Silica-supported chromium catalyst and method for producing the same

Abstract

Disclosed are compositions, such as catalyst precursors or catalysts, comprising a Cr-coated silica support having specifically defined contents of Na and Al, such that the resulting Cr / silica catalyst has an increased MI potential. In embodiments, the disclosed catalyst composition comprises a silica-containing substrate made using an alkali-coagulated gel and containing a catalytically active metal consisting of Cr, with less than 50 ppm of Al impurities and less than 800 ppm of Na in the catalyst composition. The disclosed composition has an increased MI potential compared to catalysts having higher Al contents, lower Na:Al ratios, or both. Also disclosed are methods of making the disclosed compositions and methods of using the same to prepare polyethylene.

Description

[0001] The present invention generally relates to silica catalysts for the production of polyethylene. More specifically, the present invention relates to chromium / silica (Cr / SiO2) catalysts for the production of high-density polyethylene (HDPE). Also disclosed are methods for making such catalysts using an alkali-coagulated gel, and methods for using such catalysts in HDPE applications, particularly small blown HDPE applications. Background of the Invention

[0003] The production of polyethylene, such as high-density polyethylene (HDPE), is a multi-billion dollar industry that typically utilizes supported catalysts in the polymerization process. Specifically, the supported chromium oxide catalysts used in the industry generally comprise chromium oxide and a support containing one or more of silica, titania, thoria, alumina, zirconium oxide, or aluminophosphates.

[0004] One particularly useful catalyst is known as a Phillips-type catalyst, which comprises chromium oxide supported on silica gel (e.g., Cr / SiO). This type of catalyst is a versatile polymerization catalyst for the production of HDPE, primarily because it produces HDPE with a broad molecular weight distribution that is particularly suitable for blow molding applications. Commercial silica supports for Phillips-type catalysts are typically derived from inorganic silicates, with sodium silicate being the most widely used. Consequently, commercial Phillips-type catalysts have varying levels of sodium impurities, depending on the washing procedures used to produce the silica support.

[0005] It has been found that adding large amounts of alkali or alkaline earth metal salts to chromium oxide catalysts increases the melt index ("MI") potential and activity of the catalyst when the catalyst is activated at a temperature below the sintering temperature of the catalyst. For example, U.S. Patent Nos. 5,444,132 and 5,284,811 to Witt et al. disclose impregnation of silica-titania or aluminophosphate supports (previously purified to remove alkali metal salt by-products generated by support manufacture) with 50-500 μmol of alkali or alkaline earth metal salt per gram of catalyst (equivalent to 1,150-11,500 ppm Na in the catalyst) using a non-aqueous impregnation solution. However, this large amount of Na will cause the catalyst to sinter at 750-850°C, which is a typical activation temperature for small blow molding (SBM) resin production.

[0006] Other studies have shown that alkali metal doping has a similar effect on the performance of Cr / silica catalysts in ethylene polymerization. In one such study, the lowest Na content disclosed was 0.2 mmol / g catalyst, which is equivalent to 4,600 ppm Na. See, for example, J. Catal., Vol. 176, 344-351 (1998). Adding sodium at these levels, while increasing the MI potential of catalysts activated at lower temperatures, sintered the catalyst at higher activation temperatures, thereby reducing the catalyst MI potential. The maximum MI achievable with these high levels of Na doping at any given activation temperature is lower than the MI of catalysts without Na doping but activated at higher temperatures.

[0007] Prior art focused on improving the MI potential of catalysts often teaches the effect of high levels of Al in the finished catalyst, but fails to recognize the effect of Al impurities in the support on the resulting catalyst. For example, studies on supported chromium catalysts describe the addition of Al to Cr / silica catalysts to increase polymer MI. See Marsden, "Advances in supported chromium catalysts," Plastics, Rubber and Composites Processing and Applications 21 (1994) 193-200; and U.S. Patent No. 4,119,773. These references describe that the relationship between Al and MI potential depends on many variables associated with Al incorporation, including the content, the nature of the Al compound, and the manner in which the Al is added.

[0008] Cr / silica catalysts, such as those described in the aforementioned references, are commercially used to produce HDPE for small-scale blow molding applications. These catalysts are typically activated at high temperatures, such as 750-850°C, to achieve polymer MI targets. Because these activation temperatures are very close to the sintering temperature of the Cr / silica catalysts, the ability to enhance the catalyst's MI potential by further increasing the activation temperature is very limited. While modification of the Cr / silica catalysts with metals such as Al and Ti can significantly improve the catalyst's MI potential, these modifications also broaden the molecular weight distribution of the resulting HDPE and increase catalyst manufacturing costs. Both are undesirable for SBM applications.

