DOBDC series MOFs

Figures The ID cylindrical channels present in the M/DOBDC structure. M = Zn, Ni, Co, Mg, Mn, or Fe Metal center green polyhedral C gray ball O red ball. COj isotherms for four of the DOBDC series MOFs are included, as well. (Adapted with permission from Refs. 54 and 46. Copyright (2012 and 2008) American Chemical Society.)...

Instead of the surface area or the free volume, the authors found that MOFs such as Mg/DOBDC and Ni/DOBDC with a high density of open metal sites are promising candidates for CO capture from flue gas in, which CO partial pressure is about 0.1 atm. Caskey et al. [43] found that metal substitution in the DOBDC series can significantly impact their CO capacities in the low-pressure region. The metal substitution effect m be caused by the differences in the ionic character of the metal-oxygen bonds in the DOBDC-series MOFs. Liu et al. found that Ni/DOBDC has a higher CO capacity than NaX and 5A zeolites at 0.1 atm, and 25° C. In addition, water does not affect CO adsorption in the Ni/DOBDC as much as in NaX and 5A zeolites, and it is much easier to remove water from Ni/DOBDC by heat regeneration [44]. Therefore, the Ni/DOBDC can adsorb more CO than traditional zeolites under the same moist conditions. 

This selectivity results from the UMCs (iron or copper) located in the active sites of metalloproteins. Similarly, the UMCs, especially unsaturated iron centers, will significantly facilitate this process in MOFs as well. The M-MOF-74 series are renowned for their high density of UMCs, and Fe-MOF-74 was reported as an efficient adsorbent for O2 storage and O2/N2 separation owing to the existence of unsaturated Fe(II) centers. This MOF was synthesized by air-free procedures, and it has a BET surface area of 1360m -g . After a full activation, Fe-MOF-74 [also known as Fe2(dobdc)] preferentially binds O2 over N2 at all temperatures. 

---