Effect of surface hydroxylation on the catalytic activity of a Cr(II)/SiO2 model system of Phillips catalyst

Cr/SiO2 systems (0.1 wt% of Cr) in the form of high surface area glasses have been synthesized. After proper activation in the 823-1023 K range, they transformed to Cr(VI)/SiO2 and then to Cr(II)/SiO2 due to CO reduction (performed at 623 K). Subsequently, C2H4 polymerization activity on these samples has been followed. Surprisingly, ethylene polymerization proceeds faster on samples obtained at 823 K, those ones coming from activation at 1023 K resulting less performing. UV-Vis and FT-IR spectroscopies revealed that the progressive increase of activation temperature leads to even more defined and protruding Cr(II) species at highly de-hydroxylated silica surface: this opens new questions on features of active sites and on the role of silica in preparing such species. Finally, the obtained experimental data have been used as references to validate models and methods adopted to simulate the Cr(II)/SiO2 system, foreseeing to further investigation of the reactivity of Cr(II) toward ethylene. (C) 2015 Elsevier Inc. All rights reserved.