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Zeolites

Zeolites are crystalline solidsstructures made of silicon, aluminum and oxygen that form a framework withcavities and channels inside where cations, water and/or small molecules mayreside. They are often also referred to as molecular sieves. Many ofthem occur naturally as minerals, and are extensively mined in many parts ofthe world finding applications in industry and medicine. However, most ofzeolites have been made synthetically some of them made for commercial usewhile others created by scientists to study their chemistry. At present, thereare 191 unique zeolite frameworks identified[1],and over 40 naturally occurring zeolite frameworks are known.

Zeolites were introduced in 1954as adsorbents for industrial separations and purifications. Because of theirunique porous properties, zeolites are used now in a variety of applicationswith world production estimated to be in the range of 2.5 million to 3 millionmetric tons (Mt) in 2008 year [2].They are used in petrochemical cracking, watersoftening and purification, in the separation and removal of gases andsolvents, agriculture, animal husbandry and construction.

Zeolites are crystallinealuminosilicates with open 3D framework structures built of SiO4 and AlO4 tetrahedra linked to each otherby sharing all the oxygen atoms to form regular intra-crystalline cavities andchannels of molecular dimensions. A defining feature of zeolites is that theirframeworks are made up of 4-coordinated atoms forming tetrahedra. Thesetetrahedra are linked together by their corners and make a rich variety ofbeautiful structures. The framework structure may contain linked cages,cavities or channels, which are big enough to allow small molecules to enter.The system of large voids explains the consistent low specific density of thesecompounds. In zeolites used for various applications, the voids areinterconnected and form long wide channels of various sizes depending on thecompound. These channels allow the easy drift of the resident ions andmolecules into and out of the structure. The aluminosilicate framework isnegatively charged and attracts the positive cations that reside in cages tocompensate negative charge of the framework. Unlike most other tectosilicates[3], zeolites have largeer cages in their structures.

Related materials and discoveryof new zeotypes

Zeolite-like materials havestructures similar to zeolites but elements other than Si, Al and O can bepresent in them. For example, the isoelectronic substitution of 2Si → Al + Phas led to the discovery of a wide range of AlPO structures [4-6]and related phosphate materials [7-8].Considerable success have been done recently on making tetrahedral frameworkswith the congeners of Al and Si in the next row of the periodic table, namely,Ga and Ge [9-14], and only recently the first chiralgermanosilicate ITQ-37 has the been discovered with a zeolite-like frameworkconsisting entirely of tetrahedrally coordinated positions occupied by germaniumand silicon [15]. Although hundreds of laboratories aretrying to synthesize new materials with novel zeolite framework structures,only 191 zeolite framework types have been approved by the International Zeolite Association(IZA) Structure Commission (IZA-SC).

The Atlas of Zeolite StructureTypes published by the IZA Structure Commision assigns a three letter code tobe used for a known framework topology irrespective of composition. The codesare normally derived from the name of the zeolite or "type material",e. g. LTA for Linde zeolites A, FAU for molecular sieves with a faujasitetopology, e.g. zeolites X and Y, MOR for the mordenite topology, MFI for theZSM-5 and silicate topologies. The more detailed information on topology ofzeolites and related germanates compound is included in the Reticular ChemistryStructure Resource [16] and the Database of Periodic PorousStructures [17].

Industrially important zeolites

The naturally occurring zeolitesare an important group of minerals for industrial and other purposes [18].The discovery in 1957 of largedeposits of relatively high purity zeoliteminerals in volcanic tuffs in the western United States and in a number ofother countries represents the beginning of the commercial natural zeolite era.Prior to that time there was no recognized indication that zeolite mineralswith properties useful as molecular sieve materials occurred in large deposits.Commercialization of the natural zeolites chabazite, erionite, and mordenite asmolecular sieve zeolites commenced in 1962 with their introduction as newadsorbent materials with improved stability characteristics. The applicationsof clinoptiolite in radioactive waste recovery and in waste water treatmentduring the same period of the 60's were based not only on superior stabilitycharacteristics but also high cation exchange selectivity for cesium,strontium, and for ammonium ion.

The well known and industriallyimportant zeolites have been discovered in 1950-1970 and may be classified intothree groups according to Al/Si ratio in their frameworks [18]:

"Low-silica" oraluminium rich zeolites A and X (ratio Si/Al ≈ 1).

Zeolites A (Fig. 1) and X(the most common commercial adsorbents) discovered by R. M. Milton atthe Union Carbide Corporation Laboratories represent a fortunate optimum incomposition, pore volume, and channel structure. Both zeolites are nearly"saturated" in aluminium in the framework composition with a molarratio of Si/Al ≈ 1, which is considered as highest aluminum content possible intetrahedral alumosilicate frameworks. As a consequence they contain the maximumnumber of cation exchange sites balancing the framework aluminum, and thus thehighest cation contents and exchange capacities. These compositionalcharacteristics combined give them the most highly heterogeneous surface knownamong porous materials, due to exposed cationic charges nested in analuminosilicate framework which results in high field gradients. Their surfaceis highly selective for water, polar and polarizable molecules which serves asthe basis for many applications particularly in drying and purification.

