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Metal-organic framework compound sets methane storage record

Sol-gel synthesis boosts the capacity of a common porous material

by Mark Peplow, special to C&EN
December 11, 2017

Chunks of blue material with a thin yellow-green layer on their exposed surfaces.
Credit: Immaterial Labs
These 1-cm3 chunks of HKUST-1 can adsorb huge amounts of methane.

A porous material that sucks up record-breaking amounts of methane could pave the way to more economical natural-gas-powered vehicles (Nat. Mater. 2017, DOI: 10.1038/nmat5050).

Cars powered by methane emit less CO2 than gasoline guzzlers, but they need expensive tanks and compressors to carry the gas at about 250 atm. Certain metal-organic framework (MOF) compounds—made from a lattice of metal-based nodes linked by organic struts—can store methane at lower pressures because the gas molecules pack tightly inside their pores.

So MOFs, in principle, could enable methane-powered cars to use cheaper, lighter, and safer tanks. But in practical tests, no material has met a U.S. Department of Energy (DOE) gas storage target of 263 cm3 of methane per cm3 of adsorbent at room temperature and 64 atm, enough to match the capacity of high-pressure tanks.

A team led by David Fairen-Jimenez at the University of Cambridge has now developed a synthesis method that endows a well-known MOF with a capacity of 259 cm3 of methane per cm3 under those conditions, at least 50% higher than its nearest rival. “It’s definitely a significant result,” says Jarad A. Mason at Harvard University, who works with MOFs and other materials for energy applications and was not involved in the research. “Capacity has been one of the biggest stumbling blocks.”

Only about two-thirds of the MOF’s methane was released when the pressure dropped to 6 atm, a minimum pressure needed to sustain a decent flow of gas from a tank. But this still provides the highest methane delivery capacity of any bulk adsorbent.

A crystal structure that looks like a six-pointed start shows the chemical structure of HKUST-1.
Credit: Science
A view of the crystal structure of HKUST-1. Red is oxygen, blue is copper, green is carbon.

The MOF, HKUST-1, contains copper nodes connected by 1,3,5-benzenetricarboxylate linkers. Theoretical calculations suggest that HKUST-1 should be able to hold 270 cm3 of methane per cm3 at 64 atm. But MOFs are typically made as powders and then compressed into pellets for practical applications. That treatment tends to collapse the MOF’s internal structure, reducing its gas capacity by one-third.

To overcome this limitation, Fairen-Jimenez’s team makes lumps of HKUST-1 using a sol-gel process. The researchers mix the MOF’s precursors in ethanol, centrifuge the particles that form, and then allow them to dry overnight at room temperature. During that time, linker molecules form bridges between the particles to create a monolithic 1-cm3 block that is durable and much more dense than powdered HKUST-1.

Fairen-Jimenez estimates that a car’s gas tank would need about 60 kg of this MOF to operate. So far, he and his team can produce hundreds of grams of high-capacity HKUST-1 using a continuous flow version of their synthesis method, and their spin-out company, Immaterial Labs, aims to achieve kilogram-scale production next year.

In 2013, BASF showcased demonstration vehicles fitted with MOF-based gas storage tanks. But the high cost of the MOF, along with low oil prices, has stymied a commercial roll out. Nevertheless, a techno-economic analysis coauthored by Mason estimates that scaling up production of common MOFs such as HKUST-1 could slash costs to less than $20 per kg, particularly if cheaper solvents and continuous flow methods were used (Energy & Fuels 2017, DOI: 10.1021/acs.energyfuels.6b02510).


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