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

Abstract

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.

Recent Advances of Solid‐State NMR Spectroscopy for Microporous Materials

“…2 H NMR offers a straightforward method for the investigation of methane adsorption locations and dynamics within MOFs pores. [ 78 ] Methane adsorption behavior on several representative MOFs has been examined by VT 2 H NMR spectroscopy. [ 77 ] Figure 7c displayed the static VT 2 H static NMR spectra of singly deuterated methane (CH 3 D) adsorbed on α‐Mg 3 (HCO 2 ) 6 .…”

Photochromic Free MOF‐Based Near‐Infrared Optical Switch

“…Next to high porosity, defined structures and chemical versatility, these materials may feature stimuli‐responsive structure, which, in turn, allows for reversible/irreversible structural transformation such as crystal‐to‐amorphous or crystal‐to‐crystal ones . The reversible transformations induced by external stimuli such as mechanical constrains, guest uptake, temperature, UV‐VIS light exposure and others are utilized for controllable gas adsorption, separation and catalysis . The defined structural flexibility expands the utility of MOFs as “smart” materials for remotely controlled sensors or communication devices such as memory devices and optical switchers.…”

Research on the effects of hydrothermal synthesis conditions on the crystal habit of MIL-121

“…Chen et al . [ 33 ] suggested that the thermal trigger decarboxylation method obtained hierarchical porous MOF to improve the adsorption capacity of MIL-121 to CO 2 , C 2 H 2 , C 2 H 4 and CH 4 gases. In this paper, the control of MIL-121 morphology and size was investigated by adjusting the hydrothermal reaction conditions.…”

“…According to the literature [32], the surface area of MIL-121 is only 162 m 2 g −1 , which is extremely detrimental to its application to gas adsorption separation. Chen et al [33] suggested that the thermal trigger decarboxylation method obtained hierarchical porous MOF to improve the adsorption capacity of MIL-121 to CO 2 , C 2 H 2 , C 2 H 4 and CH 4 gases. In this paper, the control of MIL-121 morphology and size was investigated by adjusting the hydrothermal reaction conditions.…”