Chapter 1: Zeolite Science and Perspectives Free

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 ζέω=to boil and λíθος=stone).¹  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.²  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;³  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.⁴  Later on, F. Grandjean showed that dehydrated chabazite adsorbs ammonia, air, mercury, sulfur, and other species,⁵  behavior later confirmed by R. Seeliger and K. Lapkamp.⁶  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.⁷  This fundamental property of zeolites was studied in detail by J. W. McBain, who coined the term “molecular sieve”.⁸  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 temperatures >373 K, while branched isomers (e.g. i-butane and i-octane) were totally excluded.⁹  Based on these and other observations on the adsorption behavior, R. M. Barrer classified zeolites into three groups.¹⁰ 

Following the discovery that soils undergo ion-exchange when contacted with solutions of ammonium salts¹¹  and that ammonium or potassium are exchanged for calcium,¹²  in 1858 H. Eichhorn first reported that this phenomenon reversibly occurs also in natrolite and chabazite.¹³ 

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. Taylor¹⁴  followed by those of natrolite, davynite-cancrinite,¹⁵  and sodalite¹⁶  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)Si_n−(a+2b)O_2n]·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,¹⁷  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.¹⁸  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”.¹⁹ 

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.²⁰  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:

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)²²  and in South Italy (Neapolitan yellow tuff rich in phillipsite and chabazite, Napoli, 1958).²³  In the 1960s, an extensive exploration campaign promoted by US companies (mainly UOP) led to identification of several deposits in western USA.²⁴  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.²⁵ 

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)).^26,27 

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.²⁸  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 1937.²⁹  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,³⁰  by high temperature conversion of mineral phases in strong alkaline solution.^31–33 

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.³⁴  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.^35,36 

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.³⁷  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,³⁸  which only some years later was recognized to be a high-silica phase with Si/Al=1.7.³⁹  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 beta⁴⁰  and ZSM-5,⁴¹  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).⁴² 

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).⁴³  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,⁴⁴  and their compositional variants (e.g. silico-alumino-phosphates, SAPOs, metallo-alumino-phosphates, MeAPO, metallo-silico-alumino-phosphates, MeAPSOs, etc.).⁴⁵  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 thomsonite⁴⁶  and later by R. M. Barrer et al. who synthesized thomsonite, zeolite A, faujasite, and harmotome containing Ga and/or Ge in the framework.⁴⁷  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.

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.^48,49  In the first of these reviews, they state that:

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.

Over the past decades, the synthesis of zeolites has been tackled by focusing on parameters that from time to time were considered worthy of attention. Among them, the nature and the characteristics of the organic additives (improperly called templates, more correctly structure directing agents, SDAs) are, by far, the most extensively investigated. Literally hundreds of different organic compounds (mostly quaternary ammonium cations, but even amines, oxygenates, or quaternary phosphonium cations) were used in hydrothermal syntheses with the primary purpose of preparing new zeolite materials. The detailed listing and analysis of the organic additives used so far is beyond the scope of this chapter and interested readers may refer to selected reviews for the most recent advances on this topic.^50–53  On the other hand, it is useful to examine the role of the organic additives in order to draw some considerations on the perspectives on their utilization.

As stated above, the introduction of organic additives has allowed disengaging from one of the main limitations of the inorganic syntheses, i.e. the low Si/Al ratio of the framework. In fact, as correctly hypothesized by R. M. Barrer and P. J. Denny,³⁷  the relatively large organic molecules compensate a lower number of negative framework charges than the small inorganic cations (Na⁺, K⁺, …) can do. This means that a lower number of trivalent metal ions (e.g. Al³⁺) are incorporated in the framework, whose maximum concentration can be modulated by varying the size of the organic molecule.

Another important aspect concerns the fact that most of the zeolites known so far have been synthesized in the presence of an organic additive, whose role has long been debated. M. E. Davis and R. F. Lobo argued that organic additives could play different roles.⁵⁴  They can simply act as void fillers, stabilizing the inorganic structure towards successive transformations into thermodynamically more stable systems (Ostwald's law of successive transformations). This is probably the role played by small neutral organic molecules, such as 1-propanol,⁵⁵  1-propanamine,⁵⁵  pentaerythritol,⁵⁵  piperazine,⁵⁶  and 1,6-hexanediol^55–57  in the synthesis of ZSM-5. Alternatively, an organic additive can act as a true template capable of promoting the growth of a zeolite phase, whose pore system reproduces the size and shape of the organic molecule itself. The template concept was introduced to explain the role of quaternary ammonium cations, such as TEA and TPA in the synthesis of zeolite beta and ZSM-5, respectively. This induced great expectations in researchers, who recognized the possibility of designing new microporous systems using suitable organic molecules. Unfortunately, these expectations were soon lowered because the desired correspondence zeolite structure↔organic additive rarely occurs; the cases of the triquaternary ammonium cation for ZSM-18 (MEI)⁵⁸  and [18]-crown-[6] for hexagonal faujasite (EMT)⁵⁹  are well-known exceptions. More commonly, it is observed that, depending on the synthesis conditions and/or on the composition of the reaction mixture, an organic additive promotes the crystallization of different zeolite phases, behaving as a structure directing agent (SDA). The lack of specificity of a SDA imposes a time consuming screening activity, necessary to explore the influence of several synthesis parameters and identify the fields of existence of the different phases. To address the problem in a rational manner, several authors have carried out systematic investigations on particular classes of organic molecules, trying to identify the properties that they would have in behaving as SDAs. For example, H. Gies et al. systematically investigated the synthesis of porosils (i.e. pure silica zeolites with framework composition SiO₂) using 61 molecules different in size, shape, and chemical character.^60,61  On the basis of the structural features of the 13 porosils obtained, they concluded that size and shape of the SDA determine the size and shape of the void (cages or channel-like), with a remarkably good geometrical fit between the guest molecules and the host framework. This means that the van der Waals interactions with the framework atoms should be maximized without appreciable deviation from the equilibrium molecular conformation.

