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 SiO_3/2F species in the framework to increase the electropositivity of the tetrahedral Ti active sites. A stronger hydrogen-bond was thus formed between H^end in Ti-O^α-O^β-H^end 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.