Catalysis in halogen exchange reactions

A halogen exchange process which comprises heating a mixture formed from ingredients comprising (i) at least one finely-divided essentially anhydrous alkali metal fluoride, (ii) at least one haloaromatic compound having at least one halogen atom of atomic number greater than 9 on an aromatic ring, and (iii) an aminophosphonium catalyst, at one or more reaction temperatures at which at least one said halogen atom of said haloaromatic compound is replaced by a fluorine atom, wherein said catalyst ingredient is a tetra(dihydrocarbylamino)phosphonium halide which has been subjected to a pretreatment process which comprises:

• a) contacting an impure tetra(dihydrocarbylamino)phosphonium halide with a liquid cyclic ether under conditions effective to dissolve the tetra(dihydrocarbylamino)phosphonium halide and leave at least a portion of said impurities remaining in the solid state;
• b) separating the solids and the liquid phase of a) from each other;
• c) mixing an anhydrous non-solvent for the tetra(dihydrocarbylamino)phosphonium halide with the liquid phase from b) to precipitate the tetra(dihydrocarbylamino)phosphonium halide;
• and
• d) separating the precipitated tetra(dihydrocarbylamino)phosphonium halide solids and the liquid phase of c) from each other.