A cross-section through a flexible MOF shows how the chemical structure shifts when methane is absorbed. The pressure and energy associated with the phase transition can be tuned either chemically or by application of mechanical pressure. (Jarad Mason graphic) Click to enlarge.

The flexible MOF collapses when the methane is extracted to run the engine, but expands when the methane is pumped in at only moderate pressure (500-900 psi), within the range produced by a home compressor. By contrast, compressed natural gas (CNG) vehicles compress natural gas into an empty tank under 250 atmospheres (3,600 psi). Liquefied natural gas (LNG) vehicles operate at lower pressures but require significant insulation in the tank system to maintain the natural gas at -162 ˚C (-260 ˚F) so that it remains liquid.

With the new flexible MOF, “You could potentially fill up at home,” said Jeffrey Long, a UC Berkeley professor of chemistry who led the project.

The driving range of an adsorbed natural gas (ANG) vehicle is determined primarily by the volumetric usable CH₄ capacity of the adsorbent, which is defined as the difference between the amount of CH₄ adsorbed at the target storage pressure (generally 35–65 bar) and the amount that is still adsorbed at the lowest desorption pressure (generally 5.8 bar). With few exceptions, adsorbents that have been investigated in the context of natural gas storage exhibit classical Langmuir-type adsorption isotherms, where the amount of CH₄ adsorbed increases continuously, but at a decreasing rate, as the pressure is raised.

Consequently, it has proved difficult to develop adsorbents with the higher usable capacities needed for a commercially viable ANG storage system. In pursuit of a new strategy for boosting usable capacity, we endeavoured to design an adsorbent with an ‘S-shaped’ or ‘stepped’ CH₄ adsorption isotherm, where the amount of CH₄ adsorbed would be small at low pressures but rise sharply just before the pressure reaches the desired storage pressure. Stepped isotherms have been observed for many flexible metal–organic frameworks that exhibit ‘gate-opening’ behaviour, whereby a non-porous structure expands to a porous structure after a certain threshold gas pressure is reached, but none of these materials have exhibited characteristics beneficial for CH₄ storage applications.

If, however, a responsive adsorbent could be designed to expand to store a high density of CH₄ at 35–65 bar, and to collapse to push out all adsorbed CH₄ at a pressure near 5.8 bar, then it should be possible to reach higher usable capacities than have been realized for classical adsorbents.

—Mason et al.

The new flexible MOFs deliver capacities beyond what was thought possible with rigid MOFs, said Long. Among the other advantages of flexible MOFs, Long said, is that they do not heat up as much as other methane absorbers, so there is less cooling of the fuel required.

The flexible MOF material could perhaps even be placed inside a balloon-like bag that stretches to accommodate the expanding MOF as methane is pumped in, so that some of the heat given off goes into stretching the bag.

In order to advance on-board natural gas storage, Ford Motor Company teamed up with UC Berkeley on this project, with funding from the Advanced Research Projects Agency–Energy (ARPA-E) of the US Department of Energy. Ford is a leader in CNG/propane-prepped vehicles with more than 57,000 sold in the US since 2009, more than all other major US automakers combined.

According to Mike Veenstra, of Ford’s research and advanced engineering group in Dearborn, Michigan, Ford recognized that ANG has the potential to lower the cost of on-board tanks, station compressors and fuel along with serving to increase natural gas-powered vehicle driving range within the limited cargo space.

Natural gas storage in porous materials provides the key advantage of being able to store significant amounts of natural gas at low pressures than compressed gas at the same conditions. The advantage of low pressure is the benefit it provides both on-board the vehicle and off-board at the station. In addition, the low-pressure application facilitates novel concepts such as tanks with reduced wall thicknesses along with conformable concepts which aid in decreasing the need to achieve the equivalent volumetric capacity of compressed CNG at high pressure.

—Mike Veenstra, principal investigator of the ARPA-E project

Long has been exploring MOFs as gas adsorbers for a decade, hoping to use them to capture carbon dioxide emitted from power plants or store hydrogen in hydrogen-fueled vehicles, or to catalyze gas reactions for industry. Last year, however, a study by UC Berkeley’s Berend Smit found that rigid MOFs have a limited capacity to store methane. Long and graduate student and first author Jarad Mason instead turned to flexible MOFs, noting that they behave better when methane is pumped in and out.Long and his colleagues are also now developing flexible MOFs to store hydrogen.

UC Berkeley co-authors are Julia Oktawiec, Mercedes Taylor, Jonathan Bachman and Miguel Gonzalez. To perform structural and thermodynamic studies of the MOFs with and without methane, the team collaborated with Matthew Hudson and Craig Brown of NIST; Julien Rodriguez and Philip Llewellyn of Aix-Marseille University in France; Antonio Cervellino of the Paul Scherrer Institute in Villigen, Switzerland; and Antonietta Guagliardi and Norberto Masciocchi of the To.Sca.Lab in Como, Italy.

Resources

• Jarad A. Mason, Julia Oktawiec, Mercedes K. Taylor, Matthew R. Hudson, Julien Rodriguez, Jonathan E. Bachman, Miguel I. Gonzalez, Antonio Cervellino, Antonietta Guagliardi, Craig M. Brown, Philip L. Llewellyn, Norberto Masciocchi & Jeffrey R. Long (2015) “Methane storage in flexible metal–organic frameworks with intrinsic thermal management” Nature doi: 10.1038/nature15732