Effects of catalyst composition on the ionic liquid catalyzed isobutane/2-butene alkylation

Article abstract of DOI:10.1016/j.molcata.2014.11.020

Ionic liquid catalyzed isobutane/2-butene alkylation modified with metal compounds was studied. The effect of catalyst composition on the alkylation selectivity was investigated. ²⁷Al NMR, ESI-MS, and FTIR spectra reveal that the catalytic selectivity of the modified ionic liquid is probably determined by the catalyst composition rather than by the acid strength. The complexation of transition metal with 2-butene can increase the internal isobutane-to-olefin ratio of feed during the alkylation reaction, which results in the better selectivity of the modified ionic liquid. The best ionic liquid catalysts were those containing CuAlCl₄ complexes, giving the alkylate with 87.5 wt% trimethylpentanes and a calculated research octane number (RON) of 100.5.

Full text of DOI:10.1016/j.molcata.2014.11.020

Journal of Molecular Catalysis A: Chemical 398 (2015) 133–139
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Effects of catalyst composition on the ionic liquid catalyzed
isobutane/2-butene alkylation
∗
Ying Liu , Rui Li, Hongjuan Sun, Ruisheng Hu
School of Chemistry and Chemical Engineering, University of Inner Mongolia, Hohhot, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 September 2014
Received in revised form
Ionic liquid catalyzed isobutane/2-butene alkylation modified with metal compounds was studied. The
effect of catalyst composition on the alkylation selectivity was investigated. Al NMR, ESI-MS, and FTIR
27
spectra reveal that the catalytic selectivity of the modified ionic liquid is probably determined by the
catalyst composition rather than by the acid strength. The complexation of transition metal with 2-butene
can increase the internal isobutane-to-olefin ratio of feed during the alkylation reaction, which results
in the better selectivity of the modified ionic liquid. The best ionic liquid catalysts were those containing
CuAlCl4 complexes, giving the alkylate with 87.5 wt% trimethylpentanes and a calculated research octane
number (RON) of 100.5.
2
0 November 2014
Accepted 21 November 2014
Available online 4 December 2014
Keywords:
Alkylation
Isobutane
©
2014 Elsevier B.V. All rights reserved.
2
-Butene
Ionic liquid
1
. Introduction
of IL can be readily tuned by using different organic cations or
inorganic anions. Compared with solid acids, most IL can maintain
high catalytic activity during the whole alkylation reaction without
carbon deposition problems. Despite no IL has reached indus-
trial application, the composite IL [Et NH]Cl–AlCl –CuCl, which
Alkylation of isobutane with C –C5 alkenes is an important
3
industrial process for the production of high-octane gasoline with
low residual vapor pressure. To date, only sulfuric acid (H SO ) and
2
4
3
3
hydrofluoric acid (HF) have been extensively commercially applied
for the alkylation due to economic considerations [1,2]. However,
acid sludge treatments and safety protection may be the most dif-
ficult problems that restrict the commercial application of the two
acids [3]. Due to the growing awareness of environmental issues,
it is necessary to use some safer and easily regenerable materials
to catalyze the reaction. Molecular sieves and heteropolyacids are
known to possess high catalytic activities and selectivities. These
solid acids have been studied as potential catalysts for alkylation
of isobutane with alkenes for years. Despite the potential benefits
presented by the solid acids, they have still not achieved commer-
cial use as alkylation catalysts due to the fast catalyst decay [4]. In
order to enhance the lifetime of solid catalysts, supercritical fluid
like supercritical isobutane has been used as media for solid acid
catalyzed alkylation [5].
is a chloroaluminate IL modified with cuprous chloride, has been
reported for the pilot-scale alkylation test [1].
It is well known that acid strength and catalyst composition will
affect the formation of t-butyl cation (t-C4+) and the rate of hydride
transfer [7]. Thus, the acidity and composition of IL are probably
the most important parameters affecting alkylation. Recently, influ-
ences of the IL acidity on isobutane/2-butene alkylation have been
studied. When some metal salts are mixed with the chloroalumi-
nate IL [Et NH]Cl–AlCl , it is supposed that the change of Lewis
3
3
acidic site in the IL that results in the alkylate quality increases
dramatically [8,9]. Zhang et al. [10] also found that the addition
of benzene to [Et NH]Cl–AlCl3 could obviously improve the alky-
3
late composition. They consider that benzene leads to modify the
Lewis acidity of [Et NH]Cl–AlCl , and the modification is presum-
3
3
ably beneficial for alkylation because it inhibits the side reactions.
Additionally, it is believed that increasing the concentration of HCl
or the molar fraction of AlCl3 in [BMIm]Cl–AlCl3 will increase its
acidity. Such an increase leads to an increase in the butene conver-
sion and a decrease in the rate of side reactions. Adding water in
an acidic molten salt can play a similar role to HCl [11]. For non-
chloroaluminate based IL, a dramatic enhancement of the catalytic
Liquid superacid catalysts such as ionic liquids (IL) are also
receiving increased attention [1,6]. The main advantages arising
from the use of IL are their clean catalytic properties for alkyla-
tion processes. Additionally, the physical and chemical properties
−
effect was observed when the IL containing the SbF6 anion [12,13].
∗ Corresponding author. Tel.: +86 471 4992981.
E-mail address: celiuy@imu.edu.cn (Y. Liu).
Using the binary mixture of acidic imidazolium IL and sulfuric or
http://dx.doi.org/10.1016/j.molcata.2014.11.020
1
381-1169/© 2014 Elsevier B.V. All rights reserved.

