1112
Guoqin Wang et al. / Chinese Journal of Catalysis 39 (2018) 1110–1120
stretching vibration, MIM), 1460 (C−H bending vibration, CH2),
1220 (SO2 stretching vibration, −SO3H), 1170, 1039 (S=O
stretching vibration, −SO3H), 759 (C−C rocking vibration,
(CH2)n, n ≥ 4), 580 (−SO3H, absorption peak).
flame ionization detector (FID), respectively. The ionic liquid
could be reused in the next run without any additional treat‐
ment. The concentrations of the reactant and products were
calculated from the areas of the corresponding chromatograph
peaks.
1
[BIMBs]HSO4. H NMR (400 MHz, D2O): δ 8.70 (d, J = 27.3
Hz, 1H), 7.36 (s, 2H), 4.07 (dd, J = 14.6, 5.1 Hz, 4H), 2.78 (m,
2H), 1.81 (d, J = 49.1 Hz, 2H), 1.67 (d, J = 23.5 Hz, 2H), 1.59 (d, J
= 2.6 Hz, 2H), 1.16 (d, J = 2.1 Hz, 2H), 0.77 (m, 3H). 13C NMR
(101 MHz, D2O): δ 135.20 (s), 122.41 (d, J = 21.6 Hz), 121.74
(m), 50.07 (s), 49.17 (d, J = 40.4 Hz), 48.60 (m), 31.18 (s), 28.09
(s), 20.96 (s), 18.75 (s), 12.61 (s). IR (KBr, ν/cm–1): 3148 (C−H,
MIM), 2965 (C−H, CH2), 1714 (C=C, MIM), 1565 (C=N, MIM),
1466 (C−H, CH2), 1223 (SO2, −SO3H), 1167, 1036 (S=O, −SO3H),
751 (C−C rocking vibration, (CH2)n, n ≥ 4), 578 (−SO3H).
[HIMBs]HSO4. 1H NMR (400 MHz, D2O): δ 8.69 (m, 1H), 7.41
(m, 2H), 4.10 (dt, J = 22.3, 7.0 Hz, 4H), 2.78 (dd, J = 38.1, 30.4 Hz,
2H), 1.89 (m, 2H), 1.75 (m, 2H), 1.60 (m, 2H), 1.17 (s, 6H), 0.74
(d, J = 6.7 Hz, 3H). 13C NMR (101 MHz, D2O): δ 135.16 (s),
122.35 (d, J = 19.7 Hz), 49.99 (s), 49.59 (s), 48.91 (s), 30.22 (s),
29.00 (s), 28.06 (s), 24.90 (s), 21.68 (s), 20.89 (s), 13.14 (s). IR
(KBr, ν/cm–1): 3150 (C−H, MIM), 2933 (C−H, CH2), 1652 (C=C,
MIM), 1565 (C=N, MIM), 1465 (C−H, CH2), 1185, 1040 (S=O,
−SO3H), 731 (C−C rocking vibration, (CH2)n, n ≥ 4), 570
(−SO3H).
According to previous studies [41,42], taking isobutene as
an example, the conversion (X) of light olefins and selectivity
(S) of products can be calculated using the following equations:
X
isobutene = (2cdimers + 3ctrimers + 4ctetramers) / (cisobutene out
+
2cdimers + 3ctrimers + 4ctetramers) 100%
S
dimers = cdimers / ∑coligomers 100%
trimers = ctrimers / ∑coligomers 100%
tetramers = ctetramers / ∑coligomers 100%
S
S
3. Results and discussion
3.1. Acidity analysis of SO3H‐functionalized ionic liquids
To compare the acidity of synthetic ionic liquids, the acid
strengths of SO3H‐functionalized ionic liquids were examined
by a PerkinElmer Lambda 35 UV/VIS spectrometer according
to previously reported methods [43,44]. For comparison pur‐
poses, the acidities of the ionic liquids were determined using
4‐nitroaniline (pKa value of 0.99) as the indicator in ethanol
and the Hammett function (H0) could be calculated using the
following equation Eq. (1).
