Novel porous organocatalysts for cycloaddition of CO2 and epoxides

Article abstract of DOI:10.1039/c9ra05466a

Three classes of organosilicas (DMO, OMOs and PMOs) containing immobilized multi-hydroxyl bis-(quaternary ammonium) iodide salts were prepared and tested in the cycloaddition of CO₂ and epoxides. Owing to its higher surface area, pore volume an

Full text of DOI:10.1039/c9ra05466a

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Novel porous organocatalysts for cycloaddition of
CO2 and epoxides†
Cite this: RSC Adv., 2019, 9, 24527
*
Joel M. Kolle and Abdelhamid Sayari
Three classes of organosilicas (DMO, OMOs and PMOs) containing immobilized multi-hydroxyl bis-
(quaternary ammonium) iodide salts were prepared and tested in the cycloaddition of CO2 and epoxides.
Owing to its higher surface area, pore volume and optimum nucleophilicity of the iodide ion, OMO-2
with two hydroxyl groups was found to be the most active catalyst. For substrates that are easy to
activate such as propylene oxide, 1,2-epoxybutane and epichlorohydrin, excellent yields and selectivities
were obtained under mild reaction conditions (0.5 MPa CO2, 50 ꢀC and 10–15 h). Moreover, OMO-2
showed very good catalytic properties (yield $ 93% and selectivity $ 98%), and excellent chemical and
textural stability in the synthesis of 1,2-butylene carbonate over 5 cycles.
Received 16th July 2019
Accepted 31st July 2019
DOI: 10.1039/c9ra05466a
rsc.li/rsc-advances
ionic liquids,7 and traditional heterogeneous catalysts9 were
reported to be active towards CO2 cycloaddition. On their own,
Introduction
the last group of catalysts showed low reactivity, and as such
received less attention compared to free and immobilized
homogeneous catalysts. The latter may consist of a single
catalytic component, or a combination of a catalyst and a co-
catalyst.8 Examples of one-component catalyst include tribu-
tylpropylammonium iodide immobilized on silica,10 quaternary
ammonium11 and phosphonium12 salts. Two-component cata-
lysts include benzylbromide/DMF,13 tetrabutylammonium salt/
EDTA,14 and polymer-supported quaternary onium salts/
aqueous solutions of metal salts.15 These catalysts are not
highly popular as they increase the complexity of the process,
leading to tedious catalyst separation/recovery and product
purication steps. Accordingly, to achieve simplied and cost-
effective chemical processes, one-component catalysts are
preferred.
One-component catalysts can be mono- or bifunctional in
nature. Mono-functional catalysts such as alkyl quaternary
ammonium16 or phosphonium17 halides play a single role,
which consists of opening the epoxide ring, leading to the
insertion of CO2. These extensively investigated catalysts were
found to exhibit good to excellent yields in the cycloaddition of
CO2 and epoxides.18,19 Moreover, one-component bifuntional
homogeneous or immobilized catalysts may contain specic
functional groups, such as –COOH, –NH2 and –OH or metals
Among greenhouse gases (GHGs), carbon dioxide (CO2) is the
main anthropogenic contributor to global warming and climate
change.1,2 Carbon capture is recognized as one of the most
promising strategies to mitigate the adverse effects of CO2
emissions.3,4 Previously viewed largely as an industrial waste
product, CO2 is increasingly being valued as a cheap, readily
available and non-toxic C1 source for the synthesis of chemicals
and fuels.5,6 Accordingly, CO2 utilization is gaining popularity as
a complementing tool to CO2 capture and sequestration
towards GHG mitigation. However, the inherent thermody-
namic stability of CO2 leads to low reactivity. As such, high CO2
pressure and reaction temperature are generally required for its
transformation. Nonetheless, combining CO2 with highly reac-
tive substrates such as epoxides and aziridines, in the presence
of suitable catalysts, provides a pathway to overcome this energy
barrier. The catalytic conversion of CO2 and epoxides to cyclic
carbonates (CCs) is one of many reactions using CO2 as an
alternative C1 feedstock. Application of CCs as aprotic polar
solvents, active pharmaceutical ingredients, and monomers in
the production of ne chemicals makes them an important
group of organic materials in the chemical industry. In the
generalized mechanism for the synthesis of CCs, regioselective
CO2 insertion is achieved by a combination of epoxide activa-
tion by Y through oxygen atom coordination and ring-opening
by a nucleophile, Xꢁ (Scheme 1).
A variety of homogeneous catalysts such as ionic liquids7 and
metal complexes;8 immobilized catalysts including supported
Centre for Catalysis Research and Innovation (CCRI), Department of Chemistry,
University of Ottawa, Ottawa, Ontario, Canada K1N 6N5. E-mail: Abdel.Sayari@
uottawa.ca
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra05466a
Scheme 1 Synthesis of cyclic carbonate from epoxide and CO2.
