JPS628091B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) This invention relates to a method for producing a carbonic ester, such as diphenyl carbonate, by transesterifying dimethyl carbonate with a hydroxy compound, such as phenol. Diphenyl is used as a raw material for polycarbonate synthesis and is of great industrial importance. (Prior art and its problems) Regarding the method of transesterifying dimethyl carbonate with a hydroxy compound in the presence of a transesterification reaction catalyst to produce a carbonate ester, for example, Patent Publication Showa et al.
56-42577 etc. Transesterification The transesterification reaction in which dimethyl carbonate is transesterified with a hydroxy compound to produce carbonic ester in the presence of a transesterification catalyst is an equilibrium reaction, and in order to proceed with the reaction, the methanol produced or the carbonic ester produced must be sequentially removed. There is a need. For such reactions, a reactive distillation method is generally used in which a reactive distillation column is added to the reactor, and methanol, which has a boiling point lower than the carbonate ester produced at the top of the column, is distilled off while the reaction is carried out in the reactor at the bottom of the column. Adopted. However, in this transesterification, the methanol produced forms an azeotrope with the raw material dimethyl carbonate, so the raw material dimethyl carbonate is also distilled off together with the methanol, which poses a problem in terms of efficiency. As a method to solve this problem,
-48732, a method has been proposed for preventing the distillation of dimethyl carbonate by adding an azeotrope with methanol. The general selection of an azeotrope forming agent in such a system is as follows: (1) The azeotropic temperature with methanol is lower than the azeotropic temperature between methanol and dimethyl carbonate, and it facilitates the separation of dimethyl carbonate. This is done due to the large temperature difference and (2) high concentration of methanol in the azeotrope with methanol from an energy standpoint, but furthermore, the reaction rate in this transesterification reaction is extremely slow. Therefore, from the viewpoint of improving the usage efficiency of the raw material dimethyl carbonate, an azeotrope-forming agent that does not hold as much dimethyl carbonate in the distillation column as possible, that is, (4) The fact that its boiling point is lower than that of dimethyl carbonate and (5) that it does not form an azeotrope with dimethyl carbonate are also important factors when selecting an azeotrope-forming agent. However, most azeotrope-forming agents that satisfy up to item (4) of the selection conditions above also form an azeotrope with dimethyl carbonate. Therefore, in practice, the concentration of dimethyl carbonate in the azeotrope with dimethyl carbonate is as low as possible. (In the case of a three-component azeotrope of methanol, dimethyl carbonate, and an azeotrope-forming agent, an azeotrope-forming agent in which the concentration of dimethyl carbonate in the three-component azeotrope is as low as possible) is preferred. The azeotrope forming agent described in JP-A-54-48732 has a problem in this respect. For example, it satisfies most of the selection conditions for the azeotrope forming agent. n-hexane contains about 20 percent by weight of dimethyl carbonate in the azeotrope with dimethyl carbonate, and as a result, a large amount of dimethyl carbonate is retained in the distillation column of the reactive distillation column, that is, in the non-reaction zone. Furthermore, it is thought that a three-component azeotrope of methanol, n-hexane, and dimethyl carbonate exists, and a large amount of dimethyl carbonate is contained in the distillate. Therefore, there is a large amount of unreacted dimethyl carbonate at the end of the reaction, resulting in poor efficiency. (Objective of the Invention) Therefore, the object of the present invention is to more efficiently remove and recover methanol produced in a method for producing carbonic ester by transesterifying dimethyl carbonate with a hydroxy compound in the presence of a transesterification catalyst. More specifically, as an azeotrope forming agent with methanol, its azeotropic temperature is higher than that of methanol and dimethyl carbonate, and the difference is large, and methanol in the azeotrope The object of the present invention is to find an azeotrope-forming agent that has a high concentration of dimethyl carbonate and a low concentration of dimethyl carbonate in the azeotrope with dimethyl carbonate. (Structure of the Invention) That is, the present invention uses benzene as an azeotrope-forming agent with methanol in a method for producing a carbonic ester by transesterifying dimethyl carbonate with a hydroxy compound in the presence of a transesterification catalyst. It consists of distilling off methanol as a mixture with benzene. The hydroxy compound used in the present invention is an organic compound represented by the general formula R(--OH) m (wherein R is an alkyl group or an aryl group, and m is an integer of 1 or more), such as methanol,
Examples include aliphatic alcohols such as propanol and allyl alcohol butanol, alicyclic alcohols such as cyclohexanol and methylcyclohexanol, phenol, and cresol. Of course 2
It is also possible to use more than one type of hydroxy compound in combination. (Effects of the Invention) The method of the present invention uses benzene as an azeotrope forming agent with methanol in a method for producing carbonic ester by transesterifying dimethyl carbonate with a hydroxy compound in the presence of a transesterification catalyst. As a result, methanol can be easily and effectively removed and recovered, making it possible to produce carbonate ester more economically. That is, benzene used in the present invention forms an azeotrope with methanol, so the azeotropic temperature is 58°C.
