Organic-inorganic hybrids of imidazole complexes of zinc (II) for catalysts in the glycerolysis of urea

Article abstract of DOI:10.1166/jnn.2014.8237

Bis(alkylimidazole) complexes of zinc, (Rlm)₂ZnX₂, were prepared by a metal insertion reaction. The synthesized (Rlm) ₂ZnX₂ exhibited good catalytic performance during synthesis of glycerol carbonate (GC) from glycerol and urea. (HEIm) ₂ZnCI₂ with a hydroxyl group exhibited the highest GC yield during glycerolysis of urea owing to incorporation of acid-base bifunctional active sites. (Elm)₂ZnX₂ catalysts based on different halide anions showed increased reactivity as Cl^- < Br < l^-, which is the order of nucleophilicity The effects of reaction parameters such as temperature, reaction time, catalyst loading, and degree of vacuum on the reactivity were also investigated. Copyright

Full text of DOI:10.1166/jnn.2014.8237

Article
Journal of
Nanoscience and Nanotechnology
Vol. 14, 4632–4638, 2014
Copyright © 2014 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Organic–Inorganic Hybrids of Imidazole Complexes of
Zinc (II) for Catalysts in the Glycerolysis of Urea
∗
Dong-Woo Kim and Dae-Won Park
School of Chemical and Biomolecular Engineering, Pusan National University, Busan 609-735, Republic of Korea
Bis(alkylimidazole) complexes of zinc, (RIm) ZnX , were prepared by a metal insertion reaction. The
2
2
synthesized (RIm) ZnX exhibited good catalytic performance during synthesis of glycerol carbonate
2
2
(
GC) from glycerol and urea. (HEIm) ZnCl with a hydroxyl group exhibited the highest GC yield
2 2
during glycerolysis of urea owing to incorporation of acid-base bifunctional active sites. (EIm) ZnX
2
2
−
−
−
catalysts based on different halide anions showed increased reactivity as Cl < Br < I , which is
the order of nucleophilicity. The effects of reaction parameters such as temperature, reaction time,
catalyst loading, and degree of vacuum on the reactivity were also investigated.
Keywords: Imidazole, Complex, Zinc, Glycerol Carbonate, Glycerol, Urea.
1
. INTRODUCTION
been prepared by the reaction of glycol with phosgene;
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Cyclic carbonates are useful sol IvPe :n t1s 1 a7 n. 2d 4 c4h . e1 m7 .i 2c a3 l8i nOt en r :- Tue h, o1 w3 e Ov ecr t, 2b e0 c1 a 5u s1e 6 o: 1f 8t h: 2e 2high toxicity and corrosive nature
Copyright: American Scientific Publishers
mediates because they have low toxicity and biodegrad-
ability and a high boiling point. These compounds have
many applications as processing agents for the production
of polyacrylonitrile fibers, diluents for polyurethanes and
epoxy resins, accelerants in dyeing and printing, additives
in fuel, lubricants and hydraulic fluids, separation of car-
bon dioxide and hydrogen sulfide, and components of elec-
trolytes used in lithium-ion rechargeable batteries, metal
of phosgene, alternative methods for the preparation of
GC such as the transesterification of dialkyl or alkylene
6
carbonates with glycerol have been explored. A method
based on the reaction of glycerol with carbon monoxide
and oxygen under high pressure in the presence of a cat-
7
alyst is also known. However, a more cost-effective and
simpler method involving the use of a highly safe material
8
is desired. Aresta et al. investigated the direct carboxy-
1
ꢀ2
ꢀ
extractions, etc.
lation of glycerol with carbon dioxide (5 MPa) at 177 C
Glycerol carbonate (4-hydroxymethyl-1,3-dioxolan-2-
one; GC) is a cyclic carbonate relatively new to the
chemical industry that has been receiving increased
interest owing to its large potential. Inexpensive glycerol
carbonate could serve as a source of new polymeric mate-
rials such as glycidol, a high-value component in the pro-
using transition-metal alkoxides. However, the yield of GC
by this method was found to be very low.
An alternative method is the carbonylation of urea,
9
–11
a reaction that was recently described in the literature.
The main advantage of this method is that the reactants,
glycerol and urea, are inexpensive and easily available raw
materials that are not explosive or poisonous. In addition,
the ammonia that is usually generated when GC is syn-
thesized from urea and glycerol can be easily converted to
urea in the presence of carbon dioxide.
