CA1126290A - Carbonation of alkali metal phenates - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A process for carbonating a solid, dry alkali metal phenate with carbon dioxide under pressure to form the corresponding alkali metal carboxylate, where the temperature during the first step is maintained at below 135°C until at least 25 mole percent of the theoretical amount of C02 required for complete carbonation is absorbed by the phenate, the temperature is then raised to above 135°C and carbonation resumed.

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Description

112~290 CARBONATION OF
ALKALI METAL PHENATES

This invention relates generally to the car-bonation of alkali metal phenates and relates more par-ticularly to the carbonation of sodium phenate whichproduces the sodium salt of salicylic acid.

It is well-known that hydroxy aromatic car-boxylic acids can be prepared by the reaction of alkali metal phenates with carbon dioxide in the absence of water. See Lindsey et al., Chemical Reviews, 57:583--620 (1957). In this reaction, dry finely divided alkali metal phenate is contacted with carbon dioxide at super-atmospheric pressures and temperatures of from about 100C to about 300C over a period of hours to produce -~
the corresponding carboxylic acid derivative. Under these conditions, however, the alkali metal phenate has a tendency to cake or agglomerate into larger particles resulting in inefficient mixing, lower yields of the acid salt product, and localized heating in excess of the desired temperature range leading to the formation of undesirable by-products. Various techniques are employed in the art to avoid this agglomeration, as is illustrated in British Patent 1,205,447.

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~ -2- 1126290 An inert solvent or suspension system can be employed to disperse the alkali metal phenate into smaller particles which are more efficiently carbonated, as illustrated in U.S. Patent 2,824,892 and British Patents 734,622 and 738,359. However, such techniques necessitate the difficult removal and recovery of sol-vents from the product. This solvent removal/recovery step is relatively expensive and limits the use of such solvents in industrial carbonation processes.

The more common practice in industry has been to employ rotary ball mills in the carbonation of sodium phenate to produce salicylic acid. In this method, loose pieces of iron or stainless steel are employed inside the rotating mill to grind the aggregate particles into a smaller, more reactive particle size. These ball mills, however, are difficult to maintain, can contaminate the product with metal fragments, re~uire a large vessel to compensate for the volume occupied by the grinding medium, and are very noisy. Further, the removal of the carbon-ated product from the mill is difficult and time-consum-ing because the product is not free-flowing. Therefore, it is necessary to rotate the mill during removal of the product.

The foregoing prior art methods for maintain-ing the relatively higher surface area and higher reac-tivity of the alkali metal phenate are relatively ineffec-tive or uneconomical. It would be desirable to provide an economical method of carbonation whereby the phenate could be maintained in a state where it can be readily carbonated, and whereby the carbonted product can be con-veniently removed from the carbonation vessel.

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The practice of this invention is useful to efficiently prepare hydroxy aromatic carboxylic acids from phenols. This carbonation process is particularly useful to prepare salicyclic acid from phenol.

The present invention is a method for carbon-ating a dry, alkali metal phenate in the solid phase with carbon dioxide under pressure to prepare the alkali metal carboxylate of the corresponding phenol, characterized in that the carbonation is a two-step process comprising:

(a) in the first step, reacting carbon dioxide with a finely divided solid alkali metal phenate at a temperature less than 135C until at least 25 percent of the stoichimetric amount of carbon dioxide is absorbed by the phenate; and (b) in the second step, raising the temperature to above 135C and resuming the carbonation of the phenate.

The present invention is also directed to a process for preparing sodium salicylate comprising:

(a) in a first step, contacting with carbon dixoide and agitating with a centrifugal solids agi-tation means for finely divided sodium phenate having a surface area of at least about 2 square meters per gram, said sodium phenate being maintained at a tem-perature less than 135C until a molar quantity of carbon dioxide equivalent to 40 to 75 percent of the moles of said phenate present prior to carbonation is absorbed by the sodium phenate; and (b) in a second step, elevating the temperature of the sodium phenate above 135C and introducing carbon dioxide as necessary to effect a pressure of about 40 to about 500 psia (2.82-35.3 kg/cm2).

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, ~ - 3a ~ 6~90 Surprisingly, practice of the present inven-tion can produce the alkali metal salt of the hydroxy aromatic carboxylic acid as a comparatively free-flowing product in good yield, using a high pressure reaction vessel equipped merely with a centrifugal agitation means for solids, such as a ribbon blender or other centrifugal mixer consisting of a rotor with a suitable mixing ele-ment. The practice of the present invention permits the use of mixing means for solids which are more efficient than the crude rotary ball mills employed by the prior art in dry carbonation processes. Heretofore, the use , of these relatively more efficient mixing means was inhibited by the tendency of the alkali metal phenate to agglomerate during carbonation.

