Simon Podkolzin - Methane Activation over Lanthanum

Methyl Chloride Production from Methane over Lanthanum-Based Catalysts.

Simon G. Podkolzin¹, Eric E. Stangland¹, Mark E. Jones ¹, Elvira Peringer ², and Johannes A. Lercher ²

Journal of the American Chemical Society 129(9), 2569-2576, 2007

Publisher: American Chemical Society, CODEN: JACSAT ISSN: 0002-7863

Link to full text on publisher's server

Abstract

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 → CH₃Cl + H₂O

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 oxidative chlorination.

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.

Address:

[1] The Dow Chemical Company, Core Research and Development, Midland, Michigan 48674, USA

[2] Department of Chemistry, Technische Universität München, 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.

CH₄ CH₃Cl CH₂Cl₂ CHCl₃ In a non-catalytic (radical) gas-phase reaction, reactivity increases with the partial charge on carbon: M1^δ+ < M2^δ+ < M3^δ+ 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: CH₄ + 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 chlorination.
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 underlying metal.

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 hydrocarbons".

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