The catalytic oligomerization of short-chain olefins to long-chain olefins through C-C bond formation is important for the production of high-value chemicals like polyethylene, surfactants, and detergents. Homogeneous catalytic systems are limited by complex solvent and catalyst recovery processes, and the desired higher selectivity to linear olefin isomers necessitate the design of novel heterogeneous catalysts with lower Brønsted acid site activities which are known to facilitate double-bond and skeletal isomerization. Metal-organic frameworks (MOFs) are promising alternatives as catalyst supports which can be used to design tunable single atom or low nuclearity transition metal catalysts. In this work, using Kohn-Sham density functional theory (DFT), we have investigated the catalytic activity of different single-atom transition metals (Ni²⁺ and Co²⁺) supported by the UiO-66 framework, a MOF with Zr₆ nodes. In particular, the 1-butene dimerization following the Cossee-Arlmann reaction mechanism by these Mⁿ⁺/UiO-66 catalysts is studied. The computed free energy barriers for this mechanism indicate that C-C bond formation is the rate-limiting step with Ni²⁺/UiO-66 predicted to be a higher-performing catalyst than Co²⁺/UiO-66 (ΔG^‡_Co/UiO66 = 30.2 kcal/mol, ΔG^‡_Ni/UiO66 = 20.2 kcal/mol) in agreement with the experimental findings of this work. Furthermore, we predict a higher selectivity towards linear octenes by Ni²⁺/UiO-66 and a higher selectivity towards the branched octenes by Co²⁺/UiO-66. Moreover, a mechanism for the in-situ formation of the metal-hydride active site is proposed.

Laura Löbbert

Appearances