Supported double-center metallocene catalyst as well as preparation method and application thereof

Abstract

The invention relates to a supported double-center metallocene catalyst, which comprises a modified silicon-based carrier, a chromium-metallocene metal and other metallocenes, and the modified silicon-based carrier is a silicon-based carrier modified by an aluminum-based substance and a fluorine-containing substance. According to the catalyst, the carrier is modified, so that the chromium-metallocene metal can be better loaded, the catalyst is used for preparing polyethylene, and the polyethylene has wider molecular weight distribution. In the preparation process of polyethylene, under the condition that alkylaluminoxane is not used, bimodal polyethylene with wide molecular weight distribution can also be obtained.

Description

Technical Field

[0001] The invention belongs to the technical field of polyolefin catalysts, and particularly relates to a preparation method of a supported dual-center metallocene catalyst and application thereof. Background Art

[0002] As one of the five major synthetic resins, polyethylene has become the largest variety of general synthetic resins due to its good wear resistance, chemical resistance and corrosion resistance. Polyethylene products can be divided into low-density polyethylene (LDPE), high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), ultra-high molecular weight polyethylene (UHMWPE), etc. according to density, molecular weight and other classification standards.

[0003] The improvement of catalyst technology capabilities is an important factor in the sustainable development of the polyethylene industry. According to different compositions, polyethylene catalysts can be divided into Ziegler-Natta catalysts, chromium catalysts, metallocene catalysts and non-metallocene catalysts.

[0004] Among them, the catalyst for preparing bimodal polyethylene has two peaks, high and low, in molecular weight because it carries two different active components on the same carrier. The obtained bimodal polyethylene has improved processing performance while ensuring superior mechanical properties. Therefore, the dual-active component catalyst used to prepare bimodal polyethylene has received widespread attention from academia and industry.

[0005] Chinese patent application CN201811245581.X discloses a supported chromium / vanadium dual-center catalyst for preparing high-density polyethylene. After inorganic chromium metal and inorganic vanadium metal are loaded on a carrier, two alkyl aluminums are used to pre-reduce the chromium active center and the vanadium active center, respectively, and during the polymerization process, the polymerization activity is increased by distributing and adding a molecular weight regulator or a comonomer, and the density of the polyethylene product is adjusted.

[0006] Chinese patent application CN201811035473.X discloses a supported chromium / metallocene bimetallic catalyst. Before the inorganic chromium and metallocene are loaded, the carrier is activated with titanium metal. The catalyst prepared by this method has excellent hydrogen regulation responsiveness, and the polymerization product exhibits excellent bimodal polyethylene performance.

[0007] In the study of bimodal polyethylene, many different supported dual-center catalysts have been reported, mainly focusing on dual-center catalysts supported by inorganic / organic chromium compounds and other metal compounds or metallocene compounds. However, when these catalysts are applied to polyethylene, the molecular weight distribution of polyethylene is not wide enough, resulting in great processing difficulties. Summary of the invention

[0008] The first object of the present application is to provide a supported dual-center metallocene catalyst, which can better load the chromium metallocene by modifying the carrier.

[0009] The second object of the present application is to provide a supported dual-center metallocene catalyst, which includes a chromium metallocene and other metallocene dual-active metals, for the preparation of polyethylene, the polyethylene having a wider molecular weight distribution. In the polyethylene preparation process, bimodal polyethylene with a wide molecular weight distribution can also be obtained without using alkylaluminoxane.

[0010] The third object of the present application is to provide a method for preparing a supported dual-center metallocene catalyst, which can better load the chromium metallocene onto the carrier.

[0011] On the one hand, a supported dual-center metallocene catalyst of the present application includes: a modified silicon-based carrier, a chromium metallocene and other metallocenes, wherein the modified silicon-based carrier refers to a silicon-based carrier modified by an aluminum-based substance and a fluorine-containing substance.

[0012] The other metallocenes include titanium metallocenes.

[0013] The present application modifies the silicon-based carrier by using aluminum-based substances and fluorine-containing substances, so that the chromocene metal can be better loaded.

[0014] On the other hand, the preparation method of the above-mentioned supported dual-center metallocene catalyst includes: modifying the silicon-based carrier with an aluminum-based substance and a fluorine-containing substance to obtain a modified silicon-based carrier; mixing the modified silicon-based carrier with a chromium metallocene solution and other metallocene solutions, and obtaining a supported dual-center metallocene catalyst after post-treatment.

