KR20220149711A - Chromium-on-silica catalyst and method for preparing same - Google Patents

Abstract

A composition such as a catalyst precursor or catalyst comprising a Cr coated silica support having specifically defined levels of Na and Al so that the resulting Cr/silica catalyst has an increased MI potential is disclosed. In one embodiment, the disclosed catalyst composition is prepared using a base-cured gel and is silica-containing comprising a catalytically active metal composed of Cr, along with less than 50 ppm of Al impurities and less than 800 ppm of Na of the catalyst composition. includes a description. The disclosed compositions have increased MI potential compared to catalysts with higher Al content, lower Na:Al ratio, or both. Methods of making the disclosed compositions, and methods of making polyethylene using them, are also disclosed.

Description

Chromium-on-silica catalyst and method for preparing same

[0001] The present disclosure relates generally to silica catalysts for the production of polyethylene. More particularly, the present disclosure relates to chromium/silica (Cr/SiO ₂ ) catalysts used in the production of high density polyethylene (HDPE). Also disclosed are methods of preparing such catalysts using base-set gels and methods of using such catalysts in HDPE applications, in particular small blow molded HDPE applications.

[0002] The production of polyethylene, such as high-density polyethylene (HDPE), is a multi-billion dollar industry that typically uses supported catalysts in polymerization processes. In particular, supported chromium oxide catalysts used in industry generally comprise a support containing chromium oxide and one or more of silica, titania, thoria, alumina, zirconia or aluminophosphate.

One particularly useful catalyst is referred to as a Phillips-type catalyst comprising chromium oxide (eg, Cr/SiO ₂ ) supported on silica gel. Catalysts of this type are popular polymerization catalysts for the production of HDPE, mainly because they produce HDPE with a broad molecular weight distribution, which is particularly suitable for blow molding applications. Commercial silica supports for Phillips-type catalysts are typically prepared 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 procedure used to prepare the silica support.

[0004] It has been found that the addition of large amounts of alkali or alkaline earth salts to a chromium oxide catalyst increases the melt index (“MI”) potential and activity of the catalyst when the catalyst is activated at a temperature below its sintering temperature. For example, U.S. Pat. Nos. 5,444,132 and 5,284,811 (both to Witt et al.) disclose silica-titania or aluminophosphate supports (pre-purified to remove alkali metal salt byproducts from support preparation) as non- Using an aqueous impregnation solution, impregnation with 50 to 500 μmol of alkali or alkaline earth metal salt per gram of catalyst, equivalent to 1,150 to 11,500 ppm of Na in the catalyst, is disclosed. However, this large amount of Na will cause catalytic sintering at 750 to 850° C., which is a typical activation temperature used to make small blow molding (SBM) resins.

[0005] Another study showed a similar effect of alkali metal doping 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 corresponds to 4,600 ppm Na (see, eg, J. Catal., Vol 176, 344-351 (1998)). The addition of this level of sodium decreases the catalyst MI potential by sintering the catalyst at a higher activation temperature while increasing the MI potential of the catalyst activated at the lower temperature. The maximum MI achievable with this high level of Na doping at any given activation temperature is lower but activated at a higher temperature than for a catalyst without Na doping.

[0006] Prior art focusing on increasing the catalyst MI potential often teaches the effect of high levels of Al in the final catalyst, but does not 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 (Marsden, "Advances in supported chromium catalyst", Plastics, Rubber and Composites Processing). and Applications 21 (1994) 193-200; and US Pat. No. 4,119,773). These references state that the relationship between Al and MI potentials, including the content and nature of the Al compound and the manner in which it is added, is dependent on a number of variables related to Al incorporation.

[0007] As described in the above reference, Cr/silica catalysts are used commercially in the production of HDPE for small blow molding applications. These catalysts are typically activated at high temperatures, such as temperatures in the range of 750 to 850° C., to achieve the polymer MI target. Since this activation temperature is very close to the sintering temperature of the Cr/silica catalyst, the ability to boost the catalyst MI potential by further increasing the activation temperature is very limited. Although modifying the Cr/silica catalyst with metals such as Al and Ti can substantially increase the catalytic MI potential, this modification also broadens the molecular weight distribution of the produced HDPE as well as increases the cost of preparing the catalyst. Both are undesirable for SBM applications.

