FR2671802A1 - Preparation of a catalyst based on a chromium oxide compound and use of the catalyst - Google Patents

Abstract

Process for the preparation of a catalyst based on a chromium oxide compound and used in combination with a granular support based on refractory oxide. This process is characterised in that, in a first stage, the granular support is impregnated with at least one organometallic compound of a metal belonging to group IIa or IIIa of the Periodic Classification of the elements and with at least one chromium compound, and in that, in a second stage, a heat treatment is carried out to activate the impregnated support obtained at the end of the first stage, so as to obtain an active catalyst for the polymerisation of alpha -olefins. The invention also relates to the use of the catalyst obtained for the polymerisation or copolymerisation of alpha -olefins, especially ethylene.

Description

 The present invention relates to a process for the preparation of a catalyst based on a chromium oxide compound, combined with a granular support based on refractory oxide and activated by a heat treatment. It also relates to the use of this catalyst for the polymerization or copolymerization of alphaolefins, and more particularly of ethylene.

 It is already known to polymerize ethylene, alone or in admixture with other alpha-olefins in the presence of catalysts such as taking a chromium oxide compound, associated with a refractory oxide-based granular support and activated by heat treatment. These catalysts, used in the polymerization of alpha-olefins and in particular ethylene, have been described in numerous patents; such as para. for example, British Patent Nos. 790,195 and 804,641.

 It is known that these catalysts can be prepared by depositing a chromium compound such as a chromium oxide, or a compound that can be converted by calcination into chromium oxide, on an oxide-based granular support. refractory, then by activation using a heat treatment-at a temperature of at least 25O0C and at most equal to the temperature at which the granular support begins to sinter, so that at the end of the heat treatment the chromium compound is at least partially in the hexavalent state. There are also many methods for modifying these catalysts including incorporating titanium compounds, fluorine or organometallic compounds.

 In particular, according to British Patent GB 1429174, it is known to prepare a catalyst based on a chromium oxide compound and containing titanium by means of a process in which a support based on a refractory oxide is subjected to a first heat treatment in the presence of a titanium compound, then impregnated with a chromium compound and finally subjected to a second thermal treatment, called activation, to produce an active catalyst.According to the patent application. EP-364 635 it is also known to modify a catalyst based on a chromium oxide compound associated with a granular support, by combining it with an oxide of a member of group IIa of the periodic table. elements, after having been previously activated using a heat treatment. However, the two catalysts obtained by the processes described above do not make it possible to manufacture polyethylenes having a low melt index and a narrow molecular weight distribution. Furthermore, they also do not allow to manufacture polyethylenes having a high impact resistance.

 The present invention relates to a process for producing a catalyst based on a chromium oxide compound and associated with a granular support. The catalyst obtained is suitable for the polymerization of alpha-olefins and in particular makes it possible to manufacture polyethylenes having a low melt index and a molecular weight distribution that is narrower than that obtained with a chromium oxide catalyst of composition. identical but prepared according to another method. Moreover, the polyethylenes obtained have a better impact resistance.

The subject of the present invention is therefore a process for preparing a catalyst based on a chromium oxide compound and associated with a granular support based on refractory oxide, characterized in that in a first stage the granular support with at least one organometallic compound of a metal belonging to the group
IIa or IIIa of the periodic table of the elements and with at least one chromium compound and in that in a second step an activation heat treatment of the impregnated support obtained at the end of the first step is carried out.

 The present invention is therefore based on the surprising principle that the catalyst must contain a granular support, chromium and a metal of the fIa or IIIa group, and that before use it must be activated by a heat treatment whereas it contains in advance these three constituents. In the opposite case, if it is activated when it does not contain these three constituents, it has been observed that a catalyst is obtained which does not have the desired properties and in particular which is not very suitable for the manufacture of polyethylene with high impact resistance.

