Methyl Chloride Production from Methane over Lanthanum-Based Catalysts.
Simon G. Podkolzin 1, Eric E.
Stangland 1, Mark E. Jones 1,
Elvira Peringer 2, and Johannes A.
Publisher: American Chemical Society, CODEN: JACSAT ISSN: 0002-7863
The mechanism of selective production of methyl chloride
by a reaction of methane, hydrogen chloride and oxygen over lanthanum-based
catalysts was studied.
CH4 + HCl + 0.5 O2
The results suggest that methane activation proceeds through
oxidation-reduction reactions on the surface of catalysts with an irreducible
metal – lanthanum, which is significantly different from known mechanisms for
Activity and spectroscopic measurements show that lanthanum oxychloride (LaOCl),
lanthanum trichloride (LaCl3) and lanthanum phases with an
intermediate extent of chlorination are all active for this reaction. The
catalyst is stable with no noticeable deactivation after 3 weeks of testing.
Kinetic measurements suggest that methane activation proceeds on the surface
of the catalyst. Flow and pulse experiments indicate that the presence of
hydrogen chloride is not required for activity, and its role appears to be
limited to maintaining the extent of catalyst chlorination. In contrast, the
presence of gas-phase oxygen is essential for catalytic activity.
Density-functional theory calculations suggest that oxygen can activate
surface chlorine species by adsorbing dissociatively and forming OCl surface
species, which can serve as an active site for methane activation. The
proposed mechanism, thus, involves changing of the formal oxidation state of
surface chlorine from -1 to +1 without any changes in the oxidation state of
the underlying metal.
 The Dow Chemical Company, Core Research and
Development, Midland, Michigan 48674, USA
 Department of Chemistry, Technische
Lichtenbergstrasse 4, D-85747 Garching, Germany
Selective methane activation is challenging because methane derivatives are
more reactive than methane itself.
For example, in the case of methane chlorination, chloromethanes are more
reactive than methane.
In a non-catalytic (radical) gas-phase reaction, reactivity increases
with the partial charge on carbon:
M1δ+ < M2δ+
As a result, selectivity to methyl chloride in methane chlorination
falls precipitously with increasing methane conversion.
For traditional catalysts, the selectivity trend in
methane activation is the same as for the radical gas-phase chemistry,
because surface chlorine has a negative charge. It is progressively easier
for negatively charged chlorine to react with with positively charged
carbon of chloromethanes.
Traditional chlorination catalysts work through a
reduction-oxidation cycle of a transition metal supported on an metal
oxide support. Such systems usually have 3 components:
Reducible active metal: for example, Cu, Co, Fe
Alkali promoter: for example, Li, Na, K, Cs
Stabilizer: for example, LaCl3
A stabilizer is required due to volatility of transition
metal chlorides. And even with a stabilizer, traditional catalysts are
unstable at the temperatures needed for methane activation. In addition,
such catalysts promote the conversion of HCl to Cl2 (Deacon
reaction); and then Cl2 reacts with methane in the gas-phase,
bringing the overall reaction selectivity closer to that observed in
gas-phase non-catalytic chemistry.
Our work shows that catalysts based only on lanthanum,
are stable and selective for methane hydroclorination:
+ HCl + 0.5 O2 → CHCl3 + H2O
Lanthanum is not a reducible metal - it does not change its oxidation
state - at usual reaction conditions. Due to its irreducibility, lanthanum
has been previously thought to be an inert catalyst component used as a
stabilized for volatile transition metal chlorides . The mechanism of
methane activation over lanthanum catalysts, therefore, should be
significantly different from the redox cycle for transition metals, such
as copper, shown above.
Reaction pulses with the full feed and with a mixture of CH4
and O2 (without HCl) monitored by a mass spectrometer.
Comparison of calculated and experimental LaOCl Raman
spectra. Evolution of in-situ Raman spectra with the extent of catalyst
|Results of reaction pulses and
in-situ Raman spectra suggest that:
- All lanthanum surfaces with Cl are active for CH4
activation to CH3Cl;
- Gas-phase O2 is required for CH4 activation, a gas-phase
source of Cl is not;
- Methane can be activated with practically 100% selectivity;
- If the surface is not regenerated with a source of chlorine, the
catalyst bulk transforms from LaOCl to LaCl3.
|Reaction kinetic measurements as a
function of oxygen partial pressure without methane in the feed show that
elemental chlorine is produced at higher oxygen partial pressures. The
rate of chlorine evolution, however, is significantly lower than the rate
of methyl chloride production when methane is present in the feed, and in
addition, these two rates are not correlated. Therefore, it can be
concluded that at the usual reaction conditions methane activation is
mainly a surface reaction, which is not influenced by the formation of
gas-phase chlorine radicals or reactions with gas-phase chlorine.
Proposed reaction mechanism for oxidative chlorination
of methane based on DFT calculations. Numbers next to element symbols show
partial atom charges calculated with the Hirshfeld method.
|Possible reaction mechanism based
on catalyst characterization and kinetic measurements was evaluated with
DFT calculations. Calculations suggest that:
- Gas-phase O2 can adsorb dissociatively, react with
surface Cl and form OCl surface species.
- In OCl surface species, Cl has a partial positive charge
δ+ (the formal oxidation state changes
from -1 to +1).
- Methane reacts with OCl to form CH3Cl and surface OH.
The proposed new mechanism for methane activation, thus, involves a
chlorine redox cycle - changing of the formal oxidation state of surface
chlorine from -1 to +1 without any changes in the oxidation state of the
Reaction mechanism studies suggest that it is preferable to operate methane
oxidative chlorination at high methane feed reactions in order to maintain
high selectivity to methyl chloride and potentially achieve complete HCl
conversion. Advantages of such operating conditions are described in the patent application
WO 2006 118935 by Simon Podkolzin et al.
"Oxidative halogenation of C1 hydrocarbons to halogenated C1
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