Spherical V-MCM-48: the synthesis, characterization and catalytic performance in styrene oxidation

Abstract

A series of spherical, vanadium-containing, mesoporous V-MCM-48 catalysts with different V/Si atomic ratios were synthesized by a direct hydrothermal method. The structure and morphology of the samples and the states of vanadium in the materials were systematically characterized by XRD, N₂ physisorption, FE-SEM, HRTEM, ICP, UV-vis, ⁵¹V MAS-NMR, ESR and XPS. The possible formation mechanism of spherical V-MCM-48 was proposed based on the Stöber process. Simultaneously, the catalytic activities of the samples were evaluated in the oxidation of styrene to benzaldehyde using H₂O₂ as oxidant. The results show that the V-MCM-48 samples have regular spherical morphology, homogeneous dispersion and highly ordered cubic mesostructure. Characterization results about the coordination states of vanadium species demonstrated that most of vanadium existed as tetracoordinated V⁴⁺ species in a silicate framework and a small amount of vanadium as isolated V⁵⁺ species on the surface. The prepared spherical V-MCM-48 exhibits much higher catalytic activity in the catalytic conversion of styrene to benzaldehyde. Spherical morphology contributes significantly to the improved catalytic performance of materials. V⁵⁺ species on the surface and/or produced from the oxidation of some V⁴⁺ in the framework act as active sites. Furthermore, the reaction parameters such as vanadium content in V-MCM-48, molar ratio of H₂O₂/styrene, temperature, time and solvents were optimized.