| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | February 22, 2000 |
| PATENT TITLE |
Preparation of aromatic hydroxycarboxylic acids and dialkali metal salts thereof |
| PATENT ABSTRACT | Alkali metal aryloxides may be speedily dried by removing water while the aryloxide is molten. Such dried alkoxides are useful as chemical intermediates. Molten alkali metal aryloxides may be contacted with carbon dioxide to quickly produce the dialkali metal salts of aromatic hydroxycarboxylic acids, which upon acidification yield the corresponding aromatic hydroxycarboxylic acids. Aromatic hydroxycarboxylic acids are useful as chemical intermediates and as monomers for polymers. Solid metal aryloxides may be reacted with carbon dioxide in a reactor in which the contents, even when pasty, may be agitated so that the reaction is more rapid than in prior methods |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | April 4, 1997 |
| PATENT REFERENCES CITED | A.S. Lindsey, et al., Chem. Rev., 57, 583-620, 1957. |
| PATENT PARENT CASE TEXT | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. A process for producing a dialkali metal salt of an aromatic hydroxycarboxylic acid, comprising, contacting, with agitation and at a temperature of at least about 275.degree. C. and of about or above the melting point of an alkali metal aryloxide, said alkali metal aryloxide with carbon dioxide. 2. The process of claim 1 wherein said alkali metal aryloxide is potassium phenoxide, sodium phenoxide, potassium 2-phenylphenoxide or potassium 2-naphthoxide. 3. The process of claim 1 wherein said alkali metal aryloxide is potassium phenoxide. 4. The process of claim 1 wherein said alkali metal aryloxide has a melting point of less than about 320.degree. C. 5. A process for producing a dialkali metal salt of an aromatic hydroxycarboxylic acid, comprising: (a) removing water from an alkali metal aryloxide while said alkali metal aryloxide is molten; and (b) contacting, with agitation and at a temperature of at least about 275.degree. C. and of about or above the melting point of said alkali metal aryloxide, said alkali metal aryloxide with carbon dioxide. 6. The process of claim 5 wherein said alkali metal aryloxide is potassium phenoxide, sodium phenoxide, potassium 2-phenylphenoxide or potassium 2-naphthoxide. 7. The process of claim 5 wherein said alkali metal aryloxide is potassium phenoxide. 8. The process of claim 5 wherein said alkali metal aryloxide has a melting point of less than about 320.degree. C. 9. The process of claim 5 wherein said alkali metal aryloxide becomes solid and then melts during drying. 10. The process of claim 5 wherein said alkali metal aryloxide remains molten while drying. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
FIELD OF THE INVENTION This invention concerns processes for drying alkali metal aryloxides and/or reacting alkali metal aryloxides with carbon dioxide. These processes are useful for the preparation of aromatic hydroxycarboxylic acids. BACKGROUND OF THE INVENTION Aromatic hydroxycarboxylic acids are used as chemical intermediates for the synthesis of drugs and antimicrobials, as well as monomers for preparing polyesters. For instance o-hydroxybenzoic acid (OHBA, salicylic acid) is used as a chemical intermediate, for instance to make aspirin, while p-hydroxybenzoic acid (PHBA) is used to make parabens and is also used as a monomer in making polyesters. They can be prepared using alkali metal aryloxides. Alkali metal aryloxides are usually prepared by the reaction of an aryl hydroxy compound such as phenol with an alkali metal containing base, such as sodium or potassium hydroxide. This may be done in the presence of water, and in addition water is produced in the reaction. These aryloxides are useful as chemical intermediates for a variety of chemical processes such as the Kolbe-Schmitt process to make hydroxy-carboxylic acids, see for instance, A. S. Lindsey, et al., Chem. Rev., vol. 57, p. 583-620 (1957). For some of these processes it is desirable to remove essentially all of the water from the aryloxide. This is typically done by heating the solid aryloxide (or initially an aqueous solution of the aryloxide which eventually becomes solid) while applying a vacuum and/or passing an inert dry gas over the aryloxide. However, this is quite time-consuming, and therefore expensive, and thus improved drying methods would be advantageous. After the alkali metal aryloxide is dried it may be reacted with carbon dioxide to (usually) form a dialkali metal