A single centre aluminium(iii) catalyst and TBAB as an ionic organo-catalyst for the homogeneous catalytic synthesis of styrene carbonate

Article abstract of DOI:10.1039/c3cy01015e

A non-symmetrical aluminium complex containing a single metal centre has been synthesised and used as a catalyst in the cycloaddition reaction of carbon dioxide to styrene oxide to yield a cyclic carbonate. The reaction proceeds to around 70% conversion using the catalyst alone; however, it can be increased to 90% conversion with the addition of tetrabutylammonium bromide (TBAB) as a co-catalyst. Interestingly, the reaction proceeds as well with the organo-catalyst TBAB alone as it does with the aluminium catalyst alone. This offers the potential for reduced cost processing and scale-up of the reaction through elimination of the aluminium catalyst due to reduced catalyst costs and ease of separation of TBAB as a salt from the reaction mixture. The rate constants for the aluminium catalysed reactions were determined as a function of process temperature in the range 80-150°C, as well as the activation energies for the reactions determined from the Arrhenius plots. The activation energy for the aluminium catalyst alone was determined to be 34 kJ mol ^-1, while adding TBAB as a co-catalyst reduced the activation energy by 11 kJ mol^-1. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3cy01015e

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A single centre aluminium(III) catalyst and TBAB as
an ionic organo-catalyst for the homogeneous
catalytic synthesis of styrene carbonate†
Cite this: Catal. Sci. Technol., 2014,
4, 1622
*
Somsak Supasitmongkol and Peter Styring
A non-symmetrical aluminium complex containing a single metal centre has been synthesised and used
as a catalyst in the cycloaddition reaction of carbon dioxide to styrene oxide to yield a cyclic carbonate.
The reaction proceeds to around 70% conversion using the catalyst alone; however, it can be increased
to 90% conversion with the addition of tetrabutylammonium bromide (TBAB) as a co-catalyst. Interestingly,
the reaction proceeds as well with the organo-catalyst TBAB alone as it does with the aluminium catalyst
alone. This offers the potential for reduced cost processing and scale-up of the reaction through elimination
of the aluminium catalyst due to reduced catalyst costs and ease of separation of TBAB as a salt from the
reaction mixture. The rate constants for the aluminium catalysed reactions were determined as a function of
process temperature in the range 80–150 °C, as well as the activation energies for the reactions determined
from the Arrhenius plots. The activation energy for the aluminium catalyst alone was determined to be
Received 4th December 2013,
Accepted 1st February 2014
DOI: 10.1039/c3cy01015e
www.rsc.org/catalysis
34 kJ mol−1, while adding TBAB as a co-catalyst reduced the activation energy by 11 kJ mol−1
.
any reaction in which it is involved. The formation of cyclic
carbonates is a key area that has attracted considerable inter-
est. Cyclic carbonates are valuable products as they can be
used as solvents in chemical reactions and as constituents of
batteries. Furthermore, ring opening polymerisation reactions
give rise to polycarbonates which are valuable structural and
functional materials. Their high strength and transparency
have made them useful as replacement materials for security
glass applications and in the packaging of foodstuffs. CO2
insertion is also beneficial as it eliminates the use of highly
toxic phosgene which is used in conventional carbonate and
polycarbonate syntheses.7 Furthermore, the formation of cyclic
carbonates is an example of an atom efficient reaction with
each atom present in the two reactants being incorporated
into the final product.
Several methods have been reported for the synthesis
of cyclic carbonates from terminal epoxides by homoge-
neous cycloaddition of carbon dioxide with a variety of cata-
lysts. Catalyst systems based primarily on the highly active
(salen)metal-based catalyst system have been reported. Among
such transition-metal complexes, chromium,8 cobalt,9,10 zinc11
and magnesium12 have been reported. Examples of such
complexes are given in Fig. 1. In particular, salen complexes
of aluminium have received considerable interest in the
synthesis of cyclic carbonates, mainly due to the low environ-
mental impact of aluminium, its high Earth abundance and
its high catalytic activity.13 Studies have reported the synthesis
of cyclic carbonates using different symmetrical aluminium
salen complexes and Lewis bases as co-catalysts, including
1. Introduction
Carbon dioxide utilization (CDU) from waste gas streams
or even the atmosphere has received growing attention over
the recent years. Indeed the World Economic Forum1 has
identified CDU as one of the top ten emerging technologies
for 2012 and 2013. There has been much debate as to
whether CDU can play a significant role in climate change
mitigation because of the vast quantities of carbon dioxide
emitted globally each year. The amount of CO2 that could be
captured and reused has been estimated at various levels
up to approximately 10% of emissions.2 Carbon dioxide is
already used widely in the food industries and increasingly in
the field of CO2-enhanced oil recovery (CO2-EOR). However,
with the exception of a recent paper by Jaramillo et al.,3
accurate life cycle analyses over the latter have yet to be
demonstrated as there are no defined and agreed boundary
conditions for the process.2 Regardless of climate mitigation,
CDU can provide an alternative chemical feedstock with
decreased reliance on petrochemical derivatives.4–6
Carbon dioxide is relatively stable and so unreactive having
a standard heat of formation (ΔfH°) of −394 kJ mol−1. There-
fore, catalysts are needed to reduce the activation energy of
UK Centre for Carbon Dioxide Utilization, Department of Chemical & Biological
Engineering, The University of Sheffield, Sir Robert Hadfield Building,
Sheffield S1 3JD, UK. E-mail: p.styring@sheffield.ac.uk
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cy01015e
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attack at the epoxide, initiating the attack at the carbon atom
of carbon dioxide by the bromo-alkoxide. The lone pair of
electrons of one oxygen atom of the carbon dioxide weakly
coordinates with the central metal ion in the (salen)AlCl
complex, leading to a linear carbonate form. The linear car-
bonate is then transformed into the cyclic carbonate via spon-
taneous intramolecular cyclic elimination.
