A high-activity chromium-based polyethylene catalyst and its preparation method

Abstract

The present invention relates to a high-activity chromium-based polyethylene catalyst and a preparation method thereof. The high-activity chromium-based polyethylene catalyst comprises an inorganic carrier modified with alkyl aluminum and / or alkoxy aluminum, and chromium and titanium elements sequentially loaded on the modified inorganic carrier. The preparation method comprises the following steps: (1) inorganic carrier modification: modifying the inorganic carrier with alkyl aluminum and / or alkoxy aluminum; (2) chromium loading; (3) titanium loading; and (4) high-temperature calcination: further calcining the modified inorganic carrier powder loaded with chromium and titanium elements in a dry non-reducing gas environment at a calcination temperature of 300 to 950° C. for a calcination time of 0.5 to 48 hours to obtain the chromium-based polyethylene catalyst. Compared with the prior art, the chromium-based catalyst prepared by the present invention has an activity increased by more than 10 times, significantly reducing the production cost of chromium-based polyethylene products, reducing the impurity content of the products, and improving product quality.

Description

Technical Field

[0001] The present invention relates to the technical field of polyethylene catalysts, and in particular to a high-activity chromium-based polyethylene catalyst and a preparation method thereof. Background Art

[0002] Chromium-based polyethylene catalysts are among the most important catalysts in the industrial production of polyethylene, with the Phillips catalyst being the most representative. These catalysts typically consist of hexavalent chromium as the active component, supported on a carrier such as porous silica gel or modified silica gel. They offer significant advantages, including high catalytic activity, a simple process, and the absence of a co-catalyst. J. P. Hogan and R. L. Bank reported a silica-gel-supported chromium oxide catalyst in patent US2825721, which later became known as the first generation of Phillips catalysts. Other patents, such as US4294724, US4295997, US4528338, US5401820, and US6388017, have investigated modifications to these supported chromium oxide catalysts, leading to the development of the Phillips catalyst.

[0003] Chromium-based catalysts can produce polyethylene products with high molecular weights and wide molecular weight distributions. This results in the final product possessing excellent mechanical and processing properties, and is widely used in high-performance products such as pipes and hollow parts. However, this type of catalyst also has some limitations, such as relatively low utilization of the active component, difficulty in adjusting molecular weight, and limited adaptability of the comonomer. Furthermore, because they contain hexavalent chromium, special attention must be paid to environmental protection requirements during use. Low-activity catalysts will result in large amounts of catalyst residues remaining in the raw materials, reducing raw material performance and limiting their application areas.

[0004] In recent years, research on chromium-based catalysts has focused on support modification technologies, the development of new co-catalysts, the optimization of active site structures, and the regulation of product performance. By developing new support materials, improving the structure of active components, and enhancing catalytic efficiency, researchers are striving to develop a new generation of more efficient and environmentally friendly chromium-based catalysts to meet growing market demand. These advances have not only boosted catalyst performance but also provided new directions for the sustainable development of the polyethylene industry.

[0005] To address the relatively low utilization rate of active components, Chinese patent CN112409514A uses ultrasonic technology during the catalyst loading stage to reduce carrier breakage and increase loading uniformity, thereby improving activity. Chinese patent CN103159875A enhances the polymerization activity of the catalyst through a new catalyst preparation process. Chinese patent CN108976322A improves catalyst activity by simultaneously introducing vanadium and chromium into a porous inorganic carrier.

[0006] However, the above methods can only increase the catalyst activity to within 500 gPE / gCAT / h. Summary of the Invention

[0007] The purpose of the present invention is to provide a high-activity chromium-based polyethylene catalyst and a preparation method thereof, so as to improve the activity of the catalyst.

[0008] The purpose of the present invention can be achieved through the following technical solution: a high-activity chromium-based polyethylene catalyst, comprising an inorganic carrier modified with alkyl aluminum and / or alkoxy aluminum and chromium and titanium elements sequentially loaded on the modified inorganic carrier.