3
ꢀ4
duction of a number of polymers. Its high functionality,
together with the versatile and well-investigated reactiv-
ity of its hydroxyl functions, is the basis for a variety of
derivatives. Indeed, many polyglycerols have been com-
mercialized for applications that range from cosmetics to
The reaction of urea with alkylene glycol was first
5
1
2
controlled drug release.
reported by Su and Speranza; however, the decomposi-
1
3
The main methods for the preparation of GC are based
on the reaction of glycerol with (a) phosgene, a dialkyl car-
bonate, or an alkylene carbonate, (b) carbon monoxide and
oxygen or carbon dioxide, or (c) urea. GC has traditionally
tion of urea was severe. Doya et al. increased the yield of
products by using zinc, magnesium and lead, or their com-
14
pounds as catalysts under vacuum. Ball et al. investigated
the reaction of primary and secondary alcohols with urea
to form carbonate and found that the reaction proceeded
in two steps. Moreover, the combination of a weak Lewis
∗
Author to whom correspondence should be addressed.
4
632
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 6
1533-4880/2014/14/4632/007
doi:10.1166/jnn.2014.8237

Kim and Park
Organic–Inorganic Hybrids of Imidazole Complexes of Zinc (II) for Catalysts in the Glycerolysis of Urea
O
acid with a Lewis base improved the carbonate formation.
1
5
O
Corma et al. reported that a balanced bifunctional acid-
base catalyst in which the Lewis acid activates the car-
bonyl of the urea and the conjugated basic site activates
the hydroxyl group of the glycerol were the most active
and selective catalysts for the synthesis of GC from urea
and glycerol.
HO
OH
O
O
2NH3
H N
NH2
OH
2
HO
Glycerol Carbonate
Glycerol
Urea
Scheme 1. Synthesis of glycerol carbonate.
Ionic liquids are well known as environmentally benign
media for catalytic processes or chemical extraction
because of their outstanding properties, which include neg-
ligible vapor pressure, excellent thermal stability, and spe-
cial characteristics as opposed to conventional organic and
ꢀ
1
(
1
00 C for 24 h under vacuum. The products were named
Im) ZnCl (ZnCl , N-imidazole), (MIm) ZnCl (ZnCl ,
2 2 2 2 2 2
-methylimidazole), (EIm) ZnCl (ZnCl , 1-ethylimida-
zole), (BIm) ZnCl (ZnCl , 1-butylimidazole), (HEIm)
2 2 2
2
2
2
2
ZnCl (ZnCl , 1-(2-hydroxyethyl)imidazole), (EIm) ZnBr
1
6–24
2
2
2
2
inorganic solvents.
The ionic liquids have recently
(
1
ZnBr2, 1-ethylimidazole) and (EIm) ZnI
-ethylimidazole).
(ZnI2,
2
2
been successfully applied as immobilizing reagents for
transition metal catalysts, combining the advantages of
both homogeneous and heterogeneous catalysis and lead-
ing to improved process performance.25–27 With the
2.3. Characterization of (RIm)
The H nuclear magnetic resonance (NMR) spectra were
obtained on a Varian 500 MHz. Fourier transform infrared
2
ZnX Catalyst
2
1
purpose of turning the acidic character, ionic liquids con-
2
8–30
taining metals are recently receiving greater attention.
(FT-IR) spectra were obtained on an AVATAR 370 Thermo
Beyond their application as mere reaction media, the metal
containing ionic liquid possessing a weak base as anion
have been successfully used as catalysts themselves, e.g.,
−1
Nicolet spectrophotometer with a resolution of 4 cm .
X-ray photoelectron spectroscopy (XPS) of the catalysts
was conducted with the Theta Probe AR-XPS System
X-ray source using monochromated Al Kꢂ radiation (hv =
3
1ꢀ32
in Friedel-Crafts alkylation.
In this study, imidazolium based zinc containing ionic
1486.6 eV).
liquid ((RIm) ZnX ꢁ, which is a bifunctional acid-base
2
2
catalyst in which the zinc ions serve as the acid center
and the anion as the base, was prepared by metal insertion
2
.4. Synthesis of Glycerol Carbonate from
Delivered by Publishing Technology to: Florida State University, College of Medicine
reaction and the catalytic performance of (RIm) ZnX for
Glycerol and Urea
IP: 117.244.17.238 On: Tue, 13 Oct 2015 16:18:22
2
2
the synthesis of glycerol carbonate from glycerol and urea
The synthesis of GC from the reaction of glycerol with
urea was carried out in a 50 mL glass reactor equipped
with a magnetic stirrer and condenser (Scheme 1).
