JPH01211558A - Production of aromatic polycarbamate - Google Patents

Abstract

PURPOSE:To increase simultaneously the conversion of the feedstock and the selectivity to 4,4'-MDU, by adding a methylenation agent in portions under specific agents in the condensation reaction between N-phenyl carbamate and a methylenation agent in the presence of an aqueous acid. CONSTITUTION:When the condensation reaction between an N-phenyl carbamate and formaldehyde is carried out in the presence of an aqueous acid at 60-120 deg.C, preferably at 80-100 deg.C, less than 85% of the formaldehyde to be added (preferably 0.4-0.7mol. per mol. of the N-phenyl carbamate) is added at first at a molar ratio of 0.2-0.4, to effect the reaction. Then, the reaction mixture is cooled and combined with the rest of formaldehyde at a temperature at least 10 deg.C lower than the reaction temperature (preferably 50-60 deg.C), and heated against up to the reaction temperature to effect the reaction whereby the selectivity of 4,4'-methylenediphenyl dicarbamate is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing polymethylene polyphenyl polycarbamate (hereinafter abbreviated as polycarbamate) using N-phenyl carbamate and a methylenating agent as raw materials.

More specifically, in a method for producing polycarbamate by allowing a methylenating agent such as formaldehyde to act on N-phenyl carbamate in the presence of an acid aqueous solution as a catalyst, the raw material conversion rate is improved to such an extent that a recovery step for unreacted raw materials is not necessary. The present invention relates to an improved method for producing polycarbamates that can be used to produce polycarbamates with high selectivity to the desired product.

Polycarbamate is a substance useful as an intermediate raw material for pharmaceuticals, agricultural chemicals, and chemical products, and in particular, polymethylene polyphenyl polyisocyanate (hereinafter referred to as
Since it is converted into polyisocyanate (simply abbreviated as polyisocyanate), it is useful as an intermediate for polyisocyanate production.

Polyisocyanates, especially the dinuclear methylene diphenyl diisocyanate (MDI), are useful substances as raw materials for the production of polyurethane elastomers and coating materials, and the amount used in this application is problematic due to their high volatility and strong toxicity. At present, it has surpassed tolylene diisocyanate (TDI), and is being mass-produced on an industrial scale.

(Prior Art) Conventionally, aromatic isocyanates have generally been industrially produced by reducing an aromatic nitro compound with hydrogen to obtain an aromatic amine, which is then treated with phosgene to form an isocyanate. However, this method is complicated, and
Problems include the use of toxic phosgene and the production of large amounts of hydrogen chloride. Therefore, methods for producing aromatic incyanate i that do not use phosgene have been actively researched for the past 20 years.

Methods that do not use phosgene are broadly divided into ∎direct method and ∎carbamate-mediated method.

The first direct method involves reacting aromatic nitro compounds with carbon monoxide in the presence of a palladium-based catalyst in an inert solvent to directly produce aromatic isocyanate compounds, but the reaction conditions are harsh. However, there are drawbacks such as low catalyst productivity and the tendency for side reactions to occur together. Furthermore, fatally, it is difficult to apply this method to the production of polyisocyanate having a polynuclear structure such as MDI.

The second carbamate-mediated method involves reacting an aromatic nitro compound and an alcohol with carbon monoxide in the presence of a platinum group metal catalyst or a selenium catalyst to obtain an intermediate aromatic carbamate, which is then heated. This is a method to obtain aromatic isocyanate by decomposition.

The polycarbamate production method of the present invention is carried out in the production of the above-mentioned polyisocyanate by this second carbamate-mediated method. In this method, as shown in the reaction formula below, N-phenyl carbamate (■) is crosslinked by condensation with a methylenating agent such as formaldehyde in the presence of an acid catalyst to produce polycarbamate (■). .

(In the formula, m is 0 or an integer of 1 to 6, R is a carbon number of 1 to 6
lower alkyl group).

This method is described, for example, in U.S. Patent No. 2.946.
Numerous proposals have been made, including No. 768 and Japanese Patent Application Laid-Open No. 55-81850.

