JP4333489B2 - Transesterification of aliphatic dihydroxy compounds with dimethyl carbonate - Google Patents

Description

  In the present invention, for example, an aliphatic dihydroxy compound such as 1,6-hexanediol and 1,4-cyclohexanediol and dimethyl carbonate (hereinafter sometimes abbreviated as DMC), and methanol produced as a by-product by a gas separation membrane are used. The present invention relates to a method for transesterification while selectively withdrawing out of the reaction system. The present invention also relates to a method for producing an oligo or polycarbonate diol, comprising a step of transesterifying while selectively extracting methanol, which is a by-product of an aliphatic dihydroxy compound and dimethyl carbonate, out of the reaction system by a gas separation membrane. . Oligo or polycarbonate diol is a very useful compound as a raw material for polyurethane, adhesives, paints and the like.

  As a method for producing oligo or polycarbonate diol, a method of transesterifying an aliphatic dihydroxy compound and dimethyl carbonate is known. Since this reaction is an equilibrium reaction, it is preferable to distill off by-produced methyl alcohol out of the reaction system in order to promote the reaction. However, methanol forms an azeotropic methanol-dimethyl carbonate mixture containing about 20 to 30% by weight of dimethyl carbonate depending on the pressure. Dimethyl carbonate is also distilled out of the reaction system. For this reason, a method of using extra dimethyl carbonate in advance or a method of supplementing a decrease in dimethyl carbonate with new dimethyl carbonate has been performed. For this reason, there has been a problem that the reaction rate (conversion rate) of dimethyl carbonate is lowered. Further, since the distilled azeotropic methanol-dimethyl carbonate mixture is difficult to separate, there is a problem that it is not easy to purify and reuse methanol or dimethyl carbonate.

In Patent Document 1, a polycarbonate diol having a molecular terminal at an alkyl group is generated from a dialkyl carbonate and an aliphatic dihydroxy compound, and then the polycarbonate diol is reacted with an aliphatic dihydroxy compound to obtain a polycarbonate diol having a small number of alkyl groups at the molecular terminal. A method has been proposed. In this reaction, it is described that unreacted dialkyl carbonate is distilled together with by-produced alcohol, resulting in a loss, and this loss should be taken into account in the initial charging.
In Patent Document 2, dimethyl carbonate and an aliphatic dihydroxy compound are transesterified while extracting a by-product mixture of methanol and dimethyl carbonate to produce a polycarbonate diol prepolymer having a molecular terminal alkyl group of 5% or less, Next, a method has been proposed in which the polycarbonate diol prepolymer is subjected to a polycondensation reaction while extracting an aliphatic hydroxy compound under reduced pressure to produce a polycarbonate diol having a hydroxyl group at the molecular end.
In Patent Document 3, an aliphatic oligocarbonate diol having a conversion rate of dimethyl carbonate of 80% or more in a multistage transesterification process from dimethyl carbonate and an aliphatic dihydroxy compound is reused in the reaction. A manufacturing method has been proposed. In addition, it is described that the terminal alkyl group oligomer obtained by the transesterification reaction is converted into a terminal hydroxy group oligomer by heating, stirring in vacuum, and uncapping to remove alcohol using nitrogen as a stripping gas. Yes.

Japanese Patent Laid-Open No. 62-187725 JP 2001-270938 A Special table 2003-535936 gazette

  An object of the present invention is to provide a method for transesterifying an aliphatic dihydroxy compound and dimethyl carbonate while selectively extracting methanol produced as a by-product out of the reaction system by a gas separation membrane. Also, an improved oligo or polycarbonate diol production comprising a step of transesterifying an aliphatic dihydroxy compound and dimethyl carbonate while selectively extracting methanol as a by-product out of the reaction system by a gas separation membrane. The purpose is to propose a method.

The present invention relates to a method of transesterifying an aliphatic dihydroxy compound and dimethyl carbonate while selectively extracting by-product methanol using a gas separation membrane, and in particular, a reaction of transesterifying an aliphatic dihydroxy compound and dimethyl carbonate. A mixed vapor containing by-product methanol and raw material dimethyl carbonate is generated from the mixed solution, the mixed vapor is brought into contact with a gas separation membrane, methanol is selectively permeated and extracted, and non-permeated dimethyl carbonate is a reaction mixed solution. The present invention relates to a method for transesterification characterized in that
In the above method, the gas separation membrane has a permeation ratio of methanol vapor to dimethyl carbonate vapor at 120 ° C. (P ′ _MeOH / P ′ _DMC ) of 2 or more, and the gas separation membrane permeates methanol vapor at 120 ° C. The speed (P ′ _MeOH ) is 1 × 10 ⁻⁵ cm ³ (STP) / cm ² · sec · cmHg or more, and the gas separation membrane is an asymmetric polyimide hollow fiber membrane that does not dissolve in parachlorophenol.

  Further, the present invention relates to a method for producing an oligo or polycarbonate diol, comprising a step of transesterifying an aliphatic dihydroxy compound and dimethyl carbonate while selectively extracting by-product methanol using a gas separation membrane.

  In the present invention, methanol is selectively extracted out of the reaction system, and dimethyl carbonate remains selectively in the reaction system, so that a high reaction rate (high conversion rate) can be achieved without additional supply of dimethyl carbonate as a raw material. The transesterification reaction between the aliphatic dihydroxy compound and dimethyl carbonate can be performed. Further, the methanol extracted out of the reaction system has a high purity of 80% by weight or more, particularly 85% by weight or more, and can be easily recovered and reused.

  The present invention relates to a method of transesterifying an aliphatic dihydroxy compound and dimethyl carbonate while selectively extracting by-product methanol out of the reaction system by a gas separation membrane, and in particular, transesterification of the aliphatic dihydroxy compound and dimethyl carbonate. In the reaction, a mixed vapor containing byproduct methanol and raw material dimethyl carbonate is generated from the reaction mixture, the mixed vapor is brought into contact with the gas separation membrane, methanol is selectively permeated and extracted out of the reaction system, On the other hand, it relates to a method of condensing non-permeated highly concentrated dimethyl carbonate and returning it to the reaction mixture.

