The present invention relates to a process for preparing polymerization catalyst of olefins in accordance with the preamble of claim 1.

A catalyst prepared according to the invention contains a catalytically active chromium compound bonded to an inorganic support material. The preparation comprises vaporizing a suitable chromium compound and reacting the reagent vapours with an inorganic support at a temperature above the condensation temperature of the chromium compound.

The conventional Phillips-type polymerization catalyst, invented at the beginning of the 1950's, contains chromium(VI)oxide adsorbed on a silica gel support. A prior art method for the preparation of such chromium-containing catalysts includes the impregnation of an inorganic support with a chromium compound, typically an aqueous solution of chromium trioxide, followed by calcination of the support by heating in-an oxidizing atmosphere at an elevated temper¬ ature (400 °C or higher) . Finally, the catalyst is activated by reduction with hydrogen, carbon monoxide or ethene, whereby unsaturated chromium compounds, which are active during polymerization, are formed on the support material surface.

A drawback of the conventional preparation method of catalysts is the need for several different phases of preparation. The preparation of the catalyst is extremely sensitive to processing conditions and thus requires strict control.

Another drawback is associated with the use of solvents. Namely, the solvents used in preparation often are capable of affecting the support material thus causing changes in its surface structure.

In addition to the above-mentioned drawbacks, impurities are contained in solvents used in the preparation that can adversely affect the performance of the catalyst.

Further, in connection with the above-described preparation methods of heterogeneous catalysts it is difficult to control the adsorption of catalytically active materials onto the support material surface, particularly when the surface of particle in the powdered support material used is structurally nonhomogeneous.

Due to these factors, the prior art methods are not easily applicable to a controlled adsorption of a metal or metal compound.

Moreover, in the prior art methods, the dispersion of catalytically active materials on the support material may be poor.

Also known in the art are methods for preparing chromium containing heterogeneous catalysts by adsorbing chromium onto the surface of an inorganic support from the vapour phase.

Thus, US Patent Specification No. 4,362,654 teaches a method for depositing metallic chromium in a vacuum evaporator onto a silica surface. The catalyst obtained is subsequently activated by heating in an oxidizing atmosphere at a temperature in the range from 300 to 1000 °C to achieve an activated polymerization catalyst.

The above-described method has several drawbacks. For instance, metal is not selectively bonded to the support but instead, the chromium is deposited by condensation onto the support material particles. Chromium adsorption on bonding sites available in the pores is not controllable, because the actual chemical bond between the chromium metal and the support is obtained only by postactivation. The regulation

of the catalyst chromium content is not easily achieved, since a major part of the evaporated metal is condensed onto the walls of the reaction vessel. Therefore, the method fails to solve the above-mentioned problems.

Described in the US Patent Specification No. 4,439,543 and in the paper by McDaniel, M.P., Journal of Catalysis, 26. (1982), pp. 17 - 28, is a method based on the injection of a known quantity, 0.5 - 4 ml, of chromyl chloride solution as such or diluted with carbon tetrachloride into a gas stream, and further, routing the stream onto a silica gel support. The silica gel is heated in oxidizing conditions at 400 - 1000 °C in order to remove a portion of the hydroxyl groups. The injected chromyl halide is reacted with silica so as to deposite both chromium and halogen onto the catalyst.

Finally, the chromium is subjected to reduction by carbon monoxide, which was found to improve the catalyst activity.

The present invention is based on the concept of introducing the precursor of the catalytically active metal in vapour phase in an equal, or preferably, in excess amount in relation to the number of available binding sites on the support material surface. The support temperature is maintained above the condensation temperature of the vapour and simultaneously sufficiently high so as to attain the thermal activation energy necessary for generation of chemical bonds between the catalytically active material or its compound and the support material.

A method of the foregoing type is described in the Finnish Patent Specification No. 84608. This kind of a method differs from the prior art methods in several respects. The chemical bonds are formed between the metal or metal compound and the support, while no physically adsorbed metal or metal compound exists on the support material surface during the process. This goal is achieved by selecting so high a reaction temperature that no metal or metal compound exists in liquid or solid state. The invention also differs

from the prior art since the formation of the surface bond is determined by the content or the number of surface sites available to the catalytically active component or its precursor in the gas phase. The metal compound is used in at least an equal amount and, preferably, in excess of the available binding sites on the support material surface, contrastingly to the metering of a certain amount of chromium compound as necessary in the US Patent No. 4,439,543 and the impregnation methods. A third essential difference from conventional technology is that the method in accordance with the present invention makes it possible advantageously to prepare catalysts in which two or more catalytically active components are bonded from vapour phase onto the support material surface. Even the adsorption order of the components can be interchanged.

The FI Patent Specification No. 84608 discloses the use of chromyl chloride as a chromium containing reagent. Surprisingly, it has now been found that particularly good results are obtained when, instead of chromyl chloride, chromium acetylacetonate is employed as a volatile regent. In the following, the abbreviation Cr(acac) ₃ will be used for the said compound.

