JP2012131728A - Method for preparing dimethyl terephthalate from polyester - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for efficiently and economically preparing dimethyl terephthalate, even when moisture is contained in a reaction raw material for preparing dimethyl terephthalate.SOLUTION: The method for preparing dimethyl terephthalate comprises: carrying out a depolymerization reaction of polyester having an alkylene terephthalate unit using alkylene glycol, thereby obtaining a depolymerized solution containing bis(ω-hydroxyalkyl)terephthalate; and carrying out transesterification reaction of the bis(ω-hydroxyalkyl)terephthalate contained in the depolymerized solution with methanol. Here, the transesterification reaction is carried out using a catalyst containing 0.01-10 wt.% calcium based on the weight of the polyester having the alkylene terephthalate unit used in the depolymerization reaction. At the initiation of the transesterification reaction, the reaction liquid contains 0.0001-2.3 wt.% moisture based on the weight of the transesterification reaction liquid.

Description

  The present invention relates to a method for producing dimethyl terephthalate, and more particularly to a method for producing dimethyl terephthalate from a polyester product.

  Polyester, such as polyethylene terephthalate, is widely used as a fiber, film, resin, etc. due to its excellent properties, but effective utilization of polyester waste such as fiber, film, resin, etc. generated in these manufacturing processes is costly. It is a big issue not only from the aspect but also from environmental issues. In addition, regarding polyester products, disposal of waste generated after use of the product (for example, empty plastic bottles after use) becomes an environmental problem.

  Various proposals have been made by material recycling, thermal recycling, and chemical recycling. Among these, for material recycling, for example, polyester resin scraps such as PET bottles are collected mainly by local governments and actively reused. However, material recycling is usually cascade recycling, and the quality decreases with each recycling. In addition, thermal recycling that converts polyester waste into fuel has the advantage of reusing the combustion heat of polyester waste, but the calorific value is relatively low, and it is nothing but burning a large amount of polyester waste. There is a problem of polyester raw material loss, which is not preferable from the viewpoint of resource saving.

  In contrast, in chemical recycling, polyester waste is recycled to raw material monomers, so there is little degradation in quality due to recycling, and it is excellent as closed-loop recycling. Most of the chemical recycling targets resin waste and film waste. For example, polyester waste was obtained after depolymerization with excess ethylene glycol (hereinafter sometimes abbreviated as EG). There have been proposed a method of directly polycondensing bis (β-hydroxyethyl) terephthalate (hereinafter sometimes abbreviated as BHET) to obtain a regenerated polyester (for example, see Patent Documents 1 and 2). In these methods, BHET is obtained as a polyester raw material. However, since BHET is difficult to purify by distillation, the purity of the obtained BHET tends to be low. On the other hand, as a method of obtaining a high-purity polyester raw material from polyester waste, high-purity dimethyl terephthalate (hereinafter abbreviated as DMT) is obtained by depolymerizing polyester waste with EG and transesterifying the obtained BHET with methanol. There is a method of obtaining the above (see, for example, Patent Document 3). However, since EG and methanol have high hygroscopicity, water may enter the reaction system, and sodium carbonate used as a catalyst in the transesterification reaction is easily deactivated by the water present in the reaction system. There's a problem.

JP-A-48-061447 JP 2005-330444 A Japanese Patent No. 4067306

  The present invention provides a method for producing dimethyl terephthalate efficiently and economically in the production of dimethyl terephthalate even when water is contained in the reaction raw material.

  As a result of intensive studies in view of the above-described prior art, the present inventors have completed the present invention. That is, in the present invention, a polyester having an alkylene terephthalate unit is subjected to a depolymerization reaction with an alkylene glycol to obtain a depolymerization solution containing bis (ω-hydroxyalkyl) terephthalate, and then the bis contained in the depolymerization reaction solution. In the transesterification of (ω-hydroxyalkyl) terephthalate with methanol, a catalyst containing 0.01 to 10% by weight of calcium in a weight ratio with respect to the polyester having an alkylene terephthalate unit used in the depolymerization reaction was used. The present invention relates to a method for producing dimethyl terephthalate, characterized in that the amount of water contained in the reaction liquid at the start of the exchange reaction is 0.0001 to 2.3% by weight with respect to the weight of the transesterification reaction liquid. is there.

