Homogeneous and silica-supported zinc complexes for the synthesis of propylene carbonate from propane-1,2-diol and carbon dioxide

Article abstract of DOI:10.1039/c6cy00134c

Three organozinc complexes have been synthesised and found to catalyse the carbonylation of propylene glycol with carbon dioxide to form propylene carbonate. A similar tethered organozinc complex was supported onto high loading aminopropyl functionalised hexagonal mesoporous silica and was also found to be catalytically active.

Full text of DOI:10.1039/c6cy00134c

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Homogeneous and silica-supported zinc complexes for the
synthesis of propylene carbonate from propane-1,2-diol and
carbon dioxide.
Received 00th January 20xx,
Accepted 00th January 20xx
James W. Comerford, Sam J. Hart, Michael North* and Adrian C. Whitwood
DOI: 10.1039/x0xx00000x
Three organozinc complexes have been synthesised and found to catalyse the carbonylation of propylene glycol with
carbon dioxide to form propylene carbonate. A similar tethered organozinc complex was supported onto high loading
aminopropyl functionalised hexagonal mesoporous silica and was also found to be catalytically active.
www.rsc.org/
homogeneous catalysts reported to date,12 it lacked reusability
which reduced its catalytic effectiveness.
Introduction
Five-membered ring cyclic carbonates such as ethylene
carbonate and propylene carbonate are of growing
O
1
2
HO
R
OH
catalyst
O
catalyst
+ CO2
O
O
commercial importance due to their wide range of
applications. These include: use as electrolytes for lithium ion
batteries,1 monomers for polymer synthesis,2 intermediates in
the production of dimethyl carbonate3 and anhydrous
ethylene glycol,4 and green polar aprotic solvents with the
potential to replace traditional solvents such as DMF, DMSO,
NMP and acetonitrile.5
+ CO2
R
- H2O
R
1: R = H
2: R = Me
Scheme 1: Routes to cyclic carbonates
Few heterogeneous catalysts have been developed for the
carbonylation of diols with carbon dioxide, with two of the
most active being reported by Tomishige; potassium iodide
supported on zinc oxide and cerium oxide used with 2-
cyanopyridine as co-catalyst/dehydrating agent.13 Despite
some impressive conversions, the very specific particle sizes of
the cerium oxide required for high activity and the mandatory
use of 2-cyanopyridine makes the process expensive and
difficult to implement on an industrial scale. As such, we
report herein our investigation into the possibility of
immobilising the previously reported zinc triflate catalyst by
coordination with tethered bidentate ligands and our
discovery of three homogeneous organozinc complexes with
good catalytic activity. Many homogeneous zinc-ligand
complexes of this type have been reported, often as initiators
for the ring-opening polymerisation of lactides,14,15 but such
complexes have not been reported as catalysts for the
synthesis of cyclic carbonates.
The commercially most important route for the synthesis
of cyclic carbonates is the addition of carbon dioxide to
epoxides (Scheme 1).6 Despite the 100% atom economy of this
synthesis and the mild reaction conditions applicable to some
of the more active aluminium catalysts,7 use of volatile
epoxides as starting materials is problematic due to their
toxicity and explosive potential. Consequently, the use of 1,2-
diols as substrates for the synthesis of five-membered cyclic
carbonates has been highlighted as a particularly attractive
route with urea, alkyl carbonates and carbon dioxide being
used as green carbonyl sources.8 Diols tend to have low
toxicity and can often be sourced from renewable bio-derived
resources, thus reducing environmental impact and moving
away from dependence on oil based feedstocks. However,
direct use of 1,2-diols and carbon dioxide to synthesise cyclic
carbonates is thermodynamically unfavourable9 and requires
high reaction temperatures, pressures and a water scavenger
to shift the equilibrium towards formation of product.10 We
recently reported that zinc triflate is a highly active catalyst for
this reaction under relatively mild reaction conditions.11
Although zinc triflate is significantly more active than other
Results and discussion
Building on our previous publication,11 the initial focus was
directed at assessing how co-ordination of zinc triflate to a
variety of ligands would affect the catalytic activity of the
complex. To investigate this, 5 mol% of ligand 3-14 (chart 1)
was added to 5 mol% of zinc triflate and used for the synthesis
of propylene carbonate from propylene glycol and carbon
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dioxide, giving the results shown in Table 1. The relatively mild
reaction conditions (135 oC, 40 bar CO2 pressure) were
developed during our previous study.
