Preparation method of high-yield methyl isocyanate

Abstract

The invention discloses a preparation method of high-yield methyl isocyanate, and belongs to the technical field of organic synthesis. The method comprises the following steps: taking aluminum isopropoxide as a raw material, preparing an aluminum oxide porous carrier through template pore-forming, and loading a zinc-based active component after PVP hydroxyl reinforced modification to prepare the supported zinc-based catalyst. Reacting methyl formate with a methylamine aqueous solution under the action of the catalyst under the conditions of 180-220 DEG C and 0.3-0.5 MPa, introducing nitrogen in the whole process, and rectifying and purifying to obtain the methyl isocyanate. Through collaborative design of the catalyst, the reaction activation energy barrier is reduced, the side reaction is inhibited, and high-yield and high-purity synthesis of methyl isocyanate is realized. Compared with a catalyst-free process, the raw material conversion rate is remarkably increased, and the method has both safety and economy and is suitable for large-scale industrial production.

Description

Technical Field

[0001] This invention belongs to the field of organic synthesis technology, specifically relating to a method for preparing methyl isocyanate with high yield. Background Technology

[0002] Methyl isocyanate is an organic synthetic intermediate containing highly reactive isocyanate groups (-NCO), playing an irreplaceable role in the synthesis of polyurethane resins, preparation of pesticide technicals, and synthesis of pharmaceutical intermediates. Its derivatives are widely used in building insulation, automobile manufacturing, biopharmaceuticals, and many other industries, with market demand continuing to grow.

[0003] Currently, the mainstream industrial preparation of methyl isocyanate is the phosgene process, which involves the acylation reaction of methyl formate with phosgene at high temperatures to obtain the target product. However, this method has several insurmountable drawbacks: First, phosgene is a highly toxic gas, requiring stringent safety standards for the sealing of production equipment and the operation process; second, hydrogen chloride byproducts are generated during the reaction, which not only corrodes the reaction equipment but also triggers a series of side reactions, resulting in a low yield of methyl isocyanate; third, the separation of residual chlorinated impurities in the product is difficult, seriously affecting the quality of downstream products.

[0004] To address the drawbacks of the phosgene method, researchers have developed non-phosgene synthesis routes such as urea pyrolysis and dimethyl carbonate. However, urea pyrolysis requires high temperature and pressure conditions, resulting in high energy consumption, poor product selectivity, and a tendency to generate amine ester byproducts. The dimethyl carbonate method faces challenges such as low reactant conversion rates, easy catalyst deactivation, and high costs for product separation and purification, making it difficult to achieve large-scale industrial application.

[0005] In non-phosgene process research, catalyst performance is the core bottleneck restricting technological breakthroughs. While traditional homogeneous catalysts exhibit high catalytic activity, they suffer from difficulties in recovery, low reusability, and high production costs. Heterogeneous supported catalysts, though easily separated and recovered, generally suffer from uneven dispersion of active components, weak interfacial bonding with the support, and insufficient exposure of active sites. During high-temperature reactions, active components are easily detached and lost, while the support pores are easily blocked by carbon deposits, leading to a rapid decline in catalytic efficiency. This makes it impossible to simultaneously achieve high yields of methyl isocyanate and long-term stable catalyst operation. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing methyl isocyanate with a high yield. This method achieves efficient and clean synthesis of methyl isocyanate by designing and preparing a customized supported zinc-based catalyst and optimizing the reaction system and process parameters.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing high-yield methyl isocyanate includes the following steps: (1) Weigh 18-20g of aluminum isopropoxide, add 80-100mL of deionized water and stir to dissolve, add 5% nitric acid to adjust the pH to 3.5, add 1.8-2g of polyethylene glycol 600 to prepare a porous alumina carrier; Aluminum isopropoxide undergoes hydrolysis under acidic conditions to generate aluminum hydroxide sol. Adjusting the pH to 3.5 with nitric acid can regulate the hydrolysis rate and prevent rapid agglomeration of aluminum hydroxide particles. Polyethylene glycol 600 serves as a template agent, forming a porous structure in the sol. After drying and sintering, a porous alumina support is obtained. The porous structure can increase the specific surface area of ​​subsequent catalysts, providing sufficient sites for loading active components.

