The distribution of framework aluminum atoms and extraframework exchanged cations in faujasite as studied by molecular dynamics, NMR simulation, neutron diffraction simulation and computer graphics
“…In chabazite, however, considerable changes in cation positions and framework geometry occur during ion exchange . It has been recognized that extraframework cations play a major role in stabilizing particular Si,Al orderings, and recent Monte Carlo (MC), molecular dynamics (MD), and quantum-chemical (QC) studies have focused on the determination of the most favorable extraframework cation distributions. − In theoretical studies of the cation's properties, the importance of allowing free relaxation of the framework was outlined. , 1 Schematic representation of the FAU zeolite framework with extraframework cation sites denoted. Oxygen atoms are not shown; they lie near the center of line segments, connecting T-atoms.…”
The electronic structure and relative stability of clusters with general composition (M
n
+)
x
/
n
H12Si12
-
x
Al
x
O18
with x = 0, 2, 4, M = H+, Li+, Na+, K+, Ca2+ forming double-six-membered rings (D6R), are studied by the
B3LYP method. In Ca2H12Si8Al4O18 clusters, maximum separation of the Al atoms is favored, which implies
a minimum number of Al−O−Si−O−Al linkages. A large number of isomers with Si,Al distributions strongly
related to the type and location of the charge-compensating cations were determined for the ratio Si/Al = 5.
All cations favor maximum separation of Al atoms within the T6O6 rings. Concurrent occupation of
extraframework cation sites in the center of the D6R and above the T6O6 window is energetically unfavorable
for Li+ and Na+ cations, whereas this is achieved with K+, in agreement with experimental data. The Si,Al
ordering with one Al atom per T6O6 ring and at the largest distance within the D6R is the most stable one
with Li+ and K+ as extraframework cations, whereas Na+ and Ca2+ favor the configuration with two Al
atoms in one T6O6 ring. The small and medium size cations Li+, Na+, and Ca2+ approach the regions of high
electron density in the D6R. The HOMOs of both the siliceous fragments and the aluminum-containing
fragments with the extraframework cations located near the four-membered ring walls consist mainly of O
2p nonbonding AOs. When the extraframework cations take up positions at the D6R center, or near the T6O6
rings, the HOMOs O 2p nonbonding character is reduced: Al 3s and Al 3p AOs are admixed to the O 2p
AOs. The maximum electron density is concentrated at the Al atoms and the Al-bonded oxygen atoms. When
the Al atoms are in one T6O6 ring of a D6R, the oxygen atoms of this ring bear a higher electron density.
Lewis basic sites appear in proximity to the Brønsted acid sites in proton balanced clusters.
“…In chabazite, however, considerable changes in cation positions and framework geometry occur during ion exchange . It has been recognized that extraframework cations play a major role in stabilizing particular Si,Al orderings, and recent Monte Carlo (MC), molecular dynamics (MD), and quantum-chemical (QC) studies have focused on the determination of the most favorable extraframework cation distributions. − In theoretical studies of the cation's properties, the importance of allowing free relaxation of the framework was outlined. , 1 Schematic representation of the FAU zeolite framework with extraframework cation sites denoted. Oxygen atoms are not shown; they lie near the center of line segments, connecting T-atoms.…”
The electronic structure and relative stability of clusters with general composition (M
n
+)
x
/
n
H12Si12
-
x
Al
x
O18
with x = 0, 2, 4, M = H+, Li+, Na+, K+, Ca2+ forming double-six-membered rings (D6R), are studied by the
B3LYP method. In Ca2H12Si8Al4O18 clusters, maximum separation of the Al atoms is favored, which implies
a minimum number of Al−O−Si−O−Al linkages. A large number of isomers with Si,Al distributions strongly
related to the type and location of the charge-compensating cations were determined for the ratio Si/Al = 5.
All cations favor maximum separation of Al atoms within the T6O6 rings. Concurrent occupation of
extraframework cation sites in the center of the D6R and above the T6O6 window is energetically unfavorable
for Li+ and Na+ cations, whereas this is achieved with K+, in agreement with experimental data. The Si,Al
ordering with one Al atom per T6O6 ring and at the largest distance within the D6R is the most stable one
with Li+ and K+ as extraframework cations, whereas Na+ and Ca2+ favor the configuration with two Al
atoms in one T6O6 ring. The small and medium size cations Li+, Na+, and Ca2+ approach the regions of high
electron density in the D6R. The HOMOs of both the siliceous fragments and the aluminum-containing
fragments with the extraframework cations located near the four-membered ring walls consist mainly of O
2p nonbonding AOs. When the extraframework cations take up positions at the D6R center, or near the T6O6
rings, the HOMOs O 2p nonbonding character is reduced: Al 3s and Al 3p AOs are admixed to the O 2p
AOs. The maximum electron density is concentrated at the Al atoms and the Al-bonded oxygen atoms. When
the Al atoms are in one T6O6 ring of a D6R, the oxygen atoms of this ring bear a higher electron density.
