Location: GLO - Global
The process “o-aminophenol, at plant, GLO" is modelled for the production of o-aminophenol from o-nitrophenol. Raw materials are modelled with a stoechiometric calculation. Emissions are estimated. Energy consumptions, infrastructure and transports are calculated with standard values.
o-Aminophenol (C6H7NO; CAS 95-55-6, 2-aminophenol, 2-hydroxyaniline, 2-amino-1-hydroxybenzene, C.I. Oxidation Base 17, C.I. 76520) forms white orthorhombic bipyramidal needles when crystallized from water or benzene. Acid – base dissociation constants are pK1 4.72 (water at 21 °C), 4.66 (1 vol % ethyl alcohol in water at 25 °C); pK2 9.66 (water at 15 °C), 9.71 (water at 22 °C) Aminophenols are made either by reduction of nitrophenols or by substitution. Reduction is accomplished with iron or with hydrogen in the presence of a catalyst. The last mentioned is the method of choice today for the production of 2- and 4-aminophenol.
The reduction of nitrophenols with iron turnings takes place in weakly acidic solution or suspension. Before the iron – iron oxide sludge is separated from the solution the product aminophenol must be made water soluble by adding sodium hydroxide. The resulting sodium aminophenolate is very susceptible to oxidation in aqueous solution; various methods are recommended for its purification (see Section Purification). Subsequent to this, the aminophenols are precipitated from acidic solution by neutralization with base and in the absence of air; reducing agents usually must be added during this procedure.
When 2-nitrophenol is reduced with iron, red insoluble colour lakes are formed as byproducts that decrease the yield. Therefore, the iron reduction of 2-nitrophenol is of minor industrial importance today.
Catalytic reduction usually takes place in solution, emulsion, or suspension in autoclaves or pressurized vessels; after the catalyst is added, the vessel is pressurized with hydrogen. Water and methanol are the preferred solvents; in water the addition of alkali hydroxide, alkali carbonate, or acid has been recommended. Nickel – preferably Raney nickel – or supported precious metals, such as platinum or palladium on activated carbon, or the oxides of these metals are used as catalysts. The catalyst life can be extended, the catalyst consumption decreased, and the product quality enhanced by adding organic solvents that are not miscible with water. The preferred hydrogen pressure is 2 MPa; the hydrogenation can also be performed at atmospheric pressure or at higher pressure up to 6 MPa. The reaction temperature does not exceed 100 – 110 °C.
Reduction of Nitrobenzene
When reducing nitrobenzene in acidic medium, the intermediate phenylhydroxylamine rearranges to 2- and 4-aminophenol before it is further reduced to aniline. The main product of this reaction is 4-aminophenol; byproducts are 2-aminophenol, aniline, and 4,4′-diaminodiphenyl ether:
In the past, metals in dilute sulfuric acid were used as reducing agents. Today, the reducing agent is hydrogen in the presence of precious-metal catalysts, e.g., palladium or platinum. Other catalysts have been suggested: molybdenum and platinum sulfide, and a platinum – ruthenium mixed catalyst. Either the catalysts are used as their oxides, or they are supported on activated carbon.
Dilute aqueous mineral acid is used as reaction medium, for example, dilute sulfuric acid; acidic salts also can be added to the reduction medium.
In a two-step process, nitrobenzene first is selectively reduced to phenylhydroxylamine with hydrogen in the presence of Raney copper and in an organic solvent, such as 2-propanol. The product rearranges to 4-aminophenol after addition of dilute sulfuric acid.
The addition of wetting agents increases the aminophenol yield; these agents must be water soluble and stable in the presence of sulfuric acid. Quaternary ammonium salts that contain at least one alkyl group with at least ten carbon atoms are suitable, e.g., dodecyltrimethylammonium chloride. The reaction usually is performed below 100 °C, either at atmospheric or at higher pressure. Hydrogen is added during the reaction as consumed. The addition of inert organic solvents further increases the yield of 4-aminophenol and the product quality.
In another variant, only 88 % of the nitrobenzene is reduced; after that, the reaction mixture consists of two phases with the precious-metal catalyst (palladium on activated carbon) remaining in the unreacted nitrobenzene phase. Therefore, phase separation is sufficient as workup, and the nitrobenzene phase can be recycled directly to the next batch. The aqueous sulfuric acid phase contains 4-aminophenol and byproduct aniline. After neutralization, the aniline is stripped, and 4-aminophenol is obtained by crystallization after the aqueous phase is purified with activated carbon.
Electrolytic reduction also is possible; this method causes less concern over pollution than metal – acid reduction systems, but it has not yet found industrial application. Electrolysis of nitrobenzene or phenylhydroxylamine in the presence of sulfuric acid or of azoxybenzene in acid solution yields specifically 4-aminophenol.
