Catalyst for improving molecular weight distribution uniformity of polyethylene resin and preparation method thereof

Abstract

The invention discloses a catalyst for improving molecular weight distribution uniformity of polyethylene resin and a preparation method thereof. According to the invention, through the design of the composite catalyst, accurate distribution and regulation of the active center are realized, and the molecular chain structure regularity of the polyethylene resin is optimized from the source. According to the application, the crystallinity uniformity can be improved by 30-50%, the scattering loss of crystal region defects to light rays is remarkably reduced, the light transmittance of the film can be expected to be improved by 15-20%, and the strict requirements of optical packaging, agricultural films and the like on the light transmittance performance are met. Accurate regulation and control of the active center of the composite catalyst promotes synergistic optimization of molecular chain entanglement density and orientation degree to form an ordered entanglement network structure, the transverse and longitudinal elongation at break of a polyethylene film prepared from the catalyst can reach 500% or above, the impact strength is improved by 25%, the tensile strength is improved by 18%, and the polyethylene film has good mechanical properties. The problem of performance imbalance of excessive rigidity and insufficient toughness in the traditional process is effectively solved, and the application space of the material in the fields of express packaging, heavy-load films and the like is widened.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of polyolefin catalysis, and particularly relates to a catalyst for improving the uniformity of the molecular weight distribution of polyethylene resin and a preparation method thereof. BACKGROUND

[0002] In the double-loop slurry polyethylene process, the Ziegler-Natta catalyst and triethyl aluminum cooperate to realize the bimodal molecular weight distribution, which brings balance between rigidity and processing fluidity to the film material. However, in the process, the non-uniformity of the catalyst particle size leads to the dispersion of the polymer chain segment distribution, reduces the uniformity of the crystallinity, and weakens the light transmission performance of the film. At the same time, the chain entanglement and orientation difference causes uneven stress distribution during horizontal and vertical stretching, limiting the improvement of the breaking elongation. Although the rigidity is improved due to the increase of the crystallinity and the intermolecular force, the imbalance between the optical and mechanical properties needs to be solved through catalyst modification, fine adjustment of process parameters, and optimization of post-processing, etc.

[0003] However, the existing double-loop slurry polyethylene process using the Ziegler-Natta catalyst and triethyl aluminum system to produce bimodal film material has the following objective shortcomings: Optical performance defects: the non-uniformity of the catalyst particle size leads to the dispersion of the polymer chain segment distribution, reduces the uniformity of the crystallinity, and significantly weakens the light transmission performance of the film, limiting its application in the field of high light transmission demand such as optical packaging.

[0004] Mechanical performance short board: the chain entanglement and orientation difference cause uneven stress distribution during horizontal and vertical stretching of the film, resulting in low breaking elongation, which is difficult to meet the application scenarios with high requirements for flexibility and tensile performance.

[0005] Performance imbalance problem: although the process can give the film good rigidity and processing fluidity, the optical and mechanical properties are difficult to be optimized simultaneously, and there is a performance imbalance phenomenon, which cannot meet the diversified demand of the downstream for the comprehensive performance of the film. SUMMARY

[0006] The purpose of the present application is to solve the above-mentioned problems, and to provide a catalyst for improving the uniformity of the molecular weight distribution of polyethylene resin and a preparation method thereof.