For each typical reaction, (RIm) ZnX catalyst, glycerol
Copyright: American Scientific Publishers
by carbonylation under solvent free conditions was inves-
tigated. The effects of reaction time, reaction temperature,
degree of vacuum and amount of catalyst were discussed
to increase our understanding of the reaction mechanism
along with the morphological effects of (RIm) ZnX .
2
2
(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 to remove ammonia as a by-product.
Analysis of the products and reactants was performed
using a gas chromatograph (HP 6890N) equipped with an
FID and a capillary column (HP-INNOWAX, polyethylene
glycol). Tetraethylene glycol (TEG) was used as an inter-
nal standard. The conversion and selectivity were calcu-
lated based on the assumption that glycerol was a limited
reactant.
2
2
2
2
1
. MATERIALS AND METHODS
.1. Materials
-alkylimidazoles (N-imidazole, 1-methyl, 1-ethyl, 1-butyl
and 1-(2-hydroxyerthyl) imidazole) and metal halides (zinc
chloride, zinc bromide, zinc iodide, magnesium chloride
and copper chloride) were purchased from Sigma-Aldrich
with high purity (>99%). Glycerol (>99%), urea (>
9
9%), glycerol carbonate (>99%), and methanol (>99%)
were obtained from Sigma-Aldrich. All materials were
used without further purification.
3. RESULTS AND DISCUSSION
3.1. Characterization of Catalysts
2
.2. Synthesis of (RIm) ZnX Catalyst
The NMR data of the (RIm) ZnX samples are as follows:
2
2
2
2
The zinc containing ionic liquid catalyst based on
-alkylimidazole, (RIm) ZnX , was prepared by a metal
insertion reaction as follows. For the synthesis of
Bis(N-imidazolium)zinc
chloride
((Im) ZnCl ).
2 2
1
1
H-NMR (400 MHz, DMSO-d ꢁ: ꢃ = 7ꢄ25 (s, CH, 1 H),
2
2
6
2
7
8.14 (s, CH, 1 H), 13.45 (s, NH, 1 H). Bis(1-methyli-
1
(
RIm) ZnX , an ethanol solution (100 mL) containing
midazolium)zinc chloride ((MIm) ZnCl ). H-NMR
2
2
2
2
1
-alkylimidazole (40 mmol) was added to an ethanol
(400 MHz, DMSO-d ꢁ: ꢃ = 3ꢄ74 (t, NCH , 3 H), 7.03
6
3
solution containing zinc halide (20 mmol). This mixture
was stirred for 2 h at 50 C, after which the mixture
was filtered. A crystal solid was obtained after drying at
(s, CH, 1 H), 7.37 (s, CH, 1 H), 8.06 (s, CH, 1 H).
ꢀ
Bis(1-ethylimidazolium)zinc chloride ((EIm) ZnCl ).
2
2
1
H-NMR (400 MHz, DMSO-d ꢁ: ꢃv = 1ꢄ36 (t, CH , 3 H),
6
3
J. Nanosci. Nanotechnol. 14, 4632–4638, 2014
4633

Organic–Inorganic Hybrids of Imidazole Complexes of Zinc (II) for Catalysts in the Glycerolysis of Urea
Kim and Park
4
1
.10 (t, NCH , 3 H), 7.06 (s, CH, 1 H), 7.48 (s, CH,
3
(
(
Im) ZnCl
2 2
Nquaternery
H), 8.14 (s, CH, 1 H). Bis(1-bytulimidazolium)zinc
EIm) ZnCl
Ntertiary
2
2
1
chloride ((BIm) ZnCl ). H-NMR (400 MHz, DMSO-
(HEIm) ZnCl
2
2
2
2
d ꢁ: ꢃ = 0ꢄ87 (t, CH , 3 H), 1.23 (m, CH , 2 H), 1.72
6
3
2
(
m, CH , 2 H), 4.08 (t, NCH , 3 H), 7.06 (s, CH,
2
3
1
H), 7.46 (s, CH, 1 H), 8.14 (s, CH, 1 H). Bis[1-
(
2-hydroxyethyl)imidazolium]zinc chloride ((HEIm)2
1
ZnCl ). H-NMR (400 MHz, DMSO-d ꢁ: ꢃ = 3ꢄ68
2
6
(t, CH , 3 H), 4.12 (t, NCH , 3 H), 5.03 (s, OH, 1 H),
3
3
7
.05 (s, CH, 1 H), 7.42 (s, CH, 1 H), 8.08 (s, CH, 1 H).