  The aliphatic dihydroxy compound used in the transesterification reaction of the present invention is not particularly limited. For example, 1,3-propanediol, 1,4-butanediol, 1,3-butanediol, 1,5 -Carbon chains such as pentanediol, 1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 1,12-dodecanediol A straight chain or a carbon chain such as 3-methylpentane-1,5-diol, 2-ethylhexane-1,6-diol, neopentyl glycol, 2-methyl-1,8-octanediol is branched. 1,3-cyclohexanediol, 1,4-cyclohexanediol, 2,2′-bis (4-hydroxycyclohexyl) propyl Carbon chains such as bread and 1,4-cyclohexanedimethanol containing alicyclic structures, and carbon chains such as diethylene glycol, triethylene glycol, tetraethylene glycol polyethylene glycol, dipropylene glycol, and polypropylene glycol are ether-bonded. Specific examples include those containing.

In the transesterification reaction of the present invention, no catalyst may be used, but a catalyst (transesterification catalyst) used in a normal transesterification reaction can be suitably used. Examples of the catalyst include alkali metal hydroxides (lithium hydroxide, sodium hydroxide, potassium hydroxide, etc.), alkali metal carbonates (lithium carbonate, sodium carbonate, potassium carbonate, etc.), and alkali metal carboxylates. (Lithium acetate, sodium acetate, potassium acetate, etc.), alkali metal compounds such as alkali metal alkoxides (lithium methoxide, sodium methoxide, potassium t-butoxide, etc.) and hydroxides of alkaline earth metals (magnesium hydroxide, etc.) ), Alkaline earth metal compounds such as alkaline earth metal alkoxide (magnesium methoxide, etc.), aluminum such as aluminum alkoxide (aluminum ethoxide, aluminum isopropoxide, aluminum s-butoxide, etc.), aluminum acetylacetonate, etc. Zinc compounds, zinc carboxylates (such as zinc acetate), zinc compounds such as zinc acetylacetonate, manganese carboxylates (such as manganese acetate), manganese compounds such as manganese acetylacetonate, and nickel carboxylic acids Nickel compounds such as salts (such as nickel acetate), nickel acetylacetonate, antimony carboxylates (such as antimony acetate), antimony compounds such as antimony alkoxide, zirconium alkoxides (such as zirconium propoxide and zirconium butoxide), zirconium acetyl Zirconium compounds such as acetonate, titanium compounds such as titanium alkoxide (tetraethoxytitanium, tetraisopropoxytitanium, tetrabutoxytitanium, etc.) and organotin compounds (dibutyltinoxy) , It may be mentioned dibutyltin diacetate, dibutyltin dilaurate, etc.) and the like. Each carboxylic acid preferably has 2 to 30 carbon atoms, and each alkoxide preferably has 1 to 30 carbon atoms in the alkoxy group. Among these catalysts, titanium compounds or organotin compounds are preferable, but titanium alkoxides (tetraethoxy titanium, tetraisopropoxy titanium, tetrabutoxy titanium, etc.) are more preferable. Of the titanium alkoxides, tetrabutoxy titanium is particularly preferred.
The amount of the catalyst used is not particularly limited, but it is usually used at a rate of 1 to 20000 ppm, more preferably 10 to 5000 ppm, based on the weight with respect to the total amount of dimethyl carbonate and aliphatic dihydroxy compound at the start of the reaction.

  In the transesterification reaction of the present invention, dimethyl carbonate is not limited, but is about 0.1 to 10 mol, particularly about 0.5 to 2 mol, usually 0.6, with respect to 1 mol of the aliphatic dihydroxy compound. About 1.7 mol is used. When the dimethyl carbonate is in excess, the resulting oligo or polycarbonate diol has an increased proportion of alkyl group (-O-CO-O-R) ends, and when the aliphatic dihydroxy compound is in excess, the resulting oligo or polycarbonate diol hydroxyl. The proportion of base ends increases.

  In the transesterification reaction of the present invention, the reaction temperature is not limited, but is preferably 80 to 210 ° C and usually 100 to 200 ° C. The pressure during the reaction is not particularly limited, but is preferably set corresponding to the reaction temperature in the range of 0.1 to 1 MPa. These conditions are set so that a mixed vapor containing by-product methanol and raw material dimethyl carbonate is generated from the reaction mixture. The reaction can be performed under an inert gas atmosphere such as air, carbon dioxide, or nitrogen, but is preferably performed under an inert gas atmosphere such as nitrogen. The reaction time (corresponding to the residence time when the reaction is carried out continuously) varies depending on the reaction temperature and the amount of catalyst, but is usually preferably in the range of 1 to 30 hours.

  In the transesterification reaction of the present invention, a mixed vapor containing by-product methanol and dimethyl carbonate is generated from the reaction mixture. It is not possible to generate only methanol vapor. The mixed vapor is brought into contact with the non-permeate side surface of the gas separation membrane. While the mixed vapor is in contact with the gas separation membrane, the methanol vapor selectively permeates the gas separation membrane and moves to the permeate side of the membrane due to the partial pressure difference between the methanol vapors on both sides of the gas separation membrane. It is pulled out of the system. Even if the dimethyl carbonate vapor contacts the gas separation membrane, it does not easily permeate the membrane and remains on the non-permeating side of the membrane, that is, in the reaction system. In the present invention, the dimethyl carbonate vapor remaining on the non-permeating side of the membrane is preferably cooled and condensed and returned to the reaction mixture.

In the transesterification reaction of the present invention, in order to extract the methanol vapor with good separation efficiency by bringing the mixed vapor into contact with the gas separation membrane, the selective permeability of methanol vapor to dimethyl carbonate vapor is high and the permeation rate of methanol vapor is high. It is preferable to use a gas separation membrane and to increase the effective membrane area of the gas separation membrane.
Furthermore, the partial pressure of methanol vapor contacting the gas separation membrane is increased by making the reaction system a pressurized system, and the permeation side of the gas separation membrane is reduced to increase the partial pressure difference between the methanol vapors on both sides of the gas separation membrane. It is also preferable that a gas other than methanol (for example, air, nitrogen gas, etc.) flows as a sweep gas on the permeation side of the gas separation membrane, and the permeated methanol vapor is forcibly swept from the permeation side of the gas separation membrane. It is. Furthermore, for example, it is preferable to increase the separation efficiency by reacting using, for example, a reactor equipped with a distillation column and distilling the mixed vapor to an azeotropic composition in the distillation column and then contacting with the gas separation membrane. Furthermore, before bringing the mixed vapor into contact with the gas separation membrane, it is preferable to superheat (superheat) about 2 to 3 ° C. or more, preferably 5 ° C. or more in order to maintain the separation efficiency of the gas separation membrane.

