Title of Invention

METHOD FOR PRODUCING A DIALKYL CARBONATE AND A DIOL

Abstract

A method for producing a dialkyl carbonate and a diol, comprising: (a) effecting a transesterification reaction between a cyclic carbonate and an aliphatic monohydric alcohol in the presence of a transesterification catalyst, thereby obtaining a reaction mixture containing a product dialkyl carbonate and a product diol, (b) withdrawing a dialkyl carbonate-containing liquid from the reaction mixture, followed by separation of the dialkyl carbonate from the dialkyl carbonate-containing liquid, and (c) withdrawing a diol-containing liquid from the reaction mixture, followed by separation of the diol from the diol-containing liquid, wherein the cyclic carbonate contains a cyclic ether in an amount of from 0.1 to 3, 000 ppm by weight, and the product dialkyl carbonate contains a carbonate ether of not more than 10, 000 ppm by weight.

Full Text

TITLE OF THE INVENTION Method for producing a dialkyl carbonate and a diol Field of the Invention [0001] The present invention relates to a method for producing a dialkyl carbonate and a diol. More par- ticularly, the present invention is concerned with a method for producing a dialkyl carbonate and a diol, comprising: (a) effecting a transesterification reac- tion between a cyclic carbonate and an aliphatic mono- hydric alcohol in the presence of a transesterification catalyst, thereby obtaining a reaction mixture contain- ing a product dialkyl carbonate and a product diol, (b) withdrawing a dialkyl carbonate-containing liquid from the reaction mixture, followed by separation of the dialkyl carbonate from the dialkyl carbonate-containing liquid, and (c) withdrawing a diol-containing liquid from the reaction mixture, followed by separation of the diol from the diol-containing liquid, wherein the cyclic carbonate contains a cyclic ether in an amount of from 0.1 to 3,000 ppm by weight, and the product dialkyl carbonate contains a carbonate ether in an amount of not more than 10,000 ppm by weight. In the dialkyl carbonate produced by the method of the present invention, the content of a carbonate ether (which is a conventionally unknown impurity) is reduced to a spe- cific low range. The dialkyl carbonate obtained by the method of the present invention can be used to produce a transesterification aromatic carbonate. The trans- esterification aromatic carbonate produced can be very advantageously used to produce a colorless, high mo- lecular weight aromatic polycarbonate. Background of the Invention [0002] With respect to the method for producing a dialkyl carbonate and a diol by reacting a cyclic car- bonate with an aliphatic monohydric alcohol, various proposals have been made. Most of those proposals re- late to the development of catalysts for the above re- action. Examples of such catalysts include alkali met- als or basic compounds containing alkali metals (see Patent Documents 1 and 2), tertiary aliphatic amines (see Patent Document 3), thallium compounds (see Patent Document 4), tin alkoxides (see Patent Document 5), alkoxides of zinc, aluminum and titanium (see Patent Document 6), a mixture of a Lewis acid with a nitrogen -containing organic base (see Patent Document 7), phosphine compounds (see Patent Document 8), quaternary phosphonium salts (see Patent Document 9), cyclic amidines (see Patent Document 10), compounds of zirco- nium, titanium and tin (see Patent Document 11), a solid, strongly basic anion-exchanger containing a qua- ternary ammonium group (see Patent Document 12), a solid catalyst selected from the group consisting of a tertiary amine- or quaternary ammonium group-containing ion-exchange resin, a strongly acidic or a weakly acidic ion-exchange resin, a silica impregnated with a silicate of an alkali metal or an alkaline earth metal, and a zeolite exchanged with ammonium ion (Patent Docu- ment 13), and a homogeneous catalyst selected from the group consisting of tertiary phosphine, tertiary arsine, tertiary stibine, a divalent sulfur compound and a se- lenium compound (see Patent Document 14). [0003] With respect to the method for conducting the above-mentioned reaction between a cyclic carbonate and a diol, the below-mentioned four types of methods (1) to (4) have conventionally been proposed. Hereinbelow, explanation is made with respect to such methods (1) to (4), taking as an example the production of dimethyl carbonate and ethylene glycol by the reaction between ethylene carbonate and methanol, which is a representa- tive example of reactions between cyclic carbonates and diols. [0004] First method (hereinafter referred to as "method (1)") is a completely batchwise method in which ethylene carbonate,, methanol and a catalyst are fed to an autoclave as a batchwise reaction vessel, and a re- action is performed at a reaction temperature higher than the boiling point of methanol under pressure for a predetermined period of time (see Patent Documents 1, 2 and 5 to 9). [0005] Second method (hereinafter referred to as "method (2)") is a batchwise method, using an apparatus comprising a reaction vessel provided at an upper por- tion thereof with a distillation column, in which eth- ylene carbonate, methanol and a catalyst are fed to the reaction vessel, and a reaction is performed by heating the contents of the reaction vessel to a predetermined temperature. In this method, in order to compensate for the methanol distilled as an azeotropic mixture of the methanol and the produced dimethyl carbonate, the continuous or batchwise addition of supplemental metha- nol to the reaction vessel is optionally conducted. However, irrespective of whether or not such an addi- tion of supplemental methanol is conducted, the reac- tion per se is performed only in a batch-type reaction vessel. That is, in this method, the reaction is batchwise performed under reflux for a prolonged period of time as long as 3 to 20 odd hours. (Patent Docu- ments 15, 3 and 4). [0006] Third method (hereinafter referred to as "method (3)") is a liquid flow method in which a solu- tion of ethylene carbonate in methanol is continuously fed to a tubular reactor to perform a reaction at a predetermined reaction temperature in the tubular reac- tor, and the resultant reaction mixture in a liquid form containing the unreacted materials (i.e., ethylene carbonate and methanol) and the reaction products (i.e., dimethyl carbonate and ethylene glycol) is continuously withdrawn through an outlet of the reactor. This method has conventionally been conducted in two differ- ent manners in accordance with the two types of cata- lyst used. That is, one of the manners consists in passing a mixture of a solution of ethylene carbonate in methanol and a solution of a homogenous catalyst in a solvent through a tubular reactor to perform a reac- tion, thereby obtaining a reaction mixture, and sepa- rating the catalyst from the obtained reaction mixture (see Patent Documents 11 and 14). The other manner consists in performing the reaction in a tubular reac- tor having a heterogeneous catalyst securely placed therein (see Patent Documents 12 and 13). Since the reaction between ethylene carbonate and methanol to produce dimethyl carbonate and ethylene glycol is an equilibrium reaction, the flow method using a tubular reactor cannot achieve a higher conversion of ethylene carbonate than the conversion of ethylene carbonate at the equilibrium state of reaction (the latter conver- sion depends on the composition ratio of the feedstocks fed to the reactor and the reaction temperature). For example, in Example 1 of Patent Document 11 which is directed to a continuous flow reaction method using a tubular reactor and wherein the flow reaction is con- ducted at 130 o C using a feedstock mixture having a methanol/ethylene carbonate molar ratio of 4/1, the conversion of ethylene carbonate is only 25 %. This means that large amounts of unreacted ethylene carbon- ate and unreacted methanol, which are contained in the reaction mixture, need to be separated and recovered, which in turn are recycled to the reactor. Actually, in the method disclosed in Patent Document 13, various apparatuses are used for the separation, purification, recovery and recycling of the unreacted compounds. [0007] Fourth method (hereinafter referred to as "method (4)") is a reactive distillation method in which each of ethylene carbonate and methanol is con- tinuously fed to a multi-stage distillation column to perform a reaction in a plurality of stages of the dis- tillation column in the presence of a catalyst, while continuously effecting separation between the produced dimethyl carbonate ,and the produced ethylene glycol (Patent Documents 16 to 19). [0008] Thus, the conventional methods for producing a dialkyl carbonate and a diol by reacting a cyclic car- bonate with an aliphatic monohydric alcohol, can be classified into the following four methods: (1) A completely batchwise method; (2) A batchwise method using a reaction vessel pro- vided at an upper portion thereof with a distillation column; (3) A liquid flow method using a tubular reactor; and (4) A reactive distillation method. However, the above-mentioned conventional methods (1) to (4) have their respective problems as described below. [0009] In the case of each of the completely batchwise method (1) and the flow method (3) using a tubular re- actor, the maximum conversion of a cyclic carbonate depends on the composition ratio of the feedstocks fed to the reactor and the reaction temperature. Therefore, it is impossible to convert all of the feedstocks into the products and the conversion of the cyclic carbonate becomes low. Further, in the batchwise method (2), for improving the conversion of a cyclic carbonate, the produced dialkyl carbonate must be removed using a largely excess amount of an aliphatic monohydric alco- hol, and a long reaction time is needed. [0010] In the case of the reactive distillation method (4), it is possible to perform a reaction with high conversion, as compared to the conversions in methods (1), (2) and (3). However, needless to say, even in the case of method (4), production of a dialkyl carbon- ate and a diol is performed by a reversible, equilib- rium reaction. Accordingly, even when it is possible to achieve a substantially 100 % conversion of a cyclic carbonate by method (4), it is impossible to prevent a trace amount of the cyclic carbonate from remaining unreacted in a produced diol. Therefore, for obtaining a high purity diol by method (4), in general, it is necessary to separate the cyclic