Influence of acidity of modified chloroaluminate based ionic liquid catalysts on alkylation of iso-butene with butene-2

Abstract

In the present work, aluminium chloride based ionic liquids (ILs) were used as catalysts for alkylation of iso-butane with butene-2. Some additives such as, transition metal salts, water and acidic cation exchange resins were used to improve the selectivity of the main products and the yield of reaction. A high content of the desired trimethylpentanes (TMPs) (up to 72.3%) and thus a high research octane number (RON up to 98) of the alkylate were received on using CuCl modified triethylamine hydrochloride aluminium chloride ([Et3NH]Cl/AlCl3) (molar fraction of AlCl3 is 0.6) at − 5 °C over a period of 15 min.The effectiveness of additives is assumed to explain by the formation of Brønsted acid or complexes which exhibit the Brønsted acidity or superacidity. The acidity of investigated catalytic systems was evaluated by FT-IR and confirmed that. The Brønsted and Lewis acidic sites were detected by FT-IR. The Lewis acidity can be adjusted mainly by mol fraction of AlCl3 while the Brønsted acidity can be increased by using the suitable amounts of additives. This is a valuable guide to explain the mechanism and to select the catalysts for alkylation.

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Short Communication
Influence of acidity of modified chloroaluminate based ionic liquid catalysts on
alkylation of iso-butene with butene-2☆
Thi Le Thuy Bui
a,
⁎, Wolfgang Korth
b
, Andreas Jess
b
a
Department of Oil Refinery and Petrochemical Engineering, Hanoi University of Mining and Geology, Vietnam
b
Department of Chemical Engineering, University of Bayreuth, 95440 Bayreuth, Germany
abstractarticle info
Article history:
Received 26 August 2011
Received in revised form 6 March 2012
Accepted 13 March 2012
Available online 23 March 2012
Keywords:
Alkylation
Ionic liquids
Acid cation exchange resins
Metal transition salts
Acidity
FT-IR
In the present work, aluminium chloride based ionic liquids (ILs) were used as catalysts for alkylation of iso-
butane with butene-2. Some additives such as, transition metal salts, water and acidic cation exchange resins
were used to improve the selectivity of the main products and the yield of reaction. A high content of the de-
sired trimethylpentanes (TMPs) (up to 72.3%) and thus a high research octane number (RON up to 98) of the
alkylate were received on using CuCl modified triethylamine hydrochloride aluminium chloride ([Et
3
NH]Cl/
AlCl
3
) (molar fraction of AlCl
3
is 0.6) at −5 °C over a period of 15 min.
The effectiveness of additives is assumed to explain by the formation of Brønsted acid or complexes which
exhibit the Brønsted acidity or superacidity. The acidity of investigated catalytic systems was evaluated by
FT-IR and confirmed that. The Brønsted and Lewis acidic sites were detected by FT-IR. The Lewis acidity
can be adjusted mainly by mol fraction of AlCl
3
while the Brønsted acidity can be increased by using the suit-
able amounts of additives. This is a valuable guide to explain the mechanism and to select the catalysts for
alkylation.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Currently, there is a general trend to use environmentally friendly
products and to develop clean and efficient catalytic processes which
minimise the harmful waste. Alkylation of iso-butane with butene
produces alkylates which have high octane number and exhibit
clean burning characteristics [1]. However, the industrially used cata-
lysts for alkylation are “non-green”because of highly corrosion and
environmentally detrimental effects [2,3].
Ionic liquids (ILs) are low melting salts (b100 °C) and exhibit a
new group of solvents and catalysts [4–7]. Their properties can be ad-
justed to be suitable for respective applications by changing the ions.
Therefore, they are intensively discussed as environmentally friendly
and less hazardous solvents or catalysts in chemical industry [8–10].
The adjustable acidity of aluminium chloride based ILs makes them
interesting catalysts for strong acid catalysed processes, especially
for alkylation [11].
Some works used ILs as catalysts for alkylation of iso-butane with
butene-2 and reported promising results [12–15]. Some additives
were found to bring significant improvements of the selectivity of
the reaction. In this research, some additives (water, acidic exchange
resin, transition metal salts (e.g. CuCl, CuCl
2,
and NiCl
2
) were mixed
with ionic liquids 1-Methyl-3-octylimidazolium bromide aluminium
chloride ([OMIM]Br-AlCl
3
) and triethylamine hydrochloride alumini-
um chloride ([Et
3
NH]Cl/AlCl
3
). These mixtures show higher selectivi-
ty and produce higher quality alkylate compared to neat ionic liquids.
