S. Saravanamurugan, A. Riisager / Catalysis Today 200 (2013) 94–98
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Scheme 1. The structures of the SO3H-ILs.
previous reports, we have shown that sulfonic acid functionalized
ILs were promising catalyst for the dehydration of fructose and
glucose to make ethyl levulinate and ethyl-d-glucopyranoside in
ethanol, respectively [3].
3.7 (s, 3H; N CH3), 7.25 (s, 1H; CH), 7.35 (s, 1H; CH), 8.6 (s, 1H;
N
CH N); 13C NMR (75.5 MHz, D2O): ı/ppm = 20.7, 27.9, 35.5,
38.3, 48.6, 49.9, 121.9, 123.4, 135.7; Td > 300 ◦C.
1-Methyl-3-(4-sulfobutyl)imidazolium
In the present study, we report the synthesis of a series
ammonium-based ILs (SO3H-ILs) with the cations: 1-methyl-3-
(4-sulfobutyl)-imidazolium ([BMIm-SO3H][X], X = HSO4, NTf2, OMs
and TfO), 1-(4-sulfobutyl)pyridinium ([BPyr-SO3H][HSO4]) and
N,N,N-triethyl-4-sulfobutanaminium ([NEt3B-SO3H][HSO4]). The
structures of the SO3H-ILs are depicted in Scheme 1. The SO3H-ILs
were tested as catalysts in the conversion of the trioses DHA and
GLA into PADA using methanol as solvent under autogenic pres-
sure. The influence of reaction parameters such as reaction time,
temperature and concentration of DHA was optimized.
bis((trifluoromethyl)sulfonyl)amide ([BMIm-SO3H][NTf2]): 1H NMR
(300 MHz, D2O): ı/ppm = 1.5–1.6 (m, 2H; CH2), 1.8–1.9 (m, 2H;
CH2), 2.7–2.8 (t, 2H; CH2-SO3H), 3.7 (s, 3H, N CH3), 7.25 (s, 1H;
CH), 7.35 (s, 1H; CH), 8.6 (s, 1H; N CH N); 13C NMR (75.5 MHz,
D2O): ı/ppm = 20.7, 27.9, 35.2, 48.6, 49.8, 117.0, 121.9, 123.3,
135.3; Td > 200 ◦C.
N,N,N-triethyl-4-sulfobutaneammonium
hydrogensulfate
([NEt3B-SO3H][HSO4]): 1H NMR (300 MHz, D2O): ı/ppm = 1.0–1.2
(t, 9H; 3CH3), 1.5–1.8 (m, 4H; 2CH2), 2.7–2.8 (t, 2H; CH2-SO3H),
3.0–3.2 (m, 8H; 4CH2 N); 13C NMR (75.5 MHz, D2O): ı/ppm = 6.5,
19.9, 21.0, 49.9, 52.3, 55.9; Td > 300 ◦C.
1-(4-Sulfobutyl)pyridinium
hydrogensulfate
([BPyr-
SO3H][HSO4]): 1H NMR (300 MHz, D2O): ı/ppm = 1.5–1.7 (m,
2H; CH2), 1.9–2.1 (m, 2H; CH2), 2.7–2.8 (t, 2H; CH2-SO3H), 4.4–4.6
(t, 3H; N CH2), 7.8–8.0 (t, 2H; 2CH), 8.4 (t, 2H; 2CH), 8.7 (d, 1H;
CH); 13C NMR (75.5 MHz, D2O): ı/ppm = 20.5, 28.9, 49.6, 60.7,
127.9, 143.8, 145.3; Td > 300 ◦C.
2. Experimental
2.1. Synthesis and characterization of SO3H-ILs
1-methylimidazol (99%, Sigma–Aldrich) or pyridine (>99%,
Sigma–Aldrich) or triethylamine (>99.5%, Fluka) (0.2 mol) and 1,4-
butanesultone (99%, Aldrich, 0.2 mol) were charged in a 100 ml
round bottomed flask. The mixture was then stirred at 40–80 ◦C
for 10 h. The solid zwitterion formed was recovered by filtration,
washed repetitively with diethyl ether until all unreacted reac-
tants were completely removed (confirmed by NMR) and then dried
under reduced pressure (15 mbar, 50 ◦C) overnight. A stoichiomet-
ric amount of acid (98% H2SO4, >98% TfOH, 95% HNTf2 or >99.5%
MsOH, Sigma–Aldrich) was subsequently added drop wise to the
respective zwitterion and the mixture stirred at 80 ◦C for 6 h. The
obtained viscous ionic liquids were finally purified by extractive
washing with diethyl ether and finally dried under reduced pres-
sure (15 mbar, 50 ◦C) overnight. Yields were above 95% for all ILs.
