Metal free synthesis of ethylene and propylene carbonate from alkylene halohydrin and CO2 at room temperature

Article abstract of DOI:10.1039/c9ra06765e

Herein we describe a metal free and one-pot pathway for the synthesis of industrially important cyclic carbonates such as ethylene carbonate (EC) and propylene carbonates (PC) from molecular CO₂ under mild reaction conditions. In the actual synthesis, the alkylene halohydrins such as alkylene chloro- or bromo or iodohydrin and organic superbase, 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) reacted equivalently with CO₂ at room temperature. The syntheses of cyclic carbonates were performed in DMSO as a solvent. Both 1,2 and 1,3 halohydrin precursors were converted into cyclic carbonates except 2-bromo- and iodoethanol, which were reacted equivalently with DBU through n-alkylation and formed corresponding n-alkylated DBU salts instead of forming cyclic carbonates. NMR analysis was used to identify the reaction components in the reaction mixture whereas this technique was also helpful in terms of understanding the reaction mechanism of cyclic carbonate formation. The mechanistic study based on the NMR analysis studies confirmed that prior to the formation of cyclic carbonate, a switchable ionic liquid (SIL) formed in situ from alkylene chlorohydrin, DBU and CO₂. As a representative study, the synthesis of cyclic carbonates from 1,2 chlorohydrins was demonstrated where the synthesis was carried out using chlorohydrin as a solvent as well as a reagent. In this case, alkylene chlorohydrin as a solvent not only replaced DMSO in the synthesis but also facilitated an efficient separation of the reaction components from the reaction mixture. The EC or PC, [DBUH][Cl] as well as an excess of the alkylene chlorhydrin were separated from each other following solvent extraction and distillation approaches. In this process, with the applied reaction conditions, >90% yields of EC and PC were achieved. Meanwhile, DBU was recovered from in situ formed [DBUH][Cl] by using NaCl saturated alkaline solution. Most importantly here, we developed a metal free, industrially feasible CO₂ capture and utilization approach to obtain EC and PC under mild reaction conditions.

Full text of DOI:10.1039/c9ra06765e

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PAPER
Metal free synthesis of ethylene and propylene
carbonate from alkylene halohydrin and CO2 at
room temperature†
Cite this: RSC Adv., 2019, 9, 34023
a
ab
*
*
and Jyri-Pekka Mikkola
Santosh Govind Khokarale
Herein we describe a metal free and one-pot pathway for the synthesis of industrially important cyclic
carbonates such as ethylene carbonate (EC) and propylene carbonates (PC) from molecular CO2 under
mild reaction conditions. In the actual synthesis, the alkylene halohydrins such as alkylene chloro- or
bromo or iodohydrin and organic superbase, 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) reacted
equivalently with CO2 at room temperature. The syntheses of cyclic carbonates were performed in
DMSO as a solvent. Both 1,2 and 1,3 halohydrin precursors were converted into cyclic carbonates except
2-bromo- and iodoethanol, which were reacted equivalently with DBU through n-alkylation and formed
corresponding n-alkylated DBU salts instead of forming cyclic carbonates. NMR analysis was used to
identify the reaction components in the reaction mixture whereas this technique was also helpful in
terms of understanding the reaction mechanism of cyclic carbonate formation. The mechanistic study
based on the NMR analysis studies confirmed that prior to the formation of cyclic carbonate,
a
switchable ionic liquid (SIL) formed in situ from alkylene chlorohydrin, DBU and CO2. As
a representative study, the synthesis of cyclic carbonates from 1,2 chlorohydrins was demonstrated
where the synthesis was carried out using chlorohydrin as a solvent as well as a reagent. In this case,
alkylene chlorohydrin as a solvent not only replaced DMSO in the synthesis but also facilitated an
efficient separation of the reaction components from the reaction mixture. The EC or PC, [DBUH][Cl] as
well as an excess of the alkylene chlorhydrin were separated from each other following solvent
extraction and distillation approaches. In this process, with the applied reaction conditions, >90% yields
of EC and PC were achieved. Meanwhile, DBU was recovered from in situ formed [DBUH][Cl] by using
NaCl saturated alkaline solution. Most importantly here, we developed a metal free, industrially feasible
CO2 capture and utilization approach to obtain EC and PC under mild reaction conditions.
Received 27th August 2019
Accepted 15th October 2019
DOI: 10.1039/c9ra06765e
rsc.li/rsc-advances
carbamates etc. Further, they nd use as electrolytes for
batteries, precursors for polymeric materials and as fuel addi-
tives.4 The traditional industrial production of cyclic carbonates
1. Introduction
Cyclic carbonates, especially ethylene and propylene carbonate
are industrially important chemicals considering their wide-
spread application.1 Being non-toxic and bio-degradable and
having high boiling points and aprotic polar nature, these cyclic
carbonates are preferentially replacing other harmful organic
solvents such as dimethylformamide (DMF), hexamethylphos-
phoramide (HMPA), N-methyl-2-pyrrolidone (NMP), dimethyla-
cetamide and acetonitrile (ACN).1–3 Cyclic carbonates have also
been successfully implemented as intermediates upon
synthesis of ne chemicals such as dialkyl carbonates, glycols,
follows the phosgene route. However, due to the toxic and
corrosive nature of phosgene as well as production of large
amounts of chlorinated salts and solvents, this process is not
found to be sustainable in terms of environmental impact and
work safety. As an alternative approach, the synthesis of cyclic
carbonates from epoxides and carbon dioxide (CO2) was
invented as an elegant, non-toxic and atom-efficient carbon
capture and utilization (CCU) pathway (Fig. 1).5 This pathway
a
˚
Technical Chemistry, Department of Chemistry, Chemical-Biological Centre, Umea
˚
University, SE-90187 Umea, Sweden. E-mail: santosh.khokarale@umu.se
bIndustrial Chemistry & Reaction Engineering, Department of Chemical Engineering,
˚
˚
Johan Gadolin Process Chemistry Centre, Abo Akademi University, FI-20500 Abo-
Turku, Finland
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra06765e
Fig. 1 Synthesis of cyclic carbonates from epoxides and CO2.
