Isobutane alkylation using acidic ionic liquid catalysts

Article abstract of DOI:10.1016/j.catcom.2012.04.022

Alkylation of isobutane with butene (C₄ cut from an industrial alkylation feed) was catalysed by a class of strongly acidic ionic liquids. A dramatic enhancement of the catalytic effect was observed for the ionic liquids containing the SbF₆^- anion. The optimal alkylate was obtained using the acidic ionic liquid [HMIm][SbF₆] (47.9 wt.%)/CF₃SO₃H (52.1 wt.%) as the catalyst. The C ₈ selectivity was as high as 80% and the research octane number (RON) was 95. These results were better than those obtained by using either neat H₂SO₄ or CF₃SO₃H as the catalyst.

Full text of DOI:10.1016/j.catcom.2012.04.022

Catalysis Communications 26 (2012) 68–71
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Isobutane alkylation using acidic ionic liquid catalysts
a,b
b,
a
b
Xueqi Xing , Guoying Zhao ⁎, Jianzhong Cui , Suojiang Zhang
a
Department of Chemistry, Tianjin University, Tianjin, 300072, PR China
Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Alkylation of isobutane with butene (C4 cut from an industrial alkylation feed) was catalysed by a class of
Received 11 December 2011
Received in revised form 1 April 2012
Accepted 20 April 2012
strongly acidic ionic liquids. A dramatic enhancement of the catalytic effect was observed for the ionic liquids
−
containing the SbF
47.9 wt.%)/CF SO
6
anion. The optimal alkylate was obtained using the acidic ionic liquid [HMIm][SbF
6
]
(
3
3
H (52.1 wt.%) as the catalyst. The C
8
selectivity was as high as 80% and the research octane
Available online 27 April 2012
number (RON) was 95. These results were better than those obtained by using either neat H
as the catalyst.
2 4 3 3
SO or CF SO H
Keywords:
Ionic liquid
Alkylation
Isobutane
Catalyst
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
isobutane alkylation by themselves [10]. Therefore, Olah et al. [11]
synthesised acidic ionic liquid catalysts with the general formula HF/
+
−
The alkylation of isobutane with butene is an important process in
n n
[amineH ] [F ]
for isobutane alkylation by immobilising HF with
the petroleum industry. The products are often used as a premium
blending stock for reformulated gasoline since they have high octane
number, low vapour pressure, and low contents of aromatics, alkenes
and sulfur. However, commercial alkylation processes catalysed by
concentrated sulfuric acid or by hydrogen fluoride are gradually
being restricted because of environmental pollution and safety prob-
lems [1]. Thus, a genuine quest has been spurred to explore safer,
cleaner, and more cost-effective alternatives.
amines, which had formed the basis for UOP's commercial alkylation
process. Tang et al. [10] have explored binary mixtures of ionic liquids
(ILs) and mineral acids as catalysts for isobutane alkylation.
Kim et al. [12] reported that the presence of ILs containing a weak-
−
−
ly coordinating anion (SbF
6 6
or PF ) dramatically enhanced the cata-
lytic activity of Sc(OTf) , which was due to the in-situ formation of
3
a strong Lewis acid through anion exchange. Soltner et al. [13]
reported that trifluoromethanesulfonic anhydride could react with
Acidic ionic liquids have been extensively applied as catalysts in
organic and refinery chemistry due to their unique properties, such as
negligible vapour pressure, good solvating ability, and tuneable acid
types (Lewis or Brønsted) and strength [2–11]. Owing to their standout
acidity, chloroaluminate-based ionic liquids have been investigated as
catalysts in isobutane alkylation [3–9]. In 1994, Chauvin et al. [3]
reported that the alkylation of isobutane with 2-butene could be effi-
ciently catalysed by the ionic liquid 1-butyl-3-methylinidazolium
chloroaluminate. Since then, various chloroaluminate-based ionic
liquids have been investigated as catalysts for isobutane-butene alkyl-
ation by Yoo et al. [4], Xu et al. [5–7], Zhang et al. [8], Bui et al. [9] and
so on. However, Lewis acid chloroaluminate ionic liquids are extremely
sensitive to moisture and hydrolyse to release HCl in contact with traces
of water [10]. Brønsted acid ionic liquids have also been utilised for
isobutane-butene alkylation [10,11]. But, the acidity of common
Brønsted acid ionic liquids is usually too low to efficiently catalyse
superacidic solutions AF/SbF
5
(A=H or D) to form the corresponding
+
−
3
salts [CF SO
3 2
A ]
[SbF
6
]
, the highest acidity of which in H scale
0
could reach to −18.5. Herein, a series of acidic ionic liquids with a weakly
−
coordinating SbF
6
anion, including 1-alkyl-3-methylimidazolium
n
hexafluoroantimonate ([C MIm]-SbF
6
, where n=4, 6 or 8) and 1-
Py]-SbF , where n=2, 4, 6
alkylpyridinium hexafluoroantimonate ([C
n
6
or 8), was coupled with trifluoromethanesulfonic acid (TFSA) to syn-
thesise a class of acidic ionic liquid catalysts. The catalytic performance
of these catalysts for isobutane-butene alkylation was then investigated.
In addition, the reaction parameters, such as the composition of the acidic
ionic liquid catalyst, the chain length of the cations and the reaction tem-
perature were optimised.
2. Experimental
2.1. Instrumentation and reagents
All chemicals (AR grade) were purchased commercially and were
used as received unless otherwise noted. TFSA (>99.9%) was pur-
chased from the 718th Research Institute of China Shipbuilding
⁎
Corresponding author. Fax: +86 10 82627080.
E-mail address: gyzhao@home.ipe.ac.cn (G. Zhao).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.04.022

