Chromium oxide catalysts for ethylene polymerization

Abstract

The present invention relates to a solid catalyst system comprising a chromium compound, a metal compound, an aluminum compound and a silica support, wherein the silica support has an average particle size in the range of > 20 to < 50 pm, a pore volume in the range of > 1.7 ml / g to < 3 ml / g, and a surface area in the range of > 400 m 2 / g to < 800 m 2 / g, and wherein the aluminum alkoxide compound has the formula R1-Al-OR2, wherein R1 is selected from (C1-C8)alkyl, and OR2 is selected from (C1-C8)alkoxy.

Description

[0001] The present invention relates to supported chromium oxide catalyst systems for the production of polyethylene in a fluidized bed gas phase reactor.

[0002] Ethylene polymerization using supported chromium based catalysts is for example disclosed by Kevin Cann in "Comparison of silylchromate and chromium oxide based olefin polymerisation catalysts" (Macromolecular Symp, 2004, 213, 29-36).

[0003] Chromium oxide based catalysts, which are commonly referred to in the literature as "Phillips catalysts", can be obtained by calcination of chromium compounds supported on inorganic oxide carriers in a non-reducing atmosphere (mainly dry air is required). Various adjustments to the chromium oxide catalyst system are made by adding different kinds of modifiers to the chromium oxide system, for example titanium and aluminium compounds, to enhance the molecular weight and molecular weight distribution. Chromium oxide catalysis and ethylene polymerization with this particular catalyst are disclosed on pages 61-64 of "Handbook of Polyethylene" by Andrew Peacock. The properties of the silica carrier, the chromium loading and the activation method can all influence the chemical state of the chromium supported on the silica catalyst and the performance of the chromium oxide in the polymerization process. For example, the activity of the catalyst generally increases with increasing activation temperature, while the molar mass of the polymerization product can decrease or the high load melt index (HLMI) can increase. The influence of the activation conditions on the catalyst properties is disclosed in Advances in Catalysis, Mc Daniel, Vol. 33, 48-98, 1985. Generally, the activation takes place at elevated temperatures, for example temperatures higher than 450°C, preferably temperatures between 450 and 850°C. The activation can take place in different atmospheres, for example in dry air. Generally, the activation takes place at least partially under an inert atmosphere, preferably an inert atmosphere consisting of nitrogen. The activation time after reaching the maximum temperature can last from a few minutes to several hours. The activation time is at least 1 hour, but it can be advantageous to activate for longer.

[0004] Pullukat et al. (Journal of Polymer Science; Polymer chemistry Edition; vol 18, 2857-2866; 1980) disclose a thermally activated ethylene polymerization catalyst containing chromium and titanium on silica.

[0005] The ethylene polymerization catalysts containing chromium, titanium and / or alkyl aluminum on silica (silica support) suffer from the fact that they cannot produce polyethylene (PE) with increased resin bulk density values and with high production rates in the ethylene polymerization process in gas phase reactor processes.

[0006] The production of resin fluff (resin in the form of polymer powder produced by the reactor before compounding) with reduced resin bulk density has a negative impact on the bed weight of the gas phase reactor, causing a reduction in the droplet size of the reactor and thus a reduction in the production rate for the same number of reactor droplets.

[0007] Furthermore, the reduced resin bulk density has a negative impact on the upper and lower fluidization bulk densities inside the reactor, which forces the operation to reduce the superficial gas velocity (SGV) of the reactor to avoid resin carry over, which leads to fouling of the distributor plate and the cooler. It is also known that reduced SGV operation reduces the momentum flux inside the reactor bed, resulting in poor conversion and thus reduced production rate. In addition to this, low SGV operation reduces the ability to reject heat.

[0008] It is an object of the present invention to provide an improved silica supported chromium oxide catalyst for ethylene polymerization in a fluidized bed gas phase process, which obtains a HDPE resin with high resin bulk density, obtains an increased bed weight and increased upper and lower fluidization bulk densities, such that the droplet size of the reactor is increased and the production rate is increased. It is a further object of the present invention to provide an improved silica supported chromium oxide catalyst with high production rate.

