Method for preparing dimethyl terephthalate by methyl esterification of crude terephthalic acid

Abstract

The invention discloses a method for preparing dimethyl terephthalate by methyl esterification of crude terephthalic acid, which comprises the following steps: washing and drying the crude terephthalic acid, and mixing with methanol to prepare crude terephthalic acid-methanol slurry; mixing one half of the crude terephthalic acid-methanol slurry with a catalyst, and carrying out a first-stage esterification reaction; and adding the other half of the crude terephthalic acid-methanol slurry and a catalyst into the first-stage esterification reaction product, adding a water absorbent at the same time, and carrying out second-stage esterification reaction to obtain dimethyl terephthalate. According to the invention, the water absorbent is added to remove water generated by esterification reaction in time, so that the esterification reaction balance is promoted to move towards the direction of generating dimethyl terephthalate, and the hydrolysis of dimethyl terephthalate is inhibited; the reaction process has the advantages of high safety, simple process, no involvement of corrosive catalysts and no generation of acid-containing wastewater.

Description

Technical Field

[0001] The invention relates to the technical field of organic chemical industry, in particular to a method for preparing dimethyl terephthalate by methyl terephthalate of crude terephthalate. Background Art

[0002] Dimethyl terephthalate (DMT) is an important chemical raw material intermediate, which is widely used in the manufacture and production of polyester resins, polyester fibers, films, high-strength polyester insulating paints and engineering plastics. It is a polyester raw material with a very large amount of use. The preparation of dimethyl terephthalate is usually completed with purified terephthalic acid (PTA) and methanol as raw materials under the action of a catalyst and certain reaction conditions. Purified terephthalic acid, one of the raw materials for the production of dimethyl terephthalate, is produced by oxidation reaction with para-xylene (PX) as raw material under a catalyst and appropriate reaction conditions. The product obtained by the above oxidation reaction is crude terephthalic acid, which usually contains a small amount of impurities such as p-carboxybenzaldehyde (4-CBA). The crude terephthalic acid needs to be further refined to obtain purified terephthalic acid, which is then used in the preparation and production of dimethyl terephthalate. If crude terephthalic acid is directly used as raw material and dimethyl terephthalate is obtained by esterification reaction with methanol, the crude terephthalic acid refining process can be omitted, which will greatly reduce the corresponding equipment cost and process cost. Therefore, the development of a reaction process for preparing dimethyl terephthalate by methyl esterification of crude terephthalic acid has great significance and industrial value.

[0003] CN114516793A discloses a method for producing terephthalic acid esters by continuous esterification. The method comprises preparing solid powdered purified terephthalic acid and part of the esterification product recycled from an esterification tower or a flash tank into a slurry, reacting with superheated alkyl alcohol vapor in the esterification tower, using or not using a catalyst in the esterification process, partially recycling the generated material back to the slurry mixing kettle, and partially sending the material to the downstream as a product, and the whole process enables continuous and stable transportation of purified terephthalic acid solid material. However, the method requires heating the alkyl alcohol to 240-285°C and increasing the pressure to 2.5-4.0MPa to form superheated alkyl alcohol vapor, which is then introduced into the esterification tower for reaction. The process consumes a lot of energy and has a high pressure, which poses a potential safety hazard.

[0004] CN1320593A discloses a process for producing dimethyl terephthalate. In this process, terephthalic acid, methanol and catalyst sulfate are first put into a reactor to react for a period of time, then the excess methanol and water are evaporated under reduced pressure, and then the same amount of methanol and sulfate as the first reaction is added for a second reaction. After the reaction is completed, the material is unloaded, drained, dried and vacuum distilled to obtain dimethyl terephthalate. This method needs to remove the excess methanol and water by vacuum evaporation after the first reaction, which is complicated and increases energy consumption.

[0005] CN104072374A discloses a method for preparing dimethyl terephthalate. The method uses crude terephthalic acid and methanol as reaction raw materials, concentrated sulfuric acid as a catalyst, and performs an esterification reaction at a certain temperature and pressure to prepare dimethyl terephthalate. The method uses highly corrosive concentrated sulfuric acid as a catalyst, has extremely high corrosion resistance requirements for equipment, and produces acid-containing wastewater during the process, which is less environmentally friendly and increases the cost of wastewater treatment.

[0006] Based on the above existing needs and deficiencies in the prior art, there is an urgent need to develop an efficient, environmentally friendly and economical reaction process for preparing dimethyl terephthalate from crude methyl terephthalate. Summary of the invention

[0007] The present invention aims to overcome the above problems existing in the prior art and provides a method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate. The method timely removes water generated by the esterification reaction by adding a water absorbent, promotes the esterification reaction equilibrium to move toward the direction of generating dimethyl terephthalate, and inhibits the hydrolysis of dimethyl terephthalate. The reaction process has the advantages of high safety, simple process, no corrosive catalyst involved and no acid-containing wastewater generated.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions: A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) washing and drying crude terephthalic acid and mixing it with methanol to prepare a crude terephthalic acid-methanol slurry; (2) mixing half of the crude terephthalic acid-methanol slurry with a catalyst to carry out a first stage esterification reaction; (3) adding the other half of the crude terephthalic acid-methanol slurry and the catalyst to the product of the first stage esterification reaction, and simultaneously adding a water absorbent to carry out a second stage esterification reaction to obtain dimethyl terephthalate; The catalyst described in step (2) and step (3) is one or a combination of two or more of tin compounds, zinc compounds, antimonides, titanium compounds, alkaline earth metal salts, H-ZSM-5 molecular sieves, and polyferric sulfate.

