NO792343L - PROCEDURE FOR PREPARING A CATALYST SYSTEM FOR POLYMERIZATION OF OLEFINES - Google Patents

Description

This invention relates to olefin polymerization catalysts and methods for the polymerization of olefins in the presence of chromium-containing catalysts deposited on supports.

. The use of chromium compounds in the polymerization of olefins

is well known. US. patent documents 2,825,721 and 2,951,816 describe the use of chromium trioxide which is deposited on a support of an inorganic material such as silicon dioxide, aluminum oxide or combinations of silicon dioxide and aluminum oxide, and which is activated by heating in a reducing atmosphere, for polymerization

of olefins. However, when a catalyst system of this type is used in processes such as the well-known "particulate form" process, resins are obtained which, although useful for many purposes, are unsatisfactory for some applications due to failure of certain properties, such as the melt index.

Attempts to improve the properties of polyolefins produced using heat-activated supported chromium oxide catalysts have been made by adding various compounds to the supported chromium oxide before the heat activation is carried out. For example, US shows. patent document 3,622,522 that an alkoxide of gallium or tin can be added to a chromium oxide deposited on a support before the heat activation (and compares with aluminum isopropbxyd, which gives worse results). In US patent 3,715,321 it is proposed to add a compound of a metal of group II-a or group III-B to chromium oxide on a support before the heat treatment, while US patent 3,780,011 describes the addition of alkyl esters

of titanium, vanadium and boron and US patent 3,484,428 describes the addition of alkylboranes to such a catalyst.

It is likewise known to use other chromium compounds in the production of olefin polymerization catalysts. Such compounds include various silyl chromate and polyalicyclic chromate esters as described in e.g. in the US. patents 3,324,095, 3,324,101, 3,642,749 and 3,704,287. Other such catalysts are described in US. patents 3,474,080, 3,985,676 and 4,100,104. These catalysts can be used together with organometallic reducing agents such as e.g. trialkyl aluminum compounds.

It is in the US. patent documents 3,984,351 and 4,049,986 explained that the properties of olefin polymers, e.g. the melting index, can be substantially improved by using a catalyst prepared by depositing an organophosphoryl chromium compound and an aluminum compound such as aluminum sec-butoxide on an inorganic support material and heat activating the material deposited on the support in a non-reducing, preferably oxygen-containing atmosphere, at a temperature from approx. 300°C and up to the decomposition temperature of the carrier. The material obtained, which is preferably combined with a metallic and/or non-metallic reducing agent, provides a catalyst system capable of providing polymers with improved flow and shear properties in addition to a higher melt index.

It is an aim of the present invention to provide a new method for the production of olefin polymerization catalysts and a polymerization process which makes use of these and which leads to the formation of polymers with a high melt index and other desired characteristics, such as variations in the molecular weight distribution.

It has thus been shown that an effective olefin polymerization catalyst can be prepared by depositing an organo-phosphoryl chromium product and a non-moist aluminum-containing product obtained by reacting an aluminum alkoxide with an aluminum salt of a lower aliphatic monocarboxylic acid, and an inorganic support material and heating of carrier material in a non-reducing atmosphere at a temperature from approx. 300°C and up to the decomposition temperature of the carrier material.

The preparation of the organophosphoryl chromium compound which is deposited on an inorganic support material is carried out as described in US. patent krift3 .985 .676. This patent document is incorporated into the present description by reference.

It is also within the scope of the invention to deposit the non-volatile aluminum-containing reaction product on the support in a subsequent first activation step and then to deposit the chromium compound on the support with a subsequent second activation step such as in the double activation process described in US. patent specification 4,100,104 which is referred to above. Dual activation of the present catalysts has been demonstrated

to give a further increase in the melt index of the polymers.

If desired, the heat-activated catalysts can be treated with a metallic and/or non-metallic reducing agent as described in US. patent document 3,984,351.

