Role of cation-anion cooperation in the selective synthesis of glycidol from glycerol using DABCO-DMC ionic liquid as catalyst

Article abstract of DOI:10.1039/c4ra04290e

Transesterification of dimethyl carbonate with glycerol has been investigated using 1,4-diazabicyclo [2.2.2] octane (DABCO) based ionic liquid as a catalyst. DABCO reacted with dimethyl carbonate to form ionic liquid as the reaction progressed. Though the basicity of DABCO based ionic liquid was lower than that of DABCO, the catalytic activity and selectivity to glycidol was higher with DABCO based ionic liquid as a catalyst, indicating that basicity may not be the only criterion in deciding activity and selectivity of the reaction. The cooperative effect of the cation and anion of the ionic liquid is responsible for the observed results. The best results (97% glycerol conversion with 83% selectivity to glycidol and 17% selectivity to glycerol carbonate) were obtained using DABCO based ionic liquid as a catalyst. A plausible mechanism involving the role of both the cation and anion of the ionic liquid has been proposed. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra04290e

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Role of cation–anion cooperation in the selective
synthesis of glycidol from glycerol using DABCO–
DMC ionic liquid as catalyst†
Cite this: RSC Adv., 2014, 4, 32127
*
Mudassir K. Munshi, Swapna M. Gade, Vilas H. Rane and Ashutosh A. Kelkar
Transesterification of dimethyl carbonate with glycerol has been investigated using 1,4-diazabicyclo [2.2.2]
octane (DABCO) based ionic liquid as a catalyst. DABCO reacted with dimethyl carbonate to form ionic
liquid as the reaction progressed. Though the basicity of DABCO based ionic liquid was lower than that
of DABCO, the catalytic activity and selectivity to glycidol was higher with DABCO based ionic liquid as a
catalyst, indicating that basicity may not be the only criterion in deciding activity and selectivity of the
reaction. The cooperative effect of the cation and anion of the ionic liquid is responsible for the
observed results. The best results (97% glycerol conversion with 83% selectivity to glycidol and 17%
selectivity to glycerol carbonate) were obtained using DABCO based ionic liquid as a catalyst. A plausible
mechanism involving the role of both the cation and anion of the ionic liquid has been proposed.
Received 8th May 2014
Accepted 3rd July 2014
DOI: 10.1039/c4ra04290e
www.rsc.org/advances
with DMC using various homogeneous as well as heterogeneous
catalysts, factors governing the selectivity pattern (GC or GD) are
1. Introduction
not well understood. Almost all the literature, except recently
published results,4 report GC with very high selectivity. Also,
strong basicity of the catalyst is attributed to high activity and
selectivity obtained to GC as the product. It should be noted
that, most of the literature reports on the synthesis of GD
involve decarboxylation of GC at very high temperature (170–
200 ꢀC) and high vacuum using catalysts similar to those
reported for the synthesis of GC from GLY.5 For this reaction
also, activity and selectivity to GD is mostly attributed to the
basicity of the catalyst. Thus, the role of catalyst in the product
distribution is not well investigated. With this aim we analyzed
the available literature on transesterication of GLY with DMC
Glycerol (GLY) is a multifunctional molecule with a potential to
undergo a wide range of reactions, leading to a number of value
added chemicals.1 In the past few years extensive research has
been carried out on the catalytic conversion of GLY to number
of industrially important chemicals.2 Transesterication of GLY
with dimethyl carbonate (DMC) is one of the important reac-
tions resulting in the formation of glycerol carbonate (GC) and
glycidol (GD) as the products (see Scheme 1).3
Both GC and GD are important products with a variety of
applications; however most of the literature reports deal with
selective synthesis of GC as the product. Although, signicant
amount of research has been carried out on the reaction of GLY
Scheme 1 Catalytic conversion of GLY to GC and GD.
Chemical Engineering and Process Development Division, National Chemical
Laboratory, Pune 411008, India. E-mail: aa.kelkar@ncl.res.in; Fax: +91 20
25902621; Tel: +91 20 25902544
Fig. 1 Transesterification of GLY with DMC using DABCO as catalyst.
