Formation and ion-exchange chemistry of new oxides with the hexagonal MoO 3 structure

Abstract

In dilute acid solution, layered oxides AVMoO 6 (A=Li, Na) with the “brannerite” structure do not undergo the expected ion-exchange reaction to produce HVMoO 6, and instead participate in an unusual “leaching reaction” that leads to the formation of new compounds with the stoichiometry A 0.13V 0.13Mo 0.87O 3 (A=H·2H 2O, Na·2H 2O). These well crystallized products have the “hexagonal MoO 3” structure, which contains one-dimensional tunnels with free diameters approaching 3.5 Å. Complete dehydration of the product of the Li brannerite reaction, (H·2H 2O) 0.13V 0.13Mo 0.87O 3 produces V 0.13Mo 0.87O 2.935, which retains the “hexagonal MoO 3” structure up to 455°C and has empty tunnels. This microporous phase readily undergoes a variety of ion-insertion reactions. Hexagonal (Li x H 1− x ) 0.13V 0.13mO 0.87O 3· nH 2O has also been prepared by direct precipitation from lithium molybdovanadate solutions. (H·2H 2O) 0.13V 0.13Mo 0.87O 3 and (Li·2H 2O) 0.13V 0.13mo 0.87O 3 have been obtained from these precipitates by ion-exchange. Upon heating, Li 0.13V 0.13Mo 0.87O 3 undergoes an exothermic transformation in which the dehydrated Li ions “hop” from the tunnel sites to trigonal prismatic sites located in the framework.