RSC Adv., 2019, 9, 24527–24538 | 24527
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(e.g. Zn, Co and Al). Bifunctionality implies that the catalytic precursor
(organotrialkoxysilanes,
bridged-
or
poly-
material plays two roles in the course of the reaction, namely; silsesquioxanes) over a structure directing agent is a one-pot
(1) activation of the epoxide by the functional groups,20 synthesis method to obtain ordered mesoporous organosilicas
rendering it more susceptible to undergo cycloaddition – Y in (OMOs) – Scheme 2b. Furthermore, a distinctive group of
Scheme 1, and (2) opening of the epoxide ring by the halide mesoporous organocatalysts, with a unique homogeneous
ion,21 similar to the mono-functional catalysts, Xꢁ in Scheme 1. distribution and high loading of covalently linked organic
The above functional groups activate the epoxides via hydrogen functionalities within the silica framework are periodic meso-
bonding, while epoxide activation by metal occurs through the porous organosilicas (PMOs). These materials are derived from
formation of metalalkoxides via Lewis acid interactions.22,23 the hydrolytic sol–gel polycondensation of organic bridged-
Examples of metal-free one-component bifunctional catalysts silsesquioxanes (R[Si(OR0)3]n; n $ 2) in the presence of surfac-
include free and supported hydroxylalkyl ammonium salts11 or tant templates without addition of silica precursors – Scheme
phosphonium17 salts. Regardless of the design, type or func- 2c.
tionality of the catalyst, the ring opening of all epoxides requires
Signicant progress in the synthesis of CCs from CO2 and
a nucleophile. The iodide ion was reported as one of the most epoxides over mesoporous organocatalysts derived by the
efficient owing to its exceptional leaving group ability, stem- graing of aminosilane followed by quaternization (Scheme 2a)
ming from its bulky nature and lower electronegativity.
has been reported in a number of reviews.7,28 Although OMOs
In terms of sustainable and low-cost processes, highly active containing ammonium halides,29,30 and PMOs containing
metal-free one-component bifunctional catalysts with stable melamine31 and urea32 moieties were reported as active catalysts
active sites and easy reusability are attractive. Many such cata- in the synthesis of CCs, OMOs and PMOs received noticeably
lysts, covalently anchored onto a variety of solid materials were less attention compared to DMOs. Considering the higher
reported in the literature.24 In some cases, these hybrid solids surface area of OMOs compared to DMOs, and the signicantly
demonstrate similar or even better activities compared to their higher organic content achievable with PMOs, organocatalysts
homogeneous counterparts.17,25 Porous inorganic solids such as with greater surface area and active sites can be obtained from
mesoporous SBA-15 silica26,27 was used as support for one- OMOs and PMOs respectively. If such active sites can be readily
component porous hybrid solid (PHS) catalysts for the accessed, a signicant enhancement in catalytic performance
synthesis of cyclic carbonates. Compared to organic supports (i.e. higher reaction rates, selectivity and yield) can be expected.
such as polystyrene, they offer better thermal and mechanical In addition, higher active site density is expected to enhance
stability, enhanced chemical interaction between the support catalytic activity. In this sense, homogeneous33 and heteroge-
and the active organic species and high loading of active sites, neous34 organocatalysts containing two active sites per molecule
thereby enhancing production. Moreover, mesoporous inor- were found to be more effective than those containing only one.
ganic supports alleviate molecular traffic within the pore In this contribution, we present four groups of novel immobi-
system, which could enhance both the rate and selectivity of the lized multi-hydroxyl bis-(quaternary ammonium) salts prepared
reaction.
via three routes: (1) graing of a bridged-SQ on silicas, followed
The main preparation method for PHS entails post-synthesis by quaternization, (2) co-condensation of (i) bridged-SQ and (ii)
surface functionalization of a silica; typically through graing poly-SQ with TEOS, and (3) PMOs derived from (i) bridge-SQ and
of an aminosilane or phosphine–silane, followed by quaterni- (ii) poly-SQ. Furthermore, their activity in the synthesis of CCs
zation to afford an immobilized quaternary ammonium or from CO2 and epoxides under mild reaction conditions was
phosphonium salt – Scheme 2a. Oen, such organosilicas discussed. The reusability of the most active catalyst was also
consist of a disordered distribution of the organic moieties; investigated to better understand the catalyst performance
hence these solids may be referred to as disordered mesoporous (yield and selectivity), chemical stability (resistance to leaching)
organosilicas (DMO). Alternatively, the co-condensation of and structural stability (textural properties) with reuse.
a silica network former (e.g. TEOS or TMOS) and an organosilica
Experimental
Materials
N,N0-Bis(2-hydroxyethyl)-N,N0-bis(trimethoxysilylpropyl)ethyl-
enediamine – BHBPD (66–68% in methanol) was purchased
from Gelest. Chromatography grade silica gel mesh 230–400
(hereaer referred to as SiL), 2-iodoethanol (99%), ammonium
iodide (99.5%), P123 (average Mn ꢂ 5800), tetraethyl orthosili-
cate – TEOS (98%), 3-chloropropyltrimethoxysilane – 3CPTS
(97%), styrene oxide (97%), propylene oxide – PO (99%), 1,2-
epoxybutane – 1,2-EB (99%), epichlorohydrin (99%), cyclo-
hexene oxide – CHO (98%), 3-glycidoxypropyltrimethoxysilane
(GPS), triethylamine – TEA (99%) and KCl (99%) were purchased
from Sigma Aldrich. Ethyl acetate (99.9%), diethyl ether – Et2O
(99.8%), ethanol (99%), methanol (99%), hydrochloric acid
Scheme 2 Synthesis of (a) DMO via surface grafting and quaterniza-
tion (b) OMO via the co-condensation route (c) periodic mesoporous
organosilicas – PMO.