Scheme 2: Formation of acetate by-products
but in all these cases no propylene carbonate was formed.
Entries 2 and 3 confirm the enhanced catalytic activity
associated with zinc triflate compared to zinc acetate, an
effect which is consistent with a need to maximise the Lewis
acidity of the zinc ion. The use of ligands 3-14 with zinc triflate
caused notable reductions in the conversion to propylene
carbonate compared to the use of zinc triflate without ligand.
This is also consistent with a reduction in Lewis acidity of the
zinc ion due to the presence of an electron donating ligand,
though steric effects may also have an influence.
Chart 1: Structures of ligands 3-14
The majority of the reactions catalysed by zinc triflate and
a potentially bidentate ligand gave conversions of 30-32%,
though the highest (39%) was achieved using methylenebis-
Table 1: Zinc complexes as homogeneous catalysts for propylene carbonate synthesis.
Entry
1
Zinc salt
-
Liganda
2 (%)
0
By-products (%)b
3,5-dimethylpyrazole
lower conversions and no propylene carbonate formation was
observed when using either 2,2’-dipyridyl ketone (entry 8) or
5 (entry 7). Monodentate ligands gave
-
-
0
2
Zn(OAc)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OAc)2
Zn(OTf)2
Zn(OAc)2
12
42
30
32
32
39
0
10
40
29
30
30
40
0
6
3
-
p-[(2-pyridylmethylene)amino]-phenol 13 (entries 15 and 16).
The inhibition of catalytic activity seen when using oxygen
containing ligands suggests the ketone/phenol competitively
binds with the zinc, preventing its interaction with carbon
dioxide and/or diol. Varying the zinc counter ion (acetate vs
triflate) did not change the lack of conversion observed with
ligand 13 (entries 15 and 16). However, removal of the oxygen
containing groups from the ligands by substituting 2,2’-
4
5c
3
3
6
4
7
5
8
6
9
7
30
27
26
26
30
31
0
28
26
26
25
28
29
0
10
11
12
13
14
15
16
17
18
8
9
10
11
12
13
13
14
14
dipyridyl ketone
6 with 2,2’-dipyridyl-N-propyl imine 7 and
substituting p-[(2-pyridylmethylene)amino]-phenol 13 with 2-
(N-phenylformimidoyl)-pyridine 12 restored the catalytic
activity of the zinc complexes, giving 30% and 31% conversion
respectively, (entries 9 and 14). Increasing the ratio of 2,2’-
0
0
31
3
30
1
bipyridine ligand
3 to zinc triflate from 1:1 to 2:1 had no
significant effect on the conversion (entries 4 and 5) which is
consistent with a 1:1 zinc-ligand species being the active
catalyst. The importance of the zinc counter ion in the
complexes is shown by entries 17 and 18 in which the triflate
complex gave ten times high conversion than the acetate
complex. This is- consistent with the need to have a counterion
which is a good leaving group to reduce competition for
coordination zinc between the counterion and the propylene
glycol.
a) Control reactions showed that ligands 3–14 themselves had no catalytic
activity. b) Combined yield of mono- and di-esters of propylene glycol. c) 10 mol%
of ligand 3 used.
Acetonitrile was found to be the most effective solvent and
chemical water trap, though the initially formed acetamide by-
product reacted with some of the diol starting material to give
a mixture of mono and di-acetate by-products 15-17 (Scheme
2).
