Aluminophosphate fluoride

Figure 2.19 The structure of the aluminophosphate fluoride UT-6 is based on the CHA framework but includes coordinated fluoride ions linking octahe-drally coordinated aluminium cations (white octahedra).

Figure 5.12 Metal amine complexes can act as templates for the crystallisation of framework aluminophosphates [Ni(tetramethylcyclam)] templates the larger B cage of the STA-7 structure (the figure, left, shows the experimentally determined position of the tetramethylcyclam complex, shown without hydrogen atoms) [Ni(diethylenetriamine)2] directs the synthesis of the aluminophosphate fluoride, UT-6 (right).

Yang, Y, Pinkas, J, Schaefer, M, Roesky. H, W. Molecular model for aluminophosphates containing fluoride as a structure-directing and mineralizing agent, Angew, Chem.. Int. Ed, 1998, 37, 2650-2653,... 

The zirconium fluoride phosphate has a microporous 3D structure similar to that found in some aluminophosphate molecular sieves. 

Although an alcohol instead of water was used in the solvothermal system, a small amount of water was still unavoidably introduced due to the use of 85 wt% of phosphoric acid in water as phosphorus source and boehmite (6-coordinated Al) as A1 source. If a fluoride source was used as mineralization agent, a small amount of water could be introduced by the fluorine source as well. Studies indicate that the small amount of water played an important role in the formation of the structures of aluminophosphates from solvothermal systems.[74]... 

Hydrothermal synthesis of microporous compounds in the presence of fluoride source refers to the hydrothermal or solvothermal crystallization of aluminosilicate zeolites or microporous aluminophosphate such as AlP04-n series in the presence of a fluoride source. The successful introduction of fluoride ion into the hydrothermal or solvothermal synthesis of microporous materials paves the way for the introduction of other complex-ion or chelation agent s to the hydrothermal crystallization of microporous compounds. 

NaOH was involved in the synthesis. After systematic studies, W.Q. Pang and S.L. Qiu developed a general approach to growing large single crystals of zeolites and related microporous materials from fluorine ion synthesis systems. Later, J.L. Guth and W.Q. Pang expanded the fluoride source hydrothermal synthesis approach to the synthesis of micro-porous aluminophosphates and other metal phosphates. 

Synthesis of Microporous Aluminophosphates and other Metal Phosphates in the Presence of Fluoride Source... 

The use of other supports such as aluminas or aluminophosphates produces a similar Cr(VI) saturation behavior, although the exact saturation levels may differ somewhat for each temperature. Addition of promoters, to improve activity or modify polymerization behavior, such as fluoride, sulfate, phosphate, titania, etc., can also affect the thermal stability of the Cr(VI) species. These subjects are addressed separately in later sections. 

The activities of diarenechromium(O) compounds are particularly sensitive to the support [650-653]. For example, dicumenechromium(O) provides little or no activity when deposited onto calcined silica from hexane at temperatures below 100 °C. Deposited onto aluminophosphate or fluoride-treated alumina at 25 °C, however, it can be quite active. Alternatively, if the silica is first impregnated with acidic compounds or ions (e.g., Al, V, H3PO4, H2SO4) and then calcined, the dicumenechro-mium(0) is activated to produce a polymerization catalyst [648]. Numerous other reagents also "activate the silica in this way. Even some acidic carbon blacks form active catalysts when treated with dicumenechromium(O) at 25 °C. 

For example, the trimethylsilylmethyl derivative of chromium(II) is well suited to this purpose. Although it produces a highly active catalyst on aluminophosphate or fluoride-treated alumina supports, it is barely active on silica by itself. Nevertheless, when added to silica-supported Cr(II) oxide, the result is a highly active catalyst that produces branched polymer. In addition to reacting with silanol groups, the chromium alkyl may also react with chromium oxide to again produce mono-attached species, such as is shown in Scheme 44. Coordination between one Cr atom and its chromium or oxide neighbor also seems likely. 

E. Vanadium Silicophosphates, Aluminophosphates, Gallophosphosphates, Borophosphates, and Phosphate Fluorides... 

Additional elements or functional groups may be incorporated into the anionic framework of the VOPO class in order to increase framework stability or to provide additional stmctural flexibility. The presence of aluminum in the framework, for example, results in enhanced thermal stability of the phases. Borate serves not only as a framework constituent but as an effective solubilizer of the metal oxide components. Addition of fluoride to silicates and aluminophosphates and gaUophosphates (188-191) results in enhanced mineralization and induces crystallization in neutral and acidic pH. While VOPO phases incorporating such additional constituents are relatively unexplored, the few... 

Huoride may adopt a number of roles in the hydrothermal synthesis of phosphate-based materials. In addition to its mineralizing effect, fluoride may have a catalytic role, as manifested in the synthesis of AIPO4-I4A, which requires fluoride, although it is not incorporated into the framework (213). However, the most extensive use of fluoride has been in the synthesis of new aluminophosphate and gallophosphate architectures that directly incorporate the fluoride into the framework (214, 215). While fluoride incorporation into vanadium phosphate structures remains relatively unexplored, the phases studied to date reveal profound structural influences concomitant to incorporation of fluoride into the anionic scaffolding. 

MAS NMR Studies. Fluoride can play an important part in hydrothermal syntheses, where it can act as a mineralising agent in the syntheses of both silicates and aluminophosphates, and in many cases can remain attached to the framework cations such as silicon or (in the case of AlPOs, aluminium) in the as-prepared material. The F nucleus has attractive NMR properties (abundant, spin large chemical shift range) and is readily studied to give information on its presence in dilferent environments, the type of sites it adopts, and its mobility at dilferent temperatures. Examples are given in Section 5.4.2 of the study of fluorine in as-prepared pure silica polymorphs of zeolites. 

The frameworks of some inorganic microporous solids show changes in dimensions and symmetry, for example as a result of dehydration or cation relocation much greater flexibility is observed in some MOFs where flexible organic linkers can change configuration and adapt to adsorption. In addition, the frameworks of aluminophosphates can tolerate redox behaviour of included transition metal cations and both AlPOs and silicates show variation in coordination around framework cations e.g. aluminium atoms in AlPOs, titanium atoms in zeotypic titanosilicates, and both aluminium and silicon atoms in fluoride-containing, as-prepared aluminophosphates or silicas, respectively). 

The use of fluoride ions in the preparation of reaction mixture has led to the formation of new molecular sieves such as the extra-large pore gal-lophosphate cloverite (133), a few novel aluminophosphate phases (134,135), the chabazite-like AIPO4-34 (136), or to the crystallization of large crystals with fewer defects (137). 

Though crystallization of the aluminophosphate materials generally prefers acidic conditions, the use of fluoride as a mineralizing agent has been found to offer further flexibility in the synthesis of AIPO4 phases. Generally, the introduction of fluoride to an aluminophosphate gel results in a more rapid formation of the crystalline product. Fluoride ion addition may also increases the size of the crystals produced [82]. In some cases centimeter long crystals have been obtained [83]. 

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