Process for purification of carboxylic acids

Carboxylic acids produced by carbonylation and having iodide and oxidisable impurities are purified by contacting with hydrogen peroxide and recovering the purified acid by distillation or evaporation. Preferably a strong acid such as sulphuric acid is used as a catalyst. Product contamination by sulphur from sulphuric acid and by excess peroxide may be reduced by the use of metal salts in the recovery step.

The present invention relates to the purification of carboxylic acids and 
in particular to a process for removing iodide and oxdisable impurities 
from carboxylic acids, preferably acetic acid. 
The liquid phase carbonylation of methanol is a well known industrially 
operated process and is widely operated commercially. The carbonylation 
process, which is typically catalysed by rhodium and methyl iodide, is 
described in detail in, for example, UK patent number GB 1233121. European 
patent application number EP 87870 describes a modification of this 
process in which acetic acid is coproduced with acetic anhydride, from 
mixtures of methanol, methyl acetate and water under anhydrous conditions. 
A problem with acetic acid produced by processes such as those described in 
GB 1233121 and EP-A-87870 is that even after purification by conventional 
distillation it still contains oxidisable impurities and relatively large 
amounts (typically 100 ppb-2 ppm) of iodide impurities. For certain 
applications, e.g. in the subsequent conversion of the acetic acid into 
vinyl acetate, these impurities are detrimental and need to be removed. 
UK patent GB 1201260 relates to purification of formic acid by treatment 
with hydrogen peroxide to reduce colour-forming impurities. Soviet patent 
number SU 1437363 relates to purification of acetic acid being regenerated 
in the production of cellulose acetates and acetic anhydride by treating 
with hydrogen peroxide in the presence of polyphosphoric acid or a salt 
thereof. Japanese laid open patent application number H2-134342 relates to 
decolourisation of acetic acid with hydrogen peroxide or percarboxylic 
acid. None of these publications addresses the problem of removing iodide 
impurities from carboxylic acids. 
European patent publication number EP-A-0013551 relates to a process for 
removing iodine from organic compounds in which the compounds are treated 
at 50.degree. to 200.degree. C. with oxidising agents such as oxygen or 
hydrogen peroxide and the reaction mixture is brought into contact with an 
adsorption agent for example active carbon. The compounds envisaged for 
treatment are in particular the products of oxidative acylation of 
olefins. EP-A-0013551 does not describe purification of carboxylic acids 
produced by carbonylation to remove iodide and oxidisable impurities. 
EP-A-0013551 does not describe recovering purified carboxylic acid from 
hydrogen-peroxide-treated-acid by distillation or evaporation. 
UK patent number GB 1293774 describes a process for the purification of 
carboxylic acids contaminated with trace amounts of halides by treating 
with an inorganic oxidising agent and subjecting the treated acid to 
distillation. According to GB 1293774, the oxidising agent contains an 
alkali metal and/or chromium or a metal of Groups VIIB of the periodic 
table according to Deming. The preferred treating agents contain sodium or 
potassium and manganese or chromium; potassium and sodium permanganate 
being especially preferred. 
European patent number EP 0217191B describes a process for the separation 
of iodine and its compounds from the carbonylation products obtained in 
the carbonylation of dimethyl ether, methyl acetate or methanol. The 
process of EP 0217191B comprises treating the carbonylation products at 
temperatures of 20.degree. C. to 200.degree. C. with peracetic acid, 
diacetyl peroxide or compounds forming these under the reaction conditions 
and then separating the product by distillation. This is said to reduce 
iodine contents to less than 20 ppb in the carbonylation products. 
Such processes are not entirely satisfactory. The problem to be solved 
therefore is to provide a process for removing iodide and oxidisable 
impurities from carboxylic acids which have been prepared by 
carbonylation. 
