Loewenstein’s rule

The assignment given is such (see Table III) that the peak at -88ppm is associated with silicon linked (within the tetrahedral manifold) via oxygen to 2 silicons and 1 aluminium whilst the peak at -93ppm is associated with silica linked exclusively to other silicons (25-27) Using an analysis based on Loewenstein s rule and widely used in zeolite struct jral analyses (28) we have shown elsewhere (20) that for this particular synthetic heidellite the (Si/Al). ratio is 11.5 ... 

Evidently, arrangements where cwo A1 atoms share a common 0 atom are much less stable than those cluster models where Loewenstein s rule is followed. 

In order to interpret the results of the following correctly, we must consider that they obey Loewenstein s rule [16] which forbids the occurrence of Al-O-Al linkages in tetrahedrally bonded aluminosilicates. As a result ... 

For this group of zeolites, according to Loewenstein s rule, only an enrichment in Si can occur, with respect to the ideal ratio i.e. the Si/Al ratio cannot be less than 1.0. 

This can be explained if we assume that the "extended Loewenstein s rule , formulated to interpret the behavior of zeolite Y, is valid for all zeolites. 

In these zeolites the Si/Al ratio is normally very near 1.5 [33]. Therefore, disorder is due to an Al->Si substitution between A1 and Sil. In this case, according to Loewenstein s rule. Si must occupy both A1 sites, and the Si atom substituted by A1 in Sil will occupy one Si2 site, the other Si2 site being occupied by the residual A1 (see Fig. lb). Therefore the crystal structure refinement must give an Al-fraction in site Sil, equal to the Si-fraction in site A1 and two times the Al-fraction in site Si2. These ratios have been found when disorder is low, as in some natrolites, in mesolite and scolecite but when the Al-fraction in Sil reaches values about 10 %, the disorder in the 4-ring formed by the T2 sites increases till it reaches complete disorder, with the same Al-fraction both in Tl and T2 (see Table 4 and Fig. 2). Curiously, complete disorder, which is not present in the majority of so-called disordered zeolites, has sometimes been found in these zeolites which are often taken as an example of order. 

Goosecreekite shows an interesting feature, unique to ordered zeolites. One of the Si-rich tetrahedra is surrounded by four other Si tetrahedra. Moreover, only one of the six independent Si sites has an Al-content remarkably different from zero ( 10 %]. This tetrahedron is bonded with two Si and both the Al tetrahedra, where (according to Loewenstein s rule) there is an Si-content near 10 %. 

The positions of Si and A1 atoms have always been assigned by applying Loewenstein s rule, which forbids the presence of an A1—O—A1 linkage in the structure. A corollary of this rule is that when the Si/Al ratio is 1, the amount of... 

The chart in Figure 2.26 shows that the most likely Si environment for zeolite A is Si(OAl)3(OSi). However, we know that the Si/Al ratio is 1, so this coordination is not possible without systematically breaking Loewenstein s rule. It was spectroscopic work on this structure (and on zeolite ZK-4) that eventually confirmed the Si(OAl)4 structure of zeolite A with strict alternation of Si and A1 and led to the extended ranges shown in the chart. 

As long as one is dealing with faujasite-type zeolites, there is a reasonable separation between the spectral regions for Si(OAl), Si(lAl), etc. However, for other types of zeolites there can be cause for ambiguity. This has led in the past to the conclusion that in some zeolites there is violation of Loewenstein s rule,62 which, briefly stated, says that Al-O-Al bridges are absent in zeolites. Current evidence, however, provides strong support that the rule is obeyed.56 57 63... 

Single-crystal X-ray results (9) point to strict alternation of Si and Al (the 4 0 ordering scheme) in accordance with Loewenstein s rules (10), but this model was recently challenged on the basis of 29si NMR measurements (1, 2) and our discovery that the sodium derivative can be rhombohedral (11,12). However, the controversy has now been resolved and the correctness of the X-ray model reaffirmed (13,14). A consequence of the Si, Al ordering is that the lattice parameter of the cubic cell is ca. 24.6 A, not 12.3 A as reported in earlier work, a feature reflected in weak superlattice reflections. The space group is Fm3c. 

A High Resolution Silicon-29 NMR and Neutron Powder Diffraction Study of Na-A Zeolite Loewenstein s Rule Vindicated... 

In a series of recent papers the structure of zeolite A has been re-examined with particluar reference to the Si and A1 ordering, and a different arrangment has been proposed,(5, 6 ) based mainly upon results from three different techniques, electron diffraction, (5, 7) 29Si-NMR(5, ], j ) and neutron diffraction, C5, 9 ) and is at variance with Loewenstein s rule. (10)... 

If the twin boundaries constitute true mirror planes, it can be shown that, across these planes Loewenstein1s rule must be contravened. If, however, the mirror planes merely reflect the tetrahedral sites of the faujasitic framework and not the actual tenants of such sites Loewenstein s rule need not be... 

Obviously, x is another way to define the Si/Al ratio, which is equal to (1-x)/x. When x1 vanishes, Loewenstein s rule is... 

Engelhardt, et al., (3.) and Melchior, et al., ( L) have pointed out that the first moment of the NMR spectrum is equal to 4/(Si/Al), provided Loewenstein s rule is obeyed. He can extend this important relationship to the following ... 

Loewenstein s rule implies that x1 = 0, which substituted in Equation (1) gives (3., 4.)... 

The following relationship allows direct calculation of the density of second-nearest-neighbor A1 pairs in a zeolite obeying Loewenstein s rule ... 

Unlike Loewenstein s rule which predicts a vanishing x for Si/Al ratios larger than 1, Dempsey s rules do not directly provide an exact value of x2 except at the lowest A1 concentrations (Si/Al 5) where next-nearest neighbors can completely be avoided to give x 0. In fact, x2 as well as the finer details of the NMR intensity distribution, depend on the particular topology (i.e., the connectivity scheme) of the zeolite under consideration. The following sections describe a method to simulate NMR spectra for any zeolite topology with the... 

Random distributions in a 20 x 20 square planar lattice according to Loewenstein s rule and Dempsey s rules. The Al s are indicated Ijv dots the Si s are left blank. Also shown are simulated Si spectra and Xg values obtained from 32 x 32 square planar lattices. 

Pair correlation function of Si/Al site occupancies in a square planar lattice with randomized A1 distribution according to Loewenstein s rule ( — —) and Dempsey s rules (---o---). 

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