[0009] For at least the aforementioned reasons, there is a need for Cr / silica catalysts with enhanced MI potential to allow for a sufficiently wide difference between the activation temperature and the sintering temperature required to achieve the resin's MI target. This enhanced MI potential would impart some flexibility to the catalyst activation and polymerization process without significantly altering the molecular weight distribution of the resulting HDPE. Catalysts with enhanced MI potential would also allow for the production of HDPE with a higher MI than currently achievable using commercially available Cr / silica catalysts, to meet additional market demands, such as HDPE homopolymers with an MI above 1.0 g / 10 min for SBM applications. To address the aforementioned needs, the present application describes a Cr-only catalyst having enhanced MI potential compared to catalysts currently on the market for small-scale blow molding HDPE resin production. Summary of the Invention

[0010] Disclosed is a composition, such as a catalyst precursor or catalyst, exhibiting enhanced MI potential, comprising a Cr-coated silica support having defined contents of an alkali metal or alkaline earth metal (including, for example, Na, Mg, or Ca) and Al. In one embodiment, disclosed is a catalyst composition comprising a Cr-coated silica support having defined contents of Na and Al, such that the resulting Cr / silica catalyst has enhanced MI potential. In one embodiment, the present application relates to a catalyst composition comprising a silica-containing substrate containing a catalytically active metal consisting of Cr. In one embodiment, the catalyst comprises Al in an amount less than 50 ppm and Na in an amount less than 800 ppm of the catalyst composition. In one embodiment, the amount of Na and Al is present in a Na:Al molar ratio greater than 5, greater than 10, greater than 20, or even greater than 30, such as a molar ratio of 10-40. It is understood that the molar ratio may comprise any combination of these endpoints, such as 5 to 10, 5 to 20, 5 to 30, etc., or a range of 5 to 40, 10 to 40, 20 to 40, or other combinations thereof.

[0011] Also disclosed is a method for preparing the disclosed composition comprising a Cr-coated support having specifically defined contents of Na and Al, such that the resulting Cr / silica catalyst exhibits enhanced MI potential. In one embodiment, the method comprises reacting a metal silicate, such as sodium silicate, with an acid to form a hydrosol that is converted to a hydrogel precursor, and substantially leaching out Al impurities associated with the sodium silicate by washing to reduce the amount of Al impurities in the resulting catalyst precursor to an amount of less than 50 ppm.

[0012] The method further comprises aging the hydrogel precursor to form a hydrogel having a hydrogel having a hydrogel content greater than 200 m 2 / g, such as greater than 250m 2In one embodiment, the hydrogel exhibits a surface area of about 300 m 2 The aging process includes mixing the hydrogel precursor with a neutral or alkaline aqueous solution to form an aqueous dispersion exhibiting a neutral or alkaline pH. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a flow chart illustrating the general steps used in the alkaline coagulation gel process to make a silica support and a Cr / silica catalyst according to an embodiment of the present invention.

[0015] Figure 2 Is compared to Figure 1 Figure 1 is a graphical representation of the Al leaching curve of a hydrogel precursor soaked in acid water at temperatures between 20 and 90° C. The total amount of acid (including the acid used to form the gel and the acid used for acidification before soaking) is about 20% more than the Na2O in the sodium silicate used to form the gel.

[0016] Figure 3 Is compared based on Figure 1 Graph of the melt index (MI) of homopolymers and copolymers prepared with Cr / silica catalysts having various Al contents and Na:Al ratios made by the method described in . Detailed Description of the Invention

[0018] As used herein, silica gels are described as either “acid-set gels” or “base-set gels.” Both types of gels are produced via an acid-base reaction using a silicate, such as sodium silicate, and an inorganic acid, such as sulfuric acid.

[0019] An "acid-set gel" is a gel formed by an acid-base reaction that is non-stoichiometric but utilizes more acid than base, such as a higher volume or weight percentage of acid.

[0020] An "alkali-set gel" is a gel formed by the acid-base reaction in a non-stoichiometric amount but utilizing more base than acid, such as a higher volume or weight percentage of base.

[0021] As used herein, "Cr-only catalyst" refers to a catalyst to which no other multivalent metals having an oxidation state of +3 or higher are added.

[0022] The term "composition" is sometimes referred to as a "catalyst composition" and is intended to describe an unactivated catalyst precursor, as well as an activated catalyst. For example, after Cr impregnation / drying, the product typically contains a Cr compound having Cr in the +3 oxidation state. This product is a catalyst precursor because it has no catalytic activity for ethylene polymerization. The catalyst precursor must be activated in an oxidizing atmosphere (such as heating in a fluidized bed reactor in dry air) to convert the Cr from the +3 oxidation state to the +6 oxidation state. The activated product is more accurately referred to as a catalyst.

[0023] As used herein, "cold water washing" means that the washing water temperature is 55°C or lower.

[0024] As used herein, "hydrosol" refers to a mixture of metal silicate and acid in liquid form.

[0025] As used herein, "hydrogel precursor" refers to an unwashed and / or unaged hydrogel.

[0026] As used herein, "hydrogel" refers to a hydrogel that has been washed and aged.

