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°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°C. Interestingly, this enhanced activity was observed at two separate, relatively narrow, ranges of temperatures: one around 650°C and the other around 800°C. Brönsted and Lewis acid sites attributed to extraframework cationic aluminum were quantified with Fourier transform infrared of adsorbed pyridine. The Lewis/Brönsted ratios of the enhanced-activity samples were high compared those of the low-activity samples; the trend in variation of the Lewis/Brönsted 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 Brönsted and Lewis acid sites that is responsible for the enhanced cracking activity.