High-temperature Shilov-type methane conversion reaction: Mechanistic and kinetic studies

Article abstract of DOI:10.1016/S1872-2067(15)60966-4

Traditional Shilov reactions (performed in aqueous solution with a PtCl₂ catalyst) for methane conversion suffer from catalyst deactivation at high temperatures (> 100 °C), therefore only very low conversion rates have been achieved. In this paper, we show that Shilov-type C-H activations are achievable at much higher temperatures (～200 °C) by addition of concentrated aqueous solutions of Cl^- to inhibit Pt catalyst precipitation. Various chloride-based ionic liquids also stabilized the Pt catalyst at mild reaction temperatures (～140 °C). Under high-pressure conditions (> 25.5 MPa), achieved using a specially designed sealed gold-tube reactor, very high methane conversion rates (> 90%) were obtained; this is attributed to the improved methane solubility in aqueous solution. Deuterium isotope (H/D) exchange between methane and water was used to examine the reaction reactivity and selectivity. Multiply D-substituted products were observed, indicating that multiple C-H activations occurred. A comprehensive network reaction that included all the chain reactions was set up to clarify the reactivities and product selectivities of the methane activation reactions. The reaction network consisted of a series of parallel first-order reactions, which can be described by the Arrhenius equation. The kinetic parameters such as the frequency factor, activation energies, and stoichiometric coefficients were obtained by fitting the experimental data. Because all four C-H bonds in a methane molecule are equivalent, multiple substitutions during methane conversion cannot be avoided. Our studies indicate that mono-substituted and di-substituted methane isotopologue generations have similar activation energies, suggesting that the highest mono-substitution selectivity cannot be greater than 50%.

Full text of DOI:10.1016/S1872-2067(15)60966-4

Chinese Journal of Catalysis 36 (2015) 1777–1784
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
High-temperature Shilov-type methane conversion reaction:
Mechanistic and kinetic studies
Shujuan Kang a, Qisheng Ma b,c, Weiqun Chen d, Guanyi Chena,e,*, Yongchun Tang d,#
a School of Environment Science and Engineering, Tianjin University, Tianjin 300072, China
b ChemEOR Inc., Covina CA, 91722, USA
c SHEETA Global Technology Corporation, Covina CA 91722, USA
d Power Environmental Energy Research Institute (PEER), Covina CA, 91722, USA
e State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Traditional Shilov reactions (performed in aqueous solution with a PtCl2 catalyst) for methane con-
version suffer from catalyst deactivation at high temperatures (> 100 °C), therefore only very low
conversion rates have been achieved. In this paper, we show that Shilov-type C–H activations are
achievable at much higher temperatures (~200 °C) by addition of concentrated aqueous solutions
of Cl− to inhibit Pt catalyst precipitation. Various chloride-based ionic liquids also stabilized the Pt
catalyst at mild reaction temperatures (~140 °C). Under high-pressure conditions (> 25.5 MPa),
achieved using a specially designed sealed gold-tube reactor, very high methane conversion rates
(> 90%) were obtained; this is attributed to the improved methane solubility in aqueous solution.
Deuterium isotope (H/D) exchange between methane and water was used to examine the reaction
reactivity and selectivity. Multiply D-substituted products were observed, indicating that multiple
C–H activations occurred. A comprehensive network reaction that included all the chain reactions
was set up to clarify the reactivities and product selectivities of the methane activation reactions.
The reaction network consisted of a series of parallel first-order reactions, which can be described
by the Arrhenius equation. The kinetic parameters such as the frequency factor, activation energies,
and stoichiometric coefficients were obtained by fitting the experimental data. Because all four C–H
bonds in a methane molecule are equivalent, multiple substitutions during methane conversion
cannot be avoided. Our studies indicate that mono-substituted and di-substituted methane isotopo-
logue generations have similar activation energies, suggesting that the highest mono-substitution
selectivity cannot be greater than 50%.
Received 20 March 2015
Accepted 17 August 2015
Published 20 October 2015
Keywords:
Shilov reaction
Catalyst deactivation
Reaction mechanism and kinetics
Reactivity and selectivity
Deuterium-isotope exchange
© 2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
bon-based feedstocks. NG is considerably more efficient and
cleaner than other fossil fuels, because of its higher H/C ratio
Natural gas (NG), which is primarily composed of methane
(typically 85%–90%), is one of the most abundant hydrocar-
and British thermal unit value. However, the economic value of
NG is greatly underplayed, partly because its gaseous nature
* Corresponding author. Tel: +86-22-87402072; Fax: +86-22-27407956; E-mail: chen@tju.edu.cn
# Corresponding author. Tel: +1-626-858-5077; Fax: +1-626-858-9250; E-mail: tang@peeri.org
This work was partially supported by US Department of Energy (DE-EE0003461) and financially supported by the National Basic Research Program
of China (973 Program, 2012CB215303).
