MATERIALS CHEMISTRY FRONTIERS View Article Online RESEARCH ARTICLE View Journal | View Issue Physicochemical surface-structure studies of highly active zirconocene polymerisation Cite this: Mater. Chem. Front., 2020, 4, 3226 catalysts on solid polymethylaluminoxane activating supports† Alexander F. R. Kilpatrick, Nicholas H. Rees, Zoe¨ R. Turner, Jean-Charles Buffet and Dermot O’Hare * Physicochemical surface-structure studies of highly active slurry-phase ethylene polymerisation catalysts has been performed. Zirconocene complexes immobilised on solid polymethylaluminoxane (sMAO) (sMAO–Cp2ZrX2), have been investigated using SEM-EDX, diffuse reflectance FT-IR (DRIFT) and high field (21.1 T) solid state NMR (ssNMR) spectroscopy. The data suggest a common surface-bound cationic 91 Received 14th July 2020, methylzirconocene is the catalytically active species. Zr solid sate NMR spectra of sMAO–Cp2ZrCl2 and Accepted 7th September 2020 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. sMAO–Cp2ZrMe2 are consistent with a common surface-bound Zr environment. However, variation of DOI: 10.1039/d0qm00482k the s-donor (X) groups on the metallocene precatalyst leads to significant differences in polymerisation activity. We report evidence for X group transfer from the precatalyst complex onto the surface of the rsc.li/frontiers-materials aluminoxane support, which in the case of X = C6F5, results in a 38% increase in activity. Introduction benzoic acid.10 We have recently reported the laboratory scale synthesis and detailed characterisation of sMAO,11 and Methylaluminoxane (MAO) is the most commonly used activa- demonstrated its function as a solid-phase support, scavenger This article is licensed under a tor and co-catalyst for transition metal containing, single-site and activator in slurry-phase ethylene polymerisation.12–14 complexes in olefin polymerisation.1,2 The combination of Observed activities are significantly higher than for other supports; MAO, an inert inorganic carrier material (most commonly for example sMAO-rac-ethylenebis(1-permethylindenyl)zirconium silica) and a precatalyst complex have been described by Severn dichloride is at least three times more active than its silica- Open Access Article. Published on 07 September 2020. Downloaded 9/23/2021 11:36:49 PM. 3 À1 À1 À1 as the ‘‘Holy Trinity’’ of supported single-site catalysts. How- supported MAO counterpart (5365 vs. 1649 kgPE molZr h bar ever, there is also growing interest in single-site catalysts in respectively).15,16 particle forming polymerisation processes that are free from an Furthermore, we have shown that reaction of sMAO with inert inorganic carrier.4 These require an activating support tris(pentafluorophenyl)borane or pentafluorophenol produces material, which can be combined with the precatalyst complex highly active modified-sMAO supports which show enhanced in a single synthetic step.5–9 This process has several advan- polymerisation activity with both rac-ethylenebis(1-indenyl) 17 tages from an industrial viewpoint, as it reduces the time and zirconium dichloride, rac-(EBI)ZrCl2, and a range of unsym- energy-intensive drying steps and hence can lower manufactur- metrical ansa-bridged permethylindenyl complexes.18 ing costs. Furthermore, these ‘self-supported’ catalyst systems Characterisation of sMAO using multinuclear NMR spectro- have an advantage over silica-supported systems as the complex scopy in solution and in the solid state reveals an aluminoxane loading can be increased significantly, which leads to corre- structure that features ‘‘structural’’ and ‘‘free’’ AlMex units in spondingly higher polymerisation activities. addition to benzoate residues. Total X-ray scattering measure- In 2013, Tosoh Finechem Corporation reported, in the patent ments on sMAO allow comparisons to be made with simulated literature, an insoluble form of solid methylaluminoxane (sMAO) data from DFT modelled structures of MAO. Of these, the best formed via controlled hydrolysis of trimethylaluminium with fit to the experimental X-ray scattering data resulted from TMA- capped nanotubular and spherical cage structures, (AlOMe)9Á Chemistry Research Laboratory, Department of Chemistry, University of Oxford, 19,20 (AlMe3)3 and (AlOMe)20Á(AlMe3)n, n = 1, 2, respectively. 