Over the past thirty years, natural gas production and consumption has steadily increased and is projected to continue through at least 2050. In the transportation sector, the difficulty in utilizing methane, the main component of natural gas, as opposed to gasoline is that methane is a gas with a density of 6.56 x 10-4 g/cm3 at STP whereas gasoline is a liquid with a density of 0.77 g/cm3. This results in dramatically lower volumetric energy density for natural gas. In order to increase this, both liquified natural gas (LNG) and compressed natural gas (CNG) systems have been investigated. However, the former requires the use of cryogen and the latter prohibitively high storage pressures. Adsorbed natural gas (ANG) systems have also received considerable recent attention with metal-organic frameworks showing the greatest promise. In terms of cost, scalability, and capacity, HKUST-1, with a volumetric uptake of approximately 250 cm3/cm3 is one of the best adsorbents reported. However, its capacity is significantly lower than the U.S. Department of Energy (DOE) target for methane storage by adsorbent-based materials of 350 cm3/cm3. In order to address the shortcomings of metal-organic frameworks we have begun investigating a related class of porous materials in porous coordination cages. These particular porous cages are a group of materials which utilize bimetallic paddlewheels and organic linkers to form zero-dimensional cages. The pore windows of these cages can be tuned at the angstrom scale via ligand functionalization. Utilizing carbazole as the base ligand, we have prepared a series of porous cages with a variety of metal cations. The pore window of the carbazole-based cages are the optimal size to store four methane molecules per window with eight windows per cage, totaling 32 methane molecules per cage. Functionalization of the ligand in the N-position with substrates ranging from alkyl chains, cyclic, aromatic, and heteroaromatic ring systems allows for the tuning of properties exhibited by these cages including thermal stability, crystallinity and solubility.

High-Pressure methane storage in carbazole-based porous cages

256th National Meeting (2018)

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