Effect of Introducing Thallium in Microporous Vanado-molybdate with Orthorhombic or Trigonal Structures and Catalytic Properties

Thallium-containing mixed vanadium and molybdenum oxides have been synthesized using a microwave-assisted hydrothermal method. The pH, the nature of the metal precursors, and the use of a structuring agent were shown to be key parameters of the synthesis; depending upon these parameters, trigonal (Tri) or orthorhombic (M1) structures were obtained. Both structures have been characterized by X-ray diffraction (XRD) and scanning and transmission electron microscopies (SEM and TEM, respectively). The orthorhombic phase has lattice parameters of a = 2.108 nm, b = 2.656 nm, c = 0.3997 nm and space group Pba2, and the trigonal phase, lattice parameters of a = 2.143 nm, c = 0.4004 nm and space group P3. In the orthorhombic phase, both hexagonal and heptagonal channels are partially but consistently occupied by thallium cations. The trigonal phase also exhibited both hexagonal and heptagonal channels; hexagonal channels were occupied by thallium cations, whereas the heptagonal channels appeared almost empty. The catalytic properties of the solids have been studied for the oxidative dehydrogenation of ethane and the oxidation of propene to acrylic acid. The orthorhombic phase was inactive; the trigonal phase, while not very active, had a high selectivity to ethylene. Although the occupancy rate of the heptagonal sites is probably low in the later phase, it strongly attenuates its activity.