[0008] (2) Take 3~5g of the porous alumina carrier prepared in step (1), mix it with 60~80mL of anhydrous ethanol, 0.5~1g of polyvinylpyrrolidone K30 (PVP) and 0.1mL of nitric acid, and add 3~5mL of 25% ammonia water to perform hydroxyl-enhanced modification on the porous alumina carrier to obtain hydroxyl-enhanced porous alumina carrier; Anhydrous ethanol is used as a dispersion medium, which allows PVP to be uniformly adsorbed on the surface of the alumina support. As a surface modifier, the polar groups on the molecular chain of PVP can combine with the hydroxyl groups on the surface of alumina, and the addition of ammonia water induces the enrichment of hydroxyl groups on the surface of the support. The addition of nitric acid can inhibit the aggregation of alumina support during the modification process.

[0009] (3) Take 2~3g of the hydroxyl-enhanced porous alumina support prepared in step (2), add 40~60mL of 0.5~0.8mol / L zinc chloride aqueous solution, add 2~3mL of 25% ammonia water dropwise, and load the zinc-based active component onto the hydroxyl-enhanced porous alumina support to obtain a supported zinc-based catalyst. Zinc chloride aqueous solution serves as the zinc source, while ammonia water can adjust the pH of the system, promoting the hydrolysis of zinc ions to generate zinc hydroxide precursors, which then bind to the alumina carrier via hydroxyl groups.

[0010] (4) Methyl formate and aqueous methylamine were added to a reaction apparatus containing the supported zinc-based catalyst, the reaction temperature and pressure were controlled, nitrogen was introduced throughout the process, and methyl isocyanate was obtained by distillation after the reaction was completed.

[0011] As a further preferred embodiment of the present invention, the specific operating conditions for preparing the porous alumina carrier in step (1) are as follows: stirring at 60~80℃ for 3~4h to obtain a transparent alumina sol, and then drying and sintering to obtain the porous alumina carrier.

[0012] As a further preferred embodiment of the present invention, the specific operating conditions for the hydroxyl-strengthening modification in step (2) are as follows: the mixed system is stirred at 50~60℃ for 2~3h, ammonia water is added dropwise and stirred for 10~15min, and the gel is taken out and dried at 110~120℃ for 2~3h.

[0013] Low-temperature drying at 110~120℃ can preserve the hydroxyl structure on the carrier surface. The enhanced hydroxyl groups can form stable coordination bonds with the subsequent zinc active components, thereby improving the loading strength of the active components.

[0014] As a further preferred embodiment of the present invention, the specific operating conditions of the zinc-based active component loaded in step (3) are as follows: the mixed system is stirred at 40~50℃ for 3~5h, heated to 120~150℃ for hydrothermal crystallization for 6~8h, filtered, washed, dried and then calcined at 500~600℃ for 4~5h.

[0015] Stirring at 40~50℃ can make the zinc hydroxide precursor uniformly dispersed in the carrier pores and surface; hydrothermal crystallization at 120~150℃ can promote the regular growth of precursor crystals and enhance catalytic activity; calcination at 500~600℃ can convert zinc hydroxide into zinc oxide crystal phase with high catalytic activity, while removing residual PVP and other organic matter and clearing the carrier pores.

[0016] As a further preferred embodiment of the present invention, the specific parameters of the reaction in step (4) are as follows: the amount of methyl formate is 20~25 mmol, the methylamine aqueous solution is a 40% methylamine aqueous solution and the amount is 25~30 mmol, the amount of the supported zinc-based catalyst is 5~8 g, the reaction temperature is 180~220℃, and the reaction pressure is 0.3~0.5 MPa.