[0007] The technical scheme adopted by the present application is as follows: a catalyst for improving the uniformity of the molecular weight distribution of polyethylene resin, comprising: a multi-level pore SiO2 / MgCl2 core-shell structure carrier 100 parts by weight, titanium tetrachloride (TiCl4) 5 parts by weight, zirconium tetrachloride (ZrCl4) 3 parts by weight, alkyl aluminum alkoxide (MAO) 25 parts by weight, ethyl benzoate (DONOR) 6 parts by weight, nano-MgO modifier 4 parts by weight, and organosilane coupling agent 3 parts by weight. In a preferred embodiment, a method for preparing a catalyst for improving the uniformity of the molecular weight distribution of polyethylene resin comprises the following steps: S1: vacuum drying the multi-level porous SiO2 / MgCl2 core-shell structured carrier for 2 hours under a nitrogen atmosphere to remove surface adsorbed water. The dried carrier will be used for the modification treatment in step S2; S2: adding a nano-sized MgO modifier and an organosilane coupling agent to the carrier dried in step S1, and stirring in a 40°C constant temperature water bath for 3 hours to uniformly graft the modifier onto the surface of the carrier. The pore size distribution data of the modified carrier will directly affect the selection of the concentration of the metal salt solution in step S3; S3: adding the modified carrier treated in step S2 to a mixed solution of titanium tetrachloride and zirconium tetrachloride, and reacting at 80°C in an oil bath for 4 hours. The uniform loading of the bimetallic active center is achieved by controlling the stirring rate at 300 rpm. The stirring rate needs to be adjusted according to the specific surface area data of the carrier obtained in step S2; S4: adding an alkylaluminoxane solution dropwise to the suspension obtained in step S3, and continuing to stir under nitrogen protection for 1 hour to form a primary catalyst complex. This step needs to be started within 30 minutes after the reaction in step S3 is completed to avoid deactivation of the metal active center; S5: adding ethyl benzoate electron donor to the reaction system in step S4 in proportion, and heating to 95°C for 2 hours of compounding reaction. The optimal amount of electron donor is determined by monitoring the viscosity change of the system online. The viscosity data will be used as a basis for setting the mixing intensity in step S6; S6: transferring the composite system obtained in step S5 to a high-pressure reaction kettle, and performing supercritical fluid treatment at a pressure of 1.2 MPa for 1.5 hours to allow the active components to fully penetrate into the pores of the carrier. The treatment pressure parameter needs to refer to the compressive strength data of the carrier determined in step S2; S7: spray drying the material treated in step S6, with the inlet temperature controlled at 120°C and the outlet temperature at 60°C to obtain a catalyst powder with uniform particle size distribution. The drying process parameters need to match the metal loading amount in step S3; S8: subjecting the catalyst powder obtained in step S7 to programmed temperature activation under nitrogen protection, increasing the temperature from room temperature to 180°C at a rate of 5°C / min, and maintaining the temperature for 3 hours. The activated catalyst needs to be immediately transferred to the high-pressure reaction kettle used in step S4 for the next polymerization reaction.

[0008] In a preferred embodiment, in step S1, the multi-level porous SiO2 / MgCl2core-shell structure carrier is placed in a vacuum drying oven under a nitrogen atmosphere, the drying temperature is set to 120°C, and the carrier is treated under a vacuum degree of 0.08 MPa for 2 hours to completely remove the surface adsorbed water and residual organic solvents. The dried carrier is immediately transferred to an inert gas protected dryer to cool to room temperature, and the surface hydroxyl content is controlled to be below 0.5 mmol / g, which directly determines the grafting efficiency of the modifier in step S2.

[0009] In a preferred embodiment, in step S2, the dried carrier after step S1 is proportionally added with nano-sized MgO modifier and organosilane coupling agent, and the mixed system is transferred to a three-necked flask with a mechanical stirring device, stirred at a stirring rate of 200 rpm for 3 hours under a constant temperature water bath at 40°C, and the zeta potential of the system is measured every 30 minutes during the stirring to ensure that the modifier is uniformly grafted on the surface of the carrier. After the modification is completed, the specific surface area and pore size distribution of the carrier are measured by static nitrogen adsorption method, and the mesopore content needs to reach more than 60%, which is used to adjust the initial concentration of the metal salt solution in step S3.

[0010] In a preferred embodiment, in step S3, the modified carrier after step S2 is added to the previously prepared mixed solution of titanium tetrachloride and zirconium tetrachloride according to a solid-liquid ratio of 1:10, wherein the concentration of titanium tetrachloride is 0.5 mol / L and the concentration of zirconium tetrachloride is 0.3 mol / L. The mixed suspension is transferred to an oil bath reactor with a reflux condenser device, the oil bath temperature is set to 80°C with a temperature control accuracy of ±1°C, and the stirring rate is 300 rpm for continuous reaction for 4 hours. During the reaction process, the particle dispersion degree is monitored by an online laser particle size analyzer, and the reaction is stopped when the particle size distribution variation coefficient of the system decreases to below 5%. The dispersion degree index will directly affect the dropping rate of alkylaluminoxane in step S4.

[0011] In a preferred embodiment, in step S4, the system temperature is reduced to 60°C within 30 minutes after the reaction in step S3 is completed, and a 1.0 mol / L alkylaluminoxane solution is slowly added to the suspension through a constant pressure dropping funnel under nitrogen protection, and the dropping rate is controlled at 2 mL / min. The stirring rate is kept unchanged during the dropping process. After the dropping is completed, the stirring is continued for 1 hour to form a stable primary catalyst complex. At this time, the oxidation-reduction potential of the system is measured to ensure that it is maintained in the range of -350 mV to -400 mV, and this potential value will be used as a basis for judging the amount of electron donor added in step S5.

[0012] In a preferred embodiment, in step S5, the ethyl benzoate electron donor is added to the primary catalytic complex system of step S4 in proportion at one time, the temperature is raised to 95 ℃ at a rate of 5 ℃ / min using a programmed temperature device, and the viscosity change of the system is monitored in real time through a rotary viscometer during the process. When the viscosity reaches 2000 mPa·s, the stirring speed is adjusted to 400 rpm to prevent local overheating. The reaction is continued at 95 ℃ for 2 hours. The reaction is stopped when the residual amount of free electron donor is less than 0.1 wt% as determined by high performance liquid chromatography. At this time, the viscosity data of the system will be used to set the supercritical treatment pressure of step S6.