[0008] For at least the above reasons, there is a need for a Cr/silica catalyst with an increased MI potential to allow a sufficiently wide difference between the activation temperature and its sintering temperature required to achieve the resin MI target. This increased MI potential will provide some flexibility to the catalyst activation and polymerization process without substantially altering the molecular weight distribution of the prepared HDPE. Catalysts with increased MI potential are also higher than currently achievable with commercially available Cr/silica catalysts to meet additional market demands, such as HDPE homopolymers with MI greater than 1.0 g/10 min for SBM applications. It will enable the production of HDPE with MI. To address the above needs, the present application describes a Cr-only catalyst with an increased MI potential compared to catalysts currently on the market for making small blow molded HDPE resins.

[0009] A composition comprising a Cr coated silica support having a defined level of an alkali or alkaline earth metal (e.g., comprising Na, Mg, or Ca) and Al exhibiting increased MI potential, e.g., a catalyst precursor , or a catalyst. In one embodiment, a catalyst composition comprising a Cr coated silica support having defined levels of Na and Al such that the resulting Cr/silica catalyst has an increased MI potential is disclosed. In one embodiment, the present application relates to a catalyst composition comprising a silica-containing substrate comprising a catalytically active metal composed 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 amounts of Na and Al are present in a Na:Al molar ratio of greater than 5, greater than 10, greater than 20, or even greater than 30, for example a molar ratio ranging from 10 to 40. The molar ratio may include any combination of these endpoints, such as 5 to 10, 5 to 20, 5 to 30, etc., or 5 to 40, 10 to 40, 20 to 40, or other combinations thereof.

[0010] Also disclosed is a method for preparing the disclosed composition comprising a Cr coated support having specifically defined levels of Na and Al such that the resulting Cr/silica catalyst exhibits increased MI potential. In one embodiment, the method comprises reacting a metal silicate, e.g., sodium silicate, with an acid to form a hydrosol that is transformed into a hydrogel precursor, and washing to substantially leach out Al impurities associated with sodium silicate to Al reducing the amount of impurities to an amount of less than 50 ppm in the resulting catalyst precursor.

[0011] The method further comprises the step of aging the hydrogel precursor to form a hydrogel having a surface area of greater than 200 m/g, such as greater than 250 m/g. In one embodiment, the hydrogel exhibits a surface area of approximately 300 m 2 /g. The aging process involves mixing a hydrogel precursor with a neutral or basic aqueous solution to form an aqueous dispersion exhibiting a neutral or basic pH.

1 is a flow chart showing general steps used in a base-cured gel process for preparing a silica support and a Cr/silica catalyst according to an embodiment of the present disclosure;
FIG. 2 is a graph comparing Al leaching profiles for hydrogel precursors immersed in acid water at temperatures ranging from 20 to 90° C. for the process shown in FIG. 1. FIG. The total amount of acid, including acid used for gel formation and acid for acidification prior to immersion, is about 20% excess of Na ₂ O in sodium silicate used for gel formation.
3 is a graph comparing melt index (MI) for homo and copolymers prepared from Cr/silica catalysts having various Al contents and Na:Al ratios prepared according to the process described in FIG.

[0015] As used herein, silica gel is described as "acid-set gel" or "base-set gel". Both types of gels are prepared using silicates, such as sodium silicate, and inorganic acids, such as sulfuric acid, via an acid-base reaction.

[0016] An "acid-cured gel" is a gel formed by an acid-base reaction that is not stoichiometric, but uses more acid than base, such as a greater volume or weight percent of acid.

[0017] A "base-cured gel" is a gel formed by the described acid-base reaction that is not stoichiometric but uses more base than acid, such as greater volume or weight percent.

[0018] As used herein, "Cr-only catalyst" refers to a catalyst free of other added polyvalent metals in oxidation states of +3 or greater.

[0019] The term "composition" is sometimes referred to as "catalyst composition" and is meant to describe an activated catalyst as well as an unactivated catalyst precursor. For example, after Cr impregnation/drying, the product usually contains Cr compounds with Cr in the +3 oxidation state. These products are catalyst precursors because they are not catalytically active for ethylene polymerization. These catalyst precursors must be activated in an oxidizing atmosphere (eg, heated in dry air in a fluidized bed reactor) to convert Cr from the +3 to +6 oxidation state. The activated product is more precisely referred to as a catalyst.

[0020] As used herein, “cold water wash” refers to wash water having a temperature of 55° C. or less.

[0021] As used herein, “hydrosol” refers to a mixture of an acid and a metal silicate in liquid form.

[0022] As used herein, "hydrogel precursor" refers to an unwashed and/or unaged hydrogel.

