| Attributs | Valeurs |
|---|
| type
| |
| Is Part Of
| |
| Subject
| |
| Title
| - Density Functional Studies of Alkali-Exchanged Zeolites. Cation Location at Six-Rings ofDifferent Aluminum Content
|
| has manifestation of work
| |
| related by
| |
| Author
| |
| Abstract
| - The interaction of a sodium cation and six-rings of a zeolite framework with different numbers of aluminumatoms was studied with the aid of a density functional method and cluster models. The cation preferentiallyinteracts with the oxygen atoms connected to aluminum at shortest distances of about 220 pm. For six-ringswith two or three aluminum atoms, sodium positions on each side of the rings were identified. At both locationsNa+ interacts mainly with the oxygen atoms directed toward the inside of the six-ring. From a comparison ofcalculated Na−O distances with crystallographic data and vibrational frequencies with the IR spectra of COon NaY zeolite, it is deduced that the crystallographic SII site in faujasites may actually consist of two cationpositions. The sodium location in a ring with one aluminum atom differs from that in the other clusters; it isalmost in the plane of the T atoms. The sodium cation was calculated to experience a very low-energy barrierfor crossing the six-ring (ca. 10 kJ/mol), which allows easy displacement perpendicular to the ring uponinteraction with guest molecules. The vibrational frequency of adsorbed carbon monoxide was calculated asa measure for the polarizing power of the sodium cation. Both the calculated binding energies of CO andvibrational frequency shifts are in agreement with the experimental values. According to our results, thealuminum content of the six-rings can be the reason for the asymmetric broadening of the stretching moderegion in the IR spectra of CO adsorbed on NaY zeolite at low pressure. The calculations indicate that adiscrimination of six-rings of different aluminum content via IR spectroscopy is difficult.
|
| article type
| |
| is part of this journal
| |
