Chemical and thermal stability of N-heterocyclic ionic liquids in catalytic C-H activation reactions

Article abstract of DOI:10.1002/mrc.4096

¹H-NMR spectrum analyses are applied to study the chemical and thermal stability of selected N-heterocyclic ionic liquids within the reaction system that can highly efficiently activate a C-H bond of methane and convert it into the C-O bond in methanol. Our results indicate that under such reaction conditions involving using a powerful Pt-based catalyst and strong acidic solvent, the aromatic ring of an imidazolium cation becomes unstable generating an ammonium ion (NH₄⁺). Our results also suggest that the instability of the imidazolium ring is more chemically (participation in reactions) than thermally based. Modifications of the aromatic ring structure such as pyrazolium and triazolium cations can increase the chemical/thermal stability of ionic liquids under these reaction conditions.

Full text of DOI:10.1002/mrc.4096

Research article
Received: 28 February 2014
Revised: 20 May 2014
Accepted: 23 May 2014
Published online in Wiley Online Library: 18 June 2014
(wileyonlinelibrary.com) DOI 10.1002/mrc.4096
Chemical and thermal stability of N-heterocyclic
ionic liquids in catalytic C–H activation reactions
a,b
a
c,d
d
Guanyi Chen, * Shujuan Kang, Qisheng Ma, Weiqun Chen
d
and Yongchun Tang **
1
H-NMR spectrum analyses are applied to study the chemical and thermal stability of selected N-heterocyclic ionic liquids
within the reaction system that can highly efficiently activate a C–H bond of methane and convert it into the C–O bond in
methanol. Our results indicate that under such reaction conditions involving using a powerful Pt-based catalyst and strong
+
4
acidic solvent, the aromatic ring of an imidazolium cation becomes unstable generating an ammonium ion (NH ). Our results
also suggest that the instability of the imidazolium ring is more chemically (participation in reactions) than thermally based.
Modifications of the aromatic ring structure such as pyrazolium and triazolium cations can increase the chemical/thermal
stability of ionic liquids under these reaction conditions. Copyright © 2014 John Wiley & Sons, Ltd.
1
Keywords: ionic liquid; C–H activation; chemical stability; thermal stability; H-NMR
large volumes of corrosive, volatile liquid acids that pose a severe
problem for the final separation/extraction steps, not to mention
Introduction
Ionic liquids (ILs) are charged organic salts and generally liquids
at room temperature. They are capable of dissolving many types
of compounds that might not be soluble in aqueous or organic
solvent systems. Therefore, ILs are considered as better solvents
to transport a wide range of unusual combinations of both
inorganic and organic materials into the same phase for
subsequent reactions. ILs offer several advantages over conven-
tional organic solvents to carry out organic reactions, including
very low vapor pressure, less flammability, and especially the
ability of being functionalized to suit a particular reaction. Unlike
conventional molten salts (for example, molten sodium chloride),
ILs typically melt at low temperature (below 300 °C), which are
ideal for catalytic hydrocarbon conversion reactions to be
performed. Furthermore, ILs are made of ions rather than
uncharged molecules, such that the involved reactions often
provide distinct selectivity and reactivity as compared with-
the safety issues for the industrial scale-up, and the spent liquid
treatments for the environment. Therefore, searching for ILs as
robust reaction media for catalytic hydrocarbon conversion
processes has attracted significant attention.
Nonetheless, the stability of ILs under such extreme conditions
in which the C–H bond can be active is a continual challenge.
For example, as will be illustrated in this article, commonly
used imidazolium-based ILs (e.g., 1-methyl-3-butyl-imiazolium
bisulfate) cannot be directly used in the previously mentioned
catalytic methane conversion processes. Under such extreme
conditions, not only the C–H bond on the substituted alkyl chains
will also be activated but also the ring-opening decomposition
+
4
and deep oxidation to ammonium (NH ) and carbon dioxide
(CO ) will occur. Consequently, the functionality of ILs will be
2
limited or eliminated or, in the least, generating undesired side
reactions and by-products that could be costly to remove from
the end product (e.g., methanol). Therefore, ILs that are stable
under normal conditions might become unstable under certain
environments. High temperature (thermal stability) is not the
only limiting factor, as many ILs are known to withstand very high
temperature, but the combination of strong oxidants (e.g.,
[
1–7]
conventional organic solvents.
