Highly efficient and selective C-H activation in gallium-containing molten-salt systems

Article abstract of DOI:10.1002/anie.200702909

(Chemical Equation Presented) Starting from methane: Isobutane is formed with very high selectivities when chloromethane reacts with Ga⁰ suspended in a molten-salt medium. During the reaction, hydrogen and Ga ^I and Ga^III chlorogallate species are formed. The latter can be electrochemically reduced back to Ga⁰ in a subsequent step. The chlorine liberated in this Ga⁰ regeneration can be used for the synthesis of the chloromethane feedstock from methane.

Full text of DOI:10.1002/anie.200702909

Angewandte
Chemie
DOI: 10.1002/anie.200702909
ꢀ
C H Activation
ꢀ
Highly Efficient and Selective C H Activation in Gallium-Containing
Molten-Salt Systems**
Peter Wasserscheid,* Tobias Weiß, Friederike Agel, Christiane Werth, Andreas Jess, and
Rüdiger Forster
Chloromethane has been found to react with Ga0 suspended
in an ionic liquid or molten salt medium with very high
selectivities (up to 98% of gaseous products) to form
isobutane. During the reaction, hydrogen and anionic chlor-
ogallate species of GaI and GaIII are formed. The latter can be
electrochemically reduced to Ga0 in a subsequent step. The
chlorine liberated in the regeneration of Ga0 can be used for
the synthesis of the chloromethane feedstock from methane,
thus making this new reaction a potential key step in a
fundamentally new process chain for increasing the value of
methane in a closed chlorine/chloride cycle.
Rational and efficient conversion of methane to more
useful higher hydrocarbons is a very important topic in
natural gas utilization. Today, methane activation is mostly
carried out indirectly by transformation of methane to syngas
(a mixture of CO and hydrogen), which then is converted into
higher hydrocarbons (olefins, gasoline, and waxes) by
Fischer–Tropsch synthesis. Alternatively, syngas can be used
to synthesize methanol, which later can be converted into
hydrocarbons using methanol-to-olefins (MTO) or methanol-
to-gasoline (MTG) catalysts.[1–4] In both routes, a great part of
the overall cost stems from production of syngas.[5] Another
problem of Fischer–Tropsch synthesis and methanol conver-
sion processes is that it is not possible to produce a single
product with high selectivity, as distributions of products with
different chain lengths are obtained.
1988, Taylor and Noceti published a process for the produc-
tion of gasoline from chloromethane in an MTG-like
process.[7] The byproduct HCl of the MTG-like step can be
recycled back to the production of chloromethane by oxy-
hydrochlorination, thus making the overall process attractive
for technical use. In the following years, different types of
zeolite catalysts were applied in this kind of chloromethane
transformation, showing different performance with respect
to catalytic activity, coke formation, and the distribution of
hydrocarbons produced.[8–12] Recently, the application of
SAPO-34-type molecular sieve catalysts was found to be
promising (SAPO = silicon aluminum phosphates). With this
type of catalyst, light alkenes (ethene, propene, butenes) can
be produced at temperatures of 350–5008C in 70–80%
selectivity in a temperature-dependent mix in which the
maximum selectivity for one individual product hardly
reaches 45%.[13] Only minor amounts of light alkanes are
obtained, while 15–20% of the products are heavier than C5.
Herein, we describe a new, highly efficient and selective
ꢀ
C H activation that may be the key reaction step in an
alternative process to transform methane into higher hydro-
carbons with unprecedented high selectivity. The new route
involves the reaction of chloromethane with a Group III
metal in a low oxidation state (in particular Ga0) to form
highly branched hydrocarbons, hydrogen, and chlorometa-
lates. These chlorometalates dissolve in the molten salt or
ionic liquid environment in which the reaction is carried out.
