As the transition from gas-powered to electric transportation intensifies, some researchers believe certain modes like long-haul trucks and planes might need a “bridge” technology instead of a direct conversion. University of Michigan researchers say natural gas technologies could provide that bridge, thanks to their use of compounds called metal-organic frameworks to overcome a major barrier.

“Natural gas is everywhere, and it’s seen as sort of a stepping stone fuel from gasoline to electric or hydrogen,” said Adam Matzger, professor of chemistry, in a news release. But there is a holdback, he explained: “Cost is good. Distribution is good. Storage is the problem.”

Addressing the issues

Natural gas needs to be stored under very high pressure — about 700 times atmospheric pressure. Storing material under those conditions requires specialized equipment and a lot of energy. Optimally using the pressurized fuel is another big challenge.

“If you’re actually going to use it in a vehicle, you’re not going to take it from the high pressure and bring it down to zero,” Matzger said. “Because when the pressure gets too low, you can’t operate the vehicle’s engine.”

Metal-organic frameworks, or MOFs, could have the right characteristics to store and use methane, the main component of natural gas, in vehicles. Molecules could cling to the rigid, porous MOFs to enable storage at low temperatures, and the methane could cycle at the correct pressure for the vehicle’s engine.

Scientists have the ability to design nearly an infinite number of MOFs. The U-M researchers developed algorithms to predict the compounds’ properties and screened for two different kinds of MOFs.

“Previously, when researchers searched for MOFs to store methane, they didn’t separate these two classes of MOFs,” said Alauddin Ahmed, assistant research scientist in mechanical engineering. “We separated those compounds with closed metal sites from those with open metal sites, which have a greater affinity for those methane molecules.” 

Enabling vehicle innovation

The team identified three MOFs that work particularly well for methane storage. They believe this discovery could lead to the production of vehicles equipped with lower-pressure tanks filled with one of the MOFs instead of the pricey high-pressure tanks currently used in natural gas-powered vehicles.

“We set the record for methane storage,” Matzger said. “These MOFs are better than any other methane storage material previously identified, and so that helps us figure out whether we’re getting close to a practical system.”