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2.2 Zeolites

Zeolites are crystalline, microporous, hydrated aluminosilicates of alkaline or alkaline earth metals. The frameworks are composed of [SiO₄]^4- and [A1O₄]^5-tetrahedra, which corner-share to form different open structures. The tetrahedra are linked together to form cages connected by pore openings of defined size; depending on the structural type, the pore sizes range from approximately 0.3-1 nm (Barrer, 1982).

The structure formula of zeolite is based on the crystallographic unit cell:

Mx/n [(AlO₂)x(SiO₂)y] wH₂O, where (M) is an alkali or alkaline earth cation, (n) is the valence of the cation, (w) is the number of water molecules per unit cell, x and y are the total number of tetrahedra per unit cell, and the ratio y/x usually has values of 1 to 5, though for the silica zeolite, y/x can be raging from 10 to 100. Zeolites have been well studied in terms of the relations among structure, properties and synthesis. Nowadays 180 synthetic zeolites are known. Some of the earlier synthetic zeolites include zeolites A, X, Y, L, ZSM-5 and omega (Petrov and Michalev, 2012). Zeolites are found in nature, and the zeolite mineral stilbite was first discovered in 1756 by the Swedish mineralogist A. F. Cronstedt. About 40 natural zeolites are known.

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. Table 2.4 indicates the natural zeolites. Sometimes natural zeolites are found as large single crystals, though it is very difficult to make large crystals in the laboratory. High-porosity zeolites such as faujasite (FAU), whose laboratory analogs are zeolites X/Y, are scarce. This is not surprising considering their metastable structures and conversion to more condensed forms. Also, high-porosity zeolites are formed in the laboratory under narrow synthesis compositions. Two natural zeolites that find extensive use are clinoptilolite (HEU) and mordenite (MOR) for ion-exchange (radioactive) agricultural uses and as sorbents. The catalytic activity of natural zeolites is limited by their impurities and low surface areas. Another natural zeolite, erionite (ERI), has toxicity comparable to or even worse than some of the most potent forms of asbestos, especially in causing a form of lung mesothelioma. (Auerbach et al., 2003)
Table 2.4: Nomenclature of Zeolites and Molecular Sieves (Auerbach et al., 2003)



2.2.1 Structure

The elementary building units of zeolites are SiO₄ and AlO₄ tetrahedra. Adjacent tetrahedra are linked at their corners via a common oxygen atom, and this result in an inorganic macromolecule with a structurally distinct three-dimensional framework. It is evident from this building principle that the net formulae of the tetrahedra are SiO₂ and AlO₂, i.e. one negative charge resides at each tetrahedron in the framework which has aluminum in its center. The chemical composition of a zeolite can hence be represented by a formula of the type

where A is a cation with the charge m, (x+y) is the number of tetrahedra per crystallographic unit cell and xly is the so-called framework silicon/aluminum ratio nsi/nAI (or simply Si/Al) (Weitkamp, 1999).

Fig. 2.5 shows the structures of four selected zeolites along with their respective void systems and pore dimensions.

Fig 2.5: Structures of four selected zeolites (from top to bottom: faujasite or zeolites X, Y; zeolite ZSM-12; zeolite ZSM-5 or silicalite-1; zeolite Theta-1 or ZSM-22) and their micropore systems and dimensions (Weitkamp, 1999)

Chemical formula:Na₂Al₂Si₃O₁₀.2(H₂O)

Figure 2.6: Crystals of the zeolite mineral, natrolite (Chippindale, 2008).