[0009] The invention divides the methyl esterification reaction of crude terephthalic acid into two stages, and adds a water absorbent in the second stage of the esterification reaction, so as to timely remove the water generated by the esterification reaction, promote the esterification reaction balance to move toward the direction of generating dimethyl terephthalate, and inhibit the hydrolysis of dimethyl terephthalate, thereby improving the conversion rate of crude terephthalic acid and the selectivity of dimethyl terephthalate. At the same time, the reaction method of the invention has the advantages of high safety, simple process, no corrosive catalyst involved and no acid-containing wastewater generated.

[0010] Preferably, in the crude terephthalic acid-methanol slurry described in step (1), the mass ratio of crude terephthalic acid to methanol is 1:2-10.

[0011] Preferably, the drying temperature after water washing in step (1) is 100 to 200° C., and the drying time is 4 to 20 hours.

[0012] Preferably, the mass of the catalyst added in step (2) is 2 to 10% of the mass of the crude terephthalic acid in the crude terephthalic acid-methanol slurry.

[0013] Preferably, the reaction temperature of the first stage esterification reaction in step (2) is 150-300° C., the reaction pressure is 1-10 MPa, and the reaction time is 1-10 h.

[0014] Preferably, the mass of the catalyst added in step (3) is 2 to 10% of the mass of the crude terephthalic acid in the crude terephthalic acid-methanol slurry.

[0015] Preferably, the mass of the water absorbent added in step (3) is 0.1 to 3% of the total mass of the crude terephthalic acid-methanol slurry added in steps (2) and (3).

[0016] Preferably, the reaction temperature of the second stage esterification reaction in step (3) is 150-300° C., the reaction pressure is 1-10 MPa, and the reaction time is 1-10 h.

[0017] Preferably, the catalyst described in step (2) and step (3) is composed of H-ZSM-5 molecular sieve, titanium dioxide and polyferric sulfate. The present invention uses H-ZSM-5 molecular sieve, titanium dioxide and polyferric sulfate as a catalyst for the reaction of methyl esterification of terephthalic acid to dimethyl terephthalate. Unrefined crude terephthalic acid can be used as a reaction raw material, eliminating the refining process of terephthalic acid and reducing production costs. The catalyst of the present invention exhibits excellent catalytic performance in the process of catalyzing the methyl esterification of crude terephthalic acid to dimethyl terephthalate, and the crude terephthalic acid conversion rate and dimethyl terephthalate selectivity can reach more than 98%. The catalyst of the present invention is basically non-corrosive to equipment and does not produce acid-containing wastewater and toxic by-products.

[0018] Preferably, the water absorbent in step (3) is one or a combination of two or more of silica gel, calcium chloride and molecular sieve desiccant.

[0019] Therefore, the present invention has the following beneficial effects: (1) The process is simple, does not involve corrosive catalysts, does not produce acidic wastewater, and is environmentally friendly; (2) During the second stage of the esterification reaction, a water absorbent is added to promptly remove the water generated by the esterification reaction, thereby promoting the equilibrium of the esterification reaction to move toward the formation of dimethyl terephthalate and inhibiting the hydrolysis of dimethyl terephthalate, thereby increasing the conversion rate of crude terephthalic acid and the selectivity of dimethyl terephthalate. DETAILED DESCRIPTION

[0020] The present invention is further described below in conjunction with specific implementation methods.

[0021] In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in the industry. The methods in the following embodiments, unless otherwise specified, are conventional methods in the art.

[0022] Overall embodiment: A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) washing and drying crude terephthalic acid and mixing it with methanol to prepare a crude terephthalic acid-methanol slurry; (2) mixing half of the crude terephthalic acid-methanol slurry with a catalyst to carry out a first stage esterification reaction; (3) adding the other half of the crude terephthalic acid-methanol slurry and the catalyst to the product of the first stage esterification reaction, and simultaneously adding a water absorbent to carry out a second stage esterification reaction to obtain dimethyl terephthalate; The catalyst described in step (2) and step (3) is one or a combination of two or more of tin compounds, zinc compounds, antimonides, titanium compounds, alkaline earth metal salts, H-ZSM-5 molecular sieves, and polyferric sulfate.

[0023] As a specific implementation manner, in the crude terephthalic acid-methanol slurry described in step (1), the mass ratio of crude terephthalic acid to methanol is 1:2-10; preferably, the mass ratio of crude terephthalic acid to methanol is 1:2-5.

[0024] As a specific implementation, the drying temperature after washing in step (1) is 100-200° C., and the drying time is 4-20 hours; preferably, the drying temperature is 120-160° C., and the drying time is 8-16 hours; As a specific implementation manner, the mass of the catalyst added in step (2) is 2 to 10% of the mass of the crude terephthalic acid in the crude terephthalic acid-methanol slurry added; preferably, the mass of the catalyst added is 2 to 6% of the mass of the crude terephthalic acid in the crude terephthalic acid-methanol slurry added.

[0025] As a specific implementation manner, the reaction temperature of the first stage esterification reaction in step (2) is 150-300°C, the reaction pressure is 1-10 MPa, and the reaction time is 1-10 h; preferably, the reaction temperature of the first stage esterification reaction is 150-250°C, the reaction pressure is 1-3 MPa, and the reaction time is 1-3 h.

