Efficient synthesis of glycerol carbonate/glycidol using 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU) based ionic liquids as catalyst

Article abstract of DOI:10.1039/c3ra47433j

Transesterification of dimethyl carbonate with glycerol to glycerol carbonate has been catalyzed efficiently using basic ionic liquids as catalysts. Activity of all the ILs tested is very high and the best result (96% conversion with 82% selectivity to glycerol carbonate and 18% selectivity to glycidol) was obtained using IL1 as catalyst. The effect of catalyst loading has significant influence on the selectivity pattern. The higher activity of the ionic liquid is explained with a plausible mechanism based on the co-operative effect of both cation and anion.

Full text of DOI:10.1039/c3ra47433j

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Efficient synthesis of glycerol carbonate/glycidol
using 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU)
based ionic liquids as catalyst†
Cite this: RSC Adv., 2014, 4, 17124
Received 9th December 2013
Accepted 19th February 2014
Mudassir K. Munshi, Pradeep S. Biradar, Swapna M. Gade, Vilas H. Rane
and Ashutosh A. Kelkar*
DOI: 10.1039/c3ra47433j
www.rsc.org/advances
Transesterification of dimethyl carbonate with glycerol to glycerol polyurethanes, polyamides, surfactants, lubricating oils and
3
carbonate has been catalyzed efficiently using basic ionic liquids as other chemicals. The reaction of phosgene with GLY is the
4
catalysts. Activity of all the ILs tested is very high and the best result conventional method for the synthesis of GC. However, the
(
96% conversion with 82% selectivity to glycerol carbonate and 18% toxicity of phosgene is the major problem with the conventional
selectivity to glycidol) was obtained using IL1 as catalyst. The effect of method. In order to overcome this drawback of the conven-
catalyst loading has significant influence on the selectivity pattern. The tional method, four new routes have been developed for
higher activity of the ionic liquid is explained with a plausible mech- conversion of GLY to GC. Namely, oxidative carbonylation of
anism based on the co-operative effect of both cation and anion.
2
GLY, direct carboxylation of GLY using CO , reaction of GLY
with urea and transesterication of dialkyl carbonate with GLY.
Among all the routes, the route based on the transesterication
of dialkyl carbonate with GLY is one of the most popular.
Introduction
Continuous reduction in fossil fuel resources and increasing Numerous heterogeneous (metal oxides, mixed metal oxides,
concerns about climate change are major issues for the hydrotalcites, supported hydroxyapatite and Sn-complexes) as
sustainable development of society. The use of renewable well as homogeneous (inorganic metal salts, quaternary
resources is one of the solutions for the aforementioned issues. ammonium salts and ionic liquids) catalysts have been
3
In the last couple of decades much attention has been devoted proposed for this route. Among the catalysts reported Sn-
to applying green catalytic processes to convert biorenewable complexes and basic ionic liquids showed high activity for GC
feed stocks to commodity chemicals and clean fuels. Glycerol synthesis, but their catalytic activities expressed as turnover
(
GLY), one of the renewable resources, is obtained as a by- number (TON) were in the range of 100–200 (TON ¼ 200 for 1,3-
ꢀ
5
product from the hydrolysis of fats, soap-manufacturing dichlorostannoxane at 100 C aer 120 minute, TON ¼ 100 for
1
ꢀ
6
processes, and production of biodiesel. Biodiesel manufacture BMIM-2-CO
2
at 74 C aer 80 min and TON ¼ ꢁ100 for [TMA]
ꢀ
7
is also increasing day-by-day and relatively cheap GLY is [OH] at 80 C aer 90 min ).
expected to be available in large quantities. One of the major
From the literature it was observed that the maximum TON
bottlenecks in the enhancement of biodiesel production is the reported was 200 and also the scope of catalysts investigated is
utilization of GLY, which needs to be converted into value limited. In particular, the use of basic ionic liquids as catalysts
added products in an economical way. Research efforts to nd is very limited and there is a scope for use of other basic ionic
new applications for conversion of GLY as a low-cost feed stock liquids as catalysts for this reaction.
