Influence of Ph2PN(R)P(Ph)N(R)H (R=c-Hex, iPr) Ligand Substituents, Modifier, and Process Parameters on Reaction Kinetics of a Chromium-based Catalyst for the Selective Trimerization of Ethylene

• Published in: ChemCatChem 2014-02, Vol.6 (2), p.514-521
• Main Authors: Heinig, Stefan, Wöhl, Anina, Müller, Wolfgang, Al-Hazmi, Mohammed H., Müller, Bernd H., Peulecke, Normen, Rosenthal, Uwe
• Format: Article
• Language: English
• Subjects: Chemical industry; Chromium; Emissions; ethylene; homogeneous catalysis; Ligands; N,P ligands; oligomerization; P ligands; substituents effects
• ISSN: 1867-3880; 1867-3899
• DOI: 10.1002/cctc.201300652

The influence of ligand substituents on a homogeneous, highly selective catalyst system for the ethylene trimerization to 1‐hexene, consisting of chromium(III)acetylacetonate as a chromium source, tetraphenylphosphonium chloride ([Ph4P]Cl) or dodecyltrimethylammonium chloride ([CH3(CH2)11N(CH3)3]Cl) as a modifier and chlorine source, Ph2PN(iPr)P(Ph)N(iPr)H (L0) as a ligand (Ph=phenyl, iPr=isopropyl), and triethylaluminum as an activator, was investigated. It was shown that the phosphonium or ammonium salts act as both chlorine source and as modifier for the activator triethylaluminum. Additionally, a systematic parameter study using Ph2PN(c‐Hex)P(Ph)N(c‐Hex)H (L1) as the ligand (c‐Hex=cyclohexyl) was performed and important performance figures for catalyst systems using L0 and L1 ligands, respectively, were compared under different reaction conditions. As a result, both catalyst systems demonstrate very similar kinetics at identical ethylene pressures, temperatures, and ligand concentrations. Additionally, the influence of the modifiers on the performance of the catalyst systems at different ethylene pressures and temperatures was studied in detail. The results suggest that the modifier has a greater influence on the reaction mechanism than expected. Moreover, the influence of the modifier on the catalytic performance was found to be more significant than the impact of the ligand’s chemical nature. Spurred on by the additive: Small changes in the ligand structure and variation of the additive (phosphonium versus ammonium chloride) for an efficient ethylene trimerization catalyst system provoke unexpected strong effects on the kinetics of the reaction. Surprisingly, the high selectivity for 1‐hexene is not affected.