[0026] As a specific implementation manner, the mass of the catalyst added in step (3) is 2 to 10% of the mass of the crude terephthalic acid in the crude terephthalic acid-methanol slurry added; preferably, the mass of the catalyst added is 2 to 6% of the mass of the crude terephthalic acid in the crude terephthalic acid-methanol slurry added.

[0027] As a specific implementation manner, the mass of the water absorbing agent added in step (3) is 0.1 to 3% of the total mass of the crude terephthalic acid-methanol slurry added in steps (2) and (3); preferably, the mass of the water absorbing agent added is 0.5 to 1.5% of the total mass of the crude terephthalic acid-methanol slurry added in steps (2) and (3).

[0028] As a specific implementation manner, the reaction temperature of the second stage esterification reaction in step (3) is 150-300°C, the reaction pressure is 1-10 MPa, and the reaction time is 1-10 h; preferably, the reaction temperature of the second stage esterification reaction is 150-250°C, the reaction pressure is 1-3 MPa, and the reaction time is 1-5 h.

[0029] As a specific implementation, the water absorbent in step (3) is one or a combination of two or more of silica gel, calcium chloride, and molecular sieve desiccant.

[0030] As a specific implementation, the catalysts described in step (2) and step (3) are composited from H-ZSM-5 molecular sieve, titanium dioxide and polyferric sulfate.

[0031] As a specific embodiment, the method for preparing the catalyst described in step (2) and step (3) comprises the following steps: A) mixing H-ZSM-5 molecular sieve with an organic acid solution, stirring the reaction, filtering, washing, drying, and then calcining the product to obtain a modified H-ZSM-5 molecular sieve; B) mixing the modified H-ZSM-5 molecular sieve with titanium dioxide to obtain a mixed powder; mixing the mixed powder with deionized water, stirring evenly, performing a hydrothermal reaction, filtering and drying the product to obtain a composite carrier; C) dissolving polyferric sulfate in an organic solvent to form an impregnation solution; impregnating the composite carrier in the impregnation solution and then drying to obtain the catalyst.

[0032] When the H-ZSM-5 molecular sieve, titanium dioxide and polyferric sulfate are compounded, the H-ZSM-5 molecular sieve is first treated with an organic acid and roasted. The acid treatment can effectively remove non-stock aluminum species in the pore channel of the H-ZSM-5 molecular sieve, and play a role in dredging the pore channel; at the same time, the acid treatment can also expand the pores of the H-ZSM-5 molecular sieve to a certain extent, so that the active sites in the pore channel participate more in the methylation reaction. Then the modified H-ZSM-5 molecular sieve is mixed with titanium dioxide for hydrothermal reaction to obtain a composite carrier. Compounding under hydrothermal conditions can promote the uniform mixing of the two at the micrometer or even nanometer scale. It is beneficial to improve the active sites of the catalyst. Finally, the polyferric sulfate is loaded on the composite carrier, which can further improve the catalytic activity of the catalyst in the reaction of preparing dimethyl terephthalate by methylation of crude terephthalate.

[0033] As a specific implementation, in step A), the silicon-aluminum ratio of the H-ZSM-5 molecular sieve is 30-120; preferably, the silicon-aluminum ratio of the H-ZSM-5 molecular sieve is 40-100.

[0034] As a specific implementation, the organic acid solution in step A) is one or a combination of two or more of citric acid solution, oxalic acid solution, tartaric acid solution, acetic acid solution, malic acid solution, and succinic acid solution.

[0035] As a specific implementation, the concentration of the organic acid solution is 0.001-0.5 mol / L; preferably, the concentration of the organic acid solution is 0.1-0.2 mol / L.

[0036] As a specific implementation, in step A), the mass ratio of H-ZSM-5 molecular sieve to organic acid solution is 1:5 to 1:100; preferably, the mass ratio of H-ZSM-5 molecular sieve to organic acid solution is 1:10 to 1:50.

[0037] As a specific implementation, the calcination temperature in step A) is 400-600° C., and the calcination time is 2-8 hours; preferably, the calcination time is 4-6 hours.

[0038] As a specific implementation manner, the temperature during the stirring reaction in step A) is 25-90° C., and the stirring time is 10-120 min; preferably, the temperature during the stirring reaction is 50-90° C., and the stirring time is 30-90 min.

[0039] As a specific implementation, the drying temperature in step A) is 80-120° C., and the drying time is 2-15 hours; preferably, the drying time is 6-12 hours.

[0040] As a specific implementation manner, in step B), the mass ratio of the modified H-ZSM-5 molecular sieve and titanium dioxide is 1:1 to 5:1; the mass ratio of the mixed powder to deionized water is 1:2 to 1:50; preferably, the mass ratio of the modified H-ZSM-5 molecular sieve and titanium dioxide is 1:1 to 3:1; the mass ratio of the mixed powder to deionized water is 1:5 to 1:25.

[0041] As a specific implementation, the titanium dioxide described in step B) is one or both of anatase titanium dioxide and diamond titanium dioxide; the particle size of the titanium dioxide is 50 to 1000 nm.

[0042] As a specific implementation manner, the hydrothermal reaction temperature in step B) is 50-90° C., and the hydrothermal reaction time is 2-10 h; preferably, the hydrothermal reaction time is 4-6 h.

