Process for preparing a procatalyst for olefin polymerization
The present invention relates to a process for preparing a procatalyst of a Ziegler-Natta catalyst composition which is suitable for production of olefin polymers.
Ziegler-Natta catalyst compositions essentially comprise a procatalyst based on compounds of a transition metal belonging to groups IVB to VIB of the Periodic Table of Elements and a cocatalyst based on organometallic compounds of elements of groups IA to IIIA of the Periodic Table of Elements. The polymerization of olefins such as ethylene, propylene and higher olefin monomers using Ziegler-Natta catalysts is widely utilized. These catalysts provide polyolefins possessing the desired characteristics of these polymers in high yield.
Various proposals have been made in the art for preparing a carrier and subsequently a catalyst for olefin polymerization. From EP-A-0504744 A2, a catalyst for producing a stereospecific polyolefin is known, which is prepared by reacting metal magnesium, a hydroxylated organic compound and an oxygen-containing organic compound of titanium, resulting in a homogeneous solution which is then gradually reacted with an aluminium halide, an electron-donating compound, a titanium halide and finally with a chlorine containing silicon compound but no solid support is used. It is emphasized that this catalyst is capable of producing a highly stereospecific polymer having a good configuration of particles in good yield in the (co) polymerization of an α-olefin having at least three carbon atoms.
U.S. patent 5,310,716 sets forth a supported solid component of a catalyst for the stereospecific polymerization of
propylene and other α-olefins which is obtained by making a non-activated silica interact in succession with a magnesium alkyl, a halogenating agent such as silicon tetrachloride, a titanium tetrahalide and a Lewis base. According to EP-A2-0525608, a catalyst system for producing olefin polymers is disclosed which comprises (A) a hydrocarbon-insoluble solid procatalyst obtained by reacting a uniform hydrocarbon solution containing an organomagnesiu compound, an organotitanium compound an a polyalkyltitanate with a halogenating agent and (B) an organoaluminium compound.
A more recent disclosure, WO 99/05187 relates to a high activity catalyst composition for the (co) polymerization of olefins, comprising a catalyst precursor and an organoaluminium cocatalyst, wherein the catalyst precursor consists of a silica carrier material, a dialkylmagnesium compound, a tetraalkyl orthosilicate, a chlorine containing silicon compound and a titanium compound. The catalyst composition shows a high activity for polymerizing olefins such as ethylene and so reduces the metal catalyst residues in the final polymer to a low level.
Furthermore, EP-859013 and EP-859014 each disclose basically the same procedure for the production of polymerization catalysts, in which the silica support is first impregnated directly with magnesium chloride in the presence of electron donors followed by further impregnation of the support with magnesium alkyls in the presence of silicon tetrachloride. Obviously the last step also serves to halogenate the procatalyst complex.
These inventions further relate to a process for the preparation of polyolefins by (co) polymerization of olefins,
wherein a supported catalyst as described above is used. The polymerization is conducted according to conventional methods, operating a liquid phase, either in the presence or in the absence of an inert hydrocarbon diluent, or in gas phase.
EP-0688794 Al attempts to eliminate problems created by the addition of chain transfer agents such as hydrogen to the polymerization reaction. For controlling the molecular weight of the resulting polymer, preferably hydrogen is used as chain transfer agent as no foreign atoms remain in the growing molecule. The unique feature of the disclosed procatalyst lies in its good balance in activity in a very wide range of molar mass regulating hydrogen partial pressures used in the olefin polymerization .
However, the employment of this type of catalyst family often creates problems involving a high amount of residues in the final polymeric product. That disadvantage is caused by high amounts of residues in the catalysts. Furthermore, said residues are responsible for a decreased catalytic activity of the catalyst itself. Metal alcoholates constitute the majority of said residues. These alcoholates are derived from the reaction between an alkoxy-magnesium compound and the halogen containing metal alkyl compound used for impregnation of the solid carrier support. Typically employed alkoxy-magnesium compounds such as bis-2-ethyl-l-hexanolate magnesium have to be soluble in inert diluents like pentane and moreover these alkoxy compounds must only possess a limited reduction power. These remarks make clear that useful alkoxy-magnesium compounds are restricted to derivates of higher and/or branched alcohols which means that high molecular weight metal alcoholates are left in the catalyst causing above-mentioned inhomogeneities in the polymer. Furthermore, the amount of
said alkoxy magnesium compounds, which can be added to the pores inside the carrier, is also restricted.
