BRPI0819120B1 - CATALYST PRECURSOR FOR AN OLEFIN POLYMERIZATION CATALYST, AND METHOD FOR PREPARING THE SAME - Google Patents

Abstract

CATALYST PRECURSOR FOR AN OLEFIN POLYMERIZATION CATALYST, METHOD FOR PREPARING SAME, OLEFIN POLYMERIZATION CATALYST, AND METHOD FOR POLYMERIZING ONE OR MORE C2 TO C8 α-ALKENES. A method for preparing a catalyst precursor for an olefin polymerization catalyst involves the use of aqueous or alcoholic solutions of a chromium salt and boric acid and aluminum carboxylate for deposition onto an inorganic support material such as silica xerogel. The chromium salt, aluminum carboxylate, and boric acid are sufficiently soluble for deposition from a single solution to be effective. The catalyst precursor can be activated by calcination to form a catalyst for homo- or copolymerization of (Alpha)-olefins that has a productivity and melt flow index for the resulting polymer or copolymer that is comparable to results obtained with catalysts prepared by prior art organometallic routes. Activation of the catalyst precursor gives reduced levels of toxic or noxious vapors during activation compared to the use of organometallic sources of chromium or aluminum.

Description

[001] The invention relates to inorganic porous particles, particularly silica particles, carrying chromium and aluminum compounds, and methods for their preparation. How particles, particularly silica particles, can be activated to form catalyst particles for use in the polymerization of olefins (α-alkenes).

[002] Typical activated catalysts for use in the polymerization of olefins comprise chromium oxide loaded onto a porous inorganic support. For many uses, it is preferred that the aluminum oxide also be present, either as part of the inorganic support itself (e.g., where the support is a silica-alumina co-gel support), or also loaded, with the chromium oxide, into the pore structure of the porous inorganic support.

[003] Incorporation of oxide mixtures into the molecular structure of the inorganic support tends to weaken the structure and make it more difficult to achieve high porosities for the porous inorganic support material. For this reason, it is desirable to load alumina with the pore structure (i.e., deposited on the pore surfaces) of the porous inorganic support material together with the chromium oxide.

[004] US 3,984,351 describes olefin polymerization catalysts prepared by depositing a chromium compound and an aluminum compound onto an inorganic support material and heating the support material in a non-reducing atmosphere to a temperature above 300°C. The resulting material is then combined with a metal or non-metal reducing agent, preferably a boron-containing compound, to give a catalyst system for use in the polymerization of olefins. Silica xerogels are mentioned as preferred support materials. The catalysts are prepared by depositing chromium and aluminum compounds onto the inorganic support. The compounds mentioned are organic compounds and the deposition is from an inert organic solvent. The catalyst system described provides improved olefin polymer characteristics, such as improved melt indices.

[005] GB 1 575 352 describes the preparation and use of supported chromium-containing catalysts for the polymerization of one or more α-alkenes. In the process described, a chromium 1,3-diketol compound, such as chromium acetyl acetonate, is reacted with an organometallic compound of a group IIA or group IIIA metal of the periodic table of elements, with aluminium or magnesium preferred. For example, triisobutyl aluminium may be used. The addition of the organometallic compound to the chromium compound solubilizes the latter in aliphatic or cycloaliphatic solvents, such as heptane, allowing the use of the resulting solution for impregnation of a porous inorganic support material. The metal and chromium compounds may then be deposited onto surfaces having the pore structure of the inorganic support material by evaporation of the solvent. The preferred porous inorganic support materials are silicas.

[006] After removal of the solvent, the impregnated particles (i.e., particles of the porous inorganic support impregnated with metal and chromium compounds) need to be activated in order to make them usable as catalyst particles. Generally, the carrier particles are sold and transported in an inactivated form (referred to in this report as the inactivated catalyst particles), and require activation before they are usable as catalyst particles for olefin polymerization. Activation is accomplished by heating the inactivated catalyst particles at an elevated temperature, such as 200 to 1200°C, for a time of a few seconds, but typically for several hours, in a non-reducing atmosphere, such as nitrogen, inert gas or carbon dioxide, or preferably in an oxidizing atmosphere such as air or oxygen, so that the chromium is converted to state VI chromium. Once activated, the catalyst is used immediately, or stored in a dry inert atmosphere until use.

[007] The process of GB 1 575 352 allows the simultaneous impregnation of an inorganic porous carrier material with both chromium and aluminium compounds. However, the method requires the chromium and aluminium compounds to be dissolved in an aliphatic and/or cycloaliphatic solvent, leading to potential flammability and solvent recovery problems when evaporating the solvent after impregnation. Furthermore, the chromium and aluminium compounds present on the inactivated catalyst particles are complex organometallic compounds. When the inactivated catalyst particles are subjected to activation by heating in a non-reducing or oxidizing atmosphere, degradation of these compounds leads to the formation of potentially toxic and odorous vapors, leading to a requirement for their containment, scrubbing and emission control. Furthermore, some solutions of chromium and aluminium compounds have been found to have high viscosities, and thus are unsuitable for solution impregnation into the pore structure of a porous inorganic support material.

[008] WO 99/12978 describes catalysts prepared by a process comprising a first stage during which a porous inorganic oxide support is impregnated with a chromium compound and a second, optional stage, during which the product of the first stage is impregnated with either a titanium compound or an aluminium compound. The chromium compound is a chromium oxide or a compound, which can be converted to chromium oxide by calcination, such as chromium nitrate, sulphate, carbonate, acetate, acetylacetonate, ammonium chromate or tert-butyl chromate. The aluminium compounds mentioned include acetyl acetate, acetylacetonate, alkoxy or alkyl types. The solvents to be used are not specified, but the aluminium compounds mentioned should not be significantly soluble in polar solvents. There are potential problems that may arise from carrying out the chromium compound impregnation and the aluminum compound impregnation as separate steps by a method such as that described in WO 99/12978. The first impregnation, followed by solvent removal, may lead to partial or complete blocking of the pore structure of the porous inorganic carrier material, possibly making it more difficult, or impossible, for a second impregnation to be carried out effectively. Furthermore, even if such blocking does not occur, the catalytic behavior of the chromium and aluminum mixture may be modified because the chromium and aluminum are not co-mixed at an atomic level in the catalyst precursor. Instead, separate particles of aluminum compound may be deposited on top of the chromium compound already present. This may lead to a change in catalytic activity and a possible reduction in the melt index for the polymer produced using the activated catalyst.

[009] US 4,814,308 describes a supported catalyst loaded with chromium, phosphorus and aluminum, each in oxide form, used in combination with a co-catalyst. The supported catalyst is formed by impregnating the porous support particles using organometallic compounds of chromium and aluminum in an organic solvent. The co-catalyst is alkyllithium and boronalkyl. A comparative example, using aluminum nitrate and phosphoric acid and chromium nitrate to deposit chromium and aluminum onto a porous silicate carrier, produces a catalyst precursor which, when activated and used for olefin polymerization, has low productivity and low melt flow rate compared to the organometallic route.

