Metal-Organic Frameworks can capture toxic air pollutants

The air that we breathe

According to the World Health Organisation1 (WHO), 92% of people worldwide live in places with poor air quality, and outdoor air pollution causes 4.2 million deaths a year, with another 3.8 million caused by indoor air pollution.

Figures published by Public Health England2 showed that the health and social care costs of air pollution in England alone were £42.88 million in 2017 and could reach £5.3 billion by 2035.

There are many different types of air pollution, which arise from a wide range of sources. In the UK, five types are of particular concern: 

  1. The mixture of liquid droplets and solid particles found in the air is called particulate matter (PM). Some PM comes from natural sources (e.g., pollen, sea spray and desert dust), but it also includes dust from car exhausts, brakes and tyres and smoke. PM is classified according to size, with PM2.5 (particles less than 2.5 micrometres across) able to reach and damage the lungs and other organs.
  2. Volatile organic compounds (VOCs) are a range of organic molecules that display similar behaviour in the atmosphere. These include vapours from household products such as air fresheners, cleaning products and perfumes, as well as petrol and solvents.
  3. Ammonia (NH3) gas is mainly released from agricultural sources such as slurry, other rotting farm waste and fertilisers.
  4. Nitrogen oxide (NOx) gases, including nitrogen dioxide (NO2), are mainly created by burning fossil fuels. 
  5. Sulphur dioxide (SO2) is an acidic gas that can irritate airways, particularly in people with asthma.

SO2 and NO2 are both reactive, corrosive gases. Removing them from the air is challenging but would have enormous benefits for human health. 

Can we clean up our act with MOFs?

Metal-Organic Frameworks (MOFs) are sponge-like materials that can adsorb and hold “guest” molecules. By fine-tuning their properties – pore size and geometry, framework topology and chemical functionality – they can be tailored for specific applications, including gas adsorption, separation, catalysis, substrate binding and delivery. MOFs containing open metal sites (OMSs), in particular, can provide highly selective adsorption of target gases. 

However, stable MOFs with OMSs are rare, as are MOF materials that can reversibly adsorb SO2 and NO2. While there are already over 100,000 known MOFs (and over half a million structures have so far been predicted), screening each one individually for its suitability for this application would be time-consuming and costly. A far better approach is to improve our understanding of the mechanism of active sites within capture materials so that we can design or discover new functional MOF materials. This in itself is a challenging task, as host-guest interactions are often dynamic processes, where multiple binding sites of similar energies affect the movement of guest molecules in the pores.

Using synchrotron techniques, an international team of researchers has described the synthesis, crystal structure and gas adsorption and separation properties of a unique {Ni12}- wheel-based MOF that exhibits high isothermal uptake of SO2 and NO2.

Using single crystal X-ray diffraction (SCXRD) at the Advanced Light Source in California and infrared (IR) single crystal micro-spectroscopy at Diamond’s B22 beamline, the team performed dynamic breakthrough experiments that confirmed the selective retention of SO2 and NO2 at low concentrations under dry conditions. Their results show, at a crystallographic resolution, a detailed molecular mechanism with reversible coordination of SO2 and NO2 at the six open Ni(II) sites on the {Ni12}-wheel and at oxygen atom and ligand sites. 

Read more on the Diamond website

Image: Artist’s impression of the unique {Ni12}- wheel-based MOF in action, exhibiting high isothermal uptake of SO2 and NO2.

Credit: Dr Sihai Yang