Method for Continuously Preparing Diphenylmethane Diisocyanate and Modified Products Thereof

Abstract

The present invention belongs to the technical field of fine chemical engineering, and particularly relates to an integrated process for continuously producing diphenylmethane diisocyanate (MDI) and its modified products based on triphosgene (BTC). The present invention discloses an HG catalyst, which is prepared by using a compound template agent through a combined microwave-hydrothermal method and supercritical CO2 drying technology. The present invention also simultaneously discloses a method for continuously preparing diphenylmethane diisocyanate and its modified products: using formaldehyde and aniline as raw materials, under the action of the HG catalyst, Ph-Amine is prepared; then, using Ph-Amine and BTC as raw materials for a three-stage continuous isocyanation reaction, and finally through solvent recovery and purification, pure MDI and mixed MDI are respectively obtained, and the mixed MDI is used to prepare modified MDI.

Description

Technical Field

[0001] The present invention belongs to the technical field of fine chemical industry, and particularly relates to an integrated process for the continuous production of diphenylmethane diisocyanate (MDI) and its modified products based on triphosgene (BTC). Background Art

[0002] Diphenylmethane diisocyanate (MDI), chemical formula: C 15 H 10 N2O2, a white to light yellow solid; its application fields include being used as polyurethane materials, adhesives, etc.

[0003] In traditional MDI preparation processes, the synthesis of amine intermediates often has problems such as low catalyst efficiency, many by-products, and long reaction cycles. Moreover, in the MDI synthesis process, there are also problems such as high HCl residue and low solvent recovery rate. In addition, the existing processes have high requirements for equipment and it is difficult to achieve continuous production.

[0004] As the mainstream synthesis process of MDI, the traditional phosgene method has long faced multiple technical bottlenecks (US 4,010,198; CN103664806A). Firstly, there are significant defects in process safety: the high corrosiveness and leakage risk of highly toxic phosgene (COCl2) force enterprises to configure complex safety facilities, directly resulting in an increase in equipment investment cost by more than 40%. Secondly, the by-product control system is inefficient: the local concentration gradient of amine substances and phosgene in the batch reaction process triggers uncontrollable side reactions, resulting in the content of trimers and biurets in the product being as high as 12% - 18%, and the content of NCO groups (isocyanate groups) being suppressed to 28% - 31%, seriously affecting the reaction activity of the product. Thirdly, there is insufficient environmental compliance: the solvent recovery rate of traditional processes is generally lower than 80%, the absorption efficiency of hydrogen chloride (HCl) tail gas is less than 65%, and the chemical oxygen demand (COD) of wastewater exceeds 500 mg / L, presenting significant environmental risks.

[0005] To avoid the toxicity of phosgene, existing technologies have tried to adopt triphosgene (BTC, C3Cl6O3) substitution schemes (CN114409543B; JP 2018 - 025634A), but their industrial applications still have systematic defects. Firstly, the problem of reaction kinetic imbalance is prominent: the one-time addition of amine solution causes intense heat release (ΔT > 25°C), the side reaction rate increases by 30%, resulting in a broadening of the product molecular weight distribution (PDI > 1.5). Secondly, not only is it difficult to meet the product purity standard, but the NCO content in the resulting mixed MDI fluctuates between 29.5% - 32.5%, and the residual HCl > 100 ppm, directly causing limited downstream applications. That is, this mixed MDI needs to be modified by adding external isocyanates such as 4,4 - MDI, 2,4 - MDI, polymeric MDI, etc. to reach a certain proportion. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for continuously preparing diphenylmethane diisocyanate and its modified products.

[0007] To solve the above problems, the present invention provides an HG catalyst used in a method for continuously preparing diphenylmethane diisocyanate and its modified products. The preparation method of the HG catalyst includes the following steps:

[0008] Step 1: Mix silane coupling agent KH-550, tetraethyl orthosilicate (TEOS), and ethanol, and then heat and stir at 60 ± 10 °C for 4 ± 0.5 hours to form a silylated precursor;

[0009] The weight ratio of silane coupling agent KH-550 to tetraethyl orthosilicate (TEOS) = (0.1 ± 0.01):1;

[0010] The dosage ratio of the tetraethyl orthosilicate to ethanol is 1 g / 1 - 1.5 ml (preferably 1 g / 1.2 - 1.3 ml);

[0011] Step 2: Add aluminum isopropoxide to the silylated precursor obtained in Step 1, and stir evenly (stir for 2 ± 0.5 hours) to obtain a homogeneous solution;

[0012] The weight ratio of aluminum isopropoxide to tetraethyl orthosilicate (TEOS) in Step 1 = 0.23 - 0.24:1;

[0013] Step 3: Mix an aqueous solution of tetraethylammonium hydroxide (TEAOH) with a mass concentration of 15 - 25% (preferably 20 ± 1%) and an aqueous solution of trimethylamine (TMA) with a mass concentration of 35 - 45% (preferably 40 ± 1%) according to the molar ratio of tetraethylammonium hydroxide (TEAOH):trimethylamine (TMA) = (4 ± 0.1):1 to obtain a template agent mixture;

[0014] First, dropwise add the template agent mixture (the dropping time is about 30 ± 10 minutes) to the homogeneous solution obtained in Step 2, and then add deionized water to form a gel;

[0015] The volume ratio of the template agent mixture to ethanol in Step 1 = 48 - 52% (preferably 50%);

[0016] The volume ratio of the deionized water to ethanol in Step 1 = 2.1 - 2.2:1;

[0017] Step 4: Subject the gel obtained in Step 3 to microwave irradiation, hydrothermal treatment (hydrothermal crystallization), first washing, supercritical CO2 drying, acid treatment, second washing, and drying to obtain the HG catalyst.

[0018] As an improvement to the preparation method of the HG catalyst of the present invention: In Step 4:

[0019] The microwave irradiation is carried out at 80±10 °C and a power of 300 - 500 W for 30 - 60 minutes (it can be carried out in a microwave reactor, corresponding to 300 - 400 g of tetraethyl orthosilicate);

[0020] The hydrothermal treatment is carried out at 120±10 °C (atmospheric pressure static hydrothermal) for 12±1 hour;

[0021] Both the first washing and the second washing are ethanol washing;

[0022] The supercritical CO2 drying is carried out at 40±5 °C and a pressure of 10 ±0.5 MPa for 4±0.5 hours;

[0023] The acid treatment is carried out by soaking in a (0.5 ±0.1) mol / L oxalic acid solution at 60±10 °C for 5±0.5 hours;

[0024] The drying is carried out by vacuum drying at 60±10 °C for 3±0.5 hours.

[0025] A mesoporous zeolite catalyst with a specific surface area ≥650 m² / g and a mesoporous structure (pore diameter 2 - 5 nm) is obtained and used as the HG catalyst.

