Defense of PhD thesis: “Evaluation of the hydrogen adsorption properties of MOFs through computational chemistry techniques” By Diego Armando Gómez H.
In the last years, the efforts to develop a clean and economically viable energy source have been increased. This efforts are a response to the increasing consumption of fuels and the high impact in the environment and the society of the exploration and the use of hydrocarbons or nuclear power. One of the most promising alternatives explored today is the use of H2 as energetic vector. However, there are some limitations related with the production and storage that must be overcomed.
Focused in the problem of the H2 storage, in this research we have studied, with computational chemistry techniques, physical chemical properties that promote the hydrogen adsorption in Metal-Organic Frameworks (MOFs). From the analysis of the results, we have identified some parameters that can be used as a reference to guide the design and synthesis of new MOFs with enhanced properties for the H2 adsorption.
Along the study, the following aspects have been evaluated in detail: i. the nature of the molecular interactions between the adsorbate and the different components of the host and ii. the structural features which promote or limit the adsorption. Such aspects were studied with computational chemistry techniques such as quantum-chemistry calculations (with the semiempirical PM6 and MP2 methods), molecular dynamic and Monte Carlo simulations. The results were analyzed as a function of the physical properties of the materials selected for the study.
In a first step, the interaction of the H2 molecules with the MOF-5 was studied through quantum chemistry calculations. Given that the strongest adsorption sites were located in positions close to the metal atoms (Zn(II)), a further study was performed with four types of metallic centres commonly observed in the inorganic building units of MOFs such as the UiO-66, MFU-1b, MIL-47 y MIL-48A(Pd).
By employing quantum chemistry calculations, particularly the MP2 method, the maximum loadings of adsorbate molecules supported by the adsorption centres were estimated. With these results, the composition of each material and their physical properties, the gravimetric and volumetric uptakes were computed. As general result, it was found that a MOF with potential utility for the hydrogen storage must have a density in a range of 0.7-1.0 g/cm³, a metal atoms density ~0.004 M-atoms/Å³ and an adsorption centre whose topology and accessibility allow the adsorption of more than 3 H2 molecules per metal atom.
Finally, the structural features which promote the H2 adsorption by confinement effects were evaluated in the materials PCN-12, HKUST-1, MOF-505, NOTT-103 y NOTT-112. With this goal, the adsorption centres were located in 3D occupancy maps computed from the molecular trajectories, computed via molecular dynamic simulations. From the occupancy maps it was observed that the confinement effects play a significant role in the H2 adsorption in materials with cavities of diameters around 8.0 Å and with small windows where the weaker adsorption centres are enhanced by the overlap of the attractive potentials.