Versatile Coordination of Cyclopentadienyl-Arene Ligands and Its Role in Titanium-Catalyzed Ethylene Trimerization

Cationic titanium(IV) complexes with ansa-(η5-cyclopentadienyl,η6-arene) ligands were synthesized and characterized by X-ray crystallography. The strength of the metal-arene interaction in these systems was studied by variable-temperature NMR spectroscopy. Complexes with a C1 bridge between the cyclopentadienyl and arene moieties feature hemilabile coordination behavior of the ligand and consequently are active ethylene trimerization catalysts. Reaction of the titanium(IV) dimethyl cations with CO results in conversion to the analogous cationic titanium(II) dicarbonyl species. Metal-to-ligand backdonation in these formally low-valent complexes gives rise to a strongly bonded, partially reduced arene moiety. In contrast to the η6-arene coordination mode observed for titanium, the more electron-rich vanadium(V) cations [cyclopentadienyl-arene]V(NiPr2)(NC6H4-4-Me)+ feature η1-arene binding, as determined by a crystallographic study. The three different metal-arene coordination modes that we experimentally observed model intermediates in the cycle for titanium-catalyzed ethylene trimerization. The nature of the metal-arene interaction in these systems was studied by DFT calculations.

Sintering Behavior of TiO2Supported Model Cobalt FischerTropsch Catalysts under H2 Reducing Conditions and Elevated Temperature

The sintering of model TiO2-supported cobalt catalysts has been studied. The TiO2 supports used were anatase, P25 (85% anatase-15% rutile) and rutile. The catalysts were characterized at each stage of treatment. These treatment stages were calcination, reduction and sintering. It was found that the TiO2 support does not influence the Co3O4 crystallite and particle size as shown by powder x-ray diffraction (XRD) and transmission electron microscopy (TEM) respectively. The reduction of the cobalt catalysts and bare supports was studied by temperature programmed reduction (TPR). It was found that the bare supports were not as inert as expected. The supports showed minor reduction as seen in the H2 consumption. All the cobalt catalysts showed a two-step reduction. Sintering of anatase-supported cobalt was shown to be the most substantial with P25- and rutile-supported showing a lower sintering tendency; P25- supported cobalt being the most stable based on TEM measurements. Sintering kinetics based on the Generalized Power law Expression (GPLE) model, showed that sintering of anatase-supported cobalt is the most rapid with a large sintering rate constant. Sintering of P25-supported cobalt is the lowest, shown by the lowest sintering rate constant. The study has conclusively shown the effect of the catalyst support phase. The study has also shown that the use of high surface area supports is not necessarily the only answer to preventing sintering. The phase of the catalyst support is also important.Sasol Group Technology, National Research Foundation and the University of Pretoria.http://pubs.acs.org/journal/iecred2017-09-30hb2016Materials Science and Metallurgical Engineerin