Selective ethylene oligomerization with novel chromium diphosphinoamine catalysts

Abstract

A series of dual-site diphosphinoamine ligands with different electronic density and steric bulkiness were synthesized. The obtained ligands were characterized by $^1H$ NMR, $^{13}C$ NMR, $^{31}P$ NMR and elemental analysis. In combination with Cr(Ⅲ) and methylaluminoxane (MAO), they generate active catalytic systems for ethylene tetramerization toward 1-octene. The ethylene oligomerization tests of all ligands were conducted at 30 atm using chromium precursors, $CrCl_3(THF)_3$ and $Cr(acac)_3$, and the products were analyzed by gas chromatography (GC) and gas chromatography-mass spectroscopy (GC-MS). In the catalytic tetramerization of ethylene, diphosphinoamine ligand containing electron donating group has higher selectivity to 1-octene, than the ligand containing electron withdrawing groups. Furthermore, the relative selectivity of 1-hexene to 1-octene was increased with the ligand having more substituents at ortho-position of aryl group attached to nitrogen. Tetramerization of ethylene by the chromium diphosphinoamine catalysts at various reaction tem-perature produced α-olefin mixtures with unexpected product distribution. These catalysts produced more 1-hexene than 1-octene at the temperature below 30℃. Such a tendency is quite different from the previously reported chromium catalysts where ethylene trimerization is predominant at higher temperature. In addition, polymer-supported catalysts which incorporate diphosphinoamine ligands on the polymeric backbone were synthesized by anionic copolymerization of diphosphinoamine monomer and styrene in molar ratio of 1 to 7. The ethylene oligomerization reactions were investigated with the polymeric diphosphinomine (PNP) ligand. However, the chromium catalysts with these polymeric ligands produced a mixture of linear α-olefins (LAOs) with Schultz-Flory distribution.