New high-performance material could reduce air pollution by capturing pollutants

Scientists have developed a new scalable and low-cost material that captures pollutants from the air without absorbing unnecessary moisture.

Researchers from Imperial College London have developed a new material that can more effectively remove pollutants known as volatile organic compounds (VOCs) from the air without taking in unnecessary water molecules in the process.

VOCs are chemicals released by various indoor and outdoor processes including automobile emissions, chemical production, using products such as paints and air fresheners and activities like cooking, smoking and wood burning. These activities severely contaminate air quality and can contribute to respiratory conditions.

In the study, published in Chemical Engineering Journal, scientists from the Department of Chemical Engineering designed a new composite material with the capacity to absorb more VOCs per unit gram of material used.

They found the new composite exhibited a two times higher absorption capacity and 3.6 times higher uptake rate than activated carbon, the current commercial material used for VOC absorption in industry – representing a promising step towards reducing air pollution.

The novel material

New materials known as metal organic frameworks (MOFs) have recently emerged as a new class of absorbent for targeting, capturing, and removing VOCs due to their tuneable chemistry and their high porosity, meaning they are excellent absorbers.

A MOF comprises a metal ion cluster and a bridging linker molecule. The multiple combinations that exist for each building block mean there can be virtually infinite possibilities of the final MOF’s pore shapes, sizes and functionalities.

One type of MOF, known as MIL-101, has the potential to capture larger amounts of pollutants from the air due to its large surface area-to-volume ratio. However, the full potential of MIL-101 is limited by its high preference to stick to water instead of the desired VOCs from the atmosphere, known as hydrophilicity.

By coating the MIL-101 in a chemical known as polydimethylsiloxane (PDMS), the team were able to design a novel hydrophobic composite material that repels water and maximises the VOC absorption from humid air.

According to lead author Luqman Hakim bin Mohd Azmi, hydrophobic PDMS is highly advantageous as it permits vapour, but not liquid to pass through, derived from silicones and therefore nontoxic, whilst also sold at a much lower cost than other hydrophobic polymers.

He said: “Our research is important because the typically used carbon adsorbents in VOC elimination are nonselective and have low uptake capacity under humid conditions. The success of this modification proves that we can develop a material to solve these fundamental deficiencies.”

A step towards cleaner air

The team used a process known as chemical vapor deposition to create the new MIL-101 PDMS composites. This involved spreading the sample in a petri dish, enclosed with the PDMS solution, and heating it to 180 degrees Celsius.

However, as co-author Dr. Pavani Cherukupally explained, “It is difficult to maintain the consistency of coating using chemical vapour deposition because the precision needs to be maintained at atomic to nanoscale level. Other coating methods such as atomic layer deposition could address this challenge, but they tend to be expensive.”

Although their research shows promise, metal organic frameworks such as the novel composites studied in this paper are still currently limited by economic and scalability issues. Therefore, developing advanced manufacturing processes for scalable, cost-efficient MOFs using alternative reactant sources such as plastic waste are needed to ensure this material can address large scale air quality issues across the globe.

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‘Fabrication of MIL-101-poyimenthliiloxane composites for environmental toluene abatement from humid air’ by Azmi et al., published on 01 February 2022 in Chemical Engineering Journal, available online from 8 September 2021.