[0026] The present invention also simultaneously provides a method for continuously preparing diphenylmethane diisocyanate and its modified products (that is, provides a preparation method of MDI / modified MDI): Using formaldehyde and aniline as raw materials, under the action of the HG catalyst, Ph - Amine is prepared; then, using Ph - Amine and BTC as raw materials for a three - stage continuous isocyanation reaction, and finally through solvent recovery and purification, pure MDI and mixed MDI are obtained respectively, and the mixed MDI is used to prepare modified MDI;

[0027] Ph - Amine represents phenyl polyamine, BTC represents bis(trichloromethyl) carbonate (that is, triphosgene), and MDI represents diphenylmethane diisocyanate.

[0028] As an improvement of the method for continuously preparing diphenylmethane diisocyanate and its modified products of the present invention, it includes the following steps:

[0029] 1), Synthesize phenyl polyamine (Ph - Amine):

[0030] Condensation reaction: Under the protection of an inert gas (such as nitrogen), aniline and formaldehyde react at 40 - 60 °C for 7 - 9 hours under the action of HG catalyst; then hydrochloric acid is added, and the reaction is carried out first at 60 - 70 °C for 4 ± 0.5 hours, and then at 90 - 100 °C for 7 ± 0.5 hours; the molar ratio of formaldehyde to aniline is 0.4 - 0.55:1, and the molar ratio of hydrogen chloride to aniline is 1.0 - 1.5:1; the weight ratio of HG catalyst to aniline is 0.01 - 0.1:1 (preferably 0.04 - 0.06:1);

[0031] The obtained reaction product is neutralized and then allowed to stand for stratification. The organic phase (oil phase) obtained by stratification is distilled under reduced pressure to obtain the crude Ph-Amine (the crude product contains HG catalyst, and the amine value is 8.5 - 9.5 mg KOH / g, and the residues of aniline and moisture are both < 100 ppb of Ph-Amine);

[0032] 2), Preparation of crude MDI:

[0033] Using the crude Ph-Amine obtained in step 1) for preparation, the obtained reaction solution contains crude MDI;

[0034] 3), Solvent recovery and purification:

[0035] After the reaction solution obtained in step 2) is distilled under normal pressure (two-stage normal pressure distillation) to recover the solvent, and then pure MDI (with NCO content ≥ 33% and HCl residue ≤ 50 ppm) is obtained by short-path molecular distillation; mixed MDI is also obtained accordingly;

[0036] The mixed MDI is used for the preparation of modified MDI.

[0037] As a further improvement of the method for continuously preparing diphenylmethane diisocyanate and its modified product of the present invention, step 2) includes the following steps:

[0038] 2.1) Raw material preparation:

[0039] Preparation of BTC solution: BTC and the solvent are mixed evenly to obtain the BTC solution (which can be placed in the BTC solution storage tank for standby); the weight ratio of BTC to the solvent is 1:3 - 5 (preferably 1:4); the BTC is bis(trichloromethyl) carbonate (i.e., triphosgene);

[0040] Preparation of the mixed amine solution: Step 1): The obtained crude Ph-Amine is mixed with a solvent (which can be placed in an intermediate mixing kettle), filtered, and the obtained filter residue is the HG catalyst (which can be reused in the next batch after drying); a tin-based catalyst is added to the filtrate and mixed evenly to obtain a mixed amine solution (which can be stored in a mixed amine solution storage tank for standby). The weight ratio of the crude Ph-Amine to the solvent is 1:3 to 5 (preferably 1:4); the weight ratio of the tin-based catalyst to the crude Ph-Amine is 0.01 to 0.1:1 (preferably 0.004 to 0.006:1);

[0041] Note: The reuse times of the HG catalyst ≥ 5 times;

[0042] The solvent used for the BTC solution and the solvent used for the mixed amine solution are preferably the same solvent;

[0043] 2.2) Tertiary isocyanation reaction:

[0044] According to the weight ratio of BTC to the crude Ph-Amine being 2.3 to 2.4:1, set the dosage ratio of the BTC solution and the mixed amine solution; the following tertiary reactions are carried out in sequence:

[0045] Primary reaction: After mixing the BTC solution with 1 / 3 of the mixed amine solution, react at normal pressure, 30 to 40 °C, and a rotation speed of 250 ± 50 rpm for 2 ± 0.5 hours to obtain a primary reaction solution;

[0046] Secondary reaction: After mixing the primary reaction solution with 1 / 3 of the mixed amine solution, react under the protection of an inert gas at 60 to 70 °C, a pressure of 0.2 ± 0.05 Mpa, and a rotation speed of 250 ± 50 rpm for 2 ± 0.5 hours to obtain a secondary reaction solution;

[0047] Tertiary reaction: After mixing the secondary reaction solution with 1 / 3 of the mixed amine solution, react at 90 ± 5 °C, a vacuum condition of -0.06 ± 0.01 MPa, and a rotation speed of 250 ± 50 rpm for 2 ± 0.5 hours to obtain a tertiary reaction solution; the tertiary reaction solution contains crude MDI;

[0048] That is, the set pressure gradient is normal pressure → slightly positive pressure → vacuum.

[0049] As a further improvement of the method for continuously preparing diphenylmethane diisocyanate and its modified products of the present invention, it further includes the following step 4):

[0050] 4) Preparation of modified MDI:

[0051] The modified reaction is carried out on the mixed MDI, polyether polyol, tin catalyst and amine catalyst under the protection of an inert gas (such as nitrogen) at 80-100 °C; until the NCO content drops to 12%-18% (preferably 15±0.5%), the modified MDI (the modified MDI with a viscosity of about 1600-2000 mPa·s) is obtained;

[0052] The polyether polyol is 48-52% (preferably 50%) of the weight of the mixed MDI;

[0053] The addition amount of the tin catalyst is 0.05-0.15% (preferably 0.08-0.15%, more preferably 0.08-0.1%) of the weight of the mixed MDI; the addition amount of the amine catalyst is 0.02-0.06% (preferably 0.05-0.06%) of the weight of the mixed MDI.

[0054] As a further improvement of the method for continuously preparing diphenylmethane diisocyanate and its modified product of the present invention, the tin catalysts in the step 2.1) and the tin catalyst in the step 4) are at least any one of the following (a combination of one or more): stannous octoate, stannous neodecanoate, dibutyltin dilaurate, methyltin mercaptide, butyltin mercaptide, octyltin mercaptide;

[0055] The amine catalyst in the step 4) is A33 catalyst;

[0056] The polyether polyol in the step 4) is PPG-2000.

