Framework anionic

From Table V, the zeolite anion is more electronegative than F. Also shown in Table V are the electron occupancies in the 5s orbital of the Ag+ that is bonded to the anion. For a perfect anion, the Ag+ to which it is bonded should have an empty 5s orbital. Again, it is seen that Ag+ in AgZ has the lowest occupancy in its 5s orbital, indicating that the Z (i.e., zeolite framework anion) is the most electronegative anion. 

Whether the rhodium dicarbonyl was attached to the zeolite lattice or to an extra-framework anion such as OH, 0 or a labile ion, could be also decided upon using IR spectroscopy. Indeed lattice vibration between 1300 and 300 cm- characteristic of an NaY zeolite (16) are sensitive to the interaction of lattice oxide ions with cations. In particular, it was observed that an IR absorption band at 877 cm- grew simultaneously with the growth of CO absorptions at 2115-2048 characteristic of the dicarbonyl (13).This... 

Fig. 21.7. Section of the framework anionic-water host lattice formed by linking almost planar (H20)4F groups in (CH3)4N+(F-4H20)-. The water molecules are three-coordinated with an almost planar configuration [436]...

The replacement of framework anions (i.e. with chalcogens (e.g. S ) represents a more recent approach for generating microporous materials. The efforts to make microporous chalcogenides began with germanium or tin sulfides. However, germanium or tin sulfides do not form microporous materials similar to all-silica polymorphs of zeolites. It was later found that the incorporation of low-valent cations such as Mn + into the Ge-S composition helped to generate 3D frameworks. 

For the purposes of this article, zeolites are defined as crystalline nanoporous substances whose frameworks are inorganic oxides. This is a narrower definition than the most general, which includes organic and noncrystalline materials, and which remains open with regard to the framework anion it may perhaps be sulfide or nitride. "Nanoporous" indicates that the dimensions of the pores are of the order of 1 nanometer (1 nm =10 A), so it is exactly the right word to use in discussing zeolites, whose pores range in size from ca 0.3 to 3 nm. 

Table VII presents a summary of calorimetric measurements of the differential heat of adsorption of ammonia, water, and carbon dioxide on the sodium form of ZSM-5 zeolite. Ammonia adsorption at 416 K (97.147) shows that NaZSM-5 zeolite is weakly acidic, whereas CO adsorption (147) indicates that in addition there are some weak basic sites. It should be noted that of the two samples studied with ammonia adsorption one was 70% H exchanged and the sodium content of the other was not given. Water adsorption on NaZSM-5 displayed unusual behavior, with a steep increase in the differential heat of adsorption at high surface coverages (166). An adsorption mechanism was proposed to explain these findings in which adsorption occurs first on the hydrophilic sites, consisting of sodium cations and framework anions where water molecules are bound by dipole-field interactions. Further adsorption takes place near these sites through weak interaction with zeolite surfaces, and when the number of water molecules close to these sites exceeds a certain value, they tend to reorient by forming clathrate-like struc-...

A series of ZSM-5 samples with differing framework aluminium contents (containing tetrapropylammonium cations, [TPA]+) have been characterised by Raman spectroscopy. Difference Raman spectra reveal evidence for two distinct occluded species in samples with non-zero framework aluminium content. These species have been Identified as [TPA]+ cations associated with framework anionic sites and non-framework anions such as Br or OH-, on the basis of correlations between the integrated intensities of difference spectra and zeolite aluminium content. The relative abundance of the two forms have been determined semi-quantitatlvely and empirical evidence for [TPA]+ disordering is reported. 

Today we tend to see the influence of zeolite structure on reactivity for acid-catalyzed reactions, not only because of different intrinsic acidity, but because of the effect of the structure and the local surroundings on adsorption, and on the stabilization of the activated complex. It seems logical that the structure will determine both spatial conformation and the number of hydrogen-bonds that the protonated transition complex can form with the framework anion. This hydrogen-bond-acceptor ability is an important component of the zeolites as microcatalytic reactors... 

Metal clusters (M ,) can be roughly described as small pieces of metal,but closer studies show that their atomic and electronic structures deviate from those of bulk metals either because of intrinsic size effects (deviation from normal electronic structure at very small nuclearities) or because of their interaction with framework anions and extra-framework species including cations. Clusters can be naked or bonded to various extra-framework ligands. They may be charged as a result of incomplete reduction or of interactions (inductive and/or field effects) with framework or extra-framework species. Clusters can involve two metals homogeneously alloyed or segregated. 

The reaction between a trinuclear metal carbonyl cluster and trimetbyl amine borane has been investigated (41) and here the cluster anion functions as a Lewis base toward the boron atom, forming a B—O covalent bond (see Carbonyls). Molecular orbital calculations, supported by stmctural characterization, show that coordination of the amine borane causes small changes in the trinuclear framework. 

As each B atom contributes 1 electron to its B-Ht bond and 2 electrons to the framework MOs, the (n + 1) framework bonding MOs are just filled by the 2n electrons from nB atoms and the 2 electrons from the anionic charge. Further, it is possible (conceptually) to remove a BHt group and replace it by 2 electrons to compensate for the 2 electrons contributed by the BHi group to the MOs. Electroneutrality can then be achieved by adding the appropriate number of protons this does not alter the number of electrons in the system and hence all bonding MOs remain just filled. 

Ms " clusters have 12 framework bonding electrons as has [BsHs]- (p. 161) the anions are also isoelectronic with the well-known cation [Bis]. Similarly, the alloy NaSn. 2.23 reacts with cryptand in ethylenediamine to give dark-red crystals of [Na(ciypt)]4 [Sng] the anion is the first example of a C41, unicapped Archi-median antiprism (Fig. 10. lOc) and differs from the >3/, structure of the isoelectronic cation [Bis] + which, in the salt Bi+[Bi9] +[HfCl6]5 (p. 591), features a tricapped trigonal prism, as in [BgHg] " (p. 153). The emerald green species [Pb9] , which is stable in liquid NH3 solution, has not so far proved amenable to isolation via ciyptand-complexed cations. 

---