FR2640966A1 - Process for the preparation of pentachlorophenol - Google Patents

Abstract

The present invention relates to a process for the preparation of pentachlorophenol by chlorination of other chlorophenols, especially of dichlorophenols, trichlorophenols and of tetrachlorophenols. More precisely, the present invention relates to a process for the chlorination, by gaseous chlorine, of dichlorophenols and/or of trichlorophenols and/or of tetrachlorophenols, optionally mixed with monochlorophenols, to pentachlorophenol, characterised in that the reaction is carried out in the presence of a gallium halide. Pentachlorophenol is a compound used as a fungicide and as an agent for preserving wood.

Description

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  PROCESS FOR THE PREPARATION OF PENTACHLOROPHENOL

  The present invention relates to a process for the preparation of pentachlorophenol by chlorination of other chlorophenols, in particular of

  dichlorophenols, trichlorophenols and tetrachlorophenols.

  More precisely, the process of the invention makes it possible to prepare pentachlorophenol with a purity which is even better compared to current processes. Pentachlorophenol is a compound used as a fungicide and

wood preservative.

  The technical pentachlorophenol appeared, about twenty years ago, under the appearance of a very dark solid,

  containing tarry impurities.

  Overall there are 2 essential ways of preparing pentachlorophenol. A first type of process consists in chlorinating the benzene into hexachlorobenzene, then in hydrolyzing one of the C-Cl bonds in

  C-OH to form pentachlorophenol.

  A second type of process consists in chlorinating the phenol.

  In general, the processes involving the hydrolysis of hexachlorobenzene in an alkaline medium and at high temperature, lead to the formation of more polychlorodibenzoparadioxins such as

  byproducts, that phenol chlorination processes.

  It has been discovered in recent years that polychlorodibenzoparadioxins have toxic properties, and varying levels, depending on the number of chlorine atoms they contain

  and the position of these chlorine atoms.

  Although the polychlorodibenzoparadioxins which are liable to form in the chlorination processes of phenol are not those which are recognized as very toxic in animals, various means have been sought to eliminate these by-products as much as possible.

  or to prevent their formation as much as possible.

  Thus, it was proposed in US Patent No. 4,016,047 to eliminate polychlorodibenzo-p-dioxins, up to contents less than 25 parts per million (0.0025% by weight), by adding an impure pentachlorophenol phenolic compound or an organic phosphite,

  then by distilling said pentachlorophenol.

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  In this document, it is indicated that the contents of polychlorinated

  benzo-p-dioxins of impure pentachlorophenol are of the order of 0.2%. These contents are very high and, even separated from pentachlorophenol, polychlorodibenzo-p-dioxins nevertheless pose the problem of their destruction. We have therefore sought to limit as completely as possible the amount of polychlorodibenzo-p-dioxins formed during the chlorination of chlorophenols to pentachlorophenol, so as to obtain polychlorodibenzo-p-dioxin contents much lower than what is

  generally considered acceptable.

  Thus the industrial process which the applicant company operates makes it possible to obtain a pentachlorophenol having the angle

  excellent quality polychlorodibenzo-p-dioxins.

  However, by continuing its research effort in the field of chlorophenols, the applicant company has found a new process for chlorinating chlorophenols, which leads to a

  pentachlorophenol containing even less polychlorodibenzopara-

  dioxins. More specifically, the present invention relates to a process for chlorination by gaseous chlorine of dichlorophenols and / or trichlorophenols and / or tetrachlorophenols, optionally in mixture with monochlorophenols, into pentachlorophenol, characterized in that

  the reaction is carried out in the presence of a gallium halide.

  The gallium halide can be gallium chloride,

  gallium bromide, gallium iodide or gallium fluoride.

  Gallium chloride is preferably used.

  The dichlorophenols, trichlorophenols and tetrachlorophenols, which represent the main part of the phenolic compounds used in the process of the invention, are more particularly 2,4-dichloro-phenol, 2,6-dichloro-phenol, trichloro- 2,4,6 phenol

and 2,3,4,6-tetrachloro phenol.