Reaction conditions: GLY: 2 g, 21.73 mmol, DMC: 5.87 g, 65.19 mmol,
time: 30 min, temp: reflux (100 ꢀC oil bath temperature) catalyst: 2
mol% with respect to GLY.
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra04290e
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using various solid bases,6 nucleophilic bases7 and basic ionic small quantity. While, aer 5 h, peak due to DABCO protons
liquids4a,8 as catalysts. From the analysis of literature, and our (at d: 2.63 for –N-(CH2)6) completely disappeared and peaks
previous experience on this reaction using basic ionic liquids as only because of the product formed (ionic liquid) were seen
catalysts,4a,4c it was observed that, the change in selectivity at d (ppm): 2.95, 3H (s), d (ppm): 3.09, 6H (t), d (ppm): 3.24, 3H
pattern, is not only because of the base strength of the catalyst (s) and at d: 3.29, 6H (t).
used, but may be depending on the type of catalyst used
The total weight gain due to the formation of DABCO–DMC
(nucleophilic or ambiphilic). In the present work we wish to IL in 5 h reaction was 1.6 g. Thus, on the basis of multiplicity
1
report synthesis of GD with high selectivity using DABCO based pattern of H-NMR spectroscopy and total weight gain due to
ionic liquid (DABCO–DMC IL) as the catalyst. Comparative the formation of ionic liquid, possible expected structures of the
results on the reaction of GLY with DMC using a nucleophilic ionic liquid are as shown in Fig. 3.
base DABCO and an ambiphilic base DABCO–DMC IL as cata-
Heteronuclear coherence [Heteronuclear Single Quantum
lysts are also discussed. Further, the effect of various reaction Coherence (HSQC) and Heteronuclear Multiple-Bond Corre-
conditions (time, temperature, molar ratios, catalyst loading) lation (HMBC)] NMR spectroscopy is extremely useful tech-
on activity and selectivity has been investigated in detail using nique for the structure elucidation.11 These experiments yield
both the catalysts.
proton connectivity through quaternary carbon or hetero-
nuclei. Thus to ascertain the exact structure of the ionic
liquid 1H–13C HSQC and HMBC (see ESI†) and 1H–15N HMBC
(see Fig. 4) 2D NMR analysis was carried out. For comparison
2. Results and discussion
DABCO is a nucleophilic base and is used as a catalyst in 1H–15N HMBC 2D NMR spectrum of DABCO was also recor-
many reactions9 including esterication reaction.10 On the ded and is presented in Fig. 5. Presence of only one signal
basis of these precedents we considered DABCO as a poten- with correlation of all the 12 proton at dN (ppm): 11.9/dH 2.58
tial catalyst for the transesterication of GLY with DMC to in 1H–15N HMBC spectrum of pure DABCO clearly shows that
GC. Preliminary experiments on transesterication of GLY both the nitrogen atoms are in the same environment and
with DMC were carried out using 2 mol% DABCO (with hence give only one signal. Whereas, key correlations
1
respect to GLY) as a catalyst and the results obtained are observed in H–15N HMBC 2D NMR spectrum of ionic liquid
presented in Fig. 1.
gives two signals, one at dN (ppm): 40.7/dH 2.95 (which
As expected the catalytic activity of DABCO was good and corresponds to three protons of –CH3) and dH 3.29 (which
97% GLY conversion was obtained in only 30 min reaction corresponds to six protons of –CH2), and another at dN (ppm):
time. To our surprise, GD was obtained with 45% selectivity 9.3/dH 3.09 (which corresponds to six protons of –CH2).
and 53% selectivity was observed for GC. Experiment was Similarly, all of the carbon signals are also distinguished by
repeated three times to conrm observed selectivity pattern. the 1H–13C HSQC and HMBC analysis (see Fig. 2 and 3 in
The observed high selectivity to GD prompted us to under- ESI†). Thus based on the 2D NMR correlations established
stand the role of DABCO as a catalyst in the reaction. From and total weight gain (corresponding to quaternization at
the literature it was found that Gade and co-workers4a for the only one of the nitrogen) it is conrmed that the structure of
ꢀ
rst time reported very high selectivity to GD (78%) at 90 C ionic liquid formed is as proposed in the Fig. 3-A.