carbonate from the diol by performing a distillation under strictly con- trolled conditions. In Patent Document 20, it is at- tempted to solve this problem by hydrolyzing the unre- acted cyclic carbonate to convert it into a diol. Further, in Patent Document 21, it is attempted to solve this problem by reacting the unreacted cyclic carbonate with a diol to convert it into an ether. [0011] In addition, there have been proposed methods in which water or water having an oxygen content of not more than 100 ppm is introduced into the process for distillation purification of a diol, to thereby obtain a high purity diol having high UV transmission or low aldehyde content (see Patent Documents 22 and 23). [0012] In the method for producing a dialkyl carbonate and a diol by reacting a cyclic carbonate with an ali- phatic monohydric alcohol, the presence of a carbonate ether described in the present invention was not known in the art. The present inventors have for the first time found that a dialkyl carbonate obtained by the above-mentioned method contains a carbonate ether and that when a dialkyl carbonate contains a carbonate ether in an amount exceeding a specific level, the re- action using such a dialkyl carbonate as a raw material poses various problems. For example, it has become ap- parent that when a transesterification aromatic carbon- ate is produced from such a conventional dialkyl car- bonate and phenol, the produced aromatic carbonate will contain impurities. [0013] As can be understood from the above, no method has heretofore been proposed for producing a dialkyl carbonate and a diol from a cyclic carbonate and an aliphatic monohydric alcohol, wherein the produced dialkyl carbonate contains a carbonate ether only in a content which is reduced to a specific low range. [0014] Patent, Document 1: U.S. Patent No. 3,642,858 Patent Document 2: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Sho 54-48715 (corresponding to U.S. Patent No. 4,181,676) Patent Document 3: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Sho 51-122025 (corresponding to U.S. Patent No. 4,062,884) Patent Document 4: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Sho 54-48716 (corresponding to U.S. Patent No. 4,307,032) Patent Document 5: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Sho 54-63023 Patent Document 6: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Sho 54-148726 Patent Document 7: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Sho 55-64550 Patent Document 8: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Sho 55-64551 Patent Document 9: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Sho 56-10144 Patent Document 10: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Sho 59-106436 Patent Document 11: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Sho 63-41432 (corresponding to U.S. Patent No. 4,661,609) Patent Document, 12: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Sho 63-238043 Patent Document 13: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Sho 64-31737 (corresponding to U.S. Patent No. 4,691,041) Patent Document 14: U.S. Patent No. 4,734,518 Patent Document 15: U.S. Patent No. 3,803,201 Patent Document 16: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Hei 4-198141 Patent Document 17: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Hei 4-230243 Patent Document 18: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Hei 5-213830 (corresponding to German Patent No. 4,129,316) Patent Document 19: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. Hei 6-9507 (cor- responding to German Patent No. 4,216,121) Patent Document 20: International Publication No. WO 97/23445 Patent Document 21: International Publication No. WO 00/51954 Patent Document 22: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. 2002-308804 Patent Document 23: Unexamined Japanese Patent Ap- plication Laid-Open Specification No. 2004-131394 Disclosure of the Invention Problems to Be Solved by the Invention [0015] An object of the present invention is to pro- vide a method for producing a dialkyl carbonate con- taining a carbonate ether (which is a conventionally unknown impurity) only in an amount which is reduced to a specific low range, wherein the dialkyl carbonate can be used to produce a transesterification aromatic car- bonate which can be very advantageously used to produce a colorless, high molecular weight aromatic polycarbon- ate . Means to Solve the Problems [0016] In this situation, for solving the above -mentioned problems, the present inventors have made extensive and intensive studies. As a result, they have unexpectedly found that, in a method for producing a dialkyl carbonate and a diol, comprising effecting a transesterification reaction between a cyclic carbonate and an aliphatic monohydric alcohol in the presence of a transesterification catalyst, thereby producing a product dialkyl carbonate and a product diol, when the cyclic carbonate as a raw material contains a cyclic ether, a carbonate ether is by-produced and enters the dialkyl carborate produced. They have also found that, by reducing the cyclic ether content of the cyclic car- bonate as a raw material, the carbonate ether content of the dialkyl carbonate produced can be reduced. Spe- cifically, the present inventors have found that the above-mentioned problems can be solved by a method for producing a dialkyl carbonate and a diol, comprising: (a) effecting a transesterification reaction between a cyclic carbonate and an aliphatic monohydric alcohol in the presence of a transesterification catalyst, thereby obtaining a reaction mixture containing a product dial- kyl carbonate and a product diol, (b) withdrawing a dialkyl carbonate-containing liquid from the reaction mixture, followed by separation of the dialkyl carbon- ate from the dialkyl carbonate-containing liquid, and (c) withdrawing a diol-containing liquid from the reac- tion mixture, followed by separation of the diol from the diol-containing liquid, wherein the cyclic carbon- ate contains a cyclic ether in an amount of from 0.1 to 3,000 ppm by weight, and the product dialkyl carbonate contains a carbonate ether in an amount of not more than 10,000 ppm by weight. Based on these findings, the present invention has been completed. [0017] The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description taken in con- nection with the accompanying drawing, and the appended claims. Effects of the Invention [0018] In the dialkyl carbonate produced by the method of the present invention, the content of a carbonate ether (which is a conventionally unknown impurity) is reduced to a specific low range. The dialkyl carbonate obtained by the method of the present invention can be used to produce a transesterification aromatic carbon- ate. The transesterification aromatic carbonate pro- duced can be very advantageously used to produce a col- orless, high molecular weight aromatic polycarbonate. Brief Description of the Drawing [0019] [Fig. 1] A diagram showing the system which was used for practicing the Examples and Comparative Example of the present application. Description of Reference Numerals [0020] 1 continuous multi-stage distillation column 3, 6 preheater 4, 18, 42, 72 top of column 7, 19, 45, 75 condenser 10, 26, 43, 73 bottom of column 12, 28, 50, 80 reboiler 17 low boiling point mixture separation column 22 carbon dioxide separation column 41 diol (ethylene glycol (EG)) purification column 71 DMC separation column 2, 2', 5, 8, 9, 11, 13, 14, 16, 20, 21, 23, 24, 25, 27, 29, 30, 46, 47, 48, 49, 51, 52, 56, 76, 77, 78, 79, 81, 82 conduit Best Mode for Carrying Out the Invention [0021] According to the present invention, there is provided a method for producing a dialkyl carbonate and a diol, comprising: (a) effecting a transesterification reaction be- tween a cyclic carbonate and an aliphatic monohydric alcohol in the presence of a transesterification cata- lyst, thereby obtaining a reaction mixture containing a product dialkyl carbonate and a product diol, (b) withdrawing a dialkyl carbonate-containing liquid from the reaction mixture, followed by separa- tion of the dialkyl carbonate from the dialkyl carbon- ate-containing liquid, and (c) withdrawing a diol-containing liquid from the reaction mixture, followed by separation of the diol from the diol-containing liquid, the steps (b) and (c) being performed in either order or simultaneously, wherein: the cyclic carbonate contains a cyclic ether rep- resented by the formula (1) below in an amount of from 0.1 to 3,000 ppm by weight, and the product dialkyl carbonate contains a carbonate ether represented by the formula (2) below in an amount of not more than 10,000 ppm by weight, [0022] wherein R1 represents a divalent group repre- sented by the formula: -(CH2)m- wherein m is an integer of from 2 to 6, and at least one hydrogen atom of R1 is optionally replaced by at least one substituent group selected from the group consisting of a C1-10 alkyl group and a C6-10 aryl group, and [0023] wherein R1 is as defined above for formula (1), R2 represents a C1-12 monovalent ali- phatic group, and at least one hydrogen atom of R2 is optionally replaced by at least one substituent group selected from the group consisting of a C1-10 alkyl group and a C6-10 aryl group. [0024] For easier understanding of the present inven- tion, the essential features and various preferred em- bodiments of the present invention are enumerated below. [0025] 1. A method for producing a dialkyl carbonate and a diol, comprising: (a) effecting a transesterification reaction be- tween a cyclic carbonate and an aliphatic monohydric alcohol in the presence of a transesterification cata- lyst, thereby obtaining a reaction mixture containing a product dialkyl carbonate and a product diol, (b) withdrawing a dialkyl carbonate-containing liquid from the reaction mixture, followed by separa- tion of the dialkyl carbonate from the dialkyl carbon- ate-containing liquid, and (c) withdrawing a diol-containing liquid from the reaction mixture, followed by separation of the diol from the diol-containing liquid, the steps (b) and (c) being performed in either order or simultaneously, wherein: the cyclic carbonate contains a cyclic ether rep- resented by the formula (1) below in an amount of from 0.1 to 3,000 ppm by weight, and the product dialkyl carbonate contains a carbonate ether represented by the formula (2) below in an amount of not more than 10,000 ppm by weight, [0026] [0027] wherein R1 represents a divalent group represented by the formula: -(CH2)m- wherein m is