSS core. This synthetic procedure is also a promising approach for
the design of a wide library of hybrid functional materials. The
materials do not possess other co-catalytic species with Lewis or
Brønsted acid functionalities which still represents a challenging
aspect for the outcome of the process. The recyclability of the
catalysts was successfully verified for four consecutive runs. The
catalytic versatility was proved with a wide range of epoxides and with
the most challenging oxetane on large scale (105-210 mmol) showing
higher performances in comparison with other unmodified
imidazolium-based catalytic systems. The new hybrids based on
supported POSS nanostructures allowed the sustainable conversion
of carbon dioxide under solvents- and metal-free reaction conditions
with a full selectivity toward cyclic carbonates.
is attracting the interest of the scientific community due to the
possibility to use carbon dioxide as inexpensive, available,
nontoxic, and renewable C1 feedstock.[1] In particular, one of the
most interesting pathway to valorize CO2 is represented by
carbon dioxide fixation into epoxides for the production of cyclic
carbonates.[2] Furthermore, cyclic carbonates are compounds of
interest finding widespread applications as aprotic polar solvents,
intermediates for the synthesis of fine or bulk chemicals,
electrolyte for batteries.[3] According to green chemistry
principles,[4] the synthesis of cyclic carbonates from CO2 and
epoxides is a productive catalytic process displaying an atom
economy of 100%. In order to face the challenging
thermodynamic stability of carbon dioxide, starting materials with
a relatively high free energy, like epoxides, have to be used
combined with a proper catalyst able to reduce the activation
energy of the process. In the last decades, several catalytic
systems, working under both homogeneous and heterogeneous
conditions, have been developed for the conversion of CO2 into
cyclic carbonates by reaction with epoxides. In particular, metal
oxides,[5] metal organic frameworks (MOF),[6] metal salts,[7] metal
complexes,[8] Lewis base systems,[9] ionic liquids (ILs),[10] and
organic polymers[10c, 11] have been proposed as catalyst for this
process. Among them, ionic liquids stand out for their high
efficiency.[12] Furthermore, it is well known that the design of
heterogeneous catalysts is particularly envisaged from industrial
parties because of their simple recovery from the reaction medium
and the possibility of using them in fixed bed reactors. The evident
advantages of heterogeneous catalytic systems encouraged the
development of supported ionic liquids hybrids for the production
of cyclic carbonates. In particular, imidazolium salts have been
immobilized on several solid supports such as cross-linked
polymers,[13] silicon-based main chain polyimidazolium salts,[14]
silica,[15] and carbon nanostructures[16]. It is worth pointing out that,
for a selected heterogeneous catalyst, the overall performance
depends on several features such as chemical, thermal and
mechanical stability, a good amount of functionalization active
sites, fast mass transport of reactants and products to and from
the active sites. Moreover, in the synthesis of cyclic carbonates
the activity of ionic liquid based catalysts strongly depends on the
nucleophilicity of the anionic species towards the attack into the
epoxide as well as on its ability as leaving group.[17] Therefore,
taking into account previously mentioned catalytic systems,
Introduction
Nowadays, the development of new technologies able to reduce
CO2 emission coming from anthropogenic activities is a topic of
growing interest from both academic and industrial parties.
Carbon dioxide capture, utilization and storage processes are part
of the challenging project to mitigate CO2 environmental impact.
