Heterogeneous metallocene catalyst

Details Heterogeneous metallocene catalyst Heterogeneous metallocene catalyst

1999-04-14

2001-05-22

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...

C502S132000, C502S133000

Reexamination Certificate

active

06235671

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a heterogeneous catalyst polymerization of ethylene and the copolymerization of ethylene with lower alpha olefins such as butene-1; propylene; isobutylene; hexene-1; 4-methyl pentene-1; and octene. The invention further relates to an ethylene polymerization process which employs the catalyst.
BACKGROUND OF THE INVENTION
The use of “metallocene” catalysts (i.e. catalysts which contain two cyclopentadienyl ligands) for the polymerization of ethylene has been investigated for some 40 years, as evidenced by a series of patents which include U.S. Pat. No. 2,827,466 (Breslow, from a filing date of Sep. 27, 1955); U.S. Pat. No. 3,231,550 (Manyik, from a filing date of Jun. 4, 1964); and U.S. Pat No. 4,542,199 (Kaminsky et al). Kaminsky et al discovered that certain alumoxanes may activate zirconium-based metallocenes in a manner that produces a “single site” catalyst with spectacular productivity—a significant discovery which led to a large increase in research in this area of catalysis. Of additional relevance to the present invention, much investigation has been done on monocyclopentadienyl transition metal catalysts which are sometimes also referred to as “pseudo-metallocenes”.
When a metallocene or a pseudometallocene catalyst is employed in a slurry or gas phase polymerization, it is highly preferred to use the catalyst in a heterogeneous or “supported form”. It is also highly preferred that the catalyst does not cause reactor fouling. The art of preparing heterogeneous catalysts which do not lead to reactor fouling is not adequately understood, though it is generally accepted that the catalytic material should be very well anchored to the support so as to reduce the incidence of fouling resulting from the deposition of catalyst or cocatalyst which has dissociated from the support.
In general, heterogeneous metallocene catalysts may be grouped into three main categories:
I. Unsupported Alumoxane/Metallocene Mixtures
These catalysts may be easily prepared by evaporating the solvent or diluent from a liquid mixture of an alumoxane and a metallocene. The resulting product is a solid at room temperature due to the comparatively high molecular weight of the alumoxane. There are two disadvantages to this approach, namely cost (i.e. alumoxanes are comparatively expensive—and the alumoxane is used as an expensive “support” material) and “reaction continuity/fouling” (i.e. the alumoxane may partially melt under polymerization conditions, leading to reactor instability/fouling). U. S. Pat. No. (USP) 4,752,597 (Turner, to Exxon) illustrates this approach for the preparation of a heterogeneous metallocene catalyst.
II. Metal Oxide Supported Catalysts
These catalysts are prepared by depositing the metallocene catalyst and a cocatalyst on a very porous metal oxide support. The catalyst and cocatalyst are substantially contained within the pore structure of the metal oxide particle. This means that a comparatively large metal oxide particle is used (typically particle size of from 40 to 80 microns). The preparation of this type of supported catalyst is described in U.S. Pat. No. 4,808,561 (Welborn, to Exxon).
III. Filled/Spray Dried Catalysts U.S. Pat. Nos. 5,648,310; 5,674,795 and 5,672,669 (all to Union Carbide)
teach the preparation of a heterogeneous metallocene catalyst by spray drying a mixture which contains a metallocene catalyst, a cocatalyst and a “filler” which is characterized by having a very small particle size (less than one micron) and by being unreactive with the catalyst and cocatalyst. The examples illustrate the use of very fine particle size “fumed” silica which has been treated to reduce the concentration of surface hydroxyls. The resulting catalysts exhibit good productivity. Moreover, they offer the potential to provide a catalyst which is not prone to “hot spots” (as the catalyst may be evenly distributed, at low concentration, throughout the heterogeneous matrix). However, these catalysts suffer from the potential disadvantage of being very friable because they are prepared with a fine, “inert” filler material which does not react with I anchor to the catalyst or cocatalyst.
Friable catalyst particles lead to the formation of “fines” in the polyethylene product, and may also aggravate reactor fouling problems.
It is one object of the present invention to provide a spray dried, heterogeneous metallocene catalyst which mitigates problems associated with prior art catalysts.
SUMMARY OF THE INVENTION
In one embodiment, the present invention provides a process to prepare a spray dried olefin polymerization catalyst, said process consisting of:
I) providing a mixture of:
(a) an organometallic complex of a group 3-10 metal having at least one cyclopentadienyltype ligand;
(b) an activator;
(c) a hydrotalcite of the formula:
Mg
1−x
Al
x
(OH)
2
)
n+
A
n−
x

·m H
2
O.
wherein 0<x≦0.5; m is a positive number, and A
n−
is an anion having a valence of n; and
(d) a diluent; and
II) spray drying said mixture.
DETAILED DESCRIPTION
An essential feature of the present invention is the use of a hydrotalcite as the filler material. Hydrotalcites are well known articles of commerce which are used in the polypropylene business to neutralize metal chloride residues from Ziegler-Natta catalysts—see for example U.S. Pat. Nos. 4,284,762 and 4,347,353. Such hydrotalcites are defined in these patents by the formula:
Mg
1−x
Al
x
(OH)
2
)
n+
A
n−
x

·m H
2
O.
wherein 0<x≦0.5; m is a positive number; and A
n−
represents an anion having a valence of n.
The hydrotalcites are further described by having:
(i) a BET specific surface area of not more than 30 m
2
g, preferably not more than 20 m
2
g (where m refers to meter and g to gram); and
(ii) an average secondary particle size of not more than 5 microns and preferably (iii) a crystallite size, in the <003> direction determined by x-ray diffraction method, of at least 600 Å, preferably at least 1000 Å.
The anion (“A
n−
”) may be, for example, a carbonate, sulfate or phosphate. Hydrotalcites having a carbonate anion are commercially available, convenient to use, and hence are preferred for use in the present invention. These hydrotalcites are sometimes referred to by those skilled in the art as “magnesium aluminum hydroxy carbonates”. Some commercially available hydrotalcites which are suitable for the present invention may be treated with a surfactant as disclosed in the aforementioned U.S. Pat Nos. 4,347,353 and 4,284,762.
In a commonly assigned patent application, the use of hydrotalcite to deactivate a Ziegler-Natta catalyzed solution polymerization process is disclosed. While not wishing to be bound by any particular theory, it is believed that the hydrotalcite reacts with metal chlorides in a manner which isolates the metal chloride in the hydrotalcite structure. In the case of Ziegler-Natta catalyzed polymerizations, this has been observed to deactivate the metal chloride sufficiently to stop polymerizations and mitigate problems which may otherwise be caused by metal chloride residues in the product. However, in direct contrast, the heterogeneous hydrotalcite/metallocene system of this invention is a highly active catalyst.
Metallocene
The catalyst used in this invention is an organometallic complex of a group 3-10 metal which is characterized by having at least one cyclopentadienyl ligand. Such catalysts are well known to those skilled in the art and are generally referred to as “metallocenes” or “pseudo” metallocenes (“true” metallocene complexes are bis-cyclopentadienyl complexes; “pseudo” complexes contain only one cyclopentadienyl ligand). These catalysts are so well known to those skilled in the art that a lengthy description of them herein is considered unnecessary.
Preferred metallocenes for use in the present invention are those which contain a group 4 organometallic complex selected from titanium, zirconium or hafnium.
Particularly preferred catalysts are those defined by the formula:
wherein M is a g

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