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introduced an opportunity to use CO2 as an inexpensive,
nontoxic, and renewable carbon C1 building block for the
synthesis of valuable chemicals. However, considering its more
oxidized state and hence chemically inert nature, the CO2
molecule usually gives rise to low reactivity. Hence, for the
chemical activation of CO2 molecule and its subsequent inter-
action with epoxides, several homogeneous and heterogeneous
catalytic systems including metal based catalysts, organo-
catalysts, ionic liquids etc. were developed to synthesize cyclic
carbonates.4–8
Apart from the catalytic systems, various types of epoxide
substrates were also successfully used. These include both
petroleum (for e.g. ethylene oxide, propylene oxide and cyclo-
hexene oxide) as well as bio-based epoxides (for e.g. epichlor-
ohydrine, limonene oxide and vinylcyclohexene oxide).6,9–11
Even though the synthesis of cyclic carbonates from various
epoxides is industrially applied process but the synthesis of
epoxides accompanied with the use of toxic or costly reagents as
well as involves a tedious workup for separation.
Alternative to epoxide as a reagent, the synthesis of cyclic
carbonates was also carried out using alkylene halohydrins as
a one of the reagent with CO2. The various alkali metal
carbonates such as K2CO3, Na2CO3, Cs2CO3 etc. were used as an
catalysts for the synthesis of the cyclic carbonates from various
alkylene halohydrins. In this case the cyclic carbonates were
obtained with high yield (>90%) under mild reaction conditions
such as room temperature and atmospheric pressure.12–15 The
metal free reaction approach for the synthesis of cyclic
carbonates from alkylene halohydrins was also used where
amine such as triethylamine was used as a both activating agent
and solvent.16 Even though these reaction approaches emerged
as an alternative to the epoxide based processes but it is also
accompanied with various challenges. In case of metal
carbonate based processes involves the use of organic solvents
(e.g. dimethylformamide, DMF) and additives (e.g. amines) as
well as the generation of equivalent amount of waste such as
metal bicarbonates and metal halides. In addition, it was also
observed that the desired cyclic carbonates were formed with
slow reaction kinetics under applied reaction conditions.
Further with triethylamine-involved processes, even though
90% yield of cyclic carbonates was obtained and recovery of
amine was achieved, but the process was carried out at high
temperature and pressure. Hence, the synthesis of cyclic
carbonates from alkylene halohydrins need few improvements
where combined reaction approach involving mild reaction
conditions along with no metal waste generation can be more
preferred.
Scheme 1 Synthesis of ethylene or propylene carbonate from alky-
lene halohydrins.
and CO2 where synthesis was carried out at room temperature
and atmospheric pressure. In addition, as shown in the reaction
Scheme 1, the organic superbase DBU was recovered in the form
of its halide salt. Further, the cyclic carbonates and salt of DBU
were separated in the pure form with mild solvent extraction
and vacuum distillation approach. The DBU was also recovered
from its halide salt following alkaline solvent's treatment.
The synthesis of EC and PC was initially carried out in DMSO
(as a solvent) while in order avoid to use of DMSO, the same
process was further developed to yield a DMSO free solvent
system where alkylene halohydrins themselves were used as
solvents. The progress of the reaction as well as recovery of EC
or PC and DBU were monitored by the means of NMR spec-
troscopic techniques. The numbering and labelling for the
chemical species used and formed in this cyclic carbonate
synthesis are shown in Fig. 2.
2. Experimental
2.1 Chemicals and methods
2.1.1 Chemicals.
1,8-Diazabicyclo-[5.4.0]-undec-7-ene
(DBU, $99.0%, GC analysis), 2-chloroethanol ($99.0%), 2-bro-
moethanol (95%), 2-iodoethanol (98%), propylene chlorohydrin
(70 wt% of 1-chloro-2-propanol and 30% of 2-chloro-1-
propanol), 3-chloro-1-propanol (98%), 3-bromo-1-propanol
(98%) and D2O (99.9 atom % D), d6-DMSO (anhydrous, 99.9
atom% D) were purchased from Sigma Aldrich while ethyl
acetate ($99.0%), ethanol ($99.0%), and dimethyl sulfoxide
($99.0%) were purchased from VWR chemicals and used
further without purication. CO2 and 13C enriched CO2 gas
bottles (>99.99%) were obtained from AGA AB (Linde Group)
and used without further purication.
Herein, we are introducing one-pot and metal free synthesis
of ethylene carbonate (EC) and propylene carbonate (PC) from
molecular CO2 at room temperature. In this route, the equiva-
lent mixture of various alkylene halohydrin such as 2-chlor-
oethanol or 2-bromoethanol or 2-iodoethanol or 1-chloro-2-
propanol or 3-chloro-1-propanol or 3-bromo-1-propanol and
organic superbase DBU react with molecular CO2 to form EC or
PC (reaction Scheme 1).
The process described in this report for the synthesis of EC
Fig. 2 Chemical species used and formed in the reaction mixture in
and PC uses equivalent amounts of DBU, alkylene halohydrin cyclic carbonate synthesis.
