The Swedish mineralog Axel Fredrik Cronstedt was rather surprised in the middle of the 18th century when he heated a piece of stone with a burner. What happened? The stone started to bubble and hiss. Water adsorbed inside these stones was released as steam. He called these stones zeolites, after the ancient Greek words zeos (‘boiling’) and lithos (‘stone’).
From a chemical point of view, zeolites are crystalline, hydrated aluminium silicates that contain alkaline-earth metals as sodium, potassium, magnesium and calcium. A remarkable feature of zeolites is that pores are part of the crystal structure. This structure consists of regular channels or interconnected cavities on a (sub)nanometer scale (see image) which may contain for example water molecules. You can remove the water reversibly from these zeolites by heating the material.
Zeolites occur in nature – as in the discovery by Cronstedt – and also exist in artificial man-made form. More than two thirds of the total zeolite market consists of Zeolite Type A, which acts as a water softener in detergents. In this aluminium silicate, part of the Si4+-ions (red) is replaced by Al3+-ions (green), so the lattice at these sites is effectively charged negative. Positively charged sodium ions would like to be located in their vicinity. These sodium ions are loosely bound to the zeolite, and can be replaced easily by other positively charged ions such as calcium ions.
Detergents have the desirable feature that they contain surfactants that can ‘detach’ particles from your clothes. Hard water contains calcium ions that counteract the action of detergents by forming an attractive couple with the negatively charged surfactants. If you could remove these calcium ions from the water, the detergent will operate much better. Therefore, zeolites in detergents are used as ‘ion exchangers’: the sodium ions in the zeolite are replaced by the calcium ions from the hard water, making this water a lot softer. No fabric softener, but ‘water softere’.
You can encounter zeolites also in the chemical industry. They combine two things: a huge internal surface area as a result of the many nanopores, and catalytically active sites as part of the crystal structure. This makes these zeolites ideal for use as catalysts – which accelerate chemical reactions without being consumed. Due to the many pores, chemical compounds that are small enough can enter the zeolites, react at catalytically active sites to desired reaction products which leave the zeolite, also via the pores. Indeed a chemical reactor on nanoscale.
Crystal structure of zeolite type A (blue and yellow shapes fill the cavities)