Thus, according to the present invention there is provided a process for 
purifying impure carboxylic acid which has been prepared by the 
carbonylation of a suitable reactant in the presence of a carbonylation 
catalyst and an iodide promoter and which contains iodide and oxidisable 
impurities, which process comprises the steps of: 
(a) contacting the impure carboxylic acid with hydrogen peroxide, and 
(b) recovering purified carboxylic acid from the product of step (a) by 
distillation or evaporation. 
The process of the present invention is suitable for removing iodide and 
oxidisable impurities from carboxylic acids having up to 6 carbon atoms 
preferably acetic acid, propionic acid and/or butyric acids, more 
preferably acetic acid. 
Processes for preparing carboxylic acids by carbonylation are known in the 
art. Suitably the acid is produced by the carbonylation of an alkyl ester 
of a carboxylic acid, an alkanol, an alkyl halide and/or a dialkyl ether 
in the presence of a Group VIII metal carbonylation catalyst, for example 
rhodium or iridium and in the presence of an iodide promoter. Acetic acid 
may be prepared by carbonylation of methanol, methyl acetate, methyl 
halide and/or dimethyl ether. Suitable processes for preparing acetic acid 
are described in GB 1233121 and EP-A-87870, the contents of which are 
hereby incorporated by reference. Suitably the carboxylic acid is 
separated from the carbonylation catalyst and iodide promoter by 
distillation and/or evaporation before purification by the process of the 
present invention. Distillation to remove the bulk of the iodide and/or 
oxidisable impurities prior to contacting with hydrogen peroxide may also 
be used depending upon the concentrations of the impurities. 
The iodide impurities comprise iodine-containing compounds, for example, 
methyl iodide, butyl iodide, hexyl iodide and inorganic iodides, typically 
at 100 ppb to 2 ppm total iodine in the acid. 
The exact nature of the oxidisable impurities is not known, but their 
presence causes the carboxylic acid to have a permaganate time of less 
than 2 hours and so fail the permaganate test as herein defined. 
The process of the present invention can reduce iodide impurities from 
about 500 ppb to below 50 ppb and oxidisable impurities sufficiently for 
the carboxylic acid to pass the permanganate test. 
The hydrogen peroxide is used as an aqueous solution, preferably at 50-60% 
w/v. 
The amount of hydrogen peroxide used will depend upon the amount of iodide 
and oxidisable impurities in the carboxylic acid but will generally be in 
the range 10 to 10000 ppm by weight of the impure carboxylic acid, 
preferably 100 to 5000 ppm. 
Preferably, the impure carboxylic acid is contacted with hydrogen peroxide 
in the presence of a strong acid. Suitable strong acids for use in the 
present invention comprise sulphuric acid, methane sulphonic acid, 
para-toluene sulphonic acid, trifluoroacetic acid, phosphoric acid and 
strong acid ion exchange resins such as Amberlyst 15. A preferred strong 
acid is sulphuric acid. 
The strong acid may be used without dilution but conveniently may be used 
as a solution in the carboxylic acid. The amount of sulphuric acid used in 
step (a) may be in the range 10 to 1000 ppm. 
The temperature at which step (a) is performed should be high enough to 
prevent the carboxylic acid freezing at one extreme or boiling at the 
other. The temperature will depend upon a number of factors. In 
particular, the higher the temperature the faster the reaction between the 
hydrogen peroxide and the impurities but also the faster it decomposes. 
Typically, for acetic acid, the temperature will be in the range 
20.degree. to 118.degree. C. although higher temperatures may be used if 
the process is operated at superatmospheric pressure. 
The step (a) of the process may be operated at subatmospheric, atmospheric 
or superatmospheric pressure, preferably atmospheric pressure. 
The contact time of the impure carboxylic acid with the hydrogen peroxide 
will depend upon such factors as the concentration of impurities and 
reagents, and the temperature, but will generally be in the range 1 minute 
to 24 hours, preferably 10 minutes to 5 hours. 
