Isobutane/2-butene alkylation catalyzed by Br?nsted-Lewis acidic ionic liquids

Article abstract of DOI:10.1039/c8ra03485k

The alkylation reaction of isobutane with 2-butene to yield C₈-alkylates was performed using Br?nsted-Lewis acidic ionic liquids (ILs) comprising various metal chlorides (ZnCl₂, FeCl₂, FeCl₃, CuCl₂, CuCl, and AlCl₃) on the anion. IL 1-(3-sulfonic acid)-propyl-3-methylimidazolium chlorozincinate [HO₃S-(CH₂)₃-mim]Cl-ZnCl_{2 (x=0.67)} exhibited outstanding catalytic performance, which is attributed to the appropriate acidity, the synergistic effect originating from its double acidic sites and the promoting effect of water on the formation and transfer of protons. The Lewis acidic strength of IL played an important role in improving IL catalytic performance. A 100% conversion of 2-butene with 85.8% selectivity for C₈-alkylate was obtained under mild reaction conditions. The IL reusability was good because its alkyl sulfonic acid group being tethered covalently, its anion [Zn₂Cl₅]^- inertia to the active hydrogen, and its insolubility in the product. IL [HO₃S-(CH₂)₃-mim]Cl-ZnCl₂ had potential applicability in the benzene alkylation reaction with olefins and halohydrocarbons.

Full text of DOI:10.1039/c8ra03485k

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Isobutane/2-butene alkylation catalyzed by
Brønsted–Lewis acidic ionic liquids†
Cite this: RSC Adv., 2018, 8, 19551
a
a
Bing Bian,ab Hailong Yu,a Qiong Wu,a Zhiguo Liu,a
*
Shiwei Liu, Shuang Tan,
Fengli Yu, Lu Li, Shitao Yu, Xiuyan Songc and Zhanqian Songa
c
a
a
*
The alkylation reaction of isobutane with 2-butene to yield C8-alkylates was performed using Brønsted–
Lewis acidic ionic liquids (ILs) comprising various metal chlorides (ZnCl2, FeCl2, FeCl3, CuCl2, CuCl, and
AlCl3) on the anion. IL 1-(3-sulfonic acid)-propyl-3-methylimidazolium chlorozincinate [HO3S-(CH2)3-
mim]Cl-ZnCl2 (x¼0.67) exhibited outstanding catalytic performance, which is attributed to the appropriate
acidity, the synergistic effect originating from its double acidic sites and the promoting effect of water
on the formation and transfer of protons. The Lewis acidic strength of IL played an important role in
improving IL catalytic performance. A 100% conversion of 2-butene with 85.8% selectivity for C8-alkylate
was obtained under mild reaction conditions. The IL reusability was good because its alkyl sulfonic acid
group being tethered covalently, its anion [Zn2Cl5]ꢀ inertia to the active hydrogen, and its insolubility in
the product. IL [HO3S-(CH2)3-mim]Cl-ZnCl2 had potential applicability in the benzene alkylation reaction
with olefins and halohydrocarbons.
Received 23rd April 2018
Accepted 16th May 2018
DOI: 10.1039/c8ra03485k
rsc.li/rsc-advances
promoted metal oxides (e.g., sulfated zirconia).6–8 Both cata-
lysts are active in the alkylation reaction, but they also exhibit
1. Introduction
some disadvantages, such as a fast deactivation, blockage of
small pores, leaching (active material catalysts), incomplete
activity recovery, and high active component (active material
catalysts) and regeneration (solid acids) costs. Therefore,
research focused on alternative catalysts is desired.
The alkylation reaction of isobutane with C4-olens forms an
alkylate consisting of branched paraffin, which is used as
a high-octane and environmentally friendly blending compo-
nent for gasoline.1,2 Sulfuric acid and hydrouoric acid are used
currently as catalysts in industry. Both catalysts exhibit some
advantages, such as a high reagent conversion and a rather high
alkylate quality. They also display some shortcomings, such as
a high or extreme toxicity (hydrouoric acid), corrosivity,
harmfulness to the environment, high acid consumption (70–
100 kg H2SO4/ton alkylate), and formation of tars. Current
research is focused on the need for less hazardous and more
effective catalysts for the reaction. Recently, two main types of
solid catalysts have been considered as potential substitutes to
overcome problems associated with conventional alkylation
catalysts.3,4 The rst are active material (e.g., BF3, La, or 12-
tungstophosphoric acid) catalysts adsorbed on inert supports
such as alumina or silica3–5 and the second are solid acids (e.g.,
H-USY zeolites) and superacid catalysts based on sulfate-
As a type of green solvent/catalyst, ionic liquids (ILs) have
attracted much attention for applications in chemistry and
industry because of their chemical stability, thermal stability,
low vapor pressure, and high ionic conductivity properties.9,10
ILs, especially acidic ILs, are recognized as promising candi-
dates to overcome shortcomings of industrially employed
traditional liquid catalysts and solid catalysts in many reac-
tions, such as esterication, polymerization, and alkylation.
Recently, acidic IL catalysts also been scrutinized in the alkyl-
ation reaction of isobutane/2-butene.11–17 Chloroaluminate-
based acidic ILs were oen used as catalysts in the alkylation
reaction. The conversion of 2-butene reached 91.0%, but the
selectivity of C8-alkylate was only 50.5% and only 10% trime-
thylpentane (TMP) selectivity was obtained.13 Many other
chloroaluminate ILs were also investigated in the alkylation
reaction of isobutane/2-butene.13,14 However, the amounts of
TMP in the alkylation products were seldom greater than
75 wt%. In contrast, the TMP selectivity is close to 80% in
commercial alkylation.1 To improve the catalytic performance
of chloroaluminate IL, HCl or Brønsted acidic IL 1-methyl-3-
(4-sulfobutyl)-imidazolium hydrogen sulfate [HO3Sbmim]
[HSO4] were used as co-catalysts in the alkylation reaction of
isobutane/2-butene.15,16 The TMP selectivity increased from
aCollege of Chemical Engineering, Qingdao University of Science and Technology, No.
