Computational and Experimental Studies on the Adsorption of CO, N₂, and CO₂on Mg-MOF-74

Abstract

Adsorption of carbon monoxide, dinitrogen, and carbon dioxide on the porous metal−organic framework Mg-MOF-74 was investigated by means of a combined methodology comprising variable-temperature infrared spectroscopy and ab initio periodic DFT-D calculations using the CRYSTAL code. Both CO and N2 were found to form nearly linear (Mg2+···CO and Mg2+···NN) adsorption complexes, in contrast with CO2, which forms an angular Mg2+···OCO complex. From IR spectra recorded at a variable-temperature, the standard adsorption enthalpy (ΔH 0) was found to be −29, −21, and −47 kJ mol−1 for CO, N2, and CO2, respectively. Calculated values of ΔH 0, including an empirical correction for dispersion forces, resulted to be in a reasonably good agreement with those experimentally obtained. Calculations also showed the very significant role played by dispersion forces, which account for about one-half of the adsorption enthalpy for each of the three adsorbates, CO, N2, and CO2. The results are discussed in the broader context of the adsorption of the same gases on other MOFs and also on zeolites.

Activation and dissociation of CO2 on the (001), (011), and (111) surfaces of mackinawite (FeS): A dispersion-corrected DFT study

“…1,2 The first step in CO 2 reduction on heterogeneous catalyst surfaces is the activation of the C==O bond and charge transfer for the eventual formation of an anion radical species. 2 A significant number of studies exist in the literature regarding the adsorption, activation, and conversion of CO 2 using both heterogeneous, e.g., on metal/metal oxide surfaces [8][9][10][11] or metal-organic frameworks, [12][13][14][15] and homogeneous reactions, e.g., transition metal complexes. [16][17][18][19][20][21] More recently, transition metal sulfides have attracted significant attention for catalytic applications owing to their low cost, natural abundance, and prominent catalytic features.…”

Analyzing the frequency shift of physiadsorbed CO2in metal organic framework materials

“…7 However, when previous research yields contradictory results, it becomes difficult to gain further insight; e.g. 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.…”

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

“…[ 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. [ 63 ] While the local structure around the CO 2 adsorption site seems to agree well with theoretical efforts, there has been on-going debate within the MOF community as to whether the CO 2 molecule adsorbs in a linear or non-linear geometry. This debate is an important one, as intramolecular bending has strong implications for proposals that have been made to utilize MOFs with open metal sites for the activation and chemical conversion of CO 2 .…”

“…[ 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.…”