[0009] This object is achieved by a solid catalyst system comprising a chromium compound, a metal compound, an aluminum compound and a silica support, wherein the silica support has an average particle size in the range of > 20 to < 50 pm, a pore volume in the range of > 1.7 ml / g to < 3 ml / g, and a surface area in the range of > 400 m 2 / g to < 800 m 2 / g, and wherein the aluminum alkoxide compound has the following formula

[0010] R1-Al-OR2

[0011] wherein R1 is selected from (C1-C8)alkyl and OR2 is selected from (C1-C8)alkoxy.

[0012] According to a preferred embodiment of the present invention, the pore radius of the silica support is at least 100 Angstrom. The upper limit is 200 Angstrom.

[0013] According to a preferred embodiment of the present invention, the silica support has an average particle size in the range of > 30 to < 40 pm, a pore volume in the range of > 1.7 ml / g to < 1.9 ml / g, and a surface area in the range of > 500 m2 / g to < 600 m 2 / g range.

[0014] The surface area and pore volume of the support are determined by BET nitrogen adsorption. Test method: ASTM D 1993-03 (2013) Standard Test Method for Precipitated Silica - Surface Area by Multipoint BET Nitrogen Adsorption. See also S. J. Gregg and K. S. W. Sing, "Adsorption, Surface Area and Porosity", Academic Press, London (1982) and S. Lowell, "Introduction to Powder Surface Area", J. Wiley & Sons, New York, NY (1979). The average particle size is determined via ASTM D-192112.

[0015] The chromium compound can be selected from various organic and inorganic forms of chromium.

[0016] Preferably, the chromium compound is selected from the group consisting of chromium acetate, chromium acetylacetonate, chromium hydroxide acetate and chromium trioxide.

[0017] Most preferably, the chromium compound is chromium acetate or chromium acetylacetonate.

[0018] Preferably, the amount of chromium in the catalyst is between 0.2 wt% and 0.9 wt%. Preferably, the loading is between 0.23 wt% and 0.30 wt%.

[0019] Examples of suitable metal compounds or metal halide transition metal compounds which act as a modifier and for the synthesis of the solid catalyst component according to the present application can be represented by the formula Tm(OR 1 ) n X 4-n and Tm(R 2 ) n X 4-n wherein Tm represents a transition metal of Group IVB, VB or VIB, R 1 and R 2 represent a (C1-C20)alkyl, (C1-C20)aryl or (C1-C20)cycloalkyl group, X represents a halogen atom, preferably chlorine, and n represents a number satisfying 0 < n < 4.

[0020] The metal alkoxide compound can also be represented by the general formula Ti(OR) a X bLet R represent a hydrocarbon residue with 1-20 carbon atoms, M be a metal, more preferably a transition metal, X be a halogen atom, and a and b be numbers that satisfy 1≤a≤4, 0≤b≤3 and a+b=4 in the case of tetravalent metals.

[0021] Preferably, the metal is selected from titanium, vanadium, hafnium, and zirconium.

[0022] The most preferred metal is titanium.

[0023] According to a preferred embodiment of the present invention, the titanium compound is Ti(OR) 1 ) n X 4-n and Ti(R) 2 ) n X 4-n The compounds in which

[0024] ·R 1 and R 2 It represents (C1-C20)alkyl, (C1-C20)aryl, or (C1-C20)cycloalkyl.

[0025] ·X represents a halogen atom, preferably chlorine, and

[0026] ·n represents a number that satisfies 0≤n≤4.

[0027] Examples of suitable titanium compounds include alkoxy titanium compounds, such as tetraethoxy titanium, tetramethoxy titanium, tetrabutoxy titanium, tetrapropoxy titanium, tetraisobutoxy titanium, tetrapentoxy titanium, triethoxy chloride titanium, diethoxy dichloride titanium, trichloroethoxy titanium, methoxy titanium trichloride, dimethoxy dichloride titanium, ethoxy titanium trichloride, diethoxy dichloride titanium, propoxy titanium trichloride, dipropoxy dichloride titanium, butoxy titanium trichloride, butoxy dichloride titanium, and titanium tetrachloride.

[0028] Preferably, it is low-salt tetraisopropyl titanium dioxide.