 During the first step of the process of the invention, it is essential to prepare a granular support impregnated with at least one chromium compound and at least one organometallic compound. This impregnated support may be prepared by means of one or more impregnation operations, an impregnation operation of bringing the granular support into contact with the compound to be impregnated on the granular support. In particular, it may be prepared by placing in contact with the granular filler simultaneously with the chromium compound and the organometallic compound or with a mixture of these two compounds prepared beforehand. It can also be prepared by two contacts made one after the other, for example by putting the granular support successively in contact with the organometallic compound, then with the chromium compound or vice versa with the compound of chromium then with the organometallic compound.

 An impregnation operation is generally carried out at a temperature between 0 and 1000 C, preferably between 15 and 9500C and lasts from 2 to 120 minutes, preferably from 5 to 90 minutes. It can be carried out dry for example by introducing the compound to be impregnated in a fluidized bed containing the granular support maintained in the fluidized state by means of an upward flow of a gas. This fluidizing gas is a dry gas, ie containing less than 10 and preferably less than 2 volumes per million of water and is inert with respect to the compound to be impregnated. Most often, nitrogen is used. The compound to be impregnated can be introduced into the fluidized bed as such, in suspension or in solution in a linear or cyclic hydrocarbon having 3 to 12 carbon atoms, such as benzene, toluene, xylenes, heptane, n-hexane or cyclohexane. A solution or a suspension may contain from 0.01 to 1 mole per liter of compound to be impregnated, as well as optionally a solubilizing agent of the compound to be impregnated, such as an alcohol. Most often it is considered that an impregnation is substantially complete when all the compound to be impregnated is totally introduced into the fluidized bed reactor: Moreover, an impregnation operation can also be carried out in a solvent of the compound to be impregnated, maintained This solvent is preferably volatile and is evaporated before or during any heat treatment of the granular support.

The refractory oxide granular support can be, for example, silica, alumina, zirconium oxide, thorium oxide, titanium oxide or mixtures or coprecipitates of two or more of these oxides. It can be obtained by various known methods, in particular by precipitation of the silicon compounds, such as silica, from an alkali metal silicate solution, or by coprecipitation of a gel or a hydrogel. refractory oxide from solutions comprising at least two compounds selected from compounds of silicon, titanium, zirconium, thorium or aluminum. Such processes are in particular described in US Pat. No. 4,053,436. and 4,111,722;
The granular support advantageously has a specific surface
2 (BET) of between 50 and 1000 m 2 / g, a pore volume ranging from 0.5 to 5 ml / g and may consist of particles having a mean diameter by mass of between 30 and 250 microns, preferably between 50 and 200 microns. It has hydroxyl functions and is preferably free of free water at the time of its use in the preparation of the catalyst. For this, prior to the process of the invention, it can be freed from free water by means known in the art. themselves, such as a heat treatment at a temperature between 100 0C and 9500C.

The chromium compound may be a chromium oxide, for example, of the formula Cr0, or a chromium compound which can be converted
3 by calcination with chromium oxide during a heat treatment carried out under a non-reducing atmosphere, such as, for example, a chromium nitrate or sulphate, an ammonium chromate, a chromium carbonate, a acetate or acetylacetonate or a chromate tert-butyl. It is used in an amount of 0.005 to 1 millimole, preferably 0.02 to 0.5 millimole of chromium per gram of granular support. This quantity is impregnated with the granular support by means of a. or several impregnations.

 The organometallic compound of the group IIA or IIIA metal is preferably a magnesium, boron or aluminum compound.

Most often an organoaluminium compound is used, such as trinoctylaluminum, or tri-n-butylaluminium or an organomagnesium compound such as diisobutylmagnesium, di-n-butylmagnesium, diethylmagnesium and di (methyl-2, pentoxy) magnesium. It is also possible to use triethylboron. This organometallic compound is used in an amount of from 0.05 to 4 millimoles, preferably from 0.2 to 2 millmoles of metal per gram of granular support. This quantity is impregnated with the granular support in one or more times.

 Furthermore, prior to the catalyst activation step, the granular support can be impregnated with other compounds in addition to the organometallic compound and the chromium compound.