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The alkali metal phenates used as starting materials in the practice of this invention are alkali metal salts of any phenolic compound (i.e., a mononuclear aromatic carbocyclic compound containing at least one nuclear hydroxyl substituent). The alkali metal phen-ates suitable for the practice of the method of this invention can bear other nuclear substituents providing such substituents are inert in the process and that there is at least one reactive site for carbonation.
Inert groups include, for example, alkyl groups, halo-gen groups, amino groups, hydroxyl groups or nitro groups. Phenates containing no more than one other substituent in addition to the hydroxyl group are pre-ferred and unsubstituted phenates are most preferred.
Suitable salts of phenolic compounds include, for exam-ple, the sodium and potassium salts of phenol, cresol or chlorophenol. The phenate reactant can be a single compound or mixture of compounds (e.g., position iso-mers), if desired. The novel process is most advanta-geously employed with sodium phenate to produce highpurity sodium salicylate in good yield.

The alkali metal phenate carbonated by the method of this invention may be prepared by any of sev-eral known processes. Advantageously, all steps in which the phenate is present prior to carbonation are performed under an inert atmosphere, such as nitrogen, to prevent degradation of the oxygen-sensitive phenate. One method which can be used to produce said phenate comprises reacting an alkali metal hydroxide with a suitable phe-nolic compound in an aqueous solution or otherwise in amanner well-known in the art. Less desirably, a suit-able phenolic compound can be reacted with the alkali metal directly. The alkali metal phenate can conve-niently be extracted in a water phase and then dried.

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The finely divided particles to be carbonated by the method of this invention advantageously should have a surface area of at least about 1 sguare meter per gram, more advantageously at least about 2 square meters per gram, as determined by nitrogen adsorption as taught in Johne et al., Chem.-Inqr -Techn., 37:57 (1965). The method of carbonation of this invention is not limited to the use of finely divided alkali metal phenate pre-pared by any particular method. One convenient method of obtaining finely divided particles with an exception-ally high surface area is to introduce an atomized spray of an aqueous solution of an alkali metal phenate into a stream of hot inert gas, such as nitrogen gas, at a temperature of at least about 140C. If the phenate reac-tant is prepared by a method which produces larger parti-cles, it can be ground to a finely divided phenate prior to carbonation.

The dry finely divided alkali metal phenate can be carbonated in a continuous or batch process. The phenate reactant is normally carbonated in a high pres-sure reaction vessel equipped with a means for agitating solids and a means for heating and cooling the contents of the reaction vessel, such as a jacket containing a ;
suitable heat transfer media. Thorough mixing and tem-perature control are facilitated by using loads of phe-nate in the range of from about 25 to about 50 volume percent of the reactor capacity. The agitating means is not necessarily critical, so long as the means provide 8ufficient mixing so as to effect during carbonation suit-able heat transfer within the mass of alkali metal phenate.Suitable heat transfer is effected where substantially all of the phenate reactant is maintained at less than about 135C throughout the first step of the instant process.
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the phenate can be carbonated in a bed fluidized with carbon dioxide and optionally an inert gas such as nitro-gen to promote heat transfer. Advantageously, a centri- -fugal agitating means can be employed to promote heat transfer. The mixing element of the centrifugal agitator can take any convenient shape such as a ribbon or plow-share. A rotary ball mill is suitable, but not desirable because of the relatively inefficient mixing resulting from its use and the difficulty in removing the carbon-ated product.

Step 1 .

The first step of the carbonation reaction is carried out at a temperature less than about 135C. How-ever, because the carbonation reaction is exothermic, it lS is difficult to maintain the temperature of the contents of the reaction vessel below about 135C until the stipu-lated amount of carbon dioxide is absorbed by the phenate reactant, if carbonation is initiated at a temperature near this upper temperature limit. Carbonation is ini-tiated, as the term is employed herein, when carbon diox-ide is reacted with the alkali metal phenate.

Carbonation is preferably initiated in the first step at a temperature of from about 20C to about 110C, more preferably about 30C to about 80C. Ini-tial temperatures lower than those mentioned above areoperable, but undesirable because of the energy wasted in cooling the reactants. The alkali metal phenate and its carbonation products are advantageously mixed during carbonation so as to maintain thermal equilibrium and to eliminate regions where the desired temperature range is exceeded.