In step (b) the purified carboxylic acid is recovered by distillation or 
evaporation, preferably as a liquid side draw from a multistage fractional 
distillation column, with small heads and base bleeds. A residues 
concentrator may be used on the base bleed from the distillation column to 
reduce the volume of residues, with return of acid to the distillation 
column. 
It is possible to perform steps (a) and (b) together by adding the hydrogen 
peroxide and optional strong acid to a distillation column. Preferably the 
steps may be performed separately in sequence rather than simultaneously. 
The process of the present invention may be performed as a batch or 
continuous process. A batch process generally produces better quality 
product acid but on an industrial scale a continuous process is generally 
preferred. 
When sulphuric acid is used as the strong acid in step (a) the separated 
carboxylic acid in step (b) may contain sulphur impurities. Contamination 
of the purified carboxylic acid by these sulphur impurities may be reduced 
by the use of a suitable metal salt in the distillation/evaporation step 
(b). Suitable metal salts are carboxylate salts such as acetate salts 
and/or are nitrate salts, for example potassium, silver and/or calcium 
carboxylate e.g. acetate and/or nitrate. Suitable salts are also potassium 
and/or magnesium hydroxide and/or copper (I) oxide. Carboxylate salts may 
be added to step (b) as such or may be formed in situ by the addition of 
hydroxides. Preferred salts are potassium carboxylate, particularly 
acetate and silver nitrate. More than one salt may be used. Suitably the 
metal salt is present at from about I% w/w to about 10% w/w, preferably 
from about 1% w/w to about 7% w/w in the kettle/reboiler of the step (b) 
distillation column. 
Contamination of the purified carboxylic acid by excess peroxide may be 
reduced by the use of a suitable metal salt in the 
distillation/evaporation step (b). Suitable metals salts are salts of the 
carboxylic acid for example carboxylate salts of potassium, silver and/or 
calcium. Suitable metal salts are also salts such as potassium and/or 
magnesium hydroxide and/or copper (I) oxide. Carboxylate salts may be 
added to step (b) as such or may be formed in situ by the addition of 
hydroxide salts. A preferred metal salt is potassium carboxylate, 
particularly potassium acetate. More than one salt may be used. Suitably 
the metal salt is present at from about 1% w/w to about 15% w/w, 
preferably about 1% w/w to about 10% w/w in the kettle/reboiler of the 
step (b) distillation column/evaporator. The metal salts used to reduce 
peroxide carryover may also be used to reduce sulphur carryover if 
sulphuric acid is used in step (a).

The invention will now be illustrated by reference to the following 
examples. 
In the examples iodide impurities were measured by neutron activation 
analysis. The permanganate times were measured at room temperature by 
adding 0.1 ml of 0.02M potassium permanganate to 2 ml of sample and 10 ml 
of distilled water; the permanganate time being the time required for the 
pink colour of the permanganate to be discharged. A permanganate time of 
greater than 2 hours was taken as a pass of the test. Water concentrations 
were determined by automatic Karl Fischer titration. Sulphur contents were 
determined by X-ray diffraction (XRF) with a limit of detection of 2 ppm. 
Samples with very low (ppb) levels of iodide impurities were analysed for 
sulphur content by microcoulormetry down to 0.2 ppm. 
Continuous Treatment Experiments 
Continuous treatment of impure acetic acid prepared by carbonylation was 
studied using a heated stirred treater vessel from which liquid product 
was passed to the reboiler of a 12 actual stage distillation column. The 
distillation column had a purified acid liquid product take-off at stage 4 
counted from the base of the column. The distillation column had a pumped 
heads reflux and small head and base take-off bleeds. 
The results of treatment with and without sulphuric acid are 
TABLE 1 
__________________________________________________________________________ 
Purification of Acetic Acid According to the Present Invention 
Impure Acetic Acid Product Acetic Acid 
Exper- Peranganate Treatment Iodide 
iment Time Iodide Temp. 