The mechanism of effectiveness of additives was referred to the for-
mation of super acids or complexes which act as super acids.
The acidity of [OMIM]Br/AlCl
3
and [Et
3
NH]Cl/AlCl
3
ILs and their
mixture with additives were evaluated by FT-IR spectrum. The results
can be used to evaluate the catalytic activity and selectivity of the cat-
alytic systems for alkylation.
2. Evaluation of the acidity
The acidity of [OMIM]Br/AlCl
3
(or [Et
3
NH]Cl/AlCl
3
) depends signif-
icantly on the molar ratio of AlCl
3
to [OMIM]Br (or Et
3
NH]Cl) used to
prepare the IL. The molar fraction x of AlCl
3
is calculated by:
x¼mol of AlCl3=mol of AlCl3þmol of OMIM
½
Br
ðÞ
Ionic liquids having a molar fraction of AlCl
3
in the range of
0bxb0.5 are called basic ionic liquids, because “free”halogenide an-
ions are still present.
Catalysis Communications 25 (2012) 118–124
☆This work was presented on the Vietnam–German Conference on Catalytical and
Chemical Technology for Sustainable Development in Hanoi, February 21–23, 2011,
jointly organized by Vietnam Institute for Industrial Chemistry(VIIC), Hanoi University
of Technology (HUT), Vietnam Petroleum Institute (VPI), Hanoi University of Sciences
(HUS) and Leibniz-Institut für Katalyse (LIKAT) an der Universität Rostock, Germany.
⁎Corresponding author. Tel.: + 84 4 37520219, +84 977476139 (mobile); fax: + 84
4 37525251.
E-mail address: thuykhai2001@yahoo.com (T.L.T. Bui).
1566-7367/$ –see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.03.018
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom

Author's personal copy
On the other hand, ILs containing a molar fraction of AlCl
3
greater
than 0.5 are called Lewis acidic ionic liquids. The molar ratio is usually
in the range of 0.5bxb0.67).
The higher the molar fraction of AlCl
3,
the higher the Lewis acidity
of the melt.
The acidity of an IL can be estimated semi-quantitatively by mea-
suring IR spectroscopy of a mixture of the IL with weak Lewis bases
such as pyridine and acetonitrile. IR- bands attributed to Lewis or
Brønsted acidity are observed. The acidity can be estimated by moni-
toring the IR absorbance of new bands or the shift of bands in the
range 1450 to 1600 cm
−1
for pyridine and in the range 2250 to
2340 cm
−1
for acetonitrile.
Pyridine was used in this work to estimate the Lewis and Brønsted
acidity of ILs. Pure pyridine has a peak at 1434 cm
−1
and no peak
near 1535 cm
−1
. The presence of a band near 1450 cm
−1
indicates
the coordination between pyridine and Lewis acidic sites while a
new peak near 1535 cm
−1
indicates the formation of a pyridinium
ion due to the presence of Brønsted acidic sites [16]. The degree of
monotonic blue shift of the band near 1450 cm
−1
shows the increase
in the Lewis acidity and the height of the peak near 1535 cm
−1
rep-
resents the Brønsted acidic strength [17].
The FT-IR spectra of various samples are given in Fig. 1. Neat pyr-
idine shows a single band at 1434 cm
−1
. If pyridine is mixed with a
Lewis acid (e.g. [OMIM]Br/AlCl
3
, x>0.5), the band shifts to
1450 cm
−1
with x=0.6 and 1454 cm
−1
with x=0.65. The pres-
ence of Lewis acidic sites is confirmed in these ILs. No peak ob-
served at 1434 cm
−1
confirms that there is no pyridine in excess
after reacting with ILs. Unreacted ILs have no peak in investigated
area.
When pyridine is mixed with HCl, a new peak appears near
1538 cm
−1
confirming the formation of pyridinium cations from
pyridine and the Brønsted acid HCl. Similar but small bands ob-
served in the mixtures of [OMIM]Br/AlCl
3
(x=0.6 and 0.65) show
that Brønsted acidic sites exist which can be attributed to traces of
water. In the mixture (e), the presence of added water and acidic
exchange resin causes the formation of more Brønsted acidic sites,
therefore, a higher band near 1538 cm
−1
appears.
