Ethylene Tetramerisation: A Structure‐Selectivity Correlation

Abstract

The effect of ethylene tetramerisation ligand structures on 1‐octene selectivity is well studied. However, by‐product formation is less understood. In this work, a range of PNP ligand structures are correlated with the full product selectivity and with catalyst activity. As steric bulk on the N‐substituent increases, the product selectivity shifts from >10 % to < 3% of both C6 cyclics and C16+ by‐products. 1‐Octene peaks at ca. 70%. Thereafter, only 1‐hexene increases. Similar selectivity changes were observed for ortho‐Ph‐substituted PNP ligands. The C10‐14 selectivity was less affected by the ligand structure. The ligand effect on the changes in selectivity is explained mechanistically. Lastly, an increase in ligand steric bulk was found to improve catalyst activity and reduce polymer formation by an order of magnitude. It is proposed that steric bulk promotes formation of cationic catalytic species which are responsible for selective ethylene oligomerisation.

Chromium-Based Complexes Bearing N-Substituted Diphosphinoamine Ligands for Ethylene Oligomerization

“…Complex 9 afforded the 58.6% selectivity toward C 8 and the 24.7% selectivity toward C 6 (45.8% 1-C 6 ). These results demonstrated that the catalytic activity and the production selectivity were mainly dependent on the electron-donating property and the steric bulk in ligands 1–3 , which was in line with the literature results. , To summarize, the experimental results also revealed that complex 7 can promote the nonselective ethylene oligomerization, while both complex 8 and complex 9 are more beneficial for the selective ethylene tri/tetramerization. Since complex 8 presented the highest C 8 selectivity and the lowest PE content, it was selected to further explore the effect of reaction conditions on the catalytic selective ethylene tri/tetramerization.…”

“…The effects of reaction conditions, including the oligomerization temperature, reaction pressure, and Al/Cr molar ratio, on the catalytic selective ethylene tri/tetramerization were studied with the complex 8/MAO system to achieve the optimal catalytic response. First, the different oligomerization temperatures (40,45,50, and 55 °C) were evaluated, as shown in Figure 2a. The catalytic activity of the complex 8/ MAO system showed a volcano shape with increasing reaction temperatures from 40 to 55 °C, and the highest catalytic activity of 201.9 kg/(g Cr•h) was obtained at 45 °C.…”

“…17,41 Besides, the PE content (0.9%) was the lowest at 45 °C. Second, the effects of ethylene pressures (30,35,40, and 45 bar) on the catalytic activity and the product selectivity with the complex 8/MAO system at 45 °C were further studied. As shown in Figure 2b, the catalytic activity was remarkably increased from 73.1 to 207.7 kg/(g Cr•h) with the increase of ethylene pressures from 30 to 45 bar (Table 3, entries 2 and 5−7), which was mainly attributed to the improved solubility of ethylene in the toluene solvent.…”

Computational Assessment and Understanding of C6 Product Selectivity for Chromium Phosphinoamidine Catalyzed Ethylene Trimerization

“…54 Experimentally, Makume examined purity for ethylene tetramerization with bisphosphine Cr catalysts where the steric bulk on the N-position of the ligand backbone partially controls purity. 55 As one recent example from a computational perspective, Liu and Liu used DFT calculations to understand trimerization/tetramerization selectivity for (2,2-dipicolylamine)Cr catalysts, 56,57 and in addition to the Cr charge controlling selectivity it was proposed that steric effects impact 1-hexene selectivity.…”

A rational approach towards selective ethylene oligomerization via PNP-ligand design with an N-triptycene functionality

“…Owing to the superior material properties of the copolymers derived from 1-hexene/1-octene over other alpha olefins, the search for a catalytic system to selectively produce C 6 /C 8 a-olefins via ethylene tri-/tetramerization has witnessed a steady upsurge in recent years. [1][2][3][4][5][6][7] Most notably, following the emergence of Sasol's initial reports on tetramerization, the chromium-based pre-catalysts where the metal center is stabilized by bidentate phosphine ligands, [8][9][10][11][12][13][14][15][16][17][18] in particular bisphosphineamines (PNPs), [8][9][10][11][12] have received considerable attention. Since the N-substituents of the PNP ligands are believed to play a critical role in oligomerization performance, numerous N-functionalized PNP ligands have been developed and investigated for this purpose.…”