FR2672606A1 - Process for the preparation of a catalyst based on vanadium and titanium - Google Patents

Abstract

Process for the preparation, in a liquid hydrocarbon, of a catalyst of Ziegler-Natta type based on a vanadium compound and on a titanium compound, which is precipitated by a reduction reaction on a spherical magnesium chloride support. The precipitation is carried out by bringing (a) a reducing agent for vanadium and titanium, chosen from organometallic compounds, into contact with (b) a vanadium compound, (c) a titanium compound, both soluble in the liquid hydrocarbon, and (d) a particular spherical magnesium dichloride support and an electron-donor compound. The catalyst prepared is employed especially for manufacturing polyethylene and ethylene copolymers in suspension or in a gas phase.

Description

 The present invention relates to a process for preparing a Ziegler Natta type catalyst, based on vanadium and titanium compounds, supported on spheroidal particles of magnesium chloride. This catalyst is suitable for the polymerization of olefins such as ethylene and is particularly suitable for the manufacture of copolymers of ethylene1 in particular according to a gas phase copolymerization process.

 It is known that catalytic systems of the Ziegler Natta type consist of a catalyst comprising at least one compound of a transition metal, such as titanium, and of a cocatalyst comprising at least one organometallic compound of a metal. lightweight, such as aluminum. It is also known that the properties of these catalysts can be strongly influenced, when the transition metal compound is used with a support constituted by a solid mineral compound, such as magnesium chloride. In the technique for preparing a supported catalyst, the properties of the support and the process for preparing the catalyst, which generally consists in fixing the transition metal compound on said support, have a very great importance on the characteristics and the behavior of the catalyst. in a polymerization or copolymerization reaction of olefins.

According to European patent application EP-AO 099 772, it is known to prepare a catalyst by precipitation of a transition metal compound on a spherical support of magnesium chloride which comprises Mg-C bonded products and a donor compound of electrons in small proportion. The transition metal compound is a halogenated compound of titanium, and the precipitation of the latter on the support is carried out by a reduction reaction of the titanium compound by means of a reducing agent, such as an organometallic compound. This catalyst is used for the manufacture of polymers of ethylene.

However, it has been observed that this catalyst is not ideal for the production of ethylene copolymers having a relatively high melt index and / or a relatively high comonomer content. Furthermore, when these copolymers have a low density and a relatively low molecular weight, the powders
of copolymers obtained are sticky in nature which can disturb
their manufacture.

 According to European patent application EP-A-0155 770, it is known to prepare a catalyst by precipitation of a vanadium compound on a spherical support of magnesium chloride which comprises Mg-C bonded products and a donor compound d 'electrons in small quantity. The precipitation is carried out by a reduction reaction of the vanadium compound in the presence of this support, without addition of a reducing agent. Presumably, the reduction reaction is spontaneously initiated by the Mg-C bonded products contained in the support. The catalyst is used to manufacture polymers of ethylene with a broad molecular weight distribution. It has also been observed, however, that this process requires the use of a large amount of vanadium compound which only fixes low proportion on the support. Catalyst washing operations are generally necessary to remove excess vanadium compound which is not attached to the support, operations which are expensive and difficult because of the toxic and corrosive nature of the vanadium compounds.

 A process has now been found for manufacturing a spherical catalyst based on vanadium and titanium supported on magnesium chloride, a process which makes it possible to limit the drawbacks mentioned above. In particular, this process makes it possible to prepare a spherical catalyst based on a vanadium and titanium compound which has a high activity in the polymerization of olefins. This catalyst is particularly suitable for the manufacture of polymers of ethylene, in particular according to a gas phase polymerization process. In this case, the catalyst makes it possible to directly manufacture an ethylene polymer powder in the form of spherical and non-sticky particles, this powder having good flow properties and being easy to handle.

The subject of the present invention is a process for preparing a Ziegler Natta type catalyst based on vanadium and titanium compounds precipitated by reduction of these metals on a spherical support of magnesium chloride, process characterized in that it consists to put in contact within a liquid hydrocarbon
a) a reducing agent for titanium and vanadium chosen from
organometallic compounds, with
b) a vanadium compound and a titanium compound, both solu
in the liquid hydrocarbon and used in quantities
such as the molar ratio of the amount of titanium to that
of vanadium is greater than 1- and
c) a solid support comprising, in mole, from 80 to 99.5 9 of
magnesium dichloride substantially free from
Mg-C bonds and from 0.5 to 20% of at least one organic compound
electron donor, D, free of labile hydrogen, this support
consisting of spherical particles having a diameter
medium in mass, Dm, from 10 to 100 microns and a distribution
narrow particle size, such as the ratio of Dm to diameter
mean in number Dn of the particles is less than 2.