As can be seen in Table 1, the symmetrical aluminium
salen complexes have been reported to show catalytic activity
in cyclic carbonate synthesis at high CO2 pressure and/or high
temperature, whereas only the bimetallic aluminium salen
complex was an effective catalyst under ambient conditions.18
Catalytic cycloaddition by a non-symmetrical aluminium salen
complex has not been reported among the catalysts described
to date. Non-symmetrical salen complexes are potentially useful
if they can be functionalised with a motif that allows subse-
quent polymerisation or grafting to a pre-formed polymeric
structure, as this immobilises the catalyst so it can act hete-
rogeneously.19–21 In this study, the synthesis of styrene car-
bonate using the non-symmetrical aluminium salen–acac
hybrid complex (salenac), TBAB and a combined catalytic sys-
tem was undertaken as a function of temperature at atmo-
spheric CO2 pressure.
Fig. 1 The structure of (salen)metal complexes: (a) (salen)MX complex
and (b) bimetallic complex [(salen)M]2O catalyst discussed in Table 1.
quaternary ammonium salts, pyridine, imidazolium, 18-crown-6-KI
and N,N′-dimethylaminopyridine (DMAP).
Lu et al. have reported the effect of a co-catalyst on the cata-
lytic activity of aluminium salen complexes.14,15 They concluded
that the binary catalyst system of (salen)AlX/Bu4NBr (TBAB) showed
better catalytic activity than (salen)AlX alone. Tetrabutylammonium
bromide (TBAB) has been shown to enhance catalytic activity
in comparison with other Lewis bases as shown in Table 1.
He et al. suggested that the existence of halides in the axial
(Al–X) group enhanced the catalytic activity of (salen)AlX.12
They also proposed the mechanism for the cyclic carbonate
synthesis using the binary catalyst system which has since
been developed by North and co-workers.16–18
Cyclic carbonates are synthesised by forming the hexa-
coordinated complex between the oxygen atom of the epoxide
and the central metal in (salen)AlCl. The formation of a hexa-
coordinated complex results in a reduction of the Al–Cl bond
strength. The nucleophilicity of the Cl atom in the (salen)AlCl
complex favours interaction with the epoxide, leading to facile
cleavage of the epoxide ring (Fig. 2). At the same time, the
highly reactive anion of the quarternary ammonium salt TBAB
catalyses the opening of the epoxide ring by nucleophilic
2. Experimental methods
All chemicals and solvents were obtained from Aldrich and
Fisher Scientific and used as received without further purification.
The following reagents were used: dichloromethane (AR, Fischer),
methanol (AR, Fischer), 2,4-pentanedione (99%, Aldrich),
1,2-diaminoethane (99%, Aldrich), 2,4-dihydroxybenzaldehyde
(98%, Aldrich), diethylaluminium chloride (1.8 M solution in
toluene, Aldrich), tetrabutylammonium bromide (99%, Aldrich),
styrene oxide (98%, Aldrich) and diethyl ether (99.8%, Fisher).
CO2 (99.8%) gas was supplied by BOC Gases and dried using
an inline molecular sieve/silica gel drying tube when used.
NMR spectra were recorded using a Bruker DRX500 spectro-
1
meter at 500.3 MHz for H-NMR and 125.77 MHz for 13C-NMR.
Table 1 Summary of cyclic carbonate synthesis by (salen)metal complexes starting from propylene oxide (PO) and ethylene oxide (EO)
TOFe
(h−1
Catalyst
Conditions
Epoxide
PO
(salen)metal complex
Co-catalyst
Solvent
T (°C)
P (bar)
Time (h)
TONd
)
Ref.