[0009] Preferably, the mass ratio of the alkyl aluminum and / or alkoxy aluminum to the inorganic carrier is (0.1-10):100.

[0010] Preferably, the content of the chromium element accounts for 0.1 to 5.0% of the mass of the modified inorganic carrier.

[0011] Further preferably, the content of the chromium element accounts for 3-5% of the mass of the modified inorganic carrier.

[0012] Preferably, the content of the titanium element accounts for 0.1 to 10% of the mass of the modified inorganic carrier loaded with chromium element.

[0013] Preferably, the inorganic carrier is selected from at least one of silicon dioxide, aluminum oxide, silica gel, aluminum phosphate, titanium dioxide, zirconium oxide, magnesium oxide, calcium oxide, and inorganic clay.

[0014] More preferably, the inorganic clay includes kaolin and montmorillonite.

[0015] Preferably, the inorganic carrier has an average particle size of 0.1 μm to 100 μm and a pore volume of 0.2 cm 3 / g~10.0 cm 3 / g, specific surface area of ​​15 m 2 / g~1000 m 2 / g.

[0016] Preferably, the alkylaluminum and / or alkoxyaluminum is selected from at least one of triisobutylaluminum, tri-n-hexylaluminum, triisopropylaluminum, methyldiethylaluminum, trimethylaluminum, diethylaluminum ethoxide, diethylaluminum methoxide, dimethylaluminum ethoxide, diisopropylaluminum ethoxide, diethylaluminum propoxide, diisobutylaluminum ethoxide and methylethylaluminum ethoxide.

[0017] A method for preparing the above-mentioned highly active chromium-based polyethylene catalyst comprises the following steps:

[0018] (1) Inorganic support modification: using alkyl aluminum and / or alkoxy aluminum to modify the inorganic support to obtain a modified inorganic support;

[0019] (2) Chromium loading: a chromium salt is prepared into a solution using a solvent, and the solution is mixed with the modified inorganic carrier. After thorough mixing, the solution is heated and allowed to stand until dry, thereby obtaining a modified inorganic carrier powder loaded with chromium.

[0020] (3) Titanium loading: The modified inorganic carrier powder loaded with chromium is further mixed with an organic titanium compound solution and a co-catalyst, and after thorough mixing, the mixture is allowed to stand, and then the temperature is raised until it is dry, thereby obtaining a modified inorganic carrier powder loaded with both chromium and titanium elements;

[0021] (4) 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 calcination temperature of 300 to 950° C. for 0.5 to 48 hours to obtain the chromium-based polyethylene catalyst.

[0022] Preferably, step (1) adopts an impregnation method for blending modification.

[0023] Preferably, step (2) is heated and allowed to stand at 50°C to 110°C.

[0024] Preferably, the chromium salt in step (2) is one or more of chromium acetate, dichromate, chromate, chromium nitrate or chromium oxide.

[0025] Preferably, the content of chromium in the chromium salt in step (2) accounts for 0.1 to 5.0% of the mass of the modified inorganic carrier.

[0026] Preferably, the solvent in step (2) is water, an alcohol solvent or a ketone solvent.

[0027] More preferably, the alcohol solvent is methanol, ethanol or propanol.

[0028] More preferably, the ketone solvent is acetone or butanone.

[0029] Preferably, after step (3) is fully mixed, the mixture is allowed to stand at 0-50°C for 0-4 hours, and then the temperature is raised to 60-80°C until it is dry.

[0030] Preferably, the organic titanium compound in step (3) is at least one of tetraisopropyl titanium, tetraethyl titanium, tetrabutyl titanium, tetraisooctyl titanate, and n-butyl titanate.

[0031] Preferably, the co-catalyst in step (3) is at least one of triethylaluminum, triisobutylaluminum, and diethylaluminum chloride.

[0032] Preferably, the solvent of the organic titanium compound solution in step (3) is water, an alcohol solvent or a ketone solvent.

[0033] More preferably, the alcohol solvent is methanol, ethanol or propanol.

