Predicting Hydrogen Adsorption Abilities In Novel Orous Aromatic Frameworks

Great efforts were paid to design and synthesize novel porous aromatic frameworks forhydrogen storage. Utilizing molecular simulation methods to predict capabilities ofhydrogen storages of different materials under different thermodynamic conditions canprovide theoretical guidance to experimental researches and to reduce the experimentalperiod and money costs.Accuracy of current force field is the central issue of molecular simulation, but underlyingparameters for hydrogen adsorption were mostly derived by fitting limited experimentaldata or being directly taken from generic force field parameters, both are limited inaccuracy and reliability.Recently, interaction energies between clusters and hydrogenmolecules could be precisely calculated by Ab initio QM calculations. These calculationsare predictive because their accuracy does not depend on knowledge of existingexperimental data. However, these calculations are limited to small cluster models.Therefore, a combination of first-principle calculation and force field simulation can beachieved by developing force field parameters with high quality ab initio data. Thisapproach is of great significance because it takes advantage of both approaches andpredicts experimentally measurable data independently.In this thesis, we report force field predictions of hydrogen uptakes for PAFs materials thatcontain functional groups of divalent metallic cations and two carboxyls. The ab initiocalculations were performed using RI-MP2 method with the TZVPP basis set and BSSEcorrection on our proposed functional groups and hydrogen molecules. A force field wasdeveloped based on the ab initio energetic data. The resulting force field was applied topredict hydrogen adsorption isotherms at different temperatures and pressures using GCMC method. Each functional group of divalent metallic cations and two carboxylsgroup provided 13 (Mg) or 14 (Ca) binding sites for hydrogen molecules with averagebinding energy of ca. 8 kJ/mol per hydrogen molecule. The predicted hydrogen adsorptionresults were improved remarkably by the functional groups at normal ambient conditions,and exceeded the 2015 DOE target. This work reveals the complex relationship betweenhydrogen uptakes and surface areas, free volumes and binding energies of differentmaterials. According to the analysis on adsorption results, we succeed to establish alangmuir-based isothermal adsorption model representing a clear relationship betweenadsorption amounts and surface area and free volume, which should be quite useful forfurther design in hydrogen storage materials.