Development of ZrTFS for CO2 capture

https://doi.org/10.1021/acsami.2c07640

“The four MOFs were tested as CO₂ adsorbents at T = 0 and 25 °C and p_CO2 up to 1.2 bar. The corresponding isotherms at an ambient temperature are reported in Figure 5a, where the amounts of CO₂ adsorbed are reported as mmol(CO₂) adsorbed per mmol (MOF) to account for the material density variation upon the fluorinated linker insertion. The more conventional mmol g^–1 unit is reported in Table 1 and Figure S12. To quantify the strength of the host–guest interactions, the isosteric heat of adsorption (Q_st) of CO₂ was evaluated from the isotherms recorded at T = 0 and 25 °C, applying a variant of the Clausius–Clapeyron equation (Figure 5b). In order to validate the calculation made on two temperature points, for ZrTFS and PF-MOF1Q_st of CO₂ was also re-calculated using three temperatures (T = −20, 0 and 25 °C; Figures S13 and S20). An alternative approach for the extrapolation of the Q_st values at zero coverage is the virial fitting of the adsorption isotherms. (35) The absolute values and the general trends calculated in our samples are identical to those obtained through the Clausius–Clapeyron equation (Figures S14–S18 and Tables S3–S6). The isosteric heat of adsorption reflects the interaction strength between CO₂ and the inner pore walls of the MOFs. Finally, we estimated the CO₂/N₂ selectivity using the ratio of the initial slopes in the Henry region of the (CO₂ and N₂) adsorption isotherms measured at 25 °C (Figure S19). From the critical and comparative analysis of the results, we can state that the good performance in CO₂ adsorption depends on two factors: BET area and fluorinated linker content. The former is predominant in MOF-801, where the absence of F atoms is compensated by the high SSA value. Consequently, MOF-801 shows the best performance among the MOFs considered in this study in terms of CO₂ loading on a gravimetric basis: 2.42 (10.6 wt %) and 3.51 (15.4 wt %) mmol g^–1 at T = 25 and 0 °C, respectively. On the other hand, the introduction of TFS^2– partially reduces the accessible surface area (and the related CO₂ loading) but considerably improves the thermodynamic affinity of the material for CO₂ and its CO₂/N₂ selectivity. The latter is particularly important in the purification of postcombustion industrial flue gases, where the amount of CO₂ is very low (4–30%). The high Henry and IAST selectivity achieved by PF-MOF2 and ZrTFS, in particular, may be ascribed to the presence of fluorine-decorated ultramicropores that hamper N₂ diffusion (because of its large kinetic diameter) but favor CO₂ adsorption through the beneficial gas–fluorine interaction. On this basis, the best compromise can be found in the PF-MOF2, with a good CO₂ loading of 2.1 mmol g^–1 (9.3 wt %) at an ambient temperature and 1 bar, a Q_st value of almost 30 kJ mol^–1, and the good CO₂/N₂ selectivity of 95 (Henry)/41 (IAST). The same conclusion can be drawn from the isotherms reported in Figure 5a, where PF-MOF2 shows the highest CO₂ uptake: 3.7 mmol_CO2 mmol_MOF^–1 at p = 1 bar. In comparison with other PF-MOFs reported in the literature (Table S7), at ambient (T,p) conditions (25 °C, 1 bar), PF-MOF2 outperforms F4-UiO-66(Ce) (1.5 mmol g^–1 at 293 K), (21) but it is less efficient than F4-MIL-140A(Ce) (2.4 mmol g^–1) (25) and AlFFIVE-1-Ni/NbOFFIVE-1-Ni (2.7/2.2 mmol g^–1, respectively) published by the team of Eddaoudi in 2016 (23,24,62) or than SIFSIX-18-Ni (3.0 mmol g^–1), reported by Zaworotko and co-workers in 2019. (20)”

“Figure 5. (a) CO₂ adsorption isotherms measured at 25 °C on the four MOFs. (b) CO₂ isosteric heat of adsorption as a function of surface coverage.”