53 Zhengzhou Road, Qingdao, 266042, China. E-mail: liufusheng63@sina.com;
liushiweiqust@126.com; Fax: +86 532 84022719
bSchool of Chemical and Environmental Engineering, Shandong University of Science
and Technology, Qingdao 266510, China
cKey Laboratory of Eco-Chemical Engineering of Ministry of Education, Qingdao
University of Science and Technology, No. 53 Zhengzhou Road, Qingdao 266042,
China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra03485k
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21.3% to 52.7% by addition of 50 wt% Brønsted acidic IL solution of 1-(3-sulfonic acid)-propyl-3-ethyl-imidazolium chlo-
[HO3Sbmim][HSO4] into IL 1-methyl-3-octyl-imidazolium ride [HO3S-(CH2)3-mim]Cl. Then 0.1 mol ZnCl2 was added to the
bromo-chloroaluminate [omim]Br-AlCl3 (x¼1.5) (x represent the obtaining aqueous solution and was heated under reux for 2 h.
molar ratio of AlCl3).16 A 95.8% C8-alkylate selectivity and 90.4% The water was removed at 1 kPa at 90 ꢁC, to yield 31.4 g IL
TMP selectivity were achieved when HCl gas was bubbled into [HO3S-(CH2)3-mim]Cl-ZnCl2 (x¼0.67) (Scheme 1). The acidity of
the Lewis acidic IL triethylamine hydrochloride–aluminum the obtained IL depended on the amount of ZnCl2 added. When
chloride/cuprous chloride [(C2H5)3NH]Cl–AlCl3/CuCl.17 These the molar fraction of ZnCl2 to [HO3S-(CH2)3-mim]Cl was less
results indicate that the addition of
compound was an effective way to improve the catalytic activity acidity. Otherwise, IL was Brønsted and Lewis dual acidic. Other
and selectivity of Lewis acidic IL. However, chloroaluminate IL acidic ILs [HO3S-(CH2)3-mim]Cl-ZnCl2
a Brønsted acidic than 0.5, IL only possessed Brønsted acidity and no Lewis
,
(x¼0.50, 0.60, 0.71, or 0.75)
hydrolyzes easily to release HCl gas because it is sensitive to [HO3S-(CH2)3-mim]Cl-CuCl2 (x¼0.67), [HO3S-(CH2)3-mim]Cl-CuCl
moisture, and it is difficult to recover HCl gas together with IL (x¼0.67), [HO3S-(CH2)3-mim]Cl-AlCl3 (x¼0.67), [HO3S-(CH2)3-mim]
for recycling of the catalytic system. This causes irreversible Cl-FeCl3 (x¼0.67), and [HO3S-(CH2)3-mim]Cl-FeCl2
were
(x¼0.67)
deactivation of the catalytic system. As a result, the catalytic synthesized according to the same process. IL 1-n-butyl-3-
system can only be applied a few times with good catalytic methyl-imidazolium chlorozincinate [C4mim]Cl-ZnCl2
performance and then its performance decreases in further was synthesized according to the ref. 16.
(x¼0.67)
repetitive use. Therefore, Brønsted–Lewis acidic ILs with a good
The water content of the synthesized ILs was tested with
stability for moisture were synthesized and used in the alkyl- a coulometric Karl Fishcher titrator, using a Metrohm 831 KF
ation reaction of isobutane/2-butene. Compared with chlor- coulometer, for 0.1 g ILs samples, to ꢂ10 ppm accuracy in water
oaluminate IL and traditional catalysts, the synthesized mass content. The ILs dynamic viscosity (h) was measured at
Brønsted–Lewis acidic ILs were inert to moisture and had 0.1 MPa as a function of temperature using an electromagnetic
a synergistic effect originating from its double acidic sites. This VINCI Tech. EV1000 viscometer. The temperature of the
provides IL with an efficient catalytic performance and a good measurement chamber was controlled by an external circu-
reusability for the alkylation reaction.
lating bath and measured inside the chamber by a platinum
resistance probe to ꢂ0.01 ꢁC. Measurements were carried in the
25 to 98 ꢁC temperature range. The calibration of the viscometer
was done using certied oils by Koehler Inc. The ILs density was
measured using pycnometer. The calibration of pycnometer was
done with millipore water. Temperature was kept constant
within ꢂ0.1 ꢁC using PID controller and circulating water using
thermo static-uid bath. The partition coefficient of IL in the n-
octanol/water system (log Kow) was determined at 37 ꢁC by the
shake-ask method. The structure of ILs was conrmed using
a Nicolet 510P FT-IR spectrometer and a Bruker AV500 NMR
spectrometer. The acidities of ILs were characterized and
determined on the basis of the Hammett acidity function. As
shown in the following equation, the Hammett acidity function
is expressed as: H0 ¼ pK(I)aq + log([I]/[IH]+). Where pK(I)aq is the
pKa value of the indicator p-nitroaniline referred to as the
aqueous solution, [IH]+ and [I] are respectively the molar
concentrations of the protonated and unprotonated forms of
the indicator, which can usually be determined by UV-visible
spectroscopy.18
2. Material and methods
2.1. Materials
Isobutane (i-butane 95.2 wt%, n-butane 3.4 wt%, and others
1.4 wt%) and 2-butene (2-butene 88.5 wt%, butane 3.4 wt%,
isobutene 4.5 wt%, 1-butene 2.3 wt%, and others 1.3 wt%) were
from Dalian Airichem Specialty Gases & Chemicals Co. Ltd.,
China. 1-Methylimidazole (99.8 wt%, Zhejiang Kaile Chemicals
Co. Ltd., China), 1,3-propane sultone (1,2-oxathiolane-2,2-
dioxide, 99.7 wt%, Wuhan Fengfan Chemicals Co. Ltd.,
China), and other chemicals (analytical purity) were commer-
cially available. All materials were used without further
purication.
