Catalytic performance of polymer-supported ionic liquids in the synthesis of glycerol carbonate from glycerol and urea

Article abstract of DOI:10.1007/s11164-011-0398-4

Ionic liquids (ILs) immobilized onto a structurally modified Merrifield peptide resin (MPR) were prepared through the reaction of imidazole with alkoxylated MPR. Elemental analysis and scanning electron microscopy (SEM) observations showed that the immobilized IL groups were effectively incorporated on the support. The MPR-supported ILs (MPR-ILs) proved to be effective heterogeneous catalysts for the synthesis of glycerol carbonate (GC) from glycerol and urea. MPR-ILs with longer alkyl chain linkers and fewer sterically hindered counteranions resulted in higher conversion of glycerol. High temperature, high degree of vacuum and the presence of zinc oxide co-catalyst were favorable for glycerol conversion. The MPR-ILs could be reused in up to three consecutive runs without significant loss of their catalytic activity.

Full text of DOI:10.1007/s11164-011-0398-4

Res Chem Intermed (2011) 37:1305–1312
DOI 10.1007/s11164-011-0398-4
Catalytic performance of polymer-supported ionic
liquids in the synthesis of glycerol carbonate
from glycerol and urea
•
Dong-Woo Kim Moon-Seok Park
•
•
•
•
Manickam Selvaraj Gyung-Ah Park Sun-Do Lee
Dae-Won Park
Received: 26 May 2011 / Accepted: 18 July 2011 / Published online: 10 September 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Ionic liquids (ILs) immobilized onto a structurally modified Merrifield
peptide resin (MPR) were prepared through the reaction of imidazole with alk-
oxylated MPR. Elemental analysis and scanning electron microscopy (SEM)
observations showed that the immobilized IL groups were effectively incorporated
on the support. The MPR-supported ILs (MPR-ILs) proved to be effective hetero-
geneous catalysts for the synthesis of glycerol carbonate (GC) from glycerol and
urea. MPR-ILs with longer alkyl chain linkers and fewer sterically hindered
counteranions resulted in higher conversion of glycerol. High temperature, high
degree of vacuum and the presence of zinc oxide co-catalyst were favorable for
glycerol conversion. The MPR-ILs could be reused in up to three consecutive runs
without significant loss of their catalytic activity.
Keywords Glycerol ꢀ Glycerol carbonate ꢀ Urea ꢀ Ionic liquid ꢀ Immobilization
Introduction
Glycerol is a by-product obtained during the production of biodiesel, which is
considered to be a renewable and clean fuel for diesel engines [1–3]. An increase in
the production of biodiesel has resulted in a surplus of glycerol, owing to a sharp
Electronic supplementary material The online version of this article (doi:
10.1007/s11164-011-0398-4) contains supplementary material, which is available to authorized users.
D.-W. Kim ꢀ M.-S. Park ꢀ M. Selvaraj ꢀ G.-A. Park ꢀ D.-W. Park (&)
Division of Chemical and Biomolecular Engineering, Pusan National University,
Busan 609-735, Korea
e-mail: dwpark@pusan.ac.kr
S.-D. Lee
Zeus Oil and Chemicals Co. Ltd., Busan 617-805, Korea
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D.-W. Kim et al.
decrease in its price. Glycerol is expected to become a major platform chemical, and it
has been recently identified as an important building block for future biorefineries [4].
Glycerol carbonate (GC), which can be synthesized from glycerol, is a new
and interesting material [5]. It has low toxicity, good biodegradability, and a high
boiling point. It has been investigated as a novel component of gas-separation
membranes, polyurethane foams [6], surfactants [7], coatings, paints, and
detergents, and as a nonvolatile reactive solvent for several types of materials.
GC could thus serve as an inexpensive source of new polymeric materials for the
production of polycarbonates and polyurethanes [8].
The main methods for the preparation of GC are based on the reaction of glycerol
with (1) a carbon source (phosgene, a dialkyl carbonate, or an alkylene carbonate),
(2) carbon monoxide and oxygen or carbon dioxide, or (3) urea. Traditionally, GC
has been prepared by the reaction of glycol with phosgene. However, because of the
high toxicity and corrosive nature of phosgene, alternative methods for the
preparation of GC, such as the transesterification of dialkyl or alkylene carbonates
with glycerol have been explored [9].