(salen)AlCla
(salen)AlCl
(salen)AlCl
(salen)AlCl
(salen)AlBr
(salen)AlEt
(salen)AlCla
(salen)AlCl
(salen)AlCl
(salen)AlCl
(salen)CrClb
[(salen)Al]2Oc
Bu4NI
Bu4NBr
Bu4NCl
18-Crown-6-KI
18-Crown-6-KI
18-Crown-6-KI
—
Pyridine
1-MeIm
—
—
—
—
—
—
CH2Cl2
—
—
25
25
25
25
25
6
6
6
6
6
6
8
8
8
8
8
8
1
1
1
1
1
3
492
504
276
463
498
492
174
690
526
2220
916
24.8
61.5
63.0
34.5
57.9
62.3
61.5
174
690
526
14, 15
25
EO
110
110
110
110
100
25
150–160
150–160
150–160
150–160
8
12
Bu4NBr
DMAP
Bu4NBr
—
CH2Cl2
—
2220
916
PO
PO
8
1
8.3
16, 18
a
b
t
c
t
d
R1, R2 = H, M = Al. R1, R2 = Bu, R3, R4 = Ph, M = Cr, X = Cl. R1, R2 = Bu, M = Al. Moles of carbonate produced per mole of catalyst.
e
Moles of carbonate produced per mole of catalyst per hour.
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Fig. 2 Proposed mechanism for the (salen)AlCl–TBAB catalysed synthesis of cyclic carbonates from epoxides and carbon dioxide.
CHN analysis was performed on a Perkin Elmer 2400 CHNS/O
Series II Elemental Analyser. Chloride content was determined
by titration by the University of Sheffield Elemental Analysis
Service in the Department of Chemistry. Mass spectra were
recorded on a VG Autospec mass spectrometer using Electron
Ionisation (EI). Gas chromatography (GC) was performed on a
Varian 3900 GC with a 15 m × 25 mm i.d. capillary column
with hydrogen as the carrier gas. The conditions used for
investigating styrene oxide conversion were: hold for 2 min
at an initial temperature of 60 °C, then ramped at a rate of
15 °C min−1 from 60 to 270 °C and held at 270 °C for 5 min.
Synthesis of the AlCl(salenac)OH complex (Al1cat) (3 in Scheme 1)
The non-symmetrical aluminium salen complex was synthesised
by the reaction of the non-symmetrical Schiff-base salenac
ligand (2) with diethylaluminium chloride (Et2AlCl) as shown
in Scheme 1 in the results and discussion section. The
(salenac)OH ligand (0.08 g, 0.305 mmol) was dissolved in
dried dichloromethane (20 ml) under a nitrogen atmosphere
at ambient temperature. Diethylaluminium chloride (0.17 ml,
1.8 M solution in toluene, 0.305 mmol) was added slowly
to the stirred solution under a nitrogen atmosphere. After
stirring for 4 h at room temperature, the product was removed
by filtration and dried overnight in a vacuum oven at 80 °C
until the weight was constant. This yielded 0.089 g (91%) of a
bright yellow powder. The structure of the AlCl(salenac)OH
complex is shown in Fig. 4 and the NMR spectra in the ESI.†
1H-NMR (500.1 MHz, (CD3)2SO) δH (ppm): 1.85 (3H, –C(CH3)N–),
1.90 (3H, 3× H1), 3.30 (2H, 2× H6), 3.75 (2H, 2× H7), 4.90 (1H, H3),
6.15 (1H, H5′), 6.25 (1H, H3′), 7.15 (1H, H6′), 8.35 (1H, –NCH–)
and 9.95 (1H, –OH). 13C-NMR (125.7 MHz, CDCl3) δC (ppm):
21.52 (C1), 25.59 (–C(CH3)N–), 52.27 (C6), 55.99 (C7), 98.56
(C3), 105.00 (C5′), 105.86 (C3′), 112.90 (C1′), 135.36 (C6′), 164.18
(C9), 165.05 (C4′), 166.89 (C4), 170.66 (C2′) and 175.79 (C2).
Elemental analysis: calculated for C14H16N2O3AlCl: C 52.06,
H 4.96, N 8.68 and Cl 10.99%; found: C 51.95, H 5.03, N 8.73
and Cl 11.07%.
Synthesis of the non-symmetrical ligand (2 in Scheme 1)
A solution of 2,4-pentanedione (1.09 ml, 10.5 mmol) in
dichloromethane (8 ml) was added dropwise to a solution of
1,2-diaminoethane (1.36 ml, 20.1 mmol) in dichloromethane
(8 ml). The resulting mixture was heated under reflux for 1 h.