(760 mmHg), and there is a sufficient boiling point difference between the azeotropic temperature of methanol and dimethyl carbonate, 64°C (760 mmHg), for distillation separation, so that almost all of the raw material dimethyl carbonate is taken out of the reaction system. The reaction can be carried out efficiently without any problems. Furthermore, since the concentration of methanol in the azeotrope of methanol and benzene is as high as 38.4% by weight, the energy load in the reaction distillation column and in the separation and recovery process of methanol and benzene is small, making it economical. Furthermore, the boiling point of benzene is lower than that of dimethyl carbonate, and the concentration of dimethyl carbonate in the azeotrope of benzene and dimethyl carbonate is 1% by weight, which is much lower than other azeotrope forming agents. Since the amount of dimethyl carbonate held in the distillation column is small and, as a result, most of the charged dimethyl carbonate exists in the reaction zone at the bottom of the distillation column, it is possible to react the charged dimethyl carbonate advantageously and efficiently. The azeotrope of methanol and benzene obtained by distillation from the reactive distillation column can be easily separated and recovered by a known method. For example, it can be easily separated into a mixture of benzene, methanol, and water by countercurrent extraction with water, and the obtained benzene can also be supplied as it is to reactive distillation. The water required for countercurrent extraction can be extracted in a relatively small amount because methanol is very easily extracted by water, and as a result, a highly concentrated methanol aqueous solution can be obtained. Therefore, even if methanol and water are separated by a known general separation operation such as distillation, the energy burden is small. Furthermore, the separated water can be used again for countercurrent extraction, making it possible to recover methanol very economically. (Means for Solving Problems) The present invention will be explained in more detail below with reference to FIG. 1, but the present invention is not limited in any way. The transesterification catalyst, dimethyl carbonate, and intermediate products such as hydroxy compounds such as phenol and optionally methyl phenyl carbonate are introduced into the reactive distillation column 3 via the feed line 1. Reactive distillation operations can be carried out either batchwise or continuously, but in cases such as transesterification of dimethyl carbonate with phenol, the reaction rate is slow.
Since a continuous method requires large equipment, a batch method is preferable. The reactive distillation column is operated under pressure for the purpose of increasing the reaction rate, and the pressure is usually 0 to 10 atm, preferably 1 to 5 atm. Carrying out the reaction under higher pressure is not preferable because it may lead to decomposition of dimethyl carbonate or generation of impurities. Benzene, which is an azeotrope-forming agent with methanol, is introduced into a reactive distillation column 3 via a benzene feed conduit 2. Benzene may be introduced at any part of the reactive distillation column 3, but considering the removal efficiency of methanol from the reaction zone, it is introduced at a position as close to the reaction zone as possible, that is, toward the bottom of the reactive distillation column. It is preferable to do so. In addition, benzene is introduced in an amount equivalent to the hold amount in the distillation section of the reactive distillation column 3 before starting the reaction, regardless of whether the reactive distillation is carried out continuously or batchwise. Preference is given to continuous or semi-batchwise introduction. In this case, it is preferable to introduce an excessive amount of benzene so that the temperature of the second to fifth stages from the bottom of the distillation section of the reactive distillation column 3 is maintained at a temperature 2 to 5 degrees Celsius higher than the boiling point of benzene. is not preferable because it causes a decrease in the reaction temperature and slows down the reaction rate. Therefore, even when carrying out reactive distillation in a batch manner, it is not preferable to introduce the entire amount of benzene at the time of introducing the raw materials for the same reason. Methanol produced in the reaction zone at the bottom of the reactive distillation column forms an azeotrope with benzene in the distillation section, is taken out from the top of the reactive distillation section via the top gas extraction conduit 5, and is condensed in the condenser 6. A portion of the condensate is refluxed via the reflux line 7 to the upper part of the reactive distillation column, and the remaining condensate is taken off as distillate via the distillate line 8. In this case, it is preferable to take out the distillate amount so that the temperature at the top of the reactive distillation column does not become higher than the azeotrope temperature of methanol and benzene. The concentration can be reduced to 1% by weight or less. The produced diphenyl carbonate, intermediate products, unreacted phenol, dimethyl carbonate, and transesterification reaction catalyst are taken out via the reaction liquid withdrawal conduit 4 and purified by fractional distillation. When reactive distillation is carried out batchwise, it is also possible to carry out fractional distillation purification using a reactive distillation column directly after the reaction is completed. The distillate taken out via the distillate conduit 8 is an azeotrope of methanol and benzene, and is passed through the countercurrent extractor 9.
