Formation of Acetylene in the Reaction of Methane with Iron Carbide Cluster Anions FeC3? under High-Temperature Conditions

Article abstract of DOI:10.1002/anie.201712463

The underlying mechanism for non-oxidative methane aromatization remains controversial owing to the lack of experimental evidence for the formation of the first C?C bond. For the first time, the elementary reaction of methane with atomic clusters (FeC₃^?) under high-temperature conditions to produce C?C coupling products has been characterized by mass spectrometry. With the elevation of temperature from 300 K to 610 K, the production of acetylene, the important intermediate proposed in a monofunctional mechanism of methane aromatization, was significantly enhanced, which can be well-rationalized by quantum chemistry calculations. This study narrows the gap between gas-phase and condensed-phase studies on methane conversion and suggests that the monofunctional mechanism probably operates in non-oxidative methane aromatization.

Full text of DOI:10.1002/anie.201712463

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Accepted Article
Title: Formation of Acetylene in the Reaction of Methane with Iron-
Carbide Cluster Anions FeC3- under High-Temperature
Conditions
Authors: Hai-Fang Li, Li-Xue Jiang, Yan-Xia Zhao, Qing-Yu Liu, Ting
Zhang, and Sheng-Gui He
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COMMUNICATION
Formation of Acetylene in the Reaction of Methane with Iron-
–
3
Carbide Cluster Anions FeC under High-Temperature Conditions
*
Hai-Fang Li, Li-Xue Jiang, Yan-Xia Zhao,* Qing-Yu Liu, Ting Zhang, and Sheng-Gui He
[
6]
Abstract: The underlying mechanism for non-oxidative methane
high selectivity.
dehydrogenation by the single iron atom coordinated with one Si
and two C atoms (SiFeC ), was proposed as an important
The methyl radical, generated through
aromatization remains controversial due to the lack of experimental
evidence for the formation of the first C−C bond. For the first time, the
2
−
elementary reaction of methane with atomic clusters (FeC
3
) under
intermediate for the subsequent gas-phase formation of benzene.
Identification of reliable mechanism of methane aromatization
relies on the real-time detection of the intermediates generated in
each elementary step.
formed in elementary reactions is usually inaccessible in
condensed-phase studies.
high-temperature conditions to produce C−C coupling product has
been characterized by mass spectrometry. With the elevation of
temperature from 300 K to 610 K, the production of acetylene, the
important intermediate proposed in a monofunctional mechanism of
methane aromatization, was significantly enhanced, which can be
well rationalized by quantum chemistry calculations. This study
narrows the gap between gas phase and condensed phase studies
on methane conversion and suggests that the monofunctional
mechanism probably operates in non-oxidative methane
aromatization.
[
2a,7]
However, detection of intermediates
Gas phase study of the elementary reactions between
methane and isolated transition metal carbide clusters (M
the active species involved in non-oxidative methane
aromatization, provides an important way to reveal mechanistic
details relevant with surface chemistry. The reactions of M
q
y
x
C
),
[
8]
q
x y
C
clusters with methane have been extensively studied under
[
9]
Methane aromatization has received significant attention
spurred by the increasing demand of aromatics in chemical
industry and the high availability of methane from natural gas,
room-temperature conditions. Little attention was paid to the
q
x y
reactions of M C clusters with methane under high-temperature
conditions, which is important to narrow the gaps between the
gas phase reactions and the related surface reactions. Herein,
[
1]
shale gas, and gas hydrate. A variety of silica-alumina zeolites
modified with transition metal species, such as Mo, Fe, and Zn
have been identified as promising catalysts for non-oxidative
we report that the high-temperature reactions of iron carbide
−
3
cluster anions (FeC
