WO2022214420A1 - Chromium based catalyst for ethylene polymerization - Google Patents

Chromium based catalyst for ethylene polymerization Download PDF

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WO2022214420A1
WO2022214420A1 PCT/EP2022/058845 EP2022058845W WO2022214420A1 WO 2022214420 A1 WO2022214420 A1 WO 2022214420A1 EP 2022058845 W EP2022058845 W EP 2022058845W WO 2022214420 A1 WO2022214420 A1 WO 2022214420A1
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compound
titanium
chromium
catalyst system
solid catalyst
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PCT/EP2022/058845
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French (fr)
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Orass Hamed
Ibrahim Abdulaziz AL-GHAMDI
Vugar Aliyev
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Sabic Global Technologies B.V.
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    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F210/00—Copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond
    • C08F210/16—Copolymers of ethene with alpha-alkenes, e.g. EP rubbers

Definitions

  • This invention relates to a supported chromium based catalyst system for the production of polyethylene.
  • the invention further relates to a process for the production of polyethylene using such catalyst system and polyethylene obtained thereby.
  • the invention further relates to articles made from such polyethylene.
  • the production processes of LDPE, HDPE and LLDPE are summarised in “Handbook of Polyethylene” by Andrew Peacock (2000; Dekker; ISBN 0824795466) at pages 43- 66.
  • the catalysts can be divided in three different subclasses including Ziegler Natta catalysts, Phillips catalysts and single site catalysts.
  • the various processes may be divided into solution polymerisation processes employing homogeneous (soluble) catalysts and processes employing supported (heterogeneous) catalysts.
  • the latter processes include both slurry and gas phase processes.
  • the chromium oxide based catalyst which is commonly referred to in the literature as “the Phillips catalyst”, can be obtained by activating a chromium compound carried on an inorganic oxide carrier in a non-reducing atmosphere (mainly dry air required).
  • a non-reducing atmosphere mainly dry air required.
  • EP3715385 discloses a solid catalyst system comprising a first chromium compound, a second chromium compound, a reaction product of an alkyl aluminium compound and a nitrogen containing compound and a silicon oxide support.
  • WO2020/152275A1 discloses a solid catalyst system comprising a chromium compound, an aluminium alkoxide compound, a nitrogen containing compound and a silicon oxide support. The aluminium alkoxide compound and the nitrogen containing compound are not reacted before being mixed with the chromium compound and the silicon oxide support.
  • One of the disadvantages of known chromium oxide based catalysts is that their use in a gas phase reactor process does not lead to a polyethylene having a high resin bulk density.
  • the present invention provides a solid catalyst system comprising a chromium compound, a silicon oxide support and a compound represented by the general formula (R 4 ) 2 -N-R 3 (I) wherein
  • R 4 is represented by R 1 -AI-OR 2 wherein R 1 is selected from C1-C8 alkyl groups and OR 2 is selected from C1-C8 alkoxyl groups and R 3 is a C3-C8 linear, branched or cyclic alkyl group.
  • polyethylene produced using the solid catalyst system according to the invention has a high resin bulk density. Further, polyethylene produced using the solid catalyst system according to the invention has a high Mw and MWD.
  • solid catalyst system according to the present invention allows an easy and simplified scaling up process of its production.
  • the chromium compound may be chromium trioxide (i.e. Cr0 3 ) or any compound convertible to chromium oxide.
  • Cr0 3 chromium trioxide
  • any compound convertible to chromium oxide see U.S. Pat. Nos. 2,825,721 : 3,023.203; 3,622,251 and 4,011 ,382.
  • Suitable compounds convertible to chromium oxide include for example, chromium acetyl acetone, chromium chloride, chromium nitrate, chromium acetate, chromium acetate hydroxide, chromium sulfate, ammonium chromate, ammonium dichromate, and other soluble chromium containing salts.
  • the amount of chromium compound added to the silicon oxide support should be sufficient to obtain between 0.01% and 10%, preferably from 0.1% to 3%, by weight of chromium, calculated as metallic chromium, based on the weight of the silicon oxide support.
  • the catalyst system comprises a compound represented by (R 4 ) 2 -N-R 3 (I)
  • R 4 is represented by R 1 -AI-OR 2 wherein R 1 is selected from C1-C8 alkyl groups and OR 2 is selected from C1-C8 alkoxyl groups.
  • R 3 is a C3-C8 linear, branched or cyclic alkyl group, preferably a C3-C8 cyclic alkyl group.
  • This may be a reaction product of an alkyl aluminium compound and a nitrogen containing compound. This means that the reaction must have taken place before the alkyl aluminium compound and the nitrogen containing compound come into contact with the other elements of the solid catalyst system.
  • reaction process of the alkyl aluminium compound and the nitrogen containing compound without the presence of other components such as the chromium compound and the silicon oxide support, making it easier to provide the desired reaction product.
  • the reaction process of the alkyl aluminium compound and the nitrogen containing compound can be optimized in the absence of other components. Thus, easy and simplified scaling-up can be achieved.
  • the aluminium alkoxide compound has the formula (RVAI-OR 2 wherein R 1 is selected from C1-C8 alkyl groups and OR 2 is selected from C1-C8 alkoxyl groups.
  • suitable aluminum alkoxide compounds include diethyl aluminium ethoxide, dihexyl aluminium ethoxide, dioctyl aluminium ethoxide and dihexyl aluminium propoxide.
  • the aluminium alkoxide compound is diethyl aluminium ethoxide.
  • the molar ratio of Al to Cr in the solid catalyst system is 1.0 to 10.0, for example 2.0 to 8.0 or 3.0 to 6.0.
  • Nitrogen containing compound is a cycloalkylamine having the formula R 3 -NH , wherein R 3 is selected from C3-C8 cycloalkyl groups, preferably a C3-C8 cyclic alkyl group.
  • the nitrogen containing compound is selected from the group consisting of cyclopropylamine, cyclobutylamine, cyclopentylamine, cyclohexylamine, cycloheptylamine and cyclooctylamine, more preferably selected from cyclohexylamine and cyclooctylamine, most preferably is cyclohexylamine.
  • the molar ratio of N to Cr in the solid catalyst system is 0.5 to 5.0, more preferably 0.80 to 3.0 or 0.90 to 2.0, more preferably at least 1.10, at least 1.20, at least 1.30, or at least 1.55. This was found to result in a higher MW of the polyethylene produced using the catalyst system.
  • the molar ratio of N to Al in the solid catalyst system is 0.10 to 1.0, for example 0.20 to 0.50, preferably at least 0.27, at least 0.305, at least 0.35 or at least 0.40. This was found to result in a higher MW of the polyethylene produced using the catalyst system.
