Structure 74-5 Aluminosilicates

The liquid-phase autoxidation of cyclohexane is carried out in the presence of dissolved cobalt salts. A lot of heterogeneous catalysts were developed for this process but most catalysts lacked stability. The incorporation of cobalt ions in the framework of aluminophosphate and aluminosilicate structures opens perspectives for heterogenization of this process. CoAPO (cobalt aluminophosphate) molecular sieves were found to be active heterogeneous catalysts of this oxidation.133 Site isolation was critical to get active catalysts.134... 

Table 1.4 lists some of the major new structures reported in the 1990s. Interestingly, as organic SDAs tended to dominate discovery of new frameworks, there were no new aluminum-rich synthetic zeoUtes reported in either the 1980s or the 1990s. The new aluminosilicate structures were all high silica or pure sihca in composition. It awaited the 2000s for new aluminosilicate zeolite materials with low to medium Si/Al to be reported (see below). 

Microcalorimetry experiments with NH3 and pyridine as probe molecules indicated that insertion of Ga into the offretite aluminosilicate structure increased the overall acid sites strength of the crystals while decreasing its acid sites density [255], The observed heterogeneity of acid site strength distribution of H,Ga,Al-offretites was attributed to some extra-framework Al(Vl) and Ga(Vl) species generated during the ion exchange and calcination procedures used to prepare H-offretite crystals. 

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). 

The paper deals with some new data concerning the state of the metal after reduction and the catalytic functions of zeolite catalysts containing nickel and platinum. By using the molecular sieve selectivity in the hydrogenation of mesitylene it has been proved that metal (platinum) is contained in the volume of the zeolite crystal. The temperature dependence of the formation of nickel crystals was investigated. The aluminosilicate structure and the zeolite composition influence mainly the formation of the metal surface which determines the catalytic activity. In the hydrocracking of cumene and disproportionation of toluene a bifunctional action of catalysts has been established. Hydrogen retarded the reaction. 

The commercially available zeolite adsorbents consist of small microporous zeolite crystals, aggregated with the aid of a clay binder. The pore size distribution thus has a well-defined bimodal character, with the diameter of the intracrystalline micropores being determined by the crystal structure and the macropore size being determined by the crystal diameter and the method of pelletization. As originally defined, the term zeolite was restricted to aluminosilicate structures, which can be regarded as assemblages of SiC>2 and AIO2 tetrahedra. However, essentially... 

As a conclusion, HM++ ions are not exclusive templates for a given structure but will stabilize an aluminosilicate structure that is preliminarily favoured by other variables such as the Al content in the gel. They will act partly as counterions to the negative framework, partly as pore fillers. Obviously, they exclusively act as pore fillers in the Al free gels, in which alkali cations can be present (this work) or absent (2.). ... 

FIGURE 7-25 An Example of an Aluminosilicate Structure, Illustrated is the space-filling arrangement of truncated octahedra, cubes, and truncated cuboctahedra. (Reproduced with permission from A. F. Wells, Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, 1975, p. 1039.)... 

The stabilizing effect of the aluminosilicate layer of DAY-Saim and DAY-T can be explained by the elimination of the terminal silanol groups and the blocking of the energy-rich Si-O-Si bonds at the crystal surface, where the water molecules attack the framework. In this case, the question of stability of high-silica faujasites is transferred to the question of stability of the aluminosilicate structures and, accordingly, transferred from the alkaline to the acid mechanism of decomposition which is rate-controlling. 

An important class of minerals called aluminosilicates results from the replacement of some of the Si atoms in silicates with aluminum (Al) atoms. Aluminum is the third most abundant element in the earth s crust (8% by mass), where it occurs largely in the form of aluminosilicates. Aluminum in minerals can be a simple cation (Al ), or it can replace silicon in tetrahedral coordination. When it replaces silicon, it contributes only three electrons to the bonding framework in place of the four electrons of Si atoms. The additional required electron is supplied by the ionization of a metal atom such as sodium (Na) or potassium (K) the resulting alkali-metal ions occupy nearby sites in the aluminosilicate structure. 

The aim of the study was to analyze the suitability of the use of new supports (different aluminosilicate structures based on modified clays in zeolitic products and PIL-clays) in... 

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