[0043] As a specific implementation manner, the drying temperature of the product in step B) is 80-120° C., and the drying time is 2-15 hours; preferably, the drying time is 6-12 hours.

[0044] As a specific implementation, the concentration of the impregnation solution in step C) is 0.05-0.5 mol / L; preferably, the concentration of the impregnation solution is 0.1-0.2 mol / L; the mass ratio of the composite carrier to the polyferric sulfate in the impregnation solution is 95:5-99:1.

[0045] As a specific implementation, in step C), the composite carrier is immersed in the immersion liquid for 5 to 24 hours; preferably, the immersion time is 8 to 16 hours.

[0046] As a specific implementation, the drying temperature in step C) is 80-150° C., and the drying time is 2-15 hours; preferably, the drying time is 6-12 hours.

[0047] As a specific implementation, the organic solvent in step C) is one or a combination of two or more of ethanol, ether, chloroform and carbon tetrachloride.

[0048] Embodiment 1: A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) 2 kg of crude terephthalic acid was washed with deionized water, dried at 120° C. for 10 h, and then mixed with 4 kg of methanol and stirred to prepare a crude terephthalic acid-methanol slurry; (2) 3 kg of crude terephthalic acid-methanol slurry was added to the first reaction kettle, and 30 g of polyferric sulfate was added at the same time; under stirring conditions, the reaction temperature of the first reaction kettle was raised to 200° C., the reaction pressure was raised to 2 MPa, and the reaction was continued for 3 h, thus completing the first stage of esterification reaction; (3) The product in the first reactor of step (2) is transferred to the second reactor, and then 3 kg of crude terephthalic acid-methanol slurry and 30 g of polyferric sulfate are added to the second reactor, and 60 g of silica gel is added to the second reactor; under stirring conditions, the reaction temperature of the second reactor is increased to 200° C., the reaction pressure is increased to 2 MPa, and the reaction is carried out for 4 hours to complete the second stage esterification reaction and obtain dimethyl terephthalate; after cooling and reducing the pressure, the reaction product is analyzed.

[0049] Embodiment 2: A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) 1 kg of crude terephthalic acid was washed with deionized water, dried at 110° C. for 12 h, and then mixed with 5 kg of methanol and stirred to prepare a crude terephthalic acid-methanol slurry; (2) 3 kg of crude terephthalic acid-methanol slurry was added to the first reaction kettle, and 15 g of polyferric sulfate was added at the same time; under stirring conditions, the reaction temperature of the first reaction kettle was raised to 220° C., the reaction pressure was raised to 2 MPa, and the reaction was carried out for 3 hours to complete the first stage of esterification reaction; (3) The product in the first reactor of step (2) is transferred to the second reactor, and then 3 kg of crude terephthalic acid-methanol slurry and 15 g of polyferric sulfate are added to the second reactor, and 60 g of molecular sieve desiccant is added to the second reactor; under stirring conditions, the reaction temperature of the second reactor is increased to 210° C., the reaction pressure is increased to 2 MPa, and the reaction is carried out for 5 hours to complete the second stage esterification reaction and obtain dimethyl terephthalate; after cooling and reducing the pressure, the reaction product is analyzed.

[0050] Embodiment 3: A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) 2 kg of crude terephthalic acid was washed with deionized water, dried at 110° C. for 14 h, and then mixed with 4 kg of methanol and stirred to prepare a crude terephthalic acid-methanol slurry; (2) 3 kg of crude terephthalic acid-methanol slurry was added to the first reaction kettle, and 30 g of dibutyl titanate was added at the same time; under stirring conditions, the reaction temperature of the first reaction kettle was raised to 220° C., the reaction pressure was raised to 2.5 MPa, and the reaction was carried out for 3 hours to complete the first stage of esterification reaction; (3) The product in the first reactor of step (2) is transferred to the second reactor, and then 3 kg of crude terephthalic acid-methanol slurry and 30 g of dibutyl titanate are added to the second reactor, and 65 g of silica gel is added to the second reactor; under stirring conditions, the reaction temperature of the second reactor is increased to 220° C., the reaction pressure is increased to 2.5 MPa, and the reaction is carried out for 4 hours to complete the second stage esterification reaction and obtain dimethyl terephthalate; after cooling and reducing the pressure, the reaction product is analyzed.

[0051] Embodiment 4: A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) 2 kg of crude terephthalic acid was washed with deionized water, dried at 130° C. for 12 h, and then mixed with 4 kg of methanol and stirred to prepare a crude terephthalic acid-methanol slurry; (2) 3 kg of crude terephthalic acid-methanol slurry was added to the first reaction kettle, and 35 g of antimony acetate was added at the same time; under stirring conditions, the reaction temperature of the first reaction kettle was raised to 210° C., the reaction pressure was raised to 2 MPa, and the reaction was carried out for 3 hours to complete the first stage esterification reaction; (3) The product in the first reactor of step (2) is transferred to the second reactor, and then 3 kg of crude terephthalic acid-methanol slurry and 35 g of antimony acetate are added to the second reactor, and 60 g of silica gel is added to the second reactor; under stirring conditions, the reaction temperature of the second reactor is increased to 220° C., the reaction pressure is increased to 2 MPa, and the reaction is carried out for 5 hours to complete the second stage esterification reaction and obtain dimethyl terephthalate; after cooling and reducing the pressure, the reaction product is taken for analysis.

