Charge compensating cations

Rutile pigments, prepared by dissolving chromophoric oxides in an oxidation state different from +4 in the mtile crystal lattice, have been described (25,26). To maintain the proper charge balance of the lattice, additional charge-compensating cations of different metal oxides also have to be dissolved in the mtile stmcture. Examples of such combinations are Ni " + Sb " in 1 2 ratio as NiO + Sb202, + Sb " in 1 1 ratio as Cr202 + Sb O, and Cr " +... 

Many pigments having such substitutions have been commerciali2ed. The most important one is the Ti—Ni—Sb yeHow pigment having nickel oxide [12035-36-8], NiO, as the chromophoric component and Sb " as the charge-compensating cation. 

As mentioned above, an acidic zeolite can provide both protonic (Bronsted) and aprotonic (Lewis) sites. The Bronsted sites are typically structural or surface hydroxyl groups and the Lewis sites can be charge compensating cations or arise from extra-framework aluminum atoms. A basic (proton acceptor) molecule B will react with surface hydroxyl groups (OH ) via hydrogen bonding... 

The shape selectivity of zeolites is influenced by the location and distribution of charge-compensating cations. The charge-compensating ions other than protons are all quadrupolar. and Li NMR spectra of dehydrated LiX-1.0 identified three crystallographically distinct sites [221]. In the case NaX with Si/Al ratio of 1.23, six distinct sodium sites were identified using fast Na NMR, DOR and nutation techniques [222]. Na MQMAS has been extensively studied for zeolites X and Y [155]. Other cations like Cs and La in zeolites have also been investigated [155,... 

In the case of subsurface cation exchange, charge compensation cations are held in the solid phase within crystals in interlayer positions, structural holes, or surface... 

Early work by Boyd et al. (1947), performed on zeohtes, showed that the ion exchange process is diffusion controlled and the reaction rate is limited by mass transfer phenomena that are either film-diffusion (ED) or particle-diffusion (PD) dependent. Under natural conditions, the charge compensation cations are held on a representative subsurface solid phase as follows within crystals in interlayer... 

How exactly the molecules are oriented inside the channels depends on their specific shape and on the adsorption interaction between the dyes and the channel walls or charge compensating cations. Because of the dye s oblongness, a double-cone-like distribution in the channels is a reasonable model. This distribution is illustrated in Fig. 19a. The arrows represent the transition moments of the dyes and a describes the half-opening angle of the double cone. The hexagonal structure of the zeolite L crystal hence allows six equivalent positions of the transition moments on this double cone with respect to the channel axis. 

Macedo et al. [227] studied HY zeolites dealuminated by steaming, and found that the strength of intermediate sites decreased with increasing dealumination for Si/Al ratios varying from 8 to greater than 100. For comparison, isomorphously substituted HY, which is free of extra-framework cationic species, possesses more acid sites than conventionally dealuminated solids with a similar framework Si/Al ratio [227], This is because some of the extra-framework aluminum species act as charge-compensating cations and therefore decrease the number of potential acid sites. 

Vanadate formation (LaV04) occur also in LaY crystals (69). Removal of other charge compensating cations (such as Na+ ions) in the form of vanadates further destabilize the crystal lattice thus promoting and enhancing zeolite destruction. 

Si and A1 are the T atoms in aluminosilicate zeolites. However, other elements, such as P, Ge, Ga, Fe, B, Be, Cr, V, Zn, Zr, Co, Mn, and other metals can as well be T atoms [16,107,108]. These elements are tetrahedrally combined to form a zeolite, with or without charge compensating cations, in such a way that the electroneutrality principle is fulfilled [16,97,98],... 

Subsequently, it is possible to consider that the adsorbate-adsorbent interaction field inside these structures is characterized by the presence of sites of minimum potential energy for the interaction of adsorbed molecules with the zeolite framework and charge-compensating cations. A simple model of the zeolite-adsorbate system is that of the periodic array of interconnected adsorption sites, where molecular migration at adsorbed molecules through the array is assumed to proceed by thermally activated jumps from one site to an adjacent site, and can be envisaged as a sort of lattice-gas. 

In zeolites, the rate of molecular diffusion depends on the position of charge-compensating cations in the pore network and the structure of the framework [77-81], Since mass transport in micropo-rous media takes place in an adsorbed phase [82,83], this transport can be envisaged as activated molecular hopping between fixed sites [60,82,84] (for more details, see Section 5.9.1). 

The framework charge-compensating cations in a zeolite, which for synthetic zeolites are normally sodium ions, can be exchanged for other cations of different type and/or valency. However, care must be taken during ion exchange to avoid strongly acidic solutions which can lead to proton exchange with the zeolite metal cations or even structure collapse. For example, zeolites A, X, and Y decompose in 0.1 N HCI. The more silica-rich zeolites such as mordenite are, however, stable under such conditions. Acidity can be introduced into a zeolite in a number of different ways ... 

Schematic 1. The structure of 2 1 layered silicates. M is a monovalent charge compensating cation in the interlayer and x is thedegree of isomorphous substitution, which for the silicates of interest is between 0.5 and 1.3. The degree of isomorphous substitution is also expressed as a cation exchange capacity (CEC) and is measured in milli-equivalents/g.

The spectra in Figure 9 is somewhat different from the spectra of offretite crystals synthesized in the NaOH-KOH-TMAOH mixed base system. Wu et al.(ll), in addition to our observed band at 3615 cm , reported the appearance of acidic bands at 3690 and 3550 cm. In contrast, Mirodatos et al.(12) reported the appearance of only one, weakly acidic band at 3660 cm . The TMA content and the concentrations of different charge compensating cations may be responsible for these differences. 

The chemical transformation of Ru-complexes in faujasite-type zeolites in the presence of water and of carbon monoxide-water mixtures is reviewed and further investigated by IR, UV-VIS spectroscopic and volumetric techniques. The catalytic activity of these materials in the watergasshift reaction was followed in a parallel way. The major observations could be rationalized in terms of a catalytic cycle involving Ru(I)bis and triscarbonyl intermediates stabilized in the supercages of the faujasite-type zeolite. The turnover frequency of this cycle is found to be determined by the nature, number and position of the charge compensating cations, as well as by the nature of the ligands present in the Ru-coordination sphere. 

In view of what precedes, it has been the aim of the present work to identify the Ru-species present in faujasite-type zeolites activated under WGS-conditions, making use of the avail-albe literature data. The activation procedure of Ru(III)hex-ammine in NaY has been related to its catalytic performance as low temperature WGS-catalvst. Subsequently, the basicity of the material was related to its catalytic behavior in the same reaction, by changing the nature of the parent complex, of the charge compensating cations and of the aluminum content of the faujasite-type zeolite. 

---