Simon Podkolzin - Exchange H-Cl over Lanthanum

Catalytic Hydrogen-Chlorine Exchange between Chlorinated Hydrocarbons under Oxygen-Free Conditions.

Alwies W. A. M. van der Heijden¹, Simon G. Podkolzin ², Mark E. Jones ², Johannes H. Bitter¹, Bert M. Weckhuysen¹

Angewandte Chemie International Edition 47(27), 5002-5004, 2008

Publisher: Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim; CODEN: ACIEF5, ISSN: 1433-7851

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Abstract

This work presents a new type of catalytic systems for the metathesis between CCl₄ and CH₂Cl₂ to form CHCl₃:

CH₂Cl₂ + CCl₄ → 2 CHCl₃

Chlorinated lanthanum materials, specifically LaCl₃ supported on carbon nanofibres, were found to be highly active, selective and stable catalysts for this reaction. This is the first report that a catalyst based on an irreducible metal, such as lanthanum, can selectively activate both C-H and C-Cl bonds in the absence of either lattice or gas-phase oxygen. Yield loss to carbon oxides is prevented since oxygen is not present. In addition, the absence of unwanted C₂+ hydrocarbon products indicates that the selective catalytic chemistry dominates over any gas-phase radical reactions. DFT calculations suggest that the reaction proceeds through the formation of weakly adsorbed Cl and H species, which can be exchanged with the reactants.

Address:

[1] Inorganic Chemistry and Catalysis Group, Department of Chemistry, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands

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

Lanthanum trichloride LaCl₃ was found to be an active, selective and stable catalyst for chlorine-hydrogen exchange reactions in the absence of any gas-phase or surface lattice oxygen.

CH₂Cl₂ + CCl₄ → 2 CHCl₃

Bulk LaCl₃ structures obtained through different preparation methods are active with noticeable activity above 400°C, as shown in the Figure. Supported LaCl₃, particularly samples supported on carbon nanofibers, exhibit significant catalytic activity already above 300°C at the tested space velocities and partial pressures of the feed components: GHSV = 400 h^-1 and inlet concentrations CCl₄ = CH₂Cl₂ = 4.7 vol .

Possible steps in the reaction mechanism were evaluated with DFT calculations. The calculations suggest that neither CCl₄ nor CH₂Cl₂ is likely to adsorb molecularly on a fully chlorinated ideal surface of LaCl₃. However, it is energetically favorable for chloromethanes to split off a Cl atom and donate it to the surface.

Activity of LaCl₃ as a function of temperature. Data collected by Alwies van der Heijden at the Utrecht University.

Although splitting off and donation of an H atom to the surface is calculated to be endothermic, the process can be partially compensated by exothermic chlorination of the resulting hydrocarbon fragment. It is, therefore, reasonable to assume that the reaction proceeds in two separate H-Cl exchange steps. First, CH₂Cl₂ exchanges an H for a surface Cl (Reaction 2) and, second, CCl₄exchanges a Cl for a surface H (Reaction 3):



Reaction 2: CH₂Cl₂ (g) + Cl–LaCl₂ (s) → CHCl₃ (g) + H–LaCl₂ (s)


Reaction 3: CCl₄ (g) + H–LaCl₂ (s) → CHCl₃ (g) + Cl–LaCl₂ (s)

Calculated energies for Reaction 2 where H is exchanged for a surface lattice Cl are high at 310-324 kJ/mol, regardless of the presence or position of any neighboring defects (Cl vacancy or F-center Cl vacancy). This result suggests that gas-phase chlorinated hydrocarbons, such as CH₂Cl₂, are unlikely to exchange their H for a Cl anion from the LaCl₃ lattice. However, the calculations also suggest that the surface of LaCl₃ can have not only terminal lattice Cl, but also weakly held adsorbed Cl species (Figure 1a). These species are calculated to have a significantly smaller Hirshfeld atomic partial charge of -0.09 compared to -0.22 for surface lattice Cl anions, and they can be viewed as a build-up of an additional Cl layer for the bulk structure, so that the coordination of surface La atoms increases from 8 to 9, the same as for the bulk La atoms.

When gas-phase CH₂Cl₂ exchanges one of its H atoms for an adsorbed surface Cl atom, the products are gas-phase CHCl₃ and a surface hydride (Figure 1b). Similarly to the adsorbed surface Cl, surface H is weakly bound, and it has only a small atomic charge of -0.05. The calculated energy change for Reaction 2 is 210 kJ/mol at the adsorbed Cl coverage of 0.25 monolayer (ML). Gas-phase CCl₄ (Figure 2a) can react with the surface hydride, regenerating the adsorbed Cl species and forming gas-phase CHCl₃, as shown in Figure 2b. This Reaction 3 is predicted to be exothermic at -225 kJ/mol.

When the calculations for Reactions 2 and 3 were repeated at a higher adsorbed Cl coverage of 0.5 ML, their energetic favorability reversed. Instead of being endothermic, Reaction 2 became energetically favorable at -170 kJ/mol; and Reaction 3 became endothermic at 155 kJ/mol. This dependence of the reaction energetics on adsorbed chlorine coverage implies that as the concentration of adsorbed Cl builds up during the reaction, a substitution of these Cl species by H becomes more favorable, and, conversely, the reverse exchange becomes less favorable. The surface, therefore, is likely to maintain some equilibrium concentration of adsorbed Cl and H species: when the concentration of surface Cl exceeds the equilibrium value, the reaction of Cl to H exchange as in Figure 1a-b will be favored, and, conversely, when the concentration of surface H is relatively high, the reverse H for Cl exchange as in Figure 2a-b will be preferentially taking place.

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