Zirconium-based double-active-center catalyst as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of polyethylene catalysts, and particularly relates to a zirconium-based double-active-center catalyst as well as a preparation method and application thereof. The invention relates to a zirconium-based double-active-center catalyst, which is characterized in that a heterogeneous catalyst for successfully fixing iron and zirconium ions between clay layers is successfully prepared by using iron ion or zirconium ion exchanged clay and a ligand raw material as reactants through a one-pot method, and then the heterogeneous catalyst is mixed to prepare the zirconium-based double-active-center catalyst. The zirconium-based double-active-center catalyst has relatively high catalytic activity and can be used for preparing single-kettle bimodal polyethylene.

Description

Technical Field

[0001] The present invention belongs to the technical field of polyethylene catalysts, and particularly relates to a zirconium-based dual-active-center catalyst, a preparation method thereof, and an application thereof. Background Art

[0002] Polyethylene is the most widely used plastic in the world, accounting for 39% of the most common thermoplastics and 62% of polyolefins respectively. Currently, polyethylene is mainly divided into low-density polyethylene, high-density polyethylene, and linear low-density polyethylene. High-density polyethylene is widely used in packaging films, blow molding containers, extrusion pipes, etc. due to its high strength, good rigidity, and easy processing. Its market share occupies more than 50% of the polyethylene market. In the past 20 - 30 years, bimodal polyethylene, as a kind of high-performance polymer product, has developed rapidly and has become the high-density polyethylene variety with the fastest growth rate. Bimodal polyethylene products have many advantages such as good processing performance and mechanical properties, and both market attention and product added value play an important role in polyethylene products.

[0003] Currently, the preparation of bimodal polyethylene mainly includes: melt blending method, series reactor method, and single reactor method. Among them, in the melt blending method, parallel reactors are used to produce polymers with larger and smaller molecular weights respectively, and then the two polymers with different molecular weights are melt-blended in proportion to obtain bimodal polyethylene. However, it is difficult to ensure the homogeneity of the polymer mixture by this method. The series reactor method is to connect two reactors in series and control the polymerization reaction conditions of different reactors to prepare polyethylene with a bimodal molecular weight distribution. The series reactor method is the main polymerization process currently used, but this polymerization process is relatively complex and the cost is high. The single reactor method uses a composite catalyst. One metal center in the bimetallic catalyst has a lower chain transfer rate and better copolymerization performance, and can synthesize high molecular weight ethylene / α-olefin copolymers, while the other metal center has a faster chain transfer rate and poorer copolymerization performance, and can synthesize low molecular weight ethylene homopolymers, so that it is possible to synthesize polyethylene products with a bimodal molecular weight distribution using only one reactor. Compared with the melt blending method and the series reactor method, the single reactor method has a simple polymerization process, lower investment in process equipment and energy consumption, more convenient operation, good product homogeneity, and great production potential. Therefore, it has become a research hotspot at home and abroad and is gradually moving towards industrial application. Summary of the Invention

[0004] In view of the above problems, the present invention provides a zirconium-based dual-active-center catalyst, a preparation method thereof, and an application thereof, providing more possibilities for the single-pot synthesis of bimodal polyethylene.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a zirconium-based dual-active-site catalyst, comprising a zirconium-based catalyst and an iron-based catalyst, and the mass ratio of the zirconium-based catalyst to the iron-based catalyst is 1:(0.3 - 1.8).

[0006] In a second aspect, the present invention provides a method for preparing the zirconium-based dual-active-site catalyst, comprising the following steps: S1, preparing zirconium-based clay and iron-based clay respectively by an ion exchange method; S2, combining the zirconium-based clay and the iron-based clay with corresponding ligands respectively to prepare a zirconium-based catalyst and an iron-based catalyst; S3, mixing the zirconium-based catalyst and the iron-based catalyst evenly to obtain the zirconium-based dual-active-site catalyst.