[0015] The supported dual-center metallocene catalyst of the present application is used in the polyethylene preparation reaction to obtain polyethylene with a wider molecular weight distribution. DETAILED DESCRIPTION

[0016] The supported dual-center metallocene catalyst and application of the present invention are further described in detail below. The protection scope of the present application is not limited, and its protection scope is defined by the claims. Certain disclosed specific details provide a comprehensive understanding of each disclosed embodiment. However, those skilled in the art know that the embodiment can be implemented without using one or more of these specific details and using other materials, etc.

[0017] Unless the context requires otherwise, in the specification and claims, the terms "include" and "comprising" should be construed as having an open-ended, inclusive meaning, that is, "including, but not limited to".

[0018] The "embodiment", "one embodiment", "another embodiment" or "certain embodiments" mentioned in the specification means that the specific features, structures or characteristics described in connection with the embodiment are included in at least one embodiment. Therefore, "embodiment", "one embodiment", "another embodiment" or "certain embodiments" do not necessarily all refer to the same embodiment. Moreover, specific features, structures or characteristics may be combined in any manner in one or more embodiments. Each feature disclosed in the specification may be replaced by any alternative feature that can provide the same, equal or similar purpose. Therefore, unless otherwise specified, the disclosed features are only general examples of equal or similar features.

[0019] The present invention will be further described below in conjunction with specific examples. It should be understood that these examples are intended only to illustrate the present invention and are not intended to limit the scope of the present invention. The experimental methods in the following examples without specifying specific conditions are usually performed under normal conditions or according to the conditions recommended by the manufacturer. Unless otherwise stated, all percentages, ratios, proportions, or parts are by weight.

[0020] The general formula of the chromocene metal compound of the present application is: p1 -Cr-C p2 Among them, C p1 and C p1 is a substituted or unsubstituted cyclopentadienyl, indenyl or fluorenyl.

[0021] The substituent is selected from C 1 -C 20 Alkyl, C 1 -C 20 Alkoxy, C 1 -C 20 Silane, C 1 -C 20 Aralkyloxy or halogen or a mixture of more than one of them.

[0022] C p1 and C p1 Can be the same or different.

[0023] Unless otherwise defined, all professional and scientific terms used herein have the same meanings as those familiar to those skilled in the art. In addition, any methods and materials similar or equivalent to those described herein can be applied to the methods of the present invention. The preferred implementation methods and materials described herein are for demonstration purposes only.

[0024] A loaded dual-center metallocene catalyst comprises: a modified silicon-based carrier, a chromium metallocene and other metallocenes, wherein the modified silicon-based carrier refers to a silicon-based carrier modified by an aluminum-based substance and a fluorine-containing substance.

[0025] After the silica gel carrier is modified by an aluminum-based modifier and a fluorine-containing modifier, a F-Al-O structured silica gel carrier is formed, which is easy to load chromium metallocene and other metallocenes.

[0026] Generally, chromocene metals cannot bond with the functional groups on the surface of unmodified silica gel carriers. After the silica gel carriers are modified with aluminum-based modifiers and fluorine-containing modifiers, a F-Al-O structure is formed, which can well load chromocene metals.

[0027] In the catalyst, the loading amount of the chromocene metal is 0.1 to 5 wt %, based on the mass of the chromium metal.

[0028] In the catalyst, the loading amount of other metallocenes is 0.1 to 5 wt %, based on the mass of the metal.

[0029] Preferably, the loading amount of the titanocene metal is 0.8 to 2.0 wt %, based on the mass of the metal. More preferably, the loading amount of the chromocene metal is 0.8 to 2.0 wt %.

[0030] When the loading amount of other metallocenes is within the above preferred range (0.8-2.0 wt%), in the system for preparing polyethylene, the dispersibility of polyethylene and the comprehensive performance of catalyst activity are better.

[0031] In a preferred embodiment, the ratio of the loading amount of chromocene metal to that of titanocene metal in the catalyst is about 1:1, and the loading amount is calculated based on the mass of chromium metal.

[0032] In the silica gel supported catalyst, the Al content is 5-10wt%, and the F content is 1-5wt%.

[0033] The "loading amount" of metallocene described in this application refers to the ratio of the mass of the corresponding metal element to the mass of the catalyst.

[0034] In certain embodiments, the chemical structure of other metallocenes of the present application is shown in Formula I or Formula II below, wherein the transition metal includes a metal element of Group IVB, for example, a titanocene compound.

[0035] Formula I: C p1 -MC p2

[0036]

[0037] Among them, M is Ti.

[0038] Wherein, Cp1 and / or Cp2 and / or Cp3 are substituted or unsubstituted cyclopentadienyl, indenyl or fluorenyl, and the substituent thereof is selected from C1-C20 alkyl, alkoxy, silane, aralkyloxy or halogen;

[0039] Wherein, R1, R2 and / or R3 are respectively C1-C20 alkyl, alkoxy, silane, aralkyloxy or halogen.