) with methane produce acetylene, the
[
2]
aromatization of methane to benzene under high-temperature
conditions (typically more than 973 K). Considerable efforts have
been devoted to uncovering the detailed mechanism of benzene
formation. However, the reaction intermediates and active phase
responsible for the cyclization to produce benzene remain
important intermediate proposed in the monofunctional
mechanism of methane aromatization. This is the first study of
high-temperature reactivity of atomic clusters toward methane.
−
y
The FeC
(y = 2−5) clusters were generated by laser
in an ion trap
ablation, mass selected, and then reacted with CH
4
[
3]
controversial. Some researchers emphasized a bifunctional
mechanism that the transition-metal carbide (M ) sites formed
in the induction period activate methane and convert the resulting
reactor at temperatures from 300 K to 610 K (see Experimental
Method in Supporting information).
clusters show similar reactivity toward methane, whereas FeC
4
clusters are inert (Figure S1). The reactivity of FeC
is the main focus of this study and the time-of-flight (TOF) mass
spectra for the reactions of FeC
Figure 1. Upon the interaction of FeC
product peak assigned as FeC
whereas the other two products FeC
barely observable (Figure 1b). When the reaction temperature
increased from 300 K to 610 K (Figure 1c,d), the relative
4 2 2 2
H and FeC H products, corresponding to
reactions 2 and 3, respectively, increased significantly. The
x
C
y
[10]
−
3
−
−
−
The FeC
and FeC
5
2
3
[
4]
CH
x
intermediates into ethylene,
which reacts further to
−
and FeC
produce benzene at the Brønsted acid sites in the zeolites. An
alternative monofunctional mechanism that both the activation of
methane to form acetylene intermediate and the subsequent
−
3
with CH
4
and CD
with CH at 300 K, a
4
are shown in
−
[
5]
3
4
aromatization take place over M
x
C
y
sites was also proposed.
−
4 4
H was observed (reaction 1),
Recently, single iron atoms embedded in a silica matrix
−
2
−
2
4
H
and FeC
2
H
were
2
(Fe©SiO ) were reported to convert methane to benzene with
−
−
intensities of FeC
[a]
Dr. H.-F. Li, L.-X. Jiang, Dr. Y.-X. Zhao, Dr. Q.-Y. Liu, T. Zhang, Prof.
Dr. S.-G. He
State Key Laboratory for Structural Chemistry of Unstable and Stable
Species, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, P. R. China.
isotopic labeling experiment with CD
confirmed the above reaction channels.
4
(Figure 1e) at 610 K
E-mail: chemzyx@iccas.ac.cn, shengguihe@iccas.ac.cn
L.-X. Jiang, T. Zhang, Prof. Dr. S.-G. He
University of Chinese Academy of Sciences, Beijing 100049, P. R.
China.
Dr. H.-F. Li, L.-X. Jiang, Dr. Y.-X. Zhao, Dr. Q.-Y. Liu, T. Zhang, Prof.
Dr. S.-G. He
Beijing National Laboratory for Molecular Sciences, CAS Research/
Education Center of Excellence in Molecular Sciences, Beijing
−
−
−
−
−
−
FeC
FeC
FeC
3
3
3
+ CH
+ CH
+ CH
4
4
4
 FeC
 FeC
 FeC
4
H
4
H
2
H
4
2
2
(1)
(2)
[
[
b]
c]
+ H
+ C
2
2
H
2
(3)
−
3
The pseudo-first-order rate constants (k
different temperatures were measured (Figure S2) and shown in
Figure 2. It can be seen that the temperature dependence of the
1
) for FeC
+ CH
4
at
100190, P. R. China.
Dr. H.-F. Li
−
15
3
−1
Present address: Department of Precision Instrument, Tsinghua
University, Beijing, 100084, P. R. China.
Supporting information for this article is given via a link at the end of
the document.
rates is non-Arrhenius. The k
1
value is 1.7 × 10 cm molecule
at 300 K and increases by a factor of 2.8 when the
temperature is ramped up to 410 K. However, the k value
increases slowly from 410 K to 575K [k (575 K) = 1.2k (410 K)].
−
1
s
1
1
1
1
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From 575 K to 610 K, the k value increases rapidly again [k (610
1 1
K) = 1.4k (575 K). The non-Arrhenius behavior can be
1
interpreted on the basis of the quantum chemistry calculated
cluster structures and reaction mechanisms in the text below.
The branching ratios (BRs) of reactions 2 and 3 increase as the
increase of the temperature while the BR of reaction 1 decreases
−
3
(