[0034] More preferably, the ketone solvent is acetone or butanone.

[0035] Preferably, the content of titanium in the organic titanium compound in step (3) accounts for 0.1-10% of the mass of the modified inorganic carrier loaded with chromium.

[0036] Preferably, the mass ratio of the co-catalyst in step (3) to the modified inorganic carrier loaded with chromium element is (0.1-5):100.

[0037] The present invention has discovered through research that a catalyst obtained by sequentially loading a chromium-containing compound and then a titanium-containing compound on an inorganic carrier modified with alkylaluminum and / or alkoxyaluminum and then calcining the catalyst at high temperature can, under the synergistic action of titanium, chromium, and aluminum, produce a high polymerization activity, and can increase the polymerization activity to above 3000 gPE / gCAT / h.

[0038] Furthermore, the catalyst using the specific treatment method of the present invention can greatly stimulate the reaction activity of the loaded chromium under the synergistic effect of specific proportions of titanium, chromium, and aluminum, resulting in extremely high polymerization activity, which can be increased to 8,000 to 20,000 gPE / gCAT / h.

[0039] The present invention also discovered that when the effective concentration of chromium loading reaches 3wt% or above, polymerization activity is significantly improved. However, in order to effectively load chromium at a high concentration and act as a good active center, the support must first be treated with alkyl aluminum and / or alkoxy aluminum, and then, after loading the chromium, a titanium compound must be loaded from outside the system. If only the support itself contains aluminum, titanium, and other elements, or if there are differences in the support treatment steps, or if aluminum, titanium, and other elements are introduced by other means, even if a large amount of chromium salt is loaded, it will not effectively act as a good active center and will not improve polymerization activity.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] 1. The present invention provides a highly active chromium-based polyethylene and a preparation method thereof, which significantly enhances the polymerization activity of the chromium-based polyethylene, reduces production costs, and improves product cleanliness.

[0042] 2. The present invention provides a highly active chromium-based catalyst that can reduce the polymerization cost of chromium-based polyethylene resins and lower the content of impurities introduced by the catalyst in the resin product;

[0043] 3. The chromium-based catalyst prepared by this invention has an activity increased by more than 10 times, significantly reducing the production cost of chromium-based polyethylene products, lowering the impurity content of the products, and improving product quality;

[0044] 4. Compared with other existing methods for preparing chromium-based polyethylene catalysts, the catalyst preparation method of the present invention is simple and has industrial potential. The activity of the resulting catalyst is significantly improved, and it has good market prospects.

[0045] 5. The present invention, through the synergistic effect of titanium, chromium, and aluminum, can greatly stimulate the reactive activity of the loaded chromium, resulting in extremely high activity in the polymerization reaction, thereby improving the polymerization activity;

[0046] 6. The present invention utilizes the steps of modification of inorganic carriers with alkyl aluminum and / or alkoxy aluminum, chromium loading, titanium loading, and high-temperature calcination to effectively load the chromium element at a high concentration, thereby acting as a good active center and improving polymerization activity. DETAILED DESCRIPTION

[0047] 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.

[0048] 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.

[0049] A method for preparing a chromium-based high-activity polyethylene catalyst comprises the following steps:

[0050] (1) Support modification: using alkyl aluminum and / or alkoxy aluminum to modify the inorganic support;

[0051] (2) Chromium loading: a chromium salt is prepared into a solution using a solvent, mixed with the modified carrier, and after thorough mixing, heated at 50°C to 110°C and allowed to stand until dry to obtain a chromium-loaded modified carrier powder;

[0052] (3) Titanium loading: The modified carrier 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 heated to 60-80°C until dried to obtain a modified carrier powder loaded with both chromium and titanium.

[0053] (4) High-temperature calcination: The modified dry carrier is further calcined in a dry non-reducing gas environment at a calcination temperature of 300 to 950°C for 0.5 to 48 hours to obtain a chromium-based polyethylene catalyst.

[0054] The following describes it in detail with reference to specific embodiments.