An objective of the present invention is to provide a method for preparing a procatalyst of a Ziegler-Natta catalyst composition which has a high activity in polymerizing olefins, an improved morphology and a good balance of activity under different hydrogen partial pressures.
It has now been found according to the present invention, that dialkyl-magnesium compounds with low molecular weight alkyl residues can be employed instead of the prior art alkoxy- magnesium compounds. Metal alcoholates, formed during the process of preparation of the procatalyst, therefore consist of alkoxy-groups having a lower molecular weight meaning less residues in the final procatalyst. It has also been found that the inherent high reductive power of dialkyl-magnesium compounds can be attenuated in the process of preparing the procatalyst .
According to the present invention, reduction sinks to control the reductive power of these compounds are provided by the use of a HC1 splitting compound or oxidation in dry air. Furthermore, the replacement of the above-mentioned alcohol- derived magnesium compounds by dialkyl compounds allows to prepare Ziegler-Natta catalysts at substantially lower costs than by comparable prior art methods as there is no more a demand for high and/or branched alcohols used for the preparation of prior art alkoxy-magnesium compounds. Surprisingly, the present inventors have found that the amount of residues in the procatalyst caused by the above-mentioned metal alcoholates are decreased. Furthermore, catalysts according to the present invention produce a higher molecular
weight tail in the molecular weight distribution of the resulting polymer than the above-mentioned prior art catalysts .
Therefore, the present invention is directed to a method for the preparation of a procatalyst for olefin polymerization having a good balance of activity in different hydrogen concentrations, said method comprising the steps: a) treating silica by successively contacting said silica with a pair of compounds comprising a moderator compound and a magnesium alkyl compound, wherein said moderator compound is able of being reduced in a controlled manner by said magnesium alkyl compound, to obtain a modified silica; b) contacting said modified silica with a chlorine containing titanium compound to obtain a solid procatalyst .
In one embodiment of the inventive method, the silica is first contacted with said moderator compound in step a) and then treated with said magnesium alkyl compound.
In another embodiment of the inventive method, the silica is first contacted with said magnesium alkyl compound in step a) and then treated with said moderator compound.
All above-mentioned process steps are preferably conducted under inert gas atmosphere, particularly preferred under nitrogen or argon atmosphere.
According to the present invention, it is further preferred that the procatalyst obtained in step b) is dried in a further step c) .
According to the present invention, said silica has been preferably subjected to a thermal or chemical pretreatment in order to remove free hydroxyl groups .
The silica used as support material must have a suitable particle size distribution, a high porosity and a large specific surface area. A good result is achieved if the support material has a specific surface area between 100 and 500 m2/g silica and a pore volume of 1 - 3 ml/g silica.
Preferably said silica is chemically pretreated by silanation or by treatment with aluminium alkyls.
Preferably, the support is dried before impregnating with said catalyst components. A good result is achieved if the support is heat-treated at 100°C to 900°C for a sufficient time, and thereby the surface hydroxyl groups, in the case of silica, are reduced to below 2 mmol/g Si02. Preferably said thermal treatment is carried out under an inert atmosphere and maintained at an elevated temperature for a certain time. The temperature may be between 200 and 800 °C, more preferably between 400 and 650 °C. The inert atmosphere may be established e.g. by purging with a suitable inert gas, preferably nitrogen.
The moderator compound used in step a) or step b) , respectively, may be any compound which can eliminate the reductive power of said magnesium alkyl compound. Also, the eventual byproduct of the neutralization reaction to control the reductive power of said magnesium alkyl compound should be such that it can be easily removed from the reaction mixture
without any separate washing step, to avoid the catalyst from being contaminated by the byproducts.
Especially suitable moderator compounds are chlorine containing compounds which are able to form hydrochloric acid when contacting the acidic silica surface. In the following, the formed hydrochloric acid reacts with said magnesium alkyl compound. Such compounds are for example, chlorine containing silicon compounds, preferably SiCl4, TiCl4, ZrCl4, HfCl4 or SnCl4.