 According to the present invention, the preparation of the catalyst uses a particular support of magnesium chloride. The support is substantially free of Mg-C bonded products, which is equivalent to saying that the ratio of the number of Mg-C bonds to the number of magnesium atoms in the support is less than 0.001. The precipitation of the vanadium and titanium compounds on the support is therefore not spontaneously initiated by a reduction reaction of vanadium and titanium with a reducing agent contained in the support.

The reduction of vanadium and titanium is carried out by a reducing agent, chosen from organometallic compounds, which is brought into contact with the particular support of magnesium chloride and the compounds of vanadium and titanium. One of the surprising aspects of this preparation is linked to the fact that the vanadium and titanium compounds are very largely fixed on the solid support.

 The particular support of magnesium chloride comprises a relatively large amount of an organic electron donor compound, D. This characteristic contributes to favor the fixing of a large amount of the vanadium and titanium compounds in the support and to give to the catalyst a remarkable activity in the polymerization or copolymerization of olefins. The support comprises in mole of 80 to 99.5 r of magnesium dichloride and of 0.5 to 20% of compound D. Preferably, it comprises in mole of 80 to 95 49 of magnesium dichloride and of 5 to 20 % of compound D, and gives excellent catalysts based on vanadium and titanium for the polymerization of olefins. The best results are obtained in the manufacture of (co) polymers of ethylene, when the support used comprises in mole of 80 to 90% magnesium dichloride and 10 to 20% compound D.

 The organic electron donor compound, D, is known as such, or as a Lewis base. It is free of labile hydrogen and, therefore, cannot be chosen from water, alcohols or phenols, for example. It has a relatively low complexing power vis-à-vis magnesium dichloride. It is advantageously chosen from ethers, trioethers, sulfones, sulfoxides, phosphines, amines and amides. We prefer to use ethers.

 It has been found that the best results are obtained when the support is in the form of a homogeneous composition, that is to say a composition in which compound D is distributed in a homogeneous manner throughout the whole chloride particle. magnesium, from the heart to the periphery. As a result, to obtain such a support, it is recommended to prepare it by a method using precipitation. In this case, compound D is chosen from products capable of not reacting with the reagents used by the precipitation of the support.

 It has also been found that the support gives very efficient catalysts, capable of withstanding the enormous growth constraints during the polymerization, when it has an essentially amorphous structure, that is to say a structure where the forms of crystallinity have largely or otherwise completely disappeared. As a result, this particular form of the support can only be obtained by precipitation carried out under relatively precise conditions.

 The support is further characterized in that it consists of spherical particles having a mass average diameter of 10 to 100 microns, preferably 20 to 50 microns. The particles of the support have a very narrow particle size distribution, such that the ratio Dm / Dn of the mass average diameter, Dm, to the number average diameter, Dn, is less than 2. More particularly, the particle size distribution of these particles can be extremely narrow, such that the Dm / Dn ratio is 1.1 to 1.5; the practically total absence of particles with a diameter greater than 1.5 x Dm or less than 0.6 x Dm is noted; the particle size distribution can also be appreciated by the fact that more than 90% by weight of the particles of the same batch are included in the range Dm + 10%.

 The term “spherical particles” is understood to mean particles which are substantially spherical, that is to say the ratio of the major axis to the minor axis is equal to or less than about 1.3.

 The specific surface of the particles of the support can be from 20 to 100 m2 / g (BET), preferably from 30 to 60 m2 / g (BET), and the density of these particles can be from 1.2 to 2.1 approximately .

The support can in particular be prepared by reacting a dialkoylmagnesium compound with a chlorinated organic compound, in the presence of the electron donor compound, D. As the dialkoylmagnesium compound, one can choose a product of formula R Mg R in which
1 2 R and R are identical or different alkyl radicals, com 2 carrying from 2 to 12 carbon atoms. One of the important properties of this dialkoylmagnesium compound is to be soluble as it is in the hydrocarbon medium where the preparation of the support will be carried out. As the chlorinated organic compound, an alkyl chloride of formula is chosen.
R Cl in which R is a secondary or, preferably 3 3 rence, tertiary alkyl radical containing from 3 to 12 carbon atoms. It is preferred to use, as electron donor compound, D, an ether of formula
R OR in which R and R are identical alkyl radicals or
45 4 5 different having from 1 to 12 carbon atoms.