12 Mansfield Road, Oxford, OX1 3TA, UK. E-mail: [email protected] Our current efforts are focussed on elucidating the structure † Electronic supplementary information (ESI) available: Experimental details (general procedures, syntheses and ethylene polymerisation), additional charac- and chemical environment of the active polymerising species in terising data (solution and solid state NMR spectroscopy, DRIFTS, SEM-EDX sMAO supported catalysts. Significant progress has been made analysis), SEM images for polyethylene samples. See DOI: 10.1039/d0qm00482k in the last few years to determine the function of MAO as a 3226 | Mater. Chem. Front., 2020, 4, 3226--3233 This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2020 View Article Online Research Article Materials Chemistry Frontiers Table 1 Elemental analysis and ethylene polymerisation data for sup- ported complex–sMAO catalysts at target loading [AlsMAO]0/[Zr]0 =50 Complex Al (wt%) Zr (wt%) [AlsMAO]0/[Zr]0 Cp2ZrCl2 36.2 2.36 51.7 Cp2ZrMe2 37.0 2.32 53.9 Cp2ZrBr2 34.1 1.71 67.4 + Scheme 1 Simplified mechanism for metallocene activation by [AlMe2] Cp2Zr(C6H5)2 34.2 1.78 65.1 32,33 groups in silica–MAO, proposed by Weckhuysen and co-workers. Cp2Zr(C6F5)2 35.1 1.73 68.6 Cp2Zr(OC6F5)2 34.4 1.69 68.8 nBu Cp2ZrCl2 36.2 2.26 54.3 + rac-(EBI)ZrCl2 34.9 2.26 52.1 source of the electrophilic cation [AlMe2] , which plays a key Me2Si(C5H4)2ZrCl2 35.7 2.08 58.1 21–31 role in the activation of metallocene complexes in solution. Me2Si(C5H4)2ZrMe2 37.6 2.40 53.1 Weckhuysen and co-workers have recently proposed a similar Immobilisation conditions: sMAO–complex ([AlsMAO]0/[Zr]0 = 50), 80 1C, mechanism which operates in the genesis of the active site for 60 minutes, toluene (30 mL). All elemental analysis experiments were {1,3-nBu,Me}Cp ZrCl immobilised on silica–MAO (Scheme 1).32,33 conducted three times to ensure the reproducibility of the corres- 2 2 ponding outcome. Specifically, metallocene activation occurs by the complexation of the precursor with weak Lewis acid sites of silica–MAO, R + that of the sMAO support (40.2 wt%Al). For all sMAO–Cp 2ZrCl2 which are a source of mobile [AlMe2] groups, leading to formation of the active cationic zirconocene species stabilised catalysts, the [AlsMAO]0/[Zr]0 molar ratio determined was close to on the surface by MAO. the targeted ratio of 50, consistent with full complex immobi- This has prompted us to investigate in more detail the nature lisation. This confirms that the degree of immobilisation of of the active species generated in sMAO supported zirconocene zirconocene dichloride complexes is not affected by the differ- ent ancillary CpR ligands. This can be attributed to the high systems. We have chosen a series of zirconocene(IV) precatalyst 2 À1 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. complexes due to the relative simplicity of their ligand environ- surface area (ca. 600 m g ) and reactivity of sMAO in which all ments which allows more facile characterisation, and enables surface sites can activate zirconocene complexes. The high comparison with previous studies of the active species in homo- concentration of active ‘‘Al–Me’’ moieties is beneficial when geneous and heterogeneous single-site catalyst systems.34–37 compared to MAO-impregnated silica supports, which generally 2 À1 We have employed a combination of characterisation methods, show a lower surface area (ca. 275 m g ) and hence [AlSMAO]0/ including high field solid state nuclear magnetic resonance (ssNMR) [Zr]0 loadings below 131 cannot be achieved without complex spectroscopy, SEM-EDX elemental mapping and diffuse-reflectance leaching and significant reactor fouling. Furthermore, inert FT-IR spectroscopy. Through these fundamental investigations into carrier supports possess –OH groups which provide a pathway 32,38 the active species, we aim to establish structure/catalytic activity for complex deactivation. Comparison of the [AlsMAO]0/[Zr]0 This article is licensed under a R relationships for the immobilised complex and solid support. data for chloride and methide complexes with the same Cp ligands, Cp2ZrX2 and Me2Si(C5H4)2ZrX2 (X = Cl, Me), reveals that both chloride and methide leaving groups result in near- quantitative complex immobilisation. However, the sMAO– Open Access Article. Published on 07 September 2020. Downloaded 9/23/2021 11:36:49 PM. Results and discussion Cp2ZrX2 catalysts derived from metallocenes with aryl and Catalyst synthesis and characterisation aryloxide s-donor groups (X = Ph, C6F5,OC6F5) show slightly higher [AlsMAO]0/[Zr]0 values, suggesting the larger leaving A series of zirconocene(IV)complexes,Cp2ZrX2 (X = Cl, Me, Br, Ph, nBu group limits the extent of complex immobilisation under these C6F5,OC6F5), Cp2ZrCl2, rac-(EBI)ZrCl2,Me2Si(C5H4)2ZrCl2 and conditions. Me2Si(C5H4)2ZrMe2 were selected as precatalysts for this study. In each case, the supported complex–sMAO sample was synthesised Solid MAO is insoluble in aromatic and aliphatic hydrocar- by addition of toluene to a mixture of solid support and complex bons but is sufficiently soluble in THF-d8, to be studied by solution NMR spectroscopy. However, efforts to apply solution (initial loading [AlsMAO]0/[Zr]0