Tracing Active Sites in Supported Ni Catalysts during Butene Oligomerization by Operando Spectroscopy under Pressure

Supported Ni catalysts have been studied during the dimerization of butenes by operando electron paramagnetic resonance (EPR) and in situ X-ray absorption spectroscopy (XAS) at 353 K and up to 16 bar. Single Ni^I/Ni^II shuttles were identified as active sites, whereby the conversion of initial Ni^I to Ni^II by oxidative addition of butene is obviously faster than the re-reduction of Ni^II to Ni^I by reductive elimination of the C8 product, rendering the equilibrium percentage of Ni^I small. At p ≤ 2 bar, Ni^I single sites form inactive Ni⁰ aggregates, while this is suppressed at higher pressure (∼12 bar). A reaction mechanism is proposed.