Tetrahedral substitution

Microprobe analyses of some berthierine pellets from sedimentary rocks (Velde, e t al., 1974, and new data, Figure 30) indicate that these minerals have compositions close to those of 7 8 chlorites delimited by synthesis studies and here there is a more restricted range of silica substitution in the structure than is found in 14 8 chlorites. Thus the two polymorphs have at least different limits in tetrahedral substitutions. 

It is a matter of some significance catalytically to be able to ascertain whether the aluminum present in a natural clay or its synthetic analogue is in a state of octahedral substitution (as in montmorillonite) or whether there is some tetrahedral substitution (as in beidellite). 27A1 MAS NMR readily provides the necessary answers. For example, Diddams et at. (462) in a study of the synthesis, characterization, and catalytic performance of synthetic beidellites and their pillared analogues, monitored the fate of AI from the gel precursor to the sheet silicate and to its pillared state by 27A1 MAS NMR (see... 

When pillared smectites without tetrahedral substitution are calcined, there is no reaction between the pillars and the smectite layers. By contrast, a considerable structural transformation occurs when pillared beidellite is calcined, which has been interpreted as the growth of a three-dimensional quasi-zeolitic framework between the two-dimensional clay layers. The acidic properties of the product are comparable with those of zeolite Y and much more pronounced than those of calcined pillared smectites without tetrahedral substitution. 

In the same samples, a second absorption feature was detected that is associated with the dopant ions themselves. These ligand-field transitions allow distinction among various octahedral and tetrahedral Co2+ species and are discussed in more detail in Section III.C. The three distinct spectra observed in Fig. 4(b) correspond to octahedral precursor (initial spectrum), tetrahedral surface-bound Co2+ (broad intermediate spectrum), and tetrahedral substitutional Co2+ in ZnO (intense structured spectrum). Plotting the tetrahedral substitutional Co2+ absorption intensity as a function of added base yields the data shown as triangles in Fig. 4(b). Again, no change in Co2+ absorption is observed until sufficient base is added to reach critical supersaturation of the precursors, after which base addition causes the conversion of solvated octahedral Co2+ into tetrahedral Co2+ substitutionally doped into ZnO. Importantly, a plot of the substitutional Co2+ absorption intensity versus added base shows the same nucleation point but does not show any jump in intensity that would correspond with the jump in ZnO intensity. Instead, extrapolation of the tetrahedral Co2+ intensities to zero shows intersection at the base concentration where ZnO first nucleates, demonstrating the need for crystalline ZnO to be... 

Attapulgite and sepiolite are clay minerals with a chain structure. The former has five octahedral positions and the latter either eight or nine. Both have relatively little tetrahedral substitution. The octahedral positions in sepiolite are filled largely with Mg and those in attapulgite with approximately half Mg and half Al. 

Ross and Hendricks (1945) redefined beidellite as the aluminum-rich end member of the dioctahedral montmorillonites. Many of their samples were later found to be mixtures and for some time the concept of a high-aluminum montmorillonite was in considerable disrepute. Recently, Weir and Greene-Kelly (1962) made a careful analysis of purified material from the Black Jack Mine from Beidell, Colorado, and definitely established that it is monomineralic and an ideal Al-rich end member (Table XXVIII). They suggest that beidellites and montmorillonites should be divided at the composition at which the lattice charges from octahedral and tetrahedral substitution equal one another . The layer charge for beidellite does not appear to be any larger than for montmorillonites. 

Hectorite is similar to stevensite in having little or no tetrahedral substitution however, the octahedral sheet has a significant U content (Table XXXVIII). The hectorite from Hector, California, contains 0.33 octahedral U. A sample described by Faust et al. (1959) contains only 0.10 Li and a sample described by Bradley and Fahey (1962) contains 0.04 octahedral Li. There could presumably exist a continuous range, although the upper limit is not known. Values as high as 1.45 are reported for the trioctahedral micas (Radoslovich,1962). Layer charge is due both to Li substitution and cation deficiencies in the octahedral sheet. Appreciable F— is present proxying for OH". 

The maximum amount of Al3+ tetrahedral substitution that 2 1 clays minerals formed at low temperatures can accommodate appears to be 0.80—0.90 per four tetrahedra. While this appears to place an upper limit on the amount of R3+ octahedral substitution, it is not clear why the limit should be such a low value. The dioctahedral smectites can accommodate more substitution (R2 + for R3+) in the octahedral sheet than can the dioctahedral micas. The reverse situation exists for trioctahedral equivalents. In the latter clays octahedral R3+ increases as tetrahedral Al increases. Thus, as one sheet increases its negative charge, the other tends to increase its positive charge. This is likely to introduce additional constraints on the structure. In the dioctahedral clays substitution in either sheet affords them a negative charge and substitution in one sheet is not predicted by substitution in the other sheet thus, one might expect more flexibility. 

