Evolution of strong acidity and high-alkane-cracking activity in ammonium-treated USY zeolites

USY zeolites prepared by steam treatment of NH4-Y zeolites were treated with ammonium nitrate solutions. Treatment with ammonium nitrate solutions resulted in a significant enhancement of the octane-cracking activity of the USY zeolites. We found by infrared spectroscopy/mass spectrometry-temperature programmed desorption (IRMS-TPD) of NH3 that the prepared USY zeolites have acidity strong enough to catalyze alkane cracking. The acid strength (ΔH) values of the enhanced Brønsted OH on the USY are found to be ca. 150 kJ mol−1. A linear correlation between ΔH and the activation energy in alkane cracking was obtained when Y-type zeolites were used for the reaction. The heats of adsorption of octane and hexane were almost constant over various types of FAU zeolites. This means that the higher the strength of the acid site, the smaller the intrinsic activation energy of the alkane cracking. This study clearly indicated that treatment with NH4+ was the key to creating very strong acidity in USY zeolites. Furthermore, the origin of the high catalytic activity of the ammonium-treated USY zeolites was attributed to the evolution of strong acidity rather than changes in the adsorption energies of the alkanes, based on the measurements of acid strength, heat of alkane adsorption, and activation energy.

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► Acid properties of USY zeolites treated with an aqueous solution were quantitatively analyzed by IRMS-TPD.
► Treatment of USY zeolites with solutions of ammonium salts led the evolution of strong Brønsted acid (150 kJ mol−1).
► A linear relationship between Brønsted acid strength and apparent activation energy was observed in alkane cracking.
► Acid strength is the primary factor that accounts for change in the intrinsic alkane activation energy among FAU-type zeolites.