  • the molar ratio of Al to Cr in the solid catalyst system is 3.0 to 6.0 and the molar ratio of N to Cr in the solid catalyst system is 0.90 to 2.0.
  • Silicon oxide support A silica support that is suitable for use in the present invention has a relatively high surface area and is amorphous.
  • the silicon oxide support has an average particle diameter of 20 to 50 pm, more preferably 25 to 39 pm.
  • the average particle diameter is determined via ASTM D- 1921 12.
  • the silicon oxide support has a pore volume of 1.2 to 3.0 m 3 /kg, more preferably 1.65 to 1.95 m 3 /kg.
  • the pore volume is determined by ASTM D4284-12 (2012) “Standard Test Method for Determining Pore Volume Distribution of Catalysts and Catalyst Carriers by Mercury Intrusion Porosimetry”.
  • the silicon oxide support has a surface area of 200 to 800 m 2 /g, more preferably 300 to 700 m 2 /g.
  • the surface area of the support is determined by the BET nitrogen adsorption method. Test Method: ASTM D 1993-03 (2013) Standard Test Method for Precipitated Silica-Surface Area by Multipoint BET Nitrogen Adsorption.
  • the silicon oxide support has a pore radius of 120 to 200 Angstrom.
  • the pore radius is determined by ASTM D4284-12 (2012) “Standard Test Method for Determining Pore Volume Distribution of Catalysts and Catalyst Carriers by Mercury Intrusion Porosimetry”.
  • the catalyst system further comprises a non-chromium metal compound, i.e. a metal compound which contains a metal which is not chromium.
  • a non-chromium metal compound i.e. a metal compound which contains a metal which is not chromium.
  • This nonchromium metal compound acts as a modifier and is used for the synthesis of the solid catalyst component according to the invention.
  • the non-chromium compound is a metal halide transition metal compound and is selected from compounds represented by formulas Tm(OR 4 ) n X4- n and Tm(R 5 )nX 4 - n, wherein Tm represents a transition metal of Group IVB, VB, orVIB,
  • R 4 and R 5 is independently selected from C1-C20 alkyl groups, C1-C20 aryl groups and C1-C20 cycloalkyl groups,
  • X represents a halogen atom, preferably chlorine and n represents a number satisfying 0 ⁇ n ⁇ 4, preferably 1 ⁇ n ⁇ 4.
  • the metal in the non-chromium metal compound, Tm is selected from titanium, vanadium, hafnium and zirconium, and is most preferably titanium.
  • titanium alkoxy compounds for example tetraethoxy titanium, tetramethoxy titanium, tetrabutoxy titanium, tetrapropoxy titanium (in particular tetraisopropoxy titanium), tetraisobutoxy titanium, tetrapentoxy titanium, triethoxychloro titanium, diethoxydichloro titanium , trichloethoxy titanium, methoxy titanium trichloride, dimethoxy titanium dichloride, ethoxy titanium trichloride, diethoxy titanium dichloride, propoxy titanium trichloride, dipropoxy titanium dichloride, butoxy titanium trichloride, butoxy titanium dichloride and titanium tetrachloride.
  • titanium alkoxy compounds for example tetraethoxy titanium, tetramethoxy titanium, tetrabutoxy titanium, tetrapropoxy titanium (in particular tetraisopropoxy titanium), tetraisobutoxy titanium, tetrapentoxy titanium, tri
  • non-chromium metal compounds include for example vanadium trichloride, vanadium tetrachloride, vanadium oxytrichloride and zirconium tetrachloride.
  • the amount of the metal in the non-chromium metal compound in the solid catalyst system is between 0.1 and 10.0 % by weight, preferably in the range between 0.1 and 6.0 % by weight.
  • the weight ratio between the metal in the non-chromium metal compound in the solid catalyst system and Cr, in particular Ti:Cr is 2 to 4.
  • Process for preparation of catalyst system comprising i) providing the chromium compound and the optional non-chromium metal compound on the silica support, ii) providing the compound (I) and iii) mixing the product of step i) and the product of step ii).
  • Step iii) may be performed by mixing a solution of the product of step i) and a solution of the product of step ii).
  • the solvents of these solutions may be selected by the skilled person.
  • step ii) involves reacting the alkyl aluminium compound and the nitrogen containing compound before the alkyl aluminium compound and the nitrogen containing compound come into contact with the chromium compound and the silicon oxide support.
  • step ii) involves reacting the alkyl aluminium compound and the nitrogen containing compound in the absence of the product of step i) (the product provided by step i)).
  • the catalyst system may require activation prior to use.
  • Activation by calcination can be accomplished by heating the solid catalyst system in steam, dry air or another oxygen containing gas at temperatures up to the sintering temperature of the support.
  • Activation temperatures are typically in the range of 300° C. to 950° C., preferably from 500° C to 900° C and activation times are typically from about 10 min. to as about 72 hrs.
  • the solid catalyst system may optionally be reduced after activation using for example, carbon monoxide or a mixture of carbon monoxide and nitrogen.
  • the present invention further relates to a process for the production of polyethylene by polymerisation of ethylene and an optional comonomer in the presence of the catalyst according to the invention.
  • the polyethylene is a high density polyethylene having a density determined according to IS01183 of 945 to 970 kg/m 3 , preferably 950 to 965 kg/m 3 , more preferably 958 to 963 kg/m 3 .
  • the polyethylene may be an ethylene homopolymer or an ethylene copolymer of ethylene and a comonomer selected from the group consisting of propylene, 1 -butene, 1-pentene, 4-methyl- 1-pentene, 1 -hexene and 1-octene, preferably 1 -hexene.
  • the polyethylene has a high-load melt index (HLMI) according to ASTM D- 1238 10 Condition F at 190°C and 21.6 kg of 1.0 to 30 dg/min, more preferably 2.0 to 15 dg/min.
  • HLMI high-load melt index
  • the solid catalyst system of the present invention is added to a polymerization zone using a dry catalyst feeder.
  • the operation is often carried out under a nitrogen atmosphere and the dry catalyst is transferred to the reactor under positive nitrogen pressure.
  • a dry catalyst feeder for example, in U.S. Pat. No. 6,319,995 and U.S. Pat. No. 8,431 ,658.
  • the process may be a slurry phase or a gas phase polymerization process, preferably a gas phase polymerization process.
  • the gas phase polymerization process may use any gas phase reactor for gas phase polymerizations and may e.g. be vertically, horizontally mechanically agitated reactor or a fluidized bed reactor.