salt of aromatic hydroxycarboxylic acid which may be converted to the free hydroxyacid by reaction with a strong acid, such as sulfuric acid. This is usually done by exposing powdered alkali metal aryloxide to carbon dioxide for long periods of time, often in an apparatus in which the solid (or the resulting paste which is formed during the reaction) may be ground in order to expose new solid surface for reaction with the CO.sub.2. Thus, improved carboxylation procedures for the Kolbe-Schmitt reaction are of commercial interest. Canadian Patent 881,906 and U.S. Pat. No. 3,554,265 describe a process for drying alkali metal phenates in a spray drying column in which the drying gas enters the column at 250 to 500.degree. C. No mention is made that the temperature of the alkali metal phenoxide reaches above the melting point of the phenoxide, nor that the dry phenoxide is ever a liquid. SUMMARY OF THE INVENTION A process of this invention for drying an alkali metal aryloxide, comprises finally removing water from said alkali metal aryloxide while said alkali metal aryloxide is molten. Further, a process of this invention for producing a dialkali metal salt of an aromatic hydroxycarboxylic acid, comprises, contacting, at a temperature about or above the melting point of an alkali metal aryloxide, said alkali metal aryloxide with carbon dioxide. Another process of this invention for producing a dialkali metal salt of an aromatic hydroxycarboxylic acid, comprises: (a) finally removing water from an alkali metal aryloxide while said alkali metal aryloxide is molten; and (b) contacting, with agitation and at a temperature about or above the melting point of said alkali metal aryloxide, said alkali metal aryloxide with carbon dioxide. The present invention also discloses and claims an improved process for the production of an aromatic hydroxycarboxylic acid using a Kolbe-Schmitt reaction of a metal salt of an aromatic hydroxy compound with carbon dioxide wherein a paste is produced during the reaction wherein the improvement comprises, carrying out the reaction with carbon dioxide at a pressure of about 0.8 to 2 atmospheres, absolute, in a reactor whose contents are agitated and wherein there is sufficient free volume that a gas may pass relatively freely through the reactor and be contacted with solid and/or liquid ingredients present provided that: the agitation is sufficient that the average residence time of a nongaseous reactant in said reactor is less than about 2 hours; at least 80 mole percent of said metal salt of an aromatic hydroxy compound is reacted with said carbon dioxide in said reactor; and the gas which is unreacted exits the reactor. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of the apparatus used in Examples 1 and 5-7. FIG. 2 is a freezing point diagram of the potassium phenoxide-water system which also shows the boiling point of the solutions at various pressures of water over the system. DETAILED DESCRIPTION OF THE INVENTION The compounds being dried herein are alkali metal aryloxides. By an alkali metal is meant any one of lithium, sodium, potassium, cesium or rubidium. Potassium and sodium are preferred alkali metals, and potassium is especially preferred. By an aryloxide is meant a monovalent anion of the structure Ar--O.sup.-, wherein Ar is an aryl group or substituted aryl group wherein the oxygen is bound to a carbon atom of an aromatic ring. Without intending to limit the generality of the foregoing, useful aryl groups include phenyl, 1-naphthyl, 2-naphthyl, 2-phenylphenyl, and 3-methylphenyl. Preferred aryl groups are phenyl, 2-phenylphenyl, and 2-naphthyl and phenyl are more preferred, and phenyl is especially preferred. Alkali metal aryloxides may be prepared by the reaction of a strong base (stronger than aryloxide anion) with a corresponding aromatic hydroxy compound, Ar--OH. A suitable base is an alkali metal hydroxide, which is often also relatively inexpensive. This reaction may be carried out in water or by reacting solid alkali metal hydroxide (which usually contains some water) with a molten aromatic hydroxy compound, typically by using approximately equivalent amounts of aromatic hydroxy compound and alkali metal hydroxide. For instance, 45% aqueous KOH may be reacted with phenol. By drying herein is meant the removal of water from an alkali metal aryloxide. It is preferred that the final "dry" aryloxide have a water content of less than 2000 ppm water, preferably less than 500 ppm water, and more preferably less than 100 ppm water. In a