an integer of from 2 to 6, and at least one hydrogen atom of R1 is optionally re- placed by at least one substituent group se- lected from the group consisting of a C1-10 alkyl group and a C6-C10 aryl group, and [0028] wherein R1 is as defined above for formula (1), R2 represents a C1-12 monovalent ali- phatic group, and at least one hydrogen atom of R2 is optionally replaced by at least one substituent group selected from the group consisting of a a c1-10 alkyl group and a c6-10 aryl group. [0029] 2. The method according to item 1 above, wherein the amount of the cyclic ether in the cyclic carbonate is from 3 to 1,500 ppm by weight. [0030] 3. The method according to item 2 above, wherein the amount of the cyclic ether in the cyclic carbonate is from 10 to 1,000 ppm by weight. [0031] 4. The method according to any one of items 1 to 3 above, wherein the cyclic carbonate is ethylene carbonate. [0032] 5. The method according to any one of items 1 to 4 above, wherein the transesterification reaction is performed in a reactive distillation column. [0033] 6. A dialkyl carbonate produced by the method of any one of items 1 to 5 above, which contains a car- bonate ether represented by the formula (2) of item 1 above in an amount of from 1 to 10,000 ppm by weight. [0034] 7. The dialkyl carbonate according to item 6 above, wherein the amount of the carbonate ether in the dialkyl carbonate is from 3 to 5,000 ppm by weight. [0035] 8. The dialkyl carbonate according to item 7 above, wherein the amount of the carbonate ether in the dialkyl carbonate is from 10 to 3,000 ppm by weight. [0036] Hereinbelow, the present invention will be ex- plained in detail. The reaction performed in the present invention is a reversible, equilibrium transesterification reaction represented by the following reaction scheme (I), in which a dialkyl carbonate (C) and a diol (D) are pro- duced from a cyclic carbonate (A) and an aliphatic monohydric alcohol (B): [0037] [0038] wherein R1 and R2 are as defined above. [0039] In the present invention, the reason why the carbonate ether content of the product dialkyl carbon- ate is reduced to a specific low range has not yet been fully elucidated. However, it is considered that a carbonate ether is generated in the reaction system by the reaction between a cyclic ether and an aliphatic monohydric alcohol or a carbonate and, therefore, the amount of generated carbonate ether can be reduced by lowering the cyclic ether content of the cyclic carbon- ate. [0040] In the present invention, the transesterifica- tion reaction can be performed by a conventional trans- esterif ication reactor. With respect to the trans- esterif ication reactor used in the method of the present invention, there is no particular limitation, and there can be used any of various conventional types of reactors, such as a reactor comprising a combination of a fixed bed reactor and a distillation column, an agitation vessel, a multi-stage agitation type reactor comprising multiple agitation vessels, a multi-stage distillation column, and a combination of the above -mentioned reactors. These reactors can be used in ei- ther a batchwise manner or a continuous manner. From the viewpoint of displacing the equilibrium of the re- action toward the product system, a multi-stage distil- lation column is preferred, and a continuous multi -stage distillation column is more preferred. With re- spect to the multi-stage distillation column, there is no particular limitation as long as it is a distilla- tion column which has two or more stages of distilla- tion and which is capable of continuous distillation. In the present invention, the term "stages" is intended to include theoretical stages (theoretical plates). In the case of a distillation column having no substantive stages, such as a packed column, the value obtained by dividing the packing height by the height per theoreti- cal stage (plate) (H.E.T.P.) (height equivalent to a theoretical plate) is considered as the number of stages. [0041] Examples of such continuous multi-stage distil- lation columns include plate type columns using a tray, such as a bubble-cap tray, a sieve tray, a valve tray or a counterflow tray, and packed type columns packed with various packings, such as a Raschig ring, a Less- ing ring, a Pall ring, a Berl saddle, an Interlox sad- dle, a Dixon packing, a McMahon packing, a Heli pack, a Sulzer packing and Mellapak. Any column which is gen- erally used as a continuous multi-stage distillation column can be utilized. Further, a mixed type of plate column and packed column, which comprises both a plate portion and a portion packed with packings, can also be preferably used. When a solid catalyst which is in- soluble in the liquid phase in a distillation column is used, a packed column type distillation column, in which the solid catalyst is used in substitution for part or all of the packings, is preferably employed. As the continuous multi-stage distillation column to be used in the method of the present invention, the above -mentioned distillation columns can be used individu- ally or in combination. When used in combination, a plurality of distillation columns may be connected in series or in parallel. [0042] A cyclic carbonate used as a raw material in the method of the present invention is represented by formula (A) in reaction scheme (I) mentioned above. Examples of cyclic carbonates include alkylene carbon- ates, such as ethylene carbonate and propylene carbon- ate, 1,3-dioxacyclohexa-2-one, 1,3-dioxacyclohepta-2- one, and the like. Of these cyclic carbonates, ethyl- ene carbonate and propylene carbonate are preferred because of their good availability. Ethylene carbonate is most preferred. [0043] In the cyclic carbonate used in the present in- vention, the content of a cyclic ether represented by the formula (1) above is preferably 3,000 ppm or less. The lower the cyclic ether content of the cyclic car- bonate, the lower the carbonate ether content of the dialkyl carbonate produced. However, for producing a cyclic carbonate having extremely low cyclic ether con- tent, the cyclic carbonate must be produced by a method employing a special purification method. Therefore, in the present invention, the cyclic ether content of the cyclic carbonate is in the range of from 0.1 to 3,000 ppm, preferably from 1 to 3,000 ppm, more preferably from 3 to 1,500 ppm, most preferably from 10 to 1,000 ppm. [0044] With respect to the cyclic ether content of a cyclic carbonate obtained in the prior art, attention is drawn to the following. When ethylene carbonate (which is a cyclic carbonate) is produced from ethylene oxide (which is a cyclic ether), it is generally known that the conversion of ethylene oxide is 99.0 %, and the selectivity for ethylene carbonate is 99.2 % (see, e.g., U.S. Patent Nos. 4,166,773, 4,786,741 and 4,314,945). That is, in the prior art, the ethylene carbonate (cyclic carbonate) produced inevitably con- tains about 5,000 ppm of ethylene oxide (cyclic ether) which remains unreacted. Therefore, in the prior art, it is impossible to obtain the cyclic carbonate used in the method of the present invention, the cyclic carbon- ate having the cyclic ether content reduced to the above-mentioned specific range (i.e., 0.1 to 3,000 ppm). [0045] In the present invention, when ethylene carbon- ate is used as a cyclic carbonate, the ethylene carbon- ate can be produced by reacting ethylene oxide (which is a cyclic ether) with carbon dioxide. As an example of a method for producing ethylene carbonate which has the cyclic ether content reduced to the above-mentioned specific range, there can be mentioned a method in which ethylene carbonate is produced by reacting ethyl- ene oxide with carbon dioxide, wherein the ethylene oxide content of the ethylene carbonate produced is reduced by any of the following methods (a) to (6): (a) a method in which a so-called finisher (which is a small reactor) is additionally provided in the produc- tion plant for ethylene carbonate, and ethylene oxide is reacted with carbon dioxide at a conversion as high as 99.5 % or more while maintaining the selectivity for ethylene carbonate at 99.0 % or more; (β) a method in which, immediately after a reaction mixture containing ethylene carbonate is taken out from the reactor, a portion of the reaction mixture is distilled off to the outside of the reaction system simultaneously with evaporation-off of carbon dioxide gas; (γ) a method in which, before performing the purification of a reaction mixture containing ethylene carbonate, unreacted ethyl- ene oxide is distilled off from the reaction mixture by using a flash tank or the like; and (δ) a method in which a portion of a column top composition (containing unreacted ethylene oxide) obtained at the top of a dis- tillation column for performing the purification of ethylene carbonate is taken out of the reaction system. The above-mentioned methods (α) to (δ) may be used in- dividually; however, from the viewpoint of ease in re- ducing the cyclic ether content of a cyclic carbonate, it is preferred to use the above-mentioned methods (α) to (δ) in combination. [0046] Specific examples of cyclic ethers include eth- ylene oxide, propylene oxide, oxacyclobutane and oxacy- clopentane. [0047] An aliphatic monohydric alcohol used as another raw material in the method of the present invention is a compound which is represented by formula (B) in reac- tion scheme (I) mentioned above and has a boiling point lower than that of the diol. The type of an aliphatic monohydric alcohol which can be used in the method of the present invention varies depending on the type of the cyclic carbonate used. Examples of aliphatic mono- hydric alcohols include methanol, ethanol, propanol (isomers), allyl alcohol, butanol (isomers), 3-butene- 1-ol, amyl alcohol (isomers), hexyl alcohol (isomers), heptyl alcohol (isomers), octyl alcohol (isomers), nonyl alcohol (isomers), decyl alcohol (isomers), unde- cyl alcohol (isomers), dodecyl alcohol (isomers), cyclopentanol, cyclohexanol, cycloheptanol, cyclooctanol, methylcyclopentanol (isomers), ethyl- cyclopentanol (isomers), methylcyclohexanol (isomers), ethylcyclohexanol (isomers), dimethylcyclohexanol (iso- mers), diethylcyclohexanol (isomers), phenylcyclohexa- nol (isomers), benzyl alcohol, phenethyl alcohol (iso- mers ), phenylpropanol (isomers) and the like. The above-mentioned aliphatic monohydric alcohol may be substituted with at least one substituent, such as a halogen atom, a lower alkoxy group, a cyano group, an alkoxycarbonyl group, an