Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Plural component system comprising a - group i to iv metal...
Reexamination Certificate
2000-11-27
2004-09-21
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Plural component system comprising a - group i to iv metal...
C502S113000, C502S154000
Reexamination Certificate
active
06794326
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a supported chromium oxide catalyst, in particular a catalyst system comprising an inorganic support, a chromium oxide and a metallocene compound, and a method for the preparation of said catalyst system. The catalyst produces branched polyethylenes without the use of a copolymer, and the molecular weight can be controlled by feeding hydrogen.
BACKGROUND ART
To obtain linear polyolefins having desired properties, different catalyst systems in combinations with controlled polymerisation conditions are used. A typical supported chromium catalyst that is extensively used in commercial polymerisations of ethylene is formed by depositing a chromium compound onto a support, which is then oxidised. The oxidised catalyst precursor may be introduced as such into a polymerisation reactor, where it will be reduced in situ by the olefin monomers to its active catalytic state. It is also well known that oxidised chromium compounds may be prereduced by suitable reagents, such as hydrogen or carbon- monoxide (J. P. Hogan, J.Polym. Sci., PtA-1, 8, 2637 (1970), and references therein). The resulting chromium surface species are generally accepted to be highly reactive. A prereduced chromium-based catalyst will produce polyethylenes having a broad molecular weight distribution (MWD) and mainly long, straight chains. Such catalysts are not hydrogen-sensitive.
Another commercially important class of catalysts used to polymerise &agr;-olefins are the cyclopentadienyl transition metal catalysts, usually termed metallocene catalysts. Metallocenes contacted with Lewis acids will give effective polymerisation catalysts that produce linear polyethylenes having a narrow molecular weight distribution (MWD). Such catalysts are sensitive to the presence of hydrogen.
By adding comonomers and optionally hydrogen during the catalysed polymerisation reactions the density and chain branching of the produced polymer can be controlled. In the prior art efforts have been made to develop combined catalyst system that produce short chain branched polyethylenes, without the addition of comonomers during the polymerisation reactions.
U.S. Pat. No. 4,735,931 discloses a catalyst system prepared by first depositing a chromium compound on a predominantly silica support, activating said catalyst in an oxygen-containing atmosphere and thereafter subjecting the thus activated composition to reduction with carbon monoxide. The resulting chromium catalyst composition is then precontacted with a cocatalyst selected from trialkyl boron and dialkyl aluminium alkoxide compounds, preferably triethyl borane, and thereafter contacted with ethylene. When used in olefin polymerisations this catalyst provides in situ generation of comonomers, resulting in tough, flexible, mainly pure polyethylene resins.
U.S. Pat. No. 5,032,651 teaches the use of catalyst mixtures of two transition metal catalysts. One of the catalysts comprises chromium oxide supported on an aluminophosphate, and the other one comprises a &bgr;-stabilized tetrahydrocarbyl zirconium compound supported on an inorganic material. The catalysts may be premixed before use, or they may be fed separately to the reactor. Olefinic polymers exhibiting high environmental stress crack resistance (ESCR) are produced.
EP 206794 describes an olefin polymerisation catalyst comprising a support contacted with a Group 4b, 5b or 6b metallocene and an aluminoxane.
EP 088 562 discloses a modified polymerisation catalyst comprising a silica support with deposited chromium. Following oxidation in dry air, the chromium is modified by being contacted with a transition metal compound of Ti, V or Cr, preferably Ti. Only the use of bis-toluene titanium is exemplified, and the obtained polyethylenes have a substantial degree of branching and a medium or broad molecular weight distribution.
U.S. Pat. No. 3,378,536 discloses a process for the polymerisation of ethylene by the use of a two-component catalyst system consisting of (a) a chromium compound deposited on e.g silica, where the chromium being activated in an oxygen-containing gas at a high temperature and then reduced with CO; and (b) chromium or vanadium arene where the arene is an aromatic, optionally substituted C
6
ring. The two catalyst components are preferably fed separately to the polymerisation reactor.
It is well known to a person skilled in the art that the various processing techniques used in the manufacturing of final articles from polyethylene resins require polyethylene grades having specific rheological properties. For instance, in the extrusion blow moulding of containers polyethylenes having a broad MWD and long straight chains are typically used, while in film blowing lower density polyethylenes are preferred.
It is an object of the present invention to provide a novel catalyst system that, produces a branched polyethylene from ethylene without any addition of comonomer during the polymerisation reaction. A further object is to control the polymerisation reaction to produce polyethylenes having a desired density and molecular weight. Another object is to obtain polyethylene resins suitable for use blow moulding and film blowing processing.
It has now been found that a catalyst system comprising a prereduced chromium on silica catalyst that have been contacted with a metallocene compound fulfils the requirements above. The novel catalyst system produces a branched low density polyethylene polymer without any added comonomer. The density and molecular weight (and hence the melt flow index) of the polymer can be controlled by the addition of hydrogen to the reactor.
DISCLOSURE OF INVENTION
The present invention thus provides a catalyst system for the polymerisation of ethylene, comprising chromium oxide supported on an inorganic support, wherein
a) said support being a particulate inorganic oxide;
b) the chromium of said chromium oxide being in a reduced oxidation state, and comprising
c) a metallocene compound having the formula:
Cp
2
ZrR′R″
wherein each Cp, being equal or different, is an unsubstituted or substituted cyclopentadienyl compound, and R′ and R″, independent of each other, are selected from the group comprising alkyls having 1 to 6 carbon atoms, unsubstituted or substituted benzyl, and phenoxy substituted with alkyls having 1 to 6 carbon atoms, and R′ or R″ may be a halide.
The invention also provides a method for the preparation of the catalyst system above, comprising the steps of:
a) calcining a support being a particulate, inorganic oxide selected from the group comprising alumina, silica, titania, zirconia, magnesia, and combinations thereof,
b) joining onto the surface of said support a chromium-organic compound to obtain a catalyst precursor,
c) subjecting said catalyst precursor to oxidising conditions to obtain the chromium in an oxidised state,
d) subjecting said catalyst precursor to reducing conditions to obtain a prereduced catalyst, thus
e) reducing the oxidised chromium to obtain the main part thereof in a bivalent oxidation state,
f) contacting said reduced catalyst with a metallocene compound having the formula:
Cp
2
ZrR′R″
wherein each Cp, equal or different, is an unsubstituted or substituted cyclopentadienyl compound, and R′ and R″, independant of each other, are selected from the group comprising alkyls having 1 to 6 carbon atoms, unsubstituted or substituted benzyl, and phenoxy substituted with alkyls having 1 to 6 carbon atoms, and R′ or R″ may be a halide.
BEST MODES FOR CARRYING OUT THE INVENTION
The catalyst system of the present invention comprises a supported reduced chromium/silica catalyst contacted with a metallocene compound. In general, metallocenes based on zirconium, hafnium and titanium are preferred as metallocene catalysts. By metallocenes are meant compounds in which a metal atom or ion or complex ion is &eegr;-bonded by at least one ligand, e.g. by 1, 2 or 3 ligands or ligand components. The &eegr;-bonding ligands in such catalysts may be simple
Blom Richard
Dahl Ivar Martin
Follestad Arild
Jens Klaus-Joachim
Bell Mark L.
Borealis Technology Oy
Brown Jennine
Testa Hurwitz & Thibeault LLP
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