The resulting polycarbamate, upon pyrolysis, has the formula (I)
is converted to the corresponding polyisocyanate shown as

(where m and R have the same meanings as above).

This method is attracting attention as an advantageous method for producing polycarbamates and polyisocyanates because an excellent method for synthesizing N-phenyl carbamate as a raw material from a nitro compound or an amino compound has been developed in recent years.

Among the polyisocyanates of the general formula (Ill) above, 4,4°-methylene diphenyl diisocyanate (hereinafter abbreviated as 4.4'-MD I), which is generally called pure MDI, has the highest reactivity and is therefore a high-value product. It is. Therefore, the polycarbamate of the general formula (II), which is a raw material for producing polyisocyanate, also has 4.4"
-methylene diphenyl cicabamate (hereinafter referred to as 4.4
'-MDU) is the most desirable product.

However, in the above condensation reaction, even if the reaction conditions are adjusted so that 4.4'-MDU is obtained, the 4.4' binuclear MDU isomer outside the integral plate, that is, 2.2'-MDU and 2.4°-MDU generation, trinuclear bodies or more [general formula (
n), the formation of polycarbamate with m≧1] is unavoidable.

However, since polycarbamate has a very high boiling point,
It is very difficult to separate the condensation product into each compound by distillation and isolate the desired 4,4'-MDU.

Therefore, conventionally, distillation of the condensation product only recovers unreacted N-phenyl carbamate, and the remaining mixture of polycarbamate condensation products is directly fed to the thermal decomposition process without being separated into its constituent components. Was. Therefore, the thermal decomposition reaction products include various polyisocyanates corresponding to various polycarbamates produced in the condensation reaction, that is, in addition to the desirable 4.4'-MDI, 2.4'-
A mixture containing '- and 2.2'-MDI and a trinuclear or more polynuclear polyisocyanate was obtained, and this was purified by means such as distillation and crystallization.

Therefore, in the condensation reaction, 4.4°-MD
In order to generate as much U as possible, 4.4'-MD
It is desired to produce U with high selectivity.

On the other hand, in order to improve the overall yield of 4.4'-MDU and polycarbamate products, it is also possible to increase the reaction rate (conversion rate) of the N-phenyl carbamate raw material, that is, to reduce the amount of unreacted raw material. Needless to say, it is extremely important for industrialization.

However, conventionally proposed polycarbamate production methods have not been able to simultaneously improve selectivity and raw material conversion rate.For example, in order to obtain high selectivity,
A method of transferring the reaction intermediate to a dinuclear unit (MDU) by treating the condensation reaction product with a strong organic acid (Japanese Unexamined Patent Application Publication No. 1983-1993)
-106453), but with this method, the raw material conversion rate is not necessarily sufficient.On the other hand, it is possible to increase the raw material conversion rate to almost 100% by increasing the amount of methylene and other agents added, but this method In some cases, the selectivity is not high, and we have previously proposed a method of increasing the selectivity with a high conversion rate by adding a methylenating agent in portions (Japanese Patent Application No. 293,227/1989); However, the selectivity to the target product, especially 4.4°-MDU, is insufficient.

(Problems to be Solved by the Invention) As described above, the current situation is extremely unsatisfactory for industrial production of polycarbamates.

The purpose of the present invention is to combine a methylenating agent and N in the presence of an aqueous acid solution.
- By providing a method for producing polycarbamate that can simultaneously improve the conversion rate of raw material N-phenyl carbamate and the selectivity to desired 4.4°-MDU when producing polycarbamate from phenyl carbamate. be.

(Means for Solving Problem M) The present inventors carried out the condensation reaction in multiple stages by adding the necessary amount of methylenating agent in two or more times, and the second time was added in the same manner as the first time. By performing the subsequent addition of the methylenating agent after cooling the reaction system to a temperature lower than the condensation reaction temperature, the desired product 4,4'-MDU can be obtained with high selectivity, and at the same time, N-phenyl carbamate can be obtained. The present invention was completed based on the discovery that the reaction rate can be significantly improved.