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(37%) and toluene (99%) were obtained from Fischer. P-10 and precursor were performed in the presence of a neutral surfac-
Q-10 mesoporous silica supports were donated by Fuji Silysia tant (P123) under acidic condition. In a typical procedure, 2.0 g
Chemical Ltd. Chloroform-D was obtained from Cambridge of triblock copolymer P123 and 2 g of KCl were dissolved in
Isotope Laboratories. All chemicals were used as obtained with a mixture of 30 g distilled water and 60 g of 2 M HCl solution in
ꢀ
no further purication. Carbon dioxide (99.998%) was obtained a 250 mL Teon-liner at 40 C. Notice that the use of KCl was
from Linde AG.
shown to enhance ordering of mesoporous organosilicas.39 To
this solution, 4.3 g of TEOS was added and the mixture stirred
for 1 h to ensure the pre-hydrolysis of TEOS. Thereaer, 2 mL
(2.83 mmol) of BHBPD, corresponding to a nominal amine
loading of 2.0 mmol per gram of silica, was slowly added under
Preparation of mesoporous organosilica materials
Method A: preparation of DMOs via graing and quaterni-
zation of amine. Three commercial grade silicas (Q-10, P-10 and
SiL) and SBA-15 silica were used as supports. SBA-15 was
prepared using an established procedure.35 Graing was carried
out in toluene in the presence of water. Our group previously
demonstrated that addition of water improves both the quality
and loading of graed amine on MCM-41.36 Based on literature
reports,37 it is possible to estimate the amount of water required
to obtain a fully hydroxylated silica surface. This can be achieve
by considering the maximum number of surface silanol groups
to be 5 ꢃ 1018 SiOH per m2, and by introducing twice as many
water molecules per m2.38 Accordingly, for a typical silica with
a surface area of 300 m2 gꢁ1, the volume of water to be added is
ꢀ
stirring. The resultant mixture was further stirred at 40 C for
20 h. Finally, the mixture was transferred into an autoclave and
heated at 100 ꢀC under static condition for 2 days. The solid
products were collected by ltration, washed with water, and
dried at ambient conditions. The surfactant was removed by
reuxing in 100 mL of a 98 : 2 v/v% ratio of ethanol : conc. HCl
ꢀ
mixture at 60 C for 6 h. The powder material was collected by
ltration, washed with ethanol, and dried at ambient condi-
tions. A similar preparation, but without BHBPD, was carried
out for SBA-15 silica, herein referred to as SBA-15*. To make
sure all P123 has been removed, 1 g of the solid ethanol-
extracted material was subjected to additional Soxhlet extrac-
approximately 0.1 mL gꢁ1
.
ꢀ
tion with 100 mL of methanol at 70 C for 16 h. The obtained
A typical procedure for graing was as follows; 1 g of ther-
mally pretreated (at 130 ꢀC for 4 h) silica was dispersed in 30 mL
of toluene at room temperature, followed by addition of an
appropriate volume of water; 0.1 mL for Q-10 and P-10, 0.15 mL
for SiL, and 0.3 mL for SBA-15. The temperature of the mixture
was raised to 85 ꢀC and further stirred for 30 min. Finally, 2 mL
of BHBPD was added and the resultant mixture was reuxed at
ꢀ
solid was dried overnight in ambient condition, then at 60 C
under vacuum for 4 h.
An off-white powder material, hereaer referred to as OMO-
1, was obtained from the co-condensation of TEOS and BHBPD.
Because of the acidic conditions, OMO-1 was actually in the
form of a quaternary ammonium chloride salt. OMO-1 was
subjected to; (1) ion-exchange with ammonium iodide to obtain
the corresponding ammonium iodide salt – OMO-2 and (2)
dequaternization, regenerate the amine – OMO-3. To obtain
a QAS with additional –OH groups, OMO-3 was quaternized
with 2-iodoethanol to afford OMO-4. The structures of OMO-(1–
4) are represented in Scheme 4.
Ion-exchange was carried out as follows; 1 g of OMO-1 was
dissolved in 30 mL of methanol followed by addition of 1 mL of
ammonium iodide under a nitrogen atmosphere. The mixture
was reuxed at 65 ꢀC for 16 h. The resultant solid was recovered
by ltration, washed with methanol, then Et2O and dried under
vacuum at 70 ꢀC for 4 h to afford a yellow powder (OMO-2). To
regenerate the amine from OMO-1, 1 g of material was
dispersed in 50 mL of ethanol, followed by the addition of 1 mL
ꢀ
85 C for 16 h in an oil bath. The graed materials were then
ltered, washed with toluene, methanol, then Et2O, and dried
under vacuum at 60 ꢀC for 4 h. The obtained solids are hereaer
referred to as S@amine, where S indicates the support. The
amine-graed solids were then quaternized as follows: 1 g of
material was dispersed in 15 mL of toluene, followed by addi-
tion of 2 mL of 2-iodoethanol under a nitrogen atmosphere. The
reaction mixture was kept under reux at 70 ꢀC for 3 days. The
obtained solids were ltered off, washed with toluene, meth-
anol, then Et2O, and dried at 70 ꢀC under vacuum for 4 h to yield
pale yellow powder materials of immobilized diammonium
iodide salt. The obtained solids are hereaer referred to as
DMO-Q10, DMO-P10, DMO-SiL and DMO-SBA15 aer the cor-
responding S@amine. The synthesis of amine-graed materials
and their quaternized counterparts is represented in Scheme 3,
with Q-10 as silica support.
Method B: preparation of OMOs via co-condensation of
BHBPD or poly-SQ salt and TEOS. The synthesis of all porous
organosilica by hydrolysis and condensation with a silica
Scheme 3 Synthesis of Q-10@amine and DMO-Q10 via grafting and Scheme 4 Synthesis of OMO-1 to OMO-4 via co-condensation fol-
quaternization.
lowed by ion-exchange or quaternization.
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of triethylamine. Aer reux at 60 ꢀC for 2 h, the solid was
recovered by hot-ltration and washed with boiling ethanol,
then dried at 60 ꢀC under vacuum for 4 h to afford an off-white
powder material – OMO-3. OMO-3 was quaternized as described
in method A to afford a yellow powder – OMO-4.