3. Experiments
1-Methylimidazole supplied by BASF was purified before use. 1-
Bromooctane, CuCl and CuCl
2
were purchased from Fluka and used
without further purification. Pyridine purchased from Merck was
dried before use. Aluminium chloride was purchased from Merck
and used without further purification.
3.1. Preparation of ionic liquids (see in Fig. 2)
3.1.1. 1-methyl-3-octylimidazolium bromide ([OMIM]Br) and 1-methyl-
3-octylimidazolium bromide aluminium chloride ([OMIM] Br/AlCl
3
)
The procedures were performed as described in Ref. [17].
3.1.2. Triethylamine hydrochloride aluminium chloride ([Et
3
NH]Cl/AlCl
3
)
The reaction was performed in a Schlenk flask containing a mag-
netic stir bar under an inert atmosphere. Aluminium chloride was
slowly added under stirring to [Et
3
N]HCl and cooled with an ice
bath to avoid a temperature increase due to the exothermic reaction.
After complete addition of AlCl
3
, the cooling bath was removed and
the reaction mixture was stirred at 80 °C for 7 h and then at room
temperature for 16 h.
The ionic liquids modified with transition metals were prepared in
such a way that the metal salts (CuCl, CuCl
2
, NiCl
2
, in 0.67% mol to mol
of AlCl
3
) were added into each Lewis acidic IL at 80 °C.
3.2. Pre-treatment of resin
Two sulfonic resins (Amberlyst 15 (H), Dowex Marathon (H))
were purchased from Sigma Aldrich. The particles with a diameter
in the range of 0.35 to 0.85 mm (Dowex) and 0.35 to 1.2 mm (Amber-
lyst) were used. Before use, the resins were pre-treated by washing
with methanol, water and 4% NaOH solution, and rinsing with deio-
nised water. Then the resin was put into the protonated form with a
15% solution of H
2
SO
4
and finally rinsed with deionised water until
the washing water was free of acid. The resin was stored in deionised
water.
3.3. Alkylation experiments
The ionic liquid (or a mixture of an IL and additives) as well as two
internal standards (n-hexane and n-octane) was added into the auto-
clave with syringes. Then iso-butane was charged and the resulting
mixture was stirred for 30 min. Then 2-butene was charged in a way
that the temperature did not increase more than 2 °C. Then argon
was filled into the reactor until the pressure was obtained. The mix-
ture was stirred for the specified time (the time was counted when
2-butene was charged). After the stirring had been stopped, the reac-
tion mixture was left for an hour to separate and then the products
(upper phase) were analysed by online-gas chromatography (GC).
3.3.1. Alkylation experiments with acidiccation exchange resins as additives
Resin and catalytic IL, internal standards and iso-butane were
charged into the autoclave and stirred for 30 min for the formation
of complexes and then procedure followed the normal method.
Lewis acidity change
(υ
υ
= 1535 cm-1)
1538 1434
1450
1454
1450
Brønsted acidity change
(
υ
= 1450 cm-1)
Absorbance
Wavenumber (cm-1)
1600 1550 1500 1450 1400 1350
(e)
(d)
(c)
(b)
(a)
Fig. 1. FT-IR spectra of: (a) pure pyridine, (b) pyridine +HCl solution (36%), (c) pyri-
dine+[OMIM]Br/AlCl
3
(x=0.6), (d) pyridine + [OMIM]Br/AlCl
3
(x=0.65), (e) pyri-
dine+[OMIM]Br/AlCl
3
(x=0.6) +Dowex (0.16 g/g of IL)+ H
2
O 3% in Dowex.
Volume ratio of pyridine to IL is 5:1.
NN +
Al2Cl6Br -
NH
+Al2Cl7-
a) b)
Fig. 2. Structure of IL catalysts studied; (a): 1-Methyl-3-octylimidazolium bromide al-
uminium chloride ([OMIM]Br/AlCl
3
); (b): Triethylamine hydrochloride aluminium
chloride ([Et
3
NH]Cl/AlCl
3
).
119T.L.T. Bui et al. / Catalysis Communications 25 (2012) 118–124

Author's personal copy
3.3.2. Alkylation experiments with transition metal salts as additives
In this case, transition metal salts were mixed with catalytic ILs
during preparation IL, so that the experiments were performed fol-
lowing the procedure of normal alkylation experiments.