In addition, the various reagents used for the preparation of the support can be used under the following conditions
- the molar ratio R Cl / R MgR is 1.9 to 2.5, preferably 3 i 2
2 to 2.3,
- the D / R MgR molar ratio is 0.1 to 1.2, preferably ii 2
0.3 to 0.8.

The reaction between R MgR and R Cl, in the presence of the compound
123 electron donor, D, can be a precipitation which takes place with stirring, within a liquid hydrocarbon. The specialist knows that, in this case, physical factors, such as the viscosity of the medium, the mode and the speed of agitation and the conditions of use of the reagents can, all other things being equal, play an important role in the shape, structure, size and size distribution of the precipitated particles. However, to obtain an excellent support, characterized in particular by the presence of a large amount of the electron donor compound, D, it is recommended to carry out the reaction of precipitation at a relatively low temperature, ranging from 10 to 50 C, preferably from 15 to 35 "cl
Furthermore, it is recommended that the precipitation reaction takes place extremely slowly, for a period of at least 10 hours, preferably a period ranging from 10 to 24 hours, so as to allow proper organization of the solid product formed, in particular. in particular the insertion of a large amount of compound D and its uniform dispersion in the support thus formed.

 The process for preparing the catalyst consists in precipitating a vanadium compound and a titanium compound from a liquid hydrocarbon on the support of magnesium chloride. The liquid hydrocarbon can be one or more alkanes, such as n-hexane or n-heptane.

 The vanadium compounds and the titanium compound used for the preparation of this catalyst can have halogens, such as chlorine or bromine and alkoxyl groups so that the molar ratio of the alkoxyl groups to the halogens of these compounds is overall O to 5, preferably from O to 1. It has been observed that if this ratio is too high, the catalyst obtained has too low an activity to be used for the polymerization or copolymerization of olefins.

The vanadium compound is soluble in the liquid hydrocarbon and is generally a compound in which the vanadium is at its maximum valence, that is to say at valence 4, or in which the vanadyl group VO is at its valence maximum, that is to say at valence 3. As a vanadium compound, a compound having one of the two general formulas V (OR) X or VO (OR) X can be used
4-mm 3-nn in which R represents an alkyl group containing from 1 to 12 carbon atoms, X a halogen atom such as bromine or chloride, m an integer or fractional number ranging from O to 4 and n a whole or fractional number ranging from 0 to 3.

 Advantageously, one or more compounds chosen from vanadium tetrachloride, vanadyl trichloride, vanadyl tri-n-propoxide, vanadyl triisopropoxide, vanadyl tri-n-butoxide, tetra-n-butoxide can be used. vanadium and vanadium tetra-n-propoxide.

The titanium compound is also a product soluble in a liquid hydrocarbon and is generally a compound in which the titanium is at valence 4. As the titanium compound, a compound having the general formula Ti (OR ") X may be used in which R "
4-qq is an alkyl group containing from 1 to 12 carbon atoms, X is a halogen atom such as bromine or chlorine and q is an integer or fractional number ranging from O to 4. Among these compounds, one can use titanium tetrachloride, titanium tetraisopropoxide, titanium tetra-n-propoxide, or titanium tetra-n-butoxide.

 The amounts of vanadium and titanium compounds used to prepare the catalyst depend on the amounts of vanadium and titanium to be fixed in the support. Generally, the amount of titanium compound to be used during the preparation of the catalyst is 0.05 to 2 moles, preferably 0.1 to 1 mole per mole of support magnesium.

 The amount of the vanadium compound used is in particular such that the molar ratio of the amount of titanium used to that of vanadium is greater than 1 and preferably greater than 2. Surprisingly, it has been observed that when the catalyst has been prepared with such proportions of vanadium and titanium compounds, this catalyst becomes particularly suitable for manufacturing in the gas phase a non-tacky ethylene polymer powder, having good dry flow properties. It has also been observed that an ethylene copolymer prepared with such a catalyst has a relatively narrow distribution of molecular weights variable according to the molar ratio of the amount of titanium to the amount of vanadium in the catalyst. The remarkable properties of this catalyst in polymerization also result from the particular support used, in particular from its spherical shape and from the presence of a large amount of the electron donor compound, D, in the support.

