Solid bases aluminosilicates

Glas-ionomer cements are acid-based materials (using, e.g., polyacrylic acid), whose setting reaction involves neutralization of the acid groups by powdered solid bases (calcium fluoro-aluminosilicate glasses). Resin-modified glass ionomer cements are hybrid materials prepared by the incorporation of polymerizable components such as 2-hydroxyethyl methacrylate. ... 

Zeolites are aluminosillicate solids based on a framework of comer-sharing Si04 and A104 tetrahedra. 

As described above, zeolites can also act as solid base catalysts when the Si Al ratio is low and the extra framework cation is a large one such as The most basic common exchanged zeolite is CsX. With this material the aldol condensation of cyclooctanone with benzaldehyde gave only the monobenzylidene compound (Eqn. 10.23) 5 while reactions using piperidine, m nesium oxide or amorphous cesium aluminosilicate gave both the mono- and di-benzylidene products(Eqn. 10.12). The smaller ketones, cyclohexanone and... 

Catalysts. - The catalysts and sources of HCHO that appeared in patents are listed in Table 6. Solid bases such as hydroxides of alkali and alkaline earth metal supported on a support such as silica gel or aluminosilicate have mainly been claimed in patents to be effective as the catalysts. In addition, another type of compounds, which possess acidic property as well as basic property, are also claimed in patents, for example, PbO, Mn02, AI2O3, metal phosphates, metal borates, multicomponent oxides containing V, Nb, W, and Mo. 

Nowadays zeolites have to be defined in line with the lUPAC recommendations as a group of solids based on either aluminosilicates up to silica polymorphs, aluminophosphates or metallosilicates or phosphates with a well-defined microporous structure. As zeolite science and application is still a rapidly growing area, the reader may refer to a recently published book "Introduction to Zeolite Science and Practice" [9] for more detailed information. This book represents the latest comprehensive review on the different fields of zeolite chemistry. 

The traditional definition of a zeolite refers to microporous, crystalline, hydrated aluminosilicates with a tliree-dimensional framework consisting of comer-linked SiO or AlO tetrahedra, although today the definition is used in a much broader sense, comprising microporous crystalline solids containing a variety of elements as tetrahedral building units. The aluminosilicate-based zeolites are represented by the empirical fonmila... 

The hot-water separation process involves extremely complicated surface chemistry with interfaces among various combinations of solids (including both silica sand and aluminosilicate clays), water, bitumen, and air. The control of pH is critical. The preferred range is 8.0—8.5, achievable by use of any of the monovalent bases. Polyvalent cations must be excluded because they tend to flocculate clays and thus raise viscosity of the middlings in the separation cell. 

While they are not solvents and solutions in the usual sense of the word, it is convenient here to introduce the concept of solid acids and bases. For example, consider the class of compounds known as zeolites. These are aluminosilicate structures with variable amounts of A1( II). Si(IV), metal cations, and water (see Chapter 16). 

It is proposed that in mixed organic base-alkali systems, the presence of the organic base changes the solid-liquid equilibrium and stabilizes larger sol-like aluminosilicate species ( 25 m/ ). The alkali ion affects agglomeration of the sol particles to larger amorphous precipitate particles from 100 to 500 min size which subsequently crystallize to zeolite. 

Crystallization was followed by analyzing the solid product quantitatively by x-ray powder diffraction. Prepared mixtures of a standard sample of mordenite and the amorphous substrate of mordenite composition were used to establish a calibration curve for the quantity of mordenite based on the summation of x-ray peak intensities. For zeolites A and X, the unreacted aluminosilicate gel was used to prepare mixtures with standard samples of zeolites A and X for quantitative phase identification. 

The principle of ion exchange is based on the fact that some solid materials exchange ions when in contact with a solution. The most commonly used materials are the resin ion exchangers. These have replaced most of the older aluminosilicate or hydrated oxide materials, and are available in a number of types and particle sizes. 

Wuest et al. have also prepared a related tetrahedral tecton 8.63, which also produces a diamondoid polymeric framework. In this case, the solid-state network is seven-fold interpenetrated, with one diamondoid lattice filling much of the large cavities in those adjacent. It is possible that the interpenetration in this instance is a result of the self-complementary nature of the host, which contains an equal number of hydrogen bond donor and acceptor sites. However, even in this case small cavities exist, which are filled by two molecules of butyric acid per host formula unit. The formation of these kinds of framework materials opens entirely new possibilities for tailor-made porous materials with very large cavities, although it is unlikely that purely organic frameworks will ever rival aluminosilicate-based materials for sheer mechanical strength. 

Oxide catalysts fall into two general categories. They are either electrical insulators or they can act as semiconductors. Insulator oxides are those in which the cationic material has a single valence so they have stoichiometric M 0 ratios. The simple oxides, MgO, AI2O3 and Si02 and the more complex zeolites, which are aluminosilicates, fall into this category. These materials are not effective as oxidation catalysts and find most use as solid acids or bases.2-3... 

Figure 5.12. Pair distribution function of the first maximum derived from large-angle scattering measurement of a 50 mol% AEOj aluminosilicate glass (solid line) compared with the calculated profile (dotted line) derived from Al MAS NMR estimates of the Al site distributions. A. Based on a model in which both the 60 and 30 ppm resonances are assumed to be tetrahedral,...

In 1968, Breck and Flanigen for the first time proposed the solid-phase mechanism based on their studies of the crystallization of aluminosilicate. They found that the formation and transformation of amorphous aluminosilicate hydrogel always happened during the crystallization process of zeolites and that the composition of the hydrogel was similar to that of the resultant zeolites. 

In the spectral area with low Si/Al ratio, besides a strong peak for the species of Al(OH), several broadened, weak peaks in the range 72 80ppm could be found in the 27A1-NMR spectra, which is a typical feature of 5-coordinated intermediates. This structure is unstable in solution, but very stable in the solid phase. The aluminate compounds containing 5-coordinated Al species can be found in nature. One example is andalusite. To interpret the polymerization reaction that occurred in aluminosilicate solution, 5-coordinated aluminate was assumed to be existed. Based on the assumption of the presence of 5-coordinated aluminate, Dent Glasser and colleagues proposed another mechanism (described below) for the polymerization of silicate and aluminate ions in the solution,[41] which was named the 5-coordinated intermediate mechanism or base-delayed mechanism. The reactions will follow the steps described in Equation (5.7) ... 

---