[0052] Embodiment 5: A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) 1 kg of crude terephthalic acid was washed with deionized water, dried at 110° C. for 14 h, and then mixed with 5 kg of methanol and stirred to prepare a crude terephthalic acid-methanol slurry; (2) 3 kg of crude terephthalic acid-methanol slurry was added to the first reaction kettle, and 18 g of stannous chloride was added at the same time; under stirring conditions, the reaction temperature of the first reaction kettle was raised to 200° C., the reaction pressure was raised to 2.5 MPa, and the reaction was carried out for 3 hours to complete the first stage esterification reaction; (3) The product in the first reactor of step (2) is transferred to the second reactor, and then 3 kg of crude terephthalic acid-methanol slurry and 18 g of stannous chloride are added to the second reactor, and 65 g of silica gel is added to the second reactor; under stirring conditions, the reaction temperature of the second reactor is increased to 200° C., the reaction pressure is increased to 2.5 MPa, and the reaction is carried out for 4 hours to complete the second stage esterification reaction and obtain dimethyl terephthalate; after cooling and reducing the pressure, the reaction product is analyzed.

[0053] Embodiment 6 (using composite catalyst): A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) Preparation of composite catalyst: A) 10 g of H-ZSM-5 molecular sieve with a silicon-aluminum ratio of 60 was uniformly mixed with 100 g of 0.1 mol / L citric acid solution, stirred at 60° C. for 60 min, filtered, and the obtained solid A was washed with deionized water until neutral, dried at 90° C. for 12 h, and calcined at 500° C. for 4 h to obtain a modified H-ZSM-5 molecular sieve; B) mixing the modified H-ZSM-5 molecular sieve obtained in step A) with 5 g of anatase titanium dioxide (particle size of 100 nm) to obtain a mixed powder; adding the mixed powder to 150 g of deionized water, stirring evenly, loading into a polytetrafluoroethylene reactor, heating at 60° C. for 5 h, filtering, and drying the obtained solid at 110° C. for 12 h to obtain a composite carrier; C) dissolving polyferric sulfate in ethanol to prepare a 0.1 mol / L impregnation solution, and evenly dropping the impregnation solution onto the composite support to make the content of polyferric sulfate in the catalyst be 3 wt %, and after impregnation at room temperature for 12 h, drying the obtained catalyst preparation sample at 120° C. for 12 h to obtain the composite catalyst; (2) washing 2 kg of crude terephthalic acid with deionized water, drying at 120° C. for 10 h, and then mixing and stirring with 4 kg of methanol to prepare a crude terephthalic acid-methanol slurry; (3) 3 kg of crude terephthalic acid-methanol slurry was added to the first reaction kettle, and 30 g of the composite catalyst was added at the same time; under stirring conditions, the reaction temperature of the first reaction kettle was raised to 200° C., the reaction pressure was raised to 2 MPa, and the reaction was carried out for 3 hours to complete the first stage esterification reaction; (4) The product in the first reactor of step (3) is transferred to the second reactor, and then 3 kg of crude terephthalic acid-methanol slurry and 30 g of composite catalyst are added to the second reactor, and 60 g of silica gel is added to the second reactor; under stirring conditions, the reaction temperature of the second reactor is increased to 200° C., the reaction pressure is increased to 2 MPa, and the reaction is carried out for 4 hours to complete the second stage esterification reaction and obtain dimethyl terephthalate; after cooling and reducing the pressure, the reaction product is taken for analysis.

[0054] Example 7 (H-ZSM-5 molecular sieve is not modified with organic acid): A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) Preparation of composite catalyst: A) 10 g of H-ZSM-5 molecular sieve with a silicon-aluminum ratio of 60 and 5 g of anatase titanium dioxide (particle size of 100 nm) were mixed to obtain a mixed powder; the mixed powder was added to 150 g of deionized water, stirred evenly, and then loaded into a polytetrafluoroethylene reactor, heated at 60° C. for 5 h, filtered, and the obtained solid was dried at 110° C. for 12 h to obtain a composite carrier; B) dissolving polyferric sulfate in ethanol to prepare a 0.1 mol / L impregnation solution, and evenly dripping the impregnation solution onto the composite support to make the content of polyferric sulfate in the catalyst be 3 wt %, and after impregnation at room temperature for 12 h, drying the obtained catalyst preparation sample at 120° C. for 12 h to obtain the composite catalyst; (2) washing 2 kg of crude terephthalic acid with deionized water, drying at 120° C. for 10 h, and then mixing and stirring with 4 kg of methanol to prepare a crude terephthalic acid-methanol slurry; (3) 3 kg of crude terephthalic acid-methanol slurry was added to the first reaction kettle, and 30 g of the composite catalyst was added at the same time; under stirring conditions, the reaction temperature of the first reaction kettle was raised to 200° C., the reaction pressure was raised to 2 MPa, and the reaction was carried out for 3 hours to complete the first stage esterification reaction; (4) The product in the first reactor of step (3) is transferred to the second reactor, and then 3 kg of crude terephthalic acid-methanol slurry and 30 g of composite catalyst are added to the second reactor, and 60 g of silica gel is added to the second reactor; under stirring conditions, the reaction temperature of the second reactor is increased to 200° C., the reaction pressure is increased to 2 MPa, and the reaction is carried out for 4 hours to complete the second stage esterification reaction and obtain dimethyl terephthalate; after cooling and reducing the pressure, the reaction product is taken for analysis.