Here, the present invention provides a method for producing polymethylene polyphenyl polycarbamate by adding a methylenating agent to N-phenyl carbamate in the presence of an aqueous acid solution and carrying out a condensation reaction by adding the methylenating agent in portions. The reaction is carried out in two or more parts, each time the methylenating agent is added to the reaction system at least 10°C lower than the condensation reaction temperature, and the reaction system is heated to the reaction temperature after the addition of the methylenating agent. This is a method for producing polymethylene polyphenyl polycarbamate, which is characterized in that the condensation reaction is carried out each time.

(effect) The present invention will be explained in detail below.

The starting material for the polycarbamate production method of the present invention is N-phenylcarbamate represented by the above general formula (1). In general formula (1), R is a lower alkyl group having 1 to 6 carbon atoms. It is particularly preferable that R is methyl or ethyl, since this facilitates the subsequent thermal decomposition reaction.

As the methylenating agent, formaldehyde or a substance that generates formaldehyde is used. A substance that generates formaldehyde is a substance that generates formaldehyde by decomposition etc. under the above condensation reaction conditions,
Specific examples include trioxane, paraformaldehyde, methylal and other formals.
Usually, for economic reasons, an aqueous formaldehyde solution (formalin) is used as the methylenating agent.

Acid catalysts include inorganic acids such as sulfuric acid, hydrochloric acid, phosphoric acid, polysulfuric acid, polyphosphoric acid, boric acid, hydrobromic acid, and perchloric acid;
Lewis acids such as boron trifluoride and organic acids such as methanesulfonic acid can be used, but strong inorganic acids, especially sulfuric acid, are preferred.

In order to simplify the explanation, formaldehyde will be used as a methylenating agent, and sulfuric acid will be used as an acid catalyst.

The condensation reaction of the present invention is characterized by the divisional addition of formaldehyde and the addition of formaldehyde each time at a temperature lower than the condensation reaction temperature.In other words, the second and subsequent additions of formaldehyde are performed by cooling the reaction product. The method can be carried out in the same manner as the conventional method, except that the second and subsequent condensation reactions are carried out by raising the temperature to the condensation reaction temperature.The reaction conditions and reaction method will be briefly explained below.

This condensation reaction can be carried out in a batch, continuous or semi-continuous manner, and there are no particular restrictions on the amount of reaction components added, but in the present invention, formaldehyde is divided into a mixture of N-phenyl carbamate and sulfuric acid. The reaction is carried out by adding:

The total amount of formaldehyde added, in molar ratio to N-phenyl carbamate, is preferably in the range of 0.2 to 1.0, more preferably 0.4 to 0.7.

In the present invention, this is added in two or more portions.

The initial addition amount of formaldehyde is preferably 4.4'
- MDU In order to ensure a high selectivity to U, it is desirable that the amount of formaldehyde added be approximately 85% or less of the total amount of formaldehyde added. The amount of formaldehyde added is preferably about 0.5 mol or less, particularly preferably between about 0.2 and 0.4 mol. With this addition amount, unreacted N-7 enyl carbamate remains after the first condensation reaction.

If the initial amount of formaldehyde added is too small, the number of repetitions of the condensation reaction required to obtain the desired conversion rate will increase, resulting in a decrease in reaction efficiency. On the other hand, if the initial amount of formaldehyde added is too large, the selectivity to the desired product will decrease.

The initial addition of formaldehyde to the reaction system is carried out at a temperature lower than the predetermined condensation reaction temperature.As will be explained later, when formaldehyde is brought into contact with N-phenyl carbamate at a high temperature, 4.4 However, the selectivity to °-MDU decreases.

It is preferable to add formaldehyde at a temperature higher than the melting point of N-phenyl carbamate because it facilitates the mixing operation, but in order to improve the selectivity, it is preferable to add formaldehyde at a temperature as low as possible unless the resulting condensation product solidifies. It is advantageous to carry out the addition of formaldehyde.