aryloxycarbonyl group, an acyloxy group or a nitro group. [0048] Of these aliphatic monohydric alcohols, an al- cohol having 1 to 6 carbon atoms is preferred. More preferred are monohydric alcohols having 1 to 4 carbon atoms, i.e., methanol, ethanol, propanol (isomers) and butanol (isomers). When ethylene carbonate or propyl- ene carbonate is used as a cyclic carbonate, methanol and ethanol are preferred, and especially preferred is methanol. [0049] In the method of the present invention, the transesterification reaction is performed in the pres- ence of a transesterification catalyst. The method for causing a transesterification catalyst to be present in the reactor is not particularly limited. For example, a homogeneous catalyst which is soluble in the reaction system under the reaction conditions can be caused to be present in the transesterification reactor by con- tinuously feeding the homogeneous catalyst to the transesterification reactor. Alternatively, a hetero- geneous catalyst (solid catalyst) which is insoluble in the reaction system under the reaction conditions, can be caused to be present in the transesterification re- actor by packing the solid catalyst in the transesteri- fication reactor. The above-mentioned homogeneous and heterogeneous catalysts can be used in combination. [0050] When a homogeneous catalyst is continuously fed to the transesterification reactor, it may be fed to the reactor together with a feedstock cyclic carbonate and/or a feedstock aliphatic monohydric alcohol. Al- ternatively, the homogeneous catalyst may be fed to the reactor at a position different from that at which the feedstock is fed. Further, when a multi-stage distil- lation column is used as a transesterification reactor, the homogeneous catalyst can be fed to the distillation column at any position as long as the position is at least one theoretical stage (plate) above the column bottom. [0051] However, since the region where the reaction actually takes place in the continuous multi-stage dis- tillation column is generally below the position at which the homogeneous catalyst is fed, it is preferred that the homogeneous catalyst is fed to the distilla- tion column at a position between the top of the column and the position at which the feedstock is fed. [0052] When a heterogeneous solid catalyst is used as a catalyst, the catalyst can be packed in a desired amount at a desired position of the reactor. When a multi-stage distillation column is used as a trans- esterif ication reactor, the catalyst layer present in the column has a height which corresponds to at least one theoretical stage (plate), preferably two or more theoretical stages (plates). A catalyst which can serve as a packing for the multi-stage distillation column can also be used. [0053] As a transesterification catalyst used in the present invention, various types of known catalysts can be used. Examples of such catalysts include alkali metals or alkaline earth metals, such as lithium, so- dium, potassium, rubidium, cesium, magnesium, calcium, strontium and barium; basic compounds, such as hydrides, hydroxides, alkoxides, aryloxides and amides of alkali metals or alkaline earth metals; basic compounds, such as carbonates and hydrogencarbonates of alkali metals or alkaline earth metals and alkali metal or alkaline earth metal salts of organic acids; tertiary amines, such as triethylamine, tributylamine, trihexylamine and benzyldiethylamine; nitrogen-containing heteroaromatic compounds, such as N-alkylpyrrole, N-alkylindole, oxa- zole, N-alkylimidazole, N-alkylpyrazole, oxadiazole, pyridine, alkylpyridine, quinoline, alkylquinoline, isoquinoline, alkylisoquinoline, acridine, alkyl- acridine, phenanthroline, alkylphenanthroline, pyrimidine, alkylpyrimidine, pyradine, alkylpyradine, triazine and alkyltriazine; cyclic amidines, such as diazabicycloundecene (DBU) and diazabicyclononene (DBN); thallium compounds, such as thallium oxide, thallium halides, thallium hydroxide, thallium carbon- ate, thallium nitrate, thallium sulfate and thallium salts of organic acids; tin compounds, such as tri- butylmethoxytin, tributylethoxytin, dibutyldimethoxytin, diethyldiethoxytin, dibutyldiethoxytin, dibutylphenoxy- tin, diphenylmethoxytin, dibutyltin acetate, tributyl- tin chloride and tin 2-ethylhexanoate; zinc compounds, such as dimethoxyzinc, diethoxyzinc, ethylenedioxyzinc and dibutoxyzinc; aluminum compounds, such as aluminum trimethoxide, aluminum triisopropoxide and aluminum tributoxide; titanium compounds, such as tetramethoxy- tItanium, tetraethoxytitanium, tetrabutoxytitanium, dichlorodimethoxytitanium, tetraisopropoxytitanium, titanium acetate and titanium acetylacetonate; phospho- rus compounds, such as trimethylphosphine, triethyl- phosphine, tributylphosphine, triphenylphosphine, tributylmethylphosphonium halides, trioctylbutylphos- phonium halides and triphenylmethylphosphonium halides• zirconium compounds, such as zirconium halides, zirco- nium acetylacetonate, zirconium alkoxides and zirconium acetate; lead and lead-containing compounds, e.g., lead oxides, such as PbO, PbO2 and Pb3O4; lead sulfides, such as PbS, Pb2S3 and PbS2; lead hydroxides, such as Pb(OH)2, Pb3O2(OH)2, Pb2[PbO2(OH)2] and Pb2O(OH)2; plumbites, such as Na2PbO2, K2PbO2, NaHPbO2 and KHPbO2; plurnbates, such as Na2PbO3, Na2H2PbO4, K2PbO3, K2[Pb(OH)6], K4PbO4, Ca2PbO4 and CaPbO3; lead carbonates and basic salts thereof, such as PbCO3 and 2PbCO3 • Pb(OH)2; alkoxylead compounds and aryloxylead compounds, such as Pb(OCH3)2, (CH3O)Pb(OPh) and Pb(OPh)2; lead salts of organic acids, and carbonates and basic salts thereof, such as Pb(OCOCH3)2, Pb(OCOCH3)4 and Pb(OCOCH3)2•PbO-3H2O; organolead compounds, such as Bu4Pb, Ph4Pb, Bu3PbCl, Ph3PbBr, Ph3Pb (or Ph6Pb2), Bu3PbOH and Ph2PbO wherein Bu represents a butyl group and Ph represents a phenyl group; lead alloys, such as Pb-Na, Pb-Ca, Pb-Ba, Pb-Sn and Pb-Sb; lead minerals, such as galena and zinc blende; hydrates of these lead compounds; ion -exchangers, such as anion-exchange resins having -tertiary amino groups, ion-exchange resins having am- ide groups, ion-exchange resins having at least one type of ion-exchange group selected from the group con- sisting of sulfonate, carboxylate and phosphate groups, and strongly basic solid anion-exchangers having qua- ternary ammonium groups as ion-exchange groups; and solid inorganic compounds, such as silica, silica- alumina, silica-magnesia, aluminosilicate, gallium silicate, various types of zeolites, various types of metal -exchanged zeolites and ammonium-exchanged zeolites. [0054] Among the above-mentioned solid catalysts, strongly basic anion-exchangers having quaternary ammo- nium groups as anion-exchange groups are preferred. Examples of such anion-exchangers include strongly ba- sic anion-exchange resins having quaternary ammonium groups as anion-exchange groups, cellulose type strongly basic anion-exchangers having quaternary ammo- nium groups as anion-exchange groups, and strongly ba- sic anion-exchangers carried on an inorganic carrier which have quaternary ammonium groups as anion-exchange groups. [0055] Of these strongly basic anion-exchange resins having quaternary ammonium groups as ion-exchange groups, styrene type strongly basic anion-exchange res- ins and the like are preferred. A styrene type strongly basic anion-exchange resin is comprised of a styrene/divinyl-benzene copolymer as a base resin, and quaternary ammonium groups (type I or type II) as an- ion-exchange groups, examples of which are diagrammati- cally represented by the following formulae (II). [0056] [0057] In above formulae (II), X represents an anion. Generally, X is at least one type of anion selected from the group consisting of F-, Cl-, Br-, I-, HCO3-, CO32-, CH3CO2-, HCO2-, IO3-, BrO3- and ClO3-. It is pre- ferred that X is selected from the group consisting of Cl-, Br-, HCO3- and CO32-. With respect to the structure of the base resin of the anion-exchange resin, either a gel type or a macroreticular type (MR type) can be used. However, because of the high resistance to organic sol- vents, the MR type is preferred. [0058] Examples of cellulose type strongly basic anion -exchangers having quaternary ammonium groups as ion -exchange groups include cellulose type strongly basic anion-exchangers having ion-exchange groups of the structure represented by the formula: -OCH2CH2NR3X, which exchangers are obtained by trialkylaminoethy- lation of a part or all of the hydroxyl groups of cel- lulose. In the above formula, R represents an alkyl group, for example, a methyl group, an ethyl group, a propyl group, a butyl group, preferably a methyl group or an ethyl group; and X is as defined above. [0059] The inorganic carrier-carried strongly basic anion-exchanger usable in the present invention, which has quaternary ammonium groups as ion-exchange groups, is an anion-exchanger having quaternary ammonium groups represented by the formula -O(CH2)nNR3X wherein R and X are as defined above and n is usually an integer of from 1 to 6, preferably 2, which anion-exchanger can be prepared by the modification of a part or all of the hydroxyl groups on the surface of the inorganic carrier. Examples of inorganic carriers include silica, alumina, silica-alumina, titania and zeolite. Of these, silica, alumina and silica-alumina are preferred. Silica is most preferred. [0060] There is no limitation with respect to the method for the modification of hydroxyl groups on the surface of the inorganic carrier. For example, such a strongly basic anion-exchanger carried on an inorganic carrier can be obtained by subjecting an inorganic car- rier and an aminoalcohol represented by the formula HO(CH2)nNR2 to a dehydration reaction between them in the presence of a basic catalyst to thereby effect ami- noalkoxylation, followed by the reaction of the resul- tant aminoalkoxylated inorganic carrier with an alkyl halide represented by the formula RX', wherein X' represents a halogen atom, preferably Cl, Br or I, to thereby convert the aminoalkoxy group into a -O(CH2)nNR3X' group. The -O(CH2)nNR3X' group is further converted to a -O(CH2)nNR3X group having the desired anion X by an anion exchange reaction. When n is 2, an inorganic carrier is treated with N,N-dialkylaziridine so that the hydroxyl groups on the inorganic carrier are N,N-dialkylaminoethoxylated to obtain a -OCH2CH2NR2 group, which is then converted to a -O(CH2)nNR3X group by the above-mentioned method. [0061] Commercially available solid, strongly basic anion-exchangers having quaternary ammonium groups as ion-exchange