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.