1
[HIMBs]CH3SO3. H NMR (400 MHz, D2O): δ 8.68 (m, 1H),
7.41 (m, 2H), 4.10 (dt, J = 22.2, 7.0 Hz, 4H), 2.82 (dd, J = 14.2, 6.6
Hz, 2H), 2.68 (s, 3H), 1.90 (m, 2H), 1.74 (m, 2H), 1.61 (dt, J =
18.3, 7.7 Hz, 2H), 1.16 (s, 6H), 0.72 (d, J = 6.8 Hz, 3H). 13C NMR
(101 MHz, D2O): δ 135.15 (s), 122.36 (d, J = 19.7 Hz), 50.00 (s),
49.59 (s), 48.92 (s), 38.40 (s), 30.22 (s), 29.01 (s), 28.08 (s),
24.91 (s), 21.68 (s), 20.90 (s), 13.15 (s).IR (KBr, ν/cm–1): 3145
(C−H, MIM), 2934 (C−H, CH2), 1655 (C=C, MIM), 1565 (C=N,
MIM), 1460 (C−H, CH2), 1193, 1040 (S=O, −SO3H), 778 (C−C
rocking vibration, (CH2)n, n ≥ 4), 572 (−SO3H).
H0 = pK(A)aq + log ([A]S / [AH+]s )
(1)
At the same 4‐nitroaniline concentration (5.0 mg/L) and IL
concentration (10 mmol/L) in ethanol, we examined the H0
values. The blank solution of 4‐nitroaniline in ethanol exhibited
maximum absorption at 370 nm. When an IL was added, the
maximum absorption of 4‐nitroaniline decreased. Fig. 1(a)
shows the absorbance of the unprotonated form of
4‐nitroaniline in different ‐R group ILs. The trend is as follows:
[HIMBs]HSO4 > [BIMBs]HSO4 > [MIMBs]HSO4. The H0 values of
the three ILs were calculated (Table 1) and it could be found
that their acidities were in the following order: [HIMBs]HSO4
(1.45) > [BIMBs]HSO4 (1.67) > [MIMBs]HSO4 (1.82). The ex‐
perimental results indicate that ILs with longer −R groups may
have greater acidity. Fig. 1(b) shows the absorbance of
4‐nitroaniline in different anionic ILs. The order of absorbance
1
2[HIMBs]SO3CH2SO3. H NMR (400 MHz, D2O): δ 8.68 (m,
1H), 7.41 (m, 2H), 4.17 (d, J = 13.1 Hz, 1H), 4.08 (m, 4H), 2.75
(m, 2H), 1.90 (m, 2H), 1.74 (m, 2H), 1.59 (m, 2H), 1.16 (s, 6H),
0.72 (d, J = 6.8 Hz, 3H). 13C NMR (101 MHz, D2O): δ 135.16 (s),
122.36 (d, J = 19.8 Hz), 66.33 (s), 50.01 (s), 49.59 (s), 48.92 (s),
30.22 (s), 29.01 (s), 28.08 (s), 24.91 (s), 21.68 (s), 20.91 (s),
13.15 (s).IR (KBr, ν/cm–1): 3140 (C−H, MIM), 2934 (C−H, CH2),
1728 (C=C, MIM), 1565 (C=N, MIM), 1466 (C−H, CH2), 1220
(SO2, −SO3H) 1164, 1015 (S=O, −SO3H), 795 (C‐C rocking vibra‐
tion, (CH2)n, n ≥ 4), 575 (−SO3H).
is as follows: 2[HIMBs]SO3CH2SO3
>
[HIMBs]HSO4 >
[HIMBs]CH3SO3. From Table 1, it could be inferred that the
acidities of these three ILs were in the following order:
2[HIMBs]SO3CH2SO3 (0.14)
>
[HIMBs]HSO4 (1.45)
>
2.3. Catalytic experiments
[HIMBs]CH3SO3 (1.53). It is obvious that ILs with different ani‐
ons have different acidity values. Besides, it is noteworthy that
the acidity of 2[HIMBs]SO3CH2SO3 is very high, which may in‐
fluence the reaction activity.
All the reactions were conducted in a 100‐mL autoclave
with polytetrafluoroethylene tubing inside and magnetic stir‐
ring. In a typical reaction, 0.01 mol ionic liquid, 0.08 mol olefin,
and 6 mL of cyclohexane were successively added and allowed
to react at 120 °C/self‐pressure for 6 h. After the reaction, the
autoclave was cooled to –20 °C over 1 h and the ionic liquid was
separated from the upper organic phase by layering. The upper
organic products were qualitatively and quantitatively ana‐
lyzed by gas chromatography/mass spectrometry (GC/MS)
(Agilent 7890A/5975C) and GC (Agilent 6820 equipped with a
3.2. Effect of different ionic liquids on the reaction
Probe tests on olefin oligomerization reactions catalyzed by
different ILs (IL/isobutene = 10% molar ratio) were carried out
at 120 °C/self‐pressure for 6 h; the results are illustrated in
Table 2. It is well known that the catalytic activities of ILs are
greatly affected by their structures. When using [HIMBs]HSO4