groups can be used in the present inven- tion. When a commercially available solid, strongly basic anion-exchanger is used, it can be treated for anion-exchange with a desired anion species before it is used as a transesterification catalyst. [0062] A solid catalyst comprised of a macroreticular or gel type organic polymer or an inorganic carrier, each having bonded thereto a heterocyclic group con- taining at least one nitrogen atom, is preferably used as a transesterification catalyst. Further, the above -mentioned solid catalyst can be treated for quater- narizing a part or all of the nitrogen-containing het- erocyclic groups before it is used. [0063] The amount of the transesterification catalyst to be used in the present invention varies depending on the type thereof. The homogeneous catalyst, which is soluble in the reaction system under the reaction con- ditions, is fed continuously in an amount of from 0.0001 to 50 % by weight, based on the total weight of the raw material cyclic carbonate and the raw material aliphatic monohydric alcohol. When the solid catalyst is used, it is used generally in an amount of from 10 to 95 % by volume, preferably from 50 to 90 % by volume, based on the internal volume of the reactor. When the solid catalyst is packed in a distillation column, it is packed preferably in an amount of from 0.01 to 75 % by volume, based on the internal volume of the empty distillation column. [0064] When a multi-stage distillation column is used as a transesterification reactor, there is no particu- lar restriction with respect to the method for continu- ously feeding a cyclic carbonate and an aliphatic mono- hydric alcohol to the continuous multi-stage distillation column. Any feeding method can be used as long as the feedstocks can be contacted with the cata- lyst in a region of the distillation column which cor- responds to at least one stage, preferably at least two stages. That is, the cyclic carbonate and the ali- phatic monohydric alcohol can be continuously fed to one or more stages of the continuous multi-stage dis- tillation column through a desired number of feeding pipes at one or more desired locations as long as the above requirement is satisfied. The cyclic carbonate and the monohydric alcohol may be fed either to the same stage of the distillation column or to separate stages individually. [0065] The feedstocks are continuously fed in a liquid form, a gaseous form or a gas-liquid mixture form. In addition to the feeding of the feedstocks to the con- tinuous multi-stage distillation column as described above, additional feedstocks can be fed in a gaseous form to the lower portion of the distillation column intermittently or continuously. Also preferred is a method wherein the cyclic carbonate is continuously fed in a liquid form or a gas-liquid mixture form to a stage at a level higher than the stage where the cata- lyst is present, while the aliphatic monohydric alcohol is continuously fed to the lower portion of the distil- lation column in a gaseous form or a gas-liquid mixture form, or in a gaseous form and in a liquid form indi- vidually. In this case, some of the aliphatic mono- hydric alcohol may be contained in the cyclic carbonate. [0066] In the present invention, a small amount of a diol as a desired product may be contained in the raw materials. Further, the aliphatic monohydric alcohol may contain a concomitant dialkyl carbonate. When the aliphatic monohydric alcohol contains a concomitant dialkyl carbonate, the amount of the concomitant dial- kyl carbonate in the aliphatic monohydric alcohol is generally in the range of from 0 to 40 % by weight, preferably from 0.1 to 30 % by weight, more preferably from 1 to 20 % by weight, based on the total weight of the aliphatic monohydric alcohol and the concomitant dialkyl carbonate. [0067] The ratio of the aliphatic monohydric alcohol to the cyclic carbonate to be fed to the transesterifi- cation reactor may vary depending on the type and quan- tity of the transesterification catalyst and the reac- tion conditions, but, in general, the molar ratio of the aliphatic monohydric alcohol to the cyclic carbon- ate may be in the range of from 0.01 to 1,000. For in- creasing the conversion of the cyclic carbonate, it is preferred to feed the aliphatic monohydric alcohol in an excess amount which is 2 times or more by mole the mole of the cyclic carbonate. However, too high a con- centration of the aliphatic monohydric alcohol is unde- sirable because the size of the reaction equipment needs to be large. Therefore, it is especially pre- ferred to use the aliphatic monohydric alcohol in an amount which is 2 to 20 times by mole the mole of the cyclic carbonate. [0068] When carbon dioxide is present in a high con- centration in the transesterification reaction system of the method of the present invention, the reaction rate becomes low. Therefore, the CO2 concentration of the reaction system is generally not higher than 500 ppm by weight, preferably not higher than 200 ppm by weight, more preferably not higher than 100 ppm by weight. [0069] Also when water is present in a high concentra- tion in the transesterification reaction system of the method of the present invention, hydrolysis of a cyclic carbonate takes place simultaneously with the trans- esterif ication, resulting in a decrease in the selec- tivity for dialkyl carbonate. Therefore, the water concentration of the reaction system is generally not higher than 200 ppm by weight, preferably not higher than 100 ppm by weight. [0070] In the method of the present invention, when it is attempted to render the conversion of the cyclic carbonate close to 100 %, the reaction time needs to be prolonged, and the aliphatic monohydric alcohol needs to be used in large excess. On the other hand, when the conversion of the cyclic carbonate is too low, the size of the apparatus used for separation removal of the unreacted cyclic carbonate needs to be large. Therefore, the conversion of the cyclic carbonate is generally in the range of from 95 to 99.999 %, prefera- bly from 98 to 99.99 %, more preferably from 99 to 99.99 %. [0071] In the present invention, the dialkyl carbonate which is one of the products of the present invention is withdrawn from the transesterification reactor. In general, the dialkyl carbonate is withdrawn from the upper portion of the transesterification reactor as a low boiling point mixture in a gaseous form. The low boiling point mixture withdrawn from the upper portion of the reactor may be the dialkyl carbonate alone or a mixture of the dialkyl carbonate and the aliphatic monohydric alcohol. Further, the withdrawn gaseous mixture may also contain a high boiling point product in a small amount. [0072] When a multi-stage distillation column is used as a transesterification reactor, a withdrawal port of the continuous multi-stage distillation column for withdrawing the gaseous low boiling point mixture con- taining the dialkyl carbonate is preferably provided at a position between the position from which the feed- stocks are fed and the top of the distillation column, or in the top of the distillation column. It is more preferred to provide the withdrawal port for the low boiling point mixture in the top of the distillation column. A part of the low boiling point mixture with- drawn from the withdrawal port may be returned to the upper portion of the distillation column to thereby effect the so-called reflux operation. When the reflux ratio is increased by conducting this reflux operation, the distillation efficiency of a low boiling product into a vapor phase is increased, thereby advantageously increasing the concentration of a low boiling point product in the withdrawn gaseous component. However, too much of an increase in the reflux ratio disadvanta- geously leads to an increase in the thermal energy re- quired. Thus, the reflux ratio is generally chosen in the range of from 0 to 10, preferably from 0 to 5, more preferably from 0 to 3. [0073] By continuously feeding the low boiling point mixture withdrawn from the upper portion of the trans- esterification reactor, containing the dialkyl carbon- ate, to a dialkyl carbonate separation apparatus and continuously recovering the dialkyl carbonate from the separation apparatus, the product dialkyl carbonate can be obtained. Examples of such dialkyl carbonate sepa- ration apparatuses include a distillation type separa- tion apparatus, an extractive distillation type separa- tion apparatus, a liquid-liquid extraction type separation apparatus, a crystallization type separation apparatus, an adsorption type separation apparatus and a membrane type separation apparatus. A combination of a plurality of different or identical separation appa- ratuses may be used. Among these separation appara- tuses, a distillation type separation apparatus is es- pecially preferred. [0074] When the low boiling point mixture (containing the dialkyl carbonate and the unreacted aliphatic mono- hydric alcohol) withdrawn from the upper portion of the multi-stage distillation column is subjected to separa- tion by means of a distillation type separation appara- tus , the low boiling point mixture can be separated into various components, such as the dialkyl carbonate and the unreacted aliphatic monohydric alcohol, wherein some of the separated components are obtained in the form of one or more column top fractions containing a single component or a plurality of components and some of the separated components are obtained in the form of a column bottom liquid. As the above-mentioned column top fraction, an azeotropic mixture may be obtained depending on the types of feedstocks. After the compo- nents in the low boiling point mixture withdrawn from the upper portion of the multi-stage distillation col- umn are separated by means of a distillation type sepa- ration apparatus, one or more fractions containing the unreacted aliphatic monohydric alcohol and/or a column bottom liquid containing the unreacted aliphatic mono- hydric alcohol is then fed to the transesterification reactor. [0075] As the distillation type separation apparatus, a single continuous multi-stage distillation column or a plurality of continuous multi-stage distillation col- umns can be used, wherein each continuous multi-stage distillation column may be of the same type as used as the transesterification reactor. Explained hereinbelow is a mode of the method of the present invention in which an aliphatic monohydric alcohol and a dialkyl carbonate form a minimum boiling point azeotropic