The preparation of OMOs from BHBPD-derived salt involves
two steps; the synthesis of the salt, hereaer referred to as poly-
SQ salt, followed by its co-condensation with TEOS. The poly-SQ
salt was synthesized as follows; to a 25 mL two-neck round
bottom ask containing 10 mL of toluene, 2 mL (2.83 mmol) of
BHBPD was added. Under a nitrogen atmosphere, 2.0 equiva-
lent of 3-chloropropyltrimethoxysilane was added. The reaction
ask was sealed under nitrogen and reuxed at 85 ꢀC for 24 h.
Toluene and unreacted reagents were removed under reduced
pressure at 90 ꢀC to afford a brownish-yellow liquid. The
procedure for the co-condensation of the poly-SQ salt and TEOS
was similar to that of BHBPD and TEOS (OMO-1). An amount of
poly-SQ salt was added to a pre-hydrolyzed TEOS solution with
a nominal amine loading of 2.0 mmol per gram of silica. The
template was extracted as described for OMO-1 to afford a light
grey solid denoted OMO-5. Ion-exchange of OMO-5 with
ammonium iodide (as described for OMO-1) produced a yellow
solid, OMO-6 (Scheme 5).
Scheme 6 Synthesis of PMOs via condensation of (top) BHBPD, and
(bottom) poly-SQ salt.
The organic or nitrogen content (N-content) of all organo-
silicas was determined using thermogravimetric analysis (TGA)
on a TA Q500 instrument. Sample were heated to 120 ꢀC at
10 ꢀC minꢁ1 under owing nitrogen and held for 30 min, to
ensure the removal of physisorbed species (e.g. water and
solvents). Then the temperature was raised to 800 ꢀC under
owing nitrogen, then air at 800 ꢀC for 10 min. The organic
content was calculated based on the weight loss beyond 200 ꢀC.
IR spectroscopy. To identity the incorporated organic
species, ATR-IR measurements were recorded on a Nicolet 6700
Method C: synthesis of PMOs via condensation of BHBPD
and poly-SQ salt. To an aqueous solution of P123, prepared as
described in method B, 2 mL of BHBPD or 2.8 mL of poly-SQ salt
was added slowly under stirring. The resultant mixture was
further stirred at 40 ꢀC for 20 h, and the mixture was transferred
ꢀ
spectrometer equipped with
a liquid N2-cooled MTC-A
into an autoclave and heated at 100 C under static condition
(mercury–tellurium–cadmium) detector and a Fisher Scientic
horizontal ATR unit with a ZnSe crystal. For this, a few mg of
powder sample was homogeneously spread onto the surface of
the ATR crystal, and the IR spectra were recorded from 600 to
4000 cmꢁ1 at a resolution of 2 cmꢁ1 using 128 scans. For each
scanning, the spectrum was collected by subtracting air (back-
ground) spectrum from the original spectrum. All ATR-FTIR
experiments were run in triplicate to ensure the reproduc-
ibility of the results, and average wavenumbers were reported
herein.
for 2 days. The solid products were collected by ltration,
washed with water, and dried at ambient conditions. Surfactant
extraction, ion-exchange, dequaternization and quaternization
were carried out on the resultant solid as described for OMO-1.
All PMOs are represented in Scheme 6. PMO-1 and PMO-3 were
obtained as off-white solids; PMO-5 was light-grey; and PMO-2,
PMO-4 and PMO-6 were yellow powder materials.
Characterization of materials
Textural properties and organic content. The specic surface
area, pore volume and mean pore diameter of all materials were
determined by nitrogen adsorption–desorption measurements
at ꢁ196 ꢀC on a Micromeritics 2020 instrument, as reported
earliꢀer.10 Samples were degassed under nitrogen for 4 h at
120 C prior to analysis. Specic surface areas were calculated
using the BET method at relative pressures of 0.06 to 0.2. The
total pore volume (Vp) was recorded at p/p0 ¼ 0.99 to exclude
possible interparticle pores. The average pore size (Dp) was
determined using the density functional theory.
13C and 29Si solid state and solution NMR. Solid-state NMR
spectra were collected at room temperature using an AVANCE
III 200 MHz spectrometer equipped with a MAS probe. The
spectra were recorded using cross polarization with high power
decoupling, with samples spinning at 4.5 kHz. The number of
scans was in the range of 6000–8000. The contact time and
recycling delays were set at 10 ms and 2 s for 13C NMR respec-
tively. For 29Si NMR, the contact time and recycling delays were
set at 2 ms and 1.5 s.
1
For solution NMR measurements, 13C and H NMR spectra
were obtained using a Bruker AV 400 MHz spectrometer.
Chemical shis are reported in ppm relative to deuterated
chloroform.
Where separation was needed to isolate the cyclic carbonate
prior to NMR analysis, silica gel ash chromatography was used
with a 3 : 1 ethylacetate : ether mixture as solvent.
Cycloaddition reaction. Catalytic activity was evaluated using
the model reaction in Scheme 7.
Scheme 5 Synthesis of poly-SQ salt and OMO-5 and OMO-6 via co-
condensation followed by ion-exchange.
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The reaction conditions were chosen such that the reaction
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was signicantly away from total conversion; enabling a mean-
ingful comparison of catalytic activity and stability. In a typical
procedure, a 100 mL high pressure autoclave equipped with
a magnetic stirrer was charged with 2 mL (17 mmol) of styrene
epoxide (SO) and 2 mol% of organocatalyst (DMO, OMO or
PMO). The reactor was directly pressurized with CO2 to 0.5 MPa
and heated to 50 ꢀC under stirring for 5 h. At the end of the
reaction, the reactor was cooled with an ice bath to #15 ꢀC and
CO2 was released slowly. Then, 50 mL of dodecane (internal
standard for GC analysis) was added to the reaction mixture,
which was then suspended in diethyl ether and ltered off to
recover the catalyst. Aer removal of all volatiles using a rota-
vap, a solvent-free reaction mixture was obtained. The spent
catalyst was further stirred in acetone for 30 min, ltered off
ꢀ
and dried in a vacuum oven at 70 C for 2 h. TG analysis and
nitrogen adsorption–desorption measurements were carried
out on all spent catalysts to determine variations in N-content
and textural properties.
