Method for producing methylaluminoxane

Abstract

To provide a method for producing methylaluminoxane that is essentially free of gel-like substance.SOLUTION: A method for producing methylaluminoxane comprises the step for subjecting a reaction solution, produced through the generation of methylaluminoxane from trimethylaluminum, to membrane separation to remove the by-produced aluminum-containing gel.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing methylaluminoxane. [Background technology]

[0002] It is widely known that methylaluminoxane, a condensation product of water and trimethylaluminum, exhibits high activity in ethylene polymerization as a cocatalyst for metallocene catalysts (Non-Patent Document 1). In the reaction of water and trimethylaluminum, the formation of solids or gels in the form of trimethylaluminum that has been subjected to excessive hydrolysis cannot be avoided, resulting in a decrease in the reaction yield based on aluminum. In addition, the gels themselves accumulate in the production line or tank, hindering the smooth operation of the production plant and causing a decrease in the yield and purity of the final product. Furthermore, the presence of impurities in polyolefins leads to a decrease in the functionality of the polyolefins themselves, coloration, adverse effects on the physiological activity of the human body, etc., so it is desirable that the cocatalyst methylaluminoxane itself does not contain impurities.

[0003] As a method for removing gel-like matter, i.e., aluminum-containing gel, there is, for example, sedimentation separation (Patent Document 1). However, the method described in the above Patent Document 1 is not preferable from the viewpoint of industrial production because of poor operability. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 09-507515 [Non-patent literature]

[0005] [Non-Patent Document 1] H. Sinn, W. Kaminsky. Adv. Organomet. Chem. 18, 99 (1980). Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, it is desirable to completely eliminate the aluminum-containing gel produced as a by-product during the production of methylaluminoxane from water and trimethylaluminum in the production process. The problem to be solved by the present invention is to provide a method for producing methylaluminoxane that is essentially free of such gel-like matter. [Means for solving the problem]

[0007] The present inventors have discovered that aluminum-containing gel, which is a by-product when methylaluminoxane is produced from trimethylaluminum, can be efficiently removed by membrane separation, and have thus completed the present invention.

[0008] This application includes the following inventions. [1] A method for producing methylaluminoxane, comprising a step of cross-flow filtering a reaction solution in which methylaluminoxane has been produced from trimethylaluminum in order to remove an aluminum-containing gel produced as a by-product. [2] The manufacturing method of [1], wherein membrane separation is performed using a ceramic filter. [3] The method according to [1] or [2], characterized in that the membrane used in the membrane separation is washed with the solvent used in producing methylaluminoxane from trimethylaluminum. [4] The method according to [3], wherein the solvent is toluene. [5] Any of the manufacturing methods [1] to [4], characterized in that the membrane used in the membrane separation is washed with a solvent containing one or more polar solvents that do not react with the aluminum-containing gel and have coordination ability with aluminum. [6] Any of the manufacturing methods according to [1] to [5], characterized in that the membrane used in the membrane separation is washed with a solvent containing one or more selected from the group consisting of toluene, dimethylformamide, N-methylpyrrolidone, tetrahydrofuran, tetraglyme and 1,4-dioxane. [7] The method according to any one of [1] to [6], characterized in that the membrane used in the membrane separation is washed with a solvent having a temperature of from 40°C to the boiling point thereof. [8] The method according to any one of [1] to [7], characterized in that the membrane used in the membrane separation is washed with the solvent distilled in the reaction liquid evaporation and concentration step. Effect of the Invention

[0009] According to the present invention, there is provided a method for producing methylaluminoxane which is essentially free of gel-like matter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The present invention relates to a method for producing methylaluminoxane, which comprises a step of removing, by membrane separation, an aluminum-containing gel that is generated when trimethylaluminum is reacted with water to produce methylaluminoxane.