1
34
Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 133–139
triflic acid has the similar effects [14]. The better catalytic perfor-
mances of such catalysts maybe result from the complex acidity
and solubility effects of IL. However, the aforementioned studies
were essentially focused on discussing the effects of acidity and
acid strength, it is necessary to investigate more characteristics of
the catalyst composition to better understand the behavior of IL
during the alkylation.
Characterization of the IL anions was achieved using a nega-
tive mode of electrospray ionization mass spectrometry (ESI-MS).
ESI-MS was recorded on a Thermo Finnigan LCQDeca XP Plus
quadrupole ion trap instrument on samples dissolved in methanol.
2.3. Procedure and analysis
In this study, we report our studies on the effect of the IL
composition on the alkylation of isobutane with 2-butene. Chloroa-
luminate IL [BMIm]Cl–AlCl , [BMIm]Br–AlCl , and [Et NH]Cl–AlCl
were used as catalysts in the studies. They were also modified
with transition metal salts, metal complexes, and metallic oxides.
The catalytic selectivities of the IL catalysts were compared with
those obtained in pure strong acids under the identical reaction
conditions. The acid strength and composition of the IL were
also investigated by mean of FTIR, 27Al NMR, and electrospray
ionization-mass spectrometry (ESI-MS).
Isobutane and 2-butene were commercial products from China
National Petroleum Corporation (CNPC) with 99% purity and used
without further purification. Alkylation reactions were carried out
in a liquid-phase pressure vessel (100 mL) with a liquid inlet. A
mechanical stirrer with a fixed impeller on the shaft provided agita-
tion at 1200 r/min. The catalyst (e.g., 15 mL) was put into the reactor
at first. After stirring, the liquid feed (a mixture of isobutane/2-
butene in a 7.5:1 molar ratio) was charged into the reactor by means
of a plunger pump at a rate of 500 mL/h. The reaction temperature
was controlled at 298 K by a water bath with temperature con-
troller. When the pressure of the reactor was higher than 0.4 MPa,
we stopped the pump and stirrer. The total reaction time was about
3
3
3
3
2
. Experimental
3
0 min. The product and catalyst were decanted from the reactor
2
.1. Preparation of catalysts
and settled for 30 min. The product was fractionated to remove
isobutane and distilled in a Claisen flask. Aliquots of the hydrocar-
bon phase were withdrawn for analysis.
Sulfuric acid (H SO , 98 wt%) and trifluoromethanesulfonic acid
2
4
(
(
CF SO H) were purchased from Sinopharm Chemical Reagent
The investigation for the isobutane-to-olefin (I/O) molar ratio
of isobutane/2-butene feed in the toluene-CuAlCl4 system was
also performed in the alkylation reactor. In a typical experiment,
the toluene-CuAlCl4 (15 mL) mixture was introduced into the
reactor at room temperature. A mixture of isobutane/2-butene (ini-
tial I/O = 7.5) from the feed vessel was pumped into the stirred
(1200 r/min) reactor. The agitation was continued for 30 min,
and then the liquid outlet at reactor was opened. The excess
isobutane/2-butene mixture was vented at room temperature for
analyzing the final molar ratio of isobutane to 2-butene.
3
3
SCRC). Before use, CF SO H were distilled under dry nitrogen.
3
3
The triethylamine hydrochloride ([Et NH]Cl), chloride or
3
bromide salts of the 1-butyl-3-methyl- imidazolium cations
[BMIm]Cl, [BMIm]Br) were all obtained from Sigma–Aldrich
Chemical Company and dried under vacuum at 383 K.
Et NH]Cl–AlCl , [BMIm]Cl–AlCl , and [BMIm]Br–AlCl chloroa-
(
[
3
3
3
3
luminate IL were prepared and characterized using methods
described earlier [15]. The molar ratio of AlCl3 to organic salt is
1
.8:1 in this work.
The IL/additive systems were prepared by directly adding addi-
The alkylate samples were sent to a gas chromatograph
(Hewlett-Packard, 6890). The GC column used for separation was a
Supelco Petrocol DH capillary column (50 m × 0.1 mm × 0.1 mm).
Initial qualitative identifications of the products were accom-
plished with the help of a gas chromatograph (Hewlett-Packard,
5890 Series II) equipped with a mass spectrometer (Hewlett-
Packard, 5972 Series II column). The research octane number (RON)
of alkylate was calculated, according to the method applied in Ref.
[7,11]. The gas samples of isobutane/2-butene were also sent to the
Hewlett-Packard 6890 GC for investigating the change of I/O ratio
of feed. Except the injection mode, the analysis conditions of gas
samples were as the same as the alkylate samples.
tives to the chloroaluminate IL. Additives included transition metal
salts, metal complexes, and metallic oxides. They were added to the
IL at the beginning of the alkylation reaction. Most of IL/additive
catalysts were suspension. According to the literature preparation
[
16,17], the mixed-metal chloride CuAlCl4 can be readily synthe-
sized in high yield from the melt of CuCl and AlCl3 (<523 K).
BMIm]Cl–AlCl –CuCl, [BMIm]Br–AlCl –CuCl,
[
and
3
3
[
Et NH]Cl–AlCl –CuCl were composite IL used in this study.
3
3
They were prepared by introducing cuprous chloride to the
chloroaluminate IL [18]. In a typical method of preparing compos-
ite IL, anhydrous aluminum chloride (1.8 mol) was slowly added
to a round-bottomed flask containing dry 1-butyl-3-methyl- imi-
dazolium chloride (1 mol) under a nitrogen atmosphere at 383 K.
