Loewenstein's Rule Research Articles

A Model for Random Si/Al Distribution in a Zeolite Framework Restricted by Loewenstein's Rule

Analytical expressions are derived for a description of random Loewenstein Si/Al ordering in the faujasite zeolite framework. These formulas served as a random distribution reference in the analysis by Melchior et al. of 29Si NMR spectra of faujasites for the determination of Si/Al ordering in framework building blocks. The equations are based on a model where correlations between the Al occupancies of T-sites belonging to different four-rings are ignored. This simple approach gives reliable results for 29Si peak intensities when the Si/Al ratio is larger than 1.5.

Number of isolated aluminum atoms in the framework of dealuminated acid zeolites

Monte Carlo calculations restricted by Loewenstein rule predict correctly the variation of the intensities of the lines contributing to the29Si MAS NMR spectra in dealuminated acid ZSM5, mordenites and Y zeolites covering a domain of ratios silicon/ framework aluminum between 4 and 80. They are also used to calculate the numbers of isolated framework aluminum and the relationship with the Bronsted acidity is briefly discussed.

Study of Si and Al ordering in the framework of siliceous cubic and hexagonal faujasite zeolites

Cubic faujasites of Si/Al = 1.7 to 3.7 and hexagonal faujasites of Si/Al = 3.3 and 4.0 were synthesized hydrothermally and were compared with secondary synthetic faujasites of Si/Al = 2.7 to 7.9 derived from SiCl 4 or (NH 4) 2SiF 6 treatment. XRD, IR and solid-state NMR ( 29Si, 27Al and 23Na) were used to characterize the materials. Both the unit cell size, a 0 and the framework vibration frequencies of faujasites obey a linear relationship with the content of framework aluminum as determined by 29Si MAS NMR. Comparisons of experimental 29Si NMR with the theoretical models of Si and Al ordering suggest that the β-cage model can be applied to fit the aluminum distribution in directly synthesized cubic and hexagonal faujasites, while the aluminum distribution in the framework of the (NH 4) 2SiF 6-treated samples, CSY, is based on the random siting model with Loewenstein's rule.

Modeling the similarities and differences between the sodalite cages (?-cages) in the generic materials

Initially in this review the sodalite framework [T12O24]6− (T=Al, Si] is modeled with regular tetrahedra and disordered T atoms. Equations are given for calculating atomic coordinates from the unit cell parameter a and the T—O distancet; the expansion or contraction of the sodaliteβ-cage is related quantitatively to changes ina through the cooperative twistsφ of TO4 tetrahedra about 4 axes and changes in 0), as in sodalite examples, and is quantitatively modeled most conveniently from sodalite examples with similarβ-cage contents. The review is extended to structures with T-ordering and distorted tetrahedra. Methods are given for estimating a for sodalites from a knowledge of the cavity contents, especially the sizes of cations and anions, and so on, present. Ways of predicting cavity sites in zeolite A as a function of cation size are presented, and the principal cavity sites in the faujasiteβ-cage region are discussed. Finally the review considers isomorphous replacement of T atoms (Si or Al) by B, Be, Fe, Ga, Ge, and P; many of these substituted frameworks are stabilized by templates, or guest molecules, which reside in the cavities. Templates also stabilize Si, Al frameworks with high Si∶Al ratios. The modeling approach reviewed here is tested on a range of isomorphously substituted frameworks isotypic with sodalite; observed and calculated values of twist and <TOT angles are in good agreement except in one case. This exception concerns aluminate sodalites [Al12O24]12− with distorted tetrahedra; these are structures with Al-O-Al bridges, which violate Loewenstein's Rule.

Temperature dependence of the Si, Al distribution in ultramarines

The Si, Al ordering pattern for blue ultramarine (aluminosilicate with a ratio Si/Al ≈ 1) is analyzed by a Monte Carlo method. The influence of the synthesis temperature and the large extraframework anions on the Si, Al distribution is studied. It is found that a combination of both factors (especially the first) can explain the reported violation of Loewenstein's rule in this compound. A long-range disordered Si, Al distribution was observed for temperatures above 800 K, which correspond to the pyrolytic synthesis of ultramarine. For lower temperatures, corresponding to the regime of hydrothermal synthesis, strict alternation of Si and Al was found, in agreement with experimental results for lazurite, the natural counterpart of ultramarine.