[0017] The beneficial effects of this invention are: (1) By synergistic design of porous alumina support with hydroxyl-enhanced modification and zinc-based active components, the coordination effect of hydroxyl groups on the support surface with zinc ions is utilized to achieve high dispersion loading and strong interfacial bonding of active components; zinc-based active sites can accurately activate the functional groups of reactants and reduce the activation energy barrier of ammonolysis-dehydration reaction.

[0018] (2) The present invention uses a three-step method of template agent pore-forming, hydroxyl-enhanced modification and hydrothermal crystallization loading to prepare supported zinc-based catalysts: polyethylene glycol 600 template agent can construct a through-porous structure, providing sufficient sites for loading active components; PVP-assisted hydroxyl-enhanced modification can induce hydroxyl enrichment on the support surface, anchoring zinc ions through coordination and avoiding aggregation of active components; hydrothermal crystallization at 120~150℃ can promote the regular growth of zinc oxide crystals, and calcination at 500~600℃ can remove residual organic matter and clear the pores, ultimately achieving the dual advantages of high exposure and high activity of catalyst active sites.

[0019] (3) The non-phosgene synthesis route adopted in this invention uses methyl formate and methylamine aqueous solution as raw materials. No highly toxic phosgene is involved in the entire process, and no corrosive hydrogen chloride byproduct is generated. The zinc-based active sites can precisely activate the functional groups of the reactants, reduce the activation energy barrier of the ammonolysis-dehydration reaction, and improve the conversion rate of raw materials and the selectivity of the target product. Combined with the distillation purification process, the final methyl isocyanate obtained has high purity. Attached Figure Description

[0020] Figure 1 Images are taken using a scanning electron microscope (SEM), in which... Figure 1 (a) is a SEM image of the porous alumina support prepared in Example 1. Figure 1 (b) is a SEM image of the supported zinc-based catalyst; Figure 2 The XRD diffraction patterns of the porous alumina support and the supported zinc-based catalyst prepared in Example 1 of this invention are shown below. Figure 3 The infrared spectrum of methyl isocyanate prepared in Example 1 of this invention; Figure 4 The NMR spectrum of methyl isocyanate prepared in Example 1 of this invention is shown below. Figure 5 The fluorescence detection results of methyl isocyanate prepared in Example 1 and Comparative Example 1 of this invention; Figure 6 These are the conversion rate curves of the samples prepared in Example 1 and Comparative Example 1. Detailed Implementation

[0021] To make the above-mentioned objectives, features, and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to examples. The following content is merely an example and illustration of the concept of the present invention. Those skilled in the art can make various modifications or additions to the described specific embodiments or use similar methods to replace them, as long as they do not deviate from the concept of the invention, they should all fall within the protection scope of the present invention.

[0022] The preparation method of the present invention will be described below through specific embodiments and comparative examples.

[0023] Example 1 A method for preparing high-yield methyl isocyanate includes the following steps: (1) Weigh 18g of aluminum isopropoxide, add 80mL of deionized water and stir to dissolve; add 5% nitric acid to adjust the pH to 3.5, add 1.8g of polyethylene glycol 600, stir at 60℃ for 3h to obtain transparent alumina sol, then dry and sinter to obtain a porous alumina carrier. (2) Take 3~5g of the porous alumina carrier prepared in step (1), 80mL of anhydrous ethanol, 1g of PVP (K30) and 0.1mL of nitric acid, stir at 50℃ for 2h, then add 3mL of 25% ammonia water, stir for 10min, take out the gel, dry at 110℃ for 2h to obtain the hydroxyl-enhanced porous alumina carrier. (3) Place 2g of the hydroxyl-enhanced porous alumina support prepared in step (2) into a reaction vessel, add 40mL of 0.5mol / L zinc chloride aqueous solution, add 2mL of 25% ammonia water dropwise, stir at 40℃ for 3h; heat to 120℃ for hydrothermal crystallization for 6h, filter, wash, dry and calcine at 500℃ for 4h to obtain the supported zinc-based catalyst; (4) Add 20 mmol of methyl formate and 25 mmol of 40% methylamine aqueous solution to a loaded 5g of the supported zinc-based catalyst prepared in step (3). Control the reaction temperature to 180℃ and the reaction pressure to 0.3MPa. Nitrogen gas is introduced throughout the reaction. After the reaction is completed, methyl isocyanate is obtained by separation, purification and distillation.

