Porous Metal-Organic Frameworks for Energy Storage Applications: Design, Synthesis and Mechanism Studies

The self-assembly of metal ions and organic linkers could afford 3-dimensional (3D) porous metal-organic frameworks (MOFs). They are promising materials for clean energy applications including carbon capture, hydrogen storage and methane storage. The primary goal of this research is the synthesis and characterization of new MOFs for these applications, and their structure-property relationship studies based on both experiments and simulations.

Firstly, a stable magnesium MOF with 1-dimensional (1D) channel structure was synthesized. In-situ powder X-ray diffraction studies reveal its interesting phase transitions properties. After removing coordinated solvent at magnesium chains, this MOF can selectively adsorb CO_(2) over N_(2).

Secondly, by varying the conditions in the solvothermal reaction, five MOFs with diverse structures were synthesized from a tetratopic ligand. Hydrogen storage properties were studied for these MOFs. A list of factors including catenation, metal nodes, charge, topology and pore size are evaluated for hydrogen storage application.

In addition, four isostructural MOFs with various functionalized pore surfaces were synthesized from a series of di-isophthalate ligands. These MOFs exhibit a new network-topology and very high hydrogen uptake. They also showed reasonable adsorption selectivity of CO_(2) over CH_(4) and N_(2).

Finally, high pressure methane uptake properties have been studied both experimentally and computationally for the series of isostructural MOFs with varying functional groups. All showed very high methane storage capacity at 298 K, 65 bar. Structure-property relationships were established for these MOFs, and simulations were employed to understand the mechanism of methane storage in MOFs. The role of copper paddlewheels and other adsorption sites for methane was evaluated. By thorough studies and careful analyses of simulation and experimental data, we proposed three novel mechanisms for methane storage in MOFs. Significantly, with the help of the mechanism studies, another two MOFs were designed, synthesized and discovered to have even higher methane storage capacities.

Ligand design has been a powerful tool in synthesizing new MOFs. Besides surface area, pore size has been discovered to be a key factor for gas storage capacities of MOFs. These findings could serve as guidance for rational design of better performing materials for clean energy applications.