Gas-phase hydrogen is an important reactant in biomass conversion and is also a potentially renewable transportation fuel. In both cases, the strong H-H bond must be activated to enable efficient utilization or reversible storage for vehicular applications, requiring a catalyst for these to occur under mild conditions. In addition to being costly, conventional noble metal-based catalysts developed for petroleum refining can be highly aggressive, leading to product mixtures that require energy-consuming separation processes. Metal-Organic Frameworks (MOFs), an emerging class of nanoporous catalysts, are an alternative to these with highly tailorable reactivity. MOFs are crystalline materials assembled from metal ions coordinated to organic electron donors. Extensive experimental and computational efforts over the past decade demonstrate that MOFs have sufficient thermal and chemical stability to make them suitable for a wide range of catalytic processes. Moreover, recent techno-economic analyses show that they can be synthesized at low cost as well. Although there are now many examples of MOF-catalyzed reactions, there are almost none involving H2 as a reactant, which is surprising given the intense interest in using MOFs for hydrogen storage. Here, we show that the MOF Mg2(dobdc) (dobdc=2,5-dioxido-1,4-benzenedicarboxylate) and other MOFs with the same “IRMOF-74” topology activate H2. This allows H2 reaction with species as diverse as aryl ethers and aluminum nanoparticles. The former are common linkages in lignin and the latter form as a result of H2 release from aluminum-containing metal hydrides. Our mechanistic investigations indicate that this remarkable versatility results from the close proximity of unsaturated Mg(II) coordination sites in this framework. We present clear experimental and theoretical evidence that H2 is activated by direct interaction with these sites. In addition, these metal ions bind and orient the aromatic ether molecules to enable the hydrogenolysis reaction to occur. Finally, the reactivity can be tuned by varying the pore size, with an optimum diameter occurring at ~18 Å. Consequently, the IRMOF-74 isoreticular series is a versatile and robust multifunctional catalyst. Intriguingly, the mechanistic understanding gained during these investigations suggests that Mg2(dobdc) can be used as an analogue for some bulk metal hydrides, providing new insights into the reactivity of these complex hydrogen storage materials.

Multifunctional metal-organic framework catalysts for hydrogen activation

255th National Meeting (2018)

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