The molten salt medium also acts as the electrolyte for the
subsequent electrochemical reduction of the metal back to
the reactive oxidation state M0. The chlorine formed during
this regeneration process can be used in the first step to
produce chloromethane from methane. Because the products
contain extremely low amounts of chlorinated hydrocarbons
(below our detection limit of ca. 100 ppm), the overall process
can be regarded as a closed cycle with respect to chlorine;
methane and oxygen are converted into highly branched
hydrocarbons, hydrogen, and water using electrical energy to
overcome the thermodynamic limitations. Surprisingly, the
selectivity for isobutane can be drastically enhanced by
employing ionic liquid mixtures containing small amounts of
benzyltrimethylammonium chloride. For such mixtures, a
selectivity of the chloromethane conversion step for isobu-
tane of more than 98% has been shown (based on the analysis
of gaseous reaction products).
A true alternative for indirect methane conversion is the
use of chloromethane as the activated intermediate. Chloro-
methane is obtained both from radical chlorination of
methane and from oxyhydrochlorination[6] of methane. In
[*] Prof. Dr. P. Wasserscheid, T. Weiß, Dr. F. Agel, Dr. C. Werth
Lehrstuhl für Chemische Reaktionstechnik
Universität Erlangen-Nürnberg
Egerlandstraße 3, 91058 Erlangen (Germany)
Fax: (+49)9131-8527421
E-mail: wasserscheid@crt.cbi.uni-erlangen.de
Homepage: http://www.crt.cbi.uni-erlangen.de
Prof. Dr. A. Jess
Lehrstuhl für Chemische Verfahrenstechnik
Universität Bayreuth
95440 Bayreuth (Germany)
Dr. R. Forster
E.ON-Ruhrgas AG
Gladbecker Strasse 404
45326 Essen (Germany)
ꢀ
The C H activation reaction described herein is not
catalytic. In contrast, we describe the combination of a
stoichiometric reaction step with the electrochemical gener-
ation of highly reactive metal(0) suspended or emulsified in
an ionic reaction medium. The step in which chloromethane
[**] We gratefully acknowledge financial support from EON-Ruhrgas
AG, Essen to begin this research.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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7281

Communications
reacts with a suspension of Al0 needles or an emulsion of
greater amount of higher hydrocarbons was detected in the
liquid Ga0 in a LiCl/AlCl3 molten salt is related to work by
Sundermeyer and co-workers in which they describe the
formation of methylaluminumdichloride by treating chloro-
methane in alkali melts with elemental Al.[14–16] However, this
system has not been reported to liberate hydrocarbons in a
selective manner.
In our study, the reaction of chloromethane in gallium-
containing chloroaluminate molten salts was carried out using
a Hastelloy C autoclave with a gas entrainment stirrer
connected to a chloromethane supply with dosing by pressure
gas phase that could only originate from C H activation steps
occurring during the reaction. No chloromethane conversion
was observed without the addition of metal(0).
ꢀ
To get a more detailed look at the meltꢀs influence on this
C H activation process, the molten saltꢀs acidity was varied
(Table 2). The reaction temperature for this set of experi-
ments was increased to 1508C to ensure that the lithium
chloroaluminate reaction medium was liquid in all cases
under investigation.
ꢀ
drop (determination of VCH Cl by gas measurement). Details
of the setup and the experimental protocol are described in
the Supporting Information.
3
Table 2: Reaction of CH3Cl in lithium chloroaluminate molten salts of
different acidities (modified by the AlCl3/LiCl ratio) in the presence of
Ga0.[a]
In a typical experiment, a mixture of LiCl, AlCl3, and the
elemental Group III metal was filled into the reactor under
inert conditions, and the mixture was heated to the reaction
temperature. The reaction temperature was chosen high
enough to form a suspension (for aluminum) or an emulsion
(for gallium) of the metal in the liquid chloroaluminate.