mix- ture, and wherein dimethyl carbonate is produced by using methanol as the aliphatic monohydric alcohol. A low boiling point mixture (containing methanol and di- methyl carbonate) withdrawn from the upper portion of the transesterification reactor is continuously fed to a dimethyl carbonate separation column. A low boiling point mixture containing a minimum boiling point azeotropic mixture of methanol and dimethyl carbonate is continuously withdrawn from an upper portion of the dimethyl carbonate separation column, while continu- ously withdrawing dimethyl carbonate from a lower por- tion of the dimethyl carbonate separation column, thereby obtaining dimethyl carbonate. [0076] As the dimethyl carbonate separation column, a single continuous multi-stage distillation column or a plurality of continuous multi-stage distillation col- umns can be used, wherein each continuous multi-stage distillation column may be of the same type as used as the transesterification reactor. The dimethyl carbon- ate separation column is generally operated under re- duced pressure, atmospheric pressure or superatmos- pheric pressure, specifically in the range of from 0.5 x 105 to 50 x 105 Pa (0.51 to 51 kg/cm2) in terms of the absolute pressure. [0077] The composition of methanol/dimethyl carbonate minimum boiling point azeotropic mixture may be varied depending on the operating pressure of the dimethyl carbonate separation column. Therefore, the operating pressure of the dimethyl carbonate separation column is chosen so that the dimethyl carbonate is obtained from the lower portion of the dimethyl carbonate separation column. Specifically, an operating pressure higher than an operating pressure corresponding to the metha- nol/dimethyl carbonate ratio of the low boiling point mixture withdrawn from the upper portion of the trans- esterification reactor is chosen for the dimethyl car- bonate separation column. [0078] The dialkyl carbonate obtained by the method of the present invention contains a carbonate ether repre- sented by the above-mentioned formula (2). The lower the cyclic ether content of the cyclic carbonate, the lower the carbonate ether content of the dialkyl car- bonate produced. However, for producing a cyclic car- bonate having extremely low cyclic ether content, the cyclic carbonate must be produced by a complicated method which is not practical for commercial production. Therefore, in the present invention, the cyclic ether content of the dialkyl carbonate produced is not more than 10,000 ppm, preferably in the range of from 1 to 10,000 ppm, more preferably from 3 to 5,000 ppm, still more preferably from 10 to 3,000 ppm. [0079] The low boiling point mixture (containing a minimum boiling point azeotropic mixture of methanol and dimethyl carbonate) withdrawn from the upper por- tion of the above-mentioned dimethyl carbonate separa- tion column may be fed to the transesterification reac- tor as a feedstock usable in the present invention. [0080] In the method of the present invention, the up- per portion of the continuous multi-stage distillation column means a portion between the top of the distilla- tion column and a position at approximately half the height of the distillation column, and the upper por- tion includes the top of the column. The lower portion of the continuous multi-stage distillation column means a portion between the bottom of the distillation column and a position at approximately half the height of the distillation column, and the lower portion includes the bottom of the column. [0081] Further, in the method of the present invention, the diol produced in the transesterification reactor is obtained as the high boiling point mixture withdrawn in a liquid form from the lower portion of the trans- esterification reactor. The high boiling point mixture contains the produced diol and the unreacted cyclic carbonate, and may also contain a part of the unreacted aliphatic monohydric alcohol or both a part of the un- reacted aliphatic monohydric alcohol and a part of the produced dialkyl carbonate. [0082] The withdrawal port for withdrawing the high boiling point mixture (containing the produced diol) in the liquid form from the transesterification reactor is positioned at a lower portion of the reactor, prefera- bly at the bottom of the reactor. A part of the with- drawn liquid mixture may be recycled to the lower por- tion of the transesterification reactor in a gaseous form or a gas-liquid mixture form by heating by means of a reboiler. [0083] When a multi-stage distillation column is used as a transesterification reactor, the rate at which a liquid flows down inside the continuous multi-stage distillation column and the rate at which a vapor as- cends inside the distillation column may be varied de- pending on the type of the distillation column, and on the type of the packing in the case of a packed column. However, the distillation column is generally operated so that no flooding or weeping occurs. [0084] In the method of the present invention, the av- erage residence time of the liquid phase in the trans- esterification reactor depends on the reaction condi- tions, and the type and inner structure (for example, the types of the plate and packing) of the transesteri- fication reactor, but is generally in the range of from 0.001 to 50 hours, preferably from 0.01 to 10 hours, more preferably from 0.05 to 5 hours. [0085] The reaction temperature varies depending on the types of the raw materials and the reaction pres- sure, but is generally chosen in the range of from -20 to 350 °C, preferably from 0 to 200 °C. The reaction pressure in the transesterification reactor can be se- lected from reduced pressure, atmospheric pressure and superatomospheric pressure. The reaction pressure is generally in the range of from 1 Pa to 2 x 106 Pa, preferably from 1 x 103 Pa to 1 x 106 Pa, more prefera- bly from 1 x 104 Pa to 5 x 105 Pa, in terms of the ab- solute pressure. [0086] It is also possible to supply a part of the high boiling point mixture withdrawn in a liquid form from the lower portion of the transesterification reac- tor to the transesterification reactor, so that a part of the unreacted cyclic carbonate and/or a part of the unreacted aliphatic monohydric alcohol can be recycled to the transesterification reactor. [0087] When the high boiling point mixture (containing the produced diol) withdrawn from the lower portion of the transesterification reactor is subjected to separa- tion in the diol purification step, the high boiling point mixture can be separated into various components. In general, (i) when the high boiling point mixture contains low boiling point components (such as an ali- phatic alcohol used as a raw material), it is preferred that, before the diol purification step, an aliphatic alcohol and the like are separated from the high boil- ing point mixture by distillation or the like and, then, the separated aliphatic alcohol and the like are recy- cled to the transesterification reactor, and (ii) it is also preferred that, before the diol purification step, the unreacted cyclic carbonate contained in the high boiling point mixture is separated therefrom and, then, the remaining part of high boiling point mixture is introduced into the diol purification step. As methods for separating unreacted cyclic carbonate from the high boiling point mixture, there can be mentioned (ii-1) separation by distillation, (ii-2) hydrolysis to con- vert unreacted cyclic carbonate to a diol, and (ii-3) elimination of unreacted cyclic carbonate by the eth- erification of the cyclic carbonate with a diol. The etherification is especially preferred in the present invention. [0088] Specifically, as preferred examples of manners in which the high boiling point mixture withdrawn from the transesterification reactor is treated before sub- jected to a step for the purification of diol, the fol- lowing two modes can be mentioned. [0089] 1. A mode in which the high boiling point mix- ture (in the liquid form) withdrawn from the lower por- tion of the transesterification reactor contains the low boiling point mixture (including a part of the un- reacted aliphatic monohydric alcohol and dialkyl car- bonate), and wherein the high boiling point mixture is continuously introduced, prior to the feeding thereof to the continuous etherification reactor, to a low boiling point mixture separation column which is com- prised of a continuous multi-stage distillation column, and wherein a low boiling point mixture containing the part of the unreacted aliphatic monohydric alcohol and dialkyl carbonate which is contained in the high boil- ing point mixture is continuously withdrawn from an upper portion of the low boiling point mixture separa- tion column, while continuously withdrawing a high boiling point mixture containing the diol and the unre- acted cyclic carbonate from the low boiling point mix- ture separation column through one or more side-cut withdrawal ports provided in a side wall of the column at one or more positions thereof corresponding to one or more stages selected from the group consisting of intermediate stages and a lowermost stage of the low boiling point mixture separation column, wherein the low boiling point mixture withdrawn from the upper portion of the low boiling point mixture separation column is continuously recycled to the transesterification reactor, while continuously feeding the high boiling point mixture withdrawn through the side-cut withdrawal port of the low boiling point mix- ture separation column to the diol separation apparatus. [0090] As the low boiling point mixture separation column, a continuous multi-stage distillation column can be used, and the continuous multi-stage distilla- tion column may be of the same type as used as the transesterification reactor. [0091] 2. A mode in which the high boiling point mix- ture withdrawn from the lower portion of the trans- esterif ication reactor contains a part of the unreacted aliphatic monohydric alcohol and dialkyl carbonate, and wherein the high boiling point mixture is continuously introduced, prior to the feeding thereof to the etheri- fication reactor, to a low boiling point mixture sepa- ration column which is comprised of a continuous multi -stage distillation column, and wherein a low boiling point mixture containing the part of the unreacted ali- phatic monohydric alcohol and dialkyl carbonate which is contained in the high boiling point mixture is con- tinuously withdrawn from an upper portion of the low boiling point mixture separation column, while