GB2098228A - Olefin polymerisation catalyst - Google Patents

Abstract

A polymerisation catalyst is prepared under anhydrous conditions by sequentially (A) heating at 300 to 1200 DEG C a hydroxyl-containing refractory oxide support with a titanium compound, (B) impregnating the product with a chromium oxide or a compound calcinable thereto and a titanium compound and (C) heating at 250 to 1200 DEG C to give an active polymerisation catalyst. The refractory oxide can be, for example, silica or alumina and the titanium compounds are preferably alkyl titanates. The chromium compound can be for example tertiary butyl chromate. The catalyst can be used to prepare ethylene homo- and copolymers having a wide range of melt indices and a broad molecular weight distribution.

Description

SPECIFICATION Olefin polymerisation catalyst and process The present invention relates to a catalyst for polymerising 1 -olefins, to a polymerisation process employing the catalyst and to the polyolefin produced therefrom.

Modern low pressure processes for manufacturing polyethylene or copolymers of ethylene with other 1-olefins are capable of producing a wide range of grades of polyolefins having specified physical properties rendering each grade suitable for a particular application. For example, a grade of polyethylene suitable for injection moulding will normally have a fairly high "melt index", i.e. a relatively low average molecular weight, and a narrow molecular weight distribution, whereas a blow moulding or film grade will normally have a lower melt index and a broader molecular weight distribution.

In the well-known Phillips and Ziegler catalysed processes for polymerising 1 -olefins, the average molecular weight of the produced polyolefin can be controlled by a variety of techniques, for example, by adjusting the polymerisation temperature or by using a chain transfer agent in the polymerisation reaction, whereas the molecular weight distribution of the produced polymer is generally dependent on the nature of the catalyst employed.

UK patent No. 1429174 filed in the name of BP Chemicals International Limited relates to a polymerisation catalyst prepared by (I) heating together a support material comprising silica, alumina, zirconia or thoria or composites thereof and a defined titanium compound at a temperature in the range 1 50 tQ 1200 C and (II) incorporating in the product from step (I) under substantially anhydrous conditions a chromium compound and heating at a temperature in the range 100 to 1 2000C to produce an active polymerisation catalyst. Catalysts of this type can be used to polymerise 1-olefins, e.g.

ethylene, to form solid mouldable polymers having a wide range of melt indices and a relatively broad molecular weight distribution.

It is an object of the present invention to provide an improved catalyst and polymerisation process for the production of ethylene homopolymers and copolymers having a wide range of melt indices and a broad molecular weight distribution.

Accordingly the present invention provides a polymerisation catalyst prepared under substantially anhydrous conditions by a process comprising the following sequential steps: (A) heating together at a temperature in the range 300-1 2000C a refractory oxide support material having surface hydroxyl groups and a titanium compound, (B) impregnating the product from step (A) with an anhydrous chromium compound which is a chromium oxide or a compound calcinable thereto and with a titanium compound which may be chemically the same as or different from the titanium compound employed in step (A), and (C) heating the impregnated material at a temperature in the range 250 to 1 2000C to produce an active polymerisation catalyst.

In a preferred embodiment according to the present invention the catalyst is additionally modified by treating the active catalyst obtained from step (C) with an organometallic compound or an organoboron compound.

The refractory oxide support material can be, for example silica, silica-alumina, silica-titania, alumina, zirconia or thoria. Silica is preferrred, particularly silica having a mean particle diameter in the range 20 to 1 50 microns; and a surface area in the range 1 50 to 800 square metres per gramme. The support material must be substantially dry, i.e. free from unbound water.Adequate dryness of the support material can be achieved, for example, by heating the support material "in vacuo" at temperatures in the range 30-1 500C until further heating under these conditions produces no substantial loss in weight If desired, the product from step A can be treated with water or an alcohol to rehydroxylate the surface of the support material, the water or alcohol then being removed and step (A) being repeated on the rehydroxylated support material.

The heating and rehydroxylation steps can be carried out more than once if desired. The process of heating the "primary" support material with titanium and the rehydroxylation can be carried out, for example, as disclosed in UK patent specification No. 1495547.

