Process for the preparation of trimethylolpropane

Trimethylolpropane is prepared in such a manner that butyraldehyde is metered into a mixture of water, alkaline condensing agent and formaldehyde, which contains less than 0.1 mol of methanol relative to 1 mol of formaldehyde, at temperatures from 15.degree. to 50.degree. C., and after addition of the butyraldehyde the reaction mixture is heated up to 90.degree. C., sufficient water being present in the reaction mixture so that the reaction mixture has a content of trimethylolpropane of 5 to 25% by weight after the reaction.

The invention relates to a process for the preparation of 
trimethylolpropane by reaction of n-butyraldehyde with aqueous 
formaldehyde in the presence of alkaline condensing agents. 
It has been disclosed, in German Auslegeschrift No. 1,153,739, German 
Auslegeschrift No. 1,154,080, German Auslegeschrift No. 1,182,646 and U.S. 
Pat. No. 3,076,854 that trimethylolpropane is prepared by alkaline 
condensation of formaldehyde with butyraldehyde. The disadvantages of 
these processes are the yields, which are unsatisfactory for a large-scale 
process, and the high industrial costs necessary in some cases (several 
reaction steps, for example, in the process of German Auslegeschrift No. 
1,154,080 and German Auslegeschrift No. 1,153,739). 
Furthermore, a process has been disclosed in German Democratic Republic 
Pat. No. 134,514 in which trimethylolpropane is prepared by alkaline 
condensation of approximately stoichiometric amounts of n-butyraldehyde 
and aqueous formaldehyde in the presence of a mixture of calcium hydroxide 
and sodium hydroxide as the condensing agent and by setting up a minimum 
content of water in the reaction mixture. The disadvantage of this process 
is that the calcium formate, which is produced as an important by-product 
is contaminated with sodium formate, it being possible to remove the 
sodium formate only at relatively high industrial cost. In addition, on 
repeating the process described in the German Democratic Republic patent 
specification, the high yields of trimethylolpropane reported there could 
not be achieved (compare the Comparison Example to Example 2 of the 
process according to the invention). 
Another possibility of preparing trimethylolpropane consists of initially 
reacting butyraldehyde with formaldehyde under alkaline reaction 
conditions to give dimethylolbutanal and then catalytically hydrogenating 
the latter to give trimethylolpropane (compare, for example, German No. 
2,702,582 and U.S. Pat. No. 4,122,290). It is true that satisfactory 
yields of trimethylolpropane are achieved with this process (up to 94.5% 
of theory according to the German patent, but it is less suitable for a 
large-scale process, since the catalytic hydrogenation must be carried out 
in a high pressure apparatus, which necessitates additional industrial 
costs. 
A process for the preparation of trimethylolpropane by the reaction of 
n-butyraldehyde with aqueous formaldehyde in the presence of alkaline 
condensing agents has now been found, which is characterized in that the 
butyraldehyde is metered into a mixture of water, alkaline condensing 
agent and formaldehyde, which contains less than 0.1 mol of methanol 
relative to 1 mol of formaldehyde, at temperatures from 15.degree. to 
50.degree. C., and after addition of the butyraldehyde the reaction 
mixture is heated up to 90.degree. C., sufficient water being present in 
the reaction mixture so that the reaction mixture has a content of 
trimethylolpropane of 5 to 25% by weight after the reaction. 
In general, in the process according to the invention, the formaldehyde is 
employed in the form of an aqueous solution having a content of about 10 
to 40% by weight of formaldehyde, preferably having a content of 20 to 30% 
by weight of formaldehyde. The formaldehyde employed for this process 
should contain less than 0.1 mol of methanol, preferably 0 to 0.07 mol of 
methanol, particularly preferably 0.001 to 0.05 mol of methanol, relative 
to 1 mol of formaldehyde. 
Suitable alkaline condensing agents are those bases known and customarily 
used for the aldol condensation. Examples which may be mentioned are the 
hydroxides and/or carbonates of alkali metals and/or alkaline earth metals 
such as sodium hydroxide, potassium hydroxide, calcium hydroxide, barium 
hydroxide and/or sodium carbonate, preferably sodium hydroxide and/or 
calcium hydroxide, particularly preferably calcium hydroxide. 
In general, in the process according to the invention, the alkaline 
condensing agents are employed in an amount of about 0.5 to 1.7 mols per 
mol of n-butyraldehyde. When using alkali metal hydroxides, customarily 
about 1 to 1.7, preferably 1.2 to 1.6, mols of alkali metal hydroxide is 
used per mol of butyraldehyde and, when employing alkaline earth metal 
hydroxides, about 0.5 to 1, preferably 0.6 to 0.8 mol of alkaline earth 
metal hydroxide is employed per mol of butyraldehyde. 