2.2.2 Applications of zeolite

General Applications. Zeolites have a wide range of commercial uses (InterSun, 2007), including:

Aquaculture

• Ammonia filtration in fish hatcheries

• Biofilter media

Agriculture

• Odor control

• Confined animal environmental control

• Livestock feed additives

60

Horticulture

• Nurseries, greenhouses

• Floriculture

• Vegetables/herbs

• Foliage

• Tree and shrub transplanting

• Turf grass soil amendment

• Reclamation, revegetation, and landscaping

• Silviculture (forestry, tree plantations)

• Medium for hydroponic growing

Household Products

• Household odor control

• Pet odor control

Industrial Products

• Absorbents for oil and spills

• Gas separations

Environmental Applications. Although environmental applications of zeolites are small compared with applications of their catalytic properties, considerable research and some implementations have taken place including:

Radioactive Waste

• Site remediation/decontamination

Water Treatment

• Water filtration

• Heavy metal removal

• Swimming pools

Wastewater Treatment

• Ammonia removal in municipal sludge/wastewater

• Heavy metal removal

• Septic leach fields

2.3 Properties of Zeolite

The properties of zeolites are as follows:

2.3.1 Ion Exchange

Ion exchange is an intrinsic characteristic of most zeolites and has become one of the most commercially important properties of zeolite. It is the property that allows the replacement of cation held in the zeolite framework by an external ion present in a bulk solution or in a melt (Dyer, 1988). The ion exchange property is due to the isomorphous substitution of Si⁴⁺ by Al³⁺ in the framework creating a net negative charge as explained. Different types of cations can be used to compensate for the ionic inbalance in the zeolite framework and to maintain ionic neutrality. Zeolites are generally synthesised using Na⁺ ion as the compensating cation in the framework and in an exchanging reaction, other cations such as Mg²⁺, Ca²⁺, NH⁴⁺ and H⁺ can replace Na⁺.

An example of exchanging of sodium ion in zeolite framework by Ca²⁺ is shown below

where Z is the zeolite.

Similar reactions are used in the generation of the acid catalyst when the cations in the zeolite framework are exchanged with proton from mineral acid or ammonium hydroxide thereby producing a protonated zeolite used as catalyst. The properties of zeolites as ion exchange material is widely used and applied in detergents, waste water treatment and radionuclide separation (Jiri et al., 2007).

2.3.2 Catalysis

The catalytic property of zeolites is another important application for which zeolites have been used. About 99% of the world petrol from crude oil depends on zeolite as catalyst for its production. The catalytic characteristics of zeolites are due to the combination of intrinsic properties of zeolite. These properties are responsible for the overall behaviour of zeolite as a catalyst however the generation of active sites otherwise called Bronsted sites (figure 2.7) by ion exchanging of ammonium hydroxide followed by calcination is the most important step in the production of zeolites as catalysts. Bronsted site is known as bridging hydroxyls and are generated at oxygen bridge site near the Si-O-Al cluster where the compensated cation are represented by protons. They are the main reason why zeolite is used as an industrial catalyst. This is because of the production of hydroxyl within the zeolite pore structure where there are high electrostatic field attracting organic reactant molecules and bond rearrangement can take place especially for cracking reactions.

Figure 2.7: Bronsted acid in zeolites (Kovo, 2010)

The reaction taking place during the production of the Bronsted site are shown below

This reaction is more associated with low silica zeolite such as zeolite X and Y, however for high silica zeolite such as ZSM-5, mineral acid such as HCl or H₂SO₄ can be used to produced the protonated zeolite by direct ion exchange thereby creating the Bronsted site. Some of the important applications of zeolites as an industrial catalyst include catalytic cracking, hydrocracking, hydroisomerization, NOx reduction and xylene isomerisation (Kovo, 2010)

2.3.3 Adsorption

Adsorption can be described as a process whereby molecules of a gas or liquid material adhered to the surface of solid. These processes can be used to separate two mixtures of species depending on the affinity of the mixtures toward the solid surface. The solid surface is known as adsorbent while the adhering molecule is called adsorbate. The process of removing the adhered molecules is called desorption and this is achieved by changing the pressure and temperature of the system. These allow the reuse of the adsorbent (Kovo, 2010).