Methanol is extracted to the water side by contact with water introduced through the water feed conduit 10. At this time, the amount of water introduced from the water supply pipe 10 depends on the capacity of the countercurrent extractor, but it is relative to the amount of methanol and benzene azeotrope introduced.
0.1 to 3.0 times by weight, preferably 0.2 to 10 times by weight, is introduced. The benzene-based liquid obtained as the extraction residue after methanol is extracted to the water side usually contains 0.07 to 0.1% by weight of water, unextracted methanol, and distillate, depending on the operating conditions of the countercurrent extractor. Most of the dimethyl carbonate contained in the methanol and benzene azeotrope introduced through conduit 8 is contained, and the extraction residue is taken out through conduit 11. This extraction residue is introduced into the reactive distillation column again via the benzene feed conduit 2 either as it is or after water and methanol have been separated by a conventional separation method such as distillation. Replenishment benzene conduit 1
7, new benzene corresponding to the benzene lost is replenished. The extract obtained by extracting methanol from the benzene-methanol azeotrope is mainly composed of methanol and water, contains extremely small amounts of dimethyl carbonate and benzene, and is taken out from the bottom of the countercurrent extractor via the extract extraction conduit 12. . This extract is introduced into the separator 13 either as it is or after separating trace amounts of benzene and dimethyl carbonate by a known method such as distillation. The methanol separated by separator 13 is recovered via methanol withdrawal conduit 15. Similarly, the separated water is withdrawn via the water withdrawal conduit 14 and introduced again into the countercurrent extractor 9 together with water which is replenished via the make-up water conduit 16. Example 1 75.3 kg of phenol and 19.8 kg of dimethyl carbonate were placed in 130 reaction vessels connected to a 20-stage distillation column with a column diameter of 20 cm.
2.0 kg of benzene and 8 mol of transesterification catalyst were charged and heated. The temperature of the fifth stage from the bottom of the distillation column is 82
Benzene was charged into the reactor to maintain the temperature between ~85°C. The distillate was reacted for 36 hours while being withdrawn so as to maintain the top temperature at 58°C to 59°C. The amount of benzene charged during reactive distillation was 17.5 kg. The amount of liquid distilled during reactive distillation was 10.9 kg.
Its composition was 37.6% by weight of methanol, 61.8% by weight of benzene, and 0.54% by weight of dimethyl carbonate. The distillate obtained by purging unreacted dimethyl carbonate, benzene, and methanol until the top temperature reached 150°C was 24.6 kg.
Its composition is 6.6% by weight of methanol, 51.8% by weight of benzene, and dimethyl carbonate.
It was 41.6% by weight. The distillation column was separated from the reaction vessel and simple distillation was carried out under reduced pressure at 10 Torr until the vessel temperature reached 150°C.The distillate obtained was 63.2 kg, containing 92.6% by weight of phenol and 5.7% by weight of methylphenyl carbonate.
weight percent, consisting of 1.6 weight percent diphenyl carbonate. Further, simple distillation was carried out at a temperature of 210℃, and the resulting distillate contained 2.2% by weight of phenol, 0.4% by weight of methylphenyl carbonate, and diphenyl carbonate.
It was 97.4% by weight. The reaction rate of dimethyl carbonate was 48.4%, and the selectivity between methylphenyl carbonate and dimethyl carbonate was 95%. Example 2 754 g of phenol, 180 g of dimethyl carbonate, 40 g of benzene, and 7.5 g of dibutyltin oxide were charged into a flask set at the bottom of a 20-stage older-shot column with a column diameter of 38 mm and heated. While methanol and benzene azeotrope were distilled off from the top of the tower at 58°C to 59°C, benzene was charged into the flask so that the temperature inside the flask at the bottom of the tower was 165°C. The amount of benzene charged during the 24 hour reaction period was
It is 138g. The distillate was 133 g, and its composition was 37.6% by weight of methanol, 61.9% by weight of benzene, and 0.51% by weight of dimethyl carbonate. Fractional distillation was carried out using the same column, and the initial distillate that was distilled off under normal pressure was 157g, and its composition was methanol.