[0023] As used herein, "hydrogel" refers to a washed and aged hydrogel.

[0024] As used herein, “alcogel” refers to a hydrogel that is further washed with an organic solvent to produce a substantially water-free hydrogel, for example by replacing water with an alcohol in the hydrogel.

[0025] As used herein, "dried gel", also known as "support precursor" refers to a dried gel prior to sizing. "Support" refers to the dried gel after sizing.

[0026] As used herein, "melt index" (MI) and "High Load Melt Index" (HLMI) are measures of molten polymer fluidity, and a load of 2.16 kg and 21.6 kg at 190°C, respectively. Inversely related to molecular weight when measured according to ASTM D-1238-4 using

[0027] The "MI potential" of the Cr catalysts described herein is a function and directly proportional to the MI of the polymer prepared from the catalyst. For example, when a Cr catalyst activated under a certain temperature is used to polymerize ethylene under certain polymerization conditions, the MI potential of the Cr catalyst is directly proportional to the MI of the polymerized ethylene. The higher the MI of the polymer, the higher the catalytic MI potential.

[0028] The density of the described polymers is measured by the procedure of ASTM D-792-13.

[0029] MI, HLMI and density are measured on polyethylene pellets obtained by processing the stabilized polymer powder using a single screw extruder under nitrogen.

[0030] The level of chromium in the catalyst composition is measured by X-ray fluorescence (“XRF”) using a PANalytical Magix Pro automatic sequential spectrometer. Samples are calcined at 1000° C. in air and then prepared as fusion beads using a lithium borate flux. The fusion is typically between 1000°C and 1250°C. Cr levels are reported as weight percentage of catalyst precursor after calcination at 1000°C.

[0031] The levels of Na and Al in the catalyst composition were measured by atomic adsorption spectroscopy (AA) using a Perkin-Elmer Analyst 100 spectrometer and inductively coupled plasma ("ICP") spectroscopy using an ACTIVA ^™ HORIBA Jobin Yvon ICP-AES spectrometer, respectively. is measured by A sample of the catalyst precursor is digested with hydrofluoric acid (HF). The resulting silicon tetrafluoride (SiF ₄ ) is fumed and the residue is analyzed for Na and Al. Na and Al levels are reported as parts per million of catalyst precursor after drying at 120°C.

[0032] Surface area and pore volume are measured by Nitrogen Porosimetry using an Autosorb-6 Testing Unit from Quantachrome Corporation. The sample is first degassed at 350° C. for at least 4 hours in an Autosorb-6 degassing unit. The multi-point surface area is calculated using BET theory taking data points ranging from 0.05 to 0.30 P/P ₀ . Pore volume measurements are recorded at a P/P ₀ of 0.984 in the desorption leg. The average pore diameter is calculated using the following equation assuming cylindrical pores.

[0033] Particle size is measured by laser light scattering using a device such as a Malvern Mastersizer™ Model 2000. These instruments use the Mie theory to calculate the particle size distribution. Mie theory predicts how light is scattered by spherical particles and takes into account the refractive index of the particles. The real value used for the silica refractive index is 1.4564 and the imaginary value is 0.1. The refractive index of the water dispersant is 1.33.

Since the resin properties are determined by the catalyst properties and polymerization conditions, it is necessary to control the catalyst design in order to be able to control the resin properties. The catalyst support typically 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 can affect the resulting polymer. Other things being equal, larger pore volume (and therefore larger pores) Cr/SiO ₂ catalysts produce polymers with lower MW and higher MI.

[0035] In one embodiment, the base-cured gels described herein comprising a low Al content contain a Cr/silica catalyst with a higher Al impurity content but a higher MI potential than a base-cured gel at the same Na content. manufacture We also found that, irrespective of the Al impurity content, Cr/silica catalysts with increased Na content generally lead to higher MI potentials. It has also been found that using a Na content that is too high (eg when the Al impurity content is high) will result in catalytic sintering at a given activation temperature, typically 750 to 850°C. Therefore, it is desirable to keep the Al impurity level as low as possible. As described in the background section and as will be understood by one of ordinary skill in the art, this is generally not standard. Rather, traditional base-curing gel processes generally result in gels with higher Al impurity content.

