Aluminum atomic size

Aluminum distribution in zeolites is also important to the catalytic activity. An inbalance in charge between the silicon atoms in the zeolite framework creates active sites, which determine the predominant reactivity and selectivity of FCC catalyst. Selectivity and octane performance are correlated with unit cell size, which in turn can be correlated with the number of aluminum atoms in the zeolite framework. ... 

Unit Cell Size (UCS). The UCS is a measure of aluminum sites or the total potential acidity per unit cell. The negatively-charged aluminum atoms are sources of active sites in the zeolite. Silicon atoms do not... 

Zeolites are aluminosilicates characterized by a network of silicon and aluminum tetrahedra with the general formula Mx(A102)x(Si02)Y. The M are cations that are necessary to balance the formal negative charge on the aluminum atoms. The tetrahedra are linked to form repeating cavities or channels of well-defined size and shape. Materials with porous structures similar to zeolites but with other atoms in the framework (P, V, Ti, etc.), as a class are referred to as zeotypes. The structure committee of the International Zeolite Association (IZA http //www.iza-online.org/) has assigned, as of July 1st 2007, 176 framework codes (three capital letters) to these materials. These mnemonic codes do not depend on the composition (i.e. the distribution of different atom types) but only describe the three-dimensional labyrinth of framework atoms. 

It has already been mentioned that the formation of ultrastable Y zeolites has been related to the expulsion of A1 from the framework into the zeolite cages in the presence of steam (8,9), and the filling of framework vacancies by silicon atoms (11,12). This results in a smaller unit cell size and lower ion- exchange capacity (6). It also results in a shift of X-ray diffraction peaks to higher 20 values. Ultrastable Y zeolites prepared with two calcination steps (USY-B) have a more silicious framework than those prepared with a single calcination step (USY-A). Furthermore, since fewer aluminum atoms are left in the USY-B framework, its unit cell size and ion-exchange capacity are also lower and most of the nonframework aluminum is in neutral form (18). 

Some vibrational modes of zeolites are sensitive to the amount of aluminum in the framework [93]. The substitution of aluminum for silicon atoms in the zeolite framework changes the T-O-T bond angles (where T is a tetrahedral atom that can be either Si or Al). This is primarily due to the smaller size and different charge density of the aluminum atoms compared to silicon. This results in a shift in frequency for vibrational modes in the zeolite due to external linkages. The T-O-T asymmetric (1100-980 cm ) and symmetric (800-600 cm ) stretching modes as well as structural unit vibrations Mke double four- and double six-rings exhibit a shift to lower frequencies as the aluminum content of the framework is increased. Figure 4.19 shows this relationship for a faujasite-type framework. 

All silver crystals have the same geometric shape. Therefore, the crystalline shape of a metallic solid is a function of the size of the metal solid atoms and their electron configuration. Each metal has its own geometric crystalline shape. Aluminum atoms pack into a face-centered cubic cell. Iron s solid structure is body-centered cubic. 

The silicon-to-aluminum atomic ratio was measured in 30-50nm steps across individual zeolite particles in the size range 0.1-2um. 

The next stage of characterization focuses upon the different phases present within the catalyst particle and their nature. Bulk, component structural information is determined principally by x-ray powder diffraction (XRD). In FCC catalysts, for example, XRD is used to determine the unit cell size of the zeolite component within the catalyst particle. The zeolite unit cell size is a function of the number of aluminum atoms in the framework and has been related to the coke selectivity and octane performance of the catalyst in commercial operations. Scanning electron microscopy (SEM) can provide information about the distribution of crystalline and chemical phases greater than lOOnm within the catalyst particle. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) can be used to obtain information on crystal transformations, decomposition, or chemical reactions within the particles. Cotterman, et al describe how the generation of this information can be used to understand an FCC catalyst system. 

The detailed geometries of the surface alloys reveal that the bond lengths between the alkali and aluminum atoms are close to the sums of the corresponding metallic radii, suggesting that the bonding is essentially metallic. Several geometrical properties of the surface alloys follow from simple considerations of the size of the atomic radii. For example, in the substitutional Al(lll)—(.ys X 3)/ 30°—Li structure, Li atoms lie almost in the plane of the first A1 layer, whereas in the corresponding Rb structure, Rb atoms are located much further from the surface plane, as shown in Fig. 8. Also, the smaller Li and Na atoms are the only alkali atoms which can occupy sites not directly... 

Surprisingly, aluminum cluster reactivity with O2 depends sensitively on the cluster size. The atom and dimer are very reactive, but a sharp decrease in reactivity is observed for the trimer. Above the trimer a nearly montonic increase in reactivity occurs as the number of aluminum atoms increases. By n = 25-30 the clusters are once again nearly as reactive as the dimer. The dominant product peak is AI3O2, but it is not produced directly through chemisorption of O2 onto an AI3 cluster. The species appears to be the product... 

The protonic sites at the surface of amorphous silica are weakly acidic compared to the acid centers in crystalline aluminosilicates (e.g., zeolites). On ZSM-5, Bronsted sites are formed by protons adjacent to aluminum atoms in the tetrahedral framework. The concentration of acid sites increases with the aluminum content. The total acidity as well as the acid strength distribution can be determined by using n-butylamine and Hammet or arylmethanol indicators (25). Depending on the pKa of the indicator, a relative scale of the strength distribution is obtained (54). Results for a series of amorphous porous silicas of graduated pore size are shown in Figure 10. The acidity varies between pKa -1 and +9 and is nearly the same for all silicas studied. The results are specifically valid for this method only and cannot be compared with those derived from other methods. 

Similarly, the small size of the boron atom may account for BH3 forming a volatile dimer, while AIH3 forms a nonvolatile polymer in which each aluminum atom is surrounded by six bridging hydrogen atoms ). 

While the valence angles around the aluminum atom are indistinguishable from those in [( H3)2A1F]4, the < Al— 1—Al angle is only 91°. That this angle is less than tetrahedral is in agreement with the valence-shell electron-pair repulsion model. learly the greater size of the chlorine atom reduces the importance of repulsion between the metal atoms. 

In some ways zeolites are similar in their properties in that they also provide size and shape selectivity because of the presence of pores within the three-dimensional structure. However in the zeolites these pores are generally much larger so that larger molecules can be incorporated into the structure. Zeolites are crystalline aluminosilicates which occur both in nature and as synthetic structures [204]. The zeolite framework is made up of aluminum and silicon atoms tetrahedrally coordinated by oxygen, giving a framework stoichiometry of MO2. For each aluminum atom within the framework there is a formal charge of —1 which is compensated by counterbalancing cations within pores in the structure. Typically these may be cations such as Na" ", Ca ", NH4, or HsO". ... 

Figure 8.17 The trend in acid-base behavior of eiement oxides. The trend in acid-base behavior for some common oxides of Group 5A(15) and Period 3 elements is shown as a gradation in color (red = acidic blue = basic). Note that the metals form basic oxides and the non-metals form acidic oxides. Aluminum forms an oxide (purple) that can act as an acid or as a base. Thus, as atomic size increases, ionization energy decreases, and oxide basicity increases.

Aluminum particle size and shape play a definite role in determining glitter performance. For example, spherical atomized particles seem to produce longer delays and larger flashes. Also, larger particles tend to produce whiter flashes, while smaller particles tend to produce yellower flashes especially if the composition contains a sodium salt. 

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