Three of the systems which gave some of the highest
conversions (entries 4, 6 and 7) were chosen for further study
by preparing, isolating and characterising the complex prior to
Entries 1-3 of Table 1 are control experiments showing that
no reaction occurred in the absence of zinc salt and
benchmarking the catalytic activity of zinc acetate and triflate
in the absence of ligand. Additional control experiments were
carried out using ligands 3-14 in the absence of any zinc salt,
its use as a catalyst. Use of ligands
formation of known15 octahedral complexes of formula [Zn(
or )2(CF3SO3)2] which were characterised by X-ray
3 and 4 resulted in the
3
4
2 | J. Name., 2012, 00, 1-3
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Table 2: Selected bond lengths and angles for the zinc complexes of ligands 3–5.
Bond length (Å) or bond angle (o)
[Zn(3)2(CF3SO3)2] [Zn(4)2(CF3SO3)2]
[Zn(5)2(CF3SO3)]+
Bond
(CF3SO3)-
2.1671(18)
2.047(2)
-
Zn1–O1
Zn1–N1
2.1823(9)
2.1048(11)
2.0972(11)
-
2.1875(9)
2.1239(11)
2.1036(11)
-
Zn1–N2
Zn1–N4
2.075(2)
2.083(2)
2.080(2)
-
Zn1–N5
Zn1–N8
N1–Zn1–N2
N1–Zn1–N4
N5–Zn1–N8
78.13(4)
79.31(4)
-
-
-
-
91.12(9)
88.54(9)
[Zn(
5
)2(CF3SO3)]+ (CF3SO3)- (Figure 3). This complex could also
and zinc triflate. The
be prepared from a 1:1 ratio of ligand
5
solution state 19F NMR spectrum of this complex showed a
single signal at all temperatures between +20 and -50 oC
suggesting rapid exchange of the coordinated and free triflate
in solution. However, a solid state 19F NMR spectrum showed
two signals at -76.8 and -78.0 ppm consistent with the X-ray
structure.
Figure 1: Ortep diagram of [Zn(3)2(CF3SO3)2]
The formation of a five-coordinate complex from ligand 5 is
probably due steric crowding caused by the 3,5-methyl groups
on the bispyrazole coupled with non-planarity of the pyrazole
rings which forces one of the bridging methylenes into the
space required if the second triflate was bound to the zinc. The
Zn-N and Zn-O bond lengths of the three complexes (Table 2)
show the effect of having one of the two triflate ions
dissociated from the metal with the complex of ligand 5 having
much shorter bond lengths consistent with electron density
being withdrawn from the ligands in order to stabilise the zinc.
Dissociation of a triflate ion creates a vacant site and increases
the Lewis acidity, which may explain the higher conversions
Figure 2: Ortep diagram of [Zn(4)2(CF3SO3)2]
achieved with the zinc complex of ligand
increased steric hindrance around the metal centre. The bite
angle of the complexes of ligands and are very similar (78-
79o), being restricted by the rigid geometry of the aromatic
ring whilst the complex of ligand has two different bite
5, despite the
3
4
5
angles (88.5 and 91.1o) which are closer to 90°. This is
achievable as the N-N distance is now larger and the
methylene group between the pyrazoles also gives additional
flexibility.
To further investigate the use of pyrazole ligands,
methylenebis(3,5-di(t-butyl)pyrazole) 18 (Chart 2) was
synthesised16 in an attempt to further increase the steric
crowding around the zinc centre and possibly cause
dissociation of both triflate ions, potentially enhancing the
Lewis acidity and reactivity of the zinc. However, the resulting
complex could not be crystallised and when one equivalent of
ligand 18 was used with 5 mol% of zinc triflate to catalyse the
addition of carbon dioxide to propylene glycol, only 28%
conversion to propylene carbonate was achieved. This
suggests that the tert-butyl groups are so sterically hindering,
that either the ligand does not complex well to zinc, or the
resulting zinc complex is too hindered to catalyse the reaction.