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2.1.2 NMR analysis. The composition of the reaction
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2.2.3 Recovery of DBU. The DBU was recovered from
[DBUH][Cl] by using alkaline NaCl saturated aqueous solution
following a previously reported procedure.17 Initially, saturated
NaCl aqueous solution was prepared by mixing 36 g of NaCl in
100 ml of water under stirring. The required amount of NaCl
solution was taken to make 4 wt% solution of NaOH in a NaCl
saturated solution. The [DBUH][Cl] was added in the alkaline
NaCl saturated solution and the mixture was stirred for 30 min
at room temperature. The DBU was extracted with ethyl acetate
and separated from the aqueous phase by using a separating
funnel. The DBU was obtained from organic phase aer
removing ethyl acetate by rotation evaporator and its purity was
conrmed by NMR analysis. The % recovery of the DBU was
calculated by the eqn (2).
mixtures as well as purity of the recovered ethylene carbonate,
propylene carbonate and DBU were conrmed by the means of
NMR analysis using the Brucker's Avance 400 MHz instrument.
In these analyses, as per requirement, either glass capillary l-
led with D2O or DMSO-d6 were used as internal reference. The
obtained data was further processed with TopSpin 3.2 soware.
2.2. Synthesis and separation of MP and MMA and recovery
of DBU
2.2.1 Synthesis of ethylene or propylene carbonate with
and without dimethyl sulfoxide (DMSO) as a solvent. The
ethylene carbonate (EC) or propylene carbonates (PC) were
prepared from various halohydrins, DBU and CO2 with or
without DMSO solvent. In this synthesis, CO2 gas was bubbled
(50 ml minꢀ1) for 20 min under stirring in the DMSO solution
containing equimolar amounts of DBU and halohydrin at room
temperature. In case of DMSO free experiments, the CO2 gas
was bubbled (50 ml minꢀ1) for 20 min in chlorohydrin solution
of DBU (50 vol% of DBU in chlorohydrin, 2 ml of total reaction
mixture). In both cases, pale yellow clear solutions were ob-
tained and their compositions were further analyzed by NMR
recovered amount of DBU ðgÞ
% Recovery of DBU ¼
expected amount of DBU ðgÞ
ꢂ100
(2)
3. Result and discussion
analysis. The cyclic carbonates were also synthesized using 13
C
isotope enriched CO2 (tracer technique) following the experi-
mental procedure that previously used for the similar synthesis
with normal or non-13C enriched CO2 in DMSO. The NMR
analysis was carried out with a glass capillary lled with D2O for
the reactions performed in DMSO solvent or with DMSO-d6
where the synthesis was carried out with solution of DBU in
alkylene halohydrins.
The synthesis of cyclic carbonates such as EC or PC was
preceded by the introduction of organic superbase DBU
through the interaction between molecular CO2 and alkylene
chlorohydrins such ethylene or propylene chlorohydrins,
respectively. Initially, the synthesis of cyclic carbonates was
performed in a DMSO solvent where equivalent mixture of DBU
and alkylene chlorohydrin were dissolved.
2.2.2 Separation of EC or PC from the reaction mixture.
The separation of EC or PC and recovery of DBU was performed
only for the reactions preceded with solution of DBU in alkylene
chlorohydrin. Aer complete conversion of DBU to its salts i.e.
[DBUH][X] (X ¼ Cl, conrmed by NMR analysis) (hence forma-
tion of EC or PC), the reaction mixture added slowly in ethyl
acetate where white coloured solid of [DBUH][Cl] got precipitate
out. The [DBUH][Cl] was separated by ltration and washed 2 to
3 times with fresh ethyl acetate. The [DBUH][Cl] was further
exposed to high vacuum for complete removal of the traces of
ethyl acetate. Further, from the collected ltrate, the ethyl
acetate was separated from the EC or PC and excess of alkylene
chlorohydrin by rotation evaporator. The excess of alkylene
chlorohydrin was separated from EC or PC by high vacuum
The primary aim of the use of DBU was to activate the CO2
molecule prior to formation of cyclic carbonates. This was
facilitated via the formation of a switchable ionic liquid (SIL)
through equivalent interaction between DBU, alkylene chloro-
hydrin and CO2. The synthesis of organic superbase comprised
SIL is a promising approach for the chemical activation of CO2
and its storage under mild reaction conditions.18 In general,
being a superbase, DBU is able to abstract the proton of –OH
group of an alcohol (low and high molecular weight alcohols as
well as biomolecules such as cellulose) and the thus formed
ꢁ
distillation at 40 C. In order to avoid loss of alkylene chloro-
hydrin during the distillation, the glass collector was used
before vacuum pump where the glass collector was kept in
acetone–dry ice mixture. The purity of the obtained products i.e.
EC or PC as well as remaining [DBUH][Cl] were conrmed by
NMR analysis (with DMSO-d6 as an internal reference). Further
the % recovery of the EC or PC was calculated by the eqn (1).
recovered amount of EC or PC ðgÞ
% Yield of EC or PC ¼
expected amount of EC or PC ðgÞ
Scheme 2 Plausible mechanism for the synthesis of ethylene or
propylene carbonate from chlorohydrin and CO2. Formation of (a)
switchable ionic liquid (SIL), [DBUH][ClCH(R)CH2CO3] and, (b) ethylene
or propylene carbonate.