[0027] As used herein, "alcogel" refers to a hydrogel that has been further washed with an organic solvent, such as by replacing the water in the hydrogel with an alcohol to produce a hydrogel that is substantially free of water.

[0028] As used herein, "dried gel," also referred to as "support precursor," refers to the dried gel before sizing. "Support" refers to the dried gel after sizing.

[0029] As used herein, "melt index" (MI) and "high load melt index" (HLMI) are measures of the fluidity of a molten polymer and are inversely related to molecular weight and are measured according to ASTM D-1238-4 at 190°C using loads of 2.16 kg and 21.6 kg, respectively.

[0030] The "MI potential" of a Cr catalyst as described herein is a function of, and is proportional to, the MI of the polymer produced by the catalyst. For example, when a Cr catalyst activated at a specified temperature is used to polymerize ethylene under specified polymerization conditions, the MI potential of the Cr catalyst is proportional to the MI of the polymerized ethylene. The higher the MI of the polymer, the higher the catalyst MI potential.

[0031] The density of the polymer is measured by the procedure of ASTM D-792-13.

[0032] MI, HLMI and density were measured on polyethylene pellets obtained by processing stabilized polymer powder using a single screw extruder under nitrogen.

[0033] The chromium content of the catalyst composition was measured using X-ray fluorescence ("XRF") using a PANalytical Magix Pro automated continuous spectrometer. Samples were calcined in air at 1000°C and subsequently prepared as fused beads using a lithium borate flux. Fusion was typically performed at temperatures between 1000°C and 1250°C. Cr content is reported as a weight percentage of the catalyst precursor after calcination at 1000°C.

[0034] The contents of Na and Al in the catalyst composition were determined by atomic adsorption spectroscopy (AA) using a Perkin-Elmer Analyst 100 spectrometer and by ACTIVA TM Measured using inductively coupled plasma ("ICP") spectroscopy on a HORIBA Jobin Yvon ICP-AES spectrometer. A sample of the catalyst precursor was digested with hydrofluoric acid (HF). The resulting silicon tetrafluoride (SiF4) was evaporated, and the residue was analyzed for Na and Al. Na and Al contents are reported as parts per million of the catalyst precursor after drying at 120°C.

[0035] Surface area and pore volume were measured by nitrogen porosimetry using a Quantachrome Autosorb-6 test unit. Samples were first degassed on an Autosorb-6 degassing unit at 350°C for at least 4 hours. Multi-point surface area was calculated using BET theory, selecting data points within the P / P0 range of 0.05 to 0.30. Pore volume measurements were recorded at a P / P0 of 0.984 on the desorption leg. The average pore size was calculated using the following equation, assuming cylindrical pores.

[0036]

[0037] Particle size is determined using a Malvern Mastersizer TM The 2000 model instrument measures light by laser scattering. This instrument uses Mie theory to calculate particle size distribution. Mie theory predicts how light is scattered by spherical particles and takes into account the particle's refractive index. The real value for the refractive index of silica is 1.4564, and the imaginary value is 0.1. The refractive index for aqueous dispersions is 1.33.

[0038] Because resin properties are determined by catalyst properties and polymerization conditions, controlling catalyst design is necessary to be able to control resin properties. The catalyst support generally acts as a dispersant for the active Cr centers and directly affects the resulting polymer properties. The surface area (SA), pore volume (PV), and pore size distribution of the catalyst may affect the resulting polymer. Other things being equal, Cr / SiO catalysts with higher pore volumes (and therefore larger pores) produce polymers with lower MW and higher MI.

[0039] In one embodiment, the alkali-coagulated gel described herein and comprising a low Al content results in a Cr / silica catalyst having a higher MI potential compared to an alkali-coagulated gel having a higher Al impurity content but the same Na content. In addition, the inventors have found that, regardless of the Al impurity content, the Cr / silica catalyst with an increased Na content generally results in a higher MI potential. It has also been found that using too high a Na content (e.g., when the Al impurity content is high) will cause the catalyst to sinter at a given activation temperature of typically 750-850°C. Therefore, it is desirable to keep the Al impurity content as low as possible. As described in the Background of the Invention section, and as will be understood by those skilled in the art, this is generally not the norm. In contrast, conventional alkali-coagulated gel processes generally result in gels having a higher Al impurity content.

