Kinetic and mechanistic study of the reaction of CCl4 with prefluorided chromia to form CCl3F and CCl2F2

The reaction of CCl4 with prefluorided chromia (prefluorided to multilayer depth) has been studied using the transient techniques of temperature programmed reaction (TPR), temperature programmed desorption (TPD) and gas adsorption chromatography (GAG). The reaction has been shown to proceed by a Langmuir-Hinshelwood mechanism in which the chemisorbed CCl4 molecule exchanges its chlorine atoms with the fluoride ions on the surface of fluorided chromia. Mono and di-exchange, forming CCl3F and CCl2F2, has been shown to occur simultaneously with an activation energies of similar to 63 kJ mol(-1) for both. The facile reaction of the chemisorbed CCl4 with the surface F- ion to form CCl3F is rationalised in a four centre exchange mechanism in which the CCl4 is bonded to the fluorided chromia surface through a chlorine atom of the CCl4 molecule bonding to a surface Cr3+ ion. The C atom of CCl4 molecule bonds to a surface F- species. Di-exchange which occurs simultaneously with mono exchange, the two reactions having the same activation energy, is accounted for by the CCl4 being bonded through two Cl atoms to two surface Cr3+ ions, with the carbon being loosely associated with two surface F- ions. This configuration which has been shown to be geometrically possible on the (001) surface of CrF3 allows for simultaneous double exchange of the halogens. (C) 1998 Academic Press.