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  • 03/22/18

    When defects become features in metal–organic framework catalysis

Using the precisely engineered, reactive structure of a metal-organic framework, ethanol can be induced to dehydrate to diethyl ether (following the green arrow pathway) but not ethylene (the blocked, red-arrow pathway).
Using the precisely engineered, reactive structure of a metal-organic framework, ethanol can be induced to dehydrate to diethyl ether (following the green arrow pathway) but not ethylene (the blocked, red-arrow pathway).

The revelation of masked functionality in otherwise crystalline materials can lead to exciting opportunities for catalysis, as long as the engineered defects enhance catalytic activity and/or selectivity without compromising the stability of the bulk material. With that in mind, highly collaborative experimental and theoretical work from the Inorganometallic Catalyst Design Center reports the synthesis, characterization, and catalytic performance of zirconium-based metal–organic frameworks (MOFs) with just such revealed features.

The MOFs under study, known as UiO-66 and UiO-67, were prepared with various solvents and modifiers to intentionally engineer vacant sites on some MOF nodes; that is, undercoordinated metallic coordination sites where some linkers are missing. These MOFs are catalytically active for ethanol dehydration, producing exclusively diethyl ether but not ethylene, which contrasts with analogous bulk materials that show opposite selectivity. Theoretical studies indicate that catalysis occurs only when two such defect sites are adjacent to one another, with reaction proceeding through a bimolecular nucleophilic substitution mechanism involving two ethanol molecules. Increasing numbers of defects increase catalytic activity, but at the expense of the integrity of the MOF, which “unzips” when too many linkers are missing. This suggests new opportunities to synthetically tune MOF-based catalysts for selective ethanol dehydration while remaining robust under controlled reaction conditions. 

The work was recently published in the Journal of the American Chemical Society, and highlighted in the Science and Technology section of Chemical & Engineering News. The results derived from a tight collaboration between the groups of Professor Christopher Cramer and Professor Laura Gagliardi from the University of Minnesota, and Professor Bruce Gates from the University of California, Davis. Davis post-doctoral scholar Dong Yang, Ph.D., designed and executed the experimental work, while a team of Minnesota post-doctoral scholars, Manuel Ortuño, Ph.D., and Varinia Bernales, Ph.D., conducted the theoretical work.

This work was supported by the Inorganometallic Catalyst Design Center—an Energy Frontier Research Center based in the Department of Chemistry at the University of Minnesota.