THE PHILLIPS CATALYST (CR(VI) SILICA-GEL) MODIFIED BY (NH4)2SIF6 - ITS INFLUENCE ON THE CHROMIUM SURFACE SPECIES

Low temperature CO IR spectra of the reduced Phillips catalyst show that treatment with (NH4)2SiF6 decreases the amount of chromium(III) and the dinuclear Cr(II)-C surface species, so that the catalytically active sites consist almost exclusively of the dinuclear Cr(II)-A surface species. The IR band of terminal CO adsorbed on this Cr(II)-A species is shifted by 5 cm-1 to 2183 cm-1, which is interpreted as due to decreasing electron density at the chromium ion. These results are in agreement with polymerization measurements described in the literature, in which a narrowing of the molecular weight distribution of the polymer and a better comonomer incorporation were observed. Treatment of the Phillips catalyst with (NH4)2SiF6 decreases the amount of surface silanol groups during the activation between 600 and 800-degrees-C by some 80% but shifts the 0-H IR band of the remaining silanol groups only by 1 cm-1. It is concluded that fluorine covers almost the entire surface, but is not bonded together with OH at the same silicon atom. However, from a comparison of the IR bands from CO adsorbed on chromium supported on silica gel or amorphous AlPO4, it is concluded that fluorine is bonded to the silicon atom that binds to chromium via an oxygen atom. Modification of the (NH4)2SiF6 treated Phillips catalyst with titanium showed only a small effect.