Ion-sieve Effect Research Articles

The average structure of partially cesium exchanged nepheline hydrate I

Abstract The crystal structure of nepheline hydrate I, cesium exchanged at 80°C to an approximate composition of Cs0.84Na2.16Al3Si3O12 · 0.32 H2O, was determined by studying the main reflections in the single crystal X-ray diffraction pattern. This average structure is orthorhombic with: a = 8.132(3), b = 15.179(2), c = 5.1865(8) Å, Pnm21, Z = 2, Dm = 2.7, Dc = 2.72 g cm−3 and was refined by least-squares techniques to the final residuals, R = 0.049 and Rw = 0.072. Only small alterations were found in the framework geometry and in the environment of two independent Na+ ions which coordinate 7 framework O atoms. The Cs+ ion is located in the large void and coordinated by 7 framework O atoms in a distorted hexagonal pyramid. About 1/6 of the large voids still contain Na+ ions and two water molecules as in the starting material. A limit is observed in the ion exchange at ∼ Cs1Na2 i.e. the crystals exhibit a partial ion sieving effect. This is explained by the restricted volume of 2/3 of the cation sites and the diameter of the associated 6-ring windows of O atoms. Since the satellite reflections are situated along a*, the real structure is modulated in one dimension.

ChemInform Abstract: EXCHANGE OF AMMONIUM AND SODIUM IONS IN SYNTHETIC FAUJASITES

AbstractIonenaustausch‐ Isothermen für das Na F′ NH4‐G1eichgewicht in Zeolithen Xund Ywurden bei 298 K und einer totalen Lösungsnormalität von 0.1 g‐Äquiv.

Exchange of ammonium and sodium ions in synthetic faujasites

Ion-exchange isotherms for the Na ⇌ NH4 equilibrium in zeolites X and Y were constructed at 298 K and a total solution normality of 0.1 g-equiv. dm–3. Values for the standard free energy of exchange were calculated and shown to be consistent with the affinity sequence predicted using dielectric theory. Comparisons are made between previously reported studies on the Na ⇌ NH4 exchange in X and Y and this current work, and the differences in experimental data are discussed. Statistical thermodynamic considerations provide a greater understanding of the Na ⇌ NH4 exchange equilibrium in X and Y, and collectively the evidence suggests that high framework charge and ion-sieving factors in X are the dominant properties determining exchange selectivity and maximum exchange levels in this zeolite. In Y, the ion-sieve effect and repulsion between the entering ammonium ions are the dominant factors.

Synthetic inorganic ion-exchange materials—XXIX Ion-exchange equilibria of crystalline antimonic(V) acid with organic cations

A study of distribution coefficients as a function of concentration of hydrochloric acid indicates an “ideal” 1:1 exchange reaction for the NH 4 + H + and CH 3 NH 3 + H + systems on crystalline antimonic(V) acid(CSbA). The ion-exchange isotherms have been measured for the NH 4 +, CH 3NH 3 +, C 2H 5NH 3 + · iso C 3H 7NH 3 +, (CH 3) 3NH +, (CH 3) 4N + and (C 2 H 5 N + H + systems on CSbA at 25, 45 and 60°C. The selectivity coefficients vary with the equivalent fraction of the exchanging organic cations. The selectivity sequence decreases in the order, NH 4 + > CH 3NH 3 + > C 2H 5NH 3 + > (CH 3) 2NH 2 + which is parallel to the increasing order of van der Waals dimensions of organic cations. The ion-sieve effect is observed for (CH 3) 4N + and ( C 2 H 5) 4 N + H + systems. The dimension of the window on CSbA lattice can be evaluated to be 5–6 Å. Overall and hypothetical (zero loading) thermodynamic data have been derived for these ion-exchange reactions.

Transition metal ion exchange in zeolites. Part 3.—Ternary exchange in mordenite involving ammonium, triethanolammonium and complexed nickel (II)

Ion exchange and selectivity of ammonium mordenite have been investigated for solutions of hexaquonickel (II) chloride and hexamminenickel (II) chloride in aqueous triethanolamine. Exhaustive exchanges of mordenite with the two salt solutions resulted in final exchange levels of 7 % and 54 % respectively. The differences between the exchange isotherms, and the unusual isotherm obtained with hexaquonickel, are explained in terms of a ternary ion exchange process involving ammonium, triethanolammonium and complexed nickel ions, in which volume–steric and ion–sieve effects influence the maximum nickel uptakes.

