(39a) A Computational Investigation Of Metal-Porphyrin-Frameworks (Mpfs) As A New Nanostructured Adsorbent For Hydrogen Storage

Path Integral Grand Canonical Monte Carlo (PI-GCMC) simulations using standard force fields have been carried out to model the adsorption of hydrogen molecules on metal-porphyrin frameworks (MPFs), a new class of metal organic framework (MOF)-type materials. The results have been compared with hydrogen adsorption in MOF-5. From the simulations, we generate hydrogen adsorption isotherms at 300 K and 77 K in an MPF and MOF-5. All calculations indicate that at 300 K, the MPF adsorbs a higher weight fraction of hydrogen than MOF-5, but is still well short of practical goals. At 77 K, we predict greater adsorption in MOF-5 at low pressures, and greater adsorption at high pressures by the MPF, which can be explained in terms of the competition between energetic (dominant at low loadings) and entropic (dominant at high loadings) contributions to adsorption. Furthermore, Voronoi Tessellations have been used to calculate the accessible volume of the materials. The result is in fairly good agreement with experimental data. A statistical mechanical lattice model can model the adsorption isotherm well at 300 K (error ±0.1%) but qualitatively fails at the lower temperature. Finally, adsorption isotherms predicted by this model show that a weight fraction of adsorbed hydrogen of 6.5 wt% can be met at room temperature and pressure with a material that has an energy of adsorption of about -17.5 kJ/mole and a site volume about 5 ų.