Aluminophosphate frameworks

The introduction of phosphate into the framework has led to an extraordinary expansion of the possible pore diameters in framework molecular sieves. The discovery of VPI-5, the first 16-membered-ring aluminophosphate framework, was a landmark (8). Although it is a neutral framework, it... 

The fraction of octahedrally coordinated aluminium species is much larger in thermally treated aluminophosphate mesoporous thin films than in thermally treated powders. The difference between thin films and powders might be due to a substrate effect that could retard the formation of a well-defined aluminophosphate framework. Indeed, it has been... 

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. 

Hydration of the NH4-form of SAPO-34 and SAPO-37, that is, of materials that were ammoniated at the bridging OH groups, caused a coordination of water molecules exclusively to Al atoms in =P-O-A1= bridges. This process led to a hydrolysis of the framework (220). No hydrolysis of the silicoaluminophosphate framework occurred, provided that not only the bridging OH groups (SiOHAl), but also the aluminophosphate framework (=P-O-A1=) was covered by ammonia. The latter finding may explain the stabilizing effect of preloaded ammonia on silicoalumino-phosphates toward hydration and weak hydrothermal treatments as recently observed for H-SAPO-34 (227). 

Synthesis of transition metal containing molecular sieves (microporous as well as mesoporous) is one of the fastest developing areas in molecular sieve science, as evidenced by recent published reviews [1,2] Several transition metals have been substituted into crystalline silica or aluminophosphate frameworks to yield the corresponding metallosilicate or metalloaluminophosphate molecular sieves, However, the location of the metal species and their state always remain uncertain, despite the employment of numerous different characterization methods comprising IR, NMR and ESR spectroscopy. 

One-electron Fe redox catalysts may also be immobilized by incorporation into aluminophosphate frameworks. Dugal el al. (143) reported the oxidation of cyclohexane to give adipic acid with air as the oxidant in the presence of Fe-AlPO-31. This molecular sieve has narrow pores, with a 0.54-nm diameter. Cyclohexane is easily adsorbed in the micropores, but desorption of initial products such as cyclohexyl hydroperoxide or cyclohexanone is slow. Consequently, subsequent radical reactions occur until the cyclohexyl ring is broken to form linear products that are sufficiently mobile to diffuse out of the molecular sieve ... 

P30s. The various structures exhibit intracrystalline adsorption pore volumes from 0.04 to 0.35 cm3/g, and pore sizes from 0.3 to 0.8nm. The aluminophosphate frameworks are hydrophilic. 

A new family of crystalline molecular sieves, 2) having aluminophosphate frameworks was synthesized. Strict alternation of A1 and P on the tetrahedral nodes yields neutral oxygens in contrast to the aluminosilicate zeolites, and non-framework cations are not needed for charge balance. Whereas a microporous silica (silicalite, 3 ) with neutral oxygens is hydrophobic, the aluminophosphate sieves are moderatley hydrophilic. 

Further variation of the stmctural and catalytic properties of four-coimected 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 Zeolites) and recent reviews. ... 

This method will not only aid the prediction of hypothetical aluminophosphate frameworks, but also serve as a tool to set up the initial structural models for the solution of unknown aluminophosphate structures. 

VPI-5 is an aluminophosphate framework with very large one-dimensional pores defined by 18-member ring[22]. The crystal structure report[7j of synthesized VPI-5 revealed the possible role of water molecules as templates. The use of VPI-5 as a molecular sieve and as a catalyst primarily depend on the removal of the occluded water molecules without the destruction of the framework structure. Prasad et al.[23] reported from their TGA experiment that the seven distinct types of water molecules could be desorbed from VPI-5 in a step-wise fashion, in the temperature range of 35 to 60°C. The cluster model calculations[24] have revealed the actual electronic interaction of water molecules with VPI-5 framework. CG technique also indicated that the void volume in VPI-5 could be controlled by the partial removal of water molecules. 

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. 

It should be noted that the synthesis of the CoAPO-20 compound dealt with here was not optimized and that the parent SOD compound of AIPO4 composition is difficult to prepare. The CHA-type compound demonstrates that Co can be incorporated on fourfold coordinated sites into aluminophosphate frameworks [51]. 

Substituted aluminophosphates are of interest as acid and oxidation catalysts. Typical substitutions include M for Al (M = Mg, Mn, Fe, Co, Zn), Si for and 2Si" for Al " -i- (described in Chapters 2 and 3). In cases where the degree of substitution is at a few percent, similar methods to those for substitutional metallosilicates can be used for confirmation (measurement of unit cell dimensions, P MAS NMR for measuring the substitutions of cations for A1 and Si NMR for measuring the mode of silicon substitution). For some of the metals, much higher metal substitutions are possible than for silicates, and complete replacement of aluminium by cations such as cobalt in tetrahe-drally connected frameworks has been reported (see Chapter 2). A number of other metals have been claimed to have been substituted into the aluminophosphate framework, including Ti" and C -3+ i03,i04... 

It is possible to oxidise and reduce atoms in the framework and also those within the pores of microporous (and mesoporous) solids of appropriate chemical compositions. Although pure aluminosilicate, silicate and aluminophosphate frameworks cannot be oxidised or reduced, transition metal and some lanthanide cations within the framework can exist in different oxidation states. Also, although typical alkali, alkali metal and most lanthanide cations in extraframework positions possess no redox chemistry, transition metal cations such as nickel, copper and platinum do. In the case of the transition metals, this enables them to become important catalysts. The included sulfide species in ultramarine-related pigments described above are also prepared through the reduction of sulfate species. 

Uytterhoeven L, Mortier WJ, Geerlings P (1989) Chaige distribution and effective electronegativity of aluminophosphate frameworks. J Phys Chem Solids 50 479-486 De Proft F, Langenaeker W, Geerlings P( 1995) A non-empirical electronegativity equalization scheme theory and applications using isolated atom properties. J Mol Struct Theochem... 

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