Zeolites are crystalline aluminosilicates

1.0INTRODUCTION TO ZEOLITES

Zeolites are crystalline aluminosilicates, composed of TO4 tetrahedra (T = Si, Al) with O atoms connecting neighbouring tetrahedral, that contain pores and cavities of molecular dimensions (Breck, 1974). Many occur as natural minerals, but it is the synthetic varieties which are among the most widely used sorbents, catalysts and ion-exchange materials in the world (Barrer, 1982).

The channels are large enough to allow the passage of guest species. In the hydrated phases, dehydration occurs at temperatures 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. Zeolites are different from other porous hydrates, as they retain their structural integrity upon loss of water.

The Structure Commission of the International Zeolite Association identifies each framework with a three-letter mnemonic code (Baerlocher et al., 2001) e.g. Amicite- GSI; Faujasite- FAU etc.

1.1IMPORTANCE OF GREEN PROCESSES

In the chemical industry, the acceptability of a process is not only governed by cost and yield but in terms of eco-friendliness and pollution abatement. Choosing a more efficient catalytic route has greatly improved the efficiency of chemical processes.

Green chemistry has been defined as the design of chemical products and processes in order to reduce or eliminate the generation of hazardous substances (Armor, 1999). The principles of green chemistry listed by Armor (1999) employs future approaches to new chemical processes. It includes: efficient use of raw materials, energy efficiency, use of biodegradable products and other subtle features.

2.0 HISTORICAL DEVELOPMENT OF ZEOLITES

2.1NATURAL ZEOLITES

Research in the field of zeolite science and technology made its first steps with natural zeolites and was mostly focused on natural zeolites until the beginning of the 1950s. The history of zeolites began in 1756 when Swedish mineralogist A.F. Cronstedt discovered the first zeolite mineral, stilbite when studying its apparent properties discovered its strange behaviour upon heating although there is no certain proof of its identity. The term ‘zeolite’ was coined from two Greek words, ‘zeo’ (to boil) and ‘lithos’ (stone). On the contrary, the first zeolite, chabazite, described by Bosch D’Antic in 1792 has clear evidence in literature. Several other zeolites were discovered in the following years and around 1850, only about 20 zeolite types were reported in mineralogy books, including analcime, brewsterite, chabazite, edingtonite, epistilbite, faujasite, gismondine, gmelinite, harmotome, heulandite, laumontite, levyne, mesolite, natrolite, phillipsite, scolecite, stilbite, and thomsonite. Starting from the middle of the 19th century until about 1975, there was a moderate increment in the number of zeolites discovered (about one new type every 6-7 years) and a clear acceleration in the last twenty five-thirty years. About 40 natural zeolites are known (Tschernich, 1992). Most zeolites known to occur in nature are of lower Si/Al ratios, since organic structure-directing agents necessary for formation of siliceous zeolites are absent. Sometimes natural zeolites are found as large single crystals, though are very difficult to make in the laboratory. The catalytic activity of natural zeolites is limited by their impurities and low surface areas.

However, interests in natural zeolites shifted towards zeolite synthesis and synthetic materials, as they offered a series of advantages such as wider versatility, more open frameworks( for adsorption and catalysis),and quality in constitution and chemistry. As a result, research on natural zeolites, was mainly devoted to ion exchange process which was discovered around 1850 (Thompson, 1850; Way, 1850). Few years later, Eichhorn observed that chabazite and natrolite behaved as reversible ion exchangers. In the early decades of the 20th century, ion exchange selectivity of a variety of zeolites for peculiar cations, e.g., ammonium was performed (Barrer, 1950) and starting from the end of the 1950s, found uses in various sectors of environmental relevance, e.g., treatment of wastewaters and soil rebuilding and remediation. The most recent frontier in the application of natural zeolites is in the field of life sciences.

One of the drawbacks of natural zeolite research for application purposes is due to the limited availability of zeolite as it is a precious mineral, compared to the synthetic counterparts which could be mass produced at a lower cost (Colella, 2005).

2.1.1 Formation

The pathway of natural zeolite formation is similar to the laboratory synthesis of zeolite. Zeolite nucleation, crystallisation and crystal growth take place as a result of slow to fast cooling of warm to hot magmas(of volcanic origin), which are basic, oversaturated in silicate and aluminate species and contain alkaline and/or alkali-earth cations.

hot fluid + volcanic ash oversaturated basic magma zeolite crystals

{solution + gel)

The magma is obtained via hydrolysis of the original glassy material and is responsible for the tetrahedral coordination of aluminium and together with silicon. The main factors responsible for the structural formation are temperature, chemistry of the ash and the chemical composition of the resulting solution. Gel is formed along the process but is however not directly connected to nucleation and growth, as there is evidence that zeolite nuclei form from the oversaturated solution at the glass shards / solution interface (Aiello et al., 1980).