2.2. Preparation of ILs
Under vigorous stirring, 0.1 mol 1-methylimidazole was reacted
with 0.1 mol 1,3-propane sulfone in 50 mL solvent ethyl acetate
ꢁ
for 2 h at 50 C, and the mixture was ltered to obtain a white
MIM-PS:IR (KBr disc): n 3464, 3199, 2989, 1629, 1485, 1454,
1222, 1149, 1041, 735, 603, 518. 1H NMR (500 MHz, D2O): d 8.70
(s, 1H), 7.49 (s, 1H), 7.40 (s, 1H), 4.30 (t, 2H), 3.81 (s, 3H), 2.85 (t,
2H), 2.25 (m, 2H). 13C NMR (125 MHz, D2O): d 135.82, 123.50,
121.85, 47.41, 46.97, 35.46, 24.73. [HO3S-(CH2)3-mim]Cl: water
content: 257 ppm. Density (25 ꢁC): 1.024 g cmꢀ3. log Kow:
precipitate. The precipitate was washed three times with 30 mL
ethyl acetate and dried at 100 ꢁC for 2 h, giving 18.6 g 3-(1-
methylimidazolium-N-yl)-propane-1-sultonate (MIM-PS, yield
91.2%). MIM-PS (0.05 mol) was dissolved in 15 mL water and
was reacted with equimolar hydrochloric acid at room temper-
ature for 30 min and then at 90 ꢁC for 2 h, to obtain an aqueous
Scheme 1 Synthesis of IL [HO3S-(CH2)3-mim]Cl-ZnCl2 (x¼0.67)
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ꢀ1.87. IR (KBr disc): n 3382, 3142, 3117, 1720, 1653, 1572, 1227,
1
1171, 1029, 807, 592. H NMR (500 MHz, D2O): d 8.53 (s, 1H),
7.32 (s, 1H), 7.25 (s, 1H), 4.16 (t, 2H), 3.71 (s, 3H), 2.71 (t, 2H),
2.11 (m, 2H). 13C NMR (500 MHz, D2O): d 134.72, 123.31, 122.16,
47.75, 46.92, 35.26, 24.73. [HO3S-(CH2)3-mim]Cl-ZnCl2 (x¼0.67)
:
water content: 246 ppm. Density (25 ꢁC): 2.367 g cmꢀ3. log Kow:
ꢀ2.32. IR (KBr disc): n 3454, 3156, 3121, 2969, 1625, 1583, 1469,
1252, 1168, 1051, 837, 756, 625, 531. 1H NMR (500 MHz, DMSO):
d 8.51 (s, 1H), 7.27 (s, 1H), 7.21 (s, 1H), 4.10 (t, 2H), 3.63 (s, 3H),
2.65 (t, 2H), 2.06 (m, 2H). 13C NMR (500 MHz, DMSO): d 135.81,
121.28, 120.04, 46.57, 45.86, 34.32, 23.35. [HO3S-(CH2)3-mim]Cl-
FeCl2 (x¼0.67): water content: 185 ppm. Density (25 ꢁC):
2.738 g cmꢀ3. log Kow: ꢀ1.94. IR (KBr disc): n 3382, 3125, 3114,
2989, 1629, 1579, 1485, 1148, 1041, 837, 735, 603, 528. 1H NMR
(500 MHz, DMSO): d 8.55 (s, 1H), 7.42 (s, 1H), 7.35 (s, 1H), 4.26
(t, 2H), 3.81 (s, 3H), 2.74 (t, 2H), 2.21 (m, 2H). 13C NMR (500
MHz, DMSO): d 134.72, 122.31, 121.16, 47.25, 46.92, 34.26,
24.73. [HO3S-(CH2)3-mim]Cl-FeCl3
:
water content:
220 ppm. Density (25 ꢁC): 2.407 g cmꢀ3(x.¼lo0.g67K) ow: ꢀ2.30. IR (KBr
disc): n 3477, 3159, 3116, 2987, 1623, 1575, 1469, 1227, 1170,
1
1048, 835, 747, 621, 525. H NMR (500 MHz, DMSO): d 8.47 (s,
Fig. 1 Apparatus for the isobutane/2-butene alkylation reaction. (1)
Feed storage tank, (2) needle valve, (3) filter, (4) double-piston
metering pumps, (5) one-way valve, (6) pressure relief valve group, (7)
constant pressure nitrogen, (8) feed tank, (9) reactor, (10) anti-
exploding valve, (11) air-vent, (12) reaction mixture, (13) catalytic phase,
(14) needle valve.
1H), 7.19 (s, 1H), 7.23 (s, 1H), 3.97 (t, 2H), 3.54 (s, 3H), 2.55 (t,
2H), 1.92 (m, 2H). 13C NMR (500 MHz, DMSO): d 136.71, 124.30,
123.13, 48.727, 47.896, 36.22, 25.71. [HO3S-(CH2)3-mim]Cl-
CuCl2 (x¼0.67): Water content: 147 ppm. Density (25 ꢁC):
2.557 g cmꢀ3. log Kow: ꢀ2.35. IR (KBr disc): n 3434, 3157, 2976,
1715, 1575, 1463, 1232, 1173, 1034, 878, 749, 622, 579. 1H NMR
(500 MHz, DMSO): d 8.47 (s, 1H), 8.35 (s, 1H), 7.25 (s, 1H), 4.16
(t, 2H), 3.71 (s, 3H), 2.71 (t, 2H), 2.11 (m, 2H). 13C NMR (500 was relieved through an airvent (11) and all excluded gas was
MHz, DMSO): d 134.75, 123.34, 122.12, 47.73, 46.94, 35.28, collected into a gasbag to determine the mass (0.2 to0.1 g) of 2-
24.75. [HO3S-(CH2)3-mim]Cl-CuCl
:
water content: butene and isobutane by gas chromatography (GC) using CH3Cl
181 ppm. Density (25 ꢁC): 2.832 g cmꢀ3(x.¼lo0.g67K) ow: ꢀ2.33. IR (KBr as the internal standard. The conversion of 2-butene was
disc): n 3458, 3140, 2992, 1720, 1568, 1456, 1223, 1169, 1017, calculated by: C% ¼ m1(2-butene)/m(2-butene) ut100, where
1
882, 755, 638, 581. H NMR (500 MHz, DMSO): d 8.56 (s, 1H), m1(2-butene) and m(2-butene) were the mass of 2-butene
8.43 (s, 1H), 7.37 (s, 1H), 4.30 (t, 2H), 3.81 (s, 3H), 2.86 (t, 2H), consumed in the reaction and feed of 2-butene, respectively.
