Abstract
Sequential treatment of a previously-calcined solid oxide support (i.e. SiO2, γ-Al2O3, or mixed SiO2–Al2O3) with solutions of Cr{N(SiMe3)2}3 (0.71 wt% Cr) and a Lewis acidic alkyl aluminium-based co-catalyst (15 molar equivalents) affords initiator systems active for the oligomerisation and/or polymerisation of ethylene. The influence of the oxide support, calcination temperature, co-catalyst, and reaction diluent on both the productivity and selectivity of the immobilised chromium initiator systems have been investigated, with the best performing combination (SiO2−600, modified methyl aluminoxane-12 {MMAO-12}, heptane) producing a mixture of hexenes (61 wt%; 79% 1-hexene), and polyethylene (16 wt%) with an activity of 2403 g gCr−1 h−1. The observed product distribution is rationalised by two competing processes: trimerisation via a supported metallacycle-based mechanism and polymerisation through a classical Cossee-Arlman chain-growth pathway. This is supported by the indirect observation of two distinct chromium environments at the surface of the oxide support by a solid-state 29Si NMR spectroscopic study of the Cr{N(SiMe3)2}x/SiO2−600 pro-initiator.
Highlights
Today’s market for short-chain linear α-olefins (LAOs) is so demanding that the traditional synthetic routes for their manufacture such as Ziegler- and SHOP-type oligomerisation processes, which give rise to statistical LAO product distributions, cannot keep pace [1]
These findings suggest that more than one catalytically active chromium species may be present at the surface of the partially dehydroxylated S iO2−600 catalyst support. To investigate this possibility, the intrinsic paramagnetic nature of the oxide-immobilised chromium species has been exploited to help probe indirectly the nature of the supported transition metal species using solid-state 29Si nuclear magnetic resonance (NMR) spectroscopy. This builds upon a previous study that demonstrated that magic-angle spinning (MAS) 29Si NMR spectroscopy provides information regarding the nature of the catalytically-relevant paramagnetic chromium species present in the related long-standing commercial “chromium” on silica Phillips ethylene polymerisation catalyst [46]
Preliminary catalyst screening of Cr{N(SiMe3)2}x/oxide−600based oligomerisation pro-initiators demonstrated that catalytic performance is intimately linked to the nature of the oxide support, aluminium activator, and organic diluent
Summary
Today’s market for short-chain linear α-olefins (LAOs) is so demanding that the traditional synthetic routes for their manufacture such as Ziegler- and SHOP-type oligomerisation processes, which give rise to statistical LAO product distributions, cannot keep pace [1]. Union Carbide [4,5,6], Chevron-Phillips [7,8,9] and BP [10, 11] have each developed their own homogeneous chromium-based catalyst packages for upgrading ethylene to 1-hexene. There are relatively few examples of selective heterogeneous ethylene oligomerisation initiators having been reported [12,13,14,15,16,17,18] Such heterogeneous systems could provide several advantages over their soluble counterparts in an industrial context, which include more efficient separation of the liquid product stream from the solid catalyst, the potential for a “solvent-free” continuous flow process, and minimisation of reactor fouling [19]. Even for established homogeneous chromium-based selective olefin oligomerisation systems, where aspects of the general catalytic mechanism have been elucidated [1, 20], most notably the role of a metallacyclic reaction pathway (Scheme 1) [1, 21], there remains considerable debate
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