using tetramethylammonium hydroxide (TMA-OH) ionic
Further, melting point of ionic liquid was measured and was
liquid as the catalyst. The increased selectivity towards GD found to be in a range of 80–84 ꢀC (melting starts at 80 ꢀC, half
ꢀ
ꢀ
was attributed to the strong basicity of the ionic liquid melt at 82 C and complete melting was observed at 84 C).
catalyst. It should also be noted that DABCO has been used as
TGA analysis of the ionic liquid was carried out under
a catalyst in many reactions involving DMC as one of the nitrogen and the ionic liquid was found to be stable up to 150 ꢀC
substrates. In the N-methylation of indole with DMC using (see ESI† for TGA graph).
DABCO as a catalyst it is proposed that DMC is activated by
Aer preparing DABCO–DMC IL in pure form, compara-
DABCO to generate ion pair.9a The ion pair formed helps in tive experiments were carried out using 2 mol% of DABCO
efficient N-methylation of indole and is regenerated at the and DABCO–DMC IL and conversion-time proles for both
end by nucleophilic attack of DABCO on DMC.9a However, the reactions are presented in Fig. 6(a) and (b) respectively.
there are no reports on the isolation and characterization of From the gure it can be seen that the activity and selectivity
DABCO–DMC IL. In order to ascertain this, we attempted the pattern were signicantly different with DABCO–DMC IL as a
synthesis of DABCO–DMC IL. For this purpose, 2.02 g (18 catalyst compared to DABCO. Thus at 10 min reaction time
mmol) of DABCO was added to 9.72 g (108 mmol) of DMC GLY conversion of 77% was observed with DABCO–DMC IL,
(DABCO : DMC molar ratio of 1 : 6) and reuxed for different while just 19% conversion was observed with DABCO. At 30
time intervals (20 min and 5 h). Aer completion of the min reaction time conversion for both the catalysts was very
desired reaction time, the excess DMC was removed on rotary high (97%), however, selectivity to GD was higher with
evaporator and the NMR analysis of the residue obtained was DABCO–DMC IL (63% compared to 45% with DABCO).
carried out. NMR of pure DABCO was used for comparison
Thus lower activity at the intermediate samples (Fig. 6, 10
and the results are presented in Fig. 2. The NMR spectrum of and 20 min) with DABCO as a catalyst can be attributed to the
sample reuxed for 20 min showed the presence of DABCO as presence of DABCO in the reaction and higher activity for
a major component with peaks due to DABCO–DMC IL in DABCO–DMC IL may be attributed to ionic liquid used as the
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Fig. 2 1H-NMR spectra of A: DABCO, B: DABCO–DMC mixture refluxed for 20 min and C: DABCO–DMC mixture refluxed for 5 h.
3. Effect of reaction conditions on
activity and selectivity pattern
3.1. Effect of catalyst loading
Effect of catalyst loading on the activity and selectivity was
investigated in a range of 0.5 to 6 mol% of DABCO as well as
DABCO–DMC IL as catalysts and the results are shown in
Table 1.
Fig. 3 Possible structures of ionic liquid.
From the results it can be clearly seen that DABCO–DMC IL
is highly active with high selectivity to GD, compared to DABCO
as a catalyst at 0.5 mol% catalyst loading. Thus GLY conversion
of 96% was observed with 55% selectivity to GD with DABCO–
DMC IL compared to 71% GLY conversion and only 10%
selectivity to GD with DABCO as a catalyst.
With increase in catalyst loading from 0.5 mol% to 6 mol%
conversion of GLY increased signicantly for DABCO catalyst,
while selectivity to GD increased for both the catalysts. Best
catalyst. The results obtained clearly show that DABCO–DMC IL
is having high activity with high selectivity to GD under reux
conditions. Hence further work was carried out using this
catalyst and for comparison results obtained with DABCO is
also presented.
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results (97% GLY conversion with 83% selectivity to GD) were signicant variation in selectivity to GD. Thus the selectivity to
observed with 6 mol% DABCO–DMC IL as the catalyst. GD was in the range of 45–50% with DABCO catalyst while, that
This is one of the best results for the selective synthesis of for DABCO–DMC IL as a catalyst was in a range of 61–66%.
