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

Silver nanoparticles for the elimination of ammonia released to the atmosphere

Researchers from the ITQ-UPV-CSIC, in collaboration with ALBA, have explored the use of silver nanoparticles as catalysts for the selective catalytic oxidation of ammonia, one of the main atmospheric pollutants. Thanks to the CLÆSS beamline at ALBA, researchers proved that the active catalyst for the reaction of ammonia to nitrogen and water is metallic silver, instead of silver cations. These findings will contribute to developing new methods for the elimination of ammonia released to the atmosphere in industry and in diesel vehicles.

Ammonia is one of the main atmospheric pollutants and damages both the human health and the environment. Most ammonia emissions come from fertilizers used in agriculture, but it is also released to the atmosphere in biomass burning, fuel combustion and industrial processes, in which unreacted ammonia escapes into the atmosphere in the exhaust gases.

In the last years, more strict environmental regulations have been intensified with the aim to develop new methods for the elimination of this pollutant. The most promising technology is the selective catalytic oxidation of ammonia to nitrogen and water.

In a recent publication, researchers from the Instituto de Tecnología Química, Universitat Politècnica de València – Consejo Superior de Investigaciones Científicas (UPV-CSIC), in collaboration with ALBA, have explored the use of silver-containing zeolites (microporous aluminosilicates) for the catalytic oxidation of ammonia. Results confirmed that the active site for the reaction is the silver found in the form of metallic nanoparticles at the external surface of the zeolite, whereas silver cations (Ag+)are practically non-active.

Furthermore, the experiment proved that silver nanoparticles present in the active catalyst were dispersed and oxidized to silver cations during the reaction. These findings will allow the scientific community to develop a method for removing ammonia released to the atmosphere in industry and in diesel vehicles.

The experiment, performed at the CLÆSS beamline in the ALBA Synchrotron, allowed to study the catalysts under reaction conditions. The researchers recorded several X-ray absorption spectra (XAS) while submitting the samples to the reactive atmosphere (ammonia and oxygen) at increasing temperatures. Results showed that the silver nanoparticles formed before the reaction were dramatically modified under reaction conditions, being most of them dispersed and resulting in small clusters and cations Ag+.

Read more on the ALBA website

Image: NH3-SCO reaction pathway using Ag-Zeolites