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Radha Kishan Motkuri, a PNNL material scientist who has worked on porous materials for nearly 20 years, wondered if a new more efficient metal organic framework (MOF) synthesis process could be developed.

MOFs are nanostructured porous materials with a high capacity to absorb water, carbon dioxide, fluorocarbon refrigerants, and other gases. They are a critical ingredient in many energy-efficient and green technologies, including heating and cooling, carbon capture, natural gas storage, adsorption cooling, refrigeration, nuclear fuel recycling, and geothermal applications, to name a few. However, their commercial application often requires many kilograms of MOFs and MOFs are typically made in small batches.

Steaming Versus Boiling: A New Spin on Cooking Up MOFs

Conventional MOF synthesis involves mixing inorganic metal salts, organic linkers, and liquid solvents together, and then heating them under pressure to create small batches of MOFs. Rather than boiling the solvents, metals, and organic linkers together, Motkuri and his team use a dry gel conversion technique where the metals and organic linkers are suspended above the solvents—like broccoli is cooked in a steamer basket, suspended above water boiling on a stove. "I was concerned it might not work because the boiling point and vapor pressure of organic solvents and water are so different," said Motkuri. "But we tried it and achieved impressive results in terms of yield, time, and cost of the MOF. Even better, the method was successfully extended to the synthesis of wide variety of MOFs."

The vapor-assisted dry gel conversion method with the solvent (brown) and a suspended pouch with MOF reagents. The center images show the top view (graphic and photograph) of the autoclave used to produce the MOF in the laboratory.

After just 24 hours, the MOFs showed optimal performance; conventional synthesis takes 72 hours. And the MOF yield was approximately 70-80 percent, compared to about 40-50 percent in conventional synthesis. Physically separating metals from solvents in MOF synthesis reduces unwanted side reactions that cause impurities, and the number of unreacted or semi-reacted precursors that don't form into fully functioning MOFs. Using this method, the solvents are left perfectly clean after one batch and researchers can recycle it up to three times to produce three batches of MOF with same performance capacity. The dry gel conversion technique not only creates more MOFs faster, but it also costs less than conventional synthesis techniques. The team showed this with their general cost analysis that suggests the dry gel conversion technique lowers the price significantly at scaled up production levels.

This research was funded by Pacific Northwest National Laboratory and the U.S. Department of Energy’s Geothermal Technologies Office. For more information see the report in Scientific Reports.

PNNL Research Team: Atanu Kumar Das, Rama Sesha Vemuri, Igor Kutnyakov, Peter McGrail, and Radha Kishan Motkuri