The Journal of Physical Chemistry B
Article
methylimidazolium hydrogenosulfate, [C1C2Im][HSO4], over
extended time periods.17 This information is of great importance
in this field and must be taken into account in determining
reference data. Therein, our group recently published a critical
overview of our previously reported density studies,18 with more
than 1800 data available from the literature over temperature and
pressure ranges from 293 to 415 K and 0.1 to 40 MPa,
respectively.19 This investigation has clearly indicated again that
ethyl sulfate-based ILs are not water-stable and time-stable
compounds, since they hydrolyze at room temperature even in
presence of any trace amounts of moisture coming from the
atmosphere. Nevertheless, to date, limited information about the
reaction process between alkyl sulfate-based ILs with water is
available in the literature. It is well-known that reversible
hydrolysis reactions are often coupled with transesterifications
reactions. In that sense, Himmler et al.20 have reported a new
synthetic method to produce alkyl sulfate ionic liquids based on a
transesterification reaction between methyl sulfate or ethyl
sulfate ionic liquids and a long-chain alcohol (1-butanol, 1-
hexanol, 1-octanol, 2-methoxyethanol, 2-ethoxyethanol, 2-
butoxyethanol, diethyleneglycol monoethyl ether and diethyle-
neglycol monomethyl ether) in the presence of a Brønsted acid
catalyst (0.4 mol % methanesulfonic acid). More recently,
hydrolysis of methyl sulfate and ethyl sulfate was reported by
Ferguson et al.,21 from which it appears that a complete
hydrolysis reaction of methyl sulfate (ethyl sulfate) to the
hydrogen sulfate anion is obtained through the addition of 2
mol·L−1 of the methyl sulfate (ethyl sulfate) ILs in a aqueous
solution containing 40 mol % of sulfuric acid solution stirred at
373 K for 5 h. Nevertheless, to date, all the reported hydrolysis21
and transesterification20 reactions have been realized in the
presence of a Brønsted acid catalyst, which cannot explain the
low hydrolytic and transesterification stabilities of alkyl sulfate-
based ILs in the presence of only water or alcohols, respectively.
As density data is often used as a probe for the purity of the
investigated materials, the density of seven pure ILs {1-ethyl-3-
methylimidazolium hydrogen sulfate, [C1C2Im][HSO4]; 1-
ethyl-3-methylimidazolium methyl sulfate, [C1C2Im][MeSO4];
1-ethyl-3-methylimidazolium ethyl sulfate, [C1C2Im][EtSO4]; 1-
ethyl-3-methylimidazolium octyl sulfate, [C1C2Im][OcSO4]; 1-
butyl-3-methylimidazolium methyl sulfate, [C1C4Im][MeSO4];
tributylmethylammonium methyl sulfate, [N1444][MeSO4]; and
1-butyl-1-methylpyrrolidinium methyl sulfate, [C1C4Pyrro]-
[MeSO4]} was measured as a function of temperature from
(293 to 363) K at atmospheric pressure to first confirm previous
data published by our group for [C1C2Im][EtSO4]4,5,18,22 and to
evaluate these data with those already published by others.23−32
Second, an attempt was performed to estimate from the density
data the effective molar volume of each unknown ion by using the
methodology already published by our group,5 i.e., to extend as
the function of temperature the observed differences highlighted
by Costa et al.24 in the effective molar volume of the hydrogen
sulfate anion in comparison with those calculated in the case of
aprotic alkyl sulfate ILs. Third, the mutual solubility of
[C1C2Im][HSO4] with linear alcohols was investigated by
measuring the clear and cloud point temperatures using a
standard nephelometric method, and the chemical stability of
each studied mixture was then checked through NMR
measurements. Hydrolysis and transesterification reactions
between the studied ILs with water and with alcohols,
respectively, were then investigated by using three types of
reactoropen and sealed with or without headspacein order
to study the chemical and thermal stabilities of each binary
mixture with the temperature. Such stabilities were then verified
by using NMR spectroscopy. Finally, the hydrolysis/trans-
esterification reactions between alkyl sulfate anions and water/
alcohol were then investigated by using G3 Gaussian calculations
of each species involved as a function of temperature. These
simulations were then compared with the experimental results.