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, After carbonation is initiated, the tempera-ture of the contents of the reàction vessel are main-tained at a temperature limit below about 135C until at least about 25 percent of the stoichiometric amount of carbon dioxide necessary to effect monocarbonation is absorbed by the phenate. The amount of carbon diox-ide absorbed is conveniently approximated by subtracting the amount of carbon dioxide present at the measured temperature and pressure in the free volume in the reac-tion vessel from the amount of carbon dioxide chargedtherein. The free volume is the volume of the reaction vessel less the volume of the phenate present at its absolute density.

Advantageously, carbon dioxide is introduced into the reaction vessel at a constant rate. The intro-duction of carbon dioxide is controlled so that the tem-perature is not raised too rapidly by the exothermic car-bonation reaction. Generally, it is preferred that the temperature of the contents of the reaction vessel is limited in this first step to a temperature below about 120C until about 40 percent, more preferably below about 115C until about 50 percent of the theoretical amount of carbon dioxide necessary to achieve 100 per-cent conversion is absorbed by said phenate so as to prevent agglomeration of said phenate. In converting sodium phenate to sodium salicylate, absorption of more than about 75 percent of the theoretical amount of car-bon dioxide at a temperature less than about 135C pro-duces significantly higher amounts of the impurity sodium para-hydroxy benzoate. Hence, it is desirable in the carbonation of sodium phenate to produce sodium salicy-late to absorb an amount of carbon dioxide in the range from about 40 to 75 mole percent below a temperature of about 135C, preferably below about 120C.

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Step 2 After the alkali metal phenate has absorbed the specified amount of carbon dioxide, the temperature of the contents of the reaction vessel is increased in S the second step to effect more rapid monocarbonation of the phenate. Carbonation temperatures of from about 150C
to about 250C are generally suitable in this second step, with temperatures from about 180C to about 210C being preferred. In the carbonation of particular alkali metal phenates certain temperature ranges are preferred in the art and are generally advantageously employed to effect carbonation of specific phenates to the corresponding hydroxy aromatic carboxylate in the practice of the method of this invention. To illustrate, in the carbon-ation of sodium phenate it is preferable to maintain atemperature in the range from about 180C to about 210C
in this second step of the carbonation to produce sodium salicylate in good yield; these higher temperatures reduce the weight percentage of an undesirable by-product, 20 sodium p-hydroxybenzoate. `

The carbon dioxide pressures in Steps 1 and ~ -

2 above are not critical to the practice of this inven-tion. Generally, superatmospheric pressures are employed during the carbonation, but any carbon dioxide pressure can be employed which effects carbonation of the alkali metal phenate at an acceptable rate and which produces the hydroxy aromatic carboxylic acid in good yield. In the first step of this method pressures of from about 0.1 to about 100 pounds per s~uare inch absolute (psia) (0.007--7.0 kg/cm2) are preferred, but these pressures need not be employed immediately upon the initiation of carbona-tion due to the absorption of a substantial amount of 26,299-F

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The duration of carbonation required to effect~ubstantial conversion of the alkali metal phenate is dependent on such parameters as the carbonation tempera-ture, the pressure of carbon dioxide, the specific alkali metal phenate being carbonated, and the desired yield of the salt of the hydroxy aromatic carboxylic acid. In general, the yield of the hydroxy aromatic carboxylate will increase with increasing carbonation times up to the point at which equilibrium is reached between the phenate and carbonated species. If all other parameters remain 26,299-F

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constant, incremental units of time have a decreasing impact on yield as this equilibrium point is approached.
The duration of carbonation is not critical, so long as a suitable yield of the carbonated product is produced.
Typically a carbonation time of about 2 hours to about lO hours is suitable.

After carbonation has proceeded to the desired extent, the pressurized carbon dioxide is conveniently vented and the reaction vessel purged with an inert gas, e.g., nitrogen. The two-step process is now complete.
The alkali metal carboxylate of a phenolic compound pro-duced in this process can now be purified by conventional methods known to the art.

It is generally desirable to effect substan-tially complete monocarbonation in a single two-step car-bonation to promote efficiency of operation. However, in a less desirable embodiment of the instant two-step pro-cess a lower degree of carbonation can be effected. The unreacted phenate can be separated and recycled back into the carbonation process to increase the overall yield of the carbonated product.

In contrast to the carbonated product of prior art techniques, the hydroxy aromatic carboxylate produced by the instant two-step process is predominantly a free--flowing powder, i.e., a free-flowing particulate mass that moves readily with a continual change of place among the constituent particles in falling toward a lower cen-ter of gravity for the mass. Comparatively little agita-tion of the free-flowing product is reguired to destroy the static state of the particles and set them in motion.
Consequently, the free-flowing product can be readily removed from the reaction vessel.