Successively, Y. Kubota et al. found that hydrophobicity (evaluated in terms of phase transfer behavior of the iodide salts) and rigidity (determined by the number of ternary and quaternary C atoms) are important features for a SDA. In particular, they found that rigid, bulky, and relatively short (∼10 Å for the longest axis) molecules with moderate hydrophobicity (i.e. those having a C/N⁺ ratio in the range 11–16) are the best candidates for acting as SDAs. Conversely, molecules with strong hydrophobicity render difficult the crystallization of zeolites.⁶²  P. Wagner et al., investigating the host/guest relationships in the synthesis of cage-based SSZ-35, SSZ-36, and SSZ-39 zeolites, found that the size and shape of the 37 different cyclic and polycyclic quaternary ammonium molecules employed are fundamental for determining the porous characteristics of the crystalline products.⁶³  Other interesting examples are based on systematic variation of the structure of a parent organic molecule by introducing substituents of increasing size. Among them, it is worth mentioning quaternary imidazolinium compounds,⁶⁴  ring-substituted and spiro-piperidinium derivatives,^65–69  and diquaternary ammonium compounds.^70–75 

Besides the experimental evidence, the definition of the role of the SDAs has also benefited from the availability of computational methods specifically developed for the study of crystalline–porous systems and their interactions with organic molecules.⁷⁶  As argued by H. Gies et al., to be an effective SDA an organic molecule should be occluded in the pores without significant distortions from the equilibrium conformation, maximizing the van der Waals interactions with the framework atoms.^60,61  Several authors confirmed this hypothesis by performing molecular mechanics and dynamics calculations on different zeolite/SDA systems. For example, D. W. Lewis et al. evaluated the relative stabilization of the MFI and MEL structures containing one and two adjacent TPA and TBA ions. Besides correctly predicting the experimental SDA for the synthesis of the two zeolites, they concluded that an organic molecule favors the crystallization of a microporous phase only when the non-bonding (van der Waals) interactions with the framework atoms are maximized and the SDA molecules efficiently pack within the pores.⁷⁷  On the basis of these criteria, confirmed by several other authors, methods for predicting suitable SDAs for a given (hopefully hypothetical) zeolite framework were developed. The code ZEBEDDE (ZEolites By Evolutionary De novo DEsign)^78,79  proved to be efficient in the selections of 2-methylcyclohexylamine for DAF-4 (LEV)⁸⁰  and 4-piperidinopiperidine for DAF-5 (CHA).⁸¹  More recently, M. W. Deem et al. proposed a new methodology,⁸²  which proved to be successful in the prediction of SDAs for the synthesis of pure silica HPM-1 (STW)⁸³  and small-pore SSZ-39 (AEI)⁸⁴  zeolites. To date, these methods have proven to be efficient for the prediction of SDAs useful for the preparation of zeolites with known structures. In future, they could be further exploited for the prediction of SDAs for the preparation of new zeolites selected among those reported in the databases of hypothetical zeolite structures.^85,86 

With the increase in knowledge on the synthesis of zeolites, it was realized that the lack of specificity of the SDA implies that other parameters (e.g. composition of the reaction mixture, crystallization time, and temperature) also influence the nature of the crystalline phase. One of them is certainly the SiO₂/Al₂O₃ molar ratio in the reaction mixture. As reported above, besides the steric effect, the SDA cations compensate the negative framework charges and replace (at least partially) the alkali metal ions, thereby allowing the crystallization of materials with a higher Si/Al ratio in the framework. SDAs are also known to favor the crystallization of pure-silica zeolite phases (e.g. Silicalite-1); in these cases, the positive charge of the SDA is compensated by the presence of siloxy groups (Si–O⁻) stabilized by H-bonds in the framework.^87–89 

As each organic additive has a well-defined size/charge ratio, the number of SDA molecules hosted in the pores will, therefore, depend on the framework charge density, which, in turn will be determined by the SiO₂/Al₂O₃ ratio in the reaction mixture. To clarify this concept, we can refer to systematic investigations, performed during the 1990s, using N,N-dimethylpiperidinium (DMP) hydroxide as a SDA.⁹⁰  It was found that DMP favors the crystallization of different microporous phases (including the new ERS-7 (ESV) small pore zeolite), whose framework density decreases upon decreasing the SiO₂/Al₂O₃ ratio in the reaction mixture (Figure 1.1).

With the SiO₂/Al₂O₃ ratio fixed at 25 (i.e. the condition for crystallizing pure ERS-7), both the crystallization temperature and time influence in a significant manner the nature of the products (Table 1.2). In particular, ERS-7 crystallizes only at a relatively high temperature (≥428 K) with the consumption of ANA (at 428 K) or MOR (at 443 K), which are formed first.

This behavior, constituting a clear example of Ostwald's law of successive transformations, demonstrates the complexity that lies behind the synthesis of zeolites; the crystallization of a given phase often occurs under well-defined conditions and does not depend on a single parameter or component of the reaction mixture.

The considerations made so far refer to the classic aluminosilicate system; however, it is well assessed that Al and Si can be isomorphically substituted by other tri- and tetravalent elements. The incorporation of Ge into thomsonite^46,47  and Ga and/or Ge, zeolite A, faujasite, and harmotome⁴⁷  reported in the 1950s were considered as mere scientific curiosity until the early 1980s, when this approach was applied to modifying the catalytic properties of zeolites. In this way, B was incorporated into several known zeolite frameworks, producing materials with weaker acid strength with respect to the parent aluminosilicates.⁹¹  More interesting was the successful incorporation of Ti into the pure silica MFI framework, giving the well-known TS-1 catalyst,⁹²  which is still employed industrially today in selective oxidation processes involving H₂O₂ under mild conditions. Besides these technologically relevant results, there is another important aspect related to the isomorphous substitution process, i.e. the possibility to stabilize specific secondary building units (SBUs) that favor the crystallization of zeolites otherwise difficult to obtain in the aluminosilicate system. As theoretically predicted by G. O. Brunner and W. M. Meier, it is possible to obtain zeolites with low framework density (FD, expressed as number of tetrahedral atoms per 1000 ų) and, hence, with high microporous volume, if the framework contains large amounts of three- and four-membered rings.⁹³  This condition can hardly be satisfied in conventional zeolites, since the three- and four-membered rings are less stable than the five- and six-membered rings commonly present in siliceous and aluminosilicate frameworks. On the other hand, the small rings are stabilized by specific heteroatoms, directing the formation of frameworks unstable in the Si/Al system. In this way, zeolites containing three-membered rings are preferentially formed in the presence of divalent ions: Be²⁺ is contained in the minerals Lovdarite (LOV),⁹⁴  Nabesite (NAB),⁹⁵  Roggianite (-RON),⁹⁶  and Alflarsenite⁹⁷  as well as in synthetic OSB-1 (OSO)⁹⁸  and OSB-2 (OBW)⁹⁸  phases; Zn²⁺ favors the crystallization of VPI-7 (VSV),⁹⁹  VPI-8 (VET),¹⁰⁰  VPI-9 (VNI),¹⁰¹  and RUB-17 (RSN).¹⁰² 