[0007] Further, the specific preparation method of the zirconium-based clay in S1 is as follows: Under stirring, slowly add sodium-based clay into a zirconium tetrachloride solution, stir and react until the sodium ion concentration in the reaction solution no longer increases significantly, filter, wash the filter cake with deionized water by stirring for multiple times, and dry to obtain the zirconium-based clay; the dosage ratio of the sodium-based clay to the zirconium tetrachloride solution is (1 - 1.5) g:(10 - 25) mL; the concentration of zirconium in the zirconium tetrachloride solution is 0.3 - 0.5 mol / L; the reaction temperature is 50 - 80 °C, the drying temperature is 80 - 100 °C, and the stirring speed is 100 - 300 r / min.

[0008] Further, the specific preparation method of the iron-based clay in S1 is as follows: Under stirring, slowly add sodium-based clay into a ferric chloride solution, stir and react until the sodium ion concentration in the reaction solution no longer increases significantly, filter, wash the filter cake with deionized water by stirring for multiple times, and dry to obtain the iron-based clay; the dosage ratio of the sodium-based clay to the ferric chloride solution is (1 - 1.5) g:(15 - 30) mL; the concentration of iron in the ferric chloride solution is 0.4 - 0.8 mol / L, the reaction temperature is 50 - 80 °C, the drying temperature is 80 - 100 °C, and the stirring speed is 100 - 300 r / min.

[0009] Further, the sodium-based clay in S1 is one of sodium-based montmorillonite, sodium-based saponite, and sodium mica.

[0010] Further, the specific preparation method of the zirconium-based catalyst in S2 is as follows: Under a nitrogen atmosphere, add the zirconium-based clay, acenaphthenequinone, and 8-aminoquinoline to a solvent, heat and react for 100 - 120 h, filter, wash the filter cake, and dry to obtain the zirconium-based catalyst; the dosage ratio of the zirconium-based clay, acenaphthenequinone, 8-aminoquinoline, and the solvent is (1 - 1.5) g:(2 - 2.5) mmol:(4 - 5) mmol:(10 - 20) mL.

[0011] Further, the specific preparation method of the iron-based catalyst in S2 is as follows: under a nitrogen atmosphere, iron-based clay, acenaphthenequinone, and 8-aminoquinoline are added to a solvent, and heated and reacted for 100-120 h, filtered, the filter cake is washed, and dried to obtain the iron-based catalyst; the dosage ratio of the iron-based clay, 2,6-diacetylpyridine, 2,4,6-trimethylaniline, and the solvent is (1-1.5) g:(1.8-2.2) mmol:(3.6-4.4) mmol:(10-20) mL.

[0012] Further, the solvents are all ethanol or acetonitrile.

[0013] Further, before performing the operation of S2, both the zirconium-based clay and the iron-based clay are activated at 200-250 °C for 4-8 h.

[0014] Thirdly, the present invention provides the application of the zirconium-based dual active center catalyst for the single-pot production of bimodal polyethylene.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention obtains bimodal polyethylene by polymerizing ethylene in a single reactor with a mixed catalyst composed of two heterogeneous catalysts in which zirconium and iron complexes are fixed between the layers of layered clay minerals; the bimodal polyethylene is a mixture of linear low-density polyethylene produced by the zirconium-based catalyst and high-density polyethylene produced by the iron-based catalyst.

[0016] The present invention uses a one-pot method with clay and ligand raw materials exchanged with iron ions and zirconium ions as reactants, and successfully prepares a heterogeneous catalyst in which metal ions are successfully fixed between the clay layers. After mixing it to prepare a zirconium-based dual active center catalyst, it is used for the preparation of single-pot bimodal polyethylene, and the catalyst has high activity. Description of the Drawings

[0017] Figure 1 is the preparation route diagram of the zirconium-based dual active center catalyst; Figure 2 is the XRD pattern of the catalyst prepared in Example 3; Figure 3 is the DSC curve of polyethylene prepared by different methods, where (a) is the polyethylene prepared by the iron-based catalyst, (b) is the polyethylene prepared by the zirconium-based catalyst; (c) is the polyethylene prepared by the zirconium-based dual active center catalyst; (d) is P-mix-PE; (e) is M-mix-PE. Detailed Embodiments

[0018] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0019] Example 1 A zirconium-based dual-active-site catalyst includes a zirconium-based catalyst and an iron-based catalyst; the mass ratio of the zirconium-based clay catalyst to the iron-based clay catalyst is 1:0.3.