The excess 1,2-diaminoethane was removed from the resulting
mixture by rotary evaporation under vacuum at 60 °C for
30 min. The residue was dissolved in dichloromethane (16 ml),
then a solution of 2,4-dihydroxybenzaldehyde (1.42 g, 10.1 mmol)
in methanol (7 ml) and dichloromethane (7 ml) was added
slowly. The resulting solution was stirred at ambient tem-
perature for 30 min. The product was filtrated and dried over-
night in a vacuum oven at 80 °C until there was no further
mass loss. The product was obtained as a pale yellow powder
(1.989 g, 75%). The structure of the (salenac)OH ligand is
shown in Fig. 3. 1H-NMR (500.1 MHz, (CD3)2SO) δH (ppm):
1.85 (3H, –C(CH3)N–), 1.90 (3H, 3× H1), 3.30 (2H, 2× H6),
3.75 (2H, 2× H7), 4.90 (1H, H3), 6.15 (1H, H5′), 6.25 (1H, H3′),
7.15 (1H, H6′), 8.35 (1H, –NCH–), 9.95 (1H, Ph–OH), 10.70
(1H, –OH) and 13.65 (1H, –C–OH). Elemental analysis: calcu-
lated for C14H18N2O3: C 64.08, H 6.87 and N 10.68%; found:
C 63.52, H 6.93 and N 10.81%.
Synthesis of styrene carbonate (4) using the Al1cat catalyst
The procedure for the homogeneously catalysed insertion
reaction of carbon dioxide into styrene oxide was adapted
from the literature.18 The catalytic reaction was carried out in
reaction tubes using a Radleys Carousel parallel synthesiser.
The reaction temperature was controlled to 0.1 °C using an
IKA fuzzy logic controller unit, while the reflux was controlled
using a water cooled heat exchanger that was integrated into
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Fig. 3 Structure of the non-symmetrical Schiff-base salenac ligand.
Fig. 4 Structure of the AlCl(salenac)OH complex (Al1cat).
the synthesiser. Carbon dioxide was dried by passing it through
a dryer column (molecular sieve/self-indicating silica gel) before
being introduced into a Radleys Carousel reaction tube at 1 bar
pressure using a PTFE sparging tube directly into the stirred
solution. Agitation was achieved using a magnetic stirrer bar.
A mixture of styrene oxide (2 mmol), tetrabutylammonium
bromide (0–2.5 mol%), Al1cat catalyst (AlCl(salenac)OH complex,
0–2.5 mol%) and dichloromethane (5 ml) as a co-solvent was
loaded into a Radleys reaction tube. This was tightly sealed
and the reaction was stirred vigorously until complete dissolu-
tion of the reagents was achieved. The reaction was heated
to the desired temperature (80–150 °C) and carbon dioxide
was introduced to the reaction at atmospheric pressure by
bubbling it through the solution. The reaction was monitored
using gas chromatography until it approached steady state
conversion, typically after 48 hours. The reaction solution was
extracted into methanol in order to determine the styrene oxide
conversion by GC analysis with tetradecane as an internal
standard. To obtain pure styrene carbonate, the reaction solu-
tion was first extracted into diethyl ether and separated by
column chromatography on silica gel using hexane–ethyl
acetate (2 : 3) as an eluent. Evaporation of the combined product
fractions gave styrene oxide in 41% isolated yield as a white
solid. The melting point of 50–52 °C agreed well with the
literature values (50–54 °C).22,23 1H-NMR (500.1 MHz, CDCl3)
δH (ppm): 4.35 (1H, OCH), 4.81 (1H, OCH), 5.68 (1H, PhCHO)
and 7.2–7.5 (5H, ArH); 13C-NMR (125.7 MHz, CDCl3) δC (ppm):
71.1 (CH2), 77.9 (CPh), 125.81, 128.55, 129.16 and 135.8 (Ph),
154.78 (CO). Electron ionisation MS analysis gave the molecular
ion of styrene carbonate at m/z 164.
Kinetic parameters of styrene carbonate synthesis
The procedure for investigating the kinetic parameters for
the cycloaddition of carbon dioxide to styrene oxide was
adapted from the literature.18 Styrene oxide (0.23 ml, 2 mmol),
tetrabutylammonium bromide (6.45 mg, 1 mol), Al1cat (6.45 mg,
1 mol) and dichloromethane (5 ml) were loaded into a Radleys
reaction tube. The solution was stirred vigorously using a
magnetic stirrer and heated to the desired temperature
(80–150 °C). Carbon dioxide was introduced to the reaction
at 1 bar by bubbling through the solution. The reaction was
monitored every 3 hours using gas chromatography as described
previously. The rate constant was determined from the experi-
mental data assuming pseudo-first order reaction kinetics
and the activation energy for the process was determined
Scheme 1 Synthesis of the non-symmetric (2) ligand and the Al1cat catalyst (3) used in the studies.
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using the Arrhenius equation based on the calculated rate
constants at different temperatures.