The most interesting case, however, concerns the use of Ge, which directs the crystallization of zeolites containing the SBU double-4-ring (D4R). This peculiarity was first demonstrated with the synthesis of some microporous germanates^103,104  and later with the preparation of microporous germanosilicates. In 2000, the group of A. Corma reported that the addition of small amounts of GeO₂ to the reaction mixture reduces the crystallization time of the large-pore ITQ-7 zeolite from 7 days necessary for the pure silica phase to 12 h.^105,106  Quantum mechanical calculations evidenced that the incorporation of up to three Ge atoms stabilizes the D4R units,^106,107  because the smaller average Ge–O–Ge angle reduces the strain in the SBU, rendering it energetically stable. A systematic investigation on 16 different zeolite structures evidenced that the presence of D4R units is a sufficient condition for stabilizing germanosilicate zeolites.¹⁰⁷  Starting from these results, an intensive synthesis activity was carried out by combining the use of different SDAs and GeO₂, leading to the preparation of several new microporous germanosilicates.^51,52  Interestingly, according to the prediction of G. O. Brunner and W. M. Meier, these phases generally have low FD and multidimensional channel systems often with 14-ring or more pore openings.^51,52  Among these extra-large pore zeolites, two deserve mention: (i) ITQ-37 (-ITV), which has a 3D channels system with 30-ring openings and free dimensions 4.3×19.3 Å (Figure 1.2a),¹⁰⁸  and (ii) ITQ-43, with a complex 3D porous system including cloverleaf-like channels similar to those of cloverite (-CLO), but with 28-ring opening and free dimensions of 19.6×21.9 Å (Figure 1.2b). Notably, this is the first example of a zeolite with a hierarchical micro-mesoporous system.¹⁰⁹ 

These observations demonstrate that, indeed, Ge is able to stabilize the SBU D4R with consequent formation of zeolites with low FD and porous systems never obtained before. Because all these zeolites were obtained with SDAs of different complexity, a question arises: Which of the two components is decisive for the crystallization of these phases? Examining the synthesis of germanosilicates with UTL-type topology, O. V. Shvets et al. reported that they can crystallize in the presence of 13 different SDAs, provided that the Si/Ge ratio in the reaction mixture is close to 2. Based on this evidence, they concluded that the role of Ge prevails over that of the SDA.¹¹⁰  In other cases, crystallization of the new zeolites was achieved during a systematic investigation of different synthesis parameters, by using high-throughput experimental approaches such as that employed by the group of A. Corma, which led to the identification of the best conditions for the synthesis of, for example, ITQ-37¹⁰⁸  and ITQ-43.¹⁰⁹ 

In general, these important results are counterbalanced by some drawbacks. Indeed, Ge has a high cost and, above all, its incorporation does not impart any peculiar property (e.g. catalytic activity) to the material. Aware of this, once a new germanosilicate is obtained researchers work to reduce, if not eliminate, this expensive element in the framework, incorporating at the same time catalytically active components (e.g. Al, B). In most cases, this goal was reached, leading to the preparation of new materials with interesting catalytic properties. On the other hand, the easy tendency to hydrolysis of the [GeO₄] tetrahedra, responsible for the low hydrothermal stability of the germanosilicates, was recently found to be the key factor for developing a rational approach for the synthesis of new zeolite materials.

A very timely research subject with potentially high technological impact is two-dimensional (2D) or lamellar zeolites, i.e. materials consisting of layers with thickness limited to 2–3 nm (equivalent to 1–2 unit cells), weakly linked to each other through H-bonds between the surface silanol groups or through the interaction with organic/inorganic species hosted in interlayer spaces.^111,112  By examining these systems, F. S. O. Ramos et al. argued that the term “lamellar zeolite” is an oxymoron.¹¹³  In fact, by their nature, zeolites are 3D networks of corner-sharing [TO₄] tetrahedra with porous systems of molecular dimensions. The lamellar phases, in contrast, are zeolites whose precursors are formed by thin layers structurally related to the parent 3D structure, which forms upon calcination by topotactic condensation of the layers themselves. The first example of a 2D zeolite dates back to 1988 with the preparation of the layered EU-19 phase constituted by hydrated silica layers intercalated by piperazinium cations.¹¹⁴  Upon calcination, the EU-19 transforms to the 3D phase EU-20,¹¹⁵  an intergrowth of CAS-type and NSI-type of frameworks, with 88% and 12% stacking probability, respectively.¹¹⁶  More interesting is the case of MWW-type zeolites, whose formation through a layered precursor was unambiguously demonstrated in 1995. Examining ERB-1, the borosilicate analogue of the MCM-22 zeolite, it was found that the as-synthesized form is actually a disordered layered phase able to intercalate polar molecules and that the ordered 3D structure forms at 543 K, through the condensation of surface silanol groups.¹¹⁷  Subsequently, this behavior was confirmed for the aluminosilicate analogue, MCM-22.¹¹⁸ 

The real breakthrough arising from these results was to realize that the MWW-type layers can be used as 2D periodic building blocks for the preparation of various materials with different structural and textural properties, as summarized in Figure 1.3.

The conventional pathway consists in the calcination of the as-synthesized layered phase (MCM-22P) to form the 3D ordered structure. Alternatively, it is possible:

• to fully delaminate the 2D phase to form materials ideally constituted by isolated randomly arranged single layers;

• to permanently expand the layers with inorganic (e.g. silica, aluminosilicate oligomers) or organic–inorganic (e.g. bridged silsesquioxanes) pillars;

• to treat the layered precursor with specific monomers, which react with the surface silanols, generating ordered 3D structures (the so-called interlayer-expanded zeolites, IEZ) with a pore system larger than that of the parent zeolite.

Recognizing the versatility of these precursors, attention has turned toward the preparation of 2D materials, in particular of zeolites whose formation pathway does not involve a layered precursor. Among the different examples reported so far, two case are worthy of mention. One is the approach proposed by R. Ryoo et al., who designed specific bifunctional SDAs (e.g. H₃C(CH₂)₂₁–N(CH₃)₂–(CH₂)₆–N(CH₃)₂–(CH₂)₅CH₃, known as C_22-6-6), which contain both the quaternary ammonium cations and a long alkyl surfactant chain.¹¹⁹  These SDAs favor the crystallization of MFI nanosheets, which are 2 nm thick in the crystallographic b direction, i.e. that along which the straight 10R channels run.

