GB2275924A - Fluorination process - Google Patents

Abstract

A fluoro-aromatic compound, particularly a fluorobenzene, is prepared by contacting a chloro-aromatic compound, particularly a chlorobenzene, with hydrogen fluoride in the vapour phase in the presence of a zinc-promoted fluorination catalyst. A preferred catalyst is a chromium-containing catalyst such as chromia, chromium fluoride and/or chromium oxyfluoride.

Description

PBOCESS FOR THE PRODUCTION OF FLUORO-AROHATIC COMPOUNDS This invention relates to a process for the production of fluoro-aromatic compounds having one or more fluorine atoms substituted on an aromatic nucleus and particularly to a process for the production of fluorobenzenes and chlorofluorobenzenes.

Fluoro-aromatic compounds having one or more fluorine atoms attached directly to a benzene or other aromatic nucleus are used extensively as intermediates in the manufacture of a variety of agrochemicals such as herbicides and pesticides.

Fluorobenzenes having one or two fluorine atoms attached directly to the benzene nucleus and which may also have one or more chlorine atoms attached directly to the benzene nucleus are produced on a large scale for this purpose. However, the substitution of a fluorine atom into a benzene or other aromatic nucleus is accomplished only with difficulty and is usually achieved by processes involving expensive diazotisation reaction or the formation and decomposition of fluoroformate esters.

These known processes employ chlorobenzenes or other chloro-aromatic compounds as starting materials, the chloro-aromatics, in contrast to the fluoro-aromatics, being readily obtained by direct chlorinatign procedures. Whilst direct fluorinationl hydrofluorination of chloro-aromatic compounds is desirable, thereby avoiding the need for diazotisation reactions or the formation of fluoroformate esters, attempts to achieve this desirable result have hitherto been unsuccessful.

We have now found that one or more fluorine atoms can be introduced into aromatic compounds by reacting chloro-aromatic compounds with hydrogen fluoride in the vapour phase over a particular type of fluorination catalyst.

According to the invention there is provided a process for the production of a fluoro-aromatic compound having one or more fluorine atoms substituted in the aromatic nucleus which comprises contacting a chloro-aromatic compound having one or more chlorine atoms substituted in the aromatic nucleus with hydrogen fluoride in the vapour phase and in the presence of a zinc-promoted fluorination catalyst.

Whilst the process of the invention is applicable to the production of a wide range of fluoro-aromatic compounds containing one or more nucleus-substituted fluorine atoms, it is particularly applicable to the production of fluoro-aromatic compounds containing a single benzene nucleus, ie fluorobenzenes and for convenience and simplicity the invention is described hereinafter only in respect of the production of fluorobenzenes from chlorobenzenes. It is to be understood, however, that the invention is not limited to the production of fluorobenzenes and furthermore that the process conditions (other than the catalysts) described hereinafter may not be the preferred conditions for the production of fluoro-aromatic compounds other than fluorobenzenes.The optimum process conditions (other than the catalysts) for any particular fluoro-aromatic compounds can readily be determined by simple routine experiment based on the guidelines given hereinafter.

The temperature at which the chlorobenzene and hydrogen fluoride are contacted in the presence of the catalyst may vary within a wide range but will usually be from about 3000C to about 4000C. The optimum temperature may depend at least to some extent upon the particular fluorination catalyst employed but in the case of a zinc-promoted chromia catalyst the preferred temperature range is from about 3200C to about 3600C. Under these conditions the contact time will usually be from 5 seconds to 30 seconds, typically about 10 to 15 seconds.

The reaction may be carried out at atmospheric or subatmospheric pressure but will usually be under superatmospheric pressure. Pressures up to about 30 bar are suitable, preferably from about 5 bar to 15 bar.

The chlorobenzene starting materials may contain a single chlorine atom or it may contain more than one chlorine atom. Dichlorobenzenes, especially 1,3-dichlorobenzene, are preferred starting materials since replacement of one chlorine atom by a fluorine atom yields a chlorofluorobenzene which in addition to a nucleus substituted fluorine atom also contains a nucleus substituted chlorine atom to provide an active reaction site for the production of compounds useful as agrochemicals. When using dichlorobenzene as the starting material, the process conditions can be optimised to provide chlorofluorobenzene and difluorobenzene in various proportions and in particular to provide chlorofluorobenzene as the major or even essentially the sole product.

The catalyst is a zinc-promoted fluorination catalyst by which is meant a known fluorination catalyst of which the activity has been promoted or enhanced by the incorporation of zinc. Suitable catalysts, preferably chromium-containing catalysts and comprising chromia, chromium fluoride andlor chromium oxyfluoride and their production are described in European Patent Application No 92300563.1, Publication No 502605 Al, the disclosure of which is incorporated herein by reference. The amount of hydrogen fluoride will usually be at least one mole per chlorine atom in the starting material to be replaced by a fluorine atom and generally the hydrogen fluoride will be present in excess up to about 10 moles per mole of chlorobenzene starting materials.If desired, the hydrogen fluoride andlor the chlorobenzene starting material may be diluted with an inert gas such as nitrogen.

