Insights into the structural studies of chromium precursors in the Phillips ethylene trimerization catalyst system: a mini review

Since the discovery of selective oligomerization of ethylene to its trimer, namely hexene-1, in the mid-60s, it has stimulated continued research activity on unique catalyst systems in industries and academic institutions. The successful commercialization of a chromium-based catalyst for producing hexene-1 by Chevron-Phillips Chemical Company was first demonstrated in 2003 by establishing a plant in the Middle East and, more recently, expanding its capacity in the US. Although a few other industries have made progress in developing hexene-1 technology based on catalysts involving chromium compounds, the actual identity of the active species responsible for ethylene trimerization remains elusive. It is pertinent to note that compared to the conventional linear alpha olefin processes for 1-hexene production via non-selective ethylene oligomerization, the on-purpose ethylene trimerization process is favored owing to its high atom efficiency and reduced downstream separation costs. It is evident that despite publications in this area, there appears to be a lack of complete theoretical understanding of the factors influencing the catalytic performance of these chromium-based systems, leading to a trial-and-error approach as the main strategy in catalyst development. In this article, the intriguing but challenging aspects of structural studies carried out so far on the chromium precursors employed in commercial processes are compiled. The earlier chromium precursor complexes employed in the catalyst systems developed by Phillips Petroleum Company and the currently used chromium(iii) carboxylate precursor in the Chevron-Philips technology (CPC) have also been compared based on patented and published research. The available literature information has helped provide definitive mechanistic insights into the landmark CPC ethylene trimerization system, which combines studies involving experimental investigations of unresolved issues in addition to exclusive theoretical calculations on the Cr(iii)2-ethylhexanoate/2,5-dimethylpyrrole/alkylaluminum-based model system.