To characterize the result of each experiment, the product compo-
sition (without unconverted i-butane) was calculated. The alkylate
selectivity was defined as the ratio between the total weight of prod-
ucts (C
5+
) and the weight of converted 2-butene. For pure (true) iso-
butane–butene alkylation one should have a selectivity of 204 wt.%
whereas for butene dimerization one should have 100 wt.%. In all
experiments (including those conducted with H
2
SO
4
as catalyst),
the conversion of 2-butene was almost complete (>98%). Hydrocar-
bons with less than 5 carbon atoms were only formed in a negligible
extent, b1%. The RON was calculated based on the alkylate composi-
tion—without unconverted i-butane—and the RON of all individual
compounds.
3.4. Acidity determination
3.4.1. FT-IR measurements
Pyridine used as an indicator was dried with KOH and distilled over
a 5 Å molecular sieve and solid KOH. All IR samples were prepared by
mixing pyridine and the ILs in a given volume ratio (1:5 by vol.). The
ILwasspreadasaliquidfilm onto an ATR window. The spectra were
recorded on a FT-IR NEXUS 470 (Thermo-Nicolet) at room temperature
(spectral resolution 0.125 cm
−1
).
All the preparation of IR samples and the IR measurements were
conducted under argon. ILs were always stored under argon atmo-
sphere in Schlenk flasks. The indicator (pyridine) was also mixed
with ionic liquid when the argon flows pass through the flask.
When the IR measurement was performed, the ATR window (of
the FT-IR machine) was covered by a glass lid with an inlet and an
outlet. The inlet was connected to the argon tube and was controlled
by a valve to be sure that a constant flow of argon was continuously
passed through the lid space. Before the IR samples were poured
onto the ATR window, the outlet valve was closed and the lid was
raised. This procedure ensured that the ILs were not in contact with
humidity.
4. Results and discussion
4.1. Alkylation using neat [OMIM]Br/AlCl
3
and [Et
3
NH]Cl/AlCl
3
ionic liq-
uids as catalysts
At first, the alkylation of iso-butane with butene-2 was performed
using neat ionic liquids (Table 1, Experiments 1 and 2). The quality of
the product is higher with the [Et
3
NH]Cl/AlCl
3
catalyst than with
[OMIM]Br/AlCl
3
. This is due to the difference between their structure
and thus between their acidity. The Brønsted acidity of neat [Et
3
NH]
Cl/AlCl
3
is assumed to be higher than that of neat [OMIM]Br/AlCl
3
.
This can be seen from the results of FT-IR spectra (Fig. 3), the
Brønsted acidic site density (shown by the small band near
1538 cm
−1
) of neat [Et
3
NH]Cl/AlCl
3
is comparable with that of
[OMIM]Br/AlCl
3
with the addition of water (compare curve a and c).
The alkylate quality received in these two cases is approximately
the same (Table 1).
4.2. Alkylation using protic compound modified [OMIM]Br/AlCl
3
as catalysts
4.2.1. Alkylation using water modified [OMIM]Br/AlCl
3
as catalysts
Water was used as an additive for ionic liquids in alkylation reac-
tion and it rises the alkylate quality (TMP content and RON increase).
This is explained by the formation of super acids when water is
brought into contact with ionic liquids [17] (Scheme 1). In this
range of experiments, water was mixed with hydrocarbon feed up
to 120 ppm by weight before contacting with ionic liquids. The max-
imum alkylate quality is achieved with the added water amount of
30–90 ppm by weight of hydrocarbons. The TMPs increase from
21.3 wt.% (neat ionic liquids) to 49.2 wt.% (with 90 ppm water as ad-
ditive) and RON increases from 90.5 to 93.7, respectively (Table 2).
FT-IR spectroscopy was used to prove these assumptions for the
catalytic systems investigated. The FT-IR spectra of mixtures of an in-
dicator (i.e. pyridine) and [OMIM]Br/AlCl
3
(x=0.6) modified with
different amounts of water were measured. The formation of
Brønsted acidic sites of [OMIM]Br/AlCl
3
in the presence of added
water is indicated by the band at 1535 cm
−1
(Fig. 4). As expected,
the Brønsted acidic site density increases (shown by the enhance-
ment of peak area of the band) with increasing amount of added
water. The addition of water does not lead to the formation of Lewis
acidic sites. Therefore, the band near 1500 cm
−1
indicating Lewis
acidic site density does not grow (Fig. 4).