[0057] As a further improvement of the method for continuously preparing diphenylmethane diisocyanate and its modified product of the present invention, the step 1) includes the following steps:

[0058] 1.1), condensation reaction:

[0059] Under the protection of an inert gas (such as nitrogen), aniline and HG catalyst are added to a reaction kettle (polyamine reaction kettle), and an aqueous formaldehyde solution with a mass concentration of 35-37% is added dropwise (the dropping time is about 2±0.5 h) in a water bath at 10±2 °C, and after uniformly stirring (stirring for 30±10 minutes), the temperature is raised to 40-60 °C and the reaction is carried out for 7-9 hours; then an aqueous hydrochloric acid solution with a mass concentration of 35-37% is added dropwise (the dropping time is about 2±0.5 h), and the reaction is carried out at 60-70 °C for 4±0.5 hours and at 90-100 °C for 7±0.5 hours in sequence;

[0060] The molar ratio of formaldehyde to aniline is 0.4-0.55:1, and the molar ratio of hydrogen chloride (hydrogen chloride in hydrochloric acid) to aniline is 1.0-1.5:1; the weight ratio of HG catalyst to aniline is 0.01-0.1:1;

[0061] 1.2), neutralization treatment:

[0062] After cooling the product (transparent brown solution) obtained in Step 1.1) to room temperature, transfer it to a neutralization and separation kettle, dropwise add a NaOH solution with a mass concentration of 36 - 40% and stir (stir for 30 - 45 minutes), then carry out neutralization and separation (pH is about 13 - 14). After standing, separate the upper organic phase;

[0063] The molar ratio of NaOH to hydrogen chloride is 0.9 - 1.1:1 (preferably 1:1);

[0064] 1.3), Vacuum distillation:

[0065] Distill the organic phase obtained in Step 1.2) under a reduced pressure of 0.1 MPa, collect the fraction at 60 - 220 °C to obtain an aniline solution (the aniline solution can be recycled and reused); the material in the kettle is phenyl polyamine (Ph-Amine).

[0066] As a further improvement of the method for continuously preparing diphenylmethane diisocyanate and its modified products of the present invention,

[0067] The solvent in Step 2.1) is at least any one of dichloromethane, dichloroethane, chlorobenzene, toluene, dimethyl isooctanoate, and dimethyl butanol (a combination of one or more);

[0068] In Step 2.2): The gases generated in the first-stage reaction, second-stage reaction, and third-stage reaction are condensed. The condensed liquid (containing trace amounts of phosgene) is returned to the corresponding reaction system for the corresponding reaction, and the condensed gas (HCl gas) enters the HCl absorption tower for absorption.

[0069] That is, remove the HCl gas and recycle it.

[0070] As a further improvement of the method for continuously preparing diphenylmethane diisocyanate and its modified products of the present invention, in Step 3):

[0071] The tertiary reaction liquid obtained in Step 2) is subjected to two-stage atmospheric distillation: First, carry out atmospheric distillation (for example, at 80 - 100 °C) to collect the solvent (which can be recycled and reused); then raise the temperature to 120 - 150 °C for atmospheric distillation;

[0072] Note: The fraction collected at 120 - 150 °C is the over-fraction (the fraction with an NCO content less than 33%); the over-fraction is waste;

[0073] Subsequently, it is purified by short-path molecular distillation (180 ± 5 °C, vacuum degree ≤ 3 kPa) to obtain pure MDI (pure MDI with an NCO content ≥ 33% and HCl residue ≤ 50 ppm); the material in the kettle is mixed MDI; the NCO content in the mixed MDI product is about 26% - 33% (preferably 30% - 33%).

[0074] The present invention also provides the use of MDI or modified MDI prepared by the above method in polyurethane foams, elastomers or coatings.

[0075] The present invention is directed to the problems of low catalyst efficiency, many by-products, poor product stability, etc. in the existing MDI preparation process, and proposes a method for continuously preparing MDI and its modified products. By designing a novel catalyst and a new BTC process, the present invention significantly improves the reaction efficiency, reduces impurity residues, and at the same time, triphosgene replacing phosgene is more environmentally friendly, reduces safety hazards during large-scale production, and realizes continuous production.

[0076] The present invention has the following technical advantages:

[0077] 1. An HG catalyst is provided for synthesizing phenyl polyamine (Ph-Amine);

[0078] The HG catalyst of the present invention has a unique structure: by using a compound template agent (TEAOH + TMA) and combining microwave-hydrothermal method with supercritical CO2 drying technology, the prepared HG catalyst has a high specific surface area (specific surface area ≥ 650 m² / g) and a mesoporous structure (pore diameter 2 - 5 nm), with uniform distribution of acidic sites, significantly improving the aniline-formaldehyde condensation reaction efficiency, and can be reused more than 5 times, which can reduce the content of high-boiling substances in phenyl polyamine. It can effectively control the content of high-molecular polyamine in phenyl polyamine, and the amine value of the obtained intermediate phenyl polyamine is stable at 8.5 - 9.5 mg KOH / g, and the aniline residue and moisture are both less than 100 ppb.

[0079] Thus, the NCO in the finally obtained diphenylmethane diisocyanate (MDI) is high.

[0080] 2. During the preparation process of MDI:

[0081] 1). Triphosgene is used to replace phosgene, which is more environmentally friendly and safe.

[0082] 2). A three-stage continuous reaction system is adopted: by designing segmented temperature control (30 - 40°C → 60 - 70°C → 90 ± 5°C) and pressure gradient (atmospheric pressure → slightly positive pressure → vacuum), the staged removal of HCl is realized, so as to achieve efficient separation of hydrochloric acid gas (HCl residue ≤ 50 ppm), and at the same time, the energy consumption is reduced by more than 20%, and the NCO content ≥ 33%;

[0083] Combined with short-path molecular distillation technology (180°C / 3 kPa), it has significant differences compared with the existing BTC process (single reaction kettle + atmospheric distillation).

[0084] Precisely control the weight ratio of BTC / Ph-Amine to 2.3 - 2.4:1, and it is proved by examples that this dosage ratio has a critical control effect on the NCO content (≥33%) and HCl residue (≤50 ppm).

[0085] Note: When the HG catalyst is recycled, the NCO content ≥ 32.8%.

[0086] 3), In the present invention, by controlling the distillation amount of pure MDI, the mixed MDI in the kettle residue can directly reach the proportion requirement (that is, the NCO content of the mixed MDI is 26% - 33%), and then directly carry out the subsequent modification application.

[0087] 4), High solvent recovery rate: The two-stage atmospheric distillation combined with short-path molecular distillation technology, the comprehensive solvent recovery rate ≥ 98% (verified by GC-MS), significantly reducing the raw material cost.

[0088] 5), The present invention can prepare high-purity Ph-Amine: In the directional synthesis of phenyl polyamine Ph-Amine, the HG catalyst and staged control of aniline-formaldehyde condensation and ring-closure reaction are adopted, and the amine value of the product Ph-Amine is precisely controllable (8.5 - 9.5 mgKOH / g), the aniline residue < 100 ppb, and the moisture < 100 ppb, meeting the requirements for high-end MDI synthesis.