  The transesterification reaction of the present invention is carried out batchwise or continuously. When the production scale is large, a continuous reaction (a method in which dimethyl carbonate, an aliphatic dihydroxy compound and a catalyst are continuously supplied, methanol is continuously extracted, and a reaction solution is continuously extracted) is suitably used. Is done.

  A solvent is not particularly required in the transesterification reaction, but there is no problem in using it as long as it is inert in the reaction system. When the solvent has a low boiling point and is vaporized together with methanol and contained in the mixed vapor, a solvent that is more difficult to permeate the gas separation membrane than the dimethyl carbonate vapor is preferable.

The product obtained by the transesterification reaction of the present invention is usually an oligo or polycarbonate diol, particularly an oligocarbonate diol, in which a hydroxyl group and an alkyl group (—O—CO—O—R) are mixed.
In the present invention, the oligo or polycarbonate diol obtained by the transesterification can be further converted into a hydroxy group-terminated oligocarbonate diol using a known technique. Further, the oligocarbonate diol obtained by the transesterification can be further polymerized by a known method to obtain a polycarbonate diol.
As these known techniques, known techniques can be applied without particular limitation. For example, the obtained oligocarbonate diol is heated and stirred in a vacuum, and nitrogen is used as a stripping gas to complete the reaction while removing the alcohol. A method of obtaining a hydroxy-terminated oligocarbonate diol by capping, a method of obtaining a polycarbonate diol by further reacting the obtained oligocarbonate diol with an aliphatic dihydroxy compound to obtain a high molecular weight, and an aliphatic dihydroxy compound from the resulting oligocarbonate diol. A method for obtaining a polycarbonate diol by increasing the molecular weight by condensation polymerization while extracting is preferable.

The gas separation membrane used in the present invention is not particularly limited as long as it selectively permeates methanol vapor with respect to dimethyl carbonate vapor. For example, polymer films such as polyimide, polycarbonate, polysulfone, polyetherimide, polyamideimide, films obtained by heat-treating these polymer films, partially carbonized films or carbon films obtained by heat-treating polymer films, ceramics A film or the like can be preferably used. These films may be uniform films or non-uniform films such as asymmetric films and composite films. Furthermore, the form of the membrane may be any of a hollow fiber membrane, a flat membrane, a spiral membrane and the like.
In the present invention, the separation membrane is suitably used by being incorporated into the reaction apparatus so that the non-permeate side communicates with the reaction system and the permeate side communicates with the outside of the reaction system. At that time, from the form of a normal separation membrane module in which the space on the permeation side and the non-permeation side of the separation membrane is incorporated in a container having a mixed gas supply port, a permeate gas discharge port and a non-permeate gas discharge port. It may be a gas separation membrane device.

The gas separation membrane used in the present invention has a ratio of the permeation rate of methanol vapor to the permeation rate of dimethyl carbonate vapor at 120 ° C. (P ′ _MeOH / P ′ _DMC ) of 2 or more, particularly 3 or more, and further 4 or more. Is preferred. When the ratio of the permeation rate of methanol vapor to the permeation rate of dimethyl carbonate vapor (P ′ _MeOH / P ′ _DMC ) is less than 2, when a by-product methanol is extracted from the reaction system of the carbonation reaction, a considerable amount of dimethyl carbonate Since the carbonate vapor is also discharged out of the reaction system at the same time, it is difficult to obtain the effect of the present invention. The gas separation membrane used in the present invention is more suitable as the ratio of the methanol vapor permeation rate to the dimethyl carbonate vapor permeation rate (P ′ _MeOH / P ′ _DMC ) is larger. It is difficult to get easily.

The gas separation membrane used in the present invention has a methanol vapor permeation rate ( _P′MeOH ) at 120 ° C. of 1 × 10 ⁻⁵ cm ³ (STP) / cm ² · sec · cmHg or more, particularly 5 × 10 ⁻⁵ cm. ³ (STP) / cm ² · sec · cmHg is preferred. The gas separation membrane used in the present invention is more suitable as the methanol vapor permeation rate (P ′ _MeOH ) is larger. Usually, 100 × 10 ⁻⁵ cm ³ (STP) / cm ² · sec · cmHg is used. It is difficult to easily get beyond.

  Further, the gas separation membrane used in the present invention preferably has good solvent resistance against organic solvents such as methanol and dimethyl carbonate, particularly good solvent resistance at high temperatures. If the solvent resistance of the gas separation membrane is low, the gas separation performance deteriorates due to organic vapor such as methanol or dimethyl carbonate, so that it cannot be suitably used. In the present invention, a gas separation membrane that can be particularly preferably used is a gas separation obtained by heat-treating a gas separation membrane made of a polymer such as polyimide to make it infusible (specifically, insolubilized in parachlorophenol). It is a membrane. A gas separation membrane having a solvent resistance higher than that which does not dissolve in parachlorophenol can be suitably used in the present invention because the gas separation performance is not deteriorated by an organic vapor such as methanol or dimethyl carbonate.

  As a gas separation membrane, a hollow fiber membrane is suitable because it is generally easy to increase the effective membrane area, and an asymmetric structure consisting of an extremely thin skin layer (uniform layer) and a porous layer for further increasing the gas permeation rate. A membrane is preferred. In the present invention, an asymmetric hollow fiber gas separation membrane obtained by heat-treating an asymmetric hollow fiber gas separation membrane made of polyimide having a substituent as a gas separation membrane to be infusible (specifically, insolubilized in parachlorophenol) is used. It can be used suitably. This asymmetric hollow fiber gas separation membrane has good organic solvent resistance and excellent methanol permselectivity. Hereinafter, an asymmetric polyimide hollow fiber gas separation membrane that is not particularly soluble in parachlorophenol and that is particularly preferably used in the present invention will be described.

  The asymmetric polyimide hollow fiber gas separation membrane that does not dissolve in parachlorophenol in the present invention specifically has solvent resistance that does not dissolve even when immersed in parachlorophenol at 50 ° C. for 30 minutes. In this gas separation membrane, a polyimide having a substituent is used to form an asymmetric hollow fiber membrane, and then the polyimide skeleton and the asymmetric structure of the asymmetric hollow fiber membrane are retained, and the substituent is decomposed / desorbed or cross-linked. It can obtain suitably by heat-processing on the temperature conditions which are infusible. This heat treatment not only improves solvent resistance, but also improves separation performance such as permeation rate and permeation rate ratio for organic vapor. Furthermore, since the asymmetric structure is retained and the polyimide skeleton is retained to maintain the mechanical strength before the heat treatment, it is a practical gas separation membrane for separating and recovering organic vapor.

