Materials Chemistry: Metal-Organic Frameworks Go Commercial

Record-breaking surface areas, exceptional pore sizes, and cavernous internal channels are properties that thrust metal-organic framework (MOF) materials into the scientific spotlight just over a decade ago. Researchers quickly predicted that these porous crystals—built from metal ions or metal clusters bridged by organic linking groups—would prove useful for gas separation and storage and other applications.

Then in 2003, a team led by Omar M. Yaghi, now at the University of California, Berkeley, reported that a zinc-based MOF could reversibly store a few percent by weight of hydrogen at room temperature and more at lower temperatures (Science 2003, DOI: 10.1126/science.1083440). That development was considered a step toward the practical use of hydrogen as a fuel for electric cars, and it sparked a flurry of activity by scientists working on hydrogen storage, which continues today. A hydrogen uptake as high as 10% by weight has been reported for a copper-based MOF at high pressure and cryogenic temperature.

Researchers today have synthesized thousands of MOFs and related types of framework compounds and demonstrated that many of them are useful for storing and purifying gases, separating hydrocarbons, capturing carbon dioxide from exhaust gas streams, mediating catalytic reactions, and soaking up uranium from seawater. Most of the reports—especially ones detailing the structures and compositions of new MOFs—come from academic research groups. But MOFs are no longer just academic curiosities. A few companies, including Sigma-Aldrich, now sell lab quantities of MOFs. And BASF makes a handful of the compounds on a ton scale, although commercial applications for them remain scarce.

“We manufacture double-digit-ton quantities of some MOFs per production run,” says BASF Senior Vice President Ulrich Müller. He adds that the company’s manufacturing capabilities have already been optimized, so no additional development work is needed; the company is just waiting for demand to pick up.

One application for BASF’s MOFs is to boost the storage capacity of natural gas fuel tanks for vehicles such as buses. The extreme surface area of a MOF—thousands of square meters per gram—and its ability to adsorb methane and other natural gas molecules mean that cylinders packed with MOFs can store roughly twice as much natural gas as unfilled cylinders. The technology has been road tested on passenger vehicles and large trucks.

Müller has been closely following MOF developments coming out of academic labs since the end of the 1990s. Yet these materials continue to amaze him.

“It’s fascinating to see the way tuning and tweaking the metals and linkers can lead to new materials with properties that would have been unimaginable just a few years ago,” Müller says. Translating some of these newer materials into real-world applications seems inevitable, he adds. But as with any new technology, Müller concedes, commercialization takes time.