Methylaluminoxane-Free Chromium Catalytic System for Ethylene Tetramerization

Area of Science:

• Organometallic Chemistry
• Catalysis
• Polymer Science

Background:

• Ethylene tetramerization to 1-octene traditionally uses chromium(III) complexes with methylaluminoxane (MAO) cocatalysts.
• The high cost and excess usage of MAO present a significant economic and practical challenge in these catalytic systems.
• Development of MAO-free alternatives is crucial for improving the industrial viability of 1-octene production.

Purpose of the Study:

• To develop and characterize a novel MAO-free catalyst system for ethylene tetramerization.
• To evaluate the catalytic performance, including activity and selectivity, of new chromium(III) complexes activated by trialkylaluminum species.
• To compare the efficiency of the new system with existing MAO-based catalysts.

Main Methods:

• Synthesis of cationic chromium(III) complexes featuring PNP ligands and non-coordinating tetrakis(pentafluorophenyl)borate ([B(C6F5)4]-) anions.
• Structural characterization of key intermediates using X-ray crystallography.
• Catalytic testing of the synthesized complexes with various trialkylaluminum activators in different solvents for ethylene tetramerization.

Main Results:

• Novel cationic PNP-Cr(III) complexes, [(PNP)CrCl2(CH3CN)2]+[B(C6F5)4]-, were successfully synthesized and structurally characterized.
• These complexes, when activated by trialkylaluminum compounds (e.g., Et3Al, iBu3Al), exhibited high activity and selectivity for 1-octene production, comparable to MAO-based systems.
• A complex with long alkyl chains ([2]-Cr complex) showed exceptional activity (168 kg/g-Cr/h) in cyclohexene, with minimal deactivation and low polyethylene byproduct formation (0.3%).

Conclusions:

• The developed MAO-free chromium catalyst systems offer a promising and cost-effective alternative for 1-octene synthesis.
• The choice of PNP ligand and activator significantly influences catalytic performance and solvent compatibility.
• The [B(C6F5)4]- anion demonstrates compatibility with trialkylaluminum activators, enabling high catalytic efficiency without trityl cations.