[0055] Catalyst evaluation method: A 2L stainless steel reactor equipped with a water circulation temperature control system was used for ethylene pressure polymerization. Nitrogen was introduced into the reactor. Under a nitrogen atmosphere, 1L of n-hexane and the catalyst were added. The nitrogen was then replaced three times with ethylene. The ethylene inlet valve was adjusted to maintain a constant ethylene pressure of 1 MPa. Polymerization was conducted at 90°C. After the reaction, the polyethylene product was dried to a constant weight and weighed. The catalyst activity was then calculated.

[0056] Catalyst activity is expressed as the weight of polyethylene produced per gram of catalyst per hour (gPE / gCAT / h).

[0057] The silica gel used in the examples and comparative examples is silica gel 3020 and 955 produced by GRACE, and the specific properties are as follows:

[0058] Silica gel 3020: particle size: 20-45 μm, specific surface area: 300 m² / g, pore volume: 1.6 mL / g, pore diameter: 15-20 nm.

[0059] Silica gel 955: particle size: 35-70 μm, specific surface area: 280-300 m² / g, pore volume: 1.65 mL / g.

[0060] The specific properties of K10 montmorillonite and K30 montmorillonite used in the examples are as follows:

[0061] K10 montmorillonite: particle size: <2μm, specific surface area: 220-270 m² / g, pore volume: 0.3-0.4 mL / g.

[0062] K30 montmorillonite: particle size: <2μm, specific surface area: 200-240 m² / g, pore volume: 0.25-0.35 mL / g.

[0063] The specific properties of the clay inorganic carrier in the embodiment are as follows:

[0064] Kaolin: particle size: 0.1-4μm, specific surface area: 15-30 m² / g, pore volume: 0.2-0.3 mL / g.

[0065] Example 1

[0066] Triisobutylaluminum and silica gel 3020 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 mass. After heating the mixture at 50°C and allowing it to stand for 10 hours, the mixture formed a dry powder.

[0067] 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. The above mixture 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.

[0068] The dried powder was calcined at 300°C for 48 hours to obtain a chromium-based polyethylene catalyst. Polymerization evaluation of the catalyst revealed a polymerization activity of 3000 gPE / gCAT / h.

[0069] Example 2

[0070] Tri-n-hexylaluminum and silica gel 955 were blended at a mass ratio of 1:100 for modification. The modified support was then blended with ethanol and potassium dichromate at a mass ratio of 5:1, with the chromium content in the potassium dichromate being 1% of the modified support's mass. After heating the mixture at 60°C and allowing it to stand for 6 hours, it formed a dry powder.

[0071] The above powder was blended with tetraisooctyl titanate, propanol, and triethylaluminum. The titanium content of tetraisopropyl titanium accounted for 4% of the powder mass, the triethylaluminum accounted for 1% of the powder mass, and the powder mass accounted for 10% of the solvent. The above blend was allowed to stand at 30°C for 2 hours, then heated to 80°C and held for 1 hour until it became a dry powder.

[0072] The dried powder was calcined at 500°C for 8 hours to obtain a chromium-based polyethylene catalyst. Polymerization evaluation of the catalyst revealed a polymerization activity of 3500 gPE / gCAT / h.

[0073] Example 3

[0074] Diethylaluminum ethoxide and K10 montmorillonite were blended at a mass ratio of 10:100 for modification. The modified support was then blended with ethanol and potassium dichromate at a mass ratio of 5:1, with the chromium content in the potassium dichromate being 5% of the modified support mass. The mixture was heated at 110°C and allowed to stand for 1 hour, resulting in a dry powder.

[0075] The above powder was blended with tetrabutyl titanium, acetone, and triethylaluminum, with the titanium content of tetraisopropyl titanium accounting for 1% of the powder mass, the triethylaluminum accounting for 2% of the powder mass, and the powder mass accounting for 10% of the solvent mass. The mixture was allowed to stand at 10°C for 2 hours, then heated to 80°C and held for 1 hour until it became a dry powder.