In general, in the process according to the invention, 1 mol of 
butyraldehyde is reacted with about 3.0 to 10, preferably 3.1 to 8, 
particularly preferably 3.2 to 7, mols of formaldehyde. 
Care should be taken in the reaction according to the invention that 
sufficient water is present in the reaction mixture so that the reaction 
mixture has a content of trimethylolpropane of about 5 to 25, preferably 6 
to 17, particularly preferably 7 to 10, % by weight after the reaction. 
The necessary water content in the reaction mixture can be set up in this 
process by an appropriate excess of aqueous formaldehyde or by the 
addition of water. 
The process according to the invention is customarily carried out at 
temperatures from about 15.degree. to 90.degree. C., the start of the 
reaction initially being carried out at low temperatures, at about 
15.degree. to 50.degree. C., preferably at 20.degree. to 40.degree. C., 
and then the reaction mixture is heated to higher temperatures, say 
90.degree. C., preferably 50.degree. to 80.degree. C., to complete the 
reaction. The temperature range in which the reaction is carried out most 
advantageously depends, inter alia, on the pH of the reaction mixture and 
can easily be found by preliminary experiments. For example, when the pH 
of the reaction mixture is about 11, the reaction is initially carried out 
at 20.degree. to 30.degree. C. and the mixture is then heated to 
40.degree. to 50.degree. C. 
The reaction times necessary for the process according to the invention are 
very dependent on the reaction temperature and are generally about 5 
minutes to 2 hours. 
The process according to the invention can be carried out advantageously by 
initially introducing the aqueous formaldehyde, the desired alkaline 
condensing agent and, if desired water and metering in the n-butyraldehyde 
at temperatures from 15.degree. to 50.degree. C. However, one can also 
initially introduce only a part of the formaldehyde and add the remaining 
formaldehyde together with the butyraldehyde. In this instance, the 
butyraldehyde is advantageously added at the rate at which it is consumed 
to the initially introduced mixture. After addition of the butyraldehyde, 
the reaction mixture is heated to about 80.degree. C. to complete the 
reaction. 
After completion of the reaction, the pH of the reaction mixture, which is 
about 7 to 10, depending on the base employed in each case and the amount 
of formaldehyde employed, is adjusted to a value in the range from about 5 
to 7, preferably 5.5 to 6.5, by the addition of acids, such as formic, 
acetic, sulphuric and/or phosphoric acid. If an excess of formaldehyde is 
present in the reaction mixture, the formaldehyde can be distilled out at 
temperatures of about 120.degree. to 150.degree. C. and under a pressure 
of about 2 to 5 bar, and thus be recovered in this manner. 
The working up of the reaction mixture can be carried out in a customary 
manner by distillation or extraction (compare, for example, Ullmanns 
Enzyklopadie der technischen Chemie (Ullmann's Encyclopaedia of Industrial 
Chemistry) 4th edition, volume 7, page 231 (1974)). 
The process according to the invention can be carried out both continuously 
and also discontinuously. 
The advantage of the process according to the invention lies in the high 
yields of trimethylolpropane, relative to the butyraldehyde employed or 
the formaldehyde employed or reacted. Thus, the process can be carried out 
particularly economically on a large industrial scale. Furthermore, fewer 
by-products are formed in the process according to the invention than in 
the known processes, and thus the working up of the reaction mixture and 
the purification of the trimethylolpropane is considerably simplified. 
However, it is particularly surprising that the yields of 
trimethylolpropane, relative to the formaldehyde employed or reacted, in 
the process according to the invention could be increased further, and 
this means that the economics of the process can be regarded as being 
extremely favourable in respect of the consumption of formaldehyde. 
Trimethylolpropane is an intermediate of industrial importance for the 
preparation of plasticisers, raw materials for paints, polyesters and 
polyurethanes (compare, for example, Ullmanns Enzyklopadie der technischen 
Chemie (Ullmann's Encyclopaedia of Industrial Chemistry) 4th edition, 
volume 7, page 228 and page 231 (1974)).

The following examples are intended to elucidate the process according to 
the invention but without restricting it to these examples. 
EXAMPLE 1 
4,800 g (32 mols) of a 20% strength aqueous formaldehyde solution having a 
methanol content of 0.43% (0.64 mol) are initially introduced into a 6 l 
stirred flask and 176 g (4.4 mols) of sodium hydroxide, dissolved in 704 g 
of water, are added. The solution is maintained at 30.degree. C., while 
300 g (4.04 mols) of n-butyraldehyde (97% pure) are metered in regularly 
within 1 hour. Subsequently, the reaction mixture is heated to 48.degree. 
