Hydrolysis of methyl esters

A process for hydrolyzing methyl esters having carboxylic acid moieties containing 6 to 22 carbon atoms involves reacting said esters with water in the presence of carboxylic acid and strong acid catalysts to produce acid corresponding to reactant ester and methanol and driving the reaction toward completion by removing the methanol.

BACKGROUND OF THE INVENTION 
This invention relates to the field of converting methyl esters to 
corresponding acids. More particularly, it relates to an improved strong 
acid process for hydrolyzing methyl esters having carboxylic acid moieties 
containing 6 to 22 carbon atoms. 
It is known that these methyl esters can be hydrolyzed by reacting with 
water under conditions of high pressure and temperature (e.g. 700 psi and 
250.degree. C.). This requires very expensive equipment and presents a 
methanol flammability problem. 
As a result, consideration has been given to converting such esters to 
acids by an acidolysis reaction wherein an ester of a first carboxylic 
acid is reacted with a second carboxylic acid in the presence of strong 
acid catalyst to produce the first carboxylic acid and the ester of the 
second carboxylic acid. This is a well known reaction and is described, 
for example, in Graves U.S. Pat. No. 1,882,808. Very often, the acidolysis 
is an acetolysis, that is, the replacing (displacing) acid is acetic acid. 
In the context of methyl esters of C.sub.6 -C.sub.22 carboxylic acids, 
this means reacting such ester with acetic acid to produce C.sub.6 
-C.sub.22 carboxylic acids and methyl acetate. This reaction has the 
disadvantage in a commercial context of requiring disposal or separate 
hydrolysis of methyl acetate. Disposal is disadvantageous because consumed 
displacing acid is lost. Separate hydrolysis has the disadvantage of 
requiring a second process facility (a reactor and distillation units 
different from the reactor and distillation units used for the acetolysis) 
and also the drying of the resulting acetic acid before it can be reused. 
It is an object of this invention to provide hydrolysis of the said methyl 
esters wherein relatively mild conditions of temperature and pressure can 
be used. 
It is a further object of this invention to provide a process wherein 
displacing acid is not consumed, where no product ester need be disposed 
of, and where only a single reaction system (e.g. reaction vessel plus 
distillation means) is required. 
DESCRIPTION OF THE INVENTION 
It has been discovered that these objects and others are satisfied and 
various advantages as indicated below are obtained by this invention which 
involves an overall reaction comprising hydrolyzing methyl ester by 
reacting such with water in the presence of catalyst consisting 
essentially of particular carboxylic acid and strong acid to produce 
carboxylic acid corresponding to the ester and methanol and also involves 
driving this overall reaction toward completion by removing methanol 
product from the reaction system (the reaction zones and any fractionation 
zone as described hereafter). 
The overall reaction has the following reaction equation: 
##STR1## 
wherein 
##STR2## 
is methyl ester and R is selected from the group consisting of saturated 
and unsaturated aliphatic (the ester is described in more detail below; R 
is used with the same meaning each time it occurs and is described in more 
detail below) and wherein R.sub.1 COOH is carboxylic acid and R.sub.1 is 
an alkyl chain containing from 2 to 4 carbon atoms (R.sub.1 is used with 
the same meaning each time it occurs). 
The overall reaction is believed to occur by a two step route. The reaction 
of the first step is an acidolysis reaction and has the following reaction 
equation: 
##STR3## 
The reaction of the second step is a hydrolysis reaction and has the 
following reaction equation: 
##STR4## 
Arrows in both directions are depicted in the above two equations to 
indicate the capability for reversibility. The process of this invention 
drives the reactions to the right completely if methanol removal is total. 
