Synthesis and characterization of molybdenm(VI) oxide sulfates and crystal structures of two polymorphs of MoO_2(SO_4)

摘要

Reactions between #alpha#-MoO_3 or MoO_3.0.43H_2O and 65-95 wt% H_2SO_4 were studied from room temperature up to the boiling point of the acid. Four new compounds, MoO_2(SO_4),(modifications I, II and III) and MoO_2(SO_4. H_2SO_4.H_2O, have been isolated depending on the H_2SO_4 concentration, reaction temperature, reaction time and molybdenum source. The characterization of these phases was performed with powder X-ray diffraction. thermogravimetric analysis,differeritial thermal analysis, chemical analysis and density measurements. Both starting materials reaciwith sulfuric acid and form MoO_2(SO_4) II at temperatures above ca. 75 °C. This compound is unstable in the reaction mixture and converts into MoO_2(SO_4) I or III depending on time and temperature. The three modifications of MoO_2(SO_4) are hygroscopic and decomposition in moist air is studied for the I and II modifications. On heating, all modifications of MoO_2(SO_4) decompose into #alpha#-MoO_3 or a mixture of #alpha#-MoO_3 and #beta#-MoO_3 depending on heating rate and sample size. The three-dimensional. open-framework structures of I and II. have been determined fromsingle-crystal X-ray diffraction data. Icrystallizes in the monoclinic space group C2/c and II in the orthorhombic space group Pna2_1. Both structures are made up of MoO_6 octahedra arid SO_4 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 MoO_6 octahedra are rather distorted, as demonstrated by large yariations in the bonding Mo-O interatomic distances, which reflect the doublebond character of the binding between molybdenum and termina1 oxygen atoms pointing into the eight-membered rings. The SO_4 tetrahedra are quite regular. The structural relation to MoO_2(SO_4) III is briefly considered.