Open metal phosphates

Aufbau Principle of Complex Open-Framework Structures of Metal Phosphates with Different Dimensionalities... 

Several classes of inorganic open-framework structures have been synthesized and characterized in the past several years. While zeolitic aluminosilicates constitute the best-known class of open-framework structures,4 metal phosphates have been gaining considerable importance, and a variety of metal phosphates with open architectures have been reported in the past decade.5 The open-framework phosphates are generally synthesized under hydrothermal conditions in the presence of organic... 

The search for new zeolite-like structures was initially extended to aluminophosphate-based molecular sieves,3 and these explorations produced a variety of exotic compounds with open-framework structures, which include besides metal phosphates,4 carboxylates,5 sulfates,6 selenites, and selenates.7 Growth of this area has been rapid in the past decade necessitating new editions of the Atlas of Zeolites.2 It is noteworthy that metal—organic framework (MOF) solids have been included as zeolite types in the latest edition of the Atlas of Zeolites. 

While all the above molecular phosphates were prepared starting from phosphonic acids and phosphate esters, there are a few examples of molecular phosphates synthesized from phosphoric acid in aqueous medium. Although under hydrothermal conditions the reactions of phosphoric acid with metal ions generally result in extended open framework structures, it has been possible to isolate molecular zero-dimensional metal phosphates.4142... 

Rao, C, N, R, Natarajan. S. Choudhury, A. Neeraj,. Ayi, A. A, Aufbau Principle of Complex Open-Framework Structures of Metal Phosphates with Different Dimensionalities. Acc, Chem, Res. 2001, 34, B0-B7,... 

B) Choudhury. A, Rao, C, N, R, Understanding the Building-Up Process of Three-Dimensional Open-Framework Metal Phosphates Acid Degradation of the 3D Structures to Lower Dimensional Structures, Chem, Commun. 2003, 366-367. 

Inorganic open-framework compounds constitute an important class of materials that has attracted much attention as evidenced by the vast number of research papers published in the last few years. Although work in this area started with aluminium silicates1,2 because of their important uses in sorption and catalysis, much of the recent work pertains to the structure and characterization of open-framework metal phosphates and carboxylates. Both these families of open-framework materials have been reviewed recently.3 7 Since open-framework silicates and phosphates can essentially be... 

The hierarchy of open-framework structures in metal phosphates and oxalates... 

After providing a brief description of zeolitic structures, we discuss the hierarchy of structures of open-framework metal phosphates ranging from zerodimensional monomeric units and one-dimensional linear chains to complex three-dimensional structures. Aspects related to the likely pathways involved in the assemblage of these fascinating structures are examined, pointing out how the formation of the complex three-dimensional structures of open-framework metal phosphates involves the transformation and assembly of smaller units. Besides the role of the four-membered monomer, the amine phosphate route to the formation of the three-dimensional structures is discussed. The last step in the formation of these structures from preformed units of the desired structure is likely to be spontaneous. Our recent studies of open-framework metal oxalates have shown the presence of a hierarchy of structures. Reactions of amine oxalates with metal ions yield members of the oxalate family with differing complexity. 

Zeolites and related aluminosilicates constitute a vital family of microporous materials with immense applications in catalysis, sorption and separation processes [1-3]. The discovery of aluminophosphates is an important landmark in the science of these materials [4], All these materials are, in general, synthesized under hydrothermal conditions by making use of template molecules [2]. The template molecules are usually organic amines and they are involved in the formation of these framework structures in more ways than one. While it is difficult to pinpoint the exact manner in which the amines participate or direct the formation of these inorganic structures, it is generally believed that their size and shape are crucial in determining the pore structure. In recent years, a variety of open-framework structures formed by divalent metal phosphates... 

From the above discussions it is clear that the amine has an important role in the formation of open-framework metal phosphates. Recent studies from this laboratory have shown that in the family of zinc phosphates, almost all the members in the hierarchy of open-framework structures can be synthesized using the same structure-directing agent, triethylenetetramine (TETA) [32], In Fig. 7.18 we show all the structures obtained with TETA along with the relative concentrations of the amine and the phosphoric acid employed in the synthesis. We see that the one-dimensional (ladder) and the two-dimensional (layer) structures are obtained when the relative concentration of phosphoric acid is high with the ratio of concentrations of the acid and the amine in the range 3 1-4 1. This is in contrast to the three-dimensional structures where the ratio... 

Clearly, the monomer containing the four-membered ring emerges as the primary building block of open-framework metal phosphates. The formation of the layer structure may require free Zn2+ ions in solution, and it is possible that they are produced during the reaction by the dissociation of the monomer (1). 

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