[0013] In a preferred embodiment, in step S6, the composite system obtained in step S5 is transferred to a 500 mL high-pressure reaction kettle, supercritical carbon dioxide fluid is introduced as the treatment medium, the initial pressure is set to 1.2 MPa, and the system is treated at 60 ℃ for 1.5 hours at a stirring speed of 250 rpm. During the treatment, trace samples are collected through a kettle top sampling valve, and the distribution of active components in the pores of the carrier is observed by scanning electron microscopy. The pressure fluctuation range needs to be adjusted in real time during the treatment according to the compressive strength data of the carrier determined in step S2 to ensure that it does not exceed 80% of the compressive strength limit of the carrier. After the treatment is completed, the fluid is released by gradient pressure reduction, and the pressure reduction rate is controlled at 0.1 MPa / min to avoid collapse of the carrier structure.

[0014] In a preferred embodiment, in step S7, the material after step S6 is delivered to a centrifugal spray dryer through a peristaltic pump, the inlet air temperature is set to 120 ℃, the outlet temperature is 60 ℃, the atomizing disc rotates at 15000 rpm, and the feeding rate is 50 mL / min. The drying process is carried out in a nitrogen protective atmosphere. The dried catalyst powder needs to be sieved through a 200 mesh standard sieve, and the particles with a particle size range of 20-50 μm are collected. The loose bulk density needs to be controlled between 0.6 and 0.8 g / cm 3 The density parameter needs to be matched with the metal loading amount in step S3 to ensure the mass transfer efficiency of the subsequent polymerization reaction.

[0015] In a preferred embodiment, in step S8, the catalyst powder obtained in step S7 is loaded into a quartz tube reactor, and a programmed temperature activation process is carried out under nitrogen protection. The temperature is raised at a rate of 5 ℃ / min from room temperature to 180 ℃, and then held for 3 hours. During the process, the content changes of hydrogen chloride and organic volatile components in the tail gas are monitored online by a mass spectrometer. After the activation is completed, the reactor is naturally cooled to 80 ℃, and then quickly transferred to a vacuum dryer for storage. The activated catalyst needs to be used for polymerization reaction within 24 hours, and the active center concentration needs to reach more than 1.2 mmol / g. This index will directly determine the monomer conversion rate in the polymerization process.

[0016] In summary, by adopting the technical scheme, the application has the following beneficial effects: 1、In the application, by the design of the composite catalyst, the active center is precisely distributed and regulated, and the molecular chain structure regularity of the polyethylene resin is optimized from the source. Compared with the "random polymerization" of the traditional catalyst, the crystallinity uniformity can be increased by 30% to 50%, the scattering loss of light by the crystal defects is significantly reduced, the light transmittance of the film can be increased by 15% to 20%, and the strict requirements on the light transmittance of optical packaging, agricultural film and the like are met. The precise regulation of the active center of the composite catalyst promotes the synergistic optimization of the molecular chain entanglement density and the orientation degree, and forms an "ordered entanglement network structure". The polyethylene film prepared by using the catalyst has a transverse and longitudinal elongation at break of more than 500%, an impact strength increased by 25%, and a tensile strength increased by 18%, effectively solving the performance imbalance problem of "excessive rigidity and insufficient toughness" in the traditional process, and widening the application space of the material in the fields of express packaging, heavy load film and the like. The composite catalyst system does not need to rely on a single cocatalyst, can be compatible with various alkyl aluminum cocatalyst schemes, and directly realizes the preparation of high-performance resin by "one-step method", so that the catalyst modification, multi-stage parameter regulation and other cumbersome links in the traditional process are omitted. The process has strong adaptability and can be directly applied to the existing double-loop slurry device, and the modification cost is reduced by 60%, thereby providing a "low threshold and high return" technical upgrading path for the industry, and promoting the development of polyolefin production towards the integration of "precise catalysis-high efficiency forming-high performance".

[0017] 2、In the application, by the precise modification of the multi-level pore carrier and the synergistic loading of the bimetallic active center, the uniform distribution and controllable assembly of the catalytically active sites are realized. The vacuum drying treatment in a nitrogen atmosphere lays a stable foundation for the modification of the carrier surface, and the step-by-step grafting of the nano-MgO modifier and the organosilane coupling agent not only optimizes the pore structure of the carrier, but also constructs a surface microenvironment conducive to the uniform dispersion of the active metal. The reaction of the mixed solution of titanium tetrachloride and zirconium tetrachloride at a specific temperature and stirring rate, combined with the complex addition of alkyl aluminum oxane and benzoate electron donors, forms a catalytically active network with synergistic effect. This ordered preparation process effectively avoids the problem of fluctuation of catalytic efficiency caused by agglomeration of active centers in the traditional process.

