Influences of fluorine implantation on catalytic performance and porosity of MOR-type titanosilicate

Fluorine species were implanted into the framework of Ti-MOR by post-treatment with fluorides in order to modify the microenvironment around Ti active sites and then to improve their catalytic activity in liquid-phase oxidation. The effects of NH₄F amount, fluorination temperature, and solvent on the catalytic performance of F-Ti-MOR were investigated in detail. Methanol was found to be superior to water in fluorination. Fluorine implantation increased the electropositivity of Ti active sites through forming SiO₃/₂F units in the neighborhood, which enhanced the catalytic performance in the ammoximation of cyclohexanone remarkably. F-Ti-MOR prepared under optimized fluorination conditions showed a cyclohexanone conversion of 99% in comparison to only 30% conversion given by primitive Ti-MOR. Meanwhile, the implanted fluorine species captured the organic molecules with a relatively large dimension tightly, creating enormous steric hindrances that prevented other molecules, in particular those with bulky molecular dimensions, from diffusing into channels freely. Thus, F-Ti-MOR showed much lower activity than Ti-MOR in the hydroxylation of aromatics.