[0029] The Cr:Ti weight ratio ranges from, for example, 1:2 to 1:4.

[0030] The titanium content of the catalyst can range from 0.1 to 10 wt%, preferably from 0.1 to 6 wt%.

[0031] Other suitable metal compounds include, for example, vanadium trichloride, vanadium tetrachloride, vanadium trichloride oxide, and zirconium tetrachloride.

[0032] Preferably, the aluminum compound is an alkane aluminum oxide compound having the following formula:

[0033] R1-Al-OR2

[0034] R1 is selected from (C1-C8) alkyl groups, and

[0035] OR2is selected from (Ci-C8)alkoxy, wherein the R2group can be the same or different from the R1group.

[0036] Examples of suitable aluminum alkoxide compounds include diethyl ethoxide aluminum, dihexyl ethoxide aluminum, dioctyl ethoxide aluminum and / or dihexyl propoxide aluminum.

[0037] According to a preferred embodiment of the present application, the aluminum alkoxide compound is diethyl ethoxide aluminum.

[0038] Typically, 0.01 mole Cr, 0.08 mole Ti and 0.05 mole Al are required in terms of quantity.

[0039] The solid catalyst system is employed in a gas phase polymerization process to produce high density ethylene polymers.

[0040] The catalyst according to the present application produces polyethylene with increased resin bulk density in a gas phase process and with increased mechanical properties such as ESCR, FNCT and Izod impact strength.

[0041] The chromium catalyst according to the present application increases the space time yield (STY) of any given gas phase reactor by about 25%.

[0042] Furthermore, the increase in upper and lower fluidization bulk density will provide more space to increase the superficial gas velocity (S.G.V) which will also allow higher conversion, obtaining higher productivity and enhanced heat removal.

[0043] The catalyst according to the present application can be used to produce homopolymers and copolymers.

[0044] In the case of producing ethylene copolymers, the alpha olefin comonomer can be selected from propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene and / or 1-octene.

[0045] Preferably, the comonomer is 1-hexene.

[0046] The polyethylene obtained with the catalyst according to the present application in a gas phase polymerization has:

[0047] • High Load Melt Index (HLMI 21.6 kg) > 0.1 g / 10 min and < 30 g / 10 min (according to ISO 1133)

[0048] • M w / M n > 10 and < 18 (measured according to Size Exclusion Chromatography (SEC))

[0049] • Density > 930 kg / m 3 and < 970 kg / m3 (ISO 1183), and

[0050] • Resin bulk density > 450 and < 530 kg / m 3 (ASTM D-1895)

[0051] The silica supported chromium oxide catalyst system according to the present application is capable of generating relatively low HLMI values, for example in the range of 1-12 (MI 21.6kg ) for large blow molded articles having the desired ESCR values at elevated bed temperatures not less than 105°C.

[0052] The gas phase reactor can be any reactor suitable for gas phase polymerization, and can for example be a vertical, horizontal mechanically agitated reactor or a fluidized bed reactor.

[0053] According to a preferred embodiment of the present application, the reactor is a fluidized bed reactor.

[0054] Generally, a fluidized bed gas phase polymerization reactor employs a "bed" of polymer and catalyst fluidized by a stream of monomer, comonomer and other optional components, at least a portion of which are gaseous. Heat is generated by the enthalpy of polymerization of the monomer flowing through the bed. Unreacted monomer and other optional gaseous components exit the fluidized bed and are contacted with a cooling system to reject heat. The cooled gas stream, containing monomer, comonomer and optionally, for example, condensable liquids, is then recycled through the polymerization zone. Simultaneously, polymer product is removed from the reactor. The reactor temperature in a gas phase process can range, for example, from 30°C to 130°C. Descriptions of gas phase processes are disclosed, for example, in US 4,543,399 and US 4,588,790.

[0055] Suitable fluidized bed reactors include, for example, a bubbling fluidized bed reactor, a circulating fluidized bed reactor, a tubular fluidized bed reactor, a multi-zone fluidized bed reactor, and a flash reactor. A "fluidized bed" means that a quantity of solid particles, in this case preferably solid catalyst and / or solid catalyst to which monomer is attached, behaves as a fluid in a solid / fluid mixture. This can be achieved by placing the quantity of solid particles under appropriate conditions, for example by introducing a fluid through the solid particles at a sufficiently high velocity to suspend the solid particles and cause them to behave as a fluid. An example of a process for producing polyethylene using a fluidized bed is disclosed in US 4,882,400. Other examples of producing polyolefins using a fluidized bed are described, for example, in 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.

