CHAPTER 1
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. Chiavennite, Maricopaite);
3. are anhydrous (e.g. Ammonioleucite)
In 1993, a subcommittee of the Commission on New Minerals and Mineral Names of the International Mineralogical Association started a long and detailed work in defining an appropriate nomenclature of zeolites. Considering the above reported violations of Smith's definition, in 1997 it defined a zeolite mineral as:
"... a crystalline substance with a structure characterized by a framework of linked tetrahedra, each consisting of four O atoms surrounding a cation. This framework contains open cavities in the form of channels and cages. These are usually occupied by H2O molecules and extra-framework cations that are commonly exchangeable. The channels are large enough to allow the passage of guest species. In the hydrated phases, dehydration occurs at temperature mostly below about 400 °C and is largely reversible. The framework may be interrupted by (OH,F) groups; these occupy a tetrahedron apex that is not shared with adjacent tetrahedra".
This is the most recent and complete definition of zeolite, applicable not only to the mineral phases but also to synthetic materials.
The hydrothermal zeolites are of merely scientific interest for crystallochemical and structural studies; they do not have any economic and practical importance because of their low content in the rocks. It is only the determination of their unique properties, useful for many applications in industrial processes, environmental technologies, and products of daily life that promoted the search for commercially exploitable deposits. Starting from the 1950s, deposits of sedimentary zeolite were found; they generally occur in volcanoclastic rocks formed at low temperature and pressure through the diagenetic alteration of tuff and ignimbrite glasses. These deposits are formed by only a few zeolites (analcime, chabazite, clinoptilolite, erionite, ferrierite, laumontite, mordenite, and phillipsite), occurring in the form of small crystals (<10 µm) contained in rocks from 10–20 wt% to 60–70 wt%, the remaining material being other crystalline (feldspars, quartz, calcite, ...) and amorphous (volcanic glass) phases. Among the first deposits discovered, we can cite those located in Japan (green tuff formation, rich in clinoptilolite and mordenite, at Yokotemachi, Akita Prefecture, 1950) and in South Italy (Neapolitan yellow tuff rich in phillipsite and chabazite, Napoli, 1958). In the 1960s, an extensive exploration campaign promoted by US companies (mainly UOP) led to identification of several deposits in western USA. Today, it is virtually impossible to know exactly the number of deposits of sedimentary zeolites in the world. To give an idea, by the end of the 1970s more than 1000 sites were estimated.
From the practical point of view, only rocks with high zeolite content (>50%, according to the petrographic practice defined as zeolitites) may be of economic interest, being potentially or actually employed for various different applications (e.g. building materials, for the separation, purification and dehydration of natural gas, in the purification of domestic, agricultural, and industrial wastewaters, in zootechnics, in agriculture as well as for the removal of radioactive species spread in the environment as a consequence of accidents in nuclear power plants (e.g. Chernobyl, 1986)).
1.3 Synthesis
In 1862, H. Saint-Claire-Deville published a note entitled "Reproduction de la Lévyne", which is the first report on the hydrothermal synthesis of a zeolite, obtained by heating at 443 K an aqueous mixture of potassium silicate and sodium aluminate. Several other papers, published up to the early 1930s and describing the synthesis of other zeolites (e.g. analcime, natrolite, chabazite, heulandite, mordenite, etc.), were systematically reviewed by G. W. Morey and E. Ingerson in 1937. Characterization of the solids obtained was limited to their chemical composition and optical properties and this entailed a considerable degree of uncertainty concerning the correct identification of the crystal phases, making the results at least doubtful. Only with the development of methods for the characterization of polycrystalline materials by X-ray diffraction was the correct identification of the solid products possible.
The modern era of the zeolite synthesis dates back to the 1940s when one of the pioneers in this field, R. M. Barrer, reported the preparation of structurally related zeolites P and Q, both without any natural counterpart and later recognized as having the KFI framework topology, by high temperature conversion of mineral phases in strong alkaline solution.
Another pioneer was R. Milton, who started his research in 1949 at Union Carbide Corporation. It is quite interesting to read his historical perspective published in 1989, where he gave readers a view of the atmosphere and of the difficulties encountered by people contributing to the initial development of zeolite synthesis. In contrast to Barrer, he exploited the higher reactivity of freshly prepared aluminosilicates gels formed by using sodium aluminate and sodium silicate so as to reduce the reaction temperature to 373 K. In this way, at the end of the year, he succeeded in the crystallization of zeolites A, B (gismondine), and C (hydroxy-sodalite). One year later (1950), pure zeolite X (the synthetic counterpart of mineral faujasite), previously found as an impurity in the synthesis of zeolite C, was also obtained. In the same paper, R. Milton highlighted the difficulties encountered with the examiners of US Patent Office, who were not able to understand the novelty of zeolites: the patent applications for zeolites A and X were filed on December 24, 1953 but their publication occurred only on April 14, 1959, i.e. after a long discussion with the examiners.