[0040] The structure of other metallocenes in the present application is preferably selected from the structure of formula II. Since this type of structured metallocene is loaded on a catalyst carrier, the catalyst structure has spatial limitations. The catalyst is applied to the reaction system of polyethylene synthesis so that the molecular weight of the reactant can only enter the active center of the catalyst in a specific manner, thereby improving the selectivity and reaction efficiency of the reaction. The catalyst of the present application, because the carrier is modified, can load more two metallocene compounds and has a higher reaction activity. The bimodal polyethylene polymer produced has a wider molecular weight distribution.

[0041] On the other hand, the preparation method of the above-mentioned supported dual-center metallocene catalyst comprises: (1) modifying a silicon-based carrier with an aluminum-based substance and a fluorine-containing substance to obtain a modified silicon-based carrier; (2) mixing the modified silicon-based carrier with a chromium metallocene solution and other metallocene solutions, and obtaining a supported dual-center metallocene catalyst after post-treatment.

[0042] The silicon-based carrier is first modified with aluminum, and the aluminum-modified silica gel carrier is calcined and activated to form an -Al-O structure, which is convenient for the subsequent modification of the fluorine-containing modifier to form a F-Al-O structure, and further, it is beneficial for chromium metallocene and other metallocenes to be loaded onto the modified carrier.

[0043] In certain embodiments, Preparation method of modified silicon-based carrier , comprising: (1) mixing an aluminum-based substance solution with a silicon-based carrier under an inert gas atmosphere, reacting for a period of time, performing solid-liquid separation to obtain a first solid substance, and calcining the first solid substance to obtain a first modified silicon-based substance;

[0044] (2) In an inert gas atmosphere, the first modified silicon-based material is mixed with a fluorine-containing substance solution. After a period of reaction, solid-liquid separation is performed to obtain a second solid substance. The second solid substance is dried to obtain a modified silicon-based carrier.

[0045] The aluminum-based material of the present application includes organic matter containing aluminum element. For example, the aluminum-based material is alkyl aluminum, such as trialkyl aluminum, C1-C4 alkyl.

[0046] Specifically, the alkyl aluminum is selected from at least one of trimethyl aluminum, triethyl aluminum, triisobutyl aluminum, tripropyl aluminum, tributyl aluminum, triisopropyl aluminum, tri-sec-butyl aluminum, ethyl dimethyl aluminum, and methyl diethyl aluminum; preferably, the alkyl aluminum is selected from one of trimethyl aluminum, triethyl aluminum, and triisobutyl aluminum, or a mixture of two or more thereof.

[0047] The fluorine-containing substance is selected from at least one of ammonium fluoride, ammonium hydrogen fluoride and ammonium hexafluorosilicate.

[0048] The silicon-based carrier is silica gel, specifically granular porous silica gel.

[0049] Before being modified by aluminum-based substances, silica gel is first calcined at 150-300°C to activate the silica gel carrier and facilitate the reaction with the aluminum-based modifier.

[0050] In certain embodiments, the temperature of the aluminum-based substance solution and the silicon-based carrier is no higher than the boiling point of the solvent in the reaction system.

[0051] For example, the solvent in the reaction system is toluene, and the reaction is carried out at a reaction temperature not higher than 110°C.

[0052] Preferably, the reaction temperature is controlled at 70-90°C.

[0053] The reaction time of the aluminum-based substance solution and the silicon-based carrier is controlled within 1 h-24 h; preferably, the reaction time is controlled within 1 h-12 h; more preferably, the reaction time is controlled within 3 h-5 h.

[0054] In certain embodiments, the calcination temperature of the first solid material is controlled at 200-600°C.

[0055] The calcination time is 1-24 hours.

[0056] In certain embodiments, the amount of aluminum alkyl used is 1%-50% of the mass of the silica gel carrier. Preferably, the amount of aluminum alkyl used is 5%-10% of the mass of the silica gel carrier.

[0057] In order to ensure that the content of aluminum on the carrier is within a certain range, a corresponding amount of metallocene can be loaded onto the modified silica-based carrier.

[0058] In certain embodiments, the temperature of the first modified silicon group and the fluorine-containing substance solution is no higher than the boiling point of the solvent in the reaction system.

[0059] For example, the solvent of the reaction system is water, and the first modified silicon group reacts with the fluorine-containing substance solution at a temperature not higher than 90°C.

[0060] Preferably, the reaction temperature is controlled at 70-90°C.

[0061] The reaction time is controlled within 1h-24h.

