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
1999-06-04
2001-10-09
Wu, David W. (Department: 1713)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Plural component system comprising a - group i to iv metal...
C502S104000, C502S113000, C526S113000, C526S154000
Reexamination Certificate
active
06300272
ABSTRACT:
BACKGROUND TO THE INVENTION
The present invention relates to a process for the production of chromium-based catalysts for the production of polyethylene and to the use of such catalysts.
DESCRIPTION OF THE PRIOR ART
Polyethylene is well known for use in the manufacture of various articles. It is generally desirable for the polyethylene resin to have good processing properties whereby the polyethylene may readily be processed to form the appropriate article. In order to achieve such good processability of the polyethylene resins, it is desired that the flow properties of the polyethylene resins are improved by broadening the molecular weight distribution of the polyethylene. In addition, for some applications, it is desirable that the polyethylene resin has a higher melt index, which is an indication of lower molecular weight polyethylene polymers. A number of different catalyst systems have been disclosed for the manufacture of polyethylene, in particular high density polyethylene HDPE. It is known in the art that the physical properties, in particular the mechanical properties, of a polyethylene product vary depending on what catalytic system was employed to make the polyethylene. This is because different catalyst systems tend to yield different molecular weight distributions in the polyethylene produced. It is known to employ a chromium-based catalyst. Such a chromium-based catalyst enables the production of polyethylene having desirable physical and rheological properties. There is a continuous incentive to develop new chromium-based catalysts for the production of polyethylene resins having improved mechanical or processing properties.
There is a particular need for the development of new blow moulding HDPE with improved environmental stress crack resistance (ESCR)/rigidity compromise. This in turn allows downgauging in the manufacture of bottles, e.g. for corrosive liquids.
Such supported chromium-oxide catalysts which have been developed for the production of linear high density polyethylene resins incorporate a support which is usually a silica, or a modified silica, with a large surface area, typically greater than 200 m
2
/g, and a large pore volume, typically greater than 0.8 ml/g. The support may be modified so as to include cogels such as silica-titania or silica-alumina and by the replacement of silica by alumina or amorphous aluminum phosphates. Furthermore, the support may comprise a tergel which is produced by mixing a chromium source with a silica and a titania compound.
Amorphous aluminum phosphates with both high surface area and pore volume have been found in the art to be difficult to produce. Consequently, it has been known into art to use a process known as “poregelisation” for introducing a metal phosphate, in particular aluminum phosphate, into the silica support. EP-A-0055864 discloses such a process for introducing metal phosphates into the silica support of a chromium-based catalyst for olefin polymerisation. In contrast to standard chromium-based catalysts having a support just of silica which has been impregnated with chromium oxide, the metal phosphate supported chromium-based catalysts are characterised by an outstanding sensitivity to hydrogen. The introduction of hydrogen into the polymerisation medium induces a drastic increase in the melt flow index of the resultant polyethylene resins. In addition, the introduction of triethyl boron (TEB) cocatalysts can result in an increase of the melt flow index of the polyethylene resins, whereas with standard chromium-based catalysts incorporating a silica support the TEB induces a decrease in the melt flow index. In addition, the metal phosphate supported chromium-based catalysts can produce resins with a broad molecular weight distribution and good mechanical properties particularly improved environmental stress cracking resistance (ESCR).
However, the metal phosphate supported chromium-based catalysts disclosed in EP-A-0055864 suffer from the disadvantage that the melt index potential of the catalyst is quite low without the use of hydrogen and/or TEB in a polymerisation medium. Moreover, those catalysts suffer from the technical problem that they have relatively low activity for the polymerisation process for the manufacture of polyethylene. Also, the use of high activation temperatures or of hydrogen often results in a decrease in the mechanical properties of the resins, mainly the ESCR.
The technique of “poregelisation” disclosed in EP-A-0055864 includes the coating of a silica carrier with an amorphous aluminum phosphate by the impregnation of an aluminum phosphate gel inside the pores of the support. Chromium can be added during this step or afterwards, for example by standard impregnation techniques. A particular method disclosed in EP-A-0055864 for impregnating a silica hydrogel or xerogel with aluminum orthophosphate involves combining a source of aluminum ions and phosphate ions with a slurry of a silica hydrogel or xerogel and then evaporating the solvent by conventional techniques whereby aluminum phosphate forms in the pores of the silica. The phosphate gel can be formed in the process by neutralisation with a neutralising agent, such as ammonium hydroxide. The resultant impregnated silica is then dried and activated at elevated temperature.
A commercial poregel catalyst developed according to the process disclosed in EP-A-0055864 is available in commerce from the company Grace Davison. The catalyst is characterised by both low surface area (234 m
2
/g) and pore volume (1.15 ml/g). This results in poor catalyst activity and a low melt index potential. There is a need to improve this commercially available catalyst.
EP-A-0055864 discloses three other methods for producing a metal phosphate supported chromium-based catalyst system for olefin polymerisation.
One, method involves the production of a cogel of silica/alumina/phosphorous oxide by coprecipitation of silicate ions, aluminum ions and phosphate ions, with subsequent neutralisation to form the cogel. The neutralisation may be achieved by concentrated ammonium hydroxide.
The second method forms an aluminum phosphate matrix, with silica as a dispersed phase therein. In this method, a silica hydrogel or xerogel is combined with aluminum orthophosphate which is then precipitated.
The third method involves mixing of a silica xerogel with an aluminum orthophosphate xerogel to form a gel mixture.
WO-A-94/26790 discloses, like the cogel production method described above, a method for making a precursor gel for a catalyst support by cogelification of at least two components chosen among silica, alumina and aluminum phosphate.
EP-A-0799841 also discloses a cogelification process to form a support containing at least two constituents selected from silica, alumina and aluminum phosphate.
EP-A-0712868 and EP-A-0757063 similarly disclose cogelation processes.
SUMMARY OF INVENTION
It is an aim of the present invention to provide a process for producing a catalyst for use in the polymerisation of ethylene which at least particularly overcome the problems of the prior art as identified above. It is a further aim of the present invention to provide such a process wherein the resultant catalyst has improved melt index potential and activity in the polymerisation of ethylene, the activity being improved without requiring the catalyst to be employed either with a cocatalyst such as TEB or with hydrogen. It is a further aim of the present invention to provide such a catalyst which can yield resins with enhanced ESCR/rigidity compromise, and thus improved mechanical properties. It is yet a further aim of the present invention to provide such a catalyst which can yield resins having an increased shear response (SR) and thus improved processability.
Accordingly, the present invention provides a process for producing a chromium-based catalyst for the production of polyethylene, the process comprising providing a silica support having a surface area of at least 380 m
2
/g and a pore volume of at least 1.5 ml/g; impregnating the silica support with a liquid
Fina Research S.A.
Lu Rixia
Wheelington Jim D.
Wu David W.
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