MOF captures and converts NO2 into useful product

A METAL-organic framework (MOF) developed at the University of Manchester, UK is capable of selective and reversible capture of nitrogen dioxide (NO₂). It could allow the capture of NO₂ from exhaust streams for conversion into nitric acid, a multi-billion-dollar industry with uses including agricultural fertiliser for crops, rocket propellant, and nylon.

According to Sihai Yang, Senior Lecturer at the Department of Chemistry at Manchester, “this is the first MOF to both capture and convert a toxic, gaseous air pollutant into a useful industrial commodity”.

MOFs are three-dimensional, often porous structures which can be used to capture gases. NO₂ is a toxic air pollutant primarily produced by diesel and biofuel use and the MOF developed at Manchester, MFM-520, could aid air pollution control and reduce the negative impact of NO₂ on the environment.

MFM-520 exhibits high NO₂ uptake at very low partial pressure. It has adsorption capacity of 1.3 and 4.2 mmol/g at 0.001 and 0.01 bar, respectively, at 298K. According to Xue Han, Postdoctoral Researcher at Manchester, this indicates great potential for practical use. Yang said that interestingly “the highest rate of NO₂ uptake by this MOF occurs at around 45^oC, which is about the temperature of automobile exhausts”.

Additionally, the material can capture NO₂ during flow and in the presence of moisture and other gaseous pollutants, such as sulfur dioxide and carbon dioxide. MFM-520 has high selectivity towards NO₂, which according to Postdoctoral Researcher Xue Han is important “because in real life, we are always dealing with gas mixtures”.

Furthermore, despite the reactive nature of NO₂, the researchers found that MFM-520 could be regenerated over multiple capture-release cycles.

Once captured, the adsorbed NO₂ can be released under conventional pressure swing adsorption, regenerating MFM-520 without loss of adsorption capacity or changes in its structure for more than 125 cycles. The researchers also discovered that the captured NO₂ could be converted into nitric acid by submerging saturated MFM-520 and stirring it in air. After the process, the recovered MF could be regenerated under heating and reused without losing NO₂ capture capacity or HNO₃ production for more than ten cycles. The researchers have not yet investigated the regeneration limits of the material.

Martin Schröder, VP of Manchester and Dean of the Faculty of Engineering and Physical Sciences, said: “The global market for nitric acid in 2016 was US$2.5bn, so there is a lot of potential for manufacturers of this MOF technology to recoup their costs and profit from the resulting nitric acid production. Especially since the only additives required are water and air.”

Previously, capturing greenhouse and toxic gases from the atmosphere has been challenging because of the low concentrations involved and the presence of water, which competes and can often negatively affect the separation of targeted gas molecules from others. Additionally, researchers struggled to find practical ways to filter and convert captured gases into useful, value-added products.

MFM-520 offers potential solutions to these challenges. According to Jiangnan Li, a PhD student at the university, “the characterisation of the mechanism responsible for high, rapid uptake of NO₂ will inform future designs of improved materials to capture air pollutants”.

Further work to develop this technology includes scaling up the material’s production before practical use, and shaping and engineering the MOF particles to enable it to be incorporated into devices.

Nature Chemistry: http://doi.org/dj9c