The titanium compound employed in step (A) of the present invention can be any titanium compound capable of reacting with the surface hydroxyl groups of the support material at a temperature in the range 300--12000C. Examples of suitable titanium compounds are titanium oxychloride (TiOCI2).

titanium acetylacetonate compounds, alkanolamine titanates, titanium pi-bonded complexes (e.g.

dicyclopentadienyl titanium) and compounds having the general formulae Ti (OR)mXn, wherein m + n is 4, m is zero or an integer from 1 to 4, R is an organic hydrocarbon group having 1 to 12 carbon atoms, X is halogen or a hydrocarbon group and when the titanium compound contains more than one R or X group the groups may be the same or different. Tetravalent titanium compounds are preferred. When the titanium compound employed in step (A) of the present invention is Ti(OR)mXn, R is preferably selected from alkyl, aryl, cycloalkyl and combinations thereof, for example aralkyl and alkaryl, each group having from 1 to 12 carbon atoms and X is preferably selected from R, cyclopentadienyl, alkenyl and halogen.

Titanium compounds represented by the formula (RO)4Ti are preferred, particularly the alkyl compounds having from 1 to 6 carbon atoms in each alkyl group, for example tetraethyl titanate and tetraisopropyl titanate. The titanium acetyl acetonate compounds can be, for example, titanium diacetylacetonate di-isopropylate, titanium dichlorodiacetylacetonate or the so called "titanium acetyl acetonate" or "titanyl acetylacetonate". The alkanolamine titanate can be for example triethanolamine titanate.

The quantity of titanium compound employed in step A of the present invention is suitably sufficient to give a titanium concentration (derived from said titanium compound) in the support material in the range 0.05 to 20 wt%, preferably 0.5 to 5 wt %, based on the weight of the solid product from step (A).

The titanium compound is preferably added to the support material in step (A) in a form in which it becomes well dispersed. For example if the titanium compound is liquid it can be mixed with the support material as such, if desired. If it is a liquid or a solid it can be dissolved in a suitable nonaqueous solvent or comminuted in a non-aqueous diluent and then mixed with the support material.

Alternatively, the titanium compound can be added as a vapour if it is volatile, or carried into the support material as an aerosol in a suitable carrier gas, for example nitrogen. The titanium compound can, if desired, be formed "in situ" on the support material. For example if it is desired to employ titanium tetramethylate as the titanium compound in step (A), the support material may be impregnated with titanium tetracloride and subsequently heated with methanol until the reaction between the methanol and the titanium tetrachloride has gone substantially to completion. The progress of this reaction can be monitored, for example, by distilling the evolved hydrogen chloride from the reaction vessel and titrating with standard alkali.

The refractory oxide support material and the titanium compound are preferably heated together in step (A) at a temperature in the range 400--10000C, most preferably 500-9000C for a period of time which can range from a few minutes to several hours. The heating may be carried out, for example, by heating in a fixed bed or in a fluidised bed. Preferably the heating is carried out in a bed fluidised by dry air.

In step (B) the anhydrous chromium compound employed in the present invention can be chromium oxide (i.e. CrO3) or a compound calcinable thereto, for example chromium nitrate, chromium carbonate, chromium acetate, ammonium chromate, chromyl chloride, tertiary butyl chromate, chromium acetylacetonate or bis (cyclopentadienyl) chromium. Ditertiary butyl chromate is preferred.

The quantity of chromium compound supported on the solid product from step (A) is suitably such as to provide a chromium concentration of at least 0.1%, preferably in the range 0.2-30 wt%, most preferably 0.5 to 5 wt% based on chromium compound and the final catalyst solid together.

The titanium compound employed in step (B) is suitably selected from the classes of titanium compound listed above as being suitable for use in step (A). The titanium compounds selected in steps (A) and (B) for making a particular catalyst according to the present invention may be chemically the same or different The quantity of titanium compound employed in step (B) is suitably sufficient to provide a titanium concentration in the active catalyst obtained from step (C) in the range 0.05 to 20 wt%, preferably 0.5 to 5.0 wt%, the wt% in this case being based on the actual amount of titanium introduced in step (B) [i.e. excluding the amount introduced in step (A)], and the total weight of the solid catalyst obtained from step (C).

