Current
heterogeneous catalysts lack the fine steric and electronic
tuning required for catalyzing the selective dimerization of ethylene
to 1-butene, which remains one of the largest industrial processes
still catalyzed by homogeneous catalysts. Here, we report that a metal–organic
framework catalyzes ethylene dimerization with a combination of activity
and selectivity for 1-butene that is premier among heterogeneous catalysts.
The capacity for mild cation exchange in the material MFU-4l (MFU-4l = Zn5Cl4(BTDD)3, H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin)
was leveraged to create a well-defined and site-isolated Ni(II) active
site bearing close structural homology to molecular tris-pyrazolylborate
complexes. In the presence of ethylene and methylaluminoxane, the
material consumes ethylene at a rate of 41,500 mol per mole of Ni
per hour with a selectivity for 1-butene of up to 96.2%, exceeding
the selectivity reported for the current
industrial dimerization process.
A recently developed metal-organic framework (MOF) catalyst for the dimerization of ethylene has a combination of selectivity and activity that surpasses that of commercial homogeneous catalysts, which have dominated this important industrial process for nearly 50 years. The uniform catalytic sites available in MOFs provide a unique opportunity to directly study reaction mechanisms in heterogeneous catalysts, a problem typically intractable due to the multiplicity of coordination environments found in many solid catalysts. In this work, we use a combination of isotopic labeling studies, mechanistic probes, and DFT calculations to demonstrate that Ni-MFU-4l operates via the Cossee-Arlman mechanism, which has also been implicated in homogeneous late transition metal catalysts. These studies demonstrate that metal nodes in MOFs mimic homogeneous catalysts not just functionally, but also mechanistically. They provide a blueprint for the development of advanced heterogeneous catalysts with similar degrees of tunability to their homogeneous counterparts.
Metal-organic frameworks (MOFs) hold great promise as structurally tunable catalysts capable of high selectivity in the solid state, yet their comparatively high cost and often limited stability remain significant concerns for their commercialization as heterogeneous catalysts. Here, we report detailed X-ray absorption spectroscopy studies of Co-and Ni-MFU-4l, a class of highly selective MOF catalysts for olefin upgrading, and reveal mechanisms that lead to their deactivation. We further show that Ni-CFA-1, a more scalable and economical alternative to Ni-MFU-4l, reproduces both the local coordination structure and the high selectivity of the latter in ethylene dimerization catalysis. Under optimal conditions, Ni-CFA-1 activated by MMAO-12 achieves a turnover frequency of 37,100 per hour and a selectivity of 87.1% for 1-butene, a combination of activity, selectivity, and affordability that is unmatched among heterogeneous ethylene dimerization catalysts. Ni-CFA-1 retains its high activity for at least 12 hours in a oneliter semi-batch reactor, offering a strategy toward robust and scalable MOFs for industrial catalysis.
Supporting Information. The supporting information is available free of charge on the ACS Publication website. Experimental details, gas chromatograms, NMR data, computational methods, XAS data.
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