[0018] 3、In the application, the organic connection of each link is realized through the dynamic correlation of key process parameters. The specific surface area data of the modified carrier directly guide the selection of the concentration of the metal salt solution, the viscosity change of the reaction system provides the regulation basis for the supercritical treatment pressure, and the process parameters of the spray drying form a matching relationship with the metal loading. This interlocking process design ensures that the whole process from carrier treatment to catalyst activation is controllable, which not only avoids the invalid loss of intermediate products, but also through precise operations such as programmed temperature activation, the catalyst powder maintains a stable active state in the storage and application process, creating favorable conditions for the efficient subsequent polymerization reaction. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The flow principle diagram of the application is shown in the figure. Figure 2 The performance comparison experimental data graph of the application is shown in the figure. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical scheme and advantages of the application clearer and more apparent, the application will be further described in detail below with reference to the drawings and examples. It should be understood that the specific examples described herein are only used to explain the application and not to limit the application.

[0021] Reference Figures 1-2 , Example: A catalyst for improving the uniformity of the molecular weight distribution of polyethylene resin, comprising: 100 parts by weight of a multi-level hole SiO2 / MgCl2 core-shell structure carrier, 5 parts by weight of titanium tetrachloride (TiCl4), 3 parts by weight of zirconium tetrachloride (ZrCl4), 25 parts by weight of alkyl aluminum alkoxide (MAO), 6 parts by weight of ethyl benzoate (DONOR), 4 parts by weight of nano MgO modifier, and 3 parts by weight of organosilane coupling agent.

[0022] A preparation method of a catalyst for improving the uniformity of the molecular weight distribution of polyethylene resin, comprising the following steps: S1: vacuum drying treatment of the multi-level hole SiO2 / MgCl2 core-shell structure carrier under nitrogen atmosphere for 2 hours to remove the surface adsorbed water. The dried carrier will be used for modification treatment in step S2; S2: adding nano MgO modifier and organosilane coupling agent to the carrier dried in step S1, stirring in a 40℃ constant temperature water bath for 3 hours to make the modifier uniformly grafted on the surface of the carrier. The pore size distribution data of the modified carrier will directly affect the selection of the concentration of the metal salt solution in step S3; S3: The modified support treated in step S2 is added to a mixed solution of titanium tetrachloride and zirconium tetrachloride and reacted at 80°C in an oil bath for 4 hours. The uniform loading of bimetallic active centers is achieved by controlling the stirring rate at 300 rpm. The stirring rate needs to be adjusted according to the specific surface area data of the support obtained in step S2. S4: An alkylaluminoxane solution is added dropwise to the suspension obtained in step S3. The stirring is continued under nitrogen protection for 1 hour to form a primary catalytic complex. This step needs to be started within 30 minutes after the reaction in step S3 ends to avoid deactivation of the metal active centers. S5: Ethyl benzoate electron donor is added to the reaction system in step S4 in proportion. The temperature is raised to 95°C for 2 hours of complexing reaction. The best amount of electron donor is determined by monitoring the viscosity change of the system online. The viscosity data will be used as a basis for setting the mixing intensity in step S6. S6: The complex system obtained in step S5 is transferred to a high-pressure reaction kettle. Supercritical fluid treatment is carried out at a pressure of 1.2 MPa for 1.5 hours to make the active components fully penetrate into the pores of the support. The treatment pressure parameter needs to refer to the compressive strength data of the support determined in step S2. S7: The material treated in step S6 is subjected to spray drying. The inlet temperature is controlled at 120°C and the outlet temperature is 60°C to obtain a catalyst powder with uniform particle size distribution. The drying process parameters need to match the metal loading in step S3. S8: The catalyst powder obtained in step S7 is subjected to programmed temperature activation under nitrogen protection. The temperature is raised from room temperature to 180°C at a rate of 5°C / min and held for 3 hours. The activated catalyst needs to be immediately transferred to the high-pressure reaction kettle used in step S4 for the next polymerization reaction.

[0023] In step S1, the multi-level pore SiO2 / MgCl2 core-shell structure support is placed in a vacuum drying oven under a nitrogen atmosphere. The drying temperature is set to 120°C. The vacuum degree is 0.08 MPa. The surface adsorbed water and residual organic solvents are completely removed by continuous treatment for 2 hours. The dried support needs to be immediately transferred to a dryer under inert gas protection and cooled to room temperature. The surface hydroxyl content needs to be controlled below 0.5 mmol / g. This index will directly determine the grafting efficiency of the modifier in step S2.