[0010] Thus, there is a need for a method for impregnating chromium and aluminum compounds onto inorganic porous support materials to form catalyst precursors that overcomes some or all of the problems of the prior art. There is also a need for the activated catalyst formed from the precursors to provide a melt flow rate and productivity similar to those of prior art catalysts prepared using organometallic compounds deposited from nonpolar solvents when the activated catalyst is used for homo- or copolymerization of olefins.

[0011] It is desirable to use an aqueous or lower aliphatic alcohol solution of chromium and aluminum compounds for impregnation of the inorganic support material. The use of such solvents, rather than nonpolar higher aliphatic compounds such as heptane, could lead to fewer processing problems associated with solvent removal, such as fewer hazards during evaporation, particularly for water as a solvent. Although chromium salts such as nitrate, carboxylates (such as acetates and oxalates) and sulfate are soluble in water, lower aliphatic alcohols or mixtures thereof, it has been found that dissolving aluminum compounds together with chromium compounds to form a mixed solution is generally not possible at the concentrations required for deposition onto an inorganic support. It has also been found that when some soluble aluminum salts are used for deposition of aluminum onto an inorganic catalyst, the resulting activated catalyst, when used for polymerization of olefins, gives reduced HLMI compared to a catalyst prepared from organometallic chromium-aluminum compounds.

[0012] Thus there is a need to identify a method for including aluminum, in a dissolved form, in solutions with chromium salts dissolved in water, C1 to C4 aliphatic alcohol, or mixtures thereof, which does not lead to the precipitation of insoluble chromium or aluminum salts. There is also a need to identify a route for deposition of aluminum salts from water, C1 to C4 aliphatic alcohol, or mixtures thereof on an inorganic carrier, together with chromium salts loaded on the same inorganic carrier, so that the resulting activated catalyst has properties similar to those of prior art catalysts prepared using organometallic compounds deposited from non-polar solvents.

[0013] Furthermore, many of the chromium and/or aluminum compounds used in the prior art, particularly organometallic compounds, can produce noxious or toxic fumes during activation or calcination of catalyst precursor to form activated catalysts. This may require complex fume treatment equipment at the site where the catalyst precursor is activated.

[0014] It is an object of this invention, among others, to provide a method for preparing catalyst precursors by deposition of chromium and aluminum from aqueous or alcoholic solvent-based solutions. It is also an object of this invention to provide a method for preparing catalyst precursors that avoid the formation of harmful or toxic vapors consisting of providing catalyst precursors that, when activated, provide catalysts for homo- or copolymerization of α-alkenes (olefins) that have activities similar to prior art catalysts prepared by organometallic routes, and that can be used to form polymers with melt indices greater than or equal to the values obtained with such prior art catalysts.

[0015] Surprisingly, it has now been found that solutions of chromium and/or aluminum compounds in water, C1 to C4 aliphatic alcohol, or mixtures thereof can be prepared with sufficiently high concentrations of chromium and/or aluminum to be usable for coating or impregnating an inorganic support material to form inactivated catalyst particles (i.e., catalyst precursor particles) comprising chromium and aluminum compounds deposited on the inorganic support material. The solutions can be of sufficiently low viscosity that they can penetrate the pore structure of porous inorganic support materials. The solvent can be readily removed by evaporation to form the catalyst precursor, with reduction in problems related to atmospheric loading with volatile organic compounds or flammability, particularly if the solvent is or includes water. Furthermore, the compounds selected are such that their decomposition, together with the decomposition of any residual solvent, during catalyst activation, does not generate toxic or harmful vapors, such as those formed by some organometallic compounds and solvents used in the prior art.

[0016] Furthermore, sequential deposition of chromium then aluminum compounds, or vice versa, gives acceptable catalyst precursors with properties comparable to where the chromium and aluminum compounds are deposited together.

[0017] A first aspect of the invention provides a catalyst precursor for an olefin polymerization catalyst comprising an inorganic support material carrying a salt of chromium, aluminum carboxylate and boric acid.

[0018] A second aspect of the invention provides a method for preparing a catalyst precursor for an olefin polymerization catalyst according to the first aspect of the invention comprising: i) providing the inorganic support material, ii) depositing a chromium salt onto the inorganic support material from a first solution comprising a chromium salt in a solvent that is water, C1 to C4 aliphatic alcohol, or a mixture thereof, iii) depositing an aluminum carboxylate and boric acid onto the inorganic support material from a second solution comprising an aluminum carboxylate and boric acid in a solvent that is water, C1 to C4 aliphatic alcohol, or a mixture thereof, and iv) removing solvent to form the catalyst precursor comprising chromium salt, aluminum carboxylate and boric acid.

[0019] The chromium salt may be deposited onto the inorganic support material from the first solution, and the solvent from the first solution removed, before the aluminum carboxylate and boric acid are deposited from the second solution. The process may also be carried out by first depositing the aluminum carboxylate and boric acid from the second solution, then removing the solvent from the second solution, before depositing the chromium salt from the first solution.

[0020] Preferably, for simplicity of process, the method of the second aspect of the invention is carried out using a single solution comprising the chromium salt, the aluminium carboxylate and the boric acid.

[0021] Thus, a preferred method of the second aspect of the invention comprises the steps of: i) providing the inorganic support material, ii) providing a solution comprising chromium salt, aluminum carboxylate and boric acid in a solvent that is water, C1 to C4 aliphatic alcohol, or a mixture thereof, iii) depositing the solution onto the inorganic support material, and iv) removing the solvent to form the catalyst precursor comprising chromium salt, aluminum carboxylate and boric acid.

[0022] By "solvent removal" one of skill in the art should understand that sufficient solvent is removed to form an apparently dry, free-flowing, and easily transportable catalyst precursor. This may mean that some residual solvent may remain as part of the catalyst precursor. For example, the catalyst precursor may contain up to 15% residual solvent, preferably 10% or less, and still behave as a free-flowing powder. Any such residual solvent will be driven off when the catalyst precursor is activated to form activated catalysts. Alternatively, the solvent may be substantially completely removed from the catalyst precursor such that no unbound solvent remains after solvent removal.

[0023] The preferred aspects and embodiments of the invention detailed below apply to both the first and second aspects of the invention, as appropriate.

[0024] By "catalyst precursor" is meant a product which is suitable for handling and transport to a location where it can be activated for use as a catalyst, particularly as an olefin polymerization catalyst. Such catalysts are generally activated by heating in a non-reducing atmosphere, as described hereinbelow, shortly before use. Thus the catalyst precursor is a commercially usable material which can be used to form catalysts simply by activation or calcination.

[0025] The chromium salt is suitably carboxylate, nitrate, sulphate, chloride or a mixture thereof, but any suitable chromium salt may be used. Chromium sulphates, chlorides or carboxylates are particularly preferred salts, particularly carboxylates, as they do not produce toxic or noxious fumes on activation of the catalyst precursor. Chromium acetate is particularly preferred for this reason and also because of its high solubility in the solvents used in the method of the second aspect of the invention. Suitably, the chromium salt should have a solubility in the solvent such that the solution may comprise at least 1% by weight of chromium, expressed as the element, preferably at least 2%, more preferably at least 4%.

