Separation and purification method of dimethyl terephthalate

Abstract

The invention discloses a separation and purification method of dimethyl terephthalate, which comprises the following steps: mixing crude terephthalic acid and methanol, and adding a catalyst for esterification reaction to obtain a thermal esterification product; filtering to obtain esterification liquid A; adding a water absorbent into the esterification liquid A, and filtering to obtain esterification liquid B; adding activated carbon into the esterification liquid B, and filtering to obtain esterification liquid C; distilling the esterification liquid C, and removing and recovering methanol to obtain esterification liquid D; and sequentially carrying out two times of fractionation on the esterification liquid D, and cooling, crystallizing and drying a secondary fractionation product to obtain a separated and purified product. The water absorbent is used for removing moisture in the esterification liquid, so that the situation that the esterification efficiency is reduced when the subsequently recycled methanol is subjected to esterification reaction cyclic utilization due to the moisture in the subsequently recycled methanol is avoided; the method disclosed by the invention is simple in process, low in equipment cost, low in energy consumption and high in product purity.

Description

Technical Field

[0001] The present invention relates to the technical field of organic chemical industry, and particularly to a method for separating and purifying dimethyl terephthalate. Background Art

[0002] Dimethyl terephthalate (DMT), as an important chemical raw material intermediate, is widely used in the manufacture and production of polyester resins, polyester fibers, films, high-strength polyester insulating paints and engineering plastics; at the same time, dimethyl terephthalate can also be used to further synthesize polyethylene terephthalate (PET), and then produce and manufacture products such as polyester fabrics, engineering plastics, and PET bottles. Thus, dimethyl terephthalate occupies an important position in the chemical industry and the national economy. Dimethyl terephthalate can be prepared by the esterification reaction of purified terephthalic acid (PTA) or crude terephthalic acid (CTA) with methanol in the presence of a catalyst and certain reaction conditions. However, during the preparation of dimethyl terephthalate by the esterification reaction, some by-products are often generated, such as monomethyl terephthalate. Especially when using crude terephthalic acid as the raw material to prepare dimethyl terephthalate, more types of by-products are produced. In order to avoid the process difficulties and economic losses caused by the by-product impurities in dimethyl terephthalate to the subsequent application processes, it is necessary to separate and purify the crude dimethyl terephthalate product obtained by the esterification reaction. Therefore, developing a separation and purification process for dimethyl terephthalate has very important economic significance and industrial value.

[0003] CN1169981A discloses a method for refining dimethyl terephthalate. This method first separates high-boiling compounds and a catalyst (concentrated sulfuric acid) from the esterified product by distillation, then adds methanol to make it miscible, cools and crystallizes to precipitate dimethyl terephthalate, separates the solid product of dimethyl terephthalate by centrifugation, and finally obtains high-purity dimethyl terephthalate by vacuum sublimation. This method uses concentrated sulfuric acid as a catalyst, requires the equipment to have good corrosion resistance, and the acidic wastewater generated needs to be treated, increasing the process cost and environmental pollution.

[0004] CN104072374A discloses a method for refining dimethyl terephthalate. In this method, methanol is first distilled off from the esterification product, and then ethanol is added as a solvent to completely dissolve dimethyl terephthalate in ethanol. Activated carbon or clay is added to remove impurities, and styrene anions are added to remove iron cations. After impurity removal, the esterification solution is cooled to precipitate dimethyl terephthalate, and then the dimethyl terephthalate product is subjected to rectification operation to remove heavy components, obtaining high-purity dimethyl terephthalate. This method introduces ethanol as an organic solvent, increasing the raw material cost. In addition, substances such as activated carbon, clay, and styrene anions are added in this method, and these added substances need to be removed later, making the process flow longer. Summary of the Invention

[0005] The present invention aims to overcome the above problems existing in the prior art, and provides a method for separating and purifying dimethyl terephthalate. A water absorbent is used to remove water in the esterification solution, avoiding water in the subsequently recovered methanol, thus reducing the esterification efficiency when entering the esterification reaction for recycling. The method of the present invention has a simple process, low equipment cost, low energy consumption, and high product purity.

[0006] To achieve the above object, the present invention adopts the following technical solutions: A method for separating and purifying dimethyl terephthalate, comprising the following steps: (1) Mix crude terephthalic acid and methanol, and add a catalyst to carry out an esterification reaction to obtain a hot esterification product; (2) Filter the hot esterification product to obtain esterification solution A; (3) Add a water absorbent to esterification solution A, and filter to obtain esterification solution B; (4) Add activated carbon to esterification solution B, and filter to obtain esterification solution C; (5) Distill esterification solution C, remove and recover methanol to obtain esterification solution D; (6) Fractionate esterification solution D twice in sequence. After cooling, crystallizing, and drying the secondary fractionation product, the separated and purified product is obtained.

[0007] The separation and purification process of dimethyl terephthalate provided by the present invention removes water in the esterification product by adding a water absorbent, thus avoiding the reduction of the esterification reaction efficiency due to the water contained in the methanol recovered in the subsequent process when recycled to the esterification reaction. Moreover, the separation and purification method of dimethyl terephthalate provided by the present invention has a simple process, low equipment cost, low energy consumption, and high product purity.