The overall reaction is readily carried out in a single reaction system 
e.g. a batch or continuous reactor coupled with distillation means, which 
provides a liquid phase reaction zone, a vapor phase reaction zone, and a 
fractionation zone. When the overall reaction is carried out in a reactor 
coupled with distillation means, the liquid phase reaction zone is 
provided in the reactor, the vapor phase reaction zone is provided partly 
in the reactor and partly in the distillation means and the fractionation 
zone is provided in the distillation means, and the following occurs: The 
second reaction step occurs in the vapor phase reaction zone, that is, 
partly in the reactor and partly in the distillation means. Methanol is 
removed from the fractionation zone to drive the second reaction step 
toward the right, thereby, in turn, driving the first reaction step toward 
the right. The R.sub.1 COOH formed in the second reaction step is 
preferably caused to return to the liquid phase reaction zone so that the 
amount of R.sub.1 COOH in that reaction zone stays approximately constant 
and thereby progressively provides an increased driving force to the right 
for the acidolysis step (the first reaction step) as methyl ester is 
converted; this is in contrast to acetolysis where the driving force 
lessens as the reaction proceeds since acetic acid is used up. 
Substitution of acetic acid or formic acid for the R.sub.1 COOH does not 
provide the advantageous results described, and instead, with the 
aforedescribed single reaction system provides methyl formate or methyl 
acetate distilling off rather than methanol. These (methyl formate or 
methyl acetate), contrary to R.sub.1 COOCH.sub.3, have lower boiling 
points than methanol and consequently are distilled off instead of 
methanol requiring new displacing acid and displacing acid ester disposal 
or hydrolysis of the displacing acid ester in a second process facility 
(reactor plus distillation means). 
We turn now to the methyl ester reactant which as indicated above has the 
formula RCOOCH.sub.3 wherein R is selected from the group consisting of 
saturated and unsaturated aliphatic chains. The methyl ester has a 
carboxylic acid moiety containing from 6 to 22 carbon atoms; thus, R 
contains from 5 to 21 carbon atoms. Examples of suitable methyl esters 
include methyl caproate, methyl caprylate, methyl caprate, methyl laurate, 
methyl myristate, methyl myristoleate, methyl palmitate, methyl 
palmitoleate, methyl stearate, methyl oleate, methyl elaidate, methyl 
linoleate, methyl linolenate, methyl arachidate, methyl gadoleate, methyl 
arachidonate, methyl behenate, and methyl erucate. Suitable methyl esters 
are readily derived from fats and oils (for example, by a methanolysis 
reaction wherein refined fat or oil is reacted with excess methanol in the 
presence of sodium methoxide) such as coconut oil, corn oil, cottonseed 
oil, lard, linseed oil, olive oil, palm oil, palm kernel oil, peanut oil, 
rapeseed oil, safflower oil, sesame oil, soybean oil, sunflower oil and 
tallow; in such case, the methyl ester derived from the fat or oil is a 
mixture of methyl esters. Thus, the methyl ester reactant herein can be a 
specific methyl ester or a mixture of different methyl esters. 
We turn now to the water reactant. The water in the acidolysis reaction 
step serves as a promoter for the strong acid catalyst (it enhances the 
strong acid's catalytic activity) and thus acts to speed the reaction. 
This promoting effect is described in articles by Meade et al at pages 1-6 
of volume 39 of Journal of the American Oil Chemists' Society (January 
1962). The water also participates as a reactant (see the overall reaction 
equation and the second reaction step). In general, the water is used in 
an amount such that the molar ratio of water to methyl ester ranges from 
about 1:1 to about 25:1. Preferably, the molar ratio of water to methyl 
ester ranges from about 1.1:1 to about 17.5:1. If a molar ratio less than 
about 1:1 is utilized, the completeness of the reaction is deleteriously 
affected or the reaction rate is slowed. If a molar ratio of greater than 
about 25:1 is utilized, the disadvantage of dilution of the system 
(resulting in slowing of the reaction) can occur. 