A method based on the reaction of glycerol with carbon monoxide and oxygen at a high
pressure in the presence of a catalyst is also known [10]. However, a more cost-effective
and simpler method involving the use of a highly safe material is desired. Aresta et al. [11]
investigated the direct carboxylation of glycerol with carbon dioxide (5 MPa) at 177 °C
using transition-metal alkoxides. However, the yield of GC by this method is very low.
An alternative method is the glycerolysis of urea, a reaction that has been recently
described in the literature [12–14]. The main advantage of this method is that the
reactant, urea, is readily available and inexpensive. In addition, the ammonia formed
can be easily converted to urea, in the presence of carbon dioxide. Several catalysts
have been used, mainly based on metal oxides of variable basicity [12–14]; however,
these catalysts suffer from the drawback of being difficult to separate from the
reaction mixture, because they dissolve in or are converted into micropowders.
Ionic liquids (ILs) are well known to be environmentally benign media for
catalytic processes or chemical extraction [15]. ILs have negligible vapor pressure,
excellent thermal stability, and special characteristics as opposed to conventional
organic and inorganic solvents. In the past decade, ILs containing imidazolium and
its counteranions were widely used in many research fields such as alternative
reaction media, separation science, and material synthesis [15, 16]. Many reactions
catalyzed with ILs and showing high performance have been reported [17–20].
In previous work [21–27], we studied the performance of ILs for the synthesis of cyclic
carbonates from epoxides and carbon dioxide. Recently, IL-based heterogeneous catalysts
have been developed to synthesize cyclic carbonates, and they were found to have the
advantages of both homogeneous ILs and heterogeneous catalysts. When compared to
pure ILs, such heterogeneous catalysts show additional advantages such as reduction in the
amount of ILs employed, easy separation, and recovery of catalyst from the reaction
mixture.
In this study, structurally modified polystyrene-based Merrifield peptide resin-
supported ILs (MPR-ILs) were prepared, and their catalytic performance in the
synthesis of GC from glycerol and urea was investigated. The effect of the length of
the alkyl chain linker, counteranion, and IL loading in the resin on the reactivity of
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Catalytic performance of polymer-supported ILs
1307
the immobilized IL catalysts was examined. In order to have a better understanding
of the reaction mechanism, the effects of the zinc oxide co-catalyst, degree of
vacuum, and reaction temperature were studied. In addition, the MPR-IL was
subjected to a recycle test in order to examine the stability of this catalyst system.
Experimental
Preparation of MPR-ILs
Merrifield peptide resin-supported ILs containing alkyl chain linkers with different
lengths and different counteranions such as MPR[pmim][X], MPR[hmim][X], and
MPR[nmim][X] (pmim = 1-n-propyl-3-methylimidazolium, hmim = 1-n-hexyl-3-
methylimidazolium, and nmim = 1-n-nonyl-3-methylimidazolium) were prepared
by adopting a previously reported method [12]. Alkyl chain linkers of different
lengths were introduced by carrying out an alkoxylation reaction of Merrifield resin
(1% divinylbenzene, 4.5 mmol Cl/g; Aldrich) with 3-bromo-1-propanol, 6-bromo-1-
hexanol, 9-bromo-1-nonanol, and 6-chloro-1-hexanol in the presence of NaH in THF
to afford resins 1a, 1b, 1c, and 1d, respectively. MPR[pmim][Br], MPR[hmim][Br],
MPR[nmim][Br], and MPR[hmim][Cl] were prepared by the direct reaction of
resins 1a–d with 1-methylimidazole at 90 °C for 72 h. Further treatment of
MPR[hmim][Cl] with NaBF4 in acetone for 48 h afforded MPR[hmim][BF4].
The MPR-ILs were characterized by elemental analyzer (EA; Varioel III), 13C
NMR (Unity Inova; 400 MHz WB spectrometer) and FT-IR (Bruker 960981A).