  The glass transition temperature of the polyimide having a substituent that forms the gas separation membrane before the heat treatment must be higher than the so-called infusibilization temperature at which the substituent decomposes and desorbs or crosslinks. The temperature at which the substituent is decomposed and eliminated or crosslinked is 250 ° C. or higher, preferably 380 ° C. or higher, and the glass transition temperature of the polyimide is similarly 250 ° C. or higher, preferably 380 ° C. or higher. When the temperature of the polyimide exceeds 450 ° C., the bond forming the main chain starts to decompose, and at a higher temperature, the polyimide ring decomposes and carbonization proceeds. Therefore, the substituent decomposes and desorbs at a temperature of 450 ° C. or less. Is preferred.

  Preferable examples of the polyimide having the substituent include aromatic polyimides having a polyimide unit represented by the following chemical formula (1).

［ここで、Ａは芳香族テトラカルボン酸からカルボン酸基を除いた４価の残基である。また、Ｂは芳香族ジアミンのアミノ基を除く２価の残基であって、下記化学式（２）〜化学式（７）で示される。］

[Here, A is a tetravalent residue obtained by removing a carboxylic acid group from an aromatic tetracarboxylic acid. B is a divalent residue excluding the amino group of the aromatic diamine, and is represented by the following chemical formulas (2) to (7). ]

［ここで、Ｒ１〜Ｒ４は置換基であって、これら置換基のうち少なくとも一つは、ＳＯ_３Ｈ基、ＳＯ_３ＨＮＬ_１Ｌ_２Ｌ_３基（Ｌ_１、Ｌ_２、Ｌ_３は、それぞれ独立に、水素原子又はアルキル基である）、カルボキシル基、水酸基、ハロゲン基、アルキル基、アルコキシ基、及び、アリールオキシ基からなる群から選ばれるいずれかの基であり、その他は水素原子である。］

[Wherein R1 to R4 are substituents, and at least one of these substituents is an SO ₃ H group, an SO ₃ HNL ₁ L ₂ L ₃ group (L ₁ , L ₂ , L ₃ are respectively Independently a hydrogen atom or an alkyl group), a group selected from the group consisting of a carboxyl group, a hydroxyl group, a halogen group, an alkyl group, an alkoxy group, and an aryloxy group, and the others are hydrogen atoms . ]

［ここで、Ｒ１〜Ｒ８は置換基であって、これら置換基のうち少なくとも一つは、ＳＯ_３Ｈ基、ＳＯ_３ＨＮＬ_１Ｌ_２Ｌ_３基（Ｌ_１、Ｌ_２、Ｌ_３は、それぞれ独立に、水素原子又はアルキル基である）、カルボキシル基、水酸基、ハロゲン基、アルキル基、アルコキシ基、及び、アリールオキシ基からなる群から選ばれるいずれかの基であり、その他は水素原子である。］

[Wherein R1 to R8 are substituents, and at least one of these substituents is SO ₃ H group, SO ₃ HNL ₁ L ₂ L ₃ group (L ₁ , L ₂ , L ₃ are respectively Independently a hydrogen atom or an alkyl group), a group selected from the group consisting of a carboxyl group, a hydroxyl group, a halogen group, an alkyl group, an alkoxy group, and an aryloxy group, and the others are hydrogen atoms . ]

［ここで、Ｒ１〜Ｒ８は置換基であって、これら置換基のうち少なくとも一つは、ＳＯ_３Ｈ基、ＳＯ_３ＨＮＬ_１Ｌ_２Ｌ_３基（Ｌ_１、Ｌ_２、Ｌ_３は、それぞれ独立に、水素原子又はアルキル基である）、カルボキシル基、水酸基、ハロゲン基、アルキル基、アルコキシ基、及び、アリールオキシ基からなる群から選ばれるいずれかの基であり、その他は水素原子である。］

[Wherein R1 to R8 are substituents, and at least one of these substituents is SO ₃ H group, SO ₃ HNL ₁ L ₂ L ₃ group (L ₁ , L ₂ , L ₃ are respectively Independently a hydrogen atom or an alkyl group), a group selected from the group consisting of a carboxyl group, a hydroxyl group, a halogen group, an alkyl group, an alkoxy group, and an aryloxy group, and the others are hydrogen atoms . ]

［ここで、Ｒ１〜Ｒ８は置換基であって、これら置換基のうち少なくとも一つは、ＳＯ_３Ｈ基、ＳＯ_３ＨＮＬ_１Ｌ_２Ｌ_３基（Ｌ_１、Ｌ_２、Ｌ_３は、それぞれ独立に、水素原子又はアルキル基である）、カルボキシル基、水酸基、ハロゲン基、アルキル基、アルコキシ基、及び、アリールオキシ基からなる群から選ばれるいずれかの基であり、その他は水素原子である。］

[Wherein R1 to R8 are substituents, and at least one of these substituents is SO ₃ H group, SO ₃ HNL ₁ L ₂ L ₃ group (L ₁ , L ₂ , L ₃ are respectively Independently a hydrogen atom or an alkyl group), a group selected from the group consisting of a carboxyl group, a hydroxyl group, a halogen group, an alkyl group, an alkoxy group, and an aryloxy group, and the others are hydrogen atoms . ]

［ここで、Ｒ１〜Ｒ６は置換基であって、これら置換基のうち少なくとも一つは、ＳＯ_３Ｈ基、ＳＯ_３ＨＮＬ_１Ｌ_２Ｌ_３基（Ｌ_１、Ｌ_２、Ｌ_３は、それぞれ独立に、水素原子又はアルキル基である）、カルボキシル基、水酸基、ハロゲン基、アルキル基、アルコキシ基、及び、アリールオキシ基からなる群から選ばれるいずれかの基であり、その他は水素原子である。］

[Wherein R1 to R6 are substituents, and at least one of these substituents is SO ₃ H group, SO ₃ HNL ₁ L ₂ L ₃ group (L ₁ , L ₂ , L ₃ are respectively Independently a hydrogen atom or an alkyl group), a group selected from the group consisting of a carboxyl group, a hydroxyl group, a halogen group, an alkyl group, an alkoxy group, and an aryloxy group, and the others are hydrogen atoms . ]