[0056] The bottom of the fluidized bed reactor can comprise an inlet connected to the feeders for the reaction composition such as ethylene, nitrogen, hydrogen and comonomer.

[0057] The middle zone above the distribution plate in the reactor comprises an inlet for a polymerization catalyst which can be fed to the reactor together with nitrogen. The middle zone of the reactor further comprises an outlet to a product discharge tank. The upper zone of the reactor comprises an outlet for a top recycle stream, wherein the outlet for the top recycle stream is connected to an inlet of a compressor. The compressor comprises an outlet for compressed fluid and the outlet of the compressor is connected to an inlet of a cooling device for the compressed fluid. The cooling device comprises an outlet for providing a bottom recycle stream, which outlet of the cooling device is connected to an inlet of the reactor bottom.

[0058] A suitable example of a multi-zone fluidized bed reactor (FBR) system is a multi-zone reactor capable of operating in a condensing mode, comprising a first zone, a second zone, a third zone, a fourth zone and a distribution plate, wherein the first zone is separated from the second zone by the distribution plate, wherein the multi-zone reactor extends in a vertical direction, wherein the second zone of the multi-zone reactor is located above the first zone, and wherein the third zone of the multi-zone reactor is located above the second zone, and wherein the fourth zone of the multi-zone reactor is located above the third zone, wherein the second zone contains an inner wall, wherein at least a portion of the inner wall of the second zone is in the form of an increasing inner diameter or is a continuously open cone, wherein the inner diameter or the opening increases in a vertical direction towards the top of the multi-zone reactor, wherein the third zone contains an inner wall, wherein at least a portion of the inner wall of the third zone is in the form of an increasing inner diameter or is a continuously open cone, wherein the inner diameter or the opening increases in a vertical direction towards the top of the multi-zone reactor, wherein the maximum inner diameter of the interior of the third zone is larger than the maximum inner diameter of the inner wall of the second zone. The FBR can operate in a so-called "condensing mode", which effectively excludes the heat generated during the exothermic polymerization. In this mode, heat removal is achieved by cooling the gaseous recycle stream to a temperature below its dew point, thereby condensing at least a portion of the recycle stream to form a bottom recycle stream containing liquid and gas. The bottom recycle stream thus formed is then introduced into the fluidized bed polymerization reactor, where the liquid portion will evaporate upon exposure to the heat of the reactor, which will remove heat from the reactor and enable the feeding of one or more very highly active catalysts. Details of FBRs operating in a condensing mode are described, for example, in EP13195141A.

[0059] The ethylene composition comprising an ethylene polymer obtained from the process according to the present application can further comprise additives, such as lubricants, fillers, stabilizers, antioxidants, compatibilizers and pigments. The additives used to stabilize the polymer can be, for example, an additive package comprising a hindered phenol, a phosphite, a UV stabilizer, an antistatic agent and a stearate.

[0060] The present invention also relates to blow molded articles made from the polyethylene according to the present invention.

[0061] According to a preferred embodiment of the present invention, the polymer obtained from the process according to the present invention is used for the manufacture of fuel tanks.

[0062] Ethylene polymers can also be extruded or blow molded into articles such as pipes, bottles, IBC containers, closure and open head drums and wire and cable applications.

[0063] The present invention will be illustrated by means of the following non-limiting examples. Example

[0064] The properties of the polymers generated in the examples were determined as follows:

[0065] The high load melt index (HLMI) was determined using ASTM D-1238 Condition F using a load of 21.6 kg at a temperature of 190 °C.

[0066] The density was measured according to ASTM D-792 08.

[0067] The bulk density was measured according to ASTM D-1895.