The solution state NMR spectra did not allow the coordination
Figure 3: Ortep diagram of [Zn(
5
)2(CF3SO3)]+ (CF3SO3)-
crystallography (Figures 1 and 2) with bond lengths and angles
closely similar to those reported previously.‡ In contrast, use of
ligand 5 resulted in formation of a five-coordinate zinc species
which was characterised by X-ray crystallography‡ as
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number of the zinc ion to be determined as a single set of
signals were seen in the 1H, 13C and 19F NMR spectra, but a
solid state 19F NMR spectrum gave two signals at -78.6 and -
79.7 ppm. These are analogous to those seen for the complex
Si(OEt)4 + Si(OEt)3(CH2)3NH2
19
NH2
20
21a-c
21-23a: 19:20 = 4:1
of ligand
5 and indicate that in the solid state the complex of
21-23b: 19:20 = 2:1
21-23c: 19:20 = 1:1
N
CHO
ligand 18 is again five-coordinate with one coordinated and
TfO
N
N
one free triflate ligand.
Zn
t
But
TfO
Bu
N
N
Zn(OTf)2
N
N
N
N
But
Chart 2: Structure of ligand 18
But
18
23a-c
22a-c
Scheme 3: Synthesis of silica supported zinc complexes 21a–c
Table 3: Zinc complexes as homogeneous catalysts for propylene carbonate synthesis.
Complex
Conversion (%)
Acetates 15–17
2 yield (%)
2
[Zn(3)2(CF3SO3)2]
[Zn(4)2(CF3SO3)2]
[Zn(5)2(CF3SO3)]+ (CF3SO3)-
31
32
40
29
29
38
29
29
37
The crystals of the zinc complexes of ligands 3–5 were used
as catalysts for the synthesis of propylene carbonate from
propylene glycol (Table 3) to investigate whether the isolated
complexes had similar activity towards the carbonylation of
diols with carbon dioxide to the in situ prepared complexes
used in Table 1. The conversions for each ligand were similar
irrespective of whether the zinc complex was prepared in situ
or isolated and crystallised. This suggests that the catalytically
active species has a single triflate ligand attached to the zinc
since the in situ catalysts were prepared from a 1:1 ratio of
Figure 4: Isotherm linear plots of silicas 21a–c and 23a–c
.
zinc to ligand and the zinc complex of ligand
a single ligand complexed to the zinc.
5 crystallised with
Having established that organozinc complexes of ligands 3–
5
were active homogeneous catalysts for the synthesis of
propylene carbonate, the heterogenisation of Zn(OTf)2 was
investigated. Modification of commercial amorphous silica
with 3-aminopropyltrimethoxysilane was initially investigated,
but was found to give low loadings (< 0.16 mmol g-1) of the
amine tether. Higher loadings of active species on the silica
were necessary due to the quantity of catalyst (5 mol%)
required to achieve good conversions to propylene carbonate.
Therefore, three hexagonal mesoporous silicas 21a-c were
synthesised with aminopropyl tethers incorporated into the
structure by a co-condensation sol gel process (Scheme 3).17
Three ratios (4:1, 2:1 and 1:1) of tetraethylorthosilicate 19 to
3-aminopropyltrimethoxysilane 20 were used in the
preparation of silicas 21 to give materials with varying
quantities of amine groups on the silica surface. 2-
Pyridinecarboxaldehyde was reacted with silicas 21a–c to give
immobilised
c 18
This ligand was chosen due to the convenient synthesis of
the silica-supported imine and the good results obtained with
homogeneous imines of 2-pyridinecarboxaldehyde (12 and 14
N-propyltriethoxysilane-1-(2-pyridyl) imines 22a–
.
,
see Table 1). Silica-supported imines 22a–c were then stirred
with a solution of zinc triflate in diethyl ether for 24 hours to
give immobilised catalysts 23a–c 18
.
4 | J. Name., 2012, 00, 1-3
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lowest yield. This is most likely due to a high proportion of the
zinc triflate being trapped within the microporous network and
hence being inaccessible. Catalysts 23b and 23c gave
essentially identical yields even though the porosimetry data
suggested increased agglomeration of zinc triflate around the
pore entrances of the latter.
Silica supported zinc complex 23b was chosen to assess
catalyst reusability as it achieved the highest conversion to
Table 4: Porosimetry and ICP-MS data of silicas 21-23 and their catalytic activities.