On the other hand, the moderator compound may also be oxygen, or more preferably, a gas containing oxygen, e.g. dry air. If a chlorine containing compound is used as the moderator compound, it is preferred in step a) to contact the silica and said moderator compound. This contacting step results in the in situ formation of hydrochloric acid which further reach in step b) with said magnesium alkyl compound when the modified silica of step a) is contacted with said magnesium alkyl compound in step b) thus neutralizing the reductive power of said magnesium alkyl compound according to Scheme (I) below:
Cl
— S 1 iOH + SiX4 ► HX| + Si— O — S I i— Cl ( 1)
Cl
HC1 + FMgR ► MgCl2 (2 )
Scheme ( I )
If oxygen or a gas containing oxygen, e.g. dry air is used as the moderator compound, it is preferred first to contact the silica and the magnesium alkyl compound (step a) and contact
the thus obtained modified silica with oxygen or a gas containing oxygen as the moderator compound. According to scheme (II) below, the reductive power of said magnesium alkyl compound is first transferred to an in situ formed aluminium alkyl compound which eventually is oxidized by surrounding oxygen.
EADC + SiOH ► Si— O—A1C12 + EtH + EADCabsorbed (1)
RMgR + Si— O—A1C12 + EKDCetoβoxbBd —► MgCl2 + A1R3 (2)
A1R3 +O2 ►Al(OR)3 (3)
Scheme (II)
According to the present invention, it is suitable to use a dialkylmagnesium compound being same or different and having alkyl groups with 1 to 18, preferably 2 to 8 carbon atoms as said magnesium alkyl compound. Preferable examples of such compounds are e.g., diethylmagnesium, dibutylmagnesium, dioctylmagnesium, butyl-ethylmagnesium or butyl- octylmagnesium.
It is important that the moderator compound used in step a) does not contain any moisture or other components which may have a detrimental effect on the catalyst performance. Thus, where feasible, the moisture should be removed from this moderator compound by using the techniques known in the art, like passing it through the molecular sieves or alumina, etc.
Step a) :
According to the present invention, the contacting between a chlorine containing compound as the moderator compound and
said silica in step a) may be carried out at any convenient temperature. Most preferably the contacting is done at a room temperature, or near the room temperature, e.g. between 15 and 35 °C.
The contacting may be done either by directly mixing said chlorine containing compound with said silica or, preferably, mixing a solution of said chlorine containing compound in an inert diluent with said silica. The contacting may be conducted as "dry" mixing, where the total volume of said chlorine containing compound and the inert diluent may be such that it does not exceed the pore volume of the silica. On the other hand, the contacting may also be carried out in slurry, where the total liquid volume is sufficient to suspend said silica.
The time of contacting should be sufficient to allow a complete or nearly complete reaction between the silica and the moderator compound used in step a) . On the other hand, the contact time should not be longer than necessary, otherwise the preparation time of the catalyst batch shall be uneconomically long. The contact time may be between about 10 minutes and about 6 hours, preferably between about 30 minutes and 3 hours .
The contacting of the resulting modified silica with said magnesium alkyl compound may be carried out at any convenient temperature, but preferably at or near room temperature. Suitable temperature range is from about 15 °C to about 70 °C. The contacting can be conducted either by mixing said modified silica with said magnesium alkyl compound or, preferably, by mixing said modified silica with a solution of said magnesium
alkyl compound in an inert solvent. The contacting may be conducted as "dry" mixing, where the total volume of said magnesium alkyl compound and the inert diluent may be such that it does not exceed the pore volume of the modified silica. On the other hand, the contacting may also be carried out in slurry, where the total liquid volume is sufficient to suspend the particles of the modified silica. The time of contacting should be sufficient to allow a complete or nearly complete reaction between the modified inorganic and said magnesium alkyl compound. The contact time may be between about 1 hour and about 24 hours, preferably between about 1 hour and 6 hours.
If the moderator compound used in step a) is in a gaseous state and said silica is first contacted with said magnesium alkyl compound, it is preferred to carry out the contacting of the resulting modified silica with said gaseous moderator compound in slurry so that said silica is suspended in the diluent and let the gas bubble through the diluent under agitation.
Step b) :
The contacting of the resulting intermediate product of step a) with the chlorine containing titanium compound may be carried out at any convenient temperature, but preferably at or near room temperature. Suitable temperature range is from about 15 °C to about 70 °C.
Said chlorine containing titanium compound is a fourvalent chlorine containing titanium compound, preferably titanium tetrachloride.