continu- ously withdrawing a high boiling point mixture contain- ing the diol and the unreacted cyclic carbonate from a lower portion of the low boiling point mixture separa- tion column, followed by the etherification of the cy- clic carbonate in the withdrawn high boiling point mix- ture at the bottom portion of the low boiling point mixture separation column, wherein the low boiling point mixture withdrawn from the upper portion of the low boiling point mixture separation column is continuously recycled to the transesterification reactor, while continuously feeding the high boiling point mixture containing the diol and the ether to the diol separation apparatus. [0092] The above-mentioned etherification can be per- formed in accordance with the etherification method described in Patent Document 21. Specifically, the high boiling point mixture containing the produced diol and the unreacted cyclic carbonate is fed to an etheri- fication reactor, to thereby effect an etherification reaction between the unreacted cyclic carbonate and a part of the produced diol and produce a chain ether represented by the following formula: wherein R1 is as defined for formula (1) above. [0093] The reaction conditions for the etherification reaction in the etherification reactor may be varied depending on the presence or absence of an etherifica- tion catalyst. When an etherification catalyst is used, the etherification reaction conditions may be varied depending on the type and amount of the etherification catalyst. However, in general, the reaction tempera- ture is from 50 to 350 °C, preferably from 80 to 300 °C, more preferably from 100 to 250 °C. The reaction time may be varied depending on the presence or absence of an etherification catalyst. When an etherification catalyst is used, the reaction time may be varied de- pending on the type and amount of the etherification catalyst and the reaction temperature. However, in general, the reaction time is from 0.001 to 50 hours, preferably from 0.01 to 10 hours, more preferably from 0.02 to 5 hours, in terms of the average residence time. The reaction pressure may be varied depending on the reaction temperature. However, in general, the reac- tion pressure is from 1 x 103 to 2 x 107 Pa, preferably from 1 x 104 to 1 x 107 Pa, in terms of the absolute pressure. [0094] The conversion of the cyclic carbonate in the etherification reaction is generally from 90 to 100 %, preferably from 95 to 100 %, more preferably from 98 to 100 %. [0095] When carbon dioxide enters the transesterifica- tion reactor, the transesterification reaction is ad- versely affected, so that the reaction rate becomes low. Therefore, when the etherification reaction mixture withdrawn from the etherification reactor contains car- bon dioxide, it is preferred that the carbon dioxide is separated from the etherification reaction mixture by means of a carbon dioxide separation apparatus. [0096] In addition, there can be used the distillation separation method described in Patent Documents 22 and 23. In this method, water is fed to the distillation separation apparatus in which a reaction mixture con- taining diol is subjected to distillation separation. [0097] In the method of the present invention, it is not necessary to use a solvent. However, for the pur- poses of, e.g., (1) facilitating the reaction operation and (2) separating a dialkyl carbonate and a diol effi- ciently by performing azeotropic distillation or ex- tractive distillation, an appropriate inert solvent may be used as a reaction solvent. Examples of inert sol- vents include an ether, an aliphatic hydrocarbon, an aromatic hydrocarbon, a halogenated aliphatic hydrocar- bon and a halogenated aromatic hydrocarbon. [0098] An inert gas, such as nitrogen, helium, argon or the like, may be present in the reaction system. Further, for the purpose of promoting the distilling -off of a generated low boiling point reaction product, the above-mentioned inert gas or a gaseous form of an inert low boiling point organic compound may be intro- duced to the reaction system from a lower portion of a continuous multi-stage distillation column. Examples [0099] Hereinbelow, the present invention will be de- scribed in more detail with reference to the following Examples and Comparative Example, which should not be construed as limiting the scope of the present inven- tion. [0100] In the following Examples and Comparative Exam- ple, the yield (%) of ethylene glycol is determined, based on the amount of the charged ethylene carbonate; the selectivity (%) for ethylene glycol is determined, based on the amount of the consumed ethylene carbonate; the yield (%) of dimethyl carbonate is determined, based on the amount of the charged ethylene carbonate; and the selectivity (%) for dimethyl carbonate is de- termined, based on the amount of the consumed methanol. The positions of respective stages of a distillation column are represented by the ordinal numbers for the respective stages as counted from the top stage of the distillation column. [0101] Example 1 Using a production system as shown in Fig. 1, di- methyl carbonate (DMC) and ethylene glycol (EG) were continuously produced from ethylene carbonate (EC) and methanol (MeOH). Continuous multi-stage distillation column 1 was comprised of a 60-stage Oldershaw distil- lation column which has an inner diameter of 5 cm. EC (containing 400 ppm of ethylene oxide) was continuously- fed in a liquid form to distillation column 1 at the fifth stage thereof through conduit 2 and preheater 3 at a flow rate of 231 g/h, and an 18 % by weight solu- tion of potassium hydroxide (as a catalyst) in ethylene glycol was also continuously fed in a liquid form to distillation column 1 at the fifth stage thereof through conduit 2' at a flow rate of 1.1 g/h, while continuously feeding a mixture of MeOH and DMC (MeOH/DMC weight ratio = 97/3) in a liquid form to dis- tillation column 1 at the 30th stage thereof through conduit 5 and preheater 6 at a flow rate of 735.0 g/h, to thereby effect a transesterification reaction. Con- tinuous multi-stage distillation column 1 was operated under conditions wherein the reaction pressure and re- action temperature, each as measured at column top 4 thereof, were atmospheric pressure and 63.8 °C, respec- tively. [0102] A low boiling point mixture in a gaseous form distilled from column top 4 of distillation column 1 was condensed by condenser 7. A part of the resultant condensate was refluxed to column top 4 of distillation column 1 through conduit 8 (reflux ratio: 0.4), while the remainder of the condensate (hereinbelow, the con- densate is referred to simply as "column top condensate from column 1") was fed to DMC separation column 71 at a position 80 cm below the top of column 71 through conduit 9 at a flow rate of 803.1 g/h, wherein column 71 was comprised of a packed column type distillation column having an inner diameter of 2.5 cm and a packing height of 160 cm and having packed therein Dixon pack- ings (3 mm(|)) . The column top condensate from column 1 contained MeOH and DMC in concentrations of 6 7.9 % by weight and 32.1 % by weight, respectively. [0103] On the other hand, a liquid was withdrawn from column bottom 10 of distillation column 1 through con- duit 11 (hereinbelow, the liquid is referred to simply as "bottom liquid of column 1"), and a part of the bot- tom liquid of column 1 was heated by reboiler 12 to provide the energy required for distillation and re- turned to column bottom 10 of distillation column 1 through conduit 13, while the remainder of the bottom liquid of column 1 was withdrawn as a high boiling point mixture in a liquid form, and fed to low boiling point mixture separation column 17 at a position 100 cm below the top of column 17 through conduit 14 at a flow rate of 231.3 g/h, wherein column 17 was comprised of a packed column type distillation column having an inner diameter of 2.5 cm and a packing height of 160 cm and having packed therein Dixon packings (3 mmϕ) . The high boiling point mixture contained EG, MeOH, EC, DMC and KOH in concentrations of 70.7 % by weight, 29.1 % by weight, 0.099 % by weight, 0.04 % by weight and 0.086 % by weight, respectively. The conversion of EC in the transesterification reaction was 99.9 %. [0104] DMC separation column 71 was operated under conditions wherein the pressure of column top 72 thereof was 1.4 x 106 Pa and the temperature of column bottom 73 thereof was 205 °C. A low boiling point mix- ture in a gaseous form distilled from column top 72 of column 71 was condensed by condenser 75. A part of the resultant condensate was refluxed to column top 72 of column 71 through conduit 7 7 (reflux ratio: 2), while feeding the remainder of the condensate to continuous multi-stage distillation column 1 through conduit 78 (which is connected to conduit 5), conduit 5 and pre- heater 6. For stabilizing the composition of the liq- uid fed to continuous multi-stage distillation column 1 through conduit 5, the composition of the liquid fed to continuous multi-stage distillation column 1 through conduit 5 was gradually changed from MeOH/DMC to only MeOH. [0105] A liquid was withdrawn from column bottom 73 of DMC separation column 71 through conduit 79 (hereinbe- low, the liquid is referred to simply as "bottom liquid of column 71"), and a part of the bottom liquid of col- umn 71 was heated by reboiler 80 to provide the energy required for distillation and returned to column bottom 73 of distillation column 71 through conduit 81, while the remainder was withdrawn at a flow rate of 235 g/h from the production system through conduit 82. The bottom liquid of column 71 contained 99.9 % by weight of DMC and 1,150 ppm of carbonate ether (CH3OCH2CH2OCOOCH3). [0106] Low boiling point mixture separation column 17 was operated under conditions wherein the pressure of column top 18 thereof was atmospheric pressure and the temperature of column bottom 26 thereof was 201 °C. At the column bottom 26 of low boiling point mixture sepa- ration column 17, an etherification reaction between EC and EG was effected, thereby forming diethylene glycol (DEG). The residence time at column bottom 26 of low boiling point mixture separation column 17 was 1.5 hours. A low boiling point mixture in a gaseous form distilled from column top 18 of column 17 was condensed by condenser 19. A part of the resultant condensate was refluxed to column top 18 of column 17 through con- duit 20 (reflux ratio: 1), while feeding the remainder of the condensate to an upper portion of carbon dioxide separation column 22 