Lowenstein s rule

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 ... 

Lowenstein s rule states that the formation of Al-O-Al links is thermodynamically unfavoured. This means that the majority of zeolites (which are prepared at low temperatures using hydrothermal methods) have ordered frameworks, with no Al-O-Al links, and the highest Si Al ratio is 1 1. 

Before continuing note that when using crystallographic data obtained for Na-Rho loaded with 1 bar of CO2, our approach is able to accurately reproduce a 298 K experimental isotherm (see Fig. 2). Note that in all the simulations presented in this work, the Si/Al ratio is approximately 4 (9.8 A1 and 38.2 Si per unit ceU) in order to simulate the material of interest. The positions of the A1 atoms are chosen randomly subject to the constraint of Lowenstein s rule [24]. 

Structures of cationic positions in zeolites are determined by location of aluminum atoms in the lattice. There are two factors which can disturb the stochastic distribution over the lattice. The first one is the availability of preferential lattice points for the location of Al atoms in zeolite. It can be important especially in the stabilization due to exchange of monovalent metal cations. The second one is mutual interaction of lattice Al atoms, which, for example, is the reason of direct adjacency impossibility for the placing of Al in the lattice (Lowenstein s rule). 

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