 The process for the preparation of the catalyst consists essentially in precipitating the vanadium and titanium compounds on the support by a reduction reaction of vanadium and titanium using a reducing agent chosen from organometallic compounds, in particular from the compounds organometallics of metals belonging to groups II or III of the periodic table. It is preferred to use organoaluminum, organomagnesium or organozinc compounds. In particular, a trialkyl aluminum, such as triethyl aluminum or triisobutyl aluminum, or preferably an alkyl aluminum halide, such as diethyl aluminum chloride, can be used. The reducing agent can be used in a relatively small amount, in particular to prevent the catalyst from bursting into fine particles during its preparation. Generally the amount of reducing agent used during the preparation of the catalyst is from 0.05 to 2 moles, preferably 0.1 to 1 mole per mole of magnesium dichloride of the support. Furthermore, the catalyst can be prepared in the presence of an additional quantity of an electron donor compound identical or different from that contained in the support.

 The preparation of the catalyst within the liquid hydrocarbon is carried out by bringing the support into contact with the vanadium and titanium compounds and the reducing agent at a temperature which may be from 0 to 120 ° C., preferably from 50 to gO "C. The contacting can last approximately from 0.5 to 15 hours.

 In practice, the preparation of the catalyst can be carried out in various ways. One can, for example, simultaneously add the reducing agent, the vanadium compound and the titanium compound to a suspension of the magnesium chloride support in the liquid hydrocarbon. The mixture thus prepared can be kept stirred for a period of 0.5 to 15 hours. Another method may consist in adding, successively in one order or another, the reducing agent, the vanadium compound and the titanium compound to the magnesium chloride support suspension. In particular, in order to increase the amount of vanadium and titanium fixed in the support, it may be preferable to first bring the reducing agent into contact with the suspension of support of magnesium chloride, then add the vanadium compound and the titanium compound to this suspension. In the latter case, after bringing the reducing agent into contact with the support, the latter can be washed using a liquid hydrocarbon.

Advantageously, the vanadium and titanium compounds can be mixed before they are brought into contact with the support.

Although most of the amount of the vanadium compound and that of the titanium compound used is fixed in the support, the prepared catalyst can be washed one or more times using a liquid hydrocarbon.

 It is surprisingly observed that during the preparation of the catalyst, the morphology and the essentially amorphous structure of the support do not change. Thus, the catalyst obtained consists of particles whose physical properties are practically identical to those of the particles of the initial support. In particular, the catalyst consists of spherical particles, having a mean diameter by mass of 10 to 100 microns, preferably from 20 to 50 microns, and a very narrow particle size distribution, measured by the ratio of the mean diameter by mass to the mean diameter in number less than 2.

 The advantage of this preparation process is linked to the fact that the major part of the vanadium compound is of the titanium compound is fixed in the support. Generally it is found that more than 80% and even more than 90% of the vanadium and titanium compounds are fixed in the support.

Another characteristic of this process is to be able to fix the vanadium and titanium compounds in a homogeneous manner throughout the support, thus making the catalyst more solid later during a polymerization. Indeed, the vanadium and titanium compounds are distributed in a homogeneous manner in each support particle, from the core to the periphery thereof. It is noted that the organic electron donor compound, D, initially present in the support decreases appreciably in the catalyst. This suggests that the vanadium and titanium compounds can become fixed in the support where compound D leaves a vacant place. In addition, it is noted that the catalyst contains part of the reducing agent used during precipitation, but in a form transformed by the reduction reaction. Thus the catalyst obtained can contain per mole of magnesium dichloride from 0.01 to 1 mole of vanadium, essentially in the reduced state, from 0.01 to 1 mole of titanium essentially in the reduced state, from 0.01 to 0.1 mole of the organic electron donor compound,
D, and from 0.05 to 1 mole of reducing agent in a form transformed by the reaction of the reducing agent.

 The catalyst thus obtained may contain from 1 to 15% by weight of vanadium and titanium. It can be used to polymerize or copolymerize under industrial conditions olefins having from 2 to 12 carbon atoms, such as ethylene, propylene, butene-1, hexene-1, methyl-4 pentene-1 , or octene-1 It is particularly suitable for the manufacture of so-called high density homopolyethylenes and copolymers of ethylene and alpha-olefins containing from 3 to 12 carbon atoms which may contain by weight up to 30% of units derived from alpha-olefins. Among these copoly mothers, depending on the content of alpha-olefins, we can distinguish the so-called high density polyethylenes, whose density is greater than 0.935, the linear low density polyethylenes having a density ranging from 0.915 to 0.935 and the very low polyethylenes linear densities having a density greater than 0.880 and less than 0.915.