[0011] Methylaluminoxane is a condensation product obtained by partial hydrolysis of highly reactive trimethylaluminum, and has a structural unit represented by the formula: [(CH3)AlO]. Methylaluminoxane is generally a mixture of two or more compounds with different molecular weights, and the compound has a linear structure, a cyclic structure, or a combination thereof. The average degree of oligomerization (average degree of polymerization) of methylaluminoxane is generally 60 or less. Methylaluminoxane may also contain a branched structure or a crosslinked structure. It is believed that there is a close relationship between the composition and structure of methylaluminoxane and its stability, but as described above, methylaluminoxane has a very complicated structure, so it is difficult to identify the difference in stability of methylaluminoxane compositions from the viewpoint of the composition and structure of methylaluminoxane.

[0012] Methylaluminoxane can be obtained by reacting trimethylaluminum with water. The reaction is preferably carried out in a solvent. Examples of the solvent include aliphatic and aromatic hydrocarbons that are inactive against trimethylaluminum and methylaluminoxane. Examples of the aliphatic hydrocarbon include saturated hydrocarbon compounds such as n-pentane, n-hexane, n-heptane, n-octane, isooctane, and refined kerosene; and cyclic hydrocarbon compounds such as cyclopentane, cyclohexane, methylcyclohexane, ethylcyclohexane, and cycloheptane. Examples of the aromatic hydrocarbon include benzene, toluene, o-xylene, m-xylene, p-xylene, ethylbenzene, propylbenzene, and cumene. The solvent is preferably toluene.

[0013] The reaction temperature is generally 0°C to 100°C, preferably 10°C to 40°C.

[0014] The reaction time is generally 0.5 to 100 hours, and preferably 1 to 10 hours.

[0015] The molar ratio of trimethylaluminum to water (moles of trimethylaluminum / moles of water) is preferably 1.0 to 5.0, more preferably 1.2 to 4.0, and even more preferably 1.5 to 3.0.

[0016] It is preferable that trimethylaluminum is mixed with the solvent to form a trimethylaluminum solution, and then reacted with water. The trimethylaluminum concentration in the trimethylaluminum solution is preferably 1% by mass to 20% by mass, and more preferably 5% by mass to 15% by mass.

[0017] Water is preferably added in the form of an aqueous dispersion. By using an aqueous dispersion, the addition rate can be easily adjusted. The aqueous dispersion can be prepared by mixing the above-mentioned solvent with water. The medium of the aqueous dispersion is preferably the same as the solvent of the trimethylaluminum solution. The water concentration in the aqueous dispersion is preferably 1% by mass to 20% by mass, more preferably 5% by mass to 15% by mass.

[0018] The reaction between trimethylaluminum and water to produce methylaluminoxane is preferably carried out using a stirring device. The stirring device is not particularly limited, but it is preferable to use a high-speed homogenizer. By using the high-speed homogenizer, trimethylaluminum and water can be reacted quickly. The rotation speed of the high-speed homogenizer can be, for example, 1000 rpm to 20000 rpm.

[0019] The low molecular weight methylaluminoxane produced in the initial stage of the reaction of trimethylaluminum with water is still highly reactive with water, just like trimethylaluminum, so if the reaction is carried out in a state where the entire amount of water is already present in the reaction field, the reaction will proceed irregularly and a gel-like substance will easily be produced. Therefore, it is preferable to gradually add water or an aqueous dispersion to the trimethylaluminum solution. This allows the reaction to be carried out while controlling the amount of water coexisting with trimethylaluminum or low molecular weight methylaluminoxane, and as a result, the production of a gel-like substance can be somewhat suppressed.

[0020] As described above, the present inventors have found that the aluminum-containing gel produced as a by-product during the production of methylaluminoxane from trimethylaluminum can be efficiently removed by cross-flow filtration. The present inventors were the first to discover that a cross-flow filtration step can be introduced into the production of methylaluminoxane for the purpose of removing the aluminum-containing gel.

[0021] The aluminum-containing gel can be removed by sedimentation or centrifugation, but from the viewpoint of industrial production, membrane separation is preferred from the viewpoint of operational operability. There is no particular limitation on the membrane separation method, but examples thereof include cross-flow filtration, ultrafiltration, pressure filtration, reduced pressure filtration, and centrifugal filtration. There is no limitation on the size of the filter used, but examples thereof include filters with a captured particle size of 0.1 μm or less. The material of the filter used may be an inorganic membrane (ceramic filter) made of inorganic ceramics or an organic membrane made of an organic material, but when an organic solvent such as toluene is used to react trimethylaluminum with water, it is preferable to use a ceramic filter or the like that is durable against the solvent as the membrane material.