Synthesis and characterization of an aluminosilsesquioxane framework that violates Loewenstein's rule

Synthesis and characterization of an aluminosilsesquioxane framework that violates Loewenstein's rule

On the Loewenstein rule and mechanism of zeolite dealumination

The reason for and the peculiarities of dealumination of aluminum-rich zeolites are explained on the basis of ab initio SCF MO calculations of the energies of local chemical interactions in the zeolite lattice. As distinct from Dempsey-Mikovsky-marshall electrostatic approach (DMM), a conclusion is made that simultaneous extraction of the most closely-situated structural elements from the lattice takes place. The first step of this reaction is their mutual extracoordination according to the scheme. This step and the subsequent formation of large nuclei of hydroxoaluminum phases are regarded as a transformation of the lattice into a most energetically favorable state corresponding to the chemical composition nSiO{sub 2}{center_dot}mAl{sub 2}O{sub 3}{center_dot}kH{sub 2}O. 33 refs., 1 fig., 2 tabs.

29Si-MAS-NMR spectra of keatite-type aluminosilicates

Abstract 29Si-MAS-NMR spectra were measured on keatite-type alumino-silicates MAlSi2O6 with M = H, Li, Na and K. Data were collected at a magnetic field of 4.7 T. The shapes of the resonance lines vary in a systematic way with the type of cation. These variations essentially arise from changes in the T – O – T bond angles in the structures of the phases from HAlSi2O6 to KAlSi2O6 which are known from X-ray and neutron diffraction. The NMR spectra are modelled quantitatively using the correlation of Murdoch et al. (1988) between 29Si chemical shift and average Si – O – T bond angles. A difference between the NMR-derived Si/Al ratio and the chemical formula further suggests that Loewenstein's Al-avoidance rule does not completely hold in these phases.

Hamiltonian graph representation of zeolite frameworks and Si, Al ordering in the framework

In the first part, a topological characterization of various zeolite frameworks is performed on the basis of the Hamiltonian graph. It affords a simple representation of the framework connectivity, and presents some possibility of the classification as well as the prediction of unknown frameworks. In the second part, a simple and direct method based on the connectivity matrix is presented, by which all the possible Al distributions in a given framework are derived exhaustively under the restriction of Loewenstein's rule. An example of Al preference site determination is applied for zeolite ZSM-5 and compared with the result by the SCF-MO method.

Aluminum-27 NMR studies of the calcium tungstate aluminate sodalite and sodium chromate aluminate sodalite, Sr8[Al12O24](CrO4)2 and Ca8[Al12O24](WO4)2

The authors have studied {sup 27}Al NMR spectra of powders of strontium chromate aluminate sodalite (SACr) and calcium tungstate aluminate sodalite (CAW) by the magic angle spinning (MAS) technique and of single-crystal SACr by a static method. They show that the precision in the determination of the quadrupole coupling constant (qcc) from MAS spectra is limited by residual broadening. Both in SACr and in CAW Loewenstein's rule is violated, and the qcc's are among the largest found for Al in a tetrahedral site of an aluminosilicate framework, due to the strong tetragonal distortion that accompanies this violation. MAS spectra in cubic and tetragonal SACr are indistinguishable, compatible with a phase transition mainly due to conformational shearing of the four-membered rings of the framework. In the orthorhombic phases of CAW and SACr, at least four different Al sites can be distinguished: the MAS spectra suggest that the Al sites in the two compounds are very similar. Due to the local character of the NMR information, this does not necessarily imply that they have the same space groups. The empirical relation between {sup 27}Al chemical shifts and bond angles established in (approximately) 1/1 aluminosilicates does not hold for these pure aluminates.

