Silicate Analogues

Phosphate-substituted zeolites have attracted commercial interest in the past two decades and it is claimed that incorporation of P increases the efficiency of these cavity-containing compounds both as catalysts and as sequestering agents for Ca + and Mg-+. Although in some cases these compounds are silicates with cavity-occluded orthophosphate groups (i.e. silicate/phosphates), in others the phosphate is incorporated in the silicate framework to form genuine silicophosphates with Si-O-P linkages. An important silicate/phosphate cavity structure is NASICON (Section 12.20). 

By co-precipitation of the component oxides or hydroxides as homogeneous gels, followed by subsequent crystallisation at 80 200 C, PO4 tetrahedra can be incorporated in the silicate frameworks of various well-established zeolites. Highly crystalline specimens containing up to 25% PO4 tetrahedra have been prepared. 

FIGURE 5.54 Silicate frameworks (a) sodalite and (b) faujasite. Si or A1 at each line junction and O atoms midway between. 

FIGURE 5.55 Kaolinite layer structure Al2Si20j(0H)4. A narrow section is shown on each layer, and the latter are indicated by broken lines. 

Unlike aluminosilicate zeolites, these almninophosphate netwoiks are neutral and without ion exchange capacity. More than 40 different AIPO4 structures of this kind have been characterised. They include both two-dimensional layer and one-, two- and three-dimensional channel-type structures [130-137]. Many are analogues of zeolite structures. 

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We further illustrate the approach by reference to studies (331) of two related zeolite structures zeolite ZK-4 (isostructural with Linde A) and the highly siliceous analogue of sodalite known as TMA-sodalite. As was shown earlier (Sections III,A and III,D), the structure of zeolite A consists of a cubic array of / -cages linked through double four-membered rings so as to form larger polyhedral a-cages. The sodalite structure (Fig. 7) consists of a dense,... 

The samples used were a standard Na-A zeolite and five nitrogeneous types of zeolite A, or N-A.(18) The N-A zeolites are siliceous analogues of zeolite A, synthesized with tetramethyl-ammonium cation. The Si Al ratio varied from 0.94 (NaA) to 3.54 for the most siliceous N-A sample.t The ratios were determined by wet chemical analysis, and the structure type and absence of impurity phases were confirmed by X-ray powder diffraction techniques. Adsorption measurements (oxygen, -183°C) showed a zeolite A content of greater than 90%. 

Midgley and Rosaman (M57) concluded from DTA evidence that SO4" can be partly replaced by OH in the AFt phase of cement pastes. Pollman et al. (P22a) showed that the fully OH"-substituted phase, C AHj, exists but is easily carbonated. Material described as a silicate analogue was later shown to have also contained (C28) but a product identified by X-... 

As is shown, degradation of compounds with these kinds of linkages may lead to a variety of products. In the first case (eqn. (7a)), the products are metasilicate and an organic molecule with a fully reduced terminal carbon atom. The second case (eqn. (7b)) illustrates a hydrogenation/cleavage reaction that leaves both products in a reduced state. In the third case (eqn. (7c)), hydrolysis produces a siliceous analogue of formic acid, as was the case in... 

The hydrogen-bonded frameworks in two of the ice polymorphs (11 and v) are different from any of the 4-connected Si(Al)—0 frameworks. The (rhombohedral) structure of ice-ii has some similarity to the tridymite-like structure of ice-i, the greater density being achieved by the proximity of a fifth neighbour at 3-24 A in addition to the four nearest neighbours at 2-80 A-compare the distance of the next nearest neighbours (4-5 A) in ice-ih. The framework of ice-v also has no obvious silica or silicate analogue, in spite of the fact that the ratio of the densities of ice-v... 

The addition of small amounts of certain oxyanions such as phosphate, perchlorate, arsenate, chlorate, bromate ect. in the synthesis mixture of zeolites and their metallosilicate / silicate analogues significantly enhances the nucleation and crystallization rates, thereby reducing the overall crystallization time by as much as five times. Sensitivity enhanced high resolution liquid state Si and P NMR studies, using a specially designed probe, on low temperature (358 K) synthesis of Silicalite-1 in the presence of NaH2P04 as promoter indicate a catalytic role of the promoter. 

P-23 - Synthesis and characterization of mesoporous cerium silicate analogues of MCM-41 type molecular sieves... 

Upper spectrum low Si/Al ratio material showing peak assignments. Lower spectrum highly siliceous analogue showing two signals in the ratio of 2 1 from the T-site inequivalence. 

For quite a number of crystalline aluminosilicates the gallium- and iron-silicate analogues have been synthesized in the past as well. Therefore, it is not too surprising that the isomorphous substitution of gallium [60] or iron [61] in the MWW framework has recently been reported. Again, as in the case of zeolite ZSM-58, this offers the opportunity of fine-tuning the strength of the Bronsted-acid sites in such materials. 

Like their silicate analogues the sodium rare earth germanates of composition Na5RGc40i2 show high ionic conductivity. The alkali cations can move freely in the structure. The conductivity values observed in the germanates lie between 10 and 12 cm (Shannon et al., 1978 Hong et al., 1978). 

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