Transition metal phosphate

Following the successful introduction of transition metals into the zeolitic AlPOs and GaPOs and the parallel discovery of open framework solids with polyhedral coordinations larger than four, it was natural that workers in this field should attempt to synthesize open-framework transition metal phosphates. The presentation will follow a chronological sequence. 

---

The hydrothermal method has been employed in recent years to synthesize a variety of solids that include aluminium phosphates (ALPOs) and other microporous transition-metal phosphates and transition-metal polychalcogenides (Davis Lobo, 1992 Haushalter Mundi, 1992 Liao Kanatzidis, 1990, 1992). Unlike zeolites, synthesis of ALPOs requires acidic or mildly basic conditions and no alkali metal cations. A typical synthetic mixture for making ALPO consists of alumina, H3PO4, water and an organic material such as a quaternary ammonium salt or an amine. The hydrothermal reaction occurs around 373-573 K. The use of fluoride ions, instead of hydroxide ions as mineralizer, allows synthesis of novel microporous materials under acidic conditions (Estermann et al, 1991 Ferey et ai, 1994). 

A new class of mixed open-framework materials consisting of both the phosphate and the oxalate units has been synthesized in the presence of structuredirecting agents [51-54], The solids thus prepared usually contain sheets made on metal phosphates, which are pillared by the oxalate units. In addition to the interesting architectures exhibited by them, the phosphate-oxalates also show remarkable adsorptive and other properties. In the case of the transition metal phosphate-oxalates, interesting magnetic properties have also been observed. We will discuss some select cases of the phosphate-oxalates of iron in the following. 

Would an alkali metal phosphate or a transition metal phosphate be better for making a solution containing phosphate ion ... 

Mixed-valence transition metal phosphates have been found for Ti, V, andNb. Their crystal chemistry and properties are more closely related to those of the Mo analogs in that they have mixed frameworks of MOe octahedra that are partially or wholly isolated from one another by intervening phosphate gronps and the d electrons do not appear to be delocalized. A particnlarly rich chemistry exists for the vanadium analogs. Details of their stmctural properties have been reviewed by Borel and coworkers. ... 

Several transition metal phosphate systems are found to catalyze mild oxidations of organic molecules. Catalytic activity is due to transition metal redox couples see Oxidation Catalysis by Transition Metal Complexes and Oxide Catalysts in Solid-state Chemistry), but the phosphate groups are important in determining the underlying and surface structures. 

Sohds that can reversibly take up and lose hthium ions (accompanied by reduction or oxidation) can be used as cathode materials in rechargeable lithium batteries. Several transition metal phosphates, for example, Li3V2(P04)3, LiV0P04, and LiFeP04, that can undergo oxidative deintercalation of lithium have been investigated in recent years. 

The reduction of phosphates by carbon is a classical method, but the purity of phosphides obtained this way can be suspect. This also holds for replacement reactions, in which a redox process between a transition metal and a metal phosphide is used for the preparation of phosphides with high thermal stabihty. Binary main-group element and transition metal phosphides hke AIP, CrP, NbP, MoP, or WP can be prepared by the reaction of the powdered metals with a melt of hthium metaphosphate LiP03. Bulk samples of transition metal phosphides hke C02P or NiMoP can be obtained via reduction of metal oxide/phosphate mixtures in a mixture of 5% H2 in N2. MoP, WP, Fc2P, M2P, FeP, and RuP can be synthesized by direct reduction of the transition metal phosphates in hydrogen atmosphere between 670 and 1320 K. ... 

In spite of their highly open structures, both materials may be synthesized without structure directing agents. Since most transition metal phosphates containing template molecules typically cannot be calcined without pore collapse, template-free synthesis appears to be the most promising direction for the formation of new porous materials. The exciting properties observed in the VSB family are likely glimpses of what may occur in other transition metal phosphate systems once thermal stability problems are overcome. 

Since the incorporation of transition metals into the frameworks of zeolites or micro-porous ahiminophosphates to form heteroatom-containing molecular sieves with important application values, the synthesis, structure, and characterization of microporous transition metal phosphates have been extensively studied in the last decade. In particular, because transition metal cations possess redox and coordination features, they are a kind of catalytic material with useful applications, and promise potential... 

There are now four major classes of materials in which organic components exert a significant structural role in controlling the inorganic oxide microstructure zeolites, mesoporous oxides of the MCM-41 class, biomineralized materials, and microporous octahedral-tetrahedral or square pyramidal-tetrahedral transition metal phosphate frameworks (TMPO) with entrained organic cations. ... 

---