The identity of the synthesized sulfonic acid functionalized ionic
liquids was confirmed by NMR (Bruker AM360 NMR spectrometer,
25 ◦C). The thermal decomposition temperature (Td) of the SO3H-
ILs were measured by TGA analysis (TGA/DSC 1 apparatus, Mettler
Toledo) by heating the ionic liquid (9–18 mg) in an aluminum sam-
ple holder from 40 ◦C to 800 ◦C with a heating ramp of 20 ◦C/min
under nitrogen atmosphere.
2.2. Catalytic testing
The catalytic reactions were carried out in 15 ml ace pres-
sure tubes. 139.3 mg (1.5 mmol) of DHA (97%, Sigma–Aldrich) or
142.2 mg (1.5 mmol) of GLA (95%, Sigma–Aldrich), 0.11 mmol of
SO3H-IL, 30 mg of naphthalene (internal reference) and 4 g of
methanol (>99.9%, Sigma–Aldrich) were charged into the ace pres-
sure tube and heated under stirring at 120 ◦C (oil bath temperature)
for 24 h.
2.3. Reactant and product analysis
The reaction mixtures were subjected to GC-FID anal-
ysis (Agilent 6890N instrument, HP-5 capillary column
30.0 m × 320 m × 0.25 m) as well as HPLC-RI analysis (Agi-
lent 1200 series, 30 cm Aminex© HPX-87H column, 0.005 M H2SO4
eluent, flow rate 0.6 ml/min). A GC–MS system (Agilent 6850
GC coupled with Agilent 5975C mass detector) was used for
qualitative analysis.
Conversion of the triose sugars DHA and GLA as well as yields of
PA were determined by HPLC using standards made from commer-
cial samples. The yield of pyruvaldehyde dimethylacetal (PADA)
and 1,1,2,2 tetramethoxypropane (TMP) were calculated from GC
results on series of PADA and TMP standards with naphthalene as
internal standard.
1-Methyl-3-(4-sulfobutyl)imidazolium trifluoromethanesulfonate
([BMIm-SO3H][OTf]): 1H NMR (300 MHz, D2O): ı/ppm = 1.5–1.6 (m,
2H; CH2), 1.8–1.9 (m, 2H; CH2), 2.7–2.8 (t, 2H; CH2-SO3H), 3.7 (s,
3H; N CH3), 7.25 (s, 1H; CH), 7.35 (s, 1H; CH), 8.6 (s, 1H; N CH N);
13C NMR (75.5 MHz, D2O): ı/ppm = 20.5, 27.9, 35.5, 49.0, 50.0, 121.9,
123.4, 131.0, 135.5; Td > 200 ◦C.
1-Methyl-3-(4-sulfobutyl)imidazolium hydrogensulfate ([BMIm-
SO3H][HSO4]): 1H NMR (300 MHz, D2O) ı/ppm = 1.5–1.6 (m, 2H;
CH2), 1.8–1.9 (m, 2H; CH2), 2.7–2.8 (t, 2H; CH2-SO3H), 3.7 (s, 3H;
N
CH3), 7.25 (s, 1H; CH), 7.35 (s, 1H; CH), 8.6 (s, 1H; N CH N); 13
C
The conversion of DHA and GLA to PADA was carried out with
SO3H-ILs as catalysts in methanol at 120 ◦C and the results are
presented in Table 1. Reaction of DHA with [BMIm-SO3H][OMs]
(Table 1, entry 1) yielded 41% of PADA and 45% of TMP along with
more than 99% conversion of DHA with PA being observed as inter-
mediate by HPLC. The yields of PADA and TMP were equally good or
NMR (75.5 MHz, D2O): ı/ppm = 20.5, 27.9, 35.5, 49.0, 50.0, 121.9,
123.4, 135.5; Td > 300 ◦C.
1-Methyl-3-(4-sulfobutyl)imidazolium
methanesulfonate
([BMIm-SO3H][OMs]): 1H NMR (300 MHz, D2O): ı/ppm = 1.5–1.6
(m, 2H; CH2), 1.8–1.9 (m, 2H; CH2), 2.7–2.8 (t, 2H; CH2-SO3H),