The second example concerns the 3D–2D conversion of a zeolite structure. Starting from the evidence that Ge preferentially sites in the D4R units and taking advantage of its facile hydrolysis, W. J. Roth et al. found that the 3D UTL-type germanosilicates can be converted into a lamellar structure by mild hydrolysis.¹²⁰  The breakthrough found by these authors was that the layered structure obtained not only can be treated similarly to the other lamellar precursors but, in fact, the layers themselves can be considered as periodic building units, which, in the approach called ADOR (assembly–disassembly–organization–reassembly), constitute the building blocks for the preparation of several interesting new zeolite structures.^121,122 

One of the important concepts behind the interest in 2D zeolites is the possibility to prepare materials with enhanced accessibility of the active sites located inside the crystals, eliminating the diffusion limitations imposed by the pore size. In fact, if on one hand the small size of the pores and their regularity throughout the crystal are at the basis of the success of the zeolites as heterogeneous catalysts, on the other hand they limit the wider use of microporous materials not only in reactions involving bulky molecules but also for exploitation of the active sites located inside large μm-sized zeolite crystals. This concept can be better explained in terms of degree of utilization of a zeolite catalyst, defined by the effectiveness factor (η), i.e. ratio between the observed and the intrinsic reaction rates. This depends on the extent to which the diffusional transport limits the rate of conversion defined by the Thiele modulus (ϕ). The whole utilization of the catalyst particle is achieved when the process occurs without diffusional constraints (ϕ=0→η=1). Conversely, as the value of ϕ increases, the degree of utilization of the catalyst particle progressively decreases: for example, with ϕ=10→η=0.1, i.e. only 10% of the catalyst particle is effectively used. Intuitively, low values of η have a negative impact on the overall process, since the low utilization of the catalyst imposes large reactor volumes, while the diffusional constraints may influence the selectivity and the life of the catalyst. Given that the intrinsic rate coefficient is constant for a given reaction, to increase η it is necessary to shorten the length of the diffusion path by decreasing the dimensions of the crystals. An alternative possibility to consider is to enhance the effective diffusivity of the molecules by employing catalysts with larger pore dimensions, provided that they do not imply variations in the selectivity of the reaction.

Suitable synthesis conditions should be adopted for preparing zeolites in the form of nanocrystals, taking into account, however, that their separation from the mother liquor could be a problem.^123,124  Other specific routes to nanocrystals involve, for instance, the synthesis in confined space in which the crystallization occurs inside the pores of a solid matrix (e.g. carbon black),^125,126  or the addition of organosilanes in the reaction mixture. In this way, the group of D. P. Serrano added phenylaminopropyl-trimethoxysilane to a preformed suspension of zeolite seeds to prevent crystal growth, stabilizing in this way nanocrystals of ZSM-5,¹²⁷  beta,¹²⁸  and mordenite.¹²⁹ 

On the other hand, the discovery of the M41S family of mesostructured materials, announced in 1992 by scientists of Mobil, led to great expectations because they were considered as an extension of zeolites in the mesoporous region.^130,131  In reality, these expectations were not always met because the use of mesostructured materials as heterogeneous catalysts is limited by the lower acidic strength and thermal/hydrothermal stability with respect to zeolites.^132,133  These severe drawbacks explain why no industrial applications of mesostructured materials have been realized so far.

To overcome these problems, which intrinsically characterize all mesostructured materials and are due to the amorphous nature of the mesoporous walls, innovative strategies for the synthesis of new materials having the same properties as zeolites, while maintaining the characteristics of the mesoporous structure, were developed. These materials, ideally constituted by an ordered array of regular mesopores with crystalline microporous walls, would encompass the advantages given by the mesoporous system (i.e. efficient mass transport) and by the crystalline zeolite structure (thermal/hydrothermal stability, better control of the characteristics of the active sites, etc.). Moreover, these materials are characterized by a hierarchical porous structure with interconnected meso- and microporous systems. The preparation, characterization, and use of materials with hierarchical porous systems have been treated in several recent review articles^133–141  and books.^142,143  Here we just provide some general information, in particular focusing on the different strategies employed for the preparation of hierarchical porous systems, summarized in Figure 1.4.

These strategies can be classified into two categories: destructive approaches (which start from preformed zeolite materials and generate the mesoporosity through, for example, de-silication or de-alumination by chemical or physical methods) and constructive approaches (which include methods based on the crystallization (zeolitization) of preformed mesostructured materials, or on the use of dual-templating agents). Focusing attention on constructive approaches, various different strategies were proposed, each of them having a different degree of complexity and success. For instance, attempts at crystallizing the amorphous walls of preformed mesostructured phases through the hydrothermal treatment of materials impregnated with a SDA (e.g. TPA⁺) were usually unsuccessful. In fact, they invariably led to an at least partial collapse of the mesoporous structure with the formation of either a local organization of the walls (identified by, for example, FT-IR spectroscopy) or well-formed zeolite crystals and ultimately of composites rather than the desired mesoporous phases with crystalline walls.

More intriguing are the dual-template approaches, based on the use of mixtures of a micro-template (i.e. a classical SDA employed for the crystallization of zeolites) and a meso-template (i.e., an agent able to generate the mesoporosity). These approaches, in turn, are divided into two categories depending on the physicochemical characteristics of the meso-templates: the hard templates (i.e. solid phases of different natures such as carbon black, carbon nanotubes, polymers) and the soft templates (i.e. organized systems of molecules, such as micelles, or polymers, such as those used in the classical syntheses of mesostructured phases).

