Effective n-octane isomerization under exceptionally mild conditions using a novel class of superacidic ionic liquids

Article abstract of DOI:10.1039/c0cc02293d

Superacidic chloroaluminate ionic liquids of the general formula [cation]Cl/AlCl₃[X(AlCl₃) > 0.5] + H₂SO ₄ effectively isomerize n-octane to form branched liquid hydrocarbon isomers. Due to the highly acidic character of the ionic liquid the reaction proceeds under extremely mild conditions in a liquid-liquid biphasic reaction mode leading to a minimum of undesired cracking side-reactions.

Full text of DOI:10.1039/c0cc02293d

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Effective n-octane isomerization under exceptionally mild conditions
using a novel class of superacidic ionic liquids
Carolin Meyer and Peter Wasserscheid*
Received 1st July 2010, Accepted 19th August 2010
DOI: 10.1039/c0cc02293d
Superacidic chloroaluminate ionic liquids of the general formula
[cation]Cl/AlCl3[X(AlCl3) > 0.5] + H2SO4 effectively isomerize
n-octane to form branched liquid hydrocarbon isomers. Due to
the highly acidic character of the ionic liquid the reaction
proceeds under extremely mild conditions in a liquid–liquid
biphasic reaction mode leading to a minimum of undesired
cracking side-reactions.
liquid–liquid biphasic systems with typical unpolar organic
product mixtures thus enabling recycling of the acidic ionic
reaction phase by simple phase separation. Moreover, the
acidity of such low-melting chloroaluminate melts is tuneable
according to the adjusted [cation]Cl/AlCl3 ratio. The higher
the molar ratio of AlCl3, the more acidic is the ionic liquid.
Note that the upper limit for pure chloroaluminate systems
is X(AlCl3) = 0.66 due to melting point and solubility
restrictions.9
In recent years, the application of a number of Research
Octane Number (RON) booster compounds in automotive
fuels has been stopped (in the case of Pb-alkyles) or severely
restricted (in the case of MTBE or even aromatics) for
environmental reasons. This development has considerably
increased the interest in the large-scale refinery production
of highly branched alkanes as the latter combine high RON
with benign toxicology and ecotoxicology properties.1 Alkane
isomerization to highly branched hydrocarbons of the same
carbon number by acidic catalysis is a highly attractive way to
achieve this goal due to low feedstock cost. Technologies using
solid acids such as zeolites,2 modified alumina3 and sulfated
zirconia4 in combination with metallic sites to provide an
additional hydrogenation/dehydrogenation functionality in
the catalytic material have been reported in the literature.5
These reactions are typically performed in the temperature
range of 403 K up to 553 K with cracking and coking being
typical problems for the catalyst selectivity and lifetime.
According to recent references, hydroisomerization of n-octane
and higher alkanes using zeolithes is industrially not applicable
yet due to the high consumption of hydrogen as a consequence
of the significant hydrocracking activity.6
Acidic chloroaluminate melts are known to form superacids
when contacted with HCl. This is due to the reaction of the
chloride with the acidic anions [Al2Cl7]À and [Al3Cl10]À
forming a ‘‘naked’’ superacid proton.10 The Hammett acidity
of these protons has been determined by different groups using
both spectroscopic methods11 and test reactions12 and values
down to H0 = À18 have been reported.
