PNNL

Abstract

Steady state rates expressions can be derived to distinguish the Cossee-Arlman and metallacycle mechanisms postulated for propylene oligomerization on nickel-based catalysts based on product selectivities, where product selectivities for the former are a function of olefin pressure because sequential coordination and insertion steps lead to independent mechanistic pathways for different hexene isomers. In contrast, the metallacycle mechanism presents pressure-independent product selectivity ratios due to successive coordination prior to the kinetically relevant steps in each mechanism. In this work, steady state propylene oligomerization rates and selectivities were measured in the absence of an activator on nickel functionalized UiO-66 metal organic framework (MOF), Ni/UiO-66, to validate the Cossee-Arlman mechanism for light olefin oligomerization. In-situ NO titrations reveal that ~5% of nickel sites were active during the reaction, and thus, not all nickel sites are relevant for catalysis. Propylene dimerization was first order in propylene pressure from 5 to 500 kPa with an apparent activation energy of ~20 kJ mol-1 from 453 to 493 K. Calculated apparent activation energies with density functional theory (DFT) calculations on cluster models of Ni/UiO-66 are in agreement with experiment to corroborate the Cossee-Arlman mechanism. Ratios of hexene product selectivities are pressure-dependent on Ni/UiO-66, consistent with selectivity expressions derived for the Cossee-Arlman mechanism for propylene oligomerization. Analysis of product selectivities can be used more extensively to demarcate the Cossee-Arlman and metallacycle mechanisms for olefin oligomerization on metal-based catalysts.