Probing the Adsorption Sites for CO₂ in Metal Organic Frameworks Materials MIL-53 (Al, Cr) and MIL-47 (V) by Density Functional Theory

Abstract

A comprehensive DFT study of the possible CO2 adsorption geometries in the MIL-53 (Al, Cr) and MIL-47 hybrid organic−inorganic materials has been performed, as a preliminary step to gain a deeper understanding of the CO2 adsorption mechanism in these systems and to help explain the “breathing” effect displayed by the MIL-53 materials. This technique allows us to explore the possible spatial configurations of the CO2 molecules in the MIL-53 systems depending on the size of the pore opening at different loadings. Our results show that the replacement of the μ2-OH groups by oxo moieties in the MIL-47 material leads to fewer, weaker adsorption sites for CO2 onto the framework itself, whereas a larger number of CO2 geometries are possible in both the large and narrow versions of MIL-53. In the narrow pore form, the double interaction, where a CO2 molecule bridges the pore to simultaneously coordinate with 2 μ2-OH groups on opposite sides of a pore wall, was predicted to be the most energetic arrangement at the initial stage of adsorption. Additional configurations include coordination with both inorganic and organic parts of the framework. When the number of CO2 molecule increases, our calculations indicate that the resulting increase in the intermolecular interactions between the adsorbate molecules lead to a significant modification of the CO2 arrangement within the pore and should be an important feature in the explanation of the “breathing” of this material. In the large pore form, an interaction reflecting the opening of the pore is not possible, and so the most likely interaction is one where a CO2 interacts with a single μ2-OH group, in a number of different orientations.

Adsorption of CO2 in metal organic frameworks of different metal centres: Grand Canonical Monte Carlo simulations compared to experiments

“…An O Carboxyl -C CO2 interaction is also possible, with similar O Carboxyl -C CO2 distances to the O μ2 -C CO2 ones being predicted by the simulations. This observation is consistent with those previously obtained by our DFT calculations (Ramsahye et al 2007b). A typical illustration of this CO 2 arrangement observed at high loading is provided in Fig.…”

Low concentration CO2 capture using physical adsorbents: Are metal–organic frameworks becoming the new benchmark materials?

“…Several types of MOFs have been proposed for CO 2 capture, including (i) MOFs with open metal sites, [25][26][27][28][29][30][31][32][33][34][35][36][37] (ii) MOFs without open metals sites, [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] (iii) MOFs with narrow pore size via interpenetration 10,56,57 or shortening the size of the ligands, 10,41 and (iv) MOFs decorated with specific functional groups, including (NH 2 , OH, etc.). 23,51,[58][59][60][61][62][63][64] Functionalization of these types of MOFs may be carried out by post-synthetic modification (PSM) on the open metal sites, 47,[65][66]…”

Doping of Alkali, Alkaline-Earth, and Transition Metals in Covalent-Organic Frameworks for Enhancing CO2 Capture by First-Principles Calculations and Molecular Simulations

“…29 It is worth mentioning that, to date, many works have shown that pure DFT functionals, such as PW91 and PBE, can often give good qualitative results in evaluating noncovalent interactions such as van der Waals (vdW) interactions in molecular complexes and in the solid state. 25,54Ϫ59 For example, Wesolowski et al studied the binding energies and geometries of benzene with O 2 , N 2 , and CO and found that PW91 can give results comparable in accuracy to the high-level MP2/6-31G** method. 43 In addition, the ESP charges were obtained by using the MerzϪKollman scheme (MK).…”