The chromium compound and the titanium compound employed in step (B) may be impregnated into the product obtained from step (A) either simultaneously or separately.

The impregnation of the product from step (A) with a chromium compound can be achieved, for example, by dissolving a soluble chromium compound in an anhydrous volatile liquid, impregnating the product from step (A) with the solution and evaporating the solvent; by impregnating the product from step (A) with a liquid chromium compound, e.g. chromyl chloride; by passing the vapour of a volatile chromium compound, e.g. chromyl chloride, into a bed of the product from step (A); orby mixing together a finely divided chromium compound and said product in the presence of a small quantity of non-aqueous solvent, continuing the mixing until a substantially homogeneous mix is obtained and then evaporating the solvent Examples of solutions that can be used to impregnate the support material are tertiary butyl chromate/hexane, and chromyl chloride/chloroform.

The impregnation of the product from step (A) with the titanium compound can be achieved, for example, by mixing a liquid titanium compound, or a non-aqueous solution of a liquid or solid titanium compound with the product from step (A), or with the product from step (A) after it has been impregnated with chromium. Alternatively, for example, the vapour of a volatile titanium compound may be impregnated into the product from step (A) or into a fluidised bed thereof.

The product from step (B) is heated in step (C) to a temperature in the range 250 to 12000 C, preferably 350--9000C, most preferably 400-7000C to produce an active polymerisation catalyst.

The heat activation is preferably carried out for a period of time in the range 5 minutes to 24 hours, most preferably 30 minutes to 10 hours. It is preferred to employ heating conditions in step (C) which favour the formation of, or retention of, chromium in its hexavalent state. The heating is preferably carried out in a non-reducing atmosphere and most preferably in an oxidising atmosphere or in vacuo.

Dry air is an example of a suitable oxidising atmosphere. The heating must be performed under anhydrous or dehydrating conditions and the activated catalyst must be protected from ingress of moisture.

The activated catalyst produced from step (C) may be used as such to polymerise 1-olefins or may be modified by the addition of an organometallic or organoboron compound. Organometallic or organoboron compounds that can suitably be used to modify the catalyst of the present invention have the general formula MRaY,~, wherein M is a metal of groups 1 A, 2A, 2B or 3A of the Periodic Table (Mendeleef) or boron, R2 is a hydrocarbon group containing 1 to 10 carbon atoms, Y is hydrogen, halogen or alkoxide, q is the valency of M, p is an integer from 1 to q inclusive and when the organometallic or organoboron compound contains more than one R2 or Y group they must be the same or different.

The term "organometallic compound" as employed hereinafter includes a reference to organoboron compounds, and similarly unless indicated to the contrary the term "metal" includes boron although it is appreciated that boron is, strictly speaking, a metalloid.

In the organometallic compound in the preferred embodiment of the present invention the R2 group is preferably an alkyl, cycloalkyl or aryl group. Metal alkyls are particularly preferred.

The metal present in the organometallic compound is preferably lithium, sodium, beryllium, magnesium, calcium, zinc, cadmium, boron, aluminium or gallium. Metals alkyls particularly preferred are dibutyl magnesium, triethyl boron, triethyl aluminium, triisobutyl aluminium. The quantity of organometallic compound employed is suitably 0.1 to 100%, preferably 1 to 10 wt% based on the total weight of catalyst.

The organometallic compound can be added to the active catalyst per se, or to the active catalyst in the presence of a substance or substances forming or intended to form the ingredients of an olefin polymerisation reaction. For ease of handling, it is preferred to add the organometallic substance to the active catalyst, or to a polymerisation system employing the active catalyst, in solution in a hydrocarbon solvent, e.g. n-hexane.

