Adsorption Sites and Binding Nature of CO₂ in Prototypical Metal−Organic Frameworks: A Combined Neutron Diffraction and First-Principles Study

Abstract

We report a detailed study of CO2 adsorption in two important metal−organic framework (MOF) compounds (Mg-MOF-74 and HKUST-1). In both MOFs, the open metal ions were identified as the primary binding sites through neutron diffraction measurements. The relatively strong metal−CO2 binding was attributed to an enhanced electrostatic interaction, and vibrational mode analysis shows that the adsorbed CO2 molecule is strongly attached through one of its oxygen atoms while the rest of the molecule is relatively free. This high orientational disorder is the reason for the large apparent O−C−O bond bending angle derived from diffraction measurements. Our calculations give only a small degree of bond bending, suggesting that the CO2 adsorption on the open metal site is still largely physisorption. Interestingly, the overall metal−CO2 binding strength is right in the range which can facilitate both adsorption (CO2 capture) and desorption (MOF regeneration) under typical flue gas conditions.

Analyzing the frequency shift of physiadsorbed CO2in metal organic framework materials

“…an O-C-O angle differing from 180 • . 8 In this work we also allow non-linear distortion of the CO 2 molecule, and we find that the relaxed CO 2 molecules are only slightly bent after adsorbed within MOF74, featuring an O-C-O angle of 179.25 • in Zn-MOF74 and 178.97 • in Mg-MOF74.…”

“…For example, it has been shown that using unsaturated metal centers, such as MOF74 with open Mg, Mn, Zn, Ni, Cu, or HKUST-1 with Cu, results in higher absorption density for hydrogen and faster absorption at small partial CO 2 pressures, the latter of which is highly desirable for CO 2 capturing applications. [5][6][7][8][9][14][15][16][17][19][20][21][22] It has further been shown that iso-structural MOFs with different open metal centers can have very different absorption rates at low pressure. 7 In particular, the electronically similar metal centers Mg and Zn result in a much faster CO 2 uptake rate in Mg-MOF74 than in Zn-MOF74 at a pressure smaller than 0.1 atm.…”

“…while the frequency shift of the asymmetric stretch mode of absorbed CO 2 in Mg-MOF74 has been reported to be blue shifted in one work using IR spectra and B3LYP-D * calculation, 10 it was reported as a red shift in another work using density functional theory (DFT) with local density approximation (LDA) simulations. 8 Furthermore, a clear correlation between the frequency shifts of the absorbed molecules and other absorption properties such as the binding energy or the adsorption site is still missing, which makes it difficult to directly correlate the observed results with the physical nature of the absorption process.…”

Carbon dioxide sorption in a nanoporous octahedral molecular sieve

“…The charge and dipole interactions were found to dominate in this system. Although, it has been shown that the electrostatic interactions dominate when CO2 is near a cation [36] and vdW interactions are prominent when CO2 is far from the cation (away from its equilibrium position) [37]. Nevertheless, we incorporated the vdW-DFT scheme in the energetic calculations to compare with DFT.…”

Understanding Small‐Molecule Interactions in Metal–Organic Frameworks: Coupling Experiment with Theory

“…[ 58,59 ] While it was hypothesized from high initial isosteric heats (−47 kJ mol −1 ), [ 40 ] derived from gas-adsorption measurements, that CO 2 molecules preferentially bind at the open metal site, other methods such as diffraction, IR and Raman spectroscopy, and density functional theory (DFT) have been used to afford direct evidence of the location and orientation of CO 2 molecules binding within the pore. [60][61][62][63][64][65] From neutron powder diffraction (NPD) data, it has been found that CO 2 molecules bind in an "end-on" orientation with Mg-O(CO 2 ) distances and angles that range from 2.24 to 2.39 Å and 125 to 144°, respectively, depending on the CO 2 loading level. [ 60 ] These results agree well with DFT-derived Mg-O(CO 2 ) distances and angles computed at the B3LYP-D level to be 2.31 Å and 129°, respectively.…”