Open-framework structures

An open-framework zinc phosphate synthesized under mild hydrothermal conditions possesses two interpenetrating helical channels.414 Piperazine phosphate yields a variety of open framework structures in reaction with zinc, including linear chain, layer, and three-dimensional systems.415... 

A number of transition metal oxides can also be intercalated by lithium. One of the best known examples is VsOi3. The VgOu structure, shown in Fig. 11.19, consists of alternate double and single layers of V2OS ribbons. The layers are connected by vertex sharing of octahedral sites, and this leads to a relatively open framework structure (Wilhelmi, Waltersson and Kihlburg, 1971). Insertion of lithium into the oxide matrix... 

The concept zeolites conventionally served as the synonym for aluminosilicates with microporous host lattice structures. Upon removal of the guest water, zeolites demonstrate adsorptive property at the molecular level as a result they are also referred to as molecular sieves. Crystalline zeosils, AlPO s, SAPO s, MAPO s (M=metal), expanded clay minerals and Werner compounds are also able to adsorb molecules vitally on reproval of any of the guest species they occlude and play an Important role in fields such as separation and catalysis (ref. 1). Inclusion compounds are another kind of crystalline materials with open framework structures. The guest molecules in an inclusion compound are believed to be indispensable to sustaining the framework structure their removal from the host lattice usually results in collapse of the host into a more compact crystal structure or even into an amorphous structure. 

Recently, we also focused attention on the synthesis of aluminoarsenates and galloarsenates with open framework structures (refs. 12—14). While the aluminophosphates are labeled as AlPO s, the gallophosphates, aluminoarsenates and galloarsenates can be designated as AlPO s, AlAsO s and GaAsO s respectively. 

In the search for a new class of nanomaterials, i.e. crystalline nanoporous semiconductors, a lot of efforts are made to synthesize new compounds with a zeolite-like open framework structure consisting of typical semiconductor elements like As, Sb, Se, etc.11-41 In 1995 Wang and Liebau reported the preparation and structure of oxoselenoantimonates (III) with a zeolite-like channel structure15 61, which are related to the natural mineral cetineite.171 This mineral with composition (K Na)3+x(Sb203)3(SbS3)(0H) (2.8-x)H20 with x = 0.5, first described in 1987 by Sabelli and Vezzalini181, was found in Le Cetine mine in Tuscany, Italy. 

Molecular-sieve zeolites represent an unique class of material [12-15]. This materials are crystalline aluminosilicates with open-framework structures made up of the primary building blocks [Si04]4 and [A104]5 tetrahedra that are linked by... 

It is noteworthy that, in general, the host framework does not retain its p, porous phase upon guest release, but collapses to the structure of the apohost - the a-phase. There are a few exceptions, in which the host behaves as a zeolite retaining its structural integrity [4-6], and a copper-containing porous open-framework structure has been reported which also withstands partial chemical modification while maintaining its structure [7],... 

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... 

In the case of aluminum phosphates, Ozin et al.3b have pointed out the possibility of the transformation of a chain structure to other chains or to layered and three-dimensional open-framework structures through a hydrolysis-condensation self-assembly pathway. We have recently carried out a few interesting molecule-material... 

The discussion in the previous sections should suffice to indicate the importance of the transformations of molecular compounds to materials. Studies of such transformations are only making a beginning, and there appears to be a great future for such investigations, Thus, the recent synthesis of a sodalite-related structure from a molecular zinc phosphate is noteworthy, besides the transformation of the centrosymmetric, tetrameric, molecular alumino-phosphate to open-framework structures. Preliminary investigations53 have shown that interesting transformations also occur in metal carboxylates. Thus, molecular zinc oxalate monomers and dimers are found to transform to chain or three-dimensional structures on heating with piperazine in an aqueous medium, The chain structure transforms to a layer structure, We believe that it would... 

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,... 

Many of these sulfates, selenites and selenates have open-framework structures. 

Coordination polymers, open-framework materials, and hybrid compounds built up with various anions have been described in the literature. The most common anions employed in open-framework structures are silicates and phosphates.1-3 Metal carboxylates with a variety of structures and dimensionalities have also been described in the recent literature.4 5 In recent years, other oxyanions such as sulfate, selenate, selenite, and tellurite have also been employed to design these structures.6,7 Surprisingly, coordination polymers... 

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