In particular it can be impregnated with one or more compounds of a transition metal, different from chromium, belonging to the groups
IV, V or VI of the periodic table of elements such as titanium, vanadium, hafnium or zirconium. This transition metal compound can be used in an amount representing from 0.05 to 4 and preferably from 0.2 to 2 millimoles of transition metal per gram of granular support: According to a particularly advantageous variant of the process of the When several successive impregnation operations are carried out, an intermediate thermal treatment of the granular support can be carried out between two impregnation operations. This intermediate treatment is intended to improve the performance of the final catalyst and in particular to improve the performance of the final catalyst. impregnation then the fixing of the various compounds on the granular support.-This intermediate treatment is most often carried out at a temperature ranging from 400 to 1000 ° C., preferably from 700 to 950 ° C., and lasts approximately from 10 minutes to 24 hours, preferably from 30 minutes to 8 hours, in an indifferent manner it is carried out under a reducing, inert or oxidizing atmosphere. This atmosphere is a dry atmosphere, ie containing less than 10 and preferably less than 2 volumes per million (vpm) of water vapor. This treatment is generally carried out in a fluidized bed reactor equipped with a medium This is heated and contains the granular support in the fluidized state by means of an upward circulation of a gas. This fluidization gas is preferably nitrogen. By way of example, in order to obtain the impregnated support, the granular support can be successively impregnated with the organometallic compound, then subjected to an intermediate heat treatment and then impregnated with the chromium compound.

In a second step of the process, the support impregnated with at least. organometallic compound and the chromium compound, must be subjected to a heat treatment, called activation, under conditions such that it is converted into a catalyst-active for the polymerization of alpha-olefins. This activation treatment is most often carried out at a temperature of at least 250 ° C. and at most equal to the temperature at which the granular support begins to sinter. The activation temperature is generally between 2500C and 1200 oC and preferably between 350.et.10000C. In general, the lower the activation temperature, the higher the molecular weight of the polymer obtained with this catalyst. The duration of the activation treatment can be between 5 minutes and 24 hours, preferably between 30 minutes. and 15 hours. At the end of the activation treatment it is preferred that the chromium be at least partially in the hexavalent state. However, an active catalyst is nevertheless obtained if the chromium is in a state of valence lower than 6. The activation treatment is carried out under a non-reducing atmosphere, preferably under an oxidizing atmosphere. It is also possible to operate under vacuum. In general, the activation treatment is carried out under an oxidizing atmosphere consisting of a gaseous mixture comprising oxygen, such as, for example, air. Furthermore, the activation treatment is carried out under an anhydrous atmosphere and the resulting catalyst must be kept away from moisture.
According to one variant of the process, the activation treatment can be carried out in the presence of fluorine compounds chosen from ammonium hexafluorotitanate, tetrafluoroborate and hexafluorosilicate, and optionally in the presence of a titanium compound chosen from This method advantageously makes it possible to increase the activity of these catalysts or to modify the properties of the polyolefins produced. For example, US Pat. No. 3,130,188 describes a supported catalyst based on chromium oxide. catalyst being obtained in particular by the so-called activation operation in the presence of a fluorine compound, such as an ammonium fluorosilicate. Another method, described in British Patent No. 1.391.77t, consists in carrying out the so-called activation operation of the supported catalyst based on chromium oxide in the presence, on the one hand, of a titanium compound, such as a titanium alkoxide, for example titanium tetraisopropoxide, and on the other hand a fluorine compound, such as hexafluorotitanate, ammonium tetrafluoroborate or hexafluorosilicate. The fluorine content in these catalysts is preferably between 0.05 and 8%. in weight.

 In practice, like all the operations described above, and in particular the possible intermediate thermal treatment, the activation heat treatment can be carried out in a fluidized bed reactor containing the impregnated granular support.

Also, according to a particular embodiment of the process of the invention, the catalyst can be entirely prepared in a continuous manner, in the same fluidized bed reactor, into which the various compounds to be impregnated on the granular support are introduced and the different heat treatments. However, during the various operations to be carried out for the preparation of the catalyst, the fluidization conditions can be changed, in particular the reactor temperature and the fluidization gas. In general, as the fluidizing gas, nitrogen is used for all the operations which precede the activation heat treatment and air for the latter.