Time 
Permanganate 
Impurities 
Sulphur 
No. Reference 
(mins) Impurities 
Reagents .degree.C. 
hour 
Time (ppb) (ppm) 
__________________________________________________________________________ 
1 AC4/15 
20 ca 2 ppm 
1000 ppm H.sub.2 O.sub.2 
115 1 2-3 hours 
137 -- 
2 AC4/13 
20 ca 2 ppm 
500 ppm H.sub.2 O.sub.2 
115 1 1 hr 50 mins 
228 -- 
3 AC4/12 
20 ca 2 ppm 
250 ppm H.sub.2 O.sub.2 
115 1 1 hour 309 -- 
4 AC4/11 
20 ca 2 ppm 
100 ppm H.sub.2 O.sub.2 
115 1 0.5-1 hour 
258 -- 
5 AC4/16B 
20 ca 2 ppm 
500 ppm H.sub.2 O.sub.2 
75 1 0.5-1 hour 
265 -- 
6 AC4/17 
20 ca 2 ppm 
500 ppm H.sub.2 O.sub.2/250 ppm 
115ub.2 SO.sub.4 
1 &gt;24 hours 
94 -- 
7 AC4/27 
70 695 ppb 
250 ppm H.sub.2 O.sub.2/50 ppm H.sub.2 SO.sub.4 
115 1 &gt;6 hours 
45 &lt;2 
8 AC4/34 
10 224 ppb 
500 ppm H.sub.2 O.sub.2/50 ppm H.sub.2 SO.sub.4 
115 1 17 hours 
27 -- 
__________________________________________________________________________ 
given in Table 1. Referring to Table 1, it will be seen that treatment with 
1000 ppm hydrogen peroxide alone at 115.degree. C. produces acid which 
passes the permanganate test. Table 1 also shows the benefits of using a 
strong acid such as sulphuric acid. Thus referring to Table 1 it will be 
seen that treating impure acetic acid with 500 ppm hydrogen peroxide 
reduces iodide impurity levels from about 2 ppm to 230 ppb. If 250 ppm 
sulphuric acid is added to the 500 ppm hydrogen peroxide treatment the 
level of iodide impurities is further reduced to 94 ppb, comparable with 
that obtained using 1000 ppm hydrogen peroxide alone. The permanganate 
time is also improved by the addition of sulphuric acid. Extended 
operation with sulphuric acid .feed to the treater and with a residue 
concentrator taking residue from the distillation column base bleed and 
returning acid to the distillation column, resulted in an increase in 
sulphur impurities in the purified acid. 
To show that treatment of impure acetic acid with hydrogen peroxide and 
sulphuric acid is different from treatment with peracetic acid, 
comparative experiments A-D were performed using different levels of 
peracetic acid in place of hydrogen peroxide/sulphuric acid and the 
results are shown in Table 2. For impure acetic acid containing 224 ppb 
iodide impurities comparison of experiment AC4/34 in Table 1 with 
comparison experiment AC4/43 in Table 2 shows, that although 500 ppm 
hydrogen peroxide has a comparable active oxygen content to 1120 ppm 
peracetic acid, the presence of 50 ppm sulphuric acid causes a significant 
improvement in the permanganate time of the treated acid to 17 hours, 
compared to 2 hours for acid treated with 1140 ppm peracetic acid. 
Batch Experiments 
In batch experiments, acetic acid prepared by carbonylation was heated in a 
flask to 115.degree. C. before the treatment reagents were added via a 
syringe directly into the liquid. The mixture was then heated for 1 hour 
before 85% of the acid was distilled from the flask and analysed. A series 
of batch experiments were performed using different strong acids. The 
results are given in Table 3. Referring to Table 3 it will be seen by 
comparison of Experiment 9 
TABLE 2 
__________________________________________________________________________ 
Comparative Experiment 
Impure Acetic Acid Product Acetic Acid 
Comparative Peranganate 
Iodide 
Treatment Permanganate 
Iodide 
Experiment Time Impurities Temp. 
