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I. Cota et al. / C. R. Chimie xxx (2013) xxx–xxx
in this work, we describe the catalytic activity and
recycling of alanine-supported PILs for a series of aldol
condensation processes. The PILs of this study consist of
substituted amine cations of the form RXN+H (X is the
number of alkyl substitutions) combined with organic
anions of the form R0COO– (being of different nature R and
R0). These short aliphatic chain PILs have low cost of
preparation and only need simple synthesis/purification
processes. Moreover, the very low toxicity and the
degradability of this kind of PILs were verified [29]. Thus,
sustainable processes can be originated from their use. By
immobilization of the studied PILs on alanine, hetero-
geneous catalysts are obtained, which can be easily
recycled and reused for several consecutive runs. The
immobilization was done by impregnation of the support
material with the PIL diluted with ethanol, followed by the
removal of the solvent by evaporation.
On the other hand, it is a continuous challenge to find
new catalysts able to perform with good activities and
selectivity condensation reactions for the synthesis of
pharmaceutical and fine chemicals and thus, replacing the
homogenous catalysts used in the industrial production
[30,31]. In order to determine the catalytic activity of the
studied PILs, two condensation reactions of carbonyl
compounds were carried out. The products obtained from
these reactions are applied in pharmacological as well as
flavor and fragrance industry.
In order to obtain the supported PILs, 1 g of PIL was
dissolved in 7 mL of ethanol and, after stirring at room
temperature for 30 min, 1 g of alanine was added. The
mixture was stirred for 2 h and then, heated at 348 K under
vacuum to remove ethanol. The supported PILs, thus,
obtained were labeled hereafter as a-PILs.
The FT–IR spectra were taken by a Jasco FT/IR 680 plus
model IR spectrometer, using a NaCl disk.
The studied reactions were the condensation between
citral and acetone and the synthesis of benzylideneace-
tone. The reactions were performed in liquid phase using a
100-mL batch reactor equipped with a condenser system.
To a stirred solution of substrate and ketone (molar ratio
ketone/substrate = 4.4) was added 1 g of a-PIL, and the flask
was maintained at 333 K using an oil bath. For the citral–
acetone condensation, the reaction mixture contained
5.26 g of citral (34.55 mmol), 8.83 g of acetone
(152.03 mmol), and 1.527 g of tetradecane (7.7 mmol). In
the case of benzaldehyde–acetone condensation, the
quantities were: 3.18 g of benzaldehyde (29.96 mmol),
7.65 g of acetone (131.71 mmol), and 1.527 g of tetra-
decane (7.7 mmol). Samples were taken at regular time
periods and analyzed by gas chromatography using a flame
ionization detector and an AG Ultra
2
column
(15 m ꢀ 0.32 mm ꢀ 0.25
m
m). Tetradecane was used as
the internal standard. The reagents (citral 95%, acet-
one > 99.5%, benzaldehyde > 99%, tetradecane > 99.5%)
were purchase from Sigma–Aldrich and used without
further purification.
In our previous two articles [15,18], we performed a
complete study regarding the synthesis, the physicochem-
ical properties, the catalytic activity and the recycling of
this new family of PILs. An efficient recovery process of the
PILs was developed, but it took about 17 h and for us, it
represented an aspect of the process, which should be
improved. Therefore, the main objective of the present
study is to improve the recycling process by supporting the
PILs onto a solid support, alanine.
For the repeated runs at the end of the reaction, the
catalyst was filtered from the reaction media and washed
several times with acetone in order to eliminate the
residues adsorbed on the catalyst surface.
3. Results and discussion
Alanine was chosen as support because it is a cheap,
commercially available amino acid. Fig. 1 presents the IR
spectrum of the a-2-HEAB and, for comparison purpose,
the spectra of the free 2-HEAB and alanine.
The broad band in the 3500–2400 cmꢁ1 range, char-
acteristic of the ammonium structure of the PIL, is present
in the supported PIL as well. Also, the broad band centered
at 1600 cmꢁ1, which is a combined band of the carbonyl
stretching and N–H plane bending vibrations in the
structure of the free PIL, is also present in the supported
PIL. Based on this information, we can conclude that after
the immobilization on alanine support, the structure of the
PILs is conserved intact.
The same bands are present in the free PIL and in the
supported PIL spectra; moreover, in the supported PIL
spectra, the intensity of the bands is increased, indicating
that the immobilization of the PIL on the alanine took place
by adsorption on the support.
The catalytic activity of the alanine-supported PILs (a-
PILs) was tested in the same reactions as the free PILs, see
results of previous works [15,18].
2. Experimental
The studied PILs were 2-hydroxy ethylammonium
formate (2-HEAF), 2-hydroxy ethylammonium acetate
(2-HEAA), 2-hydroxy ethylammonium propionate (2-
HEAPr), 2-hydroxy ethylammonium butanoate (2-HEAB),
2-hydroxy ethylammonium iso-butanoate (2-HEAiB) and
2-hydroxy ethylammonium pentanoate (2-HEAPe). The
synthesis of these PILs, which is described elsewhere
[15,18], is a simple acid–base neutralization reaction,
creating the corresponding salt of ethanolamine, which in
a general form, should be expressed as described in eq. (1).
In this equation, X is the number of ethanol substitutions
into the amine compound, Y is the number of protons
(X + Y = 3) and R is the aliphatic end of the corresponding
organic acid. For example, when X = 1, Y = 2, and R = CH3,
this equation shows the chemical reaction for the reactants
monoethanolamine + acetic acid and 2-hydroxy ethylam-
monium acetate (2-HEAA) as the neutralization product.
(1) (HOCH2CH2)XNHY + HOOC–R ! (HOCH2CH2)X NHYþþ1
(–OOC–R)
The condensation reaction between citral and acetone
(Scheme 1) leads to the formation of pseudo-ionone, an
important precursor of the synthesis of vitamin A.
Commercial citral consists of two isomers, neral and
The reagents, monoethanolamine (Merck Synthesis,
> 99%) and the organic acids (Merck Synthesis, > 99%)
were used without further purification.
Please cite this article in press as: Cota I, et al. Alanine-supported protic ionic liquids as efficient catalysts for aldol