DOI: 10.1016/S1872-2067(15)60966-4 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 36, No. 10, October 2015

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hinders its transportation and use. Methane conversion to
high-value liquid chemicals and fuel products has long been
regarded as the “Holy Grail” by chemists, because this is key to
use NG as an important chemical feedstock. Recently, the in-
crease in global NG reserves and the successful extraction of
unconventional shale gas have greatly stimulated the develop-
ment of gas-to-liquid (GTL) technologies.
The Shilov system, first reported in 1972, has had significant
impacts on the development of organometallic GTL C1 chemis-
try [1–3]. The Shilov process converts alkanes, including CH4, to
other hydrocarbon chemicals such as CH3OH and CH3Cl via
three key steps.
(1) A C–H activation step – formation of a methylplatinum(II)
intermediate from the reaction of CH4 with Pt(II) (i.e.,
[PtCl6]2−).
(2) A functionalization step – oxidation of the methylplati-
num(II) species to form a methylplatinum(IV) complex.
(3) A catalyst regeneration step – nucleophilic addition at the
methyl carbon to yield an organic product, with reduction
of the Pt(IV) complex (i.e., [PtCl4]2−) back to Pt(II).
catalyst in solution, therefore a higher reaction temperature
could be used, and the CH4 conversion rate and catalytic effi-
ciency would be improved.
In this study, we focused on developing mechanistic and ki-
netic understandings of high-temperature Shilov systems, to
improve the stability and reactivity. Pt(II)-catalyzed CH4 con-
version reactions were performed using a specially designed
sealed gold-tube mini-reactor, to enable high-pressure (25.5
MPa) conditions to be used, to improve the CH4 gas solubility in
aqueous solution. The solubility of methane in aqueous solu-
tion under ambient conditions is low (0.0012 g/kg of water at
60 °C), and is enhanced 1000-fold under high pressure (0.1202
g/kg of water at 20.0 MPa, and 0.1531 g/kg of water at 30.0
MPa, at 60 °C) [17]. H/D exchange experiments were per-
formed to determine the reactivities and selectivities of the
reactions. Possible mechanisms for stabilizing the Pt catalyst in
aqueous solution are discussed. The stability at high tempera-
tures is attributed to the effect of concentrated Cl−. A compre-
hensive reaction network using a series of first-order parallel
sequential substitution reactions, described by the Arrhenius
equation, was set up to determine the detailed reaction kinetics
of the high-temperature Shilov process.
The net reaction is
R–H + [PtCl6]2− + H2O → R–OH + [PtCl4]2− + 2H+ + 2Cl− (1)
One of the most important features of the original Shilov
system is that it can activate the stable C–H bond in CH4 at
temperatures as low as 80 °C. However, further increasing the
reaction temperature (> 120 °C) often results in irreversible
disproportionation and combination of Pt(II) complexes to
form Pt(IV) and Pt(0), which destabilizes the Pt(II) catalyst as a
result of metallic Pt(0) precipitation from the aqueous solution:
2. Experimental
2.1. Materials
K2PtCl4 and H2PtCl6 were purchased from Sigma-Aldrich.
The ILs 1-methylimidazolium bisulfate ([1mim][HSO4]) and
1-methylimidazolium chloride ([1mim][Cl]) were purchased
from Fluka, and pyrazinium bisulfate ([pyrz][HSO4]) was syn-
thesized by protonation of pyrazine with sulfuric acid. For the
H/D exchange experiments, only K2PtCl4 was used as the cata-
lyst, and D-substituted chemicals, i.e., water (D2O), CH3COOH
(CD3COOD), sulfuric acid (D2SO4), and hydrogen chloride (DCl),
were purchased from Sigma-Aldrich and used as additives. For
example, 30% CD3COOD solution was obtained by mixing
CD3COOD with pure D2O. In the reactions, the only hydrogen
source was therefore activated methane.
2 [PtCl4]2− → Pt(0) + [PtCl6]2− + 2Cl−
(2)
Consequently, only low reaction temperatures can be used,
resulting in low conversion rates.