For the skeleton isomerization of n-alkanes superacidity is
required as concentrated sulfuric acid (H0 of 100% sulfuric
acid = À11.9) is known to be not acidic enough to catalyze
this reaction.13 Thus, superacidic ionic liquids of the general
type [cation]Cl/AlCl3/HCl have been tested by a few groups in
alkane isomerization. Berenblyum et al. studied C7-mixtures
(including branched C7) using [(CH3)3NH]Cl/AlCl3/HCl.14
Zhang and coworkers investigated the isomerization of
n-pentane in the system [Et3NH]Cl/AlCl3 (here some super-
acidity originated obviously from traces of water in the
system).15 A similar system using different acidic chloro-
aluminate ionic liquids with a N-containing cation and no
added Brønsted acid was also claimed earlier in a patent for
alkane isomerization under very mild conditions.16
In the patent literature, systems of the general type
[cation]Cl/AlCl3/H2SO4 have been also disclosed.17 The systems
have been claimed to isomerize a distillation cut of paraffinic
hydrocarbons (including branched starting material) at
temperatures as mild as 298 K. Unfortunately, the patent
gives no information on the role of the added sulfuric acid,
on kinetics and on the obtained product distribution. With
respect to the latter point the ratio of feedstock isomerization
vs. feedstock cracking is of utmost importance for the practical
relevance of the catalyst system and its evaluation against
state-of-the-art heterogeneous isomerization catalysts.
In contrast to hydroisomerization using solid catalysts,
homogeneous catalyzed alkane isomerization with superacidic
systems has been shown to proceed at temperatures as low as
298 K. Typical catalysts consist of a mixture of strong Lewis
acids (such as e.g. SbF5, TaF5, NbF5) and a Brønsted acid
(CF3COOH, CF3SO3H).7 However, a clear disadvantage
of these homogeneous superacids is their highly toxic and
corrosive nature that is strongly linked to the presence of free
HF in these systems.
Since the first pioneering studies by Wilkes and coworkers in
the late 70s, the use of highly acidic ionic liquids has been
studied for a large number of organic transformations including
Friedel–Crafts alkylation and acylation, cracking and isomer-
ization.8 The key feature of these ionic liquid systems that were
usually based on chloroaluminate ions is their ability to form
In this paper we report for the first time a systematic study
on the isomerization of pure n-octane, a model compound of
the light naphtha refinery cut, using superacidic ionic liquids
of the general type [cation]Cl/AlCl3/H2SO4 under extremely
mild conditions (T = 283 K up to T = 323 K). Our work
focusses on the goal to maximize the yield of liquid organic
product (LOP), i.e. to favor isomerization and to disfavor
cracking. For this purpose liquid–liquid biphasic experiments
have been carried out in autoclaves studying in detail both the
Lehrstuhl fur Chemische Reaktionstechnik,
¨
¨
Universitat Erlangen-Nurnberg, Egerlandstr. 3, D-91058 Erlangen,
¨
Germany. E-mail: Wasserscheid@crt.cbi.uni-erlangen.de;
Fax: +49 (0)9131 8527421; Tel: +49 (0)9131 8527420
c
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Chem. Commun., 2010, 46, 7625–7627 7625

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Table 1 Liquid organic product (LOP) distribution in n-octane
isomerization using the superacidic ionic liquid system [C4C1Im]Cl/
AlCl3/H2SO4 at 303 K with [C4C1Im]Cl/AlCl3 = 1/2
in the ratio of 3/8. Given the fact that the formation of each
[Al2Cl7]À ion requires two molar equivalents of AlCl3 (of
which the first is only neutralizing the chloride salt to the
neutral [AlCl4]À) one would expect a H2SO4/AlCl3 ratio of
3/16 or 0.1875 in the ionic liquid synthesis to provide the
highest acidity. This predicted ideal composition corresponds
remarkably well to the sulfuric acid ratio found to be optimum
in our catalytic experiments. Exceeding this optimum molar
ratio, more [Al2Cl7]À and [Al3Cl10]À anions are consumed by
reacting to Al2(SO4)3. In this case the molar amount of formed
HCl exceeds the molar amount of remaining acidic anions and
therefore HCl gas leaves the system and the amount of
‘‘naked’’ protons formed drops. Therefore the overall system
acidity drops again at higher sulfuric acid contents. It is
notable that in all these experiments the formation of solid
Al2(SO4)3 in the reaction system is indeed observed and has
been confirmed by isolation and analysis of the precipitate
using BaCl2. However, isolation and removal of Al2(SO4)3 was
not necessary to apply the superacidic ionic liquid system in
catalysis. The precipitation of Al2(SO4)3 is a very important
aspect to generate the superacidity in this system as the most
basic component of the system, the sulfate ion, is removed
from the system in this way.