[0024] In step S2, the nano-sized MgO modifier and organosilane coupling agent are added to the carrier dried in step S1 in proportion, and the mixed system is transferred to a three-necked flask with a mechanical stirring device. The system is continuously stirred at a stirring rate of 200 rpm in a 40℃ constant temperature water bath for 3 hours. During this period, the zeta potential of the system is measured every 30 minutes to ensure that the modifier is uniformly grafted onto the surface of the carrier. After the modification is completed, the specific surface area and pore size distribution of the carrier are measured by static nitrogen adsorption method. The mesopore content needs to reach more than 60%, and this data will be used to adjust the initial concentration of the metal salt solution in step S3.

[0025] In step S3, the modified carrier treated in step S2 is added to the pre-prepared titanium tetrachloride and zirconium tetrachloride mixed solution in a solid-liquid ratio of 1:10, wherein the concentration of titanium tetrachloride is 0.5 mol / L and the concentration of zirconium tetrachloride is 0.3 mol / L. The mixed suspension is transferred to an oil bath reactor with a reflux condenser device. The oil bath temperature is set to 80℃, the temperature control accuracy is ±1℃, and the stirring rate is 300 rpm. The reaction is continuously carried out for 4 hours. During the reaction process, the particle dispersion degree is monitored by an online laser particle size analyzer. When the particle size distribution variation coefficient of the system decreases to less than 5%, the reaction is stopped. This dispersion degree index will directly affect the dropping rate of the alkylaluminoxane in step S4.

[0026] In step S4, the system temperature is reduced to 60℃ within 30 minutes after the reaction in step S3 is completed. Under the condition of nitrogen protection, an alkylaluminoxane solution with a concentration of 1.0 mol / L is slowly added to the suspension through a constant pressure dropping funnel. The dropping rate is controlled at 2 mL / min. The stirring rate remains unchanged during the dropping process. After the dropping is completed, the stirring is continued for 1 hour to form a stable primary catalytic complex. At this time, the oxidation-reduction potential of the system needs to be measured to ensure that it is maintained in the range of -350 mV to -400 mV. This potential value will be used as a basis for judging the amount of electron donor added in step S5.

[0027] In step S5, the ethyl benzoate electron donor is added to the primary catalytic complex system of step S4 in proportion. The programmed temperature rising device is started to raise the reaction temperature to 95℃ at a rate of 5℃ / min. In this process, the viscosity change of the system is monitored in real time by a rotary viscometer. When the viscosity reaches 2000 mPa·s, the stirring rate is adjusted to 400 rpm to prevent local overheating. The reaction is continuously carried out at 95℃ for 2 hours. The reaction endpoint is determined by high performance liquid chromatography. When the residual amount of free electron donor is less than 0.1wt%, the reaction is stopped. At this time, the viscosity data of the system will be used to set the supercritical treatment pressure in step S6.

[0028] The step S6, the composite system obtained in step S5 is transferred to a 500 mL high-pressure reactor, supercritical carbon dioxide fluid is introduced as a treatment medium, the initial pressure is set to 1.2 MPa, and the treatment is carried out at 60°C under the condition of a stirring rate of 250 rpm for 1.5 hours. During the treatment, trace samples are collected through a reactor top sampling valve, and the distribution of active components in the pores of the carrier is observed by scanning electron microscopy. During the treatment, the pressure fluctuation range is adjusted in real time according to the compressive strength data of the carrier determined in step S2 to ensure that it does not exceed 80% of the compressive strength limit of the carrier. After the treatment is completed, the fluid is released by gradient pressure reduction, and the pressure reduction rate is controlled at 0.1 MPa / min to avoid collapse of the carrier structure.

[0029] In step S7, the material after step S6 is delivered to a centrifugal spray dryer by a peristaltic pump, the inlet air temperature is set to 120°C, the outlet temperature is 60°C, the atomizing disc rotates at 15000 rpm, and the feeding rate is 50 mL / min. The dried catalyst powder is sieved through a 200 mesh standard sieve, and the particles with a particle size range of 20-50 μm are collected. The loose bulk density needs to be controlled between 0.6 and 0.8 g / cm 3 The density parameter needs to match the metal loading in step S3 to ensure the mass transfer efficiency of the subsequent polymerization reaction.