Significant efforts have been devoted to improving the sta-
bilities and reactivities of Pt(II)-based catalytic systems using
different organic ligand molecules, but commercially practical
solutions have not been found [4–9]. For instance, one of the
best-known systems was developed by Periana at Catalytica in
the 1990s; the catalyst in the system is (bpym)PtCl2 [bpym =
2
 -(2,2'-bispyrimidyl)] [10–12]. This system shows excellent
efficiency and selectivity in the conversion of CH4 to CH3SO3H,
which can be further hydrolyzed to CH3OH. In contrast to the
original Shilov system, which is performed in aqueous solution,
Catalytica’s catalyst is deactivated by water and the CH3OH
produced; therefore, the conversion reaction can only be per-
formed in highly concentrated (> 96%) sulfuric acid, which
limits its commercial feasibility [13].
Ionic liquids (ILs) are superior solvents for a wide variety of
organic and inorganic compounds, and have many advantages
over conventional volatile organic solvents. ILs are therefore
ideal coordination ligands for organometallic catalysts and are
good solvents for many catalytic systems [14,15]. Our recent
studies have identified several N-heterocyclic ILs that are both
thermally and chemically stable under extreme C–H bond acti-
vation reaction conditions, which involve powerful Pt-based
catalysts at ~200 °C [16]. We anticipated that addition of se-
lected ILs to the Shilov system would reverse the Pt(0) precipi-
tation reaction by maintaining a significant amount of the Pt
2.2. Sealed gold-tube mini-reactor
A mini gold-tube (length 10 cm, inner diameter 1.09 cm)
was used as the mini-reactor. We chose gold as the material for
the mini-reactor because of its chemical inertness, and its flexi-
bility, which allows volume expansion and contraction by ex-
ternal control of the confining pressure. It ensured 100% mass
balance, and uniform temperature control was provided by a
large box furnace. Efficient contact between the gas-phase me-
thane and liquid-phase reaction media containing the catalysts
and liquid stabilizers was achieved by applying external hy-
draulic pressure.
2.3. Methods
The Pt catalyst was dissolved in various reaction media at
desired concentrations. The prepared reaction liquid (0.3 mL)

Shujuan Kang et al. / Chinese Journal of Catalysis 36 (2015) 1777–1784
1779
was loaded into the gold-tube reactor, which was presealed at
patterns using the deconvolution method.
3. Results and discussion
one end. CH4 gas (0.34 MPa, with 3% Ar as an internal stand-
ard) was also loaded into the reactor, and the other end was
sealed by arc welding. The sealed gold-tube was placed in a
stainless-steel autoclave and an external hydraulic pressure of
up to 25.5 MPa was applied. The autoclave with the gold-tube
was placed in a box furnace to maintain the required reaction
temperature through hot-air circulation. After reaction, the
sealed gold tube was inserted into a vacuum line and all gas
compounds were analyzed using gas chromatography (GC).
The remaining liquid and solid residues (if any) were collected
and analyzed. The total weights of the sealed gold-tube before
and after the reaction were recorded and compared. Typical
weight differences were within 0.1 mg, indicating no leakages
during the reaction.
3.1. Mechanistic studies – high-temperature Shilov system
Pt(0) precipitation in the Shilov system and the effects of
different stabilizers at 140 and 185 °C were investigated, with a
focus on examining the effects of Cl− concentration on Pt stabi-
lization. Table 1 summarizes the experimental results obtained
under various conditions. The total H/D exchange ratio reflects
the degree of CH4 conversion, and the relative H/D exchange
ratio for CH3D and CH4−xDx is used as an indication of product
selectivity. The turnover number (TON) was determined from
the ratio of the molar quantity of activated CH4 to that of cata-
lyst. Inductively coupled plasma-atomic emission spectroscopy
(ICPAES; Leeman Labs Prodigy) was used to confirm that the
precipitated deposit was metallic Pt(0). A piece of the opened
gold tube with precipitates was immersed in 2% HNO3 solution
in an ultrasonic washer overnight, cleaned with deionized wa-
ter, and dried under vacuum. The total weight of the solid sam-
ple was recorded. The sample was then dissolved in chloroace-
tic acid and subjected to ICP-AES to determine the concentra-
tions of the elements in the sample solution. The results
showed that the only metallic elements in the solid residue
from the precipitates were Pt (0.15 wt%, 906.1 g/g) and Au
(99.85 wt%).