Analysis of reaction mixture: about 0.2 mL of the solvent-
free reaction mixture was injected into a Varian CP-3800 GC
equipped with Agilent J&W VF-1ms capillary column (15 m
length, 0.25 mm inner diameter and 0.25 mm lm thickness)
and a ame ionization detector (FID); with helium as carrier
gas. The temperature program used for analysis was as
follows: iniꢁti1al oven temperature 80 ꢀC (1.5 min); ramp at
30 ꢀC min
and held at 250 ꢀC (3 min); injector port
ꢀ
ꢀ
temperature: 200 C; detector temperature: 250 C. For solid
reaction mixtures, 0.1 g of the mixture was dissolved in
0.2 mL of toluene, and 0.2 mL of this solution was injected
into the GC.
13C and 1H NMR spectra of liquid samples were obtained
using a Bruker AV 400 MHz spectrometer with tetramethylsilane
as the internal standard. Chemical shis are reported in ppm
relative to deuterated chloroform.
Results and discussion
Characterization of mesoporous organosilicas
As shown in Fig. 1 and S1,† the nitrogen adsorption–desorption
isotherms for the organosilica materials and SBA-15 were of
Type IV with H1 hysteresis loop, characteristic of solids with
mesopores. The corresponding textural properties and organic
content are presented in Table 1. The TGA proles of these
materials may be found in Fig. 2. Compared to their pure silica,
all organosilica materials derived by graing, i.e. DMOs, had
lower BET surface areas and pore volumes (Table 1, entries 1–8)
– an indication of immobilization of organic species within the
Fig. 1 Nitrogen adsorption–desorption isotherms of pristine and
functionalized supports.
internal surface. The SBA-15 support obtained by template
extraction in EtOH/HCl solution had a higher surface area and
pore volume compared to that derived by calcination (Table 1,
entry 7 vs. 9). Although thermal calcination in air can
completely remove organic template, it may however cause
signicant framework shrinkage stemming from further silica
Scheme 7 Model cycloaddition reaction.
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Table 1 Textural properties of solid materials
Specic S.A.
Entry
Material
(m2 gꢁ1
)
Vp (mL gꢁ1
)
Dp (nm)
N-content (mmol gꢁ1
)
1
2
3
4
5
6
7
8
Q-10
DMO-Q10
P-10
DMO-P10
SiL
DMO-SiL
SBA-15
DMO-SBA15
SBA-15*
OMO-2
OMO-4
OMO-6
PMO-2
PMO-4
PMO-6
316
160
327
170
478
182
763
146
913
449
241
270
86
1.34
0.48
1.53
0.54
0.83
0.22
1.07
0.21
1.23
0.66
0.37
0.22
0.31
0.20
0.18
18.2
11.9
21.2
12.0
9.2
4.6
7.2
5.1
6.18
6.0
5.8
4.1
15.7
30
16.7
—
1.26
—
1.22
—
1.37
—
1.41
—
9
10
11
12
13
14
15
1.22
1.29
1.22
2.38
2.37
2.11
34
60
condensation. On the other hand, template removal by solvent content, up to 80 wt%, was obtained for PMOs compared to
extraction minimizes framework shrinkage, leading to higher DMOs and OMOs (Fig. 2c).
surface area, pore volume and number of silanols compared to
its calcined counterpart.40,41
The 13C NMR of amine@Q-10 and DMO-Q10 showed peaks
around 10, 17, 51, 56, 58 and 62 ppm, which were assigned to
The capillary condensation of all DMOs and OMOs were the ve carbon nuclei of BHBPD as illustrated in Fig. 3a. As-
observed to be less sharp compared to their corresponding synthesized SBA-15 exhibited resonances attributed to –CH2–
supports and SBA-15*, respectively; an indication of decreased CH– (76 and 74 ppm) and –CH3 (18 ppm) groups of the PPO, and
pore size. This provided further evidence that immobilization of –CH2– (71 ppm) of the PEO block of P123.43 Following template
the organic moieties took place on the internal surface of the removal by solvent extraction, 13C NMR of the resultant mate-
supports. Compared to DMOs and OMOs, all PMOs had rial, SBA-15*, showed very low intensity signals between 71 and
signicantly lower surface areas, while their pore volumes were 75 ppm, and the complete disappearance of the signal at
also lower than the corresponding OMOs (Table 1, entries 10 vs. 18 ppm. Moreover, signals at 16, 50 and 59 ppm were assigned
13, 11 vs. 14 and 12 vs. 15).
to trapped methanol and ethanol molecules used during
Moreover, PMOs showed a broad pore size distribution template removal. Hence, the 8 wt% organic content of SBA-15*,
(Fig. S2†). This nding may be attributed to the higher organic as determined by TGA was assigned to the presence of small
content of PMOs, which adversely affects their textural amount of template. As with DMOs, ve carbon nuclei could be
properties.
identied in the 13C NMR of all OMOs (Fig. 3b) and PMOs
Similar organic contents were measured for all amine func- (Fig. 3c).