The catalysts produced according to the invention

is applicable for use in conventional polymerization processes and is suitable for<p>polymerization carried out under the temperature and pressure conditions commonly used in the art, such as temperatures from 30° to 250°C, preferably from 70° to 110°C, and pressure from 14 to 70 kg/cm 2 , preferably from 21 to 56 kg/cm 2 , which is used in suspension polymerisation.

The organophosphoryl compounds which are deposited on the inorganic support material are produced by reacting an organophosphorus compound with chromium trioxide in an inert solvent such as cyclohexane, n-hexane, methylene chloride, carbon tetrachlorido, etc. In this step in the production of the catalyst system, the solid CrO^ is suspended in the solvent and the organophosphorus compound is added. After a certain time, e.g. after 1 hour, the reaction between the compounds is complete and the chromium trioxide disappears. During this time the solution assumes a red-brown color. It is usually filtered to ensure that it does not contain unreacted, solid CrO^. The solution is then applied to the support material in such a way that the catalyst solution is deposited on it, which can conveniently be done by any wet coating technique, e.g. by spraying. IN

• in typical cases, the solution is added to a dispersion of the carrier. The solvent is removed from the carrier material by drying, e.g. using heat, stripping with inert gas or using reduced pressure, as these operations can be carried out separately or in combination with each other. The reaction product is thereby placed on the support material. It is considered important that organophosphoryl chromium

the reaction product is produced in advance, i.e. that the reactants react with each other before the deposition on the support is carried out. The active catalyst is therefore considered to be derived not from chromium trioxide but from the described organophosphoryl chromium reaction product.

Among the organic phosphorus compounds that can be used in the production of the organophosphoryl chromium compounds are the tri-organophosphates and the organophosphates comprising compounds such as triphenyl phosphate, tributyl phosphate, triethyl phosphate, tri-octyl phosphate, trimethyl phosphate, etc. The derivatives of mono-(dihydrogen) phosphate are also suitable. or -phosphite and di-(hydrogen)-phosphate (examples are monobutyl phosphate, dibutyl phosphate and mono-ethyl phosphite), and these materials can of course also be mixtures. Organophosphorylchromium reaction<p>roproducts can also be formed from phosphorus-based compounds such as phenylphosphoric acid, diethylphosphonate and trioctylphosphine oxide. Preferred compounds can be represented by structures of the type:

where R is alkyl, aralkyl, aryl, cycloalkyl or hydrogen, with at least one R being different from the hydrogen, and R, when this is a hydrocarbon radical, can contain up to 18 carbon atoms. To facilitate the overview, the applicable materials will be referred to as "organophosphoryl chromium" reaction products in the description and claims.

The non-volatile aluminum-containing product, which is also deposited on the inorganic support according to the invention, is produced by reacting an aluminum alkoxide of the general

formula

where n is .1,2 or 3 and R and R' each denote alkyl of up to 8 carbon atoms, with an aluminum carboxyl salt of the general formula

where R" is alkyl of up to 8 carbon atoms. Useful aluminum alkoxides include aluminum se k-butoxide, aluminum methoxyd,