typical drying process currently practiced in the art, an aqueous solution of an alkali metal aryloxide is heated to distill or vaporize off the water. Eventually the concentration of the alkali metal aryloxide becomes so high that it precipitates from the solution, and the final stages of drying are performed on the solid aryloxide. While water is being removed, a preferably dry, inert gas may be passed over the aryloxide, or the aryloxide may be subjected to a vacuum, or both, to help remove water vapor. In the drying process disclosed herein, the last stages of drying are conducted at a high enough temperature so that the alkali metal aryloxide is molten. This means this final drying (i.e., the "last" water removed) is conducted at about or above the melting point of the dry alkali metal aryloxide. In one method of drying, the temperature of the aryloxide (and the accompanying water) are continuously raised until at or above the melting point of the aryloxide. During this time, what may have started as a one phase liquid containing water and aryloxide may be converted to a solid aryloxide or a solid and liquid, and then, if a solid had formed that solid will change back to a single phase liquid as the melting point of the aryloxide is approached and/or exceeded. During this time an inert gas may be passed over the aryloxide and/or a vacuum applied. Agitation of the aryloxide, whether solid or liquid, will usually help speed drying. The final removal of water takes place when the aryloxide is molten. Another way of carrying out the drying is to heat the mixture of water and alkali metal aryloxide above the melting point of the aryloxide under atmospheric or elevated pressure, so sufficient water is present so that a single liquid phase is maintained. As the temperature approaches the melting point of the aryloxide the pressure may be lowered to allow the water to vaporize, and the drying is finished as described above. Care should be taken that the vaporization of water does not cool the liquid sufficiently so that a solid forms during the final drying stage. The above discussion on dehydration of aryloxides is illustrated by the dehydration of potassium phenoxide, and the phase diagram associated with that dehydration which is shown in FIG. 2. This Figure was generated by appropriate measurements in the laboratory, but the data was smoothed and the curves at various pressures made parallel with the assumption that Henry's Law is followed at low partial pressures of water in the vapor phase. Therefore all data on this Figure should be considered approximate. The solid line shows the freezing point of the potassium phenoxide/water system. The abscissa is the weight fraction of potassium phenoxide present (water is the other part of the weight fraction), while the ordinate is temperature in degrees Centigrade. The dashed lines show the boiling points of the water in the system at the indicated pressures (0 psig or 0 kPag is atmospheric pressure). Where the freezing point curve is above the boiling point of the water in the system, as it is at 0 kPag (0 psig) and approximately 0.93 to 0.98 mass (weight) fraction of potassium phenoxide, the system will be partially a solid as the water is vaporized from the system. Above a mass (weight) fraction of about 0.98 potassium phenoxide the system will be a liquid as the water is removed at 0 kPag (0 psig). However, if the system pressure is held at about 68.9 kPag (10 psig) or higher, it will remain a liquid throughout the removal of the water. Removal of the final amounts of water from molten alkali metal aryloxide usually proceeds rapidly, thereby making the drying process more economical. It is of course preferred that the alkali metal aryloxide not decompose appreciably during the drying, i.e., be reasonably stable at or above its melting point. Therefore it is preferred that the alkali metal aryloxide have a melting point of less than 400.degree. C., more preferably less than about 350.degree. C., and especially preferably less than about 320.degree. C. A preferred minimum drying temperature is either the melting point of the alkali metal aryloxide or 275.degree. C., whichever is higher. The temperature referred to during the (last part of the) drying is the actual temperature of the alkali metal aryloxide itself. Melting points are determined by Differential Scanning Calorimetry at a rate of 25.degree. C./min, and the melting point is taken as the extrapolated initial temperature of the melting endotherm. Preferred alkali metal