More specifically, the process in accordance with the invention is mainly characterized by what is stated in the characterizing part of claim 1.

Interestingly, it has been discovered that by bonding the catalytically active compound in the above-described manner onto the support, a heterogeneous catalyst can be achieved whose activity at even very low contents of catalytic metal is as high as the activity of such a catalyst of multiple content of catalytically active metal that has been prepared by way of conventional methods. Compared to catalysts prepared by using chromium chloride as a volatile reagent, Cr(acac) ₃ will provide polymerization catalysts having higher activities.

According to the invention, all reagents necessary for bonding the metal compound or its precursor are reacted in the reaction chamber in gaseous state, typically only one at a time. The vapour pressure of the evaporated Cr(acac) ₃ is here maintained sufficiently high and the duration of interaction with the surface of the support material sufficiently long so as to provide the compound in at least equal, preferably in excess amount in relation to the number of binding sites available on the support material surface. Naturally, the actual number of binding sites available on the support material surface itself is a crucial factor. This parameter can be modified by heat pretreatment methods.

The reaction temperature must not drop below the temperature necessary for evaporation of the compound in order to avoid condensation or sublimation of the compound. Further, the condensation of the compound on its way to the reaction space is prevented by keeping the feed piping temperature at least at the reaction temperature.

The temperature used is selected so as to avoid decomposition of Cr(acac) ₃ and subsequent condensation of the decomposition products.

Using experimental methods, it is possible to determine a temperature range, or temperature span, in which the reac¬ tion is advantageously carried out. The lower limit of such a temperature span is determined by the condensation temperature of the Cr(acac) ₃ at the applied pressure and by the activation energy necessary for establishing the desired surface bond. Obviously, the condensation temperature of the compound is not applicable as the lower temperature limit if the condensation temperature does not impart the reagent a sufficient energy to exceed the activation energy. The upper limit is determined by the temperature at which the compound chemisorbed on the support starts to show an essential rate of desorption, that is, when the equilibrium of the chemisorption-desorption reaction has shifted toward

desorption. Since the activation and desorption energies are not generally known, the appropriate temperature must be experimentally determined.

When chromium acetylacetonate is used as the reagent and silica (silicon dioxide) as the support material, it is preferred to operate the method at a temperature in the range from 160 to 300 °C. According to a preferred embodiment, the temperature is kept at 180 to 280 °C. The pretreatment to be described below is carried out at a temperature in the range from 200 to 900 °C. According to a preferred embodiment the pretreatment is performed at a higher temperature than the actual reaction, typically at about 400 to 900, whereas the raction is conducted at 180 to 200 °C.

Since, according to the invention, the number of binding sites affects the amount of active metal bonded to the support, pretreatment and/or intermediate treatment of the support is an essential factor. The pretreatment can be accomplished before the support material is loaded into the reaction vessel, or alternatively, in the reaction vessel itself prior to introducing the vapour of Cr(acac) ₃ into the reaction space.

The number of available binding sites can be affected by, e.g., a heat treatment which controls, among other factors, the number of hydroxyl groups on the support material surface, or alternatively, by chemical processing, for instance, a treatment with water vapour which increases the number of hydroxyl groups. These treatments which modify the support material can be carried out when desired during any process step prior to the removal of the catalyst from the reaction space. It is also possible to combine said treatments, for instrance, by treating the support material with water or water vapour after a heat treatment in order to introduce new hydroxyl groups on its surface.

The preheating can be performed by conventional means, one of which is heating in an air stream at, e.g., 200 to 900 °C

The number of binding sites for Cr(acac) ₃ can also be reduced by using a blocking agent which binds to isolated hydroxyl groups. Thus, for instance, hexamethyl disilazane (HMDS) may be used as a blocking agent. In the case of HMDS, it is preferred to operate the process in an inert atmosphere. According to a preferred embodiment, the HMDS treatment is carried out after preheating of the support.

The proportion of excess active material in relation to the concentration necessary to achieve a full saturation of all available binding sites on the support material surface

(customarily called a monolayer coverage) is typically 1.5 to 1000-fold, preferably 2- to 100-fold. The amount of chromium acetylacetonate necessary for a monolayer coverage can be calculated from the area of the support material determined with the help of, e.g., the BET method, and from the known molecular structure of the support material surface.

According to the method in accordance with the invention, the reaction between the vapour of Cr(acac) ₃ and the support can be carried out in air, in an inert gas atmosphere or, alternatively, under reduced pressure using advantageously an inert gas atmosphere of 1 to 10 mbar pressure. Equally, the optional pre- and posttreatment steps can be performed in the same conditions as the actual reaction. A benefit to be gained from the use of a partial vacuum is that the reaction space is kept cleaner and the diffusion rate is increased. The reaction time is principally affected by the diffusion of vapour molecules into the support material pores. The diffusion of the vapour molecules between the support material particles is very rapid in comparison with their diffusion into the pores. Therefore, the reaction time must be sufficiently long to allow the vapour containing the

metal or metal compound to reach adequate interaction with the binding sites of the support. In the tests performed, a reaction time of 1 to 4 h proved sufficient for the purpose.

