Process for preparing alkali metal salts of 3-hydroxypropionic acid

By reacting 1,3-propanediol with oxygen or an oxygen-containing gas in an aqueous alkaline solution in the presence of a palladium containing catalyst substrate, alkaline salts of 3-hydroxypropionic acid are produced in good yield when the catalyst is used in an amount that corresponds to 0.1 to 3.0% by weight of palladium, based on the 1,3-propanediol.

FIELD OF THE INVENTION 
This invention relates to a process for the production of alkali metal 
salts of 3-hydroxypropionic acid by oxidation of propane-1,3-diol in 
aqueous alkaline solution in the presence of a palladium catalyst. 
STATEMENT OF RELATED ART 
3-Hydroxypropionic acid and its salts are valuable building blocks in 
organic synthesis. 3-Hydroxypropionic acid is normally prepared by 
addition of water onto acrylic acid or by reaction of ethylene 
chlorohydrin with sodium cyanide and subsequent hydrolysis of the 
.beta.-propiolactone formed [Ullmann's Encyclopoedia of Industrial 
Chemistry, 5th Edition, Vol. A-13, pages 507-517]. Both processes involve 
the handling of toxic substances. Accordingly, a search was made for a 
process by which toxicologically safe propane-1,3-diol, which can readily 
be obtained in high yields from glycerol by fermentation, could be 
converted by oxidation into 3-hydroxypropionic acid or alkali metal salts 
thereof. 
The oxidation of propane-1,3-diol, hereinafter referred to in brief as 
diol, in presence of a noble metal catalyst is known from the literature. 
Thus, published Japanese patent application JP 56/5433 (Sanyo) claims a 
process for the production of malonic acid by reaction of propane-1,3-diol 
with oxygen or an oxygen-containing gas. The process is preferably carried 
out in the presence of a platinum group catalyst. According to the Sanyo 
application, malonic acid can be obtained in high yields using 3.3% by 
weight palladium, based on the diol.

DESCRIPTION OF THE INVENTION 
Object of the Invention 
The problem addressed by the present invention was to provide a process by 
which propane-1,3-diol could be oxidized in high yields to 
3-hydroxypropionic acid. 
Summary of the Invention 
According to the invention, this problem has been solved by a process for 
the production of an alkali metal salt of 3-hydroxypropionic acid by 
reaction of propane-1,3-diol with oxygen or an oxygen-containing gas in 
aqueous alkaline solution in the presence of a palladium-containing 
supported catalyst, characterized in that the catalyst is used in a 
quantity corresponding to 0.1 to 3.0% by weight of palladium, based on 
propane-1,3-diol. 
Description of the Preferred Embodiments 
Palladium on a solid support, for example active carbon or aluminum oxide, 
is used as the catalyst, the quantity of palladium in the catalyst being 
from 0.5 to 10% by weight and preferably from 2 to 5% by weight, based on 
the support. 
The practicability of the process according to the invention is 
attributable to the fact that the predominant formation of 
3-hydroxypropionic acid (selective oxidation of propane-1,3-diol) takes 
priority over the competitive formation of malonic acid (complete 
oxidation of propane-1,3-diol) where the low catalyst concentration 
according to the invention, based on the diol, is used. 
In one preferred embodiment of the invention, the quantity of palladium 
present in the catalyst, based on propane-1,3-diol, is between 0.1 and 
1.0% by weight. 
It has been found that high yields of 3-hydroxypropionic acid are obtained 
even when, over and above the palladium present, the catalyst additionally 
contains platinum and/or bismuth. The total quantity of platinum and/or 
bismuth should be at most twice the quantity by weight of palladium. 
Particularly good results have been obtained with a catalyst containing 4% 
by weight of palladium, 1% by weight of platinum and 5% by weight of 
bismuth on powdered active carbon. 
The catalyst is normally activated before use. This is readily done by 
dispersing the catalyst in water and subsequently contacting it with inert 
gases, for example hydrogen and/or nitrogen, to displace adhering oxygen. 
The catalyst may be repeatedly reused in the process according to the 
invention without any losses of yield having to be accepted. On the 
contrary, it has even been found that the catalyst only develops its full 
activity after it has been used at least once to three times. 
The concentration of the diol in the reaction mixture is not subject to any 
particular limitations, although a concentration of 5 to 20% by weight is 
preferred. It is of particular advantage in this regard to adjust the 
concentration of the diol in the reaction mixture to a value of 6 to 12% 
by weight and, more particularly, to a value of 8 to 10% by weight. In 
this case, the quantity of palladium present in the catalyst can be 
further reduced to 0.1 to 0.3% by weight of the same high yield. 
According to the invention, the oxidation of propane-1,3-diol is carried 
out in alkaline medium. In this way, the 3-hydroxypropionic acid formed is 
neutralized and thus protected against partial yield-reducing degradation 
by decarboxylation. The pH value of the aqueous alkaline reaction mixtures 
should be in the range from 8 to 13 and is preferably in the range from 9 
to 12. Particularly good results are obtained at pH values of 10 to 11. 