Bis(1-ethylimidazolium)zinc bromide ((EIm) ZnBr ).
2
2
1
H-NMR (400 MHz, DMSO-d ꢁ: ꢃ = 1ꢄ36 (t, CH , 3 H),
6
3
4
8
(
.12 (t, NCH , 3 H), 7.07 (s, CH, 1 H), 7.51 (s, CH, 1 H),
3
404
402
400
398
396
.18 (s, CH, 1 H). Bis(1-ehtylimidazolium)zinc iodide
1
Binding Energy / eV
Figure 2. XPS N 1s spectra of (RIm) ZnCl .
(EIm) ZnI ). H-NMR (400 MHz, DMSO-d ꢁ: ꢃ = 1ꢄ38
2
2
6
(
t, CH , 3 H), 4.15 (t, NCH , 3 H), 7.15 (s, CH, 1 H),
3
3
2
2
7
.58 (s, CH, 1 H), 8.26 (s, CH, 1 H).
The FT-IR spectral analyses of the (RIm) ZnCl
the other peak can be regarded as a signal from the posi-
tively charged nitrogen atom of the quaternary ammonium
2
2
2
samples, (Im) ZnCl , (EIm) ZnCl and (HEIm) ZnCl
2
2
2
2
2
3
4–37
(
Fig. 1), confirm that different alkyl groups were attached
moiety.
Moreover, the Cl 2p spectra of (RIm) ZnCl
2 2
to the ring nitrogen of the (RIm) ZnCl . Clearly, the
were measured to identify the bonding aspect of the halo-
gen associated with the zinc species (Fig. 3). Niedermaier
et al. reported that the Cl 2p peak at 200.2 eV is of cova-
2
2
−
1
–
C
H stretching frequency near 3110 cm and many of
−1
37
the bands observed from 1600 to 600 cm are associated
with various vibrational modes of the ring. Differences
among the alkyl groups of the (RIm) ZnCl are noted in
3
3
lently bound chlorine (Clcovꢁ, while 197.2 eV corresponds
2
2
to ionic chlorine (Cl ꢁ. The XPS data showed only one
ion
−
1
the region from 2885–2980, 3320 and 3425 cm , cor-
peak near 197.5 eV in (RIm) ZnCl , confirming that the
2
2
responding to the –C H stretching of the ethyl group
bonds between chlorine and zinc are not covalent, but ionic
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of (EIm) ZnCl , –N H stretchi In Pg : o1 f1 t7h . e2 4( I4m. 1) 7 Z. 2n 3C 8l Oa nn d: Tue i, n1 n3 a tOu rc et . 2T 0h 1e 5a f1 o 6r :e 1m 8 e: n2 t2i oned results confirm the structure
2
2
2
2
–
OH stretching of the (HEIm) ZnCl ,C r oe sp py er ci gt i hv te :l yA .merican S co i fe n( Rt i If imc )P Zu nb Xl ishc ea rt sa lysts as shown in Scheme 2.
2
2
2
2
The XPS measurement of (RIm) ZnCl allows to con-
2
2
firm the structure of the catalysts. The N 1s spectra of
RIm) ZnCl are depicted in Figure 2. The (RIm) ZnCl
(
2
2
2
2
catalysts contain two nitrogen species that originate from
the two imidazolium nitrogens near 399.5 and 401.0 eV.
The characteristic peak of nitrogen species from the amine
portion appearing around 399.5 eV could be attributed to
the imidazolium nitrogen attached to the alkyl group and
(Im) ZnCl
Clion
2
2
(
(
EIm) ZnCl
2 2
HEIm) ZnCl
2
2
(
a)
(
b)
(
c)
2885
2980
204
202
200
198
196
194
3320
Binding Energy / eV
Figure 3. XPS Cl 2p spectra of (RIm) ZnCl .