[0062] In some embodiments, the amount of fluorine-containing substance is 1%-50% of the mass of the modified silicon-based carrier. Preferably, the mass of F in the fluorine-containing substance is 1%-5% of the mass of the modified silicon-based carrier.

[0063] The amount of fluorine-containing substance used is sufficient to form F-Al-O with the first modified silicon-based --Al-O, thereby enabling the metallocene to be loaded on the modified silicon-based carrier to exert its catalytic activity.

[0064] Through the modification of the above-mentioned aluminum source of the present application, the silicon source forms an -Al-O covalent bond with the above-mentioned aluminum source, and the carrier obtained after the modification of the fluorine source, and the catalyst obtained after the chromocene metal and the titanocene metal are loaded on the carrier is applied to the polyethylene copolymerization system, which significantly improves the dispersibility of polyethylene, that is, the molecular weight distribution is wider.

[0065] After the silica gel carrier is modified by the aluminum-based modifier and the fluorine-containing modifier, a silica gel carrier with an F-Al-O structure is formed, which is easy to load chromocene metallocene and titanocene metallocene. Among them, the aluminum-based modifier plays a good activation role on the loaded chromocene metallocene and titanocene metallocene, especially increasing the stability of the active center of the chromocene metallocene, avoiding the reduction of the number of effective active centers and the catalytic efficiency, and the resulting reduction in the polymerization activity and molecular weight distribution of the bimetallic catalyst.

[0066] There are no particular limitations on the drying conditions for the second solid material, as long as the solvent is evaporated.

[0067] In certain embodiments, the second solid material is dried at a temperature controlled at 80-90°C.

[0068] In certain embodiments, the second solid material is calcined at a temperature of 200-600°C.

[0069] The calcination time is 1-24 hours.

[0070] In certain embodiments, Method for loading chromium metallocene and other metallocenes on modified silicon-based carrier ,include:

[0071] (i) mixing the modified silicon-based carrier with the chromocene metal solution, drying and calcining in an inert atmosphere after mixing for a period of time to obtain a catalyst matrix loaded with the chromocene metal;

[0072] (ii) The catalyst precursor is mixed with other metallocene solutions, and after mixing for a period of time, the mixed solution is dried to obtain a supported dual-site metallocene catalyst.

[0073] In step (1), the calcination temperature is controlled at 500-650°C.

[0074] Calcination at this temperature can, on the one hand, remove moisture and maintain the original morphology of the modified silicon-based material; on the other hand, it can remove some hydroxyl groups on the surface of the carrier, which is beneficial for the subsequent metallocene loading. Preferably, the calcination time of step (i) is 3-7 hours.

[0075] In certain embodiments, in steps (i) and (ii), in order to remove the solvent during the drying process, the drying temperature is selected to be in the range near the boiling point of the solvent.

[0076] For example, in the process of supporting metallocene, toluene is used as solvent and the drying temperature is controlled at 100-125°C.

[0077] Hexane was selected as the solvent and the drying temperature was controlled at 80-90°C.

[0078] In the preparation process of the catalyst in the present application, if a solvent is required in each step, the solvent can be selected from at least one of benzene, toluene, xylene, hexane, pentane and heptane. Preferably, the solvent is selected from at least one of toluene, hexane and heptane.

[0079] The supported dual-center metallocene catalyst of the present application can well support the metallocene composition through secondary modification of the silicon-based carrier, and has high reactivity in ethylene homopolymerization or ethylene and α-olefin copolymerization. The molecular weight of the polyethylene produced by the catalyst is bimodal and has a wide molecular weight distribution. The use of alkylaluminoxane is avoided in the preparation of the catalyst and the polymerization process, and the production cost can be strictly controlled.

[0080] The supported dual-center metallocene catalyst of the present application is applied to the reaction system for preparing polyethylene.

[0081] The polyethylene reaction system includes: ethylene homopolymerization, or ethylene and α-olefin copolymerization.

[0082] The reaction temperature is 80-100°C; preferably, the reaction temperature is 80-90°C.

[0083] The reaction system for preparing polyethylene also includes an auxiliary catalyst, such as triethylaluminum. The mass ratio of triethylaluminum to the supported dual-center metallocene catalyst is less than 1.

[0084] The above catalyst is applied to the reaction system for preparing polyethylene. Through the modification of the silicon-based carrier with aluminum and fluorine, the two metallocenes can be well loaded on the carrier. Under the synergistic effect of the two metallocenes and the modified carrier, it has higher polymerization activity in the preparation of polyethylene, and the molecular weight dispersity of the obtained polyethylene is wider.

[0085] The supported dual-site metallocene catalyst and its application of the present invention are further described below with reference to specific examples.

