MAO‐free and extremely active catalytic system for ethylene tetramerization

Abstract

The original Sasol catalytic system for ethylene tetramerization is composed of a Cr source, a PNP ligand, and MAO (methylaluminoxane). The use of expensive MAO in excess has been a critical concern in commercial operation. Many efforts have been made to replace MAO with non‐coordinating anions (e.g., [B(C6F5)4]−); however, most of such attempts were unsuccessful. Herein, an extremely active catalytic system that avoids the use of MAO is presented. The successive addition of two equivalent [H(OEt2)2]+[B(C6F5)4]− and one equivalent CrCl3(THF)3 to (acac)AlEt2 and subsequent treatment with a PNP ligand [CH3(CH2)16]2C(H)N(PPh2)2 (1) yielded a complex presumably formulated as [1‐CrAl (acac)Cl3(THF)]2+[B(C6F5)4]−2, which exhibited high activity when combined with iBu3Al (1120 kg/g‐Cr/h; ~4 times that of the original Sasol system composed of Cr (acac)3, iPrN(PPh2)2, and MAO). Via the introduction of bulky trialkylsilyl substituents such as –SiMe3, –Si(nBu)3, or –SiMe2(CH2)7CH3 at the para‐position of phenyl groups in 1 (i.e., by using [CH3(CH2)16]2C(H)N[P(C6H4‐p‐SiR3)2]2 instead of 1), the activities were dramatically improved, i.e., tripled (2960–3340 kg/g‐Cr/h; more than 10 times that of the original Sasol system). The generation of significantly less PE (<0.2 wt%) even at a high temperature is another advantage achieved by the introduction of bulky trialkylsilyl substituents. NMR studies and DFT calculations suggest that increase of the steric bulkiness on the alkyl‐N and P‐aryl moieties restrict the free rotation around (alkyl)N–P (aryl) bonds, which may cause the generation of more robust active species in higher proportion, leading to extremely high activity along with the generation of a smaller amount of PE.

Extremely Active Ethylene Tetramerization Catalyst Avoiding the Use of Methylaluminoxane: [iPrN{P(C6H4‐p‐SiR3)2}2CrCl2]+[B(C6F5)4]−

“…Preparation of the catalyst precursors discussed in a previous work. [46] 4 ] À (1.0 eq), CrCl 3 (THF) 3, and 2 were mostly soluble in methylcyclohexane. The methylcyclohexane-soluble portion (92 wt%) was taken by filtration for the use in the oligomerization studies.…”

“…In contrast, the reactor was fully filled with PE when the oligomerization was performed with the Cr-complex prepared with 1 bearing no À SiR 3 substituents (i. e., [1-CrAlCl 3 (acac) (THF)] + [B(C 6 F 5 ) 4 ] À ). [46] By lowering the reaction temperature, the 1-octene/1-hexene ratio increased accordingly, reaching 67/16 at a temperature of 60°C (entries 3-6). When methylcyclohexane solvent was replaced with toluene, the activity was significantly reduced to 1200 kg/g-Cr/h.…”

“…[29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] 4 ] À (2.0 eq), CrCl 3 (THF) 3 , and PNP ligand that exhibited high activity (up to 4000 kg/g-Cr/h) when combined with common and inexpensive alkylaluminum iBu 3 Al (Scheme 1). [46] We presumed that the chloride-bridged adduct (i. e., [(PNP)Cr(Cl)(THF)(μ 2 -Cl) 2 Al(acac)] 2 + [B(C 6 F 5 ) 4 ] À 2 ) might be formed between the cationic Cr species [(PNP)CrCl 2 ] + [B(C 6 F 5 ) 4 ] À and the cationic Al species [ClAl(acac)] + [B(C 6 F 5 ) 4 ] À . The role of aluminum part might be blocking the formation of bis(PNP)-Cr III species (i. e., [(PNP) 2 CrCl 2 ] + [B(C 6 F 5 ) 4 ] À ).…”

Replacement of the Common Chromium Source CrCl3(thf)3 with Well-Defined [CrCl2(μ-Cl)(thf)2]2

“…CrCl 3 (thf) 3 has been actively used as a component of the ethylene tetramerization catalyst system [ 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 ]. We also developed a very efficient catalyst for ethylene tetramerization ( Scheme 1 ) [ 35 , 36 , 37 ]. The catalyst shows extremely high activity (~5000 kg/g-Cr/h) and is advantageous over other reported systems in that it works with an inexpensive activator, iBu 3 Al, avoiding the use of expensive MAO in excess (A1/Cr = 300–500) [ 38 , 39 , 40 , 41 , 42 , 43 ].…”

Computer-assisted catalyst development via automated modelling of conformationally complex molecules: application to diphosphinoamine ligands

“…Both PNP and PPN molecules possess many conformations, and analysis of the wrong conformer will result in incorrect ΔG PPN . A previous DFT study of PNP ligand conformers found variation of up to 9.9 kcal/mol in calculated Gibbs free energy 25 , which would drastically impact any thermodynamic comparison with PPN isomer.…”