[0020] The preparation method of the zirconium-based dual-active-site catalyst includes the following steps: S1.1, Prepare zirconium-based clay: Under the stirring state at a speed of 100 r / min, slowly add 1 g of sodium-based clay (the used sodium-based clay is sodium-based montmorillonite) to 10 mL of zirconium tetrachloride solution with a concentration of 0.3 mol / L, heat up to 50 °C, continue stirring and reacting until the sodium ion concentration in the reaction solution no longer increases significantly, filter, wash the filter cake with deionized water by stirring for multiple times, and dry at 80 °C to obtain zirconium-based clay; S1.2, Prepare iron-based clay: Under the stirring state at a speed of 100 r / min, slowly add 1 g of sodium-based clay (the used sodium-based clay is sodium-based saponite) to 15 mL of zirconium tetrachloride solution with a concentration of 0.4 mol / L, heat up to 50 °C, continue stirring and reacting until the sodium ion concentration in the reaction solution no longer increases significantly, filter, wash the filter cake with deionized water by stirring for multiple times, and dry at 80 °C to obtain zirconium-based clay; S2.1, Prepare zirconium-based catalyst: Under a nitrogen atmosphere, calcine and activate the zirconium-based clay at 200 °C in vacuum for 4 h, and then cool it to room temperature; add the activated zirconium-based clay, acenaphthenequinone, and 8-aminoquinoline to the solvent ethanol, react at 70 °C for 100 h, filter, wash the filter cake with the reaction solvent, toluene, and hexane in sequence, and dry at 80 °C to obtain the zirconium-based catalyst; the dosage ratio of the zirconium-based clay, acenaphthenequinone, 8-aminoquinoline, and the solvent is 1 g:2 mmol:4 mmol:10 mL; S2.2, Prepare iron-based catalyst: Under a nitrogen atmosphere, calcine and activate the iron-based clay at 200 °C in vacuum for 4 h, and then cool it to room temperature; add the activated iron-based clay, 2,6-diacetylpyridine, and 2,4,6-trimethylaniline to the solvent ethanol, react at 70 °C for 100 h, filter, wash the filter cake, and dry to obtain the iron-based catalyst; the dosage ratio of the iron-based clay, 2,6-diacetylpyridine, 2,4,6-trimethylaniline, and the solvent is 1 g:1.8 mmol:3.6 mmol:10 mL; S3. Prepare the zirconium-based dual-active-site catalyst: Grind and mix the zirconium-based catalyst and the iron-based catalyst evenly to obtain the zirconium-based dual-active-site catalyst.

[0021] Example 2 A zirconium-based dual-active-site catalyst, comprising a zirconium-based catalyst and an iron-based catalyst; the mass ratio of the zirconium-based clay catalyst to the iron-based clay catalyst is 1:0.8.