[0026] Suitably, the inorganic support material is a porous inorganic oxide.

[0027] Typically, the inorganic support material is in the form of particles having a weight average particle diameter of 1 to 300 micrometers. This typical average diameter also applies to the catalyst precursor particles and activated catalyst particles, which have essentially the same particle diameter as the support material. This can be measured by light scattering particle size analysis using an instrument such as the Malvern Mastersizer™ model S, with a 300 RF lens (measurement range 0.05 - 900 μm), Malvern Mastersizer™ software v. 2.18 and a DIF 2012 scattering unit. This instrument, made by Malvern Instruments, Malvern, Worcestershire, uses Mie theory to calculate the particle size distribution. Mie theory provides how much direct sunlight is scattered by spherical particles and takes into account the refractive index of such particles. The actual value used for silica's refractive index is 1.4564 and 0.1 for the particle's imaginary refractive index (the absorption of light), with water dispersant at a refractive index of 1.33.

[0028] Suitably, such particles have a d90 of 500 μm or less, preferably 400 or less. They may have a d50 of 300 μm or less. The particles may have a d10 of 1 μm or more, preferably 10 or more. (For clarity, d90 is the diameter at which 90% by weight of the particles have a diameter less than d90 - equivalent definitions apply to d50 and d10.) Preferably, the particles have a d50 of 1 to 300 μm, more preferably 5 to 250 μm, even more preferably 25 to 150 μm.

[0029] The particles may be prepared by comminution, optionally combined with size classification by means such as sieving, or the particles may be prepared by a route such as spray drying.

[0030] A further advantage of the method of the second aspect of the invention is that the chromium and aluminium are found to be more evenly distributed over particles of varying diameters when the inorganic support material is in particulate form. Prior art methods, such as the method of GB 1 575 352, can lead to smaller particles of inorganic material with higher concentrations of chromium and aluminium, leading to potentially less efficient utilization of the catalytically active surfaces during polymerization. Also, when activated catalysts are used to catalyze polymerization, polymerization at active sites within the catalysts tends to lead to fragmentation of the support material. This fragmentation of the catalyst particles during polymerization is an important and advantageous aspect because it means that the catalyst particles do not need to be removed from the polymer. However, if large particles of unfragmented catalyst remain in the polymer, these can lead to the formation of defects in articles formed from the polymer. If large catalyst particles have fewer active sites, this can lead to poor fragmentation of these particles and consequent downstream process defects. The invention provides larger catalyst particles having the same concentration of active sites as smaller particles, assisting with the fragmentation of these larger particles and the reduction or elimination of such process defects.

[0031] Suitable inorganic support materials include oxides such as silica, alumina, mixed silica-alumina or oxides of zirconium, thorium or magnesium. Oxides of other elements may also be present as part of the atomic lattice structure of the inorganic support material. Suitably, the support material is in the form of an inorganic oxide gel, preferably a silica gel.

[0032] When the inorganic support material is a xerogel, the xerogel may be prepared by spray drying, by evaporation of water from a hydrogel, or it may be prepared by removing water from a hydrogel by solvent exchange with a water-miscible solvent such as methanol, ethanol, ethyl acetate, isopropyl alcohol, or some other suitable solvent, together with removal of the exchange solvent from the resulting solvent-exchanged gel, e.g., by evaporation. Another route for solvent removal is by azeotropic distillation with a partially water-miscible solvent, e.g., ethyl acetate. The solvent is added to the hydrogel and the mixture distilled. A solvent-rich condensate is returned to the distillation mixture while a water-rich distillate is discarded, leading to replacement of the water in the hydrogel with the solvent. The resulting gel may then have the solvent removed by evaporation or other suitable means to form a xerogel.

[0033] For xerogels prepared by water removal using water evaporation, the process generally leads to lower final pore volumes for the resulting xerogels than would a solvent exchange process to remove water. However, a xerogel prepared by a solvent exchange route is somewhat more prone to partial structural collapse (i.e., loss of porosity) than a xerogel formed by evaporative drying of a hydrogel when an aqueous or alcoholic solution (such as chromium salt and/or aluminum salt solution or solutions of the invention) enters the pore structure.

[0034] For spray drying, typically a hydrogel is comminuted to produce a dispersion in a solvent, usually water, which is fed to the spray dryer to instantaneously distill the solvent from the suspension droplets by evaporation. This forms agglomerated, typically spheroidal, particles of dry or partially dried gel, depending on the drying conditions. Partially dried gel may be further dried to form xerogel by, for example, evaporative drying or by solvent exchange, as explained above. Other combinations of such processes are possible, for example by including a further comminution step after spray drying. Spray drying is another suitable process, which may be used for the preparation of dry gel (where the solvent frozen in solid form, for example water as ice, is removed by sublimation under vacuum).

[0035] The process of the second aspect of the invention is applicable to highly porous inorganic carriers such as those dried by solvent exchange, azeotropic distillation, or freeze-drying, as well as to less porous inorganic carriers with lower porosities, such as xerogels prepared by evaporation of water.

[0036] Thus, it has been found that the process of the second aspect of the invention can produce catalyst precursors, which can be activated to form effective olefin polymerization catalysts, without the need to use complex solvent removal routes such as solvent exchange in order to produce highly porous silica xerogels. Furthermore, xerogels produced by simple evaporation of water have been found to be less susceptible to pore collapse than most porous xerogels produced by complex drying routes when contacted with an aqueous or alcoholic solution of a chromium salt, such as carboxylate and/or aluminum carboxylate for deposition of the salts. Thus the invention also allows the use of a simple xerogel preparation process together with a solvent or solvents to load or deposit chromium and aluminum that do not cause collapse in the pore structure, and still allow high loading levels of chromium and aluminum, from a solution without excessive risks of harmful, toxic or flammable vapors generated when the solvent, in this solution, is removed, or when the resulting catalyst precursor is activated by calcination.

[0037] It should be emphasized that the second aspect of the invention is also effective when used to form catalyst precursors from xerogels or other inorganic carrier particles made by other routes such as solvent exchange, azeotropic distillation, freeze drying, etc., and is not limited to use with xerogels formed by evaporative drying from a hydrogel.

[0038] A suitable porous inorganic support material is a silica xerogel. Other oxides such as titanium or aluminium, or mixtures of such oxides, may be included in the atomic lattice structure of the silica xerogel, but these may also be absent. Suitably, silica xerogels used in the invention comprise at least 90% by weight SiO2, expressed as a percentage by weight of the xerogel. This is because the presence of other oxides may lead to weakening of the silica skeletal structure, potentially making it more difficult to achieve high porosities in the pore structure of the inorganic support material. More preferably, silica xerogels comprise at least 95% by weight silica, most preferably at least 98% by weight silica. The level of SiO2 in the xerogel is suitably measured by elemental analysis using XRF, as detailed below.

[0039] In this report, unless otherwise stated, weight percentages refer to percentages as measured by elemental analysis using XRF. In other words, percentages are expressed relative to the total material measured by XRF. The elevated temperatures used in sample preparation for XRF mean that any volatile materials such as water would already have been lost prior to measurement and will not be included. For boron, which is not measurable by XRF, percentages are calculated based on material dried at 500oC for 4 hours.