[0055] Example 8 (H-ZSM-5 molecular sieve and titanium dioxide are compounded without hydrothermal treatment): A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) Preparation of composite catalyst: A) 10 g of H-ZSM-5 molecular sieve with a silicon-aluminum ratio of 60 was uniformly mixed with 100 g of 0.1 mol / L citric acid solution, stirred at 60° C. for 60 min, filtered, and the obtained solid A was washed with deionized water until neutral, dried at 90° C. for 12 h, and calcined at 500° C. for 4 h to obtain a modified H-ZSM-5 molecular sieve; B) mixing the modified H-ZSM-5 molecular sieve obtained in step A) with 5 g of anatase titanium dioxide (particle size of 100 nm) to obtain a composite carrier; C) dissolving polyferric sulfate in ethanol to prepare a 0.1 mol / L impregnation solution, and evenly dropping the impregnation solution onto the composite support to make the content of polyferric sulfate in the catalyst be 3 wt %, and after impregnation at room temperature for 12 h, drying the obtained catalyst preparation sample at 120° C. for 12 h to obtain the composite catalyst; (2) washing 2 kg of crude terephthalic acid with deionized water, drying at 120° C. for 10 h, and then mixing and stirring with 4 kg of methanol to prepare a crude terephthalic acid-methanol slurry; (3) 3 kg of crude terephthalic acid-methanol slurry was added to the first reaction kettle, and 30 g of the composite catalyst was added at the same time; under stirring conditions, the reaction temperature of the first reaction kettle was raised to 200° C., the reaction pressure was raised to 2 MPa, and the reaction was carried out for 3 hours to complete the first stage esterification reaction; (4) The product in the first reactor of step (3) is transferred to the second reactor, and then 3 kg of crude terephthalic acid-methanol slurry and 30 g of composite catalyst are added to the second reactor, and 60 g of silica gel is added to the second reactor; under stirring conditions, the reaction temperature of the second reactor is increased to 200° C., the reaction pressure is increased to 2 MPa, and the reaction is carried out for 4 hours to complete the second stage esterification reaction and obtain dimethyl terephthalate; after cooling and reducing the pressure, the reaction product is taken for analysis.

[0056] Example 9 (without adding H-ZSM-5 molecular sieve): A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) Preparation of composite catalyst: A) 5 g of anatase titanium dioxide (particle size of 100 nm) was added to 150 g of deionized water, stirred evenly, and then loaded into a polytetrafluoroethylene reactor, heated at 60° C. for 5 h, filtered, and the obtained solid was dried at 110° C. for 12 h to obtain a carrier; B) dissolving polyferric sulfate in ethanol to prepare a 0.1 mol / L impregnation solution, and evenly dripping the impregnation solution onto the carrier to make the content of polyferric sulfate in the catalyst be 3 wt %, and after impregnation at room temperature for 12 h, drying the obtained catalyst preparation sample at 120° C. for 12 h to obtain the composite catalyst; (2) washing 2 kg of crude terephthalic acid with deionized water, drying at 120° C. for 10 h, and then mixing and stirring with 4 kg of methanol to prepare a crude terephthalic acid-methanol slurry; (3) 3 kg of crude terephthalic acid-methanol slurry was added to the first reaction kettle, and 30 g of the composite catalyst was added at the same time; under stirring conditions, the reaction temperature of the first reaction kettle was raised to 200° C., the reaction pressure was raised to 2 MPa, and the reaction was carried out for 3 hours to complete the first stage esterification reaction; (4) The product in the first reactor of step (3) is transferred to the second reactor, and then 3 kg of crude terephthalic acid-methanol slurry and 30 g of composite catalyst are added to the second reactor, and 60 g of silica gel is added to the second reactor; under stirring conditions, the reaction temperature of the second reactor is increased to 200° C., the reaction pressure is increased to 2 MPa, and the reaction is carried out for 4 hours to complete the second stage esterification reaction and obtain dimethyl terephthalate; after cooling and reducing the pressure, the reaction product is taken for analysis.

[0057] Embodiment 10 (without adding titanium dioxide): A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) Preparation of composite catalyst: A) 10 g of H-ZSM-5 molecular sieve with a silicon-aluminum ratio of 60 was uniformly mixed with 100 g of 0.1 mol / L citric acid solution, stirred at 60° C. for 60 min, filtered, and the obtained solid A was washed with deionized water until neutral, dried at 90° C. for 12 h, and calcined at 500° C. for 4 h to obtain a modified H-ZSM-5 molecular sieve; B) adding the modified H-ZSM-5 molecular sieve obtained in step A) to 150 g of deionized water, stirring evenly, loading into a polytetrafluoroethylene reaction kettle, heating at 60° C. for 5 h, filtering, and drying the obtained solid at 110° C. for 12 h to obtain a carrier; C) dissolving polyferric sulfate in ethanol to prepare a 0.1 mol / L impregnation solution, and evenly dripping the impregnation solution onto the carrier to make the content of polyferric sulfate in the catalyst be 3 wt %, and after impregnation at room temperature for 12 hours, drying the obtained catalyst preparation sample at 120° C. for 12 hours to obtain the composite catalyst; (2) washing 2 kg of crude terephthalic acid with deionized water, drying at 120° C. for 10 h, and then mixing and stirring with 4 kg of methanol to prepare a crude terephthalic acid-methanol slurry; (3) 3 kg of crude terephthalic acid-methanol slurry was added to the first reaction kettle, and 30 g of the composite catalyst was added at the same time; under stirring conditions, the reaction temperature of the first reaction kettle was raised to 200° C., the reaction pressure was raised to 2 MPa, and the reaction was carried out for 3 hours to complete the first stage esterification reaction; (4) The product in the first reactor of step (3) is transferred to the second reactor, and then 3 kg of crude terephthalic acid-methanol slurry and 30 g of composite catalyst are added to the second reactor, and 60 g of silica gel is added to the second reactor; under stirring conditions, the reaction temperature of the second reactor is increased to 200° C., the reaction pressure is increased to 2 MPa, and the reaction is carried out for 4 hours to complete the second stage esterification reaction and obtain dimethyl terephthalate; after cooling and reducing the pressure, the reaction product is taken for analysis.