Transport in glassy fast-ion conductors: A stud of LiAlSiO 4 glass

The mechanism of ionic motion in LiAlSiO 4 glass is investigated using a combination of structural (vibrational spectroscopy, X-ray diffraction) and transport (ionic conductivity) methods. We find that the material is quite a good Li + conductor ( σ ≈ 10 −3 μ −1 cm −1 at 500 °C), and that the vibrational density of states deduced from spectroscopic studies agrees very well with the continuous random network model. Moreover, the tendency towards ordering seen in many aluminosilicates and described by Loewenstein's rule is discussed in the context of our spectroscopic observations, and we find that the Loewenstein rule also holds approximately in this glassy material. The rigidly alternating potential seen by Li + ions in crystalline LiAlSiO 4 (β-eucryptite) is substantially altered by the disorder present in the glass. This disorder strongly reduces correlation effects on ionic motion in the glass, as opposed to the crystal, resulting in a smaller activation energy for conductivity in the glass.

Thermal decomposition of the zeolite catalyst ZSM-5

A systematic investigation of the thermal decomposition of ZSM-5 using FT IR absorption spectrophotometry is described to determine the composition of samples subjected to isothermal anneal. Stability is investigated as a function of aluminum substitution, sodium ion exchange, and ambient humidity. Aluminum-free ZSM-5 decomposes with first-order kinetics while aluminum containing ZSM-5 does so with second-order kinetics, except at the highest temperatures where the decomposition reverts to first-order kinetics. This is consistent with the decomposition being nucleated by migration and pairwise association of the defect sites generated by aluminum leaving the framework associated with dehydroxylation. A Loewenstein rule for these defect sites is inferred. Two regimes, with activation enthalpies of 4.6 (106 kcal/mol) and 7.3 eV (168 kcal/mol), are manifested and they deduce these to be diffusion-limited and activation-limited, respectively.

Interpretation of Mössbauer Spectra of Nontronite, Celadonite, and Glauconite

A new approach to the interpretation of M6ssbauer spectra of Fe3+-phyllosilicates having vacant trans-octahedra is based on (1) crystal structure simulation methods that allow for the size and the shape ofa Fe3+-octahedron as a function of the nearest surrounding cations; and (2) calculations of electric field gradients (EFG) on Fe 3+ in terms of the ionic point-charge model Calculations were performed by direct summation within the region of radius -<50/~. Coordinates for the anions in the coordination octahedra have been assigned to take into account the nearest cationic environment. Atomic coordinates for the rest of the summation volume are those for the average unit cell. EFG calculations for cation combinations responsible for the visible quadrupole splitting Avi~ in the spectra of nontronite, "red" muscovite, and celadonite have led to good agreement between Avis and A~ c. Computer fitting of the nontronite and celadonite spectra based on EFG calculations for the rest of the possible cation combinations suggests that the distribution of tetrahedral cations in nontronite obeys the Loewenstein rule, and in celadonite, the distribution of R 3+ and R 2+ over cis-octahedra is predominantly ordered, in agreement with electron diffraction and infrared spectroscopy data. The M6ssbauer spectrum of one of the glauconites suggested the presence of celadonite-like and muscovite-like domains in its 2:1 layers.

Distribution of isomorphous substitutions in silicates by NMR spectroscopy

The use of spectroscopic methods permits in certain cases to investigate the relative distribution of cations and anions in silicate structures. In particular, in micas (phyllosilicates 2:1) with very heterogeneous composition, the use of multinuclear ( 29Si, 27Al, 1H and 19F) NMR spectroscopy has demonstrated the existence of local ordering in both the tetrahedral and octahedral sheets. From the study of NMR spectra associated to OH − and F − ions we have concluded that: a) the Fe +2 ions have not any preference to occupy M1 or M2 sites in the octahedral sheet of micas, b) the F − ions are located in homogeneous domains composed only by F − and Mg +2 ions, and c) the Fe +2 are in close association with OH − ions. As the content in Fe +2 and F − increases Fe +2 ions have a net tendency to cluster around the OH groups. The analysis of 29Si and 27Al NMR spectra showed that in the tetrahedral sheet of micas: a) each Al is surrounded by 3Si, and b) Si, Al distribution is more dispersed that imposed by Loewenstein's rule.

