Polymer-Controlled Crystallization of Molybdenum Oxides from Peroxomolybdates: Structural Diversity and Application to Catalytic Epoxidation

Abstract

The influence of polyoxyethylene-containing polymers on the crystal structure and habit of molybdenum-oxide-based products crystallized from peroxomolybdate solutions was investigated. Polyoxyethylene homopolymers of various molar masses were compared with a polyoxyethylene alkyl ether and a triblock copolymer of polyoxyethylene and polypropylene. Conventional hydrothermal synthesis at temperatures between 70 and 180 °C was compared with an ultrasonic pathway at 70 °C. The structure of the products was investigated by small- and wide-angle X-ray scattering. Different crystal phases were obtained depending on the polymer concentration and the preparation methods. At 70 °C, a compound with tentative formula MoO 3− x (O 2 ) x · n H 2 O ( n ≈ 1), showing X-ray diffraction patterns matching those of triclinic monohydrate molybdenum trioxide, was the product found in the absence of any polymer. However, small concentrations of any polyoxyethylene-containing polymer led to a monoclinic hemihydrate phase under the same conditions and temperature. At temperatures above 90 °C, the patterns of the resulting products could be indexed according to orthorhombic anhydrous MoO 3, although the blue color of certain samples indicated an oxygen deficiency. At high polymer concentrations and temperatures under 90 °C, the material crystallized in an unusual primitive cubic structure, independent of the exact type of polyoxyethylene polymer used, with a very large cubic lattice constant of ∼5 nm. However, the molar mass and the structure of the polymer do influence the lattice constants of the final crystal leading to a slight decrease with increasing molar mass. At high polymer concentrations and 180 °C, the product was identified as MoO 2 . The polymer acts not only as a structure-directing agent but also as a mild reducing agent, as judged from the nontrivial redox behavior of the molybdenum ions when the crystallization occurs in the presence of polymer. The excellent catalytic properties of representative types of the synthesized materials were demonstrated for the epoxidation of cyclohexene and cyclooctene by tert -butyl hydroperoxide.