aluminum propoxide, aluminum isopropoxide, aluminum n-butoxide, methylaluminum propoxyd, diethylaluminiummethoxyd, diisobutylaluminiummethoxyd and the like. In the preferred aluminum alkoxides, n will be 3, and among these alkoxides, aluminum iLjek-butoxide will be the most preferred. Examples of aluminum carboxylic acid salts. which can be reacted with the above-mentioned aluminum alkoxides in accordance with the invention, are aluminum acetate, aluminum formate, aluminum propionate, aluminum butyrate, aluminum isobutyrate, etc. Aluminum acetate, Al(OOCCH3)^ is preferred. The reaction between the aluminum alkoxide and the aluminum carboxylic acid salt can conveniently be carried out by heating the reactants with reflux cooling in a suitable inert solvent which is usually an anhydrous organic solvent. Such organic solvents include aliphatic hydrocarbons as well as cycloalkyl and alkylaryl hydrocarbons and their halogenated derivatives, e.g. toluene, xylene and decalin. The molar ratio between the reactants can vary within wide limits, as molar ratios between aluminum alkoxide and aluminum carboxylic acid salt from 1:10 to 10:1 are usable and molar ratios from 1:3 to 3:1 between these reactants are particularly preferred. Correspondingly, the return temperature and reaction time can vary within wide limits. For example, aluminum oxide and the aluminum carboxylic acid salt can be heated under reflux at a temperature from 50° to 250°C for a period of from 15 min. to 24 hours. Excellent results have been obtained with reflux temperatures from 120° to 200°C and reaction times from 3 to 22 hours. During the reaction, initially insoluble aluminum alkoxide dissolves slowly in the solvent, and in some cases the solvent undergoes a slightly noticeable color change. In addition to the desired, non-volatile aluminum-containing product, the reaction also produces a volatile product which serves no purpose in the invention. At present the nature of the non-volatile aluminum-containing reaction product is not known in detail, although it is considered likely that it consists of a mixture of compounds. Without wishing in any way to commit to the following possible explanation, it has been speculated whether the reaction between the aluminum alkoxide and the aluminum carboxylic acid salt does not take place in two stages, with a complex of the two reactants being formed in the first stage and second step, u-oxo-aluminum compounds are formed and fly-

strong connections.

The inorganic carrier materials which are applicable in the method according to the invention include those which are normally used in chromium catalysts deposited on a carrier for use in olefin polymerizations such as those described in US patent 2,825,721. In typical cases, these carrier materials are inorganic oxides such as silicon dioxide, aluminum oxide, mixtures of silicon dioxide and aluminum oxide, thorium dioxide, zirconium oxide and corresponding oxides which are porous, have a medium surface area and have surface hydroxyl groups. Preferred carrier materials are silica xerogels or xerogels containing silicon dioxide as the main component. Particularly preferred are the silicon dioxide xerogels described in US. patent documents 3,652,214, 3,652,215 and 3,652,216, which silicon dioxide xerogels have a surface area in the range from 200 to 500 m<2>/g and a pore volume greater than 2.0 ml/g, a larger part of the pore volume is provided by pores with a diameter in the range from 300 to 600 Angstrom units.

The catalysts produced according to the invention can be produced by depositing the organophosphoryl chromium compound and non-volatile aluminium-containing reaction product on the inorganic carrier in any way. The organophosphoryl chromium compound can be deposited by vapor coating or in a suitable inert solvent. The aluminum-containing product can be deposited in a suitable inert solvent such as those that can be used as the reaction medium in which the aluminum-containing product is prepared. Dichloromethane is a preferred solvent for this purpose. The organophosphoryl chromium compound may be applied first to the carrier, or the aluminum-containing product may be applied first, or the chromium and aluminum products may be applied together. In the usual method for producing catalyst, the carrier is first impregnated with the organophosphoryl chromium compound and then with the aluminium-containing product.

According to the alternative double activation treatment of the catalysts discussed here, the non-volatile aluminum-containing reaction product is deposited on the support first, in the same way as in the single activation process described above, and the coated support is first activated in a non-reducing atmosphere, preferably in a dry , oxygen-containing atmosphere, at temperatures from 130°C and up to the carrier's decomposition temperature. In typical cases, the support with the deposit of aluminum-containing reaction product is heated at a temperature from 13o° to 1100°C, preferably from 260° to 820°C. The time required for this initial activation varies, depending on the temperatures used, from half an hour or less to 50 hours or more. Normally the activation treatment lasts for a period of from 2 to 12 hours. The double activation is completed by the organophosphoryl chromium compound then being deposited on the aluminum-containing support in the same way as described above for the single activation process, after which the thus treated support is heated in a preferably dry oxygen-containing atmosphere, at temperatures from approx. 430°C and up to the decomposition temperature of the carrier. The activation is suitably carried out at temperatures from 430° to 1100°C,

the best results having been obtained by activation at temperatures from 840° to 990°C. The activation in this final heating operation can extend over periods of time from half an hour to 50 hours or more and usually last from 2 to 12 hours.