  • a fluidized bed gas phase polymerization reactor employs a "bed" of polymer and catalyst which is fluidized by a flow of monomer, comonomer and other optional components which are at least partially gaseous. Heat is generated by the enthalpy of polymerization of the monomers flowing through the bed. Unreacted monomers and other optional gaseous components exit the fluidized bed and are contacted with a cooling system to remove this heat. The cooled gas stream, including monomer, comonomer and optional for example condensable liquids, is then re circulated through the polymerization zone. Simultaneously, polymer product is withdrawn from the reactor.
  • the reactor temperature in a gas phase process may range between for example 30°C and 130°C.
  • Suitable fluidized bed reactors include for example a bubbling fluidized bed reactor, a circulating fluidized bed reactor, an annular fluidized bed reactor, a multi-zone fluidized bed reactor and a flash reactor.
  • fluidized bed is meant that an amount of solid particles (in this case preferably the solid catalyst and/or the solid catalyst to which the monomer is attached) in a solid/fluid mixture acts as a fluid. This can be achieved by placing the amount of solid particles under appropriate conditions, for instance by the introduction of fluid through the solid particles at a high enough velocity to suspend the solid particles and causing them to behave as a fluid.
  • the present invention further relates to the polyethylene obtainable by or obtained by the process according to the invention.
  • the polyethylene according to the present invention has a resin bulk density according to ASTM D-1895 of at least 350 kg/m 3 , more preferably at least 400 kg/m 3 .
  • the polyethylene according to the present invention has Mw according to ASTM D-6474 12 of at least 450,000, more preferably at least 500,000.
  • the polyethylene according to the present invention has MWD according to ASTM D-6474 12 of at least 30, more preferably at least 35, more preferably at least 40.
  • the present invention further relates to a composition
  • a composition comprising the high density polyethylene obtainable by or obtained by the process according to the invention.
  • the composition may further comprise additives for example lubricants, fillers, stabilisers, antioxidants, compatibilizers and pigments.
  • the additives used to stabilize the polymers may be, for example, additive packages including hindered phenols, phosphites, UV stabilisers, antistatics and stearates.
  • the invention also relates to a molded article comprising the high density polyethylene or the composition comprising the high density polyethylene according to the present invention.
  • the molded article may preferably be a blow molded article or an extrusion molded article.
  • the molded article may preferably be selected from films, pipes, bottles, IBC containers, tight head and open head drums and fuel tanks.
  • the term ‘comprising’ does not exclude the presence of other elements.
  • a description on a product/composition comprising certain components also discloses a product/composition consisting of these components.
  • the product/composition consisting of these components may be advantageous in that it offers a simpler, more economical process for the preparation of the product/composition.
  • a description on a process comprising certain steps also discloses a process consisting of these steps. The process consisting of these steps may be advantageous in that it offers a simpler, more economical process.
  • the high load melt index was determined using the procedures of ASTM D- 1238 10 Condition F using a load of 21.6 kg at a temperature of 190°C.
  • Density was measured according to ASTM D-792 08.
  • Mz and Mz+1 are higher average molecular weights (according to ASTM D-6474 12)
  • Mw weight-average molecular weight (according to ASTM D-6474 12)
  • Mn number-average molecular weight (according to ASTM D-6474 12)
  • MWD molecular weight distribution
  • Mw weight-average molecular weight
  • Mn number-average molecular weight
  • silica support 955W surface area 310 m 2 /g, pore volume 1.5 ml/g and average particle diameter 49 pm
  • chromium acetate hydroxide was then added to the silica and then slurried in 250 cm 3 of methanol (100%), which was stirred at 80°C for 30 minutes. Subsequently, drying of the methanol solvent took place at 95°C with nitrogen purge.
  • the dried chromium on silica powder was cooled down to room temperature then slurried with 250cm 3 of iso-pentane, followed by the addition of 20 cm 3 of tetraethoxy titanium Ti(OC H 5 )4 (100%). The contents were mixed at 65°C for another 10 minutes and then the solvent was dried at 95°C with nitrogen purge. Dried Chromium-Titanium on silica powder was obtained.
  • Chromium-Titanium on silica powder was cooled down to room temperature then slurried with 500cm 3 of Iso-pentane, followed by the addition of 78 cm 3 of 1M diethyl alumiumium ethoxide (0 2 H 5 ) 2 AI-00 2 H 5 solution. The contents were mixed at 40°C for another 30 minutes then drying the iso-pentane solvent at 65°C with Nitrogen purge.
  • the dried chromium on silica powder was cooled down to room temperature then slurried with 250cm 3 of iso-pentane, followed by the addition of 20 cm 3 of tetraethoxy titanium Ti(OC H (100%). The contents were mixed at 65°C for another 10 minutes and then the solvent was dried at 95°C with nitrogen purge. Dried Chromium-Titanium on silica powder was obtained.
  • Chromium-Titanium on silica powder was cooled down to room temperature then slurried with 250 cm 3 of Iso-pentane, followed by the addition of 67 cm 3 of 1M solution of di(ethyl ethoxy aluminium) cyclohexylamine (C H -AI-OC H ) C 6 HIIN.
  • the contents were mixed at 40°C for another 20 minutes then drying the iso-pentane solvent at 65°C with Nitrogen purge.
  • the obtained catalyst particles had a high flowability believed to be due to the presence of the amine compound.
  • Such catalyst flow-ability is required for the solid powder catalyst feeders used in the Fluidized Bed Gas Phase Reactors.
  • Comparative experiment 1 ethylene-hexene copolvmerization using catalyst A An autoclave with a volume of 2 liters was purged with nitrogen at 130°C for 30 minutes. After cooling the autoclave to 70°C, one liter of iso-pentane and 20 mL of 1- hexene were introduced to the reactor. Subsequently the reactor was pressurized with 15 bar ethylene. Subsequently 0.1 mmol of triethylaluminum was injected into the reactor by the means of a catalyst injection pump.
  • Example 2 ethylene-hexene copolvmerization using catalyst B (inventive ' ) Comparative experiment 1 was repeated except that catalyst B was used instaed of catalyst A. The conditions were identical except that the reactor tempeature was raised to 101 °C instead of 100 °C.
  • Catalysts C, D, E and F were prepared by following the procedure of the preparation of catalyst B except the ratio between the dried chromium-titanium on silica powder and di(ethyl ethoxy aluminium) cyclohexylamine was varied as shown in Table 1.
  • Example 2 was repeated except that catalyst C, D, E and F were used instead of catalyst B.