Commercial silica gel preparation typically uses sodium silicate, such as sodium silicate, having a SiO ₂ :Na ₂ O weight ratio of 3.2, and an acid such as sulfuric acid via an acid-base reaction. However, in the case of commercial production, the acid-base reaction is rarely stoichiometric, i.e. either more acid is used (the gel formed in this case is called an acid-cured gel) or less acid is used (in which case the gel formed is a base) -curing gel). The base-curing gel process is widely used for commercial silica gel production because the process helps to prepare spherical silica gel particles with particle sizes ranging from tens of microns to several millimeters in size. Commercial sodium silicate is prepared from sand, which usually contains various polyvalent cations such as Al as impurities. When the gel is formed under basic pH (as is the case with base-cured gels), these polyvalent cations tend to displace a small fraction of Si atoms in the SiO ₂ gel framework and thus “fix” tightly to the gel structure. As a result, base-curing processes typically produce gels of lower purity than the corresponding acid-cured gels. As high-purity sand supplies become less and less commercially available, it is important that processes be developed that allow the preparation of high-purity base-cured gels.

[0037] Disclosed herein is a catalyst composition having a support having defined levels of Na and Al, which results in a Cr/silica catalyst of increased MI potential, as described above. Without wishing to be bound by theory, it is believed that both acid and base sites on the silica support (and thus the catalyst) increase the MI potential of Cr/silica. Na introduces base sites, while Al introduces acid sites. When both Na and Al are present, they can cancel each other's effect due to acid-base neutralization.

[0038] In various embodiments, Cr/silica catalyst compositions are disclosed wherein the silica support has an Al content of less than 50 ppm, such as less than 25 ppm, or even in the range of 10-40 ppm of the catalyst composition.

[0039] Also disclosed are Cr/silica catalysts wherein the silica support has less than about 800 ppm Na impurities, for example, the final catalyst composition is 50 to 800 ppm, 200 to 800 ppm, or 200 to 700 of the catalyst composition. Na in an amount in the ppm range, such as less than about 600 ppm, or even in an amount in the range of about 50 to 600 ppm.

[0040] Cr/silica catalyst compositions are also disclosed wherein the Na:Al molar ratio is greater than 5, greater than 10, greater than 20, even greater than 30, such as in the range of from 10 to 40.

[0041] As described herein, the catalytically active metal comprises Cr which may be added to the silica support using at least one Cr 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, nitrate, sulfate, acetylacetonate, chromium trioxide, ammonium chromate, tert-butyl chromate and other soluble chromium compounds.

[0042] In preparing the catalysts described herein, a sufficient amount of at least one of the described chromium-containing compounds should be used such that the catalyst contains Cr in an amount ranging from 0.01 to 3% by weight. In certain embodiments, the catalyst contains Cr in an amount ranging from 0.1 to 2% by weight, such as in an amount ranging from 0.25 to 1.5% by weight.

[0043] In one embodiment, a process for preparing a catalyst composition comprising a Cr coated support having specifically defined levels of Na and Al is disclosed. The method comprises reacting a metal silicate, such as sodium silicate, with an acid to form a hydrosol, which is subsequently solidified into a hydrogel precursor. Because sodium silicate contains Al as an impurity, the method further comprises treating the hydrogel precursor to reduce the amount of Al impurity and to form a hydrogel. Thus, in a broader sense, a method is described which comprises preparing a gel and leaching it to reduce Al to a desired level.

[0044] As noted above, Al is adjusted in the silica support to achieve an amount of less than 50 ppm of the catalyst composition, such as an amount of less than 25 ppm. In one embodiment, Al is adjusted in the silica support to achieve an amount ranging from 10 to 40 ppm of the catalyst composition. In one embodiment, high purity silicates with very low Al content can be used. This may eliminate the need to treat the hydrogel precursor to remove Al, assuming that the Al content level in the silicate is sufficiently low. However, high purity silicates can be expensive and not readily available.

[0045] The methods described herein may also be carried out 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 contains Na in an amount ranging from 50 to 800 ppm, from 200 to 800 ppm, or from 200 to 700 ppm, such as less than about 600 ppm, or even ranging from about 50 to 600 ppm of the catalyst composition. include

[0046] The resulting Na:Al molar ratio is typically adjusted to a value as described herein.