[0058] Embodiment 11 (without adding polyferric sulfate): A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) Preparation of composite catalyst: A) 10 g of H-ZSM-5 molecular sieve with a silicon-aluminum ratio of 60 was uniformly mixed with 100 g of 0.1 mol / L citric acid solution, stirred at 60° C. for 60 min, filtered, and the obtained solid A was washed with deionized water until neutral, dried at 90° C. for 12 h, and calcined at 500° C. for 4 h to obtain a modified H-ZSM-5 molecular sieve; B) mixing the modified H-ZSM-5 molecular sieve obtained in step A) with 5 g of anatase titanium dioxide (particle size of 100 nm) to obtain a mixed powder; adding the mixed powder to 150 g of deionized water, stirring evenly, loading into a polytetrafluoroethylene reactor, heating at 60° C. for 5 h, filtering, and drying the obtained solid at 110° C. for 12 h to obtain the composite catalyst; (2) washing 2 kg of crude terephthalic acid with deionized water, drying at 120° C. for 10 h, and then mixing and stirring with 4 kg of methanol to prepare a crude terephthalic acid-methanol slurry; (3) 3 kg of crude terephthalic acid-methanol slurry was added to the first reaction kettle, and 30 g of the composite catalyst was added at the same time; under stirring conditions, the reaction temperature of the first reaction kettle was raised to 200° C., the reaction pressure was raised to 2 MPa, and the reaction was carried out for 3 hours to complete the first stage esterification reaction; (4) The product in the first reactor of step (3) is transferred to the second reactor, and then 3 kg of crude terephthalic acid-methanol slurry and 30 g of composite catalyst are added to the second reactor, and 60 g of silica gel is added to the second reactor; under stirring conditions, the reaction temperature of the second reactor is increased to 200° C., the reaction pressure is increased to 2 MPa, and the reaction is carried out for 4 hours to complete the second stage esterification reaction and obtain dimethyl terephthalate; after cooling and reducing the pressure, the reaction product is taken for analysis.

[0059] Comparative Example 1 (without adding water absorbent): A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) 2 kg of crude terephthalic acid was washed with deionized water, dried at 120° C. for 10 h, and then mixed with 4 kg of methanol and stirred to prepare a crude terephthalic acid-methanol slurry; (2) 3 kg of crude terephthalic acid-methanol slurry was added to the first reaction kettle, and 30 g of polyferric sulfate was added at the same time; under stirring conditions, the reaction temperature of the first reaction kettle was raised to 200° C., the reaction pressure was raised to 2 MPa, and the reaction was continued for 3 h, thus completing the first stage of esterification reaction; (3) The product in the first reactor of step (2) is transferred to the second reactor, and then 3 kg of crude terephthalic acid-methanol slurry and 30 g of polyferric sulfate are added to the second reactor; under stirring conditions, the reaction temperature of the second reactor is increased to 200° C., the reaction pressure is increased to 2 MPa, and the reaction is carried out for 4 hours to complete the second stage esterification reaction and obtain dimethyl terephthalate; after cooling and reducing the pressure, the reaction product is taken for analysis.

[0060] Comparative Example 2 (water absorbing agent is added during the first stage of esterification reaction): A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate comprises the following steps: (1) 2 kg of crude terephthalic acid was washed with deionized water, dried at 120° C. for 10 h, and then mixed with 4 kg of methanol and stirred to prepare a crude terephthalic acid-methanol slurry; (2) 3 kg of crude terephthalic acid-methanol slurry was added to the first reaction kettle, and 30 g of polyferric sulfate and 60 g of silica gel were added at the same time; under stirring conditions, the reaction temperature of the first reaction kettle was raised to 200° C., the reaction pressure was raised to 2 MPa, and the reaction was carried out for 3 hours to complete the first stage of esterification reaction; (3) The product in the first reactor of step (2) is transferred to the second reactor, and then 3 kg of crude terephthalic acid-methanol slurry and 30 g of polyferric sulfate are added to the second reactor; under stirring conditions, the reaction temperature of the second reactor is increased to 200° C., the reaction pressure is increased to 2 MPa, and the reaction is carried out for 4 hours to complete the second stage esterification reaction and obtain dimethyl terephthalate; after cooling and reducing the pressure, the reaction product is taken for analysis.

[0061] The test results of the conversion rate of terephthalic acid and the selectivity of dimethyl terephthalate in the above examples and comparative examples are shown in Table 1.

[0062] Table 1: Test results of terephthalic acid conversion and dimethyl terephthalate selectivity.