2.34 (m, 2H). 13C NMR (500 MHz, DMSO): d 138.56, 124.32, The liquid phase separated into two phases because of the
120.12, 46.80, 44.87, 34.36, 22.42.
product insolubility in the catalytic system. The upper layer
(10.9 in0.1 g) was decanted from the catalyst layer and was
analyzed by GC to determine the result of the alkylation reac-
2.3. Alkylation apparatus and process
tion. The product selectivity was calculated from WS/WALL
ꢃ
The alkylation apparatus is shown in Fig. 1. Reactions were 100, where WS is the amount of one of the products and WALL is
carried out in batch mode in a 75 mL pressure reactor with the total amount of products, including hydrocarbons with ve
stirring at 600 rpm. Before the experiment, a mixture (11.3 g) of to seven carbon atoms (C5–7), TMP, dimethylhexane (DMH),
isobutane and 2-butene at a certain molar ratio (I/O ¼ 10 : 1) hydrocarbons with more than nine carbon atoms (C9+), and
and stored in the feed storage tank (1) was pumped into the feed others. The IL layer (4.3 , 0.1 g) was reused directly in the recycle
tank (8) using a double-piston metering pumps (4). The catalyst- experiments to establish its reusability. When solid catalysts
containing IL [HO3S-(CH2)3-mim]Cl-ZnCl2
(3.0 g) and such as AlCl3 and ZnCl2 were used in the alkylation, they were
(x¼0.67)
water (1.3 g) were added into the reactor (9). The reactor was directly added into the pressure reactor in glove box, and other
sealed and air was excluded using the high-pressure nitrogen. operations were similar to the above process. All experiments
Aer stirring, the reactor was heated to 80 ꢁC, and the high- were repeated ve times to determine the reproducibility of the
pressure nitrogen (6) and feed tank (14) valves were opened. results.
The mixture of isobutane and 2-butene in the feed tank (8) was
The gas and upper layer of the liquid samples were charac-
forced into the reactor and reacted for 4 h at 80 ꢁC. The pressure terized qualitatively using GC-MS on a HP6890/5973 GC-MS
in the autoclave was maintained at 2.0 MPa using high-pressure equipped with a PONA capillary column (50 m ꢃ 0.25 mm ꢃ
nitrogen to keep the reactant and the product in the liquid 0.25 mm) and their quantitative analyses were determined using
phase during the reaction. Aer the reaction was complete, the GC on a HP6890 GC equipped with a PONA capillary column
reactor was cooled in an ice water bath. The reactor pressure (50 m ꢃ 0.25 mm ꢃ 0.25 mm). The injector and detector
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temperatures were 250 ꢁC and 300 ꢁC, respectively. The TMP/DMH reached 15.2 (Entry 6), which indicates that
temperature program of the GC oven was as follows: hold at Brønsted–Lewis acidic IL [HO3S-(CH2)3-mim]Cl-ZnCl2 was an
40 ꢁC for 2 min, increase to 60 ꢁC at 2ꢁC minꢀ1, increase to efficient catalyst in the alkylation reaction. The good catalytic
120 ꢁC at 1ꢁC minꢀ1, increase to 180 ꢁC at 2ꢁC minꢀ1, and performance may be attributed to the appropriate acid strength
nally, hold at 180 ꢁC for 15 min. Qualitative analysis was of the IL and the synergistic effect originating from the
conducted based on the holding time of the peak and the Brønsted and Lewis acids of IL. Both are responsible for the
contents of the reactants and products were indicated by the GC ability of IL to protonate the olen.20 As shown in Table 1,
ChemStation system according to the area of each chromato- Hammett acidity of the [HO3S-(CH2)3-mim]Cl-ZnCl2 and [HO3S-
graph peak.
(CH2)3-mim]Cl-FeCl3 were less than the others (Entries 6–11),
indicating that the acid strength of both is stronger. So their
abilities to protonate olen are stronger and the conversion of
2-butene was higher. Otherwise, to conrm the synergistic effect
on the catalytic performance, Brønsted acidic IL [HO3S-(CH2)3-
mim]Cl and Lewis acidic IL [C4mim]Cl-ZnCl2 were selected and
used as catalysts for comparison (Entries 3 and 5). The results
show that both catalytic performances were not as good as that
of IL [HO3S-(CH2)3-mim]Cl-ZnCl2 (Entry 6), which suggests that
the synergistic effect originates from the Brønsted and Lewis
acids of IL and is important in the alkylation reaction. To clarify
the Brønsted and Lewis acids of IL [HO3S-(CH2)3-mim]Cl-ZnCl2,
the acidity of IL was determined using Fourier transform
infrared (FT-IR) spectroscopy with pyridine as the probe,21 and
the results are shown in Fig. 2. Compared with the FT-IR spectra
of pure pyridine (Fig. 2a) and IL [HO3S-(CH2)3-mim]Cl-ZnCl2
3. Results and discussion
3.1. Effect of different catalysts on reaction results
As shown in Table 1, when H2SO4 was used as a catalyst in the
alkylation reaction (Entry 1), the conversion of 2-butene was
100%, but most of the product was C5–7 and C9 hydrocarbons
+
generated from the cracking and oligomerization reaction,
respectively. This may be because the catalytic activity of H2SO4
is too strong. It has been reported that there is an induction
period before the alkylation reactions starts, the duration of the
period is inversely related to the acid strength and may affect
the formation of reaction precursors.19 The C8-alkylate product