2
2
3425
1327
1535
3110
R
EtOH
R
X
N
N
N
ZnX2
N
Zn
N
N
1095
2
R
5
0 ºC, 2 h
X
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm–1
)
–R: methyl, ethyl, butyl, 2-hydroxyethyl
X: Cl, Br, I
–
Figure 1. FT-IR spectra of (RIm) ZnCl : (a) (Im) ZnCl ,
2
2
2
2
(
b) (EIm) ZnCl , (c) (HEIm) ZnCl .
Scheme 2. Preparation of (RIm) ZnX .
2 2
2
2
2
2
4
634
J. Nanosci. Nanotechnol. 14, 4632–4638, 2014

Kim and Park
Organic–Inorganic Hybrids of Imidazole Complexes of Zinc (II) for Catalysts in the Glycerolysis of Urea
Table I. The reactivity of the metal containing ionic liquids
RIm) ZnX in the glycerolysis of urea.
Table III. The effect of catalyst loading in the glycerolysis of urea.
(
2
2
Amount of catalyst (mmol)
.1
XG (%)
SGC (%)
YGC (%)
Catalyst
XG (%)
SGC (%)
YGC (%)
0
67.2
64.1
80.9
83.6
87.2
87.9
61.2
73.6
74.2
85.5
88.7
89.3
41.1
47.1
60.1
71.4
77.3
78.5
(
(
(
Im) ZnCl2
90.1
87.3
83.6
89.6
89.7
85.5
80.7
78.3
71.4
0.2
0.3
0.5
2
MIm) ZnCl2
2
EIm) ZnCl2
2
1
1
.0
.5
(
(
BIm) ZnCl2
74.2
92.7
80.4
93.4
59.7
86.6
2
HEIm) ZnCl2
2
(
(
(
EIm) ZnCl2
83.6
87.1
90.3
85.5
85.1
92.7
71.4
74.1
83.8
2
Notes: Reaction conditions: Urea = 50 mmol, Glycerol = 50 mmol, cat. = x mmol
(EIm) ZnCl , Temp. = 140 ꢀC, Degree of vacuum = 14.7 kPa, Reaction time = 6 h.
EIm) ZnBr2
2
2
2
EIm) ZnI2
2
Notes: Reaction conditions: Urea = 50 mmol, Glycerol = 50 mmol, cat./glycerol =
hydrophilic character of (HEIm) ZnCl may also assist the
2
2
ꢀ
1
mol%, Temp. = 140 C, Degree of vacuum = 14ꢄ7 kPa, Reaction time = 6 h.
39
adsorption of the hydrophilic glycerol to the catalyst.
The effect of the anion of (EIm) ZnX on reactivity is
2
2
3
.2. Evaluation of Reactivity of (RIm) ZnX Catalysts
2 2
also shown in Table I. Comparison of the catalytic activi-
ties of the (EIm) ZnX catalysts based on different counter
Table I shows the conversion of glycerol, and the selec-
tivity and yield of GC using (RIm) ZnX catalysts hav-
2
2
2
2
anions revealed that the conversion of glycerol and yield
−
−
−
ing various functional groups and counteranions under a
of GC increased as Cl < Br < I . The effect of the
counteranion on the reactivity is attributed to the nucle-
ophilicity of the anion, and the order of the nucleophilicity
vacuum pressure of 14.7 kPa after 6 h of reaction at
ꢀ
1
40 C. A glycerol conversion of 90.1% was achieved
−
−
− 40ꢀ41
using (Im) ZnCl . When compared to (Im) ZnCl that
also increases as Cl < Br < I ꢄ
2
2
2
2
does not possess a functional group in the free N of the
imidazole, all of the (RIm) ZnCl catalysts having an alkyl
The effect of catalyst loading on the yield of GC was
ꢀ
2
2
investigated using (EIm) ZnCl as the catalyst at 140 C
2
2
group exhibited lower catalytic activity for the glycerolysis
of urea. Changing the alkyl group of (RIm) ZnCl from
for 6 h with a reduced pressure of 14.7 kPa. As shown
in Table III, the yield of GC and glycerol conversion
increased continuously as the amount of catalyst increased
2
2
methyl ((MIm) ZnCl ꢁ to butyl ((BIm) ZnCl ꢁ resulted in
2
2
2
2
a decrease in the conversion of glycerol, probably because
up to 2 mol%. However, the yield remained constant as
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of the steric hindrance of the b IuP l k: y1 1b 7u . t2y 4l 4g. r1 o 7u . p2 ,3 w8 hOi cn h: Tuet ,h e1 3a mO oc ut n2t 0o 1f 5c a1 t 6a :l 1y s8 t: 2i n2 creased further.