"Intermediate silica"zeolites: zeolite Y, mordenite, zeolite L, natural zeolites (ratio Si/Al = 2 ÷5).

In the early 1950's it wasrecognized by Union Carbide Laboratories scientists that the tetrahedralaluminum positions in the zeolite frameworks provide a site of instability forattack by acid and water vapor of steam that make synthetic zeolites A and Xless stable than their natural analogs, which have superior stabilitycharacteristics reflecting higher Si/Al molar ratio of 3-5. Therefore, zeoliteswith higher content of silicon were needed, primarily to improve stabilitycharacteristics, both thermal and to acids. The third commercially importantmolecular sieve zeolites type Y, with an Si/Al ratio from 1.5 to 3.0, and aframework topology like that of zeolites X and the rare zeolites mineralfaujasite, represented the first successful discovery in that row of compoundsmade by D. W. Breck [18].Besides improvement in stability over the more aluminous X, the differences incomposition and structures had a striking, unpredicted effect on propertiesmaking zeolites Y based catalysts valuable in many important catalyticapplications involving hydrocarbon conversion since their initial commercialintroduction in 1959 [19].

Figure 1. A representation of thezeolite A structure (LTA) as an assembly of framework's cages (tiles). Center ofa tile is the center of a void in the framework. Voids are connected withadjacend ones through the large "windows" which are faces of tiles.

The next commercially successfulsynthetic zeolite introduced in the early 1960's was a large pore mordenite (Fig. 2)with ratio Si/Al ≈ 5. The improvement in thermal, hydrothermal, and acidstability coupled with its specific structural and compositionalcharacteristics resulted in application of mordenite as an adsorbent andhydrocarbon conversion catalyst [19].Type L zeolites (Fig. 3), discovered in the early 50's byD. W. Breck and N. A. Acara with a Si/Al = 3.0have unique framework topology. They were adapted as commercial catalysts inselective hydrocarbon conversion reactions.

Figure 2. The zeolite mineralmordenite (MOR): SiO4 polyhedra are represented as yellow tetrahedra; AlO4 polyhedra are aqua colored ones.

"High silica" zeolites:zeolite beta, ZSM-5 (ratio Si/Al ≥ 10).

The most recent stages in thequest for more siliceous molecular sieve compositions was achieved in the late1960's and the early 1970's with the synthesis at the Mobil Research andDevelopment Laboratories of the "high silica zeolites" [18]. First inthat row was zeolite beta (Fig. 4) discovered byR. L. Wadlinger, G. T. Kerr and E. J. Rosinski,and later ZSM-5 (Fig. 5) discovered by R. J. Argauer andG. R. Landolt. These are molecular sieve zeolites with Si/Al ratiousfrom 10 to 100 or higher, with different surface characteristics. In contrastto the "low" and "intermediate" silica zeolites,representing heterogeneous hydrophilic surfaces within a porous crystal, thesurface of the high silica zeolites is more homogeneous with anorganophilic-hydrophobic selectivity [19].They adsorb stronger the less polar organic molecules and only weakly interactwith water and other polar molecules.

Figure 3. Tiling representationof the structure of the zeolite L (LTL). Blue tiles are channels in thestructure running along direction of the crystallographic c axis.

In addition to this novel surfaceselectivity, the high silica zeolite compositions still contain a smallconcentration of aluminum in the framework and the accompanying stoichiometriccation exchange sites. Thus, their cation exchange properties allow theintroduction of acidic OH- groups via the well known zeolite ion exchangereactions, essential to the development of acid hydrocarbon catalysisproperties.

Applications

The properties of the porousmaterials depend both on the pore structures and the chemistry of theframework. The continuously increasing demands for materials with highlyspecific chemical and physical properties as zeolites have inspired scientiststo make new porous materials with unique structures [11-15].

Figure 4. Illustration of thechannel system in the zeolite beta (BEA).

Ion Exchange

Cation exchange properties oftraditional aluminosilicate zeolites arise from the isomorphous positioning ofaluminium in tetrahedral coordination within their Si/Al frameworks [20].This imposes a net negative charge of the framework (Si+4 → Al3+) counterbalanced by cations heldwithin the cavities and channels. Ionic character of bonding betweeninterstitial cations and the framework provide facile cation exchange forzeolites with open frameworks, where cations often readily exchanged for othercations in aqueous solution, though in some of the narrow-pored frameworks,such as natrolite, cation replacement is slow and difficult.