[0086] The reagents used in the following examples, such as triethylaluminum, trimethylaluminum, triisobutylaluminum, ammonium hexafluorosilicate, bis(cyclopentadiene)chromium, bis(cyclopentadiene)titanium dichloride, bis(cyclopentadiene)dimethyltitanium, dimethylsilyl(tert-butylamine)(tetramethylcyclopentadiene)titanium dichloride, etc., are all commercially available products. Other solvents are selected from chemically pure grades.

[0087] Example 1

[0088] Under nitrogen atmosphere, the silica gel carrier is placed in a fluidized bed, heated to 200°C, and roasted for 6 hours, and the silica gel is cooled naturally in nitrogen to obtain a heat-activated silica gel carrier. Take 6.0g of the heat-activated silica gel carrier, add it to a toluene solution of triethylaluminum of a certain concentration, the aluminum loading (by the mass of Al) is 5wt%, and after continuous stirring and reaction at 80°C for 4 hours, wash it with a certain amount of toluene and n-hexane in turn, and dry it to obtain an alkyl aluminum-modified silica gel carrier, place the obtained alkyl aluminum-modified silica gel carrier in a fluidized bed, heat it to 600°C, roast it for 8 hours and then cool it naturally, then place it in a certain concentration of ammonium hexafluorosilicate aqueous solution, the fluorine loading (by the mass of F) is 4wt%, and after continuous stirring and reaction at 80°C for 4 hours, wash it and dry it to obtain a secondary modified silica gel carrier, place the obtained secondary modified silica gel carrier in a fluidized bed, heat it to 600°C, roast it for 8 hours and then cool it naturally, and store it in a nitrogen atmosphere for use.

[0089] 3.0 g of the secondary modified silica gel carrier was weighed, placed in a di(cyclopentadiene) chromium toluene solution of a certain concentration, the chromium loading (by mass of Cr) was 1.2 wt%, immersed and stirred at a temperature of 40° C. for 5 hours, heated to 120° C. in a nitrogen atmosphere, continued to stir and dry for 12 hours, then heated to 600° C. in a fluidized bed with the silica gel carrier loaded with di(cyclopentadiene) chromium for 6 hours, and naturally cooled. The prepared catalyst matrix was placed in a di(cyclopentadiene) titanium dichloride toluene solution of a certain concentration, the titanium loading (by mass of Ti) was 1.2 wt%, immersed and stirred at a temperature of 40° C. for 4 hours until the reaction was complete, then heated to 120° C., and dried for 6 hours in a nitrogen atmosphere to obtain a supported dual-center metallocene catalyst.

[0090] Example 2

[0091] The process steps and process conditions for preparing the supported dual-center metallocene catalyst in this embodiment refer to Example 1, except that the bis(cyclopentadiene)titanium dichloride in Example 1 is replaced by dimethylsilyl(tert-butylamine)(tetramethylcyclopentadiene)titanium dichloride (CGC-Ti) in this embodiment. The amount of dimethylsilyl(tert-butylamine)(tetramethylcyclopentadiene)titanium dichloride (CGC-Ti) added is sufficient to satisfy the titanium loading (based on the mass of Ti) of 1.2 wt%.

[0092] Example 3

[0093] The process steps and process conditions for preparing the supported dual-center metallocene catalyst in this embodiment refer to Example 1, except that the bis(cyclopentadienyl)titanium dichloride in Example 1 is replaced by bis(cyclopentadienyl)titanium dimethyl in this embodiment. The amount of bis(cyclopentadienyl)titanium dimethyl added is sufficient to satisfy the titanium loading (based on the mass of Ti) of 1.2 wt%.

[0094] Example 4

[0095] The process steps and process conditions for preparing the supported dual-center metallocene catalyst in this embodiment refer to Example 1, except that the amount of bis(cyclopentadiene)titanium dichloride added in Example 1 is different. In this embodiment, the amount of bis(cyclopentadiene)titanium dichloride added satisfies that the titanium loading (based on the mass of Ti) is 0.6wt%.

[0096] Example 5

[0097] The process steps and process conditions for preparing the supported dual-center metallocene catalyst in this embodiment refer to Example 1, except that the amount of bis(cyclopentadiene)titanium dichloride added in Example 1 is different. In this embodiment, the amount of bis(cyclopentadiene)titanium dichloride added satisfies the titanium loading (based on the mass of Ti) of 2.4 wt%.