  • Catalyst G was prepared by following the procedure of the preparation of catalyst A except the ratio between the dried chromium-titanium on silica powder and di(ethyl ethoxy aluminium) cyclohexylamine was varied as shown in Table 2.
  • Catalysts H, I, J and K were prepared by following the procedure of the preparation of catalyst B except the ratio between the dried chromium-titanium on silica powder and di(ethyl ethoxy aluminium) cyclohexylamine was varied as shown in Table 2.
  • Example 7 (comparative ' ) and Examples 8, 9, 10 and 11 (inventive ' )
  • Example 2 was repeated except that catalyst H, I, J and K were used instead of catalyst B.
  • the ethylene copolymer according to the invention has a higher MW and MWD than the ethylene copolymer of the comparative experiment.
  • Figures 1-4 shows graphs obtained by GPC analysis of the ethylene copolymer of Examples 8-11.

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Abstract

The invention relates to a solid catalyst system comprising a chromium compound, a silicon oxide support and a compound represented by the general formula (R4)2-N-R3 (I), wherein R4 is represented by R1-Al-OR2 wherein R1 is selected from C1-C8 alkyl groups and OR2 is selected from C1-C8 alkoxyl groups and R3 is a C3-C8 linear, branched or cyclic alkyl group.

Description

Chromium based catalyst for ethylene polymerization
This invention relates to a supported chromium based catalyst system for the production of polyethylene. The invention further relates to a process for the production of polyethylene using such catalyst system and polyethylene obtained thereby. The invention further relates to articles made from such polyethylene.
The production processes of LDPE, HDPE and LLDPE are summarised in “Handbook of Polyethylene” by Andrew Peacock (2000; Dekker; ISBN 0824795466) at pages 43- 66. The catalysts can be divided in three different subclasses including Ziegler Natta catalysts, Phillips catalysts and single site catalysts. The various processes may be divided into solution polymerisation processes employing homogeneous (soluble) catalysts and processes employing supported (heterogeneous) catalysts. The latter processes include both slurry and gas phase processes.
Use of supported chromium oxide-based catalyst systems for producing polyethylene is well-known. The polymerisation of ethylene using supported chromium based catalysts was first formulated by Hogan & Banks in 1952, then further explored by many workers including Kevin Cann “Comparison of silyl chromate and chromium oxide based olefin polymerisation catalysts” (Macromolecular Symp, 2004, 213, 29-36) and in US7504463.
The chromium oxide based catalyst, which is commonly referred to in the literature as “the Phillips catalyst”, can be obtained by activating a chromium compound carried on an inorganic oxide carrier in a non-reducing atmosphere (mainly dry air required). A number of modifications has been done to the chromium oxide catalyst system by the addition of different types of modifiers to enhance the molecular weight and molecular weight distribution, like the addition of Titanium and Aluminium compounds to the Chromium oxide systems.
Further Pullukat et al. (Journal of Polymer Science; Polymer chemistry Edition; vol18, 2857-2866; 1980) discloses thermally activated ethylene polymerisation catalysts which contain chromium and titanium on silica. EP3715385 discloses a solid catalyst system comprising a first chromium compound, a second chromium compound, a reaction product of an alkyl aluminium compound and a nitrogen containing compound and a silicon oxide support.
WO2020/152275A1 discloses a solid catalyst system comprising a chromium compound, an aluminium alkoxide compound, a nitrogen containing compound and a silicon oxide support. The aluminium alkoxide compound and the nitrogen containing compound are not reacted before being mixed with the chromium compound and the silicon oxide support.
One of the disadvantages of known chromium oxide based catalysts is that their use in a gas phase reactor process does not lead to a polyethylene having a high resin bulk density.
Production of resin fluff with a low resin bulk density has a negative impact on the Gas Phase Reactor’s Bed weight, causing a reduction in the Drop size of the reactor and hence lowering the production rate for the same number of reactor drops. Also, reduced resin bulk density have a negative impact on the Upper and Lower Fluidized Bulk Density inside the reactor, which forces operation to reduce the Superficial Gas Velocity (SGV) of the Reactor to avoid resin carry over, which causes Distributor Plate and Cooler fouling. It is also well known that running at reduced Superficial Gas Velocity (SGV) reduces the Momentum flux inside the reactor’s bed, causing poor conversion, hence reducing the production rate. Further, running at low SGV reduces the capacity of heat removal.
Accordingly, it is an objective of the present invention to provide a solid catalyst system which results in the production of polyethylene with a high resin bulk density.
Accordingly, the present invention provides a solid catalyst system comprising a chromium compound, a silicon oxide support and a compound represented by the general formula (R4)2-N-R3 (I) wherein
R4 is represented by R1-AI-OR2 wherein R1 is selected from C1-C8 alkyl groups and OR2 is selected from C1-C8 alkoxyl groups and R3 is a C3-C8 linear, branched or cyclic alkyl group.
It was surprisingly found that polyethylene produced using the solid catalyst system according to the invention has a high resin bulk density. Further, polyethylene produced using the solid catalyst system according to the invention has a high Mw and MWD.
Further, the solid catalyst system according to the present invention allows an easy and simplified scaling up process of its production.
Figure imgf000004_0001
The chromium compound may be chromium trioxide (i.e. Cr03) or any compound convertible to chromium oxide. For compounds convertible to chromium oxide see U.S. Pat. Nos. 2,825,721 : 3,023.203; 3,622,251 and 4,011 ,382.
Suitable compounds convertible to chromium oxide include for example, chromium acetyl acetone, chromium chloride, chromium nitrate, chromium acetate, chromium acetate hydroxide, chromium sulfate, ammonium chromate, ammonium dichromate, and other soluble chromium containing salts.
The amount of chromium compound added to the silicon oxide support should be sufficient to obtain between 0.01% and 10%, preferably from 0.1% to 3%, by weight of chromium, calculated as metallic chromium, based on the weight of the silicon oxide support.
Figure imgf000004_0002
According to the invention, the catalyst system comprises a compound represented by (R4)2-N-R3 (I)
R4 is represented by R1-AI-OR2 wherein R1 is selected from C1-C8 alkyl groups and OR2 is selected from C1-C8 alkoxyl groups.
R3 is a C3-C8 linear, branched or cyclic alkyl group, preferably a C3-C8 cyclic alkyl group. This may be a reaction product of an alkyl aluminium compound and a nitrogen containing compound. This means that the reaction must have taken place before the alkyl aluminium compound and the nitrogen containing compound come into contact with the other elements of the solid catalyst system.
