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Ethene polymerization in toluene has been studied in the temperature range -7 to +97°C and pressure range 0.28 to 9 bar, using two different L 2ZrCl2/methylaluminoxane (MAO) catalyst systems. With bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2, L ) Cp), the average activity over 1 h increases with temperature between 10 and 97°C. With bis(pentamethylcyclopentadienyl)zirconium dichloride (Cp*2ZrCl2, L ) Cp*), a maximum average activity over 1 h is observed at 45°C. If propagation and deactivation effects are separated through kinetic modeling, the activity corresponding to chain propagation is found to increase in the whole temperature range for both catalysts. The molecular weight is higher with L ) Cp* than with L ) Cp below 80°C. Above 80°C, the opposite is observed. With L ) Cp*, the molecular weight increases with increasing ethene pressure up to about 2 bar, where it levels off. With L ) Cp, the molecular weight is independent of pressure between 0.28 and 9 bar. The ratio between vinyl and trans-vinylene unsaturation is approximately 6:1 with L ) Cp and 1:1 with L ) Cp*, both slightly increasing with increasing ethene pressure. As the temperature is increased, the relative vinyl content decreases with L ) Cp and increases with L ) Cp*. On the basis of density-functional calculations, we present a reaction scheme consistent with most of the experimental results. This reaction scheme, in which different agostic interactions play a crucial role, assumes a Cossee-like mechanism for chain propagation, chain termination via hydrogen transfer to a coordinated monomer (for both catalysts) or to the metal (for L ) Cp*), and chain isomerization via partial hydrogen transfer to the metal, relative rotation of the olefin and the hydride, and reinsertion of the coordinated olefin. The calculated activation energy for propagation is 25-35 kJ/mol for L ) Cp*, in fair agreement with the experimental value of 17 kJ/mol. For L ) Cp, we calculate an activation energy of 10-20 kJ/mol, whereas the experimentally derived value is 61 kJ/mol. The poor agreement for L ) Cp may indicate that the polymerization is influenced by the surrounding solvent and MAO. The calculated difference in activation energy between chain propagation and termination is larger for L ) Cp* than for L ) Cp, in qualitative agreement with the stronger temperature dependence of the molecular weight observed with L ) Cp*. Chain isomerization is found to be easier, relative to termination, with L ) Cp* than with L ) Cp. This may account for the large amount of trans-vinylene unsaturation observed when Cp* 2ZrCl2 is used as catalyst.

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Size-dependent phenomena at the nanoscale influence many applications, notably in the science of heterogeneous catalysis. In cobalt-based Fischer-Tropsch synthesis (FTS), the size of Co nanoparticles (NPs) dictates to a high degree catalyst's performance in terms of activity, selectivity, and stability. Here, a highly dispersed Re/Co/γ-AlO catalyst with high Co surface area per gram of catalyst was exposed to industrially relevant FTS conditions and monitored in situ by synchrotron X-ray radiation. X-ray absorption near-edge structure spectra were obtained on the cobalt K edge and Re L edge of the working catalyst. The experimental results demonstrate development of tetrahedrally coordinated Co forming at the expense of metallic Co. The structure of the oxide resembles CoAlO and appears at the onset (first 5-10 h) of the reaction. Reoxidation of Co is more pronounced close to the outlet of the reactor, where higher pHO is anticipated. The state of the Re promoter does not change during the FT process. We propose that reoxidation of small Co NPs is followed by spreading of Co oxide that leads to the formation of CoAlO phases. Hence, in order to avoid an irreversible loss of the active phase during process start-up, catalyst design should be restricted to Co NPs larger than 5.3 nm.

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