[0022] The preparation method of the zirconium-based dual-active-site catalyst described above includes the following steps: S1.1. Prepare zirconium-based clay: Under the stirring state at a speed of 150 r / min, slowly add 1.1 g of sodium-based clay (the sodium-based clay used is sodium-based saponite) to 14 mL of zirconium tetrachloride solution with a concentration of 0.4 mol / L, heat up to 55 °C, and continue stirring and reacting until the sodium ion concentration in the reaction solution no longer increases significantly. Filter, stir and wash the filter cake with deionized water for multiple times, and dry at 85 °C to obtain zirconium-based clay. S1.2. Prepare iron-based clay: Under the stirring state at a speed of 150 r / min, slowly add 1.1 g of sodium-based clay (the sodium-based clay used is sodium-based saponite) to 18 mL of zirconium tetrachloride solution with a concentration of 0.5 mol / L, heat up to 55 °C, and continue stirring and reacting until the sodium ion concentration in the reaction solution no longer increases significantly. Filter, stir and wash the filter cake with deionized water for multiple times, and dry at 85 °C to obtain zirconium-based clay. S2.1. Prepare the zirconium-based catalyst: Under a nitrogen atmosphere, calcine and activate the zirconium-based clay in a vacuum at 210 °C for 5 h, and then cool to room temperature; add the activated zirconium-based clay, acenaphthenequinone, and 8-aminoquinoline to the solvent ethanol, react at 72 °C for 100 h, filter, wash the filter cake successively with the reaction solvent, toluene, and hexane, and dry at 85 °C to obtain the zirconium-based catalyst; the dosage ratio of the zirconium-based clay, acenaphthenequinone, 8-aminoquinoline, and the solvent is 1.1 g:2.1 mmol:4.2 mmol:12 mL. S2.2. Prepare the iron-based catalyst: Under a nitrogen atmosphere, calcine and activate the iron-based clay in a vacuum at 210 °C for 5 h, and then cool to room temperature; add the activated iron-based clay, 2,6-diacetylpyridine, and 2,4,6-trimethylaniline to the solvent ethanol, react at 72 °C for 100 h, filter, wash the filter cake, and dry to obtain the iron-based catalyst; the dosage ratio of the iron-based clay, 2,6-diacetylpyridine, 2,4,6-trimethylaniline, and the solvent is 1.1 g:1.9 mmol:3.8 mmol:12 mL. S3. Prepare the zirconium-based dual-active-site catalyst: Grind and mix the zirconium-based catalyst and the iron-based catalyst evenly to obtain the zirconium-based dual-active-site catalyst.

[0023] Example 3 A zirconium-based dual-active-site catalyst, comprising a zirconium-based catalyst and an iron-based catalyst; the mass ratio of the zirconium-based clay catalyst to the iron-based clay catalyst is 1:1.

[0024] The preparation method of the described zirconium-based dual-active-site catalyst (as Figure 1 shown) includes the following steps: S1.1, Prepare zirconium-based clay: Under the stirring state at a speed of 200 r / min, slowly add 1.3 g of sodium-based clay (the sodium-based clay used is sodium mica) to 18 mL of zirconium tetrachloride solution with a concentration of 0.4 mol / L, heat up to 65 °C, continue stirring and reacting until the sodium ion concentration in the reaction solution no longer increases significantly, filter, wash the filter cake with deionized water by stirring for multiple times, and dry at 90 °C to obtain zirconium-based clay; S1.2, Prepare iron-based clay: Under the stirring state at a speed of 200 r / min, slowly add 1.3 g of sodium-based clay (the sodium-based clay used is sodium mica) to 22 mL of zirconium tetrachloride solution with a concentration of 0.6 mol / L, heat up to 65 °C, continue stirring and reacting until the sodium ion concentration in the reaction solution no longer increases significantly, filter, wash the filter cake with deionized water by stirring for multiple times, and dry at 90 °C to obtain zirconium-based clay; S2.1, Prepare zirconium-based catalyst: Under a nitrogen atmosphere, calcine and activate the zirconium-based clay in vacuum at 225 °C for 6 h, and then cool to room temperature; add the activated zirconium-based clay, acenaphthenequinone, and 8-aminoquinoline to the solvent acetonitrile, react at 75 °C for 110 h, filter, wash the filter cake successively with the reaction solvent, toluene, and hexane, and dry at 100 °C to obtain the zirconium-based catalyst; the dosage ratio of the zirconium-based clay, acenaphthenequinone, 8-aminoquinoline, and the solvent is 1.2 g: 2.3 mmol: 4.6 mmol: 15 mL; S2.2, Prepare iron-based catalyst: Under a nitrogen atmosphere, calcine and activate the iron-based clay in vacuum at 225 °C for 6 h, and then cool to room temperature; add the activated iron-based clay, 2,6-diacetylpyridine, and 2,4,6-trimethylaniline to the solvent acetonitrile, react at 75 °C for 110 h, filter, wash the filter cake, and dry to obtain the iron-based catalyst; the dosage ratio of the iron-based clay, 2,6-diacetylpyridine, 2,4,6-trimethylaniline, and the solvent is 1.2 g: 2 mmol: 4 mmol: 15 mL; S3, Prepare zirconium-based dual-active-site catalyst: Grind and mix the zirconium-based catalyst and the iron-based catalyst evenly to obtain the zirconium-based dual-active-site catalyst.

