Ion exchange reactions of potassium brannerite, K 0.8(V 0.8Mo 1.2)O 6

Abstract

The reactions of the layered brannerite systems in acidic media are highly dependent upon the chemistry of the alkali ion that occupies the inter-layer sites. The sodium and lithium isomorphs, (Na, Li) [VMo]O 6, participate in an unusual leaching reation and form Mo-rich phases with the hexagonal MoO 3 structure; potassium brannerite, K 0.8[V 0.8M 1.2]O 6, undergoes a partial ion-exchange reaction. In weak acids half the inter-layer potassium ions are readily exchanged by protons and form K 0.4H 0.4(·2H 2O)[V 0.8Mo 1.2]O 6. During the subsequent thermolysis, the elimination of oxygen from the structure is accommodated by a reduction in the coordination of the (Mo,V) cations within the brannerite layers. After complete dehydration, a reduction stabilizes a new reduced brannerite phase with a composition close to K 0.5[VMo]O 5.5. The remaining potassium ions can be replaced by ion exchanging in NH 4Cl/HCl solutions. During heating, the product of the ammonium exchanges, {;NH 4) 0.4H 0.4(·2H 2O)}[V 0.8Mo 1.2]O 6·0.4H 2O, undergoes a series of dehydration, deammoniation and reduction reactions. The final product of the thermolysis, V 0.8Mo 1.2O 5.2, has a structure closely related to the β-( V 1− x Mo x ) 3 O 8 phases.