tionalized supports and DMOs (Fig. 2a). The calculated organic
The 29Si CP MAS NMR spectra showed only Qn species for
content of Q-10@amine, P-10@amine, SiL@amine and SBA- SBA-15*, Qn and Tn for DMOs and PMOs, and only Tn for PMOs
15@amine were 1.59, 1.70, 1.64 and 1.80 mmol gꢁ1 respec- (Fig. 3d). The characteristic resonances in the range of ꢁ89 to
tively – accounting for a graing efficiency between 80 and 90%. ꢁ118 ppm were ascribed to Qn units, with the peaks at about
This nding illustrates that despite the smaller surface areas of ꢁ97, ꢁ102.1 and ꢁ110.9 ppm corresponding to Q2 [Si(HO)2(-
P-10 and Q-10 silicas compared to SiL and SBA-15, their larger OSi)2], Q3 [Si(HO)(OSi)3] and Q4 [Si(OSi)4] species, respectively
pore volume and pore diameter enabled them to accommodate (Fig. 3d(i)). All DMOs, OMOs and PMOs showed two intense
as much BHBPDs as SiL and SBA-15.
signals at ca. ꢁ58 and ꢁ66 ppm, attributed to Si-species cova-
The N-content per gram of silica support for both S@amine lently bonded to carbon atoms, namely T2 [C–Si(OH)(OSi)2] and
solids and DMOs being the same, the yield of the quaternization T3 [C–Si(OSi)3] respectively (Fig. 3d(i)–(iv)).44 These signals arise
reactions was evaluated to be between 92 and 97%. This nding from immobilization of BHBPD onto the silica surface and
conrms the structure of DMOs, i.e. as immobilized symmet- framework of silica for DMOs and OMOs/PMOs, respectively.
rical diammonium salts. The weight lost below 200 ꢀC of SBA-15 Furthermore, for some OMOs and PMOs, a weak shoulder at ca.
was <2% compared to ca. 10% for SBA-15* (Fig. 2b), suggesting 50 ppm, attributed to T1 [C–Si(OH)2(OSi)] species was also
the presence of some P123 surfactant (vide infra). It is known observed. The presence of T1 and T2-signals is an indication that
that template removal via solvent extraction does not the condensation process was not complete for these materials.
completely remove the surfactant.42 Hence, the N-content of all Hence, the calculated nitrogen content based on TGA results,
OMOs and PMOs were obtained taking into account the pres- which assumes 100% condensation, may be slightly
ence of about 8 wt% of surfactant. Relatively higher organic overestimated.
24532 | RSC Adv., 2019, 9, 24527–24538
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adsorbed water, whose physisorption was enhanced by the ionic
nature of the –N–I bond. Furthermore, the spectra of all orga-
nosilica materials showed –CH2– stretching vibrations of the
propyl chain characterized by weak absorbance at ca. 3030 and
2980 cmꢁ1, –CH2– bending at 1550 cmꢁ1 and C–N stretching at
1450 cmꢁ1 45 These ndings are consistent with the presence of
.
BHBPD on the silica supports. The absence of a band at
960 cmꢁ1 suggests that the amount of uncondensed Si–OH
groups was minimal; which is in agreement with 29Si MAS NMR
data. It is worth noting that bands at slightly different wave-
numbers were observed for PMOs and OMOs/DMOs, suggesting
differences in the chemical environment between these two
groups of materials.
Catalytic activity
The catalytic activity of iodide-containing organosilicas was
evaluated using the cycloaddition of CO2 and styrene oxide (SO)
to styrene carbonate (SC) under neat conditions. Because of its
low activity, SO provides a more meaningful comparison than
a highly active epoxide such as 1,2-epoxybutane (1,2-EB). The
effect of temperature, time and CO2 pressure on the selectivity
and yield is presented in Table 2, entries 1–11. At 100 ꢀC, 4 h and
1 MPa CO2, yields and selectivity $ 97% were obtained (Table 2,
entry 1) with DMO-SiL as catalysts. In an earlier study, we re-
ported similar ndings in the presence of N,N,N-tributyl-N-
propylammonium iodide-functionalized mesoporous silicas.10
These results are in agreement with other reports where
immobilized ammonium46 and phosphonium17 salts were used.
To obtain yields signicantly below 100%, the following
conditions were; 50 ꢀC, 5 h and 0.5 MPa CO2. As seen in Table 2,
entries 2–11, conversion was found to be <50% for all catalysts.
Amongst the DMOs, the highest yield was obtained with DMO-
P10 and DMO-SiL (Table 2, entries 3 and 4). This may be
attributed to smaller particle size of the silicas supports P-10 (40
mm) and SiL (40–60 mm) compared to Q-10 (75–150 mm) and
SBA-15 (<150 mm).47
With respect to OMOs, a 45% yield of SC was obtained
with OMO-2 (Table 2, entry 6). However, OMO-4 consisting of
four CO2-activating hydroxyl groups, exhibited lower activity
(Table 2, entry 7), suggesting that increasing the number of
hydroxyl groups from two to four was not benecial to OMOs.
A similar nding by Cheng et al.,48 was attributed to an
enhancement in hydrogen bond formation between the
halide and additional OH groups, leading to decreased
nucleophilicity or epoxide ring-opening ability of the iodide
ion. Alternatively, the observed decrease in activity could
originate from increase diffusion-constraints with additional
ethylhydroxyl groups, such that any benet in CO2 activation
due to extra hydroxyl groups was suppressed by slow reaction
kinetics. This nding suggests that similar to homogeneous
systems,49 there is an optimal number of hydroxyl groups
required for efficient activation of epoxides. A combination of
diffusion-constrain and a high ratio of hydroxyl groups to
organics may be at the origin of the low yield obtained with
OMO-6 (Table 2, entry 8). Interestingly, OMO-4 with four
hydroxyl groups, showed superior catalytic activity to all
Fig. 2 TGA profile of (a) amine-functionalized supports and DMOs, (b)
OMOs and (c) PMOs.
