LaCl₃-based catalysts for oxidative chlorination of CH₄.

Elvira Peringer², Simon G. Podkolzin¹, Mark E. Jones¹, Roberta Olindo² and Johannes A. Lercher²

Topics in Catalysis 38(1-3), 211-220, 2006

Publisher: Springer Netherlands, ISSN: 1022-5528 (Print) 1572-9028 (Online)

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Abstract

LaCl₃ is an active, selective and stable catalyst for oxidative chlorination of methane to methyl chloride.

CH₄ + HCl + 0.5 O₂ → CH₃Cl + H₂O

Selective conversion to methyl chloride can be achieved by limiting methane conversion, for example, by using an excess of methane in the feed. Methylene chloride and carbon monoxide are the main side products at higher methane conversion levels. Transient OCl⁻ anion, formed by oxidation of Cl⁻ in LaOCl and LaCl₃ with molecular oxygen, is proposed to be the active site for the initial step of methane activation. CO_x formation is proposed to proceed through the formation of adsorbed multiply substituted chloromethanes.

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

Scanning Electron Microscope images of (a) Lanthanum oxychloride LaOCl initial catalyst and (b) chlorinated catalyst form, which is essentially lanthanum trichloride LaCl₃. Microscopy data obtained at the Technische Universität München (TUM).

It is more convenient for handling and loading to prepare the catalyst as lanthanum oxychloride LaOCl because trichloride is very hygroscopic. Typical lanthanum oxychloride LaOCl samples prepared by precipitation from an aqueous solution have the surface area of about 40 m²/g, but samples from organic solvents can have surface areas of up to 120 m²/g. On chlorination, however, the morphology of LaOCl changes significantly, and the surfaces area collapses to about 4 m²/g.

Reaction selectivity as a function of methane conversion at 748 K with CH₄:HCl:O₂:He:N₂ = 2:2:1:4:1 and total flow of 5-25 sccm. Data collected at TUM.

At low methane conversion (below 5 mol %), the reaction is practically 100% selective to methyl chloride CH₃Cl. The product distribution at higher methane conversion levels suggests that overchlorination to methylene chloride CH₂Cl₂ and chloroform CHCl₃ leads to formation of carbon oxides. Interestingly, the formation of carbon tetrachloride CCl₄ is not observed even at methane conversion levels of up to 40 mol %, presumably due to the low stability and ease of decomposition of CCl₄ to CO₂.

Effect of feed composition on the reaction rate at 4.8 kPa CH₄, 2.4 kPa O₂, 24 kPa HCl with balance He with a total flow rate of 14 sccm at 675 K.

(1) Reaction rate with the full feed: CH₄+O₂+HCl.

(2) Reaction stops abruptly without gas-phase O₂. Gas-phase oxygen is required for methane activation.

(3) Reaction rate recovers rapidly to its original value after oxygen reintroduction.

(4) Without HCl in the feed, the reaction rate decreases but can be sustained at a lower level for a significant period of time. During this time, chlorine for the reaction is supplied by the catalyst. The chlorine is supplied not only by the catalyst surface but also by the bulk through bulk chlorine-oxygen diffusion, and the catalyst gradually changes from LaCl₃ to LaOCl.

(5) Reaction rates drops practically to zero without gas-phase O₂.

(6) Reaction rate recovers rapidly to its original value once the full feed is restored. The catalyst remains stable after multiple hours on stream.

These experiments suggest that methane activation proceeds on the surface of the catalyst by chlorine species that are reactive only in the presence of gas-phase oxygen. DFT calculations suggest that molecular oxygen can dissociate on the surface of LaOCl and LaCl₃ and form transient OCl^- species where chlorine changes its formal oxidation state from -1 to +1. These transient OCl^- species can activate methane with the formation of CH₃Cl and surface OH.

    - Additional experiments performed at the Technische Universität München are described in the Ph.D. Thesis of Elvira Peringer "Oxidative chlorination of methane over LaCl₃-based catalysts".

    - Advantages of reaction conditions at high methane feed ratios are described in the patent application WO 2006 118935 by Simon Podkolzin et al. "Oxidative halogenation of C₁ hydrocarbons to halogenated C₁ hydrocarbons".

    - Additional reaction mechanism studies are described in the 2007 paper by Simon Podkolzin et al. in the Journal of the American Chemical Society.

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