Catalyst precursor particles, their preparation and use
A method for preparing a catalyst precursor for an olefin polymerization catalyst involves the use of aqueous or alcoholic solutions of a chromium salt and of boric acid and aluminum carboxylate for deposition onto an inorganic support material, such as a silica xerogel. The chromium salt, aluminum carboxylate and boric acid are sufficiently soluble for deposition from a single solution to be effective. The catalyst precursor can be activated by calcination to form a catalyst for homo- or co-polymerisation of α-olefins which has productivity and melt flow index for the resulting polymer or copolymer which is comparable to results obtained with catalysts prepared by prior art organometallic routes. The activation of the catalyst precursor gives reduced levels of toxic or noxious fumes during activation compared to use of organometallic sources of chromium or aluminum.
1. A method for preparing a catalyst precursor for an olefin polymerization catalyst, the catalyst precursor comprising an inorganic support material carrying a chromium salt, aluminium carboxylate and boric acid, the method comprising
i) providing an inorganic support material,
ii) depositing a chromium salt onto the inorganic support material from a first solution comprising a chromium salt in a solvent which is water, C 1 to C 4 aliphatic alcohol, or a mixture thereof,
iii) depositing an aluminium carboxylate and boric acid onto the inorganic support material from a second solution comprising an aluminium carboxylate and boric acid in a solvent which is water, C 1 to C 4 aliphatic alcohol, or a mixture thereof, and
iv) removing solvent to form the catalyst precursor comprising chromium salt, aluminium carboxylate and boric acid, wherein the inorganic support material is a silica xerogel comprising at least 90% by weight of SiO 2 .
2. The method of claim 1 wherein the chromium salt is carboxylate, sulphate, chloride or a mixture thereof.
3. The method of claim 1 wherein the chromium salt is acetate.
4. The method of claim 1 wherein the inorganic support material is in the form of particles having a weight mean particle diameter from 1 to 300 micrometers.
5. The method of claim 1 wherein the silica xerogel is prepared by evaporation of water from a silica hydrogel.
6. The method of claim 1 wherein the catalyst precursor comprises the chromium salt, aluminium carboxylate and boric acid within a pore structure of the inorganic support material.
7. The method of claim 1 wherein the aluminium carboxylate is aluminium acetate.
8. The method of claim 1 wherein the catalyst precursor comprises, as carried material, from 0.01 to 3% by weight of chromium expressed as the element, from 0.1 to 8% of aluminium expressed as the element and from 0.005 to 2% of boron expressed as the element.
9. The method of claim 1 wherein the chromium salt is deposited onto the inorganic support material from the first solution, and solvent is removed prior to the aluminium carboxylate and boric acid being deposited onto the inorganic support material from the second solution.
10. The method of claim 1 wherein the aluminium carboxylate and boric acid are deposited onto the inorganic support material from the second solution, and solvent is removed prior to the chromium salt being deposited onto the inorganic support material from the first solution.
11. The method of claim 1 comprising the steps of:
i) providing an inorganic support material,
ii) providing a solution comprising chromium salt, aluminium carboxylate and boric acid in a solvent which is water, C 1 to C 4 aliphatic alcohol, or a mixture thereof,
iii) depositing the solution onto the inorganic support material, and
iv) removing the solvent to form the catalyst precursor comprising chromium salt, aluminium carboxylate and boric acid.
12. A method for preparing an olefin polymerisation catalyst comprising heating a catalyst precursor prepared by the method of claim 1 in a non-reducing atmosphere at a temperature from 200 to 1200° C. for a time period from 30 minutes to 15 hours.