[0089] 6), High product consistency, suitable for large-scale production.

[0090] 3, The modified MDI has excellent performance: In the in-situ modification process of polyether polyol, by precisely blocking the polyether polyol (preferably controlling the NCO content to 15 ± 0.5%), the obtained modified MDI has low viscosity (1900 ± 200 mPa·s) and high reaction activity, and is applicable to polyurethane foam, high-precision polyurethane injection molding and spraying processes. Brief Description of the Drawings

[0091] The following further describes the specific embodiments of the present invention in conjunction with the drawings.

[0092] Figure 1 It is a schematic diagram of the device for the three-stage isocyanation reaction of the present invention. Detailed Description of the Invention

[0093] The following further describes the present invention in conjunction with specific examples, but the protection scope of the present invention is not limited thereto:

[0094] BTC: Triphosgene, that is, bis(trichloromethyl) carbonate.

[0095] Ph-Amine: Phenyl polyamine,

[0096] MDI: Diphenylmethane diisocyanate.

[0097] Catalyst Preparation Example 1. Preparation of HG catalyst. The following steps are carried out successively:

[0098] 1). Add 36 g of silane coupling agent KH-550 to a mixed solution of 360 g of tetraethyl orthosilicate (TEOS) and 450 mL of ethanol, and stir at 60 °C for 4 hours to form a silylated precursor;

[0099] 2). Add 85.5 g of aluminum isopropoxide to the silylated precursor obtained in step 1), and continue stirring for 2 hours to obtain a homogeneous solution;

[0100] 3). Mix an aqueous solution of tetraethylammonium hydroxide with a mass concentration of 20% and an aqueous solution of trimethylamine with a mass concentration of 40% according to a molar ratio of tetraethylammonium hydroxide: trimethylamine = 4:1 to obtain a template mixture;

[0101] Dropwise add 221 mL of the template mixture (the dropping time is about 30 minutes) to the homogeneous solution obtained in step 2); after the addition of the template mixture is completed, add 960 mL of deionized water (at this time, the pH is about 10 - 11) to form a gel;

[0102] 4). Transfer the gel to a microwave reactor, and irradiate it under microwave at 80 °C and a power of 500 W for 60 minutes, and then carry out static hydrothermal treatment at 120 °C and normal pressure for 12 hours;

[0103] 5). Filter the hydrothermal product obtained in step 4), wash the obtained filter cake with ethanol 3 times (the purpose of washing is to remove the template residue - tetraethylammonium hydroxide and trimethylamine)), transfer it to a supercritical CO2 drying device, and dry it at 40 °C and a pressure of 10 MPa for 4 hours;

[0104] 6). Immerse the dried product in a 0.5 mol / L oxalic acid solution at 60 °C for 5 hours, wash it with ethanol until neutral (i.e., after secondary washing), and then dry it under vacuum at 60 °C for 3 hours to obtain HG catalyst (about 100 g).

[0105] The specific surface area of the HG catalyst is 650 m² / g, and there is a mesoporous structure with pore diameters of 2 - 5 nm.

[0106] Catalyst Comparative Example 1. Cancel the use of "trimethylamine (TMA)" in step 3) of Catalyst Preparation Example 1, that is, use only TEAOH as the template; replace the "template mixture" with "an aqueous solution of tetraethylammonium hydroxide with a mass concentration of 20%", and the rest is the same as Catalyst Preparation Example 1. The obtained product is named HG catalyst A. Its specific surface area is about 520 m² / g, and there is a mesoporous structure with pore diameters of 4 - 10 nm.

[0107] Catalyst Comparative Example 2: In step 4) of Catalyst Preparation Example 1, cancel "microwave irradiation for 60 minutes under the conditions of 80°C and a power of 500 W", that is, directly subject the gel to hydrothermal treatment at 120°C under normal pressure for 13 hours; the rest is the same as Catalyst Preparation Example 1. The obtained catalyst is named HG Catalyst B. Its specific surface area is about 550 m² / g, and there is a mesoporous structure with pore diameters of 1 - 3 nm.

[0108] Catalyst Comparative Example 3: Change "supercritical CO2 drying" in step 5) of Catalyst Preparation Example 1 to ordinary drying: dry at 40°C and normal pressure for 4 hours; the rest is the same as Catalyst Preparation Example 1. The obtained catalyst is named HG Catalyst C. Its specific surface area is about 400 m² / g, and there is a bimodal pore structure with pore diameters of 1 - 2 nm and >10 nm.

[0109] Example 1: A method for preparing diphenylmethane diisocyanate (MDI) and its modified products, which successively undergoes the following steps:

[0110] I. Synthesis of phenyl polyamine (Ph-Amine)

[0111] 1.1) Condensation reaction:

[0112] Under nitrogen protection, add 1019 g of aniline (10.9 mol) and 50 g of HG catalyst (obtained from Catalyst Preparation Example 1) to a polyamine reaction kettle. Slowly add a 37% aqueous formaldehyde solution (containing 4.54 mol of formaldehyde) dropwise (the dropping time is about 2 h) in a 10°C water bath, stir for 30 minutes, then raise the temperature to 40 - 60°C and react for 8 hours; subsequently, add a 37% hydrochloric acid solution (containing 10.95 mol of hydrogen chloride) dropwise (the dropping time is about 2 h), and react at 60 - 70°C for 4 hours and at 90 - 100°C for 7 hours in sequence to obtain a transparent brown solution;

[0113] That is, in this case, the molar ratio of formaldehyde:aniline = 0.42:1; the molar ratio of hydrogen chloride:aniline = 1.005:1;

[0114] 1.2) Neutralization treatment:

[0115] After cooling the transparent brown solution obtained in step 1.1) to room temperature, transfer it to a neutralization and separation kettle, add a 38% NaOH solution (containing 10.95 mol of sodium hydroxide) dropwise to adjust the pH to about 13 - 14, stir for 30 minutes, let it stand and then separate layers, and separate the upper organic phase;

[0116] That is, in this case, the molar ratio of NaOH:hydrogen chloride in step 1) = 1:1;

[0117] 1.3) Vacuum distillation:

[0118] The organic phase was distilled under reduced pressure of 0.1 MPa until no more distillate was produced, and the distillation was stopped. The fraction at 60 - 220 °C was collected to obtain an aniline solution, which could be recycled and reused. 924 g of crude phenyl polyamine (Ph-Amine) was obtained in the kettle. The crude phenyl polyamine (Ph-Amine) contained HG catalyst. The amine value of Ph-Amine was 8.9 mg KOH / g, and the residual aniline and moisture were both less than 100 ppb.

