Method for purification of a benzoic acid

The invention relates to a method for the purification of benzoic acid, which is obtainable through oxidation of the corresponding toluene derivative, by treating it with a --NH.sub.2 or --NHR containing component in which R represents a carbon containing group, said component being applied to a support.

The invention relates to a method for the purification of a benzoic acid, 
whether or not substituted, which is obtainable through oxidation of the 
corresponding toluene derivative, by treating the benzoic acid with a 
--NH.sub.2 or --NHR containing component in which R represents a carbon 
containing group. The invention also relates to a method for the 
preparation of a benzoic acid salt. 
Such a method for the purification of a benzoic acid is known from 
EP-A-163212. A benzoic acid obtained in the oxidation of a toluene is 
purified, i.e. impurities that interfere with the further processing and 
application of the benzoic acid are removed in a distillative treatment of 
the crude benzoic acid in the presence of aliphatic amines of the formula 
HNR.sup.1 R.sup.2, R.sup.1 being an H or a hydroxyalkyl or aminoalkyl 
residual group and R.sup.2 being a hydroxyalkyl or aminoalkyl residual 
group, mentioned alkyl groups consisting of 1-6 carbon atoms. Salts 
derived from the amines may also be used. However, it has been found that 
the use of the described method leads to a product from which the 
impurities originally contained in the crude benzoic acid are at least 
partly removed but that the product is slightly, and undesirably, 
contaminated with N-containing compounds. These compounds are either the 
amines used or products derived therefrom, formed in the distillative 
treatment. This is even more the case when an excess of amine is used. 
Furthermore, there is a formation of undesired odours in the purified 
product. In addition, these N-containing compounds may cause problems in 
the further processing of the benzoic acid. A further disadvantage of the 
method described above is that the bottoms remaining after the treatment 
are contaminated with nitrogen-containing compounds foreign to the process 
and may therefore not be recirculated to the benzoic acid preparation 
section as such. 
The invention provides a method that yields a purified benzoic acid that 
does not present the aforementioned drawbacks. This is achieved by using a 
purifying agent in which--and this characterizes the invention--the 
--NH.sub.2 or --NHR containing component is applied to a support. 
By applying the --NH.sub.2 or --NHR containing component to a support a 
heterogeneous purifying agent is obtained that can easily be separated 
from the purified benzoic acid. It is also possible to regenerate the 
purifying agent after use and to make it suitable for re-use. The 
purifying agent according to the described state of the art, on the 
contrary, can usually be used once only. Because of this, the problems of 
the undesired contamination with N-containing compounds do not occur. 
Furthermore, it appears that the formation of odour is circumvented with 
using this process. 
The use of an N-containing, basic component to purify a benzoic acid is 
based on the discovery that it is primarily acids or anhydrides derived 
therefrom that constitute the contaminations in a benzoic acid. The 
benzoic acid which is obtained through catalytic oxidation of toluene 
contains, for example, phthalic acid and phthalic anhydride as undesired 
contaminations. 
It has thus been found that purifying agents that contain a primary 
(--NH.sub.2) or secondary (--NHR) group, are very suitable for purifying 
such a benzoic acid. Tertiary amino groups are per se also capable of 
removing such impurities but their effectivity is very limited. 
The heterogeneous purifying agent to be used for said purification must 
meet a number of requirements. One of the main requirements is that the 
agent is thermally and chemically stable in the benzoic acid to be 
treated. A man skilled in the art can easily find out, whether or not the 
amino group bearing heterogeneous purifying agent meets this requirement. 
In general, this means that when treating liquid benzoic acid it will have 
to be possible to use such an agent at temperatures of over 120.degree. 
C., in particular at temperatures of 125.degree.-180.degree. C. In 
addition to the benzoic acid as such, it is also possible to use solutions 
of benzoic acid. One of the reasons for using solutions may be that the 
treatment temperature may be lowered (melting temperature vs. dissolution 
temperature). The corresponding toluene is a pre-eminently suitable 
solvent because it is already present in the oxidative preparation of the 
benzoic acid. Purifying agents that are partly dissolved by the reaction 
medium, i.e. the solvent or the benzoic acid under the process conditions 
to be applied are therefore not suitable. As "support" is therefore meant 
here, a for the process stable heterogeneous agent. 