This allows controlling the reaction process of the alkyl aluminium compound and the nitrogen containing compound without the presence of other components such as the chromium compound and the silicon oxide support, making it easier to provide the desired reaction product. The reaction process of the alkyl aluminium compound and the nitrogen containing compound can be optimized in the absence of other components. Thus, easy and simplified scaling-up can be achieved.
Figure imgf000005_0001
The aluminium alkoxide compound has the formula (RVAI-OR2 wherein R1 is selected from C1-C8 alkyl groups and OR2 is selected from C1-C8 alkoxyl groups.
Examples of suitable aluminum alkoxide compounds include diethyl aluminium ethoxide, dihexyl aluminium ethoxide, dioctyl aluminium ethoxide and dihexyl aluminium propoxide.
Most preferably, the aluminium alkoxide compound is diethyl aluminium ethoxide.
Preferably, the molar ratio of Al to Cr in the solid catalyst system is 1.0 to 10.0, for example 2.0 to 8.0 or 3.0 to 6.0.
Nitrogen containing compound The nitrogen containing compound is a cycloalkylamine having the formula R3-NH , wherein R3 is selected from C3-C8 cycloalkyl groups, preferably a C3-C8 cyclic alkyl group.
More preferably, the nitrogen containing compound is selected from the group consisting of cyclopropylamine, cyclobutylamine, cyclopentylamine, cyclohexylamine, cycloheptylamine and cyclooctylamine, more preferably selected from cyclohexylamine and cyclooctylamine, most preferably is cyclohexylamine. Preferably, the molar ratio of N to Cr in the solid catalyst system is 0.5 to 5.0, more preferably 0.80 to 3.0 or 0.90 to 2.0, more preferably at least 1.10, at least 1.20, at least 1.30, or at least 1.55. This was found to result in a higher MW of the polyethylene produced using the catalyst system.
Preferably, the molar ratio of N to Al in the solid catalyst system is 0.10 to 1.0, for example 0.20 to 0.50, preferably at least 0.27, at least 0.305, at least 0.35 or at least 0.40. This was found to result in a higher MW of the polyethylene produced using the catalyst system.
In particularly preferred embodiments, the molar ratio of Al to Cr in the solid catalyst system is 3.0 to 6.0 and the molar ratio of N to Cr in the solid catalyst system is 0.90 to 2.0.
Silicon oxide support A silica support that is suitable for use in the present invention has a relatively high surface area and is amorphous.
Preferably, the silicon oxide support has an average particle diameter of 20 to 50 pm, more preferably 25 to 39 pm. The average particle diameter is determined via ASTM D- 1921 12.
Preferably, the silicon oxide support has a pore volume of 1.2 to 3.0 m3/kg, more preferably 1.65 to 1.95 m3/kg. The pore volume is determined by ASTM D4284-12 (2012) “Standard Test Method for Determining Pore Volume Distribution of Catalysts and Catalyst Carriers by Mercury Intrusion Porosimetry”.
Preferably, the silicon oxide support has a surface area of 200 to 800 m2/g, more preferably 300 to 700 m2/g. The surface area of the support is determined by the BET nitrogen adsorption method. Test Method: ASTM D 1993-03 (2013) Standard Test Method for Precipitated Silica-Surface Area by Multipoint BET Nitrogen Adsorption.
See also references “Adsorption, Surface Area and Porosity” by S.J. Gregg and K.S.W. Sing, Academic Press, London (1982) and “Introduction to Powder Surface Area” by S. Lowell, J. Wiley & Sons, New York, NY, (1979).
Preferably, the silicon oxide support has a pore radius of 120 to 200 Angstrom. The pore radius is determined by ASTM D4284-12 (2012) “Standard Test Method for Determining Pore Volume Distribution of Catalysts and Catalyst Carriers by Mercury Intrusion Porosimetry”.
Figure imgf000007_0001
Preferably, the catalyst system further comprises a non-chromium metal compound, i.e. a metal compound which contains a metal which is not chromium. This nonchromium metal compound acts as a modifier and is used for the synthesis of the solid catalyst component according to the invention.
Preferably, the non-chromium compound is a metal halide transition metal compound and is selected from compounds represented by formulas Tm(OR4)nX4-n and Tm(R5)nX4- n, wherein Tm represents a transition metal of Group IVB, VB, orVIB,
R4 and R5 is independently selected from C1-C20 alkyl groups, C1-C20 aryl groups and C1-C20 cycloalkyl groups,
X represents a halogen atom, preferably chlorine and n represents a number satisfying 0 < n < 4, preferably 1< n < 4.
Preferably, the metal in the non-chromium metal compound, Tm, is selected from titanium, vanadium, hafnium and zirconium, and is most preferably titanium.
Examples of suitable titanium compounds include titanium alkoxy compounds for example tetraethoxy titanium, tetramethoxy titanium, tetrabutoxy titanium, tetrapropoxy titanium (in particular tetraisopropoxy titanium), tetraisobutoxy titanium, tetrapentoxy titanium, triethoxychloro titanium, diethoxydichloro titanium , trichloethoxy titanium, methoxy titanium trichloride, dimethoxy titanium dichloride, ethoxy titanium trichloride, diethoxy titanium dichloride, propoxy titanium trichloride, dipropoxy titanium dichloride, butoxy titanium trichloride, butoxy titanium dichloride and titanium tetrachloride.
Other suitable non-chromium metal compounds include for example vanadium trichloride, vanadium tetrachloride, vanadium oxytrichloride and zirconium tetrachloride.
Preferably, the amount of the metal in the non-chromium metal compound in the solid catalyst system, in particular the amount of Ti in the solid catalyst system, is between 0.1 and 10.0 % by weight, preferably in the range between 0.1 and 6.0 % by weight.
Preferably, the weight ratio between the metal in the non-chromium metal compound in the solid catalyst system and Cr, in particular Ti:Cr, is 2 to 4. Process for preparation of catalyst system The invention further relates to a process for the preparation of the catalyst system according to the invention, comprising i) providing the chromium compound and the optional non-chromium metal compound on the silica support, ii) providing the compound (I) and iii) mixing the product of step i) and the product of step ii).
Each of these steps i)-iii) may be suitably performed by the skilled person. Step iii) may be performed by mixing a solution of the product of step i) and a solution of the product of step ii). The solvents of these solutions may be selected by the skilled person.
Preferably, step ii) involves reacting the alkyl aluminium compound and the nitrogen containing compound before the alkyl aluminium compound and the nitrogen containing compound come into contact with the chromium compound and the silicon oxide support.
Preferably, step ii) involves reacting the alkyl aluminium compound and the nitrogen containing compound in the absence of the product of step i) (the product provided by step i)).