Until the end of the 1950s, the zeolite syntheses were performed in the purely inorganic system, which imposes a major constraint on the Si/Al ratio of the framework (always very low). An important milestone in the history of zeolites was achieved in 1961, when R. M. Barrer and P. J. Denny succeeded in crystallizing N-A (zeolite A), N-X (zeolite X), and N-Y (zeolite Y) by adding tetramethylammonium hydroxide (TMA-OH) to the reaction mixture. In the same year, G. T. Kerr and G. T. Kokotailo at Mobil Oil Corp. reported the synthesis of ZK-4, isostructural with zeolite A, which only some years later was recognized to be a high-silica phase with Si/Al=1.7. There is no way of knowing if these authors fully understood the implications and potential arising from the use of quaternary ammonium cations in zeolite synthesis. However, after a few years the world of zeolites was revolutionized with the synthesis of some phases that, even today, have a high scientific as well as technological importance. We can refer to zeolite beta and ZSM-5, prepared in the presence of tetraethyl- (TEA-OH) and tetrapropylammonium hydroxide (TPA-OH), respectively. Beta was the first high-silica zeolite, with a Si/Al ratio ranging from 5 to 100, while ZSM-5 was the first case of a zeolite having a pure silica end-member (Silicalite-1).
These results have given rise to an explosion of studies aimed at preparing new zeolite structures, characterized by different pore architectures and sizes, obtained by using organic additives of increasing complexity and by applying advanced synthesis procedures. The number of zeolites is still increasing and the actual portfolio of crystalline microporous structures consists of 232 framework types (note that there were 201 in October 2012) and 22 families of disordered frameworks (i.e. intergrowths of two or more different but structurally related frameworks) officially recognized by the Structure Commission of the International Zeolite Association (IZA-SC). In addition to these, there are several other microporous phases whose structures are still unknown or, if known, have not yet been officially approved by IZA-SC. These data, however, refer only to the framework topologies known today and do not coincide with the number of materials available. In fact, one of the main characteristics of the zeolites is their variable stoichiometry and nature of the chemical elements constituting the frameworks. Virtually all the synthetic zeolites, in fact, can crystallize with variable Si/Al ratio in the framework and each variation produces materials with different properties. Consider, for example, zeolites X and Y, both with the same FAU topology, but characterized by different acid strength, hydrothermal stability, etc. Moreover, Al and/or Si can be replaced (at least partially) by other elements as in the case of the class of crystalline microporous aluminophosphates (AlPOs), discovered in 1982 by Union Carbide Corporation, and their compositional variants (e.g. silico-alumino-phosphates, SAPOs, metalloalumino-phosphates, MeAPO, metallo-silico-alumino-phosphates, MeAPSOs, etc.). On the other hand, it is well known that conventional zeolites may undergo isomorphous substitution, as reported first by J. R. Goldsmith in 1952, who successfully replaced some of the Si atoms with Ge in thomsonite and later by R. M. Barrer et al. who synthesized thomsonite, zeolite A, faujasite, and harmotome containing Ga and/or Ge in the framework. The result of the versatility of the zeolite framework is the huge number of materials with different characteristics and properties available today. It is interesting to examine in more detail how it has come to this.
1.3.1 Role of the Organic Additives
It is has been well assessed that zeolites are mostly prepared by hydrothermal synthesis at moderate temperature (353–523 K) under autogenous pressure. C. C. Cundy and P. A. Cox quite recently wrote two comprehensive reviews, tracing the history of the synthesis of zeolites and examining in detail the relevant phenomena related to the crystallization process in the hydrothermal environment. In the first of these reviews, they state that:
"It is unfortunately fairly common to see in the scientific literature statements to the effect that this process is still at an empirical stage, or poorly understood, or even steeped in some form of alchemical mystery. There is also a tendency to evoke special explanations for some of the phenomena observed, as if they were somehow outside the legitimate realm of classical orthodoxy. Such implications are misleading. Although we do not yet have a complete and detailed understanding of this area of science, a great deal is already established with a fair degree of certainty."
In practice, they argue that what happens during the reaction and the influence of the different parameters on it are well known. What still is insufficient is the ability to predict the conditions necessary for obtaining a given phase, in other words to design the synthesis of new materials.