The conversion of carbon dioxide into valuable chemical products
[a]
C. Calabrese, Prof. Francesco Giacalone, Prof. M. Gruttadauria
Department of Biological, Chemical and Pharmaceutical Sciences
and Technologies
University of Palermo
Viale delle Scienze, Ed. 17
90128 Palermo (Italy)
E-mail: michelangelo.gruttadauria@unipa.it
C. Calabrese, Prof. Carmela Aprile
Laboratory of Applied Materials Chemistry (CMA)
University of Namur
61 rue de Bruxelles
5000 Namur (Belgium)
[b]
[c]
imidazolium counterions play
a key role in the reaction
mechanism by promoting epoxide ring opening followed by the
CO2 insertion. Furthermore, the design of hybrid materials able to
combine the high activity and selectivity achieved by
homogeneous catalysts and the intrinsic advantages of
heterogeneous ones is still a challenging task. A promising
catalyst for the synthesis of cyclic carbonates is represented by
imidazolium functionalized polyhedral oligomeric silsesquioxanes
(POSS-Imi).[18] Polyhedral oligomeric silsesquioxanes (POSS)
E-mail: carmela.aprile@unamur.be
Dr. L. F. Liotta
Istituto per lo Studio dei Materiali Nanostrutturati
ISMN-CNR via Ugo La Malfa 153
90146 Palermo (Italy)
5000 Namur (Belgium)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201801351
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are usually considered as the smallest hybrid particles of silica.[19]
POSS are a class of organic-inorganic hybrid molecules made up
of an inorganic nanocaged silicate core surrounded by organic
functional groups dangling in a three dimensional arrangement.
The general formula of these hybrid molecules is (R-SiO1.5)n,
where n is commonly 6, 8, 10 and 12, the ratio O/Si is 1.5 and R
is the vertex functional group (hydrogen, alkyl, alkene, aryl,
arylene, etc.). POSS family is attracting the attention of the
scientific arena owing to their wide range of applications in
materials science ranging from soft electronics to
nanomedicine.[1a, 20] The modular POSS molecular structure can
be easily tuned to get a broad spectrum of properties.[21] In
addition, POSS structures display high chemical and thermal
stability together with a rigid molecular skeleton that can be
functionalized with a plethora of organic side groups. Imidazolium
modified POSS structures were used as homogeneous catalyst
for the CO2 conversion into cyclic carbonate showing improved
catalytic performances compared to the unsupported 1-butyl-3-
methylimidazolium salt. This behavior was ascribed to a proximity
effect due to the higher local concentration of imidazolium active
sites surrounding the inorganic silsesquioxane core. However, the
recovery of the catalyst from the reaction mixture was tricky.
Herein, in the light of the promising results achieved with
imidazolium functionalized POSS molecules in terms of activity,
we propose a broad series of heterogeneous catalysts based on
imidazolium modified POSS units grafted into amorphous silica
and mesostructured SBA-15. All the solids were extensively
characterized and tested as the sole catalyst in the synthesis of
cyclic carbonates by reaction between CO2 and epoxides in
solvent free reaction conditions. Moreover, the catalytic
performances were stepwise improved, by finely tuning POSS-Imi
units in terms of nucleophilic active species (Cl-, Br-, I-) and
imidazolium alkyl chain length.
Results and Discussion
A novel series of imidazolium modified POSS hybrids was
designed and prepared to be applied as heterogeneous catalytic
systems for the synthesis of cyclic carbonates from carbon
dioxide and epoxides. In order to combine the proximity effect of
imidazolium moieties surrounding the inorganic nanocage
structure[18] together with the benefits of heterogeneous catalysis,
functionalized POSS units were covalently grafted onto different
solid supports, namely amorphous silica and mesostructured
SBA-15.
Scheme 1. Synthesis of imidazolium functionalized POSS hybrids.
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A straightforward modular synthesis was developed using
triethoxy-3-(2-imidazolin-1-yl)propylsilane as linker to connect
POSS units to the solid support allowing to obtain dangling
imidazolium modified supported nanostructures. In particular, the
general synthetic route of these hybrids, reported in Scheme 1,
started from the reaction between triethoxy-3-(2-imidazolin-1-
yl)propylsilane and octakis(3-chloropropyl)-octasilsesquioxane
(compounds 1a, POSS-Cl) to give compound 2a which was
directly used for the grafting reaction onto two different supports
to give materials 3a (SiO2-Cl) and 4a (SBA-Cl). In this context, it
is worth to observe that the above synthetic strategy represents a
promising approach for the design of a wide library of hybrid
functional materials. The latter materials were in turn reacted with
All the solid state 13C CP-MAS NMR experiments were performed
using the TOSS (total suppression side band) pulse sequence to
remove the spinning side bands while leaving the isotropic signals.
