5
04
R. Ratti et al. / Catalysis Communications 11 (2010) 503–507
future [16], especially with amines or quaternary ammonium salts
which provides a simple method for the preparation of organic–
inorganic hybrid materials. Recently, Kim et al. have reported the
preparation of cationic nanoclays by immobilizing ionic liquids
as a modifier in the interlayers of clays [17]. Supported ionic-liquid
films (SILF) of nanometric thickness containing bis(oxazoline)–
copper complexes in clay has been used as recoverable catalysts
for enantioselective cyclopropanation reaction [18].
Transesterification of b-ketoesters is an important organic
transformation and has been catalyzed by a number of homo-
genous and heterogeneous catalysts which include Bronsted and
Lewis acids, bases, anion exchange resins, DMAP, titanium tetralk-
oxide, zeolites, clays, to list a few [19]. All previously reported pro-
cedures are flawed with one drawback or another, like volatility
and toxicity of acid catalysts, long reaction times, hostility of cata-
lysts to environmental issues, and their non-recoverability.
In continuation to our program for the development of acid cat-
alysts based on clays and ionic liquids [20,21], we herein report the
synthesis and characterization of sulphonic acid functionalized io-
nic liquid exchanged MMT-clay nanocomposite 2 and its applica-
tion as a solid acid catalyst for transesterification of b-ketoesters.
The cationic part of 1 was exchanged into the interlayer spacing
of MMT clay to get catalyst 2 as a dry powder. The immobilization
is by electrostatic interactions between the negatively charged
interlayers and positively charged cation as shown in Fig. 1.
+
Fig. 2. XRD patterns of (a) Na –MMT clay, (b) catalyst 2.
2
.2. Catalyst characterization
+
The Na –MMT clay and catalyst 2 were characterized by using
XRD, BET, and TGA analysis to determine the structure, surface
area, and thermal stability respectively. The Bronsted acidity was
determined by titration method. The powder XRD patterns of
Na –MMT clay and catalyst 2 are presented in Fig. 2.
The characteristic peak at 7.44° corresponds to d001 spacing of
1.86 Å in case of Na –MMT clay. Similarly, the peak at 6.07° cor-
+
+
1
responds to a d001 spacing of 14.52 Å in case of catalyst 2 indicating
2
. Experimental
a substantial expansion of the interlayers due to exchange of smal-
+
ler Na ions with the bulkier sulphonic acid functionalized cation.
2.1. Preparation of catalyst 2
The nitrogen adsorption–desorption isotherms of MMT and cat-
alyst 2 at 77 K were evaluated using an adsorption analyzer. The
BET surface area and micropore volume increased from 11.89 to
The sulphonic acid functionalized ionic liquid 1 was prepared
by ring opening of 1,3 propane sultone through the nucleophilic
attack of 1-methyimidazole and subsequent anion exchange with
triflic acid according to the reported procedure [22] and character-
2
3
1
2
4.37 m /g and 0.0452 to 0.0646 cm /g for MMT clay and catalyst
respectively, which further supports the intercalation process
À1
and remained the same after the reaction.
ized by IR and NMR analysis [IR (KBr, cm ) 3445, 2361, 1848,
637, 1426, 1043, 796, 623; 1H NMR (D
The cation exchange in catalyst 2 was determined by finding the
1
2
7
2
O) d: 1.9–2.05 (m,2H),
Bronsted acidity using titration method which increased from
.62 (t, 2H, J = 7.16 Hz), 3.59 (s, 3H), 4.05 (t, 2H, J = 7.16 Hz),
.14–7.22 (d, 2H), 8.43 (s,1H)].
+
0
.130 to 1.1 mmol H /g of clay for MMT clay and catalyst 2 respec-
tively as shown in Table 1.
To a solution of 1 (0.486 g, 1.5 mmol) in absolute ethanol
+
TGA analysis of catalyst 1 and 2 showed that catalyst 1 was
thermally stable up to 280 °C after which it degraded rapidly while
catalyst 2 showed thermal stability up to 300 °C and gradually de-
graded and continued up to 840 °C which is probably because the
ionic liquid 1 is sandwiched in the layers of MMT clay in catalyst 2.
The solid catalyst 2 was characterized by IR analysis before and
(
20 ml) was added Na –MMT clay (1.0 g) in small fractions with
vigorous stirring at room temperature and stirring continued for
8 h. The solid was filtered and washed thoroughly with ethanol
4 Â 10 ml) to remove any residual sulphonic acid functionalized
ionic liquid 1. The product was finally dried under vacuum for
4 h to get a free flowing solid and is denoted as catalyst 2.
4
(
2
after the reaction, which showed the presence of a band at
À1
1
427 cm characteristic of (S@O) stretching vibrations of –SO
3
H
groups, similar to the one observed in catalyst 1 and is as shown
in Figs. 3a and 3b.
2.3. Methods and materials
1H NMR was recorded in CDCl
on a 400 MHz Bruker instru-
3
ment using TMS as the internal standard. IR spectra were recorded
+
on a Bio-Rad-Win-IR spectrometer. Na –montmorillonite clay
+
(
Na –MMT) with cation exchange capacity of 120 meq/100 g clay,
was provided by Kunimine Co. Ltd., Japan. XRD analysis was done
using Rigaku D-Max IIIC using Ni-filtered Cu-K radiation. BET sur-
a
face area and pore volume were determined using quantachrome
autosorb automated gas sorption system. TGA analysis was carried
out using TGA Mettler Toledo system. Bronsted acidity was deter-
+
mined by taking Na –MMT clay and catalyst 2 (250 mg each) sep-
arately and suspended in aqueous NaOH (20 ml, 0.086 N) for 24 h
and filtered. The filtrate containing unreacted NaOH was titrated
Fig. 1. Sulphonic acid functionalized ionic liquid exchanged clay nanocomposite 2.
2 4
against standard H SO (0.1 N) using phenolphthalein as an