New MOFs soak up carbon dioxide from wet flue gas

Generally speaking, ‘flue gas’ refers to any gas coming out of a pipe, exhaust or chimney that is a product of combustion in a fireplace, oven, furnace, boiler or steam generator. But the term is more commonly used to describe the exhaust vapors exiting the flues of factories and power plants. Iconic though they may be, these flue gases contain significant amounts of carbon dioxide (CO₂), a major greenhouse gas contributing to global warming.

One way to ameliorate the polluting impact of flue gases is to take the CO₂ out of them and store it in geological formations or recycle it; there is, in fact, an enormous amount of research under way to find novel materials that can capture CO₂ from these flue gasses. Metal-organic frameworks (MOFs) are among the most promising of these materials, but most MOFs require the ‘wet’ flue gas to be dried first, which is technically feasible but also very expensive – and thus unlikely to be implemented commercially.

In a strange twist of nature – or design chemistry – materials that are good at capturing CO₂ have proven to be even better at capturing water, which renders them of little use with wet flue gasses. It seems that in most of these materials, CO₂ and water compete for the same adsorption sites – the areas in the material's structure that actually capture the target molecule.

Now, scientists led by Berend Smit at EPFL (Ecole Polytechnique Fédérale de Lausanne) Valais Wallis in Switzerland has designed a new material that prevents this competition, isn’t affected by water, and can capture CO₂ out of wet flue gases more efficiently than currently available commercial materials. The scientists report their work in a paper in Nature.

In what Smit calls ‘a breakthrough for computational materials design’, the scientists used an out-of-the-box approach – the tools of drug discovery – to overcome some of the difficulties with material design.

When pharmaceutical companies search for a new drug candidate, they first test millions of molecules to see which ones will bind with a target protein related to the disease in question. The ones that bind are then compared to determine what structural properties they all share. A common motif is established, and that forms the basis for designing and synthesizing actual drug molecules.

Using this approach, the EPFL scientists computer-generated 325,000 materials whose common motif is the ability to bind CO₂. All the materials belong to the family of metal-organic frameworks (MOFs) – popular and versatile materials that Smit and his team have been researching for years.

To narrow down the selection, the scientists then looked for common structural motifs among those MOFs that are very good at binding CO₂ but not water. This subclass was then further narrowed by adding parameters of selectivity and efficiency, until the researchers' MOF-generation algorithm finally settled on 35 materials that show better CO₂ capturing ability from wet flue-gas than current commercially available materials.

"What makes this work stand out is that we were also able to synthesize these materials," says Smit. "That allowed us to work with our colleagues to show that the MOFs actually adsorb CO₂ and not water, actually test them for carbon capture, and compare them with existing commercial materials." This part of the study was carried out in collaboration with researchers at the University of California Berkeley, the University of Ottawa in Canada, Heriot-Watt University in the UK and the Universidad de Granada in Spain.

"The experiments carried out in Berkeley showed that all our predictions were correct," says Smit. "The group in Heriot-Watt showed that our designed materials can capture carbon dioxide from wet flue gasses better than the commercial materials."

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.