Temperature resolved FTIR spectroscopy of Cr2+/SiO2catalysts: acetylene and methylacetylene oligomerisation

Abstract

As is well known, the Cr²⁺/SiO₂ system is an efficient catalyst for ethylene polymerisation already at RT (even if in the industrial process the running temperature is 380 K). For this reason it is the ideal system for in situ spectroscopic investigations on a genuinely working catalyst (Adv. Catal., 2001, 46, 265). Many questions are still uncertain as to the initiation, propagation and termination steps of the polymerisation mechanism. On a pre-reduced sample polymerisation commences very quickly and there is no way to record initial stages. When the experiments are performed at RT the phenomenon is too fast and IR spectroscopy, even in the time-resolved mode, failed up to now in the identification of the species formed during the initiation of the ethylene polymerisation step on the Cr²⁺/SiO₂ Phillips catalyst. We present results related to time-resolved FTIR spectroscopy at variable temperature of acetylene and methylacetylene oligomerisation on a model Phillips catalyst. These experiments have been highly informative on the nature of the active sites because we have observed that acetylene and methylacetylene result in the immediate formation of benzene and 1,3,5-trimethylbenzene, respectively, without the evidence of any measurable intermediate product. This implies that the active Cr sites are able to coordinate simultaneously three monomers and thus must exhibit a high unsaturative coordination. The results of these experiments could be an insight of chromium species active in the Phillips catalyst.