In CP-MAS 13C NMR spectra the characteristic signals of the
carbon atoms of imidazolium ring were observed in the range δ =
122-137 ppm, whereas the aliphatic ones resonated in the range
δ = 10–52 ppm. The weak signal located at δ = ~158 ppm can be
attributed to the C2 imidazolidinium carbon atom. CP-MAS 29Si
NMR spectra showed the presence of Q4[(SiO)4Si] (δ = -118 ppm)
and Q3[(SiO)3SiOH] (δ = -110 ppm) units. The signal at δ = −75
ppm corresponded to the completely condensed T3 silicon units
[R-Si(OSi)3] of both POSS nanocage and the organosilane silicon
acting as linker between the solid support and the POSS itself,
1-methylimidazole allowing to obtain the corresponding 5a and 6a. whereas the signal at δ = −67 ppm was ascribed to T2 [R-
Materials 5a and 6a, endowed with chloride as counterion, were
employed to study the effect of the solid supports on the catalytic
performances. Then, amorphous silica was used, as support, to
investigate the influence of the nucleophilic species in the final
catalytic materials. For doing so, octakis(3-bromopropyl)-
octasilsesquioxane (1b, POSS-Br) and octakis(3-iodopropyl)-
octasilsesquioxane (1c, POSS-I) were prepared by halogen
exchange reactions starting from POSS-Cl.[22] Then, both POSS-
Br and POSS-I were grafted into amorphous silica to obtain the
final imidazolium hybrids 5b and 5c. Moreover, in order to
investigate the effect of the length of imidazolium alkyl side chain,
a further hybrid was prepared via reaction between solid 3a and
1-decylimidazole to give material SiO2-decim-Cl (5d). The grafting
of imidazolium modified POSS units into the solid support was
proved by means of solid state NMR. Solid state 13C and 29Si
cross polarization - magic angle spinning (CP-MAS) NMR
experiments were carried out to characterize all the final hybrids
(Figure 1).
Si(OSi)2OR] silicon atoms bridging bulky silica to POSS units.
Combustion chemical analysis (C, H, N) allowed to estimate the
catalyst loadings of all the solids, as showed in Table S1.
Thermogravimetric analysis (TGA) allows to estimate the thermal
stability of the hybrid materials (Figure 2, Figure S1). Moreover,
from TGA data it is interesting to observe that the degradation of
organic moieties starts at 250°C. On this ground, the good thermal
stability of the solids emerged as promising feature for their
repeated use under heating conditions.
Figure 2. TGA profiles of imidazolium functionalized POSS hybrids.
Surface textural properties of the materials were investigated by
means of N2 adsorption/desorption measurements (Table 1). In
particular, the specific surface area (SSA) was analyzed by
Brunauer-Emmett-Teller (BET) method,[23] whereas pore volumes
and pore size distributions (PSD) were estimated by applying
Barrett-Joyner-Halenda (BJH) method[24] using the adsorption
isotherms. A clear decrease in surface area and cumulative pore
volume of amorphous silica and mesostructured SBA was
observed after the grafting of imidazolium modified POSS units.
All the supported POSS hybrids displayed a type IV isotherm
(Figure S2) according to the IUPAC classification with a H2
hysteresis loop.
Figure 1. Solid state 13C CP-MAS TOSS (on the left) 29Si CP-MAS (on the right)
NMR spectra of imidazolium functionalized POSS hybrids.
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competitive and higher performances in comparison with other
catalytic systems recently reported in literature (Table 2) such as
18]
1-butyl-3-methylimidazolium chloride (BmImCl)[12b,
or
Table 1. Textural properties of silica oxide and supported POSS hybrids.
pyrazolium bromide (HEPzBr)[25] unsupported ionic liquids
(entries 3,4), and heterogeneous catalysts based on polymer
supported ammonium chloride salts (PSCQNCl),[26] imidazolium
chloride[15b] and tetrafluoroborate[27] active sites supported onto
silica (SiO2ImCl, SiO2ImBF4), or imidazolium salts bearing
bromide as counterion grafted onto carbon nanohorns
(CNHsImBr),[16c] and benzyl chloride polymer (BCPImBr)[28]
(entries 5-9). The above catalytic systems do not present other
co-catalytic species with Brønsted or Lewis acid functionalities.