The polymers can in particular be manufactured in suspension or in the gas phase in a fluidized bed reactor and / or mechanically stirred. The catalyst is used in the presence of a cocatalyst, chosen from the organometallic compounds of a metal belonging to groups II to III of the Periodic Table of Elements and optionally in the presence of an activator chosen from halogenated hydrocarbons. The catalyst and the cocatalyst are generally used in proportions such that the molar ratio of the amount of metal of the cocatalyst to the amount of vanadium and titanium in the catalyst is between 0.5 and 50. The (co) polymerization reaction can be carried out at a temperature of between approximately 0 ° C. and 120 ° C., preferably between 60 ° C. and 100 ° C., under a total pressure ranging from 0.1 to 5 MPa. The catalysts prepared according to the invention can be used in the state or after having undergone an olefin prepolymerization operation, carried out in one or more stages in the gas phase and / or in suspension in a liquid hydrocarbon medium. The prepolymerization operation results in increasing the particle size of the while retaining the morphology of the latter, which consists in bringing the catalyst and the cocatalyst into contact with one or more olefins. be continued while maintaining a suitable activity in the catalyst until 10 to 500g and preferably 30 to 250 g of polyolefin per millimole of transition metal.

During the (co) polymerization reaction, a regular development of the (co) polymer particles is observed, the spheroidal shape of which is preserved and the particle size distribution remains narrow. One can obtain in particular an ethylene polymer, consisting of a powder which consists of spheroidal particles and which has good dry flow properties and a high bulk density, generally between 0.3 and 0.5
3 g / cm. The polymer has a relatively narrow distribution of molecular weights, characterized by a ratio of the weight average molecular weight, Mw, to the number average molecular weight, Mn, of between 3 and 11 and more particularly between 3.5 and 7. Furthermore, it may contain a very low content of vanadium and titanium, generally less than 15 parts by weight per million.

i Diamètre moyen en nombre

Method for determining the mean diameters in mass (Dm) and in number (Dn) of particles
According to the invention, the mean diameters in mass (Dm) and in number (Dn) of the support or catalyst particles are measured from microscopic observations. The principle of the measurement consists in obtaining, from the experimental study by optical microscopy of a population of particles, a table of numbers where is given the number (n) of particles belonging to each class (i) of diameters , each class (i) being characterized by an intermediate diameter (d) lying between the limits of said class. According to the French standard approved NF X 11-630 of June 1981, Dm and Dn are provided by the following formulas i Average diameter in mass :

i Number average diameter

The Dm / Dn ratio characterizes the particle size distribution; it is sometimes called "particle size distribution width". The measurement by the OPTOMAX image analyzer is carried out using an inverted microscope which allows the examination of the suspensions of
o support particles, or catalyst particles with a magnification between 16 and 200 times. A television camera takes the images given by the inverted microscope and transmits them to a computer which analyzes the images received line by line and point by point on each line, in order to determine the dimensions or diameters of the particles, then to classify them.

Measuring the distribution of molecular weights
The molecular weight distribution of a copolymer is calculated according to the ratio of the average molecular weight by weight,
Mw, at number average molecular weight, Mn, of the copolymer, from a molecular weight distribution curve obtained using a WATERS 150 gel permeation chromatograph
(R)
C (High Temperature Size Exclusion Chromatograph), the operating conditions being as follows
- solvent: trichloro-1,2,4 benzene
- solvent flow: 1 ml / minute
(R)
- three SHODEX AT 80 M / S columns
- temperature: 1500C
- sample concentration: 0.1% by weight
- injection volume: 500 microliters
- detection by a refractometer integrated into the chromatograph
- calibration using high density polyethylene sold by
(R)
BP CHEMICALS SNC under the trade name RIGIDEX
6070: Mw = 65,000 and Mw / Mn = 4, IF = 6 and a polyethylene
2
high density having: Mw = 210,000 and Mw / Mn = 17.5.

 The following nonlimiting examples illustrate the invention.

