Framework substitution

It has been suggested in the case of offretite that minima of the electrostatic potential calculated for the Al-substituted framework, in the absence of... 

Several authors [129,130,190] reported on Fe modified MCM-41 and HMS. Yuan et al. [190] interpreted their FTIR and EPR data on the basis of Fe incorporation in the silicate "framework". Tuel and Gontier [130] found that the EPR spectrum of Fe-HMS is comprised of three signals one of which corresponds to iron-substituted framework sites. 

Taking into account the electronegativity of the framework one could increase the basicity of a zeolite by substituting framework oxygens by nitrogens. Then, two different types of strong basic sites could be developed [28]... 

There is stiU a dispute as to whether the catalytic activity of iron-containing zeotype materials, for example, Fe-ZSM-5, should be attributed to isomorphously substituted framework iron or to extra-framework iron oxide or iron hydroxide species that are highly dispersed in the material. These extra-framework iron species are present for two reasons, either because they were not incorporated into the framework during the synthesis or because they were ejected from the framework during postsynthesis treatments (such as calcination or other heat treatments). The unresolved issue of the origin of catalytic activity continues to be the subject of research, whereby state-of-the-art characterization techniques are being applied. 

The Kd values for Cs+ -Na+ exchange are Nuclear Tank Wastes. However, substitution of 25 mol% Nb(V) for Ti(fV) eliminates half the Na+ from the tunnels. The Cs+ in the Nb substituted framework forms a twelve coordinate compound2, with eight framework oxygens and four water molecules. Because of this high coordination number the selectivity for Cs+ is sufficiently enhanced to remove Cs+ from nuclear waste solutions. 

Ma and coworkers recently utilized a double arylation strategy for an efficient synthesis of benzofuran-3-carboxylates 69 (Scheme 9.24) [125]. In the first step of this domino reaction, the Cu( I)-catalyzed chemoselective C-arylation of (3-ketoesters 67 vtith ortho-bromoiodobenzenes 66 provided the key o-bromoaryl-substituted eno-lates 68. These reactive intermediates vide supra) readily underwent the second Cu (I)-catalyzed intramolecular O-arylation, furnishing the desired benzofurans 69. The reaction tolerates a variety of substituents on both coupling partners 66 and 67, allowing for the preparation of tri- and tetra-substituted frameworks in good yields. 

For aluminophosphate-type materials, their demonstrated ability to contain several substituted framework cations chosen from a dozen elements opens opportunities to introduce transition metals in tetrahedral environments accessible to reactant molecules. Thus these materials may contribute to catalysis by direct interaction with reactant molecules, providing catalytic paths in chemical reactions similar to those in homogeneous catalysis. The prevailing steric restrictions may help to carry out new and even novel reaction steps such as the formation of optically active molecules. 

Both stages involve more than one step and these steps differ in detail among the various carboxylic acid derivatives and for different reaction conditions This chapter is organized to place the various nucleophilic acyl substitutions into a common mechanis tic framework and to point out the ways m which individual classes differ from the rest... 

Calixarenes (from the Latin ca/ x) may be understood as artificial receptor analogues of the natural cyclodextrins (96,97). In its prototypical form they feature a macrocycHc metacyclophane framework bearing protonizable hydroxy groups made from condensation of -substituted phenols with formaldehyde (Fig. 15b). Dependent on the ring size, benzene derivatives are the substrates most commonly included into the calix cavity (98), but other interesting substrates such as C q have also been accommodated (Fig. 8c) (45). 

Titanium Silicates. A number of titanium siUcate minerals are known (160) examples are Hsted in Table 19. In most cases, it is convenient to classify these on the basis of the connectivity of the SiO building blocks, eg, isolated tetrahedra, chains, and rings, that are typical of siUcates in general. In some cases, the SiO units may be replaced, even if only to a limited extent by TiO. For example, up to 6% of the SiO in the garnet schorlomite can be replaced by TiO. In general, replacement of SiO by TiO bull ding blocks increases the refractive indices of these minerals. Ti has also replaced Si in the framework of various zeofltes. In addition, the catalytic activity of both titanium-substituted ZSM-5 (TS-1) and ZSM-11 (TS-2) has received attention (161), eg, the selective oxidation of phenol, with hydrogen peroxide, to hydroquinone and catechol over TS-1 has been operated at the 10,000 t/yr scale in Italy (162). 

The benzene ring has a fully symmetrical (Deh) planar hexagonal carbon framework. Heteroatom substitution upsets this symmetry, but except in certain special cases, as for instance the thiabenzenes , the planarity of the ring is preserved. Although it is known... 

The general mechanistic framework outlined in this section must be elaborated by other details to fully describe the mechanisms of the individual electrophilic substitutions. The question of the identity of the active electrophile in each reaction is important. We have discussed the case of nitration, in which, under many circumstances, the electrophile is the nitronium ion. Similar questions arise in most of the other substitution reactions. 

The lack of a uniform order of relative reactivity of the halogens in reactions of certain nucleophiles with nitro- and polynitro-phenyl halides led Parker and Read to propose a one-stage mechanism for some aromatic nucleophilic substitutions. An alternative explanation within the framework of the two-stage S Ar2 mechanism had been proposed earlier. A range of mechanisms has been considered in the past by Chapman, who properly points out that only in a limited number of examples is the evidence for the two-stage mechanism compelling even though the balance of evidence favors it. 

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