Synthesis of ZSM-5 zeolite in fluoride media: an innovative approach to tailor both crystal size and acidity
ZSM-5 zeolites were prepared at neutral pH by fluoride-mediated synthesis. The degree of crystallinity, the specific surface area and the crystal size are higher than those of the zeolites prepared under alkaline conditions. Moreover, by varying the F/Si ratio from 0.3 to 1.6, the H-[F]-ZSM-5 zeolites with crystal size ranging from 10 to 75 [mu]m and the specific surface areas from 60 up to 350 m2/g were obtained.Temperature programmed desorption of pyridine was applied to characterize the zeolite acidity. A decrease of the number of Bronsted acid sites with a higher F/Si ratio in the starting synthesis solution was observed. Based on the Hammett acidity function H0, a model has been developed to correlate the Bronsted acidity of the resulting zeolitic material with the amount of fluoride anions introduced. Thus, ZSM-5 zeolites with tailored properties both at the molecular and microscopic levels are obtained.