The use of soft meso-templates can be seen as a method of imparting either at the same time (one-pot) or by organizing the previously prepared zeolite seeds (two-pot) the micro-mesoporosity to the materials. In reality, these approaches did not lead to the preparation of materials with the desired properties, in terms of thermal/hydrothermal stability and structural characteristics, such as those obtained by the use of hard solid templates. These approaches directly derive from the so-called “confined space synthesis” method in which the crystallization of zeolite nanocrystals occurs in the porosity of a carbon matrix impregnated with the reactant mixture.^144,145  When an excess of reactant mixture is employed with respect to the carbon matrix, large zeolite crystals grow, embedding the carbon particles, which are burned off by controlled calcination, leaving the mesoporosity at the interior of the crystals. The hard templates originally used were composed of spherical particles with controlled average dimensions (carbon black),¹⁴⁶  carbon nanotubes,¹⁴⁷  or nanofibers.¹⁴⁸  More sophisticated and complex approaches were successively developed. Among them, it is worth mentioning the use of carbon replicas obtained by mild carbonization of sucrose impregnated into the mesopores of 3D mesostructured materials, followed by dissolution of the inorganic phases.^149–151  Through the use of these carbon replicas, depending on the severity of the hydrothermal treatment, ordered mesoporous materials possessing microporous characteristics due to the presence of a local zeolitic organization of the amorphous walls or highly crystalline zeolites with disordered mesopores were obtained. More recently, confined crystal growth within the mesopores of carbon replicas of an ordered assembly of size-tunable silica nanoparticles led to the formation of uniformly shaped zeolite nanocrystals regularly organized in large domains with regular mesoporosity.^152,153  These apparently simple approaches are excellent examples of materials engineering but are so time consuming and expensive that the same authors admit that the materials obtained can be used only for advanced applications for which their high costs could be tolerated.

Another interesting approach concerns the recrystallization of preformed zeolites in the presence of a surfactant, a topic dealt with in detail in a recent comprehensive review by I. I. Ivanova and E. E. Knyazeva.¹³⁸  Briefly, this approach is based on a simple concept: the zeolite crystals are partially destroyed with an alkali solution and the zeolite fragments extracted from the crystals generate mesoporosity and are reassembled in a mesostructured phase with the help of the surfactant. The treatment conditions (OH⁻/zeolite ratio, temperature, time) determine the degree of dissolution of the zeolite crystals and, ultimately, influence the characteristics of the final products. Low dissolution degrees favor the formation of mesoporous crystals coated with a thin film of mesostructured phases, while composites of co-crystallized phases are obtained upon increasing the dissolution degree. Finally, when the dissolution of the zeolite crystals is complete, mesostructured materials with mesoporous walls formed by assembly of zeolite fragments are produced.¹³⁸  The recrystallization method is considered as the most versatile and promising route to the preparation of micro-mesoporous hierarchical systems, as confirmed by examining the characteristics of the materials in term of hierarchy factor (HF), defined quite recently by J. Pérez-Ramírez et al. as the product of the relative micropore volume (V_micro/V_total) and the relative mesopore surface area (S_meso/S_BET).¹⁵⁴  In practice, the HF provides an empirical measure of the quality of the hierarchical porous system, since the higher its value the higher the enhancement of the mesoporous surface area without a significant penalization of the micropore volume. This means that the generation of the mesopores occurs without affecting in a significant manner the zeolite characteristics of the material, a very important feature when considering the advantages of maintaining the crystalline nature of the solid.

More recently, the recrystallization method has been applied effectively for preparing mesostructured zeolites consisting of the short-range reorganization of preformed zeolite crystals in the presence of a surfactant to generate regular mesoporosity without losing crystallinity. This method, originally employed on various different zeolites (e.g. ZSM-5, mordenite, zeolite Y),¹⁵⁵  was successively optimized for the preparation of mesostructured zeolite USY used in the formulation of novel FCC catalysts.¹⁵⁶  Compared to a conventional FCC catalyst, use of the mesoporous USY led to a significant improvement of high value products (gasoline, light olefins) and a reduction of coke, dry gases, and LPG formation. These results encouraged the researcher to develop at a commercial level the technology for the preparation of mesoporous USY zeolites also through a commercial partnership with a leading FCC catalyst producer. The new catalyst was also tested in a commercial FCC unit, confirming the results obtained in the laboratory.¹⁵⁷  This is the first and, at the best of our knowledge, unique example of industrial application of a mesoporous zeolite, appropriately designed and optimized during a fruitful collaboration between academia and industry.

As reported above, research aimed at synthesizing new crystalline porous materials (whether new framework topologies or known materials with innovative features) is the focus of several research groups around the world. This thriving activity has not only purely scientific purposes but finds its motivation in the high technological interest in zeolites and related materials, which find applications in several areas. Among them, we give a short introduction to the three main areas of application (heterogeneous catalysis, adsorption/separations, and ion exchange), which will be treated in much more detail in other chapters of this book. To give an idea of the technological importance of these materials, it is sufficient to say that the overall consumption of zeolites is estimated to be ∼5×10⁶ metric tons per year, with the consumption of synthetic zeolites amounting to ∼1.8×10⁶ metric tons per year, as detergents (73%), heterogeneous catalysts (17%), and adsorbents/desiccants (10%).¹⁵⁸ 

Most transformation processes are based on the use of catalysts, which are necessary for accelerating the speed of conversion and for improving selectivity towards the desired products. Over recent decades, the introduction of zeolite catalysts has allowed the phasing out of homogeneous catalytic systems based on mineral acids and chloro-derivatives, bringing relevant economic and environmental benefits deriving from the substantial improvements in the processes yield and selectivity, the quality of the products, and the energy efficiency.

There are several reasons behind the success of zeolites as heterogeneous catalysts. Some of these (e.g. thermal stability, regenerability, life, easy separation from the reaction environment) are common to all solid catalysts, while others are specific to this family of materials. In particular, their crystalline nature allows us to stabilize the tetrahedral coordination of the heteroatoms in the framework. In the case of trivalent elements (e.g. Al, Ga, B, etc.) this generates a negative framework charge that, when compensated by the proton, gives rise to Brønsted acid sites. The presence of structural microporosity open to the exterior of the crystal means that a large part (if not all) of the [TO₄] tetrahedra and, consequently, the active sites of the catalyst are exposed on the pore walls and thus accessible to reagents. In other words, in a zeolite catalyst there is the maximum exploitation of the active component, with an obvious positive impact on its catalytic performance. Precisely the fact that the active sites are located within the micropores is the distinctive feature of zeolites. Indeed, the micropores with well-defined and constant dimensions in the entire crystal can, in many cases, be decisive in defining the reaction path, a concept universally known as shape selectivity. P. B. Weisz and V. J. Frilette first introduced this fundamental concept in 1960, when they observed that with zeolite catalysts the reaction is governed by the mass transfer within the zeolites themselves.¹⁵⁹  These and other observations further integrated by the studies of S. M. Csicsery¹⁶⁰  led to definition of the three classical theories of shape selectivity in zeolite catalysts, schematized in Figure 1.5.