  Most often the process will apply to 2,4-dichloro-phenol, to 2,6-dichloro-phenol, to 2,4,4-trichloro-phenol and to their mixtures,

  which are more readily available than 2,3,4,6-tetrachlorophenol.

  It is very interesting to use industrial mixtures, resulting from the chlorination of phenol or monochlorophenols and essentially consisting of 2,4,4,6-trichloro-phenol, 2,4-dichloro-phenol and 3-dichloro-2,6-phenol , as well as small amounts of 2-chloro-phenol,

  of 4-chloro-phenol and of 2,3,4,6-tetrachloro-phenol.

  Indeed, these industrial mixtures are very suitable in the context of the present process and are much more economical than the

  pure dichlorophenols or trichlorophenols.

  os The chlorination of the chlorophenols defined above takes place in the molten state, that is to say at a temperature located between the melting point of the chlorophenol (s) being worked on and approximately 180 C. The point

  The melting point of the chlorophenol (s) is generally between 70 and 80%.

  As the chlorination progresses, the temperature must be raised to maintain the reaction mixture at

liquid state.

  Preferably the temperature of the reaction mixture at each instant of the chlorination will be that which makes it possible to maintain the mixture at

  the liquid state while being as low as possible.

  The amount of gallium halide used can vary widely. Generally 0.01 to 5% by weight of halide will be used

  of gallium relative to the weight of the chlorophenol (s) involved.

  Preferably this amount will be from 0.05% to 2% by weight per

weight.

  The amount of chlorine used depends essentially on the chlorophenol (s) used, as well as the desired conversion rate. One of the advantages of using gallium halide lies in the excellent rate of fixation of chlorine compared to the chlorine used. The quantity of chlorine introduced will therefore generally be equal to or greater than the stoichiometric quantity required & the

  transformation of committed chlorophenols into pentachlorophenol.

  The excess of chlorine used to obtain a total or practically total conversion of the chlorophenols to pentachlorophenol will be less

  important only in processes using aluminum chloride.

  Most often, although it is not imperative, the chlorine is introduced by bubbling into the reaction medium and the part which possibly does not react immediately emerges from the apparatus o

the reaction takes place.

  -4- The pressure in the reactor is therefore usually equal or

  slightly higher than atmospheric pressure.

  Chlorine can be used alone or be diluted with an inert gas, such as nitrogen for example. The presence of an inert gas allows, if necessary, to increase the gas flow without correspondingly increasing

  the amount of chlorine introduced in a given time.

  Chlorine can also be formed in situ, from hydrochloric acid, by the addition of an oxidizing compound, such as

hydrogen peroxide for example.

  The pentachlorophenol obtained can be optionally purified by any known method, for example by treatment in solution using

carbon black.

  The following examples illustrate the invention.

EXAMPLE 1

  In a 150 cm3 glass reactor comprising a glass paddle stirrer, a temperature probe, a chlorine inlet tube, immersed in the reaction medium, a condenser connected to a washing column and to a flask containing an aqueous solution of sodium hydroxide, the following are charged: - 49.4 g of 2,4,4,6-trichloro phenol (0.25 mol) - 0.05 g of GaCl 3 (0.1% by weight relative to the trichlorophenol). It is heated to 80 ° C., then with stirring, chlorine is introduced.

with a flow rate of 5 liters / hour.

  The chlorination lasts 137 minutes; the amount of chlorine introduced is 0.50 mol, i.e. the stoichiometric amount

  to chlorinate the trichlorophenol introduced into pentachlorophenol.

  The temperature at the end of chlorination is 173 C.

  The final reaction mixture is determined by chromatography

  high pressure liquid (HPLC) after purging the device with nitrogen.