2.4. Analyses
A Hewlett-Packard 6890 GC with a J&W GS-GASPRO column,
thermal conductivity detector, and mass selective detector was
used to determine the gas composition and for D-isotopic anal-
yses. The amount of methane gas was deduced from the inter-
nal standard Ar integration, and the methane isomers were
identified from their characteristic ion masses, i.e., 20 for CD4,
19 for CHD3, 18 for CH2D2, and 17 for CH3D. All standard me-
thane isomer samples (16-CH4, 17-CHD3, 18-CH2D2, 19-CH3D,
and 20-CD4) were previously analyzed using the same GC-MS
instrument to determine their fragmentation patterns, and
stored for use as internal references. The methane isomer
compositions were derived from the sample fragmentation
3.1.1. Effects of chelation with carboxylic anions
Shilov suggested that addition of CH3COOH might signifi-
Table 1
H/D exchange experiments for Pt-catalyzed high-temperature Shilov methane conversion reactions.
Catalyst
D/H exchange ratio
TON
C–H
Ionic
liquid
Temp. Time
Entry
Reaction media
Conc.
(mmol/L)
50
(oC)
(h)
Compound
CH4-xDx CH3D D/H
1
2
3
4
5
6
7
8
9
K2PtCl4
K2PtCl4
K2PtCl4
K2PtCl4
D2O
—
—
—
—
—
—
—
—
—
140
140
140
140
140
140
140
140
185
4
4
12
5
12
12
4
7.2
13.0
13.1
18.0
19.4
31.4
2.8
3.3
5.1
5.0
9.9
7.3
8.3
2.0
1.0
22.5
10.5
18.1
18.1
27.9
26.7
39.7
4.8
0.5
2.1
2.1
2.6
3.2
0.8
0.5
0.5
37.7
20
20
50
D2O + CD3COOD (30%)
D2O + CD3COOD (30%)
D2O + F3COOD (30%)
D2O + CD3COOD (30%)
H2PtCl6
20
K2PtCl4 + H2PtCl6
K2PtCl4
20 + 100
20
D2O + CD3COOD (30%)
D2O + CD3COOD (30%) + DCl (8.4 mol/L)
D2O + CD3COOD (30%) + D2SO4 (1 mol/L)
D2O + CD3COOD (30%) + DCl (8.4 mol/L)
K2PtCl4
K2PtCl4
20
20
4
24
2.9
66.5
3.9
89.0
10
11
12
13
14
15
16
17
18
19
20
K2PtCl4
K2PtCl4
K2PtCl4
K2PtCl4
K2PtCl4
K2PtCl4
K2PtCl4
H2PtCl6
K2PtCl4
K2PtCl4
H2PtCl6
20
D2O + CD3COOD (30%) + D2SO4 (4.2 mol/L)
D2O (30%) + DCl (8.4 mol/L)
CD3COOD (30%) + DCl (8.4 mol/L)
D2O
—
185
185
185
140
140
140
140
140
4
24
4
4
4
35.5
54.1
22.6
1.9
2.0
2.2
18.0
11.8
11.2
1.8
1.7
1.7
53.5
65.9
33.8
3.7
3.7
3.9
5.9
30.6
3.8
0.1
0.1
0.4
0.5
1.0
0.4
1.6
2.7
20
20
50
50
—
—
[pyrz][HSO4]a
[pyrz][HSO4]b
[1mim][Cl]a
[1mim][Cl]b
[1mim][Cl]b
[1mim][HSO4]a 140
[1mim][Cl]b
[1mim][Cl]b
D2O
20
D2O + CD3COOD (30%)
D2O + CD3COOD (30%)
D2O + CD3COOD (30%)
D2O + CD3COOD (30%)
D2O + CD3COOD (30%)
D2O + CD3COOD (30%)
4
20
20
20
20
4
12
4
4
4
3.1
6.2
2.2
9.5
2.2
3.5
1.8
5.0
5.3
9.7
4.0
14.5
24.7
185
185
20
15.9
8.8
a The ionic liquid and Pt-catalyst were heated at 150 °C for 30 min, then added into the CD3COOD solution after cooling down.
b The ionic liquid, Pt-catalyst, and CD3COOD solution were mixed together at room temperature.

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Shujuan Kang et al. / Chinese Journal of Catalysis 36 (2015) 1777–1784
concentration, using ILs such as [1mim][Cl].