X. Xing et al. / Catalysis Communications 26 (2012) 68–71
69
complete, the autoclave was slowly depressurised and the gas hydro-
carbon phase was collected for analysis. The liquid phase was allowed
to stand for 30 min to separate the catalyst and alkylate layers. The
alkylate was then neutralised with a saturated NaHCO
washed with water, and finally dried with anhydrous MgSO
3
solution,
. The
4
products were immediately analysed by GC (detailed information is
in the supporting information).
3
. Results and discussion
3.1. Isobutane-butene alkylation catalysed by acidic ionic liquids
Scheme 1. The structure of the cations and anions of the ionic liquids.
n 6
The effect of the composition of the acidic ionic liquid [C MIm]-SbF /
TFSA on the quality of alkylate was studied (Table 1) at the optimised
catalyst/substrate ratio 0.5 (Fig. 2S in the supporting information). The
Industry Corporation. Isobutane (>99.7%) was purchased from
Linggas, LTD. Butene (C
4
cut from an industrial alkylation feed) was
alkylate quality catalysed by the [BMIm][SbF
maximum corresponding to a roughly 49.7 wt.% [BMIm][SbF
which meant it was of the optimal acidity for isobutane alkyl-
ation (Fig. 3S in the supporting information). For this maximum, the
6
]/TFSA passed through a
] content,
supplied by Sinopec Beijing Yanshan Company. NMR spectra were
recorded on a JEOL ECA-600 spectrometer. ESI–MS analyses were per-
formed on a Bruker micro TOF-QII spectrometer.
6
[
BMIm][SbF
higher C selectivity (76.2%), research octane number (RON, 92.6)
and yield of alkylate (184.6%) than neat H SO , TFSA or [BMIm][SbF
Entries 1, 2 and 6). In contrast, a small amount of viscous oligomers
detailed information is in the supporting information) was produced
SO ] (50.0 wt.%)/TFSA (50.0 wt.%) was used as
the catalyst (Entry 13). This is probably because the acidity of [BMIm]
CF SO ] (50.0 wt.%)/TFSA (50.0 wt.%) is below the lower limit of the
optimal acidity range for isobutane alkylation, which is usually around
8.0 to −13.0 in the H scale. The results indicate there may be a
synergetic catalytic effect between [BMIm][SbF ] and TFSA. According
to the references [12,13], we speculate that a strongly acidic salt
CF SO ][SbF ] may be formed through anion exchange between
TFSA and [BMIm][SbF ], which dramatically enhances the acidity and
the catalytic performance of [BMIm][SbF ]/TFSA.
The optimal selectivity and trimethylpentanes (TMP)/
dimethylhexanes (DMH) ratio were achieved with the [HMIm][SbF
47.9 wt.%)/TFSA (52.1 wt.%) (Entry 8). The literature has reported that
better quality alkylate in terms of C selectivity and RON were obtained
by using [BMIm]Cl-AlCl (86.8 wt.% and 96.9 wt.% respectively) [3] or
(C NH]Cl-AlCl /CuCl (98.1 wt.% and 99.7 wt.% respectively) [6].
6
] (49.7 wt.%)/TFSA (50.3 wt.%) catalyst (Entry 4) gave a
8
2
.2. Preparation of the ionic liquids
2
4
6
]
(
(
The ILs (Scheme 1) were synthesised according to the methods in
