Single-site heterogeneous Cr-based catalyst for the selective trimerisation of ethylene

TAC-Cr3+/SiO2 complexes are highly active and selective ethylene trimerisation catalysts and possess single-site catalytic behaviour, an unusual property for heterogeneous catalysts.     

Controlled assembly of a heterogeneous single-site ethylene trimerization catalyst as probed by X-ray absorption spectroscopy

X-ray absorption spectroscopy at the Cr K- and L(2,3)-edges was used to study the assembling process of a heterogeneous Cr-based single-site catalyst. The starting point was a Phillips-type system with monochromate species anchored on a silica surface, which was first reduced to a variety of different surface Cr(II) species. The reduced sample was modified with a 1,3,5-tribenzylhexahydro 1,3,5-triazine (TAC) ligand in the presence of CH(2)Cl(2) as solvent to yield a heterogeneous single-site Cr-based catalyst active in the trimerization of ethylene. The molecular structure of the resultant catalytic material consists of distorted octahedral Cr(III) species. The extended X-ray absorption fine-structure (EXAFS) spectroscopy fitting procedure in R space up to 2.5 A showed that the synthesis leads to coordination with a TAC ligand. The fit also shows that it was possible to complete the six-fold environment around Cr(III) with two oxygen atoms and one chloride ligand. This chloride ligand is formed in a redox process from the solvent and is responsible for the oxidation of surface Cr(II) to Cr(III). The obtained geometry and the local environment of the surface complex are discussed in light of its homogeneous counterpart and confirm the single-site characteristics of the prepared catalytic material.     

Detecting functional magnetic resonance imaging activation in white matter: Interhemispheric transfer across the corpus callosum

Background  

It is generally believed that activation in functional magnetic resonance imaging (fMRI) is restricted to gray matter. Despite this, a number of studies have reported white matter activation, particularly when the corpus callosum is targeted using interhemispheric transfer tasks. These findings suggest that fMRI signals may not be neatly confined to gray matter tissue. In the current experiment, 4 T fMRI was employed to evaluate whether it is possible to detect white matter activation. We used an interhemispheric transfer task modelled after neurological studies of callosal disconnection. It was hypothesized that white matter activation could be detected using fMRI.