ABSTRACT
Zeolites are characterized by “open” structures. They include channels and cavities of molecular dimensions, normally filled with water and containing extra-framework cations. This peculiarity has attracted the attention of researchers and technologists since the 1950s. In fact, the presence of extended inner surfaces, of the order of many hundred m2/g, including acid sites, makes them the most important heterogeneous catalysts employed nowadays. The availability of wide inner spaces (up to about 50%), when water is removed by heating, allows their use as physical sorbents and molecular sieves. The substantial weakness of the bonds linking the extra-framework cations to the alumino-silicate framework, enables zeolites to be employed as cation exchangers. Also, very useful is their elevated thermal stability, ranging from several hundred degrees centigrade to about 1000°C according to the zeolite type. For all the above reasons, zeolites represent today a unique resource in several sectors of industrial, environmental, and social relevance.[2]
Natural zeolites are a subgroup of the zeolite family, comprising zeolite minerals and zeolite-rich rocks.[3] For about 200 years (they were discovered in 1756), natural zeolites were believed to be minerals of no practical significance, because of their exiguity in hydrothermal occurrences (vugs and cavities of basalts and other traprock formations), the only known occurrences at that time. Starting from the late 1950s, upon the discovery of enormous deposits of zeolite-rich rocks of sedimentary origin, commonly referred to as zeolitic tuffs, natural zeolites have become an important commodity for the solution of environmental and agricultural problems.[4,5]
Among the known natural zeolites (some 60 types), those present in sedimentary formations, which are considered
Ul tra
so un
d –
Zo o
of practical use, are around 10.[4] Tests or applications in animal science concern, however, not more than three or four types, namely clinoptilolite, mordenite, phillipsite, and chabazite. This entry is substantially focused on clinoptilolite, which is the most abundant zeolite type in sedimentary formations. Its presence is signaled in a few hundred deposits spread all over the world. Details on its structure (HEU framework) can be found in the “Atlas of Zeolite Framework Types,” published by the Structure Commission of the International Zeolite Association,[6] whereas an updated, comprehensive review of its properties and applications is reported elsewhere.[7]
In the following, referring to materials used in experiments, mention will be made only to the main zeolite component; actually, these rocks contain other minor crystalline and amorphous minerals, which are not necessarily inert.
