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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)