ꢂ 100
(1)
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alkoxide anion subsequently interacts with CO2 and form SIL
i.e. [DBUH][alkyl carbonate] (reaction Scheme 2a).19
In this report, as shown in the reaction Scheme 2, the
synthesis of cyclic carbonates from DBU, alkylene chlorohydrin
and CO2 was initially expected to occur in two steps. The process
involves the formation of SIL i.e. [DBUH][ClCH(R)CH2CO3] (R ¼
–H or –CH3) at room temperature, followed by the synthesis of
cyclic carbonates through release of [DBUH][Cl] under thermal
treatment with or without use of catalysts. However, based on
the NMR analysis of the reaction mixture, it was observed that
during bubbling of CO2 in the reaction mixture, the synthesis of
cyclic carbonate took place in line with a one-pot process. As
1
shown in Fig. 3a and b, the H NMR spectra reveals that aer
Fig. 4 13C NMR spectra for (a) equivalent mixture of DBU and ethylene
chlorohydrin, (b) reaction mixture after bubbling of CO2 in DBU and
ethylene chlorohydrin, and (c) commercially available ethylene
carbonate in a DMSO solvent. (NMR analysis with D2O capillary).
interaction of DBU and ethylene chlorohydrin with CO2 at room
temperature, the two doublets signals of four proton atoms
(3.88–4.10 ppm) as well as singlet for the proton in hydroxyl
group (6.59 ppm, broad) in ethylene chlorohydrin disappeared.
On the other hand, two new signals with the chemical shis
4.85 (intense) and 11.11 ppm (broad) were observed. Further,
aer comparing the 1H NMR spectra of the reaction mixture
with the 1H NMR spectra of commercially available EC, the
similar intense signal at the 4.85 ppm was also observed for the
commercially available EC. The singlet for the 4 proton atoms in
the EC appeared as all these protons have identical chemical
environment. The signal observed at 11.11 ppm is related to the
proton attached to the quaternized and sp2-hybridised nitrogen
atom in protonated DBU i.e. [DBUH]+ cation.20 The occurrence
of the EC and [DBUH]+ cation were also further conrmed from
the 13C NMR analysis.
NMR spectra of the reaction mixture and commercially avail-
able EC, it was observed that the signals appearing at 65.94 and
156.40 ppm, respectively, belong to the aliphatic (C-a0 and C-b0)
and carbonyl carbon (denoted by star) of the EC molecule,
respectively (Fig. 4b and c). Further, aer the interaction of DBU
and ethylene chlorohydrin with CO2, the carbon atoms in the
DBU molecule, at position 6 and 9, got shielded while the
carbon atom at position 7 was deshielded which conrmed the
1
formation of [DBUH]+ cation.20 Hence, based on both H and
13C NMR spectra, the EC was formed aer equivalent interac-
tion between DBU, ethylene chlorohydrin and CO2 molecule in
a one-pot process.
The 13C NMR spectra for the reaction mixture obtained aer
reaction of DBU, ethylene chlorohydrin and CO2 in DMSO is
shown in Fig. 4. The signals for the non-identical carbon atoms
denoted by C-a and C-b in ethylene chlorohydrin were observed
at 47.36 and 62.31 ppm, respectively (Fig. 4a and b). Aer
bubbling of CO2 in the DMSO solution of the equivalent mixture
of DBU and ethylene chlorohydrin, similar to 1H NMR spectra,
the signals for C-a and C-b disappeared and new signals at 65.94
and 156.40 ppm were observed. Aer comparison between the
Based on the NMR analysis, even though EC formed in the
reaction mixture, it was not conrmed that the synthesis was
preceded either through two sub-steps mentioned in the
Scheme 2 or other alternative pathways. Therefore, in order to
conrm the mechanism of the formation of EC, CO2 was
bubbled though equivalent mixture of DBU and ethanol (non-
chlorinated analogue of ethylene chlorohydrin). As shown in
Fig. S1,† although SIL i.e. [DBUH][CH3CH2CO3] formed aer
interaction of equivalent mixture of DBU and ethanol with CO2,
unlike in the EC synthesis, the isolated signals for the carbon
atoms C-a0 and C-b0 in the [CH3CH2CO3]ꢀ anion were observed
in the 13C NMR spectra. Hence, due to absence of leaving group
i.e. Cl atom at the carbon atom C-a in ethanol, the subsequent
intermolecular ring closing for EC synthesis aer CO2 capture
was not observed. However, as [DBUH][CH3CH2CO3] formed in
this control experiment and also [DBUH][Cl] formed in the EC
synthesis process, we can conclude that the formation of EC
proceeds through the two sub-step comprised concerted
mechanism shown in reaction Scheme 2. Hence, as shown in
the reaction Scheme 2, initially it seems that the SIL forms in
the reaction mixture. Further due to presence of Cl atom as
a good leaving group, the negatively charged oxygen atom in the
carbonate anion induced a nucleophilic attack at the carbon
atom to which Cl atom is attached and formation of EC took
place through the release of [DBUH][Cl]. Identical mechanism
Fig. 3 1H NMR spectra for (a) equivalent mixture of DBU and ethylene
chlorohydrin, (b) reaction mixture after bubbling of CO2 in DBU and
ethylene chlorohydrin, and (c) commercially available ethylene
carbonate in DMSO solvent. (NMR analysis with D2O capillary).