(a)
(b)
3.1.3. Effects of ILs
The addition of various ILs (Table 1, entries 13–18) resulted
in virtually no Pt precipitation at 140 °C, indicating that select-
ed ILs stabilized the Pt catalyst. However, the overall H/D ex-
change rates also decreased. Possible explanations are that the
added ILs do not improve the CH4 solubility and/or formation
of Pt–IL complexes decreases the catalytic activity (as in the
case of Catalytica’s system, which required a temperature of
~220 °C). The selected ILs did not stabilize the Pt catalyst at
185 °C (Table 1, entries 19 and 20).
Fig. 1. Inner surface of gold tube after reactions with coated film (a) and
clean surface (b).
cantly increase methane conversion (~30 times) in low-tem-
perature, low-conversion reactions. He attributed this to che-
late formation between carboxylic anions and the Pt catalyst,
which would prevent Pt precipitation. Without the addition of a
chelating agent, the H/D exchange ratio was quite small in pure
D2O, even when we applied a high external pressure (3700 psi)
to increase methane solubility in the aqueous phase (Table 1,
entry 1). Addition of 30% CD3COOD significantly promoted
H/D exchange (Table 1, entry 2), even for small amounts of
catalyst; this is in general agreement with Shilov’s observa-
tions. However, at 150 °C, extensive Pt(0) precipitation still
occurred. This was supported by the finding that performing
the reaction under the same conditions for a longer time (12 h,
Table 1, entry 3) did not significantly improve the TON, sug-
gesting that the catalyst was deactivated within 4 h, and the
coating Pt(0) layer did not catalyze C–H activation. However,
CF3COOD enhanced the reactivity to a lesser extent, even at a
higher catalyst concentration and longer reaction time (Table 1,
entry 4, 150 °C, 5 h). This is probably because the stronger
electrophilicity of F atoms makes the chelating ability of the
F3COO group lower than that of the D3COO group. Changing the
catalytic system from K2PtCl4 to H2PtCl6 or their mixture (Table
1, entries 5 and 6) did not improve the Pt stability. These re-
sults suggest that the addition of CH3COOH is not enough to
prevent Pt(0) precipitation at high temperatures. The substan-
tial increases in the conversion could arise from the enhanced
CH4 solubility.
3.1.4. Effect of Cl− concentration
The maximum Cl− effect was determined from a series of
H/D exchange experiments at various DCl concentrations (Ta-
ble 2, entries 21–27, and Fig. 2). All reactions were conducted
at 200 °C and an external hydraulic pressure of 25.5 MPa for 4
h in 30% CD3COOD with 20 mmol/L K2PtCl4 as the catalyst. The
results clearly show that the amount of silver coating decreased
with the increasing Cl− concentration, and the conversion also
decreased. These results suggest that a highly concentrated Cl−
solution effectively prevented Pt(0) precipitation at high tem-
peratures, but the activity of the Pt catalyst was also reduced.
Table 2
High-temperature Shilov reactions with addition of various concentra-
tions of Cl−.
D/H exchange ratio
CH4-xDx CH3D D/H
DCl concentra-
tion (mol/L)
Entry Stabilizer
Coating
DCl + D2O
DCl + D2O
DCl + D2O
DCl + D2O
DCl + D2O
DCl + D2O
DCl + D2O
21
22
23
24
25
26
27
0.7
2.8
4.2
4.9
5.6
6.3
61.0
60.7
61.1
65.5
58.8
43.3
29.0
33.7
31.1
90.0 Silver
94.4 Silver
92.2 Silver
34.5 100.0 Slightly
29.8
20.5
10.0
88.6
63.8
32.7
No
No
No
8.4
22.7
Reaction system consisted of K2PtCl4 (20 mmol/L) + CD3COOD (30%) +
CH4, and reaction temperature was 185 °C.
100
3.1.2. Effects of acid stabilizers
Total
The addition of excess DCl and D2SO4 at 140 °C can also ef-
fectively prevent Pt(0) precipitation (Table 1, entries 7 and 8).
Even at a higher temperature (185 °C) and a longer reaction
time (24 h), the addition of DCl stabilizes the Pt catalyst in
aqueous media, but D2SO4 does not (Table 1, entries 9 and 10).
