Ion exchange in synthetic zeolites. Part 1.—Ammonium and some of its alkyl derivatives in Linde Sieves X and Y

Abstract

A study has been made of the exchange adsorption of NH⁺₄, CH₃NH⁺₃, C₂H₅NH⁺₃, iso-C₃H₇NH⁺₃, n-C₄H₉NH⁺₃, (CH₃)₂NH⁺₂, (C₂H₅)₂NH⁺₂, (CH₃)₃NH⁺ and (C₂H₅)₃NH⁺ by Linde Sieves X and Y. For steric reasons, none of these ions could effect a complete replacement of the Na⁺ ions initially present in the zeolite so that the exchange reaction was confined to the large cavities in the crystal. The maximum extent of exchange decreased with an increase in molecular weight and polarizability of the cations but was always below the limit imposed by the space requirement of the respective ions. This decrease was also greater for the di- and trialkyl derivatives than for the monoalkylammonium ions of comparable molecular weight and was more pronounced in X than in Y. A similar relationship was obtained between the standard free energy of exchange and molecular weight. These observations have been interpreted in terms of the increased difficulty of packing the larger and more branched species in the spherical large cavities of the crystal and of the affinity of the organic ion for the zeolite. The importance of affinity of the cations in determining the upper limit to exchange was further shown by the observation that for a given alkylammonium ion this limit decreased with an increase in the affinity of the inorganic ion which it replaced from the zeolite, the order being Li > Na > K > Ag > Tl(I).

It is suggested that there was an equilibrium distribution of the exchangeable ions over the different crystallographic sites, this distribution being governed by the nature of the entering alkyl-ammonium ion and of the cation initially present. If the affinity of the organic ion for the zeolite is relatively great, varying proportions of the inorganic ions may vacate their places inside the network of small cavities and enter the large cavities. For the larger alkylammonium ions having a low affinity for the zeolite or when the ions to be replaced were strongly bond to the zeolite framework (Ag⁺ and Tl⁺), the limit to exchange may fall below that corresponding to the minimum population of exchangeable ions inside the large cavities. A correlation was also found between the upper limit to exchange and the ionic potential (for inorganic ions) and the dipole moment of the corresponding alkylamine indicating that electrostatic attraction was of greater importance than van der Waals forces in determining exchange.