Rule, Loewenstein

This equation is based on the assumption that homologous M-O-M (M = Al, Ga...) pairs do not exist. For aluminosilicates this is known as the Al-O-Al avoidance rule introduced by Loewenstein (Loewenstein rule) [69]. 

The Si and A1 ions were arranged according to the Loewenstein rule (28) as is shown in Figure 1. As mentioned in the literature (29-32), half of the Si sites and half of the Si sites are occupied, and care was taken that the Si and the S/ sites were not occupied at the same double six-membered ring. The S2 sites are fully occupied. For the ions which cannot be localized... 

It now seems that the Loewenstein rule is obeyed by all known zeolites, at least on a spatially averaged basis. It is of course possible that occasional A1—O—A1 linkages are present as structural defects. 

Alkali and alkaline-earth aluminosilicates are insoluble, and Muller et al. (138) resorted to tetramethylammonium (TMA) aluminosilicates to measure 27A1 chemical shifts in aluminosilicate solutions. Solutions with different Si/Al ratios and the pure TMA aluminate solution were studied. The molar ratio TMAOH Si Al varied from 3 0 2 to 9 6 2. Theoretically there are 15 distinct Q"(mSi) units with Q = Al (n 0-4 m 0-n). However, dimeric aluminate anions are found only in very concentrated solutions and even then in very small quantities, which led the authors to suggest that the Loewenstein rule is obeyed in aluminate and aluminosilicate anions. The exclusion of Al—O—Al linkages limits the number of possibilities to five... 

The analysis of 29Si MAS NMR spectra of layer silicates with a wide range of tetrahedral compositions (Si/Al ratio 2.7-1.7) indicates (483, 486) that the distribution of Si and Al in these materials is indeed determined by (i) the local balance of charges, and (ii) the Loewenstein rule. In muscovite, phlogopite, and vermiculite, aluminum is randomly distributed in... 

It is only on these that the central silicon atom can be isomorphously substituted by aluminium without violating the Loewenstein rule (22) which prohibits Al-O-Al linkages and which is obeyed in all hydrothermally synthesised zeolites. On the other hand, all four remaining kinds of Si(nAl) sites (with n= 1, 2, 3 and 4) can be generated in the course of A1 substitution of Si(OAl) sites, as demonstrated by an increase in the intensity of the NMR signal at about -100 ppm and the accompanying decrease for the Si(OAl) signal at -105 ppm. It is therefore of interest to consider quantitatively the populations of the various types of site involved in the reaction. 

We studied the influence of the aluminum content and the location of the OH group in zeolites with model six-rings containing one (Al-1), two (Al-2) or three (Al-3) aluminum atoms [128,129]. For the cluster Al-2, both possible relative positions of the two Al atoms in the rings allowed by the Loewenstein rule [130] were considered,/>ora (Al-2p) and meta (Al-2m). The initial positions of the T and O atoms in the ring were taken from mean crystallographic values... 

The lower bound for the Al 2p binding energies for hydrothermally prepared f Al framework aluminosilicates is determined by the Loewenstein rule, which prohibits Al—O—Al bonding. This means that the Si/Al ratio must always be greater than 1.0. Thus, for zeolite Na—A and certain feldspathoids with this 1.0 Si/Al ratio, Al 2p is ca. 73.5 eV (Table VII) [37]. 

The Si/Al ratio, (jc — y)/y, cannot be lower than 1. Pairs of A104 tetrahedra are forbidden by the Loewenstein rule. The extra-framework cations are easily exchanged. Zeolites can aquire catalytic properties if transition metal cations occupy extraframework positions. Recently the use of Cu or Co exchanged zeolites in catalytic decomposition and reduction of NO,r was reported. However, zeolites can also be modified by isomorphous substitution of Si " by other tetrahedrally coordinated cations. For example, the substitution of Ti into... 

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