Phosphate-based Microporous Materials

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]. 

The use of molecular sieves as catalysts or catalyst components for synthesis of intermediates and fine chemical has increased impressively over the last two decades. A large number of reactions has been explored over a growing number of microporous materials. Also the level of imderstanding of the catalytic chemistry and the structure-activity relationships has greatly improved. Since the first review of Venuto and Landis in 1968 [1] and the one of Venuto in 1994 [2] the discovery of medium pore zeolites such as ZSM-5 [3] and of phosphate based molecular sieves [4] had the largest impact on the field. 

New materials that expand the microporous region of the zeohte are also interesting. In this group it is possible to find wide pore zeolite such as ITQ-21 which is accessible through six circular 0.74 nm openings [51], and extra-large-pore materials, like the phosphate-based VPI-5 or the more stable silicas UTD-1 and CIT-5 [52] with 0.8-1.2 nm openings. 

Zinc-phosphite and -phosphate based microporous materials are crystalline open framework materials with potential industrial applications. These were characterised by P MAS NMR and for the first time Zn NMR. In this work the local structure around the Zn centres in several representative microporous zinc phosphites and zinc phosphates was characterised by acquiring natural abundance Zn solid-state NMR spectra at ultrahigh magnetic field of 21.1 T. 

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. 

Especially, microporous aluminium phosphates and metal substituted aluminium phosphates can be routinely manufactured [185]. Synthetic microporous zeoUtes are nowadays of central importance industrially, as they are the powerful acid-base catalysts. These microporus materials have usually pore diameters in the range from 0.4 to 1.4 nm. By means of the ion-exchange of alkali metals for proton compounds acid catalysts whose acidity can be many times higher than that of sulfonic acid can be obtained [182]. An important feature of zeolite catalysts deals with their three-dimensional framework of channels and cavities, to give possibility for selection of reactants and products due to the... 

Finally, the ionothermal synthesis route developed since 2004 by Morris et al. [187] was not successful for the silica-based materials but the ionic liquids themselves can act as classical OSDAs in aqueous media. Thus, by using the fluoride route, Zones et al. synthesized pure silica zeolites of TON, ITW and MTT topologies using the OSDAs l,3-dimethyl-37/-imidazol-l-ium, l,2,3-trimethyl-3//-imidazol-l-ium and 1,3-diisopro-pyl-3//-imidazol-l-ium, respectively [12]. Then, in 2009, we reported the synthesis of IM-16 (UOS) a germanosilicate prepared with the ionic liquid 3-ethyl-l-methyl-3//-imidazol-l-ium as OSD A in aqueous media. This microporous material possesses a new topology built from d4r and mtw composite building units [165]. The ionothermal synthesis route, up to now, was only successful in the formation of new phosphate-based microporous materials, as mentioned in the next paragraph. 

The past nearly six decades have seen a chronological progression in molecular sieve materials from the aluminosilicate zeolites to microporous silica polymorphs, microporous aluminophosphate-based polymorphs, metallosilicate and metaHo-phosphate compositions, octahedral-tetrahedral frameworks, mesoporous molecular sieves and most recently hybrid metal organic frameworks (MOFs). A brief discussion of the historical progression is reviewed here. For a more detailed description prior to 2001 the reader is referred to [1]. The robustness of the field is evident from the fact that publications and patents are steadily increasing each year. 

Nowadays, the term zeolite includes all microporous solids based on silica and exhibiting crystalline walls, as well as materials where a fraction of Si atoms has been substituted by another element, T, such as a trivalent (T = Al, Fe, B, Ga,. ..) or a tctravalent (T = Ti, Ge,...) metal. Crystalline microporous phosphates are known as zeotypes or as related microporous solids (14, 54). At present, there are 179 confirmed zeoHtc framework types. For the structure types, three-letter codes are used, which were adopted from the name of the first material reported with a specific stmcturc. As an example, FAU is given for the structure of faujasite and its synthetic equivalents X and Y, and MFl for the stracture of ZSM-5 or silicalite-1 (105). Figure 9.11 shows prominent examples of zeolite firameworks, for example, FAU, LTA, and MFI types (pentasil). 

The use of organic additives represents a major breakthrough in the generation of new microporous phosphates. Unlike the materials prepared by d Yvoire, some of the phases obtained were analogous to the known zeolites. Table 2 provides a listing of the aiuminophosphate materials that have been crystallized to date and for which structures have been determined. Included in the list are materials analogous to known silicate-based phases as well as phases in which the structure is unique. 

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