[0040] Commercial silica gel production typically uses sodium silicate, such as sodium silicate with a SiO2:Na2O weight ratio of 3.2, and an inorganic acid such as sulfuric acid via an acid-base reaction. However, for commercial production, the acid-base reaction is almost never stoichiometric, and either more acid is used (in which case the gel formed is referred to as an acid-coagulated gel) or less acid is used (in which case the gel formed is referred to as an alkali-coagulated gel). The alkali-coagulated gel process is widely used in commercial silica gel manufacturing because it is beneficial for producing silica gel particles with a spherical shape and a particle size of tens of microns to several millimeters. Commercial sodium silicate is produced from sand that typically contains various multivalent cations such as Al as impurities. When forming a gel at an alkaline pH (as in the case of alkali-coagulated gels), these multivalent cations tend to replace a small portion of the Si atoms in the SiO2 gel framework, and are therefore firmly "locked" into the gel structure. Therefore, compared to the corresponding acid-coagulated gel, the alkali-coagulation process typically produces a gel with lower purity. As commercially available supplies of high purity sand become increasingly scarce, it is important to develop processes that allow the production of alkali-coagulated gels having high purity.

[0041] In view of the foregoing, disclosed herein is a catalyst composition having a support with defined Na and Al contents that results in a Cr / silica catalyst with enhanced MI potential. While not wishing to be bound by theory, it is believed that both acidic and basic sites in the silica support (and therefore the catalyst) can enhance the MI potential of the Cr / silica. Na introduces basic sites while Al introduces acidic sites. When both Na and Al are present, they can cancel each other's effects due to acid-base neutralization.

[0042] In various embodiments, Cr / silica catalysts are disclosed wherein the silica support has an Al content of less than 50 ppm of the catalyst composition, such as less than 25 ppm or even in an amount between 10 and 40 ppm.

[0043] Also disclosed are Cr / silica catalysts wherein the silica support has less than about 800 ppm of Na impurity, e.g., the final catalyst composition may comprise Na in an amount from 50 to 800 ppm, 200 to 800 ppm, or 200 to 700 ppm of the catalyst composition, such as less than about 600 ppm, or even in an amount of about 50-600 ppm.

[0044] Also disclosed are Cr / silica catalyst compositions wherein the Na:Al molar ratio is higher than 5, higher than 10, higher than 20, or even higher than 30, such as in the molar ratio of 10-40.

[0045] As described herein, the catalytically active metal comprises chromium, which may be added to the silica support using at least one chromium compound. In one embodiment, the chromium compound is chromium oxide or a compound that can be converted to chromium oxide by calcination. For example, the chromium-containing compound may be a water-soluble compound or an organic solvent-soluble compound. Non-limiting examples include chromium acetate, chromium nitrate, chromium sulfate, chromium acetylacetonate, chromium trioxide, ammonium chromate, tert-butyl chromate, and other soluble chromium compounds.

[0046] In the preparation of the catalysts described herein, at least one of the chromium-containing compounds should be used in an amount sufficient to allow the catalyst to contain Cr in an amount of 0.01 to 3 wt %. In certain embodiments, the catalyst contains Cr in an amount of 0.1 to 2 wt %, such as 0.25 to 1.5 wt %.

[0047] In one embodiment, a method for preparing a catalyst composition comprising a Cr-coated support having specifically defined Na and Al contents is disclosed. The method comprises reacting a metal silicate, such as sodium silicate, with an acid to form a hydrosol that subsequently solidifies into a hydrogel precursor. Because the sodium silicate contains Al as an impurity, the method further comprises treating the hydrogel precursor to reduce the amount of Al impurity and forming a hydrogel. Thus, in a broader sense, a method is described that comprises preparing a gel and leaching Al to reduce the Al content to a desired level.

[0048] As previously described, the Al is adjusted in the silica support to achieve an amount of less than 50 ppm of the catalyst composition, such as less than 25 ppm. In one embodiment, the Al is adjusted in the silica support to achieve an amount of 10-40 ppm of the catalyst composition. In one embodiment, a high purity silicate with a very low Al content can be used. Assuming the Al content in the silicate is low enough, this can eliminate the need to treat the hydrogel precursor to remove Al. However, high purity silicates can be expensive and not readily available.

[0049] The methods described herein can also be performed to adjust the Na content in the final catalyst composition to an amount of less than 800 ppm of the catalyst composition. In certain embodiments, the final catalyst composition comprises Na in an amount of 50 to 800 ppm, 200 to 800 ppm, or 200 to 700 ppm of the catalyst composition, such as less than about 600 ppm, or even in an amount of about 50-600 ppm.

[0050] The resulting Na:Al molar ratio is typically adjusted to the values described herein.

[0051] In one embodiment, the method further comprises aging the hydrogel precursor to form a hydrogel having a molecular weight greater than 200 nm. 2 / g, or greater than 250m 2 / g, or such as about 300m 2 / g of surface area of the hydrogel, wherein aging comprises mixing the hydrogel precursor with a neutral or alkaline solution to form an aqueous dispersion having a pH of at least 6, such as about 8-9, and maintaining the dispersion at a temperature of 70-100°C for a period of 4-36 hours.