Reactant shape selectivity occurs when a zeolite firstly acts as a molecular sieve, adsorbing from a mixture the molecules whose dimensions are compatible with the pore openings and that will react within the pores. The other molecules, too large to be adsorbed, are eluted without any reaction. In this way, zeolite A selectively dehydrates n-butanol to n-butenes in the presence of i-butyl-alcohol, which does not react; the same zeolite A, after incorporation of Pt, selectively hydrogenates 1-butene to n-butane, but not 2-methylpropene.¹⁵⁹  Note that, operating with non-zeolite acid (silica-alumina) or bi-functional (Pt/silica-alumina) catalysts, respectively, the reactions take place on both linear and branched isomers with similar rates.

A zeolite displays product shape selectivity (PSS) when the reactants are adsorbed and among the different products formed within the pores (mainly in the cages or in channels intersections) only that or those sterically compatible with the pore openings are eluted. The others undergo further reaction to less hindered species, which are eluted, or to bigger molecules that remain trapped in the pores and contribute to catalyst deactivation (coking). A classic example of PSS is the disproportionation of toluene on ZSM-5 that selectively leads to benzene and p-xylene.¹⁶¹ 

The most intriguing theory concerns the transition state shape selectivity (TSSS). In this case, the steric control of the pores is imposed on the most hindered species formed during the reaction, i.e. the transition state. In this way, among the different possible reaction pathways, only that or those involving a transition state sterically compatible with the dimensions of the pore close to the active site take place, while the others are inhibited.¹⁶⁰  For instance, TSSS is invoked to explain the lack of bulky 1,3,5-trimethylbenzene isomer in the transalkylation of m-xylene over mordenite.^162,163  This bimolecular reaction proceeds via a diphenylmethane transition state and mordenite does not have enough space to host the transition state involved in the formation of 1,3,5-trimethylbenzene, which is not found in the reaction products, which are mainly the 1,2,4-isomer.

Beside these classic experimentally demonstrated theories, other types of shape selectivity were proposed to account for some particular situations. They include:

• molecular traffic control – specific to zeolites with ab intersecting system of channels with different dimensions;^164,165 

• windows effect – invoked to explain the different diffusivity of n-paraffins of increasing length in some zeolites such as chabazite and zeolite T (i.e. ERI/OFF intergrowth);¹⁶⁶ 

• pore mouth and key–lock selectivity – proposed to account for the high selectivity of 1D medium pore zeolites in the hydroisomerization of long chain n-paraffins;¹⁶⁷ 

• nest effect – useful for interpreting the shape selectivity variations derived from the presence of active sites on the external surface of the zeolite crystals, including those located at the pore mouths.¹⁶⁴ 

The interested reader can refer to a relatively recent review in which all these types of shape selectivity are treated in some detail.¹⁶⁸  We want to emphasize here that these properties are specific for crystalline-porous solids, and are not found in other heterogeneous catalytic systems. At the same time, it is important to clarify that a crystalline-porous material cannot be defined as a shape selective catalyst in itself, because these properties depend on the reaction and on the species involved, which should have dimensions comparable to those of the pores. Independently of that, there are several commercial processes that take advantage of the peculiar properties of zeolite-based catalysts. In 1999, K. Tanabe and W. F. Hölderich published a comprehensive survey of industrial processes based on the use of solid acid–base catalysts, highlighting the primary role played by zeolites. They examined a total of 127 industrial processes, concluding that 74 of the 180 solid catalysts employed include at least a zeolite phase.¹⁶⁹  A more detailed examination of these catalysts evidenced that only a few zeolite framework types are effectively used, with FAU, MFI, MOR, and Beta being the most extensively employed. A decade later, W. Vermeiren and J.-P. Gilson gave a more detailed overview of the commercial processes using zeolite-based catalysts, updating the number of framework types effectively used.¹⁷⁰ 

The global consumption of synthetic zeolites is estimated to be ∼3×10⁵ metric tons per year and ∼95% of them are FAU-type zeolites employed in the formulation of catalysts for the fluid catalytic cracking (FCC) process. Other important refining and petrochemical processes make use of specific zeolites (e.g. Beta, MWW, and MOR for the cumene and ethylbenzene processes) but the yearly consumption is much lower, essentially because of the longer life (years) of these catalysts compared to those for FCC (months). As an example, it has been estimated that a FCC plant operating in the medium-size Schwedt refinery (230 kbbl per d) has an inventory of 200 metric tons of catalyst and an annual consumption of 1000 metric tons of catalyst. On the other hand, an isomerization unit of the same refinery has an inventory of 90 metric tons of catalyst and an annual consumption of 10 metric tons of catalyst.¹⁷¹ 

Besides these important data, it is interesting to understand the reason why only a small number (actually 13)¹⁷⁰  of the 232 framework types officially recognized are commercially employed as catalysts. In fact, one would expect that the wide range of structures with different pore size and dimensionality of the porous system (from 1D to 3D) favors the use of zeolites in a wider number of processes. However, there are some obstacles, which currently limit the achievement of this objective. The most important concern the high costs of the materials, in turn determined by long synthesis procedures, by the use of expensive raw materials (e.g. Ge), or complex and commercially unavailable SDAs. In addition, many new zeolite structures are still poorly studied and the possibility of tuning their composition and morphology is still to be demonstrated. This, however, should not be considered a reason to interrupt successful work on the synthesis of new structures or the modification/optimization of known materials.

Separations processes are critical in industry. Several technological options are available, including distillation, crystallization, extraction, membrane separation, absorption, and adsorption. The use of one or the other option depends on the properties of the chemical species contained in the mixture; when two or more technologies prove to be effective for a given separation, selection is made on the basis of the complexity of the operations and, finally, of the costs.

