Reaction kinetics of the ethene tetramerization catalyst system CrCl3(THF)3, Ph2PN(iPr)PPh2 and MAO: The unexpected and unusual formation of odd-numbered 1-olefins

This paper describes a kinetic investigation of the ethene tetramerization catalyst system comprising CrCl3(THF)3, a bis(diphenylphosphino)isopropylamine Ph2PN(iPr)PPh2 as ligand and MAO as catalyst activator. The main focus of the experimental study lied on the influence of the ligand to chromium molar ratio on the catalyst performance and the selectivity towards 1-octene. It was found that a sub-stoichiometric ligand/Cr ratio leads to a Schulz–Flory product distribution, while unusual odd-numbered 1-olefins as by-products were formed.

Graphical abstractFor the ethene tetramerization catalyst system of CrCl3(THF)3, Ph2PN(iPr)PPh2 and MAO the ligand/Cr ratio influences remarkably the product distribution. With sub-stoichiometric ligand/Cr ratios the selective process towards 1-hexene and 1-octene changes into a Schulz–Flory-like distribution and odd numbered 1-olefins are formed. This has consequences for the understanding of the catalytic system. Most likely, with the ratio of 0.5 binuclear complexes “(PNP)Cr2” (0.5:1) or higher aggregates with bridging PNP ligands are formed, showing a completely different behavior regarding kinetics and selectivity. The selectivity towards 1-hexene and 1-octene can only be reached by adjustment of a stoichiometric ratio of ligand to chromium, either by a monometallic center like “(PNP)Cr” (1:1) or a bimetallic center like “(PNP)2Cr2” (1:1). Figure optionsDownload full-size imageDownload as PowerPoint slide