Effect of Pore Size, Solvation, and Defectivity on the Perturbation of Adsorbates in MOFs: The Paradigmatic Mg2(dobpdc) Case Study

Mg2(dobpdc) (H4-dobpdc = 4,4′-dihydroxy-(1,1′-biphenyl)-3,3′-dicarboxylic acid) is an attractive metal organic framework (MOF) because of its unique performances in carbon dioxide capture when combined with aliphatic amines. We have adopted this material as a paradigmatic case for the study of the effect of pore size, availability of open metal sites, and structural defectivity on the sorptive properties of MOFs, with the aim of enabling the design of better sorbents and better tools for their characterization. In this study, the adsorption of CO2, CO, and N2 has been investigated by means of infrared spectroscopy and high-quality periodic quantum mechanical B3LYP-D* calculations. Comparison with literature data on the isomorph MOF-74-Mg, characterized by the shorter dobdc linker, allowed the verification of a small, although appreciable effect of the pore size on the perturbation of the adsorbates. Although it is generally observed that the interaction energy with adsorbates decreases with increasing pore size, Mg2(dobpdc) represents an exception in the IRMOF-74 family, and its interaction energy with adsorbates is greater than that of the smaller pore member of the family. The origin of this counterintuitive behavior was found in the increase of the dispersion energy component and in the lower framework deformation. The other typical aspects that can influence the interaction with guest molecules were investigated: the presence of residual solvents competing for adsorption and structural damage. For what concerns solvation, the affinity for solvents with different polarities was tested. Selective capping of the main adsorption sites (Mg2+) was achieved by preadsorbing CH3OH on the open metal sites. IR spectroscopy of CO adsorption at 100 K was revealed to be able to detect the presence of molecules precoordinated to Mg2+ and then to check the quality of activation procedures for MOFs with open Mg2+ sites. Because of Mg2(dobpdc) air-sensitivity, a full IR characterization was also performed after damage by exposure to a water-saturated atmosphere. Surprisingly, in spite of the drastic structural collapse verified by nitrogen volumetry (decrease of 83% in the surface area), the effect of damage on the infrared spectrum of the MOF was negligible. Similarly, the only change observed in the spectra of the probe molecules was a slight decrease in their intensity after damaging. This means that IR spectroscopy is not a reliable technique to evidence the degradation of this MOF, unlike what reported for other systems.