Aluminophosphates four-coordinated

Br0nsted Acidity in Substituted Four-Coordinated Aluminophosphates... 

In the compositional family of four-coordinated aluminophosphate materials, many structure types and topologies are available for the catalyst designer [2]. Among the substitution possibilities with ions of different valence, the use of Si to achieve thermally stable solids is obvious. Substitution with occurs mainly for A1 yielding neutral frameworks. With Si, isomorphic substitution... 

In the aluminophosphate family, three main structural classes with different metal to phosphorus ratios are known. The layered aluminophosphates normally have P/Al ratios of 4 3 and 3 2 and a new series with P/Al ratio of 2 1 has been reported [25]. The templates play an important role in the formation of these materials and there have been some attempts to rationalize the formation of these solids [25]. The two-dimensional architecture is generally formed by strictly alternating P04 and A104 tetrahedra. Of the four oxygen atoms coordinated to each tetrahedral phosphate, three are shared with adjacent A1 atoms,... 

The Al MASNMR spectrum of VPI-5 has also been reported earlier (7). The spectrum contains a major resonance near 37 ppm, a neighboring resonance at 28 ppm, and a small resonance below 0 ppm. The chemical shifts of these resonances are consistent with other hydrated aluminophosphate molecular sieves that contain tetrahedrally coordinated aluminum that are linked to four phosphorus atoms through bridging oxygen atoms (10,11). 

The studies presented here deal with the coordination of cobalt atoms in cobalt aluminophosphates (CoAPOs). The compounds investigated include those of CHA topology (CoMnAPO-34) [51] and of SOD topology [42].In the case of the CoMnAPO, a perfect fit to the filtered EXAFS function of the first coordination shell is obtained with four oxygen atoms at a distance Rco-o of 1-93 A. This result is consistent with the presumed incorporation of the Co atoms into the alu-minophosphate framework [51]. 

Several hundreds of synthetic zeolites (crystalline mieroporous aluminosilieates) and zeotypes (crystalline mieroporous ferrisilicates, gallosilicates, titanosilicates, isomorphously substituted aluminophosphates, etc.) have been sueeessfully synthesized in recent decades. All these mieroporous materials have tetrahedral coordination of their eentral atoms (Si, Al, P, Fe, etc.), which are interconnected with four oxygen bridges to form a three-dimensional crystal structure. These structures exhibit regular micropores with dimensions up to 1.0 nm and cavities, high surface areas, and adsorption capacities, and shape selectivity toward reactants, products, and transition states.. 

Mesoporous Aluminophosphates. The structure classification of amorphous mesoporous aluminophosphates is based on the pore arrangement that can be hexagonal, cubic, or disordered. Aluminum species in these materials are usually present as both four- and six-coordinated species, whereas phosphate units being always tetrahedral. The Al/P ratio differs from unity due to the uncompleted condensation of the mesoporous framework (16). 

In the as-synthesized CrAPO-5, the chromium(III) is octahedrally coordinated within the framework (four framework oxygen atoms and two water molecules are in the Ciflll) coordination sphere). During calcination, oxidation of chromium(III) to chromiumCVI) occurs leading to the formation of diox-ochromium(VI), which is still bonded to the internal aluminophosphate lattice. The acidic P-OH groups derived from the decomposition of template balance the charges (216). 

The abihty of the aluminiun to attain six coordination when associated with phosphate ligands results in the aluminophosphate molecular sieves exhibiting some unusual thermal properties. As discussed previously, D Yvoire s (n = 1 -6) materials were hydrated species with some of the aliunimun in octahedral coordination, four points attached to the phosphate in the framework, and the two remaining coordination sites occupied by molecules of water. The labihty of these water molecules is dependent on the structure. AIPO4S prepared fi-om organic-containing systems can also exhibit expanded coordination around the aluminum. 

Al and P solid-state NMR spectra obtained at high field for the aluminophosphate AlPO-36 show that the hydration process produces both 5- and 6-coordinate A1 sites. Al MQ MAS spectra of the hydrated form show at least four tetrahedral aluminium sites, as well as one octahedral and one five coordinate site. ... 

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