Microporous Transition Metal Phosphates

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 

Synthetic Approach of Zinc Phosphates and Exploration of New Synthetic Route 

Synthesis of Zinc Phosphates and Discussion of Dimension Buildup Mechanism 

More recently, open-framework metal phosphates (metal = Ti, Mo, V, Fe, Co, etc.) with redox features have been continually reported. Cheetham summarized them in 1999.[11] 

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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). 

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. ... 

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... 

Both aluminum oxide and zirconium oxide are catalytically interesting materials. Pure zirconium oxide is a weak acid catalyst and to increase its acid strength and thermal stability it is usually modified with anions such as phosphates. In the context of mesoporous zirconia prepared from zirconium sulfate using the S+X I+ synthesis route it was found that by ion exchanging sulfate counter-anions in the product with phosphates, thermally stable microporous zirconium oxo-phosphates could be obtained [30-32]. Thermally stable mesoporous zirconium phosphate, zirconium oxo-phosphate and sulfate were synthesized in a similar way [33, 34], The often-encountered thermal instability of transition metal oxide mesoporous materials was circumvented in these studies by delayed crystallization caused by the presence of phosphate or sulfate anions. 

Recently, renewed attention has been given to so-called soft chemistry methods of synthesis of new metastable materials [9]. The synthesis of new microporous materials containing transition metals in the framework is of growing interest due to the expected catalytic redox properties [10]. The microporous titanium(IV) silicates [11] discovered have already proven the concept by showing very good catalytic activities and are widely used nowadays [12]. Similarly, hydrothermally synthesized titanium phosphates with open-finmework or layered structures are attracting attention as potential materials with similar properties [13]. 

The higher the active surface area of the catalyst, the greater the number of product molecules produced per unit time. Therefore, much of the art and science of catalyst preparation deals with high-surface-area materials. Usually materials with 100- to 400-m /g surface area are prepared from alumina, silica, or carbon and more recently other oxides (Mg, Zr, Ti, V oxides), phosphates, sulfides, or carbonates have been used. These are prepared in such a way that they are often crystalline with well-defined microstructures and behave as active components of the catalyst system in spite of their accepted name supports. Transition-metal ions or atoms are then deposited in the micropores, which are then heated and reduced to produce small metal particles 10-10" A in size with virtually all the atoms located on the surface... 

The closed-shell nature of aluminosilicates renders them ineffective for certain reactions favoured by transition (d-block) elements. Haushalter has made efforts to prepare stable shape-selective microporous solids involving molybdenum phosphates [15]. These solids are prepared hydrothermally in aqueous HjPO in the presence of cationic templates along with anionic octahedral-tetrahedral frameworks containing Mo in oxidation state less than 5+ and possessing Mo-Mo bonds. Some of these contain around 40 vol% accessible internal void space. There is rich chemistry in these systems and there is considerable potential for applications. Based on this approach one may indeed discover novel microporous and catalytic oxide systems. Several open-framework metal phosphates [16] and carboxylates [17] with different connectivities have been prepared by hydrothermal synthesis. 

High oxidation state transition-metal oxide ions isolated and sparsely distributed within the Al + sublattice of open-structure metal microporous alumino-phosphate (MAlPOs) solids (M = Co +, Mn +, Fe +) function as powerful redox, catalytically active centers in the selective oxyfunctionalization of alkanes. Important chemical commodities are also conveniently prepared by using such microporous catalysts in solvent free conditions, and using oxygen or air as oxidants [179,180]. 

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