High catalytic activity under such conditions is still challenging. On
the other hand, much better performances can be obtained when
co-catalytic species are present.[29] The higher catalytic activity of
5a and 6a can be ascribed to the proximity effect due to the
increased local concentration of imidazolium moieties
surrounding POSS nanocage skeleton. It is worth to note that
supported POSS materials allowed to a good combination
between the positive features of POSS nanostructures and the
intrinsic advantages of heterogeneous catalytic systems such as
their easy recovery from the reaction mixture together with the
possibility to reuse them for consecutive runs.
Material
BET[a]
m2/g
Pore Volume
cm3/g
PSD[b]
nm
424
250
285
279
203
634
272
0.60
0.28
0.38
0.37
0.22
1.39
0.87
4.6
SiO2
5a
3.7
3.9
5b
3.5
5c
3.6
5d
15.6
14.3
SBA-15
6a
[a] BET p/p0 range: 0.05-0.3. [b] PSD calculated on the adsorption curve.
Materials 5a and 6a were analyzed by means of transmission
electron microscopy (TEM) in order to collect morphological
information. However, TEM pictures (Figure S3) did not reveal
any appreciable difference compared to pristine solid supports.
The amorphous morphology of SiO2 and the bidimensional
hexagonal mesostructure of SBA-15 were further observed for 5a
and 6a, respectively.
Table 2. Conversion of styrene oxide into cyclic carbonate.
Once characterized, imidazolium modified POSS hybrids were
tested as catalysts for the synthesis of cyclic carbonates via
reaction of CO2 with epoxides under solvent free conditions, at
150 °C, and without using any co-catalytic species with Lewis acid
properties. It is worth to mention that such reaction is highly
exothermic. In this context, from an industrial point of view a
reaction temperature around 150 °C could be evaluated as a
beneficial condition allowing to recover the reaction heat as
steam.[1b] To minimize the energy consumption, reaction
temperatures <100 °C display indeed lower heat exchange
efficiencies. The catalytic performances of the solids were
evaluated in terms of turnover number (TON, defined as moles of
epoxide converted/moles of supported imidazolium halide),
productivity (P, calculated as grams of cyclic carbonates per
grams of catalyst), recyclability and versatility. In order to study
the effect of the solid support, two catalytic tests were carried out
with materials 5a and 6a in the reaction between carbon dioxide
and styrene oxide (Table 2, entries 1, 2). From this preliminary
investigation, imidazolium modified POSS grafted into amorphous
silica (SiO2-mim-Cl 5a) resulted more active than the analogous
material prepared by supporting POSS into SBA-15 (SBA-mim-
Cl 6a). This behavior can be accounted to the specific textural
properties of the final materials (Table 1). By comparison with 6a,
the solid 5a displayed lower values in terms of both pore volume
and pore size distribution, whereas it presented a higher catalyst
loading (1.048 vs 0.753 mmol/g, Table S1). On this basis, the
better catalytic activity of 5a could be explained by a higher local
concentration of imidazolium modified POSS units which resulted
in improved intermolecular interactions. Both 5a and 6a showed
Entry
Catalyst
T
[°C]
t
[h]
TON
TOF[a]
Reference
1[b]
2[b]
3
5a
150
150
150
140
150
150
160
150
140
3
3
3
4
3
5
4
3
5
477
306
326
68
159
102
109
17
This work
This work
Ref[18]
6a
BmImCl
HEPzBr
SiO2ImCl
PSCQNCl
SiO2ImBF4
CNHsImBr
BCPImBr
4
Ref[25]
5
28
9
Ref[15b]
Ref[26]
6
75
15
7
53
13
Ref[27]
8
106
323
35
Ref[16c]
Ref[28]
9
65
[a] Turnover frequency (TOF, calculated as TON/reaction time). [b] Reaction
conditions: styrene oxide (172 mmol), CO2 (Pi 4 MPa), catalyst 5a and 6a,
150 °C, 3h.