  According to the present invention, in the production of dimethyl terephthalate, dimethyl terephthalate can be produced efficiently and economically even if water is contained in the reaction raw material.

The present invention will be described below based on embodiments.
Specific examples of the polyester having an alkylene terephthalate unit in the present embodiment include polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyhexamethylene terephthalate, poly-1,4-cyclohexylene dimethylene terephthalate, and the like. it can. More specifically, it is preferable that the product mainly contains polyethylene terephthalate, and the polyester may contain a different material. For example, in a fiber product, the product is used for dyeing polyethylene terephthalate fiber. In addition to the dye, other fiber materials such as nylon and cotton, which are included in the form of blends, and other plastic components used for the purpose of surface modification may be included. Here, “mainly” means 80% by weight or more, preferably 90% by weight or more based on the polyester.

  Specifically, the alkylene glycol in the present embodiment is ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, hexamethylene glycol, diethylene glycol, triethylene glycol, bis (trimethylene). ) Glycol) [alias: bis (3-hydroxypropyl) ether], bis (tetramethylene) glycol [alias: bis (4-hydroxybutyl) ether], 1,4-cyclohexanedimethanol and the like. Among these, it is preferable to use an alkylene glycol having the same chemical structural formula corresponding to the alkylene portion of the polyalkylene terephthalate unit, and it is more preferable to use ethylene glycol. The alkylene glycol used in the depolymerization step is preferably used 0.5 to 20 times by weight, more preferably 1.0 to 10 times by weight based on the weight of the polyalkylene terephthalate.

  In the present invention, an excess molar amount of alkylene glycol is added to a product (polyester) containing polyalkylene terephthalate, depolymerized, and bis (ω-hydroxyalkyl) terephthalate (hereinafter sometimes abbreviated as BHAT) and their products. A depolymerized solution containing an oligomer component can be obtained. In the depolymerization reaction, it is preferable to use a known depolymerization catalyst within a known catalyst concentration range and perform the depolymerization reaction in an excess of alkylene glycol heated to 120 to 230 ° C. When the temperature of the alkylene glycol is less than 120 ° C., the depolymerization time becomes very long and it is not efficient. On the other hand, when the temperature exceeds 230 ° C., the polyester is a polyester fiber, and thermal decomposition of a different material contained in the polyester fiber waste becomes remarkable, and the nitrogen compound generated by decomposition is dispersed in a useful component to be recovered. In the subsequent process for recovering useful components, separation becomes difficult. In the depolymerization reaction, the pressure during the reaction can be maintained at 0.1 to 0.5 MPa. Types of depolymerization catalysts include lithium acetate, sodium acetate, potassium acetate, rubidium acetate, zinc acetate, manganese acetate, lithium carbonate, sodium carbonate, potassium carbonate, rubidium carbonate, lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, Rubidium bicarbonate, lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, lithium alkoxide, sodium alkoxide, potassium alkoxide, rubidium alkoxide, magnesium carbonate, calcium carbonate, magnesium hydroxide, calcium hydroxide, magnesium alkoxide, calcium It is preferable to use at least one compound selected from the group consisting of alkoxides. The addition amount of these depolymerization catalysts is preferably 0.01 to 10.0% by weight and more preferably 0.01 to 5.0% by weight with respect to the polyalkylene terephthalate.