[0024] Example 2 A method for preparing high-yield methyl isocyanate includes the following steps: (1) Weigh 19g of aluminum isopropoxide, add 90mL of deionized water and stir to dissolve; add 5% nitric acid to adjust the pH to 3.5, add 1.9g of polyethylene glycol 600, stir at 70℃ for 3.5h to obtain transparent alumina sol, then dry and sinter to obtain a porous alumina carrier. (2) Take 4g of the porous alumina carrier prepared in step (1), 70mL of anhydrous ethanol, 0.7g of PVP (K30) and 0.1mL of nitric acid, stir at 55℃ for 2.5h, then add 4mL of 25% ammonia water, stir for 12min, take out the gel, dry at 115℃ for 2.5h to obtain the hydroxyl-enhanced porous alumina carrier. (3) Place 2.5g of the hydroxyl-enhanced porous alumina support prepared in step (2) into a reaction vessel, add 50mL of 0.6mol / L zinc chloride aqueous solution, add 2.5mL of 25% ammonia water dropwise, stir at 45℃ for 4h; heat to 130℃ for hydrothermal crystallization for 7h, filter, wash, dry and calcine at 550℃ for 4.5h to obtain the supported zinc-based catalyst; (4) 22 mmol of methyl formate and 27.5 mmol of 40% methylamine aqueous solution were added to a loaded 6g of the supported zinc-based catalyst prepared in step (3). The reaction temperature was controlled at 200℃ and the reaction pressure was 0.4MPa. Nitrogen gas was introduced throughout the reaction. After the reaction was completed, methyl isocyanate was obtained by separation, purification and distillation.

[0025] Example 3 A method for preparing high-yield methyl isocyanate includes the following steps: (1) Weigh 20g of aluminum isopropoxide, add 100mL of deionized water and stir to dissolve; add 5% nitric acid to adjust the pH to 3.5, add 2g of polyethylene glycol 600, stir at 80℃ for 4h to obtain transparent alumina sol, then dry and sinter to obtain porous alumina carrier. (2) Take 5g of the porous alumina carrier prepared in step (1), 80mL of anhydrous ethanol, 1g of PVP (K30) and 0.1mL of nitric acid, stir at 60℃ for 3h, then add 5mL of 25% ammonia water, stir for 15min, take out the gel, dry at 120℃ for 3h to obtain the hydroxyl-enhanced porous alumina carrier. (3) Place 3g of the hydroxyl-enhanced porous alumina support prepared in step (2) into a reaction vessel, add 60mL of 0.8mol / L zinc chloride aqueous solution, add 3mL of 25% ammonia water dropwise, stir at 50℃ for 5h; heat to 150℃ for hydrothermal crystallization for 8h, filter, wash, dry and calcine at 600℃ for 5h to obtain the supported zinc-based catalyst; (4) Add 25 mmol of methyl formate and 30 mmol of 40% methylamine aqueous solution to a loaded 8g of the supported zinc-based catalyst prepared in step (3). Control the reaction temperature to 220℃ and the reaction pressure to 0.5MPa. Nitrogen gas is introduced throughout the reaction. After the reaction is completed, methyl isocyanate is obtained by separation, purification and distillation.

