Fully diphosphinoamine (PNP)-derived chromium-based porous organic polymers for pore-confined ethylene tetramerization with high 1-octene selectivity

Porous organic polymers (POPs) are promising heterogeneous catalyst supports. However, their application in ethylene tetramerization for high-value 1-octene (1-C 8 ) is limited by nonuniform metal coordination and poorly controlled pore-confined reactions. Here, one-step solvent thermal polymerization was carried out by vinyl functionalized full diphosphine (PNP) ligand to synthesize POPs derived from the PNP ligand and used as a support for chromium-based catalysts. The characterization confirms that the interaction between POPs, modified methylaluminoxane (MMAO) and Cr(III) chloride tetrahydrofuran complex promotes the formation of Cr(III) active sites, and the chemical coordination environment of the support determines the concentration of these Cr(III) species. The adjustable POPs framework can selectively produce linear alpha-olefins (C 4 , C 6 and C 8 ) with different carbon numbers. Notably, POP-2, which exhibited a larger pore size, lower steric hindrance and a higher density of Cr(III) active sites, showed exceptional catalytic performance in the ethylene tetramerization reaction under 1 MPa and 45℃ conditions, achieving 51.54% of C 8 selectivity (1-C 8 accounted for 97.6%), C 6 /C 8 total selectivity of 88.11%, and high catalytic activity of 8.08 × 10 5 g(mol·Cr) −1 h −1 . Whereas the high-steric-hindrance POP-3 system is predisposed to trimerization. The calculation of density functional theory (DFT) revealed that the tetramerization pathway is energetically preferred over the trimerization pathway in the POPs/Cr(III) system. These results further clarified that product carbon number distribution is dually governed by the Gibbs free energy barriers of trimerization/tetramerization and the steric hindrance of Cr(III) active centers. Collectively, these findings highlight the remarkable application potential of POPs as tunable matrices for engineering Cr(III) sites and fabricating high-selectivity ethylene oligomerization catalysts.