Example 1
Preparation of a magnesium chloride support
In a 5 1 stainless steel reactor, fitted with a stirring system rotating at 325 revolutions / minute and containing 2 moles of dibutylmagnesium dissolved in 3 1 of n-hexane, the following are introduced at ambient temperature (20 ° C.), and under a nitrogen atmosphere, 204 ml (1 mole) of diisoamyl ether (EDIA). The reactor is maintained at 25 C. 484 ml (4.4 moles) of tert-butyl chloride are introduced therein over 12 hours. The mixture is then kept under stirring for 3 hours at 25 "C. The solid product obtained is washed four times with 2 liters of n-hexane. This gives 2 moles of magnesium chloride in the form of spherical particles of diameter medium = 35 microns of particle size distribution Dm / Dn = 1.6 and having an EDIA / Mg molar ratio = 0.15 and a Cl / Mg molar ratio = 2.

Example 2
Preparation of a catalyst
In a glass reactor of one liter, equipped with a stirring system rotating at a speed of 250 revolutions per minute, a suspension of the support (A) prepared at room temperature and under a nitrogen atmosphere Example 1, containing 0.1 mole of magnesium dichloride in 150 ml of n-hexane. 100 ml of a solution containing 0.2 mol / liter of diethylaluminum chloride in n-hexane are added to this suspension, kept stirred and at room temperature (25 "C) over one hour. then stirred for 2 hours at 3500. At the end of this time, the suspension kept stirred is heated to a temperature of 50 "C. and in 4 hours, 50 ml of a solution containing 0.16 mol / liter of trichloride are simultaneously added of vanadyl in n-hexane and 50 ml of a solution containing 0.24 mol / liter of titanium tetrachloride in n-hexane. A new suspension is thus obtained which is kept stirred for a period of 2 hours at a temperature of 80 C. Then the stirring is stopped and the catalyst (B) obtained is allowed to settle.

After elimination of the supernatant liquid phase, the catalyst (B) is subjected to 2 successive washes using each time 500 ml of n-hexane at 50 ° C., then at ambient temperature to 3 successive washes using each time 500 ml of n-hexane. The catalyst is isolated and stored under a nitrogen atmosphere. Its characteristics are as follows
- Vanadium / Magnesium molar ratio: 0.055
- Titanium / Magnesium molar ratio: 0.090
33
- molar ratio V i / Magnesium: 0.12
- Aluminum / Magnesium molar ratio: 0.12
Example 3 Preparation of a catalyst
The procedure is exactly as in Example 2, except that after adding the solution of diethylaluminum chloride in n-hexane to the suspension of magnesium chloride, 50 ml of a solution containing 0.08 mol is used / l of vanadyl trichloride instead of 0.16 mole / l in n-hexane and 50 ml of a solution containing 0.32 mole / l instead of 0.24 mole / l of titanium tetrachloride in n The characteristics of the catalyst obtained are as follows:
- Vanadium / magnesium molar ratio: 0.03
- Titanium / Magnesium molar ratio: 0.13
3+ 3+
- molar ratio V + Ti / Magnesium: 0.145
- Aluminum / Magnesium molar ratio: 0.13
Example 4 1. Preparation of an ethylene prepolymene
2 liters of n-hexane, 12 millimoles of sorting are introduced into a 5-liter stainless steel reactor fitted with a stirring system rotating at 750 revolutions per minute and maintained at 60 "C. -n-octylaluminium and an amount of catalyst prepared in Example 3, containing 4 millimoles of transition metal (vanadium and titanium), then 1 liter of hydrogen, measured under normal temperature and temperature conditions, is introduced into the reactor. pressure, then for 4 hours of ethylene at a constant flow rate of 80 g / h. At the end of this time, and after returning to room temperature, the contents of the reactor are transferred to a rotary evaporator to remove all of the solvent Under these conditions, a dry prepolymer is recovered which is in the form of a powder having good flow properties and which is stored under nitrogen.

2. Polymerization of ethylene in the gas phase
200 g of a stainless steel reactor fitted with a stirring device for dry powder, rotating at a speed of 250 revolutions per minute, are introduced under nitrogen atmosphere as feed powder, 200 g of an inert and perfectly anhydrous polyethylene powder, 2 millimoles of triethylaluminum, an amount of the prepolymer previously prepared containing 0.1 millimole of transition metal, and finally of hydrogen so as to obtain a relative partial pressure of hydrogen of 0.15 MPa. The reactor is then heated to 80 ° C. and ethylene is introduced into it so as to obtain a relative total pressure of 0.5 MPa which is kept constant throughout the polymerization reaction. After 4 hours of reaction, 700 g of polyethylene having the following characteristics are recovered
- transition metal content: 10 ppm
- apparent density: 0.44 g / cm3
- melt flow index measured at 1900C under a load of 2.16 kg
- IF / 190: 1 g / 10 minutes 2, s6
Example 5 1. Preparation of an ethylene prepolymer
The procedure is exactly as in Example 4, except that the catalyst prepared in Example 2 is used instead of the catalyst prepared in Example 3.