The catalytic cycloaddition was conducted initially at three
different temperatures: 25, 80 and 110 °C. There was no activity
at 25 °C, either with Al1cat alone or as a mixed catalyst system
with TBAB (entries 1 & 2 in Table 2). The reaction was carried
out in dichloromethane as the solvent. While this may seem a
curious choice of solvent for a reaction at elevated tempera-
tures and 1 bar pressure, it was used because it solubilized
both the catalyst complex and the organic substrate. CO2 was
sparged with constant flow directly into the solution, with the
reaction tube being pressure equalised by placing an identical
diameter syringe needle to the sparger inlet as the outlet in
the silicone septum. Dichloromethane vapour condensed back
into the reactor using the Radleys Carousel reflux unit and
there was no noticeable loss of solvent at the end of the reac-
tion. When the temperature was increased to 80 °C, a conver-
sion of 77% was observed with added TBAB (entry 3) or 48%
for Al1cat alone (entry 4). Good conversion was achieved at
110 °C under a number of conditions. For the reaction with
2 mol% each of Al1cat and TBAB (entry 5), 92% conversion
was achieved, while for 1 mol% of each (entry 6) the conversion
was comparable at 90%. When 1 mol% Al1cat complex was
used without added TBAB at 110 °C, reactivity was decreased
with lower styrene oxide conversion of 73% (entry 7), relative
to that of the Al1cat and TBAB binary system (90% conversion)
at the same temperature. This result followed a similar trend
to those reported by Lu et al.,14,15 although the symmetrical
aluminium salen complex and TBAB were reported in the
literature. Interestingly, the catalytic reactivity of TBAB alone
was comparable to the Al1cat system, with 70% conversion
(entry 8). This shows that the aluminium catalyst may well be
unnecessary for cyclic carbonate synthesis which will not only
reduce costs but also simplify the post-reaction separation
process, especially if supported TBAB-type reagents can be used.
All of the results of both the binary catalyst and catalyst
alone showed increased styrene oxide conversion with increasing
temperature, with no conversion at room temperature. Further-
more, the catalytic activity of the Al1cat complex can be com-
pared with a symmetrical aluminium salen complex ((salen)AlCl)
and a dimetallic aluminium salen complex ([(salen)Al]2O) as
shown in Table 3.
3. Results and discussion
The ligand used in this study was prepared by adapting
the synthetic procedure described by Styring et al.19–21 which
was used in the synthesis of other non-symmetrical metal
complexes. The ligand was dried thoroughly before forming
the coordinated complex with diethylaluminium chloride.
The general procedure is shown in Scheme 1.
Styrene carbonate was synthesised via the catalytic cycload-
dition of carbon dioxide to styrene oxide in a homogeneous
reaction (Scheme 2). The mixture was stirred at elevated tem-
peratures (80–150 °C). After reaction for 48 h, the mixture was
worked up to give a yield of 41% of styrene carbonate as a
white solid. The melting point (mp 50–52 °C) agreed well with
the range of melting points (50–54 °C) of styrene carbonate
reported in the literature.22,23 The NMR analysis was consistent
with that for styrene carbonate reported in the literature.16,22,23
Catalytic activity of the non-symmetrical aluminium
salenac complex
A summary of the catalytic behaviour of the Al1cat complex
in the styrene carbonate synthesis from styrene oxide and
CO2 is reported in Table 2. The symmetrical (salen)AlCl
complex was also studied for comparison. Previous studies at
100 bar CO2 pressure and 80 °C showed 19% conversion to
the cyclic carbonate with a TON of 59.13 However, at 1 bar
pressure we observed no conversion at temperatures up to
110 °C.
Although styrene carbonate can be produced in a shorter
reaction time using (salen)AlCl in comparison with Al1cat
reported here, (salen)AlCl suffered from low catalyst activity
Scheme 2 Cyclic styrene carbonate (4) synthesis through CO2 insertion
into styrene oxide (3).
Table 2 Effect of reaction conditions and catalysts on the cycloaddition of CO2 to styrene oxidea
Entry
Catalyst (mol%)
Co-catalyst (mol%)
Temp (°C)
Conversionb (%)
1
2
3
4
5
6
7
8
Al1cat (2.5)
Al1cat (1.0)
Al1cat (1.0)
Al1cat (1.0)
Al1cat (2.0)
Al1cat (1.0)
Al1cat (1.0)
—
TBAB (2.5)
TBAB (1.0)
TBAB (1.0)
—
TBAB (2.0)
TBAB (1.0)
—
25
25
80
0
0
77
48
92
90
73
70
80
110
110
110
110
TBAB (1.0)
a
b
Reaction conditions: styrene oxide (2 mmol), CO2 1 bar and co-solvent (DCM) 5 ml for 48 h. Conversions were calculated from GC analysis
data based on styrene oxide by using tetradecane as internal standard.