Time Time Impurities 
No. Reference 
(mins) (ppb) Reagents .degree.C. 
hours 
(mins) (ppb) 
__________________________________________________________________________ 
A AC4/43 
10 224 1400 ppm CH.sub.3 CO.sub.3 H 
115 2 120 30 
B AC4/42 
10 224 400 ppm CH.sub.3 CO.sub.3 H 
115 2 ca 40 N/A 
C AC4/40 
10 224 280 ppm CH.sub.3 CO.sub.3 H 
115 2 ca 20 N/A 
D AC4/41 
10 224 280 ppm CH.sub.3 CO.sub.3 H 
100 2 ca 20 N/A 
__________________________________________________________________________ 
TABLE 3 
______________________________________ 
Exper- Product Acetic Acid 
iment Permanganate Time 
No. Treatment (hours) 
______________________________________ 
9 500 ppm H.sub.2 O.sub.2 
&gt;6 
10 140 ppm H.sub.2 O.sub.2 
2.5 
11 140 ppm H.sub.2 O.sub.2 50 ppm H.sub.2 SO.sub.4 
9.5 
12 140 ppm H.sub.2 O.sub.2 50 ppm H.sub.3 PO.sub.4 
2 
13 140 ppm H.sub.2 O.sub.2 250 ppm H.sub.3 PO.sub.4 
6 
14 70 ppm H.sub.2 O.sub.2 
0.7 
15 70 ppm H.sub.2 O.sub.2 50 ppm MSA 
3.5 
16 70 ppm H.sub.2 O.sub.2 50 ppm TFA 
0.8 
17 70 ppm H.sub.2 O.sub.2 1% Amberlyst 15 
1.5 
Comp. P 
140 ppm H.sub.2 O.sub.2 1% acetic anhydride 
2.3 
______________________________________ 
MSA = Methane sulphonic acid 
TFA = Trifluoroacetic acid 
Amberlyst 15 = strong acid ion exchange resin 
with Experiment 2 in Table 1 that a batch process produces a better quality 
product than does a continuous process. 
Additives to Reduce Sulphur Impurities 
A series of batch experiments were performed by batch distilling the 
reboiler contents from a continuous treatment distillation column or a 
solution prepared to simulate such contents by heating together acetic 
acid, hydrogen peroxide and sulphuric acid. The batch distillations were 
performed in the absence and presence of various metal salts. Kettle and 
distillate fractions were analysed for sulphur content during the 
distillation and the results are shown in Tables 4(a) to (f). The results 
show all the salts tested, potassium acetate, silver nitrate, calcium 
nitrate, magnesium hydroxide and copper (I) oxide to be effective in 
reducing sulphur contamination of the purified acid. 
Two continuous experiments were performed using the previously described 
continuous treatment apparatus. Impure acetic acid containing 224 ppb 
iodide and having a permanganate time of 0.2 hour was treated at 
115.degree. C. with 500 ppm hydrogen peroxide and 50 ppm sulphuric acid in 
the treater with a residence time of 1 hour before being passed to the 
distillation column. A metal salt (75 ppm potassium acetate or 20 ppm 
silver nitrate) was added to the distillation column reboiler. The results 
are shown in Table 5. The iodide impurity removal in the presence of 
silver nitrate was marginally better than in the presence of potassium 
acetate. A solid (analysed by IR to be a sulphate salt) was found in the 
distillation column reboiler. The heads bleed from the distillation column 
contained less than 2 ppm sulphur when the salts were used. 