4.2.2. Alkylation using acidic ionic exchange resin modified [OMIM]Br/
AlCl
3
as catalysts
Some experiments [17] showed that Dowex has a positive effect
on the alkylation, especially when being used together with water
and the optimum water content in this case is 3% to resin (or
Table 1
Alkylate distribution with neat ionic liquids and water modified [OMIM]Br/AlCl
3
as cat-
alysts (−5 °C; 600 kPa; P/O ratio (mol) =13; stirring time =60 min; molar fraction of
AlCl
3
(x)=0.6; IL/2-butene mol ratio =0.4).
No. 1 2 3
Catalyst [OMIM]Br/AlCl
3
[Et
3
NH]Cl/AlCl
3
[OMIM]Br/AlCl
3
and
30 ppm H
2
0 in HCs
Product (wt.% C)
C
5
–C
7
12.9 21.6 18.3
TMPs 21.3 42.1 41.4
Other C
8
2.0 4.6 4.2
C
9+
63.8 31.7 36.1
Yield to butene-2 160 167 163
RON 90.5 94.5 93.7
The entries in bold emphasis show the main products (TMPs) of alkylation and the
important property of alkylation products (RON).
Brønsted acidity change (υ = 1535 cm-1)
More active centers
Absorbance
Wavenumber (cm-1)
1600 1550 1500 1450 1400 1350
(c)
(b)
(a)
Fig. 3. FT-IR spectra of mixture of pyridine+IL with: (a) [OMIM]Br/AlCl
3
(x=0.6);
(b) [Et
3
NH]Cl/AlCl
3
(x =0.6); (c) [OMIM]Br/AlCl
3
(x =0.6) +30 ppm H
2
O in HCs). Vol-
ume ratio of pyridine to IL is 1:5.
120 T.L.T. Bui et al. / Catalysis Communications 25 (2012) 118–124

Author's personal copy
225 ppm to HCs). So that in the range of experiments showed in
Table 3, the content of water was kept constantly (3 wt.% of resin)
and the amount of resin was varied. The results in Table 3 show
that alkylation dominates when the equivalent ratio of SO
3
H groups
to [Al
2
Cl
6
Br]
−
is about 0.5 (57.8 wt.% of TMPs and RON 95.2 are ob-
served). It is necessary to employ an excess amount of [Al
2
Cl
6
Br]
−
to saturate the resin. This ensures that there are still free Lewis acidic
sites (Al
2
Cl
6
Br
−
) and some of them can combine with cocatalyst HCl
(liberated from the reaction of resin or water with the IL) [13] to form
superacidic sites. The polymerization starts to dominate at ratios
equivalent to unity as well as below or above 0.5 because in both
cases less superacidic sites are formed [13,17].
The acidity of catalytic systems was evaluated by the FT-IR spec-
troscopy. The Brønsted acidic site density in this case also increases
with increasing amount of resin (or water) (Fig. 5). This is in agree-
ment with the discussion above that the presence of both of them
in ionic liquid leads to the formation of the Brønsted super acidic
sites.
However, the alkylate quality in this case is much higher com-
pared to the experiments using only water (about 0.4 wt.% of resin)
(Table 2). This is due to the complexes, which are expected to be
formed from the reaction of resin or water with the IL [17], stay in
the pore of the resin. In experiments measuring FT-IR spectrum, it is
assumed that only the liquid parts were measured, therefore, the
acidity of polymeric superacids has not been yet accounted.
4.3. Alkylation using metal salts modified ILs as catalysts
4.3.1. Alkylation using metal salts modified [OMIM]Br/AlCl
3
ILs as catalysts
In this section, alkylation of iso-butane with 2-butene was per-
formed using a mixture of [OMIM]Br/AlCl
3
and CuCl or CuCl
2
as cata-
lysts. Copper (I) and (II) salts were chosen as cocatalytic compounds
for the following reasons:
It is known from literature that pure aluminium chloride has no
catalytic activity for reactions following a carbonium ion mechanism
like alkylation, isomerisation and that a promoter or cocatalyst such
as an alkyl halide or water is required [18].