Molar ratio H2SO4/AlCl3
0.0
0.03
0.14
0.18
0.21
0.75
Yieldsk (LOP) (%)
iso-C4
n-Butane
iso-C5
n-Pentane
iso-C6
n-Hexane
iso-C7
n-Heptane
Iso-C8
1.5
4.6
0.0
7.4
0.0
6.4
0.0
5.6
0.0
6.0
34.8
2.4
53.6
11.2
7.3
0.0
11.3
1.1
10.2
0.7
7.8
0.0
10.1
28.1
4.6
71.9
14.0
8.4
0.0
13.4
1.4
13.1
0.9
8.5
0.0
10.8
22.8
7.5
77.2
14.0
7.3
0.0
11.5
1.1
11.1
0.8
7.5
0.0
10.0
26.9
4.6
73.1
13.7
1.5
0.1
1.9
0.2
1.5
0.1
0.7
0.0
1.5
75.5
0.2
24.5
6.1
0.0
2.7
0.0
2.5
0.0
2.0
0.0
2.4
69.4
1.3
30.6
7.8
n-Octane
C > 8
X(n-C8)a (%)
S(iso-C8)b (%)
Conditions: m(IL) = 30 g; m(n-octane) = 30 g; treact. = 4 h.
a
b
Conversion of n-octane. Selectivity for branched C8 products;
the rest to obtain 100% reflects gaseous C5 and C4 components of
the product mixture; Xn-C8 = (nn-C8,0 À nn-C8)/nn-C8,0; Yk
=
(LOP)
nk (LOP)|nn-C8|/nn-C8,0|nk (LOP)|; Sk (LOP) = Yk (LOP)/Xn-C8
.
influence of the system’s sulfuric acid concentration and the
effect of different reaction temperatures.
Note that the here described catalytic system [cation]Cl/
AlCl3/H2SO4 is much easier and safer to work with compared
to the earlier reported [cation]Cl/AlCl3/HCl systems (see
ref. 10–12 and 14). The handling of toxic and highly corrosive
HCl gas is avoided and accurate dosing of the Brønsted-acidic
component to the system is not influenced by a gas–liquid
mass transport step or solubility issues. The superacidic ionic
liquid system is furthermore truly catalytic (TOF = 11.7 hÀ1
and TON = 47 for the result in Table 1 with a H2SO4/AlCl3
ratio of 0.18) and has been shown to be recyclable (80% of the
original TOF demonstrated in a repetitive batch experiment
using a recycling procedure with short air contact).
Table 1 displays results of a first series of experiments in
which the H2SO4 to AlCl3 ratio was varied from 0 (pure
chloroaluminate melt) to 0.75. All experiments were carried
out using a 1-butyl-3-methylimidazolium chloride ([C4C1Im]Cl)
to AlCl3 ratio of 0.5.
From Table 1 it is evident that the addition of the Brønsted
acid H2SO4 to the Lewis-acidic chloroaluminate system
increases n-octane conversion up to the H2SO4/AlCl3 ratio
of 0.18, while at even higher sulfuric acid concentration the
catalytic activity decreases again. This indicates a maximum of
the acidity in this specific composition correlating to a maximum
n-octane conversion of 77.2% and a yield of branched C4–C8
alkanes of 54.2% (LOPC4–C8 selectivity = 70.2%). The yield
to branched C6–C8 alkanes is 32.4%. These findings can be
understood in the light of the known mechanism of alkane
isomerization. The latter is considered to proceed as a chain
reaction including an initiation step that requires a superacidic
proton to form the first carbocation via direct hydride
abstraction.18 The highest catalytic activity found for the
molar ratio range 0.14 o H2SO4/AlCl3 o 0.21 can be
rationalized by the reactions and equilibria present in the ionic
liquid system to form superacid, ‘‘naked’’ protons (1 and 2).
