Tuning product selectivity in catalytic ethylene tetramerization systems

Abstract

The catalytic oligomerization of ethylene into linear alpha olefins (LAOs) is an important industrial process that yields valuable commodity products whose utility depends on the LAO chain length. While selective LAO prodn. is desirable in terms of avoiding energy-intensive sepns. and disentangling the economics of producing and commercializing each LAO, most large-scale industrial processes instead yield complex mixts. On the other hand, ethylene oligomerization catalysts which operate under matallacycle-based mechanisms have enabled selective formation of LAOs. These Cr-based catalysts have been most successfully applied in ethylene trimerization systems, while tetramerization catalysts yield markedly lower selectivities for the target 1- octene. Optimization of catalytic ethylene tetramerization systems has been hampered by our limited understanding of the catalyst activation process and of the identity of the different catalytic species which presumably give rise to different oligomeric and polymeric products. By identifying these active species, and understanding which conditions promote their formation, one could in principle redesign the catalytic system so that the selectivity of the desired product is increased. Herein we show that, indeed, interactions between different activator species and the supporting ligand can give rise to relatively active and undesired ethylene polymn. catalysts. By identifying and controlling these interactions, we show that the selectivity and activity of the ethylene tetramerization system can be substantially improved. In doing so, we provide the first rational framework to explain the effects of ligand modification on activity and selectivity for undesired byproducts, which should guide the design of future ligands. Furthermore, we rationalize the necessity for large excesses of activator based on its effects on interactions with the supporting ligand, thus suggesting a venue to ultimately lower the required excess. Finally, we introduce an alternative activation process that minimizes polyethylene and which can be readily applied wherein moisture is challenging to exclude.