Loewenstein's Rule Research Articles

Structural model for amorphous aluminosilicates.

An analytical model is developed for the composition-dependent structure of the amorphous aluminosilicate materials (M2O)x(Al2O3)y(SiO2)1-x-y and (MO)x(Al2O3)y(SiO2)1-x-y, where 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1. The model is based on a simple set of reactions and contains a single adjustable parameter p (0 ≤ p ≤ 1). The latter is found from 27Al solid-state nuclear magnetic resonance (NMR) experiments in the regime where R = x/y ≥ 1, aided by new experiments on the magnesium and zinc aluminosilicate systems. The parameter p decreases linearly as the cation field strength of M+ or M2+ increases, as per the observation previously made for the degree of aluminum avoidance [Lee et al., J. Phys. Chem. C 120, 737 (2016)]. The results indicate that as the cation field strength increases, there are less fourfold coordinated aluminum atoms to contribute toward the glass network, and Al-O-Al bonds become more prevalent in a progressive breakdown of Loewenstein's aluminum avoidance rule. The model gives a good account of the composition-dependent fraction of non-bridging oxygen (NBO) atoms for R ≥ 1, as assessed from the results obtained from solid-state NMR experiments. An extension of the model to (M2O3)x(Al2O3)y(SiO2)1-x-y glasses leads, however, to an excess of NBO atoms, the proportion of which can be reduced by invoking network-forming fivefold coordinated Al atoms and/or oxygen triclusters. The model provides a benchmark for predicting the structure-related properties of aluminosilicate materials and a starting point for predicting the evolution in the structure of these materials under the extreme conditions encountered in the Earth's interior or in processes such as sharp-contact loading.

Condensation and growth of amorphous aluminosilicate nanoparticles via an aggregation process.

The precipitation of zeolite nanoparticles involves the initial formation of metastable precursors, such as amorphous entities, that crystallize through non-classical pathways. Here, using reactive force field-based simulations, we reveal how aluminosilicate oligomers grow concomitantly to the decondensation of silicate entities during the initial step of the reaction. Aluminate clusters first form in the solution, thus violating the Loewenstein rule in the first instant of the reaction, which is then followed by their connection with silicate oligomers at the terminal silanol groups before reorganization to finally diffuse within the silicate oligomers to form stable amorphous aluminosilicate nanoparticles that do obey the Loewenstein rule. Our results clearly indicate that aluminate does not serve as the nucleation center for the growth of aluminosilicates in a nucleation-like process but rather proceeds via an aggregation process. The coexistence of aluminosilicate oligomers and small silicate entities induces a phase separation that promotes the precipitation of zeolites with aging.

To Every Rule There is an Exception: A Rational Extension of Loewenstein's Rule

Loewenstein's rule, which states that Al−O−Al motifs are energetically unstable, is fundamental to the understanding and design of zeolites. Here, using a combination of electronic structure calculations and lattice models, we show under which circumstances this rule becomes invalid and how it can be rationally extended using the chabasite framework for demonstration.

To Every Rule There is an Exception: A Rational Extension of Loewenstein's Rule

AbstractLoewenstein's rule, which states that Al−O−Al motifs are energetically unstable, is fundamental to the understanding and design of zeolites. Here, using a combination of electronic structure calculations and lattice models, we show under which circumstances this rule becomes invalid and how it can be rationally extended using the chabasite framework for demonstration.

The Loewenstein rule: the increase in electron kinetic energy as the reason for instability of Al–O–Al linkage in aluminosilicate zeolites

Problem of Al–O–Al linkage in aluminosilicate materials or Al-avoidance is discussed for two zeolite structures (phillipsite and brewsterite) exchanged with Mg2+ cations. All models are fully optimized at periodic Hartree–Fock and hybrid density functional theory levels (the CRYSTAL code). Their properties are then calculated at the periodic level with the same basis sets. The reasons for the instability of the zeolite structures including the Al–O–Al moieties are interpreted on the basis of cell energy decomposition. This destabilization comes from an increase in kinetic energy if the Al–O(Mg)–Al moieties are present in zeolites. This effect is discussed in parallel with already known variational evidences in favor of dominating role of kinetic energy for stability of molecular systems.