[0008] Preferably, the catalyst in step (1) comprises a composite support and ferric polysulfate supported on the composite support; the composite support is formed by hydrothermal reaction of modified H-ZSM-5 molecular sieve and titanium dioxide; the modified H-ZSM-5 molecular sieve is obtained by treating H-ZSM-5 molecular sieve with an organic acid solution and then calcining; in the composite support, the mass ratio of the modified H-ZSM-5 molecular sieve to titanium dioxide is 1:1 to 5:1; the mass ratio of the composite support to ferric polysulfate is 95:5 to 99:1.

[0009] The present invention combines H-ZSM-5 molecular sieve, titanium dioxide and ferric polysulfate as a catalyst for the reaction of methyl esterification of terephthalic acid to dimethyl terephthalate. Crude terephthalic acid without purification can be used as the reaction raw material, eliminating the purification process of terephthalic acid and reducing the production cost. 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. The conversion rate of crude terephthalic acid and the selectivity of dimethyl terephthalate can both reach more than 98%. Moreover, the catalyst of the present invention has basically no corrosion to equipment, and does not produce acid-containing wastewater and toxic by-products, which is convenient for subsequent separation and purification.

[0010] When the present invention combines H-ZSM-5 molecular sieve, titanium dioxide and ferric polysulfate, the H-ZSM-5 molecular sieve is first treated with an organic acid and then calcined. The acid treatment can effectively remove the non-framework aluminum species in the pore channels of the H-ZSM-5 molecular sieve, playing a role in dredging the pore channels; at the same time, the acid treatment can also expand the pores of the H-ZSM-5 molecular sieve to a certain extent, enabling more active sites in the pore channels to participate in the methyl esterification reaction. Then the present invention mixes the modified H-ZSM-5 molecular sieve with titanium dioxide and conducts a hydrothermal reaction to obtain a composite support. The composite under hydrothermal conditions can promote the uniform mixing of the two at the micron or even nanoscale, which is beneficial to enhancing the active sites of the catalyst. Finally, ferric polysulfate is loaded on the composite support, which can further enhance the catalytic activity of the catalyst for the reaction of methyl esterification of crude terephthalic acid to dimethyl terephthalate.

[0011] Preferably, in step (1), the mass ratio of crude terephthalic acid to methanol is 1:2 to 10, and the mass of the catalyst is 2 to 10% of the mass of crude terephthalic acid.

[0012] Preferably, in step (1), the reaction temperature of the esterification reaction is 150 to 300 °C, the reaction pressure is 1 to 10 MPa, and the reaction time is 1 to 10 h.

[0013] Preferably, in step (3), the water absorbent is one or a combination of two or more of silica gel, anhydrous calcium chloride, and molecular sieve desiccant; the mass of the added water absorbent is 10 to 50% of the mass of crude terephthalic acid.

[0014] Preferably, the mass of the activated carbon added in step (4) is 5-50% of the mass of the crude terephthalic acid.

[0015] Preferably, the distillation temperature in step (5) is 65-100 °C.

[0016] Preferably, the methanol recovered in step (5) is condensed and then returned to the esterification reaction in step (1) as a reaction raw material for recycling.

[0017] Preferably, in step (6), the low-boiling compounds below 250 °C are removed by the first fractionation; the high-boiling compounds above 290 °C are removed by the second fractionation.

[0018] Preferably, the final cooling temperature for cooling crystallization in step (6) is 15-40 °C; during drying, hot inert gas is used for purging and drying, and the temperature of the hot inert gas is 50-130 °C.

[0019] Therefore, the present invention has the following beneficial effects: (1) The separation and purification process of dimethyl terephthalate provided by the present invention removes the moisture in the esterification product by adding a water absorbent, thereby avoiding the reduction of the esterification reaction efficiency when the methanol recovered in the subsequent process contains moisture and is recycled to the esterification reaction. (2) The separation and purification process of dimethyl terephthalate provided by the present invention has the advantages of simple process, low equipment cost, low energy consumption, and high product purity. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a flow chart of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention will be further described below in conjunction with the drawings and the specific embodiments.

[0022] In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in this industry. The methods in the following embodiments, unless otherwise specified, are conventional methods in this field.

[0023] General Embodiment: A method for separating and purifying dimethyl terephthalate, the process is as Figure 1 shown in, and includes the following steps: (1) Mix crude terephthalic acid and methanol, add a catalyst and carry out an esterification reaction to obtain a hot esterification product; (2) Filter the hot esterification product to obtain an esterification solution A; (3) Add a water absorbent to the esterification solution A, and filter to obtain an esterification solution B; (4) Add activated carbon to the esterification solution B, and filter to obtain an esterification solution C; (5) Distill the esterification liquid C, and after removing and recovering methanol, obtain the esterification liquid D; (6) Fractionate the esterification liquid D twice in sequence. After cooling, crystallizing, and drying the secondary fractionation product, obtain the separated and purified product.

[0024] As a specific embodiment, the catalyst in step (1) includes a composite support and polymeric ferric sulfate supported on the composite support; the composite support is formed by hydrothermal reaction of modified H-ZSM-5 molecular sieve and titanium dioxide; the modified H-ZSM-5 molecular sieve is obtained by treating H-ZSM-5 molecular sieve with an organic acid solution and then calcining; in the composite support, the mass ratio of the modified H-ZSM-5 molecular sieve to titanium dioxide is 1:1 to 5:1; the mass ratio of the composite support to polymeric ferric sulfate is 95:5 to 99:1.

[0025] As a specific embodiment, the silica-alumina ratio in the H-ZSM-5 molecular sieve is 30 to 120; preferably, the silica-alumina ratio in the H-ZSM-5 molecular sieve is 40 to 100.

