Nature of Surface Sites in Ziegler-Natta Pre-Catalysts from Quanti-tative Analysis of Ti K-edge X-Ray Absorption Spectra

Ziegler-Natta catalysts, discovered in 1950s, are still nowadays of highest importance for the industrial production of polyethylene and polypropylene. Despite being the workhorse of polymer industry and despite the years of extensive research, the improvement of Ziegler-Natta catalysts has been mostly empirical. In particular, the coordination surrounding of Ti sites in pre-catalysts, key for the formation of active sites upon AlEt3 activation, has been highly debated. In this work we prepared a series of pre-catalysts by increasing the concentration of BCl3 modifier in the synthesis, known to affect the amount of Cl- and alkoxo-ligands in the Ti local surrounding as well as the activity in ethylene polymerization. We develop a methodology, relying on a library of theoretical XAS spectra for DFT-optimized structural models and benchmarked on a series of well-defined molecular compounds. Quantitative analysis of XAS data allows us to evaluate the relative fractions of Ti sites in fully Cl-surrounding as well as containing alkoxide and O-donor ligands. The analysis shows that the number of Ti-O bonds (initially mostly present in alkoxo-form) decreases upon treatment with BCl3 up to B/Ti ratio of 2, coherent with earlier proposals, and increases afterwards, due to O-donor ligands, likely related to B-alkoxo species coordinated to Ti. The catalytic activity of these pre-catalysts after activation with AlEt3 in ethylene polymerization passes through the maximum at B/Ti ratio of ~2, pointing to its relation to the detrimental effect of O-based ligands on the activity. Thus, the approach developed allows to address and quantify the metal speciation in such complex systems as Ziegler-Natta pre-catalysts and relate this to their catalytic activity, a first step towards establishing quantitative structure-activity relationships.