TABLE 4 
______________________________________ 
Batch Testing of Metal Salts to Reduce Sulphur Impurities 
Sulphur in kettle 
Sulphur in Distillate Fraction 
Fraction 
ppm ppm 
______________________________________ 
(a) Experiment 18 - No Metal Salt 
3 approx 350 2 
6 approx 570 &lt;2 
9 approx 850 2 
12 approx 1100 3 
14 1710 11 
18 4430 44 
21 34000 550 
______________________________________ 
(b) Experiment 19 - 1% Potassium Acetate Added 
1 500 &lt;2 
5 730 &lt;2 
10 1250 &lt;2 
14 2780 &lt;2 
16 5290 &lt;2 
______________________________________ 
(c) Experiment 20 -1% Silver Nitrate Added 
1 500 2 
5 730 2 
10 1600 &lt;2 
12 3315 &lt;2 
14 8825 &lt;2 
______________________________________ 
(d) Experiment 21 - 1% Calcium Nitrate Added 
1 500 &lt;2 
5 730 2 
10 1250 2 
14 2780 &lt;2 
16 5290 2 
______________________________________ 
(e) Experiment 22 - 1% Magnesium Hydroxide Added 
1 500 &lt; 2 
5 685 &lt;2 
10 1080 &lt;2 
14 2070 &lt;2 
16 3965 &lt;2 
______________________________________ 
(f) Experiment 23 - 1% Copper I Oxide Added 
1 500 2 
5 785 2 
10 1900 2 
12 4480 2 
______________________________________ 
TABLE 5 
______________________________________ 
Continuous Treatment - Addition of Metal Salts to Reduce 
Sulphur Impurities 
Product Acid 
Perman- 
Re- Time on 
ganate Total Sul- 
Run Reboiler boiler Stream/ 
Time Iodide 
phur 
No. Feed Contents days (hours) 
(ppb) (ppm) 
______________________________________ 
115 75 ppm 3.5% .sup. 11/2 
&gt;6 11 N/A 
KOAc KOAc 
.sup. 41/2 
&gt;6 41 N/A 
7 6 14 N/A 
9 31/2 N/A N/A 
.sup. 91/2 
6 N/A N/A 
.sup. 101/2 
3.5 N/A &lt;0.2 
11 2.5 N/A &lt;0.2 
.sup. 111/2 
2.5 23 &lt;0.2 
13 5 16 N/A 
115 RUN AVERAGES 
-- about 6 16 &lt;0.2 
116 20 ppm 1% 1 &gt;7 2 &lt;0.3 
AgNO.sub.3 
AgNO.sub.3 
3 &gt;6 5 &lt;0.3 
.sup. 61/2 
&gt;5 13 &lt;0.3 
.sup. 71/2 
6 12 &lt;0.3 
9 &gt;6 12 &lt;0.3 
11 5 16 &lt;0.3 
116 RUN AVERAGES 
-- about 6 11 &lt;0.3 
______________________________________ 
Notes 
(1) Treater conditions = Temperature = 115.degree. C. 
Residence time = 1 hour 
H.sub.2 O.sub.2 treatment = 500 ppm 
H.sub.2 SO.sub.4 treatment = 50 ppm 
(2) The run averages reflect all the analytical data obtained over the 
operating period (only some of which is shown in the Table). 
(3) Impure acetic acid feed = Total iodides by NAA = 224 ppb 
Permanganate time = 0.2 hrs 
Additives to Reduce Excess Peroxide 
A batch experiment was performed to show that metal salts can be used to 
reduce excess peroxide after the treatment of impure carboxylic acid with 
hydrogen peroxide. 
500 ml of acetic acid was batch treated with 500 ppm hydrogen peroxide and 
50 ppm sulphuric acid at 115.degree. C. for 1 hour. After this time the 
peroxide concentration was found to be 220 ppm (as hydrogen peroxide). 425 
mls of the acid was then distilled off. The peroxide concentration of the 
distillate was 199 ppm. 
An identical sample was treated in the same manner as above, where after 1 
hour the peroxide concentration was 236 ppm. Potassium acetate was then 
added to the acetic acid to yield a total concentration of 7% w/w before 
425 mls of distillate was collected, the peroxide concentration of which 
was 34 ppm. 
It is envisaged that in a continuous process the metal salt would be added 
to the distillation/evaporation stage (b) to reduce excess peroxide 
carryover into the purified acid product. Such metal salts may also reduce 
sulphur carryover if sulphuric acid is used in step (a).