After formation of chloroaluminate ionic liquid, 0.5 mol of cuprous
chloride was added to the above ionic liquid. The reaction mixture
was stirred at 383 K overnight to allow a complete homogenization
of the resulting ionic liquid.
3. Results and discussion
3.1. Alkylation reactions under identical conditions
The reaction mechanism of acid catalyzed isobutane/2-butene
alkylation is a chain process occurring through carbocation inter-
2.2. Characterization of ionic liquids
mediates [19]. The protonation of 2-butene can form t-butyl cations
+
+
(t-C4 ), which initiate the alkylation reaction. A t-C4 cation is
Acidity characterizations were carried out by infrared spec-
added to a 2-butene molecule to form a 2,2,3-trimethylpentane car-
+
+
troscopy using pyridine as the acid probe at room temperature,
along with a Bio-Rad high-resolution FTIR spectrometer (FTS-40).
All spectra were manipulated by Bio-Rad Win-IR software. The
samples were prepared by mixing pyridine and IL in a volume ratio
of 5:1, and smeared into liquid films on KBr windows.
bocation (2,2,3-TMP ). Some of 2,2,3-TMP cations will isomerize
to the corresponding 2,2,4-trimethylpentane carbocations (2,2,4-
+
TMP ). The desired product 2,2,4-trimethylpentane (2,2,4-TMP) is
mainly obtained by intermolecular hydride transfer between isobu-
+
tane and 2,2,4-TMP . The hydride transfer also generates a new
27
The Al NMR measurements were carried out on a Bruker WB-
00 AMX spectrometer to investigate the difference of anions. The
t-butyl cation that will react with another 2-butene molecule.
Many side reactions that lead to forming low RON products are
always accompanying with the main reaction. The intermediate
4
spectra were obtained at 130.32 MHz with a preacquisition delay
2
7
+
+
time of 0.5 s. The Al NMR chemical shifts were referenced to an
TMP cation often competes with t-C4 cation in the hydrocarbon-
3
+
phase and adds to another 2-butene molecule forming i-C12+, i-
external Al(H O)
standard.
2
6

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 133–139
135
Table 1
The composition of alkylates in acidic catalysts (wt%).a
Catalysts
H2SO4
CF3SO3H
[Et3NH]Cl–AlCl3
[BMIm]Cl–AlCl3
Alkanes
Light ends
TMP
21.8
36.0
8.1
13.2
20.8
89.2
9.5
49.4
19.7
6.8
14.5
89.2
12.4
37.1
11.0
10.5
28.9
86.8
19.9
43.7
10.0
7.0
19.3
90.6
DMH
2
,2,5-trimethylhexane (TMH)
Heavier ends
RON
a
Reaction conditions: initial I/O = 7:1, temperature = 303 K, agitation = 1200 r/min, and catalyst = 15 mL.
C16+ and so on. Most C12 and heavier alkanes are the results of
the hydride transfer of these large cations. Strong acid can readily
97.1
92.0
cleave C12+ and C
+
cations to the C5 –C7 and dimethylhexane
+
+
(d) [BMIm]Cl-AlCl -CuCl
3
16
+
cations (DMH ) through ␤-scissions [20]. These carbocations are
believed to be the major source of light ends (C5–C7 alkanes) and
dimethylhexanes (DMH) [21].
97.1
1
03.1
It can be easily understood that in a complex reaction such
as alkylation, the reaction conditions are going to play an impor-
tant role in determining product distribution. In order to compare
our results using chloroaluminate IL as the catalysts with con-
ventional liquid acids, firstly, alkylation reactions were performed
(c) [BMIm]Cl-AlCl3
9
7.2
9
2.1
(
b) [Et NH]Cl-AlCl -CuCl
3
3
using H SO and CF SO H catalysts. In Table 1, a comparison of the
2
4
3
3
103.1
product composition for different catalysts is given. Despite the dif-
ferent types of catalyst used, under the identical reaction condition
some common features were observed in the product composi-
tion: the predominance of C8 in a spectrum of C5–C9+ paraffins,
the low content of olefinic products, and the high selectivity for
trimethylpentanes (TMP). The close similarity in the alkylate pro-
duced from different catalysts leads to the conclusion that the same
chain mechanism is operative on different types of catalyst.
(
a) [Et NH]Cl-AlCl3
3
180
160
140
120
100
80
60
40
20
0
ppm
Fig. 1. 27Al NMR spectra of ionic liquids: (a) [Et NH]Cl–AlCl , (b)
[Et3NH]Cl–AlCl3–CuCl, (c) [BMIm]Cl–AlCl3, (d) [BMIm]Cl–AlCl3–CuCl.
3
3
When [Et NH]Cl–AlCl3 and H SO4 were employed as catalysts,
3
2
isobutane/2-butene was alkylated by [BMIm]Cl–AlCl using metal-
3
more light ends (C5–C7) and heavier ends (C9+) were observed in
the alkylates. By contrast, more TMP was formed using CF SO H or
lic oxides as additives, the same products were obtained, with the
distribution slightly shifted to a higher production of C9+ alkanes.
Results of alkylation using [Et NH]Cl–AlCl /additive as well
3
3
[
BMIm]Cl–AlCl than that obtained with H SO or [Et NH]Cl–AlCl .
3 2 4 3 3
3
3
This result may be due to the solubility of isobutane in acid phase.