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was also proposed previously in case of metal carbonate and the HSQC NMR analysis represents that the signal for the
amine catalysed processes.12–15
correlation between four equivalent protons and carbon atoms
Further, in order to conrm the origin of carbon atom in the (C-a0 and C-b0) are observed with the chemical shi at 4.85/
EC, the 13C enriched CO2 was used as a CO2 source (tracer 65.94 ppm. Further, in case of HMBC NMR analysis, the protons
technique). The 13C enriched CO2 was bubbled in the equivalent in the EC gave rise to long range coupling with the 13C enriched
mixture of DBU and ethylene chlorohydrin in DMSO and the carbonyl carbon atom in the EC and corresponding correlation
one as well as two dimensional NMR analysis of the reaction signal observed at chemical shi 4.85/156.44 ppm (Fig. S2-c†).
mixture was carried out. As shown in Fig. 5c, aer the use of 13
C
Hence, from all these observations based on the both one
enriched CO2, a similar peak pattern was observed for the and two dimensional NMR analysis, it is conrmed that the
reaction mixture aer comparing it with the NMR of the reac- synthesis of EC proceeds through simultaneous SIL formation
tion mixture when ‘normal’ (non-13C enriched) CO2 was used. followed by intermolecular ring closing reaction shown in the
However, in case of 13C enriched CO2, the signal observed for reaction Scheme 2 where chemisorbed CO2 became a part of the
the carbonyl carbon in the EC at the chemical shi 156.40 ppm structure of thus synthesised EC.
was having a high intensity compared to the signal for the C-70
Aer synthesis of EC from CO2 and ethylene chlorohydrin,
in the [DBUH]+ cation (Fig. 5c). On the contrary, in case of the the similar reaction approach was further mimicked upon
reaction mixture obtained aer use of normal CO2, the intensity synthesis of PC. In this case, propylene chlorohydrin was used
of the signal at 156.40 ppm was found comparable with the as an alkylene chlorohydrin precursor. However, unlike
intensity of the signal for C-70 in [DBUH]+ cation. In addition, it ethylene chlorohydrin, propylene chlorohydrin purchased from
was also observed that the protons in the EC showed the Sigma Aldrich comprised of its two different isomers i.e. 1-
1
doublet signal at the chemical shi 4.85 ppm in the H NMR chloro-2-propanol (70 wt%) of and 2-chloro-1-propanol
spectra when 13C enriched CO2 was used in the synthesis (ESI (30 wt%). During actual experiment, it was assumed that the
S2-a†). In this case, the carbonyl carbon atom of EC contained one type of isomer of propylene chlorohydrin was present in the
a higher percentage of 13C isotope than 12C and as the protons purchased sample container and based on the equivalent of
of EC are in adjutants to this 13C enriched carbonyl carbon DBU, accordingly the amount of the propylene chlorohydrin
atom, these protons showed doublet signal in the NMR was taken for the experiment. As shown in Fig. 6a, the 13C NMR
spectra.21 On the other hand, the signal for these protons was spectra also showed that mixed signals for the both types of
observed as a singlet at the same chemical shi in the 1H NMR isomers of propylene chlorohydrin were observed with the
of the reaction mixture when normal CO2 was used (Fig. 2b). So, signals of DBU. Further CO2 was bubbled in the equivalent
these observations proved that the EC was formed from the 13
enriched CO2 and ethylene chlorohydrin through the equivalent formation of PC was conrmed by NMR analysis. The observed
interaction with DBU.
1H and 13C NMR are shown in the ESI S3† and Fig. 6, respec-
C
mixture of DBU and propylene chlorohydrin in DMSO and the
The two dimensional correlation NMR analysis techniques tively. As depicted in Fig. 6a and b, the signals for the both the
such as 1H–1C HSQC (heteronuclear single quantum coherence) types of isomers of propylene chlorohydrin disappeared. Aer
and HMBC (heteronuclear multiple bond correlation) of the comparing the NMR spectra of both the reaction mixture and
reaction mixture was performed when 13C enriched CO2 was the commercially available PC sample, it was observed that the
used. This analysis was carried out to get more proof about the PC formed in the reaction mixture (Fig. 6b and c). Also, similar
formation of EC in the reaction mixture. As shown in Fig. S2-b,† to the EC synthesis, based on the NMR analysis, [DBUH][Cl] also
Fig. 5 13C NMR spectra for (a) equivalent mixture of DBU and ethylene Fig. 6 13C NMR spectra for (a) equivalent mixture of DBU and
chlorohydrin, reaction mixture after bubbling of (b) CO2, (c) 13C propylene chlorohydrin, (b) reaction mixture after bubbling of CO2 in
enriched CO2 in DBU and ethylene chlorohydrin, and (d) commercially DBU and propylene chlorohydrin, and (c) commercially available
available ethylene carbonate in DMSO solvent. (NMR analysis with D2O propylene carbonate in DMSO solvent. (NMR analysis with D2O in
in capillary).
capillary).
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formed in the reaction mixture (Scheme 2 and Fig. 6b). Hence, and isomers of propylene chlorohydrin through their equivalent
independent on the type of isomer, it was observed that the PC reaction with DBU.
can be synthesised from mixtures of isomers of propylene
chlorohydrin, DBU and CO2 following the reaction Scheme 2.
Moreover, two-dimensional NMR analysis such as 1H–13C
HSQC and HMBC NMR analysis was also used to highlight
Similar to 13C NMR, the 1H NMR spectra of the reaction better the existence of PC in the reaction mixture. In this case,
mixture also gave rise to similar observation where prior to CO2 the two dimensional NMR analysis of the reaction mixture was
exposure, the reaction mixture represented characteristic signal carried out in which 13C enriched CO2 was used in the PC
for DBU as well as for both the isomers of propylene chlorohy- synthesis. As depicted in Fig. S4-b,† the HSQC NMR analysis
drin (Fig. S3-a†). Aer the formation of the PC, the signals reveals the correlation signals between the proton and carbon
belonging to the both isomers of the propylene chlorohydrin atom, at the positions ‘a’ and ‘b’ and gave rise to the chemical
disappeared and new deshielded signals for the protons in the shis 4.42 or 4.95/71.33 (for two different protons at C-a0) and
PC were observed (Fig. S3-b†). Further, it was also observed that 5.26/74.57 ppm, respectively. The HMBC NMR analysis also
the two protons at the carbon atom C-a in the propylene chlo- conrmed the signals for the long range correlation between
rohydrin became non-identical aer formation PC and this was the protons atom at the position ‘a’ and ‘b’ and 13C enriched
further conrmed with commercially available PC.
carbonyl carbon in PC (Fig. S4-c†).