It has been an ongoing challenge to develop highly efficient
[
8,9]
hydrocarbon conversion processes,
wherein the C–H bond
activation is the key step of the developing catalytic chemical
reactions, to which certain kinds of ILs can play an important
[
10,11]
role.
the partial oxidation (no overoxidation to CO
CH ) to liquid products (such as methanol or formic acid) at a
One of the applications of such a C–H activation is
2
) of methane
(
4
*
Correspondence to: Guanyi Chen, Tianjin University, Weijin Road 92#,Nankai
District, Tianjin, 300072, China. E-mail: chen@tju.edu.cn
relatively low temperature ~200 °C (no high-temperature syngas
generation routes). Specifically, such reactions often require
harsh conditions such as a strongly acidic or highly oxidative
environment in conjunction with powerful metal catalysts
needed to split the strong covalent C–H bond (~105 kcal/mol)
at an elevated temperature. For instance, some effective catalytic
systems for direct methane oxidation require strong acid media
** Correspondence to: Yongchun Tang, Power Environmental Energy Research
Institute, 738 Arrow Grand Circle, Covina, California 91722, USA. E-mail:
tang@peeri.org.
a School of Environment Science and Engineering, Tianjin University, Tianjin
300072, China
b State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China
c ChemEOR Inc., Covina CA, 91722, USA
(>96% sulfuric acid to 102% oleum or concentrated selenic acid
at relatively high temperatures (180 to 220 °C) and a powerful
[
12]
[13]
transition/precious metal catalyst such as Hg(II) , Pt(II) , Au
[14]
[15]
(
III) , Pd(II) , and so on. These harsh conditions include using
d Power Environmental Energy Research Institute (PEER), Covina CA, 91722, USA
Magn. Reson. Chem. 2014, 52, 673–679
Copyright © 2014 John Wiley & Sons, Ltd.

G. Chen et al.
concentrated acids) and powerful catalysts (e.g., precious metals)
could destabilize ILs at a desired reaction temperature ~200 °C
and the acetic acid (at ~3.7 ppm) that is intentionally added to
the solution for referencing purpose. The peaks (c and d; at
(
chemical stability).
H-NMR has been applied to analyze the catalytic system that
~4.5 ppm) are attributed to the multiple –CH
been used as an indication of successful partial methane
oxidation to form methanol (CH OH) or its precursor CH SO OH.
While the peaks around 6.5–7.5 ppm (g and f) corresponded to
H-atoms attaching on the ring, the appearance of the triplet at
~5.5 ppm (e) that has a fingerprint of the tetrahedral ammonium
3
groups, which have
1
highly selectively converts methane to methanesulfonic acid
CH SO OH, which can be easily hydrolyzed to methanol) by
3
3
3
(
3
3
the powerful Pt-based catalysts in concentrated sulfuric acid
solution with addition of different ILs. While ILs with n-heterocyclic
cationic compounds containing at most one substituted methyl
group have been the focus of this study, the formation of the
ion structure is considered as the strong evidence of the
+
4
formation of NH
after the reaction. This suggests that the
+
1
NH
4
peak in H-NMR spectra is used as an indication of ring
aromatic ring was broken during the reaction in which new
substances were generated.
1
openings (decomposition). To better interpret the H-NMR
spectra, molecular modeling, based on quantum chemistry
calculations, has also been conducted comparing relative shifts
of each individual compound.