A larger amount of isobutane is considered to dissolve in CF SO H
as the composite IL [Et NH]Cl–AlCl –CuCl as catalysts are given
3
3
3
3
in Table 3. Changes in the product composition similar to the
results shown in Table 2 were observed. With the addition of tran-
sition metal compounds to [Et NH]Cl–AlCl , a lower amount of
or [BMIm]Cl–AlCl than in H SO or [Et NH]Cl–AlCl , which results
3
2
4
3
3
in fewer side reactions.
3
3
However, the obtained RON value of our catalysts was lower
than that reported in literature, which may be a result of ineffi-
cient mixing of the hydrocarbon and acid phases under our reaction
conditions. The higher reaction temperatures of the alkylation reac-
tions also lower the quality of alkylate.
heavier ends and a higher amount of TMP were obtained, which
increased the alkylate quality. The best alkylate was found again
when the composite IL catalyzed the reaction. The RON difference
between the cases of [BMIm]Cl–AlCl3 and [Et NH]Cl–AlCl3 sys-
3
tems was probably due to a lower dissolved ratio of isobutane in
[
Et NH]Cl–AlCl3 than in [BMIm]Cl–AlCl3 [22].
3
3.2. Effects of metal compounds on alkylation reaction
3.3. Effects of metal compound on IL composition
The alkylation of isobutane/2-butene was studied using mod-
ification of the IL by introducing various metal compounds.
Tables 2 and 3 show the product distribution from alkylation
To investigate the effect of metal compound on the IL com-
position, we studied the Al species of [BMIm]Cl–AlCl3 and
27
reactions using modified [BMIm]Cl–AlCl3 and [Et NH]Cl–AlCl3
[Et NH]Cl–AlCl3 systems using
Al NMR characterization. The
3
3
as catalysts, respectively. The addition of additive obviously
affected the composition of the alkylate obtained. Results of
results are shown in Fig. 1.
On the basis of their chemical shifts, the two peaks in the
−
[
BMIm]Cl–AlCl /additive and [BMIm]Cl–AlCl –CuCl are summa-
spectrum of [BMIm]Cl–AlCl3 (Fig. 1(c)) can be assigned to AlCl4
3
3
− −
(97.1 ppm) and Al Cl7 (103.1 ppm) [23,24]. In contrast, AlCl4 is
2
rized in Table 2. When metal compound additives were added to
[
(
BMIm]Cl–AlCl , TMP were the major components of the alkylate
>50 wt%). DMH, light ends, and heavier ends were obtained as the
the dominant Al species in [BMIm]Cl–AlCl –CuCl sample (Fig. 1(d)),
3
3
−
−
indicating the part of Al Cl7 be transformed to AlCl4 . The appear-
2
relatively minor components, indicating alkylation reactions were
predominant. Adding CuCl, AgCl, or ZnCl2 salt to the IL, a marked
increase in TMP was observed and the amount of DMH and heav-
ier ends was decreased. However, these additives seem to be less
affected the amount of light ends. In addition, TMP content peaked
ance of a new peak at 92.0 ppm is believed the resonance signal
−
−
of CuAlCl5 , the ion of AlCl4 coordinating with CuCl [18]. For
−
the [Et NH]Cl–AlCl3 sample, only one peak (103.1 ppm, Al Cl7
)
3
2
appears in the spectrum (Fig. 1(a)). But there are two peaks at
97.2 ppm and 92.1 ppm in the spectrum of [Et NH]Cl–AlCl –CuCl
3
3
− −
when [BMIm]Cl–AlCl –CuCl was used, which produced the lowest
(Fig. 1(b)), which are CuAlCl5 and AlCl4 , respectively.
3
amount of light ends, DMH, and heavier ends. Thus, the composite IL
It is suggested that the change in anions, and therefore the
composition of IL systems, is the main factor resulting in different
[
BMIm]Cl–AlCl –CuCl gave the alkylate with the best quality. When
3

1
36
Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 133–139
Table 2
Product composition (wt%) using [BMIm]Cl–AlCl3 as catalyst.
a
Ionic liquid
Additive
Light ends
TMP
DMH
2,2,5-TMH
Heavier ends
RON
CuCl
AgCl
ZnCl2
NiCl2
CeCl3
SnCl2
CuO
18.3
18.7
16.8
18.8
12.9
15.3
17.7
18.2
16.2
15.4
8.7
60.8
62.8
60.1
67.3
67.8
60.2
58.3
50.0
54.7
64.7
71.8
5.5
4.3
7.0
4.4
4.5
4.9
3.4
4.9
4.8
5.0
4.8
3.8
2.9
2.9
2.4
3.1
3.2
3.6
3.8
4.4
3.1
4.9
11.6
11.3
13.1
7.1
11.8
16.4
17.1
22.9
19.9
11.6
9.9
94.7
95.3
93.9
96.1
95.8
93.8
93.9
91.0
92.4
95.1
97.3
[
BMIm]Cl-AlCl3
ZnO
Tb4O7
CeO2
–
[BMIm]Cl–AlCl3–CuCl
a
Reaction conditions: initial I/O = 7:1, temperature = 303 K, agitation = 1150 r/min, IL = 15 mL, and additive = 5 g.