  Further, the depolymerization liquid can be concentrated by distilling and evaporating a part of the alkylene glycol from the depolymerization liquid obtained after the completion of the depolymerization reaction. By performing this operation, the amount of methanol used in the subsequent transesterification reaction can be reduced. Furthermore, the alkylene glycol recovered by distillation / evaporation can be effectively used, for example, again in the above-mentioned depolymerization reaction. The amount of alkylene glycol distilled / evaporated in this step is 0.1 to 1.0 times the amount of alkylene glycol added during the depolymerization reaction, or alkylene glycol and polyalkylene terephthalate. It is preferable to carry out until the weight ratio becomes 0.5 to 2.0 times. In addition, it is preferable to carry out the distillation / evaporation step at a temperature in the range of 120 to 180 ° C. in terms of high purity and the ability to obtain an alkylene glycol that can be used later in a depolymerization reaction or the like. . The pressure during the distillation / evaporation step is preferably 0.001 to 0.1 MPa.

  Next, the depolymerization solution obtained by the depolymerization reaction is reacted with methanol using a catalyst containing calcium to perform a transesterification reaction. The amount of methanol used in the transesterification reaction is preferably a large amount of methanol from the viewpoint of the equilibrium reaction because the transesterification reaction is an equilibrium reaction, but if too large, separation and purification of methanol after the reaction becomes difficult. The polyalkylene terephthalate is preferably used in an amount of 0.5 to 10.0 times by weight, more preferably 1.0 to 5.0 times by weight.

  As the catalyst containing calcium in the present embodiment, calcium oxide, calcium hydroxide, calcium carbonate, calcium carbide, hydroxyapatite, calcium chloride, calcium fluoride, calcium hypochlorite, calcium phosphate, calcium phosphide, calcium nitrate, sulfuric acid Calcium, calcium alkoxide, calcium acetate, calcium glycolate, calcium glycerate hydrate, calcium stearate, calcium 3-hydroxy-3-methylbutyrate, calcium 3-methyl-2-oxovalerate, calcium 4-aminosalicylate heptahydrate Products, 4-chlorobenzoyl-L-tryptophan calcium, bis (2,4-pentanedionato) calcium (II), and the like. Preferred are calcium oxide, calcium hydroxide, and calcium carbonate. These may be used as a catalyst as they are, or may be supported on a carrier. The carrier supporting calcium in the present embodiment is not particularly limited, and examples of the inorganic carrier include an inorganic oxide carrier, an inorganic hydroxide carrier, and an activated carbon carrier. More specifically, examples of the inorganic oxide include one or more inorganic oxides selected from silica, titania, zirconia, alumina, magnesia, zinc oxide, tin oxide, and the like, or a composite thereof. Examples of the composite of inorganic oxide include composite oxides such as magnesium aluminate and zeolites such as β-zeolite (Na). Examples of the inorganic hydroxide include titanium hydroxide, zirconium hydroxide, alumina hydroxide, magnesia hydroxide, zinc hydroxide, and tin hydroxide. Preferred examples include silica, alumina (γ-alumina, δ-alumina, θ-alumina, α-alumina), aluminum hydroxide (gibbsite, buyerite, boehmite, diaspore), and more preferably silica or alumina. It is.