As our continuous efforts to search for chemically and
thermally stable ILs under extremely harsh reaction conditions,
we have examined a series of different N-heterocyclic cations. A
few good candidates have emerged. Figure 2 illustrates the
1
H-NMR spectrum of a solution with pyrazolium bisulfate ([pyraz]
Results and discussions
[
HSO
4
]) after reaction. The overall spectrum reassembles those
] (Fig. 1), with the main exception that
identified in [1-mim][HSO
is the disappearance of NH near the 5.5-ppm region. This is an
4
indication of the ring stability during the catalytic reactions. A
similar spectrum for the 1-methylpyrazolium bisulfate [1-mpyraz]
4
Experimental results and discussions
+
1
The H-NMR spectrum of the solution after a methane conversion
reaction with additive 1-methylimidazolium bisulfate ([1-mim]
[
HSO ]) is illustrated in Fig. 1. The two large peaks (b and a) at
[HSO ] additive in solution has also been found (Fig. 3). However,
4
4
each end represent the resonances of H-atoms from sulfuric acid
the lack of peaks near the 5.5-ppm region suggests the ring stability
of the [1-mpyraz] ion under the reaction conditions.
(at ~8.8 ppm) that is used as solvent and the oxidant for reaction
1
Figure 1. H-NMR spectra of the reaction solution containing [1-min][HSO
4
].
1
Figure 2. H-NMR spectra of the reaction solution containing [pyraz][HSO
4
].
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Magn. Reson. Chem. 2014, 52, 673–679

Ionic liquid stability in methane oxidation
1
Figure 3. H-NMR spectra of the reaction solution containing [1-mpyraz][HSO
4
].
The detailed mechanism of the stability of the pyrazolium
cation (Figs 2 and 3) over the imidazolium cation (Fig. 1) under
standard acetic acid (peak areas 667 and 697 before and after
reaction). Moreover, the total amount of added acetic acid is
extremely harsh reaction conditions [i.e., in a very acidic solution
0.3494 mmol (20 μl). Therefore, the increment of the –CH peak as
3
ꢀ
(
>96% sulfuric acid) with a strong oxidant (HSO4 or SO ) and an
a result of the generation of CH OH and CH SO H can be quantified
3
3
3
3
extremely powerful Pt-based catalyst that is capable of activating
the C–H bond at a temperature ~200 °C] is still unclear. However,
it has been previously demonstrated that ILs added in the
Pt-based systems can chelate the catalyst as ligands to balance
the reactivity and stability of the catalyst [11]. Therefore, we
consider that the additional stability of pyrazolium ILs is
because of chemical means rather than thermal arguments.
This is evident from the 1H-NMR of the reaction solution with
as (132/697 ꢀ 41/667)* 0.3494 = 0.04468 mmol. Therefore, the total
produced amount of CH OH/CH SO H is 0.4468mmol (10% of the
3
3
3
product liquid was injected for the NMR measurement). The initial
methane load can be calculated from the n = PV/RT equation, in
which P = 500 psi is the pressure of compressed methane gas
3
(1psi = 6894.757 J/m ), V is the empty space inside the sealed gold
2
tube, V = π/4* d * h, where d is the inner diameter (d= 0.0043 m) of
the gold tube and h is the height of the space after subtracting the
catalyst and liquid loads (h= 0.038 m), T is the absolute tempera-
ture at which the methane gas was compressed (T= 298K), and R
the presence of the triazolium bisulfate ([triaz][HSO
4
]) IL (Fig. 4).
+
The ‘no show’ of the NH
4
peak indicates the stability of the
ꢀ
7
3
triazolium ring, yet the reducing peak near the 4.5-ppm region
suggests that the catalytic reactivity of the Pt catalyst for the
methane conversion actually be suppressed by the presence
is the thermal constant (8.314J/molK). So, V = 5.5* 10 m , and
n = 0.765 mmol. Because the amount of CO generated because
2
of overoxidation has been barely detectable in this measurement,
the total methane conversion rate can be estimated as 0.4468/
0.765 * 100 = ~58%.
of the [triaz][HSO
4
] IL.
The product yield of CH
3
3 3
OH (or CH SO H) can be quantified
1
1
from H-NMR spectra. For instance, Fig. 5 illustrates H-NMR
spectra before and after the methane conversion reaction. The
peak area at 4.5 ppm, which has been identified that it originated
1
Theoretical calculations of H-NMR spectrum
1
from the –CH
3
group, can be used to determine the amount of
Molecular modeling results of calculated H-NMR spectra at three
methanol generation. The peak areas are integrated (41 and
different levels are summarized in Table 1. Considering that these
experimental measurements were conducted in the very strong
1
32 before and after reaction) and normalized with the internal
1
Figure 4. H-NMR spectra of the reaction solution containing [triaz][HSO
4
].