The technology most widely applied is distillation, which is used in >90% of the processes of the chemical industry.¹⁷²  Distillation, however, is infeasible when the components of the mixture have very similar boiling points as in the case, for instance, of the separation of n-pentane and n-hexane from their branched and cyclic isomers, an important process for upgrading the gasoline octane number. To solve this and other similar problems, separation techniques based on the different physical affinity of different components of the mixture should be preferred. We are referring, in particular, to adsorption processes based on the preferential partitioning of substances from the gaseous or liquid phase onto the surface of a solid substrate, by accumulation or concentration phenomena. In general, the physical adsorption of molecules occurs through the weak van der Waals interactions, sometimes through the electrostatic interactions, between the adsorbate molecules and the atoms on the solid surface. Adsorbents such as activated carbon, silica gels, etc. are largely used when the species in a mixture have significantly different affinities with the surface, but cannot be used to separate mixtures of hydrocarbons with similar properties. In these particular cases, zeolites offer clear advantages because the molecules adsorbed within the pores are firstly selected based on their relative dimensions with respect to those of the pore openings. In other words, zeolites behave as molecular sieves, allowing a facile and effective separation of molecules based on their effective dimensions. The adsorption properties of zeolites can be finely tuned in several ways, extending their potential uses for the separation of molecules with different characteristics in terms of polarizability, hydrophobicity/hydrophilicity, etc., achieving excellent separations even when no steric hindrance occurs.^173,174  In this way, the effective dimensions of the pores of a given zeolite structure can be modulated by the appropriate choice of the extra-framework cations, located within the pores to balance the negative framework charge.¹⁷⁵  On the other hand, the hydrophilic/hydrophobic character of a zeolite can be tuned by changing the SiO₂/Al₂O₃ molar ratio: in low-silica zeolites (e.g. zeolites A, X, Y, mordenite, natural zeolites in general) the strong electrostatic field within zeolite cavities results in very strong interactions with polar molecules (e.g. H₂O, alcohols), while high-silica zeolites (i.e. Silicalite-1, ZSM-5) are non-polar adsorbents.

Adsorption processes based on molecular sieving, electrostatic fields, and polarizability are always reversible and this renders zeolites ideal candidates for the separation processes. Zeolites can undergo a virtually unlimited number of adsorption–desorption cycles. This accounts for their considerable economic value in separation processes, with an economic impact similar to their use in refinery and petrochemical catalytic processes.¹⁷⁶  Zeolites are also used in several other industrial fields, including treatment of natural gas, industrial gas production and purification, and the production of specialty and fine chemicals and pharmaceuticals.^177,178  Also important is the use of zeolites in environmental protection (i.e. recovery of solvents from industrial off-gases, builders for phosphate-free laundry detergents, nuclear waste management, etc.).^179,180 

Among such uses, we focus here on the remediation of contaminants from groundwater, a topic not treated in any chapter of this book. Such contamination is one of the main drawbacks of fast economic growth, since it has been estimated that ∼80% of the world's population is exposed to high levels of threat to water security.¹⁸¹  It is therefore clear that water remediation is a major challenge today. Groundwater contamination is an issue due to ineffective waste management, widespread dispersion of chemicals, or the natural release of harmful species from rocks and soils. The quality of a groundwater depends on the content of contaminants (inorganic and/or organic), whose concentrations must be reduced to within the limits imposed by legislation designed to render water available for human uses. Two main approaches are employed for this purpose:¹⁸² 

• The pump & treat (P&T) technology, which involves the pumping of groundwater on the surface and its treatment before reinjection, is the most widely used. It suffers, however, from some drawbacks related to long operating times and to the high-energy demand of the water pumping.

• The use of permeable reactive barriers (PRBs), in which remediation is made directly on the groundwater, avoiding the need and the costs of pumping it on the ground; in this case, beds of active material are sunk into the plume, forming active cells where contaminants are trapped or converted into non-hazardous compounds.

The material used to treat water is the core of all these technologies. When considering organic contaminants only, granular activated carbons (GACs) are the most popular materials employed in the P&T technology owing to their low cost and high efficiency in removing a wide range of contaminants. GACs, however, have some limits the most important being the easy adsorption of humic substances and inorganic species often contained in groundwater, which reduce the efficiency in removing other contaminants and strongly affect the possibility of regenerating the exhaust material. Moreover, GACs have poor effectiveness in removing highly soluble contaminants and/or polar molecules (e.g. alcohols, ethers (in particular, methyl tert-butyl ether (MTBE)), sugars, starches, etc.). In more innovative systems, other materials such as ion-exchange resins on polymeric supports or other specific adsorbents replace GACs.¹⁸³ 

On the other hand, depending on the type of treatment, PRBs can be classified as:

• adsorbent barriers, in which the material used is an adsorbent (e.g. GAC);

• biological barriers, which contain autochthonous bacterial colonies able to convert the organic contaminants into non-toxic compounds (bioremediation);

• chemical barriers, based on the use of materials (zero-valent metals such as Fe, Sn, Zn, Pd/Fe, Ni/Fe, etc.) able to convert the organic contaminants into less harmful compounds and to reduce some heavy metals to non-toxic species (e.g. Cr^VI to Cr^III). They have a narrow field of application (limited to some heavy metal ions and to chlorinated aliphatic compounds) and a relatively slow kinetics for the degradation reactions (which implies an increase of the residence time and, consequently, of the thickness of the barrier). On the other hand, the overall life of the material depends on the nature and concentration of the chemical species dissolved in the groundwater.¹⁸⁴ 

Biological and chemical barriers are preferable to adsorbent ones because, in principle, they do not require any regeneration. However, the use of zeolites instead of GACs may open up new opportunities for developing efficient adsorbent barriers.

Natural zeolites (e.g. clinoptilolite, heulandite, chabazite) are available in huge quantities in mineral deposits. Therefore, they are considered cheap materials since their cost amounts to less than US$1 kg⁻¹. Their peculiar characteristics (low SiO₂/Al₂O₃ molar ratio, high polarity, presence of large amounts of extra-framework cations) render them ideal for ion-exchange processes. In fact, huge amounts of natural zeolites were used to capture radionuclides after the Chernobyl (Ukraine) and Three Mile Island (USA) disasters as well as for the removal of radioactive ¹³⁷Cs and ⁹⁰Sr isotopes from nuclear industry effluents.^185,186  Other more conventional uses of natural zeolites involve the removal of heavy metals (e.g. Fe, Pb, Cd, Zn) from acid mine drainage and the purification of water and urban wastewaters from ammonium and heavy metals.¹⁸⁶ 

While quite effective for the removal of cations, zeolites as such display low affinity towards anions and non-dissociated compounds. In these cases, the adsorbents require modification or, at least, careful selection.¹⁸⁷  A quite interesting approach concerns the surface modification of natural zeolites (clinoptilolite) with bulky alkylammonium surfactants, leading to so-called surface-modified zeolites (SMZs), which have been successfully implemented up to pilot-scale level in both PRB¹⁸⁸  and P&T¹⁸⁹  configurations. Interestingly, the surface modification has a minimal influence on the cation-exchange properties of the zeolites¹⁹⁰  so that SMZs can be considered as multipurpose materials able to adsorb:

• cations at the framework exchange sites;

• anions by interaction with the cationic heads of the surfactant molecules;

• neutral species by partitioning into the hydrophobic tail bundles of the surfactant molecules.