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Table 3 Comparison of styrene carbonate synthesis by different aluminium salen complexes
TOFb
(h−1
Catalyst
Condition
Entry
1
Metal cat
Co-cat
Solvent
T (°C)
P (bar)
Time (h)
TONa
)
Ref
13
(salen)AlCl
1-MeIm
—
80
80
25
110
110
100
20
1
1
1
6
6
6
48
48
59
10
35.6
41
30
9.83
1.67
6
0.85
0.63
2
3
[(salen)Al]2O
Al1cat
Al1cat
—
Bu4NBr
—
—
CH2Cl2
CH2Cl2
18
This work
a
Moles of carbonate produced per mole of catalyst. b Moles of carbonate produced per mole of catalyst per hour.
due to the need for a co-catalyst and the requirement for
high temperature and/or high CO2 pressure (80 °C, 100 bar).
The TON value of 30 for Al1cat alone as the catalyst (entry 3)
was 3 times larger than that of (salen)AlCl along with methyl-
imidazole at 20 bar of CO2 pressure (entry 1) as shown in
Table 3. However, at the same low pressure, the dimetallic
aluminium salen complex ([(salen)Al]2O) displayed better cata-
lytic activity than the Al1cat complex. Higher temperatures
and longer reaction times (110 °C, 48 h) were required for the
catalytic reaction using Al1cat, although a TBAB co-catalyst was
also used. This could be due to an increase in the inherent
oxophilicity of the Al atom in the dimetallic salen complex.
Both aluminium ions of the bimetallic complex dominate in
activating the coordination of CO2 to the aluminium ion
and then forming styrene carbonate through intramolecular
substitution and cyclic elimination.18 While the bimetallic
complexes show higher catalytic activity than Al1cat, they
are symmetrical so any functional group is mirrored in the
molecule. By contrast, the non-symmetrical Al1cat complex
can be formed bearing a single, discrete functional group, in this
case a phenolic OH group. This allows further derivativisation
of the complex to form, for example, an ether or ester group
with a support material such as a Merrifield resin. This impor-
tant feature allows the initial homogeneous catalyst to be
supported on a solid phase, which in turn allows the catalyst
to be included in continuous flow reactors that are essential if
large volumes of CO2 are to be processed.
standard styrene carbonate is not available commercially, so
styrene oxide was used as a reference standard in analysing
the reaction kinetics. Tetradecane was used as an internal stan-
dard, and the area ratios of GC peaks at different concentration
ratios of styrene oxide/tetradecane were used in the calibration.
The reaction profile was determined by fitting the data to a
plot of the concentration ratios against peak area ratios. The
kinetic parameters were studied over a wide temperature
range from 80 to 150 °C. The data were fitted to the following
rate equations:
Rate = k[epoxide]a[CO2]b[Al1cat]c[TBAB]d
(1)
(2)
(3)
Rate = kobs[epoxide]a
d epoxide


Rate 
dt
where [epoxide], [CO2], [Al1cat] and [TBAB] are the styrene
oxide, carbon dioxide, Al1cat and TBAB concentrations,
respectively; a, b, c and d are the orders of reaction with
respect to styrene oxide, carbon dioxide, Al1cat and Bu4NBr,
respectively; t is time and kobs is the observed pseudo-first-
order rate constant for styrene oxide conversion. To simplify
the equations and solve for k, it can be assumed that the con-
centrations of Al1cat and TBAB are constant during the reac-
tion and carbon dioxide is in excess to give high dilution so
that a = 1. Thus the following pseudo-first-order kinetic rate
law is used to simplify eqn (2) to calculate the observed
pseudo-first-order rate constant (kobs) for styrene oxide con-
version.18 Combining and integrating eqn (2) and (3) as a
function of time gives the following conversion:
The fact that the Al1cat complex functions well at ambient
pressure is, we believe, not a consequence of any change in
the mechanism proposed in Fig. 2, which would be equally
valid for any salen-based aluminium(III) chloride complex. The
enhanced activity is related to the experimental setup which
intensifies the contact between the CO2 gas and the liquid
solution through active sparging of the gas continuously through
the solution.
− ln[epoxide] = kobs
t
(4)
Using the Arrhenius equation, the activation energy for
the reaction can be calculated from the relationship between
the observed rate constant and temperature:
The activation energy for styrene carbonate synthesis
Although the best results for the homogeneous cycloaddi-
tion reaction was achieved for the binary catalyst (Table 2,
entries 5 & 6), it is possible that a higher temperature pro-
motes the reactivity of the catalyst alone. To investigate the
kinetics of the reaction, gas chromatography was used to ana-
lyse styrene oxide consumption relative to the initial concen-
tration over the course of the reaction.18 It was noted that
kobs = A exp(−Ea/RT)
(5)
where A and Ea are the pre-exponential factor (min−1) and the
activation energy (kJ mol−1), respectively, R is the universal
gas constant (8.314 J mol−1 K−1), and T is the absolute tem-
perature (K).
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Fig. 5 Styrene oxide conversion as a function of reaction temperature
for the Al1cat catalyst alone (solid markers) and with added TBAB
co-catalyst (open markers).
Fig. 6 Pseudo-first order kinetic plot of styrene oxide concentration
against time for the Al1cat/TBAB binary catalyst system at 150 °C.