Magn. Reson. Chem. 2014, 52, 673–679
Copyright © 2014 John Wiley & Sons, Ltd.
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G. Chen et al.
1
2 4 2 4
Figure 5. H-NMR spectra before and after the methane conversion reaction. The reaction system is PtCl /[1-mpyraz][HSO ]/102% H SO , and the
reaction condition is 200 °C for 2.5 h. Acetic acid (20 μl) was used as the internal standard. Moreover, 10% of the produced liquids were injected for
the NMR measurement.
acidic conditions, while all computational simulations were car-
ried out in gas phase, it is not surprising that these calculated
NMR shift values do not exactly match experimental results.
Nevertheless, the relative trend of each individual compound
still provides important information for the calibration NMR
peaks. In general, the NMR shifts within one single compound
follow the trend
N–H ðringÞ > C–H ðringÞ > C–H ðmethyl on ringÞ
> C–H ðmethylÞ > O–H
An exemption occurs for the acetic acid in which the shift from
the OH is larger than that from the methyl group. This is because
of the resonance effect of the carboxylic acid group that
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Magn. Reson. Chem. 2014, 52, 673–679

Ionic liquid stability in methane oxidation
1
Table 1. Computational modeling of the H-NMR spectrum of selected compounds
1
Compounds
Molecule
H-NMR
HF/6-31G(d)
GIAO
B3LYP/6-311 +
G(2d,p) GIAO
MP2/6-31G(d)
GIAO
CH
CH
OH, 4.79
3
3
, 1.31
-2, 1.71
1.66
2.09
5.49
1.53
2.00
5.56
Acetic acid
NH-4, 2.47
0.94
2.91
0.13
2.92
2.56
0.29
Ammonium ion
Sulfuric acid
Methane
OH-2, 1.85
CH-4, 0.22
OH, ꢀ0.88
ꢀ0.66
3.41
ꢀ0.14
3.37
CH
3
-2, 2.98
Methanol
3
CH , 3.18
3.65
3.48
CH
CH
CH, 7.43
CH, 7.48
CH, 8.09
NH, 8.45
3
3
, 3.71
-2, 3.90
3.94
4.07
7.53
7.60
8.05
9.22
4.03
4.24
7.65
7.65
7.82
9.25
1-Methylimidazolium ion
CH, 7.01
CH-2, 8.60
NH-2, 9.09
7.44
8.40
9.88
7.62
8.15
10.00
Pyrozalium ion
CH
CH
CH, 6.76
CH, 8.21
CH, 8.39
NH, 8.78
3
3
-2, 3.85
, 4.04
4.10
4.23
7.15
8.02
8.20
9.52
4.22
4.39
7.35
7.76
7.96
9.69
1-Methylpyrozalium ion
(Continues)
Magn. Reson. Chem. 2014, 52, 673–679
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G. Chen et al.
Table 1. (Continued)
1
Compounds
Molecule
H-NMR
HF/6-31G(d)
GIAO
B3LYP/6-311 +
G(2d,p) GIAO
MP2/6-31G(d)
GIAO
CH-2, 9.12
NH-2, 9.49
9.36
7.57
8.85
11.00
1,2,4-Triazolium ion
Reference (tetramethylsilane)
32.5976
31.8821
32.046
stabilizes the H-atom. Another difference between the calcu-
lated/measured NMR spectra arises from the ammonium ion. It
Experimental
1
+
Computational methods
is well established that the H-NMR spectrum of NH
into three main lines because of the spin–spin coupling of
I = 1/2) with the equivalent proton, which gives rise to a
4
will split
14
N
(
The gas phase geometry of the selected molecular structure was
fully optimized at the HF/6-31G(d), density functional theory
(DFT)/Becke, 3-parameter, Lee-Yang-Par (B3LYP)/6-31 + G(2d,p),
and MP2/6-31G(d) levels. NMR results were obtained following
geometry optimization to calculate the NMR shielding tensors
total spin of I = 1 (triplet). Such an effect has not been considered
in the current computational method [gauge-independent
atomic orbital (GIAO)] that only yields a quartet degeneracy
instead of splitting.