Of course, it is preferable to use a purifying agent that ensures that the 
contaminations to be removed are irreversibly retained and that the 
benzoic acid to be purified is not or only reversibly retained by the 
purifying agent in the purification. 
As purifying agent use may be made of a weak-base ion exchanger. The 
support usually used for this purpose, which is provided with the required 
--NH.sub.2 or NHR groups, consists of a polystyrene resin that is 
cross-linked with divinylbenzene. In such a method according to the 
present invention the purifying agent is used to bind the acids and 
anhydrides present to the amine groups via a reaction rather than to 
exchange ions. Organic carriers are described in--for 
example--Coordination Chemistry reviews 1984, 59, pp. 1-139. 
Another very suitable support is silica. Such a material is available both 
in a natural form and in a synthetic form and can be provided with the 
desired --NH.sub.2 or NHR groups via methods known per se. Other inorganic 
carriers consist for instance of silica-alumina mixtures. 
As basic material for the method according to the invention use may be made 
of crude benzoic acid, which can be obtained through oxidation of the 
corresponding toluene. Here and below the term benzoic acid is understood 
to be compounds of the following structural formula: 
##STR1## 
In the simplest case R.sup.1 -R.sup.5 are hydrogen atoms. One or more of 
these hydrogen atoms may be substituted so that R.sup.2 -R.sup.5 may be 
e.g. alkyl, cycloalkyl, aralkyl, hydroxyl or carboxyl groups or, for 
example, halogenides. Preferably, the method is used for unsubstituted 
benzoic acid (R.sup.1 -R.sup.5 being hydrogen), which can be obtained 
through oxidation of unsubstituted toluene. 
The purifying agent to be used must contain --NH.sub.2 or --NHR groups, as 
indicated. When use is made of an --NHR group, R may be a C.sub.1 -C.sub.8 
alkyl, whether or not substituted, a cycloalkyl or an aryl group. A 
hydroxy or amino group, for example, may be present as a substituent. 
The method according to the invention may be applied at any place in the 
process for the preparation of benzoic acid where there is a stream 
containing benzoic acid. For example, the purifying agent may be used in a 
stream of benzoic acid that comes from the distillative processing of 
crude benzoic acid, which may come from the oxidation of the corresponding 
toluene. The processing may be effected either in batch mode or in 
continuous mode, using either a slurry process or a fixed-bed process. 
Preferably, the purifying agent is used in a distillative purification 
step, which renders a separate treatment of the distillate superfluous. It 
is particularly preferred to apply this purification treatment directly 
before the actual destillation, for instance, in a fixed bed pre-column, 
because in that case the present method prevents exposure of the purifying 
agent to extremely high temperatures (such as those in the reboilers of 
distillation columns). This enhances the stability of the agent and 
prevents contamination of the benzoic acid with components of the 
purifying agent. 
The residence time of the to be treated mixture is in general more than 5 
minutes. Even within half an hour well purified benzoic acid was obtained. 
Depending on the to be purified mixture, impurities and the heterogeneous 
purifying agent, the residence time can be adjusted. Generally a residence 
time longer than 6 hr will not be applied. Preferably the to be purified 
liquid will be treated with the purifying agent during 10 min-3 hr. 
According to the method, the purifying agent is loaded with contaminations, 
as a result of which it looses its purifying effect after some time. In 
such a situation the agent can be regenerated to remove the bound 
contaminations and render the purifying agent suitable for a next 
purification process, for which saponification with the aid of a hydroxide 
(solution) (in particular NaOH (sodium hydroxide)) or hydrolysis with the 
aid of an acid (solution) may be considered. Such regeneration takes place 
at room temperature or a slightly elevated temperature. A treatment at a 
temperature that corresponds to that used for the purification of the 
benzoic acid is also suitable. 
The quality of the benzoic acid thus purified is such that the product may 
be used as a food additive or as a starting material in the 
pharmaceuticals industry. Not only the benzoic acid but also, and this 
also is a part of the invention, the salts obtainable therefrom meet high 
purity criteria (such as described in the British Pharmacopi requirement). 
