Cleaving carboxyls: understanding thermally triggered hierarchical pores in the metal-organic framework MIL-121

Carboxylic acid linker ligands are known to form strong metalcarboxylate 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 m2/g to 908 m2/g, and enhances gas adsorption uptake for industrially relevant gases (i.e., CO2, C2H2, C2H4 and CH4). For example, CO2 uptake in hierarchically porous MIL-121 is improved 8.5 times over MIL-121, reaching 215.7 cm3/g at 195 K and 1 bar; CH4 uptake is 132.3 cm3/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, Xray diffraction, electron microscopy, and gas adsorption; we propose a mechanism for decarboxylation proceeds and which local structural features are involved. Understanding the complex relationship between 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.