Lewis basic sites appear in proximity to the Brønsted acid sites in proton balanced clusters.
“…Zeolites X and Y, which have the FAU framework topology, are such zeolites, and many studies have derived some information on the silicon/aluminum distributions in these materials by combining the local environment information from NMR, the framework topology from crystallography, other chemical information, and modeling. Various attempts have used the analysis of models of ordered subunits of the framework, − subunits plus symmetry constraints, random distributions, , Monte Carlo annealing studies with either simplified or realistic energetics, − order plus disorder models, and combinations of methods including molecular dynamics . Recently, an approach using simulated annealing has been tried on other Si/Al framework materials .…”
Section: Introduction: Silicon and Aluminum Distributions In
Faujasitementioning
confidence: 99%
“…Various attempts have used the analysis of models of ordered subunits of the framework, 2-4 subunits plus symmetry constraints, 5 random distributions, 6,7 Monte Carlo annealing studies with either simplified or realistic energetics, [8][9][10][11] order plus disorder models, 12 and combinations of methods including molecular dynamics. 13 Recently, an approach using simulated annealing has been tried on other Si/ Al framework materials. 14 In all cases, the results of the modeling have been compared to the known 29 Si NMR spectra or to the integrated intensities that give the relative populations of Si(nAl) environments.…”
Section: Introduction: Silicon and Aluminum Distributions In Faujasitementioning
Simulated annealing is applied to the determination of distributions of silicon and aluminum atoms on the
faujasite lattice from 29Si NMR data. The method is described and compared to others. The local silicon
environments, Si(nAl), n = 0−4, are reproduced to high accuracy by the technique. More detailed features
of local structure are then available, such as numbers of Al on different ring systems in the zeolite. Dempsey's
rule is found to be strictly followed in the four rings of the zeolite X sample studied, while this is not the case
for zeolite Yin agreement with previous work. The results are consistent with a possible small violation of
Lowenstein's rule in the zeolite X sample.
“…The traditional assignment of 29 Si NMR spectra of zeolites is essentially based on empirical relations derived from the dependence on geometry of the observed chemical shifts. − Moreover, the analysis of deshielding effects due to the presence of Al tetrahedra has led to linear relationships between the 29 Si chemical shifts and the number of Al neighbors . These empirical predictions have been shown to be useful in determining the distribution of aluminum in faujasite lattice …”
Section: Introductionmentioning
confidence: 99%
“…21 These empirical predictions have been shown to be useful in determining the distribution of aluminum in faujasite lattice. 22 The ab initio theoretical prediction of NMR properties started with the early work by Ditchfield, 23 who proposed the GIAO method, later implemented in nonempirical Hartree-Fock calculations 24,25 and post Hartree-Fock 26 calculations. Although this method has been applied to predict the chemical shifts of organic compounds, its application to zeolite structures has still been limited to a few applications.…”
29 Si and 27 Al NMR chemical shifts have been evaluated from the NMR shielding tensors obtained using the SOS-DFPT technique. The 29 Si NMR chemical shifts have been calculated for the nine crystallographically distinct Si sites of the zeolite-β lattice. The calculations were carried out on cluster models Si(OSiH 3 ) 4 and R 3 SiOSiR 3 (R ) OSiH 3 ) representing one (1T) and two (2T) sites in the zeolite lattice, respectively. Using the 1T models, the nine signals of the experimental spectrum have been assigned with a relative error of less than 1 ppm, the absolute error being estimated at about 5 ppm. The use of a "fragmentaveraging" technique based on the results obtained with the 2T models leads to calculated absolute NMR shifts with an accuracy of (1 ppm. The 27 Al NMR shifts of AlH(OSiH 3 ) 4 models have also been calculated, after relaxation of the local geometry. It was found that the relative positions of the 27 Al chemical shifts are in agreement with the experimental spectrum.
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