But it is also known that a mixture of aluminium chloride and cu-
pric chloride promotes this kind of reactions [19,20]. Besides mixtures
of aluminium chloride (or aluminium bromide) and metal sulfates
show also catalytic activity for the isomerisation of pentane [21,22].
Mixtures of aluminium chloride and aromatic hydrocarbons or ethers
were used to raise the selectivity for the alkylation of iso-butane with
2-butene [23].
The results of the investigations presented in Table 4 show that
both copper salts (CuCl or CuCl
2
) have positive effects on the catalytic
activity of the [OMIM]Br/AlCl
3
ionic liquid catalyst. Both salts lead to
an increase of the TMPs of the products and of the RON (compare ex-
periments 1, 2 and 3). The effect of CuCl is higher than that of CuCl
2
.
The addition of water to the [OMIM]Br/AlCl3/CuCl system leads to
a reduction of the RON. But the yield of TMPs as well as the RON are
still higher compared to a mixture of [OMIM]Br/AlCl3 and CuCl2.
When water is added to the mixture of IL and CuCl, a small part of
CuCl may hydrolyse and the active species formed from CuCl and IL
may be destroyed. Better results are obtained when only water is
added to the [OMIM]Br/AlCl3 ionic liquid (experiment 4). However,
it is assumed that a destruction of ILs can occur by the addition of
water but not by the addition of metal transition salts.
To explain the positive effects of the copper salts on the selectivity
to the main products when using aluminium chloride as catalyst, the
acid strength of AlCl
3
–CuSO
4
mixture may give some insights. The
acid strength of an AlCl
3
–CuSO
4
mixture, of pure AlCl
3
and pure
CuSO
4
was determined by measuring the reflectance spectra of sever-
al Hammett indicators adsorbed on the samples [21]. The results
Scheme 1. Reaction pathway generating superacidic acids in mixture of [OMIM]Br/
AlCl
3
ILs and water.
Table 2
Influence of water content in HC on the C
4
-alkylation using [OMIM]Br/AlCl
3
as catalysts
(−5 °C; 600 kPa; P/O ratio (mol) =13; stirring time =60 min; IL/olefin mol
ratio=0.4; molar fraction of AlCl
3
=0.6).
No. 12345
Content of water (ppm in HCs) 0 30 60 90 120
Alkylate distribution 12.8 18.3 26.3 22.6 16.2
C
5
–C
7
21.3 41.4 42.2 49.2 28.9
TMPs 2.0 4.2 8.5 9.8 3.1
Other C
8
63.9 36.1 22.0 18.5 51.8
C
9+
Yield to butene-2 160 162 162 168 164
RON 90.5 93.7 92.2 93.7 92.0
The entries in bold emphasis show the main products (TMPs) of alkylation and the
important property of alkylation products (RON).
Brønsted acidity change (υ = 1535 cm-1)
More active centers
Absorbance
Wavenumber (cm-1)
1600 1550 1500 1450 1400 1350
(c)
(b)
(a)
Fig. 4. FT-IR spectra of mixture of pyridine with: (a) OMIM]Br/AlCl
3
(x=0.6), (b)
OMIM]Br/AlCl
3
(x=0.6) +30 ppm water (in HCs), (c) OMIM]Br/AlCl
3
(x=0.6) +
120 ppm water (in HCs). Volume ratio of pyridine to IL is 1:5.
Table 3
Influence of Dowex content on alkylation of iso-butane with butene-2 using [OMIM]Br/
AlCl
3
as catalysts. (Dowex adsorbed water (3% to Dowex); −5 °C; 600 kPa; P/O ratio
(mol)=13; stirring time= 60 min; molar fraction of AlCl
3
=0.6; IL/2-butene mol
ratio=0.4).
No. 1 2 3 4
Dowex/Al
2
Cl
6
Br
−
molar ratio 0 0.08 0.16 0.32
Product (wt.% C)
C
5
–C
7
12.8 16.6 14.3 15.3
TMPs 21.3 30.3 57.8 23.8
Other C
8
2.0 6.5 7.3 9.3
C
9+
63.9 46.6 20.6 51.6
Yield to butene-2 160 167 171 170
RON 90.5 92.4 95.2 91.2
The entries in bold emphasis show the main products (TMPs) of alkylation and the
important property of alkylation products (RON).