The stabilizing effect of DCl is independent of the addition of
carboxylic acid (Table 1, entries 11 and 12), but the Cl− concen-
tration has a significant effect. These results suggest that Cl−
strongly affects Pt stabilization, as a result of the equilibrium
reaction:
Multi-exchange
Mono-exchange
80
60
40
20
0
Pt(0) + [PtCl6]2− + 2 Cl− → [PtCl4]2−
(3)
0.0
2.0
4.0
6.0
8.0
10.0
This is the reverse of the Pt(0) precipitation reaction (reac-
tion (2)). More importantly, the proton concentration and solu-
tion acidity have less effect on Pt(0) stabilization, because sul-
furic acid did not prevent Pt(0) precipitation at 185 °C. This
suggests the possibility of replacing high concentrations of H+
with more eco-friendly cations, while maintaining a high Cl−
Cl concentration (mol/L)
Fig. 2. H/D exchange rates for high-temperature Shilov reaction at
185 °C with various Cl− concentrations. Pt(0) precipitation occurred at
lower Cl− concentrations (< 5.0 mol/L). Reaction slowed down if Cl−
concentration was too high.

Shujuan Kang et al. / Chinese Journal of Catalysis 36 (2015) 1777–1784
1781
3.2. Kinetic studies of sequential H/D exchanges
We developed a precursor kinetic scheme to optimize the
kinetic parameters and stoichiometric coefficients. For exam-
ple, the reaction kinetics for production of each of the
D-substituted isotopologues (CH3D, CH2D2, CHD3, and CD4)
from CH4, represented by equation (4), will be different. Four
precursors, p-CH3D, p-CH2D2, p-CHD3, and p-CD4 are intro-
duced:
The addition of highly concentrated Cl− and use of a high-
pressure reactor enabled high CH4 conversions to be achieved.
Under these conditions, multiply substituted methane isotopo-
logues such as CH3D, CH2D2, CHD3, and CD4 were observed,
indicating that multiple C–H bond activations occurred. The
high-temperature Shilov system therefore involves a complex
network of multiple and sequential reactions.
CH4 → a11p-CH3D + a12p-CH2D2 + a13p-CHD3 + a14p-CD4 (4a)
p-CH3D → CH3D
p-CH2D2 → CH2D2
p-CHD3 → CHD3
p-CD4 → CHD4
(4b)
(4c)
(4d)
(4e)
3.2.1. Experimental data
A series of isothermal experiments were conducted at 200
°C (25.5 MPa external pressure). The reaction system consisted
of K2PtCl4 (5 mmol/L) as the catalyst, and 30% CD3COOD and
DCl (9 mol/L in D2O solution) as the reaction medium. A num-
ber of gold-tube mini-reactors loaded with the same reactants
were extracted from the box furnace at reaction times of 4, 8,
20, 24, 32, and 100 h. When the gold tube was opened after the
reactions, no silver coating was observed, indicating that severe
catalyst deactivation had not occurred, because of the addition
of excess Cl−. Individual D-substituted methane compounds
were identified using GC-MS; the data are summarized in Table
3.
Reaction (4a) is considered to be fast (low activation), and
CH4 is quickly transformed to the four precursors, which follow
different reaction kinetics, (4b), (4c), (4d), and (4e), to generate
four isotopologues. Similarly, for reactions (5) and (6),
three-precursor and two-precursor schemes, respectively,
were used. There are therefore 13 reactions in the overall reac-
tion network, i.e., five for isotopic substitutions of CH4 (one for
precursor generation, four for reactions of individual compo-
nents), four for isotopic substitutions of CH3D (one for precur-
sor generation, three for reactions of individual components),
three for isotopic substitutions of CH2D2 (one for precursor
generation, two for reactions of individual components), and
one for isotopic substitution of CHD3 to form CD4. It is worth
noting that we assume that the kinetic processes have not
achieved final equilibrium, therefore all reactions are kinetical-
ly controlled, i.e., there are no recombination reactions in-
volved.
3.2.2. Reaction network
The kinetics of the high-temperature Shilov system, which
involves multiple substitution reactions, was investigated using
a reaction network involving a number of parallel reactions:
CH4 → a11CH3D + a12CH2D2 + a13CHD3 + a14CD4
CH3D → a21CH2D2 + a22CHD3 + a23CD4
CH2D2 → a31CHD3 + a32CD4
(4)
(5)
(6)
(7)
3.2.3. Optimization
CHD3 → CD4
Optimization and parameter fitting were performed itera-
tively. The concentrations of all methane isotopologues were
computed for a given parameter set (frequency factor, activa-
tion energies, and stoichiometric coefficients). An error func-
tion summing the square of errors between the calculated and
measured concentrations was minimized using the simplex
optimization method. In theory, the frequency factor Af de-
pends on the reaction conditions such as temperature and
pressure, and reflects the degree of molecular collisions. All the
reactions considered have similar mechanisms, therefore we
assume a constant frequency factory, Af = 109, for all reactions.