[0119] II. Preparation of MDI:

[0120] 2.1) Preparation of raw materials:

[0121] Preparation of BTC solution: 2177 g of BTC was mixed evenly with 8708 g of dichloroethane and placed in the BTC solution storage tank for standby;

[0122] Preparation of mixed amine solution: 924 g of the crude Ph-Amine obtained in step 1 and 3696 g of dichloroethane were placed in the transfer mixing kettle. After filtration, a solid was obtained. The solid was dried (dried to constant weight at 100 °C) to obtain the recoverable HG catalyst for the next batch application. 4.62 g of thiol butyltin was added to the filtrate and mixed evenly, and then placed in the mixed amine solution storage tank for standby.

[0123] That is, BTC:crude Ph-Amine = 2.356:1.

[0124] 2.2) Tertiary reaction:

[0125] The device used for the tertiary reaction is as Figure 1 shown:

[0126] The outlet of mixing storage tank M01 is hermetically connected to the inlet of the first-stage reaction kettle R0201. The first-stage reaction kettle R0201 is equipped with an N2 inlet (which can be used as a protective gas in case of an accident); the top of the first-stage reaction kettle R0201 is equipped with a gas outlet, which is connected to the HCl absorption tower T0201 through condenser C0201; the bottom of the first-stage reaction kettle R0201 is equipped with a material outlet; the material outlet of the first-stage reaction kettle R0201 is hermetically connected to the inlet of mixing storage tank M02 through a peristaltic pump;

[0127] The outlet of mixing storage tank M02 is hermetically connected to the inlet of the second-stage reaction kettle R0202. The second-stage reaction kettle R0202 is equipped with an N2 inlet; the top of the second-stage reaction kettle R0202 is equipped with a gas outlet, which is connected to the HCl absorption tower T0202 through condenser C0202; the bottom of the second-stage reaction kettle R0202 is equipped with a material outlet; the material outlet of the second-stage reaction kettle R0202 is hermetically connected to the inlet of mixing storage tank M03 through a peristaltic pump;

[0128] The outlet of the mixing storage tank M03 is hermetically connected to the feed inlet of the third-stage reactor R0203. An N2 inlet (used as a protective gas in case of an accident) is provided on the third-stage reactor R0203; a gas outlet is provided at the top of the third-stage reactor R0203, and this gas outlet is connected to the HCl absorption tower T0203 after passing through the condenser C0203; a material outlet is provided at the bottom of the third-stage reactor R0203; the third-stage reactor R0203 is provided with a vacuum extraction port. For the sake of simplicity of the drawing, this vacuum extraction port is Figure 1 omitted in the

[0129] The specific process of the three-stage reaction is as follows:

[0130] A. First-stage reaction: All the BTC solution in the BTC solution storage tank is transferred into the first mixing storage tank M01 through a peristaltic pump. At the same time, 1 / 3 of the mixed amine solution in the mixed amine solution storage tank is pumped into the first mixing storage tank M01 and mixed for 30 minutes. The mixed material flows into the first-stage reactor R0201 for the first-stage reaction (atmospheric pressure), and reacts at 30 °C and a rotation speed of 250 ± 10 rpm for 2 hours. The HCl gas generated by the reaction is discharged from the gas outlet at the top of the first-stage reactor R0201, condensed by the condenser C0201, and then enters the HCl absorption tower T0201, so as to realize the recovery of HCl gas; the first-stage reaction liquid obtained from the reaction is discharged from the material outlet at the bottom of the first-stage reactor R0201 and transferred into the second mixing storage tank M02 through a peristaltic pump;

[0131] B. Second-stage reaction:

[0132] After the first-stage reaction is completed, 1 / 3 of the mixed amine solution in the mixed amine solution storage tank is pumped into the first-stage reaction liquid in the second mixing storage tank M02 and mixed for 30 minutes. The mixed material flows into the second-stage reactor R0202 for the second-stage reaction. Nitrogen is introduced into the second-stage reactor R0202, and the reaction is carried out at 60 °C, a slightly positive pressure of 0.2 ± 0.05 MPa, and a rotation speed of 250 ± 10 rpm for 2 hours. The HCl gas generated by the reaction is discharged from the gas outlet at the top of the second-stage reactor R0202, condensed by the condenser C0202, and then enters the HCl absorption tower T0202, so as to realize the recovery of HCl gas; the second-stage reaction liquid obtained from the reaction is discharged from the material outlet at the bottom of the second-stage reactor R0202 and transferred into the third mixing storage tank M03 through a peristaltic pump;

[0133] C. Third-stage reaction:

[0134] After the second-stage reaction is completed, the remaining 1 / 3 of the mixed amine solution in the mixed amine solution storage tank is pumped into the second-stage reaction liquid in the mixed storage tank M03 and mixed for 30 minutes. The mixed material flows into the third-stage reaction kettle R0203 and reacts at 90 °C, -0.06 ± 0.01 MPa vacuum condition, and 250 ± 10 rpm rotation speed for 2 hours. The generated HCl gas is discharged from the gas outlet at the top of the third-stage reaction kettle R0203, condensed by the condenser C0203, and then enters the HCl absorption tower T0203, thereby realizing the recovery of HCl gas. The third-stage reaction liquid obtained from the reaction is discharged from the material outlet at the bottom of the third-stage reaction kettle R0203.

[0135] The third-stage reaction liquid contains crude MDI.

[0136] Note: The gases generated in the first-stage reaction, second-stage reaction, and third-stage reaction are condensed. The condensed liquid (containing trace amounts of phosgene) is returned to the corresponding reaction system for the corresponding reaction, and the condensed gas (HCl gas) enters the HCl absorption tower for absorption.

[0137] 2.3) Solvent recovery and purification:

[0138] The third-stage reaction liquid discharged from the material outlet at the bottom of the third-stage reaction kettle R0203 enters the solvent recovery tower through a peristaltic pump and is subjected to atmospheric distillation at 80 - 100 °C. The fraction with a tower temperature of 80 - 85 °C is collected as the solvent dichloroethane, and the solvent recovery rate is 98.8%. Subsequently, it is recycled (i.e., the main solvent is recovered).

[0139] Then, atmospheric distillation is carried out at 120 - 150 °C; the fraction collected at 120 - 150 °C is the over-fraction (the fraction with an NCO content less than 33%). The over-fraction is waste.

[0140] The material obtained in the solvent recovery tower is named crude MDI.

[0141] The crude MDI discharged from the bottom of the solvent recovery tower enters the product distillation tower through a peristaltic pump. Subsequently, in the product distillation tower, the crude MDI is purified by short-path molecular distillation (180 ± 5 °C, ≤ 3 kPa vacuum degree) to obtain 300 g of pure MDI. In the pure MDI, the NCO content ≥ 33% and the HCl residue ≤ 50 ppm. The material in the product distillation tower is the mixed MDI product, and the NCO content of the mixed MDI is 32.55% and the weight is 700 g.

[0142] Note: The NCO content can be detected by a conventional chemical titration method.

