Zeolite Science and Perspectives

R. MILLINI AND G. BELLUSSI

Eni S.p.A., Research & Technological Innovation Dept., R&D Program Energy Transition, Via F. Maritano 26, I-20097 San Donato Milanese, Italy

Email: roberto.millini@eni.com

1.1 Historical Background

The history of zeolites began in 1756, when the Swedish mineralogist Axel F. Cronstedt described the particular properties of minerals found in a copper mine in Svappavari (Lapland, Sweden) and in an unidentified locality in Iceland: when the minerals were heated in a blow-pipe flame, they seemed to boil. For this particular property, not found in other minerals known at that time, Cronstedt coined the term zeolite (from the Greek ?e? = to boil and [TEXT NOT REPRODUCIBLE IN ASCII.] = stone). In 1772 Ignaz von Born used this term to describe cubic crystals found in Iceland (Zeolithus crystallisatus cubicus Islandiae), later defined a zeolite en cube by Jean-Baptiste Louis de Romé l'Isle (1783) and chabasie by Louis Augustin Guillaume Bosc d'Antic (1788); today it is known as chabazite. During the nineteenth century, several authors reported the discovery of new minerals classified as zeolites as well as the description of some of their basic properties. For instance, in 1857 A. Damour observed that crystals of different natural zeolites (harmotome, brewsterite, faujasite, chabazite, gmelinite, analcime, levyne) desorb water, without any apparent change of transparency and morphology. In 1896 G. Friedel examined in detail the reversible dehydration of analcime, concluding that water molecules are simply included and not chemically bonded to the aluminosilicate crystal; he also reported that zeolites (chabazite, harmotome, heulandite, and analcime), once dehydrated, abundantly absorb gaseous ammonia, carbon dioxide, hydrogen sulfide, as well as alcohol, chloroform, and benzene. Later on, F. Grandjean showed that dehydrated chabazite adsorbs ammonia, air, mercury, sulfur, and other species, behavior later confirmed by R. Seeliger and K. Lapkamp. In 1925, O. Weigel and E. Steinhoff reported the adsorption behavior of dehydrated chabazite, which readily adsorbs water, methanol, ethanol, and formic acid, but not diethyl ether, acetone, and benzene. This fundamental property of zeolites was studied in detail by J. W. McBain, who coined the term "molecular sieve". Some years later, R. M. Barrer and D. A. Ibbitson found that linear alkanes (propane, n-butane, n-pentane, and n-heptane) were rapidly adsorbed on chabazite at temperatures4373 K, while branched isomers (e.g. i-butane and i-octane) were totally excluded. Based on these and other observations on the adsorption behavior, R. M. Barrer classified zeolites into three groups.

Following the discovery that soils undergo ion-exchange when contacted with solutions of ammonium salts and that ammonium or potassium are exchanged for calcium, in 1858 H. Eichhorn first reported that this phenomenon reversibly occurs also in natrolite and chabazite.

A major boost to the studies of zeolites occurred in 1930 with the first resolution of the crystal structure of a zeolite, analcite (analcime), by W. H. Taylor followed by those of natrolite, davynite-cancrinite, and sodalite by L. Pauling. This allowed the following main characteristics of these materials to be defined:

1. a tridimensional framework built up of corner-sharing [SiO4] and [AlO4] tetrahedra;

2. the presence of regular channels and/or cages (known as micropores) with free dimensions that vary from one zeolite to another but are generally in the range 3–12 Å;

3. the negative framework charge, due to the presence of [AlO4] tetrahedra, is compensated by alkali (Na, K, ...) and/or earth-alkali (Mg, Ca, ...) cations located in the micropores; they are loosely bound to the framework and easily exchangeable by other cations;

4. the presence of water molecules in the micropores, which can be reversibly desorbed upon mild thermal treatment;

5. the following chemical composition:

(M+)a(M2+)b[Al(a+2b) Sin-(a+2b)O2n]*mH2O

The atomic ratio O/(Si+Al)=2 is typical of the class of the tectosilicates, to which zeolites belong, while according to the Lowenstein's rule, the Si/Al ratio is always = 1.

Gathering all these findings together, in 1930 M. H. Hey wrote the first general review on zeolites, highlighting the critical issues still to be clarified. Only in 1963, J. V. Smith proposed the first definition of zeolite, as "an aluminosilicate with a framework structure enclosing cavities occupied by large ions and water molecules, both of which have considerable freedom of movement, permitting ion-exchange and reversible dehydration".

1.2 Natural Zeolites

Until the 1940s, zeolites were considered minerals without any practical interest, almost exclusively studied by mineralogists, who were more interested in understanding the environments and the crystallization conditions of these phases than in their practical uses. In this period, the discovery of new zeolites concerned mainly minerals of hydrothermal origin, consisting of very large (even cm-sized) crystals occurring as minor constituents in cracks or cavities in basaltic and volcanic rocks. Generally, they are found in the form of large crystals of different morphology and color, often in association with different zeolite phases and other minerals. The latest update on natural hydrothermal zeolites lists 67 different species. Among them, it is interesting to examine the minerals discovered in the 30 years prior to 2013 (Table 1.1).

According to the definition proposed by J. V. Smith, it is clear that some of these minerals:

1. are not aluminosilicates, but contain Be or Zn instead of Al (e.g. Chiavennite, Gaultite, Nabesite) or are beryllophosphates (Pahasapaite, Weinebeneite);

2. do possess an interrupted framework (e.g....