In an initial experiment, we probed the reactivity of
electrochemically generated Al0 needles (see the Supporting
Information for preparation details) suspended in acidic
chloroaluminate ionic liquids towards chloromethane with
the aim of observing ethane as the most likely product of an
oxidative coupling reaction. Indeed, at 1208C, 75% of CH3Cl
conversion could be obtained in 34 min in an acidic chlor-
oaluminate molten salt (AlCl3/LiCl = 60:40) with ethane as
the main product (Table 1, entry 1). We decided to continue
1
2
3
4
AlCl3 (mol%)
LiCl (mol%)
time to 75% conversion [min]
methane (wt%)
ethane (wt%)
propane (wt%)
isobutene (wt%)
n-butane (wt%)
dimethylpropane (wt%)
methylbutane (wt%)
n-pentane (wt%)
hydrocarbons >C5 (wt%)
50
50
51
49
55
45
68
56.8
60
40
21
55.9
750
45.0
<1.0
2.5
502
53.5
<1.0
1.0
35.2
<1.0
1.3
<1.0
12.4
17.8
3.3
<1.0
26.9
44.5
1.1
10.1
5.1
<1.0
6.5
<0.1
<0.1
5.3
3.0
<0.1
<1.0
<1.0
<1.0
6.2
<1.0
<1.0
<1.0
<1.0
[a] 55 g AlCl3/LiCl, 35 g CH3Cl, 3.00 g Ga0, T=1508C.
Remarkably, the tendency of the reaction to form higher
hydrocarbons increases significantly with lower acidities of
the lithium chloroaluminate medium. Apart from methane
formation, which was above 45 wt% in all experiments,
isobutane was found to form in quite high selectivity in the
neutral chloroaluminate system (entry 1, Table 2). While
reaction rates are clearly enhanced with higher melt acidities,
increased propane selectivity was observed in the gaseous
reaction products in more acidic melts at the expense of
isobutane formation. In all experiments, only traces of ethane
were detected.
In a related report, Power and Waggoner describe the
reaction of trimethylaluminum and trimethylgallium with
bulky primary amines.[17] In this very fundamental, crystallo-
graphic study, the authors conclude that, despite great
structural similarity, the aminogallanes have a much greater
Table 1: Reaction of CH3Cl in an acidic lithium chloroaluminate molten
salt in the presence of Al0 or Ga0.[a]
1
2
reactive metal
Al0
75
34
39.1
45.4
9.3
1.5
Ga0
75
91
69.1
<1.0
15.0
9.6
chloromethane conversion [%]
time to 75% conversion [min]
methane (wt%)
ethane (wt%)
propane (wt%)
isobutane (wt%)
n-butane (wt%)
<1.0
4.5
<1.0
<1.0
<1.0
3.7
dimethylpropane (wt%)
methylbutane (wt%)
n-pentane (wt%)
<1.0
<1.0
<1.0
<1.0
hydrocarbons over C5 (wt%)
[a] 55 g AlCl3/LiCl=60:40, 1.35 g CH3Cl, 1.50 g Al0 or 3.00 g Ga0, T=
1208C.
ꢀ
tendency for methane elimination by C H activation than for
ꢀ
elimination by capture of N H hydrogen. For example,
although the reaction between AlMe3 and H2Ndipp (dipp =
2,6-iPr2C6H3) gave trimeric (MeAlNdipp)3, the comparable
reaction with GaMe3 gave a compound in which the isopropyl
methyl group on the aryl ring of the amine was bonded to
gallium.
further experiments with Ga0 instead of Al0 for two reasons.
On the one hand, we expected easier application of Ga0
because it is a liquid under the reaction conditions and
during the electrochemical regeneration. On the other hand,
we were interested to see whether the use of Ga0 could reduce
the large amount of methane formed with the Al0 system.
Chloromethane conversion in the presence of Ga0 was
quite different than the corresponding experiment with Al0.
While methane formation was much greater in the Ga0 case,
almost no ethane was formed. Surprisingly, a significantly
ꢀ
Inspired by this account of C H activation in the presence
of amines, we studied our molten salt system in the presence
of ammonium salts. By replacing part of the LiCl by
benzyltrimethylammonium chloride, we were able to lower
the melting point of our reaction mixture below 1008C (these
melts fulfill the well-accepted definition of ionic liquids).