Encouraged by the remarkable catalytic performance of our
optimized [cation]Cl/AlCl3/H2SO4 system at a temperature as
low as 303 K and these important practical advantages we
tested in a next series of experiments whether the isomerization
rate and selectivity could be further improved by temperature
variation. As can be seen from Fig. 1 it was found that low
temperatures are favourable to maximize the LOP and
branched LOP yields (16.7% at 283 K vs. 7.4% at 323 K),
while temperature increase leads to higher overall n-octane
conversion (53.5% at 283 K vs. 91.1% at 323 K). Compared to
classical heterogeneous isomerization catalysts operating at
temperatures higher than 400 K these results are very remarkable
as it is known that isomerization of higher-molecular weight
alkanes at higher temperatures leads to a huge amount of
cracking products.19 Another advantage of performing alkane
isomerization at very mild temperatures is in the fact that
skeleton isomerization is an equilibrium limited reaction in
which the formation of the thermodynamically more stable
branched isomers is favored by low temperatures if the reaction
is conducted till the equilibrium state.
2[Al2Cl7]À + 3H2SO4 - Al2(SO4)3(k) + 6HCl + 2[AlCl4]À
(1)
6[Al2Cl7]À + 6HCl " 12[AlCl4]À + 6‘‘H+’’
(2)
It is obvious that the highest acidity is obtained if each HCl
formed by the complexation of Al with the sulfate ion
is reacting with one of the Lewis-acidic ions [Al2Cl7]À
(or [Al3Cl10]À) to form the ‘‘naked’’ protons. In this case the
highest concentration of ‘‘naked’’ protons is generated in the
least coordinating environment. From the stoichiometry of
eqn (1) and (2) it can be assumed that the H2SO4/[Al2Cl7]À
ratio resulting in the highest ionic liquid acidity should thus be
It is furthermore very remarkable that no coke formation
was observed in any experiments using our optimized
[cation]Cl/AlCl3/H2SO4 system compared to heterogeneously
catalyzed alkane isomerizations. Applying, for example, zeolithes
c
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Notes and references
1 (a) S. I. Hommeltoft, Appl. Catal., A, 2001, 221, 421–428;
(b) H. Weyda and E. Kohler, Catal. Today, 2003, 81,
51–55; (c) J. Hancsok, S. Magyar, Z. Szoboszlai and D. Kallo,
Fuel Process. Technol., 2007, 88, 393–399.
2 (a) A. Chica and A. Corma, Chem. Ing. Tech., 2007, 79, 857–870;
(b) S. Ernst, C. Bischof, M. Hartmann and J. Weitkamp,
Proceedings of the World Petroleum Congress 16th, Institute of
Petroleum, 2000, vol. 3, pp. 27–37.
3 L. I. Ali, A.-G. A. Ali, S. M. Aboul-Fotouh and A. K.
Aboul-Gheit, Appl. Catal., A, 2001, 205, 129–146.
4 (a) T. Kimura, Catal. Today, 2003, 81, 57–63; (b) T. Løften and
E. A. Blekkan, Appl. Catal., A, 2006, 299, 250–257.
5 A. Corma and H. Garcia, Chem. Rev., 2003, 103, 4307–4365.
6 J. Weitkamp, Proceedings of the 43. Jahrestreffen Deutscher
Katalytiker, Weimar, 2010.
7 US Pat., 3766286, 1973.
Fig.
1 Yield of LOP, conversion of n-octane, selectivity of
8 (a) M. Earle, in Ionic Liquids in Synthesis, ed. P. Wasserscheid and
T. Welton, Wiley-VCH, Weinheim, 2008, vol. 1, pp. 282–367;
(b) J. Joni, M. Haumann and P. Wasserscheid, Appl. Catal., A,
2010, 372, 8–15; (c) J. Joni, D. Schmitt, P. S. Schulz, T. J. Lotz and
P. Wasserscheid, J. Catal., 2008, 258, 401–409; (d) N. Brausch,
A. Metlen and P. Wasserscheid, Chem. Commun., 2004,
1552–1553; (e) C. J. Adams, M. J. Earle, G. Roberts and
K. R. Seddon, Chem. Commun., 1998, 2097–2098.
9 H. A. Øye, M. Jagtoyen, T. Oksefjell and J. S. Wilkes, Mater. Sci.
Forum, 1991, 183, 73–75.