If desired, the catalyst according to the present invention may be modified with the organometallic compound and further modified with a polyene compound. When such polyene compound is employed it is suitably a C1 to C20 conjugated or non-conjugated polyene. Preferably the polyene contains 2 double bonds. Examples of suitable polyenes are butadiene, isoprene, 1,5-hexadiene, 1,4-hexadiene myrcene, cyclo-penadiene, dicyclopentadiene and ethylidene norbornene.

The quantity of polyene employed is suitable from 0 to 50 parts by weight, preferably 0 to 10 parts by weight per unit weight of catalyst.

The present invention further provides a process for polymerising monomeric 1 -olefin comprising contacting the monomeric 1 -olefin under polymerisation conditions with the catalyst of the present invention, optionally modified with organometallic compound or with organometallic compound and a polyene.

The monomeric 1-olefin employed in the polymerisation process of the present invention is preferably ethylene or a mixture of ethylene with one or more other comonomer 1 -olefins. When comonomer 1-olefins are employed they preferably comprise up to 40 wt%, most preferably up to 25 wt% of the total monomer. Examples of comonomer 1-olefins are propylene, 1 -butene, 1 -pentene, 1-hexane and 4-methyl-1-pentene.

The polymerisation conditions employed in the process of the present invention can be, for example, any of the conditions used in the well-known Phillips polymerisation processes described, for example, in UK patent specification 790195; 804641; 853414; 886784; and 899156.

Preferably the polymerisation conditions are the so called "particle form" process conditions. In the "particle form" process the monomeric 1-olefin is contacted with a suspension or a fluidised bed of the catalyst particles in a fluid medium under conditions such that the polymeric 1 -olefin forms as solid particies suspended in orfluidised in the fluid medium.

The fluid medium employed in particle form process conditions can be a liquid or a gas. Preferably it is a liquid. Examples of suitable liquid media are hydrocarbons which are chemically inert and nondeleterious to the modified catalyst under the reaction conditions. Preferred liquid media are paraffins or cycloparaffins having 3-30 carbon atoms per molecule, for example isopentane, isobutane, cyclohexane. Most preferably the liquid medium is isobutane.

When a liquid medium is employed in the process of the present invention preferably the concentration of monomer therein is in the range 2-30 wt% although concentrations outside this range can be employed if desired.

When the process of the present invention is under particle form process conditions the polymerisation temperature is preferably in the range 50 to 1 120C, most preferably 80 to 1 080C.

The polymerisation pressure is preferably in the range 2 to 100 bar when the fluid medium is a liquid and 1 to 60 bar when the fluid medium is a gas. The residence or reaction time can vary from a few minutes to several hours and is generally in the range 1 5 minutes to 3 hours. The particle form process can be conducted under batch or continuous polymerisation conditions. Preferably the conditions are continuous. Preferred apparatus for conducting the reaction under continuous conditions in a liquid medium is described in UK patent specification 899156.

For further details of examples of solution form and particle form process conditions and apparatus which can suitably be employed in the process of the present invention, reference may be made to UK patent specification Nos: 790195; 804641; 899156; 886784 and 853414.

If desired, the polymerisation process can be conducted in the presence of hydrogen gas to increase the melt index of the produced polymer. In general, the higher the partial pressure of hydrogen in the reaction zone, the lower becomes the molecular weight of the produced polymer.

Polyethylene and ethylene copolymers produced by the process of the present invention generally have a relatively broad molecular weight distribution rendering them suitable for a variety of applications, for example blow moulding and film-blowing. The polymers are generally produced at high productivity using the catalyst of the present invention so that for most applications it is generally unnecessary to remove catalyst residues from the polymer.

The invention is illustrated by the following Examples.

EXAMPLE 1 (a) Catalyst preparation In step A, silica support material having surface hydroxyl groups (commercially available from Davison Chemical Co. as "951 silica") (30 g) was dried overnight at 1 500C and 18" Hg vacuum, and then slurried in 40-60 petroleum ether (1 50 ml) under a nitrogen blanket. Titanium tetraisopropylate (3.7 g, 3.8 ml) was added with stirring and the petroleum ether then removed in a rotary evaporator.