The acid concentration of the aqueous sulfuric acid solution used at the inlet of the reactor is preferably 20 to 80% by weight, more preferably 40 to 60% by weight. When the supplied sulfuric acid concentration is 20% by weight or more than 80% by weight, the selectivity to the target product tends to be low.

The supply ratio of sulfuric acid aqueous solution and N-phenyl carbamate is:
The molar ratio of sulfuric acid to N-phenyl carbamate is preferably 2 or more; below this supply ratio, the effect of promoting the condensation reaction decreases. The condensation reaction can be carried out without the presence of an organic solvent, but if desired, a suitable organic solvent that does not react with the methylenating agent under the condensation reaction conditions can also be coexisting.

The condensation reaction temperature is preferably 60 to 120 t, more preferably 80 to 100 °C. If the reaction temperature is too low, the condensation reaction rate will decrease and the reaction time will become too long; on the other hand, if the reaction temperature is too high, The rate of selection to the desired product decreases.

Although the reaction time varies depending on the reaction conditions, the total reaction time is 0.1 to 20 hours, preferably 0.5 to 10 hours. This is because if the reaction time is too short, the reaction will not be completed, and if the reaction time is too long, side reactions will occur significantly.

In the present invention, as will be described later, since the condensation is repeated two or more times, the condensation reaction time each time is at least half of the above.

As a result of repeated studies on the condensation reaction between N-phenyl carbamate and a methylenating agent in the presence of an acid catalyst, the inventors of the present invention discovered that this condensation reaction consists of the following reaction. , we obtained the following findings.

The reaction between N-phenylcarbamate and formaldehyde produces a methylol compound, which is an active species, which further reacts with N-phenylcarbamate. At this time,
If the reaction occurs at the benzene ring, MDU will be generated, and if the reaction occurs at the nitrogen atom of the urethane group, the benzyl compound (It/) will be generated.

(R: lower alkyl group having 1 to 6 carbon atoms) The benzyl body is
MDU by acid in the presence of N-phenyl carbamate
It is known that it metastasizes to
(See Publication No. 53).

The optimal conditions for these two reactions are different, especially when formaldehyde is present, and the MDU due to the former is
When formaldehyde disappears, MDU production by the latter transition becomes the main activity. In addition, the influence of temperature is greater in the former, and benzyl forms are more likely to be formed at low temperatures (
Therefore, it is thought that a large proportion of MDU production is due to the rearrangement reaction. Therefore, by adding the required amount of N-phenyl carbamate in portions and repeating the condensation reaction with a temperature difference, conditions are established in which the formation of MDU by the rearrangement reaction is likely to occur.
An increase in the amount of MDU generation can be expected.

Based on the above findings, in the present invention, the condensation reaction is repeated two or more times by adding the required amount of formaldehyde in two or more times, and the reaction mixture is once condensed before the second and subsequent additions of formaldehyde. Cool to a temperature range below the reaction temperature.

Considering the above objectives, it is advantageous for this cooling temperature, that is, the temperature of the reaction system when formaldehyde is added, to be as low as possible as long as the condensation product does not solidify, but it also facilitates reactivity with formaldehyde and operation. N for
- above the melting point of the phenyl carbamate, i.e. about 5
The preferred formaldehyde addition temperature, which is preferably 0°C or higher, is at least 10°C below the condensation reaction temperature.
Lower conditions are preferred, particularly within the range of about 50-80°C, particularly about 50-60°C.

The amount of formaldehyde added is determined so that the total amount of formaldehyde added falls within an appropriate range. The addition of formaldehyde can be carried out by equally dividing the total addition amount into two or three times or more, or the first addition amount can be relatively large.

After adding formaldehyde at low temperature as above,
A process in which the reaction mixture is heated to a predetermined reaction temperature again to perform a condensation reaction, and formaldehyde is added after cooling the reaction product and the temperature is immediately raised to perform a condensation reaction.
If necessary, the process can be further repeated until a desired raw material conversion rate is obtained. As long as formaldehyde is added in such a low temperature range, the desired 4.4'-M D
No decrease in selectivity towards U occurs. Therefore, the reaction rate (raw material addition rate) can be sufficiently improved while maintaining a high selectivity to this product, and the amount of unreacted material can be reduced to several percent or less.