[0030] In step S8, the catalyst powder obtained in step S7 is loaded into a quartz tube reactor and subjected to programmed temperature activation treatment under nitrogen protection. The temperature rise rate is set to 5°C / min, and the temperature is raised from room temperature to 180°C and held for 3 hours. During this period, the mass spectrometer is used to monitor the changes in the contents of hydrogen chloride and organic volatile components in the tail gas online. After activation is completed, the reactor is naturally cooled to 80°C, then quickly transferred to a vacuum dryer for storage. The activated catalyst needs to be used for polymerization within 24 hours, and the active center concentration needs to reach more than 1.2 mmol / g. This index will directly determine the monomer conversion rate in the polymerization process Comparative Example: The traditional Ziegler-Natta catalyst preparation process is adopted. The ordinary SiO2 microsphere carrier is treated in a 100°C vacuum drying oven for 3 hours to remove surface moisture, then added into a titanium tetrachloride hexane solution and stirred in a 60°C constant temperature water bath for 5 hours to form a single metal titanium active center. After the reaction is completed, triethylaluminum cocatalyst is added, and the static activation is carried out at 80°C for 4 hours under nitrogen protection. Finally, the solid catalyst powder is obtained by vacuum drying. This process does not use carrier modification treatment, the active center loading process does not introduce electron donor regulation, and it also does not carry out supercritical fluid treatment or spray drying and other fine processing steps.

[0031] The performance comparison experimental data are shown in Figure 2 : From the above can be known: In the application, through the design of the composite catalyst, the precise distribution of active centers is realized, and the molecular chain structure regularity of the polyethylene resin is optimized from the source. Compared with the "random polymerization" of the traditional catalyst, the crystallinity uniformity can be improved by 30% to 50%, the scattering loss of light by the crystal defects is significantly reduced, and the light transmittance of the film can be improved by 15% to 20%, which meets the strict requirements of optical packaging, agricultural film and the like on the light transmittance. The precise regulation of the active centers of the composite catalyst promotes the synergistic optimization of the molecular chain entanglement density and the orientation degree, and forms an "ordered entanglement network structure". The polyethylene film prepared by using the catalyst has a transverse and longitudinal elongation at break of more than 500%, the impact strength is improved by 25%, and the tensile strength is improved by 18%, effectively solving the performance imbalance problem of "excessive rigidity and insufficient toughness" in the traditional process, and widening the application space of the material in the fields of express packaging, heavy load film and the like. The composite catalyst system does not need to rely on a single cocatalyst, can be compatible with various alkyl aluminum cocatalyst schemes, and directly realizes the preparation of high-performance resin through "one-step method" polymerization, thereby omitting the cumbersome steps such as catalyst modification and multi-stage parameter regulation in the traditional process. The process has strong adaptability and can be directly applied to the existing double-loop pipe slurry device, and the modification cost is reduced by 60%, thereby providing a "low threshold and high return" technical upgrading path for the industry, and promoting the development of polyolefin production towards the integration of "precise catalysis-high efficiency forming-high performance".

[0032] In the application, through the precise modification of the multi-level pore carrier and the synergistic loading of the bimetallic active center, the uniform distribution and controllable assembly of the catalytic active sites are realized. The vacuum drying treatment under a nitrogen atmosphere lays a stable foundation for the surface modification of the carrier. The step-by-step grafting of the nano-MgO modifier and the organosilane coupling agent not only optimizes the pore structure of the carrier, but also constructs a surface microenvironment conducive to the uniform dispersion of the active metal. The reaction of the mixed solution of titanium tetrachloride and zirconium tetrachloride under a specific temperature and stirring rate, combined with the complex addition of alkyl aluminum oxide and benzoate electron donors, forms a catalytic active network with synergistic effect. This ordered preparation process effectively avoids the catalytic efficiency fluctuation problem caused by the aggregation of active centers in the traditional process.

[0033] In the application, the dynamic correlation of key process parameters realizes the organic connection of each link. The specific surface area data after the modification of the carrier directly guide the selection of the metal salt solution concentration, the viscosity change of the reaction system provides a control basis for the supercritical treatment pressure, and the process parameters of the spray drying form a matching relationship with the metal loading. This ring-by-ring process design ensures the controllability of the whole process from carrier treatment to catalyst activation, avoids the invalid loss of intermediate products, and through precise operations such as programmed temperature activation, the catalyst powder maintains a stable active state in the storage and application process, creating favorable conditions for the efficient performance of subsequent polymerization reactions.

[0034] It is to be noted that the relational terms herein, such as first and second, and the like, are used solely to distinguish one from another entity or action without necessarily requiring or implying any actual relationship or order between such entities or actions. Moreover, the terms "comprises", "comprising", "includes", "including", or any other variation thereof, are intended to cover a non-exclusive inclusion such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by "comprises... a" does not, without more constraints, exclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.

