What makes certain zeolites better at ion exchange?

I would have thought that the bigger the pores in the zeolite, the more ions it can contain, and therefore the better the use of the zeolite for ion exchange.


However, zeolite A has small pores but its main use is as an ion exchanger, as opposed to many other zeolites which have larger pores but are not used as ion exchangers.


Therefore, why are some zeolites better than others at ion exchange? Or, what influences the ion exchange capabilities of zeolites beyond pore size?




Zeolites are *selective* ion exchangers.


What is usually required for a good zeolite is that it *selectively* absorbs the ion you care about rather than any old ions in the mixture.


That selectivity is a product of the *shape* of the holes in the 3D structure. Bigger holes will be *less* selective than smaller holes. If all the ions in the mix fit in the hole the zeolite won't reject the ones you don't want. So, ideally, the holes will be big enough for one ion but too small for another one. This allows selective exchange.


That is about it. It isn't about general equilibration of ions; it is about *selective* exchange of ions.


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The ion *exchange* mechanism *will not* simply fill the inner cavities up to the rim with cations; rather than *substitute* already present ions by others. 


For water softening, if the the free / accessible diameter of the pores of zeolite(s) is at least 3 Å, more relevant properties for ion exchange for a potential application are, for example,


* the theoretical number of sites where ions may be exchanged per formula unit (exchange capacitity)
* the strength the ions to be released (in exchange to the incoming ones) are bound to the framework
* the strength the newly bond ions are bond to the frame (often you want to regenerate / recycle the zeolite)
* if there is selectivity in binding ions
* the exchange rate between incoming and released ions
* resilience of the framework under the parameters of use (temperature, pressure, acidity of the medium; mechanical stress by weight / fluid flux; repeated regeneration)
* the absence of toxic / harmful products relased during its deployment
* cost in production and waste disposal of this material


The use of synthetic zeolites of different diameters accessible (3, 4, 5 Å, to mention just *a few*) often aims to benefit from their reactivity as Brønsted / Lewis acid as well as their selectivity due to the geometrical constrain imposed. Hence a 3 Å-zeolite may be used to trap selectively remaining small quantities of water out of ethanol used as biofuel, for example.


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