Figure 2b), suggesting the consecutive processes: FeC
+ CH
4
−
−
−
FeC
4
H
4
 FeC
4 2 2 2 2 2 2
H + H or FeC H + C H .
It is noteworthy that almost all of the reported rate constants
−
13
3
of cluster reactions with methane are greater than 10
cm
Figure 2. Rate constants (a) and branching ratios (b) at different temperatures.
−
1
−1
[11]
molecule s around 300 K, corresponding to facile activation
and conversion of methane. In sharp contrast, the reaction rates
encountered for C−H activation by the quartet species while the
−
−15
3
−1 −1
doublet species can activate methane more easily. The PESs
of the current system FeC
3 4
+ CH [(28)10 cm molecule s ]
2,4
starting from the linear structures ( IS2, Figure S7) can merge
into the PESs from the cyclic structures ( IS1, Figure 3), so only
the reaction mechanism of IS1 + CH is described below.
4
are very low. The elevation of temperature is thus required to
enhance the reaction efficiency. The reaction system in this study
can be more comparable to the condensed phase systems
2,4
[
1d,12]
operated under high-temperature conditions.
4
The reaction of IS1+CH
4
has a overall barrier of 0.58 eV that
can be hardly overcome at 300 K (Figure S14). The observed

reactivity in Figure 1b can be due to the doublet FeC
3
. Figure 3
through the Fe atom with a
binding energy of 0.20 eV. The reaction then proceeds through
2
indicates that IS1 can trap CH
4
2
2
the mechanism of oxidative addition ( I1  TS1  I2) with a
barrier of 0.05 eV, resulting in activation of the first C−H bond
and formation of the Fe−H and Fe−CH
3
bonds. The intermediate
I2 is more stable in the quartet than the doublet state, which
[
15]
results in a spin crossing process. After the spin conversion
2
4
(
TS1  CP1  I2, see Figure S8), the reaction proceeds along
4
the quartet surface. Upon a ring-opening isomerization ( I2 
4
4
4
4
I3), the C−C coupling takes place ( I3  TS3  I4) and the
4
2
4
TS3 is lower than IS1 + CH in energy by 0.03 eV. The C−C
coupling step can release enough energy to overcome the barrier
of the second C−H bond activation and generate the
4
intermediate I5, in which Fe site is coordinated with two H atoms.
4
The four-membered ring of I5 embraces the CH
2
moiety to form
4
4
five-membered ring ( I5  I6) and one of H atoms transfers from
Fe atom to the carbon ligand to form I7, the most stable
structure on the potential energy surface. The intermediate I7
has enough energy (~2 eV) to relax ( I7  I8) and then to
activate the third C−H bond of CH ( I8  I9). After a structural
rearrangement to break the Fe−C bond in I9, the fourth C−H
bond can be activated to form intermediate I11, in which the Fe
4
4
4
4
4
4
4
4
4
−
Figure 1. TOF mass spectra for the reactions of mass-selected FeC
with He at 610 K (a), with CH at 300 K (b), 410 K (c), and 610 K (d), and with
CD at 610 K (e) for around 14.5 ms. The molecule density of reactant gases is
about 2.4 × 10 molecule cm
3
cluster
atom is four-fold coordinated with one C atom and three H atoms.
4
Two of the three H atoms on Fe site can make a H unit that can
2
−
4
be evaporated to form product ions FeC H2 (P1, Figure S10,
4
1
5
−3
.
reaction 2).
In addition to the generation of dihydrogen, there is a
Density functional theory (DFT) calculations were performed
to study the structures and reaction mechanisms of FeC . In
3
literature, cyclic (IS1) and linear (IS2) structures (Figure S4) with
quartet spin multiplicities were separately proposed as the