[0040] Suitably, the inorganic support material has a porosity of 0.5 to 4.0 cm3/g or 0.5 to 3.0 cm3/g, preferably 1.0 to 3.0 cm3/g or 0.8 to 2.5 cm3/g, more preferably 1.0 to 2.0 cm3/g.

[0041] Suitably the surface area of the inorganic support material is from 100 to 1000 m2/g, for example from 200 to 800 m2/g, preferably from 200 to 700 m2/g, for example from 250 to 700 m2/g, more preferably from 250 to 500 m2/g. The surface area, together with the pore volume, is measured by nitrogen porosimetry using an ASAP2420 analyser (supplied by Micromeritics Ltd, Dunstable, Bedfordshire UK). Samples are first degassed at 270oC for at least 1 hour in the degassing station of the instrument prior to measurement. The sample tube (containing the degassed sample) is transferred to the analysis station, submerged in liquid nitrogen and a nitrogen isotherm determined. A multipoint surface area is calculated using BET theory, taking data points in the range of P/P0 0.08 to 0.20. A pore volume measurement is recorded at a P/P0 of 0.98 in the desorption leg.

[0042] When porosity and surface area measurements are performed on catalyst precursors, they were kept in air at 500°C for 4 hours prior to measurement.

[0043] The second aspect of the invention may also be practiced by using a solution comprising chromium salt, aluminum carboxylate and boric acid in a solvent that is water, C1 to C4 aliphatic alcohol, or a mixture thereof as the solvent used in a solvent exchange process for removing water from a hydrogel of inorganic support material. The exchanged and thus loaded gel may then be heated to evaporate the solvent and form the catalyst precursor, loaded with chromium salt, aluminum carboxylate and boric acid. This process is suitable for use with hydrogels such as silica hydrogels, and gives the advantage that no additional drying step is necessary, as is required when organometallic compounds are used in organic solvents for impregnation.

[0044] In another method of putting the second aspect of the invention into effect, the inorganic support material may be substantially free of unbound solvent prior to carrying out the method of the second aspect of the invention. By unbound solvent is meant solvent which may be removed by heating the inorganic support material in a vacuum oven at 140°C for 1 hour.

[0045] The chromium salt, aluminum carboxylate and boric acid are appropriately loaded within a pore structure of the inorganic support material.

[0046] This may be achieved, for example, by spraying a solution comprising the chromium salt, the aluminum carboxylate and the boric acid, in a solvent, onto the porous inorganic support material, followed by removal of the solvent. For example, if the porous inorganic support material is in the form of particles, the particles may be tumbled in a mixer, such as a double cone mixer, while the solution is sprayed onto them. Subsequently, the particles may be dried in an oven or using a fluidized bed apparatus, or passed through a flash dryer to remove the solvent. Another method could be to spray the solution onto fluidized particles in a heated fluidized bed, so that loading of the materials into the pore structure occurs concurrently with removal of the solvent. Other methods are also suitable, such as dispersing the particulate material in an excess of solution, filtering to remove the excess solution and then heating the particles by a suitable means, such as an oven or in a fluidized bed apparatus, in order to remove the solvent. Preferably, incipient wetting of the porous carrier particles is used as the impregnation method, with a solution being drawn into the pore structure of the carrier particles by capillary forces. When the method of the invention is carried out using two solutions (a first chromium salt solution and a second solution comprising aluminum carboxylate and boric acid), the same process routes detailed above may be used for the first solution, followed by repetition of the process for impregnation with the second solution (or vice versa).

[0047] By carboxylate is meant a salt of an alkyl carboxylate. Suitable alkyl carboxylates include mono-, di-, and tri-carboxylic acids having 6 carbon atoms or less per molecule, preferably 4 or less, more preferably 2 or less. Such carboxylates have been found to decompose upon activation of the catalyst precursor without excessive formation of noxious or toxic vapors.

[0048] Preferred monocarboxylates include formate, acetate, lactate, and propionate. Preferred dicarboxylates include oxalate, malate, maleate, malonate, glutarate, succinate, and the preferred tricarboxylate is citrate. Mixtures of carboxylates may also be employed.

[0049] Preferably, when the chromium salt is a carboxylate, it is chromium acetate. Preferably, the aluminum carboxylate is aluminum acetate.

[0050] It was found that the presence of boric acid with the aluminum carboxylate increases the solubility of the aluminum carboxylate in the aqueous and/or alcoholic solution, and does not lead to precipitation of the aluminum carboxylate in the concentrations necessary for deposition on the inorganic support material from the solution. It was found that the presence of boric acid in the solution of the chromium aluminum carboxylate salt increases the solubility of the aluminum carboxylate in the aqueous and/or alcoholic solution, and does not lead to precipitation of the chromium salt or the aluminum salt, thus facilitating the loading of the inorganic support material from the solution. This leads to the inorganic support material also loading boric acid after removal of the solvent.

[0051] The levels of salts present in the solution will be selected in light of the pore volume of the inorganic support material in order to obtain desirable loading levels for the catalyst precursor and the eventual activated catalyst. The levels required will depend on the impregnation method used and the porosity of the inorganic support material. The solution comprising chromium salt, aluminum carboxylate and boric acid suitably comprises from 0.05% to 2% by weight of chromium expressed as the element, e.g. from 0.1% to 1%. The solution suitably comprises from 0.1% to 4% by weight of aluminum expressed as the element, e.g. from 0.2% to 2%. The solution suitably comprises from 0.01% to 0.4% by weight of boron expressed as the element, e.g. from 0.02% to 0.2%. The upper level of chromium, aluminum or boron in the solution is not particularly important. Suitably, the ratio of aluminum to boron in the solution, expressed as aluminum/boron by weight, is from 50/1 to 3/1, preferably from 40/1 to 4/1, more preferably from 30/1 to 5/1. Suitably, the ratio of aluminum to chromium in the solution, expressed as aluminum/chromium by weight, is from 25/1 to 1/2, preferably from 8/1 to 2/3, more preferably from 4/1 to 1/1. When separate solutions are used for the chromium salt and for the aluminum carboxylate and boric acid, the same typical and preferred levels are used as detailed above.

[0052] A suitable means for incorporating boric acid into a solution involves the use of commercially available basic aluminum acetate stabilized with boric acid as the source of aluminum carboxylate of the invention. Preferably, basic aluminum acetate, stabilized with boric acid, is used as a source of aluminum carboxylate and boric acid for the invention. This material is commercially available, for example, from Sigma Aldrich in the form of a white powder typically comprising from 16 to 20 wt. % aluminum and 2.5 wt. % boron, expressed as the elements.