Table 2. ILs used in this study
Name
Abbreviation
Structure/Synthesis
1-Methylimidazolium bisulfate
4
[1-mim][HSO ]
Pyrazolium bisulfate
4
[pyraz][HSO ]
1-Methylpyrazolium bisulfate
4
[1-mpyraz][HSO ]
1,2,4-Triazolium bisulfate
4
[triaz][HSO ]
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Ionic liquid stability in methane oxidation
Table 3. Reaction system and conditions to test the chemical/
thermal stability of selected ILs
dissolved into 0.8-ml deuterated sulfuric acid (as solvent) and
0 μl acetic acid (as internal standard). The mixture (0.7ml) was
subjected to H-NMR measurements relative to the concentration
2
1
Entry
Ionic liquid
T/°C Catalyst Concentration Time
of H
2
SO
4
(%)
(h)
and shift related to acetic acid.
1
2
3
4
[1-mim][HSO
[pyraz][HSO
[1-mpyraz][HSO
[triaz][HSO
4
]
200
200
200
200
PtCl
PtCl
PtCl
PtCl
2
2
2
2
94
102
94
2.5
2.5
2.5
2.5
4
]
Conclusions
4
]
ILs, which have been often regarded as the ‘green’ and ‘de-
signer’ solvents, could have a significant impact on the
selective hydrocarbon conversion processes, in particular the
organometallic reactions in which ILs have multiple functional-
ity, which includes roles as both solvent and ligand in order to
facilitate reactions. However, ILs themselves are a type of
organic compound, in which their chemical and thermal stabil-
ity, under typical reaction conditions, can also be affected.
Consequently, the comparability and stability of ILs with the
reaction systems need to be carefully examined. Our experi-
mental and theoretical studies indicate that the ionic
imidazolium ring can be decomposed under a catalytic
reaction system with the powerful Pt-based catalyst in highly
concentrated sulfuric acid conditions, in which methane can
be activated under temperatures around 200 °C. Furthermore,
we have also demonstrated that with a slight change of the
ring structures from imidazolium to pyrazolium or triazolium,
the cationic species of the ILs will be stable under previously
mentioned conditions.
4
]
102
using the GIAO method as implemented in the GAUSSIAN g98 pro-
gram. For HF/6-31G(d) and DFT/B3LYP/6-311+ G(2d,p) calculations,
there is the internally built calibration of tetramethylsilane (TMS)
spectrum. For the MP2/6-31G(d) calculations, the NMR of TMS
was calculated at the exact same computational level and taken
as a reference.
Synthesis of ionic liquids
IL samples are synthesized from direct protonation of their N-
heterocyclic precursors with concentrated sulfuric acid. For
example, 0.04 mol of pyrazole was added to 0.08-mol (4.5ml)
concentrated sulfuric acid at ambient temperature in a small
breaker. The reaction was run at 200 °C for 24 h with continuous
stirring through a magnetic stirrer. Anhydrate ether (40 ml) was
added to form a light-yellow oily phase by precipitation, from
where the liquid portion was separated by filtration. The precipitate
was washed with ether then further dissolved in 5 ml of methanol.
It was then treated with ether to precipitate the IL-bisulfate
compound that was then vacuum-dried. Both [1-mpyraz][HSO4]
and [triaz][HSO ] can be synthesized from pyrazine and 1,2,4-
triazole. [1-mim][HSO ] was also synthesized from 1-methylimidazole.
For comparison purpose, commercially available [1-mim][HSO ] was
also purchased and tested. Table 2 lists the molecular structures of
selected ILs and their synthesis reactions.
Acknowledgements
This work is financially supported by the International Science
and Technology Cooperation Program of China under agreement
4
2010DFB64070. We would like to thank Drs Zaiwei Li and Jihong
4
Chen from the Power Environmental Energy Research Center of
California Institute of Technology for their assistance in
experiment design and performance.
4
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Magn. Reson. Chem. 2014, 52, 673–679
Copyright © 2014 John Wiley & Sons, Ltd.
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