Role of Structure and Chemistry in Controlling Separations of CO2/CH4 and CO2/CH4/CO Mixtures over Honeycomb MOFs with Coordinatively Unsaturated Metal Sites

A systematic study of the effect of coordinatively unsaturated sites (cus) in the separation of CO2/CH4 and CO/CO2/CH4 mixtures on CPO-27-M (M = Ni, Co, and Zn) and STA-12-Ni metal-organic frameworks was carried out using gravimetric and breakthrough experiments. The separation selectivity and the working capacity of these structures were evaluated as important performance indicators for CO2 separations by PSA. The results demonstrate a remarkable influence of coordinatively unsaturated sites on the selectivity and the working capacity. Particularly, the high affinity of CPO-27-Ni and CPO-27-Co for CO2 leads to a low working capacity for CO2 (because regeneration of the adsorbents is difficult) but a high CO2/CH4 selectivity. With a ternary CO/CO2/CH4 feed mixture, CPO-27-Ni and -Co prefer the adsorption of CO over CO2 due to the strong specific interaction of CO with cus. Surprisingly, STA-12-Ni does not exhibit the same behavior: it is selective for the adsorption of CO2 in a ternary mixture, just like CPO-27-Zn. Among the four MOFs tested in this study, CPO-27-Zn presents the best compromise between the working capacity and the CO2/CH4 and CO2/CO selectivities. The results are discussed in terms of the coordination chemistry of the coordinatively unsaturated metal sites, their acid-base properties, and their accessibility.