Enhancing ethylene epoxidation of a MWW-type titanosilicate/H2O2 catalytic system by fluorine implanting†
Fluorine-implanted titanosilicate Ti-MWW, with greatly enhanced catalytic performance in the liquid-phase epoxidation of ethylene with aqueous hydrogen peroxide, has been post-synthesized by fluorinating conventional Ti-MWW with NH4F. The effects of fluorination conditions, such as solvent, the amount of NH4F addition and temperature, on the zeolite structure and the catalytic performance of the resultant fluorine-implanted Ti-MWW were investigated in detail. Methanol proved to be a better solvent for fluorination than water in terms of preserving the crystalline structure and improving the selective epoxidation activity. Fluorine-implanting generated SiO3/2F species in the framework to increase the electropositivity of the tetrahedral Ti active sites. A stronger hydrogen-bond was thus formed between Hend in Ti–Oα–Oβ–Hend intermediates and the adjacent Si–F species, which were helpful to significantly increase the catalytic activity in the epoxidation of ethylene with H2O2. In addition, the fluorination also generated secondary mesopores which are useful to improve the accessibility of the active sites, making the fluorine-implanted Ti-MWW catalyst possess better reusability than the parent one.