[0052] The method may further include drying the gel to produce a dried gel (support precursor) using any common technique known in the industry, such as spray drying, flash drying, or solvent washing / drying techniques. The method may also include post-drying steps such as grinding, sieving, and / or grading the precursor into a support having a desired particle size distribution, followed by impregnation with a chromium compound to form a silica-supported Cr catalyst. A process for adjusting the Na content of the catalyst to achieve an amount of Na less than 800 ppm of the catalyst composition may be performed before or during the impregnation step, such that the amount of Na is adjusted to achieve a Na:Al molar ratio greater than 5, greater than 10, greater than 20, or even greater than 30, such as a molar ratio of 10-40.

[0053] In one embodiment, a method for producing an alkaline-coagulated gel and capturing the gel in acidic water is described. Specifically, the reaction product of sodium silicate and acid is an alkaline hydrosol having a molar ratio of H2SO4:Na2O of typically 0.7 to 0.95. The method for treating the alkaline hydrogel precursor to reduce the amount of Al impurities may include a bead method. For example, the bead method comprises spraying the alkaline hydrosol into air to solidify it into beads, and capturing the beads in an acidic solution to provide a gel dispersion having a pH below 2, such as below 1, and is carried out at a temperature below about 60°C, such as below 55°C. The gel is soaked in the acidic solution for not less than 2 hours.

[0054] Applicants have found that temperature and pH have a significant effect on leaching characteristics and report herein how leaching characteristics are affected by temperature.

[0055] Temperature: Al leaching at acidic pH has been shown to be particularly sensitive to temperature, with leaching preferably being conducted at 60°C or lower, such as 55°C or lower. Figure 2 As shown in Figure 1, of the four temperatures studied (20°C, 50°C, 70°C, and 90°C), 90°C was the least efficient for leaching, and 70°C was not very efficient. Extending the time at these temperatures did not improve leaching. Leaching was found to be most efficient at 50°C over the time period studied. Without being bound by any theory, this is presumably due to faster kinetics at 50°C than at 20°C. Leaching at 20°C was not as efficient as at 50°C over the time period studied, but it appears that more Al could be leached if given a longer time. While not being bound by any theory, the % Al leached at equilibrium showed improvement with lower temperatures, but higher temperatures are preferred for kinetic considerations.

[0056] pH: In embodiments, the pH should be maintained below approximately 3-4 until most Al impurities are removed from the system, both from the gel and from the liquid phase in contact with the gel. At pH levels above 3-4, Al will redeposit on the silica surface. Therefore, a pH <2, such as <1, is desirable for leaching Al from the gel structure into the liquid phase. However, to prevent Al redeposition on the silica surface, the pH of the liquid phase should be maintained at <3-4 until the liquid phase is substantially free of leached Al. In one embodiment, washing at ambient temperature in water acidified to a pH of approximately 3 may be the best way to minimize residual Al levels. However, such lower temperatures extend washing times, which is undesirable for commercial production. To control costs, washing can be performed at higher temperatures (<60°C) throughout the entire wash period, or at least for the first few hours, to remove most of the leached Al, followed by washing at even higher temperatures. Washing with neutral water can also be performed if more acid is used for acidification before the pre-wash. As previously stated, the pH of the gel / water dispersion should not exceed pH 3-4 until most of the leached Al has been removed from the dispersion.

[0057] While soaking and pre-washing can be performed as separate steps, in embodiments, soaking and pre-washing can be combined. For example, if pre-washing is performed at a pH of about 2 or lower and a temperature of <60°C, it has the same effect as soaking, and can be more effective than soaking, depending on whether and how often the liquid phase is refreshed.

[0058] In one embodiment, the beads captured in the acidic solution are then washed in water acidified to a pH of about 3 at ambient temperature, followed by aging in water adjusted to a pH of 8-9 using aqueous ammonia (NH4OH) and at 70-90°C for 4-36 hours to achieve a particle size greater than 200 m 2 / g, or greater than about 250m 2 / g, such as about 300m 2 / g of surface area of gel.

[0059] Optionally, the gel can be washed again after aging to further reduce alkali and alkaline earth metal impurities. To facilitate the removal of alkali and alkaline earth metals, the pH of the aged hydrogel can be lowered to about 2, and the silica hydrogel can be washed with neutral water or, in embodiments, with water acidified to a pH of about 3.

[0060] The hydrogel can be dried by one of the techniques known in the art. One suitable method is flash drying. Another suitable method is spray drying. Another suitable method is washing the hydrogel with an organic solvent and then drying the gel under vacuum. The dried gel is size-sorted to a desired particle size distribution to form a silica support.

[0061] As mentioned, the amount of Al and Na affects the MI potential of the resulting catalyst. Therefore, the method further comprises adjusting the Na content of the catalyst before (e.g., by adjusting the washing process) or during the impregnation step to achieve an amount of Na less than 800 ppm of the catalyst composition and a Na:Al molar ratio as described herein.