for the synthesis of functional derivatives have led to a number
Herein, we report our results on the use of DBU based ionic
of processes for converting GLY selectively to value added liquids as basic catalysts for transesterication of dimethyl
2
products. Glycerol carbonate (GC) and glycidol (GD) obtained carbonate (DMC) with GLY. To the best of our knowledge, this is
from GLY are important products with a wide range of appli- the rst report on the use of diazabicyclo [5.4.0] undec-7-ene
cations in the elds of pharmaceutical, membrane and polymer (DBU) based ionic liquids as catalysts for GC/GD synthesis.
industries for the production of polyesters, polycarbonates,
Experimental section
Chemicals
Chemical Engineering and Process Development Division, National Chemical
Laboratory, Pune 411008, India. E-mail: aa.kelkar@ncl.res.in; Fax: +91 20
2
5902621; Tel: +91 20 25902544
Glycerol (99%), glycidol (96%), glycerol carbonate (95%) and
1,8-diazabicyclo [5.4.0] undec-7-ene (99%) were purchased from
†
Electronic supplementary information (ESI) available: NMR and IR spectra of
ionic liquids. See DOI: 10.1039/c3ra47433j
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Aldrich, dimethyl carbonate (99%) from Spectrochem, India, carried out for 30 min. The reaction mixture was cooled and it
methanol (99%) from Merck, India and propylene glycol (99%) was diluted with N,N-dimethylformamide, and a sample was
from Loba Chemie, India. All the chemicals were used as taken out for analysis. The products were analyzed by gas
received from suppliers.
chromatography on an Agilent 6890 gas chromatograph with
The synthesized ionic liquids were characterized by means of HP-Innowax capillary column (30.0 m ꢂ 0.53 mm ꢂ 1.00 mm
nuclear magnetic resonance (NMR) spectroscopy, experiments lm thicknesses). Identication of products was carried out
were carried out on a Bruker Avance 400 wide bore spectrometer using gas chromatography-mass spectrometry (GC-MS) on an
equipped with a superconducting magnet with a eld of 9.4 T. Agilent 6890N gas chromatograph coupled to an Agilent 5973
1
3
The operating frequency for C was 75.4 MHz. The ILs were mass spectrometer.
also characterized by FT-IR spectroscopy on an Agilent Tech-
nologies Cary 600 series FT-IR spectrometer.
Results and discussion
Catalyst characterization
Typical procedure for the ionic liquids preparation
DBU derived basic ionic liquids have been prepared by bubbling The typical reaction scheme for the preparation of the ionic
9
CO into the mixture of DBU and alcohols such as methanol, liquids is as shown in Scheme 1. Based on the literature reports
2
propylene glycol or GLY. Molar amounts of DBU and alcohols and also weight gain, ionic liquid formation based on methanol
used were 1 : 1, 2 : 1 and 3 : 1, respectively, for the synthesis of was quantitative, while with propylene glycol and GLY mostly
1
3
IL1, IL2 and IL3 (1 : 1 molar amount of DBU and methanol for the monocarbonate ionic liquid is formed. The typical C and
1
IL1, 2 : 1 molar amount of DBU and propylene glycol for IL2 and
H NMR spectra of all the ionic liquids and a physical mixture of
3
: 1 molar amount of DBU and GLY for IL3). All the ionic GLY and DBU (GLY–DBU, for comparison with NMR spectrum
8
liquids were prepared as per the procedure described earlier.
of IL3) is shown in Fig. 1a and b, ESI,† and the IR spectra of
1
3
From the literature it was observed that the ILs prepared from ionic liquids are shown in Fig. 1c, ESI.† The C-NMR spectrum
triol-like GLY is mostly present in monocarbonate form, indi- of the GLY–DBU mixture showed a peak at d ¼ 162.1, which can
8
cating incomplete conversion of hydroxyl groups. However, be attributed to the central C]N group of [DBU]. With bubbling
with mono-ols like methanol the conversion was almost quan- of CO2 into the GLY–DBU mixture, the free owing mixture
9
titative, probably because of the high polarity of the alcohols.
became viscous, which is a fundamental property of ILs, indi-
The typical experimental procedure for preparation of IL3 is cating the formation of an ionic liquid.
8
given below.