Elucidation of the Al/Si Ordering in Gehlenite Ca2Al2SiO7 by Combined 29Si and 27Al NMR Spectroscopy/Quantum Chemical Calculations

We have investigated the Al/Si ordering in the pseudoisolated pairs of tetrahedral sites of the structure of crystalline gehlenite Ca2Al2SiO7 by means of 29Si and 27Al NMR and first-principles quantum mechanical calculations. 29Si NMR spectra of isotopically enriched samples enables the precise determination of the population of the two silicon sites Si–(OAl)3-n(OSi)n (n = 0, 1) and hence the amount of Al–O–Al linkages. This leads to a reliable and model-free quantification of the departure from the Loewenstein rule and to an experimental Al/Si ordering enthalpy of 50.4 ± 1.6 kJ/mol fully reproduced by the quantum mechanical calculations. The seven aluminum sites arising from the Al/Si substitutions Al–(OAl)4-p(OSi)p (0 ≤ p ≤ 4) and Al–(OAl)3-p(OSi)p (p = 0, 1) are identified by 27Al MAS, MQMAS, and {29Si}27Al HMQC experiments, with their quantification being consistent with a fully disordered arrangement of the tetrahedral pairs in the a–b plane of the structure. Assignments of those strongly overlapping...

Cyclic Alumosiloxanes and Alumosilicates: Exemplifying the Loewenstein Rule at the Molecular Level

The cyclic alumosiloxane [{LAl(μ-O)(Ph(2)Si)(μ-O)}(2)] (3) and alumosilicate [{LAl(μ-O){((t)BuO)(2)Si}(μ-O)}(2)] (4) were obtained by reaction of the appropriate R(2)Si(OH)(2) precursor (R = Ph, O(t)Bu) with [{LAl(H)}(2)(μ-O)] (1), providing a nice illustration of the Loewenstein rule at work at the molecular level.

Synthesis and in situ transformation of PST-1: a potassium gallosilicate natrolite with a high Ga content

The synthetic details of a novel potassium gallosilicate natrolite with Si/Ga = 1.28, denoted PST-1, are described. The presence of K(+) and Ga with well-defined levels of concentration in the synthesis mixture is essential for directing its crystallisation. PST-1 transforms rapidly into TNU-6 under hydrothermal conditions, behaviour that contrasts sharply with its very high thermal and hydrothermal stability, which is unusual for a material of such a high Ga content. These stability issues are discussed and rationalized based on chemical composition, likely violations of Loewenstein's rule and the temperature of dehydration of as-made K-PST-1. The crystal structure of TNU-6 has been resolved through the combined use of synchrotron X-ray and electron diffraction data; it has the BaFeGaO(4) structure type with an additional radical 3a x radical 3a "GeAlO(4)" superstructure that arises from tilting of some of the tetrahedral units in all of the 6-rings.

Density functional study of structures and mechanical properties of Y-doped α-SiAlONs

A 2 × 2 × 1 supercell of α-Si 3N 4 is used to model Y-doped α-SiAlON's (silicoalumino oxonitrides). Ab initio total energy calculations are performed at the DFT (GGA) level. A full relaxation of atomic positions is performed for stoichiometric α-Si 3N 4 and for a series of substituted structures with concentrations of Y, Al and O increasing up to the composition Y 0.5Si 9.5Al 2.5O 1.0N 15.0, similar to the observed solubility limits. The avoidance of neighboring AlX 4 (X = N,O) tetrahedra in highly doped structures indicates that the distribution of Al atoms in the framework of α-SiAlON's obeys the same Loewenstein rule as in aluminosilicate zeolite structures. An increase of the lattice parameters and cell volumes is observed with increasing degree of the doping. Changes are more pronounced for the insertion of Y than for Si-N/Al-O substitution. An introduction of a Y 3+ cation into the interstitial position in the cage, however, causes a local contraction of the structure. Via the contraction a more efficient Y–N bonding is formed, leading to the stabilization of the Y-doped structure. The pronounced increase of the cell volume and the lattice parameters with Y-doping is due to three framework Al/Si framework substitutions compensating the extraframework Y 3+ cation. The calculated bulk modulus ( B 0) ranges from 223 GPa for pure α-Si 3N 4 to 197 GPa for Y 0.5Si 9.5Al 2.5O 1.0N 15.0. The slope of the decrease of B 0 with the degree of doping depends on the Y-content. For constant concentration of Y an increasing content of O causes a linear decrease of B 0.

