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.

Metal-organic frameworks (MOFs) have shown great potential in gas separation and storage, and the design of MOFs for these purposes is an on-going field of research. Solid-state nuclear magnetic resonance (SSNMR) spectroscopy is a valuable technique for characterizing these functional materials. It can provide a wide range of structural and motional insights that are complementary to and/or difficult to access with alternative methods. In this Concept article, the recent advances made in SSNMR investigations of small gas molecules (i.e., carbon dioxide, carbon monoxide, hydrogen gas and light hydrocarbons) adsorbed in MOFs are discussed. These studies demonstrate the breadth of information that can be obtained by SSNMR spectroscopy, such as the number and location of guest adsorption sites, host-guest binding strengths and guest mobility. The knowledge acquired from these experiments yields a powerful tool for progress in MOF development.

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UV photodissociation of α-alanine was studied by parahydrogen matrix isolation infrared spectroscopy. The temporal behavior of Fourier transform infrared spectra revealed that UV irradiation at 213 nm yielded the HOCO radical as a direct photoproduct from the S 2 excited state. The concentration of HOCO quickly approached a steady state due to secondary photodissociation of HOCO to produce CO 2 + H or CO + OH. On the other hand, no photoproducts were detected by S 1 excitation at 266 nm. Irradiation of fully deuterated α-alanine at 213 nm yielded ∼2 times more cis-DOCO radicals than the lower energy isomer trans-DOCO, indicating that the conformation of the hydroxyl group is fairly well-preserved upon photodissociation of α-alanine. The present study suggests that HOCO may be a good tracer species in the search for amino acids in interstellar space.

Metal-organic frameworks (MOFs) are promising gas adsorbents. Knowledge of the behavior of gas molecules adsorbed inside MOFs is crucial for advancing MOFs as gas capture materials. However, their behavior is not always well understood. In this work, carbon dioxide (CO 2 ) adsorption in the microporous α-Zn 3 (HCOO) 6 MOF was investigated. The behavior of the CO 2 molecules inside the MOF was comprehensively studied by a combination of single-crystal X-ray diffraction (SCXRD) and multinuclear solid-state magnetic resonance spectroscopy. The locations of CO 2 molecules adsorbed inside the channels of the framework were accurately determined using SCXRD, and the framework hydrogens from the formate linkers were found to act as adsorption sites. 67 Zn solid-state NMR (SSNMR) results suggest that CO 2 adsorption does not significantly affect the metal center environment. Variable-temperature 13 C SSNMR experiments were performed to quantitatively examine guest dynamics. The results indicate that CO 2 molecules adsorbed inside the MOF channel undergo two types of anisotropic motions: a localized rotation (or wobbling) upon the adsorption site and a twofold hopping between adjacent sites located along the MOF channel. Interestingly, 13 C SSNMR spectroscopy targeting adsorbed CO 2 reveals negative thermal expansion (NTE) of the framework as the temperature rose past ca. 293 K. A comparative study shows that carbon monoxide (CO) adsorption does not induce framework shrinkage at high temperatures, suggesting that the NTE effect is guest-specific.