Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Organic compound containing
Reexamination Certificate
2000-02-11
2004-01-06
Nolan, Sandra M. (Department: 1772)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Organic compound containing
C428S036900, C502S171000, C502S256000
Reexamination Certificate
active
06673736
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved chromate ester catalyst, especially a bis (triphenylsilyl) chromate catalyst useful for the polymerization of ethylene, a method of using the improved catalysts to produce high performance polyethylene products, such as polyethylene pipe, and the high performance polyethylene products made using the method.
BACKGROUND OF THE INVENTION
The use of silyl chromate as a polymerization catalyst has been known in the art for many years. See, for example, U.S. Pat. No. 3,324,095 (Carrick et al.) and U.S. Pat. No. 3,324,101 (Baker et al.) which teach the use of a silyl chromate catalyst system for olefin polymerization. Furthermore, in U.S. Pat. No. 3,642,749 (Johnson) and U.S. Pat. No. 3,704,287 (Johnson et al.) teach silyl chromate catalyst systems and disclose the deposition of chromate ester or di-tertiary polyalicyclic catalyst on activated silica and treatment with selected organo-metallic reducing agents. However, recently chromate ester catalyst have fallen out of favor due to their limited effectiveness in polymerization processes when compared with newer catalysts, such as the new generation of catalysts used to make bi-modal polyethylene products. The present invention is directed to a new chromate ester catalyst that may be used to make very high performance polyethylene products; such as polyethylene pipe, polyethylene liner systems, and high molecular weight film. In a preferred embodiment of the invention the polyethylene product comprises a copolymer.
SUMMARY OF THE INVENTION
The present invention relates to a supported chromium containing catalyst which comprises (a) a highly porous silica support having a minimum surface area of at least about 300 m
2
/gr., an average pore volume of at least 2.0 cc/gr., and an average meso pore diameter of between about 350 A and 1000 A; and (b) a promoter comprising a chromate ester treated with a reducing agent. The catalyst of the present invention is particularly useful for the polymerization of mono unsaturated alpha-olefins having from two to eight carbon atoms in the molecule. The catalyst is especially useful for the polymerization of ethylene. The chromate ester present in the catalyst is usually a silyl ester compound such as bis (triphenylsilyl) chromate. Preferably the chromate ester will be present in an amount within the range of from about 0.15 weight percent to about 0.70 weight percent when calculated on the basis of elemental chromium metal, and more preferably will be present in an amount of between about 0.25 weight percent and 0.60 weight percent when calculated on the basis of elemental chromium metal and most preferably from about 0.45 weight percent to about 0.55 weight percent.
The support used in the catalyst of the present invention is a highly porous silica which comprises not less than about 93 weight percent silica. Preferably the support will consist essentially of silica with only traces other materials, such as impurities, present. The term “meso pore” refers to only those pores in the high silica support which have a diameter of between 20 A and 1000A. Pores present in the support smaller than 20A are referred to as micropores, while pores larger than 1000A are referred to as macropores. While not wishing to be bound by any particular theory, it is believed that the average size of the mesopores are critical to the present invention, since the mesopores are believed to be the primary sites were catalytic reactions, such as polymerization, occur in the finished catalyst. Likewise surface area and the average pore volume also have been shown to be critical limitations of the support used in the present invention. For the highly porous silica support, it is preferred that the surface area be at least 300 m
2
/gr., the average pore volume be within the range of from about 2.0 cc/gr. to about 4.0 cc/gr., and the mean meso pore diameter be within the range of from about 350 A and about 700A.
Average pore diameters given in this disclosure were measured by the adsorption isotherm method. Total pore volume may be determined by the amount of liquid nitrogen adsorbed at a partial pressure of or near 0.99. Surface area may be determined by the BET surface area method (ASTM D3663). One skilled in the art will recognize that the average pore diameter and pore volume may be somewhat different for the finished catalyst as compared to the support prior to addition of the promoter.
The present invention further relates to a continuous process for polymerizing ethylene which comprises contacting a reaction sysem comprising ethylene with a catalytic amount of a promoter comprising a chromate ester treated with a reducing agent, said promoter being deposited on a highly porous silica support having a minimum surface area of at least about 300 m
2
/gr., an average pore volume of at least about 2.0 cc/gr., and an average meso pore diameter of between about 350 A and 1000 A for a time and under process conditions selected to produce an extrudable polyethylene product. In carrying out the process of the present invention the reaction system may contain only ethylene as a reactant, but in one preferred embodiment of the invention, in addition to the ethylene, the reaction system may also contain from about 0.002 mole ratio to ethylene to about 0.020 mole ratio to ethylene of a mono unsaturated alpha-olefin having from three to eight carbon atoms in the molecule. Preferably the mono unsaturated alpha-olefin present in the reaction system will have from about four to six carbon atoms in the molecule. The process of the present invention is particularly useful in the manufacture of high performance polyethylene products, i.e., polyethylene exhibiting very high PENT, NCTL or other slow crack growth test values as compared to polyethylene products made using conventional chromate ester catalysts.
The present invention is further directed to a polyethylene product obtainable by a process which comprises contacting a feedstream containing ethylene with a catalytic amount of a promoter comprising a chromate ester treated with a reducing agent, said promoter being deposited on a highly porous silica support having a minimum surface area of at least about 300 m
2
/gr., an average pore volume of is at least about 2.0 cc/gr., and an average meso pore diameter of between about 350 A and 1000 A for a time and under process conditions selected to produce an extrudable polyethylene product. The polyethylene product may consist essentially of only ethylene homopolymer, i.e., a polymer resin containing only ethylene monomer, or the polyethylene product will preferably contain co-monomers such as other mono unsaturated olefins besides ethylene. Such co-monomers, if present will usually contain from three to eight carbon atoms in the molecule and more preferably will contain from about four to six carbon atoms in the molecule. Polyethylene products, such as polyethylene pipe, within the scope of the present invention have been demonstrated to have PENT values in excess of 1000 hours and even in excess of 2000 hours.
As used in this disclosure a polyethylene product refers to either a polyethylene resin made using the present invention, or a product formed, usually by extrusion, from the polyethylene resin. Thus as used herein the term “polyethylene product” may be used to refer to a polyethylene homopolymer resin, a polyethylene copolymer resin, or to manufactures made therefrom, such as, for example, polyethylene pipe, polyethylene liner systems, and high molecular weight film.
PENT values used in this disclosure refer to the Pennsylvania Notched Test (ASTM F1473) which measures slow crack growth in the products produced using the resin. PENT values for conventional polyethylene pipe typically fall within the range of 50 to 500 hours. High performance pipe made using bi-modal polyethylene resins typically exhibit PENT values of about 1000 hours. Thus it is particularly surprising that polyethylene pipe may be manufactured using the present invention with PENT values in excess
Kellum Gene E.
Maeger Pamela L.
Chevron Chemical Company LLC
McDermott & Will & Emery
Nolan Sandra M.
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