can be cracked or react further with 2-butene because of the
back-mixing problem. When AlCl3 was used as a catalyst in the
alkylation reaction, the conversion of 2-butene was 100%, and
the selectivity of TMP was 49.6% (Entry 2). AlCl3 exhibits no
good catalytic performance for the alkylation reaction of
isobutane/2-butene, although its catalytic performance is
excellent for many other alkylation reactions. When Brønsted
acidic IL [HO3S-(CH2)3-mim]Cl or Lewis acid ZnCl2 were used as
catalysts (Entries 3 and 4), the alkylation reaction results were
poor. This is attributed to a low acid strength for catalysts that
cannot trigger alkylation reaction to begin, which results in
their poor catalytic performances. When Brønsted–Lewis acidic
IL [HO3S-(CH2)3-mim]Cl-ZnCl2 was used, the conversion of 2-
butene was 100%, the selectivity of TMP reached 80.5%, and
(Fig. 2b), the FT-IR spectrum of pyridine/[HO3S-(CH2)3-
(x¼0.67)
mim]Cl-ZnCl2
had two new characteristic absorption
(x¼0.64)
peaks at 1529 cmꢀ1 and 1455 cmꢀ1 (Fig. 2c). Both were char-
acteristic absorption peaks of the [PyH]+ cation and the Py–
Lewis complex, which form from the reactions of pyridine with
the Brønsted and Lewis acidic centers, respectively.22 This result
indicates that [HO3S-(CH2)3-mim]Cl-ZnCl2 (x¼0.67) was Brønsted
and Lewis acidic.23
Among the results catalyzed by the Brønsted–Lewis acidic ILs
(Entries 6–11), the metal chlorides had a decisive inuence on
the catalytic performance of ILs. IL [HO3S-(CH2)3-mim]Cl-ZnCl2
showed the best catalytic performance (Entry 6), and the others
Table 1 Effect of the different catalysts on the alkylation reaction resultsa
Selectivity/%
+
Entry
Catalyst
H0
C/%
C5–7
TMP
DMH
C9
TMP/DMH
b
1
2
3
4
5
6
7
8
H2SO4
AlCl3
ꢀ11.95
ꢀ2.60
5.65
—
100
100
18.5
18.7
4.3
—
9.6
5.2
4.1
4.3
—
9.6
6.2
5.9
12.6
17.2
22.6
49.6
64.7
—
64.1
80.5
62.3
58.0
—
71.5
68.9
72.3
66.5
39.6
8.5
12.3
15.5
—
7.8
5.3
10.3
10.2
—
8.2
8.6
6.2
7.6
9.0
44.2
15.2
8.9
—
10.6
6.2
18.6
21.4
—
2.7
4.0
4.2
b
[HO3S-(CH2)3-mim]Cl
51.3
10.3
66.0
100
54.2
48.9
25.9
100
70.3
66.8
58.0
65.2
c
ZnCl2
[C4mim]Cl-ZnCl2
—
8.2
15.2
6.0
[HO3S-(CH2)3-mim]Cl-ZnCl2
[HO3S-(CH2)3-mim]Cl-CuCl2
[HO3S-(CH2)3-mim]Cl-CuCl
[HO3S-(CH2)3-mim]Cl-AlCl3
[HO3S-(CH2)3-mim]Cl-FeCl3
[HO3S-(CH2)3-mim]Cl-FeCl2
[HO3S-(CH2)3-mim]Cl-ZnCl2
[HO3S-(CH2)3-mim]Cl-FeCl3
[HO3S-(CH2)3-mim]Cl-AlCl3
1.26
2.52
2.86
ꢀ2.42
0.82
1.65
—
5.7
9
10
11
12c
13c
14c
7.9
8.7
8.0
11.7
8.8
10.2
12.1
10.8
29.1
—
—
4.4
a
Feed 11.3 g, I/O ¼ 10 : 1, catalyst 3.0 g, x (Lewis acidic metal chloride) ¼ 0.67, H2O 1.3 g, T ¼ 80 ꢁC, t ¼ 4 h. b T ¼ 0 ꢁC, H2O 0 g, the other conditions
were the same as footnote a. c H2O 0 g, the other conditions were the same as footnote a. H0 represent Hammett acidity.
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water reduces the acid strength of ILs by means of dilution and
formation of a complex with metal ions (see ESI materials Table
S2†). When no water was used, the conversion of 2-butene was
less than 70% (Entry 12), but when 30 wt% water was added to
IL, the 2-butene conversion reached 100% (Entry 6). A similar
result was obtained when IL [HO3S-(CH2)3-mim]Cl-FeCl3 was
used (Entries 10 and 13). This maybe because the addition of
water to IL reducing the viscosity of IL and increasing the
isobutane and olen solubility in IL (see ESI material Fig. S1
and Table S1†), which increases the rate of mass and the proton
transfer. So the formation of carbenium ions is enchanced.
Otherwise, the anion [Zn2Cl5]ꢀ of IL maybe react with the water
to form the dioctahedral complex,25 which increases the proton
concentration in IL phase (Scheme 2). As a result, the formation
of the carbocation and the transfer of proton between carbo-
cation and isobutane maybe accelerate. However, the result was
poor in the presence of IL [HO3S-(CH2)3-mim]Cl-AlCl3 when the
same amount of water was added (Entry 9). This occurs because
the AlCl3 can react with water and the Brønsted acidic center
and release HCl, so the Lewis acidic center and even the
Brønsted acidic center of IL were lost. This also explains why the
result was unsatisfactory for the catalyst [HO3S-(CH2)3-mim]Cl-
AlCl3 when water was not added (Entry 14).
Fig. 2 FT-IR spectra of the samples using pyridine as a probe. (a) Pure
pyridine. (b) [HO3S-(CH2)3-mim]Cl-ZnCl2 (x¼0.67). (c) Pyridine/[HO3S-
(CH2)3-mim]Cl-ZnCl2 (x¼0.67). V(pyridine) : V(IL) ¼ 1 : 2.
Table 2 shows the effect of x(ZnCl2) on the alkylation reac-
tion. The conversion of 2-butene and the selectivity of TMP
increased with increasing x(ZnCl2) (Entries 1–5). When x(ZnCl2)
was 0.67, the alkylation of isobutane/2-butene was catalyzed.
This may be explained by the fact that, when x(ZnCl2) is greater
than 0.50, IL is a Brønsted and Lewis acidic, and by increasing
x(ZnCl2), the Lewis acidity of IL increases,22 which enhances IL
catalytic activity. However, a further increase in x(ZnCl2) was
unfavorable for the alkylation reaction. When x(ZnCl2) reached
0.71, the TMP selectivity decreased to 72.4%, and the selectiv-
were poor. This is because of the difference in IL acidic strength
which is responsible for the IL ability to protonate the 2-butene.
