A method for preparing trimethylaluminum using a low-boiling solvent and recycling by-product aluminum

Abstract

This invention discloses a method for preparing trimethylaluminum using a low-boiling-point solvent and recycling by-product aluminum. Under anhydrous and oxygen-free conditions, aluminum and chloromethane are used as raw materials, and low-boiling alkanes are used as the reaction solvent. A first-stage methylation reaction is carried out in the presence of a trialkylaluminum initiator to generate methylaluminum sesquichloride. Subsequently, metallic sodium is added, and chloromethane is added during the reduction stage, allowing the newly generated aluminum from the reduction process to participate in the methylation reaction in situ again, and then be further reduced to trimethylaluminum. After the reaction is complete, inorganic salts and residual solids are removed by filtration, and trimethylaluminum is obtained by distillation. This invention utilizes the low boiling point of the solvent, which is easily separated from trimethylaluminum, reducing product heat exposure and lowering costs; the in-situ recycling of by-product aluminum through the addition of chloromethane improves aluminum utilization.

Description

Technical Field

[0001] This invention relates to the field of organometallic compound preparation, and more specifically to a method for preparing trimethylaluminum in a low-boiling-point medium. Background Technology

[0002] Trimethylaluminum is an important organoaluminum compound that can be used as a co-catalyst for olefin polymerization, a raw material for the synthesis of methylaluminoxanes, and a metal-organic precursor in chemical vapor deposition and atomic layer deposition processes. Due to its high reactivity, the preparation of trimethylaluminum usually requires strict anhydrous and oxygen-free conditions, and the reaction medium, equipment, and post-processing must take into account heat transfer, mass transfer, and safety.

[0003] One existing preparation route first reacts aluminum with chloromethane to generate methylaluminum sesquichloride, which is then reduced with metallic sodium to obtain trimethylaluminum. The reaction can be represented as follows: 2 Al + 3 CH3Cl → Al2(CH3)3Cl3 Al2(CH3)3Cl3+ 3 Na → Al(CH3)3+ Al + 3 NaCl As can be seen from the above reaction relationship, the traditional reduction step generates one part of metallic aluminum along with one part of trimethylaluminum. If this newly generated aluminum does not re-enter the reaction cycle, theoretically only about half of the aluminum, based on the initial feed, will enter the target product. In practice, it will also be affected by factors such as aluminum surface passivation, solid coating, and localized mass transfer limitation, which usually further reduces the aluminum utilization rate.

[0004] Furthermore, existing processes often use high-boiling-point hydrocarbons as the reaction medium. While high-boiling-point solvents can maintain a liquid phase at the reaction temperature, they require higher distillation temperatures or deeper vacuum levels in post-processing, easily increasing energy consumption and equipment load, and prolonging the residence time of trimethylaluminum at higher temperatures. Therefore, developing a process that uses low-boiling-point, easily recoverable solvents and allows for the continued conversion of nascent aluminum produced in the reduction step is of great significance for reducing separation costs and improving aluminum utilization. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of high energy consumption of solvent separation, insufficient utilization of newly generated aluminum, and limited overall aluminum utilization in the existing sodium reduction process for preparing trimethylaluminum. This invention provides a method for preparing trimethylaluminum that uses a low-boiling-point solvent as the reaction medium and achieves in-situ recycling of newly generated aluminum by adding chloromethane during the reduction stage.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows: This invention employs a two-stage reaction. In the first stage, under anhydrous, oxygen-free, and inert atmosphere conditions, aluminum reacts with a first portion of chloromethane in a low-boiling medium and in the presence of a trialkylaluminum initiator to form methylaluminum sesquichloride or an organoaluminum chloride system containing this component. In the second stage, metallic sodium is added for reduction, while a second portion of chloromethane is added simultaneously. This allows the newly generated aluminum to undergo a methylation reaction with the added chloromethane and be reduced again by metallic sodium, thereby enabling more initial aluminum to enter the trimethylaluminum product.