[0098] Comparative Example 1

[0099] Under nitrogen atmosphere, the silica gel carrier is placed in a fluidized bed, heated to 200°C, and roasted for 6 hours. The silica gel is cooled naturally in nitrogen to obtain a heat-activated silica gel carrier. 6.0g of the heat-activated silica gel carrier is added to a certain concentration of triethylaluminum toluene solution, and the aluminum loading (by mass of Al) is 5%. After continuous stirring and reaction at 80°C for 4 hours, it is washed with a certain amount of toluene and n-hexane in turn, and dried to obtain an alkyl aluminum-modified silica gel carrier. The obtained alkyl aluminum-modified silica gel carrier is placed in a fluidized bed, heated to 600°C, roasted for 8 hours, cooled naturally, and stored in a nitrogen atmosphere for use.

[0100] 3.0 g of the alkyl aluminum modified silica gel carrier was weighed, placed in a di(cyclopentadienyl) chromium toluene solution of a certain concentration, the chromium loading (by mass of Cr) was 0.4%, immersed and stirred at a temperature of 40° C. for 5 hours, heated to 120° C. in a nitrogen atmosphere, continued to stir and dry for 12 hours, then heated to 600° C. in a fluidized bed and calcined for 6 hours on the silica gel carrier loaded with di(cyclopentadienyl) chromium, and naturally cooled. The prepared catalyst matrix was placed in a di(cyclopentadienyl) titanium dichloride toluene solution of a certain concentration, the titanium loading (by mass of Ti) was 1.2%, immersed and stirred at a temperature of 40° C. for 4 hours until the reaction was complete, then heated to 120° C., and dried for 6 hours in a nitrogen atmosphere to obtain a supported dual-center metallocene catalyst.

[0101] Comparative Example 2

[0102] Under nitrogen atmosphere, the silica gel carrier is placed in a fluidized bed, heated to 200°C, and calcined for 6 hours. The silica gel is cooled naturally in nitrogen to obtain a heat-activated silica gel carrier. 6.0 g of the heat-activated silica gel carrier is placed in a certain concentration of ammonium hexafluorosilicate solution, with a fluorine loading amount (based on the mass of F) of 4%. After continuous stirring and reaction at 80°C for 4 hours, the fluorine-modified silica gel carrier is washed and dried to obtain a fluorine-modified silica gel carrier. The obtained fluorine-modified silica gel carrier is placed in a fluidized bed, heated to 600°C, calcined for 8 hours, cooled naturally, and stored in a nitrogen atmosphere for use.

[0103] 3.0 g of fluorine-modified silica gel carrier was weighed, placed in a di(cyclopentadienyl)chromium toluene solution of a certain concentration, the chromium loading (by mass of Cr) was 0.6%, immersed and stirred at a temperature of 40° C. for 5 hours, heated to 120° C. in a nitrogen atmosphere, continued to stir and dry for 12 hours, then heated to 600° C. in a fluidized bed and calcined for 6 hours on the silica gel carrier loaded with di(cyclopentadienyl)chromium, and naturally cooled. The prepared catalyst matrix was placed in a di(cyclopentadienyl)titanium dichloride toluene solution of a certain concentration, the titanium loading (by mass of Ti) was 1.2%, immersed and stirred at a temperature of 40° C. for 4 hours until the reaction was complete, then heated to 120° C., and dried for 6 hours in a nitrogen atmosphere to obtain a supported dual-center metallocene catalyst.

[0104] Comparative Example 3

[0105] Under nitrogen atmosphere, the silica gel carrier is placed in a fluidized bed, heated to 200°C, and roasted for 6 hours. The silica gel is cooled naturally in nitrogen to obtain a heat-activated silica gel carrier. Take 6.0g of the heat-activated silica gel carrier and add it to a certain concentration of triethylaluminum toluene solution, the aluminum loading (by mass of Al) is 5%, and after continuous stirring and reaction at 80°C for 4 hours, it is washed with a certain amount of toluene and n-hexane in turn, and dried to obtain an alkyl aluminum-modified silica gel carrier. The obtained alkyl aluminum-modified silica gel carrier is placed in a fluidized bed, heated to 600°C, roasted for 8 hours and cooled naturally, and then placed in a certain concentration of ammonium hexafluorosilicate solution, the fluorine loading (by mass of F) is 4%, and continuously stirred and reacted at 80°C for 4 hours, washed and dried to obtain a secondary modified silica gel carrier. The obtained secondary modified silica gel carrier is placed in a fluidized bed, heated to 600°C, roasted for 8 hours and cooled naturally, and stored in a nitrogen atmosphere for use.