10 (a) G. P. Smith, A. S. Dworkin, R. M. Pagni and S. P. Zingg,
J. Am. Chem. Soc., 1989, 111, 525–530; (b) G. P. Smith,
A. S. Dworkin, R. M. Pagni and S. P. Zingg, J. Am. Chem. Soc.,
1989, 111, 5075–5077.
11 H. Olivier-Bourbigou and L. Magna, J. Mol. Catal., 2002, 419,
182–183.
12 D. King, R. Mantz and R. Osteryoung, J. Am. Chem. Soc., 1996,
118, 11933–11938.
13 G. A. Olah, in Hydrocarbon chemistry, ed. G. A. Olah and
A. Molnar, Wiley-Interscience, Hoboken, NJ, 2nd edn, 2003,
pp. 160–170.
14 A. S. Berenblyum, E. A. Katsman and Y. Z. Karasev, Appl. Catal.,
A, 2006, 315, 128–134.
15 R. Zhang, X. Meng, Z. Liu, J. Meng and C. Xu, Ind. Eng. Chem.
Res., 2008, 47, 8205–8210.
16 Eur. Pat., 1 310 472, 2003.
iso-octanes at various reaction temperatures. [C4C1Im]Cl/AlCl3 = 1/2;
H2SO4/AlCl3 = 0.18; m(IL) = 30 g; m(n-octane) = 30 g; treact. = 4 h.
for the n-octane hydroisomerization at high temperatures
(T > 493 K) as reported in the literature20 leads to significant
coke formation and the related catalyst deactivation even so
hydrogen is used in this case to avoid this undesired side reaction.
In conclusion, we have demonstrated that the system
[cation]Cl/AlCl3/H2SO4 is a very versatile superacidic liquid
catalyst, in particular if the optimum H2SO4 to AlCl3 ratio of
0.1875 is applied in highly acidic chloroaluminate melts. In
contrast to the well-established superacidic ionic liquids of the
general type [cation]Cl/AlCl3/HCl, the here presented system
avoids the use of toxic and corrosive HCl gas and makes use of
in situ HCl-production and Al2(SO4)3 precipitation. Applying
the optimized [cation]Cl/AlCl3/H2SO4 n-octane conversion to high
yields of liquid organic products and even a significant amount of
iso-octanes could be achieved without the use of additional
hydrogen or added precious metals. Coke formation was not
observed due to the very mild conditions of our experiments.
With the described attractive performance features we assume
these optimized [cation]Cl/AlCl3/H2SO4 systems to be indeed
highly attractive liquid catalysts for refinery isomerization processes
of higher alkanes in a liquid–liquid biphasic reaction mode.
The authors like to acknowledge support by the German
Science Foundation in the framework of the Erlangen
excellence cluster ‘‘Engineering of Advanced Materials’’
(www.eam.uni-erlangen.de).
17 Eur. Pat., 1 402 950, 2004.
18 (a) H. S. Bloch, H. Pines and L. Schmerling, J. Am. Chem. Soc.,
1946, 68, 153; (b) F. C. Whitmore, Chem. Eng. News, 1948, 26,
668.
19 M. Bassir, B. Torck and M. Hellin, Appl. Catal., 1988, 38,
211–224.
20 (a) A. Chica and A. Corma, Chem. Ing. Tech., 2007, 79, 857–870;
(b) A de Lucas, J. L. Valverde, P. Sanchez, F. Dorado and
M. J. Ramos, Appl. Catal., A, 2005, 282, 15–24.
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7625–7627 7627