(661d) Ethylene Dimerization to n-Butene with Nickel Sulfate on Zirconia: Investigation of the Molecular Structure and Activity of Surface Nickel and Sulfate Species

The shift to lighter hydrocarbon feedstocks due to the growing abundance of natural gas has led to a global shortage of n-butene, stimulating interest in ethylene dimerization as an effective means for producing butene from natural gas derived ethylene. Industrial ethylene dimerization processes, however, frequently rely on homogenous catalysts that present several environmental and operational challenges. Consequently, there has also been interest in developing heterogeneous catalysts that can achieve comparable activity and selectivity to their homogeneous counterparts. Numerous heterogenous catalysts for ethylene dimerization have been investigated, with Ni-based catalysts typically reported to have high activity and selectivity to n-butene. This work investigates sulfated nickel zirconia as a heterogeneous ethylene dimerization catalyst, owing to its strong surface acidity and well-dispersed nickel phase. The molecular structure of the surface NiO and sulfate species, the oxidation state of the Ni active site, and the importance of sulfate-Ni interactions in catalytic activity for ethylene dimerization on sulfated nickel zirconia remain controversial. Additionally, few studies on sulfated nickel zirconia have applied in situ characterization under reaction which is vital for understanding the catalyst during operating conditions. In this work, a series of nickel sulfate zirconia catalysts were synthesized by varying the loading of the active components and the order of impregnation of the Ni and sulfate species. Critical characterization techniques including in-situ UV-vis, in-situ IR, NH₃-IR and C₂⁼-TPSR spectroscopy were utilized to investigate the nature of the active site, surface species and catalytic activity. The surface sulfate species were found to anchor at terminal hydroxyl groups while Ni²⁺ sites may be anchoring adjacent to surface hydroxyl sites or defect sites. The order of impregnation of nickel and sulfate affects the catalytic activity as well as the Bronsted to Lewis acid ratio. Higher concentrations of Bronsted acid sites decrease activity and may contribute to catalytic deactivation.