Despite their effectiveness in removing the different species, SMZs present some drawbacks, including the difficult regeneration of materials exhausted with inorganic salts, for which no suitable procedure has been developed, and the pronounced surfactant leaching.¹⁹¹  Regarding the removal of the neutral species (organic molecules), it has been proved that siliceous (e.g. silicalite-1) or high-silica zeolites (e.g. mordenite and Beta with a high SiO₂/Al₂O₃ molar ratio in the framework) are better than GACs in terms of specific capacity and stability under harsh conditions,¹⁹²  avoiding at the same time the drawbacks of SMZs.¹⁹³  Siliceous zeolites are at the base of the En-Z-Lite™ process developed by Eni and the results of one-year long test concerning a PRB built under a refinery evidenced their high efficiency and structural stability.^193,194  The PRB, constituted by a sequence of packed beds filled with more than 100 kg of silicalite-1 and siliceous mordenite, treated up to 8 m³ d⁻¹ of groundwater contaminated with ∼5 mg L⁻¹ of petroleum hydrocarbons and ∼5 mg L⁻¹ of methyl tert-butyl ether (MTBE). The results were very positive, since the concentrations of the contaminants were constantly kept below the local water management specifications for the whole duration of the test. These results demonstrated the effectiveness of siliceous zeolites in the remediation of groundwater polluted by organic compounds. Therefore, zeolites represent a valid alternative to other conventional adsorbents, which are surely cheaper but even less specific and effective in removing such contaminants. Zeolites, on the other hand, are surely more expensive but the higher initial costs for the adsorbents can be compensated by the longer life, assured by excellent stability and easy regenerability.

In this chapter, we have introduced the science and technology of zeolites, a very diverse world whose complexity is such as to make it difficult its comprehensive illustration in a few pages. However, the topics introduced here will be treated in the other chapters of this book, which will provide the reader with a comprehensive picture of the synthesis, characterization, and applications of this exciting class of materials. Exciting because, in the big world of materials, it is difficult, maybe impossible, to find such versatile systems that find applications (actual or potential) in such a wide range of technological sectors. The impressive advancements achieved from the 1950s, when the modern era of zeolite science and technology began, are due to the stronger and stronger interactions between materials science and chemical engineering. This has had an important impact on the vital industrial sectors of oil refinery and petrochemistry, enabling the development of new processes that are more efficient and environmentally friendly than the old ones. Each new discovery, every new process developed, has stimulated further research that has seen not only academia but also the major oil and chemical companies invest considerable resources in all fields of the science and technology of zeolites, from the synthesis of new materials to their modification, characterization, and application.

With the progress of research, an increasing number of original frameworks were synthesized, not necessarily in the classic aluminosilicate system so that, today, the concept of zeolite is extended to virtually all porous crystalline materials with a framework constituted by corner-sharing tetrahedra, independently of their chemical nature. Each of these framework types, in turn, can be obtained in different compositional variants so that a huge number of materials is available today.

This wide availability of materials with different characteristics and properties is important in order to expand the use of solid microporous material not only in new chemical processes but also in advanced technological sectors. The significant advances in the synthesis of microporous solids achieved during recent decades have been favored by the simultaneous development of knowledge of the phenomena that take place during the nucleation and growth of zeolites and the ability to characterize in detail their complex structure. At the same time, also very important are the studies on ion exchange, adsorption, and catalysis in which the advent of sophisticated methods of molecular modeling and computational chemistry has led to a high level of understanding of the phenomena related to a single process. Over the years, the attention of researchers has been focused on issues that, in turn, appeared more promising. As widely reported in this chapter, attention turned first to the role of the SDA, considered the key component in driving the crystallization of a zeolite phase. It was then understood that the formation of a zeolite is the result of the combination of a number of different factors, often interdependent, and cannot be easily attributed to only one of them. A recent example concerns the role of Ge in the stabilization of the double-four-ring (D4R) units that, in combination with the proper SDA, led to the crystallization of several low-density zeolites, with large and extra-large pore openings. The peculiar properties of these structures and the easy hydrolysis of Ge is at the base of the ADOR approach, which is an outstanding, probably unique, example of crystal engineering. The several new zeolites derived from this approach demonstrate that it is possible to realize the old dream of researchers, i.e. to design and synthesize new structures with the desired porous properties. This in our opinion is a significant change in zeolite synthesis that today is essentially limited to the UTL system but if extended to other frameworks may open new perspectives in the preparation of microporous solids.

Another important research topic concerns modification of the morphologic and textural properties of known zeolites. The 2D zeolites and the materials derived from their delamination, pillaring, etc., and the hierarchical zeolites with interconnected meso- and microporous systems represent some of the answers given to the need to reduce the diffusion limitations that often negatively influence the overall performances of zeolite catalysts.

There are other research lines that are still in their infancy and therefore largely unexplored, but in principle are very attractive because they are expected to lead to interesting developments. We refer, in particular, to the synthesis of hybrid organic–inorganic zeolites, i.e. silicates or metallosilicates with organic groups in the framework. The availability of such materials (of which the Eni Carbon Silicates, ECSs, represent the most important family)¹⁹⁵  could indeed open up new opportunities both in classical (catalysis and separation) and in advanced technological fields (sensors, optical devices, nonlinear optics, etc.). Another example is represented by the synthesis of chiral zeolites, whose availability would expand the use of crystalline microporous materials for applications in enantioselective catalytic and separation processes of high interest, for example, to the pharmaceutical, agrochemical, and fragrance industries. The concept of chiral zeolites is now well assessed and verified experimentally, but there are some difficult challenges to face, the most important being that related to the preparation of pure enantiomeric forms of zeolites. For more details, we refer the interested reader to some recent papers that illustrate the concept of chirality in microporous solids, the problems that exist today, and possible solutions.^196–199 

All of these activities demonstrate the vitality of research in the field of zeolites, which aims to develop new materials and processes that provide new solutions to the growing demand for efficient technologies, with lower energy consumption and a lower environmental impact than those available today.