might inhibit the approach of the glyc Ce roo pl ya rni gd h ut :r eAa mt oe rt ihc ea n Sci eTn ht ei fi cf oPl luo bw l ii sn hg e kr si netic studies of the effects of reac-
active center.
tion time, temperature, and degree of vacuum were carried
out using 1 mol% of (EIm) ZnCl and (HEIm) ZnCl to
However, (HEIm) ZnCl with a hydroxyl functional
2
2
2
2
2
2
group showed the highest glycerol conversion (92.7%)
and GC yield (86.6%). To the best of our knowledge
this is the highest GC yield ever reported for the glyc-
erolysis of urea. Some recent reports are summarized in
Table II. The OH group in (HEIm) ZnCl may activate
understand the effect of hydroxyl groups in this catalyst.
The effect of reaction time on the synthesis of GC
was studied at 140 C under a reduced pressure of
14.7 kPa. As shown in Figure 4, the conversion of glycerol
increased as the reaction progressed. After the reaction had
proceeded for 1 h, the GC selectivity was <55%, imply-
ing that substantial amounts of the reaction intermediate,
ꢀ
2
2
urea since the hydroxyl oxygen atom is an acceptor of
3
8
a weak N H···O hydrogen bond of urea. The high
Table II. Reactivity of different catalysts in the glycerolysis of urea.
ꢀ
Catalyst
Cat. amount
Temp. ( C)
Reduced pressure
XG (%)
SGC (%)
Ref.
Year
Hydrotalcite (Zn)
MgO
ZnO
Au/MgO
ZnSO4
MPR[pmim][Cl]
MPR[pmim][Br]
ZnCl2
5 wt%
5 wt%
5 wt%
1.8 wt%
1.8 wt%
5 wt%
145
145
145
150
150
140
140
150
150
130
130
130
130
130
140
4 kPa
4 kPa
4 kPa
82.0
73.0
80.0
81.0
83.0
45.6
44.8
80.4
81.3
61.0
65.0
82.0
84.0
82.0
92.7
72.0
39.0
60.0
55.1
48.1
40.0
43.0
80.2
79.8
42.1
48.8
65.6
81.5
78.7
86.6
[15]
[15]
[15]
[35]
[35]
[36]
[36]
[37]
[37]
[38]
[38]
[38]
[38]
[38]
2010
2010
2010
2011
2011
2011
2011
2012
2012
2013
2013
2013
2013
2013
N purge
2
N purge
2
14.7 kPa
14.7 kPa
2.67 kPa
2.67 kPa
3 kPa
3 kPa
3 kPa
3 kPa
3 kPa
5 wt%
2 mol%
2 mol%
5.4 wt%
5.4 wt%
5.4 wt%
5.4 wt%
5.4 wt%
1 mol%
Zn(C H O ꢁ/NH Cl
3
6
3
4
ZnO
Smectite (Zn)
Hydrotalcite (Zn/Al)
ZnCl2
ZnBr2
(
HEIm) ZnCl2
14.7 kPa
This work
2
J. Nanosci. Nanotechnol. 14, 4632–4638, 2014
4635

Organic–Inorganic Hybrids of Imidazole Complexes of Zinc (II) for Catalysts in the Glycerolysis of Urea
Kim and Park
1
00
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
Catalyst
XG SGC S(3) S(5)
S(6)
(HEIm) ZnCl
2
2
(
EIm) ZnCl
2 2
Catalyst
XG
SGC
(
EIm) ZnCl
2
2
(
HEIm) ZnCl
2 2
0
.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
100
110
120
130
140
150
160
Time (h)
Temperature (ºC)
Figure 4. Effect of reaction time on the reactivity of (EIm) ZnCl
Figure 5. Effect of reaction temperature on the reactivity of
(EIm) ZnCl and (HEIm) ZnCl (Reaction conditions: Urea = 50 mmol,
2
2
and (HEIm) ZnCl2 (Reaction conditions: Urea = 50 mmol, Glycerol =
2
2
2
2
2
ꢀ
5
1
0 mmol, cat./glycerol = 1 mol%, Temp. = 140 C, Degree of vacuum =
Glycerol = 50 mmol, cat./glycerol = 1 mol%, Degree of vacuum =
14.7 kPa, Reaction time = 6 h).
4.7 kPa).
