Metal Organic Frameworks, or MOFs for short, are hybrid organic-inorganic crystalline compounds that currently hold world records for internal surface area and gas storage capacity. MOFs, sometimes referred to as porous coordination networks or porous coordination polymers, derive their unique properties from their ability to form ordered structures with tunable pore sizes and tunable chemical environments within the pores. From this nanoporosity, some MOFs exhibit extremely high surface areas. To put it in perspective, one gram of MOF material can have more surface area than a football field!
A MOF is composed of metal ions (or clusters of ions) and organic linker molecules. Depending on the metal and linker chosen, the structure can be tuned for different applications ranging from size selective catalysis to chemical sensing to drug delivery. At the atomic scale, MOFs can be likened to a three-dimensional grid with metal ion vertices connected to each other by organic ligands.
One of the most promising initial applications for MOFs (and the application with the most research to date) is gas storage. After MOF activation where solvent is removed from the framework structure opening up the pore network, gas molecules can reversibly bind by physisorption to the exposed surfaces. Reversible uptake and release behavior combined with high surface area make MOFs well suited for improving the storage of hydrogen and natural gas.
Concurrent with the shale gas boom, improved natural gas storage opens the door for potential game-changing gas utilization applications. Natural gas vehicles (NGVs) are limited by the amount of gas (primarily methane) that can be practically stored in the on-vehicle tank. Tank size, pressure, shape, and weight are all constraints limiting the amount of natural gas onboard and thus the overall range of the vehicle. With specially designed MOFs lining the insides of a tank, more gas can be reversibly stored at lower pressures increasing driving range and improving NGV safety. Lower pressure tanks also allow for storage tank lightweighting improving gas mileage. Furthermore, storage at lower pressures reduces the complexity and cost of fueling station infrastructure making refueling at the home from domestic gas suppliers an enticing possibility.
For NGVs and other exciting MOF applications to be successful, inexpensive commercial scale production must be reached and stability issues must be addressed. MOFs haven’t skyrocketed up Gartner’s hype cycle quite so quickly or so publicly as some other recent enabling material classes (such as graphene) but momentum is building and technology is accelerating. Hopefully, the “trough of disillusionment” will be more of a blip than a cliff and the MOF-trough will quickly lead to a MOF takeoff. This is where I mercifully end the bad rhymes and give an optimistic MOF sendoff.