[0007] The low-boiling-point solvent is preferably n-hexane or n-heptane, and the amount used is preferably 0.5–2.0 L / mol aluminum, more preferably about 1.0 L / mol aluminum. The solvent can maintain the reaction liquid phase in a closed, pressure-resistant system at a temperature above its atmospheric boiling point; after the reaction is completed, it can be preferentially distilled off at a lower temperature, which facilitates separation from trimethylaluminum and recycling.

[0008] The trialkylaluminum initiator can be selected from triethylaluminum, trimethylaluminum, or a mixture thereof, preferably triethylaluminum. The amount of initiator can be 1-10 mol% of the aluminum content, preferably 3-7 mol%, more preferably 5 mol%. The initiator helps to shorten the induction period of the reaction between aluminum and chloromethane and promotes the formation of the methylaluminum chloride system.

[0009] The chloromethane added in the first stage is mainly used to generate methylaluminum sesquichloride; the chloromethane added in the second stage is mainly used to capture the newly formed aluminum during the reduction process, causing it to be remethylated in situ. The chloromethane added in the first stage can be added at 1.5 to 5 equivalents of aluminum, and the chloromethane added in the second stage can be added at 1 to 5 equivalents of aluminum, with the total amount of chloromethane being 4 to 10 equivalents of aluminum. The chloromethane added in the second stage can be added all at once, in batches, or continuously. It can be added all at once before the addition of metallic sodium, or it can be added simultaneously with metallic sodium. Preferably, it is added in batches or continuously after the metallic sodium begins to react with methylaluminum sesquichloride.

[0010] The amount of metallic sodium used can be 1.5 to 4.0 equivalents of aluminum, preferably 2.5 to 3.2 equivalents. To improve dispersion and heat transfer, sodium blocks, sodium granules, sodium flakes, or sodium dispersions can be used. The addition, reaction, and post-treatment of metallic sodium should all be carried out under an inert atmosphere and strictly anhydrous and oxygen-free conditions.

[0011] The aluminum is one or more of aluminum powder, aluminum shavings, or granular aluminum.

[0012] The reaction temperature in the first stage can be 80–130℃, preferably 110–125℃; the reaction time can be 2–20 h, preferably 4–8 h. The reaction temperature in the second stage can be 80–130℃, preferably 110–125℃; the reaction time can be 1–8 h, preferably 2–6 h.

[0013] After the reaction is complete, the mixture is cooled and the unreacted chloromethane is recovered. The solvent is then recovered by atmospheric or vacuum distillation. Since the solvent's boiling point is significantly lower than that of trimethylaluminum, it can be preferentially separated at a lower temperature. The recovered solvent, after dehydration and necessary purification, can be used for the next batch of reaction. After removing the low-boiling components, sodium chloride and unreacted solids are removed by filtration. The filtrate is then distilled or fractionated to obtain the trimethylaluminum product.

[0014] The beneficial effects of the present invention are as follows: (1) Using a low-boiling-point reaction medium allows for preferential distillation and recovery after the reaction, reducing the heat load and vacuum load required for high-boiling-point solvent separation and reducing the heat exposure time of trimethylaluminum.

[0015] (2) Adding chloromethane during the sodium reduction stage allows the newly generated aluminum to recombine into methylaluminate chloride in situ and continue to be converted into trimethylaluminate, thereby significantly improving the utilization rate of aluminum.

[0016] (3) By adding trialkylaluminum initiator, chloromethane in stages and adjusting the amount of sodium, the reaction start-up and cyclic conversion process can be improved, the first stage reaction time can be shortened and the process stability can be improved. Detailed Implementation

[0017] To enable those skilled in the art to better understand the present invention, the following embodiments further illustrate the invention. However, these embodiments do not limit the scope of the invention, and the technical means used in the embodiments are conventional means well known to those skilled in the art. In the present invention, unless otherwise specified, the experimental methods used are all conventional methods, and the materials and reagents used can be purchased commercially.