Hence, based on the NMR analysis, it was preliminary
Hence, similar to EC synthesis, the one and two dimensional
concluded that PC formed form the equivalent interaction NMR analysis conrmed that the PC was formed from the
between DBU, propylene chlorohydrin and CO2. However, in equivalent interaction between DBU, propylene chlorohydrin
order to get more conrmation about the origin of the CO2, the and CO2 where captured CO2 remained as a part of PC (reaction
13C enriched CO2 was used in the PC synthesis process and the Scheme 2).
reaction mixture analysed by one and two dimensional NMR
In order to understand the substrate scope for the DBU
analyses. As shown in Fig. 7c, aer the use of 13C enriched CO2, based synthesis of cyclic carbonates, various halohydrins such
the intensity of the carbonyl carbon of the PC was found higher as 2-bromoethanol, 2-iodoethanol, 3-chloro-1-propanol and 3-
than the intensity of the carbon atom C-70 in the [DBUH]+ bromo-1-propanol were also used. The formation of the corre-
cation. Meanwhile upon use of normal CO2, the intensities of sponding cyclic carbonates was conrmed by NMR analysis. In
the carbonyl carbon atoms in PC and C-70 of [DBUH]+ cation case of both 2-bromo- and 2-iodoethanol, it was observed that
were found nearly identical to each other (Fig. 6b). Further, aer their addition to DMSO solution of DBU, the sp2-N atom of
similar to EC synthesis, upon use of 13C enriched CO2 and the DBU molecule became quaternized (ESI S5 and S6†). This
normal CO2, the protons at the positions ‘a’ and ‘b’ in the was conrmed by the change in the chemical shis of carbon
1
synthesised PCs showed different splitting patterns in their H atoms of DBU in 13C NMR at positions 6, 7 and 9, respectively.
NMR spectra. In case of normal CO2, both protons at the posi- Further, aer bubbling CO2 in the reaction mixture, the peak
tion a0 gave rise to triplets while multiplate signal was observed positions did not change and the peak belonging to the
for the proton at position b0 (Fig. S4-a†). On the other hand, carbonyl carbon atom for the carbonate species was also not
upon use of 13C enriched CO2, the triplet of doublets and observed. Hence, these observations conrmed that the sp2-N
broadened singlet were observed for the protons at the posi- atom in the DBU molecule become alkylated (n-alkylation)
tions a0 and b0, respectively (Fig. S4-a†). Hence, the NMR anal- through nucleophilic attack on the carbon atom C-a in 2-bromo-
ysis conrmed that the PC was formed from 13C enriched CO2 or 2-iodoethanol. Therefore, the sp2-N in DBU molecule did not
remain available to abstract the proton from the hydroxyl group
in these halohydrins during the CO2 capture process. As Br and
I are considered as better leaving groups than Cl, 2-chlor-
oethanol was converted to a cyclic carbonate following the
reaction Scheme 2 whereas its bromine and iodine analogues
were not able to form cyclic carbonates and converted to n-
alkylated DBU salts. Hence, it is proven that unlike 2-chlor-
oethanol, 2-bromo 2-iodoethanol cannot be considered as
a substrate for the synthesis of cyclic carbonates following the
DBU involving synthesis approach.
In case of 1,3 halohydrin, as shown in 13C NMR (Fig. 8) and
1H (ESI S7†), it is possible to synthesise PC from 3-chloro-1-
propanol. However, unlike 2-chloroethanol and 1-chloro-2-
propanol, the rate of formation of cyclic carbonate i.e. PC was
slow. From the 13C NMR analysis, it was observed that the SIL
was formed immediately aer the interaction of 3-chloro-1-
propanol and DBU with CO2. In this case, aer the formation
of SIL, the peak for the carbonyl carbon atom in the carbonate
species was observed with the chemical shi 156.9 ppm. In
addition, the peak for the carbon atom C-7 in DBU with the
Fig. 7 13C NMR spectra for (a) equivalent mixture of DBU and
propylene chlorohydrin, reaction mixture after bubbling of (b) CO2, (c)
13C enriched CO2 in DBU and propylene chlorohydrin, and (d)
commercially available propylene carbonate in DMSO solvent. (NMR
analysis with D2O in capillary).
34028 | RSC Adv., 2019, 9, 34023–34031
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convert both 1,2 and 1,3 halohydrins to cyclic carbonates. On
the other hand, K2CO3 promoted only reactions with 1,2 hal-
ohydrins to corresponding cyclic carbonates. In this study, DBU
based reaction approach except in the case of 2-bromo- and 2-
iodo ethanol, was able to convert various types of halohydrins to
cyclic carbonates and with the faster reaction kinetics compared
to metal based processes.
Although the synthesis of both EC and PC was successfully
demonstrated following mild reaction approach, the separation
of both these cyclic carbonates from DMSO solvent is an energy
intensive process. In other word, cyclic carbonates as well as
DMSO are considered as high boiling liquids and, therefore,
their separation from each other is not feasible through distil-
lation. Also, the separation using solvent extraction can also be
a difficult as EC or PC and DMSO are soluble in most of the
organic solvents as well as in water. Further, DMSO is not
a green solvent, especially in bulk scale synthesis, when its toxic
nature as well as expensiveness are considered. Hence, rather
than searching alternative techniques for the separation of
DMSO, we decided to replace the DMSO with an excess of
alkylene chlorohydrin in the reaction mixture. We have also
previously used a similar reaction approach upon acrylic plastic
precursor synthesis where excess of methanol was used as the
solvent to replace DMSO.22 Herein we also proposed that the
methanol not only performed as a solvent media but also served
as a reagent in the process. Considering this, the synthesis of EC
or PC was carried out in the alkylene chlorohydrin solvent where
the 50 vol% solution of DBU in the ethylene or propylene
chlorohydrin was used, respectively.