Silica
Surface area
(m2 g-1)
783
Pore volumea
(cm3 g-1)
0.39
[Zn]
2 yield
(mmol g-1)b
(%)c
21a
21b
21c
23a
-
-
-
Figure 5: Pore size distributions of silicas 21–23a, 21–23b and 21–23c
268
0.71
-
167
0.56
-
-
Porosimetry analysis of silicas 21 and 23 showed that a
range of surface areas, pore size distributions and pore
volumes were present (Table 4). Use of higher ratios of 3-
aminopropyltrimethoxysilane 20 in the sol gel synthesis (21-
23b,c) generally gave a large average pore diameter and low
surface area mesoporous material. Each had a type four
adsorption isotherm, with 21-23b following a H2 type
43
0.035
1.36
1.46
0.877d
0.384e
1.49
18
26
19d
8e
25
23b
23c
66
63
0.25
0.30
a) Single point adsorption total pore volume b) Determined by ICP-MS; c)
Reaction conditions: 5 mol% equivalent zinc catalyst, propylene glycol 60 mmol,
acetonitrile (10 mL), CO2 (35 bar), 135 oC, 16 hours; d) Second use of catalyst
hysteresis loop (indicative of the material having a less well
defined porous network) and 21-23c tending towards H3 type isolated by filtration, washing with MeCN and drying at 100 oC for two hours; e)
(seen with aggregates of plate like particles).20 The rapid rise of
Third use of catalyst.
the isotherms around p/po = 1 suggests the presence of
propylene carbonate
with acetonitrile between uses and then dried in an oven at
100 oC before use. Yields of propylene carbonate
were found
2. The reisolated catalyst was washed
macroporosity in the structures, more so for 21-23c than for
21-23b
.
2
In contrast, silica 21a was found to be highly microporous
with minimal capillary condensation (mimicking a type one
adsorption isotherm), showing lower pore volume and high
surface area. After modifying each of silicas 21a–c by
formation of the 2-pyridyl imine ligand and subsequent
complexation of zinc triflate, the surface area and pore volume
decreased suggesting deposition within the porous network.
Interestingly, the total quantity of zinc immobilised on each of
the silicas was similar. Silica supported zinc complex 23a
showed a significant reduction in microporosity (Figure 5A)
along with a large reduction in surface area (Figure 4). This
implies that the microporous network had become blocked
with zinc triflate, leading to increased zinc deposition on the
external surface. Silica supported zinc complex 23b showed a
consistent and relatively broad but distinct pore size
distribution in the mesoporous region with a slight reduction
in pore volume, suggesting comparatively even dispersion of
zinc triflate throughout the material. Silica supported zinc
complex 23c showed a slight increase in pore size distribution
in the mesoporous and macroporous regions. Such an effect
has been reported where deposited material aggregates
around the entrance to the pore causing artificial widening to
occur.21 Similarly, pore volume decreased suggesting
deposition of zinc triflate within the pores.
to reduce after each use due to leaching of the zinc from the
silica caused by the polarity of the reaction mixture (propylene
glycol and acetonitrile) along with the elevated reaction
temperature used. This was supported by ICP-MS analysis of
the supported zinc catalyst which showed that the zinc loading
decreased by 0.6 mmol g-1 after each use of the catalyst. In
addition, a control experiment was carried out in which
complex 23b was suspended in acetonitrile and propylene
o
glycol and heated to 135 C under nitrogen in the absence of
carbon dioxide. XRF analysis of complex 23b before and after
this treatment showed that the zinc content reduced from
23% to 19% consistent with leaching being due to the polar
reaction mixture and not to the presence of carbon dioxide.
Conclusions
This study has shown three crystalline organozinc
complexes to be active as catalysts for the synthesis of
propylene carbonate from propylene glycol and carbon
dioxide. The most active of these was [Zn(5
)2(CF3SO3)]+
(CF3SO3)- which was shown by X-ray crystallography to possess
a five coordinate zinc and a dissociated triflate anion. This
configuration is thought to increase the catalytic activity of the
zinc by enhancing its Lewis acidity, despite the increased steric
crowding around the metal centre. Use of the even more
sterically hindered ligand 18 resulted in reduced catalytic
activity showing the need to balance steric and electronic
effects in the ligand design. Use of ligands containing either an
alcohol or ketone gave complexes with no catalytic activity,
demonstrating how oxophilic the zinc is in this system.