Ordering of aluminum in tetrahedral sites in mixed-layer 2:1 phyllosilicates by solid-state high-resolution NMR

Al and Si MAS NMR results are presented for the mixed-layer 2:1 phyllosilicate rectorite. Separate Si spectra, obtained by partially relaxed inversion-recovery experiments, for the two types of tetrahedral sheets present are consistent with one sheet being smectite-like, i.e. low aluminum occupancy of tetrahedral sites, and the other sheet being mica-like, i.e. high aluminum occupancy of tetrahedral sites. Comparison of the relative areas of the Si(0-3Al) resonances from the mica-like sheets with those predicted by computer modeling indicates a high degree of ordering of substituted aluminum. The observed areas indicate that minimization of both Al-O-Al (Loewenstein's rule) and Al-O-Si-O-Al (Dempsey's rule) interactions is important. For the tetrahedral sheets Si/Al ratios derived from Al MAS spectra indicate a much higher aluminum occupancy than determined by XRF or Si NMR.

Si, Al distribution in micas; analysis by high-resolution29Si NMR spectroscopy

The analysis of 29Si MAR-NMR spectra of micas and vermiculite shows that the two main factors that govern the Si, Al distribution in the tetrahedral sheet of these minerals are: (i) Loewenstein's rule (avoidance of Al-O-Al linkages), and (ii) local balance of charges. In muscovite, phlogopite and vermiculite, Al is randomly distributed in hexagonal ring sites (meta and para dispositions equally favoured), but complying with the two above restrictions. In margarite Si and Al are observed to alternate strictly, in agreement with previous X-ray diffraction analysis.

Synthesis of new gallosilicate zeolite with sodalite structure

A sodium gallosilicate zeolite with the sodalite structure was synthesized from the silica-rich starting gel. This gallosilicate zeolite has a very low SiO 2 to Ga 2O 3 molar ratio (less than 1.0), which means that it violates the Loewenstein's rule. Moreover, against the expectations, the unit cell of this gallosilicate zeolite is smaller than that of an aluminosilicate with the sodalite structure.

Tetrahedral cation ordering in layer silicates by 29Si NMR spectroscopy

29Si NMR spectra of layer silicates with a wide range of tetrahedral composition (Si/Al ratios from 7.7 to 2.7) indicate that Si, Al distribution is mainly controlled by the electrostatic requirement of homogeneous dispersion of Al, confirming our earlier conclusions derived from the analysis of natural samples of micas with Si/Al ratios near to 3. This requirement includes, as a partial aspect, the Loewenstein's rule which forbids the presence of Al in neighbouring tetrahedra.

The myth of Loewenstein's rule

The myth of Loewenstein's rule

Silicon and aluminium site distributions in 2:1 layered silicate clays

Although the layer charge in 2:1 phyllosilicate minerals is known to result from the replacement of tetrahedral Si or octahedral Al, Fe and Mg by ions of lower charge, there is only limited information concerning the distribution of layer charge from X-ray crystallographic data1–3. Here we use 29Si and 27Al magic angle spinning (MAS) NMR spectroscopy to examine the site distribution of tetrahedral Si and Al in a series of synthetic trioctahedral clays with (Si/Al)tetr values in the range 2.74–7.69. Analysis of the 29Si spectra shows that Loewenstein's rule for Al occupancy of the tetrahedral sheet is obeyed and that there is some short-range ordering for Al sites. The nature of the ordering is explained in part by the results of electrostatic potential energy calculations. In general, strong 27Al resonances are observed for Al in tetrahedral sites, but the resonances due to Al in octahedral sites are considerably weaker than would be expected on the basis of chemical analysis. Consequently, no quantitative analysis of the 27Al spectra is possible.