Framework structure

A Mn-containing layer of essentially the same kind occurs in chalcophanlte, ZnMn307. 3 H20, ° but with one-seventh of the metal sites unoccupied so that the composition is Mn307 instead of Mn02. Associated with each Mn layer and directly above and below the unoccupied Mn positions are the Zn ions 

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All the four essential features of the active site of chymotrypsin are thus also present in subtilisin. Furthermore, these features are spatially arranged in the same way in the two enzymes, even though different framework structures bring different loop regions into position in the active site. This is a classical example of convergent evolution at the molecular level. 

Serine proteinases such as chymotrypsin and subtilisin catalyze the cleavage of peptide bonds. Four features essential for catalysis are present in the three-dimensional structures of all serine proteinases a catalytic triad, an oxyanion binding site, a substrate specificity pocket, and a nonspecific binding site for polypeptide substrates. These four features, in a very similar arrangement, are present in both chymotrypsin and subtilisin even though they are achieved in the two enzymes in completely different ways by quite different three-dimensional structures. Chymotrypsin is built up from two p-barrel domains, whereas the subtilisin structure is of the a/p type. These two enzymes provide an example of convergent evolution where completely different loop regions, attached to different framework structures, form similar active sites. 

The constant domain has a stable framework structure composed of two antiparallel sheets comprising seven p strands, four in one sheet and three in the other. The variable domains have a similar framework structure but comprising nine p strands, five in one sheet and four in the other. The three hypervariable regions are in loops at one end of the variable domain. The variable domains from the heavy and light chains associate through their five-stranded p sheets to form a barrel with the hypervariable loop regions from both domains close together at the top of the barrel. 

Orthorhombic HBO2 consists of trimeric units B303(0H)3 which are linked into layers by H bonding (Fig. 6.26) all the B atoms are 3-coordinate. Monoclinic HBO2 is built of chains of composition [B304(0H)(H20)] in which some of the B atoms are now 4-coordinate, whereas cubic HBO2 has a framework structure of tetrahedral BO4 groups some of which are H bonded. The increase in CN of B is paralleled by an increase in density and mp. 

The framework structures and pore cross-sections of two types of zeolites are shown. (Top) A Faujasite-type zeolite has a three-dimensional channel system with pores of at least 7.4 A in diameter. A pore is formed by 12 oxygen atoms in a ring. (Bottom) ZSM-5 zeolite has interconnected channels running in one direction, with pores 5.6 A in diameter. ZSM-5 pores are formed by 10 oxygen atoms in a ring. Reprinted with permission from Chemical Engineering Progress, 84(2), February 1988, 32. 

The zeohte overgrowth has been reported for FAU on EMT zeohte [44] and MCM-41 on FAU zeohte [45]. On the other hand, in this study, zeohte layers were grown on the zeohte with the same framework structure, resulting in high coverage of ZSM-5 crystals with silicalite layers and high para-selectivity. The zeohte crystals with oriented thin layer on their external surface are expected to form a new class of shape-selective catalysts. 

The new phases were discovered by the combination of exploratory synthesis and a phase compatibility study. As commonly practised, the new studies were initially made through the chemical modification of a known phase. Inclusion of salt in some cases is incidental, and the formation of mixed-framework structures can be considered a result of phase segregation (for the lack of a better term) between chemically dissimilar covalent oxide lattices and space-filling, charge-compensating salts. Limited-phase compatibility studies were performed around the region where thermodynamically stable phases were discovered. Thus far, we have enjoyed much success in isolating new salt-inclusion solids via exploratory synthesis. 

New materials are also finding application in the area of catalysis reiated to the Chemicals industry. For example, microporous [10] materials which have titanium incorporated into the framework structure (e.g. so-calied TS-1) show selective oxidation behaviour with aqueous hydrogen peroxide as oxidizing agent (Figure 5). Two processes based on these new catalytic materials have now been developed and commercialized by ENl. These include the selective oxidation of phenol to catechol and hydroquinone and the ammoxidation of cyclohexanone to e-caproiactam. 

The location or distribution of the Mo sulfide species, that is, inside or outside the zeolite cavities, was examined by HREM, XRD [17], and pore volume measurements by using benzene as adsorbate [18]. HREM observations for MoSx/NaY possessing 2Mo/SC obviously demonstrated that no Mo sulfide spiecies were formed on the outside of the zeolite and that the framework structure of the zeolite was not destroyed at all on the accommodation of Mo sulfide species. The XRD and pore volume measurements confirmed the HREM observations. It is concluded that highly dispiersed intrazeolite Mo sulfide species are produced by using Mo(CO),. 

For example, clusters identified by IR spectra and extraction as Ir4(CO)i2 on y-Al203 were found by EXAFS spectroscopy to have an Ir-Ir coordination number of nearly 3, consistent with the tetrahedral structure of the metal frame EXAFS spectroscopy produces the equivalent result for sohd Ir4(CO)i2 [27]. EXAFS spectroscopy is the most appropriate method for determination of framework structures of supported clusters, but it is limited by the errors to clusters with at most about six metal atoms. Thus, it has been used to determine frameworks that are triangular (EXAFS first-shell metal-metal coordination number of 2), tetrahedral (EXAFS first-shell metal-metal coordination number of 3), and octahedral (EXAFS first-shell metal-metal... 

Mercury iodide offers an example of a layer structure consisting of tetrahedra sharing all of their vertices (Fig. 16.23). Much more frequent are framework structures they include the different modifications of Si02 and the aluminosilicates that are discussed in Section 12.5. Another important class of aluminosilicates are the zeolites. They occur as minerals, but are also produced industrially. They have structures consisting of certain polyhedra that are linked in such a way that hollows and channels of different sizes and shapes are present. 

The [Co(en)3]3+ ion has been shown to be a useful template for the synthesis of unusual cobalt, aluminum, gallium, and magnesium phosphates exhibiting framework,632-637 layered638-641 and chain642 structures, as well as for the hydrothermal synthesis of novel zeolites643,644 and other framework structures such as molybdenum phosphates.645... 

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... 

Investigations of cyanides and cyano complexes of Cd and Hg have augmented tremendously since about 1990, after detection of inclusion compounds of Cd(CN)2. A thorough review on transition-metal cyanides especially emphasizes the chemistry of inclusion compounds of both the Hofmann type (frameworks dominated by planar Ni(CN)4 building blocks) and the cyanocad-mate type (frameworks with tetrahedral Cd(CN)4 units).87 The structures of these inclusion compounds, but also of cyanides themselves, often topologically resemble the structures of minerals this aspect ( mineralomimetic chemistry ) is dealt with in a simultaneous survey.88 A more generic review of framework structures, with a section on cyanide inclusion compounds, is also to be mentioned.90... 

Special emphasis has been laid on structural studies of Cd compounds with dicarboxylic acids, because these combinations form coordination polymers of differing dimensionalities depending on the carboxylate component and on coligands coordinated to Cd. The simplest compound of this type, namely Cd oxalate CdC204, adopts a 3-D framework structure, with CdOe octahedra linked by oxalate groups.272 Further interesting structures containing Cd, oxalate and other components have been described recently.273,274... 

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