[0026] Comparative Example 1 A method for preparing methyl isocyanate includes the following steps: (1) Weigh 20 mmol of methyl formate and 25 mmol of 40% methylamine aqueous solution according to the raw material ratio of Example 1; (2) Add the above raw materials directly into the reactor without adding any catalyst; (3) The reaction temperature was controlled at 180℃ and the reaction pressure at 0.3MPa. Nitrogen gas was introduced throughout the process, and the reaction time was the same as in Example 1. (4) After the reaction is completed, the reaction products are treated using the same separation, purification and distillation process as in Example 1.

[0027] Figure 1(a) is a scanning electron microscope (SEM) image of the porous alumina support prepared in Example 1. It can be seen that the support exhibits a three-dimensional interconnected porous structure formed by interconnected nano-sized particles. This structure is formed by the pore-forming effect of polyethylene glycol 600 template agent and the regulation of acid hydrolysis, which provides sufficient surface sites and pore space for the subsequent loading of zinc-based active components, and greatly improves the specific surface area of ​​the support.

[0028] Figure 1 (b) is a SEM image of the supported zinc-based catalyst prepared in Example 1. Compared with the alumina support, no phenomenon of active components blocking the pores of the support was observed. At the same time, the original porous structure of the support remained intact. This indicates that after hydroxyl-enhanced modification and hydrothermal crystallization loading, the zinc-based active components were highly dispersed and strongly bound to the surface and pores of the alumina support through coordination, thus achieving full exposure of the active sites and providing a good structural basis for the catalytic reaction.

[0029] Figure 2 The XRD diffraction patterns are shown for the porous alumina support and the supported zinc-based catalyst prepared in Example 1 of this invention. XRD patterns are used to analyze the crystal phase composition and crystallinity of the materials. In the pattern of the porous alumina support, only the characteristic diffraction peaks of γ-Al₂O₃ show broadening and lower intensity, indicating that the support is a highly active amorphous or low-crystallinity Al₂O₃ phase, which is beneficial for the loading of hydroxyl groups and the binding of the active components. In the pattern of the supported zinc-based catalyst, new diffraction peaks at 31.7° and 56.6° can be seen, corresponding to the (002) and (110) crystal planes of ZnO. This demonstrates that the zinc hydroxide precursor was successfully transformed into a well-defined ZnO active phase after calcination at 500-600℃; and the moderate intensity of the ZnO characteristic peaks and the absence of impurity peaks indicate high purity of the active components and the absence of other impurity phases.

[0030] Figure 3 The image shows the infrared spectrum of methyl isocyanate prepared in Example 1 of this invention. Infrared spectroscopy allows for product structure identification through the characteristic absorption peaks of functional groups. The image shows the peak at 2275 cm⁻¹. -1 A strong and sharp characteristic absorption peak appears at 2980 cm⁻¹, which is the hallmark stretching vibration peak of the isocyanate group (-NCO), a characteristic of the core functional group of the methyl isocyanate molecule; -1 The absorption peak at the position corresponds to the stretching vibration of methyl (-CH3); the absence of absorption peaks for impurity functional groups such as hydroxyl (-OH) and amino (-NH2) in the spectrum proves that no residual raw materials or byproducts are generated in the reaction.

[0031] Figure 4The ¹H NMR spectrum of methyl isocyanate prepared in Example 1 of this invention is shown. ¹H NMR spectra accurately characterize the product structure at the molecular level. A single peak signal appears at δ=3.05 ppm, corresponding to the three hydrogen atoms on the methyl group (-CH3) bonded to the nitrogen atom in the methyl isocyanate molecule. The peak is sharp and without splitting, perfectly matching the molecular structure of methyl isocyanate. The spectrum baseline is stable, without interference from other peaks, further demonstrating the high purity of the product.

