Molecular weight adjusting method of polyethylene

Abstract

The invention relates to a method for adjusting the molecular weight of polyethylene, which is characterized in that an inorganic carrier is subjected to aluminum element modification, chromium element loading and heat treatment and then is used for catalyzing polymerization of ethylene, and the molecular weight of polyethylene is adjusted by adjusting the aluminum element content on the inorganic carrier. The molecular weight distribution of polyethylene is adjusted by adjusting the heat treatment temperature and the heat treatment time. Compared with the prior art, the method has the advantages that the means for adjusting the molecular weight and molecular weight distribution of the polyethylene is controllable, the adjusting range is wide, the polyethylene with extremely high molecular weight and extremely low molecular weight can be prepared, the molecular structure can be further finely adjusted through polymerization conditions subsequently, and a good foundation is laid for preparing the high-performance chromium polyethylene in the specific field.

Description

Technical Field

[0001] The present invention relates to the technical field of polyethylene synthesis, and in particular to a method for regulating the molecular weight of polyethylene. Background Art

[0002] Polyethylene is a thermoplastic resin produced by the polymerization of ethylene monomers. With the advancement and development of polyethylene synthesis technology, polyethylene synthesis has become more specialized. To obtain products in desired applications, a balance must be struck between processing and mechanical properties. Molecular weight and molecular weight distribution are key to regulating these two properties. This requires the regulation of the molecular structure of the resulting polyethylene resin.

[0003] Chromium-based polyethylene catalysts are one of the most important catalysts in the industrial production of polyethylene. However, the current means of controlling the molecular weight of the polymerization process of chromium-based catalysts are relatively limited.

[0004] Currently, in the polymerization process, the main molecular weight adjustment methods include adding hydrogen and oxygen to adjust the molecular weight and adjusting the reaction temperature, but the adjustment range of the molecular weight by these two molecular weight adjustment methods is relatively limited.

[0005] Chinese patent CN108976322A uses alum-modified chromium catalysts, which can adjust the molecular weight through alum, increase the high molecular weight portion, and improve the molecular weight distribution. However, no solution is given for adjusting the molecular weight distribution and the low molecular weight portion.

[0006] Chinese patent CN108203476A uses Mo-modified chromium catalyst to regulate the melt index and density, but does not specifically explore the specific rules of modification conditions, molecular weight, and molecular weight distribution.

[0007] ExxonMobil has developed a bis(2-triarylsilyl) chromate catalyst capable of producing HDPE with reduced ultra-high molecular weight fractions. This approach reduces the formation of ultra-high molecular weight fractions in HDPE by introducing at least one catalyst deactivator into the fluidized bed reactor and recycle gas line. However, this method has not yet yielded a means to control the molecular weight distribution. Summary of the Invention

[0008] The purpose of the present invention is to provide a method for regulating the molecular weight of polyethylene, so as to regulate the molecular weight and molecular weight distribution of the polymerized polyethylene resin.

[0009] The objectives of the present invention can be achieved through the following technical solutions: a method for regulating the molecular weight of polyethylene, wherein an inorganic carrier is modified with aluminum, loaded with chromium and heat-treated and then used to catalyze the polymerization of ethylene, the molecular weight of the polyethylene is adjusted by adjusting the aluminum content on the inorganic carrier, and the molecular weight distribution of the polyethylene is adjusted by adjusting the heat treatment temperature and heat treatment time.

[0010] Preferably, the inorganic carrier is selected from substances containing aluminum and titanium.

[0011] More preferably, the inorganic carrier is selected from at least one of aluminum oxide, silica gel, aluminum phosphate, titanium dioxide, and inorganic clay.

[0012] More preferably, the inorganic clay includes kaolin and montmorillonite.

[0013] Preferably, the inorganic carrier has an average particle size of 0.1 μm to 100 μm and a pore volume of 0.5 cm 3 / g~10.0cm 3 / g, specific surface area of 15m 2 / g~1000m 2 / g.