[0022] As the inorganic film, it is preferable to use ceramic materials such as aluminum oxide, zirconium oxide, and titanium oxide, as well as stainless steel, glass, etc. As the organic film, it is preferable to use any one of polyethylene, tetrafluoroethylene, polyvinylidene fluoride, polypropylene, cellulose acetate, polyacrylonitrile, polyimide, polysulfone, polyethersulfone, aromatic polyamide, and polyvinyl alcohol, and although there is no limitation, examples thereof include polypropylene, polyethylene, polyethylene terephthalate, nylon, PTFE, and PES.

[0023] A particularly preferred membrane separation method is cross-flow filtration using a ceramic filter, which is less likely to clog. The separation membrane used in cross-flow filtration may be an ultrafiltration membrane or a reverse osmosis membrane, as long as it has a hollow fiber shape.

[0024] In a preferred embodiment of the present invention, the aluminum-containing gel attached to the separation membrane is removed by washing with a washing liquid. Preferably, the separation membrane is washed by backwashing in which the washing liquid flows in the opposite direction to the filtration direction of the separation membrane. A polar solvent capable of dissolving the aluminum-containing gel is preferably used as the washing liquid. Here, the polar solvent refers to a solvent having a coordination ability with aluminum. Examples of the solvent having a coordination ability with aluminum include aromatic hydrocarbons such as benzene, toluene, and xylene, amides such as dimethylformamide and formamide, amines such as N-methylpyrrolidone and 2-pyrrolidone, ethers such as tetrahydrofuran, tetraglyme, and 1,4-dioxane, carbonates such as dimethyl carbonate and diethyl carbonate, and carbamates such as methyl carbamate and ethyl carbamate. The washing liquid may be the same or different from the solvent used when, for example, trimethylaluminum is reacted with water to produce methylaluminoxane. Preferably, the cleaning solution is one or a combination of two or more polar solvents such as toluene, dimethylformamide, N-methylpyrrolidone, tetrahydrofuran, tetraglyme, 1,4-dioxane, etc., which have high solubility for aluminum-containing gel. Toluene is particularly preferred. Toluene is a solvent that is frequently used when reacting trimethylaluminum with water to produce methylaluminoxane, but it is well known that the use of toluene as a solvent also results in the generation of aluminum-containing gel, and a person skilled in the art would consider that it is not suitable for dissolving aluminum-containing gel. However, contrary to such knowledge, the present inventor surprisingly found that aluminum-containing gel is dissolved in toluene and can be effectively used to wash a separation membrane for removing aluminum-containing gel.

[0025] From the viewpoint of cycleability, the cleaning solution is preferably the same as the solvent used when producing methylaluminoxane by reacting trimethylaluminum with water. As described later, it is preferable that the methylaluminoxane reaction solution produced by reacting trimethylaluminum with water is subjected to a concentration step such as a vacuum drying step. If the above solvent and the cleaning solution are the same, the solvent distilled and recovered in the drying and concentration step of the methylaluminoxane reaction solution can be recycled as the cleaning solution, and as a result, the amount of polar solvent used in the entire production process of methylaluminoxane can be reduced, providing the advantage of being environmentally friendly.

[0026] Furthermore, it is preferable to set the temperature for washing so that the solubility of the aluminum-containing gel in the washing solution used is high. This allows the amount of washing solution used for washing the separation membrane to be minimized as much as possible, providing the advantage of being environmentally friendly. Therefore, in the washing process of the separation membrane, it is preferable to use a temperature set at a high temperature in order to increase the solubility of the aluminum-containing gel, as long as the separation membrane shows tolerance or durability. For example, when toluene is used as the washing solution, it is preferable to carry out the washing process of the separation membrane at a temperature of 25°C to the boiling point or lower, for example, 40°C to the boiling point or lower, preferably 40 to 60°C, in consideration of the solubility of the aluminum-containing gel.