Turning now to the carboxylic acid catalyst, as indicated above, it has the 
formula R.sub.1 COOH wherein R.sub.1 is an alkyl chain containing from 2 
to 4 carbon atoms. Thus, the carboxylic acid catalyst is selected from the 
group consisting of propionic acid, butyric acid, valeric acid and 
mixtures thereof. Propionic acid has a significant advantage over the 
others from the standpoints of cost, availability and odor and is 
therefore highly preferred. As indicated above, the carboxylic acid 
catalyst participates in the reaction (see the reaction equation of the 
first reaction step set forth above). It is referred to herein as a 
catalyst because it is not consumed and because it promotes the overall 
reaction and thus meets the traditional definition of a catalyst. 
Initially, it functions to drive the acidolysis step to the right and to 
compatibilize the reaction mixture. It is used in an amount such that 
molar ratio of carboxylic acid catalyst to methyl ester ranges from about 
1:1 to about 20:1, preferably from about 5:1 to about 17.5:1. If the lower 
limit of about 1:1 is not met, the reaction rate is slowed. If no 
carboxylic acid catalyst is used, the overall hydrolysis reaction takes 
days or stringent conditions of temperature and pressures as described 
above are necessary. If the upper limit of about 20:1 is exceeded, the 
disadvantage of separating and/or moving a large volume of carboxylic acid 
catalyst to reuse it, can occur. 
The strong acid catalyst can be, for example, any of those known for use to 
catalyze acidolysis reactions. The acids can be inorganic or organic, but 
are not carboxylic. Suitable inorganic acids are those having pK.sub.a 
values below about 4.0 at room temperature in aqueous solution (see 
Moeller, Inorganic Chemistry, John Wiley & Sons (1952) at pages 314 and 
315). Specific examples of such acids are sulfuric acid which is a 
preferred strong acid catalyst and hydrochloric acid, perchloric acid, 
nitric acid, phosphoric acid, and hydrofluoric acid. Organic acids 
suitable for strong acid catalysts herein are noncarboxylic acids having 
pK.sub.a values below 2.0 in water at room temperature (see Handbook of 
Chemistry and Physics, 58th edition, Chemical Rubber Publishing Company at 
pages D-150 et seq.). Examples of suitable organic acids are methane 
sulfonic acid, naphthalene sulfonic acid, trifluoromethyl sulfonic acid, 
and p-toluene sulfonic acid. Solid strong acids such as strong acid cation 
exchange resins of the gel or macroreticular types (e.g., Amberlite IR 
120, Amberlyst 15, and XN1010, all available from Rohm and Haas), and 
supported transition metal catalysts as described in U.S. Pat. No. 
4,032,550 can also be employed. Mixtures of strong acid catalysts can be 
used. When a liquid strong acid catalyst is used, the amount of it used 
generally ranges from about 1% to about 50% by weight of ester reactant, 
and preferably ranges from about 3.5% to about 20% by weight of ester 
reactant. A very preferable liquid catalyst is sulfuric acid used in an 
amount ranging from about 3.5 to about 20% by weight of ester reactant. 
When a solid strong acid catalyst such as a strong acid cation exchange 
resin is used, the amount used generally ranges from about 20 to about 120 
grams per mole of ester reactant and preferably ranges from about 40 to 
about 70 grams per mole of ester reactant. If the general lower limits on 
strong acid catalyst set forth above are not complied with, reaction rate 
is slowed. If the general upper limits on strong acid catalyst set forth 
above are exceeded, the disadvantages include increased recycling needs, 
increased cost, excessive discoloration, and increased occurrence of side 
reactions. When the reaction is carried out in a reactor coupled with a 
distillation means as described above, the strong acid catalyst is 
preferably used in both liquid phase and vapor phase reaction zones; if 
the reaction is also carried out on a batch basis, it is preferred to use 
strong acid catalyst in the liquid phase reaction zone in the reactor in a 
preferred amount as recited above (about 3.5% to about 20% by weight of 
ester reactant for a liquid strong acid catalyst and about 40 to about 70 
grams per mole of ester reactant for solid strong acid catalyst), and 
strong acid catalyst comprising strong acid cation exchange resin in the 
vapor phase reaction zone in the distillation means in an amount ranging, 
for example, from about 10 grams to about 40 grams per 100 gram charge of 
ester reactant. 
