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International audience; Brought to life more than half a century ago and successfully applied for high-value petrochemical intermediates production, nickel-catalyzed olefin oligomerization is still a very dynamic topic, with many fundamental questions to address and industrial challenges to overcome. The unique and versatile reactivity of nickel enables the oligomerization of ethylene, propylene and butenes into a wide range of oligomers that are highly sought-after in numerous fields to be controlled. Interestingly, both homogeneous and heterogeneous nickel catalysts have been scrutinized and employed to do this. This rare specificity encouraged us to interlink them in this review so as to open up opportunities for further catalyst development and innovation. An in-depth understanding of the reaction mechanisms in play is essential to being able to fine-tune the selectivity and achieve efficiency in the rational design of novel catalytic systems. This review thus provides a complete overview of the subject, compiling the main fundamental/industrial milestones and remaining challenges facing homogeneous/heterogeneous approaches as well as emerging catalytic concepts, with a focus on the last 10 years.

International audience; The reactivity of a phenoxy-imine-ether system (FI)TiCl3/MAO was studied toward selective ethylene trimerization. This system was shown to either trimerize or polymerize ethylene depending on the reaction temperature. Its selectivity switches from a significant production of the trimerization product, 1-hexene (85 wt %, 520-450 kg1-hexene gTi-1 h-1) between 30 and 40 °C, to a moderate polyethylene formation (70-80 wt %, 60-70 kgpolyethylene gTi-1 h-1) at higher reaction temperature (T > 60 °C). Polymerization was investigated based on an original "polymer-to-catalyst" strategy aiming at identifying the active species responsible for this side reaction. Using DSC, SEC and high temperature 13 C NMR analyses, polyethylenes were found to exhibit high molar masses (> 10 5 g mol-1) and a low 1-hexene content (< 1 mol %) at any temperature. Kinetic studies support that trimerization and polymerization species are generated from the catalyst precursor at 40 °C but a parallel process may occur at higher temperature. The increase dispersity to 4.6 at 80 °C suggests a change from single to multisite catalysis. The poor comonomer incorporation ability of the active species is reminiscent of a molecular Ziegler Natta or a bulky postmetallocene catalyst.

International audience; Oligomerization applications still raise numerous issues regarding process optimization as well as new catalyst development. This requires an important testing capacity, which cannot be reached with standard tools and methods (typically stirred tank reactors) in a short time. Using microreactors, which generally allow fast analysis times, is expected to minimize liquid holdup and the duration of experiments. The reaction of interest is the two-phase dimerization of ethylene using a liquid catalyst that is converted into butenes (70-80%), hexenes (20-30%) and heavier products up to C12 (5-10%). Performing this reaction in a microreactor can be challenging as the gas-liquid segmented flow will evolve as a function of the residence time in the reactor, starting from high gas-liquid ratios and long bubbles at the entrance, to a complete disappearance of the gas phase at the outlet when the reactant is totally converted and dissolved in the liquid phase. In order to understand this evolving gas-liquid flow, a hydrodynamic study was conducted in a tubular microreactor (ID: 500 µm, length: 1 m) under the operating conditions of the oligomerization reaction (typically 45°C and 20 bar). A bubble train was generated by introducing ethylene (gas phase) and a solvent (liquid phase) in a T-junction. The injection of the liquid phase catalyst was mimicked by introducing the solvent in a second T-junction, downstream of the generation of the bubble train. The flow generation in the T-junction and the stability of the gas-liquid flow after the second liquid injection were firstly studied. Flow patterns and typical characteristics of the flow (bubble and slug length, bubble velocity, film thickness) were monitored using a camera and image processing techniques. Both the gas and liquid flow rates were varied in the range Q G = 0-500 NmL/h, Q L = 0.1-2 mL/h (corresponding to U G =XXX and UL xxx) in order to represent the evolution of gas-liquid ratio during the reaction and to cover the entire conversion range in our system.This detailed characterization of the flow allows identification of the operating range that is favorable, in terms of hydrodynamics, for the reaction and of the limits of operability of the system