Gas Storage Frameworks

Recently I came across an article which describes the antibacterial use of nano-scale silica xerogels to deliver NO (which was stored/adsorbed within the xerogel framework) directly to bacterial cells at infection sites in order to kill the bacteria. According to the report, the relatively unreactive silica does not cause the same problems as other NO delivery methods (such as chemical compounds which react or decompose to generate the NO). Using porous inorganic materials as storage containera for gases is not a new idea. Zeolites, alumino-silicate frameworks with porous structures, have routinely been used in this capacity. By varying the zeolite properties such as the size of the internal channels, the diameters of the pore openings, the type and number of cross channels, and the number of acid sites, zeolites can be made to selectively store molecules of various types. They are often used as hydrocarbon traps, and I’ve used them to adsorb water from organic solvents.

There have recently been several articles promoting the use of new porous framework materials for specific gas storage applications. Chemists at UCLA have been working on ZIFs (zeolitic imidazolate frameworks) as a CO₂ storage material. Led by Omar Yaghi, who also invented MOFs (metal organic frameworks) in the 1990s, the team has been busy generating as many structurally different variants as possible and measuring their ability to selectively capture different gas molecules. Their latest material, ZIF-69, is particularly good at holding onto CO₂, opening up the possibility of using this material to sequester the CO₂ released by powerplants. Perhaps even more intriguing, and an area that I expect to see a lot of work in the future, would be the use of these framework materials as fuel storage devices. Both hydrogen and ammonia (an alternative source of hydrogen) have potential as non-hydrocarbon fuels, but their pressurized storage can be tricky. Designing porous materials which allow the storage of these gases at much lower pressures would have huge benefits. Although the potential is there, this is not a slam dunk by any means. You not only have to develop a material that holds on to the gas molecules efficiently, but which also releases those molecules at the appropriate time with a minimum of energy input. I will be interested to see where this research goes.