[0018] Example 1: Under anhydrous and oxygen-free conditions, aluminum powder (10.80 g, 0.4 mol), triethylaluminum, and n-hexane were added to a pressure-resistant reaction vessel that had been dried and purged with an inert gas. The amount of triethylaluminum was 5 mol% of the aluminum molar, and the amount of n-hexane was 1.0 L / mol aluminum. Three equivalents of chloromethane were added to the reactor, which was then sealed and heated to 120 °C and reacted for 6 h. Subsequently, three equivalents of metallic sodium granules were added, followed by a second addition of three equivalents of chloromethane. The system was allowed to continue reacting at 120 °C for 4 h. After the reaction was completed, the system was cooled, and unreacted chloromethane and n-hexane were recovered sequentially. Sodium chloride and residual solids were removed by filtration, and trimethylaluminum was obtained by distillation. The yield of trimethylaluminum was 87% (based on aluminum powder), and the aluminum atom utilization rate was 87%.

[0019] Example 2: Under anhydrous and oxygen-free conditions, aluminum powder (10.80 g, 0.4 mol), triethylaluminum, and n-heptane were added to a pressure-resistant reaction vessel that had been dried and purged with an inert gas. The amount of triethylaluminum was 5 mol% of the aluminum molar, and the amount of n-heptane was 1.0 L / mol aluminum. Three equivalents of chloromethane were added to the reactor, which was then sealed and heated to 120 °C and reacted for 6 h. Subsequently, three equivalents of metallic sodium granules were added, followed by a second addition of three equivalents of chloromethane. The system was allowed to continue reacting at 120 °C for 4 h. After the reaction was completed, the system was cooled, and unreacted chloromethane and n-heptane were recovered sequentially. Sodium chloride and residual solids were removed by filtration, and trimethylaluminum was obtained by distillation. The yield of trimethylaluminum was 84% ​​(based on aluminum powder), and the aluminum atom utilization rate was 84%.

[0020] Example 3: Under anhydrous and oxygen-free conditions, aluminum powder (10.80 g, 0.4 mol), trimethylaluminum, and n-heptane were added to a pressure-resistant reaction vessel that had been dried and purged with an inert gas. The amount of trimethylaluminum was 5 mol% of the aluminum molar, and the amount of n-heptane was 1.0 L / mol aluminum. Three equivalents of chloromethane were added to the reactor, which was then sealed and heated to 120 °C and reacted for 6 h. Subsequently, three equivalents of metallic sodium granules were added, and in the second stage, three equivalents of chloromethane were added in three batches every 0.5 hours. The system continued to react at 120 °C for 4 h. After the reaction was completed, the system was cooled, and unreacted chloromethane and n-heptane were recovered sequentially. Sodium chloride and residual solids were removed by filtration, and trimethylaluminum was obtained by distillation. The yield of trimethylaluminum was 87% (based on aluminum powder), and the aluminum atom utilization rate was 87%.

[0021] Comparative Example 1: No additional chloromethane added Under anhydrous and oxygen-free conditions, aluminum powder (10.80 g, 0.4 mol), triethylaluminum, and n-heptane were added to a pressure-resistant reaction vessel that had been dried and purged with an inert gas. The amount of triethylaluminum was 5 mol% of the aluminum molar, and the amount of n-hexane was 1.0 L / mol aluminum. Three equivalents of chloromethane were added to the reactor, which was then sealed and heated to 120 °C and reacted for 6 h. Subsequently, three equivalents of metallic sodium granules were added, and the system was allowed to react for another 4 h at 120 °C. After the reaction was completed, the system was cooled, and unreacted chloromethane and n-hexane were recovered sequentially. Sodium chloride and residual solids were removed by filtration, and trimethylaluminum was obtained by distillation with a yield of 80% (based on the theoretical yield of sesquimethylaluminum chloride) and an aluminum atom utilization rate of 40%.