[0014] Preferably, the method for adjusting the molecular weight of polyethylene comprises the following steps:

[0015] (1) Inorganic support modification: using alkyl aluminum and / or alkoxy aluminum to modify the inorganic support to obtain a modified inorganic support;

[0016] (2) Chromium loading: a chromium salt is prepared into a solution using a solvent, mixed with the modified inorganic carrier, and after thorough mixing, heated at 50°C to 110°C and allowed to stand until dry to obtain a modified inorganic carrier powder loaded with chromium element;

[0017] (3) High-temperature calcination: The modified inorganic carrier powder loaded with chromium element is further calcined in a dry non-reducing gas environment at a temperature of 300 to 950° C. for 0.5 to 48 hours to obtain a chromium-based polyethylene catalyst;

[0018] (4) Using the chromium-based polyethylene catalyst to catalyze polyethylene polymerization.

[0019] Further preferably, the alkyl aluminum and / or alkoxy aluminum in step (1) is at least one of triisobutyl aluminum, tri-n-hexylaluminum, triisopropyl aluminum, methyl diethyl aluminum, trimethyl aluminum, diethyl aluminum ethoxide, diethyl aluminum methoxide, dimethyl aluminum ethoxide, diisopropyl aluminum ethoxide, diethyl aluminum propoxide, diisobutyl aluminum ethoxide and methyl ethyl aluminum ethoxide.

[0020] Further preferably, the mass ratio of the alkyl aluminum and / or alkoxy aluminum to the inorganic carrier in step (1) is (0.1-10):100.

[0021] Further preferably, in step (1), the alkyl aluminum and / or alkoxy aluminum is used to modify the inorganic support by an impregnation method.

[0022] Further preferably, the chromium salt in step (2) is one or more of chromium acetate, dichromate, chromate, chromium nitrate or chromium oxide.

[0023] Further preferably, the chromium content in the chromium salt in step (2) accounts for 0.1 to 5.0% of the mass of the modified inorganic carrier.

[0024] Further preferably, the solvent in step (2) is water, an alcohol solvent or a ketone solvent.

[0025] More preferably, the alcohol solvent is methanol, ethanol or propanol.

[0026] More preferably, the ketone solvent is acetone or butanone.

[0027] Preferably, the height and composition of the high molecular weight portion of the polyethylene obtained by polymerization are controlled by adjusting the aluminum content in the chromium-based polyethylene catalyst, and the molecular weight distribution is further adjusted by adjusting the heat treatment (i.e., roasting) temperature and heat treatment (i.e., roasting) time.

[0028] Preferably, the inorganic carrier is modified with aluminum, loaded with chromium and titanium, and heat-treated before being used to catalyze the polymerization of ethylene. The molecular weight of the polyethylene is adjusted by adjusting the content of aluminum and titanium on the inorganic carrier, and the molecular weight distribution of the polyethylene is adjusted by adjusting the heat treatment (i.e., calcination) temperature and heat treatment (i.e., calcination) time.

[0029] Further preferably, the method for adjusting the molecular weight of polyethylene comprises the following steps:

[0030] S1 Inorganic Support Modification: Modifying the inorganic support with alkyl aluminum and / or alkoxy aluminum to obtain a modified inorganic support;

[0031] S2 Chromium loading: Use a solvent to prepare a chromium salt solution, mix it with the modified inorganic carrier, and after thorough mixing, heat and stand at 50°C to 110°C until dry to obtain a modified inorganic carrier powder loaded with chromium element;

[0032] S3 Titanium loading: The modified inorganic carrier powder loaded with chromium is further mixed with an organic titanium compound solution and a co-catalyst. After thorough mixing, the mixture is allowed to stand at 0-50°C for 0-4 hours, and then the mixture is heated to 60-80°C until dried to obtain a modified inorganic carrier powder loaded with both chromium and titanium elements.

[0033] S4 high-temperature calcination: the modified inorganic carrier powder loaded with chromium and titanium elements is further calcined in a dry non-reducing gas environment at a temperature of 300 to 950° C. for 0.5 to 48 hours to obtain a chromium-based polyethylene catalyst;

[0034] S5 uses the chromium-based polyethylene catalyst to catalyze polyethylene polymerization.