On the other hand, if the entire amount of formaldehyde is added at the beginning, the raw material will be converted as the amount added increases.

rate can be improved, but selection rate cannot be improved.
Further, even if formaldehyde is added in portions, if the second addition is carried out at the condensation reaction temperature, the selectivity will still decrease.

The condensation reaction product obtained by the method of the present invention can be recovered by phase separation of the acid aqueous solution by a conventional method.The method of the present invention improves the conversion rate to the extent that it is not necessary to recover unreacted raw materials. Since the reaction product can be
The unreacted N-phenyl carbamate can be used as it is in the thermal decomposition process without the usual distillation recovery.

Thermal decomposition converts the polycarbamate product to the corresponding polyisocyanate, and since the polycarbamate product of the present invention contains a high proportion of 4.4"-MDU, this pyrolysis converts the polycarbamate product to the corresponding polyisocyanate. (4,
Polyisocyanate products containing high amounts of 4'-MD I) can be produced.

The invention will now be illustrated by examples. In the examples, % is by weight unless otherwise specified.

1-2 and 1-5 In a 500 cd round-bottom separable flask equipped with a thermometer, stirrer, reflux condenser, and addition funnel, add 33.0 g of ethyl N-phenyl carbamate (EP C) and 195.9 g of 50% sulfuric acid. g and heated under stirring. When the temperature reached 50°C, a predetermined amount of 37% formaldehyde aqueous solution was added from the dropping funnel. After completion of the 0 dropwise addition, the mixture was heated to 90°C with stirring, and a condensation reaction was carried out at this temperature for a predetermined time.

After the first reaction was completed, in the example an additional amount of formaldehyde aqueous solution was added dropwise to the reaction product. This addition of formaldehyde is done after the temperature inside the flask has fallen below 55°C.
After the additional dropwise addition was completed, the mixture was heated to 90° C. for a predetermined time to further perform the condensation reaction. In Example 1, condensation was carried out twice,
In Example 2, the condensation was repeated three times.

After completion of all reactions, the obtained condensation product was mixed with toluene and
After dissolving in o cj and separating the aqueous layer, the toluene solution was analyzed for unreacted RPC and generated 4,4°-MDU by high performance liquid chromatography. The analysis results were analyzed by adding formaldehyde (I (CHO)!i (I
IcHO/[! PC molar ratio) and reaction time are shown in Table 1 below.

Table 1 (Effects of the Invention) As is clear from the results of the Examples, the method of the present invention produces a desirable polycarbamate product, 4,4'-M
Even if the condensation reaction is repeated so that DU can be obtained with a high selectivity and the raw material conversion rate is nearly 100%, this decrease in selectivity does not occur.In contrast, if the condensation reaction is performed once In all of the comparative examples carried out in 1, the selectivity of 4.4'-MDU was low, and this selectivity tended to decrease as the raw material conversion rate increased.

Thus, in the method of the present invention, 4,4°-MD
It is possible to simultaneously improve the selectivity of I and the raw material conversion rate, and to efficiently produce a polycarbamate product having a composition suitable as an intermediate raw material for various chemical products and also as a polyisocyanate raw material for polyurethane production. can. Furthermore, by repeating the condensation reaction, N-phenyl carbamate can be almost completely converted, eliminating the need for distillation equipment for recovering unreacted raw materials. Product quality is also improved since deterioration of carbamates is also avoided.

Claims (1)

[Claims] (1) A method for producing polymethylene polyphenyl polycarbamate by adding a methylenating agent to N-phenyl carbamate in the presence of an aqueous acid solution and carrying out a condensation reaction,
By adding the methylenating agent in portions, the condensation reaction can be carried out in two steps.
Divide the process into two or more times, and add the methylenating agent each time.
Polymethylene polyphenyl polycarbamate, which is carried out in a reaction system at least 10°C lower than the condensation reaction temperature, and each condensation reaction is carried out by raising the temperature of the reaction system to the reaction temperature after adding the methylenating agent. Production method.