Temperature and time are two factors which differentiate natural zeolitisation from laboratory synthesis.

2.1.2 Physico-chemical properties

i.Cation exchange: The ion exchange properties of natural zeolites depend on their chemistry which ismainly in terms of selectivity. Selectivity depends on the framework topology, ion size and shape, charge density on the anionic framework, ion valence and electrolyte concentration in the aqueous phase (Barrer et al., 1978).

ii.Reactions with alkalis: Oncein alkaline environments, zeolites become unstable as they tend to transform, similarlyto glassy systems, into more stable phases, usually into other framework silicates (Goto and Sand, 1988). The interaction of zeolite-rich materials with Ca(OH)2 give rise to calcium silicates and aluminates, which upon hydration are able to harden in both aerial and aqueous environments. This behaviour makes them to be known as pozzolanic materials

Thermal properties: Heating of zeolite powder induces physical and chemical changes, which have been shown to include water loss (which causes expansion on heating), decomposition and gas evolution, phase transition, structure breakdown, re-crystallisation, melting etc (Colella, 1998). This property enables zeolite tuff stones to display good sound-proofing and heat insulation and serve as good building materials. Depending on zeolite nature, chemical composition and rock constitution, the tuff expands as a result of quick heating at temperatures of 1250°C or above, inadvertently followed by a rapid quenching to room temperature.

2.2SYNTHETIC ZEOLITES

Early work could be traced back to the claimed synthesis of levynite by St Claire Deville in 1862 as there were no reliable methods for fully identifying and characterising the products. The origin of zeolite synthesis however, evolved from the work of Richard Barrer and Robert Milton which commenced in the late 1940s. The first synthetic zeolite unknown as a natural mineral later found to have the KFI structure (Baerlocher et al., 2001 ) was discovered by Barrer when investigating the conversion of known mineral phases under the action of strong salt solutions at fairly high temperatures (ca. 170-270 °C). Robert Milton was the first person to use freshly precipitated aluminosilicate gels to carry out reactions under milder conditions. This led to the discovery of zeolites A and X (Milton et al., 1989). Initially, the synthesis of zeolites required the use of only inorganic reactants but was however expanded in 1961 to include quaternary ammonium cations leading to the discovery of silica-rich phases (high-silica zeolites). Subsequently, more synthetic zeolites have been discovered (Baerlocher et al., 2001), as well as zeolite-like or zeolite-related materials (Szostak, 1989) known as zeotypes- represented by microporous alumino- and gallo phosphates (AlPO4s and GaPO4s) and titanosilicates.

Studies on understanding zeolite synthesis have continued to be carried out upto the present day (Table 1). This has been due to discoveries of new materials, advances in synthetic procedures, innovations in theoretical modelling methods and, especially, by the development of new techniques for the investigation of reaction mechanisms and the characterisation of products.

Table 1: Evolution of materials development in the zeolite field

Source: Flanigen (1980)

2.2.1Mechanism of Hydrothermal Synthesis

Experimental observations of a typical hydrothermal zeolite synthesis

Due to its chemical reactivity and low cost, amorphous and oxide-like Si and Al which make up the microporous framework are mixed with a cation source usually, in a basic water-based medium. The resulting aqueous mixture is then heated in a sealed autoclave at above 100˚C allowing the reactants to remain amorphous for sometime (induction period) after which crystalline zeolites are detected (Figure 2). Gradually, an approximately equal mass of zeolite crystals which is recovered by filtration, washing and drying replaces all the amorphous materials (Cundy and Cox, 2005).The bond type created in the crystalline zeolite product (e.g. zeolite A or ZSM-5) which contains Si-O-Al linkages is similar to that present in its precursor oxides, therefore the enthalpy change is not great. This process reduces nucleation rates, thereby forming larger crystals.

Reactivity of the gel, temperature and pH affect the rate of zeolite formation as an increase in pH and temperature leads to increase in the rate of formation of zeolite crystals. In their mother liquors, the zeolitic phases are metastable, thereby transforming the initial zeolite into an undesired thermodynamically more stable phase (Ullmann, 2002).

2.2.3 Synthesis from Clay minerals

Kaolin and metakaolin (calcining kaolin at 500-700°C) are two important clays used for the production of the zeolites NaA, NaX, and NaY (Breck, 1974; Barrer, 1978) because binder-free extrudates and granules which offer advantages in adsorption technology are produced.