The evaporated reagent can be brought to the reaction space as such, or alternatively, using an inert carrier gas such as nitrogen or noble gases.

In the catalyst according to the invention, the support materials advantageously used are inorganic oxides such as silicon oxide (silica) , aluminium oxide, thorium oxide, zirconium oxide, magnesium oxide, titanium oxide, or mixtures thereof.

The catalysts in accordance with the invention can be catalysts containing chromium alone, or alternatively, catalysts containing aluminium compounds in addition to the chromium. The chromium and aluminium compounds can be introduced in a free order. Thus, the aluminium compounds, such as aluminium trichloride, can be introduced before or after the bonding of Cr(acac) ₃ .

The excess of the chromium acetylacetonate used in each reaction step is removed, still in gas phase, before the next step by, e.g., flushing the reaction space with an inert gas. As mentioned above, it is essential that condensation of the reagent be avoided at all times during the process.

After bonding of the active metal compound, the ligands of the bonded chromium compounds are removed, e.g., by heat treatment. In most cases, no separate heat treatment will be needed, but the ligands are removed in connection with a typical activation operation, which comprises heating the catalyst in an oxidizing atmosphere at 400 to 1000 °C. Preferably, the activation is carried out in the same reaction vessel as the reaction, while it is also possible to perform the activation in a separate reaction vessel

later at any instant prior to polymerization. The oxidizing conditions can be attained with the help of an oxygen- containing gas such as air.

In the following, the invention will be examined in more detail with the help of working examples.

Example 1A

Silica support material (EP 10, Crosfield Catalysts) was loaded in an amount of 5 to 8 g into a reaction vessel after preheating in air for 16 h at different temperatures. Physisorbed water, if any, was removed by heating in an inert atmosphere (nitrogen flow) in a fixed support bed at a pressure in the range from 50 to 120 mbar chromium acetyl¬ acetonate was vaporized at 180 to 200 °C and conducted to the reaction vessel during 1 to 3 h in nitrogen flow. The reaction vessel was kept at a temperature of 200 °C during the bonding of Cr. After the reaction, the support bed was purged with nitrogen for 1 to 2 hours to remove any unreacted reagent.

The samples thus prepared were further calcinated at atmospheric pressure in a fludized bed in air or nitrogen/air atmosphere at different temperatures.

The obtained catalysts were used for ethene polymerization at 105 °C temperature and a total pressure of 4000 kPa in an autoclave of 3 liter volume. The hydrocarbon diluent in the polymerization process was isobutane. Table 1A presents the processing conditions of the catalyst and the polymerization results.

Table 1A. Properties of catalysts prepared according to the invention and polymerization results

MIR ₂₁ = high load melt index

Activity calculated as polyethene/g catalyst*h

Example IB (reference)

Polymeration catalysts containing chromium were prepared as explained in example 1 of FI Patent Specification No. 84608.

The obtained catalysts were used for ethene polymerization as explained in example 1A. The catalyst characteristics and polymerization results are shown in table IB below.

Table IB. The activities of catalysts prepared according to the prior art

Example 2

The number of binding sites for the Cr compound was reduced by using a blocking agent known to bind to isolated hydroxyl groups. Thus, Cr acetylacetonate was bonded to silica EP 10 preheated at 200 °C and treated with the blocking reagent HMDS (hexamethyl disilazane) at 175 °C. The Cr/silica sample was calcinated in the same reaction vessel at a pressure in the range from 80 to 130 mbar with dry synthetic air in a fixed bed.

Table 2 shows the processing parameters of the catalyst, Cr concentration and polymerization results.

Table 2. Processing parameters of the catalysts

Sample Preheating Activation Cr Cr(VI) PE MIR ₂₁ τ/°c τ/°c wt-% wt-% g/g*h

I 200 650 0.7 0.6 3100 3.8

Example 3

Catalysts were prepared as in Example 1A except that the activation or calcination of the catalysts was carried out in the same reaction vessel and not in a separate fluidized bed as in Example 1A.

The obtained catalysts were for ethene polymerization as described in Example 1A.

Tables 3A and 3B below present the processing conditions of the catalysts and the polymerization results. Table 3A illustrates the effect of Cr(acac)3 reaction temperature on the properties of the catalysts. The samples were pre- calcined at 560 °C and oxidation was carried out at 400 °C.

Table 3A. Effect of Cr(acac)3 reaction temperature on the properties of the catalysts

MI = melt index

FRR = flow rate ratio

Table 3B illustrates the effect of the silica precalcination (pretreatment) temperature on the properties of the catalysts. The reactions between the supports and the chromium reagents were carried out at 200 °C.

Table 3B. Effect of silica precalcination temperature

MFR = melt flow ratio

The results in Tables 3A and 3B show that when using relatively mild activation conditions it is possible to affect the product properties by changing the pretreatment temperature.