[0027] The production line for methylaluminoxane may be a batch type or a continuous type, so the washing of the separation membrane may be appropriately performed according to the type. For example, when the production line is a batch type, washing may be performed for each production batch, or may be performed every several batches, for example, every 2, 3, 4, or 5 batches. Alternatively, washing may be performed when a decrease in the filtration flow rate is observed. Even when the production line is a continuous type, the timing of the washing process of the separation membrane is arbitrary, for example, washing may be performed at a predetermined interval, or, for example, the filtration flow rate may be monitored appropriately or at any time, and the washing process may be performed when the flow rate becomes lower than a predetermined amount.

[0028] The methylaluminoxane reaction solution produced by reacting trimethylaluminum with water is preferably subjected to a concentration step, which may be performed before or after removal of the aluminum-containing gel by membrane separation. The concentration equipment is not particularly limited, but is preferably a natural circulation type, calandria type, forced circulation type, or jacket coil type concentration equipment.

[0029] The concentration is preferably carried out under reduced pressure. The concentration pressure is preferably 1 kPaA (absolute pressure) to 50 kPaA, more preferably 2.6 kPaA to 7.5 kPaA.

[0030] The concentration is generally carried out at a temperature range of 0°C (273K) to 110°C (383K). The concentration temperature is preferably 60°C (333K) or lower, more preferably 40°C (313K) or lower. In one embodiment, the concentration temperature is 0°C (273K) to 60°C (333K), preferably 20°C (293K) to 40°C (313K).

[0031] The concentration time is not particularly limited, but is preferably 1 hour to 100 hours, and more preferably 8 hours to 20 hours.

[0032] The concentration may be continuous concentration or batch concentration. In one embodiment, the batch concentration is carried out after the continuous concentration. The continuous concentration is a concentration carried out while maintaining the volume of the crude methylaluminoxane constant, and the concentration is carried out while continuously supplying the crude methylaluminoxane to the concentration tank. The batch concentration can be carried out, for example, for the purpose of precisely adjusting the concentration of the methylaluminoxane composition. In an embodiment in which the continuous concentration and the batch concentration are carried out, the concentration time means the total time of the continuous concentration and the batch concentration.

[0033] Methylaluminoxane is highly reactive with oxygen, so it is generally stored under a nitrogen gas atmosphere. Methylaluminoxane can be used directly in solution for olefin polymerization. Methylaluminoxane can also be dried by spray drying or the like and used in powder form for olefin polymerization.

[0034] All documents mentioned herein are incorporated by reference in their entirety.

[0035] The following examples of the present invention are for illustrative purposes only and do not limit the technical scope of the present invention. The technical scope of the present invention is limited only by the claims. The present invention can be modified, for example, by adding, deleting, and replacing the constituent elements of the present invention, without departing from the spirit of the present invention. EXAMPLES

[0036] The present invention will be described in more detail below with reference to examples, but these are not intended to limit the present invention in any way.

[0037] Example 1 24.9 kg of the methylaluminoxane reaction solution contained 0.124 kg (0.50 wt%) of aluminum-containing gel, which was then filtered through a ceramic membrane filter (membrane pore size 0.1 μm, membrane area 0.06 m) manufactured by NGK. 2 A portion of the reaction liquid fed from the 30 L slurry tank by a filtration pump was filtered through a membrane and sent to a 30 L filtrate tank, and the remainder was returned to the slurry tank through a circulation line. After the membrane filtration, 20.5 kg of clear methylaluminoxane filtrate was obtained in the filtrate tank. Since the filtration performance had decreased, the ceramic membrane filter used in the membrane filtration was backwashed with toluene at 25°C to remove the cake layer that had adhered to it. Backwashing was performed in six batches, using 0.1 kg of toluene per batch, and the filtration performance returned to the level before use.

[0038] Example 2 The same operation as in Example 1 was carried out, except that the backwashing of the ceramic membrane filter used in the filtration was carried out at 40° C. After carrying out the backwashing for 5 batches, the filtration performance returned to the same as before use.

[0039] Example 3 The same operation as in Example 1 was carried out, except that the backwashing of the ceramic membrane filter used in the filtration was carried out at 60° C. After carrying out the backwashing in four batches, the filtration performance returned to the same as before use.