the literature [14–16]. Pyridine or 1-methylimidazole (1.0 eq) in
ethyl acetate was mixed with an alkyl halide (1.2 eq) at room temper-
ature under a nitrogen atmosphere and then refluxed for 20–72 h to
produce the crude product [R-MIm][X] or [R-Py][X]. After a series of
post-treatment steps, the purified [R-MIm][X] or [R-Py][X] (1.0 eq)
when the [BMIm][CF
3
3
[
3
3
−
0
was anion-exchanged with NaSbF
50 mL) for 72 h. The filtrate was washed with water until no halogen
ions were detected by titration with a concentrated solution of
AgNO . Then the filtrate was washed two more times with water
6 2 2
(1.2 eq) in CH Cl (about
6
1
[
3
3
H
2
6
3
6
and the solvent was then removed by evaporation. Finally the prod-
uct was dried at 100 °C in vacuum for 48 h before use. Yields were
generally 70–80% (characterisation data is given in the supporting
information).
6
C
8
6
]
(
8
2
.3. Typical alkylation experiment
3
[
2
H
5
)
3
3
In a typical alkylation reaction, the autoclave was charged with
5 mL of catalyst, pressurised to 0.5 MPa with argon and cooled to
the desired temperature. Approximately 50 mL of the premixed
hydrocarbon feed with a 10:1 isobutane/olefin (I/O) molar ratio was
pumped into the reactor with stirring at a speed of 1000 r/min. The
reaction was stirred continuously for 10 min. After the reaction was
However, pure 2-butene and a high I/O ratio (15 [3] or 30 [6]) were
used in their studies, which has a positive effect on improving the quality
of alkylate. In addition, our results are different from those reported for
imidazolium-based acidic ionic liquids where the ILs with an octyl
group ([OMIm]) had the best catalytic performance [4]. Therefore, the
effect of the chain length on the alkylate quality was investigated.
2
Table 1
The distribution of alkylate catalysed by H
SO
2 4
, CF
3
3
SO H and [C
n
MIm][SbF
]/TFSAa.
6
Entry
ILs
IL (wt.%)
Acid (wt.%)
C
5
7
–C (wt.%)
C
8
selectivity (wt.%)
C
9+ (wt.%)
TMP/DMH
RON
Yield of alkylate (%)
b
1
2
None
None
100.0
100.0
70.3
50.3
33.3
0.0
68.2
52.1
31.1
72.6
53.2
35.9
50.0
10.5
22.6
6.7
7.2
3.8
–
49.6
32.2
64.9
76.2
59.6
–
39.9
45.2
28.4
16.6
36.6
–
5.0
1.3
4.5
5.4
5.2
–
89.8
85.0
90.5
92.6
89.8
–
154.3
95.0
176.6
184.6
177.1
0.0
168.2
180.6
172.9
159.5
176.6
161.4
0.0
c
3
4
5
6
7
8
9
1
1
1
1
[BMIm][SbF
[BMIm][SbF
[BMIm][SbF
[BMIm][SbF
[HMIm][SbF
[HMIm][SbF
[HMIm][SbF
[OMIm][SbF
[OMIm][SbF
[OMIm][SbF
6
6
6
6
]
]
]
]
29.7
49.7
66.7
100.0
31.8
47.9
68.9
27.4
46.8
64.1
50.0
6
]
]
]
18.7
4.9
5.1
11.8
8.3
4.5
–
48.6
80.1
76.5
63.8
70.1
70.7
–
32.6
14.5
19.4
24.4
21.7
24.9
–
2.5
7.2
5.4
4.2
6.1
5.3
–
87.8
95.6
92.6
90.7
92.1
91.7
–
6
6
0
1
2
3
6
]
]
]
6
6
3 3
[BMIm][CF SO ]
a
b
c
Reaction conditions: volume of hydrocarbon feed (molar I/O=10)=50 mL, volume of catalyst=25 mL, P=0.5 MPa, T=10 °C, agitation=1000 r/min, reaction time=10 min.
Entry 1 acid=H SO (96.8 wt.%).
Entries 2–13 acid=CF SO H.
2
4
3
3