  - Transformation rate (TT) of 2,4,4,6-trichlorophenol: 100% - Yield (RR) into 2,3,4,6-tetrachlorophenol: 8.8% - RR into pentachlorophenol: 79.6% - 5 - The polychlorodibenzoparadioxins are dosed in the final reaction mixture; the result is expressed in

  hexachlorodibenzoparadioxin (HCDD).

  We find 0.00023% of HCDD (i.e. 2.3 parts (HCDD) by weight

per million).

EXAMPLE 2

  Example 1 is repeated using 0.49 g of GaCl3 (1% in

  weight relative to 2,4,6,6-trichloro phenol).

  0.500 mol of chlorine is introduced (in 137 min).

  The temperature at the end of chlorination is 166 C.

  The following results are obtained: - TT of 2,4,6,6-trichloro phenol: 100% RR in 2,3,4,6-tetrachloro phenol: 3.6% - RR in pentachlorophenol: 80,4%% of polychlorodibenzoparadioxins: 0.00013% HCDD

(1.3 parts by weight per million).

COMPARATIVE TEST A

  Example 2 is repeated under the same conditions, but replacing GaCl3 with AlC13 (same amount 0.49 g)

  0.500 mol of chlorine is introduced (in 137 min).

  The temperature at the end of chlorination is 82 C.

  The following results are obtained: - TT of 2,4,6,6-trichloro-phenol: 99.6% - RR of 2,3,4,6-tetrachloro-phenol: 75.2% - RR of pentachlorophenol: 20, 8% On notes that with the same amount of A1C13, we set a

  much lower percentage of chlorine used than with GaCl3.

COMPARATIVE TEST B

  Example 2 is repeated, replacing 0.49 g of GaCl3 with 0.89 g

  of AlCl3 (1 ', 8% by weight relative to the trichlorophenol).

  0.684 mol of chlorine is introduced (in 184 min), which represents 1.34 times the stoichiometric amount to transform the

  trichlorophenol to pentachlorophenol.

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  The temperature at the end of chlorination is 174 C.

  The following results are obtained: - TT of 2,4,6,6-trichloro phenol: 100% RR of 2,3,4,6-tetrachloro phenol: 7.6% - RR of pentachlorophenol: 84% of polychlorodibenzoparadioxins: 0, 00027% HCDD (2.7

parts by weight per million).

  It is noted that more AlC13 is needed than GaC13 to achieve an equivalent yield of pentachlorophenol, but that a higher amount of polychlorodibenzoparadioxins is formed with AlC13 than with

  GaC13, mime if this quantity remains very small.

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Claims (8)

  1. Process for chlorination with chlorine gas, of dichlorophenols and / or trichlorophenols and / or tetrachlorophenols,   optionally mixed with monochlorophenols, in pentachloro-   phenol, characterized in that the reaction is carried out in the presence of a gallium halide.   2. Method according to claim 1, characterized in that the gallium halide is gallium chloride, the bromide of   gallium, gallium iodide or gallium fluoride.   3. Method according to claim 1, characterized in that the gallium halide is gallium chloride,   4. Method according to one of claims 1 & 3, characterized in   what it applies to 2,4-dichloro-phenol, to 2,6-dichloro-phenol, to   2,4,6,6-trichloro phenol and mixtures thereof.   5. Method according to one of claims 1 & 4, characterized in   what it applies to industrial mixtures derived from the chlorination of phenol or monochlorophenols and consisting essentially of   2,4,4,6-trichloro phenol, 2,4-dichloro phenol and 2,6-dichloro phenol.   6. Method according to one of claims 1 to 5, characterized in   what it is implemented at a temperature between the point of   fusion of the chlorophenol (s) and around 180 C.   7. Method according to one of claims 1 to 6, characterized in   that the amount of gallium halide used is 0.01 to 5% by   weight of gallium halide relative to the weight of the chloro (s) committed phenols.   8. Method according to one of claims 1 to 7, characterized in   what the amount of gallium halide used is from 0.05% to 2%   by weight relative to the weight of the chlorophenol (s) involved.