[0035] More preferably, the organic titanium compound in step S3 is at least one of tetraisopropyl titanium, tetraethyl titanium, tetrabutyl titanium, tetraisooctyl titanate, and n-butyl titanate.

[0036] More preferably, the co-catalyst in step S3 is at least one of triethylaluminum, triisobutylaluminum, and diethylaluminum chloride.

[0037] More preferably, the solvent of the organic titanium compound solution in step S3 is water, an alcohol solvent or a ketone solvent.

[0038] Preferably, the alcohol solvent is methanol, ethanol or propanol.

[0039] Preferably, the ketone solvent is acetone or butanone.

[0040] More preferably, the mass ratio of titanium in the organic titanium compound to the modified inorganic carrier powder loaded with chromium element in step S3 is (0.1-10):100, and the mass ratio of the co-catalyst to the modified inorganic carrier powder loaded with chromium element is (0.1-5):100.

[0041] Further preferably, the height and composition of the high molecular weight portion of the polyethylene obtained by polymerization are controlled by adjusting the aluminum content in the chromium-based polyethylene catalyst, the height and composition of the low molecular weight portion of the polyethylene obtained by polymerization are controlled by adjusting the titanium content in the chromium-based polyethylene catalyst, and the molecular weight distribution is further adjusted by adjusting the heat treatment (i.e., calcination) temperature and heat treatment (i.e., calcination) time.

[0042] More preferably, the content of the high molecular weight portion of the polyethylene obtained is increased by increasing the aluminum content in the chromium-based polyethylene catalyst, and the content of the high molecular weight portion of the polyethylene obtained is reduced by reducing the aluminum content in the chromium-based polyethylene catalyst.

[0043] More preferably, the content of the low molecular weight portion of the polyethylene obtained is increased by increasing the titanium content in the chromium-based polyethylene catalyst, and the content of the low molecular weight portion of the polyethylene obtained is reduced by reducing the titanium content in the chromium-based polyethylene catalyst.

[0044] More preferably, the molecular weight distribution of the obtained polyethylene is narrowed by increasing the heat treatment temperature and the heat treatment time, and is broadened by decreasing the heat treatment temperature and the heat treatment time.

[0045] Preferably, the method for adjusting the molecular weight of polyethylene comprises the following steps:

[0046] The inorganic carrier is modified with aluminum, loaded with chromium and titanium, and heat-treated to catalyze the polymerization of ethylene. The molecular weight of the polyethylene is adjusted by adjusting the content of aluminum and titanium on the inorganic carrier. The molecular weight distribution of the polyethylene is adjusted by adjusting the heat treatment (i.e., calcination) temperature and heat treatment (i.e., calcination) time. The molecular weight of the polyethylene is further adjusted by adjusting the polymerization temperature and / or adding oxygen during the polymerization process.

[0047] More preferably, the molecular weight of polyethylene is reduced by increasing the polymerization temperature during the polymerization of polyethylene, and the molecular weight of polyethylene is reduced by adding oxygen.

[0048] The present invention provides a method for preparing a chromium-based polyethylene catalyst and a molecular weight controllable technology. The catalyst is prepared by modifying, loading, and calcining an inorganic carrier. During the preparation process, the resin molecular weight and molecular weight distribution are adjusted by modification with aluminum elements, loading with chromium elements and titanium elements, and heat treatment, thereby obtaining a structurally controllable wide molecular weight distribution polyethylene resin with a molecular weight distribution Mw / Mn of 8 or more.

[0049] The molecular weight adjustable technology of the present invention specifically includes:

[0050] (1) High molecular weight regulation: By adjusting the concentration of alkyl aluminum and / or alkoxy aluminum in the chromium-based polyethylene catalyst, the height and composition of the high molecular weight portion of the polyethylene obtained by polymerization can be controlled.

[0051] (2) Low molecular weight regulation: By adjusting the organic titanium concentration during the preparation of the chromium-based catalyst, the height and composition of the low molecular weight portion of the polyethylene obtained by polymerization can be controlled.

