Enhancement of n-Hexane Cracking Activity over Modified ZSM-12 Zeolites

A series of ZSM-12 samples were prepared by calcining them at different temperatures that are much higher than what is normally used. The catalytic activities of these samples were tested using cracking of n-hexane at 400 (degree) C. It was found that the cracking activities of samples calcined at certain temperatures were unusually high: 3-4 times greater than that of the regular sample, which was calcined at 550 (degree)C. Interestingly, this enhanced activity was observed at two separate, relatively narrow, ranges of temperatures: one around 650 (degree)C and the other around 800 (degree)C. Bronsted and Lewis acid sites attributed to extraframework cationic aluminum were quantified with Fourier transform infrared of adsorbed pyridine. The Lewis/Bronsted ratios of the enhanced-activity samples were high compared those of the lowactivity samples; the trend in variation of the Lewis/Bronsted ratios of the samples with calcination temperature closely mirrored the trend observed for the catalytic activities. This is an indication that there is a synergistic interaction between the Bronsted and Lewis acid sites that is responsible for the enhanced cracking activity.