Generally, reaction temperatures ranging from about 90.degree. C. to about 
140.degree. C. are utilized and very preferably the relatively mild 
reaction temperatures ranging from about 105.degree. C. to about 
125.degree. C. are utilized. The temperature should be sufficient so that 
methanol can be removed to drive the overall reaction toward completion. 
When a reactor coupled with a distillation means is the reaction system, 
and the reaction is carried out at about atmospheric pressure and 
propionic acid is the carboxylic acid catalyst, the temperature is 
preferably adjusted so that the temperature at the outlet of the 
fractionation zone is about 65.degree. C. which is the boiling point of 
methanol at atmospheric pressure or so that the temperature at the outlet 
of the fractionation zone is sufficient for separation of methanol from 
methyl propionate (below 80.degree. C.). 
The overall reaction is readily and preferably carried out at atmospheric 
pressure. If desired, subatmospheric or superatmospheric pressures can be 
utilized. 
The time for reaction is dependent on several factors. Generally, 
increasing amounts of water, carboxylic acid catalyst and/or strong acid 
catalyst increase reaction rate. In general, in batch processing, 
relatively high percentage conversions (50-100%) are readily obtained in 
about one to four hours. In continuous processing, residence times are 
chosen for a particular combination of reaction parameters so as to obtain 
high percentage conversions. 
Carboxylic acid product is readily obtained from a resultant reaction 
mixture as follows. If such resultant reaction mixture is heterogeneous, 
the layer containing such product is, for example, separated, washed and 
distilled or fractionally crystallized for final purification. If a 
resultant reaction mixture is homogeneous, the carboxylic acid product is 
readily recovered by adding sufficient water to form a heterogeneous 
mixture and proceeding as for a heterogeneous mixture. Any carboxylic acid 
catalyst or strong acid recovered by layering or washing or filtration (if 
the strong acid is a solid) is readily recycled. Methanol which is 
recovered during the reaction can be used for methanolysis of 
triglycerides to obtain methyl ester reactant. 
As indicated above, the process of this invention is readily carried out 
batchwise or continuously. When the process is a batch process, the 
amounts specified above are those used, that is introduced, into the batch 
reactor system. When the process is continuous, the amounts specified 
above are those maintained. For a batch process suitable equipment 
includes a reaction vessel or pot containing the reactants and catalysts 
communicating with a fractionation column thereabove containing at its 
lower end strong acid cation exchange resin. For a continuous reaction, 
the reactor can be, for example, the same as the batch reactor but 
containing means for continuous addition of reactants and continuous 
removal of fatty acid and methanol. 
The term "fatty acid" is used herein to mean carboxylic acid corresponding 
to carboxylic acid moiety in ester reactant. 
The invention is illustrated by the following specific examples.

EXAMPLE I 
Methyl laurate (Procter & Gamble Company Stock No. CE 1295; 95% minimum 
C.sub.12 methyl esters, 82.9 grams, 0.39 moles), propionic acid (151.6 
grams, 2.05 moles), water (9.4 grams, 0.52 moles) and concentrated 
sulfuric acid (3.7 grams, 0.04 moles) were placed in a mantle-heated flask 
connected to a 30 cm. distillation column and take-off head. The bottom 
one-fourth of the distillation column was packed with Amberlite IR 120 
(obtained from Rohm & Haas Co.) ion exchange resin in the acid form (about 
10 grams of strong acid cation exchange resin). The flask was heated for 
two hours with the contents holding at 117.degree.-118.degree. C. until 
the end of the reaction when the temperature reached 151.degree. C. 
Methanol was taken off at the head during the reaction. Methanol taken off 
corresponded to a 97.6% conversion to lauric acid indicating very high 
reaction completeness. 
