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Researchers improve natural gas storage for transportation

28 October 2015

Metal–organic frameworks with an extraordinarily large internal surface area and flexible gas-adsorbing pores show promise for automotive natural gas storage.

Space-filling models of cobalt-bdp MOF in the collapsed state (left) and CH4-expanded state (right). The purple, grey, blue, and white spheres represent Co, C, N, and H atoms, respectively (image courtesy of the researchers)

Researchers at the Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a variety of metal–organic frameworks (MOFs) with an extraordinarily large internal surface area that also feature flexible gas-adsorbing pores.

This flexibility gives the MOFs a high capacity for storing methane, which in turn has the potential to help make the driving range of an adsorbed-natural-gas (ANG) car comparable to that of a typical petrol-powered car.

“Our flexible MOFs can be used to boost the usable capacity of natural gas in a tank, reduce the heating effects associated with filling an ANG tank, and reduce the cooling effects upon discharging the gas from the ANG tank,” says Jeffrey Long, a chemist with Berkeley Lab’s Materials Sciences Division and the University of California (UC) Berkeley who is leading this research. 

“This ability to maximise the deliverable capacity of natural gas while also providing internal heat management during adsorption and desorption demonstrates a new concept in the storage of natural gas that provides a possible path forward for ANG applications where none was envisioned before.”

The key to the success of the MOFs developed by Long and his colleagues is a 'stepped' adsorption and desorption of methane gas.

“Most porous materials that would be used as adsorbents exhibit classical Langmuir-type isotherm adsorption, where the amount of methane adsorbed increases continuously but with a decreasing slope as the pressure is raised so that, upon discharging the methane down to the minimum delivery pressure, much of it remains in the tank,” Long says.

“With our flexible MOFs, the adsorption process is stepped because the gas must force its way into the MOF crystal structure, opening and expanding the pores. This means the amount of methane that can be delivered to the engine - the usable capacity - is higher than for traditional, non-flexible adsorbents.”

In addition, Long says, the step in the adsorption isotherm is associated with a structural phase change in the MOF crystal that reduces the amount of heat released upon filling the tank, as well as the amount of cooling that takes place when methane is delivered to accelerate the vehicle.

“Crystallites that experience higher external pressures will have a greater free energy change associated with the phase transition and will open at higher pressures,” Long says. “Our results present the prospect of using mechanical pressure, provided, for example, through an elastic bladder, as a means of thermal management in an ANG system based on a flexible adsorbent.”

To test their approach, Long and his colleagues used a cobalt-based MOF hybrid that goes by the name 'cobalt-bdp' or Co(bdp) for cobalt (benzenedipyrazolate). In its most open form, cobalt-bdp features square-shaped pores that can flex shut like an accordion when the pores are evacuated. 

Combined gas adsorption and in situ powder X-ray diffraction experiments performed under various pressures of methane at 25°C showed that there is minimal adsorption of methane by the cobalt-bpd MOF at low pressures, then a sharp step upwards at 16bar, signifying a transition from a collapsed, non-porous structure to an expanded, porous structure.

This transition to an expanded phase was reversible. When the methane pressure decreased to between 10bar and 5bar, the framework fully converted back to the collapsed phase, pushing out all of the adsorbed methane gas.

Long says that it should be possible to design MOF adsorbents of methane with even stronger gas binding sites and higher-energy phase transitions for next generation ANG vehicles. He and his group are working on this now and are also investigating whether the strategy can be applied to hydrogen, which poses similar storage problems.

A paper describing this work is published in the journal, Nature.


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