Multivalent ions, alkali metals

Surface acidity and catalytic activity. Faujasitic zeolites exchanged with multivalent ions demonstrate significant catalytic activity for reactions involving carbonium ion mechanisms, in contrast to the inactivity of the alkali metal ion-exchanged forms. Several possible sources of the observed activity were proposed initially. Rabo et al. (202, 214) suggested that electrostatic fields associated with the multivalent ions were responsible for the catalytic activity. Lewis acid sites were proposed as the seat of catalytic activity by Turkevich et al. (50) and by Boreskovaet al. (222). Br0nsted acid sites formed by hydrolysis of the multivalent metal ions were proposed as the catalytic centers by Venuto et al. (219) and by Plank (220). 

What is the simplest adsorption model that can adequately define sorption in the system of interest for purposes of our study The simpler the model, the less information is needed to parameterize it. The distribution coefficient model requires only entry of the mass of sorbent in contact with a volume of water and a value for K,. Pesticide adsorption can often be modeled adequately using a simple K ) approach (cf. Lyman et al. 1982). For smectite and ver-miculite clays and zeolites that have dominantly pH-independent surface charge, ion-exchange or power-exchange models may accurately reproduce adsorption of the alkaline earths and alkali metals. If the system of interest experiences a wide range of pH and solution concentrations, and adsorption is of multivalent species by metal oxyhydroxides, then an electrostatic model may be most appropriate. 

Assuming a pK value of 2 to be the threshold for the carbonate-ion stability in molten salts in a C02-free atmosphere, it may be concluded that in halide melts consisting, even partially, of lithium salts and more acidic multivalent cations (Ca2+, Mg2+, Ln3+, etc.) carbonate ion is unstable and undergoes complete decomposition with the formation of the equivalent quantity of oxide ions in the melt. As for the other alkali metal halides studied, similar behaviour of CO2-ions can be expected only for CsCl, CsBr, and KBr melts at temperatures exceeding 800 °C. Of course, instability of carbonate ion in the melts does not mean an automatic disappearance of the oxygen admixture from the melt, since oxide ions arise instead of CO2- owing to their complete breakdown. This means that only for the melts characterized by a low pK value of the equilibrium (1.2.3), the carbohalogenation method of purification is the most suitable. 

Catalysts considered in the present discussion cover a wide spectrum of solids reducible multivalent metal oxides as well as non reducible basic compounds Reducible metal oxides possess some inherent problems whereas these problems are less for the alkali ions promoted alkaline earth oxides. Alkaline earth oxides seem to be more suitable for working at low partial pressure of oxygen. By doping alkaline earth oxides with alkali metal compounds it is conceivable that O species can be stabilized for dissociative absorption of methane. Reducible metal oxides will tend to transform into lower valent oxides or even upto metallic state partly under applied reaction conditions specially at low partial pressure of O2. Both activity and selectivity will be deteriorated. But for the non reducible basic oxides structural changes will be quite different. They will tend to reach an equilibrium state in the surface level amongst the oxide, hydroxide and carbonate phases on reacting with evolved H2O and CO. Both the lattice distortion and the formation of O species can occur in the alkali earth oxides in doping with alkali ions as they can not build a mixed oxide lattice. 

Nanofiltration. The nanofiltration process is, along with baclq)ulse filtration (Section 7.5.4.3), another example of the new possibilities in processing that are due to recent developments in the field of membrane technology. At conditions intermediate between those used in ultrafiltration and RO, this technology can selectively reject multivalent ions such as 804 while passing monovalent ions [147-149]. The process therefore can remove sulfate and other multivalent anions selectively from alkali chloride brines. Since the equivalent amount of alkali metal ions is held back by electrostatic forces, the net effect is the removal of M2SO4. 

---