All materials showed IR absorption bands (Fig. 4 and S4†) at
approximately 640, 800 and 1080 cmꢁ1, which was attributed to
Si–O–Si bonds of the silicate network. The broad band centered
at ca. 3320 or 3440 cmꢁ1 were assigned to the O–H stretching
vibrations of silanol groups, hydroxyl groups of BHBPD and/or
adsorbed water. A band at 1650 cmꢁ1 conrmed the presence of
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Fig. 3 13C CP-MAS NMR spectra of (a) (i) Q10@amine, (ii) DMO-Q10: (b) (i) as-synthesized SBA-15, (ii) SBA-15*, (iii) OMO-2, (iv) OMO-4, (v) OMO-
6: (c) (i) as-synthesized SBA-15, (ii) SBA-15*, (iii) PMO-2, (iv) PMO-4, (v) PMO-5 and (d) 29Si MAS NMR of; (i) SBA-15*, (ii) OMO-2, (iii) OMO-4, (iv)
OMO-6, (v) PMO-2, (vi) PMO-6.
DMOs. This nding was attributed to the larger surface area
and moderate pore volume of the latter, leading to enhanced
accessibility to active sites and improved mass transport.
PMOs were found to be the least active compared to DMOs
and OMOs, with PMO-2 the most active of all PMOs – 24%
yield (Table 2, entry 9). The inferior catalytic performance of
PMOs was attributed to their low surface area, pore volume
and disordered pore structure. Accordingly, OMO-2, the most
active catalyst, was used to optimize the reaction conditions
and material reusability.
Optimization of reaction time
Using SO as substrate, an increase in yield (44.9 to 76.9%)
was observed when the reaction time was raised from 5 to
15 h (Table 2, entry 6 vs. 12). Compared to 1,2-EB for
instance, SO is signicantly more difficult to activate, and
expected to take signicantly longer reaction times under
mild conditions to attain high yields. However, prolonged
reaction times may lead to the formation of side products,
ultimately reducing product yield and selectivity. Hence,
optimization of reaction time was carried out with 1,2-EB. As
seen in Fig. 5, the yield of 1,2-butylene carbonate (1,2-BC)
Fig. 4 IR adsorption spectra of (a) SBA-15, (b) DMO-1, (c) OMO-4, (d)
PMO-2, (e) PMO-4 and (f) PMO-6.
was found to peak aer 15 h (97.2%), which is 25% more
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Table 2 A comparison of the catalytic performance of different organosilica materialsa
Spent cat.
Catalyst
Type
Reaction condition
Reaction quantication
Yield/g cat.
(%/g)
Entry
N-content (mmol gꢁ1
)
T (ꢀC)
t (h)
PCO (MPa)
Yield (%)
Sel. (%)
Retained N-content (%)
2
1
2
3
4
5
6
7
8
DMO-SIL
DMO-Q10
DMO-P10
DMO-SiL
DMO-SBA15
OMO-2
OMO-4
OMO-6
PMO-2
PMO-4
1.41
1.22
1.26
1.41
1.37
1.24
1.29
1.22
2.38
2.37
2.11
1.24
100
50
50
50
50
50
50
50
50
50
50
50
4
5
5
5
5
5
5
5
5
1.0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
96.8
26.2
31.8
31.6
24.3
44.9
38.3
25.8
23.9
21.2
20.0
76.9
98.5
94.0
98.6
98.5
98.0
96.1
97.8
97.0
98.5
98.9
98.2
98.5
401.7
94.0
117.7
131.1
98.1
154.9
145.1
92.5
167.2
148.3
131.7
291.6
96.4
96.5
93.5
96.8
94.9
95.2
97.6
95.7
95.7
97.1
93.8
96.1
9
10
11
12
5
5
15
PMO-6
OMO-2
a
Reaction conditions; 2 mol% catalysts based on N-content, 17 mmol styrene oxide (SO).
than was obtained for SC under similar conditions (Table 2, recycling experiments with 1,2-EB as substrate. As shown in
entry 12). Moreover, no noticeable change in selectivity Table 3, no signicant changes in product yield and selectivity
($97.5%) was observed aer 20 h. Nonetheless, a slight were observed over ve cycles. Although a 7% decrease in
decrease in yield was recorded aer 20 h, suggesting the organic content was observed compared to the fresh catalyst
onset of the formation of side products beyond 15 h. Hence, (Table 3, entry 1 vs. 2), the N-content of all spent catalyst was
optimum reaction conditions for the synthesis of 1,2-BC relatively constant, indicating the stable nature of OMO-2. This
using OMO-2 were; 2 mol% catalyst, 0.5 MPa CO2, 50 ꢀC and nding is consistent with the textural properties of fresh and
15 h.
spent catalysts being comparable.
Scope of substrates
Reusability studies
The scope of OMO-2 was investigated using four additional
substrates, namely propylene oxide (PO), epichlorohydrin
(ECH), cyclohexene oxide (CHO) and 3-glycidyloxypropyl-
trimethoxysilane (GPS) to synthesize propylene carbonate
(PC), 4-(chloromethyl)-1,3-dioxolan-2-one (4-CDO) and hex-
One of the most attractive attributes of heterogeneous catalysts
is their reusability. In this regard, OMO-2 was subjected to
ahydrobenzo[d][1,3]dioxol-2-one
(HDO)
and
4-((3-
(trimethoxysilyl)propoxy)methyl)-1,3-dioxolan-2-one (4-PDO),
respectively. An excellent yield (98.2%) of PC was obtained
aer 10 h under mild reaction conditions (Table 4, entry 1).