HPC (2,3-dihydroxypropyl carbamate) (3), remained in the
reaction mixture. As the reaction progressed, HPC was
and (HEIm) ZnCl . The conversion of glycerol con-
2
2
tinuously increased as the temperature increased from
ꢀ
ꢀ
further conver Dt e ed li vt oe r Ge dC b ay n Pd ut bh lei s hs ei nl eg c tTi ve i ct yh nt oo wl o agr yd t Go :CFlori d1 a0 0St aC tet oU n1 i 6v 0e rs Ci t y. , TC h oe ll es ge lee cot if v Mi t ye dt ioc wi na er d GC produc-
production continuously increas Ie Pd : t1o 1 79 3. 2%4 .4 .A1 s7 . s2 h 3o 8w On ni n: Tue t, i o1 n3 Oi n cc tr e2 a 0s 1e d5 1w 6i :t 1h 8 :t 2e m2 perature, reaching a maximum
Copyright: American Scientific Publishers
ꢀ
of 93.4% for (HEIm) ZnCl at 140 C; however, the
2 2
Scheme 3, the main by-products (other than HPC) were
2-oxo-1,3-dioxolan-4-yl) methyl carbamate (ODMC) (6)
(
selectivity decreased as the temperature increased above
ꢀ
and 4-(hydroxymethyl)oxazolidium-2-one (5).
150 C, probably because of the transformation of GC to
Figure 5 shows the effects of reaction temperature
on the synthesis of GC in the presence of (EIm) ZnCl
ODMC (6) or the polymerization of GC and/or glycerol.
At 160 C, the selectivity of GC was very small (close
ꢀ
2
2
O
O
O
O
O
HO
OH
HO
O
NH2
H N
NH2
2
OH
OH
OH
Glycerol carbonate (4)
Glycerol (1)
Urea (2)
Glycerol urethane (3)
O
O
O
O
O
NH
O
O
OH
NH2
-(hydroxymethyl)oxazolidin-2-one (5) (2-oxo-1,3-dioxolan-4-yl)methyl carbamate (6)
Scheme 3. Possible reaction pathways for the glycerolysis of urea.
4
4
636
J. Nanosci. Nanotechnol. 14, 4632–4638, 2014

Kim and Park
Organic–Inorganic Hybrids of Imidazole Complexes of Zinc (II) for Catalysts in the Glycerolysis of Urea
Park et al.43 who used FT-IR and 1H NMR analysis
to demonstrate that there were strong hydrogen bonding
interactions between the hydroxyl group of glycerol and
100
the chloride anion of NH Cl.
4
8
6
4
2
0
0
0
0
4
. CONCLUSIONS
Glycerolysis of urea using (RIm) ZnX catalysts was eval-
2
2
uated herein, illustrating the importance of the struc-
ture and functionality of the (RIm) ZnX catalysts on
their reactivity. The incorporation of zinc halide into
the bis(imidazolium) salt structure is advantageous for
the glycerolysis of urea owing to the introduction of
acid-base bifunctional active sites. (RIm) ZnX exhibited
2
2
Catalyst
XG
SGC
2
2
(
EIm) ZnCl
2
2
good catalytic activity even in the absence of a solvent.
(RIm) ZnX with a hydroxyl group and more nucleophilic
(
HEIm) ZnCl
2 2
2
2
counteranions exhibited better reactivity for the synthesis
of GC. High temperature, long reaction time, and a high
degree of vacuum were favorable for the high conversion
of glycerol.
1
.1 14.7 27.9
54.3
101.3 150 mL/min
N2 purge
Degree of vacuum (kPa)
Figure 6. Effect of degree of vacuum on the reactivity of (EIm) ZnCl
2
2
and (HEIm) ZnCl2 (Reaction conditions: Urea = 50 mmol, Glycerol =
2
Acknowledgments: This study was supported by the
Ministry of Education of Korea through National Research
Foundation (2012-001507) and LINC.
ꢀ
5
0 mmol, cat./glycerol = 1 mol%, Temp. = 140 C, Reaction time = 6 h).
to 4%). The formation of polymers from GC or glycerol
has been reported to be one of the main factors responsible
1
5
for the low selectivity toward GC.
Figure 6 shows the effects of the degree of vacuum on
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Received: 29 April 2013. Accepted: 29 May 2013.
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Copyright: American Scientific Publishers
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638
J. Nanosci. Nanotechnol. 14, 4632–4638, 2014