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S.-D. Lee et al. / Catalysis Today 232 (2014) 127–133
Table 1
catalytic applications of the ionic liquids (ILs) immobilized on MMT
[16].
Elemental analysis results of immobilized IL onto MMT.
Ionic liquids are proven to be environmentally benign media for
catalytic processes or chemical extraction [17]. ILs have negligible
vapor pressure, excellent thermal stability, and special character-
istics as opposed to conventional organic and inorganic solvents.
Quaternary ammonium salts are some of typical ILs. They can be
widely used as catalysts in many organic syntheses [18–20].
We studied the performance of immobilized ILs on MCM-41 or
Merrifield peptide resin for the synthesis of GC [21,22]. However,
the use of expensive pore-directing agents and large amounts of
organic solvents for template removal make these catalysts imprac-
tical for commercial production. Moreover, the ordering of the
mesopores often decreases because of the use of such agents and
solvents. These findings emphasize the need for a low-cost and
commercially viable type of clay as a support.
In this study, ammonium salt type ILs were ion-exchanged with
a commercial MMT to investigate the performance of this hetero-
geneous catalyst in the synthesis of GC from glycerol and urea. The
effects of the ionic liquid cation structure, reaction time, tempera-
ture, and degree of vacuum are discussed for a better understanding
of the reaction mechanism. A recycle test of Q-MMT was also per-
formed to assess the stability of the catalyst.
Catalyst
CHNO from elemental analysis
N-contenta
(mmol/g)
C (wt%)
H (wt%)
N (wt%)
O (wt%)
MMT
0.26
11.64
20.40
30.94
39.17
1.43
3.18
4.44
6.19
7.40
0.56
1.08
1.20
1.36
1.20
9.29
9.86
8.18
7.34
6.97
–
TBA-MMT
THA-MMT
TOA-MMT
TDA-MMT
0.77
0.86
0.97
0.86
a
Amount of ionic liquid immobilized onto MMT.
50 mA) was used to calculate the d-spacing of the samples. The
pore morphology of the samples was examined by high-resolution
transmission electron microscopy (HRTEM) on a Jem-3010 with
an accelerating voltage of 200 kV. The samples were prepared by
ultrasonic dispersion, using absolute alcohol as a solvent.
Solid-state 27Al MAS-NMR experiments were performed over
a Bruker DSX-300 spectrometer at a frequency of 78.19 MHz. A
standard 4-mm double-bearing Bruker MAS probe was used for
all experiments. The nitrogen adsorption–desorption isotherms of
the samples at the temperature of liquid nitrogen were measured
by using a BET apparatus (Micromeritics ASAP 2020).
2. Experimental procedures
2.4. Synthesis of glycerol carbonate from glycerol and urea
2.1. Materials
In the synthesis of GC, the reaction of glycerol with urea was
performed in a 50-mL autoclave reactor equipped with a mag-
netic stirrer. For each typical reaction, the Q-MMT catalyst, glycerol
(50 mmol), and urea (50 mmol) were charged into the reactor.
When the desired temperature was attained, the reaction was ini-
tiated by stirring under vacuum or under nitrogen purging. The
products and reactants were analyzed by a gas chromatograph (HP
6890N) equipped with an FID and a capillary column (HP-5, 5%
phenyl methyl siloxane). Tetraethylene glycol was used as an inter-
nal standard. The selectivity to GC was measured on the basis of
glycerol as a limited reactant.
Na+-MMT, a hydrated aluminum silicate with sodium as the pre-
dominant exchangeable cation (trade name: Kunipia-F, Kunimine
Industrial Company), is obtained in powder form with a typical par-
ticle size of less than 2 m. The cation exchange capacity (CEC) of
Na+-MMT, as reported by the supplier, is 120 mequiv./100 g clay
and the pH value of a 10% aqueous suspension is 10. Ionic liq-
uids are used for the modification of MMT by ion exchange, which
included tetrabutylammonium chloride (TBAC), tetrahexylammo-
nium chloride (THAC), tetraoctylammonium chloride (TOAC), and
tetradodecylammonium chloride (TDAC). They were obtained from
Sigma–Aldrich and were used as obtained.
3. Results and discussion
2.2. Preparation of Q-MMT
Quaternary ammonium salts embedded MMT hybrids (referred
to hereafter as Q-MMT) with different alkyl chain lengths were pre-
pared by the ion-exchange method. In a 250-mL beaker, MMT-Na+
(1.0 g, 1.2 mequiv./g) was placed and 100-mL deionized water was
added. The mixture was vigorously stirred with a magnetic stirrer
and heated to 30 ◦C to obtain a swollen Na+-MMT slurry. In a sep-
arate vessel, the quaternary ammonium salts (QX, 1.2 mmol) were
dissolved in 20 mL ethanol and then poured into the vessel con-
taining the Na+-MMT slurry. The mixture was stirred vigorously at
30 ◦C for 12 h and then allowed to cool to room temperature. The
solid product was collected and washed thoroughly with deionized
water to remove unreacted QX. The end product, Q-MMT, was dried
at 60 ◦C for 24 h before analysis.
Elemental analysis data of MMT ion-exchanged with different
tetra-alkyl ammonium chlorides (TBAC, THAC, TOAC, and TDAC)
are listed in Table 1. The amount of attached QX was in the range
of 0.20–0.88 mmol/g, indicating that the QX was effectively immo-
bilized on the MMT by ion exchange. The amount of attached QX
increased with an increase in the length of alkyl chains, for exam-
ple, from butyl (TBA-MMT) to dodecyl (TDA-MMT), probably due
to the increase in the interlayer distance of the MMT.
WXRD results for MMTs modified with ILs are presented in Fig. 1.
Because the used samples were randomly oriented powders, the
X-ray powder diffraction patterns showing the basal (0 0 1) reflec-
tion are one of the main identification sources for the clay group.
shifted to lower diffraction angles due to the presence of the qua-
ternary salt. The angle 2ꢀ shifted to the left with the increasing alkyl
chain length from TBA-MMT (d) to TDA-MMT (a). As indicated in
Table 2, TDA-MMT having the bulkiest cation showed the largest
interlayer distance of 3.51 nm. Kim et al. [11] also reported that the
d-spacing increased from 1.28 to 1.61 nm when a propyl group was
substituted with a butyl group in the case of MMT modified with
alkyl-3-methylimidazolium halide.
2.3. Characterization
Fourier transform infrared spectra (FT-IR) of untreated clays,
QX, and modified clays were obtained using a Bruker A.M GMBH
(960981(A)). The percentage nitrogen in the untreated and mod-
ified MMTs was determined using a Vario-EL III CHN Elemental
Analyzer, which involved combustion in a pure oxygen environ-
ment to convert the sample elements to simple gases such as
CO2, H2O, and N2. Wide-angle X-ray diffraction (WXRD) with a
Rigaku D/Max 2500 diffractometer with Cu-K␣ radiation (40 kV,
The N2 adsorption–desorption isotherm for THA-MMT is shown
in Fig. 2. The presence of mesopores is confirmed. The BET surface