Cation exchange is exploited inwater softening, where alkali metals such as Na+ or K+ in zeolite framework arereplaced by Ca2+ and Mg2+ ions from water. Many commercialwashing powders thus contain substantial amounts of zeolites that enhancewashing efficiency. LTA have the largest scale production of synthetic zeolitesfor use as "builders" in domestic and commercial detergents to removethe calcium and magnesium "hardness" [21].

Figure 5.The ZSM-5 zeolite (MFI).The framework is represented by tiles assembly showing straight channels in thestructure.

The unique ion exchangeproperties of zeolites can also be used for cleaning up of commercial wastewater containing heavy metals and nuclear effluents containing radioactiveisotopes. In a similar way zeolites can absorb ions and molecules and thus actas a filter for odor control and toxin removal.

Interstitial cations in zeolitescan be exchanged to fine-tune the pore size of zeolites. For example, thesodium form of zeolite A has a pore opening of approximately 4 Å(4A molecularsieve). If Na+ is exchanged with the larger K+, the pore opening is reduced toapproximately 3 Å; Ca2+ replaces 2 Na+, thus, the pore openingincreases to approximately 5 Å. Ion exchange with other cations is sometimesused for particular separation purposes.
Another potential application of zeolites is a drug delivery, when water in thestructure is substituted by other liquid compound. Such treated zeolites act asa delivery system for the new fluid.

Adsorption and Separation

Adsorption and separation arebased on chromatographic processes which happen on the surface of zeolitecrystals and are determined both by different migration speed of variouscompounds along the surface of adsorbent due to diversity in the intensity oftheir interactions with the surface and due to steric effects [19].

The shape-selective properties ofzeolites are the basis for their use in molecular adsorption. The abilitypreferentially to adsorb certain molecules, while excluding others, has openedup a wide range of molecular sieving applications. Sometimes it depends merelyon the size and shape of pores controlling access into the zeolites; in othercases different types of molecule enter the zeolite, but some diffuse throughthe channels more quickly, leaving others stuck behind, as in the purificationof para-xylene by the zeolites X or Y [19].

Cation-containing zeolites areextensively used as desiccants due to their high affinity for water, and also findapplications in gas separation, where molecules are differentiated on the basisof their electrostatic interactions with the metal ions. Conversely,hydrophobic silica zeolites preferentially absorb organic solvents. Zeolitescan thus separate molecules based on differences of size, shape and polarity.

Catalysis

Zeolites have the ability to actas catalysts for chemical reactions which take place within the internalcavities. Essentially, zeolites have two properties which make themparticularly suitable as starting materials for the preparation of catalysts [22]:

• They are cation exchangers, hence it is possible to     introduce a large variety of cations with different catalytic properties     into their intracrystalline pore system, which in turn offers the     opportunity to create different catalytic properties, e. g. in acid-     or metal-catalyzed reactions;
  
• Zeolites are crystalline porous materials with pore dimensions     in the same order as the dimensions of simple molecules; hence they     possess molecular sieving properties when the shape and size of a     particular pore system exert a steric influence on the reaction,     controlling the access of reactants and products.
  

In the case of shape-selectivecatalysis in zeolites, the combination of both properties exploited to controlthe selectivity of catalytically conducted reactions.

Using zeolites as catalysts havemany advantages since can be recovered and recycled with greater ease and lowcost, leading to less waste and fewer byproducts, often function with higheractivity, may combine several catalytic steps, reduce environment pollution bysubstitution of homogeneous catalysts used in the traditional chemical industry(mineral acids, salts, heavy metals).

Hydrogen-exchanged zeolites,whose framework-bound protons give rise to very high acidity are exploited inmany organic reactions, including crude oil cracking, isomerisation and fuelsynthesis. Because of high selectivity of zeolites, they are often the mostefficient and cost-effective method for a number of refinery conversions [19,22].

Metal-exchanged zeolites canserve as oxidation or reduction catalysts, e. g. Ti-ZSM-5 in theproduction of caprolactam, and Cu-zeolites in NOx decomposition. They have beenemployed on diesel vehicles as a less costly and more effective option for NOx removal than the three-waycatalytic converter.

Zeolites find and increasingapplication in production of petrochemicals, often replacing environmentallyunfriendly catalysts. Zeolite catalysts typically yield fewer impurities, havehigher capacity, give greater unit efficiency, and afford higher selectivity.Unlike the more hazardous acid catalysts that have been used in the past, e.g.,solid phosphoric acid, hydrofluoric acid, etc., zeolites are non-hazardous andregenerable.

Acknowledgement

Content of the web page has beendeveloped by Max Peskov, StockholmUniversity.

References

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这篇文章里面的 due to exposed cationic charges nested in analuminosilicate framework which results in high field gradients.最后的高场梯度是什么意思