Deducing CO2 motion, adsorption locations and binding strengths in a flexible metal–organic framework without open metal sites†
Abstract
Microporous metal–organic frameworks (MOFs) have high surface areas and porosities, and are well-suited for CO2 capture. MIL-53 features corner-sharing MO4(OH)2 (M = Al, Ga, Cr, etc.) octahedra interconnected by benzenedicarboxylate linkers that form one-dimensional rhombic tunnels, and exhibits an excellent adsorption ability for guest molecules such as CO2. Studying the behavior of adsorbed CO2 in MIL-53 via solid-state NMR (SSNMR) provides rich information on the dynamic motion of guest molecules as well as their binding strengths to the MOF host, and sheds light on the specific guest adsorption mechanisms. Variable-temperature 13C SSNMR spectra of 13CO2 adsorbed within various forms of MIL-53 are acquired and analyzed. CO2 undergoes a combination of two motions within MIL-53; we report the types of motion present, their rates, and rotational angles. 1H–13C CP SSNMR experiments are used to examine the proximity of 1H atoms in the MOF to 13C atoms in CO2 guests. By replacing 1H with 2H in MIL-53, the location of the CO2 adsorption site in MIL-53 is experimentally confirmed by 1H–13C CP SSNMR. The binding strength of CO2 within these MIL-53 MOFs follows the order MIL-53-NH2 (Al) > MIL-53-NH2 (Ga) > MIL-53 (Al) > MIL-53 (Ga).