[0047] In one embodiment, the method further comprises aging the hydrogel precursor to form a hydrogel having a surface area of greater than 200 m/g, or greater than 250 m/g, or such as about 300 m/g, , wherein the aging comprises mixing the hydrogel precursor with a neutral or basic solution at a temperature ranging from 70 to 100° C. for a time ranging from 4 to 36 hours to form an aqueous dispersion having a pH of at least 6, such as about 8 to 9. includes doing

[0048] The method further comprises drying the gel using any conventional technique known in the industry, such as spray drying, flash drying, or solvent-wash/drying techniques to prepare a dried gel (support precursor). can do. The process also includes a post-drying step, e.g., milling, sieving and/or fractionating the precursor into a support of the desired particle size distribution, which is subsequently impregnated with a chromium compound to form a Cr on silica catalyst. may include doing The process of adjusting the Na content of the catalyst is carried out before or during the impregnation step so that the amount of Na achieves a Na:Al molar ratio of greater than 5, greater than 10, greater than 20, or even greater than 30, such as a molar ratio ranging from 10 to 40; Amounts of Na less than 800 ppm of the catalyst composition can be achieved.

[0049] In one embodiment, a process for making a base-cured gel and a catch gel in acid water is described. In particular, the reaction product of sodium silicate and acid is a basic hydrosol, typically having a H ₂ SO ₄ :Na ₂ O molar ratio in the range of 0.7 to 0.95. A method of treating the basic hydrogel precursor to reduce the amount of Al impurities may include a bead process. For example, the bead process sprays a basic hydrosol into the air to solidify it into beads, and captures the beads in an acidic solution to a pH of less than 2, such as less than 1, at a temperature of less than about 60° C., such as less than 55° C. and providing a gel dispersion of The gel is immersed in this acidic solution for a period of at least 2 hours.

[0050] Applicants have found that temperature and pH have a significant effect on leaching properties and report herein how the leaching properties are affected by temperature.

[0051] Temperature : Al leaching at acidic pH has been shown to be particularly sensitive to temperature, and leaching is preferably carried out at 60 °C or lower, such as 55 °C or lower. As shown in Figure 2, among the four temperatures studied (20°C, 50°C, 70°C and 90°C), leaching was not most efficient at 90°C and not very efficient at 70°C. Extended time does not improve leaching at these temperatures. Leaching was found to be most efficient at 50° C. within the period studied. Without wishing to be bound by any theory, this is probably due to the faster kinetics at 50°C than at 20°C. Although leaching at 20 °C is not as efficient as at 50 °C within the period studied, it appears that more Al can be leached given a longer time. While not wishing to be bound by any theory, the % Al leached at equilibrium has been shown to improve at lower temperatures, although higher temperatures are preferred for kinetic considerations.

[0052] pH : In one embodiment, the pH should be maintained below about 3-4 until most of the Al impurities are removed from the system from the gel as well as the liquid phase in contact with the gel. At a pH above 3-4, Al will redeposit on the silica surface. Thus, while a pH <2, such as <1, is preferred for Al leaching from the gel structure into the liquid phase, to prevent Al redepositing on the silica surface, the pH of the liquid phase is such that the liquid leached Al is substantially present. It should be kept at < 3 to 4 until it does not. In one embodiment, washing in water acidified to pH about 3 at ambient temperature may be the best way to reduce residual Al content to the lowest level. However, these lower temperatures prolong washing times, which is undesirable for commercial production. For cost control, the wash can be performed at a higher temperature (<60° C.) during the entire wash, or it can remove most of the leached Al for at least the first few hours and then wash at a much higher temperature. If more acid is used for acidification prior to pre-washing, washing can also be performed with neutral water. As mentioned above, the pH of the gel/water dispersion should not exceed pH 3-4 before most of the leached Al is removed from the dispersion.

[0053] Soaking and pre-washing may be performed as separate steps, but in one embodiment, soaking and pre-washing may also be combined. For example, if the pre-wash is performed at a pH of about 2 or less and a temperature of <60° C., it has the same effect as immersion and may be more efficient than immersion, depending on whether and how often the liquid phase is regenerated. .

[0054] In one embodiment, beads entrapped in acidic solution are subsequently aged in water pH adjusted to 8-9 with aqueous NH ₄ OH and at 70-90° C. for 4-36 hours, It is washed in water acidified to a pH of about 3 at ambient temperature to achieve a gel having a surface area of greater than 200 m/g, or greater than about 250 m/g, such as about 300 m/g.

Optionally, the gel may 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 water acidified to a pH of about 3 in one embodiment.

[0056] The hydrogel may be dried by any one of the techniques known in the art. One suitable method is flash drying. Another suitable method is spray drying. Another suitable method is to wash the hydrogel with an organic solvent and then dry the gel under vacuum. The dried gel is sized to the desired particle size distribution to form the silica support.