100
191.00
A: Al Cl (OH) H O +CuAlCl
-
915.41
2
2
4
2
5
-
B: Al Cl +CuAlBrCl
3
9
4
-
-
C: 2[CuAlBrCl ]+Al Cl (OH)
3
3
2
8
D: 3[CuAlBrCl ]+Al Cl (OH)
8
3
3
2
80
60
40
20
1
175.50
1893.83
6
55.42
952.55
C
D
226.83
A
B
712.74
8
40.75
1
115.75
1444.76 1617.37
172.99
4
76.82
200
400
600
800
1000
1200
1400
1600
1800
2000
m/z
(a)
1
430.65
1
00
-
1
706.15
A: CuAlCl +Al Cl O (OH)
B
4
3 3 3
-
B: 5>CuAlCl @+CuAlCl
4
5
80
60
40
20
0
1
785.65
A
4
85.52
9
15.48
2
26.75
6
55.50
1
175.58
1200
1
852.45
4
43.10
6
13.25
600
2
00
400
800
1000
1400
1600
1800
2000
m/z
(b)
Fig. 2. ESI-MS spectra of: (a) [BMIM]Br–AlCl3–CuCl (b) Et3NHCl–AlCl3–CuCl.

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 133–139
137
Table 3
Product composition (wt%) using [Et3NH]Cl–AlCl3 as catalyst.a
Ionic liquid
Additive
Light ends
TMP
DMH
2,2,5-TMH
Heavier ends
RON
ZnCl2
CuCl
CeCl3
CuCl2
ZnO
CuO
CeO2
–
10.8
8.1
55.6
61.8
59.2
49.3
44.9
45.6
52.2
68.0
7.1
16.2
15.7
12.1
12.4
13.3
15.0
8.3
6.9
5.1
5.9
5.7
7.4
12.9
6.4
5.8
19.6
8.8
6.0
17.9
24.5
13.6
9.4
92.0
92.8
93.2
90.3
88.4
90.6
91.9
95.1
13.2
14.9
10.8
14.6
17.0
7.9
[
Et3NH]Cl–AlCl3
[Et3NH]Cl–AlCl3–CuCl
10.0
a
Reaction conditions: initial I/O = 7:1, temperature = 303 K, agitation = 1150 r/min, IL = 15 mL, and additive = 5 g.
-
1
-1
catalytic performances of the systems. The higher amount of TMP
1535 cm
1437 cm
obtained for the composite IL may be attributed to the appearance
−
of CuAlCl5
.
(
f) [Et NH]Cl-AlCl3
3
More investigation had been done to further study the com-
position of IL. Unlike the IL samples for Al NMR detection, the
IL samples for ESI-MS characterization had been used to catalyze
27
(e) [Et NH]Cl-AlCl -CuCl
3 3
the alkylation reaction. For the [BMIm]Br–AlCl –CuCl sample, some
3
(d) [BMIm]Cl-AlCl -CuCl
3
−
new complex species like CuAlBrCl4 and CuAlBrCl were detected
3
(
c) [BMIm]Cl-AlCl3
−
in the spectrum (Fig. 2(a)). CuAlCl5 and CuAlCl4 species were also
identified in the ESI-MS characterization of [Et NH]Cl–AlCl –CuCl
3
3
(
b) [BMIm]Cl-ZnCl2
a) pure pyridine
sample (Fig. 2(b)).
.4. Effects of metal compound on acid strength
For H SO and HF catalysts, the composition of the acids is not
(
3
1600
1550
1500
1450
1400
1350
-
1
2
4
Wavenumber (cm )
the only factor affecting the quality of the alkylate. The change in
acid strength will also result in producing the alkylates with dif-
ferent RON values. Thus, Bui [25] and Huang [9] are convinced that
Fig. 3. FTIR spectra of ionic liquids using pyridine as an acidity probe: (a) pure
pyridine, (b) [BMIm]Cl–ZnCl2, (c) [BMIm]Cl–AlCl3, (d) [BMIm]Cl–AlCl3–CuCl, (e)
[
Et3NH]Cl–AlCl3–CuCl, (f) [Et3NH]Cl–AlCl3.
adding metal chlorides to [Et NH]Cl–AlCl3 will greatly affect the
3
acid strength and enhance the catalytic performance of the IL. The
higher amount of TMP, obtained for [Et NH]Cl–AlCl /CuCl system,
those obtained for the IL without Cu-Al complex. By increasing
the amount of complex additive from 2.5 to 5 g results, for the
[Et3NH]Cl-AlCl3/CuAlCl4 system, in an increase of the TMP amount
from 71.7 to 87.5. [BMIm]Cl-AlCl3/CuAlCl4 has a similar trend, but
the TMP of [BMIm]Br-AlCl3/CuAlCl4 system is slightly increased
from 70.1 to 83.1 due to the higher acid strength of [BMIm]Br-AlCl3
[15].
The potential recyclability of [BMIm]Cl-AlCl3 with CuAlCl4 as
additive was also investigated in a continuous alkylation process.
During the process, the alkylation product with excess isobutane
was pumped out of the reactor every 0.5 h for analysis. The results,
given in Fig. 4, show that the catalyst can be maintained good
catalytic performance less than 3 h. The catalyst’s poor TMP selec-
tivities after 3 h may be due to the deactivation of chloroaluminate
IL by the water from isobutane.
3
3
is likely to be a lower acid strength than that of the [Et NH]Cl–AlCl .
