One-pot synthesis of ordered mesoporous Cu-KIT-6 and its improved catalytic behavior for the epoxidation of styrene: Effects of the pH value of the initial gel

Abstract

The heterogeneously copper-catalyzed oxidative cleavage of styrene was studied using copper-doped mesoporous KIT-6 (Cu-KIT-6x) prepared via pH adjustment (where x is the pH: 1.43, 2.27, 3.78, 3.97, 4.24 or 6.62). Variations in the catalyst structure and morphology with pH values were characterized by X-ray power diffraction, nitrogen adsorption-desorption analysis, transmission electron microscopy and X-ray photoelectron spectroscopy. As the pH value applied during the initial synthesis, the resulting Cu-KIT-6x exhibited different structural, textural and surface characteristics, especially in terms of specific copper species and copper content. At a pH value of 3.78, approximately 4.6 wt% copper(II) was successfully incorporated into the framework of the initial KIT-6, in the form of –Cu–O–Si– groups. The catalytic performance of each catalyst was evaluated by following the epoxidation of styrene, employing tert-butyl hydroperoxide as the oxidant and CH3CN as the solvent. A significant styrene conversion of 43.5% with 86.6% selectivity for the desired styrene epoxide was obtained over the Cu-KIT-63.78. A higher Cu content, an ordered cubic Ia3d mesoporous architecture and various specific textural characteristics all combined to endow the Cu-KIT-63.78 with high catalytic activity and good stability.