(600c) Is Catenation Beneficial for Hydrogen Storage In Metal-Organic Frameworks?

Metal-organic frameworks, or MOFs, are a new class of materials that have shown promise for gas adsorption, separations, and catalysis. These frameworks self-assemble from organic linker molecules and metal corners to form stable, nanoporous crystals. One particularly exciting application for MOFs is hydrogen storage. To help reduce our dependence on foreign oil, the Department of Energy has set forth gravimetric and volumetric targets for hydrogen storage to make it a viable alternative to hydrocarbon fuels. Previous experiments using MOFs have met some of these suggested targets, although only at cryogenic temperatures. These materials can be constructed from a wide variety of organic linkers, which allows for rational design of the pore environment. There has been substantial debate as to whether smaller or larger pores are desirable for hydrogen storage.

An interesting phenomenon of MOFs is catenation, where two separate frameworks self-assemble within each other. Catenated MOFs effectively decrease the pore, which should increase the heat of adsorption of hydrogen. However, the presence of a second framework occupies a significant amount of free volume, which is then unavailable for hydrogen molecules. In order to clarify whether catenation is useful for hydrogen storage, three isoreticular MOFs (IRMOFs -1, -10, and -16) were selected as candidates for hydrogen adsorption. These IRMOFs have benzenedicarboxylate, biphenyldicarboxylate, and triphenyldicarboxylate ligands, respectively, which lead to uniform increases in pore size. For each IRMOF, hypothetical interwoven and interpenetrated structures were generated on the computer by copying atoms of the original framework and translating these positions along the [111] direction. The interwoven configuration minimized the distance between both frameworks without atomic overlap. The interpenetrated configuration maximized the distance between the two frameworks, shifting the second framework exactly one half of the cavity length in the x, y, and z directions. Grand canonical Monte Carlo simulations were performed at 77 K and room temperature. The simulation results demonstrate that catenation can be beneficial for improving hydrogen storage in metal-organic frameworks at cryogenic temperatures and low pressures but not necessarily at room temperature. One should be cautious in extrapolating from results at low temperatures and low pressures (where catenation is beneficial due to the increased heats of adsorption) to higher temperatures and pressures (where the reduction in free volume from catenation decreases storage capacity). This work examined three examples of the IRMOF series, but the essential physical insights should apply to other structures as well. For hydrogen storage applications at ambient temperature, our results show that, for the three IRMOFs studied, catenation does not improve adsorption and, in fact, decreases gravimetric uptake significantly.