The solid product, still under nitrogen, was calcined for 5 hours at 6500C in a bed fluidised with 900 ml/min of dry air. The calcined solid contained approximately 2% by weight titanium.

In step (B) the calcined solid produced from step (A) was cooled under nitrogen and slurried in 40-60 petroleum ether (1 50 ml). It was then impregnated with 1% by weight chromium as t-butyl chromate with the petroleum ether again being removed on the rotary evaporator. The solid was then reslurried under nitrogen in 40-60 petroleum ether (1 50 ml) before being impregnated with titanium tetraisopropylate (7.4 g, 7.7 ml) to give another 4% by weight titanium on the catalyst. In step (C) the product was finally activated by heating for 5 hours at 5000C in a bed fluidised with 900 ml/min of dry air. The activated catalyst was then stored under nitrogen. The activated catalyst contained 1% by weight Cr and 6% by weight Ti.

(b) Polymerisation Polymerisations were carried out in a 2.3 litre stainless steel stirred autoclave. The reactor was purged with nitrogen, baked out for 2 hours at 1 1 OOC, then cooled to the required polymerisation temperature. The catalyst prepared as above was charged to the reactor followed by 1 litre of dry isobutane containing 0.2 ml of a 10% solution of triethyl aluminium in n-hexane.

The reactor was maintained at the required polymerisation temperature and ethylene was added to bring the total pressure inside the reactor to 41.4 bar. Ethylene was then added continuously throughout the reaction to maintain this pressure. Polymerisation and polymer property data are given in Table 1.

EXAMPLES 2-9 (a) Catalyst preparation The method of preparation was as described in Example 1 except that a range of calcination temperatures of between 6500C and 8000C was used, the level of titanium added in step (B) was between 1% and 4% by weight, and activation temperatures of between 400"C and 5000C were used.

Full details of these conditions are given in Table 1.

(b) Polymerisation All polymerisations were carried out as described in Example 1.

The activity of the catalyst is quoted as kilograms of polyethylene per kilogram of catalyst per hour. The Kd value is determined by a method similar to that given in Sabia, R, J Appl. Polymer Sci.

1963, 7, 347. Kd is a measure of polymer shear response and, generally, Kd increases with breadth of polymer molecular weight distribution. The My2.16 is the 'melt index' and the My21.6 the 'high load melt index' determined according to ASTM method 1238 using 2.16 kg and 21.6 kg loads respectively. The units are grammes per 10 minutes.

TABLE 1

Catalyst Polymerisation Polymer Properties Step A Ti-Step B Step C Catalyst Time Temp. Activity Density Ex. ( C) (wt.%) ( C) (g) (minst) ( C) (kg/kg.h) MI2-16 MI21-6 Kd (kg/m ) 1 650 4 500 0.302 57 104 1429 0.19 13.5 7.3 958 2 650 4 400 0.299 60 104 1238 0.18 16.4 9.1 959 3 650 1 500 0.300 60 104 1167 0.12 10.4 6.5 959 4 650 1 400 0.314 36 104 2229 0.05 5.0 9.0 957 5 800 4 500 0.327 56 104 1740 0.11 9.5 7.4 960 6 800 4 400 0.319 60 104 1104 0.14 12.8 9.3 958 7 800 1 500 0.325 44 103 1687 0.12 10.6 6.7 957 8 800 1 400 0.302 50 104 1502 0.12 14.2 6.0 958 9 750 2 475 0.375 40 104 1656 0.13 9.3 7.5 957 Organometallic compound: 56 mg/g catalyst of triethylaluminium in Examples 1-8.

45 mg/g catalyst of triethylaluminium in Examples 9.

Claims (22)

1. A polymerisation catalyst prepared under substantially anhydrous conditions by a process comprising the following sequential steps: (A) heating together at a temperature in the range 300-1 2000C a refractory oxide support material having surface hydroxyl groups and a titanium compound, (B) impregnating the product from step (A) with an anhydrous chromium compound which is chromium oxide or a compound calcinable thereto and with a titanium compound which may be chemically the same as or different from the titanium compound employed in step (A), and (C) heating the impregnated material at a temperature in the range 250 to 1 2000C to produce an active polymerisation catalyst.