With increasing Lewis acidity of the metal chloride, IL Lewis
acidity becomes stronger, and results in a higher IL catalytic
activity.24 When IL [HO3S-(CH2)3-mim]Cl-CuCl2 and [HO3S-
(CH2)3-mim]Cl-CuCl were used in the alkylation, the conversion
+
of 2-butene was less than 55%, and the selectivity of C9 was
more than 18% (Entries 8 and 9). This is because the catalytic
activity of IL was too low, so the oligomerization of 2-butene
occurred easily. When IL [HO3S-(CH2)3-mim]Cl-FeCl3 was used,
the conversion of 2-butene was 100%, but the selectivity of TMP
was only 71.5%, and the selectivities of C5–7 and C9+ were 9.6%
and 7.9%, respectively (Entry 10). This may occur because the
Lewis acid strength of IL [HO3S-(CH2)3-mim]Cl-FeCl3 is too
strong, and improves the cracking and oligomerization reac-
tion. Using IL [HO3S-(CH2)3-mim]Cl-FeCl2 as catalyst, the
conversion of 2-butene was only 70.3%, and the selectivity of
TMP was 68.9%, indicating its poor catalytic performance
(Entry 11). This maybe because of its low acid strength to make
it hard to trigger alkylation reactions begins. As a result, the
alkylation reaction is difficult to proceed smoothly, and the
oligomerization reaction increased. The above results indicate
that too strong or too low an acid strength is unfavorable for
TMP formation. The optimum acid value is between 0.82–1.65.
Interestingly, the addition of water to IL [HO3S-(CH2)3-mim]
Cl-ZnCl2 catalyst promoted the alkylation reaction, Although
+
ities of C5–7 and C9 increased to 7.9% and 11.0%, respectively
(Entry 4). When x(ZnCl2) was 0.75, the TMP selectivity decreased
+
to 58.5%, and the selectivities of C5–7 and C9 increased to
Table 2 Effect of x(ZnCl2) on the alkylation reaction resultsa
Selectivity/%
+
Entry x(ZnCl2) H0
C/% C5–7 TMP DMH C9
TMP/DMH
1
2
3
4
5
0.50
0.60
0.67
0.71
0.75
2.87
1.73
1.26
0.20
61.2
86.4
100
100
5.3 63.2 14.3
14.2
8.9
6.2 15.2
11.0 11.3
18.6 7.3
4.4
8.9
4.6 75.4
5.2 80.5
7.9 72.4
8.5
5.3
6.4
8.0
ꢀ1.64 100 12.3 58.5
a
Feed 11.3 g, I/O ¼ 10 : 1, catalyst [HO3S-(CH2)3-mim]Cl-ZnCl2 3.0 g,
H2O 1.3 g, T ¼ 80 ꢁC, t ¼ 4 h.
Scheme 2 The formation of the dioctahedral complex from the reaction of anion [Zn2Cl5]ꢀ and water.
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absorption peak appeared at 2318 cmꢀ1 in the spectrum
(Fig. 3c), which is the characteristic absorption peak of the CN–
Lewis complex that originated from a reaction of the C^N
group of acetonitrile and the Lewis acidic center. With
increasing x(ZnCl2) to 0.71, the characteristic absorption peak
moved to 2327 cmꢀ1 (Fig. 3e). When x(ZnCl2) to 0.75, the
characteristic absorption peak was 2331 cmꢀ1 (Fig. 3f) which
indicates that the IL Lewis acidity was strengthened.
Based on the reaction results, a mechanism for the alkyl-
ation of isobutane/2-butene in the presence of the Brønsted–
Lewis acidic IL was proposed (see Fig. 4) and the synergistic
effect between double acidic sites was investigated. 2-Butene
was protonated and adsorbed on the IL Brønsted and Lewis acid
sites, respectively. On the Brønsted acidic site, a carbocation
from 2-butane formated which was necessary to initiate the
alkylation reaction. On the Lewis site, the high electronegativity
of the p-electron cloud of 2-butene coordinated weakly with the
Lewis acid site, so the adsorption of both occurred. The car-
bocation addition reaction and the adsorbed 2-butene occurred
to yield the sec-carbocation. A tertiary carbocation with good
stability was formed by a rearrangement reaction. Thereaer,
the transfer reaction of the negative hydrogen between the
tertiary carbocation and isobutane occurred, and DMH and
tertiary carbocation of isobutene formed. The tertiary carboca-
tion initiated the next round of alkylation reactions and yielded
Fig. 3 FT-IR spectra of the ILs using acetonitrile as a probe. (a) Pure
acetonitrile, (b) acetonitrile/[HO3S-(CH2)3-mim]Cl-ZnCl2 (x¼0.50). (c)
Acetonitrile/[HO3S-(CH2)3-mim]Cl-ZnCl2
[HO3S-(CH2)3-mim]Cl-ZnCl2 (x¼0.67). (e) Acetonitrile/[HO3S-(CH2)3-
mim]Cl-ZnCl2 (f) Acetonitrile/[HO3S-(CH2)3-mim]Cl-ZnCl2
. (d) Acetonitrile/
(x¼0.60)
.
(x¼0.71)
(x¼0.75). V(acetonitrile) : V(IL) ¼ 1 : 2 in the samples of (b–f).
12.3% and 18.6%, respectively (Entry 5). The above results may
be because IL catalytic activity is too high when x(ZnCl2) reaches
more than 0.71, which improves the cracking and oligomeri-
zation reaction. To clarify the effect of x(ZnCl2) on IL [HO3S-
(CH2)3-mim]Cl-ZnCl2, the Lewis acidity of IL was determined
using FT-IR spectroscopy with acetonitrile as the probe,26 and
the results are shown in Fig. 3. When x(ZnCl2) was 0.60, a new
+
TMP. The product of C12 was also formed by the oligomeriza-
tion reaction of the C8 carbocation with 2-butene. The proposed
mechanism shows that Lewis and Brønsted acidities play
a synergistic role in the alkylation of isobutane/2-butene.
Fig. 4 Proposed mechanism of the alkylation with Brønsted–Lewis acidic IL.
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satisfactory. Aer that, the selectivity of TMP decreased to
64.1%, TMP/DMH was 14.9, and the selectivities of C5–7 and
C9+ were more than 11% (Entry 8). This indicates that a high
reaction temperature accelerates side reactions such as the
cracking and oligomerization reaction, and results in an
increase in selectivities for C5–7 and C9+. Therefore, the
optimal reaction temperature was 80 ꢁC. The effect of reac-
tion time on the reaction is also shown in Table 3 (Entries 2, 9
and 10). When the reaction time was 3 h, the conversion of 2-
butene was 86.7% and the reaction was not complete. The
alkylation reaction was complete at 4.0 h (Entry 2). With
prolonged reaction time, the selectivity for TMP decreased to
3.2. Effect of reaction conditions on reaction results
Table 3 shows the effect of reaction conditions on the alkyl-
ation reaction. The conversion of 2-butane and TMP/DMH
increased with increasing IL [HO3S-(CH2)3-mim]Cl-ZnCl2
dosage from 1.5 g to 3.0 g (Entries 1 and 2). When IL
(x¼0.67)
dosage was 3.0 g, the conversion of 2-butene was 100%.