Scanning electron microscopy (SEM; Hitachi S-4200) was used to investigate the
surface morphology of MPR-ILs.
Synthesis of glycerol carbonate
The synthesis of GC from the reaction of glycerol with urea shown in the
Supplementary Scheme S1 was carried out in a 50-mL autoclave reactor equipped
with a magnetic stirrer. For each typical reaction, the MPR[amim][X] catalyst, glycerol
(50 mmol), and urea (50 mmol) were charged into the reactor. When the desired
temperature was attained, the reaction was initiated by stirring under vacuum or under
nitrogen purging. Analysis of the products and reactants was performed by using a gas
chromatograph (HP 6890N) equipped with a FID and a capillary column (HP-5, 5%
phenyl methyl siloxane). Tetraethylene glycol (TEG) was used as an internal standard.
The selectivity to GC was calculated on the basis of glycerol as a limited reactant.
Results and discussion
Characterization of the immobilized ILs
Elemental analysis data of different MPR[amim][X] (amim = 1-n-alkyl-3-methy-
limidazolium) are listed in Table 1. The amount of the attached IL was in the range
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D.-W. Kim et al.
1.8–3.4 mmol/g, indicating that the imidazole was effectively immobilized onto the
Merrifield resin through the alkyl chain linker. The amount of the attached IL
increased when the alkyl chain linker was changed from propyl (MPR[pmim][Br])
to hexyl (MPR[hmim][Br]) but decreased when nonyl (MPR[nmim][Br]) was used,
probably because of the steric hindrance caused by the long alkyl chain.
The FT-IR spectra of the Merrifield resin and MPR[hmim][Cl] are shown in
Supplementary Fig. S1. The peak due to the chloromethyl group in the Merrifield
resin at 1,260 cm-1 almost disappeared in MPR[hmim][Cl]. On the other hand, the
broad peaks at 2,400–3,500 cm-1 indicated that the imidazolium group was
attached to the Merrifield resin. Supplementary Fig. S2 shows the 13C CP-MAS
NMR spectra [d = 28–31 (d–h), 38 (m), 42 (a), 52 (i), 74 (c), 131 (b, b0, j, k), 139
(l), 148 (b, b0)]. The SEM image in Supplementary Fig. S3a shows the spherical
shape of MPR[hmim][Cl]. The EPMA data (Supplementary Fig. S3b) confirmed the
immobilization of [hmim][Cl] on the MPR.
Reactivity test of the immobilized IL catalyst
Table 2 shows the time-variant conversion of glycerol and the selectivity to GC for
MPR[hmim][Br] at 140 °C under a vacuum of 14.7 kPa. The conversion increased
continuously up to 6 h; however, the selectivity to GC showed maximum at 3 h and
decreased slightly at a longer reaction time, probably because of the decomposition
of GC.
Table 3 shows the conversion of glycerol, selectivity to GC, and turnover number
(TON) of GC for MPR-ILs of different alkyl chain linkers and counteranions under
a vacuum of 14.7 kPa after 3 h of reaction at 140 °C. When the alkyl chain linker
was changed from propyl (MPR[pmim][Br]) to nonyl (MPR[nmim][Br]), the TON
of GC increased, mainly for the following reason. The electron-donating ability of
the nonyl group was stronger than that of the propyl group, and, hence, the electron
density was concentrated around the center of the active site in nonyl
(MPR[nmim][Br]). Increasing the bulkiness of the alkyl chain forces the Br- ion
away from the cation more easily, thereby resulting in less electrostatic interaction
between the anion and the cation [28, 29]. These results are in good agreement with
previous reports on the effects of the alkyl chain length [28–31].
