Olefin Oligomerization on Nickel-Based Metal Organic Framework Catalysts

Abstract

Olefin oligomerization on nickel-based catalysts to produce linear alpha olefins, comonomers for polyethylene synthesis, requires the use of cocatalysts, involves transients during induction periods, and is plagued by catalyst deactivation. The need for cocatalysts and the observed induction period is ascribed to the formation of a nickel-alkyl intermediate which initiates propagation sequences described by the Cossee-Arlman mechanism as opposed to the metallacycle mechanism to form oligomers. This work investigates two nickel metal organic frameworks (MOFs), Ni/UiO-66 and Ni-MIL-127, that activate onstream devoid of cocatalysts and uniquely maintain stable oligomerization rates. The structures of both catalysts evolve under different chemical and thermal environments to generate catalytically relevant nickel species for olefin oligomerization, as evinced by transient and steady state rate measurements and spectroscopic methods. The active nickel species are enumerated with in-situ NO titrations to demonstrate that not all nickel species in the material are active for olefin oligomerization. Through analysis of steady state kinetics and product selectivities and comparison with experimental and computed activation energies with density functional theory on cluster models of Ni/UiO-66 and Ni-MIL-127, the Cossee-Arlman mechanism is postulated over the metallacycle mechanism for olefin oligomerization. These studies, which can be used to guide the synthesis of tunable MOFs for olefin oligomerization through a combination of experiment and theory, describe how the structures of MOFs change by undergoing active site generation and defect formation to enable distinct stability characteristics for olefin oligomerization catalysis.