[0106] Weigh 3.0 g of the secondary modified silica gel carrier, place it in a certain concentration of chromium acetate distilled water solution, the chromium loading (by mass of Cr) is 1.2%, soak and stir at 40°C for 5 hours, heat to 120°C in a nitrogen atmosphere, continue to stir and dry for 12 hours, then heat the silica gel carrier loaded with chromium acetate to 600°C in a fluidized bed and calcine for 6 hours, and cool naturally. Place the prepared catalyst matrix in a certain concentration of di(cyclopentadiene)titanium dichloride toluene solution, the titanium loading (by mass of Ti) is 1.2%, soak and stir at 40°C for 4 hours until the reaction is complete, then heat to 120°C, and dry for 6 hours in a nitrogen atmosphere to obtain a supported dual-center metallocene catalyst.

[0107] Comparative Example 4

[0108] The difference between the preparation method of the catalyst in this comparative example and that in Example 1 is that the preparation of the carrier is different, and other loaded metallocenes refer to Example 1. α-alumina and silica are mixed in a mass ratio of 15:85, dried at a temperature of 90°C for 8 hours, and then the mixture is placed in a certain concentration of ammonium hexafluorosilicate aqueous solution, the fluorine loading amount (by mass of F) is 4wt%, and after continuous stirring and reaction at 80°C for 4 hours, it is washed and dried, and then calcined at a temperature of 450°C for 3 hours to obtain a fluorine-modified alumina-silica carrier, which is naturally cooled and stored in a nitrogen atmosphere for standby use. The obtained carrier is loaded with chromocene metal and titanolocene metal by the method of Example 1 to obtain a loaded dual-center metallocene catalyst.

[0109] Experimental Example 1

[0110] In this experimental example, polyethylene was prepared using the supported dual-center metallocene catalysts prepared in Examples 1-5 and Comparative Examples 1-4, and 200 mg of each catalyst was weighed to conduct polymerization experiments.

[0111] Before starting the polymerization experiment, the stainless steel polymerization reactor was heated to 120°C and maintained under vacuum for 1 hour. Then, it was replaced with high-purity nitrogen three times, and then replaced with ethylene gas three times. Finally, the ethylene pressure in the reactor was controlled at about 0.1MPa. 200mL of dehydrated and deoxygenated n-heptane and 1.5mL of triethylaluminum solution (1.0mmol / mL) diluted with n-heptane as solvent were added to the reactor in sequence. The reactor temperature was raised to 90°C, and the ethylene pressure was increased to 1.0MPa to start the polymerization reaction. The reaction ended after 1 hour. The polymerization product was collected, dried, weighed, and analyzed.

[0112] The polymers were characterized as follows:

[0113] The weight average molecular weight (Mw), number average molecular weight (Mn) and polymer dispersibility index (PDI) of the polymer are determined by high temperature gel permeation chromatography. This experiment uses PL-GPC220 high temperature gel permeation chromatograph for determination. The experiment uses 1,2,4-trichlorobenzene and the polymer to be tested to form a polymer solution with a mass fraction of 0.1% to 0.3%, and heats and shakes at 160°C for 4 hours to form a uniform solution. Polystyrene is used as the standard sample and measured at a temperature of 160°C and a solvent flow rate of 1.0mL / min. Parameters k = 40.6, α = 0.727.

[0114] "Polymerization activity" refers to the mass of polyethylene that can be prepared under the action of 1g of catalyst within 1 hour, and the unit is gPE / gcat·h. The polymer dispersibility index (PDI) is determined by measuring the weight average molecular weight (Mw) and number average molecular weight (Mn) of the polymer. PDI is equal to the ratio of Mw to Mn. The larger the PDI value, the wider the molecular weight distribution of the polymer.

[0115] Table 1 Polymerization results of supported dual-site metallocene catalysts It can be seen from the data in Table 1 that the supported dual-center metallocene catalyst prepared by the method of the present application has the characteristics of high polymerization activity and wide molecular weight distribution of bimodal polymerization products. In Comparative Examples 1 and 2, since only the silica gel carrier is aluminum-modified or only the silica gel carrier is fluorine-modified, the reduction in the amount of chromocene loading directly leads to the reduction in polymerization activity and the narrowing of molecular weight distribution. In Comparative Example 3, since the metallocene chromium is replaced by organic metal chromium, the polymerization activity does not change significantly, but the molecular weight distribution of the polyethylene product obtained is significantly narrowed.

Claims

1. A supported dual-center metallocene catalyst comprising: Modified silicon-based carriers, chromium metallocenes and other metallocenes, wherein the modified silicon-based carrier refers to a silicon-based carrier modified by aluminum-based substances and fluorine-containing substances; Preferably, the other metallocene refers to metallocene compounds of transition metal elements other than chromium; More preferably, the other metallocene comprises a titanocene compound.

