Cavities and channels

Recently, the PER of phenyl acetate and phenyl benzoate has been studied within zeolites of the faujasite and pentasil families, which are structurally very different. [277,278] It was found that for phenyl acetatet in faujasites X and Y, the predominating product is the ortho isomer, whereas in the pentasils ZSM-5 and ZSM-11, the preferred product is the para isomer. These differences have been explained taking into account the different sizes and shapes of the cavities and channels of the two families. The faujasites are made up of channels of 8 A diameter, leading to supercages of 13 A diameter, where the reaction is expected to occur, whereas the pentasils are made up of narrow channels of 6 A diameter. In the X and Y faujasites, the ortho position would be favored due to the restricted mobility, whereas in the channels of ZSM-5 and ZSM-11, there would be a shape limitation because the ortho isomer does not fit but the para isomer does (Table 15). With phenyl benzoate inside X or Y zeolites, the same... 

Zeolites, which are aluminosilicates that can be regarded as being derived from AI2O3 and SiC>2, function as acidic catalysts in much the same way (Section 7.3). In addition, they catalyze isomerization, cracking, alkylation, and other organic reactions. A structurally related class of micro-porous materials based on aluminum phosphate (AIPO4) has also been developed (Section 7.7) like zeolites, they have cavities and channels at the molecular level and can function as shape-selective catalysts. 

Zeolite Structures These are crystalline, microporous solids that contain cavities and channels of molecular dimensions (3 A to 10A) and sometimes are called molecular sieves. Zeolites are used principally in catalysis, separation, purification, and ion exchange The fundamental building block of a zeolite is a tetrahedron of four oxygen atoms surrounding a central silicon atom (i.e.. (Si04)4-). From the fundamental unit, numerous combinations of secondary building units (polygons) can be formed. The corners of these polyhedra may he Si or A1 atoms.2... 

Figure 3.76 (a) The X-ray crystal structure of [Cs([18]crown-6)2]+e, the first electride complex, (b) cavities and channels in the structure in the a-b plane. (Reprinted with kind permission from Elsevier). 

Ultrasonic machining, also known as ultrasonic impact grinding, uses ultra-sonically induced vibration delivered to a tool to create accurate cavities and channels of many shapes [146]. It can be used to form deep cavities as small as 250 pm in diameter (with an accuracy of 50 pm) in both hard and brittle materials such as glass, quartz, polymers, ceramics and metals. This technique may be useful for fabrication of large masters. 

The bifunctional zeolite catalysts are composed of both acid sites and metal clusters. The preparation methods of these catalysts encompasses three steps ion exchange, calcination, and reduction. The ion-exchange process is carried out with aqueous solutions of salts or, more commonly, of complexes of the metals that will be incorporated into the zeolite cavities and channels. 

Now, to explain the operation of the PFIEBR, it is proposed that the interdiffusion in the adhering liquid thin layer is the rate-determining step, then, it is possible to consider that n = 1 in Equation 7.33, since for this transport process the diffusion rate, k, is proportional to concentration [38], This approach is based on the assumption that states that the rate-determining process during the dynamic ionic exchange in zeolite columns determines the diffusion in the zeolite secondary porosity, that is, the transport process in the macro- and mesoporosities formed by the matrix inserted between zeolite crystals and the diffusion in the zeolite primary porosity, that is, in the cavities and channels which constitute the zeolite framework [38], This fact is experimentally justified later. With the help of Equations 7.33 through 7.35, we obtain ... 

The existence of the carbon monoxide dimer is currently accepted as a weakly bound molecular complex [CO CO] [124], Besides, it has been found that weakly bound molecular clusters of carbon monoxide dimers and cyclic trimers, confined in zeolite cavities and channels, are stabilized under the zeolite framework cationic field [125], These structures are similar in geometric configuration to those reported in Figure 9.14 however these are not covalently bonded, but are only weakly bound. 

Several functional properties can be expected from the materials of coordination polymers owing to their extended nature and high thermal stability and the existence of cavities and channels. Here we describe some of the properties that have been explored using above-described layered structures. 