[0068] The polymer molecular weight and its distribution (MWD) were determined by Polymer Labs 220 Gel Permeation Chromatography (GPC). The chromatograph was run at 150 °C using 1,2,4-trichlorobenzene as solvent at a flow rate of 0.9 ml / min. A refractive index detector was used to collect the signal for molecular weight. The software used was Cirrus by Polyab for the molecular weight determined from GPC. The calibration for the HT-GPC used Hamielec type calibration and a broad standard and fresh calibration was used for each sample group.

[0069] Mz and Mz+1 are higher average molecular weights (according to ASTM D-6474 12)

[0070] Mw: Weight average molecular weight (according to ASTM D-6474 12)

[0071] Mn: Number average molecular weight (according to ASTM D-6474 12)

[0072] MWD (Molecular Weight Distribution) is the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) (according to ASTM D-6474 12)

[0073] Catalyst synthesis

[0074] 200 g of dry silica carrier at 200 °C was placed into a three-necked round-bottom flask equipped with a condenser and a mechanical stirrer. Then, 4.7 g of chromium acetate hydroxide was added to the silica, and the mixture was stirred at 250 cm⁻¹. 3 The silica powder was prepared in 100% methanol and stirred at 80°C for 30 minutes. Afterwards, the methanol solvent was dried by purging with nitrogen at 95°C. The dried chromium on the silica powder was cooled to room temperature and then dried using a 250cm³ slurry. 3 The isopentane was used for pulping, followed by the addition of 41 cm. 3 Tetraethoxytitanium Ti(OC2H5)4 (100%). The contents were mixed at 65°C for an additional 10 minutes, and then the solvent was dried by purging with nitrogen at 95°C.

[0075] To activate the chromium catalyst, the dry catalyst powder is placed in a calciner and the following procedure is followed:

[0076] - Heat from ambient temperature to 400°C under a flow of N2, then hold for 20 minutes.

[0077] - Switch from N2 to airflow at 400℃

[0078] - Heating from 400℃ to 800℃ in dry air

[0079] - Keep at 777°C in dry air for 4 hours

[0080] - Cool to room temperature, then switch to N2 purging.

[0081] The dried chromium-titanium coating on the silica powder was cooled to room temperature, and then subjected to a 250 cm... 3 The isopentane was used for pulping, followed by the addition of 17cm 3 Diethylalumina (C2H5)2Al-OC2H5 (98%). The contents were mixed at 45°C for an additional 10 minutes, and then the solvent was dried by purging with nitrogen at 85°C.

[0082] Table 1: Overview of the prepared catalyst system.

[0083]

[0084] *In the preparation of the catalyst, diethylalumina (C2H5)2Al-OC2H5 (98%) is not used; instead, TEAL is used.

[0085] In the preparation of Comparative Example B of the sample catalyst, TEAL was used instead of diethyl ethoxyalumina (C2H5)2Al-OC2H5 (98%).

[0086] Ethylene copolymerization

[0087] A 2 liter volume autoclave was purged with nitrogen for 30 minutes at 130°C. After cooling the autoclave to 70°C, 1 liter of isopentane with 10 ml of 1-hexene was introduced into the reactor, then the reactor was pressurized with 15 bar of ethylene.

[0088] Then 0.1 mmol of TEAL was injected into the reactor by means of a catalyst injection pump.

[0089] Then 0.2 g of catalyst according to table 1 slurried in 20 cm3of isopentane was injected. 3 Isopentane solvent. The reactor temperature was raised to 101 °C. Ethylene polymerization was carried out for 60 minutes; ethylene was supplied as needed to maintain the total reactor pressure at 20 bar.

[0090] Table 2: PE properties - results from small slurry reactor.

[0091] For the polymerization, the corresponding catalyst system of table 1 was used.

[0092]

[0093] From table 2 it can be seen that inventive example I achieved an excellent resin bulk density, while at the same time the catalyst productivity was higher than for comparative examples A to E.

[0094] Table 3: Mechanical properties of PE resins.

[0095]

[0096] Inventive example I obtained a resin very suitable for fuel tank grade. The resin properties were far better than for sample comparative examples A and B.

[0097] Data concerning the application of the catalyst according to comparative example A and according to inventive example I to a fluidized bed gas phase polymerization are summarized in table 4.

[0098] Table 4: Gas phase polymerization - conditions and resin properties.