The adsorption mechanism in zeolite depends on several factors such as the pore size of the zeolite, the ion exchange, the physical and chemical composition of adsorbate. These mechanisms include:

• 
  Equilibrium selective adsorption
  
• 
  Rate selective adsorption
  
• 
  Shape selective adsorption
  
• 
  Ion exchange
  
• 
  Reactive adsorption
  

The adsorption properties of zeolites whichever mechanism controlled the process is broadly influenced by some specific properties of zeolites. Adsorption is an important characteristic of all zeolites as it directly influences the use of the material as a catalyst and separator. Adsorption in zeolite has both economic and environmental benefit because of their uses in oil refining and petrochemical over several years as catalysts just like it application in pollution abatement. Some of the important uses of the adsorption and molecular sieving properties of zeolite

include drying agent (zeolite 4A are used as general purpose drying agent such as gas drying column in GC), gas separation (pollution control) and separation of bulk mixture such as i-paraffin/n-paraffin system. Other specific utilization of adsorption properties of zeolites include:

• 
  Aromatic removal from linear paraffin in the C10-C15 range used in linear alkyl
  
• 
  benzene production
  
• 
  Nitrogenate removal
  
• 
  Oxygenate removal
  
• 
  Sulphur removal
  

2.4 Synthesis of Zeolite
2.4.1 Settling time for refining kaolin

Kaolin obtained naturally is usually fractionated to enrich the kaolinite content and reduce other unwanted clay mineral before application in manufacturing materials such as zeolites. The most common and simplest method of enriching the kaolin content of raw kaolin sample is fractionation by sedimentation (Bergaya and Lagaly, 2006). The refining process of kaolin is clearly divided into two groups namely removal of foreign material by chemical method and refinement by sedimentation to remove larger impurities especially quartz which is trapped within the mineral aggregates.

However addition of chemicals in the treatment process can impair the properties of the parent material, therefore, the use of chemical treatment should be a last resort (Chipera and Bish, 2001).

Even though there are several other methods such as selective flocculation, flotation, delamination, ultrasonic treatment that can be used to process raw kaolin, fractionation by sedimentation is the most common procedure used for kaolin processing to obtain highly pure kaolin at laboratory level (Chipera and Bish, 2001).Sedimentation is based on the principle that a particle with different mass and density will settle at different terminal velocity in a given viscous media (Pabst et al., 1999).

Stokes’ law enables the separation based on the clay particle size which is assumed to have a spherical shape (Bergaya and Lagaly, 2006). The law is based on the relationship between the Stokes’ frictional force 3πηvd acting on dispersed particles at constant velocity v , and the difference between the gravitational force mg = ρVg and the buoyancy force ρ_oVg and given in equation below

Here

v is the particle volume =

ρ is the density of the particle

ρ_o is the density of the solvent

g is the gravitational acceleration = 9.81 m/s²

η is the viscosity of solvent

d is particle diameter

v is settling velocity

The settling velocity can also be expressed as a function of distance travelled by particle and time ie

By substituting we have time of settling as:



2.4.2 Metakaolinization of Kaolin

Kaolin in its natural state is less reactive and forms hydrosodalite when reacted with sodium hydroxide. In the synthesis of zeolites, kaolin needs to be made reactive through a process called metakaolinization or dehydroxylation before zeolitization can take place. The process of metakaolinization involves the loss of hydrogen and framework oxygen (Breck, 1974). This process is usually performed by a method of calcination of kaolin to form metakaolin between the temperatures of 550^oC to 900^oC. Metakaolinization directly involves the loss of hydroxyl group and this is followed by rearrangement of the octahedral layer to tetrahedral orientation in the calcined clay (Klinowski and Joao, 1990). Kaolin is an aluminosilicates and the Al³⁺ ion can be in the form of IV and VI coordination state and it is essential in zeolite formation that Al³⁺ is in IV coordination, hence metakaolin with IV Al coordination and amorphous in nature is a necessary path toward zeolite synthesis from kaolin (Granizo et al., 2000). The transition of kaolin when heated in a furnace circulating with air has been well reported (Breck, 1974) and found to have a number of technological uses. This reaction series includes the formation of highly disordered metakaolin formed at around 550-900^oC when kaolin is air calcined as shown below:

Metakaolin

On further heating, a new phase is obtained at around 925^oC called spinel, a defected alumina-silica structure

A more stable material called mullite and cristobalite are formed at a temperature of 1050^oC

2.4.3 Synthesis and Dealumination of metakaolin to form zeolites

Highly siliceous zeolites are well-known to be strong Bronsted acids and to possess unique catalytic properties. There are two methods to synthesize zeolites with high Si :A1 ratio:

1. 
   synthesis from aluminosilicates using hydrothermal methods
   
2. 
   By the dealumination of zeolites with low Si:AI ratio.
   

The siliceous zeolites synthesized by the latter method are isostructural with the original zeolites. The synthetic zeolite of the faujasite structure, Na-Y zeolite, has been successfully dealuminated using SiCl₄ (Beyer and Belenykaja, 1980; Klinowski et al., 1981) and halogen-containing reagents (metal halides, oxyhalides, acid halides, etc.) (Fejes et al., 1980) at high temperature.

The products obtained by calcination or "thermal activation" of kaolinite can be used to synthesize zeolites or "molecular sieves" (Breck, 1974) by hydrothermal treatment (HT) in alkaline medium. The value of the molar Si/A1 ratio in metakaolinite (a solid more reactive than kaolinite) is equal to unity, and then corresponds to the composition of zeolite A. When metakaolinite recrystallizes at high temperature giving rise to spinel, mullite or y-alumina structures, the side formation of reactive amorphous silica changed the Si/A1 ratio in the gel and theoretically other zeolites should be obtained, e.g. faujasite (Flank, 1970).

Table 2.5: values of molar ratios according to type of zeolite required (Howell et al., 1965)


When kaolin –type clay is heated, it will undergo several transitions. The first of these takes place at about (550-600°C), and produces the disordered metakaolin phase (metastable phase) by an endothermic dehydroxylation reaction (Corma, 1995). The metakaolin is then stable to about 935-950°C where it rearranges to give a defect aluminum–silicon spinel, which is also referred to as a γ-alumina type structure. The so called γ-Al2O3 phase converts to mullite 3Al₂O₃.2SiO₂ at 1050°C, as shown in the following equations (Breck, 1974)

Zeolite Y with SiO₂/Al₂O₃ molar ratio of 3.53 was synthesized from kaolin (clay mineral locally found) under hydrothermal condition at atmospheric pressure. The effects of various factors (the amount of water volume added, ageing and crystallization condition) on the structure of the samples were extensively investigated. The samples were characterized by X-ray Diffraction (XRD), Scanning Electron Microscope (SEM) and analyzed by gravimetric method. The results show that the typical zeolite Y can be prepared with a molar composition of 6SiO₂:Al₂O₃:9Na₂O:249H₂O by ageing at 50˚C for 24 hours and crystallized at 100˚C for 48 hours (Mu and Mya, 2008).

2.5 Crystallization of Zeolites

Crystallization is the (natural or artificial) process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. The crystallization process consists of two major events, nucleation and crystal growth. Nucleation is the step where the solute molecules dispersed in the solvent start to gather into clusters, on the nanometer scale (elevating solute concentration in a small region), that become stable under the current operating conditions. The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size. Nucleation and growth continue to occur simultaneously while the supersaturation exists. Supersaturation is the driving force of the crystallization; hence, the rate of nucleation and growth is driven by the existing supersaturation in the solution. (Wikipedia, 2015).