[0062] In an embodiment, a method for preparing a composition is disclosed, comprising: reacting sodium silicate containing Al as an impurity with an acid to form a hydrosol; forming the hydrosol into a gel precursor; treating the gel precursor to reduce the amount of Al; aging the gel precursor to form a gel having a viscosity greater than 200 m / s. 2 / g, such as 300m 2 / g of surface area of a hydrogel, wherein aging comprises mixing the gel precursor with a neutral solution or an alkaline solution to form a hydrogel having a pH of at least 6; and drying to produce a dried gel, followed by impregnation of the dried gel with a chromium compound to form a Cr on silica catalyst composition. There are many optional processing steps that can be used, including grinding the hydrogel to form particles of a desired size, or sieving / classifying the dried gel to produce a support having a desired particle size distribution.

[0063] The particles of the catalyst precursor according to the present invention may have a d90 (90% by volume of the particles have a diameter less than this diameter) of 500 μm or less, for example 400 μm or less. In certain embodiments, the particles may have a d50 of 300 μm or less. The particles may also have a d10 of 1 μm or greater, for example 10 μm or greater. In certain embodiments, the particles have a d50 of 1 to 300 μm, 5 to 250 μm, or 25 to 150 μm. The particles may be prepared by comminution combined with size classification, by means such as screening or air classification, or by routes such as spray drying followed by size classification.

[0064] The ethylene polymerization catalyst is obtained or obtainable from the composition described herein by heating the catalyst precursor composition at a temperature of 200 to 1200° C. in a non-reducing atmosphere, such as an oxidizing atmosphere, for an activation period of 30 minutes to 15 hours, such as 400 to 850° C. for about 4 hours to 12 hours.

[0065] The resins produced by the catalysts described herein are particularly suitable for small blow molding applications. As previously mentioned, in order to produce resins using the disclosed catalysts, the catalyst must first be activated using a thermal step, such as in a fluidized bed reactor. In one embodiment, the catalyst can be activated in dry air in a fluidized bed reactor, such as at a temperature of 750 to 850° C., for example, 800 to 850° C., for 30 minutes to 15 hours, such as 6 hours.

[0066] The process according to the invention can be applied to prepare copolymers of polyethylene and ethylene, wherein the combined ethylene is present in an amount of at least 25 mole %, such as at least 50 mole %, or at least 75 mole %. The copolymer can be prepared from a mixture of ethylene and one or more C3 to C8 alpha-olefins.

[0067]

[00146] Features and advantages of the catalysts and methods disclosed herein are illustrated by the following examples, which should not be construed as limiting the scope of the invention in any way. Example

[0068] Examples 1 and 2 below describe alkali-set gels having low Al content prepared from standard purity sodium silicate and high purity sodium silicate, respectively.

[0069] Example 1: Preparation of Alkali-Coagulated Gel with Low Al Content from Standard Purity Sodium Silicate

[0070] This article provides a method for producing an alkali-coagulated gel of a silica support for a silica-supported Cr catalyst according to the present invention. The method starts with sodium silicate containing Al as an impurity. Figure 1 By a series of steps as shown in the flow chart of FIG, the Al and Na contents are reduced to acceptable levels and ratios in the resulting catalyst.

[0071] A dilute sodium silicate solution with a SiO2:Na2O weight ratio of 3.3 is first reacted with dilute sulfuric acid to form a hydrosol with the following composition: 12 wt% SiO2; 0.8 molar ratio of H2SO4:Na2O. The sodium silicate solution contains approximately 400 ppm Al based on the weight of the SiO2. Therefore, the resulting hydrosol is alkaline.

[0072] The hydrosol is sprayed into the air, where it breaks down into droplets and solidifies into beads with a diameter of several millimeters, which are then captured in solution. Unlike the conventional bead method used to prepare Comparative Catalyst 1, which utilizes water or a solution (such as an aqueous solution of ammonium sulfate, sodium bicarbonate, etc.) to buffer the pH of the bead / solution system at an alkaline pH, the inventive method for producing a gel with reduced Al content uses an aqueous acid solution to capture the beads. Thus, the total acid used to form the beads according to this example (including the acid in the capture solution) results in a gel dispersion with a molar ratio of H2SO4:Na2O of approximately 1.2 and a pH <1. The hydrogel precursor beads are immersed in this acid solution at ambient temperature for ≥6 hours.

[0073] The beads were washed at ambient temperature with water acidified to a pH of about 3, resulting in beads that were essentially free of Na and Al. After the washing step, aqueous ammonia (NH4OH) was then added to the solution to raise the pH to about 9. Aging was performed at 70°C for about 16 hours to achieve a solution having a pH of about 300 m 2 / g of surface area of gel.

[0074] Acid is then added to lower the pH to about 2. The beads are then washed with water acidified to a pH of about 3. The beads are further washed with methanol until substantially free of water (such as <2 wt% water) and dried to produce a support precursor, and then size sorted to a d50 of about 100 μm to produce a support.