As reported by Anugwom et al., one new peak was observed
at d ¼ 159.2 just before the peak of the central C]N group of
1
3
DBU–glyceryl carbonate IL
[DBU] in C-NMR analysis of IL3, along with all other peaks;
this new peak could be because of the carbonate carbon of the
ionic liquid. Similarly, peaks due to carbonate carbon are also
seen at d ¼ 163.7 and 160.8 for IL1 and IL2 (see Fig. 1a, ESI†).
The DBU glyceryl carbonate ionic liquid was prepared by
bubbling CO through a mixture of DBU–GLY (molar ratio 3 : 1).
2
DBU (6 g) 39.41 mmol and GLY (1.2 g) 13.13 mmol were used for
preparing a 3 : 1 molar mixture. Thereaer, a narrow-gauge
1
H-NMR signals of the GLY–DBU mixture and IL3 showed broad
8
and complex patterns, as reported by Anugwom et al. and
hence assignment of signals to respective protons is difficult.
Similarly the H NMR for IL1 and IL2 were broad and complex
glass tube was inserted and CO was bubbled through the liquid
2
8
until there was no more weight increase. The reaction was
notably exothermic and the solution was stirred mechanically
throughout the bubbling cycle. During the course of the reac-
tion the liquid became viscous. Other ionic liquids were also
prepared starting from DBU and other alcohols such as meth-
anol and propylene glycol as reactants by following the same
procedure. All the ionic liquids prepared were characterized by
NMR and IR analysis.
1
and assignment of protons is difficult (see Fig. 1b, ESI†).
The FTIR spectra of all ionic liquids give a broad spectral
ꢃ1
response at ꢁ3400 cm due to N–H stretching vibrations. The
ꢃ1
bands at 2931 and 2861 cm were from the C–H stretching
vibrations in the ring and from the alcohol. The strong band at
ꢃ1
1
650 cm , which is comparable to the n (C]N) group of the
ꢃ1
DBU ring, and a band at 1630 cm can be assigned to the n
ꢃ
ꢃ1
(
1
COO ) group of the ionic liquid. And bands at 1324 cm and
Transesterication of DMC with GLY
ꢃ1
106 cm can be due to the asymmetric and symmetric C–O–C
The transesterication of DMC with GLY was carried out in a 50
ml round bottom ask equipped with a reux condenser under
vigorous stirring. In a typical run, 0.2173 mmol (1 mol %) of
catalyst with respect to GLY was charged to the 50 ml round
bottom ask containing GLY (2 g, 21.73 mmol) and DMC (5.87
g, 65.19 mmol). The reaction was carried out at reux temper-
stretching vibrations (see Fig. 1c, ESI†).
ꢀ
ature by keeping the oil bath temperature at 100 C for the
selected reaction time. During the course of the reaction
ꢀ
temperature decreased from 88 to 71 C as the reaction pro-
gressed. The drop in temperature was because of the formation
of methanol as the reaction progressed. Standard reaction was Scheme 1 Preparation of ionic liquids.
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1
3
a
Thus, from the IR and C NMR analysis it can be said that Table 2 Effect of catalyst loading on activity and selectivity
the presence of peaks at d ¼ 160.8, 163.9 and 159.2 for IL1-3 in
1
3
ꢃ1
Catalyst loading GLY conversion GD selectivity GC selectivity
the C-NMR spectrum and a band at n ¼ 1630 cm in IR
(mmol)
(%)
(%)
(%)
TON
ꢃ
analysis conrms the presence of the COO group and the
formation of ionic liquids.
0.01
53
63
66
87
96
97
92
3
4
97
96
89
82
82
61
44
1152
456
286
171
96
0
0
0
0
0
.03
.05
.11
.22
.44
11
18
18
39
56
Catalyst screening
The ionic liquids prepared were screened for the reaction with a
48
30
30 minute reaction time using 2.5 wt% catalyst loading and the 0.66
results obtained are presented in Table 1.
From the results it can be seen that all the ionic liquids minute; oil bath temperature: 100 C. TON: number of moles of GLY
prepared were active for the reaction and the best results (96%
a
Reaction conditions: GLY: 21.73 mmol; DMC: 65.19 mmol; time: 30
ꢀ
consumed/no of moles of GLY charged.
conversion of GLY with 82% selectivity to GC and 18% selec-
tivity to GD) were obtained with IL1 at 2.5 wt% loading. GLY
conversion with IL2 and IL3 prepared using propylene glycol
and GLY was marginally lower with very high selectivity to GC
The typical reaction progress and variations in conversion
and selectivity pattern as a function of time using IL1 as catalyst
is shown in Fig. 1. From the gure it can be seen that activity of
the catalyst is high and ꢁ89% GLY conversion is achieved with a
(90 and 92%, respectively). Since higher GLY conversion with
good selectivity to GC (82%) was obtained with IL1, further work
was carried out using IL1 as a catalyst.