[0052] (3) Calcination temperature and time control: The molecular weight distribution can be further adjusted by adjusting the calcination temperature and time.

[0053] Research has shown that the concentrations of both aluminum and titanium compounds during catalyst preparation significantly influence molecular weight. When using an aluminum-containing carrier, the higher the complexity of the aluminum compound in the carrier, the wider the molecular weight distribution of the resulting polyethylene in the high molecular weight fraction.

[0054] The higher the complexity of the aluminum alkyl and / or aluminum alkoxide added during the preparation process, the wider the molecular weight distribution of the high molecular weight portion of the resulting polyethylene. Increasing the concentration of the aluminum alkyl and / or aluminum alkoxide also increases the content of the high molecular weight portion.

[0055] If titanium loading is further performed during the preparation process to increase the organic titanium concentration, the content of the low molecular weight part will increase.

[0056] By adjusting the aluminum and titanium elements, the molecular weight of the polymer can be adjusted. After completing the initial molecular weight adjustment, the molecular weight distribution of the resulting polyethylene can be further fine-tuned through high-temperature calcination. Experiments have shown that higher calcination temperatures and longer calcination times narrow the molecular weight distribution of the resulting polyethylene, while lower calcination temperatures and shorter calcination times produce polyethylene with a wider molecular weight distribution.

[0057] Based on the above, the molecular weight of the resulting polyethylene can be further fine-tuned during ethylene polymerization by adjusting the polymerization temperature and the influence of external influencing substances on the catalyst. The higher the polymerization temperature during the polymerization reaction, the lower the molecular weight of the resulting polyethylene will be. The maximum polymerization temperature is 110°C. Adding oxygen to the polymerization reaction can also further adjust the molecular weight of the polyethylene, further reducing it.

[0058] Compared with the prior art, the present invention has the following beneficial effects:

[0059] 1. The present invention, through the development of catalysts, controls the molecular weight of the polyethylene obtained by polymerization from the catalyst stage to obtain polyethylene with controllable structure;

[0060] 2. The present invention can regulate the molecular weight and molecular weight distribution of the polymerized polyethylene resin through the catalyst preparation process, thereby obtaining polyethylene with a specified structure and a wide molecular weight distribution;

[0061] 3. The present invention can adjust the high molecular weight fraction, low molecular weight fraction, and molecular weight distribution of polyethylene obtained by chromium-based catalyst polymerization through catalyst treatment to obtain the target polyethylene resin, laying a good foundation for the development of high-performance polyethylene resins.

[0062] 4. During the catalyst preparation process, the present invention can adjust the content of aluminum and titanium elements and the high-temperature calcination conditions to achieve the adjustment of the molecular weight and distribution of the polyethylene obtained by polymerization;

[0063] 5. The present invention allows for controllable adjustment of polyethylene molecular weight and molecular weight distribution over a wide range, enabling the preparation of both very high and very low molecular weight polyethylene. The molecular structure can also be further fine-tuned through subsequent polymerization conditions, laying a solid foundation for the preparation of high-performance chromium-based polyethylene in specific fields.

[0064] 6. The present invention can prepare polyethylene with a molecular weight range of 10,000 to 10 million, which has a wide range and has good application prospects in the field of polyethylene resins. DETAILED DESCRIPTION

[0065] The embodiments of the present invention are described in detail below. The following embodiments are implemented based on the technical solutions of the present invention, and provide detailed implementation methods and specific operating processes. However, the protection scope of the present invention is not limited to the following embodiments.

[0066] Unless otherwise specified, the reagents, methods, instruments and equipment used in the present invention are conventional reagents, methods, instruments and equipment in the art. Unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0067] Standard polymerization conditions: Ethylene pressure polymerization reaction apparatus is a 2L stainless steel reactor equipped with water circulation temperature control. Nitrogen is flushed into the reactor. 1L of n-hexane and catalyst are added under a nitrogen atmosphere. The nitrogen is then replaced three times with ethylene. The ethylene inlet valve is adjusted to maintain a constant ethylene pressure of 1 MPa. Polymerization is carried out at 60°C to 115°C. After the reaction, the reactor is dried to a constant weight and weighed to obtain the polyethylene product.