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Experiments were carried out at both 1208C (for comparison
with results without the ammonium salt) and 1008C (to
benefit from the melting point suppression), and the results
are summarized in Table 3.
reaction mechanism (for details of sample preparation and
spectra, see the Supporting Information). The measurements
were carried out at 1508C to ensure that all samples were in a
liquid state. We first investigated molten salt media without
ammonium salts. Depending on the amount of Ga0 added to
the reaction, the 71Ga NMR spectrum shows signals in the
typical ranges for GaI and GaIII species (for low initial Ga0/
chloromethane ratios) or only signals in the typical range for
GaI species (for high initial Ga0/chloromethane ratios).
However, 71Ga NMR spectra of GaI are known to be much
sharper than those of GaIII,[18–20] so that some GaIII might also
have been formed in the latter case. Moreover, the acidity of
the melt had a pronounced influence on the detection of the
different Ga species, with GaIII appearing much more
pronounced in neutral melts. These results confirmed our
mechanistic expectations that some kind of oxidative addition
process of chloromethane to Ga0 is the starting point of the
reaction, with subsequent formation of GaIII. The NMR
spectroscopic investigation of the ionic liquids containing
benzyltrimethylammonium chloride revealed that no major
decomposition or aromatic alkylation of the ammonium
cation takes place during the reaction (the sample was taken
after experiment entry 2, Table 3). Apart from the unchanged
ammonium cation, the 1H NMR spectrum shows only a small
number of signals in the alkyl range representing reaction
intermediates still dissolved in the melt.
Table 3: Reaction of CH3Cl in lithium chloroaluminate/benzyltrimethyl-
ammonium chloride ionic liquids in the presence of Ga0.[a]
1
2
3
4
5
AlCl3 (mol%)
LiCl (mol%)
[Me3NCH2C6H5]Cl (mol%)
reactive metal
temperature [8C]
60
39
1
60
30
10
60
30
10
60
25
15
60
20
20
Ga0
120
26
Ga0
120
16
Ga0
120
17
Ga0
100
50
Ga0
100
230
time to 75% conversion [min]
methane (wt%)
18.2
2.6
5.0
2.0
1.3
ethane (wt%)
<0.1 <0.1 <0.1 <0.1 <1.0
propane (wt%)
isobutane (wt%)
n-butane (wt%)
1.8
70.1
10.0
2.1
87.7
1.0 <1.0 <0.1
93.0 97.5 98.5
7.6 <1.0 <1.0 <0.1
dimethylpropane (wt%)
methylbutane (wt%)
n-pentane (wt%)
hydrocarbons >C5 (wt%)
0.0 <0.1 <0.1 <0.1 <0.1
<1.0 <0.1 <0.1 <0.1 <0.1
<1.0 <0.1 <0.1 <0.1 <0.1
<0.1 <0.1 <0.1 <0.1 <0.1
[a] 55 g AlCl3/LiCl, 1.35 g CH3Cl, 3.00 g Ga0.
However, the true role of the ammonium salt is not yet
fully understood. The 71Ga NMR spectrum of the ionic liquid
obtained from entry 2, Table 3 showed only one clear signal in
the typical GaI range. It is not clear whether the ammonium
cation takes part in the reaction directly (e.g. as a ligand
precursor) or whether it allows for lower reaction temper-
atures and modulates the meltꢀs acidity to suppress detri-
mental cracking. Preliminary experiments with other ammo-
nium salts revealed an important role for the aromatic ring of
the ammonium cation, as non-aromatic ammonium cations
Surprisingly, the addition of only 1 mol% of the ammo-
nium salt had a drastic effect on the reaction compared to the
same experiment without the ammonium salt (entry 1,
Table 3 vs. entry 2, Table 1). While the addition of small
amounts of the ammonium salt had a rate-accelerating effect,
the selectivity for methane decreased significantly while the
selectivity for isobutane increased substantially. This effect
became even more pronounced by adding 10 mol% of the
ammonium salt, for which an isobutane selectivity of 87.7%
in the gaseous reaction products could be realized (Table 3,
entry 3). By lowering the temperature to 1008C, the selectiv-
ity for isobutane could even be increased to 98.5% by
increasing the amount of ammonium salt to a mixture with
AlCl3/LiCl/[Me3NCH2C6H5]Cl = 60:20:20 (entry 5, Table 3;
for detailed conversion/yield and time/conversion plots, see
the Supporting Information). However, reaction rates sig-
nificantly decreased for ammonium salt contents above
10 mol%, probably because of a buffering effect of the
ammonium salt on the meltꢀs acidity at higher concentrations.