  The amount of calcium supported when calcium is supported on the carrier is not particularly limited, but from the viewpoint of efficiently producing dimethyl terephthalate, it is preferably 5.0 to 30% by weight, preferably 10 to 25% by weight based on the total amount of the catalyst. % Is more preferable. The method for producing the calcium-supported catalyst is not particularly limited, but may be dried and then calcined after impregnating the carrier with a calcium-containing liquid. When the amount of the calcium-containing liquid in the catalyst production is large, the catalyst may be produced by an incipient wetness method in which the impregnation and drying are repeated or an evaporation to dryness method. Further, the temperature at which the carrier is impregnated and the temperature at which the catalyst is dried in the process of producing the catalyst are not particularly limited. Although the temperature of the said baking is not specifically limited, 300-700 degreeC is preferable and 500 degreeC is more preferable. Also, the atmosphere at the time of firing is not particularly limited, such as an air atmosphere, an inert gas atmosphere such as nitrogen or argon, a reducing gas atmosphere such as hydrogen, etc., but the atmosphere at the time of firing is preferably an air atmosphere. . The amount of the catalyst used in the transesterification reaction is 0 by weight ratio with respect to the polyester having an alkylene terephthalate unit used in the depolymerization reaction, in other words, to the polyester having a polyalkylene terephthalate unit used in the depolymerization step. The amount is preferably 0.01 to 10.0% by weight, and more preferably 0.1 to 9.0% by weight for the purpose of allowing the reaction to proceed efficiently and sufficiently. In addition to the catalyst containing calcium as a catalyst in the transesterification reaction, other known alkaline earth metal compounds or alkali metal compounds may be added. Specifically, zinc acetate, manganese acetate, tin acetate, lithium acetate, sodium acetate, potassium acetate, rubidium acetate, cesium acetate, magnesium acetate, strontium acetate, barium acetate, lithium carbonate, sodium carbonate, potassium carbonate, rubidium carbonate, Cesium carbonate, magnesium carbonate, strontium carbonate, barium carbonate, lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, rubidium bicarbonate, lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide, hydroxide Magnesium, strontium hydroxide, barium hydroxide, lithium alkoxide, sodium alkoxide, potassium alkoxide, rubidium alkoxide, magnesium alkoxide, strontium alkoxide Preferable to use at least one compound selected from the group. The addition amount of these compounds is preferably the above-mentioned use amount including the catalyst containing calcium. In the production method of the present invention, when a catalyst containing calcium is used in the transesterification reaction with an alcohol such as methanol as compared with a conventional catalyst, methanol is used even if water is present within a predetermined range. It is considered that the deprotonation from is proceeded effectively. As a result, since the nucleophilic substitution reaction for the ester bond of the depolymerized reaction product proceeds effectively even in the presence of water, the transesterification reaction proceeds efficiently.

  The temperature of the transesterification reaction in the present embodiment is preferably 60 to 90 ° C. because a high pressure facility is required if the reaction temperature is too high, and the reaction rate is slow if the reaction temperature is too low. The amount of water contained in the reaction liquid at the start of the transesterification reaction is reduced with respect to the weight of the reaction liquid at the start of the transesterification because the yield of dimethyl terephthalate decreases if the amount of water contained is too much. The weight ratio is preferably 0.0001 to 2.3% by weight, and more preferably 0.001 to 2.1% by weight. More preferably, it is 0.05 to 2.0% by weight, still more preferably 0.08 to 1.5% by weight, and most preferably 0.1 to 1.0% by weight.

  If the water content exceeds 2.3% by weight, dimethyl terephthalate cannot be produced in a sufficient yield even by the production method of the present invention. After the transesterification reaction, dimethyl terephthalate can be produced by solid-liquid separation of the produced dimethyl terephthalate crystals by solid-liquid separation means such as centrifugation. When a high-purity product is desired, the purity of dimethyl terephthalate can be increased by purification by distillation.

  Further, after transesterification, by adding a residue obtained by removing methanol and ethylene glycol from the residual liquid from which dimethyl terephthalate has been separated to the depolymerization reaction, the calcium component and BHAT flowing into the residual liquid are added. And active ingredients such as dimethyl terephthalate can be recovered. This recycling process is suitable when it is desired to increase the amount of dimethyl terephthalate produced for a given amount of input polyalkylene terephthalate.

  Hereinafter, the content of the present invention will be described more specifically with reference to examples, but the present invention is not limited thereto. The product was analyzed by gas chromatograph (GC) using a flame ionization detector. “GC-17A” manufactured by Shimadzu Corporation was used for GC, and capillary column “TC-5HT” (column inner diameter 0.25 mm, length 30 m) manufactured by GL Sciences Inc. was used for the GC column for analysis. For measurement of moisture, a Karl Fischer moisture meter CA-06 manufactured by Mitsubishi Chemical Corporation was used.