[0068] Molecular weight and molecular weight distribution test method: using high temperature gel chromatography.

[0069] The specific properties of the K10 montmorillonite used in the examples are as follows:

[0070] K10 montmorillonite: particle size: <2μm, specific surface area: 220-270m 2 / g, pore volume: 0.3-0.4mL / g.

[0071] The specific properties of the clay inorganic carrier in the embodiment are as follows:

[0072] Kaolin: particle size: 0.1-4μm, specific surface area: 15-30m 2 / g, pore volume: 0.2-0.3mL / g.

[0073] Example 1

[0074] Triisobutylaluminum and K10 montmorillonite were blended at a mass ratio of 0.1:100 for modification. Ethanol and potassium dichromate were mixed at a mass ratio of 5:1 and then blended with the modified support. The chromium content in the potassium dichromate was 0.1% of the modified support's mass. After heating the mixture at 50°C and allowing it to stand for 10 hours, the mixture formed a dry powder.

[0075] The above powder was blended with tetraisopropyl titanium, ethanol, and triethylaluminum, with the titanium content of tetraisopropyl titanium accounting for 0.1% of the powder mass, the triethylaluminum accounting for 1% of the powder mass, and the powder mass accounting for 10% of the solvent mass. The above blend was allowed to stand at 0°C for 4 hours, then heated to 80°C and held for 1 hour until it became a dry powder.

[0076] The dried powder was calcined at 300°C for 0.5 hours to obtain a chromium-based polyethylene catalyst. The catalyst was evaluated for polymerization, and the resulting polyethylene had a weight-average molecular weight of 360,000 and a molecular weight distribution (Mw / Mn) of 16.4.

[0077] Example 2

[0078] The other conditions were the same as in Example 1. The treated dry powder was calcined at 950°C for 48 hours to obtain a chromium-based polyethylene catalyst. Polymerization evaluation of the catalyst revealed that the polyethylene obtained had a weight-average molecular weight of 320,000 and a molecular weight distribution (Mw / Mn) of 8.4.

[0079] Example 3

[0080] Other conditions were the same as in Example 1, except that the mass ratio of triisobutylaluminum to K10 montmorillonite was adjusted from 0.1:100 to 10:100 for blending modification. The catalyst was evaluated for polymerization, and the resulting polyethylene had a weight-average molecular weight of 1.54 million and a molecular weight distribution Mw / Mn of 29.4.

[0081] Example 4

[0082] Other conditions were the same as those in Example 1, except that the catalyst carrier was changed from K10 montmorillonite to kaolin. The catalyst was subjected to polymerization evaluation, and the polyethylene obtained by polymerization had a weight average molecular weight of 640,000 and a molecular weight distribution Mw / Mn of 24.4.

[0083] Example 5

[0084] Other conditions were the same as in Example 1, except that the titanium content of the tetraisopropyl titanium powder was adjusted from 0.1% to 10%. The catalyst was evaluated for polymerization, and the polyethylene obtained by polymerization had a weight-average molecular weight of 180,000 and a molecular weight distribution Mw / Mn of 28.1.

[0085] Example 6

[0086] Other conditions were the same as in Example 1, except that the mass ratio of triisobutylaluminum to K10 montmorillonite was adjusted from 0.1:100 to 10:100 for blending, and the titanium content of the tetraisopropyl titanium powder was adjusted from 0.1% to 10%. Polymerization evaluation of the catalyst revealed polyethylene with a weight-average molecular weight of 1.32 million and a molecular weight distribution (Mw / Mn) of 69.3.

[0087] Example 7

[0088] Other conditions were consistent with those in Example 5. The catalyst prepared in Example 5 was subjected to polymerization temperature increased to 110° C. during the polymerization evaluation process to obtain polyethylene with a weight-average molecular weight of 110,000 and a molecular weight distribution Mw / Mn of 29.2.

[0089] Example 8

[0090] Other conditions were consistent with those in Example 5. 30 ppm of oxygen was added to the catalyst prepared in Example 5 during the polymerization evaluation process to obtain polyethylene with a weight-average molecular weight of 190,000 and a molecular weight distribution Mw / Mn of 30.2.

