Journal
ACS APPLIED MATERIALS & INTERFACES
Volume 11, Issue 7, Pages 7014-7021Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsami.8b20450
Keywords
CO2/CO separation; metal-organic frameworks; hexagonal OH clusters; unsaturated metal sites; CO2 and CO adsorption sites
Funding
- Korea Institute of Energy Research (KIER) [B9-2441]
- Center for Advanced Meta-Materials (CAMM) - Ministry of Science and ICT (MSIT) as a Global Frontier Program [2019M3A6B3030636]
- National Research Foundation of Korea (NRF) [2016R1C1B2010838]
- MSIT
- POSTECH
- National Research Foundation of Korea [2016R1C1B2010838] Funding Source: Korea Institute of Science & Technology Information (KISTI), National Science & Technology Information Service (NTIS)
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Isostructural [M-2(DOBDC)(EG)(2)) (M = Mg, Co, Ni) frameworks are first synthesized by controlling the pH* in the reaction medium. Coordinated ethylene glycols form a hexagonal OH cluster, which works as a template to grow single crystals with high crystallinity. After the liberation of solvated molecules, [M-2(DOBDC)] shows notably higher surface areas than the reported values and completely different CO2 and CO separation properties depending on the kinds of unsaturated metal. Therefore, breakthrough experiments using a CO2/CO mixed gas show that Mg-MOF has a longer breakthrough time for CO2 than for CO, whereas Co/Ni-MOFs have longer breakthrough times for CO than for CO2. Apart from CO2 and CO, other gases such as CH4, H-2, and N-2 were almost not adsorbed at all in these materials at 298 K. To reveal the role of unsaturated metal sites, CO2 and CO adsorption sites are unequivocally determined by single-crystal X-ray diffraction analysis. One of very interesting discoveries is that there are two CO2 and CO adsorption positions (sites A and B) in the hexagonal channels. Site A is the unsaturated metal center working as Lewis acidic sites, and site B is the secondary adsorption site located between two A sites. A close inspection of crystal structures reveals that unsaturated Co(II) and Ni(II) sites adsorb both CO2 and CO, whereas the unsaturated Mg(II) sites strongly capture only CO2, not CO. Density functional theory calculations elucidate the discrepancy in CO affinity: Co(II) and Ni(II) form strong pi-back-donating bonds with CO via electron transfer from the d orbitals of the transition metals to the antibonding molecular orbitals of CO, whereas Mg(II) does not participate in electron transfer or orbital overlap with CO. This observation provides new insight into the synthesis of novel functional materials with high CO2/CO separation performance.
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