Lowenstein rule

Another difficulty is that if three prisms are connected in a particular way to form part of a single sodalite cage, six nearest neighbors of the fourth prism are determined. In accordance with Lowensteins rule this may restrict the structure of the fourth prism. Lowensteins rule means that there are constraints involved in the ways the prisms can connect that are not considered here. However, this will not restrict the possible distributions of a large enough crystal and should not affect the results of this work. 

In this last example, we mentioned that zeolite framework silicon atoms could be substituted. This substitution is naturally found in zeolites. Silicon atoms can be replaced with +in valence atoms, such as Ar or or +V valence atoms, such as Then, cations or anions are introduced to neutralize the charge of the framework (see Figure 2). The only known limitation of the silicon substitutions is the Lowenstein rule which states that substitutions cannot be found into two adjacent tetrahedra. The presence of cations or anions within zeolites allows the zeolite crystals to be used in ion exchanged processes. 

According to the Lowenstein rule, the centers of TO tetrahedra are occupied by silicon atoms and alxuninxmi in a Si Al ratio which can vary... 

The fourth neighbor is generated when the prisms are connected. Since each aluminum is connected to a silicon according to Lowenstein s rule, there are Qs - QA silicon atoms available for connecting to another silicon, where Qs = number of silicon atoms and QA = number of aluminum atoms. 

It has also been observed that zeolite A (10,11,12) and Losod (7) both with Si/Al = 1 have chemical shifts of -88.9 ppm, outside of the range proposed for Si-4A1. At first, this was attributed to S1-3A1 ordering in violation of Lowenstein s rule. However, recent neutron diffraction work on zeolite A (14) and NMR studies of its high silica analogue ZK-4, (8,13) have shown that the single resonance in zeolite A does correspond to S1-4A1. The unusual chemical shift for zeolite A may be a consequence of the strained double four rings in the structure and the presence of one nearly linear T-O-T angle. 

NMR compositions were calculated from the relative peak intensities assuming Lowenstein s rule and using the relation... 

Theoretical and experimental approaches of the framework Si/Al ratio effect on the acidity led to the conclusion that the strength of the protonic sites of zeolites is influenced by the presence of neighbors (11, 12). Each framework Al atom has 4 Si atoms (Lowenstein s rule) in the first surrounding layer (nearest neighbors) and, depending on the zeolite topology, 9-12 Al or Si atoms in the second layer (Next-... 

Hass, E. C., P. G. Mezey, and P. J. Plath (1981). A nonempirical molecular orbital study on Lowenstein s rule and zeolite composition. J. Molec. Struct. 76, 389-99. 

In a zeolite, where the framework comprises TO4 tetrahedra (T silicon or aluminium), the chemical shift of the silicon, compared to tetramethylsilane adopted as reference, varies with the number of aluminium atoms located in the second co-ordination sphere (cf. Fig. 13.6). From the NMR spectrum, it is possible to obtain the concentration of each of these configurations and, by applying Lowenstein s rule (no Al-O-AI links), calculate the atomic Si/Al ratio of the framework using the formula ... 

This method has been applied to generate aluminophosphates with a specified stoichiometry. According to Lowenstein s rule, two criteria must be satisfied during the calculation of the possible combinations for a specified stoichiometry ... 

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