aryloxides are potassium phenoxide, sodium phenoxide, and potassium 2-naphthoxide, and potassium phenoxide is especially preferred. The drying process may be carried out in any vessel suitable for use with the materials and temperatures involved. Stainless steel is particularly suitable. If part of the drying process is to be done under pressure and/or vacuum, the vessel should be so rated. It may be desirable to be able to stir the liquid and/or solid which may be present in the vessel, to improve heat transfer and promote vaporization of the water from the aryloxide. The vessel and its contents may be heated by the usual methods, such as a condensing vapor such as Dowtherm.RTM. heat transfer fluid (available from Dow Chemical Co., Midland, Mich.), electrically, or by hot oil. The dry aryloxide which is produced may be cooled and solidified, and used in a traditional Kolbe-Schmitt-type process, or may remain molten and undergo reaction with CO.sub.2 to form a dialkali metal salt of an aromatic hydroxycarboxylic acid. The chemical reaction involved in the Kolbe-Schmitt synthesis may be represented as: ArOH+MOH.fwdarw.ArOM+H.sub.2 O (1) 2ArOM+CO.sub.2 .fwdarw.M.sub.2 (OArCO.sub.2)+ArOH (2) M.sub.2 (OArCO.sub.2)+2H.sup.+ .fwdarw.HOArCO.sub.2 H+2M.sup.+(3) M in the above equations in an alkali metal, and Ar is an aryl, substituted aryl, substituted arylene, or arylene group, as appropriate. Equation (1) is simply the formation of an alkali metal aryloxide whose drying is described above. The "dry" aryloxide is reacted with CO.sub.2 to usually form a dialkali metal salt of an aromatic hydroxycarboxylic acid. This salt may then be acidified to form an aromatic hydroxycarboxylic acid (sometimes referred to herein as the "free" aromatic hydroxycarboxylic acid, which means it is not a salt). As noted above, the reaction of equation (2) has been carried out by exposing solid alkali metal aryloxide to CO.sub.2, typically in the temperature range of about 150-250.degree. C. Usually some form of agitation or grinding is provided to expose all of the solid to the CO.sub.2. The desired dialkali metal salt of the aromatic hydroxycarboxylic acid is a solid under these conditions, but the aromatic hydroxy compound formed is a liquid which often eventually boils or vaporizes off in the CO.sub.2 (or other gas) stream. Thus the reaction often goes from a solid powder to a paste to a solid powder, and that combined with the difficulty of completely reacting a solid with a gas requires the reaction to be run for long periods of time to complete it. It has been found that much shorter reaction times can be obtained if molten alkali metal aryloxide is reacted (contacted) with the CO.sub.2, particularly with agitation. In this scenario, the initial material is a liquid which usually turns to a paste as the solid dialkali metal salt of the aromatic hydroxycarboxylic acid and the liquid aromatic hydroxy compound are formed. This usually gradually turns to a dry solid as the aromatic hydroxy compound is boiled or volatilized from the mixture. Since it is desired to react the alkali metal aryloxide while it is in the molten state, the contacting with should be carried out about at or above the melting point of the alkali metal aryloxide. Since it will usually be desirable to eventually cool the resultant dialkali metal salt of the aromatic hydroxy-carboxylic acid, up to the last 20 mole percent of the reaction with CO.sub.2 may be carried out below the melting point of the alkali metal aryloxide, although it is preferred that at least 90 mole percent, more preferably at least 98 mole percent of this reaction be carried out about at or above the melting point of the alkali metal aryloxide. In the contacting with CO.sub.2, it is of course preferred that the alkali metal aryloxide or any of the other materials present during the process not decompose appreciably, i.e., be reasonably stable at the process temperature. Therefore it is preferred that the alkali metal aryloxide have a melting point of less than 400.degree. C., more preferably less than about 350.degree. C., and especially preferably less than about 320.degree. C. A preferred minimum temperature is either the melting point of the alkali metal aryloxide or 275.degree. C., whichever is higher. In the reaction with CO.sub.2, useful aryl groups include phenyl, 1-naphthyl, 2-naphthyl, 2-phenylphenyl, and 3-methylphenyl. Preferred aryl groups are phenyl, 2-phenylphneyl and 2-naphthyl, phenyl and 2-naphthyl are more preferred, and phenyl is especially