Oxidation of the diol is carried out at a constant pH value of the reaction 
mixture throughout the reaction. The constant pH value may be controlled, 
for example, by coupling a pH meter, which continuously monitors the pH 
value of the reaction mixture, to a metering unit containing the 
corresponding alkali metal hydroxide. The alkali metal hydroxides used are 
preferably sodium hydroxide and potassium hydroxide, more particularly in 
the form of aqueous solutions. The concentration of the aqueous alkali 
metal hydroxide used is not subject to any particular limitation, although 
it is clear that the use of highly dilute solutions is uneconomical in 
regard to optimal utilization of the reactor. For practical reasons, 
therefore, 20 to 50% by weight of aqueous alkali metal hydroxides will be 
used. In the case of NaOH, it has proved to be practical to use a 30% by 
weight, i.e. 10-normal, solution. 
In the process according to the invention, oxidation of the diol is 
preferably carried out at temperatures of 40.degree. to 55.degree. C. 
Higher temperatures do not produce any significant increases in yield and 
actually involve the danger of dehydration of the 3-hydroxypropionic acid 
to acrylic acid. 
In the process according to the invention, the reaction mixture is 
contacted with oxygen or an oxygen-containing gas, for example air. This 
may readily be done, for example, by injecting air into the reaction 
mixture with stirring. It has been found that a flow rate of 30 normal 
liters of air per hour, based on 700 to 1000 ml reaction mixture, is 
particularly suitable. At lower flow rates, for example 10 normal liters 
air per hour, the reaction time is significantly extended and the reaction 
mixture can undergo unwanted yellowing; at higher flow rates, the removal 
of catalyst from the reactor is too high. 
The reaction may be carried out under pressures of 1.0 to 1.5 bar, but is 
preferably carried out under a slight excess pressure of the order of 1.01 
to 1.06 bar. 
In kinetic studies, it was found that the consumption of alkali metal 
hydroxide in the process according to the invention is dependent on time. 
The consumption of alkali metal hydroxide initially increases linearly as 
a function of time, reaching a plateau value at the end of the reaction. 
On the basis of this observation, therefore, the end point of the reaction 
may readily be determined from the fact that no more alkali metal 
hydroxide is needed to keep the pH value constant. At this stage, the 
reaction is terminated and the reaction mixture is worked up in the usual 
way beginning with removal of the catalyst by filtration. 
The alkali metal salt of 3-hydroxypropionic acid obtained may be used 
either directly or after further concentration in the form of an aqueous 
solution. If desired, the alkali metal salt may be converted by 
acidification, for example by means of an acidic ion exchanger, into the 
free 3-hydroxypropionic acid, which may optionally be purified by 
distillation. 
The following Examples are intended to illustrate the invention. 
Examples 
1. Reagents 
The following catalysts were used: 
a) 5% by weight Pd on active carbon with a water content of 52.5% by 
weight, commercially available under the name of Escat 10 (Engelhard). 
b) 4% by weight Pd, 1% by weight Pt and 5% by weight Bi on active carbon 
with a water content of 59.3% by weight, commercially available under the 
name of Cef 196 raw (Degussa AG). 
2. Test apparatus 
The oxidations were carried out in a 2 liter pressure autoclave equipped 
with a turbine stirrer [1400 revolutions per minute]. Air and 30% by 
weight sodium hydroxide were introduced into the reaction mixture from 
below. NaOH was continuously introduced at such a rate that the pH value 
of the mixture remained constant. The pH was controlled through a metering 
dispenser (Dulcometer, manufacturer: Prominent) equipped with a resistance 
thermometer (Pt-100). The waste gas passed through a cooler (deposition of 
condensate), a buffer vessel, a water-filled washing bottle, a drying 
tower, a throughflow meter and an oxygen analyzer (Servomex 570, 
manufacturer: Buhler). Due to variations in air pressure, the instrument 
was recalibrated before each test. The consumption of oxygen was 
continuously recorded as a function of time by a connected recorder. The 
air throughput was adjusted by a precision control valve to a value of 30 
normal liters per hour. The pressure inside the reactor was 1.06 bar. 
3. Test descriptions 
The quantities of catalyst shown in all the Examples and Comparison 
Examples are based on dry matter. 
Example 1 (E1) 
2.13 g of catalyst (Escat 10) were dispersed in 300 ml of water and 
activated overnight under hydrogen, a quantity of about 350 ml being taken 
up. The prepared catalyst was transferred to the autoclave together with 
77.6 g (1 mole) of propane-1,3-diol and another 500 ml water. The 
autoclave was closed and the reaction mixture was heated under nitrogen to 
50.degree. C. The reaction was initiated by the simultaneous introduction 
of sodium hydroxide and air. NaOH was continuously introduced at such a 
rate that the pH value remained constant at 11. The air throughput was 30 
normal liters per hour. As soon a constant value had been reached for the 
total amount of sodium hydroxide solution introduced as well as for the 
therewith connected total consumption of oxygen, the reaction was 
terminated, the reaction mixture was drained off by blowing out the NaOH 
feed line, and the yield was determined by weighing. After cooling of the 
mixture, the catalyst was filtered off through a suction filter and the 
samples were analyzed by HPLC (Shodex Ionpak C-811: cation exchange phase, 
0.1% by weight aqueous phosphoric acid as eluent, Ri detection). The 
yields of 3-hydroxypropionic acid [in % of the theoretical] are shown 
together with other data in line 1 of Table 1. 