[0063] It can be seen from the data in Table 1 that the method of the present invention in Examples 1 to 11 can obtain high terephthalic acid conversion and dimethyl terephthalate selectivity. In addition, the catalyst composed of acid-modified H-ZSM-5 molecular sieve, titanium dioxide and polyferric sulfate in Example 6 has higher catalytic activity for the reaction of preparing dimethyl terephthalate from crude terephthalic acid methyl esterification, and higher terephthalic acid conversion and dimethyl terephthalate selectivity than the single catalyst in Examples 1 to 5. In the catalyst of Example 7, the H-ZSM-5 molecular sieve is not modified with organic acid. Due to the decrease in the protonic acid concentration and pore volume in the catalyst, the terephthalic acid conversion and dimethyl terephthalate selectivity are both reduced compared with those in Example 6. In the catalyst of Example 8, the modified H-ZSM-5 molecular sieve is directly blended with titanium dioxide as a composite carrier, and the two are not hydrothermally composited. The H-ZSM-5 molecular sieve and titanium dioxide are mixed unevenly, and the terephthalic acid conversion rate and dimethyl terephthalate selectivity are also reduced compared with Example 6. In the catalyst of Example 9, only titanium dioxide is used as a carrier, and titanium dioxide is not composited with H-ZSM-5 molecular sieve. The proton acid concentration in the catalyst is too low, resulting in a decrease in the terephthalic acid conversion rate and dimethyl terephthalate selectivity. In the catalyst of Example 10, only modified H-ZSM-5 molecular sieve is used as a carrier, and titanium dioxide is not composited. The Lewis acid concentration in the catalyst is too low, which also leads to a decrease in the terephthalic acid conversion rate and dimethyl terephthalate selectivity. In Example 11, polyferric sulfate is not loaded on the composite carrier. Since the activation energy of the esterification reaction cannot be effectively reduced and the adsorption rate of the reactant molecules is reduced, the terephthalic acid conversion rate and the dimethyl terephthalate selectivity are reduced compared with those in Example 6.

[0064] In Comparative Example 1, no water absorbent was added during the esterification reaction. The water generated during the first and second esterification reactions caused partial hydrolysis of dimethyl terephthalate, resulting in a decrease in the terephthalic acid conversion rate and dimethyl terephthalate selectivity. In Comparative Example 2, a water absorbent was added during the first esterification reaction. Although the water absorbent could absorb the water generated during the first esterification reaction, the water generated during the second esterification reaction caused partial hydrolysis of dimethyl terephthalate generated during the first and second esterification reactions, resulting in a decrease in the terephthalic acid conversion rate and dimethyl terephthalate selectivity.

[0065] The above content is a further detailed description of the technical solution of the present invention in combination with a specific preferred implementation method, and should not be understood as a limitation on the applicable scope of the present invention. For ordinary technicians in the field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the present invention, which should be deemed to fall within the scope of patent protection determined by the claims submitted for the present invention.

Claims

1. A method for preparing dimethyl terephthalate by methyl esterification of crude terephthalic acid, characterized in that it comprises: Follow these steps: (1) washing and drying crude terephthalic acid and mixing it with methanol to prepare a crude terephthalic acid-methanol slurry; (2) mixing half of the crude terephthalic acid-methanol slurry with a catalyst to carry out a first stage esterification reaction; (3) adding the other half of the crude terephthalic acid-methanol slurry and the catalyst to the product of the first stage esterification reaction, and simultaneously adding a water absorbent to carry out a second stage esterification reaction to obtain dimethyl terephthalate; The catalyst in step (2) and step (3) is one or a combination of two or more of tin compounds, zinc compounds, antimonides, titanium compounds, alkaline earth metal salts, H-ZSM-5 molecular sieves, and polyferric sulfate.

2. The method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate according to claim 1, characterized in that: In the crude terephthalic acid-methanol slurry described in step (1), the mass ratio of crude terephthalic acid to methanol is 1:2-10.

3. The method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate according to claim 1, characterized in that: The drying temperature after washing in step (1) is 100-200°C, and the drying time is 4-20h.

4. The method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate according to claim 1, characterized in that: The mass of the catalyst added in step (2) is 2-10% of the mass of the crude terephthalic acid in the crude terephthalic acid-methanol slurry.

5. The method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate according to claim 1 or 4, characterized in that: The reaction temperature of the first stage esterification reaction in step (2) is 150-300° C., the reaction pressure is 1-10 MPa, and the reaction time is 1-10 h.

6. The method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate according to claim 1, characterized in that: The mass of the catalyst added in step (3) is 2-10% of the mass of the crude terephthalic acid in the crude terephthalic acid-methanol slurry.

7. The method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate according to claim 1, characterized in that: The mass of the water absorbent added in step (3) is 0.1-3% of the total mass of the crude terephthalic acid-methanol slurry added in steps (2) and (3).

8. The method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate according to claim 1, 6 or 7, characterized in that: The reaction temperature of the second stage esterification reaction in step (3) is 150-300° C., the reaction pressure is 1-10 MPa, and the reaction time is 1-10 h.

9. The method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate according to claim 1, characterized in that: The catalysts described in step (2) and step (3) are composited from H-ZSM-5 molecular sieve, titanium dioxide and polyferric sulfate.

10. The method for preparing dimethyl terephthalate by methyl esterification of crude terephthalate according to claim 1, characterized in that: The water absorbent in step (3) is one or a combination of two or more of silica gel, calcium chloride and molecular sieve desiccant.

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