Metal-substituted aluminum phosphate

The influence of both bivalent and trivalent metal substituent from a range of metal cation (Co, Mn, Mg, Fe, and Cr) on the acidic property (both Bronsted and Lewis) of metal-substituted aluminum phosphate MeAlPOs is monitored [91]. The influence of the environment of the acid site is studied both by localized cluster and periodic calculations to propose that the acidity of AlPOs can be predictable with accuracy so that AlPO material with desired acidity can be designed. A semiquan-titative reactivity scale within the domain of HSAB principle is proposed in terms of the metal substitutions using DFT. It is observed that for the bivalent metal cations Lewis acidity linearly increases with ionic size, whereas the Bronsted acidity is solely dependent on the nearest oxygen environment. Intramolecular and intermolecular interactions show that once the active site of the interacting species is identified, the influence of the environment can be prescribed. Mg" -doped AlPO-34 exhibits highest Bronsted acidity, whereas Cr -doped species shows lowest acidity. Fe -Fe -doped AlPO-34 shows highest Lewis acidity, whereas Mn", Mg" shows lowest acidity. 

These structures are unique for several reasons. First, they represent three new multidimensional 12-MR systems, which are rare even among zeolites. Second, the amount of framework substitution by metals such as Mn2+ and Mg2+ was unknown prior to this series. Also, the ease of forming both gallium and aluminum phosphates appear to be comparable. Finally, it would appear the charge-matching approach has proven to be a successful strategy for the synthesis of new molecular sieves. It is not clear whether these materials are thermally or hydrothermally stable but they do represent novel pore structures that should impart some unusual properties. 

Aluminum Phosphate-Based Molecular Sieves. Although still in the early stages of development, aluminum phosphate-based molecular sieves have shown great promise as a microporous catalyst and an adsorbent material. They were first reported in 1982 with a neutral aluminum phosphate (AlP04)-n framework (15), Si, other metals (such as Be, Mg, Ti, Mn, Cr, Fe, Co, and Zn), and other elements (such as Li, B, Ga, Ge, and As) have subsequently been incorporated into the AIPO4 framework these substitutions have allowed additional applications for the catalyst. Some of the applications reported are catalytic dewaxing, hydrocracking, methanol conversion, and toluene alkylation. 

In addition to aluminosilicates, crystalline microporous materials can be phosphate-based. The aluminophosphate (A1P04) framework is electroneutral (analogue of Si02), and the aluminum and/or phosphorus tetrahedral atoms can be substituted by a number of metal and non-metal atoms that result in producing charged frameworks [1-3], e.g. Si+4 substitution for P+5. In addition, numerous other metal oxide, and nitride based microporous materials have been reported recently [4, 5]. 

What the high concentration of fluoride in the solution does is to induce the formation of a soluble tetracoordinated state of aluminum, which has the same geometry, size, and coordination as a phosphate. Among many metals tested in the presence of fluoride, Sternweis and Gilman [14] found that only beryllium could substitute for aluminum for the activation of adenylate cyclase. This fact reinforced the assumption that aluminum acts through its tetrahedral phosphate-like complex AlF4 1, because all beryllium complexes are tetracoordinated [16]. 

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