TABLE 1 
__________________________________________________________________________ 
Propane-1,3-diol 
Water 
Cat..sup.a) 
Pd.sup.b) 
Temp. 
Time.sup.c) 
Yield.sup.d) 
Ex. [g] [mmoles] 
[g] [g] [%] 
[.degree.C.] 
[mins.] 
[%] 
__________________________________________________________________________ 
E1 77.6 
1000 800 2.13 
0.14 
50 1075.sup.e) 
70.1 
E2 77.6 
1000 800 3.80 
0.24 
50 490 
74.7 
E3 77.6 
1000 800 5.32 
0.34 
50 415 
68.4 
E4 77.6 
1000 800 10.64 
0.69 
50 330 
51.5 
C1 77.6 
1000 800 21.28 
1.37 
50 580 
23.5 
E5 23.3 
300 700 13.10 
2.81 
40 190 
65.6 
C2 23.3 
300 700 13.10 
2.81 
60 220 
29.0 
__________________________________________________________________________ 
.sup.a) Escat 10; the quantities of catalyst shown are dry weights 
.sup.b) % by weight palladium, based on propane1,3-diol 
.sup.c) Reaction time in minutes 
.sup.d) Yield of 3hydroxypropionic acid in % of the theoretical 
.sup.e) Air throughput: 10 normal liters per hour 
Examples 2 to 4 (E2 to E4) 
Example 1 was repeated with different quantities of catalyst. The results 
are set out in Table 1. It can be seen that optimal yields of 
3-hydroxypropionic acid are obtained in particular at low catalyst 
concentrations. 
Comparison Example 1 (C1) 
Example 1 was repeated with a distinctly larger quantity of catalyst. 
Particulars are set out in Table 1. It can be seen that the increase in 
the quantity of catalyst is accompanied by a drastic reduction in the 
yield of 3-hydroxypropionic acid. 
Example 5 (E5) 
13.1 kg of catalyst (Escat 10) were dispersed in 300 ml of water and 
activated overnight under hydrogen, a quantity of about 900 ml being taken 
up. The prepared catalyst was transferred to the autoclave together with 
23.3 g (300 mmoles) of propane-1,3-diol and another 400 ml of water. The 
autoclave was closed and the reaction mixture was heated under nitrogen to 
40.degree. C. The remaining procedure was as in Example 1. The yield of 
3-hydroxypropionic acid was 65.6%, cf. Table 1. 
Comparison Example 2 (C2) 
Example 5 was repeated at 60.degree. C. The yield of 3-hydroxypropionic 
acid was 29%, cf. Table 1. 
Example 6 (E6) 
13.1 kg of catalyst (Cef 196 raw) were dispersed in 300 ml of water and 
activated overnight under hydrogen, a quantity of about 900 ml being taken 
up. The prepared catalyst was transferred to the autoclave together with 
31.1 g (400 mmoles) of propane-1,3-diol and another 631 ml of water. The 
autoclave was closed and the reaction mixture was heated under nitrogen to 
50.degree. C. The remaining procedure was as in Example 1. The yield of 
3-hydroxypropionic acid was 81.8%, cf. Table 2. 
Example 7 to 10 (E7 to E10) 
Example 6 was repeated with different concentrations of propane-1,3-diol in 
the reaction mixture and hence indirectly with different ratios of 
catalyst to diol. Particulars and also the yields of 3-hydroxypropionic 
acid are set out in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Propane-1,3-diol 
Water 
Cat..sup.a) 
Pd.sup.b) 
Met.sup.c) 
Temp. 
Time.sup.d) 
Yield.sup.e) 
Ex. 
[g] [mmoles] 
[g] 
[g] [%] 
[%] 
[.degree.C.] 
[mins.] 
[%] 
__________________________________________________________________________ 
E6 31.1 
400 931 17.46 
2.2 
3.4 
50 140 81.8 
E7 23.3 
300 700 13.10 
2.3 
3.3 
50 140 73.8 
E8 46.6 
600 700 13.10 
1.1 
1.7 
50 240 77.2 
E9 69.8 
900 700 13.10 
0.8 
1.1 
50 300 78.2 
E10 
93.1 
1200 700 13.10 
0.6 
0.8 
50 570 70.5 
__________________________________________________________________________ 
.sup.a) Cef 196 raw; the quantities of catalyst shown are dry weights 
.sup.b) % by weight palladium, based on propane1,3-diol 
.sup.c) % by weight platinum + bismuth (based on propane1,3-diol 
.sup.d) Reaction time in minutes 
.sup.e) Yield of 3hydroxypropionic acid in % of the theoretical