Periodic Density Functional Calculation on the Brønsted Acidity of Modified Y-Type Zeolite
journal contributionposted on 09.04.2009 by Katsuki Suzuki, Takayuki Noda, German Sastre, Naonobu Katada, Miki Niwa
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Periodic density functional calculations on Brønsted acidities of Ca2+-, Ba2+-, 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 (Eads) was enhanced and the vibrational frequency of the acidic O1H group (νOH) 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 27Al 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.