Activation
The catalyst system may require activation prior to use. Activation by calcination can be accomplished by heating the solid catalyst system in steam, dry air or another oxygen containing gas at temperatures up to the sintering temperature of the support. Activation temperatures are typically in the range of 300° C. to 950° C., preferably from 500° C to 900° C and activation times are typically from about 10 min. to as about 72 hrs. The solid catalyst system may optionally be reduced after activation using for example, carbon monoxide or a mixture of carbon monoxide and nitrogen.
Process for producing polyethylene The present invention further relates to a process for the production of polyethylene by polymerisation of ethylene and an optional comonomer in the presence of the catalyst according to the invention. Preferably, the polyethylene is a high density polyethylene having a density determined according to IS01183 of 945 to 970 kg/m3, preferably 950 to 965 kg/m3, more preferably 958 to 963 kg/m3.
The polyethylene may be an ethylene homopolymer or an ethylene copolymer of ethylene and a comonomer selected from the group consisting of propylene, 1 -butene, 1-pentene, 4-methyl- 1-pentene, 1 -hexene and 1-octene, preferably 1 -hexene.
Preferably, the polyethylene has a high-load melt index (HLMI) according to ASTM D- 1238 10 Condition F at 190°C and 21.6 kg of 1.0 to 30 dg/min, more preferably 2.0 to 15 dg/min.
In the process according to the invention, the solid catalyst system of the present invention is added to a polymerization zone using a dry catalyst feeder. The operation is often carried out under a nitrogen atmosphere and the dry catalyst is transferred to the reactor under positive nitrogen pressure. For methods of feeding a dry catalyst to a polymerization reactor is described for example, in U.S. Pat. No. 6,319,995 and U.S. Pat. No. 8,431 ,658.
The process may be a slurry phase or a gas phase polymerization process, preferably a gas phase polymerization process.
The gas phase polymerization process may use any gas phase reactor for gas phase polymerizations and may e.g. be vertically, horizontally mechanically agitated reactor or a fluidized bed reactor.
In general, a fluidized bed gas phase polymerization reactor employs a "bed" of polymer and catalyst which is fluidized by a flow of monomer, comonomer and other optional components which are at least partially gaseous. Heat is generated by the enthalpy of polymerization of the monomers flowing through the bed. Unreacted monomers and other optional gaseous components exit the fluidized bed and are contacted with a cooling system to remove this heat. The cooled gas stream, including monomer, comonomer and optional for example condensable liquids, is then re circulated through the polymerization zone. Simultaneously, polymer product is withdrawn from the reactor. The reactor temperature in a gas phase process may range between for example 30°C and 130°C. A description of a gas phase process is disclosed in for example US 4,543,399 and US 4,588,790. Suitable fluidized bed reactors include for example a bubbling fluidized bed reactor, a circulating fluidized bed reactor, an annular fluidized bed reactor, a multi-zone fluidized bed reactor and a flash reactor. With ‘fluidized bed’ is meant that an amount of solid particles (in this case preferably the solid catalyst and/or the solid catalyst to which the monomer is attached) in a solid/fluid mixture acts as a fluid. This can be achieved by placing the amount of solid particles under appropriate conditions, for instance by the introduction of fluid through the solid particles at a high enough velocity to suspend the solid particles and causing them to behave as a fluid. An example of a process using a fluidized bed for producing polyolefins is disclosed in US 4,882,400. Other examples of processes using a fluidized bed for producing polyolefins are described in, for example, US 3,709,853; 4,003,712; 4,011 ,382; 4,302,566; 4,543,399; 4,882,400; 5,352,749; 5,541 ,270; 7,122,607, and 7,300,987.
The present invention further relates to the polyethylene obtainable by or obtained by the process according to the invention.
Preferably, the polyethylene according to the present invention has a resin bulk density according to ASTM D-1895 of at least 350 kg/m3, more preferably at least 400 kg/m3.
Preferably, the polyethylene according to the present invention has Mw according to ASTM D-6474 12 of at least 450,000, more preferably at least 500,000.
Preferably, the polyethylene according to the present invention has MWD according to ASTM D-6474 12 of at least 30, more preferably at least 35, more preferably at least 40.
The present invention further relates to a composition comprising the high density polyethylene obtainable by or obtained by the process according to the invention. The composition may further comprise additives for example lubricants, fillers, stabilisers, antioxidants, compatibilizers and pigments. The additives used to stabilize the polymers may be, for example, additive packages including hindered phenols, phosphites, UV stabilisers, antistatics and stearates.
The invention also relates to a molded article comprising the high density polyethylene or the composition comprising the high density polyethylene according to the present invention. The molded article may preferably be a blow molded article or an extrusion molded article. The molded article may preferably be selected from films, pipes, bottles, IBC containers, tight head and open head drums and fuel tanks.
It is noted that the invention relates to all possible combinations of features described herein, preferred in particular are those combinations of features that are present in the claims. It will therefore be appreciated that all combinations of features relating to the composition according to the invention; all combinations of features relating to the process according to the invention and all combinations of features relating to the composition according to the invention and features relating to the process according to the invention are described herein.
It is further noted that the term ‘comprising’ does not exclude the presence of other elements. However, it is also to be understood that a description on a product/composition comprising certain components also discloses a product/composition consisting of these components. The product/composition consisting of these components may be advantageous in that it offers a simpler, more economical process for the preparation of the product/composition. Similarly, it is also to be understood that a description on a process comprising certain steps also discloses a process consisting of these steps. The process consisting of these steps may be advantageous in that it offers a simpler, more economical process.
When values are mentioned for a lower limit and an upper limit for a parameter, ranges made by the combinations of the values of the lower limit and the values of the upper limit are also understood to be disclosed.
The invention is now elucidated by way of the following examples, without however being limited thereto.
The properties of the polymers produced in the examples were determined as follows:
The high load melt index (HLMI) was determined using the procedures of ASTM D- 1238 10 Condition F using a load of 21.6 kg at a temperature of 190°C.
Density was measured according to ASTM D-792 08.
Bulk density was measured according to ASTM D-1895. Polymer molecular weight (MW) and its distribution (MWD) were determined by Polymer Labs 220 gel permeation chromatograph (GPC). The chromatograms were run at 150°C using 1 ,2,4-trichlorobenzene as the solvent with a flow rate of 0.9 ml/min. A refractive index detector is used to collect the signal for molecular weights. The software used is Cirrus from Polyab for molecular weights from GPC. The calibration of the HT-GPC uses a Hamielec type calibration with broad standard and fresh calibration with each sample set.