［ここで、Ｒ１〜Ｒ８は置換基であって、これら置換基のうち少なくとも一つは、ＳＯ_３Ｈ基、ＳＯ_３ＨＮＬ_１Ｌ_２Ｌ_３基（Ｌ_１、Ｌ_２、Ｌ_３は、それぞれ独立に、水素原子又はアルキル基である）、カルボキシル基、水酸基、ハロゲン基、アルキル基、アルコキシ基、及び、アリールオキシ基からなる群から選ばれるいずれかの基であり、その他は水素原子である。］

[Wherein R1 to R8 are substituents, and at least one of these substituents is SO ₃ H group, SO ₃ HNL ₁ L ₂ L ₃ group (L ₁ , L ₂ , L ₃ are respectively Independently a hydrogen atom or an alkyl group), a group selected from the group consisting of a carboxyl group, a hydroxyl group, a halogen group, an alkyl group, an alkoxy group, and an aryloxy group, and the others are hydrogen atoms . ]

  Specific examples of A include 3,3 ′, 4,4′-biphenyltetracarboxylic acid, 2,3,3 ′, 4′-biphenyltetracarboxylic acid, and 2,2 ′, 3,3′-biphenyltetracarboxylic acid. The carboxylic acid group is removed from tetracarboxylic acids such as acid, 2,2-bis (3,4-dicarboxyphenyl) hexafluoropropane, 1,4,5,8-naphthalenetetracarboxylic acid and pyromellitic acid. A valent residue can be preferably mentioned. In the present invention, A is 3,3 ′, 4,4′-biphenyltetracarboxylic acid, 2,3,3 ′, 4′-biphenyltetracarboxylic acid, 2,2 ′, 3,3′-biphenyltetracarboxylic acid. Particularly preferred is a tetravalent residue obtained by removing a carboxylic acid group from the tetracarboxylic acid.

  Specific examples of B include 1,3-diaminobenzene-4-sulfonic acid and its triethylammonium salt, 1,3-diaminobenzene-4,6-disulfonic acid and its triethylammonium salt, 3,5-diaminobenzoic acid 2,5-dichloro-p-phenylenediamine, 2,5-dimethyl-p-phenylenediamine, 2-trifluoromethyl-p-phenylenediamine, benzidine-2,2′-disulfonic acid and its triethylammonium salt, 2 , 2 ′, 5,5′-tetrachlorobenzidine, 3,3′-dihydroxybenzidine, 2,2′-dimethyl-4,4′-diaminobiphenyl, 2,2′-di (trifluoromethyl) -4, 4′-diaminodiphenyl, o-dianisidine, 3,3′-dicarboxy-4,4′-diaminobiphenyl, , 3 ′, 5,5′-tetrabromo-6,6′-dimethylbenzidine, 3,3′-dimethyl-4,4′-diaminodiphenylsulfone, 3,3′-dimethyl-4,4′-diaminodiphenylmethane, 2,2-bis (3-amino-4-methylphenyl) hexafluoropropane, 2,2-bis (3-amino-4-hydroxyphenyl) hexafluoropropane, 2,8-dimethyl-3,7-diaminodibenzo Preferable examples include divalent residues excluding amino groups from diamines such as thiophene-5,5-dioxide, 4,4′-diaminodiphenyl ether-2,2′-disulfonic acid and triethylammonium salt thereof.

The substituent of the polyimide having the substituent must be decomposed and eliminated at a temperature lower than the glass transition temperature of the polyimide. Specifically, the SO ₃ H group, triethylammonium sulfonate, tributyl sulfonate SO ₃ HNL ₁ L ₂ L ₃ group such as ammonium (L ₁ , L ₂ and L ₃ are each independently a hydrogen atom or an alkyl group), carboxyl group, hydroxyl group, halogen group such as Cl and Br, C Are preferably an alkyl group having 1 to 3, an alkoxy group having 1 to 3 C, an aryloxy group such as phenoxy, a trifluoromethyl group, and the like.

  The asymmetric polyimide hollow fiber gas separation membrane that does not dissolve in parachlorophenol that can be suitably used in the present invention has a skin layer thickness of 10 to 200 nm and a porous layer thickness of 20 to 200 μm. preferable. If the thickness of the skin layer is less than 10 nm, it is difficult to produce, and if it exceeds 200 nm, the gas permeation rate decreases, which is not preferable. On the other hand, if the porous layer is less than 20 μm, the mechanical strength becomes small and impractical, and if it exceeds 200 μm, the permeation rate of the porous gas becomes small.

  The asymmetric polyimide hollow fiber gas separation membrane that does not dissolve in parachlorophenol that can be suitably used in the present invention is formed by first forming an asymmetric polyimide hollow fiber gas separation membrane with a polyimide having a substituent, and then the asymmetric polyimide hollow fiber separation membrane. Can be suitably obtained by a method of heat-treating under a temperature condition in which the asymmetric structure is maintained and the substituent is decomposed / desorbed or cross-linked so as to be infusible. Hereinafter, the manufacturing method will be described.

  Approximately equimolar amounts of an aromatic tetracarboxylic acid compound such as tetracarboxylic dianhydride and an aromatic diamine compound are dissolved in a solvent such as phenol and heated at a reaction temperature of 100 to 250 ° C, preferably 130 to 200 ° C. Then, polymerization imidization is carried out while removing water or alcohol to be eliminated. At the reaction temperature, the polycondensation reaction and the imidization reaction proceed to obtain a polyimide solution dissolved in a phenolic solvent in one step. The reaction time is about 10 to 60 hours. At that time, the amount of aromatic tetracarboxylic acid compound and aromatic diamine compound used in the solvent is such that the concentration of polyimide in the solvent is 5 to 50% by weight, particularly when the polymerization solution is used directly for the production of hollow fiber membranes and the like. The concentration is preferably 10 to 40% by weight, more preferably 12 to 25% by weight, and the rotational viscosity (measured at 100 ° C.) of the polymerization solution is preferably 10 to 8000 poise, particularly preferably 100 to 3000 poise. . If the rotational viscosity is too high or too low, spinning and film formation become difficult, which is not preferable. The solvent used for the polymerization imidation of the polyimide is preferably a phenol solvent such as phenol, catechol or parachlorophenol, particularly parachlorophenol, which has good solubility and handleability.