[0143] Description: In the present invention, the distillation amount of pure MDI is controlled by controlling the parameters of short-path molecular distillation, so that the mixed MDI in the still residue meets the proportion requirements (i.e., the NCO content of the mixed MDI is 26% - 33%), and then directly proceed with the subsequent modification application;

[0144] III. Preparation of modified MDI:

[0145] Add 350 g of polyether polyol (such as PPG - 2000), 0.7 g of stannous octoate, and 0.35 g of A33 catalyst to 700 g of the mixed MDI product. The addition amount of stannous octoate is 0.1% of the mixed MDI, and the addition amount of A33 catalyst is 0.05% of the mixed MDI. React at 80 - 100 °C under nitrogen protection until the NCO content drops to 15% to obtain modified MDI with a viscosity of 1800 mPa·s (25 °C).

[0146] Table 1 Key data of Example 1

[0147]

[0148] Example 2: Recycling of HG catalyst:

[0149] First recycling: Change the "50 g of HG catalyst" in step 1.1) of Example 1 to the recyclable HG catalyst obtained in step 2.1) of Example 1. Since there may be slight damage to the HG catalyst, supplement fresh HG catalyst until the total amount is 50 g; the rest is the same as Example 1.

[0150] And so on, that is, for subsequent batches of recycling, use the recyclable HG catalyst from the previous batch and supplement fresh HG catalyst until the total amount is 50 g.

[0151] A total of 5 times of recycling were carried out, and the results are as shown in Table 2 below.

[0152] Table 2 Key data of Example 2

[0153]

[0154] In summary, it can be seen that the "recyclable HG catalyst" obtained in step 2.1) of the present invention can indeed be recycled.

[0155] Example 3 - 1: Change the "37% formaldehyde aqueous solution containing 4.54 mol of formaldehyde" in step 1.1) of Example 1 involving "synthesis of phenyl polyamine (Ph - Amine)" to "37% formaldehyde aqueous solution containing 6.06 mol of formaldehyde", that is, change the molar ratio of formaldehyde:aniline to about 0.55:1; the rest is the same as Example 1.

[0156] Example 3-2: In step 1.1) of "synthesizing phenyl polyamine (Ph-Amine)" in Example 1, change the "37% aqueous formaldehyde solution containing 4.54 mol of formaldehyde" to "37% aqueous formaldehyde solution containing 4.36 mol of formaldehyde", that is, change the molar ratio of formaldehyde to aniline to 0.4:1; the rest is the same as in Example 1.

[0157] Comparative Example 1-1: In step 1.1) of "synthesizing phenyl polyamine (Ph-Amine)" in Example 1, change the "37% aqueous formaldehyde solution containing 4.54 mol of formaldehyde" to "37% aqueous formaldehyde solution containing 6.81 mol of formaldehyde", that is, change the molar ratio of formaldehyde to aniline to approximately 0.62:1; the rest is the same as in Example 1.

[0158] Comparative Example 1-2: In step 1.1) of "synthesizing phenyl polyamine (Ph-Amine)" in Example 1, change the "37% aqueous formaldehyde solution containing 4.54 mol of formaldehyde" to "37% aqueous formaldehyde solution containing 3.89 mol of formaldehyde", that is, change the molar ratio of formaldehyde to aniline to approximately 0.36:1; the rest is the same as in Example 1.

[0159] Example 4-1: In step 1.1) of "synthesizing phenyl polyamine (Ph-Amine)" in Example 1, change the "37% hydrochloric acid containing 10.95 mol of hydrogen chloride" to "37% hydrochloric acid containing 10.90 mol of hydrogen chloride", that is, the molar ratio of aniline to hydrogen chloride is 1:1; and extend the stirring time for neutralization treatment in step 1.2) from "30 minutes" to "45 minutes", the rest is the same as in Example 1.

[0160] Example 4-2: In step 1.1) of "synthesizing phenyl polyamine (Ph-Amine)" in Example 1, change the "37% hydrochloric acid containing 10.95 mol of hydrogen chloride" to "37% hydrochloric acid containing 16.35 mol of hydrogen chloride", that is, the molar ratio of aniline to hydrogen chloride is 1:1.5; and increase the amount of sodium hydroxide used for neutralization treatment in step 1.2), that is, still ensure the molar ratio of NaOH to hydrogen chloride in step 1) is 1:1; the rest is the same as in Example 1.

[0161] The comparison of the above cases is shown in Table 3 below:

[0162] Table 3

[0163]

[0164] Example 5-1: In step 2.1) of "Preparation of MDI" in Example 1, change "2177 g of BTC and 8708 g of dichloroethane" to "2124 g of BTC and 8496 g of dichloroethane", and keep the amount of crude Ph-Amine unchanged, still 924 g. Therefore, the weight ratio of BTC:crude Ph-Amine = 2.30:1; the rest is the same as in Example 1.

[0165] Example 5-2: In step 2.1) of "Preparation of MDI" in Example 1, change "2177 g of BTC and 8708 g of dichloroethane" to "2218 g of BTC and 8872 g of dichloroethane", and keep the amount of crude Ph-Amine unchanged, still 924 g. Therefore, the weight ratio of BTC:crude Ph-Amine = 2.40:1; the rest is the same as in Example 1.

[0166] Comparative Example 2-1: In step 2.1) of "Preparation of MDI" in Example 1, change the amount of crude Ph-Amine from "924 g" to "990 g", and keep the amount of BTC unchanged, that is, BTC:crude Ph-Amine = 2.20:1 (similar to the existing dosage ratio); the rest is the same as in Example 1.

[0167] Comparative Example 2-2: Change the amount of crude Ph-Amine in step 2.1) of "Preparation of MDI" in Example 1, and keep the amount of BTC unchanged, so that BTC:crude Ph-Amine = 2.50:1 (similar to the existing dosage ratio). The rest is the same as in Example 1.

[0168] The comparison of the above cases is shown in Table 4 below:

[0169] Table 4

[0170]

[0171] Comparative Example 3: Compared with Example 1, referring to Example 8 in Patent CN 112292413 B, change "the staged removal of HCl in the three-stage reaction system is achieved through a temperature-pressure gradient (atmospheric pressure → slightly positive pressure → vacuum)" in Example 1 of the present invention to "single reaction kettle + atmospheric distillation";

[0172] Specifically as follows: Add the mixed amine solution to the reaction kettle, keep the temperature at 145°C, introduce the BTC solution in 28 h, and react at 145°C to 150°C for 3-4 h. Distill at 120°C, and the resulting product is a mixture. This mixture cannot be accurately designed for application formulations, and the yield and conversion data of the mixture cannot be determined, and the process is not rigorous.