The most effective catalysts have been found to be those containing the organophosphoryl chromium compound in such an amount that the amount of Cr is from 0.25 to 2.5% by weight, calculated on the weight of the support, preferably from 0.5 to 12.5% by weight, although also amounts outside these ranges also provide usable catalysts. The aluminum-containing product should be added in a sufficient amount to give from 0.1 to 10% by weight of aluminum, calculated on the weight of the carrier, preferably from 0.5 to 5.5% by weight, although amounts outside these ranges can also be used for the production of usable catalysts.

After the organophosphoryl chromium compound and the aluminum-containing product have been deposited on the inorganic support, the support is heated in a non-reducing atmosphere, preferably in an oxygen-containing atmosphere, at a temperature from 300°C up to the salting temperature of the support. In typical cases, the materials deposited on the carrier are heated at a temperature from 500° to 1000°C. The warm-up time may vary, depending on the temperatures used, e.g. from half an hour or less to 50 hours or more. Normally, the heating lasts from 2 to 12 hours. The non-reducing atmosphere, which is preferably air or other oxygen-containing gas, should be dry and should preferably be dehumidified to a few ppm of water to achieve maximum catalyst activity. Generally, air used in the present process will

manner, is dried to a water content of less than 2-3 ppm.

The heated, supported, chromium-containing and aluminum-containing catalyst produced according to the invention can, if desired, be used in combination with metallic and/or non-metallic reducing agents. Examples of metallic reducing agents are trialkylaluminum compounds such as triethylaluminum, triisobutylaluminium, alkylaluminum halides, alkylaluminum alkoxides, dialkylzinc, dialkylmagnesium and metal borohydrides including the borohydrides of the alkali metals, especially of sodium, lithium and potassium, and of magnesium, beryllium and aluminium. The non-metallic reducing agents include alkylboranes such as triethylborane, triisobutylborane and trimethylborane and borohydrides such as diborane, pentaborane, hexaborane and decaborane.

The present heat-treated catalysts can be combined with the metallic or non-metallic reducing agent before being fed to an olefin polymerization reactor, or these two components can be fed separately to an olefin polymerization reactor.

When determining the amount of metallic or non-metallic reducing agent in relation to the amount of chromium compound used in the present catalyst systems, the possibilities for variation are quite large, but certain guidelines should be followed with a view to achieving a good yield, favorable polymer properties and economical material use. When using metallic and/or non-metallic reducing agents together with a sufficient amount of chromium compound to give approx. 1 wt% Cr, calculated on the weight of the carrier, is e.g. the parameters below representative. The atomic ratios are based on a calculation of the metal in the metallic reducing agent and/or the non-metal in the non-metallic reducing agent versus the chromium content of the chromium compound on the support.

For a catalyst material containing approx. 1 wt% Cr, calculated on the weight of the support, is thus the preferred amount of an organometallic reducing agent, such as triisobutylaluminum ("TIBAL"), approx. 11.4% by weight, which corresponds to an atomic ratio AI/CR of approx. 3/1. The preferred range for the Al/Cr atomic ratio is from 0.5/1 to 8/1, corresponding to from 1.9 to 30% by weight "TIBAL". The practical limits of the amount of "TIBAL", expressed in the atomic ratio Al/Cr, are from 0.1/1 to 20/1 and, expressed in weight%, from 0.4 to 75.

Another example of an organometallic reducing agent for use in connection with the present catalyst material is triethylaluminum. If here, too, the starting point is a catalyst material containing approx. 1 wt% Cr, calculated on the weight of the carrier, the preferred amount of triethylaluminum (TEA) is approx. 6.6% by weight, calculated on the weight of the carrier, which gives an atomic ratio Al/Cr of approx. 3/1. The preferred range for the Al/Cr atomic ratio is from 0.5/1 to 8/1, corresponding to from 1.1 to 18% by weight

(TEA). The practical limits for the amount of TEA are, expressed as the atomic ratio Al/Cr, from 0.1/1 to 20/1 and, expressed in weight%, from 0.22 to 44.