The formation of hydrogen could be confirmed by GC for
these highly selective reactions, thus indicating the strongly
ꢀ
led to much less selective C H activation.
Apart from the chain-growth step, we also investigated
the electrochemical reduction of the metal(III) species
formed during the reaction. For the chloroaluminate system
both without organic additives and with benzyltrimethylam-
monium chloride, the electrochemical regeneration of the
reactive metal (Al0 or Ga0) could be achieved in a simple
electrochemical chamber. Al0 grows as needles on the
cathode and can be removed from the electrode surface
either by mechanical abrasion or by reversion of polarity. As
Ga0 is a liquid under the reaction and electrolysis conditions
(m.p. = 29.88C), it deposits on the corresponding electrode as
small, immiscible droplets that can be collected easily at the
bottom of the electrolysis chamber and pumped back into the
chain-growth reactor (see the Supporting Information for
details).
ꢀ
C H-activating nature of the reaction system. Moreover,
product analysis with a chloride-sensitive GC method (see the
Supporting Information) did not show any chlorinated
hydrocarbons above our detection limit of 100 ppm. Further-
more, a control experiment at 1208C, in which the ionic
medium (AlCl3/LiCl/[Me3NCH2C6H5]Cl = 60:30:10) was
brought into contact with Ga0 without adding chloromethane,
did not reveal any detectable decomposition products of the
ammonium salt.
A particularly attractive use of the reaction/regeneration
cycle would be the direct use of methane. In the first step of
such a scheme, the methane is converted into chloromethane.
On the basis of the stoichiometry of the overall process, we
propose a combination of oxyhydrochlorination and radical
chlorination according to Equations (1) and (2), resulting in
Equation (3) for the overall chlorination of methane:
In view of these interesting results, we used 71Ga and
1H NMR spectroscopy measurements to investigate the
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2 CH4 þ O2 þ 2 HCl ! 2 CH3Cl þ 2 H2O
ð1Þ
ð2Þ
ð3Þ
Experimental Section
All chemicals where handled using standard Schlenk techniques.
Conversion of CH3Cl: Before starting an experiment, the equip-
ment was dried in an oven overnight at 708C. To prepare the molten
salt medium, the chemicals for the chloroaluminate system were
mixed under argon in the desired ratio, keeping a constant mass of
55 g for all compositions. The components of the melt were filled into
a Hastelloy C autoclave (see Figure S-1 in the Supporting Informa-
tion); for melts containing benzyltrimethylammonium chloride, ice
cooling was necessary to avoid hotspots owing to the highly
exothermic formation of the ionic liquid. The reactive metal (1.50 g
Al0 as needles or 3.00 g Ga0 in small pieces) was then added.
After closing the autoclave, the whole system was flushed with
argon for 20 min. Then the reactor was pressurized with He (10 bar).
If the pressure in the reactor kept constant for 30 min, the He was
discharged and the autoclave was heated to the desired temperature
(100–1508C) with slow stirring (100 rpm). The reaction temperature
was kept constant for at least one hour to ensure complete melting of
all chemicals. Then the reactor was filled with 600 mLN (1.35 g) of
chloromethane by pressure drop. The amount of chloromethane was
measured by a volume flow indicator.
