Aluminophosphates framework structured materials

Aluminophosphate-Based Molecular Sieves In 1982 a major discovery of a new class of aluminophosphate molecular sieves was reported by Wilson et al. [26]. By 1986 some 13 elements were reported to be incorporated into the aluminophosphate frameworks Li, Be, B, Mg, Si, Ti, Mn, Fe, Co, Zn, Ga, Ge and As [27]. These new generations of molecular sieve materials, designated AlP04-based molecular sieves, comprise more than 24 structures and 200 compositions. 

The most obvious approach to obtain controlled mesoporosity is the extension of zeolite-like three-dimensional framework structures to larger pore-sizes. Zeolite-like compounds in the micro- and mesoporous range are the aluminophosphate VPI-5, cloverite and ULP (MCM-41). These new materials are characterized by pore diameters between 1.3 nm and 20 nm. 

Zeolites and related aluminosilicates constitute a vital family of microporous materials with immense applications in catalysis, sorption and separation processes [1-3]. The discovery of aluminophosphates is an important landmark in the science of these materials [4], All these materials are, in general, synthesized under hydrothermal conditions by making use of template molecules [2]. The template molecules are usually organic amines and they are involved in the formation of these framework structures in more ways than one. While it is difficult to pinpoint the exact manner in which the amines participate or direct the formation of these inorganic structures, it is generally believed that their size and shape are crucial in determining the pore structure. In recent years, a variety of open-framework structures formed by divalent metal phosphates... 

A novel class of crystalline, microporous aluminophosphate phases has been discovered. It represents the first class of molecular sieves with framework oxide compositions free of silica. The new class of materials encompasses some fourteen reported three-dimensional microporous framework structures, and six two-dimensional layer-type structures. The three-dimensional structures include structural analogues of the zeolites sodalite and erionite-offre-tite. The novel phases can be synthesized hydro-thermally in the presence of organic amines and quaternary ammonium templates. The template is entrapped or clathrated within the crystallizing aluminophosphate network. After thermal decomposition of the template the three-dimensional molecular sieves have the general composition of Al303 1.0 ... 

Aluminophosphates (AIPO4S) are a class of materials which, like the aluminosilicate zeolites, assume open framework structures containing channels of molecular dimensions with molecular sieve properties. Since the AIPO4 structures contain equal numbers of AIO4 and PO4 units there is no necessity for charge-balancing extraframework cations and consequently no sites to provide acid catalytic properties. 

The synthesis of zeolites and zeolitic materials has been pursued for nearly 50 years (2), and the literature is filled with reports of structures, methods of preparation, and uses for these materials. The substitution of aluminum or silicon in the framework structure has been performed using many main group elements (3.41 as well as some transition metals (3.5). New families of molecular sieves which are based on an aluminophosphate framework have been reported recently, some of which are also microporous (6.7). Of the various new materials which have been reported, this review will focus on crystalline borosilicate molecular sieves. 

Further variation of the structural and catalytic properties of four-connected tetrahedral frameworks is obtained by the substitution of silicon or metal cations, giving materials known as SAPO s and MeAPO s, respectively. More than twenty metal aluminophosphate frameworks have been identified with Mg, Mn, Fe, Co, or Zn substituents. These give the possibility of framework redox activity (e.g. Fe +/Fe +) in catalysis as well as the usual Bronsted acidity. For further information about zeolitic and microporous phosphate frameworks see Porous Inorganic Materials and Zeolite and recent reviews. 

However, the incorporation of metal cations whose valence is different from that of A1 or P leads to the formation of electronically unsaturated sites, as schematically shown in Figure 3. This addition of aliovalent metal cations into the lattice of AlPO-n generates solid acidity and ion-exchange sites. There are numerous reports on the incorporation of many different metal cations into the lattice of AlPO-n. Table 2 summarizes the reported isomorphous substituted AlPO-n. The family of AlPO-n substituted with metal cations is generally called metal aluminophosphates (MeAPO-n). The typical metal cations substituted into AlPO-n are Li, B, Be, Mg, Ti, Mn, Fe, Co, Zn, Ga, Ge, Si, and As. The Si-substituted AlPO-n is called a silicoaluminophosphate and denoted as SAPO-n, where n also means the framework structure, and it is distinct from the MeAPO-n materials.SAPO-n exhibits both structural diversity and compositional variation. In particular, the crystal structure of SAPO-n is of greatest interest, because the distribution of the Si atom in the framework is quite complicated. Some crystal structures, such as SAPO-40, are only found in SAPO-n and not in AlPO-n or zeolite. The mole... 

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.

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). 

There are only a few reports on the preparation of open-framework hybrid aluminophosphate-based materials, since the majority of the studies on hybrid aluminophosphates resulted in formation of chain- or layered-structures (98). In 2000, Kedamath and co-workers reported the first framework structure of aluminophosphate-oxalate possessing large circular channels similar to that of... 

The characteristics of aluminophosphate molecular sieves include a univariant framework composition with Al/P = 1, a high degree of structural diversity and a wide range of pore sizes and volumes, exceeding the pore sizes known previously in zeolite molecular sieves with the VPI-5 18-membered ring material. They are neutral frameworks and therefore have nil ion-exchange capacity or acidic catalytic properties. Their surface selectivity is mildly hydrophilic. They exhibit excellent thermal and hydrothermal stability, up to 1000 °C (thermal) and 600 °C (steam). 

The first part of the book documents the history, structure, chemistry, formulation and characterizations of zeolites in Chapters 1-4. The past 60 years have seen a progression in molecular sieve materials from aluminosilicate zeolites to micro-porous silica polymorphs, microporous aluminophosphate-based polymorphs, metallosihcate and metallophosphate compositions, octahedral-tetrahedral frameworks, mesoporous molecular sieves and, most recently, hybrid metal organic frameworks (MOFs). 

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