MATERIALS SCIENCE AND CHEMISTRY

Metal-Organic Frameworks (MOFs) are a new class of materials that in the last decade have seen a paramount growth and are expected to have huge impact on the development of next-generation technologies. They consist of inorganic nodes (i.e. a metal ion or a cluster) connected through organic linkers to form a porous 3D framework. The combination of different nodes and linkers makes MOFs very versatile materials with interesting and promising applications in many fields, including: gas adsorption, catalysis, drug delivery, nonlinear optics.

This project has focused on the ab initio modeling of the adsorptive capacity of the so-called giant MOFs, in particular of MIL-100 (see Figure 1) because of the very large size of the pores (meso-pores: 25–30 Å) and a corresponding huge surface area (3,340 m² g⁻¹) which is three times larger than for commonly used adsorbent materials.

MIL-100, with thousands of atoms in the unit cell (2,788 atoms in the primitive cell that is the one adopted for calculation), represents a tremendous challenge for current ab initio modelling. The use of Tier-0 computer resources provided by the Partnership for Advanced Computing in Europe (PRACE), such as HPC system SuperMUC of the Leibniz Supercomputing Centre in Garching near Munich, was essential to tackle this challenging problem. The periodic ab initio code CRYSTAL was used for the calculations (ab initio hybrid HF/DFT functionals augmented with a London-type correction).

Among the hottest topics in MOFs research in recent years has been the carbon dioxide capture. MIL-100 is characterized by the presence of open metal sites (OMS) exposed at the inner surface of the pores (see Figure 2) that greatly enhance its adsorptive properties. The adsorption of CO₂ was then investigated for the isoreticular family of MIL-100 with different metals, namely: Al, Sc, Cr and Fe. Computed results were in qualitatively good agreement with experimental data thus confirming that the interaction of CO₂ decreases along the series: Cr > Sc > Fe > Al. In contrast with experiment, results for different OMS showed that adsorption sites are rather homogeneous. This could be explained by the presence of defective sites in the real MOF structure.