121T.L.T. Bui et al. / Catalysis Communications 25 (2012) 118–124

Author's personal copy
show that the acid strength of the AlCl
3
–CuSO
4
mixture is between
H
o
=−14.52 and H
o
=−13.75. AlCl
3
has H
o
values between
−13.75 and −13.16 while CuSO
4
shows no acid strength. Thus, the
increase of acid strength of aluminium halide can be due to a forma-
tion of complexes with cupric sulfate.
On the basis of the similarities in the structure and properties, the
positive effect of metal transition salts on the selectivity of alkylation
with IL catalysts has been inferred to have an analogous pathway. Ac-
tive species were suggested to form in the mixture of ionic liquid and
CuSO
4
. These species result in the increase in the Hammett acidity
and therefore the selectivity of catalyst.
The acidity of mixtures of [OMIM]Br/AlCl
3
and copper (I) and (II)
chlorides was evaluated by FT-IR in this work to find how the acidity
of ILs is changed in the presence of metal salts.
The rise of the band at 1535 cm
−1
and the shift of band
1450 cm
−1
to 1454 cm
−1
show that both Brønsted acidic site density
and Lewis acidity increase when the IL is mixed with CuCl (Fig. 6).
However, only the shift of band 1450 cm
−1
to 1454 cm
−1
is observed
in the case of using CuCl
2
as additive. These can explain the higher
quality alkylate received in the former case.
FT-IR measurement shows that the effect of the copper salts is based
on a modification of the acidity. This explains the increase of the alkylate
quality when transition metal salts are used as additives. However, the
increase of the total acidity of the mixture is mostly caused by the rise
of Lewis acidity. Therefore, the alkylation rate is only slightly affected.
4.3.2. Alkylation using metal salt modified Et
3
NHCl/AlCl
3
ILs as catalysts
In this work some metal transition salts were also used as addi-
tives for Et
3
NHCl/AlCl
3
catalyst. The results in Table 5 showed that
metal chlorides such as: CuCl, CuCl
2
and NiCl
2
had an influence on al-
kylation with Et
3
NHCl/AlCl
3
IL catalysts. TMPs content of products
and RON increased when metal salts were used as additives. CuCl
was found to have the highest influence among investigated salts
on alkylation so that in other experiment the AlCl
3
/Et
3
NHCl molar
ratio was increased to see if the alkylate quality can be improved
but no further improvement was observed. Therefore, any further in-
crease or decrease of this ratio will not bring any advantage.
As mentioned before, transition metal salts were used for a long
time to increase the selectivity of aluminium chloride in reactions
with alkanes. The complexes of aluminium chloride and metal salts
were used for alkylation of iso-butane with olefins [24]. Some metal
salts decrease the formation of undesired products and increase the
quality of alkylate. Roebuck found that aluminium chloride reacts
with metal chlorides to form complexes [23]. These compounds sup-
press the side reactions. The ability to form such complexes affects
the inhibition capacity of salts [23].
Y. Liu et al. reported that CuCl and IL based on CuCl (Et
3
NHCl/CuCl)
were inactive with alkylation of iso-butane with butene. The mea-
surement acid strength of Et
3
NHCl/CuCl by some indicators (e.g.
anttraquinone, nitrobenzene, O-nitrochlorobenzene) showed that
they have no strong acid site [25]. CuCl, CuCl
2
, NiCl
2
have potential
to form the complexes with some compounds: amine, phosphine,
ammonia, etc. [26–29].
On the basis of the similarities in the structure, it is conjectured
that there was a combination of CuCl with Et
3
NHCl in acidic chloroa-
luminium ILs, such a combination would raise the Brønsted acidic site
density and reduce the Lewis acidic site density. The Brønsted acidic
sites inhibit side reactions of cracking, polymerisation and
Brønsted acidity change (υ = 1535 cm-1)
More active centers
Absorbance
Wavenumber (cm-1)
1600 1550 1500 1450 1400 1350
(c)
(b)
(a)
(d)
Fig. 5. FT-IR spectra of mixture of pyridine +[OMIM]Br/AlCl
3
(x =0.6) with: (a) no additives,
(b) Dowex (0.08 g/g of IL) +30 μl water (3%), (c) Dowex (0.16 g/g of IL)+ 60 μlwater(3%),
(d) Dowex (0.32 g/g of IL) +120 μl water (3%). Volume ratio of pyridine to IL is 1:5.
