Density Functional Studies of Alkali-Exchanged Zeolites. Cation Location at Six-Rings of Different Aluminum Content

Abstract

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.

FTIR study of low-temperature CO adsorption on Mn-ZSM-5 and MnY zeolites. Effect of the zeolite matrix on the formation of Mn2+(CO) geminal species

“…Cations in Y zeolites are located in S I , S II and S II 0 positions, but, especially at low temperatures, cations in S II positions only are accessible for adsorption. Although not all details are well-known for all systems, it is believed that these cations are coordinated to three oxygen atoms from the six-ring windows [32]. A detailed XRD study of Mn 2þ ions in faujasite and their CO sorption complexes has been recently published [33].…”

Analytical evaluation of pseudopotential matrix elements with Gaussian-type solid harmonics of arbitrary angular momentum

“…The results of our benchmark calculations together with the characteristic dimensions of the compounds investigated are given in Table VIII. Beside the sample systems already mentioned, it contains benchmarks for a zeolite model cluster: AlSi 5 O 6 H 12 [51], a gold thiolate cluster: Au 13 (SH) 8 [52], and a model for an active site of a Mo-S enzyme: [(HOCHO)(SH)OMoS 2 C 2 H 2 ] − [53]. The implementation of the pseudopotentials are fully compatible with the special design of PARA-GAUSS to efficiently use a large number of processors in parallel.…”

Theoretical investigation of site-specific characteristics of CO adsorption complexes in the Li+-FER zeolite

“…[22,[27][28][29][30][31][32][33][34][35][36]); however, in majority of cases the small cluster models were used and only one type of cation coordination (corresponding to one particular site in zeolite) was investigated. There are only few systematic theoretical investigations of alkali metal sites in high-silica zeolite.…”

“…The correlation between interaction energy and shift of the CO frequency was also discussed. The interaction of CO with alkali metal cations in zeolites was also investigated using a small cluster model of various sizes (clusters containing one or more framework T atoms) [30,31]. It is now well established that the interaction of alkali metal cation with CO can be reliably described at the density function theory (DFT) level employing either hybrid or GGA type exchange-correlation functionals [29,38].…”