Note: Descriptions are shown in the official language in which they were submitted.
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.
26,299-F
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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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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.
For example, in an operable but less desirable embodiment,
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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.
26,299-F
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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
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carbon dioxide. Also during this first step, the car-
bon dioxide pressure will usually fluctuate due to the
interruption of the carbon dioxide flow into the reaction
vessel to prevent the temperature from exceeding about
S 135C. In the second step of this carbonation, higher
carbon dioxide pressures in some cases increase the reac-
tion rate with a consequent improvement in yield for a
given reaction time. In the carbonation of specific
alkali metal phenates, certain ranges of carbon dioxide
pressure may be preferred in the art and can advanta-
geously be employed in the second step of the method of
this invention to carbonate said phenate. To illustrate,
in the carbonation of sodium phenate the rate of carbon-
ation suddenly increases at a temperature in the range of
135C to about 150C as can be observed as a sudden drop
in the carbon dioxide pressure. Advantageously, less
than a minute after the drop in pressure is observed the
flow of carbon dioxide into the reaction vessel can be
increased so as to increase the pressure and cause more
rapid carbonation of the phenate. Preferably, the ulti-
mate pressure in the sodium phenate carbonation is in
the range from about 40 to about 500 pounds psia (2.82-
-35.2 kg/cmZ) and is employed slowly over a period of at
least about 20 minutes or more to avoid the agglomera-
tion of the product.
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
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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
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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
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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.
26,299-F
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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
215C
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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
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