As can be seen in Fig. 5, the styrene oxide concentra-
tion decreased with reaction time, when CO2 was intro-
duced in the reaction mixture containing Al1cat alone or the
Al1cat/TBAB binary system at different temperatures. An induction
period can be observed in the reaction catalysed by Al1cat
alone or the Al1cat/TBAB binary system. For both catalyst
systems the induction period was reduced with increasing
temperature. The Al1cat/TBAB binary system showed a shorter
induction period than Al1cat alone at the same temperature.
A similar behaviour has been described previously in a work
reported by North and Pasquale18 who studied the cyclic
carbonate synthesis by using a combination of the dimetallic
aluminium salen complex [(salen)Al]2O and TBAB. They
reported that the induction period disappeared when a high
concentration of TBAB was used in the reaction mixture. The
shorter induction period and faster approach to steady state
in the binary catalyst system indicate that TBAB is acting as a
catalytic enhancer for styrene carbonate synthesis in this
study. This can be clearly observed in the activation energy
values of both catalytic systems reported in Table 4.
The observed rate constant was determined from the slope
of a linear plot of the natural logarithm of the changing
sample concentration with time at different temperatures
based on eqn (4). Fig. 6 shows the determination of the
observed rate constant for styrene oxide conversion by the
Al1cat and TBAB catalysts at 150 °C. This was repeated for
other reaction temperatures and catalyst compositions. The
results indicate the reaction to be a pseudo-first-order overall
under each of the different conditions.
The activation energies for the CO2 insertion reaction into
styrene oxide catalysed by Al1cat and the Al1cat/TBAB binary
catalyst system were determined over the range 80–150 °C
by fitting the data from a plot of the natural logarithm of
the observed pseudo-first order rate constant (ln kobs) against
the reciprocal absolute temperature (K/T). Fig. 7 shows the
Arrhenius plots for both catalytic systems and the data derived
from the studies are summarised in Table 4.
Table 4 The kinetic parameters of styrene oxide cycloaddition with
the different catalysts
Kinetic parametersb
The observed first-
order rate constant,a Ea
A
Catalyst
T (°C) kobs (×10−3) (min−1
)
(kJ mol−1
)
(min−1
)
R2
Al1cat/TBAB 80
0.53 0.06
0.87 0.15
1.20 0.10
2.20 0.20
0.23 0.10
0.40 0.10
0.73 0.15
1.73 0.25
23
34
1.3
0.91
110
140
150
Al1cat
80
110
140
150
18.0
0.90
a
The average and standard deviation of the observed first-order
kinetic rate constants of all of the samples obtained from the three
b
Fig. 7 Arrhenius plots for the cycloaddition of carbon dioxide to
styrene oxide using Al1cat alone (■) and the AL1cat/TBAB co-catalyst
system (♦).
experiments at each temperature. Kinetic parameters and the
R2 values of the Al1cat catalyst alone obtained from the linear
Arrhenius plot of ln kobs against 1/T.
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Fig. 8 Mechanism for the TBAB catalysed cycloaddition of CO2 to a terminal epoxide.
For the homogeneous catalyst Al1cat alone, the activa-
tion energy was calculated to be 34 kJ mol−1. When the
Al1cat/TBAB binary catalyst system was used this value
decreased to 23 kJ mol−1. The data show that TBAB acts by
reducing the activation energy value by 11 kJ mol−1 in the
catalysed reaction. From the observation of the induction
period and activation energy data, it can be concluded that
styrene oxide cyclic carbonation can be catalysed by Al1cat,
while the TBAB acts as a catalytic promoter or a co-catalyst in
this system. This is consistent with many reports, although
the symmetrical aluminium salen complexes were used in the
binary catalyst system studied by He et al.,12 Lu et al.14,15 and
North and Pasquale.18 The mechanism of cyclic carbonate
catalysis has been well established in the presence of a binary
catalyst using TBAB as a co-catalyst and this is shown in
Fig. 2. This mechanism correlates well with the behaviour of
the catalysed reaction observed in this study. The central
metal in Al1cat coordinates with an oxygen atom of epoxide,
activating it towards a nucleophilic attack by the bromide ion
from TBAB. This results in elimination of the chloride ion
from the aluminium atom. The cleavage of the epoxide ring
results in the formation of a linear bromo-alkoxide coordi-
nated with the aluminium centre. The oxygen coordinated to
the aluminium is activated and can attack the carbon dioxide
molecule, which is essentially a nucleophilic attack of the oxy-
gen lone pair of electrons at a carbonyl group. The carbonate
anion formed then undergoes an intramolecular cyclisation
reaction, eliminating the bromide ion back into the catalytic
cycle. The displaced chloride ion then reattaches at the
aluminium centre to eliminate the cyclic carbonate product.