 The present invention also relates to a catalyst containing at least one granular support, chromium and a group IIa or IIIa metal, which can be obtained according to the method of the invention described previously. Such a catalyst can be used under industrial conditions to polymerize or copolymerize an alpha-olefin having 2 to 12 carbon atoms. It is particularly suitable for the polymerization of ethylene and the copolymerization of ethylene with at least one alpha-olefin having from 3 to 12 carbon atoms such as propylene, butene-1, 1-octene, methyl -4 pentene-1, or even decene-1.

Thus, it has been found that this catalyst is well suited to the manufacture of ethylene polymers having from 0 to 30. by weight of at least one alpha-olefin ,. among which are high density polyethylenes, linear low density polyethylenes and very low linear density polyethylenes. The polyethylenes manufactured have a density ranging from 0.900 to 0.965, a melt index measured at 190 ° C. under a load of 2.16 kg, which can range from 0.005 to 2 and preferably from 0.05 to 1 g. 10 min and a relatively narrow molecular weight distribution, for a chromium oxide catalyst, characterized by a ratio of the weight-average molecular weight Mw to the number-average molecular weight Mn ranging from 4 to 15 and more particularly from 4.5 to - 6.5.By the way, they have good resistance to impact. Accordingly, the invention includes polymers containing at least one alphaolefin having 2 to 12 carbon atoms which can be prepared using the catalyst of the invention. These polymers can be used to produce finished objects, in particular by extrusion blowing.
The catalyst - can be used in suspension or preferably in the gas phase in a fluidized bed reactor and / or stirred mechanically. It is advantageously used in the presence of a second organometallic compound of a metal belonging to groups I and III of the periodic table of elements such as an organomagnesium, organozinc or organoaluminium compound. The catalyst and the second organometallic compound are generally used. in proportions such that the molar ratio of the amount of metal of the second organometallic compound to the amount of chromium of the catalyst is between 0.005 and 5. The reaction of (co) polymerization of an alpha-olefin can be carried out at a temperature of between about 0.degree. C. and 120.degree. C., preferably between 65.degree. C. and 115.degree. C. and in a gaseous phase under a pressure ranging from 0.1 to 5 MPa. The catalyst prepared according to the invention can be used in the state or after undergoing an olefin prepolymerization operation, carried out in one or more gas phase stages and / or in suspension in a liquid hydrocarbon medium. The prepolymerization operation leads to increasing the particle size of the catalyst while maintaining the morphology of the latter. It consists in bringing the catalyst and the second organometallic compound into contact with one or more alpha-olefins. The prepolymerization reaction can be continued while maintaining the catalyst with a suitable activity until 10 to 500 g, preferably 30 to 250 g of polymer per millimole of chromium are obtained.

Measuring the repetition of molecular weights
In the examples and the description the molecular weight distribution of a polymer is calculated according to the ratio of the weight average molecular weight, Mw, to the number average molecular weight, Mn, of the polymer, from a curve of molecular weight distribution obtained by means of a
(R) WATERS 150 C Brand Gel Permeation Chromatograph (High)
Temperature Size - Exclusion Chromatograph), the operating conditions being as follows
- solvent, trichloro-1,2,4 benzene
- solvent flow rate: 1 ml / minute - (R)
- three columns of brand SHODEX E AT 80 M / S
- temperature: 150 C
- sample concentration: 0.1% by weight
- injection volume: 500 microliters
- detection by a refractometer integrated in the chromatograph
- calibration using a high density polyethylene sold
by BP CHEMICALS SNC under the trade name
(R)
RIGIDEX 6070: Mw = 65,000 and Mw / Mn = 4, IF2 = 6, and
of a high density polyethylene having: Mw = 210,000 and
Mw / Mn = 17.5.

 The following nonlimiting examples illustrate the present invention.

Example 1
Preparation of a catalyst (A)
It operates in a fluidized bed reactor consisting essentially of a vertical cylinder 75 cm in height and 5 cm in diameter surmounted by a plenum chamber. This reactor is provided with a fluidization sole consisting of porous quartz disposed in the lower part of the cylinder, a heating resistor and a supply of fluidization gas which circulates at a rate of rise of 12 cm / s.