As mentioned, the amount of Al and Na affects the MI potential of the resulting catalyst. Thus, the method can be used, for example, prior to or during the impregnation step, for example by a washing process, to achieve Na in an amount of less than 800 ppm of the catalyst composition, and a Na:Al molar ratio as described herein. It further comprises adjusting the Na content of the catalyst.

[0058] In one embodiment, sodium silicate comprising Al as an impurity is reacted with an acid to form a hydrosol; causing the hydrosol to form a gel precursor; treating the gel precursor to reduce the amount of Al; Aging the gel precursor to form a hydrogel having a surface area greater than 200 m/g, such as 300 m/g, wherein the aging mixes the gel precursor with a neutral or basic solution to form a hydrogel having a pH of at least 6. let; Disclosed is a method for preparing a composition comprising drying to produce a dried gel and impregnating it with a chromium compound to prepare a Cr on silica catalyst composition. A number of optional processing steps may be used, including milling the hydrogel to form particles of the desired size, or sieving/classifying the dried gel to produce a support of the desired particle size distribution.

[0059] The particles of the catalyst precursor according to the present disclosure may have a d90 (a diameter in which 90% by volume of the particles have a smaller 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 1-300 μm, 5-250 μm, or 25-150 μm. The particles may be prepared by grinding combined with size classification by means such as sieving or air classification, or the particles may be prepared by a route such as spray-drying followed by size classification.

[0060] The ethylene polymerization catalyst reacts the catalyst precursor composition in a non-reducing atmosphere, such as an oxidizing atmosphere, at a temperature of 200° C. to 1200° C. for an activation period of 30 minutes to 15 hours, for example, at a temperature of 400 to 850° C. for about 4 hours. It can be obtained or obtainable from the compositions described herein by heating for an activation period of from 12 hours to 12 hours.

[0061] The resins prepared by the catalysts described herein are particularly suitable for small blow molding applications. As mentioned above, in order to prepare a resin using the disclosed catalyst, 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, e.g., at a temperature in the range of 750 to 850 °C, e.g., at 800 to 850 °C for a time ranging from 30 minutes to 15 hours, such as 6 hours. have.

[0062] The method according to the present disclosure is applicable to the preparation of 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 may be prepared from a mixture of ethylene and one or more C3 to C8 α-alkenes.

[0063] The 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 present disclosure in any way.

Example

[0064] Examples 1 and 2 below describe base-cured gels with low Al content prepared from standard purity and high purity sodium silicate, respectively.

Example 1: Preparation of low Al content base-cured gels from standard purity sodium silicate

[0065] Provided herein is a base-cured gel process for preparing a silica support for a Cr on silica catalyst according to the present disclosure. This process started with sodium silicate containing Al as an impurity. Through the series of steps shown in the flow chart of FIG. 1, the Al and Na contents were reduced to acceptable levels and ratios in the obtained catalyst.

[0066] A dilute sodium silicate solution of SiO ₂ :Na ₂ O in a weight ratio of 3.3 was first reacted with diluted sulfuric acid to form a hydrosol having the following composition: 12 wt % SiO ₂ ; A molar ratio of H ₂ SO ₄ :Na ₂ O of 0.8. The sodium silicate solution contains about 400 ppm Al by weight of SiO ₂ . As a result, the hydrosol obtained was basic.

[0067] The hydrosol is sprayed into the air, where it breaks down into droplets and solidifies into beads with a diameter of several millimeters before being entrapped in solution. In contrast to the traditional bead process that uses water or a solution that buffers the pH of the bead/solution system at basic pH (e.g., an aqueous solution of ammonium sulfate, sodium bicarbonate, etc.) used to prepare Comparative Catalyst 1, the present invention The process for preparing a gel with a reduced Al content of , used an aqueous solution of acid to trap the beads. Thus, the total acid used to form the beads according to this example, including the acid in the capture solution, resulted in a H ₂ SO ₄ :Na ₂ O molar ratio of about 1.2 and a gel dispersion of pH < 1. The hydrogel precursor beads were immersed in this acid solution at ambient temperature for ≧6 hours.

The beads were washed with water acidified to pH about 3 at ambient temperature, leaving the beads substantially free of Na and Al. After the washing step, aqueous NH ₄ OH was added to the solution to raise the pH to about 9. Aging was performed at 70° C. for about 16 hours to achieve a gel with a surface area of about 300 m 2 /g.

Then, acid was added to lower the pH to about 2. The beads were then washed with water acidified to pH about 3. The beads were further washed with methanol until substantially free of water (eg <2 wt. % water), dried to prepare the support precursor, and then sized to a d50 of about 100 μm to create the support.