Fig. 8 13C NMR spectra for (a) equivalent mixture of DBU and 3-
chloro-1-propanol in DMSO, reaction mixture of DBU and 3-chloro-1-
propanol in DMSO after bubbling of CO2 for (b) 20 min, (c) 45 min and,
(d) 1 h. (NMR analysis with D2O in capillary).
chemical shi 161.4 ppm disappeared while new peak
belonging to the carbon atom at position 7 (C-70) in protonated
DBU was observed at 165.8 ppm. Simultaneously, it was also
observed that in 20 min PC was formed in the reaction mixture
where the peak for the corresponding carbonyl carbon atom was
observed at 148.9 ppm (shown by lled star). Further, with an
increase in time, the amount of propylene carbonate increased
steadily in the reaction mixture as the peak height at chemical
shi 148.9 ppm increased. However, even aer 1 h of reaction,
complete conversion of SIL to cyclic carbonate was not
observed. Hence, these observations conrmed that the
immediate formation of SIL between DBU, 1-chloro-3-propanol
and CO2 occurred immediately, but the subsequent cyclisation
in the anion of SIL to form propylene carbonate was found as
a slow step in this synthesis compared 1,2 chlorohydrins. The
reason could be a different position of leaving group i.e. Cl atom
in the 1-chloro-3-propanol molecule compared other 1, 2 chlo-
rohydrins. Compared to 1,2 chlorohydrins, having one more
methylene group at the position ‘b’ in 1-chloro-3-propanol
increases the electron density at the carbon atom at position
‘a’ to which Cl atom is attached. This renders the Cl atom as
a less good leaving group which reduces further the rate of
formation of propylene carbonate compared to the 1,2 chloro-
hydrins. The use of 3-bromo-1-propanol, on other hand, gave
rise to a faster reaction rate than 3-chloro-1-propanol where in
20 minutes it was converted to PC (ESI S8†). In this case, Br is
considered as a better leaving group than Cl and, therefore,
beyond the structural similarities, the Br containing 1,3 hal-
ohydrin showed faster reaction kinetics in the synthesis of PC
compared to its Cl containing analogue.
Aer exposure of the solution of DBU in alkylene chlorohy-
drin to CO2, the formation of EC or PC was conrmed by NMR
analysis. The 13C and 1H NMR spectra for the synthesis of EC in
the ethylene chlorohydrin solution are shown in Fig. 9 and in
the ESI Fig. S9,† respectively. Similar to synthesis in DMSO, the
NMR analysis conrms that EC was formed when the solution
of DBU in ethylene chlorohydrin reacted with CO2. The 13C
NMR analysis of the reaction mixture gave rise to isolated
Hence, similar to previously reported metal based reaction
approach, organic superbase DBU was also used in the
synthesis of cyclic carbonates using various types of halohydrin
substrates. In this case, both 1,2 and 1,3 chloro- and bromo-
hydrin except 2-bromo- and 2-iodoethanol were found as useful
substrates in the synthesis following the reaction Scheme 1. In
case of 1,3 halohydrins, the previous studies involving metal
carbonate have controversial results where Cs2CO3 was able to
Fig. 9 13C NMR spectra for (a) 50 vol% solution of DBU in the ethylene
chlorohydrin and, (b) reaction mixture after bubbling of CO2 in 50 vol%
solution of DBU in the ethylene chlorohydrin (NMR analysis in DMSO-
d6).
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signals for both EC and [DBUH][Cl] as well as for the unreacted separation of the reaction fractions such as cyclic carbonates
1
ethylene chlorohydrin. Similar to synthesis of EC, H and 13C and [DBUH][Cl] from each other.
NMR analysis also conrmed the formation of PC aer inter-
DBU was recovered from the [DBUH][Cl] applying alkaline
actions between the solution of DBU in propylene chlorohydrin solvent treatment and solvent extraction process. The purity
1
and CO2 (Fig. 10 and S10†). As shown in Fig. 10, the NMR and the recovery of the DBU was determined by the H NMR
spectra conrm that the signals for the PC, [DBUH][Cl] and analysis and the eqn (2) (Experimental section). The NMR
unreacted isomers of the propylene chlorohydrin were analysis conrmed that pure DBU was recovered while 76%
observed. Hence, it can be concluded that DMSO free synthesis recovery was achieved (Fig. S12-b†). However, as mentioned in
of the EC or PC can proceed where alkylene chlorohydrins can a previous report, the recovery of the DBU using alkaline solvent
be used as both solvents and reagents in these processes.
treatment from its cationic form is inconsistent and not totally
Further, as discussed previously, excess of alkylene chloro- optimized.17 However, considering the price as well as the
hydrins were used in order to facilitate the separation of importance of DBU in the activation of CO2 under mild reaction
components in the reaction mixture aer the formation of cyclic conditions, more work needs to be performed in order to reach
carbonates. The separation of the EC or PC, [DBUH][Cl] and the higher recovery of the DBU from [DBUH][Cl].
