Zeolites are crystalline, hydrated aluminosiliciates that possess framework structures containing regular channels and or cavities. The cavities contain H2O molecules and cations. Zeolites are environmentally friendly compared to phosphates and the development if industrial processes in which they can replace other harmful acid catalysts are advantageous. In zeolites, the cavities are much larger and can accommodate not only cations but also molecules such as H2O and CO2, MeOH and hydrocarbons. Commercially and industrially, zeolites are extremely important. The Al:Si ratio varies widely among zeolites; Al rich systems are hydrophilic and their ability to take up H2O leads to their use as lab drying agents (molecular sieves). Different zeolites contain different sized cavities and channels permitting a choice of zeolite to affect selective molecular adsorption. Silicon rich systems are hydrophobic. Catalytic uses of zeolites are widespread. Electrical neutrality upon al for Si replacement can also be achieved by converting O- to a terminal OH group. These groups are strongly acidic which means that such zeolites are excellent for ion exchange materials and have applications in water purification and washing powders
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The structure of zeolites is based on SiO4 tetrahedra which can share corners, the number of shared corners is also used to classify the different types of zeolites. The aluminosiliciate structure forms a framework with pores of molecular dimensions 5-15A. these pores can also lead to cages and channels.
Cations are hydrated and exchangeable
Proton compensation possible leading to acidity
Sources Si: sio2 and silicates
Sources al: al2o3 and aluminates
Sources cations: naoh
Dissolved in highly alkaline solutions to make gel
Heated under pressure 100-200: zeolite slowly crystallizes out
Structure highly sensitive to details pH composition si/al ratio cations
Use of templates organic cations eg TMA+ to direct structure to particular architecture post synthesis template occupies cages, calcined off (burned)
However naturally occurring zeolites also do exist for example clinoptilolite
APPLICATIONS IN INDUSTRY
The uses of zeolites derive from their special properties: They can interact with water to absorb or release ions (ion exchange); they can selectively absorb ions that fit the cavities in their structures (molecular sieves); they can hold large molecules and help them break into smaller pieces (catalytic cracking).
Industrial applications make use of synthetic zeolites of high purity, which have larger cavities than the natural zeolites. These larger cavities enable synthetic zeolites to absorb or hold molecules that the natural zeolites do not. Some zeolites are used as molecular sieves to remove water and nitrogen impurities from natural gas. Because of their ability to interact with organic molecules, zeolites are important in refining and purifying natural gas and petroleum chemicals. The zeolites are not affected by these processes, so they are acting as catalysts. Zeolites are used to help break down large organic molecules found in petroleum into the smaller molecules that make up gasoline, a process called catalytic cracking. Zeolites are also used in hydrogenating vegetable oils and in many other industrial processes involving organic compounds.
One of the 'low tech' uses of zeolites in industry is for water softening. Hard water contains mg2+ and ca2+ ions which complex with the stearate ions in soaps, producing insoluble scum in household baths and basins. Calcium ions and magnesium ions are removed through the exchange of loosely bound (hydrated) cations. Hard water is filtered through a zeolite (which typically contain Na+ ions), which absorb the calcium ion and release the sodium ions into the water. When the zeolite can absorb no more calcium, it can be reused by submerging it in sodium chloride solution, which forces out the calcium ions and replaces them with sodium.
A more 'high tech' use of zeolites is the removal and storage of radioactive ions. The same principle as water softening is used in the removal of radioactive cations such as Cs+ and Sr2+ and heavy metal cations such as Cu2+ and Pb2+. These radioactive ions are present in recycling waters of atomic power stations or as environmental contaminants after accidents at atomic power stations. These can be very dangerous if not dealt with. The water is passed through an exchange column containing the zeolite, where the cation in the structure is exchanged for one of the radioactive/heavy metal ions. The water is then safe, with the radioactive material contained solely in the zeolite structure where it can be dealt with safely.
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By using certain natural zeolites, however, researchers have made headway in the drying and purification of acid gases. Mordenite and chabazite, for example, can withstand the rigors of continuous cycling in acid environments and have been used to remove water and carbon dioxide from sour natural gas. Union Carbide Corporation (now UOP Corporation, Tarrytown, NY) marketed an AW-500 product (natural chabazite-rich tuff from Bowie, AZ) for removing HCl from reformed H2 streams (pH , 2), H2O from Cl2, and CO2 from stack gas emissions (29). NRG Corporation (Los Angeles, CA;ref. 30) used a pressure-swing adsorption process with Bowie chabazite to remove polar H2O, H2S, and CO2 from low-BTU(British thermal unit) natural gas and developed a zeolite adsorption process for purifying methane produced by decaying garbage in a Los Angeles landfill (Fig. 5). A pressure-swing adsorption process using natural Mordenite was developed in Japan to produce high-grade O2 from air (T. Tamura, unpublished work; refs. 31 and 32). Domine´ and Ha¨y (33) showed that the quadrupole moment of nitrogen is apparently responsible for its adsorption by a dehydrated zeolite in preference to oxygen, resulting in a distinct separation of the two gases for a finite length of time. Similar processes use synthetic Ca Azeolite to produce O2 in sewage-treatment plants in several countries. In Japan, small zeolite adsorption units generateO2-enriched air for hospitals, in fish breeding and transportation, and in poorly ventilated restaurants. Modifying the surface of clinoptilolite with long-chain quaternary amines allowed it to adsorb benzene, toluene, and xylene in the presence of water, a process that shows promise in the cleanup of gasoline and other petroleum spills (34-36).These hydrophilic products can be treated further with additional amine to produce anion exchangers capable of taking up chromate, arsenate, selenate, and other metal oxyanions from aqueous solutions.