Structural variations in the solid solution series of sodalite-type |(Eu x Ca2–x )4(OH)8|[(Al2+x Si1–x )4O24]-SOD with 0 ≤ x ≤ 1, determined by X-ray powder diffraction and 27Al MAS NMR spectroscopy

Abstract The mineral bicchulite, |Ca8(OH)8|[Al8Si4O24]-SOD, has sodalite-(SOD)-type structure. It is one of the few minerals contradicting Loewenstein’s rule, as it contains twice as much Al as Si at the topologically unique tetrahedrally coordinated position, with both species being randomly distributed. The solid solution series |(EuxCa2–x )4(OH)8|[(Al2+x Si1–x )4O24]-SOD with 0 ≤ x ≤ 1 and Δx = 0.125, was synthesized in order to clarify, whether the continuous violation of Loewenstein's rule is possible in an aluminosilicate framework structure like SOD, or whether the reported examples of SOD-type compounds with an Al : Si-stoichiometry of 2 : 1 represent a preferred composition on the tetrahedrally coordinated site of the structure. Here, we report on the variation of the lattice parameter and of some structural parameters in the series, determined by X-ray powder diffraction and results of 27Al MAS NMR spectroscopy. We found Loewenstein's rule to be systematically and continuously violated in this series. The observed structural variations mainly reflect stronger electrostatic interactions between the cage cations and the sodalite framework with increasing (Al + Eu)-content. While the evaluation of the X-ray diffraction data indicate a global adaptation of the sodalite framework to the cage guest size, the significant line-broadening of the NMR spectra with increasing (Al + Eu)-content points towards local adaptations of the framework to the shape of the cage guests.

Structural Variations in the Solid‐Solution Series LnxCa2−xAl[Al1+xSi1−xO7] with 0 ≤ x ≤ 1 and Ln = La, Eu, Er

AbstractGehlenite, Ca2Al[AlSiO7], has melilite‐type structure with space group $P{\bar 4}{\rm 2}{\rm _{1}}m$. It contains two topologically distinct positions coordinated tetrahedrally by oxygen. One is completely occupied by Al3+, whereas the other one contains Al3+ and Si4+. Normally, the Al3+ molar fraction in the second tetrahedrally coordinated position does not exceed xAl = 0.5, i.e. the so‐called Loewenstein‐rule is obeyed. In this contribution the structural variations in the melilite‐type compounds of the compositions LaxCa2−xAl[Al1+xSi1−xO7], EuxCa2−xAl[Al1+xSi1−xO7] and ErxCa2−xAl[Al1+xSi1−xO7] are discussed. All members of the solid solution except the end‐members violate Loewenstein's rule. Rietveld refinements against X‐ray powder diffraction patterns confirm that the compounds have space group $P{\bar 4}{\rm 2}{\rm _{1}}m$, without changes in the Wyckoff‐positions of the ions compared to gehlenite.

Systematic violation of Loewenstein's rule established

Systematic violation of Loewenstein's rule established

29Si NMR Study of Structural Ordering in Aluminosilicate Geopolymer Gels

A systematic series of aluminosilicate geopolymer gels was synthesized and then analyzed using 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) in combination with Gaussian peak deconvolution to characterize the short-range ordering in terms of T-O-T bonds (where T is Al or Si). The effect of nominal Na2O/(Na2O + K2O) and Si/Al ratios on short-range network ordering was quantified by deconvolution of the 29Si MAS NMR spectra into individual Gaussian peaks representing different Q4(mAl) silicon centers. The deconvolution procedure developed in this work is applicable to other aluminosilicate gel systems. The short-range ordering observed here indicates that Loewenstein's Rule of perfect aluminum avoidance may not apply strictly to geopolymeric gels, although further analyses are required to quantify the degree of aluminum avoidance. Potassium geopolymers appeared to exhibit a more random Si/Al distribution compared to that of mixed-alkali and sodium systems. This work provides a quantitative account of the silicon and aluminum ordering in geopolymers, which is essential for extending our understanding of the mechanical strength, chemical and thermal stability, and fundamental structure of these systems.

