Alcohol amination
The Alkoholaminierung is a chemical-industrial process and nowadays the most important method for preparing lower aliphatic amines . The basic principle of this reaction is the catalytic amination of alcohols with ammonia , as well as primary or secondary amines, to form a product mixture of mono-, di- and trialkylamines. In addition to common alcohols, more highly functionalized compounds such as. B. amino alcohols , polyols , fatty alcohols and polyetherols are aminated.
Process variants
The aliphatic amines are among the most important organic intermediates in the chemical industry . Particularly short-chain amines are mainly produced by alcohol amination in various process variants. The latter differ mainly in the type of catalyst, the temperature and pressure ranges and the type of implementation (gas or liquid phase). These conditions depend in particular on the physical and chemical properties of the alcohols used. The following table gives an overview of the different process variants:
| Process variant | execution | Temperature (° C) | Pressure (bar) | catalyst | Products |
|---|---|---|---|---|---|
| Amination of short-chain alcohols C 1 | Gas phase without H 2 | 250-450 | 10-40 | Acid mixed oxides of Si, Al, Ti, W, zeolites | Methylamines |
| Amination of short-chain alcohols C 2 -C 6 | Gas / liquid phase with H 2 | 150-250 | 30-80 | Metal oxides of Ni, Cu, Co, Fe, Ru, Re | Alkylamines C 2 -C 6 |
| Amination of long-chain alcohols C 8 -C 15 | Liquid phase with H 2 | 180-250 | 180-300 | Metal oxides of Ni, Cu, Co, Fe, Ru, Re | Fatty amines C 8 -C 15 |
| Amination of amino alcohols | Liquid phase with H 2 | 140-230 | 150-250 | Metal oxides of Ni, Cu, Co, Fe, Ru, Re | Ethylene amines, propylene amines etc. |
Procedure overview
The reaction of the alcohols with ammonia - depending on the alcohol - is carried out in the gas or liquid phase over acidic oxidic catalysts or over transition metal oxide catalysts with dehydrogenation and hydrogenation functions , usually in the presence of hydrogen . A product mixture of primary , secondary and tertiary amines is always formed . Formally, the following equation results as an overview reaction:
The catalysts used are primarily transition metal oxides (preferably nickel , copper and cobalt oxides ) on an oxidic support material (Al 2 O 3 , SiO 2 , TiO 2 , ZrO 2 ) and acidic mixed oxide catalysts based on aluminum and silicon (aluminosilicates, zeolites ). The latter, however, are limited to the synthesis of methylamines from methanol , since from an alkyl radical of two carbon atoms, the dehydration of the alcohols used to give the corresponding alkene increasingly occurs as a competitive and side reaction . Consequently, transition metal oxides are used as catalyst in these cases, which enable both dehydrogenation and hydrogenation . Further details are given in the Reaction Mechanism section . Furthermore, the reaction is carried out in the presence of hydrogen (except for the methylamines). This ensures that any side reactions such as imide or nitrile formation are suppressed and protects the catalyst from possible deactivation through deposits of nitrogen- or carbon-containing coking products .
Reaction mechanism
Concerning the study of the mechanism of alcohol amination, numerous mechanistic studies have been conducted in recent years. Different working groups came to different results. The assumption that is most widespread today about the correct reaction mechanism was already recognized by Schwoegler and Adkins (1939) and largely confirmed by further studies.
According to this mechanism, in the rate-determining step, the alcohol is first dehydrogenated by the transition metal oxide catalyst to form the corresponding carbonyl compound ( aldehyde or ketone ) (1) . Nucleophilic ammonia then accumulates on these with the formation of a hemiaminal (2) as an intermediate. Water is eliminated via a transition state and an imine (3) or enamine (4) is formed, which are in equilibrium due to the imine-enamine tautomerism . In a final step, the imine / enamine is then hydrogenated again over the catalyst to give the corresponding amine (5) .
The amination of a primary alcohol was explained in the overview scheme, but can be applied analogously to secondary alcohols. Tertiary alcohols cannot be converted into amines by this process, since oxidation (dehydrogenation) to the corresponding carbonyl compounds is not possible. This mechanism cannot explain the formation of the tertiary alkylamine either, since secondary amines cannot form imines with carbonyl compounds. According to Baiker et al. (1983) the formation of the trialkylamine can be explained when a secondary amine adsorbed via the nitrogen atom reacts with an alcohol molecule adsorbed on the catalyst surface. This intermediate step takes place during the dehydrogenation of the alcohol to the carbonyl compound by means of an α-H abstraction.
Process description
In the industrial production of C 2 -C 6 -alkylamines is by amination of short-chain alcohols, ammonia , the corresponding alcohol and optionally hydrogen via a vaporizer (1) , the heat exchanger ( economizer ) (2) and a superheater (3) in a fixed bed reactor down . In most cases this reactor is designed as a tube or tube bundle reactor . After the reaction, the gaseous product stream, consisting of ammonia, hydrogen, process water and the alkylamines (mono-, di- and trialkylamine), is passed through the heat exchanger (2) into a cooler (4) and then from gaseous hydrogen in the separating vessel (5) freed. The latter is returned to the reaction stage. The liquid product stream then reaches the bottom of the ammonia column (6) , in which ammonia is separated off at the top and is returned to the reaction stage together with the hydrogen. The bottom of the column is now passed into the monoalkylamine column (7) in which the low-boiling monoalkylamine is separated off. The remaining product mixture of di- and trialkylamine and water is further separated in the dialkylamine column (8) and the dialkylamine is drawn off overhead. In a final step, the process water is removed in a separator (9) and the trialkylamine is freed from residual water in the drying column (10) .
The process flow diagram shown shows a simplified scheme for carrying out the reaction in the gas phase.
literature
- Roland Dittmeyer, Wilhelm Keim, Gerhard Kreysa, Alfred Oberholz (eds.): Winnacker • Küchler: Chemical technology - processes and products - organic intermediate compounds, polymers . 5th edition. tape 5 . Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2005, ISBN 978-3-527-30770-8 .
Individual evidence
- ↑ Peter Roose, Karsten Eller, Erhard Henkes, Roland Rossbacher, Hartmut Höke: Amines, Aliphatic. In: Ullmann's Encyclopedia of Industrial Chemistry . Wiley ‐ VCH Verlag GmbH & Co. KGaA., September 30, 2015, doi : 10.1002 / 14356007.a02_001.pub2 .
- ^ Andreas Karl Rausch: Heterogeneously catalyzed hydroamination of ethanol. Carl von Ossietzky University of Oldenburg, diploma thesis, p. 13, ( Link ).