**Author details**

*Ab initio* simulations was performed to calculate the exchange barrier (activation energy) and study the reaction pathway for the H2O → CO2 and SO2 → CO<sup>2</sup> exchange processes at the primate adsorption site of the open metal center. It was found that hydrogen bonding of H2O or NH3 molecules with the nearby oxygen of the organic linker facilitates the positioning of the H2O oxygen atom toward the metal center and displacing the preadsorbed CO2 molecule as shown in **Figure 13**. However, SO2 (and other molecules without H atoms) are not able to do so and remain bound at a distant site of carbon ring from metal center. In order to displace the CO2 molecules at metal site, SO2 needs to break away from the attractive force of the initial adsorption sites to move to the meter center and overcomes a high energy barrier (~20 kJ/mol)

This important scientific finding revolutionized the understanding of MOF coadsorption by establishing that the displacement of one molecule by another within porous materials is a complex process that the energetics consideration alone cannot successfully predict. In other words, the binding energy at the most favorable adsorption site is not a sufficient indicator of the molecular stability in MOFs and kinetics of exchange process must be considered.

Vibrational spectroscopy has been proved to be the very informative technique to investigate the interaction of small gas molecules with metal organic frameworks. By examining subtle changes in the spectra of both adsorbate and adsorbent, insightful details regarding the adsorption mechanism are revealed. With the help of theoretical calculation, which provides direct access to many properties of the system, the experimental models are validated and a

For H2, although free molecule is IR inactive, the stretching mode is activated and becomes observable once the molecule is polarized by binding to the surface. A wealth of information for the interaction details, i.e., binding site and geometry, interaction potential can be extracted

For CO<sup>2</sup> molecules, both the perturbation of stretching and bending mode convey important information for the nature of interaction. For physical adsorption with lower binding energy (<50–60 kJ/mol), the stretching mode suffers a small shift (<15 cm−1) compare to gas phase value and the bending mode is spitted due to the loss of degeneracy. If the molecules are chemically adsorbed with a high adsorption heat over 60–70 kJ/mol, IR adsorption features of new species such as carbamate can be observed. The structural modifications for functional groups are

For the reactive molecules such as H2O, O2, H2S, SO, and NO adsorbing into MOFs, the crystalline structure is strongly modified and even become degraded. By examining the difference spectra before and after adsorption, the weak point of the complicated MOFs structure can be identified and reaction pathway can be also unveiled, which is crucial to

than water molecules (~13 kJ/mol) to remove the preoccupied CO2 molecules.

complete understanding of the adsorption behaviors can be derived.

by analyzing the peak position, intensity, and width.

reflected by tracking the spectroscopic signatures.

design robust structure.

**3. Conclusion**

3214 Metal-Organic Frameworks

Kui Tan\* and Yves Jean Chabal

\*Address all correspondence to: kuitannk@gmail.com

Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas, USA