In Situ Disorder−Order Transformation in Synthetic Gallosilicate Zeolites with the NAT Topology

Here, we report that synthetic gallosilicate molecular sieves with the NAT topology and Si/Ga ratios close to but slightly higher than 1.50 undergo an in situ transformation under their crystallization conditions. The materials have been studied ex situ by using powder X-ray diffraction, elemental and thermal analyses, and multinuclear MAS NMR. The transformation is characterized by a change in the distribution of Si and Ga of the NAT framework, from a quite (but not completely) disordered phase to a very highly (but not completely) ordered one, accompanied by a change from tetragonal to orthorhombic symmetry. During most of the solution-mediated transformation, no noticeable signs of fresh precipitation, phase segregation, or changes in the chemical composition are detected. Intermediate materials show variations in the degree of Si-Ga ordering and orthorhombic distortion and are not physical mixtures of the disordered and ordered phases. Ab initio calculations strongly suggest a preferential siting of Si in the tetrahedral sites involved in a smaller number of 4-rings in the NAT topology (i.e., the low multiplicity site). The cost of violations of Loewenstein's rule has also been calculated. For this topology and chemical composition the preferential siting and Loewenstein's rule drive together the system to the ordered configuration. A Monte Carlo sampling procedure affords a reasonable model for the initial, mainly disordered state, which fits well within the experimental disorder-order series.

Distribution of Si and Al in Clintonites: A Combined NMR and Monte Carlo Study

The distribution of silicon and aluminum atoms over the tetrahedral sheets of clintonites (aluminum-rich phyllosilicates 2:1) has been studied by 29Si and 27Al magic-angle-spinning NMR spectroscopies. According to the 29Si NMR spectra, no silicon atoms are present in adjacent tetrahedra; however, several environments have been detected in the 27Al NMR signal, proving that these aluminosilicates do not adhere to Loewenstein's rule. The atom distribution has been simulated by a Monte Carlo procedure. From the results of these simulations, a further dispersion of silicon atoms over the tetrahedral sheets has been found, indicating a homogeneous dispersion of the electrostatic charge over the phyllosilicate layers. In contrast, a maximum dispersion of charges must be discarded in the layers of studied clintonites. Finally, an analysis of structural factors controlling this kind of cation distribution is given.

Characteristics of MOR-Framework Zeolites Synthesized in Fluoride-Containing Media and Related Ordered Distribution of Al Atoms in the Framework

MOR type zeolites with Si/Al ratios of 5.19−9.00 have been synthesized in the presence of fluoride ion (denoted by MS-F). Although MS-F has the same framework topology as conventionally synthesized mordenite (MS-C), the characteristics of MS-F, such as crystal morphology, adsorptivity for benzene and thermal stability, are quite different from MS-C. Solid-state 29Si MAS NMR measurements revealed that the number of Si atoms with a Si(2Al) environment in MS-F is less than that in MS-C of the same Si/Al ratio. Based upon the results of NMR measurements and those of benzene sorption and thermal stability, the ordered distribution of Al atoms in the framework of MS-F was determined by using the connectivity-configuration matrixes method under the restriction of Loewenstein's rule and the 2Al/5-ring avoidance rule. It is suggested that Al atoms are located at T1, T2, and T4 sites in MS-F, which is different from the case of MS-C occupying T1, T2, T3, and T4 sites. The differences in the characteristics between MS-F and MS-C can be explained by the different distribution of Al atoms in their framework.

Preparation of Lithium Oligosiloxane Aluminates and Acid Strength of Hydroxy Groups in a Molecular Aluminum Oligosiloxane

The Loewenstein rule, according to which the AlO4 tetrahedra in aluminum silicates should always be separated by silicon atoms, is apparently not valid for molecular aluminum silicates such as 2, because the linkage of the AlO4 tetrahedra does not change when the four acidic protons of 1 are replaced by lithium atoms.

