Single-site, Ni-modified Wells–Dawson-type polyoxometalate for propylene dimerization

Abstract

Olefin oligomerization is an essential step in the production of liquid fuels, chemicals, and chemical precursors. Here, we report gas phase propylene oligomerization catalyzed by isolated Ni²⁺ sites substituted on the lacunary defect of a Wells–Dawson polyoxometalate (K₈P₂W₁₇O₆₁·Ni²⁺) supported on SBA-15 (Ni-POM-WD/SBA-15). The Ni-POM-WD/SBA-15 catalyst exhibited high product selectivity for linear propylene dimers (>76%) relative to branched propylene dimers (<24%). The linear dimer selectivity was independent of the overall propylene conversion between 0.6–5% but was dependent on reaction temperature. The propylene dimerization activation energy was measured as 44.5 kJ mol⁻¹, which is consistent with the reported values for Ni²⁺ exchanged-zeolites for propylene oligomerization. Further, the measured dimerization reaction rate order was a strong function of the initial propylene partial pressure and transitioned from second order to first order at higher propylene partial pressures. The catalyst was fully regenerated after reaction by applying a thermal regeneration step in helium. Transient, time-on-stream catalyst performance measurements showed the catalyst had a mean life of ∼0.6–1.15 h during three reaction cycles and had a slightly increased initial propylene consumption rate with each cycle.