The reactor being maintained at 600 ° C. and the fluidization gas
being dry nitrogen containing less than 2 vpm of water, it introduces
50 g of silica sold under the trademark EP 10 (R) by the company
CROSFIELD (Great Britain). After this introduction, the reactor
is heated at 200 C during; 2 hours then is cooled to 600C. AT
this temperature is charged with 5,7 g of dibutylmagnesium in
minutes. One hour after this introduction, the reactor is heated
at 9000C for 1 hour then is cooled to 60 "C. At this temperature
a pre-prepared suspension containing 100
ml of cyclohexane, 0.28 g of tert-butanol and 0.7 g of acetylacetonate
of chromium (III) in 75 minutes.At the end of this time, the reactor is
heated at 3000C for 1 hour. Then the fluidization gas is
replaced by air containing less than 2 vpm of water, and the reactor
is heated at 815 C for 4 hours, then at 3000C for one
hour. At the end of these 5 hours, the fluidizing gas is replaced
by dry nitrogen and the reactor is kept at room temperature
ambient temperature of 20 C. Under these conditions, a
catalyst (A) active and ready to use that is stored under nitrogen and away from moisture.

Example 2
Preparation of a catalyst (B)
It operates in a reactor identical to that used in Example 1.

 With the reactor maintained at 60 ° C., and the fluidization gas being dry nitrogen containing less than 2 vpm of water, 50 g of silica sold under the trademark SG 332 R by the company GRACE (United States) are introduced therein. ) -. After this introduction, the reactor is heated at 2000 ° C. for 2 hours and then cooled to 60 ° C. At this temperature, 8.25 g of di (2-methylpentoxy) magnesium are introduced in the course of 15 minutes. after this introduction, the reactor is heated at 99 ° C. for 4 hours and then cooled to 60 ° C. At this temperature, a suspension prepared beforehand and containing 100 ml of cyclohexane, 1.42 g of tert-butanol and 3.36 g of After this time, the reactor is heated at 300 ° C. for 1 hour, and the fluidization gas is replaced by air containing less than 2 vpm of carbonyl acetate (III) in 75 minutes. water and the reactor is maintained at 4000C for 4 hours, then at 3000C for 1 hour At the end of these 5 hours, the fluidization gas is replaced by dry nitrogen and the reactor is maintained at the ambient temperature of 20 C. Under these conditions we recover a catalyst (B), active and ready to use oi that is stored under nitrogen and protected from moisture.

Example 3
Preparation of a catalyst (C)
The procedure is exactly as in Example 2 except that 13.6 g of tri-n-octylaluminium is used in the presence of 8.25 g of di (2-methylpentoxy) magnesium. Under these conditions is recovered a catalyst (C) active and ready to use that is stored under nitrogen and protected from moisture.

Example 4
Preparation of a catalyst (D)
The procedure is as in Example 2, except that a mixture containing 6.8 g of tri-n-octylaluminum and 4.1 g of di (2-methylpentoxy) magnesium instead of 8 g is used. 25 g of di (2-methylpentoxy) magnesium. In these conditions we recover. an active and ready-to-use catalyst (D) which is stored under a nitrogen atmosphere and protected from moisture.

Example 5
Preparation of a catalyst (E)
The procedure is exactly as in Example 2, except that 2.8 g of triethylboron are used instead of 8.25 g of (methyl-2, pentoxy) magnesium. Under these conditions, an active catalyst (E) is recovered and ready for use that it keeps under a nitrogen atmosphere and protected from moisture.

Example 6 (comparative)
Preparation of a catalyst (F)
It operates exactly as in Example 1, except that it does not use dibutylmagnesium and the fact that one does not perform the heat treatment for one hour at 900 C. In these conditions it recovers an active and ready-to-use catalyst (F) which is stored under nitrogen and protected from moisture.

