Charge-balancing cations

We alluded earlier to the variety of structural modifications which may he observed in sheet silicates. Clearly it is a matter of considerable in jortance to he able to determine if, for example, the aluminium content within a clay arises p a ely from octahedral substitution (as in montmorillonite) or whether there is some tetrahedral component (as in heidellite). a1 MASNMR readily provides the necessary answers. Figvire 1 illustrates the a1 spectrum for a synthetic heidellite material with Na as charge balancing cation. Aluminium in two distinct chemical environments is observed, with chemical shifts corresponding to octahedrally and tetrahedrally co-ordinated aluminium. 

Supported palladium oxide is the most effective catalyst used in total methane oxidation and in catalytic oxidation of VOCs [1-5]. However, the activity of the conventional catalysts is not sufficient [5-6]. Recently, the Pd-zeolite catalysts have attracted considerable attention due to their high and stable CH4 conversion efficiency [4-8]. In this work, the effect of the preparation method, the nature of the charge-balancing cations, the palladium loading and the pre-treatment gas nature on the texture, structure and catalytic activity of the Pd-ZSM-5 solids are investigated. 

The N2-physisorption characterisation results show that, no significant variations (less than 5%) are observed on the BET surface area, the total pore volume and the micropore volume of the different Pd-ZSM-5 catalysts, when the preparation method, the pretreatment gas, the charge-balancing cations and the palladium loading are modified. This result suggests that the ZSM-5 texture is stable with respect to the preparative parameter variations and that the observed activity differences are not related to any... 

Marquez, F., Marti, V., Palomares, E., Garcia, H. and Adam, W. (2002). Observation of azo chromophore fluorescence and phosphorescence emissions from DBH by applying exclusively the orbital confinement effect in siliceous zeolites devoid of charge-balancing cations. J. Am. Chem. Soc. 124, 7264-7265... 

For weak acceptors like charge balancing cations (Na, K, etc.), the interaction with the lone pair on the nitrogen is weak and results in a absorbance band around 1440cm . For stronger acceptors like extra-framework aluminum atoms (Lewis... 

The protons released are presumably available to compensate for the loss of the charge balancing cations within the zeolite. In conventional syntheses, the phtha-lonitrile condensation normally requires the nucleophilic attack of a strong base on the phthalonitrile cyano group [176, 177]. This function is presumably accommodated by the Si-O-Al (cation) basic sites within the ion-exchanged faujasite zeolites [178, 179]. The importance of this role is perhaps emphasized by the widespread use of alkali metal exchanged faujasites, particularly the more basic NaX materials of higher aluminium content [180, 181] as hosts for encapsulated phthalocyanine complexes. 

The sorptive, catalytic, and ion-exchange properties of zeolites depend strongly on the kind, position, and mobility of the charge-balancing cations. Since chemical shifts and multiplicities of lines are related to site occupancy and their widths to cationic mobility, NMR can in principle provide important information on the nature of the intracrystalline environment. 

The size of the tactoids is a function of the nature of the charge balancing cation, of the ionic strength within the suspension, and of the microcrystalline morphology. 

Ammonium zeolites are transformed into acid zeolites by the decomposition of ammonium cations the zeolite is modified to its acid form by exchanging sodium or any other charge-balancing cation present in the zeolite for ammonium, as follows [18] ... 

A. Substitution in octahedral-tetrahedral sheets, charge balancing cations present... 

If the charge balancing cation in a zeolite is then the material is a solid acid that can reveal shape selective properties due to the confinement of the acidic proton within the zeolite pore architecture. An example of shape selective acid catalysis is provided in Figure 5.3.7. In this case, normal butanol and isobutanol were dehydrated over CaX and CaA zeolites that contained protons in the pore structure. Both the primary and secondary alcohols were dehydrated on the X zeolite whereas only the primary one reacted on the A zeolite. Since the secondary alcohol is too large to diffuse through the pores of CaA, it cannot reach the active sites within the CaA crystals. 

With MnAPO-5, the activity in the aerobial oxidation of cyclohexane to Ol/One increased linearly with the number of redox-active sites, since the reaction involves cycling between Mn(II) and Mn(III). Therefore, the reaction requires cation sites able to reversibly form charge-balancing cationic species [24f[. 

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