2. The supported dual-site metallocene catalyst according to claim 1, characterized in that In the silica gel supported catalyst, the Al content is 5-10wt%, and the F content is 1-5wt%.

3. The supported dual-center metallocene catalyst according to claim 1 or 2, characterized in that: In the catalyst, the loading amount of the chromocene metal is 0.1 to 5 wt %, based on the mass of the chromium metal; In the catalyst, the loading amount of other metallocenes is 0.1 to 5 wt % based on the mass of the metal.

4. A method for preparing a supported dual-center metallocene catalyst, comprising: (1) Modifying the silicon-based carrier with an aluminum-based substance and a fluorine-containing substance to obtain a modified silicon-based carrier; (2) The modified silicon-based carrier is mixed with a chromium metallocene solution and other metallocene solutions, and after post-treatment, a supported dual-center metallocene catalyst is obtained; Preferably, the amount of alkyl aluminum is 1%-50% (preferably, 5%-10%) of the mass of the silica gel carrier by aluminum element; The amount of the fluorine-containing substance used is such that the mass of the F element accounts for 1%-50% (preferably, 1%-5%) of the mass of the modified silicon-based carrier.

5. The preparation method according to claim 4, characterized in that: The preparation method of the modified silicon-based carrier comprises: (1) mixing an aluminum-based substance solution with a silicon-based carrier under an inert gas atmosphere, reacting for a period of time, performing solid-liquid separation to obtain a first solid substance, and calcining the first solid substance to obtain a first modified silicon-based carrier; (2) In an inert gas atmosphere, the first modified silicon-based material is mixed with a fluorine-containing substance solution, and after a period of reaction, solid-liquid separation is performed to obtain a second solid substance, and the second solid substance is dried to obtain a modified silicon-based carrier; Preferably, the aluminum-based material comprises an aluminum alkyl; preferably, a trialkyl aluminum, a C1-C4 alkyl. More preferably, the alkylaluminum is selected from at least one of trimethylaluminum, triethylaluminum, triisobutylaluminum, tripropylaluminum, tributylaluminum, triisopropylaluminum, tri-sec-butylaluminum, ethyldimethylaluminum, and methyldiethylaluminum; preferably, the alkylaluminum is selected from one or a mixture of two or more of trimethylaluminum, triethylaluminum, and triisobutylaluminum; Preferably, the fluorine-containing substance is selected from at least one of ammonium fluoride, ammonium hydrogen fluoride and ammonium hexafluorosilicate.

6. The preparation method according to claim 4 or 5, characterized in that: The temperature of the aluminum-based substance solution and the silicon-based carrier is not higher than the boiling point of the solvent in the reaction system; Preferably, the solvent in the reaction system is toluene, and the reaction is carried out at a reaction temperature not higher than 110°C; More preferably, the reaction temperature is controlled at 70-90°C.

7. The preparation method according to claim 4 or 5, characterized in that: The temperature of the first solid material roasting is controlled at 200-600°C; The temperature of the second solid material roasting is controlled at 200-600°C; Preferably, the calcination time of the first solid or the second solid is 1-24 hours.

8. The preparation method according to any one of claims 4 to 7, characterized in that: Methods for supporting chromium metallocene and other metallocenes on modified silicon-based carriers, including: (i) mixing the modified silicon-based carrier with the chromocene metal solution, drying and calcining in an inert atmosphere after mixing for a period of time to obtain a catalyst matrix loaded with the chromocene metal; (ii) mixing the catalyst precursor with other metallocene solutions, mixing for a period of time and then drying to obtain a supported dual-site metallocene catalyst; Preferably, in step (i), the calcination temperature is controlled at 500-650°C; More preferably, the calcination time in step (1) is 3-7 hours.

9. The preparation method according to any one of claims 4 to 7, characterized in that: The chemical structures of other metallocenes are shown in Formula I or Formula II below. Formula I: C p1 -MC p2 Among them, M is Ti. Cp1 and / or Cp2 and / or Cp3 are substituted or unsubstituted cyclopentadienyl, indenyl or fluorenyl, and the substituents thereof are selected from C1-C20 alkyl, alkoxy, silane, aralkyloxy or halogen; R1 and / or R2 and / or R3 are C1-C20 alkyl, alkoxy, silane, aralkyloxy or halogen; Preferably, the chemical structure of the other metallocene is as follows: Formula II.

10. A supported dual-site metallocene catalyst according to any one of claims 1 to 3 is applied to a reaction system for preparing polyethylene; Preferably, the polyethylene reaction system comprises: Ethylene homopolymerization, or ethylene and α-olefin copolymerization; Preferably, the reaction temperature is 80-100°C; more preferably, the reaction temperature is 80-90°C.

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