Such salts can be obtained by reacting the purified benzoic acid in a 
manner known per se with an alkali or an alkaline-earth metal compound. 
This applies to the preparation of sodium benzoate in particular. 
The invention will be illustrated with the following examples and 
comparative experiments, without introducing a limitation thereto. 
COMATIVE EXPERIMENT A 
100 grams of benzoic acid, contaminated with 0.3 wt. % phthalic acid and 
phthalic anhydride (PA) was treated in a Vigreux distillation column, at a 
pressure of 2000 Pa and a top temperature of 150.degree. C. The 
distillation was carried out for 3 hours in the presence or absence of a 
soluble amino compound, after which the products obtained were 
chromatographically (HPLC) analysed to determine the phthalic acid and 
phthalic anhydride contents. The nitrogen content was also determined. The 
results obtained are shown in Table I. 
TABLE I 
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distillate residue 
Purifying PA N N 
Exp. No. 
agent molar ratio*.sup.) 
wt. % wt. % wt. % 
______________________________________ 
A-1 -- -- 0.27 -- -- 
A-2 NH.sub.3 5.0 0.04 0.06 0.31 
A-3 ethanol- 3.6 &lt;0.01 0.03 0.20 
amine 
______________________________________ 
*.sup.) the ratio of the amount of purifying agent and the amount of PA.

EXAMPLE I 
1.0 gram of weak-base ion exchanger (Merck II, Trade name 4766, particle 
size 0.3-0.9 mm) was added to 50 grams of melted benzoic acid containing 
0.3 wt. % PA. 
The treatment took place at a temperature of 130.degree. C. for 1 hour. The 
PA and nitrogen contents were determined of the liquid thus obtained. The 
results are shown in Table II. 
TABLE II 
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purified benzoic acid 
PA N 
Exp. % water (1) wt. % ppm 
______________________________________ 
I-1 39 &lt;0.01 &lt;100 
I-2 3.2 (2) &lt;0.01 &lt;100 
I-3 13.6 (3) &lt;0.01 &lt;100 
______________________________________ 
(1) water content of the fresh exchanger 
(2) water content of the exchanger reduced through azeotropic distillatio 
with the aid of toluene 
(3) the same, only now through suppression with the aid of ethanol 
EXAMPLE II 
Streams of 140 and 64 grams/hour of benzoic acid containing 0.3 wt. % PA 
were passed through a double-walled column filled with 20 grams of dry 
exchanger (Merck II), at a temperature of 140.degree. C. 
Before the test the exchanger was dried for one night in a vacuum stove at 
60.degree. C. The streams obtained were analysed in the same way as above. 
The results are shown in Table III. 
TABLE III 
______________________________________ 
purified benzoic acid 
Residence time 
PA N 
Example (min) wt. % ppm 
______________________________________ 
II-1 30 &lt;0.01 &lt;100 
II-2 65 &lt;0.01 &lt;100 
______________________________________ 
EXAMPLE III 
Example II was repeated using 100 grams/hour of benzoic acid that contained 
0.13 wt. % PA. This was passed over the Merck II exchanger (27 grams). The 
benzoic acid obtained, which contained &lt;0.01% PA and &lt;100 ppm nitrogen, 
was used to prepare sodium benzoate. The sodium benzoate thus obtained met 
the British Pharmocopi requirement for sodium benzoate. 
EXAMPLE IV 
A purifying agent consisting of a silica support modified with 
N-3(trimethoxysilylpropyl)ethylenediamine (total base content 0.51 
mgeq/gram) was prepared according to the procedure described in Inorg. 
Chim. Acta, 30(1978) pp. 51-58. 
A benzoic acid containing 0.31 wt. % PA was treated at 140.degree. C. with 
a 5-fold excess of this purification agent for 60 minutes. After this 
period the liquid contained &lt;0.01% PA and &lt;100 ppm nitrogen. 
A test in which the benzoic acid was dissolved in toluene and a temperature 
of 90.degree. C. was used yielded an analogous result. 
The same result was obtained when a silica support, similarly modified with 
3-aminopropyltrimethoxysilane or N-methyl-3-aminopropyltrimethoxysilane, 
was used as a purifying agent.