Air pollution is a serious global problem, contributing to 8 million deaths annually, with 92% of people living in areas that fail to meet the WHO guidelines of air quality. Air pollutants come in a variety of guises, and from a wide variety of sources. Some are easier to tackle than others, but sulphur dioxide (SO2) and nitrogen dioxide (NO2) gases are both extremely toxic and challenging to clean up. One potential solution is to use metal-organic frameworks (MOFs) to adsorb these gases. MOFs are sponge-like materials that can selectively capture target molecules. However, more than 100,000 known MOFs exist, and it's not feasible to screen each one for its ability to filter each target molecule. A better approach is to develop an understanding of the mechanism of active sites within capture materials. In work recently published in Angewandte Chemie International Edition, an international team of researchers used synchrotron InfraRed and X-rays to investigate the host-guest binding dynamics of a nickel-based MOF on adsorption of NO2 and SO2. Their results are of critical importance for the design and discovery of new functional sorbent materials.
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:
SO2 and NO2 are both reactive, corrosive gases. Removing them from the air is challenging but would have enormous benefits for human health.
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.
Dr Mark Frogley, Senior Beamline Scientist on B22 comments,
For the infrared measurements on B22 we look at the gas binding dynamics through selective gas adsorption experiments. As well as directly detecting the adsorbed gas species in the framework, this technique shows that specific vibrations of molecules in the MOF structure and vibrations of the gas molecules change significantly when there is a strong interaction between the host and guest. This gives rich information about the binding dynamics of each gas with the MOF and can also help us understand the competition for binding between different gases which leads to selective adsorption of particular gases from a mixture.
Dr Sihai Yang at the University of Manchester has a long relationship with Diamond. He won the 2011 Diamond PhD Investigator Award, rewarding outstanding synchrotron research by Early Career Scientists. He says:
This MOF is very exciting because it can selectively remove NO2 and SO2 from a mixture of other gases. Not only that, but it can preferentially remove SO2, which is very unusual. We’ve also been working on recovering the gases once they have been captured, as both SO2 and NO2 are valuable feedstocks for chemicals manufacturing – particularly sulphuric acid and nitric acid.
Dr Yang has worked with many beamlines at Diamond and is currently participating in a four-year collaboration with the staff at B22 to develop a new gas sample environment. A joint Manchester-Diamond PhD student is assisting with that project, with Dr Mark D Frogley co-supervisor from the MIRIAM beamline B22.
As Dr Yang explains,
The MIRIAM beamline B22 with its current setup is excellent for carrying out experiments with inert gases. However, to work with reactive/corrosive gases, we need a new sample environment, and that's what we're working on together. When it's complete, it will be invaluable to continue our collaborative research on this project and will also be useful for other users who work with (e.g.) catalysis. We're hoping to make the new sample environment available later this year.
To find out more about the MIRIAM beamline B22 or discuss potential applications, please see www.diamond.ac.uk/B22 or contact Principal Beamline Scientist Dr Gianfelice Cinque: gianfelice.cinque@diamond.ac.uk.
1. Air Pollution, World Health Organization.
2. New tool calculates NHS and social care costs of air pollution, Public Health England, May 2018.
Han Z et al. A {Ni12}‐Wheel‐Based Metal-Organic Framework for Coordinative Binding of Sulphur Dioxide and Nitrogen Dioxide. Angewandte Chemie International Edition (2022): e202115585. DOI:10.1002/anie.202115585.
Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.
Copyright © 2022 Diamond Light Source
Diamond Light Source Ltd
Diamond House
Harwell Science & Innovation Campus
Didcot
Oxfordshire
OX11 0DE
Diamond Light Source® and the Diamond logo are registered trademarks of Diamond Light Source Ltd
Registered in England and Wales at Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom. Company number: 4375679. VAT number: 287 461 957. Economic Operators Registration and Identification (EORI) number: GB287461957003.
Science