A Combination of the Monte Carlo Method and Molecular Mechanics Calculations: A Novel Way To Study the Ti(IV) Distribution in Titanium Silicalite-1

A combination of the Metropolis Monte Carlo method and molecular mechanics calculations is used to study the Ti(IV) distribution in titanium silicalite-1 (TS-1). Calculations are carried out in which the Ti atoms are placed at crystallographically different T-sites rather than at symmetry-related T-positions. In this way, also the effect of already incorporated Ti atoms on the accommodation of new Ti atoms can be studied. It is shown that the Ti atoms are distributed over all crystallographically different lattice positions rather than located at one preferred T-site. The distribution, however, is not random: T2 and T12 are clearly favored. Modeling of the Ti distribution at loadings above the experimentally determined maximum limit of 21/2 Ti atoms per unit cell seems questionable. The framework symmetry is related to both the location of the Ti atoms and the Ti loading. There are much less Ti atoms present as neighbors than can be expected from a random substitution, which might indicate that Loewenstein's rule is dependent not only on electrostatic effects but also on framework deformations.

Aluminosilicate and borosilicate single 4-rings: Effects of counterions and water on structure, stability, and spectra

Rings containing four tetrahedrally coordinated atoms connected by bridging O atoms in a single four ring (S4R) geometry have been used to model the properties of ring species occurring in aluminosilicate and borosilicate crystals, glasses, and melts. We recently established a molecular basis for the Al avoidance, or “Loewenstein's rule,” using molecular quantum mechanical methods applied to the non-Loewenstein paired, i.e., Al … Al … Si … Si …, and the Loewenstein alternating, i.e., Al … Si … Al … Si …, geometric isomers of the Si 2Al 2O 4H 8 −2 molecular anion. We here extend this model to the Si 2Al 2O 4H 8 −2 and Si 2B 2O 4H 8 −2 molecules neutralized by various counterions such as H +, CH 3 +, Na +, and Ca 2+. Our calculations show that the paired geometry isomer can be strongly stabilized by coordination of counterions to the underbonded bridging O in the AlOAl or BOB linkages. For the Si 2Al 2 … case, coordination of H + or Ca 2+ to the underbonded O actually makes the paired geometry isomer more stable than the alternating geometry one. This result helps us to explain why the energy penalty for AlOAl bonds is smaller in Ca aluminosilicates than in Na aluminosilicates. For the Si 2B 2O 4H 8(CH 3) 2 and Si 2B 2O 4H 8Ca molecules the paired geometry is again the more stable. Underbonded O atoms in BOB linkages are found to be stabilized more strongly by counterions than are those in AlOAl linkages. This provides an explanation for the different T,T′ ordering patterns in anorthite, CaAl 2Si 2O 8, and danburite, CaB 2Si 2O 8. Our results also indicate that it may be possible to systematically synthesize gas phase and condensed phase species in which the Al avoidance rule is violated. We have also used the S4R species to model rings in Na aluminosilicate glasses and their reaction with H 2O. Calculated NMR shieldings are reduced (deshielded) by about 5–6 ppm for both Si and Al when two Na + ions are coordinated to Si 2Al 2O 12H 8 −2. Reaction of H 2O with Si 2Al 2O 12H 8Na 2 does not cause hydrolysis of the SiOAl bond or protonation of the bridging O, rather the H 2O coordinates strongly to Na + while H-bonding to the bridging O. This reaction is exoergic by about 35 kJ/mol and produces a slight shielding of the Si and a reduction in the quadrupole coupling constant of the Al. No single species was found to be consistent with all the NMR spectral data for hydrous aluminosilicate glasses.

Solid-state NMR Studies on AlPO<sub>4</sub>-5 Molecular Sieve with Al/P>1

Solid-state NMR. spectroscopy was employed to investigate the coordination state of P and Al, as well as their linkages in a series of newly synthesized aluminophosphate molecular sieve AlPO4-5 with Al/P>1. Both P-31 and Al-27 NMR provides evidence that Al-O-Al linkages are present in the framework of these molecular sieves, which implies that the ordering : rang cut of P and Al does not obey the well-known Loewenstein rule. Apart from the usually observed NMR signals, a broad peak at ca. delta=10 is present in the Al-27 MAS spectra of the calcined and dehydrated AlPO4-5 molecular sieves. Doe to its large second-order quadrupole shift, an isotropic chemical shift of ca. delta=24 was estimated. Hence, we attribute this signal to 4-coordinated aluminium Al[nAl, (4-n)P)] (n=1,2,3,4). Similarly, an additional broad signal at delta=-24 was observed in the corresponding P-31 MAS spectra and is due to the P coordination state in which its second coordinated shell contains Al PETAL (4-n)P](second shell) (n=1,2,3,4).