However, when IL dosage was 4.5 g, TMP/DMH was 16.3, but
the selectivity of TMP decreased to 73.4%, and the selectiv-
+
ities for C5–7 and C9 increased to 10.2% and 9.6%, respec-
tively (Entry 3). This is attributed to too much catalyst usage
that increases the number of catalytic active centers, the
dissolved quantity and mass transfer of reactants in ILs. So
the cracking and oligomerization reactions were favored. The
mass fraction of water in ILs had an obvious inuence on the
reaction (Entries 2, 4–6). Upon increasing the water amount
in IL, the results of the alkylation reaction were more satis-
fying. When the mass fraction of water in IL was more than
30 wt%, the results of the alkylation reaction became poor
(Entry 6). This may be that the water use decreased the acid
strength and the viscosity of IL (see ESI material Fig. S1 and
Table S2†), which improves the mass transfer of reactants
and the formation of the TMP. Otherwise, the water maybe
+
73.0%, and the C5–7 and C9 selectivities increased to 7.2%
and 12.4%, respectively (Entry 10). This may be because of
the TMP cracks and oligomerization for a longer reaction
+
time, which forms C5–7 and C9 products. So the optimal
reaction time was 4.0 h. Some literature reports that the
alkylation reaction of isobutane/2-butene occured easily
from ꢀ5 ꢁC to 15 ꢁC within a short time.1 This is attributed to
the difference in acidic strength of the used catalyst. Cata-
lysts used in the literature were chloroaluminate ILs with
a super acidic catalyst, and its catalytic activity was higher
than that of IL used in this paper. However, chloroaluminate
ILs are usually sensitive to moisture and hydrolyze easily to
form HCl, which results in the irreversible deactivation of the
catalyst system.27 The effect of I/O molar ratio on the reaction
results is also given in Table 3 (Entries 2, 11 and 12). With
increasing I/O molar ratio, the 2-butene conversion and TMP
ꢀ
react with the anion Zn2Cl5 of IL to form the dioctahedral
complex, which promotes the formation of protons (Scheme
2). The reaction temperature had a signicant effect on the
alkylation reaction (Entries 2, 7 and 8). The alkylation reac-
tion of isobutane/2-butene is exothermic, so a low reaction
temperature favors the formation of C8-alkylate and TMP and
also inhibits the side reactions. When the reaction temper-
ature was 70 ꢁC, the total selectivity of the C8-alkylate was
more than 90%, but the conversion of 2-butene was only
86.7% (Entry 7). It is indicated the alkylation reaction did not
react completely. This maybe because that, when the reaction
temperature was low, the mass transfer of the reagents and
the reaction speed are low. The conversion of 2-butene
increased with increasing reaction temperature. When the
reaction temperature reached 80 ꢁC (Entry 2), 2-butene had
reacted completely, and TMP/DMH was 15.2. The results were
+
selectivity increased, and the C9 selectivity decreased. This
indicates that increasing I/O ratio favors TMP formation, but
disfavors the formation of light and heavy ends.28 Based on
the above results, the optimal reaction conditions were ob-
tained as follows: feed 11.3 g, I/O ¼ 10 : 1, IL [HO3S-(CH2)3-
mim]Cl-ZnCl2 (x¼0.67) 3.0 g, H2O 1.3 g, reaction temperature
80 ꢁC, and reaction time 4 h. Under the above reaction
conditions, the conversion of 2-butene and the TMP selec-
tivity were 100% and 80.5%, respectively. And TMP/DMH was
more than 15.2.
Table 3 Effect of the reaction conditions on the alkylation reaction resultsa
Selectivity/%
+
Entry
IL/g
Water/wt%
T/ꢁC
t/h
I/O
C/%
C5–7
TMP
DMH
C9
TMP/DMH
1
2
3
4
5
6
7
8
1.5
3.0
4.5
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
30
30
30
10
20
40
30
30
30
30
30
30
80
80
80
80
80
80
70
90
80
80
80
80
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
3.0
5.0
4.0
4.0
10
10
10
10
10
10
10
10
10
10
5
88.5
100
100
100
100
91.2
86.7
100
86.7
100
100
100
3.2
5.2
10.2
8.3
6.4
3.6
2.0
11.3
4.1
7.2
6.4
6.0
83.2
80.5
73.4
72.4
77.9
81.3
83.8
64.1
82.4
73.0
62.5
82.1
6.9
5.3
4.5
4.9
5.4
7.7
6.4
4.3
5.5
4.5
18.2
5.1
2.8
6.2
9.6
11.4
7.5
4.6
5.3
17.6
5.5
12.4
10.2
4.2
12.1
15.2
16.3
14.8
14.4
10.6
13.1
14.9
15.0
16.2
3.4
9
10
11
12
15
16.1
a
Feed 11.3 g, catalyst [HO3S-(CH2)3-mim]Cl-ZnCl2 (x¼0.67), H2O 1.3 g.
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Table 4 Reusability of IL [HO3S-(CH2)3-mim]Cl-ZnCl2 (x¼0.67)
phase is attributed to the alkyl sulfonic acid group being teth-
ered covalently to IL cation and the Lewis acidic center of IL
being inert and stable to water or the alkyl sulfonic acid group.
Fig. 5 shows the IR spectra of the unused IL [HO3S-(CH2)3-mim]
Cl-ZnCl2 (x¼0.67) and the seven repeatedly used IL [HO3S-(CH2)3-
mim]Cl-ZnCl2 (x¼0.67). The peaks in both spectra are similar,
which indicates that IL structure was unchanged even aer
seven cycles. The IR result conrms its good stability. Addi-
tionally, the product and IL phase are mutually insoluble and
easily separated. The catalyst is not lost easily in the separation
process, which is supported by the water/n-octanol partition
coefficient of IL (the partition coefficient of the IL in the water/n-
octanol system at 37 ꢁC was 207.3). Meanwhile, it is conrmed
that the heavy products were almost no retained in catalytic
phase of IL aer its several runs. Therefore, IL and its acid active
sites are not easily lost and its reusability is good.