[0053] The catalyst precursor of the first aspect of the invention, or prepared by the second aspect of the invention, suitably comprises, as carrier material, typically deposited on the surfaces of the catalyst precursor, from 0.01 to 3% by weight of chromium expressed as the element, preferably from 0.1 to 2%, more preferably from 0.25 to 1.5%. The catalyst precursor suitably comprises, as carrier material, typically deposited on the surfaces of the catalyst precursor, from 0.1 to 8% of aluminium, expressed as the element, preferably from 0.2 to 4%, more preferably from 0.5 to 2.5%. The catalyst precursor suitably comprises, as carrier material, typically deposited on the surfaces of the catalyst precursor, from 0.005 to 2% of boron, expressed as the element, preferably from 0.05 to 0.5%, more preferably from 0.1 to 0.3% by weight. The term "on surfaces" means to include the surfaces within any catalyst precursor pore structure, in addition to any external surfaces. Chromium and aluminum levels are conveniently measured by XRF (X-ray fluorescence) using a Phillips PW2400 instrument. Samples for analysis are prepared as molten beads using a lithium borate flux. Melting is typically between 1000°C and 1250°C.

[0054] Boron, which cannot be measured by XRF, is conveniently measured in solution using ICP (inductively coupled plasma) spectrometry. Typically an extract is generated by adding 10 cm3 of nitric acid to 1 g of catalyst sample, boiling and subsequently diluting to 250 cm3. Prior to analysis of the solution by ICP this extract is filtered using a 0.45 μm filter. Boron values are determined by comparison with boron standards in a corresponding acid matrix, using a Varian inductively coupled plasma-optical emission spectrometer (ICP-OES). The value obtained is corrected to a base value in the dry state by measuring the weight, equilibrium weight loss at 500°C for a catalyst precursor sample of equal origin. The boron level is expressed as the weight of boron relative to the weight of dry material at 500°C.

[0055] The amounts of chromium, aluminum, and boron present as carrier material (i.e., typically deposited on the surfaces of the catalyst precursor) are determined by comparison with the levels measured in the inorganic support material prior to impregnation to form the catalyst precursor. The levels present as carrier material are obtained by subtracting any contribution of Cr, Al, or B already within the structure of the inorganic support material. The terms "present as carrier material" and "deposited on surfaces" include material on surfaces contained within any pore structure of the inorganic support material.

[0056] The solvent used to provide a solution of chromium salt, aluminum carboxylate and boric acid is preferably water, a C1 to C4 aliphatic alcohol, or a mixture thereof. This is to avoid noxious, flammable or toxic vapors when the solvent is removed. Water, methanol, ethanol, isopropanol and mixtures thereof are preferred solvents, with water, methanol and ethanol and mixtures thereof particularly preferred and water, methanol and mixtures thereof especially preferred. Water is particularly useful due to its lack of toxicity and flammability. Other solvents may also be used or incorporated into the preferred solvents at levels such that they do not interfere with the performance of the invention.

[0057] Preferably, the solvent is removed by evaporation to leave the chromium salt, aluminum carboxylate and boric acid deposited on the inorganic support material. As explained above, "removal" means that sufficient solvent is removed so that a free-flowing and readily transportable catalyst precursor is obtained, and as much as 15% or 10% or less by weight of solvent may still be present, expressed as the weight percentage of the catalyst precursor, after removal of the solvent. Alternatively, the solvent may be substantially removed so that no unbound solvent is present. By unbound solvent is meant solvent which may be removed by heating the inorganic support material in a vacuum oven at 140°C for 1 hour.

[0058] A third aspect of the invention provides an olefin polymerization catalyst, obtained or obtainable by heating a catalyst precursor according to the first aspect of the invention, or prepared by the method of the second aspect of the invention, in a non-reducing atmosphere, at a temperature of 200 to 1200°C, preferably of 300 to 1100°C, more preferably of 400 to 900°C for a period of time of 30 minutes to 15 hours, preferably up to 8 hours, most preferably up to 6 hours.

[0059] Preferably, the non-reducing atmosphere is an oxygen-containing atmosphere such as air or oxygen or mixtures thereof. This process is known as activation or calcination of the catalyst precursor to form the active olefin polymerization catalyst.

[0060] For prior art catalysts, particularly those where the catalyst precursor has been loaded with organometallic chromium and aluminum compounds liberated from aliphatic solvents, this activation process can lead to the formation of volatile harmful and/or toxic components in the chromium and aluminum compounds decomposed during the activation process. The present invention avoids the formation of such undesirable activation byproducts.

[0061] Typically, the activated olefin polymerization catalyst comprises levels of chromium, aluminum and boron as carrier material similar to the preferred levels detailed above for the catalyst precursor. Measurement techniques for Cr, Al and B involve pre-treatment prior to or during analysis so that the levels measured for the catalyst precursor and activated catalysts are substantially equal. Thermally activated diffusion during the activation/calcination process for the catalyst can cause the charged Al, Cr to diffuse into the structure of the inorganic support material rather than being present on its surfaces. The levels of chromium and aluminum are appropriately measured by XRF as detailed above. Boron is also measured as detailed above. The amounts present as carrier material in the catalyst are obtained by subtracting the measured amounts of Cr, Al and B already in the inorganic support material prior to impregnation to form a catalyst precursor.

[0062] A fourth aspect of the invention relates to a method for polymerizing one or more C2 to C8 α-alkenes, characterized in that the polymerization is carried out in the presence of an olefin polymerization catalyst according to the third aspect of the invention. Mixtures of alkenes or alkenes in combination with other monomers may be employed. The use of the catalysts prepared using the methods of the invention and from the catalyst precursors of the invention is not particularly restricted to alkene polymerization, but is particularly suitable for this purpose. Co-catalysts may be used in combination with the catalysts prepared from precursors of the invention or by the method of the invention. Typically, after activation of the catalyst, it is cooled to room temperature and stored ready for use in the polymerization. The catalyst prepared using the invention can be used in a variety of homo- or co-polymerization pathways, for example for the production of polyethylenes, by process pathways such as solution, suspension-loop, solution CSTR (continuous flow stirred tank) or gas phase polymerization.

[0063] Specific embodiments of the present invention will now be described, by way of example only.

Examples

[0064] In the examples detailed below, the high load melt index (HLMI) and melt index (Ml) were determined in accordance with ASTM D-1238 using loads of 21.6 kg and 2.16 kg respectively at 190 oC.

[0065] The silica xerogel support materials used were verified to contain no measurable amounts of chromium, aluminum or boron, as measured by the methods detailed hereinabove, prior to performing the deposition experiments detailed below.

Example 1 (comparative - heptane organometals - homopolymerization - 700°C).

[0066] 100 g of a silica xerogel support (pure silica) was prepared by evaporative drying of a hydrogel. The support has a surface area and pore volume by nitrogen porosimetry, measured as detailed above using a Micromeritics ASAP 2420 analyzer, of 381 m2/g and 1.76 cm3/g.

[0067] The support particles had the following particle diameter distribution, as measured using a Malvern Mastersizer™: d90 of 169.3 μm, d50 of 114.6 μm, and d10 of 63.4 μm. (d90 is the diameter at which 90 wt. % of particles have a diameter smaller than d90 - equivalent definitions apply to d50 and d10.