“Dimethyl terephthalate production” shown in the following Examples and Comparative Examples is a value calculated based on the following formula (1).
Formula (1): Production amount of dimethyl terephthalate = (cake weight after solid-liquid separation) × (purity of dimethyl terephthalate in cake (% by weight)) / 100

[Example 1]
100 g of polyethylene terephthalate, 400 g of EG, and 3 g of sodium carbonate as a depolymerization catalyst were charged into a three-necked flask equipped with a reflux tube and subjected to a depolymerization reaction with stirring at a reaction temperature of 200 ° C. and normal pressure for 3 hours to obtain a depolymerization solution containing BHET. Thereafter, 300 g of EG was distilled off in the same reactor under the conditions of a temperature of 160 ° C. and a pressure of 13.3 kPa to concentrate the depolymerization solution. Next, 0.37 g of calcium oxide (0.37% by weight based on polyethylene terephthalate, hereinafter the same in Examples 3, 4 and 6), 176 g of methanol and 1 g of pure water were added to the reactor as a transesterification catalyst, A part of the reaction solution (0.2 g) was sampled for the amount measurement, and the water content was measured. As a result, the water concentration was 0.37% by weight. The remaining reaction solution was subjected to a transesterification reaction under normal pressure at a reaction temperature of 75 to 80 ° C. for 1 hour. At this time, precipitation of DMT crystals was confirmed 1 minute after the start of the transesterification reaction. After the reaction, the reaction solution is cooled to 10 ° C. and then suction filtered to separate the solid and liquid. The obtained cake is washed and filtered twice with 100 g of methanol, dried at 90 ° C. all day and night, and a cake containing dimethyl terephthalate as a main component. Got. The production amount of dimethyl terephthalate was 81.8 g. The results are shown in Table 1.

[Example 2]
Dimethyl terephthalate was produced in the same manner as in Example 1 except that the amount of pure water added during the transesterification reaction was changed from 1 g to 7 g. At this time, precipitation of DMT crystals was confirmed 25 minutes after the start of the transesterification reaction, and the production amount of dimethyl terephthalate was 44.5 g. Moreover, the water concentration contained in the reaction liquid at the start of the transesterification reaction was 2.0% by weight. The results are shown in Table 1.

[Example 3]
Dimethyl terephthalate was produced in the same manner as in Example 1 except that 0.37 g of calcium oxide was replaced with 0.49 g (0.49 wt%) of calcium hydroxide as a transesterification catalyst. At this time, precipitation of DMT crystals was confirmed 5 minutes after the start of the transesterification reaction, and the production amount of dimethyl terephthalate was 82.1 g. Moreover, the water concentration contained in the reaction liquid at the start of the transesterification reaction was 0.39% by weight. The results are shown in Table 1.

[Example 4]
Dimethyl terephthalate was produced in the same manner as in Example 1 except that the amount of calcium oxide used as the transesterification catalyst was changed from 0.37 g to 0.1 g (0.10 wt%). At this time, precipitation of DMT crystals was confirmed 20 minutes after the start of the transesterification reaction, and the production amount of dimethyl terephthalate was 70.5 g. Moreover, the water concentration contained in the reaction liquid at the start of the transesterification reaction was 0.38% by weight. The results are shown in Table 1.

[Example 5]
Dimethyl terephthalate was produced in the same manner as in Example 1 except that pure water was not added during the transesterification reaction and that EG and methanol were used to absorb moisture. At this time, precipitation of DMT crystals was confirmed 1 minute after the start of the transesterification reaction, and the production amount of dimethyl terephthalate was 83.1 g. Moreover, the water concentration contained in the reaction liquid at the start of the transesterification reaction was 0.12% by weight. The results are shown in Table 1.