[0022] Comparative Example 2: No triethylaluminum was added as an initiator Under anhydrous and oxygen-free conditions, aluminum powder (10.80 g, 0.4 mol) and n-hexane were added to a pressure-resistant reaction vessel that had been dried and purged with an inert gas. The amount of n-hexane was 1.0 L / mol aluminum. Three equivalents of chloromethane were added to the reactor, which was then sealed and heated to 120 °C and reacted for 20 h. Subsequently, three equivalents of metallic sodium granules were added, followed by a second addition of three equivalents of chloromethane. The system was then allowed to react at 120 °C for another 4 h. After the reaction was complete, the system was cooled, and unreacted chloromethane and n-hexane were recovered sequentially. Sodium chloride and residual solids were removed by filtration, and trimethylaluminum was obtained by distillation. The yield of trimethylaluminum was 78% (based on aluminum powder), and the aluminum atom utilization rate was 78%.

Claims

1. A method for preparing trimethylaluminum using a low-boiling-point solvent and recycling by-product aluminum, characterized in that, Includes the following steps: (1) Under anhydrous, oxygen-free and inert atmosphere conditions, aluminum, low-boiling alkanes and trialkylaluminum initiators are added to a pressure-resistant reaction vessel, chloromethane is introduced, and the first-stage methylation reaction is carried out at 80-130℃ to obtain a reaction system containing methylaluminum sesquichloride. (2) Add metallic sodium to the reaction system and carry out the second-stage reduction reaction at 80-130°C. At the same time as adding metallic sodium, add chloromethane so that the newly generated aluminum generated in the reduction reaction is at least partially converted in situ into methylaluminate chloride and continues to participate in the reduction reaction, to obtain a reaction solution containing trimethylaluminum. (3) After the reaction is completed, the mixture is cooled, filtered to remove sodium chloride and residual solids, and then separated by distillation to obtain trimethylaluminum.

2. The method according to claim 1, characterized in that, The trialkylaluminum initiator is triethylaluminum, trimethylaluminum, or a mixture of the two; the amount of the trialkylaluminum initiator added is 1 to 10 mol of aluminum.

3. The method according to claim 1 or 2, characterized in that, The low-boiling alkane is n-hexane or n-heptane, and the amount added is 0.5–2.0 L / mol aluminum.

4. The method according to any one of claims 1 to 3, characterized in that, The molar ratio of chloromethane to aluminum added in step (1) is 1.5 to 5:1, the molar ratio of chloromethane to aluminum added in step (2) is 1 to 5:1, and the molar ratio of the total amount of chloromethane added in steps (1) and (2) to aluminum is 4 to 10:

1.

5. The method according to any one of claims 1 to 4, characterized in that, The molar ratio of sodium to aluminum is 1.5 to 4.0:

1.

6. The method according to any one of claims 1 to 5, characterized in that, The chloromethane added in step (2) can be added in one go, in batches, or continuously.

7. The method according to any one of claims 1 to 6, characterized in that, The first stage reaction temperature is 110–125℃ and the reaction time is 4–8 h; the second stage reaction temperature is 110–125℃ and the reaction time is 2–6 h.

8. The method according to any one of claims 1 to 7, characterized in that, The aluminum is one or more of aluminum powder, aluminum shavings, or granular aluminum; the sodium metal is sodium block, sodium granules, sodium flakes, or sodium dispersion dispersed in an inert hydrocarbon medium.

9. The method according to any one of claims 1 to 8, characterized in that, In step (3), chloromethane is recovered first, and then the solvent is recovered by atmospheric distillation or vacuum distillation. The recovered solvent is dehydrated and reused in subsequent reaction batches. After the solvent is removed, the material containing trimethylaluminum is distilled.

10. The method according to any one of claims 1 to 9, characterized in that, The molar ratio of total aluminum, chloromethane, and metallic sodium was 1.0:6.0:3.

0. The amount of triethylaluminum initiator was 5 mol% of the aluminum content, and the amount of low-boiling alkane was 1.0 L / mol aluminum. In the first stage, 3 equivalents of chloromethane were added and reacted at 120℃ for 6 h. In the second stage, 3 equivalents of chloromethane were added again and reacted at 120℃ for 4 h. The utilization rate of aluminum was 87%.