70
X. Xing et al. / Catalysis Communications 26 (2012) 68–71
Fig. 2. Effect of reaction temperature on the alkylate quality. Conditions: 50 mL C
4
feed;
2
1
5 mL [HMIm][SbF
0 min.
] (47.9 wt.%)/CF SO H (52.1 wt.%) catalyst; 0.5 MPa; 1000 r/min;
6 3 3
Fig. 1. The alkylation catalysed by [EPy][SbF
6
] (50 wt.%)/TFSA (50 wt.%), [BPy][SbF
6
]
]
(
(
50 wt.%)/TFSA (50 wt.%), [HPy][SbF
50 wt.%)/TFSA (50 wt.%). Conditions: 50 mL
000 r/min; 10 min; 10 °C.
6
] (50 wt.%)/TFSA (50 wt.%) and [OPy][SbF
6
C
4
feed; 25 mL catalyst; 0.5 MPa;
1
to 7.2% and from 11.5% to 42.3% respectively. This was caused by an
accelerated rate of polymerisation and cracking. Conversely, the C
8
selectivity decreased from 85.6% (at −5 °C) to 45.2% (at 30 °C). This
trend is similar to alkylation catalysed by sulfuric acid [17,18] or by
chloroaluminate-based ionic liquids [4].
3
.2. The effect of cation chain length in [C
n 6
Py][SbF ] (n=2, 4, 6 or 8) on
the alkylate quality
As shown in Fig. 1, the yield of alkylate catalysed by [C
n
Py]-SbF
6
/
3.4. Reusability of catalyst
TFSA (n=2, 4, 6 or 8) increased as the length of the alkyl group
increased from 2 to 6 and then decreased for n=8. Maximum yield
6 3 3
The reusability of the [HMIm][SbF ] (47.9 wt.%)/CF SO H (52.1 wt.%)
(
171.8%), highest C
obtained with [HPy]-SbF
sults obtained for the [C MIm]-SbF
8
selectivity (79%) and best RON (94) were
/TFSA, which is in agreement with the re-
/TFSA catalyst. This may be main-
catalyst was studied at the conditions of 0.5 catalyst/substrate ratio, 0 °C
and 10 min. The alkylate distribution and the RON are displayed in Fig. 3.
The quality and yield of alkylate varied slowly in the four runs and
then dropped dramatically after the fifth run. The amount of heavy
ends (C9+) started to increase in the fifth run, and reached 47 wt.% by
the eighth run. The colour of catalyst changed from light yellow into
dark brown because of the accumulation of acid soluble oils (ASOs)
(Fig. 4 S in the supporting information). In addition, the volume of the
catalyst was reduced from 25 to 22 mL in the eighth run, which may be
due to the vaporisation of catalyst or its dissolution/entrainment in the
alkylate during the batch operation. These may be the main reasons for
the reduction of the acidity. Vice versa, low acidity of the catalyst acceler-
ated the formation of heavy ends and ASOs.
6
n
6
ly attributed to a complex interplay of the acidity of the coupled
catalysts and the solubility of isobutane, the reaction intermediates
and products in the coupled catalysts. The longer the chain length
of the alkyl group of ionic liquid is, the higher the solubility of hydro-
carbon is in the ionic liquid and its solution [17]. A high isobutane
solubility increases the rate of the hydrogen transfer from the isobu-
tane to the C
desired C
+
8
carbonium, which accelerates the generation of the
8
fraction and inhibits side reactions, such as cracking and
oligomerisation. However, high alkylate solubility is not beneficial
for the production and separation of alkylate. The coupled catalyst
6
[HPy][SbF ]/TFSA which has a moderate chain length of alkyl group,
probably has the most suitable solubility for isobutane and the reac-
tion intermediates, thus produces the best quality and the maximum
amount of alkylate.
4. Conclusions
Alkylation of isobutane with industrial butene feed using a class of
strongly acidic ionic liquid catalysts had been investigated. The ionic
3
.3. Effect of reaction temperature on the alkylate quality
liquid catalyst showed superior performance over pure CF
3 3
SO H and
H
2
SO . A dramatic enhancement in catalytic effect was observed for
4
−
The effect of temperature on the alkylate quality was investigated by
the ILs containing SbF
CF
6
anion compared with the ILs containing
selectivity of up to 80%
−
using [HMIm][SbF
6
] (47.9 wt.%)/CF
see Fig. 2). As the temperature increased, the amount of light ends
–C products) and heavy ends (C9+ products) increased from 1.7%
3
SO
3
H (52.1 wt.%) as the catalyst
3
SO
3
anion. The optimal alkylate with a C
8
(
(
and RON of 95 was obtained using the acidic ionic liquid [HMIm]
[SbF ] (47.9 wt.%)/CF SO H (52.1 wt.%) as the catalyst. A complex
C
5
7
6
3
3

X. Xing et al. / Catalysis Communications 26 (2012) 68–71
71
Acknowledgements
This work was supported by the National Basic Research Program
of China (973 Program, grant no. 2009-CB219901), Key Program of
National Natural Science Foundation of China (grant no. 21036007)
and Beijing Natural Science Foundation (grant no. 2122052).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2012.04.022.
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