The synthesis, structure, and reactions of metastable (vanadium,molybdenum) oxides with the hexagonal molybdenum trioxide structure

Although many layered compounds undergo proton exchange reaction in acid, layered brannerites A(VMo)O$\sb6$ (A = Li, Na) are two exceptions. Previous studies showed that in dilute HCl, the Li and Na brannerites participate in an unusual acid-leaching reaction and form new compounds $\rm A\sp\prime\sb{0.13}(V\sb{0.13}Mo\sb{0.87})$O${\rm\sb3{\cdot} nH\sb2O}$ $\rm (A\sp\prime=H,Na;\ n\approx 0.26)$ with the hexagonal MoO$\sb3$ structure. By dehydrating $\rm H\sb{0.13}(V\sb{0.13}Mo\sb{0.87})$O${\rm\sb3{\cdot}nH\sb2O,}$ a new microporous form of this structure, $\rm (V\sb{0.13}Mo\sb{0.87})O\sb{2.935}$ which promises potential applications as catalyst and insertion host, could be obtained. However, the mechanism of the reaction and the properties of these new compounds were not investigated. In this thesis, we explored these areas using various techniques such as TGA, DSC, SEM/EDAX, AA, IR, XRD, neutron diffraction and Rietveld refinement method, etc. Our studies indicate that the leaching of Li brannerite proceeds via a conventional dissolution-precipitation mechanism, while for the Na isomorph the crystallography of the brannerite solute affects the formation and the chemistry of the hexagonal product. The vanadium-stabilized hexagonal MoO$\sb3$ compounds can also be directly precipitated from acidified aqueous A-V-Mo (A = Li, Na) solutions. Hexagonal precipitates from Li-V-Mo solutions, $\rm (Li\sb{x}H\sb{0.13-x})(V\sb{0.13}Mo\sb{0.87})$O${\rm\sb3{\cdot} nH\sb2O}$ $(0\le {\rm x}\le 0.13),$ undergo ion-exchange reactions in chloride solutions permitting the formation of a series of hexagonal isomorphs $\rm A\sp{m+}\sb{0.13/m}(V\sb{0.13}Mo\sb{0.87})$O${\rm\sb3{\cdot} nH\sb2O}$ with A = H, Li, Na, K, NH$\sb4$ and Ca. The protonic form of hexagonal MoO$\sb3$ obtained through the solution precipitation route is identical to the one prepared from Li brannerite. However, hexagonal precipitates from Na-V-Mo solutions, $\rm Na\sb{0.167}V\sb{x}Mo\sb{1-x-y}$(osbM) sbyH sbx + 6y-0.167$O$sb3 cdotnH sb2O$ $(0 le x le 0.14, 0.01 le y le 0.87, n approx 0.33),$ are quite different from those produced by the leaching of Na brannerite in both composition and thermal stability. The microporous form of hexagonal $MoO sb3, (V sb0.13Mo sb0.87)O sb2.935$ which is stable up to 455$ sp circ$C, readily undergo ion insertion reactions with Li and H forming $A sbx(V sb0.13Mo sb0.87)O sb2.935$ with $0 le x le 1.67$ for Li and $0 le x le 1.13$ for H. Although the framework structures of the $\rm A\sp{m+}\sb{0.13/m}(V\sb{0.13}Mo\sb{0.87})$O$\rm\sb3{\cdot} nH\sb2O$ are all the same, the location and the coordination geometry of the A cations inside the channels is strongly correlated to the ionic size of A and for small cations such as H, Li, Na and Ca, the presence of water in the channels is critical in the stabilization of the structure