[Example 6]
(Precursor solution preparation process)
11.7846 g of calcium nitrate tetrahydrate was dissolved in 50 mL of distilled water and lightly stirred to make the solution uniform to prepare a precursor solution.
(Supporting process)
The prepared precursor solution is dropped onto 7.2016 g of γ-alumina serving as a carrier until the carrier is evenly wetted using a dropper, stirred with a glass rod, and then heated to about 80 ° C. to remove moisture. Flew and dried. The same operation was performed until the precursor solution disappeared. Thereafter, the solid impregnated with the precursor solution was dried overnight in a constant temperature dryer at 110 ° C.
(Baking process)
The dried solid was calcined in a muffle furnace in air at 500 ° C. for 3 hours to obtain a Ca (20) / γ-alumina catalyst (the number in parentheses immediately after the metal element symbol is the support in the total catalyst weight). % By weight of calcium).
(Production of dimethyl terephthalate)
As a transesterification catalyst, dimethyl terephthalate was produced in the same manner as in Example 1 except that 0.37 g of calcium oxide was replaced with 5 g (5.0% by weight) of Ca (20) / γ-alumina prepared above. . At this time, precipitation of DMT crystals was confirmed 5 minutes after the start of the transesterification reaction, and the production amount of dimethyl terephthalate was 82.5 g. Moreover, the water concentration contained in the reaction liquid at the start of the transesterification reaction was 0.37% by weight. The results are shown in Table 1.

[Comparative Example 1]
Dimethyl terephthalate was produced in the same manner as in Example 1 except that 0.37 g of calcium oxide was changed to 0.7 g of sodium carbonate as a transesterification catalyst. However, no precipitation of DMT crystals was observed even after 1 hour from the start of the transesterification reaction, and the experiment was stopped because the reaction was insufficient. The water concentration contained in the reaction solution at the start of the transesterification reaction was 0.37% by weight. The results are shown in Table 1.

[Comparative Example 2]
Dimethyl terephthalate was produced in the same manner as in Example 1 except that the amount of pure water added during the transesterification reaction was changed from 1 g to 10 g. However, no precipitation of DMT crystals was observed even after 1 hour from the start of the transesterification reaction, and the experiment was stopped because the reaction was insufficient. The water concentration contained in the reaction solution at the start of the transesterification reaction was 2.7% by weight. The results are shown in Table 1.

[Comparative Example 3]
Dimethyl terephthalate was produced in the same manner as in Example 1 except that 0.37 g of calcium oxide was changed to 0.27 g of magnesium oxide as a transesterification catalyst. However, no precipitation of DMT crystals was observed even after 1 hour from the start of the transesterification reaction, and the experiment was stopped because the reaction was insufficient. The water concentration contained in the reaction solution at the start of the transesterification reaction was 0.37% by weight. The results are shown in Table 1.

  As shown in Table 1, it was confirmed that by using a catalyst containing calcium as the transesterification catalyst, dimethyl terephthalate can be produced efficiently and economically even if water is contained in the reaction raw material.

  According to the present invention, in the production of dimethyl terephthalate, by using a catalyst containing calcium as a transesterification catalyst, dimethyl terephthalate can be efficiently and economically produced even if moisture is contained in the reaction raw material. And its industrial significance is great.

Claims (3)

  A polyester having an alkylene terephthalate unit is subjected to a depolymerization reaction with alkylene glycol to obtain a depolymerization solution containing bis (ω-hydroxyalkyl) terephthalate, and then bis (ω-hydroxyalkyl) contained in the depolymerization reaction solution. When transesterifying terephthalate with methanol, a reaction containing 0.01 to 10% by weight of calcium in a weight ratio with respect to the polyester having an alkylene terephthalate unit used in the depolymerization reaction is performed at the start of the transesterification reaction. A method for producing dimethyl terephthalate, characterized in that the amount of water contained in the liquid is 0.0001 to 2.3% by weight with respect to the weight of the transesterification reaction liquid.   The method for producing dimethyl terephthalate according to claim 1, wherein the catalyst containing calcium is a catalyst containing calcium oxide, calcium hydroxide or calcium carbonate.   The method for producing dimethyl terephthalate according to claim 1 or 2, wherein the catalyst containing calcium is supported on at least one selected from alumina and silica.