[0099] Sample Inventive Example I-G Comparative Example A-G Catalyst Catalyst Inventive Example I Catalyst Comparative Example A Bed temperature 106℃ 103℃ ΔT 4.9℃ 4.3℃ Total reactor pressure 20.6 20.7 Ethylene partial pressure 15.4 bar 15.4 bar [C6 / C2 molar ratio] 0.0037 0.004 [H2 / C2] 0.015 0.014 Fluidized bulk density 380 kg / m 3 ]] 220 kg / m 3 ]] Superficial gas velocity 0.44 m / sec 0.42 m / sec Resin bulk density 477 kg / m 3 ]] 318 kg / m 3 ]] Ash <100 ppm 110 ppm Density 945 kg / m 3 ]] 945 kg / m 3 ]] HLMI 6.8 6.7 Average particle size 0.53 mm 0.55 mm Fines 0.2％ 1.8％

[0100] Space time yield calculation:

[0101] Defined as the production per unit volume: STY = productivity (lb / h) / volume (ft) 3

[0102] Residence time according to comparative example:

[0103] Residence time = bed weight / productivity

[0104] Residence time = 93 kg / (40 kg / h) = 2.35 hours

[0105] For the invention, the bed weight increased to 118 kg

[0106] To maintain the same residence time of 2.35, the productivity was increased to 51 kg / h

[0107] For the residence time of the invention: 118 kg / 51 kg / h = 2.32

[0108] For the comparative catalyst, the space time yield = 40 kg (88 lb) / 6.8 ft 3 = 12.9

[0109] For the catalyst according to the invention, the space time yield = 51 kg (112.2 lb) / 6.8 ft 3 = 16.5

[0110] This resulted in an increase in space time yield of 27.9%

[0111] As can be seen from Table 4, inventive example I-G shows a much higher resin bulk density than comparative example A-G. In addition, inventive example I-G shows a much higher yield than comparative example A-G.

Claims

1. A solid catalyst system comprising a chromium compound, a metal compound, an aluminum compound and a silica support, wherein the silica support has an average particle size in the range of > 30 to < 40 pm, a pore volume in the range of > 1.7 ml / g to < 1.9 ml / g, and a surface area in the range of > 500 m 2 / g to < 600 m 2 / g, and wherein the aluminum compound is selected from the group consisting of diethylaluminum ethoxide, dihexylaluminum ethoxide, dioctylaluminum ethoxide and / or dihexylaluminum propoxide, wherein the metal of the metal compound is selected from the group consisting of titanium, vanadium, hafnium and zirconium.

2. The catalyst system according to claim 1, wherein the chromium compound is selected from the group consisting of chromium trioxide, chromium acetylacetonate, chromium acetate and / or chromium hydroxide acetate.

3. The catalyst system according to claim 1, wherein the metal compound is represented by the formula Tm(OR 1 ) n X 4-n and Tm(R 2 ) n X 4-n wherein Tm represents a transition metal of Group IVB, VB or VIB, R 1 and R 2 represent a C1-C20 alkyl group, a C1-C20 aryl group or a C1-C20 cycloalkyl group, X represents a halogen atom, and n represents a number satisfying 0 < n < 4.

4. The catalyst system of claim 3, wherein in the formula Tm(OR 1 ) n X 4-n and Tm(R 2 ) n X 4-n X represents chlorine.

5. The catalyst system according to claim 1, wherein the metal of the metal compound is titanium.

6. The catalyst system of claim 5, wherein the titanium compound is a compound according to the formula Ti(OR 1 ) n X 4-n and Ti(R 2 ) n X 4-n wherein • R 1 and R 2 denote C1-C20alkyl, C1-C20aryl or C1-C20cycloalkyl, • X represents a halogen atom, and • n represents a number satisfying 0 < n < 4.

7. The catalyst system according to claim 6, wherein in the formula Ti(OR 1 ) n X 4-n and Ti(R 2 ) n X 4-n X represents chlorine.

8. The catalyst system according to claim 1, wherein the aluminum compound is diethylaluminum ethoxide.

9. An ethylene gas phase polymerization process, wherein the catalyst is the catalyst system according to any one of claims 1 to 8.

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