Triethylboron (TEB) may be selected as the preferred example of a non-metallic reducing agent for use in conjunction with the present catalyst material. If, in this case too, one relies on a catalyst material containing approx.

1 wt% Cr, calculated on the weight of the carrier, the preferred amount of TEB is approx. 5% by weight, calculated on the weight of the carrier, which gives an atomic ratio B/Cr of approx. 2.7/1. The preferred range for the B/Cr atomic ratio is from 0.1/1 to 10/1, corresponding to 0.19-19% TEB. The limits for the applicable quantities are, expressed in the atomic ratio B/Cr, from 0.01/1 to 20/1 and, expressed in weight%, from 0.02 to

38, calculated on the weight of the carrier.

The production of catalysts according to the invention is described in the following examples: 1. Production of non-volatile, aluminum-containing reaction products from aluminum sec.-butoxide and aluminum acetate

Example A: 4.43 g of aluminum acetate (21.7 mmol) was added to a flask along with 65.5 g of CH 2 Cl 2 . 5.34 g of aluminum sec.-butoxide

(21.7 mmol) was added, and the contents of the flask were heated at reflux under a nitrogen atmosphere. After heating with reflux for 3 hours, the clear, gray solution was stirred at room temperature overnight. The volatiles were removed in vacuo, yielding 9.30g of a clear, viscous residue (weight loss of 0.47g, calculated on the weight of non-volatile starting materials). Assuming no loss of aluminum , the residue contains the original ones

43.4 mmol Al.

Example B: 4.06 g (19.9 mmol) of aluminum acetate, 5.15 g (20.9 mmol) of aluminum sec.butoxide and 52.4 g of toluene were brought together as indicated above. Within 15 minutes from the start of heating with reflux cooling, the acetate had gone into solution,

and a pale yellow viscous solution was obtained. As the heating with reflux cooling continued, the color became more intense, while the viscosity decreased. After 6 hours of heating with reflux cooling, the volatile compounds were removed in vacuo, whereby 7.78 g of a gray, viscous residue (weight loss 1.4 3 g) containing 14.1% aluminum was obtained.

Example C: 6.49 g (31.8 mmol) of aluminum acetate, 7.92 g (31.8 mmol) of aluminum sec'-butoxide and 157.4 g of xylene were used. The acetate dissolved easily during heating with reflux cooling. The temperature of the condensing volatiles (peak temperature) dropped to 134°C during 5 hours of reflux heating. Distillation of 108 g of volatile material caused the reflux temperature to rise to 138.5°C. 76.7 g of fresh xylene was added, and heating with reflux cooling was continued. After 22 hours of heating with reflux cooling, 95.3 g of distillate was collected, and the remaining volatile compounds were removed in vacuo after cooling. The viscous,

gray residue weighed 9.13 g (weight loss 5.28 g) and contained 18.8% aluminum.

Example D: 4.74 g (23.2 mmol) of aluminum acetate, 11.45 g (46.4 mmol) of aluminum sec.-butoxide and 85.25 g of xylene were used. When the cloudy mixture was heated, the mixture assumed a yellow color. At the boiling point the turbidity disappeared rapidly and a clear yellow solution was obtained. After 5 hours of heating with reflux cooling, 26.7 g of distillate were removed (peak temperature increased from 135° to 138°C). No fresh xylene was added. After 22 hours of heating-refluxing, the mixture assumed a gray color, and a further 31.2 g of distillate was collected (peak temperature increased from 136.5° to 138°C). After cooling, the remaining volatile compounds were removed in vacuo. The viscous, grey-green residue weighed 12.7 g (weight loss 4.02 g) and contained 15.4% aluminium.

