Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Phosphorus or compound containing same
Reexamination Certificate
2001-01-23
2002-07-23
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Phosphorus or compound containing same
C502S210000, C502S214000, C502S239000, C502S242000, C502S256000, C502S309000, C502S349000, C423S306000
Reexamination Certificate
active
06423663
ABSTRACT:
The present invention relates to a highly active catalyst for producing polyethylene having good processability and good mechanical properties. The present invention further relates to a process for producing said catalyst and to the use of such a catalyst.
For polyethylene, and for high density polyethylene (HDPE) in particular, the molecular weight distribution (MWD) is a fundamental property which determines the properties of the polymer, and thus its applications. 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; it is in particular desirable that the polyethylene resin has a high melt index, which is an indication of low molecular weight polyethylene polymers in the polyethylene resin. Good physical properties can be obtained with polyethylene having a high molecular weight. These high molecular weight molecules, however, render the polymer more difficult to process. The broadening of the molecular weight distribution permits an improvement in the processing of polyethylene at high molecular weight while keeping its good physical properties.
The molecular weight distribution can be completely defined by means of a curve obtained by gel permeation chromatography. Generally, the molecular weight distribution (MWD) is more simply defined by a parameter, known as the dispersion index D, which is the ratio between the average molecular weight by weight (Mw) and the average molecular weight by number (Mn). The dispersion index constitutes a measure of the width of the molecular weight distribution. For most applications, the molecular dispersion index varies between 10 and 30.
A variaty of catalyst systems are known for the manufacture of polyethylene. It is known in the art that the physical properties, in particular the mechanical properties, of a polyethylene resin can vary depending on what catalyst 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 i.e. a catalyst known in the art as a “Phillips 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 Phillips catalysts for the production of polyethylene resins having improved mechanical or processing properties.
Such supported chromium-oxide Phillips catalysts which have been developed for the production of linear high density polyethylene resins incorporate a support which is usually a 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 aluminium phosphates. Furthermore, the support may comprise a tergel which is produced by mixing a chromium source with the silica and titania compound.
EP-A-0712868 discloses a catalytic system for the polymerisation of olefins comprising a catalytic solid based on chrome deposited on a support comprising silica, alumina and aluminium phosphate, together with an organoaluminium co-catalyst. Titanium dioxide may be present in the support. The support may be prepared by a co-precipitation technique.
U.S. Pat. No. 4,727,124 and EP 0250860 disclose the preparing of a supported catalyst which is laden with chromium, phosphorous and titanium and has a silicate carrier. The silicate carrier is mixed successively with suspensions incorporating chromium trioxide, a phosphate or phosphite and a titanate prior to the suspension being evaporated to dryness.
Amorphous aluminium phosphates with both high surface area and high pore volume have been found in the art to be difficult to produce. Consequently, it has been known to use a process known as “poregelisation” for introducing a metal phosphate, in particular an aluminium phosphate, into the silica support. EP-A-0,055,864 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 silica support 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) cocatalyst 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).
The metal phosphate supported chromium-based catalysts disclosed in EP-A-0,055,864 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.
The technique of “poregelisation” disclosed in EP-A-0,055,864 includes the coating of a silica carrier with an amorphous aluminium phosphate by the impregnation of an aluminium 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-0,055,864 for impregnating a silica hydrogel or xerogel with aluminium orthophosphate involves combining a source of aluminium ions and phosphate ions with a slurry of a silica hydrogel or xerogel and then evaporating the solvent by conventional techniques whereby aluminium phosphate forms 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-0,055,864 is available in commerce from the company Grace Davison. The catalyst is characterised by a surface area of 234 cm
2
l/g and a pore volume of 1.15 ml/g; it has a poor catalyst activity and a low melt index potential. There is a need to improve this commercially available catalyst.
EP-A-0,055,864 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, aluminium ions and phosphate ions, with subsequent neutralisation to form the cogel. The neutralisation may be achieved by concentrated ammonium hydroxyde.
The second method forms an aluminium phosphate matrix, with silica as a dispersed phase therein. In this method, a silica hydrogel or xerogel is combined with aluminium orthophosphate which is then precipitated.
The third method involves mixing of a silica xerogel with an aluminium orthophosphate xerogel to form a gel mixture.
WO-A-94/26798 also discloses a cogelation process in which a catalyst support containing at least two components chosen among silica, alumina and aluminium phosphate is formed as a gel which is then washed and dried to form a powder which is then calcined. Titanium may be present in the support.
These known catalysts are often used with a triethylboron (TEB) cocatalyst. They are quite sensitive to the introductio
Bell Mark L.
Fina Research S.A.
Hailey Patricia L.
Jackson William D.
LandOfFree
Titanated chromium/silica-aluminophosphate catalyst does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Titanated chromium/silica-aluminophosphate catalyst, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Titanated chromium/silica-aluminophosphate catalyst will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2863056