Structure-performance relationships of Cr/Ti/SiO2 catalysts modified with TEAl for oligomerisation of ethylene: tuning the selectivity towards 1-hexene

A study on the influence of H-2 and the degree of titanation of a Cr/SiO2 Phillips polymerisation catalyst on the selective oligomerisation of ethylene induced by pre-contacting the catalyst with triethylaluminium (TEAl) is presented. Ethylene oligomerisation reactions were performed at 373 K and 1 bar, inside a quartz reactor of a specially designed operando experimental setup, which allowed examination of the catalysts by UV-vis-NIR diffuse reflectance spectroscopy, while the gas phase was continuously monitored by online mass spectroscopy and subsequently analysed by gas chromatography. With this combination of techniques, it was possible to acquire detailed insight into the different distributions of produced oligomers, which were highly dependent on the catalyst structure. The addition of small amounts of Ti significantly changes the electronic environment of Cr oligomerisation sites by the formation of Cr-O-Ti-O-Si linkages, favouring beta-H transfer and increasing the selectivity towards butene at the expense of 1-hexene. Moreover, we show that ethylene oligo-/polymerisation reactions follow at least two different pathways, i.e. metallacycle for olefinic species with a broken Schulz-Flory distribution, and Cossee-Arlman for other hydrocarbon species.