Each of silicas 23a–c were used as catalysts for the
synthesis of propylene carbonate (Table 4). These
heterogeneous catalysts were found to give lower yields of
carbonate
2
than the homogeneous complexes. All gave a
(18–26%) with 23a giving the
similar yield of compound
2
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Similarly, the choice of the zinc counter ion is important for
good catalytic activity with triflate being far superior to
acetate.
to a predried stainless steel Parr reactor. The vessel was
o
pressurised to 5 bar with CO2 and heated to 135 C. Once this
temperature had been reached, the reactor was pressurised to
40 bar. After 16 hours, the reactor was cooled to room
temperature and the pressure released. Acetonitrile was
removed under reduced pressure and a sample of the residue
was dissolved in d6-DMSO for 1H NMR analysis. The residue
was purified by flash column chromatography using CH2Cl2 as
Three aminopropyl functionalised silicas were prepared
and modification of the amine groups to give pyridyl imine
ligands followed by immobilisation of zinc triflate onto these
silicas was successful, although deposition of the zinc varied
between the materials. Porosimetry data suggested that the
microporosity of silica 23a caused the majority of zinc to
aggregate on the external silica surface. Conversely, silica 23c
eluent to give propylene carbonate
liquid. H(CDCl3) 4.85-4.73 (m, 1H), 4.49 (ddt
1H), 3.95 (ddt 8.5, 7.2, 1.3 Hz, 1H), 1.40 (dt
2 as a colourless
δ
J
J
8.4, 7.6, 1.2 Hz,
6.3, 1.4 Hz, 3H).
synthesised
using
the
highest
ratio
of
3-
J
δ
C(CDCl3) 155.2, 73.8, 70.8, 19.4. IR (ATR): 2991 and 1785 cm-1.
aminopropyltrimethoxysilane to tetraethylorthosilicate was
found to have increased quantities of zinc triflate aggregated
around the pore entrances. Silica 23b was found to have
HRMS (ESI-TOF) m/z: [M+Na]+ Calcd. for C4H6O3Na 125.0209;
Found 125.0212.
minimal macroporosity and
a reproducible pore size
Synthesis of [Zn(5)2(CF3SO3)]+ (CF3SO3)-.
distribution post modification suggesting relatively uniform
dispersion of zinc throughout the material. All of the modified
silicas were found to be catalytically active for the synthesis of
propylene carbonate from propylene glycol and carbon
dioxide. However, upon reuse silica 23b was found to leach
zinc, which may be attributed to the very polar reaction
mixture.
Ligand 5 (0.22 g, 1.1 mmol) was stirred with anhydrous zinc
triflate (0.18 g, 0.5 mmol) in dry MeCN (2 mL). After 16 hours,
the complex was precipitated by addition of Et2O. For X-ray
diffraction analysis, the complex was crystallised from MeCN
by vapour diffusion of Et2O; the crystals were filtered and
washed with Et2O.
6H), 1.71 (s, 6H).
δ
H(CD3CN) 6.30 (s, 2H), 6.11 (s, 2H), 2.45 (s,
δC(CD3CN) 152.53, 144.4, 107.7, 56.9, 30.0,
11.6, 10.3.
δF(CD3COCD3) -79.18 (s) at 298 K; -79.33 (s) at 223
K. F(solid state) -76.8 and -78.0. Mp 235.4–236.9 C. IR(ATR):
Experimental
o
δ
All reagents were commercially available (Alfa Aeser,
Sigma-Aldrich, Fluka, TCI or Acros) and were used as received.
1559, 1466, 1389, 1306, 1278, 1254, 1228, 1205, 1152, 1030,
1019, 685, 636 and 574 cm-1. HRMS (ESI-TOF) m/z: [M+] Calcd.
for C23H32F3N8O3SZn 621.1556; Found 621.1553.
