On the active site of the Phillips catalyst for ethylene polymerization

Abstract

The FTIR spectra of CO adsorbed on the chromium(II)-A surface compound (the active site of the Phillips catalyst) before and after short polymerization with ethylene have been reexamined. No difference in the CO spectra of the room temperature triplet at 2190, 2185, and 2179 cm −1 was found for that part of chromium which reacts with ethylene compared to the CO spectra of the unreacted surface compound. The same result was observed with the low temperature CO IR triplet at 2120, 2100, and 2035 cm −1. After low temperature polymerization with ethylene a new CO IR band at 2176.82 cm −1 was observed. After high temperature reaction with ethylene a symmetrical and rather sharp CO IR band at 2171.52 cm −1 was noted at room temperature instead. It is concluded that in both cases rather well-structured chromium surface compounds are formed. The CO IR bands of the room temperature triplet shifted from 2190.79, 2184.74 and 2178.69 cm −1 to 2192.28, 2185.03, and 2180.76 cm −1 after seven oxidation (800 °C, O 2)/reduction (350 °C, CO) cycles. The CO IR bands of the low temperature triplet (−145 °C) separated into two triplets with higher (2121.12, 2102.06, and 2040.37 cm −1) and lower (2117.12, 2098.47 and 2031.34 cm −1) positions than the original one. It is concluded that the surface structure of the support has a rather large influence on the electronic density of the chromium(II) surface ions.