[0026] As a specific embodiment, the organic acid solution 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.

[0027] As a specific embodiment, the concentration of the organic acid solution is 0.001 to 0.5 mol / L; preferably, the concentration of the organic acid solution is 0.1 to 0.2 mol / L.

[0028] As a specific embodiment, the mass ratio of the H-ZSM-5 molecular sieve to the organic acid solution is 1:5 to 1:100; preferably, the mass ratio of the H-ZSM-5 molecular sieve to the organic acid solution is 1:10 to 1:50.

[0029] As a specific embodiment, the calcination temperature after organic acid treatment is 400 to 600 °C, and the calcination time is 2 to 8 h; preferably, the calcination time is 4 to 6 h.

[0030] As a specific embodiment, the titanium dioxide is one or both of anatase titanium dioxide and rutile titanium dioxide; the particle size of the titanium dioxide is 50 to 1000 nm.

[0031] As a specific embodiment, the preparation method of the catalyst includes the following steps: A) Mix the H-ZSM-5 molecular sieve with the organic acid solution, stir and react, then filter, wash, and dry the product, and then calcine to obtain the modified H-ZSM-5 molecular sieve; B) Mix the modified H-ZSM-5 molecular sieve with titanium dioxide to obtain a mixed powder; mix the mixed powder with deionized water, stir evenly, and then carry out a hydrothermal reaction. Filter and dry the product to obtain a composite support. C) Dissolve the polyferric sulfate in an organic solvent to form an impregnation solution; impregnate the composite support in the impregnation solution and then dry it to obtain the catalyst for the methyl esterification reaction of terephthalic acid.

[0032] As a specific embodiment, 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.

[0033] As a specific embodiment, the drying temperature in step A) is 80-120°C, and the drying time is 2-15 h; preferably, the drying time is 6-12 h.

[0034] As a specific embodiment, the mass ratio of the mixed powder to deionized water in step B) is 1:2-1:50; preferably, the mass ratio of the mixed powder to deionized water is 1:5-1:25.

[0035] As a specific embodiment, 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.

[0036] As a specific embodiment, the drying temperature of the product in step B) is 80-120°C, and the drying time is 2-15 h; preferably, the drying time is 6-12 h.

[0037] As a specific embodiment, 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.

[0038] As a specific embodiment, the impregnation time of the composite support in the impregnation solution in step C) is 5-24 h; preferably, the impregnation time is 8-16 h.

[0039] As a specific embodiment, the drying temperature in step C) is 80-150°C, and the drying time is 2-15 h; preferably, the drying time is 6-12 h.

[0040] As a specific embodiment, the organic solvent in step C) is one or a combination of two or more of ethanol, ether, chloroform, and carbon tetrachloride.

[0041] As a specific embodiment, in step (1), the mass ratio of crude terephthalic acid to methanol is 1:2 to 10, and the mass of the catalyst is 2 to 10% of the mass of the crude terephthalic acid; preferably, the mass ratio of crude terephthalic acid to methanol is 1:2 to 6, and the mass of the catalyst is 2 to 6% of the mass of the crude terephthalic acid. As a specific embodiment, in step (1), the reaction temperature of the esterification reaction is 150 to 300 °C, the reaction pressure is 1 to 10 MPa, and the reaction time is 1 to 10 h; preferably, the reaction temperature of the esterification reaction is 150 to 250 °C, the reaction pressure is 1 to 3 MPa, and the reaction time is 1 to 3 h.

[0042] As a specific embodiment, in step (3), the water absorbent is one or a combination of two or more of silica gel, anhydrous calcium chloride, and molecular sieve desiccant; the mass of the added water absorbent is 10 to 50% of the mass of the crude terephthalic acid; preferably, the mass of the added water absorbent is 10 to 30% of the mass of the crude terephthalic acid.

[0043] As a specific embodiment, in step (4), the mass of the added activated carbon is 5 to 50% of the mass of the crude terephthalic acid; preferably, the mass of the added activated carbon is 5 to 30% of the mass of the crude terephthalic acid. As a specific embodiment, in step (5), the distillation temperature is 65 to 100 °C; preferably, the distillation temperature is 70 to 90 °C.

[0044] As a specific embodiment, the methanol recovered in step (5) is condensed and returned to the esterification reaction in step (1) for recycling as a reaction raw material.

[0045] As a specific embodiment, in step (6), the low-boiling compounds below 250 °C are removed by the first fractionation; the high-boiling compounds above 290 °C are removed by the second fractionation.

[0046] As a specific embodiment, in step (6), the final cooling temperature of the cooling crystallization is 15 to 40 °C; preferably, the final cooling temperature of the cooling crystallization is 15 to 30 °C.

[0047] As a specific embodiment, in step (6), hot inert gas purging drying is used during drying, and the temperature of the hot inert gas is 50 to 130 °C; preferably, the temperature of the hot inert gas is 90 to 130 °C.

[0048] As a specific embodiment, the hot inert gas is one or a combination of two or more of nitrogen, argon, and helium.

