Revisiting the Crystal Structure of Metal–Organic Framework UTSA-16 for Chemical Consistency

The metal-organic framework UTSA-16, based on K+, Co2+, and citrate as the organic linker and first reported in 2005, is one of the most attractive candidates as a solid adsorbent for postcombustion CO2 capture. Here, we revisit the crystal structure of UTSA-16 on the basis of clear evidence that the amount of potassium has been systematically underestimated in previous literature reports dealing with crystallographic characterization. By combining CHN elemental analysis, inductively coupled plasma optical emission spectroscopy, and thermogravimetric analysis, we find that the actual chemical formula for UTSA-16 is K2Co3(cit)2 & centerdot;8H2O, with a K/Co ratio twice as large as the one assumed in previous literature. Single crystal X-ray diffraction analysis leads to identifying two extraframework positions occupied by previously overlooked K+ ions, which are likely to play a key role in the adsorption behavior of UTSA-16.