[0091] Example 9

[0092] Other conditions were the same as those in Example 1, except that titanium loading was not performed during the preparation process of Example 1, thereby obtaining a chromium-based polyethylene catalyst. The catalyst was evaluated for polymerization, and the polyethylene obtained by polymerization had a weight-average molecular weight of 570,000 and a molecular weight distribution Mw / Mn of 10.1.

[0093] Comparative Example 1

[0094] Based on the carrier modification in Example 1, titanium loading was first performed and then chromium loading was performed, and then high-temperature calcination was performed. Other conditions were consistent with those in Example 1. The obtained catalyst was found to be inactive after polymerization evaluation.

[0095] Comparative Example 2

[0096] Other conditions were the same as those in Example 1, except that titanium loading was not performed during the preparation process of Example 1. The obtained catalyst was subjected to polymerization evaluation by increasing the polymerization temperature to 110° C., and polyethylene with a weight average molecular weight of 390,000 and a molecular weight distribution Mw / Mn of 23.1 was obtained.

[0097] Comparative Example 3

[0098] Other conditions were the same as those in Example 1, except that titanium loading was not performed during the preparation process of Example 1. 30 ppm of oxygen was added to the obtained catalyst during the polymerization evaluation process, and polyethylene with a weight average molecular weight of 430,000 and a molecular weight distribution Mw / Mn of 19.8 was obtained.

[0099] Table 1 Key parameters and performance data of each embodiment and comparative example

[0100]

[0101]

[0102]

[0103] As can be seen from the table above, without titanium or aluminum modification and high-temperature calcination to adjust the catalyst, the ability to adjust molecular weight through methods such as increasing the temperature and adding oxygen during the polymerization process is relatively limited. However, titanium or aluminum modification of the catalyst and high-temperature calcination can effectively adjust the molecular weight distribution and level of the polymerization, resulting in a resin with specific requirements.

[0104] The above description of the embodiments is intended to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above-described embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention, without departing from the scope of the present invention, should be within the scope of protection of the present invention.

Claims

1. A method for regulating the molecular weight of polyethylene, characterized in that: The inorganic carrier is modified with aluminum, loaded with chromium and heat-treated, and then used to catalyze the polymerization of ethylene. The molecular weight of polyethylene is adjusted by adjusting the aluminum content on the inorganic carrier, and the molecular weight distribution of polyethylene is adjusted by adjusting the heat treatment temperature and time.

2. The method for regulating the molecular weight of polyethylene according to claim 1, wherein The inorganic carrier is selected from materials containing aluminum and titanium, and has an average particle size of 0.1 micron to 100 microns and a pore volume of 0.5 cm 3 / g~10.0cm 3 / g, specific surface area of 15m 2 / g~1000m 2 / g.

3. The method for regulating the molecular weight of polyethylene according to claim 1, wherein The following steps are involved: (1) Inorganic support modification: using alkyl aluminum and / or alkoxy aluminum to modify the inorganic support to obtain a modified inorganic support; (2) Chromium loading: a chromium salt is prepared into a solution using a solvent, mixed with the modified inorganic carrier, and after thorough mixing, heated at 50°C to 110°C and allowed to stand until dry to obtain a modified inorganic carrier powder loaded with chromium element; (3) High-temperature calcination: The modified inorganic carrier powder loaded with chromium element is further calcined in a dry non-reducing gas environment at a temperature of 300 to 950° C. for 0.5 to 48 hours to obtain a chromium-based polyethylene catalyst; (4) Using the chromium-based polyethylene catalyst to catalyze polyethylene polymerization.

4. The method for regulating the molecular weight of polyethylene according to claim 3, characterized in that: The alkyl aluminum and / or alkoxy aluminum in step (1) is at least one of triisobutyl aluminum, tri-n-hexylaluminum, triisopropyl aluminum, methyl diethyl aluminum, trimethyl aluminum, diethyl aluminum ethoxide, diethyl aluminum methoxide, dimethyl aluminum ethoxide, diisopropyl aluminum ethoxide, diethyl aluminum propoxide, diisobutyl aluminum ethoxide and methyl ethyl aluminum ethoxide, and the mass ratio of the alkyl aluminum and / or alkoxy aluminum to the inorganic carrier is (0.1 to 10):

100.

