DFT Study of MOF801 and ZrTFS based CO2 capture

“To gain insight into the effect of the fluorinated ligand on the MOF structural properties and CO₂ adsorption ability, DFT simulations in the gas phase at the [PBE/MOLOPT] level of theory were carried out. As a first step in the computational analysis, the structures of MOF-801 and ZrTFS were optimized. For MOF-801, the starting geometry was the defect-free crystal lattice with formula [Zr₆O₄(OH)₄(FUM)₆], while for ZrTFS the initial guess [Zr₆O₄(OH)₄(TFS)₆] was considered. To evaluate the effect of the presence of both linkers in the same solid phase on the CO₂ adsorption performance from a theoretical viewpoint, a hypothetical (simplified) structure of minimal formula [Zr₆O₄(OH)₄(FUM)₄(TFS)₂] (PF-MOF) was also built in silico and subsequently optimized. Albeit this computational model cannot be considered fully representative of the real exchanged materials PF-MOF1 and PF-MOF2, it is a useful reference for a comparison between a mixed-linker MOF and its homolinker counterparts. Indeed, the design of computationally representative defective structures is too challenging, as the linker is placed onto the defects randomly. On these models, a joint variable-cell and atomic position optimization was performed (see the Computational Details section). As shown in Table 2, the optimized lattice parameters are in perfect agreement with the experimentally determined values, with the reproduction of the slight unit cell size increment moving from pure fumarate to the fluorinated systems. These MOFs feature tetrahedral and octahedral pores interconnected by triangular windows, with a lattice structure similar to that of UiO-66. (63) The windows are smaller than the pores (Figure 6 and Table 2), and they could be responsible for the percolation of the gas inside the MOF. The gas adsorption process is driven by two main factors: the host–guest chemical affinity and the (high) accessible SSA. Another interesting factor that deeply influences the adsorption performance is the gas percolation and diffusion inside the material. (64) Literature evidence suggests that MOFs featuring high CO₂ chemical affinity but with very small pore windows are poor adsorbents. (65,66) To compare the window and pores of the different models, the surface of the window is defined as the area of the triangle defined by the three hydrogen (or fluorine) atoms, as indicated in Figure S21a, while the pore dimension is evaluated as the distance between opposite hydrogen or fluorine atoms in Figure S21b. The average values of the cavities and window size are summarized in Table 2. Moving from MOF-801 to ZrTFS, the octahedral pore size does not change significantly, while the window area and the tetrahedral pore size are considerably reduced. Moreover, the mixed-linker PF-MOF shows a variable pore width (between 6 and 9 Å; Table 2) that depends on the presence of either TFS^2– or FUM^2–. To gain deeper insights into the preferential CO₂ adsorption sites in these materials, CO₂ was introduced in the computational model and located in four different lattice positions (Figure 6): in the center of the octahedral cavity (1), close to the zirconium ions (2), in the center of the tetrahedral pore (3), and in the middle of the pore windows (4). Afterward, the ensemble was reoptimized. The CO₂ adsorption energy (ΔE_ads) is then calculated as the energy difference between the optimized [MOF + CO₂] ensemble and the separated components; a negative value indicates a favorable interaction, while the opposite holds for positive ΔE_ads values. The most favorable adsorption site is the tetrahedral pore, even if it is slightly smaller than the octahedral one. This energy stabilization comes from a hydrogen bond interaction between CO₂ and a hydroxyl group on the inorganic cluster (Figure 6), which is known to be the most favorable adsorption site for polar and quadrupolar species in UiO-66. (67) Apparently, there is no simple correlation between F-functionalization and MOF–CO₂ interaction. We can only state that the interaction of CO₂ with the linker F atoms is not particularly strong, at least at the computational level used here. To better understand the role of fluorine atoms in increasing the affinity with CO₂, the Mulliken partial charges on F species were evaluated as already reported in the literature for the SIFSIX family MOFs. (68) In particular, the SiF₆^2– anion was taken as the benchmark model, obtaining a partial F charge of −0.76e, in agreement with the values found for similar systems. (68) Subsequently, the partial charge on the fluorine atoms of the H₂TFS linker was calculated at the same level of theory, obtaining an average value of −0.25e. This clearly indicates a reduced C–F bond polarization in H₂TFS if compared to the Si–F bond polarization in SiF₆^2–. Consequently, these results suggest a lower CO₂ affinity of ZrTFS with respect to a general SIFSIX system. If the (ΔE_ads)_T values found for the different materials are compared, an increase in the interaction energy is found when moving from MOF-801 to ZrTFS, in perfect agreement with the Q_st trend reported in Table 1. It is to be noted that the window site in ZrTFS is associated with a positive (ΔE_ads) value, which suggests that CO₂ diffusion through the windows is unfavorable, explaining the low adsorption capacity of this MOF.”

“Figure 6. Top: optimized DFT structure of ZrTFS. The blue and the orange spheres represent the octahedral and tetragonal pores, respectively; the yellow curved line represents the window between the two pores; the numbers indicate the different sites taken into account for the evaluation of CO₂ adsorption enthalpies (ΔE_ads) in reference to Table 2. Bottom: detail of the hydrogen bonds between CO₂ and the OH group for (a) MOF-801, (b) PF-MOF, and (c) ZrTFS.”