Table 4
Effect of metal salts on alkylate distribution and RON of products using [OMIM]Br/AlCl
3
as catalysts (content of water in HCs =30 ppm; (−5 °C; 600 kPa; P/O ratio (mol) =13;
stirring time =60 min; IL/olefin mol ratio =0.4; molar fraction of AlCl
3
=0.6).
No. 1 2 3 4
Catalyst IL IL + CuCl IL+CuCl
2
IL+CuCl +H
2
0
Product (wt.% C)
C
5
–C
7
12.9 14.2 11.3 19.0
TMPS 21.3 32.1 26.9 32.0
Other C
8
2.0 2.4 2.1 2.7
C
9+
63.8 51.3 59.7 46.3
Yield to butene-2 160 163 164 164
RON 90.5 92.6 91.2 92.1
The entries in bold emphasis show the main products (TMPs) of alkylation and the
important property of alkylation products (RON).
Lewis acidity change
(υ near 1450 cm-1)
Absorbance
Wavenumber (cm-1)
1600 1550 1500 1450 1400 1350
(c)
(b)
(a)
Fig. 6. FT-IR spectra of mixture of pyridine +[OMIM]Br/AlCl
3
(x =0.6) with: (a) no salt,
(b) CuCl (1.7% mol to used AlCl
3
), (c) CuCl
2
(1.7% mol to used AlCl
3
). Volume ratio of
pyridine to IL is 1:5.
122 T.L.T. Bui et al. / Catalysis Communications 25 (2012) 118–124

Author's personal copy
isomerisation and catalyze the alkylation. The results of acidity mea-
surement by FT-IR (Fig. 7) support this assume. The formation of
the Brønsted acidic sites in mixtures of Et
3
NHCl/AlCl
3
and transition
metal salts can be assumed as follows: there was an exchange be-
tween metal and hydrogen ions so that metal ion could combine
with nito by the combination of unshared electron pair of nitrogen
atom and the unfilled (free) orbital of transition metal. H
+
released
could combine with AlCl
4
−
or Al
2
Cl
7
−
to form Brønsted acidic sites
(Scheme 2).
5. Conclusion
The alkylation of iso-butane with butene-2 was carried out with
modified [OMIM]Br/AlCl
3
and Et
3
NHCl/AlCl
3
catalysts under the suit-
able conditions.
Some additives such as: water, acidic cationic exchange resin and
transition metal salts were used. Resins were found to improve dra-
matically the selectivity and the yield of the alkylation with [OMIM]
Br/AlCl
3
catalysts. The increase of acidity of IL, especially of Brønsted
acidity was observed when additives were mixed with ILs.
Using transition metal salts raises the selectivity of ionic liquids
and thus the alkylate quality, especially when Et
3
NHCl/AlCl
3
is used.
Metal salts raise the acidity of [OMIM]Br/AlCl
3
but mostly the Lewis
acidity, therefore, the alkylate quality increases but not so much.
The explanations for the effective mechanism of additives were
assumed for each case. FT-IR spectrum showed to be an efficient
method to evaluate the acidity of ILs.
It was supposed that when cationic exchange resin is in contact with
[OMIM]Br/AlCl
3
ILs, some complexes which have highly Brønsted acid-
ity are formed. Moreover, the presence of water which is adsorbed in
resin results also in the formation of Brønsted acidic sites. It is those
Brønsted acids which speed upthe reaction and increase the selectivity
of the reaction. When metal salts are mixed with Et
3
NHCl/AlCl
3
there is
a rearrangement of ions in ILs which results in the increase in
the Brønsted acidic sites. In this case, the alkylate quality increases
dramatically.
Abbreviations
[OMIM]Br/AlCl
3
1-n-octyl-3-methylimidazolium bromide aluminium chloride
[Et
3
NH]Cl/AlCl
3
Triethylamine hydrochloride aluminium chloride
IL ionic liquid
TMPs trimethylpentanes
RON research octane number
DMHs dimethylhexanes
HC hydrocarbon
P paraffin
O olefin
RON research octane number
Acknowledgments
Financial support from the Vietnamese government and the Ministry
of the Education and Training is gratefully acknowledged.
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Wavenumber (cm-1)
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NHCl/AlCl
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.
123T.L.T. Bui et al. / Catalysis Communications 25 (2012) 118–124

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