−
competing reaction for
2 2
C H formation (reaction 3). After
activation of the third C−H bond of methane to form intermediate
4
I9, one H atom can transfer from the Fe site to a C atom of C
3
4
4
4
−
[13]
ligand to form a [C
which both of the two C atoms in [C
ligand. For further desorption of C
2
H
2
] unit in intermediate I13 ( I9  I13), in
] unit originate from the C
molecule, the formed [C
lowest-lying energy isomers of FeC
3
.
The gas phase ion
indicated that both cyclic and
can be produced in the experiments.
The adopted M06L functional (see Theoretical Method in
[
14]
2
H
2
3
chromatography expeiments
−
y
2
H
2
2 2
H ]
linear strucuters of FeC
4
4
4
unit in I13 should coordinate with Fe site ( I13  I14).
4
4
4
Therefore, the resulting intermediate I14 has enough internal
Supporting Information) predicts that the quartet IS1 and IS2
structures are close in energy (0.14 eV) and the doublet IS1 and
IS2 are above the IS1 and IS2 by 0.27 and 0.09 eV,
respectively (Figure S4). As a result, both IS1 and IS2 can be
produced by our experiment and IS1 and IS2 may also be
populated through thermal equilibrium (Figure S5).
2
energy to break [H
2
C
2
−C
2
H] bond to generate the intermediate
4
2
4
4
I15 with isolated [HC=CH] unit, which can be evaporated to
−
4
4
produce the experimentally observed FeC
Figure S11). The pathway for incorporation of the C atom from
CH into [HC=CH] unit is also considered (Figures S12 and S13).
2 2
H product ions (P2,
2
2
4
4
From the intermediate I7, the H atom bonded with Fe can
transfer to the adjacent C atom of [CH] moiety to form
The potential energy surfaces (PESs) for methane activation
2,4
2,4
4
by all of the four species ( IS1 and
IS2) were explored
intermediate I16 with two equivalent [CH ] units. Then, the third
2
4
(
Figures 3 and S7). High overall positive barriers (>0.5 eV) are
C−H bond of methane is activated to form intermediate I17, the
2
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Angewandte Chemie International Edition
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COMMUNICATION
4
structure of which is similar to that of I7. Further transformation
(+0.08 eV) in activation of the first C−H bond can be surmounted
by the kinetic energy and the vibrational energy of the reactants
(Evib = 0.06 eV at 300 K, see also Figure S14). As a result, the
4
of I17 to generate HC≡CH product follows the pathway similar to
4
I7 → P2 + C
2 2
H .
reactions 13 are thermodynamically and kinetically favorable at
2
In the case that the reaction starts with IS1 + CH
4
, all of the
−
3
00 K. The energy of FeC
4
H
2
−
+ H
2
(P1, −0.83 eV) is significantly
reaction intermediates, transition states, and the spin conversion
point (CP1) are lower in energy than the separated reactants
lower than that of FeC
qualitatively consistent with the higher branching ratio of FeC H
4 2
2
H
2
+C H (P2, −0.17 eV), which is
2 2
−
2
(
Figure 3). In IS2 + CH
4
(Figure S7), the tiny positive barrier
−
2 2
over that of FeC H (Figure 2b).
−
−
−
−
3 4 3 4 2 2 2
Figure 3. DFT calculated potential energy profiles (a) and structures (b) for FeC (IS1) + CH . The isomeric structures of FeC , FeC H , and FeC H are given in
Figures S4, S10, and S11, respectively. The transition states and some intermediates can be found in Figure S6. The zero-point vibration corrected energies (∆H
0
4
in eV) with respect to IS1 + CH
4
are given. Bond lengths are given in pm.