EXAMPLE II 
Methyl laurate (same feedstock as Example I, 87.1 grams, 0.41 moles), 
propionic acid (207.3 grams, 2.8 moles), water (25.2 grams, 1.4 moles) and 
Amberlyst 15 (strong acid macroreticular cation exchange resin obtained 
from Rohm & Haas, 24.0 grams dry weight) in the acid form were added as 
described below to a round bottom flask equipped with 30 cm. distillation 
column (attached to a fractionating take-off head) having the bottom 8 cm. 
packed with methanol soaked Amberlyst 15 (about 10 grams). The Amberlyst 
15 added to the flask was previously soaked in methyl laurate for 21 
hours, filtered, and then added to the methyl laurate reactant already in 
the flask. This was heated to 100.degree. C. The propionic acid and water 
were heated to 80.degree. C. and then added to the ester and Amberlyst 15 
already in the flask. Heat was applied to the flask via a heating mantle 
and the pot temperature reached 96.degree. C. when methanol collection 
began. In two hours of heating, methanol collection continued and the pot 
temperature gradually increased to 132.degree. C. At this time, 10.5 
milliliters of methanol was collected, representing 61.3% of theoretical 
yield and indicating over 50% conversion of methyl laurate to lauric acid. 
EXAMPLE III 
Methyl laurate (same feedstock as in Example I, 84.6 grams, 0.40 moles), 
propionic acid (210.2 grams, 2.84 moles), water (25.4 grams, 1.4 moles) 
and XN1010 (Rohm & Haas Co. experimental macroreticular resin of the 
polystyrene sulfonic acid type, having an ion exchange capacity of 3.3 
meq/gram and a surface area of 570 m.sup.2 /gram, 24.2 grams dry weight) 
were added as described below to a mantle-heated flask equipped with a 30 
cm. distillation column (the bottom 8 cm. of which were packed with 
methanol soaked XN1010 in an amount of about 10 grams) attached to a 
fractionating take-off head. The XN1010 catalyst was previously soaked in 
methyl laurate for 21 hours, filtered, added to the methyl laurate 
reactant and heated to 100.degree. C. The propionic acid and water were 
mixed, heated to 80.degree. C. and added to the flask. Heat was applied to 
the flask, and the contents reached a maximum of 121.degree. C. over a 65 
minute heating period. Methanol was taken off at the head during the 
reaction. Work-up of the product and isolation of lauric acid gave a 91.1% 
yield. 
EXAMPLE IV 
Methyl oleate (100 grams, 0.338 moles), propionic acid (400.4 grams, 5.4 
moles), water (50 grams, 2.8 moles) and concentrated sulfuric acid (20 
grams, 0.2 moles) were placed in a mantle-heated flask connected to a 30 
cm. distillation column (the bottom 8 cm. of which were packed with 
approximately 10 grams of strong acid cation exchange resin) attached to a 
take-off head. The flask was heated for two hours to provide a reaction 
temperature of approximately 105.degree. C. Methanol was collected at the 
take-off head during the reaction. Analysis indicated 98% conversion to 
oleic acid. 
EXAMPLE V 
Methyl oleate (100 grams, 0.338 moles), propionic acid (250.3 grams, 3.38 
moles), concentrated sulfuric acid (12 grams, 0.12 moles), distilled water 
(100 grams, 5.56 moles), and Covi-Ox T-50 antioxidant (0.087 grams) are 
charged to a mantle-heated round bottom flask connected to a 30 cm. 
distillation column fitted with a take-off head. The bottom 8 cm. of the 
distillation column is packed with approximately 10 grams of XN1010 ion 
exchange resin (described in Example III) in pellet form. The distillation 
column is designed to allow vapor flow upward through the resin packing 
but liquid downflow is returned without passing through the resin packing. 
The flask is heated to between 105.degree. C. and 110.degree. C. for three 
hours. Methanol is taken off at the take-off head during reaction. Sample 
workup and analysis indicates a 99% yield of oleic acid. 