preferred. Preferred alkali metal aryloxides are potassium phenoxide, sodium phenoxide, potassium 2-phenylphenoxide, potassium 2-naphthoxide, potassium phenoxide and potassium 2-naphthoxide are more preferred and potassium phenoxide is especially preferred. Preferred products (expressed as their parent aromatic hydroxycarboxylic acids, they are originally produced as their dialkali metal salts) are salicylic acid, p-hydroxy-benzoic acid, 3-phenyl-4-hydroxybenzoic acid, and 6-hydroxy-2-napthoic acid, and p-hydroxybenzoic acid is especially preferred. Suitable materials of construction for the CO.sub.2 reaction are the same as described above for the drying of alkali metal aryloxides. Useful types of equipment for this reaction include, but are not limited to, a ribbon blender, the LIST-CRP and LIST ORP, made by LIST AG, Arisdorf, Switzerland, and available in various sizes, a Littleford VT Series vacuum dryer made by Littleford Day, Inc., Florence, Ky., U.S.A., or a Reactotherm RT made by Krauss-Maffei Verfahrenstechnik Gmbh, Munchen, Germany. The reaction may be run in a batch, semicontinuous or continuous manner. For instance a modified screw conveyor reactor may be used. Screw conveyor reactors are known in the art, see for instance B. Elvers, et al., Ed., Ullmann's Encyclopedia of Industrial Chemistry, Vol. B4, 5th Ed., VCH Verlagagesellschaft, Weinhem, 1992, p. 111-112. These are usually used in reactions where solids and liquids are involved. By a modified screw conveyor reactor is meant such a reactor wherein there is sufficient free volume that a gas may pass relatively freely through the reactor and be contacted with the solid and/or liquid ingredients present or its equivalent. Such a reactor may have single or multiple rotating shaft(s), or other agitating aids, such as high speed choppers. By an equivalent is meant any reaction apparatus that can move the liquid and/or solid reactants through the reactor at an appropriate rate (to give the desired residence time) while at the same time exposing fresh surfaces of the solid and/or liquid to the gaseous atmosphere, in this case containing CO.sub.2. It should also have provision for adding one or more gases at one or more points along or through the reaction vessel, and preferably also have a port for removing any excess gas that is added to the system, and/or any volatiles generated during the process. Where a "screw-type" conveyor reactor is used, the "screws" of the reactor need not be solid screws, but may be segmented screws with open spaces between the screw shaft and the periphery of the screw. The screw or agitator shafts may be cored so that heat transfer fluid or CO.sub.2 and/or other gas may be passed through. In the case of the CO.sub.2 and/or other gas, there may be holes in the shaft(s) so that gas can pass through into contact with the reacting mass. As mentioned above, the reaction of CO.sub.2 with two equivalents of the alkali metal aryloxide yields one equivalent of the dialkali metal salt of the aromatic hydroxycarboxylic acid and one equivalent of the original aromatic hydroxy compound. In practice, the aromatic hydroxy compound is usually above its melting point at the process temperature and forms a paste with the dialkali metal salt formed and the solid (below its melting point) alkali metal aryloxide. This paste formation results in difficulty in mixing the CO.sub.2 with the starting alkali metal aryloxide, thereby slowing the reaction. In addition, it is believed that the reactivity of the alkali metal aryloxide with CO.sub.2 is reduced in the presence of the free aromatic hydroxy compound. The formation of this paste is believed responsible for the long overall process cycles, usually at least several hours, experienced in the typical commercial Kolbe-Schmitt process, such as in a ball mill. In order to overcome this problem, a reactor which has an agitator that agitates the contents is desirable. The agitator and agitator drive (i.e., the power for the agitator) must be such that the agitator is sufficiently powerful to mix the contents of the reactors, especially in the paste phase of the reaction, so that relatively rapid reaction rates with CO.sub.2 are maintained. Reactors such as are described above (when reacting molten alkali metal aryloxides) are suitable, and modified screw conveyor reactors (see above) are a preferred type of reactor. One such line of suitable modified screw conveyor reactors is the LIST-CRP and LIST ORP, made by LIST AG, Arisdorf, Switzerland, and available in various