GD at lower temperature starting from GLY and DMC as reac-
tants. Also, results clearly show the difference in activity of
DABCO and DABCO–DMC IL. Results indicate that the role of
catalyst may not be only as a strong base, but combined effect of
basicity and interaction of cation and anion from the DABCO–
DMC IL. Previous literature reports also indicate similar
behaviour,12 however, in this work we could show the effect by
comparison of the results with DABCO and DABCO–DMC IL as
catalysts.
3.3. Effect of temperature
Effect of temperature on the conversion of GLY was studied at
ꢀ
ꢀ
ꢀ
three different temperatures (70 C, 80 C and 100 C [oil bath
temperature]) using DABCO–DMC IL as the catalyst using
standard reaction conditions and the results obtained are pre-
sented in Fig. 8(a)–(c) respectively. The conversion was found to
increase signicantly with increase in temperature from 70 to
100 ꢀC. From the gures it can be clearly seen that the catalytic
activity increases with increase in reaction temperature
whereas, selectivity pattern was not signicantly affected by a
change in reaction temperature. The catalytic activity of DABCO
was very low (less than 5%) at 70 ꢀC and only 16% GLY
conversion was observed at 80 ꢀC aer 30 min reaction time and
hence conversion time proles are not presented. However,
comparison of the results observed with DABCO–DMC IL clearly
indicates lower activity with DABCO as a catalyst.
3.2. Effect of GLY to DMC molar ratio
The effect of GLY : DMC mole ratio on the GLY conversion and
selectivity for both DABCO as well as DABCO–DMC IL was
studied by varying GLY : DMC molar ratio in the range of 1 : 1,
1 : 2 and 1 : 3 and the results are presented in Fig. 7(a) and (b)
respectively.
GLY quantity of 21.73 mmol was used in the experiment
carried out with GLY : DMC ratio of 1 : 1. In other experiments,
the quantity of DMC was varied. From the results it was
observed that GLY conversion was low (47%) when DABCO was
used as a catalyst and 68% with DABCO–DMC IL as a catalyst
aer 30 min reaction time when the reaction was carried out at a
GLY : DMC ratio of 1 : 1. This is expected since this is an
equilibrium controlled reaction and excess DMC is necessary to
achieve high conversion. Thus GLY conversion increased with
increase in DMC concentration, and high conversion (97%) was
observed for both DABCO as well as DABCO–DMC IL catalysts at
GLY : DMC ratio of 1 : 3. However, the selectivity pattern was
not affected signicantly by the change in GLY : DMC ratio for
both the catalysts. But comparison of the selectivity pattern
observed for DABCO and DABCO–DMC IL as catalysts shows
4. Reaction mechanism
Based on the published reports,4a,9a and the results obtained in
this study, a plausible mechanism consisting of (1) DABCO–
DMC IL formation and (2) one pot synthesis of GD by the
reaction of DMC with GLY using DABCO–DMC ionic liquid as
the catalyst has been proposed (see Scheme 2). For the forma-
tion of ionic liquid DABCO functions as a nucleophilic catalyst
and reacts with DMC and generates an ion pair.9a The pH of
aqueous 0.035 molar solutions of both DABCO and DABCO–
DMC IL was measured at 17 ꢀC and was found to be 11 and 9.1
respectively. Thus DABCO–DMC IL is less basic than the parent
DABCO. The observed results clearly show that higher activity
and better selectivity towards GD observed with DABCO–DMC
Fig. 4 1H–15N HMBC NMR spectrum of ionic liquid.
32130 | RSC Adv., 2014, 4, 32127–32133
Fig. 5 1H–15N HMBC NMR spectrum of pure DABCO.
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Fig. 6 Conversion time profiles with DABCO and DABCO–DMC IL as catalysts. (a): Catalyst (DABCO). (b): Catalyst (DABCO–DMC IL). Reaction
conditions: GLY: 2 g, 21.73 mmol, DMC: 5.87 g, 65.19 mmol, temp: reflux (100 ꢀC oil bath temperature) catalyst: 2 mol% with respect to GLY.
