Tetrahedral aluminum

According to El-Mashri et al.,190 the A106 A104 ratio determines the hydration capacity of anodic oxides. Tetrahedral sites are hydrated easily to form a boehmite-like structure, which is known to be composed of double layers of Al-centered octahedra, weakly linked by water molecules to other layers.184 As the oxide formed in H3P04 contains about 70% tetrahedral aluminum bonds, its hydration ability should be higher than that of the oxide formed in tartrate solution. However, this has not been found in practice, which is interpreted by El-Mashri et al. as being due to some reduction of A104 by incorporated phosphate species. 

Figure 2 (Left) shows the 27Al NMR spectra for the aluminosilicates. All of them displayed a tetrahedral incorporation of aluminum inside the silica network. That is corroborated by the signal at 55 ppm [9, 10] which also become more intense with the decreasing of Si/Al ratio. Octahedral aluminum was observed just for the samples with the lowest Si/Al ratio. Tetrahedral aluminum gives place to strong Bronsted acid sites, which were identified by the interaction of these groups with pyridine that generates a...

Thermal dealumination. The method involves calcination of the ammonium (or hydrogen) form of the zeolite at relatively high temperatures (usually over 500°C) in the presence of steam. This results in the expulsion of tetrahedral aluminum from the framework into non-framework positions, but does not remove the aluminum from the zeolite. The process consists essentially in a high-temperature hydrolysis of Si-O-Al bonds and leads to the formation of neutral and cationic aluminum species (Figure 1A). 

Aluminum isopropoxide is shown as a monomer in Scheme 14, but it is known to exist in several oligomeric forms (114). Freshly distilled Al(OIV)3 is mainly trimeric and is slowly transformed to a tetrameric form (109) in which a central octahedral aluminum atom is surrounded by three tetrahedral aluminum atoms (112). 

The A1 NMR spectra of three steam-dealuminated HY zeolites are shown in Figure 1. They Sjt w one intense line centered at 60 ppm (tetrahedral aluminum, A1 ) d two more lines at ppm and 0... 

Al MAS-NMR spectra of as-shynthesized samples of Si/Al 10 and 30 show a peak at 54-56 ppm corresponding to tetrahedral aluminum (15). After calcination in air, a peak centered at O ppm corresponding to octahedral aluminum (15) appears in the spectrum of the sample with the lowest Si/Al ratio. When the samples are exchanged two times with NHJ ions and calcined again, the octahedral aluminum signal also appears in the spectrum of the zeolite with Si/Al=30. 

Zecchina (123) mentioned the appearance of two complex groups of OH-stretching bands at 3800 to 3780 cm-1 and 3740 to 3690 cm-1, respectively, on surfaces of rj- and 7-Al2 03. The first group of bands was assigned to hydroxyls belonging to the coordination sphere of tetrahedral Al3+ ions, and the second group to hydroxyls shared by an octahedral and a tetrahedral aluminum ion. 

NMR peaks in the range of -187 to -194 ppm (19F-NMR) and 49 to 52 ppm (27Al-NMR) were observed for the TFA salts prepared via anhydrous routes. Thus, they correspond to tetrahedral aluminum coordinated by four fluorine atoms. This is demonstrated, for example, in the coupling for [PSH] [TFA] in CD3CN, which was observed as a sextet from 19F-27Al in the 19F-NMR, and a quintet from 27Al-19F in the solid state 27Al-NMR. 

AIPO4-5 has one- dimensional pores of wider and narrower parts whose widths are 1.137 and 1.002 nm, respectively. Also well-crystalline samples are available to provide reliable experimental data. The framework of AIPO4-5 consists of alternate tetrahedral aluminum and phosphorous atoms bridged by oxygen atoms and thus is electrically neutral. The pores of AIPO4-5 are not interconnected and form one-dimensional channels parallel to the crystallographic c -axis [42]. 

Figure 16 is a plot of versus Af, the calculated tetrahedral aluminum content of the pyroxene per formula unit, based on microprobe... 

The collapse to an amorphous phase is the most pronounced effect accompanying treatments of zeolites at elevated temperatures. In addition, heating results in changes in the framework composition, caused by movement of the tetrahedral aluminum into extra-lattice positions. 

AI2O3 samples calcined at various temperatures was studied as tetrahedral aluminum (AF) concentration could be linked to acidic dehydrating sites on the alumina surface [4]. 

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