[0076] The dried powder was calcined at 400°C for 6 hours to obtain a chromium-based polyethylene catalyst. Polymerization evaluation of the catalyst revealed a polymerization activity of 5500 gPE / gCAT / h.

[0077] Example 4

[0078] Dimethylaluminum ethoxide and K30 montmorillonite were blended at a mass ratio of 3:100 for modification. Ethanol and chromium acetate were mixed at a mass ratio of 3:1 and then blended with the modified support. The chromium content in the potassium dichromate was 2% of the modified support mass. After heating the mixture at 100°C and allowing it to stand for 2 hours, it formed a dry powder.

[0079] The above powder was blended with tetrabutyl titanium, butanone, and triethyl aluminum. The titanium content of tetraisopropyl titanium accounted for 1% of the powder mass, the triethyl aluminum accounted for 2% of the powder mass, and the powder mass accounted for 10% of the solvent mass. The above mixture was allowed to stand at 20°C for 3 hours, then heated to 80°C and held for 1 hour until it became a dry powder.

[0080] The dried powder was calcined at 950°C for 6 hours to obtain a chromium-based polyethylene catalyst. Polymerization evaluation of the catalyst revealed a polymerization activity of 4000 gPE / gCAT / h.

[0081] Example 5

[0082] A mixture of diisobutylaluminum ethoxide and methylethylaluminum ethoxide was blended with kaolin for modification. The weight ratio of the two mixtures was 1:1, and the mixture accounted for 5% of the weight of the kaolin. Acetone and chromium nitrate were mixed at a weight ratio of 3:1 and then blended with the modified support. The chromium content of the potassium dichromate was 2% of the weight of the modified support. After heating the mixture at 100°C and allowing it to stand for 2 hours, the mixture became a dry powder.

[0083] A blend of tetraethyl hexyl titanate and n-butyl titanate was prepared at a 1:1 mass ratio. This powder was then blended with a titanium-containing blend, butanone, and triethylaluminum. The titanium content of the titanium-containing blend accounted for 2% of the powder mass, the triethylaluminum accounted for 5% of the powder mass, and the powder accounted for 10% of the solvent mass. The mixture was allowed to stand at 20°C for 1 hour, then heated to 80°C and held for 2 hours until it became a dry powder.

[0084] The dried powder was calcined at 500°C for 5 hours to obtain a chromium-based polyethylene catalyst. The catalyst was evaluated for polymerization activity, which reached 5000 gPE / gCAT / h.

[0085] Comparative Example 1

[0086] The same carrier modification, chromium loading and calcination methods as in Example 1 were used, but titanium loading was not performed. The remaining preparation conditions were the same as in Example 1 to obtain a catalyst. The obtained catalyst was evaluated to have an activity of 350 gPE / gCAT / h.

[0087] Comparative Example 2

[0088] Based on the carrier modification of Example 1, titanium loading was first performed and then chromium loading was performed, and then high-temperature calcination was performed. The other preparation conditions were the same as those of Example 1. The obtained catalyst was evaluated as inactive by polymerization.

[0089] Comparative Example 3

[0090] The support was not modified with alkyl aluminum, and the other preparation conditions were the same as those in Example 1. The obtained catalyst was evaluated by polymerization, and the activity was 200 gPE / gCAT / h.

[0091] Comparative Example 4

[0092] In Example 1, the support was not modified with alkyl aluminum. Instead, a mixture of ethanol and potassium dichromate at a mass ratio of 5:1 was blended with the modified support. Triisobutylaluminum was then blended with the mixture at a mass ratio of 0.1:600. All other preparation conditions were the same as in Example 1. The resulting catalyst was evaluated by polymerization and found to be inactive.

[0093] Table 1 Key parameters and performance data of each embodiment and comparative example

[0094]

[0095] The table above shows that using aluminum alkyls or aluminum alkoxides (such as triisobutylaluminum and diethylaluminum ethoxide) to modify the support significantly improves catalyst activity. The selection of chromium and titanium sources and their loading order have a significant impact on catalyst activity. Failure to load titanium (Comparative Example 1) or unmodified support (Comparative Example 3) significantly reduces catalyst activity. Loading titanium before chromium (Comparative Example 2) completely deactivates the catalyst.