[0040] Example 4 The same operation as in Example 1 was carried out, except that the backwashing of the ceramic membrane filter used in the filtration was carried out at 80° C. After carrying out the backwashing in four batches, the filtration performance returned to the same as before use.

[0041] Example 5 The same operation as in Example 1 was carried out, except that the ceramic membrane filter used in the filtration was backwashed with dimethylformamide at 25° C. After two batches of backwashing, the filtration performance returned to that before use.

[0042] Example 6 The same operation as in Example 1 was carried out, except that the ceramic membrane filter used in the filtration was backwashed with N-methylpyrrolidone at 25° C. After carrying out the backwashing in two batches, the filtration performance returned to that before use.

[0043] Example 7 The same operation as in Example 1 was carried out, except that the ceramic membrane filter used in the filtration was backwashed with tetrahydrofuran at 25° C. After two batches of backwashing, the filtration performance returned to that before use.

[0044] Example 8 The same operation as in Example 1 was carried out, except that the ceramic membrane filter used in the filtration was backwashed with tetrahydrofuran at 40° C. After two batches of backwashing, the filtration performance returned to that before use.

[0045] Example 9 The same operation as in Example 1 was carried out, except that the ceramic membrane filter used in the filtration was backwashed with tetraglyme at 25° C. After two batches of backwashing, the filtration performance returned to that before use.

[0046] Example 10 The same operation as in Example 1 was carried out, except that the ceramic membrane filter used in the filtration was backwashed with 1,4-dioxane at 25° C. After two batches of backwashing, the filtration performance returned to that before use.

[0047] Example 11 The same operation as in Example 1 was carried out, except that the ceramic membrane filter used in the filtration was backwashed with toluene / dimethylformamide (1 / 1 (v / v)) at 25° C. After two batches of backwashing, the filtration performance returned to that before use.

[0048] Comparative Example 1 The same operation as in Example 1 was carried out, except that the ceramic membrane filter used in the filtration was backwashed with hexane. Twelve batches of backwashing were required to restore the filtration performance before use. Comparative Example 2 When 24.7 kg of a methylaluminoxane reaction liquid having the same composition as in Example 1 was subjected to dead-end filtration, the filter cloth became clogged during the process, making filtration impossible. Comparative Example 3 When 25.6 kg of a methylaluminoxane reaction solution having the same composition as in Example 1 was allowed to settle and separate, it took 30 hours for the gel to settle.

[0049] From the type of solvent used for backwashing in the above Examples and Comparative Examples, the temperature, and the number of ceramic membrane filter backwash batches required to restore the filtration performance before use, it can be seen that, among the solvents used, polar solvents are preferable as cleaning liquids.

[0050] Although the embodiments of the present invention have been described above, the present invention is not limited to these, and the materials used, various conditions, etc. can be appropriately changed without departing from the spirit of the invention.

Claims

1. A method for producing methylaluminoxane, comprising a step of cross-flow filtering a reaction solution in which methylaluminoxane has been produced from trimethylaluminum to remove an aluminum-containing gel by-product.

2. 2. The method according to claim 1, wherein the membrane separation is carried out using a ceramic filter.

3. 3. The method according to claim 1, wherein the membrane used in the membrane separation is washed with the solvent used in producing methylaluminoxane from trimethylaluminum.

4. 4. The method according to claim 3, wherein the solvent is toluene.

5. 3. The method according to claim 1, wherein the membrane used in the membrane separation is washed with a solvent containing one or more polar solvents that do not react with the aluminum-containing gel and have a coordinating ability to aluminum.

6. 3. The method according to claim 1, wherein the membrane used in the membrane separation is washed with a solvent containing one or more selected from the group consisting of toluene, dimethylformamide, N-methylpyrrolidone, tetrahydrofuran, tetraglyme, and 1,4-dioxane.

7. 3. The method according to claim 1, wherein the membrane used in the membrane separation is washed with a solvent at a temperature of from 40° C. to the boiling point thereof.

8. 3. The method according to claim 1, wherein the membrane used in the membrane separation is washed with the solvent distilled off in the reaction solution evaporation and concentration step.