There are a total of nine stoichiometric coefficients and 10 ac-
tivation energies that need to be optimized (we assume that the
three precursor generation reactions are fast, and their activa-
tion energies are small, i.e., 20 kcal/mol). The commercially
available Kinetics05 and OptimRx1.35.0 software packages
were used for the optimization procedure.
If the reactions are assumed to be first order, the rate con-
stant k of each reaction can be determined from the Arrhenius
equation:
k = Afexp(−Ea/RT)
(8)
where Af is the frequency factor, Ea is the activation energy, T is
the absolute temperature, and R is the gas constant. Because
nearly 100% mass balance can be achieved with our experi-
mental setup, we consider the redistributions of five different
methane isotopologues to be kinetically controlled by reactions
(4) to (7). The stoichiometric coefficients aij can be determined
from the optimized reaction kinetics, based on the experi-
mental data listed in Table 3.
Table 3
Results of isothermal high-temperature Shilov reactions.
H/D exchange ratio (‰)
Time
(h)
Entry
CH3D
34.70
81.05
CH2D2
CHD3
0.00
29.10
CD4 CH4-yDy/CH4
3.2.4. Kinetics of high-temperature Shilov reactions
28
29
30
31
32
33
4
8
14.30
68.85
0.00
0.00
49.00
179.00
227.10
909.80
875.10
1000.00
The optimized kinetic parameters are listed in Table 4, and
the theoretical predictions made based on optimized kinetic
fitting with the experimental data are shown in Fig. 3.
First, the activation energy of CH4 → CH2D2 (29.9 kcal/mol)
is nearly equal to that of CH4 → CH3D (29.9 kcal/mol), but much
less than those of CH4 → CHD3 (39.7 kcal/mol) and CH4 → CD4
(40.0 kcal/mol), indicating that the primary generation reac-
20
24
32
100
197.70 164.50 144.10
86.90
149.30 193.40 302.30 264.80
180.10 183.80 263.10 248.10
0.00
0.00 130.70 869.30
Reaction system consisted of K2PtCl4 (5 mmol/L) + CD3COOD (30%) +
DCl (9 mol/L in D2O) + CH4, and reaction temperature was 200 °C.

1782
Shujuan Kang et al. / Chinese Journal of Catalysis 36 (2015) 1777–1784
coefficients of CH4→CHD3 and CD4 are much smaller (< 0.1%)
Table 4
than those of CH3D and CH2D2. CH3D and CH2D2 will therefore
both be initially generated from CH4. After more than 20 h, fur-
ther isotopic replacements occur such that CHD3 and CD4 are
generated from CH3D and CH2D2. After 100 h, all multi-substi-
tuted compounds also become reactive, and the overall process
is no longer kinetically controlled. The thermodynamic equilib-
rium effect becomes predominant; our results indicate that
CH3D and CD4 are more energetically stable than CH2D2 and
CHD3.
Reactions in methane H/D exchange experiments for kinetic fitting, and
preliminary fitting results.
Ea
Reaction
(kcal/mol)
CH4→a11CH3D-p1+a12CH2D2-p1+a13CHD3-p1+a14CD4-p1 E04 = 20 (4)
a11 = 0.561, a12 = 0.437, a13 = 0.00207, a14 = 0.0000142
CH3D-p1→CH3D
CH2D2-p1→CH2D2
CHD3-p1→CHD3
CD4-p1→CD4
E4a= 29.9 (4a)
E4b = 29.9 (4b)
E4c = 39.7 (4c)
E4d = 40.0 (4d)
E05 = 20 (5)
CH3D→a21CH2D2-p2+a22CHD3-p2+a23CD4-p2
a
21 = 0.396, a22 = 0.591, a23 = 0.013
4. Conclusions
CH2D2-p2→CH2D2
CHD3-p2→CHD3
CD4-p2→CD4
E5a = 29.8 (5a)
E5b = 29.5 (5b)
E5c = 31.1 (5c)
The original Pt-catalyzed Shilov system for CH4 activation
can be stabilized at high temperature (~200 °C) by the inclu-
sion of excess Cl− in the aqueous solution. The CH4 conversion
rate can be further improved by enhancing the gas solubility
using a high-pressure reactor. The mechanism of Pt stabiliza-
tion with concentrated Cl− is attributed to an equilibrium shift
for the reaction Pt(0) + PtCl62− + 2 Cl− ↔ 2 PtCl42−; this inhibits
Pt(0) precipitation. Selected chloride-based ILs also stabilize
the Pt catalyst at a lower reaction temperature (~140 °C). The
carboxylic group from CH3COOH or CF3COOH can chelate with
the Pt catalyst, but the stabilization effects are smaller than that
of Cl−.