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The product of the instant two-step process, which is an alkali metal salt of a hydroxy carboxylic acid, can be readily acidified in a manner known to the art. Conveniently, said salt can be acidified with a strong mineral acid, such as sulfuric acid or hydrochloric acid, in an aqueous solution. While the foregoing methods of acidification of the alkali metal salt are convenient, the instant carbonation proces~ is not limited by any par-ticular method of acidification following carbonation.

The hydroxy aromatic carboxylic acid can be `
recovered after acidification and purified by methods well-known in the art. Generally, the acids are precipi-tated from a cold aqueous solution, collected and dried.
Salicylic acid can be readily purified by sublimation as is illustrated in U.S. Patent Nos. 1,987,301 and 1,987,382.

The following examples illustrate the invention.

Procedure in Exam~les Sodium phenate is carbonated with carbon diox-ide under pressure in a ten-cubic foot (0.28 m3) reaction vessel e~uipped with a stainless steel ribbon blender driven by a motor and a heating jacket. The reaction ves-sel is heated by means of steam introduced into the reac-tion vessel jacket from a source pressurized at 165 psia (11.6 kg/cm2). Cooling of the reaction vessel is effected by introducing water into the jacket. The reaction vessel is also connected to vacuum and carbon dioxide lines and to five internal temperature sensing devices positioned just above the phenate prior to agitation, and in the phe-nate. The solids temperatures reported in the experi-ments are taken from a thermocouple kept free of any 26,299 F

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insulating phenate by the mixing element. Care must be observed in the placement of a thermocouple in the reac-tion vessel to avoid false readings of the temperature of the phenate due to poor mixing and poor heat transfer of the phenate in contact with the thermocouple.

The sodium phenate carbonated in these experi-ments is prepared by reacting equimolar amounts of sodium hydroxide and phenol in an agueous solution at reactive conditions. The solution of sodium phenate is then spray dried in a stream of nitrogen at about 200C and the finely divided sodium phenate collected. Analysis of the phenate disclosed a surface area of from 2.5 to 3.5 square meters per gram, whereas phenate dried in a rotary ball mill typically has a surface area one-fifth as great. The phenate is loaded into the reaction vessel under vacuum and the vessel equilibrated at 100C for one hour and then -i8 equilibrated at the initial carbonation temperature.
The ribbon blender is rotated at about 60 revolutions per minute in the experiments to sweep out 20 reactor volumes per minute. The carbon dioxide line is then opened and the gas introduced slowly to prevent the exothermic reac-tion from proceeding too rapidly. After about five min-utes the flow of carbon dioxide is increased to about 1.5 percent of the theoretical amount introduced per min-ute. The reaction vessel is eventually pressurized to100 psig (7.0 kg/cm2) with the addition of carbon diox-ide occurring over a period of about five hours. The experiments indicate the heating or cooling cycle employed in the individual carbonation runs.

The carbonation product is recovered after the reactor is vented, purged with nitrogen, and cooled to less than 90C unless otherwise indicated. The product is then analyzed by liquid chromatography of the acidified 26,299-F

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~" 13 _ product for organic compounds and analyzed by neutron activation for sodium. The percentage yield in moles of sodium salicylate based on sodium phenate is then calcu-lated from the foregoing analyses on the basis of the ratio of sodium salicylate to sodium in the product. The yield of sodium para-hydroxybenzoate, an impurity which is difficult to remove, is calculated in an analogous manner.

ExamPles 1 and 2 and Comparative A
Sodium phenate is carbonated according to the foregoing description. The reactor is loaded with a -sodium phenate charge of about 70 pounds (31.8 kg) in a first run and about 100 pounds (45.4 kg) in a second and a third run.

In the first run, Comparative Example A, the carbonation is initiated at about 80C. In this first run the contents of the reaction vessel are permitted to freely exotherm effecting a maximum temperature of about 161C, at which time about 25 percent of the moles of carbon dioxide required for complete carbonation of the phenate is absorbed. .Over the carbonation period of five hours, ~1 pounds (14.1 kg) of carbon dioxide is absorbed and a maximum pressure of about 73 psia (5.15 kg/cm2) is effected. The product of this carbonation is visibly coar~e in texture and contains large lumps.