[0035] The above embodiments are only used to illustrate the technical solutions of the present application, not to limit it; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacements for part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A catalyst for improving the uniformity of the molecular weight distribution of a polyethylene resin, characterized by: It comprises: 100 parts by weight of a multi-level hole SiO2 / MgCl2 core-shell structure carrier, 5 parts by weight of titanium tetrachloride, 3 parts by weight of zirconium tetrachloride, 25 parts by weight of alkyl aluminum oxane, 6 parts by weight of ethyl benzoate, 4 parts by weight of nano MgO modifier, and 3 parts by weight of organosilane coupling agent.

2. A process for preparing a catalyst for improving the uniformity of the molecular weight distribution of polyethylene resins as claimed in claim 1, characterized in that: The method comprises the following steps: S1: The multi-level hole SiO2 / MgCl2 core-shell structure carrier is vacuum dried for 2 hours under a nitrogen atmosphere to remove surface adsorbed water. The dried carrier will be used for the modification treatment of step S2; S2: Nano MgO modifier and organosilane coupling agent are added to the carrier dried in step S1, and stirred in a 40°C constant temperature water bath for 3 hours to make the modifier uniformly grafted on the surface of the carrier. The pore size distribution data of the modified carrier will directly affect the selection of the concentration of the metal salt solution in step S3; S3: The modified carrier treated in step S2 is added to the mixed solution of titanium tetrachloride and zirconium tetrachloride, and reacted at 80°C under oil bath conditions for 4 hours. By controlling the stirring rate at 300 rpm, uniform loading of bimetallic active centers is achieved. The stirring rate needs to be adjusted according to the specific surface area data of the carrier obtained in step S2; S4: Add alkyl aluminum oxane solution to the suspension obtained in step S3 and continue stirring under nitrogen protection for 1 hour to form a primary catalyst complex. This step needs to be started within 30 minutes after the reaction in step S3 is completed to avoid deactivation of the metal active centers; S5: Add the electron donor ethyl benzoate to the reaction system of step S4 in proportion, heat to 95°C, and perform complexation reaction for 2 hours. The optimal amount of electron donor is determined by online monitoring of the viscosity change of the system. The viscosity data will be used as a basis for setting the mixing intensity in step S6; S6: Transfer the complex system obtained in step S5 to a high-pressure reaction kettle and perform supercritical fluid treatment at 1.2 MPa for 1.5 hours to make the active components fully penetrate into the carrier pores. The treatment pressure parameter needs to refer to the carrier pressure resistance data measured in step S2; S7: The material treated in step S6 is subjected to spray drying, with the inlet temperature controlled at 120°C and the outlet temperature at 60°C to obtain a catalyst powder with uniform particle size distribution. The drying process parameters need to match the metal loading in step S3; S8: The catalyst powder obtained in step S7 is subjected to programmed temperature activation under nitrogen protection, with the temperature raised from room temperature to 180°C at a rate of 5°C / min and held for 3 hours. The activated catalyst needs to be immediately transferred to the high-pressure reaction kettle used in step S4 for the next polymerization reaction.

3. The method for preparing a catalyst to improve the uniformity of molecular weight distribution of polyethylene resin as described in claim 2, characterized in that: In step S1, the multi-level hole SiO2 / MgCl2 core-shell structure carrier is placed in a vacuum drying oven under a nitrogen atmosphere, the drying temperature is set to 120°C, and the vacuum degree is 0.08 MPa. The surface adsorbed water and residual organic solvents are completely removed after 2 hours of continuous treatment. The dried carrier needs to be immediately transferred to a dryer under inert gas protection and cooled to room temperature. The surface hydroxyl content needs to be controlled below 0.5 mmol / g, which will directly determine the grafting efficiency of the modifier in step S2.

4. The method for preparing a catalyst for improving the uniformity of molecular weight distribution of polyethylene resin as described in claim 2, characterized in that: In step S2, the nano-sized MgO modifier and organosilane coupling agent are added to the carrier dried in step S1 in proportion, and the mixed system is transferred to a three-necked flask with a mechanical stirring device. The system is continuously stirred at a stirring rate of 200 rpm in a 40 ℃ constant temperature water bath for 3 hours. During this period, the zeta potential of the system is measured every 30 minutes to ensure that the modifier is uniformly grafted onto the surface of the carrier. After the modification is completed, the specific surface area and pore size distribution of the carrier are measured by static nitrogen adsorption method, wherein the mesopore ratio needs to be more than 60%. This data will be used to adjust the initial concentration of the metal salt solution in step S3.