[0049] Example 1: A method for separating and purifying dimethyl terephthalate, the steps are as follows: (1) Mix 100 g of crude terephthalic acid and 300 g of methanol evenly, add 5 g of polymeric ferric sulfate as a catalyst thereto, and react for 3 h under the conditions of a reaction temperature of 210 °C and a reaction pressure of 2.5 MPa to obtain a hot esterification product; (2) Filter the hot esterification product to remove the catalyst to obtain esterification liquid A; (3) Add 20 g of anhydrous calcium chloride as a water absorbent to esterification liquid A to remove moisture, and then filter to obtain esterification liquid B; (4) Add 20 g of activated carbon to esterification liquid B to remove impurities, and then filter to obtain esterification liquid C; (5) Perform a distillation operation on esterification liquid C at 75 °C, remove and recover methanol to obtain esterification liquid D; (6) Heat esterification liquid D for the first fractionation to remove low-boiling compounds below 250 °C to obtain a first fractionation product; heat the first fractionation product for the second fractionation to remove high-boiling compounds above 290 °C to obtain a second fractionation product; cool the second fractionation product to 25 °C to precipitate crystals, and blow-dry the crystals with hot nitrogen at 90 °C to obtain a purified dimethyl terephthalate product.

[0050] Example 2: (1) Mix 100 g of crude terephthalic acid and 350 g of methanol evenly, add 6 g of tetrabutyl titanate as a catalyst thereto, and react for 3 h under the conditions of a reaction temperature of 200 °C and a reaction pressure of 2 MPa to obtain a hot esterification product; (2) Filter the hot esterification product to remove the catalyst to obtain esterification liquid A; (3) Add 20 g of anhydrous calcium chloride as a water absorbent to esterification liquid A to remove moisture, and then filter to obtain esterification liquid B; (4) Add 15 g of activated carbon to esterification liquid B to remove impurities, and then filter to obtain esterification liquid C; (5) Perform a distillation operation on esterification liquid C at 80 °C, remove and recover methanol to obtain esterification liquid D; (6) Heat esterification liquid D for the first fractionation to remove low-boiling compounds below 250 °C to obtain a first fractionation product; heat the first fractionation product for the second fractionation to remove high-boiling compounds above 290 °C to obtain a second fractionation product; cool the second fractionation product to 20 °C to precipitate crystals, and blow-dry the crystals with hot nitrogen at 100 °C to obtain a purified dimethyl terephthalate product.

[0051] Example 3: A method for separating and purifying dimethyl terephthalate, the steps are as follows: (1) Prepare a catalyst: A) Mix 10 g of H-ZSM-5 zeolite with a silica-aluminum ratio of 60 evenly with 100 g of 0.1 mol / L citric acid solution, stir at 60 °C for 60 min, filter, wash the obtained solid A with deionized water until neutral, dry at 90 °C for 12 h, and calcine at 500 °C for 4 h to obtain the modified H-ZSM-5 zeolite; B) Mix the modified H-ZSM-5 zeolite obtained in step A) with 5 g of anatase titanium dioxide (particle size 100 nm) to obtain a mixed powder; add the mixed powder to 150 g of deionized water, stir evenly, put it into a polytetrafluoroethylene reaction kettle, heat at 60 °C for 5 h, filter, and dry the obtained solid at 110 °C for 12 h to obtain a composite support; C) Dissolve polyferric sulfate in ethanol to prepare an impregnation solution with a concentration of 0.1 mol / L, evenly drip the impregnation solution onto the composite support so that the content of polyferric sulfate in the catalyst is 3 wt%, after impregnating at room temperature for 12 h, dry the obtained catalyst preparation sample at 120 °C for 12 h to obtain the catalyst; (2) Mix 100 g of crude terephthalic acid and 300 g of methanol evenly, add 5 g of the catalyst thereto, and react at a reaction temperature of 210 °C and a reaction pressure of 2.5 MPa for 3 h to obtain a hot esterification product; (3) Filter the hot esterification product to remove the catalyst to obtain esterification liquid A; (4) Add 20 g of anhydrous calcium chloride as a water absorbent to esterification liquid A to remove water, and then filter to obtain esterification liquid B; (5) Add 20 g of activated carbon to esterification liquid B to remove impurities, and then filter to obtain esterification liquid C; (6) Perform a distillation operation on esterification liquid C at 75 °C, remove and recover methanol to obtain esterification liquid D; (7) Heat esterification liquid D for the first fractionation to remove low-boiling compounds below 250 °C to obtain a first fractionation product; heat the first fractionation product for the second fractionation to remove high-boiling compounds above 290 °C to obtain a second fractionation product; cool the second fractionation product to 25 °C to precipitate crystals, and use hot nitrogen at 90 °C to purge and dry the crystals to obtain a purified dimethyl terephthalate product.

