Iron aluminophosphate

A very unusual natural microporous iron aluminophosphate which exhibits reversible adsorption properties called cacoxenite (AlFe2405(0H)i2(P04)i7. (H20)24 ca. 51 H2O) contains a unidimensional channel system with 14.2 A pore opening [16,17]. In this structure the iron exists in octahedral coordination with aluminum occupying a distorted octahedral position within the framework. This phosphate mineral was the first known example of a microporous material with a pore size larger than 7 A. It imdergoes structural collapse when heated. 

The products of such reactions depend upon the pressure, temperature, pH, phosphate, and cation concentrations, and may be difficult to predict or rationalize. For example, the equation (3) produces an acid phosphate and a phosphate hydroxide. Microporous aluminophosphates and related phases (see Section 5.1.2) are prepared in hydrothermal bombs using hydrated cations or molecular templates such as organic amines or ammonium cations to direct the porous framework. Many new structures with metal phosphate chains, layers, or three-dimensional networks have been prepared hydrothermally in recent years, for example, templated vanadium phosphates and iron phosphates. ... 

The replacement of aluminium by bivalent cations such as Zn + and Mn2+ in aluminophosphate molecular sieves results in a negative charged framework and gives rise to the formation of acidic sites, which are known to act catalytically. Most transition metals do not fulfil the requirements for isomorphous substitution and are found to be extra-framework. There are only three well-established cases of isomorphous substitution iron, cobalt and zinc. ... 

Manganese- and iron-substituted aluminophosphates have also been synthesized but they have proven to be particularly unreactive oxidation catalysts [72]. This lack of redox activity may be a result of the stable environment of isomorphously substituted Mn " and Fe" . Interest in iron-substituted molecular sieves is derived from he fact that many redox enzymes contain iron in their active site. Hence, with this element one is obliged to adopt the alternative approach of encapsulation. 

P-39 - Structure of Fe(III) sites in iron substituted aluminophosphates a computational and X-ray spectroscopic investigation... 

Iron substituted aluminophosphate molecular sieves (Fe-AlP04-l 1, Fe-AlP04-5 and Fe-VPI-5) are catalytically active in oxidations of aromatic compounds such as hydroxylation of phenol, benzene, and naphthol, as well as epoxidation of styrene. Catalytic data show that the activities of Fe-AlP04-l 1, Fe-AlP04-5 are comparable with that of TS-1 in the oxidation of aromatic compounds. Furthermore, Fe-VPI-5 shows high activity in naphthol hydroxylation by H2O2, while TS-1 is completely inactive due to the small pore size. By comparison of various catalysts, Fe (III) in the framework is considered to be the major active site in the catalytic reactions. 

Figure 9.1 Framework structures of aluminophosphates-34, -18, -31, -5 and -36, with framework oxygen atoms shown with van der Waals radii. Once doped with cobalt, iron or manganese, these materials can act as shape selective oxidation catalysts, for example in the aerial oxidation of alkanes and aromatics.

In 1982 a major discovery of a new class of aluminophosphate molecular sieves was reported by Wilson et al. By 1986 some 13 elements were reported to be incorporated into the aluminophosphate frameworks lithium (Li), beryllium (Be), boron (B), magnesium (Mg), silicon (Si), titanium (Ti), manganese (Mn), iron (Fe), cerium (Ce), zinc (Zn), gallium (Ga), germanium (Ge), and arsenic (As). lliis new generation of molecular sieve materials designated AlP04 based molecular sieves comprise more than two dozen structures and about 200 compositions. 

Probably the most common secondary phosphate minerals produced by weathering are members of the crandallite series, Wavellite is also a common aluminophosphate weathering mineral. Iron phosphates... 

Iron. Incorporation of iron into different aluminophosphate frameworks is typically investigated using the UV-vis DRS, EPR, IR, Mossbauer, and NMR spectroscopies (Tables 9 and 10). Although Messina and co-workers in the first report on FAPO-5 supposed (based on the chemical analysis) that the substitution... 

Table 10. Structural Parameters of the First Coordination Shell around Fe Atom in the Template-Free Microporous and Mesoporous Iron-Modified Aluminophosphate Structures...

Iron-substituted mesoporous aluminophosphate was used as a novel, efficient and ecofriendly catalyst for reductive cleavage of azo dyes and reduction in nitroarenes and catalytic transfer hydrogenation of nitro and carbonyl compounds. Unlike most of the iron-containing molecular sieves, dislodgement of tetrahedral Fe(III) was not observed upon various process treatments such... 

Related compounds can be prepared using different metal atoms, like iron, ruthenium, copper or tin, and with differing guest species like ferrocene, co-baltocenixim, tetra- -butylanunonium, tetraethylanunonium, and methylvio-logen. The list of guest species su ests a strong similarity to aluminosilicate or aluminophosphate molecular sieves, since several of these molecules are typical templates for zeolite synthesis. 

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