  The polyimide asymmetric hollow fiber gas separation membrane has a dope made of a polyimide solution obtained by polymerization imidization from a hollow fiber spinning nozzle at a spinning nozzle discharge temperature of 60 to 150 ° C., and a rotational viscosity at that temperature of 10 After extruding a dope solution of 10000 poise in a dry air or nitrogen gas atmosphere to form a hollow fiber-shaped molded product, the hollow fiber-shaped molded product was kept with an alcohol solvent or water maintained at a temperature of -10 to 60 ° C. It can be suitably obtained by conducting it in a mixed solvent with an alcohol solvent, solidifying it, washing with an alcohol solvent and an aliphatic hydrocarbon solvent, and drying.

  The asymmetric hollow fiber gas separation membrane made of polyimide having a substituent thus obtained is then heat-treated. The heat treatment may be performed in an inert gas atmosphere such as nitrogen gas, but since so-called infusibilization accompanied by decomposition / desorption or cross-linking of substituents can be performed efficiently, in a gas atmosphere containing oxygen, particularly in the air. Is preferred. The heat treatment is less than the glass transition temperature of the polyimide constituting the asymmetric hollow fiber gas separation membrane and 450 ° C. or lower in order to maintain the polyimide skeleton, and higher than the temperature at which the substituent is decomposed, desorbed or cross-linked to become infusible Must be carried out in the temperature range. This temperature range is determined by the polyimide and its substituents. The temperature at which the substituent is decomposed / desorbed or crosslinked can be confirmed by pyrolysis gas chromatography / mass spectrometry or thermogravimetric mass spectrometry, or can be estimated by weight reduction or thermomechanical analysis by thermogravimetric analysis. Usually, it is 250-450 degreeC, Preferably it is 380-450 degreeC. In addition, the heat treatment may be a batch type or a continuous type, and the heat treatment time is not particularly limited, but is preferably 0.01 to 10 hours, particularly 0.1 to 2 hours from the viewpoint of treatment efficiency.

  The heat-treated asymmetric polyimide hollow fiber gas separation membrane substantially retains the polyimide skeleton and the asymmetric structure of the asymmetric polyimide hollow fiber gas separation membrane before heating. Further, it is infusibilized by decomposing / eliminating substituents or crosslinking. As a result, for example, parachlorophenol does not dissolve in the solvent that showed solubility before the heat treatment (solvent resistance is improved), and further, the organic vapor separation performance can be achieved by this heat treatment. Improved to.

  The transesterification reaction of the present invention will be described using a schematic diagram of an example of the production apparatus shown in FIG. A raw material dimethyl carbonate, an aliphatic dihydroxy compound, and, if necessary, a catalyst and a solvent are added to a reaction tank 1 equipped with a heating means and a stirring means, and stirred while heating to a predetermined temperature. A mixed vapor containing methanol vapor and dimethyl carbonate vapor by-produced from the reaction mixture is generated, superheated by the heating device 2, and introduced into the gas separation membrane device 3. The permeation side of the gas separation membrane of the gas separation device 3 is decompressed by the vacuum pump 4. In the gas separation membrane device 3, the mixed vapor contacts the gas separation membrane, and methanol vapor selectively permeates the membrane. Methanol vapor is highly concentrated on the permeate side of the membrane, and is condensed by the cooling device 5 and recovered in the recovery container 6 provided with cooling means. The mixed vapor flowing on the non-permeate side of the membrane has a high concentration of dimethyl carbonate, and is condensed by the cooling device 7 and returned to the reaction tank 1 by the pump 9 through the buffer tank 8 provided with cooling means.

  Hereinafter, the present invention will be described in more detail with reference to examples. In addition, this invention is not limited to a following example.

Abbreviations used in the following examples will be described.
s-BPDA: 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride 6FDA: 2,2-bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride PMDA: pyromellitic dianhydride Product TSN: 2,8-dimethyl-3,7-diaminodibenzothiophene-5,5-dioxide TCB: 2,2 ′, 5,5′-tetrachlorobenzidine MeOH: methanol DMC: dimethyl carbonate

The measurement method used in the following examples will be described.
(Measurement of gas separation performance for mixed steam)
Preparation of measurement hollow fiber membrane element: 10 hollow fiber membranes are bundled and cut to form a hollow fiber membrane bundle, and one end of the yarn bundle is fixed with an epoxy resin so that the end of the hollow fiber is opened, The other end was fixed with an epoxy resin so that the end of the hollow fiber was closed, and a hollow fiber membrane element for measurement having an effective length of 7.5 cm and an effective membrane area of 9.4 cm ² was manufactured.
Measurement of gas separation performance: A description will be given with reference to a schematic diagram (FIG. 2) of a gas separation membrane performance measuring apparatus. A flask 11 equipped with a heating device is charged with methanol and dimethyl carbonate at a predetermined mixing ratio so that the molar composition ratio of methanol and dimethyl carbonate in the generated mixed vapor is 7: 3, and heated with the heating device. The mixed organic vapor was generated. This mixed organic vapor was superheated by a heating device 12 for superheating to be a mixed organic vapor having an atmospheric pressure of 120 ° C., cooled and liquefied by a cooling device 15, and circulated to the flask 11. During this time, the measuring hollow fiber membrane element is not incorporated in the measuring device. The built-in part is sealed. After the preparation of the mixed organic vapor was continued for 2 hours or more, the mixed organic vapor was analyzed to confirm that the molar composition ratio of methanol to dimethyl carbonate was 7: 3. Thereafter, the measurement hollow fiber membrane element 10 is incorporated into the gas separation membrane performance measuring apparatus as shown in FIG. 2, and the permeation side (inside) of the hollow fiber membrane of the element is maintained at a reduced pressure of 0.0007 MPa by the vacuum pump 13. Gas separation started. After continuing the gas separation for 30 minutes or more as a running-in operation, the permeated gas obtained from the permeation side of the measurement hollow fiber membrane element 10 was led to the dry ice-methanol trap 14 for 30 minutes and collected as a condensate. While calculating | requiring the weight of the collected condensate, the density | concentration of each component was measured by the gas chromatography analysis method, and the quantity of each component of the permeate | transmitted organic vapor | steam was calculated | required. The permeation rate and permeation rate ratio of each organic vapor component were calculated from the amount of each organic vapor component that permeated. The amount of methanol and dimethyl carbonate charged was a sufficient amount so that the mixing ratio was not substantially changed during this measurement.