[0075] Comparative Catalyst 1 : This catalyst is a silica support containing about 330 ppm Al and 40 ppm Na, prepared by a conventional alkali coagulated bead process and impregnated with 1 wt% Cr using a methanolic solution of chromium acetate.

[0076] Catalyst A : This catalyst is a silica support modified according to the present invention, resulting in a support with a low Al content of about 20 ppm and a Na content of about 16 ppm. This support was impregnated with 1 wt% Cr using a methanolic solution of chromium acetate.

[0077] Catalyst B : This catalyst is the same silica support as Catalyst A, but impregnated with 1 wt% Cr and about 400 ppm Na using a methanolic solution containing chromium acetate and sodium formate. The properties of Comparative Catalyst 1 and Inventive Catalysts A and B are provided in Table 1.

[0078] Table 1. Catalyst properties

[0079]

[0080] These catalyst samples were evaluated for homopolymerization and copolymerization. For homopolymerization, approximately 10 g of catalyst was activated in a fluidized bed reactor in dry air. The temperature was maintained at 850° C. for 6 hours, followed by cooling. At 300° C., the air was switched to nitrogen. Approximately 0.17 g of the activated catalyst was loaded into a 2.5 L slurry polymerization reactor. Polymerization was carried out at 102° C. using 10 mol % ethylene in isobutane. The polymerization reaction was terminated when approximately 425 g of polyethylene was produced. For copolymerization, the catalyst was activated similarly, except at 815° C. Polymerization was also carried out similarly, except at 100° C. and using 5 mL of 1-hexene.

[0081] The polymerization results are summarized in Table 2 below. It is clear that when the Al content of the catalyst is reduced from 330 ppm to 19 ppm, the polymer MI increases significantly. For catalysts with low Al content, when the catalyst Na:Al molar ratio is increased from 1 to 26, the polymer MI increases further significantly. See, for example Figure 3 Graph showing MI results.

[0082] Table 2. Aggregate evaluation results

[0083]

[0084] Example 2: Preparation of alkali-coagulated gel with low Al content from high-purity sodium silicate

[0085] This example describes the method used to prepare the inventive and comparative supports. The methods used to make the different supports were identical, with the difference being that the inventive supports were made using high purity sodium silicate from PQ (trade name FS) manufactured.

[0086] The high-purity silicate used to prepare the support of the present invention contains approximately 20 ppm Al based on the weight of SiO2. The resulting beads have 12% SiO2 and a H2SO4:Na2O molar ratio of 0.8 and are trapped in an ammonium sulfate solution. Aging is performed at 70°C for 16 hours. The beads are then acidified to a pH of approximately 2 and washed with water acidified to a pH of approximately 3. The hydrogel beads are then washed with methanol, dried under vacuum, and ground / classified to the desired particle size distribution.

[0087] Comparative Catalyst 2 was prepared in the same manner as Comparative Catalyst 1, except that it was made from a different batch preparation and a different batch of silicate, for example sodium silicate of normal purity. For the comparative support, the sodium silicate used was PQ's N-clear silicate and it typically contained 300 to 400 ppm Al based on the weight of SiO2.

[0088] Catalyst C was prepared in the same manner as Comparative Catalysts 1 and 2, except that the support was prepared from a high purity silicate as described above.

[0089] Catalyst D was prepared in the same manner as Catalyst C, except that a small amount of sodium formate was added to the Cr coating solution during catalyst preparation. Catalyst properties and polymerization evaluation results are summarized in Tables 3 and 4, respectively. Polymerization evaluation was carried out in the same manner as Example 1, except that homopolymerization was carried out at 107°C instead of 102°C.

[0090] Table 3. Catalyst properties

[0091]

[0092] Table 4. Aggregate evaluation results

[0093]

[0094] As shown in Tables 3 and 4, catalysts with low Al (Catalysts C and D) resulted in polymers with higher MI than Comparative Catalyst 2. Likewise, the data show that increasing the Na:Al ratio further increases the polymer MI.

[0095] Unless otherwise indicated, all numerical values expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention.

[0096] Other embodiments of the catalysts and methods of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the embodiments disclosed herein.It is intended that the specification and examples be considered as exemplary only, with the true scope of the application being indicated by the following claims.

Claims

1. An ethylene polymerization catalyst composition comprising: A silica-containing substrate derived from an alkali-coagulated gel, the silica-containing substrate comprising: a catalytically active metal comprising Cr; Al in an amount less than 50 ppm of the catalyst composition; and Na in an amount less than 800 ppm of the catalyst composition; The alkali-coagulated gel is prepared by: forming an alkaline hydrosol having a molar ratio of H2SO4:Na2O of 0.7 to 0.95 by reacting sodium silicate containing Al as an impurity with an acid to produce an acid-base reaction, wherein the acid-base reaction is non-stoichiometric and contains more base than acid; forming the hydrosol into an Al-containing hydrogel precursor; treating the hydrogel precursor to reduce the amount of Al; aging the hydrogel precursor to form a hydrogel; and The hydrogel is dried to produce a dried gel.