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As a result of the higher activity of 5a compared to 6a, amorphous
silica was selected as solid support in order to study the effect of
the nucleophilic species. For doing so, materials 5b and 5c,
bearing imidazolium bromide and iodide moieties respectively,
were synthesized and tested. In order to compare the catalytic
performances and investigate the recyclability of the solids 5a-c,
styrene oxide was selected as target substrate (Figure 3, Table
S2). The effect of the nucleophilic species led to the overall order
of activity I– > Br– > Cl– corresponding to 5c > 5b > 5a hybrids. All
the solid displayed promising turnover number and excellent
selectivity. The recyclability of 5a, 5b, 5c was studied for four
consecutive runs without showing, in the case of 5a and 5b, any
significant decrease in the catalytic activity. However,
a
progressive decrease was observed in the case of the most active
5c.
Figure 4. TGA profiles of as-synthesized and reused 5a-c. Magnification of TGA
curves (inset).
to 85% conversion into the desired product in 3 h. Furthermore,
when the catalyst loading was scaled down to 0.006 mol%, 5b
showed an outstanding increased activity resulting in a turnover
number of 7109 (entries 1-3). Such catalytic behavior could be
ascribed to a better dispersion of the solid in the reaction medium
when a decreased amount of 5b was utilized. Furthermore, the
overall versatility of 5b was evaluated with a broad range of
epoxides such as epichlorohydrin, propylene oxide, glycidol, 1,4-
butanediol diglycidyl ether, and the less reactive cyclohexene
oxide (entries 1-9). In all the catalytic tests, a full selectivity toward
the corresponding cyclic carbonates was reached together with
excellent TON and productivity. In particular, the reaction
between CO2 and 1,4-butanediol diglycidyl ether using 0.148
mmol/g of catalyst (entry 7) gave rise to a quantitative conversion.
An additional catalytic test of 5b was performed using the most
challenging oxetane as substrate (entry 10). The synthesis of six
membered cyclic carbonates by coupling of CO2 and oxetane is
challenging because of i) the lower reactivity of four membered
ether rings compared to epoxides, and ii) a poor reaction
selectivity as the six-membered cyclic carbonates is
thermodynamically less stable than its corresponding co-polymer.
Therefore, in the reaction with CO2, oxetane ring opening step
usually requires the use of an organometallic catalyst[30] or
bicomponent catalysts composed of organometallic catalyst and
ammonium salts[31]. A simple organocatalytic approach has been
used only twice, using 2 mol% of tetraphenylstibonium iodide[32]
Figure 3. Recycling tests of materials 5a, 5b, 5c. Reaction conditions: styrene
oxide (172 mmol), CO2 (Pi 4 MPa), 200 mg of catalyst (0.166 mmol of 5a, 0.140
mmol of 5b, 0.144 mmol of 5c), 150 °C, 3 h. TON calculated on the first cycle.
In order to check the thermal stability and the catalyst loading of
reused solids, TGA measurements were performed on 5a-c after
the fourth catalytic cycle. Thermogravimetric analysis of reused
catalysts confirmed the good robustness of materials 5a-b.
Indeed, TGA profiles of reused 5a-b displayed the same weight
loss compared to those of the as synthesized materials (Figure
4) leading to rule out the possibility of any leaching phenomena
during the catalytic tests.
Conversely, TGA profile of 5c reused for four cycles compared to
that of the as-synthesized catalyst showed a loss in weight
(~2.7%) allowing to justify the decrease in the catalytic activity
during recycling experiments. Although the material 5c proved to
be the most active catalyst, the solid 5b was selected for further
investigations (Table 3) because of its higher stability after
recycling compared to 5c (Figure 3). Firstly, the potential
influence of catalyst amount and operating CO2 pressure was
evaluated by using epichlorohydrin as target epoxide (entries 1-
3). Material 5b, tested at 0.022 mol%, displayed appealing
performances in terms of TON, productivity and selectivity leading
or 3 mol% of TBAI and a hydrogen bond donor as co-catalyst[33]
.
In the latter case a mixture of oligocarbonate and six membered
cyclic carbonate was formed. Nevertheless, in the reaction
between carbon dioxide and oxetane ring, the catalytic
performances of 5b stand out from literature data in terms of TON.