2
0 minute reaction time, with 86% and 14% selectivity to GC
and GD, respectively. Conversion increased to 96% aer 30
minutes’ reaction time. Also, it is observed that with progress of
the reaction GD selectivity increases marginally from 14 to 18%
and GC selectivity decreases marginally from 86 to 82% (Fig. 1).
Thus 96% conversion of GLY was obtained with 82% GC and
Optimization of reaction conditions
The effect of reaction parameters such as catalyst amount,
reaction time, temperature, and GLY to DMC molar ratio on the
activity and selectivity were investigated using IL1 as catalyst
and the details are presented below.
The effect of catalyst loading on the activity and selectivity
was investigated in the range of 0.01–0.66 mmol and the results
are shown in Table 2.
From the results it can be seen that GLY conversion as well as
selectivity pattern is signicantly affected by a change in catalyst
loading. The GLY conversion increased from 53% to 96% and
selectivity to GC decreased from 97% to 82% with the increase
in catalyst loading from 0.01–0.22 mmol. With further increase
in catalyst loading, GLY conversion was not signicantly
affected. However, selectivity to GD increased signicantly with
an increase in catalyst loading (see Table 2). The probable
reason for the increase in GD selectivity could be mainly
because of an increase in basic sites with an increase in catalyst
loading, resulting in decarboxylation of GC formed as a
18% GD selectivity aer only 30 minutes’ reaction time.
The reaction of DMC and GLY catalyzed by IL 1 was inves-
ꢀ
ꢀ
ꢀ
tigated at three different temperatures (50 C, 75 C and 100 C)
keeping other parameters constant and the results are pre-
sented in Fig. 2. The conversion increased marginally with an
ꢀ
ꢀ
ꢀ
increase in temperature from 50 C to 75 C. Selectivity to GC
was very high (ꢁ95%) at 50 ꢀC and 75 C. However, with a
further increase in temperature very high GLY conversion 96%
was achieved with a marginal drop in GC selectivity (82%) and
ꢀ
increase in GD selectivity (18%) at 100 C (oil bath temperature).
10
Cho et al. have observed a similar trend in the trans-
esterication of GLY to GC using tetraalkylammonium salts
immobilized on MCM-41 as the catalyst. Since very high
product. Formation of CO in these experiments was conrmed
2
by passing the gas phase through a saturated barium hydroxide
solution to obtain a white precipitate of barium carbonate.
Decarboxylation of the GC to GD is well known in the presence
7
of strong bases.
a
Table 1 Screening of various ionic liquids
GLY conversion
(%)
GD selectivity
(%)
GC selectivity
(%)
Catalyst
IL1
IL2
IL3
96
87
85
18
10
8
82
90
92
a
Reaction conditions: GLY: 21.73 mmol; DMC: 65.19 mmol; catalyst: 50 Fig. 1 Effect of reaction time. Reaction conditions: GLY: 21.73 mmol;
ꢀ
ꢀ
mg (2.5 wt%); time: 30 minute; oil bath temperature: 100 C.
DMC: 65.19 mmol, catalyst: 1 mol%, oil bath temperature: 100 C.
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3
bases reported in the literature has suggested that the catalytic
performance of IL catalysts does not depend only on basicity.
Rather, ambiphilic (both nucleophilic and electrophilic)
catalysis appears more suitable to account for the results. This
1
2
is based on recent proposal by Chakraborti and Roy on
electrophile nucleophile dual activation” role of IL catalysts
in o-tert-butoxycarbonylation of 2-naphthol with Boc O using
0 mol% [bmim][BF ] as catalyst. There are a few reports on
“
2
1
4
the use of IL catalysts where both anion and cation help in
activation of substrates via ion pairs: anions and cations of ILs
may activate nucleophiles (as true nucleophiles or bases) and
1
2–14
electrophiles, respectively.