[0096] 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 high-activity chromium-based polyethylene catalyst, characterized in that: It comprises an inorganic carrier modified with alkyl aluminum and / or alkoxy aluminum and chromium and titanium elements sequentially loaded on the modified inorganic carrier; The mass ratio of the alkyl aluminum and / or alkoxy aluminum to the inorganic support is (0.1-10):100, the content of chromium element accounts for 0.1-5.0% of the mass of the modified inorganic support, and the content of titanium element accounts for 0.1-10% of the mass of the modified inorganic support loaded with chromium element; The preparation method of the high-activity chromium-based polyethylene catalyst comprises the following steps: (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, and the solution is mixed with the modified inorganic carrier. After thorough mixing, the solution is heated and allowed to stand until dry to obtain a modified inorganic carrier powder loaded with chromium. (3) Titanium loading: The modified inorganic carrier powder loaded with chromium is further mixed with an organic titanium compound solution and a co-catalyst, and after thorough mixing, the mixture is allowed to stand, and then the temperature is raised until dry, thereby obtaining a modified inorganic carrier powder loaded with both chromium and titanium elements; (4) 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 the chromium-based polyethylene catalyst; The chromium salt in step (2) is one or more of chromium acetate, dichromate, chromate, chromium nitrate or chromium oxide; The solvent in step (2) is an alcohol solvent or a ketone solvent; The co-catalyst in step (3) is at least one of triethylaluminum, triisobutylaluminum, and diethylaluminum chloride; The solvent of the organic titanium compound solution in step (3) is an alcohol solvent or a ketone solvent.

2. The high-activity chromium-based polyethylene catalyst according to claim 1, characterized in that The inorganic carrier is selected from at least one of silicon dioxide, aluminum oxide, silica gel, aluminum phosphate, titanium dioxide, zirconium oxide, magnesium oxide, calcium oxide, and inorganic clay.

3. The high-activity chromium-based polyethylene catalyst according to claim 1, characterized in that The average particle size of the inorganic carrier is 0.1 micron to 100 microns, and the pore volume is 0.2 cm 3 / g~10.0cm 3 / g, specific surface area of ​​15m 2 / g~1000m 2 / g.

4. The high-activity chromium-based polyethylene catalyst according to claim 1, characterized in that The alkylaluminum and / or alkoxyaluminum is selected from at least one of triisobutylaluminum, tri-n-hexylaluminum, triisopropylaluminum, methyldiethylaluminum, trimethylaluminum, diethylaluminum ethoxide, diethylaluminum methoxide, dimethylaluminum ethoxide, diisopropylaluminum ethoxide, diethylaluminum propoxide, diisobutylaluminum ethoxide and methylethylaluminum ethoxide.

5. The high-activity chromium-based polyethylene catalyst according to claim 1, characterized in that Step (2) heating and standing at 50°C to 110°C; After step (3) is fully mixed, the mixture is allowed to stand at 0-50° C. for 0-4 hours, and then the temperature is raised to 60-80° C. until it is dry.

6. The high-activity chromium-based polyethylene catalyst according to claim 1, characterized in that The organic titanium compound in step (3) is at least one of tetraisopropyl titanium, tetraethyl titanium, tetrabutyl titanium, tetraisooctyl titanate, and n-butyl titanate.

7. The high-activity chromium-based polyethylene catalyst according to claim 1, characterized in that The content of chromium in the chromium salt in step (2) accounts for 0.1 to 5.0% of the mass of the modified inorganic carrier; The content of titanium in the organic titanium compound in step (3) accounts for 0.1 to 10% of the mass of the modified inorganic carrier loaded with chromium; The mass ratio of the co-catalyst in step (3) to the modified inorganic carrier loaded with chromium element is (0.1-5):100.

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