[0173] Example 6-1: In Step 3 of "Modified MDI Preparation" involved in Example 1, change the "stannous octoate addition amount (weight ratio of stannous octoate to mixed MDI)" from "0.1%" to "0.08%", and change the "A33 catalyst addition amount (weight ratio of A33 catalyst to mixed MDI)" from "0.05%" to "0.06%"; the rest is the same as Example 1.

[0174] Example 6-2: In Step 3 of "Modified MDI Preparation" involved in Example 1, change the "stannous octoate addition amount (weight ratio of stannous octoate to mixed MDI)" from "0.1%" to "0.15%", and change the "A33 catalyst addition amount (weight ratio of A33 catalyst to mixed MDI)" from "0.05%" to "0.06%"; the rest is the same as Example 1.

[0175] Comparative Example 4-1: In Step 3 of "Modified MDI Preparation" involved in Example 1, change the "stannous octoate addition amount (weight ratio of stannous octoate to mixed MDI)" from "0.1%" to "0.20%", and change the "A33 catalyst addition amount (weight ratio of A33 catalyst to mixed MDI)" from "0.05%" to "0.08%"; the rest is the same as Example 1.

[0176] Comparative Example 4-2: In Step 3 of "Modified MDI Preparation" involved in Example 1, change the "stannous octoate addition amount (weight ratio of stannous octoate to mixed MDI)" from "0.1%" to "0.03%", and change the "A33 catalyst addition amount (weight ratio of A33 catalyst to mixed MDI)" from "0.05%" to "0.01%"; the rest is the same as Example 1.

[0177] Comparative Example 5: In Step 3 of "Modified MDI Preparation" involved in Example 1, cancel the use of A33 catalyst and keep the stannous octoate addition amount unchanged; the rest is the same as Example 1.

[0178] Experiment 1. A low-density flexible foam formula is as shown in Table 5 below. The modified MDI used in the formula is the modified MDI prepared in Example 1, Example 6-1, Example 6-2, Comparative Example 4-1, Comparative Example 4-2, and Comparative Example 5.

[0179] Table 5 Low-density flexible foam formula table

[0180]

[0181] Foam preparation operation process:

[0182] Add polyether polyol PPG-2000, latex polyether HF-22, deionized water, silicone oil HGD-858H and dimethyl methyl phosphate into a stainless steel stirring kettle, stir at 1200 rpm for 20 minutes until the system becomes transparent, add amine catalyst A33, and stir at 500 rpm for 5 minutes to obtain component A.

[0183] Stannous octoate was added to the modified MDI, and the mixture was stirred at a low speed of 300 rpm for 10 minutes under nitrogen protection to obtain component B.

[0184] Pour component B into component A and mix for 15 seconds using a mechanical stirrer (1500 rpm, Φ50 mm anchor blade). Immediately pour into a 30×30×15 cm mold and allow to foam freely for 5 minutes at 45°C and 70% humidity. Then cure at 60°C and 50% humidity for 20 minutes, and then demould at 80°C and 30% humidity for 180 minutes to obtain foam.

[0185] Finally, the foam was placed in a 23°C / 50%RH environment for 48 hours to obtain a low-density soft foam product. Various performance tests were performed according to ASTM D3574 standard.

[0186] Table 6 Performance comparison table

[0187]

[0188] According to Table 6 above, it can be seen that the pore structure of Example 1 is the best.

[0189] Comparative Experiment 2: The “HG catalyst obtained by using catalyst preparation example 1” in Example 1 is respectively changed to “HG catalyst A obtained by catalyst comparison example 1”, “HG catalyst B obtained by catalyst comparison example 2”, and “HG catalyst C obtained by catalyst comparison example 3”; the rest is the same as Example 1.

[0190] The results are shown in Table 7 below:

[0191] Table 7

[0192]

[0193] Finally, it should be noted that the above examples are only some specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and there are many variations. All variations that can be directly derived or associated with the content disclosed by a person skilled in the art should be considered as the protection scope of the present invention.

Claims

1. The HG catalyst used in the method for continuously preparing diphenylmethane diisocyanate and its modified products, characterized in that Preparation method of HG catalyst, comprising the following steps: Step 1: Mix silane coupling agent KH-550 with tetraethyl orthosilicate and ethanol, and heat and stir at 60±10°C for 4±0.5 hours to form a silylated precursor; The weight ratio of silane coupling agent KH-550 to tetraethyl orthosilicate is (0.1±0.01):1; The dosage ratio of tetraethyl orthosilicate to ethanol is 1g / 1~1.5ml; Step 2: Add aluminum isopropoxide to the silylated precursor obtained in Step 1, and stir evenly to obtain a homogeneous solution; The weight ratio of aluminum isopropoxide to tetraethyl orthosilicate in Step 1 is 0.23~0.24:1; Step 3: Mix an aqueous solution of tetraethylammonium hydroxide with a mass concentration of 15~25% and an aqueous solution of trimethylamine with a mass concentration of 35~45% according to the molar ratio of tetraethylammonium hydroxide:trimethylamine = (4±0.1):1 to obtain a template mixture; First, add the template mixture dropwise to the homogeneous solution obtained in Step 2, and then add deionized water to form a gel; The volume ratio of the template mixture to ethanol in Step 1 is 48~52%; The volume ratio of deionized water to ethanol in Step 1 is 2.1~2.2:1; Step 4: Subject the gel obtained in Step 3 to microwave irradiation, hydrothermal treatment, first washing, supercritical CO2 drying, acid treatment, second washing, and drying, and use the obtained mesoporous zeolite catalyst as the HG catalyst; The microwave irradiation is carried out at 80±10°C and a power of 300~500W for 30~60 minutes; The hydrothermal treatment is carried out at 120±10°C for 12±1 hour; The first washing is to remove tetraethylammonium hydroxide and trimethylamine as the template; 2. The HG catalyst used in the method for continuously preparing diphenylmethane diisocyanate and its modified products according to claim 1, characterized in that In Step 4: Both the first washing and the second washing are ethanol washing; The supercritical CO2 drying is carried out at 40±5°C and a pressure of 10 ±0.5MPa for 4±0.5 hours; The acid treatment is to soak in a (0.5 ±0.1) mol / L oxalic acid solution at 60±10°C for 5±0.5 hours; The drying is carried out by vacuum drying at 60±10°C for 3±0.5 hours.

3. A method for continuously preparing diphenylmethane diisocyanate and its modified products, characterized in that: Using formaldehyde and aniline as raw materials, under the action of the HG catalyst described in Claim 1 or 2, Ph-Amine is prepared; then, using Ph-Amine and BTC as raw materials, a three-stage continuous isocyanation reaction is carried out, and finally, through solvent recovery and purification, pure MDI and mixed MDI are obtained respectively, and the mixed MDI is used to prepare modified MDI; Ph-Amine represents phenyl polyamine, BTC represents bis(trichloromethyl) carbonate, and MDI represents diphenylmethane diisocyanate.