2 CH4 þ 2 Cl2 ! 2 CH3Cl þ 2 HCl
4 CH4 þ O2 þ 2 Cl2 ! 4 CH3Cl þ 2 H2O
Assuming that GaIII is indeed the final oxidation state of
gallium during its reaction with chloromethane in the
chloroaluminate ionic liquid, Equation (4) represents the
selective formation of isobutane and hydrogen:
12 CH3Cl þ 4 LiCl þ 4 Ga0 ! 3 i-C4H10 þ 4 Li½GaCl4ꢁ þ 3 H2
ð4Þ
The final step involves the reduction of the chlorogallate
anion back to the high energetic level of Ga0 [Eq. (5)]. The
reduction is carried out electrochemically, and the chlorine
formed in this step is recycled into the chlorination step
shown in Equation (2).
electrolysis
ƒƒƒƒƒ!
4 LiCl þ 4 Ga0 þ 6 Cl
ð5Þ
4 Li½GaCl ꢁ
4
2
The reaction was carried out with a continuous stirring rate of
1000 rpm. During the reaction, gas samples were taken for GC
analysis. After conversion of more than 95%, the reaction was
deliberately stopped.
Independent on the final oxidation state of the reactive
metal, the overall reaction is shown in Equation (6).
Electrochemical regeneration of Ga0: For electrolysis of Ga-
containing melts, the same setup as for production of Al0 needles was
used (see the Supporting Information). The electrochemical regen-
eration of Ga0 was carried out in a melt consisting of AlCl3
(60 mol%), LiCl (30 mol%), and benzyltrimethylammonium chlo-
ride (10 mol%). In order to avoid crystallization, the flask was heated
to slightly above melting temperature of the system (1058C) and
stirred with a magnetic stirrer.
electric energy
ƒƒƒƒƒƒƒ!
4 CH4 þ O2
i-C H10 þ 2 H2O þ H2
ð6Þ
4
In conclusion, we have described a new and highly
selective, albeit noncatalytic, method for C H activation in
ꢀ
molten salt or ionic liquid media. In a slightly acidic AlCl3/
LiCl/[Me3NCH2C6H5]Cl mixture, chloromethane reacts to
isobutane and hydrogen with up to 98.5% selectivity (based
on analysis of gaseous products). The new reaction may be the
key step of a new process to convert methane into higher
hydrocarbons. The thermodynamic limitation of the direct
reaction is overcome by introducing electric energy into the
reaction via highly reactive metal(0) species.
For the deposition of Ga0, 1.0–2.5 V were applied to the tungsten
electrodes. During the electrolytic regeneration Ga0, droplets formed
on the cathode and fell off. These droplets formed a visible liquid
phase at the bottom of the flask from which they could be pumped
back into the reactor.
Details of GC analytic and NMR spectroscopic investigations are
given in the Supporting Information, which also includes diagrams
with detailed results of selectivity and conversions and some NMR
spectra.
From an industrial perspective the described reaction
sequence could open up a highly attractive alternative to
increase the value of stranded gas in remote areas, since the
process can be carried out from methane as the only feedstock
(the electrical energy for the electrolysis can also be
generated from methane). The inventory of molten salt and
elemental metal would be constantly recycled in a continuous
process. Furthermore, the process does not form any byprod-
ucts that must be disposed of. In particular, no chlorinated
hydrocarbons are formed in detectable amounts. Besides the
main hydrocarbon, the only products are hydrogen, water,
and CO2 (from the power plant if it is hydrocarbon-based).
The hydrogen (from the growth step and from isobutane
dehydrogenation) could be used on-site to regain a small
portion of the required energy.
From the perspective of ionic liquid chemistry, the
reaction demonstrates the potential of ionic reaction media
to allow new and highly attractive combinations of electro-
chemical and reaction steps in closed cycles. Obviously, highly
reactive metal intermediates can be formed electrochemically
in ionic liquid media that provide quite unique reactivity
patterns. This observation could well be relevant beyond the
Received: July 2, 2007
Published online: August 31, 2007
ꢀ
Keywords: C H activation · gallium · ionic liquids ·
methane activation · molten salts
.
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