In the second run, Example 1 of this invention, the carbonation is also initiated at 80C. In this second run cooling water at a temperature of about 30~C is circu- -lated in the jacket of the reactor and the flow of carbon 26,299-F
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dioxide is controlled to maintain a maximum temperature of about 130C until about one-half of the theoretical amount of carbon dioxide is absorbed. The temperature of the contents of the reaction vessel is increased grad-ually above 130C with the carbon dioxide flow turned offuntil a sudden pressure drop is registered. The carbon dioxide flow is turned on and gradually increased so as to reach a pressure of about 80 psia (5.64 kg/cm2) in five minutes and a constant value of about 115 psia (8.1 kg/cm2) shortly thereafter. The temperature of the contents of the reaction vessel increases as the exothermic carbona-tion continues to a maximum temperature of about 205C.
The product is a finely divided powder that appears iden-tical to the starting material.

In the third run, Example 2 of this invention, the carbonation is once more initiated at about 80C. The temperature of the contents of the reactor are cooled with -30C water in the jacket to maintain a temperature of about 80C as the sodium phenate slowly absorbs about one-half of the carbon dioxide necessary to effect theoretically complete conversion. The contents of the reaction vessel are then reacted with carbon dioxide at higher tempera-tures effected by the exothermic reaction and steam heating to a maximum of about 165C. The product is a finely divided powder that appears identical to the ~tarting material.

The sodium salicylate and sodium para-hydroxy- -~
benzoate yields as mole percentages of the sodium phenate are tabulated in Table I.

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TABLE I `

Na Salicylate Na p-Hydroxybenzoate ExamPle ~%) Compara-5tive A 32.0 6.1 1 ~2.5 0.2 2 81.0 5.1 It is apparent from the data in Table I and visual examination of the carbonation product in the three runs, that the sodium phenate aggregates into large lumps when the temperature is not carefully regu-lated during carbonation, resulting in a poor yield of carbonated product. On the other hand, carbonation of ~odium phenate with centrifugal agitation according to this invention produces finely divided sodium salicylate in excellent yield. Higher maximum temperatures in the second carbonation step reduce the contamination of the product with sodium para-hydroxybenzoate.

Exam~le 2 Sodium phenate is carbonated in a first run according to the method of Example 1. In a second run according to the method of this invention, the sodium phenate is carbonated as in the first run except that the carbon dioxide pressure is increased to about 165 psia (11.6 kg/cm2) in the second step of the carbonation and the maximum temperature consequently increases to 26,2g9-F

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, ~ 16~ 62~0 A sample of the product in both of the runs is removed for analysis at the end of five hours of car-bonation in the first run and after four hours of carbon-ation in the second run. The molar percentages of sodium salicylate, of sodium para-hydroxybenzoate are tabulated in Table II.

TABLE II

Na Salicylate Na p-Hydroxybenzoate Run Number (%) (%) 10 1 82.50.20 2 76.00.18 26,299-F

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Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for carbonating a dry, alkali metal phenate in the solid phase with carbon dioxide under pressure to prepare the alkali metal carboxylate of the corresponding phenol, characterized in that the carbonation is a two-step process comprising:

(a) in the first step, reacting carbon diox-ide with a finely divided solid alkali metal phenate at a temperature less than 135°C until at least 25 percent of the stoichiometric amount of carbon diox-ide is absorbed by the phenate; and (b) in the second step, raising the tempera-ture to above 135°C and resuming the carbonation of the phenate.

2. The method of Claim 1 wherein the alkali metal phenate during carbonation is agitated by a centri-fugal agitation means for solids.

3. The method of Claim 1 wherein the alkali metal phenate is sodium phenate and the alkali metal car-boxylate of the corresponding phenol is sodium salicylate.

4. The method of Claim 3 wherein the sodium phenate during carbonation is agitated by a centrifugal agitation means for solids.

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5. The method of Claim 4 wherein the finely divided sodium phenate to be carbonated is prepared by spray drying a solution of sodium phenate in a hot nitro-gen stream to obtain a surface area of at least about 2 square meters per gram of phenate.

6. The method of Claim 5 wherein in the first step a molar amount of carbon dioxide equivalent to from 40 to 75 percent of the moles of said phenate present prior to carbonation is absorbed by said phenate before elevating the temperature above 120°C.

7. A process for preparing sodium salicylate comprising:
(a) in a first step, contacting with carbon dioxide and agitating with a centrifugal solids agi-tation means for finely divided sodium phenate having a surface area of at least about 2 square meters per gram, said sodium phenate being maintained at a tem-perature less than 135°C until a molar quantity of carbon dioxide equivalent to 40 to 75 percent of the moles of said phenate present prior to carbonation is absorbed by the sodium phenate; and (b) in a second step, elevating the tempera-ture of the sodium phenate above 135°C and introduc-ing carbon dioxide as necessary to effect a pressure of about 40 to about 500 psia (2.82-35.3 kg/cm2).

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