4. The method for continuously preparing diphenylmethane diisocyanate and its modified products according to claim 3, characterized in that Comprising the following steps: 1), Synthesis of phenyl polyamine: Under the protection of inert gas, aniline and formaldehyde react at 40~60°C for 7~9 hours under the action of the HG catalyst; then add hydrochloric acid, react first at 60~70°C for 4±0.5 hours, and then at 90~100°C for 7±0.5 hours; the molar ratio of formaldehyde to aniline is 0.4~0.55:1, and the molar ratio of hydrogen chloride to aniline is 1.0~1.5:1; the weight ratio of the HG catalyst to aniline is 0.01~0.1:1; The obtained reaction product is neutralized and then allowed to stand for layering. The separated organic phase is distilled under reduced pressure to obtain the crude Ph-Amine; 2), Preparation of crude MDI: It is prepared by using the crude Ph-Amine obtained in step 1). The obtained reaction solution contains crude MDI; 3), Solvent recovery and purification: After the reaction solution obtained in step 2) is distilled under normal pressure to recover the solvent, then pure MDI is obtained by short-path molecular distillation; Mixed MDI is also obtained accordingly; The mixed MDI is used to prepare modified MDI.

5. The method for continuously preparing diphenylmethane diisocyanate and its modified products according to claim 4, characterized in that Step 2) includes the following steps: 2.1) Raw material preparation: Preparation of BTC solution: BTC is mixed evenly with a solvent to obtain a BTC solution; The weight ratio of BTC to the solvent is 1:3 - 5; The BTC is bis(trichloromethyl) carbonate; Preparation of mixed amine solution: The crude Ph-Amine obtained in step 1) is mixed with a solvent and filtered. The obtained filter residue is the HG catalyst; A tin-based catalyst is added to the filtrate and mixed evenly to obtain a mixed amine solution. The weight ratio of the crude Ph-Amine to the solvent is 1:3 - 5; The weight ratio of the tin-based catalyst to the crude Ph-Amine is 0.01 - 0.1:1; 2.2), Tertiary isocyanation reaction: According to the weight ratio of BTC to the crude Ph-Amine being 2.3 - 2.4:1, the dosage ratio of the BTC solution and the mixed amine solution is set; The following tertiary reactions are carried out in sequence: Primary reaction: The BTC solution is mixed with 1 / 3 of the mixed amine solution and then reacted at 30 - 40 °C and a rotation speed of 250 ± 50 rpm for 2 ± 0.5 hours to obtain a primary reaction solution; Secondary reaction: The primary reaction solution is mixed with 1 / 3 of the mixed amine solution and then reacted under the protection of an inert gas at 60 - 70 °C, a pressure of 0.2 ± 0.05 MPa, and a rotation speed of 250 ± 50 rpm for 2 ± 0.5 hours to obtain a secondary reaction solution; Tertiary reaction: The secondary reaction solution is mixed with 1 / 3 of the mixed amine solution and then reacted at 90 ± 5 °C, a vacuum condition of -0.06 ± 0.01 MPa, and a rotation speed of 250 ± 50 rpm for 2 ± 0.5 hours to obtain a tertiary reaction solution; The tertiary reaction solution contains crude MDI.

6. The method for continuously preparing diphenylmethane diisocyanate and its modified products according to claim 5, characterized in that It also includes the following step 4): 4), Preparation of modified MDI: The mixed MDI, polyether polyol, tin-based catalyst, and amine catalyst are subjected to a modification reaction under the protection of an inert gas at 80 - 100 °C; Until the NCO content drops to 12% - 18%, the modified MDI is prepared; The polyether polyol is 48 - 52% of the weight of the mixed MDI; The addition amount of the tin-based catalyst is 0.05 - 0.15% of the weight of the mixed MDI; The addition amount of the amine catalyst is 0.02 - 0.06% of the weight of the mixed MDI.

7. The method for continuously preparing diphenylmethane diisocyanate and its modified product according to claim 5 or 6, characterized in that: The tin-based catalysts in step 2.1) and the tin-based catalyst in step 4) are both one or more combinations of the following: stannous octoate, stannous neodecanoate, dibutyltin dilaurate, methyltin mercaptide, butyltin mercaptide, octyltin mercaptide; The amine catalyst in step 4) is A33 catalyst; The polyether polyol in step 4) is PPG-2000.

8. The method for continuously preparing diphenylmethane diisocyanate and its modified products according to claim 7, characterized in that Step 1) includes the following steps: 1.1) Condensation reaction: Under the protection of inert gas, aniline and HG catalyst are added to the reaction kettle, and an aqueous formaldehyde solution with a mass concentration of 35-37% is added dropwise at 10±2°C. After stirring evenly, the temperature is raised to 40-60°C and the reaction is carried out for 7-9 hours; then an aqueous hydrochloric acid solution with a mass concentration of 35-37% is added dropwise, and the reaction is carried out at 60-70°C for 4±0.5 hours and at 90-100°C for 7±0.5 hours in sequence; The molar ratio of formaldehyde to aniline is 0.4-0.55:1, and the molar ratio of hydrogen chloride to aniline is 1.0-1.5:1; the weight ratio of HG catalyst to aniline is 0.01-0.1:1; 1.2) Neutralization treatment: After the product obtained in step 1.1) is cooled to room temperature, it is transferred to a neutralization and separation kettle, and a NaOH solution with a mass concentration of 36-40% is added dropwise and stirred for neutralization and separation. After standing, the upper organic phase is separated; The molar ratio of NaOH to hydrogen chloride is 0.9-1.1:1; 1.3) Vacuum distillation: The organic phase obtained in step 1.2) is subjected to vacuum distillation to collect an aniline solution; the material in the kettle is phenyl polyamine.

9. The method for continuously preparing diphenylmethane diisocyanate and its modified products according to claim 8, characterized in that: The solvent in step 2.1) is one or a combination of dichloromethane, dichloroethane, chlorobenzene, and toluene; In step 2.2): The gases generated in the first-stage reaction, second-stage reaction, and third-stage reaction are condensed, and the condensed liquid is returned to the corresponding reaction system for the corresponding reaction, and the condensed gas enters the HCl absorption tower for absorption.

10. The method for continuously preparing diphenylmethane diisocyanate and its modified products according to claim 9, characterized in that In step 3): The tertiary reaction liquid obtained in step 2) is subjected to two-stage atmospheric distillation: first, atmospheric distillation is carried out to collect the solvent; then the temperature is raised to 120-150°C for atmospheric distillation; Then, short-path molecular distillation is carried out at 180±5°C and a vacuum degree of ≤3 kPa for purification to obtain MDI; the material in the kettle is mixed MDI.

11. The application of MDI or modified MDI prepared by the method according to any one of claims 3-10 in polyurethane foam, elastomer or coating.

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