Zeolite
Zeolites are a group of several microporous, crystalline aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, and oxygen, and have the general formula ･y where is either a metal ion or H+.
The term was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that rapidly heating a material, believed to have been stilbite, produced large amounts of steam from water that had been adsorbed by the material. Based on this, he called the material zeolite, from the Greek ζέω, meaning "to boil" and λίθος, meaning "stone".
Production
The first synthetic structure was reported by Richard Barrer in 1948. Industrially important zeolites are produced synthetically. Over 200 synthetic zeolites have been reported.Synthetic zeolites hold some key advantages over their natural analogs. The synthetic materials are manufactured in a uniform, phase-pure state. It is also possible to produce zeolite structures that do not appear in nature. Zeolite A is a well-known example. Since the principal raw materials used to manufacture zeolites are silica and alumina, which are among the most abundant mineral components on earth, the potential to supply zeolites is virtually unlimited.
Zeolites occur naturally, but are also produced industrially on a large scale., 253 unique zeolite frameworks have been identified, and over 40 naturally occurring zeolite frameworks are known. Every new zeolite structure that is obtained is examined by the International Zeolite Association Structure Commission and receives a three-letter designation.
Characteristics
Properties
Zeolites are white solids with ordinary handling properties, like many routine aluminosilicate minerals, e.g. feldspar. They have the general formula where M+ is usually H+ and Na+. The Si/Al ratio is variable, which provides a means to tune the properties. Zeolites with a Si/Al ratios higher than about 3 are classified as high-silica zeolites, which tend to be more hydrophobic. The H+ and Na+ can be replaced by diverse cations, because zeolites have ion exchange properties. The nature of the cations influences the porosity of zeolites.Zeolites have microporous structures with a typical diameter of 0.3–0.8 nm. Like most aluminosilicates, the framework is formed by linking of aluminium and silicon atoms by oxides. This linking leads to a 3-dimensional network of Si-O-Al, Si-O-Si, and Al-O-Al linkages. The aluminium centers are negatively charged, which requires an accompanying cation. These cations are hydrated during the formation of the materials. The hydrated cations interrupt the otherwise dense network of Si-O-Al, Si-O-Si, and Al-O-Al linkage, leading to regular water-filled cavities. Because of the porosity of the zeolite, the water can exit the material through channels. Because of the rigidity of the zeolite framework, the loss of water does not result in collapse of the cavities and channels.
Their ability to generate voids within a rigid solid underpins the use of zeolites as catalysts and molecular sieves. They possess high physical and chemical stability due to the covalent bonding within their framework. Some are hydrophobic and thus are suited for adsorption of hydrophobic molecules such as hydrocarbons. In addition to that, high-silica zeolites are exchangeable, unlike natural zeolites, and are used as solid acid catalysts. The high silica zeolites especially are sufficiently acidic to protonate hydrocarbons. These are used for fluid catalytic cracking in petrochemical industry. Cracking catalysts become poisoned by carbonaceous residues. These residues can be combusted without major damage to the host zeolite, a further testament to their robustness. For some applications, such regenerated zeolites are called equilibrium catalysts.
Framework structure
The structures of hundreds of zeolites have been determined. Most do not occur naturally. For each structure, the International Zeolite Association gives a three-letter code called framework type code. For example, the major molecular sieves, 3A, 4A and 5A, are all LTA. Most commercially available natural zeolites are of the MOR, HEU or ANA-types.An example of the notation of the ring structure of zeolite and other silicate materials is shown in the upper right figure. The middle figure shows a common notation using structural formula. The left figure emphasizes the SiO tetrahedral structure. Connecting oxygen atoms together creates a four-membered ring of oxygen. In fact, such a ring substructure is called four membered ring or simply four-ring. The figure on the right shows a 4-ring with Si atoms connected to each other, which is the most common way to express the topology of the framework.
The figure on the right compares the typical framework structures of LTA and FAU. Both zeolites share the truncated octahedral structure . However, the way they are connected is different: in LTA, the four-membered rings of the cage are connected to each other to form a skeleton, while in FAU, the six-membered rings are connected to each other. As a result, the pore entrance of LTA is an 8-ring and belongs to the small pore zeolite, while the pore entrance of FAU is a 12-ring and belongs to the large pore zeolite, respectively. Materials with a 10-ring are called medium pore zeolites, a typical example being ZSM-5.
Although more than 200 types of zeolites are known, only about 100 types of aluminosilicate are available. In addition, there are only a few types that can be synthesized in industrially feasible way and have sufficient thermal stability to meet the requirements for industrial use. In particular, the FAU, *BEA, MOR, MFI, and FER types are called the big five of high silica zeolites, and industrial production methods have been established.
Isomorphous replacement
The scope of zeolites is greatly expanded because Si and Al in zeolites can be at least partially replaced with other atoms without major changes in the overall framework, i.e., isomorphous replacement. Germanium, iron, gallium, boron, zinc, tin, and titanium have been investigated. Other heteroatoms have been claimed, including titanium, and zinc Al atoms in zeolites can be also structurally replaced with boron and gallium.The silicoaluminophosphate type, in which Si is isomorphous with Al and P and Al is isomorphous with Si, and the gallogermanate and others are known.
Porosity
The term molecular sieve refers to a particular property of these materials, i.e., the ability to selectively sort molecules based primarily on a size exclusion process. This is due to a very regular pore structure of molecular dimensions. The maximum size of the molecular or ionic species that can enter the pores of a zeolite is controlled by the dimensions of the channels. These are conventionally defined by the ring size of the aperture, where, for example, the term "eight-ring" refers to a closed-loop that is built from eight tetrahedrally coordinated silicon atoms and eight oxygen atoms. These rings are not always perfectly symmetrical due to a variety of causes, including strain induced by the bonding between units that are needed to produce the overall structure or coordination of some of the oxygen atoms of the rings to cations within the structure. Therefore, the pores in many zeolites are not cylindrical.Natural occurrence
Some of the more common mineral zeolites are analcime, chabazite, clinoptilolite, heulandite, natrolite, phillipsite, and stilbite. An example of the mineral formula of a zeolite is: ·2H2O, the formula for natrolite.Zeolites transform to other minerals under weathering, hydrothermal alteration or metamorphic conditions. Some examples:
- The sequence of silica-rich volcanic rocks commonly progresses from:
- * Clay → quartz → mordenite–heulandite → epistilbite → stilbite → thomsonite → mesolite → scolecite → chabazite → calcite.
- The sequence of silica-poor volcanic rocks commonly progresses from:
- * Cowlesite → levyne → offretite → analcime → thomsonite → mesolite → scolecite → chabazite → calcite.
Ore mining
Synthesis
Typical procedures entail heating aqueous solutions of alumina and silica with sodium hydroxide. Equivalent reagents include sodium aluminate and sodium silicate. Further variations include the use of structure directing agents such as quaternary ammonium cations. Zeolite synthesis involves sol-gel-like processes. The product properties depend on reaction mixture composition, pH of the system, operating temperature, pre-reaction 'seeding' time, reaction time as well as the templates used. In the sol-gel process, other elements can be incorporated.Applications
Zeolites are widely used as catalysts and sorbents. In chemistry, zeolites are used as membranes to separate molecules, and as traps for molecules so they can be analyzed.Research into and development of the many biochemical and biomedical applications of zeolites, particularly the naturally occurring species heulandite, clinoptilolite, and chabazite has been ongoing.