CR2- FTIR CHARACTERIZATION AND ETHYLENE POLYMERIZATION ACTIVITY( IONSGRAFTED TO SILICA AND SILICALITE SURFACES )

The coordinative situation of Cr2+ ions grafted to the external surfac e of amorphous silica and to the internal surface of silicalite has be en studied by adsorption (at room temperature and 77 K) of different p robe molecules (CO, NO and CO/NO mixtures) by means of FTIR spectrosco py. On amorphous silica three main families (A, B, C) of Cr2+ centres are singled out. The cations with highest coordinative unsaturation ar e able to form Cr2+(CO)n (where 1 less-than-or-equal-to n less-than-or -equal-to 4) polycarbonylic complexes with CO and Cr2+(NO)n (with 2 le ss-than-or-equal-to n less-than-or-equal-to 4) polynitrosylic complexe s with NO. Mixed species formation is observed with CO/NO mixtures. Cr 2+ centres grafted to the internal surface of silicalite are in an ave rage higher coordination state and hence their tendency to form polyca rbonylic species is depressed. The Cr2+ ions on both SiO2 and silicali te polymerize ethylene giving rise to living (CH2)n polymer. The more active sites on Cr2+/silica belong to the Cr(A) family. Due to the ver y fast polymerization rate the initiation mechanism on such catalyst c annot be fully investigated even by means of time resolved FTIR spectr oscopy. On Cr2+/silicalite the spatial limitations imposed by the poro us character of the solid depress the growth of the polymeric chains, so allowing a more accurate study of the initiation steps. It is so co nfirmed that during the chain initiation no methyl groups are formed.