Heterogeneous catalysis
In chemistry, heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid, liquid, vs gas, but also immiscible liquids, e.g. oil and water. The great majority of practical heterogeneous catalysts are solids and the great majority of reactants are gases or liquids.[1] Heterogeneous catalysis is of paramount importance in many areas of the chemical and energy industries. Heterogeneous catalysis has attracted Nobel prizes for Fritz Haber and Carl Bosch in 1918, Irving Langmuir in 1932, and Gerhard Ertl in 2007.
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[edit] Concepts
In heterogeneous catalysis, the reactants diffuse to the catalyst surface and adsorb onto it, via the formation of chemical bonds. After reaction, the products desorb from the surface and diffuse away. Understanding the transport phenomena and surface chemistry such as dispersion is important. If diffusion rates are not taken into account, the reaction rates for various reactions on surfaces depend solely on the rate constants and reactant concentrations. For solid heterogeneous catalysts, the surface area of the catalyst is critical since it determines the availability of catalytic sites. Surface areas can be large, for example some mesoporous silicates have areas of 1000 m2/g. The most common approach to maximizing surface area is by the use of catalyst supports, which are the materials over which the catalysts are spread.
[edit] Classes of heterogeneous catalysts
Although the majority of heterogeneous catalysts are solids, many variations exist.
| Reacting phases | Examples | Comment |
|---|---|---|
| solid + gas | Ammonia synthesis from N2 + H2 over iron catalysts | |
| solid + solution | hydrogenation of fatty acids with nickel | used for the production of margarine |
| immiscible liquid phases | hydroformylation of propene | catalyst in aqueous phase, reactants and products mainly in nonaqueous phase |
[edit] Examples
Many examples exist, the table emphasizes large-scale industrial processes,[2] although diverse examples are known.
| Process | Reactants, product(s) | Catalyst | Comment |
|---|---|---|---|
| Sulfuric acid synthesis (Contact process) | SO2 + O2, SO3 | vanadium oxides | hydration of SO3 gives H2SO4 |
| Ammonia synthesis (Haber-Bosch process) | N2 + H2, NH3 | iron oxides on alumina | consumes 1% of world's industrial energy budget |
| Nitric acid synthesis (Ostwald process) | NH3 + O2, HNO3 | unsupported Pt-Rh gauze | direct routes from N2 are uneconomical |
| Hydrogen production by Steam reforming | CH4 + H2O, H2 + CO2 | Nickel or K2O | Greener routes to H2 by water splitting actively sought |
| Ethylene oxide synthesis | C2H4 + O2, C2H4O | silver on alumina, with many promotors | poorly applicable to other alkenes |
| Hydrogen cyanide synthesis (Andrussov oxidation) | NH3 + O2 + CH4, HCN | Pt-Rh | Related ammoxidation process converts hydrocarbons to nitriles |
| Olefin polymerization Ziegler-Natta polymerization | propylene, polypropylene | TiCl3 on MgCl2 | many variations exist, including some homogeneous examples |
| Desulfurization of petroleum (hydrodesulfurization) | H2 + R2S (idealized organosulfur impurity), RH + H2S | Mo-Co on alumina | produces low-sulfur hydrocarbons, sulfur recovered via the Claus process |
[edit] Other examples
- Reduction of nitriles for instance in a synthesis of phenethylamine with Raney nickel and ammonia:[3]
- The cracking, isomerisation and re-forming of hydrocarbons to form appropriate and useful blends of petrol.
- Catalytic converters are often used in automobiles. Three main reactions are catalysed by catalytic converters. The oxidation of carbon monoxide to carbon dioxide.
- 2CO(g) + O2(g) → 2CO2(g)
- The reduction of nitrogen monoxide back to nitrogen.
- 2NO(g) + 2CO(g) → N2(g) + 2CO2(g)
The oxidation of hydrocarbons to water and carbon dioxide. This process can occur with any of the hydrocarbons, but most commonly is performed with petrol or diesel.
- 2 C6H6 + 15 O2 → 12 CO2 + 6 H2O
- Asymmetric heterogeneous catalysis affords enantiomerically enriched compounds using chiral heterogeneous catalysts.[4]
- The vast majority of heterogeneous catalysts are based on metals or metal oxides, however, some chemical reactions can be catalyzed by carbon-based materials, e.g., oxidative dehydrogenations[5] or selective oxidations[6].
- Ethylbenzene + 1/2 O2 → Styrene + H2O
- Acrolein + 1/2 O2 → Acrylic acid
[edit] See also
[edit] References
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Wikimedia Commons has media related to: Heterogeneous Catalysis |
- ^ Gadi Rothenberg, Catalysis: Concepts and green applications, Wiley-VCH: Weinheim, ISBN 978-3-527-31824-7
- ^ Zhen Ma, Francisco Zaera "Heterogeneous Catalysis by Metals" in Encyclopedia of Inorganic Chemistry, 2006, John Wiley. doi:10.1002/0470862106.ia084
- ^ Organic Syntheses, Coll. Vol. 3, p.720 (1955); Vol. 23, p.71 (1943). http://orgsynth.org/orgsyn/pdfs/CV4P0603.pdf
- ^ Heitbaum, Glorius, Escher, Asymmetric heterogeneous catalysis, Angew. Chem. Int. Ed. 2006, 45, 4732.
- ^ Zhang, J.; Liu, X.; Blume, R.; Zhang, A.; Schlögl, R.; Su, D. S. (2008). "Surface-Modified Carbon Nanotubes Catalyze Oxidative Dehydrogenation of n-Butane". Science 322 (5898): 73–77. doi:10.1126/science.1161916.
- ^ Frank, B.; Blume, R.; Rinaldi, A.; Trunschke, A.; Schlögl, R. (2011). "Oxygen Insertion Catalysis by sp2 Carbon". Angew. Chem. Int. Ed. 50 (43): 10226–10230. doi:10.1002/anie.201103340.
