The Role of Catalyst Support, Diluent and Co-Catalyst in Chromium-Mediated Heterogeneous Ethylene Trimerisation

Sequential treatment of a previously-calcined solid oxide support (i.e. SiO(2), γ-Al(2)O(3), or mixed SiO(2)–Al(2)O(3)) with solutions of Cr{N(SiMe(3))(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 (SiO(2−600), 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 g(Cr)(−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 (29)Si NMR spectroscopic study of the Cr{N(SiMe(3))(2)}(x)/SiO(2−600) pro-initiator. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s11244-018-0891-8) contains supplementary material, which is available to authorized users.