Substitution of various groups by amino or hydroxyl groups is industrially unimportant for the production of 2- and 4-aminophenol, but this type of reaction is used for the synthesis of 2- or 4-aminophenol derivatives.
However, 3-aminophenol cannot be obtained easily by reduction. It is made mainly by the reaction of 3-aminobenzenesulfonic acid with sodium hydroxide or by the reaction of resorcinol with ammonia. Substitution of the sulfonic acid group in 3-aminobenzenesulfonic acid is accomplished by caustic soda fusion (5 – 6 h; 240 – 245 °C). The product is purified by vacuum distillation.
Alternatively, resorcinol reacts with ammonia, for example, in the presence of diammonium phosphate and arsenic pentoxide or ammonium sulfite to form 3-aminophenol. The compound also may be made by hydrolysis of 3-aminoaniline.
Generally, aminophenols can be purified by sublimation at reduced pressure and higher temperature. 3-Aminophenol may be purified by vacuum distillation; to obtain a colourless product sulfur dioxide is added during distillation or the distillate is collected under a blanket of an unreactive liquid of lower density, such as water.
Another method for purifying aminophenols is the treatment of their aqueous solutions with activated carbon. During this treatment, sodium sulfite, sodium dithionite, or disodium ethylenediaminotetraacetate is added to increase the quality and stability of the products and to chelate heavy-metal ions that would catalyze oxidation. Addition of sodium dithionite, hydrazine, or sodium hydrosulfite also is recommended during precipitation or crystallization of aminophenols.
Contaminants, which are usually present in the 4-aminophenol made by catalytic reduction, can be reduced or even removed completely by a variety of procedures: treatment with 2-propanol7; with aliphatic, cycloaliphatic or aromatic ketones; with aromatic amines; with toluene or low molecular mass alkyl acetates; with phosphoric acid, hydroxyacetic acid, hydroxypropionic acid, or citric acid; or by extraction with methylene chloride, chloroform, or nitroenzene.
Both 2- and 4-aminophenols are strong reducing agents and are employed as photographic developers under the trade names of Atomal and Ortol (2-aminophenol); Activol, Azol, Certinal, Citol, Paranol, Rodinal, Unal, and Ursol P (4-aminophenol); they may be used alone or in combination with hydroquinone. The oxalate salt of 4-aminophenol is marketed under the name of Kodelon.
The aminophenols are versatile intermediates and are employed in the synthesis of virtually every class of stain and dye. In addition, 2-aminophenol is specifically used for shading leather, fur, and hair from grays and browns to yellowish brown. 3-Aminophenol has found application as a hair colorant and as a coupler molecule in hair dyes. 4-Aminophenol is used as an intermediate in the synthesis of pharmaceuticals, as a wood stain imparting a roselike colour to timber, and as a dyeing agent for fur and feathers.
As a result of the close proximity of the amino and hydroxyl groups on the benzene ring and their ease of condensation with suitable reagents, 2-aminophenol is a principal intermediate in the synthesis of such heterocyclic systems as oxyquinolines, phenoxamines, and benzoxazoles. The last-named compounds have been used as inflammation inhibitors. 3-Aminophenol has found use as a stabilizer of chlorine-containing thermoplastics, although its major use is as an intermediate in the production of 4-amino-2-hydroxybenzoic acid, a tuberculostat. Similarly, nitrogen-substituted 4-aminophenols have long been known as antipyretics and analgesics, and the production of these derivatives is a major use of 4-aminophenol.
Frischknecht R., Jungbluth N., Althaus H.-J., Doka G., Dones R., Heck T., Hellweg S., Hischier R., Nemecek T., Rebitzer G. and Spielmann M. (2007) Overview and Methodology. Final report ecoinvent v2.0 No. 1. Swiss Centre for Life Cycle Inventories, Dübendorf, CH, retrieved from: www.ecoinvent.org.
Gendorf (2000) Umwelterklärung 2000, Werk Gendorf. Werk Gendorf, Burgkirchen as pdf-File under: http://www.gendorf.de/pdf/umwelterklaerung2000.pdf
Stephen C. Mitchell/Rosemary H. Waring: Aminophenols. Published online: 2000. In: Ullmann's Encyclopedia of Industrial Chemistry, Seventh Edition, 2004 Electronic Release (ed. Fiedler E., Grossmann G., Kersebohm D., Weiss G. and Witte C.). 7 th Electronic Release Edition. Wiley InterScience, New York, Online-Version under: DOI: 10.1002/14356007.a02_099
Undefined unit processes (UPRs) are the unlinked, multi-product activity datasets that form the basis for all of the system models available in the ecoinvent database. This is the way the datasets are obtained and entered into the database by the data providers. These activity datasets are useful for investigating the environmental impacts of a specific activity (gate-to-gate), without regard to its upstream or downstream impacts.
reduction of o-nitrophenol