[0068] 100 g of the silica xerogel support (pre-dried at 140°C) was added to a dry reaction flask equipped with a stirrer. 600 cm3 of heptane was added to the flask to form a silica/heptane suspension. 6.78 g of chromium acetylacetonate was added to another clean and dry flask containing a magnetic stirrer. 50 cm3 of heptane was added to the chromium acetylacetonate and 64.3 cm3 of a triisobutyl aluminum solution (1.0 molar solution in heptane) was added to a stirred suspension of chromium acetylacetonate. This reaction mixture was allowed to cool. Then the Cr/Al complex solution was transferred to the flask containing silica and heptane. The resulting suspension was stirred for another hour and then 6.4 cm3 of methanol was added to the suspension. Stirring was stopped and catalyst precursor particles were allowed to settle. The suspension was filtered to remove excess heptane and the residual filter cake, which was wet with solvent, was dried in a vacuum oven at 140°C to give a free-flowing green powder. The resulting catalyst precursor contained 1.06% chromium and 1.76% aluminum, expressed as the respective elements. The surface area and pore volume were determined for the catalyst precursor using the ASAP 2420 instrument and were found to be 358 m2/g and 1.66 cm3/g, respectively.

[0069] 10 g of this catalyst precursor was activated in a fluidized bed reactor using dry air as the fluidizing gas. The temperature was maintained at 700°C for 5 hours before cooling and switching to nitrogen. 0.148 g of activated catalysts was transferred to a 5 liter isobutane suspension polymerization reactor and tested under homopolymerization conditions using ethylene. The total reactor pressure was 40.3 atm and ethylene partial pressure 16.0 atm. The reactor was maintained at 104°C. 370 g polyethylene was produced and the catalytic activity was calculated as 2640 g/g/h. The polymer had a melt index (Ml) of 0.22 g/10 min, a high load melt index (HLMI) of 19.7 g/10 min and a density of 0.9611 g/cm3.

Example 2 (according to the invention - homopolymerization - 700°C).

[0070] A methanolic solution of chromium acetate and aluminum acetate was prepared by adding 1.15 g of chromium acetate and 2.84 g of basic aluminum acetate stabilized in boric acid to a beaker containing a magnetic stirrer.

[0071] Approximately 50 cm3 of methanol was added and the composition was stirred for 1 hour so that all the chromium and aluminum salts were dissolved. This chromium and aluminum solution was added to 30 g of the dried silica xerogel support as used in example 1, so that the chromium and aluminum species were impregnated into the support. Methanol was removed from the catalyst precursor thus formed by drying in a vacuum oven at 140°C for 3 hours. The resulting blue/green catalyst precursor powder had chromium and aluminum contents of 0.93% and 1.87% by weight, expressed as the respective element. Boron content was determined at 1720 ppm by weight. The surface area and pore volume determined using the Micromeritics ASAP 2420 were 364 m2 /g and 1.67 cm3 /g respectively.

[0072] 10 g of this catalyst precursor was activated in a fluidized bed reactor using dry air as the fluidizing gas. The temperature was maintained at 700°C for 5 hours before cooling and switching to nitrogen.

[0073] 0.172 g of activated catalyst was transferred to a 5 liter isobutane suspension polymerization reactor and tested under homopolymerization conditions (i.e. using ethylene as monomer) as per example 1. The total reactor pressure was 40.3 atm and ethylene partial pressure 16.0 atm. The reactor was maintained at 104°C. 431 g of polyethylene were produced and the activity calculated at 2839 g/g/h. The polymer has a Ml of 0.22 g/10 min, HLMI of 19.4 g/10 min and density of 0.9606 g/cm3.

Example 3 (comparative - sequentially Cr then Al impregnation - homopolymerization - 700°C).

[0074] An aqueous solution of chromium acetate was prepared by diluting a chromium acetate solution containing 7.68 wt% chromium with an additional 2201 cm3 of water. This solution was added to 1500 g of silica xerogel as detailed in Example 1. Water was removed to give a free-flowing catalyst with a chromium content of 0.97%. The surface area and pore volume were 380 m2/g and 1.62 cm3/g (ASAP 2420).

[0075] 70 g of this Cr-containing silica was pre-dried at 140°C for 4 hours and placed in a sealed vial. 250 cm3 of heptane was added. Under a nitrogen purge, 43 cm3 of a 1.0 molar triisobutyl aluminum solution in heptane was added. After stirring for 30 minutes the composition was filtered and the resulting filter cake dried in a vacuum oven at 140°C. The pale blue/green catalyst precursor powder generated had chromium and aluminum contents of 0.95 and 1.71%, expressed as the respective elements. The surface area and pore volume for the catalyst precursor measured using the ASAP 2420 were 375 m2/g and 1.61 cm3/g.

[0076] 10 g of this catalyst was activated in a fluidized bed reactor using dry air as the fluidizing gas. The temperature was maintained at 700°C for 5 hours before cooling and switching to nitrogen.

[0077] 0.172 g of activated catalysts were transferred to a 5 liter isobutane suspension polymerization reactor and tested under homopolymerization conditions as used in Example 1. The total reactor pressure was 39.9 atm and ethylene partial pressure 15.9 atm. The reactor was maintained at 104 °C. 447 g of polyethylene were produced and the activity calculated at 2997 g/g/h. The polymer had a MI of 0.14 g/10 min, HLMI of 14.4 g/10 min and density of 0.9602 g/cm3.

Example 4 (according to the invention - copolymerization - activation at 600°C).

[0078] An aqueous solution of chromium acetate and aluminum acetate was prepared by adding 33.5 g of chromium acetate and 96.4 g of basic aluminum acetate stabilized with boric acid to a beaker. Approximately 1.44 liters of water were added and the composition stirred for 1 hour so that all the chromium and aluminum salts were dissolved. This chromium and aluminum solution was added to 1 kg of silica support having a surface area of 377 m2/g and pore volume of 1.75 cm3/g, both parameters determined using the ASAP 2420 instrument.

[0079] Water was removed from the resulting catalyst precursor giving a blue/grey catalyst powder which had chromium and aluminium contents of 1.00% and 1.98% by weight of the respective elements. The boron content was determined at 2130 ppm. The surface area and pore volume determined using ASAP 2420 were 354 m2/g and 1.63 cm3/g respectively. The particle diameter distribution was determined for this catalyst by Malvern Mastersizer™ which indicated: d10 of 75.1 μm, d50 of 126 μm and d90 of 180.6 μm. 10 g of this catalyst precursor was activated in a fluidized bed reactor using dry air as the fluidizing gas. The temperature was maintained at 600oC for 5 hours before cooling and switching to nitrogen.

[0080] 0.180 g of activated catalysts were transferred to a 5 liter isobutane suspension polymerization reactor and tested under copolymerization conditions with 15 cm of hexene. The total reactor pressure was 40.1 atm and ethylene partial pressure 15.8 atm. The reactor was maintained at 100oC. 468 g of copolymer were produced and the activity calculated at 2643 g/g/h. The copolymer has a HLMI of 10.8 g/10 min and a density of 0.9487 g/cm3.

