Rigid chemical bonds help MOFs to capture carbon dioxide

"Rapid heat treatment is an easy and versatile technique that can vastly improve the gas-separation performance of the MOF membranes."Kumar Varoon Agrawal, EPFL

Metal-organic frameworks (MOFs) are versatile compounds that host nano-sized pores in their crystal structure. Because of these nanopores, MOFs are now used in a wide range of applications, including separating petrochemicals, mimicking DNA, and removing heavy metals, fluoride anions, hydrogen and even gold from water.

Gas separation, in particular, is of great interest to a number of industries, such as biogas production, enriching air in metal working, purifying natural gas, and recovering hydrogen from ammonia plants and oil refineries. But MOFs have a problem with gas separation.

"The flexible 'lattice' structure of metal-organic frameworks soaks up gas molecules that are even larger than its pore window making it difficult to carry out efficient membrane-based separation," explains Kumar Varoon Agrawal, who holds a chair for advanced separations at EPFL (Ecole Polytechnique Fédérale de Lausanne) Valais Wallis in Switzerland.

Now, scientists from Agrawal's lab have greatly improved the gas separation ability of MOFs by making their lattice structure rigid. They did this by using a novel ‘post-synthetic rapid heat treatment’ method, which basically involved baking a popular MOF called ZIF-8 (zeolitic imidazolate framework 8) at 360°C for a few seconds.

As the scientists report in a paper in Advanced Materials, this method drastically improved ZIF-8's gas-separation performance – specifically in 'carbon capture', a process that captures carbon dioxide emissions produced by the use of fossil fuels to prevent them from entering the atmosphere. "For the first time, we have achieved commercially attractive dioxide sieving performance for a MOF membrane," says Agrawal.

The scientists attribute this improvement to a shrinkage in the lattice parameters, which makes the chemical bonds of MOF more rigid. The essential chemical composition, bonding environment and crystallinity of the material was unaffected by the new procedure.

"Rapid heat treatment is an easy and versatile technique that can vastly improve the gas-separation performance of the MOF membranes," says Agrawal. "By making the lattice rigid, we can efficiently carry out a number of separations."

This story is adapted from material from EPFL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.