Localized structural and compositional effects on the kinetic performance of Ni-modified polyoxometalate catalysts for ethylene oligomerization

Abstract

Ethylene oligomerization is an important step for upgrading light olefins to liquid fuels or value-added chemicals such as butene monomers. Ethylene oligomerization catalyzed by mono-Ni substituted polyoxometalate (POM) catalysts is promising due to the observed selectivity toward linear butene products and stability of these well-defined materials. In this work, two approaches to tailor the catalytic properties of isolated Ni sites in POMs were experimentally tested, both of which perturbed the surrounding molecular environment of the active sites. First, structural modifications were performed to alter the size of the polyoxometalates, evaluating between Wells-Dawson and Keggin structures while maintaining the same elemental components. When comparing these two structures, it was observed that the Ni active sites were kinetically independent from the effect of POM size, even when the POM consisted of the same elements. This was evidenced by the identical product distributions and kinetic parameters observed with both structures. However, a localized kinetic effect was observed when Keggin POM structures containing different internal heteroatoms (e.g, P, Si, Al) were included in close proximity to the Ni active site. Specifically, a notable periodic trend was observed between the electronegativity of internal heteroatoms and the measured apparent activation energy for the catalyzed ethylene coupling reaction to butene.