The role of ionic liquids IL1-3 as transesterication catalyst
15
Fig. 2 Effect of reaction temperature. Reaction conditions: GLY: 21.73 Selva and co-workers have proposed mechanism for the
mmol; DMC: 65.19 mmol, catalyst: 1 mol%, time: 30 minutes.
synthesis of unsymmetrical carbonates involving parallel
requirement of both anions and cations of IL for the activation
of DMC and alcohols using methyltrioctylphosphonium salts as
ꢀ
ꢀ
conversion of GLY was obtained at 100 C, for further study the
7
basic IL catalysts. Gade and co-workers in their work on direct
reaction temperature was xed at 100 C.
synthesis of GD from GLY via GC as an intermediate have
also discussed the role of both cation as well as anion in
decarboxylation of GC to GD over tetramethyl ammonium
hydroxide IL as catalyst.
The effect of the GLY : DMC mole ratio on the GLY conver-
sion and selectivity was studied by varying the GLY : DMC molar
ratio in the range of 1 : 3, 1 : 2, 1 : 1, 2 : 1, and 3 : 1 and the
results are presented in Table 3. A GLY or DMC quantity of 21.73
mmol was used in the experiment carried out with a GLY : DMC
ratio of 1 : 1. In other experiments, the quantity of one of the
reactant was varied, keeping the quantity of other reactant
constant at 21.73 mmol. Also, conversion reported is based on
the limiting reactant. From the results it was observed that
conversion was very low (62% GLY conversion aer 30 minute
reaction time) at the GLY : DMC ratio of 1 : 1.
By combining the reaction mechanism proposed by Selva
et al. and Gade et al., a plausible reaction mechanism for the
transesterication of DMC with GLY to GC and decarboxylation
GC to GD over IL1 is shown in Scheme 2. The nitrogen–oxygen
interaction favours the coordination of the N center of IL to the
basic carboxylic oxygen of DMC resulting in DMC activation.
16
Yoshio et al. have proposed the interaction of the nitrogen
This is expected since this is an equilibrium controlled
5,11
reaction and excess DMC is used to achieve high conversion.
With increase in either GLY or DMC, conversion increased
signicantly, indicating a positive dependence on both reac-
tants. High conversion (96%) was observed at a GLY : DMC ratio
of 1 : 3 compared to only (68%) at GLY : DMC ratio of 3 : 1. The
observed results may be due to an increase in viscosity of the
reaction mixture at high GLY concentrations leading to mass
transfer issues. The selectivity pattern was not affected signi-
cantly by a change in the GLY : DMC ratio.
The superior catalytic activity of IL1-3 (Tables 1 and 2)
compared to the other conventional organic and inorganic
Table 3 Effect of GLY to DMC molar ratio on activity and selectivity
a
pattern
GLY : DMC
ratio
GLY conversion
(%)
GD selectivity
(%)
GC selectivity
(%)
b
1
1
1
2
3
: 1
: 2
: 3
: 1
: 1
62
16
15
18
23
14
84
85
82
77
86
b
71
b
96
c
78
c
68
a
ꢀ
Reaction conditions: time: 30 minute; oil bath temperature: 100 C, Scheme 2 Reaction mechanism: ionic liquid catalyzed [A] trans-
b
c
catalyst: 1 mol %. Conversion w.r.t GLY. Conversion w.r.t DMC.
esterification of DMC with GLY [B] decarboxylation of GC.
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center of the quaternary ammonium salt with the carboxyl reaction. To the best of our knowledge this is the rst report on
oxygen of the carboxylic acid in the reaction of ethylene the catalyst with very high activity (TON 1152) for trans-
carbonate with carboxylic acids to get the corresponding esters. esterication of DMC with GLY.
Nucleophilic activation can take place by the proton exchange
reaction between the anion of the ionic liquid and GLY leading
Acknowledgements
to (GLY activation) the formation of glyceroxide and methyl
15
carbamic acid as proposed by Selva et al. Once both the elec-
trophile and the nucleophile are triggered by the catalyst, glyc-
eroxide attacks the activated carbonyl carbon group, and
glyceryl methyl carbonate (intermediate) is obtained. At the
Authors would like to thank Department of Science and Tech-
nology (DST), Government of India for the nancial support to
this project. Mudassir K. Munshi and Swapna M. Gade would
like to thank CSIR, Government of India for fellowship awarded
to them. The analytical help on GC-MS by Dr Mrs S. K. Shingote
is greatly appreciated.