[0025] The prepared iron-based clay and zirconium-based clay were tested by X-ray fluorescence spectroscopy, and the exchange amounts of iron ions and zirconium ions were calculated. The results showed that the exchange amount of zirconium ions was 0.52 mmol / g, and the exchange amount of iron ions was 0.48 mmol / g; this indicated that zirconium ions and iron ions were successfully exchanged into the interlayers of sodium-based clay, realizing the replacement of interlayer cations and providing a basis for the subsequent preparation of zirconium-based catalysts and iron-based catalysts.

[0026] Figure 2 XRD patterns of the prepared catalysts are shown. From Figure 2 it can be seen that compared with the iron-based clay and zirconium-based clay without inserted ligands, the XRD patterns of the iron-based catalyst and zirconium-based catalyst showed a shift to the low-angle side and an increase in peak intensity. Calculated from the diffraction angle of the basal reflection peak, the basal plane spacing of the zirconium-based catalyst was 1.40 nm, and the basal plane spacing of the iron-based catalyst was 1.45 nm. This was because the iron-based ligand and zirconium-based ligand were inserted into the interlayers of the clay, the interlayer spacing would expand, resulting in the shift of the basal reflection peak to the low-angle side, and the stacking regularity of the layers was improved, increasing the peak intensity, indicating that the target catalysts were successfully prepared.

[0027] The specific surface areas of sodium-based clay, the prepared iron-based clay and zirconium-based clay were measured. The results showed that the specific surface area of sodium-based clay was 1.6 m 2 / g, the specific surface area of zirconium-based clay was 3.5 m 2 / g, and the specific surface area of zirconium-based clay was 2.9 m 2 / g. Although the specific surface area increased after cation exchange, the increase amplitude was not large. This indicated that the cation exchange process did not cause obvious damage to the crystal structure of sodium-based clay.

[0028] Example 4 A zirconium-based dual-active center catalyst, comprising a zirconium-based catalyst and an iron-based catalyst; the mass ratio of the zirconium-based clay catalyst to the iron-based clay catalyst is 1:1.5.

[0029] The preparation method of the described zirconium-based dual-active center catalyst comprises the following steps: S1.1, preparing zirconium-based clay: Under the stirring state at a speed of 250 r / min, 1.4 g of sodium-based clay (the used sodium-based clay is sodium-based saponite) was slowly added to 22 mL of zirconium tetrachloride solution with a concentration of 0.5 mol / L, the temperature was raised to 70 °C, and stirring was continued until the sodium ion concentration in the reaction solution no longer increased significantly. After filtration, the filter cake was washed with deionized water by stirring for multiple times and dried at 95 °C to obtain zirconium-based clay; S1.2. Preparation of iron-based clay: Under the stirring condition at a speed of 250 r / min, 1.4 g of sodium-based clay (the sodium-based clay used is sodium-based saponite) was slowly added to 26 mL of zirconium tetrachloride solution with a concentration of 0.7 mol / L, the temperature was raised to 70 °C, and the stirring reaction was continued until the sodium ion concentration in the reaction solution no longer increased significantly. After filtration, the filter cake was washed by stirring with deionized water for multiple times and dried at 95 °C to obtain zirconium-based clay; S2.1. Preparation of zirconium-based catalyst: Under a nitrogen atmosphere, the zirconium-based clay was calcined and activated under vacuum at 240 °C for 7 h, and then cooled to room temperature; The activated zirconium-based clay, acenaphthenequinone, and 8-aminoquinoline were added to the solvent acetonitrile, and the reaction was carried out at 75 °C for 115 h. After filtration, the filter cake was washed successively with the reaction solvent, toluene, and hexane and dried at 110 °C to obtain the zirconium-based catalyst; The dosage ratio of the zirconium-based clay, acenaphthenequinone, 8-aminoquinoline, and the solvent is 1.3 g: 2.4 mmol: 4.8 mmol: 18 mL; S2.2. Preparation of iron-based catalyst: Under a nitrogen atmosphere, the iron-based clay was calcined and activated under vacuum at 240 °C for 7 h, and then cooled to room temperature; The activated iron-based clay, 2,6-diacetylpyridine, and 2,4,6-trimethylaniline were added to the solvent acetonitrile, and the reaction was carried out at 75 °C for 115 h. After filtration, the filter cake was washed and dried to obtain the iron-based catalyst; The dosage ratio of the iron-based clay, 2,6-diacetylpyridine, 2,4,6-trimethylaniline, and the solvent is 1.3 g: 2.1 mmol: 4.2 mmol: 18 mL; S3. Preparation of zirconium-based dual-active-center catalyst: The zirconium-based catalyst and the iron-based catalyst were ground and mixed evenly to obtain the zirconium-based dual-active-center catalyst.