Example 7 (comparative)
Preparation of a catalyst (G)
We operate exactly as in Example 2, except that
11.8 g of titanium tetraisopropylate are used instead of 8.25 g
of di (2-methylpentoxy) magnesium and the fact that a
solution containing 100 ml of cyclohexane, 1.42 g of tert-butanol and
0.96 g chromium trioxide (CrO) instead of the suspension containing
100 ml cyclohexane, 1,423 g tert-butanol and 3.36 g
of chromium (III) acetylacetonate. In these conditions, we recover
CG catalyst) active and ready for use that is kept under
nitrogen atmosphere and away from moisture.

Examples 8 to 14
Polymerization of ethylene
In a stainless steel reactor maintained under a pressure of 0.1
MPa nitrogen, a volume of 2.6 liters and equipped with an agitator for
dry powder rotating at 350 rpm, is introduced as a
filler powder 200 g of a perfectly anhydrous polyethylene and
from a previous reaction, 0.19 millimole
triethylaluminium and then 200 mg of catalyst selected from the catalysts (A), å (G) examples i to 7. The reactor is then
heated to 100 C and introduced hydrogen to obtain
a total pressure of 1.5 MPa and then ethylene, a flow
constant of 200 g / hour for 2 hours. At the end of this time, we
recovering about 600 g of polyethylene whose flow index
IF IF measured at 190 ° C., with a load of 2.16 kg and expressed in g / 10
2.16
minutes is shown in Table 1.

Table 1

<tb><SEP> | <SEP> I <SEP> - <SEP> I <SEP> I
<tb> Examples <SEP> Catalyst <SEP> IF <SEP> 2.16 <SEP> Mw / Mn
<tb> 8 <SEP> A <SEP> 0.55 <SEP> 5.8
<tb> 9 <SEP> B <SEP> 0.30- <SEP>
<tb> 10 <SEP> C <SEP> 0.35 <SEP>
<tb><SEP> I <SEP> I <SEP> I <SEP> I
<tb> 11 <SEP> D <SEP> 0.35 <SEP>
<tb> 12 <SEP> E <SEP> 0.25 <SEP>
<tb><SEP> I <SEP> I <SEP> I <SEP> I
<tb> 13 <SEP> (comparative) <SEP> F <SEP> 0.60 <SEP> 7
<tb> 14 <SEP> (comparative) <SEP> G <SEP> 6.5 <SEP>

Claims (13)

 1. Process for preparing a catalyst based on a compound  of chromium oxide and associated with a granular support based on oxide  refractory characterizes; in that in a first step one impregnates  granular suport with at least one organometallic compound of a  metal belonging to group lia or IIIa of the classification periodi  than elements and with at least one chromium compound, and in that  in a second step, a heat treatment is carried out  of activation of the impregnated support obtained at the end of the first step:  2. Method according to claim 1, characterized in that the  organometallic compound is selected from magnesium compounds,  boron or aluminum.  3. Process according to claim 1 or 2, characterized in that  the chromium compound is a chromium oxide or a compound that can be  converted to chromium oxide by calcination during treatment  thermal performed in a non-reducing atmosphere.  4. Method according to any one of claims 1 to 3,  characterized in that the granular carrier is silica or  alumina or a mixture of these two refractory oxides.  5. Method according to claim 1, characterized in that the  impregnated support is prepared by means of several  gnation.  6. Method according to claim 5, characterized in that  two impregnation operations is carried out an intermediate heat treatment of the granular support. 7. Process according to claim 6, characterized in that the intermediate heat treatment is carried out at a temperature ranging from 4000C to 10000C. 8. Process according to any one of Claims 1 to 7, characterized in that the granular support is impregnated with a compound to be impregnated at a temperature of between 0 and 1000 ° C. 9. Method according to any one of claims 1 to 8, characterized in that the second step of the process is carried out at a temperature between 250 and 12000C. 10. Catalyst containing at least one granular support, chromium and a metal of group ixia or IIIa, which can be obtained by the method according to any one of claims 1 to 9. 11. Use of the catalyst according to claim 10 for polymerizing. or copolymerizing an alpha-olefin having 2 to 12 carbon atoms. Polymer containing at least one alpha-olefin having 2 to 12 carbon atoms which can be prepared using the catalyst of claim 10. Finished articles made with a polymer according to claim 12.