(Measurement of solvent resistance)
After three hollow fiber membranes cut to a length of 2 cm were completely immersed in 20 ml of parachlorophenol adjusted to a temperature of 50 ° C. and held for 30 minutes, the hollow fiber membranes were taken out and visually observed. Observed. In the case of dissolution during immersion, “x” indicates that the hollow fiber membrane shape is maintained but swelling is observed, and “Δ” indicates that no change is observed.

(GPC analysis)
Measured with Tosoh Corporation GPC equipment SC-8010 type (column: shodex KF-800D1 + KF-805L2, solvent: THF, temperature: 40 ° C., detector: differential refraction system), number in terms of standard polystyrene Average molecular weight and weight average molecular weight were determined.

⁽¹ H-NMR analysis method)
¹ H-NMR analysis was performed using JNM AL400 (400 MHz, temperature: room temperature, solvent: CDCl ₃ , reference material: TMS) of an FT-NMR apparatus manufactured by JEOL Ltd. Here, chemical _{shifts, -C H 2 OH: 3.65ppm,} -OC H 3: 3.77ppm, -C H 2 -OCOO-C H 2 -: a 4.15 ppm.

[Reference Example 1]
A gas separation membrane was produced as follows.
(Preparation of polyimide solution having substituents)
0.0315 mol s-BPDA, 0.0280 mol 6FDA, 0.0105 mol PMDA, 0.0355 mol TSN, and 0.0355 mol TCB together with 215 g of parachlorophenol, heating device and agitator Weigh in a separable flask equipped with a nitrogen gas inlet tube and a discharge tube, conduct a polymerization imidization reaction at a temperature of 170 ° C. for 15 hours with stirring in a nitrogen gas atmosphere, and dissolve in parachlorophenol A polyimide solution having a substituent with a polyimide concentration of 17% by weight was prepared.

(Production of asymmetric hollow fiber membrane made of polyimide having substituents)
This solution was filtered through a 400 mesh stainless steel wire mesh to obtain a dope for spinning. This dope is charged into a spinning device equipped with a hollow fiber spinning nozzle (circular opening outer diameter: 1000 μm, circular opening slit width: 200 μm, core opening outer diameter: 400 μm), and hollow fiber spinning nozzle. The hollow fiber-shaped molded product is then discharged into a primary coagulation bath at a temperature of 0 ° C. composed of a 70 wt% aqueous ethanol solution, and a secondary coagulation bath (coagulation solution) provided with a pair of guide rolls. The solidification was completed by reciprocating between guide rolls in a 70 wt% ethanol aqueous solution (temperature: 0 ° C.), and the wet asymmetric hollow fiber membrane was wound around a bobbin. The take-up speed was 10 m / min. This asymmetric hollow fiber membrane was thoroughly washed in ethanol, and then the ethanol was replaced with isooctane. Then, isooctane was evaporated and dried at 100 ° C. to obtain an asymmetric hollow fiber membrane composed of a polyimide having a substituent.

(Production and performance of infusibilized asymmetric hollow fiber gas separation membrane)
The asymmetric hollow fiber membrane composed of the polyimide having the substituent is heat-treated in an air atmosphere oven at a temperature of 400 ° C. for 30 minutes to obtain an asymmetric gas separation membrane having an outer diameter of about 400 μm and an inner diameter of about 200 μm. Obtained. The obtained infusible asymmetric gas separation membrane has solvent resistance and characteristics of the gas separation membrane. The solvent resistance is ○, and the permeation rate of methanol vapor (P ′ _MeOH ) is 11.3 × 10 ⁻⁵ cm ³ (STP ) / Cm ² · sec · cmHg, and the ratio of the permeation rate of methanol vapor to dimethyl carbonate vapor (P ′ _MeOH / P ′ _DMC ) was 10.6.

[Example 1]
A gas separation membrane device 3 composed of an infusible asymmetric hollow fiber gas separation membrane produced in Reference Example 1 and having an effective membrane area of 1370 cm ² was incorporated into the reaction apparatus as shown in FIG. Here, 1 is a reaction tank that can be heated and stirred, 2 is a heating means for superheating, 4 is a vacuum pump, 5 is a cooling device for cooling and condensing and trapping the permeated methanol vapor, 6 A recovery container having a cooling means, 7 is a cooling device for cooling and condensing the mixed vapor after methanol vapor is selectively removed, and refluxing it to the flask, 8 is a buffer tank having the cooling means, and 9 is a liquid It is a pump. Reference numeral 3 denotes a gas separation membrane device, in which a hollow fiber inner space and a hollow fiber outer space of the hollow fiber gas separation membrane are separated with a hollow fiber separation membrane interposed therebetween, and the hollow fiber inner space is interposed via a cooling device 5. The hollow fiber outer space communicates with the vacuum pump 4 so that the pressure is reduced, and the hollow fiber communicates with the inside of the reaction system, and the mixed vapor generated in the reaction tank 1 is superheated by the heating means 2 and then the hollow fiber. It arrange | positions so that the outer surface of a separation membrane may be contacted.

1.66 moles of 1,6-hexanediol, 2.66 moles of dimethyl carbonate (1.0 times moles relative to 1,6-hexanediol), and 0.001 mole of catalyst tetrabutoxytitanium are charged into the reaction vessel 1. The temperature of the reaction mixture was raised to 110 ° C. while stirring. The inside of the reaction tank 1 was an atmosphere of nitrogen gas at atmospheric pressure, and the permeation side of the gas separation membrane was decompressed to 0.0007 MPa by a vacuum pump. As the reaction progressed, the by-produced methanol vapor selectively permeated through the gas separation membrane, and was liquefied by the cooling device 5 and recovered in the recovery container 6 equipped with cooling means. The mixed vapor flowing on the non-permeate side of the separation membrane has a high concentration of dimethyl carbonate, and is condensed by the cooling device 7 and returned to the reaction tank 1 by the pump 9 through the buffer tank 8 provided with cooling means. . Further, as the reaction progressed, the temperature of the reaction mixture was gradually raised to 180 ° C. Since the reflux of dimethyl carbonate disappeared after a reaction time of 10 hours, the reaction was terminated.
The obtained reaction product was 392 g. From GPC analysis and ¹ H-NMR analysis, the number average molecular weight was 1053, the weight average molecular weight was 2442, and the terminal was a hydroxy group and an alkyl group (—O—CO—O—R). Was an oligocarbonate diol mixed. ^{A 1} H-NMR chart is shown in FIG.
Further, 128.9 g of a mixed solution consisting of 90% by weight of methanol and 10% by weight of dimethyl carbonate was recovered in the recovery container 6 equipped with a cooling means during the reaction from gas chromatography analysis. From this result, the reaction rate of dimethyl carbonate was 95%.