Mz and Mz+1 are higher average molecular weights (according to ASTM D-6474 12) Mw: weight-average molecular weight (according to ASTM D-6474 12)
Mn: number-average molecular weight (according to ASTM D-6474 12)
MWD (molecular weight distribution) is the ratio of weight-average molecular weight (Mw) to number-average molecular weight (Mn), (according to ASTM D-6474 12)
Preparation of catalyst A (comparative)
To a three-necked round bottom flask equipped with a condenser and a mechanical stirrer, 100 g of dried silica support 955W (surface area 310 m2/g, pore volume 1.5 ml/g and average particle diameter 49 pm) was placed at 200°C. 2 g of chromium acetate hydroxide was then added to the silica and then slurried in 250 cm3 of methanol (100%), which was stirred at 80°C for 30 minutes. Subsequently, drying of the methanol solvent took place at 95°C with nitrogen purge.
The dried chromium on silica powder was cooled down to room temperature then slurried with 250cm3 of iso-pentane, followed by the addition of 20 cm3 of tetraethoxy titanium Ti(OC H5)4 (100%). The contents were mixed at 65°C for another 10 minutes and then the solvent was dried at 95°C with nitrogen purge. Dried Chromium-Titanium on silica powder was obtained.
For chromium catalyst activation the dried catalyst powder was placed in a calciner and the following sequence was followed:
Ramp from ambient to 400°C in under N2 flow then hold for 20 minutes
At 400°C switch from N2 to Air flow
Ramp from 400°C to 800°C under dry Air
Hold at 800°C for 4 hours under Dry Air
Cool to room temperature then switch to N2 purge. The activated Chromium-Titanium on silica powder was cooled down to room temperature then slurried with 500cm3 of Iso-pentane, followed by the addition of 78 cm3 of 1M diethyl alumiumium ethoxide (02H5)2AI-002H5 solution. The contents were mixed at 40°C for another 30 minutes then drying the iso-pentane solvent at 65°C with Nitrogen purge.
ICP Elemental Analysis: 0.51 wt% Cr, 3.9 wt% Ti and 1.05 wt% Al, [AI]:[Cr] = 3.96
Preparation of catalyst B (inventive)
To a three-necked round bottom flask equipped with a condenser and a mechanical stirrer, 100 g of dried silica support (0.71 wt% Cr; surface area 350 m2/g, pore volume 1.7 ml/g and average particle diameter 33 pm) obtained from AGC, Japan was placed. The flask was placed in oil bath and heated up to 150°C under continuous stirring with Nitrogen flow to pre-dry the Cr on Silica.
The dried chromium on silica powder was cooled down to room temperature then slurried with 250cm3 of iso-pentane, followed by the addition of 20 cm3 of tetraethoxy titanium Ti(OC H (100%). The contents were mixed at 65°C for another 10 minutes and then the solvent was dried at 95°C with nitrogen purge. Dried Chromium-Titanium on silica powder was obtained.
For chromium catalyst activation the dried catalyst powder was placed in a calciner and the following sequence was followed:
Ramp from ambient to 400°C in under N2 flow then hold for 20 minutes
At 400°C switch from N2 to Air flow
Ramp from 400°C to 700°C under dry Air
Hold at 700°C for 4 hours under Dry Air
Cool to room temperature then switch to N2 purge.
In a 250 ml 3 neck round bottom flask, fitted with a stirrer and placed in an oil bath, 47.7 cm3 of 1 M solution of Diethyl Aluminium Ethoxide was added to the flask followed by the addition of 13.6 cm3 of 1 M solution of Cyclohexylamine, allowed to mix for 30 minutes at 65°C. Reaction gives off Ethane gas in a form bubbles. Di(ethyl ethoxy aluminium) cyclohexylamine (C H -AI-OC H ) C6HnN was obtained. The dried Chromium-Titanium on silica powder was cooled down to room temperature then slurried with 250 cm3 of Iso-pentane, followed by the addition of 67 cm3 of 1M solution of di(ethyl ethoxy aluminium) cyclohexylamine (C H -AI-OC H ) C6HIIN. The contents were mixed at 40°C for another 20 minutes then drying the iso-pentane solvent at 65°C with Nitrogen purge.
ICP Elemental Analysis: 0.7 wt% Cr, 3.1 wt% Ti and 1.4 wt% Al, [AI]:[Cr] = 3.5
The obtained catalyst particles had a high flowability believed to be due to the presence of the amine compound. Such catalyst flow-ability is required for the solid powder catalyst feeders used in the Fluidized Bed Gas Phase Reactors.
Comparative experiment 1 : ethylene-hexene copolvmerization using catalyst A An autoclave with a volume of 2 liters was purged with nitrogen at 130°C for 30 minutes. After cooling the autoclave to 70°C, one liter of iso-pentane and 20 mL of 1- hexene were introduced to the reactor. Subsequently the reactor was pressurized with 15 bar ethylene. Subsequently 0.1 mmol of triethylaluminum was injected into the reactor by the means of a catalyst injection pump.
This was followed by injection of 0.25 g of catalyst A after being slurried in 20 cm3 of Iso-pentane solvent. The reactor temperature was raised to 100°C. Ethylene polymerization was carried out for 60 minutes; with ethylene supplied on demand to maintain the total reactor pressure at 20 bar.
280 liter of ethylene were consumed and 190 grams of ethylene- 1 -hexene copolymer was recovered giving a catalyst productivity of 760 g PE/g cat h at 200 psig.
The characteristics of the obtained polyethylene were as follows
• Density: 0.956 kg/m3
• Resin Bulk density: 331 kg/m3
• Fines level: 1.1 %
Figure imgf000014_0001
Example 2: ethylene-hexene copolvmerization using catalyst B (inventive') Comparative experiment 1 was repeated except that catalyst B was used instaed of catalyst A. The conditions were identical except that the reactor tempeature was raised to 101 °C instead of 100 °C.
240 liter of ethylene were consumed and 167 grams of polyethylene was recovered giving a catalyst productivity of 668 g PE/g cat h at 200 psig.
The characteristics of the obtained polyethylene
• Density: 0.959 kg/m3
• Resin Bulk density: 439 kg/m3
• Fines level: 0.2 %
• HLMI: 3.2
By comparison of comparative experiment 1 and example 2, it can be understood that a higher resin bulk density is obtained by using the catalyst system according to the invention comprising di(ethyl ethoxy aluminium) cyclohexylamine (catalyst A) compared to a catalyst system comprising diethyl alumiumium ethoxide (catalyst B).