Selectivity/%
+
Entry
C/%
C5–7
TMP
DMH
C9
TMP/DMH
1
2
3
4
5
6
7
100
100
100
100
100
100
100
5.2
5.5
5.3
5.7
4.8
6.0
4.5
80.5
82.4
81.5
78.2
80.8
77.6
80.3
5.3
5.4
5.1
6.0
6.3
7.5
6.2
6.2
4.4
5.5
7.2
5.6
5.7
6.6
15.2
15.3
16.0
13.0
12.8
10.3
13.0
3.3. Reusability of IL
To investigate the possibility of recycling the IL phase, recycling
experiments were conducted under optimized reaction condi-
tions and the results are shown in Table 4. The IL [HO3S-(CH2)3-
mim]Cl-ZnCl2
was reused seven times without any
3.4. General applicability of IL catalytic performance for
benzene alkylation reactions
(x¼0.67)
obvious decrease in reaction conversion and product selectivity.
Therefore, the IL phase showed an excellent reusable perfor-
mance in the alkylation reaction. The good reusability of the IL
To illustrate the general applicability of IL [HO3S-(CH2)3-mim]
Cl-ZnCl2
in other alkylation reactions, it was used to
(x¼0.67)
catalyze the alkylation reaction of benzene. Table 5 shows the
alkylation reactions results. The kinds of alkylation reagents
had an obvious inuence on the reaction results. When olens
and halohydrocarbons were used as alkylation reagents, the
conversion of benzene was nearly 100%, and the target product
selectivities were also satisfying (Entries 2–8). In the alkylation
reactions of propylene, benzyl chloride, and 2-chloropropane,
the selectivities for the target products were greater than 90%.
To clarify the synergistic effect of the Brønsted and Lewis acids
on IL catalytic performance, the alkylation reaction of benzene
with propylene was selected as the model reaction (Entries 10
and 11). When ILs [HO3S-(CH2)3-mim]Cl and [C4mim]Cl-ZnCl2
were used as catalysts, the benzene conversion was less than
80%, which indicates that both ILs exhibit poor catalytic
performance in alkylation. So the synergistic effect can improve
the catalytic performance of IL. When methanol was used in the
alkylation, the reaction result was poor. The conversion of
Fig. 5 FT-IR spectra of [HO3S-(CH2)3-mim]Cl-ZnCl2 (x¼0.67). (a) The
unused IL. (b) The seven repeatedly used IL.
Table 5 The benzene alkylation results over ILsa
Entry
Alkylation reagents
Nrea
T/ꢁC
t/h
Product
C/%
S/%
1
2
3
4
5
6
7
8
Propylene
Propylene
2-Butene
1-Hexene
1-Dodecene
Benzyl chloride
Bromoethane
2-Chloropropane
Methanol
4
240
240
240
80
40
60
240
80
240
240
240
240
0.25
0.25
0.25
0.5
0.5
0.5
2
2
4
0.25
0.25
4
Cymene
Cymene
78.2
100
100
100
100
100
98.2
100
60.2
76.0
69.1
71.6
99.2
90.2
85.0
72.5
66.3
94.7
76.0
93.2
55.6
95.4
92.6
58.4
10
10
8
8
10
4
4
4
10
10
4
sec-Butylbenzene
1-Methylpentylbenzene
2-Phenyl isomer
Diphenylmethane
Ethylbenzene
Cymene
Toluene
Cymene
Cymene
Toluene
9
10b
11c
12d
Propylene
Propylene
Methanol
a
Benzene 5.0 g, IL [HO3S-(CH2)3-mim]Cl-ZnCl2 (x¼0.67) 0.25 g. b The catalyst was IL [HO3S-(CH2)3-mim]Cl. c The catalyst was [C4mim]Cl-ZnCl2. d The
catalyst was [HO3S-(CH2)3-mim]Cl-FeCl3 (x¼0.67). Nrea: n(benzene) : n(alkylation reagent). C/%: the conversion of benzene. S/%: the selectivity of the
product in the table.
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benzene and the selectivity of toluene were less than 61% (Entry
9), but the results were better than those of the zeolite catalyst.29
4 M. J. Janik, R. J. Davis and M. Neurock, J. Catal., 2006, 244,
65.
When IL [HO3S-(CH2)3-mim]Cl-FeCl3
was used in the
5 C. Sievers, J. S. Liebert, M. M. Stratmann, R. Olindo and
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(x¼0.67)
alkylation, the benzene conversion increased to 71.6%, and the
toluene selectivity reached 58.4% (Entry 12). It is seen that, with
increase in Lewis acidic strength, the IL catalytic activity is
enhanced. In conclusion, the synthesized double acidic ILs
exhibit a high activity for some alkylation reactions, and the
high catalytic performance may be attributed to the special
double-acid characteristics. The research will be further in-
depth studied.
9 Q. H. Zhang, S. G. Zhang and Y. Q. Deng, Green Chem., 2011,
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4. Conclusions
The alkylation reaction of isobutane/2-butene was investigated
in the presence of Brønsted–Lewis acidic ILs. IL [HO3S-(CH2)3-
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exhibited an outstanding catalytic
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(x¼0.67)
performance with 100% conversion of 2-butene and 80.5%
selectivity for TMP. The separated IL phase could be reused
seven times without any obvious decrease in catalytic perfor-
mance because of its stability and insolubility. The synergistic
effect originating from the Brønsted and Lewis acid sites of IL
and the water addition enhanced the IL catalytic performance
signicantly. Otherwise, the double acidic IL with good catalytic
performance may have a potential applicability in other alkyl-
ation reactions.
17 H. Xing, T. Wang, Z. Zhou and Y. Dai, J. Mol. Catal. A: Chem.,
2007, 264, 53.
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Conflicts of interest
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There are no conicts to declare.
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (31370570 and 21476120), the
Taishan Scholar Program of Shandong Province (ts201511033),
the Key R&D Project of Shandong (2017GGX40106 and
2015GSF116008), and the People's Livelihood Science and
Technology Project of Qingdao (173383NSH). The research
supported by Ecological Chemical Shandong Province Collab-
orative Innovation Center and Key Laboratory of Multiphase
Fluid Reaction and Separation in Shandong Province. The
authors are grateful for support.
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RSC Adv., 2018, 8, 19551–19559 | 19559