Doubling the reaction temperature and pressure led to a very
good yield (96%) aer just 3 h (Table 4, entry 2). The observed
marginal decreases in yield and selectively were attributed to
the formation of side products, which are facilitated at higher
temperatures.
Similar high yields were obtained in the synthesis of 1,2-BC
and 4-(chloromethyl)-1,3-dioxolan-2-one under mild reaction
conditions (Table 4, entries 3 and 5). As with the synthesis of PC,
similar high yields of 1,2-BC and 4-CDO were obtained at 4 h,
but at higher temperature and pressure (Table 4, entries 4 and
6). It is worth noting the high yield and selectively of these cyclic
Fig. 5 Optimization of reaction time. Reaction conditions; 2 mol%
catalysts, 22.7 mmol 1,2-EB.
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Table 3 Reusability of OMO-2 in the synthesis of 1,2-butylene carbonate
N-content (mmol gꢁ1
)
Yield (%)
Sel. (%)
BET SAa (m2 gꢁ1
)
Vpb (mL gꢁ1
)
Dp (nm)
c
Entry
Reaction
1
2
3
4
5
Run 1d
Run 2e
Run 3
Run 4
Run 5
1.22
1.14
1.11
1.13
1.12
96.4
94.7
95.1
93.2
94.3
98.2
98.4
98.5
97.8
98.0
449
469
453
446
456
0.66
0.72
0.68
0.69
0.61
6.0
5.9
5.9
6.0
6.2
a
BET surface area. b Pore volume. c Pore diameter. d Fresh catalyst. e First reuse.
carbonates under the current mild reactions condition using However, 97% yield was obtained by doubling the
a metal-free catalyst. Compared to other organocatalaysts reaction temperature and pressure, while limiting the
immobilized on inert support, namely; ammonium salts on reaction to 4 h (Table 4, entry 8). Furthermore, reacting CHO
nanocrystalline zeolites,50 imidazole-based ionic liquid (IL) on and GPS under the current mild reaction conditions led to
graphene oxide51 and covalent organic frameworks,52 tri- very low yields (Table 4, entries 9 and 11). Nonetheless,
phosphonate cavitand on SBA-15,53 phosphonium-based poly- signicantly higher yields were obtained at higher temper-
meric IL,54 phosphonium salt on polystyrene,55 OMO-2 was ature and pressure (Table 4, entries 10 and 12). This is not
signicantly more active.
surprising as these three epoxides are much more difficult to
As noted earlier, under the current mild reaction condi- activate (due to steric hindrance) than the rst three in
tions, the yield of SC was limited to 77% (Table 4, entry 7). Table 4.
Table 4 Yield and selectivity of various epoxides with CO2 using OMO-2
Yield
Entry Epoxide
Cyclic carbonate
PCO (MPa) T (ꢀC) t (h) (%)
Sel. (%)
2
1
2
0.5
1.0
50
100
10
3
98.2
96.0
99.1
97.2
3
4
0.5
1.0
50
100
15
4
96.4
95.5
98.2
97.4
5
6
0.5
1.0
50
100
15
4
95.3
94.8
97.5
97.0
7
8
0.5
1.0
50
100
15
4
76.9
97.5
98.5
98.6
9
10
0.5
1.0
50
100
15
15
3.3
70.0
96.7
85.2
11
12
0.5
1.0
50
100
15
24
10.2
85.1
96.4
95.3
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16 Y. Kumatabara, M. Okada and S. Shirakawa, ACS Sustain.
Chem. Eng., 2017, 5, 7295–7301.
Conclusions
17 J. Steinbauer, L. Longwitz, M. Frank, J. Epping, U. Kragl and
T. Werner, Green Chem., 2017, 19, 4435–4445.
18 M. North, R. Pasquale and C. Young, Green Chem., 2010, 12,
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19 G. Fiorani, W. Guo and A. W. Kleij, Green Chem., 2015, 17(3),
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E. K. Jeong, H. S. Kim and M. Cheong, Appl. Catal. B
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A series of novel DMOs, OMOs and PMOs were prepared,
characterized and screened against the cycloaddition reaction
of CO2 and epoxides. Results showed that OMOs were the most
active class of all materials. Furthermore, OMO-2, containing
two hydroxyl groups per immobilized molecule was found to be
the most active material. For substrates which are easy to acti-
vate such as PO, 1,2-EB and ECH, excellent yield and selectivity
were obtained under mild reaction conditions (0.5 MPa CO2,
50 ꢀC and 10–15 h). However, higher CO2 pressure and
temperature were required with substrates which are more
difficult to activate, e.g. SO, CHO and GPS. Furthermore, OMO-2
proved to be both chemically and structurally stable aer ve
cycles of 1,2-EB synthesis.
´
22 C. Martın, G. Fiorani and A. W. Kleij, ACS Catal., 2015, 5,
1353–1370.
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24 A. J. Kamphuis, F. Picchioni and P. P. Pescarmona, Green
Chem., 2019, 21, 406–448.
Conflicts of interest
25 T. Sakai, Y. Tsutsumi and T. Ema, Green Chem., 2008, 10,
337.
There are no conicts to declare.
26 F. Adam, J. N. Appaturi and E. P. Ng, J. Mol. Catal. A Chem.,
2014, 386, 42–48.
Acknowledgements
27 B. Chatelet, L. Joucla, J.-P. Dutasta, A. Martinez and
V. Dufaud, J. Mater. Chem. A, 2014, 2, 14164.
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Financial support from the Natural Sciences and Engineering
Research Council of Canada (NSERC) is gratefully
acknowledged.
30 S. Udayakumar, S. W. Park, D. W. Park and B. S. Choi, Catal.
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