5. The method for regulating the molecular weight of polyethylene according to claim 3, characterized in that: The chromium salt in step (2) is one or more of chromium acetate, dichromate, chromate, chromium nitrate or chromium oxide, the chromium content in the chromium salt accounts for 0.1-5.0% of the mass of the modified inorganic support, and the solvent is water, an alcohol solvent or a ketone solvent; The alcohol solvent is methanol, ethanol or propanol, and the ketone solvent is acetone or butanone.

6. The method for regulating the molecular weight of polyethylene according to claim 1, characterized in that: By adjusting the aluminum content in the chromium-based polyethylene catalyst, the height and composition of the high molecular weight portion of the polyethylene obtained by polymerization can be controlled, and the molecular weight distribution can be further adjusted by adjusting the heat treatment temperature and heat treatment time.

7. The method for regulating the molecular weight of polyethylene according to claim 1, wherein: The inorganic carrier is modified with aluminum, loaded with chromium and titanium, and heat-treated to catalyze the polymerization of ethylene. The molecular weight of the polyethylene is adjusted by adjusting the content of aluminum and titanium on the inorganic carrier, and the molecular weight distribution of the polyethylene is adjusted by adjusting the heat treatment temperature and time.

8. The method for regulating the molecular weight of polyethylene according to claim 7, characterized in that: The following steps are involved: S1 Inorganic Support Modification: Modifying the inorganic support with alkyl aluminum and / or alkoxy aluminum to obtain a modified inorganic support; S2 Chromium loading: Use a solvent to prepare a chromium salt solution, mix it with the modified inorganic carrier, and after thorough mixing, heat and stand at 50°C to 110°C until dry to obtain a modified inorganic carrier powder loaded with chromium element; S3 Titanium loading: The modified inorganic carrier powder loaded with chromium is further mixed with an organic titanium compound solution and a co-catalyst. After thorough mixing, the mixture is allowed to stand at 0-50°C for 0-4 hours, and then the mixture is heated to 60-80°C until dried to obtain a modified inorganic carrier powder loaded with both chromium and titanium elements. S4 high-temperature calcination: the modified inorganic carrier powder loaded with chromium and titanium elements is further calcined in a dry non-reducing gas environment at a temperature of 300 to 950° C. for 0.5 to 48 hours to obtain a chromium-based polyethylene catalyst; S5 uses the chromium-based polyethylene catalyst to catalyze polyethylene polymerization.

9. The method for regulating the molecular weight of polyethylene according to claim 8, characterized in that: The organic titanium compound in step S3 is at least one of tetraisopropyl titanium, tetraethyl titanium, tetrabutyl titanium, tetraisooctyl titanate, and n-butyl titanate; the co-catalyst is at least one of triethyl aluminum, triisobutyl aluminum, and diethyl aluminum chloride; and the solvent of the organic titanium compound solution is water, an alcohol solvent, or a ketone solvent; The alcohol solvent is methanol, ethanol or propanol, and the ketone solvent is acetone or butanone; The mass ratio of titanium in the organic titanium compound to the modified inorganic carrier powder loaded with chromium element is (0.1-10):100, and the mass ratio of the co-catalyst to the modified inorganic carrier powder loaded with chromium element is (0.1-5):

100.

10. The method for regulating the molecular weight of polyethylene according to claim 7, characterized in that: By adjusting the aluminum content in the chromium-based polyethylene catalyst, the height and composition of the high molecular weight portion of the polyethylene obtained by polymerization can be controlled. By adjusting the titanium content in the chromium-based polyethylene catalyst, the height and composition of the low molecular weight portion of the polyethylene obtained by polymerization can be controlled. The molecular weight distribution can be further adjusted by adjusting the heat treatment temperature and heat treatment time.

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