Kaolin Metakaolin

Depending on the zeolite, the clay is shaped and, SiO2and seed crystals are added and while in the preformed shape, the zeolite crystallises. Alternatively, zeolite is formed when the binder component of metakaolin undergoes hydrothermal treatment with sodium hydroxide solution (Goytisolo et al., 1973; Chi and Hoffman, 1977). Using ultrasonic radiation, reaction rate is enhanced and there is energy saving and lower production cost due to lower temperatures. This process is less often used as it could cause odor of the product due to impurities present in clay e.g. iron

2.2.2Industrial Zeolite Synthesis

Zeolite synthesis is an extremely broad area of research and due to differences in the preparation of each zeolite type, two representative zeolite types, TPA-ZSM-5 and zeolite Na-A, are chosen for a more detailed presentation of the synthesis {Table 2} (Jansen, 2001).

Table 2: Synthesis mixtures, physical & chemical properties of the representative zeolites

Source: Jansen (2001)

The composition of zeolite product can be expressed by the cation type and its overall Si/Al ratio. In the preparation of zeolite, nucleation is the rate determining step which is influenced by a range of factors dependent on the temperature of the reaction mixture.

Low Temperature Reaction Mixture: Here, the reaction mixture is prepared at low temperature, < 60˚C providing adequate aging. Sol particles which are homogenously dispersed or aggregated are present in the (alumina) silicate gel phase. If OH- is the mineralising agent, the pH of the liquid phase will range between 8-12 (Jansen, 2001). At relatively high pH, the monomeric ions of Si-species are abundant which is released via hydrolysis (Figure 4).

At high pH, condensation occurs when the nucleophilic deprotonated silanol group on monomeric neutral species is attacked (Figure 5). The acidity of the silanol group depends on the number and type of substituents on the silicon-atom (Jansen, 2001).

Temperature raise of the reaction mixture from <60˚C to <200˚C depends on the autoclave size, pattern of agitation and viscosity of the reaction mixture. Gel re-arrangement, dissolution of gel into monomeric silicates and dissociation of silicates, and degradation of quaternary ammonium ions take place (Jansen, 2001).

High Temperature Reaction Mixture: At this temperature, zeolites are formed from amorphous material which involves, reorganisation of the low temperature synthesis mixture, nucleation and precipitation (crystallisation). During the induction period, gel and species in solution rearrange from a continuous changing phase of monomers and clusters which disappears through hydrolysis and condensation, in which nucleation occurs (Jansen, 2001). The process particles become stable and nuclei forms, followed by crystallisation which could occur in metastable solid, highly dispersed or dense gel forms.

Product quality, reaction time and yield influence efficient production of zeolites by optimising their composition.

2.2.2 Secondary Synthesis Methods

Catalytic or adsorbent properties that cannot be achieved by direct synthesis utilise post-synthesis (secondary) treatments to increase catalytic activity, shape selectivity or porosity and thermal/hydrothermal stability. Dealumination and ion exchange are used to carry out these modifications.

Dealumination

The zeolite structure is selectively dealuminated by acid solutions, washing out aluminium out of the crystal, as was observed for zeolite A. However, for higher silica containing materials (clinoptilolite), a fully decationated structure is produced after continuous acid treatment. The metal ion is replaced with H3O+ followed by (Al+3 + H3O+) removal, generating a hydroxyl nest.

Aluminium is removed from the framework but not the crystal by hydrothermal dealumination. The heterogeneity in the concentration of the framework and non-framework of aluminium depends on the type of modification used. Hydrothermal treatment causes the amorphous aluminium to collect on the crystal surface which through fluorosilicate treatment can reduce aluminium centred acid sites. Often, a secondary pore system is generated and hydroxyl nests can be annealed. In order to enhance the catalytic properties as well as stability, silicon, aluminium and other metal ions are introduced into the framework (Szostak, 2001). Other methods of producing thermally and hydrothermally stable cracking catalysts include: use of EDTA, SiCl4 vapor, and (NH4)2SiF6.

Acid mediated dealumination process via aluminium extraction and generation of hydroxyl nest (Szostak, 2001)

Ion Exchange

This is an important technique in pore-size engineering for the production of zeolitic adsorbents (Breck, 1974). Ion exchange used in the production of Brønsted acid sites has major importance in the synthesis of solid acid catalysts (Ullmann, 2002). Ion exchange can be achieved also, for certain intermediate-silica and high-silica zeolites (e.g., mordenite) by treatment with mineral acids although involves the risk of dealuminating the zeolite framework (McDaniel and Maher, 1976). An indirect route via an ion exchange with ammonium salt solutions must be followed, producing the “ammonium form” calcined at ca. 400°C to liberate ammonia and give the hydrogen form (Ullmann, 2002). When cations to be exchanged are positioned inaccessible cages, a sieve effect is produced.

pH is an important factor in ion exchanging of highly charged transition metal ions in order to prevent metal hydroxide precipitation especially at low pH.