[0030] Example 5 A zirconium-based dual-active-center catalyst includes a zirconium-based catalyst and an iron-based catalyst; The mass ratio of the zirconium-based clay catalyst to the iron-based clay catalyst is 1:1.8.

[0031] The preparation method of the described zirconium-based dual-active-center catalyst includes the following steps: S1.1. Preparation of zirconium-based clay: Under the stirring condition at a speed of 300 r / min, 1.5 g of sodium-based clay (the sodium-based clay used is sodium-based montmorillonite) was slowly added to 25 mL of zirconium tetrachloride solution with a concentration of 0.5 mol / L, the temperature was raised to 80 °C, and the stirring reaction was continued until the sodium ion concentration in the reaction solution no longer increased significantly. After filtration, the filter cake was washed by stirring with deionized water for multiple times and dried at 100 °C to obtain zirconium-based clay; S1.2, Preparation of iron-based clay: Under the stirring state at a speed of 300 r / min, 1.5 g of sodium-based clay (the sodium-based clay used is sodium-based montmorillonite) was slowly added to 30 mL of zirconium tetrachloride solution with a concentration of 0.8 mol / L, the temperature was raised to 80 °C, and stirring was continued until the sodium ion concentration in the reaction solution no longer increased significantly. After filtration, the filter cake was washed with deionized water by stirring multiple times and dried at 100 °C to obtain zirconium-based clay; S2.1, Preparation of zirconium-based catalyst: Under a nitrogen atmosphere, the zirconium-based clay was calcined and activated in vacuum at 250 °C for 8 h, and then cooled to room temperature; the activated zirconium-based clay, acenaphthenequinone, and 8-aminoquinoline were added to the solvent ethanol, and the reaction was carried out at 78 °C for 120 h. After filtration, the filter cake was washed with the reaction solvent, toluene, and hexane in sequence and dried at 120 °C to obtain the zirconium-based catalyst; the dosage ratio of the zirconium-based clay, acenaphthenequinone, 8-aminoquinoline, and the solvent was 1.5 g: 2.5 mmol: 5 mmol: 20 mL; S2.2, Preparation of iron-based catalyst: Under a nitrogen atmosphere, the iron-based clay was calcined and activated in vacuum at 250 °C for 8 h, and then cooled to room temperature; the activated iron-based clay, 2,6-diacetylpyridine, and 2,4,6-trimethylaniline were added to the solvent ethanol, and the reaction was carried out at 78 °C for 120 h. After filtration, the filter cake was washed and dried to obtain the iron-based catalyst; the dosage ratio of the iron-based clay, 2,6-diacetylpyridine, 2,4,6-trimethylaniline, and the solvent was 1.5 g: 2.2 mmol: 4.4 mmol: 20 mL; S3, Preparation of zirconium-based dual-active-center catalyst: The zirconium-based catalyst and the iron-based catalyst were ground and mixed evenly to obtain the zirconium-based dual-active-center catalyst.

