Aldolisation-dehydration process

Details Aldolisation-dehydration process Aldolisation-dehydration process

1994-10-06

1995-07-18

Lone, Werren B.

Organic compounds -- part of the class 532-570 series

Organic compounds

Oxygen containing

568458, 568463, 568464, 568465, C07C 4574, C07C 4572, C07C 4566, C07C 4721

Patent

active

054343135

DESCRIPTION:

BRIEF SUMMARY
This application is a 371 of PCT/GB93/00729, filed Apr. 7, 1993.
This invention relates to a process for the production of unsaturated aldehydes by aldolisation followed by dehydration.
Aldolisation is a well known process in which aldehydes undergo dimerisation, typically in the presence of an alkali metal hydroxide catalyst, according to the following equation: ##STR1## where R is a hydrogen atom, an alkyl group or an aryl group. This aldol can then undergo dehydration to form an unsaturated aldehyde which has twice as many carbon atoms as the starting aldehyde of formula R.CH.sub.2.CHO, according to the following equation: ##STR2## For example, n-butyraldehyde forms ethyl propyl acrolein (2-ethylhex-2-enal) by the reactions: ##STR3## This reaction is commercially practised on a large scale as part of a process for production of the tonnage chemical, 2-ethylhexanol. This can be obtained by hydrogenation of ethyl propyl acrolein: ##STR4## This alcohol, i.e. 2-ethylhexanol, is widely used as a plasticiser alcohol for the production of di-(2-ethylhexyl)phthalate which is a plasticiser in common use for plasticising polyvinyl chloride.
Plasticiser alcohols have to satisfy stringent purity criteria, including low acidity, low water content, lack of colour, and low "sulphuric acid colour". In this last mentioned test a sample of the plasticiser alcohol is reacted with concentrated sulphuric acid and the colour of the resulting solution is measured against a set of standard solutions, using visual or instrumental methods.
In the presence of an iso-aldehyde a mixed aldol product can be formed. For example, iso-butyraldehyde can react with n-butyraldehyde as follows: ##STR5##
For a description of the mechanism of the aldolisation reaction reference may be made, for example, to the textbook "Organic Chemistry" by I. L. Finar, Second Edition, published by Longmans (1954), and to pages 127 and 128 thereof in particular.
A competing reaction to aldolisation is the Cannizzaro reaction:
Since higher molecular weight aldehydes, such as n-butyraldehyde, and aqueous solutions have limited mutual solubility, the aldolisation reaction is a two phase liquid reaction. Presumably aldehyde undergoes mass transfer into the aqueous phase, and rapidly aldolises in the aqueous phase, and then the aldolisation products transfer back into the organic phase.
Various authors have studied the kinetics of aldolisation, including: seq.; also Chem. Abs. 1968, 69, 85912); and (see also Chem. Abs. 1964, 60, 7833).
A typical current design of aldolisation plant for production of ethyl propyl acrolein from n-butyraldehyde involves three reaction stages connected in series. The first reaction stage comprises a pumped loop in which fresh n-butyraldehyde is mixed with recycled material and make up quantities of sodium hydroxide solution and then pumped in turn through a static mixer and a cooler. Most of the reaction mixture is recycled but a portion is taken to the second stage. The reaction temperature in the first stage is 120.degree. C. and the residence time is about 10 minutes. In the second stage the temperature is again 120.degree. C. and the residence time is about 20 minutes. This second stage consists of a tank reactor through which is circulated by an external pump a mixture of organic and aqueous phases. The third stage is similar to the second stage. Material drawn off from the third stage is fed to a settler from which the upper organic phase comprising ethyl propyl acrolein is recovered for hydrogenation, whilst the lower aqueous phase is recycled to the first stage. A purge stream is taken from this aqueous recycle stream in order to remove the water formed in the process and to control the level of sodium n-butyrate, formed as a result of the Cannizzaro reaction, in the circulating aqueous liquor. This purge stream presents an environmental problem since the sodium hydroxide present in it must be neutralised and the neutralised stream subjected to biological treatment to reduce its biological oxygen demand (BOD), before it can b

REFERENCES:
patent: 3248428 (1966-04-01), Porter, Jr. et al.
patent: 4408079 (1983-10-01), Merger et al.
patent: 4943663 (1990-07-01), Diekhaus et al.
patent: 5243081 (1993-09-01), Ishino et al.

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