[0052] Example 4: A method for separating and purifying dimethyl terephthalate, the steps are as follows: (1) Prepare the catalyst: A) Mix 10 g of H-ZSM-5 zeolite with a silica-alumina ratio of 90 evenly with 100 g of 0.1 mol / L oxalic acid solution, stir at 70 °C for 70 min, filter, wash the obtained solid A with deionized water until neutral, dry at 110 °C for 12 h, and calcine at 550 °C for 4 h to obtain modified H-ZSM-5 zeolite; B) Mix the modified H-ZSM-5 zeolite obtained in step A) with 2 g of anatase titanium dioxide (particle size of 150 nm) to obtain a mixed powder; add the mixed powder to 200 g of deionized water, stir evenly, put it into a polytetrafluoroethylene reaction kettle, heat at 80 °C for 5 h, filter, and dry the obtained solid at 110 °C for 12 h to obtain a composite support; C) Dissolve ferric poly sulfate in ether to prepare an impregnation solution of 0.1 mol / L, evenly drip the impregnation solution onto the composite support so that the content of ferric poly sulfate in the catalyst is 1 wt%, after impregnation at room temperature for 12 h, dry the obtained catalyst preparation sample at 110 °C for 12 h to obtain the catalyst; (2) Mix 100 g of crude terephthalic acid and 400 g of methanol evenly, add 5 g of the catalyst thereto, and react at a reaction temperature of 200 °C and a reaction pressure of 2.5 MPa for 3 h to obtain a thermal esterification product; (3) Filter the thermal esterification product to remove the catalyst to obtain esterification liquid A; (4) Add 30 g of anhydrous calcium chloride as a water absorbent to esterification liquid A to remove water, and then filter to obtain esterification liquid B; (5) Add 20 g of activated carbon to esterification liquid B to remove impurities, and then filter to obtain esterification liquid C; (6) Perform a distillation operation on esterification liquid C at 80 °C, remove and recover methanol to obtain esterification liquid D; (7) Heat esterification liquid D for the first fractionation to remove low-boiling compounds below 250 °C to obtain a first fractionation product; heat the first fractionation product for the second fractionation to remove high-boiling compounds above 290 °C to obtain a second fractionation product; cool the second fractionation product to 20 °C to precipitate crystals, and use hot nitrogen at 90 °C to blow and dry the crystals to obtain a purified dimethyl terephthalate product.

[0053] Example 5: A method for separating and purifying dimethyl terephthalate, the steps are as follows: (1) Prepare a catalyst: A) Mix 10 g of H-ZSM-5 zeolite with a silica-alumina ratio of 85 evenly with 150 g of 0.1 mol / L citric acid solution, stir at 80 °C for 60 min, filter, wash the obtained solid A with deionized water until neutral, dry at 110 °C for 10 h, and calcine at 550 °C for 4 h to obtain modified H-ZSM-5 zeolite; B) Mix the modified H-ZSM-5 zeolite obtained in step (1) with 10 g of diamond-type titanium dioxide (particle size of 100 nm) to obtain a mixed powder; add the mixed powder to 200 g of deionized water, stir evenly, then load it into a polytetrafluoroethylene reaction kettle, heat at 70 °C for 5 h, filter, and dry the obtained solid at 110 °C for 10 h to obtain a composite support; C) Dissolve polyferric sulfate in ethanol to prepare an impregnating solution with a concentration of 0.1 mol / L, evenly drip the impregnating solution onto the composite support to make the content of polyferric sulfate in the catalyst 5 wt%, after impregnating at room temperature for 15 h, dry the obtained catalyst precursor sample at 100 °C for 14 h to obtain the catalyst; (2) Mix 100 g of crude terephthalic acid and 330 g of methanol evenly, add 4 g of the catalyst thereto, and react at a reaction temperature of 210 °C and a reaction pressure of 2 MPa for 3 h to obtain a thermal esterification product; (3) Filter the thermal esterification product to remove the catalyst to obtain esterification liquid A; (4) Add 30 g of molecular sieve desiccant as a water absorbent to esterification liquid A to remove water, and then filter to obtain esterification liquid B; (5) Add 15 g of activated carbon to esterification liquid B to remove impurities, and then filter to obtain esterification liquid C; (6) Perform a distillation operation on esterification liquid C at 85 °C, remove and recover methanol to obtain esterification liquid D; (7) Heat esterification liquid D for the first fractionation to remove low-boiling compounds below 250 °C to obtain a first fractionation product; heat the first fractionation product for the second fractionation to remove high-boiling compounds above 290 °C to obtain a second fractionation product; cool the second fractionation product to 25 °C to precipitate crystals, and use hot nitrogen at 110 °C to purge and dry the crystals to obtain a purified dimethyl terephthalate product.

[0054] Example 6 (H-ZSM-5 zeolite not modified with organic acid): A method for separating and purifying dimethyl terephthalate, the steps are as follows: (1) Prepare a catalyst: A) Mix 10 g of H-ZSM-5 zeolite with a silica-alumina ratio of 60 and 5 g of anatase titanium dioxide (particle size of 100 nm) to obtain a mixed powder; add the mixed powder to 150 g of deionized water, stir evenly, then load it into a polytetrafluoroethylene reaction kettle, heat it at 60 °C for 5 h, filter, and dry the obtained solid at 110 °C for 12 h to obtain a composite support; B) Dissolve polyferric sulfate in ethanol to prepare an impregnation solution with a concentration of 0.1 mol / L. Drop the impregnation solution evenly onto the composite support so that the content of polyferric sulfate in the catalyst is 3 wt%. After impregnation at room temperature for 12 h, dry the obtained catalyst preparation sample at 120 °C for 12 h to obtain the catalyst; (2) Mix 100 g of crude terephthalic acid and 300 g of methanol evenly, add 5 g of the catalyst thereto, and react at a reaction temperature of 210 °C and a reaction pressure of 2.5 MPa for 3 h to obtain a thermal esterification product; (3) Filter the thermal esterification product to remove the catalyst to obtain esterification liquid A; (4) Add 20 g of anhydrous calcium chloride as a water absorbent to esterification liquid A to remove water, and then filter to obtain esterification liquid B; (5) Add 20 g of activated carbon to esterification liquid B to remove impurities, and then filter to obtain esterification liquid C; (6) Perform a distillation operation on esterification liquid C at 75 °C. After removing and recovering methanol, obtain esterification liquid D; (7) Heat esterification liquid D for the first fractionation to remove low-boiling compounds below 250 °C to obtain a first fractionation product; heat the first fractionation product for the second fractionation to remove high-boiling compounds above 290 °C to obtain a second fractionation product; cool the second fractionation product to 25 °C to precipitate crystals, and use hot nitrogen at 90 °C to blow-dry the crystals to obtain a purified dimethyl terephthalate product.

