Density functional studies of alkali-exchanged zeolites. Cation location at six-rings of different aluminum content

The interaction of a sodium cation and six-rings of a zeolite framework with different numbers of aluminum atoms was studied with the aid of a density functional method and cluster models. The cation preferentially interacts with the oxygen atoms connected to aluminum at shortest distances of about 220 pm. For six-rings with two or three aluminum atoms, sodium positions on each side of the rings were identified. At both locations Na⁺ interacts mainly with the oxygen atoms directed toward the inside of the six-ring. From a comparison of calculated Na-O distances with crystallographic data and vibrational frequencies with the IR spectra of CO on NaY zeolite, it is deduced that the crystallographic SII site in faujasites may actually consist of two cation positions. The sodium location in a ring with one aluminum atom differs from that in the other clusters; it is almost in the plane of the T atoms. The sodium cation was calculated to experience a very low-energy barrier for crossing the six-ring (ca. 10 kJ/mol), which allows easy displacement perpendicular to the ring upon interaction with guest molecules. The vibrational frequency of adsorbed carbon monoxide was calculated as a measure for the polarizing power of the sodium cation. Both the calculated binding energies of CO and vibrational frequency shifts are in agreement with the experimental values. According to our results, the aluminum content of the six-rings can be the reason for the asymmetric broadening of the stretching mode region in the IR spectra of CO adsorbed on NaY zeolite at low pressure. The calculations indicate that a discrimination of six-rings of different aluminum content via IR spectroscopy is difficult.