3
3
The FTIR analysis can investigate the change of Lewis acidi-
ties of IL by using pyridine as a molecular probe [26]. The degree
−
1
of monotonic blue shift of the band near 1437 cm
shows the
increase in the Lewis acid strength and the height of the peak
−1
near 1535 cm represents the Brønsted acid strength [25]. Further
study on the role of metal chloride on the acidity of IL was carried
out using the same method (Fig. 3). The wavenumber of the band
−
1
−1
near 1437 cm increases from 1441.49 cm for [BMIm]Cl–AlCl3
−
(
Fig. 3 (c)) to 1442.80 cm 1 for [BMIm]Cl–AlCl –CuCl (Fig. 3(d)),
3
indicating that the Lewis acid strength slightly decreases. But
the [Et NH]Cl–AlCl and [Et NH]Cl–AlCl –CuCl samples almost
3
3
3
3
−
1
have the same wavenumbers near 1437 cm , which are 1442.01
Fig. 3(f)) and 1441.98 cm (Fig. 3(e)), respectively. According to
−
1
(
these results, the product distribution of the comparison IL sam-
ples should have very little difference. However, as Tables 2 and 3
shown, more TMP were obtained with the modified IL when
compared to the net IL. In addition, the wavenumber bands for
3
.6. CuAlCl4 and 2-butene adsorption
Transition metal complexes like CuAlCl4 and AgAlCl4 can be
−
BMIm]Cl–AlCl and [BMIm]Cl–ZnCl (Fig. 3(b)) near 1437 cm are
3 2
1
[
used as the selective separating agent for light olefins and paraffins
separations [27]. The formation of electron donor/acceptor com-
plexes between alkenes and some transition metals has long been
known. Herberhold [28] satisfactorily explained the nature of the
weak chemical bonding of olefin-metal complexes.
also very close, but [BMIm]Cl–ZnCl2 cannot catalyze the alkylation
reaction due to its weak acid strength. The Et NHCl–AlCl –CuCl
3
3
and Et NHCl–AlCl samples have almost identical peak areas
3
3
−1
at 1535 cm , implying that they have the same Brønsted acid
strength.
The relationship between CuAlCl4 and 2-butene adsorption
was examined by charging the liquid-phase isobutane/2-butene
3.5. Effect of CuAlCl4 on alkylate composition
into the CuAlCl /toluene solution. The condition of the 2-butene
4
adsorption test was identical to the alkylation reaction (15 mL
toluene, 0.4 MPa, agitation = 1200 r/min, and the initial I/O ratio of
feed = 7.5). Here, toluene was used as a solvent, and a series of
different weights of CuAlCl4 was used to investigate the CuAlCl4
adsorption ability for the olefin. As Fig. 5(a) shown, the final I/O
ratio of isobutane/2-butene feed was increased when increasing
The complex CuAlCl4 was used as the additive of IL to study
its effect on the alkylate composition. Both little (2.5 g) and more
5 g) amount of CuAlCl4 were investigated and compared with
those of net chloroaluminate IL systems (Table 4). The quality of
alkylates was high for the IL/CuAlCl4 systems when compared to
(

1
38
Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 133–139
100
Table 5
a
Effects of metal compounds on 2-butene adsorption.
90
80
70
60
50
40
30
20
10
0
Solvent
Toulene
Additive
Final I/O ratio of isobutane/2-butene feed
ZnCl2
CuCl
CeCl3
NiCl2
13.8
15.1
20.2
17.6
15.3
31.3
29.4
28.1
25.7
26.2
SnCl2
AgAlCl4
AgBF4
AgClO4
Ni(AlCl4)2
b
ZnCl2–AlCl3c
a
Reaction
conditions:
initial
I/O = 7:1,
temperature = 283 K,
agita-
tion = 1200 r/min, toluene = 15 mL, additive = 5 g.
b
Preliminary experiments indicate that Ni(AlCl4)2 formed readily when NiCl2 and
◦
Al2Cl6 are mixed at 125 C.
c
◦
When ZnCl2 and AlCl3 are mixed at 210 C, the reddish-orange compound with
0
1
2
3
4
5
monoclinic needle structure will be formed.
Continuous alkylation time (h)
Fig. 4. Relationship between TMP and continuous alkylation process of
BMIm]Cl–AlCl3: I/O = 7:1, temperature = 283 K, agitation = 1150 r/min, IL = 15 mL,
and CuAlCl4 = 5 g.
employed to promote the desired reaction. We would like to inves-
tigate how I/O ratio affected the alkylation product when using IL
as catalysts.
[
When isobutane was alkylated with 2-butene in a batch reactor
283 K, 1200 r/min, and 15 mL [BMIm]Cl–AlCl3 catalyst), the trend
the amount of CuAlCl4 in the toluene-CuAlCl4 system. This result
suggests that the toluene-CuAlCl4 system could adsorb the olefin
just because of the complexation of Cu(I)-butene.
(
of I/O ratio vs. alkylate distribution of [BMIm]Cl–AlCl3 was plot-
ted in Fig. 5(b). With I/O ratio increasing, TMP markedly increased
and a maximum in TMP yield was found at I/O = 33. When high I/O
ratios are fed, 2-butene has a higher probability of reacting with an
isobutane molecule to form the desired product than of performing
butene-butene polymerization [29].