Molecular sieving separation of n alkane from branched by zeolite A, channels in zeolites selectively separate
Differential sorption separation of n2/o2 by li zeolite Y
Pass air over a system containing li+. N2 will interact more strongly therefore will pass through more slowly li+ must be accessible.
Applications in catalysis include (i) a selective-forming catalyst developed by Mobil Corporation using natural erionite-clinoptilolite (37); (ii) a hydrocarbon conversion catalyst for the disproportionation of toluene to benzene and xylene, employing a hydrogen-exchanged natural Mordenite (38); (iii)
a catalyst using cation-exchanged clinoptilolite from Tokaj, Hungary, for the hydromethylation of toluene (39); and (iv)clinoptilolite catalysts for the isomerization of n-butene, the dehydration of methanol to dimethyl ether, and the hydration of acetylene to acetaldehyde (40)
Shape selective catalysis chemistry controlled by pore structure
Acid catalysis, bronsted acid centres attack sorbed organics carbocations
Cracking isomerisationa and alkylation
Mechanise acid attack carbonation breaks into smaller molecules controlled by shape selectivity
Framework and extra framework metal ion affect Redox reactions transition metal cations
Reactivity controlled by pore architecture shape selective catalysis
Controlled and selective oxidation very important in industry
The biggest user of solid acid catalysts is the petrochemicals industry, accounting for most of the 130 current industrial processes that employ them. Of these processes, around 40% use zeolites as acidic frameworks to catalyse a range of reactions, including alkylations, catalytic cracking reactions and isomerisations. Acylation reactions are also widely carried out using solid acid catalysts. Here, there is a dramatic environmental benefit, in that, without the use of solid acids; stoichiometric quantities of mineral acids are required, resulting in the production of a large volume of waste that must be disposed of, along with increasing the hazardous nature of the process.
Due to pore size limitations, zeolites cannot be used to process heavy fuels, as they cannot penetrate beyond the surface of the materials. While this means alternative solid acid catalysts are used instead, this size-limited diffusion can be used to selectively catalyse the transformation of small reactants while leaving bulky components unchanged.
Solid base catalysts are also used in the petrochemicals industry in the butadiene alkylation of o-xylene and the steam cracking of olefins (UOP), but there are few other examples of their use. This is apparently due to a lack of research rather than a lack of applicability.
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Solid Acid Catalysts
The alkylation of toluene and other aromatic compounds is carried out on an industrial scale using solid acid catalysts to produce a range of starting materials and platform chemicals. Zeolites are often used, as well as being employed in butane isomerisation reactions, olefin oligomerisation and the production of synthetic hydrocarbons from the reduction of methanol.
DuPont have recently announced availability of a solid fluorinated sulphonic acid catalyst for reactions that require superacid catalysts. This material is much easier to handle than conventional superacids, has low volatility issues and can be recycled. Solid acid sulphonated resins have also been commercially available for some time, including Amberlyst-15 and Nafion-H. These can be used in a range of reactions including alkylation, acylation, nitration and esterification.
Acidic clays are used in about 5% of all industrial solid acid-catalysed processes, generally in systems where zeolites or solid oxides are not applicable. These have the advantage of being relatively low-cost, but reaction rates tend to be slower due to lower reactant diffusion. Unlike zeolites however, small and bulky molecules can undergo reaction. This is an advantage where large reactant molecules require transformation, but clays do not allow for shape-selective synthesis.
Shape-selective synthesis is achieved using zeolites or porous oxides or phosphates. Here, choice of the correct solid acid catalyst can lead to the selective reaction of small molecules, while larger molecules remain unaltered due to being unable to enter the pores of the catalyst. This can be especially useful in oligomerisation reactions, where oligomers with narrow molecule weight distributions can be produced.
Technology Issues and the Future
The use of zeolites in the petroleum industry for many years has helped to push forward the development of solid acid and base catalysts for use in alternative industries. They are now industrially and commercially ready for uptake, but their use remains limited, mainly due to a lack of understanding of their reaction mechanisms, lack of track record in the fine chemicals industry or general lack of academic testing of the materials in reactions that are of concern to industry.
Solid acids and bases could potentially find widespread application in synthesis, where the removal of liquid acids and bases and subsequent treatment of products and wastewater streams would reduce waste, cost, pollution and toxicity. However, major changes in reactor designs, processing and understanding will be required. The costs of these changes may initially outweigh the financial benefits, restricting the use of solid catalysts to new processes rather than retrofitting to old syntheses.
Fundamental research is required into the kinetics and mechanisms of reactions on solid catalysts surfaces. Novel analytical techniques will also need to be developed in order to study these environments. Further study into a wider range of applications for solid acid and base catalysts would help to convince industry of their applicability.
Traditional industrial catalysis either relies on homogeneous organometallic catalysts or heterogeneous catalysis, where the catalyst is traditionally a solid while the reactants are liquids or gases. Homogeneous catalysis is industrially problematic as often it is difficult to recover and recycle the catalyst and trace amounts of catalyst may remain in the product. The advantages of the technique are that reaction rates and product selectivities are generally higher than heterogeneous systems. The reverse is true for heterogeneous catalysis, where uncontaminated product isolation and catalyst reuse is facile, but reaction rates and selectivities can be low. Alternative catalysis methods to overcome cost, separation, selectivity, toxicity and environmental issues have been investigated for many years in academia and some technologies are now becoming employed industrially. Here, industrial and academic examples of solid catalysts and zeolites are discussed. These have the advantages that they:
• Are relatively cheap
• Offer easy product separation
• Can reduce waste production
• Are easily handled and stored
• Can be tailored to suit the chemistry in which they are used.