[0055] Example 7 (H-ZSM-5 zeolite and titanium dioxide without hydrothermal composite): A method for separating and purifying dimethyl terephthalate, the steps are as follows: (1) Prepare the catalyst: A) Mix 10 g of H-ZSM-5 zeolite with a silica-alumina ratio of 60 and 100 g of a 0.1 mol / L citric acid solution evenly, stir at a temperature of 60 °C for 60 min, filter, wash the obtained solid A with deionized water until neutral, dry it at 90 °C for 12 h, and calcine it at 500 °C for 4 h to obtain a modified H-ZSM-5 zeolite; B) Mix the modified H-ZSM-5 zeolite obtained in step A) with 5 g of anatase titanium dioxide (particle size of 100 nm) to obtain a composite support; C) Dissolve ferric poly sulfate in ethanol to prepare an impregnating solution with a concentration of 0.1 mol / L. Drop the impregnating solution evenly onto the composite support so that the content of ferric poly sulfate in the catalyst is 3 wt%. After impregnation at room temperature for 12 h, dry the obtained catalyst preliminary sample at 120 °C for 12 h to obtain the catalyst; (2) Mix 100 g of crude terephthalic acid and 300 g of methanol evenly, add 5 g of the catalyst thereto, and react at a reaction temperature of 210 °C and a reaction pressure of 2.5 MPa for 3 h to obtain a hot esterification product; (3) Filter the hot esterification product to remove the catalyst to obtain esterification liquid A; (4) Add 20 g of anhydrous calcium chloride as a water absorbent to esterification liquid A to remove water, and then filter to obtain esterification liquid B; (5) Add 20 g of activated carbon to esterification liquid B to remove impurities, and then filter to obtain esterification liquid C; (6) Perform a distillation operation on esterification liquid C at 75 °C. After removing and recovering methanol, obtain esterification liquid D; (7) Heat esterification liquid D for the first fractionation to remove low-boiling compounds below 250 °C to obtain a first fractionation product; heat the first fractionation product for the second fractionation to remove high-boiling compounds above 290 °C to obtain a second fractionation product; cool the second fractionation product to 25 °C to precipitate crystals, and use hot nitrogen at 90 °C to purge and dry the crystals to obtain a purified dimethyl terephthalate product.

[0056] Example 8 (without adding H-ZSM-5 molecular sieve): A method for separating and purifying dimethyl terephthalate, the steps are as follows: (1) Prepare the catalyst: A) Add 5 g of anatase titanium dioxide (particle size of 100 nm) to 150 g of deionized water, stir evenly, then load it into a polytetrafluoroethylene reaction kettle, heat it at 60 °C for 5 h, filter, and dry the obtained solid at 110 °C for 12 h to obtain the support; B) Dissolve ferric poly sulfate in ethanol to prepare an impregnating solution with a concentration of 0.1 mol / L. Drop the impregnating solution evenly onto the support so that the content of ferric poly sulfate in the catalyst is 3 wt%. After impregnation at room temperature for 12 h, dry the obtained catalyst preliminary sample at 120 °C for 12 h to obtain the catalyst; (2) Mix 100 g of crude terephthalic acid and 300 g of methanol evenly, add 5 g of the catalyst thereto, and react at a reaction temperature of 210 °C and a reaction pressure of 2.5 MPa for 3 h to obtain a hot esterification product; (3) Filter the hot esterification product to remove the catalyst to obtain esterification liquid A; (4) Add 20 g of anhydrous calcium chloride as a water absorbent to the esterification liquid A to remove moisture, and then filter to obtain the esterification liquid B; (5) Add 20 g of activated carbon to the esterification liquid B to remove impurities, and then filter to obtain the esterification liquid C; (6) Perform a distillation operation on the esterification liquid C at 75 °C. After removing and recovering methanol, obtain the esterification liquid D; (7) Heat the esterification liquid D for the first fractionation to remove low-boiling compounds below 250 °C to obtain the first fractionation product; heat the first fractionation product for the second fractionation to remove high-boiling compounds above 290 °C to obtain the second fractionation product; cool the second fractionation product to 25 °C to precipitate crystals, and use hot nitrogen at 90 °C to purge and dry the crystals to obtain the purified dimethyl terephthalate product.