2.3 CHARACTERISATION OF ZEOLITES

In order to determine the relationships between the physical and physicochemical as well as sorptive and catalytic properties of zeolites, it is important to know the structural, chemical and catalytic characteristics of zeolites. Several standard techniques are employed in zeolite characterisation. The most common of which is X-ray diffraction used in determining the structure and purity of zeolites. Others include: x-ray fluorescence spectroscopy (XRF) or atomic absorption spectrometry, used to analyze elemental composition, sorption analysis to study the pore system, IR-spectroscopy, typically using adsorbed probe molecules to characterize the acid sites, scanning electron microscopy (SEM), for determining the size and morphology of zeolite crystallites, high-resolution transmission electron microscopy (HRTEM), nuclear magnetic resonance (NMR) spectroscopy, temperature programme desorption (TPD) and many others (Schüth, 2005).

3.0 GENERAL APPLICATIONS OF ZEOLITES

Zeolites are used primarily in 3 major applications: ion-exchange, adsorbents, and catalysts. Natural zeolites play an important role in bulk mineral applications.

Adsorbent applications:

Common adsorbent applications focus on removal of small polar molecules and bulk separations, by more aluminous zeolites and based on molecular sieving processes respectively (Table 3).

Table 3: Zeolite commercial applications as adsorbents

Source: Flanigen (1980).

Catalyst applications:

Zeolites have the greatest use in catalytic cracking. They also play a role in hydroisomerisation, hydrocracking and aromatics processing. The strong acidity of zeolites plays a role in hydrocarbon processing. Asides this, they are finding increasing use in synthesis of fine chemicals and organic intermediates in isomerisation reactions, nucleophilic substitution and addition etc.

Table 4: Zeolite applications in Catalysis

Source: Flanigen (1980); Galarneau et al (2001).

Ion-exchange applications:

Zeolite properties are directly exploited in several applications such as in the detergent industry, where zeolites are used for water softening or ‘building’, animal food supplementation and in the treatment of wastewater (Townsend and Coker, 2001). Zeolite A has selectivity for Ca2+, thereby providing a unique advantage. Also, natural zeolites can be used to remove of Cs+ and Sr 2+radioisotopes through ion-exchange (Payra and Dutta, 2003).

Table 5: Applications and advantages of Ion-exchange

Source: Flanigen (1980)

Other Applications:

Zeolites also play important roles in health-related applications (such as antibacterial agents, vaccine adjuvants, drug delivery, bone formation, biosensors and enzyme mimetics), oil refining, and petrochemical processes. Zeolite powders are used for odor removal and as plastic additives. Zeolitic membranes offer the possibility of organic transformations and separations coupled into one unit (Payra and Dutta, 2003).

3.1 ZEOLITES AND THE ENVIRONMENT

Nearly all applications of zeolites are driven by environmental concerns, from cleaning toxic (nuclear) wastes, to treatment of wastewater, thereby reducing pollution. Zeolites have now been used to replace harmful phosphate builders in powder detergents due to water pollution risks. Zeolite catalysts help to save energy as they make chemical processes more efficient, minimising un-necessary waste and by-products. When used as solid catalysts and redox catalysts/sorbents, they reduce the need for corrosive liquid acids and remove atmospheric pollutants, (such as engine exhaust gases and ozone-depleting CFCs) respectively (Bell, 2001). In wastewater, zeolites (clinoptilolite, mordenite) are used to remove ammonia and ammonium ions (Townsend and Coker, 2001), as well as heavy metal cations and transition metals.

3.2 ZEOLITE CATALYSTS IN GREEN CHEMISTRY

Zeolite catalysts have contributed to the design and synthesis of novel materials and development of new methodologies in organic synthesis, displacing the conventional and waste generating reagents thereby maximising atom utilization and reducing waste generated (E-factor).

Zeolites play an important role in acid-catalyzed reactions such as acylation, alkylation, isomerisation and condensation, cyclisation and electrophilic aromatic substitution.

Acylation of aromatic substrates: used in fine chemicals manufacture although has proven unsuccessful in less reactive aromatic compounds due to adsorption imbalance, unless performed in vapor phase using H-ZSM-5 (Singh and Pandey, 1997).

4.0 CONCLUSION

Due to the role zeolites play mainly as catalysts in the environment as well as in chemical industry, the efficiency of the zeolite catalysts has been greatly improved. The yield and selectivity of the zeolite process is quantitative and in addition, reduces energy requirements, capital costs and complexity of equipments.

Over the years, the synthesis process of zeolites have encompassed the principles of green chemistry as described in the report which has included waste prevention, energy efficiency, fewer environmental impacts, safer solvents, renewable materials, process intensification, catalysis and reduction in capital cost.

Though present techniques seem to apply some of the principles of green chemistry, further research is still being employed to improve the overall process.

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