Periodic Density Functional Calculation on the Brønsted Acidity of Modified Y-Type Zeolite

Periodic density functional calculations on Bronsted acidities of Ca 2+ -, Ba 2+ -, and Al(OH) 2+ -exchanged Y-type zeolites were performed. When a divalent cation was stabilized in the 6-membered ring window (sites I', II, and II'), the adsorption energy of ammonia (E ads ) was enhanced and the vibrational frequency of the acidic O1H group (ν OH ) located in the supercage was decreased. Those calculated values agreed with the corresponding experimental values obtained by the ammonia infrared spectroscopy/mass spectroscopy-temperature programmed desorption (IRMS-TPD) method. Al in Al(OH) 2+ is located in sites I' and II' coordinated to the surrounding three oxygen atoms in the 6-membered ring, and those were stabilized as tetracoordinated extra-framework Al (EFAL) species. This configuration of EFAL species has been supported by 27 Al MQ MAS NMR measurement at a high magnetic field (16 T). High electron-withdrawing potentials of cations at the sodalite cage enhanced the strength of the acidic O1H group located in supercage, and thus, the catalytic activity of paraffin cracking was improved.