Example 5 (comparative - in Al - copolymerization - activation at 600°C)

[0081] An aqueous solution of chromium acetate was prepared by adding 33.5 g of chromium acetate to a beaker. Approximately 1.44 liters of water were added and the composition stirred for 1 hour so that all of the chromium acetate was dissolved. This chromium solution was added to 1 kg of silica support as described in Example 4.

[0082] Water was removed from the resulting catalyst precursor giving a blue/grey catalyst powder having a chromium content of 1.03%. The surface area and pore volume determined using ASAP 2420 were 376 m2/g and 1.69 cm3/g, respectively. The particle diameter distribution was determined for this catalyst by Malvern Mastersizer™ which indicated: d10 of 75.4 μm, d50 of 126.2 μm and d90 of 181.3 μm. 10 g of this catalyst precursor was activated in a fluidized bed reactor using dry air as the fluidizing gas. The temperature was maintained at 600°C for 5 hours before cooling and switching to nitrogen.

[0083] 0.178 g of activated catalysts were transferred to a 5 liter isobutane suspension polymerization reactor and tested under copolymerization conditions with 15 cm of hexene. The total reactor pressure was 40.0 atm and ethylene partial pressure 15.8 atm. The reactor was maintained at 100°C. 459 g of copolymer were produced and the activity calculated at 2630 g/g/h. The copolymer has a HLMI of 5.7 g/10 min and a density of 0.9466 g/cm3.

Example 6 (according to the invention - copolymerization - activation at 600°C).

[0084] A methanolic solution of chromium acetate and aluminum acetate was prepared by adding 5.14 g of chromium acetate and 13.67 g of basic aluminum acetate stabilized with boric acid to a beaker containing a magnetic stirrer.

[0085] Approximately 240 cm3 of methanol was added and the composition stirred for 1 hour so that all the chromium and aluminum salts were dissolved. This chromium and aluminum solution was added to 150 g of silica support having a surface area of 383 m2/g and pore volume of 1.75 cm3/g, both parameters determined using ASAP2420 instrument, so that the chromium and aluminum species were impregnated within the silica. Excess methanol was removed from the catalyst by drying in a vacuum oven at 140°C for 3 hours. The resulting blue/gray catalyst powder had chromium and aluminum contents of 1.03% and 1.92% by weight of the respective elements. The boron content was determined at 1600 ppm. The surface area and pore volume determined using ASAP 2420 were 361 m2/g and 1.63 cm3/g, respectively.

[0086] Particle diameter distribution was determined by Malvern Mastersizer™ and indicated d10 of 65.5 μm, d50 of 118.4 μm and d90 of 176.4 μm.

[0087] 10 g of this catalyst was activated in a fluidized bed reactor using dry air as the fluidizing gas. The temperature was maintained at 600oC for 5 hours before cooling and switching to nitrogen.

[0088] 0.193 g of activated catalysts were transferred to a 5 liter isobutane suspension polymerization reactor and tested under copolymerization conditions with 15 cm3 of hexene. The total reactor pressure was 40.0 atm and ethylene partial pressure was 15.8 atm. The reactor was maintained at 100°C. 490 g of copolymer were produced and the activity calculated at 2340 g/g/h. The copolymer had a HLMI of 8.6 g/10 min and a density of 0.9503 g/cm3.

Example 7 (comparative - heptane organometals - copolymerization - activation at 600°C).

[0089] 150 g of silica support (pre-dried at 140°C) with a surface area and pore volume by ASAP 2420 of 393 m2/g and 1.73 cm3/g respectively, were added to a dry reaction flask equipped with a stirrer. 900 cm3 of heptane was added to the flask to form a silica/heptane suspension. 9.51 g of chromium acetylacetonate was added to another clean and dry flask containing a magnetic stirrer. 80 cm3 of heptane was added to the chromium acetylacetonate and 100.1 cm3 of 1.0 molar solution of triisobutyl aluminum in heptane was added to the stirred chromium acetylacetonate suspension. This reaction mixture was allowed to cool. Then the Cr/AI complex solution was transferred to the flask containing silica and heptane. The resulting suspension was stirred for an additional hour and then 9.6 cm3 of methanol was added to the suspension. Stirring was stopped and the catalyst particles were allowed to settle. The suspension was filtered to remove excess heptane and the residual solvent-wet filter cake was dried in a vacuum oven at 140 °C to give a free-flowing green powder. The resulting catalyst precursor contained 0.99 wt % chromium and 1.86 wt % aluminum as the respective elements. The surface area and pore volume were determined using an ASAP 2420 instrument and were 375 m2/g and 1.63 cm3/g respectively.

[0090] 10 g of this catalyst precursor was activated in a fluidized bed reactor using dry air as the fluidizing gas. The temperature was maintained at 600°C for 5 hours before cooling and switching to nitrogen.

[0091] 0.190 g of activated catalysts were transferred to a 5 liter isobutane suspension polymerization reactor and tested under copolymerization conditions (15 cm of hexene). The total reactor pressure was 40.1 atm and ethylene partial pressure was 15.9 atm. The reactor was maintained at 100°C. 484 g of copolymer were produced and the activity was calculated at 2215 g/g/h. The copolymer has a HLMI of 8.0 g/10 min and a density of 0.9508 g/cm3.

Example 8 (comparative - aluminum and chromium nitrates used - copolymerization - activation at 600°C)

[0092] A methanolic solution of chromium and aluminum nitrate was prepared by adding 7.54 g of Cr(NO3).9H2O and 25.71 g of Al(NO3)3.9H2O to a beaker containing a magnetic stirrer. Approximately 143 cm3 of methanol was added and the composition stirred for 1 hour so that all of the chromium and aluminum salts were dissolved. The chromium/aluminum solution was added to 100 g of silica having a surface area of 377 m2/g and pore volume of 1.75 cm3/g, both parameters determined using an ASAP 2420 instrument.

[0093] Excess methanol was removed from the catalyst precursor by drying in a vacuum oven at 140°C for 3 hours. The resulting blue/green catalyst precursor had chromium and aluminum contents of 1.06% and 1.96% by weight of the respective elements. The surface area and pore volume determined for the precursor using ASAP2420 were 362 m2/g and 1.64 cm3/g respectively.

[0094] Particle size distribution was determined by Malvern Mastersizer™ and indicated d10 of 73.8 μm, d50 of 125.2 μm and d90 of 180.4 μm.

[0095] 10 g of this catalyst was activated in a fluidized bed reactor using dry air as the fluidizing gas. The temperature was maintained at 600oC for 5 hours before cooling and switching to nitrogen.

[0096] 0.185 g of activated catalyst was transferred to a 5 liter isobutane suspension polymerization reactor and tested under copolymerization conditions (using 15 cm3 of hexene). The total reactor pressure was 40.1 atm and ethylene partial pressure was 16.0 atm. The reactor was maintained at 100°C. 485 g of copolymer was produced and the activity calculated at 2666 g/g/h. The copolymer had a HLMI of 6.4 g/10 min and a density of 0.9508 g/cm3.

Example 9 (comparative - heptane organometals)

[0097] 100 g of a silica xerogel support (pure silica) was prepared by freeze drying a hydrogel. The support had a surface area and pore volume by nitrogen porosimetry, measured as detailed above using a Micromeritics ASAP 2420 analyzer, of 321 m2/g and 2.32 cm3/g.

