Layer-lattice structure

The crystal structure of many compounds is dominated by the effect of H bonds, and numerous examples will emerge in ensuing chapters. Ice (p. 624) is perhaps the classic example, but the layer lattice structure of B(OH)3 (p. 203) and the striking difference between the a- and 6-forms of oxalic and other dicarboxylic acids is notable (Fig. 3.9). The more subtle distortions that lead to ferroelectric phenomena in KH2PO4 and other crystals have already been noted (p. 57). Hydrogen bonds between fluorine atoms result in the formation of infinite zigzag chains in crystalline hydrogen fluoride... 

In203 has the C-type M2O3 structure (p. 1238) and InO(OH) (prepared hydrothermal ly from In(OH)3 at 250-400°C and 100-1500 atm) has a deformed rutile structure (p. 961) rather than the layer lattice structure of AIO(OH) and GaO(OH). Crystalline In(OH)3 is best prepared by addition of NH3 to aqueous InCl3 at 100° and ageing the precipitate for a few hours at this temperature it has the simple Re03-type structure distorted somewhat by multiple H bonds. 

The predominantly ionic alkali metal sulfides M2S (Li, Na, K, Rb, Cs) adopt the antifluorite structure (p. 118) in which each S atom is surrounded by a cube of 8 M and each M by a tetrahedron of S. The alkaline earth sulfides MS (Mg, Ca, Sr, Ba) adopt the NaCl-type 6 6 structure (p. 242) as do many other monosulfides of rather less basic metals (M = Pb, Mn, La, Ce, Pr, Nd, Sm, Eu, Tb, Ho, Th, U, Pu). However, many metals in the later transition element groups show substantial trends to increasing covalency leading either to lower coordination numbers or to layer-lattice structures. Thus MS (Be, Zn, Cd, Hg) adopt the 4 4 zinc blende structure (p. 1210) and ZnS, CdS and MnS also crystallize in the 4 4 wurtzite modification (p. 1210). In both of these structures both M and S are tetrahedrally coordinated, whereas PtS, which also has 4 4... 

The members of this group possess a layered-lattice structure, and this is reflected in the platy habit of their crystals and in the perfect cleavage (OOOlj parallel to the layers. The minerals are flexible and soft (2-25). Limited isomorphism is foimd between Mg and Mn. The small number of available analyses, however, influences the record of compositional varia tion in the species. 

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

Among all layered silicate clays, the smectite family of 2 1 layer lattice structures are preeminent in their ability to adsorb organic molecules and to catalyze their chemical transformations. All metal oxides in the soil environment may exhibit some degree of surface reactivity. However, the adsorptivity and reactivity of typical smectites are facilitated by their relatively high internal surface areas 700 m2/g) and external surface areas (10-50 m2/g). 

Crowell s Expression for Lattice Sum. The Original Application. Crowell (2) has shown how to evaluate the lattice sums, s n, for a layer-lattice structure by an analytical method. The lattice is approximated by a set of layer planes, each with a uniform distribution of matter, and separated by the interplanar distance, d. The lattice sum is approximated by integrating over the planes and forming the sum of the resulting terms. Let the adsorbate, at a distance z from the surface, be separated by a distance Sy from any point in the mtn plane below the surface. The vertical distance of the adsorbate from this plane is (z + md) let y be the projection of s on the mth plane. Then the summation of sj1 for the mth plane is... 

The adsorbent was chosen purposefully as one likely to give a near-homotattic surface, because of the hexagonal layer-lattice structure of the crystal the surface constituent of low heterogeneity is therefore identified with the basal plane of... 

The layer-lattice structure has often been compared with that of graphite, but in fact there are important differences. All the atoms in graphite are identical, and there is a relatively large inherent interlayer attraction caused by the interplanar n electron pairs. In molybdenum disulphide there are two different atomic species and the attraction between molybdenum and sulphur is powerful covalent bonding, but between lattice layers there is only very weak van der Waals attraction. Thus in any... 

We have been investigating the use of imogolite as a pillaring agent for smectite clays with layer lattice structures ". The regular intercalation of the tubes within the layered host results in the formation of a tubular silicate-layered silicate (TSLS) complex. These new nanocomposite materials may be viewed as pillared clays in which the pillars themselves are microporous. Significantly, the TSLS structure is thermally stable up to 450 C when montmorillonite is selected as the layered host . 

Because several spatial stacking arrangements are possible there are several kaolin minerals, each with the same chemical composition, namely Al2Si205(0H)4, but with different properties. Nacrite, dickite, kaolinite, halloysite, and livesite are well recognized species. No positive evidence has so far been published linking other trivalent cations with a single layer lattice structure, but it has been suggested that iron(iii) can replace aluminium in part in the kaolin lattice. 

A number of mechanisms have been offered to explain isotaeticity. This author favors the approach advanced by Cossee (37-33), and extended by others (2,34), which attributes stereoregularity to the special geometry arising from active sites which are adjacent to crystal defects in a solid substrate. Titanium trichloride is a layer-lattice structure (35) consisting of layers of titanium ions interspersed with layers of chloride ions, with the specific structure shown in Fig. 8b. 

Assuming a hexagonal structure for the underlying lattice defines one possible distribution of pillaring cations. It is of interest to consider how different distributions of pillaring cations influence the reaction efficiency. Consider first a distribution of cations that leads to a layered lattice structure built up of triangular lattice arrays. The number of pathways available to the diffusing species in the one-layer system is v = 6 for the two-layer assembly, all sites are of valency v = 1 and, for the three-layer assembly, lattice sites in the upper and lower layers are of valency v — 7, whereas sites in the middle layer are of valency v = 8. For a distribution of cations... 

Many inorganic solids crystallize in layer-lattice structures cadmium iodide is frequently cited as the type example. The cadmium ions are arranged hexagonally in sheets, each with a sheet of hexagonally arranged iodide ions above and below. The separation of adjacent sheets of... 

Graphite is a lamellar solid with the layer-lattice structure shown in Fig. 19-11. The distance between neighboring carbon atoms in each layer is 0.142 nm, very close to the 0.140 nm for the C-C distance in benzene the distance between layers is 0.335 nm. This long interlayer spacing was the basis of an early postulate [46] that the low friction observed for the rubbing of graphite against solid surfaces in ordinary ambient air could be ascribed to weak interatomic forces between the... 

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