IL as a catalyst are not only because of the basicity of IL, and the catalyst, glyceroxide attacks the activated carbonyl carbon
there must be role of both cations and anions in the formation group, and glyceryl methyl carbonate (intermediate) is obtained
of GD as a product. There are few reports on the use of ILs as as the product (see structure A in Scheme 2).
catalysts where both anion and cation help in activation of
Glyceryl methyl carbonate undergoes similar reaction
substrates.12,13 In light of the literature reports a plausible sequence followed by cyclization leading to the formation of GC
mechanism involving the role of both cation and anion in one as the nal product. With this catalyst completes trans-
pot synthesis of GD from GLY and DMC is shown in Scheme 2. esterication cycle and enters the catalytic cycle for decarbox-
The nitrogen–oxygen interaction favours the co-ordination of ylation of GC formed as the product. In this case cationic centre
the N center of IL to the basic carbonyl oxygen of DMC resulting of ionic liquid can interact with oxygen of the carbonyl group of
in DMC activation. Nucleophilic activation can take place by the GC making the bond weaker. Further interaction of anionic
proton exchange reaction between anion of the ionic liquid and centre of IL with more substituted SP3 hybridized alkylene
GLY leading to (GLY activation) the formation of glyceroxide. carbon of GC can lead to decarboxylation and formation of GD
Once both the electrophile and the nucleophile are triggered by as the product. The proposed mechanism is speculative and
Table 1 Effect of catalyst loading. Reaction conditions: GLY: 2 g, 21.73 mmol, DMC: 5.87 g, 65.19 mmol, temp: reflux (100 ꢀC oil bath
temperature) time: 30 min
DABCO
DABCO–DMC IL
Cat mol%
GLY conv.%
GD sel%
GC sel%
Cat mol%
GLY conv.%
GD sel%
GC sel%
0.5
1
2
4
6
71
79
97
96
97
10
17
45
68
78
90
83
53
32
22
0.5
1
2
4
6
96
96
97
97
97
55
65
66
78
83
44
35
34
19
17
Fig. 7 Effect of GLY to DMC molar ratio. (a): Catalyst (DABCO). (b): Catalyst (DABCO–DMC IL). Reaction conditions: GLY: 2 g, 21.73 mmol, temp:
reflux (100 ꢀC oil bath temperature) catalyst: 2 mol% with respect to GLY, time: 30 min.
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Fig. 8 Effect of reaction temperature using DABCO–DMC IL as catalyst: (a): temp: 70 ꢀC. (b): temp: 80 ꢀC. (c): temp: reflux (100 ꢀC oil bath temp).
Reaction conditions: GLY: 2 g, 21.73 mmol, DMC: 5.87 g, 65.19 mmol, catalyst: 2 mol% with respect to GLY.
Scheme 2 Proposed reaction mechanism.
further work is necessary to understand the mechanism of the was active and selective catalyst for the synthesis of GD as the
reaction.
product compared to DABCO. Observed results indicated that
basicity may not be the only criteria in deciding activity and
selectivity of the reaction. Best results (97% conversion of GLY
with 83% selectivity to GD and 17% selectivity to GC) were
obtained using DABCO–DMC IL as the catalyst. A plausible
mechanism has been proposed based on the results obtained.
5. Conclusions
Selective synthesis of GD from GLY and DMC was investigated
using DABCO–DMC IL as the catalyst. DABCO–DMC IL was
synthesized and characterized by NMR analysis. It was observed
that DABCO was the major component aer 20 min reaction
time, while aer 5 h DABCO was completely converted to
DABCO–DMC IL. Basicity of DABCO–DMC IL was found to be
6. Experimental
6.1. Materials
lower than DABCO. Effect of different reaction conditions on Glycerol, glycidol, glycerol carbonate, and 1,4-diazabicyclo
the activity and selectivity clearly showed that DABCO–DMC IL [2.2.2] octane (DABCO) were purchased from Sigma Aldrich.
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Dimethyl carbonate, dimethyl formamide were purchased from
Spectrochem, India, and methanol was purchased from Merck,
India. All the chemicals were used as received from suppliers.
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Acknowledgements
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 NMR analysis by Dr P. R. Raj-
mohanan is greatly appreciated.
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