Example E: 6.23 g (30.9 mmol) of aluminum acetate, 15.22 g (61.8 mmol) of aluminum sec. "butoxide and 95.4 g of decalin were heated to reflux temperature. At the reflux temperature (peak temperature 153°C) a clear, yellow solution was obtained. After 2 hours of heating with reflux cooling, the yellow color had disappeared and a water-clear solution was obtained (peak temperature 150°C). After 5.5 hours heating with reflux cooling, 37.8 g of distillate was collected (the peak temperature increased from 150° to 189°C. After 22 hours of heating with reflux cooling, a gray solution was obtained. 54.8 g of distillate was collected (the peak temperature increased from 159° to 189° C).The remaining volatile compounds were removed in vacuo to give a white solid weighing 10.32 g (weight loss 11.22 g) and containing 24.2% aluminum.

The data for the reaction products from Examples A-E are listed in Table I below:

II. Preparation of heat-activated catalyst A

A silica gel with a pore volume of approx. 2.5 ml/g prepared

according to US. patent 3,652,215 is added to a 200 ml three-necked round flask fitted with a stirrer, nitrogen inlet and a Y-tube with a water cooler. The flask is kept under a nitrogen atmosphere during the deposition operation. Dichloromethane is then added to the flask containing the silica gel, and stirring is started to ensure uniform wetting of the gel. A dichloromethane solution of the product of the reaction between CrO^ and triethyl phosphate prepared as described in US. patent document 3,985,676 is then added to the flask in sufficient quantity to obtain a dry, coated catalyst with approx. 1 wt% Cr. The surface liquid is removed by filtration, and the coated gel is dried in a rotary evaporator at a temperature of up to approx. 4o°C, in a vacuum of 20-40mm Hg.

Dichloromethane is then added to the flask containing the chromium material deposited, on a carrier as described in the step above (approx. 8 g dichloromethane or g silica gel), and the stirrer is started to ensure uniform wetting of the gel. A solution of dichloromethane and the non-volatile reaction product from example A above is prepared in a pressure equalizing dropping funnel, and the dropping funnel is prepared the stirred flask is connected. The aluminum-containing solution is gradually added to the flask at a rate of approx. 10 g of solution per minute. After the addition of the solution has been completed, the slurry is stirred in the flask for approx. 1 hour. The surface liquid is removed by filtration, and the coated gel is dried in a rotary evaporator at temperatures up to approx. 40°C using a vacuum of 20-40 mm Hg. The amount of aluminum compound added depends on the desired percentage of aluminum, which in turn is conditioned by the desired properties of the olefin polymers to be produced using the finished catalyst.

To heat activate the catalyst material, the supported catalyst is placed in a cylindrical container, where it is fluidized with dry air at a linear velocity of 30-40 m per second. hour, while it is heated to a temperature of 900°C and held at this temperature for 6 hours. After cooling under a nitrogen atmosphere, the activated, carrier-deposited catalyst is obtained in the form of a powder.

In the same manner as described above, heat-activated catalysts IIB-IIE were prepared with each of the non-volatile aluminum-containing reaction products from Examples B-E.

The catalysts IIA IIE were used in the polymerization of ethylene with the results listed in the following table II: Reaction conditions

The polyethylene resins obtained by the polymerizations

for which the main data are given in Table II, were analyzed by gel permeation chromatography. The results are listed in

the following table III: 1 - Heat-activated product of the reaction between chromium oxide and triethyl phosphate and aluminum sec.-butoxide modifier deposited on silica gel to an aluminum content of 3.7% was used as catalyst.

2 - Mn is number average molecular weight.

3 - Mw is weight average molecular weight.

4 - R is the ratio Mw/Mn.

As the above data show, polyethylenes prepared using the present catalysts generally have narrower molecular weight distributions than the nolyethylenes prepared using the comparative catalyst, as shown by the R values in Table III. Moreover, each of the<p>olyethylenes produced using the new catalysts exhibits a set of properties that is significantly different from the properties of the product produced using the control catalyst, as Table III clearly shows. In order to substantiate the manner of the melt index MI and the narrow molecular weight distribution MWD and the reproducibility of the results obtained with the present catalyst systems, reaction product B was re-prepared and used for the preparation of an identical catalyst system.

