Catalytic Light Olefin Oligomerization via Transition Metal Substituted Polyoxometalate Catalysts

In this dissertation, a new molecular level platform that can bear single transition metal active sites was introduced. The new platform was developed by utilizing a known synthetic method of lacunary defect formation on well-defined polyoxometalate (POM) structures and subsequent metalation of formed defective vacancy. On the lacunary defects of Wells Dawson phosphor-tungstate structures, single Ni²⁺ active sites were isolated. Upon dispersion of Ni substituted Wells Dawson polyoxometalate (Ni-POM-WD) on SBA-15, substituted Ni sites successfully converted ethylene to linear butene species and heavier oligomers. Ni-POM-WD catalysts yielded resembling kinetic parameters to those of other solid nickel catalysts such as Ni²⁺ exchanged zeolites. The robust stability of Ni-POM-WD catalysts was shown by multiple regenerated cycles ethylene oligomerization without structural degradation. We investigated the influence of molecular surroundings on substituted Ni sites by changing the size and elemental compositions of site bearing polyoxometalates. It was shown that the size of polyoxometalate structure does not perturb the kinetic nature of substituted Ni sites. On the other hand, the activation energy of Ni sites was found to be correlated with the electronegativity of internal heteroatoms. Moreover, three Ni sites were isolated on one polyoxometalate structure. Mass normalized ethylene consumption rate was increased due to the increased per POM Ni concentration. More interestingly, the ratio of 1-butene/2-butene was able to be changed by facilitating re-adsorption and isomerization of 1-butene with structurally controlled Ni site proximity. Other known transition metal active sites (Cr, Co) for light olefin coupling reaction were isolated on lacunary defects and their catalytic performance was evaluated.