Ligands 5, 7, 12–14 and 18 were prepared by a literature
procedure.22 The zinc triflate used was anhydrous; see
supplementary material for further information. Carbon
dioxide was purchased from BOC. 1H and 13C NMR spectra
were recorded on a Jeol Oxford 400 at resonance frequencies
of 400 MHz and 100 MHz respectively. 19F NMR spectra were
recorded on a Bruker AVIIIHD500 spectrometer at 470 MHz.
Solid state 19F spectra were recorded on a Bruker Avance III HD
400 at 377 MHz. Fluorine spectra are referenced relative to
CFCl3. X-ray crystallography was performed on a single-crystal
Oxford Diffraction SuperNova with dual Mo & Cu sources at
110 K and a Bruker D8 powder diffractometer equipped with a
Cu source at room temperature. Porosity data was obtained
using a Micrometrics ASAP 2020 surface area and porosity
analyser. Infrared spectroscopy of organic compounds was
performed at a resolution of 1 cm-1 using a Specac Quest ATR
FTIR with a Bruker Vertex 70 Infrared Spectrometer. DRIFTS
analysis of inorganic materials and heterogeneous catalysts
was performed on a Bruker Equinox 55 using a resolution of 2
cm-1 and a KBr dilution of 10:1. Melting points were obtained
using a Gallenkamp melting point apparatus. Mass spectra
were obtained using a Bruker micrOTOF time of flight mass
spectrometer with ESI and APCI sources and an Agilent 1200
series LC system.
Synthesis of complex from ligand 18 and Zn(OTf)2.
Ligand 18 (0.050 g, 0.134 mmol) was stirred with
anhydrous zinc triflate (0.024 g, 0.067 mmol) in dry Me2CO (5
mL). After 16 hours, the solvent was evaporated in vacuo to
leave a pale yellow solid.
δ
H((CD3CO)2) 6.46 (s, 2H), 5.97 (s, 2H),
C((CD3CO)2) 158.9, 152.8, 101.2,
F((CD3CO)2) -78.97 (s). F(solid
1.30 (s, 18H), 1.19 (s, 18H).
δ
δ
65.3, 31.7, 31.6, 29.9, 28.8.
δ
state) -78.6 and -79.7. Mp 130.1–131.9 oC. IR(ATR): 2964,
1637, 1543, 1462, 1363, 1313, 1282, 1228, 1181, 1029, 1016,
800, 774, 766 and 632 cm-1. HRMS (ESI-TOF) m/z: [M+] Calcd.
for C51H91N8OZn [(18)2ZnOMe]+ 895.6607; Found 895.6189.
Synthesis of aminopropyl functionalised silicas 21a–c.
To a solution of n-dodecylamine (5.08 g, 27.5 mmol)
dissolved in water–ethanol (53 ml water, 46 ml ethanol) were
added, at room temperature, separately but simultaneously,
tetraethylorthosilicate and 3-aminopropyltrimethoxysilane in
quantities such that a total of 0.1 mole of silicon was added
with the ratio required to give the desired degree of
functionalisation. The initially clear solution became a thick
white paste after 18 hours. The white solid was filtered and
extracted with ethanol using a Soxhlet extractor (16 hours
overnight). The remaining solid was collected and dried at 110
°C to give the aminopropyl functionalised silicas 21a–c as white
solids. For 21a: DRIFTS 10:1 KBr dilution: 3737, 3363, 3293,
2939, 2868 and 1070 cm-1. BET surface area: 783 m²/g, Single
point adsorption total pore volume at P/Po = 0.98: 0.39 cm³/g,
Synthesis of propylene carbonate using homogeneous complexes
derived from zinc triflate.
Propylene glycol (4.56 g, 60 mmol), MeCN (10 mL), zinc
triflate (1.06 g, 3 mmol,) and ligand 3–14 (3 mmol) were added
6 | J. Name., 2012, 00, 1-3
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BJH adsorption average pore radius: 1.1 nm. For 21b: DRIFTS:
3755, 3364, 3299, 2936, 2864 and 1051 cm-1. BET surface area:
268 m²/g, Single point adsorption total pore volume at P/Po =
0.99: 0.71 cm³/g, BJH adsorption average pore radius: 5.2 nm.