C. and maintained at this temperature for 1 hour. The reaction product 
(5,980 g) thus obtained contains 8.56% of trimethylolpropane (3.82 mols) 
and 9.7% of formaldehyde (19.3 mols). The yield of trimethylolpropane is 
94.5% relative to n-butyraldehyde employed and 90.3% relative to reacted 
formaldehyde. The unreacted formaldehyde is recovered in a customary 
manner by distillation under pressure. 
Comparison Example to Example 1 (carried out according to Example 1 of 
German Auslegeschrift No. 1,182,646) 
20% strength aqueous formaldehyde in an amount of 4,800 g (32 mols) having 
a commercial methanol content of 4.1% (corresponding to 6.1 mols) are 
placed in a stirred flask of volume 6 l and 176 g (4.4 mols) of sodium 
hydroxide, dissolved in 700 g of water, are added. 296 g (3.98 mols) of 
n-butyraldehyde (97% pure) are metered in regularly to this solution, at 
an initial temperature of 28.degree. C., with stirring in the course of 1 
hour. Due to the reaction taking place exothermically, the temperature 
rises to 49.degree. C. After addition of the n-butyraldehyde the reaction 
product is heated to 57.degree. C. and maintained at this temperature for 
1 hour. The reaction product (5,972 g) is analyzed as in Example 1. It 
contains 7.95% of trimethylolpropane (3.54 mols) and 9.6% of formaldehyde 
(19.1 mols). The yield of trimethylolpropane is 88.9% relative to the 
n-butyraldehyde employed and 82.3% relative to the reacted formaldehyde. 
EXAMPLE 2 
200 g of calcium hydroxide are added to 4,400 g of an aqueous formalin 
solution, which contains 3.4% of formaldehyde (5 mols) and 0.08% of 
methanol (0.1 mol), and the mixture is stirred. 800 g of formalin, which 
contains 30% of formaldehyde (8 mols) and 0.7% of methanol (0.2 mol) are 
pumped regularly into this mixture at 30.degree. C. within 1 hour. 
Simultaneously, and also in the course of 1 hour, 300 g of 97% pure 
n-butyraldehyde (4.036 mols) are metered in. During this, the mixture is 
stirred vigorously and the temperature is maintained at 30.degree. C. The 
reaction mixture (5,700 g) is then heated to 60.degree. C. The reaction 
product thus obtained contains 8.62% of trimethylolpropane (3.66 mols), 
corresponding to a yield of 90.7% of trimethylolpropane, relative to the 
butyraldehyde employed and 84.5% of theory, relative to the formaldehyde 
employed. 
Comparison Example to Example 2 (carried out according to Example 2 of 
German Democratic Republic Patent Specification No. 134,514) 
2,454 g (12.3 mols) of a 15% strength aqueous formaldehyde solution, having 
a commercial methanol content of 3.1% (2.4 mols), were initially 
introduced, at 20.degree. C., into a stirred flask of volume 6 l. In the 
course of 80 minutes, 299 g (4.02 mols) of butyraldehyde (97% pure) and 
1,830 g (1.1 mols) of a 1.8% strength aqueous formaldehyde solution were 
pumped in regularly and 146 g (1.97 mols) of calcium hydroxide powder and 
40.9 ml of a 20% strength sodium hydroxide solution (0.2 mol) were metered 
in. During this, the reaction temperature was limited to a maximum of 
40.degree. C. The reaction mixture obtained after completion of the main 
reaction was maintained at 65.degree. C. for 45 minutes. The reaction 
product (4,762 g) thus obtained contained 9.94% of trimethylolpropane 
(3.53 mols) corresponding to a yield of trimethylolpropane of 88.3%, 
relative to the n-butyraldehyde employed and 79% relative to the 
formaldehyde employed. 
EXAMPLE 3 
5,200 g of 18.5% strength aqueous formaldehyde solution (32 mols), which 
contained 0.4% of methanol (0.65 mol), were initially introduced into a 
stirred flask of volume 6 l at 25.degree. C. and 200 g of calcium 
hydroxide powder (2.7 mols) were added. In the course of 1 hour, with 
vigorous stirring, 300 g of (97% pure) n-butyraldehyde (4.036 mols) were 
metered in regularly, the reaction temperature being maintained at 
25.degree. C. by cooling. After addition of the butyraldehyde, the 
reaction mixture was heated to 45.degree. C. and maintained at this 
temperature for 1 hour. The reaction product was analysed. It contained 
8.86% of trimethylolpropane and 10.1% of formaldehyde, corresponding to a 
yield of trimethylolpropane of 93.4% relative to the butyraldehyde 
employed, and a yield of 88.3% relative to the reacted formaldehyde.