Then, the conversion of CO2 and styrene oxide into styrene
carbonate was chosen as benchmark reaction to check the
influence of imidazolium alkyl chain length by comparison
between the solids bearing imidazolium moieties with methyl and
decyl alkyl side chain (5a and 5d). Hybrid 5d was tested for four
consecutive runs and results compared with those of hybrid 5a
(Figure 5, Table S2).
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Table 3. Synthesis of cyclic carbonates catalyzed by 5b[a]
.
Entry
Substrate
Catalyst
amt [mg]
Conv.
[%]
S
t
TON
P
[mol%]
0.020
[%]
[h]
1
60
40
16
40
40
40
85
61
42
55
51
41
>99
>99
>99
>99
>99
>99
3
4250
4575
7875
4125
3825
3075
402
433
745
292
313
309
2
0.013
0.005
0.013
0.013
0.027
3
3
3
4
3
5[b]
3
6[c]
16
7[c]
200
0.133
99
>99
16
1485
149
8[d]
200
200
200
0.081
0.093
0.067
53
30
16
>99
>99
>99
3
651
321
240
74
32
19
9[e]
16
16
10[f]
[a] Conversion (Conv.), Selectivity (S), reaction time (t), Productivity (P, calculated as grams of cyclic carbonate/grams of catalyst). Reaction conditions: epoxide
(210 mmol), CO2 (Pi 4 MPa), catalyst 5b, 150 °C. [b] Reaction conditions: glycidol (210 mmol), CO2 (Pi 4 MPa), catalyst 5b, 100 °C. [c] Reaction conditions: 1,4-
butanediol diglycidyl ether (105 mmol), CO2 (Pi 4 MPa), catalyst 5b, 150 °C. [d] Reaction conditions: styrene oxide (172 mmol), CO2 (Pi 4 MPa), catalyst 5b,
150 °C. [e] Reaction conditions: epoxide (150 mmol), CO2 (Pi 4 MPa), catalyst 5b, 150 °C. [f] Reaction conditions: oxetane (210 mmol), CO2 (Pi 4 MPa), catalyst
5b, 150 °C.
Catalyst 5d proved to be slightly less active than 5a in terms of
TON, indicating that the major role in the catalytic activity is
probably played by the local arrangement due to the presence of
the nanocage. In both 5a and 5d catalytic tests, the activity of the
solid displayed a decreasing from the first to the second cycle
followed by a stabilization in the next cycle. This behavior is
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probably due to thermal condensation phenomena that took place
during the catalytic cycle.
reasons of the recyclability and high activity of such catalytic
materials.
Experimental Section
Spectroscopic and analytical methods
Chemicals and solvents were purchased from commercial suppliers to be
used without further purification. Combustion chemical analysis was
carried out on
a PerkinElmer 2400 Series II Elemental Analyzer.
Thermogravimetric analysis was performed under oxygen flow from 100 to
1000°C with a heating rate of 10°C min-1 in a Mettler Toledo TGA STAR
system. Solid state CP-MAS-TOSS 13C-NMR spectra were recorded at
room temperature, on a Bruker Avance 500 Spectrometer operating at
11.7 T, using a contact time of 2 ms, a spinning rate of 5 KHz and a Bruker
probe of 4mm. CP-MAS 29Si-NMR spectra were recorded at room
temperature, on a Bruker Avance 500 Spectrometer operating at 11.7 T,
using a contact time of 2ms, a spinning rate of 8 KHz and a Bruker probe
of 4mm. Liquid state 1H-NMR spectra were collected on a JEOL 400
spectrometer. TEM micrographs were taken on a Philips TECNAI 10 at 80-
100 kV. N2 adsorption–desorption measurements were carried out at 77 K
by using a volumetric adsorption analyzer (Micromeritics Tristar 3000).
Before the analysis, the samples were pretreated at 150°C for 16 h under
reduced pressure (0.1 mbar). The BET method was applied in the
p/p0=0.05–0.30 range to calculate the specific surface area, whereas the
pore size distributions were estimated from the adsorption isotherm using
the BJH method.