+
ꢃ
same time an alkoxide exchanged IL, namely {[DBUH ] RO },
formed as an intermediate reacts with methyl carbamic acid
formed in the rst step, yielding methanol and regenerating the
ionic liquid. The existence of alkoxide exchanged ILs is known
17
in the literature. Methyl glyceryl carbonate undergoes a
similar reaction sequence, which followed by cyclization leads
to the formation of GC as the nal product.
Notes and references
1 A. Behr, J. Eilting, K. Irawaldi, J. Leschinski and F. Lindner,
Green Chem., 2008, 10, 13–30.
It may be noted that at very low catalyst loading (0.01 and
0
.03 mmol) the selectivity to GC is very high (>95%). However,
2 F. Jerome, Y. Pouilloux and J. Barrault, ChemSusChem, 2008,
1, 586–613.
3 M. O. Sonnati, S. Amigoni, E. P. Taffin de Givenchy,
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A. A. Kelkar, Catal. Commun., 2012, 27, 184–188.
8 I. Anugwom, P. Maki-Arvela, P. Virtanen, P. Damlin,
R. Sjoholm and J. P. Mikkola, RSC Adv., 2011, 1, 452–457.
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X. Li, P. Pollet, R. Wang, C. A. Eckert, C. L. Liotta and
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10 H. Cho, H. Kwon, J. Tharun and D. Park, J. Ind. Eng. Chem.,
with an increase in the catalyst loading decarboxylation of GC
leads to an increase in GD formation and at a catalyst loading of
0.66 mmol, 56% selectivity to GD is obtained (Table 2). An
increase in IL loading will increase the interaction of IL with GC,
leading to decarboxylation of GC. Gade et al. have proposed the
interaction of cationic and anionic centres of IL with GC leading
to efficient decarboxylation reaction and high (78%) selectivity
to GD at higher catalyst loading in their work on the synthesis of
GC/GD by the transesterication of DMC with GLY using
7
+
ꢃ
[TMA] [OH] ionic liquid as catalyst. Likewise, in the present
+
work the [DBUH] centre can interact with the oxygen of the
carbonyl group of GC, making the bond weaker. Further inter-
action of the anion with the more substituted sp hybridized
alkylene carbon of GC can lead to decarboxylation and forma-
tion of GD as the product as shown in Scheme 2. The proposed
mechanism is speculative and further work is necessary to
understand the mechanism of the reaction.
3
2010, 16, 679–683.
1
1
1 J. Li and T. Wang, Chem. Eng. Process., 2010, 49, 530–534.
2 A. K. Chakraborti and S. R. Roy, J. Am. Chem. Soc., 2009, 131,
6902–6903.
Conclusion
Transesterication of DMC with GLY has been investigated
using DBU based ILs as catalysts. All the ILs were prepared as 13 M. Selva, M. Fabris, V. Lucchini, A. Perosa and M. Noe, Org.
per the literature procedure and characterized by FT-IR and Biomol. Chem., 2010, 8, 5187–5198.
NMR analysis. Catalytic activity of all the ILs screened is very 14 V. Lucchini, M. Noe, M. Selva, M. Fabris and A. Perosa,
high and detailed work was carried out using IL1 as a catalyst. Chem. Commun., 2012, 48, 5178–5180.
GLY conversion increased with an increase in catalyst loading, 15 M. Selva, M. Noe, A. Perosa and M. Gottardo, Org. Biomol.
temperature of the reaction and also with increase in GLY or Chem., 2012, 10, 6569–6578.
DMC concentration. Catalyst loading was found to have a 16 T. Yoshino, S. Inaba, H. Komura and Y. Ishido, J. Chem. Soc.,
signicant effect on the selectivity pattern. Thus, highest Perkin Trans. 1, 1977, 1266–1272.
selectivity (56%) to GD was achieved with a catalyst loading of 17 V. Eta, P. Makiarvela, E. Salminen, T. Salmi, D. Y. Murzin
0.66 mmol. A plausible mechanism has been proposed for the
and J. P. Mikkola, Catal. Lett., 2011, 141, 1254–1261.
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