Zeolitic Open-framework Structures

Since the discovery of neutral framework aluminophosphate molecular sieves AlP04-n, a large number of anionic framework aluminophosphates have been reported. Jilin group has carried out systematic research in this field.1144-1471 Recently, they have built up a 

JDF-20 ([(Et3NH)2] [A15P6024H] 2H20)[ 190,1911 has an Al/P ratio of 5/6. To date, JDF-20 has the largest pore opening in microporous ahiminophosphates. It crystallizes in the monoclinic space group C2/c with a = 32.035 A, b = 14.308 A, c = 8.852 A, and [I = 104.65°. The structure is constructed from alternation of A104 tetrahedra and P04 tetrahedra in which all the A1 vertices are shared by adjacent P vertices one third of P 

H-bonds. The structure of JDF-20 also features a series of comer-sharing and edgesharing 4-ring chains. 

A1PO-CJB1 ([(CH2)6N4H3]3+[A112P13052]3-)[1941 has an Al/P ratio of 12/13. It is the first anionic aluminophosphate molecular sieve possessing Bronsted acidity. It crystallizes 

There are three kinds of layered aluminophosphates with AlP2Og3 stoichiometry. [NH4]3[Co(NH3)6]3[Al2(P04)4]2[156] crystallizes in the orthorhombic space group Abal, 

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For microporous compounds with special compositions, calcination effects are even more severe. As compared with zeolites, these compounds have lower thermal stability. Strictly speaking, most of them are nonporous since removal of the occluded guest molecules by calcination usually results in collapse. This is due to strong H-bonds with the framework, coordination bonds, and sometimes the templating molecule is shared with the inorganic polyhedra. Relevant examples of low-stability microporous compounds with interesting structural features are zeolitic open-framework phosphates made of Ga [178], In [179], Zn [180], Fe [181],... 

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. 

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],... 

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. 

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... 

Ramamurthy and coworkers have utilized zeolites modified with chiral organic compounds [17,18]. Zeolites are crystalline aluminosilicates with open framework structures. In this approach, the zeolite is first loaded with a chiral inductor and the compound to be photolyzed is then added in a second, separate adsorption step. Asymmetric induction ensues as a result of the close proximity enforced between reactant and chiral inductor in the confined space of the zeolite supercage. The zeolite method has the disadvantage that the size of the substrate is limited by the pore size of the zeolite being used. Most of the work using the chirally modified zeolite approach was compared with the ionic chiral auxiliary method by Scheffer and coworkers. The enantiodifferentiations by the zeolites are usually low to moderate. 

Zeolites natural and synthetic hydrated aluminosilicates with open framework structures enclosing cavities and channels in which reside mobile cations and water molecules... 

Mesoporous molecular sieves materials5-8 designated M41S (which include the MCM-41 class of materials) have made a further major impact on the area of synthesis of porous materials. A variety of open framework structures that are mesoporous have recently been reviewed by Thomas.9 Activated charcoal, MCM-41, mesoporous tungsten oxide, and substituted MCM-41 materials are mentioned. This article primarily emphasizes potential applications of such materials and possible mechanisms of reaction. The mesoporous sysems are compared briefly to microporous materials such as zeolites, ALPOs, MeALPOs and SAPOs. 

Aluminophosphates (AIPO4S) are a class of materials which, like the aluminosilicate zeolites, assume open framework structures containing channels of molecular dimensions with molecular sieve properties. Since the AIPO4 structures contain equal numbers of AIO4 and PO4 units there is no necessity for charge-balancing extraframework cations and consequently no sites to provide acid catalytic properties. 

Besides zeolites, a diverse range of microporous materials with novel open-framework structures have been discovered. The framework atoms of microporous materials have expanded to include most of the elements in the periodic table.151 The framework elements are not limited to A1 and Si atoms alone, and the primary building units are not only confined to tetrahedra. This chapter will mainly describe the structural characteristics of zeolites and some zeolitic open-framework materials. 

Y. Zhou, H. Zhu, Z. Chen, M. Chen, Y. Xu, H. Zhang, and D. Zhao, A Large 24-Membered-ring Germanate Zeolite-type Open-framework Structure with Three-dimensional Intersecting Channels. Angew. Chem., Int. Ed., 2001, 40, 2166-2168. 

The zeolites are a group of hydrated aluminosilicate minerals with open framework structures of (Si,Al)04 tetrahedra, charge-balanced mostly by Ca, Na, or (cf. Mumpton 1977). A general... 

The open framework structure of the zeolites allows small molecules to be absorbed into their structures. The size and shape of the molecules absorbed depends on the structure of the zeolite, and hence the geometry of its pores. For example, zeolite A readily absorbs water but ethanol is excluded. Figure 7.17 demonstrates the effect of cavity size on the molecules which can be absorbed. 

What is a zeolite (2) These materials possess open framework structures constructed of Si04 and AIO4 tetrahedra linked through oxygen... 

Zeolite Any of the minerals of the zeolite group. Aluminosilicate minerals with an open framework structure that allows for easily reversible hydration, gas adsorption, and either cation or anion exchange. 

In most syntheses, a gel is initially prepared that contains framework-building inorganic species, typically aluminates, silicates, phosphates or metal-oxyani-ons, available in a reactive form. There are also syntheses that result in crystallisation from clear solution, and others that proceed under essentially dry gel conditions (without added water). The sources of the reagents can determine the products that form in general more reactive sources are favoured. In addition, for zeolites and related solids, structure directing species must be present in the gel that can stabilise the open framework structures relative to denser phases of the same composition. Hydrated metal cations and/or organic cations perform this function in zeolite preparations. 

Zeolites. Zeolites are crystalline inorganic polymers made of aluminosilicates and have open framework structures. Natural zeolites (faujasites) have pores of sufficient size to be useful in petroleum refining. Synthetic faujasite-type zeolites are now available in large commercial quantities and have become an important catalyst in the petroleum industry. Comprehensive reviews on the application of zeolite catalysis are available (ll lS). 

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