For the same reaction of CO2 and styrene oxide catalysed
by calcined Mg–Al mixed oxides, the activation energy was
internal pores of the solid therefore dominates the reaction
kinetics.
One important feature of the catalytic studies was the
observation that a reaction was observed with TBAB alone at
110 °C, with a conversion comparable with Al1cat alone. This
is potentially important as it means that the aluminium cata-
lyst can be eliminated from processes making the scale-up
of the process significantly less expensive in terms of both
materials and process/separation costs, whilst also offering
the opportunity to form immobilized analogues as heteroge-
neous catalysts. Similar results have also been reported by
Caló et al.25 for a variety of alkenes, using TBAB as both the
catalyst and solvent. Cyclic carbonate formation directly from
alkenes has been reported previously; however, the process is
a multi-phase system using CO2 at elevated pressures.26 The
proposed mechanism is shown in Fig. 8. The mechanism is
remarkably similar to the Al1cat catalyst process and offers
an explanation as to why the metal catalyst combined with
TBAB has lower activation energy compared to Al1cat alone.
The aluminium catalyst activates the epoxide towards attack
of the bromide ion. However, in the metal-free system, the
nucleophilic bromide ion attack at the epoxide can still occur
without prior activation of the epoxide. Cycloaddition of carbon
dioxide with concurrent bromide ion elimination can then
occur to yield the cyclic carbonate, with the bromide ion
re-joining the catalytic cycle as TBAB.
Conclusions
The novel non-symmetrical aluminium(III) salenac complex
(Al1cat) exhibits catalytic activity toward the cycloaddition of
CO2 into styrene oxide at atmospheric pressure. It displayed
better catalytic activity than the symmetrical aluminium
salen complex reported by Alvaro et al.13 at low CO2 pressure.
However, a high temperature was required in order to achieve
high yields. The activation energy for Al1cat was found to be
34 kJ mol−1, which was reduced to 23 kJ mol−1 for the binary
catalyst system (Al1cat/TBAB) at reaction temperatures in the
range 80–150 °C. Importantly, catalysis using TBAB alone was
found to be as effective as using Al1cat alone at 110 °C. This
has significant implications in the industrialisation of the
process as it allows the aluminium complex to be eliminated
from the reaction, relying only on an organic salt to carry out
the catalysis which can be readily separated post-reaction.
Future studies will investigate the use of immobilised TBAB
reported to be 78 kJ mol−1 24
It has been reported that the
.
calcined Mg–Al mixed oxides exhibit good catalytic activity
due to the presence of both acid and strong basic sites
on their surfaces. However, the styrene carbonate formation
in this case required a larger activation energy than Al1cat
(34 kJ mol−1), especially the binary Al1cat/TBAB catalyst
(23 kJ mol−1). This may be due to the fact that the homoge-
neous catalyst complex, Al1cat alone or the binary catalyst
system, is uniformly distributed in the reaction medium and
offers good mass transfer. By contrast, only the surface atoms
of the calcined heterogeneous Mg–Al mixed oxides are active
in the reaction medium and so the catalytic activity of the
mixed oxides depends on surface coverage and reaction of the
solid. The diffusion of reactants and products through
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analogues, solvent-less systems and scaled up processes to further
improve the technology readiness of the reaction and process.
9 X.-B. Lu, B. Liang, Y.-J. Zhang, Y.-Z. Tian, Y.-M. Wang,
C.-X. Bai, H. Wang and R. Zhang, J. Am. Chem. Soc., 2004,
126, 3732–3733.
10 W.-M. Ren, G.-P. Wu, F. Lin, J.-Y. Jiang, C. Liu, Y. Luo and
X.-B. Lu, Chem. Sci., 2012, 3, 2094–2102.
11 W. M. Shen, W.-L. Duan and M. Shi, J. Org. Chem., 2003,
68, 1559–1562.
12 R. He, X.-B. Lu and X.-J. Feng, Appl. Catal., A, 2002, 234, 25–33.
13 M. Alvaro, C. Baleizao, E. Carbonell, M. El Ghoul, H. GarcÍa
and B. Gigante, Tetrahedron, 2005, 61, 12131–12139.
14 X.-B. Lu, Y.-J. Zhang, K. Jin, L.-M. Luo and H. Wang,
J. Catal., 2004, 227(2), 537–541.
Acknowledgements
We thank the Royal Thai Government and the National Metal
and Materials Technology Center (MTEC) for funding the PhD
studentship to SS, and the Engineering and Physical Sciences
Research Council (EPSRC) for its funding of the C-Cycle Research
Programme (EP/E010318/1) and the CO2Chem Network
(EP/H035702/1 and EP/K007947/1).
15 X.-B. Lu, Y.-J. Zhang, B. Liang, X. Li and H. Wang, J. Mol.
Catal. A: Chem., 2004, 210, 31–34.
16 M. North, J. Melendez and R. Pasquale, Eur. J. Inorg. Chem.,
2007, 3323–3327.
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