Microporous AlPOs/SAPOs

Besides the conventional zeolites, several novel zeolite analogues such as the ALPOs (aluminophosphates), MeALPOs (divalent-metal (Me) substituted aluminophos-phates), SAPOs (silicon substituted aluminophosphates) and so on have been synthesized (Davis Lobo, 1992). Wilson et al. (1982) first reported the synthesis of microporous ALPOs. ALPO synthesis differs from zeolite synthesis in that it involves acidic or mildly basic conditions and no alkali metal ions. Some members in the ALPO... 

Zeolites and zeotypes can be defined as microporous crystalline structures (Figure 1 and Table 1) in which the framework is formed by tetrahedral of silica, in which there is isomorphic substitution with trivalent or tetravalent elements such as for instance Al, Ge, B, Fe, Cr, Ge, Ti, etc. [1]. Similar types of structure can also be achieved with the framework formed by Al and P, with or without other transition metal elements [2]. These types of structure are denoted as AlPOs, SAPOs, and MEAPOs, depending on the composition of the framework [3]. 

The acidity of thermally stable mesoporous aluminophosphates (AlPO) and sili-coaluminophosphates (SAPO) has also been stndied by microcalorimetry [245]. By contrast with microporous crystalline alnminophosphate molecnlar sieves, mesoporous compounds are amorphous and characterized by Al/P ratios greater than 1. These particularities are responsible for a strong Lewis acidity, making these mesoporous materials more acidic than the microporons analognes, with an amonnt of strong acid sites that increases with the silicon content. 

The concept zeolites conventionally served as the synonym for aluminosilicates with microporous host lattice structures. Upon removal of the guest water, zeolites demonstrate adsorptive property at the molecular level as a result they are also referred to as molecular sieves. Crystalline zeosils, AlPO s, SAPO s, MAPO s (M=metal), expanded clay minerals and Werner compounds are also able to adsorb molecules vitally on reproval of any of the guest species they occlude and play an Important role in fields such as separation and catalysis (ref. 1). Inclusion compounds are another kind of crystalline materials with open framework structures. The guest molecules in an inclusion compound are believed to be indispensable to sustaining the framework structure their removal from the host lattice usually results in collapse of the host into a more compact crystal structure or even into an amorphous structure. 

Of the zeolitic materials, AlPO s cut a conspicuous figure because of their structural diversity and the incorporation of other elements into their frameworks. The recently developed VPI-5 (refs. 2, 3) announced the feasibility of synthesis of micoporous structures with windows comprising rings of over 12-T. All AlPO s, SAPO s and MAPO s form a family of microporous structures constructed by or essentially by A1(I) and P(V). Some of them are isostructural with zeolites but a majority have novel structures. The primary building units (PBU) centred by P(V) are invariantly PO4 whereas those centred by A1(I) are AlO in most cases and AIO5 or even AlOs in a few cases. So far all AlPO s, SAPO s and MAPO s have been synthesized exclusively in the presence of amines or... 

Oxides of transition metals can act as acid-base or redox catalysts. Oxides of non-transition metals (AI2O3, SiOj) are, however, good acid-base catalysts. There is a large family of aluminosilicate zeolitic acids (e.g. H -ZSM-5, H-mordenite). Micropor-ous aluminium phosphates (ALPOs) can be modified to yield acidic SAPOs (Si replaces... 

The recent descriptions of the ALPO-n, SAPO-n and MeAPO-n families of microporous materials illustrate that hydrothermal syntheses can afford a wide and diverse range of four-coordinate framework structures based on nearregular tetrahedra [1,2]. As building blocks, octahedra and tetrahedra can also be combined, in various proportions, into a variety of structure types [3,4]. Reflecting the conditions used for conventional synthesis [3,4], most of these structures are condensed, with little accessible pore volume. There are, however, examples of both synthetic [5-7] and natural materials [8-11] that have microporous crystalline structures. Further, the formation chemistry of silicates and aluminosilicates [12,13] illustrates that the more open structures are generally produced under relatively mild conditions. Open octahedral-tetrahedral structures with large pore systems might therefore also be accessible under appropriate low temperature hydrothermal conditions. 

Mesoporous molecular sieves materials5-8 designated M41S (which include the MCM-41 class of materials) have made a further major impact on the area of synthesis of porous materials. A variety of open framework structures that are mesoporous have recently been reviewed by Thomas.9 Activated charcoal, MCM-41, mesoporous tungsten oxide, and substituted MCM-41 materials are mentioned. This article primarily emphasizes potential applications of such materials and possible mechanisms of reaction. The mesoporous sysems are compared briefly to microporous materials such as zeolites, ALPOs, MeALPOs and SAPOs. 

Based on the quite remarkable results obtained with TS-1 we expect that many more examples of microporous solids modified by isomorphous substitution with redox metal ions in the crystal lattice will be described in the future (see Fig. 12). Indeed, the scope for developing unique oxidation catalysts based on the concept of site-isolation in zeolites, silicalites, alpos and sapos is enormous [37], In addition to varying the redox metal the size and hydrophobicity of the cavity can be tuned by, for example, varying Si/Al ratios to provide a variety of unique heterogeneous catalysts for liquid phase oxidation. 

Since 1982, a new class of microporous materials has grown in importance [25c] these are known as the AlPOs, the SAPOs, MeAPOs and ElAPOs being derived by isomorphous... 

Metal Aliminophosphates (MeAPO), In the metal aluminophosphate (MeAPO) family, the framework composition contains metal (Me), Al, and phosphorus (P). The Me species includes the divalent forms of Co, Fe, Mg, Mn, and Zn and trivalent Fe. As in the case of SAPO, the MeAPO s exhibit both structural diversity and even more extensive compositional variation. Seventeen microporous structures have been reported, and 11 of these have never before been observed in zeolites. Structure types crystallized in the MeAPO family include framework topologies related to the zeolites, for example, 34 (CHA) and 35 (LEV), and to the AlPO/s, for example, - 5 and -11, as well as novel structures, for example, -36 (0.8 nm pore) and -39 (0.4 nm pore). The MeAPO s represent the first demonstrated incorporation of divalent elements into microporous frameworks. 

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