For 21c: DRIFTS: 3736, 3364, 3299, 2932, 2859 and 1049 cm-1.
BET surface area: 167 m²/g, Single point adsorption total pore
volume at P/Po = 0.98: 0.56 cm³/g, BJH adsorption average
pore radius: 6.5 nm.
Acknowledgements
We gratefully acknowledge financial support from the
European Union Seventh Framework Programme FP7–NMP–
2012 under grant agreement number 309497 and Dr. Richard
Heyn of SINTEF for helpful suggestions.
Notes and references
‡
Crystallographic data has been submitted to CCDC and
compounds 3-5 given codes CCDC-1448143, CCDC-1448144
and CCDC-1448145 respectively.
Synthesis of silica supported ligands 22a–c.
Aminopropyl functionalised silica 21a–c (4 g) was added to
a round bottom flask along with 2-pyridinecarboxaldehyde
(12–22 mmol depending on the aminopropyl loading) and
absolute ethanol (40 mL). The mixture was heated to reflux for
16 hours, then cooled to room temperature and filtered. The
resulting solid was washed three times with hot ethanol and
dried under reduced pressure at 50 oC until a free flowing
powder formed. For 22a: DRIFTS: 3759, 3365, 3293), 2978,
1
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:
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1439 and 1054 cm-1. For 22c: DRIFTS: 3744, 3278, 2935, 2887,
1650, 1590, 1589, 1470, 1437 and 1056 cm-1.
3
Synthesis of silica supported zinc complexes 23a–c.
4
5
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Silica supported ligand 22a–c and anhydrous zinc triflate
(7.27 g, 20 mmol) were added to Et2O (30 mL) and stirred at RT
for 48 h. Then the solid was filtered and washed four times
with Et2O. The resulting solid was dried under vacuum to give
23a–c as pale yellow solids. ICP-MS analysis showed zinc
2
, 1739–1742; J. A. Castro-Osma, J. W. Comerford, S.
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concentrations of 1.36, 1.46, 1.49 mmol g-1 for 23a,b and
c
5
respectively. For 23a: DRIFTS: 3748, 3254, 2982, 1654, 1605,
1572, 1483 and 1037 cm-1. BET surface area: 43 m²/g, Single
point adsorption total pore volume at P/Po = 0.98: 0.035
6
7
J. W. Comerford, I. D. V. Ingram, M. North and X. Wu,
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Wilhelm, M. H. Anthofer, W. A. Herrmann and F. E.
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cm³/g, BJH adsorption average pore radius: 2.8 nm. For 23b
:
DRIFTS: 3746, 3254, 2982, 1655, 1603, 1572, 1450 and 1036
cm-1. BET surface area: 66 m²/g, Single point adsorption total
pore volume at P/Po = 0.99: 0.25 cm³/g, BJH adsorption
average pore radius: 8.1 nm. For 23c: DRIFTS: 3746, 3254,
2982, 1655, 1603, 1573, 1449 and 1027 cm-1. BET surface area:
63 m²/g, Single point adsorption total pore volume at P/Po =
0.98: 0.30 cm³/g, BJH adsorption average pore radius: 10.2
nm.
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Catalyst leaching test.
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Propylene glycol (4.6 g, 60 mmol), dry acetonitrile (10 mL)
and of catalyst 23b (5 mol% equivalent) were added to a dry
stainless steel Parr reactor. The reactor was sealed, evacuated
and refilled with N2 three times and charged to 5 bar with N2.
8
9
o
It was then heated to 135 C achieving a final pressure of 10
bar. After 16 hours, the reactor was cooled to room
temperature and the pressure released. The catalyst was
filtered and washed four times with acetonitrile before being
analysed by XRF. The % zinc content was found to reduce from
23.0% before the leaching test to 18.8% after the leaching test.
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ARTICLE
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Graphical abstract
Homogeneous and silica-supported zinc triflate complexes
catalyse the synthesis of propylene carbonate from propane-1,2-
diol and carbon dioxide.
8 | J. Name., 2012, 00, 1-3
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