Zeolites are normally crystallised from aqueous alkaline gels containig sources of silica, alumina and cations. Crystallisation can take weeks to accomplish, under pressure and at temperatures up to 200 °C. Small changes in mixture concentration, pressure and temperature can change the properties of the final material. Variation of the inorganic base can result in a range of products, and interesting effects can be achieved from the use of organic templating cations, eg. tetraalkylammonium cations, NR4+.
Using large organic templating cations high-alumina zeolites are more favourably formed. The use of templates is quite sophisticated, and zeolites can be tailored to certain specifications.
Zeolite structures can also be modified after synthesis, the simplest being the exchange of extra-framework species. The Si/AL ratio can also be changed by dealumination precedures which involve steaming, acid treatment and ammonium exchange. Other atoms, such as B, Ga, Fe and Ti can also be introduced into the zeolite framework.