Table 1 Elemental analysis results of MPR[amim][X]
Catalyst
CHN from elemental analysis
Amount of
imidazole (mmol/g)
N (wt%)
C (wt%)
H (wt%)
MPR[pmim][Br]
MPR[hmim][Br]
MPR[nmim][Br]
MPR[hmim][Cl]
MPR[pmim][Cl]
9.5
5.9
7.0
5.6
7.5
64.0
72.9
68.9
69.5
69.2
5.5
7.2
4.1
4.3
7.4
3.4
2.1
2.5
1.9
2.7
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Catalytic performance of polymer-supported ILs
1309
Table 2 Effects of reaction
Reaction time (h)
Conv. (%)
Sel. (%)
Yield (%)
time on the synthesis of GC
1
25.1
28.5
43.0
50.3
59.1
63.6
57.6
61.4
81.7
77.3
76.0
72.8
14.4
17.5
35.2
38.9
45.0
46.3
2
Reaction conditions:
urea = 50 mmol,
3
4
5
6
glycerol = 50 mmol,
cat. = MPR[pmim][Br] 0.23 g,
temp. = 140 °C,
press. = 14.7 kPa
Table 3 Effects of ionic liquid structure on the synthesis of GC
Catalyst
Conv. (%)
Sel. (%)
Yield (%)
TONa
MPR[nmim][Br]
MPR[hmim][Br]
MPR[pmim][Br]
MPR[pmim][Cl]
43.5
39.6
43.0
45.6
99.0
81.2
81.7
87.8
43.0
33.7
35.2
40.0
37.4
34.9
22.5
45.8
Reaction conditions: urea = 50 mmol, glycerol = 50 mmol, cat. = MPR[amim][X] 0.23 g, temp. =
140 °C, press. = 14.7 kPa, reaction time = 3 h
a
TON = moles of GC produced/moles of IL
By comparing the activities of the counteranions, it was found that the TON of
GC for MPR-IL wit Cl- was higher than Br-. The effect of the counteranion on the
reactivity is due to the combination of nucleophilicity and steric hindrance, where
the Br- counterion has both higher steric hindrance and higher nucleophilicity than
Cl- [32, 33]. The lower activity of Br- when compared to that of Cl- suggests that
the steric hindrance of MPR[amim][X] plays a crucial role in the synthesis of GC.
Table 4 shows the effects of reaction temperature on the synthesis of GC with
MPR[hmim][Br] as the catalyst. The rate of conversion of glycerol increased as
the temperature increased from 80 to 180 °C. Selectivity to GC increased with
temperature, reaching a maximum at 140 °C and decreased at temperatures higher
than 150 °C, probably because of the decomposition of GC to glycerol or the
polymerization of GC and/or glycerol. The formation of polymers from GC or
glycerol is considered the main factor responsible for the low selectivity to GC.
Table 5 shows the effects of the degree of vacuum on the reactivity of the
immobilized catalyst, MPR[hmim][Br]. The conversion of glycerol increased as
the degree of vacuum increased from 101.3 to 1.33 kPa. A high degree of vacuum
could enhance the removal of the formed ammonia gas, thereby resulting in
the acceleration of the forward reaction of glycerol and urea. The selectivity to GC
increased continuously when the degree of vacuum increased from 101.3 to
1.33 kPa, since a high degree of vacuum may inhibit the backward decomposition
of GC to glycerol and the polymerization of GC and/or glycerol. It is also interesting
to note that nitrogen purging of ammonia is as effective as operation under a high
degree of vacuum.
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D.-W. Kim et al.
Yield (%)
Table 4 Effects of reaction
temperature on the synthesis
of GC
Temp. (°C)
Conv. (%)
Sel. (%)
100
120
130
140
150
160
180
11.5
25.6
29.6
43.0
53.4
57.8
62.1
45.7
40.7
65.9
81.7
70.5
61.2
0
5.3
10.4
19.5
35.2
37.6
35.4
0
Reaction conditions:
urea = 50 mmol,
glycerol = 50 mmol,
cat. = MPR[pmim][Br] 0.23 g,
press. = 14.7 kPa, reaction
time = 3 h
Table 5 Effects of reaction
pressure on the synthesis of GC
Press. (kPa)
Conv. (%)
Sel. (%)
Yield (%)
101.3
53.2
21.2
24.2
44.4
43.0
51.4
44.8
47.9
70.3
48.2
81.7
92.3
96.0
10.2
17.0
21.4
35.2
47.8
43.0
Reaction conditions:
urea = 50 mmol,
26.7
14.7
glycerol = 50 mmol,
cat. = MPR[pmim][Br] 0.23 g,
temp. = 140 °C, reaction
time = 3 h
1.3
N2 purge
The effects of the co-catalyst on the synthesis of GC with MPR[hmim][Cl] as
the catalyst were also studied, and the results are listed in Table 6. With 0.23 g of
MPR[hmim][Cl], the conversion of glycerol and the selectivity to GC were 52.5 and
80.2%, respectively. With a combination of zinc oxide with MPR[hmim][Cl], the
conversion of glycerol and selectivity to GC were much higher than those observed
when using MPR[hmim][Cl] alone. However, further increasing the amount of zinc
oxide did not increase the yield of GC; rather, it decreased the selectivity to GC.
