A systematic study of the effect of coordinatively unsaturated sites (cus) in the separation of CO 2 /CH 4 and CO/CO 2 /CH 4 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 CO 2 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 CO 2 leads to a low working capacity for CO 2 (because regeneration of the adsorbents is difficult) but a high CO 2 /CH 4 selectivity. With a ternary CO/CO 2 /CH 4 feed mixture, CPO-27-Ni and -Co prefer the adsorption of CO over CO 2 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 CO 2 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 CO 2 /CH 4 and CO 2 /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.
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García et al. (2012) studied this question.