[0057] Example 9 (without adding titanium dioxide): A method for separating and purifying dimethyl terephthalate, the steps are as follows: (1) Prepare the catalyst: A) Mix 10 g of H-ZSM-5 molecular sieve with a silica-alumina ratio of 60 evenly with 100 g of 0.1 mol / L citric acid solution, stir at a temperature of 60 °C for 60 min, filter, wash the obtained solid A with deionized water until neutral, dry at 90 °C for 12 h, and calcine at 500 °C for 4 h to obtain the modified H-ZSM-5 molecular sieve; B) Add the modified H-ZSM-5 molecular sieve obtained in step A) to 150 g of deionized water, stir evenly and then load it into a polytetrafluoroethylene reaction kettle, heat at 60 °C for 5 h, filter, and dry the obtained solid at 110 °C for 12 h to obtain the carrier; C) Dissolve polyferric sulfate in ethanol to prepare an impregnation solution with a concentration of 0.1 mol / L, evenly drip the impregnation solution onto the carrier so that the content of polyferric sulfate in the catalyst is 3 wt%, after impregnating at room temperature for 12 h, dry the obtained catalyst preparation sample at 120 °C for 12 h to obtain the catalyst; (2) Mix 100 g of crude terephthalic acid and 300 g of methanol evenly, add 5 g of the catalyst thereto, and react at a reaction temperature of 210 °C and a reaction pressure of 2.5 MPa for 3 h to obtain a hot esterification product; (3) Filter the hot esterification product to remove the catalyst to obtain the esterification liquid A; (4) Add 20 g of anhydrous calcium chloride as a water absorbent to the esterification liquid A to remove moisture, and then filter to obtain the esterification liquid B; (5) Add 20 g of activated carbon to the esterification liquid B to remove impurities, and then filter to obtain the esterification liquid C; (6) Distill the esterification liquid C at 75 °C, remove and recover methanol, and obtain the esterification liquid D; (7) Heat the esterification liquid D for the first fractionation to remove the low-boiling compounds below 250 °C to obtain the first fractionation product; heat the first fractionation product for the second fractionation to remove the high-boiling compounds above 290 °C to obtain the second fractionation product; cool the second fractionation product to 25 °C to precipitate crystals, and use hot nitrogen gas at 90 °C to purge and dry the crystals to obtain the purified dimethyl terephthalate product.

[0058] Example 10 (without adding polyferric sulfate): A method for separating and purifying dimethyl terephthalate, the steps are as follows: (1) Prepare the catalyst: A) Mix 10 g of H-ZSM-5 molecular sieve with a silica-alumina ratio of 60 and 100 g of 0.1 mol / L citric acid solution evenly, stir at 60 °C for 60 min, filter, wash the obtained solid A with deionized water until neutral, dry at 90 °C for 12 h, and calcine at 500 °C for 4 h to obtain the modified H-ZSM-5 molecular sieve; B) Mix 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; add the mixed powder to 150 g of deionized water, stir evenly and then load it into a polytetrafluoroethylene reaction kettle, heat at 60 °C for 5 h, filter, and dry the obtained solid at 110 °C for 12 h to obtain the catalyst; (2) Mix 100 g of crude terephthalic acid and 300 g of methanol evenly, add 5 g of the catalyst thereto, and react at a reaction temperature of 210 °C and a reaction pressure of 2.5 MPa for 3 h to obtain a hot esterification product; (3) Filter the hot esterification product to remove the catalyst to obtain the esterification liquid A; (4) Add 20 g of anhydrous calcium chloride as a water absorbent to the esterification liquid A to remove moisture, and then filter to obtain the esterification liquid B; (5) Add 20 g of activated carbon to the esterification liquid B to remove impurities, and then filter to obtain the esterification liquid C; (6) Distill the esterification liquid C at 75 °C, remove and recover methanol, and obtain the esterification liquid D; (7) Heat the esterification liquid D for the first fractionation to remove the low-boiling compounds below 250 °C to obtain the first fractionation product; heat the first fractionation product for the second fractionation to remove the high-boiling compounds above 290 °C to obtain the second fractionation product; cool the second fractionation product to 25 °C to precipitate crystals, and use hot nitrogen gas at 90 °C to purge and dry the crystals to obtain the purified dimethyl terephthalate product.

[0059] Comparative Example 1 (without adding water absorbent): A method for separating and purifying dimethyl terephthalate, the steps are as follows: (1) Mix 100 g of crude terephthalic acid and 300 g of methanol evenly, add 5 g of polyferric sulfate as a catalyst thereto, and react at a reaction temperature of 210 °C and a reaction pressure of 2.5 MPa for 3 h to obtain a hot esterification product; (2) Filter the hot esterification product to remove the catalyst to obtain esterification liquid A; (3) Add 20 g of activated carbon to esterification liquid A to remove impurities, and then filter to obtain esterification liquid B; (5) Perform a distillation operation on esterification liquid B at 75 °C, remove and recover methanol to obtain esterification liquid C; (6) Heat esterification liquid C for a first fractionation to remove low-boiling compounds below 250 °C to obtain a first fractionation product; heat the first fractionation product for a second fractionation to remove high-boiling compounds above 290 °C to obtain a second fractionation product; cool the second fractionation product to 25 °C to precipitate crystals, and blow-dry the crystals with hot nitrogen at 90 °C to obtain a dimethyl terephthalate product after separation and purification.

[0060] Detect the purity of the dimethyl terephthalate products obtained after separation and purification in the above examples and comparative examples, and the results are shown in Table 1.