EXAMPLE VI 
Coconut-derived methyl esters (CE810, Procter & Gamble Company; typical 
analysis: 3.9% C.sub.6, 56.2% C.sub.8, 39.2% C.sub.10, 0.7% C.sub.12 ; 50 
grams; (0.3 moles), propionic acid (110.9 grams, 1.5 moles), concentrated 
sulfuric acid (2 grams, 0.02 moles), and water (6 grams, 0.33 moles) were 
placed in a mantle-heated flask connected to a 30 cm. distillation column 
(the bottom 8 cm. of which were packed with approximately 10 grams of 
strong acid cation exchange resin) fitted with a take-off head. The flask 
was heated for 2 hours to provide a reaction temperature of approximately 
110.degree. C. Methanol was collected at the take-off head during the 
reaction. Analysis indicated 91% conversion to fatty acids. 
EXAMPLE VII 
Example VI was repeated except that the amounts were as follows: 
Coconut-derived methyl esters (CE810), 100 grams, 0.6 moles; propionic 
acid (354.9 grams, 4.8 moles); concentrated sulfuric acid (10 grams, 0.1 
moles); and water (12 grams, 0.67 moles). Analysis indicated 94% 
conversion to fatty acids. 
EXAMPLE VIII 
Coconut-derived methyl esters (CE810, Procter & Gamble Company, described 
in Example VI, 100 grams, 0.6 moles), propionic acid (354.9 grams, 4.8 
moles) concentrated sulfuric acid (4 grams, 0.04 moles) and distilled 
water (20 grams, 1.11 moles) are charged to a mantle-heated, round bottom 
flask connected to a 30 cm. distillation column fitted with a take-off 
head. The bottom 8 cm. of the distillation column is packed with 
approximately 10 grams of XN1010 ion exchange resin (described in Example 
III) in pellet form. The distillation column is designed to allow vapor 
flow through the resin packing but liquid downflow is returned without 
passing through the resin packing. The flask is heated up to 120.degree. 
C. Heat is applied for two hours. Methanol is taken off at the take-off 
head during the reaction. Sample workup and analysis shows a 97.7% yield 
of the fatty acids. 
EXAMPLE IX 
Methyl linoleate (50 grams, 0.17 moles), propionic acid (175.8 grams, 2.37 
moles), concentrated sulfuric acid (6 grams, 0.06 moles) and water (30 
grams, 1.66 moles) were placed in a mantle-heated flask connected to a 30 
cm. distillation column (the bottom 8 cm. of which were packed with 
approximately 10 grams of strong acid cation exchange resin) fitted with a 
take-off head. The flask was heated for three hours to provide a reaction 
temperature of approximately 105.degree. C. Methanol was collected at the 
take-off head during the reaction. Analysis indicated 98% conversion to 
linoleic acid. 
EXAMPLE X 
Safflower methyl esters (50 grams, 0.17 moles), propionic acid (126 grams, 
1.7 moles), p-toluene sulfonic acid (20.6 grams, 0.12 moles) and water 
(3.6 grams, 1.9 moles) were placed in a mantle-heated flask connected to a 
30 cm. distillation column (the bottom 8 cm. of which were packed with 
approximately 10 grams of strong acid cation exchange resin) fitted with a 
take-off head. The flask was heated for 3 hours to provide a reaction 
temperature of approximately 105.degree. C. Methanol was collected at the 
take-off head during the reaction. Analysis indicated 99.3% conversion to 
safflower fatty acids. 
In the above examples the portion of the distillation column packed with 
resin constitutes vapor phase reaction zone. 
When in the above examples, equivalent amounts of butyric acid, or valeric 
acid are substituted for part of or all the propionic acid, substantially 
equal yields and conversions are obtained. 
While the foregoing describes preferred embodiments of the invention, 
modifications will be readily apparent to those skilled in the art. The 
scope of the invention is intended to be defined by the following claims.