epistilbite

Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a metal ion or H⁺. These positive ions can be exchanged for others in a contacting electrolyte solution. exchanged zeolites are particularly useful as solid acid catalysts.

The term ''zeolite'' 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 , meaning "to boil" and , meaning "stone".

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 the aluminosilicate members of the family of microporous materials, and mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a metal ion or H⁺. The value of x (Si/Al molar ratio) is greater than 1 and y is the number of water molecules in the formula unit. Zeolites have microporous structures with a typical diameter of 0.3–0.8 nm and accommodate a wide variety of cations, such as Na⁺, K⁺, Ca²⁺, Mg²⁺ and others. These positive ions are often loosely held and can readily be exchanged for others in a contacting electrolyte solution. Cation exchanged zeolites possess different acidity and catalyze different reactions.. The Si/Al ratio is greater than 1 because zeolites have no Al-O-Al bond (Löwenstein rule).

They are formed by the linking of the corner oxygen atoms of AlO₄ and SiO₄ tetrahedra to form covalent network structures. The general formula of zeolite, , where the part is ionic bond-like and the part is covalent bond-like. Zeolites therefore have both ionic crystal and covalent crystal properties, and the balance of these properties depends on the Si/Al ratio (x).

Si/Al ratios below about 3 correspond to natural zeolites and some synthetic zeolites such as A-type and X-type zeolites. They are useful as ion-exchange agents because of their high ion-exchange capacity. Commercially available molecular sieve adsorbents often belong to this group.

Zeolites with a Si/Al ratios higher about 3 are classified as high-silica zeolites, which are rarely found in nature and are synthesized industrially. They possess high physical and chemical stability due to the large colvalent bonding contribution. They have excellent hydrophobicity and are suited for adsorption of builky, 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 acidity is strong enough to protonate hydrocarbons and high-silica zeolites are used in acid catalysis processes such as fluid catalytic cracking in petrochemical industry.

Apart from the adsorption, catalytic, and other properties of zeolites, it also shows several interesting properties, e.g. window effect, single-file diffusion, and Levitation Effect.

The Levitation Effect is an anomalous diffusion phenomenon observed in all systems with van der Waals interactions.

Framework structure

As of December 2018, the framework structures of 253 different zeolites or their analogues are known, nearly 200 of which can only be synthesized artificially. For each structure, the International Zeolite Association (IZA) gives a three-letter code called framework type code (FTC). For example, the major molecular sieves, 3A, 4A and 5A, are all LTA (Linde Type A). 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 (blue bold line). 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 (left) and FAU (right). Both zeolites share the truncated octahedral structure ( sodalite cage) (purple line). However, the way they are connected (yellow line) 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 (0.41 nm) and belongs to the small pore zeolite, while the pore entrance of FAU is a 12-ring (0.74 nm) and belongs to the large pore zeolite, respectively. Materials with a 10-ring are called medium pore zeolites, a typical example being ZSM-5