3.7. Influence of isobutane-to-olefin ratio
The isobutane-to-olefin ratio (I/O) of feed is another key aspect
in alkylation reaction. Because the conditions that favor alkylation
also favor polymerization of olefins, an excess of isobutane is always
It can be easily understood the results listed in Table 4. When
[Et NH]Cl–AlCl /CuAlCl or [BMIm]Cl–AlCl /CuAlCl4 was used as
3
3
4
3
Light ends
TMP
DMH
2,2,5-tri-Me-hexane
Heavy ends
3
3
3
3
2
2
2
2
2
1
1
1
1
1
6
4
2
0
8
6
4
2
0
8
6
4
2
0
8
6
3
3
28
2
4
2
1
2
3
7
0
10
20
30
40
50
60
70
80
90
100
0
2
4
6
8
10
(
b) Product distribution of [BMIm]Cl-AlCl , wt%
3
(
a) The amount of CuAlCl in toluene (g)
4
Fig. 5. Relationship between I/O and product distribution of [BMIm]Cl–AlCl3.
Table 4
Effect of CuAlCl4 on the product composition (wt%).
a
IL system
[Et3NH]Cl–AlCl3
[BMIm]Br–AlCl3
[BMIm]Cl–AlCl3
CuAlCl4 (g)
None
2.5
5
10
None
2.5
5
None
2.5
5
10
Alkanes
Light ends
TMP
13.8
49.8
7.9
8.2
71.7
6.8
2.2
87.5
5
3.8
64.9
4.7
19.3
51.9
9.4
11.3
70.1
4.6
4.1
83.1
4.7
17.5
53.1
6.4
6
10.5
70.6
7.3
2.9
85.2
4.6
4.5
65.4
6.4
DMH
2
,2,5-TMH
3.1
5.5
1.6
4.9
3.2
2.7
1.5
3.6
0.9
5.7
Heavier ends
RON
25.4
90.0
7.9
96.4
3.7
100.5
21.7
93.1
16.1
91.8
11.2
96.1
6.6
98.6
17
92.1
8
96.3
6.4
98.8
18
93.6
a
Reaction conditions: initial I/O = 7:1, temperature =283 K, agitation = 1150 r/min, IL = 15 mL.

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 133–139
139
Table 6
Effects of transition metal complexeson product composition (wt%) of [BMIm]Cl–AlCl3.
a
Ionic liquid
Trational metal complex
light ends
TMP
DMH
2,2,5-TMH
Heavier ends
RON
[BMIm]Cl–AlCl3
AgAlCl4
AgBF4
AgClO4
Ni(AlCl4)2
ZnCl2–AlCl3
2.8
3.3
4.5
9.6
9.6
83.4
81.3
78.6
75.9
77.8
5.7
4.6
5.4
3.3
1.7
1.6
4.1
2.9
2.9
2
6.5
6.7
8.6
8.3
8.9
98.3
98.2
97.9
97.3
97.7
a
Reaction conditions: initial I/O = 7:1, temperature = 283 K, agitation = 1200 r/min, IL = 15 mL, and transition metal complex = 5 g.
the alkylation catalyst, 2-butene could be adsorbed by CuAlCl4
which increased the I/O ratio of the alkylation process. As the for-
Adding a small amount of transition metal compound cannot signif-
icantly change the acid strength of the IL, but can increase the ratio
of I/O of the reaction system. It is clear that the I/O of hydrocarbon
phase will significantly affect the product composition of IL, and
CuAlCl4 in IL can helpincrease the I/Oratio due to the Cu(I)-butene
complexation. The trace amounts of Al-transition metal species like
mation of complex between 2-butene and CuAlCl , the hydride
4
transfer between isobutane and TMP+ becomes more easily and
generates more TMP product. However, further adding more
CuAlCl4 (10 g) to the IL decreases the quality of alkylates (Table 4).
This could be explained by a lower interfacial area between the IL
and hydrocarbon phases where alkylation reactions occurred [30].
CuAlCl , CuAlBrCl3 are detected in the IL catalysts, which adsorb
4
the 2-butene in the alkylation process. The better TMP selectivity
of the modified IL can be attributed to the formation of complexes
between 2-butene and transition metals.
3.8. Metal compounds and 2-butene adsorption
Munson et al. [31] have reported the separation of olefins
Acknowledgement
from paraffins using IL solutions. In Table 5, comparable trends of
-butene adsorption are observed for transition metal-toluene sys-
2
We thank the National Natural Science Foundation of China for
financial support (No. 21266015, No. 20806036).
tems. Although most transition metal compounds are not dissolved
in toluene, toluene can be used as an IL simulant. Under identical
conditions of the alkylation reaction, the systems were all found
give a higher I/O ratio after operation, indicating that some other
metal compounds could adsorb 2-butene of feed.
References
[
1] H. Olivier-Bourbigou, L. Magna, D. Morvan, Appl. Catal. A: Gen. 373 (2010)
–56.
2] A. Corma, A. Martinez, Catal. Rev. Sci. Eng. 35 (1993) 483–570.
1
According to the aforementioned chain mechanism, higher
I/O ratio of feed will decrease the reaction probabilities of
[
[3] W.Z. Sun, Y. Shi, J. Chen, Z.H. Xi, L. Zhao, Ind. Eng. Chem. Res. 52 (2013)
15262–15269.
[4] J. Weitkamp, Y. Traa, Catal. Today 49 (1999) 193–199.
C8 + CH3CH = CHCH3 → C , C16+ because of more isobutane
+
+
12
+
molecules around C cations. It seems to be an explanation for
8
[
5] C.J. Lyon, V.R. Sarsani, B. Subramaniam, Ind. Eng. Chem. Res. 43 (2004)
809–4814.
[6] Z.C. Zhang, Adv. Catal. 49 (2006) 153–237.
the better selectivities of composite IL and IL/metal compound sys-
tems. Commercially, internal circulation consists of circulating the
reactor contents rapidly and introducing fresh isobutane into the
circulating stream. Internal I/O ratios may be as high as 1 000:1[2].