2. Copolymerization of ethylene and butene-1 in the gas phase
Into a 2.5 liter stainless steel reactor, fitted with a stirring system for dry powder, rotating at 250 revolutions per minute, 150 g of a polymer powder originating from a previous polymerization and having been stored under nitrogen, 2 millimoles of triethylaluminum then an amount of prepolymer prepared previously and containing 0.1 millimole of transition metal (vanadium and titanium).

The reactor is then heated to 80 ° C. and a quantity of hydrogen is introduced into it so as to obtain a relative pressure of 0.012 MPa and a gas mixture containing 95% ethylene and 5% butene-1 in mole so as to a total relative pressure of 0.4 MPa is obtained. During the reaction, this mixture of ethylene and butene-1 is introduced into the reactor so as to maintain a constant pressure. After 3 hours of reaction, 680 g of a polymer powder having the following characteristics - density: 0.916 - apparent density: 0.41 g / ml - melt index measured at 1900C under a load of 2.16 kg
3.1 g / 10 minutes - mass diameter of the particles Dm: 420 microns - ratio Dm / Dn of the mean diameter in mass to the mean diameter in
number: 2

Claims (1)

CLAIMS 1. Process for the preparation of a Ziegler-Natta type catalyst based on vanadium and titanium compounds precipitated by reduction of these metals on a spherical support of magnesium chloride, process characterized in that it consists in contact within a liquid hydrocabure a) a reducing agent for titanium and vanadium chosen from organometallic compounds, with b) a vanadium compound and a titanium compound, both of which are soluble in liquid hydrocarbon and used in amounts such that the molar ratio of the amount of titanium to that of vanadium is greater than 1, and  c) a solid support comprising from 80 to 99.5% dichlo by mole  magnesium rure substantially free from loose product  Mg-C sounds and from 0.5 to 20% of at least one organic compound  electron donor, D, free of labile hydrogen, this support  consisting of spherical particles having a diameter  medium in mass, Dm, from 10 to 100 microns and a distribution  narrow particle size, such as the ratio of Dm to diameter  mean in number Dn of the particles is less than 2. 2. Method according to claim 1, characterized in that the support comprises in mole from 80% to 95% of magnesium dichloride and from 5 to 20% of the organic electron donor compound, D. 3. Method according to claim 1, characterized in that the vanadium compound and the titanium compound have alkoxyl groups OR and halogens X so that the molar ratio of the alkoxide groups to the halogens of these compounds is generally from 0 to 5. 4. Method according to claim 3, characterized in that the vanadium compound corresponds to one of the two general formulas  V (OR) X or VO (OR) X  4-mm 3-nn formulas in which R represents an alkyl radical having from 1 to 12 carbon atoms, X a halogen atom, m an integer or fractional number ranging from O to 4 and n an integer or fractional number ranging from O to 3. 5. Method according to claim 3, characterized in that the titanium compound corresponds to the general formula Ti (OR) X in 4-p P which R represents an alkyl radical having from 1 to 12 carbon atoms, X an atom d 'halogen and p a whole or fractional number ranging from 0 to 4. 6. Method according to claim 1, characterized in that the reducing agent for vanadium and titanium is chosen from organoaluminum, organomagnesium and organozinc compounds. 7. Method according to claim 1, characterized in that 0.05 to 2 moles of the reducing agent are brought into contact per mole of magnesian dichloride of the support. 8. Method according to claim 1, characterized in that 0.05 to 2 moles of the titanium compound are brought into contact per mole of magnesian dichioride of the support. 9. Method according to claim 1, characterized in that the contacting is carried out at a temperature ranging from 0 C to 1200C for a period ranging from 0.5 to 15 hours. 10. Use of the catalyst according to any one of claims 1 to 9, for the manufacture of homopolyethylenes and of copolymers of ethylene and of alpha-olefins comprising from 3 to 12 carbon atoms.