Preparation of catalysts C, D, E and F (inventive')
Catalysts C, D, E and F were prepared by following the procedure of the preparation of catalyst B except the ratio between the dried chromium-titanium on silica powder and di(ethyl ethoxy aluminium) cyclohexylamine was varied as shown in Table 1.
Examples 3, 4, 5 and 6 (inventive)
Example 2 was repeated except that catalyst C, D, E and F were used instead of catalyst B.
Mn, Mw, MWD, Mz and Mz+1 of the polymers obtained by examples 2-6 are shown in Table 1.
Table 1
Figure imgf000015_0001
Preparation of catalyst G (comparative') and catalysts H, I, J and K (inventive')
Catalyst G was prepared by following the procedure of the preparation of catalyst A except the ratio between the dried chromium-titanium on silica powder and di(ethyl ethoxy aluminium) cyclohexylamine was varied as shown in Table 2.
Catalysts H, I, J and K were prepared by following the procedure of the preparation of catalyst B except the ratio between the dried chromium-titanium on silica powder and di(ethyl ethoxy aluminium) cyclohexylamine was varied as shown in Table 2.
Example 7 (comparative') and Examples 8, 9, 10 and 11 (inventive')
Example 2 was repeated except that catalyst H, I, J and K were used instead of catalyst B.
Mn, Mw, MWD, Mz and Mz+1 of the polymers obtained by examples 7-11 are shown in Table 2.
Table 2
Figure imgf000016_0001
It can be understood that the ethylene copolymer according to the invention has a higher MW and MWD than the ethylene copolymer of the comparative experiment.
Figures 1-4 shows graphs obtained by GPC analysis of the ethylene copolymer of Examples 8-11.

Claims

1. A solid catalyst system comprising a chromium compound, a silicon oxide support and a compound represented by the general formula
(R4)2-N-R3 (I) wherein R4 is represented by R1-AI-OR2 wherein R1 is selected from C1-C8 alkyl groups and OR2 is selected from C1-C8 alkoxyl groups and
R3 is a C3-C8 linear, branched or cyclic alkyl group.
2. The solid catalyst system according to claim 1, wherein the chromium compound is selected from chromium trioxide, chromium acetyl acetone, chromium chloride, chromium nitrate, chromium acetate, chromium acetate hydroxide, chromium sulfate, ammonium chromate and ammonium dichromate.
3. The solid catalyst system according to any one of the preceding claims, wherein the molar ratio of Al to Cr in the solid catalyst system is 1.0 to 10.0, for example 2.0 to 8.0 or 3.0 to 6.0.
4. The solid catalyst system according to any one of the preceding claims, wherein the compound (I) is a reaction product of an alkyl aluminium compound and a nitrogen containing compound, wherein the aluminium alkoxide compound has the formula (R1)2-AI-0R2 and the nitrogen containing compound has the formula R3-NH2, wherein the reaction product has been prepared by a reaction which has taken place before the alkyl aluminium compound and the nitrogen containing compound came into contact with the chromium compound and the silicon oxide support.
5. The solid catalyst system according to claim 4, wherein the aluminium alkoxide compound is selected from the group consisting of diethyl aluminium ethoxide, dihexyl aluminium ethoxide, dioctyl aluminium ethoxide and dihexyl aluminium propoxide, preferably diethyl aluminium ethoxide, and/or the nitrogen containing compound is selected from the group consisting of cyclopropylamine, cyclobutylamine, cyclopentylamine, cyclohexylamine, cycloheptylamine and cyclooctylamine, more preferably selected from cyclohexylamine and cyclooctylamine, most preferably is cyclohexylamine.
6. The solid catalyst system according to any one of the preceding claims, molar ratio of N to Cr in the solid catalyst system is 0.5 to 5.0, more preferably 0.80 to 3.0 or 0.90 to 2.0, more preferably at least 1.10, at least 1.20, at least 1.30, or at least 1.55.
7. The solid catalyst system according to any one of the preceding claims, the molar ratio of N to Al in the solid catalyst system is 0.10 to 1.0, for example 0.20 to 0.50, preferably at least 0.27, at least 0.305, at least 0.35 or at least 0.40.
8. The solid catalyst system according to any one of the preceding claims, wherein the silicon oxide support has an average particle diameter of 20 to 50 pm determined via ASTM D-1921 12, a pore volume of 1.2 to 3.0 m3/kg determined by ASTM D4284-12 (2012) and/or a surface area of 200 to 800 m2/g determined by the BET nitrogen adsorption method ASTM D 1993-03 (2013).
9. The solid catalyst system according to any one of the preceding claims, further comprising a non-chromium metal compound, preferably represented by Tm(OR4)nX4-n or Tm(R5)nX4-n, wherein
Tm represents a transition metal of Group IVB, VB, or VIB,
R4 and R5 is independently selected from C1-C20 alkyl groups, C1-C20 aryl groups and C1-C20 cycloalkyl groups,
X represents a halogen atom, preferably chlorine and n represents a number satisfying 0 < n < 4, preferably 1< n < 4.
10. The solid catalyst system according to claim 9, wherein the non-chromium metal compound is a titanium alkoxy compound selected from the group consisting of tetraethoxy titanium, tetramethoxy titanium, tetrabutoxy titanium, tetrapropoxy titanium (in particular tetraisopropoxy titanium), tetraisobutoxy titanium, tetrapentoxy titanium, triethoxychloro titanium, diethoxydichloro titanium , trichloethoxy titanium, methoxy titanium trichloride, dimethoxy titanium dichloride, ethoxy titanium trichloride, diethoxy titanium dichloride, propoxy titanium trichloride, dipropoxy titanium dichloride, butoxy titanium trichloride, butoxy titanium dichloride and titanium tetrachloride.
11. The solid catalyst system according to claim 10, wherein the weight ratio of Ti:Cr is 2 to 4.
12. A process for the preparation of the solid catalyst system according to any one of the preceding claims, comprising i) providing the chromium compound and the optional non-chromium metal compound on the silica support, ii) providing the compound (I), involving reacting the alkyl aluminium compound and the nitrogen containing compound before the alkyl aluminium compound and the nitrogen containing compound come into contact with the chromium compound and the silicon oxide support and iii) mixing the product of step i) and the product of step ii)..
13. A process for the production of polyethylene by polymerisation of ethylene and an optional comonomer in the presence of the solid catalyst system according to any one of claims 1-11.
14. The polyethylene obtained by or obtainable by the process according to claim 13.
15. An article comprising the polyethylene according to claim 14, preferably selected from films, pipes, bottles, IBC containers, tight head and open head drums and fuel tanks
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