2. The ethylene polymerization catalyst composition according to claim 1, wherein Al is present in an amount of 10-40 ppm of the catalyst composition.

3. The ethylene polymerization catalyst composition according to claim 1, wherein Na is present in an amount of 50 to 800 ppm of the catalyst composition.

4. The ethylene polymerization catalyst composition according to claim 1, wherein the Na:Al molar ratio is higher than 5.

5. The ethylene polymerization catalyst composition according to claim 4, wherein the Na:Al molar ratio is 10-40.

6. The ethylene polymerization catalyst composition according to claim 1, wherein the catalyst has a molecular weight of 250 to 400 m 2 / g of surface area.

7. The ethylene polymerization catalyst composition according to claim 1, wherein Cr is present in an amount of 0.1 to 2.0 wt%.

8. The ethylene polymerization catalyst composition according to claim 1, wherein Cr is added from at least one compound selected from the group consisting of chromium acetate, chromium nitrate, chromium sulfate, chromium acetylacetonate, chromium trioxide, ammonium chromate, t-butyl chromate and other soluble chromium compounds.

9. A method for preparing the ethylene polymerization catalyst composition of claim 1, comprising: forming an alkaline hydrosol having a molar ratio of H2SO4:Na2O of 0.7 to 0.95 by reacting sodium silicate containing Al as an impurity with an acid to produce an acid-base reaction, wherein the acid-base reaction is non-stoichiometric and contains more base than acid; forming the hydrosol into an Al-containing hydrogel precursor; treating the hydrogel precursor to reduce the amount of Al; aging the hydrogel precursor to form a hydrogel; drying the hydrogel to produce a dried gel; and The support is impregnated with a solution containing a chromium compound, followed by drying to form Cr on the silica catalyst composition.

10. The method of claim 9, further comprising at least one additional step selected from the group consisting of: pulverizing the hydrogel precursor prior to further processing; washing the hydrogel with an organic solvent to remove water from the hydrogel to form an alcohol gel; grinding the dried gel; and sieving / classifying the dried gel to produce a carrier having a desired particle size distribution.

11. The method of claim 9, wherein the hydrogel precursor is treated to reduce the amount of Al to less than 50 ppm of the composition.

12. The method of claim 9, wherein the hydrogel precursor is treated to reduce the amount of Al to an amount of 10-40 ppm of the composition.

13. The method of claim 9, further comprising adjusting the Na content of the composition to achieve an amount of Na of less than 800 ppm of the composition.

14. The method of claim 13, wherein the Na content of the composition is adjusted prior to or simultaneously with impregnation of the dried gel with the chromium compound.

15. The method of claim 9, wherein the amounts of Na and Al are adjusted so that the resulting Na to Al molar ratio is higher than 5.

16. The method of claim 9, wherein the support is impregnated with an aqueous solution or an organic solvent solution of a chromium compound to form Cr on silica catalyst composition, wherein the chromium compound comprises: Chromium acetate, chromium nitrate, chromium sulfate, chromium acetylacetonate, chromium trioxide, ammonium chromate, tert-butyl chromate, or other chromium compounds soluble in water or the organic solvent used.

17. The method of claim 9, wherein Cr is present in an amount of 0.1-2 wt%.

18. The method of claim 9, wherein aging the hydrogel precursor forms a hydrogel having a thickness greater than 200 m 2 / g of surface area of a hydrogel, wherein the aging comprises mixing the hydrogel precursor with a neutral or alkaline solution to form a hydrogel dispersion having a pH of at least 6.

19. The method of claim 9, wherein treating the hydrogel is performed at a temperature of 55°C or less so as to leach Al in an amount less than 50 ppm of the catalyst composition.

20. The method of claim 9, wherein treating the alkaline hydrogel precursor to reduce the amount of Al impurities comprises an alkali coagulation gel method comprising spraying the hydrosol into air to solidify the hydrosol, and capturing the solidified hydrogel precursor beads in an acidic solution such that a molar ratio of the total acid used, including the acid for bead formation and the acid in the capturing solution, to Na2O in the metal silicate solution is greater than 1.

2.

21. The method of claim 9, wherein the sodium silicate is a high purity sodium silicate containing sufficiently low amounts of Al that the step of treating the hydrogel precursor to remove Al is eliminated.

22. A process for polymerizing ethylene using the catalyst composition of claim 9, further comprising activating the catalyst using one or more thermal steps.

23. The method of claim 22, wherein the one or more thermal steps comprise heating the catalyst at a temperature of 200 to 1200°C for an activation period of 30 minutes to 15 hours.

24. The method of claim 23, wherein the one or more thermal steps are performed in dry air in a fluidized bed reactor.

25. The process of claim 22, wherein the ethylene is combined with one or more C3 to C8 alpha-olefins and used to produce HDPE for small blow molding applications.

Images