MATERIALS AND METHODS
■
Materials. Chemicals. The reagents used for the synthesis of
ILs were supplied by Aldrich [tributylamine (99%), 1-
butylpyrrolidine (99%), 1-butylimidazole (99%), 1-methylimi-
dazole (99%), dimethyl sulfate (99%), diethyl sulfate (99%),
benzyl alcohol (99%), butylamine (99%), N,N-diisopropylami-
noethanol (99%) and 2,2,3,3,4,4,4-heptafluoro-1-butanol
(98%)]. Toluene (HPLC grade) and ethyl acetate (HPLC
grade) were supplied by Riedel-de-Haen. All chemicals and
reagents were used without any further purification. 1-Ethyl-3-
methylimidazolium hydrogen sulfate and the corresponding
octyl sulfate were kindly supplied by BASF and Merck,
respectively, and were used after drying under high vacuum (1
Pa) at 333 K overnight.
General Procedure for the Synthesis of Alkyl Sulfate-Based
ILs. All ILs were prepared according to the following procedure:
dialkyl sulfate (0.2 mol) dissolved in anhydrous toluene (50 mL)
was added slowly dropwise to a precooled solution of amine (0.2
mol) in anhydrous toluene (150 mL). The mixture was
continually cooled in an ice-bath under nitrogen with care
being taken to maintain the reaction temperature below 298 K.
After complete addition of the dialkyl sulfate, the reaction
mixture was then stirred at room temperature for 4 h. The upper
organic phase of the resulting mixture was decanted, and the
lower IL phase was washed with ethyl acetate (3 × 20 mL). After
washing, the remaining ethyl acetate and any residual water was
removed by heating under reduced pressure. All ILs were
obtained with more than 98% of purity. Their structures were
confirmed by comparison of 1H NMR available in the literature
(see Supporting Information, Figures S1−S6).23−26
1-Ethyl-3-methylimidazolium Methyl Sulfate, [C1C2Im]-
1
[MeSO4]. H NMR (300 MHz, DMSO capillary, ppm): 9.38
(s, 1H, H-2), 7.57 (s, 2H, H-4 and H-5), 4.31 (q, J = 7.2 Hz, 2H,
NCH2CH3), 4.01 (s, 3H, NCH3), 3.70 (s, 3H, OCH3), 1.55 (t, J =
7.2 Hz, 3H, NCH2CH3).
1-Butyl-3-methylimidazolium Methyl Sulfate, [C1C4Im]-
1
[MeSO4]. H NMR (300 MHz, DMSO capillary, ppm): 9.06
(s, 1H, H-2), 7.77 (s, 1H, H-4), 7.68 (s, 1H, H-5), 4.14 (q, J = 7.4
Hz, 2 H, NCH2CH2), 3.87 (s, 3H, NCH3), 3.42 (s, 3H, OCH3),
1.67 (m, 2H, NCH2CH2), 1.09 (m, 2H, N(CH2)2CH2), 0.64 (t, J
= 7.1 Hz, 3H, N(CH2)3CH3).
1-Butyl-1-methylpyrrolidinium Methyl Sulfate, [C1C4pyrro]-
[MeSO4]. 1H NMR (300 MHz, DMSO capillary, ppm, δ): 3.66
(s, 3H, OCH3), 3.39 (m, 4H, NCH2), 3.23 (m, 2H, NCH2), 2.93
(s, 3H, NCH3), 2.11 (m, 4H, NCH2CH2), 1.66 (m, 2H,
NCH2CH2), 1.29 (sextuplet, J = 7.4 Hz, 2H, N(CH2)2CH2), 0.85
(t, 3H, J = 7.4 Hz, N(CH2)3CH3).
Tributylmethylammonium Methyl Sulfate, [N1444][MeSO4].
1H NMR (300 MHz, DMSO, ppm, δ): 3.39 (s, 3H, OCH3), 3.19
(m, 6H, NCH2), 2.96 (s, 3H, NCH3), 1.59 (m, 6H, NCH2CH2),
1.32 (q, J = 7.5 Hz, 6H, N(CH2)2CH2), 0.95 (t, J = 7.3 Hz, 9H,
N(CH2)3CH3).
1-Ethyl-3-methylimidazolium Ethyl Sulfate, [C1C2Im]-
1
[EtSO4]. H NMR (300 MHz, DMSO capillary, ppm): 9.39 (s,
1H, H-2), 7.73 (s, 2H, H-4 and H-5), 4.34 (q, J = 7.4 Hz, 2H,
NCH2CH3), 4.12 (q, J = 7.0 Hz, 2H, OCH2CH3), 4.06 (s, 3H,
1939
dx.doi.org/10.1021/jp312241h | J. Phys. Chem. B 2013, 117, 1938−1949