Production of polyethylene

Abstract

The present invention is directed to the use of aluminum alkyl activators and co-catalysts to improve the performance of chromium-based catalysts. The aluminum alkyls allow for the variable control of polymer molecular weight, control of side branching while possessing desirable productivities, and may be applied to the catalyst directly or separately to the reactor. Adding the alkyl aluminum compound directly to the reactor (in-situ) eliminates induction times.

1 . A supported chromium catalyst comprising:
 chromium oxide,   a silica-containing support comprising silica having:
 a pore volume of about 0.9 to about 1.8 cm 3 /g and a surface area of about 245 to about 590 m 2 /g; 
   and,   an organoaluminum compound;   wherein said supported chromium catalyst is activated at about 400 to about 860° C.   
   
   
       2 . The supported chromium catalyst of  claim 1  wherein said organoaluminum compound is added in situ. 
   
   
       3 . The supported chromium catalyst of  claim 1  wherein said organoaluminum compound is an alkyl aluminum alkoxide compound. 
   
   
       4 . The supported chromium catalyst of  claim 3  wherein said alkyl aluminum alkoxide compound is diethyl aluminum ethoxide, diethyl aluminum methoxide, dimethyl aluminum ethoxide, di-isopropyl aluminum ethoxide, diethyl aluminum propoxide, di-isobutyl aluminum ethoxide, methyl ethyl aluminum ethoxide, or a combination thereof. 
   
   
       5 . The supported chromium catalyst of  claim 3  formed by the in-situ addition of said alkyl aluminum alkoxide compound. 
   
   
       6 . The supported chromium catalyst of  claim 5  wherein said alkyl aluminum alkoxide compound is diethyl aluminum ethoxide, diethyl aluminum methoxide, dimethyl aluminum ethoxide, di-isopropyl aluminum ethoxide, diethyl aluminum propoxide, di-isobutyl aluminum ethoxide, methyl ethyl aluminum ethoxide, or a combination thereof. 
   
   
       7 . The supported chromium catalyst of  claim 1  wherein said supported chromium catalyst is activated at about 600 to about 860° C. 
   
   
       8 . The supported chromium catalyst of  claim 1  further comprising titanium tetraisopropoxide. 
   
   
       9 . The supported chromium catalyst of  claim 1  wherein said organoaluminum compound is an alkyl aluminum compound. 
   
   
       10 . The supported chromium catalyst of  claim 9  wherein said alkyl aluminum compound is selected from the group consisting of triethyl aluminum, tri-isobutyl aluminum, and tri-n-hexyl aluminum. 
   
   
       11 . The supported chromium catalyst of  claim 9  formed by the in situ addition of said alkyl aluminum compound. 
   
   
       12 . The supported chromium catalyst of  claim 11  wherein said alkyl aluminum compound is triethyl aluminum. 
   
   
       13 . A process for producing an ethylene polymer comprising the steps of contacting ethylene under polymerization conditions with the supported chromium catalyst of  claim 1 . 
   
   
       14 . The process of  claim 13  further comprising controlling catalyst productivity, reaction induction time, and polymer molecular weight of the resulting ethylene polymer by the addition of an organoaluminum compound in an amount to effect a final ratio of aluminum to equivalents of chromium of from 0.1:1 to about 10:1. 
   
   
       15 . The supported chromium catalyst of  claim 1  wherein the silica has a pore volume of about 1.1 to about 1.8 cm 3 /g and a surface area of about 245 to about 375 m 2 /g. 
   
   
       16 . The supported chromium catalyst of  claim 1  wherein the silica has a pore volume of about 0.9 to about 1.4 cm 3 /g and a surface area of about 390 to about 590 m 2 /g. 
   
   
       17 . The supported chromium catalyst of  claim 1  wherein said organoaluminum compound is present in an amount sufficient to produce a gas phase polymerization reaction temperature which is at least 2.5° C. higher than a comparable gas phase polymerization reaction temperature obtained when polymerizing the same olefins with the same chromium catalyst system under the same polymerization conditions to produce a polymer having the same molecular weight and density at the same space-time-yield value, in the absence of said organoaluminum compound. 
   
   
       18 . A supported chromium catalyst comprising:
 silylchromate, a silica-containing support comprising silica selected from the group consisting of silica having:
 (a) a pore volume of about 1.1 to about 1.8 cm 3 /g and a surface area of about 245 to about 375 m 2 /g; 
 (b) a pore volume of about 2.4 to about 3.7 cm 3 /g and a surface area of about 410 to about 620 m 2 /g; and 
 (c) a pore volume of about 0.9 to about 1.4 cm 3 /g and a surface area of about 390 to about 590 m 2 /g; 
 and, 
   an organoaluminum compound;   wherein said supported chromium catalyst is dehydrated at about 400 to about 860° C.   wherein said silylchromate is loaded onto said silica-containing support at a loading of about 0.15 to about 1.0 weight percent chromium.   
   
   
       19 . The supported chromium catalyst of  claim 18  wherein said supported chromium catalyst is formed by in-situ addition of at least a portion of said organoaluminum compound to said silylchromate and said silica containing support in a polymerization rector under polymerization conditions.