through conduit 21. From conduit 23 provided at the bottom of carbon dioxide separation column 22, nitrogen gas was introduced into the conden- sate, thereby bubbling the condensate with the nitrogen gas. The nitrogen gas entraining carbon dioxide was discharged from conduit 24 provided at the top of col- umn 22. The resultant carbon dioxide-free liquid ob- tained in column 2 2 was withdrawn from conduit 2 5 pro- vided at a lower portion of column 22, and recycled to continuous multi-stage distillation column 1 at the 30th stage thereof at a flow rate of 70.2 g/h. [0107] A liquid was withdrawn from the bottom of low boiling point mixture separation column 17 (hereinbelow, the liquid is referred to simply as "bottom liquid of column 17"). The bottom liquid of column 17 contained EG and DEG in concentrations of 67.6 % by weight and 17.3 % by weight, respectively. The bottom liquid of column 17 was withdrawn at a flow rate of 163.9 g/h through conduit 30 as an etherification reaction mix- ture, while the remainder of the bottom liquid of col- umn 17 was heated by reboiler 28 and returned to column bottom 26 of column 17 through conduit 29. The etheri- fication reaction mixture contained EG and DEG in con- centrations of 99.7 % by weight and 0.17 % by weight, respectively (EC was not detected). [0108] The etherification reaction mixture was fed to EG purification column 41 at a position 90 cm below the top of column 41 through conduit 30, wherein column 41 was comprised of a packed column type distillation col- umn having an inner diameter of 2.5 cm and a packing height of 120 cm and having packed therein Dixon pack- ings (3 mmϕ) . [0109] EG purification column 41 was operated under conditions wherein the pressure of column top 42 thereof was 4,000 Pa (30 torr) and the temperature of column bottom 43 thereof was 123.5 °C. A distillate was withdrawn in a liquid form through conduit 56 pro- vided at a position 50 cm below the top of column 41 at a flow rate of 160.9 g/h, to thereby obtain a side-cut distillate. A mixture distilled from column top 42 of column 41 was condensed by condenser 45. A part of the resultant condensate was refluxed to column top 42 of column 41 through conduit 47 (reflux ratio: 2), while withdrawing the remainder of the condensate through conduit 48. A liquid was withdrawn from the bottom of EG purification column 41 through conduit 49 (hereinbe- low, the liquid is referred to simply as "bottom liquid of column 41"). The bottom liquid of column 41 con- tained EG and DEG in concentrations of 65.2 % by weight and 20.3 % by weight, respectively. 2.8 g of the bot- tom liquid of column 41 was withdrawn every two hours from the production system through conduit 52, while the remainder of the bottom liquid of column 41 was returned to column bottom 43 of column 41 through re- boiler 50 and conduit 51. [0110] From the above data, it can be seen that the yield of DMC (containing 1,150 ppm of carbonate ether) was 99.8 %, the selectivity for DMC was not lower than 99 %, and EG was obtained in a yield of 99.8 %. [0111] The above-obtained DMC and phenol were sub- jected to a transesterification reaction, thereby ob- taining diphenyl carbonate (DPC). 235 g of the ob- tained DPC and 228 g of bisphenol A were charged into a vacuum reactor equipped with a stirrer. The atmosphere in the vacuum reactor was purged with nitrogen gas, and the temperature was gradually elevated from 180 °C to 220 °C while stirring. Thereafter, the vacuum reactor was hermetically closed. Then, polymerization was per- formed while stirring at 100 rpm under a pressure of 8,000 Pa for 30 minutes and, then, under a pressure of 4,000 Pa for 90 minutes. Subsequently, the temperature was elevated to 270 °C, and further polymerization was performed under a pressure of 70 Pa for 1 hour, thereby obtaining an aromatic polycarbonate. The obtained aro- matic polycarbonate was colorless and transparent, which is good. The number average molecular weight was 10,500. [0112] Comparative Example 1 Production of DMC and EG was performed in substan- tially the same manner as in Example 1 except that the ethylene oxide content of EC used as a raw material was changed to 5,000 ppm. As a result, there were obtained DMC (containing 15,300 ppm of carbonate ether) and EG, wherein the yield of DMC was 96.4 %, the selectivity for DMC was 95.2 %, and the yield of EG was 9 4.3 %. From the obtained DMC, DPC was produced in the same manner as in Example 1. From the obtained DPC, an aro- matic polycarbonate was produced in the same manner as in Example 1. The obtained aromatic polycarbonate was discolored to assume a yellow color and had a number average molecular weight of 6,300. A comparison be- tween the results of Example 1 and the results of this Comparative Example 1 shows that, from a cyclic carbon- ate containing a large amount of a cyclic ether, a dialkyl carbonate containing a high concentration of a carbonate ether is obtained, and that, from such a dialkyl carbonate containing a high concentration of a carbonate ether, it is impossible to obtain highly re- active diphenyl carbonate necessary for producing a high molecular weight aromatic polycarbonate. [0113] Example 2 Production of DMC and EG was performed in substan- tially the same manner as in Example 1 except that the ethylene oxide content of EC used as a raw material was changed to 1,200 ppm. As a result, there were obtained DMC (containing 3,400 ppm of carbonate ether) and EG, wherein the yield of DMC was 99.6 %, the selectivity for DMC was not lower than 99 %, and the yield of EG was 99.6 %. From the obtained DMC, DPC was produced in the same manner as in Example 1. From the obtained DPC, an aromatic polycarbonate was produced in the same man- ner as in Example 1. The obtained aromatic polycarbon- ate was colorless and had a number average molecular weight of 10,200. [0114] Example 3 Production of DMC and EG was performed in substan- tially the same manner as in Example 1 except that the ethylene oxide content of EC used as a raw material was changed to 2,000 ppm. As a result, there were obtained DMC (containing 5,600 ppm of carbonate ether) and EG, wherein the yield of DMC was 99.2 %, the selectivity for DMC was not lower than 99 %, and the yield of EG was 99.1 %. From the obtained DMC, DPC was produced in the same manner as in Example 1. From the obtained DPC, an aromatic polycarbonate was produced in the same man- ner as in Example 1. The obtained aromatic polycarbon- ate was colorless and had a number average molecular weight of 9,100. INDUSTRIAL APPLICABILITY [0115] In the dialkyl carbonate produced by the method of the present invention, the content of a carbonate ether (which is a conventionally unknown impurity) is reduced to a specific low range. The dialkyl carbonate obtained by the method of the present invention can be used to produce a transesterification aromatic carbon- ate. The transesterification aromatic carbonate pro- duced can be very advantageously used to produce a col- orless, high molecular weight aromatic polycarbonate. WE CLAIM: 1. A method for producing a dialkyl carbonate and a diol, comprising: (a) effecting a transesterification reaction be- tween a cyclic carbonate and an aliphatic monohydric alcohol in the presence of a transesterification cata- lyst , thereby obtaining a reaction mixture containing a product dialkyl carbonate and a product diol, (b) withdrawing a dialkyl carbonate-containing liquid from said reaction mixture, followed by separa- tion of the dialkyl carbonate from the dialkyl carbon- ate-containing liquid, and (c) withdrawing a diol-containing liquid from said reaction mixture, followed by separation of the diol from the diol-containing liquid, said steps (b) and (c) being performed in either order or simultaneously, wherein: said cyclic carbonate contains a cyclic ether rep- resented by the formula (1) below in an amount of from 0.1 to 3,000 ppm by weight, and said product dialkyl carbonate contains a carbon- ate ether represented by the formula (2) below in an amount of not more than 10,000 ppm by weight. wherein R1 represents a divalent group repre- sented by the formula: -(CH2)m- wherein m is an integer of from 2 to 6, and at least one hydrogen atom of R1 is optionally replaced by at least one substituent group selected from the group consisting of a C1-10 alkyl group and a C6-10 aryl group, and wherein R1 is as defined above for formula (1), R2 represents a C1-12 monovalent ali- phatic group, and at least one hydrogen atom of R2 is optionally replaced by at least one substituent group selected from the group consisting of a C1-10 alkyl group and a C6-10 aryl group. 2. The method as claimed in claim 1, wherein the amount of said cyclic ether in said cyclic carbonate is from 3 to 1, 500 ppm by weight. 3. The method as claimed in claim 2, wherein the amount of said cyclic ether in said cyclic carbonate is from 10 to 1,000 ppm by weight. 4. The method as claimed in any one of claims 1 to 3, wherein said cyclic carbonate is ethylene carbonate. 5. The method as claimed in any one of claims 1 to 4, wherein said transesterification reaction is performed in a reactive distillation column. 6. A dialkyl carbonate produced by the method of any one of claims 1 to 5, which contains a carbonate ether represented by the formula (2) as claimed in claim 1 in an amount of from 1 to 10,000 ppm by weight. 7. The dialkyl carbonate as claimed in claim 6, wherein the amount of said carbonate ether in the dialkyl carbonate is from 3 to 5,000 ppm by weight. 8. The dialkyl carbonate as claimed in claim 7, wherein the amount of said carbonate ether in the dialkyl carbonate is from 10 to 3,000 ppm by weight. A method for producing a dialkyl carbonate and a diol, comprising: (a) effecting a transesterification reaction between a cyclic carbonate and an aliphatic monohydric alcohol in the presence of a transesterification catalyst, thereby obtaining a reaction mixture containing a product dialkyl carbonate and a product diol, (b) withdrawing a dialkyl carbonate-containing liquid from the reaction mixture, followed by separation of the dialkyl carbonate from the dialkyl carbonate-containing liquid, and (c) withdrawing a diol-containing liquid from the reaction mixture, followed by separation of the diol from the diol-containing liquid, wherein the cyclic carbonate contains a cyclic ether in an amount of from 0.1 to 3, 000 ppm by weight, and the product dialkyl carbonate contains a carbonate ether of not more than 10, 000 ppm by weight.

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