The contacting can be conducted either by mixing said intermediate product with said chlorine containing titanium compound or, preferably, by mixing said intermediate product with a solution of said chlorine containing titanium compound in an inert solvent. The contacting may be conducted as "dry" mixing, where the total volume of said chlorine containing titanium compound and the inert diluent may be such that it does not exceed the pore volume of said silica. On the other hand, the contacting may also be carried out in slurry, where the total liquid volume is sufficient to suspend said silica. The time of contacting should be sufficient to allow a complete or nearly complete reaction between said intermediate product of step a) and said chlorine containing titanium compound. The contact time may be between about 1 hour and about 24 hours, preferably between about 1 hour and 6 hours. As the reductive power of the magnesium alkyl has been neutralized by the above described steps, no reduction of TiCl4 takes part.
Another aspect of the present invention relates to the use of said procatalyst for the production of olefin polymers, preferably polyethylene or polypropylene.
According to yet another aspect of the present invention, olefin polymers obtainable in a process making use of said procatalyst are provided.
In another embodiment of the present invention, the use of olefin polymers produced in the inventive process making use of said inventive procatalyst is provided for preparing molded articles.
Preparation Examples
Chemicals: Magala 7,5 in heptane has the CAS-nr 1191-47-5/97- 93-8 and the composition 7, 5 [ (n-C4H9) 2Mg] : (C2H5) 3A1.
Example 1 :
An amount of 17.9 grams of silica ES747JR, which had been treated for 6 hours at 600 °C to remove the hydroxyl groups from the surface, was introduced into a glass reactor vessel under nitrogen atmosphere. Onto the silica was added 2,67 ml (1.3 mmol/g silica) silicon tetrachloride diluted in 7 ml pentane at room temperature. The reaction was allowed to proceed for 1 hour at room-temperature, after which 29,8 ml 20wt-% (2.0 mmol/g silica) of BOMAG A (Butyl-octyl-magnesium, supplied by Witco) was introduced on the silica at room temperature. 75 ml pentane was added. The reaction was allowed to proceed for 3h at 20-40 °C, after which the product was dried by purging with nitrogen at 44-60 °C. The product was cooled down to room temperature. 1,97 ml TiCl4 diluted in 3 ml pentane was added at 20 °C. The product was stirred at 45 °C for 3 hours. The product was dried during 30 minutes.
The thus obtained catalyst contained 3,1 wt-% Mg, 4,3 wt-% Ti and 14,8 wt-% Cl.
Example 2 :
Ethylene was polymerized in a 3 dm3 autoclave reactor which had been carefully purged with nitrogen to remove oxygen and moisture. Then, 1.8 dm3 pentane was charged into the reactor. The reactor was heated to 90 °C. 57.3 mg of the catalyst prepared according to Example 1 and 0.5 ml of a solution containing 10% by weight triethylaluminium in heptane were introduced into the reactor. Then, hydrogen was added from a
500 ml vessel so that the pressure of the hydrogen in the vessel was reduced by 500 kPa . Then ethylene feed was started and the pressure in the reactor was adjusted to 1440 kPa (abs) . Ethylene was continuously introduced into the reactor and the polymer was collected, dried and analysed. In total
298 grams of polyethylene having MFR5 of 0.67 g/10 min and bulk density of 380 g/dm3 was obtained.
Example 3 : 4.21 grams of ES747JR calcinated at 600 °C for 6h was added into a synthesis reactor. 4.864 grams of 17.5 wt-% EADC (1.6 mmol/g carrier) was added slowly to the silica at room temperature. The impregnated silica was stirred for 2 hours at 25 °C. Further 9.32 g of 8.8 wt-% of MAGALA 7,5 (1.4 mmol/g carrier) in heptane was added to the impregnated silica.
Pentane was added to reach slurry phase. The suspension was stirred for 1.5 hours at 45 °C. Air was dried by molecular sieves. 1000 ml of the dried air was purged through the synthesis reactor. The synthesis reactor was closed and left at 45°C overnight. The reactor was cooled down to room temperature and the precursor was washed two times with pentane. Finally 0.559 g (0.7 mmol/g carrier) TiCl4 was added to the precursor in pentane slurry. The precursor was stirred 5 hours at 70 °C. The catalyst was dried at 70 °C during 3 hours.
The calculated metal contents were 2.82 wt~& Al, 13,9 wt-% Cl, 2.22 wt-% Mg and 2.19 wt-% Ti.
Example 4 :
The procedure of Example 2 was repeated, except that the catalyst prepared according to Example 3 was used. The
activity of the catalyst was found to be 4.1 kg PE/g catalyst/hour. The MFR2 of the polymer was 0.47 g/10 min, and the MFR2ι was 14.1 g/10 min. The bulk density of the polymer was 422 g/dm3.