[0070] Comparative Catalyst 1 : This catalyst was a silica support comprising approximately 330 ppm Al and 40 ppm Na prepared from a traditional base-cured bead process and impregnated with 1 wt % Cr using a methanol solution of chromium acetate .

[0071] Catalyst A : This catalyst was a silica support modified according to the present invention, resulting in a support with low Al content (about 20 ppm) and Na content (about 16 ppm). This support was impregnated with 1 wt % Cr using a methanol solution of chromium acetate.

[0072] Catalyst B : This catalyst was the same silica support as Catalyst A except that it was impregnated with 1 wt % Cr and about 400 ppm Na using a methanol solution containing chromium acetate and sodium formate. The properties of Comparative Catalyst 1 and Inventive Catalysts A and B are provided in Table 1 below.

Table 1. Catalyst properties

[0073] These catalyst samples were evaluated for both homo- and copolymerization. For homo-polymerization, about 10 g of catalyst was activated in dry air in a fluidized bed reactor. The temperature was maintained at 850° C. for 6 hours before cooling. Air was converted to nitrogen at 300°C. About 0.17 g of activated catalyst was charged to a 2.5 L slurry polymerization reactor. Polymerization was carried out at 102° C. with 10 mol % ethylene in isobutane. The polymerization reaction was terminated when about 425 g of polyethylene had been prepared. For copolymerization, the catalysts were activated similarly except at 815°C. Polymerization was also performed similarly at 100° C. except that 5 mL of 1-hexene was used.

Polymerization results are summarized in Table 2 below. As is evident, the polymer MI increases substantially when the Al content of the catalyst is reduced from 330 ppm to 19 ppm. The polymer MI further substantially increases when the catalyst Na:Al molar ratio is increased from 1 to 26 for catalysts with low Al content. See, for example, Figure 3 which shows a plot of the MI results.

Table 2 Polymerization evaluation results

Example 2: Preparation of low Al content base-cured gel from high purity sodium silicate

[0075] This example describes the method used to prepare both the inventive and comparative supports. The process used to prepare these different supports is the same except that the support of the present invention was prepared using high purity sodium silicate from PQ (trade name ^CRYSTAL® FS).

[0076] The high purity silicate used to prepare the support of the present invention contains about 20 ppm Al by weight of SiO ₂ . Beads formed with a molar ratio of 12% SiO ₂ and H ₂ SO ₄ :Na ₂ O of 0.8 were captured in an ammonium sulfate solution. Aging was performed at 70° C. for 16 hours. The beads were then acidified to pH about 2 and washed with water acidified to pH about 3. The hydrogel beads were subsequently washed with methanol, dried under vacuum and milled/sorted to the desired particle size distribution.

[0077] Comparative Example 2 was prepared in the same manner as Comparative Example 1 except that it was prepared from different production batches and different batches of silicate, for example sodium silicate of normal purity. For the comparative support, the sodium silicate used is an N-clear silicate of PQ, which typically contains 300 to 400 ppm Al based on the weight of SiO ₂ .

[0078] Catalyst C was prepared in the same manner as in Comparative Examples 1 and 2 except for the support prepared from high-purity silicate as described above.

Catalyst D was prepared in the same manner as Catalyst C, except that a small amount of Na formate was added to the Cr coating solution during catalyst preparation. The catalyst properties and polymerization evaluation results are summarized in Tables 3 and 4, respectively. Polymerization evaluation was performed in the same manner as in Example 1, except that homo-polymerization was performed at 107°C instead of 102°C.

Table 3. Catalyst properties

Table 4. Polymerization evaluation results

[0080] As shown in Tables 3 and 4, catalysts with low-Al (catalysts C and D) produce polymers with higher MI (than Comparative Example 2). In addition, the data show that increasing the Na:Al ratio further increases the polymer MI.

[0081] Unless otherwise indicated, all numbers expressing amounts of ingredients, reaction conditions, etc. used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless otherwise indicated, the numerical parameters set forth in the following specification and appended claims are approximations which may vary depending upon the desired properties to be attained by the present disclosure.

[0082] Other embodiments of the catalysts and methods of the present disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the embodiments disclosed herein. The specification and examples are to be regarded as illustrative only, the true scope of the present application being indicated by the following claims.