[0098] The support particles had the following particle diameter distribution as measured using Malvern Mastersizer™: d90 of 182.8 μm, d50 of 76.5 μm, and d10 of 12.9 μm. 25 g of the silica xerogel support (pre-dried at 140°C) was added to a dry reaction vial equipped with a stirrer. 150 cm of heptane was added to the vial to form a silica/heptane suspension. 1.82 g of chromium acetylacetonate was added to another clean, dry vial containing a magnetic stirrer. 20 cm3 of heptane was added to chromium acetylacetonate and 17.3 cm3 of a 1.0 molar solution of triisobutyl aluminum in heptane was added to the stirred chromium acetylacetonate suspension. This reaction mixture was allowed to cool. The Cr/Al complex solution was then transferred to the flask containing silica and heptane. The resulting suspension was stirred for an additional hour and then 1.7 cm3 of methanol was added to the suspension. Stirring was stopped and the catalyst precursor particles were allowed to settle. The suspension was filtered to remove excess heptane and the residual filter cake, which was wetted with solvent, was dried in a vacuum oven at 140 °C to give a free-flowing green powder. The resulting catalyst precursor contained 1.05% chromium and 1.73% aluminum expressed as the respective elements. The surface area and pore volume were determined for the catalyst precursor using the ASAP 2420 instrument and were 301 m2/g and 2.21 cm3/g, respectively.

Example 10 (according to the invention)

[0099] A methanolic solution of chromium acetate and aluminum acetate was prepared by adding 1.07 g of chromium acetate and 2.85 g of basic aluminum acetate stabilized with boric acid to a beaker containing a magnetic stirrer.

[00100] Approximately 74 cm3 of methanol was added and the composition was stirred for 1 hour so that all the chromium and aluminum salts were dissolved. This solution of chromium acetate, aluminum acetate and boric acid was added to 30 g of the silica xerogel support, as used in example 9, so that the chromium, aluminum and boron species were impregnated into the support. Methanol was removed from the catalyst precursor thus formed by drying in a vacuum oven at 140°C for 3 hours. The resulting blue/green catalyst precursor powder had chromium and aluminum contents of 0.96% and 1.76% by weight, expressed as the respective element. Boron content was determined to be 1380 ppm by weight (0.138%). The surface area and pore volume determined using Micromeritics ASAP 2420 were 306 m2/g and 2.13 cm3/g, respectively.

[00101] It should be noted that all examples 1 to 3 relate to homopolymerization equivalent to 700°C with catalysts having similar particle sizes, pore volumes, surface areas and similar loadings of Cr and Al. Example 1 was a catalyst prepared according to the prior art. Example 2, using a catalyst prepared according to the invention, gave homopolymer density and melt indices that were similar to those resulting from the use of the prior art catalyst. Example 3, in which the catalyst was prepared by deposition of Cr from aqueous solution followed by deposition of Al by an organometallic route, gave a homopolymer where the melt indices were considerably lower than for examples 1 or 2, demonstrating the different nature of the catalyst prepared using this route. Activation of the catalyst for example 1 also led to the formation of noxious fumes.

[00102] Examples 4 to 8 all related to copolymerization at 600°C. Again, the particle sizes, surface areas and pore volumes for the catalysts were substantially equal, as were the Cr and Al loadings, except for example 5 where only Cr was present (no Al). Example 7 (comparative) corresponded to the use of a catalyst prepared according to the prior art using an organometallic route. It gave a copolymer of HLMI value 8.0. Examples 4 and 6 used catalysts prepared by embodiments of the method of the invention, with Cr/Al deposited from aqueous, methanol and aqueous solutions, respectively. These gave HLMI values of 10.8 and 8.6 respectively, showing that catalysts prepared according to the invention can match or exceed the HLMI values for prior art catalysts. Example 5 (comparative), without aluminum, did not give the required HLMI value of 8.0 or more. Neither did Example 8 (comparative), where the catalyst was prepared using deposition of chromium and aluminum nitrates from methanolic solution, give an adequate HLMI (the value was only 6.4). Furthermore, activation of the catalysts for Examples 7 and 8 led to the formation of noxious vapors.

[00103] Examples 9 and 10 relate to an embodiment of the method of the second aspect of the invention for forming catalyst precursors on high porosity inorganic carriers which have been made by a freeze-drying route. Although a small reduction in porosity is noted arising from the application of the method, it is apparent that the method is still applicable to such high porosity carriers without carrier collapse.

[00104] Thus, it can be seen that activated catalysts prepared using precursors according to the invention give the same or higher melting indices for polymers as prior art catalysts prepared from precursors made by a conventional organometallic route. Also, the catalysts have similar activities but do not result in the formation of harmful vapors during catalyst activation from the catalyst precursors of the invention. Furthermore, the method of the invention does not lead to a substantial loss in porosity for the resulting catalyst precursor compared to prior art methods for precursor formation.

[00105] It should be appreciated that modifications to the examples may be made without departing from the scope of the invention as defined in the appended claims. For example, the xerogel support may be prepared by grinding a hydrogel to form an aqueous suspension, followed by spray drying rather than by direct evaporative drying of a hydrogel.

Claims (8)

1. A catalyst precursor for an olefin polymerization catalyst comprising a silica xerogel comprising at least 90 wt.% SiO2 and carrying within a pore structure of the silica xerogel a salt of chromium, aluminum carboxylate and boric acid, wherein the chromium salt is chromium carboxylate. 2. Catalyst precursor according to claim 1, characterized in that the chromium salt is chromium acetate. 3. Catalyst precursor according to any one of the preceding claims, characterized in that the aluminum carboxylate is aluminum acetate. 4. Catalyst precursor according to any one of the preceding claims, characterized in that the silica xerogel is in the form of particles having a weight average particle diameter of 1 to 300 micrometers. 5. Catalyst precursor according to any one of the preceding claims, characterized in that the catalyst precursor comprises, as charged material, from 0.01 to 3% by weight of chromium, expressed as the element, and from 0.1 to 8% by weight of aluminum, expressed as the element, and from 0.005 to 2% by weight of boron, expressed as the element. 6. A method for preparing a catalyst precursor for an olefin polymerization catalyst as defined in any one of claims 1 to 5, comprising: i) providing an inorganic support material, which is a silica xerogel comprising at least 90 wt. % SiO2 having a porosity of 0.5 to 4.0 cm3/g, ii) providing a solution comprising a chromium salt, aluminum carboxylate and boric acid in a solvent that is water, C1 to C4 aliphatic alcohol, or a mixture thereof, wherein the chromium salt is chromium carboxylate, iii) depositing the solution onto the silica xerogel, and iv) removing the solvent to form the catalyst precursor comprising chromium salt, aluminum carboxylate and boric acid within the pore structure of the silica xerogel. 7. Method according to claim 6, characterized in that the solvent is removed by evaporation. 8. The method of claim 6, wherein the inorganic support material is a silica xerogel prepared by evaporating water from a silica hydrogel.