METHOD FOR PRODUCING POLYMERISATES OF ETHYLENE BY MEANS OF A SILICONE EXEROGEL / CHROMTRIOXIDE CATALYST.

摘要

1. A process for the preparation of an ethylene homopolymer or an ethylene copolymer containing minor amounts of copolymerized C3 -C12 -alpha-monoolefins by polymerization of the monomer or monomers at from 30 to 150 degrés C, and under from 2 to 150 bar, using a silica xerogel/chrominum trioxide catalyst which is obtained when (1) a finely divided silica xerogel is first prepared by a method in which (1.1) a silica hydrogel which contains from 10 to 25 percent by weight of solid (calculated as silicon dioxide), is substantially spherical and has a particle diameter of from 1 to 8 mm is employed as a starting material, this hydrogel being obtained by a method in which (1.1.1) a sodium or potassium waterglass solution is introduced into a stream of an aqueous mineral acid subjected to angular momentum, the introduction being effected both longitudinally and tangentially with respect to the stream, (1.1.2) the resulting silica hydrosol is sprayed in the form of drops into a gaseous medium, (1.1.3) the atomized hydrosol is allowed to solidify in the gaseous medium to give a hydrogel, and (1.1.4) the resulting, substantially spherical hydrogel is freed from salts by washing, without prior aging, (1.2) not less than 60 % of the water present in the hydrogel (1.1) is extracted using an organic liquid from the series consisting of the C1 -C4 -alkanols and/or C3 -C5 -alkanones, (1.3) the resulting gel which has been dewatered and has been treated with the organic liquid is dried until no further weight loss occurs in the course of 30 minutes at 180 degrees C under a reduced pressure of 10/750 bar (10 mm Hg) (xerogel formation), and (1.4) the xerogel thus obtained is converted to particles having a diameter of from 20 to 2,000 mu m, (2) this xerogel (1) is then loaded with the desired amount of chromium from a 0.05 - 5 percent strength by weight solution of chromium trioxide in a C3 -C5 -alkanone or from a 0.05 - 15 percent strength by weight solution, in a C1 -C4 -alkanol, of a chromium compound which is converted to chromium trioxide under the conditions of stage (3), the particular solvent being permitted to contain no more than 20 percent by weight of water and being evaporated during the procedure, and (3) the resulting product (2) is finally kept in an anhydrous stream of gas containing more than 10 vol percent of oxygen for from 10 to 1,000 minutes at from 400 to 1,100 degrees C, wherein, in addition to the catalyst, a co-catalyst is employed which consists of an open-chain or cyclic alumoxane of the formula R2 AlO[RAlO]m AlR2 ou [RAlO]n respectively, there R is C1 -C12 -alkyl, m is from 0 to 50 and n is from 2 to 20, with the proviso that the atomic ratio of chromium in the catalyst to aluminum in the cocatalyst is from 1:1 to 1:300.