Figure 5. Recycling tests of materials 5a and 5d. Reaction conditions: styrene
oxide (172 mmol), CO2 (Pi 4 MPa), 200 mg of catalyst (0.166 mmol of 5a, 0.183
mmol of 5d), 150 °C, 3 h. TON calculated on the first cycle.
In order to verify such hypothesis, a 29Si CP-MAS NMR
experiment was carried out on the reused catalyst 5a after the
fourth cycle (Figure S11). The 29Si spectrum of 5a reused for four
cycles compared to that of the as-synthesized catalyst showed an
increase in the Q4/Q3 and T3/T2 ratio allowing to confirm a
structural modification during the reaction.
Preparation of SBA-15
Mesoporous SBA-15 was prepared starting from tetraethyl orthosilicate
(TEOS) as silica source, Pluronic P123 (EO20PO70EO20) as template, and
mesitylene as swelling agent, according to a published procedure.[35]
Conclusions
Synthesis of POSS 1a-c
In conclusion, we have prepared and characterized a series of
hybrid materials based on imidazolium modified polyhedral
oligomeric silsesquioxanes (POSS-Imi) in order to be employed
as heterogeneous organocatalysts for the conversion of epoxides
and CO2 into cyclic carbonates in solvent-free reaction conditions.
The main goal of this research was to maintain the high catalytic
activity of unsupported imidazolium modified POSS, due to the
proximity effect of the imidazolium units linked to the POSS
nanocage, with the benefits of heterogeneous catalysis, in terms
of recyclability, without the need of other co-catalytic species with
Brønsted or Lewis acid functionalities. All the solids were easily
prepared following tailored procedures designed to study the
influence of the solid support (SiO2 vs SBA-15) and the effect of
both nucleophilic species (Cl-, Br-, I-) and imidazolium alkyl side
chain length. Such new hybrid materials were easily recyclable as
well as highly active toward the formation of cyclic carbonates
even with the less reactive oxetane, showing higher
performances in terms of turnover number, productivity and
selectivity in comparison with other unmodified imidazolium-
based catalytic systems described in literature.[34] The combined
features due to the good thermal stability of the solids and the
local arrangement of the imidazolium units are, probably, the main
POSS-Cl,[36] POSS-Br and POSS-I[22] were synthesized according to
literature procedures.
Synthesis of materials 3a-c, 4a
A selected polyhedral oligomeric silsesquioxane (1 eq., 0.675 mmol of 1a-
c) was transferred in a two-necked round bottom flask. Anhydrous toluene
(10 mL) and trethoxy-3-(2-imidazolin-1-yl)propylsilane (1.5 eq.) were
added to POSS. The reaction mixture was refluxed under Argon
atmosphere for 24 h. Then, the support (1.8 g of SiO2 or SBA-15) was
added to the reaction mixture without a prior isolation of intermediates 2a-
c. The obtained suspension was refluxed under Argon atmosphere for 72
h. The solids were recovered by centrifugation and washed with toluene,
dichloromethane, and diethyl ether. The obtained materials (3a-c, 4a)
were dried at 60 °C under vacuum.
Synthesis of materials 5a-d, 6a
1-methylimidazole or 1-decylimidazole (15 eq. respect to halopropyl
moieties of 3a-c and 4a) were added to a suspension of materials 3a-c
and 4a (2.4 g) in toluene (25 mL). The reaction mixtures were refluxed for
72 h. The final solids 5a-d and 6a were recovered by centrifugation and
washed with toluene, dichloromethane, hot methanol and diethyl ether.
The final materials were dried at 60°C under vacuum.
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10.1002/cctc.201801351
ChemCatChem
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Entry for the Table of Contents
Layout 1:
FULL PAPER
PossAbility: POSS-based materials
display great ability in the conversion
of CO2 into cyclic carbonates in
terms of turnover number,
productivity, selectivity and
recyclability without the need of
other co-catalytic species with
Brønsted or Lewis acid
Carla Calabrese, Leonarda F. Liotta,
Francesco Giacalone, Michelangelo
Gruttadauria*, Carmela Aprile*
Page No. – Page No.
Supported POSS Based Materials as
Highly Active Organocatalysts for
the Conversion of CO2
functionalities.
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