Aluminum atoms, distribution

The ground state distribution of electrons in the aluminum atom is lT2T2 3T3/). The oxidation state of aluminum is +3, except at high temperatures where monovalent species such as AIQ, AIF, and AI2 have been spectrally identified At lower temperatures, these compounds disproportionate... 

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. ... 

Since the first structure determination by Wadsley [56] in 1952 there has been confusion about the correct cell dimensions and symmetry of natural as well of synthetic lithiophorite. Wadsley determined a monoclinic cell (for details see Table 3) with a disordered distribution of the lithium and aluminium atoms at their respective sites. Giovanoli et al. [75] found, in a sample of synthetic lithiophorite, that the unique monoclinic b-axis of Wadsley s cell setting has to tripled for correct indexing of the electron diffraction patterns. Additionally, they concluded that the lithium and aluminum atoms occupy different sites and show an ordered arrangement within the layers. Thus, the resulting formula given by Giovanelli et al. 

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. 

The calculation permits a transformation of the 29Si NMR intensities to give distributions of aluminum atoms in faujasite with implications for the numbers of strong and weak acid sites available. 

The random A1 siting method of reference (7) was used to compute 29Si NMR intensities for comparison with experimental results reported in reference (2). The results in Table II show clearly some discrepancy between the experimental and calculated results. The variance a2 ranges from 35 to 329. The discrepancy is greatest at higher Si/Al ratios where the experimental distribution is much sharper than is expected of the maximum probability distribution of silicon and aluminum atoms. These results imply some ordering of the aluminum atoms in the lattice. 

We have shown that four of the nine possible prisms containing three or four aluminum atoms are sufficient to describe the 29Si MASNMR data. Are all four necessary It is difficult to answer that question since distributions derived from each of the four identified structures can be approximated by combinations of other stuctures. For example there is a combination of Na a1 and N3°Ma3 that can give the same relative distribution of silicon environments as Na a0. 

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. 

There are several aluminum boride phases known. One of the more common, AIB12, is employed in the manufacture of lightweight armor, as a catalyst for organic reactions, and as a economical replacement for diamond abrasive. It has a complex stmcture composed of B12 icosahedra, B19 units (twinned icosahedra), and single boron atoms in a 2 1 1 ratio. The aluminum atoms are distributed statistically over all the boron sites. AIB12 is synthesized by direct combination of the elements at 1100 °C or by reaction of aluminum with Na4B40y at 1100 °C. 

Mordenite is a naturally occurring zeolite that has a Si Al ratio near ten. Synthetic mordenites also have this same narrow range Si Al ratio, a fact that suggests that the aluminum atoms are distributed in an orderly manner in this zeolite rather than in the presumed random orientation in the other zeolites. The central feature of mordenite is a slightly oval shaped 12-ring with a 0.67 x 0.7 run... 

Figure 2.13 shows SIMS profiles of the boron and aluminum atoms at different temperatures (2000 and 2200 °C) after diffusion for 10 min into porous SiC layer with a thickness of 2.7 pm. It is clearly seen that the distribution of aluminum atoms in a porous layer is practically uniform and constant for both temperatures, while the distribution of boron atoms is uniform only for the lower temperature of diffusion. At the higher temperature, the maximum concentration of boron atoms is greater near the surface than that after diffusion at lower temperature, and yet it gradually decreases within the porous layer. Note that the concentration of aluminum in the porous layer is the same for both temperatures of diffusion, while the concentrations of boron atoms fit their values for different temperatures only at the interface porous layer-substrate. Once... 

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. 

Structure A can easily give up its weakly bound proton and hence function as a Bronsted acid. Structure B possesses electron-accepting properties by virtue of the aluminum atom which contains an empty -orbital. Reversion of the aniline spectrum to that of benzene will occur when the excess electron density distributed to the ring by the amino group becomes localized on the nitrogen atom. Thus proton addition to the amino group in aniline produces the anilinium ion whose... 

The X-ray mapping of both catalysts reveals a homogeneous distribution of aluminum and silicon atoms for both supports related to the use of sol-gel method. However, the platinum atom distribution is more heterogeneous in relation with the high active phase percentage leading to large platinum particles disclosed also by TEM measurements. ... 

All transitional aluminas crystallize in disordered (and distorted) defect spinel lattices. The unit cell contains a cubic close-packed array of 32 0 ions. Electroneutrality demands that of the 24 sites in the cation sublattice only 21f are occupied, leaving 2 vacant positions. In essence, the various transitional aluminas differ in the uniformity of the anionic stacking and in the distribution of aluminum atoms over the octahedral and tetrahedral sites. In contrast to the AIO(OH) and Al(OH)3 precursors and to a-Al203, the transitional aluminas show a significant fraction of tetrahedral aluminum sites, which are readily identified and quantitated by Al MAS-NMR 58,59], The fraction of four-coordinate aluminum sites decreases in the order t] 7 8 0, in accordance... 

The somewhat random distribution of atoms required in the amorphous structure of the silica-alumina cracking catalyst makes it unreasonable to suggest a single exact distance between the aluminum atoms in the active sites along the edges of the catalyst ribbon. Furthermore, catalysts formed by impregnation of silica gel with alumina may be represented as forming by the condensation of aluminum hydroxyls with hydroxyls on adjacent... 

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