The crystallization step is the most important process in zeolite synthesis and it involves a condensation reaction taking place between polysilicate and aluminate in a strongly basic solution. Furthermore crystallization of zeolites is a complicated process but can be divided into two major steps. These are nucleation and crystal growth of zeolites. Nucleation is the most important stage in zeolite crystallization. It involves the formation of aggregation of unstable nuclei from the supersaturated solution prepared from the initial precursor and with time become large enough to form stable nuclei from where crystal growth takes place. Nucleation can either be homogenous or heterogeneous in nature depending on the whether it occurs spontaneously or induced by the presence of impurities. Nucleation and crystal growth are promoted by several parameters such the extent of time of incubation and the history of the system. Again the size of the crystal can be controlled based on the choice of conditions of the crystallization. (Kovo, 2010).
2.5.1 Crystallization Process and Formation Mechanism of Zeolites

Studies on the crystallization process and formation mechanism of zeolites are very important not only because of their theoretical significance but also due to practical values. Even though numerous zeolite structures have been successfully synthesized, it is still necessary to rationally design and synthesize more and more zeolite structures with specific architectures and properties, which requires a fuller understanding of the crystallization process and formation mechanism of zeolites. (Xu et al., 2007). The understanding of the mechanism of zeolite formation will help in the rational design of zeolite of specific structure and properties. The actual mechanism of zeolite formation throughout the entire process of crystallization is still subject of much speculation with three routes currently accepted for zeolite formation.(Kovo, 2010).

2.5.1.1 Solid Hydrogel Transformation Mechanism

Another name for solid hydrogel transformation mechanism is solid-phase mechanism, while solution-mediated transport mechanism is also called liquid-phase mechanism. The main difference in explaining the formation process of zeolites by these two mechanisms lies in whether the liquid component is involved during the crystallization of zeolites. The views of these two mechanisms are opposite to each other and have their own experimental supporting evidence. To date, the liquid-phase mechanism has more experimental support than does the solid-phase mechanism.(Xu et al., 2007).

This mechanism of zeolite formation suggests that zeolite synthesis take place as result of direct transformation of the solid gel phase used in zeolitization. In this mechanism it is understood that liquid component of the aluminosilicate gel take no part in the reaction toward zeolite formation. In a word, in the solid-phase mechanism, it is believed that neither the dissolution of solid gel nor the direct involvement of the liquid phase happened for the nucleation and growth of zeolite crystals during the crystallization process of zeolites. The nucleation and growth of zeolite crystals came from the structural rearrangement of the framework of solid aluminosilicate gel under hydrothermal crystallization conditions. The mechanism originally proposed by Breck and Flanigen when they noticed that the resultant zeolite phase and the hydrogel always have the same composition during zeolite synthesis. It was later explained that zeolite crystallization take place via structural rearrangement of the framework of the solid aluminosilicate hydrogel under hydrothermal conditions.(Kovo, 2010; Xu et al., 2007).

Fig 2.8 Illustration of solid-phase mechanism adapted from Xu et al., 2007

2.5.1.2 Solution-mediated Transport Mechanism

This mechanism is almost a direct opposite of the solid hydrogel system because the concept of the mechanism is that nucleation and crystal growth take place in solution. (Kovo, 2010). Zhdanov and colleagues for the first time discussed in detail the solution-mediated transport mechanism. They believed that

1) Nucleation happened in the solution or at the interface of the solution and solid gel;

2) The further growth of zeolite nuclei consumed the silicate and aluminate ions in solution;

3) The solution supplied the soluble structural units for the growth of zeolite crystal; and

4) The consumption of the liquid component during the crystallization process resulted in the continuous dissolution of solid gel. (Xu et al., 2007)

The liquid-phase mechanism is illustrated in Figure 2.9. An equilibrium is formed between the component of the aluminosilicates gel following mixing of the source material. Increase in the temperature of the mixture lead to shift in equilibrium between the solid gel and the liquid phase causing an increase in the concentration of polysilicate and aluminate. This thereby formed the required zeolite nuclei and followed by crystal growth. Continuous dissolution of the gel happens because of consumption of the polysilicate and aluminate ion in the liquid phase.(Kovo, 2010).

Fig 2.9 Illustration of Solution-mediated Transport Mechanism adapted from Xu et al., 2007