Polymerizations were carried out on a somewhat larger scale. The polyethylene resins obtained using catalysts produced according to the invention were compared to polyethylenes produced with catalysts that have hitherto been used in olefin polymerization. The properties of the polyethylenes are given in Table IV.

The polyethylene resins prepared using the new catalysts (Examples 19 and 20) showed a significantly narrower molecular weight distribution MWD than the corresponding polyethylenes prepared using known catalyst systems (Examples 18 and 21), confirming the above results.

Claims (16)

1. Method for producing a catalyst system for olefin polymerization, characterized in that a) an organophosphoryl chromium reaction product of chromium trioxide and an organic phosphorus compound is deposited on a solid, inorganic carrier material, b) a non-volatile aluminum-containing product obtained by reacting an aluminum alkoxide with an aluminum carboxylic acid salt is deposited on said carrier material, and c) the organophosphoryl chromium reaction product and the non-volatile aluminium-containing reaction product deposited on the support material is heat activated in a non-reducing atmosphere at a temperature from 300°C up to the support material's decomposition temperature. 2. Method according to claim 1, characterized in that a compound of the formula is used as the organic phosphorus compound where R is alkyl, aralkyl, aryl, cycloalkyl or hydrogen, at least one of the substituents R being different from hydrogen. 3. Method according to claim 1 or 2, characterized in that the non-volatile aluminum-containing reaction product is first deposited on the support material and the thus treated support is heat activated in a non-reducing atmosphere at a temperature from 130°C up to the support's cleavage temperature, after which the organophosphoryl chromium reaction product is deposited on the support and the support thus treated is again heat activated but at a temperature from 300°C and up to the support's cleavage temperature. 4. Method according to claims 1-3, characterized in that a carrier material containing silica gel is used. 5. Method according to claim 4, characterized in that a silica xerogel with a surface area in the range from 200 to 500 m<2>/g and a pore volume greater than 2.0 ml/g is used as silica gel, with a larger proportion of the pore volume being made up of nores with a diameter in the range from 300 to 600 Angstrom units. 6. Method according to claims 1-5, characterized in that a phosphoryl chromium reaction product obtained by reacting chromium trioxide with triethyl phosphate is used. 7. Method according to claims 1-6, characterized in that a lower aliphatic monocarboxylic acid is used as carboxylic acid. 8. Method according to claims 1-7, characterized in that a non-volatile aluminum-containing product obtained by reaction of aluminum sec.-butoxide with aluminum acetate is obtained. 9. Method according to claims 1-8, characterized in that a metallic and/or non-metallic reducing agent is added to the heat-activated catalyst. 10. Method according to claim 8, characterized in that the reducing agent used is a metallic one chosen from triethylaluminium, triisobutylaluminium, alkylaluminium halides, alkylaluminium alkoxides, dialkylzinc, dialkylmagnesium and metal borohydride. 11. Method according to claim 9, characterized in that the reducing agent used is a non-metallic one selected from among alkylboranes and borohydrides. . 12. Method according to claim 11, characterized in that triethylborane is used as alkylborane. 13. Method according to claims 1-12, characterized in that the organophosphoryl chromium reaction product is used in a sufficiently large amount to give from 0.25 to 2.5 wt% Cr, calculated on the weight of the carrier material, and that the deposited, non-volatile aluminum-containing compound is used in an amount sufficient to provide from 0.10 to 10% by weight of Al, calculated on the weight of the support material. 14. Process for the polymerization of an olefin, characterized in that the olefin is brought into contact with a catalyst system prepared according to the method according to claims 1-13. 15. Method according to claim 15, characterized in that the hydrogen gas is present in the reaction medium. 16. Method according to claims 1-16, characterized in that the olefin is ethylene.