[Example 2]
The same reaction as in Example 1 was carried out except that 2.66 mol of 1,6-hexanediol and 2.93 mol of dimethyl carbonate (1.1 times mol to 1,6-hexanediol) were used.
The obtained reaction product was 367 g, and from GPC analysis and ¹ H-NMR analysis, the number average molecular weight was 1785, the weight average molecular weight was 4228, and the terminal was a hydroxy group and an alkyl group (—O—CO—O—R). Was an oligocarbonate diol mixed. ^{The 1} H-NMR chart is shown in FIG.
Further, 151.4 g of a mixed solution consisting of 88% by weight of methanol and 12% by weight of dimethyl carbonate was recovered in the recovery container 6 equipped with a cooling means during the reaction from gas chromatography analysis. From this result, the reaction rate of dimethyl carbonate was 95%.

Example 3
The reaction was carried out in the same manner as in Example 1, except that 2.66 mol of 1,6-hexanediol and 3.19 mol of dimethyl carbonate (1.2 times mol with respect to 1,6-hexanediol) were used.
The obtained reaction product was 379 g, the number average molecular weight was 1996 and the weight average molecular weight was 4845 from GPC analysis and ¹ H-NMR analysis, and the terminal was a hydroxy group and an alkyl group (—O—CO—O—R). Was an oligocarbonate diol mixed. ^{A 1} H-NMR chart is shown in FIG.
Further, 157.1 g of a mixed liquid consisting of 90% by weight of methanol and 10% by weight of dimethyl carbonate was recovered in the recovery container 6 equipped with a cooling means during the reaction from gas chromatography analysis. From this result, the reaction rate of dimethyl carbonate was 93%.

[Comparative Example 1]
1 except for the reflux line consisting of the heating means 2, the gas separation membrane device 3, the cooling device 7, the buffer tank 8 and the liquid pump 9 and the vacuum pump 4 from the reaction apparatus as shown in FIG. A device was used that was changed so that the carbonate mixed vapor was extracted out of the reaction system by a cooling device 5 for cooling and condensing and trapping, and a recovery vessel 6 equipped with cooling means.

1.66 moles of 1,6-hexanediol, 2.66 moles of dimethyl carbonate (1.0 times moles relative to 1,6-hexanediol), and 0.001 mole of catalyst tetrabutoxytitanium are charged into the reaction vessel 1. The temperature of the reaction mixture was raised to 110 ° C. while stirring. The reaction tank 1 was filled with nitrogen gas at atmospheric pressure, and the generated methanol-dimethyl carbonate mixed vapor was extracted out of the reaction system. As the reaction progressed, the temperature of the reaction mixture was gradually raised to 180 ° C. The reaction was completed in 10 hours after the reflux of dimethyl carbonate disappeared.
The obtained reaction product was 381 g. From GPC analysis and ¹ H-NMR analysis, the number average molecular weight was 586, the weight average molecular weight was 1321, and the terminal was a hydroxy group and an alkyl group (—O—CO—O—R). Was an oligocarbonate diol mixed. ^{A 1} H-NMR chart is shown in FIG.
Further, 122.2 g of a mixed solution consisting of 68% by weight of methanol and 32% by weight of dimethyl carbonate was recovered in the recovery container 6 equipped with a cooling means during the reaction from gas chromatography analysis. From this result, the reaction rate of dimethyl carbonate was 83%.

  The present invention is an improved method of transesterification of an aliphatic dihydroxy compound and dimethyl carbonate, and an oligo or polycarbonate diol can be obtained in a high yield using a relatively small amount of dimethyl carbonate. Furthermore, the methanol extracted out of the reaction system has a high purity and can be easily recovered and reused.

The schematic diagram for demonstrating the outline of an example of the manufacturing apparatus used by this invention Schematic schematic diagram of an apparatus for measuring the gas separation performance of a gas separation membrane ¹ H-NMR chart of the reaction product of Example 1 ¹ H-NMR chart of the reaction product of Example 2 ¹ H-NMR chart of the reaction product of Example 3 ¹ H-NMR chart of the reaction product of Comparative Example 1

Explanation of symbols

1: Reaction tank provided with heating means and stirring means 2: Heating means for superheating mixed steam 3: Gas separation membrane device 4: Vacuum pump 5: Cooling device 6: Recovery container 7 with cooling means: Cooling Apparatus 8: Buffer tank provided with cooling means 9: Liquid pump 10: Gas separation membrane apparatus 11: Heatable and stirred flask 12: Heating apparatus 13 for superheating mixed vapor 13: Vacuum pump 14: Cooling apparatus (trap)
15: Cooling device 16: Side pipe opening

Claims (5)

A mixed vapor containing by-product methanol and the raw material dimethyl carbonate is generated from a reaction mixture for transesterification of an aliphatic dihydroxy compound and dimethyl carbonate, and the mixed vapor is brought into contact with a gas separation membrane. A method of transesterifying an aliphatic dihydroxy compound and dimethyl carbonate , wherein the non-permeated dimethyl carbonate is returned to the reaction mixture. The method according to claim 1, wherein the gas separation membrane has a permeation rate ratio (P ′ _MeOH / P ′ _DMC ) of methanol vapor to dimethyl carbonate vapor at 120 ° C. of 2 or more. Permeation rate of methanol vapor at 120 ° C. of the gas separation membrane (P _'MeOH) is the following claims 1-2, characterized in that it is ^{1 × 10 -5 cm 3 (STP} ) / cm 2 · sec · cmHg or more The method according to any one. The method according to any of the claims 1-3, characterized in that the gas separation membrane is asymmetric polyimide hollow fiber membranes which does not dissolve the para-chlorophenol. A mixed vapor containing by-product methanol and the raw material dimethyl carbonate is generated from a reaction mixture for transesterification of an aliphatic dihydroxy compound and dimethyl carbonate , and the mixed vapor is brought into contact with a gas separation membrane. A process for producing an oligo or polycarbonate diol, comprising a step of transesterifying and removing non-permeated dimethyl carbonate while returning to the reaction mixture .

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