Tiny compounds might store methane in cars

Lower pressures mean tanks can be lighter and made to fit cars better, Michael Deem says. They may also offer the possibility that customers can tank up from household gas supply lines. (Credit: atTRAZIONE MOTORI/Flickr)

Cars that run on natural gas are touted as efficient and environmentally friendly, but getting enough gas onboard to make them practical is a hurdle.

Tiny synthetic molecules called metal organic frameworks (MOFs) are one possible storage solution.

MOFs are nanoscale compounds of metal ions or clusters known as secondary building units (SBUs) and organic binding ligands, or linkers. These linkers hold the SBUs together in a spongy network that can capture and store methane molecules in a tank under pressure. As the pressure is relieved, the network releases the methane for use.

Because there are tens of thousands of possible MOFs, it’s a daunting task to synthesize them for testing. Researchers have turned to using computers to model candidates with the right qualities.

A team led by Rice University bioengineer Michael Deem went a step further; they used a custom algorithm to not only quickly design new MOF configurations able to store compressed natural gas—also known as methane—with a high “deliverable capacity,” but ones that can be reliably synthesized from commercial precursor molecules.

And here’s a handy bonus: the algorithm also keeps track of the routes to synthesis.

Deem and his colleagues at Rice, the Lawrence Berkeley National Laboratory, and the University of California-Berkeley reported their results in the Journal of Physical Chemistry C.

“MOFs are being commercialized for methane storage in vehicles now,” Deem says.

Fill up at home?

The advantages to using MOF as a storage medium are many and start with increased capacity over the heavy, high-pressure cylinders in current use. The study found 48 MOFs that beat the best currently available, a compound called MOF-5, by as much as 8 percent.

The program adhered to standard US Department of Energy conditions that an ideal MOF would store methane at 65 bar (atmospheric pressure at sea level is one bar) and release it at 5.8 bar, all at 298 kelvins (about 77 degrees Fahrenheit). That pressure is significantly less than standard CNG tanks, and the temperature is far higher than liquid natural gas tanks that must be cooled to minus 260 degrees F.

Lower pressures mean tanks can be lighter and made to fit cars better, Deem says. They may also offer the possibility that customers can tank up from household gas supply lines.

The Deem group’s algorithm was adapted from an earlier project to identify zeolites. The researchers ran Monte Carlo calculations on nearly 57,000 precursor molecules, modifying them with synthetic chemistry reactions via the computer to find which would make MOFs with the best deliverable capacity—the amount of fuel that can be practically stored and released for use.

“Our work differs from previous efforts because we’re searching the space of possible MOF linkers specifically for this deliverable capacity,” Deem says.

The researchers hope to begin real-world testing of their best MOF

“We’re very keen to work with experimental groups, and happy to collaborate,” Deem says. “We have joint projects under way, so we hope some of these predicted materials will be synthesized very soon.”

The DOE Office of Basic Energy Sciences supported the research.

Source: Rice University