[0032] Preparation of polyethylene: Seven groups of experiments were set up, and the catalysts were respectively: 1 mg of the zirconium-based dual-active-center catalyst prepared in Examples 1-5, 1 mg of the iron-based catalyst prepared in Example 3, and 1 mg of the zirconium-based catalyst prepared in Example 3. Then the catalysts were respectively added to 2 mL of toluene to prepare toluene suspensions.

[0033] The specific preparation operation of polyethylene is as follows: In a 120 mL autoclave, nitrogen was first introduced to exhaust the air in the autoclave, 50 mL of n-hexane, 1 mL of toluene solution of triethylaluminum activator with a concentration of 0.2 mol / L, and 2 mL of toluene suspension containing the catalyst were added. Then the gas in the system was replaced with ethylene, the ethylene pressure was adjusted to 0.8 MPa, the temperature was raised to 55 °C, and the polymerization reaction was carried out for 60 min. After the reaction was completed, ethylene was cut off to stop the reaction. Then the mixture in the reaction kettle was poured into 10% hydrochloric acid-ethanol to precipitate the product, the polymer was filtered out, washed alternately with water and ethanol three times, and dried in vacuum at 60 °C to constant weight to obtain a solid polymer, that is, polyethylene. The prepared polyethylene was tested and analyzed, and the results are shown in Table 1; Table 1. Polyethylene test results As can be seen from Table 1, compared with single-site zirconium-based catalysts and iron-based catalysts, the zirconium-based dual-active-site catalyst used in the present invention has higher polymerization activity, the prepared polyethylene has a wider molecular weight distribution, and the melt flow rate is also significantly increased.

[0034] The polyethylene obtained from the iron-based catalyst and the zirconium-based catalyst was physically mixed and melt-mixed to obtain P-mix-PE and M-mix-PE respectively. Then, the polyethylene prepared in Example 3, the polyethylene prepared by the zirconium-based catalyst, the polyethylene prepared by the iron-based catalyst, P-mix-PE, and M-mix-PE were used to obtain DSC curves with a differential scanning analyzer, and the results are as Figure 3 shown.

[0035] From Figure 3 it can be seen that for the polyethylene prepared by using the zirconium-based catalyst alone, due to "chain walking", methyl branches are generated, a melting peak appears at 114 °C, and a shoulder peak appears near 107 °C; the polyethylene prepared by the iron-based catalyst has a melting peak of high-density polyethylene at 135 °C. This shows that the two catalysts respectively produce polyethylenes with different structures and melting points; in the DSC curve of the polyethylene prepared by using the zirconium-based dual-active-site catalyst, melting peaks corresponding to the PEs obtained by the two individual catalysts appear, and a new peak also appears at 125 °C. By comparing the DSC curves of P-mix-PE and M-mix-PE, it is found that the physically mixed polyethylene only shows the melting peaks of the respective individual polyethylenes, while the melt-mixed polyethylene shows a new peak at 126 °C in addition to the peaks in the low melting point region. This shows that when the polyethylene prepared by the zirconium-based dual-active-site catalyst is first melted, the partially melted two polyethylenes are mixed at the microscopic level, and after cooling, a polyethylene crystal with different crystallization physical properties from the individual polyethylenes is formed, thus resulting in a new peak at 125 °C, indicating that the prepared polyethylene has a unique microscopic structure and thermal properties. Considering the molecular weight and molecular weight distribution, the prepared polyethylene is bimodal polyethylene.

[0036] Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A zirconium-based dual-active-site catalyst, characterized in that: It includes a zirconium-based catalyst and an iron-based catalyst, and the mass ratio of the zirconium-based catalyst to the iron-based catalyst is 1:(0.3 - 1.8).