sizes. Another type of suitable reactor is a Reactotherm RT made by Krauss-Maffei Verfahrenstechnik Gmbh, Munchen, Germany. The reactor should also have provision for adding one or more gases at one or more points along or through the screw conveyor system, and especially preferably also have a port for removing any excess gas that is added to the system, and/or any volatiles generated during the process. It is preferred that the agitator of the reactor be essentially self cleaning, i.e., that essentially all material that may stick to the screw or barrel surfaces is wiped or otherwise removed from the surfaces so that it does not remain in the reactor for an excessive period of time. As mentioned above, the drive for the reactor should be powerful enough to overcome any drag that may be caused by the solids or any pastes that may form inside the reactor, so as to constantly mix the solid and/or liquids with the gas phase in the reactor, and not restrict the rate of reaction of CO.sub.2. Most Kolbe-Schmitt reactions are run at elevated temperatures, so there should usually be a means of heating the reactor. Such means are known to those skilled in the art, and include steam or other condensing vapors, hot liquid, electrical heat, etc. The reaction may be run in a batch, semi-batch or continuous manner, and preferably the process is run in a continuous manner. Also preferably, the solid alkali metal aryloxide is fed at a first end of the reactor and continuously moves through the reactor to the second end of the reactor, where it exits the reactor. Preferably at least some of the CO.sub.2 is added at the second end of the reactor and moves towards the first end of the reactor (preferably counter-currently to the movement of the solids and liquids in the reactor). More CO.sub.2 may be added at other points along the reactor. Preferably the amount of CO.sub.2 added is about stoichiometric (the amount required to react with the metal aryloxide added) to about 5 times the stoichiomctric amount, more preferably about 2 to 4 times the stoichiometric amount. Optionally, an inert gas such as nitrogen may also be added to the reactor. Excess CO.sub.2 and/or the inert gas can help remove byproduct aromatic hydroxy compound which is formed during the reaction, thereby helping to reduce paste formation in the reactor. As mentioned above, it is believed that presence of aromatic hydroxy compound reduces the reaction rate of alkali metal aryloxide with CO.sub.2, and therefore removal of the aromatic hydroxy compound is believed to result in an overall shorter process time. It is also noted that many Kolbe-Schmitt reactions are run at temperatures above the atmospheric boiling points of many of the aromatic hydroxy compounds which are used. Thus even without the addition of excess CO.sub.2 and inert gas to the reactor, much of the aromatic hydroxy compound may be removed by simple vaporization in many cases, so long as there is a vent port on the reactor. The heat of the reaction may be utilized to help vaporize the free aromatic hydroxy compound, further reducing cycle time and improving thermal efficiency. It is preferred that the average residence time of the alkali metal aryloxide (or its reaction product with CO.sub.2) in the reactor is less than about 1.5 h, more preferably less than about 1 hr, especially preferably less than about 0.5 h, and highly preferably less than about 0.25 h. In less than 2 h, or any of the times mentioned above, it is preferred that at least 80 mole percent, more preferably at least 90 mole percent, and especially preferably at least 95 mole percent, and most preferably at least 98 mole percent of the original alkali metal aryloxide has reacted with CO.sub.2. By residence in a batch reactor is simply meant the reaction time for the batch. Average residence time in semi-batch and continuous reactions have their usual meanings. Preferably, the interior of the reactor is at the ambient atmospheric pressure, plus or minus 20.0 kPa (0.2 bar). Pressures at or above the ambient pressure minimize the leakage of atmospheric gases such as oxygen or moisture into the reactor. Making such commercial reactors gas tight can be expensive and difficult. Preferably the alkali metal is sodium or potassium, more preferably potassium. The preferred aryloxide anions for the process in the agitated reactor are the same as those listed above for the process involving drying and/or reaction of molten alkali metal aryloxides. |
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