Claims (25)

A catalyst composition comprising:
a silica-containing substrate derived from a base-set gel;
The silica-containing substrate is
catalytically active metals including Cr;
Al in an amount of less than 50 ppm of the catalyst composition; and
and Na in an amount less than 800 ppm of the catalyst composition. The catalyst composition of claim 1 , wherein Al is present in an amount ranging from 10 to 40 ppm of the catalyst composition. The catalyst composition of claim 1 , wherein Na is present in an amount ranging from 50 to 800 ppm of the catalyst composition. The catalyst composition of claim 1 , wherein the Na:Al molar ratio is greater than 5. 5. The catalyst composition of claim 4, wherein the Na:Al molar ratio ranges from 10 to 40. The catalyst composition of claim 1 , wherein the catalyst has a surface area in the range of 250 to 400 m 2 /g. The catalyst composition of claim 1 , wherein Cr is present in an amount ranging from 0.1 to 2.0% by weight. The catalyst composition of claim 1 , wherein Cr is added from at least one compound selected from chromium acetate, nitrate, sulfate, acetylacetonate, chromium trioxide, ammonium chromate, tert-butyl chromate and other soluble chromium compounds. A method for preparing a catalyst composition comprising:
performing an acid-base reaction by reacting sodium silicate comprising Al as an impurity with an acid to form a hydrosol, the acid-base reaction being non-stoichiometric and comprising more base than acid;
allowing the hydrosol to form an Al-containing hydrogel precursor;
reducing the amount of Al by treating the hydrogel precursor;
Aging the hydrogel precursor to form a hydrogel;
drying the hydrogel or alcohol gel to prepare a dried gel; and
A method comprising the step of impregnating a support with a solution containing a chromium compound and then drying it to form a Cr on silica catalyst composition. 10. The method of claim 9, further comprising the steps of: crushing the hydrogel precursor before further processing; washing the hydrogel with an organic solvent to remove water from the hydrogel to form an alcogel; milling the dried gel; and at least one additional step selected from sieving/classifying the dried gel to produce a support of a desired particle size distribution. 10. 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. 10. The method of claim 9, wherein the hydrogel precursor is treated to reduce the amount of Al to an amount ranging from 10 to 40 ppm of the composition. 10. 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 according to claim 13, wherein the Na content of the composition is adjusted before or simultaneously with the impregnation of the dried gel with the chromium compound. 10. The method according to claim 9, wherein the amounts of Na and Al are adjusted such that the resulting Na to Al molar ratio is greater than 5. 10. The method of claim 9, wherein the support is impregnated with an aqueous solution of a chromium compound or an organic solvent solution to form a Cr on silica catalyst composition, wherein the chromium compound is chromium acetate, nitrate, sulfate, acetylacetonate, chromium trioxide, ammonium A process comprising chromate, tert-butyl chromate or other chromium compound soluble in water or in the organic solvent used. 10. The method of claim 9, wherein Cr is present in an amount ranging from 0.1 to 2% by weight. 10. The method of claim 9, wherein aging of the hydrogel precursor forms a hydrogel having a surface area greater than 200 m 2 /g, wherein the aging neutralizes the hydrogel precursor to form a hydrogel dispersion having a pH of at least 6. A method comprising mixing with a basic solution. 10. The method of claim 9, wherein treating the hydrogel is performed at a temperature of 55° C. or less to leach Al in an amount of less than 50 ppm of the catalyst composition. 10. The method of claim 9, wherein treating the basic hydrogel precursor to reduce the amount of Al impurities comprises a base-curing gel process, wherein the base-curing gel process solidifies the hydrosol by spraying the hydrosol into the air. and capturing the solidified hydrogel precursor in an acidic solution such that the total acid used, including the acid for bead formation and the acid in the capture solution, has a molar ratio to Na ₂ O in the metal silicate solution of greater than about 1.2. How to. 10. The method of claim 9, wherein said sodium silicate is a high purity sodium silicate containing a sufficiently low amount of Al to eliminate the step of treating the hydrogel precursor to remove Al. 10. A method for polymerizing ethylene using the catalyst composition according to claim 9, said method further comprising activating the catalyst with one or more thermal steps. 23. The method of claim 22, wherein the at least one thermal step comprises heating the catalyst at a temperature ranging from 200°C to 1200°C for an activation period ranging from 30 minutes to 15 hours. 24. The method of claim 23, wherein the at least one thermal step is performed in dry air in a fluidized bed reactor. 23. The method of claim 22, wherein ethylene is found in combination with one or more C3 to C8 α-alkenes and is used to produce HDPE for small blow molding applications.

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