The activity of the catalyst tends to decrease during its use and regeneration/reactivation of the catalyst may be desirable. Any known catalyst regenerationlreactivation techniques may be employed, for example heating the used catalyst in air to burn off carbon and organic compounds deposited on the catalyst surface followed by treating the catalyst with hydrogen fluoride to restore the activity of the catalyst. A preferred regenerationlreactivation technique comprises heating the used catalyst with a mixture of air or other oxidising agent and hydrogen fluoride, as is described in European Patent Application No 91308191.5, Publication No 475693 Al, the disclosure of which is incorporated herein by reference.

In carrying out the process according to the invention, the (chloro)- fluorobenzene product is separated, for example by distillation and unreacted chlorobenzene and hydrogen fluoride can be recycled.

Benzene is usually produced as a by-product and this can be recovered from the product stream and chlorinated to produce the chlorobenzene starting material for the present process. The process may be operated continuously.

The invention is illustrated but in no way limited by the following examples.

EXAMPLE 1 The equipment comprised a nitrogen and HF feed system, a liquid saturator, a reactor and a batch potassium carbonate scrubbing system. The reactor comprised a i" external diameter, U-tube Inconel reactor charged with 10g of catalyst of particle size 0.5-1.5 mm.

The catalyst, a zinc-promoted chromia (zinc chromite) containing 8Z wlw of zinc and prepared by a technique involving co-precipitating zinc and chromium hydroxides, was charged to the U-tube reactor and the reactor was located in an oven fitted with a rapid air circulator to maintain a constant and controlled reactor temperature. The catalyst was dried with a 20 mlslminute nitrogen supply for 60 minutes. The nitrogen flow was terminated and HF was fed to the reactor at 60 mlslmin to prefluorinate the catalyst at 3000C. A nitrogen flow of 100 mlslmin was added to the reactor vent to avoid condensation of vapours. After 16 hours of prefluorination, the catalyst was ready for evaluation.

The monochlorobenzene feed system comprised a " diameter saturator through which was passed a 20 mllmin nitrogen flow. The chlorobenzene saturated nitrogen was then mixed with the HF feed and passed to the reactor system.

The reaction products were passed into a 2 litre batch scrubber containing potassium carbonate. After approximately one hour of operation the reactor off-gas was diverted to a sample vessel for collection and analysis.

The above sample from the reactor was injected onto a megabore DB-1 column housed in a Varian 3400 gas chromatograph operating at 400C. The chlorobenzene conversion was found to be approximately 6.8Z and the fluorobenzene yield was approximately 4.6Z. A minor amount of benzene was detected in the product with traces of dihalobenzenes.

EXAMPLE 2 The equipment and general procedures were as described in Example 1.

The (62-3) zinc chromite catalyst which was used in Example 1 was cleaned of surface carbon deposits by-heating for 16 hrs at 3800C in an HF:Air stream of molar ratio 20:1. The air flow was terminated and the catalyst was cooled to 3000C. Dichlorobenzene was then passed over the catalyst with the 60 mlslmin of HF feed. The l,3-dichlorobenzene was fed by passing 20 mlslmin of nitrogen through å "diameter saturator. The dichlorobenzene saturated nitrogen was then mixed with the HF feed and passed to the reactor system. The reaction products were passed into a 2 litre batch scrubber containing potassium carbonate. The reaction products were sampled over a period of 65 minutes; the results are shown below.

Dichlorobenzene conversion was about 23Z.

In a comparative example 10g of an equivalent pure chromia catalyst was prefluorinated in HF at 3000C and fed with dichlorobenzene, as described above. The reaction products were sampled periodically and the results are shown below.

Reaction Z Yield Z Yield Time (minus) 1-chloro-3-fluorobenzene Chlorobenzene 15 14.1 3.3 45 9.6 2.5 65 10.2 2.5 Comparison 15 0.1 10.3 45 0.2 7.5

Claims (10)

1. CLAIMS 1. A process for the production of a fluoro-aromatic compound having one or more fluorine atoms substituted in the aromatic nucleus which comprises contacting a chloro-aromatic compound having one or more chlorine atoms substituted in the aromatic nucleus with hydrogen fluoride in the vapour phase and in the presence of a zinc-promoted fluorination catalyst.
2. 2. A process as claimed in claim 1 which is carried out at a temperature in the range of 300"C to 4000C.
3. 3. A process as claimed in claim 1 or claim 2 which is carried out under superatomospheric pressure.
4. 4. A process as claimed in claim 3 wherein the pressure is from'5 bar to 15 bar.
5. 5. A process as claimed in any one of claims 1 to 4 wherein the catalyst is a chromium-containing catalyst.
6. 6. A process as claimed in any one of the preceding claims wherein the amount of hydrogen fluoride is at least 1 mole per chlorine atom in the starting material to be replaced.
7. 7. A process as claimed in claim 6 wherein the amount of hydrogen fluoride is up to 10 moles per mole of the starting material.
8. 8. A process as claimed in any one of the preceding claims wherein the starting material is diluted with an inert gas.
9. 9. A process as claimed in any one of the preceding claims in which unreacted starting materials are recycled.
10. 10. A process as claimed in any one of the preceding claims wherein the starting material is a chlorobenzene.