(608a) MOF-Supported Transition Metal Catalysts for 1-Butene Dimerization: A Mechanistic Study

The catalytic dimerization of short-chain olefins to long-chain olefins through C-C bond formation is important in the production of high-value chemicals such as lubricants, surfactants, and detergents. Homogeneous catalytic systems are limited by complex solvent and catalyst recovery processes, and the desire to selectively form linear dimers necessitates 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 are promising alternatives as catalyst supports which can be used to design tunable low nuclearity transition metal catalysts. In this work, we have investigated the catalytic activity of different single-atom transition metals (Ni²⁺, Co²⁺, and Cu²⁺) supported by the UiO-66 for 1-butene dimerization using Kohn-Sham density functional theory (DFT).

Simulated extended X-ray absorption fine structure (EXAFS) spectra for DFT-optimized cluster models of M²⁺/UiO-66 catalysts, with varying aqua coordination hypothesized both for the fresh and activated catalysts, show good agreement with the experimental EXAFS spectra. The results suggest the presence of undercoordinated single-atom transition metal sites in the activated M²⁺/UiO-66 catalysts. The computed enthalpies of 1-butene adsorption on activated M²⁺/UiO-66 (~50-80 kJ/mol depending on the metal) showed agreement within ~15% of that determined from calorimetry experiments. The hydride-mediated Cossee-Arlmann reaction mechanism was found to be the most favorable route energetically for 1-butene dimerization. The computed free energy barriers for the rate-determining step predict the Ni²⁺/UiO-66 catalyst to be a higher-performing catalyst than Cu²⁺/UiO-66 and Co²⁺/UiO-66 catalysts (ΔG^‡_Ni/UiO66 = 93.8 kJ/mol, ΔG^‡_Cu/UiO66 = 104.2 kJ/mol, ΔG^‡_Co/UiO66 = 126.3 kJ/mol at 473 K) in agreement with our experimental findings. We observe a higher selectivity of the Ni²⁺/UiO-66 catalyst toward linear octenes. Lastly, a mechanism for the in-situ formation of the M-H active site in the presence of 1-butene is proposed that allows to avoid the use of co-catalysts or alkyl-aluminum activators.