excess of the alkylene chlorohydrins was achieved following the
Hence, in this report, compared to previous study where
separation approaches mentioned in the experimental section. metal carbonates were used as an activating agent for cyclic
The composition of the [DBUH][Cl] obtained through its carbonate synthesis from halohydrins, the similar synthesis was
precipitation aer addition of ethyl acetate in the reaction performed at room temperature and atmospheric pressure with
mixture was conrmed by 1H NMR analysis. As shown in the ESI faster reaction kinetics and no use of the metal based catalysts
S12-a,† pure form of [DBUH][Cl] was recovered from the reac- or co-catalysts. As it was described previously, even though
tion mixture aer the synthesis of the cyclic carbonates. Aer metal based synthesis approach for cyclic carbonate synthesis
the removal of the [DBUH][Cl], the remaining ethyl acetate can be applied for various types of halohydrin substrates, it is
solution was further processed for the recovery of the excess of also accompanied with the generation of the 2 equivalents of
alkylene chlorohydrin and EC or PC. The ethyl acetate was metal salts as a waste per equivalent of cyclic carbonate
removed using rotation evaporator followed by alkylene chlo- (MHCO3 and MCl or MBr where M ¼ K, Na or Cs). In this case,
rohydrins were separated from the EC or PC using high vacuum authors also did not describe the recovery of these salts as well
ꢁ
distillation process at 40 C. The composition of the recovered as regeneration of active metal carbonate catalysts. Unlike these
EC or PC were analysed by the 1H NMR analysis (Fig. S11†) while metal based processes, the generation of waste did not occur in
their recovery was calculated by eqn (1) (Experimental section). DBU based reaction approach except the formation of easily
The 1H NMR spectra of the EC and PC showed that pure cyclic recoverable by-product such as [DBUH][Cl] salt. Besides that,
carbonates were recovered where recovery levels of 92 and 96%, the recovered DBU salt can be converted to DBU with alkaline
respectively, were reached. The high recovery as well as purity of solvent treatment. Hence, the route proposed in this report for
these cyclic carbonates was achieved because of their high the synthesis of cyclic carbonates from various chlorohydrins
boiling points compared to respective alkylene chlorohydrins. can be more preferable compared to the metal based approach.
Therefore, high vacuum distillation at 40 ꢁC allowed efficient
separation of alkylene chlorohydrins from cyclic carbonates.
4. Conclusions
Hence, use of alkylene chlorohydrin solution of the DBU instead
of DMSO in the cyclic carbonates synthesis facilitated the A one-pot and metal free synthesis of the both ethylene
carbonate (EC) and propylene carbonates (EC) from molecular
CO2 was successfully carried out under mild reaction condi-
tions. Various Cl, Br and I comprised 1,2 and 1,3 alkylene hal-
ohydrins and organic superbase DBU reacted with CO2 to form
cyclic carbonate and halide salts of DBU. The NMR based
mechanistic studies conrmed that the switchable ionic liquid
(SIL) was formed in situ from DBU, alkylene halohydrin and CO2
and was further converted to EC or PC through intermolecular
ring closing reaction. In the case of 1,2 halohydrins, 2-chlor-
oethanol and 1-chloro-2-propanol were converted to EC and PC,
respectively, whereas 2-bromo and 2-iodoethanol reacted
equivalently with DBU and formed n-alkylated of DBU salts
instead of cyclic carbonates. Both 1,3 halohydrins such as 3-
chloro-1-propanol and 3-bromo-1-propanol could be converted
to PC. However due to having Br as a good leaving species
compared to Cl, 3-bromo-1-propanol gave rise to faster reaction
kinetics compared to 3-chloro-1-propanol during the formation
Fig. 10 13C NMR spectra for (a) 50 vol% solution of DBU in the
propylene chlorohydrin and, (b) reaction mixture after bubbling of CO2
of PC. In order to avoid the use of DMSO as a solvent, the EC or
PC synthesis from 1,2 chlorohydrins was further carried out
in 50 vol% solution of DBU in the propylene chlorohydrin (NMR
analysis in DMSO-d6).
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using chlorohydrins as a solvent. In this case, the chlorohydrins
not only performed as a solvent media but also served as
reagents in the process. Further, the use of chlorohydrins as
a solvent also enabled efficient separation of the reaction
components through mild experimental approaches. In this
process, pure forms of EC or PC were obtained where 92 and
96% of recovery of cyclic carbonates was achieved, respectively.
The organic superbase DBU was also recovered with high purity
from [DBUH][Cl] using alkaline solvent treatment whereupon
the recovery was 76%. Hence, in this report DBU superbase
involved synthesis of EC and PC from CO2 was carried out using
various types of halohydrins. The synthesis was further
successfully upgraded using halohydrins as both solvent media
and reagent. Being metal free, one pot and mild reaction
condition approach as well as having control over the waste
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Conflicts of interest
There are no conicts to declare.
13 Z. Xi, Y. Xiangui, C. Tong, Z. Yi and W. Gongying, Chin. J.
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14 J.-L. Wang, L.-N. He, X.-Y. Dou and F. Wu, Aust. J. Chem.,
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Acknowledgements
We wish to thanks Dr Tobias Sparrman and Dr Mattias
¨
Hedenstrom for their assistance in the NMR analysis. This work
is part of activities of the Technical Chemistry, Department of
16 J. R. O. Gomez, O. G.-J. Aberasturi, C. A. R. Lopez,
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˚
Chemistry, Chemical-Biological Centre, Umea University, Swe-
den as well as the Johan Gadolin Process Chemistry Centre at
˚
Abo Akademi University in Finland. The Swedish Research
Council (Drn: 2016-04090), Bio4Energy programme, Kempe
Foundations and Wallenberg Wood Science Center under
auspices of Alice and Knut Wallenberg Foundation are grate-
fully acknowledged for funding this project.
19 P. G. Jessop, S. M. Mercer and D. J. Heldebrant, Energy
Environ. Sci., 2012, 5, 7240–7253.
20 I. Anugwom, P. Maki-Arvela, P. Virtanen, P. Damlin,
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