[0061] Table 1: Purity test results of dimethyl terephthalate products. Serial number Purity of dimethyl terephthalate (%) Example 1 99.1 Example 2 99.2 Example 3 99.9 Example 4 99.9 Example 5 99.8 Example 6 99.5 Example 7 99.6 Example 8 99.5 Example 9 99.6 Example 10 99.2 Comparative Example 1 76.7

[0062] As can be seen from the data in Table 1, by using the method of the present invention in Examples 1 to 10, dimethyl terephthalate products with high purity can be obtained. In Examples 3 to 5, a catalyst composed of an acid-modified H-ZSM-5 molecular sieve, titanium dioxide, and polyferric sulfate is used. Compared with the single catalyst used in Examples 1 to 2, it can improve the conversion rate of reactants and the selectivity of the target product. The impurities contained in the system after the reaction are less, and it is easy to separate and purify to obtain a high-purity target product. In the catalyst of Example 6, the H-ZSM-5 molecular sieve is not modified with an organic acid. Due to the decrease in the proton acid concentration and pore volume in the catalyst, both the conversion rate of reactants and the selectivity of the target product decrease. The impurities contained in the system after the reaction increase, resulting in a slight decrease in the purity of the target product after separation and purification compared with that in Example 3. In the catalyst of Example 7, the modified H-ZSM-5 molecular sieve and titanium dioxide are directly blended as a composite support without hydrothermally compounding them. The mixing of the H-ZSM-5 molecular sieve and titanium dioxide is uneven, and both the conversion rate of reactants and the selectivity of the target product also decrease. The purity of the target product after separation and purification slightly decreases. In the catalyst of Example 8, only titanium dioxide is used as the support without compounding it with the H-ZSM-5 molecular sieve. The proton acid concentration in the catalyst is too low, and the impurities contained in the system after the reaction increase. The purity of the target product after separation and purification also slightly decreases. In the catalyst of Example 9, only the modified H-ZSM-5 molecular sieve is used as the support without compounding titanium dioxide. The Lewis acid concentration in the catalyst is too low, and the purity of the target product after separation and purification also decreases. In Example 10, polyferric sulfate is not loaded on the composite support. Due to the inability to effectively reduce the activation energy of the esterification reaction and the decrease in the adsorption rate of reactant molecules, both the conversion rate of reactants and the selectivity of the target product decrease. The impurities contained in the system after the reaction increase, resulting in a slight decrease in the purity of the target product after separation and purification.

[0063] In Comparative Example 1, no water absorbent is added to the esterification solution. Since water promotes the hydrolysis of dimethyl terephthalate, a considerable part of dimethyl terephthalate hydrolyzes to form other impurities, resulting in a significant decrease in the purity of the target product after separation and purification.

[0064] The above content is a further detailed description of the technical solution of the present invention in combination with specific preferred embodiments, and should not be construed as a limitation on the scope of implementation of the present invention. For those of ordinary skill in the art to which the present invention pertains, without departing from the premise of the present invention, several simple deductions or substitutions can still be made, which should all be regarded as belonging to the scope of patent protection determined by the claims submitted for the present invention.

Claims

1. A method for separating and purifying dimethyl terephthalate, characterized in that it comprises: Follow these steps: (1) crude terephthalic acid and methanol are mixed, and a catalyst is added to carry out an esterification reaction to obtain a thermal esterification product; (2) filtering the hot esterification product to obtain esterification liquid A; (3) Adding a water absorbent to the esterification liquid A and filtering to obtain the esterification liquid B; (4) Adding activated carbon to the esterification liquid B and filtering to obtain the esterification liquid C; (5) distilling the esterification liquid C to remove and recover methanol to obtain the esterification liquid D; (6) The esterification liquid D is fractionated twice in sequence, and the secondary fractionation products are cooled, crystallized and dried to obtain a separated and purified product.

2. The method for separating and purifying dimethyl terephthalate according to claim 1, wherein: The catalyst in step (1) comprises a composite carrier and polyferric sulfate supported on the composite carrier; the composite carrier is composited by a modified H-ZSM-5 molecular sieve and titanium dioxide through a hydrothermal reaction; the modified H-ZSM-5 molecular sieve is prepared by calcining the H-ZSM-5 molecular sieve after being treated with an organic acid solution; in the composite carrier, the mass ratio of the modified H-ZSM-5 molecular sieve to the titanium dioxide is 1:1-5:1; and the mass ratio of the composite carrier to the polyferric sulfate is 95:5-99:

1.

3. The method for separating and purifying dimethyl terephthalate according to claim 1 or 2, characterized in that: In step (1), the mass ratio of crude terephthalic acid to methanol is 1:2-10, and the mass of the catalyst is 2-10% of the mass of the crude terephthalic acid.

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

5. The method for separating and purifying dimethyl terephthalate according to claim 1, wherein: The water absorbent described in step (3) is one or a combination of two or more of silica gel, anhydrous calcium chloride, and molecular sieve desiccant; the mass of the added water absorbent is 10-50% of the mass of the crude terephthalic acid.

6. The method for separating and purifying dimethyl terephthalate according to claim 1, wherein: The mass of activated carbon added in step (4) is 5-50% of the mass of crude terephthalic acid.

7. The method for separating and purifying dimethyl terephthalate according to claim 1, characterized in that: The distillation temperature in step (5) is 65-100°C.

8. The method for separating and purifying dimethyl terephthalate according to claim 1 or 7, characterized in that: The methanol recovered in step (5) is condensed and returned to the esterification reaction in step (1) for recycling as a reaction raw material.

9. The method for separating and purifying dimethyl terephthalate according to claim 1, characterized in that: In step (6), the first fractionation removes low boiling point compounds below 250°C; and the second fractionation removes high boiling point compounds above 290°C.

10. The method for separating and purifying dimethyl terephthalate according to claim 1 or 9, characterized in that: The final cooling temperature of the cooling crystallization in step (6) is 15-40°C; during drying, hot inert gas is used for purging and drying, and the temperature of the hot inert gas is 50-130°C.

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