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Carboxylic acid linker ligands are known to form strong metal-carboxylate bonds to afford many different variations of permanently microporous metal-organic frameworks (MOFs). A controlled approach to decarboxylation of the ligands in carboxylate-based MOFs could result in structural modifications, offering scope to improve existing properties or to unlock entirely new properties. In this work, we demonstrate that the microporous MOF MIL-121 is transformed to a hierarchically porous MOF via thermally triggered decarboxylation of its linker. Decarboxylation and the introduction of hierarchical porosity increases the surface area of this material from 13 to 908 m 2 /g and enhances gas adsorption uptake for industrially relevant gases (i.e., CO 2 , C 2 H 2 , C 2 H 4 , and CH 4 ). For example, CO 2 uptake in hierarchically porous MIL-121 is improved 8.5 times over MIL-121, reaching 215.7 cm 3 /g at 195 K and 1 bar; CH 4 uptake is 132.3 cm 3 /g at 298 K and 80 bar in hierarchically porous MIL-121 versus zero in unmodified MIL-121. The approach taken was validated using a related aluminum-based MOF, ISOMIL-53. However, many specifics of the decarboxylation procedure in MOFs have yet to be unraveled and demand prompt examination. Decarboxylation, the formation of heterogeneous hierarchical pores, gas uptakes, and host-guest interactions are comprehensively investigated using variable-temperature multinuclear solid-state NMR spectroscopy, X-ray diffraction, electron microscopy, and gas adsorption; we propose a mechanism for how decarboxylation proceeds and which local structural features are involved. Understanding the complex relationship among the molecular-level MOF structure, thermal stability, and the decarboxylation process is essential to fine-tune MOF porosity, thus offering a systematic approach to the design of hierarchically porous, custom-built MOFs suited for targeted applications.

Cleaving Carboxyls: Understanding Thermally Triggered Hierarchical Pores in the Metal–Organic Framework MIL-121