In this study, although no fresh isobutane was charged into the
IL/metal chloride system, the adsorption of the 2-buteneby the
metal chloride that might dramatically increase the internal I/O
ratio (Table 5) and produce the high quality alkylate (as the data
listed in Tables 2 and 3). Similarly, the alkylates with higher RON
values (Table 6) were obtained by[BMIm]Cl–AlCl3/metal complex
systems just because the transition metal complexes could increase
the I/O ratios.
4
[
7] G.A. Olah, P. Batamack, D. Deffieux, B. Török, Q. Wang, Á. Molnár, G.K.S.
Prakash, Appl. Catal. A: Gen. 146 (1996) 107–117.
[
8] T.L.T. Bui, W. Korth, S. Aschauerb, A. Jessb, Green Chem. 11 (2009) 1961–1967.
[9] C.P. Huang, Z.C. Liu, C.M. Xu, B.H. Chen, Y.F. Liu, Appl. Catal. A: Gen. 277 (2004)
1–43.
4
[
[
10] J. Zhang, C.P. Huang, B.H. Chen, P.J. Ren, M. Pu, J. Catal. 249 (2007) 261–268.
11] Y. Chauvin, A. Hirschauer, H. Olivier, J. Mol. Catal. 92 (1994) 155–165.
[12] X.Q. Xing, G.Y. Zhao, J.Z. Cui, S.J. Zhang, Catal. Commun. 26 (2012) 68–71.
[
[
[
13] P. Cui, G.Y. Zhao, H.L. Ren, J. Huang, S.J. Zhang, Catal. Today 200 (2013) 30–35.
14] S. Tang, A.M. Scurto, B. Subramaniam, J. Catal. 268 (2009) 243–250.
15] K. Yoo, V.V. Naboodiri, R.S. Varma, J. Catal. 222 (2004) 511–519.
[16] R.M. Sullivan, H.M. Liu, D.S. Smith, J.C. Hanson, D. Osterhout, M. Ciraolo, C.P.
Grey, J.D. Martin, J. Am. Chem. Soc. 125 (2003) 11065–11079.
[17] J.D. Martin, B.R. Leafblad, R.M. Sullivan, P.D. Boyle, Inorg. Chem. 37 (1998)
−
−
Reaction CuAlCl5 → CuAlCl + Cl must have been taken place
4
1341–1346.
−
in the alkylation of the composite IL, because the CuAlCl5
[18] Y. Liu, R.S. Hu, C.M. Xu, H.Q. Su, Appl. Catal. A: Gen. 346 (2008) 189–193.
[19] J.E. Hofmann, A. Schriescheim, J. Am. Chem. Soc. 84 (1962) 953–957.
and CuAlCl4 were detected in the ESI-MS spectrum of the used
[
20] G.A. Olah, T. Mathew, A. Goeppert, B. Torok, I. Bucsi, X.Y. Li, Q. Wang, E.R.
Marinez, P. Batamack, R. Aniszfeld, G.K. Prakash, J. Am. Chem. Soc. 127 (2005)
5964–5969.
[
Et3NH]Cl AlCl3–CuCl (Fig. 2). Such a transition helpsmore 2-
buteneto beadsorbed by CuAlCl . However, the best alkylate quality
4
obtained with [Et3NH]Cl–AlCl3–CuCl (Table 3) wasstill lower
[21] L.F. Albright, Industrial and laboratory alkylations, in: ACS Symposium Series,
Vol. 55, ACS, Washington DC, 1977.
than that obtained using [Et3NH]Cl–AlCl /CuAlCl (RON = 100.5) or
3
4
[
22] K.W. Li, R.E. Eckert, L.F. Albright, Ind. Eng. Chem. Proc. Des. Dev. 9 (1970)
41–446.
[23] J.S. Wilkes, J.S. Frye, G.F. Reynolds, Inorg. Chem. 22 (1983) 3870–3872.
[
BMIm]Br–AlCl3 (RON = 98.6) as catalyst. This could be explained by
4
a lower amount of CuAlCl4 in the composite IL, which results in a
lower I/O ratio than that of the IL/CuAlCl4 system.
[
[
[
24] J.L. Gray, G.E. Maciel, J. Am. Chem. Soc. 103 (1981) 7147–7151.
25] T.L.T. Bui, W. Korth, A. Jess, Catal. Commun. 25 (2012) 118–124.
26] Y.L. Yang, Y. Kou, Chem. Commun (2004) 226–227.
4
. Conclusions
[27] D.J. Safarik, R.B. Eldridge, Ind. Eng. Chem. Res. 37 (1998) 2571–2581.
[
28] M. Herberhold, Metal Complexes: Part II: Specific Aspects, Elsevier, New York,
974.
29] L.F. Albright, Ind. Eng. Chem. Res. 42 (2003) 4283–4289.
1
Study of the effects of catalyst composition and acidity on
[
IL catalyzed isobutane/2-butene alkylation, shows better product
composition (indicated by the amount of TMP of the product) can
be obtained when the IL systems modified with metal compounds.
[30] B. Doshi, L.F. Albright, Ind. Eng. Chem. Proc. Des. Dev. 15 (1976) 53–60.
31] C.L. Munson, L.C. Boudreau, M.S. Driver, W.L. Schinski, US Patent 6623 659 B2
2003), to Chevron U.S.A. Inc. 1–8.
[
(