Ion exchange in synthetic zeolites. Part 2.—Thermodynamic formalism for incomplete exchange

In zeolites, ion-exchange phenomena can be limited by steric or ion-sieve effects. A thermodynamic study of the ion-exchange in these systems, using the Gaines and Thomas equation, requires the application of adequate normalization and correction procedures to the experimental exchange isotherms. Such a procedure is developed, and a practical demonstration is given for the exchange of sodium ions by n-alkylammonium ions in synthetic faujasite. The correctness of the method is substantiated by a thermodynamic development, in which the formulae are derived starting from the Gibbs-Duhem equation.

Exchange of sodium in clinoptilolite by organic cations

A study has been made of ion-exchange equilibria and kinetics in which Na-clinoptilolite was converted to mixed Na-alkylammonium clinoptilolites. The Na + exchanged with ▪H 4, CH 3 ▪H 3, (CH 3) 2 ▪H 2, (CH 3) 3 ▪H, C 2H 5 ▪H 3, n- C 3 H 7 ▪ H 3 , iso-C 3H 7 ▪H 3, and n- C 4 H 9 ▪ H 3 . Tetramethyl-ammonium and tert-butylammonium ions were excluded from the channel system in clinoptilolite by an ion sieve action, but of the above eight exchangeable ions ▪H 4, CH 3 ▪H 3, (CH 3) 2 ▪H 2, C 2 ▪ 5 ▪H 3, and n- C 3 H 7 ▪ H 3 replaced all the Na, while with (CH 3) 3 ▪H, iso-C 3H 7 ▪H 3, and n- C 4 H 9 ▪ H 3 only partial exchange occurred. Exchange isotherms were reasonably well described by the equation: log 10 K = log 10 K a + C(2 N B z − 1), where K is the mass action quotient, K a the thermodynamic equilibrium constant, C a coefficient, and N B z the cation fraction of the entering organic ion, B. Activity coefficients F A z and F B z of the ions A and B in the zeolite are thus approximated by Kielland's equation: (14) log 10 F A z = C( N B z ) 2; log 10 F B z = C( N A z ) 2. The ion sieve effect, and the groups of ions which can displace all or only some of the total Na can be reconciled with the free dimensions of the windows giving access to the two kinds of channel in clinoptilolite if this is assumed to have an aluminosilicate framework iso-structural with that of heulandite. (7) The numbers of water molecules displaced from the zeolite by the organic ions vary linearly with the extent of exchange and with the volume of the organic ion. Kinetics of exchange have been studied and their characteristics compared with those expected for possible rate-controlling mechanisms. The kinetics then proved fully compatible with intraparticle diffusion, but not with intracrystalline or film diffusion.

Ion exchange in felspathoids as a solid-state reaction

A quantitative investigation has been made of ion-exchange diffusion and equilibria in the felspathoids, basic cancrinite (M2O. Al2O2. 2.4SiO2. 0.6MOH.xH2O), basic sodalite (M2O. Al2O3. 2.5SiO2. 0.34MOH.xH2O) and K and Rb analcite (M2O. Al2O3. 4SiO2). The isotherm contours were of three kinds: an ‘ideal’ form obeying the mass action lawK= (BcAs/AcBs) (Na–Li, Na–K , Li–K and Na–Ag in basic sodalite); a sigmoid form obeying the equation log10K = log10(BcAs/AcBs) + C(1 — 2Bc) where the constantCtakes a negative value (Na–Li in basic cancrinite); and a form exhibiting hysteresis (Na–Ag and Li–Ag in basic cancrinite and K–Rb in analcite). The hysteresis was shown to be due to limited mutual solid solubility of the end-members of the exchange and to an associated difficulty in nucleating crystallites of one growing phase on or in a matrix of the other. This effect is most strikingly found in the Rb–K exchange in analcite, for which various scanning loops were traversed. A quantitative approach to the theory of the above types of exchange isotherm has been given, and applied to the present and to earlier results obtained with crystalline exchangers. This gives a possible theoretical basis of the equation log10K = log10(BcAs/AcBs) +C(1 — 2Bc) and for Kielland’s equation log10fAc=CB2c. Examination of selectivity coefficients shows that the crystalline exchangers may possess very high selectivities towards one alkali metal ion as against another or for heavy metal ions such as Ag or Tl. These selectivities may change radically as one crystal is substituted for another. Ion sieve effects, partial or complete, were observed and can bring about a Donnan membrane hydrolysis of salts even of strong acids or bases, such as CsCl. Comparison has been made of the exchange diffusion coefficients evaluated in these laboratories for several kinds of crystal. A feature of exchanges where the temperature variable has been studied is the normally small temperature coefficient of the exchange equilibria and so a small value of the heat of exchange. A model has been proposed which regards the reactions as an interchange of ions between two inert dielectrics. This interpretation provides a simple explanation of the observed behaviour.