[0032] Figure 5 The fluorescence detection results are for methyl isocyanate prepared in Example 1 and Comparative Example 1 of this invention. The fluorescence detection is based on the specific fluorescence derivatization reaction of methyl isocyanate with acetylacetone-formaldehyde to achieve quantitative analysis. The detection conditions are pH=5.6 and excitation wavelength λ. ex =436nm, emission wavelength λ emm =504nm. In this detection system, the content of methyl isocyanate is positively correlated with the fluorescence intensity, while reaction byproducts (such as urethane compounds) and unreacted raw materials (methyl formate, methylamine) show no obvious fluorescence response. As can be seen in the figure, the fluorescence intensity peak of Example 1 is higher than that of Comparative Example 1, indicating that the methyl isocyanate synthesized by the method used in this invention has higher purity.

[0033] Figure 6 These are the conversion rate curves of the samples prepared in Example 1 and Comparative Example 1.

[0034] The conversion rate curve reflects the rate of feedstock consumption and the reaction equilibrium state during the reaction process. In the figure, the feedstock conversion rate of Example 1 (with catalyst) reached 78.2% after 200 min of reaction and tended to equilibrium; while the conversion rate of Comparative Example 1 (without catalyst) was only 52.5% in the same time period, and the curve slope was extremely low, indicating a slow reaction rate. This is because the active sites of the zinc-based catalyst can precisely activate the carbonyl group of methyl formate and the amino group of methylamine, lowering the activation energy barrier of the ammonolysis-dehydration reaction and accelerating the reaction process.

Claims

1. A process for the preparation of high yield methyl isocyanate, characterized by, The method comprises the following steps: (1) 18-20 g of aluminum isopropoxide is weighed, stirred and dissolved in 80-100 mL of deionized water, 5% nitric acid is added dropwise to adjust the pH to 3.5, and 1.8-2 g of polyethylene glycol 600 is added to prepare an alumina porous carrier; (2) 3-5 g of the alumina porous carrier prepared in step (1) is mixed with 60-80 mL of anhydrous ethanol, 0.5-1 g of PVP and 0.1 mL of nitric acid, 3-5 mL of 25% ammonia water is added dropwise, the alumina porous carrier is subjected to hydroxyl strengthening modification, and a hydroxyl-strengthened porous alumina carrier is obtained; (3) 2-3 g of the hydroxyl-strengthened porous alumina carrier prepared in step (2) is added to 40-60 mL of a 0.5-0.8 mol / L zinc chloride aqueous solution, 2-3 mL of 25% ammonia water is added dropwise, a mixed system is obtained, and the reaction is carried out to obtain a supported zinc-based catalyst; (4) Methyl formate and a methylamine aqueous solution are added to a reaction device loaded with the supported zinc-based catalyst, the reaction temperature and pressure are controlled, nitrogen is introduced throughout the reaction, and after the reaction is completed, methyl isocyanate is obtained by rectification and purification.

2. The method of claim 1, wherein the high yield of methyl isocyanate is produced by, The specific operation conditions for preparing the alumina porous carrier in step (1) are as follows: stirring at 60-80°C for 3-4 h to obtain a transparent alumina sol, drying, sintering, and then obtaining an alumina porous carrier.

3. The method of claim 1, wherein the method is characterized by, The specific operation conditions for the hydroxyl strengthening modification in step (2) are as follows: the mixed system is stirred at 50-60°C for 2-3 h, ammonia water is added dropwise and stirred for 10-15 min, and then the gel is taken out and dried at 110-120°C for 2-3 h.

4. The method of claim 1, wherein the method is characterized by, The specific parameters of the reaction in step (3) are as follows: the mixed system is stirred at 40-50°C for 3-5 h, the temperature is raised to 120-150°C, hydrothermal crystallization is carried out for 6-8 h, and then filtration, washing, drying, and calcination at 500-600°C for 4-5 h are carried out.

5. The method of claim 1, wherein the high yield of methyl isocyanate is produced by, The specific parameters of the reaction in step (4) are as follows: the amount of methyl formate is 20-25 mmol, the methylamine aqueous solution is a 40% methylamine aqueous solution and the amount is 25-30 mmol, the amount of the supported zinc-based catalyst is 5-8 g, the reaction temperature is 180-220°C, and the reaction pressure is 0.3-0.5 MPa.

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