2. The preparation method of a zirconium-based dual-active center catalyst according to claim 1, characterized in that: It includes the following steps: S1. Prepare zirconium-based clay and iron-based clay respectively by the ion exchange method; S2. Combine the zirconium-based clay and the iron-based clay with corresponding ligands respectively to prepare a zirconium-based catalyst and an iron-based catalyst; S3. Mix the zirconium-based catalyst and the iron-based catalyst evenly to obtain a zirconium-based dual-active center catalyst.

3. The preparation method of a zirconium-based dual-active center catalyst according to claim 2, characterized in that, The specific preparation method of the zirconium-based clay in S1 is as follows: Under stirring, slowly add sodium-based clay into zirconium tetrachloride solution, stir and react until the sodium ion concentration in the reaction solution no longer increases significantly, filter, wash the filter cake with deionized water by stirring for multiple times, and dry to obtain zirconium-based clay; the dosage ratio of the sodium-based clay to the zirconium tetrachloride solution is (1 - 1.5) g:(10 - 25) mL; the concentration of zirconium in the zirconium tetrachloride solution is 0.3 - 0.5 mol / L; the reaction temperature is 50 - 80 °C, the drying temperature is 80 - 100 °C, and the stirring speed is 100 - 300 r / min.

4. The preparation method of a zirconium-based dual-active center catalyst according to claim 3, characterized in that: The specific preparation method of the iron-based clay in S1 is as follows: Under stirring, slowly add sodium-based clay into ferric chloride solution, stir and react until the sodium ion concentration in the reaction solution no longer increases significantly, filter, wash the filter cake with deionized water by stirring for multiple times, and dry to obtain iron-based clay; the dosage ratio of the sodium-based clay to the ferric chloride solution is (1 - 1.5) g:(15 - 30) mL; the concentration of iron in the ferric chloride solution is 0.4 - 0.8 mol / L, the reaction temperature is 50 - 80 °C, the drying temperature is 80 - 100 °C, and the stirring speed is 100 - 300 r / min.

5. The preparation method of a zirconium-based dual-active-site catalyst according to claim 4, characterized in that: The sodium-based clay in S1 is one of sodium-based montmorillonite, sodium-based saponite, and sodium mica.

6. The preparation method of a zirconium-based dual-active center catalyst according to claim 2, characterized in that: The specific preparation method of the zirconium-based catalyst in S2 is as follows: Under a nitrogen atmosphere, add zirconium-based clay, acenaphthenequinone, and 8-aminoquinoline to a solvent, heat and react for 100 - 120 h, filter, wash the filter cake, and dry to obtain a zirconium-based catalyst; the dosage ratio of the zirconium-based clay, acenaphthenequinone, 8-aminoquinoline, and the solvent is (1 - 1.5) g:(2 - 2.5) mmol:(4 - 5) mmol:(10 - 20) mL.

7. The preparation method of a zirconium-based dual-active-site catalyst according to claim 6, characterized in that: The specific preparation method of the iron-based catalyst in S2 is as follows: Under a nitrogen atmosphere, add iron-based clay, acenaphthenequinone, and 8-aminoquinoline to a solvent, heat and react for 100 - 120 h, filter, wash the filter cake, and dry to obtain an iron-based catalyst; the dosage ratio of the iron-based clay, 2,6-diacetylpyridine, 2,4,6-trimethylaniline, and the solvent is (1 - 1.5) g:(1.8 - 2.2) mmol:(3.6 - 4.4) mmol:(10 - 20) mL.

8. The preparation method of a zirconium-based dual-active-site catalyst according to claim 7, characterized in that: The solvents are all ethanol or acetonitrile.

9. The preparation method of a zirconium-based dual-active-site catalyst according to claim 8, characterized in that: Before performing the operation of S2, both the zirconium-based clay and the iron-based clay are activated at 200 - 250 °C for 4 - 8 h.

10. The application of a zirconium-based dual-active center catalyst according to claim 1, characterized in that: It is used for single-reactor production of bimodal polyethylene.

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