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
Publisher: Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim; CODEN: ACIEF5, ISSN: 1433-7851
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.
 Inorganic Chemistry and Catalysis Group, Department of Chemistry, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands
 The Dow Chemical Company, Core Research and Development, Midland, Michigan 48674, USA
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, CCl4 exchanges a Cl for a surface H (Reaction 3):
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.