The hydrothermal dioctahedral chlorites have considerably less tetrahedral substitution than those formed in sediments. The former would appear to be a stable phase and the latter a metastable phase. The tetrahedral composition of the hydro-thermal specimens is similar to that for the other dioctahedral clays and represents a reasonable fit between the tetrahedral and octahedral sheets. 

Tetrahedral Al3 + values range from 0.10 to 0.76 per two tetrahedral positions and average 0.49 for the ten selected samples. The amount of tetrahedral substitution of the larger Al3+ for the smaller Si4+ is appreciably greater than has been reported for the 2 1 clays, although the higher values are in the range (0.9 1.5 per four tetrahedral positions) reported for the trioctahedral micas (Foster, 1960) and chlorites (Brown and Bailey,1962). 

Lewis Acid Sites. Many other mechanisms (66, 85) are best described in terms of the more general concept of aprotic, or Lewis acidity which is defined in terms of the capacity to donate or share pairs of electrons. Aprotic acid sites are commonly derived from the coordinatively unsaturated cations at crystal edges or adsorbed on crystal faces, from deydration of hydroxylated surfaces, and from deamination or deamination and dehydration of silica-aluminum catalysts or similarly treated clays having extensive tetrahedral substitution (130, 132). Formation of Lewis acid sites by deamination or deamination-dehydration is dependent on inversion of the basal oxygens of the aluminum-substituted tetrahedron away from the surface, in order to expose the aluminum (131). 

Flee et al. (13) showed that, upon calcining, there is a reaction between the pillars and the clay surface in tetrahedrally substituted smectites (e.g. beidellite). This reaction did not occur in smectites without tetrahedral substitution. The pillared I/M do not show the remarkable hydrothermal... 

After the elimination of water, the carbonyl oxygen has been substituted for the =NOH group in the final product. This compound is called an oxime. This reaction can also be performed with base catalysis. This type of tetrahedral substitution reaction may be performed with many nitrogen derivatives, such as hydrazine, NH2NH2. Write down the product that is formed from the reaction of... 

The mineralogy that comprises the clay component of the soil is also critical to contamination of soils by biological and chemical threat agents. For example, ricin was sorbed to four different clay minerals as described above (Figure 4.2). Kaolinite, a 1 1 clay mineral, sorbed very little ricin. Sepiolite, a fibrous clay mineral, sorbed much more ricin. Illite, a tetrahedrally substituted 2 1 clay mineral, sorbed similar amounts of ricin as the sepiolite. The octahedrally substituted clay mineral, montmorillonite, sorbed much greater quantities than all of the other clay minerals. Even with this clay mineral, the cation that was dominantly sorbed to the clay made a difference. The Na-saturated montmorillonite sorbed more than the Ca-saturated montmorillonite. This is thought to be due to the swelling of the interlayers of the clays. Na-saturated clays swell more so than do Ca-saturated ones. Similar results are shown with aflatoxin Bj (Jaynes et al. 2007). 

The supposition of a large barrier to rotation about the C(i7)-C(20) bond, in pregnanes with C-20 tetrahedrally substituted, is said to be supported by the isolation of two distinct rotamers of the compound (5), when the 20-acetal... 

Based on layer lattice thickness estimates from Chapter 4, what is the relative attraction for a dehydrated K+ ion residing directly on the mineral surface of a tetrahedrally substituted 2 1 mineral when compared to an octahedrally substituted 2 1 mineral ... 

Scheme 2 The Fischer projection formula for a tetrahedrally substituted carbon atom.

Attention was turned to an approach (Scheme 10) that avoided tetrahedral substitution at positions 1 and 8a, utilizing the stable methoxycyclopropane moiety. Alcohol 74, after Jones oxidation, reductive alkylation with lithium m-methoxyphenylacetylide, and catalytic hydrogenation yielded the a-alcohol 81 in 44% yield, together with a 46% yield of the epimeric 3-alcohol. Unfortunately, only the a-alcohol could be dehydrated to the desired endocyclic olefin (82), the 3 tdcohol forming only the exocyclic olefin, in poor yield. The remainder of the transformations were carried out on olefin 82 to give the alcohol 83. As expected from the model studies, protolysis followed by acid catalyzed cycli-zation yielded the desired BC trans isomer, providing the first sample of the tetracyclic ketone 69. The overall yield was 3.3% in 15 steps. 

Layered silicates dispersed as a reinforcing phase in polymer matrix are one of the most important forms of hybrid organic-inorganic nanocomposites. MMT, hectorite, and saponite are the most commonly used layered silicates. Layered silicates have two types of structure tetrahedral-substituted and octahedral substituted figure 6. In the case of tetrahedrally substituted layered silicates the negative charge is located on the surface of silicate layers, and hence, the polymer matrices can react interact more readily with these than with octahedrally-substituted material. 

Plee, D., Gatineau, L., and Fripiat, J. J. 1987. Pillaring processes of smectites with and without tetrahedral substitution. Clays Clay Minerals 35 81-88. 

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