The catalytic activity of zinc oxide for the glycerolysis of urea was also reported by
Climent et al. [34]. Its catalytic performance was explained by its balanced acid–
base properties. Although strong Lewis acid and base sites mainly promote
polymerization reactions, a cooperative effect between weak Lewis acid metal sites
that can activate the carbonyl group of urea and the Lewis base sites that can
activate glycerol could be responsible for the high catalytic performance of Al/Zn
oxides [34].
Recycling experiments were carried out to investigate the stability of the
immobilized IL catalyst. In each cycle, the used catalyst was separated by filtration,
washed with methanol to remove the products adhering to the surface of catalyst,
dried at room temperature, and then reused directly for the next run without
regeneration. Table 7 shows the activity of the reused MPR[hmim][Cl] catalyst.
The conversion of glycerol decreased slightly; however, the selectivity to GC was
almost the same as before. This means that the catalyst can be reused at least up to
three consecutive times without any considerable loss of its activity.
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Catalytic performance of polymer-supported ILs
1311
Table 6 Effects of ZnO co-catalyst on the synthesis of GC
Catalyst
Conv. (%)
Sel. (%)
Yield (%)
0.115 g MPR[pmim][Cl]
46.7
52.5
74.8
78.3
77.7
77.7
80.4
80.2
37.5
42.1
76.8
73.9
78.6
69.9
0.23 g MPR[pmim][Cl]
0.115 g MPR[pmim][Cl] ? 0.115 g ZnO
0.115 g MPR[pmim][Cl] ? 0.23 g ZnO
0.23 g MPR[pmim][Cl] ? 0.115 g ZnO
0.23 g MPR[pmim][Cl] ? 0.23 g ZnO
102.6
94.4
101.2
89.9
Reaction conditions: urea = 50 mmol, glycerol = 50 mmol, cat. = MPR[pmim][Cl], co-cat. = ZnO,
temp. = 140 °C, press. = 14.7 kPa, reaction time = 3 h
Table 7 Recycle test of MPR[pmim][Cl] in the synthesis of GC
Run
Conv. (%)
Sel. (%)
Yield (%)
Fresh
1st
50.4
49.5
48.3
45.2
80.9
81.8
81.0
80.5
40.8
40.5
39.1
36.4
2nd
3rd
Reaction conditions: urea = 50 mmol, glycerol = 50 mmol, cat. = MPR[pmim][Cl] 0.15 g, temp. =
140 °C, press. = 14.7 kPa, reaction time = 3 h
Conclusions
Ionic liquids were successfully immobilized onto a structurally modified Merrifield
peptide resin (MPR) through the reaction of imidazole with alkoxylated MPR. In the
synthesis of GC from glycerol and urea, the immobilized IL on MPR showed good
catalytic activity even in the absence of any solvent. ILs with long alkyl chain
structures and fewer sterically hindered counteranions exhibited better reactivity for
the synthesis of GC. High temperature, long reaction time, and high degree of
vacuum were favorable for the high conversion of glycerol. The combination of zinc
oxide co-catalyst with the immobilized IL resulted in improved reactivity because
of the cooperative action of the Lewis acid sites of zinc oxide. MPR-supported ILs
can be easily recovered and reused without any considerable loss of their initial
activity.
Acknowledgments This study was supported by the Ministry of Education, Science and Technology
(2010-0015657) and the Small and Medium Business Administration of Korea (S1071254). M. Selvaraj
thanks the support of Pusan National University Grant (2008).
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