Process for the polymerisation of olefins

Abstract

The present invention relates to a process for polymerisation of olefins, in particular gas phase polymerisation of olefins, with the aid of a supported chromium oxide based catalyst.

1 - 14 . (canceled) 
     
     
         15 . Process for the polymerisation, preferably the gas phase polymerisation, of at least one alpha olefin containing from 2 to 12 carbon atoms in the presence of a supported chromium oxide based catalyst characterised in that
 the supported chromium oxide based catalyst is a titanium modified supported chromium oxide based catalyst;   the chromium oxide based catalyst is supported on a refractory oxide; and   the titanium modified supported chromium oxide based catalyst has been subjected to a 2 steps thermal treatment wherein
 the first step consists in bringing the catalyst under an inert atmosphere to a temperature comprised between 600 and 900° C. and then keeping the catalyst during a holding time of less than or equal to 8 hours under an inert atmosphere at a temperature comprised between 600 and 900° C., and 
 the second step consists in the treatment of the catalyst coming from step 1 under an oxidizing atmosphere, preferably air, at a temperature comprised between 400 and 700° C. during a holding time comprised between 2 and 10 hours, and 
 wherein the maximum temperature of step 2 is always lower than the maximum temperature of step 1 and wherein the difference between the maximum temperature of step 1 and the maximum temperature of step 2 is comprised between 50 and 250° C. 
   
     
     
         16 . Process according to  claim 15  wherein the refractory oxide support for the chromium oxide based catalyst used in the process of the present invention is spherical and/or spheroidal and is preferably chosen amongst silica, alumina, aluminophosphate, metal oxides such as oxides of titanium, zirconium, boron, zinc, magnesium, and the like, or combinations thereof. 
     
     
         17 . Process according to  claim 16  wherein the spherical and/or spheroidal refractory oxide support consists of silica. 
     
     
         18 . Process according to  claim 16  wherein the spherical and/or spheroidal refractory oxide support has a pore volume higher than 1 mL/g. 
     
     
         19 . Process according to  claim 16  wherein the spherical and/or spheroidal refractory oxide support has a specific surface area ranging from 100 to 800 m 2 /g. 
     
     
         20 . Process according to  claim 15  wherein the chromium oxide based catalyst contains 0.3 to 2% by weight of chromium, preferably 0.3 to 1% by weight of chromium, more preferably between 0.3 and 0.7% by weight of chromium, and most preferably between 0.4 and 0.6% by weight of chromium. 
     
     
         21 . Process according to  claim 15  wherein the chromium oxide based catalyst contains between 0.5 and 5% by weight of titanium, preferably between 1.5 and 4%, most preferably between 2 and 4%, and most preferably between 2 and 3% by weight of titanium. 
     
     
         22 . Process according to  claim 15  wherein the holding time during the first thermal treatment is less than or equal to 6 hours, particularly less than or equal to 4 hours; and/or said holding time is at least 30 minutes, particularly at least 1 hour, more particularly at least 1.5 hours. 
     
     
         23 . Process according to  claim 15  wherein the temperature described in the first thermal treatment step is at least 650° C., preferably at least 700° C., for example at least 730° C.; and/or said temperature is less than or equal to 850° C., for example less than or equal to 800° C. 
     
     
         24 . Process according to  claim 15  wherein the second step has a holding time of at least 3 hours, particularly at least 4 hours; and/or said holding time is less than or equal to 9 hours, particularly less than or equal to 8 hours, for example less than or equal to 7 hours. 
     
     
         25 . Process according to  claim 15  wherein the range of temperatures described in the second step is comprised between 500 and 700° C., preferably comprised between 600 and 700° C. 
     
     
         26 . Process according to  claim 15  wherein the maximum temperature of step 2 is always lower than the maximum temperature of step 1 and wherein the difference between the maximum temperature of step 1 and the maximum temperature of step  2  is comprised between  50  and  200 ° C.; preferably, such difference in temperatures is of at least 80° C., preferably at least 100° C. 
     
     
         27 . Process according to  claim 15  wherein step 1 is performed under nitrogen. 
     
     
         28 . Process according to  claim 15  wherein the total duration of any thermal treatment (non oxidising and oxidising steps) above 300° C. of our titanium modified supported chromium oxide based catalyst is always less than 24 hours, preferably less than 22 hours, more preferably less 20 hours and even more preferably less than 18 hours.