2. A polymerisation catalyst as claimed in claim 1 wherein the heating in step A is carried out at a temperature in the range 400--10000C.

3. A polymerisation catalyst as claimed in claim 1 or 2 wherein the heating in step A is carried out at a temperature in the range 500900 C.

4. A polymerisation catalyst as claimed in any one of the preceding claims wherein the refractory oxide support material is silica, silica-alumina, silica-titania, alumina, zirconia orthoria.

5. A polymerisation catalyst as claimed in any one-of the preceding claims wherein the titanium compound employed in step A is titanium oxychloride, a titanium acetylacetonate compound, an alkanolamine titanate, a titanium pi-bonded complex or a compound, having the general formula Ti(OR)mXn wherein m + n is 4, m is zero or an integer from 1 to 4, R is an organic group having 1 to 12 carbon atoms and X is halogen or a hydrocarbon group.

6. A polymerisation catalyst as claimed in any one of the preceding claims wherein the titanium compound employed in step A is a tetraalkyl titanate.

7. A polymerisation catalyst as claimed in any one of the preceding claims wherein the quantity of titanium compound employed in step A is sufficient to give a titanium concentration in the support material in the range 0.5 to 5 wt% based on the weight of the solid product from step A.

8. A polymerisation catalyst as claimed in any one of the preceding claims wherein the anhydrous chromium compound employed is ditertiary butyl chromate.

9. A polymerisation catalyst as claimed in any one of the preceding claims wherein the titanium compound employed in step B is titanium oxychloride, a titanium acetylacetonate compound, an alkanolamine titanate, a titanium pi-bonded complex, or a compound having the general formula Ti(OR)mXn wherein m + n is 4, m is zero or an integer from 1 to 4, R is an organic group having 1 to 12 carbon atoms and X is halogen or a hydrocarbon group.

10. A polymerisation catalyst as claimed in any one of the preceding claims wherein the titanium compound employed in step (B) is a tetraalkyl titanate.

11. A polymerisation catalyst as claimed in any one of the preceding claims wherein the quantity of titanium compound employed in step (B) is sufficient to provide a titanium concentration in the active catalyst obtained from step (C) in the range 0.5 to 5.0 wt%, the wt% in this case being based on the actual amount of titanium introduced in step (B) (excluding the amount added in Step (A), and the total weight of solid catalyst obtained from step (C).

12. A polymerisation catalyst as claimed in any one of the preceding claims wherein the heating in step (C) is carried out at a temperature in the range 400--7000C.

1 3. A polymerisation catalyst as claimed in any one of the preceding claims wherein the active catalyst obtained in step (C) is modified by the addition of an organometallic compound or an organoboron compound.

14. A polymerisation catalyst as claimed in claim 13 wherein the organometallic or organoboron compound is dibutyl magnesium, triethyl boron, triethyl aluminium or triisobutyl aluminium.

1 5. A polymerisation catalyst as claimed in claim 13 or 14 wherein the catalyst is further modified by the addition of a polyene compound.

1 6. A polymerisation catalyst as claimed in claim 1 5 wherein the polyene is a C1 to C20 conjugated or non-conjugated diene.

17. A polymerisation catalyst substantially as described in any one of the Examples.

1 8. A process for polymerising monomeric 1-olefin comprising contacting the monomeric 1-olefin under polymerisation conditions with the polymerisation catalyst claimed in any one of the preceding claims.

19. A process as claimed in claim 1 8 wherein the monomeric 1 -olefin is ethylene or a mixture of ethylene with one or more comonomer 1 -olefins.

20. A process as claimed in claim 1 8 or 19 wherein the polymerisation conditions are particle form process conditions.

21. A process for polymerising ethylene substantially as described in any one of the Examples.

22. Polyolefin prepared by the process claimed in any one of claims 1 8-21.