Synthesis and characterization of molybdenum(VI) oxide sulfates and crystal structures of two polymorphs of MoO2(SO4)

Abstract

Reactions between α-MoO3 or MoO3·0.43H2O and 65–95 wt% H2SO4 were studied from room temperature up to the boiling point of the acid. Four new compounds, MoO2(SO4), (modifications I, II and III) and MoO2(SO4)· H2SO4·H2O, have been isolated depending on the H2SO4 concentration, reaction temperature, reaction time and molybdenum source. The characterization of these phases was performed with powder X-ray diffraction, thermogravimetric analysis, differential thermal analysis, chemical analysis and density measurements. Both starting materials react with sulfuric acid and form MoO2(SO4) II at temperatures above ca. 75 °C. This compound is unstable in the reaction mixture and converts into MoO2(SO4) I or III depending on time and temperature. The three modifications of MoO2(SO4) are hygroscopic and decomposition in moist air is studied for the I and II modifications. On heating, all modifications of MoO2(SO4) decompose into α-MoO3 or a mixture of α-MoO3 and β-MoO3 depending on heating rate and sample size. The three-dimensional, open-framework structures of I and II, have been determined from single-crystal X-ray diffraction data. I crystallizes in the monoclinic space group C2/c and II in the orthorhombic space group Pna21. Both structures are made up of MoO6 octahedra and SO4 tetrahedra and contain layers of eight- and four-membered rings of alternating, corner-sharing octahedra and tetrahedra. These layers are linked (also via Mo–S bridges) to form a three-dimensional framework. The MoO6 octahedra are rather distorted, as demonstrated by large variations in the bonding Mo–O interatomic distances, which reflect the double-bond character of the binding between molybdenum and terminal oxygen atoms pointing into the eight-membered rings. The SO4 tetrahedra are quite regular. The structural relation to MoO2(SO4) III is briefly considered.