4
The doublet FeC
3
can activate and transform methane at
Alternatively, it can transform to separated products ( I3 →
−
−
3
00 K (Figure 1b) while the reaction efficiency is very low (on
the order of 10 ). The high energy barrier ( TS3 I3 = 0.95 eV)
of the tight transition state TS3 (G298 = +0.56 eV) for C−C
coupling is the bottleneck to form the H
products (reactions 2 and 3). In addition, the spin conversion in
the C−H activation ( TS1  CP1  I2) may also slow down the
formation of I3 that is the most possible candidate of the
experimentally observed adsorption complex FeC
). The above two rate limiting processes ( I3 → TS3 and TS1
I2) can be enhanced by elevation of reaction temperature,
which is consistent with the significant increase of rate constant
from 300 K to 410 K (Figure 2a). Note that at 300 K, the quartet
is unlikely to contribute to the observed reactivity because
4
very tiny percentage (10 ) of CH molecules have enough
FeC
4
H
2
+ H
2
or FeC
2
H
2
+ C
2
−
H
2
) or dissociate back to
+ CH ). The temperature
6
4
4
4
separated reactants ( I3 → FeC
3
4
4
increase should always favour the formation of separated
products and the total rate constant of reactions 2 and 3
increase during the entire termperature range (from 300 K to
610 K, Figure S3). However, the increased temperature
disfavors the stablization of adsorption complex. As a result,
reaction 1 can have a negative temperature dependence (from
410 K to 610 K in Figure S3). The coexistence of multiple
species and the negative termperature dependence of the
association reaction are responsible for the non-Arrhenius
temperature dependence of the total rate constants shown in
Figure 2a.
2 2 2
and C H elimination
2
4
4

4
H
4
(reaction
4
4
2
1
→
4

FeC
3

8
In literature, the gas phase reactions of methane with
kinetic energy to overcome the high reaction barrier (0.550.58
eV, Figure S14). At high temperatures (~600 K), the quartet
may react with a fair amount of CH (0.03%0.09%),
4
which is consistent with a sharp increase of the rate constant
from 575 K to 610 K in the experiment (Figure 2a).
[
9a,b,d,e]
+
+
transition-metal carbide cluster cations
AuC , and CuC ) and the anions
(TaC1−14 , MoC ,
+
+
[9c,f]
−
6
−

(FeC
and Ta
2
C
4
) have
FeC
3
been studied by mass spectrometric experiments under
room-temperature conditions. The generation of ethylene has
been identified for many of the cations and the production of
+
[9a]
During the gas phase experiment, the adsorption complex
I3 can be stabilized through the collisions with the bath gas.
acetylene was only observed for the TaC
y
.
The cluster
4
anions are generally much less reactive than the corresponding
3
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Angewandte Chemie International Edition
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COMMUNICATION
[
16]
cations in the reactions with methane.
The carbide cluster
Keywords: methane transformation • metal carbide cluster •
high temperature reaction • mass spectrometry • density
functional calculations
anions were reported to dehydrogenate methane to produce
dihydrogen at room temperature facilitated by the dissociative
[
17]
[9f]
CO adsorption or by the cooperation of two metal atoms,
while no C−C coupling product was observed. In this study, the
production of C−C coupling product, acetylene, was evidently
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[
1d,18]
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2c,19]
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2b,5a]
[20]
suggested by Mériaudeau,
and Ha,
emphasized
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[
21]
5215−5219.
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phase methyl radical that undergoes series reactions to form
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[
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This work was financially supported by the Chinese Academy of
Sciences (No. XDA09030101) and the National Natural Science
Foundation of China (Nos. 91645203, 21573247, and
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4
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Angewandte Chemie International Edition
10.1002/anie.201712463
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
The generation of C−C coupling
Hai-Fang Li, Li-Xue Jiang, Yan-Xia
Zhao,* Qing-Yu Liu, Ting Zhang, and
Sheng-Gui He*
product of acetylene in the high
temperature reaction of methane with
−
3
atomic cluster species (FeC
) has
Page No. – Page No.
been identified, providing a molecular
level origin of the monofunctional
Formation of Acetylene in the
methanism
for
non-oxidative
Reaction
of
Methane
with
–
methane aromatization proposed in
condensed phase systems.
Iron-Carbide Cluster Anions FeC
3
under High-Temperature Conditions
5
This article is protected by copyright. All rights reserved.