Zirconium-Based MOF for CO2 capture

https://doi.org/10.3390/molecules26247620

“The carbon dioxide adsorption isotherms are presented in Figure 7. The shape of the sorption isotherms follows the same trends, regardless of the material studied. As expected, the CO₂/MOF affinity is not very high, as denoted by the rather moderate slope at low relative pressure. Additionally, the inflexion of the curves, which also reflects this affinity, is not very marked. This is more especially true in the case of Muc-Zr MOF and MOF-801, which suggests that these materials weakly interact with carbon dioxide molecules. It can be noted that up to 20 kPa in the case of MOF-801 and Muc-Zr, the adsorbed amount increases in linear fashion versus the relative pressure. This is an indication that the surface appears as homogeneous for CO₂ molecules. In contrast, in the case of MIP-202, there is a noticeable inflexion in the sorption isotherm located at p = 10 kPa. This inflexion can be interpreted as the change between two sorption regimes [53]. This shows the influence of high interaction sites (extra-framework anions (Cl⁻) and amine functions) as well as the impact of the framework on the CO₂ adsorption mechanism.”

“Figure 7. Carbon dioxide adsorption isotherms at 298 K by the prepared MOFs. (Red) Muc-Zr MOF, (green) MOF-801, (blue) MIP-202; (top left) experimental data, (top right) predicted data. Experimental data reported as per square meter. (bottom left) Simulated data reported as per square meter (bottom right).”

“It can be deduced that CO₂ has two different sorption regimes in MIP-202. In MIP-202, extra-framework anions and amino groups borne by the aspartic acid ligand likely are the main interaction for carbon dioxide. Once these sites are saturated, sorption can occur at higher pressures on the other sites in this MOF. In terms of theoretical adsorbed amounts, MOF-801 exhibits a slightly better sorption capacity, compared to MIP-202, which is consistent with the similar theoretical pore volumes (0.32 cm³/g for MOF-801 and 0.27 cm³/g for MIP-202. This is also consistent with the difference observed in terms of specific surface areas of these two materials. The morphological factors invoked above are therefore also responsible for the higher sorption capacity, (i) lower particle size, and (ii) higher external surface area. In general terms, the adsorbed amounts obtained are consistent with the literature. For instance, Grajciar et al. obtained a theoretical CO₂ adsorbed amount of ~130 mg.g⁻¹ in CuBTC at 1 bar. This result is not far from our own results as CuBTC has a higher specific surface area [54]. Sun et al. prepared a PEBA matrix in which MOF-801 nanocrystals were incorporated. They reported an adsorbed amount of 5 cm³·g⁻¹ at p/p° = 0.1 when CO₂ was adsorbed on the MOF-801 crystals alone, which is less than the adsorbed amount obtained in our work [38]. Our results are also consistent with the theoretical sorption isotherms (Figure 7, top right). Indeed, the comparison of saturation capacities for Muc-Zr-MOF and MIP-202 between experimental and theoretical results show a very good agreement, while those for MOF-801 are drastically different due to the presence of mesopores. Differences between experimental and theoretical results at low loadings for MIP-202 regarding the highest affinity for CO₂ can be explained by the use of extra-framework anions (Cl⁻) which are free of solvent and therefore directly available for adsorption in molecular simulations, while they can be already impacted by impurities (water or solvent molecules) present in the pores, which is consistent with its low experimental pore volume. However, the fair consistency between experiments and simulations validates the force fields developed here for the solids. The interaction involved can be better appreciated by plotting the adsorbed amount per square meter in order to discard the influence of the extent of the surface area onto the sorption properties of the materials (Figure 7, bottom). The adsorption isotherm obtained with MIP-202 exhibits a higher affinity, and the inflexion corresponding to the change of sorption process is more pronounced than in the case of the other materials. It is even more striking when looking at Figure 7 (bottom right), where the simulated sorption isotherms have plotted. The differences observed between the adsorbed amounts at 100 kPa can be related to the strong affinity of MIP-202 in contrast to other MOFs (as illustrated below by the theoretical adsorption enthalpy at low coverage), the effect of the confinement (MOF-801 has smaller pores compared to MUc-Zr-MOF) and probably the density/accessibility of the more energetical sites (strongly dependent from the size of the linkers). This representation therefore highlights the interpretations suggested above. The sorption capabilities of our Zr-based MOFs compared well with those of other MOFs, as highlighted by Li et al. and Ghanbari et al. in their reviews [1,17].”