Karle IL, Duesler E (1977) Arrangement of water molecules in cavities and channels of the lattice of [PheVal6] antamanide dodecahydrate. Proc Natl Acad Sci USA 74 2602 - 2606... 

These principles of crystal engineering and supramolecular synthesis have thus far been used to design, isolate, and characterize network structures from relatively small molecular components. In the context of coordination polymer networks, a recent review indicates how wide the range of chemical components and accessible network motifs has become.48b However, the scale of these structures is such that cavities and channels are on the order of 1 nm and, to date, each cavity is identical. Careful selection of appropriate substrates or components and ever more control over crystal packing will offer the potential for rational design of an even more extensive array of modular (i.e. binary, ternary, or even higher order) structures than those that are currently available. In particular, judicious choice of supermolecules... 

By direct action of NH3 on HgO one obtains Hg2N0H-2H20 (Millon s base), which is a clathrate. The framework is made of (Hg2N)m and the cavities and channels contain the OH ions and water molecules. In addition to the chloride mentioned above, many other salts of Millon s base, Hg2NX-nH20, with X = N03, CIO4, Br , I-, and n = 0-2, are known. These compounds are a type of ion exchanger. 

Considering the special bioactive and biocatalytic functions of copper(II) carboxylate complexes,[9] along with the magnetic properties and various coordination fashions of Cu(II) ion,[10-12] we focused our work on the Cu(II) and H4btec system. Based on the layer structure of [Cu2(btec)(H20) -4H20] (1),[13] in which the Cu(ll) atom with bi-pyramidal configuration is coordinated by two monodentate carboxyl O atoms and three aqua molecules, part of the coordinated aqua molecules of Cu(II) is possible to be replaced by some bridging spacers, such as pyrazine, 4,4 -bipyridine (4,4 -bpy) and the related species, and to construct 2-D and 3-D open-frameworks with variable cavities and channels. 

Most of the supramolecular synthetic strategies rely upon the preprogramming of synthons, in order to allow a directed-assembly approach. In fact, the electronic coupling, the dehnition of cavities and channels for ion and/or substrate diffusion, the peculiar catalytic or photochemically active sites environment, among other aspects, can be dehned by the structure and chemical nature of the building blocks. 

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

It has been shown by using a methodology combining molecular dynamics and an energy minimization technique 60) that in the PER pores (cavities and channels), the formation of Cti olefin intermediates is inhibited. These theoretical results agree well with the experimental indications of restricted transition state shape selectivity. Indeed, for materials such as zeolites and MeAPOs, most of the sites are expected to be located inside the micro-pores. Molecular sieves with large mesopores and/or large external surface... 

Measurements of molecular density and intrinsic viscosity of PAMAM dendrimers indicate an unusual variation with dendrimer generation. Minimum density and maximum intrinsic viscosity were observed at around G4, which suggests that the fully developed dendrimers have a high accessible internal surface area in a solvent-filled intramolecular free volume that may consist of internal cavities and channels. Similar findings were reported by Mourey et al. for PBE monodendrons (based on dihydroxybenzyl alcohol) and tridendrons produced by coupling these dendrons to a trifunctional core, l,l,l-tris(4 -hydro-xyphenyl)ethane prepared by the convergent method. A maximum intrinsic viscosity occurred at G3 for tridendrons and G5 for monodendrons, consistent with the model developed by Lescanec and Muthukumar... 

Blue ultramarine (with the chromogenic group 83, herein generally represented as Sx) is formed by slow (up to 20 days ) oxidation of the sulfur in the cavities and channels in the zeolite cage ... 

Based on detailed experimental and theoretical data, the free eleetrons in these complexes have been shown to exist in the cavities and channels formed by the crystal packing of the metal-chelate units . 

H. Li, A. Laine, M. O Keeffe, and O.M. Yaghi, Supertetrahedral Sulfide Crystals with Giant Cavities and Channels. Science, 1999, 283, 1145-1147. 

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