5. The method for preparing a catalyst for improving the molecular weight distribution uniformity of polyethylene resin as described in claim 2, characterized in that: In step S3, the modified carrier treated in step S2 is added to the pre-prepared mixed solution of titanium tetrachloride and zirconium tetrachloride in a solid-liquid ratio of 1:10, wherein the concentration of titanium tetrachloride is 0.5 mol / L and the concentration of zirconium tetrachloride is 0.3 mol / L. The mixed suspension is transferred to an oil bath reactor with a reflux condenser. The oil bath temperature is set to 80 ℃ with a temperature control accuracy of ±1 ℃. The stirring rate is 300 rpm and the reaction is continued for 4 hours. During the reaction, the particle dispersion degree is monitored by an online laser particle size analyzer. When the particle size distribution variation coefficient of the system decreases to less than 5%, the reaction is stopped. This dispersion degree index will directly affect the dropping rate of the alkylaluminoxane in step S4.

6. The method for preparing a catalyst for improving the uniformity of molecular weight distribution of polyethylene resin as described in claim 2, characterized in that: In step S4, the system temperature is reduced to 60 ℃ within 30 minutes after the reaction in step S3 is completed. Under the condition of nitrogen protection, the alkylaluminoxane solution with a concentration of 1.0 mol / L is slowly added to the suspension through a constant pressure dropping funnel. The dropping rate is controlled at 2 mL / min. During the dropping process, the stirring rate remains unchanged. After the dropping is completed, the stirring is continued for 1 hour to form a stable primary catalytic complex. At this time, the oxidation-reduction potential of the system is measured to ensure that it is maintained in the range of -350 mV to -400 mV. This potential value will be used as a basis for judging the amount of electron donor added in step S5.

7. The method for preparing a catalyst for improving the uniformity of molecular weight distribution of polyethylene resin as described in claim 2, characterized in that: In step S5, the ethyl benzoate electron donor is added to the primary catalytic complex system of step S4 in proportion. The programmed temperature rising device is started to raise the reaction temperature to 95 ℃ at a rate of 5 ℃ / min. During this process, the viscosity change of the system is monitored in real time by a rotary viscometer. When the viscosity reaches 2000 mPa·s, the stirring rate is adjusted to 400 rpm to prevent local overheating. The reaction is continued at 95 ℃ for 2 hours. The reaction endpoint is determined by high performance liquid chromatography. When the residual amount of free electron donor is less than 0.1 wt%, the reaction is stopped. At this time, the viscosity data of the system will be used to set the supercritical treatment pressure in step S6.

8. The method for preparing a catalyst for improving the uniformity of molecular weight distribution of polyethylene resin as described in claim 2, characterized in that: In the step S6, the composite system obtained in the step S5 is transferred into a 500 mL high-pressure reactor, supercritical carbon dioxide fluid is introduced as the treatment medium, the initial pressure is set to 1.2 MPa, and the treatment is carried out at 60°C under the condition of a stirring rate of 250 rpm for 1.5 hours. During the treatment, trace samples are collected through a reactor top sampling valve, and the distribution of the active components in the pores of the carrier is observed by using a scanning electron microscope. During the treatment, the pressure fluctuation range is adjusted in real time according to the compressive strength data of the carrier determined in the step S2, so as to ensure that the pressure fluctuation range does not exceed 80% of the compressive strength limit of the carrier. After the treatment is completed, the fluid is released by a gradient pressure reduction method, and the pressure reduction rate is controlled at 0.1 MPa / min to avoid the collapse of the structure of the carrier.

9. The method for preparing a catalyst for improving the uniformity of molecular weight distribution of polyethylene resin as described in claim 2, characterized in that: The material after step S6 is delivered into a centrifugal spray dryer by a peristaltic pump, the inlet temperature is set to 120℃, the outlet temperature is set to 60℃, the rotating speed of the atomizing disc is set to 15000rpm, the feeding rate is set to 50mL / min, the drying process is carried out under a nitrogen atmosphere, the dried catalyst powder is sieved by a 200 mesh standard sieve, and the particles with a particle size ranging from 20 to 50μm are collected, and the loose bulk density is controlled to be between 0.6 and 0.8g / cm 3 .

10. The method for preparing a catalyst for improving the uniformity of molecular weight distribution of polyethylene resin as described in claim 2, characterized in that: In the step S8, the catalyst powder obtained in the step S7 is loaded into a quartz tube reactor, and a programmed temperature activation treatment is carried out under the protection of nitrogen. The temperature rising rate is set to 5°C / min, the temperature is raised from room temperature to 180°C, and then the temperature is kept for 3 hours. During the treatment, the content changes of hydrogen chloride and organic volatile components in the tail gas are monitored online by using a mass spectrometer. After the activation is completed, the reactor is naturally cooled to 80°C, and then is quickly transferred into a vacuum dryer for storage. The activated catalyst needs to be used for a polymerization reaction within 24 hours, and the active center concentration of the activated catalyst needs to reach 1.2 mmol / g or more.

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