CH2D2→a31CHD3-p3+a32CD4-p3
a31 = 0.682, a32 = 0.318
CHD3-p3→CHD3
CD4-p3→CD4
E06 = 20 (6)
E6a =29.9 (6a)
E6b = 28.9 (6b)
E07 = 30.4 (7)
CHD3→CD4
tions from CH4 to CH3D and CH2D2 occur simultaneously. Sec-
ondly, the activation energies of CH3D→CH2D2 (29.8 kcal/mol),
CH3D→CHD3 (29.5 kcal/mol), and CH3D→CD4 (31.1 kcal/mol),
CH2D4→CHD3 (29.9 kcal/mol) and CH2D2→CD4 (28.9 kcal/mol),
and CHD3→CD4 (30.4 kcal/mol) are all very close to or less than
those of the first generations of CH2D2 and CHD3. This suggests
that because all four C–H bonds are equivalent, the activation
conditions for the second, third, and fourth C–H bonds are the
same as those for the first C–H bond. Thirdly, the stoichiometric
CH4 is a highly symmetric (tetrahedral symmetry, Td) mole-
cule in which all four C–H bonds are equivalent. A catalytic sys-
tem with reaction conditions under which a C–H bond can be
activated will probably also activate the other three C–H bonds,
100
100
CH3D
CH2D2
CH3D-Exp
CH2D2-Exp
CH2D2-Theo
80
80
60
CH3D-Theo
60
40
20
0
40
20
0
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (h)
Time (h)
100
80
60
40
20
0
100
80
60
40
20
0
CHD3
CD4
CHD3-Exp
CHD3-Theo
CD4-Exp
CD4-Theo
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (h)
Time (h)
Fig. 3. Kinetic fitting results based on reaction network, and experimental data.

Shujuan Kang et al. / Chinese Journal of Catalysis 36 (2015) 1777–1784
1783
Graphical Abstract
Chin. J. Catal., 2015, 36: 1777–1784 doi: 10.1016/S1872-2067(15)60966-4
High-temperature shilov-type methane conversion reaction: Mechanistic
and kinetic studies
Precursor
CH2D2
CH3D
29.9 kcal/mol
Precursor
Shujuan Kang, Qisheng Ma, Weiqun Chen, Guanyi Chen *, Yongchun Tang*
Tianjin University, China; ChemEOR Inc, USA; SHEETA Global Technology Corpora-
tion, USA; Power Environmental Energy Research Institute (PEER), China
29.9 kcal/mol
Precursor
CHD3
CH4
CD4
Kinetic studies indicate that multiple substitutions in methane conversion are
inevitable. Mono- and di-substituted methane isotopologue generation reactions
have similar activation energies. The highest mono-substitution selectivity cannot
exceed 50%.
and such sequential reactions often lead to poor product selec-
tivity. We used a high-pressure reactor and excess Cl− to main-
tain the catalytic activity at higher reaction temperatures and
achieved nearly 100% CH4 conversion in a Shilov-type reaction.
We were therefore able to study the detailed mechanism and
kinetics of the reaction, including C–H over-activation reac-
tions. A reaction network consisting of a series of parallel
chemical reactions was established, and the kinetic parameters
(activation energies and stoichiometric coefficients) were op-
timized by fitting the experimental data.
is possible. Even if higher substitution reactions can be pre-
vented, the best achievable CH3Cl selectivity is probably 50%.
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The following conclusions were drawn.
1. The H atoms in CH4 are substituted with D atoms to give
CH3D or CH2D2 via a first-order substitution reaction. The the-
oretical fitting results gave similar activation energies (~29.9
kcal/mol), reflecting the experimental observation of early
generation of both CH3D and CH2D2.
2. Direct tri- or tetra-substitutions of CH4 to form CHD3 and
CD4 require much higher energies (39.7 kcal/mol and 40.0
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Page numbers refer to the contents in the print version, which include
both the English version and extended Chinese abstract of the paper. The
online version only has the English version. The pages with the extended
Chinese abstract are only available in the print version.
6. It is difficult to achieve high selectivity for singly substi-
tuted methane (i.e., CH3Cl), not only because of possible higher
substitution reactions, but also because the formation of CH2Cl2