2.0% by weight, benzene 61.0% by weight, and dimethyl carbonate 37.0% by weight.
Further, fractional distillation was carried out at a top pressure of 10 Torr, and the amounts of phenol, methylphenyl carbonate, and diphenyl carbonate obtained were 597 g, 151 g, and 72 g, respectively. Therefore, the reaction rate of the charged dimethyl carbonate was 67.3%, and the reaction rate of methylphenyl carbonate to diphenyl carbonate was 98%. Comparative Example 1 Phenol was added to a flask in the same column as in Example 2.
753g, dimethyl carbonate 180g, n-hexane 40g,
7.5 g of dibutyl soot oxide was charged and heated. While methanol and n-hexane azeotrope were distilled off from the top of the column at 50°C to 51°C, n-hexane was charged into the flask so that the temperature inside the flask at the bottom of the column was 165°C. 151 g of n-hexane was charged during the 24 hour reaction period. The distillate was 141 g, and its composition was 24.5 percent by weight of methanol, 66.8 percent by weight of n-hexane, and 8.6 percent by weight of dimethyl carbonate. Fractional distillation was carried out using the same column, and the initial distillate that was distilled off under normal pressure was 221 g, and its composition was 0.3% by weight of methanol, n-hexane,
56.3% by weight, and 43.4% by weight of dimethyl carbonate. Furthermore, fractional distillation was carried out at a tower top pressure of 10 Torr to produce 642 g of phenol and 111 g of methyl phenyl carbonate.
g, and 40 g of diphenyl carbonate were obtained. The reaction rate of dimethyl carbonate is 46.7%, methyl phenyl carbonate,
The selectivity for diphenyl carbonate was 98%. Example 3 Cyclohexanol was placed in a 500 ml flask set at the bottom of a 20-stage older-shot column with a column diameter of 38 mm.
201 g, dimethyl carbonate 45.5 g, benzene 40 g, and tetramethoxy titanate 0.35 g were charged and heated. Methanol and benzene azeotrope from the top of the column at 58℃
While distilling at ~59℃, the temperature at the 17th stage from the top of the column rose.
Benzene was added so that the temperature was between 82°C and 85°C. After 8 hours of reaction, purging distillation was carried out until the top temperature reached 160°C. 41 g of benzene was charged during 8 hours of reactive distillation. The distillate that was distilled off is
65g, its composition is 36.0% by weight of methanol, 63.5% by weight of benzene, and dimethyl carbonate.
It was 0.5% by weight. In addition, the distillate obtained by purging distillation after 8 hours of reactive distillation weighed 47 g, and its composition was methanol 10.0% by weight,
Benzene 84.2% by weight, dimethyl carbonate 4.1%
The weight percent of cyclohexanol was 1.7 weight percent. The liquid remaining in the flask is 216
The composition was 52.1% by weight of cyclohexanol, 5.9% by weight of methylcyclohexyl carbonate, and 41.8% by weight of dicyclohexyl carbonate. The conversion rate of dimethyl carbonate is 95 percent. Example 4 In a flask in the same column as in Example 3, 116.6 g of allyl alcohol, 45.9 g of dimethyl carbonate, and 50.3 g of benzene were added.
g and 0.35 g of tetramethoxy titanate were charged, heated, and reacted for 9.5 hours in the same manner as in Example 3, followed by expulsion distillation until the top temperature reached 98°C. The amounts and compositions of each liquid are shown in Table 1. The conversion rate of dimethyl carbonate was 93.8%. 【table】

[Brief explanation of the drawing]

FIG. 1 is a process diagram showing one embodiment of the present invention. 1... Raw material feeding conduit, 2... Benzene feeding conduit, 3... Reactive distillation column, 4... Reaction liquid withdrawal conduit, 5... Tower gas removal conduit, 6... Condenser, 7... Reflux liquid conduit , 8... distillate conduit, 9...
...Countercurrent extractor, 10...Water charging conduit, 11...
Raffinate extraction conduit, 12...Extract liquid extraction conduit, 13...Separator, 14...Water removal conduit,
15... Methanol extraction conduit, 16... Replenishment water conduit, 17... Replenishment benzene conduit.

Claims (1)

[Claims] 1 A method for producing carbonic esters by transesterifying dimethyl carbonate with a hydroxy compound in the presence of a transesterification reaction catalyst, in which benzene is used as an azeotrope forming agent with methanol, and methanol is mixed with benzene. A method for producing carbonate ester, characterized by distillation.